Id Xy Research Articles

Anomaly in the Complex Conductivity of Overdoped Y1−x Ca x Ba2Cu3O7−δ Thin Films from THz Spectroscopy

We measured the complex conductivity of Ca-doped YBCO thin films in the THz frequency range. The films were measured using both Time domain and Frequency domain methods for THz spectroscopy. We show that a subgap exists in the overdoped samples of 5% and 10% Ca doping. The subgap appears as a sharp decrease in the real part of conductivity at frequencies equivalent to gap energy of 1 meV and is more prominent with increased doping. We suggest that this decrease in conductivity is related to a \(d_{x^{2}-y^{2}}\)-wave pairing symmetry with an imaginary part of is or id xy . The imaginary part of the conductivity shows the well-known 1/ω behavior, but its ωσ 2 product shows a dip in the spectrum at about ∼1 meV.

Ultrasound attenuation and collective modes in mixed d x2-y2 + id xy state of unconventional superconductors

We discuss two methods used for the study of unconventional superconductors: ultrasound attenuation and collective modes. These two methods, as well as microwave absorption, turn out to be coupled and have become very important now. Within models built by path integration technique we analyze some recent ideas concerning possible realization of the mixture of different d-wave states in high temperature superconductors (HTSC).We specifically consider the mixture of d x2-y2 and d xy states. We study the collective mode spectrum and the ultrasound attenuation in the mixed state, and show that each of the two methods allows us to distinguish a pure d-wave state from a mixed one. They also allow us to identify the type of pairing and order parameter in unconventional superconductors, including the presence and topology of gap nodes, the magnitude of the gap, and degree of admixture in the mixed state.

MAGNETIC FIELD AND IMPURITY EFFECTS IN PSEUDOGAP STATE OF CUPRATES

Both impurities and magnetic field can be used as a tool to investigate the properties of superconducting state of cuprates. Recent STM tunneling experiments on impurity states in superconducting state provide a new set of results that can be compared with the expectations for d-wave pairing in cuprates. We will argue that impurities and magnetic field could be used to test the nature of the pseudogap state (PG). i) We will present the extension of the theory of impurity on the pseudogap regime. The impurity such as Zn or Ni, will generate resonance state in PG state on very general grounds. Single vs double impurity peak will appear depending on the superconducting vs non-superconducting origin of the PG. ii) For the particular proposal of the d- density wave PG (DDW) we found that out of plane magnetic field will induce complex, proportional to the field, id xy -wave density wave component. Physical properties of this, previously not considered, state will be discussed.

Tunneling conductance and broken time reversal symmetry state near the interface of a normal metal/d-wave superconductor

We discuss tunneling conductance spectra in normal metal/d-wave superconductor junctions in the presence of broken time reversal symmetry state (BTRSS) at the interface of d-wave superconductor based on the quasiclassical Green's function formalism. Our calculation is performed for two types of BTRSS; d+is-wave and d+id xy -wave state. We show that the amplitudes of the subdominant components are quite sensitive to the transparency of the junctions, and they are enhanced for low transmissive junctions. The zero-bias conductance peak splits into two peaks only at junctions with small transmissivity.

Field-induced d x2− y2+ id xy state and marginal stability of high- Tc superconductor

It is shown that the complex d xy component is generated in d-wave superconductor in the magnetic field. As one enters superconducting state at finite field, the normal to superconducting transition occurs into bulk d x 2− y 2 + id xy (d+ id′) state. The driving force for the transition is the linear coupling between magnetic field and non-zero magnetization of the d+ id′ condensate. The external magnetic field violates parity and time reversal symmetries and the nodal quasiparticle states respond by generating the id xy component of the order parameter, with the magnitude estimated to be on the order of a few Kelvins. Parity ( P) and time reversal ( T) symmetries are violated in this state.

A method of distinction of a mixed d x2− y2+id xy state of HTSC from the pure d-wave state

Recent experiments and theoretical considerations show that in HTSC the mixture of different d-wave states is realized. We calculate for the first time the collective mode spectrum in a mixed state d x 2− y 2 +id xy of HTSC. We use the model of d-pairing for superconductive and superfluid fermi-systems (HTSC, HFSC, etc., developed by P.N. Brusov and N.P. Brusova within path integration technique earlier. There are 10 collective modes in considered state: five of them are high frequency and five others seem to be Goldstone-like ones. In spite of the fact that spectra in both states d x 2− y 2 and d xy are identical the spectrum in mixture d x 2− y 2 +id xy state turns out to be quite different from them. Thus the probe of the spectrum in ultrasound and/or microwave absorption experiments could be used to distinguish the mixture — of two d-wave states from pure d-wave state.

Phase transition from a dx2− y2 to dx2− y2+ id xy superconductor

The temperature dependencies of specific heat and spin susceptibility of a coupled d x 2− y 2 + id xy superconductor in the presence of a weak d xy component are investigated in the tight-binding model (1) on square lattice and (2) on a lattice with orthorhombic distortion. As the temperature is lowered past the critical temperature T c, first a less ordered d x 2− y 2 superconductor is created, which changes to a more ordered d x 2− y 2 + id xy superconductor at T c1(< T c). This manifests in two second order phase transitions identified by two jumps in specific heat at T c and T c 1. The temperature dependencies of the superconducting observables exhibit a change from power-law to exponential behavior as temperature is lowered below T c 1 and confirm the new phase transition.

MAGNETIC-FIELD-INDUCED id xy ORDER IN A dx2− y2 SUPERCONDUCTOR

The interaction between planar quasiparticles in a d x 2− y 2 superconductor and quantized vortices associated with a magnetic field perpendicular to the plane is shown to induce a pair potential with d xy symmetry, out of phase with the d x 2− y 2 order. A microscopic calculation of a process involving quasiparticle scattering by the supercurrent around a vortex and Andreev reflection from its core is presented. Other processes that also lead to an id xy pair potential are discussed. It is argued that such a fully gapped state may be the high-field low-temperature phase observed by Krishana and Ong et al. in magnetothermal conductivity measurements of superconducting single-crystal Bi-2212.