92 research outputs found
Spiral order in the honeycomb iridate Li2IrO3
The honeycomb iridates A2IrO3 (A=Na, Li) constitute promising candidate
materials to realize the Heisenberg-Kitaev model (HKM) in nature, hosting
unconventional magnetic as well as spin liquid phases. Recent experiments
suggest, however, that Li2IrO3 exhibits a magnetically ordered state of
incommensurate spiral type which has not been identified in the HKM. We show
that these findings can be understood in the context of an extended
Heisenberg-Kitaev scenario satisfying all tentative experimental evidence: (i)
the maximum of the magnetic susceptibility is located inside the first
Brillouin zone, (ii) the Curie-Weiss temperature is negative relating to
dominant antiferromagnetic fluctuations, and (iii) significant second-neighbor
spin-exchange is involved.Comment: 5 pages, 5 figures, selected as an Editors' suggestio
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
Finite-temperature phase diagram of the Heisenberg-Kitaev model
We discuss the finite-temperature phase diagram of the Heisenberg-Kitaev
model on the hexagonal lattice, which has been suggested to describe the
spin-orbital exchange of the effective spin-1/2 momenta in the Mott insulating
Iridate Na2IrO3. At zero-temperature this model exhibits magnetically ordered
states well beyond the isotropic Heisenberg limit as well as an extended
gapless spin liquid phase around the highly anisotropic Kitaev limit. Using a
pseudofermion functional renormalization group (RG) approach, we extract both
the Curie-Weiss scale and the critical ordering scale (for the magnetically
ordered states) from the RG flow of the magnetic susceptibility. The
Curie-Weiss scale switches sign -- indicating a transition of the dominant
exchange from antiferromagnetic to ferromagnetic -- deep in the magnetically
ordered regime. For the latter we find no significant frustration, i.e. a
substantial suppression of the ordering scale with regard to the Curie-Weiss
scale. We discuss our results in light of recent experimental susceptibility
measurements for Na2IrO3.Comment: 4+e pages, 5 figure
Renormalization group analysis of competing quantum phases in the J1-J2 Heisenberg model on the kagome lattice
Recent discoveries in neutron scattering experiments for Kapellasite and
Herbertsmithite as well as theoretical calculations of possible spin liquid
phases have revived interest in magnetic phenomena on the kagome lattice. We
study the quantum phase diagram of the S=1/2 Heisenberg kagome model as a
function of nearest neighbor coupling J1 and second neighbor coupling J2.
Employing the pseudofermion functional renormalization group, we find four
types of magnetic quantum order (q=0 order, cuboc order, ferromagnetic order,
and Sqrt{3}x\Sqrt{3} order) as well as extended magnetically disordered regions
by which we specify the possible parameter regime for Kapellasite. In the
disordered regime J2/J1<<1, the flatness of the magnetic susceptibility at the
zone boundary which is observed for Herbertsmithite can be reconciled with the
presence of small J2>0 coupling. In particular, we analyze the dimer
susceptibilities related to different valence bond crystal (VBC) patterns,
which are strongly inhomogeneous indicating the rejection of VBC order in the
RG flow.Comment: 4+e pages, 3 figures; 2 pages of supplementary materia
Three-band Hubbard model for NaIrO: Topological insulator, zigzag antiferromagnet, and Kitaev-Heisenberg material
NaIrO was one of the first materials proposed to feature the
Kane-Mele type topological insulator phase. Contemporaneously it was claimed
that the very same material is in a Mott insulating phase which is described by
the Kitaev-Heisenberg (KH) model. First experiments indeed revealed Mott
insulating behavior in conjunction with antiferromagnetic long-range order.
Further refined experiments established antiferromagnetic order of zigzag type
which is not captured by the KH model. Since then several extensions and
modifications of the KH model were proposed in order to describe the
experimental findings. Here we suggest that adding charge fluctuations to the
KH model represents an alternative explanation of zigzag antiferromagnetism.
Moreover, a phenomenological three-band Hubbard model unifies all the pieces of
the puzzle: topological insulator physics for weak and KH model for strong
electron-electron interactions as well as a zigzag antiferromagnet at
intermediate interaction strength.Comment: 5 pages, 3 figures; v2 (as published): added discussion about kinetic
energy scale C; more realistic values of C shift the zigzag AFM phase to
larger values of
Topological spinon bands and vison excitations in spin-orbit coupled quantum spin liquids
Spin liquids are exotic quantum states characterized by the existence of
fractional and deconfined quasiparticle excitations, referred to as spinons
and visons. Their fractional nature establishes topological properties such as
a protected ground-state degeneracy. This work investigates spin-orbit coupled
spin liquids where, additionally, topology enters via nontrivial band
structures of the spinons. We revisit the Z2 spin-liquid phases that have
recently been identified in a projective symmetry-group analysis on the square
lattice when spin-rotation symmetry is maximally lifted [J. Reuther et al.,
Phys. Rev. B 90, 174417 (2014)]. We find that in the case of nearest-neighbor
couplings only, Z2 spin liquids on the square lattice always exhibit trivial
spinon bands. Adding second-neighbor terms, the simplest projective symmetry-
group solution closely resembles the Bernevig-Hughes-Zhang model for
topological insulators. Assuming that the emergent gauge fields are static, we
investigate vison excitations, which we confirm to be deconfined in all
investigated spin phases. Particularly, if the spinon bands are topological,
the spinons and visons form bound states consisting of several spinon-Majorana
zero modes coupling to one vison. The existence of such zero modes follows
from an exact mapping between these spin phases and topological p+ip
superconductors with vortices. We propose experimental probes to detect such
states in real materials
Quantum spin liquids in frustrated spin-1 diamond antiferromagnets
Motivated by the recent synthesis of the spin-1 A-site spinel NiRhO, we investigate the classical to quantum crossover of a
frustrated - Heisenberg model on the diamond lattice upon varying the
spin length . Applying a recently developed pseudospin functional
renormalization group (pf-FRG) approach for arbitrary spin- magnets, we find
that systems with reside in the classical regime where the
low-temperature physics is dominated by the formation of coplanar spirals and a
thermal (order-by-disorder) transition. For smaller local moments =1 or
=1/2 we find that the system evades a thermal ordering transition and forms
a quantum spiral spin liquid where the fluctuations are restricted to
characteristic momentum-space surfaces. For the tetragonal phase of
NiRhO, a modified -- exchange
model is found to favor a conventionally ordered N\'eel state (for arbitrary
spin ) even in the presence of a strong local single-ion spin anisotropy and
it requires additional sources of frustration to explain the experimentally
observed absence of a thermal ordering transition.Comment: 11 pages, 14 figure
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