Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder

Isothermal remanent magnetization decay,M _r(t), and ‘in-field’ growth of zero-field-cooled magnetization,M _ZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25T _c to 1.25T _c (whereT _c is the Curie temperature, determined previously for the same sample from static critical phenomena measurements) for a nearly ordered intermetallic compound Ni₃Al, which is an experimental realization of a three-dimensional (d = 3) ferromagnet with weak quenched random-exchange disorder. None of the functional forms ofM _r(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions and closely reproduce such data in the present case. The most striking features of magnetic relaxation in the system in question are as follows: Aging effects are absent in bothM _r t andM _ZFC(t) at all temperatures in the temperature range covered in the present experiments. A cross-over in equilibrium dynamics from the one, characteristic of a pured = 3 ferromagnet with complete atomic ordering and prevalent at temperatures away from T_c, to that, typical of ad = 3 random-exchange ferromagnet, occurs asT → T_c. The relaxation times τ₁(T)(τ₁ ^′(T)) and τ₂(T)(τ₂ ^′(T)) exhibit logarithmic divergence at critical temperatures and ; and both increase with the external magnetic field strength,H, such that at any given field value, . The exponent characterizing the logarithmic divergence in τ ₁ ^′ (T) and τ ₂ ^′ T possesses a field-independent value of ≃16 for both relaxation times. Of all the available theoretical models, the droplet fluctuation model alone provides a qualitative explanation for some aspects of the present magnetic relaxation data     

Dynamics of quark-gluon plasma from field correlators

It is argued that strong dynamics in the quark-gluon plasma and bound states of quarks and gluons is mostly due to nonperturbative effects described by field correlators. The emphasis in the paper is made on two explicit calculations of these effects from first principles—one analytic using gluelump Green’s functions and another using independent lattice data on correlators. The resulting hadron spectra are investigated in the range T _c ≤ T < 2T _c . The spectra of charmonia, bottomonia, light s mesons, glueballs, and quark-gluon states calculated numerically are in general agreement with lattice MEM data. The possible role of these bound states in the thermodynamics of quark—gluon plasma is discussed. The text was submitted by the authors in English.     

Symmetry and optical properties of wurtzite nanostructures with the <Emphasis Type="Italic">c</Emphasis> axis in the layer plane

In wurtzite-based quantum wells and superlattices with the c axis parallel to the layer plane, this plane is parallel either to a symmetry plane of the wurtzite lattice (type I structures, the 〈110〉 growth direction) or to a glide plane containing the c axis (type II structures, the 〈100〉 growth direction). In both cases, the space symmetry of the structure depends on the parity of the number of monolayers within the slab(s). The point symmetry is C _2v except for the type II structures with odd monolayer number(s). The latter structures have the σ _v point symmetry and can have a built-in electric field. Quite different selection rules, depending on the structure symmetry, govern electron optical transitions and exciton radiative recombination. The major part of the proceedings of the XIII Feofilov Symposium was published in Fizika Tverdogo Tela (Physics of the Solid State), 2008, Vol. 50, No. 9. The text was submitted by the authors in English.     

Crystal structure of ice accretions

Summary The crystal size in accretions grown in an icing wind tunnel at ambient temperatures −18≥T _a ≥−27°C and different substrate temperatureT _s <0°C is studied. For constantT _s the mean area of crystal sections shows an approximately exponential dependence onT _a which may be written as , where ΔT≃−T _a represents the droplet supercooling. On the other hand, for fixedT _a , the crystal size is observed to decrease markedly withT _s . By considering previous results on the orientation of crystals forming the accretions, it is shown that the value of the mean angle formed by the crystalc-axis with the growth direction depends as well both onT _a andT _s . The exponential dependence of andT _a and the observed correspondence between the behaviour of and are correlated with a nucleation mechanism, accompanied by a process of orientation selection, which would be responsible of both the crystal size and the orientation. The application of the crystal structure analysis to the determination of the growth conditions of hailstones is discussed.

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Riassunto Si studia la dimensione di un cristallo in accrezioni cresciute in una galleria a vento congelante a temperature ambienti −18≥T _a ≥−27 °C e diversa temperatura di substratoT _s <0 °C. PerT _s costante, l’area media delle sezioni di cristallo mostra una dipendenza daT _a approssimativamente esponenziale che potrebbe essere scritta così: , dove ΔT≃−T _a rappresenta il superraffreddamento della gocciolina. D’altra parte, perT _a fisso, si osserva che la dimensione del cristallo decresce marcatamente conT _s . Considerando precedenti risultati sull’orientamento dei cristalli che formano le accrezioni, si mostra che anche il valore dell’angolo medio formato dall’assec del cristallo con la direzione di accrescimento dipende sia daT _a che daT _s . La dipendenza esponenziale di daT _a e la corrispondenza osservata tra il comportamento di e di sono collegate con il meccanismo di nucleazione, accompagnato da un processo di selezione dell’orientamento, che dovrebbe essere responsabile sia della dimensione che dell’orientamento del cristallo. Si discute l’applicazione dell’analisi della struttura del cristallo alla determinzione delle condizioni di accrescimento dei chicchi di grandine.

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To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.

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Comisión Nacional de Energía Atómica and Consejo Nacional de Investigaciones Científicas y Técnicas.     

Thermal expansion anomalies in TbVO4 due to the cooperative Jahn-Teller effect

The thermal expansion anomalies in TbVO₄ due to the cooperative Jahn-Teller effect were studied experimentally and theoretically. Characteristic magnetoelastic anomalies were observed in the curves of Δa/a and Δc/c of the cell parameters at T<T _c. Calculations of the magnetoelastic contribution to Δa/a and Δc/c from fully symmetric and and low-symmetry ɛ^δ modes were performed using the general crystal-field formalism, and the values of the magnetoelastic coefficient B ^δ were obtained from spectroscopic and spontaneous-deformation data. It is shown that the thermal expansion of TbVO₄ in both the tetragonal and orthorhombic phases can be described well on the basis of a general model based on a single set of interaction parameters. Fiz. Tverd. Tela (St. Petersburg) 40, 1663–1666 (September 1998)     

On the role of the Coulomb interaction in the mechanism of d-wave Cooper pairing of charge carriers in high-T c superconductors

The question of the effect of the structure of the anisotropic quasi-two-dimensional electron spectrum of high-T _c superconductors on the character of the screening of the Coulomb interaction and the symmetry of the superconducting order parameter is studied. Calculations of the polarization operator of electrons are performed on the basis of the single-particle band spectrum extracted from angle-resolved photoemission spectroscopy data. It is shown that the static screened Coulomb repulsion has a minimum at small momentum transfers. This corresponds to an effective electron-electron attraction in the -wave channel of Cooper pairing of the charge carriers on account of their interaction with the long-wavelength charge-density fluctuations. This attraction together with the anisotropic electron-phonon interaction increase the critical superconducting transition temperature T _c with increasing hole density and can give quite high values of T _c while at the same time suppressing the isotope effect, in qualitative agreement with the experimental data for underdoped hole-type cuprate metal-oxide compounds. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 703–710 (25 May 1999)     

The Ginzburg-Landau expansion and the slope of the upper critical field in disordered superconductors

It is shown that the slope of the upper critical field in superconductors with d pairing drops rapidly with increasing concentration of normal impurities, while in superconductors with anisotropic s pairing increases and reaches the well-known asymptotic level characteristic for the isotropic case. This difference makes it possible, in principle, to employ measurements of H _c 2 in disordered superconductors as an experimental method for determining the type of pairing in high-T _c superconductors and systems with heavy fermions. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 347–352 (10 March 1996)     

Optical studies of phase transitions in the (NH4)3VO2F4 crystal

(NH₄)₃VO₂F₄ crystals were grown, and polarization-optical studies and measurements of birefringence were conducted on crystal plates of various cuts over a wide temperature range. Phase transitions were detected at temperatures T _1↑ = 417 K, T _3↑ = 211 K, and T _4↑ = 205 K (on heating) and at T _1↓ = 413 K, T _3↓ = 210 K, and T _4↓ = 200 K (on cooling). The transitions are accompanied by anomalies of the birefringence and by twinning. The sequence of changes in the phase symmetry is assumed to be as follows: cubic Fm m ↔ orthorhombic Immm (I222₁) ↔ monoclinic 112/m) ↔ triclinic P . Near temperatures T ₂ ≈ 240–250 K, an additional anomaly of the birefringence is observed, with the crystal retaining the orthorhombic symmetry. Original Russian Text ? S.V. Mel’nikova, A G. Kocharova, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 562–564.     

Effect of heavy quark symmetry on the mass difference ofB-system in minimal left right symmetric model

An estimation of the mass difference of system with heavy quark symmetry formalism is presented. The effective Hamiltonian describing the transition (whereh=b forB _d ⁰ -system) is considered in a manifest left right symmetric (MLRS) model along with contribution from neutral Higgs boson. We use the spin and flavor symmetry for heavy quarks to obtain the transition matrix element 〈B _d ⁰ |ℋ_eff(x)| _d ⁰ 〉 in terms of Isgur-Wise function. Assuming thatB _d ⁰ and states are at rest, we find that Isgur-Wise function turns out to be unity. However using the experimental values of ΔM _K and as input, we find thatM _R=835 GeV andM _H⩾2·9 TeV.     

Anisotropic structure of the gap in high-T c superconductors: Competition between s-and d-type symmetry

We show that the anisotropic structure, observed in angle-resolved photoemission spectroscopy experiments, of the gap in high-T _c superconductors based on layered cuprate metal-oxide compounds is the result of the strong anisotropy of the electronic spectrum in the plane of the layers, an anisotropy caused by the hybridization between the overlapping broad and anomalously narrow bands. Depending on the values of the electron-phonon coupling constants and the Coulomb repulsion, which in certain conditions is balanced almost perfectly by the attraction caused by the electron-plasmon interaction, either the or the s _xy symmetry is realized in the superconducting order parameter. When the initial C _v4 symmetry of the band spectrum of the Bi(2212) single crystal with a superlattice or the Y(123) single crystal with one-dimensional chains is broken, an anomalous temperature dependence of the anisotropic-gap width Δ is observed, and this dependence differs dramatically from the standard Δ (T) dependence of the BCS theory. Zh. éksp. Teor. Fiz. 111, 298–317 (January 1997)     

Dependence ofT c on doping in oxygen-hole induced Jahn-Teller mediated superconductors

A bipolaronic model of high-T _c superconductivity, that is based upon the idea that a doped oxygenhole reduces the separation between the and levels of the neighbouring Cu-ion and hence causes a Jahn-Teller mixing between the two levels, is suggested. Within the BCS scheme, the bipolaron Hamiltonian is investigated for the superconducting order parameter and the critical temperatureT _c as a function of hole-doping. Under suitable approximation for the bipolaronic kinetic energy, the hole-doping dependence ofT _c shows a universal behaviour. For temperatures close toT _c and forT _c 1, obeys the BCS type of relation with pre-factor of (1–T/T _c )² related to the range of superconducting-phase against the doping axis.     

Superconductivity of strongly correlated electrons in copper and ruthenium oxides within the t-J-I model

We propose a t-J-I model with direct ferromagnetic exchange I to explain the superconductivity of copper oxides and the ruthenate Sr₂RuO₄ on the basis of the analysis of the electronic structure of these substances. We analyze the possible p-and d-type superconducting solutions. Solutions of the s type with singlet pairings are impossible in the strong-electron-correlations regime, and p-type solutions correspond to triplet superconductivity and is formed near the ferromagnetic instability threshold in ruthenates. The solution with the symmetry near the antiferromagnetic instability threshold corresponds to copper oxides. We also discuss the reason for the high values of the superconducting transition temperature (T _c ∼100 K) in copper oxides and the low values (T _c ∼1 K) in ruthenates. Zh. éksp. Teor. Fiz. 116, 655–670 (August 1999)     

Superconductor-insulator duality for an array of Josephson wires

A novel model system is proposed for the study of superconductor-insulator transitions that is a regular lattice whose each link consists of a Josephson-junction chain of N ≫ 1 junctions in sequence. The theory of such an array is developed for the case of semiclassical junctions with the Josephson energy E _J larger compared to the Coulomb energy E _C = e ²/2C of the junctions. An exact duality transformation is derived that transforms the Hamiltonian of the proposed model into a standard Hamiltonian of a JJ array. The nature of the ground state is controlled (in the absence of random offset charges) by the parameter q ≈ N ² exp with the superconductive state corresponding to small q < q _c . The values of q _c are calculated for magnetic frustrations f = 0 and f = 1/2. The temperature of the superconductive transition T _c (q) and q < q _c is estimated for the same values of f. In the presence of strong random offset charges, the T = 0 phase diagram is controlled by the parameter ; the critical value and the critical temperature at zero magnetic frustration are estimated. The text was submitted by the authors in English.     

NMR dynamics in disordered magnets

The excitation dynamics of site diluted magnets can be described at low energies (long length scales) by magnons, and above a crossover frequency, ω_c, (short length scales) by fractons. The density of fracton states is given by , where is the fracton dimensionality. Dilution gives rise to a characteristic length ξ∝(p−p _c)^ν, wherep _c is the critical concentration for (magnetic) percolation. The crossover frequency ω_c is proportional to ξ^-1[1+(θ/2)], where θ is the rate at which the diffusion constant decays with distance for diffusion on an equivalent network. A fractal dimensionD describes the density of magnetic sites on the infinite network, and . For percolating networks, for all dimensions ≥2. Neutron scattering structure factor measurements by Uemura and Birgeneau compare well with calculations using fracton concepts. Magnons are extended at low energies, while the fracton states are geometrically localized, with a wave function envelope proportional to exp . Here, is the fracton length scale at frequency ω. The exponentd _ϕ lies between 1 andd _min, the chemical length index (of the order of 1.6 in three dimensions). The localization of the magnetic excitations causes a spread in the NMR relaxation rates. A given nuclear moment will experience only a limited set of fracton excitations, resulting in an overall non-exponential decay of the NMR relaxation signal. When strong cross-relaxation is present, the relaxation will be exponential, but the temperature dependence will be strongly altered from the concentrated result.     

Identifying functions determined by linear functional operators

The new identifying problem is formulated for general linear functional operators F = Σc _j F ○ a _j which significantly generalizes in particular the well-known Ulam stability problem. The results obtained can be very useful when processing experimental data of any kind as they enable to determine with high precision the structure of a compactly supported Banach-valued function F by using a rather restricted information concerning F. Dedicated to the memory of Leonid Volevich     

Photoelasticity in quasicrystals

The maximum number of non-vanishing and independent second order photoelastic coefficients required by the seven pentagonal and the two icosahedral point groups 5(C₅), (S₁₀), (C_5h ), m2(D_5h ), 52(D₅), 5m(C_5v ), 2m(D_5d ); 235(I), 2/m (I _h )—that describe the quasicrystals symmetry groups in two and three dimensions—is obtained. The schemes of non-vanishing and independent coefficients have been calculated and listed. Finally the results of this group-theoretical study are briefly discussed.     

Effect of a magnetic field on the yield point of NaCl crystals

A constant magnetic field is found to have a substantial effect on the macroplasticity of NaCl crystals when they are being actively strained at a constant rate during magnetic treatment. We have measured the dependence of the yield point σ _y on the magnetic induction B=0–0.48 T and the strain rate . It is shown that this magnetic effect has a threshold character and is observed only for B>B _c , where B _c grows with increasing as . The lower the strain rate , the larger the relative decrease in the yield point σ _y (B)/σ _y (0) at fixed field B>B _c . At small enough strain rates the threshold field B _c ceases to depend on and goes constant. A theoretical model is proposed which is in good agreement with the observed regularities. The model is based on the competition between thermally activated and magnetically stimulated depinning of dislocations from paramagnetic impurity centers. Zh. éksp. Teor. Fiz. 115, 951–958 (March 1999)     

MD simulation of concentrated polymer solutions: Structural relaxation near the glass transition

We examine by molecular dynamics simulations the relaxation of polymer-solvent mixtures close to the glass transition. The simulations employ a coarse-grained model in which polymers are represented by bead-spring chains and solvent particles by monomers. The interaction parameters between polymer and solvent are adjusted such that mixing is favored. We find that the mixtures have one glass transition temperature T _g or critical temperature T _c of mode-coupling theory (MCT). Both T _g and T _c (> T _g decrease with increasing solvent concentration . The decrease is linear for the concentrations studied (up to = 25%. Above T _c we explore the structure and relaxation of the polymer-solvent mixtures on cooling. We find that, if the polymer solution is compared to the pure polymer melt at the same T, local spatial correlations on the length scale of the first peak of the static structure factor S(q) are reduced. This difference between melt and solution is largely removed when comparing the S(q) of both systems at similar distance to the respective T _c. Near T _c we investigate dynamic correlation functions, such as the incoherent intermediate scattering function (t), mean-square displacements of the monomers and solvent particles, two non-Gaussian parameters, and the probability distribution P(ln r;t) of the logarithm of single-particle displacements. In accordance with MCT we find, for instance, that (t) obeys the time-temperature superposition principle and has relaxation times which are compatible with a power law increase close (but not too close) to T _c. In divergence to MCT, however, the increase of depends on the wavelength q, small q values having weaker increase than large ones. This decoupling of local and large-length scale relaxation could be related to the emergence of dynamic heterogeneity at low T. In the time window of the relaxation an analysis of P(ln r;t) reveals a double-peak structure close to T _c. The first peak correponds to “slow” particles (monomer or solvent) which have not moved much farther than 10% of their diameter in time t, whereas the second occurs at distances of the order of the particle diameter. These “fast” particles have succeeded in leaving their nearest-neighbor cage in time t. The simulation thus demonstrates that large fluctuations in particle mobility accompany the final structural relaxation of the cold polymer solution in the vicinity of the extrapolated T _c.     

Electrical conductivity and structural behaviour of Al65Cu20Ru15 quasicrystalline alloy

The low temperature conductivity σ (T) and the magnetoresistance (up to 8 T) of a phason disorder free pure Al₆₅Cu₂₀Ru₁₅ quasicrystalline alloy, prepared by melt casting, has been investigated in the temperature range from 1.3 K to 200 K. In the temperature range 1.3 to 20 K the conductivity exhibits a -variation, whereas for the range 20 to 200 K a linearT-dependence has been observed. The analysis of magnetoresistance data reveals the presence of weak localization and electron-electron interaction effects. The positive nature of the magnetoresistance indicates the presence of spin orbit scattering. The σ∞ behaviour prevailing over such a wide temperature range has been observed for the first time for Al₆₅Cu₂₀Ru₁₅. A quantitative analysis of the -variation in terms of electron-electron interaction yields a very low density of states at the Fermi level.