Session S49: Exotic Collective Excitations and Topology-Driven Dynamics in Quantum Materials

Live

We study non-local hydrodynamic response of electron and combined electron-phonon fluids. This hydrodynamic regime is governed by infinite set of kinetic modes that describe scattering events in different angular harmonic channels. The scattering rates of these modes leads to anomalous, Lévy-flight-like phase space diffusion and allows us to obtain expressions for the non-local heat and charge conductivities. We discuss Dirac, Fermi, and electron-phonon fluids, including the crossover from hydrodynamic to ballistic transport.   

Live

In the last few years, a number of experiments carried out with very intense THz fields, either in transmission or in pump-probe configuration, have shown the possibility to excite superconducting collective modes, like amplitude (Higgs) and phase (Goldstone) excitation of the order parameter, via light pulses. The signatures of the superconducting excitations manifest either in third-harmonic generation, in the case of narrowband pulses, or in well-defined oscillations of the transmitted field as a function of the pump-probe delay, in the case of broadband pulses. In both cases, one is essentially measuring the properties of the non-linear optical kernel [1,2]. Besides collective electronic excitations, in the superconducting state also the BCS response, connected to the continuum of quasiparticle excitations, can lead to pronounced resonances. How to distinguish and characterise the different possible processes has been the subject of an intense experimental and theoretical research. In this talk I will review the current state of the art, and I will discuss some recent results on the role of disorder[3] and of the layered structure[4], as the one of cuprates, for what concerns the excitations of superconducting phase modes.

[1] F. Giorgianni, T. Cea, C. Vicario, C. P. Hauri, W. K. Withanage, X. Xi, and L. Benfatto, Nat. Phys. 15, 341 (2019).
[2] M. Udina, T. Cea and L. Benfatto, Phys. Rev. B 100, 165131 (2019)
[3] G. Seibold, M. Udina, C. Castellani and L. Benfatto,arXiv:2010.12507
[3] F. Gabriele, M. Udina and L. Benfatto, arXiv:2006.02296.   

Live

I will show that two-particle correlation functions in 2D Fermi liquids have a nontrivial topological structure. The structure manifests itself in unconventional zero-sound collective modes: “hidden” and “mirage” modes. A hidden mode resides outside the particle-hole continuum already for attractive interaction. It does not appear as a sharp peak in the dynamical susceptibility χ(q,ω), yet determines the long-time transient response of a Fermi liquid. A mirage mode emerges for strong enough repulsion. Unlike the conventional zero sound, it does not correspond to a pole of χ(q,ω), yet gives rise to a peak in the particle-hole susceptibility. I will discuss how these features are associated with the existence of a two-sheet Riemann surface defined by χ(q,ω). The hidden modes reside below the branch cut gluing the sheets, and the mirage modes reside on an unphysical sheet of the Riemann surface.   

Live

Motivated by the recent development of terahertz pump-probe experiments, we investigate the short-time dynamics in superconductors with multiple attractive pairing channels and competing nematic order. Studying a single-band and multiband superconductors, 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 non-equilibrium 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. In the rotationally symmetric state, we show that the Bardasis-Schrieffer mode, corresponding to the subdominant pairing, hybridizes with the nematic collective mode and merges into a single in-gap mode, with the mixing vanishing only close to the phase boundaries. For the d-wave ground state, we find that nematic interaction suppresses the damping of the collective oscillations in the short-time dynamics. Additionally, we find that even inside the nematic s+d-wave superconducting state, a Bardasis-Schrieffer-like mode leads to order parameter oscillations that strongly depend on the competition between the two pairing symmetries. We discuss the connection of our results to the recent pump-probe experiments on high-Tc superconductors.
[1] Marvin A. Mueller, Pengtao Shen, Maxim Dzero, Ilya M. Eremin, Phys. Rev. B 98 024522 (2018)
[2] Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin,Phys. Rev. B 100 , 140501(R) (2019)
[2] Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, arXiv:2011.01042   

Live

Plasmon modes in moire graphene, as a result of strong interactions, can pierce through the particle-hole continuum of the flat band, reimerge above it and acquire strong over-the-band character [1]. We will discuss various implications of this interesting behavior. One is the opportunity to realize plasmons that are not subject to Landau damping. Eliminating damping is central for the ongoing quest for low-loss plasmons and dissipationless light–matter coupling. These modes feature enhanced optical coherence and spatial interference, directly testable by state-of-the-art near-field techniques. Another implication is the appearance of novel optical plasmon excitations [2]. These modes are uniquely sensitive to the correlated insulating order in a flat band, becoming strongly dipole-active and developing a gap within the correlated insulator gap. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical and microwave measurements, offering a convenient diagnostic of the correlated states. Lastly, we will consider superconductivity enabled by these excitations [3]. The new pairing mechanism relies on the high-energy degrees of freedom, the far-out pairs in the empty band positioned above the conduction band, and is fully repulsion-dominated.

1. Cyprian K Lewandowski and LL, PNAS 116, 20869 (2019)
2. Ali Fahimniya, Cyprian K Lewandowski and LL, arXiv:2011.02982
3. Zhiyu Dong and LL, to be published