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Coherent Transmutation of Electrons Into Fractionalized Anyons

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Coherent Transmutation of Electrons Into Fractionalized Anyons Coherent Transmutation of Electrons into Fractionalized Anyons Maissam Barkeshli,1 Erez Berg,2 and Steven Kivelson3 1Station Q, Microsoft Research, Santa Barbara, California 93106, USA 2Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, 76100, Israel 3Department of Physics, Stanford University, Stanford, California 94305, USA Electrons have three quantized properties – charge, spin, and Fermi statistics – that are directly responsible for a vast array of phenomena. Here we show how these properties can be coherently and dynamically stripped from the electron as it enters certain exotic states of matter known as a quantum spin liquid (QSL). In a QSL, electron spins collectively form a highly entangled quantum state that gives rise to emergent gauge forces and fractionalization of spin, charge, and statistics. We show that certain QSLs host distinct, topologically robust boundary types, some of which allow the electron to coherently enter the QSL as a fractionalized quasiparticle, leaving its spin, charge, or statistics behind. We use these ideas to propose a number of universal, conclusive experimental signatures that would establish fractionalization in QSLs. Introduction – A notable example of emergence in experiments in ZnCu3(OH)6Cl2 (Herbertsmithite) [21], physics is fractionalization, where the long-wavelength, and also a number of experimental observations of the or- low-energy excitations of a quantum phase of matter pos- ganic compound κ-(ET)2Cu2(CN)3 [22]. We will build on sess quantum numbers that are fractions of those of the recent results that demonstrate that gapped fractional- microscopic constituents. In a fractional quantum Hall ized phases support topologically distinct types of gapped (FQH) state for example, the emergent quasiparticle ex- boundaries [12, 15]. These results imply that the Z2 citations carry fractional electric charge and “anyonic” sRVB state necessarily supports exactly two topologi- quantum statistics. In a quantum spin liquid (QSL), the cally distinct types of gapped boundaries, referred to as electron fractionalizes at low energies into two quasipar- the e edge and the m edge [43], that are separated by a ticles – a spinon and a holon – that independently carry topological quantum phase transition. the spin and charge of the electron [1–8]. When an elec- tron is injected into such systems, it can decay into frac- The Z2 sRVB state is an insulating, spin-rotationally tionalized components, but a direct quantum mechanical invariant gapped spin liquid state. That it is a legitimate conversion of an electron to a single fractionalized quasi- state of matter has been proven by constructing model particle has conventionally been thought to be impossi- Hamiltonians with exact sRVB ground-states[6, 7, 23, ble. Consequently, the question of how to experimentally 24]. At low energies, there are four topologically distinct detect fractionalization in a QSL, even in principle, has types of elementary quasiparticle excitations: (i) topolog- remained a major challenge. It is particularly timely to ically trivial excitations, which can be created with local revisit this question, given that there are now a number operators, (ii) spinons and holons, which carry spin-1/2 of materials that have recently been shown to exhibit and charge 0, or spin 0 and charge 1, respectively, (iii) anomalous properties that may indicate that they are visons, which do not carry spin or charge and which have QSLs - for a review, see [9]. mutually semionic statistics with respect to the spinons and holons, and (iv) the composite of a spinon or holon Here we show that some QSLs allow electrons to co- with a vison. Because electrons always carry both unit herently enter through their boundary as a fractional- electric charge and spin-1/2, the spinons and holons can- ized quasiparticle, leaving behind their charge, spin, or not individually be created by any local combinations even Fermi statistics. We show that this leads to univer- of electron operators, and therefore must be topological sal experimental signatures that could provide incontro- excitations. That spinons and holons are topologically arXiv:1402.6321v2 [cond-mat.str-el] 11 Nov 2014 vertible evidence of fractionalization. Our considerations equivalent follows from the fact that one can be con- are based on recent theoretical breakthroughs regarding verted into the other by the local operation of adding or the physics of two-dimensional (2D) topologically ordered removing an electron. Moreover, the spinons and holons states associated with extrinsic line and point defects [10– can be either bosonic or fermionic, depending on detailed 19], of which robust boundary phenomena are a special energetics. case. We focus primarily on explaining these phenomena in The Z2 sRVB state can be understood at low energies the context of the simplest gapped 2D QSL, the Z2 short- in terms of a Z2 lattice gauge theory. In this language, ranged resonating valence bond state (sRVB) state, and the spinons and holons carry the Z2 gauge charge, while explaining how it can be distinguished from nonfraction- the visons are the Z2 fluxes. As a result, the spinons alized magnetic insulators, or even from QSLs with dif- and holons are sometimes referred to as the e particles ferent types of fractionalization. This type of QSL has and the visons as the m particles. This state can also been proposed[20] to explain recent neutron scattering be described at long wavelengths using Abelian Chern- 2 Simons (CS) field theory [25, 26]: Edge physics Edge type Spin/charge conserved m 1 IJ I J Heisenberg exchange to collinear SDW m L = K ǫµνλaµ∂ν aλ + Lmatter, (1) 4π Heisenberg exchange J > Jc to N.C. SDW e Magnetic field B > Bc at edge of XXZ system e where µ,ν,λ =0, 1, 2 are 2+1D space-time indices; ǫµνλ c 0 2 Pair tunneling tpair >tpair to superconductor e is the Levi-Civita tensor, K = ; I,J = 1, 2; and 2 0 TABLE I: Summary of conditions under which an e or m Lmatter describes the fractionalized quasiparticles, which edge can be realized in the Z2 sRVB. Aside from the XXZ I are minimally coupled to the U(1) gauge fields a . The case with an applied magnetic field B, the QSL is assumed to visons carry unit charge under a1, while the spinons and be SO(3) spin rotationally invariant. The first two cases can, holons both carry unit charge under a2. The CS term in principle, also be gapless, instead of realizing the gapped c binds charges to fluxes in such a way as to properly cap- m edge. Jc, Bc and tpair are the critical Heisenberg exchange, ture the nontrivial mutual statistics between spinons or magnetic field, and pair-tunneling strength needed to realize the e edge, respectively. holons and visons. Clever numerical studies of spin 1/2 frustrated Heisen- berg models[27–30] provided evidence for gapped spin- to a local operator on the edge. There are thus two ba- liquid ground states. However, it is not yet clear whether sic types of local terms, δLZ2 = λm cos(2φ)+ λe cos(2θ), the ground state in those models is a Z2 sRVB state or with coupling constants λe,m, that effectively backscatter a certain competing QSL, the “doubled semion” state, counterpropagating modes and can induce an energy gap 2 0 which is characterized instead by K = . on the edge. [44] 0 −2 Because φ and θ are conjugate, the cosine terms can- Each topologically ordered phase, characterized by a not simultaneously pin their arguments, so there are two matrix K, can support topologically distinct types of distinct phases. Where |λ | is the dominant coupling, gapped boundaries. A classification of such gapped edges m heiφi 6= 0 and heiθi = 0, implying that the m-particles [12, 15], when applied to the Z sRVB state, predicts ex- 2 are condensed on the edge. Conversely, if |λ | is domi- actly two topologically distinct types of gapped edges. e nant, the e particles are condensed on the edge: heiθi 6=0 As we explain below, these correspond to whether the e and heiφi = 0. The two phases, referred to as the m edge or the m particles are condensed along the boundaries. and e edge, respectively, are topologically distinct. In the The doubled semion state, in contrast, possesses only one absence of any additional global symmetries, there is a type of gapped boundary (see Supplemental materials). single quantum critical point between these two gapped The edge theory can be derived by starting with the phases in the Ising universality class. [See the Supple- Abelian CS theory (1). [For an alternative explanation, mental Materials for additional discussion.] see Supplemental Materials.] It is well-known [26] that In the presence of spin rotation and charge conserva- on a manifold with a boundary, Eq. (1) is only gauge- tion symmetries, the modes φ and θ must represent either invariant if the gauge transformations are restricted to be low-energy spin or charge fluctuations (but not both), zero on the boundary. This implies that on the boundary, depending on the physical situation. If they describe the gauge fields correspond directly to physical degrees charge fluctuations, then the charge density is given by of freedom. One can derive an edge Lagrangian [26] in ρ = 1 ∂ φ, and h = eiθ+inφ creates a holon which is terms of scalar fields φ : c π x I bosonic for even integer n and fermonic for odd n. The 2iθ 1 operator e is a topologically trivial excitation that L = KIJ ∂xφI ∂tφJ − VIJ ∂xφI ∂xφJ , (2) carries charge 2 and no spin and is therefore physically 4π equivalent to a Cooper pair. Alternatively, if the boson where VIJ is a positive-definite velocity matrix. The modes describe spin fluctuations, then the spin density z 1 ± ±i2θ ±iθ+inφ number of left (or right) movers is given by the num- is S = 2π ∂xφ, and S = e .
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