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 J. Akimitsu, T. Sasaki, T. Nagata Dept. of Physics, Aoyama-Gakuin University, Tokyo, Japan Single crystals y=3, 5.2 x=0 – 11.5 B. Gorshunov et al., Phys.Rev.B 66, 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 www.ifs.hr/real_science

Motivation/Questions Doped spin-ladders: SC and CDW, finite spin gap Motivation/Questions Dagotto et al., PRB 1992 What is the mechanism of charge transport in nonisovalently substituted LaySr14-y-xCaxCu24O41 ? Hole pairs Spin singlets J>>J|| Doping What is the difference between charge transport in LaySr14-y-xCaxCu24O41 and Sr14-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

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

 X-ray absorption spectroscopy (NEXAFS) Where are holes ? x or y!  Stoichiometry parent compound Sr14Cu24O41 y=0 dh=6 holes/f.u. Sr/Ca substitution – isovalent Sr14-xCaxCu24O4 y=0 dh=6 holes/f.u. La substitution – nonisovalent LaySr14-y-xCaxCu24O41 y≠0 dh=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 nC 6-y hole/f.u. in chains !!! nL  0 hole/f.u. in ladders nC +nL = dh=6-y 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

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 X-ray difraction, T=50K: Cox et al., PRB 1998 5 holes/10 sites 2cC 2cC Antiferromagnetic dimers pattern: spin gap in chains by NMR, inelastic neutron scattering Takigawa al., PRB 1998 Regnault et al., PRB 1999; Eccleston et al., PRL 1998 T= 5K: 6 holes/10 sites Charge is ordered

Experimental techniques 1. Physikalisches Institut Universität 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-100 MW 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

dc conductivity 2D= 3200 K 2D= 4200 K T< Tc 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 T< Tc Mott’s variable range hopping T> Tc  Nearest neighbor hopping dc hopping length > Localization length a-1≈1Å

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

Question: Result: Answer: What is the mechanism of the charge transport in LaySr14-y-xCaxCu24O41? Result:  dc conductivity follows Mott’s VRH law for 1D system, above Tc changes 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 as a 1D disorder driven insulator, i.e. the transport is due to hopping of holes localized in chains. Note: for dh=6 (y=0) the chains cross over into a charge-ordered (CO) state.

What is the difference between charge transport in LaySr14-y-xCaxCu24O41 and Sr14-xCaxCu24O41? dc transport in chains vs. dc transport in 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. RoomTemperature conductivity in La-substituted, dh≤5 materials, is at least 3 orders of magnitude smaller than in isovalently substituted dh=6 materials

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

Conclusion: dh=6-y, all holes in the chains Charge transport active subsystem - Chains: 1D disorder driven 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.