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Phase-sensitive harmonic measurements of microwave nonlinearities in cuprate thin films Dragos I. Mircea,1 Hua Xu,2 and Steven M. Anlage3

Summary: Phase-sensitive harmonic measurements of microwave nonlinearities in cuprate thin films
Dragos I. Mircea,1 Hua Xu,2 and Steven M. Anlage3
1San Jose Research Center, Hitachi Global Storage Technologies, 3403 Yerba Buena Rd., San Jose, California 95135, USA
2Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
3Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park,
Maryland 20742, USA
Received 22 June 2009; revised manuscript received 13 September 2009; published 5 October 2009
Investigations of the intrinsic electromagnetic nonlinearity of superconductors give insight into the funda-
mental physics of these materials. Phase-sensitive third-order harmonic-voltage data u~3f = u3f exp i 3f are
acquired with a near-field microwave microscope on homogeneous YBa2Cu3O7- thin films in a temperature
range close to the critical temperature Tc. As temperature is increased from below Tc, the harmonic magnitude
exhibits a maximum while the phase /2 in the superconducting state goes through a minimum. It is found that
samples with doping ranges from near optimal =0.16 to underdoped =0.47 exhibit different behavior in
terms of both the harmonic magnitude and phase. In optimally doped samples, the harmonic magnitude reaches
its maximum at a temperature TM slightly lower than that associated with the minimum of phase Tm and drops
into the noisefloor as soon as Tm is exceeded. In underdoped samples TM is shifted toward lower temperatures
with respect to Tm and the harmonic-voltage magnitude decreases slower with temperature than in the case of
optimally doped samples. A field-based analytical model of u~3f is presented, where the nonlinear behavior is
introduced as corrections to the low-field linear-response complex conductivity. The model reproduces the
low-temperature regime where the 2 nonlinearity dominates, in agreement with published theoretical and


Source: Anlage, Steven - Center for Superconductivity Research & Department of Physics, University of Maryland at College Park


Collections: Materials Science