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arXiv:1712.03598 · Science Advances

Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy

Fabio Novelli, Jonathan O. Tollerud, D. Prabhakaran, Jeffrey A. Davis

Published Feb 28, 2020• 15 citations
arXivHTMLPDF

Abstract

Quantum materials displaying intriguing magnetic and electronic properties could be key to the development of future technologies. However, it is poorly understood how the macroscopic behavior emerges in complex materials with strong electronic correlations. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 to 60 meV. These results reveal that the states in this energy range are correlated with the optically excited states at ~1.5 eV and point to nontrivial interactions between quantum many-body states on the different energy scales. In revealing these dynamics and correlations, we demonstrate that multidimensional coherent spectroscopy can interrogate complex quantum materials in unprecedented ways.

DOI
10.1126/sciadv.aaw9932

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Quantum materials displaying intriguing magnetic and electronic properties could be key to the development of future technologies. However, it is poorly understood how the macroscopic behavior emerges in complex materials with strong electronic correlations. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 to 60 meV. These results reveal that the states in this energy range are correlated with the optically excited states at ~1.5 eV and point to nontrivial interactions between quantum many-body states on the different energy scales. In revealing these dynamics and correlations, we demonstrate that multidimensional coherent spectroscopy can interrogate complex quantum materials in unprecedented ways.
Abstract Strongly correlated solids are complex and fascinating quantum systems, where new electronic states continue to emerge, especially when interaction with light triggers interplay between them. In this interplay, a sub-laser-cycle electronic response is particularly attractive as a tool for the ultrafast manipulation of matter at the petahertz scale. Here we introduce a new type of nonlinear multidimensional spectroscopy, which allows us to unravel charge and energy flows in strongly correlated systems interacting with few-cycle infrared pulses and the complex interplay between different correlated states evolving on the sub-femtosecond timescale. We demonstrate that the sub-cycle spectroscopy of a single-particle electronic response is extremely sensitive to correlated many-body dynamics and provides direct access to many-body response functions. For the two-dimensional Hubbard model under the influence of ultrashort, intense electric-field transients, we resolve the sub-femtosecond pathways of charge and energy flows between localized and delocalized many-body states and the creation of a highly correlated state surviving after the end of the laser pulse. Our findings open the way towards a regime of imaging and manipulating strongly correlated materials at optical rates, beyond the multicycle approach employed in Floquet engineering, with the sub-cycle response being a key tool for accessing many-body phenomena.
The primary consideration in developing new material platforms for quantum applications is to optimize coherence. Despite its importance, decoherence processes remain challenging to experimentally interrogate and quantify. In this Perspective, we first introduce the concept of decoherence in quantum systems and conventional techniques to assess decoherence at optical frequencies. We then introduce multidimensional coherent spectroscopy as a unique probe capable of revealing the full complexity of decoherence dynamics in realistic circumstances. To contextualize the techniques discussed here, demonstrative examples in two prototypical quantum systems, namely colloidal nanocrystals and vacancy centers in diamond, are provided.
Multidimensional coherent spectroscopy (MDCS) has been established in quantum chemistry as a powerful tool for studying the nonlinear response and nonequilibrium dynamics of molecular systems. More recently, the technique has also been applied to correlated electron materials, where the interplay of localized and itinerant states makes the interpretation of the spectra more challenging. Here we use the Keldysh contour representation of effective models and nonequilibrium dynamical mean field theory to systematically study the MDCS signals of prototypical correlated lattice systems. By analyzing the current induced by sequences of ultrashort laser pulses we demonstrate the usefulness of MDCS as a diagnostic tool for excitation pathways and coherent processes in correlated solids. We also show that this technique allows to extract detailed information on the nature and evolution of photo-excited nonequilibrium states.
Abstract Optical multidimensional coherent spectroscopy provides information about the structure and excitation dynamics in a broad range of material systems. This book focuses on systems of interest in physics, including atomic vapors, semiconductor nanostructures and two-dimensional materials. An introduction to ultrafast spectroscopy and the fundamental concepts of multidimensional coherent spectroscopy applies to all material systems. Relevant experimental implementations, including phase-stabilized crossed-beams, frequency-tagged fully-collinear, three-dimensional and frequency-comb based methods. The use of the method to study atomic vapors, semiconductor quantum wells, semiconductor quantum dots and two-dimensional materials are described in separate chapters. The final chapters survey the use of the method in some other areas and speculate on future directions.
The quantitative description of correlated electron materials remains a modern computational challenge. We demonstrate a numerical strategy to simulate correlated materials at the fully ab initio level beyond the solution of effective low-energy models and apply it to gain a detailed microscopic understanding across a family of cuprate superconducting materials in their parent undoped states. We uncover microscopic trends in the electron correlations and reveal the link between the material composition and magnetic energy scales through a many-body picture of excitation processes involving the buffer layers. Our work illustrates a path toward a quantitative and reliable understanding of more complex states of correlated materials at the ab initio many-body level.
Over the past 15 years, researchers have extended the multidimensional techniques which originated with NMR in the 1970s to infrared and visible coherent spectroscopy. These advances have dramatically enhanced the temporal resolution from the microsecond to the femtosecond regime. NMR spectroscopists have developed principles for the design of pulse sequences that enhance selected spectral features and reveal desired dynamical events. Extending these principles to the optical regime offers numerous opportunities for narrowing the line shapes in specific directions, unraveling weak cross-peaks from otherwise congested spectra, and controlling the interferences between quantum pathways. We can achieve these enhancements by shaping the spectral and temporal profiles of the pulses. Pulse polarization shaping may lead to unique probes of time-dependent chirality. In this Account, we compare two types of signals. The first, the photon echo, is generated in the direction -k(1) + k(2) + k(3), and the second, double quantum coherence, is detected at +k(1) + k(2) - k(3). Here k(1), k(2), and k(3) are the wave vectors of the three incoming pulses in chronological order. We illustrate the novel information extracted from these signals by simulations of three physical systems. In the first system, spectra of GaAs semiconductor quantum wells provide a direct look at many-body electron correlation effects. We directly observe specific projections of the many-electron wave function, which we can use to test the quality of various levels of computational techniques for electronic structure. Secondly, the spectra of photosynthetic aggregates reveal couplings between chromophores, quantum coherence signatures of chromophore entanglement, and energy-transfer pathways. Using some fundamental symmetries of pulse polarization configurations of nonlinear signals, we can construct superpositions of signals designed to better distinguish among various coherent and incoherent exciton transport pathways and amplify subtle variations among different species of the Fenna-Matthews-Olson (FMO) antenna complex. Both of the first two applications require femtosecond pulses of light in the visible range. The third application demonstrates how resonant core spectroscopy may be used to generate core excitations that are highly localized at selected atoms. Such signals can monitor the motions of valence electron wavepackets in real space with atomic spatial resolution. These future X-ray applications will require attosecond bright X-ray sources, which are currently being developed in several labs. Common principles underlie these techniques in coherent spectroscopy for spins, valence electrons, and core electronic excitations, spanning frequencies from radiowaves to hard X-rays.