Entanglement Spectra of Heisenberg Ladders of higher Spin

We study the entanglement spectrum of Heisenberg spin ladders of arbitrary spin length S in the perturbative regime of strong rung coupling. For isotropic spin coupling the the entanglement spectrum is, within first order perturbation theory, always proportional to the energy spectrum of the single chain with a proportionality factor being also independent of S. A more complicated situation arises for anisotropic ladders of higher spin S>=1 since here even the unperturbed ground state has a nontrivial entanglement spectrum. Finally we discuss related issues in dimerized spin chains

Finite Correlation Length Scaling in Lorentz-Invariant Gapless iPEPS Wave Functions

It is an open question how well tensor network states in the form of an infinite projected entangled pair states (iPEPS) tensor network can approximate gapless quantum states of matter. Here we address this issue for two different physical scenarios: i) a conformally invariant $(2+1)d$ quantum critical point in the incarnation of the transverse field Ising model on the square lattice and ii) spontaneously broken continuous symmetries with gapless Goldstone modes exemplified by the $S=1/2$ antiferromagnetic Heisenberg and XY models on the square lattice. We find that the energetically best wave functions display {\em finite} correlation lengths and we introduce a powerful finite correlation length scaling framework for the analysis of such finite-$D$ iPEPS states. The framework is important i) to understand the mild limitations of the finite-$D$ iPEPS manifold in representing Lorentz-invariant, gapless many body quantum states and ii) to put forward a practical scheme in which the finite correlation length $\xi(D)$ combined with field theory inspired formulae can be used to extrapolate the data to infinite correlation length, i.e. to the thermodynamic limit. The finite correlation length scaling framework opens the way for further exploration of quantum matter with an (expected) Lorentz-invariant, massless low-energy description, with many applications ranging from condensed matter to high-energy physics.Comment: 16 pages, 11 figure

Exotic phenomena in doped quantum magnets

We investigate the properties of the two-dimensional frustrated quantum antiferromagnet on the square lattice, especially at infinitesimal doping. We find that next nearest neighbor (N.N.) J2 and next-next N.N. J3 interactions together destroy the antiferromagnetic long range order and stabilize a quantum disordered valence bond crystalline plaquette phase. A static vacancy or a dynamic hole doped into this phase liberates a spinon. From the profile of the spinon wavefunction around the (static) vacancy we identify an intermediate behavior between complete deconfinement (behavior seen in the kagome lattice) and strong confinement (behavior seen in the checkerboard lattice) with the emergence of two length scales, a spinon confinement length larger than the magnetic correlation length. When a finite hole hopping is introduced, this behavior translates into an extended (mobile) spinon-holon boundstate with a very small quasiparticle weight. These features provide clear evidence for a nearby "deconfined critical point" in a doped microscopic model. Finally, we give arguments in favor of superconducting properties of the doped plaquette phase.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of International Conference "Highly Frustrated Magnets", Osaka (Japan), August 2006). 6 pages, 5 figures Display problems with Figure 2 fixe

The S=1/2S=1/2 Kagome Heisenberg Antiferromagnet Revisited

We examine the perennial quantum spin-liquid candidate $S=1/2$ Heisenberg antiferromagnet on the kagome lattice. Our study is based on achieving Lanczos diagonalization of the Hamiltonian on a $48$ site cluster in sectors with dimensions as a large as $5 \times 10^{11}$. The results reveal novel intricate structures in the low-lying energy spectrum. These structures by no means unambiguously support an emerging consensus of a $\mathbb{Z}_2$ spin liquid ground state, but instead appear compatible with several scenarios, including four-fold topological degeneracy, inversion symmetry breaking and a combination thereof. We discuss finite-size effects, such as the apparent absence of ETH, and note that while considerably reduced, some are still present for the largest cluster. Finally, we observe that an XXZ model in the Ising limit reproduces remarkably well the most striking features of finite-size spectra.Comment: 8 pages, 5 figure