[en] We studied the persistent photoconductivity in underdoped metallic YBa₂Cu₃O_x (x∼6.6) by visible and UV excitation above and below T_c. The results show that the photodoping effect exists also when the sample is in the superconducting state and that its efficiency is approximately the same as at low temperatures above T_c. The dependence of the effect on the temperature where the photodoping is performed is essentially the same for visible light and UV radiation, i.e., a relatively small effect at low temperatures but a significantly larger effect near room temperature. We also observed that the efficiency is somewhat smaller with UV radiation, in contrast to previous results in semiconducting samples

No abstract available

[en] Full text: This work deals with transport measurements on HTSC at very high levels of current density under dissipative conditions. In order to avoid the resulting temperature rise at high current densities, the measurements are carried out using short-duration pulse currents, instead of the continuous current of the conventional dc four-probe method. Our experimental setup allows to detect voltage pulses in a four-probe arrangement as short as 50 ns, at current densities ranging up to 8 MA/cm². This is about 100 times faster than any other previously reported pulse-current measurement. The presented data concern the in-plane resistivity of optimally doped and oxygen depleted YBCO thin films, recorded while varying temperature at fixed current, or as a function of electrical field at fixed temperature. The experimental dependencies, consistent to one another, are accounted for by a model based on the recently developed theory of superconducting fluctuations in high electrical fields. (author)

[en] Superconducting compounds of the family Hg-Ba-Ca-Cu-O have evoked intensive research efforts since the current record-holder for the highest critical temperature of a superconductor belongs to this class of materials. Thin films of the compound with two adjacent copper-oxide layers and a critical temperature of about 120 K were prepared by a two-step process that starts with the pulsed-laser deposition of precursor films and subsequent annealing in mercury-vapour atmosphere. Hg-Ba-Ca-Cu-O is a quasi-two dimensional material with high anisotropy, and, like some other high-temperature superconductors, exhibits a specific anomaly of the Hall effect, a double-sign change of the Hall coefficient close to the super-conducting transition. We have investigated this phenomenon by measurements of the Hall effect at different angles between the magnetic field direction and the crystallographic c axis. The results are discussed in terms of various contributions to the Hall effect by normal-state carriers, superconducting fluctuations, and dissipative vortex motion. (author)

[en] The critical current density of a superconductor as a function of temperature and magnetic field is a key quantity for the design of superconductor applications. Tremendous efforts are currently undertaken to enhance the critical current in technical materials by improving the pinning forces. The ultimate thermodynamic limit for carrying a superconducting current is named the depairing current and usually cannot be measured directly. Our measurements are carried out using short-pulse currents with about 50 ns duration in order to avoid thermal heating and eventually destruction of the samples that result inevitably from such high dissipation. In addition, ultrathin films of YBa₂Cu₃O_7-x are used, patterned to microbridges to allow for an effective heat transfer into the MgO substrate. The depairing current is directly measured and is in good accordance with the values predicted by Ginzburg-Landau theory. In intense currents the critical temperature is not only shifted to lower temperatures, as seen in conventional d.c. measurements, but the shape of the transition changes significantly. The latter effect is attributed to a reduction of the fluctuation paraconductivity with increasing electric field and is compared to our recent theoretical work. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] A novel geometry for a sampling calorimeter employing inorganic scintillators as an active medium is presented. To overcome the mechanical challenges of construction, an innovative light collection geometry has been pioneered, that minimises the complexity of construction. First test results are presented, demonstrating a successful signal extraction. The geometry consists of a sampling calorimeter with passive absorber layers interleaved with layers of an active medium made of inorganic scintillating crystals. Wavelength-shifting (WLS) fibres run along the four long, chamfered edges of the stack, transporting the light to photodetectors at the rear. To maximise the amount of scintillation light reaching the WLS fibres, the scintillator chamfers are depolished. It is shown herein that this concept is working for cerium fluoride (CeF_3) as a scintillator. Coupled to it, several different types of materials have been tested as WLS medium. In particular, materials that might be sufficiently resistant to the High- Luminosity Large Hadron Collider radiation environment, such as cerium-doped Lutetium- Yttrium Orthosilicate (LYSO) and cerium-doped quartz, are compared to conventional plastic WLS fibres. Finally, an outlook is presented on the possible optimisation of the different components, and the construction and commissioning of a full calorimeter cell prototype is presented

[en] Thin films of the high-temperature superconductor HgBa₂CaCu₂O₆ have been prepared on SrTiO₃ substrates by pulsed-laser deposition of precursor films and subsequent annealing in mercury-vapor atmosphere. The microstructural properties of such films can vary considerably and have been analyzed by x-ray analysis and atomic force microscopy. Whereas the resistivity is significantly enhanced in samples with coarse-grained structure, the Hall effect shows little variation. This disparity is discussed based on models for transport properties in granular materials. We find that, despite of the morphological variation, all samples have similar superconducting properties. The critical temperatures T _c ∼ 121.2 K …122.0 K, resistivity, and Hall data indicate that the samples are optimally doped. The analyses of superconducting order parameter fluctuations in zero and finite magnetic fields yield the in-plane ξ _ab(0) ∼ 2.3 nm …2.8 nm and out-of-plane ξ _c(0) ∼ 0.17 nm …0.24 nm Ginzburg–Landau coherence lengths at zero temperature. Hall measurements provide estimates of carrier scattering defects in the normal state and vortex pinning properties in the superconducting state inside the grains. (paper)

[en] The resistive transition from the superconducting to the normal state in very thin films of optimally doped YBa₂Cu₃O_7-δ was measured using sub-μs current pulses and current densities up to 7 MA cm^-2. Very thin films allow for a good thermal diffusion into the MgO substrate and minimize the magnetic self field generated by the current. The depairing current was determined as a function of temperature and it was found that it agrees well with the predictions from Ginzburg-Landau theory. It is demonstrated that the depairing current has a different dependence on a magnetic field than the critical current determined by a low-voltage criterion

[en] We present studies of the time dependent electrical resistivity of YBa₂Cu₃O_x during visible light excitation (photodoping). Illumination with a green semiconductor laser changes the resistivity in a non-exponential way, which is well described by a stretched-exponential function with parameters β (dispersion parameter) and τ (time constant). β showed an unusual behaviour: at temperatures below 220 K β was about 0.3 to 0.4, but increased near room temperature to 0.5 to 0.6. For comparison with results obtained by red-laser excitation we discuss the dependence of the resistivity on the cumulative photon dose, thereby introducing a ''photon dose constant'' N_e which corresponds to τ. The results indicate that green-light excitation is somewhat more effective than red-light excitation at low temperatures

[en] Resistivity measurements on ultrathin (36 and 42 nm thick) YBCO films were performed with a novel pulsed-current technique that allows for application of current densities up to several MA/cm² with minimal self-heating even in the resistive state. Using current pulses as short as 50 ns we found that the onset of resistance in the I-V curves takes place at substantially higher current densities as compared to results achieved with the commonly used d.c. technique. Both the temperature dependence and the absolute values for such critical current measurements indicate that indeed the depairing current has been probed. The results are in quantitative agreement with the predictions from Ginzburg-Landau theory. Thus, our measurements can determine the ultimate thermodynamic limit for dissipation-less current transport in a high-temperature superconductor that is more than an order of magnitude larger than previous d.c. technique results