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Register a new userUltrafast photoinduced nonthermal melting due to local dynamic instability
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Laser-induced nonthermal melting in semiconductors has been studied for several decades, but the melting mechanism is still under debate. Based on real-time time-dependent density functional theory (rt-TDDFT) simulation, we reveal that the rapid nonthermal melting induced by photoexcitation in silicon originates from a local dynamic instability rather than a homogeneous inertial mechanism. Due to this local dynamic instability, any initial small random displacements can be amplified, create a local self-trapping mechanism for the excited carrier. This carrier self-trapping will amplify the initial randomness, cause locally nonthermal melting spots. Such locally melted spots gradually diffuse to the whole system achieving overall nonthermal melting within 200 fs. We also found that the initial hot carrier cooling towards the anti-bonding state is essential in order to realize this dynamic instability. This causes different cooling time depending on the excitation laser frequency, in accordance with the experimental observations. Our study provides an exquisite detail for the nonthermal melting mechanism.
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We review recent progress in utilizing ultrafast light-matter interaction to control the macroscopic properties of quantum materials. Particular emphasis is placed on photoinduced phenomena that do not result from ultrafast heating effects but rather emerge from microscopic processes that are inherently nonthermal in nature. Many of these processes can be described as transient modifications to the free-energy landscape resulting from the redistribution of quasiparticle populations, the dynamical modification of coupling strengths and the resonant driving of the crystal lattice. Other pathways result from the coherent dressing of a materials quantum states by the light field. We discuss a selection of recently discovered effects leveraging these mechanisms, as well as the technological advances that led to their discovery. A road map for how the field can harness these nonthermal pathways to create new functionalities is presented.
Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.
Quasi-two-dimensional transition metal dichalcogenides (TMDs) are a key platform for exploring emergent nanoscale phenomena arising from complex interactions. Access to the underlying degrees of freedom on their natural time scales motivates the use of advanced ultrafast probes that directly couple to the self-organised electronic and atomic structural order. Here, we report the first ultrafast investigation of TaTe2, which exhibits unique charge and lattice structural order characterised by a transition upon cooling from stripe-like trimer chains into a $(3 times 3)$ superstructure of trimer clusters. Utilising MeV-scale ultrafast electron diffraction, the TaTe2 structural dynamics is resolved following intense pulsed laser excitation at 1.2 eV. We observe a rapid $approx!1.4$ ps melting of the low-temperature ordered state, followed by recovery of the clusters via thermalisation into a hot superstructure persisting for extended times. Density-functional calculations indicate that the initial quench is triggered by Ta trimer bonding to nonbonding transitions that destabilises the clusters, unlike melting of charge density waves in other TaX2 compounds. Our work paves the way for further exploration and ultimately directed manipulation of the trimer superstructure for novel applications.
We report femtosecond optical pump and x-ray diffraction probe experiments on SnSe. We find that under photoexcitation, SnSe has an instability towards an orthorhombically-distorted rocksalt structure that is not present in the equilibrium phase diagram. The new lattice instability is accompanied by a drastic softening of the lowest frequency A$_g$ phonon which is usually associated with the thermodynamic Pnma-Cmcm transition. However, our reconstruction of the transient atomic displacements shows that instead of moving towards the Cmcm structure, the material moves towards a more symmetric orthorhombic distortion of the rock-salt structure belonging to the Immm space group. The experimental results combined with density functional theory (DFT) simulations show that photoexcitation can act as a state-selective perturbation of the electronic distribution, in this case by promoting electrons from Se 4$p$ Sn 5$s$ derived bands from deep below the Fermi level. The subsequent potential energy landscape modified by such electronic excitation can reveal minima with metastable phases that are distinct from those accessible in equilibrium. These results may have implications for optical control of the thermoelectric, ferroelectric and topological properties of the monochalcogenides and related 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, Ge2Sb2Te5 has been widely used in optical data storage and non-volatile 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 and TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the correlated local structure in the averaged rock-salt phase of Ge2Sb2Te5. The ultrafast suppression of the local Peierls distortions gives rise to a local structure change from the rhombohedral to the cubic geometry within ~ 0.3 ps. Our work provides new microscopic insights into contributions of the correlated local structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the correlated local structure in the subunit cell and the link between the correlated disorder and physical properties in functional materials with complex microstructures.
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