Observation of crystallization slowdown in supercooled parahydrogen and orthodeuterium quantum liquid mixtures

Abstract

We report a quantitative experimental study of the crystallization kinetics of supercooled quantum liquid mixtures of parahydrogen (pH 2 ) and orthodeuterium (oD 2 ) by high spatial resolution Raman spectroscopy of liquid microjets. We show that in a wide range of compositions the crystallization rate of the isotopic mixtures is significantly reduced with respect to that of the pure substances. To clarify this behavior we have performed path-integral simulations of the nonequilibrium pH 2 -oD 2 liquid mixtures, revealing that differences in quantum delocalization between the two isotopic species translate into different effective particle sizes. Our results provide experimental evidence for crystallization slowdown of quantum origin, offering a benchmark for theoretical studies of quantum behavior in supercooled liquids. Understanding the stability of supercooled liquids with respect to crystallization is a fundamental open problem in condensed matter physics [1]. In this regard, since crystallization competes with glass formation, a knowledge of the mechanisms that govern the crystal growth in supercooled liquids is considered an important step to elucidate the nature of the glass transition [2-6]. So far, experimental studies aiming at providing microscopic insights into the dynamics and crystallization of supercooled liquids have been largely based on the use of colloidal suspensions [7,8], where the large particle size allows one to follow the crystal growth on the laboratory time scale. However, diverse drawbacks such as polydispersity and sedimentation often make the experimental data from these systems difficult to interpret [8,9]. Accessing the details of the crystallization process in simple atomic and molecular counterparts, on the other hand, remains an experimental challenge due to relevant time scales that are orders of magnitude shorter.Theoretical studies have shown that the inclusion of quantum effects adds a further degree of complexity in the behavior of supercooled liquids, leading to novel exotic phenomena such as superfluidity [10,11] or enhanced dynamical slowing down [12][13][14]. Yet again, the difficulties in supercooling a quantum liquid to very low temperatures have so far precluded possible experimental studies of the interplay of quantum effects and structural transformations in nonequilibrium bulk liquids. Here we address these challenges reporting on the experimental investigation of the crystallization kinetics of supercooled liquid mixtures of the isotopic species pH 2 and oD 2 , showing that their quantum nature has a profound impact on the crystallization process.* grisenti@atom.uni-frankfurt.de Binary liquid mixtures exhibit, in general, properties that differ fundamentally from their corresponding pure substances, and mixing a few components is, in particular, a common strategy to hinder crystallization. Indeed, classical binary systems of particles that interact via a simple LennardJones (LJ) pair potential have been widely employed as the simplest theoretical models to inve...

Mixing effects in the crystallization of supercooled quantum binary liquids

“…2 are qualitatively similar to those reported recently for pH 2 -oD 2 mixtures, 12 yet both the slowdown of crystal growth and the earlier onset of freezing upon mixing appear here strongly enhanced.…”

“…The overall picture that emerges from our crystallization data is that the inclusion of a small amount of impurity atoms reduces, on one side, the time required for the formation of a critical crystallite and, on the other side, slows down the subsequent crystal growth. The reduced nucleation time, which has been previously observed in pH 2 -oD 2 mixtures, 12 can be now understood qualitatively with the aid of the rotational Raman spectra. In fact, we have seen (Fig.…”

“…5 a summary of the growth rate data by plotting the total crystallization time of the solvent (either pH 2 or oD 2 ), relative to the pure case, as a function of the impurity content for the pH 2 -Ne and oD 2 -Ne mixtures, as well for the pH 2 -oD 2 mixtures. 12 One can distinguish two limiting cases, represented on one side by the barely significant increase of the crystallization rate of oD 2 upon dilution with pH 2 (red squares) and, on the other side, by the strong dependence on the Ne impurity content of the crystallization rate of pH 2 (green circles).…”

“…12 The most striking observation in those studies was the strong dependence of the crystallization rate on composition: starting with a pure pH 2 system, the crystal growth was found to slow down considerably with increasing amount of oD 2 , reaching a maximum in the crystallization time at about 20% oD 2 content. This behavior may appear surprising because the intermolecular interaction potential is isotope independent.…”

“…14 Here, we have extended the above studies to supercooled pH 2 -Ne and oD 2 -Ne liquid mixtures, addressing also relevant structural aspects of the crystallization process in the Ne mixtures, as well as in diluted pH 2 -oD 2 mixtures, which have been not taken into account in our recent work. 12 In principle, a Ne atom can be viewed as an isotopic impurity when mixed to pH 2 or oD 2 because its interatomic spherical potential is very similar to those of pH 2 and oD 2 . Indeed, the LJ potential parameters (potential well depth ϵ and interaction length σ) for the H 2 -H 2 interaction are σ = 2.96 Å and ϵ = 34.2 K, while for H 2 -Ne, they are σ = 2.8745 Å and ϵ = 35.49 K (Ref.…”