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Fractons: New Pathway to Topological Order in 3D

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Fractons: New Pathway to Topological Order in 3D Fractons: new pathway to topological order in 3D Olga Petrova Junior Research Chair Ecole´ normale sup´erieure Spin Phenomena Interdisciplinary Center July 10, 2018 1 / 23 Fractons: new pathway to topological order in 3D Outline and motivation Non-conventional magnetic order - Regular magnets - Non-conventional (but not yet topological) magnets: examples in 1D and 3D Topological order - From 2D to 3D - 2D example: toric code Our fracton model OP, N. Regnault, Phys. Rev. B 96, 224429 (2017) Spin Phenomena Interdisciplinary Center July 10, 2018 2 / 23 Non-conventional magnetic order Regular magnets Outline 1 Non-conventional magnetic order Regular magnets Spin liquids 2 Topological order 3 Fracton topological order Spin Phenomena Interdisciplinary Center July 10, 2018 3 / 23 Non-conventional magnetic order Regular magnets Regular magnets Spontaneous symmetry breaking Ground state: lower symmetry than Hamiltonian P - Example: ferromagnet H = −J n Sn · Sn+1 Long range order Spin wave in a ferromagnet: Excitations: spin waves Magnons: quantized spin waves - Linear (AFM) or quadratic (FM) dispersion Inelastic neutron scattering: branches Magnons in FM, AFM: Spin Phenomena Interdisciplinary Center July 10, 2018 4 / 23 Non-conventional magnetic order Spin liquids Outline 1 Non-conventional magnetic order Regular magnets Spin liquids 2 Topological order 3 Fracton topological order Spin Phenomena Interdisciplinary Center July 10, 2018 5 / 23 Non-conventional magnetic order Spin liquids Non-conventional magnetic order: spin liquids Spontaneous symmetry breaking Ground state: lower symmetry than Hamiltonian Ground state does not break any symmetries Disordered but local constraints Different from a paramagnet! Excitations: spin waves Magnons: quantized spin waves Fractionalized excitations Magnon fractionalizes into multiple quasiparticles ) Momentum distributed in multiple ways ) Inelastic neutron scattering: continuum Spin Phenomena Interdisciplinary Center July 10, 2018 6 / 23 Non-conventional magnetic order Spin liquids Heisenberg antiferromagnetic chain XXZ antiferromagnetic spin 1=2 chain (∆ 1) X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 n \Magnon": flip a spin 2 upset bonds ∆ ! 1: stays like this Magnon splits into two spinons x x y y σn σn+1 + σn σn+1 give spinons dynamics Neutron scattering: continuum! Spin Phenomena Interdisciplinary Center July 10, 2018 7 / 23 Non-conventional magnetic order Spin liquids Heisenberg antiferromagnetic chain XXZ antiferromagnetic spin 1=2 chain (∆ 1) X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 n \Magnon": flip a spin 2 upset bonds ∆ ! 1: stays like this Magnon splits into two spinons x x y y σn σn+1 + σn σn+1 give spinons dynamics Neutron scattering: continuum! Spin Phenomena Interdisciplinary Center July 10, 2018 7 / 23 Non-conventional magnetic order Spin liquids Heisenberg antiferromagnetic chain XXZ antiferromagnetic spin 1=2 chain (∆ 1) X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 n \Magnon": flip a spin 2 upset bonds ∆ ! 1: stays like this Magnon splits into two spinons x x y y σn σn+1 + σn σn+1 give spinons dynamics Neutron scattering: continuum! Spin Phenomena Interdisciplinary Center July 10, 2018 7 / 23 Non-conventional magnetic order Spin liquids Heisenberg antiferromagnetic chain XXZ antiferromagnetic spin 1=2 chain (∆ 1) X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 n \Magnon": flip a spin Stone et al., PRL 91, 037205 (2003) 2 upset bonds model ∆ ! 1: stays like this Magnon splits into two spinons x x y y σn σn+1 + σn σn+1 give spinons dynamics Neutron scattering: continuum! Spin Phenomena Interdisciplinary Center July 10, 2018 7 / 23 Non-conventional magnetic order Spin liquids Heisenberg antiferromagnetic chain XXZ antiferromagnetic spin 1=2 chain (∆ 1) X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 n \Magnon": flip a spin Stone et al., PRL 91, 037205 (2003) 2 upset bonds experimental data ∆ ! 1: stays like this Magnon splits into two spinons x x y y σn σn+1 + σn σn+1 give spinons dynamics Neutron scattering: continuum! Spin Phenomena Interdisciplinary Center July 10, 2018 7 / 23 Non-conventional magnetic order Spin liquids From 1D chain to a 3D model X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 ; ∆ ! 1 n Antiferromagnet in 1D: neighbors are misaligned pyrochlore lattice Antiferromagnet on 3D pyrochlore lattice: neighbors are 4 spins on a tetrahedron Frustration : Ising spins on a triangle Each tetrahedron: two up, two down Many degenerate configurations Each is disordered, but local constraints! Pauling residual entropy (like water ice!) SPIN ICE Spin Phenomena Interdisciplinary Center July 10, 2018 8 / 23 Non-conventional magnetic order Spin liquids Fractionalization in spin ice X x x y y z z H = σn σn+1 + σn σn+1 + ∆σnσn+1 ; ∆ ! 1 n Antiferromagnet in 1D: magnon ! two spinons pyrochlore lattice Spin flip: two excited tetrahedra Fractionalizes into two quasiparticles Dipolar spin terms in H: ≈ Coulomb interaction SPIN ICE Magnetic monopoles Spin Phenomena Interdisciplinary Center July 10, 2018 9 / 23 Non-conventional magnetic order Spin liquids Spin ice: classical and quantum Classical spin ices: Ho2Ti2O7, Dy2Ti2O7 X z z Spin flips from thermal fluctuations H = Jjj σnσn+1 n Quantum spin ice (many candidate materials!) X x x y y X z z H = J? σn σn+1 + σn σn+1 + Jjj σnσn+1 n n Lifts the extensive ground state degeneracy Ground state is featureless, but not the excitations! `'Hydrogenic states of monopoles in diluted quantum spin ice", OP, R. Moessner, S. L. Sondhi PRB 92, 100401 (2015) `'Mesonic excited states of magnetic monopoles in quantum spin ice", OP, arXiv:1807.02193 Spin Phenomena Interdisciplinary Center July 10, 2018 10 / 23 Non-conventional magnetic order From 1D to 3D What changes when we go 1D ! higher dimensions? Examples we discussed: - Richer physics, but ≈ same phenomena (Frustration is not specific to 3D) Topological order: - 2D vs. 3D: qualitatively different Spin Phenomena Interdisciplinary Center July 10, 2018 11 / 23 Topological order Outline 1 Non-conventional magnetic order Regular magnets Spin liquids 2 Topological order 3 Fracton topological order Spin Phenomena Interdisciplinary Center July 10, 2018 12 / 23 Topological order Topological order: anyons Will discuss what is topological on the next slide! Exchange two particles: pick up a phase eiφ Double exchange ≈ closed loop: ei2φ 3D: can lift the loop and shrink it to a point ) ei2φ = 1 ) eiφ = ±1, bosons and fermions 2D: cannot shrink the loop ) eiφ can in principle take any value ) anyons Anyons are a sign of topological order Pointlike anyons: only in 2D Non-identical particles: mutual exchange statistics Spin Phenomena Interdisciplinary Center July 10, 2018 13 / 23 Topological order Topological order: anyons Will discuss what is topological on the next slide! Exchange two particles: pick up a phase eiφ Double exchange ≈ closed loop: ei2φ 3D: can lift the loop and shrink it to a point ) ei2φ = 1 ) eiφ = ±1, bosons and fermions 2D: cannot shrink the loop ) eiφ can in principle take any value ) anyons Anyons are a sign of topological order Pointlike anyons: only in 2D Non-identical particles: mutual exchange statistics Spin Phenomena Interdisciplinary Center July 10, 2018 13 / 23 Topological order Topological order: anyons Will discuss what is topological on the next slide! Exchange two particles: pick up a phase eiφ Double exchange ≈ closed loop: ei2φ 3D: can lift the loop and shrink it to a point ) ei2φ = 1 ) eiφ = ±1, bosons and fermions 2D: cannot shrink the loop ) eiφ can in principle take any value ) anyons Anyons are a sign of topological order Pointlike anyons: only in 2D Non-identical particles: mutual exchange statistics Spin Phenomena Interdisciplinary Center July 10, 2018 13 / 23 Topological order Topological order: anyons Will discuss what is topological on the next slide! Exchange two particles: pick up a phase eiφ Double exchange ≈ closed loop: ei2φ 3D: can lift the loop and shrink it to a point ) ei2φ = 1 ) eiφ = ±1, bosons and fermions 2D: cannot shrink the loop ) eiφ can in principle take any value ) anyons Anyons are a sign of topological order Pointlike anyons: only in 2D Non-identical particles: mutual exchange statistics Spin Phenomena Interdisciplinary Center July 10, 2018 13 / 23 Topological order Topological degeneracy What is topological? Ground state degeneracy depends on topology - Example: non-degenerate on a sphere, but 4-fold on a torus Closely related to the existence of anyon excitations Can tell what kind of order we have from topological degeneracy! Locally indistinguishable ground states Transform via global operations Stable to local noise Applications in quantum computing Spin Phenomena Interdisciplinary Center July 10, 2018 14 / 23 Topological order Example: the toric code Wen plaquette model: X.-G. Wen, PRL 90, 016803 (2003) Kitaev toric code: A. Kitaev, Ann. Phys. 303, 2 (2003) σx = X^; σz = Z^ Spins 1/2 on the sites of a square lattice X ^ ^ x z x z H = − Qxy ; Qxy = σ1 σ2 σ3 σ4 p All Q^xy commute ) exactly solvable Qxy = ±1; ground state: all Qxy = +1 Excitations Qxy = −1: spin flip ! excitation pair Act on ground state with σx or σz Qxy = −1 at the ends of string operators Two flavors of plaquettes: e and m Spin Phenomena Interdisciplinary Center July 10, 2018 15 / 23 Topological order Anyons in the toric code e and m are bosons Mutual statistics: take e around m in a loop e and m strings anticommute when they cross e and m are mutual anyons: e goes around m ) phase = −1 GS degeneracy on a torus: 4-fold - N spins; N plaquettes; NQxy values - GS: all Qxy = +1, but they are not all linearly independent - 4 locally indistinguishable ground states Spin Phenomena Interdisciplinary Center July 10, 2018 16 / 23 Fracton topological order Fracton topological order Fracton topological order in 3D 2D topological order (Sub)extensive topological Finite topological GS degeneracy degeneracy, 2αL Excitations: pointlike anyons Excitations: immobile fractons Anyons at the endpoints of strings Immobile = energy penalty to move Composite particles mobile in 2D/1D 3D topological order Can construct 3D fracton phases from layered 2D topological models Finite topological GS degeneracy Excitations: loop-like anyons H.
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