Nernst effect - Iowa State University
Transcript of Nernst effect - Iowa State University
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Nernst effect
590B
Makariy A. Tanatar
November 21, 2008
•Nernst effect
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Thermopower at QCP is log divergingNernst effect is bigWhat does this mean?
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Nernst-Ettingshausen effect (1stNE)
dxdT
BNEy
1=
ZxBPIdydT = Albert von Ettingshausen
1850 - 1932
Walther Hermann Nernst1864-1941
Thesis 1887: Electromotive forces produced by magnetism in heated metal plates
Third law of thermodynamics (Nobel prize 1920)Nernst glowerBernstein-Siemens-Nernst electric piano
Graz 1887Ludwig Boltzman and coworkers
Nernst-Ettingshausen effect (2nd NE)
NE B
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BdIBRV HH =
+VD
q
FL
EH IE
Hall effect
+VH
-VH
Transverse voltageLongitudinal currentMagnetic field
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+q
FL
EN Q=κ∆T
T+∆T T
+VN
-VN∧ dxdT
BEN Zy
/=
Nernst effect or 1st Nernst-Ettingshausen effect
B
Transverse voltage Longitudinal heat currentMagnetic field
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+q
FL
T-∆T
B
I
κρ
2
2
21
dI
dBIPT −=∆
d-sample thicknessκ- thermal conductivityρ- electrical resistivity
Transverse thermal gradient Longitudinal electrical current Magnetic field
T
Ettingshausen effect or 2nd Nernst-Ettingshausen effect
Can be used for thermoelectric cooling
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+q
FL
EN QT+∆T T
+VN
+VS-VS
-VN
+
+
+
∧
Closer lookNernst effect B Hall effect: Lorenz force
Flowing charges in B
NE Steady state: no charge flow!Es compensates gradT force N=0?We met this situation before.Where?
Magnetoresistance, cyclotron orbit curvature is compensated by Hall voltageto first degree effect is zero
Nernst effect is second order effectDISTRIBUTION of velocities of carriers, qEs compensates average vD, but not hotter and colder than average
In SIMPLE metals for “good scattering”N is negligibly small
B
+VD
q
FL
EH I
+
-
E
V+V-
Weak field MR
ES
gradT
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Nernst effect in gold ~0.1 nV/KT
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Anisotropy notably increases Nernst effect
YBCO “Increased” N~5nV/KT!
Rapid rise on approaching SC Tc
“Bad scattering”
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Measurement of Nernst effect
Typical numbers Metals Good scattering ???Anisotropic scattering 5 nV/kT
V<10 nV
Semiconductorsn~1016-1024 m-3
V~1-100 µV
Very demanding measurements from thermal stability and electrical noise point of view
Ideologically similar toThermopower measurements
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Measurements
Nernst signal is defined as odd part of Sxy in field
Sxy=Snonequipotential+SMR+SN
Measurements in positive and negative fields, Sxy(H)-Sxy(-H)=2SN
Fixed temperature +H to –H sweepTime consuming
Strict requirements for T-drift, Sxy(δT)<<Sxy(H)
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Why bother measuring Nernst effect?
•Additional insight into multiple carrier conductors
•Anomalous scattering, easy to detect sharp features in
•Exotic scattering in magnetic systemsεεσ
∂∂ )(
K. Behnia
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Nernst effect in superconductors
N is big in the mixed state of SC Flux quanta respond to grad T by thermal diffusiongenerate Seebeck and Nernst voltages
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Nernst effect in superconductors
Same big in high-Tc cupratesBut in a broader T-range
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Claims: Nernst effect is too big for a metalSuperconducting vortices above TcPreformed pairs scenario
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B
I+ VD
q
VD
-q-
FL
E
Can Nernst effect be big otherwise?
To have big MR you need VH=0
Q. What should we have =0to get big Nernst effect?
No compensation for cyclotronorbit curvature
A. S =0Compensation of different carrier types
When S=0 there is no restoring force and there is always a current of two carrier types in the same direction
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B
+ VD
q -q-
FL
Q
T+∆T T
+VN
∧FL
VD
Contrary to Hall effectContributions of +q and –qSum up in ambipolar Nernst effect
Q. What is the difference?-VN
Ambipolar Nernst effect
One carrier type: Nernst is second order effectTwo carrier types: Nernst effect is FIRST order effect
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Number of unknowns=number of equations To solve transport completely you want to measureSingle carrier type: resistivity +HallSecond carrier type: +Seebeck +Nernst
Nernst effect Ge: big
100µV/KT
SemiconductorsFirst observationKrylova TV, Mochan IV, J. Tech. Phys. (USSR) 25, 2119 (1955)
N is very big in narrow gap semiconductors
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Not negligible evenin metallic systems!
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Nernst effect is big when weak field MR is big: Compensation+ high mobility
Bi metal 200% MR in 2T at room temperature
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K. Behnia group
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Reading:GeneralJ. M. Ziman Principles of the theory of solids
Nernst effect in superconductors, reviewR. P. Huebener, Supercond. Sci. Technology 8, 189 (1995).
Nernst effect in exotic materialsK. Behnia, virtual lecture at hvar05.ifs.hr/workshop/work_lectures.html
Review articles on physics of thermal and thermoelectric phenomenaN. Hussey Adv. Phys. 51, 1685 (2002).K. Behnia, D. Jaccard, J. Flouquet, JPCM 16, 5187 (2003)