Magnetotransport of single crystalline NbAs Ghimire, N J; Luo, Yongkang; Neupane, M ...
Journal of physics. Condensed matter,
04/2015, Letnik:
27, Številka:
15
Journal Article
Recenzirano
Odprti dostop
We report transport measurement in zero and applied magnetic field on a single crystal of NbAs. Transverse and longitudinal magnetoresistance in the plane of this tetragonal structure does not ...saturate up to 9 T. In the transverse configuration (H c, I ⊥ c) it is 230 000% at 2 K. The Hall coefficient changes sign from hole-like at room temperature to electron-like below ∼150 K. The electron carrier density and mobility calculated at 2 K based on a single band approximation are 1.8 × 1019 cm−3 and 3.5 × 105 cm2 Vs−1, respectively. These values are similar to reported values for TaAs and NbP, and further emphasize that this class of noncentrosymmetric, transition-metal monopnictides is a promising family to explore the properties of Weyl semimetals and the consequences of their novel electronic structure.
The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related ...effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, E bind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.
In this study, via angular Shubnikov–de Haas (SdH) quantum oscillations measurements, we determine the Fermi surface topology of NbAs, a Weyl semimetal candidate. The SdH oscillations consist of two ...frequencies corresponding to two Fermi surface extrema: 20.8 T (α pocket) and 15.6 T (β pocket). The analysis, including a Landau fan plot, shows that the β pocket has a Berry phase of π and a small effective mass of ~ 0.033 m0, indicative of a nontrivial topology in momentum space, whereas the α pocket has a trivial Berry phase of 0 and a heavier effective mass of ~ 0.066 m0. From the effective mass and the β -pocket frequency, we determine that the Weyl node is 110.5 meV from the chemical potential. An electron-hole compensation effect is discussed in this system, and its impact on magnetotransport properties is addressed. Finally, the difference between NbAs and other monopnictide Weyl semimetals is also discussed.
The change in resistance of a material in a magnetic field reflects its electronic state. In metals with weakly- or non-interacting electrons, the resistance typically increases upon the application ...of a magnetic field. In contrast, negative magnetoresistance may appear under some circumstances, e.g., in metals with anisotropic Fermi surfaces or with spin-disorder scattering and semimetals with Dirac or Weyl electronic structures. Here we show that the non-magnetic semimetal TaAs2 possesses a very large negative magnetoresistance, with an unknown scattering mechanism. Density functional calculations find that TaAs2 is a new topological semimetal ℤ2 invariant (0;111) without Dirac dispersion, demonstrating that a negative magnetoresistance in non-magnetic semimetals cannot be attributed uniquely to the Adler-Bell-Jackiw chiral anomaly of bulk Dirac/Weyl fermions.
Weyl fermions are a recently discovered ingredient for correlated states of electronic matter. A key difficulty has been that real materials also contain non-Weyl quasiparticles, and disentangling ...the experimental signatures has proven challenging. Here we use magnetic fields up to 95 T to drive the Weyl semimetal TaAs far into its quantum limit, where only the purely chiral 0th Landau levels of the Weyl fermions are occupied. We find the electrical resistivity to be nearly independent of magnetic field up to 50 T: unusual for conventional metals but consistent with the chiral anomaly for Weyl fermions. Above 50 T we observe a two-order-of-magnitude increase in resistivity, indicating that a gap opens in the chiral Landau levels. Above 80 T we observe strong ultrasonic attenuation below 2 K, suggesting a mesoscopically textured state of matter. These results point the way to inducing new correlated states of matter in the quantum limit of Weyl semimetals.
Abstract
Ferromagnetic (FM) order in a two-dimensional kagome layer is predicted to generate a topological Chern insulator without an applied magnetic field. The Chern gap is largest when spin ...moments point perpendicular to the kagome layer, enabling the capability to switch topological transport properties, such as the quantum anomalous Hall effect, by controlling the spin orientation. In TbMn
6
Sn
6
, the uniaxial magnetic anisotropy of the Tb
3+
ion is effective at generating the Chern state within the FM Mn kagome layers while a spin-reorientation (SR) transition to easy-plane order above
T
S
R
= 310 K provides a mechanism for switching. Here, we use inelastic neutron scattering to provide key insights into the fundamental nature of the SR transition. The observation of two Tb excitations, which are split by the magnetic anisotropy energy, indicates an effective two-state orbital character for the Tb ion, with a uniaxial ground state and an isotropic excited state. The simultaneous observation of both modes below
T
S
R
confirms that orbital fluctuations are slow on magnetic and electronic time scales < ps and act as a spatially-random orbital alloy. A thermally-driven critical concentration of isotropic Tb ions triggers the SR transition.
The khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), is a serious pest of stored products and is the only stored product insect pest that triggers a quarantine response when it ...is found in the United States. The larvae of T. granarium feed on a wide range of dry food products of plant and animal origin, including cereals, dried fish, and museum specimens. In this study, we evaluated the residual efficacy of two pyrethroid insecticides, deltamethrin and cyfluthrin, applied on concrete, wood, painted wood, vinyl flooring tile, and metal surfaces using small and large T. granarium larvae. Residual efficacy of two insect growth regulators (IGRs), methoprene and pyriproxyfen was also evaluated on concrete, metal, and wood surfaces. In both studies, larvae were exposed with provision of a food source on the treated surfaces and residual assays were conducted at 0 months (1 d), 1, 2, and 3 months post treatment. In general, both of the pyrethroids provided a high level of control of T. granarium larvae, though small larvae were much more susceptible than large larvae. The IGRs were comparatively less effective, with more larval survival and adult emergence of exposed larvae compared with the pyrethroids. Residues of the pyrethroids and IGRs were most persistent on the metal surface. Results can be used to help to control and eradicate infestations of T. granarium when they are detected in the United States.
Magnetotransport of single crystalline YSb Ghimire, N J; Botana, A S; Phelan, D ...
Journal of physics. Condensed matter,
06/2016, Letnik:
28, Številka:
23
Journal Article
Recenzirano
Odprti dostop
We report magnetic field dependent transport measurements on a single crystal of cubic YSb together with first principles calculations of its electronic structure. The transverse magnetoresistance ...does not saturate up to 9 T and attains a value of 75 000% at 1.8 K. The Hall coefficient is electron-like at high temperature, changes sign to hole-like between 110 and 50 K, and again becomes electron-like below 50 K. First principles calculations show that YSb is a compensated semimetal with a qualitatively similar electronic structure to that of isostructural LaSb and LaBi, but with larger Fermi surface volume. The measured electron carrier density and Hall mobility calculated at 1.8 K, based on a single band approximation, are 6.5×1020 cm−3 and 6.2×104 cm2 Vs−1, respectively. These values are comparable with those reported for LaBi and LaSb. Like LaBi and LaSb, YSb undergoes a magnetic field-induced metal-insulator-like transition below a characteristic temperature Tm, with resistivity saturation below 13 K. Thickness dependent electrical resistance measurements show a deviation of the resistance behavior from that expected for a normal metal; however, they do not unambiguously establish surface conduction as the mechanism for the resistivity plateau.
We report the effect of hydrostatic pressure on the magnetotransport properties of the Weyl semimetal NbAs. Subtle changes can be seen in the profiles with pressure up to 2.31 GPa. The Fermi surfaces ...undergo an anisotropic evolution under pressure: the extremal areas slightly increase in the - plane, but decrease in the -() plane. The topological features of the two pockets observed at atmospheric pressure, however, remain unchanged at 2.31 GPa. No superconductivity can be seen down to 0.3 K for all the pressures measured. By fitting the temperature dependence of specific heat to the Debye model, we obtain a small Sommerfeld coefficient mJ (mol·K2)−1 and a large Debye temperature, K, confirming a 'hard' crystalline lattice that is stable under pressure. We also studied the Kadowaki-Woods ratio of this low-carrier-density massless system, cm mol2 K2 J−2. After accounting for the small carrier density in NbAs, this indicates a suppressed transport scattering rate relative to other metals.
Electron-hole (e−h) compensation is a hallmark of multiband semimetals with extremely large magnetoresistance (XMR) and has been considered to be the basis for XMR. Recent spectroscopic experiments, ...however, reveal that YSb with nonsaturating magnetoresistance is uncompensated, questioning the e−h compensation scenario for XMR. Here we demonstrate with magnetoresistivity and angle-dependent Shubnikov–de Haas (SdH) quantum oscillation measurements that YSb does have nearly perfect e−h compensation, with a density ratio of ∼0.95 for electrons and holes. The density and mobility anisotropy of the charge carriers revealed in the SdH experiments allow us to quantitatively describe the magnetoresistance with an anisotropic multiband model that includes contributions from all Fermi pockets. We elucidate the role of compensated multibands in the occurrence of XMR by demonstrating the evolution of calculated magnetoresistances for a single band and for various combinations of electron and hole Fermi pockets.
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