Experimental Specific Heat Data Research Articles

Quantum Order by Disorder in Frustrated Diamond Lattice Antiferromagnets

We present a quantum theory of frustrated diamond lattice antiferromagnets. Considering quantum fluctuations as the predominant mechanism relieving spin frustration, we find a rich phase diagram comprising of six phases with coplanar spiral ordering in addition to the Néel phase. By computing the specific heat of these ordered phases, we obtain a remarkable agreement between (k, k, 0) spiral ordering and the experimental specific heat data for the diamond lattice spinel compounds MnSc2S4, Co3O4, and CoRh2O4, i.e., specific heat data is a strong evidence for (k, k, 0) spiral ordering in all of these materials. This prediction can be tested in future neutron scattering experiments on Co3O4 and CoRh2O4, and is consistent with existing neutron scattering data on MnSc2S4. Based on this agreement, we infer a monotonically increasing relationship between frustration and the strength of quantum fluctuations.

Monte Carlo calculations of the magnetocaloric effect in gadolinium

In this work we discuss the magnetocaloric effect in metallic gadolinium. We use a model Hamiltonian of interacting 4f spins and treat the 4f spin–spin interaction both in the mean field approximation and in the Monte Carlo simulation. The calculations show that the mean field approximation yields reasonable results for the magnetocaloric potentials Δ S and Δ T ad but it fails in explaining the experimental data of specific heat at the magnetic ordering temperature. On the other hand, our theoretical results show that the Monte Carlo calculation describes well not only the magnetocaloric potentials Δ S and Δ T ad but also the specific heat capacity.

A Model for Separation and Melting of Deoxyribonucleic Acid in Replication and Transcription Processes

In order to explain the denaturation and melting in replication and transcription of deoxyribonucleic acid (DNA) at physiological temperature, we propose a dynamical model on the basis of structure and motion of DNA under the actions of energy released in hydrolysis of adenosine triphosphate (ATP) and enzymes. The model admits three degrees of freedom per base pair: two displacement variables associated with the vibrations of hydrogen atom in the hydrogen bonds and base (nucleotide), respectively, and an angular variable describing the rotation of base. The important role of hydrogen atom in the hydrogen bonds is stressed in this model. The Hamiltonian of the intact double helix system is given first, subsequently the equations of motion and solutions of a melting system are developed. The solutions represent the excited states formed by the displacements of hydrogen atoms and bases and the rotations of bases, arising from the energy absorbed by DNA, respectively. The results obtained show that the displacements of hydrogen atoms increase with time and along the helix. Once the displacements of hydrogen atoms and rotations of bases reach to certain limits, the hydrogen bonds are disrupted, and the two double helical strands of DNA are separated, a melting state in the replication or transcription process occurs. The properties of thermal motion of hydrogen atoms in this state at physiological temperature is further studied by using a transfer integral method and the thermodynamic properties such as free energy and entropy of the thermally excited state are obtained in the process. Values of characteristic parameters and critical temperature of melting in transcription process are derived. Finally the validity of the theory was demonstrated through comparison between experimental and theoretical data of specific heat and force of phase transition. This investigation is helpful to understand mechanism and essence of the replication or transcription process and promote quantitative description of these processes.

Structural, thermodynamic, and transport properties of Laves-phaseZrMn2from x-ray and neutron diffraction and first principles

Recently, the prediction of structural degeneracy for the ground state of $\mathrm{Zr}{\mathrm{Mn}}_{2}$ by ab initio methods has stimulated research on this system. We investigated the structural and thermodynamic properties by single crystal and powder x-ray diffraction (at zero and high pressure) as well as by neutron powder diffraction, demonstrating that the C14 Laves phase structure is stable down to a temperature of $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and up to applied pressure of $56\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The bulk modulus and enthalpy of formation (measured by high-temperature drop calorimetric experiments) of $\mathrm{Zr}{\mathrm{Mn}}_{2}$ are found to be $150\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $\ensuremath{-}66.6\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.3em}{0ex}}{(\mathrm{mol}\phantom{\rule{0.3em}{0ex}}\text{of}\phantom{\rule{0.3em}{0ex}}\mathrm{f.u.})}^{\ensuremath{-}1}$, respectively, in nice agreement with the ab initio calculated results. The experimental bulk thermal-expansion data of $\mathrm{Zr}{\mathrm{Mn}}_{2}$ resembles that of a typical metal. However, the temperature-dependent electrical resisitivity shows that there is a power law with $n\ensuremath{\approx}1.1$ at low temperature quite different from simple metals which originates probably from scattering of the charge carriers on the frustrated Mn moments. The phonons have been investigated in detail by comparing results of ab initio calculations with neutron spectra and experimental specific-heat data.

He4 State Equation Below 0.8 K

This work develops the Helmholtz potential A(ρ, T) for He4 below 0.8 K. Superfluid terms, related to temperature and momentum gradients, are neglected with negligible loss of accuracy in the derived state properties (specific heats, first sound velocity, expansivity, compressibility, etc.). Retained terms are directly related to a bulk fluid compressibility plus phonon and roton excitations in this quantum fluid. The bulk fluid compressibility is found from the empirical equation c13 ≈ c103 + b; P, where c1 is the velocity of first sound, P is the pressure, and c10 and b are constants; this empirical equation is found to apply also to other helium temperature ranges and to other fluids. The phonon excitations lead to a single temperature-dependent term in A(ρ ,T) up to 0.3 K, with only two more terms added up to 0.8 K. The roton potential, negligible below about 0.3 K, is a single term first derived 60 years ago but little used in more recent work. The final A(ρ ,T) is shown to fit available experimental specific heat data to about ±2% or better. The magnitude of the pressure-independent Gruneisen parameter below 0.3 K is typical of highly compressed normal liquids. Extension of the equation above 0.8 K is hampered by lack of data between 0.8 and 1.2 K

Chiral and helical phase transitions in quasi-1D molecular magnets

Abstract We review recent experimental data obtained with quasi-one-dimensional (1D) molecular magnets Gd(hfac) 3 NITR. For Gd(hfac) 3 NITiPr the results can be coherently explained only in terms of the Villain’s conjecture: a chiral order at intermediate temperatures and helical order at low temperatures. In the case of weakly frustrated molecular magnetic chain compounds Gd(hfac) 3 NITPh, new specific heat measurements show the presence of a phase transition to the three-dimensional (3D) helical order, observed at T ∼ 0.63 K. In the temperature range 25–250 mK the experimental specific heat data are reproduced very well by a simple spin wave theory.

Interpretation of experimental data near λ-transition point in liquid helium

The recently published experimental data for specific heat C_p of liquid helium in zero gravity conditions very close to the lambda-transition have been discussed. We have shown that these data allow different interpretations. They can be well interpreted within the perturbative RG approach and within our recently developed theory, as well. Allowing the logarithmic correction, the corresponding fits lie almost on top of each other over the whole range of the reduced temperatures t (for bin averaged data) 6.3 x 10^{-10} < t < 8.8 x 10^{-3}. However, the plot of the effective exponent alpha_eff(t) suggests that the behaviour of C_p, probably, changes very close to the lambda-transition temperature. To clarify this question, we need more accurate data for t<10^{-7}. In addition, we show that the experimental data for superfluid fraction of liquid helium close to the critical point within 3 x 10^{-7} < t < 10^{-4} can be better fit by our exponents nu=9/13, Delta=5/13 than by the RG exponents (nu approximately 0.6705 and Delta about 0.5). The latter ones are preferable to fit the whole measured range 3 x 10^{-7} < t < 10^{-2} where, however, remarkable systematic deviations appear. Our estimated value 0.694 +/- 0.017 of the asymptotic exponent nu well agrees with the theoretical prediction nu=9/13.

Structural, transport, and thermal properties of the single-crystalline type-VIII clathrateBa8Ga16Sn30

We report the electrical resistivity, Hall coefficient, thermoelectric power, specific heat, and thermal conductivity on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and R_H over a wide temperature range indicate that electrons dominate electrical transport properties. Both rho(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 uV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10^19 cm^(-3). Hall mobility u_H shows a maximum of 62 cm^2/Vs around 70 K. The analysis of temperature dependence of u_H suggests a crossover of dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.

Ab initiocalculations of superconductivity in palladium under pressure

We have calculated the phonon dispersion and the electron-phonon interaction in palladium at ambient and under pressure using ab initio methods and evaluated superconducting transition temperature on the basis of the BCS theory. At ambient pressure, we have evaluated the electron-phonon coupling constant to be $\ensuremath{\lambda}=0.377$ by the use of the density-functional perturbation theory, and estimated the spin fluctuation effect in relation to the experimental data of specific heat. The vanishing of the superconducting transition at ambient pressure is resulted from the spin fluctuation effect. The spin fluctuation effect reduces with increasing pressure, and the superconductivity appears under pressure.

Possible pairing mechanisms ofPuCoGa5superconductor

We examine possible pairing mechanisms of superconductivity in PuCoGa$_5$ based on spin-fluctuations or phonons as mediating bosons. We consider experimental data of specific heat C(T) and resistivity $\rho(T)$ as input to determine a consistent scattering boson with the superconducting transition temperature of 18.5K in PuCoGa$_5$. Irrespective to the type of boson, the characteristic boson frequency is found to be $\sim 150 K$ from the resistivity fitting. The spin fluctuation model is most consistent with the experimental resistivity, successfully explaining the anomalous temperature dependence ($\sim \frac{T^2}{150 K +T}$) at low temperatures as well as the saturation behavior at high temperatures. Assuming that the pairing state is non s-wave, the large residual resistivity $\rho_{imp} \sim 20 \mu \Omega cm \sim 120 K$ suggests that an ideally pure sample of PuCoGa$_5$ would have a maximum T$_c$ of 39 K.

The Field-Induced Magnetic Ordering Transition in TlCuCl3

In the first part we investigate in detail if the Bose-Einstein condensation scenario for magnons can quantitatively explain the observed field-induced magnetic ordering in TlCuCl$_3$. We use a bond-operator approach to map the spin system onto hard-core bosons and exactly account for the hard-core constraint in the dilute limit. We solve the hard-core model within the Hartree-Fock-Popov approximation and discuss its validity and the consequences of this approximation for the critical properties. In the second part the effects of spin-phonon and spin-orbit coupling are discussed within this framework. We show that the experimental magnetization and specific heat data are well described if a certain type of anisotropy is included. We also present predictions for the quasiparticle gap which might be tested in the future.

Electronic structure and superconductivity of MgB2

Results of ab initio electronic structure calculations on the compound, MgB 2, using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume,c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated, D(E F) and the experimental specific heat data. TheT c is found to be 37 K. We use a parametrized description of the calculated band structure to obtain the T =0 K values of the London penetration depth and the superconducting coherence length. The penetration depth calculated by us is too small and the coherence length too large as compared to the experimentally determined values of these quantities. This indicates the limitations of a theory that relies only on electronic structure calculations in describing the superconducting state in this material and implies that impurity effects as well as mass renormalization effects need to be included.

Phonon spectrum of YBCO obtained by specific heat inversion method for realdata

In this paper, the phonon spectrum of YBCO is obtained from experimentalspecific heat data by an exact inversion formula with a parameter for eliminatingdivergences. The results can be compared to those of neutron inelasticscattering, which can only be carried out in a few laboratories. Somekey points of specific heat-phonon spectrum inversion (SPI) theory anda method of asymptotic behaviour control are discussed. An improvedunique existence theorem is presented, and a universal function set fornumerical calculation of SPI is calculated with high accuracy, which makes theinversion method applicable and convenient in practice. This is the firsttime specific heat-phonon SPI has been realized for a concrete system.

Electronic structure of MgB2

Results of ab initio electronic structure calculations on the compound MgB2 using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (E F) and the experimental specific heat data. The T c is found to be 24.7 K.

Specific heat of dipolar magnet KEr(MoO4)2 in external magnetic field

In this paper we present theoretical studies of Ising dipolar system in the paramagnetic regime. Mean-field approximation with the nearest-neighbour interaction involved in one direction has been applied. The corresponding theoretical predictions of specific heat are compared with experimental specific heat data of the title compound. The influence of further interactions is discussed.

Orthogonal-to-unitary ensemble crossover in the electronic specific heat of metal nanoclusters

We present a theoretical and experimental study on the influence of a magnetic, field on the energy-level statistics in metal nanoparticles. Based on the random-matrix theory, a gradual field-induced crossover behavior is predicted from the orthogonal to the unitary ensemble. Experimental data of the electronic specific heat of metal nanoparticles for different field sin the quantum size temperature regime compare favorably with these theoretical (analytical) predictions.

A concrete realization of specific heat-phonon spectrum inversion for YBCO

In this Letter, a phonon spectrum of YBCO is obtained from its experimental specific heat data by an exact inversion formula with eliminating divergence parameter [Dai Xianxi, Xu Xinwen and Dai Jiqiong, Proceedings of Beijing International Conference on High T c Superconductivity, Sept. 4–8 Beijing, China, (1989) 521], [Dai Xianxi, Xu Xinwen and Dai Jiqiong, Phys. Lett. A 147 (1990) 445]. The results are comparable to that from neutron inelastic scattering. Some key points of specific heat-phonon spectrum inversion (SPI) theory as well as a method of asymptotic behavior control are discussed. An improved unique existence theorem is presented. A universal function set for the numerical calculation in SPI is obtained, which will make the inversion method applicable and convenient in practice. This is the first time to realize the specific heat-phonon spectrum inversion in a concrete system.

Lattice dynamics and e-ph coupling in BEDT-TTF superconductors

The crystal structures and phonon frequencies of α- and β- phases of (BEDT-TTF) 2I 3 salts are computed by the Quasi Harmonic Lattice Dynamics (QHLD) method. Experimental specific heat data of the β- phase can be reproduced only by introducing also low frequency intra-molecular vibrations. The strength of the electron-phonon ( e-ph) coupling is evaluated.

Specific heat fluctuations in the vicinity of the spin-Peierls transition of CuGeO3

We present an analysis of experimental specific heat data measured in the vicinity of the spin-Peierls transition of a CuGeO3 single crystal. The aim of the investigation was to analyse the type of the transition, including the character of the fluctuations, and to give a proper determination of the critical temperature. We used and successively compared the Gaussian and critical fluctuations models to fit the experimental points detected in magnetic fields up to 21 T. Our results present new information about the influence of a magnetic field on fluctuations in the CuGeO3 compound. Within the framework of the Gaussian fluctuations model, there is no significant magnetic field dependence of the fluctuations. Using this model, the critical region is estimated to be as small as 10 mK in zero field, which excludes the possibility of observation of critical fluctuations in the present experiments. In spite of this, a critical fluctuations type of analysis can also be carried out, with a critical exponent value lying between the three-dimensional XY and the Heisenberg universality classes. We also note the absence of broadening of the fluctuations regime under magnetic field. Taking into account the scatter of the data, the two models are equally suitable in the fitting process.

Pseudo-quaternary boro-carbides: a two sub-lattice model

We consider recently studied boro-carbides RE(Ni 1− x M x ) 2B 2C, with emphasis in RE=Y and Ho. Ho compounds exhibit in some cases spiral magnetic order. We extended a recent previous work on Heusler compounds to study the specific heat of Y boro-carbides doped with Co and to calculate the coupling between Ho moments in neighbouring planes. The effect of impurity disorder is included in the electronic structure via the Coherent Potential Approximation (CPA) and in the absence of experimental data for the pseudoquaternary specific heat, we restricted ourselves to the dilute limit and determine the impurity potential via Friedel's sum rule. The electron–phonon coupling ( λ( x)) is extracted from the measured superconducting transition temperature, using the Allen–Dines formula. The existent specific heat data for Y compounds is used to fix the parameters of the two sub-lattice model. Finally we study the change, with transition metal concentration, of the angle between moments of the spiral magnetic order.