Reduced s-d model with antiferromagnetically coupled impurity spins

An antiferromagnetic equivalent-neighbour Heisenberg interaction H_i between impurity spins is added to the reduced s-d Hamiltonian H_r previously introduced by simplifying the Kondo s-d exchange Hamiltonian H_K. Asymptotic mean-field theory is developed for H_r + H_i, in the presence and absence of external magnetic field, and applied to (La_1−xCe_x)Al₂ alloys. Specific heat c_i(T) and zero-field susceptibility χ_i(0,T) curves for (La_1−xCe_x)Al₂ are depicted. The coupling constants of H_r + H_i and conduction bandwidth are adjusted so that T_c temperatures for x = 0.2, 0.1 are equal to the experimental values. c_i(T) exhibits a jump at T_c and is decreasing for T < T_c. χ_i(0,T) has a first order pole at T_c which corresponds to the maximum of experimental susceptibility and χ_i(0,0) > 0. These results improve those obtained earlier on the grounds of H_r theory.     

Chiral symmtry of Wilson fermions at finite temperature

Chiral properties of Wilson fermions at finite temperature are investigated by numerical methods. We show that when the quark mass is properly defined, for a given β and K (hopping parameter), its value is independent of whether the system is in the high-temperature phase or in the low-temperature phase. The temperature dependence of the pion screening mass at the chiral limit where the quark mass vanishes is consistent with the physical picture that the spontaneously broken chiral symmetry is recovered at T = T_c.     

Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Ising spin glass on a D = 3 hierarchical lattice: Effects of tailed coupling distributions

Within a real-space renormalization-group framework, we approach the cubic lattice through a D = 3 diamond-like hierarchical lattice. The model is a standard, nearest-neighbor, Ising spin glass with coupling constants {J_ij} distributed according to the family of continuous probability distributions P_q(J_ij) ∝ 1/[1 + (q − 1)J_ij²/2J²]^{1/(q − 1)} (if 1 + (q − 1) J_ij²/2J² > 0, and zero otherwise; q ). Such distributions, which arise naturally in the treatment, within the recently proposed nonextensive thermostatistics, of anomalous diffusion, reproduce the usual, Gaussian case, for q → 1. Moreover, they present a second moment J_ij² proportional to (5 − 3q)⁻¹ for q < 5/3, diverging for q ≥ 5/3, but keeping a finite width at midheight. In the limit q → 3, P_q(J_ij) collapses with the abscissa, and so the width at midheight diverges. We compute the q-dependence of the spin-glass critical temperature T_c. We show numerically that T_c does not scale with J_ij^21/2 (contrary to the usual belief), but rather with the width at midheight of P_q(J_ij). Our results suggest that T_c vanishes as −1/q when q → −∞; furthermore, we verified that T_c diverges exponentially when q approaches 3 from below.     

Raman active collective mode in underdoped cuprates

We report experimental observation of Bardasis–Schrieffer-type s-symmetry collective mode in the d-wave high T_c superconductors. The mode persists above T_c in the pseudogap phase of the underdoped superconductors — direct spectroscopic evidence of the correlations in the particle–particle channel above T_c.     

The Hadron to quark-gluon transition in relativistic heavy-ion collisions

In this paper we construct a scenario for the QCD transition from the hadron phase to the quark/gluon phase using physical models for these phases. The hadron phase is modeled by a spectrum of hadrons with masses which drop (with a common scaling factor) towards zero at chiral symmetry restoration. The number of hadronic effective degrees of freedom is limited by the number of microscopic degrees of freedom in the quark/gluon phase. This limitation can be imposed either by fiat or through the introduction of a temperature-dependent excluded volume. Given that the number of degrees of freedom in hadrons and in quarks and gluons are roughly equal, the QCD phase transition is inhibited by the bag constant. The only phase transition seen in lattice-gauge calculations, once low-mass quarks are included, is the restoration of chiral symmetry which occurs at the relatively low temperature of ˜ 150 MeV. At present, lattice gauge calculations do not have the resolution to determine the properties of the higher hadronic states accurately. They do, however, demonstrate that chiral restoration takes place in the (ρ. a₁), ( +)), ( −)) and (π, σ) systems by yielding “screening masses” for chiral partners which are distinct for T < T _xSR and identical for T>T _xSR. Further, within numerical accuracy, these “screening masses” are consistent with pure thermal energies and show no evidence of remaining bare masses once chiral symmetry is restored. These, and other lattice-gauge results, will be discussed in the light of our scenario. We shall also consider the consequences of our picture for relativistic heavy-ion experiments.     

Influence of high magnetic fields on the low temperature heat capacity of the itinerant electron ferromagnet Sc3In

The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at T_c becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above T_c the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields ( ≈ 5 T). Our results show that the spin fluctuations in Sc₃In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, T_s, of this itinerant electron ferromagnet is about the same as T_c.     

The static and dynamic lattice effects in La1−xCaxMnO3

The structural and magnetic behavior of the perovskite insulator La_0.9Ca_0.1MnO₃ were studied as a function of temperature from 15 to 300 K by neutron powder diffraction. Although this compound shows an anomalous response of the lattice parameters around T_c (150 K), the behavior of the oxygen/manganese Debye-Waller factors is in clear contrast to its “colossal magnetoresistance” (CMR) counterpart La_0.65Ca_0.35MnO₃. We speculate that the difference is intimately associated with the metal-insulator transition in the latter compound.     

Planar spin system with annealed bond disorder

This is a study of the equilibrium statistical mechanics of a mixed system of planar spins and Ising spins. The Ising spins σ_r are situated (one on each of the bonds connecting the planar spins s_r so that the planar spins interact only via Ising spins. For a one-dimensional lattice the partition function and the two-spin correlation functions (σ_rσ_r¹ and S_r·S_r¹) are calculated ex actly in terms of modified Bessel functions.

For a two-dimensional anisotropic generalization of the model an equation is given for the critical temperature and the equation is shown to have one and only one solution T_c such that 0<T_c<∞.     

Reaching the unreachable: critical scaling at the superconducting transition

The large coherence length of conventional superconductors causes the critical region near T_c to be unobservably small, leading to the impression that mean-field theory can adequately explain properties near the transition. The high-temperature superconductors, however, have such strongly type-II character that the critical, or at least an intermediate critical, region is experimentally accessible. We demonstrate the applicability of XY-like critical scaling to magnetization, heat capacity and conductivity data and through the fitting parameters, make contact with a strong-coupling calculation of the critical temperature. Further, the scaled conductivity data show clear evidence for a thermodynamic transition along the freezing line H_m(T)∝(1 − T/T_c)^4/3.     

Current–phase relationship measurements in the flow of superfluid He through a single orifice

We report accurate measurements of the current-phase relationship in the flow of superfluid ³He through a single orifice. While ideal sinusoidal 2π-periodic Josephson behavior prevails close to T_c, increasingly strong π-periodic admixture is usually observed at lower temperatures.     

Tracking the formation of 2223 BiSrCaCu oxygen compounds by resistivity and susceptibility measurements reaching Tc(0)=115 K

Unsubstituted and lead-antimony substituted 2223 BiSrCaCu Oxygen compounds have been synthesized and compared by resistivity R(T) and susceptibility χ(T) measurements and chemical analysis. The R(T) and χ(T) measurements show that interface effects occur which are reflected in the R(T) characteristics. The chemical analysis of samples with a resistance zero T_c(0)=114 K and a diamagnetic onset at 115 K yields a severe loss of lead from nominally 40 at% down to about 3 at%, whereas the T_c(0)=110 K samples contain up to about 30 at% of lead. It is assumed that for the formation of the 115 K phase lead oxide acts as a catalyst not being incorporated to a considerable amount into the lattice.     

A lattice calculation of thermal dilepton rates

Using clover improved Wilson fermions we calculate thermal vector meson correlation functions above the deconfinement phase transition of quenched QCD. At temperatures 1.5 T_c and 3T_c they are found to differ by less than 15% from that of a freely propagating quark–anti-quark pair. This puts severe constraints on the dilepton production rate and in particular rules out a strong divergence of the dilepton rate at low energies. The vector spectral function, which has been reconstructed using the Maximum Entropy Method, yields an enhancement of the dilepton rate over the Born rate of at most a factor two in the energy interval 4ω/T8 and suggests that the spectrum is cut-off at low energies by a thermal mass threshold of about (2–3)T.     

The impact of nuclear magnetism on superconductivity

The influence of nuclear magnetism on the superconducting critical field B_c has been measured in the nuclear magnets Al, AuIn₂, and Sn, at ultralow temperatures, 17 μKT1 K. The materials have been chosen in respect to their distinctive Korringa constants κ(Al, AuIn₂, Sn)=(1.8, 0.1, 0.05) Ks. Both in the weakly and in the strongly coupled nuclear magnets, Al and Sn, a reduction of B_c is observed being proportional to the nuclear magnetization at T4 mK. However, using the high resolution of the experiment performed on Sn, the decrease of B_c at cooling down to 70 μK is found to be nonmonotonic. The features of the recently measured B_c(T) curve of Sn which are not understood yet as well as the formerly observed results of the interplay of nuclear ferromagnetism and superconductivity in AuIn₂ have motivated ongoing experiments which try to clear up the pair breaking contribution of nuclear magnetism.     

On the Curie temperature of a spin one XY ferromagnet with uniaxial anisotropy

A spin one XY ferromagnet with uniaxial anisotropy has been investigated, using Green's function technique in random phase approximation (RPA). The Green functions associated with the anisotropy energy are treated without decoupling. A set of coupled equations have been obtained to find the critical temperature T_c and (S^Z)² at T_c as function of the uniaxial anisotropy parameter D. T_c and (S^Z)² at T_c are found to increase with D. The results are compared with the earlier results obtained in the Narath type of RPA.     

Inhomogeneous Tl NMR line broadening in Tl2Ba2CuOx single crystals

Temperature and angular dependences of ²⁰⁵Tl line width, Δ , and transverse relaxation time, τ, have been measured in two crystals of Tl₂Ba₂CuO_x with T_c = 32 and 115 K, respectively. In the normal state both Δ and τ have been found to be temperature-independent and not to vary with varying T_c, implying that the size of inhomogeneous broadening is also invariable. Analysis of the line width angular dependence shows that the inhomogeneous broadening originates, most probably, from local distortions associated with Cu substitution for Tl. A conclusion is that variations in T_c are not related to this substitution.     

The SU(3) deconfining transition in the presence of quarks

We perform a Monte Carlo simulation of the full theory of QCD at finite temperature. The effect of closed quark loops is taken in account by employing the pseudo-fermion method. Our preliminary data, show that for three flavors the scenario given by the pure gauge theory is unchanged: the theory still undergoes a first order phase transition at the critical coupling β_c = 6/g_c² = 5.2 ± 0.1 (T_c/Λ_L = 171 ± 28). Increasing the number of flavors we observe stronger ordering effects and the critical coupling decreases.     

The contribution of Bc mesons to the search for B → τ ντ decays at LEP

We study the contribution of B_c mesons to the search for B → τν_τ decays. We find that at LEP the contributions from B_u and B_c mesons can be comparable. This observation can have a relevant impact on the extraction of constraints on new physics (such as charged-Higgs contributions) from current LEP limits on B → τν final states. Inclusion of the B_c contribution can reduce the current L3 limit on tan β/M_H from 0.38 GeV⁻¹ (90% CL) down to 0.27 GeV⁻¹ (90% CL).     

Influence of magnetization on lattice constants of 3d transition metal alloys

A simple relation is found in 3d transitional metal alloys between the lattice constant and the magnetization, which can be described with an equation: a(x) = a₀^A· (1 − x) + a₀^B. x + C μ(x). It is proposed that studies of lattice constants at high temperatures may serve as an experimental method to detect the existence of localized moments above T_c. The anomalous thermal expansion of the Invar alloy is explained as a result of the collapse of localized moments above T_c.     

Correlations in one-dimensional quantum impurity problems with an external field

We study response functions of integrable quantum impurity problems with an external field at T = 0 using non-perturbative techniques derived from the Bethe ansatz. We develop the first steps of the theory of excitations over the new, field-dependent ground state, leading to renormalized (or “dressed”) form factors. We obtain exactly the low-frequency behaviour of the dynamical susceptibility χ″ (ω) in the double-well problem of dissipative quantum mechanics (or equivalently the anisotropic Kondo problem), and the low-frequency behaviour of the AC noise S_t(ω) for tunnelling between edges in fractional quantum Hall devices. We also obtain exactly the structure of singularities in χ″ (ω) and S_t (ω). Our results differ significantly from previous perturbative approaches.