Infrared investigation of the low-temperature structural and magnetic transitions
in the spin-ladder candidate „ DT-TTF…
2
Au„ mnt…
2
R. Wesolowski and J. T. Haraldsen
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee37996, USA
J. L. Musfeldt
Department of Chemistry, University of Tennessee, Knoxville, Tennessee37996, USA
T. Barnes
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee37996, USA
and Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
M. Mas-Torrent and C. Rovira
Institut de Cie ´ncia dels Materials de Barcelona, Campus de la U. A. B., E-08193 Bellaterra, Spain
R. T. Henriques and M. Almeida
Departamento de Quimica, ITN, P-2686 Sacaven Codex, Portugal
Received 20 June 2003; revised manuscript received 12 August 2003; published 3 October 2003
We report measurements of the variable temperature infrared response of the organic spin-ladder candidate
dithiophentelrathiafulvalene gold maleonitrile dithiolate (DT-TTF)
2
Aumnt
2
. The 220 K structural transi-
tion is driven by massive symmetry breaking along the rung direction, whereas the 70 K magnetic transition is
associated with a change in symmetry of the vibronically activated A
g
modes in the rail direction. From
molecular dynamics simulations, we assign the most important intramolecular vibrational modes involved in
each transition. From an analysis of the charge-transfer behavior, we find that the localization of unpaired
electrons in the interacting DT-TTF double chains is modified at these transition temperatures as well. To
motivate future inelastic neutron-scattering studies, we have also calculated the spectrum of low-lying magnon
excitations expected in this material, assuming an isotropic Heisenberg spin-ladder model. We estimate a gap
of 0.6 meV for the one-magnon mode.
DOI: 10.1103/PhysRevB.68.134405 PACS numbers: 75.10.Pq, 78.68.+m, 33.20.Tp, 72.80.Le
I. INTRODUCTION
Quantum spin ladders have attracted considerable interest
as intermediaries between one-dimensional 1D chains and
two-dimensional 2D square lattices.
1–3
Additional interest
has arisen from theoretical studies of the t -J model, which
find that hole-doped spin-ladders can support
superconductivity.
3,4
Thus, investigation of spin ladder mate-
rials allows a study of the relation between dimensionality
and magnetic interactions, and may also lead to the identifi-
cation of new families of high-T
c
analog superconductors.
5
Whereas most ladder systems of current interest are struc-
tural ladders „SrCu
2
O
3
, Sr
2
Cu
3
O
5
, Cu14Sr
24
O
41
,
CuCl
4
MeCN and Ni(4,4' -Bipy)
2.5
(H
2
O)
2
(ClO
4
)
2
1.5(4,4' -Bipy)2(H
2
O…,
1,4,6 – 8
, several other lad-
derlike compounds are formed by the coupling of molecular
building blocks. (DT-TTF)
2
Au(mnt)
2
,
2,9–12
Ph(NH
3
)
(18-crown-6) Ni(dmit)
2
,
13
and (5IAP)
2
CuBr
4
2H
2
O
Ref. 14–16 are three such examples. Of course, these ma-
terials should only be regarded as spin-ladder candidates,
pending inelastic neutron scattering experiments.
17,18
(DT-TTF)
2
Au(mnt)
2
and (5IAP)
2
CuBr
4
2H
2
O are particu-
larly attractive ladder candidates because they belong to
families of compounds that share the same basic ladderlike
structure, with tuneable magnetic properties depending on
the nature of the counterion.
2,9–12,14–16
The fact that
(DT-TTF)
2
Au(mnt)
2
is an organic solid with delocalized
charge and spin
2,9–12
brings additional features to an already
fascinating area, which to date has been dominated by
cuprates.
5
(DT-TTF)
2
Au(mnt)
2
is a semiconducting, organic
charge-transfer salt formed from two building-block mol-
ecules, the donor DT-TTF dithiophentetrathiafulvalene and
the acceptor Au(mnt)
2
gold maleonitrile dithiolate.
2,9–12
The 300 K structure, shown in Fig. 1, is monoclinic. The b
direction is defined by two columns of paramagnetic DT-
TTF stacks, related by a screw axis and separated by one
stack of diamagnetic Au(mnt)
2
.
19
Note that this material is
not a structural ladder; the molecular building blocks may,
however, interact magnetically as a spin ladder, due to long-
range intermolecular interactions. Since the spin resides on
the DT-TTF molecule,
2,9–12
charge transfer occurs along the
b direction. Because the DT-TTF stacks are dimerized, the
charge transfer actually takes place between dimers; these
interactions form the rails of the ladder. The rungs of the
proposed ladder lie along the c direction and are formed by
close S ••• S contacts between DT-TTF molecules from two
adjacent stacks.
9,10,20
The rail and rung magnetic interaction
strengths are estimated from susceptibility measurements to
be J
=82 K and J
=142 K;
2,9–12
their ratio =J
/ J
0.6 places (DT-TTF)
2
Au(mnt)
2
in the theoretically inter-
PHYSICAL REVIEW B 68, 134405 2003
0163-1829/2003/6813/1344058/$20.00 ©2003 The American Physical Society 68 134405-1