Photoinduced Structural Phase Transitions Research Articles

Photoinduced Electronic and Spin Topological Phase Transitions in Monolayer Bismuth.

Ultrathin bismuth exhibits rich physics including strong spin-orbit coupling, ferroelectricity, nontrivial topology, and light-induced structural dynamics. We use abinitio calculations to show that light can induce structural transitions to four transient phases in bismuth monolayers. These light-induced phases exhibit nontrivial topological character, which we illustrate using the recently introduced concept of spin bands and spin-resolved Wilson loops. Specifically, we find that the topology changes via the closing of the electron and spin band gaps during photoinduced structural phase transitions, leading to distinct edge states. Our study provides strategies to tailor electronic and spin topology via ultrafast control of photoexcited carriers and associated structural dynamics.

Atomic-scale view of the photoinduced structural transition to form sp3-like bonded order phase in graphite

Photoexcitation of solids often induces structural phase transitions between different ordered phases, some of which are unprecedented and thermodynamically inaccessible. The phenomenon, known as photoinduced structural phase transition (PSPT), is of significant interest to the technological progress of advanced materials processing and the fundamental understanding of material physics. Here, we applied scanning tunnelling microscopy (STM) to directly characterise the primary processes of the PSPT in graphite to form a sp3-like carbon nano-phase called diaphite. The primary challenge was to provide microscopic views of the graphite-to-diaphite transition. On an atomic scale, STM imaging of the photoexcited surface revealed the nucleation and proliferation processes of the diaphite phase; these were governed by the formation of sp3-like interlayer bonds. The growth mode of the diaphite phase depends strongly on the photon energy of excitation laser light. Different dynamical pathways were proposed to explain the formation of a sp3-like interlayer bonding. Potential mechanisms for photon-energy-dependent growth were examined based on the experimental and calculated results. The present results provide insight towards realising optical control of sp2-to-sp3 conversions and the organisation of nanoscale structures in graphene-related materials.

Transient dynamics of the phase transition in VO2 revealed by mega-electron-volt ultrafast electron diffraction

Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO2 have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesize freestanding quasi-single-crystal VO2 films and examine their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples is observed in our quasi-single-crystal samples. Our findings provide essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO2.

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO2.

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic (M1-) to rutile (R-) VO2 and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.

Optoelectronic dynamic memristor systems based on two-dimensional crystals

Abstract Optical modulation of resistive switching in an optoelectronic memristor allows it to be optically controlled at ultra-high speed and ultra-low power consumption. Optical memristive systems with memory elements switched by electromagnetic radiation can be used in optically reconfigurable and tunable neural networks for neuromorphic computing and brain-inspired artificial intelligence systems. Two-dimensional (2D) crystals with unique electrical and optical properties demonstrate tremendous potential in the creation of highly efficient information and sensor systems for real-time data monitoring and processing. In this paper, we consider optoelectronic structures based on graphene, graphene oxide, and molybdenum disulfide for dynamic memristive signal processing. 2D optoelectronic memristor structures exhibit multiple states, which can be monitored in a wide range of optical excitations and used as sensors for pattern recognition and image processing. An optoelectronic dynamic memristive structure with quantum dots (QDs), controlled by ultrafast photoinduced structural phase transitions, is promising for creating information systems with stochastic data processing. The phase transition, controlled by charge and temperature, results in tunable periodic oscillations due to two state variables that exhibit chaotic behavior similar to that which occurs in a biological neuron network.

Real Time Quantum Dynamics of Spontaneous Translational Symmetry Breakage in the Early Stage of Photo-Induced Structural Phase Transitions

Real time quantum dynamics of the spontaneous translational symmetry breakage in the early stage of photo-induced structural phase transitions is reviewed and supplementally explained, under the guide of the Toyozawa theory, which is exactly in compliance with the conservation laws of the total momentum and energy. At the Franck-Condon state, an electronic excitation just created by a visible light, is in a plane wave state, which is extended all over the crystal. While, after the lattice relaxation having been completed, it is localized around a certain lattice site of the crystal, as a new excitation. Is there a sudden shrinkage of the excitation wave function, in between? No! The wave function never shrinks, but only the spatial (or inter lattice-site) quantum coherence (interference) of the excitation disappears, as the lattice relaxation proceeds. This is nothing but the spontaneous breakage of translational symmetry.

Multiple timescales in the photoswitching kinetics of crystalline thin films of azobenzene-trimers

Functional materials that exhibit photoinduced structural phase transitions are highly interesting for applications in optomechanics and mechanochemistry. It is, however, still not fully understood how photochemical reactions, which are often accompanied by molecular motion, proceed in confined and crystalline environments. Here we show that thin films of azobenzene trimers exhibit high structural order and determine the crystallographic unit cell. We demonstrate that thin film can be switched partially reversibly between a crystalline and an amorphous phase. The time constant of the photoinduced amorphisation as measured with real-time x-ray diffraction (220 s) lies between the two time constants (120 s and 2870 s) of the ensemble photoisomerisation processes that are measured via optical spectroscopy. Our observation of a photoinduced shrinking of the crystalline domains indicates a cascading process, in which photoisomerisation starts at the surface of the thin film and propagates deeper into the crystalline layer by introducing disorder and generating free volume. This finding is important for the rapidly evolving research field of photoresponsive thin films and smart crystalline materials in general.

In situ imaging of the dynamics of photo-induced structural phase transition at high pressures by picosecond acoustic interferometry

Picosecond acoustic interferometry is used to monitor in time the motion of the phase transition boundary between two water ice phases, VII and VI, coexisting at a pressure of 2.15 GPa when compressed in a diamond anvil cell at room temperature. By analyzing the time-domain Brillouin scattering signals accumulated for a single incidence direction of probe laser pulses, it is possible to access ratios of sound velocity values and of the refractive indices of the involved phases, and to distinguish between the structural phase transition and a recrystallization process. Two-dimensional spatial imaging of the phase transition dynamics indicates that it is initiated by the pump and probe laser pulses, preferentially at the diamond/ice interface. This method should find applications in three-dimensional monitoring with nanometer spatial resolution of the temporal dynamics of low-contrast material inhomogeneities caused by phase transitions or chemical reactions in optically transparent media.

Ultrafast electron crystallography of the cooperative reaction path in vanadium dioxide.

Time-resolved electron diffraction with atomic-scale spatial and temporal resolution was used to unravel the transformation pathway in the photoinduced structural phase transition of vanadium dioxide. Results from bulk crystals and single-crystalline thin-films reveal a common, stepwise mechanism: First, there is a femtosecond V−V bond dilation within 300 fs, second, an intracell adjustment in picoseconds and, third, a nanoscale shear motion within tens of picoseconds. Experiments at different ambient temperatures and pump laser fluences reveal a temperature-dependent excitation threshold required to trigger the transitional reaction path of the atomic motions.

Photoinduced dynamics of spinel MnV2O4

By pump-probe optical reflectivity measurements, we studied the photoinduced dynamics of MnV${}_{2}$O${}_{4}$, which is a ${d}^{2}$ Mott insulator and exhibits a simultaneous spin and orbital ordering accompanied by a structural phase transition at 57 K. We compared the present experimental results with the results of the steady-state optical measurement of the same compound and the steady-state and pump-probe optical measurements of Al-doped MnV${}_{2}$O${}_{4}$, in which the orbital ordering and structural phase transition are absent but the magnetic ordering exists. We found that the photoinduced dynamics of MnV${}_{2}$O${}_{4}$ can be separated into three components: (1) the suppression of the Mott-excitation peak, (2) the melting of the orbital ordering, and (3) the melting of the magnetic ordering. Furthermore, we found that the lattice distortion associated with the photoinduced structural phase transition from tetragonal to cubic induces a strain wave whose propagation velocity is discernibly smaller than that of a conventional shockwave.

Quantum and Spontaneous Breakage of Translational Symmetry due to Light Excitation in Photo-Induced Structural Phase Transitions

Real time quantum dynamics of the spontaneous translational symmetry breakage due to light excitation in the early stage of photo-induced structural phase transitions is reviewed under the guide of the Toyozawa theory, which is in exact compliance with the conservation law of the total momentum. At the Franck–Condon state, an electronic excitation just created by a visible photon is in a plane wave state, extended all over the crystal. While, after the lattice relaxation having been completed, it is localized as a new excitation. So, is there the shrinkage of the excitation wave function? No! The wave function never shrinks, but only the spatial (or inter lattice-site) quantum coherence (interference) of the excitation disappears, as the lattice relaxation proceeds. This is the breakage of translational symmetry.

Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Sp3 domain in graphite by visible light and photoinduced phase transitions

Photoinduced structural phase transition (PSPT)s are reviewed in connection with recent experimental results. There are two key concepts: the hidden multi-stability of the ground state, and the proliferations of optically excited states. Taking the ionic (I)-neutral (N) phase transition in an organic charge-transfer (CT) crystal TTF-CA, as an example, we, briefly look back the essence of its PSPT, in terms of the CT exciton and the N-domain proliferation. Next, we are concerned with the discovery of a new photoinduced phase with inter-layer σ-bonds in a graphite. We will see the mechanism of this nonequilibrium phase transition, in terms of the proliferation of photo-generated inter-layer CT excitations in the visible region. At the Franck-Condon state, the resultant electron-hole pair is quite unstable, being easily dissipated into the two-dimensional electronic continuum, as plus and minus free carriers. However, by a small probability, the electron and the hole are bound as an inter-layer CT exciton. This exciton self-localizes, contracting the inter-layer distance and buckling the six membered ring of graphite, only around it. Thus a tiny sp 3 nano-domain appears.

Early history and theoretical problems of photo-induced structural phase transitions

Early history and theoretical problems of photo-induced structural phase transitions are reviewed, in connection with various materials involved in this phenomenon. We also review new measurement methods that exceed the conventional visible photon by photon spectroscopy.

Theory of Photoinduced Phase Transitions: From Semiclassical to Quantum Aspects

We review recent progress of theoretical studies for the photoinduced phase tran- sitions (PIPTs) to clarify what the PIPTs are. There are two types of the PIPTs: (a) global change via optically excited states and (b) new material phase creation in optically excited states. First, concerning (a), photoinduced structural phase transitions via excited electronic states are discussed using a minimal one-dimensional model composed of localized electrons and lattices. We show that the global structural change by photoexcitation only at a single site is possible under the adiabatic or diabatic approximation. This dynamics of the domain bound- aries (domain walls) is called the “photoinduced domino process,” which is the photoinduced nucleation in nonequilibrium first-order phase transition. Second, concerning (b), we discuss quantum orders of electron-hole (e-h) systems, which are optically excited states of insulators consisting of many electrons and holes in two bands. In particular, the “exciton Mott transi- tion,” i.e., the “from-insulator-to-metal” transition of the e-h systems as the particle density increases is introduced. We stress that this transition depends strongly on dimensionality of the system.

Anisotropy of inter-exciton interaction and pattern formation dynamics in photoinduced structural phase transitions

We study theoretically nonlinear nonequilibrium real-time dynamics of an optically excited many-exciton system in a two-dimensional insulator, and search for conditions under which excitons can proliferate efficiently to produce a macroscopic photoinduced structural phase transition (PIPT). To study this problem theoretically, we take a many-exciton system coupling strongly with Einstein-phonons, and numerically calculate the proliferation of excitons and the resultant domain patterns.In particular, we focus on the anisotropy of inter-exciton interaction, and compare a strong anisotropic case to a weak one. We have concluded that the anisotropic case leads to very efficient proliferation, and makes the final PIPT extensive and successful compared to isotropic cases.

Theory for pattern formation dynamics in photoinduced structural phase transitions

We theoretically study formation dynamics of exciton domain patterns organized in optically excited 2-dimensional many-exciton system which is strongly coupled with an Einstein-phonon, and also immersed in a reservoir. It is shown that the exciton may proliferate quite rapidly by several successive photon-pulse excitation, and the resultant pattern becomes something like a fjord structure. Furthermore, the further proliferation can be occurred by filling up this structure even when the excitation is already turned off. We also investigate the effect of anisotropy of inter-exciton interaction on the efficiency of the proliferation. Consequently, we conclude that the strong anisotropy leads to the very efficient proliferation and makes the resultant photoinduced structural phase transition vast and successful, as compared with isotropic cases.

Theory for photoinduced structural phase transitions and their dynamics in a 2-D insulating crystal

We theoretically study non-linear non-equilibrium real-time dynamics of domain formation by photogenerated excitons in a 2-D insulating crystals, and search for successful conditions under which photoinduced structural phase transition (PSPT) occur. Our model is a many-exciton system, coupling strongly with Einstein phonons, and also interacting with a reservoir. Using this model, we numerically calculate its time-evolution dynamics, full-quantum mechanically. It is found that the anisotropy of inter-exciton interactions is essential for a large domain nucleation, and also to make the PSPT successful, as compared with the isotropic cases.

Nonlinearity and macroscopic oscillations in photoinduced structural phase transitions

We theoretically investigate the phenomena of photoinduced structural phase transitions, using a two-chain system with an electron-lattice interaction as a prototype model. Our first concern is the nonlinearity, which is defined as that in the photoconverted fraction with respect to absorbed light intensity. In our model, the photoconversion is assumed to occur from one phase of a charge-density-wave ground state at half filling to the other phase that is degenerate with the former. The nonlinearity is successfully reproduced as an almost ${I}^{2}$ behavior with I as the light intensity. The second concern is the macroscopic oscillations occurring during the process of photoconversion. We show that our model develops also this feature, in a simplified manner and clarify that they are essentially different from linear modes that own only small oscillation amplitudes.

A model of Photoinduced Regular-Dimerized Stack Transitions

We present a model of photoinduced regular-dimerized stack transitions and discuss theoretically the behavior of the organic molecular mixed-stack charge-transfer crystal, Tetrathiafulvalene-Chloranil (TTF-CA). We investigate both the photoinduced structural phase transitions and the thermal phase transition between the regular-stack neutral phase at high temperatures and the dimerized-stack ionic phase at low temperatures. Our model is based on a simple phenomenological insight that the molecules have three stable displacements in the crystal. The cooperative effect is accounted for by introducing harmonic coupling between the molecules. It is the first attempt to include not only the photo-excitation but also the temperature effect on an equal footing.