Outline Motivation Quantum Hall Eect The Model Conclusion
AdS/QHE
A holographic approach to describing quantum Hall plateaux transitions
Allan Bayntun
National Taiwan University Taipei, Taiwan
December 31, 2010
Outline Motivation Quantum Hall Eect The Model Conclusion
Motivation
Strongly Correlated Electron Systems Quantum Hall Eect
QHE Properties Symmetries of QHE The Model
Quantum Hall-ography Conclusion
Outline Motivation Quantum Hall Eect The Model Conclusion
Why condensed matter at all?
• Many recent outstanding problems in condensed matter are related to strongly correlated electron systems
• Any theoretical tool capable of calculating quantities in strongly interacting systems (of real electrons) would be of great value to these problems
• Moreover, many experiments (and theories) in condensed matter are interested in critical phenomena of these systems
• These are very close to CFTs, and so the AdS/CFT dictionary could be the right tool
Outline Motivation Quantum Hall Eect The Model Conclusion
Why condensed matter at all?
• Many recent outstanding problems in condensed matter are related to strongly correlated electron systems
• Any theoretical tool capable of calculating quantities in strongly interacting systems (of real electrons) would be of great value to these problems
• Moreover, many experiments (and theories) in condensed matter are interested in critical phenomena of these systems
• These are very close to CFTs, and so the AdS/CFT dictionary could be the right tool
Outline Motivation Quantum Hall Eect The Model Conclusion
Why condensed matter at all?
• Many recent outstanding problems in condensed matter are related to strongly correlated electron systems
• Any theoretical tool capable of calculating quantities in strongly interacting systems (of real electrons) would be of great value to these problems
• Moreover, many experiments (and theories) in condensed matter are interested in critical phenomena of these systems
• These are very close to CFTs, and so the AdS/CFT dictionary could be the right tool
Outline Motivation Quantum Hall Eect The Model Conclusion
Why condensed matter at all?
• Many recent outstanding problems in condensed matter are related to strongly correlated electron systems
• Any theoretical tool capable of calculating quantities in strongly interacting systems (of real electrons) would be of great value to these problems
• Moreover, many experiments (and theories) in condensed matter are interested in critical phenomena of these systems
• These are very close to CFTs, and so the AdS/CFT dictionary could be the right tool
Outline Motivation Quantum Hall Eect The Model Conclusion
Hopes and Dreams of AdS/CMT
Graphene
Quantum Critical Theories
High-Tc Superconductivity
Quantum Hall Eect
Outline Motivation Quantum Hall Eect The Model Conclusion
Hopes and Dreams of AdS/CMT
Graphene
Quantum Critical Theories
High-Tc Superconductivity
Quantum Hall Eect
Outline Motivation Quantum Hall Eect The Model Conclusion
Hopes and Dreams of AdS/CMT
Graphene
Quantum Critical Theories
High-Tc Superconductivity
Quantum Hall Eect
Outline Motivation Quantum Hall Eect The Model Conclusion
Hopes and Dreams of AdS/CMT
Graphene
Quantum Critical Theories
High-Tc Superconductivity
Quantum Hall Eect
Outline Motivation Quantum Hall Eect The Model Conclusion
Hopes and Dreams of AdS/CMT
Graphene
Quantum Critical Theories
High-Tc Superconductivity
Quantum Hall Eect
Outline Motivation Quantum Hall Eect The Model Conclusion
Why the (Fractional) Quantum Hall Eect (FQHE)?
• Any strongly correlated system would be remarkable if completely described by AdS space.
• There are many possible gravity duals (even more from a string phenomenological setting)
• Best guide is to use symmetry principles to constrain our possible gravity duals.
• FQHE has a large amount of (fairly unique) symmetries and dualities
Outline Motivation Quantum Hall Eect The Model Conclusion
Why the (Fractional) Quantum Hall Eect (FQHE)?
• Any strongly correlated system would be remarkable if completely described by AdS space.
• There are many possible gravity duals (even more from a string phenomenological setting)
• Best guide is to use symmetry principles to constrain our possible gravity duals.
• FQHE has a large amount of (fairly unique) symmetries and dualities
Outline Motivation Quantum Hall Eect The Model Conclusion
Why the (Fractional) Quantum Hall Eect (FQHE)?
• Any strongly correlated system would be remarkable if completely described by AdS space.
• There are many possible gravity duals (even more from a string phenomenological setting)
• Best guide is to use symmetry principles to constrain our possible gravity duals.
• FQHE has a large amount of (fairly unique) symmetries and dualities
Outline Motivation Quantum Hall Eect The Model Conclusion
Why the (Fractional) Quantum Hall Eect (FQHE)?
• Any strongly correlated system would be remarkable if completely described by AdS space.
• There are many possible gravity duals (even more from a string phenomenological setting)
• Best guide is to use symmetry principles to constrain our possible gravity duals.
• FQHE has a large amount of (fairly unique) symmetries and dualities
Outline Motivation Quantum Hall Eect The Model Conclusion
The Classical Hall Eect
Lorentz force on charge carriers in a current.
Dene resistivity matrix as Ei = ρijJj
As T → 0, we expect the ohmic resistivity ρxx→ 0,
classically.
What about Hall resistivity?
0 = Fx = qEx− qvyBz= qEx−VnJyBz
Since ρxyJy = Ex we see ρxy = Bρz. The hall resistivity is linear in Bz.
Outline Motivation Quantum Hall Eect The Model Conclusion
The Classical Hall Eect
Lorentz force on charge carriers in a current.
Dene resistivity matrix as Ei = ρijJj
As T → 0, we expect the ohmic resistivity ρxx→ 0,
classically.
What about Hall resistivity?
0 = Fx = qEx− qvyBz= qEx−VnJyBz
Since ρxyJy = Ex we see ρxy = Bρz. The hall resistivity is linear in Bz.
Outline Motivation Quantum Hall Eect The Model Conclusion
The (Fractional) Quantum Hall Eect
Instead of a straight line, we see successive plateaux as we increase the magnetic eld.
These quantized conductivities (resistivities) are given by σxy = ρ−1xy = ρ
B ≡ νeh2,
where ν is quantized to be a fraction in general with odd denominator.
Outline Motivation Quantum Hall Eect The Model Conclusion
The (Fractional) Quantum Hall Eect
Instead of a straight line, we see successive plateaux as we increase the magnetic eld.
These quantized conductivities (resistivities) are given by σxy = ρ−1xy = ρ
B ≡ νeh2,
where ν is quantized to be a fraction in general with odd denominator.
Outline Motivation Quantum Hall Eect The Model Conclusion
The (Fractional) Quantum Hall Eect
Instead of a straight line, we see successive plateaux as we increase the magnetic eld.
These quantized conductivities (resistivities) are given by σxy = ρ−1xy = ρ
B ≡ νeh2,
where ν is quantized to be a fraction in general with odd denominator.
Outline Motivation Quantum Hall Eect The Model Conclusion
The (Fractional) Quantum Hall Eect
Instead of a straight line, we see successive plateaux as we increase the magnetic eld.
These quantized conductivities (resistivities) are given by σxy = ρ−1xy = ρ
B ≡ νeh2,
where ν is quantized to be a fraction in general with odd denominator.
Outline Motivation Quantum Hall Eect The Model Conclusion
Properties of FQHE
Semi-Circle Law
Semi-circles are swept out in the conductivity plane departing from plateau. Also, states ν = p/q and ν0 = r/sare only connected if and only if |ps − rq| = 1.
Outline Motivation Quantum Hall Eect The Model Conclusion
Properties of FQHE
Duality
Across the critical magnetic eld, the ohmic resistance enjoys the relationship
ρxx(νc− ∆ν) ∝ 1 ρxx(νc+ ∆ν)
Outline Motivation Quantum Hall Eect The Model Conclusion
Duality and the Modular Group - SL(2, Z)
• Dene a complex conductivity σ ≡ σxy+ iσxx
• These properties imply that σ has the relation σ → aσ + b
cσ + d
• This describes the duality relating the various plateaux for integers a, b, c, and d for |ad − bc| = 1
• The observed odd-denominator quantum hall states restricts c to be even
• This breaks SL(2, Z) to Γ0(2)
Outline Motivation Quantum Hall Eect The Model Conclusion
Duality and the Modular Group - SL(2, Z)
• Dene a complex conductivity σ ≡ σxy+ iσxx
• These properties imply that σ has the relation σ → aσ + b
cσ + d
• This describes the duality relating the various plateaux for integers a, b, c, and d for |ad − bc| = 1
• The observed odd-denominator quantum hall states restricts c to be even
• This breaks SL(2, Z) to Γ0(2)
Outline Motivation Quantum Hall Eect The Model Conclusion
Duality and the Modular Group - SL(2, Z)
• Dene a complex conductivity σ ≡ σxy+ iσxx
• These properties imply that σ has the relation σ → aσ + b
cσ + d
• This describes the duality relating the various plateaux for integers a, b, c, and d for |ad − bc| = 1
• The observed odd-denominator quantum hall states restricts c to be even
• This breaks SL(2, Z) to Γ0(2)
Outline Motivation Quantum Hall Eect The Model Conclusion
Duality and the Modular Group - SL(2, Z)
• Dene a complex conductivity σ ≡ σxy+ iσxx
• These properties imply that σ has the relation σ → aσ + b
cσ + d
• This describes the duality relating the various plateaux for integers a, b, c, and d for |ad − bc| = 1
• The observed odd-denominator quantum hall states restricts c to be even
• This breaks SL(2, Z) to Γ0(2)
Outline Motivation Quantum Hall Eect The Model Conclusion
Duality and the Modular Group - SL(2, Z)
• Dene a complex conductivity σ ≡ σxy+ iσxx
• These properties imply that σ has the relation σ → aσ + b
cσ + d
• This describes the duality relating the various plateaux for integers a, b, c, and d for |ad − bc| = 1
• The observed odd-denominator quantum hall states restricts c to be even
• This breaks SL(2, Z) to Γ0(2)
Outline Motivation Quantum Hall Eect The Model Conclusion
Duality and the Modular Group - SL(2, Z)
To further motivate this group, we can see RG ow lines consistent with commuting with the group Γ0(2)
From this, we see both the semicircles associated with moving away from a plateaux, as well as ow towards various
odd-denominator states as T → 0
Outline Motivation Quantum Hall Eect The Model Conclusion
Deep IR - Fractionally Charged Anoyons
The nal ingredient we would like is knowledge of the system in the IR. It is generally understood that the mechanism for fractional plateaux is from the charge-carries having fractional statistics.
This is accomplished by deforming an action by
S[ψ, A] → S[ψ, A + a] − e2 2ϑ
Z
d3xµνλaµ∂νaλ
ie we couple the system to a statistics eld with a Chern-Simons term
Outline Motivation Quantum Hall Eect The Model Conclusion
Deep IR - Fractionally Charged Anoyons
The nal ingredient we would like is knowledge of the system in the IR. It is generally understood that the mechanism for fractional plateaux is from the charge-carries having fractional statistics.
This is accomplished by deforming an action by
S[ψ, A] → S[ψ, A + a] − e2 2ϑ
Z
d3xµνλaµ∂νaλ
ie we couple the system to a statistics eld with a Chern-Simons term
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Review: Our CMT Ingredient List
We want a duality group relating the plateaux that the RG ow commutes with
• Look for bulk actions which are SL(2, R) invariant (which we can later break)
We need fractional statistics and therefore a Chern-Simons term on the boundary theory
• A bulk axion, χ, is simply the Chern-Simons coeecient on the boundary in the AdS/CFT dictionary
Finally, we require a nite Ohmic conductivity at nite
temperature in the boundary theory to see the semicircles in the conductivity plane
• Require a `probe' system that can dissipate energy into a background, avoiding a Drude peak in conductivity
Outline Motivation Quantum Hall Eect The Model Conclusion
Quantum Hall-ography - The Model
Background bulk action:
Sgrav= − Z
d4x√
−g
1 2κ2
R − 6
L2 + 1 2
∂µφ∂µφ + e2φ∂µχ∂µχ
+ SLifshitz
With SLifshitzsuch that the background near-horizon metric is ds2 = L2
−h(v)dt2
v2z + dv2
v2h(v) +dx2+ dy2 v2
with a temperature T = |h4πv0(vz−1h)|
h and h(vh) = 0.
If we dene
τ = χ + ie−φ,
this action has the familiar SL(2, R) symmetry found in string theory,
τ → τ0 = aτ + b
cτ + d |ad − bc| = 1
Outline Motivation Quantum Hall Eect The Model Conclusion
Quantum Hall-ography - The Model
Background bulk action:
Sgrav= − Z
d4x√
−g
1 2κ2
R − 6
L2 + 1 2
∂µφ∂µφ + e2φ∂µχ∂µχ
+ SLifshitz
With SLifshitzsuch that the background near-horizon metric is ds2 = L2
−h(v)dt2
v2z + dv2
v2h(v) +dx2+ dy2 v2
with a temperature T = |h4πv0(vz−1h)|
h and h(vh) = 0. If we dene
τ = χ + ie−φ,
this action has the familiar SL(2, R) symmetry found in string theory,
τ → τ0 = aτ + b
cτ + d |ad − bc| = 1
Outline Motivation Quantum Hall Eect The Model Conclusion
Quantum Hall-ography - The Model
Background bulk action:
Sgrav= − Z
d4x√
−g
1 2κ2
R − 6
L2 + 1 2
∂µφ∂µφ + e2φ∂µχ∂µχ
+ SLifshitz
With SLifshitzsuch that the background near-horizon metric is ds2 = L2
−h(v)dt2
v2z + dv2
v2h(v) +dx2+ dy2 v2
with a temperature T = |h4πv0(vz−1h)|
h and h(vh) = 0. If we dene
τ = χ + ie−φ,
this action has the familiar SL(2, R) symmetry found in string theory,
τ → τ0 = aτ + b
cτ + d |ad − bc| = 1
Outline Motivation Quantum Hall Eect The Model Conclusion
Quantum Hall-ography - The Model
Probe-brane action:
Sgauge= − T Z
d4x
q
−det gµν+ `2e−φ/2Fµν −√
−g
−1 4
Z d4x√
−gχFµνF˜µν
We use the standard AdS/CFT dictionary to calculate the current (and therefore conductivity)
Ja= δSgauge δAa|v=0,
which gives us an ohmic conductivity of σxx ∝ T−2/z at low temperatures.
Outline Motivation Quantum Hall Eect The Model Conclusion
Conductivity plane
So have we reproduced the quantum hall eect?
Outline Motivation Quantum Hall Eect The Model Conclusion
Conductivity plane
So have we reproduced the quantum hall eect?
From this, the only plateaux-like behaviour is for σ = τ. But for Imτ → 0 this corresponds to strong coupling, where we expect SL(2, R)to be broken.
Outline Motivation Quantum Hall Eect The Model Conclusion
Conductivity plane
So have we reproduced the quantum hall eect?
However, we can map areas of weak coupling to regions of strong coupling by the surviving SL(2, R) symmetry (or one of the sub- groups). Here we can see plateaux-like behaviour.
Outline Motivation Quantum Hall Eect The Model Conclusion
A Bonus Gift!
From dimensional arguments, the conductivity will be of the functional form
σ = σ
ρ T2/z, B
T2/z
,
which we can rewrite (for small changes in lling fraction) as
σ ' σ
∆ν, ∆B T2/z
Experimentally, the exponent of T measured experimentally (∼ 0.42) closely corresponds to a dynamical critical exponent of z = 5, which is the exponent corresponding to our `probe' system being the background!
Outline Motivation Quantum Hall Eect The Model Conclusion
A Bonus Gift!
From dimensional arguments, the conductivity will be of the functional form
σ = σ
ρ T2/z, B
T2/z
,
which we can rewrite (for small changes in lling fraction) as
σ ' σ
∆ν, ∆B T2/z
Experimentally, the exponent of T measured experimentally (∼ 0.42) closely corresponds to a dynamical critical exponent of z = 5, which is the exponent corresponding to our `probe' system being the background!
Outline Motivation Quantum Hall Eect The Model Conclusion
A Bonus Gift!
From dimensional arguments, the conductivity will be of the functional form
σ = σ
ρ T2/z, B
T2/z
,
which we can rewrite (for small changes in lling fraction) as
σ ' σ
∆ν, ∆B T2/z
Experimentally, the exponent of T measured experimentally (∼ 0.42) closely corresponds to a dynamical critical exponent of z = 5, which is the exponent corresponding to our `probe' system being the background!
Outline Motivation Quantum Hall Eect The Model Conclusion
SL(2, R) - From Bulk Fields to Boundary Conductivities
Dene
Gµν = −2 δS δFµν. SL(2, R)acts as
Gµν →aGµν− b ˜Fµν
Fµν →c ˜Gµν+ dFµν τ →aτ + b
cτ + d
AdS/CFT says
√−gGva|v=0= Ja. Transforming the conductivity equation
Ja= σabEb, by SL(2, R) induces
σ → aσ + b cσ + d.
Any SL(2, R) invariant theory (with the symmetry group acting on the Maxwell eld) has an SL(2, R) invariant boundary conductivity!
Outline Motivation Quantum Hall Eect The Model Conclusion
SL(2, R) - From Bulk Fields to Boundary Conductivities
Dene
Gµν = −2 δS δFµν. SL(2, R)acts as
Gµν →aGµν− b ˜Fµν
Fµν →c ˜Gµν+ dFµν τ →aτ + b
cτ + d
AdS/CFT says
√−gGva|v=0= Ja. Transforming the conductivity equation
Ja = σabEb, by SL(2, R) induces
σ → aσ + b cσ + d.
Any SL(2, R) invariant theory (with the symmetry group acting on the Maxwell eld) has an SL(2, R) invariant boundary conductivity!
Outline Motivation Quantum Hall Eect The Model Conclusion
SL(2, R) - From Bulk Fields to Boundary Conductivities
Dene
Gµν = −2 δS δFµν. SL(2, R)acts as
Gµν →aGµν− b ˜Fµν
Fµν →c ˜Gµν+ dFµν τ →aτ + b
cτ + d
AdS/CFT says
√−gGva|v=0= Ja. Transforming the conductivity equation
Ja = σabEb, by SL(2, R) induces
σ → aσ + b cσ + d.
Any SL(2, R) invariant theory (with the symmetry group acting on the Maxwell eld) has an SL(2, R) invariant boundary conductivity!
Outline Motivation Quantum Hall Eect The Model Conclusion
Summary
• AdS/CFT is an emerging tool to compute strongly correlated electron systems in condensed matter.
• One system in condensed matter that has resisted
theoretical tools is the fractional quantum hall eect. Every reason to try to apply hall-ography.
• We have found a plausible model in AdS/CFT that captures some essential features of the quantum hall eect as well as a possible experimental prediction.
• Even if this is the wrong model, there's recourse in the very general arguments with SL(2, R) as a symmetry that should be present in any QHE model.
Outline Motivation Quantum Hall Eect The Model Conclusion
Summary
• AdS/CFT is an emerging tool to compute strongly correlated electron systems in condensed matter.
• One system in condensed matter that has resisted
theoretical tools is the fractional quantum hall eect. Every reason to try to apply hall-ography.
• We have found a plausible model in AdS/CFT that captures some essential features of the quantum hall eect as well as a possible experimental prediction.
• Even if this is the wrong model, there's recourse in the very general arguments with SL(2, R) as a symmetry that should be present in any QHE model.
Outline Motivation Quantum Hall Eect The Model Conclusion
Summary
• AdS/CFT is an emerging tool to compute strongly correlated electron systems in condensed matter.
• One system in condensed matter that has resisted
theoretical tools is the fractional quantum hall eect. Every reason to try to apply hall-ography.
• We have found a plausible model in AdS/CFT that captures some essential features of the quantum hall eect as well as a possible experimental prediction.
• Even if this is the wrong model, there's recourse in the very general arguments with SL(2, R) as a symmetry that should be present in any QHE model.
Outline Motivation Quantum Hall Eect The Model Conclusion
Summary
• AdS/CFT is an emerging tool to compute strongly correlated electron systems in condensed matter.
• One system in condensed matter that has resisted
theoretical tools is the fractional quantum hall eect. Every reason to try to apply hall-ography.
• We have found a plausible model in AdS/CFT that captures some essential features of the quantum hall eect as well as a possible experimental prediction.
• Even if this is the wrong model, there's recourse in the very general arguments with SL(2, R) as a symmetry that should be present in any QHE model.
Outline Motivation Quantum Hall Eect The Model Conclusion
Acknowledgements
Cli Burgess Brian Dolan Sung-Sik Lee In addition to several people in the condensed
matter-holography community that have contributed great reviews on the subject (Sean Hartnoll, Joe Polchinski, John McGreevy, Gary Horowitz, etc.)