Specific heat of small vanadium particles in the normal and superconducting state

The specific heat C of ultrafine vanadium particles of various diameters (2.9–13 nm) has been measured in the temperature interval 1.5–12 K and in magnetic fields up to 3.5 T. Both the vibrational and electronic contributions to C in the normal state are strongly enhanced as compared to the bulk behavior. For not too small particles (> ～10nm), the vibrational specific heat can be interpreted in terms of the discrete phonon spectrum of free elastically vibrating small spheres while, at low temperatures, the vibrational specific heat of the smallest particles is predominantly due to Einstein modes which are attributed to low-frequency vibrations of weakly bound surface atoms. Level quantization does not appear to play a detectable role in the electronic specific heat of the normal state. Rather, the observed enhancement must be attributed to an increased electronic density of states at (100) surfaces of bcc metals or to electronic states of substoichiometric V-oxides. The transition range to superconductivity progressively broadens with decreasing particle size due to fluctuations. In this temperature range, the electronic specific heat behaves in qualitative agreement with theoretical predictions.     

Specific heat excess in Se and Se.84 Ge.16 glasses: Relationship to far infra-red absorption

Whereas far infra-red measurements reveal a strong modification in the vibrational density of states and consequently of the structure between pure vitreous selenium and the Se_.84Ge_.16 alloy, the specific heat data in the temperature range 1.3–20 K show that the excess characteristic of the amorphous phase remains almost unchanged in temperature. An interpretation of this excess as mainly due to modes originating in transverse acoustic bands (by analogy with the crystalline state) fails for these glasses.     

Heat capacity of Tb2Cu2O5 in the temperature range 379–924 K

The heat capacity of Tb₂Cu₂O₅ in the temperature range 379–924 K has been measured using differential scanning calorimetry. It has been shown that the obtained dependence C _p = f(T) can be described by a combination of the Debye and Einstein functions.     

The specific heat of CeAl2 between 0.02 and 1 K

We report specific heat measurements on a CeAl₂ single crystal between 0.02 and 1 K. Above 0.08 K, we found C₀ = γT + βT³ with γ = (130±0.5) mJ/K²mole and β = (142±1) mJ/K⁴mole in good agreement with previous results above 0.3 K. Below 0.08 K, an excess specific heat C_N = αT^?2 with α = (6.4±1) mJK/mole was detected and interpreted in terms of hyperfine splitting of the Al²⁷ nuclear states. Our results suggest that in CeAl₂ (complex) antiferromagnetism coexists with the Kondo effect at least down to 20 mK.     

Specific heat of A-15 Nb3Si produced by explosive compression

The specific heat from 1.2 to 23 K has been measured on a new high T_c superconductor, A-15 Nb₃Si. The sample was prepared by explosive compression and has an onset of bulk superconductivity at 18.0 K, with a transition width of 0.7 K. The density of states for pure A-15 Nb₃Si implied from the specific heat data is 0.94 ± 0.20 states/eV-atom, ΔC/γ T_c is 2.0 ± 0.2.     

Low temperature specific heat of single-domain and polydomain ferroelectric NaNO2

The specific heat of a NaNO₂ sample has been measured between 2 K and 40 K in both single-domain and polydomain states. In this region the specific heat of the single domain sample follows exactly the T³ dependence. A clear excess contribution which in this temperature range has a temperature dependence between T and T² has been detected for the polydomain sample. It is attributed to domain walls.     

Specific heat of 1T-Ta0.93Ti0.07S2 in the Anderson localized states

The specific heat of 1T-Ta_0.93Ti_0.07S₂ in the Anderson localized states has been measured from 0.2 to 5.0 K in magnetic fields up to 60 kOe. Below 3.5 K, a Schottky type excess specific heat was observed, depending on the magnetic field. This excess specific heat is explained on the basis of both the Coulomb interactions between different Anderson localized states as well as in the same state.     

Vibrational spectroscopy of Cm–H,C β –C β stretching vibrations of Nickel metalloporphyrins: An algebraic approach

An algebraic model of coupled anharmonic oscillators is introduced, capable of describing the stretching vibrations of medium and large polyatomic molecules. This model is applied to the calculation of C_m–H and C _β –C _β vibrational modes of nickel octaethyl porphyrins and nickel porphyrins molecules. The model appears to describe the data accurately.     

Low-temperature specific heats of lanthanum and yttrium phosphate glasses

Summary The specific heats of (R₂O₃) _x (P₂O₅)_1−x glasses containing high concentrations of La³⁺ and Y³⁺ ions have been measured between 1.5K and 30K. It is shown that, in addition to the usual Debye contribution, there is an excess specific heat arising from localized vibrational states which has been discussed in terms of two distinct models. The first predicts a maximum in the temperature dependence of the excess specific heat associated with the crossover frequency from phonon to fracton behaviour. The phonon-fracton density of states used to fit the excess specific heat gives rise to model parameters having the same magnitudes as those found previously for other glasses including samarium phosphates. The second model, formulated on the basis of soft vibrations in glasses, predicts a minimum in the excess specific heat, which is also observed. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Capanello, Italy, July 4–8, 1994.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

Lattice dynamics investigations of SiGe core-shell nanowires

We present results from lattice dynamics calculations on the phonon modes and specific heat of SiGe core-shell nanowires. The results show that phonon dispersion relation of SiGe nanowires consists of four acoustic branches. The frequency of the first optical mode at Γ point shifts to low frequency as the Ge concentration is increasing. There are three strong peaks in the spectra of density of states. The peaks at 9.0 THz and 15.0 THz can be attributed to the high frequency Ge-Ge and Si-Si bond vibration. The broad peak around 3.0 THz of pure silicon nanowire corresponds to the transverse acoustic branch of bulk silicon. Moreover, specific heat of SiGe nanowires increases (decreases) with the increase of the concentration x at low (high) temperature. The specific heat at 300 K can be fitted by C _V = x ² C _Ge + (1 − x)C _Si, where C _Ge and C _Si are specific heat of pure germanium and silicon nanowires respectively.     

Mechanism for the metal-conducting behavior of a CaB4 single crystal

The temperature-dependent resistivity of a pure CaB₄ single crystal has been observed to demonstrate metal-conducting behavior. The normal Hall-coefficient measurement is indicative of the electrons as the conducting carriers with a room-temperature density of 4.7×10²¹ cm^?3. The observed metal-conducting behavior can be understood using a simple model, in which the Ca ions are treated as independent Einstein harmonic oscillators embedded in a Debye rigid boron network. The ab initio calculations of band structure and density of states corroborate the experimentally observed metal-conducting behavior of CaB₄, showing that it originates mainly from the p orbital of B atom and the hybridized d orbital of Ca atom.     

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.     

High Field ESR System and Its Application to the Study of Quantum Spin Systems

Developments of the high field ESR system in Kobe University is presented. Using Gunn oscillators and backward traveling oscillators (BWO), we can cover the frequency region from 30 to 1183.6 GHz with the use of InSb detector. Pulsed magnetic field up to 30 T is available and we are now trying to extend the field up to 40 T. Temperature range is from 1.8 to 300 K. Using this system, we studied S=1/2 ladder like system Cu₂(C₅H₁₂N₂)₂Cl₄, and found a new magnetic transition at 10.1 T at 1.8 K. The temperature dependence of ESR in Cu₂(C₅H₁₂N₂)₂CI₄ shows g-shift below 8 K which corresponds to the maximum of the magnetic susceptibility. The g-shift below 8 K suggests the increase of the quantum fluctuation in the system, and the role of the quantum fluctuation in Cu₂(C₅H₁₂N₂)₂CI₄ is discussed.     

A thermodynamic theory of mechanical relaxation due to energy transfer between strain-sensitive and strain-insensitive modes in polymers

A thermodynamic theory has been developed on the relaxation strength of mechanical relaxation due to energy-transfer between strain-sensitive (intermolecular) modes and strain-insensitive (intramolecular) modes. Assuming that the strain-insensitive modes exhibit an overwhelmingly large heat capacity and function as a heat reservoir of constant temperature during the relaxation process, the relaxation strength ΔG, the difference between instantaneous and equilibrium moduli, is given as ΔG = TG² _Tα² ₁/C_z1, where T is the absolute temperature, G_T is the isothermal elastic modulus, and ₁ and C_z1 are the thermal expansion coefficient and the specific heat at constant strain of the strainsensitive modes, respectively. This assumption is reasonable for a polymeric solid in which backbone chains have a lot of vibrational degrees of freedom whose energy is rather insensitive to intermolecular distance and, on the contrary, the potential for localized motion such as motion of side chains is highly sensitive to intermolecular distance.

In a special case where the strain-sensitive modes are a set of vibrators with an angular frequency ω, ΔG = γ²C_z1T, where γ is the Grüneisen constant, defined by the strain-derivative of ω, = ?? In ω?z. The case where the strainsensitive modes are a set of rotators in an n-fold symmetrical potential can be treated as an extension of the above case. In the case where the strain-sensitive modes involve the transition between two states, ΔG in the present theory is reduced to that of the well-known two-state transition theory. Agreement is obtained between theory and experiment for a-methyl relaxation of poly(methy1 methacrylate).     

Dynamic compactification with stabilized extra dimensions in cubic Lovelock gravity

In this paper the dynamic compactification in Lovelock gravity with a cubic term is studied. The ansatz will be of space–time where the three dimensional space and the extra dimensions are constant curvature manifolds with independent scale factors. The numerical analysis shows that there exist a phenomenologically realistic compactification regime where the three dimensional hubble parameter and the extra dimensional scale factor tend to a constant. This result comes as surprise as in Einstein–Gauss–Bonnet gravity this regime exists only when the couplings of the theory are such that the theory does not admit a maximally symmetric solution (i.e. “geometric frustration”). In cubic Lovelock gravity however there always exists at least one maximally symmetric solution which makes it fundamentally different from the Einstein–Gauss–Bonnet case. Moreover, in opposition to Einstein–Gauss–Bonnet Gravity, it is also found that for some values of the couplings and initial conditions these compactification regimes can coexist with isotropizing solutions.     

Specific heat of Ce x La1 − x B6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality Ce _x La_{1 ? x} B₆ (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4–300 K. LaB₆ samples with various boron isotope compositions (¹⁰B, ¹¹B, ^nat B) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature Θ_E ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature Θ_D ≈ 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5?2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy ΔE = 92–98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.     

Low temperature specific heat and thermal conductivity of bulk metallic glass (Cu50Zr50)94Al6

The low temperature specific heat and thermal conductivity of (Cu₅₀Zr₅₀)₉₄Al₆ bulk metallic glass have been studied experimentally. A low temperature anomaly in the specific heat is observed in this alloy. It is also found that in addition to Debye oscillators, the localized vibration modes whose vibration density of state has a Gaussian distribution should be considered to explain the low temperature phonon specific heat anomaly. The phonon thermal conductivity dependence on temperature for the sample does not show apparent plateau characteristics as other glass materials do; however, the influence of the resonant scattering from the localized modes on the lattice thermal conductivity is prominent in the bulk metallic glass at low temperatures.     

The specific heat of Y0.7Th0.3C1.58

The specific heat of the novel high temperature superconductor Y_0.7Th_0.3C_1.58 (T_c = 17.0 K) has been measured between 4 and 22 K. Unlike the other known high temperature superconductors (T_c > 16 K) which have either an A-15 or a NaCl-type structure, this material forms in the b.c.c., Pu₂C₃-type, structure. The Debye temperature, θ_D, is 346 K and the linear term coefficient, γ, of the specific heat has the value 4.66 mJ/mole-K². Thus the electronic density of states, N(0), which is proportional to γ, is quite low. The energy gap, 2Δ/kT_c, on the other hand has an anomalously high value of 5.8. Comparisons between these parameters of Y_0.7Th_0.3C_1.58 and those for some A-15 and NaCl-type superconductors are made.     

EPR observation of the spin pair correlation above Tc in lightly alkali metal-doped C60 fullerene

The existence of two temperatures: T_p-Cooper pairing and T_c-Bose–Einstein condensation in high temperature superconductors has been stipulated in a lightly potassium-doped C₆₀ by Magnetically Modulated Microwave Absorption. This doping level corresponds to the carrier density greater than the critical one: x>x₁. In case of rubidium lightly doped C₆₀, where the carrier density x was smaller than the critical one: x<x₁, anomalous EPR temperature dependence was observed. The characteristic temperature of bound electron pair formation T_p≈65 K and the energy gap 2Δ/k=30 K were estimated from the temperature dependence of the EPR signal intensity in non-superconducting state. These results suggest that the liquid fermions–liquid bosons transition can be observed as the opening of the spin gap at temperature T_p postulated in Micnas–Ranninger–Robaszkiewicz theory.