24 research outputs found
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Light-induced charge density wave in LaTe3
When electrons in a solid are excited with light, they can alter the free
energy landscape and access phases of matter that are beyond reach in thermal
equilibrium. This accessibility becomes of vast importance in the presence of
phase competition, when one state of matter is preferred over another by only a
small energy scale that, in principle, is surmountable by light. Here, we study
a layered compound, LaTe, where a small in-plane (a-c plane) lattice
anisotropy results in a unidirectional charge density wave (CDW) along the
c-axis. Using ultrafast electron diffraction, we find that after
photoexcitation, the CDW along the c-axis is weakened and subsequently, a
different competing CDW along the a-axis emerges. The timescales characterizing
the relaxation of this new CDW and the reestablishment of the original CDW are
nearly identical, which points towards a strong competition between the two
orders. The new density wave represents a transient non-equilibrium phase of
matter with no equilibrium counterpart, and this study thus provides a
framework for unleashing similar states of matter that are "trapped" under
equilibrium conditions
Dynamical slowing down in an ultrafast photo-induced phase transition
Complex systems, which consist of a large number of interacting constituents,
often exhibit universal behavior near a phase transition. A slowdown of certain
dynamical observables is one such recurring feature found in a vast array of
contexts. This phenomenon, known as critical slowing down, is well studied
mostly in thermodynamic phase transitions. However, it is less understood in
highly nonequilibrium settings, where the time it takes to traverse the phase
boundary becomes comparable to the timescale of dynamical fluctuations. Using
transient optical spectroscopy and femtosecond electron diffraction, we studied
a photo-induced transition of a model charge-density-wave (CDW) compound,
LaTe. We observed that it takes the longest time to suppress the order
parameter at the threshold photoexcitation density, where the CDW transiently
vanishes. This finding can be quantitatively captured by generalizing the
time-dependent Landau theory to a system far from equilibrium. The experimental
observation and theoretical understanding of dynamical slowing down may offer
insight into other general principles behind nonequilibrium phase transitions
in many-body systems
Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals
Nonradiative processes limit optoelectronic functionality of nanocrystals and
curb their device performance. Nevertheless, the dynamic structural origins of
nonradiative relaxations in nanocrystals are not understood. Here, femtosecond
electron diffraction measurements corroborated by atomistic simulations uncover
transient lattice deformations accompanying radiationless electronic processes
in semiconductor nanocrystals. Investigation of the excitation energy
dependence shows that hot carriers created by a photon energy considerably
larger than the bandgap induce structural distortions at nanocrystal surfaces
on few picosecond timescales associated with the localization of trapped holes.
On the other hand, carriers created by a photon energy close to the bandgap
result in transient lattice heating that occurs on a much longer 200 ps
timescale, governed by an Auger heating mechanism. Elucidation of the
structural deformations associated with the surface trapping of hot holes
provides atomic-scale insights into the mechanisms deteriorating optoelectronic
performance and a pathway towards minimizing these losses in nanocrystal
devices.Comment: 17 pages, 4 figure
Evacetrapib and Cardiovascular Outcomes in High-Risk Vascular Disease
BACKGROUND:
The cholesteryl ester transfer protein inhibitor evacetrapib substantially raises the high-density lipoprotein (HDL) cholesterol level, reduces the low-density lipoprotein (LDL) cholesterol level, and enhances cellular cholesterol efflux capacity. We sought to determine the effect of evacetrapib on major adverse cardiovascular outcomes in patients with high-risk vascular disease.
METHODS:
In a multicenter, randomized, double-blind, placebo-controlled phase 3 trial, we enrolled 12,092 patients who had at least one of the following conditions: an acute coronary syndrome within the previous 30 to 365 days, cerebrovascular atherosclerotic disease, peripheral vascular arterial disease, or diabetes mellitus with coronary artery disease. Patients were randomly assigned to receive either evacetrapib at a dose of 130 mg or matching placebo, administered daily, in addition to standard medical therapy. The primary efficacy end point was the first occurrence of any component of the composite of death from cardiovascular causes, myocardial infarction, stroke, coronary revascularization, or hospitalization for unstable angina.
RESULTS:
At 3 months, a 31.1% decrease in the mean LDL cholesterol level was observed with evacetrapib versus a 6.0% increase with placebo, and a 133.2% increase in the mean HDL cholesterol level was seen with evacetrapib versus a 1.6% increase with placebo. After 1363 of the planned 1670 primary end-point events had occurred, the data and safety monitoring board recommended that the trial be terminated early because of a lack of efficacy. After a median of 26 months of evacetrapib or placebo, a primary end-point event occurred in 12.9% of the patients in the evacetrapib group and in 12.8% of those in the placebo group (hazard ratio, 1.01; 95% confidence interval, 0.91 to 1.11; P=0.91).
CONCLUSIONS:
Although the cholesteryl ester transfer protein inhibitor evacetrapib had favorable effects on established lipid biomarkers, treatment with evacetrapib did not result in a lower rate of cardiovascular events than placebo among patients with high-risk vascular disease. (Funded by Eli Lilly; ACCELERATE ClinicalTrials.gov number, NCT01687998 .)
Role of Equilibrium Fluctuations in Light-Induced Order
Engineering novel states of matter with light is at the forefront of materials research. An intensely studied direction is to realize broken-symmetry phases that are "hidden" under equilibrium conditions but can be unleashed by an ultrashort laser pulse. Despite a plethora of experimental discoveries, the nature of these orders and how they transiently appear remain unclear. To this end, we investigate a nonequilibrium charge density wave (CDW) in rare-earth tritellurides, which is suppressed in equilibrium but emerges after photoexcitation. Using a pump-pump-probe protocol implemented in ultrafast electron diffraction, we demonstrate that the light-induced CDW consists solely of order parameter fluctuations, which bear striking similarities to critical fluctuations in equilibrium despite differences in the length scale. By calculating the dynamics of CDW fluctuations in a nonperturbative model, we further show that the strength of the light-induced order is governed by the amplitude of equilibrium fluctuations. These findings highlight photoinduced fluctuations as an important ingredient for the emergence of transient orders out of equilibrium. Our results further suggest that materials with strong fluctuations in equilibrium are promising platforms to host hidden orders after laser excitation