5 research outputs found
Anomalous Excitation Spectra of Frustrated Quantum Antiferromagnets
We use series expansions to study the excitation spectra of spin-1/2
antiferromagnets on anisotropic triangular lattices. For the isotropic
triangular lattice model (TLM) the high-energy spectra show several anomalous
features that differ strongly from linear spin-wave theory (LSWT). Even in the
Neel phase, the deviations from LSWT increase sharply with frustration, leading
to roton-like minima at special wavevectors. We argue that these results can be
interpreted naturally in a spinon language, and provide an explanation for the
previously observed anomalous finite-temperature properties of the TLM. In the
coupled-chains limit, quantum renormalizations strongly enhance the
one-dimensionality of the spectra, in agreement with experiments on Cs_2CuCl_4.Comment: 4 pages, 5 figures. New Fig. 3 with higher-order series data, paper
shortened, references updated, one added (Ref. 28), minor changes otherwise.
Published versio
Excitation spectra and ground state properties of the layered spin-1/2 frustrated antiferromagnets Cs_2CuCl_4 and Cs_2CuBr_4
We use series expansion methods to study ground- and excited-state properties
in the helically ordered phase of spin-1/2 frustrated antiferromagnets on an
anisotropic triangular lattice. We calculate the ground state energy, ordering
wavevector, sublattice magnetization and one-magnon excitation spectrum for
parameters relevant to Cs_2CuCl_4 and Cs_2CuBr_4. Both materials are modeled in
terms of a Heisenberg model with spatially anisotropic exchange constants; for
Cs_2CuCl_4 we also take into account the additional Dzyaloshinskii-Moriya (DM)
interaction. We compare our results for Cs_2CuCl_4 with unpolarized neutron
scattering experiments and find good agreement. In particular, the large
quantum renormalizations of the one-magnon dispersion are well accounted for in
our analysis, and inclusion of the DM interaction brings the theoretical
predictions for the ordering wavevector and the magnon dispersion closer to the
experimental results.Comment: 10 pages, 8 figure
Excitation spectra of the spin-1/2 triangular-lattice Heisenberg antiferromagnet
We use series expansion methods to calculate the dispersion relation of the
one-magnon excitations for the spin-1/2 triangular-lattice nearest-neighbor
Heisenberg antiferromagnet above a three-sublattice ordered ground state.
Several striking features are observed compared to the classical (large-S)
spin-wave spectra. Whereas at low energies the dispersion is only weakly
renormalized by quantum fluctuations, significant anomalies are observed at
high energies. In particular, we find roton-like minima at special wave-vectors
and strong downward renormalization in large parts of the Brillouin zone,
leading to very flat or dispersionless modes. We present detailed comparison of
our calculated excitation energies in the Brillouin zone with the spin-wave
dispersion to order 1/S calculated recently by Starykh, Chubukov, and Abanov
[cond-mat/0608002]. We find many common features but also some quantitative and
qualitative differences. We show that at temperatures as low as 0.1J the
thermally excited rotons make a significant contribution to the entropy.
Consequently, unlike for the square lattice model, a non-linear sigma model
description of the finite-temperature properties is only applicable at
extremely low temperatures.Comment: 14 pages, 11 figure
Ground-state fidelity of Luttinger liquids: A wave functional approach
We use a wave functional approach to calculate the fidelity of ground states
in the Luttinger liquid universality class of one-dimensional gapless quantum
many-body systems. The ground-state wave functionals are discussed using both
the Schrodinger (functional differential equation) formulation and a path
integral formulation. The fidelity between Luttinger liquids with Luttinger
parameters K and K' is found to decay exponentially with system size, and to
obey the symmetry F(K,K')=F(1/K,1/K') as a consequence of a duality in the
bosonization description of Luttinger liquids.Comment: 13 pages, IOP single-column format. Sec. 3 expanded with discussion
of short-distance cut-off. Some typos corrected. Ref. 44 in v2 is now
footnote 2 (moved by copy editor). Published versio
Finite-size geometric entanglement from tensor network algorithms
The global geometric entanglement (GE) is studied in the context of newly developed tensor network algorithms for finite systems. For onedimensional quantum spin systems it is found that, at criticality, the leading finite-size correction to the global GE per site behaves as b/n, where n is the size of the system and b a given coefficient. Our conclusion is based on the computation of the GE per spin for the quantum Ising model in a transverse magnetic field and for the spin-1/2 XXZ model. We also discuss the possibility of coefficient b being universal