Anisotropy of the low-field critical point of the melting line
of twinned YBa
2
Cu
3
O
7 À
single crystals
J. Figueras, T. Puig, and X. Obradors
Institut de Cie `ncia de Materials de Barcelona, CSIC, Campus U.A.B., 08193 Bellaterra, Spain
A. Erb and E. Walker
Department de Physique de la Matie `re Condense `e, Universite ´ de Gene `ve, 24 quaie Ernest-Ansermet, CH-1211 Gene `ve 4, Switzerland
Received 28 September 2001; published 7 February 2002
We demonstrate that the anisotropy of the low-field critical point ( H
lcp
) of the vortex lattice melting line in
a twinned YBa
2
Cu
3
O
7-
single crystal follows the intrinsic anisotropic behavior of the melting line when the
disordered phase is originated by point defects. When the vortices are pinned at twin boundaries an upwards
shift of H
lcp
is observed. The contribution of twin boundary pinning has been found to match with the change
in the vortex elastic energy barriers thus supporting the identification of the low-field phase as a disordered
solid vortex phase where vortex pinning overcomes the vortex-vortex interaction.
DOI: 10.1103/PhysRevB.65.092505 PACS numbers: 74.25.Fy, 74.60.Ge, 74.72.Bk
In clean superconducting crystals the vortices present a
first-order transition from a liquid to an ordered solid state.
This transition has been characterized by several authors by
means of thermodynamic,
1
magnetic,
2
and transport
3
mea-
surements. When some kind of disorder is introduced in the
lattice, however, a complex behavior appears which is con-
trolled by the balance between vortex-vortex interaction, the
pinning energy with defects, and thermal activation. One im-
portant consequence of disorder is that the lattice melting
line is actually bounded by an upper critical field H
ucp
and a
low critical field H
lcp
where the transition becomes second
order. The influence of random point defects, such as oxygen
vacancies,
4
electron or proton irradiation,
5
planar defects
such as twin boundaries,
6,7
and columnar defects generated
by ion irradiation,
8
on the features of the magnetic phase
diagram of YBa
2
Cu
3
O
7 -
, has been widely investigated,
however a coherent picture is still lacking. The nature of the
transition among the liquid and solid vortex states below the
low-field critical point remains still an open issue.
9
In par-
ticular, the anisotropy of H
lcp
and its dependence on the dif-
ferent types of defects are not known. The low critical field
may be shifted by more than two orders of magnitude, when
the crystal is irradiated with protons.
5
When point defects are
created by electron irradiation, however, the low-field critical
point remains insensible. The influence of twin boundaries
remains also extremely puzzling.
4,6,10
It was suggested that
H
lcp
was not modified by twin boundaries but this is an un-
expected behavior because twin boundaries lead to a second-
order Bose-glass transition,
11
characterized by a cusp in the
‘‘irreversibility line’’ and in the vortex activation energy for
magnetic-field orientations close to the c axis.
12,13
This be-
havior would be also inconsistent with a recent study of the
influence on H
lcp
of linear defects created by ion irradiation.
8
That work showed that H
lcp
is pushed up while the Bose-
glass regime develops. It appears then a fundamental prob-
lem to determine the intrinsic anisotropy of the critical points
and their interaction with planar defects behaving as corre-
lated disorder.
14
In this paper we address these fundamental open ques-
tions for the low-field critical point. From anisotropic mag-
netoresistance measurements we determine the intrinsic an-
gular dependence of the low critical point which follows that
of the melting temperature. In addition, since we are using a
twinned high-purity YBa
2
Cu
3
O
7 -
single crystal, we dem-
onstrate that planar defects rise from the low-field critical
point. Finally, we quantify the relationship between the pin-
ning energy and the temperature shift of the low-field critical
point. Our results support the idea that the disordering of the
vortex lattice is generated because the energy pinning at the
existing defects overcomes the vortex-vortex interaction re-
sponsible for the flux lattice melting.
High quality single crystals with 99.995% purity were
grown using BaZrO
3
crucibles with a self-flux technique de-
scribed elsewhere.
15
The crystals were oxygen annealed at
650 °C during 125 h in an oxygen pressure P O
2
=100 bar and they displayed a superconducting transition
temperature T
c
93.1 K with T
c
0.4 K which should
correspond to 0.08.
4,6,15
Polarized optical microscopy ob-
servations showed that the crystals are heavily twinned with
a mean twin separation d
tb
500 nm. The electrical resistiv-
ity was measured with a standard four-probe method and the
current was injected parallel to the 100 axis. Magnetic
fields up to 9 T were rotated from the c axis to the ab plane
maintaining the perpendicularity between the magnetic field
and the current.
In Fig. 1a the temperature dependence of the magnetore-
sistance is displayed at different magnetic fields applied at
15° from the c axis. These measurements show the typical
‘‘kink’’ at /
n
0.2 where
n
is the normal-state resistiv-
ity denoting the first-order character of the transition. A
clearer identification of the transition is achieved in the tem-
perature dependence of d / dT , as shown in the inset of Fig.
1a, where a sharp peak hatched area in the figure emerges
over a broad background. The melting transition is defined as
the high-temperature peak onset
9
which is also identified as
the high-temperature point where the second derivative of
the resistivity reaches a zero value.
As can be observed, the sharp peak only appears at high
magnetic fields while at low fields the broad peak indicates
PHYSICAL REVIEW B, VOLUME 65, 092505
0163-1829/2002/659/0925054/$20.00 ©2002 The American Physical Society 65 092505-1