Ultrafast Phase Transition Research Articles

Photo-induced ultrafast phase transition in twisted bilayer graphene

Photo-induced ultrafast phase transition in twisted bilayer graphene

Ultrafast phase transitions in polyamorphic materials triggered by swift heavy ion impacts

We report unexpectedly fast phase transitions inside the track as pressure and temperature develop after a swift heavy ion impact. By means of molecular dynamics simulations we reveal the origin of the core-shell fine structure observed in a-${\mathrm{Si}}_{}{\mathrm{O}}_{2}$ and a-${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$, as well as the dense core in a-Si. The fine structure of the track is composed of two distinct amorphous phases that result from the different pressure levels in the core and in the shell during solidification. We link this behavior to the tetrahedral network present in these materials.

Electrical Breakdown Spectroscopy of Nano-/Micro-Thermites

We describe the optical and electrical analysis of a micro-explosion of silicon and micro-thermite micro-bridges with a spectral temporal resolution within a 2 ms time range. The intensity of peaks and their mean lifetimes are calculated to identify the contributing atomic and molecular species. Singly ionised atoms and molecules were the main contributor to spectroscopic signature of emission. It is shown that micro-bridges can be used to characterise small (safe) < 10 mg amounts of energetic materials. Use of ions at low ionisation numbers of Si as well as Cu-oxides with different metal-to-oxygen ratios allows the thermal conditions for micro-explosions and a higher temperature of the discharge plasma to be engineered. Temperatures of 5000 ± 500 K (the black-body radiation fit) were achieved with simple micro-bridge junctions of 10–30 Ω resistance initiated with 100–250 V (triggered capacitor discharge). The demonstrated approach can be applied in material science research concerning ultra-fast melting, phase transitions, and detailed steps leading towards detonation (exponentially growing exothermic chemical reactions).

Terahertz Driven Reversible Topological Phase Transition of Monolayer Transition Metal Dichalcogenides.

This paper shows how terahertz light can drive ultrafast topological phase transitions in monolayer transition metal dichalcogenides (TMDs). The phase transition is induced by the light interaction with both electron and phonon subsystems in the material. The mechanism of such a phase transition is formulated by thermodynamics theory: the Gibbs free energy landscape can be effectively modulated under light, and the relative stability between different (meta‐)stable phases can be switched. This mechanism is applied to TMDs and reversible phase transitions between the topologically trivial 2H and nontrivial 1T′ phases are predicted, providing appropriate light frequency, polarization, and intensity are applied. The large energy barrier on the martensitic transformation path can be significantly reduced, yielding a small energy barrier phase transition with fast kinetics. Compared with other phase transition schemes, light illumination has great advantages, such as its non‐contact nature and easy tunability. The reversible topological phase transition can be applicable in high‐resolution fast data storage and in‐memory computing devices.

Raman spectra and elastic light scattering dynamics of V3O5 across insulator–metal transition

The observation of Raman-active mode softening is a powerful tool to understand phase transition mechanisms in solids. The evolution of the phonon spectrum of technologically important correlated oxide V3O5 attracts much attention for the steady-state regime and for ultrashort temporal scales since thermal contribution to the insulator-to-metal (IMT) transition is excluded during the first few picoseconds. Here, we show theoretical modeling of V3O5 phonon spectrum and experiment. It is shown that the IMT is a first-order phase transition. Photoinduced transition was observed by a elastic light scattering technique, showing the principal difference in V3O5 surface dynamics across IMT for thermally and ultrafast light-induced phase transitions. The ultrafast IMT is accompanied primarily by short-range optical phonon interactions with a negligible contribution by acoustic phonons, while the acoustic phonons provide a key contribution to the transformation of surface geometry upon thermally induced IMT. Scattering imaging of surface autocorrelation function upon IMT shows the rise of optical homogeneity in the lateral direction of the surface in the steady-state regime and on a subpicosecond time scale.

Ultrafast phononic switching of magnetization

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal. Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only.

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.

Sub‐Picosecond Response Time of a Hybrid VO2:Silicon Waveguide at 1550 nm

AbstractHybrid material systems are a promising approach for extending the capabilities of silicon photonics. Given the weak electro‐optic and thermo‐optic effects in silicon, there is intense interest in integrating an ultrafast‐switching phase‐change material with a large refractive index contrast into the waveguide, such as vanadium dioxide (VO2). It is well established that the phase transition in VO2 thin films can be triggered by ultrafast, 800 nm laser pulses, and that pump‐laser fluence is a critical determinant of the recovery time of thin films irradiated by femtosecond pulses. However, thin‐film experiments are not reliable guides to a VO2:Si system for all‐optical, on‐chip switching because of the differences in VO2 optical constants in the telecommunication band, and the complex sample geometry and alignment issues in a waveguide geometry. This paper reports the first demonstration that the reversible, ultrafast photoinduced phase transition in VO2 can achieve sub‐picosecond response when small VO2 volumes are integrated into a silicon waveguide as the active element. The result suggests that VO2 can be pursued as a strong candidate for waveguide switching with sub‐picosecond on‐off times.

Ultrafast spin-nematic and ferroelectric phase transitions induced by femtosecond light pulses

Optically induced phase transitions of the manganite ${\mathrm{Pr}}_{1/3}{\mathrm{Ca}}_{2/3}\mathrm{Mn}{\mathrm{O}}_{3}$ have been simulated by using a model Hamiltonian that captures the dynamics of strongly correlated charge, orbital, lattice, and spin degrees of freedom. Its parameters have been extracted from first-principles calculations. Beyond a critical intensity of a femtosecond light pulse, the material undergoes an ultrafast and nonthermal magnetic phase transition from a noncollinear to collinear antiferromagnetic phase. The light-pulse excites selectively either a spin-nematic or a ferroelectric phase, depending on the light polarization. The behavior can be traced to an optically induced ferromagnetic coupling between Mn trimers, i.e., polarons which are delocalized over three Mn sites. The polarization guides the polymerization of the polaronic crystal into distinct patterns of ferromagnetic chains determining the target phase.

Element-Specific Magnetization Dynamics of Complex Magnetic Systems Probed by Ultrafast Magneto-Optical Spectroscopy

The vision to manipulate and control magnetism with light is driven on the one hand by fundamental questions of direct and indirect photon-spin interactions, and on the other hand by the necessity to cope with ever growing data volumes, requiring radically new approaches on how to write, read and process information. Here, we present two complementary experimental geometries to access the element-specific magnetization dynamics of complex magnetic systems via ultrafast magneto-optical spectroscopy in the extreme ultraviolet spectral range. First, we employ linearly polarized radiation of a free electron laser facility to demonstrate decoupled dynamics of the two sublattices of an FeGd alloy, a prerequisite for all-optical magnetization switching. Second, we use circularly polarized radiation generated in a laboratory-based high harmonic generation setup to show optical inter-site spin transfer in a CoPt alloy, a mechanism which only very recently has been predicted to mediate ultrafast metamagnetic phase transitions.

Natural Orbital Branching Scheme for Time-Dependent Density Functional Theory Nonadiabatic Simulations.

Real time-time-dependent density functional theory (rt-TDDFT) has now been used to study a wide range of problems, from optical excitation to charge transfer, to ion collision, and to ultrafast phase transition. However, conventional rt-TDDFT Ehrenfest dynamics for nuclear movement lacks a few critical features to describe many problems: the detail balance between state transitions, decoherence for the wave function evolution, and stochastic branching of the nuclear trajectory. There are many-body formalisms to describe such nonadiabatic molecular dynamics, especially the ones based on mixed quantum/classical simulations, like the surface hopping and wave function collapsing schemes. However, there are still challenges to implement such many-body formalisms to the rt-TDDFT simulations, especially for large systems where the excited state electronic structure configuration space is large. Here we introduce two new algorithms for nonadiabatic rt-TDDFT simulations: the first is a Boltzmann factor algorithm which introduces decoherence and detailed balance in the carrier dynamics but uses mean field theory for nuclear trajectory. The second is a natural orbital branching (NOB) formalism, which uses a time-dependent density matrix for electron evolution and a natural orbital set to collapse the wave function upon. It provides the features of decoherence, detailed balance, and trajectory branching. We have tested these methods for a molecule radiolysis decay problem. We found that these methods can be used to study such radiolysis problems in which the molecule is broken into many fragments following complex electronic structure transition paths. The computational time of NOB is similar to that of the original plain rt-TDDFT simulations.

Aurore: A platform for ultrafast sciences

We present the Aurore platform for ultrafast sciences. This platform is based on a unique 20 W, 1 kHz, 26 fs Ti:sapphire laser system designed for reliable operation and high intensity temporal contrast. The specific design ensures the high stability in terms of pulse duration, energy, and beam pointing necessary for extended experimental campaigns. The laser supplies 5 different beamlines, all dedicated to a specific field: attosecond science (Aurore 1), ultrafast phase transitions in solids (Aurore 2 and 3), ultrafast luminescence in solids (Aurore 4), and femtochemistry (Aurore 5). The technical specifications of these five beamlines are described in detail, and examples of the recent results are given.

Sub-cycle atomic-scale forces coherently control a single-molecule switch.

Scanning probe techniques can leverage atomically precise forces to sculpt matter at surfaces, atom by atom. These forces have been applied quasi-statically to create surface structures1-7 and influence chemical processes8,9, but exploiting local dynamics10-14 to realize coherent control on the atomic scale remains an intriguing prospect. Chemical reactions15-17, conformational changes18,19 and desorption20 have been followed on ultrafast timescales, but directly exerting femtosecond forces on individual atoms to selectively induce molecular motion has yet to be realized. Here we show that the near field of a terahertz wave confined to an atomically sharp tip provides femtosecond atomic-scale forces that selectively induce coherent hindered rotation in the molecular frame of a bistable magnesium phthalocyanine molecule. Combining lightwave-driven scanning tunnelling microscopy21-24 with ultrafast action spectroscopy10,13, we find that the induced rotation modulates the probability of the molecule switching between its two stable adsorption geometries by up to 39 per cent. Mapping the response of the molecule in space and time confirms that the force acts on the atomic scale and within less than an optical cycle (that is, faster than an oscillation period of the carrier wave of light). We anticipate that our strategy might ultimately enable the coherent manipulation of individual atoms within single molecules or solids so that chemical reactions and ultrafast phase transitions can be manipulated on their intrinsic spatio-temporal scales.

Does Vo2 Host a Transient Monoclinic Metallic Phase?

Ultrafast phase transitions induced by femtosecond light pulses present a new opportunity for manipulating the properties of materials. Understanding how these transient states are different from, or similar to, their thermal counterparts is key to determining how materials can exhibit properties that are not found in equilibrium. In this paper, we reexamine the case of the light-induced insulator-metal phase transition in the prototypical, strongly correlated material VO$_2$, for which a nonthermal Mott-Hubbard transition has been claimed. Here, we show that heat, even on the ultrafast timescale, plays a key role in the phase transition. When heating is properly accounted for, we find a single phase-transition threshold corresponding to the thermodynamic structural insulator-metal phase transition, and we find no evidence of a hidden transient Mott-Hubbard nonthermal phase. The interplay between the initial thermal state and the ultrafast transition may have implications for other transient states of matter.

Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy.

Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe_{2}. Wepresent a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.

Molecular Dynamics Simulation of Ultra-Fast Phase Transition in Water Nanofilms

Abstract Molecular dynamics simulations are used to explore explosive boiling of thin water films on a gold substrate. In particular, water films of 0.7, 1.6, and 2.5 nanometer thickness were examined. Three different surface wettabilities with contact angles of 11 deg, 47 deg, and 110 deg were simulated along with substrate temperatures of 400 K, 600 K, 800 K, and 1000 K. The 11 and 47 deg contact angles were obtained using a Morse interaction potential between the water film and gold substrate while the 47 and 110 deg contact angles were obtained via a Lennard-Jones potential. Evaporation was the first mode of phase change observed in all cases and explosive boiling did not occur until the substrate reached a temperature of 800 K. When explosive boiling was present for all three contact angles, it was consistently shown to occur first for the surface with a 47 deg contact angle and Lennard-Jones potential. These results suggest that explosive boiling onset is strongly dependent on the particularities of the interaction potential. For instance, the Morse potential is smoother when compared to the Lennard-Jones potential, but has more interaction sites per molecule—two hydrogen atoms and one oxygen atom versus one oxygen atom. Thus, even when the water film reaches a higher temperature with the Morse potential, explosive boiling onset is delayed as more interaction sites have to be disrupted. These results suggest that contact angle alone is insufficient and both the interaction strength and the number of atoms interacting at the interface must be considered when investigating trends of explosive boiling with surface wettability.

Large non-thermal contribution to picosecond strain pulse generation using the photo-induced phase transition in VO2

Picosecond strain pulses are a versatile tool for investigation of mechanical properties of meso- and nano-scale objects with high temporal and spatial resolutions. Generation of such pulses is traditionally realized via ultrafast laser excitation of a light-to-strain transducer involving thermoelastic, deformation potential, or inverse piezoelectric effects. These approaches unavoidably lead to heat dissipation and a temperature rise, which can modify delicate specimens, like biological tissues, and ultimately destroy the transducer itself limiting the amplitude of generated picosecond strain. Here we propose a non-thermal mechanism for generating picosecond strain pulses via ultrafast photo-induced first-order phase transitions (PIPTs). We perform experiments on vanadium dioxide VO2 films, which exhibit a first-order PIPT accompanied by a lattice change. We demonstrate that during femtosecond optical excitation of VO2 the PIPT alone contributes to ultrafast expansion of this material as large as 0.45%, which is not accompanied by heat dissipation, and, for excitation density of 8 mJ cm−2, exceeds the contribution from thermoelastic effect by a factor of five.

A role of a picosecond strain in an ultrafast optically-driven phase transition in VO2 nanostructures

We report on experimental picosecond acoustic studies of an ultrafast photoinduced insulator-to-metal and structural transition in VO2 nanostructures epitaxially grown on Al2O3 substrates with different orientations. Applying a pump-probe technique with combined excitation of a sample with picosecond strain and femtosecond laser pulses we demonstrate that dynamical strain of moderate amplitude of 0.1% has a pronounced impact on ultrafast photoinduced phase transition in VO2 nanohillocks. This enables novel path for controlling such transitions at picosecond and nanometer scales. Our experiments also allowed characterizing elastic and photo-elastic properties of the photo-induced metallic phase in VO2 and to relate them to the properties of the equilibrium phase. Furthermore, we demonstrate the generation of picosecond strain pulses upon laser-induced excitation of thin epitaxial VO2.

Sequential insulating-metal-insulating phase transition of NbO2 by doping photoexcited carrier

A large number of free carriers are introduced in the photoexcitation process and the complex interaction between photoexcited carriers and lattice can lead to an ultrafast phase transition in the solid. Niobium dioxide is an important optical material, which exhibiting a threshold switching phenomenon that can be explained by insulating-metal phase transition. It can be used as an ultrafast optical switches for telecommunication and optoelectronic devices. Here, we investigated the effects of injecting photoexcited carriers on consecutive insulator-metal-insulator phase transition of NbO2 by applying the GGA and GGA + U energy functions on the basis of density functional theory (DFT). Compared with electron injection, the effect of hole doping on phase transition in NbO2 is significantly more effective. The length of Nb-Nb dimer (long or short) is nearly unchanged upon electron doping, while the length of Nb-Nb_l is gradual shortened from 3.366 to 3.039 Å and the Nb-Nb_s is gradual stretched from 2.733 to 3.035 Å upon hole doping below a critical level of doping 12 holes per conventional cell. The structure phase changed from a body-center tetragonal (bct) to a rutile. Upon a higher hole doping, the structure kept the rutile structure until the number of doping holes reached 28 and it began to change to bct structure. We studied the electronic properties of NbO2 at a certain carriers concentration and reported a controlled and reversible insulator-metal-insulator phase transition by carries doping. The coexistence of the strong electron correlation and the Nb-Nb dimerization due to lattice distortion obscures the nature of the phase transition in NbO2. By analyzing the evolutions of lattice parameters and electronic properties of NbO2 at different charge carrier concentration, we got a conclusion that this sequential phase transition of NbO2 is not just a Mott phase transition or Peierls phase transition, but a collaborative of Peierls-Mott phase transition. Our work confirms that the observed structural transformations are driven primarily by hole doping, and the dominant lattice distortion caused by hole doping is the off-centering of Nb atoms in NbO6 octahedral.

Ultrafast Structural Dynamics of Nanoparticles in Intense Laser Fields.

Femtosecond laser pulses have opened new frontiers for the study of ultrafast phase transitions and nonequilibrium states of matter. In this Letter, we report on structural dynamics in atomic clusters pumped with intense near-infrared (NIR) pulses into a nanoplasma state. Employing wide-angle scattering with intense femtosecond x-ray pulses from a free-electron laser source, we find that highly excited xenon nanoparticles retain their crystalline bulk structure and density in the inner core long after the driving NIR pulse. The observed emergence of structural disorder in the nanoplasma is consistent with a propagation from the surface to the inner core of the clusters.