Paraconductivity of underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ thin-film superconductors using high magnetic fields

The contribution of superconducting fluctuations to the conductivity, or paraconductivity, is studied in the underdoped regime of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ cuprates. A perpendicular magnetic field up to $50\mathrm{T}$ is applied to suppress the superconductivity and obtain the normal state resistivity, which is then used to calculate the paraconductivity. Surprisingly enough, it is consistent with a two-dimensional Aslamazov-Larkin regime of Gaussian fluctuations close to the critical temperature. At higher temperature, the paraconductivity exhibits a power-law decrease in temperature (as $T^{-\alpha }$), as was previously shown for underdoped $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ samples. Our observations are not consistent with the existence of Kosterlitz-Thouless fluctuations. This tends to indicate that the superconducting pair amplitude is not already defined above $T_C$ in the pseudogap state.

Figure 1

(Color online) $ab$-plane resistivity as a function of temperature of the LSCO thin film under high magnetic field. The marine triangles are the data at $47\mathrm{T}$; the blue circles are the data at $43\mathrm{T}$ and the red line is the data at $0\mathrm{T}$. The green (dashed) line is an interpolation of the data at $47\mathrm{T}$, of the form $\rho \left[\mu \Omega \mathrm{cm}\right]=490.3\ln (80.3∕\mathrm{T}\left[\mathrm{K}\right])+7.54\mathrm{T}\left[\mathrm{K}\right]$.

Figure 2

(Color online) Green markers: Paraconductivity $\Delta \sigma$ as a function of the reduced temperature $ϵ=\log \left(\frac{T}{T_C}\right)$ for sample LS644, with $T_C=19.0\mathrm{K}$. The error bars are due to uncertainty in the sample thickness and magneto resistance of the normal state. The data are also plotted for $T_C=18.8\mathrm{K}$ (dash-dot-dots) and $T_C=19.2\mathrm{K}$ (dash-dots). Marine blue dotted line: 2D AL model with $d=0.66\mathrm{nm}$. The dashed lines are for the Halperin-Nelson model (Ref. 18) with different values of $T_C^0$. (From bottom to top, $T_C^0=20$, 25, 30, 40, and $50\mathrm{K}$.)

Figure 3

(Color online) Paraconductivity $\Delta \sigma$ as a function of the temperature. Green circles, LS644 $(x=0.09)$; red dots (inset), LS388 $(x=0.08)$. After the 2D AL regime (blue dashed line), a linear regime is observed, which corresponds to a power law in $T^{-\alpha }$ with the exponent $\alpha \approx 3$.