CHEprl02.pdf

Strongly Correlated

-Wave Superconductivity in the

-Type Infinite-Layer Cuprate

C.-T. Chen,

P. Seneor,

N.-C. Yeh,

R. P. Vasquez,

L. D. Bell,

C. U. Jung,

J. Y. Kim,

Min-Seok Park,

Heon-Jung Kim,

and Sung-Ik Lee

Department of Physics, California Institute of Technology, Pasadena, California 91125

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109

National Creative Research Initiative Center for Superconductivity and Department of Physics,

Pohang University of Science and Technology, Pohang 790-784, Korea

(Received 17 November 2001; published 16 May 2002)

Quasiparticle tunneling spectra of the electron-doped (

-type) infinite-layer cuprate Sr

0.9

0.1

CuO

reveal characteristics that counter a number of common phenomena in the hole-doped (

-type) cuprates.

The optimally doped Sr

0.9

0.1

CuO

with

K exhibits a momentum-independent superconduct-

ing gap

13.0

1.0

meV that substantially exceeds the BCS value, and the spectral characteristics

indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The re-

sponse to quantum impurities in the Cu sites also differs fundamentally from that of the

-type cuprates

with

-wave pairing symmetry.

PACS

numbers:

74.72

.–h, 74.50

.+r, 74.62.Dh

The predominantly

pairing symmetry [1,2], the

existence of spin

uctuations in the CuO

planes [3,4],

and the pseudogap phenomena [3

–

5] in the underdoped

and optimally

-type cuprates have been widely con-

ceived as essential to high-temperature superconductivity.

However, recent scanning tunneling spectroscopic studies

have shown that the pairing symmetry may be dependent

on the hole-doping concentration, with

mixed symmetries in certain overdoped cuprates such

[6].

Furthermore, whether

the pairing symmetry is

wave in the one-

layer

-type cuprates such as Nd

1.85

0.15

CuO

and

1.85

0.15

CuO

remains controversial [7,8], and it has

been suggested that the pairing symmetry in the one-layer

-type cuprates may change from

, depending

on the electron doping level [9]. The nonuniversal pairing

symmetries in cuprate superconductors imply that the sym-

metry is likely the result of competing orders rather than a

suf

cient condition for pairing. Nonetheless, an important

consequence of either

-wave pairing

is that the resulting nodal quasiparticles can interact

strongly with the quantum impurities in the CuO

planes

[10,11], such that a small concentration of impurities

can give rise to strong suppression of superconductivity

and modi

cation of the collective Cu

spin excitations

[6,12

–

17]. In addition, Kondo effects could be induced

by nonmagnetic impurities through breaking the nearest-

neighbor antiferromagnetic Cu

-Cu

interaction [18].

Such strong response to nonmagnetic impurities is in sharp

contrast to conventional

-wave superconductivity [19,20].

Despite signi

cant progress in the studies of cuprate

superconductivity, the research on the simplest form of

cuprates, the in

nite-layer system Sr

CuO

(

La,

Gd, Sm), has been limited [21

–

23] due to the dif

culties in

making single-phase samples with complete superconduct-

ing volume. Recently, Jung

et al.

[24] have demonstrated

single-phase samples of Sr

0.9

0.1

CuO

with nearly

100%

superconducting

volume

and

a sharp

superconducting

tran-

sition temperature at

K, thus enabling reliable

spectroscopic studies of the pairing symmetry and the ef-

fects of quantum impurities. These single-phased in

nite-

layer cuprates are

-type with

mmm

symmetry, which

differ signi

cantly from other cuprates in that no excess

charge reservoir block exists between consecutive CuO

planes except a single layer of Sr(La), as illustrated in

Fig. 1(a), suggesting stronger CuO

interplanar coupling.

Furthermore, the

-axis superconducting coherence length

0.53

is found to be longer than the

-axis lattice

constant

0.347

[25], in stark contrast to other

cuprate superconductors with

. Hence, the super-

conducting properties of the in

nite-layer system are ex-

pected to be more three-dimensional, as opposed to the

quasi-two-dimensional nature of all other cuprates. In this

Letter, we report experimental

ndings based on the scan-

ning tunneling spectroscopy studies of pure in

nite-layer

samples and those with a small concentration (

) of ei-

ther magnetic or nonmagnetic quantum impurities. A num-

ber of surprising results are found and compared with the

established phenomena in other cuprates.

The samples studied in this work included high-

density granular materials of Sr

0.9

0.1

CuO

(SLCO),

0.9

0.1

0.99

0.01

(

Zn-SLCO),

and

0.9

0.1

0.99

0.01

(

Ni-SLCO) [24].

X-ray

diffraction (XRD) con

rmed the single-phase nature of all

samples, and both XRD and scanning electron microscopy

[24] revealed random grain orientation and a typical grain

size of a few micrometers in diameter. Magnetization

studies revealed nearly

100%

superconducting volume

for all samples, with

K for SLCO and

Zn-

SLCO, and

K for

Ni-SLCO. Structurally, the

nite-layer system with up to

Zn or Ni substitu-

tions was stoichiometrically homogeneous [24]. However,

the superconductivity appeared to be sensitive to the type

of impurities. While nonmagnetic Zn had little effect on

FIG. 1. (a) Comparison of the structure of the in

nite-layer

system Sr

CuO

(

La, Gd, Sm), with those of the one-

layer

-type (

-phase) and one-layer

-type (

-phase) cuprates.

(b) A representative surface topography of an area of SLCO

with subnanometer

atness. The typical area with atomic-scale

atness where most tunneling spectra were taken was greater

than (

nm), and the work function of the spectra was

0.1

eV. (c) A zoom-out view of the region shown in part (b)

(indicated by the dashed box) over an area (

nm).

Also shown in the lower left corner is a grain boundary.

for up to

concentration, strong suppression of

already occurred with

Ni, and nearly complete sup-

pression of

was reached with only

Ni [24]. Thus,

the global response of SLCO to impurities appeared dif-

ferent from that in the

-type cuprates [6,12

–

17] and was

similar to that in conventional superconductors [19,20].

Quasiparticle tunneling spectra were taken using a low-

temperature scanning tunneling microscope on hundreds

of randomly oriented grains for the three different in

nite-

layer samples, so that a range of different quasiparticle

momenta relative to the crystalline axes of the local grains

could be sampled. The sample surface was prepared ac-

cording to the chemical etching method described else-

where [26], and a nearly stoichiometric surface with no

discernible chemical residue was con

rmed with the x-ray

photoemission spectroscopy [26]. A typical surface topog-

raphy of the pure SLCO sample for our spectroscopic stud-

ies with subnanometer

atness is exempli

ed in the left

panel of Fig. 1(b), and a zoom-out view of this area is il-

lustrated in Fig. 1(c). Con

rming the local

atness for the

tunneling spectra was to ensure that the average momen-

tum of the incident quasiparticles relative to the crystalline

axes of a grain was well de

ned. A set of representative

differential conductance

vs biased voltage

spectra for such a

at area is given in Fig. 2(a). In general,

all spectral characteristics revealed long-range (

nm)

FIG. 2. (a) Representative

quasiparticle spectra of

SLCO taken at

4.2

K. The curves correspond to spectra taken

1.5

nm equally spaced locations within one grain and have

been displaced vertically for clarity except the lowest curve. Left

inset: a typical spectrum taken at

4.2

K (solid line) compared

with the corresponding high-voltage background (dashed line).

Right inset: comparison of a typical spectrum taken at

4.2

with one taken slightly above

. (b) A spectrum normalized

relative to the high-voltage background given in the left inset

of (a), together with a BCS theoretical curve for the normal-

ized DOS at

0.1

and a corresponding

-axis tunnel-

ing spectrum for a pure

-wave superconductor (thin solid

line). Left inset: a normalized

-axis tunneling spectrum of

an optimally doped YBa

(

92.5

0.5

K). Right

inset: a typical spectrum for the

Zn-SLCO sample taken

4.2

spatial homogeneity within each grain and small variations

in the superconducting gap value (

13.0

1.0

meV)

from one grain to another. Here

was de

ned as the

conductance peak-to-peak separation in the spectra. This

observation was in sharp contrast to our previous

ndings

of strongly momentum-dependent spectra in the

-type

cuprates with

pairing symmetry [6]. The absence

of the zero bias conductance peak (ZBCP) [6], known

as a hallmark for unconventional pairing symmetry, for

over 1000 spectra provided additional support for a fully

gapped Fermi surface.

Despite suggestive evidence for

-wave pairing symme-

try, the unusually large ratio of

7.0

as com-

pared with the BCS ratio of 3.5 was indicative of strong

coupling effects. Moreover, the commonly observed

“

sat-

ellite features

”

in the quasiparticle spectra of

-type

cuprate superconductors [6], as exempli

ed in the left in-

set of Fig. 2(b), were invisible in SLCO. The satellite fea-

tures in

-type cuprates were associated with quasiparticle

damping by many-body interactions such as the collective

spin excitations [6,27,28]. Thus, the absence of satellite

features in SLCO is consistent with weakened spin

uc-

tuations as the result of diluted antiferromagnetic coupling

due to the presence of Cu

introduced by electron dop-

ing. In addition,

was found to completely vanish above

, with no apparent energy scale associated with any

depression of the density of states (DOS) at

,as

shown in the right inset of Fig. 2(a), and the tunneling

spectra were nearly temperature independent from just

above

110

K. The absence of any spectroscopic

pseudogap in the

-type in

nite-layer system was dis-

tinctly different from the

ndings in optimally doped and

underdoped

-type cuprates [5] and was independently

veri

ed by the NMR studies on similar samples [29].

By normalizing a typical spectrum in Fig. 2(a) relative

to the background conductance shown in the left inset of

Fig. 2(a), we compared the quasiparticle DOS of SLCO

with the BCS theoretical curve, as illustrated in Fig. 2(b).

The spectral weight of SLCO for quasiparticle energies at

was smaller than the BCS prediction, whereas

additional DOS appeared for

and the DOS ap-

proached 0 at the Fermi level (i.e.,

). Such behavior

cannot be accounted for by the simple inclusion of disorder

in the BCS weak-coupling limit, because the latter would

have only broadened the width of the conductance peaks

and also increased the DOS near

substantially. The

spectra also differed fundamentally from those of pure

-wave cuprates [6] because of the lack of discernible

gap variations and of the absence of ZBCP in all spectra

taken on random grain orientations. Even in a special

case of

-axis tunneling,

would have been

a positive constant in a

-wave superconductor, as

simulated by the thin solid line in Fig. 2(b), which is in

contrast to the

nding of

in SLCO.

Interestingly, recent Knight shift data from NMR studies

of similar SLCO samples have revealed much smaller

normal-state DOS at the Fermi level as compared with

those of other cuprates [29], which corroborates the inap-

plicability of weak-coupling theory to SLCO. We there-

fore suggest strongly correlated

-wave pairing in the

nite-layer system based on the empirical

ndings of

momentum-independent quasiparticle spectra, absence of

ZBCP, and

for all grain orientations.

In the case of

Zn-SLCO, the spectral characteristics

also revealed long-range spatial homogeneity in the

spectra and a similar gap value (

13.0

2.5

meV)

for randomly sampled areas in different grains, as ex-

empli

ed in the right inset of Fig. 2(b). Given that the

average separation among Zn impurities is

1.8

1.6

, our exhaustive spectral studies should have

covered a signi

cant number of Zn impurities. However,

no signi

cant local variations were found in the spectra

of the

Zn-SLCO, which differed fundamentally from

our observation of atomic-scale spectral variations in a

YBa

0.9934

0.0026

0.004

6.9

single crystal near

nonmagnetic Zn or Mg impurities using the same apparatus

[6]. Nevertheless, the conductance peaks in

Zn-SLCO

were signi

cantly broadened relative to pure SLCO, with

an increase in the DOS for

, as shown in the right

inset of Fig. 2(b). These features suggest that Zn impu-

rities resulted in reduced quasiparticle lifetime while re-

taining

, similar to the response of conventional

-wave

superconductors [19,20].

In contrast, two types of spectra were observed in

Ni-SLCO, as illustrated in Fig. 3(a). The majority spec-

tra (

90%

) exhibited suppressed coherence peaks, large

zero bias residual conductance, strong electron-hole spec-

tral asymmetry, and gradual spatial evolution over a long

range. In contrast, the minority spectra (

10%

) exhib-

ited sharp spectral peaks, small zero bias conductance,

and varying electron-hole spectral asymmetry over a short

range (

nm), as exempli

ed in the inset of Fig. 3(a) for

two representative minority spectra. The signi

cant spec-

tral asymmetry implied different phase shifts in the elec-

tronlike and holelike quasiparticle states as the result of

broken time-reversal symmetry [20,30], which may be re-

sponsible for the global suppression of the superconducting

phase coherence and thus a reduction in

Assuming homogeneous Ni-impurity distributions, the

average Ni-Ni separation would be

1.8

nm in the

plane and

1.6

nm along the

axis in each grain.

The impurity wave function with poor screening from

the carriers would have extended over a coherence vol-

ume

[20,30]. Given the coherence lengths

4.8

nm and

0.53

nm [25],

30%

volume probabil-

ity in each grain could be considered as under signi

cantly

weaker impurity in

uence. In the limit of completely ran-

dom grain orientation in

Ni-SLCO, the STM studies

of the grain surfaces would have

20%

probability for

nding surface regions with weak impurity in

uence and

spatial extension over a short range (

0.5

nm) along the

axis. This simple estimate is in reasonable agreement

-20

-0.5

0.0

(b)

(1)-(0)

-40-30-20-10 0 10203040

0.0

0.4

0.8

1.2

(0)

(2)

(1)

(a)

normalized dI/dV

V(mV)

(0) Pure SLCO

(1) Majority

(2) Mino

rity

-20 0 20 40

-30 -15 0

15 3

dI/dV (nS)

(c)

V(mV)

FIG. 3. (a) Main panel: comparison of a normalized majority

spectrum of

Ni-SLCO and that of pure SLCO at

4.2

K. The

normalization was made relative to the background conductance

shown by the dashed line in part (c). Inset: two minority spectra

with different electron-hole asymmetry. (b) Spectral difference

of the majority spectra relative to that of the pure SLCO. (c) A

series of spectra taken on the same grain of

Ni-SLCO at

nm apart. The conductance of all curves except the lowest

one has been displaced up for clarity.

with our observation of

10%

minority spectra with short-

range (

nm) spatial homogeneity. However, due to the

lack of direct information for the Ni distribution on the

sample surface, the true origin for two types of spectra in

Ni-SLCO remains uncertain.

Considering the spectral difference between the major-

ity spectrum of

Ni-SLCO and that of pure SLCO, as

shown in Fig. 3(b), we

nd that the spectral characteris-

tics resemble the

ndings in Ref. [20] and are representa-

tive of the impurity-induced state. On the other hand, the

slowly varying majority spectra of

Ni-SLCO, as shown

in Fig. 3(c), were possibly the result of strong overlapping

in the Ni-impurity wave functions and of weak screen-

ing effects due to low carrier density in SLCO, which

differed markedly from the rapidly diminishing impurity

effects away from an isolated Mn or Gd atom on the

surface of Nb [20], and also from the strong atomic-scale

spectral variations near Ni impurities in the

-type cuprate

[17]. The contrast in the spa-

tial extension of the Ni-impurity effects may be attributed

to the variation in the impurity coupling strength and range,

and also to the degree of impurity screening by carri-

ers. We suggest that the strongly interacting Ni impurities

Ni-SLCO are analogous to a Kondo alloy, which

cannot be explained by the Abrikosov-Gor

’

kov theory for

magnetic impurities in BCS superconductors [30].

The parent materials of all

-type cuprates are Mott

insulators with strong on-site Coulomb repulsion [3,4].

Thus, the formation of

-wave pairing symmetry is

energetically favorable in reducing the Coulomb repulsion

while retaining the quasi-two-dimensionality. On the other

hand, the strong three-dimensional coupling in the in

nite-

layer system could favor

-wave pairing symmetry by com-

pensating the resulting increase in the Coulomb repulsion

with a large gain in the condensation energy. Thus, the

pairing symmetry of cuprate superconductors may be de-

pendent on the speci

c structures and various competing

energy scales. Similarly, the pseudogap phenomena may

be due to competing orders and need not be universal for

all cuprates.

A recent study of the angular-resolved photoemission

spectroscopy (ARPES) on three different families of

-type cuprates suggested that an abrupt change of the

electron velocity in the

meV energy range was

ubiquitous and might be associated with the longitudinal

optical oxygen phonon modes in the CuO

planes [31].

Such changes in ARPES approximately coincided with

“

dip

”

feature in the quasiparticle tunneling spectra

of some cuprates [6]. However, our tunneling spectra of

SLCO revealed a dip energy at

meV, and that of

YBa

0.9934

0.0026

0.004

6.9

meV, much

smaller than the energy

meV for pure YBa

6.9

[6]. Thus, the only ubiquitous features among all cuprates

appear to be the strong electronic correlation and the

background antiferromagnetism of Cu

ions in the CuO

planes.

The research at Caltech was supported by NSF Grant

No. DMR-0103045 and the Caltech President

’

s Fund. Part

of the work was performed by the Center for Space Mi-

croelectronics Technology, Jet Propulsion Laboratory, and

was sponsored by NASA. The work at Pohang University

was supported by the Ministry of Science and Technology

of Korea through the Creative Research Initiative Program.

[1] C. C. Tsuei and J. R. Kirtley, Rev. Mod. Phys.

969

(2000), and references therein.

[2] D. J. VanHarlingen, Rev. Mod. Phys.

515 (1995).

[3] J. Orenstein and A. J. Millis, Science

288

468 (2000), and

references therein.

[4] S. Sachdev, Science

288

475 (2000), and references

therein.

[5] T. Timusk and B. Statt, Rep. Prog. Phys.

61 (1999),

and references therein.

[6] N.-C. Yeh

et al.,

Phys. Rev. Lett.

087003 (2001), and

references therein.

[7] L. Alff

et al.,

Phys. Rev. Lett.

2644 (1999).

[8] C. C. Tsuei and J. R. Kirtley, Phys. Rev. Lett.

182

(2000), and references therein.

[9] J. A. Skinta

et al.,

Phys. Rev. Lett.

207005 (2002).

[10] A. V. Balatsky, M. I. Salkola, and A. Rosengren, Phys.

Rev. B

15 547 (1995).

[11] M. E. Flatte, Phys. Rev. B

14 920 (2000).

[12] H. Alloul

et al.,

Phys. Rev. Lett.

3140 (1991).

[13] Y. Sidis

et al.,

Phys. Rev. Lett.

5900 (2000).

[14] S. H. Pan

et al.,

Nature (London)

403

746 (2000).

[15] W. A. MacFarlane

et al.,

Phys. Rev. Lett.

1108 (2000).

[16] K. Ishida

et al.,

Phys. Rev. Lett.

531 (1996).

[17] E. W. Hudson

et al.,

Nature (London)

411

920 (2001).

[18] A. Polkovnikov, S. Sachdev, and M. Vojta, Phys. Rev. Lett.

296 (2001).

[19] P. W. Anderson, J. Phys. Chem. Solids

26 (1959).

[20] A. Yazdani

et al.,

Science

275

1767 (1997).

[21] J. D. Jorgensen

et al.,

Phys. Rev. B

14 654 (1993).

[22] T. Siegrist, S. M. Zahurak, D. W. Murphy, and R. S. Roth,

Nature (London)

334

231 (1988).

[23] M. G. Smith

et al.,

Nature (London)

351

549 (1991).

[24] C. U. Jung

et al.,

Physica (Amsterdam)

366C

299 (2002);

Phys. Rev. B

172501 (2002).

[25] M.-S. Kim

et al.,

cond-mat /0102420.

[26] R. P. Vasquez, C. U. Jung, J. Y. Kim, M. S. Park, H. J. Kim,

and S. I. Lee, J. Phys. Condens. Matter

7977 (2001).

[27] A. V. Chubukov, N. Gemelke, and A. Abanov, Phys.

Rev. B

R6467 (2000), and references therein.

[28] C. L. Wu, C. Y. Mou, and D. Chang, Phys. Rev. B

172503 (2001).

[29] G. V. M. Williams

et al.,

cond-mat /0111421.

[30] A. A. Abrikosov and L. P. Gor

’

kov, Sov. Phys. JETP

1243 (1961).

[31] A. Lanzara

et al.,

Nature (London)

412

510 (2001).