Non-centrosymmetric Weyl semimetal state and strain effect in the twisted-brick phase transition metal monochalcogenides.

Weyl semimetals are a class of gapless electronic excitation topological quantum materials upon breaking time-reversal or inversion symmetry. Here, we demonstrate the existence of the Weyl semimetal state in the non-centrosymmetric twisted-brick phase MoTe theoretically. The topological properties and strain effects of MoTe have been systematically studied based on first-principles calculations and the Wannier-based tight-binding method. In the absence of spin-orbit coupling (SOC), MoTe exhibits gapless nodal loop states related to the mirror reflection symmetry. When the SOC is turned on, the two nodal loops split into 22 pairs of Weyl points (WPs) with opposite chirality. When the effect of uniaxial (εz) strain is taken into account, the Weyl semimetal phase of MoTe shows great robustness and striking tunable topological strength. In particular, the total number of WPs changes significantly under strain. MoTe under +4% and +8% uniaxial strains have only four pairs of WPs with a relatively large separation in momentum space. These results show that MoTe under weak strain is a promising partly ideal type I Weyl semimetal candidate, while the isolog structure WTe both opens a direct gap with and without SOC, showing a compensated semimetal state.

Noncentrosymmetric Weyl phase and topological phase transition in bulk MoTe.

Ideal topological materials are those stable materials with less nontrivial band crossing near the Fermi surface and a long Fermi arc. By means of first-principles calculations, here we present that the 3D monochalcogenide molybdenum telluride (Pm-MoTe) without an inversion center shows a type-II Weyl semimetal (WSM) phase which cannot checked by symmetry index method. A total of eight Weyl points (WPs) are found in different quadrants of the Brillouin zone (BZ) of Pm-MoTe, which guarantee a long Fermi arc. The WSM phase is robust against the spin-orbit coupling (SOC) effect because of mirror symmetry and time reversal symmetry. It is also found that a topological phase transition can be tuned by strain. For different types of strain, the number of WPs can be effectively modulated to a minimum number, and their energies could be closer to Fermi level. These findings propose a promising material candidate that partly satisfies the ideal WSM criteria and extends the potential applications of the tunable topological phase.

Leakage of Majorana bound states in an inhomogeneous topological nanowire.

We present an exact solution of the continuum Bogolyubov-de-Gennes Hamiltonian for Majorana bound states (MBSs) generated in a superconductor-semiconductor hybrid topological nanowire. The full energy spectra that include the band states and in-gap states could be obtained. We show that for relatively short wire length, the zero energy mode could be induced even in the topological trivial regime, which also indicates oscillatory dependence on the chemical potential. With the increase of the Zeeman field, the MBSs are almost fully spin-polarized and do not localize at the wire ends gradually. We also extend our discussion to the property of Majorana modes in an inhomogeneous nanowire, in which a local gate voltage is applied to one end of the nanowire. It is found that the local potential barrier or well could modulate the Majorana energy splitting periodically. The leakage of MBSs to the potential region is exponentially suppressed for the barrier case. A potential well could induce near-zero-energy bound states and these states merge with MBSs, leading to the delocalization of MBSs. In the potential well region, both the spin-up and spin-down components of the trivial states account for a significant proportion, which can be detected experimentally.

Effects of Se substitution and transition metal doping on the electronic and magnetic properties of a MoSxSe2-x/h-BN heterostructure.

The van der Waals heterostructures created by stacking two monolayer semiconductors have been rapidly developed experimentally and exhibit various unique physical properties. In this work, we investigate the effects of Se atom substitution and 3d-TM atom doping on the structural, electronic, and magnetic properties of the MoSe2/h-BN heterostructure, by using first-principles calculations based on density functional theory (DFT). It is found that Se atom substitution could considerably enhance the band gaps of MoSe2/h-BN heterostructures. With an increase in the substitution concentration, the energy band changes from an indirect to a direct band gap when the substitution concentration exceeds a critical value. For 3d-TM atom doping, it is shown that V-, Mn-, Fe-, and Co-doped systems exhibit a half-metallic state and magnetic behavior, while there is no spin polarization in the Ni-doped case. The results provide a theoretical basis for the development of diluted magnetic semiconductors and spin devices based on the MoSxSe2-x/h-BN heterostructure.

Magnetic impurity in topological nodal loop semimetals.

We investigate the Kondo effect of a spin-1/2 magnetic impurity in a topological nodal loop semimetal, in which band touchings form a nodal loop. The Fermi surface of a nodal loop semimetal is a torus or a drum-like structure, which is determined by chemical potential. When the chemical potential µ lies at the nodal loop ([Formula: see text]), the magnetic impurity and the conduction electrons form bound states only if their coupling exceeds a critical value. As the chemical potential is tuned away from the nodal loop, the Fermi surface becomes a torus or drum-like structure and the impurity and the host material always favor a bound state due to the finite density of state. Due to the anisotropic dispersion relationship in the energy band, the spatial spin-spin correlations [Formula: see text]([Formula: see text]) are of power-law decay with the decay rates proportional to [Formula: see text] and [Formula: see text] in different directions, respectively. The product [Formula: see text] and [Formula: see text] oscillates in coordinate space and the period is enhanced gradually as the Fermi surface evolves from a torus surface into a drum-like structure.

Disorder-induced suppression of the zero-bias conductance peak splitting in topological superconducting nanowires.

We investigate the effect of three types of intrinsic disorder, including that in pairing energy, chemical potential, and hopping amplitude, on the transport properties through the superconducting nanowires with Majorana bound states (MBSs). The conductance and the noise Fano factor are calculated based on a tight-binding model by adopting a non-equilibrium Green's function method. It is found that the disorder can effectively lead to a reduction in the conductance peak spacings and significantly suppress the peak height. Remarkably, for a longer nanowire, the zero-bias peak could be reproduced by weak disorder for a finite Majorana energy splitting. It is interesting that the shot noise provides a signature to discriminate whether the zero-bias peak is induced by Majorana zero mode or disorder. For Majorana zero mode, the noise Fano factor approaches zero in the low bias voltage limit due to the resonant Andreev tunneling. However, the Fano factor is finite in the case of a disorder-induced zero-bias peak.

Super-Poissonian shot noise as a probe of spin bias in mesoscopic systems.

It is proposed that super-Poissonian shot noise can be used to probe and measure the spin bias in mesoscopic systems. Current shot noise through a quantum dot coupled to two conducting leads is theoretically investigated when a pure spin bias is applied. It is found that super-Poissonian shot noise may be induced when the dot level is located within the spin bias window. This further demonstrates the dependence of shot noise on the dot-lead coupling asymmetry and the spin-flip scattering.