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T. J. Newman

Publications and source records attributed to T. J. Newman.

29 records · Page 2Linked to original sources
arXiv1998

Continuum theory of vacancy-mediated diffusion

We present and solve a continuum theory of vacancy-mediated diffusion (as evidenced, for example, in the vacancy driven motion of tracers in crystals). Results are obtained for all spatial dimensions, and reveal the strongly non-gaussian nature of the tracer fluctuations. In integer dimensions, our results are in complete agreement with those from previous exact lattice calculations. We also extend our model to describe the vacancy-driven fluctuations of a slaved flux line.

cond-mat.stat-mech↗
arXiv1998

Diffusive persistence and the `sign-time' distribution

We present a new method for extracting the persistence exponent theta for the diffusion equation, based on the distribution P of `sign-times'. With the aid of a numerically verified Ansatz for P we derive an exact formula for theta in arbitrary spatial dimension d. Our results are in excellent agreement with previous numerical studies. Furthermore, our results indicate a qualitative change in P above d ~ 36, signalling the existence of a sharp change in the ergodic properties of the diffusion field.

cond-mat.stat-mech↗
arXiv1997

Mixed phases in U(N) superconductivity

We consider a general class of type-II superconductors which are described by N complex order parameters (OP), with an overall U(N) symmetry for the energy functional. In the lowest energy state, each OP adopts its own particular lattice structure, making the spatial variation of the overall condensate very complex. We present detailed results for the case N=2. We also treat the limit N -> infinity, which allows us to resolve a recent controversy, by showing that all previous studies are fundamentally flawed.

cond-mat.supr-con↗
arXiv1997

Non-universal exponents in interface growth

We report on an extensive numerical investigation of the Kardar-Parisi-Zhang equation describing non-equilibrium interfaces. Attention is paid to the dependence of the growth exponents on the details of the distribution of the noise. All distributions considered are delta-correlated in space and time, and have finite cumulants. We find that the exponents become progressively more sensitive to details of the distribution with increasing dimensionality. We discuss the implications of these results for the universality hypothesis.

cond-mat.stat-mech↗
arXiv1997

Dynamical Scaling in Dissipative Burgers Turbulence

An exact analysis is performed for the two-point correlation function C(r,t) in dissipative Burgers turbulence with bounded initial data, in arbitrary spatial dimension d. Contrary to the usual scaling hypothesis of a single dynamic length scale, it is found that C contains two dynamic scales: a diffusive scale l_{D} \sim t^{1/2} for very large r, and a super-diffusive scale L(t) \sim t^{a} for r \ll l_{D}, where a = (d+1)/(d+2). The consequences for conventional scaling theory are discussed. Finally, some simple scaling arguments are presented within the `toy model' of disordered systems theory, which may be exactly mapped onto the current problem.

cond-mat.stat-mech↗
arXiv1996

Directed lines in sparse potentials

We present a continuum formulation of a (d+1)-dimensional directed line interacting with sparse potentials (i.e. d-dimensional potentials defined only at discrete longitudinal locations.) An iterative solution for the partition function is derived. The impulsive influence of the potentials induces discontinuities in the evolution of the probability density P(x,t) of the directed line. The effects of these discontinuities are studied in detail for the simple case of a single defect. We then investigate sparse columnar potentials defined as a periodic array of defects in (2+1) dimensions, and solve exactly for P. A non-trivial binding/unbinding transition is found.

cond-mat↗
arXiv1996

Strong-coupling behaviour in discrete Kardar-Parisi-Zhang equations

We present a systematic discretization scheme for the Kardar-Parisi-Zhang (KPZ) equation, which correctly captures the strong-coupling properties of the continuum model. In particular we show that the scheme contains no finite-time singularities in contrast to conventional schemes. The implications of these results to i) previous numerical integration of the KPZ equation, and ii) the non-trivial diversity of universality classes for discrete models of `KPZ-type' are examined. The new scheme makes the strong-coupling physics of the KPZ equation more transparent than the original continuum version and allows the possibility of building new continuum models which may be easier to analyse in the strong-coupling regime.

cond-mat↗
arXiv1995

Strong coupling probe for the Kardar-Parisi-Zhang equation

We present an exact solution of the {\it deterministic} Kardar-Parisi-Zhang (KPZ) equation under the influence of a local driving force $f$. For substrate dimension $d \le 2$ we recover the well-known result that for arbitrarily small $f>0$, the interface develops a non-zero velocity $v(f)$. Novel behaviour is found in the strong-coupling regime for $d > 2$, in which $f$ must exceed a critical force $f_c$ in order to drive the interface with constant velocity. We find $v(f) \sim (f-f_c)^{α(d)}$ for $f \searrow f_{c}$. In particular, the exponent $α(d) = 2/(d-2)$ for $2 4$, indicating that for this simple problem, there exists a finite upper critical dimension $d_u=4$. For $d>2$ the surface distortion caused by the applied force scales logarithmically with distance within a critical radius $R_{c} \sim (f-f_{c})^{-ν(d)}$, where $ν(d) = α(d)/2$. Connections between these results, and the critical properties of the weak/strong-coupling transition in the noisy KPZ equation are pursued.

cond-mat↗
arXiv1995

Critical Fluctuations and Disorder at the Vortex Liquid to Crystal Transition in Type-II Superconductors

We present a functional renormalization group (FRG) analysis of a Landau-Ginzburg model of type-II superconductors (generalized to $n/2$ complex fields) in a magnetic field, both for a pure system, and in the presence of quenched random impurities. Our analysis is based on a previous FRG treatment of the pure case [E.Brézin et. al., Phys. Rev. B, {\bf 31}, 7124 (1985)] which is an expansion in $ε= 6-d$. If the coupling functions are restricted to the space of functions with non-zero support only at reciprocal lattice vectors corresponding to the Abrikosov lattice, we find a stable FRG fixed point in the presence of disorder for $1 4$ and so the physical case ($n = 2$) is likely to have a first order transition. We speculate that the recent experimental findings that disorder removes the apparent first order transition are consistent with these calculations.

cond-mat↗
arXiv1994

Non-equilibrium Dynamics of Finite Interfaces

We present an exact solution to an interface model representing the dynamics of a domain wall in a two-phase Ising system. The model is microscopically motivated, yet we find that in the scaling regime our results are consistent with those obtained previously from a phenomenological, coarse-grained Langevin approach.

cond-mat↗