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1. www.ifs.hr/real_science Nonisovalent La substitution in LaySr14-y-xCaxCu24O41: switching the transport from ladders to chains dc resistivity T. Vuletić, T. Ivek, B. Korin-Hamzić, S. Tomić Institut za fiziku, Zagreb, Croatia Low-frequency dielectric spectroscopy B. Gorshunov, P. Haas, T. Rõõm, M. Dressel 1.Physikalisches Institut, Universität Stuttgart, Germany Microwave/Optical spectroscopy Single crystals y=3, 5.2 x=0 – 11.5 J. Akimitsu, T. Sasaki, T. Nagata Dept. of Physics, Aoyama-Gakuin University, Tokyo, Japan B. Gorshunov et al., Phys.Rev.B66, 060508(R) (2002) T. Vuletić et al., Phys.Rev.B 67, 184521 (2003) T. Vuletić et al.,Phys.Rev.Lett.90, 257002 (2003) T. Vuletić et al.,submitted to Phys.Rev.Lett. (2004); cond-mat/0403611

2. J>>J|| Doped spin-ladders: SCand CDW, finite spin gap Hole pairs Spin singlets Doping Motivation/Questions Dagotto et al., PRB 1992 What is the mechanism of charge transport in nonisovalently substituted LaySr14-y-xCaxCu24O41 ? What is the difference between charge transport in LaySr14-y-xCaxCu24O41 andSr14-xCaxCu24O41? What is the phase diagram of LaySr14-y-xCaxCu24O41? Sr14-xCaxCu24O41 composite chain/ladder inherently doped x10 & p35 kbar: SC Uehara et al., JPSJ 1996

3. b=12.9 Å CuO2 chains Cu2O3 ladders Sr/Ca/La a=11.4 Å cL cC 10·cChains≈7·cLadders≈27.5 Å Crystallographic structure Sr14Cu24O41: parent compound Sr/Ca/La substituted materials are isostructural composite structure: two interpenetrating sub-systems: (Sr/Ca/La) – (Cu2O3) subsystem& CuO2subsystem  nearly commensurate at 7·c(ladders) ≈10·c(chains) McCarron et al., Mat.Res.Bull. 1988

4. Where are holes ? x or y! •  Stoichiometry parent compound Sr14Cu24O41y=0dh=6 holes/f.u. Sr/Ca substitution – isovalent Sr14-xCaxCu24O4y=0dh=6 holes/f.u. La substitution – nonisovalent LaySr14-y-xCaxCu24O41y≠0dh=6-y holes/f.u. • X-ray absorption spectroscopy (NEXAFS) Nücker et al., PRB 2000 • Optical measurements Osafune et al., PRL 1997 Electron Spin Resonance Kataev et al., PRB 2001 y=0, x=0 or x≠0 nC max. 5 hole/f.u. in chains nL min. 1 hole/f.u. in ladders !!! nC +nL = dh=6 y≠0 nC 6-y hole/f.u. in chains !!! nL  0 hole/f.u. in ladders nC +nL =dh=6-y

5. Complementary spin/charge arrangement in CHAINS magnetic susceptibility, T=20-400K:Motoyama et al., PRL 1997 Curie paramagnetism of free spins in chains y≠0 Example y=4  dh=6-y=2 10 sites =8 free spins + 2holes NMR, spin gap, y=1,2 polycrystal ? No charge ordering Kumagai al., PRB 1997 y=0 5 holes/10 sites X-ray difraction, T=50K: Cox et al., PRB 1998 Antiferromagnetic dimers pattern: spin gap in chains by NMR, inelastic neutron scattering 2cC 2cC Takigawa al., PRB 1998 Regnault et al., PRB 1999; Eccleston et al., PRL 1998 6 holes/10 sites T= 5K: Charge is ordered

6. Experimental techniques 1. Physikalisches InstitutUniversität Stuttgart Zagreb temperature range: 2 K -700 K dc transport 4 probe measurements: lock-ins for 1 mW-1 kW dc current source/voltmeter 1 W-100MW 2 probe measurements: electrometer in V/I mode, up to 30 GW lock-in and current preamp, up to 1 TW ac transport – LFDS (low-frequency dielectric spectroscopy) 2 probe measurements: lock-in and current preamp, 1 mHz-1 kHz impedance analyzers, 20Hz-10MHz

7. T< Tc Mott’s variable range hopping T> Tc  Nearest neighbor hopping dc conductivity La3Sr3Ca8Cu24O41 Tc = 300 K La5.2Ca8.8Cu24O41 T0=2.9·104 K d=1 Tc = 330 K T0=5·104 K d=1 2D= 3200 K 2D= 4200 K dc hopping length > Localization length a-1≈1Å

8. ac conductivity in La3Sr3Ca8Cu24O41 No collective response, no CDW ac hopping length nco- crossover frequency: ac hopping overcomes dc Quasi-optical microwave/FIR: hopping in addition to phonon Hopping dies out

9. Question: What is the mechanism of the charge transport in LaySr14-y-xCaxCu24O41? Result:  dc conductivity follows Mott’s VRH law for 1D system, • above Tcchanges to nearest-neighbor hopping, (simple activation) •  dc hopping distance larger than localization length, standard for VRH • hopping contribution is observed in ac conductivity when ac hopping distance becomes shorter than dc distance Answer: • for hole count dh≤5 (y≥1), the chains behave asa 1D disorder driven insulator,i.e. the transport is due to hopping of holes localized in chains. Note: fordh=6 (y=0) the chains cross over into a charge-ordered (CO) state.

10. What is the difference between charge transport in LaySr14-y-xCaxCu24O41 andSr14-xCaxCu24O41? dc transport in chains vs. dc transportin ladders In isovalently substituted materials the CO phase in the chains coexists with the CDW in the ladders. RT HT phase, above the CDW, is a Mott insulator. Transition is well defined. RoomTemperatureconductivity in La-substituted, dh≤5 materials, is at least 3 orders of magnitude smaller than in isovalently substituteddh=6 materials

11. Phase Diagram of LaySr14-y-xCaxCu24O41 Y1Sr5Ca8Cu24O41 Motoyama et al., PRL 1997 RT • Hole count can not account for rRT: • 2 orders of magnitude decrease for dh≤5 • 3 orders of magnitude decrease • between dh≤5 and dh=5 rRT dependence on substitution nonisovalent  strong isovalent weak Ca-content in x=8 similar to La3,La5.2,Y1 Energy and temperature scales dependence on substitution nonisovalent  weak isovalent  strong

12. Conclusion: dh=6-y, all holes in the chains Charge transportactive subsystem - Chains:1D disorderdriven insulator, the transport is due to hopping of localized holes dh=6, at least one hole in the ladders Charge transport active subsystem - Ladders: q1D system with mobile carriers where the electron-electron interaction moves the system towards Mott and/or charge-ordered insulator, CDW ground state Unresolved issues: • The way how the transport switches from the • chains to the ladders in 5<dh <6 range • Is there a phase transition from La-substituted to La-free materials? • How the phase diagram of the former merges with • the one of the latter should be resolved by a • further study of materials with the total hole count 5<dh <6.