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We investigate the surface oxidation in a van der Waals ferromagnet Fe3-xGeTe2, a material widely utilized for spintronic applications. While known for its relative air-insensitivity, exposure to air during the handling process (e.g. device or heterostructure fabrication) can lead to reduction or disappearance of its magnetic signal. Comparison of x-ray absorption and x-ray magnetic circular dichroism spectra between pristine and air-exposed Fe3-xGeTe2 confirm a naturally oxidized surface layer on the material. The surface oxide layer has predominantly Fe3+ content. X-ray absorption spectroscopy done on micron-sized exfoliated Fe3-xGeTe2 flakes reveal that the change in the surface chemical properties can be quite significant for thin flakes. The surface modulation of Fe3-xGeTe2 can lead to inaccuracies in characterizing its interfacial magnetic and spin transport properties, and complicate device and heterostructure fabrication processes.

Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state with no energy barrier to create emergent, fractionalized spinon excitations that carry spin but no charge. Materials that realize this kind of spin liquid are expected to have a low-energy behaviour described by a spinon Fermi surface. Directly imaging the resulting spinons, however, is difficult due to their chargeless nature. Here we use scanning tunnelling spectroscopy to image density waves consistent with the predictions of spinon density modulation arising from a spinon Fermi surface instability in single-layer 1T-TaSe2. We confirm the existence of a triangular lattice of localized spins in this material by contacting it with a metallic 1H-TaSe2 substrate and measuring the Kondo effect. Spectroscopic imaging of isolated single-layer 1T-TaSe2 reveals long-wavelength super-modulations at Hubbard band energies, consistent with the predicted behaviour of itinerant spinons. These super-modulations allow the direct experimental measurement of the spinon Fermi wavevector, in good agreement with theoretical predictions for a two-dimensional quantum spin liquid. Some quantum spin liquids are expected to have an effective Fermi surface of fractionalized spinon excitations. The two-dimensional spin liquid candidate 1T-TaSe2 has charge density modulations that may be caused by an unstable spinon Fermi surface.

Strong electron correlation can induce Mott insulating behaviour and produce intriguing states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunnelling microscopy and angle-resolved photoemission. Our results indicate that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by unusual orbital texture. Interlayer coupling weakens the insulating phase, as shown by reduction of the energy gap and quenching of the correlation-driven orbital texture in bilayer and trilayer 1T-TaSe2. This establishes single-layer 1T-TaSe2 as a useful platform for investigating strong correlation physics in two dimensions. The electrons that contribute to the Mott insulator state in single-layer 1T-TaSe2 are shown to also have a rich variation in their orbital occupation. As more layers are added, both the insulating state and orbital texture weaken.

The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor1-4. In this scheme, 'on' is the ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator5-9, and 'off' is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits4, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far6-8. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases3,10-16. Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal3,17 Na3Bi are two-dimensional topological insulators with bulk bandgaps greater than 300millielectronvolts owing to quantum confinement in the absence of electric field. On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25millielectronvolts), suggest that ultrathin Na3Bi is suitable for room-temperature topological transistor operation.

Recent advances in two-dimensional (2D) crystals make it possible to realize an ideal interface structure that is required for device applications. Specifically, a p-n junction made of 2D crystals is predicted to exhibit an atomically well-defined interface that will lead to high device performance. Using angle-resolved photoemission spectroscopy, a simple surface treatment was shown to allow the possible formation of such an interface. Ta adsorption on the surface of a p-doped SnSe shifts the valence band maximum towards higher binding energy due to the charge transfer from Ta to SnSe that is highly localized at the surface due to the layered structure of SnSe. As a result, the charge carriers of the surface are changed from holes of its bulk characteristics to electrons, while the bulk remains as a p-type semiconductor. This observation suggests that the well-defined interface of a p-n junction with an atomically thin {\it n}-region is formed between Ta-adsorbed surface and bulk., 4 figures

Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1T'-WSe2. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1T'-WSe2 and a semiconducting domain of 1H-WSe2 in contiguous single layers. The QSHI nature of single-layer 1T'-WSe2 is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states., This research was supported by the VdW Heterostructure program (KCWF16) (STM spectroscopy and QPI mapping) funded by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contract No. DE-AC02-05CH11231. Support was also provided by National Science Foundation award EFMA1542741 (surface treatment and topographic characterization). The work at the ALS (sample growth and ARPES measurements) is supported by the Office of Basic Energy Sciences, US DOE under Contract No. DE-AC02-05CH11231. The work at the Stanford Institute for Materials and Energy Sciences and Stanford University (ARPES measurements) was supported by the Office of Basic Energy Sciences, US DOE under contract No. DE-AC02-76SF00515. S. T. acknowledges the support by CPSF-CAS Joint Foundation for Excellent Postdoctoral Fellows. H. R. acknowledges fellowship support from NRF, Korea through Max Planck Korea/POSTECH Research Initiatives No. 2016K1A4A4A01922028 and No. 2011-0031558. A.P. and O.V.Y. acknowledge support by the ERC Starting grant “TopoMat” (Grant No. 306504) (theoretical formalism development). Q.W. acknowledges support from NCCR Marvel (hybrid functional calculations). First-principles calculations were performed at the Swiss National Supercomputing Centre (CSCS) under project s832 and the facilities of Scientific IT and Application Support Center of EPFL. The work at Nanjing University (Y.Z.) is supported by the Fundamental Research Funds for the Central Universities N°. 020414380037 (surface structure analysis). The SIMES work is supported by DOE BES, Division of Materials Sciences. M.M.U. acknowledges support by Spanish MINECO under grant no. MAT2017-88377-C2-1-R (data analysis).

We present the electronic characterization of single-layer 1H-TaSe2 grown by molecular beam epitaxy using a combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory calculations. We demonstrate that 3 × 3 charge-density-wave (CDW) order persists despite distinct changes in the low energy electronic structure highlighted by the reduction in the number of bands crossing the Fermi energy and the corresponding modification of Fermi surface topology. Enhanced spin-orbit coupling and lattice distortion in the single-layer play a crucial role in the formation of CDW order. Our findings provide a deeper understanding of the nature of CDW order in the two-dimensional limit.

SnTe is a prototypical topological crystalline insulator, in which the gapless surface state is protected by a crystal symmetry. The hallmark of the topological properties in SnTe is the Dirac cones projected to the surfaces with mirror symmetry, stemming from the band inversion near the L points of its bulk Brillouin zone, which can be measured by angle-resolved photoemission. We have obtained the (111) surface of SnTe film by molecular beam epitaxy on BaF2(111) substrate. Photon-energy-dependence of in situ angle-resolved photoemission, covering multiple Brillouin zones in the direction perpendicular tothe (111) surface, demonstrate the projected Dirac cones at the Gamma_bar and M_bar points of the surface Brillouinzone. In addition, we observe a Dirac-cone-like band structure at the Gamma point of the bulk Brillouin zone,whose Dirac energy is largely different from those at the Gamma_bar and M_bar points., 8 pages, 4 figures

A combination of photoemission and scanning tunnelling spectroscopy measurements provide compelling evidence that single layers of 1T'-WTe2 are a class of quantum spin Hall insulator. A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling1,2. By investigating the electronic structure of epitaxially grown monolayer 1T'-WTe2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulk bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large bandgap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).

Superconductivity in monolayer niobium diselenide (NbSe2) on bilayer graphene is studied by electrical transport. Monolayer NbSe2 is grown on bilayer graphene by molecular beam epitaxy and capped with a selenium film to avoid degradation in air. The selenium capped samples have TC = 1.9 K. In situ measurements down to 4 K in ultrahigh vacuum show that the effect of the selenium layer on the transport is negligible. The superconducting transition and upper critical fields in air exposed and selenium capped samples are compared. Schematic of monolayer NbSe2/bilayer graphene with selenium capping layer and electrical contacts.

A scanning tunnelling microscopy study demonstrates that one-dimensional charge density waves can form at twin boundaries in a monolayer transition metal dichalcogenide.

We investigate the electronic structure of BaMnBi2 and BaZnBi2 using angle-resolved photoemission spectroscopy and first-principles calculations. Although they share similar structural properties, we show that their electronic structure exhibit dramatic differences. A strong anisotropic Dirac dispersion is revealed in BaMnBi2 with a decreased asymmetry factor compared with other members of AMnBi2 (A = alkali earth or rare earth elements) family. In addition to the Dirac cones, multiple bands crossing the Fermi energy give rise to a complex Fermi surface topology for BaZnBi2. We further show that the strength of hybridization between Bi-p and Mn-d/Zn-s states is the main driver of the differences in electronic structure for these two related compounds.

Polarized Raman scattering spectra of the K_xCo_{2-y}Se_2 single crystals reveal the presence of two phonon modes, assigned as of the A_{1g} and B_{1g} symmetry. Absence of additional modes excludes the possibility of vacancy ordering, unlike in K_xFe_{2-y}Se_2. The ferromagnetic (FM) phase transition at T_c\approx 74 K leaves a clear fingerprint on the temperature dependence of the Raman mode energy and linewidth. For T>T_c the temperature dependence looks conventional, driven by the thermal expansion and anharmonicity. The Raman modes are rather broad due to the electron-phonon coupling increased by the disorder and spin fluctuation effects. In the FM phase the phonon frequency of both modes increases, while an opposite trend is seen in their linewidth: the A_{1g} mode narrows in the FM phase, whereas the B_{1g} mode broadens. We argue that the large asymmetry and anomalous frequency shift of the B_{1g} mode is due to the coupling of spin fluctuations and vibration. Our density functional theory (DFT) calculations for the phonon frequencies agree rather well with the Raman measurements, with some discrepancy being expected since the DFT calculations neglect the spin fluctuations., 9 pages, 4 figures, 3 tables

The interaction between a magnetic impurity, such as cerium (Ce) atom, and surrounding electrons has been one of the core problems in understanding many-body interaction in solid and its relation to magnetism. Kondo effect, the formation of a new resonant ground state with quenched magnetic moment, provides a general framework to describe many-body interaction in the presence of magnetic impurity. In this Letter, a combined study of angle-resolved photoemission (ARPES) and dynamic mean-field theory (DMFT) on Ce-intercalated graphene shows that Ce-induced localized states near Fermi energy, $E_{\rm F}$, hybridized with the graphene $\pi$ band, exhibit gradual increase in spectral weight upon decreasing temperature. The observed temperature dependence follows the expectations from the Kondo picture in the weak coupling limit. Our results provide a novel insight how Kondo physics emerges in the sea of two-dimensional Dirac electrons., Comment: 17 pages, 4 figures

The interaction between graphene and substrates provides a viable route to enhance the functionality of both materials. Depending on the nature of electronic interaction at the interface, the electron band structure of graphene is strongly influenced, allowing us to make use of the intrinsic properties of graphene or to design additional functionalities in graphene. Here, we present an angle-resolved photoemission study on the interaction between graphene and a platinum substrate. The formation of an interface between graphene and platinum leads to a strong deviation in the electronic structure of graphene not only from its freestanding form but also from the behavior observed on typical metals. The combined study on the experimental and theoretical electron band structure unveils the unique electronic properties of graphene on a platinum substrate, which singles out graphene/platinum as a model system investigating graphene on a metallic substrate with strong interaction.

Three-dimensional (3D) topological Dirac semimetals (TDSs) are rare but important as a versatile platform for exploring exotic electronic properties and topological phase transitions. A quintessential feature of TDSs is 3D Dirac fermions associated with bulk electronic states near the Fermi level. Using angle-resolved photoemission spectroscopy (ARPES), we have observed such bulk Dirac cones in epitaxially-grown {\alpha}-Sn films on InSb(111), the first such TDS system realized in an elemental form. First-principles calculations confirm that epitaxial strain is key to the formation of the TDS phase. A phase diagram is established that connects the 3D TDS phase through a singular point of a zero-gap semimetal phase to a topological insulator (TI) phase. The nature of the Dirac cone crosses over from 3D to 2D as the film thickness is reduced.

We report on the evidence for the multiband electronic transport in $\alpha$-YbAlB$_{4}$ and $\alpha$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Multiband transport reveals itself below 10 K in both compounds via Hall effect measurements, whereas anisotropic magnetic ground state sets in below 3 K in $\alpha$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Our results show that Sr$^{2+}$ substitution enhances conductivity, but does not change the quasiparticle mass of bands induced by heavy fermion hybridization., Comment: 5 pages, 4 figures

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

Monolayer transition metal dichalcogenides (TMDCs) in the 1T′ structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T′ phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T′-WTe2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T′-MoTe2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemission spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T′-MoTe2 on bilayer graphene a semimetal.

Layered transition metal dichalcogenides (TMDs) are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single 2D layer of NbSe2 by means of low temperature scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3x3 CDW order in NbSe2 remains intact in 2D. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of {\Delta} = 4 meV at the Fermi energy, which is accessible via STS due to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing new insight into CDW formation and superconductivity in this model strongly-correlated system., Comment: Nature Physics (2015), DOI:10.1038/nphys3527

We report electron doping effects by Ni in KxFe2-δ-yNiySe₂ (0.06 ≤ y ≤ 1.44) single crystal alloys. A rich ground state phase diagram is observed. Thus, a small amount of Ni (~ 4%) suppressed superconductivity below 1.8 K, inducing insulating spin glass magnetic ground state for higher Ni content. With further Ni substitution, metallic resistivity is restored. For high Ni concentration in the lattice the unit cell symmetry is high symmetry I4/mmm with no phase separation whereas both I4/m + I4/mmm space groups were detected in the phase separated crystals when concentration of Ni < Fe. The absence of superconductivity coincides with the absence of crystalline Fe vacancy order.

Spin-$1/2$ Heisenberg antiferromagnets ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ and ${\mathrm{Cs}}_{2}{\mathrm{CuBr}}_{4}$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$, yielding $J/{k}_{B}=4.7(2)\text{ }\text{ }\mathrm{K}$, ${J}^{\ensuremath{'}}/{k}_{B}=1.42(7)\text{ }\text{ }\mathrm{K}$, [${J}^{\ensuremath{'}}/J\ensuremath{\simeq}0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural ${\mathrm{Cs}}_{2}{\mathrm{CuBr}}_{4}$, we obtain $J/{k}_{B}=14.9(7)\text{ }\text{ }\mathrm{K}$, ${J}^{\ensuremath{'}}/{k}_{B}=6.1(3)\text{ }\text{ }\mathrm{K}$, [${J}^{\ensuremath{'}}/J\ensuremath{\simeq}0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

We have successfully synthesized a layered iron oxychalcogenide BaFe${}_{2}$Se${}_{2}$O single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the $c$ axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the $b$ axis and are bridged by oxygen along the $a$ axis. Physical property measurements indicate that BaFe${}_{2}$Se${}_{2}$O is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long-range antiferromagnetic transition at 240 K, corresponding to the peak in specific-heat measurement, and two transitions at 115 K and 43 K where magnetic susceptibility drops abruptly. The magnetic entropy up to 300 K is much smaller than the expected value for Fe${}^{2+}$ in tetrahedral crystal fields and the M\"ossbauer spectrum indicates that long-range magnetic order is unlikely at 294 K. Our results suggest that BaFe${}_{2}$Se${}_{2}$O is a magnetic insulator at the borderline between a long-range antiferromagnetic spin ordering and possible spin dimerization.

We report on the emergence of an Electronic Griffiths Phase (EGP) in the doped semiconductor FeSb$_{2}$, predicted for disordered insulators with random localized moments in the vicinity of a metal-insulator transition (MIT). Magnetic, transport, and thermodynamic measurements of Fe(Sb$_{1-x}$Te$_{x}$)$_{2}$ single crystals show signatures of disorder-induced non-Fermi liquid behavior and a Wilson ratio expected for strong electronic correlations. The EGP states are found on the metallic boundary, between the insulating state ($x = 0$) and a long-range albeit weak magnetic order ($x \geq 0.075$)., 5 pages, 4 figures

Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.

We present the upper critical fields ${\ensuremath{\mu}}_{0}{H}_{c2}(T)$ and Hall effect in $\ensuremath{\beta}$-FeSe single crystals. The ${\ensuremath{\mu}}_{0}{H}_{c2}(T)$ increases as the temperature is lowered for fields applied parallel and perpendicular to (101), the natural growth facet of the crystal. The ${\ensuremath{\mu}}_{0}{H}_{c2}(T)$ for both field directions and the anisotropy at low temperature increase under pressure. Hole carriers are dominant at high magnetic fields. However, the contribution of electron-type carriers is significant at low fields and low temperature. Our results show that multiband effects dominate ${\ensuremath{\mu}}_{0}{H}_{c2}(T)$ and electronic transport in the normal state.

Polarized Raman scattering spectra of superconducting K$_x$Fe$_{2-y}$Se$_2$ and nonsuperconducting K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$ single crystals were measured in a temperature range from 10 K up to 300 K. Two Raman active modes from the $I4/mmm$ phase and seven from the $I4/m$ phase are observed in frequency range from 150 to 325 cm$^{-1}$ in both compounds, suggesting that K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$ single crystal also has two-phase nature. Temperature dependence of Raman mode energy is analyzed in terms of lattice thermal expansion and phonon-phonon interaction. Temperature dependence of Raman mode linewidth is dominated by temperature-induced anharmonic effects. It is shown that change of Raman mode energy with temperature is dominantly driven by thermal expansion of the crystal lattice. Abrupt change of the A$_{1g}$ mode energy near $T_C$ was observed in K$_x$Fe$_{2-y}$Se$_2$, whereas it is absent in non-superconducting K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below critical temperature., 13 pages, 6 figures