arXiv subjects
Publications and source records attributed to Durga P. Choudhury.
The nonlinear surface impedance of a thin superconducting strip carrying a microwave current has been calculated numerically from first principles based upon flux penetration due to a current-induced critical state. The calculations approach analytic results in the appropriate limit. The technique is useful for design of passive superconducting microwave circuits.
Measurement of harmonic power on a suspended stripline microwave resonator patterned out of thin film YBa_2Cu_3O_7 (YBCO) has been carried out as a function of temperature and microwave power. The third harmonic power P_3 shows a subtle nonlinear dependence on the fundamental power P_1 on a log-log scale. Fit to a straight line yields slopes of P_3 vs. P_1 between 1.5 to 3, contrary to elementary calculations that predict a slope of exactly 3. It is shown that third harmonic power generated from higher order terms in the nonlinear impedance could account for the discrepancy quantitatively without resorting to any additional ad-hoc assumptions. Results of measurement of fifth harmonic power is presented as a confirmation. A small second harmonic power is also observed that does not show anomalous power dependence within the experimental accuracy.
We report the observation of novel behaviour in the radio frequency (rf) and microwave response of DyNi_2B_2C over a wide range of temperature (T) and magnetic field (H) in the antiferromagnetic (AFM) and superconducting (SC) states. At microwave frequencies of 10 GHz, the T dependence of the surface impedance Z_s=R_s+iX_s was measured which yields the T dependence of the complex conductivity σ_1-iσ_2 in the SC and AFM states. At radio frequencies (4 MHz), the H and T dependence of the penetration depth λ(T,H) were measured. The establishment of antiferromagnetic order at T_N=10.3 K results in a marked decrease in the scattering of charge carriers, leading to sharp decreases in R_s and X_s. However, R_s and X_s differ from each other in the AFM state. We show that the results are consistent with conductivity relaxation whence the scattering rate becomes comparable to the microwave frequency. The rf measurements yield a rich dependence of the scattering on the magnetic field near and below T_N. Anomalous decrease of scattering at moderate applied fields is observed at temperatures near and above T_N, and arises due to a crossover from a negative magnetoresistance state, possibly associated with a loss of spin disorder scattering at low fields, to a positive magnetoresistance state associated with the metallic nature. The normal state magnetoresistance is positive at all temperatures for μ_0H>2T and at all fields for T>15K. Several characteristic field and temperature scales associated with metamagnetic transitions (H_M1(T), H_M2(T)) and onset of spin disorder H_D(T), in addition to T_c, T_N and H_c2(T) are observed in the rf measurements.
The non-linear microwave surface impedance of patterned YBCO thin films, was measured using a suspended line resonator in the presence of a perpendicular DC magnetic field of magnitude comparable to that of the microwave field. Signature of the virgin state was found to be absent even for relatively low microwave power levels. The microwave loss was initially found to decrease for small applied DC field before increasing again. Also, non-linearities inherent in the sample were found to be substantially suppressed at low powers at these applied fields. These two features together can lead to significant improvement in device performance.