Geometric responses of Quantum Hall systems

Transcription

1 Geometric responses of Quantum Hall systems Alexander Abanov December 14, 2015 Cologne Geometric Aspects of the Quantum Hall Effect

2 Fractional Quantum Hall state exotic fluid Two-dimensional electron gas in magnetic field forms a new type of quantum fluid It can be understood as quantum condensation of electrons coupled to vortices/fluxes Quasiparticles are gapped, have fractional charge and statistics The fluid is ideal no dissipation! Density is proportional to vorticity! Transverse transport: Hall conductivity and Hall viscosity, thermal Hall effect Protected chiral dynamics at the boundary

3 Transverse transport Signature of FQH states quantization and robustness of Hall conductance σ H j i = σ H ɛ ij E j, σ H = ν e2 h - Hall conductivity. Are there other universal transverse transport coefficients? Hall viscosity: transverse momentum transport Thermal Hall conductivity: transverse energy/heat transport What are the values of the corresponding kinetic coefficients for various FQH states? Are there corresponding protected boundary modes?

4 Acknowledgments and References A. G. Abanov and A. Gromov, Phys. Rev. B 90, (2014). Electromagnetic and gravitational responses of two-dimensional noninteracting electrons in a background magnetic field. A. Gromov and A. G. Abanov, Phys. Rev. Lett. 113, (2014). Density-Curvature Response and Gravitational Anomaly. A. Gromov and A. G. Abanov, Phys. Rev. Lett. 114, (2015). Thermal Hall Effect and Geometry with Torsion. A. Gromov, G. Cho, Y. You, A. G. Abanov, and E. Fradkin, Phys. Rev. Lett. 114, (2015). Framing Anomaly in the Effective Theory of the Fractional Quantum Hall Effect. A. Gromov, K. Jensen, and A.G. Abanov, arxiv: (2015). Boundary effective action for quantum Hall states.

5 Essential points of the talk Induced Action encodes linear responses of the system Coefficients of geometric terms of the induced action universal transverse responses. Hall conductivity, Hall viscosity, thermal Hall conductivity. These coefficients are computed for various FQH states. Framing anomaly is crucial in obtaining the correct gravitational Chern-Simons term!

6 Induced action Partition function of fermions in external e/m field A µ is: Z = DψDψ e is[ψ,ψ ;A µ] = e is ind[a µ] with S[ψ, ψ ; A µ ] = [ d 2 x dt ψ i t + ea 0 1 2m + interactions ( i e c A ) 2 ] ψ Induced action encodes current-current correlation functions j µ = δs ind δa µ, j µ j ν = δ2 S ind δa µ δa ν,... + various limits m 0, e 2 /l B,...

7 Induced action [phenomenological] Use general principles: gap+symmetries to find the form of S ind Locality expansion in gradients of A µ Gauge invariance written in terms of E and B Other symmetries: rotational, translational,... S ind = ν 4π AdA + d 2 x dt [ ɛ 2 E2 1 ] 2µ B2 + σb E +... Find responses in terms of phenomenological parameters ν, ɛ, µ, σ,... Compute these parameters from the underlying theory. For non-interacting particles in B with ν = N see AA, Gromov Any functional of E and B is gauge invariant, but...

8 Chern-Simons action S CS = ν 4π AdA

9 Linear responses from the Chern-Simons action In components S CS = ν 4π = ν 4π AdA ν d 2 x dt ɛ µνλ A µ ν A λ 4π [ ] d 2 x dt A 0 ( 1 A 2 2 A 1 ) +... Varying over A µ ρ = δs CS δa 0 = ν 2π B, j 1 = δs CS δa 1 = ν 2π E 2, We have: σ H = ν 2π and σ H = ρ B Streda formula.

10 Properties of the Chern-Simons term Gauge invariant in the absence of the boundary (allowed in the induced action) Not invariant in the presence of the boundary Leads to protected gapless edge modes First order in derivatives (more relevant than F µν F µν, B 2 or E 2 at large distances) Relativistically invariant (accidentally!!!) Does not depend on metric g µν (topological, does not contribute to the stress-energy tensor)

11 Elastic responses: Strain and Metric Deformation of solid or fluid r r + u(r) u(r) - displacement vector u ik = 1 2 ( ku i + i u k ) - strain tensor u ik plays a role of the deformation metric deformation metric g ik δ ik + 2u ik with ds 2 = g ik dx i dx k stress tensor T ij - response to the deformation metric g ij

12 Stress tensor and induced action Studying responses Microscopic model S = S[ψ] Introduce gauge field and metric background S[ψ, A, g] Integrate out matter degrees of freedom and obtain and S ind [A, g] Obtain E/M, elastic, and mixed responses from δs ind = dx dt g (j µ δa µ + 12 ) T ij δg ij Elastic responses = gravitational responses Important: stress is present even in flat space!

13 Quantum Hall in Geometric Background (by Gil Cho) e e e e = electron = magnetic field

14 Geometric background For 2+1 dimensions and spatial metric g ij we introduce spin connection ω µ so that 1 gr 2 = 1 ω 2 2 ω 1 gravi-magnetic field, E i = ω i i ω 0 gravi-electric field, For small deviations from flat space g ik = δ ik + δg ik we have explicitly ω 0 = 1 2 ɛjk δg ij ġ ik, ω i = 1 2 ɛjk j δg ik Close analogy with E/M fields A µ ω µ!

15 Geometric terms of the induced action Terms of the lowest order in derivatives S ind = ν 4π Geometric terms: [ AdA + 2 s ωda + β ωdω]. AdA Chern-Simons term (ν: Hall conductance, filling factor) ωda Wen-Zee term ( s: orbital spin, Hall viscosity, Shift) ωdω gravitational CS term (β : Hall viscosity - curvature, thermal Hall effect, orbital spin variance) In the presence of the boundary β can be divided into chiral central charge c and s 2 (Bradlyn, Read, 2014). The latter does not correspond to an anomaly. (Gromov, Jensen, AA, 2015)

16 The Wen-Zee term Responses from the Wen-Zee term S W Z = ν s ωda. 2π Emergent spin (orbital spin) s ν (A + sω)d(a + sω) 4π Wen-Zee shift for sphere δn = νs; S = 2 s ν s 2π A 0dω δρ = ν s ν s dω δn = d 2 x gr = ν sχ = ν s(2 2g) 2π 4π Hall viscosity (per particle) η H = s 2 n e: ν s Bν s ωda 2π 2π ω s 0 = n e sω 0 = n e 2 ɛjk δg ij ġ ik

17 Hall viscosity Gradient correction to the stress tensor T ik = η H (ɛ in v nk + ɛ kn v ni ), where v ik = 1 2 ( iv k + k v i ) = 1 2ġik strain rate (a) Shear viscosity (b) η H Hall viscosity picture from Lapa, Hughes, 2013 Avron, Seiler, Zograf, 1995

18 Obtaining the effective field theory for FQH states Reduce problem to noninteracting fermions with ν - integer interacting with statistical Abelian and non-abelian gauge fields. Can be done, e.g., by flux attachment or parton construction (Zhang, Hansson, Kivelson, 1989; Wen, 1991; Cho, You, Fradkin, 2014) Integrate out fermions and obtain the effective action S[a, A, g] using the results for free fermions. (AA, Gromov, 2014) Integrate out statistical gauge fields taking into account the framing anomaly. (Gromov et al., 2015) Obtain the induced action S geom ind [A, g] and study the corresponding responses.

19 Flux attachment in geometric background Cho, You, Fradkin, 2014 Flux attachment is based on the identity: { Db Da exp i 2p bdb i 1 4π 2π adb + i L } (a + pω) Here L is a link. The last term pω is due to the framing regularization (Polyakov, 1988). Integrating over b (wrong!) { 1 Da exp i ada + i (2p)2π L } (a + pω) = 1 = 1 Integrating over b (correct!) { 1 Da exp i ada + i (a + pω) i 1 (2p)2π 48π L } ωdω = 1 Integrating over b (correct!) { 1 1 }

20 Explicit calculation for free fermions at ν = N Starting point S[ψ, ψ ; A µ, g ij ] = dt d 2 x [ g ψ i ] D t 2 2m gij (D i ψ) (D j ψ). Straightforward computation at ν = N (Gromov, AA, 2014) S (geom) eff = N 4π ( AdA + N Adω + 2N or equivalently (as a sum over Landau levels) S (geom) eff = N n=1 ) ωdω [ 1 4π (A + s nω)d(a + s n ω) c ] 48π ωdω s n = 2n 1 2, c = 1.

21 Geometric effective action for ν = 1 [ S (geom) 1 eff = (A + 12 ) 4π ω d (A + 12 ) ω 1 ] 48π ωdω Is there an intuitive way to obtain this result?

22 The Wen-Zee construction for ν = 1 Integrate out fermions but leave currents j = 1 2π da (Wen, Zee, 1992) S[a; A, ω] = 1 [ ada + 2 (A + 12 ) ] 4π ω da. (Wen, Zee, 1992) + framing anomaly: solve for a: a = ( A ω) substitute back into the action take into account framing anomaly (Gromov et.al., 2015) S ind = 1 (A + 12 ) 4π ω d (A + 12 ) ω 1 48π ωdω

23 Digression: the quantum Chern-Simons theory The partition function for Chern-Simons theory in the metric background (Witten, 1989) { Da exp i k } { c ada = exp i tr (ΓdΓ + 23 )} 4π 96π Γ3 { } c = exp i ωdω, 48π where c = sgn(k) and the last equality is correct for our background. We specialized Witten s results to the Abelian CS theory The result is obtained from the fluctuation determinant det(d) The dependence on metric comes from the gauge fixing dv φd µ a µ Action does not depend on metric, path integral does: anomaly (framing anomaly)

24 Consistency check (for ν = 1) Basic idea: two routes to effective action. Route 1: Route 2: 1 4π S 0 [ψ; A, g] 1 4π (A + 12 ) ω d (A + 12 ) ω 1 48π ωdω S 0 [ψ; A, g] S 0 [ψ; A + a + ω, g] + 1 8π ada 1 48π ωdω Introducing A = A + a + ω (A 12 ) ω d 1 4π (A + 12 ω ) d (A 12 ω ) π ωdω + 1 8π ada 1 48π ωdω (A + 12 ) ω π ωdω π ωdω Consistent only if the framing anomaly is taken into account (twice)!

25 Obtaining the effective field theory for FQH states Reduce problem to noninteracting fermions with ν - integer interacting with statistical Abelian and non-abelian gauge fields. Can be done, e.g., by flux attachment or parton construction (Zhang, Hansson, Kivelson, 1989; Wen, 1991; Cho, You, Fradkin, 2014) Integrate out fermions and obtain the effective action S[a, A, g] using the results for free fermions. (Gromov, AA, 2014) Integrate out statistical gauge fields taking into account the framing anomaly. (Gromov et al., 2015) Obtain the induced action S geom ind [A, g] and study the corresponding responses.

26 Example: Laughlin s states Flux attachment for Laughlin s states ν = 1 2m+1 [ 2m S 0 [ψ, A + a + mω, g] 4π bdb + 1 ] 2π adb Integrating out ψ, a, b ( S geom 1 1 ind = A + 2m + 1 ω 4π 2m Coefficients ν = ) ( d A + 2m + 1 ω 2 1 2m + 1, s =, c = 1. 2m ) 1 48π ωdω

27 Other states Geometric effective actions have been obtained for: Free fermions at ν = N Laughlin s states Jain series Arbitrary Abelian QH states Read-Rezayi non-abelian states The method can be applied to other FQH states A. Gromov et.al., PRL 114, (2015). Framing Anomaly in the Effective Theory of the Fractional Quantum Hall Effect.

28 Consequences of the gravitational CS term S gcs = c 96π tr (ΓdΓ + 23 ) Γ3 = c 48π ωdω. 1. From CS and WZ term [shift] (Wen, Zee, 1992) n = ν 2π B + ν s 4π R N = ν(n φ + sχ) From WZ and gcs term [Hall viscosity shift] (Gromov, AA, 2014 cf. Hughes, Leigh, Parrikar, 2013) η H = s 2 n c R 24 4π 2. Thermal Hall effect [from the boundary!] (Kane, Fisher, 1996; Read, Green, 2000; Cappelli, Huerta, Zemba, 2002) K H = c πk2 B T 6.

29 Some recent closely related works Geometric terms from adiabatic transport and adiabatic deformations of trial FQH wave functions (Bradlyn, Read, 2015; Klevtsov, Wiegmann, 2015) QH wave functions in geometric backgrounds (Can, Laskin, Wiegmann, 2014; Klevtsov, Ma, Marinescu, Wiegmann, 2015) Newton-Cartan geometric background and Galilean invariance (Hoyos, Son, 2011; Gromov, AA, 2014; Jensen, 2014) Thermal transport in quantum Hall systems (Geracie, Son, Wu, Wu, 2014; Gromov, AA, 2014; Bradlyn, Read, 2014)

30 Main results Response functions can be encoded in the form of the induced action for FQHE. S ind = ν [ (A + sω)d(a + sω) + βωdω] 4π c tr [ΓdΓ + 23 ] 96π Γ3 +..., where ν is the filling fraction, s is the average orbital spin, β is the orbital spin variance, and c is the chiral central charge. The coefficients ν, s, β, c are computed for various known Abelian and non-abelian FQH states. Framing anomaly is crucial in obtaining the correct gravitational Chern-Simons term!

31 Quantum Hall at the Edge (by Gil Cho)