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Enhanced thermoelectricity Enhanced thermoelectricity in the correlated semiconductor FeSb 2 Peijie SUN Max Planck Institute for Chemical Physics of Solids Dresden, Germany Acknowledgement: N. Oeschler, F. Steglich (MPI, Dresden) S Johnsen B
Peijie SUN Max Planck Institute for Chemical Physics of Solids Dresden, Germany
S Johnsen B B Iversen (Aarhus Univ Denmark)
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2
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Hulliger, Nature, 198 (1963) 1081; J. Solid State Chem. 5 (1972) 144
Thermally activated paramagnetism, (narrow gap and narrow band model applicable)
Mandrus, PRB 1995
Eg ~ 350 K
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Petrovic, PRB 72 (2005) 045103; Fan et al, J. Solid State Chem. 5 (1972) 136
Optical spectral weight recovers above 1eV
Perucchi, Eur. Phys. J. B., 2006
Largest S in d based systems largest PF so far known Largest S in d-based systems, largest PF so far known
Dimensionless figure of merit
ZT = T S2 σ / к Large power factor PF= S2 σ
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Reducing к while keeping high PF ?
Large power factor PF S σ Small thermal conductivity к
Samples preparation Vapor transport (FeSb2), self-flux (RuSb2) FeSb2 Vapor transport (FeSb2), self flux (RuSb2) Crystal characterization Powder x-ray, Laue diffraction
Home-made cryostat (1.5K-RT, 0-7T)
Thermopower, S = Vx/|∆T| N t ffi (L/W) V /B|∆T|
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Nernst coeffi., v = (L/W) ·Vy /B|∆T|
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1
10
10
2
10
10 10
1
10
10
10
3
10 10 10
10
)
10
10
10
10
ρ (Ω-cm)
RH| (m3/C) 10
10 10
10
ρ (Ω cm)
RH| (m
3/C)
10
10
10
10
10
ρ
|R 10
10 10
10
ρ
|R
1 10 100 10 10 T (K) 400 0.0 0.1 0.2 10 10 T
D bl E 52 K d 350 K ( (E /2T)) Double gap, Eg = 52 K and 350 K (ρ = ρ0 exp (Eg/2T))
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10
10
3
10
m
2/Vs)
10
10
µH (m
1 10 100
10
T (K)
negligibly small mobility of a second sub-band One band analysis possible
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Large power factor
600 700
Large power factor
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300 400 500 600 (µW/cmK
2)
(mV/K)
ν (µV/KT)
10 20 30 40 50 60 70 80 100 200 PF FeSb2
S
N(T), B = 0.5 T N(T), B = 2 T
ν
10 20 30 40 50 60 70 80 T (K)
1 10 100
T (K)
>> Classical Upper limit of semiconductor Eg/2eTmax ~ 1500 µV/K
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10 K 20 K 6 6 K
S (T) = kB/e ·Eg/2T + C (1)
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)
10 K 20 K 6.6 K
Thermal activation of S, in agreement with T-activated ρ and RH in this T-range.
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H
g However, Eg~ 840 K
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g
>> 52 K (from ρ and RH)
0.05 0.1 0.15 1/T (1/K) 0.05
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Non-degenerate model (2) Degenerate model (3)
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10
1
10
K)
30 x (2)
15 x (3)
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S| (mV/
(2)
(3)
10
1
|S
m*=m0
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T (K)
10 100 1
A factor is needed to optimize the description quantitatively.
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100
FeSb2
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EF (K)
10
E
1 10 100 1
T (K) ( )
The measuremental temperatures are not very different with EF
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100000 10000 FeSb2 AAFa-batch 1000
(µV/K)
100
S measured
|S| (
10 100 10
non-deg. model m*=0.1m0 factor 40
T (K) Assuming m* = 0.1m0, the same discussion works. T (K)
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M t l/d t i d
Two origin of Nernst signal
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4
(b)
Metal/degenerate semicond.
10
2
10
3
(b)
/KT)
S tanθH/B 10 10
1
|ν| (µV/
Intrinsic/compensated semicond. bi l ff t ( iti i l)
1 10 100 10
10
N (B=0.5T) N (B= 2T)
T (K)
Huge Nernst signal for the first mechanism!
T (K)
Enhanced ∂τ/∂E accompanying onset of the gaps probably account for the double peaks
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Delves, Rep. Prog. Phys. 1965 Wang, et al, PRB 2001
400 10
2
10
3
∼T
3/2
300 10
1
10
µm)
200 10
10 κ (W/m lp, le (µ
100
3
10
10 κ
10 100 10
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Extremely small electron mfp compared to that of phonon indicates large room for optimizing ZT
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g p g
6 4
2
emu/mo FeSb2 ( 10
R Sb
χ (
RuSb2
100 200 300 400
T (K)
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10000
10
2
1000
m)
FeSb2
10
1
K)
FeSb2
10 100
F (µW/K2cm 10
|S| (mV/K
RuSb2
1
PF
RuSb2
10
10
10 20 30 40 50 60 0.1
T (K) 1 10 100 10
T (K)
300
Huge S and PF below 30 K in FeSb2 relative to RuSb2, despite the similar n and even larger к of the latter
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When T = Const.
A five fold enhanced slope in 2D system, relevance to FeSb2 ?
19 Ohta et al, Nature material 2007
Huge S and PF are observed in FeSb2, while low values in RuSb2. Classical electron-diffusion model qualitatively describes the observed S. An enhancing factor is needed for quantitative explanations. Large mobility contributes to the large PF as well.
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