Electron momentum density in Cu<Subscript>0.9</Subscript>Al<Subscript>0.1</Subscript>

A reconstruction technique based on the solution of the Radon transform in terms of Jacobi polynomials is used to obtain the 3D electron momentum density, ϱ(p), from nine high-resolution Compton profiles (CPs) for a Cu_0.9Al_0.1 disordered alloy single crystal. The method was also applied to theoretical CPs computed within the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) first-principles scheme for the same nine orientations of the crystal. The experimental ϱ(p) is in satisfactory agreement with the theoretical ϱ(p), shows most details of the Fermi surface (FS) and exhibits electron correlation effects. We comment on the map of the FS obtained by folding the reconstructed ϱ(p) into the first Brillouin zone, which yields the occupation number density, ϱ(k). A test of the validity of data via a consistency condition (within our reconstruction algorithm) as well as the propagation of experimental noise in the reconstruction of both ϱ(p) and ϱ(k) are investigated. Received: 24 October 2001 / Accepted: 20 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +48-71/344-10-29, E-mail: samsel@int.pan.wroc.pl     

Low-temperature behavior of specific heat curves in ruthenocuprate RuSr<Subscript>2</Subscript>Gd<Subscript>1.5</Subscript>Ge<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10 − δ</Subscript>: Superconducting,magnetic and crossing-point effects

Magnetic and superconducting properties of polycrystalline samples of RuSr₂Gd_1.5Ce_0.5Cu₂O_{(10 − δ)}, asprepared (by solid-state reaction) and annealed in pure oxygen at different pressure are presented. Specific heat and magnetization were investigated in the temperature range 1.8–300 K with a magnetic field up to 8 T. Specific heat, C (T), shows a jump at the superconducting transition (with onset at T ≈ 37.5 K) and a Schottky-type anomaly below 20 K. It is found that curves C(T) taken for different values of magnetic field have the same crossing point (at T _* ≈ 2.7 K) for all samples studied. At the same time, C(H) curves taken for different temperatures have a crossing point at a characteristic field H _* ≈ 3.7 T. These effects are manifestations of the crossing-point phenomenon, which is supposed to be inherent for strongly correlated electron systems.     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

Dynamics in molecular crystals: An application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently?

The synthesis of a specific isotopomer, C₆D₄H(ortho)-H(ortho)D₄C₆ of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other. Between the incommensurate (IC) phase transition of biphenyl at 38 K andT=250 K, the prominent namical mode consists of oscillatory twists ϕ(t) of the two rings. The data allow us to infer the mean square, (φ²), of these twists. (φ²) is found to grow linearly withT for 50<T<200 K. From the slope of (φ²) vs.T the frequency (the wave number[(v)\tilde]\tilde v) is derived. The result is[(v)\tilde] = 20\tilde v = 20 cm⁻¹. ForT<38 K, the spectra give direct evidence of the IC phase transition and its nature (stripelike rather than quiltlike). The temperature dependence of the magnitude of the order parameter of the IC phase is obtained.     

Specific heat of KTiOPO4 within the 80–300 K range

The specific heat of the KTiOPO₄ crystal has been measured with a vacuum adiabatic calorimeter within the 80–300 K range. A peak-shaped anomaly in the specific heat indicating a phase transition has been revealed in the C _p (T) curve at T≅279 K. Numerical integration of smoothened experimental C _p (T) curves yielded the thermodynamic functions of KTiOPO₄, namely, the entropy, enthalpy, and reduced Gibbs energy. The entropy and enthalpy of the observed transition have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 497–498 (March 1999)     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

Effect of γ irradiation on the low-temperature electrical conductivity and dielectric properties of TlGaSe2 crystals

Electrical conductivity and dielectric properties of single-crystal TlGaSe₂ have been studied as a function of γ irradiation dose in the 100–280 K range including the existence of an incommensurate phase. Anomalies in the form of maxima have been observed in the σ=f(T), tan δ=f(T), and ɛ=f(T) curves at the points of transition from the paraphase to incommensurate (IC) phase, T _i, and from the IC to commensurate phase, T _c. The increase in the quantities σ, tan δ, and ɛ observed initially with increasing irradiation dose is followed by their strong decrease and disappearance of the anomalies. It has been established that γ irradiation does not affect the phase transition temperatures T _i and T _c. Fiz. Tverd. Tela (St. Petersburg) 40, 1328–1331 (July 1998)     

Analysis of the temperature dependence of the heat capacity of solid tin in the vicinity of its melting temperature

Summary The experimental temperature dependence of the heat capacityC _p(T) of solid tin (Sn) in its premelting region 402.07–502.22 K was investigated and two regions with different temperature dependences ofC _p(T) were found. In the far (from the melting temperatureT _m) region 402.07–485.88 K the experimentalC _p(T) of Sn is described by the standard vacancy model. In the close region 485.88–502.22 K it is described by the formation of complicated volumetric defects in the crystalline lattice of solid Sn near itsT _m.     

Ferrimagnetic phase transition in the system Li0.5Fe2.52−x GaxO4 in the vicinity of the multicritical point of the x-T phase diagram

In connection with the problem of identifying magnetic states in the vicinity of x ₀ (the multicritical point of the x-T diagrams of spin-glass systems) a study has been made of properties that can be exploited to determine the presence of a thermodynamic phase transition at the Curie point T _C and the distinctive features of the transition, specifically, the temperature dependence of the magnetic part of the specific heat C _m(T), the temperature dependence of the low-field magnetization σ _H(T), and (with a view toward examining critical behavior in a magnetic field) the magnetization isotherms σ _H(T). The investigated object is the system of dilute ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄, in which every type of magnetic state has spatially inhomogeneous cluster structures. The results obtained for a sample with x=1.45 indicate that the classical criteria of a ferrimagnetic second-order phase transition at T _C=(97±2) K occur for x∼x ₀. The results of similar investigations for a sample with x=1.6, which exists in the cluster spin-glass state for T<T _f=22 K and in an uncorrelated cluster state of the superparamagnetic type for T>T _f, are also given for comparison with the preceding case. Zh. éksp. Teor. Fiz. 114, 2065–2077 (December 1998)     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Free Energies of Dilute Bose Gases: Upper Bound

We derive an upper bound on the free energy of a Bose gas at density ϱ and temperature T. In combination with the lower bound derived previously by Seiringer (Commun. Math. Phys. 279(3): 595–636, 2008), our result proves that in the low density limit, i.e., when a ³ ϱ≪1, where a denotes the scattering length of the pair-interaction potential, the leading term of Δf, the free energy difference per volume between interacting and ideal Bose gases, is equal to 4pa(2r²-[r-r_c]²₊)4\pi a(2\varrho^{2}-[\varrho-\varrho_{c}]^{2}_{+}). Here, ϱ _c (T) denotes the critical density for Bose–Einstein condensation (for the ideal Bose gas), and [⋅]₊=max {⋅,0} denotes the positive part.     

Oxygen diffusion in C60 films

We have performed detailed resistivity measurements as a function of temperature in the range from 12 to 300 K on oxygen loaded C₆₀ films. We observe that two ordering phase transitions (i.e.,T ₀=260 K andT _g =90 K) are present in (T), which, in addition, strongly depends on the oxygen content. We find a decrease of both ordering temperatures with increasing oxygen concentrations. The mechanisms of oxygen diffusion are greatly enhanced in the ordered phase on heating. Finally, the transition to a glassy state atT _g is detected as a point of reversibility of the resistivity curve as a function of temperature.     

Characteristic features of the magnetic ordering of Dy3+ ions in a monoclinic RbDy(WO4)2 single crystal

The investigation of the specific heat of a RbDy(WO₄)₂ single crystal at temperatures 0.2–2.5 K and in magnetic fields up to 2 T are reported. The temperature dependence of the specific heat near T _N=0.818 K is compared with the predictions for different models. The 2D Ising model describes satisfactorily C(T) below T _N, while for T>T _N none of the theoretical models agree with the behavior of C(T) of RbDy(WO₄)₂. The H-T phase diagram for H∥c is complicated and possesses a triple point, where regions of existence of three magnetic phases converge. The magnetic ordering is analyzed from the standpoint of the Jahn-Teller nature of the structural phase transitions occurring in RbDy(WO₄)₂ at higher temperatures. It is shown that the form of the phase diagram depends on the direction of the vector H, for the general case of an arbitrary direction of H, two phase transitions can occur with increasing field. Fiz. Tverd. Tela (St. Petersburg) 41, 491–496 (March 1999)     

Investigation of thermal expansion,phase diagrams,and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides

The thermal expansion along the principal crystallographic axes of the (NH₄)₂WO₂F₄ and (NH₄)₂MoO₂F₄ oxyfluorides has been studied. The anomalous behavior of α _i (T) due to the phase transitions has been revealed at T ₁ = 271.4 K and T ₂ ≈ 180 K for the molybdate and at T ₁ = 201.5 K and T ₂ ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσ _i , which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The p-T and α _i -T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−17 K for the molybdate and ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−13 K for the tungstate.     

Thermal physical properties of the La0.825Sr0.175MnO3 single crystals

The heat capacity (C _P), the thermal diffusion (η), the thermal conductivity (κ), and the electrical resistance of the La_0.825Sr_0.175MnO₃ single crystal have been measured in the temperature range 80–350 K in magnetic fields to 40 kOe. Dependences C _P(T), κ(T), and η(T) have anomalies near T _C, which are suppressed in magnetic field. The minima in dependences κ(T) and η(T) near T _C are explained by the phonon scattering on fluctuations of the magnetic order parameter. Dependences κ(T) and η(T) have anomalies near T _S = 200 K related to the structural transition from the rhombohedral (R) to the orthorhombic (O*) phase.

     

Dielectric properties of (NH4)2SO4 crystals in the range of electronic excitations

Spectra of the real and imaginary parts of the pseudo‐dielectric permittivity, 〈?₁〉(E) and 〈?₂〉(E), of ferroelectric ammonium sulfate crystals, (NH₄)₂SO₄, have been measured in the range of electronic excitations 4.0 to 9.5 eV by ellipsometry using synchrotron radiation. Temperature dependences of the corresponding susceptibilities, 〈χ₁〉(T) and 〈χ₂〉(T), obtained for the photon energy E = 8.5 eV, related to excitations of oxygen p‐electrons, reveal sharp peak‐like temperature changes near the Curie point T_C = 223 K. The large temperature‐dependent increase of the imaginary part of the susceptibility χ₂(T), together with a simultaneous decrease of the real part of the susceptibility χ₁(T), take place at the phase transition. These anomalies have been ascribed mainly to the SO₄ group of the crystal structure.     

Temperature dependence of the heat capacity and saturation magnetization of dilute reentrant ferrimagnetic spinels

Using model objects consisting of dilute reentrant spinels Li_0.5Fe_2.5−x Ga_xO₄ with x=1.0–1.2, this paper describes the temperature dependence of the magnetic contribution to the heat capacity, C _m (T), at H=0 and temperatures exceeding the freezing temperature (T _f ∼10 K) and, for T⩾4.2 K, the low-field magnetization σ _H (T) and the saturation magnetization σ _S (T), as well as the magnetization isotherms σ _T (T) in fields of up to 10 kOe. The specific features of the behavior of the overall characteristics of C _m (T) and σ _S (T) are found for the states of a frustrated ferrimagnetic that occur in the conditions considered here (H=0 and T>T _f or T⩾4.2 K and strong magnetic fields) and are discussed in connection with changes in the magnetic excitation spectrum caused by local breakdowns of collinear spin ordering and frustrations. Zh. éksp. Teor. Fiz. 113, 1339–1349 (April 1998)     

Magnetic properties of RPd5Al2 (R=Y, Ce, Pr, Nd, Sm, Gd)

Polycrystalline samples of a new rare-earth series RPd₅Al₂ crystallizing in the tetragonal ZrNi₂Al₅-type structure have been prepared. Their physical properties by electrical resistivity ρ, magnetic susceptibility χ, magnetization M and specific heat C_p measurements are reported. The ingots are composed of elongated grains preferentially aligned in the c direction; therefore, measurements were conducted parallel and perpendicular to the grains. Antiferromagnetic ordering appears in R=Ce, Nd, Gd, and Sm at low temperatures. CePd₅Al₂ has two AFM transitions at 4.1 and 2.9 K and ρ(T) indicates a Kondo metal behavior with large anisotropy. In PrPd₅Al₂ no magnetic transition was observed down to 0.4 K. The C_p(T) shows a broad peak around 13 K due to the CEF effect, suggesting a non-magnetic singlet ground state. In NdPd₅Al₂, χ(T) shows anisotropy and the C_p(T) shows a sharp peak at 1.2 K. The magnetic entropy at 3 K is very close to Rln2, indicating a Kramers doublet ground state. In SmPd₅Al₂, C_p(T) shows a magnetic transition at 1.7 K. C_p(T) for GdPd₅Al₂ shows a peak at 6 K, followed by a broad anomaly around 3 K. Within this series, T_N's for CePd₅Al₂ and NdPd₅Al₂ clearly deviate from the relation predicted by de Gennes scaling, which is ascribed to the CEF effect.     

Magnetotransport properties of a layered La1.6Ca1.4Mn2O7 polycrystal

Magnetotransport properties of a two-layered La_1.6Ca_1.4Mn₂O₇ polycrystal have been examined as a function of temperature and applied field. It was found that the magnetic transition temperature (T_c) is about 70 K higher than the insulator–metal transition temperature (T_p). Two peaks were observed on both the temperature dependence of the imaginary part of the ac magnetic susceptibility χ^′′(T ) and that of the magnetoresistance MR(T). One is slightly below T_p∼107 K and the other is near T_c∼170 K. Below 70 K, the MR ratio increases with decreasing temperature. Around and above T_p but below T_c, the magnetization shows some indication of saturation, whereas the MR ratio shows no indication of saturation. The magnetotransport properties can be explained by considering the anisotropy exchange interactions along the a–b plane and the c direction, and the low-temperature MR can be attributed to the effects of the nearly fully spin-polarized carriers’ tunneling through the insulating (La,Ca)₂O₂ layers between the adjacent MnO₂ bilayers. Received: 18 September 2000 / Accepted: 20 February 2001 / Published online: 26 April 2001