Ab initio study on the high superconducting transition temperature in calcium under high pressure

For calcium in the phases IV and V, we estimated the superconducting transition temperature T _c by the use of the Allen–Dynes formula. Setting the effective screened Coulomb repulsion constant μ* at 0.1 in the formula, we obtained T _c =23.42 K at 100 GPa for Ca-IV and T _c =15.87 K at 120 GPa for Ca-V. In order to clarify the origin of such high values of T _c , first, we investigated the band character of electrons and found that the high T _c is not necessarily related to the so called s–d transfer. Then we analyzed the electron–phonon coupling at each phonon mode in Ca-V where the highest T _c in elements has been experimentally observed. As a result, we discovered that an optical mode at the Γ point has the strongest electron–phonon coupling. Such phonon mode can exist only in the complex crystal structure of Ca-V, and the result shows that the high T _c seems to be closely linked with the complex crystal structures like Ca-IV and Ca-V.     

First-principles study on superconductivity of the gold–indium alloy under high pressure

The superconductivity of gold–indium alloys has been investigated using first-principles calculations based on the density functional theory. At ambient pressure, the calculated superconducting transition temperature (T _c) is 0.04 μ K in pure gold, but T _c dramatically increases by substituting indium atoms for gold atoms. The gold–indium alloy having 12.5 atomic percent indium (Au_0.875In_0.125) shows T _c of 0.1 K, and Au_0.75In_0.25 marks 1.7 K. The dramatic increase in T _c owing to the alloying effect is caused by the enhancement of the electron–phonon coupling. The superconductivity of gold is predicted to be drastically weakened with increasing pressure and virtually disappear at 10 GPa, but it continues up to at least 30 GPa by the inclusion of indium atoms.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Anharmonicity in aluminum hydride at high pressures

Aluminum hydride has been predicted to be a superconductor with a transition temperature of 24 K at 110 GPa, in disagreement with the experimental observation. In this work, it is shown that the bulk of the electron–phonon coupling can be associated with modes that are highly anharmonic according to frozen phonon calculations. This large anharmonicity could partially explain the origin of the disagreement between previous predictions and experiments.     

Theoretical investigation of superconductivity in ternary silicide NaAlSi with layered diamond-like structure

We have investigated the electronic structure, phonon modes and electron–phonon coupling to understand superconductivity in the ternary silicide NaAlSi with a layered diamond-like structure. Our electronic results, using the density functional theory within a generalized gradient approximation, indicate that the density of states at the Fermi level is mainly governed by Si p states. The largest contributions to the electron–phonon coupling parameter involve Si-related vibrations both in the x–y plane as well as along the z-axis in the x–z plane. Our results indicate that this material is an s-p electron superconductor with a medium level electron–phonon coupling parameter of 0.68. Using the Allen–Dynes modification of the McMillan formula we obtain the superconducting critical temperature of 6.98 K, in excellent agreement with experimentally determined value of 7 K.     

Physical and mechanical properties of C60 under high pressures and high temperatures

The physical and mechanical properties of a C₆₀ fullerene sample have been investigated under high pressure–high temperature conditions using a designer Diamond Anvil Cell. Electrical resistance measurements show evidence of C₆₀ cage collapse at 20 GPa, which leads to the formation of an insulating phase at higher pressure. Energy dispersive X-ray diffraction (EDXD) data indicated that the characteristic fcc reflections gradually decrease in intensity and eventually disappear above 28 GPa. A C₆₀ sample was laser-heated at a pressure of 35 GPa to a temperature of 1910±100 K and, subsequently, decompressed to ambient conditions. The photoluminescence spectra and the Raman spectrum of the pressure–temperature-treated sample were measured at a low temperature of 80 K. Raman peak at 1322.3 cm^?1 with full-width half-maximum of 2.9 cm^?1 was observed from the sample, which is attributed to the hexagonal diamond phase in the sample. The room temperature photoluminescence spectra showed a symmetric emission band centered in the red spectral range with a peak at 690 nm. The structural analysis of the pressure–temperature-processed C₆₀ sample using EDXD method showed strong internal structure orientation and a phase close to hexagonal diamond. Mechanical properties such as hardness and Young’s modulus were measured by nanoindentation technique and the values were found to be 90±7 and 1215±50 GPa, respectively and these values are characteristic of sp³-bonded carbon materials.     

Suppression of charge ordering and thermal hysteresis of electronic transport and magnetisation in La0.5Ca0.5Mn1−xNixO3

A series of polycrystalline La_0.5Ca_0.5Mn_1?xNi_xO₃ (x = 0.00, 0.025, 0.050, 0.075, 0.100 and 0.125) was synthesised using solid state reaction. Measurements in a cooling and warming cycle between 300 and 80 K were carried out to study the Ni-doping effects on the electrical resistivity, thermopower and magnetisation of single-phase La_0.5Ca_0.5Mn_1?xNi_xO₃. Partial substitution of Ni for Mn leads to the suppression of charge ordering state, the evidence of which is shown by the dramatic decrease in electrical resistivity and thermal hysteresis width in electrical resistivity, thermopower and magnetisation. However, the magnitude of both electrical resistivity and thermopower increases with increasing Ni content. This can be attributed to an increase in the Mn⁴⁺ concentration, which favours the antiferromagnetic state and leads to a gradual disappearance of ferromagnetic double exchange interaction. Besides, the metal–nonmetal transition temperature decreases with increasing Ni content until x = 0.075, which might arise from increased electron–phonon coupling due to less ordered spins at temperatures above ferromagnetic transition. For samples with x greater than 0.075, no metal–nonmetal transition is observed due to the suppression of double exchange mechanism.     

Enhanced Fröhlich interaction of semiconductor cuprous oxide films determined by temperature‐dependent Raman scattering and spectral transmittance

Anomalous low temperature behaviors in cuprous oxide (Cu₂O) film grown on quartz substrate have been investigated by temperature‐dependent Raman and transmittance spectra. The longitudinal optical components of two Γ₁₅‐ phonon modes become sharper and more intense at a low temperature. It can be found that the highest‐order electronic transition located at 6.4 eV exhibits a minimum transmittance near 200 K. Correspondingly, the variations from phonon intensity ratios reveal obvious anomalies with the decreasing temperature, indicating the existence of strong electron–phonon coupling mediated by Fröhlich interaction in the Cu₂O films below the temperature of 200 K. Copyright © 2012 John Wiley & Sons, Ltd.     

A new high-pressure polymorph of Ti2O3: implication for high-pressure phase transition in sesquioxides

The post-corundum phase transition has been investigated in Ti₂O₃ on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti₂O₃ was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti₂O₃ can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a=7.6965 (19) Å, b=2.8009 (9) Å, c=7.9300 (23) Å, V=170.95 (15) Å³ at 19 GPa, and a=7.8240 (2) Å, b=2.8502 (1) Å, c=8.1209 (3) Å, V=181.10 (1) Å³ at ambient conditions. The Birch–Murnaghan equation of state yields K ₀=206 (3) GPa and K′₀=4 (fixed) for corundum phase, and K ₀=296 (4) GPa and K′₀=4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti₂O₃ can be identified as Th₂S₃-type rather than U₂S₃-type structure. The transition from corundum-type to Th₂S₃-type structure accompanies the drastic change of the form of polyhedron: from TiO₆ octahedron in the corundum-type to TiO₇ polyhedron in the Th₂S₃-type structures.     

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.     

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of MgB₂ ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ∼590 cm⁻¹ related to the E _2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas, beyond this region, the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ∼5 GPa exhibits a change in the slope, as well as a “hysteresis” effect in the frequency vs. pressure behavior. These singularities in the E _2g mode behavior under pressure support the suggestion that MgB₂ may undergo a pressure-induced topological electronic transition.     

Electrical properties and conduction mechanisms in La2/3Ca1/3MnO3 thin films prepared by pulsed laser deposition on different substrates

Perovskite manganite La_2/3Ca_1/3MnO₃ thin films were directly grown on MgO(100), Si(100) and glass substrates by pulsed laser deposition. From the XRD patterns, the films are found to be polycrystalline, single-phase orthorhombic. The metal–insulator transition temperature is 209 K for LCMO/MgO, 266 K for LCMO/Si and 231 K for film deposited on the glass substrate. The conduction mechanism in these films is investigated in different temperature regimes. Low-temperature resistivity data below the phase transition temperature (T _P) have been fitted with the relation \( \rho = \rho_{0} + \rho_{2} T^{2} + \rho_{4.5} T^{4.5} \) , indicating that the electron–electron scattering affects the conduction of these materials. The high-temperature resistivity data (T > T _P) were explained using variable-range hopping (VRH) and small-polaron hopping (SPH) models. Debye temperature values are 548 K for LCMO/Cg, 568 K for LCMO/Si and 508 K for LCMO/MgO thin films. In all thin films, the best fitting in the range of VRH is found for 3D dimension. The density of states near the Fermi level N (E _F) for LCMO/MgO is lower due to the prominent role of the grain boundary in LCMO/MgO and increase in bending of Mn–O–Mn bond angle, which decreases the double exchange coupling of Mn³⁺–O²–Mn⁴⁺ and in turn makes the LCMO/MgO sample less conducting as compared to the other films.     

Raman scattering study of temperature and hydrostatic pressure phase transitions in Rb2KTiOF5 crystal

Raman spectra of Rb₂KTiOF₅ crystal were obtained and analyzed in the temperature range from 77 to 297 K and under hydrostatic pressure up to 4.2 GPa (at T = 295 K). The experimental results were compared with quantum‐chemical simulation of TiOF₅ pseudo‐octahedron. To interpret effects of lattice ordering, phonon spectra of several ordered phases of Rb₂KTiOF₅ were calculated within ab initio generalized Gordon–Kim model, and ordering of TiOF₅ molecular groups were simulated within Monte Carlo approach. The spectra exhibited orientation disordering in the cubic phase under ambient conditions. Cooling below the phase transition temperature (215 K) leads to partial ordering of the structure. The isotropic perovskite‐like phase was found to undergo first‐order transition into a low‐symmetry anisotropic phase at about 1 GPa. Further compression up to 4.1 GPa did not show any effects associated with phase transitions. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

Pressure and temperature dependence of optical phonons in La0.75Ca0.25MnO3

We present the results of extensive studies on Raman and infrared active phonons in the La_0.75Ca_0.25MnO₃ manganite over a wide temperature (100–320 K) and pressure (0–14 GPa) range. The analysis of the temperature dependent data allows to identify a clear spectroscopic signature of the insulator to metal transition. Indeed the abrupt reduction of the effective electron–phonon interaction on entering the metallic phase determines a change in slope in the temperature dependence of the Jahn–Teller phonon line width. The analysis of the pressure dependent data shows that the octahedral Jahn–Teller distortion, and consequently the electron–phonon interaction, is strongly reduced only in the low-pressure regime. At very high pressure, the onset of a pressure-activated localizing mechanism efficiently contrasts the natural delocalizing tendency of pressure. We finally guess that this effect could be attributed to charge-localizing antiferromagnetic interactions activated by the strong lattice compression.     

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb₂AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb₂AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E_2g (ω₁) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E_2g (ω₂) and A_1g (ω₄) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E_2g (ω₂) and A_1g (ω₄) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of Ge-Sn at high pressure and high temperature in a laser-heated diamond anvil cell

Ge–Sn compound is predicted to be a direct band gap semiconductor with a tunable band gap. However, the bulk synthesis of this material by conventional methods at ambient pressure is unsuccessful due to the poor solubility of Sn in Ge. We report the successful synthesis of Ge–Sn in a laser-heated diamond anvil cell (LHDAC) at ~7.6 GPa &; ~2000 K. In situ Raman spectroscopy of the sample showed, apart from the characteristic Raman modes of Ge TO (Г) and β-Sn TO (Г), two additional Raman modes at ~225 cm^?1 (named Ge–Sn₁) and ~133 cm^?1 (named Ge–Sn₂). When the sample was quenched, the Ge–Sn₁ mode remained stable at ~215 cm^?1, whereas the Ge–Sn₂ mode had diminished in intensity. Comparing the Ge–Sn Raman mode at ~225 cm^?1 with the one observed in thin film studies, we interpret that the observed phonon mode may be formed due to Sn-rich Ge–Sn system. The additional Raman mode seen at ~133 cm^?1 suggested the formation of low symmetry phase under high P–T conditions. The results are compared with Ge–Si binary system.     

Study of spin–phonon coupling in LiFe1 − xMnxPO4olivines

LiFe_{1 − x}Mn_xPO₄ olivines are promising material for improved performance of Li‐ion batteries. Spin–phonon coupling of LiFe_{1 − x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature‐dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm⁻¹ is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature‐dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin–phonon coupling quickly decreases. Our analyses show that the quick decrease of spin–phonon coupling is due to decrease of the strength of spin–phonon coupling, but not change of spin‐ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin–phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li‐ion batteries. Copyright © 2015 John Wiley & Sons, Ltd.     

Doping effect on the phase transition temperature in ferroelectric SrBi2−xNdxNb2O9 layer‐structured ceramics: a micro‐Raman scattering study

The temperature dependence of Raman spectra for SrBi_2−xNd_xNb₂O₉ ceramics (x from 0 to 0.2) has been studied in a wide temperature range from 80 to 873 K. It is found that the peak position of the A_1g[Nb] phonon mode at 207 cm^–1, which is directly associated with the distortion of NbO₆ octahedron, decreases with increasing Nd composition, while the A_1g[O] phonon mode at 835 cm^–1 increases. Moreover, both the peak position and intensity of the A_1g[Nb] phonon mode reveal strong anomalies around the ferroelectric to paraelectric phase transition temperature. It indicates that the phase transition temperature decreases from about 710 to 550 K with increasing Nd composition, which is due to the fact that the introduction of Nd ions in the Bi₂O₂ layers reduces the distortion extent of NbO₆ octahedron. Copyright © 2011 John Wiley & Sons, Ltd.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.