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1.
Adv Mater ; : e2106401, 2021 Dec 27.
Artigo em Inglês | MEDLINE | ID: mdl-34958699

RESUMO

Understanding the phase transitions and domain evolutions of mesoscale topological structures in ferroic materials is critical to realize their potential applications in next-generation high-performance storage devices. Here, we study the behaviors of a mesoscale supercrystal with 3-D nanoscale periodicity and rotational topology phases in a PbTiO3 /SrTiO3 superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. We construct a phase diagram of temperature versus polar state, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasing process of a supercrystal returning to a classical a-twin structure. Under an in-plane electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms to a new type of stripe-like supercrystal. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings provide a theoretical understanding which may be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature. This article is protected by copyright. All rights reserved.

2.
Nano Lett ; 21(20): 8554-8562, 2021 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-34623164

RESUMO

As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. Understanding the coupling with its topological phase requires studying the interactions of carriers with the lattice on time scales down to the subpicosecond regime. Here, we investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films by time-resolved diffraction using an X-ray free-electron laser. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Our theoretical calculations suggest that the band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insights into the topological phase control by light-induced structural change on ultrafast time scales.

3.
Nat Commun ; 12(1): 5744, 2021 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-34593814

RESUMO

Next-generation terahertz (THz) sources demand lightweight, low-cost, defect-tolerant, and robust components with synergistic, tunable capabilities. However, a paucity of materials systems simultaneously possessing these desirable attributes and functionalities has made device realization difficult. Here we report the observation of asymmetric spintronic-THz radiation in Two-Dimensional Hybrid Metal Halides (2D-HMH) interfaced with a ferromagnetic metal, produced by ultrafast spin current under femtosecond laser excitation. The generated THz radiation exhibits an asymmetric intensity toward forward and backward emission direction whose directionality can be mutually controlled by the direction of applied magnetic field and linear polarization of the laser pulse. Our work demonstrates the capability for the coherent control of THz emission from 2D-HMHs, enabling their promising applications on the ultrafast timescale as solution-processed material candidates for future THz emitters.

4.
Sci Rep ; 11(1): 19322, 2021 Sep 29.
Artigo em Inglês | MEDLINE | ID: mdl-34588533

RESUMO

A fundamental understanding of materials' structural dynamics, with fine spatial and temporal control, underpins future developments in electronic and quantum materials. Here, we introduce an optical transient grating pump and focused X-ray diffraction probe technique (TGXD) to examine the structural evolution of materials excited by modulated light with a precisely controlled spatial profile. This method adds spatial resolution and direct structural sensitivity to the established utility of a sinusoidal transient-grating excitation. We demonstrate TGXD using two thin-film samples: epitaxial BiFeO3, which exhibits a photoinduced strain (structural grating) with an amplitude proportional to the optical fluence, and FeRh, which undergoes a magnetostructural phase transformation. In BiFeO3, structural relaxation is location independent, and the strain persists on the order of microseconds, consistent with the optical excitation of long-lived charge carriers. The strain profile of the structural grating in FeRh, in comparison, deviates from the sinusoidal excitation and exhibits both higher-order spatial frequencies and a location-dependent relaxation. The focused X-ray probe provides spatial resolution within the engineered optical excitation profile, resolving the spatiotemporal flow of heat through FeRh locally heated above the phase transition temperature. TGXD successfully characterizes mesoscopic energy transport in functional materials without relying on a specific transport model.

5.
Phys Rev Lett ; 127(9): 097402, 2021 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-34506196

RESUMO

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO_{3} thin film by an above-band-gap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343 K in the better ordered single-phase a/c regime. Phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

6.
Nano Lett ; 21(16): 6938-6945, 2021 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-34428905

RESUMO

Direct optical probing of the antiferromagnetic order parameter in atomically thin samples is challenging, for example, via magneto-optical spectroscopy, due to the lack of net magnetization. Here, we report zigzag-antiferromagnetism (AFM) induced optical linear dichroism (LD) in layered transition-metal thiophosphate FePS3 down to the monolayer limit. The observed LD is giant despite having the optical wave vector parallel to the Néel vector. The LD is at least one order of magnitude larger than those reported in other antiferromagnetic systems, where the optical wave vector is orthogonal to the Néel vector. The large LD enables the probe of 60° orientated zigzag-AFM domains. The optical anisotropy in FePS3 originates from an electronic anisotropy associated with the zigzag direction of the AFM order and is independent of the spin-pointing direction. Our findings point to a new optical approach for the investigation and control of zigzag or stripe magnetic order in strongly correlated systems.

7.
Nature ; 592(7854): 376-380, 2021 04.
Artigo em Inglês | MEDLINE | ID: mdl-33854251

RESUMO

The collective dynamics of topological structures1-6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.

8.
J Synchrotron Radiat ; 26(Pt 5): 1790-1796, 2019 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-31490171

RESUMO

A multimodal imaging instrument has been developed that integrates scanning near-field optical microscopy with nanofocused synchrotron X-ray diffraction imaging. The instrument allows for the simultaneous nanoscale characterization of electronic/near-field optical properties of materials together with their crystallographic structure, facilitating the investigation of local structure-property relationships. The design, implementation and operating procedures of this instrument are reported. The scientific capabilities are demonstrated in a proof-of-principle study of the insulator-metal phase transition in samarium sulfide (SmS) single crystals induced by applying mechanical pressure via a scanning tip. The multimodal imaging of an in situ tip-written region shows that the near-field optical reflectivity can be correlated with the heterogeneously transformed structure of the near-surface region of the crystal.

9.
Phys Rev Lett ; 123(4): 045703, 2019 Jul 26.
Artigo em Inglês | MEDLINE | ID: mdl-31491252

RESUMO

Above-band-gap optical illumination of compressively strained BiFeO_{3} induces a transient reversible transformation from a state of coexisting tilted tetragonal-like and rhombohedral-like phases to an untilted tetragonal-like phase. Time-resolved synchrotron x-ray diffraction reveals that the transformation is induced by an ultrafast optically induced lattice expansion that shifts the relative free energies of the tetragonal-like and rhombohedral-like phases. The transformation proceeds at interfaces between regions of the tetragonal-like phase and regions of a mixture of tilted phases, consistent with the motion of a phase boundary. The optically induced transformation demonstrates that there are new optically driven routes towards nanosecond-scale control of phase transformations in ferroelectrics and multiferroics.

10.
Phys Rev Lett ; 123(9): 097601, 2019 Aug 30.
Artigo em Inglês | MEDLINE | ID: mdl-31524450

RESUMO

Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing-down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced transition of a model charge-density-wave (CDW) compound LaTe_{3}. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing-down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

11.
Struct Dyn ; 6(1): 014502, 2019 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-30868087

RESUMO

Due to the strong dependence of electronic properties on the local bonding environment, a full characterization of the structural dynamics in ultrafast experiments is critical. Here, we report the dynamics and structural refinement at nanosecond time scales of a perovskite thin film by combining optical excitation with time-resolved X-ray diffraction. This is achieved by monitoring the temporal response of both integer and half-integer diffraction peaks of LaVO3 in response to an above-band-gap 800 nm pump pulse. We find that the lattice expands by 0.1% out of plane, and the relaxation is characterized by a biexponential decay with 2 and 12 ns time scales. We analyze the relative intensity change in half-integer peaks and show that the distortions to the substructure are small: the oxygen octahedral rotation angles decrease by ∼0.3° and La displacements decrease by ∼0.2 pm, which directly corresponds to an ∼0.8° increase in the V-O-V bond-angles, an in-plane V-O bond length reduction of ∼0.3 pm, and an unchanged out-of-plane bond length. This demonstration of tracking the atomic positions in a pump-probe experiment provides experimentally accessible values for structural and electronic tunability in this class of materials and will stimulate future experiments.

12.
J Phys Condens Matter ; 31(25): 255801, 2019 Jun 26.
Artigo em Inglês | MEDLINE | ID: mdl-30897558

RESUMO

The recovery of antiferromagnetic and lattice order of Sr2IrO4 upon laser excitation was measured by time-resolved x-ray diffraction on nanosecond time scales. The in situ measurements of both magnetic and lattice order parameters allow direct comparison of their time evolutions without ambiguity. We found that the magnetic order recovers with two time constants. The fast sub-nanosecond recovery is associated with the re-establishment of three dimensional antiferromagnetic order while the slow sub-nanosecond recovery agrees with the lattice cooling on tens of nanoseconds. The strong oscillating behavior of magnetic order during the long time recovery may be related to complicated dynamics of defect-pinned magnetic domains.

13.
Langmuir ; 34(37): 10828-10836, 2018 09 18.
Artigo em Inglês | MEDLINE | ID: mdl-30145906

RESUMO

Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved X-ray reflectivity studies show that the reconfiguration of a densely packed rhenium-azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

14.
Phys Rev Lett ; 120(20): 207207, 2018 May 18.
Artigo em Inglês | MEDLINE | ID: mdl-29864340

RESUMO

We show that a femtosecond spin-current pulse can generate terahertz (THz) transients at Rashba interfaces between two nonmagnetic materials. Our results unambiguously demonstrate the importance of the interface in this conversion process that we interpret in terms of the inverse Rashba Edelstein effect, in contrast to the THz emission in the bulk conversion process via the inverse spin-Hall effect. Furthermore, we show that at Rashba interfaces the THz-field amplitude can be controlled by the helicity of the light. The optical generation of electric photocurrents by these interfacial effects in the femtosecond regime will open up new opportunities in ultrafast spintronics.

15.
Nat Commun ; 9(1): 1799, 2018 05 04.
Artigo em Inglês | MEDLINE | ID: mdl-29728580

RESUMO

The coupling of ordered electronic phases with lattice, spin, and orbital degrees of freedom are of central interest in strongly correlated systems. Their interplay has been intensively studied from femtosecond to picosecond time scales, while their dynamics beyond nanoseconds are usually assumed to follow lattice cooling. Here, we report an unusual slowing down of the recovery of an electronic phase across a first-order phase transition. Following optical excitation, the recovery time of both transient optical reflectivity and X-ray diffraction intensity from the charge-ordered superstructure in a La1/3Sr2/3FeO3 thin film increases by orders of magnitude as the sample temperature approaches the phase transition temperature. In this regime, the recovery time becomes much longer than the lattice cooling time. The combined experimental and theoretical investigation shows that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition.

16.
ACS Appl Mater Interfaces ; 10(15): 12781-12789, 2018 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-29521082

RESUMO

The effect of above-band gap photons on the domains of the BiFeO3 (BFO) thin film was investigated via piezoresponse force microscopy and Kelvin probe force microscopy. It is found that under above-band gap illumination, the relaxation time of the polarization state was significantly extended, while the effective polarizing voltage for the pristine domains was reduced. We propose that this photoinduced domain stabilization can be attributed to the interaction between photogenerated surface charges and domains. Importantly, a similar phenomenon is observed in other ferroelectric (FE) materials with an internal electric field once they are illuminated by above-band gap light, indicating that this photoinduced stabilization is potentially universal rather than specific to BFO. Thus, this study will not only contribute to the knowledge of photovoltaic (PV) phenomena but also provide a new route to promote the stability of PV and FE materials.

17.
Phys Rev Lett ; 120(9): 096101, 2018 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-29547337

RESUMO

Using time- and spatially resolved hard x-ray diffraction microscopy, the striking structural and electrical dynamics upon optical excitation of a single crystal of BaTiO_{3} are simultaneously captured on subnanoseconds and nanoscale within individual ferroelectric domains and across walls. A large emergent photoinduced electric field of up to 20×10^{6} V/m is discovered in a surface layer of the crystal, which then drives polarization and lattice dynamics that are dramatically distinct in a surface layer versus bulk regions. A dynamical phase-field modeling method is developed that reveals the microscopic origin of these dynamics, leading to gigahertz polarization and elastic waves traveling in the crystal with sonic speeds and spatially varying frequencies. The advances in spatiotemporal imaging and dynamical modeling tools open up opportunities for disentangling ultrafast processes in complex mesoscale structures such as ferroelectric domains.

18.
Nano Lett ; 17(12): 7696-7701, 2017 12 13.
Artigo em Inglês | MEDLINE | ID: mdl-29086574

RESUMO

Visualizing the dynamical response of material heterointerfaces is increasingly important for the design of hybrid materials and structures with tailored properties for use in functional devices. In situ characterization of nanoscale heterointerfaces such as metal-semiconductor interfaces, which exhibit a complex interplay between lattice strain, electric potential, and heat transport at subnanosecond time scales, is particularly challenging. In this work, we use a laser pump/X-ray probe form of Bragg coherent diffraction imaging (BCDI) to visualize in three-dimension the deformation of the core of a model core/shell semiconductor-metal (ZnO/Ni) nanorod following laser heating of the shell. We observe a rich interplay of radial, axial, and shear deformation modes acting at different time scales that are induced by the strain from the Ni shell. We construct experimentally informed models by directly importing the reconstructed crystal from the ultrafast experiment into a thermo-electromechanical continuum model. The model elucidates the origin of the deformation modes observed experimentally. Our integrated imaging approach represents an invaluable tool to probe strain dynamics across mixed interfaces under operando conditions.

19.
Nano Lett ; 17(12): 7761-7766, 2017 12 13.
Artigo em Inglês | MEDLINE | ID: mdl-29119791

RESUMO

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.

20.
Phys Rev Lett ; 119(5): 057601, 2017 Aug 04.
Artigo em Inglês | MEDLINE | ID: mdl-28949700

RESUMO

The nanodomain pattern in ferroelectric-dielectric superlattices transforms to a uniform polarization state under above-band-gap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

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