topological quantum matter, 7.5 ectsphysics.gu.se/~tfkhj/topo/topologicalquantummatter.pdf ·...
TRANSCRIPT
Topological Quantum Matter, 7.5 ECTSphysics.gu.se/~tfkhj/topomatter.html
Part 1: 10 double hour lectures 12/9 - 3/10, 27/10 - 31/10 (tentatively)Part 2: 5 guest lectures 10/11 - 12/12
Examination: homework problems on part 1, project based on one of the guest lectures
Literature: Lecture notes downloadable from the course homepage.Additional text/references will be made available during the course.
Masters students: To get credit for the course, please contactBengt-Erik Mellander, [email protected], after completion ofthe examination.
Concepts in Topological Quantum Matter, 4.5 ECTSphysics.gu.se/~tfkhj/topomatter/
Part 1: 10 double hour lectures 12/9 - 3/10, 27/10 - 31/10 (tentatively)Part 2: 5 guest lectures 10/11 - 12/12
Examination: homework problems on part 1, project based on one of the guest lectures
Literature: Lecture notes downloadable from the course homepage.Additional text/references will be made available during the course.
Masters students: To get credit for the course, please contactBengt-Erik Mellander, [email protected], after completion ofthe examination.
What is topological quantum matter & why do we care?
Before the era of topology in physics...
Standard paradigm of emergent order:spontaneous symmetry breaking
What is topological quantum matter & why do we care?
Some examples:
Crystals break the translational and rotational symmetry of free space
What is topological quantum matter & why do we care?
Crystals
Liquid crystals break rotational but not translational symmetry
What is topological quantum matter & why do we care?
Crystals
Liquid crystals
Magnets break time-reversal symmetry and the rotational symmetry of spin space
What is topological quantum matter & why do we care?
Crystals
Liquid crystals
Magnets
Superconductors break a gauge symmetry
What is topological quantum matter & why do we care?
Crystals
Liquid crystals
Magnets
Superconductors
... and many more examples
What is topological quantum matter & why do we care?
Crystals
Liquid crystals
Magnets
Superconductors
... and many more examples
At high temperature, entropy dominates and leads to a disordered state.
At low temperature, energy dominates and leads to an ordered state.
Lev Landau http://www-groups.dcs.st-and.ac.uk/~history/PictDisplay/Landau_Le...
1 av 1 06-01-16 14.40
The ordered state breaks one or more symmetries ”spontaneously”.
Lev Landau
What is topological quantum matter & why do we care?
In 1980, the ”Landau paradigm” was challenged by the discovery of the (integer) quantum Hall effect (von Klitzing et al.)
Electrons confined to a plane and in a strong magnetic field show, at low enough temperature, plateaus in the “Hall conductance”:
to a precision of at least 9 decimal places!
There is no symmetry breaking. What type of ordercauses this precise quantization?
TOPOLOGICAL ORDER!
Topological quantum matter
”Symmetry-protected topological matter”
Unique groundstate protected by atopological invariant (Chern number,Z2-index,...), ordinary electron excitations,short-range quantum entanglement:
integer quantum Hall effect,topological insulators,Chern insulators,topological superconductors,...
Groundstate degeneracies on higher-genus manifolds, fractionalized excitations,long-range quantum entanglement:
fractional quantum Hall effect,quantum spin liquids,...
”Topologically ordered matter” (proper)
Topological quantum matter
”Topologically protected matter”
Unique groundstate protected by atopological invariant (Chern number,Z2-index,...), ordinary electron excitations,short-range quantum entanglement:
integer quantum Hall effect,
topological insulators,Chern insulators,topological superconductors,...
Groundstate degeneracies on higher-genus manifolds, fractionalized excitations,long-range quantum entanglement:
fractional quantum Hall effect,quantum spin liquids,...
”Topologically ordered matter” (proper)
The topological insulatorsprovide the best ”port of entry” to the study of topological quantum matter.This is how we shall go about it in the course!
”An electrical insulator is a material whose internal electric charges do not flow freely... examples include glass, paper, Teflon, plastics, ceramic materials,... ”
?
Ordinary insulators have nothing to do with topology, but topological insulators do!?
2D topological insulators... taking off from the quantum Hall effect
no channel for backscattering
B“skipping current”
quantization
chiral edge states
perfect conductance along the edge
2D topological insulators... taking off from quantum Hall effect
B
perfect conductance along the edge von Klitzing et al., PRL (1980)
2D topological insulators... taking off from quantum Hall effect
Is this kind of physics possible without a magnetic field?
a bulk insulator with perfectlyconducting edge states
Duncan Haldane
Well..., at least one doesn’t need anet magnetic field... PRL, 1988
Charlie Kane Gene Mele
In fact, one can do away with themagnetic field altogether! PRLs, 2005
To see how this is possible, consider a Gedanken experiment... Bernevig & Zhang, PRL (2006)
spin-orbit interaction
1
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
H =
�dx [Hc +Hs]
Hi =vi
2[(∂xϕi)
2+(∂xϑi)
2]−
mi
πacos(
�2πKiϕi), (1)
∆R = γ1 sin(q0a)
with vi and Ki functions of g1τ , g2τ and g4τ
Λ ∼ bandwidth (2)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (3)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (4)
R†τ and L
†τ create excitations at the Fermi points of the
right- and left-moving branches with spin projection τ
(5)
L+
vF = 2a
�t2 + γ
20 and ∆R = γ1 sin(q0a)
Hτ =−ivF
�:R
†τ (x)∂xRτ (x) ::L
†τ (x)∂xLτ (x) :
�
−2∆R cos(Qx)�e−2ik0
F (x+a/2)R
†τ (x)Lτ (x)+H.c.
�,(6)
τ = ±
q0
HR =−i
�
n,µ,ν
(γ0 + γ1 cos (Qna))
�c†n,µσ
yµνcn+1,ν−H.c.
�
γj = αja−1
(j = 0, 1)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (7)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (8)
Hint = g1− :R†τLτL
†−τR−τ : + g2τ :R
†+R+L
†τLτ :
+g4τ
2(:R
†+R+R
†τRτ : +R↔ L) (9)
g2τ ≡g2τ − δτ+g1τ (10)
with vi and K functions of g1τ , g2τ , g3τ (11)
compare with an integer quantum Hall system
Lorentz force
1
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
H =
�dx [Hc +Hs]
Hi =vi
2[(∂xϕi)
2+(∂xϑi)
2]−
mi
πacos(
�2πKiϕi), (1)
∆R = γ1 sin(q0a)
with vi and Ki functions of g1τ , g2τ and g4τ
Λ ∼ bandwidth (2)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (3)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (4)
R†τ and L
†τ create excitations at the Fermi points of the
right- and left-moving branches with spin projection τ
(5)
L+
vF = 2a
�t2 + γ
20 and ∆R = γ1 sin(q0a)
Hτ =−ivF
�:R
†τ (x)∂xRτ (x) ::L
†τ (x)∂xLτ (x) :
�
−2∆R cos(Qx)�e−2ik0
F (x+a/2)R
†τ (x)Lτ (x)+H.c.
�,(6)
τ = ±
q0
HR =−i
�
n,µ,ν
(γ0 + γ1 cos (Qna))
�c†n,µσ
yµνcn+1,ν−H.c.
�
γj = αja−1
(j = 0, 1)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (7)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (8)
Hint = g1− :R†τLτL
†−τR−τ : + g2τ :R
†+R+L
†τLτ :
+g4τ
2(:R
†+R+R
†τRτ : +R↔ L) (9)
g2τ ≡g2τ − δτ+g1τ (10)
with vi and K functions of g1τ , g2τ , g3τ (11)
1
B = ∇×A
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
H =
�dx [Hc +Hs]
Hi =vi
2[(∂xϕi)
2+(∂xϑi)
2]−
mi
πacos(
�2πKiϕi), (1)
∆R = γ1 sin(q0a)
with vi and Ki functions of g1τ , g2τ and g4τ
Λ ∼ bandwidth (2)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (3)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (4)
R†τ and L
†τ create excitations at the Fermi points of the
right- and left-moving branches with spin projection τ
(5)
L+
vF = 2a
�t2 + γ
20 and ∆R = γ1 sin(q0a)
Hτ =−ivF
�:R
†τ (x)∂xRτ (x) ::L
†τ (x)∂xLτ (x) :
�
−2∆R cos(Qx)�e−2ik0
F (x+a/2)R
†τ (x)Lτ (x)+H.c.
�,(6)
τ = ±
q0
HR =−i
�
n,µ,ν
(γ0 + γ1 cos (Qna))
�c†n,µσ
yµνcn+1,ν−H.c.
�
γj = αja−1
(j = 0, 1)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (7)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (8)
Hint = g1− :R†τLτL
†−τR−τ : + g2τ :R
†+R+L
†τLτ :
+g4τ
2(:R
†+R+R
†τRτ : +R↔ L) (9)
g2τ ≡g2τ − δτ+g1τ (10)
To see how this is possible, consider a Gedanken experiment...
spin-orbit interaction
1
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
H =
�dx [Hc +Hs]
Hi =vi
2[(∂xϕi)
2+(∂xϑi)
2]−
mi
πacos(
�2πKiϕi), (1)
∆R = γ1 sin(q0a)
with vi and Ki functions of g1τ , g2τ and g4τ
Λ ∼ bandwidth (2)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (3)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (4)
R†τ and L
†τ create excitations at the Fermi points of the
right- and left-moving branches with spin projection τ
(5)
L+
vF = 2a
�t2 + γ
20 and ∆R = γ1 sin(q0a)
Hτ =−ivF
�:R
†τ (x)∂xRτ (x) ::L
†τ (x)∂xLτ (x) :
�
−2∆R cos(Qx)�e−2ik0
F (x+a/2)R
†τ (x)Lτ (x)+H.c.
�,(6)
τ = ±
q0
HR =−i
�
n,µ,ν
(γ0 + γ1 cos (Qna))
�c†n,µσ
yµνcn+1,ν−H.c.
�
γj = αja−1
(j = 0, 1)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (7)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (8)
Hint = g1− :R†τLτL
†−τR−τ : + g2τ :R
†+R+L
†τLτ :
+g4τ
2(:R
†+R+R
†τRτ : +R↔ L) (9)
g2τ ≡g2τ − δτ+g1τ (10)
with vi and K functions of g1τ , g2τ , g3τ (11)
compare with an integer quantum Hall system
Lorentz force
1
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
H =
�dx [Hc +Hs]
Hi =vi
2[(∂xϕi)
2+(∂xϑi)
2]−
mi
πacos(
�2πKiϕi), (1)
∆R = γ1 sin(q0a)
with vi and Ki functions of g1τ , g2τ and g4τ
Λ ∼ bandwidth (2)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (3)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (4)
R†τ and L
†τ create excitations at the Fermi points of the
right- and left-moving branches with spin projection τ
(5)
L+
vF = 2a
�t2 + γ
20 and ∆R = γ1 sin(q0a)
Hτ =−ivF
�:R
†τ (x)∂xRτ (x) ::L
†τ (x)∂xLτ (x) :
�
−2∆R cos(Qx)�e−2ik0
F (x+a/2)R
†τ (x)Lτ (x)+H.c.
�,(6)
τ = ±
q0
HR =−i
�
n,µ,ν
(γ0 + γ1 cos (Qna))
�c†n,µσ
yµνcn+1,ν−H.c.
�
γj = αja−1
(j = 0, 1)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (7)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (8)
Hint = g1− :R†τLτL
†−τR−τ : + g2τ :R
†+R+L
†τLτ :
+g4τ
2(:R
†+R+R
†τRτ : +R↔ L) (9)
g2τ ≡g2τ − δτ+g1τ (10)
with vi and K functions of g1τ , g2τ , g3τ (11)
1
B = ∇×A
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
H =
�dx [Hc +Hs]
Hi =vi
2[(∂xϕi)
2+(∂xϑi)
2]−
mi
πacos(
�2πKiϕi), (1)
∆R = γ1 sin(q0a)
with vi and Ki functions of g1τ , g2τ and g4τ
Λ ∼ bandwidth (2)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (3)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (4)
R†τ and L
†τ create excitations at the Fermi points of the
right- and left-moving branches with spin projection τ
(5)
L+
vF = 2a
�t2 + γ
20 and ∆R = γ1 sin(q0a)
Hτ =−ivF
�:R
†τ (x)∂xRτ (x) ::L
†τ (x)∂xLτ (x) :
�
−2∆R cos(Qx)�e−2ik0
F (x+a/2)R
†τ (x)Lτ (x)+H.c.
�,(6)
τ = ±
q0
HR =−i
�
n,µ,ν
(γ0 + γ1 cos (Qna))
�c†n,µσ
yµνcn+1,ν−H.c.
�
γj = αja−1
(j = 0, 1)
ϕc ≡ (ϕ+ + ϕ−)/
√2 (7)
ϕs ≡ (ϕ+ − ϕ−)/
√2 (8)
Hint = g1− :R†τLτL
†−τR−τ : + g2τ :R
†+R+L
†τLτ :
+g4τ
2(:R
†+R+R
†τRτ : +R↔ L) (9)
g2τ ≡g2τ − δτ+g1τ (10)
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To see how this is possible, consider a Gedanken experiment...
| >
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Two copies of a quantum Hall system, bulk insulator with helical edge states
2D topological insulator
single Kramers pair
x
How does Nature do it?Also by spin-orbit interactions!
x
”s-band”
”p-band”
EF
ordinary insulator
local band structure
x
How does Nature do it?Strong atomic spin-orbit interactions!
x
topological insulatorordinary insulator ordinary insulator
EF
”s-band”
”p-band”
conducting edge stateslocal band structure
x
How does Nature do it?Strong atomic spin-orbit interactions!
x
topological insulator
EFconducting edge states
local band structure
First proposed by Kane and Mele for graphene (2005)C. L. Kane and E. J. Mele, PRL 95, 226801 (2005)
Bernevig et al. proposal for HgTe quantum wells (2006)B. A. Bernevig, T. A. Hughes, and S. C. Zhang, Science 314, 1757 (2006)
Experimental observation by König et al. (2007)M. König et al., Science 318, 766 (2007)
Experimental realizations...
The helical edge states are stable against time-reversal invariant perturbations,no disorder backscattering
edge states
HgTe quantum well | >–
+| >
1
G =2e
2
h
L ∼ 1µm
L ∼ 20µm
g
g ∼ 1/2v
ξ < L
α(±2kF )
±2kF
z⊥ ≡ 2g
√CK
z� ≡ 4K − 2
γ = (α(2kF ) + α(−2kF ))/2
∂xθ
ψ↑ = η↑ exp�i√
π[φ + θ]�/
√2πκ
ψ↓ = η↓ exp�−i√
π[φ− θ]�/
√2πκ
H0 + Hd + Hf + HR
O(α2)
α(x) = �α(x)�+
�
n
α(kn)eikxn
Ψ↑↓(x)=ψ↑↓(x)e±ikF x
Ψ↓(x)=ψ↓(x)e−ikF x
HR = α(x)(z × k) · σ
B = ∇×A
E = E(x, y, 0)
(E × k) · σ = Eσz(kyx− kxy)
A =B
2(y,−x, 0)
A · k ∼ eB(kyx− kxy)
G = νe2
h
Mc ≈ 100 meV
D. Grundler, Phys. Rev. Lett. 84, 6074 (2000)
W. Hausler, L. Kecke, and A. H. MacDonald, Phys.
Rev. B 65, 085104 (2002)
�α0 � 2× 10−11
eV m
Λ � 0.5 eV
vF � 1× 106
m/s
Kc + Ks � 1.8
HR
ballistic transport
”symmetry
-protecte
d”
topologic
al phase
König et al. (2007)
normal band gap
large samples(inelastic scatteringfrom the bulk)
Fermi level notinside the inverted gap
What is ”topological” about a topological insulator?
Warm up... Gauss-Bonnet theorem
1
1
2π
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
Pierre Ossian Bonnet, 1819-1892
1
�
MKdS = 2(1− g)
K = κmin · κmax
1
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
What is ”topological” about a topological insulator?
Warm up... Gauss-Bonnet theorem
1
1
2π
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
1
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
1
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
1
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
1
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
1
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
What is ”topological” about a topological insulator? 1
1
2π
�
MKdS = 2(1− g)
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
Generalized Gauss-Bonnet theorem
”Chern theorem”
Berry curvature
Chern numberShiing-Shen Chern, 1911-2004
What is ”topological” about a topological insulator?
1
H = −t
�
n,α
c†n,αcn+1,α +
µ
2
�
n,α
c†n,αcn,α −i
�
n,α,β
c†n,α
�γD σx
αβ+γR σyαβ
�cn+1,β (1)
− iγ�R
�
n,α,β
cos(Qna)c†n,ασyαβcn+1,β +
µ�
2
�
n,α
cos(Qna)c†n,αcn,α (2)
+�
n,n�;α,β
V (n− n�)c†n,αc
†n�,βcn�,βcn,α + h. c. (3)
SA(n,N)− SA(n− 1, N − 1) = Simp
whereJK = J1 = J2 = Kondo couplingn = number of sites in A incl. the impurity siteN = total number of sites in A+B
HRashba = αkxσy
EF
HZeeman = (µBgB/2)σx
SA(JK , n,NL, NR)− SA(1, n− 1, NL, NR) = Simp
whereNL and NR are the number of sites in the left and right blue block, respectively
1
2π
�
MF · dS = C
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
Berry curvature
Chern number
un,k(r) = � r |un,k �
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
band index
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Brillouin zone (BZ)
Electron wave function in a crystal
1
H = −t
�
n,α
c†n,αcn+1,α +
µ
2
�
n,α
c†n,αcn,α −i
�
n,α,β
c†n,α
�γD σx
αβ+γR σyαβ
�cn+1,β (1)
− iγ�R
�
n,α,β
cos(Qna)c†n,ασyαβcn+1,β +
µ�
2
�
n,α
cos(Qna)c†n,αcn,α (2)
+�
n,n�;α,β
V (n− n�)c†n,αc
†n�,βcn�,βcn,α + h. c. (3)
SA(n,N)− SA(n− 1, N − 1) = Simp
whereJK = J1 = J2 = Kondo couplingn = number of sites in A incl. the impurity siteN = total number of sites in A+B
HRashba = αkxσy
EF
HZeeman = (µBgB/2)σx
SA(JK , n,NL, NR)− SA(1, n− 1, NL, NR) = Simp
whereNL and NR are the number of sites in the left and right blue block, respectively
1
2π
�
MF · dS = C
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
Berry curvature
Chern number
un,k(r) = � r |un,k �
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Electron wave function in a crystal
Michael Berry
1
H = −t
�
n,α
c†n,αcn+1,α +
µ
2
�
n,α
c†n,αcn,α −i
�
n,α,β
c†n,α
�γD σx
αβ+γR σyαβ
�cn+1,β (1)
− iγ�R
�
n,α,β
cos(Qna)c†n,ασyαβcn+1,β +
µ�
2
�
n,α
cos(Qna)c†n,αcn,α (2)
+�
n,n�;α,β
V (n− n�)c†n,αc
†n�,βcn�,βcn,α + h. c. (3)
SA(n,N)− SA(n− 1, N − 1) = Simp
whereJK = J1 = J2 = Kondo couplingn = number of sites in A incl. the impurity siteN = total number of sites in A+B
HRashba = αkxσy
EF
HZeeman = (µBgB/2)σx
SA(JK , n,NL, NR)− SA(1, n− 1, NL, NR) = Simp
whereNL and NR are the number of sites in the left and right blue block, respectively
1
2π
�
MF · dS = C
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
Berry curvature
Chern number
un,k(r) = � r |un,k �
Berry curvature Berry, RSPSA (1984)
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Electron wave function in a crystal
1
H = −t
�
n,α
c†n,αcn+1,α +
µ
2
�
n,α
c†n,αcn,α −i
�
n,α,β
c†n,α
�γD σx
αβ+γR σyαβ
�cn+1,β (1)
− iγ�R
�
n,α,β
cos(Qna)c†n,ασyαβcn+1,β +
µ�
2
�
n,α
cos(Qna)c†n,αcn,α (2)
+�
n,n�;α,β
V (n− n�)c†n,αc
†n�,βcn�,βcn,α + h. c. (3)
SA(n,N)− SA(n− 1, N − 1) = Simp
whereJK = J1 = J2 = Kondo couplingn = number of sites in A incl. the impurity siteN = total number of sites in A+B
HRashba = αkxσy
EF
HZeeman = (µBgB/2)σx
SA(JK , n,NL, NR)− SA(1, n− 1, NL, NR) = Simp
whereNL and NR are the number of sites in the left and right blue block, respectively
1
2π
�
MF · dS = C
K = κmin · κmax
radius =1
κ
g = 0 g = 1 g = 2 g = 3
⊂ M
Berry curvature
Chern number
un,k(r) = � r |un,k �
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n=1
� filled bands
�
M=BZFn(k) · dk = C, C ∈ Z
1
2π
� filled bands�
n=1
�
M=BZFn,z(k) dkx dky = C
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Berry curvature Berry, RSPSA (1984)
vanishing Chern number
odd undertime reversal
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Electron wave function in a crystal
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n=1
� filled bands
�
M=BZFn(k) · dk = C, C ∈ Z
1
2π
� filled bands�
n=1
�
M=BZFn,z(k) dkx dky = C
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Berry curvature Berry, RSPSA (1984)
odd undertime reversal
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Electron wave function in a crystal
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n=1
� filled bands
�
M=BZFn(k) · dk = C, C ∈ Z
1
2π
� filled bands�
n=1
�
M=BZFn,z(k) dkx dky = C
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
broken time-reversal
symmetry (like in the
quantum Hall effect)
David Thouless
integer quantum Hall effect: ”TKNN invariant”Thouless et al., PRL (1982)
Berry curvature Berry, RSPSA (1984)
odd undertime reversal
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
EBZFn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Electron wave function in a crystal
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n=1
� filled bands
�
M=BZFn(k) · dk = C, C ∈ Z
1
2π
� filled bands�
n=1
�
M=BZFn,z(k) dkx dky = C
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
Berry curvature Berry, RSPSA (1984)
What if we identify time-reversed points in the BZ?
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = ∇k ×An(k)
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
The parity of the Chern number is unique!
1
ψn,k(r) = un,k(r)eik·r
Γ A B C
An(k) = −i�un,k | ∇k | un,k�
Fn(k) = −i∇k × �un,k | ∇k | un,k�
1
2π
�
n=1
�
M=BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�
BZFn(k) · dk = C, C ∈ Z
1
2π
�
n
�Fn(k) · dk = C
C =
�0 mod 2 ordinary insulator1 mod 2 topological insulator
”Z2 topological invariant”,counts the number of Kramers pairs at the edge of the topological insulator(bulk-boundary correspondence)
Kane & Mele, PRL (2005)
Why all the hoopla?
https://www.youtube.com/watch?v=HBuLMrzgbgM
• Future electronics/spintronics:
• New physics in hybrid structures: Majorana fermions, magnetic monopoles, ”dyons”,...
• An inroad to the general study of topological quantum matter
Why all the hoopla?
An inroad to the general study of topological quantum matter...
2D topological insulator
”Periodic tables” for symmetry-protected topological matter:
Now for some serious work on the blackboard!