introduction to the quantum hall effect and topological...
TRANSCRIPT
Introduction to the Quantum Hall Effect andTopological Phases
Mark O. Goerbig
Ecole du GDR “Physique Mesoscopique, Cargese, November 2016
Historical Introduction
What is the common point between
• graphene,
• quantum Hall effects
• and topological insulators?
... and what is it?
The 1920ies: Band Theory
• quantum treatment of (non-interacting) electrons in aperiodic lattice
• bands = energy of the electrons as a function of aquasi-momentum
1950-70: Many-Body Theory
• Physical System described by a local order parameter(a) ∆k = 〈ψ†
−k,↑ψ†k,↓〉 (superconductivity)
(b) Mµ(r) =∑
τ,τ ′〈ψ†τ (r)σ
µτ,τ ′ψτ ′(r)〉 (ferromagnetism)
• Ginzburg-Landau theory of second-order phase transitions(1957)
∆ = 0(disordered)
↔ ∆ 6= 0(ordered)
• symmetry breaking(a) broken (gauge) symmetry U(1)(b) broken (rotation) symmetry O(3)
• emergence of (collective) Goldstone modes(a) superfluid mode, with ω ∝ |k|(b) spin waves, with ω ∝ |k|2
The Revolution(s) of the 1980ies
3 essential discoveries:
• integer quantum Hall effect (1980, v. Klitzing, Dorda,Pepper)
• fractional quantum Hall effect (1982, Tsui, Störmer,Gossard)
• high-temperature superconductivity (1986, Bednorz, Müller)
Integer Quantum Hall Effect (I)
8 12 160 4Magnetic Field B (T)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
ρ xy
(h/e
)2
0
0.5
1.0
1.5
2.0
ρΩ
xx(k
)
2/3 3/5
5/9
6/11
7/15
2/53/74/9
5/11
6/13
7/13
8/15
1 2/3 2/
5/7
4/5
3 4/
Vx
VyIx
4/7
5/34/3
8/57/5
123456
Magnetic Field B[T]
[mesurement by J. Smet et al., MPI-Stuttgart]
QHE = plateau in Hall res. & vanishing long. res.
Integer Quantum Hall Effect (II)
Quantised Hall resistance at low temperatures
RH =h
e21
n
h/e2: universal constantn: quantum number (topological invariant)
• result independent of geometric and microscopic details
• quantisation of high precision (> 109)
⇒ resistance standard: RK−90 = 25 812, 807Ω
Fractional Quantum Hall Effect
partially filled Landau level → Coulomb interactions relevant
1983: Laughlin’s N -particle wave function
• no (local) order parameter associated with symmetrybreaking
• no Goldstone modes• quasi-particles with fractional charges and statistics
1990ies : description in terms of topological (Chern-Simons)field theories
The Physics of the New Millenium
• simulation of condensed-matter models with optical lattices(cold atoms)
• 2004 : physics of graphene (2D graphite)
• 2005-07 : topological insulators
Graphene – First 2D Crystal
• honeycomb lattice =two triangular (Barvais) lattices
AB
B
B
e3
e1
2e
band structure
Band Structure and Conduction Properties (Bis)
I II I II
gap
metal (2D)
energy
Fermi
level
momentum
electron metal hole metal
Fermi
levelinsulator (2D)
semimetal (2D) Fermi
level
graphene (undoped)Fermi
level
Fermilevel
energy
density
of states
Topological Insulators
generic form of a two-band Hamiltonian:
H = ǫ0(q)1+∑
j=x,y,z
ǫj(q)σj
• Haldane (1988): anomalous quantum Hall effect → quantumspin Hall effect (QSHE)
• Kane and Mele (2005): graphene with spin-orbit coupling• Bernevig, Hughes, Zhang (2006): prediction of a QSHE in
HgTe/CdTe quantum wells• König et al. (2007): experimental verification of the QSHE
⇒ 3D topological insulators (mostly based on bismuth):surface states ∼ ultra-relativistic massless electrons
Outline of the Classes
Mon : Introduction and Landau quantisation
Tue : Issues of the IQHE; Introduction to the Berry phase
Thu : Simple models for topological insulators
Further Reading
• D. Yoshioka, The Quantum Hall Effect, Springer, Berlin (2002).
• S. M. Girvin, The Quantum Hall Effect: Novel Excitations and BrokenSymmetries, Les Houches Summer School 1998http://arxiv.org/abs/cond-mat/9907002
• G. Murthy and R. Shankar, Rev. Mod. Phys. 75, 1101(2003).http://arxiv.org/abs/cond-mat/0205326
• M. O. Goerbig, Quantum Hall Effectshttp://arxiv.org/abs/0909.1998
• B. A. Bernevig, Topological Insulators and TopologicalSuperconductors, Princeton UP (2013).
1. Introduction To the Integer Quantum Hall
Effect and Materials
Classical Hall Effect (1879)
B
I
longitudinal Hallresistance resistance
C1
C4
C2 C3
C5C6
2D electron gas_ _ _ _ _ _
++ + + ++
Quantum Hall system :2D electrons in a B-field
Hal
l res
ista
nce
magnetic field B
RH(b)
Hall resistance:
RH = B/enel
Drude model (classical stationary equation):
dp
dt= −e
(
E+p
m×B
)
− p
τ= 0
Resistivity and Conductivity
σ0Ex = −enel
mpx −
enel
mpy(ωCτ)
σ0Ey =enel
mpx(ωCτ)−
enel
mpy
Ohm’s law : E = ρj with current density j = −enelv = −enelp/m
⇒ resistivity/conductivity tensor
ρ =1
σ0
(
1 ωCτ−ωCτ 1
)
σ = ρ−1 =σ0
1 + (ωCτ)2
(
1 −ωCτωCτ 1
)
Link with mobility µ = eτ/m: ωCτ = µB
Hall resistivity : ρH = ωCτ/σ0 = B/enel
Shubnikov-de Haas Effect (1930)H
all r
esis
tanc
e
magnetic field B
long
itudi
nal r
esis
tanc
e
Bc
(a)
Den
sity
of s
tate
s
EnergyEF
hωC
(b)
oscillations in longitudinal resistance→ Einstein relations σ0 ∝ ∂nel/∂µ ∝ ρ(ǫF )→ Landau quantisation (into levels ǫn)
σ0 ∝ ρ(ǫF ) ∝∑
n
f(ǫF − ǫn)
Quantum Hall Effect (QHE)
8 12 160 4Magnetic Field B (T)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
ρ xy
(h/e
)2
0
0.5
1.0
1.5
2.0ρ
Ωxx
(k)
2/3 3/5
5/9
6/11
7/15
2/53/74/9
5/11
6/13
7/13
8/15
1 2/3 2/
5/7
4/5
3 4/
Vx
VyIx
4/7
5/34/3
8/57/5
123456
Magnetic Field B[T]
QHE = plateau in RH & RL = 0
1980 : Integer quantum Hall effect (IQHE)1982 : Fractional quantum Hall effect (FQHE)
Metal-Oxide Field-Effect Transistor (MOSFET)
conductionband
acceptorlevels
valenceband
conductionband
acceptorlevels
valenceband
conductionband
acceptorlevels
valenceband
E
z
FE
z
F
E
z
F
(a)
(b) (c)
VVG
G
metal oxide(insulator)
semiconductor
metal oxide(insulator)
semiconductor metal oxide(insulator)
II
I
VG
z
z
E
E
E
1
0
metaloxide
semiconductor
2D electrons
usually silicon-based materials (Si/SiO2 interfaces)
GaAs/AlGaAs Heterostructure
dopants
AlGaAs
z
EF
GaAs
dopants
AlGaAs
z
EF
GaAs(a) (b)
2D electrons
Impurity levels farther away from 2DEG (as compared toSi/SiO2)
⇒ enhanced mobility (FQHE)
Electronic Mesurement of Graphene
SiO
Si dopé
V
2
g
Novoselov et al., Science 306,p. 666 (2004)
2. Landau Quantisation and Integer
Quantum Hall Effect
Landau quantisation (reminder of first class)
• 2D electrons in continuum limit (|q|a≪ 1)
H(px, py) p = ~q
• Peierls substitution : p → Π = p+ eA(r) for a/lB ≪ 1
H(px, py) → HB(Πx,Πy)
• Quantum mechanics
[x, px] = [y, py] = i~ → [Πx,Πy] = −i~2
l2B
⇒ Ladder operators [a, a†] = 1
Πx =~√2lB
(
a† + a)
Πy =~√2ilB
(
a† − a)
Simple Landau levels
Schrödinger fermions :
HB|n〉 = ǫn|n〉, a†a|n〉 = n|n〉, ǫn = ~ωC(n+ 1/2)
Dirac fermions (graphene) :
HB =√2~vFlB
(
0 aa† 0
)
, ψn =
(
unvn
)
, ǫλ=±,n = λ~vFlB
√2n
Simple Landau levels
Schrödinger fermions :
HB|n〉 = ǫn|n〉, a†a|n〉 = n|n〉, ǫn = ~ωC(n+ 1/2)
Dirac fermions (graphene) :
HB =√2~vFlB
(
0 aa† 0
)
, ψn =
(
unvn
)
, ǫλ=±,n = λ~vFlB
√2n
Eigenstates :
ψn=0 =
(
0|n = 0〉
)
ψλ,n= =1√2
(
|n− 1〉λ|n〉
)
Infrared Transmission Spectroscopy
10 20 30 40 50 60 70 80
0.96
0.98
1.00
B
E
2L3L
2L
3L
0L
1L
Be2cE1 ~1L
1E
1E
A
B
C
D
B
E
2L3L
2L
3L
0L
1L
Be2cE1 ~1L
1E
1E
A
B
C
D
(D)(C)
(B)
Rel
ativ
e tra
nsm
issi
on
Energy (meV)
(A)
0.4 T1.9 K
0.0 0.5 1.0 1.5 2.00
10
20
30
40
50
60
70
80 )(32 DLL )(23 DLL
)(12 CLL )(21 CLL
)(01 BLL )(10 BLL
)(21 ALL
Tran
sitio
n en
ergy
(meV
)
sqrt(B)
10 20 30 40 50 60 70 80 900.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
Rel
ativ
e tra
nsm
issi
on
Energy (meV)
1 T
0.4T
2T4T
10 20 30 40 50 60 70 80 90
0.99
1.00
0.7T
0.2T
0.3T
0.5T
Grenoble high−field group: Sadowski et al., PRL 97, 266405 (2007)
transition C
transition B
rela
tive
tran
smis
sion
rela
tive
tran
smis
sion
Energy [meV]
Energy [meV]
Tra
nsm
issi
on e
nerg
y [m
eV]
Sqrt[B]
selectionrules :
λ, n→ λ′, n±1
Edge States
ymaxn+1
ν = n ν =
n−1
yymaxymaxn n−1
n+1
n
n−1
(a)
(b)
y
xν = n+1
µ
LLs bended upwards atthe edges (confinementpotential)
chiral edge states⇒ only forward scattering
ν= n+1 ν= n ν= n−1
Four-terminal Resistance Measurement
I I
R ~
56
2 3
41
R ~ µ − µ = µ − µ
3µ − µ = 02
5
L
H
µ = µµ = µ2 LL3
µ = µ = µ6 5 R
3 R L
: hot spots [Klass et al, Z. Phys. B:Cond. Matt. 82, 351 (1991)]
IQHE – One-Particle Localisation
n
ε
(n+1)
ν
NL
(a)
density of states
RxyxxR
B=n
h/e n2
FE
IQHE – One-Particle Localisation
n
ε
n
ε(b)
(n+1)
ν
NL
(a)
density of statesdensity of states
RxyxxR
B
EF
RxyxxR
B=n
h/e n2
FE
IQHE – One-Particle Localisation
n
ε
n
ε
n
ε(b) (c)
(n+1)
ν
NL
(a)
density of states density of states density of states
extended states
localised states
RxyxxR
B
EF
Rxy
B
xx
EF
R
h/e (n+1)
h/e n2
2
RxyxxR
B=n
h/e n2
FE
IQHE in Graphene Novoselov et al., Nature 438, 197 (2005)
Zhang et al., Nature 438, 201 (2005)
V =15V
Density of states
B=9T
T=30mK
T=1.6K
∼ ν
∼ 1/ν
Graphene IQHE:
R = h/e
at = 2(2n+1)
at = 2n
ν
ν
H ν2
(no Zeeman)
Usual IQHE:
g
Percolation Model – STS Measurement
2DEG on n-InSb surface Hashimoto et al., PRL 101, 256802 (2008)
(a)-(g) dI/dV for different values of sample potentials (lower spinbranch of LL n = 0)
(i) calculated LDOS for a given disorder potential in LL n = 0
(j) dI/dV in upper spin branch of LL n = 0
Towards topological band theory
• Berry-ology :
– Berry connexion : ~An(k) = iψ†n(k)∇kψn(k)
– Berry curvature : ~Bn(k) = ∇k × ~An(k)
– Berry phase : γn =∮
dk · ~An(k)
→ Chern number : Cn = 1
2π
∮
BZdk · ~An(k)
Towards topological band theory
• Pseudo-Chern number of a (massive) Dirac point :
C =1
2ξsgn(m)
Towards topological band theory
• Pseudo-Chern number of a (massive) Dirac point :
C =1
2ξsgn(m)
• Remarks :– half integer (not a true toplogical invariant) due to
non-compact support k ∈ R2
– each Dirac point contributes ±1/2 to total Chern number→ Dirac points (on a lattice) come in pairs to get integer
Chern numbers ! (fermion doubling)– Haldane model (CK = CK′) : C = CK + CK′ = ±1– Kane-Mele model : C↑ = −C↓
Non-local transport in QSHE (I)
CdTe/HgTe quantum wells [Roth et al., Science 2009]
I1
2 3
4
56
V
I1
2 3
4
56
V
-0.5 0.0 0.5 1.0 1.5 2.00
5
10
15
20
25
30
35
40
R (
kΩ)
V* (V)
(1 x 0.5) µm2
(2 x 1) µm2
R14,14=3/2 h/e2
R14,23=1/2 h/e2
Non-local transport in QSHE l’EHQS (II)
CdTe/HgTe quantum wells [Roth et al., Science 2009]
Fig. 4
0.0 0.5 1.0 1.5 2.00
5
10
15
20
25
R (
kΩ)
V* (V)
I: 1-4
V: 2-3
1
3
2
4
R14,23=1/4 h/e2
R14,14=3/4 h/e2
3D topological insulators (I)
Frist generation based on Bi1−xSbx alloys [Hasan and Kane, RMP 2010]
→ band inversion above critical Sb concentration xc ≃ 0.04
3D topological insulators (II)
• closing of (∼ mass) gap at band inversion
⇒ Dirac fermions at surface of 3D toplogical insulator (∼ edgestates in 2D) :
Hsurface = vp · σ
p : momentum in surfaceσ : characterises true spin
⇒ single Dirac point (contrary to graphene with 4)
3D topological insulators (III)
2nd generation : Bi2Se3, Bi2Te2, Sb2Te3 [Zhang et al., 2009]
(ab initio calculations)
3D topological insulators (IV)
ARPES measurements of de Dirac fermions at a Bi2Se3 surface[Hsieh et al., 2009]
→ change of Fermi level by chemical doping (absorption of NO2)