All-optical seeding of a light-induced phase transition with correlated disorder

Ultrafast manipulation of vibrational coherence is an emergent route to control the structure of solids. However, this strategy can only induce long-range correlations and cannot modify atomic structure locally, which is required in many technologically-relevant phase transitions. Here, we demonstrate that ultrafast lasers can generate incoherent structural fluctuations which are more efficient for material control than coherent vibrations, extending optical control to a wider range of materials. We observe that local, non-equilibrium lattice distortions generated by a weak laser pulse reduce the energy barrier to switch between insulating and metallic states in vanadium dioxide by 6%. Seeding inhomogeneous structural-fluctuations presents an alternative, more energy efficient, route for controlling materials that may be applicable to all solids, including those used in data and energy storage devices

Ultrafast Suppression of the Ferroelectric Instability in KTaO3_3

We use an x-ray free-electron laser to study the ultrafast lattice dynamics following above band-gap photoexcitation of the incipient ferroelectric potassium-tantalate, \kto. % We use ultrafast near-UV (central wavelength 266\,nm and 50 fs pulse duration) laser light to photoexcite charge carriers across the gap and probe the ultrafast lattice dynamics by recording the x-ray diffuse intensity throughout multiple Brillouin zones using pulses from the Linac Coherent Light Source (LCLS) (central wavelength 1.3\,\AA\, and $< 10$~fs pulse duration). We observe changes in the diffuse intensity that we conclude are associated with a hardening of the soft transverse optical and transverse acoustic phonon branches along $\Gamma$ to $X$ and $\Gamma$ to $M$. Using ground- and excited-state interatomic force constants from density functional theory (DFT) and assuming the phonon populations can be described by a time-dependent temperature, we fit the quasi-equilibrium thermal diffuse intensity to the experimental time-dependent intensity. We obtain the instantaneous lattice temperature and density of photoexcited charge carriers as a function of time delay. The DFT calculations demonstrate that photoexcitation transfers charge from oxygen $2p$ derived $\pi$-bonding orbitals to Ta $5d$ derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.Comment: 8 pages, 4 figure

Ultrafast measurements of mode-specific deformation potentials of Bi2_2Te3_3 and Bi2_2Se3_3

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi$_2$Te$_3$ and Bi$_2$Se$_3$, following the excitation of coherent A$_{1g}$ optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A$_{1g}$-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES.We find the coupling in Bi$_2$Te$_3$ and Bi$_2$Se$_3$ to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling

All-optical seeding of a light-induced phase transition with correlated disorder

International audienceUltrafast manipulation of vibrational coherence provides a route to control the structure of solids. However, this strategy can only induce long-range correlations and cannot modify atomic structure locally, which is a requirement for many technological applications such as non-volatile electronics. Here we demonstrate that ultrafast lasers can generate incoherent structural fluctuations that are more efficient for material control than coherent vibrations, extending optical control to a wide range of materials. We observe that local non-equilibrium lattice distortions generated by a weak laser pulse reduce the energy barrier to switch between insulating and metallic states in vanadium dioxide. Seeding inhomogeneous structural fluctuations presents an alternative, more energy-efficient, route for controlling materials that may be applicable to all solids, including those used in data- and energy-storage devices. Controlling phase transitions in solids is crucial for many applications. Ultrafast laser pulses have now been shown to enable the energy-efficient generation of structural fluctuations in VO2 by harnessing the correlated disorder in the material

Ultrafast lattice disordering can be accelerated by electronic collisional forces

International audienceThe Born-Oppenheimer approximation is the prevailing assumption for interpreting ultrafast electron dynamics in solids. Evidence now suggests that collisions between electrons and lattice not captured by this approximation play an important role. In the prevalent picture of ultrafast structural phase transitions, atomic motion occurs in a slowly varying potential energy surface adiabatically determined by fast electrons. However, this ignores non-conservative forces caused by electron-lattice collisions, which can substantially influence atomic motion. Most ultrafast techniques only probe the average structure and are less sensitive to random displacements and therefore do not detect the role played by non-conservative forces in phase transitions. Here we show that the lattice dynamics of the prototypical insulator-metal transition of vanadium dioxide cannot be described by potential energy alone. We use the sample temperature to control the preexisting lattice disorder before ultrafast photoexcitation across the phase transition and our ultrafast diffuse scattering experiments show that the fluctuations characteristic of rutile metal develop equally fast (120 fs) at initial temperatures of 100 and 300 K. This indicates that additional non-conservative forces are responsible for the increased lattice disorder. These results highlight the need for more sophisticated descriptions of ultrafast phenomena beyond the Born-Oppenheimer approximation as well as ultrafast probes of spatial fluctuations beyond the average unit cell measured by diffraction

Ultrafast Measurements of Mode-Specific Deformation Potentials of Bi₂Te₃ and Bi₂Se₃

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi₂Te₃ and Bi₂Se₃, following the excitation of coherent A_1g optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A_1g-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES. We find the coupling in Bi₂Te₃ and Bi₂Se₃ to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.ISSN:2160-330