Orbital character of the spin-reorientation transition in TbMn6Sn6.

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 TbMn6Sn6, the uniaxial magnetic anisotropy of the Tb3+ 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 TSR = 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 TSR 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.

Quantum limit transport and destruction of the Weyl nodes in TaAs.

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.

Anomalous electronic structure and magnetoresistance in TaAs2.

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.

Magnetotransport of single crystalline YSb.

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 [Formula: see text] cm(-3) and [Formula: see text] cm(2) 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 T m, 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.

'Hard' crystalline lattice in the Weyl semimetal NbAs.

We report the effect of hydrostatic pressure on the magnetotransport properties of the Weyl semimetal NbAs. Subtle changes can be seen in the ρ(xx)(T) profiles with pressure up to 2.31 GPa. The Fermi surfaces undergo an anisotropic evolution under pressure: the extremal areas slightly increase in the k(x)-k(y) plane, but decrease in the k(z)-k(y)(k(x)) 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 γ(0) = 0.09(1) mJ (mol·K(2))(-1) and a large Debye temperature, Θ(D) = 450(9) 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, R(KW) = 3.2 × 10(4) µΩ cm mol(2) K(2) J(-2). After accounting for the small carrier density in NbAs, this R(KW) indicates a suppressed transport scattering rate relative to other metals.

Magnetic structure of the antiferromagnetic Kondo lattice compounds CeRhAl4Si2 and CeIrAl4Si2.

We have investigated the magnetic ground state of the antiferromagnetic Kondo-lattice compounds CeMAl4Si2(M = Rh, Ir) using neutron powder diffraction. Although both of these compounds show two magnetic transitions T(N1) and T(N2) in the bulk properties measurements, evidence for magnetic long-range order was only found below the lower transition T(N2). Analysis of the diffraction profiles reveals a commensurate antiferromagnetic structure with a propagation vector k = (0, 0, 1/2). The magnetic moment in the ordered state of CeRhAl4Si2 and CeIrAl4Si2 were determined to be 1.14(2) and 1.41(3) µ(B) Ce(-1), respectively, and are parallel to the crystallographic c-axis in agreement with magnetic susceptibility measurements.

Magnetotransport of single crystalline NbAs.

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 × 10(19) cm(-3) and 3.5 × 10(5) cm(2) 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.

Investigation of the physical properties of the tetragonal CeMAl4Si2 (M = Rh, Ir, Pt) compounds.

The synthesis, crystal structure and physical properties studied by means of x-ray diffraction, magnetic, thermal and transport measurements of CeMAl4Si2 (M = Rh, Ir, Pt) are reported, along with the electronic structure calculations for LaMAl4Si2 (M = Rh, Ir, Pt). These materials adopt a tetragonal crystal structure (space group P4/mmm) comprised of BaAl4 blocks, separated by MAl2 units, stacked along the c-axis. Both CeRhAl4Si2 and CeIrAl4Si2 order antiferromagnetically below TN1 = 14 and 16 K, respectively, and undergo a second antiferromagnetic transitition at lower temperature (TN2 = 9 and 14 K, respectively). CePtAl4Si2 orders ferromagnetically below TC = 3 K with an ordered moment of µsat = 0.8 µB for a magnetic field applied perpendicular to the c-axis. Electronic structure calculations reveal quasi-2D character of the Fermi surface.

Magnitude of the magnetic exchange interaction in the heavy-fermion antiferromagnet CeRhIn5.

We have used high-resolution neutron spectroscopy experiments to determine the complete spin wave spectrum of the heavy-fermion antiferromagnet CeRhIn5. The spin wave dispersion can be quantitatively reproduced with a simple frustrated J1-J2 model that also naturally explains the magnetic spin-spiral ground state of CeRhIn5 and yields a dominant in-plane nearest-neighbor magnetic exchange constant J0=0.74(3) meV. Our results pave the way to a quantitative understanding of the rich low-temperature phase diagram of the prominent CeTIn5 (T=Co, Rh, Ir) class of heavy-fermion materials.

Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy.

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.