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arXiv:cond-mat/0208334 · Physica C Superconductivity

Spin collective mode and quasiparticle contributions to STM spectra of d-wave superconductors with pinning

Anatoli Polkovnikov, Subir Sachdev A, Matthias Vojta C

Published May 1, 2003• 19 citations
arXivHTMLPDF
DOI
10.1016/s0921-4534(02)02605-9

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QUASIPARTICLES IN SUPERFLUIDS AND SUPERCONDUCTORS

Jonathan B. Curtis

Quasiparticle descriptions are a powerful tool in condensed matter physics as they provide an analytical treatment of interacting systems. In this thesis we will apply this tool to theoretically describe two systems: a superconductor interacting with cavity photons and a flowing Bose-Einstein condensate forming a sonic black hole. First we will consider a two-dimensional s-wave BCS superconductor coupled to microwave cavity photons. We show how a nonequilibrium occupation of the photons can induce a nonequilibrium distribution of superconductor Bogoliubov quasiparticles, yielding an enhancement of the superconducting gap. The analytic dependence of this enhancement is provided in terms of the photon spectral and occupation functions, offering a large parameter space over which enhancement exists. Next, we analyze the equilibrium properties of a similar superconductor-cavity structure which has strong sub-dominant d-wave pairing interaction. In this case there is a collective mode known as the Bardasis-Schrieffer mode, which is essentially an uncondensed d-wave Cooper pair. We show that by driving an external supercurrent through the sample the Bardasis-Schrieffer mode can be hybridized with cavity photons, forming exotic Bardasis-Schrieffer-polaritons. We then turn to consider a flowing Bose-Einstein condensate. In the presence of inhomogeneous flow, the long-wavelength motion of quasiparticles can be mapped onto the kinematics of matter fields in a curved spacetime. This mapping allows for the simulation of a black hole and its interactions with quantum fields via analogy. We show that in the case of a step-like jump in the condensate flow the emission of analogue Hawking radiation is accompanied by evanescent modes which are pinned to the event horizon. Finally, we generalize this setup to include pseudo-spin half spinor Bose condensates. In this case, we show that the analogue spacetime the quasiparticles experience can be of the exotic Newton-Cartan type. Newton-Cartan gravity -- the geometric formulation of Newtonian gravity -- is realized when the Goldstone mode disperses quadratically as opposed to linearly. The nature of the analogue spacetime is controlled by the presence or absence of an easy-axis anisotropy in the boson spin-exchange interaction. We conclude by arguing that this Newton-Cartan spacetime can be experimentally realized in current platforms.

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We present additional details of the scanning tunneling microscopy (STM) spectra predicted by the model of pinning of dynamic spin collective modes in d-wave superconductor proposed by Polkovnikov et al. (Phys. Rev. B 65, 220509 (2002)). Along with modulations at the twice the wavevector of the spin collective mode, the local density of states (LDOS) displays features linked to the spectrum of the Bogoliubov quasiparticles. The former is expected to depend more strongly on an applied magnetic eld or the doping c oncentration. The spin collective mode and the quasiparticles are distinct, co-existing, low energy excitations of the d-wave superconductor (strongly coupled only in some sectors of the Brillouin zone), and should not be viewed as mutually exclusive sources of LDOS modulation.
Motivated by the recent development of terahertz pump-probe experiments, we investigate the short-time dynamics in superconductors with multiple attractive pairing channels. Studying a single-band square lattice model with a spin-spin interaction as an example, we find the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. We show that the polarization and intensity of the pulse can be used to control the symmetry of the nonequilibrium state as well as frequencies and relative intensities of the contributions of different collective modes. We find particularly strong signatures of the Bardasis-Schrieffer mode in the dynamics of the quasiparticle distribution function. Our work shows the potential of modern ultrafast experiments to address the collective excitations in unconventional superconductors and highlights the importance of subdominant interactions for the nonequilibrium dynamics in these systems.
Quasiparticle descriptions are a powerful tool in condensed matter physics as they provide an analytical treatment of interacting systems. In this thesis we will apply this tool to theoretically describe two systems: a superconductor interacting with cavity photons and a flowing Bose-Einstein condensate forming a sonic black hole. First we will consider a two-dimensional s-wave BCS superconductor coupled to microwave cavity photons. We show how a nonequilibrium occupation of the photons can induce a nonequilibrium distribution of superconductor Bogoliubov quasiparticles, yielding an enhancement of the superconducting gap. The analytic dependence of this enhancement is provided in terms of the photon spectral and occupation functions, offering a large parameter space over which enhancement exists. Next, we analyze the equilibrium properties of a similar superconductor-cavity structure which has strong sub-dominant d-wave pairing interaction. In this case there is a collective mode known as the Bardasis-Schrieffer mode, which is essentially an uncondensed d-wave Cooper pair. We show that by driving an external supercurrent through the sample the Bardasis-Schrieffer mode can be hybridized with cavity photons, forming exotic Bardasis-Schrieffer-polaritons. We then turn to consider a flowing Bose-Einstein condensate. In the presence of inhomogeneous flow, the long-wavelength motion of quasiparticles can be mapped onto the kinematics of matter fields in a curved spacetime. This mapping allows for the simulation of a black hole and its interactions with quantum fields via analogy. We show that in the case of a step-like jump in the condensate flow the emission of analogue Hawking radiation is accompanied by evanescent modes which are pinned to the event horizon. Finally, we generalize this setup to include pseudo-spin half spinor Bose condensates. In this case, we show that the analogue spacetime the quasiparticles experience can be of the exotic Newton-Cartan type. Newton-Cartan gravity -- the geometric formulation of Newtonian gravity -- is realized when the Goldstone mode disperses quadratically as opposed to linearly. The nature of the analogue spacetime is controlled by the presence or absence of an easy-axis anisotropy in the boson spin-exchange interaction. We conclude by arguing that this Newton-Cartan spacetime can be experimentally realized in current platforms.