Microwave absorption in high-Tc superconductors studied by the EPR method

Dependences of the microwave absorption on temperature, magnetic field and microwave power obtained for the high-T _c superconductors are presented. Shape of the magnetically modulated microwave absorption, low-field phase diagram and overheating of the Josephson junction system induced by microwave irradiation are discussed. The model of the Josephson junction system interacting with microwaves has been used to explain the behavior of the high-T _c superconductors in low magnetic field.     

Microwave studies of Bi(2212) HTSC single crystals at nitrogen temperatures

Absorption of microwave power by single-phase Bi(2212) crystals with a broad (15 K) phase transition has been studied. A temperature-localized drop of microwave power absorption at the beginning of the superconducting transition at 80 K has been established. A possible mechanism of this absorption feature based on the concept of synchronization of Josephson junctions near T _c is discussed.     

Microwave irradiation on a-axis oriented Y123/Pr123 two-stacked Josephson junctions device

Two-stacked submicron Josephson junctions devices were fabricated in a-axis oriented YBaCu₃O₇ and PrBa₂Cu₃O₇ (Y123/Pr123) multi layered thin films using focused ion beam milling technique. The transition temperature and critical current density (J_c) of the device are about 83 K and 5 × 10⁵ A/cm² at 20 K, respectively. The device was irradiated with external microwave of 10 GHz and studied at 20 K. The microwave induced voltage steps are observed in I–V characteristics. The supercurrent branch become resistive above a certain microwave power and also the J_c was suppressed as we increased the microwave power. Magnetic field modulation of critical current shows periodicity of about 2000 gauss correspond to the Josephson junctions in the stack.     

Dissipative flow of Josephson and Abrikosov fluxons in high Tc superconductors

A model is presented for the field dependent microwave absorption in high T_c superconductors; fluxon motion in Josephson juctions and in bulk material causes the absorption. Junction dissipation is demonstrated by microwave induced dc voltage across YBaCuO samples which has a magnetic field dependence.     

Anomalous dependence of the critical current of 45° grain boundaries in YBa2Cu3O7−x on an applied magnetic field

Grain boundaries grown by the biepitaxial technique do not show Fraunhofer-type critical current I _c vs. magnetic field H _a dependences, which are a hallmark of standard Josephson junctions. To clarify the reason for this unusual behavior, we have fabricated asymmetric 45° grain boundaries using the bicrystal technique and analyzed their I _c (H _a) characteristics. These characteristics crisply show a number of remarkable features, which suggest that due to the particular orientation of these junctions, their I _c (H _a) dependence is intrinsically non-Fraunhofer. Conventional models, relying on standard tunneling and a superconducting order parameter with s-wave symmetry, do not account for the I _c (H _a) characteristics observed.     

Effect of deoxygenation on the weak-link behavior of YBa2Cu3O7−δ superconductors

A systematic study of the weak-link behavior for YBa₂Cu₃O_7−δ polycrystalline samples has been done using the electrical resistivity and AC susceptibility techniques. The experiments were performed with two samples of similar grain, a sample of well-coupled grains, and a deoxygenated sample in such a way that the oxygen mostly comes from the intergrain region. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's critical state model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor normal–metal superconductor (SNS) for well-coupled samples to superconductor insulator normal–metal–superconductor (SINS) type of junctions for the deoxygenated sample. These results are interpreted in terms of oxygen depletion from grain boundaries, which in turn decreases the intergranular Josephson coupling energy with a concomitant decrease of pinning of the intergranular vortices.     

Microwave absorption in 2HNbSe2 and critical fluctuations above Tc

Large three-dimensional critical fluctuations above T_c have been observed in 2HNbSe₂ compounds by microwave absorption.     

Investigation of Josephson effect in weakly-coupled high-temperature superconductors using MW scanner

The possibility of using a differential technique for high-temperature (Hi-T _c) superconductors is shown. The critical current and step-by-step detected current dependence on the sample incident microwave power in powderYBa ₂ Cu ₃ O ₇ is observed. The obtained result suggest nonstationary effect Josephson takes place.     

Critical current of an inhomogeneous Josephson junction at an intergrain boundary with a random distribution of dislocations

The critical current J _c(θ) of an intergrain boundary is calculated as a function of the contact misorientation angle θ of the granules. It is assumed that the ordering parameter is suppressed in regions near boundaries with an enhanced mechanical stress induced by randomly distributed surface dislocations. The stress distribution function is determined using a probabilistic approach. Assuming that the weak coupling at the boundary is Josephson coupling, an analytic expression is found for the angular dependence J _c(θ) (for tilt and twist boundaries). The magnitude of the residual critical current of a boundary in a strong magnetic field is estimated. Fiz. Tverd. Tela (St. Petersburg) 40, 393–402 (March 1998)     

Critical currents in Josephson junctions with microinhomogeneities attracting solitons

The dependence of the critical zero-voltage current (γ_c) on the external magnetic field h in one-dimensional Josephson junctions with local attractive inhomogeneities is studied numerically and analytically. A main effect for the overlap geometry consists in forming near h=0 a cross-shaped structure in γ_c(h), due to the soliton and antisoliton localization on the inhomogeneity. Together with the state existing in homogeneous junctions they form a collection of three states (tristability) in a fairly large interval of h.     

Mechanisms of direct microwave absorption in Y1Ba2Cu3O7−x

Direct microwave absorption was studied as a function of magnetic field up to 8 kG and in a temperature range between 10 K and 150 K in Y₁Ba₂Cu₃O_7−x. Large microwave absorption was observed below T_c over a wide magnetic field range. The microwave power absorption is approximately described by I=A(T) H+C(T), where A(T) and C(T) are temperature dependent constants. A(T) is proportional to the square root of the microwave power, while C(T) shows a different behaviour. We propose different origins for A(T) and C(T), namely fluxons in grains and weak Josephson links. Moreover, also flux trapping was observed in the superconducting state. Possible mechanisms for the absorption are discussed.     

Effective decrease in the exchange energy in <Emphasis Type="Italic">S</Emphasis>-(<Emphasis Type="Italic">FN</Emphasis>)-<Emphasis Type="Italic">S</Emphasis> Josephson structures

The critical current I _c of S-(FN)-S Josephson structures has been calculated as a function of the distance L between superconducting (S) electrodes using the Usadel quasiclassical equations for the case of specifying the supercurrent in the direction parallel to the interface between the ferromagnetic (F) and normal (N) films of the composite weak-link region. It has been shown that, owing to the interaction between F and N films, both the typical decrease scale I _c(L) and the period of the critical current oscillations can be much larger than the respective quantities for the SFS junctions. The conditions have been determined under which these lengths are on the order of the effective depth ζ_N of superconductivity penetration to a normal metal.     

Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics $E(j)_{H_{treat} } = const$ of ceramic (granular) YBa₂Cu₃O_6.95 samples preliminarily magnetized in different transverse magnetic fields H _treat have been measured in a zero field (H _ext = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 ≤ H _trap ≤ H _c2J (where H _trap is the magnetic field trapped as a result of the pretreatment in H _treat and H _c2J is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance ρ_A(H _treat) _{j = const} of superconducting grains. It has been established that the magnetoresistance ρ_A of the superconducting grains is significantly lower than the magnetoresistance ρ_J for the Josephson medium. The dependence of ρ_A on H _treat and on the transport current density j has been investigated. It has been shown that the dependences ρ_A(H _treat) _{j = const} exhibit a clearly pronounced tendency to saturation, ρ_satur, and the value of ρ_satur increases with increasing j. It has been found that the lower critical field H _c1A of the superconducting grains strongly depends on the transport current density.     

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa₂Cu₃O_6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H _ext with a strength of up to H _ext ≈ 500 Oe for low transport current densities (0.1 A/cm² ≤ j ≤ 0.6 A/cm²). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j) _Hext=const) and H _ext(ρ(H _ext) _{j = const}). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H _ext ≥ H _c1g , where H _c1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H _{ext = const} and ρ(H _ext) _{j = const} has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H _c1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field H _c2J .     

Phase transitions in hybrid SFS structures with thin superconducting layers

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature T_c of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers d_s is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature T_c at the reduction of the thickness d_s is detected and is attributed to the 0-π transition.     

Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa2Cu3O7 − δ high-temperature superconductor

The temperature dependences of the electrical resistivity ρ(T) _{H = const} have been measured in external magnetic fields H _ext (0 ≤ H _ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa₂Cu₃O_{7 ? δ} high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T _min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) _{H = 0} for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H _ext) _{T = const}. A comparative analysis of the specific features in the behavior of the curves ρ(H) _{T = const} for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H _c1g , H _c2g ) and Josephson weak links (H _c2J ) have been determined, and the H-T phase diagrams of granular YBa₂Cu₃O_{7 ? δ} HTSCs have been recovered.     

Particle size effect of a.c. superconducting properties in compound

Using a new starting material of Ba₂Cu₃O₅ and a three step heat treatment, single phase Tl₂Ca₁Ba₂Cu₂O₈ high- superconducting samples have been prepared, possessing the onset- and critical temperatures K and K. The morphology dependent value of is 17 Oe, 8 Oe and 5 Oe at 77 K in the case of bulk, crushed and powdered materials, respectively. The a.c. susceptibility, r.f. susceptibility and microwave absorption properties show a significant dependence on the particle size with a sharp change in the interval between 750 μm and 1200 μm. These experiments provide characteristic parameters for intergrain material (treated as 3D Josephson network) as mm, Oe and A/cm² at 77 K. The data are controlled by modulated microwave absorption measurements. The results obtained can be explained well both by the finite size junction model and cavity mode absorption model. The Josephson network is determined unambiguously by metallic S-N-S weak links. Received 10 October 1998     

Magnetoresistance hysteresis in granular HTSCs as a manifestation of the magnetic flux trapped by superconducting grains in YBCO + CuO composites

Hysterestic behavior of the magnetoresistance of granular HTSCs and its interaction with the magnetic hysteresis are studied by measuring magnetoresistance R(H) and critical current I _c(H) of composites formed by HTSC Y_0.75Lu_0.25Ba₂Cu₃O₇ and CuO. A network of Josephson junctions is formed in such composites, in which the nonsuperconducting component plays the role of barriers between HTSC grains. Hysteretic dependences R(H) of magnetoresistance are studied in a wide range of transport current density j and are analyzed in the framework of the two-level model of a granular superconductor, in which dissipation takes place in the Josephson medium and the magnetic flux can be pinned both in grains and in the Josephson medium. The interrelation between the hysteresis of critical current I _c(H) and the evolution of the hysterestic dependence R(H) of the magnetoresistance upon transport current variation is demonstrated experimentally. The effect of the magnetic past history on the hysteretic behavior of R(H) and the emergence of a segment with a negative magnetoresistance are analyzed. It is shown for the first time that the R(H) dependences are characterized by a parameter that is independent of the transport current, viz., the width of the R(H) hysteresis loop.     

Application of MMMA method: Dispersed superconductors and magnetic nanolayers

Microwave nonresonant dissipation observed around zero field makes a powerful method in characterizing superconducting state (to determineT _C,H _C1,H _CJ parameters). This review shows several successful applications of magnetically modulated microwave absorption (MMMA). MMMA enables to study MgB₂ superconducting nanoregions embedded in the Mg host structure. Application of MMMA to study intercalation of fullerence enabled the observation of a new superconducting phase. The pressure effect dT _C/dp has been studied below the percolation threshold in YBaCuO/PST composites. The local temperature of the Josephson junction measured by the MMMA technique showed that a granular superconductor consists of two phases: host phase and Josephson junction system. MMMA has been applied lately to separate the giant magnetoresistance from the magnetization effect in differential hysteresis loops.     

Tunable supercurrent in superconductor/normal metal/superconductor Josephson junctions

When two superconductors are connected by a weak link a supercurrent flows determined by the difference in the macroscopic quantum phases of the superconductors. Originally, this phenomenon was discovered by Josephson for the case of a weak link formed by a thin tunnel barrier. The supercurrent I is related to the phase difference ϕ through the Josephson current–phase relation, I = I_csin ϕ, with I_c, the critical current, depending on the properties of the weak link. A similar relation holds for weak links consisting of a normal metal, a semiconductor or a constriction . In all cases, the phase differenceϕ =  0 when no supercurrent flows through the junction, and ϕ increases monotonically with increasing supercurrent until the critical current is reached. Using nanolithography techniques we have succeeded in making and studying a Josephson junction with a normal metal weak link, in which we have direct access to the microscopic current-carrying states inside the link. We find that the fundamental Josephson relation can be changed fromI = I_csin ϕ toI = I_csin(ϕ + π), i.e. to a π -junction, by suitably controlling the energy distribution of the current-carrying states in the normal metal. This fundamental change in the way these Josephson junctions behave has potential implications for their use in superconducting electronics as well as (quantum) logic circuits based on superconductors.