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We have performed extensive molecular dynamics simulations to study noise power spectra of density and potential energy fluctuations of a Lennard-Jones model of a fluid in the supercritical region. Emanating from the liquid-vapor critical point, there is a locus of isobaric specific heat maxima, called the Widom line, which is often regarded as an extension of the liquid-vapor coexistence line. Our simulation results show that the noise power spectrum of the density fluctuations on the Widom line of the liquid-vapor transition exhibits three distinct $1/f^{\gamma}$ behaviors with exponents $\gamma$= 0, 1.2 and 2, depending on the frequency $f$. We find that the intermediate frequency region with an exponent $\gamma \sim$ 1 appears as the temperature approaches the Widom temperature from above or below. On the other hand, we do not find three distinct regions of $1/f^{\gamma}$ in the power spectrum of the potential energy fluctuations on the Widom line. Furthermore, we find that the power spectra of both the density and potential energy fluctuations at low frequency have a maximum on the Widom line, suggesting that the noise power can provide an alternative signature of the Widom line.

We provide a general scheme to predict and derive the contribution to the noise spectrum of a stochastic sequence of pulses from the distribution of pulse parameters. An example is the magnetization noise spectra of a 2D Ising system near its phase transition. At $T\le T_c$, the low frequency spectra is dominated by magnetization flips of nearly the entire system. We find that both the predicted and the analytically derived spectra fit those produced from simulations. Subtracting this contribution leaves the high frequency spectra which follow a power law set by the critical exponents., Comment: 4 pages, 5 figures. We improved text and included a predicted noise curve in Figure 4. 2 examples from Figure 3 are removed

Dilute dipolar systems in three dimensions are expected to undergo a spin glass transition as the temperature decreases. Contrary to this, we find from Wang-Landau Monte Carlo simulations that at low concentrations $x$, dipoles randomly placed on a cubic lattice with dipolar interactions do not undergo a phase transition. We find that in the thermodynamic limit the ``glass'' transition temperature $T_g$ goes to zero as $1/\sqrt{N}$ where $N$ is the number of dipoles. The entropy per particle at low temperatures is larger for lower concentrations ($x=4.5%$) than for higher concentrations ($x=20%$).

We have performed a Monte Carlo study of a classical three dimensional Coulomb system in which we systematically increase the positional disorder. We start from a completely ordered system and gradually transition to a Coulomb glass. The phase transition as a function of temperature is second order for all values of disorder. We use finite size scaling to determine the transition temperature T_C and the critical exponent nu. We find that T_C decreases and that nu increases with increasing disorder. We also observe changes in the specific heat, the single particle density of states, and the staggered occupation as a function of disorder and temperature., Comment: 14 pages, Latex, 21 encapsulated postscript figures, to be published in Phys. Rev. B

We show that a glass transition, signaled by a peak in the specific heat vs. temperature, can occur because a glassy system that shows no signs of aging progresses so slowly through the energy landscape that the time needed to obtain an accurate estimate of the thermodynamic averages exceeds the observation time. We find that below the glass transition temperature of a three dimensional binary mixture of soft spheres, the specific heat increases with measurement time spans orders of magnitude longer than previously recognized equilibration times., Comment: 4 pages, Latex, 3 encapsulated postscript figures

We introduce a new quantity to probe the glass transition. This quantity is a linear generalized compressibility which depends solely on the positions of the particles. We have performed a molecular dynamics simulation on a glass forming liquid consisting of a two component mixture of soft spheres in three dimensions. As the temperature is lowered (or as the density is increased), the generalized compressibility drops sharply at the glass transition, with the drop becoming more and more abrupt as the measurement time increases. At our longest measurement times, the drop occurs approximately at the mode coupling temperature $T_C$. The drop in the linear generalized compressibility occurs at the same temperature as the peak in the specific heat. By examining the inherent structure energy as a function of temperature, we find that our results are consistent with the kinetic view of the glass transition in which the system falls out of equilibrium. We find no size dependence and no evidence for a second order phase transition though this does not exclude the possibility of a phase transition below the observed glass transition temperature. We discuss the relation between the linear generalized compressibility and the ordinary isothermal compressibility as well as the static structure factor., Comment: 18 pages, Latex, 26 encapsulated postscript figures, revised paper is shorter, to appear in Phys. Rev. E

At low temperatures electron hopping in a three dimensional Coulomb glass produces fluctuations in the single particle density of states and hence in the resistivity. This results in a low frequency resisitivity noise spectrum which goes as (1/f)^(alpha) where alpha is very close to 1. This holds down to extremely low frequencies., Comment: 5 pages, Latex, 1 postscript figures

We introduce a new quantity to probe the glass transition. This quantity is a linear generalized compressibility which depends solely on the positions of the particles. We have performed a molecular dynamics simulation on a glass forming liquid consisting of a two component mixture of soft spheres in three dimensions. As the temperature is lowered, the generalized compressibility drops sharply at the glass transition. Our results are consistent with the kinetic view of the glass transition, but not with an underlying second order phase transition., Comment: 5 pages, Latex, 4 postscript figures, to be published in Phil. Mag. B, some corrections to previous version