Order Parameter In High-temperature Superconductors Research Articles

Spontaneous currents in Josephson junctions between unconventional superconductors and d-wave qubits (Review)

The modern physics of superconductivity can be called the physics of unconventional superconductivity. The discovery of the d-wave symmetry of the order parameter in high-temperature superconductors and the triplet superconductivity in compound Sr2RuO4 has caused a huge stream of theoretical and experimental investigations of unconventional superconductors. In this review we discuss some novel aspects of the Josephson effect which are related to the symmetry of the order parameter. The most intriguing of them is spontaneous current generation in an unconventional weak link. The example of a Josephson junction in the form of a grain boundary between two disorientated d-wave or f-wave superconductors is considered in detail. Josephson current–phase relations and the phase dependences of the spontaneous current that flows along the interface are analyzed. The spontaneous current and spontaneous phase difference are manifestations of the time-reversal symmetry (𝒯 ) breaking states in the system. We analyzed the region of appearance of 𝒯-breaking states as function of temperature and mismatch angle. A review of the basics of superconducting qubits with emphasis on specific properties of d-wave qubits is given. Recent results in the problem of decoherence in d-wave qubits, which is the major concern for any qubit realization, are presented.

Collective properties of superconductors with nontrivial pairing

Three-and two-dimensional models of p-and d-pairing are constructed for superconductors and superfluid quantum liquids using the functional integration formalism. In these models, the collective excitation spectra are calculated for superconductors with nontrivial pairing (such as high-temperature superconductors (HTSC) and heavy-fermion superconductors (HFSC)) for p-and d-pairing. Both three-and two-dimensional systems are considered. Some of recent ideas concerning the realization of the mixture of different states in HTSC are considered. In particular, the mixture of states \(d_{x^2 - y^2 } + id_{xy} \) is analyzed. The obtained results of calculations of collective excitation spectra in superconductors with nontrivial pairing may be used for determining the type of pairing and the order parameter in HTSC and HFSC and also for interpreting the experimental results on ultrasound and microwave absorption in these system.

Temperature-dependent transport characteristics of quasiballistic normal-metal–superconductor junctions

We study the temperature effect on the zero-bias conductance and current fluctuations in a two-terminal heterostructure with a mesoscopic normal region connected to normal and superconducting reservoirs. For a quasiballistic regime, the shape of the conductance-voltage characteristic and its temperature behavior for an s-wave superconductor strongly depends on the transmission properties of the normal interlayer. On the contrary, in the low-frequency limit, the near-zero-voltage current noise power versus temperature is insensitive to the nature of the transition region but dramatically changes for different pairing states of the superconducting electrode. The relevant experiment could serve as a diagnostic tool to identify the symmetry of the superconducting order parameter in high-temperature superconductors.

Anisotropic impurity scattering effects onTcandHc2inYBa2Cu3Ox

of the samples. Furthermore, Tc decreases slower in irradiated YBa2Cu3Oy than in YBa2(Cu12xZnx) 3Oy . This may be well explained by the model that the scattering due to in-plane oxygen defects is more anisotropic than that due to Zn impurities. The different behavior of the reduced slopes ( dHc2 /dT)T c /(dHc2 /dT)T c0 in these two types of samples can also be understood in this context. @S0163-1829~99!08309-5# The symmetry of the order parameter in high-temperature superconductors ~HTSC’s ! has been actively debated. While there are now substantial evidences supporting the anisotropic pairing symmetry with line nodes and sign changes, which is consistent with d-wave symmetry, 1,2 this scenario still faces challenges for quantitative reconciliation between certain experiments and the theoretical models. One of these problems is Tc suppression rate due to nonmagnetic impurities in HTSC’s. In d-wave superconductors, nonmagnetic impurity scattering may lead to pair breaking and extreme Tc suppression. 3‐6 Furthermore, assuming that the change of the carrier concentration caused by impurities is negligible, most d-wave pair breaking models 3‐6 lead to an approximately universal dependence of Tc suppression on the residual resistivity r 0 independent of the types of impurities and the details of the pairing mechanism. However, the observed T c suppression rates dTc /dr0 due to different types of impurities or defects are not universal, 7‐10 though they all qualitatively follow the predicted Abrikosov-Gorkov function. 11 Especially, the comprehensive studies of Tc suppression due to in-plane oxygen defects in YBa2Cu3Oy ~YBCO! lead to a Tc suppression rate which is at least twice slower than the simple d-wave pair breaking model predicts. 7,8,12,13 Several

TO PROBE THE PHASE OF THE ORDER PARAMETER IN HIGH-TEMPERATURE SUPERCONDUCTORS BY USING r.f. SQUID

Based on the theory of r.f. SQUID,the principle for a phase-sensitive test of the symmetry of the order parameter in high-temperature superconductors by using r.f. SQUID in the inductive mode(β≤1,without the limit of β1)is given.We also investigate (calculating and discussing) a system with β=0.7 and offer the range of its r.f. bias flux,under which the r.f. SQUID can be used to probe the phase of the pairing state.

On competition between s- and d-symmetry types of the order parameter in high-temperature superconductors

It is shown that strong anisotropy of the quasi-two-dimensional electron spectrum in high-temperature superconductors and the competition between attraction and repulsion in the electron–electron interaction can lead either to anisotropic s-, or d-type of Cooper pairing depending on the system parameters. In the case when attraction prevails on the entire Fermi surface (e.g., due to electron–phonon interaction), the sxy- or s*-symmetry in the superconducting order parameter is advantageous from the energy point of view. At the same time, the dx2−y2-symmetry of the gap is observed in the case of repulsion in the entire Brillouin zone (which is typical, for example, of the electron–magnon interaction) as well as in the case of effective attraction on the regions of Fermi surface with the maximum density of states. In the latter case, the superconducting transition temperature Tc is always higher than in the former case, indicating the important role of additional attraction in the mechanism of high-temperature superconductivity.

Change of parity of the order parameter in high-temperature superconductors due to impurities.

The dependence of the critical temperature on impurity concentration is calculated for a model of a tetragonal high-${T}_{c}$ superconductor described in a previous paper [A. A. Abrikosov, Phys. Rev. B 52, R15 738 (1995)] under conditions when the odd ($d$ symmetry) order parameter is favorable for a clean substance. The critical temperature decreases rapidly with increasing impurity concentration according to the general pair-braking formula and tends to zero at a critical concentration. Since, however, the critical temperature for the phase with an even order parameter is only weakly influenced by impurities, it eventually exceeds the ${T}_{c}$ of the odd phase, and this means a phase transition to the even phase. The ${T}_{c}$ curve is expected to have a kink and a long tail.

IcRn product in c-axis Josephson junctions involving high-temperature superconductors.

The Josephson coupling between a conventional and a high-temperature superconductor is investigated theoretically for the case in which the high-${\mathit{T}}_{\mathit{c}}$ superconductor with its c axis perpendicular to the junction has a layer with a significantly depleted order parameter. If that layer is in contact with the junction barrier, it is demonstrated that the product of the junction resistance ${\mathit{R}}_{\mathit{n}}$ and the critical current ${\mathit{I}}_{\mathit{c}}$ can be substantially reduced from the standard Ambegaokar-Baratoff value. The result is discussed in connection with recent experiments aimed at exploring the symmetry of the order parameter in high-temperature superconductors.

Dependence of the Josephson coupling of unconventional superconductors on the properties of the tunneling barrier.

The Josephson coupling between a conventional and an unconventional superconductor is investigated as a function of the properties of the tunneling barrier. A simple model is adopted for the tunneling probability and it is shown that its variation dramatically affects the ${\mathit{I}}_{\mathit{c}}$${\mathit{R}}_{\mathit{n}}$ product of an s-d, as opposed to an s-s junction. Based on these conclusions, experiments are proposed to probe the symmetry of the order parameter in high-temperature superconductors.

Probing spatial correlations with nanoscale two-contact tunneling.

Interference effects on the transport through two localized tunnel junctions on the surface of a well-grounded sample reveal intrinsic spatial correlations characteristic of the uncoupled sample. Differential conductances of the two-junction probe are related to the spatial correlations of both normal and superconducting samples. For a superconducting sample the gap anisotropy strongly affects the results. This may serve as a sensitive probe of the order parameter in high-temperature superconductors.

Persistent supercurrents in composite rings of high-Tc and conventional superconductors

Abstract We have measured substantial supercurrents in rings formed by segments of epitaxial Y 1 Ba 2 Cu 3 O 7 and lead thin films. The supercurrent density across the planar interface of the conventional and high- T c superconductor has been found to be larger than 600 A/cm 2 at 4.2 K. Our experimental observations can place powerful constraints on the up to now unknown nature of the superconducting order parameter in high-temperature superconductors.