Wave Gap Symmetry Research Articles

D-wave Pair Symmetry in the Superconductivity of κ-(BEDT-TTF)2X

The superconducting phase of organic superconductors κ-(BEDT-TTF) 2 X was studied by the electron tunneling using scanning tunneling microscope (STM). Tunneling both at the conducting plane and lateral surfaces was performed on single crystals of κ-(BEDT-TTF) 2 Cu(NCS) 2 . Tunneling conductance curve obtained at the conducting plane is explained by the d -wave gap symmetry. The temperature dependence of tunneling spectra is also consistent with the d -wave symmetry. Tunneling spectra at lateral surfaces varies depending on the tunneling direction, suggesting the in-plane anisotropy of the gap. The directional dependence is also consistent with the d -wave symmetry with line nodes π/4 from the k b - and k c -axes. It indicates that the superconducting pair wave function has the d x 2 - y 2 -wave symmetry. (If we take the same conventions for the magnetic Brillouin zone as high- T c cuprates with the d x 2 - y 2 -wave symmetry, the gap symmetry of κ-(BEDT-TTF) 2 Cu(NCS) 2 is the d x y -wave symmetry.) Proto...

Antiferromagnetic exchange and spin-fluctuation pairing in cuprate superconductors

Abstract A microscopic theory of superconductivity is formulated within an effective p - d Hubbard model for a CuO 2 plane. By applying the Mori-type projection technique, the Dyson equation is derived for the Green functions in terms of Hubbard operators. The antiferromagnetic exchange caused by interband hopping results in pairing of all carries in the conduction subband and high T c proportional to the Fermi energy. Kinematic interaction in intraband hopping is responsible for the conventional spin-fluctuation pairing. Numerical solution of the gap equation proves the d -wave gap symmetry and defines T c doping dependence. Oxygen isotope shift and pressure dependence of T c are also discussed.

Magnetic relaxation measurement of the infinite-layer superconductor Sr 0.9La 0.1CuO 2

The relaxation of the irreversible magnetization in the superconducting state for the grain-aligned electron-doped infinite-layer superconductor Sr 0.9La 0.1CuO 2 was measured. The normalized flux creep rate, S( T), and the current-dependent activation barrier, U( j), were obtained. Based on S( T) and U( j), three different magnetic relaxation behaviors were observed in the superconducting state, and this is quite different from what is seen in the highly anisotropic cuprate superconductors. Even compared to the absolute value of S( T) for the most isotropic superconductor YBa 2Cu 3O 7− δ among the cuprate superconductors, the absolute value of S( T) for Sr 0.9La 0.1CuO 2 is quite small. Along with the recent observation of a striking s-wave gap symmetry and the absence of any significant spin fluctuation in this electron-doped cuprate, which is quite different from the behavior seen in hole-doped cuprates, the magnetic properties of Sr 0.9La 0.1CuO 2 are not very similar to those of the hole-doped cuprates. U( j) has a H − α form with α≈0.2. This value is quite different from the predictions of the Yeshurun and Malozemoff model and the Tinkham model where α=1.

Doping dependence of the superconducting gap by Andreev reflection in Au/La 2− xSr xCuO 4 point-contact junctions

We studied the doping dependence of the superconducting gap in La 2− x Sr x CuO 4 (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at T c A close to the bulk T c. The fit of the conductance curves with the BTK–Tanaka–Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δ s follows very well the T c vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T ∗. This confirms the different origin of these two energy scales at T< T c.

Temperature and junction-type dependency of Andreev reflection in MgB 2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal–MgB 2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of MgB 2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2 Δ/ k B T c=3.3. In out-of-plane, high-pressure point-contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Δ=2.6±0.2 meV (clean limit).

Fermi surface and gap parameter in high- Tc superconductors: the Stripe Quantum Critical Point scenario

We study the single-particle spectral properties of electrons coupled to quasicritical charge and spin fluctuations close to a stripe-phase, which is governed by a Quantum Critical Point near optimum doping. We find that spectral weight is transferred from the quasiparticle peak to incoherent dispersive features. As a consequence, the distribution of low-laying spectral weight is modified with respect to the quasiparticle Fermi surface. The interplay of charge and spin fluctuations reproduces features of the observed Fermi surface, such as the asymmetric suppression of spectral weight near the M points of the Brillouin zone. Within the model, we also analyze the interplay between repulsive spin and attractive charge fluctuations in determining the symmetry and the peculiar momentum dependence of the superconducting gap parameter. When both spin and charge fluctuations are coupled to the electrons, we find d x 2− y 2 -wave gap symmetry in a wide range of parameter. A crossover d- vs. s-wave symmetry of the gap may occur when the strength of charge fluctuations increases with respect to spin fluctuations.

The isotope effect coefficient dependence on nonstoichiometry for single CuO layer superconductors

Considering that the variation of the critical temperature can be explained if the impurity potential acts only by the non-spin-flip part, in the presence of either a pure d-wave or anisotropic s-wave gap symmetry, and inserting the pair-breaking time dependence on temperature, we obtain a quantitative expression for the isotope effect coefficient α as a function of measured critical temperature T c maximal critical temperature T and degree of anisotropy of the energy gap. Our result predicts the achievement of a large range of values in α for different ratios T/ T c. Introducing the dependence of the critical temperature and pair-breaking time on dopant content for a single CuO layer superconductors, we can describe the variation of the isotope effect coefficient with nonstoichiometry for these materials. The result for the case of anisotropic s-wave gap symmetry is in good agreement with the experimental data.

Influence of a Van Hove singularity on the electronic specific heat of high-T c superconductors

We study the influence of a logarithmic Van Hove singularity on the electronic specific heat C e of a 2D superconductor. The theoretical results are obtained for an isotropic s-wave gap parameter or an anisotropic d x 2− y 2 -wave gap parameter. The specific heat jump at T c observed in high-T c superconductors can be reproduced by considering both gap parameter symmetries. However the very low temperature behavior of C e as observed in a single crystal of YBa 2Cu 3O 7−δ is only consistent with a gap parameter of d x 2− y 2 wave type.

Grazing incidence infrared reflectivity of La1.85Sr0.15CuO4 and NbN.

Infrared reflectivity measurements, using p-polarized light at a grazing angle of incidence, show an increased sensitivity to the optical conductivity of highly reflecting superconducting materials. We demonstrate that when this measurement technique is applied to the conventional s-wave superconductor NbN, the results are in perfect agreement with BCS theory. For the in-plane response of a ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ single crystal, in the superconducting state, we find a reduction of the optical conductivity in the frequency range below 20 meV. The observed frequency dependence excludes an isotropic s-wave gap, but agrees well with model calculations assuming a d-wave order parameter.