Persistent Spin Currents Research Articles

From paramagnetic to diamagnetic charge and spin currents in a Hubbard necklace

We investigate persistent charge and spin currents in a Hubbard necklace driven by Aharonov-Bohm and Aharonov-Casher (AC) fluxes, respectively, by paying specific attention to the roles of interactions affecting the charge and spin sectors in different ways. Interacting electrons at half filling induce ``diamagnetic'' spin currents at lower AC fluxes whereas charge currents display paramagnetic responses for noninteracting electrons. We find that for systems away from half filling the characteristic roles of spin and charge currents are interchanged, that is, the charge currents exhibit a change from paramagnetic to diamagnetic currents, while the spin currents remain ``paramagnetic'' states.

Directional Dependence of Spin Currents Induced by Aharonov–Casher Phase

We have calculated the persistent spin current of an open ring induced by the Aharonov–Casher phase. For unpolarized electrons there exist no persistent charge currents, but persistent spin currents. We show that, in general, the magnitude of the persistent spin current in a ring depends on the direction of the direct current flow from one reservoir to another. The persistent spin current is modulated by the cosine function of the spin precession angle. The nonadiabatic Aharonov–Casher phase gives anomalous behaviors. The Aharonov–Anandan phase is determined by the solid angle of spin precession. When the nonadiabatic Aharonov–Anandan phase approaches a constant value with the increase of the electric field, the periodic behavior of the spin persistent current occurs in an adiabatic limit. In this limit the periodic behavior of the persistent spin current could be understood by the effective spin-dependent Aharonov–Bohm flux.

Spiral state and persistent spin current in a system with strong Hund coupling

An effective Hamiltonian for systems with strong Hund coupling is presented, which contains a novel type of interaction between the spin current operator and the curl of two localised spins. We obtain a solution in the case that localised spins form a spiral state. In this state there exists a persistent spin current, but no electric current. In the equilibrium case, the spiral state can only exist in a one-dimensional ring or another circuit form. Our finding rules out the spiral state as a stable magnetic structure in a one-dimensional open chain or higher-dimensional system with strong Hund coupling. A possible experiment which could verify or falsify the assumption of strong Hund coupling is proposed.

Aharonov - Bohm - Casher oscillations in strongly correlated electron systems at finite temperature

Persistent charge and spin currents due to Aharonov - Bohm and Aharonov - Casher interferences of correlated electrons moving along a mesoscopic ring are discussed at finite temperature. As a function of applied flux the ground-state persistent currents have the shape of a generalized saw-tooth, i.e. they consist of piecewise straight segments. The periods and amplitudes of the oscillations are associated with the properties of the Fermi surface of the elementary excitations (two Dirac seas), namely the group velocities and the matrix of dressed generalized charges (Luttinger parameters). The temperature reduces the amplitudes of oscillation by smearing the Fermi surface in a similar way to that for the de Haas - van Alphen effect in 3D metals. The amplitude of higher harmonics decreases more quickly with T than the fundamental one, changing the saw-tooth to a more sinusoidal form with much smaller amplitude. The controlling parameters are the ratios of the thermal energy to the level spacings in the ring. The results are discussed in the context of the exact Bethe ansatz solutions for the Hubbard chain and the supersymmetric t - J model.

Finite-size effects in a metallic multichannel ring with Kondo impurity: Persistent currents and magnetoresistance.

We consider a spin-\textonehalf{} impurity interacting with conduction electrons in two different orbital channels via an isotropic spin exchange. The exchange is the same for both channels, but a crystalline field breaks the symmetry between the orbital channels. This corresponds to a splitting of the conduction electron ${\ensuremath{\Gamma}}_{8}$ into two doublets in the quadrupolar Kondo effect, or to the electron-assisted tunneling of an atom in a double-well potential in an external magnetic field. Another possible realization could be a quantum dot coupled to two equal rings of the same length subject to an electrostatic potential difference. We consider the Bethe ansatz equations for this model and derive the tower structure of the finite-size corrections to the ground-state energy. These results are used to discuss the Aharonov-Bohm-Casher interference pattern in the persistent charge and spin currents, and the magnetoresistivity due to the scattering of electrons off the impurity.

Period and amplitude halving of persistent spin currents via the Aharonov-Casher effect

We study the parity (electron number) dependence of persistent currents due to the Aharonov-Casher effect for a system of noninteracting many electrons in a one-dimensional mesoscopic ring in the presence of a cylindrically symmetric electric field. The persistent charge and spin currents, and the magnetization for the ground-state of the system are calculated. We find the period and amplitude halving of the persistent spin current due to the Aharonov-Casher effect, which is dual to that of the persistent charge current due to the Aharonov-Bohm effect.

Aharonov-Casher effect in the Heisenberg spin chain with many impurities.

The Aharonov-Casher effect is a quantum interference arising in mesoscopic rings involving particles with magnetic moments. The wave function acquires a phase due to an electric-field flux F enclosed by the ring and gives rise to flux-dependent oscillations in persistent spin currents. In the ground state of the spin-1/2 Heisenberg chain the period and amplitude of the oscillations are associated with the properties of the Fermi surface of the elementary excitations of the spin chain (spinons). We consider an antiferromagnetic spin-1/2 Heisenberg ring with a concentration x of impurity spins S. The calculations are performed within the framework of Bethe's ansatz. Due to the finite concentration of impurities the ground state consists of two (rather than one) Dirac seas, corresponding to spinons and 2S strings of rapidities. The persistent spin current reveals two periods of oscillation associated with the properties of the two Fermi seas. The interference pattern as a function of F is discussed for several external magnetic fields.

Persistent spin currents induced by the Aharonov-Casher effect in mesoscopic rings.

We investigate the Aharonov-Casher effect in a mesoscopic ring in the presence of a cylindrically symmetric electric field. The Aharonov-Casher phase is obtained as a function of the tilt angle of the applied electric field. The exact energy spectrum of the system is obtained by using a gauge transformation. We demonstrate the persistent spin current induced by the Aharonov-Casher effect at finite temperature.

Persistent currents in normal metal mesoscopic ring in an inhomogeneous magnetic field

We have performed analytical calculations and computer simulations to study the persistent currents in one-dimensional normal metal mesoscopic ring in an inhomogeneous magnetic field. In the absence of disorder, the general exact solution is found. It is shown that the system supports persistent equilibrium spin and charge currents, even in the absence of Aharonov-Bohm type flux. The effects of disorder are to round off discontinuity in the charge current-flux characteristices and to reduce the amplitude of the current. In strong disorder, this characteristics is very similar to the AB case, but with a displacement.

Persistent spin and mass currents and Aharonov-Casher effect.

Spin-orbit interaction produces persistent spin and mass currents in the ring via the Aharonov-Casher effect. The experiment in $^3He-A_1$ phase, in which this effect leads to the excitation of mass and spin supercurrent is proposed.

Persistent currents from Berry's phase in mesoscopic systems.

The quantum orbital motion of electrons in mesoscopic normal-metal rings threaded by a magnetic flux produces striking interference phenomena such as persistent currents due to the Aharonov-Bohm effect. Similarly, when a quantum spin adiabatically follows a magnetic field that rotates slowly in time, the phase of its state vector acquires an additional contribution known as the Berry phase. We explore the combination of these two quantum phenomena by examining the interplay between orbital and spin degrees of freedom for a charged spin-1/2 particle moving in a mesoscopic ring embedded in a classical, static inhomogeneous magnetic field, i.e, a texture. As a consequence of its orbital motion through the texture, the spin experiences, via the Zeeman interaction, a varying magnetic field. This results in a Berry---or geometric---phase, leading to persistent (i.e., equilibrium) currents of charge and spin. These mesoscopic phenomena are related to (but should be distinguished from) the conventional persistent currents that result from magnetic flux through a ring. We develop a path-integral approach to decouple the orbital and spin motion and, by using an adiabatic approximation, we compute the equilibrium expectation values of the persistent charge and spin currents and the magnetization. We find that the persistent currents depend on the texture in a striking manner through a geometric phase (related to a surface area characterizing the texture) and a geometric vector (related to the projections of this area). In the special case of a cylindrically symmetric texture we use a spectrum obtained by Kuratsuji and Iida to obtain exact results that confirm, independently, the validity of the path-integral approach in the adiabatic limit. We discuss the connection between the geometric vector and quantum-mechanical correlations, and examine quantum fluctuations and the zero-point energy.

Berry's phase and persistent charge and spin currents in textured mesoscopic rings.

We consider the motion of electrons through a mesoscopic ring in the presence of a classical, static, inhomogeneous, magnetic field. Zeeman interaction between the electron spin and this texture couples spin and orbital motion, and results in a Berry phase. As a consequence, the system supports persistent equilibrium spin and charge currents, even in the absence of conventional electromagnetic flux through the ring. We mention the possibility of analogous persistent mass and spin currents in normal $^{3}\mathrm{He}$.