Aharonov-Casher Oscillations Research Articles

Aharonov-Casher effect and quantum transport in graphene based nano rings: A self-consistent Born approximation

In this study, Rashba coupling induced Aharonov-Casher effect in a graphene based nano ring is investigated theoretically. The graphene based nano ring is considered as a central device connected to semi-infinite graphene nano ribbons. In the presence of the Rashba spin-orbit interaction, two armchair shaped edge nano ribbons are considered as semi-infinite leads. The non-equilibrium Green’s function approach is utilized to obtain the quantum transport characteristics of the system. The relaxation and dephasing mechanisms within the self-consistent Born approximation is scrutinized. The Lopez-Sancho method is also applied to obtain the self-energy of the leads. We unveil that the non-equilibrium current of the system possesses measurable Aharonov-Casher oscillations with respect to the Rashba coupling strength. In addition, we have observed the same oscillations in dilute impurity regimes in which amplitude of the oscillations is shown to be suppressed as a result of the relaxations.

Anomalous Oscillations due to Aharonov–Bohm and Aharonov–Casher Effects of the One-Dimensional Hubbard Ring in the Strong Coupling Limit

We investigate anomalous oscillations due to the Aharonov–Bohm (AB) and Aharonov–Casher (AC) effects of the one-dimensional Hubbard ring with flux in the strong coupling limit. By using the exact diagonalization method and the Shiba transformation, we examine the energies of the ground-state and a few excited states in the presence of the flux producing the AB or AC effect, where the transformation not only reverses the sign of the interaction U but also exchanges the role between the AB and AC effects in the model Hamiltonian. We systematically classify the AB and AC oscillations by using the number of minima Nmin of the ground-state energy as a function of a normalized phase shift ϕ for 0 ≤ ϕ < 1, and clarify the close relationship between the AB and AC effects. For example, it is shown that Nmin is given by NL − Ne (NL − N↑ + N↓) for the AB (AC) effect in the very strong attraction, where NL, Ne, N↑, and N↓ are the system size, the total number of electrons, the number of electrons with up-spin, and th...

Aharonov-Bohm conductance of a disordered single-channel quantum ring

We study the effect of weak disorder on tunneling conductance of a single-channel quantum ring threaded by magnetic flux. We assume that the temperature is higher than the level spacing in the ring and smaller than the Fermi energy. In the absence of disorder, the conductance shows sharp dips (antiresonances) as a function of magnetic flux. We discuss different types of disorder and find that the short-range disorder broadens antiresonances, while the long-range one leads to the appearance of additional resonant dips. We demonstrate that the resonant dips have essentially non-Lorentzian shape. The results are generalized to account for the spin-orbit interaction, which leads to splitting of the disorder-broadened resonant dips, and consequently, to coexisting of two types of oscillations (both having the form of sharp dips): Aharonov-Bohm oscillations with magnetic flux and Aharonov-Casher oscillations with the strength of the spin-orbit coupling. We also discuss the effect of the Zeeman coupling.

Experimental Demonstration of Spin Geometric Phase: Radius Dependence of Time-Reversal Aharonov-Casher Oscillations

A geometric phase of electron spin is studied in arrays of InAlAs/InGaAs two-dimensional electron gas rings. By increasing the radius of the rings, the time-reversal symmetric Aharonov-Casher oscillations of the electrical resistance are shifted towards weaker spin-orbit interaction regions with their shortened period. We conclude that the shift is due to a modulation of the spin geometric phase, the maximum modulation of which is approximately 1.5 rad. We further show that the Aharonov-Casher oscillations in various radius arrays collapse onto a universal curve if the radius and the strength of Rashba spin-orbit interaction are taken into account. The result is interpreted as the observation of the effective spin-dependent flux through a ring.

High-temperature Aharonov-Bohm-Casher interferometer

We study theoretically the combined effect of the spin-orbit and Zeeman interactions on the tunneling electron transport through a single-channel quantum ring threaded by magnetic flux. We focus on the high-temperature case (temperature is much higher than the level spacing in the ring) and demonstrate that spin-interference effects are not suppressed by thermal averaging. In the absence of the Zeeman coupling, the high-temperature tunneling conductance of the ring exhibits two types of oscillations: Aharonov-Bohm oscillations with magnetic flux and Aharonov-Casher oscillations with the strength of the spin-orbit interaction. For weak tunneling coupling, both oscillations have the form of sharp periodic antiresonances. In the vicinity of the antiresonances, the tunneling electrons acquire spin polarization, so that the ring serves as a spin polarizer. We also demonstrate that the Zeeman coupling leads to appearance of two additional peaks, both in the tunneling conductance and in the spin polarization.

Spin interference of holes in silicon nanosandwiches

Spin-dependent transport of holes is studied in silicon nanosandwiches on an n-Si (100) surface which are represented by ultranarrow p-Si quantum wells confined by δ-barriers heavily doped with boron. The measurement data of the longitudinal and Hall voltages as functions of the top gate voltage without an external magnetic field show the presence of edge conduction channels in the silicon nanosandwiches. An increase in the stabilized source-drain current within the range 0.25–5 nA subsequently exhibits the longitudinal conductance value 4e2/h, caused by the contribution of the multiple Andreev reflection, the value 0.7(2e2/h) corresponding to the known quantum conductance staircase feature, and displays Aharonov-Casher oscillations, which are indicative of the spin polarization of holes in the edge channels. In addition, at a low stabilized source-drain current, due to spin polarization, a nonzero Hall voltage is detected which is dependent on the top gate voltage; i. e., the quantum spin Hall effect is observed. The measured longitudinal I–V characteristics demonstrate Fiske steps and a negative differential resistance caused by the generation of electromagnetic radiation as a result of the Josephson effect. The results obtained are explained within a model of topological edge states which are a system of superconducting channels containing quantum point contacts transformable to single Josephson junctions at an increasing stabilized source-drain current.

Suppression of Aharonov-Casher spin interference in an InGaAs ring array

An Aharonov-Casher (AC) spin interference is studied in a gate-fitted InGaAs ring array. The AC interference period is consistent with gate voltage dependence of Rashba spin-orbit interaction parameter in a single subband occupied region. However, the AC interference amplitude and oscillations are suppressed above a particular gate voltage where the second subband is occupied. This suppression is attributed to the decoherence induced by inter-subband scattering.

Quantum spin Hall effect in nanostructures based on cadmium fluoride

Tunneling current-voltage (I-V) characteristics and temperature dependences of static magnetic susceptibility and specific heat of the CdBxF 2- x/pCdF 2-QW/CdBxF 2- x planar sandwich structures formed on the surface of an nCdF 2 crystal have been studied in order to identify superconducting properties of the CdBxF 2- x δ barriers confining the ptype CdF 2 ultranarrow quantum well. Comparative analysis of current-voltage (I-V) characteristics and conductance-voltage dependences (measured at the tempera� tures, respectively, below and above the critical temperature of superconducting transition) indicates that there is an interrelation between quantization of supercurrent and dimensional quantization of holes in the pCdF 2 ultranarrow quantum well. It is noteworthy that detection of the Josephson peak of current in each hole subband is accompanied by the appearance of the spectrum of the multiple Andreev reflection (MAR). A high degree of spin polarization of holes in the edge channels along the perimeter of the pCdF 2 ultranarrow quantum well appears as a result of MAR and makes it possible to identify the quantum spin Hall effect I-V characteristics; this effect becomes pronounced in the case of detection of nonzero conductance at the zero voltage applied to the vertical gate in the Hall geometry of the experiment. Within the energy range of super� conducting gap, the I-V characteristics of the spin transistor and quantum spin Hall effect are controlled by the MAR spectrum appearing as the voltage applied to the vertical gate is varied. Beyond the range of the superconducting gap, the observed I-V characteristic of the quantum spin Hall effect is represented by a quantum conductance staircase with a height of the steps equal to e 2 /h; this height is interrelated with the Aharonov-Casher oscillations of longitudinal and depends on the voltage applied to the vertical gate.

Aharonov-Casher oscillations of spin current through a multichannel mesoscopic ring

The Aharonov-Casher (AC) oscillations of spin current through a two-dimensional ballistic ring in the presence of Rashba spin-orbit interaction and external magnetic field has been calculated using the semiclassical path-integral method. For classically chaotic trajectories the Fokker-Planck equation determining dynamics of the particle spin polarization has been derived. On the basis of this equation an analytic expression for the spin conductance has been obtained taking into account a finite width of the ring arms carrying large number of conducting channels. It was shown that the finite width results in a broadening and damping of spin-current AC oscillations. We found that an external magnetic field leads to appearance of new nondiagonal components of the spin conductance, allowing thus by applying a rather weak magnetic field to change a direction of the transmitted spin-current polarization.

Spin-polarized oscillations of conductance through an aharonov-casher ring with a quantum gate

Spin-polarized oscillations in conductance is studied through a mesoscopic Aharonov-Casher (AC) ring with a quantum gate that is tuned by an external magnetic field. Both the conductance and its spin polarization at zero temperature are calculated as a function of the textured electric field, the magnetic field, and Fermi energy. It is found that for some special Fermi energies, spin-up electrons are driven into perfect transmission or reflection states, unaffected by the electric field when Zeeman energy of the incident electrons aligns with one level of the isolated stub or is larger than Fermi energy. This brings about AC oscillations of spin-down conductance . It shows that periodic oscillations of the conductance appear in the adiabatic region of quantum phase and in the nonadiabatic region. Anomalous behavior of the conductance oscillation is dependent on the difference between the tilt angle of spin and the electric field.

Transmission modulated by quantum gate in aharonov-casher ring

We study the electron transmission tuned by quantum gate in an Aharonov-Casher (AC) ring. Transmission probability is obtained as a function of the normalized textured electric fields and Fermi energy. We find that modulating electron wavefunction in the stub can drastically affects electron transmission through the ring system. As Fermi energy crosses every eigenenergy of the isolated stub, the phases of both anomalous and periodic oscillations of the AC conductance generally have abrupt change by π. On two sides of several special Fermi energies, both anomalous and periodic oscillations have no abrupt phase change and are in phase. The detailed characteristics of the anomalous oscillations are dependent on the difference between the tilt angle of the spin and that of textured electric field. By modulating external magnetic field applied to the stub, we present AC oscillations of spin-polarized conductance within adiabatic limit.

MICROSCOPIC FRACTIONAL PERSISTENT CURRENT OSCILLATIONS IN A METALLIC RING WITH THE ANDERSON IMPURITY

Microscopic spin and charge persistent currents in a metallic ring with the Anderson magnetic impurity are studied. It is shown that the impurity changes caused by the external magnetic and electric fluxes, the Aharonov-Bohm and Aharonov-Casher oscillations (magnitudes and periods), contrast with the normal metallic case. Spin-charge separation is the reason for the fractional Φ0/N (N is the number of electrons) period of the Aharonov-Bohm oscillations and F0/(N−2M) (M is the number of paired electrons) period of the Aharonov-Casher oscillations.