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Superconductors are perfect conductors of electricity only at zero frequency (DC). At any non-zero frequency (AC) they dissipate the charge carrier kinetic energy into heat, although this conversion is often quite small. Nevertheless, much can be learned about the basic physics of superconductors by measuring their response to AC fields. Consider a superconductor exposed to a time-varying magnetic field parallel to its surface. The response of the superconductor is complex in the sense that currents are induced that are both in-phase and in-quadrature (90 degrees out of phase) with the alternating magnetic field. The out-of-phase (with B) currents are dissipative and arise from the absorption of photons by quasiparticles excited out of the ground state. The in-phase response is due to the Meissner Effect in which the superconductor actively excludes magnetic field (B) from its interior to maintain the condition B = 0. This exclusion takes the form of a diamagnetic screening current induced in the near-surface region ("penetration depth") of the material. The strength of this current is dictated by the fraction of the charge carriers that condense into the ground state that constitute the "super-fluid." The surface impedance (Z_s = R_s + i X_s) and compex conductivity (s = s₁ - i s₂) are used to quantify the complex response of a superconductor to AC fields.

Our early work focused on measurements of the magnetic penetration depth in high-T_c cuprate superconductors. The goal was to do "gap spectroscopy" to learn whether or not the cuprate superconductors posses nodes in the energy gap on the Fermi surface. Nodes can arise if interesting and unusual higher angular momentum pairing states exist in the material. We examined two cuprate superconductors, the hole-doped Y-Ba-Cu-O material and the electron-doped Nd-Ce-Cu-O material. See papers 9, 17, 18, 21, 31, 35, 79. We have developed a new type of gap spectroscopy of unconventional superconductors, as outlined in paper 161.

We also developed a cavity perturbation technique to measure the surface resistance (R_s) of superconductors (i.e. the dissipative part of the surface impedance). This was applied to further corroborate the results obtained through penetration depth measurements. See papers 21, 31, 35, 79. Halbritter BCS R_s calculation.

An altogether novel method to measure the absolute value of the magnetic penetration depth and surface resistance of superconducting thin films was developed and patented by the group. This method makes use of the variation in resonant frequency and quality factor of a superconducting resonator as one of its dimensions is changed with nanometer precision. See papers 26, 69, 75, 85, and patent 2.

Novel studies were made of proximity-coupled superconductor/normal-metal bilayer thin films. In this case the normal metal can be induced into a kind of superconducting state due to its close proximity to a bone fide superconductor. This affects the surface impedance in a dramatic manner at low temperatures as the superconducting correlations are enhanced in the normal metal. See papers 33, 41, 46, and 50.

The intellectual merit of this work is an improved understanding of the basic physics of superconductors. Other broad impacts include continued support for training of graduate and undergraduate students. This work is supported by the National Science Foundation and the Maryland Center for Nanophysics and Advanced Materials.

Here is a nice summary of applications of superconductivity

Some relevant papers: (All papers can be downloaded from the full publication list)
9. Steven M. Anlage, Hsuan Sze, Howard J. Snortland, Shuichi Tahara, Brian Langley, Chang-Beom Eom, M. R. Beasley and Robert Taber, "Measurements of the magnetic penetration depth in YBa₂Cu₃O_7-d thin films by the microstrip resonator technique," Appl. Phys. Lett. 54 , 2710 (1989).

17. B. W. Langley , S. M. Anlage, R. F. W. Pease and M. R. Beasley, "Magnetic Penetration Depth Measurements of Superconducting Thin Films by the Microstrip Resonator Technique," Rev. Sci. Instrum. 62, 1801 (1991).

18. Steven Anlage, Brian Langley, Guy Deutscher, J. Halbritter and M. R. Beasley, "Measurements of the Temperature Dependence of the Magnetic Penetration Depth in YBa₂Cu₃O_7-d Superconducting Thin Films," Phys. Rev. B (Rapid Communications) 44, 9764 (1991).

21. D. H. Wu , J. Mao, S. N. Mao, J. L. Peng, X. X. Xi, T. Venkatesan, R. L. Greene, and Steven M. Anlage, "Temperature Dependence of the Magnetic Penetration Depth and Surface Resistance of Nd_1.85Ce_0.15CuO_4-y Superconducting Thin Films and Single Crystals," Phys. Rev. Lett. 70, 85 (1993).

26. M. Pambianchi, S. M. Anlage, E. S. Hellman, E. H. Hartford, M. Bruns, and S. Y. Lee, "Penetration Depth, Microwave Surface Resistance, and Gap Ratio in NbN and Ba_1-xK_xBiO₃ Thin Films," Appl. Phys. Lett., 64, 244 (1994).

31. Steven M. Anlage, Dong-Ho Wu, Jian Mao, Sining Mao, X. X. Xi, T. Venkatesan, J. L. Peng, and R. L. Greene, "The Electrodynamics of Nd_1.85Ce_0.15CuO_4-y: Comparison with Nb and YBa₂Cu₃O_7-d," Phys. Rev. B 50, 523 (1994).

33. Michael S. Pambianchi, Jian Mao, and Steven M. Anlage, "Magnetic Screening in Proximity-Coupled Superconductor / Normal-Metal Bilayers," Phys. Rev. B 50, 13659 (1994).

35. Jian Mao, D. H. Wu, J. L. Peng, R. L. Greene, and Steven M. Anlage, "Anisotropic Surface Impedance of YBa₂Cu₃O_7-d Single Crystals," Phys. Rev. B (Rapid Communications) 51, 3316 (1995).

41. Michael S. Pambianchi, S. N. Mao, and Steven M. Anlage, "Microwave Surface Impedance of Proximity-Coupled Nb/Al Bilayer Films," Phys. Rev. B 52, 4477 (1995).

46. M. S. Pambianchi, Lie Chen, and Steven M. Anlage, "Complex Conductivity of Proximity-Superconducting Nb /Cu Bilayers," Phys. Rev. B 54, 3508-3513 (1996).

50. M. S. Pambianchi, C. Kwon, T. Venkatesan, and Steven M. Anlage, "Surface Impedance of YBa₂Cu₃O₇ /Y_0.6Pr_0.4Ba₂Cu₃O₇ Bilayers: Possible Evidence for the Proximity Effect," Phys. Rev. B 54, 15 513 (1996).

69. Vladimir V. Talanov, Lucia V. Mercaldo, and Steven M. Anlage, "Measurement of the Absolute Penetration Depth and Surface Resistance of Superconductors using the Variable Spacing Parallel Plate Resonator," IEEE Trans. Appl. Supercond. 9, 2179-2182 (1999) .

75. Vladimir V. Talanov, Lucia V. Mercaldo, Steven M. Anlage, and John H. Claassen "Measurement of the Absolute Penetration Depth and Surface Resistance of Superconductors and Normal Metals with the Variable Spacing Parallel Plate Resonator," Rev. Sci. Instrum. 71, 2136-2147 (2000).

79. J. David Kokales, Patrick Fournier, Lucia V. Mercaldo, Vladimir V. Talanov, Richard L. Greene, and Steven M. Anlage, " Microwave Electrodynamics of Electron-Doped Cuprate Superconductors," Phys. Rev. Lett. 85, 3696-3699 (2000) .

85. K. S. Harshavardhan, H. M. Christen, S. D. Silliman, V. V. Talanov, S. M. Anlage, M. Rajeswari, and J. Claassen, "Low-loss YBa₂Cu₃O₇ films on flexible, polycrystalline-yttria-stabilized zirconia tapes for cryoelectronic applications," Appl. Phys. Lett. 78, 1888-1890 (2001).

161. A. P. Zhuravel, B. G. Ghamsari, C. Kurter, P. Jung, S. Remillard, J. Abrahams, A. V. Lukashenko, A. V. Ustinov, Steven M. Anlage, “Imaging the Anisotropic Nonlinear Meissner Effect in Nodal YBa₂Cu₃O_7-δ Thin-Film Superconductors,” Phys. Rev. Lett. 110, 087002 (2013). pdf