Quantum phase transitions in bilayer SU(N) anti-ferromagnets

We present a detailed study of the destruction of SU(N) magnetic order in square lattice bilayer anti-ferromagnets using unbiased quantum Monte Carlo numerical simulations and field theoretic techniques. We study phase transitions from an SU(N) N\'eel state into two distinct quantum disordered "valence-bond" phases: a valence-bond liquid (VBL) with no broken symmetries and a lattice-symmetry breaking valence-bond solid (VBS) state. For finite inter-layer coupling, the cancellation of Berry phases between the layers has dramatic consequences on the two phase transitions: the N\'eel-VBS transition is first order for all $N\geq5$ accesible in our model, whereas the N\'eel-VBL transition is continuous for N=2 and first order for N>= 4; for N=3 the N\'eel-VBL transition show no signs of first-order behavior

Domain wall dynamics in a two-component Bose-Mott insulator

We model the dynamics of two species of bosonic atoms trapped in an optical lattice within the Mott regime by mapping the system onto a spin model. A field gradient breaks the cloud into two domains. We study how the domain wall evolves under adiabatic and diabatic changes of this gradient. We determine the timescales for adiabaticity, and study how temperature evolves for slow ramps. We show that after large, sudden changes of the field gradient, the system does not equilibrate on typical experimental timescales. We find interesting spin dynamics even when the initial temperature is large compared to the super-exchange energy. We discuss the implication of our results for experiments wishing to use such a two-component system for thermometry, or as part of a cooling scheme.Comment: 6 pages, 5 figures Minor typographical errors corrected. Figure labels changed. Added concluding statement

Peak Effect in Superconductors: Melting of Larkin Domains

Motivated by the recent observations of the peak effect in high-$T_c$ YBCO superconductors, we reexamine the origin of this unusual phenomenon. We show that the mechanism based on the $k$-dependence (nonlocality) of the vortex-lattice tilt modulus $C_{44}({\bf k})$ cannot account for the essential feature of the peak effect. We propose a scenario in which the peak effect is related to the melting of Larkin domains. In our model, the rise of critical current with increasing temperature is a result of a crossover from the Larkin pinning length to the length scale set by thermally excited free dislocations.Comment: 13 pages, 2 figures, REVTE

Hyperuniform long-range correlations are a signature of disordered jammed hard-particle packings

We show that quasi-long-range (QLR) pair correlations that decay asymptotically with scaling $r^{-(d+1)}$ in $d$-dimensional Euclidean space $\mathbb{R}^d$, trademarks of certain quantum systems and cosmological structures, are a universal signature of maximally random jammed (MRJ) hard-particle packings. We introduce a novel hyperuniformity descriptor in MRJ packings by studying local-volume-fraction fluctuations and show that infinite-wavelength fluctuations vanish even for packings with size- and shape-distributions. Special void statistics induce hyperuniformity and QLR pair correlations.Comment: 10 pages, 3 figures; changes to figures and text based on review process; accepted for publication at Phys. Rev. Let

Thermal Casimir drag in fluctuating classical fields

A uniformly moving inclusion which locally suppresses the fluctuations of a classical thermally excited field is shown to experience a drag force which depends on the dynamics of the field. It is shown that in a number of cases the linear friction coefficient is dominated by short distance fluctuations and takes a very simple form. Examples where this drag can occur are for stiff objects, such as proteins, nonspecifically bound to more flexible ones such as polymers and membranes.Comment: 4 pages RevTex, 2 figure

The phase diagram of 2D polar condensates in a magnetic field

Spin one condensates in the polar (antiferromagnetic) phase in two dimensions are shown to undergo a transition of the Ising type, in addition to the expected Kosterlitz--Thouless (KT) transition of half vortices, due to the quadratic Zeeman effect. We establish the phase diagram in terms of temperature and the strength of the Zeeman effect using Monte Carlo simulations. When the Zeeman effect is sufficiently strong the Ising and KT transitions merge. For very strong Zeeman field the remaining transition is of the familiar integer KT type.Comment: 4 pages, 7 figure

Cyclic phase in F=2 spinor condensate: Long-range order, kinks, and roughening transition

We study the effect of thermal fluctuations on homogeneous infinite Bose-Einstein condensate with spin F=2 in the cyclic state, when atoms occupy three hyperfine states with $m_{F}=0, \pm 2$. We use both the approach of small-amplitude oscillations and mapping of our model on the sine-Gordon model. We show that thermal fluctuations lead to the existence of the rough phase in one- and two-dimensional systems, when presence of kinks is favorable. The structure and energy of a single kink are found. We also discuss the effect of thermal fluctuations on spin degrees of freedom in F=1 condensate.Comment: 6 pages, 1 figure; final version, accepted for publication in Phys. Rev.

Lateral and normal forces between patterned substrates induced by nematic fluctuations

We consider a nematic liquid crystal confined by two parallel flat substrates whose anchoring conditions vary periodically in one lateral direction. Within the Gaussian approximation, we study the effective forces between the patterned substrates induced by the thermal fluctuations of the nematic director. The shear force oscillates as function of the lateral shift between the patterns on the lower and the upper substrates. We compare the strength of this fluctuation-induced lateral force with the lateral van der Waals force arising from chemically structured adsorbed monolayers. The fluctuation-induced force in normal direction is either repulsive or attractive, depending on the model parameters.Comment: 9 pages, 9 figure

Exact Constructions of a Family of Dense Periodic Packings of Tetrahedra

The determination of the densest packings of regular tetrahedra (one of the five Platonic solids) is attracting great attention as evidenced by the rapid pace at which packing records are being broken and the fascinating packing structures that have emerged. Here we provide the most general analytical formulation to date to construct dense periodic packings of tetrahedra with four particles per fundamental cell. This analysis results in six-parameter family of dense tetrahedron packings that includes as special cases recently discovered "dimer" packings of tetrahedra, including the densest known packings with density $\phi= 4000/4671 = 0.856347...$. This study strongly suggests that the latter set of packings are the densest among all packings with a four-particle basis. Whether they are the densest packings of tetrahedra among all packings is an open question, but we offer remarks about this issue. Moreover, we describe a procedure that provides estimates of upper bounds on the maximal density of tetrahedron packings, which could aid in assessing the packing efficiency of candidate dense packings.Comment: It contains 25 pages, 5 figures

Unconventional string-like singularities in flat spacetime

The conical singularity in flat spacetime is mostly known as a model of the cosmic string or the wedge disclination in solids. Its another, equally important, function is to be a representative of quasiregular singularities. From all these of views it seems interesting to find out whether there exist other similar singularities. To specify what "similar" means I introduce the notion of the string-like singularity, which is, roughly speaking, an absolutely mild singularity concentrated on a curve or on a 2-surface S (depending on whether the space is three- of four-dimensional). A few such singularities are already known: the aforementioned conical singularity, two its Lorentzian versions, the "spinning string", the "screw dislocation", and Tod's spacetime. In all these spacetimes S is a straight line (or a plane) and one may wonder if this is an inherent property of the string-like singularities. The aim of this paper is to construct string-like singularities with less trivial S. These include flat spacetimes in which S is a spiral, or even a loop. If such singularities exist in nature (in particular, as an approximation to gravitational field of strings) their cosmological and astrophysical manifestations must differ drastically from those of the conventional cosmic strings. Likewise, being realized as topological defects in crystals such loops and spirals will probably also have rather unusual properties.Comment: Draft. References and comments are welcome. v2. Section 3 is intact, the rest is made briefer and clearer. A couple of references are added. v3. Insignificant correstions. The published versio