P. Singha Deo

Vortex Phase Diagram for Mesoscopic Superconducting Disks

Solving numerically the 3D nonlinear Ginzburg-Landau (GL) equations, we study equilibrium and nonequilibrium phase transitions between different superconducting states of mesoscopic disks which are thinner than the coherence length and the penetration depth. We have found a smooth transition from a multivortex superconducting state to a giant vortex state by increasing both the disk thickness and the magnetic field. A vortex phase diagram is obtained which shows, as a function of the magnetic field, a reentrant behavior between the multivortex and the giant vortex state.

Quantum rings for beginners: energy spectra and persistent currents

Abstract Theoretical approaches to one-dimensional and quasi-one-dimensional quantum rings with a few electrons are reviewed. Discrete Hubbard-type models and continuum models are shown to give similar results governed by the special features of the one-dimensionality. The energy spectrum of the many-body states can be described by a rotation–vibration spectrum of a “Wigner molecule” of “localized” electrons, combined with the spin-state determined from an effective antiferromagnetic Heisenberg Hamiltonian. The persistent current as a function of the magnetic flux through the ring shows periodic oscillations arising from the “rigid rotation” of the electron ring. For polarized electrons the periodicity of the oscillations is always the flux quantum Φ 0 . For nonpolarized electrons the periodicity depends on the strength of the effective Heisenberg coupling and changes from Φ 0 first to Φ 0 /2 and eventually to Φ 0 / N when the ring gets narrower.

Persistent currents in the presence of a transport current.

We have considered a system of a metallic ring coupled to two electron reservoirs. We show that in the presence of a transport current, the persistent current can flow in a ring, even in the absence of a magnetic field. This is purely a quantum effect and is related to the current magnification in the loop. These persistent currents can be observed if one tunes the Fermi energy near the antiresonances of the total transmission coefficient or the two-port conductance.

EFFECT OF IMPURITIES ON THE CURRENT MAGNIFICATION IN MESOSCOPIC OPEN RINGS

We have considered an open system consisting of a metallic ring coupled to two electron reservoirs. We have recently shown that in the presence of a transport current, circulating currents can flow in such a ring even in the absence of magnetic field. This is related to the current magnification effect in the ring. In our present work we have studied the effect of impurity on the current magnification. We find that the presence of impurity can enhance the current magnification in the loop significantly and thus lead to large circulating currents in a certain range of Fermi energies. This is in contrast to the known fact that impurities can only decrease the persistent currents in closed ring in the presence of magnetic flux.

Hysteresis in mesoscopic superconducting disks: The Bean-Livingston barrier

Depending on the size of mesoscopic superconducting disks, the magnetization can show hysteretic behavior which we explain by using the Ginzburg-Landau (GL) theory and properly taking into account the demagnetization effects due to geometrical form factors. In large disks the hysteresis is due to the Bean-Livingston surface barrier while in small disks it is the volume barrier which is responsible for it. Although the sample magnetization is diamagnetic (negative) we show that the measured magnetization can be positive at certain fields as observed experimentally and which is a consequence of both the demagnetization effect and the experimental setup.

Current-spin-density-functional study of persistent currents in quantum rings

We present a numerical study of persistent currents in quantum rings using current spin density functional theory (CSDFT). This formalism allows for a systematic study of the joint effects of both spin, interactions and impurities for realistic systems. It is illustrated that CSDFT is suitable for describing the physical effects related to Aharonov-Bohm phases by comparing energy spectra of impurity-free rings to existing exact diagonalization and experimental results. Further, we examine the effects of a symmetry-breaking impurity potential on the density and current characteristics of the system and propose that narrowing the confining potential at fixed impurity potential will suppress the persistent current in a characteristic way. PACS numbers: 73.20.Dx, 73.23.Ra, 71.15.Mb

Phase of Aharonov-Bohm oscillations: Effect of channel mixing and fano resonances

We show that in a ring with a gated arm, for the Aharonov-Bohm oscillations, there is a certain energy scale, which coincides with the interchannel separation, in which the phase of wave-function in the gated arm can abruptly change by π. For a ring with a quantum dot in one arm, the same mechanism can produce a phase slip of π between two consecutive resonances of the dot.

Friedel sum rule for a single-channel quantum wire

Elastic scattering in a quantum wire has several novel features not seen in one dimension (1D), 2D, or 3D. In this work we consider a single channel quantum wire as its application is inevitable in making devices based on quantum interference effects. We consider a point defect or a single delta function impurity in such a wire and show how some of these novel features affect Friedel-sum-rule (FSR) in a way, that is quite unlike in 1D, 2D, and 3D.

Quantum current magnification in a multichannel mesoscopic ring

We have studied the current magnification effect in a multi-channel open mesoscopic ring. We show that the current magnification effect is robust even in the presence of several propagating modes inspite of mode mixing and cancellation effects. The magnitude of circulating currents in the multichannel regime can be much larger than that in a single channel case. Impurities can enhance or degrade the current magnification effect depending sensitively on the system parameters. Circulating currents are mostly associated with Fano resonances in the total transport current. We further show that system-lead coupling qualitatively changes the current magnification effect.

NOVEL INTERFERENCE EFFECTS IN MULTIPLY CONNECTED NORMAL METAL RINGS

We have investigated the magnetoconductance of a normal metal loop connected to ideal wires in the presence of magnetic flux. The quantum mechanical potential, V, in the loop is much higher than that in the connecting wires (V=0). The electrons with energies less than the potential height on entering the loop propagate as evanescent modes. In such a situation, the contribution to the conductance arises from two non-classical effects, namely, Aharonov-Bohm effect and quantum tunneling. For this case we show that, on application of a small magnetic field, the conductance initially always decreases, or small field magnetoconductance is always negative. This is in contrast to the behavior in the absence of the barrier, wherein the small field magnetoconductance is either positive or negative depending on the Fermi energy and other geometric details. We also discuss the possibility of a better switch action based on quantum interference effects in such structures.