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Mott physics in organic charge-transfer salts

Temperature. Mott physics in organic charge-transfer salts. - (BEDT-TTF) 2 X. anomalous metal. Mott insulator. Fermi-liquid. AF order. superconductor. Pressure. Michael Lang J.W. Goethe-Universität Frankfurt. Collaborations.

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Mott physics in organic charge-transfer salts

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  1. Temperature Mott physics in organic charge-transfer salts -(BEDT-TTF)2X anomalous metal Mott insulator Fermi-liquid AF order superconductor Pressure Michael Lang J.W. Goethe-Universität Frankfurt

  2. Collaborations Experiment: Mariano de Souza* Phys. Institute, Goethe-Univ., FFM Rudra S. Manna * State Univ. Sao Paulo, Brazil Andreas Brühl Christian Strack Sebastian Köhler Ulrich Tutsch Jens Müller Phys. Institute, Goethe-Univ., FFM Gerd Schönhense Phys. Institute, Gutenberg-Univ., Mainz Katja Medjanik Hans J. Elmers Theory: Lorenz Bartosch Institute f. Theor. Phys., Goethe-Univ., FFM Harald Jeschke Roser Valentί Samples: Dieter Schweitzer University Stuttgart John Schlueter Argonne Nat. Lab. SFB/TR49 (Frankfurt-Kaiserslautern-Mainz) “Condensend Matter Systems with Variable Many-Body Interactions“

  3. P ~ 200 bar anomalous metal T* (P0,T0) param. insulator TMI metal -(ET)2Cu[N(CN)2]Cl TN AFM insulator superconductor superconductor Kino, Fukuyama, JPSJ 65, 2158 (’96) Kanoda, Hyperfine Int. 104, 235 (’97) Lefebvre et al., PRL 85, 5420 (’00) Limelette et al., PRL 91, 016401 (‘03) Fournier et al., PRL 90, 127002 (‘03) Kagawa et al., Nature 436, 534 (’05) Materials on the verge of the Mott transition P ~ 20 kbar (V1-xCrx)2O3 param. insulator param. metal T (K) AFM insulator McWham et al., PRB 7, 1920 (’73) Limelette et al., Science 302, 89 (‘03) Georges et al., J. Phys. 114, 165 (’04) fundamental aspects : - universality of the Mott transition (order parameter, analogy to liquid-gas trans.?) - role of lattice degrees of freedomin the Mott transition - anomalous states next to the Mott transition

  4. T gas liquid solid double occupancy (n = 1)  order parameter  Ising universality class !? p Analogy to liquid-gas transition !? Insulator Metal W. F. Brinkman, T. M. Rice, PRB 7, 1508 (1973) holon C. Castellani et al., PRL 43, 1957 (1979) doublon high density (“liquid“) low density of doublons (“gas“)

  5. outline • Lattice effects at the Mott transition 2) Mott criticality 3) Probingtheanomalous metallic stateby Hard X-rayphotoemissionspectroscopy

  6. -(BEDT-TTF)2X X- [(BEDT-TTF)2]+ EF X = Cu[N(CN)2]Br, Cu[N(CN)2]Cl: • strong dimerization: one hole/dimer • UW : strong-correlation regime • - triangular dimer structure t‘~ t

  7. t t t‘ frustrating interactions -(BEDT-TTF)2X X t‘/t Cu[N(CN)2]Br 0.68 0.42 Cu[N(CN)2]Cl 0.72 0.44 Cu(NCS)2 0.84 0.58 Cu2(CN)3 1.06 ~ 0.83 spin liquid ext. Hückel ab initio T. Mori et al., Chem. Soc. Jpn 72, 179 (’99) Komatsu et al., JPSJ 65, 1340 (’96) Kandpal et al., PRL 103, 067004 (’09) Nakamura et al., JPSC 78, 083710 (‘09).

  8. X = Cu2(CN)3 spin liquid X = Cu[N(CN)2]Cl (afm) X = Cu(NCS)2 (superconductor) X = Cu[N(CN)2]Br (superconductor) Minimal model for -(BEDT-TTF)2X 2D frustrated Hubbard model Kino, Fukuyama, JPSJ 65, 2158 (1996) Cellular dynamical mean-field theory: Kyung, Tremblay PRL 97, 046402 (2006) U/t spin liquid antiferromagnet d-wave superconductor metal t‘/t Ab initio-derived U/t and t‘/t values: Kandpal et al., PRL 103, 067004 (2009)

  9. Effect of (chemical) pressure -(BEDT-TTF)2X anomalous metal Cu[N(CN)2]Cl T* (P0,T0) param. insulator TMI metal TN AFM insulator Cu[N(CN)2]Br superconductor superconductor W/U X = Cu[N(CN)2]Cl Cu[N(CN)2]Br Pressure: increase the bandwidth W  bandwidth-controlled Mott transition

  10. CH2 CD2 -(D8-ET)2Cu[N(CN)2]Br (“D8-Br“): crossing TMI at ambient pressure ! A. Kawamoto et al., PRB 55, 14140 (‘97) “D8-Br“ Effect of (chemical) pressure -(BEDT-TTF)2X anomalous metal T* (P0,T0) param. insulator TMI metal TN AFM insulator superconductor superconductor W/U X = Cu[N(CN)2]Cl Cu[N(CN)2]Br

  11. -(D8-ET)2Cu[N(CN)2]Br resistivity #2 TMI~ 16 K #1 #3 #3 #1 - M-I transition TMI 16 K - percolative (not bulk) superconductivity

  12. High-resolution dilatometrie 30 mm resolution: L  1/100 Å ( L/L  10-10 for L = 10 mm)

  13. (T0, P0) literature T* T* Tg TMI afm insulator afm insulator Sweep rate: ±1.5 K/h sc T* D8-Br #1  La/La #1 Thermal expansion on -(D8-ET)2Cu[N(CN)2]Br • glass-liketransitionatTg 77 K • J. Müller et al., PRB 65, 144521 (02) • A.U.B. Wolter et al., PRB 75, 104512 (07) #1 • peak anomaly at T* = 30 K • (indicative of 2nd-order transition) - phasetransitionat TMI  14 K (1st-order) a/a TMI M. de Souza et al., PRL 99, 037003 (07)

  14. 1) Directional-dependent lattice effects M. de Souza et al.,PRL 99, 037003 (07) D8-Br A2907#1 T* - volume expansion V/V ~ 0.04 % below TMI - striking in-plane anisotropy! - unexpected out-of-plane expansion! • Intricate coupling of -electrons to lattice degrees of freedom • intra-dimer degrees of freedom !?

  15. (T0, P0) literature T* TMI afm insulator afm insulator sc D8-Br #1 2) Critical fluctuations close to (P0, T0) D8-Br #1 #3  = 0.5 preliminary analysis based on the assumption:   C ( = /C = const.)  = 0.3 M. de Souza, et al., PRL 99, 037003 (07) M. de Souza et al.,PRL 99, 037003 (07) ~ • “unusual (large) critical exponent“  = (0.8  0.15) ?! • “inhomogeneous broadening“ T0  1.7 K ?!

  16. Finite-T critical end point: T dependent ! T p, T : independent variables p Criticality at 2nd-order end-point (P0, T0) T Tc = Tc(p):   = const. p

  17. Criticality at 2nd-order end-point (P0, T0) Scaling form for the Gibbs free energy for the 2D Ising universality class: s  (T0,p0)  large(T) anomaly + sign change expected

  18. Thermodynamics:  large lattice deformations accompany the Mott critical end point Scaling for 2D Ising universality class L. Bartosch, M. de Souza, M.L., PRL 104, 245701 (10) #1 #3 - Expansitity data consistent with 2D Ising universality class - “D8-Br“ crystals are situated 20-30 bar off P0 - For P = P0:  4 times bigger (T) anomaly expected !

  19. - -(ET)2Cu[N(CN)2]Cl Ass.: conductivity (T)    t “unconventional Mott criticality“: (,,) = ( 2, 1, 1) cf. 2D Ising: (15,1/8, 1.75) (F. Kagawa et al.,Nature 436, 534 (05); Nature Phys. 5, 880 (09)) • (T) = (, energy density) (S. Papanikolaou et al., Phys. Rev. Lett. 100, 026408 (2008)) • for dominant coupling to energy density: •  conductivity data on -(ET)2Cu[N(CN)2]Cl consistent with • 2D Ising universality class Literature results - (V1-xCrx)2O3 “universality of liquid-gas transition“ (3D Ising) (P. Limelette et al., Science 302, 89 (03))

  20. Summary -(BEDT-TTF)2X anomalous metal T* (P0,T0) param. insulator TMI metal TN AFM insulator superconductor W/U superconductor • 1) Lattice effects at the Mott transition: • strongly anisotropic lattice effects which do not match the dimer-dimer electronic • structure  intricate role of the lattice degrees of freedom (intra-dimer effects !?) • 2) Mott criticality : • strongly T-dependent Grüneisen parameter incl. sign change at the 2nd-order • critical end point  large lattice deformations around (P0, T0)! • - Thermal expansion data are consistent with a 2D Ising universality class

  21. HAXPES > 10 nm escape depth Hard X-ray photoemission spectroscopy UPS 0.5 nm information depth 1.6 nm bulk sensitivity

  22. Core-level spectroscopy Sulphur 2s HAXPES: local spectroscopic probe in the BEDT-TTF layers idea: use interactions of core hole with “local electronic environment“ via (“shake-up processes“) to gain access to the electronic states in the valence shell

  23. Core-level spectroscopy anomalous metal T* param. insulator TMI metal TN AFM insulator superconductor superconductor U/W -(ET)2Cu[N(CN)2]Br T< T* : single-line spectrum T  T*: occurrence of satellites A, B, B‘ at higher binding energies

  24. Core-level spectroscopy T  T* • sudden decrease of main line intensity to 84% • accompanied by satellites A, B, B‘, C with T-dependent intensities “T* anomalies“

  25. EF (i) Correlated metal regime T  40 K: single-line spectrum “(cf. simple metals) (itinerant electrons do not couple to the core hole) Intensity consistent with a “correlated metal“ state all three valence electrons per dimer are itinerant (“coherent band state“)

  26. EF HOMO-1 HOMO A HOMO-1 B Ligand state (ii) Anomalous metal regime T  T*: core-level satellites (cf. open d-shell systems) “partial localization of electrons on dimer sites“ main line satellites A B opens new excitations channels: (shake up into empty states) Instanteneous rearrangements of electrons in HOMO and HOMO-1 (incl. ligand) states required energy  lowering of the electrons‘ kinetic energy

  27. speculation:(3x3)-R30° HOMOs C C Si So  reduction of 1/3 of 50 % 16 % expected Si ( 50% of all S) (ii) Anomalous metal regime Drop in main-line intensity: 100%  84%  formation of a superstructure!?

  28. - predicted by theory (compressional Hubbard model) as response to the lattice expansion through the Mott transition Hassan et al., PRL 94, 036402 (05) Acoustic anomaly sound velocity “crossovers“ (P0,T0) P > P0 param. insulator P0 P < P0 TMI metal TN AFM insulator superconductor superconductor T0 Fournier et al., PRL 90, 127002 (03) -Cl • large softening of c22 ( planes) upon approaching (P0,T0) • “diverging electronic compressibility!“

  29. Background contribution D8-Br H8-Br background: non-critical electronic + phonon contributions

  30. H8-Br T* = 37 K Tc= 11K (T1T)-1 J. Müller, M.L. et al., PRB 65, 144521 (2002) T* = 37 K (H8-Br) coincides with “pseudogap“ phenomena T* Mayaffre et al. ’94, Kawamoto et al. ’95, DeSoto et al. ‘95 Evolution of T* anomaly with chemical pressure D8-Br A2907#1 D8-Br A2995#3 T* = 29.6K T* = 30.1K TMI ~ 14.1K TMI ~ 13.5K M. de Souza, et al., PRL 99, 037003 (07)

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