4,765 research outputs found
Low-energy singlet and triplet excitations in the spin-liquid phase of the two-dimensional J1-J2 model
We analyze the stability of the spontaneously dimerized spin-liquid phase of
the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet
excitation, corresponding to breaking of a singlet bond, is found to be stable
in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy
collective singlet mode, which is closely related to the spontaneous violation
of the discrete symmetry. Both modes are gapped in the quantum disordered phase
and become gapless at the transition point to the Neel ordered phase
(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and we
argue that the disappearance of dimer order is related to the vanishing of the
singlet gap. We also present exact diagonalization data on a small (4x4)
cluster which indeed show a structure of the spectrum, consistent with that of
a system with a four-fold degenerate (spontaneously dimerized) ground state.Comment: 4 pages, 4 figures, small changes, published versio
Single hole dynamics in dimerized spin liquids
The dynamics of a single hole in quantum antiferromagnets is influenced by
magnetic fluctuations. In the present work we consider two situations. The
first one corresponds to a single hole in the two leg t-J spin ladder. In this
case the wave function renormalization is relatively small and the
quasiparticle residue of the S=1/2 state remains close to unity. However at
large t/J there are higher spin (S=3/2,5/2,..) bound states of the hole with
the magnetic excitations, and therefore there is a crossover from
quasiparticles with S=1/2 to quasiparticles with higher spin.
The second situation corresponds to a single hole in two coupled
antiferromagnetic planes very close to the point of antiferromagnetic
instability. In this case the hole wave function renormalization is very strong
and the quasiparticle residue vanishes at the point of instability.Comment: 12 pages, 3 figure
Bound states of magnons in the S=1/2 quantum spin ladder
We study the excitation spectrum of the two-leg antiferromagnetic S=1/2
Heisenberg ladder. Our approach is based on the description of the excitations
as triplets above a strong-coupling singlet ground state. The quasiparticle
spectrum is calculated by treating the excitations as a dilute Bose gas with
infinite on-site repulsion. We find singlet (S=0) and triplet (S=1)
two-particle bound states of the elementary triplets. We argue that bound
states generally exist in any dimerized quantum spin model.Comment: 4 REVTeX pages, 4 Postscript figure
Low-lying excitations and magnetization process of coupled tetrahedral systems
We investigate low-lying singlet and triplet excitations and the
magnetization process of quasi-1D spin systems composed of tetrahedral spin
clusters. For a class of such models, we found various exact low-lying
excitations; some of them are responsible for the first-order transition
between two different ground states formed by local singlets. Moreover, we find
that there are two different kinds of magnetization plateaus which are
separated by a first-order transition.Comment: To appear in Phys.Rev.B (Issue 01 August 2002). A short comment is
adde
Spin 1/2 Magnetic Impurity in a 2D Magnetic System Close to Quantum Critical Point
We consider a magnetic impurity in a spin liquid state of a magnetic system
which is close to the quantum phase transition to the magnetically ordered
state. There is similarity between this problem and the Kondo problem. We
derive the impurity Green's function, consider renormalizations of the magnetic
moments of the impurity, calculate critical indexes for the magnetic
susceptibilities and finally consider specific heat and magnetic interaction of
two impurities.Comment: 9 pages, 9 figure
New quantum phase transitions in the two-dimensional J1-J2 model
We analyze the phase diagram of the frustrated Heisenberg antiferromagnet,
the J1-J2 model, in two dimensions. Two quantum phase transitions in the model
are already known: the second order transition from the Neel state to the spin
liquid state at (J_2/J_1)_{c2}=0.38, and the first order transition from the
spin liquid state to the collinear state at (J_2/J_1)_{c4}=0.60. We have found
evidence for two new second order phase transitions: the transition from the
spin columnar dimerized state to the state with plaquette type modulation at
(J_2/J_1)_{c3}=0.50(2), and the transition from the simple Neel state to the
Neel state with spin columnar dimerization at (J_2/J_1)_{c1}=0.34(4). We also
present an independent calculation of (J_2/J_1)_{c2}=0.38 using a new approach.Comment: 3 pages, 5 figures; added referenc
Magnetic Impurity in the two-dimensional Heisenberg Antiferromagnet
We analyze the ground state properties of the two-dimensional quantum
antiferromagnet with a S=1/2 Kondo impurity. Perturbation theory around the
strong Kondo coupling limit is developed and the results compared with studies,
based on exact diagonalization of small clusters. We find that at intermediate
coupling the impurity is partially screened and the magnetization locally
suppressed. A local singlet between the impurity and the host spin is formed
asymptotically.Comment: 12 REVTex pages, 4 Postscript figure
- …