25 Years of studies on onedimensional Mo blue bronzes New developments

1
25 Years of studies on one-dimensional Mo blue
bronzes New developments

• Claire SCHLENKER, Jean DUMAS, Jacques MARCUS,
• LEPES, CNRS Grenoble

2
Short history

• Mo bronzes known by chemists since 1960
• 1st synthesis of blue bronze K 0.30 MoO3 A. Wold
  et al, MIT 1964 - Bouchard et al., Cornell U 1967
• Crystal structure Graham and Wadsley, Australia
  1966 layer structure
• 1st physicalstudies Perloff et al., Brown U.
  1969 anisotropy - Fogle and Perlstein, John
  Hopkins U. 1972 metal-insulator transition
  attributed to excitons

3
New studies since1980

• Search for exotic materials with bipolarons like
  Ti4O7 Mo blue bronze good candidate
• Transport properties, Brussetti et al. 1981, CNRS
  Grenoble strong anisotropy, quasi 1D material
• Optical reflectivity, Travaglini et al. ETH
  Zurich 1981 quasi 1D metal
• X-ray diffuse scattering studies, Pouget et al.,
  LPS Orsay 1983 Metal-insulator transition is a
  Peierls transition
• Nonlinear conductivity, Dumas et al. CNRS
  Grenoble 1983 charge density wave transport
• Since 1983, hundreds of articles on nonlinear
  transport, structure, thermodynamics,excitations
  etc.. on A 0.30 MoO3

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Outline

• I - Basic properties and Peierls transition
• II - Non linear transport
• III - Spectroscopy studies and excitations
• IV - Vizualisation of CDW, CDW strain and defects
  - New developments
• V - Perspectives

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I - Basic properties and the Peierls transition

• Metallic oxide conduction band empty in MoO3
  partially filled in K 0.30 MoO3 due to electron
  transfer from K - 3 conduction electrons per
  primitive unit cell KMo10O30
• Layered crystal structure and quasi
  one-dimensional electronic structure
• Fermi surface nesting leading to Peierls
  instability and semiconducting charge density
  wave state below the Peierls temperature
• A 0.30 MoO3 A K, Rb, very similar properties

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Quasi one-dimensionnal conductor

• Layers of MoO6 octahedra separated by the alkali
  ions
• Infinite chains of clusters of 10 MoO6 octahedra
  along the 1D b monoclinic axis
• Electron transfer along b through the Mo-2 and
  Mo-3 octahedra inside the clusters
• 3 conduction electrons per primitive cell
  K3Mo10O30

K
Mo1
Mo2 Mo3
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Anisotropic resistivity - Peierls transition

• Higher conductivity along the 1D b-axis
• Metal-semiconductor transition at
• Tp 180K
• Complete gap opening at Tp

8
Optical reflectivity (Travaglini et al., 1981)

• Polarized reflectivity strong anisotropy at 300
  K, metallic //b, semiconductor ??1D conductor
• At low temperature, semiconductor
• Peierls gap 0.15 eV

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Peierls structural instability

• X-ray diffuse scattering (J.P. Pouget et al.
  1983)
• TgtTp elongated diffuse segments corresponding to
  2D anisotropic fluctuations in layer plane
• TltTp satellite peaks at qbb, qb 0.72 at 300K
• qb ? 0.75 at low temperature

10
Modulated structure

• Schutte and De Boer, 1993
• Largest modulation amplitudes on the Mo(2) and
  Mo(3) sites, perpendicular to the 1D b axis
• Displacements of 0.03 and 0.05 Â

MoO6 layers // to b and 102
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Band structureDensity functional theory (Mozos
et al.2002)
Fermi surface nesting qb0.75 b 2 kF

• 2 Mo dxz-O pz bonding and antibonding bands at
  the Fermi level - 3 electrons per primitive cell,
  for 2 bands on the average 3/4 filled (Canadell
  and Whangbo, 1991)
• Consistent with ARPES (Gweon et al.) and
  diffraction

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Low temperature specific heat

• Konaté 1984, Dalhauser et al. 1986
• Requardt et al. 1997
• Odin et al., Lorenzo and Requardt 2002
• 0.1KltTlt0.6K Low energy excitations due to
  disorder in the CDW
• 0.7KltTlt3.5K acoustical phonon contribution, T3
  law
• 2KltTlt30K Bump in C/T3 Phason contribution or low
  energy phonon modes?

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II - Nonlinear transport (Dumas et al., 1983)

• Voltage-current curve, threshold field
  Et 0.14 V/cm Depinning of the CDW for Egt Et
• Pulses for Elt Et
• Noise for Egt Et

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Current oscillations (narrow band noise)
Spectral analysis of the voltage at 77 K, Rb 0.30
MoO3 Noise frequency vs CDW current J CDW /F
nc(T) e Vd F noise frequency (washboard
frequency) nc(T) electron concentration in the
CDW Vd drift velocity of the CDW
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Hysteresis and metastability
Differential resistance vs current. 1 virgin
state Remanent resistance vs current applied
during cooling (TRR) or vs Imax for Isothermal
Remanent Resistance (IRR) Larger effects in Fe
doped crystals role of defects Similarities with
spin glasses
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Nonlinear transport and metastability
Peak width
V
t

• Low temperature properties depend on cooling
  conditions applied field, quenching
• Transport CDW voltage fluctuations (Dumas et
  al., 1984)
• X ray superlattice peak width (Fleming et al.,
  1985)
• Role of CDW defects (dislocations..)

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Low temperature nonlinear conductivity

• G. Mihaly and G.X. Tessema, 1986
• G. Mihaly, P. Beauchêne et al., 1988
• Large threshold field at low T - Very low damping
  due to freezing of carriers

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Temperature dependence of the threshold field
Dumas et al., 1993 2 Different regimes Low T
Et2 10 V/cm High T Et1 0.1V/cm 2 behaviours
at Tgt100K depending on the sample quality
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And many other studies on nonlinear transport

• Bouffard et al. Mihaly et al.
• Cava et al. Sambongi et al.
• Fleming et al. Tessema et al.
• Jamet et al. Tsutsumi et al.
• Janossy, Kriza et al. Wang and Ong
• Maeda et al.

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III - SPECTROSCOPIES AND EXCITATIONS - 1. NMR

• Rb 0.30 MoO3 NMR Berthier et al., 1985
• Lineshape of the 87Rb first order quadrupole
  satellite
• Establishes that the CDW is incommensurate below
  Tp
• Lineshape depends on cooling/ heating cycles
  around Tp metastability attributed to coupling
  of CDW with defects .

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NMR study of the CDW velocity

• Janossy et al., 1987
• Motional narrowing variation of the NMR
  lineshape vs CDW current- Fixed field, E 15 Et,
  CDW velocity swept by changing T
• Oscill. Freq. of the nuclei?d Vd/?, Vd drift
  velocity of the CDW
• lt?d gt/JCDW11kHz/A.cm-2 consistent with the
  density of cond. electrons in the CDW, lt?dgt is
  the noise frequency F

jCDW A cm-2
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Conclusion of the NMR studies

• All available electrons condensed in the
  incommensurate CDW
• A large number of domains are involved in the
  sliding process
• A distribution of CDW velocities accounts for the
  NMR lineshape and noise inhomogeneous CDW sliding

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2 - Angular resolved electron spectroscopy

• Veuillen et al., 1987
• 1st studies of ARUPS on samples cleaved in UHV,
  (-2 0 1) surface
• Conduction band peak
• angular dependent, consistent with kF 0.375 b
• ? angle between the direction of collection and
  the normal to the surface

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Further ARPES studies

• Dardel et al., 1992
• Gweon et al., 1996
• Fedorov et al. 2000, observation of the
  temperature dependent Fermi wave vectors
• Perfetti et al., 2002, lineshapes indicate heavy
  quasi particles consistent with mobile polarons
• Ando et al., 2005, 2 conduction bands ?

Fermi energy intensity map 200K showing the
bonding (red) and antibonding (blue) Mo4d-O 2p
bands
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Digression on polarons and bipolarons

• ARPES results?mobile small polarons?
• Original studies of the blue bronze search for
  localized bipolarons, as in Ti4O7
• Strong electron phonon coupling model (Aubry and
  Quémerais, 1989)?effective bipolarons with
  wavefunction overlap, incommensutate CDW become
   non analytical  above a critical electron
  phonon coupling, the phason gap is finite
• Quantum fluctuations destabilize Fröhlich
  incommensurate CDW, only bipolaronic CDW can
  survive!
• CDW models should be revisited?

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3 - Optical properties CDW excitations

• CDW excitations
• Amplitude mode or amplitudon, does not move the
  CDW nodes and can be observed by Raman scattering
• Phase mode or phason corresponds to an
  oscillation of the CDW phase, can be observed by
  reflectivity studies

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Raman scattering amplitudons
Raman scattering spectra, showing the amplitude
mode and the T dependence of the coupling
coefficient - Travaglini et al., 1983
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Far infra-red optical reflectivity phasons

• Travaglini and Wachter, 1984
• Temperature dependence of the far IR optical
  conductivity attributed to a phase mode

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4 - Inelastic neutron and x-ray scattering of the
CDWexcitations
Pouget et al., 1991
Kohn Anom.
Neutron scattering on a crystal of 5 cm3 (M.
Sato) dispersion of low-lying phonon branches at
230 K, Kohn anomaly at q2kF. Softening when T?Tp
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Neutron scattering Amplitude mode

• Pouget et al., 1991
• Observation of the amplitude below TP, consistent
  with optical Raman scattering data

frequency
damping
Temperature dependence of the amplitude mode
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Dispersion of the phase mode

• Hennion et al., 1992
• Longitudinal acousticlike phase mode velocity
  along b increases below TP
• Reduction of screening of the Coulomb forces
  below TP
• CDW mass 150 me, consistent with microwave
  conductivity data

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Inelastic x-ray scattering (ESRF) amplitude and
phase mode

• Ravy et al., 2004
• Low energy excitations can be measured with
  synchrotron radiation
• Dispersion curves of the phase mode consistent
  with neutron data

Lines correspond to neutron scattering data
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IV- Vizualisation of CDW, CDW strain and defects

• Many attempts to visualize the CDW, the CDW
  strain under current and the CDW defects
• Electromodulated transmission (Itkis and Brill,
  1995)
• STM (Brun, Wang et al., 2005)
• Coherent x-ray diffraction (Le Bolloch et al.,
  2005)
• X-ray topography (Baruchel et al., 2004)

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Electromodulated IR transm.

• Itkis and Brill, 1994
• Electromodulated transmission affected by an
  electric field above Et, due to different
  screening effects near the contacts
• Spatial dependence for different voltages
• Imaging of CDW strains betwen the contacts
• Consistent with CDW polarization and with
  nucleation of phase defects (CDW dislocat.?)

??/?
2
1
0
Dist. from the contact (mm)
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Elastic and plastic deformations of CDW

• Feinberg and Friedel, 1988 - CDW dislocations
• Ong and Maki, 1985
• Brazovskii, 1996 - model of solitons
• Aubry and Quémerais, 1989 - strong phonon
  coupling limit, bipolarons, defects
  discommensutations

Distortion of the CDW near a contact, the CDW is
 broken  above a bending angle ?C
36
1 - STM observation of surface CDW

• Brun, Wang et al., 2005
• Low temperature studies in an UHV STM system on
  in situ cleaved (-2 01) surfaces of Rb 0.3 MoO3
• Observation of CDW modulation of 0.1 Â along b
• Inhomogeneity of the CDW vector on the surface
  due to Rb distribution
• Possibility of observing CDW dislocations

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Origin of the STM CDW images

• Machado-Charry et al., 2006, analysis by density
  funcional theory
• STM sensitive mostly to the MoO6-I octahedra,
  higher on the surface, although the CDW electron
  density is on the II and III octahedra
• Due to strong hybridization between 3 types of
  octahedra

38
CDW dislocation observed by coherent x ray
diffraction - SR

• Le Bolloch et al., 2005
• 2D diffraction pattern of a CDW satellite
  reflection (5,-1,-3)qC, as a function of a 20
  ?m beam position
• Speckle patterns for some positionsinterf.
  fringes attributed to a local. phase deformation
  of the CDW
• Consistent with a single CDW screw dislocation
  located several ?m below the surface

Model of the CDW screw dislocation
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Synchrotron radiation diffraction imaging - x ray
topography

• Objective observation ofCDWdefects,dislocations
• Imaging at T81K simultaneously from a
  fundamental (Bragg) and from a satellite
  reflection
• a) subgrain structure visible on the fundam.
  reflection since a small part of the sample is in
  diffraction position b) most of
  the sample is imaged on the satellite reflection,
  possibly due to the small coherence length of the
  CDW
• White line under the contact change ?q/q 10-3,
  why?

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V - Perspectives

• Microscopic understanding of CDW transport
  requires the observation of defects in the
  sliding regime the blue bronze is a good
  candidate
• STM, coherent x-ray diffraction and imaging are
  adequate techniques
• Mesoscopic studies should be performed on the
  blue bronze since the screening length is much
  longer than in NbSe3. Thin single crystals
  needles are good candidates.
• And the nature of the phase transition, weak or
  strong electron phonon coupling should be
  revisited

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Main contributors and collaborations

• Collaborations
• J. Allen et al., U.Michigan
• M. Almeida et al,Lisbonne
• S. Bouffard et al.
• C. Berthier et al.,Grenoble
• H. Canadell, Barcelone
• M. Grioni et al, EPFL
• S. Jandl et al., Sherbrooke
• A. Janossy, G. Mihaly, Budapest
• M. Marezio et al.Grenoble
• J.P. Pouget et al, Orsay
• P. Wachter et al.,ETHZur.
• Z.Z.Wang et al., LPN
• M. Whangbo, U. N. Carol.

• Thesis students
• Elisabeth Bervas
• K. Konaté
• A. Arbaoui
• P. Beauchêne
• LEPES
• J. Dumas
• R. Buder
• R. Chevalier
• C. Escribe-Filippini
• D. Feinberg
• H. Guyot
• J. Marcus
• J.Y. Veuillen