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  1. Abstract We study helical structures in spin-spiral single crystals. In the continuum approach for the helicity potential energy the simple electronic band splits into two non-parabolic bands. For low exchange integrals, the lower band is described by a surface with a saddle shape in the direction of the helicity axis. Using the Boltzmann equation with the relaxation due to acoustic phonons, we discover the dependence of the current on the angle between the electric field and helicity axis leading to the both parallel and perpendicular to the electric field components in the electroconductivity. The latter can be interpreted as a planar Hall effect. In addition, we find that the transition rates depend on an electron spin allowing the transition between the bands. The electric conductivities exhibit nonlinear behaviors with respect to chemical potential µ . We explain this effect as the interference of the band anisotropy, spin conservation, and interband transitions. The proposed theory with the spherical model in the effective mass approximation for conduction electrons can elucidate nonlinear dependencies that can be identified in experiments. We find the excellent agreement between the theoretical and experimental data for parallel resistivity depending on temperature at the phase transition from helical tomore »ferromagnetic state in a M n P single crystal. In addition, we predict that the perpendicular resistivity abruptly drops to zero in the ferromagnetic phase.« less
    Free, publicly-accessible full text available November 3, 2023
  2. CrBr 3 is a layered van der Waals material with magnetic ordering down to the 2D limit. For decades, based on optical measurements, it is believed that the energy gap of CrBr 3 is in the range of 1.68–2.1 eV. However, controversial results have indicated that the band gap of CrBr 3 is possibly smaller than that. An unambiguous determination of the energy gap is critical to the correct interpretations of the experimental results of CrBr 3 . Here, we present the scanning tunneling microscopy and spectroscopy (STM/S) results of CrBr 3 thin and thick flakes exfoliated onto highly ordered pyrolytic graphite (HOPG) surfaces and density functional theory (DFT) calculations to reveal the small energy gap (peak-to-peak energy gap to be 0.57 ± 0.04 eV; or the onset signal energy gap to be 0.29 ± 0.05 eV from d I /d V spectra). Atomic resolution topography images show the defect-free crystal structure and the d I /d V spectra exhibit multiple peak features measured at 77 K. The conduction band – valence band peak pairs in the multi-peak d I /d V spectrum agree very well with all reported optical transitions. STM topography images of mono- and bi-layer CrBr 3more »flakes exhibit edge degradation due to short air exposure (∼15 min) during sample transfer. The unambiguously determined small energy gap settles the controversy and is the key in better understanding CrBr 3 and similar materials.« less