Composite Dirac liquids

Transcription

1 Composite Dirac liquids Composite Fermi liquid non-interacting 3D TI surface Interactions Composite Dirac liquid ~ Jason Alicea, Caltech David Mross, Andrew Essin, & JA, Physical Review X 5, (2015)

2 O(100) plateaus observed to date. How might we most efficiently capture these topological phases?

3 ν = 1/2 (no plateau!) O(100) plateaus observed to date. How might we most efficiently capture these topological phases?

4 Composite Fermi liquid at ν = 1/2 Half-filled Landau level Landau Level Energy ~! c One state/flux quantum

5 Composite Fermi liquid at ν = 1/2 Half-filled Landau level Landau Level Energy ~! c One state/flux quantum electron

6 Composite Fermi liquid at ν = 1/2 Half-filled Landau level Landau Level Energy ~! c One state/flux quantum electron = statistical flux quanta composite fermion

7 Composite Fermi liquid at ν = 1/2 Half-filled Landau level Landau Level Energy ~! c One state/flux quantum k y = electron statistical flux quanta composite fermion composite fermion Fermi sea! k x

8 Composite Fermi liquid at ν = 1/2 Composite Fermi liquid k y = electron statistical flux quanta composite fermion composite fermion Fermi sea! k x

9 Composite Fermi liquid at ν = 1/2 Quantum oscillations of composite Fermi sea! k y = electron statistical flux quanta composite fermion composite fermion Fermi sea! k x

10 Composite Fermi liquid at ν = 1/2 Composite fermion Cooper pairs Pairing instability of composite Fermi sea yields non-abelian Moore-Read fractional quantum Hall state! e/4 non-abelian anyon co-propagating Majorana + charge edge modes k y = electron statistical flux quanta composite fermion composite fermion Fermi sea! k x

11 Composite Fermi liquid at ν = 1/2 Composite fermion Cooper pairs e/4 non-abelian anyon co-propagating Majorana + charge edge modes Pairing instability of composite Fermi sea yields non-abelian Moore-Read fractional quantum Hall state! Moral: emergent metals are interesting and provide efficient way of capturing exotic topologically ordered gapped phases. k y = electron statistical flux quanta composite fermion composite fermion Fermi sea! k x

12 Anomalous 3D TI surface physics 3D topological insulator: protected by time-reversal, U(1) particle conservation symmetries Insulating bulk; assumed symmetric throughout

13 Anomalous 3D TI surface physics 3D topological insulator: protected by time-reversal, U(1) particle conservation symmetries Gapless surface with single Dirac cone Impossible band structure in strict 2D systems with time-reversal, where only even # of Dirac cones can appear.

14 Anomalous 3D TI surface physics 3D topological insulator: protected by time-reversal, U(1) particle conservation symmetries Gapless surface with single Dirac cone Integer quantum Hall state with half-integer Hall conductance Ferromagnet Impossible in strict 2D, since would imply fractionalization, topological order.

15 Anomalous 3D TI surface physics 3D topological insulator: protected by time-reversal, U(1) particle conservation symmetries Gapless surface with single Dirac cone Integer quantum Hall state with half-integer Hall conductance Up Ferromagnet Down Ferromagnet xy = e2 Magnetic domain walls bind integer quantum Hall edge states Impossible in strict 2D, since would imply fractionalization, topological order.

16 Anomalous 3D TI surface physics 3D topological insulator: protected by time-reversal, U(1) particle conservation symmetries Gapless surface with single Dirac cone Integer quantum Hall state with half-integer Hall conductance Up Ferromagnet Cooper pair Down Ferromagnet xy = e2 s-wave superconductor Time-reversalinvariant cousin of topological spinless p+ip superconductor Impossible in strict 2D, since superconductor would always break T.

17 Without interactions, this is the full story: -Symmetry implies massless electron Dirac cone -broken symmetry implies anomalous gapped phases. Does symmetry imply surface metallicity more generally?

18 Without interactions, this is the full story: -Symmetry implies massless electron Dirac cone -broken symmetry implies anomalous gapped phases. Does symmetry imply surface metallicity more generally? No!

19 T-Pfaffian Pfaffianantisemion

20 T-Pfaffian Clearly break time reversal symmetry in 2D, but not on surface! Pfaffianantisemion

21 T-Pfaffian Clearly break time reversal symmetry in 2D, but not on surface! Pfaffianantisemion Composite Dirac liquids = new correlated gapless surface states that unify these works.

22 Composite Dirac liquids: phenomenology non-interacting 3D TI surface electronic Dirac cone

23 Composite Dirac liquids: phenomenology Interactions strip electric charge from surface Dirac cone non-interacting 3D TI surface Composite Dirac liquid electronic Dirac cone Dirac cone for emergent neutral fermions! -Charge sector gapped, so surface is electrical insulator -Neutral fermions yield metallic thermal transport similar to original Dirac cone -Wild deviation from Weidemann-Franz law!

24 Composite Dirac liquids: phenomenology Interactions strip electric charge from surface Dirac cone Composite Fermi liquid non-interacting 3D TI surface Composite Dirac liquid ~ k y electronic Dirac cone Dirac cone for emergent neutral fermions! composite fermion Fermi sea k x -Charge sector gapped, so surface is electrical insulator -Neutral fermions yield metallic thermal transport similar to original Dirac cone -Wild deviation from Weidemann-Franz law!

25 Composite Dirac liquids: phenomenology Interactions strip electric charge from surface Dirac cone Composite Fermi liquid non-interacting 3D TI surface Composite Dirac liquid ~ k y electronic Dirac cone Dirac cone for emergent neutral fermions! composite fermion Fermi sea k x Cooper pair neutral fermions [preserves U(1)] Cooper pair composite fermions time-reversal-invariant cousin of Moore-Read (T-Pfaffian)! non-abelian Moore-Read FQH state

26 Accessing composite Dirac liquids In an ideal world: H Dirac + H interactions Composite Dirac liquid 3D TI surface

27 Accessing composite Dirac liquids In an ideal world: H Dirac + H interactions Composite Dirac liquid (too hard) 3D TI surface Instead, will cheat and follow route with virtues of physical transparency, analytical control.

28 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) 3D TI surface

29 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) xy = e2 Insert ferromagnets w/alternating magnetization xy = e2

30 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) xy = e2 electron tunneling Insert ferromagnets w/alternating magnetization xy = e2 electronic Dirac cone

31 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) xy = e2 Insert ferromagnets w/alternating magnetization xy = e2 Quasi-1D deformation allows rigorous analytical progress a la Teo & Kane.

32 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) xy = e2 Insert ferromagnets w/alternating magnetization xy = e2 xy = e2 xy = e2 Insert 2D FQH states xy = e2

33 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) xy = e2 Insert ferromagnets w/alternating magnetization xy = e2 xy = e2 xy = xy = e2 e2 Insert 2D FQH states Charge flow is non-chiral, so interactions can gap charge sector but heat transport is chiral, so must have neutral modes left over!

34 Accessing composite Dirac liquids Key technical idea: relax time-reversal to weaker antiferromagnetic symmetry (states still impossible in 2D) Neutral fermions but heat transport is chiral, so must have neutral modes left over!

35 Accessing composite Dirac liquids Composite Dirac liquid! neutral fermion tunneling neutral Dirac cone Neutral fermions but heat transport is chiral, so must have neutral modes left over!

36 Nested Composite Dirac Liquids Interactions strip electric charge from surface Dirac cone non-interacting 3D TI surface Composite Dirac liquid electronic Dirac cone Dirac cone for emergent neutral fermions! Cooper pair neutral fermions [preserves U(1)] time-reversal-invariant cousin of Moore-Read (T-Pfaffian)

37 Nested Composite Dirac Liquids non-interacting 3D TI surface Interactions strip electric charge from surface Dirac cone Composite Dirac liquid Same interactions, but for neutral fermions Nested Composite Dirac liquid electronic Dirac cone Dirac cone for emergent neutral fermions! 2nd generation neutral Dirac cone Cooper pair neutral fermions [preserves U(1)] Cooper pair neutral fermions [preserves U(1)] time-reversal-invariant cousin of Moore-Read (T-Pfaffian) symmetric topological order w/more anyons (Pfaffian-antisemion)

38 Nested Composite Dirac Liquids non-interacting 3D TI surface Interactions strip electric charge from surface Dirac cone Composite Dirac liquid Same interactions, but for neutral fermions Nested Composite Dirac liquid electronic Dirac cone Dirac cone for emergent neutral fermions! 2nd generation neutral Dirac cone Topological orders captured previously, now in one unified framework! Cooper pair neutral fermions [preserves U(1)] time-reversal-invariant cousin of Moore-Read (T-Pfaffian) Cooper pair neutral fermions [preserves U(1)] symmetric topological order w/more anyons (Pfaffian-antisemion)

39 Outlook Interplay between interactions, symmetry-protected topological phases rich topic. neutral Dirac cone Composite Dirac liquid Provide unifying view on lots of physics, including quantum Hall; accessible via controlled analytical methods that deal with physical electrons. Naturally extends to weak topological insulators. (D. Mross, A. Essin, JA, A. Stern, in preparation) Extension to 3D topological superconductors? y Isotropic implementations? z x Quasi-realistic Hamiltonians for such states?