Searcharxiv⌕ Search

arXiv subjects

C. S. Adams

Publications and source records attributed to C. S. Adams.

85 records · Page 5Linked to original sources
arXiv2001

A Fast Semi-Implicit Finite Difference Method for the TDGL Equations

We propose a finite-difference algorithm for solving the time-dependent Ginzburg-Landau (TDGL) equation coupled to the appropriate Maxwell equation. The time derivatives are discretized using a second order semi-implicit scheme which, for intermediate values of the Ginzburg-Landau parameter $κ$, allows time-steps two orders of magnitude larger than commonly used in explicit schemes. We demonstrate the use of the method by solving a fully three-dimensional problem of a current-carrying wire with longitudinal and transverse magnetic fields.

cond-mat.supr-con↗
arXiv2001

Vortex dynamics, pinning, and critical currents in a Ginzburg-Landau type-II superconductor

The dynamics of vortices in a type-II superconductor with defects are studied by solving the time-dependent Ginzburg-Landau equations in two and three dimensions. We show that vortex flux tubes are trapped by volume defects up to a critical current density where they begin to jump between pinning sites along static flow channels. We study the dependence of the critical current on the pinning distribution and find for random distributions a maximum critical current equal to a few percent of the depairing current at a pinning density three times larger than the vortex line density. Whereas for a regular triangular pinning array, the critical current is significantly larger when the pinning density matches the vortex line density.

cond-mat.supr-con↗
arXiv2000

Damping and revivals of collective oscillations in a finite-temperature model of trapped Bose-Einstein condensation

We utilize a two-gas model to simulate collective oscillations of a Bose-Einstein condensate at finite temperatures. The condensate is described using a generalized Gross-Pitaevskii equation, which is coupled to a thermal cloud modelled by a Monte Carlo algorithm. This allows us to include the collective dynamics of both the condensed and non-condensed components self-consistently. We simulate quadrupolar excitations, and measure the damping rate and frequency as a function of temperature. We also observe revivals in condensate oscillations at high temperatures, and in the thermal cloud at low temperature. Extensions of the model to include non-equilibrium effects and describe more complex phenomena are discussed.

cond-mat.stat-mech↗
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↗
arXiv2000

Motion of an object through a dilute quantum fluid

We simulate the motion of a massive object through a dilute Bose-Einstein condensate by numerical solution of the non-linear Schrödinger equation coupled to an equation of motion for the object. Under a constant applied force, the object accelerates up to a maximum velocity where a vortex ring is formed which slows the object down. If the applied force is less than a critical value, the object becomes trapped within the vortex core. We show that the motion can be described using the time-independent quantum states, and use these states to predict the conditions required for vortex scattering.

cond-mat↗
arXiv2000

Vortex shedding and drag in dilute Bose-Einstein condensates

Above a critical velocity, the dominant mechanism of energy transfer between a moving object and a dilute Bose-Einstein condensate is vortex formation. In this paper, we discuss the critical velocity for vortex formation and the link between vortex shedding and drag in both homogeneous and inhomogeneous condensates. We find that at supersonic velocities sound radiation also contributes significantly to the drag force.

cond-mat↗
arXiv2000

Dissipation and Vortex Creation in Bose-Einstein Condensed Gases

We solve the Gross-Pitaevskii equation to study energy transfer from an oscillating `object' to a trapped Bose-Einstein condensate. Two regimes are found: for object velocities below a critical value, energy is transferred by excitation of phonons at the motion extrema; while above the critical velocity, energy transfer is via vortex formation. The second regime corresponds to significantly enhanced heating, in agreement with a recent experiment.

cond-mat.stat-mech↗
arXiv1999

Vortex line and ring dynamics in trapped Bose-Einstein condensates

Vortex dynamics in inhomogeneous Bose-Einstein condensates are studied numerically in two and three dimensions. We simulate the precession of a single vortex around the center of a trapped condensate, and use the Magnus force to estimate the precession frequency. Vortex ring dynamics in a spherical trap are also simulated, and we discover that a ring undergoes oscillatory motion around a circle of maximum energy. The position of this locus is calculated as a function of the number of condensed atoms. In the presence of dissipation, the amplitude of the oscillation will increase, eventually resulting in self-annihilation of the ring.

cond-mat.stat-mech↗
arXiv1999

Vortex structures in dilute quantum fluids

Vortex structures in dilute quantum fluids are studied using the Gross-Pitaevskii equation. The velocity and momentum of multiply quantized vortex rings are determined and their core structures analysed. For flow past a spherical object, we study the encircling and pinned ring solutions, and determine their excitation energies as a function of the flow velocity for both penetrable and impenetrable objects. The ring and laminar flow solutions converge at a critical velocity, which decreases with increasing object size. We also study the vortex solutions associated with flow past a surface bump which indicate that surface roughness also reduces the critical velocity. This effect may have important implications for the threshold of dissipation in superfluids and superconductors.

cond-mat↗
arXiv1998

Pressure drag in linear and nonlinear quantum fluids

We study the flow of a weakly-interacting Bose-Einstein condensate around an obstacle by numerical solution of the Gross-Pitaevskii equation. We observe vortex emission and the formation of bow waves leading to pressure drag. We compare the drag law with that of an ideal Bose gas, and show that interactions reduce the drag force. This reduction can be explained in terms of a `collisional screening' of the obstacle.

cond-mat.stat-mech↗
arXiv1998

Output coupling and flow of a dilute Bose-Einstein condensate

We solve the Gross-Pitaevskii equation to model the behaviour of a weakly-interacting Bose condensate. Solutions are presented for the eigenstates in one- and two-dimensional harmonic traps. We include the effect of gravity and coupling to a second condensate to simulate an output coupler for Bose condensed atoms, and find that the output pulse shape is in qualitative agreement with experiment. We also model flow under gravity through a constriction, demonstrating excitation and separation of eigenmodes in the condensate.

cond-mat.stat-mech↗
arXiv1997

Vortex formation in weakly-interacting inhomogeneous Bose condensates

We solve the time-dependent Gross-Pitaevskii in 2-D to simulate the flow of an object through a dilute Bose condensate trapped in a harmonic well. We demonstrate vortex formation and analyze the process in terms of the accumulation of phase-slip and the evolution of the fluid velocity.

cond-mat.stat-mech↗