298 research outputs found
Ground states of a frustrated quantum spin chain with long-range interactions
The ground state of a spin-1/2 Heisenberg chain with both frustration and
long-range interactions is studied using Lanczos exact diagonalization. The
evolution of the well known dimerization transition of the system with
short-range frustrated interactions (the J1-J2 chain) is investigated in the
presence of additional unfrustrated interactions decaying with distance as
1/r^a. It is shown that the continuous (infinite-order) dimerization transition
develops into a first-order transition between a long-range ordered
antiferromagnetic state and a state with coexisting dimerization and critical
spin correlations at wave-number k=\pi/2. The relevance of the model to real
systems is discussed.Comment: 4 pages, 5 figures, final published versio
Energy and multipartite entanglement in multidimensional and frustrated spin models
We investigate the relation between the entanglement properties of a quantum
state and its energy for macroscopic spin models. To this aim, we develop a
general method to compute energy bounds for states without certain forms of
multipartite entanglement. Violation of these bounds implies the presence of
these types of multipartite entanglement. As examples, we investigate the
Heisenberg model in different dimensions, the Ising model and the XX model in
the presence of a magnetic field. Finally, by studying the Heisenberg model on
a triangular lattice, we demonstrate that our techniques can be applied also to
frustrated systems.Comment: 9 pages, 6 figures, v2: small change
Electro-disintegration following beta-decay
I show that the disintegration of weakly-bound nuclei and the ionization of
weakly-bound atomic electrons due to their interaction with leptons from beta
decay is a negligible effect.Comment: 3 pages, 1 figure, to be published in the Physical Review
Hardcore dimer aspects of the SU(2) Singlet wavefunction
We demonstrate that any SU(2) singlet wavefunction can be characterized by a
set of Valence Bond occupation numbers, testing dimer presence/vacancy on pairs
of sites. This genuine quantum property of singlet states (i) shows that SU(2)
singlets share some of the intuitive features of hardcore quantum dimers, (ii)
gives rigorous basis for interesting albeit apparently ill-defined quantities
introduced recently in the context of Quantum Magnetism or Quantum Information
to measure respectively spin correlations and bipartite entanglement and, (iii)
suggests a scheme to define consistently a wide family of quantities analogous
to high order spin correlation. This result is demonstrated in the framework of
a general functional mapping between the Hilbert space generated by an
arbitrary number of spins and a set of algebraic functions found to be an
efficient analytical tool for the description of quantum spins or qubits
systems.Comment: 5 pages, 2 figure
Ground state projection of quantum spin systems in the valence bond basis
A Monte Carlo method for quantum spin systems is formulated in the basis of
valence bond (singlet pair) states. The non-orthogonality of this basis allows
for an efficient importance-sampled projection of the ground state out of an
arbitrary state. The method provides access to resonating valence-bond physics,
enables a direct improved estimator for the singlet-triplet gap, and extends
the class of models that can be studied without negative-sign problems. As a
demonstration, the valence bond distribution in the ground state of the 2D
Heisenberg antiferromagnet is calculated. Generalizations of the method to
fermion systems are also discussed.Comment: 4+ pages, accepted for publication in Phys. Rev. Let
Orbital-controlled magnetic transition between gapful and gapless phases in the Haldane system with t2g-orbital degeneracy
In order to clarify a key role of orbital degree of freedom in the spin S=1
Haldane system, we investigate ground-state properties of the t2g-orbital
degenerate Hubbard model on the linear chain by using numerical techniques.
Increasing the Hund's rule coupling in multi-orbital systems, in general, there
occurs a transition from an antiferromagnetic to a ferromagnetic phase. We find
that the antiferromagnetic phase is described as the Haldane system with spin
gap, while in the ferromagnetic phase, there exists the gapless excitation with
respect to orbital degree of freedom. Possible relevance of the present results
to actual systems is also discussed.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
Neel order in the two-dimensional S=1/2 Heisenberg Model
The existence of Neel order in the S=1/2 Heisenberg model on the square
lattice at T=0 is shown using inequalities set up by Kennedy, Lieb and Shastry
in combination with high precision Quantum Monte Carlo data.Comment: 4 pages, 1 figur
Master equation approach to computing RVB bond amplitudes
We describe a "master equation" analysis for the bond amplitudes h(r) of an
RVB wavefunction. Starting from any initial guess, h(r) evolves (in a manner
dictated by the spin hamiltonian under consideration) toward a steady-state
distribution representing an approximation to the true ground state. Unknown
transition coefficients in the master equation are treated as variational
parameters. We illustrate the method by applying it to the J1-J2
antiferromagnetic Heisenberg model. Without frustration (J2=0), the amplitudes
are radially symmetric and fall off as 1/r^3 in the bond length. As the
frustration increases, there are precursor signs of columnar or plaquette VBS
order: the bonds preferentially align along the axes of the square lattice and
weight accrues in the nearest-neighbour bond amplitudes. The Marshall sign rule
holds over a large range of couplings, J2/J1 < 0.418. It fails when the r=(2,1)
bond amplitude first goes negative, a point also marked by a cusp in the ground
state energy. A nonrigourous extrapolation of the staggered magnetic moment
(through this point of nonanalyticity) shows it vanishing continuously at a
critical value J2/J1 = 0.447. This may be preempted by a first-order transition
to a state of broken translational symmetry.Comment: 8 pages, 7 figure
Light-Front Dynamic Analysis of Bound States in Scalar Field Model
The light-front dynamics (LFD) of the scalar field model theory is analyzed
to solve the two-body bound-state problem. The light-front two-body bound-state
equation is extended to the full LFD kernel including the ladder, cross-ladder,
stretched-box, and particle-antiparticle creation/annihilation effects to study
the contributions of higher Fock-states. The light-front two-body equation is
also modified by the term corresponding to the self-energy corrections and
counter-terms. Using the variational principle, we obtain the numerical result
of the binding energy B versus the coupling constant \alpha\ for various mass
ratios of the constituent particles including the cases of non-zero exchange
particle mass. We also discuss the correlation between the mass spectrum and
the corresponding bound-state wavefunction.Comment: 21 pages, 13 figures. Submitted to Physical Review
Flavor-twisted boundary condition for simulations of quantum many-body systems
We present an approximative simulation method for quantum many-body systems
based on coarse graining the space of the momentum transferred between
interacting particles, which leads to effective Hamiltonians of reduced size
with the flavor-twisted boundary condition. A rapid, accurate, and fast
convergent computation of the ground-state energy is demonstrated on the
spin-1/2 quantum antiferromagnet of any dimension by employing only two sites.
The method is expected to be useful for future simulations and quick estimates
on other strongly correlated systems.Comment: 6 pages, 2 figure
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