Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

The effect of ferrimagnetic ordering in insulating component of composites HTSC+Yttrium Iron Garnet on its transport properties

Composites Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Fe₅O₁₂ modeling random network superconductor-ferrimagnetic-superconductor have been prepared and their transport properties (temperature dependences of resistivity ρ and critical current density j_c, current-voltage characteristics) have been studied. Below the superconducting transition temperature T_c, the ρ(T) dependences exhibit a kink at some temperature T_m. The crossover of current-voltage characteristics from ohmic-like behavior in range T_m/T_c to non-linear one below T_m is observed. Transport properties of the ‘benchmark’ composites Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Al₅O₁₂ are typical for network superconductor-insulator-superconductor Josephson junction. The behavior observed for Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Fe₅O₁₂ composites is attributed to peculiarities of supercurrent carriers tunnelling through ferrimagnetic barriers.     

Phase transitions and vortex structure evolution in two-level high-temperature granular superconductor YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 – δ</Subscript> under temperature and magnetic field

Temperature dependences of the resistivity ρ(T) of samples of granular high-temperature superconductor YBa₂Cu₃O_{7 – δ} are measured at various transverse external magnetic fields at 0 < H _ext < 1900 Оe in the temperature range from the upper Josephson critical temperature of “weak bonds” T _c2J to temperatures slightly exceeding the superconducting transition temperature T _c . Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds,” and temperature and field dependences of the magnetic contribution to the resistivity \(\left[ {\Delta \rho \left( {T,H} \right) = \rho {{\left( T \right)}_{{H_{ext}} = const}} - \rho {{\left( T \right)}_{{H_{ext}} = 0}}} \right]\). It is shown that the behavior of the magnetic contribution to the resistivity Δρ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains T _c1g (H _ext) is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of T _c1g : the Josephson vortex decreases, and the Abrikosov vortex density increases.     

Nature of unusual spontaneous and field-induced phase transitions in multiferroics RMn2O5

Complex magnetic, magnetoelectric and magnetoelastic studies of spontaneous and field-induced phase transitions in TmMn₂O₅ were carried out. In the vicinity of spontaneous phase transition temperatures (35 and 25 K) the magnetoelectric and magnetoelastic dependences demonstrated the jumps of polarization and magnetostriction induced by the field ∼150 kOe. These anomalies can be attributed to the influence of magnetic field on the conditions of incommensurate-commensurate phase transition at 35 K and the reverse one at 25 K. In b-axis dependences the magnetic field-induced spin-reorientation phase transition was also observed below 20 K. Finally the magnetoelectric anomaly associated with metamagnetic transition is observed below the temperature of rare-earth subsystem ordering at relatively small critical fields of 5 kOe. This variety of spontaneous and induced phase transitions in RMn₂O₅ stems from the interplay of three magnetic subsystems: Mn³⁺, Mn⁴⁺, R³⁺. The comparison with YMn₂O₅ highlights the role of rare earth in low-temperature region (metamagnetic and spin-reorientation phase transitions), while the phase transition at higher temperatures between incommensurate and commensurate phases should be ascribed to the different temperature dependences of Mn³⁺ and Mn⁴⁺ ions. The strong correlation of magnetoelastic and magnetoelectric properties observed in the whole class of RMn₂O₅ highlights their multiferroic nature.     

Setup for studying nonlinear magnetic properties of high-temperature superconductors with the aid of magnetization harmonics

A setup for measuring cophasal and quadrature components of higher harmonics of an electromotive-force signal of the response of a high-temperature superconductor makes it possible to study nonlinear magnetic properties of superconductors in variable magnetic fields of up to 1 kOe and constant magnetic fields of up to 10 T in the temperature range of 5?C300 K. This setup was used to measure the temperature dependences of the absolute values of the real and imaginary parts of the first and third harmonics of the magnetization of textured Yba₂Cu₃O_{7 ? x} polycrystalline samples in the temperature range of 77?C220 K at various values of variable and constant magnetic fields. An analysis of resulting data made it possible to reveal the presence of different dynamical modes of the magnetic flux in YBa₂Cu₃O_{7 ? x} that were dominant in different temperature ranges. The nonlinearity of the magnetization of YBa₂Cu₃O_{7 ? x} (the appearance of higher harmonics) was observed up to temperatures in the range of T = 103?C112 K, which were substantially higher than the temperature of the transition of this compound to a superconducting state. The observed feature in the magnetization of YBa₂Cu₃O_{7 ? x} was associated with the emergence of a pseudogap state in this compound.     

Lattice vibrations of α′-NaV2O5

We have measured far infrared reflectance and transmittance spectra as well as Raman scattering spectra of α′-NaV₂O₅ single crystals for all the principal polarizations. The temperature range above the phase transition temperature T _c =35 K was investigated, mainly. On the basis of this experimental study and of the lattice dynamics calculations we conclude that the symmetry of NaV₂O₅ in the high-temperature phase is described by the centrosymmetric D _2h ¹³ space group. This conclusion leads to important physical consequences concerning the interpretation of one-dimensional magnetic properties of NaV₂O₅ and of the phase transition at 35 K considered earlier to be an ordinary spin-Peierls transition. The assignment of the observed phonons is given. Values of dielectric constants are obtained from the infrared data. Asymmetric shapes of several infrared lines and higher-order infrared vibrational spectra are discussed. The crystal field energy levels of the 3d electron localized at the V⁴⁺ site have been calculated in the framework of the exchange charge model using the values of effective charges obtained from the lattice dynamics calculations. According to the results of these calculations, the broad optical bands observed earlier in the vinicity of 1 eV can be interpreted as phonon assisted d-d transitions. Zh. éksp. Teor. Fiz. 115, 2170–2189 (June 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Andreev reflection and experimental temperature dependences of the critical current in heterogeneous high-temperature superconductors (polycrystals and related composites)

The temperature dependences of the critical current of Y_3/4Lu_1/4Ba₂Cu₃O₇ polycrystalline high-temperature superconductors subjected to annealing for different times and those of Y_3/4Lu_1/4Ba₂Cu₃O₇ + BaPbO₃ composites were measured. The results obtained were analyzed in terms of the Gunsenheimer-Schüssler-Kümmler theory considering the Andreev reflection of carriers in a “superconductor-normal metal-superconductor” (S-N-S) junction. Good agreement between the experimental and theoretical temperature dependences of the critical current over a wide temperature range for polycrystals with both natural and artificially created boundaries (composites) made it possible to estimate the effective length of grain boundaries in the high-temperature superconductors. It was revealed that the critical current density measured at 4.2 K is an exponential function of the length of grain boundaries in samples of both types.     

Magnetization harmonics for YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis></Subscript> textured polycrystals above the superconducting transition temperature

The temperature dependences of the third and other higher magnetization harmonics for YBa₂Cu₃O_{7 ? x} textured polycrystals are experimentally investigated in the temperature range 77–120 K. It is revealed that the magnetization of the YBa₂Cu₃O_{7 ? x} textured polycrystals exhibits a nonlinear behavior up to temperatures considerably higher than the superconducting transition temperature. The observed nonlinearity of the magnetization of the YBa₂Cu₃O_{7 ? x} textured polycrystals is attributed to the pseudogap state that appears for this compound at a temperature T ～ 102 K.     

梯形化合物NaV2O4F电子结构的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理赝势平面波方法, 通过自旋极化的广义梯度近似(GGA)电子结构计算对梯形化合物NaV₂O₄F进行了研究. 考虑了四种假想的自旋有序态,计算结果表明该化合物的磁基态具有二维反铁磁(AFM)结构, 即沿梯阶和梯腿方向都表现为AFM作用. 能带结构显示NaV₂O₄F为绝缘体材料, 带隙约为1.0eV. 方锥体中的晶体场劈裂使得VO₄F方锥体中的 V^{4+ 关键词： 2O₄F')" href="#">NaV₂O₄F 梯形化合物 第一性原理计算 电子结构}     

Magnetostriction of the spin-Peierls cuprate CuGeO3

The isothermal, longitudinal magnetostriction of the spin-Peierls cuprate CuGeO₃ has been measured along the three orthorhombic directions up to 14 Tesla by means of a high resolution capacitance dilatometer. For all three axes we observe anomalies at both the dimerized/incommensurate (D/I) and the dimerized/uniform (D/U) transition whose sizes and signs differ. A precise H-T phase diagram is determined from the field and temperature dependence of the lattice constants, which roughly agrees with theoretical predictions. At the D/I transition the magnetostriction shows a jumplike behavior and a hysteresis indicating a first order transition. From the jumps of the magnetostriction at the D/I transition we estimate considerable, uniaxial stress dependences of the critical magnetic field H _c, which correlate with those of the spin-Peierls transition temperature. A finite magnetostriction is also resolved within the high temperature uniform phase of CuGeO₃ for all three lattice directions. These data show a pronounced, strongly anisotropic spin-lattice coupling in CuGeO₃ and allow to derive the uniaxial pressure dependences of the magnetic susceptibility. Based on our findings the relevance of different structural parameters for the magnetic exchange interaction is discussed.     

Temperature dependence of the critical current of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> films

The temperature dependence of the critical current of YBa₂Cu₃O_7?δ films is studied experimentally. The performed analysis allows separating two components of the critical current owing to pinning of vortices both on defects in the volume of the superconductor and on oxygen vacancies in the CuO₂ planes. The established temperature dependences of components make it possible to correctly describe the behavior of the total critical current in the studied temperature range from 4.2 K to the irreversibility temperature.     

23Na NMR study of the local order in the Na1/2Bi1/2TiO3 structure in a weak magnetic field

The orientation dependences of the second-order quadrupole shifts of the central component in the ²³Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na_1/2Bi_1/2TiO₃ structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] _p direction and a small displacement statistically or dynamically disordered over six equally probable [100] _p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] _p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.     

The influence of chromium impurity centers on the critical properties of a weakly polar ferroelectric Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript>

The Cr³⁺ EPR spectra of Li₂Ge₇O₁₅ (LGO) crystals are analyzed in the temperature range of the ferroelectric phase transition. The temperature dependence of the local order parameter is determined from the measured splittings of the EPR lines in the polar phase. The experimental critical exponent of the order parameter β=0.31 in the range from the phase transition temperature T _C to (T _C -T) ～ 40 K corresponds to the critical exponent of the three-dimensional Ising model. Analysis of the available data demonstrates that, away from the phase transition temperature T _C , the macroscopic and local properties of LGO crystals are characterized by a crossover from the fluctuation behavior to the classical behavior described in terms of the mean-field theory. The temperature dependence of the local order parameter for LGO: Cr crystals does not exhibit a crossover from the Ising behavior (β=0.31) to the classical behavior (β=0.5). This is explained by the defect nature of Cr³⁺ impurity centers, which weaken the spatial correlations in the LGO host crystal. The specific features of the critical properties of LGO: Cr³⁺ crystals are discussed within a microscopic model of structural phase transitions.     

Dielectric properties of lead zirconate titanate films synthesized through oxidation of metal layers

Films of the composition Pb(Zr _x Ti_1?x )O₃ are prepared by magnetron sputtering of metal layers onto titanium substrates with subsequent heat treatment in an oxygen atmosphere. The electrical properties of the samples prepared are investigated using impedance spectroscopy in the frequency range from 10² to 5 × 10⁵ Hz at temperatures of 300–750 K. The temperature dependences of the permittivity and the dielectric loss tangent at different frequencies exhibit a behavior typical of ferroelectrics and indicate the occurrence of a ferroelectric phase transition at temperatures close to T = 663 K. Analysis of the imaginary part of the electric modulus has revealed two possible relaxation mechanisms. The activation energy for dc electrical conduction in the paraelectric phase is determined.     

Intensities of f-f transitions in Pr3+ and Dy3+ in glasses in the near-IR spectral range

The absorption spectra of Pr³⁺ and Dy³⁺ ions in three glass matrices (SiO₂-P₂O₅-GeO₂, Al₂O₃-B₂O₃-SiO₂, and LiB₃O₅) are thoroughly studied in the near-IR spectral range (4600–14 300 cm^?1). The temperature dependences of the intensity, the width, and the location of the absorption bands observed are investigated for the first time. It is shown that the f-f transitions in the studied glasses are allowed by static odd distortions in the environment of the rare-earth ions and these distortions decrease with an increase in temperature. A comparative analysis of the absorption band parameters and their temperature behavior in different materials makes it possible to determine the differences in magnitudes, symmetries, and dispersions of the distortions in the nearest environment of rare-earth ions in different glasses.     

Dielectric properties and specific heat of the (1 ? x)PbNi1/3Nb2/3O3-xPbTiO3 ferroelectric ceramics

The temperature dependences of the permittivity and specific heat of a mixed (1 ? x)PbNi_1/3Nb_2/3O₃-xPbTiO₃ system (where x = 0.3, 0.4, 0.5) have been studied over a wide temperature range 290?C700 K. It has been shown that the composition with x = 0.3 undergoes a diffuse phase transition at T _m ?? 315 K. A specific heat anomaly that is characteristic of the phase transition has been revealed at T ?? 315 K in all the compositions under study.     

Investigation of the Josephson coupling through a magnetoactive barrier (ferrimagnet, paramagnet) in Y3/4Lu1/4Ba2Cu3O7 + Y3(Al1 − x Fe x )5O12 composites

The transport properties of two-phase composites consisting of a high-temperature superconductor and a nonsuperconducting component with magnetic ordering are analyzed. These composites are considered as a network of “superconductor-magnetoactive insulator-superconductor” weak links of the Josephson type. Substituted garnets Y₃(Al_{1 ? x} Fe _x )₅O₁₂ (x = 0, ..., 1.0) are used as a magnetoactive component. The composites under investigation contain 92.5 vol % Y_3/4Lu_1/4Ba₂Cu₃O₇ (the high-temperature superconductor) and 7.5 vol % Y₃(Al_{1 ? x} Fe _x )₅O₁₂ (x = 0, ..., 1.0). It is shown that an increase in the iron content in the Y₃(Al_{1 ? x} Fe _x )₅O₁₂ garnet leads to a reduction of the Josephson coupling strength: the temperature range in which the electrical resistance of the composites is equal to zero is reduced, and the critical current density at a temperature of 4.2 K decreases exponentially. For composites in which the iron content in the Y₃(Al_{1 ? x} Fe _x )₅O₁₂ garnet is higher than 0.1, the temperature dependence of the electrical resistance R(T) at temperatures below the transition point T _C of high-temperature superconductor crystallites has a portion in the range T _m -T _C where the resistance R(T) is independent of the transport current and the magnetic field strength. Below the temperature T _m , the dependences of the electrical resistance R(T) of the composites are nonlinear functions of the current and involve a considerable contribution from magnetoresistance. This behavior is characteristic of a network of Josephson junctions. The temperature T _m decreases with an increase in the iron content in the Y₃(Al_{1 ? x} Fe _x )₅O₁₂ garnet. The appearance of the above feature in the temperature dependences of the electrical resistance R(T) is interpreted as complete suppression of the Josephson coupling in the temperature range above T _m due to the interaction of supercurrent carrier pairs with magnetic moments of iron atoms in the dielectric barriers separating high-temperature superconductor grains.     

Technical magnetization of Tb3Fe5O12 garnet ferrite near the magnetic compensation point

The domain structure and technical magnetization of a single-crystalline Tb₃Fe₅O₁₂ garnet ferrite sheet near its magnetic compensation point (T _c ≈ 250 K) are studied. It is found that above this temperature the domain structure persists and considerably influences the field and temperature dependences of the Faraday effect. It is shown that the behavior of Tb₃Fe₅O₁₂ during technical magnetization near T _c is not fully described by the existing thermodynamic theory of the domain structure. The features of the domain structure and technical magnetization of Tb₃Fe₅O₁₂ are related to the transition of the magnetic structure to a non-collinear phase.     

Magnetic and electrical properties of bismuth cobaltite Bi24(CoBi)O40 with charge ordering

The compound Bi₂₄(CoBi)O₄₀ has been synthesized using the solid-phase reaction method. The temperature and field dependences of the magnetic moment in the temperature range 4 K < T < 300 K and the temperature dependences of the EPR line width and g-factor at temperatures 80 K < T < 300 K have been investigated. The electrical resistivity and thermoelectric power have been measured in the temperature range 100 K < T < 1000 K. The activation energy has been determined and the crossover of the thermoelectric power from the phonon mechanism to the electron mechanism with variations in the temperature has been observed. The thermal expansion coefficient of the samples has been measured in the temperature range 300 K < T < 1000 K and the qualitative agreement with the temperature behavior of the electrical resistivity has been achieved. The electrical and structural properties of the compound have been explained in the framework of the model of the electronic-structure transition with inclusion of the exchange and Coulomb interactions between electrons and the electron-phonon interaction.     

Phase transitions in xPbZr0.53Ti0.47O3-(1 − x)Mn0.4Zn0.6Fe2O4 magnetoelectric composites

The elastic, inelastic, and dielectric properties of the magnetoelectric composite xPbZr_0.53Ti_0.47O₃-(1 ? x)Mn_0.4Zn_0.6Fe₂O₄ (PZT-MZF) are studied in the temperature range from room temperature to 673 K. The influence of the ferroelectric PZT phase on the magnetic phase transition and the magnetic MZF phase on the ferroelectric phase transition is revealed. It is established that, as the PZT content increases, the degree of diffuseness of the phase transition decreases and a gradual crossover from a pronounced relaxor behavior to a more ordered ferroelectric behavior occurs.