Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
SGM Group
Stefan Heun
NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore Piazza San Silvestro 12, 56127 Pisa, Italy
e-mail: [email protected]
Nanomaterials Characterization by low-temperature Scanning Probe Microscopy
SGM Group
Stefan Heun
NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore Piazza San Silvestro 12, 56127 Pisa, Italy
e-mail: [email protected]
Inner structure of edge channels in the quantum Hall regime probed by scanning gate microscopy
Outline
• Introduction to quantum Hall physics
• Basics of Scanning Gate Microscopy
• Inner structure of edge channels in the
quantum Hall regime by SGM
Outline
• Introduction to quantum Hall physics
• Basics of Scanning Gate Microscopy
• Inner structure of edge channels in the
quantum Hall regime by SGM
Further reading:
Klaus von Klitzing: The quantized Hall effect, Nobel lecture, 1985 (http://nobelprize.org/nobel_prizes/physics/laureates/1985/klitzing.pdf)
R. J. Haug: Edge-state transport and its experimental consequences in high magnetic fields, Semicond. Sci. Technol. 8 (1993) 131.
2 Dimensional Electron System
see also: Horst L. Stoermer, Nobel Lecture, December 8, 1998
Hall bar geometry
K. von Klitzing et al.: Physik Journal 4 (2005) No.6 p.37.
Measurement of longitudinal and transversal resistance
Drude model
vmBvEe
dt
vdmF
(=0; steady state)
zz
x
y
xy
x
xxx
Bne
B
j
E
constj
E
.
1
0
yxcy
xycx
jjE
jjE
0
0
Evnejm
ne
m
eBzc
,,
2
0with
Transverse current is zero:
0yj It follows:
Ashcroft / Mermin: Solid State Physics.
Hall effect
K. von Klitzing et al.: Physik Journal 4 (2005) No.6 p.37.
Drude model:
Rxy = proportional to B
Rxx = costant in B
Quantum Hall Effect
K. von Klitzing et al.: Physik Journal 4 (2005) No.6 p.37.
Behaviour observed at
low temperature in high
mobility samples in high
B ( ):
Plateaux in Rxy
Minima in Rxx
Quantum Hall Effect:
deviations from Drude
model observed around
B = nh / ne
n: filling factor
1c
The Nobel prize in Physics
1985: integer QHE 1998: fractional QHE
http://nobelprize.org/
Landau Levels
Single particle Hamiltonian for an electron in an external magnetic field:
Landau-Gauge: with
For the wavefunction can be written as:
It follows:
rVAepm
H
2
2
1
0,,0 BxA
zeBB
xeyxyiky ,
xxxmm
pxE c
x
2
0
22
2
1
2
0rV
R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Landau Levels
This is the equation of a harmonic oscillator with angular frequency
center of cyclotron orbit at and magnetic length
Drift velocity
meBc
2
0 Bykx
)(
25
TB
nm
eBB
xxxmm
pxE c
x
2
0
22
2
1
2
2
1 jE cj
Landau Levels 0 yjy dkdEv
R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Landau Levels
Degeneracy of each
Landau Level:
The number of filled
Landau Levels for a given
carrier concentration ns is
then:
n: filling factor
R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Bh
eBn deg
eB
hn
n
n ss deg
n
Landau Levels
At the edges of the sample, a boundary confinement can be added by a small
V(x) which does not depend on y.
221 Bycyj kVjkE
B. I. Halperin, Phys. Rev. B 25, 2185 (1982).
Introduction to edge channels
x y x
see also: R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Introduction to edge channels
x y x
see also: R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Introduction to edge channels
x y
edge: 1D “one-way” conductor, skipping orbits
bulk: insulator x
see also: R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Introduction to edge channels
x y
edge: 1D “one-way” conductor, skipping orbits
bulk: insulator
Quantization of energy levels (Landau levels)
Gap for excitation in the bulk
Transport via edge states
x
see also: R. J. Haug, Semicond. Sci. Technol. 8 (1993) 131.
Edge states
compressibility:
incompressible:
sch n
sch n
K. von Klitzing et al.: Physik Journal 4 (2005) No.6 p.37.
incompressibilities
Example Semiconductor
n
VB
CB
n*
n* independent of B
Edge states
D. B. Chklovskii et al.: PRB 46 (1992) 4026.
compressibility:
incompressible:
sch n
sch n
Edge states
D. B. Chklovskii et al.: PRB 46 (1992) 4026.
compressibility:
incompressible:
sch n
sch n
SGM Group
Non-interacting vs. interacting picture
D. B. Chklovskii et al.: PRB 46 (1992) 4026.
• The self consistent potential due to e-e interactions modifies the edge structure •For any realistic potential the density goes smoothly to zero. •Alternating compressible and incompressible stripes arise at the sample edge
Incompressible stripes: •The electron density is constant •The potential has a jump
Compressible stripes: •The electron density has a jump •The potential is constant
SGM Group
Non-interacting vs. interacting picture
D. B. Chklovskii et al.: PRB 46 (1992) 4026.
• The self consistent potential due to e-e interactions modifies the edge structure •For any realistic potential the density goes smoothly to zero. •Alternating compressible and incompressible stripes arise at the sample edge
Incompressible stripes: •The electron density is constant •The potential has a jump
Compressible stripes: •The electron density has a jump •The potential is constant
Each of these edge channels carries M. Büttiker, Phys. Rev. B 38, 9375 (1988)
VheheI 2
How to probe edge channels?
S. Roddaro et al., Phys. Rev. Lett. 90 (2003) 046805.
How to probe edge channels?
S. Roddaro et al., Phys. Rev. Lett. 90 (2003) 046805.
How to probe edge channels?
S. Roddaro et al., Phys. Rev. Lett. 90 (2003) 046805.
How to probe edge channels?
V
t
r 0
V
I(V)
Vg
S. Roddaro et al., Phys. Rev. Lett. 90 (2003) 046805.
Quantum Point
Contact (QPC)
V
t
r 0
V
I(V)
Vg
S. Roddaro et al., Phys. Rev. Lett. 90 (2003) 046805.
we induce backscattering
by reducing this distance
Current conservation:
t + r = 1
How to probe edge channels?
Hot topics
• The quantum Hall Effect as an Electrical
Resistance Standard
RK = 25812.807572Ω
Hot topics
• The quantum Hall Effect as an Electrical
Resistance Standard
• Quantum Hall Interferometry
Y. Ji et al., Nature 422 (2003) 415.
Hot topics
• The quantum Hall Effect as an Electrical
Resistance Standard
• Quantum Hall Interferometry
• Quantum Computing
http://stationq.ucsb.edu/research.html
Hot topics
• The quantum Hall Effect as an Electrical
Resistance Standard
• Quantum Hall Interferometry
• Quantum Computing
• Basic Research
Outline
• Introduction to quantum Hall physics
• Basics of Scanning Gate Microscopy
• Inner structure of edge channels in the
quantum Hall regime by SGM Further reading:
M. A. Topinka, B. J. LeRoy, S. E. J. Shaw, E. J. Heller, R. M. Westervelt, K. D. Maranowski, and A. C. Gossard: Imaging coherent electron flow from a quantum point contact, Science 289 (2000) 2323.
M. A. Topinka, B. J. LeRoy, R. M. Westervelt, S. E. J. Shaw, R. Fleischmann, E. J. Heller, K. D. Maranowski, and A. C. Gossard: Coherent branched flow in a two-dimensional electron gas, Nature 410 (2001) 183.
Scanning Gate Microscopy
AFM with conductive tip
Tip at negatively bias
(local gate - locally
depletes the 2DEG), no
current flows
SGM performed in
constant height mode
(10-50 nm above
surface), no strain
M. A. Topinka et al.:
Science 289 (2000) 2323.
Scanning Gate Microscopy
AFM with conductive tip
Tip at negatively bias
(local gate - locally
depletes the 2DEG), no
current flows
SGM performed in
constant height mode
(10-50 nm above
surface), no strain
A. Iagallo et al.:
Nano Reserach,
in press.
Coherent branched flow of
electrons
M. A. Topinka et al., Nature 410 (2001) 183.
G: 0.00e2/h 0.25e2/h
Tip-induced backscattering
N. Paradiso et al., Physica E 42 (2010) 1038.
Tip-induced backscattering
N. Paradiso et al., Physica E 42 (2010) 1038.
Coherent branched flow of
electrons
M. A. Topinka et al., Nature 410 (2001) 183.
G: 0.00e2/h 0.25e2/h
source-drain current
SGM Group
The SGM @NEST lab in Pisa
• AFM non-optical detection scheme (tuning fork)
• With vibration and noise isolation system
• 3He insert (cold finger base temp. :300 mK)
• 9 T cryomagnet
Tip at negative bias (moveable gate locally depletes the 2DEG)
M. A. Topinka et al.: Science 289 (2000) 2323. Pioneering work by:
SGM performed in constant height mode (10-50 nm above surface), no strain
Setup:
Oct 2007: First cooldown
AFM Head
• Positioner (range in xyz > 6 mm) allow to locate features on the sample with µm precision
• Scan range:
• 42µm x 42µm x 2µm @ RT
• 8.5µm x 8.5µm x 1.4µm @ 300 mK
• Temperature measurement (RuO2) close to sample
• Drift < 1 nm / h
• Temperature stability Delta T / T < 5% for hours, even at max. B field
• Noise in z at 300 mK: 1nm
Tuning Fork
Sample holder for transport
measurements
Base mounted on AFM scanner Contact via pogo pins
Chip carrier
holds sample
Sample holder mounted on scanner
Tip – sample geometry
Outline
• Introduction to quantum Hall physics
• Basics of Scanning Gate Microscopy
• Inner structure of edge channels in the
quantum Hall regime by SGM
Further reading:
N. Paradiso et al.: Spatially resolved analysis of edge-channel equilibration in quantum Hall circuits, Phys. Rev. B 83 (2011) 155305.
N. Paradisoet al.: Selective control of edge-channel trajectories by scanning gate microscopy, Physica E 42 (2010) 1038.
Hall-bar samples
1900 m x 300 m
N. Paradiso et al., Physica E 42 (2010) 1038.
Hall-bar samples
N. Paradiso et al., Physica E 42 (2010) 1038.
Hall-bar samples
N. Paradiso et al., Physica E 42 (2010) 1038.
Hall-bar samples
N. Paradiso et al., Physica E 42 (2010) 1038.
Hall-bar samples
N. Paradiso et al., Physica E 42 (2010) 1038.
SGM Group
Conductance quantization in QPCs
1D confinement
In 1D systems the current is carried by a finite number of modes (arising from confined subbands) . Each mode contributes two quantum of conductance.
H. van Houten and C. Beenakker: Physics Today, July 1996, p. 22
2e2/h
SGM Group
Conductance quantization in QPCs
1D confinement
In 1D systems the current is carried by a finite number of modes (arising from confined subbands) . Each mode contributes two quantum of conductance.
2e2/h
First we fix the mode number (QPC setpoint), then we start scanning the biased tip at a fixed height.
SGM Group
Conductance quantization in QPCs
1D confinement
In 1D systems the current is carried by a finite number of modes (arising from confined subbands) . Each mode contributes two quantum of conductance.
First we fix the mode number (QPC setpoint), then we start scanning the biased tip at a fixed height.
source-drain current 2DEG
The split gates define a constriction by depleting the 2DEG underneath
The biased tip creates a depletion spot that we use to backscatter the electrons passing through the constriction
SGM Group
QPC at 3rd plateau 3rd plateau
600nm600nm
�5.50
�
0.00
G = 0
G = 6 e2/h
600nm
SGM Group
QPC at 2nd plateau 2nd plateau
�5.50
�
0.00
G = 0
G = 6 e2/h
600nm
SGM Group
QPC at 1st plateau 1st plateau
�5.50
�
0.00
G = 0
G = 6 e2/h
SGM Group
Branched flow and interference fringes
400nm 100nm
Fringe periodicity: lF/2=20 nm
• QPC conductance G = 6 e2/h (3rd plateau) • Tip voltage Vtip = -5 V, height htip = 10 nm • see also M. A. Topinka et al., Nature 410 (2001) 183.
SGM in the Quantum Hall regime
Tip voltage Vtip = -5 V, height = 30 nm
N. Paradiso et al., Physica E 42 (2010) 1038.
Magnetoresistance
N. Paradiso et al., Physica E 42 (2010) 1038.
SGM Group
Selective control of edge channel trajectories by SGM
• Bulk filling factor n=4
• B = 3.04 T
• 2 spin-degenerate edge channels
• gate-region filling factors g1 = g2 = 0
�2.90
�
0.00
4.0 e2/h
0.0 e2/h
4.0 e2/h
0.0 e2/h
600nm
N. Paradiso et al., Physica E 42 (2010) 1038.
SGM technique: we select individual channels from the edge of a quantized 2DEG, we send them to the constriction and make them backscatter with the biased SGM tip.
SGM Group
How we probe incompressible stripes
-100 0 100 200 300 400 500 600 700 800
0
1
2
3
4
co
nd
uc
tan
ce
(e
2/h
)
tip position (nm)
tip position
Self-consistent potential
ħωc
Landau levels inside the constriction
tip induced potential
SGM Group
How we probe incompressible stripes
-100 0 100 200 300 400 500 600 700 800
0
1
2
3
4
co
nd
uc
tan
ce
(e
2/h
)
tip position (nm)
tip position
backscattering
SGM Group
How we probe incompressible stripes
-100 0 100 200 300 400 500 600 700 800
0
1
2
3
4
co
nd
uc
tan
ce
(e
2/h
)
tip position (nm)
tip position
SGM Group
How we probe incompressible stripes
-100 0 100 200 300 400 500 600 700 800
0
1
2
3
4
co
nd
uc
tan
ce
(e
2/h
)
tip position (nm)
tip position
backscattering
Energy gap: ħω=5.7 meV Plateau width: 60 nm Incompr. stripe width: ≈30nm
Can we exploit the non-trivial edge structure?
The picture of a QH device emerging from the experiments
A “bus” of compressible and incompressible channels
Is it possible to study and image the
microscopic details of the inter-channel
interactions?
SGM Group
SGM Group
Studying inter-channel equilibration
d
Two co-propagating edge channels originating from two ohmic contacts at different potential
n=2
g=1
SGM Group
Studying inter-channel equilibration
devices with fixed interaction length d: elusive determination of the microscopic details of
the equilibration mechanisms
d
n=2
g=1
SGM Group
The oppurtunity of the Scanning Gate Microscopy
Our technique allows to selectively control the channel trajectory
Our idea: exploit the mobile depletion spot induced by the SGM to continuously tune d
nbulk = 4 two spin degenerate edges
SGM tip
n=0
n=2
n=4 N. Paradiso et al., PRB 83 (2011) 115305.
SGM Group
The oppurtunity of the Scanning Gate Microscopy
Our idea: exploit the mobile depletion spot induced by the SGM to continuously tune d
nbulk = 4 two spin degenerate edges
SGM tip
n=0
n=2
n=4 N. Paradiso et al., PRB 83 (2011) 115305.
Our technique allows to selectively control the channel trajectory
SGM Group
The oppurtunity of the Scanning Gate Microscopy
Our idea: exploit the mobile depletion spot induced by the SGM to continuously tune d
nbulk = 4 two spin degenerate edges
SGM tip
n=0
n=2
n=4
Each channel carries 2e2/h units of conductance
N. Paradiso et al., PRB 83 (2011) 115305.
Our technique allows to selectively control the channel trajectory
SGM Group
The oppurtunity of the Scanning Gate Microscopy
Our technique allows to selectively control the channel trajectory
Our idea: exploit the mobile depletion spot induced by the SGM to continuously tune d
N. Paradiso et al., PRB 83 (2011) 115305.
SGM Group
The oppurtunity of the Scanning Gate Microscopy
Our technique allows to selectively control the channel trajectory
Our idea: exploit the mobile depletion spot induced by the SGM to continuously tune d
N. Paradiso et al., PRB 83 (2011) 115305.
No eq. Full eq.
A 2 e2/h 1 e2/h
B 0 e2/h 1 e2/h
SGM Group
Experimental setup
SEM micrograph of the device
source bias V=VAC + VDC
tip
Scheme of the electronic setup
edge-selector gate
reflected component
transmitted component
SGM Group
Imaging the inter-channel equilibration
Source bias: VAC=50V, VDC=0mV
The profiles of GB(d) along the trajectory show a strict dependance on the local details
Imaging the edge channel equilibration
1
0
[e2/h]
SGM Group
Imaging the inter-channel equilibration
The profiles of GB(d) along the trajectory show a strict dependance on the local details
We can directly image the potential induced by the most important defects by means of a
scan at zero magnetic field
correlation found
Source bias: VAC=50V, VDC=0mV
Imaging the edge channel equilibration
SGM scan at zero magnetic field
1
0
[e2/h]
-1 0 1 2 3 4 5 6 7
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
dif
fere
nti
al c
on
du
cta
nc
e G
B (
e2/h
)
position (m)
Experimental data
Tight binding simulations
SGM Group
Tight binding simulations
Pictorial model for the disorder potential
tip potential
“big impurities” potential
background potential
scattering centers
SGM map of the inter-channel equilibration in another device
1
0
[e2/h]
Simulations made by the theoretical group of Scuola Normale Superiore (Pisa, Italy) D. Venturelli, F. Taddei, V. Giovannetti and R.Fazio
N. Paradiso et al., PRB 83 (2011) 115305.
SGM Group
Nonlinear regime
The backscattered current is a function of the local imbalance V(x) that depends on the specific scattering process.
SGM Group
Two mechanisms for the inter-channel scattering
For low bias the only relevant mechanism is the elastic scattering induced by impurities, which determines an ohmic behavior (linear I-V)
At high bias (Δμ≈ħωc) vertical transition with photon emission are enabled (threshold and saturation)
SGM Group
Impact of the electron heating
Electron heating due to injection of hot carriers:
The relaxation of hot carriers induces a dramatic temperature increase. This is why the transition is smoothened and the threshold voltage reduced for high d
SGM Group
Conclusions
We explore the use of Scanning Gate Microscopy to selectively control edge channel trajectories
We built size-tunable QH circuits to directly image the equilibration between imbalanced co-propagating edges
The comparison with the SGM scan at zero magnetic field reveals a correlation between the local potential and steps in the GB(d) curve
Shift of the threshold voltage for the onset of photon emission is explained by a simple model for the electron heating
Control of the edge channel trajectory allows us to study their structure
Coworkers
R. Fazio V. Giovannetti F. Taddei D. Venturelli
N. Paradiso S. Roddaro G. Biasiol (IOM) L. Sorba F. Beltram
Funding