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Sh
Topology in e-h Double Layers
Rui-Rui Du
Rice University
Shoucheng Zhang Memorial Workshop May 2-5,.2019
路漫漫其修远兮吾将上下而求索
I shall take a long Journeyto seek eternal truth
1989
1992
Composite Fermions
Du, Stormer, Tsui, Pfeiffer, West 1993
A Gift from Nature
Infrared Detectors
FQHELaser
Transistor
AlAsGaAs
Eg
Ge Si
InAs
GaSb
IQHE
QSHE
Exciton
Insulator
OUTLINES1. Introduction
2. Materials and Devices
3. Quantum Spin Hall Effect
4. Large Gap TI
Quantized Conductance of Edge States
Phys. Rev. Lett. 2011, Knez et al, Evidence for edge modes
Phys. Rev. Lett. 2012, Knez et al, Perfect Andreev reflection of edge-SC junction
Phys. Rev. Lett. 2015, Du et al, Quantized edge states for small samples
Phys. Rev. Lett. 2014, Stanton et al,
Phys. Rev. Lett. 2014, Knez et al.
Phys. Rev. Lett. 2015, Li et al
Phys. Rev. Lett. 2017, Du, et al , N.Comm. 2017, Du et al
Date: 01/09/2011 (04:54:27 PM CDT)
From: sczhang <[email protected]>
Cc: [email protected], Chaoxing Liu <[email protected]>
Dear Rui and Ivan,
I read your beautiful paper over and over
again, and simply can not withhold
my excitement about this important
breakthrough in the field!
Eg1
Large
Gap
Eg2
Small
Gap
Eg3
Negative
Gap
Mott, Philos Mag 1961
Keldysh, Yu.V., Fizika ,1964
Halperin & Rice, Rev Mod Phys
1968
+e
-e
Exciton Insulators
EF
EFTopological
Exciton Insulator
w/ edge states
OUTLINES
1. Introduction
2. Materials and Devices
3. Quantum Spin Hall Effect
4. Large Gap TI
10
Liu, et al, PRL 100, 236601(2008)
6.1 A Family
1. Band parameter
DE = E1 – H1
tuned by width, x, gates
2. DE < 0 inverted -0.15 eV to 0
Tunneling mix e-h and opens
a gap at finite k-vector
p
n SCSC
NTT, ETH
Wafer and Device
11
n-Inv
n-Metal
p-Metal
TI
NI
p-Inv
Two Limits of Coupling
Strong
Tunneling
1- D
Dirac Fermions
Majorana
Pair
Mini Gap
S-Superconductor+
Weak Tunneling
Strong Coulomb
2D
Excitons
Exciton
Insulator
Counter flow
Superfluid
-e
+e
E
Iin
Iout
OUTLINES
1. Introduction
2. Materials and Devices
3. Quantum Spin Hall Effect
4. Large Gap TI
Corbino Disk
rBulk >> 10 MW
Minigap ~ 66 K
Mobility gap
~ 26 K
Truly Insulating Bulk
15
Lingjie Du
Conductance Plateau up to 4 K
16
Non-Local Ede Transport: H-Bar
17Lingjie Du
18
SQUID Imaging of InAs/GaSb Edge Modes
Stanford: Spanton, Moler
Rice: Lingjie Du, RRDPRL 14
50 mm
Current(Horizontal)
Current(Vertical)
APS March Meeting 2018, LA, California, USA
QSHE vs. Exciton Condensation
A competition between even- and odd-parity exciton
condensate
the parity of order parameter
o For small A, topologically trivial s-
wave exciton condensate phase
o For large A, topologically nontrivial
QSH insulator phase with a p-wave
exciton order parameter
Phys. Rev. Lett.112,
176403(2014)
Coupling term between two layers
odd-parity even-parity
Editors' ChoicePhysics
Probing an excitonic insulator
1.Jelena Stajic
Science 22 Dec 2017:
Vol. 358, Issue 6370, pp. 1552-1553
DOI: 10.1126/science.358.6370.1552-g
Bohr Rad. ~ 45A
Intra. Dis. ~ 100-300A
Binding E. ~100K
THz Absorption Spectrum
Transport Measurements
Conductance Oscillations
in the Gap of Inverted InAs/GaSb
Frequency Jump from Metallic to Gap Regimes
QSH to QH Topological Phase Transition
OUTLINES
1. Introduction
2. Materials and Devices
3. Quantum Spin Hall Effect
4. Large Gap TI
Strained QWS
Strain Effects on the Band Structures
9.5 nm InAs/4 nm Ga0.75In0.25Sb
InAs/GaSb InAs/In0.25Ga0.75Sb
20% Indium
~ 1% strain in InxGa1-xSb
0 % 60 K
25% 130 K
32% 250 K
40% 480 K
Enhanced Bulk Hybridization Gaps
20% 25% 32%
B//
0
25
32%
B//
Quantized Edge Transport in 10 um Length
Quantized in a 10um Hall Bar
April 2018
All based on InAS/GaSb and InAs/InGaSb
Gap > 50 meV
Large Fermi velocity
Epitaxial Transfer Process
• (a,b,c) The epitaxially grown, single-crystal
InAs films are patterned with PMMA and
wet etched into nanoribbon (NR) arrays. (d)
A subsequent selective wet etch of the
underlying AlGaSb sacrificial layer. InAs
NRs are partially released. (e,f) transfer of
NRs by using an elastomeric PDMS slab
resulting the formation of InAs NR arrays on
Si/ SiO2 substrates.
Partially released InAs nanoribbons
(a) (b) (c)
(f) (e) (d)
姚思齐,韩中东
Marcus, Palmstrøm groups Hard Gap
SiO2GaInSbInAs
Al Al
Our Group
Double-Layer Excitonic Condensations
Strong
Tunneling
1- D
Dirac Fermions
Majorana
Pair
Mini Gap
S-Superconductor+
Weak Tunneling
Strong Coulomb
2D
Excitons
Exciton
Insulator
Counter flow
Superfluid
-e
+e
E
Iin
Iout
SUMMARY1) Quantum Spin Hall Effect inInAs/GaSb
2) Ground States becomes Excitoinc Insulator
3) Large gap TI is within reach
4) Next : Carry on
A clean laboratory for
electron/hole many-particle physics
1. Materials: Strained-Layer InAs/GaInSb
2. Large-Gap QSHI
3. Double-Layer Excitonic Condensation
40
6.1 A Family
1. Band parameter
DE = E1 – H1
tuned by width, x, gates
2. DE < 0 inverted -0.15 eV to 0
Tunneling mix e-h and opens
a gap at finite k-vector
Liu et al, PRL 100, 236601 (2008)
Infrared Detectors
p
n SCSC
NTT, ETH
Wafer and Device
41
n-Inv
n-Metal
p-Metal
TI
NI
p-Inv
C. Liu et al, PRL 100 236601
(2008)Phase Diagram
Three Regimes of InAs/GaSb System
Shallowly InvertedDeeply Inverted
(semimetal,
bulk still conducting)
Non-
inverted
(Normal
Insulator)
This is the regime
for QSHE !
Quantum Spin Hall Insulators (QSHIs)
L. Du et al, PRL 114 096802 (2015)
InAs/GaSb QWs in Shallowly
Inverted Regime
Larger samples:
Coherent length ~ 4 mm
Some backscattering
Small sample
44
* Quantized conductance for small samples
* Non-quantized conductance for large samples
* Conductance does not change under high B//
•Low velocity of edge modes
uF ~ 2 x104 m/s
kF ~ 1 x 106/cm
•Extreme. long scattering time
t=l/uF = 4mm/(2x104m/s)~200 ps
•Tunable mode dispersion
Properties of InAs/GaSb Edge States
HgTe
5 x105 m/sg = e2/ uF e
WHY Large-gap 2D Topological Insulators
• Recover single particle
properties
“Plain Vanilla QSHI”
• More insulating bulk gaps
• Long coherent length
Strained QWS
Strain Effects on the Band Structures
9.5 nm InAs/4 nm Ga0.75In0.25Sb
InAs/GaSb InAs/In0.25Ga0.75Sb
20% Indium
~ 1% strain in InxGa1-xSb
0 % 60 K
25% 130 K
32% 250 K
40% 480 K
Enhanced Bulk Hybridization Gaps
20% 25% 32%
B//
0
25
32%
B//
“Coherent Length” increase
with less inverted band:
Correlated with increased VF
- Less interacting 1D Electronic system
- Move away from Luttinger
Coherent length increases with increasing VF
Quantized Edge Transport in 10 um Length
Quantized in a 10um Hall Bar
TRS Protected Helical Edge States
9.5 nm InAs/4 nm Ga0.75In0.25Sb
SiO2
Compatible with III-V CMOS Technology
Marcus, Palmstrøm groups Hard Gap
SiO2GaInSbInAs
Al Al
Our Group
3. Double-Layer Excitonic Condensations
Strong
Tunneling
1- D
Dirac Fermions
Majorana
Pair
Mini Gap
S-Superconductor+
Weak Tunneling
Strong Coulomb
2D
Excitons
Exciton
Insulator
Counter flow
Superfluid
-e
+e
E
Iin
Iout
57
Mott, Philos Mag 1961
Knox, 1963
Keldysh, Yu.V., Fizika ,1964
Jerome, Rice, Kohn, 1967
Halperin & Rice, Rev Mod Phys, 1968
The Exciton Analog of BCS Superconductivity
Electron-Hole Pairing Cooper Pairing
58
BEC
BCSBCS
dDilute limit: 5 x 1010/cm2
Reasonably high mobility 105 cm2/Vs
Electron-hole symmetry
9.5nm
10nm
5nm
Observation
T > 4 K
Main Peak
Onset 40 K
Gap 90 K
Side Peak
Onset 15 K
Gap 25 K
Large Energy Scale
61
THz Absorption Shows e-h Pair-breaking Excitation
Summary for Large Gap QSHI in Strained Layer QWs
• The strained InAs/Ga1-xInxSb QWs offers
Larger bulk insulating gap up to 50 meV
Z2 2D topological insulator
Double-layer exciton condensation
• Future work:
• SC Proximity + Edge States + FM
Insulator
• Counter-flow experiments
QSH Edge
Majorana Platform