Superconductivity of lanthanum and cerium at high pressures

A theoretical model involving the presence of an inner 4f shell is developed here to explain the pressure dependence of the superconducting transition temperature of lanthanum and the presence of a superconducting high pressure phase of cerium. In this model, only the conduction electrons are assumed to participate in the superconductivity mechanism, while the 4f electrons tend to inhibit superconductivity. As the 4f character decreases with increasing pressure in lanthanum, it is possible to account for the anomalous pressure dependence of the superconducting transition temperature by assuming a valency a little less than three at normal pressure. The anomalous behaviour of cerium, magnetic at room temperature and normal pressure and superconducting above 50 kbar, is also discussed here.     

Superconductivity and phase stability of selenium at high pressures

Hexagonal Se shows no indication of a transformation to a metallic, superconducting phase up to 160 kbar. Amorphous Se transforms at about 130 kbar to an unstable metallic, superconducting state which anneals slowly at room temperature toward a non-metallic, non-superconducting phase. Monoclinic Se behaves much like amorphous Se. X-ray diffraction indicates that all samples are in the hexagonal phase after release of pressure.     

Superconductivity of BCC Zr and Zr-Nb alloys at high pressures

The superconducting transition temperature T _c of bcc Zr is measured at pressures to 64 GPa. The T _c value gradually decreases as pressure is increased. For the Zr-Nb alloys, there are found anomalies in the T _c (P) dependences at rather low pressures. The anomalies are discussed within the available theoretical models. We assume on the basis of the T _c (P) experimental data for the Zr-Nb alloys that the T _c (P) curve for bcc zirconium has a maximum in the metastability region.     

Superconductivity in doped polyethylene at high pressure

In this work we study the pressure-dependent phase diagram of polyethylene (H₂C)_x from 50 to 200 GPa. Low-symmetry, organic polymeric phases, that are dynamically stable and thermodynamically competitive with elemental decomposition, are reported. Electronic structure calculations reveal that the band gap of the lowest energy polymeric phase decreases from 5.5 to 4.5 eV in the 50–200 GPa range, but metalization occurs only for pressures well above 500 GPa. The possibility of metalization via doping was also investigated, observing that it can be achieved through boron substitution at carbon sites. We report a sizable electron-phonon coupling (λ ? 0.79) in this metallic phase, with an estimated superconducting transition temperature of about 35 K. However, a rather narrow domain of stability is found; most of the dopant elements render the polymeric phases unstable and induce amorphization. This suggests that doping under pressure, though presenting an alternative route to find high temperature superconductors, would be challenging to achieve experimentally.     

Superconductivity of tin selenide at pressures up to 70 GPa

We report the discovery of superconductivity in SnSe with T _c around 4.5 K at pressures exceeding 58 GPa. It is conjectured that the high-pressure superconducting modification has the CsCl structure. Fiz. Tverd. Tela (St. Petersburg) 39, 236 (February 1997)     

Iron at high negative pressures

With the object of verifying the presence of a region of anomalous iron compressibility at negative pressures, as predicted by the ab initio calculations, the reflection of compression pulses from the surfaces of iron single crystals was detected. No evidence of the expected formation of rarefaction shock waves was observed in the range of attained tensile stresses up to 7.6 GPa. The breaking stresses achieved 25-50% of the theoretical iron ultimate strength for a load duration of ～10^?8 s. The dependence of breaking strength on the extension rate did not reveal any singularities in the region of assumed anomaly in iron compressibility.     

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.     

Spectroscopy at very high pressures—XXV: The IR spectrum of ferrocene at high pressures

The IR spectrum of ferrocene has been examined up to 50kbar under approximately hydrostatic conditions in a gasketed diamond anvil cell, and also under shear stress without gaskets. The results provide evidence in support of Duecker and Lippincott's claim of a phase transition at about 11.5 ± 0.5 kbar which is sluggish except under shear stress. Thermal strain and phonon self-energy contributions to the temperature-induced frequency shifts were analysed but no general pattern emerged.     

Thermopower of calcium at high pressures

Calcium at megabar pressures undergoes numerous structural transitions and has a complex phase diagram. At the same time, according to the recent theoretical investigations, an anomalous behavior of many physical properties, including a transition to the state of a narrow-gap semiconductor, can be expected even in the region of stability of the normal-pressure phase of calcium with the fcc structure at moderate pressures P ～ 5–15 GPa. Data on the thermopower of calcium in the pressure range up to 9 GPa have been reported. The thermopower in this pressure range is positive, has a smooth maximum at 5–6 GPa, and decreases quite rapidly at higher pressures. The absolute values of the thermopower (5–12 μV/K) indicate that calcium in this pressure range is a metal. The difference between the thermopowers in the direct and inverse passages in the range of 5–7.5 GPa is fairly noticeable (～10%). The possible reasons for such an anomalous behavior, as well as new calculations of the band structure of calcium, have been discussed.     

Polyhedral carbon nanoparticles at high pressures

JETP Letters - The solid-phase transformations of polyhedral nanoparticles at a pressure of 8.0 GPa and various temperatures have been investigated by X-ray diffraction, small-angle X-ray...     

Polymorphism of InS at high pressures

In situ X-ray diffraction has been used to study high-pressure polymorphism of InS up to ～ 13 Gpa in the 293–573 K temperature range. The phase transition InS I?arr2;InS II is found under isothermal compression at p_t = 7.5 ± 0.5 GPa and T = 293 K; at p_t = 6.0 ± 0.5 GPa and T = 573 K. InS II crystallizes in the structural type of Hg₂Cl₂: $\text{a = 3.823 ± 0.008}\text{A}\text{?}$; $\text{c = 10.868 ± 0.030}\text{A}\text{?}$; c/a = 2.843; Z = 4; D_4h¹⁷(I4/mmm); V_p/V₀ = 0.85; p = 10 GPa, T = 293 K. X-ray powder data indicate a continuous change of the orthorhombic structure of InS I with increasing pressure associated with the transition to the tetragonal phase InS II.     

Polyhedral carbon nanoparticles at high pressures

The solid-phase transformations of polyhedral nanoparticles at a pressure of 8.0 GPa and various temperatures have been investigated by X-ray diffraction, small-angle X-ray scattering, and transmission electron microscopy. It has been found that the graphene layers of the inner cavities of polyhedral particles are transformed into onion-like structures at temperatures above ～1000°C. This transformation gives rise to the formation of hybrid-type sp2 carbon nanoparticles, which combine the outer polyhedral shape with the quasispherical onion-like core. Polyhedral nanoparticles smaller than ～40 nm are completely transformed into onion-like particles at 1600°C.     

Spectrum of helium at high pressures

The effect of high pressures on an atom is frequently simulated by enclosing the atom in an impenetrable spherical box. The spectrum of such a confined helium atom placed at the centre of a spherical box is investigated. A model potential is used to calculate the energies of twelve excited states and thereby the transition wavelengths for a range of values of the radius of the confining sphere. Applications of results are discussed. Received 29 August 2002 Published online 10 December 2002 RID="a" ID="a"e-mail: ypvsj@uottawa.ca     

Superconductivity at 108 K in YBa2Cu4O8 at pressures up to 12 Gpa

Abstract

Resistive measurements in a cryogenic diamond anvil cell show that the T, of YBa₂Cu₄O₈ can be increased by almost 30 K by applying a pressure of 12 GPa. The pressure derivative is, however, not constant. At p < 5 GPa, dT_c/dp ～5 K/GPa. At higher pressures dT_c/dp decreases gradually and T_c saturates at ～108 K. This behavior can be reproduced by a phenomenological model where T, depends only on the number of holes in the CuO₂-planes.     

Superconductivity in an organic insulator at very high magnetic fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor lambda-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe3+ ions. We further argue that the Fe3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state.     

Hypersonic-absorption and sound-speed in water at high pressures

Speed and attenuation of hypersound (frequency from 5.5 to 6.7 GHz) in water of 25°C have been measured by Brillouin scattering at pressures ranging up to 1.75 kb. The results of the experiment are in good agreement with ultrasonic data.     

Elastic phase transitions in metals at high pressures

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-?to fourth-order elastic constants. The second-,third-?and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P?≈?69?GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.     

Superconductivity of vanadium and vanadium-titanium alloys at high pressure

The effect of pressure on the superconducting transition temperature T _c of vanadium and V₉₄Ti₆, V₈₅Ti₁₅, V₆₇Ti₃₃, and V₄₈Ti₅₂ (at %) bcc alloys has been studied. It has been found that the T _c(P) dependence of pure vanadium is close to linear in the pressure ranges 0–14 and 23–32 GPa, whereas dT _c/dP decreases to zero with a pressure increase in the 14–23 GPa range. The T _c(P) curves for all alloys are nonmonotonic and have two features in the respective pressure ranges of 3–11 and (a peak-shaped feature) 15–25 GPa.     

Optical properties of methanol at high pressures

The pressure dependence of the refractive index of methanol has been measured in a diamond-anvil cell up to 5·6 GPa at room temperature. These experiments cover the liquid phase of methanol (below 3·6 GPa) as well as the ‘superpressed fluid’ phase above 3·6 GPa. We have extracted the polarizability of methanol from our experimental data and the equation of state by using the Lorentz-Lorenz equation.