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M. Leadbeater

Publications and source records attributed to M. Leadbeater.

16 recordsLinked to original sources
arXiv2004

Josephson tunnelling of a phase-imprinted Bose-Einstein condensate in a time-dependent double-well potential

This paper discusses the feasibility of experimental control of the flow direction of atomic Bose-Einstein condensates in a double-well potential using phase-imprinting. The flow is induced by the application of a time-dependent potential gradient, providing a clear signature of macroscopic quantum tunneling in atomic condensates. By studying both initial state preparation and subsequent tunneling dynamics we find the parameters to optimise the phase induced Josephson current. We find that the effect is largest for condensates up to a few thousand atoms, and is only weakly-dependent on trap geometry.

cond-mat.other↗
arXiv2003

Josephson Current Counterflow in Atomic Bose-Einstein Condensates

Josephson weak links in superconductors can be engineered such that the phase difference across the junction is $π$ ($π$-junction),leading to the reversal of the current. The conditions under which the analogous effect of supercurrent counterflow can be achieved in a double-well atomic Bose-Einstein condensate are investigated. It is shown that this effect is observable for condensates up to a few thousand atoms, which are initially prepared in an anti-symmetric '$π$-state' and subsequently subjected to a uniformly increasing magnetic field gradient. This effect is found to be only weakly-dependent on trap geometry, and can be observed in both attractive and repulsive condensates.

cond-mat↗
arXiv2003

Deformation of dark solitons in inhomogeneous Bose-Einstein condensates

A dark soliton becomes unstable when it is incident on a background density gradient, and the induced instability results in the emission of sound. Detailed quantitative studies of sound emission are performed for various potentials, such as steps, linear ramps and gaussian traps. The amount of sound emission is found to be a significant fraction of the soliton energy for typical potentials. Continuous emission of sound is found to lead to an apparent deformation of the soliton profile. The power emitted by the soliton is shown to be parametrised by the square of the displacement of the centre of mass of the soliton from its density minimum, thus highlighting the significance of the inhomogeneity-induced soliton deformation.

cond-mat↗
arXiv2003

Soliton-sound interactions in quasi-one-dimensional Bose-Einstein condensates

Longitudinal confinement of dark solitons in quasi-one-dimensional Bose-Einstein condensates leads to sound emission and reabsorption. We perform quantitative studies of the dynamics of a soliton oscillating in a tight dimple trap, embedded in a weaker harmonic trap. The dimple depth provides a sensitive handle to control the soliton-sound interaction. In the limit of no reabsorption, the power radiated is found to be proportional to the soliton acceleration squared. An experiment is proposed to detect sound emission as a change in amplitude and frequency of soliton oscillations.

cond-mat↗
arXiv2002

Creation of solitons and vortices by Bragg reflection of Bose-Einstein condensates in an optical lattice

We study the dynamics of Bose-Einstein condensates in an optical lattice and harmonic trap. The condensates are set in motion by displacing the trap and initially follow simple semiclassical paths, shaped by the lowest energy band. Above a critical displacement, the condensate undergoes Bragg reflection. For high atom densities, the first Bragg reflection generates a train of solitons and vortices, which destabilize the condensate and trigger explosive expansion. At lower densities, soliton and vortex formation requires multiple Bragg reflections, and damps the center-of-mass motion.

cond-mat↗
arXiv2002

Decay of superfluid turbulence via Kelvin wave radiation and vortex reconnections

The elementary processes involved in the decay of superfluid turbulence in the limit of low temperature are studied by numerical simulations of vortex ring collisions. We find that small vortex rings produced by reconnections eventually annihilate in a collision where all their energy is converted into Kelvin waves and sound. We show that sound emission due to Kelvin waves also leads to a loss of vortex line length but this dissipation mechanism alone is too small to account for the experimentally observed decay.

cond-mat↗
arXiv2002

Josephson spectroscopy of a dilute Bose-Einstein condensate in a double-well potential

The dynamics of a Bose-Einstein condensate in a double-well potential are analysed in terms of transitions between energy eigenstates. By solving the time-dependent and time-independent Gross-Pitaevskii equation in one dimension, we identify tunnelling resonances associated with level crossings, and determine the critical velocity that characterises the resonance. We test the validity of a non-linear two-state model, and show that for the experimentally interesting case, where the critical velocity is large, the influence of higher-lying states is important.

cond-mat↗
arXiv2000

Sound emission due to superfluid vortex reconnections

By performing numerical simulations of superfluid vortex ring collisions we make direct quantitative measurements of the sound energy released due to vortex reconnections. We show that the energy radiated expressed in terms of the loss of vortex line length is a simple function of the reconnection angle. In addition, we study the temporal and spatial distribution of the radiation and show that energy is emitted in the form of a rarefaction pulse. The pulse evolves into a sound wave with a wavelength of 6-8 healing lengths.

cond-mat.soft↗
arXiv1999

Semiclassical Theory of Integrable and Rough Andreev Billiards

We study the effect on the density of states in mesoscopic ballistic billiards to which a superconducting lead is attached. The expression for the density of states is derived in the semiclassical S-matrix formalism shedding insight into the origin of the differences between the semiclassical theory and the corresponding result derived from random matrix models. Applications to a square billiard geometry and billiards with boundary roughness are discussed. The saturation of the quasiparticle excitation spectrum is related to the classical dynamics of the billiard. The influence of weak magnetic fields on the proximity effect in rough Andreev billiards is discussed and an analytical formula is derived. The semiclassical theory provides an interpretation for the suppression of the proximity effect in the presence of magnetic fields as a coherence effect of time reversed trajectories, similar to the weak localisation correction of the magneto-resistance in chaotic mesoscopic systems. The semiclassical theory is shown to be in good agreement with quantum mechanical calculations.

cond-mat.supr-con↗
arXiv1999

Numerical results for two interacting particles in a random environment

Much evidence has been collected to date which shows that repulsive electron-electron interaction can lead to the formation of particle pairs in a one-dimensional random energy landscape. The localization length λ_2 of these pair states is finite, but larger than the localization length λ_1 of the individual particles. After a short review of previous work, we present numerical evidence for this effect based on an analysis of the interaction matrix elements and an application of the decimation method. The results based on the decimation method for a two-dimensional disordered medium support the localization-delocalization transition of pair states predicted recently.

cond-mat.dis-nn↗
arXiv1999

Non-linear conductivity and quantum interference in disordered metals

We report on a novel non-linear electric field effect in the conductivity of disordered conductors. We find that an electric field gives rise to dephasing in the particle-hole channel, which depresses the interference effects due to disorder and interaction and leads to a non-linear conductivity. This non-linear effect introduces a field dependent temperature scale $T_E$ and provides a microscopic mechanism for electric field scaling at the metal-insulator transition. We also study the magnetic field dependence of the non-linear conductivity and suggest possible ways to experimentally verify our predictions. These effects offer a new probe to test the role of quantum interference at the metal-insulator transition in disordered conductors.

cond-mat.dis-nn↗
arXiv1999

Lévy flights in quantum transport in quasi-ballistic wires

Conductance fluctuations, localization and statistics of Lyapunov exponents are studied numerically in pure metallic wires with rough boundaries (quasi-ballistic wires). We find that the correlation energy of conductance fluctuations scales anomalously with the sample dimensions, indicating the role of Lévy flights. Application of a magnetic field deflects the Lévy flights which reduces the localization length. This deflection also breaks the geometrical flux cancellation and restores the usual Aharonov-Bohm type magneto-conductance fluctuations.

cond-mat.mes-hall↗
arXiv1998

Sub-gap conductance in ferromagnetic-superconducting mesoscopic structures

We study the sub-gap conductance of a ferromagnetic mesoscopic region attached to a ferromagnetic and a superconducting electrode by means of tunnel junctions. In the absence of the exchange field, the ratio $r= γ/ ε_T$ of the two tunnel junction resistances determines the behaviour of the sub-gap conductance which possesses a zero-bias peak for $r\gg 1$ and for $r\ll 1$ a peak at finite voltage. We show that the inclusion of the exchange field leads to a peak splitting for $r\ll 1$, while it shifts the zero-bias anomaly to finite voltages for $r\gg 1$.

cond-mat.mes-hall↗
arXiv1997

Large-scale superconductivity-induced conductance suppression in mesoscopic normal-superconducting structures

Experiments on hybrid superconducting normal-metal structures have revealed that even in the absence of tunnel junctions the onset of superconductivity can lead to a decrease in the electrical conductance by an amount many orders of magnitude greater than $e^2/h$. In this Letter we provide a theory of this phenomenon which shows that it originates from an instability in 4 - probe conductance measurements which is absent from 2-probe measurements. We compare the zero-bias,zero-temperature 4-probe conductances $G_{N}$ and $G_{S}$ of a normal diffusive metal in contact with a superconductor in both the normal (N) and superconducting (S) states respectively. In the absence of tunnel barriers, the ensemble average of the difference $δG=G_{S}-G_N$ vanishes, in agreement with quasi-classical theory. However we also predict that there exists macroscopic sample specific fluctuations in $δG$, which lie beyond quasi-classical theory and allow large negative values of $δG$ to occur.

cond-mat.supr-con↗
arXiv1995

Resonant Andreev Scattering In Phase-Coherent, Superconducting Nanostructures

Analytic predictions for resonant transport in three generic structures are presented. For a structure comprising a normal (N) contact - normal dot (NDOT) - superconducting (S) contact, we predict that finite voltage, differential conductance resonances are destroyed by the switching on of superconductivity in the S-contact. In the weak coupling limit, the surviving resonances have a double-peaked line-shape. Secondly, we demonstrate that resonant Andreev interferometers can provide galvonometric magnetic flux detectors, with a sensitivity in excess of the flux quantum. Finally, for a superconducting dot (SDOT) connected to normal contacts (N), we show that the onset of superconductivity can increase the sub-gap conductance, in contrast with the usual behaviour of a tunnel junction.

cond-mat↗