Intro slide

1
Charge order in organic conductors

• Compounds are M2X-
• Case ¼-filled CO may be important for the g.s of
  the insulators ET, TMTTF
• What about the conductors (and superconductors)?

NMR Ferenc Zamborszky Weiqiang Yu David
Chow Pawel Wzietek Sylvie Lefebvre Molecules and
crystals Craig Merlic Andreas Baur Dean
Tantillo Barakat Alavi
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my family is from this part of the world.
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kappa phase kappa-(ET)2X, XCu(NCS)2
Distinction of highest Tcs Cu(NCS)2 10K CuN(
CN)2Br 11K CuN(CN)2Cl 13K (w/ pressure)
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kappa phase kappa-(ET)2X,
Lefebvre, et al. (2001)
ET2X phase diagram from Cl salt
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C. Hotta, JPSJ (2003).
dimerization
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Simplified model Extended Hubbard on anisotropic
triangular lattice horizontal and vertical t, V
C. Hotta JPSJ (2003)
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Charge-ordering in q-ET2X, XRbZn(SCN)4 Miyagawa
, et al. PRB (2000)
TMI
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Structure of (TMTCF)2X
TMTCFTMTTF,TMTSF XClO4, PF6, Br, etc.
(TMTTF)2PF6
(TMTSF)2PF6
(adapted from Bourbonnais and Jerome)
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Emery Mott-Hubbard gap in (TMTTF)2X
1/2-filled TMTTF stack
1/4-filled TMTSF stack
1/2-filling quasi-1D ----gt turns on 4kF Umklapp
scattering Effective for localizing carriers so
long as t? not too large
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13C spectrum in (TMTTF)2AsF6, signature of CO is
emergence of inequivalent sites
B at magic angle
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• Some questions to ask (is the phase diagram, as
  drawn, telling the story?)
• What is nature of MI transition?
• What is impact of CO on ground state?
• SP vs. AF
• commensurate magnetism
• LOOK AT AsF6 (ground state SP), SbF6 (ground
  state comm. AF)
• 3. What is nature of conducting phases of ET,
  TMTSF salts?

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Order parameters for two compounds (TMTTF)2PF6,
(TMTTF)2AsF6
Tco(PF6)65K Tco(AsF6)103K
CO transition is probably continuous
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F. Zamborszky, et al., PRB 2002
Charge disproportionation ratio approx. 31 ??.25
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H. Javadi, et al. (1988)
CO is ubiquitous to TMTTF materials
? Origin of metal-insulator (structureless)
transition in (TMTTF)2SbF6 unknown for more than
15 years (who cares about insulators?)
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OP probably breaks inversion symmetry in MF6
salts
Divergence of real part of electric
susceptibility ce(q0,w0) observed see
Monceau, et al. (PRL, 2001)
(Ising) symmetry-breaking OP that leads to
divergent ce(q0)
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1D (or Q1D) Extended Hubbard model _at_ 1/4 filling,
T0 consistent with CO seen by experiments Seo
and Fukuyama, JPSJ (1997), mean-field
approximation Clay, et al., PRB (2002) cluster
calc. Ground state AF with charge
disproportionation
Clay, et al., PRB (2002)
Milas estimate from optical conductivity
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From Laversanne, et al., J. Phys. Lett. 45, L393
(1984)
AsF6,PF6
SbF6
CO
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Competition between CO/SP phases in (TMTTF)2AsF6
high-pressure experiments
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CO/SP coexistence
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Kuwabara, Seo, and Ogata, JPSJ (2003)
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SbF6 salt CO at higher T AF (comm.) at lower
T Ask CO, comm. related? More generally, does
it fit into P.D.?
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?/2
?
t1/2
t1/2
t2
preparation
detect
Here echo ampl.cos(?ct1/2)
T200K, P1 bar
2D technique used to eliminate internuclear
dipolar field
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2D FT, ambient pressure
TCO156K
T200K
T125K
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Applied pressure and the (TMTTF)2SbF6 phase
diagram
comm. AF order parameters in strongly CO system
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25K
High pressure (at edge of where CO disappears)
2K
At lower temperature, line broadens. 2D
experiment demonstrates some molecules see no
paramagnetism (like SP phase)
Low T phase is singlet
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Possible reason for suppression of CO
SbF6 counterion sees a previously unknown broken
symmetry?
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x10
28
x10
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Summary of TMTTF results CO ubiquitous to
insulators Order parameter in MF6 salts probably
breaks inversion symmetry CO competes with
Spin-Peierls state, when it is strong a comm. AF
ground state is stabized
(TMTTF)2PF6
(TMTSF)2PF6
(TMTTF)2SbF6
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Should ask is V relevant to conductors? ETs
first
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Charge-ordering in q-ET2X, XRbZn(SCN)4 Miyagawa
, et al. PRB (2000)
Raman, X-ray measurements (e.g., Yamamoto, et
al., PRB (2002)) Horizontal stripes
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J. Merino and R.H. McKenzie (PRL 87, 237002
(2001)) Ignore details. Square lattice extended
Hubbard model

• Main results
• Large V/t gives the checkerboard phase (AF)
  anisotropy in V,t produces stripes (McKenzie, et
  al., PRB 64, 085109 (2001))
• At edge of checkerboard phase, find an
  attractive pairing interaction with dxy symmetry

33
From Merino and MacKenzie
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CO fluctuations in TMTTF salts persist up to
order 100-200K. What about TMTSF (i.e., the
superconductors)? too bad that the accessible
time scales (using NMR) are long. So try some
disorder in (TMTSF)2PF6 and see what
happens (TMTSF)2(PF6)1-x(AsF6)x
35
Low-field studies B1.3T
Line broadening is inhomogeneous (quasi-static
spatial variation of local hyperfine
fields) Field dependence B ---most likely
inhomogeneous carrier density
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J. Haase, et al. (2000)
Magnetic broadening of NMR lines in
La1.85Sr0.15CuO4 result from spin-density correla
tions on short length scales
??freezing of stripes??
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Summary ¼-filled (or nearly so) M2X susceptible
to CO Interactions relevant to CO have important
role in ground state of insulators in TMTTF, ET
families Presumably, could stabilize different
CO phases in ET with uniaxial stress along
different directions. What intermediate, ground
state properties emerge? If spin and/or charge
fluctuations important to SC, then in general, V
should be important in that physics, too.