Low-temperature Thermodynamic Properties Research Articles

Low-Temperature Properties of the Randomly Depleted Heisenberg Ladder

Low-temperature thermodynamic properties of the spin-1/2 Heisenberg ladder system with non-magnetic impurities are discussed using an effective Hamiltonian for the impurity-induced spins in the background of a spin liquid with a gap. It is shown that the uniform susceptibility shows Curie-like behavior in high-, intermediate-, and low-temperature regimes, with three different Curie constants. The ratio of specific heat to temperature, C/T, is divergent in the low-temperature limit, reflecting a singular distribution of the effective couplings among the impurity spins.

Low temperature thermodynamic and magnetic properties of fermion-like fractional statistics

The low-temperature properties of the internal energy, the specific heat and the Pauli susceptibility for an ideal gas of particles obeying to a fermion-like fractional statistics are considered. The Sommerfeld expansions of this quantities are calculated in tems of the central moments of the derivative of the density of entropy with respect to the occupational number.

Slow heat release below 1 K in density-wave systems

Low-temperature thermodynamical properties of Charge (Spin)-Density Wave (CDW/SDW) systems are characterized by a glassy behaviour, somewhat reminiscent of structural glasses, the deep origin of disorder being here related to the random pinning of the electronic DW subsystem by impurities or defects. A remarquable property is the strong non-exponential energy relaxation following a small heat perturbation, atT<1 K. We report on an investigation of this effect in TaS3 [CDW], and in (TMTSF)2PF6 [SDW]. A comparison is done with the slow heat release effects observed in similarT-range in structural glasses.

Mean-field theory of the spin-Peierls state under magnetic field: Application to CuGeO3.

A mean-field theory of the spin Peierls systems based on the two dimensional dimerized Heisenberg model is proposed by introducing an alternating bond order parameter. Improvements with respect to previous mean-field results are found in the one-dimensional limit for the ground state and the gap energies. In two dimensions, the analysis of the competition between antiferromagnetic long range order and the spin-Peierls ordering is given as a function of the coupling constants. We show that the lowest energy gap to be observed does not have a singlet-triplet character in agreement with the low temperature thermodynamic properties of CuGeO3.

A self-consistent phase diagram for supercooled water

MANY of the low-temperature thermodynamic properties of water, such as heat capacity and isothermal compressibility, exhibit anomalous behaviour that tends to diverge in the supercooled state1,2. On the basis of molecular dynamics simulations3,4, these phenomena have recently been attributed to the influence of a critical point terminating the temperature–pressure coexistence line separating low- and high-density amorphous ices. But the fact that water tends to lose its anomalous behaviour5 in the pressure range predicted for the new critical point poses problems for such an interpretation. Moreover, the phase diagram derived from these simulations contrasts sharply with another conjecture, whereby it is argued that at atmospheric pressure no thermodynamically continuous path exists between low-density amorphous ice and normal water6. Here I report the results of a series of molecular dynamics simulations at constant (approximately atmospheric) pressure, which show that both ideas can be reconciled by relocating the critical point to negative pressures. The resulting phase diagram is not only simpler, but also accounts for the transition of water from a fragile to a strong liquid in the supercoooled region7,8.

A generalization of Haldane's state-counting procedure and π-deformations of statistics

We consider the generalization of Haldane's state-counting procedure to describe all possible types of exclusion statistics which are linear in the deformation parameter g. The statistics are parametrized by elements of the symmetric group of the particles in question. For several specific cases we determine the form of the distribution functions which generalizes results obtained by Wu. Using them we analyze the low-temperature behavior and thermodynamic properties of these systems and compare our results with previous studies of the thermodynamics of a gas of g-ons. Various possible physical applications of these constructions are discussed.

The pϱT properties of ethanol + hexane

The pϱT behavior of ethanol, hexane and its mixture has been investigated with the aim to cover the whole region from low pressure-low temperature thermodynamic excess properties up to the critical curve. The mixture exhibits a positive azeotrope which persists over the whole range. The positive excess volume shows a remarkable temperature dependence. Whereas the thermodynamic evaluation and the comparison to model calculations will be the subject of a future paper, all experimental results are put together in this paper.

Glassy-like thermodynamical properties of (TMTSF) 2PF 6

We report on specific heat measurements of (TMTSF) 2PF 6 between 0.1 and 25 K. In addition to the small anomaly at the SDW transition temperature, this SDW compound exhibits all characteristics of the low temperature thermodynamical properties of CDW systems. These typical features are additional excitations to regular phonons contributing to the specific heat below 0.5 K which are reminiscent of low-energy excitations in glasses and the enthalpy relaxation exhibiting aging effects such as in other disordered systems. Finally, the role of the isothermal annealing and the kinetics of measurements on the specific heat anomaly in the range 2.5–5 K are indicative of a glass transition in the general frame of freezing of supercooled liquids.

Low-temperature properties of anisotropic superconductors with kondo impurities

We present a self-consistent theory of superconductors in the presence of Kondo impurities, using large-$N$ slave-boson methods to treat the impurity dynamics. The technique is tested on the s-wave case and shown to give good results compared to other methods for $T_K > T_c$. We calculate low temperature thermodynamic and transport properties for various superconducting states, including isotropic s-wave and representative anisotropic model states with line and point nodes on the Fermi surface.

Propagating and localized vibrational modes in Ni-Zr glasses

Numerical investigations of the vibrational eigenmodes of amorphous NixZr100-x alloys are presented. Structural models are prepared by molecular dynamics simulations of the quenching processes, based on interatomic forces derived using a tight-binding-bond approach. The vibrational properties are investigated via a direct diagonalization of the dynamical matrix for N=729-atom models, and via recursion calculations of the vibrational spectral functions, partial and total dynamical structure factors and vibrational densities of states for large N=2916-atom models. The static structure of the NixZr100-x glasses is characterized by a pronounced chemical and topological short-range order (SRO). We investigate in detail the manifestation of the SRO in the partial dynamic spectral functions and structure factors SIJ(k, omega ). We discuss the possibility of measuring partial dynamic structure factors using inelastic neutron scattering and demonstrate that our results are in good agreement with the existing experimental data on the total dynamical structure factors. We show that, although most eigenmodes are extended, localized modes can be found at the upper and lower edges of the frequency spectrum. Of particular interest is the prediction of low-energy localized modes, which have a profound influence on the low-temperature thermodynamic properties.

Spin polarons in solid3He: Suggestions for further experiments

A proposal is made for a simple way to test experimentally whether anomalies in the low temperature thermodynamic properties of solid3He are due to metastable trapped vacancies in spin polarons. The idea is to perform susceptibility measurements before and after applying a strong magnetic field for some time, with the aid of a SQUID or NMR. If this magnetic annealing reduces the susceptibility, it provides evidence for the existence of metastable trapped vacancies in the original sample. With this procedure it may also be possible to clarify the issue of the existence of a “vacancy solid.” Finally, the effects of the tendency of vacancies to cluster are discussed.

Spectral and magnetic interplay in quantum spin chains: Stabilization of the critical phase due to long-range order.

The anisotropic quantum XY spin chain in a modulating magnetic field incommensurate with the periodicity of the chain is studied. Using Jordan-Wigner transformation, the model is mapped to a tight-binding model in fermions. In the absence of symmetry-breaking terms, the fermion model is identical with the well-known Harper equation exhibiting extended and localized phases with the onset of transition being a critical point with fractal spectrum and wave functions. The effect of O(2) symmetry breaking is to fatten the critical point resulting in a critical phase with power-law localization existing in a finite window of size determined by the anisotropy. In this three-phase spectral diagram, the transition from critical to localized phase is accompanied by the magnetic transition to long-range order. Furthermore, the scaling properties of the fractal spectrum vary in the critical phase. This leads to interesting consequences on the low-temperature thermodynamical properties of the system.

Low-temperature heat capacity and thermodynamic properties of four boron nitride modifications

A short review of the low-temperature heat capacity studies of four boron nitride (BN) modifications is presented. C p versus ( T) dependences of high-ordered hexagonal and disordered (turbostratic) modifications were studied by adiabatic calorimetry. The influence of disordering on the heat capacity of boron nitride is shown. The values of thermodynamic properties (heat capacity, entropy, enthalpy, and formation enthalpy) of four BN modifications is reported.

Thermodynamics of local-pair superconductors with anisotropic lattice structures.

The effective-pseudospin model of the local-electron-pair superconductor, being equivalent to a hard-core charged Bose gas on a lattice, is analyzed for the case of anisotropic lattice structures and arbitrary electron concentration with the use of self-consistent random-phase approximation. The evolution of the ground-state phase diagrams, the low-temperature thermodynamic properties, and the superconducting critical temperature (${\mathit{T}}_{\mathit{c}}$), as a function of the lattice anisotropy and electron concentration (n), are determined. We also discuss the superfluid properties within the framework of the Landau quasiparticle model. In the quasi-two-dimensional case and for low concentrations, the changeover from ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}${\mathit{n}}^{2/3}$ to ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}n is found to occur for ${\mathit{J}}_{\mathrm{\ensuremath{\perp}}}$/${\mathit{J}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}2}$, with ${\mathit{J}}_{\mathrm{\ensuremath{\parallel}}}$ and ${\mathit{J}}_{\mathrm{\ensuremath{\perp}}}$ being the intraplanar and interplanar pair hopping amplitude parameters, respectively. It turns out that the increasing lattice anisotropy (i.e., reduction of interplanar or interchain couplings) extends the range of stability of the homogeneous superconducting phase and suppresses the charge-density-wave state. The results for the ground-state superconducting order parameter are in good agreement with recent Monte Carlo and exact-diagonalization studies for the two-dimensional square lattice. The relationship between the pseudospin and bosonic descriptions of local-electron-pair superconductor in the dilute limit is also further analyzed.

Phases of URu2Si2.

The tetragonal heavy-fermion system ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ may well be an unconventional superconductor, as evidenced by its upper critical field and its low-temperature thermodynamic properties. Under the assumption that it belongs to the E representation of the ${\mathit{D}}_{4\mathit{h}}$ point group, we enumerate the phase diagrams which are possible in the H-T plane. We take into account the possible coupling of superconductivity to the antiferromagnetic order parameter.

Polar pyramidal molecules on a graphite substrate

We study the energetics of single CH 4, CH 3F, CH 3Cl, CF 3H, CF 3Cl, and CF 4 molecules physisorbed on a graphite substrate using the Lennard-Jones 6–12 and Coulomb pair potentials. The ground state orientations and positions of these molecules have been obtained. In contrast to CH 4 and CF 4, for the dipolar molecules we found a shift in the preferred adsorption site slightly off of the atop site. This shift removes the existing orientational degeneracy of the adsorption potential associated with the three-fold rotational symmetry of the graphite lattice, but introduces a new degeneracy due to the three possible directions of the shift. The softening of the potential well due to this off-center displacement should show up in the low-temperature thermodynamic properties.

Bosonic mean-field theory of quantum Heisenberg spin systems: Bose condensation and magnetic order.

We discuss the mean-field approach to the quantum Heisenberg ferromagnet and antiferromagnet on bipartite lattices using the Schwinger boson representation, with the constraints imposed on the average. The low-dimensional results of Arovas and Auerbach [Phys. Rev. B. 38, 316 (1988)] are derived simply by using a Hartree-Fock decomposition and the Peierls variational principle. We study the models below their critical temperatures in three dimensions (and at zero temperature in d=2 dimensions) by identifying magnetic ordering with Bose condensation of the Schwinger bosons. This novel interpretation enables us to compute the low-temperature (in the ordered regime) thermodynamic properties and dispersion relations, which agree with the results of spin-wave theory. We also extract critical properties that are related to those of the spherical model. A brief discussion of the limitations of the approach is also presented.

Low-temperature thermodynamic properties of amorphous sputtered Zr 100-xCux alloys. Effect of structural relaxation

Low-temperature thermodynamic properties of amorphous sputtered Zr 100-xCux alloys. Effect of structural relaxation

Erratum - Low-temperature thermodynamic properties of amorphous sputtered Zr100 - xCux alloys. Effect of structural relaxation

We report on low-temperature specific heat measurements of superconducting amorphous Zr 100-x Cu x (19≤x≤64) alloys prepared by sputtering that we compare to corresponding alloys obtained by fast liquid-quenching technique. Whereas the superconducting transition temperature T c is very close for these two kinds of amorphous alloys, indicating that it is almost insensitive to the higher degree of structural disorder inherent to sputtering, both the electronic coefficient γ and the lattice βT 3 contribution are larger for the sputtered alloys. The common character of an increasing value of γ with the Zr content is considerably enhanced for the sputtered alloys. However, such surprisingly high γ values do not lead to any anomalous behaviour for the superconductivity process, as proved by the complete electronic condensation below T c . All thermodynamic parameters are sensitive to structural relaxation, at variance with the liquid-quenched alloys, whereas the T c depression is of the same magnitude in both kinds of alloys Mesures de chaleur specifique a basse temperature d'alliages Zr 100-x Cu x (19≤x≤64) prepares par pulverisation cathodique et comparaison aux alliages correspondant prepares par ultra-trempe. On observe que le coefficient electronique γ et la contribution du reseau BT 3 sont plus eleves pour les alliages « pulverises» alors que la temperature de transition supraconductrice est tres voisine pour les deux types d'alliages

Effect of structural relaxation on the thermodynamic properties of amorphous superconducting ZrCu alloys

We report on the effect of structural relaxation on the low-temperature thermal properties of superconducting amorphous Zr 100− x Cu x alloys prepared by sputtering. Using the fact that low-temperature annealing causes changes to many physical properties depending both on temperature and on time, without change of chemical composition, we performed an extensive and systematic study of low-temperature thermodynamic and superconducting properties of sputtered amorphous Zr 100− x Cu x ( x = 19, 23, 27, 32, 38, 48 at.%) in either their “as-prepared” or annealed state. We compare our results with those for liquid-quenched ZrCu alloys for x ⩾ 25 at.%. We intend to try to connect the degree of structural disorder to the magnitude of changes, on annealing or aging, of physical properties, mainly the Debye temperature θ D , and the density of electronic states at the Fermi level N( E F ).