Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K05: Optically Driven Correlated Electron Systems: TheoryInvited
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Sponsoring Units: DCMP Room: LACC 152 |
Wednesday, March 7, 2018 8:00AM - 8:36AM |
K05.00001: Properties of transient superfluids Invited Speaker: Aditi Mitra While properties of equilibrium superfluids are well understood, recent pump probe experiments make it imperative to understand transport in a highly non-equilibrium setting. We first address the following question: If fermions are initially in a normal phase, and the interactions are quenched so that the system is near a superconducting critical point, what are the transport properties in the transient regime? Through a quantum kinetic equation that accounts for scattering off of the temporally growing critical fluctuations, we show the appearance of aging physics in the optical conductivity, and the gradual appearance of a highly conducting channel indicated by the growth in time of the |
Wednesday, March 7, 2018 8:36AM - 9:12AM |
K05.00002: Electronic Squeezing of Pumped Phonons: Negative U and Transient Superconductivity Invited Speaker: Dante Kennes Advances in light sources and time resolved spectroscopy have made it possible to excite specific atomic vibrations in solids and to observe the resulting changes in electronic properties but the mechanism by which phonon excitation causes qualitative changes in electronic properties, is still under debate. Here, we show that the dominant symmetry-allowed coupling between electron density and dipole active modes implies an electron density-dependent squeezing of the phonon state which provides an attractive contribution to the electron-electron interaction, independent of the sign of the bare electron-phonon coupling and with a magnitude proportional to the degree of laser-induced phonon excitation. Reasonable excitation amplitudes lead to non-negligible attractive interactions that may cause significant transient changes in electronic properties including superconductivity. The mechanism is generically applicable to a wide range of systems, offering a promising route to manipulating and controlling electronic phase behavior in novel materials. Building on these results we analyze the non-equilbrium response of the electronic system and discuss implications for experimentally accessible observables, such as optical conductivity. |
Wednesday, March 7, 2018 9:12AM - 9:48AM |
K05.00003: Theory of pump-probe spectroscopy: Ultrafast laser engineering of ordered phases and microscopic couplings Invited Speaker: Michael Sentef Intense femtosecond laser pulses, spanning a large range of photon energies from the X-ray to the THz regime, allow for controlled excitations ("pump") and monitoring ("probe") of the nonequilibrium dynamics of all the relevant microscopic degrees of freedom in solids. The field of ultrafast materials science is currently evolving from measuring time constants - for instance for the decay of hot electrons via phonon emission - towards ultrafast laser engineering of nonthermal phases of matter with novel properties. Notable examples include light-induced superconducting-like behavior [1], ultrafast switching to hidden ordered states [2], or time-reversal symmetry-broken Floquet states in topological insulators [3]. |
Wednesday, March 7, 2018 9:48AM - 10:24AM |
K05.00004: Nonequilibrium Materials Design of Frustrated Mott Insulators Invited Speaker: Martin Claassen Spurred by recent progress in melting, enhancement and induction of electronic order out of equilibrium, a promising prospect concerns accessing transient steady states of periodically-driven quantum systems, to affect electronic properties. In this talk, I will discuss how light can provide a handle to dynamically break symmetries in a class of frustrated Mott insulators with competing orders and tune the underlying magnetic interactions. In a Mott antiferromagnet, optical pumping below the charge gap can drive the system into Floquet steady states of a transient effective spin Hamiltonian, whereas heating remains suppressed for sufficiently short pulse widths. On the Kagome lattice, circularly-polarized light dynamically breaks parity and time-reversal symmetry and transiently induces a staggered scalar spin chirality contribution to the effective spin dynamics, revealing a stable photo-induced chiral spin liquid - the elusive ν=1/2 fractional quantum Hall effect of spins - in proximity to the equilibrium state. I will then discuss how transient manipulation of Mott insulators could serve as a tool to investigate dynamics of frustrated spin systems in pump-probe experiments. |
Wednesday, March 7, 2018 10:24AM - 11:00AM |
K05.00005: Non-equilibrium steady states and and transient dynamics of correlated electron systems Invited Speaker: Martin Eckstein Femtosecond laser technology has opened the possibility to probe and control the dynamics of complex condensed matter phases on ultra-short timescales. Non-equilibrium Green's functions and non-equilibrium dynamical mean-field theory [1] provide a versatile approach to study those out of equilibrium states in correlated matter from a microscopic perspective. In this talk, I will focus on the fundamental question how long-range electronic order, such as magnetic and orbital order or superconductivity, can be manipulated, enhanced, or even induced out of equilibrium. We will show that in excitonic condensates, which are similar to BCS superconductors, the condensate fraction can be enhanced in an electronically excited non-equilibrium state, even when the energy of the state corresponds to temperatures above the critical temperature. Furthermore, we study the steady state regime, in which the action of the laser on the electrons, or the driving of phonons which couple to the electrons, can be understood in terms of a Floquet-Hamiltonian with modified interactions. A Floquet Green's function approach allows us to address the steady state in which driving is balanced by dissipation [2], and to see to what extent nontrivial phases of the Floquet Hamiltonian such as superconductivity can be realized. |
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