Thomas Dahm

Theory of intermodulation in a superconducting microstrip resonator

The penetration depth and surface resistance of a superconductor depend upon the superfluid current density. This dependence gives rise to nonlinear mixing in a superconducting microstrip resonator. Here we discuss the problem of intermodulation in which two signals at ω1 and ω2, laying within the pass band of a microstrip cavity resonance, mix and generate a signal at 2ω1−ω2. An expression relating the power generated at 2ω1−ω2 to the power transmitted at ω1 and ω2 is given. We focus on the high-Tc superconductors where it is believed that the order parameter has dx2−y2 symmetry. We find for a resonator with a large unloaded Q that intermodulation arises dominantly from the reactive nonlinear inductance of the superconducting film.

Strength of the spin-fluctuation-mediated pairing interaction in a high-temperature superconductor

Although spin fluctuations are believed to have an important role in the mechanism responsible for high-temperature superconductivity, it has been unclear whether the strength of their coupling with fermionic quasiparticles is sufficiently strong. Systematic analysis of angle-resolved photoemission and neutron spectra suggests it is. Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely1. Here, we analyse the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin–fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations2,3,4,5,6,7 and ‘kinks’ in the fermionic band dispersions along the nodal direction8,9,10,11,12. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.

NMR Relaxation and Resistivity from Rattling Phonons in Pyrochlore Superconductors

We calculate the temperature dependence of the NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T2 at low temperatures and following a square root T behavior at high temperatures. Both results are in qualitative agreement with recent observations on beta-pyrochlore oxide superconductors.

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2.

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.

Nonlinear current response of a d-wave superfluid

Despite several efforts the nonlinear Meissner effect in d-wave superconductors, as has been discussed by Yip and Sauls in 1992, has not been verified experimentally in high-T(c) superconductors at present. Here, we reinvestigate the nonlinear response expected in a d-wave superconductor. While the linear /(H) over right arrow/ field dependence of the penetration depth, predicted by Yip and Sauls, is restricted by the lower critical field and can be masked by nonlocal effects, we argue that the upturn of the nonlinear coefficient of the quadratic field dependence is more stable and remains observable over a broader range of parameters. We investigate this by studying the influence of nonmagnetic impurities on the nonlinear response. We discuss the difficulties of observing this intrinsic d-wave signature in present day high-T(c) films and single crystals. [S0163-1829(99)02841-6].

Theory of microwave intermodulation in a high‐Tc superconducting microstrip resonator

We calculate the intermodulation power for a superconducting microstrip resonator arising from the intrinsic microscopic nonlinearities. We focus on high‐Tc superconductors where it is believed that the order parameter has dx2−y2 symmetry. A characteristic upturn of the intermodulation power is found at low temperatures. We discuss different ways to reduce intermodulations by changing the geometry of the microstrip.

UNUSUAL POWER DEPENDENCE OF TWO-TONE INTERMODULATION IN HIGH-TC SUPERCONDUCTING MICROWAVE RESONATORS

We study the power dependence of two-tone intermodulation arising in high-T-c superconducting microwave resonators as a function of each tone separately. In the regime where the intermodulation power varies as the square of the input power for equally strong tones, we observe unexpected behavior of the power dependence on the individual tones. We show that this behavior can be understood in terms of a nonlinear inductance which varies linearly with the magnitude of the input current. These findings provide a consistent phenomenological picture of the unusual power dependence and should place constraints on a microscopic description of nonlinear behavior in high-T-c films where the intermodulation products vary like the square of the input power

Anomalous Coherence Peak in the Microwave Conductivity of c-Axis Oriented MgB2 Thin Films

The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by bandstructure calculations. In addition, the observation of a coherence peak indicates that the pi-band is in the dirty limit and dominates the total conductivity of our films

Intrinsic limits on the Q and intermodulation of low power high temperature superconducting microstrip resonators

Cuprate superconducting thin films are being used to make compact low power microwave devices. In recent years, with improved materials and designs, there has been a steady improvement in device performance, notably an increase in resonator Q and a decrease in the nonlinear intermodulation. It is important to understand how much improvement can be expected. Here we discuss the intrinsic limiting behavior that one might achieve with a perfect film, review the present status, and discuss what the limiting behavior implies for high temperature superconducting filters. Our analysis indicates that increases in unloaded Q to ∼106 and decreases in intermodulation by a factor of ∼104, compared with today’s values, might be achieved.

Microwave intermodulation of a superconducting disk resonator

We calculate the third order intermodulation products for the TM010 mode of a thin film disk resonator and discuss the particular case of a 1 GHz resonator in order to obtain a quantitative idea of the performance that can be obtained. The linear and nonlinear microwave responses of a TM010 disk resonator are then compared to that of an equivalent half wavelength microstrip resonator. This analysis allows one to independently quantify the contributions to the nonlinear device performance from the material properties, device size and field configuration.