Torque magnetometry on single-crystal high temperature superconductors near the critical temperature: a scaling approach

Angular-dependent magnetic torque measurements performed near the critical temperature on single crystals of HgBa_{2}CuO_{4+y}, La_{2-x}Sr{x}CuO_{4}, and YBa_{2}Cu_{3}O_{6.93} are scaled, following the 3D XY model, in order to determine the scaling function dG^{\pm}(z)/dz which describes the universal critical properties near T_{c}. A systematic shift of the scaling function with increasing effective mass anisotropy \gamma = (m_{ab}*/m_{c}*)^{1/2} is observed, which may be understood in terms of a 3D-2D crossover. Further evidence for a 3D-2D crossover is found from temperature-dependent torque measurements carried out in different magnetic fields at different field orientations \delta, which show a quasi 2D "crossing region'' (M*,T*). The occurrence of this "crossing phenomenon'' is explained in a phenomenological way from the weak z dependence of the scaling function around a value z = z*. The "crossing'' temperature T* is found to be angular-dependent. Torque measurements above T_{c} reveal that fluctuations are strongly enhanced in the underdoped regime where the anisotropy is large, whereas they are less important in the overdoped regime.Comment: 9 pages, 10 figures, submitted to PR

Josephson junction between anisotropic superconductors

The sin-Gordon equation for Josephson junctions with arbitrary misaligned anisotropic banks is derived. As an application, the problem of Josephson vortices at twin planes of a YBCO-like material is considered. It is shown that for an arbitrary orientation of these vortices relative to the crystal axes of the banks, the junctions should experience a mechanical torque which is evaluated. This torque and its angular dependence may, in principle, be measured in small fields, since the flux penetration into twinned crystals begins with nucleation of Josephson vortices at twin planes.Comment: 6 page

Synthesis and Bulk Properties of Oxychloride Superconductor Ca2-xNaxCuO2Cl2

Polycrystalline samples and submillimeter size single crystals of Na-doped Ca2CuO2Cl2 have been synthesized under high pressure. A series of experiments showed that the Na content depends not only on the pressure during the synthesis but also on the synthesis temperature and time. From a comparison of the Na-CCOC data with those of structurally related La214 cuprate superconductors we concluded that chlorine at the apical site is less effective that oxygen in supplying charge carriers to the CuO2 plans. As a result, the coupling between the CuO2 planes is weakened, the transition temperature Tc is reduced and the anisotropic nature is enhanced.Comment: 7 pages, 7 figures, 1 table, presenthed at the Eucas 2007 conference. Accepted for "Journal of Physics: Conference Series (JPCS)" 2008 and European News Forum, Issue 3 (2008

Dynamical Induction of s-wave Component in d-wave Superconductor Driven by Thermal Fluctuations

We investigated the mutual induction effects between the d-wave and the s-wave components of order parameters due to superconducting fluctuation above the critical temperatures and calculated its contributions to paraconductivity and excess Hall conductivity based on the two-component stochastic TDGL equation. It is shown that the coupling of two components increases paraconductivity while it decreases excess Hall conductivity compared to the cases when each component fluctuates independently. We also found the singular behavior in the paraconductivity and the excess Hall conductivity dependence on the coupling parameter which is consistent with the natural restriction among the coefficients of gradient terms.Comment: 10 pages, 4 figures included, submitted to J.Phys.Soc.Jp

Identification of the bulk pairing symmetry in high-temperature superconductors: Evidence for an extended s-wave with eight line nodes

we identify the intrinsic bulk pairing symmetry for both electron and hole-doped cuprates from the existing bulk- and nearly bulk-sensitive experimental results such as magnetic penetration depth, Raman scattering, single-particle tunneling, Andreev reflection, nonlinear Meissner effect, neutron scattering, thermal conductivity, specific heat, and angle-resolved photoemission spectroscopy. These experiments consistently show that the dominant bulk pairing symmetry in hole-doped cuprates is of extended s-wave with eight line nodes, and of anisotropic s-wave in electron-doped cuprates. The proposed pairing symmetries do not contradict some surface- and phase-sensitive experiments which show a predominant d-wave pairing symmetry at the degraded surfaces. We also quantitatively explain the phase-sensitive experiments along the c-axis for both Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and YBa_{2}Cu_{3}O_{7-y}.Comment: 11 pages, 9 figure

Spatial Symmetry of Superconducting Gap in YBa2Cu3O7-\delta Obtained from Femtosecond Spectroscopy

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa2Cu3O7-\delta thin films are reported. This bulk-sensitive spectroscopy, combining with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By the orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.Comment: 8 pages, 4 figures. To be published in Physical Review B, Rapid Communication

Numerical studies of the phase diagram of layered type II superconductors in a magnetic field

We report on simulations of layered superconductors using the Lawrence-Doniach model in the framework of the lowest Landau level approximation. We find a first order phase transition with a $B(T)$ dependence which agrees very well with the experimental ``melting'' line in YBaCuO. The transition is not associated with vortex lattice melting, but separates two vortex liquid states characterised by different degrees of short-range crystalline order and different length scales of correlations between vortices in different layers. The transition line ends at a critical end-point at low fields. We find the magnetization discontinuity and the location of the lower critical magnetic field to be in good agreement with experiments in YBaCuO. Length scales of order parameter correlations parallel and perpendicular to the magnetic field increase exponentially as 1/T at low temperatures. The dominant relaxation time scales grow roughly exponentially with these correlation lengths. We find that the first order phase transition persists in the presence of weak random point disorder but can be suppressed entirely by strong disorder. No vortex glass or Bragg glass state is found in the presence of disorder. The consistency of our numerical results with various experimental features in YBaCuO, including the dependence on anisotropy, and the temperature dependence of the structure factor at the Bragg peaks in neutron scattering experiments is demonstrated.Comment: 25 pages (revtex), 19 figures included, submitted to PR

MgB2 single crystals: high pressure growth and anisotropic properties

Single crystals of MgB2 with a size up to 1.5x0.9x0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements in fields of 1-5 Oe show sharp transitions to the superconducting state at 37-38.6 K with width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that Hc2//c is very low (24 kOe at 15 K), while Hc2//ab increases up to 140 kOe at 15 K. The upper critical field anisotropy gamma = Hc2//ab/ Hc2//c was found to be temperature dependent (decreasing from 6 at 15 K to 2.8 at 35 K). The effective anisotropy gamma_eff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter being rapidly suppressed in a magnetic field. Torque magnetometry investigations show a pronounced peak effect, indicating an order-disorder transition of vortex matter. Decoration experiments and STS visualise a hexagonal vortex lattice. STS spectra evidence two gaps (3 meV/6 meV) with direction dependent weight. Magneto-optic investigations with H//c show a clear signature of the smaller of the two gaps, disappearing in fields higher than Hc2//c.Comment: 17 pages pdf only, 15 figures integrated (higher resolution photographs available on request); submitted to Supercond. Sci. Technol. (Proceedings of Boromag conference

Basal-Plane Magnetic Anisotropies of High-kappa d-Wave Superconductors in a Mixed State: A Quasiclassical Approach

We study the basal-plane anisotropies of reversible magnetization and torque in a mixed state of layered d-wave superconductors based on the quasiclassical version of the BCS-Gor'kov theory. Both the longitudinal magnetization ($M_L$) and torque ($\tau$) show fourfold oscillations as a function of the field angle $\chi$. The relationship between the node position and the oscillatory patterns shown by $M_L$ and $\tau$ is clarified. It is also shown that the sign of the $\tau (\chi)$-oscillation does not change between $H_{c1}$ and $H_{c2}$, while the sign of the $M_L (\chi)$-oscillation changes. The newly obtained result for $\tau$ indicates that the torque experiment can allow us to detect the in-plane anisotropies of $H_{c2}$ even in a material with strong fluctuations such as cuprate or organic superconductors, where the $H_{c2}$ itself cannot be determined experimentally.Comment: 10 pages, 9 figure