Charge ordering in quasi one-dimensional semiconductor (NbSe 4 ) 3 I

1. Charge ordering in quasi one-dimensional semiconductor (NbSe4)3I D. Starešinić, K. Biljaković 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 • n=2,10/3: metallic with CDW below RT • n=3, M=Nb: narrow band semiconductor at RT • n=3: 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. 1/t ~ (T-Tc) 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. Dvoršek et al., submitted [3] M. Očko, 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= 5∙10-9 - 6∙10-8 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-a=0.8

8. Characteristic parameters • enegligible, De follows low frequency Re e • t0 activated growth with Dt=0.11 eV • nearly follows activated growth of rdc (Dr=0.13 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