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Quantum Critical Point in the Quasi 2D Conductor, (Me-DH-TTP) 2 AsF 6

ISCOM2005, September 11-16, 2005 Key West, Florida. Quantum Critical Point in the Quasi 2D Conductor, (Me-DH-TTP) 2 AsF 6. Kensuke Kobayashi , Syuma Yasuzuka, Tsutomu Nakanishi, Keiichi Yokogawa, Harukazu Yoshino, Keizo Murata Osaka City University, Japan Hiroyuki Nishikawa

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Quantum Critical Point in the Quasi 2D Conductor, (Me-DH-TTP) 2 AsF 6

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  1. ISCOM2005, September 11-16, 2005 Key West, Florida Quantum Critical Point in the Quasi 2D Conductor, (Me-DH-TTP)2AsF6 Kensuke Kobayashi, Syuma Yasuzuka, Tsutomu Nakanishi, Keiichi Yokogawa, Harukazu Yoshino, Keizo Murata Osaka City University, Japan Hiroyuki Nishikawa University of Tsukuba, Japan

  2. Outline • Introduction and Purpose Metal-insulator transition in organic conductors. • Experimental Generation of high pressure up to 4.0GPa. • Results and Discussion Detail of insulating phase. Nature of metallic phase. P-T Phase diagram. • Summary

  3. 2AsF6 QCP ? Main Results Sudden disappearance of insulating state. Effect of quantum and thermal fluctuations near Pc ?

  4. U : on-site Coulomb repulsion V : inter-site Coulomb repulsion How do charge fluctuations affect metallic phase? Metal-Insulator transition Introduction and Purpose W : Bandwidth Pressure control vs ½ filled k-type Mott insulator ¼ filled q- and a- types Charge ordered (CO) insulator T • Fermi liquid(FL) theory : r T2 • Non Fermi liquid(NFL) behavior by spin fluctuation : r T Metallic Quantum critical region? CO W / V QCP?

  5. 0.5 kz Z M EF ky G Y -0.5 G M Y G Z M H. Nishikawa et al. Synth. Met. (2001) k-type donor arrangement (Me-DH-TTP)2AsF6 BDH-TTP Me-DH-TTP Reducing p-electronsUincrease Band structure calculation Small dimerization gap: ~17 meV 1/4 filled? (~170 meV for k-ET salts) Q2D Fermi surface

  6. Resistivity was measured under 0-4.0 GPa. Experiment 0.3-1.2 GPa 1.8-4.0 GPa piston cylinder ~3.0 GPa cubic anvil 1.8-8.0 GPa

  7. Resistivity under various pressures (Me-DH-TTP)2AsF6 • Insulator phase is suppressed by applying pressure. • Tmin shifts to lower temperature with increasing pressure. • MI transition becomesmuch sharper with increasing pressure. • No hysteresis. • Metallic phase above 2.5 GPa. • Sudden disappearance of insulating phase near Pc = 2.4 GPa. a V I c b Tmin = 44 K

  8. Temperature dependence ofd(logr)/d(1/T) Not crossover But phase transition? TM-I Not Mott insulator But CO insulator? Clear peaks were observed even at lower pressure.

  9. Nature of metallic phase near Pc (I) Crossover from Fermi liquid to non-Fermi liquid at T*. rT a 0 2.5 GPa, T > 30 K : a = 1.0 T < 30 K :a = 1.9 T* = 30 K 4.0 GPa, T > 83 K :a = 1.5 T < 83 K :a = 2.1 T* = 83 K 2.5 GPa NFL : a < 2 T* 4.0 GPa FL : a ~ 2

  10. Nature of metallic phase near Pc (II) T < 10 K Fitting to r⊥ = r0 + AT 2 Pc = 2.4 GPa “A” at 2.5 GPa is about six times larger than at 4.0 GPa. Divergence of temperature coefficient A at Pc.

  11. P-T phase diagram of (Me-DH-TTP)2AsF6 • Fluctuation region is narrowing toward Pc. • TMI and T* approach 0 K at Pc = 2.4 GPa. • Crossover from FL to NFL at T*. • FL region extends to higher temperature as pressure increases. r T TMI T* 0 K r T2 Possible existence of “quantum critical region” near Pc.

  12. Origin of the Crossover from FL to NFLin the Metallic State ? (1) Temperature-induced dimensional crossover from 2D to 3D. • S. Fujimoto & K. Yamada showed rT2 for 2D FL state (JPSJ 1991). • Thermal confinement model cannot explain rT behavior above T*. • Energy scale of T*(=t⊥/kB ~130 K ) is much higher than observed T*. (2) Crossover from the anomalous metal with the charge fluctuations to simple 3D FL state. • In heavy fermion system, NFL behavior (rT ) is frequently observed near QCP for AF phase. • Thermal and Quantum fluctuations may be responsible for the sudden disappearance of the M-I transition at Pc. • W / V may determinethe “ Energy scale of T* ”. NFL behavior (rT) could be caused by the charge fluctuations associated with the insulating state.

  13. Possibility of tricritical point in (Me-DH-TTP)2AsF6 (DI-DCNQI)2Ag (Q1D) T. Itoh et al. Phys. Rev. Lett. (2004) (Me-DH-TTP)2AsF6 (Q2D)Present work 2nd order 1st order? Tricritical point (19±1 kbar, 21.5±1.0 K) Tricritical point may appear close to Pc=2.4 GPa.

  14. Summary • Reduced p system(Me-DH-TTP)2AsF6 with strong on-site Coulomb interaction was metallized by applying pressure. • The M-I transition suddenly disappears near Pc = 2.4 GPa. • Crossover from FL to NFL above Pc. • Divergence of quadratic temperature coefficient A near Pc suggests the presence of strong thermal and quantum fluctuations near Pc. • Thermal confinement model cannot explain NFL (r∝T) behavior above Pc. • Sudden disappearance of the insulating phase may be due to the thermal and quantum fluctuations near Pc.

  15. (Me-DH-TTP)2AsF6 BDT-TTP, 880 S/cm (Metallic) (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) Reducing p-electrons →enhancement U (U : on-site Coulomb repulsion) BDH-TTP, 49 S/cm (Metallic) (2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene) (Me-DH-TTP)2AsF6 (BDH-TTP)2AsF6 Metal Insulator Ea ~ 30 meV anisotropy r/ r//~100 Me-DH-TTP, 1.4 S/cm (nonmetallic) (2-methyl-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene )

  16. 2AsF6 In-plane resistivity and phase diagram of (Me-DH-TTP)2AsF6under 3.2-22.4 kbar

  17. ESR results of (Me-DH-TTP)2AsF6 290 K 245 K 200 K 150 K 100 K 80 K 60 K 40 K 35 K 32 K 27 K 24 K 21 K H. Nishikawa Powder sample 40 K H / mT Antiferromagnetic ordering below ~40 K

  18. 0.5 kz Z M 0.5 EF ky G Y EF -0.5 G M Y G Z M -0.5 Z G M Y G M Band structure calculation (BEDT-TTF)2Cu(NCS)2 (Me-DH-TTP)2AsF6 ~170 meV ~17 meV Q2D Fermi surface

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