observation of two dielectric transitions at 110 and 60 K in the microwave response of
nonsuperconducting YBa
2
Cu
3
O
6.0
crystals. The transitions are characterized by a change in polarizability and
presence of loss peaks, associated with overdamped dielectric modes. An explanation is presented in terms of
changes in polarizability of the apical O atoms in the BaO layer, affected by lattice softening at 110 K, due
to change in buckling of the CuO layer. The onset of another mode at 60 K strongly suggests an additional
local lattice change at this temperature. Thus microwave dielectric measurements are sensitive indicators of
lattice softening which may be relevant to superconductivity.
DOI: 10.1103/PhysRevB.63.092508
PACS number s : 74.72.Bk, 74.25.Nf, 77.22.Gm, 74.80.Dm
It was recognized soon after the discovery of the high-
temperatures superconductors that the cuprates are structur-
ally similar to the ferroelectric perovskites.
1
The basic per-
ovskite ABO
3
structure occurs in ferroelectrics like BaTiO
3
and incipient ferroelectrics or quantum paraelectrics such as
SrTiO
3
, as well as in subunits of the superconductor
YBa
2
Cu
3
O
6.0
x
. The implications of this structural similar-
ity received early support from the observation of large di-
electric response
2,3
in the insulating parent compound
YBa
2
Cu
3
O
6.0
. Furthermore, theoretical models have been
proposed which include the possible competition between
ferroelectricity and superconductivity.
46
In this paper we show some striking dielectric properties
of single crystals of insulating YBa
2
Cu
3
O
6.0
which seem to
have a strong bearing on the superconductivity of doped
YBa
2
Cu
3
O
6.0
x
. Traditionally the information on lattice dy-
namics has been obtained from inelastic neutron, x-ray ab-
sorption ne structure XAFS , Raman, and infrared spec-
troscopy measurements. Our microwave measurements
probe consequences of lattice modes on the long wavelength
(q
0) dielectric properties, and its very high sensitivity
leads to the observation of features not detected by other
techniques.
In addition to large dielectric strengths
10
2
10
3
,
consistent with previous measurements, we report the pres-
ence of two dielectric transitions at 110 and 60 K. These
transitions are accompanied by the onset of polarization
modes indicated by the presence of dielectric loss peaks be-
low the transition temperatures. The transitions arise from
structural distortions occurring at these temperatures, such as
buckling of the CuO plane leading to the 110 K transition,
which affect the electrodynamic response. Thus precision
microwave measurements are shown to be a sensitive probe
of lattice effects, complementing other traditional probes of
lattice dynamics. Taken together with numerous reports of
lattice effects at or near the superconducting transition tem-
perature T
c
,
7
the present results demonstrate the importance
of charge and lattice dynamics in the high-temperature su-
perconducting oxides.
Ultrapure single crystals of YBa
2
Cu
3
O
6.0
were prepared
in contamination-free BaZrO
3
crucibles. The high quality of
these single crystals has been extensively documented in a
wide range of measurements, including structural and trans-
port studies. A brief list of these reports can be found in Ref.
8.
The high sensitivity microwave measurements were car-
ried out in a Nb superconducting cavity resonant at 10 GHz
in the TE
011
mode. The sample is placed at the center of the
cavity at a maximum of the microwave magnetic eld H .
We introduce an electromagnetic susceptibility
H
(T)
H
(T)
i
H
(T) which is related to the measured param-
eters, the shift in cavity resonant frequency
f (T) and the
resonance width
f (T) by
f (T)
i f (T)
g
H
(T)
i
H
(T) , where g is a geometric factor. A detailed analysis
of the relevant cavity perturbation for general sample condi-
tions including lossy dielectric and metallic or superconduct-
ing states, has been recently carried out by us.
9
We are able
to directly measure the conductivity
tot
or the dielectric
permittivity
tot
where
tot
i
0 tot
). The analysis
shows that for arbitrary conductivity, H
T
3
2
1
3/z 2
3 cot z
/z ,
1
where z ka
k
0
a
tot
.
Note that we use time dependences e
i t
. In the limit z 1,
H
(T)
(1/10)(k
0
a)
2
(
tot
1) for a lossy dielectric.
The dielectric permittivity
tot
was extracted from the data
using Eq. 1 .
tot
includes both bound polarization ( ) and
free charge conductivity
contributions, i.e.,
tot i /
0
. In the present case the conductivity is negligible
and
tot
. We have earlier carried out extensive measure-
ments
of
the
surface
impedance
of
a
variety
of
superconductors,
10
metals, and insulators, and demonstrated
the validity of these measurements.
PHYSICAL REVIEW B, VOLUME 63, 092508
0163-1829/2001/63 9 /092508 4 /$15.00
©2001 The American Physical Society
63
092508-1 The
dielectric
permittivity
(T)
and
(T)
of
YBa
2
Cu
3
O
6.0
are shown in Fig. 1. Here H
c axis, so that
the displacement currents are in the ab plane, i.e., we are
measuring in-plane dielectric permittivity
ab
. The large mi-
crowave dielectric permittivity observed in the present com-
position seems to be a characteristic of some perovskite ox-
ides. Such large dielectric strengths
10
2
10
3
in
nonmetallic insulating YBa
2
Cu
3
O
6.0
x
were reported by Rey
et al.,
2
for ceramic samples which were quenched to retain
the oxygen homogeneity. It is worth remarking that the
present crystals are also quenched from high temperature and
this may be an important requirement for the observation of
this effect. We have observed similar response in the micro-
wave dielectric permittivity of another YBa
2
Cu
3
O
6.0
crystal
Fig. 2, bottom panel obtained from a different batch, con-
rming the presence of the dielectric transitions reported
here.
The data in Fig. 1 can be analyzed in terms of three di-
electric modes,
, each of which is well de-
scribed by a Debye relaxation form with respect to the tem-
perature dependence:
i
, ,
i</i>0
T
1
i
i
T .
2 appears to represent the low T tail of a high-
temperature
process,
with
0
(T)
160,
and
with
a
relaxation time
(T)
6.5
10
13
sec
1
exp(1000/T) char-
acterized by an activation energy of 1000 K. The process
is dominant between 300 K and approximately 180 K, below
which it freezes out quasistatically as the dipole relax-
ation rate becomes extremely slow. A residual temperature
independent dielectric contribution
465
i</i>125 remains at
all temperatures. We believe is the contribution which has
been measured by several previous investigators
2,3
on non-
metallic YBa
2
Cu
3
O
6.0
and represents a polarization mode
formed at high temperatures T
300 K.
and
indicate
the onset of two dielectric modes which turn on below tran-
sition temperatures T
c
60 K and T
c
110 K. We describe
these modes with the following parameters:
0
(T)
60 1
(T/T
c
) , T
c
110 K and
(T)
4
10
10
(sec
K)/(T 200) , for the process, and
0
(T)
280 1
(T/T
c
) , T
c
60 K and
(T)
2.5
10
10
(sec
K)/(T 5), for the process.
0
and
0
are similar to order parameters which grow at
temperatures below a transition. As T is lowered, both
(T)
and
(T) increase initially due to the growing polarization.
However below a characteristic temperature both
(T) and
(T) begin to decrease because the dipoles are no longer
able to follow the microwave eld. The peak temperature
T
p
25 K is determined by the condition
(T
p
)
1,
although the peak for
is at a higher T than for
. The
peaks are so-called dielectric loss peaks. Identical arguments
hold for (T) also. Here the peak is much broader and
occurs at T
p
75 K.
For the
and
processes, the temperature dependence is
too broad to be described by an activated relaxation rate. We
have found that a relaxation rate which is linear in T, i.e.,
,
1
(T)
a
,
(T
T
( , )0
), describes the data very well as
seen in Fig. 1, with a
0.4
10
10
(sec
K)
1
, T
0
5 K
FIG. 1. Dielectric constants
and
at 10 GHz of
YBa
2
Cu
3
O
6.0
. Note the onset of dielectric response at T
cA
60 K
and T
cB
110 K. The solid lines represent
as de-
scribed in the text.
FIG. 2. Dielectric constants
at 10 GHz of YBa
2
Cu
3
O
6.0
for
samples 1 and 2. Note the onset of dielectric modes at T
c
60 K
and T
c
110 K are present in both the samples.
BRIEF REPORTS
PHYSICAL REVIEW B 63 092508
092508-2 and a
0.25
10
10
(sec
K)
1
and T
0
200 K. Such re-
laxation rates with linear T dependences are well known in
the copper oxide superconductors.
We note that similar large dielectric constants have been
observed in other copper oxides. In Bi
2
Sr
2
(Dy,Y,Er)Cu
2
O
8
,
the parent compound of Bi:2212, large in plane
10
3 10
5
were reported
11
. It is also important to note that the
dielectric modes discussed here bear a strong similarity to
the numerous modes observed in the dielectric response of
the perovskite SrTiO
3
Ref. 12 at 65, 37, and 16 K. The
dielectric loss peaks reported here are similar to those ob-
served in La
5/3
Sr
1/3
NiO
4
Ref. 13 and Sr
14
Cu
24
O
41
.
14
The present results indicate that at 110 Ref. 15 and 60
K, two polarization onset transitions occur in YBa
2
Cu
3
O
6.0
.
We note that the 110 and 60 K onsets cannot arise from any
contamination of the sample by a superconducting phase,
since then the contribution should be diamagnetic negative
), opposite to what is observed.
A scenario leading to such dielectric transitions can be
arrived at starting with the so-called Bilz model for
ferroelectricity,
16
which is based upon the nonlinear polariz-
ability of oxygen, and originally developed for perovskite
structures. This applies to a displacive-type ferroelectric
where dipole moments are induced during the phase transi-
tion so that soft mode con