Thermodynamics and Transport in Iron-based superconductors

Maxim G. Vavilov, University of Wisconsin-Madison, DMR 0955500

Recent discovery of novel iron-pnictide superconductors has stimulated massive theoretical and experimental research aimed at unveiling their unique properties. This research has high potential for applications in superconducting technologies.We developed a theoretical model of iron-pnictides and applied this model to•Analyze the effect of disorder on the phase diagram and specific heat (top figure).1 In particular, our model explains why in some samples the disorder reduces the temperature for transition to a spin-density wave (SDW) phase but increases the temperature of the transition to a superconducting (SC) phase.•Evaluate the magnetic penetration depth, in the coexistence phase of the spin-density wave and superconductivity (bottom figure).2 We provided a quantitative description for a monotonic increase of the magnetic penetration depth as doping drives the system from a pure SC towards a pure SDW.

References: 1aVavilov, Chubukov, PRB 81, 174538 (2010); 1bFernandes, Vavilov, Chubukov, PRB 85, 140512(R) (2012). 2Kuzmanovski, Vavilov, Supercond. Sci. & Tech. 25 084001 (2012).

Phase diagram (temperature vs. doping) in clean (dashed) and disordered (solid) materials, from Ref. 1b.

Dependence of the superfluid density, ~-2, on the temperature of SC transition, Tc, from Ref. 2. Both Tc anddecrease as doping decreases. Solid and dashed lines represent different values of due to anisotropy.

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Temperature, Tc/Tc0

Other research projects in the group:

•We described a system of quantum dots interacting with microwave radiation in a high-quality resonator. In particular, the radiation may cause electric current to flow through a double quantum dot, the effect is known as photovoltaic effect [to be published with C. Xu]; see figure, three curves represent evolution of the current with decoherence rate, changing from small (blue) to large (red).

•We analyzed interaction of quantum dots with metallic gates. The gates are sources of evanescent wave Johnson noise that may significantly suppress the coherence time of quantum dot systems.3

•We investigated dynamics of a realistic quantum system (current biased Josephson junction) driven by external perturbation to characterize the Berry curvature of the system. The goal is to find optimal conditions for experimental observation of the Berry curvature [with Poudel and Glaudell].

References: 3Langsjoen, Poudel, Vavilov, Joynt, PRA 86, 010301 (2012).

Thermodynamics and Dynamics of Superconducting Devices

Maxim G. Vavilov, University of Wisconsin-Madison, DMR 0955500

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Microwave frequency detuning, GHz

Currently, two undergraduate and three PhD students are involved in projects supported by DMR award 0955500.

Outreach Activity highlights:•Set an activity table during Physics Fair about superconductivity and superconducting devices;•Undergraduate colloquium “Quantum Superconducting Circuits.”