Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3

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Collective Charge Excitations below the
Metal-to-Insulator Transition in BaVS3

• Tomislav Ivek, Tomislav Vuletic, Silvia Tomic
• Institut za fiziku, Zagreb, Croatia
• Ana Akrap, Helmuth Berger, László Forró
• Ecole Polytechnique Fédérale, Lausanne,
  Switzerland
• T. Ivek et al., Phys. Rev. B 78, 035110 (2008).

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Out line

• Chain sulfide BaVS3
• Low-frequency dielectric spectroscopy complex
  dielectric function in the insulating phase of
  BaVS3
• Nature of the insulating phase ground state?
• Collective excitations of the orbital ordering

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BaVS3

• Consists of VS3 chains separated by Ba atoms
• Neighboring VS6 octahedra share a face, stack
  along c-axis
• Room Temperature primitive hexagonal unit
• 2 formula units per primitive cell
• At 240 K transition to orthorhombic structure
• At 70 K monoclinic structure
• Internal distortion of VS6 octahedra
• Tetramerization of V4 chains

S
Ba
V
Lechermann et al., PRB 76, 085101 (2007)
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BaVS3

• 2 electrons in
• a wide A1g band (dz2)
• narrow Eg1, Eg2 bands (et2g)

• Filling of bands governed by Coulomb repulsion,
  local Hunds rule coupling
• A1g, Eg1 close to half-filling
• Metal-to-insulator phase transition at TMI70 K
• Diffuse x-ray scattering Fagot et al., PRL 90,
  196401 (2003)
• pretransition fluctuations up to 170 K
• qc 2kF (A1g) superstructure
• characteristic for a Peierls transition and
  Charge Density Wave ground state
• No charge disproportionation in anomalous x-ray
  scattering! - Fagot et al., PRB 73, 033102 (2006)
• Magnetic transition at T?30 K incommensurate
  magnetic ordering (Nakamura et al., J. Phys. Soc.
  Jpn. 69, 2763 (2000), Mihály et al., PRB 61,
  R7831 (2000))

Lechermann et al., PRB 76, 085101 (2007) LDA
DMFT

• Nature of MI transition?
• Ground state?

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Samples

• Needle-like single crystals grown along c-axis,
  hexagonal cross-section
• 3 x 0.25 x 0.25 mm3
• Important quality check suppression of
  insulating phase at 20 kbar
• Contacts
• evaporated 50 nm chrome
• evaporated 50 nm gold
• DuPont silver paint 6838 cured at 350C for 10
  min in vacuum

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Low-Frequency Dielectric Spectroscopy
Ivek et al., PRB 78, 035110 (2008)

• 0.01 Hz 10 MHz
• Complex conductivity -gt
• Complex dielectric function
• Insulating phase
• single symmetrically widened overdamped loss peak
• reminiscent of a Charge Density Wave phason
  response (Littlewood, PRB 36, 3108 (1987))

• What is the connection of this relaxation with
  the MI transition?

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Metal-InsulatorPhase Transition

• TMI 67K peak in dc resistivity derivation
• dc gap 2?500 K corresponds to the optical gap
  (Kézsmárki et al., PRL 96, 186402 (2006))
• Peak in ?e at the same T!
• Screening by free charge carriers

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CDW Phasons?

• Do we have a long-wavelength, phason response?
• Screening by free charge carriers Littlewood
• Unexpected ?e behavior
• CDW ?e(T)const.107
• Lack of a significant non-linear dc conductivity
  no sliding
• Another DW phason fingerprint a narrow microwave
  pinned mode
• no experimental results

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Hopping conduction?

• Cross-over frequency far above the observed
  dielectric response
• Optical conductivity not enhanced compared to dc
  values
• Not a candidate

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Ferroelectric nature of the MI transition?

• Below TMI noncentrosymmetric
• structure with a polar axis in the
• reflection plane of VS3 chains
• High polarizability of electron
• system coupled to V4 displacements
• could induce high ?e
• BVS (Fagot et al., Solid State Sci. 7, 718
  (2005)) some charge disproportionation at low T
• But, overestimated due to a nonsymmetric V4
  environment, thermal contraction, imprecise
  atomic coordinates (Foury-Leylekian (2007))
• Charge redistribution not larger than 0.01e
  (Fagot et al., PRB 73, 033102 (2006))
• FE cannot explain our dielectric results

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Orbital ordering?

• No charge modulation in the insulating phase
• Fagot et al., Lechermann et al. modulation of
  orbital occupancy
• 51V NMR and NQR measurements suggest an orbital
  ordering below TMI that is fully developed only
  at Tx (Nakamura et al., PRL 79, 3779 (1997))
• Magnetic susceptibility (Mihály et al., PRB 61,
  R7831 (2000)) lack of magnetic long-range order
  between TMI and T?
• Magnetic anisotropy (M. Miljak, unpublished) AF
  domain structure below T?

Fagot et al., PRB 73, 033102 (2006)
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Interpretation in the context ofOrbital Order

• Primary order parameter for the MI phase
  transition
• 1D Charge Density Wave instability
• Orbital ordering transition happens at TMI,
  driven via structural changes, tetramerization
• Domains of OO gradually develop in size with
  lowering temperature
• OO coupled with spin degrees of freedom, drives
  the spin-ordering into an AF-like ground state
  below 30K domains persist!
• Short-wavelength excitations of domain walls

• ?e collective excitation density, i.e. number
  of domain walls
• Domains consolidate number of domain walls
  diminishes with cooling
• ?e decreases only down to T?
• Below that a long-range spin ordering is
  established and ?e stays constant

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Conclusion

• BaVS3 system with orbital degeneracy
• Metal-Insulator transition at TMI67 K
• Magnetic transition at T?30 K
• Low-Frequency Dielectric Spectroscopy the
  observed mode cannot be assigned to phason
  excitations
• Density of excitations decreases from TMI with
  decreasing T, becomes constant under T?
• Short-wavelength excitations lt-gt Orbital Ordering

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Hopping
Dyre and Schroeder, Rev.Modern Physics 72, 873
(2000)
b) frequency marking the onset of ac conduction
ncross is roughly proportional to the dc
conductivity Barton-Nakajima-Namikawa
relation connects sdc and dielectric loss peak
frequency t0-1 sdc ? t0-1
- BaVS at low T sdc ? 10-5 10-6 W-1cm-1 ?
ncross expected at gt 1 MHz - For BaVS simple
calculation yields ncross (25 K) 360 MHz and
ncross (50 K) 3.8 GHz
T.Vuletic et al., Physics Reports 428, 169 (2006).
c) t00 ? 1ns is too long to be attributed to
quasi-particles
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Contacts
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Low-Frequency Dielectric Spectroscopy

• Complex conductivity as a function of frequency

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Low frequencies, high impedances

• Lock-in current preamplifier
• Voltage output
• Measuring the current
• 10 mHz 3 kHz
• Resistances up to 1 TO

sample
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Autobalancing bridge

• 10 Hz up to 100 MHz
• Resistances up to 1 GO
• Virtual ground avoids capacitive coupling to
  ground
• Lc is kept at 0 potential by a feedback loop

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Dana analysis

• We measure complex admittance YGiB as a
  function of frequency
• After subtracting the background, complex
  dielectric function is given by

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Havriliak-Negami model dielectric function

• ?? ?(0)-?(?) dielectric strength
• ?0 mean relaxation time
• (1-?) relaxation time distribution width

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