Universality of frequency and field scaling of the conductivity measured by ac susceptibility of a YBa2Cu3O7 film

A temperature-controlled cryogen free cryostat integrated with transceiver-mode superconducting coil for high-resolution magnetic resonance imaging

Small-sized High Temperature Superconducting (HTS) radiofrequency coils are used in a number of micro-magnetic resonance imaging applications and demonstrate a high detection sensitivity that improves the signal-to-noise ratio. However, the use of HTS coils could be limited by the rarity of cryostats that are suitable for the MR environment. This study presents a magnetic resonance (MR)-compatible and easily operated cryogen-free cryostat based on the pulse tube cryocooler technology for the cooling and monitoring of HTS coils below the temperature of liquid nitrogen. This cryostat features a real-time temperature control function that allows the precise frequency adjustment of the HTS coil. The influence of the temperature on the electrical properties, resonance frequency (f₀), and quality factor (Q) of the HTS coil was investigated. Temperature control is obtained with an accuracy of over 0.55 K from 60 K to 86 K, and the sensitivity of the system, extracted from the frequency measurement from 60 K to 75 K, is of about 2 kHz/K, allowing a fine retuning (within few Hz, compared to 10 kHz bandwidth) in good agreement with experimental requirements. We demonstrated that the cryostat, which is mainly composed of non-magnetic materials, does not perturb the electromagnetic field in any way. MR images of a 10 × 10 × 15 mm³ liquid phantom were acquired using the HTS coil as a transceiver with a spatial resolution of 100 × 100 × 300 µm³ in less than 20 min under experimental conditions at 1.5 T.

Loss of superconducting phase coherence in YBa(2)Cu(3)O(7) films: vortex-loop unbinding and Kosterlitz-Thouless phenomena

The linear conductance G' + iG(") parallel to the CuO(2) planes of flat d = 50-600 nm thin films is measured between 30 mHz and 800 kHz. The decay of the phase-coherent superfluid density signals the proliferation of 3D vortex loops which terminates at T*<T(c), where omegaG"(T*) attains 35(5) nH(-1) being independent of d. We ascribe this novel transition to the blowout of loops which reach the film thickness. Above T(c), G(omega) appears to be determined by unbound vortex lines thermally excited in a 2D phase-incoherent superfluid showing features previously being associated with a Kosterlitz-Thouless transition.