Semimetallic Behavior in Fe2VAl: NMR Evidence

We report the results of a 27Al and 51V nuclear magnetic resonance study of Fe2VAl at temperatures between 4 and 550 K. This material has been a subject of current interest due to indications of possible heavy fermion behavior. The low-temperature NMR relaxation rate follows a Korringa law, indicating a small density of carriers at the Fermi level. At elevated temperatures, the shifts and relaxation rates go over to a thermally activated response, a semiconductorlike behavior, consistent with separate low-lying bands removed from the Fermi-level. These results are consistent with recent electronic structure calculations, and can explain both the reported activated resistivity as well as the Fermi cutoff exhibited in photoemission studies. While we observe nonstoichiometric samples of (Fe1−xVx)3Al to be magnetic, the x=0.33 composition is nonmagnetic, with narrow NMR linewidths

Spin fluctuations in Al3V: An NMR study

We report the results of a nuclear magnetic resonance (NMR) study of Al3V at temperatures between 4 and 300 K. 51V and 27Al Knight shifts exhibit a Curie-Weiss behavior corresponding to an effective moment Peff≃0.32μB per V, indicating intrinsic weak magnetism localized in vanadium orbitals. The T1 exhibits a T‾‾√ dependence at higher temperatures, indicative of nearly antiferromagnetic behavior of itinerant electrons. We have fit the observed behavior within the self-consistent renormalized model for itinerant magnetism, which provides good agreement with the measurements. At about 80 K, the Knight shifts and spin-lattice relaxation rates exhibit a crossover from Curie-Weiss magnetism to Korringa behavior. Comparing previously reported specific heat and transport results with our NMR results, we find that the Wilson ratio and Kadowaki-Woods relation are close to those observed for highly correlated electron compounds, despite the relatively small enhancement of γ and χ

NMR study of trialuminide intermetallics

We present a systematic study of the DO22-structure trialuminide intermetallic alloys using 27Al NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times on Al3Ti, Al3V, Al3Nb, and Al3Ta have been identified. Knight-shift tensors were isolated by observation of quadrupole satellite lines and fitting to the central-transition powder patterns. The results are associated with the local electronic density of states for each crystallographic site. Universally small isotropic Knight shifts and long T1’s are consistent with low Fermi-surface densities of states indicating the importance of Fermi-surface features for the phase stability of these alloys. Larger anisotropic Knight shifts occurring at aluminum site I indicate strong hybridization at this site, and the electric-field-gradient tensors confirm the strong ab plane bonding configuration. Local-moment magnetism is found in Al3V, yet electrically this material appears very similar to the other DO22 aluminides

Atomic-scale study of type-II Dirac semimetal PtTe2 surface

Dirac semimetals (DSM) host linear bulk bands and topologically protected surface states, giving rise to exotic and robust properties. Platinum ditelluride (PtTe2) belongs to this interesting group of topological materials. Here, we employ scanning tunneling microscopy (STM) in combination with first-principles calculations to visualize and identify the native defects at the surface of a freshly cleaved PtTe2 crystal. Around these defects, short-wavelength electron density oscillations are observed. Fourier transform analysis of the energy-dependent quasiparticle interference patterns is in good agreement with our calculated joint density of states, demonstrating the singular properties of the surface of this type-II DSM. Our results evidence the power of STM in understanding the surface of topological material

Terahertz nonlinear hall rectifiers based on spin-polarized topological electronic states in 1T-CoTe2

The zero-magnetic-field nonlinear Hall effect (NLHE) refers to the second-order transverse current induced by an applied alternating electric field; it indicates the topological properties of inversion-symmetry-breaking crystals. Despite several studies on the NLHE induced by the Berry-curvature dipole in Weyl semimetals, the direct current conversion by rectification is limited to very low driving frequencies and cryogenic temperatures. The nonlinear photoresponse generated by the NLHE at room temperature can be useful for numerous applications in communication, sensing, and photodetection across a high bandwidth. In this study, observations of the second-order NLHE in type-II Dirac semimetal CoTe2 under time-reversal symmetry are reported. This is determined by the disorder-induced extrinsic contribution on the broken-inversion-symmetry surface and room-temperature terahertz rectification without the need for semiconductor junctions or bias voltage. It is shown that remarkable photoresponsivity over 0.1 A W−1, a response time of approximately 710 ns, and a mean noise equivalent power of 1 pW Hz−1/2 can be achieved at room temperature. The results open a new pathway for low-energy photon harvesting via nonlinear rectification induced by the NLHE in strongly spin–orbit-coupled and inversion-symmetry-breaking systems, promising a considerable impact in the field of infrared/terahertz photonicsPID2019–109525RB-I00, CEX2018-000805-M, EU’s H2020 NFFA-Europe (n. 654360), and NFFA-Europe-Pilot (10100741

Two-dimensional superconductivity and magnetotransport from topological surface states in AuSn4 semimetal

Surface states of topological semimetals may give rise to unusual transport properties and topological superconductivity. Here, the H-T phase diagram of AuSn4 is experimentally established, displaying 2D superconductivity, Bose metal behavior, and normal-state magnetotransport driven by surface states

Pressure-dependent modifications in the LaAuSb2 charge density wave system

Hydrostatic pressure response of LaAuSb2 charge density wave (CDW) system is investigated using electrical transport, x-ray diffraction (XRD), and density-functional theory (DFT). Resistivity data at ambient pressure evidence a clear CDW transition at 83 K. X-ray diffraction at ambient pressure reveals that in plane and out of plane axes show opposite behavior with decreasing temperature, in particular, out of plane c axis develops a distinct change at ∼250 K, much above the observed CDW transition at 83 K. The CDW transition shifts to low temperature with increasing pressure. Our resistivity data indicate a complete suppression of the CDW transition at ∼3.6 GPa. High-pressure XRD revealed a change from the linear trend for the out of plane (c) and the in plane (a) lattice parameters for pressure above 3.8 GPa. With compression, DFT indicated an anomaly in the c/a ratio around 8 GPa. The calculated electronic structure also indicated minor changes in the band structure in this pressure range. In addition, high-pressure DFT structural investigations reveal the LaAuSb2 system to be stable up to pressures as high as 150 GP

Tin Diselenide (SnSe2) Van der Waals Semiconductor: Surface Chemical Reactivity, Ambient Stability, Chemical and Optical Sensors

Tin diselenide (SnSe2) is a layered semiconductor with broad application capabilities in the fields of energy storage, photocatalysis, and photodetection. Here, we correlate the physicochemical properties of this van der Waals semiconductor to sensing applications for detecting chemical species (chemosensors) and millimeter waves (terahertz photodetectors) by combining experiments of high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy with density functional theory. The response of the pristine, defective, and oxidized SnSe2 surface towards H2, H2O, H2S, NH3, and NO2 analytes was investigated. Furthermore, the effects of the thickness were assessed for monolayer, bilayer, and bulk samples of SnSe2. The formation of a sub-nanometric SnO2 skin over the SnSe2 surface (self-assembled SnO2/SnSe2 heterostructure) corresponds to a strong adsorption of all analytes. The formation of non-covalent bonds between SnO2 and analytes corresponds to an increase of the magnitude of the transferred charge. The theoretical model nicely fits experimental data on gas response to analytes, validating the SnO2/SnSe2 heterostructure as a suitable playground for sensing of noxious gases, with sensitivities of 0.43, 2.13, 0.11, 1.06 [ppm]−1 for H2, H2S, NH3, and NO2, respectively. The corresponding limit of detection is 5 ppm, 10 ppb, 250 ppb, and 400 ppb for H2, H2S, NH3, and NO2, respectively. Furthermore, SnSe2-based sensors are also suitable for fast large-area imaging applications at room temperature for millimeter waves in the THz range