Abstract-Collective modes and terahertz near-field response of superconductors

Zhiyuan Sun, M. M. Fogler, D. N. Basov, and Andrew J. Millis

https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.023413

We theoretically study the low-energy electromagnetic response of Bardeen-Cooper-Schrieffer–type superconductors focusing on propagating collective modes that are observable with terahertz near-field optics. The interesting frequency and momentum range is $\omega <2\Delta$ and $q<1/\xi$, where $\Delta$ is the gap and $\xi$ is the coherence length. We show that it is possible to observe the superfluid plasmons, amplitude (Higgs) modes, Bardasis-Schrieffer modes, and Carlson-Goldman modes using the terahertz near-field technique, although none of these modes couple linearly to far-field radiation. Coupling of terahertz near-field radiation to the amplitude mode requires particle-hole symmetry breaking, while coupling to the Bardasis-Schrieffer mode does not and is typically stronger. For parameters appropriate to layered superconductors of current interest, the Carlson-Goldman mode appears in the near-field reflection coefficient as a weak feature in the subterahertz frequency range. In a system of two superconducting layers with nanometer-scale separation, an acoustic phase mode appears as the antisymmetric density fluctuation mode of the system. This mode produces well-defined resonance peaks in the near-field terahertz response and has strong anticrossings with the Bardasis-Schrieffer and amplitude modes, enhancing their response. In a slab consisting of many layers of quasi-two-dimensional superconductors, realized for example in samples of high-$T_c$ cuprate compounds, many branches of propagating Josephson plasmon modes are found to couple to the terahertz near-field radiation.

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Abstract-Band Structure and Terahertz Optical Conductivity of Transition Metal Oxides: Theory and Application to [Math Processing Error]

Hung T. Dang, Jernej Mravlje, Antoine Georges, and Andrew J. Millis

Phys. Rev. Lett. 115, 107003 – Published 3 September 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.107003

Density functional plus dynamical mean field calculations are used to show that in transition metal oxides, rotational and tilting ([Math Processing Error]-type) distortions of the ideal cubic perovskite structure produce a multiplicity of low-energy optical transitions which affect the conductivity down to frequencies of the order of 1 or 2 mV (terahertz regime), mimicking non-Fermi-liquid effects even in systems with a strictly Fermi-liquid self-energy. For [Math Processing Error], a material whose measured electromagnetic response in the terahertz frequency regime has been interpreted as evidence for non-Fermi-liquid physics, the combination of these band structure effects and a renormalized Fermi-liquid self-energy accounts for the low frequency optical response which had previously been regarded as a signature of exotic physics. Signatures of deviations from Fermi-liquid behavior at higher frequencies ([Math Processing Error]) are discussed.

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