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Register a new userNernst effect of epitaxial YBCO films doped with Ca and Zn
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We report Nernst effect measurements for some crystalline films of Ca and Zn-doped yttrium barium copper oxide grown by pulsed laser deposition. We argue that our results and most of the published data for LSCO are consistent with the theory of Gaussian superconducting fluctuations.
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Nb-doped SrTiO$_{3}$ epitaxial thin films have been prepared on (001) SrTiO$_{3}$ substrates using pulsed laser deposition. A high substrate temperature ($>1000^{circ}{C}$) was found to be necessary to achieve 2-dimensional growth. Atomic force microscopy reveals atomically flat surfaces with 3.9 AA $ $ steps. The films show a metallic behavior, residual resistivity ratios between 10 and 100, and low residual resistivity of the order of 10$^{-4}$$Omega$cm. At 0.3 K, a sharp superconducting transition, reaching zero resistance, is observed.
We describe an alternating current method to measure the Nernst effect in superconducting thin films at low temperatures. The Nernst effect is an important tool in the understanding superconducting fluctuations and, in particular, vortex motion near critical points. However, in most materials, the Nernst signal in a typical experimental setup rarely exceeds a few $mu$V, in some cases being as low as a few nV. DC measurements of such small signals require extensive signal processing and protection against stray pickups and offsets, limiting the sensitivity of such measurements to $>$5nV. Here we describe a method utilizing a one-heater-two-thermometer setup with the heating element and thermometers fabricated on-chip with the sample, which helped to reduce thermal load and temperature lag between the substrate and thermometer. Using AC heating power and 2$omega$ measurement, we are able to achieve sub-nanovolt sensitivity in 20-30nm MoGe thin films on glass substrate, compared to a sensitivity of $sim$9nV using DC techniques on the same setup.
Measurements of the nonlinear flux-flow resistivity $rho$ and the critical vortex velocity $rm v^*_phi$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov cite{LO} are reported along the node and antinode directions of the d-wave order parameter in the textit{a-b} plane of epitaxial $YBa_2Cu_3O_{7-delta}$ films. In this pinning-free regime, $rho$ and $rm v^*_phi$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $rho$ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of $YBa_2Cu_3O_{7-delta}$.
We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf sputtering on KTaO3 substrates with x = 0.12. The as-grown deposits are insulating and a series of superconducting films with various Tc(R=0) up to 26 K have been obtained by in-situ oxygen reduction. Transport measurements in the ab plane of these samples have been undertaken. We report original results on the temperature dependence of the Hall effect and on the anisotropic magnetoresistance (T > Tc). We discuss the magnitude of upper critical fields and anisotropy, the Hall effect, which presents changes of sign indicative of the existence of two types of carriers, the normal state magnetoresistance, negative in parallel magnetic field, a possible signature of spin scattering. These properties are compared to those of hole-doped cuprates, such as BiSr(La)CuO with comparable Tc.
The iron-based superconductor Ba$_{1-x}$K$_x$Fe$_text{2}$As$_text{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial Ba$_{1-x}$K$_x$Fe$_text{2}$As$_text{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates (CaF$_text{2}$, SrF$_text{2}$, and BaF$_text{2}$) and a low growth temperature (350$-$420$^circ$C). Our epitaxial thin film grown on CaF$_text{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by ~2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density ($J$$_text{c}$) determined by the magnetization hysteresis loop is as high as 2.2 MA/cm$^text{2}$ at 4 K under self-field. In-field $J$$_text{c}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of Ba$_{1-x}$K$_x$Fe$_text{2}$As$_text{2}$.
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