D-wave superconductors

It was shown then that all these observed features can be described self-consistently by Fermi-liquid model for quasiparticles in clean d-wave superconductor with resonant intralayer scattering [14]. The superconducting gap is expressed as A([Pg.185]

In this paper, we concentrate on the /j+SR measurements and determine if YBa2Cu307 is a bulk. v-wave (nodeless) superconductor, as determined in Refs. 1-3, or a d-wave superconductor, whose order parameter A(k), changes sign as a function of k, as claimed in Ref. 8. In making this determination, we show that the features observed in the single-crystal data of Ref. 8 are actually due to temperature-activated fluxon de-pinning, an effect which is not readily observable in strongly pinned systems such as the early powder samples or the early heavily-twinned crystals. 

EFFECTS OF DISORDER WITH FINITE RANGE ON THE PROPERTIES OF D-WAVE SUPERCONDUCTORS... 

Here, we shall assume that we are dealing with d-wave superconductors. Since for unconventional superconductors there is no qualitative difference between these two types of scattering, we shall confine ourselves to the study of potential scattering. Even with this limitation there is a wide range of theoretical predictions as regards rc-suppression, density of states, transport properties etc, depending on the way disorder is modelled and depending on the analytical and numerical approximations employed to derive experimentally verifiable conclusions. [Atkinson et al., 2000]... 

Keywords Quantum bit, Josephson junction, d-wave superconductors. 

If a conventional superconductor (S) described by a s-wave order parameter symmetry (OPS) is put together with a non conventional superconductor (D), described by a pure d-wave OPS, to form two junctions in a superconducting loop, as indicated in Fig. 4, a self 7r — frustrated loop is achieved [van Harlingen 1995], Indeed, one of the two SD junctions behaves as a conventional "0" junction, since the Josephson coupling is between the positive lobe of the d-wave superconductor (white color in Fig. 4) and the S electrode on the contrary, the other junction is a V junction, because the coupling is now between the S electrode and the negative lobe. As a consequence, a shift of 7r along the loop is achieved and the device is self-frustrated by a half flux quantum. 

Figure 4. SD 7r-loop obtained by a s-wave and a d-wave superconductor. The two junctions composing the loop are indicated by black rectangles. The positive lobe of the d-wave is white.

Fig. 16.48 This residual density of states was pointed out theoretically to appear in the unitarity limit scattering by non-magnetic impurities in p- or d-wave superconductors in a heavy fermion study.49 From this result it became possible to explain the BCS-like temperature dependence of the penetration depth, A,50 which supported strongly the. 9-wave pairing model in high-7 , superconductors at an early stage, in terms of the d-wave + impurity model.51 53...

The residual absorption found in the vast majority of cuprates is not consistent with s-wave symmetry of the superconducting order parameter. Since the DOS inside the gap region of an s-wave superconductor is exactly zero at T = 0, the dissipative part of the conductivity must vanish for a) < 2A, in clear contrast with the experimental data. There have been several attempts to explain the residual absorption in the superconducting state assuming d-wave symmetry. In a d-wave superconductor, the DOS is finite at all finite... 

Carbotte et al. (1995) analyzed the complex conductivity in the presence of disorder in the weak Bom-scattering limit assuming different symmetries of the order parameter. Their main result is that the data presented in fig. 20 are not consistent with an s-wave gap but can be qualitatively accounted for within a d-wave model. In particular, disordered d-wave superconductors are expected to reveal an enhancement of the spectral weight of the normal component in the superconducting state response. The frequency dependence of the conductivity at T and its evolution with disorder are remarkably similar in the experimental data by Basov et al. (1994b) and in theoretical calculations by Jiang et al. (1996). 

It is pointed out that the current-voltage characteristics below the gap are consistent with tunneling into d-wave superconductors. The tunneling quasiparticle current I V) is calculated using the expression... 

On the other hand, the substitution of Zn (content 1%) showed Tc 79K, and the gap value 2App w 50 meV was almost the same value as that of the pure sample. However, the conductance at zero bias was increased. This increase, estimated to a residual DOS of 0.4, was in good agreement with the results of NMR experiments (Ishida et al. 1993). Their results were compared with theoretical results of impurity effects in d-wave superconductors in the unitary limit (Balatsky and Salkola 1996, Salkola et al. 1996), though they did not claim that the zero-bias anomaly observed in Zn-substituted Bi2212 is explained by their prediction. These variations in the substituent atoms seem to indicate that Co, Zn and Ni affect the electronic states in different ways. 

Tanuma et al. (1998) calculated the tunneling density of states on the uneven surface of d-wave superconductors and showed that their calculation reproduced various types of anomalous features observed in the actual tunneling experiments, such as ZBCP, double-peak and multiple-dip structures, and a suppressed superconducting gap. They claimed that the wide variety of experimental data showing these features are namral for anisotropic superconductors, and hence, should not be rejected as unidentified spectra observed on degraded surface or in bad junctions (Kashiwaya et al. 1994a). 

Wollman et al. (1993, 1995) reported detailed measurements of the magnetic field dependence of the Josephson critical current for Y123/Au/Pb junctions formed on Y123 crystals. The observed results of the dependences for the edge junction and the corner junction are as shown in fig. 28. Double peak stmetures were seen for the comer junction, as expected from equation (5), indicating that Y123 is a d-wave superconductor. 

The mechanism responsible for the formation of Cooper pairs in the superconductive state remains unsolved. Extensive spin-polarized inelastic neutron-scattering experiments have revealed a 41 meV resonance in the spin-excitation spectrum of the superconductive copper oxides that has caught theoretical attention [317]. Carbotte et al. [318] have noted that if these spin excitations are strongly coupled to the charge carriers, they should also be seen as a peak in the optical conductivity. They therefore calculated a((o) for a d-wave superconductor with inelastic scattering from the neutron data. Comparison with a-axis optical-conductivity data [319] showed that the... 

Recently, much attention has been paid to the so-called anisotropic gap state superconductor. At T = 0, Cs(0)/yT<- exhibits (T/T ) temperature dependence in the case of gap function with point node, while (T/T<-) temperature dependence is observed in the case of gap function with line node. As low-energy excitation is possible, one can observe the power law T dependence at the low temperature in specific heat for p and d wave superconductors. The measurements of specific heat give extremely fruitful information about the superconducting gap complementary with other measurements, such as NMR. Even if a superconductor is not bulk, zero resistance may be observed when there is a continuous superconducting current path inside the sample. By using specific heat measurements, it is possible to determine whether superconducting behavior occurs in the bulk or not. We note that it is extremely important to check the bulk nature of pressure-induced SC by specific heat measurements imder pressure. 

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