Antiferromagnetism-superconductivity competition in electron-doped
  cuprates triggered by oxygen reduction
Antiferromagnetism-superconductivity competition in electron-doped cuprates
triggered by oxygen reduction
P. Richard,1, ∗ M. Neupane,1 Y.-M. Xu,1 P. Fournier,2 S. Li,3 Pengcheng Dai,3, 4 Z. Wang,1 and H. Ding1
Department of Physics, Boston College, Chestnut Hill, MA 02467
Département de physique, Université de Sherbrooke, Sherbrooke, Québec, Canada, J1K 2R1
Department of Physics and astronomy, The University of Tennessee, Knoxville, TN 37996
Neutron Scattering Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
(Dated: October 25, 2018)
We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-
reduced Pr2−xCexCuO4 and Pr1−xLaCexCuO4 electron-doped cuprates. In contrast to the common
belief, neither the band filling nor the band parameters are significantly affected by the oxygen
reduction process. Instead, we show that the main electronic role of the reduction process is to
remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap
is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.
PACS numbers: 74.72.Jt, 74.25.Jb, 74.62.Dh, 79.60.-i
Even though most of the work has been focused on
hole-doped cuprates, the understanding of their electron-
doped counterparts is essential for obtaining a univer-
sal picture of high-Tc superconductivity. To achieve this
goal, it is necessary to first solve the main mystery that
holds since the discovery of the T’-structure electron-
doped cuprates RE2−xCexCuO4 and RE1−xLaCexCuO4
(RE = Pr, Nd, Sm, Eu): why is superconductivity in
these compounds achieved only when a tiny amount
of oxygen (∼ 1%) is removed from the as-grown (AG)
samples following a post-annealing process (reduction)
[1, 2, 3, 4, 5]? In fact, the AG samples, even with suf-
ficient electron-doping by adding Ce, are antiferromag-
netic (AF) insulators at low temperature. Far from be-
ing a simple materials issue, the understanding of the
microscopic origin of the reduction process that triggers
superconductivity may shed light on other related ques-
tions in high-Tc superconductivity.
Long considered as the microscopic explanation of
the reduction process, the removal of extraneous oxygen
atoms located above Cu (apical oxygen) has been ruled
out by recent Raman and crystal-field infrared transmis-
sion studies [6, 7]. Indeed, these studies revealed two
main defects appearing with the oxygen reduction, which
have been tentatively assigned to out-of-plane and in-
plane oxygen vacancies, the latter being the only one
observed at optimal doping. In parallel, a (RE,Ce)2O3
impurity phase epitaxial to the CuO2 planes appears in
reduced superconducting samples but disappears in re-
oxygenated nonsuperconducting samples [8, 9, 10, 11].
Based on this phenomenon, it has been proposed recently
that the Cu excess released during the formation of the
(RE,Ce)2O3 impurity phase fills Cu vacancies and makes
the remaining structure more stoichiometric [11]. Which
of these structural defects has the most significant impact
on the electronic properties is still under intense debate
∗Electronic address: richarpi@bc.edu
and calls for a better characterization of the electronic
band structure before and after the reduction process.
In contrast to the widespread belief that the reduction
process in the electron-doped cuprates can be consid-
ered as an independent degree of freedom for carrier dop-
ing, a recent systematic study of the Hall coefficient in
Pr2−xCexCuO4 thin films with various oxygen contents
showed that the carrier mobility rather than their con-
centration is modified by the reduction process [12]. In
particular, the annealing process leads to the delocaliza-
tion of holelike carriers, most likely due to the suppres-
sion of the long-range AF order [10, 13]. Understanding
how the reduction process can tune the competition be-
tween the AF and superconducting states and modify the
electronic band structure is thus crucial.
In this letter, we present the first systematic angle-
resolved photoemission spectroscopy (ARPES) study of
the impact of the reduction process in the electron-doped
cuprates. We show that neither the electronic filling nor
the band structure parameters are significantly changed
by the reduction process. Instead, the reduction process
suppresses long-range AF order and fills up a leading edge
gap (LEG) which has two components. While the nodal
LEG is of AF origin, the nature of the antinodal LEG
remains unclear.
High-quality Pr1.85Ce0.15CuO4 single crystals have
been grown by the flux technique. Some of the non-
superconducting AG samples have been annealed in an
argon environment at temperatures between 850 and 925
oC for a typical period of 5 days encapsulated in a poly-
crystalline matrix [14] and are referred in the text as
reduced samples. The reduced samples exhibit super-
conducting transitions around 24 K. Using the floating
zone technique, high-quality Pr0.88LaCe0.12CuO4 single
crystals have also been grown and have been annealed
as described in Ref. [11]. The samples have been stud-
ied by ARPES using the PGM and U1-NIM beamlines
of the Synchrotron Radiation Center (Stoughton, WI)
with photon energies of 73.5 and 22 eV. The data have
been recorded at 40 K using a Scienta SES-2002 ana-
http://arxiv.org/abs/0704.0453v1
mailto:richarpi@bc.edu
lyzer with a 30 meV energy resolution. The samples
have been cleaved in situ in a vacuum better than 10−10
Torr. Although this letter focuses on the data obtained
on Pr2−xCexCuO4, similar results have been obtained for
the Pr1−xLaCexCuO4 samples.
In order to transform AG samples into superconduc-
tors, the reduction process must affect the electronic
structure and especially the excitations near the Fermi
energy (EF). Fig. 1 compares the constant energy in-
tensity plots (CEIPs) of the reduced (a,b) and AG (d,e)
samples in momentum space, as obtained by ARPES.
Bright spots indicate regions with large photoemission
intensity. The CEIPs centered at -100 meV with 20 meV
energy integration window (b,e) are quite similar, and
one can easily distinguish the X(±π,±π)-centered hole-
like pockets. This contrasts with the CEIPs centered at
EF (a,d). Contrary to the reduced sample (Fig. 1a),
the intensity at EF is strongly suppressed in the AG
sample (Fig. 1d). Nevertheless, underlying Fermi sur-
face (FS) contours can be extracted in both cases and
the results are reproduced in Figs. 1c and f for the re-
duced and AG samples, respectively. Surprisingly, the
data extracted for the reduced and the AG samples can
be fitted, within uncertainties, with the same band pa-
rameters µ = 0.05 eV, t = −1.1 eV and t′ = 0.32
eV, using the simple effective tight-binding (TB) model
E − µ = t/2[cos(kx) + cos(ky)] + t
′ cos(kx) cos(ky).
According to Luttinger theorem, the introduction of
extra negative carriers following the reduction would lead
to smaller X-centered holelike pockets and thus to at least
an increase of µ, which is not observed. Actually, the un-
derlying FS contours coincide with a doping of x ≈ 0.15
in both cases. This is a strong evidence that the re-
duction process modifies neither the band filling nor the
shape of the band dispersion sufficiently to induce the
dramatic changes observed in the transport properties
[12]. Instead, the CEIPs at EF indicate that the anneal-
ing process removes a LEG that is present at EF in the
AG samples. This assertion is confirmed by Fig. 1h,
which compares the electron distribution curves (EDCs)
of AG and reduced samples at different k-locations given
in Fig. 1g. All the AG sample spectra have their leading
edge shifted towards higher binding energies as compared
with the corresponding reduced sample spectra and thus
have much weaker intensities at EF.
Now one asks the question: how can the reduction pro-
cess suppress the LEG observed in the AG samples? We
first checked that the samples were not charged by in-
creasing the photon flux, which had no influence on the
leading edge shift in our experiment. The most likely
candidate to explain this mystery is the AF ordering,
which exists in AG samples. It is known that AF is sup-
pressed in the reduced samples [10, 13]. This effect is
clearly observable by ARPES. We plotted in Figs. 2a
and 2b the electronic dispersion as measured along the
lines indicated in Fig. 2c for the reduced (dashed) and
AG (solid) samples, respectively. In addition to the main
band branches, indicated with dashed arrows, the spec-
FIG. 1: (Color online) a-b and d-e) CEIPs (20 meV integra-
tion) of the ARPES data obtained at 40 K using a 73.5 eV
photons with a A||Γ-X polarization. c and f) Underlying FS
contours associated with the reduced and AG samples. The
experimental data are represented by circles while the points
indicated by diamonds have been obtained by symmetry oper-
ations. The data have been fitted by an effective tight-binding
model (see the text). h) Comparison of the EDCs of reduced
(solid) and AG (dashed) at the locations given in panel g.
trum of the AG sample shows features that are not ob-
served in the reduced one. As indicated directly by solid
arrows on Fig. 2, these features correspond to the AF-
induced folding (AIF) band. Such features, observed in
the first as well as in the second Brillouin zones (BZs), are
responsible for the M(-π,0)-centered electron FS pockets
emphasized in Fig. 2d, which shows the AG CEIP at -50
meV obtained with 22 eV photons.
In the presence of an AIF band, the hybridization of
the main and AIF bands opens a gap at locations coin-
ciding with the magnetic BZ boundary [15]. The energy
position of the center of the gap between the upper and
lower hybridized bands depends on the k-location along
that boundary, defined by M(0,π) and equivalent points.
Hence, along the (0,π)→(π,0) direction, it occurs below
EF between the M points and the hot spots, which are
defined as the k-locations where the intersection occurs
exactly at EF. On the other hand, between the hot spots,
the intersection occurs always above EF and the upper
hybridized band can never cross EF and therefore cannot
be observed by low temperature ARPES, whereas the
top of the lower hybridized band is pushed down. When
the AF gap is large enough, the small holelike FS pocket
around (π/2,π/2) is gapped out.
In order to check this scenario, we investigated the
band dispersion of reduced and AG samples along the
nodal (Γ-X) direction. The results are given in Fig. 3.
Figs. 3a and b show the EDCs of the reduced and AG
samples, respectively. Besides the clear leading edge shift
FIG. 2: (Color online) Comparison of the reduced (a) and AG
(b) spectra obtained along the lines given in c. MB (dashed
arrows) and AIF (solid arrows) indicate the main and AIF
band structures, respectively. The number next to MB or
AFI indicates the zone in which the band is detected. d) CEIP
at -50 meV (30 meV integration) of a Pr1.85Ce0.15CuO4 AG
sample obtained at 22 eV. The suppression along the vertical
Γ-M direction is due to ARPES selection rules.
observed for the AG sample as compared to the reduced
one, the EDC maxima of the AG sample exhibit a bend-
ing back characteristic of hybridization: from the top to
the bottom of Fig. 3b, the EDC maxima first move closer
to EF, and then move away, with a decrease of intensity.
A contrast in the shape of the momentum distribution
curves (MDCs) of the reduced and AG samples, which are
given respectively in Figs. 3c and d, is also observed. The
asymmetric shape of the AGMDCs suggests the presence
of a band folded along the AF boundary (vertical dashed
line). This effect is clearly seen on the corresponding
second momentum-derivative intensity (SMDI) plots dis-
played in Figs. 3e and f for the reduced and AG samples,
respectively. The position of the MDC peaks corresponds
to the minimum in the SMDI plots (bright spots). In con-
trast to the situation shown in Fig. 3e, Fig. 3f exhibits
an additional band whose dispersion is the reflection of
the main band with respect to the AF boundary. Using
the position of the MDCs, we extracted the main band
dispersion and reported it on Fig. 3f, along with its re-
flection across the AF boundary. We also reported on
Fig. 3f the position of the EDC maxima associated with
the AG sample. These maxima, which coincide with the
renormalized dispersion band, support the hybridization
scenario and indicate that the portion of the FS around
(π/2,π/2) is suppressed in the AG samples.
Fig. 4a, which compares the k-dependence of the lead-
ing edge shift for the AG and reduced samples, provides
additional evidence that an AF hybridization gap is sup-
pressed after the reduction process. While no clear lead-
ing edge shift is observable for the reduced sample, an
anisotropic LEG is observed for the AG sample. Hence,
FIG. 3: (Color online) Comparison of the nodal dispersion
(second zone) between reduced (top panels) and AG samples
(bottom panels). a-b) EDCs from -0.87π/a (bottom) to -
0.62π/a (top). c-d) MDCs between - 80 meV (bottom) and 0
meV (top). e-f) SMDI plots. The vertical dashed line corre-
sponds to the AF boundary, while the solid lines and the dots
correspond to the unhybridized main and AF bands, and to
the hybridized band, respectively.
a maximum is observed around the hot spot, as expected
from the hybridization scenario [15]. In order to illus-
trate further the AF scenario, we plotted in Figs. 4b-e
simulations of the nodal dispersion obtained using the fit
parameters given above, with various AF gap sizes. We
introduced a broadening to mimic realistic results and re-
moved the Fermi function for sake of clarity. For a large
gap, the band folding is clearly observable and the lower
hybridized band never crosses EF. As a consequence, a
large leading edge shift is recorded. This leading edge
shift decreases as the gap becomes smaller, and it dis-
appears when the lower band crosses EF, as illustrated
in 4d. Our simulations indicate that a LEG of 20 meV
along the nodal direction can be produced by a 60 meV
AF LEG at the hot spot with a proper linewidth, in
agreement with our observation in Fig. 4a.
While the experimental results for the nodal region can
easily be described by an AF hybridization gap, such a
model alone fails to explain the LEG observed for the
antinodal region, where the main and AIF bands inter-
sect below EF. In particular, the original band position
at M is ∼ 300 meV below EF, thus a small LEG (∼ 20
meV) cannot be produced by a simple AF hybridization
effect. Nevertheless, AF order may still be responsible
for the antinodal LEG. It has been predicted that the
whole FS of the Nd1.85Ce0.15CuO4 superconducting sam-
ples, including the antinodal region, is pseudogapped due
to paramagnons in the semiclassical regime [16]. Even
though this is in apparent contradiction with our exper-
imental data, which indicate no antinodal LEG for the
reduced samples, this idea may be valid for the AG sam-
ples, for which the AF correlations are much stronger.
FIG. 4: (Color online) a) k-dependece of the leading edge
shift. Lines are guides for the eye. b-e) Simulations of the
band dispersion in the presence of an AF gap. We used the
TB parameters defined in the text and introduced a band
broadening in order to mimic real data.
We now turn to a critical question: how can a small
amount of extra oxygen atoms induce AF order in the
AG samples? Literature provides two opposite scenarios
involving CuO2 plane defects and the competition be-
tween the AF and superconducting states. It has been
suggested that in-plane oxygen vacancies in the reduced
samples can suppress the AF order, affect the band pa-
rameters and induce superconductivity [6, 7]. However,
the present study indicates that the band parameters
are not modified significantly by the reduction process.
Moreover, the antinodal LEG in this scenario would be
more likely an indirect consequence of magnetic fluctu-
ations such as paramagnons [16]. The lack of theoreti-
cal study on the subject leaves open the possibility that
charge disorder induced by oxygen vacancies can suppress
the AF order and promote superconductivity.
An opposite scenario is based on a recent neutron study
suggesting a deficiency of Cu in AG Pr1−xLaCexCuO4
samples that is healed after the reduction process through
the formation of a (RE,Ce)2O3 impurity phase [11]. It
has been argued that, in hole-doped cuprates, a Cu va-
cancy, like a nonmagnetic Zn impurity, would suppress
local superconducting phase coherence and at the same
time induce a staggered paramagnetic S=1/2 local mo-
ment extending over a few unit cells [17]. Such impurity
or Cu vacancy induced local magnetism has been recently
observed by in-plane 17O NMR in the superconducting
state of the optimally hole-doped YBa2Cu3O7 with dilute
Zn impurities [18]. If the amount of Cu vacancies in the
AG samples, although small, is sufficient, it is possible to
establish AF long-range order by quantum percolation of
the AF regions. In addition, the strong scattering of the
Cu vacancies in CuO2 planes of AG samples may produce
a localization gap (or Coulomb gap) that could explain
the observed antinodal LEG. We note that the residual
resistivity (∼ 500 µΩcm) at the superconductor-insulator
transition, found in re-oxygenated Pr1.83Ce0.17CuO4 thin
films [12], corresponds to the two-dimensional (2D) re-
sistance ρ2D0 ≈ 8.3 kΩ/� per CuO2, close to the uni-
versal 2D value h/4e2 ≃ 6.5kΩ/� [19, 20, 21]. Simi-
lar results were also found in the Zn-substituted hole-
doped cuprates [22]. Moreover, the 2%-4% of Zn sub-
stitution needed to suppress completely superconductiv-
ity in La2−xSrxCu1−zZnzO4 [22] is similar to the value
of 1.2% to 2.3% Cu vacancies estimated in the AG and
non-superconducting Pr0.88LaCe0.12CuO4 samples [11].
We caution that there are other possible explanations
to account for the antinodal LEG, such as a charge den-
sity wave induced by the nesting of two sides of the M-
centered electron pockets. However, it would then be
hard to explain why both the long-range AF and this
charge density wave are suppressed in the reduced sam-
ples. The unexpected presence of the antinodal LEG calls
for further theoretical and experimental studies.
We are indebted to A.-M. S. Tremblay and D. Sénéchal
for useful discussions. We acknowledge support from
NSF DMR-0353108, DOE DEFG02-99ER45747 and DE-
FG02-05ER46202. This work is based upon research con-
ducted at the Synchrotron Radiation Center supported
by NSF DMR-0537588. P.F. acknowledges the support
of NSERC (Canada), FQRNT (Québec), CFI and CIAR.
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