Metastable surface phase for NaxC60

Na_xC₆₀ thin films with Na concentration 0 ? x ? 3 were investigated using angle-dependent photoelectron spectroscopy. For low doping we observed two distinct fulleride phases coexisting with regions of pristine C₆₀. One of these fulleride phase is predominantly formed close to the surface and disappears after annealing and further addition of Na. At higher doping a phase with stoichiometry of x = 3 is formed.     

Chiral superconducting phase transition in 3-K phase of Sr2RuO4

We have investigated the transports of micro-fabricated sample of 3-K phase superconductivity (T_c  3 K) in Sr₂RuO₄–Ru eutectic system in order to clarify the pairing symmetry. Up to now, pure Sr₂RuO₄ (T_c = 1.5 K: 1.5-K phase) is widely recognized to be a spin-triplet odd-parity superconductor. However, the enhancement mechanism of T_c up to 3 K and the pairing symmetry of the 3-K phase have not been cleared yet. By using micro fabrication technique with focused ion beam, we have succeeded to extract individual superconducting channels for the 3-K phase in which only a few pieces of Ru inclusions are contained. Multiple kink structures observed in differential resistance–current (dV/dI − I) characteristics indicate serially connected superconducting filaments in the 3-K phase. We confirm that the 3-K phase is an odd-parity superconductor similar to pure Sr₂RuO₄ from the monotonous temperature dependence of the critical currents. In addition, we observed a quite unusual hysteresis in dV/dI − I below 2 K, which suggests the internal degrees of freedom in the superconducting state: the most probably the chiral p-wave state.     

Preparation and physical properties of CuxWO3

We report on the study of WO₃ doped with Cu using sol-gel (Cu_xWO₃^d) and impregnation (Cu_xWO₃ⁱ) methods. All materials are well crystallized and exhibit single phases whose crystallite size ranges from 17 to 100 nm depending on Cu amount and the preparation technique. The conductivity dependence on temperature demonstrates semiconductor behavior and follows the Arrhenius model, with activation energies, E_σ, commonly in the range 0.4-0.6 eV. Moreover, the thermopower study shows that Cu_xWO₃^d is mainly of p-type conductivity, whereas Cu_xWO₃ⁱ is n-type. The mechanism of conduction is attributed to a small polaron hopping. The doping process is found to decrease the interband transition down to 520 nm depending on the preparation conditions. The photoelectrochemical characterization confirms the conductivity type and demonstrates that the photocurrent J_ph increases with Cu-doping. Taking into consideration the activation energy, the flat band potential and the band gap energy, the band positions of each material are proposed according to the preparation method and Cu amount.     

Structural phase transition in superconducting BaPb0.75Bi0.25O3

The room temperature crystallographic properties of distorted perovskite-type system BaPb_1-xBi_xO₃ were examined by using X-ray diffraction analysis. In addition, structural temperature changes were investigated for superconducting BaPb_0.75Bi_0.25O₃ crystals with T_c=11.4K. It was found that the room temperature structure of BaPb_1-xBi_xO₃ has an orthorhombic symmetry in the range of O≤×?0.9 and a monoclinic one in 0.9?×≤1, and that superconducting BaPb_0.75Bi_0.25O₃ shows a structural transition from an orthorhombic to a monoclinic phase at about 160K.     

Ultrasonic propagation near the ferroelectric phase transition temperature in RbH2PO4

Ferroelectric phase transition in RbH₂PO₄ is studied by the propagation of longitudinal ultrasonic waves along the polar axis. The velocity data are analyzed to obtain the temperature dependence of the soft mode frequency in the ferroelectric phase. Unlike KH₂PO₄ and KD₂PO₄, the attenuation in RbH₂PO₄ does not exhibit a relaxation type behaviour.     

The effect of pressure on the superconducting transition temperature in TaSe3

The transition temperature of TaSe₃ is found to decrease for pressures up to 10 kbar at a rate -7.5 ± 0.3×10^-5 °K/bar. This unusually large decrease can only be explained by both a pressure induced lattice stiffening and a decrease in the Fermi level density of states.     

Low temperature phase transition in the chain-like compounds NbSe3 and TaSe3

New transitions have been observed at low temperature by electrical measurements on the linear trichalcogenides NbSe₃ and TaSe₃. The resistivity of TaSe₃ drops near zero below 1.5 K which may indicate a superconducting transition. The resistivity of NbSe₃ drops below 2.2 K by 30 to 75% of its residual value at 4.2 K. However for TaSe₃ and NbSe₃ initial susceptibility measurements do not show any increase in diamagnetism indicating a flux expulsion. In addition the transitions below the critical temperatures are strongly non-linear with current density higher than 10^-3 A mm^-2.     

Tight-binding calculation of the optical conductivity in NaxCoO2

Optical conductivity in Na_xCoO₂ is calculated with tight binding approximation. The calculated gross features agree with the experimental findings. Interband transitions between nearly degenerate t_2g bands bring about absorption in the mid-infrared and near infrared regions. Structures observed at higher energies are also quite well reproduced with the present calculation. Possible effects of electron correlation are also briefly discussed.     

Anomalies of the phonon properties in multiferroic BiFeO3 thin films near the magnetic phase transition temperature

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO₃ thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO₃ and strong anharmonic spin-phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.     

Oscillatory behaviour of ultrasonic attenuation in KH2PO4 near the phase transition temperature

Oscillations in ultrasonic attenuation have been observed in KH₂PO₄ near the ferroelectric phase transition temperature for longitudinal ultrasonic wave propagation along the polarization axis. Oscillations are present both above and below the transition temperature; the periods of oscillations are different in the two phases for the same ultrasonic frequency and the same ∣T ? T_c∣. The temperature dependence of the oscillations shows that they are due to the phase transition.     

Structure transformations during phase transitions in the K3WO3F3 oxyfluoride

The structures of three phases of the K₃WO₃F₃ crystal have been determined from X-ray diffraction data obtained for a powder sample. The profile and structural parameters have been refined according to the technique implemented in the DDM program. The results obtained have been discussed using the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic displacements and allows the interpretation of the experimental data. The sequence of structural transformations is found to be as follows: $ Fm\bar 3m\xrightarrow[{(\eta _1 ,0,0)}]{{11 - 10(\Gamma _4^ - )}}I4mm\xrightarrow[{(\eta _1 ,\eta _2 ,0)}]{{11 - 10(\Gamma _4^ - )}}Cm $ Fm\bar 3m\xrightarrow[{(\eta _1 ,0,0)}]{{11 - 10(\Gamma _4^ - )}}I4mm\xrightarrow[{(\eta _1 ,\eta _2 ,0)}]{{11 - 10(\Gamma _4^ - )}}Cm .     

Proton mobility in HxWO3 measured by proton magnetic resonance

An NMR study for a powder sample of H_xWO₃ prepared by the reduction of WO₃ with zinc grains and a dilute HCl is reported and the temperature dependence of the correlation time and the proton mobility estimated from the linewidth of proton magnetic resonance (PMR) is presented. The correlation time can be described by a single exponential line $\text{τ = τ}{}_0\text{exp}\text{(}\text{E}\text{kT}\text{)}$, where τ₀ = 1.1 × 10^-10 sec and E = 0.13 eV. The proton mobility is compared with the one obtained for an amorphous film by bleaching current measurement.     

Critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.7Pb0.05Na0.25MnO3

Critical behavior in La_0.7Pb_0.05Na_0.25MnO₃ has been investigated by dc magnetization measurements. Magnetic data indicate that the compound exhibits a continuous (second-order) paramagnetic (PM) to ferromagnetic (FM) phase transition. Estimates of critical exponents yield δ=4.80±0.01, γ=1.296±0.002 and β=0.344±0.007 (consistent with both the predictions for the three-dimensional-Heisenberg model and with those reported for materials when the FM transition is ascribed to the double exchange (DE) mechanism as a major origin) with T_C=334.54±0.08. The critical exponent γ is slightly inferior than predicted from the 3D Heisenberg model. Such a difference may be due, within the context of the quenched disorder, to the presence of some alterations of short-range magnetic order of FM clusters in the PM phase. The temperature variation in the effective exponent (γ_eff) is similar to those for disordered ferromagnets.     

LaAlO3衬底高温超导线性相位滤波器

报道了带宽为10 MHz,80%以上带宽内群时延失真小于20 ns的高温超导线性相位滤波器.该滤波器制作于LaAlO₃衬底上,采用了一种新的谐振器结构以消除寄生耦合的影响,使得滤波器不仅具有很好的线性相位特性,而且具有对称的幅频响应特性.滤波器实测带宽为9.85 MHz,回波损耗约16 dB,带外抑制大于80 dB,群时延失真（80%以上带宽内）约20 ns,其他实测指标也与仿真结果符合很好. 关键词： 高温超导 滤波器 自均衡 线性相位     

High temperature Raman scattering and phase transition in EuAlO3

Polarized Raman spectra of EuAlO₃ have been obtained at various temperatures from 77–1500 K. Three phonon modes are observed to soften in frequency with increasing temperature. Discontinuity in the frequency temperature dependence appears at 1420 K. This result is consistent with a first order phase transition. Analysis of the Raman data shows evidence of a coupling between the soft mode of Bg(XY) symmetry and one mode of same symmetry.     

Temperature dependence of acoustic modes in KNbO3 near the orthorhombic-tetragonal phase transition

The wave vector and temperature dependences of transverse acoustic modes propagating along the [001] and polarized along the [100] orthorhombic axis in KNbO₃ have been studied by means of coherent inelastic neutron scattering methods. The results are consistent with previous elastic constant and neutron scattering measurements. The mechanism of the orthorhombic-tetragonal phase transition is discussed in the light of all these data.     

Spontaneous magnetization on the superconducting transition in UPt3

Theories predict that spontaneous magnetization occurs in the superconducting B phase in UPt₃ because of broken time-reversal symmetry. The strength of the spontaneous magnetization depends on whether it comes from spin degeneracy or orbital degeneracy. We measured the internal field due to the spontaneous magnetization on the supercondicting transition to the B phase for a UPt₃ sample in the magnetic fields very close to zero, using a SQUID magnetometer. The upper limit of the spontaneous internal field was 0.007 gauss. This result seems to favor the theory of the orbital degeneracy.     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Thermochemistry of the cubic and hexagonal lithium tungsten bronze phases LixWO3

The enthalpies of formation of hexagonal WO₃, hexagonal and cubic Li_xWO₃ have been determined by solution calorimetry. The ambient-temperature materials h-Li_0.44WO₃ and c-Li_xWO₃ (x=0.51, 0.71) were prepared by n-butyl lithiation in hexane. The thermochemistries of the lithium insertion compounds were examined and related to structural features.     

Pressure dependence of the superconducting transition temperature of Th4H15

Measurements of the superconducting transition temperature of Th₄H₁₅ under hydrostatic pressure up to 28 kbar are reported. The initial linear increase is found to be $\text{+42}\text{mK}\text{kbar}$.