Ground state and excitations of disordered boson systems

After an introduction to the dirty bosons problem, we present a gaussian theory for the ground state and excitations. This approach is physically equivalent to the Bogoliubov approximation. We find that ODLRO can be destroyed with sufficient disorder. The density of states and localization of the elementary excitations are discussed.     

Slave boson theories of correlated electron systems

Slave boson theories of various models of correlated fermions are critically reviewed and several new results are presented. In the example of the Andersen impurity model the limitations of slave boson mean field theory are discussed. Self-consistent conserving approximations are compared with results obtained from the numerical renormalization group. The gauge field theory of the t-J-model is considered in the quasistatic approximation. It is shown that weak localization effects can give valuable information on the existence of gauge fields. Applications of the slave-boson approach due to Kotliar and Ruckenstein to the Hubbard model are also discussed.     

The nonlinear thermoelectric effect in disordered metallic systems

The nonlinear thermoelectric effect in a disordered metal due to the nonlocal corrections to the thermoelectric current is investigated. We have found that the electron-electron interaction makes the thermoelectric response nonlinear at low temperatures even if the applied electric field or temperature gradient is constant. Both the heat current induced by the electric field and the electric current induced by the temperature gradient are discussed.The authors are grateful to A. V. Sergeev for valuable discussions.     

Superconductivity in transition metal-germanium systems

The variation in the superconducting properties of various binary alloys of the transition metal-germanium system was surveyed by studying sputter-deposited samples prepared under various conditions. Radiofrequency cosputtering was used to form samples of the form T _x Ge _1–x , where T represents a transition metal, Y, La, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and Re. The primary interest has been to study the formation of A-15 compounds. While sputtered V ₃ Ge hadT _cs close to bulk sample values, Nb ₃ Ge (T _c=21.7 K) and Mo ₃ Ge (T _c=5.7 K) showed a considerable enhancement inT _c over the bulk values. The formation of the metastable A - 15 modification of Ta ₃ Ge, Ti ₃ Ge, Zr ₃ Ge, and Hf ₃ Ge was attempted, without success. Superconductivity was observed in samples of ZrGe ₂ , HfGe ₂ , YGe ₂ , and LaGe ₂ , the highestT _c being that of ZrGe ₂ (T _c=8.0 K). Annealing studies indicated that superconducting ZrGe₂ and HfGe₂ were metastable, depending on the conditions of synthesis. Superconductivity was also observed in mixtures of semiconducting Ge and metallic NbGe ₂ , the highestT _c observed being 16 K. Several explanations are considered to account for the observedT _cbehavior among the various alloys.Supported in part by the National Science Foundation.     

Magnetic imaging of a supercooling glass transition in a weakly disordered ferromagnet

Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems where replica symmetry is broken. Although magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin-glass picture, our results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition. Our data also suggest that accommodation strain is important in the kinetics of the phase transition. This cooperative freezing idea has been applied to structural glasses including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free-energy landscape.     

Effective conductivity of heterogeneous systems with disordered structure

A model reflecting the disordered character of the structure of a heterogeneous system is proposed, together with a method for calculating the coefficients of generalized conductivity of compounds, eutectics, composites, and solutions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 4, pp. 648–655, April, 1978.     

Microwave studies of chaotic billiards and disordered systems

Many of the concepts developed for the quantum mechanics of classically chaotic or disordered systems are notes genuinely quantum mechanical in origin but can be applied to classical waves as well. This opens up the opportunity of an experimental approach to questions which before were accessible by theory only. As an illustration, microwave results on spectral level dynamics, on field distributions in three-dimensional resonators and on linewidths in open billiards are presented.     

Thin3He films on a disordered substrate: A Metal-Insulator transition

The heat capacity of thin³He films and of liquid³He underpressure up to 28 bar in porous Vycor glass has been measured at temperatures 10–600 mK. An evolution from solid (Insulator) to liquid (Metal) behaviour has been observed with increasing the film thickness from one to three atomic layers. At low temperatures the second layer has a temperature independent specific heat. Using a model of glassy second layer of³He on a rough substrate, that has a broad uniform distribution of the logarithm of nuclear exchange interaction, we calculate its specific heat and susceptibility and compare them with the available experimental data.On leave from the Institute of Solid State Physics, Russ. Acad. Sci., Chernogolovka 142432 Moscow distr., Russia     

Effect of electron-electron interaction on thermal conductivity of disordered systems

We present a calculation of a correction to the thermal conductivity due to the inclusion of electron-electron interaction in a disordered metallic system. We find that, to the first order in electron-electron interaction, the Wiedemann-Franz law is not satisfied.     

Analysis of intermediate boson spectra from FIR data for HTSC and heavy fermion systems

Calculations of the effect of intermediate bosons on the optical properties of the normal state are used to analyze the spectra of these bosons. For high temperature superconductors these spectra are in agreement with the standard electron-phonon model (the end of the spectrum is near 800 cm^–1) with a moderate coupling constant =1–1.5. For the heavy fermion compound UPt₃ the spectrum of intermediate bosons is consistent with neutron data for spin fluctuations.This work was partially supported by ISF and RFFI (No 94-02-04186a) grants.     

A simple model for studying the magnetism of an actinide impurity in a disordered transition host

We present a theoretical model able to describe the occurrence of localized magnetism in disordered systems such as A _x B_1–x Ac, where A and B are two transition elements and Ac is an actinide impurity. The disordered host is treated within the coherent potential approximation (CPA); thed level of the actinide impurity creates a Slater-Koster-liked scattering potential and thef states of the actinide impurity are broadened by thed–f hybridization. The occurrence of magnetic-nonmagnetic transitions as a function ofx is discussed within the Hartree-Fock approximation. In the limiting casesx=0 andx=1, this model improves the previous theories for the magnetism of an actinide impurity in a pure transition host and in the general case it permits some predictions, especially for the system Pd _x Th_1–x Pu.Part of this work is included in the thesis to be submitted by J.R.I.-S. to the Faculté des Sciences d'Orsay.     

DO22 to L12 transition in intermetallic systems

Ternary additions of metals such as chromium, manganese, iron, cobalt, nickel, copper and zinc to tetragonal (DO₂₂) Al₃Ti are known to lead to stabilization of the cubic (L1₂) structure. This DO₂₂ to L1₂ transition has been studied in the Al-Ti-Ni system using X-ray diffraction and scanning electron microscopy. The results show that nickel substitution has no significant effect on the lattice parameter (and therefore on the tetragonality) of the DO₂₂ phase and that the solid solubility of nickel in the DO₂₂ phase is very limited. The L1₂ phase precipitates out on addition of nickel to Al₃Ti, its amount increasing with increasing nickel content of the alloy. The compositions of the DO₂₂ and L1₂ phases do not change significantly with the alloy composition. These results are discussed in terms of theories of structural transitions in ordered alloys. Similar transitions have been reported in transition metal-based systems. An analysis of the transition in intermetallic systems is presented.     

Discussion of a model for the prewetting transition of liquid helium on a disordered substrate

We discuss the prewetting transition on a disordered surface by mapping the system onto the random-field Ising model (RFIM). It is argued that the nonequilibrium properties of this model can be employed to interpret experiments with liquid helium on weak-binding alkali metal substrates. According to results of numerical simulations of the RFIM, prewetting transitions should appear as continuous and hysteretic changes of the coverage. The variation of these rounded transitions with the disorder strength is discussed, and a comparison with experimental prewetting transitions of liquid helium on alkali metal surfaces is presented.     

Quantum oscillations of the superconducting transition temperature in Al-SiO systems

We report measurements on the superconducting transition temperature T _cand the normal resistance R _nof quench-condensed pure and granular aluminum films. The changes of the superconducting transition temperature are characteristically dependent on the coating film thickness and are inversely dependent on the Al film thickness. The observed oscillations of the transition temperature are interpreted in the framework of the theory of quantum oscillations developed by Kagan and Dubovskii.This work was supported in part by the Deutsche Forschungsgemeinschaft.     

Observation of superconducting transition temperature oscillations in metal-nonmetal film systems

For technetium-carbon, molybdenum-carbon, and vanadium-carbon thin-film layer structures an oscillatory dependence of the superconducting transition temperature on carbon layer thickness is found. The oscillations take place for thin metal layers (d<300 Å) and for very small thicknesses of the nonmetal film (d _C<30 Å). In addition to the oscillations ofT _c , oscillations of the normal-state electrical resistance remaining up to room temperature are found in the layered samples. A correlation is observed between critical temperature and resistance changes: Maxima in the curveT _c (d _C) correspond to minima in (d _C). The amplitude of the critical temperature oscillations is 10–15% of theT _c value and depends on the metal layer thickness, increasing with decreasingd approximately as 1/d. Also found in the thin-film layer structures are oscillations of the critical magnetic fieldsH _c andH _c correlating withT _c oscillations. A mutual influence of nonmetal layers located on opposite sides of the metal film was observed.     

Analysis of developed transition road safety barrier systems

Road safety barriers protect vehicles from roadside hazards by redirecting errant vehicles in a safe manner as well as providing high levels of safety during and after impact. This paper focused on transition safety barrier systems which were located at the point of attachment between a bridge and roadside barriers. The aim of this study was to provide an overview of the behavior of transition systems located at upstream bridge rail with different designs and performance levels. Design factors such as occupant risk and vehicle trajectory for different systems were collected and compared. To achieve this aim a comprehensive database was developed using previous studies. The comparison showed that Test 3–21, which is conducted by impacting a pickup truck with speed of 100 km/h and angle of 25° to transition system, was the most severe test. Occupant impact velocity and ridedown acceleration for heavy vehicles were lower than the amounts for passenger cars and pickup trucks, and in most cases higher occupant lateral impact ridedown acceleration was observed on vehicles subjected to higher levels of damage. The best transition system was selected to give optimum performance which reduced occupant risk factors using the similar crashes in accordance with Test 3–21.