We report the picosecond spin current generation from the interface between a heavy metal and a vicinal antiferromagnet insulator Cr2O3 by laser pulses at room temperature and zero magnetic field. It is converted into a detectable terahertz emission in the heavy metal via the inverse spin Hall effect. The vicinal interfaces are apparently the source of the picosecond spin current, as evidenced by the proportional terahertz signals to the vicinal angle. We attribute the origin of the spin current to the transient magnetic moment generated by an interfacial nonlinear magnetic-dipole difference-frequency generation. We propose a model based on the in-plane inversion symmetry breaking to quantitatively explain the terahertz intensity with respect to the angles of the laser polarization and the film azimuth. Our work opens new opportunities in antiferromagnetic and ultrafast spintronics by considering symmetry breaking.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) was observed in the bilayer consisting of a heavy metal and a ferromagnetic insulator such as Pt vertical bar Tm3Fe5O12 (TIG), but the source and origin are still controversial. Here, we quantitatively investigate the iDMI strength in Pt vertical bar TIG by inserting a thin Y3Fe5O12 layer and/or a thin Cu layer between Pt and TIG. Our results suggest the iDMI contributed by both the Pt vertical bar TIG and the TIG vertical bar substrate interfaces. At the Pt vertical bar TIG interface, we find that Pt is essential for the strong iDMI. The disentangled iDMIs for Fe-Fe and Tm-Fe pairs are comparable, revealing that the Tm ions provide not only an additional Tm-Fe iDMI, but also the spin-orbit coupling to enhance the iDMI. At the TIG vertical bar substrate interface, the iDMI is attributed to the lattice-mismatch-induced deformation of the electron cloud for the ferromagnetic ions.

We present a theoretical study of the Kondo effect for a magnetic atom placed inside nanocorrals using Green's function calculations. Based on the standard mapping of the Anderson impurity model to a one-dimensional chain model, we formulate a weak-coupling theory to study the Anderson impurities in a hosting bath with a surface state. With further taking into account the multiple scattering effect of the surrounding atoms, our calculations show that the Kondo resonance width of the atom placed at the center of the nanocorral can be significantly tuned by the corral size, in good agreement with recent experiments [Q. L. Li et al., Phys. Rev. B 97, 035417 (2018)]. The method can also be applied to the atom placed at an arbitrary position inside the corral where our calculation shows that the Kondo resonance width also oscillates as the function of its separation from the corral center. The prediction is further confirmed by the low-temperature scanning tunneling microscopy studies where a one-to-one correspondence is found. The good agreement with the experiments validates the generality of the method to the system where multiadatoms are involved.

We address the long-term controversy on the fundamental question of the role of the surface state on the Kondo effect with Co adatoms on a Ag(111) surface. The width of the Kondo resonance oscillates with the same period of half Fermi wavelength of the surface state. But the amplitude increases for a Co adatom placed next to another Co adatom, at the vicinity of a step edge, and quantum confined within nanocorrals. A greater than three times enhancement of the resonance width can be achieved when quantum confinement is introduced. The experimental results are described quantitatively utilizing an analytical model where the contributions of the bulk and surface states are weighted by their exchange values with the magnetic impurity. These findings clarify the role of the surface state on the Kondo effect and pave a pathway to tailor the Kondo effect via quantum confinement of the surface state.

By using the non-equilibrium Green's function technique, we investigate the electronic transport properties in an Aharonov—Bohm interferometer coupling with Majorana fermions. We find a fixed unit conductance peak which is independent of the other factors when the topological superconductor is grounded. Especially, an additional phase appears when the topological superconductor is in the strong Coulomb regime, which induces a new conductance resonant peak compared with the structure of replacing the topological superconductor by a quantum dot, and the conductance oscillation with the magnetic flux reveals a 2π phase shift by raising (lowering) a charge on the capacitor.

A single-molecule magnet (SMM) coupled to two normal metallic electrodes can both switch spin-up and spin-down electronic currents within two different windows of SMM gate voltage. Such spin current switching in the SMM tunnel junction arises from spin-selected single electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. Since it is not magnetically controlled but all-electrically controlled, the proposed spin current switching effect may have potential applications in future spintronics.

We report the theoretical investigation of the electrochemical capacitance of a double-quantum-dot device. Analytic result is obtained using the discrete potential approximation. We find that electrochemical capacitance can be tuned by the interdot coupling t. For large t, the electrochemical capacitance can be negative.

We report theoretical investigations of dynamic conductance of molecular systems in the metal-molecule-metal device configuration. The quantum coherent ac transport may be mediated by resonant states extending the entire molecule, or mediated by localized states within the molecule itself. The latter is characterized by tiny features in the dc conductance, but the dissipative part of dynamic conductance can be enhanced by several orders of magnitude as the ac frequency is increased. This phenomenon can be understood from an analytical model.

We investigate the nonequilibrium transport properties of a quantum dot in the normal-metal and superconductor hybrid system, where the Andreev reflection plays the important role. The electron spin-flip effect in quantum dot is explicitly taken into account. By using the nonequilibrium Green's function (NEGF) approach, we first give a general current formula which is valid for any bias and temperature, then we discuss the linear conductance in terms of the local density of states. The various resonant peaks appear for the different spin-flip strengths in quantum dot. Finally, we numerically calculate the current-bias characteristic when the bias is smaller than the superconductor gap.

We investigate the heat current and spin current through a carbon-nanotube-based molecular quantum pump. We have derived a general expression for the heat current at finite frequency so that the heat current can be calculated order by order in pumping amplitudes. We have applied our theory to a carbon-nanotube-based quantum pump. The heat current generated during the parametric pumping has been calculated at small frequencies for finite pumping amplitude. At finite frequencies, we have calculated the heat current to the second order in pumping amplitudes. The photon assisted process is clearly observed in the heat current. In the presence of magnetic field, the carbon-nanotube-based quantum pump can function as a spin pump, a molecular device by which a dc pure spin current without accompanying charge current is generated at zero bias voltage via a cyclic deformation of two device parameters. The pure spin current is achieved when the Fermi energy is near the resonant level of the quantum pump. We find that the pure spin current is sensitive to system parameters such as pumping amplitude, external magnetic field, and gate voltage.

We theoretically put forward a spin injector, which consists of a three-terminal ferromagnetic-metal (FM) nonmagnetic-semiconductor (NS)-superconductor (SC) mesoscopic hybrid system. This device can inject not only the spin-up current but also the pure spin current into the NS lead. The crossed Andreev reflection plays a key role in this device. Such a spin injector may be realized within the reach of the present-day technology.

We report the investigation of the spin-valve effect through a resonant level between a ferromagnetic electrode in the presence of an external ac bias. We use the current conserving and gauge invariant theory developed by B\"uttiker to calculate the dynamic conductance. Specifically, we have calculated the tunneling magnetoresistance (TMR) ratio as a function of various system parameters such as the angle between magnetization of the left and right leads, ac frequency, and the Fermi energy. We found that the TMR ratio can be modulated by ac frequency. At large frequency, the TMR ratio can be negative.

We report a theoretical investigation of shot noise of spin current without an accompanying charge current. For a two-probe spin pump, both cross- and autocorrelations are needed to characterize the noise. The corresponding Fano factors measure the spin unit of the quasiparticles in the spin current. The shot noise also detects open channels for spin transport, and can have qualitatively different behavior compared with the shot noise of charge current.

It is well known that the particles in a beam of bosons obeying Bose-Einstein statistics tend to cluster (bunching effect), while the particles in a degenerate beam of fermions obeying Fermi-Dirac statistics expel each other (antibunching effect). Here we investigate the statistical correlation effect for a three-terminal normal-metal-superconductor-superconductor hybrid mesoscopic system. By using a nonequilibrium Green's-function technique, we obtain a positive cross correlation when the external voltage is smaller than the gap energy, which demonstrates bosonic behavior. In the larger voltage limit, the cross correlation becomes negative due to the contribution of the quasiparticles. At large voltages, the oscillation between fermionic and bosonic behavior of cross correlation is also observed in the strong-coupling regime as one changes the position of the resonant levels.

We report on a theoretical analysis of transport characteristics of a spin-valve system formed by a quantum dot connecting to two ferromagnetic electrodes whose magnetic moments are oriented at an angle theta with respect to each other. We pay special attention to the effects of a finite on-site Coulomb repulsion U. Using the Keldysh nonequilibrium Green's functions we derived a formula for the current in general terms of bias, temperature, and the parameters theta,U. We have studied the local density of states and nonlinear conductance of this device in the Kondo regime at different polarization angle theta. Our results suggest that the Kondo peaks in the local density of states and in the conductance can be modulated by theta.

We have extended the previous parametric electron pumping theory to include the spin-polarized pumping effect. Specifically, we consider a parametric pump consisting of a nonmagnetic system with two ferromagnetic leads whose magnetic moments orient at an angle theta with respect to each other. In our theory, the leads can be maintained at different chemical potentials. As a result, the current is driven due to both the external bias and the pumping potentials. When both theta and the external bias are zero, our theory recovers the known theory. In particular, two cases are considered: (i) in the adiabatic regime, we have derived the pumped current for an arbitrary pumping amplitude and external bias and (ii) at finite frequency, the system is away from equilibrium, and we have derived the pumped current up to quadratic order in pumping amplitude. From our numerical results we found that the pumped current can be modulated by the angle theta, showing interesting spin-valve effects.

We investigate the heat flow in the parametric quantum pump. Using the time dependent scattering matrix theory, we have developed a general theory for the pumped heat current at finite pumping amplitude and frequency. We have applied our theory to a double barrier structure and studied pumped heat current in both the weak and strong pumping regimes as different system parameters vary. By comparing the pumped heat current and the power of Joule heat generated in the system, we found that the double barrier structure can function as an optimal pump in the strong pumping regime.

We investigate the parametric electron pumping of a double barrier structure in the presence of a superconducting lead. The parametric pumping is facilitated by cyclic variation of the barrier heights ${x}_{1}$ and ${x}_{2}$ of the barriers. In the weak-coupling regime, there exists a resonance line in the parameter space ${(x}_{1}{,x}_{2})$ so that the energy of the quasibound state is in line with the incoming Fermi energy. Levinson et al. found recently that the pumped charge for each pumping cycle is quantized with $Q=2e$ for normal structure when the pumping contour encircles the resonance line. In the presence of a superconducting lead, we find that the pumped charge is quantized with the value $2e.$

We report a first-principles theory for analyzing the parametric electron pump at finite frequency. The pump is controlled by two pumping parameters with phase difference phi. In the zero-frequency limit. our theory recovers the well-known adiabatic result that the pumped current I(phi) similar to sin phi. At a finite frequency, it predicts I(phi = 0) not equal 0 while I(phi = pi) approximate to 0, consistent with recent experimental data. We discuss a possible mechanism behind the nonzero pumped current at phi = 0 from photon-assisted processes.

We report a theoretical analysis of parametric electron pump through a quantum dot in the Kondo regime. In the adiabatic regime, we have derived the expression for pumped current in the Kondo regime using nonequilibrium Green's function. The pumped current versus different system parameters such as gate voltage, pumping amplitude, as well as the phase difference between two pumping forces are calculated and interesting physics are revealed.