Persistent currents in superconducting quantum interference devices

Starting from the reduced dynamical model of a two-junction quantum interference device, it shown that a quantum analog of the system can be exhibited. This quantum model extends the well-known properties of the device when its characteristic dimensions are of the order of mesoscopic length scales. By finding eigenvalues of the corresponding Hamiltonian operator, the persistent currents flowing in the ring have been obtained. The resulting quantum analog of the overdamped two-junction quantum interference device can be seen as a supercurrent qubit operating in the limit of negligible capacitance and finite inductance.     

Microwave-enhanced critical currents in superconducting microbridges explained by the electric-field induced quasiparticle-pair inequilibrium

A supercurrent through a superconducting microbridge gives rise to a minimum in the density of Cooper pairs in the middle of the bridge. Application of a microwave electric field creates a non-equilibrium situation and therefore smears the Cooper pair distribution. This allows for a higher critical current and explains the microwave-enhanced supercurrent observed in thin-film microbridges.     

Persistent current in superconducting nanorings

The superconductivity in very thin rings is suppressed by quantum phase slips. As a result, the amplitude of the persistent current oscillations with flux becomes exponentially small, and their shape changes from sawtooth to a sinusoidal one. We reduce the problem of low-energy properties of a superconducting nanoring to that of a quantum particle in a sinusoidal potential and show that the dependence of the current on the flux belongs to a one-parameter family of functions obtained by solving the respective Schr?dinger equation with twisted boundary conditions.     

Persistent currents in mesoscopic rings

The discrepancy between measured values of the persistent current in mesoscopic rings and theoretical calculations based upon the model of independent electrons moving in a random potential is discussed. Some attempts at including the Coulomb interaction between electrons are reviewed, and results of model calculations are presented.     

Limiting currents in fermionic superconducting strings

It is usually assumed that the critical current in a fermionic superconducting string is of order em, where m is the in vacuo mass of the charge carrier. It is argued here that in reality the critical current is of order em²R, where R is the string's radius of curvature. This has some important consequences: in particular, scattering amongst the charge carriers can be shown to limit the current to less than roughly 10⁹ GeV.     

Aharanov-Bohm currents in thin superconducting cylinders

The Aharanov-Bohm effect is the influence of classically inaccessible electromagnetic fields on quantum wave functions. In this paper we consider the Ginsburg-Landau (GL) equations for the stationary states of a thin, superconducting cylinder in the presence of a curl-free, static electromagnetic potential corresponding to zero fields. We solve the GL equations explicitly to obtain self-consistent solutions for the current density, the induced field and the free energy in a well-defined and accessible approximation. The analysis makes quantitative predictions which can, in principle, be experimentally tested to provide a clear and convincing demonstration of the Aharanov-Bohm effect.     

Persistent currents in mesoscopic connected rings

We report measurements of the low temperature magnetic response of a line of 16 GaAs/GaAlAs connected mesoscopic rings whose total length is much larger than l(straight phi). Using an on-chip micro-SQUID technology, we have measured a periodic response, with period h/e, corresponding to persistent currents in the rings of a typical amplitude of 0.40+/-0.08 nA per ring. Direct comparison with measurements on the same rings but isolated is presented.     

Persistent currents in three-dimensional shell-doped nanorings

The persistent current in three-dimensional (P× N²) nanorings as a function of the unit cell number (P), the channel number (M = N²), surface disorder (ξ ), and temperature (T) is theoretically investigated in terms of rotational symmetry. On the whole, the typical current increases linearly with \sqrt M but decreases exponentially with P, while wide fluctuations exist therein. In the presence of surface disorder, the persistent current decreases with ξ in the regime of weak disorder but increases in the regime of strong disorder. In addition, it is found that the persistent current in perfect rings decreases exponentially with temperature even at T < T^*     

Persistent currents in interacting lattice models

We present the results of an exact calculation of the averaged persistent current and its root mean square value in interacting disordered 1D rings. While the averaged persistent current exhibits a variety of interesting behaviors depending on the disorder and the strength of electron-electron interactions, the r.m.s. value depends very weakly on the strength of interaction. In general we observe that no dramatical increase of the averaged current or its r.m.s. value as a result of interactions is possible for these systems.     

Persistent photoconductivity in ZnO nanostructures induced by surface oxygen vacancy

ZnO nanowall networks grown on SiO₂/Si substrate were found to exhibit persistent photoconductivity (PPC). The relaxation rate of the persistent photocurrent is enhanced by a higher oxygen level in the ambient suggesting that PPC is closely related to the ZnO surface. Surface modification with hydrogen peroxide can significantly reduce the PPC relaxation time, implying that surface oxygen deficiency is responsible for the effect. The transition between the neutral and the metastable singly ionized states of the surface oxygen vacancy is suggested to account for the phenomenon and it is supported by the temperature and wavelength dependence of the PPC. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Persistent currents in one-dimensional aperiodic mesoscopic rings

In the tight-binding approximation, we have investigated the behaviour of persistent currents in a one-dimensional Thue-Morse mesoscopic ring threaded by a magnetic flux. By applying a transfer-matrix technique, the energy spectra and the persistent currents in the system have been numerically calculated. It is shown that the flux-dependent eigenenergies form “band” structures and the energy gaps will enlarge if the site energy increases. Actually, the site energy and the filling-up number of electrons are two important factors which have much influence upon the persistent current. Increment of the site energy in the system will lead to a dramatic suppression of the currents. When the highest-occupied energy level is on the top of the band, the total current is limited; otherwise, the persistent current increases by several orders of magnitude. Generally, this kind of large scale change in the magnitude of the current can easily be observed in the vicinity of band gaps. The parity effect in the Thue-Morse ring is also discussed. Received 22 January 2001 and Received in final form 25 October 2001     

Pressure induced effects on the Fermi surface of superconducting 2H-NbSe2

The pressure dependence of the critical temperature T(c) and upper critical field H(c2)(T) has been measured up to 19 GPa in the layered superconducting material 2H-NbSe2. T(c)(P) has a maximum at 10.5 GPa, well above the pressure for the suppression of the charge density wave (CDW) order. Using an effective two-band model to fit H(c2)(T), we obtain the pressure dependence of the anisotropy in the electron-phonon coupling and Fermi velocities, which reveals the peculiar interplay between CDW order, Fermi surface complexity, and superconductivity in this system.     

Persistent spin currents in mesoscopic Heisenberg rings

We show that at low temperatures T an inhomogeneous radial magnetic field with magnitude B gives rise to a persistent magnetization current around a mesoscopic ferromagnetic Heisenberg ring. Under optimal conditions, this spin current can be as large as gmicro(B)(T/ variant Planck's over 2pi )exp([-2pi(gmicro(B)B/delta)(1/2)], as obtained from leading-order spin-wave theory. Here g is the gyromagnetic factor, micro(B) is the Bohr magneton, and delta is the energy gap between the ground-state and the first spin-wave excitation. The magnetization current endows the ring with an electric dipole moment.     

Persistent currents in two dimensions: New regimes induced by the interplay between electronic correlations and disorder

Using the persistent current I induced by an Aharonov-Bohm flux in square lattices with random potentials, we study the interplay between electronic correlations and disorder upon the ground state (GS) of a few polarized electrons (spinless fermions) with Coulomb repulsion. being the total momentum, we show that in the continuum limit. We use this relation to distinguish between the continuum regimes, where the lattice GS behaves as in the continuum limit and I is independent of the interaction strength U when is conserved, and the lattice regimes where I decays as U increases. Changing the disorder strength W and U, we obtain many regimes which we study using the map of local currents carried by three spinless fermions. The decays of I characterizing three different lattice regimes are described by large U perturbative expansions. In one of them, I forms a stripe of current flowing along the axis of the diamagnetic Wigner molecule induced by large electronic correlations. This stripe of current persists in the continuum limit. The quantum melting of the diamagnetic molecule gives rise to an intermediate “supersolid” regime where a paramagnetic correlated pair co-exists with a third particle, before the total melting. The concepts of stripe and of supersolid which we use to describe certain regimes exhibited by three spinless fermions are reminiscent of the observations and conjectures done in other fields at the thermodynamic limit (stripe for high-Tc cuprates, supersolid for Helium quantum solids).