J. Betz

Giant voltages upon surface heating in normal YBa2Cu3O7−δ films suggesting an atomic layer thermopile

Experiments are reported which show that temperature gradients perpendicular to the surface of epitaxial normal conducting YBa2Cu3O7−δ films give rise to large transversal voltages between contacts on the film surface. The temperature gradients have been produced by pulsed laser irradiation and by continuous heating of the films by heater wires. To explain the large lateral voltages, an atomic layer thermopile is proposed, which may be formed by the layered structure of the material.

Heat propagation in high Tc films investigated by optical response measurements

The optical response of granular Tl‐Ba‐Ca‐Cu‐O films has been used to investigate thermal properties of the films. An analysis of the response using a heat transfer model yields a thermal diffusivity D=10−3 cm2/s at 150 K which rises to 6×10−3 cm2/s at a temperature of 30 K and allows for an estimation of the boundary resistance Rbd≊10−3 K cm2/W between film and substrate. The dependence of the response time on film thickness obtained from the heat transfer model is compared with published data indicating that in many experiments the observed response is mainly bolometric in origin.

Use of high Tc superconductors for far‐infrared Fabry–Perot resonators

In this letter we report on the application of high Tc superconductors for fabrication of far‐infrared Fabry–Perot resonators. Thin films of YBa2Cu3O7−δ deposited on MgO substrates by the laser ablation technique were used as reflectors. Two films on two different MgO plates were arranged parallel at a distance determining the lowest order resonance. We demonstrate operation of a resonator in a frequency range (up to about 300 cm−1) in which MgO is transparent at low temperature.

Thermopile effect due to laser radiation heating in thin films of high-Tc materials

We report on the observation of a thermopile effect caused by laser radiation heating in thin films of high-Tc material. The thermopile effect is due to a transverse Seebeck effect. The transverse Seebeck effect is observable first because of the anisotropy of YBa2Cu3O7−δ, and secondly because it is possible to grow off-axis epitaxial films. The thermopile effect can be used for developing detectors for laser radiation.

Superconducting Energy Gap from Infrared Response Measurements in Granular Tl2Ba2CaCu2O8 Films

The resistance response of polycrystalline Tl2Ba2CaCu2O8 films to short laser pulses has been investigated for a wide range of far-infrared frequencies. For temperatures T ll Tc a drastic change in response time is observed at a critical frequency νc simeq 4 THz. The time constant increases from about 1 ns at ν νc. The critical frequency is attributed to a superconducting energy gap with 2Δ(0)/kB Tc simeq 2.

Infrared Detection by Tl-Ba-Ca-Cu-O superconducting Films

The response of polycrystalline Tl−Ba−Ca−Cu−O superconducting thin films on short laser pulses has been investigated for radiation between 10 μm and 500 μm wavelength. Fast signals with time constants less than 1 ns were observed for wavelengths longer than about 100 μm whereas for shorter wavelengths only a bolometric signal could be detected.

Preparation of Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba2Ca2Cu3Ox films

Preparation of superconducting Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba2Ca2Cu3Ox thin films is reported. From a sintered Ba2Ca2Cu3Ox bulk sample we prepared using a pulsed Nd:YAG laser, Ba-Ca-Cu-O thin films on sapphire and SrTiO3 single-crystal substrates. Subsequently the films were loaded with Tl by simultaneously annealing the films together with a sintered Tl2Ba2Ca2Cu3O10 sample both enclosed in a small stainless steel box; in our procedure Tl contamination was reduced to a minimum. Tc values near 100 K and critical currents of 5·103 A/cm2 at 77 K were obtained.

Damping of the oscillations of a permanent magnet levitating between high‐Tc superconductors

A permanent magnet (spherical shape, radius 100 μm) is levitating inside a superconducting parallel plate capacitor made of YBCO. Translational oscillations of the magnet at its resonance frequency (∼150 Hz) can be excited and detected. The damping of the oscillations is investigated as a function of temperature and oscillation amplitude. Close to T c we observe a steep minimum of the damping. The data can be described by a linear frictional force which decreases near T c , and a quadratic one which diverges at T c . These frictional forces are attributed to flux flow phenomena in the high‐T c superconductor.

Characterization of YBCO films by torque magnetometry

Abstract YBaCuO films with c- or a-axis texture were prepared by laser deposition and sputtering. From rotational hysteresis losses critical current densities and pinning forces are calculated. The torque relaxation gives activation energies in the range of 0.21 to 0.37 eV. Characteristic differences of critical current densities and activation energies are related to the film microstrusture.

Giant Voltages Upon Surface Heating in Normal YBa2Cu3O7−δ Films Explained by an Atomic-Layer Thermopile

Large voltage signals have been observed beetween lateral contacts on epitaxial YBa2Cu3O7−δ films, if the films were heated by pulsed laser irradiation or by heater wires. Our investigations show that the voltaic signals are thermoelectric in origin. A model is proposed which explains the voltages by an atomic layer thermopile formed by the layered structure of the material.