Optical properties of Nd
1.85
Ce
0.15
CuO
4
C. C. Homes
*
and B. P. Clayman
Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
J. L. Peng and R. L. Greene
Center for Superconductivity Research, Department of Physics and Astronomy, University of Maryland, College Park, Maryland 20742
Received 26 March 1997
The ab -plane reflectance of a Nd
1.85
Ce
0.15
CuO
4
single crystal ( T
c
=23 K has been measured from 35 to
9500 cm
-1
at temperatures above and below T
c
, and the optical properties calculated from a Kramers-Kronig
analysis. A rich phonon spectrum is observed, and there are a number of c -axis infrared and Raman modes that
are observed at low temperature which are believed to be activated by disorder; several of these modes show
evidence for electron-phonon coupling. The normal-state optical conductivity may be described by a Drude-
like component and an overdamped midinfrared component. The Drude-like component narrows rapidly; at 30
K the mean free path is estimated to be 750 Å. Below T
c
the Drude carriers collapse into the condensate; the
plasma frequency of the function is determined to be
pS
10 000 cm
-1
. The small coherence length
(
0
70 Å places this material into the clean limit (
0
/ l 1). The London penetration depth is determined to
be
ab
=1600100 Å, which places it well off the Uemura line. Estimates of the electron-phonon coupling
from normal-state transport measurements of
tr
0.5, and the absence of Holstein sidebands below T
c
,
indicate that the carriers that participate in superconductivity are weakly coupled to the phonons. The small
values for the penetration depth and the electron-phonon coupling constant suggest that the superconductivity
in this material is not due to the electron-phonon mechanism, and is different than in other hole-doped
superconducting cuprates. S0163-18299700434-7
I. INTRODUCTION
In most high-temperature high-T
c
) superconducting cu-
prates, such as YBa
2
Cu
3
O
7 -
and La
2 -x
Sr
x
CuO
4
,
the charge carriers are doped holes. However, in
Nd
2 -x
Ce
x
CuO
4
, where superconductivity is induced by
substituting Nd
3 +
with Ce
4 +
, the CuO
2
planes are believed
to be doped with electrons
1
as well as holes.
2
Another im-
portant difference between Nd
2 -x
Ce
x
CuO
4
and other cu-
prate superconductors is the coordination of the planar cop-
per atoms.
3
In orthorhombic YBa
2
Cu
3
O
7 -
and tetragonal
La
2 -x
Sr
x
CuO
4
apical oxygen atoms sit above and below the
copper atoms, yielding a nearly octahedral coordination.
However, Nd
2 -x
Ce
x
CuO
4
is a modified tetragonal ( D
4 h
17
)
structure which is composed of two-dimensional sheets of
Cu-O layers with no apical oxygen atoms, resulting in cop-
per atoms with square coordination. Within the NdCeO
layers in between the CuO
2
planes, the NdCe and O atoms
are not coplanar. The resistivity perpendicular to the CuO
2
planes is much higher than that in the planes (
ab
c
), and
there is some evidence for an incipient metal-insulator tran-
sition along the c axis in these materials at low temperature.
4
The Nd
2 -x
Ce
x
CuO
4
system has attracted a great deal of
interest because of its possible conventional BCS s -wave
pairing in the superconducting state, as opposed to the un-
conventional d -wave behavior proposed for the hole-doped
cuprates.
5,6
The microwave surface impedance measure-
ments on both thin films and single crystals have shown
evidence for a conventional BCS s -wave behavior with a gap
of 2 4 k
B
T
c
.
7–9
Tunneling measurements have also
shown a resemblance to conventional superconductors.
10
However, the magnetic-field dependence of the specific-heat
anomaly
11,12
and thin-film transmission
13
of
Nd
2 -x
Ce
x
CuO
4
both show a non-BCS-like behavior.
Whether or not this system can be considered as a con-
ventional BCS-type superconductor is an important question,
given the strong evidence that the other hole-doped cuprates
are not. Infrared techniques have long been acknowledged as
a powerful method for probing the electronic properties of
metals and superconductors,
14,15
and may provide informa-
tion about the nature of superconductivity in
Nd
2 -x
Ce
x
CuO
4
. While there have been a number of reports
on the infrared
13,16–22
and Raman
23–28
properties of ceram-
ics, thin films, and single crystals of Nd
2 -x
Ce
x
CuO
4
, to
date there has been no investigation of the far-infrared opti-
cal properties of single crystals.
In this paper we report on the optical properties of
Nd
1.85
Ce
0.15
CuO
4
( T
c
=23 K over a wide frequency range,
at temperatures above and below T
c
. While the optical prop-
erties cannot make an absolute determination of the nature of
the gap, they show a detailed phonon spectrum and they
indicate that the coupling between the carriers that become
superconducting and the phonons is very small, suggesting
that the pairing mechanism is not phonon mediated. The
small value for the penetration depth also separates this ma-
terial from other cuprate superconductors.
II. EXPERIMENT AND SAMPLE PREPARATION
Large, single crystals of Nd
1.85
Ce
0.15
CuO
4
were grown
from a CuO-based flux using a directional solidification
technique.
29
A mixture of high-purity 99.9% starting mate-
rials of Nd
2
O
3
, CeO
2
, and CuO were heated rapidly to just
PHYSICAL REVIEW B 1 SEPTEMBER 1997-I VOLUME 56, NUMBER 9
56 0163-1829/97/569/552510/$10.00 5525 © 1997 The American Physical Society