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Register a new userMagnetic torque anomaly in the quantum limit of the Weyl semi-metal NbAs
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Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue physicists due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we report a large anomaly in the magnetic torque of the Weyl semi-metal NbAs upon entering the quantum limit state in high magnetic fields, where topological corrections to the energy spectrum become dominant. The quantum limit torque displays a striking change in sign, signaling a reversal of the magnetic anisotropy that can be directly attributed to the topological properties of the Weyl semi-metal. Our results establish that anomalous quantum limit torque measurements provide a simple experimental method to identify Weyl- and Dirac- semi-metals.
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Weyl semi-metal is the three dimensional analog of graphene. According to the quantum field theory, the appearance of Weyl points near the Fermi level will cause novel transport phenomena related to chiral anomaly. In the present paper, we report the first experimental evidence for the long-anticipated negative magneto-resistance generated by the chiral anomaly in a newly predicted time-reversal invariant Weyl semi-metal material TaAs. Clear Shubnikov de Haas oscillations (SdH) have been detected starting from very weak magnetic field. Analysis of the SdH peaks gives the Berry phase accumulated along the cyclotron orbits to be {pi}, indicating the existence of Weyl points.
We performed a series of high-pressure synchrotron X-ray diffraction (XRD) and resistance measurements on the Weyl semimetal NbAs. The crystal structure remains stable up to 26 GPa according to the powder XRD data. The resistance of NbAs single crystal increases monotonically with pressure at low temperature. Up to 20 GPa, no superconducting transition is observed down to 0.3 K. These results show that the Weyl semimetal phase is robust in NbAs, and applying pressure is not a good way to get a topological superconductor from a Weyl semimetal.
Charged excitons (trions) are essential for the optical spectra in low dimensional doped monolayers (ML) of transitional metal dichalcogenides (TMDC). Using a direct diagonalization of the three-body Hamiltonian, we explore the low-lying trion states in four types of TMDC MLs. We show that the trions fine structure results from the interplay between the spin-valley fine structure of the single-particle bands and the exchange interaction between the composing particles. We demonstrate that by variations of the doping and dielectric environment, trion energy fine structure can be tuned, leading to anti-crossing of the bright and dark states with substantial implications for the optical spectra of TMDC MLs.
Weyl semimetals are materials where electrons behave effectively as a kind of massless relativistic particles known asWeyl fermions. These particles occur in two flavours, or chiralities, and are subject to quantum anomalies, the breaking of a conservation law by quantum fluctuations. For instance, the number of Weyl fermions of each chirality is not independently conserved in parallel electric and magnetic field, a phenomenon known as the chiral anomaly. In addition, an underlying curved spacetime provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial-gravitational anomaly, which remains experimentally elusive. However, the presence of a mixed gauge-gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field, even in flat spacetime, opening the door to experimentally probe such type of anomalies in Weyl semimetals. Using a temperature gradient, we experimentally observe a positive longitudinal magnetothermoelectric conductance (PMTC) in the Weyl semimetal NbP for collinear temperature gradients and magnetic fields (DT || B) that vanishes in the ultra quantum limit. This observation is consistent with the presence of a mixed axial-gravitational anomaly. Our work provides clear experimental evidence for the existence of a mixed axial-gravitational anomaly of Weyl fermions, an outstanding theoretical concept that has so far eluded experimental detection.
Interplay of spin, charge, orbital and lattice degrees of freedom in oxide heterostructures results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices with enhanced functionalities. The paradigm example is the interface between the two band insulators LaAlO3 and SrTiO3 (LAO/STO) that hosts two-dimensional electron system (2DES). Apart from the mobile charge carriers, this system exhibits a range of intriguing properties such as field effect, superconductivity and ferromagnetism, whose fundamental origins are still debated. Here, we use soft-X-ray angle-resolved photoelectron spectroscopy to penetrate through the LAO overlayer and access charge carriers at the buried interface. The experimental spectral function directly identifies the interface charge carriers as large polarons, emerging from coupling of charge and lattice degrees of freedom, and involving two phonons of different energy and thermal activity. This phenomenon fundamentally limits the carrier mobility and explains its puzzling drop at high temperatures.
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