Temperature-dependent criticality in random 2D Ising models

We consider 2D random Ising ferromagnetic models, where quenched disorder is represented either by random local magnetic fields (random-field Ising model) or by a random distribution of interaction couplings (random-bond Ising model). In both cases, we first perform zero- and finite-temperature Monte Carlo simulations to determine how the critical temperature depends on the disorder parameter. We then focus on the reversal transition triggered by an external field and study the associated Barkhausen noise. Our main result is that the critical exponents characterizing the power law associated with the Barkhausen noise exhibit a temperature dependence in line with existing experimental observations.

Тепловые и магнитокалорические свойства манганитов La-=SUB=-0.7-=/SUB=-Sr-=SUB=-0.3-x-=/SUB=-Ba-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-

The results of a study of the effect of partial substitution of Sr2+ ions by Ba2+ ions on the thermophysical and magnetocaloric properties of manganite La0.7Sr0.3-xBaxMnO3 (x = 0; 0.02; 0.05 and 0.10) in the temperature range 100-400 K and in a magnetic field up to 1.8 T. It is shown that such a substitution leads to a significant decrease in TC. In the behaviour of the thermal diffusivity (T) and thermal conductivity (T) near TС, minima were found that are associated with both the scattering of phonons by local distortions of the crystal lattice and by spin fluctuations. The absolute values of thermal conductivity decrease with an increase in the disorder parameter. The magnetocaloric effect exhibits a weak dependence on the substitution level (x).

Nonequilibrium opinion dynamics on triangular, honeycomb, and Kagome lattices

The Ising model on all Archimedean lattices exhibits spontaneous ordering. Three examples of these lattices, namely triangular ([Formula: see text]), honeycomb [Formula: see text] and Kagome [Formula: see text] lattices, are considered to study the kinetic continuous opinion dynamics model (KCOD) through extensive Monte Carlo simulations. The order/disorder phase transition is observed in all lattices for the KCOD. The estimated values of the critical disorder parameter are [Formula: see text], [Formula: see text], and [Formula: see text] for [Formula: see text], [Formula: see text] and [Formula: see text] lattices, respectively. The critical exponents [Formula: see text], [Formula: see text] and [Formula: see text] for the model are [Formula: see text], [Formula: see text], and [Formula: see text]; [Formula: see text], [Formula: see text], and [Formula: see text]; [Formula: see text], [Formula: see text], and [Formula: see text], for [Formula: see text], [Formula: see text] and [Formula: see text] lattices, respectively. These results agree with the majority-vote model on ([Formula: see text]), ([Formula: see text]), and [Formula: see text] lattices but are different from KCOD model results on square lattices [Formula: see text].

Optical Properties of Ge23.5Pb20S56.5 Glassy System

The spectra of refractive index and absorption coefficient in thin films of Ge23.5Pb20S56.5 glassy system were calculated on the base of analysis of transmission spectra. The refractive index has values between 2.0 and 2.6 in the spectral range between 500 nm and 1200 nm. The existence of indirect allowed transitions with energy Eg = 1.70 ± 0.01 eV and phonon energy of wph= 0.11 eV at room temperature was found. For values of the absorption coefficient a < 103см –1 the fundamental absorption edge is described by exponential function known as Urbach-rule. The static structure disorder parameter wo, is equal to 0.06 eV.

Thermal behaviour of alum-(K) KAl(SO4)2·12H2O from in situ laboratory high-temperature powder X-ray diffraction data: thermal expansion and modelling of the sulfate orientational disorder

The present work analyses the thermal behaviour of alum-(K), KAl(SO4)2·12H2O, by in situ laboratory high-temperature powder X-ray diffraction data from 303 K to melting, which starts at 355 K and is completed, due to kinetics, at 359 K. The calculated a0 linear thermal expansion coefficient is of 14.68(11) × 10–6 K–1 within the investigated thermal range. The k disorder parameter, describing the extension of the orientational disorder of the sulfate group, has been found to decrease from ∼0.70 to ∼0.65 just before melting. It has been demonstrated that the occurrence of the disorder implies the coexistence of K+ ions in both six- and seven-fold coordination. This is necessary for assigning a reasonable bond-valence sum of 0.81 valence units (vu) to the 'average' K+ ion a instead of 0.66 vu, which is obtained in the case of six-fold coordination alone. We can describe the temperature dependence of k from 93–355 K by means of the empirical equation k = 0.798(12) + 2.5(11) × 10–4 T – 1.9(2) × 10–6T2, which includes reference low-temperature data. Bond-valence analysis has shown that, on cooling, an increase of the k disorder parameter and shortening of the K–O2 bond distance act together to maintain constancy in the bond-valence sum at the K site, stabilizing the structure. Therefore, the need for keeping the 'average' K+ ion at a reasonable bond-valence sum appears to be the driving force for the ordering process involving the sulfate group.

Upper Critical Field in Electron-Doped Superconductor with Nonstoichiometric Disorder near Antiferromagnetic-Superconducting Phase Boundary

Using the resistivity method it was found that temperature dependence of the upper critical field for underdoped Nd1.86Ce0.14CuO4+δ have an anomalous upward curvature of Hc2(T) dependence and can be consistently explained by the two-band/two-gap model of a dirty superconductor. Near antiferromagnetic-superconducting phase boundary the critical temperature remains constant with the change of the disorder parameter and the slope of Bc2 increases with increasing of the disorder parameter. This behavior is completely different from dependencies for pure superconducting phase at optimal doping region. This difference may indicate the change of the type of the paring: from the predominance of the anisotropic s-wave component (may be due to unstable competition between antiferromagnetic (AF) and superconducting (SC) regions) in underdoped (x=0.14) region to the prevalence of d-wave part in optimal doped regions (x=0.15) because of residual spin fluctuations.

Effects of Surface Disordering on Optical & Mechanical Properties of Ar+ Implanted Polycarbonate

The surface disordering produced in polycarbonate specimens by 130 keV Ar+ ions has been investigated. The effect of argon ions on the surface structure of polycarbonate specimens has also been studied. The polycarbonate specimens were implanted with 130 keV Ar+ ions in the dose range of 1×1014-1×1016 ions cm-2. The change in the Urbach energy (disorder parameter) after implantation has been estimated using Urbach edge method by applying UV-Visible spectroscopic technique. Optical energy gaps of virgin as well as implanted specimens have also been calculated using UV-Visible spectroscopic technique. A clear enhancement in Urbach energy (disorder parameter) from 0.61 eV (virgin sample) to 1.38 eV (at a fluence of 1×1016 Ar+ cm-2) and a drastic decline in optical energy gap (4.1 eV to 0.63 eV) with increasing implantation dose has been observed. This decrease in optical energy gap has been found to have linear dependence on the increase in the Urbach energy which point towards the formation of disordered structures in the implanted layers of polycarbonate. The structural changes produced due to implantation have been studied using Attenuated Total Reflectance-Fourier Transform Infrared spectroscopic technique. Furthermore, Knoop microhardness has been found to be enhanced 14 times (at a load of 9.8 mN) after implantation. The possible correlation of the increase in Knoop surface hardness with the structural changes observed as a result of implantation has been established and discussed.