Prediction of superconductivity of Ta2AlC: in situ Raman spectrometry and density functional investigations

In this paper, in situ Raman spectra of Ta₂AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line‐width of E_2g (ω₃) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line‐widths of E_2g (ω₁) and A_1g (ω₄) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta₂AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.     

Point contact spectroscopy in Ga single crystals

The current-voltage characteristics of point contacts in Ga single crystals have been measured in the liquid helium temperature range. At least six singular structures were found in energy range between 6 to 31 mV of the spectra d²V/dI². They are in good agreement with the structure of the phonon density-of-states determined from the inelastic neutron scattering experiment by Reichardt et al. The obtained spectra were used to estimate the electron-phonon coupling parameter.     

Scattering parameters from an analysis of the hall electron mobility in Ga1−xAlxAs alloys

Hall mobilities in high purity epitaxial layers of n-type Ga_1?xA_xAs alloys with room-temperature electron concentrations in the range (5–10) × 10¹⁵ cm^?3 have been measured at 300 K for alloy compositions in the range 0? x ? 0.78. Models for the variation of various material parameters with composition are developed. The acoustic deformation potentials for the Π and X conduction band minima, the intervalley coupling constants for electron scattering among the X-X and L-X minima and the longitudinal optical phonon temperature for these scatterings and the polar optical scattering in the X minima have been determined from a theoretical fit to the experimental data. The electron mobilities in the Π, L and X minima are found to decrease with the composition in the range 0≦ x ? 0.45, show a minimum at x ? 0.50 and then increase again with composition. These mobility variations are reflected in the minimum in the Hall mobility at x ? 0.5 due to intense non-equivalent electron scattering involving the L and x minima. The minimum in the electron mobilities in the Π and X minima have been identified to occur mainly due to the space charge and intervalley scattering among the L and X minima, respectively. The number of equivalent x minima in Ga_1?x Al_x As alloys are estimated to be three and are, thus, situated at the zone edge.     

Phonon spectrum and interaction between nanotubes in single-walled carbon nanotube bundles at high pressures and temperatures

The Raman spectra of single-walled carbon nanotubes at temperatures up to 730 K and pressures up to 7 GPa have been measured. The behavior of phonon modes and the interaction between nanotubes in bundles have been studied. It has been found that the temperature shift of the vibrational G mode is completely reversible, whereas the temperature shift of radial breathing modes is partially irreversible and the softening of the modes and narrowing of phonon bands are observed. The temperature shift and softening of radial breathing modes are also observed when samples are irradiated by laser radiation with a power density of 6.5 kW/mm². The dependence of the relative frequency Ω/Ω₀ for G ⁺ and G ^? phonon modes on the relative change A ₀/A in the triangular lattice constant of bundles of nanotubes calculated using the thermal expansion coefficient and compressibility coefficient of nanotube bundles shows that the temperature shift of the G mode is determined by the softening of the C-C bond in nanotubes. An increase in the equilibrium distances between nanotubes at the breaking of random covalent C-C bonds between nanotubes in bundles of nanotubes is in my opinion the main reason for the softening of the radial breathing modes.     

Puzzling features of heavy-ion fission at sub-barrier energies

Elastic and inelastic scatterings cross-sections of¹²C+¹²C in the energy range 11–24 MeV/n have been analyzed within the coupled channels framework including appropriate symmetrization. The set obtained withr ₀=1.01 fm,r _I =0.86 fm for seven incident energies gave the best agreement with the data and could be, therefore, considered as really significant when compared with other sets. Coupled channels calculations have been made to fit the data and it has been found that they require a reduction of the imaginary potential. The total reaction cross-sectionσ _R (E) determined from this analysis is in good agreement with a direct and recent measurement of σ _R . Quadrupole deformation parameterβ ₂ has been extracted and compared with those obtained for proton and alpha scatterings. Some small energy dependence of β₂ over this energy range has been found, ¦β ₂¦ ～0.39–0.47. Finally, these calculations further confirm that β₂ depends on the probe used in the various scattering experiments. For composite projectile such asα or¹²C,β ₂ is clearly smaller than that one for nucleon as projectile.     

Brillouin scattering by pure-transverse phonons in GaSe and GaS

Brillouin scattering experiments in the layer-type semiconductors GaSe and GaS by the pure-transverse phonon domains have been carried out at room temperature. Weak resonant enhancement and cancellation have been found for both materials in the region of transparency. The non-diagonal elastic constants C₁₂ have also been determined to be 3.77 × 10¹¹ dyn cm^?2 (GaSe) and 5.35 × 10¹¹ dyn cm^?2 (GaS) by measuring the sound velocities of the phonon domains.     

Anomalous low-frequency phonons in ZrB<Subscript>12</Subscript>

The influence of pressure and temperature on low-frequency lines (100–300 cm^–1) in Raman spectra of ZrB₁₂ and LuB₁₂ single crystals has been investigated. These spectral features have been identified as one-phonon and two-phonon excitations of the acoustic branches of the phonon spectrum. It has been found that the observed spectra of ZrB₁₂ single crystals are more structured and indicate the development of phonon anomalies with a decrease in the temperature. Despite the fact that the low-frequency features in the phonon spectrum of ZrB₁₂ are characterized by a high value of the isothermal Grüneisen parameter, their isobaric Grüneisen parameter has a negative value. This indicates large contributions from the fourth-order anharmonicity, which significantly exceed the volume effects. The appearance of narrow lines in the frequency range from 155 to 175 cm^–1 at temperatures T < 100 K suggests the possibility of a structural transition with an incomplete softening of the lattice.     

Low-temperature thermal conductivity of tellurium-doped bismuth

Phonon thermal conductivities κ₂₂ (?T ‖ C1) and κ₃₃ (? T ‖ C₃) of tellurium-doped bismuth with an electron concentration in the range 1.8 × 10¹⁹ ≤ n_L ≤ 1.4 × 10²⁰ cm^?3 were studied in the temperature interval 2 < T < 300 K. The temperature dependence of the phonon thermal conductivity obtained on doped bismuth samples of both orientations exhibits two maxima, one at a low temperature and the other at a high temperature. The effect of various phonon relaxation mechanisms on the dependence of both phonon thermal conductivity maxima on temperature, impurity concentration, and electron density is studied.     

Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells

In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n=2 subband in GaAs/Al_0.37Ga_0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm.     

Thermal expansion and kinetic coefficients of crystals

Electrical (ρ) and thermal (W) resistivities and thermal expansion coefficient (β) of Cu, Zn, Al, Pb, Ni, β-brass, Al₂O₃, NaCl, Si, SiO₂(∥), and SiO₂(⊥) were simultaneously measured with standard four-probe, absolute steady-state, and quartz dilatometer techniques. Measurements of Ni and β-brass were performed at temperatures from 300 to 1100 K and measurements of all other samples were made between 90 and 500 K. This temperature range includes the range below and above the Debye temperature (T_D). The total uncertainties of the specific electrical and thermal resistivities and thermal expansion coefficient (TEC) measurements are 0.5%, 3.0%, and (1.5-4.0%), respectively. The universal linear relationship between the electrical and thermal resistivities and βΤ over the wide temperature range was found experimentally. Using the Landau criterion for convection development for ideal phonon and electron gases in the solids, the universal relations, ρ^ph/ρ^*≡βT and W^ph/W^*≡βT (where ρ^ph is the phonon electrical resistivity, is the characteristic electrical resistivity, W^ph is the phonon thermal resistivity, and W^*=k_BG/qc_p is the characteristic thermal resistivity) between relative phonon electrical and phonon thermal resistivities and βΤ were derived. The derived universal relations provide a new method for estimating the kinetic coefficients (electrical and thermal resistivities) from TEC measurements.     

Oscillator parameters of transverse phonons in <Emphasis Type="Italic">a</Emphasis>-domain lead titanate

The temperature behavior of polarized Raman spectra of a-domain PbTiO₃ single crystals has been investigated. The spectra obtained are fit by a damping harmonic oscillator model with allowance for a complex background structure. A full set of oscillator parameters of all phonon modes in the frequency range 12 < ν < 1200 cm^?1 is obtained and their temperature dependences in the range from 300 to 800 K are presented.     

First-principles investigation of the thermodynamic properties of two-dimensional MoS2

Density-functional perturbation theory has been applied to investigate the thermodynamic properties of two-dimensional MoS₂ within the Perdew-Burke-Ernzerhof genealized gradient approximation (PBE-GGA). The Murnaghan’s isothermal equation of state has been derived for the monolayer, giving how the pressure and the total energy of the monolayer evolve versus the volume of the unit cell. The temperature-dependent behavior of some thermodynamic quantities, including the Helmholtz free energy, total energy, Debye temperature, mean square displacements, specific heat, entropy and number of microstates, have been determined as well as their functional forms within the quasi-harmonic approximation, by calculating the partial phonon density of states as a fundamental quantity. The value of 45.6 cm${}^{?1}$ has been found for the phonon band gap in the frequency range, separating the acoustic and the optical phonon bands in fair agreement with the literature. The results broadly support the view that the validity of the Debye T³-law for low-temperature specific heat of solids is violated for the case of two-dimensional MoS₂ and therefore, a T²-law is proposed, accurately describing the behavior of the isochoric specific heat from 0 to 60 K.     

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Dispersive effects and correction term in two-mode phonon conduction model for Ge

A complete analysis of the phonon conductivity κ, still lacking in the literature, is presented in the two-mode conduction model for Germanium. First a method is derived from which the correction term κ_c of the Callaway model is separated into its longitudinal and transverse parts and then the effects of strong phonon dispersion and the role of longitudinal and transverse phonons on κ_c are studied. For this purpose we have also proposed some new empirical expressions for the three phonon relaxation rates τ_3ph⁻¹'s which are valid in the entire temperature range. This improvised model, when applied simultaneously to the phonon conductivity data of both normal and enriched Ge, yields some new results. These are (i) κ_c neglected by the earlier workers in the two-mode phonon conduction model, gives a substantial contribution beyond the conductivity maximum and (ii) the longitudinal phonons are the major carriers of heat at high temperature.     

Effect of pressure on the temperature dependence of the thermal conductivity of InSb

The dependence of the thermal conductivity of indium antimonide on temperature (in the range 300–450 K) and hydrostatic pressure (up to 0.4 GPa) has been investigated. It is shown that the phonon thermal conductivity λ_ph obeys the law T ^?n (n ≥ 1). Hydrostatic pressure affects the magnitude and temperature dependence of the thermal conductivity of InSb: with an increase in pressure, the thermal conductivity increases, while the parameter n in the dependence λ_ph ≈ T ^?n decreases.     

Homogeneous He-He solid solutions in the pre-separation region

Temperature dependences of the pressure P(T) in homogeneous solid ³He-⁴He mixtures have been studied experimentally in the wide range of concentrations (35.0%, 62.0%, 68.3%, 74.1%, 75.0%, and 89.3% ³He) above and below the equilibrium phase separation temperature T_s. An anomalous behaviour of the pressure in the vicinity of T_s is found for all investigated samples. With decreasing temperature, as T_s is approached, the pressure increases instead of expected reduction due to decrease in the phonon contribution (P_ph∼T⁴). Such an increase in pressure continues in the metastable region below T_s until the mixture separates. Theoretical interpretation of the observed effects based on a rigorous thermodynamic approach is proposed. The found experimentally pressure behaviour can be described only with the consistent account for fluctuations in the impurity subsystem which near T_s dominates over phonon contribution into the pressure. The obtained theoretical results are in good quantitative agreement with the experimental data. Density fluctuations in the concentrated mixtures give rise to a spontaneous formation of impuriton nano-clusters containing several hundreds of atoms. The fluctuation can be rigorously interpreted as a nucleus of the second phase in the pre-separated homogeneous solid mixture. The estimated size of the fluctuation nano-clusters agrees with the corresponding value for second phase nuclei obtained from the Lifshits-Slesov phenomenological theory of homogeneous nucleation.     

Field-domain dynamics in negative-effective-mass terahertz oscillators

We have theoretically studied electric-field-domain dynamics and current self-oscillations in dc-biased negative-effective-mass (NEM) p ⁺ pp ⁺ diodes. The formation and traveling of electric-field domains in the diodes are investigated in detail with a realistic treatment of the scatterings contributions from carrier-impurity, carrier-acoustic phonon, and carrier-optic phonon within the balance-equation theory. The interesting patterns of the spatiotemporal electric-field domains are shown as a gray density plot with the applied bias as a controlling parameter. It is found that, the applied bias could largely influence the current patterns and self-oscillating frequencies, which lie in the THz range for the NEM p ⁺ pp ⁺ diode with a submicrometer p-base. The NEM p ⁺ pp ⁺ diode may therefore be developed as an electrically tunable THz-frequency oscillator.Received: 18 July 2003, Published online: 23 December 2003PACS: 73.61.Ey III-V semiconductors - 73.50.Fq High-field and nonlinear effects - 85.30.Fg Bulk semiconductor and conductivity oscillation devices (including Hall effect devices, space-charge-limited devices, and Gunn effect devices) - 85.30.De Semiconductor-device characterization, design, and modeling     

Spontaneous polarization and piezoelectric effects on intraband relaxation time in a wurtzite GaN/AlGaN quantum well

Spontaneous (SP) and piezoelectric (PZ) polarization effects on the intraband relaxation time for a wurtzite (WZ) GaN/AlGaN quantum well (QW) are investigated theoretically as functions of the sheet carrier density and well thickness. The self-consistent (SC) model with the SP and PZ polarizations shows that linewidths for carrier–carrier and carrier–phonon scatterings are significantly reduced compared to those for the flat-band (FB) model without SP and PZ polarization. In particular, line-widths for the e–h and h–e scatterings are reduced by about two orders of magnitude at a sheet carrier density as low as 2×10¹² cm^-2 compared to the case of the FB model. This is attributed to the decrease of the matrix element due to the spatial separation between electron and hole wave functions. In the case of the e–e and h–h scatterings, the reduction of linewidths is mainly attributed to the decrease of the scattering matrix element due to the increase of the inverse screening length. Linewidths for e–h and h–e scatterings gradually increase with the sheet carrier density since the screening field increases, while linewidths for the other scatterings are almost independent of the sheet carrier density. The SC model also shows that linewidths for the carrier–carrier and carrier–phonon scatterings are nearly, constant irrespective of well thickness except for e–h and h–e scatterings. In the case of e–h and h–e scatterings, linewidths greatly decrease with the well width because of the increase of the spatial separation of wave functions. Received in final version: 13 July 2000 / Accepted: 13 July 2000 / Published online: 16 August 2000     

Anharmonic effects of phonons with Kohn anomalies

In a quasi-one-dimensional conductor, the phonons with anomaly at the Fermi diameter, 2k_F, cause the phonons at 4k_F to soften via the three-phonon coupling, mainly due to the phonon modulation of the Coulomb interaction between tight-binding electrons on different sites. The 2k_F phonons also cause strong second-order scatterings of X-rays or neutrons. These results may explain the recent observations of phonon anomalies at 4k_F in TTF-TCNQ.     

Low temperature specific heat of Ni-Co and Ni-Fe alloys

Values of the low temperature specific heat of f.c.c. Ni-Co and Ni-Fe alloys have been determined in the 1.2–8 K range with a relative accuracy of 10^-3 and analysed to separate the electron, phonon, magnon and hyperfine terms. Results are in good agreement with former data^5–7 and with theory: Gomes' model⁴ of a d-d tight-binding scattering in the dilute case, Hasegawa's^12,13 CPA calculations are both consistent with experiments.