Static hole in a two-dimensional antiferromagnet

The energetics of static hole in a 2D antiferromagnetic (AF) is studied. AF is described by magnon operators in the LSW approximation, while the holes by fermionic operators. The energy spectrum of the magnons in the presence of the hole is determined from the perturbed Green’s function.     

Low-temperature thermodynamics of a one-dimensional interacting Fermi system

Using the exact solution of theSU(2) Thirring model, we derive an infinite set of coupled nonlinear integral equations describing the thermodynamic properties of a one-dimensional Fermi gas at a given temperature and magnetic field. The limiting case ofT=0 is considered, in which the magnetic field dependence of the ground state energy is calculated for both repulsive and attractive interactions.     

Phase transition of an itinerant-electron antiferromagnet

A weak-coupling theory of the itinerant-electron antiferromagnet forT>T _c is described, Vertex and self-energy parts are treated self-consistently using Ward-Pitaevskii identities. The weak-coupling criterion is that the scalar three-vertex must be suitably small. This condition is not satisfied right down toT _c ; however, it is satisfied in a region nearT _c where the corrections to the bare electron-electron interaction are not small. Expressions are derived for the specific heat, temperature derivative of the electrical resistivity, etc., which are all proportional to the weakly singular form[T _c /(T–T _c )] ⁿ , withh«1. An analogous theory applies for superconductors. In this case the weak-coupling exponenth is approximately equal to(T _c /E _F )². This quantity is typically 10^–8–10^–7 for superconductors, and in the weak-coupling temperature regime the lambda curve singularity would not be observable experimentally. However, for an itinerant-electron antiferromagnet such as chromium, the quasicubic shape of the electron and hole Fermi surfaces causes enhancement ofh to a value of order 10^–1. For chromium the theory is consistent with experimental results and with a recent theory by Dzyaloshinskii and Kats (using a different method).     

Transformation of the equations of thermodynamics

A simplified method is proposed and studied for deriving the equations of thermodynamics through automatic conversion (transformation) of an equation into a series of other equations through a replacement of variables on the basis of appropriate codes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 6, pp. 1092–1097, December, 1969.     

Solitons in the one-dimensional antiferromagnet in a magnetic field

The one-dimensional classical planar antiferromagnet in an applied magnetic field in the plane of the planar spin is analyzed theoretically. Solving the transfer matrix equation, we calculate the transverse and the longitudinal spin correlation lengths. The dynamical structure factors are also determined. The transverse spin fluctuations are dominated by the multisoliton term, while the longitudinal spin fluctuations are dominated by a single magnon term.This work was supported in part by the National Science Foundation under grant DMR 76-21032.Guggenheim Fellow 1979-80. On sabbatical leave from Department of Physics, University of Southern California, Los Angeles California.Laboratoire associé au CNRS.     

On the classification of the low energy excitations in the doped antiferromagnet

A long-wavelength, low energy hamiltonian is derived to describe the dynamic of a single hole in a quantum antiferromagnet in two or higher spatial dimensions. In thisexactly solvable limit anew kind of symmetry is important to classify the elementary spin excitations of a single hole in the t — J model. This symmetry is hidden at finite size or at short wavelength. The resulting classification has important consequences for understanding the physics of doped antiferromagnets. Fermi liquid like s = 1/2 and charge one excitations are still well defined in a quantum antiferromagnet, but are not a complete set to describe the low energy physics.     

Low-temperature synthesis of graphene

A new method is proposed for the low-temperature production of graphene with high yield. The method is based on the low-temperature synthesis and decomposition of graphite intercalation compounds with sodium ammoniates. The proposed technology retains high efficiency, which is typical of graphene synthesis via graphite oxidation and thermal expansion, and offers new advantages by ensuring high purity and structural perfection of the product.     

Studies of the hyperfine enhanced nuclear antiferromagnet Cs2NaHoCl6

A neutron scattering study of the hyperfine enhanced nuclear spin system Cs₂NaHoCl₆ has been carried out following adiabatic demagnetization cooling. The nuclear spin system was cooled to about 2 mK, belowT _N =4.8 mK. We did not observe any antiferromagnetic diffraction peaks in the (1 ) reciprocal lattice plane. The anisotropy of the magnetic susceptibility above and below the Néel temperature was also measured. The magnetic structure is discussed.     

Vibrational thermodynamics of materials

The literature on vibrational thermodynamics of materials is reviewed. The emphasis is on metals and alloys, especially on the progress over the last decade in understanding differences in the vibrational entropy of different alloy phases and phase transformations. Some results on carbides, nitrides, oxides, hydrides and lithium-storage materials are also covered.Principles of harmonic phonons in alloys are organized into thermodynamic models for unmixing and ordering transformations on an Ising lattice, and extended for non-harmonic potentials. Owing to the high accuracy required for the phonon frequencies, quantitative predictions of vibrational entropy with analytical models prove elusive. Accurate tools for such calculations or measurements were challenging for many years, but are more accessible today. Ab initio methods for calculating phonons in solids are summarized. The experimental techniques of calorimetry, inelastic neutron scattering, and inelastic X-ray scattering are explained with enough detail to show the issues of using these methods for investigations of vibrational thermodynamics. The explanations extend to methods of data analysis that affect the accuracy of thermodynamic information.It is sometimes possible to identify the structural and chemical origins of the differences in vibrational entropy of materials, and the number of these assessments is growing. There has been considerable progress in our understanding of the vibrational entropy of mixing in solid solutions, compound formation from pure elements, chemical unmixing of alloys, order-disorder transformations, and martensitic transformations. Systematic trends are available for some of these phase transformations, although more examples are needed, and many results are less reliable at high temperatures. Nanostructures in materials can alter sufficiently the vibrational dynamics to affect thermodynamic stability. Internal stresses in polycrystals of anisotropic materials also contribute to the heat capacity. Lanthanides and actinides show a complex interplay of vibrational, electronic, and magnetic entropy, even at low temperatures.A “quasiharmonic model” is often used to extend the systematics of harmonic phonons to high temperatures by accounting for the effects of thermal expansion against a bulk modulus. Non-harmonic effects beyond the quasiharmonic approximation originate from the interactions of thermally-excited phonons with other phonons, or with the interactions of phonons with electronic excitations. In the classical high temperature limit, the adiabatic electron-phonon coupling can have a surprisingly large effect in metals when temperature causes significant changes in the electron density near the Fermi level. There are useful similarities in how temperature, pressure, and composition alter the conduction electron screening and the interatomic force constants. Phonon-phonon “anharmonic” interactions arise from those non-harmonic parts of the interatomic potential that cannot be accounted for by the quasiharmonic model. Anharmonic shifts in phonon frequency with temperature can be substantial, but trends are not well understood. Anharmonic phonon damping does show systematic trends, however, at least for fcc metals.Trends of vibrational entropy are often justified with atomic properties such as atomic size, electronegativity, electron-to-atom ratio, and mass. Since vibrational entropy originates at the level of electrons in solids, such rules of thumb prove no better than similar rules devised for trends in bonding and structure, and tend to be worse. Fortunately, the required tools for accurate experimental investigations of vibrational entropy have improved dramatically over the past few years, and the required ab initio methods have become more accessible. Steady progress is expected for understanding the phenomena reviewed here, as investigations are performed with the new tools of experiment and theory, sometimes in integrated ways.     

Relativistic thermodynamics of continua

We attempt to construct a relativistic model of a dissipative continuum that complies with the laws of both mechanics and thermodynamics, without restricting ourselves to the fluid case.     

Chemical thermodynamics of the vapor-oxygen gasification of graphite

A thermodynamic analysis of the vapor-oxygen gasification and combustion of graphite with a variation of the oxidizer excess α has been performed. Chemical reactions proceeding in the process of gasification and combustion, as well as components of flammable gases, which are the combustion products, have been determined. The channels of the distributing of the energy of exothermal and endothermal processes have been revealed. The energy of exothermal reactions compensates the endothermal effect of the reaction and increases the physical heat of the system. Expenditures on the endothermal reaction where flammable gases are produced are not irretrievable and transverse to the calorific power of the flammable gases. The effect of the oxidizer concentration on the adiabatic temperature at partial combustion of graphite has been analyzed. The thermal efficiency has been found as the ratio of the total thermal energy in the combustion of flammable gases and their physical heat to the spent energy, which is determined by the calorific power of graphite. The thermal efficiency is about 100%.     

Effects of impurities on the phase transition of an itinerant electron antiferromagnet

Theory of the phase transition of an itinerant electron antiferromagnet is similar to superconductivity theory, including singular contributions to the specific heat and other properties at the transition temperature. The effects of a small concentration of normal impurities, which are pair breaking for the antiferromagnet, are calculated for temperatures nearT _c . The principal effect of normal homovalent impurities is temperature smearing of the phase transition due to inhomogeneous electron-impurity scattering. This temperature smearing removes the singularities, leaving peaks with finite amplitudes which depend on the impurity concentration in a simple manner. Recent experimental results for chromium are discussed.     

Critical review of the thermodynamics of the Na-Al-O system

The free energies of formation of various phases in the Na-Al-O systems are critically reviewed for the composition range between pure alumina and sodium-aluminate. The results clearly indicate that in the temperature range between 600 and 1100 K neither-alumina nor'-alumina (Na₂O.5Al₂O₃) are stable with respect to sodium; the most stable phase in that range being sodium aluminate. Least square fitting and statistical analysis of all the available data on the free energy of formation of-alumina deemed accurate yields G _form ⁰ (from oxides) = –8772+1.53T (J/mole of O) (600 T 1200K) If the composition of-alumina is assumed to be Na₂0.11Al₂O₃ then G _form ⁰ (Na_1/17Al_11/17O, from elements) = –565100+109.6T (J mole of O) (600 T 1200 K) If the composition is assumed to be 19 then: G _form ⁰ (Na_1/14Al_9/14O, from elements) = –564000+109.6T (J/mole of O) (600 T 1200K) Based on the only study to date, the best estimate for the free energy of formation of-alumina is G _form ⁰ (Na_1/8Al_5/8O, from the elements) = –567860+114T (J/mole of O) (600 T 1200 K)The uncertainty (95% confidence band) is estimated to be about ±200 J/mole of O. Based on these results-alumina should be thermodynamically stable at lower temperatures, but become unstable at temperatures >900 K and should, kinetics permitting, dissociate into-alumina and sodium aluminate. Experimental evidence, however exists that conclusively shows that-alumina is thermodynamically stable at much higher temperatures than 900 K which leads to the conclusion that G °_form for-alumina reported to date is too high and should be more negative.     

On the thermodynamics of phase transitions in the crystals

To take into account the surface effects in a crystal during a phase change a suitable model of a continuum with an interface is proposed. In this model the interface S exhibits characteristics which vary discontinuously across curves belonging to S. General laws of balance are employed to derive the local equations and jump conditions in statical and dynamical situations. The equilibrium problem of a crystal in the presence of its melting or vapor is discussed and it is proved that the effects of surface tension can be pointed out only in non linear elasticity.     

Some problems concerning the thermodynamics of irreversible processes

Au approximate solution is obtained to the linearized Boltzmann equation, on the basis of which the transmission laws are then generalized. The hyperbolic equation of heat conduction is also derived.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 5, pp. 935–939, May, 1973.     

Exergy representation in the thermodynamics of irreversible processes

It is proposed to use the exergy of a system as the thermodynamic Lagrangian. The corresponding variational principle is formulated and its relationship to other variational principles of nonequilibrium thermodynamics is demonstrated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 32, No. 2, pp. 336–341, February, 1977.