Nonthermal Phase Transition Research Articles

First-Order Rhombohedral-to-Cubic Phase Transition in Photoexcited GeTe.

Photoexcited GeTe undergoes a nonthermal phase transition from a rhombohedral to a rocksalt crystalline phase. The microscopic mechanism and the nature of the transition are unclear. By using constrained density functional perturbation theory and by accounting for quantum anharmonicity within the stochastic self-consistent harmonic approximation, we show that the nonthermal phase transition is strongly first order and does not involve phonon softening, at odds with the thermal one. The transition is driven by the closure of the single particle gap in the photoexcited rhombohedral phase. Finally, we show that ultrafast x-ray diffraction data are consistent with a coexistence of the two phases, as expected in a first order transition. Our results are relevant for the understanding of phase transitions and bonding in phase change materials.

Following the Nonthermal Phase Transition in Niobium Dioxide by Time-Resolved Harmonic Spectroscopy.

Photoinduced phase transitions in correlated materials promise diverse applications from ultrafast switches to optoelectronics. Resolving those transitions and possible metastable phases temporally are key enablers for these applications, but challenge existing experimental approaches. Extreme nonlinear optics can help probe phase changes, as higher-order nonlinearities have higher sensitivity and temporal resolution to band structure and lattice deformations. Here the ultrafast transition from the semiconducting to the metallic phases in polycrystalline thin-film NbO_{2} is investigated by time-resolved harmonic spectroscopy. The emission strength of all harmonic orders shows a steplike suppression when the excitation fluence exceeds a threshold (∼11-12 mJ/cm^{2}), below the fluence required for the thermal transition-a signature of the nonthermal emergence of a metallic phase within 100±20 fs. This observation is backed by full abinitio simulations as well as a 1D chain model of high-harmonic generation from both phases. Our results demonstrate femtosecond harmonic probing of phase transitions and nonthermal dynamics in solids.

Light-Induced Nonthermal Phase Transition to the Topological Crystalline Insulator State in SnSe.

Femtosecond pulses have been used to reveal hidden broken symmetry states and induce transitions to metastable states. However, these states are mostly transient and disappear after laser removal. Photoinduced phase transitions toward crystalline metastable states with a change of topological order are rare and difficult to predict and realize experimentally. Here, by using constrained density functional perturbation theory and accounting for light-induced quantum anharmonicity, we show that ultrafast lasers can permanently transform the topologically trivial orthorhombic structure of SnSe into the topological crystalline insulating rocksalt phase via a first-order nonthermal phase transition. We describe the reaction path and evaluate the critical fluence and possible decay channels after photoexcitation. Our simulations of the photoexcited structural and vibrational properties are in excellent agreement with recent pump-probe data in the intermediate fluence regime below the transition with an error on the curvature of the quantum free energy of the photoexcited state that is smaller than 2%.

Free electron laser stochastic spectroscopy revealing silicon bond softening dynamics

Time-resolved x-ray emission/absorption spectroscopy (Tr-XES/XAS) is an informative experimental tool sensitive to electronic dynamics in materials, widely exploited in diverse research fields. Typically, Tr-XES/XAS requires x-ray pulses with both a narrow bandwidth and subpicosecond pulse duration, a combination that in principle finds its optimum with Fourier transform-limited pulses. In this work, we explore an alternative experimental approach, capable of simultaneously retrieving information about unoccupied (XAS) and occupied (XES) states from the stochastic fluctuations of broadband extreme ultraviolet pulses of a free electron laser. We used this method, in combination with singular-value decomposition and Tikhonov regularization procedures, to determine the XAS/XES response from a crystalline silicon sample at the ${L}_{2,3}$ edge, with an energy resolution of a few tens of meV. Finally, we combined this spectroscopic method with a pump-probe approach to measure structural and electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after photoexcitation with an optical laser pulse at 390 nm allowed us to observe perturbations of the band structure, which are compatible with the formation of the predicted precursor state of a nonthermal solid-liquid phase transition associated with a bond softening phenomenon.

Electronic excitation induced non-thermal phase transition of tungsten

The electronically excited states of tungsten (W) material tuned by ultrafast laser irradiation cause intriguing jumps between different Born-Oppenheimer surfaces. This may give rise to a structural phase transformation of BCC-to-FCC or BCC-to-HCP on a very short time scale, which is still debated. By performing calculations on W with both density functional theory and a tight-binding approach, a martensitic phase transformation from BCC phase to HCP phase in bulk W is predicted at a threshold (15,500 ± 500 K) of electronic temperatures, where the HCP phase is found to be more stable than both BCC and FCC phases under intense electronic excitations. Additionally, it is proposed that the non-thermal phase transition could potentially be probed through experimental measurements of the c/a ratio of system in the future. The mechanism underlying the BCC-to-HCP transformation in the electronically excited bulk W system is analyzed under the level of electronic structure. It is found that the irradiation-induced changes of the density of states at the Fermi level could drive the structural phase transformation. This provides new insight into the atomic-level non-thermal phase transition process of W under intense electronic excitations.

Multiphysics modeling of femtosecond laser–copper interaction: From electron dynamics to plasma eruption

The femtosecond laser ablation of metals is a complex and violent nonequilibrium process, and numerous studies have sought to reveal the evolution of a single physical phenomenon, such as laser-induced periodic surface micro-nanostructures or plasma eruptions. By considering the multiphysics scenarios of energy and heat transfer, structural mechanics, hydrodynamics, and nucleation dynamics, a femto-nanosecond and nano-micrometer multiscale framework combining electron–phonon-coupled heat transfer, lattice deformation, phase transition, and plasma eruption was constructed to describe the heat and mass transfer mechanism of femtosecond laser–copper interaction. A multiphysics model was proposed in this study to simulate the ablation process with different laser fluences. Ablation occurs at low near-threshold fluences primarily via a combination of the thermal phase transition process of melting and thermoplastic deformation coupled with the nonthermal phase transition process of hot electron explosion. Marangoni convection and non-uniform nucleation at the solid–liquid interface create micro-nano structures on the surface of the ablation crater. At a high laser fluence, plasma plumes are emitted via gasification and eruption, and as the material is heated to decrease its density, the surface is broken into a micro-column structure, and then the micro-columns fracture and erupt to form micro-nano structures and plasma plumes. Numerical results offer a better understanding of surface topography modifications and plasma plume evolution and promote the application of femtosecond laser precision fabrication in the fields of aviation, mechanics, electronics, and materials engineering.

Optical Properties of V2O5 Thin Films on Different Substrates and Femtosecond Laser-Induced Phase Transition Studied by Pump-Probe Method.

Vanadium pentoxide can undergo a reversible phase transition by heating above 260 °C; its non-thermal phase transition, as well as ultrafast dynamical processes, is still not known. Here, femtosecond laser-induced phase transition properties in V2O5 thin films were first explored using femtosecond time-resolved pump–probe spectroscopy. The results show that the phase transient processes occur on a 10−15–10−13 temporal scale. The phase transition and recovery properties are dependent on both the substrates and pump laser energy densities. We propose the oxygen vacancies theory to explain the results, and we provide valuable insights into V2O5 films for potential applications.

Time-resolved structural dynamics of the out-of-equilibrium charge density wave phase transition in GdTe3.

We use ultrafast electron diffraction to study the out-of-equilibrium dynamics of the charge density wave (CDW) phase transition in GdTe3, a quasi-two-dimensional compound displaying a unidirectional CDW state. Experiments were conducted at different incident fluences and different initial sample temperatures below Tc. We find that following photo-excitation, the system undergoes a non-thermal ultrafast phase transition that occurs in out-of-equilibrium conditions. The intrinsic crystal temperature was estimated at each time delay from the atomic thermal motion, which affects each Bragg peak intensity via the Debye Waller factor. We find that the crystal temperature stabilizes with a 6 ps timescale in a quasi-equilibrium state at temperature . We then relate the recovery time of the CDW and its correlation lengths as a function of . The charge density wave is suppressed in less than a picosecond while its recovery time increases linearly with incident fluence and initial temperature. Our results highlight that the dynamics is strongly determined by the initial sample temperature. In addition, the transient CDW phase recently observed along the transverse direction in LaTe3 and CeTe3 is not observed in GdTe3.

Selective ultrashort laser annealing of amorphous Ge/Si multilayer stacks

We report on single-short laser crystallization of Ge/Si multilayer stacks consisting of alternating amorphous nanosized films of silicon and germanium using near- and mid-infrared femtosecond and picosecond laser pulses. The phase composition of the irradiated stacks was investigated by the Raman scattering technique. Several non-ablative regimes of crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of one- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are analysed based on theoretical estimations and comparison with experimental data. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point. The possibility of ultrafast non-thermal phase transition in germanium in the studied regimes is also discussed.

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.

Nonthermal phase transitions in irradiated oxides

It is predicted theoretically that various oxides (Al2O3, MgO, SiO2 and TiO2) under ultrafast excitation of the electronic system exhibit nonthermal phase transitions. In the bulk, Al2O3 transiently forms a superionic phase via nonthermal phase transition, MgO and SiO2 disorder, TiO2 experiences solid–solid phase transition while thermal effects lead to melting. In the finite-size samples and near-surface regions, MgO undergoes solid–solid phase transition at lower doses than those required for atomic disorder. All studied oxides but TiO2, if allowed to expand, exhibit a lower damage threshold, whereas in TiO2 expansion releases the stress and prevents solid–solid phase transition thereby increasing the damage threshold up to the melting one. The results suggest that a nonthermal phase transition is a general response of oxides to sufficiently high ultrafast electronic excitation. A comparison with nonadiabatic simulations demonstrates that Born–Oppenheimer approximation systematically overestimates damage thresholds, and in some cases misses a phase transition entirely.

Superionic State in Alumina Produced by Nonthermal Melting

Density functional–based molecular dynamics reveals a transient superionic state in Al2O3 produced by nonthermal phase transition under extreme electronic excitation. At electronic temperatures above Te ≈ 2.75 eV, the oxygen sublattice exhibits fluid behavior, whereas the aluminum sublattice is in a solid state. This state exists up to Te ≈ 3.25 eV, where Al2O3 turns metallic–superionic; at Te ≈ 3.75 eV, the aluminum sublattice disorders. Quenching the superionic state under pressure >400 GPa freezes it into a metastable mixed amorphous–crystalline phase where the oxygen subsystem is disordered solid, whereas the aluminum one is ordered.

О фотоиндуцированной гетероструктуре в пленке двуокиси ванадия

A non-thermal photoinduced semiconductor-metal phase transition occurring over a time Δt <1ps in a vanadium dioxide film on an aluminum substrate is theoretically investigated. It is shown that under the action of a short laser pulse in the VO2 film a structure of metal and semiconductor layers parallel to the substrate is formed. The dependence of the layer thickness on the energy density W of the laser pulse is obtained. A diagram is constructed that determines the number of layers depending on the energy density W of the laser pulse and the thickness a of the film. A comparison with experimental data is made.

Phase-change like process through bond switching in distorted and resonantly bonded crystal

Although some methods to improve phase-change memory efficiency have been proposed, an effective experimental approach to induce a phase-change like process without external heat energy has not yet been reported. Herein we have shown that GeTe is a prototype phase-change material, which can exhibit a non-thermal phase-change-like process under uniaxial stress. Due to its structural characteristics like directional structural instability and resonance bonding under 1% uniaxial stress, we observed that bond switching in the GeTe film between short and long bonds is possible. Due to this phase change, GeTe displays the same phase-change as crystal layer rotation. Crystal layer rotation has not been observed in the conventional phase change process using intermediate states, but it is related to the structural characteristics required for maintaining local coordination. Moreover, since the resonance bonding characteristics are effectively turned off upon applying uniaxial stress, the high-frequency dielectric constant can be significantly decreased. Our results also show that the most significant process in the non-thermal phase transition of phase-change materials is the modulation of the lattice relaxation process after the initial perturbation, rather than the method inducing the perturbation itself. Finally, these consequences suggest that a new type of phase-change memory is possible through changes in the optical properties under stress.

Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta2NiSe5

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the \Gamma\ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.

Dissipative Dicke model with collective atomic decay: Bistability, noise-driven activation, and the nonthermal first-order superradiance transition

The Dicke model describes the coherent interaction of a laser-driven ensemble of two level atoms with a quantized light field. It is realized within cavity QED experiments, which in addition to the coherent Dicke dynamics feature dissipation due to e.g. atomic spontaneous emission and cavity photon loss. Spontaneous emission supports the uncorrelated decay of individual atomic excitations as well as the enhanced, collective decay of an excitation that is shared by $N$ atoms and whose strength is determined by the cavity geometry. We derive a many-body master equation for the dissipative Dicke model including both spontaneous emission channels and analyze its dynamics on the basis of Heisenberg-Langevin and stochastic Bloch equations. We find that the collective loss channel leads to a region of bistability between the empty and the superradiant state. Transitions between these states are driven by non-thermal, markovian noise. The interplay between dissipative and coherent elements leads to a genuine non-equilibrium dynamics in the bistable regime, which is expressed via a non-conservative force and a multiplicative noise kernel appearing in the stochastic Bloch equations. We present a semiclassical approach, based on stochastic nonlinear optical Bloch equations, which for the infinite-range Dicke Model become exact in the large-$N$-limit. The absence of an effective free energy functional, however, necessitates to include fluctuation corrections with $\mathcal{O}(1/N)$ for finite $N<\infty$ to locate the non-thermal first-order phase transition between the superradiant and the empty cavity.

Time-dependent correlation functions in open quadratic fermionic systems

We formulate and discuss the explicit computation of dynamic correlation functions in open quadratic fermionic systems which are driven and dissipated by Lindblad jump processes that are linear in canonical fermionic operators. Dynamic correlators are interpreted in terms of a local quantum quench where the pre-quench state is a non-equilibrium steady state, i.e. a fixed point of the Liouvillian. As an example, we study the XY spin 1/2 chain and quadratic Majorana chains with boundary Lindblad driving, whose dynamics exhibit asymmetric (skewed) light cone behaviour. We also numerically treat the two-dimensional XY model and the XY spin chain with additional Dzyaloshinskii–Moriya interactions. The latter exhibits a new non-thermal phase transition which can be understood in terms of bifurcations of the quasi-particle dispersion relation.Finally, considering in some detail the periodic quadratic fermionic ring with dissipation at a single (arbitrary) site, we present analytical expressions for first order corrections (in the strength of dissipation) to the spectrum and non-equilibrium steady state (NESS) correlation functions.

Molecular dynamics simulations of a femtosecond-laser-induced solid-to-solid transition in antimony

We performed ab initio molecular dynamics (MD) simulations to describe the ultrafast dynamics of laser-excited antimony on a supercell consisting of 864 atoms. For low laser fluences (represented in our theory by moderate electronic temperatures), we obtain the well-known oscillations of the crystal planes in the [111] direction, corresponding to the large amplitude coherent A $$_{1\rm g}$$ phonon. For large fluences (high electronic temperature) below the melting threshold, simulations suggest a possible transition from the initial, Peierls-distorted A7 structure into a structure without Peierls distortion. However, fluctuations due to finite size effects prevent a clean demonstration of such a nonthermal phase transition. Therefore, and based on the ab initio results, we derived an analytical potential depending on the electronic temperature and used it to perform large-scale MD simulations in supercells containing up to 10 $$^6$$ atoms. The potential can clearly reproduce the nonthermal phenomena and the excitation of the A $$_{1\rm g}$$ coherent phonon observed in the ab initio results. Most importantly, due to the minimization of finite size effects, our large-scale simulations predict a clean nonthermal transition from the Peierls-distorted A7 structure into a structure without Peierls distortion.

Carrier-Multiplication-Induced Structural Change during Ultrafast Carrier Relaxation and Nonthermal Phase Transition in Semiconductors.

While being extensively studied as an important physical process to alter exciton population in nanostructures at the fs time scale, carrier multiplication has not been considered seriously as a major mechanism for phase transition. Real-time time-dependent density functional theory study of Ge_{2}Sb_{2}Te_{5} reveals that carrier multiplication can induce an ultrafast phase transition in the solid state despite that the lattice remains cold. The results also unify the experimental findings in other semiconductors for which the explanation remains to be the 30-year old phenomenological plasma annealing model.

Ab initio investigation of photoinduced non-thermal phase transition in β-cristobalite**Project support by the National Natural Science Foundation of China (Grant Nos. 11374217 and 11547158).

Using the linear-response method, we investigate the phonon properties of β-cristobalite crystal under electronic excitation effect. We find that the transverse-acoustic phonon frequency becomes imaginary as the electron temperature is increased, which means that the lattice of β-cristobalite becomes unstable under intense laser irradiation. In addition, for the optic phonon mode, the LO(H)–TO(H) splitting disappears when the electronic temperature reaches a certain value, corresponding to the whole transverse-acoustic phonon branches becoming negative. It means that the electronic excitation destroys the macroscopic electric field of β-cristobalite. Based on the calculated phonon band structures, some thermodynamic properties are calculated as a function of temperature at different electronic temperatures. These investigations provide evidence that non-thermal melting takes place during a femtosecond pulse laser interaction with β-cristobalite.