92 research outputs found

    Spiral order in the honeycomb iridate Li2IrO3

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    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

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    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

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    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

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    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 Na2_2IrO3_3: Topological insulator, zigzag antiferromagnet, and Kitaev-Heisenberg material

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    Na2_2IrO3_3 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

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    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

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    Motivated by the recent synthesis of the spin-1 A-site spinel NiRh2_{\text 2}O4_{\text 4}, we investigate the classical to quantum crossover of a frustrated J1J_1-J2J_2 Heisenberg model on the diamond lattice upon varying the spin length SS. Applying a recently developed pseudospin functional renormalization group (pf-FRG) approach for arbitrary spin-SS magnets, we find that systems with S3/2S \geq 3/2 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 SS=1 or SS=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 NiRh2_{\text 2}O4_{\text 4}, a modified J1J_1-J2J_2^--J2J_2^\perp exchange model is found to favor a conventionally ordered N\'eel state (for arbitrary spin SS) 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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