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Unusual spin-Peierls physics in the spin ½ quantum magnet TiOCl

in collaboration with: M. Hoinkis S. Glawion G. Berner Würzburg M. Sing M. Klemm S. Horn J. Deisenhofer Augsburg J. Hemberger A. Krimmel A. Loidl .

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Unusual spin-Peierls physics in the spin ½ quantum magnet TiOCl

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  1. in collaboration with: M. HoinkisS. Glawion G. Berner WürzburgM. SingM. KlemmS. Horn J. Deisenhofer AugsburgJ. HembergerA. KrimmelA. Loidl S. van Smaalen (Bayreuth)C. Kuntscher (Stuttgart)R. Valenti (Frankfurt) L. Pisani (Frankfurt)T. Saha-Dasgupta (Kolkata)H. Benthien (Marburg)E. Jeckelmann (Mainz/Hannover) electron07, 08-02-2007 Unusual spin-Peierls physics in the spin ½ quantum magnet TiOCl R. ClaessenUniversität Würzburg, Germany

  2. c b Ti O Cl a electron07, 08-02-2007 Unusual spin-Peierls physics in the spin ½ quantum magnet TiOCl • Why TiOCl ? • Phase diagram and spin-Peierls physics • Electronic structure • Driving TiOCl metallic

  3. (b) (a) c t t´ b a Ti O Cl b a 50mm a c b TiOCl: A low-dimensional Mott insulator

  4. (b) (a) c t t´ b a Ti O Cl b a TiOCl: A low-dimensional Mott insulator • configuration: Ti 3d1 • 1e-/atom: Mott insulator • local spin s=1/2

  5. (b) (a) c t t´ b a Ti O Cl b a TiOCl: A low-dimensional Mott insulator • configuration: Ti 3d1 • 1e-/atom: Mott insulator • local spin s=1/2 • frustrated magnetism, resonating valence bond (RVB) physics ? ?

  6. c b Ti O Cl a electron07, 08-02-2007 • Why TiOCl ? • Phase diagram and spin-Peierls physics • Electronic structure • Driving TiOCl metallic

  7. High T Bonner-Fisher behavior characteristic for 1D AF spin chains Low T spin gap formation of spin singlets due to a spin-Peierls transition ? Magnetic susceptibility

  8. High temperature phase electronic origin of 1D behavior ? band theory (LDA+U): • susceptibility of a 1D spin-1/2 chain • exchange constant: J ~ 660 K Seidel et al. (2003)Valenti et al. (2004)

  9. Low temperature phase PRB 71, 100405(R) (2005),with S. van Smaalen et al (U Bayreuth)

  10. Low temperature phase dimerization and magnetoelastic coupling  spin-Peierls instability ! PRB 71, 100405(R) (2005),with S. van Smaalen et al (U Bayreuth)

  11. Intermediate phase (orbital) fluctuations ? Raman scattering: anomalous phonon line broadening NMR: pseudogap in spin excitation spectrum specific heat: entropy at Tc1 not fully released

  12. Specific heat capacity: (orbital) fluctuations ? S << R ln 2 PRB 72, 012420 (2005)with J. Hemberger et al (U Augsburg)

  13. Intermediate phase (orbital) fluctuations ? Raman scattering: anomalous phonon line broadening NMR: pseudogap in spin excitation spectrum specific heat: entropy at Tc1 not fully released LDA+U: phonon-induced admixture of dxz/dyz to dxy ground state ? most likely not orbitals! photoemission: no admixture from other d-orbitals @ 300 KPRB 72, 125127 (2005) optical spectroscopy:pure dxy ground state up to 100 KRückkamp et al., PRL 95, 097203 (2005)

  14. Intermediate phase X-ray diffraction:scan neighborhood of commensurate (C) peaks incommensurate spin-Peierls state PRB 73, 172413 (2006)with A. Krimmel et al (U Augsburg)

  15. Phase diagram complex spin-Peierls transition: Tc2 - second order transition into incommensurate phase Tc1 – first order lock-in transition induced by frustrated interchain interaction ? Rückamp et al., PRL 95, 097203 (2005)

  16. b a Frustrated interchain interaction

  17. b a Frustrated interchain interaction  (partial) restoration of RVB physics ?  exotic ground state, if driven into metallic state ?

  18. c b Ti O Cl a electron07, 08-02-2007 • Why TiOCl ? • Phase diagram and spin-Peierls physics • Electronic structure • Driving TiOCl metallic

  19. Ti 3d O 2p / Cl 3p Valence band: photoemission vs. theory PRB 72, 125127 (2005)with T. Saha-Dasgupta, R. Valenti et al. T = 370 K

  20. Ti 3d PDOS: photoemission vs. theory PRB 72, 125127 (2005) T = 370 K cluster = Ti dimer T. Saha-Dasgupta, R. Valenti, A. Lichtenstein, et al., submitted

  21. Angle-resolved photoemission (ARPES) PRB 72, 125127 (2005) T = 300 K

  22. k ARPES on Ti 3d band PRB 72, 125127 (2005)

  23. ARPES on Ti 3d band PRB 72, 125127 (2005) 1D Hubbard model DDMRGH. Benthien, E. Jeckelmann

  24. TiOCl vs. TiOBr: effective dimensionality? • TiOBr: J ~ 375 K (55% of TiOCl) • bandwidth more 2D than 1D • b-axis bandwidth scales with t, not J • dispersion reflects charge, not spin dynamics !

  25. c b Ti O Cl a electron07, 08-02-2007 • Why TiOCl ? • Phase diagram and spin-Peierls physics • Electronic structure • Driving TiOCl metallic

  26. O Ti Cl van der Waals-gap Lix(THF)yHfNCl Nature 392, 580 (1998) TiOCl: Doping and exotic superconductivity ? various routes towards doping: • cation substitution: (Ti,Sc)OCl, (Ti,V)OCl • anion substitution: Ti(O,N)Cl • intercalation: donorsacceptors

  27. In situ doping of TiOCl with Na angle-integrated PES @ 370 K • new states in theMott gap • but not (yet?) metallic

  28. X  X X  X k|| k|| In situ doping of TiOCl with Na fresh cleave Na-doped • new states in theMott gap • but not (yet?) metallic energy relative to chem. potential (eV)

  29. TiOCl: pressure-induced metal-insulator transition ? infrared absorption under pressure PRB 74, 184402 (2006)with C. Kuntscher et al (U Stuttgart)

  30. Summary • TiOX (X=Cl,Br) ideal candidates for 2D RVB physics ?- spin-½ system with magnetically frustrated topology • Instead: unconventional quasi-1D spin-Peierls instability- non-ordered 1D spin chain  incommensurate state  dimerized state- incommensurability suggests frustrated interchain interaction • Electronic structure from photoemission- electronic dispersion: 2D vs. 1D behavior- ARPES dispersions qualitatively not understood- local ground state: pure Ti 3dxy (no orbital fluctuations) • Towards a metallic state- doping with alkaline metals: new states in the gap, not (yet) metallic- bandwidth-controlled MIT at high pressure ?

  31. THE END

  32. TiOCl: Pressure induced metal-insulator transition ? infrared absorption C. Kuntscher et al., submitted absorption edge

  33. Specific heat capacity: (orbital) fluctuations ? S << R ln 2 PRB 72, 012420 (2005)with J. Hemberger et al (U Augsburg)

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

  35. 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

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

  37. TiOCl: ARPES on Ti 3d band PRB 72, 125127 (2005) 1D Hubbard model(DDMRG) quasi-1D dispersion along b-axis cannot explain exp.tl dispersion !(WHAT IS MISSING ??)

  38. Valence band: photoemission vs. theory PRB 72, 125127 (2005) with T. Saha-Dasgupta, R. Valenti et al. Ti 3d photoemission vs. theory Ti 3d O 2p / Cl 3p Cluster-DMFTT. Saha-Dasgupta et al.

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