Investigation of the hyperfine interaction in the antiferromagnetic CsMnI3

The lower branch of the resonance spectrum of the quasi-one-dimensional triangular antiferromagnetic CsMnI₃ has been investigated experimentally. This branch possesses a gap due to the dynamic hyperfine interaction. The temperature dependence of the energy gap was studied in detail at several frequencies. A theoretical calculation of the corresponding spectrum of coupled electron-nuclear spin oscillations was performed in the “hydrodynamic approximation” with an empirical correction for thermal fluctuations of the antiferromagnetic system. The good agreement between the calculation and experimental data makes it possible to determine the zero-point spin reduction in the antiferromagnetic. Zh. éksp. Teor. Fiz. 112, 1893–1898 (November 1997)     

Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and thermal expansion coefficient of SmB₆, Sm_0.8B₆, and Sm_1–x La_xB₆ (x=0.1 and 0.2) are measured in the temperature range T=4–300 K. The dispersion curves of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation gap in the electron density of states in both nonstoichiometric and lanthanum-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the temperature variation of the valence and, like the magnetic susceptibility, reflect features of the f-electron excitation spectrum. It is found that lanthanum doping does not lead to significant changes in the anomalies in the phonon spectrum of SmB₆. It is established that the homogeneous intermediate-valent state of the samarium ion is fairly stable and is maintained when the perfection of the Sm sublattice is violated. Zh. éksp. Teor. Fiz. 115, 1024–1038 (March 1999)     

Phase transition in liquid 3He in an aerogel at zero temperature

An expansion of the thermodynamic potential in powers of the order parameter of the superfluid phase transition is found for liquid ³He in an aerogel at T=0. The discontinuity in the compressibility (sound velocity), which is the analog of the discontinuity in the specific heat for second-order temperature phase transitions, is calculated. The magnitude of the critical quantum fluctuations (zero-point vibrations) of the order parameter is estimated. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 655–660 (25 November 1997)     

Quantum temperature fluctuations in the magnetization of antiferromagnetic semimetals

It is shown that in semimetallic, low-temperature antiferromagnetic materials located in a quantizing magnetic field, the part of the band magnetization M _∼ which oscillates in H can have a nonmonotonic temperature dependence. This non-Fermi liquid behavior will show up experimentally in the form of quantum temperature fluctuations of the magnetization when the decrease with rising temperature is oscillatory, rather than the usual monotonic decrease. It is shown that the magnetization from an individual spin electron (or hole) subband has the form of weakly damped periodic oscillations as a function of T ². This result makes it possible to develop an efficient method for studying the electronic structure of antiferromagnetic semimetals based on an examination of the quantum temperature fluctuations. Calculations show that quantum temperature fluctuations can be observed, for example, in the cerium monopnictides CeP and CeAs, which are strongly correlated, antiferromagnetic, compensated semimetals with low Neel temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 1674–1680 (September 1998)     

Electronic structure and magnetic phase transition in MnSi

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature T _S (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.     

Effect of thermal expansion on the elastic moduli and Debye temperature of paramagnetic lutetium

The temperature dependence of the Young’s modulus E of paramagnetic lutetium has been studied. It has been shown that an important reason for the dependence E(T) is thermal expansion of the crystal lattice, which also leads to a change in the Debye temperature Θ. The effect of this factor is also revealed in the thermodynamic properties of metals. In particular, we have shown that there is another contribution to variation of the total specific heat of lutetium, associated with the Θ(T) dependence and comparable with the electronic contribution. Fiz. Tverd. Tela (St. Petersburg) 40, 1581–1584 (September 1998)     

Equidistant spectrum of localized states due to fluctuations of the parameters in quantum-well structures

Narrow equidistant peaks are recorded in the microphotoluminescence spectra of GaAs/Al_xGa_1−x As superlattices with thin barrier layers. An approximate method, consisting of the reduction of a three-dimensional problem to a one-dimensional problem, is developed for calculating the electronic spectrum of quantum dots of arbitrary shape. It is shown on the basis of this method that the observed peaks may be due to the formation of electronic states of a new type, which are produced as a result of the overlapping of fluctuations of the thicknesses of adjacent wells in structures with thin barriers. The equidistant spectrum of the new states is accounted for by the nearly quadratic dependence of the diameter of the overlap regions on the coordinate in the plane of the layers. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 260–265 (25 February 1996)     

A quantitative determination of magnetic fluctuations in CuGeO3

A polarised neutron scattering investigation has been carried out on a powder sample of CuGeO₃ within the temperature range of 1.5 K to 600 K. The magnetic scattering has been separated from all other contributions by using polarised neutrons and polarisation analysis and placed onto an absolute scale. At low temperatures the long wavelength components of the paramagnetic response are suppressed consistent with the formation of Cu dimers in which the magnetic moments are correlated antiferromagnetically. This form of the scattering persists to temperatures well above the dimerisation temperature T _sp ∼ 14 K. However as the temperature is raised the intensity of the long wavelength spin fluctuations increases and above ∼150 K they are the dominant feature in the wave vector dependence of the response. At all temperatures the observed scattering extrapolates smoothly to the Q = 0 value given by the uniform susceptibility. Consequently the thermal variation of the uniform susceptibility arises from the evolution of the long wavelength magnetic fluctuations. At large wave vectors the energy dependence of the scattering revealed that the response occurs below 16 meV in agreement with the reported maximum magnetic excitation energy at the zone boundary in the ground state. However the total magnetic scattering is significantly less than that expected for a local moment system suggesting that the spectrum of thermal and quantum fluctuations overlap. Received 30 May 2000 and Received in final form 22 March 2001     

Suppression of nonlocal thermal conductivity in a turbulent plasma

It is shown that anomalous suppression of the electronic thermal conductivity occurs in a weakly collisional plasma as a result of electron scattering not only by charged particles but also by low-frequency turbulent fluctuations. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 579–582 (25 April 1996)     

Temperature evolution of electronic and lattice configurations in highly ordered trans-polyacetylene

We have studied the temperature evolution of the optical absorption and Raman scattering spectra of a trans-cis blend of “soluble” polyacetylene in a polyvinylbutyral/butanol mixture. On decreasing temperature a reversible restructuring of the electronic and lattice systems of trans-(CH)_x occurs: the electronic energy gap contracts at the high rate of 0.4 meV/K, and the vibrational modes are modified. The experimental data obtained are interpreted in terms of a peculiar interaction of π-conjugated electrons with lattice fluctuations. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 613–617 (10 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Fluctuating coulomb field near ionic dielectric surfaces

Analytical and numerical studies of the energy density spatial distribution of a fluctuating electric field near the model ionic crystal surface as a function of temperature are carried out. It is established that the fluctuating field energy density decreases as h ^−3.3 with increasing distance h from the crystal surface. The fluctuating field energy increases with temperature and differs from zero at −273°C due to quantum zero-point vibrations of ions in the lattice.     

On the electronic spectrum of low-dimensional structures with the symmetry of borders

The features of the electronic spectrum in low-dimensional (quasi-one-dimensional) structures with symmetry of borders have been considered. It has been shown that, in the spinless case, the conical massless (Dirac) form of the spectrum (ɛ(k) ∝ ± |k|) appears in groups with glide planes. This spectrum is a consequence of forced degeneracy, which appears due to the joint action of the lattice symmetry elements and time reversal invariance. For the spinor representations, the conical shape of the spectrum appears in all groups.     

X-ray diffraction investigations of the crystallographic parameters and thermal expansion of β-BaB2O4 crystals

The lattice parameters a and c of β-BaB₂O₄ crystals have been measured in the temperature range 80–300 K by the x-ray diffraction method. The thermal expansion coefficients α are calculated from the measured values of the parameters. A substantial anisotropy of the thermal expansion is found. It is shown that the thermal expansion coefficient α _c along the c axis is an order of magnitude greater than the thermal expansion coefficient α _a in a plane perpendicular to this axis. It is established that α _a becomes negative in the temperature range 80–190 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1038–1040 (June 1997)     

Anomalous influence of spin fluctuations on the heat capacity and entropy in a strongly correlated helical ferromagnet MnSi

The influence of spin fluctuations on the thermodynamic properties of a helical ferromagnet MnSi has been investigated in the framework of the Hubbard model with the electronic spectrum determined from the first-principles LDA + U + SO calculation, which is extended taking into account the Hund coupling and the Dzyaloshinskii–Moriya antisymmetric exchange. It has been shown that the ground state of the magnetic material is characterized by large zero-point fluctuations, which disappear at the temperature T* (<T _c is the temperature of the magnetic phase transition). In this case, the entropy abruptly increases, and a lambdashaped anomaly appears in the temperature dependence of the heat capacity at constant volume (C _V (T)). In the temperature range T* < T < T _c , thermal fluctuations lead to the disappearance of the inhomogeneous magnetization. The competition between the increase in the entropy due to paramagnon excitations and its decrease as a result of the reduction in the amplitude of local magnetic moments, under the conditions of strong Hund exchange, is responsible for in the appearance of a “shoulder” in the dependence C _V (T)).     

Long-range orientational order in a two-dimensional degenerate system of dipoles on a square lattice

It is shown that in a planar degenerate system on a square lattice with dipole-dipole interactions of electric or magnetic moments the thermodynamic fluctuations destroy the Goldstone mode in the spectrum of orientational oscillations. This gives rise to layered antiferroelectric structures characterized by a nonzero long-range orientational order parameter, which in the low-temperature limit is proportional to exp(−T|lnT|). Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 623–627 (25 April 1996)     

Spin fluctuations and properties of the thermoelectric power of nearly ferromagnetic iron monosilicide

The thermoelectric power of nearly ferromagnetic iron monosilicide, which passes through an electronic semiconductor-metal transition with increasing temperature, is investigated theoretically. The results of this investigation indicate that a sizable paramagnon-related increase in charge carriers can occur in nearly ferromagnetic semiconductors, and that spin fluctuations can modify the electronic spectrum and thereby renormalize the diffusion component of the thermoelectric power. The transition from semiconductor to metal decreases the paramagnon component sharply and the thermoelectric power changes sign, which agrees with experimental data for iron monosilicide. Fiz. Tverd. Tela (St. Petersburg) 41, 1054–1056 (June 1999)     

Kinetics of muonium formation in liquid helium

The spatial distribution of electron-muon pairs in superfluid helium (He-II) is determined using a new algorithm for reconstructing the muonium (Mu) formation probability. It is shown that because a gap is present in the excitation spectrum of He-II the thermalization time of muons and secondary electrons increases with decreasing temperature. As a result, the average distance in the electron-muon pairs increases and, correspondingly, the muonium formation rate decreases. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 251–256 (10 February 1999)     

Fluctuation-induced broadening of Mn2+ EPR lines in the incommensurate phase of Rb2ZnCl4 crystals

The EPR spectra of Mn²⁺ ions in Rb₂ZnCl₄ crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase. When these crystals are cooled below the transition temperature T _i =304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the vicinity of T _i are caused by amplitude fluctuations. Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999)     

Spin fluctuations and Curie temperature in the compounds R2M17 with nonmagnetic elements

The magnetic properties of materials for permanent magnets based on binary compounds R₂M₁₇ (R=Y, Sm; M=Fe, Co), also including additions of the nonmagnetic elements N, Al, and Si, are investigated a the theory of dynamical fluctuations of the electronic spin density. It is shown that the Curie temperature is determined by the ratio of the exchange splitting energy (proportional to the magnetization at T=0) and the rms value of the fluctuations (proportional to the local spin susceptibility). The fluctuations are much larger in iron compounds than in cobalt alloys. This results not only in quantitative differences in their characteristics but also in a qualitatively different change in the properties of these materials on nitriding. Fiz. Tverd. Tela (St. Petersburg) 41, 77–83 (January 1999)     

Thermal boundary layer for non-Newtonian fluid

Summary Analytical and numerical solutions for the momentum and thermal boundary layer equations of a non-Newtonian power law fluid are presented. The flow is assumed to be under the influence of an external magnetic fieldB (x) applied perpendicular to the surface and an electric fieldE(x) perpendicular toB(x) and the direction of the longitudinal velocity in the boundary layer. For the power law fluid it is assumed that the shear stress is proportional to then-th power of the velocity gradient andn is called the flow index. The variations of the velocity fieldf′, the temperature field θ, the shear stress on the surfaceτ _W , the displacement thicknessδ ₁ and the momentum thicknessδ ₂ with the magnetic-field parameter γ, the flow indexn, the heat transfer indexS and the Prandtl number Pr are studied. It is found that, if the outer flow velocityU(x) (potential flow) is proportional to the arc lengthx raised to a powerm, then the similarity solution for the thermal boundary layer equation is possible only whenm=1/3, which represents flow past a wedge of included angle π/2. It is established that the temperature of the wedge increases with the increase of γ, Pr,S and the decrease ofn. In general the magnetic field can be used as a heater for the surface of the wedge.