The bolometric response of YBa2Cu3O7-(YBCO) hot-electron bolometers (HEBs) to near-infrared radiation was studied. Devices were fabricated from a 50 nm thick film and had in-plane areas of 10 × 10 µm2, 2 × 0.2 µm2, 1 × 0.2µm2 and 0.5 × 0.2 µm2. We found that nonequilibrium phonons cool down more effectively for the bolometers with smaller area. For the smallest bolometer the bolometric component in the response is 10 dB less than for the largest one.
Flux noise and ac susceptibility measurements have been performed on epitaxial YBa 2 Cu 3 O 7 films to investigate dynamical properties of vortex fluctuations close to the resistive transition. The validity of the fluctuation-dissipation theorem is verified by the proportionality between flux noise and ac susceptibility results. The zero-field flux-noise spectrum ͓S ͑f͔͒ can be characterized as follows. Below a temperature dependent frequency f 0 ͑T͒, the flux noise spectrum is frequency independent. For frequencies f Ͼ f 0 ͑T͒, the flux noise spectrum follows f −x , with x Ϸ 1.5-1.75. The characteristic frequency f 0 exhibits a dramatic decrease close to the resistive transition; f 0 decreases by 3-4 orders of magnitude in a temperature interval of ⌬T Ϸ 1 K. Moreover, the flux noise spectrum scales as fS = g(f / f 0 ͑T͒), where g͑·͒ is a scaling function. The influence of a weak perturbing magnetic field ͑H dc ͒ on the characteristics of the flux noise spectrum has been investigated. While the general characteristics of the flux noise spectrum remain, except for a change in frequency dependence at f Ͼ f 0 , the characteristic frequency f 0 exhibits a dramatic increase with increasing applied field; f 0 increases by 3-4 orders of magnitude on increasing the field from zero to a few oersteds. Moreover, it is found that f 0 is proportional to H dc y , with y Ϸ 1.15. The results are interpreted in terms of thermally generated vortex-antivortex fluctuations in combination with fluctuations of field-generated vortices.
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