Singular temperature dependence of the equation of state of superconductors with spin–orbit interaction in the low-temperature region

The equation of state is investigated for a thin superconducting film in a longitudinal magnetic field and with strong spin-orbit interaction at the critical point. As a first step, the state with the maximal value of the magnetic field for a given value of spin–orbit interaction at T = 0 is chosen. This state is investigated in the low-temperature region. The temperature contribution to the equation of state is weakly singular.     

Influence of irradiation defects on charge‐density‐wave systems

The consequences of controlled irradiation-induced disorder have been examined in experiments carried out on several low-dimensional conductors that show charge-density wave phenomena. The most prominent action of the irradiation-induced defects is the pinning of the charge-density-wave. It is effective at concentrations starting from the ppm level as evidenced by the effect on the threshold of the collective (sliding mode) conductivity. At higher concentrations the defects start to influence the stability of the charge-density-waves and simultaneously the effect on the structural coherence of incommensurate or commensurate charge-density-wave phases becomes evident in the diffraction and electron microscope studies on irradiated samples. Defects perturb strongly the usual normal metal-incommensurate-commensurate charge-density-wave transition sequence in spite of the fact that they do not suppress completely the instability towards the condensed charge-density-wave phase. The existence of metastable pinned configurations and memory effects in association with defects has also been clearly demonstrated.     

Survival of charged ρ condensation at high temperature and density

The charged vector ρ mesons in the presence of external magnetic fields at finite temperature T and chemical potential μ have been investigated in the framework of the Nambu-Jona-Lasinio model.We compute the masses of charged ρ mesons numerically as a function of the magnetic field for different values of temperature and chemical potential.The self-energy of the ρ meson contains the quark-loop contribution,i.e.the leading order contribution in 1/N_C expansion.The charged ρ meson mass decreases with the magnetic field and drops to zero at a critical magnetic field eB_c,which indicates that the charged vector meson condensation,i.e.the electromagnetic superconductor can be induced above the critical magnetic field.Surprisingly,it is found that the charged ρ condensation can even survive at high temperature and density.At zero temperature,the critical magnetic field just increases slightly with the chemical potential,which indicates that charged ρ condensation might occur inside compact stars.At zero density,in the temperature range 0.2 — 0.5 GeV,the critical magnetic field for charged ρ condensation is in the range of 0.2 — 0.6 GeV~2,which indicates that a high temperature electromagnetic superconductor might be created at LHC.     

Transport properties of YBa2−xLayCu3On high temperature superconductors

The lattice parameters of the defect and La(III) substituted YBa_2–xLa_yCu₃O_n (0.05 x0.4,0y0.2) superconductors prepared in presence of the K₂CO₃ flux indicate substitution of La(III) on Ba(II) sites and intercalation of oxygen on 0(5) sites. This is manifested by a lowering ofT _c values. The presence of metastable phases with a sharp drop in resistivity around 200 K, which have been reported by Suzuki et al. (1989) for samples of similar composition, was not confirmed. The conductivity measurements show an increase of resistivity above 673 K, which is a consequence of gradual deoxygenation and the loss of metallic character. The full reversibility of this process in the bulk sample is modified by the oxygen diffusion into the sample.     

Hydrogen dominant metallic alloys: high temperature superconductors?

The arguments suggesting that metallic hydrogen, either as a monatomic or paired metal, should be a candidate for high temperature superconductivity are shown to apply with comparable weight to alloys of metallic hydrogen where hydrogen is a dominant constituent, for example, in the dense group IVa hydrides. The attainment of metallic states should be well within current capabilities of diamond anvil cells, but at pressures considerably lower than may be necessary for hydrogen.     

Interface superconductivity – Possible origin of high critical temperature in layered superconductors

Superconducting transition temperatures T_c of the YBCO film interfaces were measured inductively. It was observed that the interface-T_c is always higher then the surface-T_c probably because of the substrate influence. Deposition of the silver over-layer on the film surface enhances also its critical temperature. In the annealed YBCO/Ag bilayers magnetic properties of the interface were observed. This two dimensional structure reminds the layered microstructure of the high-temperature superconductors and one can suppose that enhanced superconductivity of the interfaces is the origin of the high critical temperatures in layered superconductors.     

A μSR study of spin density wave condensation in Cr85Mo15

Zero field SR spectra from Cr₈₅Mo₁₅ are well described by the sum of a lightly damped (0.02s^–1<₁<0.2s^–1) and a heavily damped (2s^–1<₂<15s^–1) exponential. The temperature dependence of these components is discussed in relation to the condensation of the incommensurate spin density wave and the onset of the antiferromagnetic state in this Cr-like alloy below T_N=120K. Evidence is presented for the nucleation of the spin density wave at temperatures greater than 1.5T_N.     

Scaling of ac susceptibility and the nonlinear response function of high-temperature superconductors

The amplitude-dependent ac susceptibility of high-temperature superconductors is shown to obey some empirical scaling relations. We try to analyze this behavior by extending a dc nonlinear response function of mixed state to the ac cases. The derived equations for critical current and ac susceptibility X(T) agree with the scaling relations of experimental data.     

Low density flux fluids in high —T c superconductors: The influence of disorder

We analyze the influence of thermal and frozen-in disorder on the flux line (FL) density close to the lower critical fieldH _c1. Arguments which consider the steric repulsion of fluctuating FLs give with the roughness exponent of a single FL andd the space dimensionality. We show by a phenomenological scaling approach and a renormalization group treatment, that this is correct only fordd _c =2/–1, i.e. for . Ford>d _c the steric FL repulsion at scales more than some critical one is irrelevant and . For disordered superconductorsd _c =2 and ford=2, 3. We also found the melting line for a FL lattice in the presence of frozen-in impurities close toH _c1.     

Zero temperature spin wave damping in spin polarized 3He: does it exist?

Previous spin echo experiments at equilibrium polarizations in 3He- 4He mixtures have confirmed the prediction of zero temperature polarization-induced spin wave damping in Fermi liquids. We have measured the damping of spin waves in dilute 3He, spin polarized by a 4He circulating dilution refrigerator. The maximum polarization is almost a factor of 5 higher than the equilibrium polarization in a magnetic field of 10.54 T at temperatures between 10 and 25 mK. The spin wave damping is much smaller than expected on the basis of the spin echo experiments and shows that the existence of polarization-induced spin wave damping is an open question.     

Phonon density of states and phonon dispersion of superconducting MgB2

For the superconductor MgB2, we have calculated the phonon density of states （DOS）, phonon dispersion and Eliashberg function throughout the Brillouin zone （BZ）, using an empirical potential model. The calculated values are consistent with the theoretical and experimental values. The calculated results show our empiricalpotential model is available for MgB2.     

Phonon density of states and phonon dispersion of superconducting MgB2

For the superconductor MgB2, we have calculated the phonon density of states (DOS), phonon dispersion and Eliashberg function throughout the Brillouin zone (BZ), using an empirical potential model. The calculated values are consistent with the theoretical and experimental values. The calculated results show our empirical potential model is available for MgB2.     

Pressure and temperature induced high spin–low spin phase transition: Macroscopic and microscopic consideration

The behavior under pressure of the high spin–low spin phase transition in the coordination compounds containing 3d ions is analyzed using thermodynamic and microscopic approaches. For thermodynamic approach the mean field model with interactions between spin-crossover molecules is considered. Microscopic model takes into account the interaction of d electrons of the transition metal ions with full symmetric distortions of the ligands. The relationship of the thermodynamic interaction parameters with microscopic ones is installed and shown how the quantum–mechanical interactions form the cooperativity of the system. Within the microscopic model the temperature and pressure dependences of the high spin fraction in 2-D compounds {Fe(3-Fpy)₂[M(CN)₄]} (M=Pd, Pt) are simulated and microscopic parameters are evaluated. It is concluded that different experimental behaviors of the temperature and pressure induced spin transitions are determined by different variations of the inelastic and elastic energies under pressure, and vibrational component of the free energy drives the ST equally with electronic part.     

On the Anderson localization and “universal degradation” of the critical temperature in superconductors

We give an analytical formula for the critical temperature of a superconductor taking into account the enhanced Coulomb repulsion which will be considered as frequency dependent.     

Time‐decay of magnetisation in high-Tc superconductors

An extension of Bean's model, incorporating the observed exponential decay, with increasing field, of critical current density in the high-T_c superconductors, is developed. It is used for explaining the various features of the isothermal magnetisation curves. Using this model, a calculation is presented for the time-decay of magnetisation after the sample is exposed to a magnetic field, and also after this applied field is switched off. The field dependence of the decay rate in these two conditions is calculated, and then compared with available results on the 90 K and the 40 K class of oxide superconductors. The results, in the limit of field-independent current density, also agree qualitatively with experimental results for the heavy fermion superconductor CeCu₂Si₂.     

Quantum diffusion of light particles in the vortex state of type‐II superconductors

The paper investigates the modification of the quasiparticle excitation spectrum of type‐II superconductors caused by the formation of a flux‐line lattice and its effects on the tunnelling rate of light positively charged interstitial particles. It is shown that near the upper critical field B_c2 the BCS–Gorkov equations predict for the spectral density J(\omega) (giving the probability of an energy exchange \hbar\omega between particle and the electron system) “superohmic” behaviour \propto \omega^3/2 at low frequencies with a prefactor that depends on the position of the tunnelling centre relative to the flux‐line lattice and on the angle between jump vector and the flux‐lines. As in normal‐conducting metals, at higher frequencies J(\omega)=2K_el\omega, is predicted. At low temperatures T the superohmic J(\omega) leads to hopping rates \nu \propto T^-2. At higher temperatures a crossover to the Kondo law ( ) is expected. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of the spin–orbit coupling on magnetic and superconducting properties in iron-based superconductors

We analyze the magnetic properties through two-orbital Hubbard model with the spin–orbit coupling (SOC) interaction in the iron-based superconductors. With the help of the Ising approximation for the Hund’s coupling between the itinerant electrons and the localized spins, we give a self-consistent account of the various magnetic orders observed in pnictides and the pairing symmetry. We also calculate the local density of states (LDOS) of the vortex state when a magnetic field is applied. The LDOS without SOC shows no resonant peak at the vortex core center in the superconducting state, while it shows an obvious resonant peak when SOC is applied.     

Crystallographic aspects related to the high pressure–high temperature phase transformation of boron nitride

Crystallographic relations between different forms of boron nitride (BN) appearing at the high pressure–high temperature structural phase transformation have been revealed by high-resolution transmission electron microscopy (HRTEM). As starting materials, crystalline hexagonal BN (hBN) with different degrees of crystallinity, or with defects intentionally introduced, were used. Cubic BN (cBN) is formed only as a minor component, the rest consisting of different forms of sp ² bonded BN: hBN, compressed, monoclinic deformed hBN, or turbostratic BN (tBN). The small cBN crystallites (300–400?nm) contain many defects such as twins, stacking faults and nanoinclusions of other BN forms: tBN, rhombohedral BN (rBN) and wurtzite BN (wBN). The cBN phase grows epitaxially on the basal plane of hBN. The nucleation sites for cBN are revealed by HRTEM. They consist of nanoarches (sp ³ hybridized, highly curved nanostructures), frequently observed at the edges of the hBN crystallites in the starting materials. Based on HRTEM observations of specimens not fully transformed, a nucleation and growth model for cBN is proposed which is consistent with existing theoretical and experimental models.     

Thermoelectric transport and spin density of graphene nanoribbons with Rashba spin–orbit interaction

In the present paper, we have theoretically investigated thermoelectric transport properties of armchair and zigzag graphene nanoribbons with Rashba spin–orbit interaction, as well as dephasing scattering processes by applying the nonequilibrium Green function method. Behaviors of electronic and thermal currents, as well as thermoelectric coefficients are studied. It is found that both electronic and thermal currents decrease, and thermoelectric properties been suppressed, with increasing strength of Rashba spin–orbit interaction. We have also studied spin split and spin density induced by Rashba spin–orbit interaction in the graphene nanoribbons.     

Effects of discharge current and voltage on the high density of metastable helium atoms

Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10^{-2}－66.7Pa. The highest metastable density of 3.8×10^{16}m^{-3} is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.