Temperature dependence of the energy GAP in GaAs

It is shown that electron-phonon self-energy effects are as important as the Debye-Waller screening of the pseudopotential in the temperature dependence of the band gap when the valence band is degenerate.     

Temperature dependence of the thermal conductivity for tellurium dioxide

The thermal conductivity of tellurium dioxide is reported as a function of temperature. At 77K, the thermal conductivity is 0.09 W/cm-K and it decreases approximately as the inverse of the temperature to a value of 0.02 W/cm-K at 300K. The anisotropy between the a- and c-axes is less than 20% over the entire experimental temperature range, with the a-axis more conductive than the c-axis. By making some simplifying assumptions, the phonon mean free paths and relaxation times are deduced.     

Temperature dependence of phase shift in ferrite phase shifters

Reciprocal ferrite phase shifters exhibit a pronounced dependence of phase shift upon temperature. In a sample of magnesium-copper-chromium ferrite it is shown that the temperature dependence of the phase shift is basically determined by the temperature dependence of the microwave magnetic permeability.In conclusion the authors wish to thank G. I. Yudin for making the measurements of phase shift and its dependence on the field for various temperatures.     

Temperature dependence of the superconductivity energy gap

The possibility of including the triplet representation ofSU ₃ in the usual octet model without loosing the integrity of charges is discussed.     

Pr0.7Ca0.15Ba0.15MnO3 manganite: Electron paramagnetic resonance,conductivity and susceptibility

We present a systematic experimental study of single phase Pr_0.7Ca_0.15Ba_0.15MnO₃ manganites. Ferromagnetic insulating state is observed in wide temperature range between the Curie point and peak resistivity temperature. Two approaches for the analysis of the electron paramagnetic resonance (EPR) line-width (ΔH) were considered in details. The first approach is based on the proportionality between the temperature dependencies of resistivity and inverse ΔH. The second approach is based on the model where ΔH∝1/Tχ_dc (χ_dc is a static magnetic susceptibility). For the studied Pr_0.7Ca_0.15Ba_0.15MnO₃ manganites the resistivity, associated with the grain boundary scattering, follows Mott law; the EPR line-width, suggested to be an intrinsic property, is well described with polaron hopping model.     

Temperature dependence of diffuse field phase

Diffuse fields, which have scattered from microstructure or reflected from walls so much as to prohibit conventional analyses, are usually examined by means of the time evolution of their ultrasonic spectral energy density. The phase information is usually discarded as resisting analysis. The phase, while unpredictable is, however, robust; according to theory it remains constant if source and receiver are not disturbed. Nevertheless, in practice we do observe slow drifts of phase over time scales of minutes. Here we examine the hypothesis that the phase drifts are due to temperature fluctuations. Temperature changes on cooling from 40 degrees C to room temperature were monitored and compared with changes in diffuse field phase. It was found that the reverberant ultrasonic field in a 7 cm aluminum block evolves with temperature in a manner that is in accord with published data on the temperature dependence of the ultrasonic velocities. Our 1 MHz transient source gives rise to a complex waveform that is observed to undergo an almost pure dilation. The precision with which this shift can be measured approaches 20 ns. This is remarkable when compared with the 100 ms travel time of the signal. Thus the temperature dependence of elastic wave speed is measured with a precision limited by the precision of one's thermometer. The signal is also found to suffer some distortion which, it is suggested, is related to the different rates of change of longitudinal and shear speeds. The corresponding prediction for the degree of distortion is found to be in accord with measurements.     

Temperature dependence of the fundamental energy gap in GaAs

We show that the temperature dependence of the fundamental gap energy E₀ of GaAs can be theoretically described within the empirical local pseudopotential approach for the band structure. For the calculation we take into account the thermal expansion effect and two contributions of the deformation potential type electron-phonon interaction. These are the Debye-Waller part, which shifts the gap to lower energies with increasing temperature, and the self-energy part, which partly cancels the shift caused by the Debye-Waller part.     

Temperature dependence of the gap in the density of states near the Fermi level in a hole doped manganite

In this paper, we report a model-based quantitative analysis of temperature dependent scanning tunneling spectroscopy (STS) data taken on epitaxial thin films of the hole doped manganite La_0.7Ca_0.3MnO₃. The film, grown on lattice matched NdGaO₃ substrate, has a ferromagnetic transition temperature T_c=268 K. The analysis allows us to evaluate how the tunneling curve evolves across the transition temperature. We find that there is a gap Δ in the density of states (DOS), which peaks at T≈T_c. The gap closes in the ferromagnetic state following the evolution of the magnetization. The gap closing is gradual and not sudden at T=T_c. Above T_c the gap reduces from the peak value and reaches a limiting value of ≈75 meV for T/T_c≥1.1 which is close to the value of 60 meV seen from transport experiments.     

Parameter dependence of optical conductivity in antiferromagnetic phase of iron pnictides

We investigate the optical conductivity of iron pnictides in antiferromagnetic state by mean-field calculation in a five-band Hubbard model, focusing on its anisotropic behavior by examining several states calculated with different Hund coupling J, such as the states with a low or high magnetization and with or without a strong orbital ordering. In addition, we investigate the J dependence of the Dirac cone structure, which is crucial for the low energy excitation. In our calculations, a weakly ordered state with no orbital ordering shows the anisotropy of optical conductivity in accord with experiments. We conclude that the low energy part of the optical conductivity is relevant to the Dirac electron structure rather than the orbital ordering.     

Electrical conductivity and cation distribution in nickel manganite

A study has been made of the oxygen loss from nickel manganite at temperatures between 800 and 1220°C in atmospheres with oxygen partial pressures which varied from 0.12–100% oxygen using isothermal gravimetry. An atmosphere-temperature-composition diagram has been constructed which is used in conjunction with high temperature electrical conductance measurements to determine the concentration of conductance cations present in NiMn₂O₄. A comparison of results with those reported in the literature leads to a formula for the cation valence distribution in nickel manganite of Ni²⁺_0.35Mn²⁺_0.65 (Ni²⁺_0.65Mn³⁺_0.70Mn⁴⁺_0.65) O^2-₄ at 900°C. An equation for the variation of the nickel inversion parameter with temperature has also been derived.     

Period dependence of coherent oscillations in manganite superlattices

In three-component superlattices of [(NdMnO3)_n/(SrMnO3)_n/(LaMnO3)_n]_m consisting of nonferroelectric and antiferromagnetic layers, the generation of coherent phonons in the system is investigated by the relaxation process of the superlattice using a pump–probe technique. The coherent phonons are related to MnO₆ octahedra in the layers. While the frequency of the phonons is 49 GHz without regard to the period of each layer, the amplitude of the phonons is influenced by the period of the superlattices. The superlattices have the maximum amplitude of the acoustic phonons at a period of (5, 12) together with strong emergent interfacial effects such as ferromagnetism and ferroelectricity, which are not present in bulk materials. This implies a possible correlation of the lattice structure with interface-induced properties.     

Temperature dependence of coherent oscillations in Josephson phase qubits

We experimentally investigate the temperature dependence of Rabi oscillations and Ramsey fringes in superconducting phase qubits. In a wide range of temperatures, we find that both the decay time and the amplitude of these coherent oscillations remain nearly unaffected by thermal fluctuations. In the two-level limit, coherent qubit response rapidly vanishes as soon as the energy of thermal fluctuations k(B)T becomes larger than the energy level spacing variant Planck's over h omega of the qubit. In contrast, a sample of much shorter coherence times displayed semiclassical oscillations very similar to Rabi oscillation, but showing a qualitatively different temperature dependence. Our observations shed new light on the origin of decoherence in superconducting qubits. The experimental data suggest that, without degrading already achieved coherence times, phase qubits can be operated at temperatures much higher than those reported till now.     

Temperature dependence of ferromagnetic anisotropy energy in cubic crystals

The magnetic anisotropy constant K₁ for cubic ferromagnetic crystals has been discussed based on the general expressions derived by Yang for hexagon crystals. By matching the experimental data, we obtained

$\text{K}{}_1\text{(T,H)}\text{K}{}_1\text{(0,0)}\text{=?6.14}\text{I}\text{?}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{(T,H) + 3.36}\text{I}\text{?}\text{9}\text{2}\text{(T,H) + 4.88m}{}^2\text{(T,H) ?1.10[}\text{I}\text{?}\text{5}\text{2}\text{(T,H)]}{}^2$

for nickel, and $\text{K}{}_1\text{(T,H)/K}{}_1\text{(0,0)=I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ for iron, where Î_{$\text{5}\text{2}$} and Î_{$\text{9}\text{2}$} are the hyperbolic bessel functions of order 2 and 4 respectively and m is the reduced magnetization. Both expressions have a theoretical basis.     

Temperature dependence of energy dissipation in type I superconductors

Energy dissipation due to the movement of the magnetic flux in superconducting tin was studied by means of the torsion pendulum technique. The study was made as a function of the mean velocity of vortices, applied magnetic field and temperature. A phenomenological expression for the energy dissipation is proposed. This model explained our experimental results as well as these obtained previously by Houston and Smith.