Effective two-dimensional thickness for the Berezinskii-Kosterlitz-Thouless-like transition in a highly underdopedLa2−xSrxCuO4

Abstract

The nature of the superconducting transition in highly underdoped thick films of La2−xSrxCuO4 (x = 0.07 and 0.08) has been investigated using the in-plane transport measurements. The contribution of superconducting fluctuations to the conductivity in zero magnetic field, or paraconductivity, was determined from the magnetoresistance measured in fields applied perpendicular to the CuO2 planes. Both the temperature dependence of the paraconductivity above the transition and the nonlinear current-voltage (I − V ) characteristics measured across it, exhibit the main signatures of the Berezinskii-Kosterlitz-Thouless (BKT) transition. The quantitative comparison of the superfluid stiffness, extracted from the I − V data, with the renormalization-group results for the BKT theory, reveals a large value of the vortex-core energy. This finding is confirmed by the analysis of the paraconductivity obtained using different methods. The results strongly suggest that the characteristic energy scale controlling the BKT behavior in this layered system corresponds to the superfluid stiffness of a few layers.

Possible observation of the Berezinskii-Kosterlitz-Thouless transition in boron-doped diamond films

“…The b value can be related to the vortex core energy here expressed with respect to the value predicted by the XY model. 23,24 We find µ/µ XY = 2.37 and 3.27 for samples B5 and B4 respectively, thus the sample showing higher BKT transition temperature has a lower vortex core energy. This can again be related to the microstructure of the films if we take into account that vortex pinning, as expected for granular samples, can reduce the vortex core energy by an amount related to the pinning energy.…”

“…23 Fig. 2(a) shows a comparison of the phase transition from normal to superconducting state for the three samples plotted as resistance vs. temperature.…”

“…2(a) shows a comparison of the phase transition from normal to superconducting state for the three samples plotted as resistance vs. temperature. The solid red curve is a fit using the following interpolation formula [23][24][25] adapted from the HalperinNelsons equation which is valid for the paraconductivity region between T BKT and the mean field temperature T C . As the BKT transition was not pronounced in sample B2.5 an estimated temperature of 0.1 K was used from the extrapolation of data in Fig.…”

“…Both of these discrepancies have before been observed when fitting the Halperin-Nelson formula. The deviation from theory above the mean field critical point has been explained to arise due to formation of a pseudo-gap 23 allowing for quasiparticle excitations which affects the conductivity. Also deviations at temperatures above the mean field critical point are expected due to the breakdown of the interpolation scheme.…”

Evidence for correlated dynamics near the Berezinskii-Kosterlitz-Thouless-like transition in highly underdopedLa2−xSrxCuO4

“…Layered superconductors with weak interlayer Josephson coupling, such as underdoped cuprates, present an alternative route for the exploration of the BKT behavior [16][17][18][19][20]. We report a study of the dynamics near the SC, BKT-like transition, in which a) measurements are performed on highly underdoped, atomically smooth thick films of La 2−x Sr x CuO 4 (LSCO), a prototypical cuprate, and b) resistance (R) noise spectroscopy, a relatively uncommon technique, is used to probe the dynamics on orders-of-magnitude longer time scales than those explored previously in other systems near the BKT transition.…”