1. 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, (R) (2002)
T. Vuletić et al., Phys.Rev.B 67, (2003)
T. Vuletić et al., Phys.Rev.Lett. 90, (2003)
T. Vuletić et al., submitted to Phys.Rev.Lett. (2004); cond-mat/

2. 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

3. 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

4.  X-ray absorption spectroscopy (NEXAFS)
Where are holes ? x or y!
 Stoichiometry
parent compound Sr14Cu24O y=0 dh=6 holes/f.u.
Sr/Ca substitution – isovalent Sr14-xCaxCu24O 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

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= 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

6. 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

7. 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

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: 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.

10. 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

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 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.