Pressure-induced transformations of low-energy excitations in glasses (original) (raw)
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Pressure Effects on Relaxation in a Polymer Glass: A Persistent Spectral Hole Burning Study
Optics and Spectroscopy, 2005
The influence of hydrostatic pressure in the range of some kilobars on low-temperature (T < 20 K) relaxation in a polymer (polystyrene) glass after optical excitation of a probe chromophore in it is studied using two different kinds of spectral hole burning experiments-under isothermal-isobaric and in temperature cycling conditions. In the first case, the temperature dependence of the hole width reflects the dynamics of interaction of the electronic transition in a probe molecule with soft localized vibrational modes and with two-level systems, whereas, in the second case, the observed residual hole broadening after the temperature cycle arises from activated (overbarrier) transitions in almost symmetric double-well soft potentials. It is shown that both these processes are essentially suppressed by the applied hydrostatic pressure (the hole width in the first case and its increment in the second case are both reduced about twofold at 5 kbar). An extension of the soft potential model for glasses is proposed explaining in a coherent manner both effects. Its essential points are the presence in the potential of an extra term linear in pressure and the soft coordinate and an assumption about asymmetric distribution of the cubic anharmonicity parameter ξ in the potential.
Chemical Physics, 1997
We report time-resolved spectral hole-burning experiments on bacteriochlorophyll-a (BChl-a) doped into the glass triethylamine (TEA) at ambient pressure (Δp=0) and at a pressure of Δp=3.4 GPa. We observe a number of remarkable effects: (a) from the change in the temperature dependence of the “effective” optical homogeneous linewidth Γhom′, we conclude that local order is introduced in TEA under high pressure;
Intrinsic Quantum Excitations of Low Temperature Glasses
Physical Review Letters, 2001
Several puzzling regularities concerning the low temperature excitations of glasses are quantitatively explained by quantizing domain wall motions of the random first order glass transition theory. The density of excitations agrees with experiment and scales with the size of a dynamically coherent region at Tg, being about 200 molecules. The phonon coupling depends on the Lindemann ratio for vitrification yielding the observed universal relation l/λ ≃ 150 between phonon wavelength λ and mean free path l. Multilevel behavior is predicted to occur in the temperature range of the thermal conductivity plateau.
Thermal origin of quasi-localised excitations in glasses
2019
Key aspects of glasses are controlled by the presence of excitations in which a group of particles can rearrange. Surprisingly, recent observations indicate that their density is dramatically reduced and their size decreases as the temperature of the supercooled liquid is lowered. Some theories predict these excitations to cause a pseudo-gap in the spectrum of quasi-localised modes of the Hessian, D_L(ω), while others predict a gap in D_L(ω) that grows upon cooling. To unify these views and observations, we generate glassy configurations of controlled gap magnitude ω_c at temperature T=0, using so-called 'breathing' particles, and study how such gapped states respond to thermal fluctuations. We find that (i) the gap always fills up at finite T with D_L(ω) ≈ A_4(T) ω^4 and A_4 ∼(-E_a / T) at low T, (ii) E_a rapidly grows with ω_c, in reasonable agreement with a simple scaling prediction E_a∼ω_c^4 and (iii) at larger ω_c excitations involve fewer particles, as we rationalise, ...
Influence of Pressure on Quasielastic Scattering in Glasses: Relationship to the Boson Peak
Physical Review Letters, 2009
We report unexpectedly strong variations in the quasielastic scattering (QES) intensity in glasses under pressure. Analysis of the data reveals strong correlations between pressure-induced changes in the QES intensity and the intensity of the boson peak. This observation emphasizes a direct relationship between these two components of the fast dynamics. In addition, we observe changes of the QES spectral shape that can be interpreted as pressure-induced variations in the underlying energy landscape.
Energy landscapes of model glasses. II. Results for constant pressure
The Journal of Chemical Physics, 2003
New geometry optimization techniques are introduced for characterizing local minima, transition states, and pathways corresponding to enthalpy surfaces at constant pressure. Results are obtained for comparison with the potential energy surfaces of model glass formers studied in previous work. The constant pressure condition, where the the box lengths of the simulation cell vary, makes the enthalpy surface less rugged than the potential energy surface corresponding to the same mean density. Analysis of barrier heights as a function of pressure provides insight into transport and relaxation processes. Elementary rearrangements can be separated into ''diffusive'' and ''nondiffusive'' processes, where the former involve changes in the nearest-neighbor coordination of at least one atom, and the latter do not. With increasing pressure the barrier heights for cage-breaking rearrangements rise, while those for cage-preserving rearrangements appear relatively unchanged. The ''strong'' or ''fragile'' character of the system can therefore change with pressure because the barriers encountered vary in a systematic fashion. The geometric mean normal mode frequencies of a binary Lennard-Jones system decrease with increasing potential energy for constant pressure, rather than increase as they do at constant volume, in agreement with a simple model.
Quantum Phenomena in Structural Glasses: The Intrinsic Origin of Electronic and Cryogenic Anomalies
The Journal of Physical Chemistry Letters, 2011
The structural glass transition is often regarded as purely a problem of the classical theory of liquids. The dynamics of electrons enters only implicitly, through the interactions between ionic cores or molecules. Likewise, zero-point effects tied to the atomic masses hardly affect the typical barriers for liquid rearrangements. Yet, glasses do exhibit many quantum phenomenaelectronic, optical, and cryogenic peculiarities that seem to have universal characteristics. These anomalies of the glassy state are uncommon or strongly system dependent in crystals and amorphous solids not made by a quasi-equilibrium quench of a melt. These clearly quantum phenomena include midgap electronic states in amorphous semiconductors, the two-level systems, and the Boson peak. Here, we discuss how these quantum phenomena found in glasses are not merely consequences of any kind of disorder but have universal characteristics stemming from the structural dynamics inherent in the glass transition itself. The quantum dynamics at cryogenic temperatures and electronic dynamics are related to the transition states for relaxational motions above the glass transition temperature, which are partially frozen when the sample is quenched.
Ultrastable glasses portray similar behaviour to ordinary glasses at high pressure
Scientific Reports, 2016
Pressure experiments provide a unique opportunity to unravel new insights into glass-forming liquids by exploring its effect on the dynamics of viscous liquids and on the evolution of the glass transition temperature. Here we compare the pressure dependence of the onset of devitrification, T on , between two molecular glasses prepared from the same material but with extremely different ambient-pressure kinetic and thermodynamic stabilities. Our data clearly reveal that, while both glasses exhibit different dT on /dP values at low pressures, they evolve towards closer calorimetric devitrification temperature and pressure dependence as pressure increases. We tentatively interpret these results from the different densities of the starting materials at room temperature and pressure. Our data shows that at the probed pressures, the relaxation time of the glass into the supercooled liquid is determined by temperature and pressure similarly to the behaviour of liquids, but using stability-dependent parameters.