Kinetic theory of adsorption and desorption

In this paper we use stochastic methods to discuss adsorption and desorption. The paper derives generalized coefficients of sticking and accomodation depending on surface temperatureT _s and gas temperatureT _g and shows, that for additive Markov processes, these kinetic coefficients are identical. Furthermore, exact solutions of the kinetic equations for certain simple transition probabilitiesP(, ) are found and an approximation method for more complicatedP(, ) is given. The comparison of the theory with experimental results for noble gas-metal systems indicate a quadratic relationship between the first moment of the transition probabilityP(, ) and the well depth of the physisorption system.     

Theory for superconductivity in amorphous transition metals

A theory is presented for superconductivity in amorphous transition metals. It is shown that in contrast to simple metals for transition metals the changes in the phonon spectrum, in the electronic density of states and in the electronic matrix elements which result from strong lattice disorder can enhance as well as decreaseT _c. The numerical results for the superconducting transition temperatureT _c of amorphous 4d-and 5d-transition metals agree well with the experimental results.     

Time dependent Ginzburg-Landau equations for strong coupling superconductors

We derive time dependent Ginzburg-Landau equations for strong coupling superconductors. It is shown that due to a certain separability of the order parameter the equation for it’s time dependent fluctuations is again of diffusion type. Strong coupling effects show up only in the numerical coefficients of the diffusion equation. We apply our findings to the problem of electrical resistivity in strong coupling superconducting materials above the transition temperatureT _c.     

The investigation of magnetite microcrystals

Four samples of Fe₃O₄ microcrystals with average particle size of 72, 88, 100, 100Å have been investigated by Mössbauer spectroscopy, accompanied with x-ray diffraction and transmission electron microscopy. The experimental results show that the anisotropy energy constantK increases as the particle size decreases. It is found that a sudden drop ofK exists around Verwey transition temperatureT _v andK remains constant in temperature well aboveT _v . The Verwey temperatureT _v for Fe₃O₄ microcrystals with different particle size is almost the same as that of bulk magnetite.     

Magnetic phase diagram of UCoAl

Temperature T and pressure P dependences of the Landau coefficients for UCoAl are discussed by taking into account the magneto-volume effects and anisotropic spin fluctuations based on the Landau–Ginzburg theory. The P-dependence of the mean square amplitude of spin fluctuations is also discussed. A magnetic phase diagram is obtained with the parameters estimated from the observed magnetization curves under high pressures. The P-dependences of the characteristic temperatures T₀ and T_max, where the metamagnetic transition disappears and the susceptibility reaches a maximum, are estimated and compared with the observed results.     

Effect of hadronic hard-core radius on the phase transition to an interacting quark-gluon plasma

The effect of volume corrections on the equations of state for the hadron gas by treating nucleons and antinucleons as hard-core particles or bags is studied. Its consequences on the critical values of temperatureT and chemical potential μ _B of the phase transition from a gas of finite sized hadrons to an interacting quark matter are explored.     

Effects of low temperature irradiation with heavy ions on superconductor niobium

To study the effects of heavy ion irradiation at low temperature on type II superconductor Nb, the transition temperatureT _c , the normal state residual resistivityρ _B , the transition widthΔT _ph using oxygen ions of 25 MeV and subsequent thermal annealing were measured. The samples were held at temperatures <20 K during irradiation in a cryostat for in situ measurements. The maximum oxygen fluence was about 2·10¹⁵ cm^?2 corresponding a relatively high defect concentration. The heavy ion irradiation experiments are described. The critical temperatureT _c decreases with increasing residual resistivityρ _B . In agreement with the theory and experiments, the gap anisotropy parameter is 〈a ²〉=0.008, subsequent annealing shows a hysteresis ofT _c versusρ _B . The resistivity saturation value Δρ_BS = 2.55 μΩ cm was obtained and different recovery stages were found. Significant broadening of transition width during irradiation was observed.T _c andΔT _ph anneal to 60% in the temperature interval of (60–90) K. Oxygen induced effects as a simulation method of high neutron damage are compared with irradiation measurements using neutrons and deuterons.     

Mean-field theory for interacting rotons in superfluid helium

It is shown within the framework of mean-field theory that the gas of roton excitations in superfluid helium becomes unstable at a critical temperatureT _c if the roton-roton interaction is attractive. The instability is characterized by a singularity of the specific heat and signals the transition into the normal fluid state. The roton energy gap remains finite atT _c .     

Der Einfluß von Spinfluktuationen auf die Sprungtemperatur eines Zweibandsupraleiters

The superconducting transition temperatureT _c of a two band superconductor in the presence of magnetic and nonmagnetic impurities as well as exchange fields is calculated using a new (8 × 8)-matrix formalism for the electron Green's function. In particular we investigate the influence of spin correlations onT _c near a magnetic phase transition. It is shown that in the strong interband phonon coupling limit the system behaves essentially as a one band superconductor. In the weak coupling limit we find typical deviations from one band theories.     

Nonlinearity in cooperative systems-dynamical Bethe-Ising model

The dynamics of the short-range order as well as the long-range order in the nonlinear cooperative system is investigated specifically for a kinetic Ising model in the Bethe approximation. The phenomena of critical slowing down near the transition temperatureT _c and anomalous fluctuation belowT _c are directly related to the instability of the long-range order. The dynamics of the short-range order is essentially a fast mode and is noncritical. However, through the nonlinear coupling the short-range order is also influenced by the critical behavior of the long-range order.     

On heat diffusion mode of structural phase transition by two-fluid model

With the help of the two-fluid model developed by Götze and Michel for phonons it is shown for a simple model Hamiltonian that in the low temperature phase the optical soft mode becomes isothermal, the heat diffusion mode is dominant near the transition temperatureT _c and the quasiparticle interaction is of great importance in determining the thermodynamic quantities nearT _c. Green function techniques are applied to describe the two-fluid model functions in a microscopic way. The simplest approximations are discussed for the model equations describing nonequilibrium phenomena of the soft optical phonon mode in the low temperature phase. The quasiparticle interaction operator can be related to the interaction operator between quasiparticles and the condensed mode. This relation enables one to understand the behaviour of the thermodynamic quantities near the transition temperature on a microscopic way. The first order displacive phase transition is also discussed.     

Exact quantum partition function of the BCS model

The exact calculation of the reduced BCS model quantum partition function (QPF) in the thermodynamic limit is carried out by the path integration method. The expression for the QPF and the phase transition temperatureT _c in the regular phase coincide with the results of Bogolyubov. In the nonregular phase a temperature singularity appears in the expression for the QPF: the QPF diverges in the region of temperaturesT _c which are smaller than some critical temperatureT _c ^* , and it turns out that in all casesT _c ^* > T _c and the differenceT _c ^* –T _c is not small. The interpretation of the temperatureT _c ^* is given.     

Pressure dependence of the Kondo resistance anomaly and the pair breaking effect in La-Ce alloys

The pressure dependence of the parr breaking effect and of the resistance anomaly was measured in LaCe alloys. The results indicate that the maximum in the pressure dependent pair breaking effect is due to a monotonic shift of the Kondo temperatureT _k with pressure from valuesT _k > ?T _c0 toT _k ?T _c0, whereT _c0 is the superconducting transition temperature of pure lanthanum.     

On the roughening transition in the Potts model (q=2, 3) in two and three dimensions

Roughening-respectively depinning-transitions of interfaces between regions of different phases are considered in theq-state Potts model (q=2, 3) in two and three dimensions. For two dimensions the depinning temperatureT _R is obtained from transfer matrix calculations. In three dimensions we have used a low temperature expansion in a solid-on-solid type approximation to determine the roughening temperatureT _R and the critical exponents for some moments of the interface profile nearT _R .     

What can we learn from the finite temperature renormalization group?

The renormalization group for finite temperature quantum field theories is studied, in particular for λ?⁴. It is shown that the “high” temperature limit can only be discussed perturbatively ifT dependent renormalization schemes are implemented. Zero temperature renormalization schemes or renormalization at some fixed reference temperatureT _o are both inadequate as they imply perturbative expansions about fixed points of the renormalization group which are associated with a zero temperature system and a system at temperatureT _o respectively.T dependent schemes give rise to an expansion about the true fixed point of the system, the resulting renormalization group allows the entire crossover between high and low temperature behaviour to be investigated.     

Pseudo-discontinuities in the isomer shift near magnetic transitions

It is shown that a relatively small non-linear behavior of the velocity drive system used in Mössbauer effect studies yields pseudo-discontinuities in the isomer shift near the critical temperatureT _c of a magnetic-paramagnetic transition. The anomaly of the⁵⁷Fe isomer shift in iron is attributed to this effect.     

Pion condensation in heavy ion collisions

We calculate the pion spectrum in dense nuclear matter for finite temperatures. The critical temperatureT _c(ρ) that marks the beginning of a second order phase transition due to pion condensation is given in a phase diagram. We show that in heavy ion collisions, pion condensation should occur, leading to an enhancement in the formation of nuclear shock waves.     

Theory for superconductivity in Pd-H and Pd-D systems

A theory is presented explaining the recently observed superconductivity at 9 °K in Pd-H and at 11 °K in Pd-D as due to the quenching of the spin-fluctuations and as due to the decrease of the Coulomb pseudo-potential μ present in Pd. That one observes for PdD a larger superconducting transition temperatureT _c than for PdH is explained by the larger lattice expansion resulting for adding H to Pd. The observed non-monotonic dependence ofT _c on the Hydrogen concentration is explained by the fact that approximately λ ∝N(0) for smallN(0), whereN(0) is the electronic density of states at the Fermi-energy and where λ denotes the electron-phonon coupling constant. Superconducting transition temperatures are estimated for PtH and RhH.     

Limiting temperature ofπ andK 0 inpp and\bar pp collisions

The limiting temperatureT ₀ of secondaries from \(pp/\bar pp\) collisions is estimated on the basis of the Lorentz contraction factor, to be compared with the critical temperatureT _c determined by the specific heat. It is found, in terms of the critical exponent law,T ₀≈T _c ≈215 MeV for π^? andK ⁰.