Superconductivity well above room temperature in compressed MgH6

It has been suggested that hydrogen-rich systems at high pressure may exhibit notably high super-conducting transition temperatures. One of the more interesting theoretical predictions was that hydrogen sulfide can be metallized and the high-temperature superconducting state can be induced. A record critical temperature (203 K) was later confirmed for H₃S in an experiment. In this paper, we investigated, within the framework of the Eliashberg formalism, the properties of compressed MgH₆, which is expected to be a very good candidate for room-temperature superconductivity. This applies particularly to the pressure range from 300 to 400 GPa, where the transition temperature is close to 400 K. Moreover, the estimated thermodynamic properties and the resulting dimensionless ratios exceed the predictions of the Bardeen–Cooper–Schrieffer theory. This behavior is attributed to the strong electron–phonon coupling and retardation effects existing in hydrogen-dominated materials under high pressure.     

Structural and magnetic phase transitions in Pr<Subscript>0.15</Subscript>Sr<Subscript>0.85</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of Pr_0.15Sr_0.85MnO₃ manganite have been studied by means of powder X-ray and neutron diffraction in the temperature range 10–400 K at high external pressures up to 55 and 4 GPa, respectively. A structural phase transition from cubic to tetragonal phase upon compression was observed, with large positive pressure coefficient of transition temperature dT _ct /dP = 28(2) K/GPa. The C-type antiferromagnetic (AFM) ground state is formed below T _N ≈ 260 K at ambient pressure. While at ambient pressure the structural and magnetic transition temperatures are close, T _ct ~ T _N , upon compression they become decoupled with T _N ≪ T _ct due to much weaker T _N pressure dependence with coefficient dT _N /dP = 3.8(1) K/GPa.     

Structure,thermal behavior,and dielectric properties of new cesium hydrogen sulfate arsenate: Cs<Subscript>2</Subscript>(HSO<Subscript>4</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)

Synthesis and structural characterization by single-crystal X-ray diffraction method, thermal behavior, and electrical proprieties are given for a new compound with a superprotonic phase transition Cs₂(HSO₄)(H₂AsO₄). The title compound crystallizes in the monoclinic system with the P2₁/n space group. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (10ī). The existence of O–H and (S/As)–O bonds in the structure at room temperature has been confirmed by IR and Raman spectroscopy in the frequency ranges 4000–400 cm^?1and 1200–50 cm^?1, respectively. Differential scanning calorimetry analysis of the superprotonic transition in Cs₂(HSO₄)(H₂AsO₄) showed that the transformation to high temperature phase occurs at 417 K by one-step process. Thermal decomposition of the product takes place at much higher temperatures, with an onset of approximately 534 K. The superprotonic transition was also studied by impedance and modulus spectroscopy techniques. The conductivity in the high temperature phase at 423 K is 1.58 × 10^?4 Ω^?1 cm^?1, and the activation energy for the proton transport is 0.28 eV. The conductivity relaxation parameters associated with the high disorder protonic conduction have been examined from analysis of the M”/M”_max spectrum measured in a wide temperature range. Transport properties of this material appear to be due to the proton hopping mechanism.     

Phonon linewidths in YNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN₂B₂C was chosen as a test example because it is a conventional superconductor with a fairly high T _c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron-phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.      

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe₃(BO₃)₄ was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.     

High-pressure effect on the ferroelectric-paraelectric transition in PbTiO<Subscript>3</Subscript>

The crystal structure of lead titanate PbTiO₃ was investigated by energy dispersive X-ray diffraction at high pressures up to 4 GPa in a temperature range of 300–950 K. At the ambient conditions, the PbTiO₃ structure is tetragonal with the space group P4mm (ferroelectric phase). A structural phase transition into the cubic phase with a space group Pm[`3]mPm\bar 3m is observed at T = 747 K. It was found that the phase transition temperature decreases upon applying the high pressure with the coefficient dT _C /dP = -65 K/GPa. Dependences of parameters and volume of the unit cell on the pressure and temperature was found, and the bulk modulus and thermal expansion coefficients for the tetragonal and cubic phases of lead titanate have been calculated.     

Transport and magnetic properties of a Zn<Subscript>0.1</Subscript>Cd<Subscript>0.9</Subscript>GeAs<Subscript>2</Subscript> + 10 wt % MnAs composite with magnetic clusters at high pressure

The temperature (T = 77–420 K) dependences of the electrical resistivity and the magnetization, the magnetic-field (H ≤ 5 kOe) and pressure (P ≤ 7 GPa) dependences of the resistivity, the Hall coefficient, and the magnetization have been measured in the Zn_0.1Cd_0.9GeAs₂ + 10 wt % MnAs composite with the Curie temperature T _C = 310 K. The magnetoresistive effect has been observed at high hydrostatic pressure to 7 GPa. At nearly room temperature, the pressure dependence of the magnetization demonstrated a transition from the ferromagnetic to paramagnetic state at P ~ 3.2 GPa that was accompanied by the semiconductor–metal phase transition.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

Room temperature synthesis of single-crystal Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles with superparamagnetic property

Single-crystal Fe₃O₄ nanoparticles with uniform size and relatively better monodispersity have been successfully synthesized via a facile room temperature coprecipitation route in the present of poly(vinyl pyrrolidone) (PVP). This method does not require high temperature, expensive and toxic starting materials, complicated procedure and toxic organic solvents. The magnetic properties of as-prepared samples were recorded on a superconducting quantum interference device magnetometer. Its blocking temperature is 140 K. The hysteresis loops of single-crystal Fe₃O₄ nanoparticles at 300 K and 10 K show the transition from superparamagnetic to ferromagnetic behavior. And the maintenance of high saturation magnetization ascribes to the single-crystalline nature of these Fe₃O₄ nanoparticles. PACS 75.50.K; 75.70.C     

Electronic transition and the metallization effect in the BiFeO<Subscript>3</Subscript> crystal at high pressures

The optical absorption spectra from bismuth ferrite (BiFeO₃) have been studied at high pressures up to 60 GPa in diamond anvil cells. An electronic transition at which the energy of the optical absorption edge decreases sharply from ～1.5 eV to zero has been observed at room temperature in a pressure range of 45–55 GPa. This indirectly indicates a insulator-metal transition. The observed electronic transition correlates with the recently revealed structural and magnetic transitions induced by high pressures in this crystal. The behavior of the optical absorption edge with decreasing the pressure is completely reversible in correlation with the reversibility of the magnetic transition. The “smearing” of the structural transition in pressure is caused by thermal fluctuations between the high-spin state and low-spin state of the Fe³⁺ ions near the transition.     

Piezo-optic effect in Cu<Subscript>6</Subscript>PS<Subscript>5</Subscript>Br crystals

The effect of hydrostatic (P=10–400 MPa) and uniaxial σ=0–5.8 MPa) pressures on birefringence Δn of Cu₆PS₅Br single crystals at the wavelength λ=0.6328 μm has been studied below the temperature of the ferroelastic phase transition (T<268 K). It is found that Δn linearly depends on pressure. The obtained data are analyzed.     

Magnetic properties of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> under applied hydrostatic pressure

Pressure effects on magnetic properties of two La_0.7Ca_0.3MnO₃ nanoparticle samples with different mean particle sizes were investigated. Both the samples were prepared by the glycine-nitrate method: sample S—as-prepared (10 nm), and sample S₉₀₀—subsequently annealed at 900 °C for 2 h (50 nm). Magnetization measurements revealed remarkable differences in magnetic properties with the applied pressure up to 0.75 GPa: (i) for S sample, both transition temperatures, para-to-ferromagnetic T _C = 120 K and spin-glass-like transition T _f = 102 K, decrease with the pressure with the respective pressure coefficients dT _C/dP = −2.9 K/GPa and dT _f/dP = −4.4 K/GPa; (ii) for S₉₀₀ sample, para-to-ferromagnetic transition temperature T _C = 261 K increases with pressure with the pressure coefficient dT _C/dP = 14.8 K/GPa. At the same time, saturation magnetization M _S recorded at 10 K decreases/increases with pressure for S/S₉₀₀ sample, respectively. Explanation of these unusual pressure effects on the magnetism of sample S is proposed within the scenario of the combined contributions of two types of disorders present in the system: surface disorder introduced by the particle shell, and structural disorder of the particle core caused by the prominent Jahn–Teller distortion. Both disorders tend to vanish with the annealing of the system (i.e., with the nanoparticle growth), and so the behavior of the sample S₉₀₀ is similar to that previously observed for the bulk counterpart.     

Electrical conduction and polarization in PbWO<Subscript>4</Subscript> crystals

The regularities of ion-electron processes in an undoped PbWO₄ single crystal upon transition to a quasi-equilibrium state in an external dc electric field with a linear variation in the temperature in the range 290–600 K are investigated using different methods. The total conductivity, thermally stimulated polarization current, and thermally stimulated depolarization current are measured. It is assumed that the temperature dependence of the conductivity can be described within the theory of small-radius polarons. The thermally stimulated polarization (depolarization) currents are interpreted in terms of the space-charge (peaks of the current in the range 400–550 K) and dipole (peaks of the current in the range 290–370 K) mechanisms of generation of a polarization charge in the sample. The inference is drawn that the dominant contribution to the dipole polarization is made by dipolons, namely, doubly charged (cation-anion) vacancy pairs coupled through electrostatic interaction. The basic parameters of relaxation phenomena and charge transfer are calculated.     

Metallic liquid hydrogen and likely Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metallic glass

Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al₂O₃ might be a metallic glass at ∼ 300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H₂ and in sapphire (single-crystal Al₂O₃), electrons are delocalized, which leads to formation of energy bands in fluid H and probably in amorphous Al₂O₃. The high strength of sapphire causes shock dissipation to be absorbed primarily in entropy up to ∼400 GPa, which also causes the 300-K isotherm and Hugoniot to be virtually coincident in this pressure range. Above ∼400 GPa shock dissipation must go primarily into temperature, which is observed experimentally as a rapid increase in shock pressure above ∼400 GPa. The metallization of glassy Al₂O₃, if verified, is expected to be general in strong oxide insulators. Implications for Super Earths are discussed.     

Structural and magnetic ordering of CrNb<Subscript>3</Subscript>S<Subscript>6</Subscript> single crystals grown by gas transport method

Paramagnetic layered semiconductor NbS₂ doped with some transition metals can transform into ferromagnetic material. That is why such materials are promising candidates for spintronic devices. It is found that only at certain concentrations of a doping metal T crystallographic ordering is possible, which is essential for magnetic ordering of ternary compounds TNbS₂. In particular, CrNb3S6 crystals are studied, which form almost completely ordered superstructure with intercalated Cr between NbS₂ layers. The main difficulty in crystal growth is reaching stoichiometry of the compound. This problem is solved in the developed method of two-staged gas transport chemical reaction. This new approach provides growth of CrNb₃S₆ single crystals of several millimeters in diameter and 0.3–0.5 mm thickness. X-ray phase analysis (XRD) of powders is performed to identify all phases involved in synthesis and growth of the crystals. High frequency absorption in external periodic magnetic field as a function of temperature and intensity of magnetic field is used to estimate the temperature of ferromagnetic transition in CrNb₃S₆ single crystals. The Curie temperature is estimated as 115 K. Growth of CrNb₃S₆ crystals from vapor phase is studied in detail and full analysis of phase transitions during growth is given. It has been shown that using of high frequency absorption in the crystal provides reliable estimation of the point of ferromagnetic transition in this semiconductor. The authors are grateful to the Physical Science Department of Russian Academy of Sciences for financial support of the studies in the frameworks of the program “Physics of new materials and structures” (project no. 00-12-10).     

Multiple magnetic transitions in single crystals of Ce<Subscript>12</Subscript>Fe<Subscript>57.5</Subscript>As<Subscript>41</Subscript> and La<Subscript>12</Subscript>Fe<Subscript>57.5</Subscript>As<Subscript>41</Subscript>

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce₁₂Fe_57.5As₄₁ and La₁₂Fe_57.5As₄₁. The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds. Both compounds exhibited multiple magnetic orders within 2–300 K and metamagnetic transitions at various fields. Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce₁₂Fe_57.5As₄₁ and La₁₂Fe_57.5As₄₁, respectively, followed by antiferromagnetic type spin reorientations near Curie temperatures. The magnetic properties underwent complex evolution in the magnetic field for both compounds. An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce₁₂Fe_57.5As₄₁. The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure. A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce₁₂Fe_57.5As₄₁. A temperature-field phase diagram was present for these two rare earth systems. In addition, a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150–300 K, which is rarely found in 3D-based compounds. It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Galvanomagnetic properties of polycrystalline manganese selenide Gd<Subscript>0.2</Subscript>Mn<Subscript>0.8</Subscript>Se

The electrical and galvanomagnetic properties of the Gd_0.2Mn_0.8Se solid solutions are investigated in zero magnetic field and in a field of 13 kOe in the temperature range of 80–400 K. The negative magnetoresistance below room temperature and hysteresis of the I–V characteristics are found. The change in the magnetoresistance sign and thermopower with increasing temperature is established. The carrier type is determined from the Hall constant; the difference between the thermopower and Hall coefficient signs at high temperatures is established. The experimental data are explained using the model of orbital ordering and spin-orbit interaction.     

Dielectric dispersion and magnetic properties of Ba-modified Pb(Fe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>

The polycrystalline samples of Ba-modified Pb(Fe_1/2Nb_1/2)O₃ (i.e., (Pb_1-xBa_x)(Fe_1/2Nb_1/2)O₃ PBFN, with x=0,0.05,0.07) were synthesized by a mechanosynthesis (i.e., high-energy ball milling) route followed by a mixed oxide method. Structural analysis provides the information on formation of single-phase orthorhombic structure on substitution of a small amount (x=0.07) of Ba at the Pb-site of Pb(Fe_0.50Nb_0.50)O₃ (PFN). The ferroelectric–paraelectric phase transition in PFN was observed at 383 K, which decreases on increasing Ba-concentration in PBFN. Detailed studies of dielectric properties of PBFW show the following: (i) diffuse phase transition, (ii) low loss tangent, (iii) low activation energy, and (iv) low frequency dielectric dispersion. An anomaly in the ac conductivity was found very close to phase transition temperature. The activation energy is found to decrease from 0.19 to 0.01 eV on increasing Ba-concentration to 7% (x=0.07). Temperature field-dependent magnetization measurements of all the samples showed antiferromagnetic transition at ∼15 K (for x=0.07). PBFN sample showed a slight increase in the coercivity (i.e., from 400 Oe (PFN) to 500 Oe (PBFN, for x=0.07) at 2 K. PACS 61.10.Nz; 68.37.Hk; 75.50.Ss; 75.60.Ej; 77.22.Ch; 77.22.Gm     

Resistive switching in β-SrV<Subscript>6</Subscript>O<Subscript>15</Subscript>

We investigate the pressure and temperature behavior of current-dependent resistivity of β-SrV₆O₁₅. We observe a switching between states of different resistivities in the insulating state of β-SrV₆O₁₅. In the low pressure phase, the resistive switching appears at temperatures below the semiconductor-insulator transition. In the high pressure phase, under ~1.6 GPa, the switching appears in the temperature range of the phase transition. The existence of switching may imply an important role of strontium off-stoichiometry for the electrical transport in β-SrV₆O₁₅. No electric-field-induced enhancement of the conductivity is observed. However, the conduction is significantly nonlinear under ~1.6 GPa, indicating that the charge order pattern in the high pressure phase is considerably different from that of the low pressure phase.