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Localization 2011
Augutst 4-7, 2011; POSTECH
Department of Physics, POSTECH
Quantum Transport and Superconductivity Laboratory
Thermoelectric Transport in Bilayer Graphene
Seung-Geol Nam
Hu-Jong LeeDong-Keun Ki
Stefan Ketteman
POSTECHPOSTECHAMS, WCUJacobs Univ. Bremen
- Introduction Thermoelectric power (TEP)
TEP in monolayer graphene
TEP in bilayer graphene
- Results and Discussion – studies on bilayer Gate-voltage and temperature dependence
Deviation from Mott’s relation
Magneto-thermoelectric power
- Conclusion
Outline
f(E)
E
f(E)
E
Thermoelectric Power
V
EF
Th Tc
Mott relation
- Mott relation ; valid for TEP generated by diffusive carriers
Compare
Mott Relation of TEP
For weak e-p interaction in graphene; - TEP linearly depends on T- Phonon-drag component nonlinear T dependence
TEP details of EF(n),band structure of graphene near EF
TEP Meaurements on Monolayer Graphene
Checkelsky and Ong, PRB (2009)
- Gate dependent TEP and sign changes at CNP- Linear dispersion relation
P. Wei, et. al., PRL (2009)Zuev, Chang, and Kim,
PRL (2009)
- TEP increases linearly with T – weak e-p interaction1 1
thbg
Vn V
hole-like
electron-like
Bilayer Graphene
1
CNP
tight-binding model withinterlayer hopping (1)
How do thermoelectric properties change in bilayer graphene ?
1 ~ 0.39 eV (graphite)
A2 B2 A2 B2
A1 B1 B1
A1
v
v
Interlayer hopping energy
McCann and Fal’ko, PRL 96, 086805(2006)
1
- Introduction Thermoelectric effect
TEP in monolayer graphene
TEP in bilayer graphene
- Results and Discussion Gate-voltage and temperature dependence
Deviation from Mott’s relation
Magneto-thermoelectric effect
- Conclusion
Outline
Measurement Configuration
Ti/Au (3-17 nm)
Main Results
Resistance Thermoelectric power
- Resistance near the CNP is T-dependent - Sign changes in TEP @ CNP –ambipolar nature of carriers- Insulating behavior due to thermally excited
carriers
TEP > 0hole regime
TEP < 0electron regime
mobility = 2000 ∼ 4000 cm2/Vs
- Low e-p scattering rate in bilayer graphene
Comparison to Monolayer Graphene
R(T) changes
thermally activated carriers
Zuev et al. PRL 102, 096807 (2009)
general trends are similar
Monolayer graphene Bilayer graphene
Fit of TEP at Low Temperatures
TEP is consistent with the hyperbolic band structure of bilayer graphene.
T=30 K
Nam et al., PRB 82, 245416 (2010)
Fit at Different Temperatures
Fixed parameters;
At high T and low carrier density, a deviation appears.
P. Wei, et. al.PRL 102, 166808 (2009)
- For T up to 50 K; semiclassical Boltzmann theory valid- Deviation above 140 K – in contrast to monolayer graphene
Temperature Dependence for Different VBG
Y. M. Zuev, et. al., PRL 102, 096807 (2009)
guide line
Deviation from Mott’s Formula
E. H. Hwang, et. al., PRB 80, 235415 (2009)
(TF ~ 330 K at V = 10 V)
(V = VBG – VCNP)Bilayer
(TF ~ 1150 K at V = 10 V)
Monolayer
Mott’s relation; valid only at low T ≪ TF (~ 0.2TF)
Zuev et al. PRL 102, 096807 (2009) - Deviation from Mott’s relation at an onset temp. T*
(gate-dependent) – due to failure of Sommerfeld expansion at high T
- TEP saturates near the onset temp. T*
Temperature Dependence
guide line
consistent with theoretical prediction Hwang, et. al., PRB 2009
- upswing of T/TF potentially leads to new physics- further clarified with cleaner graphene by overcoming the effect of electron-hole puddles.
Nam et al., PRB 82, 245416 (2010)
T = 15 K, B = 7 T
Sxx in High Magnetic Field
Bilayergraphene
EF
DO
SE
8-fold degeneracy
Sxx in Magnetic Field
T = 15 K, B = 7 T
kBln2/e ~ 59.6 V/K
1 ln 2 11 2
Bnk
en
; monolayer
; bilayermax
,
0
ln 2 1
( )
x Bxx n n
mm
E kST e f E
- thermal broadening of Landau levels- disorder
1. Mott relation; valid in bilayer graphene at low temperatures
Conclusion
- Hyperbolic dispersion relation - Interlayer hopping term 1
2. Deviation from Mott’sbehavior;
- Saturation of TEP at high T- Saturation T is gate-dependent
- Validity of Mott’s formulaTF of bilayer ≪ TF of monolayer
~ Em
3. In high magnetic field, oscillatory behavior of TEP is observed
Reduction of maximum of Sxx - disorder effect?
Conclusion
T = 15 K, B = 7 T
Sxy in Magnetic Field – Nernst Effect
P. Wei, et. al.PRL 102, 166808 (2009) T = 15 K, B = 7 T
Z. Zhu, et. al., Nature Phys. 6, 26 (2010)
Two dimensional band
graphite 2D(graphene)
Sxy =−Ey/|∇T|
band dispersion along c axis
• (ln2)(kB/e) ~ 60 V/K• 1/(m+1/2) dependence를 보이지만 값은 작은데 disorder가있을 때 Sxx는 이론값보다 줄어
들 수 있습니다.
Increasing disorder
3. In high magnetic field, oscillatory behaviors of TEP and the Nernst signals are also observed
No anomalous sign change of TEP near CNP in bilayer graphene
Nernst signal points to 2D nature of the bilayer graphene
Conclusion
T = 15 K, B = 7 T
T = 15 K, B = 7 T
D.-K. Ki
S. Kettemann
H.-J. Lee S.-G. Nam
Collaborators
n=0 n=1 n=m
Monolayer Graphene
n=0+,0- n=1 n=m
Bilayer Graphene
n=0 n=1 n=m
2DEG
T = 15 K, B = 7 T
Sxx in High Magnetic Field Bilayer
graphene
EFD
OS
E
8-fold degeneracy
No anomalous sign change near the CNP
Checkelsky and Ong, PRB 80, 081413(R) (2009)
Zuev, Chang, & Kim, PRL 102, 096807 (2009)
Thermoelectric Power
Saturation of TEP
E. H. Hwang, et. al., PRB 80, 235415 (2009)
~ Emin monolayer graphene;
- Scattering due to unscreened charged impurities
Why is it saturated ?
- Further theoretical work is required
Zuev et al. PRL 102, 096807 (2009)
- Deviation from Mott’s relation at a certain onset T, (gate-dependent)
- TEP saturates near the onset T.
Comparison to Monolayer Graphene
Zuev et al. PRL 102, 096807 (2009)
Morozov et al. PRL 100, 0166027 (2008)
Finite DOS at CNP
Monolayer
Bilayer
~ 1012 cm-2 at RT
Due to the finite DOS in bilayer graphene at the CNP, TF of the system is almost an order of magnitude smaller than that of the monolayer graphene for a given charge carrier density.
M. F. Craciun, et. al., Nature Nanotech. 4,
383 (2009)
J. B. Oostinga, et. al., Nature Mater. 7, 151
(2007)
From Graphine to Graphite
Two dimension Three dimension
Bilayer Trilayer ·· ·· ·· ·· ··??
Z. Zhu, et. al., Nature Phys. 6, 26 (2010)
S. Ghosh, et. al., arXiv:1003.5247
(2010)
electron
hole
Manchester Univ.Columbia Univ.
Exfoliation
TEP
TEP measured TEP measured