Tailoring Matter on the Molecular Level: organic solids as models to study physics in reduced dimensions Martin Dressel 1. Physikalisches Institut der Universit ä t Stuttgart, Germany Outline N. Drichko, M. Dumm, D. Faltermeier, S. Kaiser, 1. Organic Conductors Y. Sun, S. Yasin basics and development Universit ä t Stuttgart, Germany 2. Competing Interactions charge order C. Meziere, P. Batail CNRS, Universite d ’ Angers, France charge fluctuations, superconductivity J. Schlueter 3. Electronic Correlations Argonne National Laboratory, U.S.A. localization R. Lyubovskaya Mott transition, charge dynamics RUS, Chernogolovka, Russia 4. Outlook J. Merino Universidad Autonoma, Madrid, Spain R. McKenzie UQ, Brisbane, Australia A. Greco Rosario, Argentina
Organic Conductors basics Organic materials: 19 Mio. compounds containing carbon Aromatic rings: delocalized π -orbitals e.g. benzene . . . naphthalene
Organic Conductors basics Organic solids are in general insulators because the bonds are saturated, the electronic bands are filled Requirements for electrical conductivity: • overlap of orbitals: band formation • add or extract electrons: partially filled bands - electrical field effect - charge transfer salts
Organic Conductors charge transfer salts TTF-TCNQ The structure consists of TTF and TCNQ stacks. TTF is a strong electron donor, TCNQ is an electron acceptor. Tetracyanoquinodimethane Tetrathiofulvalene N C C C C N S S C C C C C C TCNQ C C TTF C C S S N C C C C N Along the stacks the π -orbitals overlap leading to one-dimensional conductivity.
Organic Conductors charge transfer salts (TMTTF) 2 PF 6 The structure consists of TMTTF stacks as electron donors, separated by inorganic acceptors. Tetrametyl-tetraselenafulvalene C C Se Se C C C C TMTSF C C C C Se Se a c Along the stacks the π -orbitals overlap leading to one-dimensional conductivity. b c
Organic Conductors crystal growth Growth of single crystals by electro-crystallization from solution. typical size: 1 to 5 mm (TMTSF) 2 PF 6 5 mm
Organic Conductors development Starting point: 1964 W.A. Little predicts excitonic superconductivity in organic polymer chains Tetracyanoquinodimethane N C C C C N C C C C TCNQ 1973 F. Wudl, A. Heeger N C C C C N 1dim organic conductor TTF-TCNQ Tetrathiofulvalene S S C C C C TTF C C S S 1979 K. Bechgaard, D. Jerome Tetrametyl-tetraselenafulvalene 1 dim organic superconductor C C Se Se C C (TMTSF) 2 ClO 4 C C TMTSF C C C C Se Se 1984 E.B. Yagubskii Bis(ethylene-dithio)tetrathiofulvalene S S 2 dim organic superconductor S S C C C C (BEDT-TTF) 2 X C C BEDT-TTF C C C C S S S S Ishiguro, Yamaji, Saito, 1998
Organic Conductors radical cation salts (BEDT-TTF) 2 X Bis(ethylene-dithio)tetrathiofulvalene S S S S C C C C C C BEDT-TTF C C C C S S S S The structure consists of BEDT-TTF layers as electron donors, separated by sheets of inorganic acceptors. b a c σ c / σ a ~ 0.5 anisotropy within the plane σ b / σ a ~ 10000 perpendicular to the plane
Organic Conductors radical cation salts (BEDT-TTF) 2 X The layered arrangement of the organic molecules, separated by anions, leads to a two-dimensional electronic system. • The bandwidth depends on the overlap integral between neighboring molecules W = 8t ≈ 1 eV for these compounds. • The band-filling depends on the stoichiometry. • The on-site (U) and inter-site (V) Coulomb interactions depend on the molecule U eff = 0.5 eV strong influence of electron-electron correlations
Ordering Phenomena low-dimensional systems Competing interactions: lattice degree of freedom electron-electron interaction charge degree of freedom electron-phonon interaction spin degree of freedom spin-phonon interaction orbital degree of freedom spin-spin interaction … Localization, metal-insulator transition, antiferromagnetic ordering, … Ordering patterns in low-dimensional systems k-space phenomena: Fermi-surface instabilities: CDW, SDW real space phenomena: spin-Peierls, spin order, charge order, Wigner crystal
Ordering Phenomena low-dimensional systems Charge order in two dimensions 1/4-filled systems homogeneous horizontal stripes vertical stripes diagonal stripes charge distribution checker board homogeneous charge order charge order 1/5 filling charge distribution frustration fluctuations
Ordering Phenomena low-dimensional systems Charge order in two dimensions 1/4-filled systems homogeneous horizontal stripes vertical stripes diagonal stripes charge distribution checker board homogeneous charge order charge order 1/5 filling charge distribution frustration fluctuations
Optical Reflection Measurements experimental setup Fourier transform infrared spectrometer Bruker IFS 113v Bruker IFS 66v Infrared microscope Bruker Hyperion Frequency range: 15 cm -1 – 25 000 cm -1 (2 meV – 3 eV) Temperature range: 1 K ≤ T ≤ 300 K Magnetic field: Hydrostatic pressure: B ≤ 12 Tesla p < 7 GPa Absolute values of reflectivity by Au evaporation method. size of the surfaces: (0.5 – 1 mm) 2 with IR microscope: (150 μ m) 2
Quasi-Two-Dimensional Organic Conductors optical properties Small in-plane anisotropy: reflectivity is higher in the direction of larger overlap. 1,0 c Deviations from a simple metallic behavior. 0,8 Reflectivity R min The spectral weight is defined as 0,6 ω ∞ 2 = ∫ a σ ω ω = p I ( ) d 0,4 R max σ 1 8 0 where the plasma is given by 0,2 π ρ ⎧ ⎫ 2 2 16 td e ω 2 = 0,0 ⎨ ⎬ sin -1 ) h ⎩ ⎭ p V 2 -1 cm m α -(BEDT-TTF) 2 NH 4 Hg(SCN) 4 T = 300 K 600 Conductivity ( Ω The width of the conductance band is typically 0.8-1 eV 400 (overlap integrals t about 0.1 eV). This is comparable to Coulomb interaction U. 200 0 1000 2000 3000 4000 5000 6000 M. Dressel and N. Drichko, Chem. Rev. 104, 5689 (2004) -1 ) Wavenumber (cm N. Drichko et al., Phys. Rev. B 74, 235121 (2006)
Molecular Vibtrations in BEDT-TTF salts The intra-molecular vibrations are 1550 a measure of the localized charge. ν 2 (A g ) 1500 1450 ν 3 ( A g ) ν 3 (A g ) Vibrational frequency (cm -1 ) ν 27 (B 1u ) 1400 ν 2 = 1554.2 cm − 1 1350 The phonon frequency shifts down 1000 when electrons are taken off. ν 6 (A g ) 950 900 Raman shift in α -(BEDT-TTF) 2 NH 4 Hg(SCN) 4 0.0 0.5 1.0 1.5 2.0 Average charge per BEDT-TTF molecule (+e) three vibrational peaks indicates three different sites. electrons on molecule intensity (a.u.) 1486 1462 1510 1350 1400 1450 1500 1550 1600 Raman shift (cm-1) horizontal stripes
Molecular Vibtrations in BEDT-TTF salts The intra-molecular vibrations are 1550 a measure of the localized charge. ν 2 (A g ) 1500 1450 ν 27 (B 1u ) ν 3 (A g ) Vibrational frequency (cm -1 ) ν 27 (B 1u ) 1400 1 1350 IR active molecular vibrations are intense 1000 only for polarization E ⊥ conducting layers ν 6 (A g ) 950 900 IR mode in β″ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 0.0 0.5 1.0 1.5 2.0 Average charge per BEDT-TTF molecule (+e) splitting below 150 K indicates charge order Δρ = 0.2e electrons on molecule 60 004K 050K 100K 150K σ ( Ω−1 cm-1) 40 200K 300K 20 0 1400 1420 1440 1460 1480 1500 Wavenumber (cm-1) vertical stripes S. Kaiser et al., arXiv:0812.3732
Collective Modes in charged ordered systems The inter-molecular vibrations are lattice vibrations, which become IR active due to charge order: collective excitations. Collective CO mode in β″ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 appears below 150 K. S. Kaiser et al., arXiv:0812.3732
Quasi-Two-Dimensional Organic Conductors electronic correlations • α -(BEDT-TTF) 2 X 2:1 stoichiometry: insulator, metal, superconductor 1/4-filled system: hole carriers E k - π /a + π /a 0 • κ -(BEDT-TTF) 2 X 2:1 stoichiometry, dimerized: metal, superconductor 1/2-filled upper band: hole carriers E k + π /2a - π /a - π /2a + π /a 0
Quasi-Two-Dimensional Organic Conductors (BEDT-TTF) 2 X salts Proposed Phase Diagrams ¼ filled compounds ½ filled compounds V / W U / W electronic correlations (on-site U, inter-site V) _____________________________________ Tuning parameters : bandwidth W
Metal-Insulator Transition bandwidth control by anion substitution What are the dynamical properties close to the metal-insulator transition? We investigated the temperature dependent optical conductivity of κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br x Cl 1-x with x = 0%, 40%, 73%, 85%, and 90%. Changing size of the anions changes the Cl Br p overlap integral t : ‘chemical pressure’ in-plane dc resistivity 100000 κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl is κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br x Cl 1-x 10000 a semiconductor at room temperature 1000 100 which becomes a Mott insulator below 100 K. 10 ρ / ρ 300K At low temperature it orders magnetically, 1 under slight pressure it superconducts. 0.1 20% Br 0.01 40% Br κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br is 1E-3 70% Br 80% Br 1E-4 metallic for T ≤ T* ≈ 50 K. 85% Br 1E-5 90% Br Organic superconductor with maximum T c = 12 K. 1E-6 0 50 100 150 200 250 300 T (K) D. Faltermeier et al., Phys. Rev. B 76 , 165113 (2007); M. Dumm et al. Phys. Rev B (2009)
Optical Properties of κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br x Cl 1-x Br content
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