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In this study, high pressure synchrotron powder X-ray diffraction is used to investigate the compression of two high pressure polymorphs of \ce{CrSb2}. The first is the \ce{CuAl2}-type polymorph with an eight-fold coordinated Cr, which can be quenched to ambient conditions from high-pressure high-temperature conditions. The second is the recently discovered \ce{MoP2}-type polymorph, which is induced by compression at room temperature, with a seven-fold coordinated Cr. Here, the assigned structure is unambiguously confirmed by solving it from single-crystal X-ray diffraction. Furthermore, the electrical properties of the \ce{MoP2}-type polymorph were investigated theoretically and the resistance calculations under pressure were accompanied by resistance measurements under high pressure on a single crystal of \ce{CrSb2}. The calculated electronic band structure for the \ce{MoP2}-type phase is discussed and we show that the polymorph is semimetallic and possesses type-I Weyl points. No further phase transitions were observed for the \ce{CuAl2}-type structure up to 50 GPa and 40 GPa for the \ce{MoP2}-type structure. Even though the \ce{CuAl2}-phase has the highest coordination number of Cr, it was found to be less compressible than the \ce{MoP2}-phase having a seven-fold coordinated Cr, which was attributed to the longer Cr-Sb distance in the \ce{CuAl2}-type phase. The discovery of a type-I Weyl semimetallic phase in \ce{CrSb2} opens up for discovering other Weyl semimetals in the transition metal di-pnictides under high pressure., Comment: 20 pages, 4 tables, 6 figures

Understanding how light modifies long-range order is key to improve our ability to control material functionality on an ultrafast timescale. Transient spatial heterogeneity has been proposed in many materials, but isolating the dynamics of different regions experimentally has been challenging. Here we address this issue and measure the dynamics of orbital order melting in the layered manganite, La0.5Sr1.5MnO4, and isolate the surface dynamics from the bulk for the first time. Bulk measurements show orbital order is rapidly suppressed, but the correlation length surprisingly increases. However, the surface dynamics, show a stronger suppression and a significant decrease in correlation length. By isolating the surface changes, we find that light preferentially melts a less ordered surface and the loss of long-range order is likely driven by the formation of local and disordered polarons. Melting the disordered surface effectively increases the average correlation of the bulk probed volume, resolving the contradictory response. These results show that surface scattering methods are necessary to understand both surface and bulk dynamics in heterogeneous materials., Comment: 22 pages, 8 figures

The electronic structure of NdTe$_3$ in the charge density wave phase (CDW) is investigated by angle-resolved photoemission spectroscopy. The combination of high-quality crystals and careful surface preparation reveals subtle and previously unobserved details in the Fermi surface topology, allowing an interpretation of the rich and unexplained quantum oscillations in the rare earth tritellurides RTe$_3$. In particular, several closed Fermi surface elements can be observed that are related to CDW-induced replicas of the original bands, leading to the curious situation in which a CDW does not only remove Fermi surface elements but creates new ones that are observable in transport experiments. Moreover, a large residual Fermi surface is found in the CDW gap, very close to the position of the gapped normal-state Fermi surface. Its area agrees very well with high-frequency quantum oscillations in NdTe$_3$ and its presence is explained by strong electron-phonon coupling combined with the quasi one-dimensional character of the CDW. Finally, we identify the origin of the low-frequency $\alpha$ quantum oscillations ubiquitous for the lighter R elements in the RTe$_3$ family and responsible for the high mobility in these compounds.

Misfit compounds are thermodynamically stable stacks of two-dimensional materials, forming a three-dimensional structure that remains incommensurate in one direction parallel to the layers. As a consequence, no true bonding is expected between the layers, with their interaction being dominated by charge transfer. In contrast to this well-established picture, we show that interlayer coupling can strongly influence the electronic properties of one type of layer in a misfit structure, in a similar way to the creation of modified band structures in an artificial moir\'e structure between two-dimensional materials. Using angle-resolved photoemission spectroscopy with a micron-scale light focus, we selectively probe the electronic properties of hexagonal NbSe$_2$ and square BiSe layers that terminate the surface of the (BiSe)$_{1+\delta}$NbSe$_2$ misfit compound. We show that the band structure in the BiSe layers is strongly affected by the presence of the hexagonal NbSe$_2$ layers, leading to quasi one-dimensional electronic features. The electronic structure of the NbSe$_2$ layers, on the other hand, is hardly influenced by the presence of the BiSe. Using density functional theory calculations of the unfolded band structures, we argue that the preferred modification of one type of bands is mainly due to the atomic and orbital character of the states involved, opening a promising way to design novel electronic states that exploit the partially incommensurate character of the misfit compounds.

While parallel segments in the Fermi level contours, often found at the surfaces of topological insulators (TIs) would imply "strong" nesting conditions, the existence of charge density waves (CDWs) - periodic modulations of the electron density - has not been verified up to now. Here, we report the observation of a CDW at the surface of the Bi$_2$Te$_2$Se(111), below $\approx 350\,$K by helium atom scattering, and thus experimental evidence of a CDW involving Dirac topological electrons. Deviations of the order parameter observed below $180\,$K and a low temperature break of time reversal symmetry suggest the onset of a spin density wave with the same period as the CDW in presence of a prominent electron-phonon interaction originating from the Rashba spin-orbit coupling.

The bulk band structure of the topological insulator \sbte~ is investigated by angle-resolved photoemission spectroscopy. Of particular interest is the dispersion of the uppermost valence band with respect to the topological surface state Dirac point. The valence band maximum has been calculated to be either near the Brillouin zone centre or in a low-symmetry position in the $\bar{\Gamma}-\bar{M}$ azimuthal direction. In order to observe the full energy range of the valence band, the strongly p-doped crystals are counter-doped by surface alkali adsorption. The data show that that the absolute valence band maximum is likely to be found at the bulk $\Gamma$ point and predictions of a low-symmetry position with an energy higher than the surface Dirac point can be ruled out., Comment: 6 pages, 4 figures

We report on temperature-dependent soft X-ray absorption spectroscopy (XAS) measurements utilizing linearly polarized synchrotron radiation to probe magnetic phase transitions in iron-rich Fe1+yTe. X-ray magnetic linear dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces, start to increase monotonically below the N\'eel temperature TN = 57 K. This increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment of Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was achieved by a [100]-oriented biasing field favoring a single-domain state during cooling across TN. Our specific heat and magnetization measurements confirm the bulk character of this AFM phase transition. On longer time scales, however, we observe that the field-biased AFM state is highly unstable even at the lowest temperature of T = 3 K. After switching off the biasing field, the XMLD signal decays exponentially with a time constant {\tau} = 1506 s. The initial XMLD signal is restored only upon repeating a cycle consisting of heating and field-cooling through TN. We explain the time effect by a gradual formation of a multi-domain state with 90 deg rotated AFM domains, promoted by structural disorder, facilitating the motion of twin-domains. Significant disorder in our Fe1+yTe sample is evident from our X-ray diffraction and specific heat data. The stability of magnetic phases in Fe-chalcogenides is an important material property, since the Fe(Te1-xSex) phase diagram shows magnetism intimately connected with superconductivity., Comment: Conference proceeding for JEMS 2020

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator (TI) Bi$_2$Se$_3$ with particular emphasis on the low-energy region below 10 meV that has been difficult to resolve experimentally. By applying inelastic helium atom scattering (HAS), the entire phonon dispersion was determined and compared with density functional perturbation theory (DFPT) calculations. The intensity of the phonon modes is dominated by a strong Rayleigh mode, in contrast to previous experimental works. Moreover, also at variance with recent reports, no Kohn-anomaly is observed. These observations are in excellent agreement with DFPT calculations. Besides these results, the experimental data reveal$-$via bound-state resonance enhancement$-$two additional dispersion curves in the gap below the Rayleigh mode. They are possibly associated with an excitation of a surface electron density superstructure that we observe in HAS diffraction patterns. The electron-phonon coupling paramenter $\lambda$ = 0.23 derived from our temperature dependent Debye-Waller measurements compares well with values determined by angular resolved photoemission or Landau level spectroscopy. Our work opens up a new perspective for THz measurements on 2D materials as well as the investigation of subtle details (band bending, the presence of quantum well states) with respect to the electron-phonon coupling.

We present the result of an \textit{ab initio} search for new Dirac materials among inverse perovskites. Our investigation is focused on the less studied class of lanthanide antiperovskites containing heavy $f$-electron elements in the cation position. Some of the studied compounds have not yet been synthesized experimentally. Our computational approach is based on density functional theory calculations which account for spin-orbit interaction and strong correlations of the $f$-electron atoms. We find several promising candidates among lanthanide antiperovskites which host bulk Dirac states close to the Fermi level. Specifically, our calculations reveal massive three-dimensional Dirac states in materials of the class A$_3$BO, where A=Sm, Eu, Gd, Yb and B=Sn, Pb. In materials with finite magnetic moment, such as Eu$_3$BO (B=Sn, Pb), the degeneracy of the Dirac nodes is lifted, leading to appearance of Weyl nodes., Comment: 9 pages, 5 figures

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi$_2$Te$_3$. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi$_2$Te$_3$(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.

VS2 is a challenging material to prepare stoichiometrically in the bulk, and the single layer has not been successfully isolated before now. Here we report the first realization of single-layer VS2, which we have prepared epitaxially with high quality on Au(111) in the octahedral (1T) structure. We find that we can deplete the VS2 lattice of S by annealing in vacuum so as to create an entirely new two-dimensional compound that has no bulk analogue. The transition is reversible upon annealing in an H2S gas atmosphere. We report the structural properties of both the stoichiometric and S-depleted compounds on the basis of low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and scanning tunneling microscopy experiments., Comment: 18 pages, 6 figures

We have determined an atom-surface interaction potential for the He$-$Bi$_2$Te$_3$(111) system by analysing ultrahigh resolution measurements of selective adsorption resonances. The experimental measurements were obtained using $^3$He spin-echo spectrometry. Following an initial free-particle model analysis, we use elastic close-coupling calculations to obtain a three-dimensional potential. The three-dimensional potential is then further refined based on the experimental data set, giving rise to an optimised potential which fully reproduces the experimental data. Based on this analysis, the He$-$Bi$_2$Te$_3$(111) interaction potential can be described by a corrugated Morse potential with a well depth $D=(6.22\pm0.05)~\mathrm{meV}$, a stiffness $\kappa =(0.92\pm0.01)~\mathrm{\AA}^{-1}$ and a surface electronic corrugation of $(9.6\pm0.2)$% of the lattice constant. The improved uncertainties of the atom-surface interaction potential should also enable the use in inelastic close-coupled calculations in order to eventually study the temperature dependence and the line width of selective adsorption resonances.

The investigation of the magnetic phase transitions in the parent compounds of Fe-based superconductors is regarded essential for an understanding of the pairing mechanism in the related superconducting compounds. Even though the chemical and electronic properties of these materials are often strongly inhomogeneous on a nanometer length scale, studies of the magnetic phase transitions using spatially resolved experimental techniques are still scarce. Here, we present a real space spin-resolved scanning tunneling microscopy investigation of the surface of Fe$_{1+y}$Te single crystals with different excess Fe content, $y$, which are continuously driven through the magnetic phase transition. For Fe$_{1.08}$Te, the transition into the low-temperature monoclinic commensurate antiferromagnetic phase is accompanied by the sudden emergence of ordering into four rotational domains with different orientations of the monoclinic lattice and of the antiferromagnetic order, showing how structural and magnetic order are intertwined. In the low-temperature phase of Fe$_{1.12}$Te one type of the domain boundaries disappears, and the transition into the paramagnetic phase gets rather broad, which is assigned to the formation of a mixture of orthorhombic and monoclinic phases.

We studied the temperature dependence of the diagonal double-stripe spin order in one and two unit cell thick layers of FeTe grown on the topological insulator Bi_2Te_3 via spin-polarized scanning tunneling microscopy. The spin order persists up to temperatures which are higher than the transition temperature reported for bulk Fe_1+yTe with lowest possible excess Fe content y. The enhanced spin order stability is assigned to a strongly decreased y with respect to the lowest values achievable in bulk crystal growth, and effects due to the interface between the FeTe and the topological insulator. The result is relevant for understanding the recent observation of a coexistence of superconducting correlations and spin order in this system., Comment: 16 pages and 4 figures

The electronic structure of thin films of FeTe grown on Bi$_2$Te$_3$ is investigated using angle-resolved photoemission spectroscopy, scanning tunneling microscopy and first principles calculations. As a comparison, data from cleaved bulk \FeTe taken under the same experimental conditions is also presented. Due to the substrate and thin film symmetry, FeTe thin films grow on Bi$_2$Te$_3$ in three domains, rotated by 0$^{\circ}$, 120$^{\circ}$, and 240$^{\circ}$. This results in a superposition of photoemission intensity from the domains, complicating the analysis. However, by combining bulk and thin film data, it is possible to partly disentangle the contributions from three domains. We find a close similarity between thin film and bulk electronic structure and an overall good agreement with first principles calculations, assuming a p-doping shift of 65~meV for the bulk and a renormalization factor of around 2. By tracking the change of substrate electronic structure upon film growth, we find indications of an electron transfer from the FeTe film to the substrate. No significant change of the film's electronic structure or doping is observed when alkali atoms are dosed onto the surface. This is ascribed to the film's high density of states at the Fermi energy. This behavior is also supported by the ab-initio calculations.

We have studied the topological insulator Bi$_2$Te$_3$(111) by means of helium atom scattering. The average electron-phonon coupling $\lambda$ of Bi$_2$Te$_3$(111) is determined by adapting a recently developed quantum-theoretical derivation of the helium scattering probabilities to the case of degenerate semiconductors. Based on the Debye-Waller attenuation of the elastic diffraction peaks of Bi$_2$Te$_3$(111), measured at surface temperatures between $110~\mbox{K}$ and $355~\mbox{K}$, we find $\lambda$ to be in the range of $0.04-0.11$. This method allows to extract a correctly averaged $\lambda$ and to address the discrepancy between previous studies. The relatively modest value of $\lambda$ is not surprising even though some individual phonons may provide a larger electron-phonon interaction. Furthermore, the surface Debye temperature of Bi$_2$Te$_3$(111) is determined as ${\rm \Theta}_D = (81\pm6)~\mbox{K}$. The electronic surface corrugation was analysed based on close-coupling calculations. By using a corrugated Morse potential a peak-to-peak corrugation of 9% of the lattice constant is obtained., Comment: 9 pages, 4 figures

The crystal structure of CeN was investigated up to pressures of 82 GPa, using diamond anvil cell powder X-ray diffraction in two experiments with He and Si-oil as the pressure transmitting media. In contrast to previous reports, we do not observe the B2 (CsCl type) structure at high pressure. Instead, the structural phase transition, starting at 65 GPa, from the ambient rock salt B1 structure results in a distorted CsCl-like B10 structure, irrespective of the pressure medium. Our result unambiguously confirms two recent density functional theory (DFT) studies predicting the B10 phase to be stable at these pressures, rather than the B2 (CsCl type) phase previously reported. The B10 structure appears to approach the B2 structure as pressure is increased further, but DFT calculations indicate that an L1 0 structure (AuCu type) is energetically favored., Comment: 35 pages, preprint

We present our investigations on the superconducting properties of monolayers of FeSe$_{0.5}$Te$_{0.5}$ grown on the 3D topological insulator Bi$_{2}$Se$_{1.2}$Te$_{1.8}$ using low temperature scanning tunneling spectroscopy (STS). While the morphology and the overall transition temperature resemble those of similarly doped bulk crystals, the spatially resolved spectroscopic data at 1.1K shows a much larger spatial inhomogeneity in the superconducting energy gaps. Despite the gap inhomogeneity all the spectra can be fitted with a two-fold anisotropic s-wave gap function. The two-fold nature of the gap symmetry is evident from the Bogoliubov quasiparticle interference (QPI) pattern which shows distinct C$_{2}$ symmetric scattering intensities. We argue that the gap inhomogeneity emerges as a result of intrinsic disorder in our system similar to disordered conventional superconductors. Even though most of our findings clearly differ from the current understanding of the corresponding bulk system, it provides an ideal platform to study unconventional superconductivity in Fe chalcogenides thinned down to a single layer and in close proximity to a topological insulator.

Establishing the relation between the ubiquitous antiferromagnetism in the non-superconducting parent compounds of unconventional superconductors and their superconducting phase is believed to be important for the understanding of the complex physics in these materials. Going from the bulk systems to thin films strongly affects the phase diagram of unconventional superconductors. For Fe$_{1+y}$Te, the parent compound of the Fe$_{1+y}$Se$_{1-x}$Te$_x$ superconductors, bulk sensitive neutron diffraction has revealed an in-plane oriented bicollinear antiferromagnetic structure. Here, we show by spin-resolved scanning tunneling microscopy that on the surfaces of bulk Fe$_{1+y}$Te, as well as on thin films grown on the topological insulator Bi$_2$Te$_3$, the spin direction is canted both away from the surface plane and from the high-symmetry directions of the surface unit cell, while keeping the bicollinear magnetic structure. Our results demonstrate that the magnetism at the Fe-chalcogenide surface markedly deviates from a simple in-plane oriented bicollinear antiferromagnetic structure, which implies that the pairing at the surface of the related superconducting compounds might be different from that in the bulk.

The structural and electronic properties of FeSe ultra-thin layers on Bi$_{2}$Se$_{3}$ have been investigated with a combination of scanning tunneling microscopy and spectroscopy and angle-resolved photoemission spectroscopy. The FeSe multi-layers, which are predominantly 3-5 monolayers (ML) thick, exhibit a hole pocket-like electron band at \bar{\Gamma} and a dumbbell-like feature at \bar{M}, similar to multi-layers of FeSe on SrTiO$_{3}$. Moreover, the topological state of the Bi2Se3 is preserved beneath the FeSe layer, as indicated by a heavily \it{n}-doped Dirac cone. Low temperature STS does not exhibit a superconducting gap for any investigated thickness down to a temperature of 5 K.

The discovery of high-temperature superconductivity in Fe-based compounds [1,2] has triggered numerous investigations on the interplay between superconductivity and magnetism [3] and, more recently, on the enhancement of transition temperatures through interface effects [4]. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture of this relationship remains elusive [1] due to the lack of data with atomic spatial resolution [5-7]. Here, we present a spin-polarized scanning tunneling spectroscopy (SP-STS) study of ultrathin FeTe$_{1-x}$Se$_x$ (x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level indicating superconducting correlations up to Tc ~ 6 K for one unit cell thin FeTe layers grown on Bi2Te3 substrates, in contrast to the non-superconducting FeTe bulk compound [8]. Moreover, SP-STS reveals that the energy gap spatially coexists with bicollinear AFM order. This finding opens novel perspectives for theoretical studies of competing orders in Fe-based superconductors as well as for experimental investigations of exotic phases in heterostructures of topological insulators and superconducting layers.

Near-surface two-dimensional electron gases on the topological insulator Bi$_2$Te$_2$Se are induced by electron doping and studied by angle-resolved photoemission spectroscopy. A pronounced spin-orbit splitting is observed for these states. The $k$-dependent splitting is strongly anisotropic to a degree where a large splitting ($\approx 0.06$ \AA$^{-1}$) can be found in the $\bar{\Gamma}\bar{M}$ direction while the states are hardly split along $\bar{\Gamma}\bar{K}$. The direction of the anisotropy is found to be qualitatively inconsistent with results expected for a third-order anisotropic Rashba Hamiltonian. However, a $\mathbf{k} \cdot \mathbf{p}$ model that includes the possibility of band structure anisotropy as well as both isotropic and anisotropic third order Rashba splitting can explain the results. The isotropic third order contribution to the Rashba Hamiltonian is found to be negative, reducing the energy splitting at high $k$. The interplay of band structure, higher order Rashba effect and tuneable doping offers the opportunity to engineer not only the size of the spin-orbit splitting but also its direction., Comment: 5 pages, 4 figures

Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillations, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present the first direct measurement of surface-dominated conduction on an atomically clean surface of bulk-insulating Bi$_2$Te$_2$Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm$^{2}$V$^{-1}$s$^{-1}$ at 30 K at a carrier concentration of 8.71(7)$\times 10^{12}$ cm$^{-2}$.