Positron-annihilation lifetime in the high-Tc Oxides

The superconducting transition affects the positron-annihilation lifetime in the high-T_c oxides. This effect is due to the unusual properties of the oxides in the normal state and, in particular, to the small Fermi energy. The value of the shift also depends on the interlayer distance.     

Contributions of muon spin rotation/relaxation/resonance to the understanding of problems in magnetism — Current and future

During these 25 years following the start of muon spin rotation/relaxation/resonance (μSR) studies on condensed matter, various new aspects of magnetic materials have been explored by using high sensitivity to the microscopic dynamical behaviour of the host magnetism, specifically, the studies on magnetic ordering in strongly-correlated electron systems, elementary excitations in conducting polymers and protein, etc. Intense (more than 10⁶/cm·s) and ultra-slow (below keV) muon beams to be available at future accelerator facilities may open new areas such as spin dynamics studies of surface magnetism, cluster particles, etc.     

Femtosecond Pump-Probe Polarization Dependent Investigation of Relaxation Dynamics in YBa2Cu4O8

Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YBa₂Cu₄O₈ as a function of temperature and polarization. We compare our results with the data obtained on YBa₂Cu₃O_7– and show that similar two-component relaxation is present in both cases. A strong polarization anisotropy of the picosecond response is observed below T _c and interpreted with the aid of a simple model which considers the anisotropy of the probe transition matrix elements.     

Critical temperature of a strongly disordered two-dimensional superconductor

Using the Eliashberg equations, we calculated the critical temperature of a two-dimensional superconductor with strong disorder. The retarded Coulomb interaction and the magnetic correlations have been included in the renormalization function Z(). The critical temperature decreases with the concentration of impurities, in agreement with the experiments.     

Convergence of Theory and Experiments on Electron-Lattice Interactions in Cuprates: An Overview of SILS

A brief overview of some of main highlights of the Symposium on Localised and Itinerant States (SILS) is presented.     

Energy scale of the pairing model with energy-dependent density of state

The critical temperature of a pairing model for HT _c S has been calculated using an energy-dependent density of states. We showed that the problem which is connected with the van Hove scenario is very sensitive to the energy scale and to the behavior of the density of states at low and high energy.     

On the Possibility of s+d Wave Superconductivity Within a Two-Band Scenario for High Tc Cuprates

A variety of different experimental results show substantial evidence that the order parameter in high-temperature superconducting copper oxides is not of pure d-wave symmetry, but that an s-wave component exists, which especially shows up in experiments that test the c-axis properties. These findings are modeled theoretically within a two-band model with interband interactions, where the superconducting order parameters in the two bands are allowed to differ in symmetry. It is found that the coupling of order parameters with different symmetries (s+d) leads to substantial enhancements of the superconducting transition temperature T _c as compared to order parameters with only s-wave symmetry. An additional enhancement factor of T _c is obtained from the coupling of the bands to the lattice where moderate couplings favor superconductivity while too strong couplings lead to electron (hole) localization and consequently suppress superconductivity.     

Disorder and plasmonic effects in the two-dimensional model for high-critical-temperature superconductors

The effect of disorder on the phononic-plasmonic microscopic model for high-critical-temperature superconductors has been studied. The dynamic screening of the Coulomb interaction due to the disorder results in a decrease inT _c . For a two-dimensional electronic system the plasmon modes are strongly affected by disorder. Taking this effect into consideration, we show thatT _c can decrease but there is also an additional effect to the phonon-plasmon attractive contribution.     

Effect of superconducting fluctuations on the nmr relaxation rate of high-Tc superconductors

The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T ₁, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T _c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T₁ atT _c and briefly discuss possible reasons why this peak is not observed.     

Complementary Analysis of the Field Perturbation Theory of Superconductivity

We reexamine the Nambu–Gorkov perturbation theory of superconductivity. We suggest that any field perturbation theory of superconductivity should be based on the Bogoliubov–Valatin (BV) quasi-particles. We show that two such different fields (and two additional analogous fields) may be constructed on the basis of this suggestion. The Nambu field is only one of them. For the field that is different than Nambu’s, the coherence field, the interaction is given by means of two interaction vertices that are based on the Pauli matrices τ₁ and τ₃. Consequently, the Hartree integral for the off-diagonal pairing self-energy may be finite, and in some cases large. We interpret the results in terms of conventional superconductivity and also discuss briefly the possible implications to HTSC.     

Thermal expansion in the Ba1?xKxBiO3 system

A negative thermal expansion was measured for Ba_1–x K _x BiO₃. This anomaly decreases with increasingx, still exists atx=0.4, and disappears atx=0.5.     

Observation of a Large Spin–Orbit Interaction in Photoexcited InxGa1?xAs/InyAl1?yAs Heterostructures

Far-infrared magneto-optical absorption measurements of photoexcited InGaAs/InAlAs heterostructures are carried out. The resonance peak positions of cyclotron resonance due to spin-up and -down electrons can be determined from modulated absorption signals quantitatively. The photopulse excitation by band gap light changes the resonance positions. The amplitude of the spin splitting is found to depend on the excitation intensity and delay time. The change in the spin splitting energy is dominated by the modulation of confinement potential by photoexcitation.     

Photoluminescence and spin relaxation of MnZnO/GaN-based light-emitting diodes

This study investigates the spin relaxation of GaN-based light-emitting diodes with an MnZnO film by examining its photoluminescence (PL) and time-resolved magnetization modulation photoluminescence. PL measurements reveal that the application of a magnetic field produced a clear difference between the intensities of the right (σ⁺) and left (σ⁻) circular polarization components. The circular polarization was identified as P_circ = [I(σ⁺) − I(σ⁻)] / [I(σ⁺) + I(σ⁻)], where I(σ⁺) and I(σ⁻) are the intensities of the σ⁺ and σ⁻ components, respectively. The PL polarization was 3.6% in a 0.5 T magnetic field. In a magnetic field, the photo-ionized lifetime and spin-polarized lifetime values were approximately 13.64 and 54.54 ns, respectively. The right-circular-spin-polarization lifetime and the left-circular-spin-polarization lifetime values were about 39.09 and 40.01 ns, respectively.     

Boosting the superconducting spin valve effect in a metallic superconductor/ferromagnet heterostructure

Superconducting spin valves based on the superconductor/ferromagnet (S/F) proximity effect are considered to be a key element in the emerging field of superconducting spintronics. Here, we demonstrate the crucial role of the morphology of the superconducting layer in the operation of a multilayer S/F1/F2 spin valve. We study two types of superconducting spin valve heterostructures, with rough and with smooth superconducting layers, using transmission electron microscopy in combination with transport and magnetic characterization. We find that the quality of the S/F interface is not critical for the S/F proximity effect, as regards the suppression of the critical temperature of the S layer. However, it appears to be of paramount importance in the performance of the S/F1/F2 spin valve. As the morphology of the S layer changes from the form of overlapping islands to a smooth case, the magnitude of the conventional superconducting spin valve effect significantly increases. We attribute this dramatic effect to a homogenization of the Green function of the superconducting condensate over the S/F interface in the S/F1/F2 valve with a smooth surface of the S layer.

Open image in new window

     

High-Tc superconductor in an a.c. field

The impedance of the layered high-T _c oxide is evaluated. The response of the S-N proximity system is studied. The mechanism of intrinsic losses due to the low-lying collective mode is proposed.     

Phase transitions in the Ba1?xKxBiO3 system and other oxide superconductors

The mechanisms of dielectric-to-metal and metal-to-superconductor phase transitions are proposed. The antiphase-ordered array of heavy holes localized on the apical oxygen-bismuth bonds creates a dielectric gap on the Fermi surfce. With doping by potassium, light carriers are introduced, which reduce the Coulomb potential of ordered heavy holes and dielectric gap. Heavy holes tunneling between the neighboring apical O^–1 and O^–2 ions produce local charge fluctuations. Their interactions with short-wavelength optical phonons yield the coupled tunneling-phonon modes. Those local boson modes lead to the pairing of light-conducting carriers and to superconductivity with a short coherence length. The local pairing may explain the observed upward curvature ofH _c2(T) in the range fromT _c =30 K down to 4 K and up to 16 T. After obvious rearrangements, the mechanism can be applied for all bismuth oxide-based and copper oxide-based superconductors.     

Nuclear relaxation in the superconducting state of (MDT-TTF)2AuI2

We report the results of our attempt to measure the proton nuclear relaxation rate, 1/T ₁, in the superconducting state of the title material. The relaxation rate in the superconducting state at a field of 1 T was found much longer than that in the normal state, but it became clear that the dominant contribution came from the normal core region. The nuclear relaxation at zero field was examined by using the field cycling technique. An ln(t) term in the relaxation curve was observed at low temperatures, suggesting the contribution of the creeping motion of vortices. We discuss the possibility to determine the intrinsic temperature dependence of 1/T ₁ in the superconducting state.     

Lattice instability in the normal state of La2?xSrxCuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Magnetic correlations and disorder in a two-dimensional fermi liquid: Possible important effects for high-Tc superconductors

The influence of magnetic correlations and disorder in a two-dimensional Fermi liquid is considered in connection with the occurrence of the superconducting state. The disorder effect and magnetic correlations work against the superconducting state but at the same time are competitive phenomena. We show that the critical temperature can decrease due to the magnetic correlations and disorder, but the effect of correlations is reduced by the disorder. The electronic mass enhancement has been calculated taking into consideration the magnetic correlation effects and disorder. Its dependence on the concentration impurities, in agreement with the experimental data, shows the importance of the localization effects due to disorder. These results are in agreement with the experimental data obtained for the high critical temperature superconductors.     

New Ru-Based Layered Cuprates with 1232 Structure in the RuSr2(LnCe2?xSrx)Cu2Oz (Ln = Sm, Eu, and Gd) System

New Ru-based layered cuprates with 1232 structure have been synthesized in the RuSr₂(LnCe_2–x Sr _x )Cu₂O _z (Ln = Sm, Eu, and Gd) system. Nearly single-phase samples are obtained in the composition range from x = 0.0 to 0.2 for Ln = Sm and Eu, and at the composition of x = 0.2 for Ln = Gd. Each of the samples of Ln = Sm, Eu, and Gd with x = 0.2 crystallizes in a tetragonal lattice and lattice parameters a and c are 0.3845 and 1.700 nm, 0.3842 and 1.699 nm, and then 0.3839 and 1.698 nm, respectively. Each of them shows some anomalies at a low temperature in addition to a semiconductor-like temperature dependence of the resistivity. Especially, the sample with Ln = Gd shows two resistivity dropping phenomena at about 16 and 22 K. Then, all of nearly single-phase samples show diamagnetic signals originating from superconductivity, and also rapid increase of magnetization due to a transition from paramagnetic state to ferromagnetic one below 80 K.