scholarly journals Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition

Science ◽  
2018 ◽  
Vol 362 (6416) ◽  
pp. 821-825 ◽  
Author(s):  
C. W. Nicholson ◽  
A. Lücke ◽  
W. G. Schmidt ◽  
M. Puppin ◽  
L. Rettig ◽  
...  
Keyword(s):  
Reaction Pathway ◽  
Energy Landscape ◽  
Real Space ◽  
Nonequilibrium Dynamics ◽  
Test Case ◽  
Chemical Bonds ◽  
Ab Initio Simulations ◽  
Potential Energy Landscape ◽  
Microscopic Interactions ◽  
Space Description

Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.

scholarly journals A localization transition underlies the mode-coupling crossover of glasses

2019 ◽  
Vol 7 (6) ◽  
Author(s):  
Daniele Coslovich ◽  
Andrea Ninarello ◽  
Ludovic Berthier
Keyword(s):  
Mode Coupling ◽  
Characteristic Temperature ◽  
Energy Landscape ◽  
Mean Field ◽  
Real Space ◽  
Stationary Points ◽  
Crossover Temperature ◽  
Localization Transition ◽  
Finite Dimensional ◽  
Potential Energy Landscape

We study the equilibrium statistical properties of the potential energy landscape of several glass models in a temperature regime so far inaccessible to computer simulations. We show that unstable modes of the stationary points undergo a localization transition in real space close to the mode-coupling crossover temperature determined from the dynamics. The concentration of localized unstable modes found at low temperature is a non-universal, finite dimensional feature not captured by mean-field glass theory. Our analysis reconciles, and considerably expands, previous conflicting numerical results and provides a characteristic temperature for glassy dynamics that unambiguously locates the mode-coupling crossover.

scholarly journals The intimate relationship between structural relaxation and the energy landscape of monatomic liquid metals

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Franz Demmel ◽  
Louis Hennet ◽  
Noel Jakse
Keyword(s):  
High Temperature ◽  
Velocity Field ◽  
Liquid State ◽  
Liquid Metals ◽  
Energy Landscape ◽  
Experimental Studies ◽  
Transport Parameter ◽  
Arrhenius Behavior ◽  
Liquid Aluminium ◽  
Potential Energy Landscape

AbstractThe characteristic property of a liquid, discriminating it from a solid, is its fluidity, which can be expressed by a velocity field. The reaction of the velocity field on forces is enshrined in the transport parameter viscosity. In contrast, a solid reacts to forces elastically through a displacement field, the particles are trapped in their potential minimum. The flow in a liquid needs enough thermal energy to overcome the changing potential barriers, which is supported through a continuous rearrangement of surrounding particles. Cooling a liquid will decrease the fluidity of a particle and the mobility of the neighbouring particles, resulting in an increase of the viscosity until the system comes to an arrest. This process with a concomitant slowing down of collective particle rearrangements might already start deep inside the liquid state. The idea of the potential energy landscape provides an attractive picture for these dramatic changes. However, despite the appealing idea there is a scarcity of quantitative assessments, in particular, when it comes to experimental studies. Here we present results on a monatomic liquid metal through a combination of ab initio molecular dynamics, neutron spectroscopy and inelastic x-ray scattering. We investigated the collective dynamics of liquid aluminium to reveal the changes in dynamics when the high temperature liquid is cooled towards solidification. The results demonstrate the main signatures of the energy landscape picture, a reduction in the internal atomic structural energy, a transition to a stretched relaxation process and a deviation from the high-temperature Arrhenius behavior of the relaxation time. All changes occur in the same temperature range at about $$1.4 \cdot T_{melting}$$ 1.4 · T melting , which can be regarded as the temperature when the liquid aluminium enters the landscape influenced phase and enters a more viscous liquid state towards solidification. The similarity in dynamics with other monatomic liquid metals suggests a universal dynamic crossover above the melting point.

scholarly journals Enhancing ductility in bulk metallic glasses by straining during cooling

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Rodrigo Miguel Ojeda Mota ◽  
Ethen Thomas Lund ◽  
Sungwoo Sohn ◽  
David John Browne ◽  
Douglas Clayton Hofmann ◽  
...  
Keyword(s):  
Metallic Glasses ◽  
Energy Landscape ◽  
Bulk Metallic Glasses ◽  
Bulk Glass ◽  
Liquid Cooling ◽  
Fictive Temperature ◽  
Cooling Method ◽  
Potential Energy Landscape ◽  
Major Shortcoming ◽  
Processing Techniques

AbstractMost of the known bulk metallic glasses lack sufficient ductility or toughness when fabricated under conditions resulting in bulk glass formation. To address this major shortcoming, processing techniques to improve ductility that mechanically affect the glass have been developed, however it remains unclear for which metallic glass formers they work and by how much. Instead of manipulating the glass state, we show here that an applied strain rate can excite the liquid, and simultaneous cooling results in freezing of the excited liquid into a glass with a higher fictive temperature. Microscopically, straining causes the structure to dilate, hence “pulls” the structure energetically up the potential energy landscape. Upon further cooling, the resulting excited liquid freezes into an excited glass that exhibits enhanced ductility. We use Zr44Ti11Cu10Ni10Be25 as an example alloy to pull bulk metallic glasses through this excited liquid cooling method, which can lead to tripling of the bending ductility.

scholarly journals An energy landscape approach to locomotor transitions in complex 3D terrain

2020 ◽  
Vol 117 (26) ◽  
pp. 14987-14995 ◽  
Author(s):  
Ratan Othayoth ◽  
George Thoms ◽  
Chen Li
Keyword(s):  
Potential Energy ◽  
Complex Terrain ◽  
Statistical Physics ◽  
Energy Landscape ◽  
Three Dimensional ◽  
Single Mode ◽  
Physical Interaction ◽  
Landscape Approach ◽  
3D Terrain ◽  
Potential Energy Landscape

Effective locomotion in nature happens by transitioning across multiple modes (e.g., walk, run, climb). Despite this, far more mechanistic understanding of terrestrial locomotion has been on how to generate and stabilize around near–steady-state movement in a single mode. We still know little about how locomotor transitions emerge from physical interaction with complex terrain. Consequently, robots largely rely on geometric maps to avoid obstacles, not traverse them. Recent studies revealed that locomotor transitions in complex three-dimensional (3D) terrain occur probabilistically via multiple pathways. Here, we show that an energy landscape approach elucidates the underlying physical principles. We discovered that locomotor transitions of animals and robots self-propelled through complex 3D terrain correspond to barrier-crossing transitions on a potential energy landscape. Locomotor modes are attracted to landscape basins separated by potential energy barriers. Kinetic energy fluctuation from oscillatory self-propulsion helps the system stochastically escape from one basin and reach another to make transitions. Escape is more likely toward lower barrier direction. These principles are surprisingly similar to those of near-equilibrium, microscopic systems. Analogous to free-energy landscapes for multipathway protein folding transitions, our energy landscape approach from first principles is the beginning of a statistical physics theory of multipathway locomotor transitions in complex terrain. This will not only help understand how the organization of animal behavior emerges from multiscale interactions between their neural and mechanical systems and the physical environment, but also guide robot design, control, and planning over the large, intractable locomotor-terrain parameter space to generate robust locomotor transitions through the real world.

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