Role of electronic excitation in the amorphization of Ge-Sb-Te alloys

Light-Induced Complete Reversal of Ferroelectric Polarization in Sliding Ferroelectrics

Previous experiments have provided evidence of sliding ferroelectricity and photoexcited interlayer shear displacement in two-dimensional materials, respectively. Herein, we find that a complete reversal of vertical ferroelectric polarization can be achieved within an astonishing 0.5 ps in h-BN bilayer by laser illumination. Comprehensive analysis suggests that ferroelectric polarization switching originates from laser-induced interlayer sliding triggered by selective excitation of multiple phonons. The interlayer electron excitation from the p_{z} orbitals of the upper layer N atoms to the p_{z} orbitals of the lower layer B atoms produces desirable and directional interlayer forces activating the in-plane optical TO-1 and LO-1 phonon modes. The atomic motions driven by the coupling of TO-1 and LO-1 modes are coherent with ferroelectric soft mode, thus modulating the dynamical potential energy surface and resulting in ultrafast ferroelectric polarization reversal. Our work provides a novel microscopic insight into ultrafast polarization switching in sliding ferroelectrics.

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 ab initio 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.

Ab Initio Molecular Dynamics Study of Electron Excitation Effects on UO2 and U3Si

In this study, an ab initio molecular dynamics method is employed to investigate how the microstructures of UO2 and U3Si evolve under electron excitation. It is found that the U3Si is more resistant to electron excitation than UO2 at room temperature. UO2 undergoes a crystalline-to-amorphous structural transition with an electronic excitation concentration of 3.6%, whereas U3Si maintains a crystalline structure until an electronic excitation concentration reaches up to 6%. Such discrepancy is mainly due to their different electronic structures. For insulator UO2, once valence U 5f electrons receive enough energy, they are excited to the conduction bands, which induces charge redistribution. Anion disordering is then driven by cation disordering, eventually resulting in structural amorphization. As for metallic U3Si, the U 5f electrons are relatively more difficult to excite, and the electron excitation leads to cation disordering, which eventually drives the crystalline-to-amorphous phase transition. This study reveals that U3Si is more resistant to electron excitation than UO2 under an irradiation environment, which may advance the understanding of related experimental and theoretical investigations to design radiation-resistant nuclear fuel uranium materials.

Optically Induced Multistage Phase Transition in Coherent Phonon-Dominated a-GeTe

Ultrafast photoexcitation can decouple the multilevel nonequilibrium dynamics of electron-lattice interactions, providing an ideal probe for dissecting photoinduced phase transition in solids. Here, real-time time-dependent density functional theory simulations combined with occupation-constrained DFT methods are employed to explore the nonadiabatic paths of optically excited a-GeTe. Results show that the short-wavelength ultrafast laser is capable of generating full-domain carrier excitation and repopulation, whereas the long-wavelength ultrafast laser favors the excitation of lone pair electrons in the antibonded state. Photodoping makes the double-valley potential energy surface shallower and allows the insertion of A_1g coherent forces in the atomic pairs, by which the phase reversal of Ge and Te atoms in the ⟨001⟩ direction is activated with ultrafast suppression of the Peierls distortion. These findings have far-reaching implications regarding nonequilibrium phase engineering strategies based on phase-change materials.

Electronic state evolution of oxygen-doped monolayer WSe2 assisted by femtosecond laser irradiation

Electronic states are significantly correlated with chemical compositions, and the information related to these factors is especially crucial for the manipulation of the properties of matter. However, this key information is usually verified by after-validation methods, which could not be obtained during material processing, for example, in the field of femtosecond laser direct writing inside materials. Here, critical evolution stages of electronic states for monolayer tungsten diselenide (WSe₂) around the modification threshold (at a Mott density of ∼10¹³ cm^-2) are observed by broadband femtosecond transient absorption spectroscopy, which is associated with the intense femtosecond-laser-assisted oxygen-doping mechanism. First-principles calculations and control experiments on graphene-covered monolayer WSe₂ further confirm this modification mechanism. Our findings reveal a photochemical reaction for monolayer WSe₂ under the Mott density condition and provide an electronic state criterion to in situ monitor the degrees of modification in monolayer transition metal dichalcogenides during the femtosecond laser modification.

Ultrafast Temporal-Spatial Dynamics of Phase Transition in N-Doped Ge2Sb2Te5 Film Induced by Femtosecond Laser Pulse Irradiation

Element-doped phase change material (PCM) could improve the performances, e.g., better thermal stability, higher electrical resistance, and faster crystallization speed; thus, the influence of the doping element needs to be further investigated. In this paper, a femtosecond laser, which could realize the ultrafast phase transition rate of PCM between amorphization and crystallization, was used to explore the properties of nitrogen-doped Ge₂Sb₂Te₅ (GST), and a bond effect was proposed. The pure GST and different nitrogen contents of doped GST films were investigated by femtosecond laser pulse excitation through a pump-probe shadowgraph imaging technique. The results showed that the element-doped films could change photon absorption because of the increase in free carriers. This caused the faster rate of reflectivity to change in the irradiated area by the laser beam as the more nitrogen doped. When the nitrogen content increased, the crystallization evolution became harder because it enhanced the bond effect, which suppressed crystalline grain growth and improved the thermal stability. Based on the analysis in the paper, the desired performances of PCMs, e.g., ultrafast dynamics, crystallization evolution, and thermal stability, could be controlled according to the demands by modifying the bond effect.

Photoexcitation-Induced Nonthermal Ultrafast Loss of Long-Range Order in GeTe

Nonadiabatic quantum molecular dynamics is used to investigate the evolution of GeTe photoexcited states. Results reveal a photoexcitation-induced picosecond nonthermal path for the loss of long-range order. A valence electron excitation threshold of 4% is found to trigger local disorder by switching Ge atoms from octahedral to tetrahedral sites and promoting Ge-Ge bonding. The resulting loss of long-range order for a higher valence electron excitation fraction is achieved without fulfilling the Lindemann criterion for melting, therefore utilizing a nonthermal path. The photoexcitation-induced structural disorder is accompanied by charge transfer from Te to Ge, Ge-Te bonding-to-antibonding, and Ge-Ge antibonding-to-bonding change, triggering Ge-Te bond breaking and promoting the formation of Ge-Ge wrong bonds. These results provide an electronic-structure basis to understand the photoexcitation-induced ultrafast changes in the structure and properties of GeTe and other phase-change materials.

Photoinduced Ultrafast Transition of the Local Correlated Structure in Chalcogenide Phase-Change Materials

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, Ge_{2}Sb_{2}Te_{5} has been widely used in optical data storage and nonvolatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation, and time-dependent density-functional theory molecular dynamic simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rocksalt phase of Ge_{2}Sb_{2}Te_{5}. The randomly oriented Peierls distortion among unit cells in the averaged rocksalt phase of Ge_{2}Sb_{2}Te_{5} is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in the individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within ∼0.3  ps. In addition, the impact of the carrier relaxation and the large number of vacancies to the ultrafast structural response is quantified and discussed. Our Letter provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures.

Low-loss ultrafast and non-volatile all-optical switch enabled by all-dielectric phase change materials

All-optical switches show great potential to overcome the speed and power consumption limitations of electrical switching. Owing to its nonvolatile and superb cycle abilities, phase-change materials enabled all-optical switch (PC-AOS) is attracting much attention. However, realizing low-loss and ultrafast switching remains a challenge, because previous PC-AOS are mostly based on plasmonic metamaterials. The high thermal conductance of metallic materials disturbs the thermal accumulation for phase transition, and eventually decreases the switching speed to tens of nanoseconds. Here, we demonstrate an ultrafast switching (4.5 ps) and low-loss (2.8 dB) all-optical switch based on all-dielectric structure consisting of Ge₂Sb₂Te₅ and photonic crystals. Its switching speed is approximately ten thousand times faster than the plasmonic one. A 5.4 dB on-off ratio at 1550 nm has been experimentally achieved. We believe that the proposed all-dielectric optical switch will accelerate the progress of ultrafast and energy-efficient photonic devices and systems.

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All-dielectric phase change materials are used to achieve low loss all optical switch

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Only 15 nm phase change film is used for laser induced ultrafast switching

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Up to 7.4 dB switching contrast can be realized in the Near Infrared Spectrum

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Nano-hole array metasurface enables polarization insensitive optical filtering

Light-Tunable Charge Density Wave Orders in MoTe_{2} and WTe_{2} Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure, and band gap. Moreover, optical pumping of the semimetallic 1T^{'} phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

Orbital-selective electronic excitation in phase-change memory materials: a brief review

Ultrafast laser-induced phase/structural transitions show a great potential in optical memory and optical computing technologies, which are believed to have advantages of ultrafast speed, low power consumption, less heat diffusion and remote control as compared with electronic devices. Here, we review and discuss the principles of orbital-selective electronic excitation and its roles in phase/structural transitions of phase-change memory (PCM) materials, including Sc0.2Sb1.8Te3 and GeTe phases. It is demonstrated, that the mechanism can influence the dynamics or results of structural transitions, such as an ultrafast amorphization of Sc0.2Sb1.8Te3 and a non-volatile order-to-order structural transition of GeTe. Without thermal melting, these structural transitions have the advantages of ultrafast speed and low power consumption. It suggests that the orbital-selective electronic excitation can play a significant role in discovering new physics of phase change and shows a potential for new applications.

Superconducting Phase Induced by a Local Structure Transition in Amorphous Sb_{2}Se_{3} under High Pressure

Superconductivity and Anderson localization represent two extreme cases of electronic behavior in solids. Surprisingly, these two competing scenarios can occur in the same quantum system, e.g., in an amorphous superconductor. Although the disorder-driven quantum phase transition has attracted much attention, its structural origins remain elusive. Here, we discovered an unambiguous correlation between superconductivity and density in amorphous Sb_{2}Se_{3} at high pressure. Superconductivity first emerges in the high-density amorphous (HDA) phase at about 24 GPa, where the density of glass unexpectedly exceeds its crystalline counterpart, and then shows an enhanced critical temperature when pressure induces crystallization at 51 GPa. Ab initio simulations reveal that the bcc-like local geometry motifs form in the HDA phase, arising from distinct "metavalent bonds." Our results demonstrate that HDA phase is critical for the incipient superconductive behavior.

A Comparative Study of Electron Radiation Responses of Pu2Zr2O7 and La2Zr2O7: An abinitio Molecular Dynamics Study

In this study, the response of Pu₂Zr₂O₇ and La₂Zr₂O₇ to electronic radiation is simulated, employing an ab initio molecular dynamics method. It is shown that Pu₂Zr₂O₇ undergoes a crystalline-to-amorphous structural transition with 0.3% electronic excitation, while for La₂Zr₂O₇, the structural amorphization occurs with 1.2% electronic excitation. During the microstructural evolution, the anion disorder further drives cation disorder and eventually results in the structural amorphization of Pu₂Zr₂O₇ and La₂Zr₂O₇. The difference in responses to electron radiation between Pu₂Zr₂O₇ and La₂Zr₂O₇ mainly results from the strong correlation effects between Pu 5f electrons and the smaller band gap of Pu₂Zr₂O₇. These results suggest that Pu₂Zr₂O₇ is less resistant to amorphization under local ionization rates that produce a low level of electronic excitation, since the level of the concentration of excited electrons is relatively low in Pu₂Zr₂O₇. The presented results will advance the understanding of the radiation damage effects of zirconate pyrochlores.

Effect of structure and composition on the electronic excitation induced amorphization of La2Ti2-xZrxO7 ceramics

Understanding the response of ceramics operating in extreme environments is of interest for a variety of applications. Ab initio molecular dynamic simulations have been used to investigate the effect of structure and B-site (=Ti, Zr) cation composition of lanthanum-based oxides (La₂B₂O₇) on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La₂Ti₂O₇ occurs for a lower degree of electronic excitation than for cubic pyrochlore La₂Zr₂O₇. While in each case the formation of O₂-like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO₆ octahedra in the monoclinic phase can promote such molecule formation, while such octahedral rotation is not possible in the cubic phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La₂TiZrO₇) is found to be more resistant in the monoclinic than in the cubic phase, which may be related to the lower bandgap of the cubic phase. These results illustrate the complex interplay of structure and composition that give rise to the radiation resistance of these important functional materials.

Directional Forces by Momentumless Excitation and Order-to-Order Transition in Peierls-Distorted Solids: The Case of GeTe

Time-dependent density-functional theory molecular dynamics reveals an unexpected effect of optical excitation in the experimentally observed rhombohedral-to-cubic transition of GeTe. The excitation induces coherent forces along [001], which may be attributed to the unique energy landscape of Peierls-distorted solids. The forces drive the A_{1g} optical phonon mode in which Ge and Te move out of phase. Upon damping of the A_{1g} mode, phase transition takes place, which involves no atomic diffusion, defect formation, or the nucleation and growth of the cubic phase.

Facile fabrication of flexible graphene FETs by sunlight reduction of graphene oxide

We reported here a facile fabrication of flexible graphene-based field effect transistors (FETs) by sunlight reduction of graphene oxide (GO) as channel material. As a mask-free and chemical-free method, sunlight photoreduction of GO without the use of any complex equipments is simple and green. The resultant FET demonstrated excellent electrical properties (e.g., an optimized I_on/I_off ratio of 111, hole mobility of 0.17  cm²  V^-1 s^-1), revealing great potential for development of flexible microelectrics. Additionally, since the reduced GO channel could be fabricated by sunlight treatment between two pre-patterned electrodes, the process features post-fabrication capability, which makes it possible to integrate graphene-based devices with given device structures.

Giant lattice expansion by quantum stress and universal atomic forces in semiconductors under instant ultrafast laser excitation

Electronic excitation induced stress and force may provide a new route to manipulate the structure of materials using ultrafast lasers.

Electronic excitation induced hydrogen-bond adjustment and lattice control in organic–inorganic hybrid cubic perovskites: a fixed occupation molecular dynamics study

Fixed occupation first-principles study reveals the effect of electronic excitation on lattice of cubic perovskite MAPbI₃. With excitations, the hydrogen bonding between MA molecules and inorganic lattice is weakened and the cubic symmetry is recovered.

Initial Atomic Motion Immediately Following Femtosecond-Laser Excitation in Phase-Change Materials

Despite the fact that phase-change materials are widely used for data storage, no consensus exists on the unique mechanism of their ultrafast phase change and its accompanied large and rapid optical change. By using the pump-probe observation method combining a femtosecond optical laser and an x-ray free-electron laser, we substantiate experimentally that, in both GeTe and Ge_{2}Sb_{2}Te_{5} crystals, rattling motion of mainly Ge atoms takes place with keeping the off-center position just after femtosecond-optical-laser irradiation, which eventually leads to a higher symmetry or disordered state. This very initial rattling motion in the undistorted lattice can be related to instantaneous optical change due to the loss of resonant bonding that characterizes GeTe-based phase change materials. Based on the amorphous structure derived by first-principles molecular dynamics simulation, we infer a plausible ultrafast amorphization mechanism via nonmelting.

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.

Vacancy Structures and Melting Behavior in Rock-Salt GeSbTe

Ge-Sb-Te alloys have been widely used in optical/electrical memory storage. Because of the extremely fast crystalline-amorphous transition, they are also expected to play a vital role in next generation nonvolatile microelectronic memory devices. However, the distribution and structural properties of vacancies have been one of the key issues in determining the speed of melting (or amorphization), phase-stability, and heat-dissipation of rock-salt GeSbTe, which is crucial for its technological breakthrough in memory devices. Using spherical aberration-aberration corrected scanning transmission electron microscopy and atomic scale energy-dispersive X-ray mapping, we observe a new rock-salt structure with high-degree vacancy ordering (or layered-like ordering) at an elevated temperature, which is a result of phase transition from the rock-salt phase with randomly distributed vacancies. First-principles calculations reveal that the phase transition is an energetically favored process. Moreover, molecular dynamics studies suggest that the melting of the cubic rock-salt phases is initiated at the vacancies, which propagate to nearby regions. The observation of multi-rock-salt phases suggests another route for multi-level data storage using GeSbTe.

Exploring long-wave infrared transmitting materials with AxBy form: First-principles gene-like studies

Long-wave infrared (8-12 μm) transmitting materials play critical roles in space science and electronic science. However, the paradox between their mechanical strength and infrared transmitting performance seriously prohibits their applications in harsh external environment. From the experimental view, searching a good window material compatible with both properties is a vast trail-and-error engineering project, which is not readily achieved efficiently. In this work, we propose a very simple and efficient method to explore potential infrared window materials with suitable mechanical property by first-principles gene-like searching. Two hundred and fifty-three potential materials are evaluated to find their bulk modulus (for mechanical performance) and phonon vibrational frequency (for optical performance). Seven new potential candidates are selected, namely TiSe, TiS, MgS, CdF2, HgF2, CdO, and SrO. Especially, the performances of TiS and CdF2 can be comparable to that of the most popular commercial ZnS at high temperature. Finally, we propose possible ranges of infrared transmission for halogen, chalcogen and nitrogen compounds respectively to guide further exploration. The present strategy to explore IR window materials can significantly speed up the new development progress. The same idea can be used for other material rapid searching towards special functions and applications.

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived (>100 ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.

Coherent gigahertz phonons in Ge₂Sb₂Te₅ phase-change materials

Using ≈40 fs ultrashort laser pulses, we investigate the picosecond acoustic response from a prototypical phase change material, thin Ge2Sb2Te5 (GST) films with various thicknesses. After excitation with a 1.53 eV-energy pulse with a fluence of ≈5 mJ cm(-2), the time-resolved reflectivity change exhibits transient electronic response, followed by a combination of exponential-like strain and coherent acoustic phonons in the gigahertz (GHz) frequency range. The time-domain shape of the coherent acoustic pulse is well reproduced by the use of the strain model by Thomsen et al 1986 (Phys. Rev. B 34 4129). We found that the decay rate (the inverse of the relaxation time) of the acoustic phonon both in the amorphous and in the crystalline phases decreases as the film thickness increases. The thickness dependence of the acoustic phonon decay is well modeled based on both phonon-defect scattering and acoustic phonon attenuation at the GST/Si interface, and it is revealed that those scattering and attenuation are larger in crystalline GST films than those in amorphous GST films.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons

Multicomponent chalcogenides, such as quasi-binary GeTe-Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 10(6) write-erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump-pump-probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.

Understanding Phase-Change Memory Alloys from a Chemical Perspective

Phase-change memories (PCM) are associated with reversible ultra-fast low-energy crystal-to-amorphous switching in GeTe-based alloys co-existing with the high stability of the two phases at ambient temperature, a unique property that has been recently explained by the high fragility of the glass-forming liquid phase, where the activation barrier for crystallisation drastically increases as the temperature decreases from the glass-transition to room temperature. At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown. In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons. Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

Ultrafast time-resolved electron diffraction revealing the nonthermal dynamics of near-UV photoexcitation-induced amorphization in Ge2Sb2Te5

Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices. These phase transitions are achieved by laser irradiation via thermal processes. Recent studies have suggested the potential of nonthermal phase transitions in the chalcogenide glass material Ge2Sb2Te5 triggered by ultrashort optical pulses; however, a detailed understanding of the amorphization and damage mechanisms governed by nonthermal processes is still lacking. Here we performed ultrafast time-resolved electron diffraction and single-shot optical pump-probe measurements followed by femtosecond near-ultraviolet pulse irradiation to study the structural dynamics of polycrystalline Ge2Sb2Te5. The experimental results present a nonthermal crystal-to-amorphous phase transition of Ge2Sb2Te5 initiated by the displacements of Ge atoms. Above the fluence threshold, we found that the permanent amorphization caused by multi-displacement effects is accompanied by a partial hexagonal crystallization.

Photo-induced optical activity in phase-change memory materials

We demonstrate that optical activity in amorphous isotropic thin films of pure Ge₂Sb₂Te₅ and N-doped Ge₂Sb₂Te₅N phase-change memory materials can be induced using rapid photo crystallisation with circularly polarised laser light. The new anisotropic phase transition has been confirmed by circular dichroism measurements. This opens up the possibility of controlled induction of optical activity at the nanosecond time scale for exploitation in a new generation of high-density optical memory, fast chiroptical switches and chiral metamaterials.

Electronic excitation induced amorphization in titanate pyrochlores: an ab initio molecular dynamics study

The response of titanate pyrochlores (A₂Ti₂O₇, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O₂-like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations.

Regulating energy transfer of excited carriers and the case for excitation-induced hydrogen dissociation on hydrogenated graphene

Understanding and controlling of excited carrier dynamics is of fundamental and practical importance, particularly in photochemistry and solar energy applications. However, theory of energy relaxation of excited carriers is still in its early stage. Here, using ab initio molecular dynamics (MD) coupled with time-dependent density functional theory, we show a coverage-dependent energy transfer of photoexcited carriers in hydrogenated graphene, giving rise to distinctively different ion dynamics. Graphene with sparsely populated H is difficult to dissociate due to inefficient transfer of the excitation energy into kinetic energy of the H. In contrast, H can easily desorb from fully hydrogenated graphane. The key is to bring down the H antibonding state to the conduction band minimum as the band gap increases. These results can be contrasted to those of standard ground-state MD that predict H in the sparse case should be much less stable than that in fully hydrogenated graphane. Our findings thus signify the importance of carrying out explicit electronic dynamics in excited-state simulations.