ChargeDensityWave

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Charge-Density-Wave

• LU MINGTAO

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Outline

• 1. Peierls Transition
• 2. DC Characteristics
• quasi-particle
• collective excitation
• 3. Negative Resistance
• 4. Explanations
• 5. Conclusion

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Peierls Transition

• The two degenerate ground state of polyacetylene
• approx. 0.08 Å difference between CC and CC
  bond lengths

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Peierls Transition

• n(x,t)n0?ncos(2kFxf(x,t)), kFpNe/a

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Why one-dimension
Brillouin zone and Fermi surface
1D The Brillouin zone and
Fermi surface always overlap with each other
2D
3D
The Brillouin zone and Fermi surface are not
fully match with each in 2D and 3D
Nesting charge
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One-dimensional materials
NbSe3
K0.3MoO3
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DC characteristics
DC characteristics describe the response of CDW
to the applied dc electric field
1) Nonlinear dc response
2) Narrow band noise
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Single particle model
The velocity of the single particle is modulated
by a frequency of ?0

• Washboard potential

The motion of the single particle
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Quasi-particle Mattucks quasi-horse
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Quasi-particle
Entry
Free propagation
Exit
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Collective mode
Collective mode can be measured by optical
methode Phase mode is IR active Amplitude mode is
Raman active
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Negative resistance
When current is larger than 3.5µA, a negative
absolute resistance is observed The dash line is
the average of different segments. It matches
with the I-V curve measured in long distance.
The CDW and quasi-particles are driven by
different force
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Explanations

• Phase slip and amplitude collapse occur at the
  strong pinning center.
• The CDW is driven by the electric potential as
  well as
• the quasi-particle is driven by
  electrochemical potential .
• A vortex may occurs at the strong pinning center

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Conclusion

• Normally, CDW behaves as a semiconductor.
  Different samples show diverse dc and ac
  characteristics. In some samples, we may get
  hysteresis, switching or negative differential
  resistance.
• There is some similarity between CDW and BCS
  superconductivity. CDW has its priority because
  it is one-dimensional.
• The NR could be gotten in a length scale less
  than
• 1µm. The origin of NR is still not clear.
• The quasi-particle and CDW are driven by
  different force.
• The macroscopic defect gives a vortex of the CDW
  motion around the strong pinning center.

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Acknowledgement

• Thanks to my supervisor
• Prof. P.H.M. van Loosdrecht