The a-b plane microwave surface impedance of a high-quality Bi 2 Sr 2 CaCu 2 O 8 single crystal ͑T c ഠ 93 K͒ has been measured at 14.4, 24.6, and 34.7 GHz. The surface resistance at low temperature is the lowest yet reported, is comparable with the best YBa 2 Cu 3 O 72d data, and has a characteristic v 2 frequency dependence. The change in penetration depth, Dl ab ͑T ͒, has a strong linear term at low temperature which is consistent with a gap with line nodes on the Fermi surface. The real part of the microwave conductivity displays a broad peak at low temperature, similar to that observed in YBa 2 Cu 3 O 72d . [S0031-9007(96)00735-1]
We have measured the London penetration depth λ(T) of
CeCoIn5, a quantum-critical metal where superconductivity arises
from a non-Fermi-liquid normal state. Using a tunnel diode
oscillator with a novel coil geometry designed to avoid spurious
contributions to λ(T), we have established the existence
of intrinsic and anomalous power law behaviour at low
temperature. A systematic analysis raises the possibility that
the unusual observations might be due to an extension of quantum
criticality into the superconducting state.
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