Phase-sensitive harmonic measurements of microwave nonlinearities in cuprate thin films

Abstract

Investigations of the intrinsic electromagnetic nonlinearity of superconductors give insight into the fundamental physics of these materials. Phase-sensitive third-order harmonic-voltage data ũ3f=|u3f|exp(iϕ3f) are acquired with a near-field microwave microscope on homogeneous YBa2Cu3O7−δ 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 π/2 in the superconducting state goes through a minimum. It is found that samples with doping ranges from near optimal (δ=0.16) to underdoped (δ=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 ũ3f 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 σ2 nonlinearity dominates, in agreement with published theoretical and experimental results. Additionally the model identifies Tm as the temperature where the order-parameter relaxation time becomes comparable to the microwave probing period and reproduces semiquantitatively the experimental data.

Source-reported materials — not catalogue approval

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