Orientational glasses

Abstract

This review summarizes experimental evidence for the freezing of reorienting moments in solids. The moments may be of dipolar or quadrupolar nature, or both; they belong to one of the constituents of a mixed-crystal solid. Extensive results are reported for the following systems: KC1 doped with hydroxyl, potassium tantalate doped with Li, Na and Nb, alkali halide cyanides and alkali-alkali cyanides, rubidium ammonium dihydrogen phosphate, solid ortho-para hydrogen and argon-nitrogen mixtures. These have clearly glass-like properties. In other systems, results are limited to one or two methods hinting at glass formation; some of those are also reported. Clustering phenomena and the slow-down of reorientations at the freezing temperature are observed in susceptibility measurements and by local probing on nuclear spins. The modulation of the structure by cluster formation is revealed by diffraction experiments. These phenomena are confronted with model predictions and numerical simulations. l , 2. IntroductionWe apply the term 'orientational glass' to a solid consisting of a regular lattice some of whose sites are occupied by constituents containing a dipole or quadrupole moment. These moments have orientational degrees o f freedom; they interact with one another and, below some freezing temperature Tf, their motion slows and they freeze into a configuration devoid of long-range order. A typical example is KC1 doped with OH molecules that replace C1 at random sites. At ! 0 K the dipoles associated with the OH molecules reorient rapidly, whereas at 0-1 K their orientations are static on experimental time scales, yet there is no macroscopic (spontaneous) polarization. Early investigations of this orientational-glass prototype (of the subspecies 'dipole glass') were performed by Kfinzig et al. (1964). Brout (1965)considered its short-range order and pointed out the analogy with what was to become the spin-glass prototype, Cu doped with Mn, where Mn has a spin of ~ and therefore contains magnetic degrees of freedom.The use of the word 'glass' suggests a similarity with what we call ~canonical glasses' (the prototype of which is fused silica, SiO2), in order to distinguish them from orientational and spin glasses. There are indeed orientational degrees of freedom in SiO2, in addition to rapid freezing of the moments and a low-temperature configuration devoid of order. Owing to the lack of an underlying lattice, however, it is impossible to associate moments at site i with interactions between moments at sites i and j, which is a pre-requisite for modelling a glass in terms of a Hamiltonian.Spin-glass models are based almost entirely on the existence of a Hamiltonian containing interactions between spins and their coupling to a magnetic field. The interactions chosen to model spin glasses are, in general, totally unspecific for spins. They are usually probabilistic distributions o f interaction strengths with a variance Var~j much larger than their average ~j and with both quantities independent of i j. The picture of spi...

Excess electron transport in cryoobjects

“…The first term in (2) is the signal from all muoniums formed up to the time t including the phase shift in muon state (the constant 1/2 appears as only triplet spin state of muonium is seen [14], the second term is the muon signal. From Eq.…”

“…To observe the effects of disorder on electron transport without the complications of electron-electron interactions, one must therefore study electron dynamics in a disordered insulating host [13]. Orientational glasses formed by random mixtures of molecular (N 2 , CH 4 , CD 4 ) and atomic (Ar, Kr) species [14] offers a unique opportunity for such studies.…”

Tunneling dynamics in cryocrystals: localization and delocalization

“…All the studies on muon and muonium localization so far have been focused on crystals with weak disorder [1]. In this section we present experimental studies of muonium tunneling dynamics under conditions of strong disorder in orientational glass [22,23].…”

“…Orientational glasses formed by random mixtures of molecular and atomic species [22] offers a unique opportunity for such studies. One of the best studied orientational glass systems is the N 2 -Ar mixture [45].…”

“…); strong enough dilution leads to a new type of phase, where the multipole moments are frozen into random directions. These glass phases are believed to result from the combined effect of the frustration of the highly anisotropic interactions between the molecules (e.g., electrostatic multipole-multipole) and the disorder introduced by the random substitution of molecular multipoles by non-interacting shperical atoms or molecules [22]. The frustration in these systems arises from the geometrical impossibility of realizing the minimum possible energy configuration for all pairs of neighboring molecular quadrupoles in close 3D lattices, and disorder simply comes from the replacement of multipole bearing molecules by non-interacting diluents such as Kr in the CH 4 -Kr system.…”