Charge ordering in quasi onedimensional semiconductor NbSe43I

1
Charge ordering in quasi one-dimensional
semiconductor (NbSe4)3I

• D. Stareinic, K. Biljakovic
• Institute of Physics, Zagreb, Croatia
• P. Lunkenheimer, A. Loidl
• University of Augsburg, Germany

2
(MSe4)nI family

• M Ta, Nb (dz2 orbitals)

• n governs band filling

• n2,10/3 metallic with CDW below RT

• n3, MNb narrow band semiconductor at RT

• n3 6 Se4 rectrangles in unit cell without
  screw symmetry

? gap in zone center

• Nb trimerization (CO) in addition

P. Gressier et al., Mat. Res., Bull. 20, 539
(1985) P. Gressier et al., Inorg. Chem. 23, 1221
(1984)
3
Structural transition at 274 K

• 2nd order displacive transition
• - relative shift of two chains
• - no center of symmetry

• pseudo Jahn-Teller effect in narrow band
  semiconductors

? increase of LT gap

• intrachain Nb atom rearrangement

• only small kink in s(T)

P. Gressier et al., Mat. Res., Bull. 20, 539
(1985) T. Sekine, M. Izumi, Phys. Rev. B 38, 2012
(1988)
4
New results

• inexplicable features in optics and ARPES 1

• fs spectroscopy 2 incomplete phonon
  softening at Tc

? purely electronic order parameter (central peak)

• huge thermopower changing sign at LT 3

1 V. Vescoli et al., Phys. Rev. Lett. 84, 1272
(2000).
2 D. Dvorek et al., submitted
3 M. Ocko, private communication
5
Dielectric spectrosopy

• broadband dielectric response 10 mHz - 100 MHz
• Wide temperature range 1.5 K - 300 K
• frequency-response analysis Novocontrol
  a-analyzer
• reflectometric technique impedance analyzer
  Agilent 4294A
• needle-shaped samples S 510-9 - 610-8 m2 l
  0.9 4.5 mm
• crystallographic c-direction
• no non-linear effects, no hysteresis
• Complex dielectric susceptibility e(n) from
  complex conductivity s(n)

P. Lunkenheimer et al., Contemporary Physics 41,
15 (2000).
6
Low frequency dielectric response

• increase of Re e below Tc
• maximum at 150 K
• huge value at maximum
• strong dispersion
• similarity to relaxor ferroelectrics (dilute with
  frustration and disorder)
• similarity to isostructural CDW systems 1

1 T. Sekine et al., Physica B 143, 158 (1986).
7
Frequency dependence

• relaxational dynamics
• strong slowing down
• described by Cole-Cole

• De amplitude
• t0 relaxation time
• narrow distribution 1-a0.8

8
Characteristic parameters

• e? negligible, De follows low frequency Re e

• t0 activated growth with Dt0.11 eV

• nearly follows activated growth of rdc (Dr0.13
  eV)
• resembles the effect of free carrier screening
  in CDW 1

1 P. B. Littlewood, Phys. Rev. B 36, 3108
(1987).
9
Wide temperature dc conductivity

• no narrow gap
• ? crossower between HT and LT activated regime 1

• Tc in the middle of the plateau
• DHT/DLT?3/2
• no nonlinear conductivity to 100 V/cm below RT

1 P. Gressier et al., J. Solid State Chem. 51,
141 (1984)
10
Structure evolution vs dc conductivity

• two stable distorted structures with different
  nodal properties at the top of valence band 1

• very small stability difference

• continuous change from (i) to (ii) would give
  qualitatively the measured r(T)
• observed RT 2 and LT 3 structures corresond
  to (i) and (ii) respectively

1 P. Gressier et al., Inorg. Chem. 23, 1221
(1984)
2 P. Gressier et al., Mat. Res., Bull. 20, 539
(1985) 3 M. Izumi et al., Synthetic Metals 19,
863 (1987)
11
JT distortion vs Nb displacement

• JT distortion selects the nodal properties at the
  top of valence band

• Nb atoms do not follow immediately

• interchain ferroelectric interaction competing
  with antiferroelectric structural ordering

12
Conclusion

• (NbSe4)3I is not so narrow band semiconductor
• transition from HT to LT activated regime
  consistent with structural changes
• JT transition does not rearrange Nb atoms within
  the chain
• Nb atoms rearrangement continuous in T
• high e and strong dispersion probably due to the
  FE and AFE competition