225 research outputs found
Lattice Pseudospin Model for Quantum Hall Bilayers
We present a new theoretical approach to the study of quantum Hall
bilayer that is based on a systematic mapping of the microscopic Hamiltonian to
an anisotropic SU(4) spin model on a lattice. To study the properties of this
model we generalize the Heisenberg model Schwinger boson mean field theory
(SBMFT) of Arovas and Auerbach to spin models with anisotropy. We calculate the
temperature dependence of experimentally observable quantities, including the
spin magnetization, and the differential interlayer capacitance. Our theory
represents a substantial improvement over the conventional Hartree-Fock picture
which neglects quantum and thermal fluctuations, and has advantages over
long-wavelength effective models that fail to capture important microscopic
physics at all realistic layer separations. The formalism we develop can be
generalized to treat quantum Hall bilayers at filling factor .Comment: 26 pages, 10 figures. The final version, to appear in PR
Magnetic order and paramagnetic phases in the quantum J1-J2-J3 honeycomb model
Recent work shows that a quantum spin liquid can arise in realistic fermionic
models on a honeycomb lattice. We study the quantum spin-1/2 Heisenberg
honeycomb model, considering couplings J1, J2, and J3 up to third nearest
neighbors. We use an unbiased pseudofermion functional renormalization group
method to compute the magnetic susceptibility and determine the ordered and
disordered states of the model. Aside from antiferromagnetic, collinear, and
spiral order domains, we find a large paramagnetic region at intermediate J2
coupling. For larger J2 within this domain, we find a strong tendency to
staggered dimer ordering, while the remaining paramagnetic regime for low J2
shows only weak plaquet and staggered dimer response. We suggest this regime to
be a promising region to look for quantum spin liquid states when charge
fluctuations would be included.Comment: 4 pages, 3 figure
Magnetic susceptibility of a CuO2 plane in the La2CuO4 system: I. RPA treatment of the Dzyaloshinskii-Moriya Interactions
Motivated by recent experiments on undoped La2CuO4, which found pronounced
temperature-dependent anisotropies in the low-field magnetic susceptibility, we
have investigated a two-dimensional square lattice of S=1/2 spins that interact
via Heisenberg exchange plus the symmetric and anti-symmetric
Dzyaloshinskii-Moriya anisotropies. We describe the transition to a state with
long-ranged order, and find the spin-wave excitations, with a mean-field
theory, linear spin-wave analysis, and using Tyablikov's RPA decoupling scheme.
We find the different components of the susceptibility within all of these
approximations, both below and above the N'eel temperature, and obtain evidence
of strong quantum fluctuations and spin-wave interactions in a broad
temperature region near the transition.Comment: 20 pages, 2 column format, 22 figure
Quantum Spin Liquids
Quantum spin liquids may be considered "quantum disordered" ground states of
spin systems, in which zero point fluctuations are so strong that they prevent
conventional magnetic long range order. More interestingly, quantum spin
liquids are prototypical examples of ground states with massive many-body
entanglement, of a degree sufficient to render these states distinct phases of
matter. Their highly entangled nature imbues quantum spin liquids with unique
physical aspects, such as non-local excitations, topological properties, and
more. In this review, we discuss the nature of such phases and their properties
based on paradigmatic models and general arguments, and introduce theoretical
technology such as gauge theory and partons that are conveniently used in the
study of quantum spin liquids. An overview is given of the different types of
quantum spin liquids and the models and theories used to describe them. We also
provide a guide to the current status of experiments to study quantum spin
liquids, and to the diverse probes used therein.Comment: 60 pages, 8 figures, 1 tabl
Semi-classical Theories of Quantum Magnets
Recent progress in magnetism has been driven by embracing the complexity associated with entangled spin, orbital, and lattice degrees of freedom and by understanding the emergent quantum behaviors of magnetic systems. Over the past decades, intense efforts have been devoted to “extreme quantum materials” comprising low-dimensional lattices of spin S = 1/2 degrees of freedom, that are candidates to host quantum spin liquid phases with no classical counterpart. Finite-spin (S ≥ 1) systems that exhibit ground states with short-ranged entanglement have not been the center of much attention because they are expected to behave semi-classically. However, as we will demonstrate in this dissertation, the traditional classical limit (1/S expansion) does not work for large classes of finite-spin systems, which still admit an accurate classical or semi-classical treatment. To address this important problem, we will exploit the fact that N -level systems admit more than one classical limit. As we will demonstrate in this dissertation, different classical limits lead to different generalizations of the so-called Landau-Lifshitz dynamics. In particular, we will introduce generalized classical spin dynamics based on coherent states of SU(N ), where N is the dimension of the local Hilbert space. This new approach also allows generalizing the semi-classical spin dynamics (1/S-expansion) from SU(2) to SU(N ), providing a better approximation to incorporate quantum effects in the spin dynamics of large classes of realistic spin Hamiltonians, including S ≥ 1 systems with large single-ion anisotropy and weakly coupled multi-spin units, such as dimers, trimers or tetramers. Besides developing the mathematical formalism, we illustrate these ideas by comparing our theoretical predictions against inelastic neutron scattering data of two realistic effective S = 1 iron-based compounds. In the last part of this dissertation, we generalize the concepts of the magnetic skyrmions by taking alternative classical limits of quantum spin systems. In particular, we report the emergence of magnetic CP2 skyrmions in realistic spin-1 models based on SU(3) coherent states
Spin liquid nature in the Heisenberg - triangular antiferromagnet
We investigate the spin- Heisenberg model on the triangular
lattice in the presence of nearest-neighbor and next-nearest-neighbor
antiferromagnetic couplings. Motivated by recent findings from
density-matrix renormalization group (DMRG) claiming the existence of a gapped
spin liquid with signatures of spontaneously broken lattice point group
symmetry [Zhu and White, Phys. Rev. B 92, 041105 (2015); Hu, Gong, Zhu, and
Sheng, Phys. Rev. B 92, 140403 (2015)], we employ the variational Monte Carlo
(VMC) approach to analyze the model from an alternative perspective that
considers both magnetically ordered and paramagnetic trial states. We find a
quantum paramagnet in the regime , framed by
coplanar (stripe collinear) antiferromagnetic order for smaller
(larger) . By considering the optimization of spin-liquid wave
functions of a different gauge group and lattice point group content as derived
from Abrikosov mean-field theory, we obtain the gapless Dirac spin
liquid as the energetically most preferable state in comparison to all
symmetric or nematic gapped spin liquids so far advocated by
DMRG. Moreover, by the application of few Lanczos iterations, we find the
energy to be the same as the DMRG result within error-bars. To further resolve
the intriguing disagreement between VMC and DMRG, we complement our
methodological approach by the pseudofermion functional renormalization group
(PFFRG) to compare the spin structure factors for the paramagnetic regime
calculated by VMC, DMRG, and PFFRG. This model promises to be an ideal test-bed
for future numerical refinements in tracking the long-range correlations in
frustrated magnets.Comment: Editors' Suggestion. 16 pages, 13 figures, 4 table
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