Glasses are ubiquitous in daily life and technology. However the microscopic mechanisms generating this state of matter remain subject to debate: Glasses are considered either as merely hyper-viscous liquids or as resulting from a genuine thermodynamic phase transition towards a rigid state. We show that third and fifth order susceptibilities provide a definite answer to this longstanding controversy. Performing the corresponding high-precision nonlinear dielectric experiments for supercooled glycerol and propylene carbonate, we find strong support for theories based upon thermodynamic amorphous order. Moreover, when lowering temperature, we find that the growing transient domains are compact -that is their fractal dimension d f = 3. The glass transition may thus represent a class of critical phenomena different from canonical second-order phase transitions for which d f < 3.The glassy state of matter, despite is omnipresence in nature and technology (1), continues to be one of the most puzzling riddles in condensed-matter physics (1, 2): For all practical purposes, glasses are rigid like crystals but they lack any long-range order. Some theories describe glasses as kinetically constrained liquids (3), becoming so viscous below the glass transition that they seem effectively rigid. By contrast, other theories (4, 5) are built on the existence of an underlying thermodynamic phase transition to a state where the molecules are frozen in well defined, yet disordered positions. This so-called "amorphous order" cannot be revealed by canonical static correlation functions, but rather by new kinds of correlations [i.e. point-to-set correlations or other measures of local order (6, 7)] that have now been detected in recent numerical simulations (7)(8)(9). In these theories, thermodynamic correlations lock together the fluctuations and response of the molecules, which collectively rearrange over some length-scale , ultimately leading to rigidity. In this thermodynamic scenario, is proportional to a power of ln(τ α /τ 0 ) where τ α is the structural relaxation time and τ 0 is the microscopic time-scale, generally smaller than 1 arXiv:1606.04079v1 [cond-mat.dis-nn] 13 Jun 2016 1ps (4, 5). Because equilibrium measurements require a time larger than τ α , they cannot be performed in the range where is very large since this would require exponentially long times. This limitation is essentially why the true nature of glasses is still a matter of intense debate.Here, we propose a pioneering strategy to unveil the existence of a thermodynamic length that grows upon cooling. Instead of only varying the temperature T , we also vary the non-linear order k of the response of supercooled liquids. This is motivated by a general, although rarely considered (10), property of critical points: At a second order critical temperature T c , the linear susceptiblity χ 1 associated to the order parameter is not the only diverging response. As a function of temperature, all the higher order responses χ 2m+1 with m ≥ 1 diverge even faster than χ ...

Finding new ionic conductors that enable significant advancements in the development of energy-storage devices is a challenging goal of current material science. Aside of material classes as ionic liquids or amorphous ion conductors, the so-called plastic crystals (PCs) have been shown to be good candidates combining high conductivity and favorable mechanical properties. PCs are formed by molecules whose orientational degrees of freedom still fluctuate despite the material exhibits a well-defined crystalline lattice. In the present work, we show that the conductivity of Li(+) ions in succinonitrile, the most prominent molecular PC electrolyte, can be enhanced by several decades when replacing part of the molecules in the crystalline lattice by larger ones. Dielectric spectroscopy reveals that this is accompanied by a stronger coupling of ionic and reorientational motions. These findings, which can be understood in terms of an optimized "revolving door" mechanism, open a new path towards the development of better solid-state electrolytes.

The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane, where only the molecular reorientational degrees of freedom freeze without long-range order, is investigated by nonlinear dielectric spectroscopy. Marked differences to canonical glass formers show up: While molecular cooperativity governs the glassy freezing, it leads to a much weaker slowing down of molecular dynamics than in supercooled liquids. Moreover, the observed nonlinear effects cannot be explained with the same heterogeneity scenario recently applied to canonical glass formers. This supports ideas that molecular relaxation in plastic crystals may be intrinsically nonexponential. Finally, no nonlinear effects were detected for the secondary processes in cyclo-octanol.

This work aims at reconsidering several interpretations coexisting in the recent literature concerning non-linear susceptibilities in supercooled liquids. We present experimental results on glycerol and propylene carbonate showing that the three independent cubic susceptibilities have very similar frequency and temperature dependences, both for their amplitudes and phases. This strongly suggests a unique physical mechanism responsible for the growth of these non-linear susceptibilities. We show that the framework proposed by two of us [BB, Phys. Rev. B 72, 064204 (2005)], where the growth of non-linear susceptibilities is intimately related to the growth of "glassy domains", accounts for all the salient experimental features. We then review several complementary and/or alternative models, and show that the notion of cooperatively rearranging glassy domains is a key (implicit or explicit) ingredient to all of them. This paves the way for future experiments which should deepen our understanding of glasses.

A joint study of the rotational dynamics and free volume in amorphous 1-propanol (1-PrOH) as a prototypical monohydroxy alcohol by electron spin resonance (ESR) or positron annihilation lifetime spectroscopy (PALS), respectively, is reported. The dynamic parameters of the molecular spin probe 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and the annihilation ones of the atomic ortho-positronium (o-Ps) probe as a function of temperature are compared. A number of coincidences between various effects in the ESR and PALS responses at the corresponding characteristic ESR and PALS temperatures were found suggesting a common origin of the underlying dynamic processes that were identified using viscosity (VISC) in terms of the two-order parameter (TOP) model and broadband dielectric spectroscopy (BDS) data.