Magnetic field effect on topological properties of Dirac semimetals PdTe2/PtTe2/PtSe2

We investigated magnetic field effect on the topological properties of transition metal dichalcogenide Dirac semimetals (DSMs) PdTe₂/PtTe₂/PtSe₂based on Wannier-function-based tight-binding (WFTB) model obtained from first-principles calculations. The DSMs PdTe₂/PtTe₂/PtSe₂undergo a transition from DSMs into Weyl semimetals with four pairs of Weyl points (WPs) in the entire Brillouin zone by splitting Dirac points under external magnetic fieldB. The positions and energies of WPs vary linearly with the strength of theBfield under thec-axis magnetic fieldB. Under thea- andb-axisBfield, however, the positions of magnetic-field-inducing WPs deviate slightly from thecaxis, and theirk_zcoordinates and energies change in a parabolic-like curve with the increasingBfield. However, the system opens an axial gap on theA-Γ axis, and the gap changes with the direction of theBfield when the out ofc-axisBfield is applied. When we further apply the magnetic field in theac,bc, andabplanes, the results are more diverse compared to the axial magnetic field. Under theacandbcplaneBfield, thek_zand energies of WPs within angleθ= [0°, 90°] andθ= [90°, 180°] are mirror symmetrically distributed. The distribution of WPs shows broken rotational symmetry under theabplaneBfield due to the difference of non-diagonal part of Hamiltonian. Our theoretical findings can provide a useful guideline for the applications of DSM materials under external magnetic field in the future topological electronic devices.

Application of magnetic field over-modulation for improved EPR linewidth measurements using probes with Lorentzian lineshape

Magnetic field modulation in CW electron paramagnetic resonance (EPR) is used for signal detection. However, it can also distort signal lineshape. In experiments where the linewidth information is of particular importance, small modulation amplitude is usually used to limit the lineshape distortion. The use of small modulation amplitude, however, results in low signal-to-noise ratio and therefore affects the precision of linewidth measurements. Recently, a new spectral simulation model has been developed enabling accurate fitting of modulation-broadened EPR spectra in liquids. Since the use of large modulation amplitude (over-modulation) can significantly enhance the EPR signal, the precision of linewidth measurements is therefore greatly improved. We investigated the over-modulation technique in EPR oximetry experiments using the oxygen-sensing probe lithium octa-n-butoxy-substitued naphthalocyanine (LiNc-BuO). Modulation amplitudes 2-18 times the intrinsic linewidth of the probe were applied to increase the spectral signal-to-noise ratio. The intrinsic linewidth of the probe at different oxygen concentrations was accurately extracted through curve fitting from the enhanced spectra. Thus, we demonstrated that the over-modulation model is also applicable to particulate oxygen-sensing probes such as LiNc-BuO and that the lineshape broadening induced by oxygen is separable from that induced by over-modulation. Therefore, the over-modulation technique can be used to enhance sensitivity and improve linewidth measurements for EPR oximetry with particulate oxygen-sensing probes with Lorentzian lineshape. It should be particularly useful for in vivo oxygen measurements, in which direct linewidth measurements may not be feasible due to inadequate signal-to-noise ratio.