Simon Wall

Ultrafast changes in lattice symmetry probed by coherent phonons.

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Optical switching in VO2 films by below-gap excitation

We study the photo-induced insulator-metal transition in VO2, correlating threshold and dynamic evolution with excitation wavelength. In high-quality single crystal samples, we find that switching can only be induced with photon energies above the 670-meV gap. This contrasts with the case of polycrystalline films, where formation of the metallic state can also be triggered with photon energies as low as 180 meV, well below the bandgap. Perfection of this process may be conducive to novel schemes for optical switches, limiters and detectors, operating at room temperature in the mid-IR.

Driving magnetic order in a manganite by ultrafast lattice excitation

Femtosecond midinfrared pulses are used to directly excite the lattice of the single-layer manganite La0.5Sr1.5MnO4. Magnetic and orbital orders, as measured by femtosecond resonant soft x-ray diffraction with an x-ray free-electron laser, are reduced within a few picoseconds. This effect is interpreted as a displacive exchange quench, a prompt shift in the equilibrium value of the magnetic- and orbital-order parameters after the lattice has been distorted. Control of magnetism through ultrafast lattice excitation may be of use for high-speed optomagnetism.

Tracking the motion of charges in a terahertz light field by femtosecond X-ray diffraction

In condensed matter, light propagation near resonances is described in terms of polaritons, electro-mechanical excitations in which the time-dependent electric field is coupled to the oscillation of charged masses. This description underpins our understanding of the macroscopic optical properties of solids, liquids and plasmas, as well as of their dispersion with frequency. In ferroelectric materials, terahertz radiation propagates by driving infrared-active lattice vibrations, resulting in phonon-polariton waves. Electro-optic sampling with femtosecond optical pulses can measure the time-dependent electrical polarization, providing a phase-sensitive analogue to optical Raman scattering. Here we use femtosecond time-resolved X-ray diffraction, a phase-sensitive analogue to inelastic X-ray scattering, to measure the corresponding displacements of ions in ferroelectric lithium tantalate, LiTaO3. Amplitude and phase of all degrees of freedom in a light field are thus directly measured in the time domain. Notably, extension of other X-ray techniques to the femtosecond timescale (for example, magnetic or anomalous scattering) would allow for studies in complex systems, where electric fields couple to multiple degrees of freedom.

Time-domain separation of optical properties from structural transitions in resonantly bonded materials

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

Photoinduced Melting of Antiferromagnetic Order in La0.5Sr1.5MnO4 Measured Using Ultrafast Resonant Soft X-Ray Diffraction

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

Analysis of published evidence on minimally invasive total hip arthroplasty.

Minimally invasive surgery has become a popular method of total hip arthroplasty. This study reviewed the literature to determine the number and quality of scientific publications analyzing different types of minimally invasive surgery approaches. The miniposterior approach has been studied the most and to our knowledge is the only approach with good quality randomized control study evidence. The overall length of follow-up and quality of reports for minimally invasive total hip arthroplasty is low.

Quantum interference between charge excitation paths in a solid-state Mott insulator

Ultrafast spectroscopy reveals the many-body effects behind the metallization of a one-dimensional Mott insulator. Unlike in ultracold gases, these femtosecond excitation studies of quantum dynamics occur at room temperature.

Biomechanical comparison of three methods of sacral fracture fixation in osteoporotic bone.

Study Design. Biomechanical cadaveric bench study. Objective. To measure motion at the fracture site in an osteoporotic cadaveric sacral insufficiency fracture model before and after fracture creation, after fixation (via 1 of 3 fixation techniques), and after cyclic loading and to compare those values with motion of the intact pelvis. Summary of Background Data. Sacral insufficiency fractures occur frequently in the elderly and pose treatment challenges. Screw fixation and sacroplasty have been proposed as possible treatments. There is little information about the stabilization provided by these treatments. Methods. We potted 18 osteoporotic cadaveric pelves, mounted them on a materials testing machine, measured sacroiliac (SI) joint motion with a vertical load applied to the lumbar spine, and created simulated sacral insufficiency fractures. Then, we measured fracture site motion under load and repaired the fracture using 1 of 3 techniques: a unilateral SI screw, a bilateral SI screw, or sacroplasty. A vertical compressive load (10–350 N) was applied cyclically at 0.5 Hz to the lumbar spine of the repaired specimens for 5000 cycles. Kinematic analysis was conducted prefracture, postfracture, postrepair, and after cyclic loading. Results. Postfracture, there was a significant increase in motion relative to the intact SI joint. After fixation, the average motion in all 3 groups was similar to that of the intact pelvis. After cyclical loading, motion increased in all groups. No significant differences were found between treatments. Conclusion. All 3 fixation methods resulted acutely in motion similar to that of the intact pelvis. Although motion increased as a function of cyclical loading, no significant differences were found between fixation methods. All 3 repair methods reduced fracture site motion, but clinical studies are needed to determine if each method relieves pain and provides sufficient fixation for fracture healing.

Strain-engineered diffusive atomic switching in two-dimensional crystals.

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb2Te3–GeTe van der Waals superlattice. The number of quintuple Sb2Te3 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb2Te3–GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.