Anisotropic optical conductivity of Sr
3
Ru
2
O
7
C. Mirri,
1
L. Baldassarre,
1
S. Lupi,
1
M. Ortolani,
2
R. Fittipaldi,
3
A. Vecchione,
3
and P. Calvani
1
1
Dipartimento di Fisica and CNR-INFM “Coherentia,” Università di Roma La Sapienza, Piazzale A. Moro, 2, I-00185 Roma, Italy
2
Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H., Albert-Einstein Strasse 15, D-12489 Berlin, Germany
3
Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno and CNR-INFM Laboratorio Regionale Supermat, Via S.
Allende, I-84081 Baronissi Salerno, Italy
Received 30 May 2008; published 31 October 2008
The optical conductivity of Sr
3
Ru
2
O
7
has been studied both in the ab planes, between 12 and 450 K,
and along the c axis between 12 and 300 K.
ab
is Drude-type but, for increasing T, it shows a crossover
around 300 K to a regime with enhanced scattering rate, probably driven by a stronger coupling with the
optical phonons. The spectral weight shows the imprints of strong correlations, but less pronounced than in a
high-T
c
material. Along the c axis, the Drude term has a much smaller plasma frequency, and a strong
absorption appears at 1 eV. These findings indicate an anisotropic metallic state where electron-electron and
electron-phonon interactions play a major role. This picture may be reconciled with the local-density calcula-
tions reported in the literature, which predict an insulating c axis, once those interactions are taken into
account.
DOI: 10.1103/PhysRevB.78.155132 PACS numbers: 74.70.Pq, 78.30.-j, 78.20.-e
I. INTRODUCTION
The Ruddlesden-Popper-type series Sr
n+1
Ru
n
O
3n+1
n =1,2, and has attracted wide interest in the last decade
due either to the unconventional superconductivity of
Sr
2
RuO
4
n =1, where the Cooper pairs are in a p-symmetry
triplet state,
1
or to the metamagnetism and quantum critical-
ity shown by Sr
3
Ru
2
O
7
n =2 at T 1 K.
2,3
In turn, SrRuO
3
n = is the only ferromagnetic material among the itinerant
4d transition-metal oxides.
4
In the present paper we focus on
the double-layered, nonsuperconducting, perovskite
Sr
3
Ru
2
O
7
, with the aim to investigate its basically unex-
plored, anisotropic, optical properties. The interest for aniso-
tropic, conducting oxides has grown in recent years after the
discovery of high-T
c
superconductivity in the cuprates,
which share with the ruthenates the perovskite structure and
the typical transition metal-oxygen layer. Moreover, single
crystals, large and pure enough along different axes, have
been grown successfully for many chemical compositions,
allowing for a detailed determination of their anisotropic be-
havior by different spectroscopic techniques.
Sr
3
Ru
2
O
7
has a quasitetragonal cell elongated along the c
axis. In the calculations of both electronic and phonon bands,
a symmetry I4 / mmm is generally assumed. However, under
rotations of the RuO
6
octahedra around the c axis, such
structure tends to transform into lower-symmetry orthorhom-
bic phases like Bbcb no. 68.
5
At room temperature, the
average values for the lattice constants of the tetragonal
cell are a = a + b / 2=0.5500 nm with a b, and
c =2.0725 nm. At 40 K a = b, and below this temperature
a b.
5
The octahedra RuO
6
are formed by the hybridized
bonds between the 2p levels of oxygen and the 4d levels of
Ru
4+
, which host four electrons. In the crystal field these
orbitals are split by an energy
6
10Dq, into a triplet t
2g
and a
doublet e
g
. In Sr
3
Ru
2
O
7
, the crystal field is strong enough
that 10Dq J, where J is the Hund energy. Therefore, in the
ground state the four electrons are in the t
2g
state, e
g
is
empty, and the total spin S is 1. In turn, the degeneracy of the
t
2g
state is removed by the Jahn-Teller distortion.
In Sr
3
Ru
2
O
7
the carriers are holes, as shown by Hall
data.
7
The resistivity is anisotropic, consistently with the
quasi-two-dimensional Fermi surface extracted from band-
structure calculations.
8
Indeed, both the ab plane resistivity
ab
T and that along c,
c
T, are metallic. However, at 300
K
c
T 40
ab
T.
9
This latter is linear down to 15 K,
while
10
ab
T T
1.5
between 15 and 6 K, T
2
below 6 K.
This change in slope, which is much more pronounced in
c
T, corresponds to a peak in the magnetic susceptibility at
16 K which was tentatively interpreted in terms of ferromag-
netic fluctuations.
9
A ferromagnetic phase is observed only
under external pressure, with a Curie temperature
T
c
70 K at 1.1 GPa.
11
Concerning the optical properties, those of
12,13
Sr
2
RuO
4
and of
14
SrRuO
3
have been studied in detail, while for
Sr
3
Ru
2
O
7
only the room-temperature optical conductivity
ab
of the ab plane can be found in the literature.
15,16
ab
appears metallic in the midinfrared and shows elec-
tronic bands above 3 eV, which are attributed to charge-
transfer CT transitions between the O 2p and the Ru t
2g
levels. Spectra at lower temperatures were reported only in
the region of the stretching mode of the Ru-O bond, to study
how its peak frequency depends on T.
The present experiment is aimed at completing our optical
knowledge of Sr
3
Ru
2
O
7
, by measuring its both with the
radiation field polarized in the ab plane between 12 and 450
K and along the c axis between 12 and 300 K, for
ranging from the far infrared to the near UV.
II. SAMPLE AND EXPERIMENT
The Sr
3
Ru
2
O
7
single crystal was grown by the floating
zone technique and characterized by x-ray diffraction and
scanning electron microscopy. Traces of the Sr
4
Ru
3
O
10
phase, as usual in these samples, were found. The reflectivity
R of the crystal was measured at nearly normal incidence
PHYSICAL REVIEW B 78, 155132 2008
1098-0121/2008/7815/1551326 ©2008 The American Physical Society 155132-1