We use a magnetic field to tune a highly anisotropic single crystal of oxygen-deficient YBa_2Cu_3O_(7-δ) with a transition temperature of 2 K through the superconductor-insulator transition. The sheet resistance scales with temperature, 0.05≤T≤1.0 K, and field, 0≤H≤94 kOe, in a manner predicted by a theory for quantum phase transitions in disordered two-dimensional superconductors

Rosenbaum, T. F.

Seidler, G. T.

Veal, B. W.

Caltech Authors - Main

PHYSICAL REVIEW B VOLUME 45, NUMBER 17 1 MAY 1992-I
Two-dimensional superconductor-insulator transition in bulk single-crystal YBazCu306 3s
G. T. Seidler and T. F. Rosenbaum
The James Franck Institute and Department of PhysicsT, he University of Chicago, Chicago, Illinois 60637
B. W. Veal
Materials Science Division, Argonne JVational Laboratory, Argonne, Illinois 60439
(Received 2l January 1992)
We use a magnetic field to tune a highly anisotropic single crystal of oxygen-deficient YBa2Cu307 —$
with a transition temperature of 2 K through the superconductor-insulator transition. The sheet resis-
tance scales with temperature, 0.05( T ~ 1.0 K, and field, 0(H (94 kOe, in a manner predicted by
a theory for quantum phase transitions in disordered two-dimensional superconductors.
The deleterious effects of disorder are amplified in
lower dimensions, where long-range coherence is, at best,
only marginally stable. A case in point is the destruction
of superconductivity in ultrathin granular' and continu-
ous metal films. The disorder is parametrized by the film
sheet resistance and the data suggest a zero-temperature
threshold for quenching global superconducting coherence
near h/4e =6.45 kO/&. Experimental variables for ad-
justing the disorder include film thickness for elemental
metals ' and composites, ' stoichiometry or doping for
compounds, including oxide superconductor single crys-
tals, " and ion bombardment.
The application of a magnetic field to a disordered su-
perconducting film leads to a fundamentally different type
of T =0 superconductor-insulator transition. ' Field-
induced vortices Bose condense, resulting in a transition
from a state with zero resistance and pinned vortices
("vortex glass" ) to a state with zero conductivity and lo-
calized Cooper pairs ("electron glass" ). The finite-
temperature scaling predictions of the Fisher theory '
have been verified in amorphous-composite InO thin
films, ' ' and values for universal critical exponents predict-
ed, but not numerically determined by the theory were
measured. We report here evidence that the field-induced
two-dimensional superconductor-insulator transition per-
sists in a highly anisotropic three-dimensional super-
conductor, namely oxygen-deficient YBa2Cu307 —s (8
=0.62). We find the same dynamical exponent describ-
ing the critical behavior of the sheet resistance in field as
measured for a-lnO„, but with a very different scale for
the transition. Our results on barely superconducting
YBa2Cu307 —b crystals permit a test of the field-induced
superconductor-insulator scenario in a regime not accessi-
ble to disordered films as well as helping elucidate the na-
ture of the incipient superconducting state in relatively
pristine, but essentially decoupled Cu02 planes.
Decreasing the oxygen content of single-crystal Y-
BazCu307 & preferentially creates vacancies in the
chains, reducing the superconducting-transition tempera-
ture T, and increasing the anisotropy. ' Moreover, when
thermally equilibrated and quenched from elevated tem-
peratures Tq below 200 C, the vacancies disorder, lower-
ing T, while keeping the total oxygen content of the crys-
tal constant. ' By adjusting Tg, T,. can be precisely con-
trolled. This unique property of YBa2Cu307 —& allowed us
to fine tune T,. suSciently close to the superconductor-
insulator boundary that a magnetic field could drive the
transition.
We started with a high-quality twinned single crystal of
YBa2Cu30q b with a superconducting-transition temper-
ature of 13 K and a 10%-90% width of 1 K as measured
in a SQUID magnetometer. The sample was heated to
Tg =150 C, quenched by immersion in liquid nitrogen,
mounted on the sample holder and top loaded into a heli-
um dilution refrigerator, resulting in a resistive-zero T;p
=2. 1 K. Four-probe magnetoresistance measurements in
the a-b plane with Hllc were made for 0.05 & T ~ 15 K
and 0~ H ~ 94 kOe using a standard lock-in technique
at 16 Hz. Input power was restricted to less than 10
watts to avoid sample heating and nonlinear I-V charac-
teristics. Two opposite faces of the sample were covered
with silver epoxy to serve as low-resistance current leads
and two 75-pm stripes were painted along c on one adjoin-
ing face to serve as the voltage leads. A heat treatment at
550'C served both to adjust the oxygen stoichiometry of
the crystal and to sinter the silver to the sample surface,
providing reliable low-resistance contacts ( ( 1 0 at
T=0.05 K in field). EITective sample dimensions were
0.6x0.6x0.3(c axis) mm . Errors due to finite contact
size and variations in the sample thickness make absolute
values of R(H, T) uncertain within 10%. Uncertainties
due to possible microcracking from quenching the crystal
have not been included. As the resistive anisotropy of
barely superconducting YBa2Cu307 —b is in excess of
3000, ' the method used here of effectively shorting out
any possible c-axis transport is crucial for obtaining the
true resistance in the a-b plane.
We show in Fig. 1 R,„(T) in a semilogarithmic plot for
selected values of H. The measured three-dimensiona1
sample resistivity was converted to a two-dimensional
resistance R,q of pairs of Cu02 planes by dividing the
thickness of the crystal by the c-axis lattice constant, '
11.8 A. The decision to use pairs of Cu02 planes as the
two-dimensional unit is based on recent measurements'
of the Hall effect in fully oxygenated YBazCu307 —$
which conclude that the pairs of Cu02 planes in each unit
cell remain strongly coupled near T, The alternative as-
sumption that individual Cu02 planes are the noninteract-
10 162 1992 The American Physical Society
1O163R-INSULATOR. . .L SUPERCONDUCTOTWO-DIMENSIONAL
13—
10 10' 10"
T for a series of
1 h 0 1lds H perpendicular to t emagnetic fie s
0.40.1
I
0.2 0.30.0
T (K)
of the sheet resistance asde endence
-'nsulatord
transition at H, . =84 kOe an
nse uence of doublingin - al unit has the co s quen lin
b 1of R, reported here, u nc uall values
e able to ma eT~0.
Io Iof R, due o
1 t d t th gl y
equilibrium m
h h h
times are re a etimes. W e
the transition or
thh H dle heat sinking a
e, we used the linear depen-
e apo o p oce tt
wh h ---d,within 1% it e
~ ~ ~ ~
st in Fig. 1. Firs,
re dc pc nden
metalliclike above T,p= .
ob dof resistive aniso r
n I upturne,„(T) II
fi 1d' tes that the con
-axis resistivi y.
this in ica
h th
-di-
Secon,
. q
r er h/4e asd tt ofod
F 11, h
mensiona
r transition.
fild o d fosistance at constan eow-
b ' decreasing o
criticalth tbtemperature, eing " g
fild be 2 on the magne ic-
causes
g.
context o ef h li hohkQ — e.
nductor-insu a o
as the critica
pe
this identi es
peco d ctngthe "true su
atin electron g aT=0 Ig
th
ortices at
d hin lass is expecte
thrature slower an
e. Further-
creasing tempe
rn which we observe.sistent wits' h the weak upturn
r redicts a scaling form,, the theory p ic s
I /=g vg ]R (cti i H —H„ I /TH T) —(h/4e )R,,q( ,
onent and cp is av is a universa I dynamical exponen8 8
nonuniviversal constant. i
dRsq
dH
h —i/z+v+RP(0)cpT
4e
v can be deter-al exponents z8v8the pro uc
I I fo Ilrn t e experimenta resu
, /dH) I t d tH„.
dH)iH„To Io -Io I .'g . q
ow a power- awFor
-t0. 1 Th" "1"'f= 1.37find rom
'
II tII lt font a rees well witthe dynamical exponent gr
10
QO
O
O
CL
10 2 10-' 10'
ma netic- e-fi ld derivative of'ng ehavior of the g
iving aevaluated a
e is a least-
tes
=1.37
sin le-}1The inset compa
osite InO, md h 0
d 1 fx nents u i(Ret'. I I), with similar e po
10 164 G. T. SEIDLER, T. F. ROSENBAUM, AND B. W. VEAL
amorphous-composite InO in field, where zava =1.26 for
a T, =0.29-K film and za vq =1.31 for a T, =0.54-K film.
The experimental values for the exponent product are con-
sistent with the theoretical constraints ' zg = 1 and
Va~ 1.
We show in the inset to Fig. 3 the correspondence be-
tween the YBa2Cu3063g and the a-lnO, data sets. Al-
though the exponents are essentially the same for the two
systems, dR,q/dH evaluated at H„ is of order I 0/Oe for
lnO„, while it is of order 0.03 0/Oe for YBazCu3063s.
This diA'erence could be due to either the fact that single-
crystal YBa2Cu306 3g„with disorder primarily in the
chains, is closer to the clean limit or that co is sensitive to
possible weak coupling between the CuO2 planes. In the
latter view, co gradually vanishing would be a natural way
to pass from the two-dimensional to the three-dimensional
limit.
Hebard and Paalanen" were able to test the prediction
H, . & T,. for a series of a-lnO, films, finding z =1 as pre-
dicted. Unfortunately, we were unable to quench our
crystal to lower T„and make an independent test of the
variation of H, with T,. Hence, the scaling behavior of
Fig. 3 with a universal value for zq va remains the strong-
est evidence that bulk single crystals of YBa2Cu307-s ex-
hibit a two-dimensional field-induced superconductor-
insulator transition. We are able to compare, however,
our result with the H, vs T, curve determined for a-InO, .
For H„84 kOe one would expect a Kosterlitz-Thouless '
transition temperature T, =1.3 K, which is close to the
temperature where we observe the onset of scaling behav-
ior. The disappearance of scaling behavior for T & 1 K
may be due to finite-temperature corrections which are of
order (T/T„) or it may reflect a crossover from two-
dimensional to three-dimensional character as T ap-
proaches T,o. Alternatively, the finite width of the super-
conducting transition may suppress artificially the true
scaling form.
Previous studies'" of the irreversibility line in oxygen-
deficient single-crystal YBa2Cu307 z found large flux
flow even for superconducting-transition temperatures as
low as 7 K. We have shown here that the magnetic field-
induced superconductor-insulator transition for single-
crystal YBapCu3063g with T,O=2. 1 K is dominated by
the Bose condensation of the magnetic vortices at H, =84
kOe and R,.q —h/2e . The scaling behavior of R,q(H, T)
at the transition is consistent with a two-dimensional
description, establishing anisotropic oxide superconduc-
tors as fertile ground for testing universal behavior at the
superconductor-insulator boundary.
We are grateful to J. W. Downey and A. P. Paulikas for
their help in sample preparation. The work at The Uni-
versity of Chicago was supported by the NSF Science and
Technology Center for Superconductivity, Grant No. 88-
09854. B.M.V. acknowledges support from U.S. DOE
BES-Materials Sciences, Contract No. W-31-109-ENG-
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Two-dimensional superconductor-insulator transition in bulk single-crystal YBa_2Cu_3O_(6.38)

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Two-dimensional superconductor-insulator transition in bulk single-crystal YBa_2Cu_3O_(6.38)

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