Linear expansion, phase separation, and magnetic inhomogeneities in La0.92Ca0.08MnO3

A relation of the thermal expansion with magnetic and magnetotransport properties has been revealed in La_0.92Ca_0.08MnO₃ single crystals in the paramagnetic state. The magnetotransport and lattice properties and the specific features in the neutron scattering characteristics of the La_0.92Ca_0.08MnO₃ single crystals have been explained by the phase separation in the paramagnetic state into magnetic inhomogeneities (clusters) with short-range (～10 Å) and long-range (>10² Å) orders. The performed investigations have demonstrated that the clusters are closely related to the crystal lattice and that the magnetic inhomogeneities in the paramagnetic region are correlated to T ～ 250–300 K ? TC.     

Effect of granularity and inhomogeneity in excess conductivity of YBa2Cu3O7−δ+xBaTiO3 superconductor

Synthesis of polycrystalline YBCO+xBaTiO₃ (x=1.0, 2.5, 5.0) superconductor has been done and the effects of granularity and inhomogeneities due to inclusions of nano-BaTiO₃ in excess conductivity are reported in this work. The phase formation, texture and grain alignments were analyzed through XRD and SEM techniques. SEM results reveal that the grain size is reduced and morphology is improved with the incorporation of nano-BaTiO₃ particles. Superconducting order parameter fluctuation (SCOPF) studies on the electrical conductivity were investigated from the resistivity vs. temperature data in the experimental domain relatively above T_c. Log(Δσ) vs. log(ε) plots show that the 2D to 3D crossover temperature (T_LD) that demarcates dimensional nature of fluctuation inside the grains is influenced by BaTiO₃ incorporation in YBCO matrix. An upward shift of T_LD in the mean field region has been observed as a consequent dominance of 3D region with increase in 1 wt% BaTiO₃ in the composites as compared to higher inclusions. It has been analyzed that microscopic inhomogeneities produced as a result of diffusion of a fraction of Ti ions into the grains affect fluctuations in the excess conductive region. The interplay of microscopic inhomogeneities produced inside the grains and mesoscopic inhomogeneities in the grain boundaries on the excess conductivity has been explained in terms of thermal fluctuations for the composites.     

Pair-breaking current of inhomogeneous superconducting films

The influence of the inhomogeneities in the cross section and in the mean free path on the pair-breaking current I_c is considered using the Ginzburg-Landau theory. In the limiting case of “point” inhomogeneities, the simple analytic dependences of I_c on the parameters of the inhomogeneities and on their concentration are found. The temperature dependence of I_c is obtained.     

Specific heat of a Kondo superconductor: (La, Ce) Al2

The specific heat of LaAl₂ and (La_1-xCe_x)Al₂ (x ? 0.0064) has been measured between 0.3 and 5 K, both in the superconducting and in the normal state. For all samples the same values for the Debye temperature as well as for the electronic specific heat coefficient have been determined. LaAl₂ shows an excellent BCS behavior. A remarkable excess specific heat at low temperatures due to the Kondo effect has been observed for all superconducting as well as for the normal conducting (La_1-xCe_x) Al₂ alloys. The specific heat jump ΔC at T_c depressed rapidly with increasing Ce concentration, allows the Kondo temperature T_K ? 1 K to be determined. ΔC vanishes at finite temperatures.     

Effect of cation doping on low-temperature specific heat of LaMnO3 manganite

We have investigated the thermodynamic properties of perovskite manganite LaMnO₃, the parent compound of colossal magnetoresistive manganites, with the Ca²⁺ doping at the A-site. As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. We have described the temperature dependence of the lattice contribution to the specific heat at constant volume (C_v(lattice)) of La_1−xCa_xMnO₃ (x=0.125, 0.175, 0.25, 0.35, 0.50, 0.67, 0.75) as a function of temperature (1 K–20 K) by means of a rigid ion model (RIM).The trends of specific heat variations with temperature are almost similar at all the composition. The Debye temperatures obtained from the lattice contributions are found to be in somewhat closer agreement with the experimental data. The specific heat values revealed by using RIM are in closer agreement with the available experimental data, particularly at low temperatures for some concentrations (x) of La_1−xCa_xMnO₃. The theoretical results at higher temperatures can be improved by including the effects of the charge ordering, van der Waals attraction and anharmonicity in the framework of RIM.     

Superconductivity and electronic specific heat in V-Mo system

The parameters of superconductivity and specific heat are determined from the low temperature specific heat measurements of V_(1−x)Mo_x (0 ⩽ x ⩽ 1) solid solutions. The coefficient of electronic specific heat γ decreases with increasing Mo concentration and shows a minimum around 80 at. %Mo. Debye temperature θ_D varies slightly over the whole composition range. The superconducting transition temperature T_c also decreases with increasing Mo concentration. The variation of T_c is explained by the variation of γ and discussed in terms of the band structure.     

B(1)(+)/actual flip angle and reception sensitivity mapping methods: simulation and comparison

Knowledge of the spatial distribution of transmission field B₁⁺ and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B₁⁺ and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo.Several B₁⁺ mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B₁⁺ mapping techniques.This study introduces a quantitative comparison between two known methods for B₁⁺/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Effect of the correlation properties of one-and three-dimensional inhomogeneities on the high-frequency magnetic susceptibility of sinusoidal superlattices

The effect of one-(1D) and three-dimensional (3D) inhomogeneities on the high-frequency magnetic susceptibility at the boundary of the first Brillouin zone of a ferromagnetic superlattice is studied. The study is performed with an earlier developed method of random spatial modulation (RSM) of the superlattice period. In this method, structural inhomogeneities are described in terms of the random-phase model, in which the phase depends on three coordinates in the general case. The frequency spacing Δν_m between two peaks in the imaginary part of the averaged Green’s function, which characterizes the gap width in the frequency spectrum at the boundary of the Brillouin zone, is calculated as a function of both the root-mean-square fluctuations γ_i and the correlation wavenumbers η_i of phase inhomogeneities (i = 1 and 3 for 1D and 3D inhomogeneities, respectively). The function Δν_m(γ₁, η₁) for 1D inhomogeneities is shown to be symmetric with respect to interchanging the variables γ ₁ ² and η₁, whereas the function Δν_m(γ₃, η₃) for 3D inhomogeneities is strongly asymmetric with respect to interchanging γ ₂ ³ and η₃. This effect is associated with the difference in form between the correlation functions of 1D and 3D inhomogeneities and can be used to determine the dimensionality of inhomogeneities from the results of spectral studies of such superlattices.     

Specific heat of the (La,Gd) Al2 system in the superconducting and normal state

Specific heat measurements between 0.5 and 4.2°K are reported for the system ($\text{La}$, Gd) Al₂ in both the superconducting and normal state. The observed specific heat jump at the superconducting transition temperature T_c is in excellent agreement with the Abrikosov-Gor'kov (AG) theory. This is in accordance with the previously reported close correspondence of the T_c vs. Gd concentration curve with the AG theory. Two very interesting features occur in the normal state specific heat. First, the Gd impurities cause a surprisingly strong enhancement of the electronic specific heat coefficient. Second, there is a large magnetic field dependent Schottky-like anomaly at low temperatures. This anomaly persists even in the superconducting state.     

A.C. specific heat of terbium and dysprosium

The dependence of a.c. specific heat measurements upon the temperature modulation amplitude ΔT has confirmed the first-order nature of the antiferromagnetic-ferromagnetic transition in single crystal terbium (T_C～221K) and polycrystalline dysprosium (T_C～85K). No peak in a.c. specific heat is obtained for 2ΔT less than the temperature hysteresis at the transition. By comparison, the peaks expected in a.c. specific heat were observed at the higher-order antiferromagnetic-paramagnetic transition in both materials for ΔT amplitudes as low as～40 mK.     

Possible anisotropy gap in La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 detected through specific heat and magnetization measurements

Magnetization and specific heat measurements, as a function of temperature, were performed on single crystals of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄, under different applied magnetic fields (H). The specific heat in La_1.35Sr_1.65Mn₂O₇ was decreased for H=9 T parallel to the crystal c axis, compared with H=0, possibly due to a suppression of spin-wave excitations (magnons) in that ferromagnetic bilayer structure. On the other hand, the applied magnetic field had no effect in the specific heat of the antiferromagnetic La_1.5Sr_0.5NiO₄. For H=9 T and below the temperature of 4 K the specific heat data, for each crystal, was well fitted by an exponential decay law. This allowed the calculation of energy gaps around 1 meV for both compounds, in close agreement with Δ=2μ_BH for an expected energy gap in the magnon spectrum. Detailed magnetization measurements showed monotonic variations below 4 K and a steep increase close to 2 K. Both magnetization and specific heat measurements suggest the existence of an anisotropy gap in the energy spectrum of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄.     

Specific heat of La(Fe<Subscript>0.873</Subscript>Co<Subscript>0.007</Subscript>Al<Subscript>0.12</Subscript>)<Subscript>13</Subscript> compound in antiferromagnetic and ferromagnetic states

The low-temperature specific heat C _p of La(Fe_0.873Co_0.007Al_0.12)₁₃ compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of T _N = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction B _C ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*_C = 23 K. The coefficient γ_FM in the low-temperature specific heat C = γT + βT ³ in the FM state differs quite insignificantly from that (γ_AFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered.     

Field-enhanced electronic specific heat of carbon nanotubes

The tight-binding model including curvature effects is used to study the effect of transverse electric field on the low-temperature electronic specific heat (C_v) for armchair and zigzag carbon nanotubes (ACNTs and ZCNTs). Electric field could effectively modulate energy dispersions of CNTs and cause a shift of electronic states toward the Fermi energy. As field strength reaches to a critical value (F_c), it induces special structures in the density of states near the Fermi energy and thus the giant specific heat. At F_cs, C_v has a value comparable to that of the phonon specific heat and reveals strongly non-linear dependence on temperature. The critical field strength and giant specific heat are closely related to nanotube's geometry. Moreover, under F_cs, the extra longitudinal magnetic flux could cause a re-enhancement in C_v for ZCNTs, whereas C_v is always diminished for ACNTs.     

Dielectric properties and specific heat of Bi<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Sm<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>FeO<Subscript>3</Subscript> multiferroics

The specific heat and dielectric permittivity of Bi_1–xSm_xFeO₃ (x = 0–0.30) multiferroics have been studied in the temperature range of 300–800 K. A slight substitution of bismuth with samarium is established to cause a considerable shift in the antiferromagnetic phase transition temperature and to an increase in the specific heat over a wide temperature range. Other anomalies typical of phase transitions have been found in the temperature dependences of the specific heat and dielectric permittivity for the compounds with x = 0.10 and 0.15 at T ≈ 735 and 495 K, respectively. The results of the studies of the specific heat have been discussed together with the data of the structural investigations.     

Specific heat studies of high-Tc superconductor YBa2Cu3O7−x

Specific heat studies of the high-T_c superconducting compound YBa₂Cu₃O_7−x with bulk transition temperature at 92K are reported. A distinct anomaly of electronic origin in the specific heat is observed with granular-like behavior corresponding to a Sommerfeld constant γ = 7±2mJ(moleCuK²)^±1 Debye temperature (φo ≈ 400K) is obtained by fitting the experimental data with the theoretical Debye specific heat.     

The low temperature specific heat of Lu-Cu-Y metallic glasses

The specific heat of a series of amorphous metallic alloys of the form Lu_xCu_0.37Y_0.63-x (x = 0, 0.1, 0.3 and 0.4) has been measured between 2 and 50 K, primarily in order to be able to determine the non-magnetic contributions of the specific heat in magnetic RE-Cu-Y amorphous alloys. The data at low temperature fit the simple form C_p = γT + βT³ from which values of γ and θ_D(0) have been determined. Consideration is given to the error that arises if Y is used rather than Lu or La in forming non-magnetic rare earth intermetallics for purposes of determining the non-magnetic contributions to the specific heat of magnetic samples. A simple procedure is described that allows a useful improvement in accuracy in estimating non-magnetic contributions below 20 K if Y is used. The method may also be useful if only a restricted range of compositions using Lu is possible.     

Specific heat of YBa2Cu3O7−x from 4.2 to 60 K

The specific heat of superconducting oxide compound, YBa₂Cu₃O_{7 ?x} , is studied using a quasi-adiabatic calorimeter from 4.2 to 60 K. The analysis of the specific heat data below 15 K gives a value of 17 mJ/mole K² for the electronic heat capacity coefficient. The value ofθ _D(0) is determined to be 397±8 K. The variation ofθ _D with temperature was calculated in the temperature range 4.2 to 60 K.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

Effect of carbon substitution on the specific heat of Fe80B20−xCx (0≤x≤8)

The specific heat of ferromagnetic metallic glasses Fe₈₀B_20?xC_x (0≤x≤8) has been measured in the temperature range between 1.5 and 10 K. It is found that the electronic specific heat coefficient is independent of the carbon concentration x. The Debye temperature has a broad peak around x = 4. Both these results are in sharp contrast to the case of FeB binary metallic glasses and could be qualitatively understood in terms of magnetic and structural properties.