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The S=1/2 quantum magnet TiOCl studied by photoemission spectroscopy Michael Sing (U Würzburg)

The S=1/2 quantum magnet TiOCl studied by photoemission spectroscopy Michael Sing (U Würzburg). M. Hoinkis (U Augsburg) J. Schäfer (U Augsburg) photoemission R. Claessen (U Würzburg) & M. Klemm (U Augsburg) crystals S. Horn (U Augsburg)

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The S=1/2 quantum magnet TiOCl studied by photoemission spectroscopy Michael Sing (U Würzburg)

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  1. The S=1/2 quantum magnet TiOCl studied by photoemission spectroscopy Michael Sing (U Würzburg) M. Hoinkis (U Augsburg) J. Schäfer (U Augsburg) photoemission R. Claessen (U Würzburg)&M. Klemm (U Augsburg) crystals S. Horn (U Augsburg) H. Benthien (U Marburg) DDMRG E. Jeckelmann (U Hannover) T. Saha-Dasgupta (S. N. Bose Centre, Kolkata, India) LDA+U/LDA+DMFT L. Pisani (U Frankfurt/M) R. Valenti (U Frankfurt/M) S. van Smaalen (U Bayreuth) structureJ. Deisenhofer (U Augsburg)J. Hemberger (U Augsburg) ESR, specific heatA. Loidl (U Augsburg)

  2. Outline: • TiOCl: the story so far… • valence band DOS • dispersions and anisotropy • orbital symmetry and fluctuations • conclusions

  3. dxz, dyz dxy eg t2g dxy dxz, dyz b a O Ti Cl TiOCl: crystal and orbital structure • Issues: • electronic structure: Ti 3d1 Mott insulator • dimensionality  1d vs. 2d? • phase transitions  spin-Peierls? orbital ordering? fluctuations? • doping with n- or p-type carriers  Luttinger liquid? RVB superconductor? from LDA+U:Seidel et al. (2003)Saha-Dasgupta et al. (2004) O Ti Cl

  4. ? TiOCl: phase transitions and fluctuations magnetic susceptibility ESR: Deisenhofer et al. (2005)Kataev et al. (2003) • high-T phase: • transition @ Tc2 = 91 K • Bonner-Fisher suscept.(s=1/2 Heisenberg chain) • fluctuations for T >> Tc2: • NMR pseudogap • anomalous phonon softening/broadening in Raman & IR • insufficient release of entropy at Tc1,2 (from heat capacity)J. Hemberger et al., cond-mat/0501517 low-T phase: first ordertransition@ Tc1 = 67 K Ti-dimerization* spin-Peierls phase* *M. Shaz et al. (2005) Tc1

  5. TiOCl: cluster calculations & optics cluster calculations & optics: Rückamp et al., cond-mat/0503409 Orbital degrees of freedom are quenched Intermediate phase: incommensurate order due to frustrated interchain interactions

  6. TiOCl: angle-integrated photoemission and DOS complete valence band DOS T = 300 K Ti 3d1 O 2p/Cl 3p LDA+U with U = 3.3 eV (R. Valenti et al.)

  7. TiOCl: angle-integrated photoemission and DOS Ti 3d only T = 300 K LDA+U with U = 3.3 eV (R. Valenti et al.) Hubbard Model (DDMRG)(H. Benthien, E. Jeckelmann) LDA+DMFT (IPT)(L. Craco et al., cond-mat/0410472) LDA+DMFT (QMC)(T. Saha-Dasgupta et al., cond-mat/0411631)

  8. b a TiOCl: angle-resolved photoemission (ARPES) complete valence bands Ti 3d O 2p/Cl 3p T = 300 K Ti 3dxy

  9. TiOCl: angle-resolved photoemission (ARPES) Ti 3d band T = 300 K

  10. TiOCl: angle-resolved photoemission (ARPES) Ti 3d band (T=300K) • quasi-1D dispersion along b-axis • a-axis dispersion • b-axis dispersion??

  11. c Evertical hn b sample Ehorizontal e- analyzer h: even v: odd dxy: evendxz/yz: odd even dxy dxz/yz TiOCl: polarization effects in ARPES  horizontal polarization probes even states (dxy)  vertical polarization probes odd states (dxz, dyz)  no sizable contribution of dxz,yz states at room temperature

  12. eg dxz,yz dxy (even) t2g dxy eg dxz/yz dxy dxz,yz (odd) t2g TiOCl: polarization effects in ARPES equilibrium RT structure: LDA+U for frozen phonon mode:T. Saha-Dasgupta et al., EPL 67, 63 (2004)  phonon-induced orbital fluctuations excluded

  13. (AR)PES on TiOCl • Results for T=300 K: • electronic ground state: almost pure Ti 3d1 • quasi-1D dispersion along b-axis ( dxy) • dispersive behavior not explained by LDA+U/DMFT, 1D single-band Hubbard model  spin-Peierls fluctuations? 1D multi-band Hubbard model? • no admixture of dxz,yz dynamical Jahn-Teller effect excluded • Open issues: • electronic structure at low T  x-ray absorption spectroscopy • doping away from Mott insulator (Ti 3d1x)  work in progress

  14. THE END

  15. susceptibility (ESR) Cl c Ti O b TiOCl: a low-dimensional s=1/2 quantum magnet • Eigene Kristallzucht (CVT) mit C1 • Charakterisierung mit A2,B2,C1,C2 • Struktur / Phasenübergängemit van Smaalen (Bayreuth) Dimerisierung in Tieftemp.-Phase: Spin-Gap entsteht durch Spin-Peierls-Übergang (1. Ordnung !)

  16. b Fluctuations • Raman scattering1 • Fluctuation regime T < 200K: • Softening of a BZ-boundary phonon • NMR2 • Pseudogap regime T < 135K: • Suppression of low frequency spin excitations • Specific heat3 • Fluctuations delay the release of the full entropy at Tc1. LDA+U4 Frozen phonon approach: Change of orbital occupancies in a distorted structure “The ground state is described by the dyz and dxz orbitals instead of dxy” 1 G. Caimi et al. (Degiorgi group), Phys. Rev. B 69 (12) 125108 (2004)2 T. Imai and F.C. Chou, cond-mat/03014253 J. Hemberger et al., cond-mat 0501517 4 T. Saha-Dasgupta et al. (Valenti group), Europhys. Lett. 67 (1) 63 (2004)

  17. TiOCl: a low-dimensional s=1/2 quantum magnet • Photoemission: "DOS" gesamtes Valenzband LDA+U: R. Valenti et al. (Frankfurt) Ti 3d LDA+U: R. Valenti et al. (Frankfurt) LDA+DMFT (IPT): Müller-Hartmann et al. (Köln) LDA+DMFT (QMC): Valenti/Lichtenstein(Frankfurt/HH)

  18. SEM 50mm a c b dissolution growth powder crystals TiOCl: crystal growth Chemical Vapor Transport

  19. O Ti eg Cl t2g TiOCl: structure • 2D layered structure • TiO4Cl2 octahedra • Ti 3d1, s = 1/2

  20. c b Cl Ti O TiOCl: low-temperature phase • XRD:* Dimerization below Tc1 • Spin-Peierls phase S = 0 * M. Shaz et al. (van Smaalen group), To appear in Phys. Rev. B.

  21. b dxy dxz, dyz a TiOCl: high-temperature phase For T > 130K,  can be fitted to the Bonner-Fisher-curve.*  1D Heisenberg chain Two posibillities for the 1D direction: * A. Seidel et al., Phys. Rev. B. 67, 020405 (2003)

  22. S Z c X a Y b b dxz,yz dxy a ~250meV TiOCl: electronic structure LDA+U* Ti 3dxy E–EF (eV) O 2p Cl 3p 3dxy 1D chain along b * R. Valenti, Universität Frankfurt

  23. Summary • LDA+U / DMFT cannot explain dispersion / shape of Ti d band • ARPES  1D direction along b axis • Polarization dependence  no pure dxy character of Ti d band

  24. Outlook • Electron doping: • LDA+DMFT*:„A nearly first-order I-M transition with rapid change in the carrier density (...) is clearly seen in the LDA+DMFT results. (...) • suitably intercalated (electron doped) TiOCl may also exhibit unconventional superconductivity upon metallization.“ • Growth of Ti1-xVxOCl • In situ Alkali doping * L. Craco et al. (Müller-Hartmann group), cond-mat/0410472

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