Multi-triplet bound states and finite-temperature dynamics in highly frustrated quantum spin ladders

Low-dimensional quantum magnets at finite temperatures present a complex interplay of quantum and thermal fluctuation effects in a restricted phase space. While some information about dynamical response functions is available from theoretical studies of the one-triplet dispersion in unfrustrated chains and ladders, little is known about the finite-temperature dynamics of frustrated systems. Experimentally, inelastic neutron scattering studies of the highly frustrated two-dimensional material SrCu$_2$(BO$_3$)$_2$ show an almost complete destruction of the one-triplet excitation band at a temperature only 1/3 of its gap energy, accompanied by strong scattering intensities for apparent multi-triplet excitations. We investigate these questions in the frustrated spin ladder and present numerical results from exact diagonalization for the dynamical structure factor as a function of temperature. We find anomalously rapid transfer of spectral weight out of the one-triplet band and into both broad and sharp spectral features at a wide range of energies, including below the zero-temperature gap of this excitation. These features are multi-triplet bound states, which develop particularly strongly near the quantum phase transition, fall to particularly low energies there, and persist to all the way to infinite temperature. Our results offer valuable insight into the physics of finite-temperature spectral functions in SrCu$_2$(BO$_3$)$_2$ and many other highly frustrated spin systems.Comment: 22 pages, 19 figures; published version: many small modification

Efficient Quantum Monte Carlo simulations of highly frustrated magnets: the frustrated spin-1/2 ladder

Quantum Monte Carlo simulations provide one of the more powerful and versatile numerical approaches to condensed matter systems. However, their application to frustrated quantum spin models, in all relevant temperature regimes, is hamstrung by the infamous "sign problem." Here we exploit the fact that the sign problem is basis-dependent. Recent studies have shown that passing to a dimer (two-site) basis eliminates the sign problem completely for a fully frustrated spin model on the two-leg ladder. We generalize this result to all partially frustrated two-leg spin-1/2 ladders, meaning those where the diagonal and leg couplings take any antiferromagnetic values. We find that, although the sign problem does reappear, it remains remarkably mild throughout the entire phase diagram. We explain this result and apply it to perform efficient quantum Monte Carlo simulations of frustrated ladders, obtaining accurate results for thermodynamic quantities such as the magnetic specific heat and susceptibility of ladders up to L=200 rungs (400 spins 1/2) and down to very low temperatures.Comment: 26 pages including 12 figures; this version: minor modifications in sections 3.3 and 4.

Field-induced gap in ordered Heisenberg antiferromagnets

Heisenberg antiferromagnets in a strong uniform magnetic field $H$ are expected to exhibit a gapless phase with a global O(2) symmetry. In many real magnets, a small energy gap is induced by additional interactions that can be viewed as a staggered transverse magnetic field $h = c H$, where $c$ is a small proportionality constant. We study the effects of such a perturbation, particularly for magnets with long-range order, by using several complimentary approaches: numerical diagonalizations of a model with long-range interactions, classical equations of motion, and scaling arguments. In an ordered state at zero temperature, the energy gap at first grows as $(cH)^{1/2}$ and then may dip to a smaller value, of order $(cH)^{2/3}$, at the quantum critical point separating the ``gapless'' phase from the gapped state with saturated magnetization. In one spatial dimension, the latter exponent changes to 4/5.Comment: 6 pages, 5 figure

Static impurities in the kagome lattice: dimer freezing and mutual repulsion

We consider the effects of doping the S = 1/2 kagome lattice with static impurities. We demonstrate that impurities lower the number of low-lying singlet states, induce dimer-dimer correlations of considerable spatial extent, and do not generate free spin degrees of freedom. Most importantly, they experience a highly unconventional mutual repulsion as a direct consequence of the strong spin frustration. These properties are illustrated by exact diagonalization, and reproduced to semi-quantitative accuracy within a dimer resonating-valence-bond description which affords access to longer length scales. We calculate the local magnetization induced by doped impurities, and consider its implications for nuclear magnetic resonance measurements on known kagome systems.Comment: 9 pages, 12 figure

Zero-Temperature Properties of the Quantum Dimer Model on the Triangular Lattice

Using exact diagonalizations and Green's function Monte Carlo simulations, we have studied the zero-temperature properties of the quantum dimer model on the triangular lattice on clusters with up to 588 sites. A detailed comparison of the properties in different topological sectors as a function of the cluster size and for different cluster shapes has allowed us to identify different phases, to show explicitly the presence of topological degeneracy in a phase close to the Rokhsar-Kivelson point, and to understand finite-size effects inside this phase. The nature of the various phases has been further investigated by calculating dimer-dimer correlation functions. The present results confirm and complement the phase diagram proposed by Moessner and Sondhi on the basis of finite-temperature simulations [Phys. Rev. Lett. {\bf 86}, 1881 (2001)].Comment: 10 pages, 16 figure

Magnetization plateaux in an extended Shastry-Sutherland model

We study an extended two-dimensional Shastry-Sutherland model in a magnetic field where besides the usual Heisenberg exchanges of the Shastry-Sutherland model two additional SU(2) invariant couplings are included. Perturbative continous unitary transformations are used to determine the leading order effects of the additional couplings on the pure hopping and on the long-range interactions between the triplons which are the most relevant terms for small magnetization. We then compare the energy of various magnetization plateaux in the classical limit and we discuss the implications for the two-dimensional quantum magnet SrCu$_2$(BO$_3$)$_2$.Comment: 8 pages, Proceedings of the HFM2008 Conferenc

A 2D quantum antiferromagnet with a four-fold degenerate valence-bond-solid ground state

We study the competition between antiferromagnetic order and valence bond solid formation in a two-dimensional frustrated spin-1/2 model. The J1-J2 model on the square lattice is further frustrated by introducing products of three-spin projectors which stabilize four dimer-product states as degenerate ground states. These four states are reminiscent of the dimerized singlet ground state of the Shastry-Sutherland model. Using exact diagonalizations, we study the evolution of the ground state by varying theratio of interactions. For a large range of parameters (J2 > 0.25J1), our model shows a direct transition between the valence-bond-solid phase and the collinear antiferromagnetic phase. For small values of J2, several intermediate phases appear which are also analyzed

Field Induced Staggered Magnetization and Magnetic Ordering in Cu2(C5H12N2)2Cl4Cu_2 (C_5 H_{12} N_2)_2 Cl_4

We present a $^2$D NMR investigation of the gapped spin-1/2 compound $Cu_2 (C_5 H_{12} N_2)_2 Cl_4$. Our measurements reveal the presence of a magnetic field induced transverse staggered magnetization (TSM) which persists well below and above the field-induced 3D long-range magnetically ordered (FIMO) phase. The symmetry of this TSM is different from that of the TSM induced by the order parameter of the FIMO phase. Its origin, field dependence and symmetry can be explained by an intra-dimer Dzyaloshinskii-Moriya interaction, as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict that the transition into the FIMO phase is not in the BEC universality class.Comment: 4 page

Evidence of columnar order in the fully frustrated transverse field Ising model on the square lattice

Using extensive classical and quantum Monte Carlo simulations, we investigate the ground-state phase diagram of the fully frustrated transverse field Ising model on the square lattice. We show that pure columnar order develops in the low-field phase above a surprisingly large length scale, below which an effective U(1) symmetry is present. The same conclusion applies to the Quantum Dimer Model with purely kinetic energy, to which the model reduces in the zero-field limit, as well as to the stacked classical version of the model. By contrast, the 2D classical version of the model is shown to develop plaquette order. Semiclassical arguments show that the transition from plaquette to columnar order is a consequence of quantum fluctuations.Comment: 5 pages (including Supplemental Material), 5 figure

Upaya Meningkatkan Keterampilan Membaca Aksara Jawa melalui Media Audio Visual pada Siswa Kelas IV SD Negeri 2 Gadingharjo Sanden Bantul

This study aimed to describe (1) students\u27 activities in the teaching of reading skills of Java letters through audio visual media and (2) the improvement of reading skills of Java letters through audio visual media among the fourth grade students of SD Negeri 2 Gadingharjo. The type of this study was classroom action research in the teaching of reading skills of Java letters. The subject of the research was the fourth grade students of SD Negeri 2 Gdaingharjo Sanden Bantul consisted of 39 students. The object of the research was reading skills of Java letters. Data collecting methods used test, observation, and documentation. This research used validity by using expert judgment. This study shows that (1) the use of audio visual could improve the students\u27 activities among the fourth grade students of SD Negeri 2 Gadingharjo. The students\u27 activities in pre cycle was 38.97%, cycle I was 67.69%, and cycle II was 89.74%. (2) The use of audio visual media could improve reading skill of Java letters among the fourth grade students of SD Negeri 2 Gadingharjo. The students\u27 reading skills show that there was an improvement from pre cycle until cycle II. The percentage of complete learning from pre cycle was 35.90% became 61.54% in cycle I and more improved in cycle II became 87.18%. It could be concluded that the use of audio visual media could improve learning activities and reading skills of Java letters