Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, which is driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. In the spatially uniform 2d electron-hole system, the instability leads to a triangular lattice pattern while, at an electron-hole interface, a periodic 1d pattern develops. We analyze the mechanism of wavelength selection, and show that the transition is abrupt (type I) for the uniform 2d system, and continuous (type II) for the electron-hole interface.Comment: 5 pages, 3 figure

Resistance to flow in alluvial channels

CER61DBS79.Includes bibliographical references.From: Transactions of the American Society of Civil Engineers, 1962

Forms of bed roughness in alluvial channels

CER60DBS3.January 1960.Includes bibliographical references

Resistance to flow in alluvial channels

CER59DBS48.November 1959.Includes bibliographical references.Presented at the New York ASCE Convention, October, 1958.This paper presents the initial results of a flume study of alluvial channels currently being conducted by the U.S. Geological Survey at Colorado State University. A detailed classification of the regimes of flow, the forms of bed roughness, and the basic concepts pertaining to resistance to now are discussed

Non-universal corrections to the level curvature distribution beyond random matrix theory

The level curvature distribution function is studied beyond the random matrix theory for the case of T-breaking perturbations over the orthogonal ensemble. The leading correction to the shape of the level curvature distribution is calculated using the nonlinear sigma-model. The sign of the correction depends on the presence or absence of the global gauge invariance and is different for perturbations caused by the constant vector-potential and by the random magnetic field. Scaling arguments are discussed that indicate on the qualitative difference in the level statistics in the dirty metal phase for space dimensionalities $d4$.Comment: 4 pages, Late

Topological universality of level dynamics in quasi-one-dimensional disordered conductors

Nonperturbative, in inverse Thouless conductance 1/g, corrections to distributions of level velocities and level curvatures in quasi-one-dimensional disordered conductors with a topology of a ring subject to a constant vector potential are studied within the framework of the instanton approximation of nonlinear sigma-model. It is demonstrated that a global character of the perturbation reveals the universal features of the level dynamics. The universality shows up in the form of weak topological oscillations of the magnitude ~ exp(-g) covering the main bodies of the densities of level velocities and level curvatures. The period of discovered universal oscillations does not depend on microscopic parameters of conductor, and is only determined by the global symmetries of the Hamiltonian before and after the perturbation was applied. We predict the period of topological oscillations to be 4/(pi)^2 for the distribution function of level curvatures in orthogonal symmetry class, and 3^(1/2)/(pi) for the distribution of level velocities in unitary and symplectic symmetry classes.Comment: 15 pages (revtex), 3 figure

Study of flow in alluvial channels: depth-discharge relations, A

CER59DBS34.Includes bibliographical references (page 42).Alluvial channel stage-discharge and depth-discharge relations were studied in a large sand bed-recirculating flume. From this study, it was found that the form of these relationships are intimately related to: 1. Regime of flow; 2. Form of bed roughness, a. Characteristics of the bed material, b. Concentration of fine sediment, c. Temperature; 3. Rate of change of discharge with time. In the range of shear where ripples and dunes develop on the bed, the stage-discharge curve for a rising stage is usually quite different from that for a falling stage. These curves are only valid for the conditions upon which they are based--no general solution is possible. In the range of shear, which develops plane bed, standing sand, and water waves, which are in phase, and antidunes, the rising and falling stage curves coincide and hold for all values of discharge associated with these forms of bed roughness. When a channel experiences a shear stress, which develops dunes at small discharges and plane bed and perhaps standing waves and antidunes at larger discharges, there is a discontinuity in the stage-discharge or depth-discharge curves particularly on the rising stage, which occurs when the dunes wash out. This is caused by the large reduction in resistance to flow, which occurs when the bed form changes from ripples or dunes to plane bed, standing waves, or antidunes, and the resultant reduction in depth even though discharge is increasing

Interpreting depositional environments of sedimentary structures

CERDFN-DBS-EVR15.January 28-30, 1965.Includes bibliographical references (page 11).Presented at the Southwest Regional Meeting, American Geophysical Union, Socorro, New Mexico, January 28-30, 1965

Flume studies using medium sand (0.45mm)

CER58DBS2.Includes bibliographical references.The results pertaining to the progress during the first year of a comprehensive study of fluvial hydraulics, specifically roughness in alluvial channels, are presented. The report is based on the data collected by using a recirculating rectangular flume of adjustable slope, 8 feet wide, 2 feet deep, and 150 feet long with an alluvial bed of sand approximately 0. 7 foot deep. A typical river sand has been utilized. Its median diameter, d, is 0.45 mm and its relative standard deviation, σ, is 1.60. A total of 45 runs have been completed over a range of bed roughness forms extending from the plane bed with no movement to antidunes. In order to achieve this range, the discharge was varied from 2 to 21 cubic feet per second, the average velocity was varied from 0.5 to 7 feet per second, the average depth of flow was varied from 0.3 to 1.0 foot, and the slope of water surface was varied from 0.00014 to 0.01. Other variables measured included: water temperature, bed roughness, suspended sediment load, and total sediment load. Terms describing channel roughness were formulated and tested based on the data collected. The results indicate, as one possibility, that the Chezy coefficient of discharge in dimensionless form C/√g is a function of parameters involving the Froude number, viscosity of fluid, fall velocity, specific weight of the sediment, median diameter of the sediment particles and slope of the water surface. The various expressions presented were formulated on the fundamental concepts of fluid mechanics, dimensional analysis, and a detailed study of the variations of the variables measured. In the two regimes of flow the following forms of bed roughness were observed. For tranquil flow regime: plane bed without movement, ripples, dunes and transition from dunes to rapid flow forms. For rapid flow regime: plane bed with movement, standing sand waves, and antidunes. These forms of bed roughness are discussed and defined in various relationships. Other data of both a laboratory and a field nature were combined with the flume data to develop a graphical relationship in which the form of bed roughness is related to size of bed material

Classical and Quantum Dynamics in a Random Magnetic Field

Using the supersymmetry approach, we study spectral statistical properties of a two-dimensional quantum particle subject to a non-uniform magnetic field. We focus mainly on the problem of regularisation of the field theory. Our analysis begins with an investigation of the spectral properties of the purely classical evolution operator. We show that, although the kinetic equation is formally time-reversible, density relaxation is controlled by {\em irreversible} classical dynamics. In the case of a weak magnetic field, the effective kinetic operator corresponds to diffusion in the angle space, the diffusion constant being determined by the spectral resolution of the inhomogeneous magnetic field. Applying these results to the quantum problem, we demonstrate that the low-lying modes of the field theory are related to the eigenmodes of the irreversible classical dynamics, and the higher modes are separated from the zero mode by a gap associated with the lowest density relaxation rate. As a consequence, we find that the long-time properties of the system are characterised by universal Wigner-Dyson statistics. For a weak magnetic field, we obtain a description in terms of the quasi one-dimensional non-linear $\sigma$-model.Comment: 16 pages, RevTe