Hysteretic transition between avalanches and continuous flow in rotated granular systems

Experiments in drums or cylinders partly filled with a granular system and rotated constantly about their horizontally aligned axis of symmetry show a hysteretic transition from discrete avalanches to continuous flow if the rotation rate is adiabatically changed. Herein, we show that this hysteresis can be explained by the impact of global Langevin-type fluctuations in a recently proposed minimal model for surface flow along granular piles. For too large magnitudes of the fluctuations corresponding to almost elastic grains, the hysteresis vanishes. This might explain why molecular dynamical simulations were not yet able to detect the hysteretic transition. (c) 1999 American Institute of Physics.     

Strong coupling theory for driven tunneling and vibrational relaxation

We investigate on a unified basis tunneling and vibrational relaxation in driven dissipative multistable systems described by their N lowest lying unperturbed levels. By use of the discrete variable representation we derive a set of coupled non-Markovian master equations. We present analytical treatments that describe the dynamics in the regime of strong system-bath coupling. Our findings are corroborated by "ab initio" real-time path integral calculations.     

Quantum features of Brownian motors and stochastic resonance

We investigate quantum Brownian motion sustained transport in both, adiabatically rocked ratchet systems and quantum stochastic resonance (QSR). Above a characteristic crossover temperature T(0) tunneling events are rare; yet they can considerably enhance the quantum-noise-driven particle current and the amplification of signal output in comparison to their classical counterparts. Below T(0) tunneling prevails, thus yielding characteristic novel quantum transport phenomena. For example, upon approaching T=0 the quantum current in Brownian motors exhibits a tunneling-induced reversal, and tends to a finite limit, while the classical result approaches zero without such a change of sign. As a consequence, similar current inversions generated by quantum effects follow upon variation of the particle mass or of its friction coefficient. Likewise, in this latter regime of very low temperatures the tunneling dynamics becomes increasingly coherent, thus suppressing the semiclassically predicted QSR. Moreover, nonadiabatic driving may cause driving-induced coherences and quantized resonant transitions with no classical analog. (c) 1998 American Institute of Physics.     

Nuclear magnetic relaxation dispersion measurement of water mobility at a silica surface

Nuclear magnetic relaxation rates are measured as a function of magnetic field strength corresponding to proton Larmor frequencies ranging from 0.01 to 42 MHz for silica gel samples with a nitroxide free radical covalently attached at the surface. The field dependence of the relaxation rate is interpreted using a translational model for the relaxation equation to yield a translational diffusion coefficient for the water, in the immediate vicinity of the radical attached to the surface, of 2.1 × 10^?6 cm² s^?1 at 278 K for Si-4000 silica.     

Nonlinear effects of colored nonstationary noise: Exact results

The master equation is derived for random systems under nonlinear time-dependent conditions. The (non-Markov) process is of such a type that with a time-dependent state transformation the dynamics can be modelled by a nonlinear but drift-free Langevin equation. The focus is on the statistical content of resulting master equation. The existence of stationary solutions and the quality of approximative results is discussed.     

Langevin description of Markov master equations II: Noise correlations

We present experimental results of dielectric and birefringence measurements on 3.5% and 8% Li∶KTaO₃ as a function of temperature. These results indicate that the 8% Li∶KTaO₃ sample undergoes a ferroelectric phase transition at 97 K. The 3.5% sample seems to undergo a distortive transition at ～75 K. Raman experiments show that the splitting of the TO₁ mode of the 8% Li sample extrapolates to zero at ～97 K supporting the conclusion that this sample undergoes a phase transition at this temperature. Raman spectra of both samples at temperatures above the phase transition or freezing temperatures exhibit in addition to second order Raman features first order Raman lines. These lines have the properties of fluctuation induced first order Raman scattering. Measurement of the integrated intensity of these lines is used to obtain the temperature dependence of the fluctuation intensity. The results are compared to the Halperin and Varma theory. The data obtained for the 3.5% Li sample partialy fits the theory. However the results for the 8% Li sample do not fit the theory. The misfit is attributed to the overlap of defect zones.