Upper critical field, penetration depth, and depinning frequency of the high-temperature superconductor ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ studied by microwave surface impedance

Temperature-dependent and magnetic-field-dependent measurements of the microwave surface impedance of superconducting ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ $\left(T_c\approx 26\text{K}\right)$ reveal a very large upper critical field $\left(B_{c2}\approx 56\text{T}\right)$ and a large value of the depinning frequency $\left(f_0\approx 6\text{GHz}\right)$; together with an upper limit for the effective London penetration depth, ${\lambda }_{\text{eff}}\le 200\text{nm}$, our results indicate a strong similarity between this system and the high-$T_c$ superconducting cuprates.

Figure 1

(Color online) Temperature dependence of the resistivity. Top left-hand side inset: Resistivity plotted versus $T^2$, showing a deviation above $T\approx 200\text{K}$. Bottom right-hand side inset: The superconducting transition.

Figure 2

(Color online) Temperature dependence of the susceptibility measured in zero-field cooled (ZFC) and field cooled (FC, 20 Oe) conditions.

Figure 3

(Color online) Temperature dependence of the complex-frequency shift in various applied magnetic fields (imaginary part in the top panel and real part in the bottom panel).

Figure 4

(Color online) Temperature dependence of the complex penetration depth (left axis) and the complex impedance (right axis) in zero applied magnetic field.

Figure 5

(Color online) The upper critical field as extracted from the temperature dependence of the complex conductivity in an applied magnetic field; the line is a fit to the data below $T_c=23.7\text{K}$ using Eq. (2).

Figure 6

(Color online) The field dependence of the depinning frequency ${\omega }_0$ at two temperatures. The driving frequency was $\omega =2\pi 12.77\text{GHz}$.