Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L19: Ultrafast Laser-Based Probes of Quantum MaterialsInvited
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Sponsoring Units: DLS Room: 207 |
Wednesday, March 4, 2020 8:00AM - 8:36AM |
L19.00001: Light induced phase transitions in charge density waves Invited Speaker: Nuh Gedik Upon excitation with an intense laser pulse, materials can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we studied light induced phase transitions in two different charge density wave (CDW) systems. First, we investigated the photo-induced melting of a unidirectional CDW in LaTe3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence dictated by the presence of topological defects in CDW. Furthermore, after the suppression of the original CDW by photoexcitation, a different, competing CDW along the perpendicular direction emerges. The timescales characterizing the relaxation of this new transient CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. Secondly, I will also report the realization of optical chiral induction and the observation of a gyrotropically ordered CDW phase in 1T -TiSe2. Our results provide a framework for understanding other photoinduced phase transitions and for unleashing novel states of matter that are “trapped” under equilibrium conditions. |
Wednesday, March 4, 2020 8:36AM - 9:12AM |
L19.00002: Quantum Beats by Lightwave Acceleration of Broken-Symmetry Supercurrents Invited Speaker: Jigang Wang Light-induced supercurrents chart a path forward for the electromagnetic design of emergent materials phases and collective modes for quantum engineering applications. In this talk, I will discuss our recent progress towards applying this new tuning knob, enabled by using single- and few-cycle THz pulses, to reveal some distinguishing features of quantum systems: Anderson pseudo-spin precessions and higher harmonics forbidden by equilibrium symmetry, hidden emergent phases that are difficult to be accessed by other tuning methods, Higgs modes in unconventional superconductors. We will also discuss how to extend THz light-driven coherence and subcycle symmetry breaking for quantum control of other complex systems including topological matter, with implications on quantum gate and sensing applications. |
Wednesday, March 4, 2020 9:12AM - 9:48AM |
L19.00003: Local Symmetry Breaking and Spin Momentum Locking in Cuprates Superconductors Invited Speaker: Alessandra Lanzara The 20th century has been dominated by the realization that symmetry and symmetry breaking are keys to much of the novel phenomena observed in physics today. Superconductivity for example requires both time-reversal and inversion symmetry, and the removal of one of these (e.g. time reversal through a magnetic field) leads to the suppression of the superconducting order. Recently however it has been realized that, even if the global symmetry of the system is retained, a local symmetry breaking can still induce a variety of novel fascinating behaviors. |
Wednesday, March 4, 2020 9:48AM - 10:24AM |
L19.00004: Femtosecond Covariance Spectroscopy to Control Multimode Quantum Correlations Invited Speaker: Daniele Fausti The vast majority of nonlinear optical spectroscopies work in an integrated mode, namely they use the mean-value signal, properly averaged over several stroboscopic repetitions. While ensuring an adequate signal-to-noise ratio, this approach relies strongly on pulse-to-pulse consistency and has thus motivated significant efforts in pursuit of perfect experimental stability. By contrast, we have developed a fundamentally different approach, named Femtosecond Covariance Spectroscopy (FCS), which identifies noise as a powerful and unique asset to access information that standard mean-value experiments miss. FCS is based on covariance rather than mean-value observables and relies on the study of multimode quantum correlations imprinted on stochastic ultrashort pulses by light-matter interactions. As a proof of principle, we have successfully applied such approach to the study of Raman-active vibrational modes excited through Impulsive Stimulated Raman Scattering (ISRS) in crystalline quartz. Nevertheless, the impact of FCS is not only limited to the field of condensed matter physics. Indeed, given the formal analogy between the quantum description of ISRS and optomechanical experiments, FCS could pave the way to a new generation of experiments in which the coupling between the electromagnetic field and the mechanical oscillator, could be reproduced by the interaction between light pulses and phonon modes. |
Wednesday, March 4, 2020 10:24AM - 11:00AM |
L19.00005: The order-disorder phase transition in VO2 revealed by an x-ray free electron laser Invited Speaker: Mariano Trigo Femtosecond laser pulses provide a promising strategy to control quantum phases of matter with potential to realize “properties on demand” [1]. While laser pulses can distort the lattice into novel structures with exotic properties not accessible in equilibrium, probing those transient structures remains a challenge. X-ray Free Electron Lasers (XFELs) hold promise to visualize the atomic and electronic structure of these transient states as they transform. I will present XFEL results on the ultrafast monoclinic (M 1 ) to rutile (R) transition of VO 2 . We used femtosecond x-ray diffuse scattering [2,3] at the Linac Coherent light Source (LCLS) at SLAC to study the structural dynamics of the transition at all length-scales. Contrary to the displacive transitions common among charge density waves (CDWs) where the dynamics occur in the vicinity of the CDW wavevector, the M 1 to R transformation in VO 2 proceeds by an ultrafast increase in the x-ray diffuse intensity spanning the Brillouin zone. The intensity momentum distribution shows that photoexcitation induces a prompt uncorrelated disordering of the vanadium dimers of the M 1 phase that reach a rutile quasi-equilibrium distribution in ~ 150 fs. Our results suggest that the pump induces a highly anharmonic potential that enables fast disordering of the lattice, which is intimately related with the high phonon entropy of the R phase. |
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