Holographic data storage in a DX-center material

We report on the optical storage of digital data in a semiconductor sample containing DX centers. The diffraction efficiency and the bit-error-rate performance of multiplexed data images are shown to agree well with a simple model of the material. Uniform storage without an exposure schedule is demonstrated. The volume sensitivity is found to be ~10^3 times that of LiNBO3:Fe. The importance of coherent addition of scattered light with diffracted light in holographic data storage is discussed

Mechanical coupling in flashing ratchets

We consider the transport of rigid objects with internal structure in a flashing ratchet potential by investigating the overdamped behavior of a rod-like chain of evenly spaced point particles. In 1D, analytical arguments show that the velocity can reverse direction multiple times in response to changing the size of the chain or the temperature of the heat bath. The physical reason is that the effective potential experienced by the mechanically coupled objects can have a different symmetry than that of individual objects. All analytical predictions are confirmed by Brownian dynamics simulations. These results may provide a route to simple, coarse-grained models of molecular motor transport that incorporate an object's size and rotational degrees of freedom into the mechanism of transport.Comment: 9 pages, 10 figure

Diffusion Enhancement in a Periodic Potential under High-Frequency Space-Dependent Forcing

We study the long-time behavior of underdamped Brownian particle moving through a viscous medium and in a systematic potential, when it is subjected to a space-dependent high-frequency periodic force. When the frequency is very large, much larger than all other relevant system-frequencies, there is a Kapitsa time-window wherein the effect of frequency dependent forcing can be replaced by a static effective potential. Our new analysis includes the case when the forcing, in addition to being frequency-dependent, is space-dependent as well. The results of the Kapitsa analysis then lead to additional contributions to the effective potential. These are applied to the numerical calculation of the diffusion coefficient (D) for a Brownian particle moving in a periodic potential. Presented are numerical results, which are in excellent agreement with theoretical predictions and which indicate a significant enhancement of D due to the space-dependent forcing terms. In addition we study the transport property (current) of underdamped Brownian particles in a ratchet potential.Comment: RevTex 6 pages, 5 figure

Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts

The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first subband, six times larger than in GaAs. We report subband spacings in excess of 10 meV, which facilitates quantum transport at higher temperatures.Comment: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in Applied Physics Letter

Symmetry of two terminal, non-linear electric conduction

The well-established symmetry relations for linear transport phenomena can not, in general, be applied in the non-linear regime. Here we propose a set of symmetry relations with respect to bias voltage and magnetic field for the non-linear conductance of two-terminal electric conductors. We experimentally confirm these relations using phase-coherent, semiconductor quantum dots.Comment: 4 pages, 4 figure

Thermal ratchet effects in ferrofluids

Rotational Brownian motion of colloidal magnetic particles in ferrofluids under the influence of an oscillating external magnetic field is investigated. It is shown that for a suitable time dependence of the magnetic field, a noise induced rotation of the ferromagnetic particles due to rectification of thermal fluctuations takes place. Via viscous coupling, the associated angular momentum is transferred from the magnetic nano-particles to the carrier liquid and can then be measured as macroscopic torque on the fluid sample. A thorough theoretical analysis of the effect in terms of symmetry considerations, analytical approximations, and numerical solutions is given which is in accordance with recent experimental findings.Comment: 18 pages, 6 figure

A lattice determination of g_A and <x> from overlap fermions

We present results for the nucleon's axial charge g_A and the first moment of the unpolarized parton distribution function from a simulation of quenched overlap fermions.Comment: Talk presented at Lattice2004(chiral), 4 pages, 4 figure