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Viewpoint on The Viewpoint on the “Theory of the superglass phase” and a proof of principe of quantum critical jamming and related phases Zohar Nussinov1 1Department of Physics, Washington University, St. Louis, MO 63160, USA (Dated: November 21, 2018) A viewpoint article on the very interesting work of Biroli, Chamon, and Zamponi on superglasses. I further suggest how additional new superglass and ”spin-superglass” phases of matter (the latter phases contain quenched disorder) and general characteristics may be proven as a theoretical proof of concept in various electronic systems. The new phases include: (1) superglasses of Cooper pairs, i.e., glassy superconductors, (2) superglass phases of quantum spins, and (3) superglasses of the electronic orbitals. New general features which may be derived by the same construct include (a) quantum dynamical heterogeneities- a low temperature quantum analogue of dynamical heterogeneities known to exist in classical glasses and spin-glasses wherein the local dynamics and temporal correlations are spatially non-uniform. I also discuss on a new class of quantum critical systems. In particular, I outline (b) the derivation of the quantum analogue of the zero temperature jamming transition that has a non-trivial dynamical exponent. We very briefly comment on (c) quantum liquid crystals. I. INTRODUCTION rithmic potential. By using the classical plasma analogy and using known results on it, Laughlin was able to make headway on the challenging many body quantum prob- In an exciting article in this issue, Biroli, Chamon, lem and construct his highly successful wavefunctions. and Zamponi, (BCZ) illustrate theoretically the possibil- The mapping used by BCZ similarly enables exact re- ity of a “superglass” phase1. This new quantum phase sults on quantum problem and a detailed correspondence forms an intriguing amorphous counterpart to the ”su- of spatial and temporal correlations between the classi- persolid” phase2,3 that has seen a surge of interest in cal and quantum systems. BCZ apply this mapping to a recent years4. Within a ”supersolid” phase, superfluidity classical system well known to exhibit glassy dynamics- can occur without disrupting crystalline order. the Brownian hard sphere problem. The quantum coun- So, what are “superglasses”? Glasses are liquids that terpart of the classical hard sphere problem is a natu- have ceased to flow on experimentally measurable time ral system containing hard sphere interactions. On the scales. By constrast, superfluids flow without any resis- classical side of the correspondence, the hard sphere sys- tance. The existence of a phase characterized by simulta- tem has been heavily investigated.9–11 When the sphere neous glassiness and superfluidity may seem like a clear packing density is slowly varied, the classical Brownian contradiction of terms. In their article, BCZ prove that hard sphere system undergoes a transition from a liquid this is not so. Interacting quantum particles can indeed at low density to an ordered crystal at high density9. form such a “super-glass” phase at very low temperature When crystallization is thwarted by a rapid increase of and high density; their work confirms the earlier numer- the packing density or by, e.g., a change of the particle ical suggestion of such a phase by Bonnsegi, Prokof’ev, 5 6 geometry, the system cannot order nicely into a crystal and Svistunov and an investigation by Philips and Wu . and instead jams into a dense amorphous glass10,11. BCZ The superglass phase is characterized by an amorphous noticed that when fused with the mapping between clas- density profile. At the same time, a finite fraction of sical and quantum systems, information on classical glass the particles flow without any resistance- as if they were forming systems such as the Brownian spheres gives rise superfluid. Thus, the ”superglass” constitutes a glassy to highly non-trivial results. In particular, the glassy counterpart to the ”supersolid” phase . phase of the classical system translates into a quantum The approach invoked by BCZ to prove the existence glass of a Bose system. Similarly, the classical solid maps of superglasses is particularly elegant. It relies on the onto a quantum bosonic crystal. The ensuing phase dia- arXiv:1203.4648v1 [cond-mat.stat-mech] 21 Mar 2012 mapping8 between viscous classical systems whose prop- gram is provided in Fig.(2) of their article. The spatio- erties are well known to new many body quantum sys- temporal correlations of the (bosonic) quantum dual can tems. In realizing the link between classical and quan- be computed by mapping to the classical system. Both tum systems to gain insight into the quantum many body the glassy and solid phases harbor a finite Bose-Einstein phases, BCZ nicely add an important new result to ear- condensate fraction. Putting all of the pieces together, lier investigations that built on such similar insights else- BCZ provide an important proof of concept of the su- where. Chester3 suggested the existence of a supersolid perglass phase in a simple and precise way. This route by relying on such a connection. In a similar fashion, may be replicated for classical systems other than the Laughlin invoked a highly inspirational analogy7 between Brownian hard sphere which also display solid and glass variational (Jastrow type) wavefunctions describing the phases. fractional quantum Hall system and a well known sys- tem of classical charged particles interacting via a loga- What physical systems may realize the new superglass 2 phase? Recent experiments4 on solid Helium 4 exhibit antiferromagnets in an external magnetic field, triplet super-solid type features and have led to a flurry of ac- states with spins aligned along the field direction can tivity. In the simplest explanation of observations, a frac- be regarded as hard core bosons. In many other sys- tion of the medium becomes, at low temperatures, a su- tems, interactions between quantum spins may also be perfluid that decouples from the measurement apparatus. mapped onto hard core type bosonic systems.2122 If a However, the required condensate fraction does not sim- solid or glassy phase appears in a classical Brownian sys- ply conform with thermodynamic measurement12. Rit- tem, then a mapping similar to that of BCZ suggests tner and Reppy13 further found that the putative super- supersolidity/superglassiness in the corresponding quan- solid type feature is accutely sensitive to the quench tum spin system. Recently, there has been much work rate for solidifying the liquid. Aoki, Keiderling, and examining supersolidity in such spin systems, e.g.,22. It Kojima discovered rich hysteresis and memory effects14. is highly natural to expect new lattice spin superglass All of these features can arise from glassy characteristics counterparts. alone12,15- precisely as in the superglass phase discussed In transition-metal compounds, the fractional filling of by BCZ. It may indeed well be that a confluence of both the 3d-shells allows for cooperative orbital ordering. This superfluid and glassy features (and their effects on elastic order has been observed in numerous compounds.23 Sim- properties, e.g., screened finite elastic shear penetration ilar to the spin and charge degrees of freedom, we may depths)16 may be at work. There may be new experi- ask whether low temperature Bose condensed glasses of mental consequences of (super-) glassy dynamics such as orbitals may appear: an orbital superglass. The work of this. For instance, such dynamics can manifest disparate BCZ allows to investigate this by knowing the dynamics relaxation times that may be probed for.12 Typical glass of hard core Bose model derived from a classical counter- formers indeed typically exhibit relaxations on two dif- part. Orbital states can be described in terms of a S=1/2 ferent time scales. pseudo-spin.23 We may, in turn, map these pseudo-spins Cold atom systems may provide another realization to hard core bosons21 and then investigate the dynamics of a superglass state. Indeed, a supersolid state of cold of these bosons by mapping the system to that of classical atoms in a confining optical lattice was very recently Brownian particles on a lattice. achieved17. It is natural to expect a superglass analog The mapping employed by BCZ also suggests a new of these cold atomic systems. quantum critical point in related systems. The classi- The super-glass phases that BCZ find and the mapping cal jamming transition24 of hard spheres/disks from a they employ may also have new manifestations elsewhere. jammed system at high density to an unjammed one at We suggest a few of these below. lower densities is a continuous transition with known crit- We may envision lattice extensions of the continuum ical exponents, both static25 and dynamic26. Replicating system investigated by BCZ: a ”lattice superglass”. For the mapping used by BCZ28, we may derive an analog charged bosons (e.g., Cooper pairs) on a lattice, such quantum system harboring a zero temperature transi- a charge superglass would correspond to a superconduc- tion with similar critical exponents. The classical zero tor with glassy dynamics. In a similar vein, a ”lattice temperature critical point (”point J”)24,25 may rear its supersolid” of Cooper pairs would correspond to a su- head anew in the form of quantum critical jamming of perconductor concomitant with well defined crystalline the bosonic systems with dynamical exponents, poten- (i.e., charge density wave) order. Indeed, in some heavy tially as high as z =4.6, as we may ascertain from those fermion compounds as well as in the cuprate and the reported for the classical jamming system26. newly discovered iron arsenide family of high temper- All of the above mentioned examples are free of ature superconductors18 there are some indications of quenched disorder. Glassiness in structural glasses and non-uniform meso-scale spatial electronic structures and the Brownian hard sphere system is not triggered by dis- glassy dynamics.
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