D-wave symmetry

Concerning the manifestation of various gaps of the model, the Ey excitations become attributed to the hot spectrum. The cold spectrum is usually considered as nongapped [28,49], If one takes account of this by multiplying the defect subsystem Aa by a d-wave symmetry factor, the cold spectrum becomes empty. At c < cp the appearance of two pseudogaps is expected. In the basic optimal doping region (c> Cp, c < c0) the spectrum... 

Thus we can infer that experimental low V, T interlayer tunneling data are consistent with Fermi-liquid picture taking into account d-wave symmetry and essentially coherent interlayer tunneling. 

Fig. 10b shows a comparison of our data with the microwave results [18]. The origin of the peak of oab(T) has been widely discussed as a result of a d-wave symmetry of the OP in BSCCO and YBCO. In particular, in a d-wave Fermi-liquid model it was shown that at low temperatures oab grows with temperature as [27] o(m—>0,T) = <70o (1 + fl2), where Ooo is a universal inplane conductivity introduced by Lee [28], Ooo = n e2 /(n mabAo) with mab the effective quasiparticle in-plane mass. 

It should be noted here that the conclusion about s-wave nature of the SC order parameter is consistent with conclusion about s-wave symmetry of the SC order parameter in the bulk and d-wave symmetry at the surface of the sample of the cuprates [17]. It was noted in [17] that most conclusions about d-wave symmetry was obtained in experiments (e.g. ARPES ones) on the cuprates in which mainly surface phenomena have been used. In this sense, the resistive measurements on the cuprates (see, e.g. [4]) are essentially bulk in the nature. In addition, the electron scattering (in resistivity measurements) is sensitive to the spin disorder in the system (magnetic contribution in the electrical resistivity appears, see Sec.l). Moreover, the electron scattering permits probe not only static magnetic order but dynamical (short-lived) ones because of short characteristic times as compared e.g. with usual neutron scattering. 

Despite the previously debated d-wave symmetry of the superconducting gap in YNi2B2C as suggested from the interpretation of specific-heat (Cp) data (Nohara et al., 1997), additional mechanisms for the T3 dependence of the electronic part of Cp and its unusual magnetic-field dependence were discussed, including the shrinking of the vortex core radius with increasing field (Nohara et al., 1999). The influence of disorder is discussed in Section 6.2. Measurements of the microwave... 

There are a variety of theoretical descriptions of the origin of the (tr.tr) resonance, ranging from a superconducting coherence effect which occurs in the firamework of an itinerant-electron picture (Ohashi and Shiba 1993, Monthoux and Scalapino 1994, Lu 1992, Bulut and Scalapino 1996, Lavagna and Stemmann 1994, Onufiieva and Rossat-Mignod 1995, Liu et al. 1995) to a collective mode associated with a multicomponent, SO(5), order parameter (Zhang 1997). In almost all descriptions of the resonance peak it is a direct consequence of the d-wave symmetry of the order parameter. 

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... 

Above all, an order parameter other than s-wave symmetry has been argued for intensively in order to explain the experimental results from, e.g., NMR, penetration depth, speeifie heat, neutron scattering, and phase-sensitive SQUID measurements. Up to now, it seems that the argument has been converging to consider that the symmetry of the order parameter in these materials essentially has d-wave symmetry. 

Differing from the conventional s-wave superconductors, the symmetry of the Cooper pair in the HTSC has been established by now to be of d-wave symmetry. Its angular dependence was determined by ARPES (angle-resolved photoemission spectroscopy) to be by Loeser et al. (1996) and (Ding et al. 1996). Furthermore, they have... 

It was difficult to interpret the latter type of spectrum in an s-wave framework, unless a rare case such as extended s-wave was assumed. In the meantime, many other observations made by different methods such as NMR, specific heat, phase-sensitive SQUID method, ARPES, etc., supported d-wave symmetry, and as a result, tunneling spectra of this kind were analyzed according to d-wave synunetry and are now regarded more positively as one category of evidence to support it. 

Therefore, as far as the symmetry of the Cooper pair is concerned, it is thought that d-wave symmetry has been well established. The differences between experimental results then seem to be attributable to surface- and/or angle-dependent phenomena. 

Figure 30 shows the Josephson critical current measured under external fields along different directions. With the field perpendicular to the twin boundary (j> 90° in the figure), the plot of the critical current exhibited the same Fraunhofer pattern as observed in an ordinary junction, except for a lower peak current. With the field parallel to the boundary = 0), a dip rather than a peak appeared at zero field B = 0), and the maximum current occurred at a field value corresponding to half-integer quantum flux, These observations indicate that the flux cancels the phase difference between the two domains and causes a current flow in the same direction. These observations indicate that Y123 favors d-wave symmetry with some s-wave admixture. 

An alternative view is to consider pairing of itinerant vibronic states. The periodicity of the travelling CDW to which the electrons are coupled would introduce a pairing of electrons of momentum k -I- q and -(k -I- q) to open a gap at the Fermi surface that has d-wave symmetry with a maximum in the ( 71, 0) and (0, 7r) directions. This approach, which has been considered by Seibold and Varlamov [320] is more consistent with the massive evidence now available for strong electron-lattice interactions in the copper oxides, as in the perovskites, at the cross-over from localized to itinerant electronic behavior. 

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