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Unification of Bosonic and Fermionic Theories of Spin Liquids on The Unification of bosonic and fermionic theories of spin liquids on the kagome lattice Yuan-Ming Lu,1, 2, 3 Gil Young Cho,1, 4 and Ashvin Vishwanath1, 2 1Department of Physics, University of California, Berkeley, CA 94720 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 3Department of Physics, The Ohio State University, Columbus, OH 43210, USA 4Department of Physics, Institute for Condensed Matter Theory, University of Illinois at Urbana-Champaign, IL 61801 (Dated: January 1, 2018) Recent numerical studies have provided strong evidence for a gapped Z2 quantum spin liquid in the kagome lattice spin-1/2 Heisenberg model. A special feature of spin liquids is that symmetries can be fractionalized, and different patterns of symmetry fractionalization imply distinct phases. The symmetry fractionalization pattern for the kagome spin liquid remains to be determined. A popular approach to studying spin liquids is to decompose the physical spin into partons obeying either bose (Schwinger bosons) or fermi (Abrikosov fermions) statstics, which are then treated within the mean-field theory. A longstanding question has been whether these two approaches are truly distinct, or describe the same phase in complementary ways. Here we show that all 8 Z2 spin liquid phases in Schwinger-boson mean-field (SBMF) construction can also be described in terms of Abrikosov fermions, unifying pairs of theories that seem rather distinct. The key idea is that for Z2 spin liquid states that admit a SBMF description on kagome lattice, the symmetry fractionalization of visions is uniquely fixed. Two promising candidate states for kagome Heisenberg model, Sachdev's Q1 = Q2 SBMF state and Lu-Ran-Lee's Z2[0; π]β Abrikosov fermion state, are found to describe the same symmetric spin liquid phase. We expect these results to aid in a complete specification of the numerically observed spin liquid phase. We also discuss a set of Z2 spin liquid phases in fermionic parton approach, where spin rotation and lattice symmetries protect gapless edge states, that do not admit a SBMF description. PACS numbers: I. INTRODUCTION glement entropy serve as fingerprints of the topological 16 order in Z2 spin liquids. Analogous results have been reported for the frustrated square lattice, although the Z2 spin liquids (SLs) are a class of disordered many- spin states which have a finite energy gap for all bulk correlation lengths in that case are not as small as in the 17{21 excitations. They differ fundamentally from symme- kagome lattice . try breaking ground states such as magnetically ordered Intriguingly, the experimentally studied spin-1=2 phases and valence bond solids, since, in their simplest kagome materials - such as herbertsmithite22 - also re- form, they preserve all the symmetries including spin ro- main quantum disordered down to the lowest temper- tation, time reversal and crystal symmetries. More im- ature scales studied, well below the exchange energy portantly they possess bulk quasiparticles obeying frac- scales. However, in contrast to the numerical studies, 1 tional statistics . For example in the most common Z2 most experimental evidences point to a gapless ground SL of a spin-1/2 system, there are three distinct types of state23,24. It is currently still under debate if the gapless- fractionalized bulk excitations2{7: bosonic spinon b with ness is an intrinsic feature25 or a consequence of impu- half-integer spin, fermionic spinon f with half-integer rities that are known to be present in these materials26. spin, and bosonic vison v (a vortex excitation of Z2 gauge Furthermore the magnetic Hamiltonian of the material theory) with integer spin. They all obey mutual semion may depart from the pure Heisenberg limit. Relating the statistics7: i.e. a bosonic spinon acquires a −1 Berry numerical results to experiments remains an important arXiv:1403.0575v3 [cond-mat.str-el] 29 Dec 2017 phase when it adiabatically encircles a fermionic spinon open question. or a vison. These statistical properties are identical to Since it preserves all symmetries of the system, is a Z2 8 those of excitations in Z2 gauge theory , hence the name SL fully characterized by its topological order? The an- \Z2 spin liquid". swer is no. In fact, the interplay of symmetry and topo- Recently, interest in Z2 SLs has been recharged by logical order leads to a very rich structure. There are numerical studies on the spin-1=2 Heisenberg model on many different Z2 spin liquids with the same Z2 topo- kagome9{12 lattice, where this state is strongly indicated. logical order and the same symmetry group, but they In particular a topological entanglement entropy13,14 of cannot be continuously connected to each other with- γ = log 2 is observed in the ground state. Just like lo- out breaking the symmetry: they are dubbed \symme- cal order parameters used to describe symmetry breaking try enriched topological (SET)" phases27{35. In a SET phases15, here fractional statistics and topological entan- phase the quasiparticles not only have fractional statis- 2 two distinct Z2 SLs in f SR can have the same PSG for tics, but can also carry projective representation of the fermionic spinons while only one of them has symmetry- symmetry group. This phenomena is dubbed \symmetry protected gapless edge modes. However, they differ in fractionalization"30,34,35, a well-known example being the the topology of spinon band structures, which provides fractional charge carried by the quasiholes (or quasielec- an interesting link between symmetry implementation trons) in the fractional quantum Hall effect36. Differ- and topological edge states. By arguing the absence of ent SET phases are characterized by different patterns of symmetry-protected gapless edge states in any SBMF symmetry fractionalization, mathematically classified by state, we show that all Z2 spin liquids in SBMF con- 30,34,35 2 the 2nd group cohomology H (Gs; A), where Gs is struction must have a trivial PSG (or symmetry fraction- the symmetry group of the system and A is the (fusion) alization pattern) for vison v. The knowledge of bosonic group of Abelian anyons in the topological order. In the spinon PSG and vison PSG in a SBMF state leads to its case of Z2 spin liquid, A = Z2 ×Z2 according the Abelian fermionic spinon PSG, with the help of proper twist fac- fusion rules summarized in (8). tors, therefore establishing the correspondence between a In the literature Z2 SLs have been constructed SBMF state and an Abrikosov-fermion Z2 SL. Applying in various slave-particle (or parton) frameworks: the these general principles to Z2 SLs on kagome lattice, we most predominant two approaches fractionalize physi- show that all 8 different Schwinger-boson (bSR) states cal spin-1/2's into bosonic spinons2,37{39 and fermionic have their partners in the Abrikosov-fermion (f SR) rep- 3,5,27,40{43 38 spinons respectively. Both approaches yield resentation. In particular Q1 = Q2 state in Schwinger variational wavefunctions with good energetics44{46 for boson representation belongs to the same SET phase 48 the kagome lattice model. It was proposed that sym- as Z2[0; π]β state in Abrikosov fermion representation. metric Z2 SLs are classified by the projective symmetry This correspondence allows us to identify the possible 27 groups (PSGs) of bosonic/fermionic spinons. However symmetry-breaking phases in proximity to Z2 SLs on it has been a long-time puzzle to understand the relation kagome lattice. In fact all 8 SBMF states have their between different PSGs in bosonic-spinon representation Abrikosov fermion counterparts, as summarized in Table (bSR) and fermionic-spinon representations (f SR)47. To II). Part of these correspondences (for 4 SBMF states be specific in the kagome lattice Heisenberg model, in with p2 + p3 = 1 in TABLEII) has been obtained previ- bSR (Schwinger-boson approach) there are 8 different Z2 ously in Ref. 51, by explicitly identifying their projected 39 38 SLs among which the so-called Q1 = Q2 state is con- wavefunctions. These results serve as a useful guide in sidered a promising candidate according to variational future studies of Z2 SLs. 44 48 calculations . Meanwhile there are 20 distinct Z2 SLs This article is organized as follows. After a brief re- in f SR (Abrikosov-fermion approach), including the so- view on symmetry fractionalization, PSG and their re- 48 called Z2[0; π]β state which is in the neighborhood of lations in Z2 SLs in section IIA-II B, we first establish energetically favorable U(1) Dirac SL45. Are these two the twist factors between PSGs of different anyons in candidate states actually two different descriptions of the a Z2 spin liquid in section IIC. In section III we show same gapped phase? If not, what are their counterparts that absence of protected edge states and defect bound in the other representation? states in a Z2 spin liquid can determine the vison PSG In this paper we establish the general connection be- (see TABLEI). These results allow us to compute the vison and fermion PSG in any SBMF state, which estab- tween different Z2 SLs in bSR and f SR. We show that Z2 SLs constructed by projecting parton mean-field states lishes to the correspondence between Schwinger-boson in bSR (Schwinger-boson representation) cannot host and Abrikosov-fermion mean-field states of Z2 spin liq- symmetry-protected gapless edge states. This impor- uids, as studied in sectionIV and summarized in TABLE tant observation allows us to determine how visons trans- II. In sectionV we analyze and argue the most promising Z spin liquid candidate for spin-1/2 kagome Heisenberg form under symmetry in Schwinger-boson Z2 SLs, and to 2 further relate a Schwinger-boson state to an Abrikosov- model, i.e.
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