Smeared phase transition in a three-dimensional Ising model with planar defects: Monte-Carlo simulations

We present results of large-scale Monte Carlo simulations for a three-dimensional Ising model with short range interactions and planar defects, i.e., disorder perfectly correlated in two dimensions. We show that the phase transition in this system is smeared, i.e., there is no single critical temperature, but different parts of the system order at different temperatures. This is caused by effects similar to but stronger than Griffiths phenomena. In an infinite-size sample there is an exponentially small but finite probability to find an arbitrary large region devoid of impurities. Such a rare region can develop true long-range order while the bulk system is still in the disordered phase. We compute the thermodynamic magnetization and its finite-size effects, the local magnetization, and the probability distribution of the ordering temperatures for different samples. Our Monte-Carlo results are in good agreement with a recent theory based on extremal statistics.Comment: 9 pages, 6 eps figures, final version as publishe

Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment

In this paper, we review theoretical and experimental research on rare region effects at quantum phase transitions in disordered itinerant electron systems. After summarizing a few basic concepts about phase transitions in the presence of quenched randomness, we introduce the idea of rare regions and discuss their importance. We then analyze in detail the different phenomena that can arise at magnetic quantum phase transitions in disordered metals, including quantum Griffiths singularities, smeared phase transitions, and cluster-glass formation. For each scenario, we discuss the resulting phase diagram and summarize the behavior of various observables. We then review several recent experiments that provide examples of these rare region phenomena. We conclude by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative Ising chain fixed, references added, v3: final version as publishe

Structure and dynamics of liquids from nmr relaxation. some selected examples

First very briefly the basic physical principles are presented from which the following results are derived. As a first group of examples a number of hydration and solvation structures around the ions F-, Na+, and l- is given. Some molecular pair configurations of maximum occurrence probability in mixtures with acetic acid and of liquid formic acid are shown. The connection with X-ray and neutron diffraction data is discussed. Then a general criterion for the existence of association in liquids is given. Among the cases of application the problem of hydrophobic association and the effect of preferential solvation are considered. In the field of molecular motions in liquids our interest is only devoted to the problem of internal motion. The molecules treated are ethanol, i-propanol, methanol and toluene, partly in mixtures with other substances. It will be shown that the current theoretical approaches do not give a satisfactory description of the observed data

NMR Relaxation study of l

Proton magnetic relaxation rates of HCON (CD3)2 and DCON (CD3) (D2H) caused by magnetic dipole-dipole interaction with 127I and 23Na in the ion I- and Na+ , respectively, have been measured. These results were obtained by the isotopic H/D substitution technique with extrapolation to the limit proton content xH → O. From these data closest distances of approach between the interacting nuclei were derived which then, with the geometry of the DMF molecule, yielded the ion-solvent configuration in the solvation sphere

A study of self-association in mixtures of methanol and cyclohexane by the nuclear magnetic relaxation method

Proton- and deuteron magnetic relaxation rates and self-diffusion coefficients have been measured in the system methanol/ cyclohexane at 60°C. The proton relaxation rates were separated into the intra- and intermolecular contributions. By means of the association parameter A strong self-association of methanol with respect to the OH— as well as the CH3-group were revealed. The results indicate that a proper counterpart to the macroscopic characteristics of this system (as e.g. positive deviations from Raoult's law and partial immiscibility) exists on a molecular scale

The effect of structure on self-diffusion in concentrated electrolytes : relationship between the water and ionic self-diffusion coefficients for structure-forming salts

Experimental self-diffusion coefficients of Mg2+ and Cl– in aqueous MgCl2 solutions at 25 °C and 10 °C are reported as a function of the salt concentration. The tracer-diffusion technique has been applied. Na+ and Cl– self-diffusion coefficients in NaCl solution at 10 °C are also reported. The results are evaluated in terms of a model connecting the ionic self-diffusion coefficients of the structure forming cations with the water self-diffusion coefficients in the same solutions

A study concerning self-association in mixtures of cyclohexane, benzene and DMSO with CC14 by the nuclear magnetic relaxation method

Proton and deuteron magnetic relaxation rates and self-diffusion coefficients have been measured in the systems cyclohexane/CCl4, benzene/CCl4 and DMSO/CCl4 at 298 K. By means of the association parameter A self-association of DMSO, weak deassociation of benzene(within experimental uncertainty) and constant values of A for cyclohexane were found. The results indicate that proper counterparts to the macroscopic characteristics of these systems exist on a molecular scale. © 1984, R. Oldenbourg Verlag, Münche