Abstract
The result of a physical measurement depends on the time scale of the experimental probe. In solidstate systems, this simple quantum mechanical principle has farreaching consequences: the interplay of several degrees of freedom close to charge, spin or orbital instabilities combined with the disparity of the time scales associated to their fluctuations can lead to seemingly contradictory experimental findings. A particularly striking example is provided by systems of adatoms adsorbed on semiconductor surfaces where different experiments – angleresolved photoemission, scanning tunneling microscopy and corelevel spectroscopy – suggest different ordering phenomena. Using most recent first principles manybody techniques, we resolve this puzzle by invoking the time scales of fluctuations when approaching the different instabilities. These findings suggest a reinterpretation of ordering phenomena and their fluctuations in a wide class of solidstate systems ranging from organic materials to hightemperature superconducting cuprates.
Introduction
Understanding the mechanisms of charge, spin or orbital ordering and the competition of different instabilities is a leitmotiv of modern solidstate physics. Charge ordering phenomena in (quasi) twodimensional transition metal oxides have recently attracted tremendous attention^{1,2,3,4,5}. Indeed, competing ordering phenomena might pave the way to superconductivity and could be key to an understanding of the unusually high transition temperatures observed in high T_{c} cuprate superconductors. The competition of different instabilities also leads to surprisingly complex phase diagrams in a number of other materials, ranging from lowdimensional organics^{6,7}, heavy fermion materials^{8} to even simple oxides^{9,10}, iridates^{11} or tellurides^{12}.
Here, we study a material that epitomizes this interplay of degrees of freedom, the timescales associated to their fluctuations and the subtleties involved when probing them experimentally. 1/3 of a monolayer of Sn atoms adsorbed on the Si(111) surface – a representative of the socalled Si αphases – forms a twodimensional triangular lattice, where three of the four valence electrons of Sn are forming covalent bonds with the silicon substrate while the remaining dangling bond results in a narrow halffilled surface band. While the electronic structure at the band theory level is thus extremely simple, manybody Coulomb correlations are a source for complex phase diagrams which have been attracting considerable interest, both experimentally^{13,14,15,16,17,18,19,20,21,22,23,24,25,26,27} and theoretically^{27,28,29,30,31,32,33,34,35,36,37,38,39,40,41}. Interestingly, despite the appealing singleorbital nature of the physics of these compounds a simple model with purely local Hubbard interactions falls short^{31,42,43} of describing the observed phase diagrams. In particular, charge ordering instabilities are driven by nonlocal interactions and closely related compounds like Pb/Si(111) or Sn/Ge(111) are found to be in symmetrybroken chargeordered (CO) phases. For Sn/Si(111), experimental results seem contradictory: angleresolved photoemission spectroscopy (ARPES) shows backfoldings of bands associated to a 3 × 3 reconstruction of the unit cell^{44}, while scanning tunneling microscopy (STM) does not yield any indication of any order. In a recent study Li et al.^{27} proposed a magneticallyordered state to be at the origin of these contradictions. However, while the proposed spin order is indeed a natural candidate, no direct experimental evidence for such an order has been found yet. Moreover, as shown below, only charge fluctuations solve an equally important puzzle raised by corelevel spectroscopies of Sn. There, local excitations of the Sn 4d core shell suggest that the ground state of the Sn/Si(111) system is composed of more than one Sn electronic configuration (i.e. valence states). A similar contradiction, also pointing towards the importance of chargedegrees of freedom, is found between ARPES and lowenergy electron diffraction (LEED) and has led to speculations about inhomogenous phases^{45}, with thermally fluctuating Snpositions^{16,17,26,46,47,48}. On the other hand it has been shown^{43} that Sn/Si(111) is in the immediate vicinity of a phase transition between a Mott insulating and a chargeordered phase. Here we show that this peculiar position is key to resolve the above puzzles: Close to the phase transition we find dynamic charge fluctuations (beyond static approximations like LDA) of 3 × 3 and R30° symmetry with lifetimes on the order of femto seconds. This lifefime is long enough for probes like ARPES and core level photoemission (cPES) to detect them while STM, as a static (i.e. time averaging) probe, is blind to such dynamic modes. Our analysis becomes quantitative when assuming a (likely) phase separation in the vicinity of the first order MottCO transition.
Results
Our starting point is the chargecharge correlation function computed within combined manybody perturbation theory and dynamical mean field theory (GW + DMFT), see upper lefthand side of Fig. 1, ref. 43 and the Methods section. This quantity suggests the presence of three major dynamic chargefluctuation symmetries, which are represented in a cartoonlike fashion in the upper righthand side of Fig. 1): a 3 × 3 chargeordered state, where three inequivalent sites are respectively doubly occupied, halffilled and empty (a “210” charge distribution), a R30° reconstructed state where stripes of empty sites alternate with stripes of doubly occupied sites and the conventional Mott insulating state where all sites are halffilled. Its time/frequency dependence indicates fluctuations of these symmetries with a characteristic timescale (i.e. quasiparticles of a specific lifetime) of the order of femto seconds, long enough for the cPES process to capture the fluctuations in a “snapshot”like measurement. We remark that for 3 × 3 chargeordered Sn/Ge(111) a charge distribution with one filled and two quarterfilled sites per unit cell has been discussed. The configurations of the manybody wave functions that would lead to such charge distribution in the symmetry broken phase are included in our GW + DMFT calculations as charge fluctuations visible in our calculated charge susceptibility.
Mathematically, we determine the ground state wave function to be a quantum superposition state (QS) composed of the three configurations with weights 0.13, 0.56 and 0.31 for the R30°, 3 × 3 and R30° configurations respectively, see Methods section. With these insights, we now turn to a discussion of the different observables measured in cPES, STM and ARPES.
Corelevel spectroscopy
The first probe we consider is core electron emission from the Sn 4dshell. Corelevel spectroscopy is a local Sn probe as sketched on the lefthand side of Fig. 1 where a 4d core electron is emitted out of the solid by an incoming photon. Due to the Coulomb interaction of this 4d core hole with the 5p valence electrons (U_{cv}) the spectrum is sensitive to the Sn valence configuration which consists of an either empty, halffilled, or full surface orbital. It is therefore a most efficient probe for chargeordered states or charge fluctuations on timescales slower than the experimental process. To arrive at a first principles description of the corelevel spectra we first determine the corelevel emission spectra corresponding to the three different valence configurations (blue, red and green curves in the central panel of Fig. 1 corresponding to singly occupied, empty and full surface orbital configurations respectively) from cluster multiplet simulations (see Methods section). The main energy scale for each contribution is the spinorbit coupling of the core hole which splits each spectrum into two main peaks associated to a core hole with total angular momentum J_{ch} = 5/2 or J_{ch} = 3/2. On a smaller energy scale (below experimental resolution), the corevalence interaction (U_{cv}) leads to multiplet splittings within each J_{ch} subspace (Note that for a filled valence shell (green) and its fully spherical charge distribution such multiplet splittings cannot occur).
If the system were homogeneously in the QS state determined above the resulting corelevel spectra would be given by the superposition of the spectra determined for the different valence states with contributions of ≈13% of the R30° phase, ≈56% of the 3 × 3 phase and ≈31% of the R30° phase. Translated into Sn valence contributions this corresponds to ≈32% halffilled and ≈68% empty/doublyoccupied sites. The resulting spectrum is plotted as yellowdashed line in Fig. 1) and is found to give an unsatisfactory description of the experimental data (black^{45}/gray^{22} dots). If on the other hand, the system were in a pure Mott insulating state, our estimate of the corelevel spectrum would be given by the contribution of the halffilled surface orbital only, broadened by the experimental resolution (blue dashed line in Fig. 1). Obviously, this assumption does not hold either, confirming our analysis of charge fluctuations contributing to the corelevel spectrum. Being an order to order transition, the Mott to CO transition is expected to be of first order. This is confirmed by the behaviour of our charge susceptibility (discussed further below). Phase separation is very likely in the vicinity of such a firstorder transition.
Indeed, closer inspection of the spectra corresponding to the two possible states and comparison to the experimental spectra shows that while neither the homogeneous QS state of the type determined above nor the Mott state yield theoretical spectra in agreement with experiment, the sum of the two spectra with weights 0.7 for the QS state and 0.3 for the Mott state, does. Such an incoherent superposition can be interpreted as simulating a spatial averaging of the two phases, that is, a state where phase separation leads to a spatial coexistence of Mottinsulating and chargeordered patches with ratio 3/7. The obviously good agreement with the experimental measurement gives support to our interpretation of the sample being in an inhomogeneous state where Mott insulating islands are embedded into a dynamical QS background and the spatial averaging done by the corelevel spectroscopy results in a weighted average of the two contributions with weights 3:7. These results yield a complementary, quantum mechanical perspective on speculations of an inhomogeneous phase^{45}, although rather as a superposition of the Mott state with the QS precursor we have described, locating Sn/Si(111) in the phase coexistence region of a firstorder phase transition between these two phases.
Comparison to the related compound Sn/Ge(111)^{26,46,47} yields further insight: experiments unambigously find Sn/Ge(111) in a fully static chargeordered phase of 3 × 3 symmetry. In this case the phase coexistence has disappeared and the QS patches have grown to macroscopic length scales at temperatures below 60 K. Moreover, for the static case the experimental timescale is irrelevant and direct comparison of cPES and STM is unproblematic. In Sn/Si(111), on the other hand, such comparison can only be made by considering the snapshotlike nature of cPES measuring the spatial average of coexisting phases. Before turning to a quantitative discussion of the time and spatial extent of the charge fluctuations, based on the chargecharge correlation function χ_{GW+DMFT} as obtained from GW + DMFT (see methods section) we revisit another experiment – complementary to STM and cPES – of Sn/Si(111) that has caused recent controversies.
Angleresolved photoemission spectroscopy
Just as in corelevel spectroscopy, the comparison of ARPES with STM surface images (suggesting the absence of any charge order) is not straightforward. More specifically, previous interpretations of ARPES spectra have assumed the breaking of some kind of spin^{27} or charge^{44} symmetry of the ground state in order to account for backfolded features of the momentumresolved spectral function. The origin of such a symmetry breaking has however been unclear so far, since there is no other experimental evidence for it: a static chargeordered state would contradict STM results and no experimental probe has found any direct indication for spinordering. In the light of the previous discussion on the corelevel spectroscopy and by comparison of experimental ARPES spectra with theoretical calculations, we will argue in the following that assuming such symmetry breaking is unnecessary when taking into account the typical timescale of the experiment: just like corelevel spectroscopy, ARPES can be understood as a “snapshot” probe that spatially averages over the surface. Hence, shortlived charge fluctuations of specific symmetry and finite but sufficient spatial extension (see next paragraph for details and quantification) will be picked up by ARPES and incoherently averaged (such an effect has been recently shown for AF spin fluctuations and their impact on ARPES experiments for the high T_{c} cuprates^{49}).
In order to provide a direct comparison between our theory and experiment we have simulated the ARPES spectrum in the R30°, the 3 × 3 and the R30°phase. In Fig. 2 we show in the bottom right panel the experimentally obtained ARPES signal along a specific path in the R30° Brillouin zone (BZ) (taken from ref. 27). Along with the experimental data, we also provide a complete map of singleparticle spectra, so as to illustrate how the total spectrum can be disentangled into its components and provide reference spectra for future experiments on Sn/Si(111) or related compounds in the X/Y(111) family (X being a group IV adatom and Y a semiconductor like Si, Ge, SiC, etc.).
If we now take the mixture (and subsequent broadening) of the three theoretically calculated spectra (left panels in in Fig. 2) with the weights determined above, the agreement between experimental spectra and theory becomes satisfactory. To be precise, we can identify certain symmetry features, i.e. backfoldings, to be related to a specific charge fluctuation. The common feature of all shown spectra is their insulating nature, i.e. a finite gap. In the R30°phase (upper left) the gap separates an upper and lower Hubbard band of the Mott insulating state In the 3 × 3 phase (center left), we find (as expected from the 3 × 3 occupations) the combination of a bandinsulating (empty and doubly occupied sites) and Mottinsulating (singly occupied sites) gap. Finally, the spectral function of the R30°stripe phase (lower left) separates the bands of the doubly occupied and empty lattice sites and, hence, represents a bandinsulating spectrum. While the momentumstructure of the R30° Hubbard bands closely resembles the dispersion of a free electron on the surface lattice, the CO phases display characteristic backfolding features in the R30° BZ. Particularly noteworthy are maxima of the spectral functions along the K − Γ and M − Γ directions (attention must be paid to the different conventions for the naming of BZ points in different publications) that were subject to discussions in previous studies^{27,44}. Finally, we remark that the difference of the total energy scale (i.e. the Fermi level) between our simulation and experiment (Δε_{F} ~ 0.15–0.2 eV) most probably originates from i) uncertainties of the theoretical/experimental determination of the Fermi level or ii) error bars of our ab initio calculated values for onsite and nonlocal interactions.
Time and spatial resolution of the charge fluctuations
In the previous two paragraphs we have seen that spectroscopies such as core electron emission and ARPES seem to suggest charge order and, hence, are in contradiction to STM images of the Sn/Si(111) surface. As alluded to before, this contradiction can be resolved by considering the typical timescales of the experiments: While the spectroscopies are spatially averaged but quasi instantaneous snapshot probes, the STM complementarily yields timeaveraged but spatially resolved information. We will now report on the details of time and spatial resolution of the charge fluctuations relevant to the phase coexistence. Indeed, the discussion on the different experiments above and their interpretation is based on a result from our theoretical abinitio treatment of Sn/Si(111) within selfconsistent GW + DMFT applied to a low energy Hamiltonian (see Methods section). The most relevant quantity for the present discussion is the chargecharge correlation function χ (q, ω) resolved in momentum q and frequency ω and its respective Fourier transforms. In our framework this quantity is selfconsistently obtained and can be employed as a sensitive probe for chargeorder instabilities. More specifically, the vicinity to a transition into an ordered phase of a specific symmetry would be signaled by the behaviour of χ (q, ω = 0) at the corresponding q vector. Intuitive insight can be obtained from the Fourier transformed correlation function in real space and time χ (R, τ) plotted in Fig. 3. With this quantity we can find the typical correlation length ξ and timescale Δτ_{0} of a charge fluctuation. Since the Mott to CO phase transition is not a second order transition, χ (R, τ) does not become continuously long range (i.e. χ (q, ω = 0) does not diverge at a given q) but the correlation length ξ → ∞ only increases up to a finite value of the order of a few lattice constants before entering the symmetrybroken phase.
In Fig. 3 we plot the calculated χ_{GW+DMFT}(R, τ) for Sn/Si(111) on the zaxis at three different imaginary time slices which correspond to averages over increasingly large timescales. Four panels show the evolution from instantaneous measurements to averages roughly over some femto seconds. x and y axes present the 2D surface indicated also by the black dots at the respective adatom sites. From these plots we can conclude that the Mott phase coexists with shortlived finite size latticecommensurate charge fluctuations. To quantify this claim we extract a correlation length at τ = 0 of about 4 l.u. (enough for backfolded bands to be occupied in the ARPES experiment^{49}). These numbers immediately resolve the spectroscopy vs. microscopy puzzle: Sn/Si(111) is found in close vicinity but not yet in a chargeordered phase (which can actually be reached by substituting the Si substrate by a Ge one). However, the ordered phase is preceded by quickly decaying charge fluctuations that can be picked up by fast corelevel and photoemission spectroscopies but not by STM.
Discussion
In this work, we have demonstrated that the apparent contradictions between STM, ARPES and corelevel spectroscopy for twodimensional systems of adatoms adsorbed on semiconductor surfaces can be resolved by considering that i) specific compounds like Sn/Si(111) are located in the phase coexistence region of the firstorder phase transition from a Mott insulator to a chargeordered insulator and ii) the timescales intrinsic to the different experiments matter: quickly decaying charge fluctuations (of specific symmetries) can be seen by fast snapshotlike spectroscopies (corelevel spectroscopy, ARPES) while slow microscopy (STM) detects only a timeaveraged image in which the charge modulations are averaged out. We have shed light on the history of controversial interpretations of Sn/Si(111) by quantifying these statements, based on first principles manybody calculations using combined manybody perturbation theory and extended dynamical meanfield theory (GW + DMFT). In order to provide a direct theoryexperiment comparison, we have computed the observables of corelevel spectroscopy and angular resolved photoemission. Moreover, we have visualized and discussed the key observable for dynamically fluctuating surface compounds: the charge susceptibility. Our analysis underlines the need for a very careful analysis of experimental results in circumstances where characteristic timescales of the material (i.e. fluctuations) and the experimental probe coincide.
Interestingly, in the related Sn/Ge(111) system, experimental discrepancies at temperatures above the 3 × 3 ordered phase have been explained with single particle theories by a freezingin of Snvibrations along the surface normal^{46} while no such freezingin is seen in Sn/Si(111). It has been speculated that physics equivalent to Sn/Ge(111) might occur only at lower temperatures^{45} which, however, has not been confirmed so far. Other paradigmatic phase transitions have also been explained by dynamical fluctuations, as in the metalinsulator transition on In/Si(111)^{50,51,52} or in the novel clusterdiffusion transition on Sn/Si(111):B^{53}. In all of these transitions, the fluctuations have been argued to correspond to classical fluctuations between inequivalent configurations. While we do not exclude backcoupling to the lattice, our study reveals that the experimental observations can be explained from the purely electronic manybody wavefunction.
It is clear that this kind of phenomenology is not restricted to adatom systems, but can be expected to occur quite generally in twodimensional systems close to competing instabilities. Most notably, observations of chargeordering fluctuations dominate the recent literature on hightemperature superconducting oxides^{54,55,56,57}, with conflicting interpretations concerning stripe or checkerboardtype chargeordering tendencies, their driving mechanism and their implications for superconductivity. These questions should be carefully reexamined in the light of our findings. The present surface systems only provide a particularly clean and tunable model system, without the complications due to disorder, mixing in of multiorbital or ligand degrees of freedom present in the cuprates. The welldefined singleorbital character of the surface systems allows for a truly first principles treatment, providing us with a unique tool for refining our understanding of competing instabilities in the proximity of different ordering phenomena. Our results and conclusions do not negate magnetic fluctuations (discussed by Li et al. ref. 27 and a very recent study by Glass et al. ref. 64) which should be considered as complementary to the discussed charge fluctuations. Reconciling both spin and charge degrees of freedom in one theoretical framework will be one of the next challenges  fortunately the materials to be tested are real and experimental data is accessible to support or falsify theoretical predictions.
Methods
Charge correlation function
The correlation function in the charge channel χ (R, τ) displayed in Fig. 3 has been obtained by spatial and temporal Fourier transformation of the charge correlation function χ (k, iω). The latter is computed from the polarization function P (k, iω) through the relation:
Here, v (k) is the Fourier transform of the interactions (a in the lattice constant and R_{i} denotes a lattice site.). The polarization function is computed in the GW + DMFT approximation^{58,59,60,61}, namely as the sum of the impurity polarization P_{imp} (iω) and of the nonlocal part of the bubble ~2 GG, more specifically
where G (q, iv) is the fully selfconsistent Green’s function from a converged GW + DMFT calculation.
The factor of 2 stems from spin degeneracy. The values of the interaction parameters are calculated within the constrained random phase approximation^{42,43}, namely U = 1.0 eV and V = 0.5 eV. A recent ab initio determination of the interaction parameters of the Si αphases (X/Si(111) with X = C, Sn and Pb) has found nonlocal interactions to be as large as 50% of the onsite ones and established a materials trend with the Sn compound being “halfway” between Mott insulating C/Si(111) and chargeordered Pb/Si(111)^{62}.
Groundstate wave function of chargeordered state
In order to determine the weight with which the “210”, stripe and Mott configurations contribute to the groundstate wave function in the chargeordered state, we have solved – by exact diagonalization – a sixsite cluster with periodic boundary conditions. Subsequent projection of the ground state on the three relevant configurations of interest results in the estimates for the coefficients shown in Fig. 4 as a function of nonlocal interaction. The qualitative behaviour of the curves shown here can be understood as follows: For small nonlocal interaction the Mottlike R30° phase is dominant and unmixed with energetically highlying configurations. However, upon increasing the nonlocal interactions some of these highlying configurations (in particular the 3 × 3 and R30° states) become lower in energy with respect to the ground state and for nonlocal interactions exceeding sim 0.34 even replace the R30° configuration as the main contribution for the ground state (note that mixing of the different configurations is driven by the gain of kinetic energy, i.e. electron hopping).
According to the cRPA calculations, the physical values for the nonlocal interactions lead to a ground state composed of the above three components with coefficients 0.13 ( R30°), 0.56 (3 × 3) and 0.31 ( R30°)  normalized values extracted from the results shown in Fig. 4.
Multiplet cluster calculations
In order to simulate the Sn 4d corelevel spectra (shown in Fig. 1) we employ full multiplet cluster calculations using the code introduced in ref. 63. As common practice for such cluster simulated spectra we estimated the strength of the core hole spinorbit coupling (SOC) and the multipole part of the corevalence interaction from atomic HartreeFock calculations^{64}. For valence and core SOC we use ζ_{5p} = 0.40 eV and ζ_{4d} = 0.41 eV; for the multipole moments of the core valence interaction we use the Slater integrals F^{1} = 0.46 eV, F^{2} = 1.47 eV and F^{3} = 0.42 eV^{65}. The monopole part of the corehole valence interaction is (in combination with the onsite U of Sn) responsible for the relative shift of the three spectra and fixed by the overall width of the spectrum. The spectral functions are calculated with exact diagonalization in the cluster limit and broadened by convoluting with a Gaussian of width 0.37 eV.
Theoretical ARPES spectra
The theoretical ARPES spectrum (topright and middleleft panels of Fig. 2) is computed as a weighted average of the spectra for the 3 symmetries :
The relative weights λ_{α} are determined from the cluster diagonalisation and the cPES spectra (see above). denote the number of electrons per unit cell for each symmetry (respectively 1, 3, 2). In the topright panel, this spectrum is broadened with a Gaussian distribution of mean deviation σ = 0.3 to account for ARPES uncertainties. The individual spectra are shown in the remaining three panels. The selfenergy of the R30° symmetry is obtained by MaxEnt analytical continuation of the imaginaryfrequency impurity selfenergy computed selfconsistently through an EDMFT scheme^{43,60}.
We find this selfenergy to be reminiscent of an atomic selfenergy with a renormalized interaction given by the selfconsistently computed effective impurity interaction as obtained from GW + DMFT. For the 3 × 3 symmetry, we take the atomic selfenergy for the halffilled band, while for the empty and full bands we take the Hartree estimates for the selfenergy:
For the symmetry, we also take Hartree estimates:
Additional Information
How to cite this article: Hansmann, P. et al. Uncertainty principle for experimental measurements: Fast versus slow probes. Sci. Rep. 6, 19728; doi: 10.1038/srep19728 (2016).
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Acknowledgements
We thank Gang Li, Ralph Claessen, Jörg Schäfer, Werner Hanke and Daniel Malterre, Yannick FagotRevurat, Alessandro Toschi, Georg Rohringer, Thomas Schaefer and Masatoshi Imada for useful discussions. We are particularly indebted to Ralph Claessen and Jörg Schäfer for providing us with the ARPES data of and allowing us to replot them. This work was supported by the French ANR under project SURMOTT, IDRIS/GENCI under project 091393 and the European Research Council under its Consolidator Grant scheme 2013 (project number 617196).
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P.H. performed the DFT, cRPA and cluster calculations. T.A. performed the GW + DMFT calculations, T.A. and P.H. performed the calculations of the kresolved spectra. S.B., A.T. and P.H. planned the study. All authors contributed to the interpretation of the results and writing of the manuscript.
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Hansmann, P., Ayral, T., Tejeda, A. et al. Uncertainty principle for experimental measurements: Fast versus slow probes. Sci Rep 6, 19728 (2016). https://doi.org/10.1038/srep19728
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