Perspective: Structural dynamics in condensed matter mapped by femtosecond x-ray diffraction

Elsaesser, Thomas; Woerner, M.

doi:10.1063/1.4855115

Cited by 66 publications

(37 citation statements)

References 98 publications

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“…At present, state-of-the-art ultrafast methods offer a near-complete characterization of the transient state 152 . Indeed, tr-ARPES provides a direct probe of the transient electronic structure 66,67,120,153 , tabletop and accelerator-based X-ray free-electron lasers readily capture momentary crystal arrangements 118,154 , nonlinear optical methods can directly interrogate the symmetries of quantum phases and their host lattices 30,[155][156][157][158] , while optical and soft X-ray techniques document electronic excitations, in some cases with elemental and orbital specificity 159 . Importantly, mesoscopic phenomena that are prevalent in quantum materials lead to new time and energy scales, as documented in various ultrafast studies 110,160 .…”

Section: Looking Into the Futurementioning

confidence: 99%

Towards properties on demand in quantum materials

2017

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Q uantum materials are on the ascent. This term embodies a vast portfolio of compounds and phenomena where ramifications of quantum mechanics are demonstrably real. Quantum materials are in the vanguard of contemporary physics in part because these systems afford an exceptional venue to uncover the many roles of symmetry, topology, dimensionality and strong correlations in macroscopic observables. Here we set out to explore the ways and means of creating new states of matter in quantum materials and manipulating their phases via external stimuli. Practical control of these properties is a precondition for exploiting quantum advantages in new photonic, electronic and energy technologies, a task of significant societal impact 1 . We will primarily focus on the following classes of quantum materials: transition metal oxides, Fe-and Cu-based high-T c superconductors, van der Waals semiconductors, topological insulators and Weyl semimetals, and, finally, graphene.The properties of quantum materials are anomalously sensitive to external stimuli. In these systems, interactions associated with spin, charge, lattice and orbital degrees of freedom are commonly on par with the electronic kinetic energy. A rather fragile balance between coexisting and competing ground states can be readily shifted via external stimuli, leading to a raft of quantum phases and transitions between them 2,3 . Furthermore, certain classes of driven quantum states (Fig. 1a and Box 1) are explicit products of coherent interaction between light and matter 4-6 . Alternatively, the properties of quantum materials can be pre-programmed by directly manipulating the electronic wavefunction and the attendant Berry phase that give rise to the anomalous velocity of electrons in a solid [7][8][9] . These complementary avenues of controls mean that investigations no longer need to be reduced to merely observing (in contrast, for example, to astrophysics). Instead, it is now feasible to attain, in a predictable fashion, 'properties on demand' by steering a quantum material towards a desirable ground, metastable or transient state. The past decade has witnessed an explosion in the field of quantum materials, headlined by the predictions and discoveries of novel Landau-symmetry-broken phases in correlated electron systems, topological phases in systems with strong spin-orbit coupling, and ultra-manipulable materials platforms based on two-dimensional van der Waals crystals. Discovering pathways to experimentally realize quantum phases of matter and exert control over their properties is a central goal of modern condensed-matter physics, which holds promise for a new generation of electronic/photonic devices with currently inaccessible and likely unimaginable functionalities. In this Review, we describe emerging strategies for selectively perturbing microscopic interaction parameters, which can be used to transform materials into a desired quantum state. Particular emphasis will be placed on recent successes to tailor electronic interaction parameters through th...

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Section: Looking Into the Futurementioning

confidence: 99%

Towards properties on demand in quantum materials

2017

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“…The frequency-resolved spectra of scattered x-rays further delineate electron from ion diagnostics (inelastic and elastic scattering), providing temperatures, densities, and ionization states. Perhaps the most novel aspect of the XRTS diagnostic is its high femtosecond time resolution, providing key insight into transient structure changes underlying physical and chemical processes [7]. Such ultrashort x-ray pulses can probe structural dynamics during phase transitions or chemical reactions.…”

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confidence: 99%

Evidence for out-of-equilibrium states in warm dense matter probed by x-ray Thomson scattering

et al. 2015

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A recent and unexpected discrepancy between ab initio simulations and the interpretation of a laser shock experiment on aluminum, probed by X-ray Thomson scattering (XRTS), is addressed.The ion-ion structure factor deduced from the XRTS elastic peak (ion feature) is only compatible with a strongly coupled out-of-equilibrium state. Orbital free molecular dynamics simulations with ions colder than the electrons are employed to interpret the experiment. The relevance of decoupled temperatures for ions and electrons is discussed. The possibility that it mimics a transient, or metastable, out-of-equilibrium state after melting is also suggested.

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“…The notion of heterodyne detection used to interpret the TRXS signal [12] was recently challenged by Mukamel and co-workers in a brief comment [14]. Note that the concept of heterodyne detection has been used to interpret the TRXS signal from an electronically excited solid [13,15], but there is no consistent quantum theory of TRXS from a nonstationary solid showing that the heterodyne detection is feasible. So why does the concept of heterodyne detection come under debate in the case of photoexcited gas-phase molecules while it is assumed to work well in the case of photoexcited crystal, as has been used by Elsaesser and co-workers for a long time [13,15]?…”

Section: Introductionmentioning

confidence: 99%

“…Note that the concept of heterodyne detection has been used to interpret the TRXS signal from an electronically excited solid [13,15], but there is no consistent quantum theory of TRXS from a nonstationary solid showing that the heterodyne detection is feasible. So why does the concept of heterodyne detection come under debate in the case of photoexcited gas-phase molecules while it is assumed to work well in the case of photoexcited crystal, as has been used by Elsaesser and co-workers for a long time [13,15]? In this article, we will explore this question and show that even though in both cases the scattering signal is based on the same mathematical structure, the information encoded in the respective signals is completely different.…”

Section: Introductionmentioning

confidence: 99%

“…Moreover, the idea of holographic (heterodyne) detection, based on the assumption of interference between the ground-state stationary charge distribution and the nonstationary excitation, was used to analyze the data in this experiment [12]. Also, plasma-based ultrashort x-ray sources have been used to image various nonequilibrium phenomena in matter using TRXS [13]. These state-of-the-art experiments have demonstrated that TRXS is an emerging and promising approach to imaging ultrafast processes in real space and in real time with atomic-scale spatiotemporal resolutions.…”

Section: Introductionmentioning

confidence: 99%

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Time-resolved ultrafast x-ray scattering from an incoherent electronic mixture

Dixit

Santra

2017

Phys. Rev. A

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Time-resolved ultrafast x-ray scattering from photoexcited matter is an emerging method to image ultrafast dynamics in matter with atomic-scale spatial and temporal resolutions. For a correct and rigorous understanding of current and upcoming imaging experiments, we present the theory of time-resolved x-ray scattering from an incoherent electronic mixture using quantum electrodynamical theory of light-matter interaction. We show that the total scattering signal is an incoherent sum of the individual scattering signals arising from different electronic states, and therefore heterodyning of the individual signals is not possible for an ensemble of gas-phase photoexcited molecules. We scrutinize the information encoded in the total signal for the experimentally important situation when pulse duration and coherence time of the x-ray pulse are short in comparison to the time scale of the vibrational motion and long in comparison to the time scale of the electronic motion, respectively. Finally, we show that in the case of an electronically excited crystal the total scattering signal imprints the interference of the individual scattering amplitudes associated with different electronic states and heterodyning is possible.

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Perspective: Structural dynamics in condensed matter mapped by femtosecond x-ray diffraction

Cited by 66 publications

References 98 publications

Towards properties on demand in quantum materials

Towards properties on demand in quantum materials

Evidence for out-of-equilibrium states in warm dense matter probed by x-ray Thomson scattering

Time-resolved ultrafast x-ray scattering from an incoherent electronic mixture

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