Term In Specific Heat Research Articles

Wilson and Kadowaki-Woods ratios in heavy fermions

Recently we have shown that a one-parameter scaling, $T_{coh}$, describes the physical behavior of several heavy fermions in a region of their phase diagram. In this paper we fully characterize this region, obtaining the uniform susceptibility, the resistivity and the specific heat in terms of the coherence temperature $T_{coh}$. This allows for an explicit evaluation of the Wilson and the Kadowaki-Woods ratios in this regime. These quantities turn out to be independent of the distance $|\delta|$ to the quantum critical point (QCP). The theory of the one-parameter scaling corresponds to a local interacting model. Although spatial correlations are irrelevant in this case, time fluctuations are critically correlated as a consequence of the quantum character of the transition.

Critical behavior inLaTiO3+δ/2in the vicinity of antiferromagnetic instability

The critical behavior in the vicinity of antiferromagnetic instability has been investigated for the filling-controlled insulator-metal transition system, ${\mathrm{LaTiO}}_{3+\ensuremath{\delta}/2},$ by transport and specific heat measurements. With the increase of nominal hole doping $\ensuremath{\delta},$ the system undergoes a phase change from an insulating phase to a metallic phase at ${\ensuremath{\delta}}_{c}^{\mathrm{IM}}\ensuremath{\approx}0.05$ and the low-temperature antiferromagnetic phase disappears at ${\ensuremath{\delta}}_{c}^{\mathrm{mag}}\ensuremath{\approx}0.08.$ The Hall coefficient for an antiferromagnetic metallic crystal of $\ensuremath{\delta}\ensuremath{\approx}0.06$ shows a broad minimum near the N\'eel temperature while that for the compound of $\ensuremath{\delta}={\ensuremath{\delta}}_{c}^{\mathrm{mag}}$ exhibits little temperature variation. At the antiferromagnetic instability point, the electronic specific heat coefficient $\ensuremath{\gamma}$ as well as the coefficient $\ensuremath{\beta}$ of ${T}^{3}$ term of specific heat takes a maximum value as a function of $\ensuremath{\delta}.$ High-pressure measurements of resistivity and Hall coefficient revealed that not only the antiferromagnetic spin correlation but also randomness effect plays an important role in producing an unconventional metallic state in the immediate vicinity of the magnetic instability point of the present compound.

Specific heat of La 1− xCa xMnO 3− δ

Detailed specific heat measurements from 4.5 to 300 K are reported on a series of polycrystalline La 1− x Ca x MnO 3− δ samples, with x=0.10, 0.33 and 0.62. The samples were also investigated using electrical transport, magnetic measurements and low temperature NMR. The x=0.10 composition does not show a well defined transition in the specific heat at the ferromagnetic ordering temperature, but the low temperature specific heat shows a ferromagnetic T 3/2 spin wave term, and no electronic linear term as expected for an insulator. The x=0.33 composition undergoes a transition to a ferromagnetic metal with a well defined peak in the specific heat, suggestive of an underlying first order transition at T c; at low temperatures a linear electronic specific heat term is observed but no spin wave ferromagnetic term. High temperature specific heat data on the x=0.62 sample shows only one anomaly, suggesting simultaneous onset of charge ordering and antiferromagnetism. Low temperature data indicates that the magnetic phase is an A-type antiferromagnetic insulator with no electronic contribution. The NMR data at 4.2 K on this compound shows a motionally narrowed peak indicating short range ferromagnetic interactions or possibly magnetic polarons at 4.2 K.

The interface vibrational contribution to the thermodynamic functions of the system of liquid and solid separated by a planar interface

We study theoretically the interface vibrational contribution to thermodynamic properties of a system of solid and liquid in contact across a planar interface. We express thermodynamic quantities such as the free energy and specific heat in terms of the interface density of vibrational states. We derive a simple analytic expression for this function in terms of the static Green's tensor of the system. In order to calculate the free energy, we modify the Debye model to apply it to the interface problem. We also obtain the dynamical Green's tensor for the system of liquid and solid separated by a planar interface.

Magnetic Field Dependence of the Low-Temperature Specific Heat of the Borocarbide Superconductor LuNi2B2C

The low temperature specific heat of the borocarbide superconductor LuNi 2 B 2 C was measured under various magnetic fields ( H ) up to 8 T. The coefficient of the T -linear term in specific heat in the mixed state shows marked deviations from the H -linear dependence expected for conventional superconductors and is found to obey a \(\sqrt{H}\) dependence. A good description of the data can be obtained by assuming the presence of a zero-gap region along lines on the Fermi surface, indicating the possibility of unconventional pairing in the superconducting state of the borocarbides.

Anomalies of the low-temperature specific heat of the cuprates La2CuO4 and La2−x MxCuO4 (M = Sr, Ba)

The low-temperature specific heat of the cuprates La2CuO4, La2−x MxCuO4 (M = Sr, Ba) in the temperature interval 2–45 K has been investigated by the technique of pulsed differential calorimetry. It is found that the coefficient of the residual linear term in the specific heat remains constant in the entire experimental temperature interval. It is shown that La atoms play a special role in the formation of the anomaly, associated with the specific nature of the interaction of these atoms with their environment, in the acoustic region of the phonon spectrum of these objects near 6 meV.

Low-Temperature Specific Heat ofLa1−xSrxMnO3+δ

The specific heat of the perovskite manganite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3+\ensuremath{\delta}}$ was measured in the doping regime $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.0$ to 0.3 where at low temperatures the material changes from a layered antiferromagnetic insulator to a ferromagnetic metal. A term in the specific heat, $C\ensuremath{\propto}{T}^{2}$, is found in ${\mathrm{LaMnO}}_{3}$ that is attributable to spin excitations in a layered antiferromagnet. Ferromagnetic samples have the expected specific heat term $C\ensuremath{\propto}{T}^{3/2}$ due to ferromagnetic spin waves. The exchange coupling ( $J$), electronic linear term ( $\ensuremath{\gamma}$), Debye temperature ( ${\ensuremath{\theta}}_{D}$), and the local field at the Mn site ( ${H}_{\mathrm{hyp}}$) are extracted from the specific heat as functions of Sr doping.

The specific heat of the solid electrolyte system CuI - AgI

The specific heat for AgI-doped CuI has been measured by the method of differential scanning calorimetry. Anomalous specific heats were observed around the transition temperature from non-superionic to superionic phases. These were well explained in terms of the Schottky-type excess specific heat.

Strong nonequilibrium effects on specific heats and thermal conductivity of diatomic gas

Abstract The specific heats and thermal conductivity in a diatomic gas with a strong vibrational nonequilibrium are studied starting from the kinetic equations for the distribution functions. Three models for the heat conductivity coefficients are considered. The first one is based on a rigorous kinetic theory treatment. In the second model the inelastic collision integrals are neglected and the thermal conductivity coefficients are expressed only in terms of standard elastic collision integrals and nonequilibrium specific heats. The third model uses the Eucken empirical formula with real gas specific heats. The results are compared with experimental data and with the results of other authors. The limits of the validity of the Eucken formula are estimated and a refinement for strong nonequilibrium is given. A simplified formula for the heat conductivity coefficient is proposed which closely agrees with the experimental data and with the exact kinetic model.

Green's-tensor approach in the theory of the surface or interface vibrational contribution to thermodynamic properties of solids.

We present a theory of the surface or interface contribution to vibrational thermodynamic properties of solids. We express the thermodynamic quantities such as the free energy and specific heat in terms of the surface or interface density of vibrational states. We derive a simple analytic expression for this function in terms of a static Green's tensor of the system. We redefine the dynamical Green's tensor compared with some earlier works, requiring that it correspond to a Hermitian differential operator. In order to calculate the free energy, we modify the Debye model to apply it to surface or interface problems. The free energy and specific heat are calculated for the cases of an isotropic solid bounded by a stress-free planar surface and two isotropic solids separated by a flat interface. \textcopyright{} 1996 The American Physical Society.

The Kondo anomaly in the low temperature specific heat of Pr doped YBCO

Low-temperature specific heat measurements on Y 1−xPr xBa 2Cu 3 O 7 as a function of temperature T for X=0.2, 0.45, 0.6 with 90mK<T< 30K are reported. The specific heat of this system consists of a nuclear Schottky anomaly, a Pr magnetic Kondo anomaly and the background contribution. The large linear term of specific heat has been found to be connected with Kondo scattering of the mobile hole carriers by Pr magnetic moments. The spin-1 character of the Kondo specific heat anomaly indicates that the Pr valence state is close to +3 in the Y 1−xPr xBa 2CuO 7 system. The Tc depression and metal-insulator transition are believed to be attributed to the localization due to the interaction of Pr 4f electrons with hole carriers in CuO 2 planes.

The fluctuation-dissipation theorem in the framework of the Tsallis statistics

In the framework of a generalized statistical mechanics introduced recently by Tsallis (1993), we derive a generalized form of the fluctuation-dissipation theorem, which expresses a relation between extended susceptibilities and equilibrium fluctuations. To achieve this, we consistently propose a generalized functional form For the instantaneous distribution function. The present theorem recovers as particular cases the corresponding generalized relations already obtained for the specific heat in terms of the generalized energy fluctuations and for the susceptibility of a magnetic system under the action of a uniform magnetic field.

The specific heat of Ba 0.6K 0.4BiO 3 below 2K

We have measured the specific heat C of a polycrystalline sample of the non-cuprate superconductor Ba 0.6K 0.4BiO 3 ( T c ≈ 27K) from T = 1.8 K down to T = 0.1 K. The data could be fitted accurately to the expression C = AT −2 + αT n + βT 3 where n = 0.53 ± 0.10. The nuclear term ( AT −2) is attributed to the presence of a crystalline electric field gradient on the Bi nuclei. The residual specific heat term ( αT n) has also been observed in the low temperature specific heat of several cuprate high- T c superconductors (0.3 ≤ n ≤ 1) suggesting a possible common origin in all of these systems.

The electronic band structure and Fermi surface of YbBiPt

The local density band structure of the heavy-fermion compound YbBiPt is calculated using the linear-muffin-tin-orbital method. The topology of the resulting Fermi surface is investigated and found to be remarkably simple. In order to explain the observed low-field linear coefficient of the specific heat in terms of single particle excitations an enhancement of the band-structure density of states at the Fermi level by a factor of 750 is required. A corresponding enhancement of the calculated cyclotron masses would predict the existence of fermion quasiparticles with masses several thousand times that of the bare electron. The feasibility of observing such highly renormalized particles by means of the de Haas-van Alphen effect is considered.

Dependence of spin-glass behaviour on atomic short-range order

The effect of short-range order (SRO) on the magnetic behaviour as well as on the low-temperature specific heat of Au Fe-alloys has been studied for three well defined states of SRO. Usual spin-glass features are observed. The freezing temperature T f and the linear specific heat term γ ∗T appear to be not sensitive to SRO whereas the amplitude of the susceptibility peak at T f is drastically increased. This behaviour corresponds astonishingly well with previous findings on Cu Mn. This might be a consequence of the coupling of second nearest neighbours in Cu Mn but of third nearest neighbours in Au Fe.

Resistivity and magnetoresistance in La 2−xSr xCuO 4−δ. Ceramic samples

We report on the magnetoresistance of La 2−xSr xCuO 4−δ as a function of temperature, in applied magnetic fields up to 5 Tesla. These measurements were carried out on five samples ranging in strontium concentration from x=0.07 to x=0.20. In an earlier work on similar samples, the results of differential specific heat experiments were reported. In this present work we derive average values for the upper critical field H c2(O) and the electronic specific heat term γ is then estimated. Analysing the results in terms of fluctuation theory we obtain estimates for the coherence length ξ(O) and finally showthat the fluctuations appear to have a two dimensional character.

Atomic short-range order and spin-glass behaviour in concentrated Cu-Mn alloys

The effect of atomic short-range order (SRO) in Cu-Mn (5-30 at.% Mn) on the spin-glass properties is studied by measuring the magnetic behaviour and the low-temperature specific heat for two well defined equilibrium states of SRO. Usual spin-glass features are observed. The freezing temperatures Tf as well as the coefficient gamma * of the linear specific heat term appear to be not sensitive to SRO, whereas the amplitude of the peak susceptibility at Tf is drastically increased. The results are compared with previous work after different thermal treatments.

Thermal conductivity, electrical resistivity and specific heat of La 1.85Sr 0.15CuO 4 superconductor

The temperature dependences of the thermal conductivity and electrical resistivity of La 1.85 Sr 0.15 CuO 4 superconductor have been measured in the temperature range from 1.8K to 260K in order to investigate the scattering mechanism of conduction. Although the thermal conductivity does not exhibit a striking increase below T c , such as in the systems Y-Ba-Cu-O(1-2-3 type) and Bi-Pb-Sr-Ca-Cu-O, the lattice thermal conductivity was dominant. The T 2 -dependence of the thermal conductivity was also observed below 10K, like as those of Y-Ba-Cu-O and Bi-Pb-Sr-Ca-Cu-O. The low temperature specific heat has been also measured in the temperature range from 1.8K to 43K and it turned out that the electronic specific heat term was little and the Debye temperature, Θ D , was about 221K.

Interaction of conduction electrons with coherent atomic displacements in HTSC: Pseudospinons

A “pseudospin ferromagnet” model is proposed which describes the coupling of conduction electrons to coherent atomic displacements in high- T c superconductors and Ti-based alloys. Non-quasiparticle states (“pseudospinons”) arising in the model yield appreciable contributions to thermodynamic and transport properties (e.g., linear term in specific heat) and induced the gapless superconductivity at finite T. Under certain conditions, new energy scale (“Kondo temperature”) may occur in the electron spectrum near E F.

Magnetic behavior of U(3d) 2Ge 2 intermetallic compounds

We have prepared and studied a series of U(3d) 2Ge 2 compounds with 3d = Mn, Fe (single crystal), Co, Ni and Cu. In addition to the previously reported magnetization and resistivity measurements, we have now measured the specific heat of this complete series. An initial attempt is made to explain the specific heat in terms of electronic, phonon and magnetic contributions.