Imprints of Nano-structured and Mott Phase in Optical Phonons and Continua in 1T-TaS2

Transcription

1 ECRYS 2014 International School-Workshop on Electronic Crystals Imprints of Nano-structured and Mott Phase in Optical Phonons and Continua in 1T-TaS2 Kristijan Velebit Laboratory for the physics of transport phenomena Institut of Physics, Zagreb

2 Co-authors Institute of Physics, Zagreb Department of Physics, Faculty of Science, University of Zagreb 1. Physikalisches Institut, Universität Stuttgart École Polytechnique Fédérale de Lausanne Eduard Tutiš Petar Popčević Neven Barišić Ana Smontara Ivo Batistić Martin Dressel László Forró Helmuth Berger

3 Bulk 1T-TaS 2 S planes Ta plane ( σω ( ))

4 Phases in bulk 1T-TaS2 (two most interesting charge density wave (CDW) phases) * Commensurate(c) phase Mott insulator stabilized by super-structuring * Nearly Commensurate (nc) phase - metal-semiconductor mixture at nano-scale turns superconducting if stabilized to low temperature (by pressure or doping)

5 dc resistivity of 1T-TaS 2 60 c-cdw/mott nc-cdw 40 ρ(t)/ρ(290k) 20 Pure 1T-TaS See: Wilson et al., 1975 Adv. Phys. T (K)

6 dc resistivity of 1T-TaS 2 60 nc-cdw 40 ρ(t)/ρ(290k) 20 Doped 1T-TaS 2 (d-tas 2 ) See: Xu et al., 2010 Phys. Rev. B T (K)

7 σω ( ) Measure optical conductivity to observe/determine: c-cdw and nc-cdw phase * Local charge redistribution in superstructure ( charge profile of stars ) nc-cdw phase * Feature of the optical response of nano-composite: broad surface plasmon * Mixing of metallic excitations and lattice oscillations from separated nano-domains produces asymmetric phonon modes

8 c/mott phase

9 c/mott phase of 1T-TaS 2 Grouping of Ta atoms in real space Charge redistribution within super-cell Brillouin zone reconstruction q2 q1 q0 G1 G2 ky kx Deformation regroups Ta-atoms into lattice of David stars (13 Ta-atoms per star) Deformation & electronic phase transition causes charge redistribution between star constituents Brillouin zone (BZ) in restructured phase is 13x smaller, with 12 (6+6) reciprocal lattice points of parent compound folding into center of new BZ

10 Zone folding and phonon modes Γ G1 G2 ky kx

11 Zone folding and phonon modes Γ G1 G2 ky kx New optically active modes only if there is charge redistribution in addition to the lattice super-structuring!

12 Optical strength of folded phonons R-space q R j, j K-space Γ G 1 G 2 ky Optics 3 f( G) q j exp M j 2 ( ir ) jg Fourier transform of charge distribution on David star kx f 3 ( ) [( ) ( ) ] 2 1, q + 13± 13 q mta + 2mS ( G ) Implication: Phonon Optical Strength Charge distribution in super-cell

13 Simple charge-spring model of 1T-TaS 2 σ(ω) [arb.u.] ω[cm -1 ]

14 Optical conductivity in c/mott phase of 1T-TaS 2 Sulphur mode of parent structure 1T-TaS 2, 23K Ta modes via superstructure S modes from superstructure interband transitions generated in superstructure

15 Ta1 Assignation of modes (motivated by DFT calculations of phonons) Ta4 S0 Ta1, Ta4 Ta2, Ta3(?) S0 f( G ) f( G 1 2) f S q0 q2 q1 Ta2 Ta3 q + 6( q + q ) = f 3 ( ) [( ) ( ) ] 2 1, q + 13± 13 q mta + 2mS ( G ) SW/SW(S0) Ta1 Ta2 Ta3 Ta4 1T-TaS2, 23K 0.44± ± ± ±0.05 1T-TaS2, 200K 0.7± ± ± ±0.06 d-tas2, 23 K 0.39± ± ± ±0.06 d-tas2, 200K 0.22± ± ± ±0.08 Charge q0 q1 q2 Err 1T-TaS2, 23K ±0.02 1T-TaS2, 200K ±0.02 d-tas2, 23 K ±0.02 d-tas2, 200K ±0.02

16 nc phase

17 c-cdw vs. nc-cdw A Spijkerman 1997: kagome-like domain super-structuring

18 Typical optical response of nc phase of 1T-TaS 2 Sulphur mode of parent structure d-tas 2, 23K Ta modes via superstructure S modes from superstructure interband transitions generated in superstructure phonon modes (most) show pronounced assymetry Quasi-constant Drude continuum Sub-interband broad feature? * Coupling to continuum? * Mechanism?

19 Mixed Effective medium theory (EMT) Effective dielectric function: f ε A( ω) ε eff ( ω) 1 q ε A( ω) + ε eff ( ω) q + (1 f ε B ( ω) ε eff ( ω) ) 1 q ε B ( ω) + ε eff ( ω) q = 0 dielectric function (complex): ε A component A ε B component B ε eff effective for mixed medium Bruggeman equation Bruggeman, D.A.G., Ann. Physik (Leipz.) 24, 636 (1935) Qazilbash et al.,prb 79, (2009) f fraction of A q depolarization factor (percolation at f q)

20 General effect of mixing the media (EMT) metal insulator EMT mix mixing generates surface plasmon effect in metallic component composite

21 nc phase Composite Dielectric Components B A Super-structured semiconductor Metal

22 nc phase: plasmonic effect in nano-textured material d-tas2, 23K exp. data Weighted Sum of Components EMT mix Plasmonic mode arising in nano-textured (nc) phase

23 Asymmetry in phonon signals due to phase mixing in nc phase (EMT) component B component A Symmetric EMT mix Weighted Sum of components EMT mix Asymmetric! EMT mix Asymmetry of phonon mode generated by mixing Background provided by surface plasmon in mixture ( Fano without Fano?)

24 σω ( ) Optics can see: ( σω ( )) * Charge redistribution within supercell * Surface plasmon feature of nano-composite (nc phase) * Asymmetric shapes of phonons ( screened by metallic nano-domains)

25 Institutions & Support ZAGREB SVEUČILIŠTE U ZAGREBU UKF 65/10