High-pressure core structures of Si nanoparticles for solar energy conversion.

We present density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si nanoparticles (NPs) with core structures based on high-pressure bulk Si phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Several mechanisms are discussed to further reduce the gap, including surface reconstruction and chemistry, excitonic effects, and embedding pressure. Experiments reported the formation of BC8 NPs embedded in amorphous Si and in amorphous regions of femtosecond-laser doped "black silicon." For all these reasons, BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion.

Dislocation glasses: aging during relaxation and coarsening.

The dynamics of dislocations is reported to exhibit a range of glassy properties. We study numerically various versions of 2D edge dislocation systems, in the absence of externally applied stress. Two types of glassy behavior are identified (i) dislocations gliding along randomly placed, but fixed, axes exhibit relaxation to their spatially disordered stable state; (ii) if both climb and annihilation are allowed, irregular cellular structures can form on a growing length scale before all dislocations annihilate. In all cases both the correlation function and the diffusion coefficient are found to exhibit aging. Relaxation in case (i) is a slow power law, furthermore, in the transient process (ii) the dynamical exponent z approximately 6, i.e., the cellular structure coarsens relatively slowly.

Reversal-field memory in the hysteresis of spin glasses.

We report a novel singularity in the hysteresis of spin glasses, the reversal-field memory effect, which creates a nonanalyticity in the magnetization curves at a particular point related to the history of the sample. The origin of the effect is due to the existence of a macroscopic number of "symmetric clusters" of spins associated with a local spin-reversal symmetry of the Hamiltonian. We use first order reversal curve (FORC) diagrams to characterize the effect and compare to experimental results on thin magnetic films. We contrast our results on spin glasses to random magnets and show that the FORC technique is an effective "magnetic fingerprinting" tool.

Using hysteresis for optimization.

We propose a new optimization method based on a demagnetization procedure well known in magnetism. We show how this procedure can be applied as a general tool to search for optimal solutions in any system where the configuration space is endowed with a suitable "distance." We test the new algorithm on frustrated magnetic models and the traveling salesman problem. We find that the new method successfully competes with similar basic algorithms such as simulated annealing.

Metastability and uniqueness of vortex states at depinning.

We present results from numerical simulations of the transport of vortices in the zero-field-cooled (ZFC) and the field-cooled (FC) state of a type-II superconductor. In the absence of an applied current I, we find that the FC state has a lower defect density than the ZFC state, and is stable against thermal cycling. On the other hand, by cycling I, surprisingly, we find that the ZFC state is the stable state. The FC state is metastable as manifested by increasing I to the depinning current I(c), in which case the FC state evolves into the ZFC state. We also find that all configurations acquire a unique defect density at the depinning transition independent of the history of the initial states.

Superconducting single-charge transistor in a tunable dissipative environment.

We study a superconducting single-charge transistor, where the coherence of Cooper pair tunneling is destroyed by the coupling to a tunable dissipative environment. Sequential tunneling and cotunneling processes are analyzed to construct the shape of the conductance peaks and their dependence on the dissipation and temperature. Unexpected features are found due to a crossover between two distinct regimes, one "environment assisted" the other "environment dominated." Several of the predictions have been confirmed by recent experiments. The model and results apply also to the dynamics of Josephson junction quantum bits on a conducting ground plane, thus explaining the influence of dissipation on the coherence.

Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors.

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.

Vortices Freeze like Window Glass: The Vortex Molasses Scenario.

We overview several recent experimental and numerical observations, which are at odds with the vortex glass theory of the freezing of disordered vortex matter. To reinvestigate the issue, we performed numerical simulations of the overdamped London-Langevin model, and use finite size scaling to analyze the data. Upon approaching the transition the initial vortex-glass-type criticality is arrested at some crossover temperature. Below this temperature the time scales continue growing very quickly, consistent with the Vogel-Fulcher form, while the spatial correlation length ξ stops exhibiting any observable divergence. We call this mode of freezing the vortex molasses scenario.