Topological Phases of Matter Out of Equilibrium

Transcription

1 Topological Phases of Matter Out of Equilibrium Nigel Cooper T.C.M. Group, Cavendish Laboratory, University of Cambridge Solvay Workshop on Quantum Simulation ULB, Brussels, 18 February 2019 Max McGinley (Cambridge) Marcello Caio (KCL/Leiden), Gunnar Moller (Kent), Joe Bhaseen (KCL)

2 Topological Invariants 1 κ da = (2 2g) 2π closed surface Gaussian curvature κ = 1 R 1 R 2 2D Bloch Bands [Thouless, Kohmoto, Nightingale & den Nijs, PRL 1982] Chern number: Berry curvature: Insulating bulk with ν = 1 2π BZ d 2 k Ω k Ω k = i k u k k u k z ν cannot change under smooth deformations ν gapless edge states

3 Topological Insulators Many generalisations when symmetries are included: [Hasan & Kane, RMP 2010] topological insulators/superconductors in all spatial dimensions bulk gap + gapless surface states Time reversal symmetry (non-magnetic system in vanishing magnetic field) Chiral (sublattice) symmetry e.g. Su-Schrieffer-Heeger model A B A B A B A B Detailed classification of topological matter at equilibrium [ARPES: Xia et al., 2008] [Here for free fermions, but also for strongly interacting systems]

4 Non-Equilibrium Dynamics? [unitary evolution] e.g. dynamical change in band topology E E E k k k ν=0 ν=? ν=1 time Preparation of topological phases? Is there a topological classification of non-equilibrium many-body states?

5 Outline Dynamics of Chern Insulators (2D) Dynamics of Topological Phases in 1D Topological Classification Out of Equilibrium

6 Outline Dynamics of Chern Insulators (2D) Dynamics of Topological Phases in 1D Topological Classification Out of Equilibrium

7 Dynamics of Chern Insulators (2D) Quench: start in ground state of H i then time evolve under E E E H f k k k ν=0 ν=? ν=1 time Time-evolving Bloch state of fermion at k u k (t) = exp( i H f k t) u k (0) Ω k (t) = i k u k (t) k u k (t) z Chern number of the many-body state is preserved [D Alessio & Rigol, Nat. Commun. 2015; Caio, NRC & Bhaseen, PRL 2015] [ topological invariant under smooth changes of the Bloch states]

8 Dynamics of Chern Insulators: Physical Consequences Obstruction to preparation of a state with differing Chern number [For slow ramps, τ L/v, deviations can be small] Chern number can be obtained by tomography of Bloch states [Two-band model: u k = cos(θ k /2) A, k + sin(θ k /2)e iϕ k B, k ] [Fläschner et al. [Hamburg], Science 2016] Topological of final Hamiltonian can be obtained by tracking the evolution of the Bloch states in time [Wang et al. [Tsinghua], PRL 2017; Tarnowski et al. [Hamburg], arxiv 2017]

9 Dynamics of Chern Insulators in Real Space NATURE PHYSIC [Caio, Möller, NRC & Bhaseen, Nat. Phys. 2019] Haldane model [Haldane, PRL 1988] a a +φ a M = 0.5 M = 0.0 M = 0.5 1M = t* 6 9 b t π 1 P) yp rαthe c(rα) = Im rα 1.6 P x( relativ maximum of s s A c s=a,b Increasing L 1.6 x C y0 t=0 t = 2.5 t=5 10 c b y t1 M = 1.0 c x c 1 t2 c +M Local Chern Marker 1.2[Bianco & Resta, PRB 2011] 1.0 log M ATURE PHYSICS M y c ram. a, Lattice geometry of the 5 The latter is attributed logl lowing the quench, and Q = Î P in the definitio at the frequency of the 2.0 Mc 1.8 M interference, the associa can be larger than the global conservation bands have slopes with Fig. c(r)3c. d 2rBy = 0contrast, th timescale correspondin bands, as shown by the c interference = J region in t 0 t 5 c 10 Topological marker c (M Mc)L1/ν v 6 Further evidence for t 4 shaped sample (light grey) with Quench dynamics involves flow of the Chern marker canslice also be obtained 3 2 Scaling of the Chern marker. a, Vertical Fig. through the phasefr mitive lattice vectors, and N!=!2L2 Nigel Cooper, Topological Phases of Matter Out of Equilibrium 4 University of Cambridge diagram 2 For a local finite-size in Fig. 1b with φ!=!π/2, showing the edges. variation of the Chern

10 Outline Dynamics of Chern Insulators (2D) Dynamics of Topological Phases in 1D Topological Classification Out of Equilibrium

11 Dynamics of Topological Phases in 1D (Free Fermions) In 1D all topological invariants can be determined from CS 1 = 1 2π BZ dk u k k u k Equivalently: Berry phase around the Brillouin zone (Zak phase) Only quantized in the presence of symmetries In 1D, topology must be protected by symmetry

12 Example: Su-Schrieffer-Heeger Model A B A B A B A B H k = ( 0 J + Je ika J + Je ika 0 ) = h(k) σ Chiral (sublattice) symmetry CS 1 = N/2 winding number N = 1 2π BZ dϕ dk dk h = (h x, h y,0) 2 h x + ih y = h(k) e iϕ(k) h y /J h x /J 2 integer Is this topological invariant preserved out of equilibrium? 2 J 0 =1.5 J J 0 =0.5 J No need to consider symmetries!

13 Symmetry-Protected Topology Out of Equilibrium Start in ground state of What if respects the symmetry? Symmetry can still be broken! [Max McGinley & NRC, PRL 2018] H f breaks symmetry topological invariant can vary [ explicit symmetry breaking ] H f H i then time evolve under H f Anti-unitary symmetries Oe iht O 1 = e +iht [ OΨ, OΦ = Φ, Ψ * ] Symmetry broken in the non-equilibrium state [ dynamically induced symmetry breaking ] Topological invariant time-varying even if symmetries respected!

14 Time-Varying CS 1 (t) : Physical Consequences [Max McGinley & NRC, PRL 2018] Could be observed in Bloch state tomography [cf. Chern number] d Directly measure via dt CS 1 (t) = j(t) = Q(t) [cf. Chern number Hall conductance out of equilibrium] Example: quenches in a generalised SSH model AIII: chiral symmetry only BDI: time-reversal, particle-hole & chiral How can we define topology out of equilibrium?

15 Outline Dynamics of Chern Insulators (2D) Dynamics of Topological Phases in 1D Topological Classification Out of Equilibrium

16 Topological Classification Out Of Equilibrium [Max McGinley & NRC, PRL 2018] Equilibrium topological phase gapless surface states Non-equilibrium topological state gapless entanglement spectrum Ψ(t) = e λ i ψ i i L ψr i Example: quenches in a generalised SSH model [Li & Haldane, PRL 2008] AIII: chiral symmetry only BDI: time-reversal, particle-hole & chiral meaningful topological classification out of equilibrium

17 Generalization to Interacting SPT Phases [Max McGinley & NRC, PRL 2018] Example: Haldane phase of a S=1 spin chain is an SPT phase that can be protected by a variety of symmetries: Time-reversal symmetry (anti-unitary) Dihedral symmetry (unitary) Anti-unitary symmetries lost dynamically TRS only TRS and dihedral

18 Generalization to other Spatial Dimensions Ten-fold way for free fermions [Time-reversal, particle-hole & chiral symmetries] [Chiu, Teo, Schnyder & Ryu, RMP 2016] symmetries spatial dimension δ Class T C S A Z 0 Z 0 Z 0 Z 0 AIII Z 0 Z 0 Z 0 Z AI þ 0 0 Z Z 0 Z 2 Z 2 BDI þ þ 1 Z 2 Z Z 0 Z 2 D 0 þ 0 Z 2 Z 2 Z Z 0 DIII þ 1 0 Z 2 Z 2 Z Z AII 0 0 2Z 0 Z 2 Z 2 Z CII 1 0 2Z 0 Z 2 Z 2 Z 0 0 C Z 0 Z 2 Z 2 Z 0 CI þ Z 0 Z 2 Z 2 Z

19 Topological Classification Out Of Equilibrium Ten-fold way for free fermions [Chiu, Teo, Schnyder & Ryu, RMP 2016] [Time-reversal, particle-hole & chiral symmetries] Non-Equilibrium Classification [Max McGinley & NRC, arxiv: ] Class Symmetries Spatial dimension d T C S A Z 0 Z 0 Z 0 Z 0 AIII Z! 0 0 Z! 0 0 Z! 0 0 Z! 0 AI Z Z 0 Z 2! 0 Z 2! 0 BDI Z 2 Z! Z Z! 0 0 Z 2! 0 D Z 2 Z 2 Z Z 0 DIII Z 2! 0 Z 2! 0 Z! Z! 0 AII 0 0 2Z 0 Z 2! 0 Z 2! 0 Z CII 1 0 2Z! 0 0 Z 2! 0 Z 2 Z! Z C Z 0 Z 2 Z 2 Z 0 CI Z! 0 0 Z 2! 0 Z 2! 0 Z! 0 Z

20 Physical Consequences of Non-Equilibrium Classification Preparation of topological states States with non-trivial index cannot be prepared on timescales short compared to the inverse level spacing L/v Stability of topologically protected boundary modes [Max McGinley & NRC, arxiv: ] Adiabatic mixing of Kramers pairs under braiding [Wölms, Stern & Flensberg, PRL 2014] Decoherence of Majorana qubit memories due to noise Retrieval of quantum information via recovery fidelity [Mazza, Rizzi, Lukin & Cirac, PRB 2013]

21 Summary There exists a topological classification of non-equilibrium quantum states, which differs from that at equilibrium: bulk-boundary correspondence applies to the entanglement spectrum. [holds also for interacting and disordered systems] For 2D systems, the Chern number is preserved under unitary dynamics. However, there can be a spatial flow of the local Chern marker. More generally, topological invariants can vary in time in 1D such variations appear as a current no obstruction to changing the topology of the state dynamically sensitivity of topologically protected degrees of freedom to noise