Yi-Hang Nie

Effect of finite Coulomb interaction on full counting statistics of electronic transport through single-molecule magnet

Abstract We study the full counting statistics (FCS) in a single-molecule magnet (SMM) with finite Coulomb interaction U. For finite U the FCS, differing from U → ∞ , shows a symmetric gate-voltage-dependence when the coupling strengths with two electrodes are interchanged, which can be observed experimentally just by reversing the bias-voltage. Moreover, we find that the effect of finite U on shot noise depends on the internal level structure of the SMM and the coupling asymmetry of the SMM with two electrodes as well. When the coupling of the SMM with the incident-electrode is stronger than that with the outgoing-electrode, the super-Poissonian shot noise in the sequential tunneling regime appears under relatively small gate-voltage and relatively large finite U, and dose not for U → ∞ ; while it occurs at relatively large gate-voltage for the opposite coupling case. The formation mechanism of super-Poissonian shot noise can be qualitatively attributed to the competition between fast and slow transport channels.

Macroscopic quantum coherence in small antiferromagnetic particles and quantum interference effects

Abstract Starting from the Hamiltonian of the uncompensated two-sublattice model of a small antiferromagnetic particle, we derive the effective Lagrangian of a biaxial antiferromagnetic particle in an external magnetic field with the help of spin-coherent-state path integrals. Two unequal level-shifts induced by tunneling through two types of barriers are obtained using the instanton method. The energy spectrum is found from Bloch theory regarding the periodic potential as a superlattice. The external magnetic field indeed removes Kramers degeneracy, however, a new quenching of the energy splitting depending on the applied magnetic field is observed for both integer and half-integer spins due to the quantum interference between transitions through two types of barriers.

Electron transport through double quantum dots with spin-polarization dependent interdot coupling

By means of Green functions within the equation of motion scheme, we theoretically investigate electron transport through two lateral quantum dots between which spin-dependent tunneling occurs. In the presence of intradot Coulomb interactions, the expressions for spin and charge currents are given under the Hartree–Fock approximation. The magnitude and polarization of the spin current can be controlled critically by adjusting the gate voltage applied over the quantum dots. The variation of the spin current and the charge current versus the gate voltage can be qualitatively explained by a spin-dependent resonant tunneling picture.

Periodic instanton and phase transition in quantum tunneling of spin systems

Abstract The quantum-classical transitions of the escape rates in a uniaxial spin model relevant to the molecular magnet Mn12Ac and a biaxial anisotropic ferromagnetic particle are investigated by applying the periodic instanton method. The effective free energies are expanded around the top of the potential barrier in analogy to the Landau theory of phase transitions. We show that the first-order transitions occur below the critical external magnetic field h x = 1 4 for the uniaxial spin model and beyond the critical anisotropy constant ratio λ = 1 2 for the biaxial ferromagnetic grains, which are in good agreement with earlier works.

TEMPERATURE DEPENDENCE OF MACROSCOPIC QUANTUM TUNNELING IN ANTIFERROMAGNETIC PARTICLES

Abstract By explicit calculation of quantum tunneling at the level of excited states in a model of an antiferromagnetic particle, it is demonstrated that the transition through the potential barrier may be subdivided into three distinct regions: the ground state tunneling region, the thermally assisted tunneling region and the thermal activation region. The numerical result that the tunneling rate is independent of temperature in the first region agrees with the experimental observation reported by Tejada and Zhang [J. Phys. Condens. Matt. 6 (1994) 263].

Thermoelectric effects of the single-spin state in the ferromagnetic-normal junction with artificial magnetic impurities

We theoretically analyze the thermoelectric properties of the single-spin state based on the resonant tunneling of electron in the ferromagnetic-normal junction with artificial magnetic impurities. The thermoelectric coefficients, such as electrical conductance G, thermal conductance K, thermopower S and effective figure of merit Y, have been calculated using the nonequilibrium Green function in the linear regime. It is found that the thermoelectric coefficients can achieve considerable values by adjusting key parameters of the hybrid mesoscopic structure, such as the level detuning, the interdot hopping coefficient, the external magnetic field and the angle θ. When the level detuning changes, the spectra of electrical conductance and thermal conductance exhibit the electronic Dicke-like effect in the low temperature. Two valleys of electrical conductance and thermal conductance are always located at the single-spin level of QD2 ( and ), and can achieve the antiresonant point by adjusting the interdot hopping coefficient. Thermoelectric coefficients can achieve considerable values near valleys because the Wiedemann–Franz law is strongly violated. Thermopower S and effective figure of merit Y can get larger values in the vicinity of by adjusting key parameters of the hybrid mesoscopic structure, such as the level detuning, the interdot hopping coefficient and the polarization. But the thermoelectric effect is reversed by changing the angle θ. When the angle θ increases, S and Y are suppressed in the vicinity of meanwhile, S and Y are enhanced in the vicinity of shows that an electron in the state can virtually tunnel into the spin-up (or spin-down) state of the ferromagnet. The amplitude of electron tunneling is (or ). G and K decrease (or increase) as the angle θ increases in the vicinity of (or . So theoretical fundament is provided for the design of the single spin device.

Non-equilibrium quantum transport of spin-polarized electrons and back action on molecular magnet tunnel-junction

We investigate the non-equilibrium quantum transport through a single-molecule magnet embedded in a tunnel junction with ferromagnetic electrodes, which generate spin-polarized electrons. The lead magnetization direction is non-collinear with the uniaxial anisotropy easy-axis of molecule-magnet. Based on the Pauli rate-equation approach we demonstrate the magnetization reversion of molecule-magnet induced by the back action of spin-polarized current in the sequential tunnel regime. The asymptotic magnetization of molecular magnet and spin-polarization of transport current are obtained as functions of time by means of time-dependent solution of the rate equation. It is found that the antiparallel configuration of the ferromagnetic electrodes and molecular anisotropy easy-axis is an effective structure to reverse both the magnetization of molecule-magnet and spin-polarization of the transport current. Particularly the non-collinear angle dependence provides useful knowledge for the quantum manipulation of m...

Macroscopic quantum phase interference in antiferromagnetic particles

The tunnel splitting in biaxial antiferromagnetic particles is studied with a magnetic field applied along the hard anisotropy axis. We observe the oscillation of tunnel splitting as a function of the magnetic field due to the quantum phase interference of two tunnelling paths of opposite windings. The oscillation is similar to the recent experimental result with Fe8 molecular clusters.

Periodic Instanton Calculations of Quantum-Classical Transitions in Spin Systems

The quantum-classical transitions of the escape rates in the molecular magnet Mn12Ac and a biaxial anisotropic ferromagnetic spin model are investigated by applying the periodic instanton method. The effective free energies are expanded around the top of the potential barrier in analogy to the Landau theory of phase transitions. We show that the first-order transitions occur below the critical external magnetic field hx = 1/4 for Mn12Ac and beyond the critical anisotropy constant ratio λ = 1/2 for the biaxial ferromagnetic grains, which is in good agreement with earlier studies.

Spin-dependent thermoelectric effect and spin battery mechanism in triple quantum dots with Rashba spin–orbital interaction*

We have studied spin-dependent thermoelectric transport through parallel triple quantum dots with Rashba spin–orbital interaction (RSOI) embedded in an Aharonov–Bohm interferometer connected symmetrically to leads using nonequilibrium Greens function method in the linear response regime. Under the appropriate configuration of magnetic flux phase and RSOI phase, the spin figure of merit can be enhanced and is even larger than the charge figure of merit. In particular, the charge and spin thermopowers as functions of both the magnetic flux phase and the RSOI phase present quadruple-peak structures in the contour graphs. For some specific configuration of the two phases, the device can provide a mechanism that converts heat into a spin voltage when the charge thermopower vanishes while the spin thermopower is not zero, which is useful in realizing the thermal spin battery and inducing a pure spin current in the device.