Session S08: Superconductivity in EM fields

Debye mechanism of giant microwave absorption in superconductors

We present a theory of microwave absorption in conventional superconductors and show that in the presence of a superfluid velocity V_s the microwave absorption coefficient can be orders of magnitude larger than at V_s = 0. The reason for this is that the V_s-dependent contribution is proportional to the inelastic scattering time rather than the elastic scattering time. The mechanism responsible for this larger contribution is similar to the Debye mechanism for microwave absorption in molecules. Further, the V_s dependance of the absorption coefficient is non-analytical, and measurement of this contribution to the absorption coefficient will give direct information about the inelastic scattering time in superconductors which in general has been difficult to directly measure. In the absence of the superfluid velocity we identify a new mechanism of non-linear conductivity with anomalously low threshold.   

A nonlinear THz study of strong light-matter coupling between plasmonic metamaterials and a superconducting Josephson Plasmon in La2-xSrxCuO4.

The Josephson plasma resonance (JPR) in layered high temperature superconducting cuprates provides a useful probe of the superconducting condensate, as well as an avenue to couple incident electromagnetic fields to the condensate. We enhance this light-matter coupling by applying interchangeable metamaterial tapes to a c-axis single crystal of La_2-xSr_xCuO₄ and perform linear THz time domain spectroscopy in reflection in order to identify anti-crossing behavior characteristic of strong coupling. We then drive the hybrid superconducting metamaterial system with large single cycle THz fields in excess of 100kV/cm in order to explore emergent nonlinear behavior of the coupled system. This work informs future possibilities of utilizing strong coupling effects in conjunction with large transient fields to achieve coherent control of a superconducting condensate and an avenue towards light enhanced superconductivity.   

Cavity Quantum Enhancement of Superconductivity

Standard superconductors are known to exhibit a number of fascinating and potentially useful phenomena when driven away from thermal equilibrium by coherent classical electromagnetic radiation. These non-equilibrium effects can often lead to an enhancement in the strength of the superconducting gap and its manifestations. We aim to understand what happens when the classical electromagnetic field is replaced by a fluctuating quantum electromagnetic field, as may be found in a microwave cavity resonator. We show that in the lossy regime non-equilibrium cavity photons can rid the superconductor of deleterious quasiparticles, thereby enhancing the superconducting gap strength. This proposed quantum enhancement of superconductivity opens the door to the study of more exotic phenomena which may arise when superconductors are subjected to cavity quantum electrodynamic environments.   

Modulating Superconductivity with Metamaterial Plasmonic Structures – Theory

In 1972, Kirzhnits et al. reformulated the BCS superconductivity theory in terms of a dielectric response function¹. The conclusion of this theory was that the strength of the Cooper pairings can be controlled by the dielectric environment. A demonstration of this effect was recently given in a metamaterial composite made of Al nanoparticles², designed to have suppressed dielectric function at the Eliashberg function maximum, leading to threefold increase of superconductivity critical temperature. In this work, we have studied theoretically, two-dimensional Babinet metamaterials made of Pb, properly designed to have the Cooper pairing strength systematically controlled (enhanced or suppressed). This work, has been coupled to the parallel effort to obtain experimental confirmation of these effects.

¹A. Kirzhnits et al, J. Low Temp. Phys., 10, 79-93 (1973).
²V. N. Smolyaninova et al., Sci. Rep., 4, 7321 (2015).   

Modulating Superconductivity with Metamaterial Plasmonic Structures – Experiment.

In 1972, Kirzhnits et al. reformulated the BCS superconductivity theory in terms of a dielectric response function,¹ concluding that the strength of Cooper pairing can be controlled by the dielectric environment. A demonstration of this effect was recently given in a metamaterial composite made of Al nanoparticles², designed to have suppressed dielectric function at the Eliashberg function maximum, leading to threefold increase of superconductivity critical temperature. We report fabrication and experimental measurements of thin film Babinet metamaterials made of Pb, designed to have the Cooper pairing strength systematically controlled (enhanced or suppressed). This work is coupled to a parallel effort to determine these effects theoretically.
¹A. Kirzhnits et al., J. Low Temp. Phys. 10, 79-93 (1973).
²V. N. Smolyaninova et al., Sci. Rep. 4, 7321(2015).   

Strong Plasmonic Enhancement of Photoelectric Quantum Efficiency of Nb using In Nano islands

The development of superconducting photocathodes is presented which explores thin film coatings to enhance the quantum efficiency (QE) of superconducting Nb above its bulk value of < 10^-6. Deposition of a 10nm layer of Mg (work function = 3.6 eV) onto Nb after UHV anneal increases QE by a factor of 10. Deposition of ultra-thin islands of In (4 nm) on top of Nb/Nb oxide or Nb/Mg/Mg oxide leads to overall enhancements of QE by up to 400 times. We attribute this latter enhancement to plasmonic effects where the stored EM fields in the In islands couple to Nb electrons   

Theory of coherent plasmon in one dimensional insulators

Recent microwave reflection measurements of Josephson junction ladders have suggested the presence of nearly coherent collective charge oscillations deep in the
insulating phase. Here we develop a qualitative understanding of such coherent charge modes by studying the local dynamical charge susceptibility of the insulating phase of the
Sine-Gordon model. By considering parameters near the non-interacting Fermion
limit where the charge operator dominantly couples to soliton-antisoliton bound states of the Sine-Gordon model, we find that the local charge susceptibility shows an array of sharp peaks
in frequency representing coherent plasma oscillations on top of an incoherent background. The strength of the coherent peaks relative to the incoherent background
increases as a powerlaw in frequency whose exponents depend on system parameters. The charge susceptibility also clearly shows the insulating gap. We then compare the controlled results in the high frequency limit to phase-slip-induced decay of
plasmons in the Josephson junction ladder.   

Application of metamaterial nano-engineering for increasing the superconducting critical temperature

In previous work, we demonstrated that the metamaterial approach to dielectric response engineering increases the critical temperature of a composite superconductor-dielectric system in the epsilon near zero (ENZ) and hyperbolic regimes. To create such metamaterial superconductors three approaches were implemented: 1) mixtures of tin and barium titanate nanoparticles of varying composition and tin and strontium titanate nanoparticles, 2) composite Al₂O₃-coated aluminium nanoparticles, and 3) thin Al/Al₂O₃heterostructures that form a hyperbolic metamaterial superconductor. IR reflectivity measurements confirmed the predicted metamaterial modification of the dielectric function thus demonstrating the efficacy of the metamaterial approach to T_c engineering. In this talk, we present specific heat data on samples of the composite Al₂O₃-coated aluminium nanoparticles showing that the normal state density of states (DOS) is similar to that of pure aluminum, thus precluding the DOS from being responsible for the observed enahanced T_c. We will also discuss other features in the specific heat results that are consistent with results of neutron scattering experiments.   

Observation of plasmon-phonons in a metamaterial superconductor using inelastic neutron scattering

Recent experiments have demonstrated that the metamaterial approach is capable of drastically increasing the critical temperature, T_c, of composite metal-dielectric epsilon near zero (ENZ) metamaterial superconductors. For example, a tripling of the critical temperature was observed in bulk Al-Al₂O₃ ENZ core-shell metamaterials. A theoretical model based on the Maxwell-Garnett effective medium approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation in the composite metal-dielectric metamaterial. We report the first observation of a hybrid plasmon-phonon excitation in Al-Al₂O₃ ENZ core-shell metamaterials using inelastic neutron scattering. These results provide strong support for this novel mechanism of superconductivity in ENZ metamaterials and provide an explanation for the 50 year old mystery of enhanced T_c in granular aluminum thin films.   

Interference experiments with superconducting microwave beam splitter

Superconducting quantum circuit is one of the most promising way for realization of quantum systems. Quite interesting effects in superconducting quantum systems can be observed involving single photons interference[1]. To conduct these type of experiments, different devices like single-photon sources and an element for entanglement generation are required. The most natural realization for such element is a beam splitter [2]. In microwave range, it is convenient to use a hybrid beam splitter [1]. It is natural to use a beam splitter on-chip for less insertion losses. In the talk, the investigation of this kind of beam splitter with proposed interference experiments will be reported.

[1] C. Lang, C. Eichler, L. Steffen, J.M. Fink, M.J. Woolley, A. Blais, and A. Wallraff. Correlations, indistinguishability and entanglement in Hong–Ou–Mandel experiments at microwave frequencies. Nature Physics, 9(6):345, 2013.
[2] Gregor Weihs and Zeilinger. Photon statistics at beam-splitters: an essential tool in quantum information and teleportation. Coherence and Statistics of Photons and Atoms, pages 262–288, 2001.
  

Transient optical response of correlated electrons

Recent discoveries of non-equilibrium higher order harmonic response of the superconducting condensate in cuprates, enhancement of the transient superconductivity in organic superconductor K₃C₆₀ and signature of the Higgs mode in BCS superconductor has attracted much attention. To understand the underlying physics behind such collective response of correlated electrons to electromagnetic field requires an understanding of response functions in the time domain. In our work, we calculate transient optical conductivity using a non-equilibrium Green’s function method. We study a phonon mediated superconducting state. In the equilibrium state, we observe the normal superconducting gap in the real part of conductivity, and the 1/ω divergence in the imaginary part. In a pump driven non-equilibrium state we observe suppression and the oscillatory recovery of the order parameter - the Higgs amplitude mode. We use different quantifiers – probe current, phonon-valley, coherence peak and the gap edge – to study the Higgs mode and the transient superconducting state. Our calculation helps in the understanding of transient superconducting state and provide a general framework to analyze the transient response of correlated electrons.   

Manifestation of vibronic dynamics in infrared spectra of Mott insulating fullerides

In order to probe the nature of Jahn-Teller dynamics in alkali-doped fullerides, the fine structure and temperature evolution of infrared spectra have been intensively used last years [1,2]. At the same time, theoretical framework to adequately extract the information on the complicated vibronic dynamics from infrared spectra is still lacking. In this work, the first-principles theory of the infrared spectra of dynamical Jahn-Teller system is developed and applied to the Mott-insulating Cs₃C₆₀ [3]. With the calculated coupling parameters for Jahn-Teller and infrared active vibrational modes, the manifestation of the dynamical Jahn-Teller effect in infrared spectra is elucidated. In particular, the temperature evolution of the infrared line shape is explained. The transformation of the latter into Fano resonance type in metallic fulleride [2] is discussed on the basis of obtained results.

[1] G. Klupp et al., Nat. Commun. 3, 912 (2012).
[2] R. H. Zadik et al., Sci. Adv. 1, e1500059 (2015).
[3] Y. Matsuda, N. Iwahara, K. Tanigaki, and L. F. Chibotaru, Phys. Rev. B 98, 165410 (2018).