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Optical investigations of the influence of electronic correlations in  -(BEDT-TTF) 2 M Hg(SCN) 4

Optical investigations of the influence of electronic correlations in  -(BEDT-TTF) 2 M Hg(SCN) 4. Natalia Drichko. 1. Physikalisches Institut, Universität Stuttgart.

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Optical investigations of the influence of electronic correlations in  -(BEDT-TTF) 2 M Hg(SCN) 4

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  1. Optical investigations of the influence of electronic correlations in -(BEDT-TTF)2MHg(SCN)4 Natalia Drichko 1. Physikalisches Institut, Universität Stuttgart M. Dressel,Ch. Kuntscher, A. Pashkin 1. Physikalisches Institut, Universität Stuttgart J. MerinoDpt. de Física Teórica de la Mat. Cond., Universidad Autónoma de Madrid A. Greco Facultad de Ciencias Exactas Ingeniería y Agrimensura e Instituto de Física Rosario, UNR-CONICET, Argentina J. Schlueter Material Science Division, Argonne National Laboratory

  2. Our interest in influence of electron-electron interactions: general features in phase diagrams of correlated solids • heavy fermions: • high-temperature superconductors • q1D and q2D organic conductors electron-electron correlations N. D. Mathur et al. Nature 395, 39 (1998)

  3. electronic correlations (on-site U, inter-site V) _____________________________________ Tuning parameters: bandwidth W Tuning the parameters of the conducting layers in q2D: • bandwidth depends on overlap intergrals between • the neighboring molecules: W~t • W is about 1eV for these compounds • on-site (U) and inter-site (V) electronic correlations: • depend on the molecule • effective U is about 0.5eV • strong influence of electron-electron correlations • band filling depends on stoichiometry

  4. Proved for ½ filled materials: Proposed for ¼ filled materials: U / W V / W _____________________________________ electronic correlations (on-site U, inter-site V) Tuning parameters: bandwidth W Quasi-two-dimensional organic conductors (BEDT-TTF)2X K. Kanoda Hyperfine interactions 104 (1997) 235 J. Merino et al. PRL 87, 237002 (2001)

  5. 1/4 – filled conductors: phase diagram of - phases. charge order metal-insulator transition  - (BEDT-TTF)2MM‘(SCN)4 Charge order fluctuations above the temperature of phase transition: NMR: R.Ciba et al. PRL 93 216405 (2004) Raman: K. Suzuki et al. Synth. Met. 135-136, 525 (2003) Transfer integral t (10-2 eV) H. Mori et al. Phys. Rev. B 57 (1998) 12023

  6. a-(BEDT-TTF)2I3 1/4 – filled conductors:-phases • charge-order MI transition • in a-(BEDT-TTF)2I3 at Tc=135 K • no evidence of structural changes • charge disproportionantion found by: • -IR spectroscopy (Moldenhauer et al. Synth.Met.60, 31 (1993)) • NMR (T. Takano et al. J. Phys.Chem.Solids 63, 393, 2001) • Raman (R Wojciechowski PRB 67 224105 (2003)) • CO pattern: stripes perpendicular to the stacks • pressure drivesit to conducting state • (N. Tajima J. Phys.Soc. Japan 69, 543 (2000)) • Evidence for charge disproportionation • above Tc: d0.05e • NMR:S. Moroto et al. J. Phys. IV France 114 339 (2004) • Raman: R Wojciechowski PRB 67 224105 (2003) (BEDT-TTF)2MHg(SCN)4 (BEDT-TTF)2NH4Hg(SCN)4superconductor (Тс~ 1 K) (BEDT-TTF)2KHg(SCN)4 (BEDT-TTF)2RbHg(SCN)4 (BEDT-TTF)2TlHg(SCN)4 metallic d.c.conductivity at ~ 8 K enter density-wave-state

  7. a-(BEDT-TTF)2I3 -(BEDT-TTF)2MHg(SCN)4 temperature dependence • Our present interest: • properties of -(BEDT-TTF)2MHg(SCN)4 • at temperatures above • the density wave state (T > 8 K ) • Importance of electron-electron interactions • for the metallic state of these compounds • Systematic change of the influence • of electronic correlations • in -(BEDT-TTF)2MHg(SCN)4 family • Approaching charge order transition • from the metallic side: increasing V/t (BEDT-TTF)2MHg(SCN)4 V/t

  8. A B p2 c4 p4 c1 p3 A‘ C c c3 p1 c2 I II Unit cell parameters and transfer integral values for -(BEDT-TTF)2MHg(SCN)4 Structure of BEDT-TTF layer a Calculated values of transfer integrals a • overlap integrals p • (in the a-axis direction) • have higher values H. Mori et al. Bull. Chem. Soc. Jpn, 63 (1990) 2183.

  9. 1 Reflectivity 0 Conductivity 100 101 102 103 104 105 • What we measure: • Reflectance measurements in the range 50-7000 cm-1 at 6-300 K • ambient pressure • Reflectance measurements in the range 800-7000 cm-1 at 300 K, • hydrostatic pressure up to 2 GPa What we can learn: If the charge localization is not complete and does not affect all carriers, Drude-like component remains which corresponds to itinerant charges; In the optical response we expect a dip in the reflectivity, pseudogap in the conductivity, and a narrow Drude contribution in the gap.

  10. -(BEDT-TTF)2NH4Hg(SCN)4 300 K Rmin c Rmax a Optical studies in the conducting plane: at 300 K similar spectra for all the -(BEDT-TTF)2MHg(SCN)4 compounds in 50-10000 cm-1 range: reflectivity is higher in the higher overlap direction Drude- peak is present at 300 K molecular vibrations: Ag vibrations of BEDT-TTF activated by coupling with electrons

  11. Drude Lorentz Lorentz can be distinguished at T < 200 K Interpretation of the electronic features in the spectra -(BEDT-TTF)2NH4Hg(SCN)4 U = 20t, different V/texact diagonalization calculations on anextended Hubbardmodelfor ¼ filled metals close to charge order t ~ 0.1 eV 1000 4000 cm-1 J. Merino Phys. Rev. B 68 245121 (2003)

  12. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I Confirming the interpretation of the spectra: application of hydrostatic pressure C. Campos et al. PRB 53 12725 (1996): at hydrostatic pressure up to 10 kbar Inter-stack transfer integrals increase in-stack transfer integrals do not change Expected: shift of the spectral weight to low frequencies with the decrease of V/t

  13. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I Reflectivity of -(BEDT-TTF)2KHg(SCN)4 under hydrostatic pressure at 300 K Estacks Conductivity received by a Drude-Lorentz fit: Estacks E|| stacks

  14. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I Cooling down: Thermal contraction Inter-stack transfer integrals increase in-stack transfer integrals do not change E. Ono et al. J. Phys. Soc. Jap (1997) 1. 2. Decrease of V/t due to increase of t Increase of the total spectral weight due to increase of the bandwidth W=8t

  15. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I Redistribution of the spectral weight on cooling -(BEDT-TTF)2NH4Hg(SCN)4 Temperature dependence of the total spectral weight Rstacks Rstacks R||stacks

  16. a A A B B p2 p2 c4 c4 p4 p4 c1 c1 A‘ A‘ C C c c p3 p3 c3 c3 p1 p1 c2 c2 II II I I a Estacks • Increase of inter-stack transfer integrals • under pressure and on cooling: • increase of the total spectral weight • decrease of V/t leads to a shift • of the spectral weight from MIR features to Drude E || stacks • In-stack transfer integrals do not change on cooling • bandwidth stays constant • V/t stays constant

  17. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I a-(BEDT-TTF)2NH4Hg(SCN)4: superconductor Tc~1 K E||stacks • Total spectral weight • is conserved • Spectral weight shifts from • high frequencies to Drude-component • on cooling down: • Drude scattering rate decreases • from 600 to ~20 cm-1 on cooling Drude spectral weight grows on T decrease MIR spectral weight decreases on T-decrease

  18. Theoretical predictions of temperature dependance: `increase` of metallic behavior at low temperatures cooling t The calculated border between metallic and charge ordered phase shifts to higher V/t values on temperature decrease

  19. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I a-(BEDT-TTF)2RbHg(SCN)4: metal, DW state below ~ 8 K E||stack • Spectral weight shifts from high frequencies • to Drude-component on cooling down • the change is less pronounced • than for a-(BEDT-TTF)2NH4Hg(SCN)4; • Increase of the spectral weight • of the feature at ~1000 cm-1 • Drude scattering rate decreases • from 1000 to ~80 cm-1 on cooling Drude spectral weight grows on T decrease MIR spectral weight decreases on T-decrease

  20. -(BEDT-TTF)2RbHg(SCN)4 -(BEDT-TTF)2NH4Hg(SCN)4 Temperature dependence of effective mass for -(BEDT-TTF)2MHg(SCN)4 , M = NH4 and Rb Calculations of mDrude/mb band mass: mDrude: Drude plasma frequency

  21. T* Temperature dependence of the Drude scattering rate • Theory predicts a change • from 1/t ~ T to1/t ~ T2 behaviour at T* ~ 0.1t • T*= 50 K for t=0.06eV • Larger temperature changes of the 1/t • for more correlated compound • Effects of electronic correlations • increase from NH4 to Rb T* V/t J. Merino Phys. Rev. B 68 245121 (2003)

  22. cooling t Rb NH4

  23. 100 K CHARGE ORDER 10 K charge fluctuations 1 K METAL I3 K Tl Rb NH4 CO Insulator -(BEDT-TTF)2X V/t

  24. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I a-(BEDT-TTF)2TlHg(SCN)4: metal, DW state below ~ 8 K In both polarizations increase of the spectral weight of the feature at ~1000 cm-1 at T<200 K E||stacks Estacks Intensity at about 1000 cm-1 increases

  25. a A B p2 c4 p4 c1 A‘ C c p3 c3 p1 c2 II I -(BEDT-TTF)2KHg(SCN)4: metal, DW state below ~8 K E||stacks In both polarizations increase of the spectral weight of the feature at ~1000 cm-1: at T < 200 K. Dressel, et al., PRL 90, 167002 (2003)

  26. t Charge order fluctuations in a-(BEDT-TTF)2MHg(SCN) M = Tl, K Growing intensity of the feature at 1000 cm-1 points of increase of CO fluctuations on temperature decrease Similarity with -phases: CO fluctuations close to MI transition

  27. 100 K 10 K 1 K METAL I3 DW K Tl Rb NH4 CO Insulator Phase diagram for a-(BEDT-TTF)2MHg(SCN)4 pressure CHARGE ORDER charge order fluctuations SC V/t

  28. Conclusions: • In the spectra of all -(BEDT-TTF)2MHg(SCN)4 compounds Drude peak is present; some spectral weight is shifted to the high frequencies due to correlation effects. • On the increase of the size of transfer integrals with the application of hydrostatic pressure and on cooling the spectral weight shifts from the finite-frequency features to the Drude-peak. • -(BEDT-TTF)2MHg(SCN)4 (M=NH4, Rb) show decrease of the effective mass of charge carriers on cooling in agreement with the shift of Vc/t ratio to lower values on temperature decrease. Temperature dependence of the scattering rate is in agreement with the theory for the scattering on the fluctuating charge order. Higher effective mass and scattering rate of Rb compound point on higher electronic correlations compared to NH4 salt. • -(BEDT-TTF)2MHg(SCN)4 (M=Tl, K) show an increase of the spectral features due to charge order fluctuations on cooling.

  29. Acknolwlegements to Alexander von Humboldt foundation

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