Exchange-biased topological transverse thermoelectric effects in a Kagome ferrimagnet.

Kagome metal TbMn6Sn6 was recently discovered to be a ferrimagnetic topological Dirac material by scanning tunneling microscopy/spectroscopy measurements. Here, we report the observation of large anomalous Nernst effect and anomalous thermal Hall effect in this compound. The anomalous transverse transport is consistent with the Berry curvature contribution from the massive Dirac gaps in the 3D momentum space as demonstrated by our first-principles calculations. Furthermore, the transverse thermoelectric transport exhibits asymmetry with respect to the applied magnetic field, i.e., an exchange-bias behavior. Together, these features place TbMn6Sn6 as a promising system for the outstanding thermoelectric performance based on anomalous Nernst effect.

Coexistence of Ferroelectriclike Polarization and Dirac-like Surface State in TaNiTe_{5}.

By combining angle-resolved photoemission spectroscopy, scanning tunneling microscopy, atomic force microscope based piezoresponse force microscopy and first-principles calculations, we have studied the low-energy band structure, atomic structure, and charge polarization on the surface of a topological semimetal candidate TaNiTe_{5}. Dirac-like surface states were observed on the (010) surface by angle-resolved photoemission spectroscopy, consistent with the first-principles calculations. On the other hand, piezoresponse force microscopy reveals a switchable ferroelectriclike polarization on the same surface. We propose that the noncentrosymmetric surface relaxation observed by scanning tunneling microscopy could be the origin of the observed ferroelectriclike state in this novel material. Our findings provide a new platform with the coexistence of a ferroelectriclike surface charge distribution and novel surface states.

Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet.

Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with κ_{xy}∼1×10^{-2} W K^{-1} m^{-1} in an insulating van der Waals ferromagnet VI_{3}. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI_{3} exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI_{3} as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics-magnonics applications.

Transport property of multi-band topological material PtBi[Formula: see text] studied by maximum entropy mobility spectrum analysis (MEMSA).

Electrical transport of both longitudinal and transverse directions carries rich information. Mobility spectrum analysis (MSA) is capable of extracting charge information from conductivity tensor, including charge types, concentration and mobilities. Using a numerical method based on maximum entropy principle, i.e., maximum entropy mobility spectrum analysis (MEMSA), mobility spectrum for [Formula: see text]-type PtBi[Formula: see text] is studied. Three hole-pockets and two electron-pockets were found, including a small hole pocket with very high mobility, which is very likely corresponding to Dirac Fermions. Benefiting from our high resolution result, we studied temperature dependence of carrier properties and explained the sign change phenomenon of Hall conductivity. We further compared the results with band structure obtained by our first principle calculation. The present results prove MEMSA is a useful tool of extracting carries' information in recently discovered Iron-based superconductors, and topological materials.

Fermi-crossing Type-II Dirac fermions and topological surface states in NiTe2.

Transition-metal dichalcogenides (TMDs) offer an ideal platform to experimentally realize Dirac fermions. However, typically these exotic quasiparticles are located far away from the Fermi level, limiting the contribution of Dirac-like carriers to the transport properties. Here we show that NiTe2 hosts both bulk Type-II Dirac points and topological surface states. The underlying mechanism is shared with other TMDs and based on the generic topological character of the Te p-orbital manifold. However, unique to NiTe2, a significant contribution of Ni d orbital states shifts the energy of the Type-II Dirac point close to the Fermi level. In addition, one of the topological surface states intersects the Fermi energy and exhibits a remarkably large spin splitting of 120 meV. Our results establish NiTe2 as an exciting candidate for next-generation spintronics devices.

Anisotropic Transport and Quantum Oscillations in the Quasi-One-Dimensional TaNiTe5: Evidence for the Nontrivial Band Topology.

The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most materials that have been studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present clear-cut evidence for Dirac Fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic, and we observe nontrivial Berry phases via quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides.

Pressure engineering of the Dirac fermions in quasi-one-dimensional Tl2Mo6Se6.

Topological band dispersions other than the standard Dirac or Weyl fermions have garnered the increasing interest in materials science. Among them, the cubic Dirac fermions were recently proposed in the family of quasi-one-dimensional (q-1D) conductors A2Mo6X6 (A = Na, K, In, Tl; X = S, Se, Te), where the band crossing is characterized by a linear dispersion in one k-space direction but the cubic dispersion in the plane perpendicular to it. It is not yet clear, however, how the external perturbations can alter these nontrivial carriers and ultimately induce a new distinct quantum phase. Here we study the evolution of Dirac fermions, in particular the cubic Dirac crossing, under external pressure in the representative q-1D Tl2Mo6Se6 via the first-principles calculations. Specifically, it is found that the topological properties, including the bulk Dirac crossings and the topological surface states, change progressively under pressure up to 50 GPa where it undergoes a structural transition from the hexagonal phase to body-centered tetragonal phase. Above 50 GPa, the system is more likely to be topologically trivial. Further, we also investigate its phonon spectra, which reveals a gradual depletion of the negative phonon modes with pressure, consistent with the more three-dimensional Fermi surface in the high-pressure phase. Our work may provide a useful guideline for further experimental search and the band engineering of the topologically nontrivial fermions in this intriguing state of matter.

Evidence for nematic superconductivity of topological surface states in PbTaSe2.

Spontaneous symmetry breaking has been a paradigm to describe the phase transitions in condensed matter physics. In addition to the continuous electromagnetic gauge symmetry, an unconventional superconductor can break discrete symmetries simultaneously, such as time reversal and lattice rotational symmetry. In this work we report a characteristic in-plane 2-fold behaviour of the resistive upper critical field and point-contact spectra on the superconducting semimetal PbTaSe2 with topological nodal-rings, despite its hexagonal lattice symmetry (or D3h in bulk while C3v on surface, to be precise). The 2-fold behaviour persists up to its surface upper critical field Hc2R even though bulk superconductivity has been suppressed at its bulk upper critical field Hc2HC≪Hc2R, signaling its probable surface-only electronic nematicity. In addition, we do not observe any lattice rotational symmetry breaking signal from field-angle-dependent specific heat within the resolution. It is worth noting that such surface-only electronic nematicity is in sharp contrast to the observation in the topological superconductor candidate, CuxBi2Se3, where the nematicity occurs in various bulk measurements. In combination with theory, superconducting nematicity is likely to emerge from the topological surface states of PbTaSe2, rather than the proximity effect. The issue of time reversal symmetry breaking is also addressed. Thus, our results on PbTaSe2 shed new light on possible routes to realize nematic superconductivity with nontrivial topology.

Angular-dependent magnetoresistance study in Ca0.73La0.27FeAs2: a 'parent' compound of 112-type iron pnictide superconductors.

We report a study of angular-dependent magnetoresistance (AMR) with the magnetic field rotated in the plane perpendicular to the current on a Ca0.73La0.27FeAs2 single crystal, which is regarded as a 'parent' compound of 112-type iron pnictide superconductors. A pronounced AMR with twofold symmetry is observed, signifying the highly anisotropic Fermi surface. By further analyzing the AMR data, we find that the Fermi surface above the structural/antiferromagnetic (AFM) transition (T s/T N) is quasi-two-dimensional (quasi-2D), as revealed by the 2D scaling behavior of the AMR, Δρ/ρ(0) (H, θ) = Δρ/ρ(0) (µ 0 Hcosθ), θ being the magnetic field angle with respect to the c axis. While such 2D scaling becomes invalid at temperatures below T s/T N, the three-dimensional (3D) scaling approach by inclusion of the anisotropy of the Fermi surface is efficient, indicating that the appearance of the 3D Fermi surface contributes to anisotropic electronic transport. Compared with other experimental observations, we suspect that the additional 3D hole pocket (generated by the Ca d orbital and As1 p z orbital) around the Γ point in CaFeAs2 will disappear in the heavily electron doped regime, and moreover, the Fermi surface should be reconstructed across the structural/AFM transition. Besides, a quasi-linear in-plane magnetoresistance with H//ab is observed at low temperatures and its possible origins are also discussed. Our results provide more information to further understand the electronic structure of 112-type IBSs.

Anti-atherosclerotic potential of baicalin mediated by promoting cholesterol efflux from macrophages via the PPARγ-LXRα-ABCA1/ABCG1 pathway.

BACKGROUND: Atherosclerosis (AS) is associated with severe cardiovascular disease. The anti-inflammatory, anti-oxidation, and lipid regulating properties of baicalin suggest potential as an anti-atherosclerotic agent. We therefore investigated whether baicalin can protect against the development of atherosclerosis in an AS rabbit model and explored the underling mechanisms in THP-1 macrophages. METHODS AND RESULTS: In vivo, treatment with baicalin markedly decreased atherosclerotic lesion sizes and lipid accumulation in AS rabbit carotid arteries. Western blotting revealed that the protein expression levels of both peroxisome proliferator-activated receptor gamma (PPARγ) and liver X receptor alpha (LXRα) were up-regulated in the baicalin group compared with the model group. In vitro, baicalin restricted oxidized-low density lipoprotein (ox-LDL)-induced intracellular lipid accumulation and foam cell formation in THP-1 macrophages. Molecular data showed that baicalin significantly increased the expression levels of PPARγ, LXRα, ATP binding cassette transporters (ABC) A1 and ABCG1. Cell transfection experiments (including PPARγ and LXRα siRNAs) suggested that the effects of baicalin are mediated by the PPARγ-LXRα signalling pathway, which stimulates the expression of ABCA1 and ABCG1. CONCLUSION: These results suggest that baicalin potentially exerts anti-atherosclerosis effects, possibly through the PPARγ-LXRα-ABCA1/ABCG1 pathway, by promoting efflux of cholesterol from macrophages and delaying the formation of foam cells.