Intrinsic and extrinsic effects in the temperature-dependent photoluminescence of semiconducting carbon nanotubes

The temperature dependence of the band gap of semiconducting carbon nanotubes was measured for ten different nanotube species. The unprecedented effectiveness in avoiding the effect of external strain, or any other effects originating from the surrounding environment, lead to an accurate measurement of the band gap temperature dependence, giving fundamental insight into the nanotube electron-phonon interaction. Small but reproducible energy shifts of the emission lines with temperature were observed, showing a moderate chirality dependence, well in agreement with recent theoretical calculations. In addition to the energy shift, a substantial narrowing of the emission lines was also observed. The removal of the temperature shift of the band gap allows the precise measurement of the effect of external strain on carbon nanotubes in different environments.     

Temperature and magnetic field dependence of superconductivity in nanoscopic metallic grains

We study pairing correlations in ultrasmall superconductor in the nanoscopic limit by means of a toy model where electrons are confined in a single, multiply degenerate energy level. We solve the model exactly to investigate the temperature and magnetic field dependence of number parity effect (dependence of ground state energy on evenness or oddness of the number of electrons). We find a different parity effect parameter to critical temperature ratio (4 rather than 3.5) which turns out to be consistent with exact solution of the BCS gap equation for our model. This suggests the equivalence between the parity effect parameter and the superconducting gap. We also find that magnetic field is suppressed as temperature increases.     

New features in the pressure dependence of photoluminescence spectra of C60 at room temperature

We present new results on the pressure dependence of the electronic band gap of molecular C₆₀ measured by photoluminescence spectroscopy up to 10 GPa at room temperature. In agreement with previous results, the energy gap decreases with increasing pressure up to about 6 GPa. For higher pressures, however, we observe an energy gap that is wider than that at 6 GPa.     

Evidence for two energy scales in the superconducting state of optimally doped (Bi,Pb)2(Sr,La)2CuO6+delta

We use angle-resolved photoemission spectroscopy to investigate the energy gap(s) in (Bi,Pb)2(Sr,La)2CuO6+delta. We find that the spectral gap has two components in the superconducting state: a superconducting gap and pseudogap. Differences in their momentum and temperature dependence suggest that they represent two separate energy scales. Spectra near the node reveal a sharp peak with a small gap below T(c) that closes at T(c). Near the antinode, spectra are broad with a large energy gap of approximately 40 meV above and below T(c). The latter spectral shape and gap magnitude are almost constant across T(c), indicating that the pseudogap state coexists with the superconducting state below T(c), and it dominates spectra around the antinode. We speculate that the pseudogap state competes with the superconductivity by diminishing spectral weight in antinodal regions, where the superconducting gap is largest.     

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.     

The isotope effect coefficient dependence on nonstoichiometry for single CuO layer superconductors

Considering that the variation of the critical temperature can be explained if the impurity potential acts only by the non-spin-flip part, in the presence of either a pure d-wave or anisotropic s-wave gap symmetry, and inserting the pair-breaking time dependence on temperature, we obtain a quantitative expression for the isotope effect coefficient as a function of measured critical temperature T_c maximal critical temperature T and degree of anisotropy of the energy gap. Our result predicts the achievement of a large range of values in for different ratios T/T_c. Introducing the dependence of the critical temperature and pair-breaking time on dopant content for a single Cu---O layer superconductors, we can describe the variation of the isotope effect coefficient with nonstoichiometry for these materials. The result for the case of anisotropic s-wave gap symmetry is in good agreement with the experimental data.     

Evidence of electromagnetic absorption by collective modes in the heavy fermion superconductor UBe(13)

We present results of a microwave surface impedance study of the heavy fermion superconductor UBe(13). We clearly observe an absorption peak whose frequency and temperature dependence scales with the BCS gap function Delta(T). Resonant absorption into a collective mode, with energy approximately proportional to the superconducting gap, is proposed as a possible explanation. Fits to the data provide a simple relation between Delta(T) and the collective mode frequency.     

Optoelectronic properties of rutile (TiO2)

We present here a study of the electronic and optical properties of rutile (TiO₂). An investigation into the energy, pressure and temperature dependence of n_eff and ε_{∞ eff} is presented. Using the calculated values of n_eff and ε_{∞ effs}, we evaluate the Penn gap. The Penn gap thus obtained is shown to be in good agreement with the reflectivity data and also with the value obtained from band structure. In addition, we also show that essentially the Varshni formula explains the temperature dependence of the energy gap for rutile fairly well.     

An EPR study of the temperature dependence of the energy gap in ytterbium dodecaboride

An EPR study of ytterbium dodecaboride (YbB₁₂) showed the presence of an energy gap with a width of 2Δ=12 meV in the energy spectrum of this Kondo insulator. The temperature dependence of the energy gap was determined by interpreting the experimental data within the framework of the exciton dielectric model: Δ(T)=72 K at an absolute zero and Δ(T)=0 at ～115 K. The temperature dependence of the EPR line-width exhibits a feature at 13–15 K, which is indicative of a finite density of states inside the gap. This can be related to the presence of impurity states or bound polaron excitations in the electron spectrum of YbB₁₂.     

The temperature dependence of the resistance and the spin density wave energy gap of antiferromagnetic chromium

The anisotropic electrical resistance of antiferromagnetic chromium was measured in the temperature range of 10 K – 200 K. The two-band model was used to analyze the temperature dependence of the resistance, considering the effective number of conduction electrons associated with the SDW energy gap.     

Temperature dependence of the schottky barrier height in gallium arsenide

The Schottky barrier height is measured in Ni-low doped n-type gallium arsenide and found to decrease with increasing temperature. The temperature dependence of the barrier height is nearly equal to that of the energy gap in GaAs.     

On the material parameters of some useful quaternary compounds relevant to optoelectronic device design

Studies of some quaternary compounds showing lattice matching condition on various substrates suitable for device design in various wavelength regions have been reported here. The compositional dependence of lowest energy band gap, dielectric constant, coefficient of thermal expansion, temperature and pressure dependence of lowest band gap energy, etc. of these compounds have been presented and finally proper concentrations suitable for device design under lattice matching condition have been mentioned.     

Field dependent specific heat and longitudinal magnetization of the quantum magnet layer Cs2CuCl4: Anisotropy effects

We have addressed the specific heat and magnetization of an anisotropic spin-1/2 triangular Heisenberg antiferromagnet Cs₂CuCl₄ in the presence of magnetic field at finite temperature. We have investigated the behavior of thermodynamic properties by means of excitation spectrum in terms of a hard core bosonic representation. The effect of in-plane anisotropy on thermodynamic properties has also been studied via the bosonic model by Green’s function approach. This anisotropy is considered for exchange constants that couple spin components perpendicular to magnetic field direction. We have found the temperature dependence of the specific heat and longitudinal magnetization in the gapped field induced spin-polarized phase for various magnetic fields and anisotropy parameters. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various anisotropy parameters. Our results show temperature dependence of specific heat includes a peak so that its temperature position goes to higher temperature with increase of magnetic field. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum.     

On the relation between the gap,the transformation temperature and the average heat of atomization of As2Te3 glasses

The average heat of atomization of semiconducting glasses on the As₂Te₃ base, containing silicon, germanium and thallium is calculated from the heats of atomization of constituting elements. The average heat of atomization is correlated with the intrinsic energy gap and the transformation temperature of the studied glasses. The linear dependence between the energy gap, the transformation temperature and the average heat of atomization is found for As₂Te₃ glasses with silicon, germanium and thallium and is correlated with the binding energy between the constituting atoms.     

Enhanced charge gap in the bilayer manganite La2-2xSr1+2xMn2O7 near x = 0.4

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3相似文献   

Picosecond thermometer in the amide I band of myoglobin

The amide I and II bands in myoglobin show a heterogeneous temperature dependence, with bands at 6.17 and 6.43 microm which are more intense at low temperatures. The amide I band temperature dependence is on the long wavelength edge of the band, while the short wavelength side has almost no temperature dependence. We compare concepts of anharmonic solid-state crystal physics and chemical physics for the origins of these bands. We suggest that the long wavelength side is composed of those amino acids which hydrogen bond to the hydration shell of the protein, and that temperature dependent bands can be used to determine the time it takes vibrational energy to flow into the hydration shell. We determine that vibrational energy flow to the hydration shell from the amide I takes approximately 20 ps to occur.     

Nonempirical investigation of the probability for radiationless transitions. II

The dependence of the probability for radiationless transitions on the temperature and energy gap is investigated using the theory of multiphonon transitions. The temperature dependence is found to be extremely weak in the case limited to interval vibrations.Translated from Izvestiya VUZ. Fizika, No. 1, pp. 23–27, January, 1972.     

Optical sum rule in metals with a strong interaction

The restricted optical sum rule and its dependence on the temperature, a superconducting gap and the cutoff energy have been investigated. As known this sum rule depends on the cutoff energy and the relaxation rate Γ(T) even for a homogeneous electron gas interacting with impurities or phonons. It is shown here that additional dependence of the spectral weight on a superconducting gap is very small in this model and this effect disappears totally when Γ=0. The model metal with a single band is considered in details. It is well known that for this model there is the dependence of the sum rule on the temperature and the energy gap even in the case when Γ=0. This dependence exists due to the smearing of the electron distribution function and it is expressed in the terms of Sommerfeld expansion. Here it is shown that these effects are considerably smaller than that of related with the relaxation rate if the band width is larger than the average phonon frequency. It is shown also that the experimental data about the temperature dependence of the spectral weight for the high-T_c materials can be successfully explained in the framework approach based on the temperature dependence of the relaxation rate.     

Local investigation of the energy gap within the incompressible strip in the quantum Hall regime

We experimentally study the energy gap within the incompressible strip at local filling factor ν _c = 1 at the quantum Hall edge for samples of very different mobilities. The obtained results indicate strong enhancement of the energy gap in comparison to the single-particle Zeeman splitting. We identify the measured gap as a mobility gap, so a pronounced experimental in-plane magnetic field dependence can both be attributed to the spin effects as well as to the change in the energy levels broadening.