Vortex glass transitions in disordered three-dimensional XY models: Simulations for several different sets of parameters

The anisotropic frustrated 3D XY model with strong disorder in the coupling constants is studied as a model of a disordered superconductor in an applied magnetic field. Simulations with the exchange Monte Carlo method are performed for frustrations f=1/5 and f=1/4, corresponding to two different values of the magnetic field along the z direction. The anisotropy is also varied. The determination of the helicity modulus from twist histograms is discussed in some detail and the helicity modulus is used in finite size scaling analyses of the vortex glass transition. The general picture is that the behavior in [Phys. Rev. Lett. 91, 077002 (2003)] is confirmed. For strong (e.g. isotropic) coupling in the z direction the helicity modulus fails to scale and it is argued that this is due to a too small effective randomness of such systems for the accessible system sizes

How much entanglement is needed to reduce the energy variance?

We explore the relation between the entanglement of a pure state and its energy variance for a local one dimensional Hamiltonian, as the system size increases. In particular, we introduce a construction which creates a matrix product state of arbitrarily small energy variance $\delta^2$ for $N$ spins, with bond dimension scaling as $\sqrt{N} D_0^{1/\delta}$, where $D_0>1$ is a constant. This implies that a polynomially increasing bond dimension is enough to construct states with energy variance that vanishes with the inverse of the logarithm of the system size. We run numerical simulations to probe the construction on two different models, and compare the local reduced density matrices of the resulting states to the corresponding thermal equilibrium. Our results suggest that the spatially homogeneous states with logarithmically decreasing variance, which can be constructed efficiently, do converge to the thermal equilibrium in the thermodynamic limit, while the same is not true if the variance remains constant.Comment: small changes to fix typos and bibliographic reference

Ground State of the Kagome Lattice Heisenberg Antiferromagnet

Using series expansions around the dimer limit, we show that the ground state of the Heisenberg Antiferromagnet on the Kagome Lattice appears to be a Valence Bond Crystal (VBC) with a 36-site unit cell, and an energy per site of $E/J=-0.433\pm0.001$. It is a honeycomb lattice of `perfect hexagons' as discussed by Nikolic and Senthil. The energy difference between the ground state and other ordered states with the maximum number of `perfect hexagons', such as a stripe-ordered state, is of order $0.001 J$. The energy of the 36-site system with periodic boundary conditions is further lowered by an amount of $0.005\pm 0.001 J$, consistent with Exact Diagonalization. Every unit cell of the VBC has two singlet states whose degeneracy is not lifted to $6th$ order in the expansion. We estimate this energy difference to be smaller than $0.001 J$. Two leading orders of perturbation theory find the lowest-energy triplet excitations to be dispersionless and confined to the `perfect hexagons'

Trimers, molecules and polarons in imbalanced atomic Fermi gases

We consider the ground state of a single "spin-down" impurity atom interacting attractively with a "spin-up" atomic Fermi gas. By constructing variational wave functions for polarons, molecules and trimers, we perform a detailed study of the transitions between each of these dressed bound states as a function of mass ratio $r=m_\uparrow/m_\downarrow$ and interaction strength. We find that the presence of a Fermi sea enhances the stability of the $p$-wave trimer, which can be viewed as a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) molecule that has bound an additional majority atom. For sufficiently large $r$, we find that the transitions lie outside the region of phase separation in imbalanced Fermi gases and should thus be observable in experiment, unlike the well-studied equal-mass case.Comment: 5 pages, 2 figure

Enlarging and cooling the N\'eel state in an optical lattice

We propose an experimental scheme to favor both the realization and the detection of the N\'eel state in a two-component gas of ultracold fermions in a three-dimensional simple-cubic optical lattice. By adding three compensating Gaussian laser beams to the standard three pairs of retroreflected lattice beams, and adjusting the relative waists and intensities of the beams, one can significantly enhance the size of the N\'eel state in the trap, thus increasing the signal of optical Bragg scattering. Furthermore, the additional beams provide for adjustment of the local chemical potential and the possibility to evaporatively cool the gas while in the lattice. Our proposals are relevant to other attempts to realize many-body quantum phases in optical lattices.Comment: 8 pages, 10 figures (significantly revised text and figures

Universality and Crossover of Directed Polymers and Growing Surfaces

We study KPZ surfaces on Euclidean lattices and directed polymers on hierarchical lattices subject to different distributions of disorder, showing that universality holds, at odds with recent results on Euclidean lattices. Moreover, we find the presence of a slow (power-law) crossover toward the universal values of the exponents and verify that the exponent governing such crossover is universal too. In the limit of a 1+epsilon dimensional system we obtain both numerically and analytically that the crossover exponent is 1/2.Comment: LateX file + 5 .eps figures; to appear on Phys. Rev. Let

VELO Module Production - Module Assembly

This note describes in detail the procedures used in the gluing of sensors to hybrid and hybrid to pedestal for the LHCb VELO detector module assembly

Competing density-wave orders in a one-dimensional hard-boson model

We describe the zero-temperature phase diagram of a model of bosons, occupying sites of a linear chain, which obey a hard-exclusion constraint: any two nearest-neighbor sites may have at most one boson. A special case of our model was recently proposed as a description of a ``tilted'' Mott insulator of atoms trapped in an optical lattice. Our quantum Hamiltonian is shown to generate the transfer matrix of Baxter's hard-square model. Aided by exact solutions of a number of special cases, and by numerical studies, we obtain a phase diagram containing states with long-range density-wave order with period 2 and period 3, and also a floating incommensurate phase. Critical theories for the various quantum phase transitions are presented. As a byproduct, we show how to compute the Luttinger parameter in integrable theories with hard-exclusion constraints.Comment: 16 page

Do the surface Fermi arcs in Weyl semimetals survive disorder?

We theoretically study the topological robustness of the surface physics induced by Weyl Fermi-arc surface states in the presence of short-ranged quenched disorder and surface-bulk hybridization. This is investigated with numerically exact calculations on a lattice model exhibiting Weyl Fermi-arcs. We find that the Fermi-arc surface states, in addition to having a finite lifetime from disorder broadening, hybridize with nonperturbative bulk rare states making them no longer bound to the surface (i.e. they lose their purely surface spectral character). Thus, we provide strong numerical evidence that the Weyl Fermi-arcs are not topologically protected from disorder. Nonetheless, the surface chiral velocity is robust and survives in the presence of strong disorder, persisting all the way to the Anderson-localized phase by forming localized current loops that live within the localization length of the surface. Thus, the Weyl semimetal is not topologically robust to the presence of disorder, but the surface chiral velocity is.Comment: Single column; 24 pages, 12 figure