Abstract
Twodimensional electronic states at surfaces are often observed in simple wideband metals such as Cu or Ag (refs. ^{1,2,3,4}). Confinement by closed geometries at the nanometre scale, such as surface terraces, leads to quantized energy levels formed from the surface band, in stark contrast to the continuous energy dependence of bulk electron bands^{2,5,6,7,8,9,10}. Their energylevel separation is typically hundreds of meV (refs. ^{3,6,11}). In a distinct class of materials, strong electronic correlations lead to socalled heavy fermions with a strongly reduced bandwidth and exotic bulk ground states^{12,13}. Quantumwell states in twodimensional heavy fermions (2DHFs) remain, however, notoriously difficult to observe because of their tiny energy separation. Here we use millikelvin scanning tunnelling microscopy (STM) to study atomically flat terraces on Uterminated surfaces of the heavyfermion superconductor URu_{2}Si_{2}, which exhibits a mysterious hiddenorder (HO) state below 17.5 K (ref. ^{14}). We observe 2DHFs made of 5f electrons with an effective mass 17 times the free electron mass. The 2DHFs form quantized states separated by a fraction of a meV and their level width is set by the interaction with correlated bulk states. Edge states on steps between terraces appear along one of the two inplane directions, suggesting electronic symmetry breaking at the surface. Our results propose a new route to realize quantumwell states in strongly correlated quantum materials and to explore how these connect to the electronic environment.
Similar content being viewed by others
Main
Heavy fermions form a unique class of quantum materials that exhibit exceptional properties related to their narrow electronicband dispersion^{12}. Previous experiments on heavy fermions showed that reducing the dimensionality leads to enhanced electronic correlations and strong coupling superconductivity^{15,16}. Furthermore, narrow surface bands with a Dirac dispersion were found in a semiconducting heavy fermion^{17}. In spite of intensive investigations, 2DHFs have not been observed at surfaces of superconducting compounds and no quantumwell states owing to lateral confinement of heavy electron states have been realized.
We investigate the heavyfermion superconductor URu_{2}Si_{2}, which exhibits correlated narrow electron bands crossing the Fermi level and undergoes a transition to the HO phase characterized by an as yet unknown order parameter^{14}. A partial gap opens in the electronic band structure below T_{HO} = 17.5 K, out of which an unconventional superconducting state develops below T_{c} = 1.5 K (refs. ^{14,18}). The surface electronic states observed until now mostly have small effective masses^{19,20,21,22}. Using STM to investigate smallsized, atomically flat terraces on the Uterminated surface of URu_{2}Si_{2} at subkelvin temperatures, we observe clearly 2DHFs with an effective mass 17 times the free electron mass, as well as quantization owing to lateral confinement and uncover the surface–bulk interaction. In Fig. 1a, we present a STM image of terraces of Uterminated surfaces on URu_{2}Si_{2} (see Extended Data Fig. 1 for the surface termination). Our starting point is the tunnelling conductance obtained in a small range of a few mV around the Fermi level and at a temperature of 0.1 K, well below the superconducting critical temperature, shown in Fig. 1b (see Extended Data Fig. 2 for a larger bias voltage range).
Quantumwell states by confinement
We focus on the tunnelling conductance along the white line shown in Fig. 1a. Along this line, we identify four terraces of different sizes, L_{1} ≈ 2 nm, L_{2} ≈ 5.5 nm, L_{3} ≈ 20 nm and L_{4} ≈ 38.5 nm (Fig. 1c). We observe a strong bias voltage and position dependence of the tunnelling conductance, which is different for each terrace as illustrated in Fig. 1d. We show in Fig. 1e representative tunnelling conductance curves at each terrace, in which we identify a set of regular peaks.
Let us analyse the terrace L_{3} (dashed rectangle in Fig. 1a). We present a symmetrized map of the tunnelling conductance in Fig. 2a (see Methods for further details). We identify a set of peaks in the tunnelling conductance, which evolve in both position and bias voltage. Subtracting the features at ε_{−} and ε_{+} (details provided in Methods and in Extended Data Figs. 3 and 4), we obtain the pattern shown in Fig. 2b, which shows the lateral quantization of 2DHFs. The quantization pattern for confined electrons resembles the Fabry–Pérot expression for an interferometer made by partially reflecting mirrors^{23} (reflection coefficient r and the phase shift ϕ are the free parameters; details in Extended Data Fig. 4 and results on different terraces in Extended Data Fig. 5). Lateral quantization results from interfering wavefunctions partially reflected at steps. Quantized levels obtained from the Fabry–Pérot expression are the white dots in Fig. 2a,b, whose position coincides well with the peaks in the conductance pattern observed in the experiment. In Fig. 2c, we plot as points the position of the peaks as a function of the wavevector k and as a line the dispersion relation E = E_{0} + ħ^{2}k^{2}/(2m*), in which m* is the effective mass and E_{0} the bottom of the band. We obtain E_{0} = −2.3 meV and m* = 17m_{0}, with m_{0} the free electron mass, that is, we find that the 2DHF is derived from a massive surface electron state. A detailed comparison of the tunnelling conductance versus position with the square of wavefunctions confined by a lateral potential leads to excellent fits, shown in Fig. 2d. The phase shift ϕ determines the position and energy of the peaks (white dots in Fig. 2a,b) and the best account of our observations is obtained with ϕ = −π. We find values around r ≈ 0.2, which slightly increase when approaching E_{0} (Fig. 2e). The low value of r is also found in surface states of simple metals; for example, r is between 0.2 and 0.4 in Ag and Cu (refs. ^{23,24,25}). However, the energy dependence (Fig. 2e) at the surface of URu_{2}Si_{2} is completely different to that in usual metals. Although r varies mildly in Cu or Ag in the range of a few eV (see dashed line in Fig. 2e and refs. ^{23,24,25}), here we observe instead that r decreases markedly in a range of a few meV (points in Fig. 2e). We can reproduce the observed dependence of r versus bias voltage assuming a potential well^{23,24} (continuous line in Fig. 2e; details provided in Methods and Extended Data Figs. 3 and 4). It is also insightful to trace the tunnelling conductance as a function of the bias voltage at the centre of the terrace in Fig. 2b (circles in Fig. 2f) and compare it to the expectation for r ≈ 1 (dashed line in Fig. 2f) and for r ≈ 0.4 (continuous line in Fig. 2f). We see that, for a reduced r, both the periodicity and shape of the tunnelling conductance are well explained in an energy range of a few meV, that is, two orders of magnitude below the energy range observed in conventional metals^{1,2,3,4,23,24,25,26}.
To further investigate the quantized levels, we have fitted each peak to a Lorentzian function, whose width Γ (black arrow in Fig. 2b) provides the lifetime τ (Fig. 2g) of the quantumwell states. Taking a twodimensional electron gas, we expect \(\hbar /\tau ={\Gamma }_{0}+( {E}_{0} /4\pi ){\left[(E+ {E}_{0} )/{E}_{0}\right]}^{2}\)\( {\rm{ln}} (E+ {E}_{0} )/{E}_{0} {\rm{ln}}(2{q}_{{\rm{TF}}}/{k}_{{\rm{F}}})1/2 \), with ħ the reduced Planck’s constant, q_{TF} = 0.0906 Å^{−1} the Thomas–Fermi screening length and k_{F} the Fermi wavevector^{27} (dashed line in Fig. 2g). Our data are not well reproduced by this expression. Taking instead \(\hbar /\tau ={\Gamma }_{0}+( {E}_{0} /4\pi ){\left[(E+ {E}_{0} )/{E}_{0}\right]}^{2}\) with E_{0} = −2.3 meV and Γ_{0} ≈ 60 μeV (continuous line in Fig. 2g), we find a much better account of our data^{27,28}. The latter expression takes into account the connection between the 2DHF and bulk states, showing that, in our experiments, the lifetime τ is set by the decay of the 2DHF into heavyfermion bulk states. Quantumwell states sense the bulk correlations, given by the quadratic energy term in ħ/τ. This has been observed in surface states of noble metals, monolayers of Pb and in Sb. However, in those cases, the energy range was three orders of magnitude above the one we discuss here^{3,6,11,28}.
From the obtained value of Γ_{0} ≈ 60 μeV, we estimate the lifetime of the ground state as τ_{0}(E_{0} = −2.3 meV) = ħ/Γ_{0} ≈ 11 ps. Similarly, the lifetime of states close to the Fermi level is τ(E = 0) = ħ/Γ(E = 0) ≈ 3 ps. We can also estimate a value for a mean free path, ℓ_{0} = v_{F}ħ/Γ(0) ≈ 0.14 μm, with v_{F} the Fermi velocity of URu_{2}Si_{2}. This value is on the same order of magnitude as those observed in ultraclean URu_{2}Si_{2} single crystals^{29}.
To vindicate the existence of a heavyfermion surface state, we performed density functional theory calculations of the surface band structure of a slab of URu_{2}Si_{2} (Extended Data Fig. 6). We find a shallow, Uderived felectron band with a flat dispersion relation compatible with our experiments around the X point of the simple tetragonal Brillouin zone. The bulk electronic spectrum is gapped in this part of the Brillouin zone^{21,22}. The rest of the Brillouin zone provides surface states with much smaller effective masses.
Onedimensional edge states
In Fig. 3, we show that the 2DHF is peculiarly modified at the steps separating terraces at which onedimensional edge states (1DESs) appear. 1DESs were previously observed in simple metals, in which a gap opens at the step and is filled with a very large density of states at E_{1DES} by the 1DES^{3,8,11,24,25,30}. The width of the conductance peak at E_{1DES}, η_{1DES}, results from inelastic scattering into bulk states^{24,25}. Here we find that the features in the tunnelling conductance completely change at a step (Fig. 3a,b). We find a high peak at E_{1DES} ≈ −0.38 meV (Fig. 3b,c) in steps, but notably only along one of the two equivalent inplane axes, that is, an inplane symmetry breaking occurs. The HO is known to cause breaking of the bodycentred translation symmetry^{14}, leading to inequivalent electronic properties in subsequent U layers, which is consistent with our measurements (details in Methods and Extended Data Fig. 7). This should reduce inelastic scattering and favour the formation of a 1DES. Here, however, we observe a rather unique situation in which the edge state is either observed or not, on two crystallographically equivalent inplane axes. This indicates spontaneous symmetry breaking of the fourfold rotational symmetry close to the surface. Such inplane symmetry breaking has been proposed^{31} for bulk URu_{2}Si_{2} but has been difficult to detect. This symmetry breaking would cause an orthorhombicity given by a tiny difference in the basalplane lattice constants a and b (\(\frac{ \,ab\, }{(a+b)}\approx 1{0}^{5}\))^{32}. Other experiments could, however, not confirm such inplane symmetry breaking^{33}, which could be favoured by defects. Recent group theory considerations and nuclear magnetic resonance (NMR) data propose that the HO state can belong to four space groups, #126, #128, #134 and #136, all having the same crystal structure as the hightemperature state^{34}. NMR experiments suggest the presence of fourfold symmetry at Ru, Si and U sites, narrowing down the most probable choice to #126 (refs. ^{34,35}). Notably, although these measurements gave absence of fourfold symmetry breaking in bulk URu_{2}Si_{2}, our STM data sensitive to individual uranium layers clearly show an inplane symmetry breaking in the 1DES. We note that a possible source of changes in the electronic structure that could also affect the 1DES is modifications of the valence of U edge atoms^{21}. Nonetheless, the symmetrybreaking in the 1DES suggests a deeper origin, which pinpoints that the 1DES serves as a sensitive probe of fundamental electronic properties of the nearsurface U lattice.
Superconductivity
At the energy range below the superconducting gap, Δ_{SC} ≈ 200 μeV, we observe that there is a large zerobias conductance (see Fig. 4a). There are indications for unconventional superconductivity in bulk URu_{2}Si_{2}, with a dwave symmetry order parameter^{29}. This can contribute to the suppression of the superconducting features in the tunnelling conductance, but it hardly leads to the zerobias conductance observed in our experiment. Similar smallsized superconducting features are found in other heavyfermion superconductors, such as CeCoIn_{5} or UTe_{2}, and remain difficult to explain^{36,37,38}. Most notably, macroscopic measurements such as specific heat or thermal conductivity provide, in all these systems, a negligible zerotemperature extrapolation of the electronic density, suggesting that the superconducting density of states at the Fermi level is very small^{12}. The 2DHF is strongly coupled to the superconducting bulk states and the proximity effect from bulk superconductivity should provide only a small amount of states at low energies. However, it is important to consider the coupling of the 2DHF to strongly energydependent resonant states giving peaks in the tunnelling conductance as well. The concomitant broadening then leads to a large zerobias tunnelling conductance. Using a model that takes this into account (see Methods and Extended Data Fig. 8), we can understand the main features of the tunnelling conductance and follow the superconducting gap with temperature (Fig. 4b,c). This solves the discrepancy between macroscopic and surface experiments and shows the relevance of twodimensional electronic states to understanding the tunnelling conductance.
In summary, we have observed 2DHFs in terraced surfaces inside the HO phase of URu_{2}Si_{2}. The 2DHF exhibits quantumwell states with energy separation of fractions of a meV when confined between steps. The 2DHF is connected to the bulk heavyfermion states. At steps, we observe a 1DES, which shows inplane electronic symmetry breaking and inequivalent electronic arrangement in subsequent U layers in the HO phase. The discovery of 2DHFs and related confined states opens new possibilities to study the interplay of quantized heavyfermion states and unconventional superconductivity, as several heavyfermion materials show unconventional superconductivity in the bulk, often coexisting with other longrange ordered phases. Apart from URu_{2}Si_{2}, there are other heavy fermions, such as CeCoIn_{5}, UBe_{13}, UPt_{3} or UTe_{2}, in which the proposed superconducting states are spinsinglet dwave or spintriplet pwave and fwave states. These could exhibit 2DHFs and the associated edge states could incorporate excitations with unique properties such as Majorana fermions following nonAbelian statistics. Furthermore, because the source of quantization is lateral confinement, correlated quantumconfined states can be obtained in nanostructures built on the surface by manipulation of adatoms or by controlling layer growth in thin films^{15,16,39,40}. This opens new avenues to generate, isolate and manipulate excitations in unconventional superconductors.
Methods
STM experiments
Single crystals of URu_{2}Si_{2} were grown by the Czochralski technique in a Tetra Arc Furnace. We scanned samples for a low residual resistivity and a high superconducting critical temperature, close to 1.5 K. Such samples were then cut in a bar shape with dimensions 4 × 1 × 1 mm^{3}, with the long distance parallel to the c axis. We mounted the samples on the sample holder of a scanning tunnelling microscope. The scanning tunnelling microscope was mounted in a dilution refrigerator. The resolution in energy of the setup was tested by measuring the superconducting tunnelling conductance with the tip and sample of swave superconductors Al and Pb down to 100 mK (ref. ^{41}). Details of imagerendering software are provided in refs. ^{42,43}. The scanning tunnelling microscope head features a lowtemperature movable sample holder, which is used to cleave the sample at cryogenic temperatures^{41,44}. At the same time, and importantly for this study, the sample holder allows modifying many times the scanning window. The terraces discussed in this work were found in three different samples, after studying hundreds of fields of view.
Surface termination in URu_{2}Si_{2}
We focus on Uterminated surfaces. In Extended Data Fig. 1a, we show the URu_{2}Si_{2} crystal unit cell highlighting the U, Si and Ru planes; their interlayer distances are indicated in units of the caxis lattice parameter. In Extended Data Fig. 1b–d, we show STM images corresponding to different surface terminations. These surfaces are all obtained after cryogenic cleaving. On the surfaces full of squareshaped terraces, we find the results obtained in the main text. An example is shown in Extended Data Fig. 1b. All the observed terraces are separated by c/2 ≈ 4.84 Å. In Extended Data Fig. 1c,d, we show terraces with a triangular shape, in which we do not observe the phenomena discussed in the main text. Here the distance between consecutive terraces is about 0.11c, about 0.39c or about 0.61c, which correspond, respectively, to the three possible distances between U–Si planes (coloured arrows in Extended Data Fig. 1a). Therefore, we see that the surfaces with terraces having a triangular shape correspond to Si layers, sometimes with a U layer in between. By contrast, the surfaces with terraces having a square shape are U terminated. Atomically resolved images inside terraces (Extended Data Fig. 1e) provide the square atomic U lattice with an inplane constant lattice of a = 4.12 Å . In Extended Data Fig. 1f, we show a typical atomicsized image on Siterminated surfaces. We do not observe atomic resolution and have sometimes seen circular defects. Defects in the Uterminated surfaces are very different, as shown in Extended Data Fig. 1g–o. We distinguish two distinct types of defect. The defects can be either pointlike protrusions (Extended Data Fig. 1g,h) or troughs (Extended Data Fig. 1i). Sometimes, defects are arranged in smallsized square or rectangular structures (Extended Data Fig. 1j–o). Most of these defects are probably because of vacancies or interstitial atoms in layers below the U surface layer.
Tunnelling conductance in the HO state
The tunnelling conductance of the HO state has been discussed in refs. ^{45,46,47,48,49}. We have reproduced the results, as shown in Extended Data Fig. 2a,b. The tunnelling conductance results from simultaneous tunnelling into heavy and light bands, as in other heavyfermion compounds^{50,51,52}. The red line in Extended Data Fig. 2a for T = 18 K follows a Fano function
in which A is a constant of proportionality, q is the ratio between two tunnelling paths and E_{Fano} is the Fano resonance energy with width \({\Gamma }_{{\rm{F}}}=2\sqrt{{(\pi {k}_{{\rm{B}}}T)}^{2}+2{({k}_{{\rm{B}}}{T}_{{\rm{K}}})}^{2}}\), T_{K} being the Kondo temperature^{45,46}. For the fit, we include an asymmetric linear background owing to the degree of particle–hole asymmetry in the light conduction band^{45,53}. To account for the thermal broadening, we convolute the result with the derivative of the Fermi–Dirac distribution. We find q = 0.8 ± 0.5, E_{Fano} = 3 ± 1 mV, Γ_{F} = 22 ± 1 mV and T_{K} = 90 ± 5 K, consistent with previous reports^{45,46}.
Inside the HO phase (red line in Extended Data Fig. 2b), we use the same Fano function, multiplied by an asymmetric BCSlike gap function with an offset δ_{E}
The resulting function is convoluted with the derivative of the Fermi–Dirac distribution function. We find δ_{E}(4.1 K) = 1.5 ± 0.5 meV and Δ_{HO}(4.1 K) = 4.0 ± 0.5 meV, consistent with previous reports^{45,46}.
Note that we also observe further features at lower temperatures and smaller bias voltages (Extended Data Fig. 2c). The red line in Extended Data Fig. 2c is a fit described below. The features above the superconducting gap can also be roughly obtained by using two Lorentzians at ε_{−} and ε_{+} and an asymmetric background. Probably, the peaks at ε_{−} and ε_{+} are because of avoided crossings in the band structure of the 2DHF at very low energies. We notice that the small feature at ε_{+} occurs at a very similar energy range as a kink in the band structure found at the surface of Thdoped URu_{2}Si_{2} (refs. ^{48,49}). In the calculations we show below, we can identify features in surface fderived bands that can be associated to such peaks in the tunnelling conductance. However, such features can form as a result of correlations elsewhere in the Brillouin zone as well.
Quantumwell states at terraces between steps
In Extended Data Fig. 3a, we show the tunnelling conductance background subtracted from Fig. 2a to obtain Fig. 2b. To carry out the background subtraction, we first identify the features at ε_{−} and ε_{+} in the conductance map. These are the lightblue regions centred at ε_{+} and the red–yellow region centred at ε_{−} in Extended Data Fig. 3a. We then identify the edge states occurring at the steps, given by the red areas at the sides of Extended Data Fig. 3a. Similar peaks are obtained on steps separating different terraces. The nature and shape of the 1DESs is discussed below. We then model these features by a set of Lorentzians and obtain the pattern shown in Extended Data Fig. 3a. We subtract this pattern from the experiment (Fig. 2a) to obtain the pattern shown in Extended Data Fig. 3b.
To model the quantumwell states, we use the Fabry–Pérot interferometer expression for the density of states g_{FP}(x, E) given by
with \(k=\sqrt{2{m}^{* }(E{E}_{0})/{\hbar }^{2}}\), m* the electronic effective mass, r the reflection amplitude, ϕ the phase and L the width of the terrace^{23}. The Fabry–Pérot interferometer is an optical resonator made of semireflecting mirrors and provides a simple and insightful way to model electronic wavefunctions confined between two wells. More information on surface band structure and on quantumwell states by confinement is provided in refs. ^{3,9,54,55,56,57,58,59,60}. We assume a symmetric potential well with L = 20 nm, r = 0.5 and ϕ = −π. The pattern generated by equation (3) is shown in Extended Data Fig. 3c. White points provide the positions of quantized levels as in Fig. 2a,b. It is not difficult to see that the structure of quantized levels is renormalized together with the electronic band structure. Smaller electronic effective masses imply larger quantized level width and separation, and vice versa. However, the simple Fabry–Pérot model does not take into account relaxation by electron–electron interactions, which lead to the extra level broadening discussed in the main text and in refs. ^{27,28,61}.
The black lines in Fig. 2d are fits to the equation (3). To account for the behaviour at the edges, we add the equation (5) for the 1DES. We use the parameters extracted for the terrace L_{3}, discussed in Extended Data Table 1. We show further examples in Extended Data Fig. 4. Note that, in Fig. 2d, we use equation (3) along with the contribution from the 1DES and contributions for ε_{−} and ε_{+} at the bias voltages at which these features are observed in the tunnelling conductance.
The 2DHF quantization was observed on the surfaces of different URu_{2}Si_{2} samples. In Extended Data Fig. 5a–c, we show the result on another sample. Notice here that terraces have different sizes. We show in Extended Data Fig. 5a the STM topography image. In Extended Data Fig. 5b, we show a height profile through the white line in Extended Data Fig. 5a. In Extended Data Fig. 5c, we represent the tunnelling conductance along the central terrace (L ≈ 57 nm) of this profile. We observe similar tunnelling conductance curves as those presented in the main text. Notice the features at ε_{−} and ε_{+}. The quantized levels are also readily observed. These occur, however, at different energy values, as the size of the terrace L is different to that of the terrace in the main text. In Extended Data Fig. 5d, we represent the values of the quantized levels found in terraces of different sizes L by different colours; we show the dispersion relation of the 2DHF as a magenta line.
In Extended Data Fig. 5e, we show as coloured points the bias voltage dependence of the energy spacing ΔE between consecutive quantized levels for terraces L_{3}, L_{4}, the terrace with length L = 57 nm (shown in Extended Data Fig. 5c) and a terrace with length L = 27 nm (not shown). We can write that \(\Delta E={E}_{n+1}{E}_{n}=\left(\frac{{\hbar }^{2}{\pi }^{2}}{2{m}^{* }{L}^{2}}\right)\left({(n+1)}^{2}{n}^{2}\right)\), with n = 1, 2, 3,…. This gives a squareroot dependence of ΔE on the energy, \(\Delta E={E}_{n+1}{E}_{n}=\left(\frac{{\hbar }^{2}{\pi }^{2}}{2{m}^{* }{L}^{2}}\right)\left(2n+1\right)\propto \sqrt{E}\), shown in Extended Data Fig. 5e. In Extended Data Fig. 5f, we plot the average value of \(\frac{\Delta E}{2n+1}\) for each terrace as a function of L. We find the expected \(\frac{1}{{L}^{2}}\) dependence.
Note that in Extended Data Fig. 5 and Fig. 1d, we do not perform any symmetrization. We can see a tendency of the quantized states to shift towards the sides of the terrace, giving intensity patterns that are slightly asymmetric. We have calculated the expected patterns for different reflection coefficients at each side of the terrace. This produces asymmetric patterns similar to those observed experimentally. However, it is difficult to separate such an asymmetry from the signal coming from the edge states in the tunnelling conductance. Lateral symmetrization thus remains the best way to analyse and understand quantum states in the terraces observed here in URu_{2}Si_{2}.
We use \(\hbar /\tau ={\Gamma }_{0}+\left( {E}_{0} /4\pi \right){\left[(E+ {E}_{0} )/{E}_{0}\right]}^{2}\) to fit the energy dependence of the lifetime^{61}. The parameter E_{0}/4π is a prefactor that fits our experiment well. The prefactor is sometimes provided as a number^{28} and has also been estimated as \(\frac{{e}^{2}{k}_{{\rm{F}}}^{2}\pi }{4\pi {{\epsilon }}_{0}\,32\,\hbar {q}_{{\rm{TF}}}}\) or, equivalently, \(\frac{e{m}^{3/2}}{32\times {3}^{5/6}{\pi }^{2/3}{{\epsilon }}_{0}^{1/2}{\hbar }^{4}{n}^{5/6}}\) (with ϵ_{0} the dielectric constant, n the electron density and q_{TF} the Thomas–Fermi screening length)^{27}. These estimations provide similar values to E_{0}/4π.
To obtain the energy dependence of the reflection coefficient, r(E), we used the model described in ref. ^{24}. To this end, we consider a onedimensional periodic array of scattering objects, each modelled by a square potential well of width b < L (L is the width of the terrace). A constant complex potential W provides confinement and coupling to the bulk states. We can then write
with \(k=\sqrt{\frac{2{m}^{* }E}{{\hbar }^{2}}}\) and \(q=\sqrt{\frac{2{m}^{* }(EW)}{{\hbar }^{2}}}\). The reflection coefficient r(E) is given by r(E) = R(E)^{2}. For the dashed line in Fig. 2e, we use typical parameters for Cu, with m* = 0.46m_{0} and W = (−2 − 1i) eV. We shift the obtained curve in energy to obtain a result within the energy range of our data. For the continuous line in Fig. 2e, we use m* = 17m_{0} and W = (−18 − 5i) meV.
Bandstructure calculations at Uterminated surfaces of URu_{2}Si_{2}
The band structure of bulk URu_{2}Si_{2} has been analysed previously in detail using DFT calculations^{62,63,64}. Relevant results coincide with angleresolved photoemission, STM and quantum oscillation studies^{22,65,66,67,68,69,70,71}.
Several surface states have been observed by angleresolved photoemission spectroscopy^{19,20,21,22}. The surface state discussed in refs. ^{19,20} is formed by a holelike band with its maximum at −35 meV and is thus far from what we observe here. At the X point of the Brillouin zone, there are no bulk states. Angleresolved photoemission spectroscopy measurements show hints of surfacelike bands with twodimensional character at these points^{21}. We have taken a closer look at the X point through DFT calculations. To this end, we built a Uterminated supercell consisting of 37 atomic layers, giving a total of ten U layers (Extended Data Fig. 6a). We performed DFT calculations using the fullpotential linearized augmented planewave method with local orbitals as implemented in the WIEN2k package^{72}. Atomic spheres radii were set to 2.5, 2.5 and 1.9 Bohr radii for U, Ru and Si, respectively. We used a 19 × 19 × 1 mesh of kpoints in the first Brillouin zone, reduced by symmetry to 55 distinct kpoints. The RK_{max} parameter was set to 6.5, resulting in a basis size of approximately 5,400 (more than 100 basis functions per atom). Spin–orbital coupling was included in the second variational step^{73} and relativistic local orbitals were included for U 6p_{1/2} and Ru 4p_{1/2} states. The basis for calculations of the spin–orbital eigenvalue problem consisted of scalarrelativistic valence states of energies up to about 5 Ry, resulting in a basis size of about 3,800. The local density approximation was used for the treatment of exchange and correlation effects^{63,74}.
In Extended Data Fig. 6b, we highlight in particular the U spinup character of the obtained surfaceprojected band structure. The spindown character is much less pronounced within the shown energy range. There are several bands inside gaps of the bulk band structure, but only those around the X point of the simple tetragonal Brillouin zone, X_{st} (see Extended Data Fig. 6c), are sufficiently shallow to provide large effective masses.
We find a surface state (upper inset of Extended Data Fig. 6b) that consists of two hybridized hole bands, forming an Mshaped feature close to the Fermi level. The dispersion relation found in our experiment (magenta line in the upper inset of Extended Data Fig. 6b) is compatible with the central part of the Mshaped feature.
1DES and HO within U layers
To analyse the 1DES at the step between two terraces, we use a onedimensional Diracfunctionlike potential at the step, V(x) = U_{0}δ(x − x_{1DES}), in which x_{1DES} is the position of the 1DES. We take U_{0} = b_{0}V_{0}, with b_{0} the width of the potential well and V_{0} the energy depth (V_{0} < 0). We add a complex potential, V(x) → (U_{0} − iU_{1})δ(x − x_{1DES}) to simulate the coupling of the 1DES to the bulk of the crystal. A schematic representation of this model is shown in Extended Data Fig. 7a. Solving the Schrödinger equation for E < 0, we obtain the Green’s function of the states in the potential well
in which \({\lambda }_{x}=\frac{{\hbar }^{2}}{{m}^{* }} {U}_{0}{ }^{1}\) is the decay length with m* the effective mass. E_{1DES} and η_{1DES} are the energy position and the energy broadening of the 1DES given by
in which δV is the height of the potential barrier of the well relative to the Fermi level.
We can now fit the tunnelling conductance at the 1DES using
convoluted with the derivative of the Fermi–Dirac distribution function. Extended Data Table 1 shows the extracted fitting parameters E_{1DES}, η_{1DES}, λ_{x} and x_{1DES} for the four different terraces L_{1} to L_{4} from Fig. 1.
From Extended Data Table 1, we see that the energy position and the energy broadening of the 1DES are independent of the terrace size, with average values of E_{1DES} = −0.52 ± 0.14 meV and η_{1DES} = 0.45 ± 0.06 meV. We also see that all the spatial features are always at the same position with respect to the step, x_{1DES} ≈ 4.0a_{0}, a_{0} being the inplane lattice constant, with a decay length λ_{x} ≈ 0.9 nm ≈ 2a_{0}. The latter indicates that 1DESs and 2DHFs couple when the decay length reaches a few interatomic distances. With the extracted average values from Extended Data Table 1 for λ_{x}, η_{1DES} and E_{1DES}, we obtain U_{0} = 5.4 meVÅ, U_{1} = 0.38 meVÅ and δV = 3.1 meV.
We can analyse the 1DES through the tunnelling conductance at a step (Extended Data Fig. 7b,c). At low bias voltages, we find a dip in the tunnelling conductance of a few nanometres at the upper side of the step (blue lines in Extended Data Fig. 7b; for example, at −1.2 mV). The dip fills with the 1DES at about E_{1DES} (red lines in Extended Data Fig. 7b at −0.4 mV) and empties again at higher bias voltages. This shows that charge depletion close to the step opens a gap in the band structure. The gap is filled at the resonant energy of the 1DES, as observed previously in metals^{30,75,76}. By normalizing the tunnelling conductance to its shape far from the step (Extended Data Fig. 7c), we can follow the decay of the 1DES into the quantumwell states of the 2DHF with the model described above (Extended Data Fig. 7a). The decay length is on the order of the inverse of the wavevector of the 2DHF.
Taking a closer look at the steps, we surprisingly find a notable inplane anisotropy of the 1DES. As we see in Extended Data Fig. 7b, the 1DES is observed when crossing steps along the dashed red line in Extended Data Fig. 7b but not along the dashed blue line, as discussed in the main text. It is useful here to take a closer look at the HO as well. As discussed in refs. ^{14,34,64,77,78,79}, there is no dipolar (magnetic) or structural order related to the HO phase. Instead, the U lattice can present some sort of longrange electronic ordering, whose actual symmetry and shape is considered as a relevant and open mystery^{14}.
Previous NMR measurements indicated the absence of fourfold inplane symmetry breaking in bulk URu_{2}Si_{2} (ref. ^{35}), whereas our STM data clearly show an inplane symmetry breaking in the 1DES. At the surface, there can be marked changes in the electronic structure owing to a modification of the valence of uranium atoms^{21,80}. Rather, the breaking of the inplane symmetry observed here in the 1DES suggests that a fundamental breaking of the nearsurface electronic properties of the U lattice is at play in the HO phase.
Interplay between superconductivity and the 2DHF
We consider several parallel conduction channels between the tip and the surface. For simplicity, we take into account tunnelling into the 2DHF and into the feature of largest size at ε_{−} (Extended Data Fig. 8a). The first channel, t_{1}, connects the tip with the 2DHF. The 2DHF is superconducting by proximity from the bulk superconductor, which we model using a coupling t_{s}. With the second channel, t_{2}, we connect the tip to other surface features. We can write the retarded Green function \({\hat{G}}^{{\rm{r}}}\) as
with
in which E_{2DHF} is the energy associated to the 2DHF and includes the shift of energy owing to HO and Fano resonance, W is an energy scale related to the normal density of states at the Fermi level by ρ(E_{F}) = 1/(πW), Δ is the superconducting gap and η is a small energy relaxation rate. We have added the self energies iΓ_{j}/2, (j = 2DHF, −, +), with Γ_{j} the width of the resonance j.
The differential conductance is calculated as
with
Here f(E, T) is the Fermi–Dirac distribution at the energy E and temperature T and \({\sigma }_{0}=\frac{2{e}^{2}}{h}\) is the quantum of conductance (with h being Planck’s constant and e the elementary charge). Notice that T is the transmission, not the density of states often used to discuss STM measurements in superconductors. Notice also that we take into account tunnelling into the 2DHF (\({G}_{11}^{{\rm{r}}}\)) and into ε_{−} (\({G}_{33}^{{\rm{r}}}\)), with mixed contributions (\({G}_{31}^{{\rm{r}}}\) and \({G}_{13}^{{\rm{r}}}\)).
To fit the tunnelling conductance curves shown in Fig. 4a,b and Extended Data Fig. 8b, we subtracted an asymmetric background (see Extended Data Figs. 2 and 8b). In Extended Data Table 2, we show the parameters used to obtain the tunnelling conductance with temperature (shown as black lines in Fig. 4a,b) from equation (11). We do not vary the parameters η = 0.018 meV, ε_{2DHF} = 12 meV, Γ_{2DHF} = 1 meV, Γ_{−} = 0.55 meV and Γ_{+} = 0.14 meV with temperature. We see that the superconducting lifetime itself is practically negligible, η = 0.018 meV ≪ Δ = 0.2 meV. The large zerobias conductance is not because of the incomplete coupling to the bulk, as t_{s} is close to one. There is further smearing coming from features at ε_{+} and ε_{−}. Γ_{+} and Γ_{−} provide the smeared superconducting density of states and a finite tunnelling conductance at zero bias. Notice that the superconducting gap vanishes at the critical temperature T_{c} but that the strongly biasvoltagedependent tunnelling conductance remains up to higher temperatures (Extended Data Fig. 8).
There are numerous evidences for dwave or more complex superconducting properties in URu_{2}Si_{2}. The differences in the superconducting density of states between these superconducting states and swave superconductivity are relatively small, particularly because the tunnelling conductance obtained with the model described here provides smeared conductance curves. The same occurs for the temperature dependence of the superconducting gap. Instead, the shape and asymmetry of the tunnelling conductance at defects might provide the connection between the properties of the superconducting 2DHF and the unconventional superconductivity of the bulk.
Results at point defects
We analyse in more detail here the tunnelling conductance at defects. We plot the tunnelling conductance obtained on two different defects in Extended Data Fig. 9a as blue and red lines. Notice the pronounced electron–hole asymmetry, which provides curves that widely differ from the curves far from defects (black line in Extended Data Fig. 9a). As discussed above, we can identify two kinds of defects: protrusions with height increases of around 15 pm, probably because of interstitial atoms located beneath the surface (Extended Data Fig. 9b,d,f,h), and troughs of around 15 pm in size, probably because of vacancies beneath the surface (Extended Data Fig. 9c,e,g,i). The defects visibly affect the tunnelling conductance. We plot the tunnelling conductance at ε_{+}, 0 mV and ε_{−} for both types of defect in Extended Data Fig. 9f,g. In Extended Data Fig. 9h,i, we show the spatial dependence of the tunnelling conductance along a crystalline axis for both types of defect. At the site of the defect, there is a pronounced electron–hole asymmetry, which is opposite for each kind of defect. Protrusions provide a substantially enhanced tunnelling conductance for empty states above the Fermi level, whereas troughs provide the opposite (Extended Data Fig. 9a). At zero bias, the troughs show a pronounced inplane anisotropy and a reduction of the superconducting gap, whereas the protrusions are inplane isotropic and show an opened superconducting gap. When leaving the defect, the usual behaviour is recovered after several nanometres (Extended Data Fig. 9h,i).
To explain the pronounced modification of the electron–hole asymmetry of the tunnelling conductance, let us consider electron correlations creating a large Fermi surface scenario at some portion of the band structure. At low temperatures, correlations provide an avoided band crossing owing to hybridized heavy and light bands. We can assume that it happens somewhere in the band structure of URu_{2}Si_{2}, for example, close to the Γ point, as suggested in refs. ^{14,18,47}. Then, we obtain at low temperatures a large Fermi surface with heavy electrons and a closelying light band with smaller wavevectors above the Fermi level. We can then assume that both kinds of defect couple to different parts of such a correlated band structure. Following our experiments, protrusions create 2DHF coupling to the small light band and a large density of states for empty states above the Fermi level (red curve in Extended Data Fig. 9a). Troughs can instead favour coupling to the heavy band, giving the opposite behaviour (blue curve in Extended Data Fig. 9a). The superconducting gap is disturbed at troughs. Generally, we expect all sorts of defects to be pair breaking, as URu_{2}Si_{2} is not a swave superconductor. However, twodimensional quantized states screen pairbreaking interactions from the underlying superconducting bulk^{81}. This suggests that the absence of ingap states in protrusions is because of screening by the 2DHF. Troughs, however, produce a strong coupling to the heavy bands that carry the heavy superconducting state and thus also contribute to pair breaking.
Ingap states at troughs have a certain inplane anisotropy (Extended Data Fig. 9g, middle panel). The anisotropy of the superconducting gap has been analysed with macroscopic measurements^{82,83,84,85}. There are indications for gap nodes, for instance, from specific heat measurements^{82,83,85}. Local vortex dynamics shows an inplane fourfold or twofold pinning potential^{86}. When measured as a function of the angle, there is a pronounced outofplane anisotropy, which has been associated to nodes along the c axis, but there is little inplane variation^{85}. Following such a nodal structure, several experiments propose a chiral k_{z}(k_{x} + ik_{y}) superconducting wavefunction^{84,85,87}. The surface states of a (k_{x} ± ik_{y}) superconductor are predicted to be chiral arc states connecting the Weyl nodes^{88}. It is so far unclear how these surface states of the superconducting phase adapt to the surface states of the normal phase, which appear as a consequence of the surfaceinduced perturbation of the crystalline potential. As mentioned, we observe an asymmetry at zero bias (Extended Data Fig. 9g, middle panel). To increase the signaltonoise ratio, we were forced to apply a fourfold symmetrization. Thus, the anisotropy might be twofold or fourfold. The chiral k_{z}(k_{x} + ik_{y}) state is inplane isotropic. However, the shape of ingap states is determined by the anisotropy of the normalstate wavefunctions, together with the symmetry of the superconducting wavefunction, so that the observed inplane anisotropy of the ingap states probably reflects the normalstate inplane anisotropy.
Data availability
All data supporting the findings of this study are available from the corresponding authors on request.
Code availability
The simulation code WIEN2k used to compute the surface electronic band structure can be obtained from http://susi.theochem.tuwien.ac.at.
References
Hasegawa, Y. & Avouris, P. Direct observation of standing wave formation at surface steps using scanning tunneling spectroscopy. Phys. Rev. Lett. 71, 1071–1074 (1993).
Crommie, M. F., Lutz, C. P. & Eigler, D. M. Imaging standing waves in a twodimensional electron gas. Nature 363, 524–527 (1993).
Echenique, P. & Pendry, J. Theory of image states at metal surfaces. Prog. Surf. Sci. 32, 111–159 (1989).
Li, J., Schneider, W.D., Berndt, R. & Crampin, S. Electron confinement to nanoscale Ag islands on Ag(111): a quantitative study. Phys. Rev. Lett. 80, 3332–3335 (1998).
Wenderoth, M. et al. Lowtemperature scanning tunneling spectroscopy as a probe for a confined electron gas. Europhys. Lett. 45, 579–584 (1999).
Bürgi, L., Jeandupeux, O., Brune, H. & Kern, K. Probing hotelectron dynamics at surfaces with a cold scanning tunneling microscope. Phys. Rev. Lett. 82, 4516–4519 (1999).
Crommie, M. F., Lutz, C. P. & Eigler, D. M. Confinement of electrons to quantum corrals on a metal surface. Science 262, 218–220 (1993).
Heller, E. J., Crommie, M. F., Lutz, C. P. & Eigler, D. M. Scattering and absorption of surface electron waves in quantum corrals. Nature 369, 464–466 (1994).
Fiete, G. A. & Heller, E. J. Colloquium: Theory of quantum corrals and quantum mirages. Rev. Mod. Phys. 75, 933–948 (2003).
Manoharan, H. C., Lutz, C. P. & Eigler, D. M. Quantum mirages formed by coherent projection of electronic structure. Nature 403, 512–515 (2000).
Kirchmann, P. S. et al. Quasiparticle lifetimes in metallic quantumwell nanostructures. Nat. Phys. 6, 782–785 (2010).
Hewson, A. C. The Kondo Problem to Heavy Fermions (Cambridge Univ. Press, 1993).
Patil, S. et al. ARPES view on surface and bulk hybridization phenomena in the antiferromagnetic Kondo lattice CeRh_{2}Si_{2}. Nat. Commun. 7, 11029 (2016).
Mydosh, J. A. & Oppeneer, P. M. Hidden order behaviour in URu_{2}Si_{2} (a critical review of the status of hidden order in 2014). Philos. Mag. 94, 3642–3662 (2014).
Shishido, H. et al. Tuning the dimensionality of the heavy fermion compound CeIn_{3}. Science 327, 980–983 (2010).
Mizukami, H. et al. Extremely strongcoupling superconductivity in artificial twodimensional Kondo lattices. Nat. Phys. 7, 849–853 (2011).
Pirie, H. et al. Imaging emergent heavy Dirac fermions of a topological Kondo insulator. Nat. Phys. 16, 52–56 (2020).
Oppeneer, P. M. et al. Spin and orbital hybridization at specifically nested Fermi surfaces in URu_{2}Si_{2}. Phys. Rev. B 84, 241102 (2011).
Boariu, F. et al. The surface state of URu_{2}Si_{2}. J. Electron Spectrosc. Relat. Phenom. 181, 82–87 (2010).
Zhang, W. et al. ARPES/STM study of the surface terminations and 5felectron character in URu_{2}Si_{2}. Phys. Rev. B 98, 115121 (2018).
Fujimori, S.I., Takeda, Y., Yamagami, H., Yamamoto, E. & Haga, Y. Electronic structure of URu_{2}Si_{2} in paramagnetic phase: threedimensional angle resolved photoelectron spectroscopy study. Electron. Struct. 3, 024008 (2021).
Denlinger, J. D. et al. Global perspectives of the bulk electronic structure of URu_{2}Si_{2} from angleresolved photoemission. Electron. Struct. 4, 013001 (2022).
Bürgi, L., Jeandupeux, O., Hirstein, A., Brune, H. & Kern, K. Confinement of surface state electrons in FabryPérot resonators. Phys. Rev. Lett. 81, 5370–5373 (1998).
Hörmandinger, G. & Pendry, J. B. Interaction of surface states with rows of adsorbed atoms and other onedimensional scatterers. Phys. Rev. B 50, 18607–18620 (1994).
Crampin, S., Boon, M. H. & Inglesfield, J. E. Influence of bulk states on laterally confined surface state electrons. Phys. Rev. Lett. 73, 1015–1018 (1994).
Gartland, P. O. & Slagsvold, B. J. Transitions conserving parallel momentum in photoemission from the (111) face of copper. Phys. Rev. B 12, 4047–4058 (1975).
Giuliani, G. F. & Quinn, J. J. Lifetime of a quasiparticle in a twodimensional electron gas. Phys. Rev. B 26, 4421–4428 (1982).
Seo, J. et al. Transmission of topological surface states through surface barriers. Nature 466, 343–346 (2010).
Kasahara, Y. et al. Superconducting gap structure of heavyfermion compound URu_{2}Si_{2} determined by angleresolved thermal conductivity. New J. Phys. 11, 055061 (2009).
Ortega, J. E., Himpsel, F. J., Haight, R. & Peale, D. R. Onedimensional image state on stepped Cu(100). Phys. Rev. B 49, 13859 (1994).
Okazaki, R. et al. Rotational symmetry breaking in the hiddenorder phase of URu_{2}Si_{2}. Science 331, 439–442 (2011).
Tonegawa, S. et al. Direct observation of lattice symmetry breaking at the hiddenorder transition in URu_{2}Si_{2}. Nat. Commun. 5, 5188 (2014).
Choi, J. et al. Pressureinduced rotational symmetry breaking in URu_{2}Si_{2}. Phys. Rev. B 98, 241113 (2018).
Harima, H. Hiddenorders of uranium compounds. SciPost, scipost_202208_00031v1 (2022).
Kambe, S. et al. Oddparity electronic multipolar ordering in URu_{2}Si_{2}: conclusions from Si and Ru NMR measurements. Phys. Rev. B 97, 235142 (2018).
Allan, M. P. et al. Imaging Cooper pairing of heavy fermions in CeCoIn_{5}. Nat. Phys. 9, 468–473 (2013).
Zhou, B. B. et al. Visualizing nodal heavy fermion superconductivity in CeCoIn_{5}. Nat. Phys. 9, 474–479 (2013).
Jiao, L. et al. Chiral superconductivity in heavyfermion metal UTe_{2}. Nature 579, 523–527 (2020).
Chatterjee, S. Heavy fermion thin films: progress and prospects. Electron. Struct. 3, 043001 (2021).
Jourdan, M., Huth, M. & Adrian, H. Superconductivity mediated by spin fluctuations in the heavyfermion compound UPd_{2}Al_{3}. Nature 398, 47–49 (1999).
FernándezLomana, M. et al. Millikelvin scanning tunneling microscope at 20/22 T with a graphite enabled stickslip approach and an energy resolution below 8 μeV: application to conductance quantization at 20 T in single atom point contacts of Al and Au and to the charge density wave of 2HNbSe_{2}. Rev. Sci. Instrum. 92, 093701 (2021).
MartínVega, F. et al. Simplified feedback control system for scanning tunneling microscopy. Rev. Sci. Instrum. 92, 103705 (2021).
Horcas, I. et al. WSXM: a software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 78, 013705 (2007).
Suderow, H., Guillamon, I. & Vieira, S. Compact very low temperature scanning tunneling microscope with mechanically driven horizontal linear positioning stage. Rev. Sci. Instrum. 82, 033711 (2011).
Schmidt, A. R. et al. Imaging the Fano lattice to hidden order transition in URu_{2}Si_{2}. Nature 465, 570–576 (2010).
Aynajian, P. et al. Visualizing the formation of the Kondo lattice and the hidden order in URu_{2}Si_{2}. Proc. Natl Acad. Sci. 107, 10383–10388 (2010).
Yuan, T., Figgins, J. & Morr, D. K. Hidden order transition in URu_{2}Si_{2}: evidence for the emergence of a coherent Anderson lattice from scanning tunneling spectroscopy. Phys. Rev. B 86, 035129 (2012).
Hamidian, M. H. et al. How Kondoholes create intense nanoscale heavyfermion hybridization disorder. Proc. Natl Acad. Sci. 108, 18233–18237 (2011).
Morr, D. K. Theory of scanning tunneling spectroscopy: from Kondo impurities to heavy fermion materials. Rep. Prog. Phys. 80, 014502 (2016).
Löhneysen, H. V., Rosch, A., Vojta, M. & Wölfle, P. Fermiliquid instabilities at magnetic quantum phase transitions. Rev. Mod. Phys. 79, 1015–1075 (2007).
Coleman, P. Heavy Fermions: Electrons at the Edge of Magnetism, Chs. 1–3, 1–217 (Wiley, 2007).
Flouquet, J. in Progress in Low Temperature Physics Vol. 15 (ed. Halperin, W. P.) 139–281 (Elsevier, 2005).
Figgins, J. & Morr, D. K. Differential conductance and quantum interference in Kondo systems. Phys. Rev. Lett. 104, 187202 (2010).
Fu, Y.S. et al. Manipulating the Kondo resonance through quantum size effects. Phys. Rev. Lett. 99, 256601 (2007).
Crommie, M. F., Lutz, C. P., Eigler, D. M. & Heller, E. J. Quantum corrals. Phys. D Nonlinear Phenom. 83, 98–108 (1995).
Crommie, M. F., Lutz, C. P., Eigler, D. M. & Heller, E. J. Quantum interference in 2D atomicscale structures. Surf. Sci. 361, 864–869 (1996).
Tamm, I. Über eine mögliche Art der Elektronenbindung an Kristalloberflächen. Z. Phys. 76, 849–850 (1932).
Shockley, W. On the surface states associated with a periodic potential. Phys. Rev. 56, 317–323 (1939).
Kevan, S. & Eberhardt, W. in Kevan, S. (ed.) AngleResolved Photoemission: Theory and Current Applications Vol. 74 (ed. Kevan, S.) 99–143 (Elsevier, 1992).
Fiete, G. A. et al. Scattering theory of Kondo mirages and observation of single Kondo atom phase shift. Phys. Rev. Lett. 86, 2392–2395 (2001).
Giuliani, G. & Vignale, G. Quantum Theory of the Electron Liquid (Cambridge Univ. Press, 2005).
Oppeneer, P. M. et al. Electronic structure theory of the hiddenorder material URu_{2}Si_{2}. Phys. Rev. B 82, 205103 (2010).
Elgazzar, S., Rusz, J., Amft, M., Oppeneer, P. M. & Mydosh, J. A. Hidden order in URu_{2}Si_{2} originates from Fermi surface gapping induced by dynamic symmetry breaking. Nat. Mater. 8, 337–341 (2009).
Ikeda, H. et al. Emergent rank5 nematic order in URu_{2}Si_{2}. Nat. Phys. 8, 528–533 (2012).
Aoki, D. et al. Highfield Fermi surface properties in the lowcarrier heavyfermion compound URu_{2}Si_{2}. J. Phys. Soc. Jpn. 81, 074715 (2012).
Onuki, H. et al. Fermi surface properties and de Haas–van Alphen oscillation in both the normal and superconducting mixed states of URu_{2}Si_{2}. Philos. Mag. B 79, 1045–1077 (1999).
Yoshida, R. et al. Signature of hidden order and evidence for periodicity modification in URu_{2}Si_{2}. Phys. Rev. B 82, 205108 (2010).
Kawasaki, I. et al. Electronic structure of URu_{2}Si_{2} in paramagnetic phase studied by soft xray photoemission spectroscopy. J. Phys. Conf. Ser. 273, 012039 (2011).
Meng, J.Q. et al. Imaging the three dimensional Fermi surface pairing near the hidden order transition in URu_{2}Si_{2} using angleresolved photoemission spectroscopy. Phys. Rev. Lett. 111, 127002 (2013).
Bareille, C. et al. Momentumresolved hiddenorder gap reveals symmetry breaking and origin of entropy loss in URu_{2}Si_{2}. Nat. Commun. 5, 4326 (2014).
SantanderSyro, A. F. et al. Fermisurface instability at the ‘hiddenorder’ transition of URu_{2}Si_{2}. Nat. Phys. 5, 637–641 (2009).
Blaha, P. et al. WIEN2k: an APW+lo program for calculating the properties of solids. J. Chem. Phys. 152, 074101 (2020).
Kuneš, J., Novák, P., Diviš, M. & Oppeneer, P. M. Magnetic, magnetooptical, and structural properties of URhAl from firstprinciples calculations. Phys. Rev. B 63, 205111 (2001).
Perdew, J. P. & Wang, Y. Accurate and simple analytic representation of the electrongas correlation energy. Phys. Rev. B 45, 13244–13249 (1992).
Namba, H., Nakanishi, N., Yamaguchi, T. & Kuroda, H. Electronic states localized at step edges on Ni(7 9 11) surfaces studied by angleresolved photoelectron spectroscopy. Phys. Rev. Lett. 71, 4027–4030 (1993).
Avouris, P. & Lyo, I.W. Observation of quantumsize effects at room temperature on metal surfaces with STM. Science 264, 942–945 (1994).
Chandra, P., Coleman, P. & Flint, R. Hastatic order in the heavyfermion compound URu_{2}Si_{2}. Nature 493, 621–626 (2013).
Broholm, C. L. et al. Strict limit on inplane ordered magnetic dipole moment in URu_{2}Si_{2}. Phys. Rev. B 89, 155122 (2014).
Harima, H., Miyake, K. & Flouquet, J. Why the hidden order in URu_{2}Si_{2} is still hidden—one simple answer. J. Phys. Soc. Jpn. 79, 033705 (2010).
Johansson, B. Valence state at the surface of rareearth metals. Phys. Rev. B 71, 6615 (1979).
Morr, D. K. & Stavropoulos, N. A. Quantum corrals, eigenmodes, and quantum mirages in swave superconductors. Phys. Rev. Lett. 92, 107006 (2004).
Fisher, R. A. et al. Specific heat of URu_{2}Si_{2}: effect of pressure and magnetic field on the magnetic and superconducting transitions. Phys. B Condens. Matter 163, 419–423 (1990).
Brison, J. P. et al. Very low temperature properties of heavy fermion materials. Phys. B Condens. Matter 199, 70–75 (1994).
Kasahara, Y. et al. Superconducting gap structure of heavyFermion compound URu_{2}Si_{2} determined by angleresolved thermal conductivity. New J. Phys. 11, 055061 (2009).
Yano, K. et al. Fieldangledependent specific heat measurements and gap determination of a heavy fermion superconductor URu_{2}Si_{2}. Phys. Rev. Lett. 100, 017004 (2008).
Iguchi, Y. et al. Local observation of linearT superfluid density and anomalous vortex dynamics in URu_{2}Si_{2}. Phys. Rev. B 103, L220503 (2021).
Kittaka, S. et al. Evidence for chiral dwave superconductivity in URu_{2}Si_{2} from the fieldangle variation of its specific heat. J. Phys. Soc. Jpn. 85, 033704 (2016).
Schnyder, A. P. & Brydon, P. M. R. Topological surface states in nodal superconductors. J. Phys. Condens. Matter 27, 243201 (2015).
Acknowledgements
We acknowledge discussions on the symmetry of HO with H. Harima. This work was supported by the Spanish State Research Agency (PID2020114071RBI00, PID2020117671GBI00 and CEX2018000805M), by the Comunidad de Madrid through the programme NanomagCOSTCM (programme no. S2018/NMT4321) and by EU (PNICTEYES ERCStG679080 and COST SUPERQUMAP CA21144). H.S., E.H. and I.G. acknowledge SEGAINVEX at UAM for the design and construction of the STM cryogenic equipment. J.A.G. and E.H. acknowledge the support of the Ministerio de Ciencia, Tecnología e Innovación de Colombia (grants 122585271058 and 784(2017)). W.J.H. and E.H. acknowledge support from the Universidad Nacional de Colombia (DIEB projects 48148, 57522 and 201010025979(2016)). J.P.B. and G.K. acknowledge support from the French National Agency for Research (ANR) within the projects FRESCO no. ANR20CE300020 and FETTOM ANR19CE300037. J.R. and P.M.O. acknowledge support from the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation (grant no. 2022.0079) and the Swedish National Infrastructure for Computing (SNIC), through grant no. 201805973.
Author information
Authors and Affiliations
Contributions
E.H. performed the study and made all experiments, with the supervision of I.G. E.H. analysed the data and compared with theory, with the supervision of W.H. and J.A.G. J.R. and P.M.O. performed the bandstructure calculations and discussed the features of surface states. The models to account for superconducting features and for the behaviour at the step were proposed by W.H. and A.L.Y. D.A. synthesized and G.K. and J.P.B. characterized the samples. D.A., J.F. and H.S. proposed the study. The manuscript was written by E.H., I.G., W.H., A.L.Y., P.M.O. and H.S., with contributions from all authors.
Corresponding authors
Ethics declarations
Competing interests
The authors declare no competing interests.
Peer review
Peer review information
Nature thanks Yifeng Yang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Fig. 1 Different surface terminations in URu_{2}Si_{2}.
a, URu_{2}Si_{2} crystal structure. U atoms are shown in green, Ru in magenta and Si in yellow. We show using the same colours the corresponding planes and indicate the distances between planes, normalized to the caxis lattice parameter. With coloured arrows, we highlight the distance between the planes observed in the STM images. b, STM topography image at a surface with terraces. As we discuss in the text, these are all Uterminated terraces. The height scale is given by the bar on the left. In the upperright inset, we show a height histogram (distances normalized to the caxis lattice constant). Notice that all peaks are located at an integer of c/2. The crystal axes are shown as white arrows. The small white square is the area shown in e. c,d, STM topography images at surfaces showing terraces with a triangular shape. These are distinct terraces. The crystal axes are shown as arrows. In the upperright inset, we show the height histogram, with distances between terraces having different sizes. Coloured arrows are as in a and identify height differences between the U and Si terraces. The small white squares in c and d provide the areas shown in f and g, respectively. We show other kinds of defect in h to o, with the colour scale given by the bars on the left of each image. All data were taken at 100 mK with a tunnelling current I_{tunnel} = 10 nA and a bias voltage V_{Bias} = 10 mV.
Extended Data Fig. 2 Tunnelling conductance of the HO state.
a, Tunnelling conductance versus bias voltage at a temperature above the HO transition temperature (17.5 K) is shown as black points. The red line is a fit based on the Fano lineshape owing to parallel tunnelling paths into light and heavy bands. b, Tunnelling conductance inside the HO state versus bias voltage is shown as black points. Notice the reduction of the bias voltage range, to highlight the features associated to HO within the Fano lineshape. The HO gap is schematically marked by an arrow. The red line is a fit as described in the text. c, At the lowest temperatures and focusing on very low bias voltages, we observe the tunnelling conductance represented as black points. The red line is a fit with a model described in Methods. We show by dashed lines the features at ε_{+}, ε_{−} and at the superconducting gap value Δ_{SC}. Temperatures at which the data were taken are provided in each panel.
Extended Data Fig. 3 Subtracted tunnelling conductance background and quantized density of states pattern.
a, Bias voltage dependence of the subtracted tunnelling conductance background versus distance. b, Bias voltage dependence of the tunnelling conductance obtained after the subtraction. c, Fabry–Pérot calculation using the parameters discussed in the text. Quantized levels are represented by white points. Colour scale from blue (low conductance) to red (high conductance) represents values given in the bars at the bottom left.
Extended Data Fig. 4 Fits of the tunnelling conductance.
We show as circles the tunnelling conductance as a function of distance for the bias voltages indicated in the legend of each panel (taken at 0.1 K). Lines are fits described in Methods, with–in addition–a nearly constant background to account for the features at ε_{−} and ε_{+} and the features of the 1DES.
Extended Data Fig. 5 Dispersion relation and quantization on different terraces.
a, STM image on a field of view containing a few Uterminated terraces (taken at 0.1 K). b, Height profile (normalized with the caxis lattice constant) along the white line in a. c, Tunnelling conductance along the central terrace (L ≈ 57 nm) of the profile shown in b (colour scale from red, high conductance, to blue, low conductance). We mark the position of the features at ε_{+} and ε_{−} with dashed white lines. d, Dispersion relation of the 2DHF (magenta line). Points are the positions of the quantized levels obtained from different terraces as described in the text (size L of each terrace is 20 nm blue, 28 nm red, 38.5 nm orange and 57 nm green). e, separation between energy levels ΔE as a function of the energy (coloured points following the colour code of d). Lines are a squareroot dependence, from \(\Delta E={E}_{n+1}{E}_{n}=\left(\frac{{\hbar }^{2}{\pi }^{2}}{2{m}^{* }{L}^{2}}\right)\left(2n+1\right)\). f, Average of ΔE (colour code as in d) as a function of the inverse of L^{2}.
Extended Data Fig. 6 DFT calculations of the surface band structure around the X point of the simple tetragonal Brillouin zone.
a, Uterminated supercell structure of URu_{2}Si_{2} used for DFT calculations. b, Band structure of URu_{2}Si_{2} in a slab calculation (blue points), described in the text, along the highsymmetry directions of the simple tetragonal Brillouin zone, Γ_{st}, M_{st} and X_{st}. The size of the points provides the U spinup character of the bands. In the upper inset, we show a zoomin around the X_{st} point. The magenta line provides the dispersion relation compatible with our experiments and the black points are the quantized levels we identified (from Fig. 2c). c, The usual Brillouin zone construction of URu_{2}Si_{2}, with the tetragonal Brillouin zone (red lines) and the simple tetragonal (st) construction (yellow lines) used to describe the lowtemperature HO phase.
Extended Data Fig. 7 1DES and HO within U layers.
a, Schematic representation of the parameters used to describe the 1DES at a step between two consecutive terraces of length L. We represent the quantized levels of the confined 2DHF on each terrace with pink colour. The dashed black (continuous red) line represents the exponential behaviour of the 1DES without (with) coupling to the quantized levels. b, Topography image along a step between two consecutive terraces formed by U layers is shown in the topleft panel. The colour scale is shown as a bar on the left, in Å. White arrows provide the inplane crystalline axis. The other panels are tunnelling conductance images (colour scale provided at the left) in the same field of view for different values of the bias voltage (taken at 0.1 K). Notice the different shape of the 1DES along steps that are located on the two inplane directions of the U atom lattice. c, Circles in red and blue represent the tunnelling conductance as a function of distance (referred to the tunnelling conductance far from the edge at x = 50 nm) at the bias voltage at which the 1DES appears V ≈ −0.4 mV, σ_{norm}(−0.4 mV) = σ(V = −0.4 mV,x) − σ(V = −0.4 mV,x = 50 nm) along the red and blue lines in the topleft panel of b. The continuous red line is the fit of the 1DES (equation (8)) plus the quantized level model (equation (3)). The bottom panel shows (black line) the STM height profile along the red and blue lines in the topleft panel of b in units of the caxis lattice constant.
Extended Data Fig. 8 Model and tunnelling conductance from 0.1 K to 4 K.
a, Schematic representation of the model and its parameters described in Methods. b, Tunnelling conductance as a function of temperature, also well above the superconducting transition temperature T_{c}. Notice how the feature at ε_{+} vanishes because of thermal smearing, whereas the more pronounced feature at ε_{−} remains up to higher temperatures. Data are shown by coloured points and the black lines are fits, described in the text.
Extended Data Fig. 9 Tunnelling conductance at atomicsized defects.
a, Tunnelling conductance obtained far from defects (black line). We show by the vertical dashed lines the features at ε_{+} and ε_{−}. Curves are taken at 0.1 K. b,c, Topographic images of the U lattice showing two types of defect. White arrows provide the inplane lattice constants. The white scale bar is also shown. The red and blue triangles provide the positions at which the red and blue curves in a were taken. d,e, Profiles obtained along the dashed lines of b and c with the same colours. f,g, Tunnelling conductance maps obtained at the bias voltages shown in each panel, using the colour scale provided on the left for protrusions and troughs, respectively. The scale bar is given in black. To increase the signaltonoise ratio, we have made a fourfold symmetrization in all these panels. h,i, Conductance across the defect site as a function of the distance from the centre of the defects in b and c, respectively, along a crystalline direction. The colour scale is given by the bar on the left of f and g.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Herrera, E., Guillamón, I., Barrena, V. et al. Quantumwell states at the surface of a heavyfermion superconductor. Nature 616, 465–469 (2023). https://doi.org/10.1038/s41586023058301
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41586023058301
This article is cited by

Twodimensional heavy fermion in a monoatomiclayer Kondo lattice YbCu2
Nature Communications (2023)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.