201 research outputs found
Two-hole bound states in modified t-J model
We consider modified model with minimum of single-hole dispersion at
the points , . It is shown that two holes on
antiferromagnetic background produce a bound state which properties strongly
differs from the states known in the unmodified 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 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 . 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
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