Clustering in a precipitate free GeMn magnetic semiconductor

We present the first study relating structural parameters of precipitate free Ge0.95Mn0.05 films to magnetisation data. Nanometer sized clusters - areas with increased Mn content on substitutional lattice sites compared to the host matrix - are detected in transmission electron microscopy (TEM) analysis. The films show no overall spontaneous magnetisation at all down to 2K. The TEM and magnetisation results are interpreted in terms of an assembly of superparamagnetic moments developing in the dense distribution of clusters. Each cluster individually turns ferromagnetic below an ordering temperature which depends on its volume and Mn content.Comment: accepted for publication in Phys. Rev. Lett. (2006). High resolution images ibide

Rayleigh-B\'{e}nard convection in a homeotropically aligned nematic liquid crystal

We report experimental results for convection near onset in a thin layer of a homeotropically aligned nematic liquid crystal heated from below as a function of the temperature difference $\Delta T$ and the applied vertical magnetic field $H$ and compare them with theoretical calculations. The experiments cover the field range 8 \alt h \equiv H/ H_{F} \alt 80 ($H_F =$ is the Fr\'eedericksz field). For $h$ less than a codimension-two field $h_{ct} \simeq 46$ the bifurcation is subcritical and oscillatory, with travelling- and standing-wave transients. Beyond $h_{ct}$ the bifurcation is stationary and subcritical until a tricritical field $h_t= 57.2$ is reached, beyond which it is supercritical. The bifurcation sequence as a function of $h$ found in the experiment confirms the qualitative aspects of the theoretical predictions. However, the value of $h_{ct}$ is about 10% higher than the predicted value and the results for $k_c$ are systematically below the theory by about 2% at small $h$ and by as much as 7% near $h_{ct}$. At $h_{ct}$, $k_c$ is continuous within the experimental resolution whereas the theory indicates a 7% discontinuity. The theoretical tricritical field $h_t^{th} = 51$ is somewhat below the experimental one. The fully developed flow above $R_c$ for $h < h_{ct}$ is chaotic. For $h_{ct} < h < h_t$ the subcritical stationary bifurcation also leads to a chaotic state. The chaotic states persist upon reducing the Rayleigh number below $R_c$, i.e. the bifurcation is hysteretic. Above the tricritical field $h_t$, we find a bifurcation to a time independent pattern which within our resolution is non-hysteretic.Comment: 15 pages incl. 23 eps figure

Power-Law Behavior of Power Spectra in Low Prandtl Number Rayleigh-Benard Convection

The origin of the power-law decay measured in the power spectra of low Prandtl number Rayleigh-Benard convection near the onset of chaos is addressed using long time numerical simulations of the three-dimensional Boussinesq equations in cylindrical domains. The power-law is found to arise from quasi-discontinuous changes in the slope of the time series of the heat transport associated with the nucleation of dislocation pairs and roll pinch-off events. For larger frequencies, the power spectra decay exponentially as expected for time continuous deterministic dynamics.Comment: (10 pages, 6 figures

Gamma-ray halos as a measure of intergalactic magnetic fields: a classical moment problem

The presence of weak intergalactic magnetic fields can be studied by their effect on electro-magnetic cascades induced by multi-TeV gamma-rays in the cosmic radiation background. Small deflections of secondary electrons and positrons as the cascade develops extend the apparent size of the emission region of distant TeV gamma-ray sources. These gamma-ray halos can be resolvable in imaging atmospheric Cherenkov telescopes and serve as a measure of the intergalactic magnetic field strength and coherence length. We present a method of calculating the gamma-ray halo for isotropically emitting sources by treating magnetic deflections in the cascade as a diffusion process. With this ansatz the moments of the halo follow from a set of simple diffusion-cascade equations. The reconstruction of the angular distribution is then equivalent to a classical moment problem. We present a simple solution using Pade approximations of the moment's generating function.Comment: 12 pages, 6 figure

Spiral Defect Chaos in Large Aspect Ratio Rayleigh-Benard Convection

We report experiments on convection patterns in a cylindrical cell with a large aspect ratio. The fluid had a Prandtl number of approximately 1. We observed a chaotic pattern consisting of many rotating spirals and other defects in the parameter range where theory predicts that steady straight rolls should be stable. The correlation length of the pattern decreased rapidly with increasing control parameter so that the size of a correlated area became much smaller than the area of the cell. This suggests that the chaotic behavior is intrinsic to large aspect ratio geometries.Comment: Preprint of experimental paper submitted to Phys. Rev. Lett. May 12 1993. Text is preceeded by many TeX macros. Figures 1 and 2 are rather lon

Singularity in the boundary resistance between superfluid 4^4He and a solid surface

We report new measurements in four cells of the thermal boundary resistance $R$ between copper and $^4$He below but near the superfluid-transition temperature $T_\lambda$. For $10^{-7} \leq t \equiv 1 - T/T_\lambda \leq 10^{-4}$ fits of $R = R_0 t^{x_b} + B_0$ to the data yielded $x_b \simeq 0.18$, whereas a fit to theoretical values based on the renormalization-group theory yielded $x_b = 0.23$. Alternatively, a good fit of the theory to the data could be obtained if the {\it amplitude} of the prediction was reduced by a factor close to two. The results raise the question whether the boundary conditions used in the theory should be modified.Comment: 4 pages, 4 figures, revte

Isentropic Curves at Magnetic Phase Transitions

Experiments on cold atom systems in which a lattice potential is ramped up on a confined cloud have raised intriguing questions about how the temperature varies along isentropic curves, and how these curves intersect features in the phase diagram. In this paper, we study the isentropic curves of two models of magnetic phase transitions- the classical Blume-Capel Model (BCM) and the Fermi Hubbard Model (FHM). Both Mean Field Theory (MFT) and Monte Carlo (MC) methods are used. The isentropic curves of the BCM generally run parallel to the phase boundary in the Ising regime of low vacancy density, but intersect the phase boundary when the magnetic transition is mainly driven by a proliferation of vacancies. Adiabatic heating occurs in moving away from the phase boundary. The isentropes of the half-filled FHM have a relatively simple structure, running parallel to the temperature axis in the paramagnetic phase, and then curving upwards as the antiferromagnetic transition occurs. However, in the doped case, where two magnetic phase boundaries are crossed, the isentrope topology is considerably more complex

The three-dimensional XY universality class: A high precision Monte Carlo estimate of the universal amplitude ratio A_+/A_-

We simulate the improved three-dimensional two-component phi^4 model on the simple cubic lattice in the low and the high temperature phase for reduced temperatures down to |T-T_c|/T_c \approx 0.0017 on lattices of a size up to 350^3. Our new results for the internal energy and the specific heat are combined with the accurate estimates of beta_c and data for the internal energy and the specific heat at \beta_c recently obtained in cond-mat/0605083. We find R_{\alpha} = (1-A_+/A_-)/\alpha = 4.01(5), where alpha is the critical exponent of the specific heat and A_{\pm} is the amplitude of the specific heat in the high and the low temperature phase, respectively.Comment: 14 pages, 4 figure

Self-organization in He4 near the superfluid transition in heat flow and gravity

We investigate the nonlinear dynamics of He4 slightly below the superffluid transition by integrating model F equations in three dimensions. When heated from above under gravity, a vortex tangle and a sheetlike phase slip are generated near the bottom plate. Then a self-organized superfluid containing high-density vortices and phase slips grows upward into an ordinary superfluid. The thermal resistance due to these defects produces a constant temperature gradient equal to the gradient of the pressure-dependent transition temperature $T_{\lambda}(p)$. In this self-organized region, the temperature deviation $T-T_{\lambda}(p)$ consists of a negative constant independent of the height and time-dependent fluctuations. Its time-average is calculated in good agreement with the experimental value (W.A. Moeur {\it et al.}, Phys. Rev. Lett. 78, 2421 (1997)).Comment: 8 pages, 7 figure

Resistive Anomalies at Ferromagnetic Transitions Revisited: the case of SrRuO_3

We show that recent resistivity data on SrRuO_3 for T->T_c are consistent with conventional theory when corrections to scaling are included and a small shift in T_c is allowed.Comment: 2 pages, 1 figure; revte