Nite Temperature Research Articles

Chiral Disorder and Random Matrix Theory with Magnetism

jeHj. Weak magnetism corresponds to jeHj 1= 2 with 1=3 fm the vacuum instanton size, while strong magnetism the reverse. Asymptotics (ultra-strong magnetism) is in the realm of perturbative QCD. We analyze weak magnetism using the concept of the quark return probability in the diusive regime of chiral disorder. The result is in agreement with expectations from chiral perturbation theory. We analyze strong and ultra-strong magnetism in the ergodic regime using random matrix theory including the eects of nite temperature. The strong magnetism results are in agreement with the currently reported lattice data in the presence of a small shift of the Polyakov line. The ultra-strong magnetism results are consistent with expectations from perturbative QCD. We suggest a chiral random matrix eective action with matter and magnetism to analyze the QCD phase diagram near the critical points under the inuence of magnetism.

Boson Fields in Ordered Magnets

Spin wave theory of magnetism reveals two severe shortcomings. First, it considers non quantized classical spins and, second, the predicted temperature power functions for the thermal decrease of the magnetic order parameter hold asymptotically at T = 0 only. As experiments unambiguously show the dynamics is di erent for magnets with integer and half-integer spin and the critical power functions at T = 0 of type ∆M ≈ T e or at T = Tc of type ≈ (Tc − T ) hold over a nite temperature range, independent of spin structure. The nite critical range unequivocally indicates that the dynamics of the spins is controlled by a eld of freely propagating bosons instead by exchange interactions. Consequently, eld theories are necessary for description of the thermodynamics of ordered magnets. The experimental indications will be discussed that the eld quanta are essentially magnetic dipole radiation emitted by the precessing magnetic moments. Since integer and half-integer spins precess di erently the generated eld quanta and the dynamics of the eld are correspondingly di erent.

Tensor Effects in Dipole Excitations at Finite Temperature

Tensor e ects in the dipole excitation of neutron-rich Ni nucleus are investigated in the framework of Skyrme Hartree Fock plus random phase approximation at nite temperature. We calculate isovector giant and pygmy dipole strengths with nite temperature random phase approximation by using di erent tensor correlations. The e ect of both tensor and nite temperature on the giant dipole resonance pygmy dipole resonance energy region is analysed. Pygmy dipole resonance calculations with di erent proton neutron tensor coupling constants are also compared with the experimental results.

On the BCS-BEC Crossover in the 2D Asymmetric Attractive Hubbard Model

We analyze the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs in the 2D asymmetric attractive Hubbard model, in the presence of the Zeeman magnetic eld (h). The broken symmetry Hartree approximation is used. We also apply the Kosterlitz Thouless scenario to determine the phase coherence temperatures. We obtain that for the spin dependent hopping integrals (t↑ = t↓) the homogeneous polarized super uid (SCM ) phase in the ground state for the strong attraction and lower lling can be stabilized. We nd a topological quantum phase transition (the Lifshitz type) from the unpolarized super uid phase (SC0) to SCM and tricritical point in the (h−μ) and spin polarization (P ) vs. attraction (U < 0) ground state phase diagrams. The nite temperatures phase diagrams for t↑ = t↓ are constructed.

Effects of Frustrating Hopping on Charge Ordered States in Itinerant Fermion Systems for Arbitrary Concentration in 2D Lattice

There is ongoing, intense, research in the eld of electron charge orderings (CO) and charge density waves phenomena, due to experimental discovery of such phases in numerous important compounds. The aim of this work is to extend recent advances in the eld by studying two simple e ective paradigmatic models used to describe CO in narrow band materials i.e. (i) a model of correlated electrons: the so-called t W model of spinless fermions with repulsive interaction W and (ii) the molecular crystal model with the coupling of electrons to intramolecular (crystal eld) vibrations in the static limit. The nite temperature phase diagrams are evaluated at arbitrary carriers concentration for several representative cases. Our calculations are performed within the (broken symmetry) HFA for d = 2 square lattice and arbitrary carriers concentration. In this contribution we focus on the e ects of next-nearest-neighbor hopping on the CO states in these systems and the problem of phase separations involving checkerboard CO with the nesting vector Q = (π, π). The results we show here are an extension of our previous work on the subject.

Thermolysis in the BaO2-NaClO4 system

The mutual influence of barium peroxide and sodium perchlorate on thermolysis of the (2 ⎯ x)BaO2-xNaClO4 system (x = 0-2.0) has been studied. It has been demonstrated that the use of the physicochemical mechanism of this influence makes it possible to carry out heterogeneous combustion in complex oxide systems in the controlled regime by changing the mutual concentration of solid inner reaction oxidizers. An explanation of the thermal and kinetic effects in the thermolysis of the peroxide-per� chlorate system has been suggested. Alkali metal chlorates and perchlorates decompose on heating to evolve oxygen. This makes them suitable for using as oxygen sources in oxidation reactions, including the selfpropagating hightemperature syn� thesis (SHS) of complex oxides, which requires the presence of a fuel and oxidizer in the reaction system (1, 2). In the SHS of complex oxide materials, solid inner reaction oxidizers, such as sodium perchlorate NaClO 4 , can be used as an alternative or complement to gaseous oxygen. Sodium perchlorate contains more than 50 wt % of oxygen, which is completely evolved at relatively low temperatures (3). However, in the com� bustion reaction, the time interval of the vigorous oxy� gen evolution from the solid oxidizer may not coincide with the intense exothermic oxidation reaction (com� bustion) of the metal component. If this is the case, the chemical reaction rate significantly decreases, which leads to incomplete reaction of the components and even to combustion failure. The kinetic parame� ters of sodium perchlorate thermolysis can be changed by both external physical actions (for example, electric field (4)) and various chemical additives. As is known, small additions (up to 4 wt %) of some metal oxides catalyze the thermolysis of potassium perchlorate and sodium chlorate (5). Inasmuch as metal oxides are components of reaction mixtures for the SHS of complex oxide materials, if the character of the influence of some oxide on perchlorate is known, it is possible to find the bath composition at which the process will be controllable and will proceed at defi� nite rates and temperatures. In SHS processes, alkali metal and alkaline earth peroxides are often used as oxide components. For example, barium peroxide BaO 2 is used in the synthesis of hightemperature superconducting ceramics (HTSC—YBa2Cu3O 7- x), barium hexaferrite (BaFe12O19), barium stannate (BaSnO 3 ), etc. In these systems, the fuel is a powder of a corresponding metal: iron, copper, and tin. Thus, to determine optimal component ratios when sodium perchlorate is used as a solid inner reaction oxidizer, it is necessary to know the mechanism of the influence of superstoichiometric NaClO4 , as a chemical addi� tive, on the synthesis in these systems. To determine this mechanism, we considered the mutual influence of NaClO4 and BaO2 on thermolysis of these com� pounds in the reaction mixture.

The Bogolubov representation of the polaron model and its completely integrable RPA-approximation

The polaron model in ionic crystal is studied in the Bogolubov representation using a special RPA-approximation. A new exactly solvable approximated polaron model is derived and described in detail. Its free energy at nite temperature is calculated analytically. The polaron free energy in the constant magnetic eld at nite temperature is also discussed. Based on the structure of the Bogolubov unitary transformed polaron Hamiltonian there is stated a very important new result: the full polaron model is exactly solvable.

The emptiness formation probability correlation in homogeneous and dimerized XX chains

We review some known properties of the iemptiness formation probabilityi correlation which we have calculated numerically for spin-1/2 XX chains with constant (homogeneous) or alternating (dimerized) nearestneighbor coupling and an external eld (in z direction) for arbitrary temperature. The long-distance asymptotic behavior of this correlation is known to be Gaussian at zero temperature and exponential at nite temperature for the homogeneous chain. By simple analytical arguments the exponential behavior at nite temperature extends to the dimerized system. Numerical results for the dimerized chain conr m the exponential decay at nite temperature and show Gaussian decay at zero temperature.

Consequences of lattice imperfections and interchain couplings for the critical properties of spin-1/2 chain compounds

To allow for a comparison of theoretical predictions for spin chains with experimental data, it is often important to take impurity effects as well as interchain couplings into account. Here we present the eld theory for nite spin chains at nite temperature and calculate experimentally measurable quantities like susceptibilities and nuclear magnetic resonance spectra. For the interchain couplings we concentrate on geometries relevant for cuprate spin chains like Sr2CuO3 and SrCuO2. The eld theoretical results are compared to experimental as well as numerical data obtained by the density matrix renormalization group.

Quantum to classical crossover in the 2D easy-plane XXZ model

Ground-state and thermodynamic properties of the spin-1/2 two-dimensional easy-plane XXZ model are investigated by both a Green's-function approach and by Lanczos diagonalization on lattices with up to 36 sites. We calculate the spatial and temperature dependences of various spin correlation functions, as well as the wave-vector dependence of the spin susceptibility for all anisotropy parameters delta. In the easy plane ferromagnetic region (-1 < delta < 0), the longitudinal correlators of spins at distance r change sign at a nite temperature T0 (delta, r). This transition, observed in the 2D case for the first time, can be interpreted as a quantum to classical crossover.

Some Exact Results of Hubbard Model at Finite Temperature

Two theorems of the Hubbard model at nite temperature are proven employing the fluctuation–dissipation theorem and particle–hole transform. The main conclusion states that for the prototype Hubbard model, the expectation value of [Formula: see text] will be of order Nλ at any temperature except those critical.

Forced harmonic oscillator with damping and thermal effects.

Nonperturbative quantum-mechanical solutions of the forced harmonic oscillator with radiation-reaction damping are obtained from a previous analysis based on stochastic electrodynamics. The transition to excited states is shown to be to coherent states which follows the classical trajectory. The quantum Wigner distribution in phase space is constructed. All the results are extended to fi- nite temperatures.