Abstract
A normal conductor placed in good contact with a superconductor can inherit its remarkable electronic properties^{1,2}. This proximity effect microscopically originates from the formation in the conductor of entangled electron–hole states, called Andreev states^{3,4,5,6,7,8}. Spectroscopic studies of Andreev states have been performed in just a handful of systems^{9,10,11,12,13}. The unique geometry, electronic structure and high mobility of graphene^{14,15} make it a novel platform for studying Andreev physics in two dimensions. Here we use a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor–graphene–superconductor junctions. The measured energy spectra, which depend on the phase difference between the superconductors, reveal the presence of a continuum of Andreev bound states. Moreover, our device heterostructure geometry and materials enable us to measure the Andreev spectrum as a function of the graphene Fermi energy, showing a transition between different mesoscopic regimes. Furthermore, by experimentally introducing a novel concept, the supercurrent spectral density, we determine the supercurrent–phase relation in a tunnelling experiment, thus establishing the connection between Andreev physics at finite energy and the Josephson effect. This work opens up new avenues for probing exotic topological phases of matter in hybrid superconducting Dirac materials^{16,17,18}.
Main
When a normal quantum conductor (N) is sandwiched between two superconductors (S), a current can flow in the absence of any voltage, due to the Josephson effect^{1}. This macroscopic supercurrent is driven by the difference between the order parameter phases of the two superconductors, φ. Microscopically, it corresponds to the coherent flow of Cooper pairs through the conductor, which is made possible by successive Andreev reflections at the N/S interfaces, where electrons (holes) are reflected as oppositespin holes (electrons) (Fig. 1a). Due to this process, resonant electron–hole states form in the central conductor, known as Andreev bound states (ABS)^{3,4,5,6}. They lie as energy levels inside the superconducting gap [−Δ, Δ], only negativeenergy states being populated in the ground state (Fig. 1b). Crucially, the Andreev energy E_{n}(φ) depends on φ, and each populated ABS carries in response a supercurrent (1/φ_{0})(∂E_{n}/∂φ), where φ_{0} = ℏ/2e is the reduced flux quantum, with ℏ the reduced Planck constant and e the elementary charge. The overall phasedependent Andreev spectrum is shown in Fig. 1c and depends on geometric and microscopic parameters^{7,8}, such as the conductor dimensions that determine the number of ABS, the scattering processes in the conductor and the contact’s transparency. Strong ABS phase dependence, and therefore a prominent proximity effect, is achieved in ballistic systems with transparent N/S interfaces^{7,8}.
Graphene (G) can exhibit low contact resistance and weak scattering when connected to superconducting electrodes^{19,20}. These properties, combined with the ability to control the number of conduction channels with a gate voltage, make graphene an ideal testbed for exploring Andreev physics in two dimensions (2D). Although the superconducting proximity effect in graphene has attracted considerable research interest^{19,20,21,22,23,24,25,26,27,28,29}, most of the experimental studies have been limited to transport measurements of dissipationless supercurrent in graphenebased (S–G–S) Josephson junctions. Accessing the energy domain while controlling the superconducting phase difference is crucial for probing ABS, and it has been performed in just a few systems, such as silver wires^{9}, carbon nanotubes^{10}, semiconductor nanowires^{11}, and atomic breakjunctions^{12,13}. However, a direct spectroscopic observation of phasedependent ABS in graphene is missing. The way ABS form in graphene still remains an open question, especially around the neutrality point, where its unique electronic spectrum should give rise to specular Andreev reflections^{27,30} and potentially to exotic ABS. Interest in such a spectroscopy has increased with recent proposals for hosting Majorana modes^{16,17,18} within proximitized graphene operated in the quantum Hall regime^{29}.
To probe Andreev physics in the energy domain, we perform tunnelling spectroscopy of graphene proximitized by superconductors (Fig. 1d). The experiment is performed using a full van der Waals heterostructure shown schematically in Fig. 1e. A monolayer graphene sheet is encapsulated between hexagonal boron nitride (hBN) crystals. The bottom one is 15 nm thick and isolates graphene from a graphite local backgate, which enables us to control electrostatically the Fermi energy of graphene with the voltage V_{g}. The top hBN crystal is just one atom thick and is used as a tunnelling barrier. Immediately on top sits a 150nmwide metallic probe made of thin graphite. The use of a graphite probe is crucial, as it limits the doping in the graphene (directly underneath the monolayer hBN barrier) owing to the small work function difference between graphite and graphene. This makes the lowdensity regime around the charge neutrality point (CNP) accessible to our investigations. The graphene sheet, with width W = 2 μm, is well connected to two superconducting aluminium electrodes, with an interlead distance L = 380 nm. These electrodes are patterned in a loop that enables us to control the phase difference φ = Φ/φ_{0} across the graphene by applying a magnetic flux Φ through the loop.
By measuring the differential conductance dI/dV of this tunnelling device at low temperature (20 mK), one can infer the local density of states (DOS) of graphene at energy eV, V being the bias voltage between the graphite probe and the superconducting electrodes (Fig. 1d, e). Such a measurement is shown in Fig. 2a for different values of the magnetic field and a constant gate voltage V_{g} = 1 V. Each trace exhibits a dip centred at zero bias and two side peaks at V = ±160 μV. We emphasize that the tunnelling process takes place between thin graphite (that is, nonsuperconductor) and the graphene flake, which is only laterally contacted by the superconductors. The graphene DOS, usually featureless within this very narrow energy range in the normal state, has dramatically changed due to the superconducting proximity effect and displays a soft induced gap Δ ∼ 160 μeV, only slightly reduced compared to that of aluminium Δ_{Al} ∼ 180 μeV. When varying the magnetic field B (Fig. 2b), the DOS oscillates with a periodicity δB = 360 μT, which corresponds to one flux quantum 2πφ_{0} threading the loop of enclosed area A = 5.7 μm^{2} (taking into account the Meissner effect). The corresponding phase φ = A(B − B_{0})/φ_{0} is shown on the top axis, where B_{0} = 250 μT is an offset magnetic field in this experiment. The gap and side peaks are most pronounced at φ = 0 and get reduced when the phase is swept towards φ = π (see Fig. 2b, top axis).
This fluxdependent DOS is consistent with a continuum of ABS modulating with phase. The graphene weak link being spatially extended indeed consists of a large number of conduction channels M ∼ 2W/λ_{F}, each containing ∼2L_{m}/πξ pairs of ABS^{8}. Here, is the Fermi wavelength of electrons in graphene (with c_{g} ≈ 2.3 fF μm^{−2} the gate capacitance per unit area) and M can be tuned with the backgate V_{g} typically between 40 and 300, ξ is the superconducting coherence length and L_{m} the effective length of channel m. To estimate the superconducting coherence length ξ, we measured in the normal regime the gatedependent differential resistance of a similarly fabricated graphene junction (see Supplementary Fig. 3). From the extracted mean free path l_{e} ∼ 140 nm < L, one can infer a lowerbound ξ ∼ 590 nm using the diffusive relation , with D = v_{F}l_{e}/2 the Einstein diffusion coefficient in 2D and v_{F} the Fermi velocity. Since the S–G–S junction is in the intermediate regime L ∼ ξ, the spectrum is complex and made out of different type of ABS. Most channels are in the shortjunction regime (L_{m} < πξ/2) and contain a single pair of ABS (solid lines in Fig. 1c). In the case of good coupling to the superconducting leads, the ABS strongly depend on phase and are responsible for the flux modulation of the DOS. The latter indeed peaks strongly at the gap when φ = 0, due to the accumulation of ABS at energies E_{n} ∼ ±Δ. At φ = π, the ABS reach their minimum energy and populate the gap, resulting in an enhanced DOS at low energy. At the same time, channels with low contact transparency provide ABS that detach from the gap edges at φ = 0 and have weaker phase modulation, thus explaining the observed soft superconducting gap. On top of that, channels with large transverse momentum are effectively longer and contain more ABS that fill the superconducting gap and which exhibit weaker phase modulation (dashed lines in Fig. 1c), contributing therefore to the finite DOS in the softgap region. Such softgap physics is expected for short and wide quasiballistic junctions^{31}. Note, however, that the measured spectra are very different from the ones observed with diffusive metallic junctions with clean interfaces^{9}, which display a strong ‘minigap’ that modulates with the phase.
To characterize further how Andreev physics manifests in graphene, we now vary the carrier density of the weak link n = c_{g} V_{g} /e. In the normal state, the differential conductance as a function of V_{g} is Vshaped with a minimum at V_{CNP} ∼ 0.05 V, which reflects the linear DOS of the Dirac cone (see Supplementary Fig. 4). Figure 3 shows the DOS in the superconducting state, as a function of both energy and phase, for various values of V_{g} (see also Supplementary Fig. 6 for a more exhaustive gallery). The energy spectra depend strongly on the graphene carrier density, which is tuned from holetype (panels a–c) through the CNP (panels d–f) to electrontype (panels g–i). Noticeably, the larger the carrier density, the stronger the DOS is modulated with phase, resulting in a larger supercurrent which is in agreement with transport measurements of Gbased Josephson junctions^{19,20,21,22,23}. In contrast, at low density n < 1 × 10^{11} cm^{−2}, close to the CNP (Fig. 3e), the phase modulation of the DOS is very weak. In this gate voltage range, which corresponds to E_{F} < 37 meV in Fermi energy, the normal state DOS also shows negligible V_{g} dependence. We attribute these effects to disorder, which causes the formation of electron–hole puddles in graphene when its energy scale is larger than the Fermi energy^{32,33}. In such a heterogeneous and disordered configuration^{25}, the mean free path is reduced and the graphene enters a longjunction regime. Moreover, the coupling of a given puddle to the superconducting leads tends to be reduced and asymmetric. This results in an overall weak phase modulation of the DOS. Note that such spatial fluctuations of Fermi energy due to disorder may also prevent us from observing exotic ABS arising from specular Andreev reflexions^{27,30}. On the contrary, at large carrier density the phase modulation of the DOS is much stronger. This is consistent with a disorder potential smaller than the Fermi energy, which results in more ABS from wellcoupled and transmitted conduction channels in the shortjunction limit. At very large carrier density n > 3 × 10^{12} cm^{−2}, one can even measure a complete closing of the induced gap and a flat DOS at φ = π (Fig. 3a, h, i, and Supplementary Fig. 8), as more ballistic ABS reach zero energy at φ = π (solid lines in Fig. 1c).
The effect of the normal scattering properties also appears as an asymmetry between the energy spectra for opposite carrier density (see Fig. 3 and Supplementary Fig. 9). When the graphene is holedoped, the phase modulation of the DOS is indeed smaller with a Vshaped induced gap, whereas it is Ushaped with a stronger phase modulation for the electrondoped case. This is because aluminium ndopes graphene underneath the contact due to their work function difference, which results in an n–p–n potential profile when V_{g} < V_{CNP}. These p–n junctions reduce the contacts’ transparency, which repel the ABS from the gap edges toward lower energy (solid lines in Fig. 1c) and weaken the phase modulation of the DOS, in good agreement with measurements of Al–G–Al Josephson junctions that show smaller supercurrent in the holedoped region^{21}.
Probing Andreev physics in the energy domain while controlling the phase difference additionally allows one to access the supercurrent spectral density J_{S}(E, φ), which quantifies the amount of supercurrent carried by Andreev states at energy E and phase φ. This quantity makes evident the link between Andreev physics and the Josephson effect. It has seldom been discussed in the literature^{8,34,35,36,37} and, to the best of our knowledge, has never been directly connected to the DOS. We propose here the heuristic definition J_{S}(E, φ) = −(1/φ_{0})∫_{0}^{E}(∂DOS/∂φ)(ε, φ)dε, which can easily be checked as being correct in the case of a set of discrete ABS. Using this definition and the factor c = 34.7 meV ^{−1} μS^{−1} to convert tunnelling conductance to DOS (see Supplementary Section 6), one can numerically compute J_{S} for each energy spectrum. Figure 4 shows both the Andreev spectrum in graphene and its corresponding supercurrent spectral density for a gate voltage V_{g} = 2.4 V. The data confirm that opposite energy ABS carry opposite supercurrent, as expected from timereversal symmetry. Going further, one can derive the supercurrent in the ground state, defined as I_{S}(φ) = (1/2)∫_{−∞}^{∞}sgn(− E)J_{S}(E, φ)dE (Fig. 4c), and thus access the Josephson current–phase relation. In this measurement, which was taken over a large range in magnetic field, both DOS, J_{S} and I_{S} oscillate with phase, the oscillations being progressively washed out at large field. At large fields, a significant flux is threading graphene due to its extended 2Dnature, which results in a spacedependent phase difference and a dephasing of ABS along the junction. Close to zero flux, the current–phase relation is strongly anharmonic (see Supplementary Fig. 11) and can be fitted by the supercurrent carried by perfectly coupled short ABS with an average transmission coefficient τ = 0.725 (solid red curve)^{6}. This confirms that ABS from wellcoupled and welltransmitted short channels are dominating the supercurrent at large carrier density.
Our tunnelling spectroscopy study provides fundamental insights into how the Josephson effect develops in graphene, and it can be extended to other 2D materials. The combined effects of disorder, geometry, and Fermi energy offer many opportunities to investigate Andreev physics in different mesoscopic superconductivity regimes. Moreover, as graphene’s extended 2D nature enables one to combine superconductivity and the quantum Hall effect^{29}, this platform is promising for the detection of Majorana modes^{18}, key ingredients for topologically protected quantum computation.
Methods
The van der Waals heterostructure is created via successive pickups and transfers using a polymerbased dry transfer technique^{38,39} (see Supplementary Information for a detailed description of the fabrication procedure). The superconducting electrodes are fabricated by electronbeam lithography and evaporation of 70 nm of aluminium, with 7 nm of titanium as a sticking layer. The sample is anchored to the mixing chamber of a dilution refrigerator at 20 mK, well below the transition temperature of aluminium (T_{C} ∼ 1.1 K). The biasing and measurement lines are heavily filtered using both discrete and distributed cryogenic lowpass filters^{40}. The measurements are made at low frequency (10–100 Hz) using roomtemperature amplification and standard lockin techniques with an excitation voltage of 5–10 μV.
Data availability.
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We acknowledge helpful discussions with W. Belzig, J. C. Cuevas, V. Fatemi, Ç. Girit, P. Joyez, A. L. Yeyati, J.D. Pillet, H. Pothier, V. Shumeiko and C. Urbina. This work has been primarily supported by the US DOE, BES Office, Division of Materials Sciences and Engineering under Award DESC0001819 and by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4541 to P.J.H. J.I.J.W. was partially supported by a Taiwan Merit Scholarship TMS0941A001. This work made use of the MRSEC Shared Experimental Facilities supported by NSF under award No. DMR0819762 and of Harvard’s CNS, supported by NSF under Grant ECS0335765.
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J.I.J.W., L.B. and P.J.H. designed the experiment. J.I.J.W. and R.P. fabricated the devices. L.B. and J.I.J.W. carried out the measurements. L.B. analysed and interpreted the data. K.W. and T.T. supplied hBN crystals. L.B. and J.I.J.W. wrote the manuscript with input from all the authors.
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Bretheau, L., Wang, JJ., Pisoni, R. et al. Tunnelling spectroscopy of Andreev states in graphene. Nature Phys 13, 756–760 (2017). https://doi.org/10.1038/nphys4110
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DOI: https://doi.org/10.1038/nphys4110
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