Two-hole bound states in modified t-J model

We consider modified $t-J$ model with minimum of single-hole dispersion at the points $(0,\pm \pi)$, $(\pm \pi,0)$. It is shown that two holes on antiferromagnetic background produce a bound state which properties strongly differs from the states known in the unmodified $t-J$ model. The bound state is d-wave, it has four nodes on the face of the magnetic Brillouin zone. However, in the coordinate representation it looks like as usual s-wave.Comment: LaTeX 9 page

Quantum impurity in an antiferromagnet: non-linear sigma model theory

We present a new formulation of the theory of an arbitrary quantum impurity in an antiferromagnet, using the O(3) non-linear sigma model. We obtain the low temperature expansion for the impurity spin susceptibilities of antiferromagnets with magnetic long-range order in the ground state. We also consider the bulk quantum phase transition in d=2 to the gapped paramagnet (d is the spatial dimension): the impurity is described solely by a topological Berry phase term which is an exactly marginal perturbation to the critical theory. The physical properties of the quantum impurity near criticality are obtained by an expansion in (d-1).Comment: 14 pages, 7 figures; (v2) added re

Excitation spectrum and ground state properties of the S=1/2 Heisenberg ladder with staggered dimerization

We have studied the excitation spectrum of the $S=1/2$ quantum spin ladder with staggered dimerization by dimer series expansions, diagrammatic analysis of an effective interacting Bose gas of local triplets, and exact diagonalization of small clusters. We find that the model has two massive phases, with predominant inter-chain (rung) or intra-chain correlations. The transition from the rung dimer into the intra-chain dimer phase is characterized by softening of the triplet spectrum at $k=\pi$. The excitation spectrum as well as the spin correlations away from and close to the critical line are calculated. The location of the phase boundary is also determined.Comment: 13 pages, 7 figure

Critical Dynamics of Singlet Excitations in a Frustrated Spin System

We construct and analyze a two-dimensional frustrated quantum spin model with plaquette order, in which the low-energy dynamics is controlled by spin singlets. At a critical value of frustration the singlet spectrum becomes gapless, indicating a quantum transition to a phase with dimer order. This T=0 transition belongs to the 3D Ising universality class, while at finite temperature a 2D Ising critical line separates the plaquette and dimerized phases. The magnetic susceptibility has an activated form throughout the phase diagram, whereas the specific heat exhibits a rich structure and a power law dependence on temperature at the quantum critical point. We argue that the novel quantum critical behavior associated with singlet criticality discussed in this work can be relevant to a wide class of quantum spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where the low-energy excitations are known to be spin singlets, as well as to the CAVO lattice and several recently discovered strongly frustrated square-lattice antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear in Phys. Rev.

Zero temperature phases of the frustrated J1-J2 antiferromagnetic spin-1/2 Heisenberg model on a simple cubic lattice

At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg antiferromagnet on a simple cubic lattice with competing first and second neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave theory. We find existence of two phases: a two sublattice Neel phase for small J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain the sublattice magnetizations and ground state energies for the two phases and find that there exists a first order phase transition from the AF-phase to the CAF-phase at the critical transition point, pc = 0.28. Our results for the value of pc are in excellent agreement with results from Monte-Carlo simulations and variational spin wave theory. We also show that the quartic 1/S corrections due spin-wave interactions enhance the sublattice magnetization in both the phases which causes the intermediate paramagnetic phase predicted from linear spin wave theory to disappear.Comment: 19 pages, 4 figures, Fig. 1b modified, Appendix B text modifie

Single hole dynamics in the t-J model on two- and three-leg ladders

The dynamics of a single hole in the t-J model on two- (2LL) and three- (3LL) leg ladders is studied using a recently developed quantum Monte Carlo algorithm. For the 2LL it is shown that in addition to the most pronounced features of the spectral function, well described by the limit of strong coupling along the rungs, a clear shadow band appears in the antibonding channel. Moreover, both the bonding band and its shadow have a finite quasiparticle (QP) weight in the thermodynamic limit. For strong coupling along the rungs of the 3LL, the low-energy spectrum in the antisymmetric channel is similar to a one-dimensional chain, whereas in the two symmetric channels it resembles the 2LL. The QP weight vanishes in the antisymmetric channel, but is finite in the symmetric one

High-resolution study of 0+ and 2+ excitations in 168Er with the (p,t) reaction

Excited states in the deformed nucleus 168Er have been studied with high-energy resolution, in the (p, t ) reaction, with the Munich Q3D spectrograph. A number of 25 excited 0+ states (four tentative) and 63 2+ states have been assigned up to 4.0 MeV excitation energy. This unusually rich characterization of the 0+ and 2+ states in a deformed nucleus, close to a complete level scheme, offers a unique opportunity to check, in detail, models of nuclear structure that incorporate many excitation modes. A comparison of the experimental data is made with two such models: the quasiparticle-phonon model (QPM), and the projected shell model (PSM). The PSM wave functions appear to contain fewer correlations than those of the QPM and than required by the data

Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order

We present spectral and optical properties of the Hubbard model on a two-dimensional square lattice using a generalization of dynamical mean-field theory to magnetic states in finite dimension. The self-energy includes the effect of spin fluctuations and screening of the Coulomb interaction due to particle-particle scattering. At half-filling the quasiparticles reduce the width of the Mott-Hubbard `gap' and have dispersions and spectral weights that agree remarkably well with quantum Monte Carlo and exact diagonalization calculations. Away from half-filling we consider incommensurate magnetic order with a varying local spin direction, and derive the photoemission and optical spectra. The incommensurate magnetic order leads to a pseudogap which opens at the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle states survive in the doped systems, but their dispersion is modified with the doping and a rigid band picture does not apply. Spectral weight in the optical conductivity is transferred to lower energies and the Drude weight increases linearly with increasing doping. We show that incommensurate magnetic order leads also to mid-gap states in the optical spectra and to decreased scattering rates in the transport processes, in qualitative agreement with the experimental observations in doped systems. The gradual disappearence of the spiral magnetic order and the vanishing pseudogap with increasing temperature is found to be responsible for the linear resistivity. We discuss the possible reasons why these results may only partially explain the features observed in the optical spectra of high temperature superconductors.Comment: 22 pages, 18 figure

Quantum magnetism in two dimensions: From semi-classical N\'eel order to magnetic disorder

This is a review of ground-state features of the s=1/2 Heisenberg antiferromagnet on two-dimensional lattices. A central issue is the interplay of lattice topology (e.g. coordination number, non-equivalent nearest-neighbor bonds, geometric frustration) and quantum fluctuations and their impact on possible long-range order. This article presents a unified summary of all 11 two-dimensional uniform Archimedean lattices which include e.g. the square, triangular and kagome lattice. We find that the ground state of the spin-1/2 Heisenberg antiferromagnet is likely to be semi-classically ordered in most cases. However, the interplay of geometric frustration and quantum fluctuations gives rise to a quantum paramagnetic ground state without semi-classical long-range order on two lattices which are precisely those among the 11 uniform Archimedean lattices with a highly degenerate ground state in the classical limit. The first one is the famous kagome lattice where many low-lying singlet excitations are known to arise in the spin gap. The second lattice is called star lattice and has a clear gap to all excitations. Modification of certain bonds leads to quantum phase transitions which are also discussed briefly. Furthermore, we discuss the magnetization process of the Heisenberg antiferromagnet on the 11 Archimedean lattices, focusing on anomalies like plateaus and a magnetization jump just below the saturation field. As an illustration we discuss the two-dimensional Shastry-Sutherland model which is used to describe SrCu2(BO3)2.Comment: This is now the complete 72-page preprint version of the 2004 review article. This version corrects two further typographic errors (three total with respect to the published version), see page 2 for detail

Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS

We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final version to appear in Physics Letters