75 research outputs found
Real-time observation of a coherent lattice transformation into a high-symmetry phase
Excursions far from their equilibrium structures can bring crystalline solids
through collective transformations including transitions into new phases that
may be transient or long-lived. Direct spectroscopic observation of
far-from-equilibrium rearrangements provides fundamental mechanistic insight
into chemical and structural transformations, and a potential route to
practical applications, including ultrafast optical control over material
structure and properties. However, in many cases photoinduced transitions are
irreversible or only slowly reversible, or the light fluence required exceeds
material damage thresholds. This precludes conventional ultrafast spectroscopy
in which optical excitation and probe pulses irradiate the sample many times,
each measurement providing information about the sample response at just one
probe delay time following excitation, with each measurement at a high
repetition rate and with the sample fully recovering its initial state in
between measurements. Using a single-shot, real-time measurement method, we
were able to observe the photoinduced phase transition from the semimetallic,
low-symmetry phase of crystalline bismuth into a high-symmetry phase whose
existence at high electronic excitation densities was predicted based on
earlier measurements at moderate excitation densities below the damage
threshold. Our observations indicate that coherent lattice vibrational motion
launched upon photoexcitation with an incident fluence above 10 mJ/cm2 in bulk
bismuth brings the lattice structure directly into the high-symmetry
configuration for tens of picoseconds, after which carrier relaxation and
diffusion restore the equilibrium lattice configuration.Comment: 22 pages, 4 figure
Carrier confinement and bond softening in photoexcited bismuth films
Femtosecond pump-probe spectroscopy of bismuth thin films has revealed strong dependencies of reflectivity and phonon frequency on film thickness in the range of 25−40 nm. The reflectivity variations are ascribed to distinct electronic structures originating from strongly varying electronic temperatures and proximity of the film thickness to the optical penetration depth of visible light. The phonon frequency is redshifted by an amount that increases with decreasing film thickness under the same excitation fluence, indicating carrier density-dependent bond softening that increases due to suppressed diffusion of carriers away from the photoexcited region in thin films. The results have significant implications for nonthermal melting of bismuth as well as lattice heating due to inelastic electron-phonon scattering.United States. Office of Naval Research (Grant N00014-12-1-0530)National Science Foundation (U.S.) (Grant CHE-1111557
The persistence of memory in ionic conduction probed by nonlinear optics
Predicting practical rates of transport in condensed phases enables the rational design of materials, devices and processes. This is especially critical to developing low-carbon energy technologies such as rechargeable batteries1,2,3. For ionic conduction, the collective mechanisms4,5, variation of conductivity with timescales6,7,8 and confinement9,10, and ambiguity in the phononic origin of translation11,12, call for a direct probe of the fundamental steps of ionic diffusion: ion hops. However, such hops are rare-event large-amplitude translations, and are challenging to excite and detect. Here we use single-cycle terahertz pumps to impulsively trigger ionic hopping in battery solid electrolytes. This is visualized by an induced transient birefringence, enabling direct probing of anisotropy in ionic hopping on the picosecond timescale. The relaxation of the transient signal measures the decay of orientational memory, and the production of entropy in diffusion. We extend experimental results using in silico transient birefringence to identify vibrational attempt frequencies for ion hopping. Using nonlinear optical methods, we probe ion transport at its fastest limit, distinguish correlated conduction mechanisms from a true random walk at the atomic scale, and demonstrate the connection between activated transport and the thermodynamics of information
The Persistence of Memory in Ionic Conduction Probed by Nonlinear Optics
Predicting practical rates of ion transport from atomistic descriptors
enables the rational design of materials, devices, and processes, which is
especially critical to developing low-carbon energy technologies such as
rechargeable batteries. The correlated mechanisms of ionic conduction,
variation of conductivity with timescale and confinement, and ambiguity in the
vibrational origin of translation, the attempt frequency, call for a direct
atomic probe of the most fundamental steps of ionic diffusion: ion hops.
However, such hops are rare-event large-amplitude translations, and are
challenging to excite and detect. Here we use single-cycle terahertz pumps to
impulsively trigger ionic hopping in battery solid electrolytes. This is
visualized by an induced transient birefringence enabling direct probing of
anisotropy in ionic hopping on the picosecond timescale. The relaxation of the
transient signal measures the decay of orientational memory, and the production
of entropy in diffusion. We extend experimental results using in silico
transient birefringence to identify attempt frequencies for ion hopping. Using
nonlinear optical methods, we probe ion transport at its fastest limit,
distinguish correlated conduction mechanisms from a true random walk at the
atomic scale, and demonstrate the connection between activated transport and
the thermodynamics of information.Comment: 41 pages, 22 figure
Cooperative photoinduced metastable phase control in strained manganite films
A major challenge in condensed matter physics is active control of quantum
phases. Dynamic control with pulsed electromagnetic fields can overcome
energetic barriers enabling access to transient or metastable states that are
not thermally accessible. Here we demonstrate strain-engineered tuning of
La2/3Ca1/3MnO3 into an emergent charge-ordered insulating phase with extreme
photo-susceptibility where even a single optical pulse can initiate a
transition to a long-lived metastable hidden metallic phase. Comprehensive
single-shot pulsed excitation measurements demonstrate that the transition is
cooperative and ultrafast, requiring a critical absorbed photon density to
activate local charge excitations that mediate magnetic-lattice coupling that,
in turn, stabilize the metallic phase. These results reveal that strain
engineering can tune emergent functionality towards proximal macroscopic states
to enable dynamic ultrafast optical phase switching and control
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In Situ X‑ray Scattering Reveals Coarsening Rates of Superlattices Self-Assembled from Electrostatically Stabilized Metal Nanocrystals Depend Nonmonotonically on Driving Force
Self-assembly of colloidal nanocrystals (NCs) into superlattices (SLs) is an appealing strategy to design hierarchically organized materials with promising functionalities. Mechanistic studies are still needed to uncover the design principles for SL self-assembly, but such studies have been difficult to perform due to the fast time and short length scales of NC systems. To address this challenge, we developed an apparatus to directly measure the evolving phases in situ and in real time of an electrostatically stabilized Au NC solution before, during, and after it is quenched to form SLs using small-angle X-ray scattering. By developing a quantitative model, we fit the time-dependent scattering patterns to obtain the phase diagram of the system and the kinetics of the colloidal and SL phases as a function of varying quench conditions. The extracted phase diagram is consistent with particles whose interactions are short in range relative to their diameter. We find the degree of SL order is primarily determined by fast (subsecond) initial nucleation and growth kinetics, while coarsening at later times depends nonmonotonically on the driving force for self-assembly. We validate these results by direct comparison with simulations and use them to suggest dynamic design principles to optimize the crystallinity within a finite time window. The combination of this measurement methodology, quantitative analysis, and simulation should be generalizable to elucidate and better control the microscopic self-assembly pathways of a wide range of bottom-up assembled systems and architectures
Ultrafast lattice disordering can be accelerated by electronic collisional forces
In the prevalent picture of ultrafast structural phase transitions, the
atomic motion occurs in a slowly varying potential energy surface determined
adiabatically by the fast electrons. However, this ignores non-conservative
forces caused by electron-lattice collisions, which can significantly 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-to-metal transition of VO2
cannot be described by a 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 the rutile metal develop equally
fast (120 fs) at initial temperatures of 100 K 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
Determination of nonthermal bonding origin of a novel photoexcited lattice instability in SnSe
Interatomic forces that bind materials are largely determined by an often
complex interplay between the electronic band-structure and the atomic
arrangements to form its equilibrium structure and dynamics. As these forces
also determine the phonon dispersion, lattice dynamics measurements are often
crucial tools for understanding how materials transform between different
structures. This is the case for the mono-chalcogenides which feature a number
of lattice instabilities associated with their network of resonant bonds and a
large tunability in their functional properties. SnSe hosts a novel lattice
instability upon above-bandgap photoexcitation that is distinct from the
distortions associated with its high temperature phase transition,
demonstrating that photoexcitation can alter the interatomic forces
significantly different than thermal excitation. Here we report decisive
time-resolved X-ray scattering-based measurements of the nonequlibrium lattice
dynamics in SnSe. By fitting interatomic force models to the excited-state
dispersion, we determine this instability as being primarily due to changes in
the fourth-nearest neighbor bonds that connect bilayers, with relatively little
change to the intralayer resonant bonds. In addition to providing critical
insight into the nonthermal bonding origin of the instability in SnSe, such
measurements will be crucial for understanding and controlling materials
properties under non-equilibrium conditions
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