Holographic Entanglement Entropy from Cond -mat to Emergent Spacetime
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Transcript of Holographic Entanglement Entropy from Cond -mat to Emergent Spacetime
Holographic Entanglement Entropy
from Cond-mat to Emergent Spacetime
Tadashi Takayanagi
Yukawa Institute for Theoretical Physics (YITP), Kyoto University
Based on arXiv 1111.1023(JHEP 2012(125)1) with N. Ogawa (RIKEN) and T. Ugajin (IPMU/YITP) + on going work with M.Nozaki (YITP) and S.Ryu (Ilinois,Urbana-S.)
Gravity Theories and Their Avatars @CCTP, Crete July 13-19, 2012
Contents
① Introduction ② Entanglement Entropy and Fermi surfaces③ Holographic Entanglement Entropy (HEE)④ Fermi Surfaces and HEE ⑤ Emergent Metric via Quantum Entanglement⑥ Conclusions
① Introduction– from cond-mat viewpoint
AdS/CFT is a very powerful method to understand strongly coupled condensed matter systems.
Especially, the calculations become most tractable in the strong coupling and large N limit of gauge theories.
In this limit, the AdS side is given by a classical gravity and we can naturally expect universal behaviors such as the no hair theorem in GR, η/s=1/4π, etc.
So, we concentrate on this limit for a while.
We would like to consider what is a universal properties for metallic condensed matter systems via AdS/CFT.
The metals are usually described by the Landau’s Fermi liquids. It is well-known that Fermi liquid states are stable against perturbations by the Coulomb forces.
However, in strongly correlated electron systems such as the strange metal phase of high Tc superconductors or heavy fermion systems etc., we encounter so called non-Fermi liquids.
Pseudo Gap
Strange Metal
Fermi Liquid
Mott Insulator
SC
Figs taken from Sachdev 0907.0008
So, one of the main purposes of this talk is to answer the question:
Can we obtain Landau’s Fermi liquids in the classical gravity limit ?
Note: Several interesting setups of (non-)Fermi liquids have already been found.
(i) Probe fermions in SUGRA b.g.
(ii) Electron stars (Lifshitz metric in the IR) ⇒ ∃ Fermi surfaces, but, not in the leading order O(N2) of the large N limit.
[Hartnoll-Tavanfar 2010]
[Rey 07, Faulkner-Liu-McGreevy-Vegh 09, Cubrovic-Zaanen-Schalm 09, DeWolfe-Gubser-Rosen 11, 12]
Systems with Fermi surfaces ⇔ Fermi liquids or non-Fermi liquids
So, we concentrate on systems with Fermi surfaces.
To make the presentation simpler, we will work for 2+1 dim. systems with Fermi surfaces. But our analysis can be generalized to higher dimensions, straightforwardly.
xk
ykFermi surface
Fermi sea
How to characterize the Fermi surfaces ?
Metals Conductivity ? ⇒
But it seems difficult to find universal results for conductivity in the gravity dual. This is because it is related to the propagation of U(1) gauge fields in AdS, whose behavior largely depends on the precise Lagrangian of gauge fields e.g. f(φ)F2 .
So we want to find a quantity whose gravity dual is closely related to the metric (i.e. gravity field). ⇒ We should look at a thermodynamical quantity !
One traditional candidate is the specific heat C.
For (Landau’s) fermi liquids, we always have the behavior
This linear specific heat can be understood if we note that we can approximate the excitations of Fermi liquids by an infinite copies of 2 dim. CFTs.
.VTSC
|| k
EFermi surface
Fk
FFk
kE
k2
//
)(CFT FL
//k
k
Fermi surfacexk
ykFermi sea
In 2d CFT, we know
In this way, we can estimate the specific heat of the Fermi liquids
However, the linear specific heat is not true for non-Fermi liquids.
This is because they have anomalous dynamical exponents z. (~infinite copies of 2d Lifshitz theory: )
.LTSC
.)( VTkLkLTSC FF
./1 VTSC z
),(~),( xtxt z
To characterize the existence of Fermi surfaces, we need to look at a property which is common to both the FL and non-FL.
⇒ The entanglement entropy is a suitable quantity.
After we concentrate on the systems with Fermi surfaces, we can distinguish between FL and non-FL by calculating the specific heat.
Divide a quantum system into two subsystems A and B:
We define the reduced density matrix for A by
taking trace over the Hilbert space of B .
A
, Tr totBA
. BAtot HHH
A BExample: Spin Chain
② Entanglement Entropy and Fermi surfaces (2-1) Definition and Properties of Entanglement Entropy
Now the entanglement entropy is defined by the von-Neumann entropy
In QFTs, it is defined geometrically:
. log Tr AAAAS
AS
BA B A
slice time:N
(2-2) Area law
EE in QFTs includes UV divergences.
In a (d+1 ) dim. QFT with a UV fixed point, the leading term of EE is proportional to the area of the (d-1) dim. boundary :
where is a UV cutoff (i.e. lattice spacing).
Intuitively, this property is understood like:
Most strongly entangled
terms),subleading(A)Area(~ 1 dA a
S
A
a
[Bombelli-Koul-Lee-Sorkin 86, Srednicki 93]
A∂A
Area Law
However, there are two known exceptions:
(a) 1+1 dim. CFT
(b) Fermi surfaces ( )∃ 1~ akF
.log3 a
lcSA
...log~1
al
alS
d
AlA
B A B
l
[Holzhey-Larsen-Wilczek 94, Calabrese-Cardy 04]
[Wolf 05, Gioev-Klich 05]
(2-3) Fermi Surfaces and Entanglement Entropy
Why do Fermi Liquids violate the area law ?
This can be understood if we remember that the Fermi liquids can be though of as infinite copies of 2d CFTs:
We will mainly assume this choice of subsystem A below.
CFT 2d ofEach CFTs 2d of #
1
.log~al
aLS
d
A
1dL B A B
l
Recently, there have been evidences that this logarithmic behavior is true also for non-Fermi liquids (e.g.spin
liquids). [Swingle 09,10, Zhang-Grover-Vishwanath 11 etc.]
Intuitively, we can naturally expect this because the logarithmic behavior does not change if we introduce the dynamical exponent z in the 2d theory as .
Therefore we find the characterization:
∃Fermi surface ⇔ Logarithmic behavior of EE
lzl z loglog
.)log()( 11
1
Fd
Fd
d
A klkLaLS
To apply the AdS/CFT, we will embed the Fermi surface in a CFT.In this case, the leading divergence still satisfies the area law.But the subleading finite term has the logarithmic behavior:
if we assume .
So, we will concentrate on the gravity dual whose entanglement entropy has this behavior in our arguments below.
1dL B A B
l
1 Fkl
Holographic Entanglement Entropy Formula [Ryu-TT 06]
is the minimal area surface (codim.=2) such that
homologous
. ~ and AA AA
N
AA 4G
)Area(S
2dAdS z
A
)direction. timeomit the (We
③ Holographic Entanglement Entropy
BA
1dCFT
. 2
1
1222
22
zdzdxdt
Rdsd
i iAdSAdS
off)cut (UV
az
• In spite of a heuristic argument [Fursaev, 06] , there has been no complete proof. But, so many evidences and no counter examples.
[A Partial List of Evidences]
Area law follows straightforwardly [Ryu-TT 06] Agreements with analytical 2d CFT results for AdS3 [Ryu-TT 06] Holographic proof of strong subadditivity [Headrick-TT 07] Consistency of 2d CFT results for disconnected subsystems [Calabrese-Cardy-Tonni 09] with our holographic formula [Headrick 10]
Agreement on the coefficient of log term in 4d CFT (~a+c) [Ryu-TT 06, Solodukhin 08,10, Lohmayer-Neuberger-Schwimmer-Theisen 09, Dowker 10, Casini-Huerta, 10, Myers-Sinha 10] A direct proof when A = round ball [Casini-Hueta-Myers 11] Holographic proof of Cadney-Linden-Winter inequality [Hayden-Headrick-Maloney 11]
④ Fermi Surfaces and HEE [Ogawa-Ugajin-TT 11]
(4-1) Setup of gravity dual
For simplicity, we consider a general gravity dual of 2+1 dim. systems. The general metric can be written as follows (up to diff.)
where f(z) and g(z) are arbitrary functions.We impose that it is asymptotically AdS4 i.e.
,)()( 22222
22 dydxdzzgdtzf
zRds
.0n whe1)( and 1)( zzgzf
(4-2) Holographic EE
Now we would like to calculate the HEE for this gravity dual.We choose the subsystem as the strip width l as before
the minimal surface condition reads
L B A B
l
yx
LylxlyxA 0 ,
22|),(
.)(')(2Area* 2
22
z
azxzg
zdzLR
zAd
S Bd
y Ax
2l
2l
*z.
/1)()(' 4
*42
*
2
zzzg
zzzx
In the end, we obtain
when the size of subsystem A is large . In this case, the minimal surface extends to the IR region deeply.
⇒ The logarithmic behavior of EE is realized just when n=1.
We identify as a characteristic scale of the Fermi energy.Note: f(z) does not affect the HEE and is still arbitrary.
...,2
11
22
nn
FFNn
NA z
lzGLRk
aGLRS
Fzl
Fz
. )( )( Surface Fermi ..2
z
zzzgeiF
(4-3) Null Energy Condition
To have a sensible holographic dual, a necessary condition is known as the null energy condition:
In the IR region, the null energy condition argues
. vector nullany for
021
N
NNRgRNNT
.1 )( ,)( 22 mzzfzzg m
At finite temperature, we expect that the solution is given by a black brane extension of our background:
The `non-extremal factors’ behave near the horizon
From this, we can easily find the behavior of specific heat:
.)(
~)( 2
2222)1(222
zdydx
zhdzdtzhzRds m
.)(~
,)(H
H
H
H
zzz
zhz
zzzh
Hzz
. 22 mTSC
Combined with the null energy condition: , we obtain
Notice that this excludes the Landau’s Fermi liquids (α=1).
In summary, we find that classical gravity duals only allow non-fermi liquids.
Comments: (i) This result might not be so unnatural as the non-Fermi liquids
are expected in strongly correlated systems.(ii) Even in the presence of perturbative higher derivative
corrections, the result does not seem to be changed.
.32 with TC
1m
(iii) Some miracle coincidences ?
AdS: No curvature singularity in the gravity dual ⇒ α=2/3 [11]Shaghoulian
CMT: Spin fluctuations: [Moriya, Hertz, Millis …. 70’-90’]
N Fermions + U(1) gauge: ⇒ α=2/3 (i.e. z=3) [Lee 09, Metlitski, and S. Sachdev 10, Mross-McFreevy-Liu-Senthil 10, Lawler-Barci-Fernandez-Fradkin-Oxman 06]
Experiment: YbRh2(Si1-xGex)2
⇒ α=2/3
Examples of heavy fermions [Pepin 11, talk at KITP]
(iv) We can embed this background in an effective gravity theory:
[Earlier works, Gubser-Rocha 09, Charmousis-Goutéraux-Kim-Kiritsis-Meyer 10,…]
if W and V behave in the large φ limit as follows [Ogawa-Ugajin-TT 11]
)].()(2216
1
VFFWRgdxS d
GEMS N
). (pzzgzzf
eRppz
AW
eR
ppV
p
p
F
p
AdS
2 ,)( ,)(
, )8(
8)(
,4
)3212(2)(
2
)2(23
22
2
22
2
2
(v) This metric can also be regarded as a generalization of Lifshitz backgrounds so that it violates the hyperscaling. [Huijse-Sachdev-Swingle 11, Dong-Harrison-Kachru-Torroba-Wang 12]
.
. /)(
1222)1(2)(
)2(2
zd
d
i izd
d
TSC
dxdrdtrrds
10 ,L ~ : 10
surface Fermi log~ : 1
law Area ofViolation 1 ,L ~ : 11
dqS d-θ
L (L)S d-θ
d q d-Sdd
qA
d-A
qA
`Landscape’ of (d+1) dim. Quantum Phases from HEE
q
A alS
qd-1 d
Area Law (QFTs)
Logarithmic(Fermi surface)
Volume Law (Non-local)
Lower dimensional structure
Not allowed
Power violation ? ( Hol. duals)∃
Can we find examples in realistic systems ?
⑤ Emergent Metric from Quantum Entanglement
(5-1) Basic Outline
In principle, we can obtain a metric from a CFT as follows:
a CFT state Information (~EE) = Minimal Areas ⇒ ⇒ metric
One candidate of such frameworks is so called the entanglement renormalization (MERA) [Vidal 05 (for a review see 0912.1651)] as pointed out by [Swingle 09].
AS )Area( A g
[cf. Emergent gravity: Raamsdonk 09, Lee 09]
(5-2) Tensor Network (TN) [See e.g. Vidal 1106.1082 and references therein]
Recently, there have been remarkable progresses in numerical algorithms for quantum lattice models, based on so called tensor product states.
Basically, people try to find nice variational ansatzs for the ground state wave functions for various spin systems. ⇒ An ansatz is good if it respects the quantum entanglement
of the true ground state.
Ex. Matrix Product State (MPS) [DMRG: White 92,…, Rommer-Ostlund 95,..]
)(M
1 2 n
1 23 n
Spinsnnn
n
MMM 21
,,,21 ,,, ])()()(Tr[
21
. or
,1,2,...,
i
i
Spin chain
MPS and TTN are not good near quantum critical points (CFTs) because their entanglement entropies are too small:
In general,
).~log( log2 CFTAA SLS
1 2 n1 2 3 4 6 7 853 1n
A A
]. of onsIntersectimin[#,log~
int
int
A
A
NNS
A A
(5-3) AdS/CFT and (c)MERA
MERA (Multiscale Entanglement Renormalization Ansatz):An efficient variational ansatz to find CFT ground states have been developed recently. [Vidal 05 (for a review see 0912.1651)].
To respect its large entanglement in a CFT, we add (dis)entanglers.
1 2 3 4 6 7 85 1 2 3 4 6 7 85
Unitary transf.between 2 spins
Calculations of EE in 1+1 dim. MERA
A= an interval (length L)
A
A
CFTs. 2d with agrees log]Bonds[#Min
LS A
L
Llog
0u
1u2u
3u
4u
A conjectued relation to AdS/CFT [Swingle 09]
0u 1u
A A
A
Min[Area]
A
.
,)(Metric 2
22222
2
22
u
u
ezwherez
xddtdzxddteds
2dAdS1dCFT
A
Bonds]Min[#
? Equivalent
)( IRuu
Now, to make the connection to AdS/CFT clearer, we would like to consider the MERA for quantum field theories.
Continuous MERA (cMERA) [Haegeman-Osborne-Verschelde-Verstraete 11]
. AdSon gravity cMERA .dim 1
. ~ scalelength :flowation renormaliz space Real
,Ω])([exp)(
2d
nt)entangleme (nostate IR
entangled)(highly state ground True
u
u
u
u
ezd
e
LsKdsiPuIR
Conjecture
K(s) : disentangler, L: scale transformation
(5-4) Emergent Metric from cMERA
We conjecture that the metric in the extra direction is given by using the idea of the quantum metric (up to a constant c):
Note: The quantum distance between two states is defined by
[Nozaki-Ryu-TT, in preparation]
.Vol
)(||)(1
0
2
2
ue d
iLdu
uu
dk
duueucdug
.|1),(2
2121 d
Comments
(1) The denominator represents the total volume of phase space at energy scale u.
(2) The operation e^{iLdu} removes the coarse-graining procedure so that we can measure the strength of
unitary transformations induced by disentanglers (bonds). ⇒ guu measures the density of bonds.
Consistent with the HEE:
uduuuA egduS
IR
)1(0~
A
B
A
IRuzu log
0UVu
(5-5) Emergent Metric in a (d+1) dim. Free Scalar Theory
Hamiltonian:
Ground state :
Moreover, we introduce the `IR state’ which has no real space entanglement.
)].()()()()([21 22 kkmkkkdkH d
.0ka
.0
).()( 0 i.e.
),()( ,0
A
xx
x
xx
S
xMixMa
xMixMaa
For a free scalar theory, the ground state corresponds to
For the excited states, becomes time-dependent. One might be tempting to guess
Indeed, the previous proposal for guu lead to
|).x|(1-(x) :function offcut a is (x) where
,.).(/)(2
)(ˆ
chaaMkeudkiuK kkud
.)2/1(u) ,0for ( ,/2
1)( 222
2
mMme
es u
u
)(s
222
222 dtgxdedugds tt
u
uuGravity
? |)(| ug ss
Density of bonds
.)( 2uguu
Explicit metric
(i) Massless scalar (E=k)
(ii) Lifshitz scalar (E=kν)
(iii) Massive scalar
222
222 dtgxdedugds tt
u
uuGravity
AdSguu pure the 41
geometry Lifshitz the 4
2
uug
222
2
222
22
2222
4
1
.)/(4
dtgxdmzz
dzds
meeg
tt
u
u
uu
Capped off in the IR
(5-6) Excited states after quantum quenches
m(t)
t
.0
021
,0
021
4/1
22
24/1
2
22
4/1
22
24/1
2
22
iktk
iktk
emk
kk
mkB
emk
kk
mkA
m0
).1||||( ,0)( 22 kkkkkk BAaBaA
Note: There is an phase factor ambiguity of (A,B). ⇒ Different choices of time slices ?
Time dependent metric from the Quantum Quench
t
Light cone
looks like a propagation of gravitational wave.
zzgz
We can also confirm the linear growth of EE: SA t.∝This is consistent with the known CFT (2d) [Calabrese-Cardy 05]and with the HEE results (any dim.).
z
[Arrastia-Aparicio-Lopez 10 , Albash-Johnson 10, Balasubramanian-Bernamonti-de Boer-Copland-Craps- Keski-Vakkuri-Müller-Schäfer-Shigemori-Staessens 10, 11,….]
(5-7) Towards Holographic Dual of Flat Space
If we consider the (almost) flat metric
the corresponding dispersion relation reads
This leads to the highly non-local Hamiltonian: [cf. Li-TT 10]
,222222 uuu
uu egdxedueds
. 21)( k
ku
ekk
kk eEeE
Eku
u
).()(2
xexdxH d
⑤ Conclusions• The entanglement entropy (EE) is a useful bridge between gravity (string theory) and cond-mat physics.
Gravity Entanglement Cond-mat. systems
• Classical gravity duals + Null energy condition ⇒ a constraint on specific heat ⇒ Non-fermi liquids ! • Questions: String theory embeddings of the NFL b.g. ? [See also solutions in Singh 10, Narayan
12]
g AreaAS
.32 with TC
• We studied the conjectured relation between AdS/CFT and MERA. We especially employed the cMERA and proposed a definition of metric in the extra dimension. This metric passes several qualitative tests. Many future problems: how to determine gtt ? ambiguity of choices time slices ? large N and strongly coupling limit ? higher spin holography ? : :