Surface state transport in double-gated and magnetized topological insulators with hexagonal warping effects.

We explore the scattering of Dirac electrons in a double-gated topological insulator in the presence of magnetic proximity effects and warped surface states. It is found that a magnetic field can shift the Dirac cone in momentum space and deform the constant-energy contour, or opens up a band gap at the Dirac point, depending on the magnetization orientation. The double gate voltage induces quantum wells and/or quantum barriers on the surface of topological insulators, generating surface resonant tunnelling states. It is found that the hexagonal warping effect can increase the electronic transport at high energies when the constant-energy contour exhibits a snowflake shape. The energy-dependent conductances in the parallel and antiparallel magnetic configurations exhibit out-of-phase oscillations due to the quantum interference of propagating waves in the region between the two magnetized segments. Although the conductance spectrum of the double-well structure is higher than that of the double-barrier structure, the magnetoresistance ratio versus the separation distance between the two magnetized barriers exhibits pronounced oscillations due to the resonant tunnelling states. We show that the surface state transport can be controlled by the exchange field and gate voltage without breaking time reversal symmetry, suggesting that the double gated and magnetized topological insulators can be utilized to achieve a large magnetoresistance ratio with a tunable sign.

Controlling the thermoelectric effect by mechanical manipulation of the electron's quantum phase in atomic junctions.

The thermoelectric voltage developed across an atomic metal junction (i.e., a nanostructure in which one or a few atoms connect two metal electrodes) in response to a temperature difference between the electrodes, results from the quantum interference of electrons that pass through the junction multiple times after being scattered by the surrounding defects. Here we report successfully tuning this quantum interference and thus controlling the magnitude and sign of the thermoelectric voltage by applying a mechanical force that deforms the junction. The observed switching of the thermoelectric voltage is reversible and can be cycled many times. Our ab initio and semi-empirical calculations elucidate the detailed mechanism by which the quantum interference is tuned. We show that the applied strain alters the quantum phases of electrons passing through the narrowest part of the junction and hence modifies the electronic quantum interference in the device. Tuning the quantum interference causes the energies of electronic transport resonances to shift, which affects the thermoelectric voltage. These experimental and theoretical studies reveal that Au atomic junctions can be made to exhibit both positive and negative thermoelectric voltages on demand, and demonstrate the importance and tunability of the quantum interference effect in the atomic-scale metal nanostructures.

Gate-controlled valley transport and Goos-Hänchen effect in monolayer WS2.

Based on a Dirac-like Hamiltonian and coherent scattering formalism, we study the spin-valley transport and Goos-Hänchen-like (GHL) effect of transmitted and reflected electrons in a gated monolayer WS2. Our results show that the lateral shift of spin-polarized electrons is strongly dependent on the width of the gated region and can be positive or negative in both Klein tunneling and classical motion regimes. The absolute values of the lateral displacements at resonance positions can be considerably enhanced when the incident angle of electrons is close to the critical angle. In contrast to the time reversal symmetry for the transmitted electrons, the GHL shift of the reflected beams is not invariant under simultaneous interchange of spins and valleys, indicating the lack of spin-valley symmetry induced by the tunable potential barrier on the WS2 monolayer. Our findings provide evidence for electrical control of valley filtering and valley beam splitting by tuning the incident angle of electrons in nanoelectronic devices based on monolayer transition metal dichalcogenides.

Electrical conductance and structure of copper atomic junctions in the presence of water molecules.

We have investigated Cu atomic contacts in the presence of H2O both experimentally and theoretically. The conductance measurements showed the formation of H2O/Cu junctions with a fixed conductance value of around 0.1 G0 (G0 = 2e(2)/h). These structures were found to be stable and could be stretched over 0.5 nm, indicating the formation of an atomic or molecular chain. In agreement with the experimental findings, theoretical calculations revealed that the conductance of H2O/Cu junctions decreases in stages as the junction is stretched, with the formation of a H2O/Cu atomic chain with a conductance of ca. 0.1 G0 prior to junction rupture. Conversely, in the absence of H2O, the conductance of the Cu junction remains close to 1 G0 prior to the junction rupture and abrupt conductance drop.