Topological effects at short antiferromagnetic Heisenberg chains

The manifestations of topological effects in finite antiferromagnetic Heisenberg chains is examined by density matrix renormalization group technique in this paper. We find that difference between integer and half-integer spin chains shows up in ground state energy per site when length of spin chain is longer than $\sim\xi$, where $\xi\sim\exp(\pi S)$ is a spin-spin correlation length, for spin magnitude S up to 5/2. For open chains with spin magnitudes $S=5/2$ to S=5, we verify that end states with fractional spin quantum numbers $S'$ exist and are visible even when the chain length is much smaller than the correlation length $\xi$. The end states manifest themselves in the structure of the low energy excitation spectrum.Comment: 4 pages, 6 figure

Impurity state in Haldane gap for S=1 Heisenberg antiferromagnetic chain with bond doping

Using a new impurity density matrix renormalization group scheme, we establish a reliable picture of how the low lying energy levels of a $S=1$ Heisenberg antiferromagnetic chain change {\it quantitatively} upon bond doping. A new impurity state gradually occurs in the Haldane gap as $J' < J$, while it appears only if $J'/J>\gamma_c$ with $1/\gamma_c=0.708$ as $J'>J$. The system is non-perturbative as $1\leq J'/J\leq\gamma_c$. This explains the appearance of a new state in the Haldane gap in a recent experiment on Y$_{2-x}$Ca$_x$BaNiO$_5$ [J.F. DiTusa, et al., Phys. Rev. Lett. 73 1857(1994)].Comment: 4 pages of uuencoded gzip'd postscrip

A class of ansatz wave functions for 1D spin systems and their relation to DMRG

We investigate the density matrix renormalization group (DMRG) discovered by White and show that in the case where the renormalization eventually converges to a fixed point the DMRG ground state can be simply written as a ``matrix product'' form. This ground state can also be rederived through a simple variational ansatz making no reference to the DMRG construction. We also show how to construct the ``matrix product'' states and how to calculate their properties, including the excitation spectrum. This paper provides details of many results announced in an earlier letter.Comment: RevTeX, 49 pages including 4 figures (macro included). Uuencoded with uufiles. A complete postscript file is available at http://fy.chalmers.se/~tfksr/prb.dmrg.p

Leveraging virtual brand community engagement and consumer brand identification as a bounce-back brand recovery strategy : role of brand endorsers

Brand community engagement in the virtual environment during a brand crisis has gained attention from firms and marketing scholars due to innovative technologies. However, there is a limited empirical insight into its antecedents and consequences. This study, grounded in signaling and social identity theories, introduces the relationship between endorser credibility, virtual brand community engagement, and consumer brand identification as a pro-active brand recovery strategy. It also examines the impact of virtual brand engagement and consumer brand identification on brand love and forgiveness. The data were collected from online automobile brand community members. The results show that celebrity endorsers' trustworthiness, attractiveness, and expertise can be used as communication signals to expedite the process of consumer brand identification and virtual brand community engagement. Consumers who strongly identify with the brand and engage in its virtual community are more likely to develop an emotional bond with the brand and forgive it following a crisis. Interestingly, the study finds that the relationships between endorser credibility attractiveness and virtual consumer brand identification, endorser credibility trustworthiness and consumer brand identification, and consumer brand identification and virtual brand community are nonlinear

Variational and DMRG studies of the Frustrated Antiferromagnetic Heisenberg S=1 Quantum Spin Chain

We study a frustrated antiferromagnetic isotropic Heisenberg $S=1$ chain using a variational ansatz and the DMRG. At $\alpha_D=0.284(1)$, there is a disorder point of the second kind, marking the onset of incommensurate correlations in the chain. At $\alpha_L=0.3725(25)$ there is a Lifshitz point, at which the excitation spectrum develops a doubly degenerate structure. These points are the quantum remnants of the transition from antiferromagnetic to spiral order in the classical frustrated chain. At $\alpha_T=0.7444(6)$ there is a first order phase transition from an AKLT phase to a next-nearest neighbor generalization of the AKLT model. At the transition, the string order parameter shows a discontinuous jump of 0.085 to 0; the correlation length and the gap are both finite at the transition. The problem of edge states in open frustrated chains is discussed at length.Comment: 37 pages, 14 figures, submitted to Phys.Rev.

Frustrated antiferromagnetic quantum spin chains for spin length S > 1

We investigate frustrated antiferromagnetic Heisenberg quantum spin chains at T=0 for S=3/2 and S=2 using the DMRG method. We localize disorder and Lifshitz points, confirming that quantum disorder points can be seen as quantum remnants of classical phase transitions. Both in the S=3/2 and the S=2 chain, we observe the disappearance of effectively free S=1/2 and S=1 end spins respectively. The frustrated spin chain is therefore a suitable system for clearly showing the existence of free end spins S'=[S/2] also in half-integer antiferromagnetic spin chains with S>1/2. We suggest that the first order transition found for S=1 in our previous work is present in all frustrated spin chains with S>1/2, characterized by the disappearance of effectively free end spins with S'=[S/2].Comment: 6 pages, 8 ps figures, uses RevTeX, submitted to PR

Pervasive and standalone computing: The perceptual effects of variable multimedia quality.

The introduction of multimedia on pervasive and mobile communication devices raises a number of perceptual quality issues, however, limited work has been done examining the 3-way interaction between use of equipment, quality of perception and quality of service. Our work measures levels of informational transfer (objective) and user satisfaction (subjective)when users are presented with multimedia video clips at three different frame rates, using four different display devices, simulating variation in participant mobility. Our results will show that variation in frame-rate does not impact a user’s level of information assimilation, however, does impact a users’ perception of multimedia video ‘quality’. Additionally, increased visual immersion can be used to increase transfer of video information, but can negatively affect the users’ perception of ‘quality’. Finally, we illustrate the significant affect of clip-content on the transfer of video, audio and textual information, placing into doubt the use of purely objective quality definitions when considering multimedia presentations

The spectral gap for some spin chains with discrete symmetry breaking

We prove that for any finite set of generalized valence bond solid (GVBS) states of a quantum spin chain there exists a translation invariant finite-range Hamiltonian for which this set is the set of ground states. This result implies that there are GVBS models with arbitrary broken discrete symmetries that are described as combinations of lattice translations, lattice reflections, and local unitary or anti-unitary transformations. We also show that all GVBS models that satisfy some natural conditions have a spectral gap. The existence of a spectral gap is obtained by applying a simple and quite general strategy for proving lower bounds on the spectral gap of the generator of a classical or quantum spin dynamics. This general scheme is interesting in its own right and therefore, although the basic idea is not new, we present it in a system-independent setting. The results are illustrated with an number of examples.Comment: 48 pages, Plain TeX, BN26/Oct/9

The Haldane gap for the S=2 antiferromagnetic Heisenberg chain revisited

Using the density matrix renormalization group (DMRG) technique, we carry out a large scale numerical calculation for the S=2 antiferromagnetic Heisenberg chain. Performing systematic scaling analysis for both the chain length $L$ and the number of optimal states kept in the iterations $m$, the Haldane gap $\Delta (2)$ is estimated accurately as $(0.0876\pm0.0013)J$. Our systematic analysis for the S=2 chains not only ends the controversies arising from various DMRG calculations and Monte Carlo simulations, but also sheds light on how to obtain reliable results from the DMRG calculations for other complicated systems.Comment: 4 pages and 1 figur

Influence of shear flow on vesicles near a wall: a numerical study

We describe the dynamics of three-dimensional fluid vesicles in steady shear flow in the vicinity of a wall. This is analyzed numerically at low Reynolds numbers using a boundary element method. The area-incompressible vesicle exhibits bending elasticity. Forces due to adhesion or gravity oppose the hydrodynamic lift force driving the vesicle away from a wall. We investigate three cases. First, a neutrally buoyant vesicle is placed in the vicinity of a wall which acts only as a geometrical constraint. We find that the lift velocity is linearly proportional to shear rate and decreases with increasing distance between the vesicle and the wall. Second, with a vesicle filled with a denser fluid, we find a stationary hovering state. We present an estimate of the viscous lift force which seems to agree with recent experiments of Lorz et al. [Europhys. Lett., vol. 51, 468 (2000)]. Third, if the wall exerts an additional adhesive force, we investigate the dynamical unbinding transition which occurs at an adhesion strength linearly proportional to the shear rate.Comment: 17 pages (incl. 10 figures), RevTeX (figures in PostScript