Linear Temperature Term Research Articles

Heat capacity and thermal expansion of the itinerant helimagnet MnSi

The heat capacity and thermal expansion of a high quality single crystal of MnSiwere measured at ambient pressure at zero and high magnetic fields. Thecalculated magnetic entropy change in the temperature range 0–30 K is less than0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A lineartemperature term dominates the thermal expansion coefficient in the range 30–150 K, whichcorrelates with an enhancement of the linear electronic term in the heat capacity. Asurprising similarity among the variations of the heat capacity, thermal expansioncoefficient and temperature derivative of the resistivity is observed through the phasetransition in MnSi. Specific forms of the heat capacity, thermal expansion coefficient andtemperature derivative of resistivity at the phase transition to a helical magnetic state near29 K are interpreted as the combination of sharp first-order features and broad peaks orshallow valleys of as yet unknown origin. The appearance of these broad satellites probablyhints at a frustrated magnetic state slightly above the transition temperature in MnSi.

Experimental study of the magnetic phase transition in the MnSi itinerant helimagnet

Magnetic susceptibility, heat capacity, thermal expansion, and resistivity of a high-quality single crystal of MnSi were carefully studied at ambient pressure. The calculated change in magnetic entropy in the temperature range 0–30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the behavior of the thermal expansion coefficient in the range 30–150 K, which correlates to a large enhancement of the linear electronic term in the heat capacity. A surprising similarity between variation of the heat capacity, the thermal expansion coefficient, and the temperature derivative of resistivity through the phase transition in MnSi is observed. Specific forms of the heat capacity, thermal expansion coefficient, and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as a combination of sharp first-order features and broad peaks or shallow valleys of yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state in MnSi slightly above the transition temperature. Present experimental findings bring the current views on the phase diagram of MnSi into question.

Heat transport inBi2+xSr2−xCuO6+δ: Departure from the Wiedemann-Franz law in the vicinity of the metal-insulator transition

We present a study of heat transport in the cuprate superconductor ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ at sub-Kelvin temperatures and in magnetic fields as high as 25 T. In several samples with different doping levels close to optimal, the linear-temperature term of thermal conductivity was measured both at zero field and in presence of a magnetic field strong enough to quench superconductivity. The zero-field data yields a superconducting gap of reasonable magnitude displaying a doping dependence similar to the one reported in other families of cuprate. The normal-state data together with the results of the resistivity measurements allows us to test the Wiedemann-Franz (WF) law, the validity of which was confirmed in an overdoped sample in agreement with previous studies. In contrast, a systematic deviation from the WF law was resolved for samples displaying either a lower doping content or a higher disorder. Thus, in the vicinity of the metal-insulator crossover, heat conduction in the zero-temperature limit appears to become significantly larger than predicted by the WF law. Possible origins of this observation are discussed.

Zero- and one-dimensional 4He Bose fluids realized in nanometer pores

To realize helium quantum fluids in clusters and in one-dimensions, we have studied 4He and 3He adsorbed in nanometer diameter pores of zeolite and FSM-16. In each cage 13 Å in diameter of Na-Y zeolite covered with helium solid layer, a few helium atoms are suggested to be Bose and Fermi clusters for 4He and 3He atoms, respectively, of which symmetry difference results in completely different eigenstates and low temperature heat capacities. In the one-dimensional (1D) pores of 18 and 28 Å in FSM-16, 4He fluid nanotubes are formed on the pore walls preplated with solid-like helium layers. Absolute 1D Bose fluid is realized at sufficiently low temperatures and appropriate coverages, where the transverse motion in the fluid tube is in a ground state. We observed a temperature-linear term of the heat capacity probably due to the 1D phonon heat capacity of the 1D interacting Bose fluids.

Random magnetic moments and spin-glass-like behaviors in the heavy-fermion compoundCeNi2Sn2

We have studied the magnetic, electric, and thermal properties of ${\mathrm{CeNi}}_{2}{\mathrm{Sn}}_{2}.$ The zero-field-cooled susceptibility ${\ensuremath{\chi}}_{\mathrm{ZFC}}(T)$ deviates from the field-cooled susceptibility ${\ensuremath{\chi}}_{\mathrm{FC}}(T)$ below 3 K. Both real $({\ensuremath{\chi}}^{\ensuremath{'}})$ and imaginary $({\ensuremath{\chi}}^{\ensuremath{''}})$ components of the ac susceptibility exhibit pronounced maxima at \ensuremath{\sim}4 and \ensuremath{\sim}5 K. At 2 K, the magnetization $M(H)$ clearly exhibits hysteresis. The zero-field cooled-magnetization ${M}_{\mathrm{ZFC}}(t)$ is frozen at 2 K in a 0.005-T field with an enormous time constant of 917 sec. In 0.005 T, zero-field-cooled resistivity ${\ensuremath{\rho}}_{\mathrm{ZFC}}(T)$ starts to deviate from field-cooled resistivity ${\ensuremath{\rho}}_{\mathrm{FC}}(T)$ at \ensuremath{\sim}8 K, and both ${\ensuremath{\rho}}_{\mathrm{ZFC}}(T)$ and ${\ensuremath{\rho}}_{\mathrm{FC}}(T)$ rapidly decrease as the temperature is below 5 K. These behaviors suggest a spin-glass phase transition at ${T}_{f}\ensuremath{\sim}3\mathrm{K}$ for ${\mathrm{CeNi}}_{2}{\mathrm{Sn}}_{2}.$ The coefficient of the term linear in temperature \ensuremath{\gamma}, in the specific heat of ${\mathrm{CeNi}}_{2}{\mathrm{Sn}}_{2}$ between 30 and 40 K, is 112 mJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$, which is much larger than those of normal metals. ${\mathrm{CeNi}}_{2}{\mathrm{Sn}}_{2}$ is a true heavy fermion with spin-glass-like behavior. In a 10-T field, the temperature dependence of ${C}_{m}/T$ exhibits a peak at \ensuremath{\sim}5 K. The easiest interpretation for this behavior is that, in a high magnetic field, there is a field-induced ferromagnetic phase transition at \ensuremath{\sim}5 K.

Nearly antiferromagnetic metal with linear spin chains: LaMn 4Al 8

Magnetic properties of the tetragonal ThMn 12-type LaMn 4Al 8 were investigated to get information on the origin of the large temperature-linear term of the low- temperature specic heat (γ=265 mJ/K 2 mol) of this compound. The susceptibility obeys the modified Curie–Weiss law with a small effective moment at high temperatures and shows a broad maximum at around 5 K. The 27Al nuclear spin-lattice relaxation rate shows the typical temperature dependence for the nearly antiferromagnetic metal. The one-dimensional-like arrangement of Mn atoms is emphasized as one of the characteristics of this compound.

Helium-4 Bose fluids formed in one-dimensional 18 A diameter pores.

Bose fluids restricted in one dimension (1D) are realized by adsorbing 4He atoms on the 1D pore walls with a diameter of about 18 A. The Bose fluid appears above an adsorbed amount after the pore walls are coated with the inert 4He atoms. Heat capacity of the fluid was observed to have a temperature-linear term at low temperatures. This corresponds to the phonon heat capacity of the Bose fluid in the 1D pores. We estimate the phonon velocity and the interaction of the 1D Bose fluid.

Giant Heat Release and Time-Dependent Thermal Expansion of Nb-Ti-D

Glasslike anomalies of low-temperature thermal properties were observed for the polycrystalline Nb37Ti63 alloy and Nb37Ti63 doped with deuterium. A giant heat release effect was found in (Nb-Ti)92D8 corresponding to a spectral density of two level systems of 7.8·1045 J−1 m−3. After rapid cooling of the sample a length relaxation with a quadratic dependence on the starting temperature was detected obeying a logarithmic time dependence. The results fit to the standard tunneling model assuming a temperature and time independent Gruneisen parameter. A constant Gruneisen parameter is compatible with a constant deformation potential γ but requires ∂λ/∂V to depend on Δ/Δ0. For both systems the thermal expansion coefficient exhibits a linear temperature term in the superconducting state dominating below 1 K.

Use of a rotational bench viscometer to study the influence of temperature and agitation speed on vinasse viscosity

Brookfield R.V.T. apparent viscosity measurements of 31.1 to 73o Brix vinasses, with and without nondissolved solids (N.D.S.), were carried out at varying rotation speeds (N) and temperatures (T). A regression analysis of this data was carried out to select the corresponding mathematical models. It was concluded that apparent Brookfield viscosity for low concentration vinasses (up to 52.4o Brix) depends only on linear and quadratic temperature terms. At higher concentrations (66o and 73o Brix) regression models for apparent viscosity, must also include quadratic as well as rotation speed-temperature interaction terms. This behavior is discussed, identifying two types of rheological behaviors and advancing a preliminary hypothesis about the role of solutes, N.D.S. and temperature.

Ce2CuGe3:A nonmagnetic atom-disorder spin glass

We have studied the magnetic, electric, and thermal properties of ${\mathrm{Ce}}_{2}{\mathrm{CuGe}}_{3}.$ The zero-field cooled susceptibility ${\ensuremath{\chi}}_{\mathrm{ZFC}}(T)$ deviates from the field-cooled susceptibility ${\ensuremath{\chi}}_{\mathrm{FC}}(T)$ below 3 K, which suggests a spin-glass phase transition at ${T}_{f}=3\mathrm{K}.$ The real $({\ensuremath{\chi}}^{\ensuremath{'}})$ and imaginary $({\ensuremath{\chi}}^{\ensuremath{''}})$ components of the ac susceptibility exhibit pronounced maxima at \ensuremath{\sim}3 K and \ensuremath{\sim}2 K, respectively. At 2 K, the magnetization $M(H)$ clearly exhibits hysteresis. Below 6 K, ${\mathrm{Ce}}_{2}{\mathrm{CuGe}}_{3}$ shows small spontaneous magnetic ordering. The specific heat $C(T)$ of ${\mathrm{Ce}}_{2}{\mathrm{CuGe}}_{3}$ presents a peak \ensuremath{\sim}4 K. Since the magnetic entropy between 2 and 4 K is much smaller than $2R\mathrm{ln}2,$ the 4-K peak of $C(T)$ is unlikely related to a long-range magnetic ordering. This 4-K peak of $C(T)$ might be due to Schottky anomaly with spin-glass contribution. The coefficient of the term linear in temperature in the specific heat \ensuremath{\gamma} of ${\mathrm{Ce}}_{2}{\mathrm{CuGe}}_{3}$ is 249.86 mJ ${\mathrm{mole}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$, which is much larger than that of normal metals. A spin-glass magnetism could result in a possible enlargement of the specific heat. At 2 K, the resistance $R(H)$ features hysteresis. This compound might be classified as a nonmagnetic atom-disorder spin glass or a reentrant spin glass.

Effects of doping on thermally excited quasiparticles in the high-Tcsuperconducting state

The physical properties of low-energy superconducting quasiparticles in high-${T}_{c}$ superconductors are examined using magnetic penetration depth and specific-heat experimental data. We find that the low-energy density of states of quasiparticles of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ scales with $(x\ensuremath{-}{x}_{c}{)/T}_{c}$ to the leading-order approximation, where ${x}_{c}$ is the critical doping concentration below which ${T}_{c}=0.$ The linear temperature term of the superfluid density is renormalized by quasiparticle interactions and the renormalization factor times the Fermi velocity is found to be doping independent.

Complex magnetic ordering in Nd 2CuSi 3

We report the magnetic susceptibility, specific heat and electrical resistivity measurements of Nd 2CuSi 3. Nd 2CuSi 3 exhibits an antiferromagnetic transition at T N = 5 K and shows spin-glass properties below 5 K. The coefficient of the term linear in temperature in the specific heat γ of Nd 2CuSi 3 is 0.753 J mole −1 K −2 which is much larger than that of normal metals. The influence of strong site disorder among Cu and Si atoms results in an inhomogeneous magnetic ordering and causes a large contribution to heat capacity near the ordering temperature.

Systematic behavior of the in-plane penetration depth ind-wave cuprates

We report the temperature T and oxygen concentration dependences of the penetration depth of grain-aligned YBa_2Cu_3O_{7-\delta} with \delta= 0.0, 0.3 and 0.43. The values of the in-plane \lambda_{ab}(0) and out-of-plane \lambda_{c}(0) penetration depths, the low temperature linear term in \lambda_{ab}(T), and the ratio \lambda_{c}(0) /\lambda_{ab}(T) were found to increase with increasing $\delta$. The systematic changes of the linear term in \lambda_{ab}(T) with T_c found here and in recent work on HgBa_2Ca_{n-1} Cu_nO_{2n+2+\delta} (n = 1 and 3) are discussed.

Giant magnetoresistance inFe1−xCox/Cr(001)trilayers

We have investigated the magnetic and magnetotransport properties of wedge-shaped ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}/{\mathrm{C}\mathrm{r}/\mathrm{F}\mathrm{e}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$ trilayers grown by molecular-beam epitaxy on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ $(11\ifmmode\bar\else\textasciimacron\fi{}02)$ substrates with the Co concentration $x$ ranging from 0 to 0.2. A completely antiferromagnetic (AFM) coupling with zero remanence is obtained in both pure Fe/Cr and alloy ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}/\mathrm{Cr}$ samples at a Cr thickness of about 10 \AA{}. The giant magnetoresistance (GMR) of the AFM coupled Fe/Cr trilayer is about 5.5% at low temperatures. With increasing Co concentration, the GMR effect, $\ensuremath{\Delta}\ensuremath{\rho}/{\ensuremath{\rho}}_{s},$ decreases drastically, having an amplitude of only 0.7% at $x=0.2$. In the pure Fe/Cr trilayer, there is a strong reduction of the GMR effect with increasing temperature. In contrast, the GMR of the samples with alloy magnetic layers is only weakly temperature dependent because of an increase of the net change of the magnetoresistance, $\ensuremath{\Delta}\ensuremath{\rho}$, with temperature. The strong decrease of the GMR effect in the alloy trilayers is tentatively ascribed to the possible loss of the band matching of the minority-spin band, which is important in the Fe/Cr superlattice. The temperature dependence of $\ensuremath{\Delta}\ensuremath{\rho}$ at finite temperatures can well be interpreted by introducing a positive linear temperature term together with a negative quadratic term, which are attributed to spin-dependent electron-phonon scattering and spin-flip electron-magnon scattering, respectively.

Computational analysis of coupled radiation-convection dissipative non-gray gas flow in a non-Darcy porous medium using the Keller-box implicit difference scheme

The effects of thermal radiation parameter (F), transpiration (γ), Eckert number (Ec), Prandtl number (Pr), buoyancy (Grashof number Gr), a Darcy parameter (Re/Gr Da) and a Forcheimmer inertial parameter (Fs Re/Gr Da) on two-dimensional free convective flow of an optically thin, near-equilibrium, non-gray gas past a vertical surface in a non-Darcy porous medium, are studied using the robust Keller finite-difference technique incorporating Newtonian quasilinearization and block-tridiagonal elimination. The Darcy–Brinkman–Forcheimmer inertial-viscous flow model is used for the momentum equation and the Cogley–Vincenti–Giles formulation is adopted to simulate the radiation component of heat transfer. The one-dimensional thermal radiation model works successfully for gases in the optically thin limit. Pseudo-similarity transformations are employed to simplify the highly non-linear partial differential equations for momentum and heat transfer into numerically manageable pseudosimilar ordinary differential equations which are solved with Keller's box method. Effectively, the radiation contribution is seen to take the form of a linear temperature term Fθ coupled with the streamwise pseudo-similar variable ξ. Local wall shear stress and local heat transfer rates are systematically computed for a wide selection of radiation parameter F values. The results are presented graphically for different gases. © 1998 John Wiley & Sons, Ltd.

Mass enhancement and spin-glass behavior inPr2CuSi3

We report magnetic-susceptibility, specific-heat, and electrical resistivity measurements of ${\mathrm{Pr}}_{2}{\mathrm{CuSi}}_{3}.$ This compound exhibits ferromagnetism at ${T}_{c}=10$ K and upon further cooling shows spin-glass properties below about ${T}_{f}=8$ K. The coefficient of the term linear in temperature in the specific heat \ensuremath{\gamma} of ${\mathrm{Pr}}_{2}{\mathrm{CuSi}}_{3}$ is 0.505 ${\mathrm{J}\mathrm{}\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$ which is much larger than that of normal metals. This compound might be classified as a nonmagnetic atom-disorder spin glass.

Low-temperature specific heat of κ-(BEDT-TTF)2Cu[N(CN)2]Br in the superconducting state

The low-temperature specific heat of single crystals of \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$ Cu[N(CN${)}_{2}$ ]Br was studied in the temperature range between 0.11 and 4.5 K. The electronic specific heat, ${\mathrm{C}}_{\mathrm{el}}$, in the superconducting state shows a quadratic temperature dependence that is most reasonably attributed to the quasiparticle excitations found in unconventional superconductors with line nodes in the gap structure. The magnetic-field dependence of the temperature-linear term in ${\mathrm{C}}_{\mathrm{el}}$ is also consistent with this picture of superconductivity.

Charge-spin separation, spin-excitation spectrum, and the normal-state properties of the copper oxide superconductors.

We show that the unusual normal-state properties of the copper oxide superconductors can be derived from the one-band t-J model with the use of a rigorously imposed constraint of single-electron occupancy and the Wigner-Jordan representation of the spin-excitation spectrum. These include the linear resistivity in the ab plane, the 1/T resistivity along the c direction, the anomalous spin-relaxation rate in the Cu site, the excessive absorption in the infrared conductivity over that expected from the Drude theory, the flat electronic Raman background, the broadening of the photoemission spectra, and the linear-bias dependence of the normal-metal--normal-metal tunneling conductance (along the ab plane). The zero-bias tunneling conductance along the ab plane is predicted to have a linear temperature dependence. A particularly interesting prediction is that the ratio of the linear temperature term of the zero-bias conductance to the linear-bias term of the zero-temperature conductance is 2T/\ensuremath{\Vert}eV\ensuremath{\Vert}, independent of all other parameters. The static spin susceptibility at low temperature is predicted to have the form A+B ln(${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$/T), where ${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$\ensuremath{\sim}J is a cutoff energy. The doping (x) dependences of various physical quantities are also predicted for the range of x large enough to allow the holons to form a band and the spins to have a disordered (liquid) ground state, but not too large to destroy the antiferromagnetic spin correlation. The imaginary part of the spin susceptibility has a wave-vector-independent component with an x dependence of x(1-x${)}^{2}$. The nuclear relaxation at the Cu site, 1/(${\mathit{TT}}_{1}$), has a 1/T component with an x dependence of x(1-x${)}^{2}$. The zero-bias tunneling conductance along the ab plane at zero temperature increases with ${\mathit{x}}^{2}$. The 1/T resistivity along the c direction at low temperature is inversely proportional to x, and the linear resistivity of the ab plane has a weak x dependence in the doping range mentioned above.

Anisotropic low-temperature heat transport in YBa2Cu3O6.9 single crystal

We report measurements of the low-temperature thermal conductivity of YBa2Cu3O7−δ (δ≃0.1) single crystals (Tc=84 K) both parallel (κa, b) and perpendicular (κc) to the CuO2 planes. Whereasκc (T) is found to be identical, within experimental resolution, with the phonon contributionκph (T),κa, b (T) contains an additional term linear in temperature,AT. We ascribeAT to the contribution of unpaired electronic carriers residing in the “chain layers”. Measurements performed in external magnetic fieldsB≦8 T support this interpretation. Our observations can be explained by an “internal multilayer” (IML) model in which it is assumed that strong superconductivity is generated within the CuO2 layers and weak superconductivity is induced in the “chain layers” by the proximity effect. The fit of the experimental results to the IML model reveals that approximately 15% of the electronic carriers remain unpaired in YBa2Cu3O7 belowT=1 K.

Anomalous magnetovolume effects of CeCu 6 at low temperatures

We report on a low-temperature magnetovolume study of single-crystalline heavy-fermion CeCu 6. Magnetostriction data ( B∥ c) taken at temperatures below 100 mK reveal two distinct anomalies at B 1=2.2 T and B 2=4.0 T, that are rapidly washed out by increasing the temperature. Thermal expansion measurements in fields up to 8 T ( B∥ c) reveal an anomalous field suppression of the term linear in temperature. The field B 1 and B 2 are likely related to a change in magnetic correlations. Their interrelation is investigated by a scaling approach.