D-wave Pairing Mechanism Research Articles

Comments on the d-wave pairing mechanism for cuprate high T c superconductors: Higher is different?

The question of pairing glue for the cuprate superconductors (SC)is revisited and its determination through the angle resolved photo-emission spectroscopy (ARPES) is discussed in detail. There are two schools of thoughts about the pairing glue question: One argues that superconductivity in the cuprates emerges out of doping the spin singlet resonating valence bond (RVB) state. Since singlet pairs are already formed in the RVB state there is no need for additional boson glue to pair the electrons. The other instead suggests that the d-wave pairs are mediated by the collective bosons like the conventional low $T_c$ SC with the alteration that the phonons are replaced by another kind of bosons ranging from the antiferromagnetic (AF) to loop current fluctuations. An approach to resolve this dispute is to determine the frequency and momentum dependences of the diagonal and off-diagonal self-energies directly from experiments like the McMillan-Rowell procedure for the conventional SC. In that a simple d-wave BCS theory describes superconducting properties of the cuprates well, the Eliashberg analysis of well designed high resolution experimental data will yield the crucial frequency and momentum dependences of the self-energies. This line of approach using ARPES are discussed in more detail in this review, and some remaining problems are commented.

Influence of structural disorder on low-temperature behavior of penetration depth in electron-doped high-TC thin films

To probe the influence of structural disorder on low-temperature behavior of magnetic penetration depth, λ(T), in electron-doped high-TC superconductors, a comparative study of high-quality Pr1.85Ce0.15CuO4 (PCCO) and Sm1.85Ce0.15CuO4 (SCCO) thin films is presented. The λ(T) profiles are extracted from conductance-voltage data using a highly-sensitive home-made mutual-inductance technique. The obtained results confirm a d-wave pairing mechanism in both samples (with nodal gap parameter Δ0/kBTC=2.0 and 2.1 for PCCO and SCCO films, respectively), substantially modified by impurity scattering (which is more noticeable in less homogeneous SCCO films) at the lowest temperatures. More precisely, Δλ(T)=λ(T)−λ(0) is found to follow the Goldenfeld–Hirschfeld interpolation formulae Δλ(T)/λ(0)=AT2/(T+T0) with T0=ln(2)kBΓ1/2Δ01/2 being the crossover temperature which demarcates pure and impure scattering processes (T0/TC=0.13 and 0.26 for PCCO and SCCO films, respectively). The value of the extracted impurity scattering rate Γ correlates with the quality of our samples and is found to be much higher in less homogeneous films with lower TC.

Probing pairing symmetry of Sm 1.85Ce 0.15CuO 4 via highly-sensitive voltage measurements: Evidence for strong impurity scattering

Using a highly-sensitive home-made mutual-inductance technique, temperature profiles of the magnetic penetration depth λ ( T ) in the optimally-doped Sm 1.85Ce 0.15CuO 4 thin films have been extracted. The low-temperature behavior of λ ( T ) is found to be best-fitted by linear Δ λ ( T ) / λ ( 0 ) = ln ( 2 ) k B T / Δ 0 and quadratic Δ λ ( T ) / λ ( 0 ) = Γ − 1 / 2 Δ 0 − 3 / 2 T 2 laws above and below T = 0.22 T C , respectively, which clearly indicates the presence of d-wave pairing mechanism dominated by strong paramagnetic scattering at the lowest temperatures. The best fits produce Δ 0 / k B T C = 2.07 and Γ / T C = 0.25 ( T C / Δ 0 ) 3 for the estimates of the nodal gap parameter and impurity scattering rate.

YBa/sub 2/Cu/sub 3/O/sub X/ submicron Josephson junctions on bicrystal substrates

A technological process based on e-beam lithography and ion beam etching of YBa/sub 2/Cu/sub 3/O/sub X/ (YBCO) film through carbon mask is applied to fabricate a series of YBCO dc SQUIDs on SrTiO/sub 3/ 30/spl deg/-bicrystal substrates. The width of Josephson junctions used in SQUIDs ranges from 0.6 /spl mu/m to 1 /spl mu/m. The character of the temperature dependence of the junctions critical current enables one to confirm the realization of d-wave pairing mechanism as well as the presence of localized states in the grain boundary region. The diffraction pattern of submicron junctions is discussed in relation to their further application in SQUIDs for task of scanning SQUID microscopy in high magnetic fields.

An intralayer pairing mechanism for the coexistence of charge- and spin-density waves induced superconductivity in LaSrCuO

Starting from an effective two-dimensional dynamic interaction that includes screening of holes as carriers by charge density fluctuations and by optical phonons, we investigate the nature of a d-wave pairing mechanism leading to superconductivity in layered La-based cuprates. We consider the La–Sr–CuO system as an ionic solid containing layers of holes as carriers with a single CuO2 layer in a unit cell, where the localized spins form an antiferromagnetic (AF) order. The electron–phonon interaction matrix element in the case of an ordinary unit cell without the local AF order yields s-wave superconductivity. While for the unit cell with AF order, the wave-vector dependence of the intralayer effective interaction potential shows the sign reversal to create d-wave pairing due to localized antiferromagnetic spin order for the screened phonons. Following the strong coupling theory, the superconducting transition temperature, the isotope exponent, coherence length and magnetic penetration depth are also estimated. The implications of the intralayer pairing model and its analysis are discussed.

A novel approach to the polaronic metallic state of cuprate superconductors and the d-wave pairing mechanism

The present novel approach consists of two stages: in the first stage the many-electron states of a CuO6 octahedron or a CuO5 pyramid are calculated by the first-principles variational method, by taking into account the local distortions of a CuO6 octahedron or a CuO5 pyramid. In the second stage a metallic state is constructed in the presence of the local AF ordering constructed by the localized spins. In this metallic state the local distortions are treated in the mean field approximation, so that a “pseudopolaronic effect” is taken into account. Based on this approach, the hole-concentration dependence of Tc and the isotope effect are calculated for LSCO, and compared with experimental results. This approach leads to d-wave pairing mechanism.

Reply to ``Comment on `Paramagnetic Meissner effect in Nb' ''

Rice and Sigrist (RS) have proposed a d-wave mechanism to explain the paramagnetic Meissner effect (PME) observed in some inhomogeneous samples of high-${\mathrm{T}}_{\mathrm{c}}$ superconductors. However, the observation of a PME in some inhomogeneous samples of Nb (an s-wave superconductor) demonstrates that a d-wave pairing mechanism is not needed to produce the effect. Moreover, the remarkable similarity between the PME observed in these Nb and in the high-${\mathrm{T}}_{\mathrm{c}}$ samples argues that the same mechanism is operable in both types of superconductors. In this Reply, new measurements on ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ and on irradiated Nb are discussed, which strongly support the notion that the PME is most likely associated with layered ${\mathrm{T}}_{\mathrm{c}}$ inhomogeneities. Further experiments to test this notion are proposed. We also discuss critically the three experiments proposed by RS to distinguish between two PME mechanisms unrelated to layered ${\mathrm{T}}_{\mathrm{c}}$ inhomogeneities.

Out-of-plane instability and electron-phonon contribution tos- andd-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model

The equilibrium structure, energy bands, phonon dispersions, and s- and d-channel electron-phonon interactions (EPIs) are calculated for the infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA and the linear-response full-potential LMTO method were used. In the equilibrium structure, oxygen is found to buckle slightly out of the plane and, as a result, the characters of the energy bands near EF are found to be similar to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in accord with previous LDA calculations for YBa2Cu3O7. This supports the common belief that the EPI mechanism alone is insufficient to explain HTSC. Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This is surprising and indicates that the EPI could enhance some other d-wave pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute significantly to the EPI, although these contributions are proportional to the static buckling and would vanish for flat planes. These numerical results can be understood from a generic tight-binding model originally derived from the LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the buckling-modes and the influence of the spin-fluctuations should be investigated.

New theory of superconducting copper oxides—from electronic structure to the d-wave pairing mechanism

Firstly, a brief review is given on recent theoretical developments made by our group as to the many-body effect including electronic structure of hole-doped cuprates. Then the mechanism of superconductivity is discussed on the basis of the above-mentioned electronic structure of LSCO. In doing so we present the results of quantitative calculation of the electron phonon coupling constants and the k , k ′ dependence of the spectral function α 2 F ( Ω, k , k ′) made on the basis of the renormalized one-electron-type band. The following important results emerge from this calculation: (1) d-wave pairing occurs even for the phonon mechanism, provided the localized spins form a local antiferromagnetie order: (2) phonon modes in which oxygen ions move vertically for CuO 2 plane contribute to the electron phonon interaction in cuprates while those like breathing modes in which oxygen and copper ions vibrate within a CuO 2 plane do not contribute significantly: (3) the electron phonon interaction in CuO 5 pyramid-type cuprates is stronger than CuO 6 octahedral-type cuprates.