Modern Physics Letters B

We firstly briefly summarize some of the most relevant recent results and open questions across rather complex electronic phase diagram of cuprates. We continue with a discussion of results on thin superconducting oxide films grown by laser ablation. Systematic studies show that BSCCO-phases and LSCO-214 exhibit conductor-like Fermi edge, whereas materials containing "chains" (like YBCO-123) are prone to very rapid surface degradation, most likely related to critical oxygen loss at the outermost layers. Recently, direct ARPES dispersion measurements on in-situ grown, strained 10UC thin LSCO-214 films (T_c = 44 K) have shown the band crossing of Fermi level well before the Brillouin zone boundary. This is in contrast to the flat band observed in unstrained single crystals — and to the band flattening predicted by band calculations for in-plane compressive strain. In spite of density of states reduction near the Fermi level, the critical temperature increases in strained films with respect to unstrained crystals; this poses further challenge to HTSC theory.

In this article we report the energy gap evolution in Bi₂Sr₂CaCu₂O_8+δ(Bi2212) on the basis of recent tunneling and Raman scattering experiments over wide temperature and hole-doping (p) ranges. It has been demonstrated that in the normal-state electronic excitations, there exist two kinds of pseudogaps (LPG and SPG) with different characteristic energies. The LPG, which is 3 to 4 times larger than the superconducting (SC) gap at T ≪ T_c, develops below ~ T_max, where the magnetic susceptibility starts to decrease because of the gradual development of antiferromagnetic spin fluctuations. On the other hand, the SPG, whose magnitude is comparable to the SC gap, develops progressively, in addition to the LPG, below the mean-field characteristic temperature T_co for d-wave superconductors, and then evolves into the SC gap below T_c, suggesting that it will be some kind of precursor of superconductivity. It has also been demonstrated that the maximal gap Δ₀ or the gap at the positions near (±π, 0) and (0,±π), determined at T ≪ T_c by tunneling technique, does not scale with T_c, while the gap around (±π/2,±π/2), determined at T ≪ T_c from the coherence peak energy in B_2g Raman spectra, scales with T_c, suggesting that the latter gap will function as an effective SC gap Δ_eff in determining T_c. Furthermore, we will report that Δ_eff is given by the product of p and Δ₀; Δ_eff ~ 5.3pΔ₀ in Bi2212, and discuss some scenarios for the SC transition which is consistent with the present result on the effective SC gap.

Properties of quasiparticles in doped cuprates formed by an oxygen hole and two adjacent copper holes are investigated on the basis of the extended Hubbard model. The ground state energy, wave functions and the polaron-phonon coupling are calculated. We also analyzed the polaron-polaron interaction via the phonon field. It was found that this interaction is highly anisotropic and can explain the experimentally observed phase separation in the strongly underdoped LaSrCuO:Mn system.

We present a renormalization equations for two-band superconductivity by using a two-band model and present phase diagrams for the two-band superconductivity. In the framework of two-band model, the present results predict that superconductivity appears, even if electron-electron interaction is positive. We discuss the possibility of a cooperative mechanism in the two-band superconductivity in relation to high-T_c superconductivity.

The variety of the normal-state magnetic properties of cuprate high-T_c superconductors is interpreted based on the self-consistent solution of the self-energy equations for the two-dimensional t-J model. The observed variations of the spin correlation length with the hole concentration x, of the spin susceptibility with x and temperature T and the scaling of the static uniform susceptibility are well reproduced by the calculated results. The nonmonotonic temperature dependence of the Cu spin-lattice relaxation rate is connected with two competing tendencies in the low-frequency susceptibility: its temperature decrease due to the increasing spin gap and the growth of the susceptibility in this frequency region with the temperature broadening of the maximum in the susceptibility.

The effect of three-center interactions on the formation of a superconducting phase with d_x²-y²-symmetry is considered using the diagram technique for Hubbard operators and irreducible Green's functions. It is shown that these interactions lead to a decrease in T_c by a factor of several tens.

Cuprate superconductivity gaps are investigated applying the interband pairing scheme to the itinerant band and a defect "band" which is created by hole doping. A pseudogap is naturally present in this model at underdoping. Vibronic hybridization of bands renormalizes the pseudogap and two superconductivity gaps. Now two pseudogaps of electronic and vibronic origin are present in the underdoped normal state as recently observed. Entering the optimally doped region (overlapping bands) there appears a joint normal state pseudogap of vibronic origin.

We present a model calculation of the paramagnetic-resonance linewidth in La_0.95Sr_0.05MnO₃. Both the temperature and the angular dependencies can be described by contributions from the crystal field due to the cooperative Jahn–Teller transition.

We analyze the temperature and angular dependence of the paramagnetic-resonance linewidth of La_0.95Sr_0.05MnO₃. The observed anisotropy can be well described by the crystal field contributions originated by the Jahn–Teller distortion. The critical behavior of the linewidth at the Jahn–Teller transition is found to be directly related with the orbital order.

Quasiparticle band structure in hole doped CuO₂ layer is calculated with account for strong electron correlations in the framework of multiband p–d model. For undoped layer we obtain the charge-transfer antiferromagnetic insulator. With doping unusual impurity-like quasiparticle appears at the top of the valence band with spectral weight proportional to doping concentration. In the overdoped regime the band structure in the paramagnetic phase results in the doping dependent Fermi surface in agreement to ARPES data.

In this report we present our study of two-dimensional Heisenberg magnets based on the picture of thermally excited skyrmions. The local order parameter, energy spectrum of elementary spin excitations above the skyrmion background, the skyrmion averaged radius and static magnetic susceptibility were obtained by the Green functions method. There was calculated the rate of nuclear relaxation caused by the interaction of nuclear spins with above mentioned spin excitations. The contribution of Moriya–Dsyloshinskii and anisotropic symmetric interactions into EPR line was considered. A comparison with NMR and neutron scattering experiments in cuprates and with the results obtained by other methods is given. We discuss the consequences of an interaction between skyrmions as the result of the spin fluctuations.

We report a range of experiments designed to test the efficacy of spin polarized injection from colossal magnetoresistive material (CMR) into high temperature superconductors (HTS). These include traditional transport measurements, studies of CMR/YBCO contacts, measurements of penetration depth and studies of the effect of spin-injection on the critical state of square thin films and rings. We have used pulsed currents to minimize heating and have made comparisons with the non-magnetic homologue of CMR LaNiO₃. Our experiments provide evidence that there is a measurable effect, but that it is small and mainly confined to 10–20 nm of the YBCO surface. Current gain of about 0.2 has been observed.

It has recently been proposed that if phase incoherent preformed pairs exist above critical temperature (T_c), Andreev reflection should be observable above T_c too. A distinctive feature of Andreev reflection is a zero bias conductance peak (ZBCP) in the conductance/voltage (G/V) characteristics. We have performed an extensive search for the ZBCP in HTS/normal-metal planar junctions including samples which have been annealed in Argon in an attempt to induce underdoping and enhancement of the pseudogap temperature. We have observed a maximum in the G/V characteristics above T_c in a number of samples and contact configurations. In some samples this behavior persists up to room temperature, but has a very broad voltage scale, much greater than the gap voltage. In YBa₂Cu₃O_x/Au and YBa₂Cu₃O_x/Ag contacts the form and amplitude of the ZBCP is close to the theoretical prediction, but the origin of this effect could also be in sample heating. One particularly curious result comes from an Au/LSMO/YBCO sample that shows a clear sub-gap ZBCP over the temperature range 15–266 K.

Flux pinning is investigated in a thin type-II superconducting film covering a ferromagnetic Co/Pt film with perpendicular anisotropy. Stable bubble domains with varying size and density can be created in the ferromagnet by means of different magnetization procedures. The flux pinning is strongly depending on this magnetic domain structure, resulting in asymmetric magnetization curves with respect to the field polarity.

Multiply connected mesoscopic superconductors are considered within the framework of the nonlinear Ginzburg–Landau theory. The two coupled nonlinear equations are solved numerically and we investigated the properties of a superconducting ring, two concentric rings, and an asymmetric ring. We find that (i) for a mesoscopic superconducting ring the flux through the hole is not quantized, (ii) two concentric mesoscopic superconducting rings are magnetically coupled and the interaction energy increases with increasing sample thickness, and (iii) in asymmetric rings, a stationary phase slip state is predicted.

Beyond the well-known Abrikosov and giant vortex configurations, new solutions to the Ginzburg–Landau model corresponding to vortices of integer and half-integer winding number are described. Phase diagrams (B_ext, Energy) and magnetization curves have been determined, aiming towards an understanding of the magnetic properties of lead nanowires and the possible consequences of such solutions with respect to the switching mechanism between vortex states in mesoscopic superconductors.

High flux velocities in a superconductor can distort the quasiparticle distribution function and elevate the electronic temperature. Close to T_c, a non-thermal distribution function shrinks the vortex core producing the well-known Larkin-Ovchinnikov flux instability. In the present work we consider the opposite limit of low temperatures, where electron-electron scattering is more rapid than electron-phonon, resulting in an electronic temperature rise with a thermal-like distribution function. This produces a different kind of flux instability, due to a reduction in condensate and expansion of the vortex core. Measurements in YBCO films confirm the distinct predictions ofthis mechanism.

Magnetic properties of the composite film structure of NiO_x/Nb are investigated by using ESR data, because NiO_x/Nb is a key structure in Super/Normal/Super of Nb/NiO_x/Nb and Nb/AlO_x/NiO_x/Nb. The ESR signals were not observed below liquid He temperature, while they were observed above about 7 K. The result is explained by temperature dependence of H_c2-curve of Nb. The suppression of Josephson current is related to the local moment observed by the X-band ESR. The existence of the local moment may explained by the model of "AF-domains" of the micro-crystals on polycrystalline film. The disappearance of ESR signals below liquid He temperature shows the change of the magnetic effect by the quasi-particle tunneling.

Nb/Au/YBa₂Cu₃O_x heterojunctions in (001) and (1 1 20)-oriented YBa₂Cu₃O_x films have been fabricated. I-V curves of both kinds of heterojunctions show pronounced superconducting current. As affected by microwave radiation Shapiro steps at voltages hf/4e arise in the I-V curves of Nb/Au/YBa₂Cu₃O_x (001) heterojunctions implying the double periodic (~sin2φ) superconducting current-phase relation. However, in the case of Nb/Au/YBa₂Cu₃O_x (1 1 20) heterojunctions Shapiro steps have been observed in the I-V curves at voltages hf/2e only supporting a sinusoidal (~sinφ) superconducting current-phase relation within measurement accuracy. A zero bias conductance peak associated with zero-energy Andreev bound states occurs in the I-V curve of the Nb/Au/YBa₂Cu₃O_x (1 1 20) heterojunctions. We discuss the experimental results in terms of the d-wave symmetrical superconducting order parameter in YBCO.

The structure of the flux-line lattice in the superconducting film with the periodic system of artificial pinning sites is studied theoretically using the London model. The existence of different vortex phases (pinned square phase, depinned deformed triangular phase, and "intermediate" phase) is revealed. The phase diagrams of the vortex lattice are obtained.

The stability of the anisotropic flux flow in the vortex-antivortex system of a superconductor is theoretically studied. The vortex motion is assumed to be governed by the power current-voltage (I-V) characteristic, E ∝ J^m. It is shown that the instability of the interface between vortices and antivortices occurs even at relatively small anisotropy of the current capability of a sample if the exponent m is large. The dispersion equation defining the dependence of the increment of the instability on the wave number is obtained and analyzed. The instability leads to the macroturbulence of the flux flow which is observed in superconductors of the system Y-123.

The suppression (collapse) of the static magnetization of an anisotropic hard superconductor under effect of ac magnetic fields was studied experimentally and theoretically. The measurements were carried out in mutually orthogonal dc and ac magnetic fields, and . Both fields were parallel to the sample surface. Superconducting Y-123 plate-like single crystals were cut from a textured ingot in such a way that the c axis was in the sample plane. The amplitude of the ac field that suppresses completely the static magnetization was shown to be dependent on the orientation of the magnetic fields with respect to the crystallographic axes of the sample. The dynamics of the collapse of the magnetization was investigated. In some cases, the magnetic moment was found to change non-monotonically with the increase of the amplitude of the ac magnetic field. An unusual phenomenon of the transition of the superconductor from the paramagnetic to diamagnetic state induced by the transverse ac field is discovered. The observed results are interpreted within the framework of the critical state model generalized to the anisotropic case.

We report on experimental investigation of the effect of flux compression in superconducting YBa₂Cu₃O_x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers. The flux compression produces positive magnetic moment (m) upon the cooling in a field from above to below the critical temperature. We found effect of compression in all measured films and multilayers. In accordance with theoretical calculations, m is proportional to applied magnetic field. The amplitude of the effect depends on the cooling rate, which suggests the inhomogeneous cooling as its origin. The positive moment is always very small, a fraction of a percent of the ideal diamagnetic response. A CMR layer in contact with HTS decreases the amplitude of the effect. The flux compression weakly depends on sample size, but sensitive to its form and topology. The positive magnetic moment does not appear in bulk samples at low rates of the cooling. Our results show that the main features of the flux compression are very different from those in Paramagnetic Meissner effect observed in bulk high temperature superconductors and Nb disks.

In this paper we focus on the comparison of the equilibrium and transport properties of singular and nonsingular vortex structures in high-temperature compounds. Using the time-dependent Ginzburg–Landau theory we study the dynamics of vortex structures in superconductors with (d+s)-wave pairing. We calculate the angular dependent correction to the viscosity tensor of a singular flux line, which appears due to the admixture of subdominant s-wave order parameter component in the vortex core. The second order phase transition between singular and nonsingular vortices in high-temperature superconductors was simulated within the time-dependent Ginzburg-Landau equations. The semiclassical analysis of the positions of gap nodes in the vortex core is performed both for singular and nonsingular flux lines.

The experimental investigations of the transport properties of the superconducting Mo/Si multilayered structures are presented. The observed anisotropy of the electrical resistivity of Mo/Si bridges in a weak magnetic fields H ≲ 5 × 10²Oe is in agreement with the data reported earlier. However, for the Mo/Si microbridges with the width 2 μm in a transverse magnetic field H ≳ 5 × 10²Oe we have found out that the temperature dependence of the in-plane resistivity R(T) demonstrates a sharp change of the resistivity at a certain critical temperature T* < T_c, where T_c is the superconducting transition temperature in a zero magnetic field. For a parallel magnetic field this step-like behavior has not been revealed; we neither observed this effect in wider bridges. The comparison of the transport properties and the nonlinear microwave characteristics is presented. A possible origin of this unusual temperature dependence of R(T) is proposed.

We report on a dynamical effect in YBa₂Cu₃O_x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers, in which variations in the magnetic field sweep rate can change, or reverse the sign of, the magnetic moment. The effect occurs in thin films having a low critical current density, either intrinsically or as a result of operation near the critical temperature. We attribute this effect to the large demagnetizing field present in the films, leading to a relaxation in the non-equilibrium vortex distribution, in which current near the edge of the film changes direction. We have investigated this effect in a variety of systems by magnetometery, and provide some magneto-optical evidence of its mechanism.

We analyze the point NS contact conductivity taking into account the depression of superconductivity at high-injection current density and Andreev reflection at the adaptive NS boundary. The dependence of the excess current on the voltage, as well as conductivity of contact at arbitrary voltage is obtained.

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB₂ (Me = Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter λ ≲ 0.1 were estimated. The data are consistent with the measured surface phonon dispersion curves. The low λ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB₂ films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB₂ a two-band model is exploited and the proximity effect in the k-space is suggested.

For MgB₂ where coexist two coupled superconductivity gaps a two-band scheme has been developed. Three interaction channels have been taken into account: a pair-transfer type σ-π-interband repulsion, a σ-intraband effective attraction of electron-phonon nature, and a σ-intraband Coulomb interaction. By means of simultaneous fitting of T_c, specific heat jump and zero-temperature gaps ratio the optimal values of interaction constants have been found. The T_c isotope effect exponent, the temperature dependencies of gaps and the thermodynamic characteristics (specific heat, H_c2) calculated agree with the experimental findings. The theoretical curve of T_c vs chemical potential reproduces qualitatively the influence of doping on superconductivity phase transition in MgB₂.

We report magnetization and transport measurements on MgB₂ in the form of powder, bulk ceramic, wire made by diffusion of Mg into B, and pulsed laser deposited thin films. Ceramic and wire forms show strong intergranular links, and we compare their properties with those of single crystals. The powder shows a magnetic moment versus temperature curve that scales with the moment at the lowest temperature, consistent with a distribution of grain sizes, on the scale of the London penetration depth. The ceramics shows anisotropic magnetization behavior, which is probably a consequence of the anisotropic compressional forces used in its manufacture. In both powder and ceramic, we have observed intriguing negative magnetic moments and steps therein upon changing temperature, well above the obvious superconducting transition. These could indicate small amounts of some higher T_c superconducting phases. However, magnetization loops measured in this regime show ferromagnetism, which we suggest is the origin of the magnetic properties above T_c. The wire shows a linear diamagnetic response up to an H_c1 of 236 Oe, has a normal state resistivity of 2.6 × 10^-6 Ω·cm just above the transition and a resistivity ratio of 21, which is also similar to those of single crystals. The thin films are composed of large crystalline platelets, have a T_c of 35 K, and are diamagnetic.

Magnetic characterisation of an intermetallic bulk superconductor, MgB₂, was carried out by use of odd-order harmonic susceptibility χ_n = χ′_n - iχ″_n (n is odd) at low fields. The simplified Kim model was found to be suitable to account for the main features of the experimental odd-order harmonic susceptibilities. Full penetration field is fitted to for temperature scaling. The experiments are in good agreement with theory.

The study of mechanical properties of crystals under exterior factors gives important information on the structure in the body of a crystal. So, for example, exterior magnetic field ~ 1–10 T brings about change in the plastic properties in dielectric crystals and polymers. The influence of pulsing magnetic field more than 10 ΔH on the change of a microhardness monocrystal C₆₀ is also revealed. In the present work the influence of another type of exterior effect in the form of pulsing laser irradiation upon mechanical parameters is studied. The mechanical parameters, the microhardness and acoustic emission, are studied under microindentation. The study is done on thick films C₆₀ on a quartz substrate; the thickness of the film C₆₀ ~ 3 μm. In order to prevent photoboosted oxidizing, the measuring of a microhardness was performed in darkness with a red light filter on the PMT-3 device. The experiments for the study of the acoustic emission (Å) were also carried out with microindentation C₆₀ and the dependences Å from the energy of laser bundle are received. An explanation is offered of a similar dependence of microhardness bound with the dislocation mechanism of plastic deformation.

The in-plane resistivity, Hall effect (T = 77 K) and SC-transition for some Nd_1.85-Ce_0.15CuO_4+δ single crystal films are investigated at different nonstoichiometric disorder (δ). It is shown that with increasing of δ the Hall-coefficient for the most of the samples differs no more than twice but the normal state resistivity increases by a factor of five. SC-transition temperature decreases with essential broadening of the transition region. The observed evolution from homogeneous metallic (and superconducting) Nd_1.85Ce_0.15CuO_4+δ system to inhomogeneous dielectric one is described as Anderson-type disorder-induced transition in a two-dimensional electron system.

The superconductivity Hamiltonian model with attraction is investigated. An asymptotic relation for average calculation is constructed in the model and the approximating Hamiltonians are presented.