Post on 23-Feb-2016
description
Sliding of a charge density wave probed by coherent X-Ray Diffraction
E. PinsolleLaboratoire de physique des solides, Orsay
Synchrotron Sources
Experiment caried out by:
E. PinsolleV. JacquesD. Le bolloc’hA. SinchenkoP. Monceau
Experiment done at the ESRF on the beamline ID10A
NbSe3 sample
Sample dimensions:2mm x 10μm x 10μm
3 types of chains:-first one with Peierls transition at 145K-second one with Peierls transition at 59K-last one with no transition.
Threshold current 0.9 mA
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Signature of CDW in X-ray diffraction
Gap opening at the fermi level.Electron density modulation
-2kF +2kF
Spawning of satellites around initial Bragg peak.
Satellite reflection (0 1.25 0)
Experimental setup
Incident X ray beam
Chain axis
Diffracted Beam CCD camera
Gold contacts
Two interest of this experiment:-Coherent beam sensitive to phase defects.-Micrometer scale beam.
Above the threshold
Bellow the threshold 2D diffraction
patternsIncreasing current values
I=0 mA
I=0.8 mA
I=1.4 mA
I=1.8 mA
I=0.2 mA
I=0.4 mA
I=0.6 mA
I=1.6 mA
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First Result:Satellite in transverse direction.
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Satellite in the transverse direction for different current value, bellow (colon 1) and above (colon 2) the threshold. (IS=0.9 mA)
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Satellite with I=1.8 mA
Satellite with I=0.2 mA
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I=0 mA
I=1.8 mA
I=1.6 mA
I=0.8 mA
I=0.6 mA
I=1.4 mAI=0.4 mA
I=0.2 mA I=0 mA
The shift of the satellite change sign when we exceed the threshold.
-No speckle and no shape change.
The shift δq= +/- 0.26 10-4 Å-1 which correspond in the real space to a change of the wave length δλ=+/-8.10-4 Å
Second Result:Satellite in
longitudinal direction.
Satellite in direction of chain axis for different current value, under (colon 1) and above (colon 2) the threshold. (IS=0.9 mA)
Speckle signature of creepBellow thresholdPhase equation:
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2 zyxv
tEe F
-We suppose to be in a stationary case. - Correlations in longitudinal direction are much higher than in transverse direction.
This lead to the following phase equation:
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2 yvEe F
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I=0 mA
I=1.4 mA
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Influence of stepsabove threshold
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I=0 mA
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I=0 mA
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Chain axis
The CDW must vanish at the interface of the step.
Nb Se3 is known to have step at the surface
If some strong pinning defects are present on chain dn, this fixe the phase on these chain (φ(dn)=0) and leads to the solution:
))((21 nn
Fn dydy
veE
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simulation from the previous theory compared to
experiment.
dn dn-2dn-1
φn Φn-1
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Speckle signature of creepBellow threshold
2kF contractionBellow the threshold.Theoretical explanation:
cntotal nnn ein nnn where
The total number of carriers is conserved. But when some current is applied we create a disequilibrium of the normal carrier densities.
xnn cn
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I=0 mA
I=1.8 mA
I=1.6 mA
I=0.8 mA
I=0.6 mA
I=1.4 mAI=0.4 mA
I=0.2 mA I=0 mA
Approximate view of the band structure
δq
Current conversionabove the threshold.
Stress experienced by the CDW
iF
iiF N
nxN
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I=0 mA
I=1.8 mA
I=1.6 mA
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I=1.4 mAI=0.4 mA
I=0.2 mA I=0 mA
Un
Chain axisGold contacts Gold contacts
δλ δq
δλ=0
P. Monceau et al. Plastic sliding of charge density waves. Physical review B
Conclusion
Thank you for your attention!
Different behaviors of the CDW under current probed by coherent X ray diffraction:
Bellow the threshold:
Along the transverse direction. We observe phase defects which is the signature of creep phenomenon.
Along longitudinal direction. Condensation of normal electrons in the CDW.
Above the threshold: Along transverse direction. Influence of
steps on the CDW.
Along longitudinal direction. Current conversion far from contacts.