Optical Response of Sliding Charge Density Waves Martin Dressel 1. Physikalisches Institut, Universit

1
Optical Response of Sliding Charge Density
WavesMartin Dressel1. Physikalisches Institut,
Universität Stuttgart, Germany
Rossendorf 19. Januar 2001

• Outline
• Blue Bronze
• Electrodynamics of CDW
• Optical Response of the Sliding CDW
• Conclusion and Outlook

• Universität Stuttgart
• B. Gorshunov, S. Haffner
• Universität Frankfurt
• B. Lommel, F. Ritter, W. Assmuss

2
Blue Bronze K0.3MoO3

• quasi one-dimensional conductor
• lattice parametera 16.25 Åb 7.56 Å c
  9.87 Å
• charge density wave ground state

conductivity(at T 300 K) ? 1500 (?cm)-1
transition temperatureTCDW 183 K threshold
fieldET 50 500 mV/cm
3
Blue Bronze K0.3MoO3charge density wave formation
room temperature conductivity? 1500
(?cm)-1 anisotropy?b ?c ?a 1000 30
1
polarized optical reflectivitystrong
anisotropymetallic behavior parallel to
chainsweak metallic behavior perpendicular to
chains
Rouxel et al. 1989
Gorshunov et al. 1993
4
Charge Density Wavesin one dimensional conductors
metallic state constant charge distributionparabo
lic energy bandsfilled up to the Fermi
wavevector metallic conductivity
CDW state spatially modulated charge
densityenergy gap at the Fermi
energy semiconducting conductivity
5
Non-Linear Conductivity field dependent
electrical transport
depinning of collective mode the CDW condensate
is pinned to impurities for E gt ET the CDW is
depinned andmoves coherently
K0.3MoO3 T 4.2 K
Grüner et al. 1994
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Low-Frequency Excitations of the CDW Ground
State frequency dependent conductivity
internal deformations of the density wave
distortion due to interaction with
impuritiesinternal polarization, relaxation
Mihaly et al. 1989
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Current Oscillations in the CDW Ground
State narrow-band noise
ac response to a driving dc current frequency f
vd/? with ? ?/kF f 1/h
2vFvd proportional to the excess current
Dumas et al. 1983
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Optical Properties of the Density Wave Ground
State frequency dependent conductivity

• two feature can be distinguished
• pinned mode resonance in the GHz
  rangecollective response of the charge density
  wave pinned to imperfections
• single particle gap in the FIRexcitations
  across the energy gap at the Fermi edge

9
Optical Properties of K0.3MoO3 fluctuation
effects paraconductivity
Conductivity due to phase fluctuations of the
electrons condensed into correlated CDW
segments. This is observed in the temperature
intervall 183 K TCDW lt T lt TMF 330
K diffusive motion free electrons oscillatory
motion pinned to impurities
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Collective CDW Excitationpinned mode resonance
decreasing conductivity as temperature lowers
below TCDW 180 K development of a pinned mode
around 4.5 cm-1 band width 1 cm-1 strong
increase in dielectric constant fluctuating CDW
seeneven at room temperature
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Joint AC-DC Experiments
Does the ac (optical) response change if a dc
voltage is applied? How do the collective CDW
excitations change when an electric field is
applied? Does the pinned mode resonance
vary? Do we see the amplitude mode? Do other
modes appear?
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Amplitude and Phase Excitations of a CDW

• There are two distinctively
• different collective excitations
• possible in the CDW ground state.
• Amplitude modeis related with a change in
  kinetic energy and thus ?A2(q0) ?1/2?2kF
• Phase modetranslational motion of the
  undistorted condensatein the ideal case it is
  gapless??(q0) 0

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Optical Response of the Collective CDW Excitations

• major contributions
• phason mode in the GHz range
• amplitude mode in the FIR
• phonons in the FIR

phason mode
amplitude mode
phonons
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Optical Properties of the CDW Ground
State collective response

Travaglini and Wachter, 1984 Ng et al.
1986 Degiorgi and Grüner, 1991
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Optical Properties of the CDW Ground State Raman
scattering
amplitude mode of the CDW at around 50 cm-1 in
K0.3MoO3
Travaglini et al. 1983
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Non-Linear Tansport field dependent conductivity
threshold field at 77 K ET 6 V/cm depinning of
charge density wave collective charge transport
I
V
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Joint AC-DC Experiments change of the optical
properties by applying an electric field
I

• How can we be sure that
• we do not heat the sample?
• measurements in both directions of polarization
• measurements at different temperature
• measurements at different applied dc field
• We observe a sudden change of the optical
  properties when EgtET.

V
focus of optical beam
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Optical Response of the Sliding CDW

• Experimental procedure
• measure the un-biased reflectivity of K0.3MoO3
• without moving the samplemeasurements of the
  ratio bias - non-biasfor increasing fields
• significant changes as the electric field
  exceeds the threshold field
• calculation of the bias-reflectivity

19
Optical Response of the Sliding CDW dependence on
applied dc field

• Experimental procedure
• measure the un-biased reflectivity of K0.3MoO3
• without moving the samplemeasurements of the
  ratio bias - non-biasfor increasing fields
• the effect increases continuous as the dc bias
  field increases

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Optical Response of the Sliding CDW

• major contributions
• phason mode in the GHz range
• amplitude mode in the FIR
• phonons in the FIR
• sliding CDW mode

phason mode
amplitude mode
phonons
sliding CDW mode
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Optical Response of the Sliding CDW

• major findings
• the phason mode does not change in frequency,in
  width, nor in spectral weight
• the amplitude mode does not change in
  frequency,but slightly looses spectral weight
• the additional modeconsists of a number of
  equally spaced ripples,which are due to
  interaction of the CDW condensate with the
  underlying lattice

22
Optical Response of Sliding CDW collective
response

• ac response to a driving dc current
• internal deformationscurrent oscillations,
  interference effectsinterference of CDW and
  pinning centerskHz and MHz range f 1/h 2vFvd
• collective responseno change of phase
  excitationsmodifications of amplitude
  excitationsripple modeinterference of CDW and
  lattice

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Conclusions
We have measured the optical response of K0.3MoO3
single crystals in the charge density wave
ground state. In the millimeter wave and
far-infrared range we found two optical modes,
which we assign to the amplitude and the phason
mode located at 2 cm-1 and 10 cm-1,
respectively. Applying an external electric
field, which exceeds the threshold of non-linear
conductivity, an new mode appears around 4 cm-1,
which exhibits an additional fine structure.We
assign this mode to the sliding charge density
condensate which interacts with the underlying
lattice. We observe no change in the phason mode
(pinned mode resonance) and only a small loss of
spectral weight in the amplitude mode.
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Fourier Transform Spectrometermodified Bruker
IFS 113 v
Spectral range 10 cm-1 10 000
cm-1 Resolution 0.03 cm-1
3 Sources

6 Detectors
Sample chamber
77 KMCT
1.2 KBolometer
6Beamsplitter
4.2 K Bolometer
77 KInSb
DTGS
Genzel-type Michelson interferometer
Reflection unit
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Coherent Source Spectrometer50 GHz - 1500 GHz ,
2 cm-1 50 cm-1
sources backward wave oscillator monochromatic,
coherent tunable, powerful lenses
polyethylene beamsplitter, polarizer free
standing wire grids detector Golay cell,
He-cooled bolometer cryostat 1.5 K 300
K magnet 0 8 Tesla split ring,
superconducting Voigt, Faraday geometry
Mach-Zehnder Interferometer
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1. Physikalisches Institut der Universität
StuttgartMartin Dressel und Boris Gorshunov
collaborations P. Haas, S. Haffner, C.
Kuntscher, T. Rõõm materialsW. Aßmus
(Frankfurt) K0.3MoO3, LiCuVO4J. Akimitsu
(Tokyo) (Ca,Sr)14Cu24O41M.Greenblatt
(Rutgers) Li0.9Mo6O17P. Monceau
(Grenoble) (TaSe4)2IC. Schlenker
(Grenoble) KxP4W8O32C.S. Jacobsen
(Lyngby) TTF-TCNQ
experimentalR. Claessen (Augsburg) Photoemissio
nC. Thomsen (Berlin) RamanS. Tomic
(Zagreb) transport, ac-response theoryA.
Muramatsu (Stuttgart) spin chain,
spin-laddersJ. Voit (Bayreuth) low-dim.
conductorsV.I. Torgashev (Rostov-on-Don) group
analysis
on-going projects LiCuVO4 finished B.Gorshunov
et al., Euro. Phys. J B 23, 427 (2001) KxP4W8O32
finished S. Haffner et al., Euro. Phys. J B
24, 123 (2001) (Ca,Sr)14Cu24O41 to be
analyzed charge density wave TTF-TCNQ to be
analyzed unusual metallic behavior, shift in
spectral weight, energy gap K0.3MoO3,
(TaSe4)2I to continue optical response of the
sliding charge density wave
intended projects (Ca,Sr)14Cu24O41 magnetic
field dependence, ac-dc response, shift in
spectral weight, energy gap SrCu2(BO3)2 B field
dependence of low-energy modes Li0.9Mo6O17 non-Fe
rmi liquid behavior
CuBi2O4 spin chains, correlations CuNb2O6
phonons