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Topological semimetals such as Dirac, Weyl or nodal line semimetals are widely studied for their peculiar properties including high Fermi velocities, small effective masses and high magnetoresistance. When the Dirac cone is tilted, exotic phenomena could emerge whereas materials hosting such states are promising for photonics and plasmonics applications. Here we present evidence that SrAgBi is a spin-orbit coupling-induced type-II three-dimensional Dirac semimetal featuring tilted Dirac cone at the Fermi energy. Near charge compensation and Fermi surface characteristics are not much perturbed by 7% of vacancy defects on the Ag atomic site, suggesting that SrAgBi could be a material of interest for observation of robust optical and spintronic topological quantum phenomena.

Identifying and understanding the mechanisms behind strong phonon–phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c-axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices.

Acta crystallographica / A 77(6), 611 - 636 (2021). doi:10.1107/S2053273321010159, We explore data reduction and correction steps and processed data reproducibility in the emerging single crystal total scattering based technique of three-dimensional differential atomic pair distribution function (3D-$\Delta$PDF) analysis. All steps from sample measurement to data-processing are outlined in detail using a CuIr$_2$S$_4$ example crystal studied in a setup equipped with a high-energy x-ray beam and a flat panel area detector. Computational overhead as it pertains to data-sampling and the associated data processing steps is also discussed. Various aspects of the final 3D-$\Delta$PDF reproducibility are explicitly tested by varying data-processing order and included steps, and by carrying out a crystal-to-crystal data comparison. We identify situations in which the 3D-$\Delta$PDF is robust, and caution against a few particular cases which can lead to inconsistent 3D-$\Delta$PDFs. Although not all the approaches applied here-in will be valid across all systems, and a more in-depth analysis of some of the effects of the data processing steps may still needed, the methods collected here-in represent the start of a more systematic discussion about data processing and corrections in this field., Published by Blackwell, Oxford [u.a.]

The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe0.55Se0.45. However, the topological features and superconducting properties are not observed uniformly across the sample surface. The understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications. Here, we combine neutron scattering, scanning angle-resolved photoemission spectroscopy, and microprobe composition and resistivity measurements to characterize the electronic state of Fe1+yTe1-xSex. We establish a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, whereas the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe0.55Se0.45 is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. Our results demonstrate the compositional control required for use of topological MZMs in practical applications.

We present a detailed investigation of the crystal structure of VI3, a two-dimensional van der Waals material of interest for studies of low-dimensional magnetism. As opposed to the average crystal structure that features R3 symmetry of the unit cell, our Raman scattering and X-ray atomic pair distribution function analysis supported by density functional theory calculations point to the coexistence of short-range ordered P31c and long-range ordered R3 phases. The highest-intensity peak, A1g3, exhibits a moderate asymmetry that might be traced back to the spin-phonon interactions, as in the case of CrI3.

Identifying material parameters affecting properties of ferromagnets is key to optimized materials that are better suited for spintronics. Magnetic anisotropy is of particular importance in van der Waals magnets, since it not only influences magnetic and spin transport properties, but also is essential to stabilizing magnetic order in the two-dimensional limit. Here, we report that hole doping effectively modulates the magnetic anisotropy of a van der Waals ferromagnet and explore the physical origin of this effect. Fe3-xGeTe2 nanoflakes show a significant suppression of the magnetic anisotropy with hole doping. Electronic structure measurements and calculations reveal that the chemical potential shift associated with hole doping is responsible for the reduced magnetic anisotropy by decreasing the energy gain from the spin-orbit induced band splitting. Our findings provide an understanding of the intricate connection between electronic structures and magnetic properties in two-dimensional magnets and propose a method to engineer magnetic properties through doping.

The name of the third author of the article by Koch et al. [Acta Cryst. (2021). A77, 611–636] is corrected.

We report our investigations on the effects of pressure on the electronic and magnetic properties of magnetic Dirac semimetal ${\mathrm{EuMnBi}}_{2}$ by using electrical transport, synchrotron x-ray diffraction, and x-ray absorption spectroscopy. The antiferromagnetic interaction due to the Eu magnetic moment in ${\mathrm{EuMnBi}}_{2}$ is enhanced under pressure up to $\ensuremath{\sim}3.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The N\'eel temperature (${T}_{N}$) is found to disappear above 4 GPa and a new transition ${T}_{t}$ at around 29 K emerges above 6 GPa. ${T}_{t}$ remains relatively constant up to 15 GPa, above which it increases with further compression, reaching $\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 22 GPa. Eu $L$-edge x-ray absorption spectroscopy revealed a valence change of Eu toward a trivalent state that begins above 6 GPa, which indicates that ${T}_{t}$ is likely related to the valence transition temperature of Eu. Sign reversal of the Hall resistivity above 7 GPa suggests that the valence change of Eu also induces a Fermi surface modification.

Topological fermions have mainly been addressed in the limit of weakly correlated systems, while the realization of Kondo-like physics arising from Dirac-Weyl fermions and implying strong correlations is rare. The noncentrosymmetric Weyl fermion $R\mathrm{AlGe}$ compounds (with $R=\mathrm{La} \mathrm{and} \mathrm{Ce}$) provide a promising playground for revealing the impact of electronic correlations driven by $f$-electron states on Weyl fermions. Here, we tackle their charge dynamics as a function of temperature. Besides spotting typical optical signatures of a type-II Weyl semimetal in both materials, we discover that electronic correlations at low temperatures renormalize the nontrivial bands hosting the Weyl nodes and lead to a reduction of the Fermi velocity as well as an enhancement of the charge carriers effective mass in CeAlGe with respect to LaAlGe, both being a fingerprint of a so-called Weyl-Kondo system.

We present a polarization-resolved, high-resolution Raman scattering study of the three consecutive charge density wave (CDW) regimes in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ single crystals, supported by ab initio calculations. Our analysis of the spectra within the low-temperature commensurate (C-CDW) regime shows $P\overline{3}$ symmetry of the system, thus excluding the previously proposed triclinic stacking of the ``star-of-David'' structure, and promoting trigonal or hexagonal stacking instead. The spectra of the high-temperature incommensurate (IC-CDW) phase directly project the phonon density of states due to the breaking of the translational invariance, supplemented by sizable electron-phonon coupling. Between 200 and 352 K, our Raman spectra show contributions from both the IC-CDW and the C-CDW phases, indicating their coexistence in the so-called nearly commensurate (NC-CDW) phase. The temperature dependence of the symmetry-resolved Raman conductivity indicates the stepwise reduction of the density of states in the CDW phases, followed by a Mott transition within the C-CDW phase. We determine the size of the Mott gap to be ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{gap}}\ensuremath{\approx}170\text{--}190$ meV, and track its temperature dependence.

Two-dimensional (2D) materials with intrinsic ferromagnetism provide unique opportunity to engineer new functionalities in nano-spintronics. One such material is CrI3, showing long-range magnetic order in monolayer with the Curie temperature (Tc) of 45 K. Here we study detailed evolution of magnetic transition and magnetic critical properties in response to systematic reduction in crystal thickness down to 50 nm. Bulk Tc of 61 K is gradually suppressed to 57 K, however, the satellite transition at T * = 45 K is observed layer-independent at fixed magnetic field of 1 kOe. The origin of T * is proposed to be a crossover from pinning to depinning of magnetic domain walls. The reduction of thickness facilitates a field-driven metamagnetic transition around 20 kOe with out-of-plane field, in contrast to the continuous changes with in-plane field. The critical analysis around Tc elucidates the mean-field type interactions in microscale-thick CrI3.

We present a comprehensive study on anisotropic magnetocaloric porperties of the van der Waals weak-itinerant ferromagnet Fe3−xGeTe2 that features gate-tunable room-temperature ferromagnetism in few-layer device. Intrinsic magnetocrystalline anisotropy is observed to be temperature-dependent and most likely favors the long-range magnetic order in thin Fe3−xGeTe2 crsytal. The magnetic entropy change ΔSM also reveals an anisotropic characteristic between H//ab and H//c, which could be well scaled into a universal curve. The peak value $$-\Delta {S}_{M}^{{\max }}$$−ΔSMmax of 1.20 J kg−1 K−1 and the corresponding adiabatic temperature change ΔTad of 0.66 K are deduced from heat capacity with out-of-plane field change of 5 T. By fitting of the field-dependent parameters of $$-\Delta {S}_{M}^{{\max }}$$−ΔSMmax and the relative cooling power RCP, it gives $$-{\rm{\triangle }}{S}_{M}^{{\max }}$$−∆SMmax ∝ Hn with n = 0.603(6) and RCP ∝ Hm with m = 1.20(1) when H//c. Given the high and tunable Tc, Fe3−xGeTe2 crystals are of interest for fabricating the heterostructure-based spintronics device.

Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the $d$-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature of one such compound, \cis . We show that this hitherto overlooked fluctuating symmetry lowering is electronic in origin and will significantly modify the energy-level spectrum and electronic and magnetic properties. The explanation is a local, fluctuating, orbital-degeneracy-lifted state. The natural extension of our result would be that this phenomenon is likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties., 10 pages 3 figures

We provide a discussion of our previously collected data (Chinotti et al., Phys. Rev. B 94, 245101 (2016)) on the electrodynamic response in YbMnBi2, a representative Weyl semimetal, and in its gapped semimetal counterpart EuMnBi2, which can be fairly well reproduced within a recent devoted theoretical treatment (S.P. Mukherjee and J.P. Carbotte, J. Phys.: Condens. Matter 29 (2017) 425301). This allows identifying and catching all the essential features of their peculiar electronic structure as imaged by the excitation spectrum.

Van der Waals (vdW) ferromagnets are an important class of materials for spintronics applications. The recent discovery of atomically vdW magnets ${\mathrm{CrI}}_{3}$ and ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ has triggered a renaissance in the area of two-dimensional (2D) magnetism. Herein we systematically studied 2H-${\mathrm{Mn}}_{0.28}{\mathrm{TaS}}_{2}$ single crystal, a 2D vdW ferromagnet with ${T}_{c}\ensuremath{\sim}82.3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and a large in-plane magnetic anisotropy. Mn $K$-edge x-ray absorption spectroscopy was measured to provide information on its electronic state and local atomic environment. The detailed magnetic isotherms measured in the vicinity of ${T}_{c}$ indicates that the spin coupling inside 2H-${\mathrm{Mn}}_{0.28}{\mathrm{TaS}}_{2}$ is of a 3D Heisenberg type coupled with the attractive long-range interaction between spins that decay as $J(r)\ensuremath{\approx}{r}^{\ensuremath{-}4.85}$. Both resistivity $\ensuremath{\rho}(T)$ and thermopower $S(T)$ exhibit anomalies near ${T}_{c}$, confirming that the hole-type transport carriers strongly interact with local moments. An unusual angle-dependent magnetoresistance is further observed, suggesting a possible field-induced novel magnetic structure.

Semiconducting CoSbS is well known for its thermoelectric performance at high temperatures but it is also of interest because of its colossal low-temperature thermopower. Here, we address the temperature dependence of its optical response over a broad spectral range, from which we reveal several ingredients determining the thermoelectric properties at cryotemperatures. We discover coherent phonon modes and with the additional support of scanning transmission electron microscopy investigations we provide evidence for in-gap impurity states driving the formation of correlated electrons in the valence band. Their implications, with respect to the high thermoelectric power at low temperatures, are discussed within the framework of the phonon-drag transport of low-mobility heavy quasiparticles.

We examine the magnetic properties of the quasi 2D ferrimagnetic single crystal Mn3Si2Te6 (MST) through critical phenomena and magnetic entropy analysis in the easy axis (H || ab) as a function of proton irradiance. Employing a modified asymptotic analysis method, we find that upon proton irradiation the critical exponents do not fall into any particular universality class but lie close to mean field critical exponents ({\gamma} = 1, \b{eta} = 0.5). The presence of long-range interactions can be safely assumed for the pristine and irradiated cases of MST examined in this work. Further analysis on the effective spatial dimensionality reveal that MST remains at d = 3 under proton irradiation transitioning from an n = 1 spin dimensionality to n = 2 and n=3 for 1 x 10^15 and 5 x 10^15 H+/cm^2, indicating an XY interaction and a Heisenberg interaction, respectively. The pair (spin-spin) correlation function reveals an increase in magnetic correlations at the proton irradiance of 5 x 10^15 H+/cm^2. In conjunction, the maximum change in magnetic entropy obtained from isothermal magnetization at 3 T is the largest for 5 x 10^15 H+/cm^2 with a value of 2.45 J/kgK at T = 73.66 K, in comparison to 1.60 J/kgK for pristine MST at T = 73 K. Magnetic entropy derived from zero-field heat capacity does not show large deviations across the proton irradiated samples. This suggests that the antiferromagnetic coupling between the Mn sites is stable even after proton irradiation. Such result implies that magnetization is enhanced through a strengthening of the super-exchange interaction between Mn atoms mediated through Te rather than a weakening of the AFM component. Overall, our study finds that the magnetic interactions are manipulated the greatest when MST is irradiated at 5 x 10^15 H+/cm^2.

We carried out a comprehensive study of electronic transport, thermal, and thermodynamic properties in $\mathrm{Fe}{\mathrm{Cr}}_{2}{\mathrm{Te}}_{4}$ single crystals. It exhibits bad-metallic behavior and anomalous Hall effect (AHE) below a weak-itinerant paramagnetic-to-ferrimagnetic transition ${T}_{c}\ensuremath{\sim}123$ K. The linear scaling between the anomalous Hall resistivity ${\ensuremath{\rho}}_{xy}$ and the longitudinal resistivity ${\ensuremath{\rho}}_{xx}$ implies that the AHE in $\mathrm{Fe}{\mathrm{Cr}}_{2}{\mathrm{Te}}_{4}$ is most likely dominated by an extrinsic skew-scattering mechanism rather than an intrinsic KL or an extrinsic side-jump mechanism, which is supported by our Berry phase calculations.

The coupling between charge and spin orderings in strongly correlated systems plays a crucial role in fundamental physics and device applications. As a candidate of multiferroic materials, LuFe2O4 with a nominal Fe2.5+ valence state has the potential for strong charge-spin interactions; however, these interactions have not been fully understood until now. Here, combining complementary characterization methods with theoretical calculations, two types of charge orderings with distinct magnetic properties are revealed. The ground states of LuFe2O4 are decided by the parallel/antiparallel coupling of both charge and spin orderings in the adjacent FeO double layers. Whereas the ferroelectric charge ordering remains ferrimagnetic below 230 K, the antiferroelectric ordering undergoes antiferromagnetic-ferrimagnetic-paramagnetic transitions from 2 K to room temperature. This study demonstrates the unique aspects of strong spin-charge coupling within LuFe2O4. Our results shed light on the coexistence and competing nature of orderings in quantum materials.

Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) measurements have previously been applied to the study of the heavy-fermion system CeCoIn5 to examine the superconducting gap structure and band dispersions via quasiparticle intereference. Here we directly measure the dispersing electron bands with angle-resolved photoelectron spectroscopy (ARPES) and compare with first-principles electronic structure calculations. By autocorrelating the ARPES-resolved bands with themselves we can measure the potential q vectors and discern exactly which bands the STM is measuring. We find that the STM results are dominated by scattering associated with a cloverleaf shaped band centered at the zone corners. This same band is also a viable candidate to host the superconducting gap. The electronic structure calculations indicate that this region of the Fermi surface involves significant contributions from the Co d electrons, an indication that the superconductivity in these materials is more three dimensional than that found in the related unconventional superconductors, the cuprates and the pnictides. publishedVersion

We report discovery of new antiferromagnetic semimetal EuZnSb$_2$, obtained and studied in the form of single crystals. Electric resistivity, magnetic susceptibility and heat capacity indicate antiferromagnetic order of Eu with $T_N$ = 20 K. The effective moment of Eu$^{2+}$ inferred from the magnetization and specific heat measurement is 3.5 $\mu_B$, smaller than the theoretical value of Eu$^{2+}$ due to presence of both Eu$^{3+}$ and Eu$^{2+}$. Magnetic field-dependent resistivity measurements suggest dominant quasi two dimensional Fermi surfaces whereas the first-principle calculations point to the presence of Dirac fermions. Therefore, EuZnSb$_2$ could represent the first platform to study the interplay of dynamical charge fluctuations, localized magnetic 4$f$ moments and Dirac states with Sb orbital character., Comment: 9 pages, 4 figures

Ferromagnetic van der Waals (vdW) insulators are of great scientific interest for their promising applications in spintronics. It has been indicated that in the two materials within this class, CrI$_3$ and VI$_3$, the magnetic ground state, the band gap, and the Fermi level could be manipulated by varying the layer thickness, strain or doping. To understand how these factors impact the properties, a detailed understanding of the electronic structure would be required. However, the experimental studies of the electronic structure of these materials are still very sparse. Here, we present the detailed electronic structure of CrI$_3$ and VI$_3$ measured by angle-resolved photoemission spectroscopy (ARPES). Our results show a band-gap of the order of 1 eV, sharply contrasting some theoretical predictions such as Dirac half-metallicity and metallic phases, indicating that the intra-atomic interaction parameter (U) and spin-orbit coupling (SOC) were not properly accounted for in the calculations. We also find significant differences in the electronic properties of these two materials, in spite of similarities in their crystal structure. In CrI$_3$, the valence band maximum is dominated by the I 5{\it p}, whereas in VI$_3$ it is dominated by the V 3{\it d} derived states. Our results represent valuable input for further improvements in the theoretical modeling of these systems., 8 pages, 4 figures

Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscopic study with electronic transport neutron, X-ray scattering, and first principle calculation, we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering, which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell Pnnm → Pm. The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb2 paves the way for the understanding and tailoring of giant thermopower in related materials.

Two-dimensional van der Waals (vdW) magnetic materials have emerged as possible candidates for future ultrathin spintronic devices, and finding a way to tune their physical properties is desirable for wider applications. Owing to the sensitivity and tunability of the physical properties to the variation of interatomic separations, this class of materials is attractive to explore under pressure. Here, we present the observation of direct to indirect band gap crossover and an insulator–metal transition in the vdW antiferromagnetic insulator CrPS4 under pressure through in-situ photoluminescence, optical absorption, and resistivity measurements. Raman spectroscopy experiments revealed no changes in the spectral feature during the band gap crossover whereas the insulator–metal transition is possibly driven by the formation of the high-pressure crystal structure. Theoretical calculations suggest that the band gap crossover is driven by the shrinkage and rearrangement of the CrS6 octahedra under pressure. Such high tunability under pressure demonstrates an interesting interplay between structural, optical and magnetic degrees of freedom in CrPS4, and provides further opportunity for the development of devices based on tunable properties of 2D vdW magnetic materials.

We report anisotropic magnetocaloric effect and critical behavior in a quasi-one-dimensional ferromagnetic ${\mathrm{CrSbSe}}_{3}$ single crystal. The maximum magnetic entropy change $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{\mathrm{max}}$ is 2.16 J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for the easy $a$ axis (2.03 J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for the hard $b$ axis) and the relative cooling power RCP is 163.1 J ${\mathrm{kg}}^{\ensuremath{-}1}$ for the easy $a$ axis (142.1 J ${\mathrm{kg}}^{\ensuremath{-}1}$ for the hard $b$ axis) near ${T}_{c}$ with a magnetic field change of 50 kOe. The magnetocrystalline anisotropy constant ${K}_{u}$ is estimated to be 148.5 kJ ${\mathrm{m}}^{\ensuremath{-}3}$ at 10 K, decreasing to 39.4 kJ ${\mathrm{m}}^{\ensuremath{-}3}$ at 70 K. The rescaled $\mathrm{\ensuremath{\Delta}}{S}_{M}(T,H)$ curves along all three axes collapse onto a universal curve, respectively, confirming the second-order ferromagnetic transition. Further critical behavior analysis around ${T}_{c}\ensuremath{\sim}70\phantom{\rule{4pt}{0ex}}\mathrm{K}$ gives that the critical exponents $\ensuremath{\beta}=0.26(1)$, $\ensuremath{\gamma}=1.32(2)$, and $\ensuremath{\delta}=6.17(9)$ for $H\ensuremath{\parallel}a$, while $\ensuremath{\beta}=0.28(2)$, $\ensuremath{\gamma}=1.02(1)$, and $\ensuremath{\delta}=4.14(16)$ for $H\ensuremath{\parallel}b$. The determined critical exponents suggest that the anisotropic magnetic coupling in ${\mathrm{CrSbSe}}_{3}$ is strongly dependent on orientations of the applied magnetic field.

This work establishes a fundamental base for understanding colossal thermopower in FeSb2. The authors uncover the FeSb2 electronic structure within ARPES, show many-body electronic structure calculations, and point out the basic ingredients responsible for its colossal thermopower

The ternary $A{\mathrm{MnBi}}_{2}$ ($A$ is alkaline as well as rare-earth atom) materials provide an arena for investigating the interplay between low-dimensional magnetism of the antiferromagnetic MnBi layers and the electronic states in the intercalated Bi layers, which harbor relativistic fermions. Here, we report on a comprehensive study of the optical properties and magnetic torque response of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{MnBi}}_{2}$. Our findings give evidence for a spin canting occurring at ${T}_{s}\ensuremath{\sim}50--100\text{ }\text{ }\mathrm{K}$. With the support of first-principles calculations we establish a direct link between the spin canting and the reconstruction of the electronic band structure, having immediate implications for the spectral weight reshuffling in the optical response, signaling a partial gapping of the Fermi surface, and the dc transport properties below ${T}_{s}$.

We present a comprehensive study on anisotropic magnetocaloric porperties of the van der Waals weak-itinerant ferromagnet Fe

We present inelastic neutron scattering (INS) measurements of magnetic excitations in YbMnBi$_2$, which reveal features consistent with a direct coupling of magnetic excitations to Dirac fermions. In contrast with the large broadening of magnetic spectra observed in antiferromagnetic metals such as the iron pnictides, here the spin waves exhibit a small but resolvable intrinsic width, consistent with our theoretical analysis. The subtle manifestation of spin-fermion coupling is a consequence of the Dirac nature of the conduction electrons, including the vanishing density of states near the Dirac points. Accounting for the Dirac fermion dispersion specific to \ymb\ leads to particular signatures, such as the nearly wave-vector independent damping observed in the experiment., Comment: 8 pages, 11 figures including both main text and supplemental material

We have examined size effect on thermal, transport, and thermodynamic properties of a ${\mathrm{CrSb}}_{2}$ single crystal. We demonstrate highly anisotropic quasi-one-dimensional electrical conductivity, quasiballistic phonons, and giant thermopower of $\ensuremath{-}6$ mV/K at 15 K. A thermopower peak is suppressed to $\ensuremath{-}1.6$ mV/K by changing crystal dimensions and shows linear dependence on the phonon mean-free path. Whereas electronic diffusion thermopower is significant, the bulk of the giant thermopower in ${\mathrm{CrSb}}_{2}$ stems from the coupling of the very long mean-free path phonons with the in-gap states.

The electric pulse-induced responses of 1T-TaS2 and 1T-TaS1.6Se0.4 crystals in the commensurate charge-density-wave (CCDW) phase in the hysteresis temperature range have been investigated. We observed that abrupt multiple steps of the resistance are excited by electric pulses at a fixed temperature forming multi metastable like states. We propose that the response of the system corresponds to the rearrangements of the textures of CCDW domains and the multi-resistance states or the nonvolatile resistance properties excited simply by electric pulses have profound significance for the exploration of solid-state devices., Comment: 5 pages, 6 figures. Supplementary Information

The contribution of bulk and surface to the electrical resistance along crystallographic \textit{b}- and \textit{c}-axes as a function of crystal thickness gives evidence for a temperature independent surface states in an antiferromagnetic narrow-gap semiconductor CrSb$_{2}$. ARPES shows a clear electron-like pocket at $\Gamma$-$Z$ direction which is absent in the bulk band structure. First-principles calculations also confirm the existence of metallic surface states inside the bulk gap. Whereas combined experimental probes point to enhanced surface conduction similar to topological insulators, surface states are trivial since CrSb$_2$ exhibits no band inversion.

Two-dimensional (2D) materials with intrinsic ferromagnetism provide unique opportunity to engineer new functionalities in nano-spintronics. One such material is CrI

We report quantum oscillation measurements of LaAlGe, a Lorentz-violating type-II Weyl semimetal with tilted Weyl cones. Very small quasiparticle masses and very high Fermi velocities were detected at the Fermi surface. Whereas three main frequencies have been observed, angular dependence of two Fermi surface sheets indicates possible two-dimensional (2D) character despite the absence of the 2D structural features such as van der Waals bonds. Such conducting states may offer a good platform for low-dimensional polarized spin current in magnetic RAlGe (R = Ce, Pr) materials., Comment: 5 pages, 3 figures

We present a comprehensive investigation of the anisotropic magnetic and magnetocaloric properties of quasi-two-dimensional weak itinerant ferromagnet trigonal ${\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ single crystals. Magnetic-anisotropy-induced satellite transition ${T}^{*}$ is observed at low fields applied parallel to the $ab$ plane below the Curie temperature (${T}_{c}\ensuremath{\sim}230\phantom{\rule{4pt}{0ex}}\mathrm{K}$). ${T}^{*}$ is featured by an anomalous magnetization downturn, similar to that in structurally related ${\mathrm{CrI}}_{3}$, which possibly stems from an in-plane antiferromagnetic alignment induced by out-of-plane ferromagnetic spins canting. Magnetocrystalline anisotropy is also reflected in magnetic entropy change $\mathrm{\ensuremath{\Delta}}{S}_{M}(T,H)$ and relative cooling power. Given the high Curie temperature, ${\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ crystals are materials of interest for nanofabrication in basic science and applied technology.

The iron antimonide FeSb$_2$ possesses an extraordinarily high thermoelectric power factor at low temperature, making it a leading candidate for cryogenic thermoelectric cooling devices. However, the origin of this unusual behavior is controversial, having been variously attributed to electronic correlations as well as the phonon-drag effect. The optical properties of a material provide information on both the electronic and vibrational properties. The optical conductivity reveals an anisotropic response at room temperature; the low-frequency optical conductivity decreases rapidly with temperature, signalling a metal-insulator transition. One-dimensional semiconducting behavior is observed along the $b$ axis at low temperature, in agreement with first-principle calculations. The infrared-active lattice vibrations are also symmetric and extremely narrow, indicating long phonon relaxation times and a lack of electron-phonon coupling. Surprisingly, there are more lattice modes along the $a$ axis than are predicted from group theory; several of these modes undergo significant changes below about 100 K, hinting at a weak structural distortion or phase transition. While the extremely narrow phonon line shapes favor the phonon-drag effect, the one-dimensional behavior of this system at low temperature may also contribute to the extraordinarily high thermopower observed in this material., 7 pages with 5 figures and one table; 9 pages of supplementary material

Synchrotron X-ray Scanning Tunneling Microscopy (SX-STM) is a novel imaging technique capable of providing real space chemically specific mapping with a potential of reaching atomic resolution. Determination of chemical composition along with ultra-high resolution imaging by SX-STM can be realized through excitation of core electrons by incident X-rays when their energy is tuned to an absorption edge of a particular atom during raster scanning, as is done in the conventional STM experiments. In this work, we provide a brief summary and the current status of SX-STM and discuss its applications for material science. In particular, we discuss instrumentation challenges associated with the SX-STM technique and present early experiments on Cu doped ZrTe3 single crystals.

We discuss our recent optical investigation of YbMnBi 2 , a representative type II Weyl semimetal, by considering a simple scheme for the electronic structure, which can be embedded within a recent theoretical approach for the calculation of the excitation spectrum. Our study allows us disentangling the generic optical fingerprints of Weyl fermions, which are in broad agreement with the theoretical predictions but also challenge the present understanding of their electrodynamic response.

Crystals of FeSb2 correlated narrow-gap semiconductor host colossal thermopower values. By tuning the impurity level here, we show that electron–phonon scattering that transfers phonon momentum to electrons is efficient only for certain optimal carrier concentration in the low-mobility band. Phonon drag acting on such states in crystals with high phonon mean free path enhances thermopower to colossal values, whereas for different carrier concentration, dominant thermal transport mechanism is electronic diffusion. This highlights the dual nature of correlated in-gap states that take part in the phonon drag but also reduce phonon mean free path.

The electric field dependence of the dielectric properties below 50 K along the chain direction of o-TaS3 has been investigated. Under a dc bias field exceeding the lower threshold E ′ T , the real part of dielectric constant ϵ 1 ( ω ) decreases possibly due to the decreasing concentration of bound solitons. We propose that the delocalization of bound solitons with the increasing applied electric field could be used to explain the reduction of ϵ 1 ( ω ) and the enhancement of the dc conductivity. This has also been confirmed by the quantitative comparison between the enhancement of the dc conductivity and the change of the dielectric spectral weight.

The newly designed $A{\mathrm{MnBi}}_{2}$ ($A=\text{alkaline}$ as well as rare-earth atom) materials based on quasi-two-dimensional bismuth layers provide a suitable playground to address the physics of Dirac fermions, in connection with magnetism and structural changes. Here, we perform an optical investigation as a function of temperature of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{MnBi}}_{2}$, which reveals a vestigial linear frequency-dependent behavior of the optical conductivity in the midinfrared, ascribed to electronic interband transitions involving Dirac bands. Furthermore, we uncover optical signatures for a partial gapping of the Fermi surface, for energy scales up to 0.2 eV, at the onset of the spin reorientation transition which also manifests as an anomaly in the dc transport data. This may reveal the inclination towards a Fermi surface instability in topological materials, possibly related to a density-wave order.

We report giant thermopower S = 2.5 mV/K in CoSbS single crystals, a material that shows strong high-temperature thermoelectric performance when doped with Ni or Se. Changes of low temperature thermopower induced by magnetic field point to mechanism of electronic diffusion of carriers in the heavy valence band. Intrinsic magnetic susceptibility is consistent with the Kondo- Insulator-like accumulation of electronic states around the gap edges. This suggests that giant thermopower stems from temperature-dependent renormalization of the non-interacting bands and buildup of the electronic correlations on cooling., 6 pages, 4 figures

We present Raman spectroscopy measurements of the van der Waals bonded ferromagnet ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$, together with lattice dynamics. Four out of eight Raman active modes are observed and assigned, in agreement with numerical calculations. The energies and linewidths of the observed modes display an unconventional temperature dependence at about 150 and 220 K, followed by the nonmonotonic evolution of the Raman continuum. Whereas the former can be related to the magnetic phase transition, the origin of the latter anomaly remains an open question.

Superconductivity in the iron pnictides emerges from metallic parent compounds exhibiting intertwined stripe-type magnetic order and nematic order, with itinerant electrons suggested to be essential for both. Here we use x-ray and neutron scattering to show that a similar intertwined state is realized in semiconducting KFe_{0.8}Ag_{1.2}Te_{2} (K_{5}Fe_{4}Ag_{6}Te_{10}) without itinerant electrons. We find that Fe atoms in KFe_{0.8}Ag_{1.2}Te_{2} form isolated 2×2 blocks, separated by nonmagnetic Ag atoms. Long-range magnetic order sets in below T_{N}≈35 K, with magnetic moments within the 2×2 Fe blocks ordering into the stripe-type configuration. A nematic order accompanies the magnetic transition, manifest as a structural distortion that breaks the fourfold rotational symmetry of the lattice. The nematic orders in KFe_{0.8}Ag_{1.2}Te_{2} and iron pnictide parent compounds are similar in magnitude and in how they relate to the magnetic order, indicating a common origin. Since KFe_{0.8}Ag_{1.2}Te_{2} is a semiconductor without itinerant electrons, this indicates that local-moment magnetic interactions are integral to its magnetic and nematic orders, and such interactions may play a key role in iron-based superconductivity.

The electric pulses induced responses of $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ in the commensurate charge-density-wave phase in the hysteresis temperature region ($160\ensuremath{\sim}210$ K) have been investigated. We observed an abrupt jump of the resistance excited by pulse, followed by a slow relaxation process in a time scale of $\ensuremath{\sim}100$ s. At a fixed temperature, the various electric pulses can drive the system to multi metastable states. We propose that the spontaneous evolution or slow relaxation of the system by pulse excitations corresponds to the rearrangements of the textures of CCDW domains.

We report pair distribution function measurements of the iron-based superconductor FeSe above and below the structural transition temperature. Structural analysis reveals a local orthorhombic distortion with a correlation length of about 4 nm at temperatures where an average tetragonal symmetry is observed. The analysis further demonstrates that the local distortion is larger than the distortion at temperatures where the average observed symmetry is orthorhombic. Our results suggest that the low-temperature macroscopic nematic state in FeSe forms from an imperfect ordering of orbital-degeneracy-lifted nematic fluctuations which persist up to at least 300 K., Comment: 6 pages, 4 figures