Nonisovalent La substitution in LaySr14-y-xCaxCu24O41: switching the transport from ladders to chainsT. Vuleti, T. Ivek, B. Korin-Hamzi, S. Tomi Institut za fiziku, Zagreb, CroatiaJ. Akimitsu, T. Sasaki, T. Nagata Dept. of Physics, Aoyama-Gakuin University, Tokyo, JapanB. Gorshunov, P. Haas, T. Rm, M. Dressel 1.Physikalisches Institut, Universitt Stuttgart, GermanyB. Gorshunov et al., Phys.Rev.B 66, 060508(R) (2002)T. Vuleti et al., Phys.Rev.B 67, 184521 (2003)T. Vuleti et al., Phys.Rev.Lett.90, 257002 (2003)T. Vuleti et al., submitted to Phys.Rev.Lett. (2004); cond-mat/0403611 dc resistivityLow-frequency dielectric spectroscopy Microwave/Optical spectroscopy

Single crystalsy=3, 5.2x=0 11.5

Motivation/QuestionsSr14-xCaxCu24O41composite chain/ladderinherently dopedx10 & p35 kbar: SC Doped spin-ladders: SC and CDW, finite spin gap Dagotto et al., PRB 1992Uehara et al., JPSJ 1996What is the difference between charge transport in LaySr14-y-xCaxCu24O41 and Sr14-xCaxCu24O41?What is the mechanism of charge transport in nonisovalently substituted LaySr14-y-xCaxCu24O41 ?What is the phase diagram of LaySr14-y-xCaxCu24O41?

Crystallographic structureSr14Cu24O41 : parent compoundSr/Ca/La substituted materials are isostructural

composite structure: two interpenetrating sub-systems: (Sr/Ca/La) (Cu2O3) subsystem & CuO2 subsystem nearly commensurate at 7c (ladders) 10c (chains)McCarron et al., Mat.Res.Bull. 1988

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 y0 dh=6-y holes/f.u.Ncker et al., PRB 2000 X-ray absorption spectroscopy (NEXAFS)Osafune et al., PRL 1997 Optical measurements Electron Spin ResonanceKataev et al., PRB 2001y=0, x=0 or x0nC max. 5 hole/f.u. in chains nL min. 1 hole/f.u. in ladders !!! nC +nL = dh=6 y0nC 6-y hole/f.u. in chains !!! nL 0 hole/f.u. in laddersnC +nL = dh=6-y

Complementary spin/charge arrangement in CHAINS2cC2cCX-ray difraction, T=50K: Cox et al., PRB 1998 5 holes/10 sitesy=0y0magnetic susceptibility, T=20-400K: Motoyama et al., PRL 1997 Curie paramagnetism of free spins in chainsExample y=4 dh=6-y=2 10 sites =8 free spins + 2holesNo charge orderingCharge is orderedAntiferromagnetic dimers pattern: spin gap in chains by NMR, inelastic neutron scatteringTakigawa al., PRB 1998Regnault et al., PRB 1999; Eccleston et al., PRL 19986 holes/10 sitesT= 5K:NMR, spin gap, y=1,2polycrystal ?Kumagai al., PRB 1997

Experimental techniquestemperature range: 2 K -700 Kdc 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 TWac transport LFDS (low-frequency dielectric spectroscopy) 2 probe measurements: lock-in and current preamp, 1 mHz-1 kHz impedance analyzers, 20Hz-10MHz Zagreb

dc conductivityTc = 300 KTc = 330 KT0=2.9104 Kd=12D=4200 KLocalization length a-11dc hopping length >T0=5104 Kd=12D=3200 KLa5.2Ca8.8Cu24O41La3Sr3Ca8Cu24O41

ac conductivity in La3Sr3Ca8Cu24O41 ac hopping lengthnco- crossover frequency: ac hopping overcomes dcQuasi-optical microwave/FIR: hopping in addition to phonon

Hopping dies out No collective response, no CDW

Question:What is the mechanism of the charge transport in LaySr14-y-xCaxCu24O41? for hole count dh5 (y1), 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.Result: dc conductivity follows Motts 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 distanceAnswer:

What is the difference between charge transport in LaySr14-y-xCaxCu24O41 and Sr14-xCaxCu24O41?dc transport in chains vs. dc transport in laddersHT phase, above the CDW, is a Mott insulator. Transition is well defined.In isovalently substituted materials the CO phase in the chains coexists with the CDW in the ladders. RoomTemperature conductivity in La-substituted, dh5 materials,is at least 3 orders of magnitudesmaller than inisovalently substituted dh=6 materialsRT

Phase Diagram of LaySr14-y-xCaxCu24O41Y1Sr5Ca8Cu24O41Motoyama et al., PRL 1997 Energy and temperature scales dependence on substitution nonisovalent weakisovalent strongHole count can not account for rRT: 2 orders of magnitude decrease for dh5 3 orders of magnitude decrease between dh5 and dh=5Ca-content in x=8 similar to La3,La5.2,Y1 rRT dependence on substitution nonisovalent strongisovalent weakRT

Conclusion: The way how the transport switches from the chains to the ladders in 5< dh