Mott physics in organic charge-transfer saltsMichael LangJ.W. Goethe-Universitt FrankfurtPressureMott insulatorAF ordersuperconductoranomalous metal-(BEDT-TTF)2XFermi-liquid

CollaborationsExperiment:Mariano de Souza*Phys. Institute, Goethe-Univ., FFMRudra S. Manna* State Univ. Sao Paulo, BrazilAndreas BrhlChristian StrackSebastian KhlerUlrich TutschJens MllerPhys. Institute, Goethe-Univ., FFM

Gerd SchnhensePhys. Institute, Gutenberg-Univ., MainzKatja MedjanikHans J. Elmers

Theory:Lorenz BartoschInstitute f. Theor. Phys., Goethe-Univ., FFMHarald JeschkeRoser Valent

Samples:Dieter SchweitzerUniversity StuttgartJohn SchlueterArgonne Nat. Lab.SFB/TR49(Frankfurt-Kaiserslautern-Mainz)Condensend Matter Systems with Variable Many-Body Interactions

Materials on the verge of the Mott transition fundamental aspects : - universality of the Mott transition (order parameter, analogy to liquid-gas trans.?) - role of lattice degrees of freedom in the Mott transition - anomalous states next to the Mott transition(V1-xCrx)2O3AFMinsulatorparam.insulatorparam.metalT (K)McWham et al., PRB 7, 1920 (73)Limelette et al., Science 302, 89 (03)Georges et al., J. Phys. 114, 165 (04) P ~ 20 kbar

C. Castellani et al.,PRL 43, 1957 (1979)Analogy to liquid-gas transition !?low density of doublons (gas)W. F. Brinkman, T. M. Rice,PRB 7, 1508 (1973)

outlineLattice effects at the Mott transition2) Mott criticality 3)Probing the anomalous metallic state by Hard X-ray photoemission spectroscopy

-(BEDT-TTF)2X X-[(BEDT-TTF)2]+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~ tEF

-(BEDT-TTF)2Xfrustrating interactionsX t/tCu[N(CN)2]Br0.68 0.42Cu[N(CN)2]Cl0.72 0.44Cu(NCS)20.84 0.58Cu2(CN)31.06 ~ 0.83 ext. Hckel ab initioKandpal et al.,PRL 103, 067004 (09)

Nakamura et al., JPSC 78, 083710 (09).T. Mori et al.,Chem. Soc. Jpn 72, 179 (99)

Komatsu et al.,JPSJ 65, 1340 (96)spin liquid

Minimal model for -(BEDT-TTF)2XKandpal et al., PRL 103, 067004 (2009)metald-wavesuperconductorantiferromagnet2D frustrated Hubbard modelCellular dynamical mean-field theory: Kyung, Tremblay PRL 97, 046402 (2006)Ab initio-derived U/t and t/t values: spinliquidU/tt/tKino, Fukuyama, JPSJ 65, 2158 (1996)

Effect of (chemical) pressureX = Cu[N(CN)2]Cl (P0,T0)metalsuperconductorparam.insulatorTMITNAFMinsulator superconductorCu[N(CN)2]Br -(BEDT-TTF)2XW/UT*anomalousmetalPressure: increase the bandwidth W bandwidth-controlled Mott transitionCu[N(CN)2]ClCu[N(CN)2]Br

X = Cu[N(CN)2]Cl (P0,T0)metalsuperconductorparam.insulatorTMITNAFMinsulator superconductorCu[N(CN)2]Br -(BEDT-TTF)2XW/UT*anomalousmetalEffect of (chemical) pressure

#3#1TMI ~ 16 K#3#1-(D8-ET)2Cu[N(CN)2]Br#2- M-I transition TMI 16 K- percolative (not bulk) superconductivityresistivity

High-resolution dilatometrie resolution: L 1/100 ( L/L 10-10 for L = 10 mm)30 mm

Thermal expansion on -(D8-ET)2Cu[N(CN)2]Br#1 glass-like transition at Tg 77 K J. Mller et al., PRB 65, 144521 (02) A.U.B. Wolter et al., PRB 75, 104512 (07)

peak anomaly at T* = 30 K (indicative of 2nd-order transition)a/aTMIM. de Souza et al., PRL 99, 037003 (07)

1) Directional-dependent lattice effectsD8-Br A2907#1 - volume expansion V/V ~ 0.04 % below TMIM. de Souza et al., PRL 99, 037003 (07) - striking in-plane anisotropy! - unexpected out-of-plane expansion! Intricate coupling of -electrons to lattice degrees of freedomintra-dimer degrees of freedom !?T*

2) Critical fluctuations close to (P0, T0)preliminary analysis based on the assumption:

C ( = /C = const.)M. de Souza, et al., PRL 99, 037003 (07)D8-Br = 0.5 = 0.3#1#3M. de Souza et al., PRL 99, 037003 (07)

Criticality at 2nd-order end-point (P0, T0)

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

Scaling for 2D Ising universality classL. Bartosch, M. de Souza, M.L., PRL 104, 245701 (10)- Expansitity data consistent with 2D Ising universality class #1#3- D8-Br crystals are situated 20-30 bar off P0 - For P = P0: 4 times bigger (T) anomaly expected !

Literature results - (V1-xCrx)2O3 universality of liquid-gas transition (3D Ising) (P. Limelette et al., Science 302, 89 (03))

Summary-(BEDT-TTF)2XW/U1) 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 Grneisen 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

Hard X-ray photoemission spectroscopy1.6 nmUPS0.5 nminformation depth

Core-level spectroscopySulphur 2s HAXPES:

local spectroscopic probein the BEDT-TTF layersidea:

use interactions of core hole with local electronic environment via (shake-up processes) to gain access to the electronic states in the valence shell

metalsuperconductorparam.insulatorTMITNAFMinsulator superconductor-(ET)2Cu[N(CN)2]Br U/WT*anomalousmetalT < T* : single-line spectrumCore-level spectroscopyT T*: occurrence of satellites A, B, Bat higher binding energies

Core-level spectroscopysudden decrease of main line intensity to 84%

accompanied by satellites A, B, B, C with T-dependent intensities

T T*T* anomalies

(i) Correlated metal regimeconsistent with a correlated metal state

all three valence electrons per dimer are itinerant (coherent band state)

T 40 K: single-line spectrum (cf. simple metals)Intensity(itinerant electrons do not couple to the core hole)

(ii) Anomalous metal regimemain linesatellites BAT T*: core-level satellites (cf. open d-shell systems) partial localization of electrons on dimer sitesopens 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

(ii) Anomalous metal regime formation of a superstructure!?Drop in main-line intensity: 100% 84%

Acoustic anomalysound velocityFournier et al., PRL 90, 127002 (03) large softening of c22 ( planes) upon approaching (P0,T0) diverging electronic compressibility!P > P0P0P < P0(P0,T0)-Cl metalsuperconductorparam.insulatorTMITNAFMinsulator superconductorT0crossovers

Background contribution background: non-critical electronic + phonon contributionsD8-BrH8-Br

TMI ~ 13.5KT* = 30.1KEvolution of T* anomaly with chemical pressureD8-Br A2907#1D8-Br A2995#3TMI ~ 14.1KT* = 29.6KM. de Souza, et al., PRL 99, 037003 (07)

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