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Pairing symmetry and spin-polarized

quasiparticle transport in high-

temperature superconducting

cuprates

Nai-Chang Yeh, Chu-Cheng Fu, Ching-Tzu Chen, Z.

Huang, Richard P. Vasquez, et al.

Nai-Chang Yeh, Chu-Cheng Fu, Ching-Tzu Chen, Z. Huang, Richard P.

Vasquez, Setsuko Tajima, "Pairing symmetry and spin-polarized quasiparticle

transport in high-temperature superconducting cuprates," Proc. SPIE 4058,

Superconducting and Related Oxides: Physics and Nanoengineering IV, (6

September 2000); doi: 10.1117/12.397825

Event: AeroSense 2000, 2000, Orlando, FL, United States

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Pairing Symmetry and Spin-Polarized Quasiparticle Transport in

High-Temperature Superconducting Cuprates

N.-C. Yeha, c.-c.

FUa,

C.-T. Chena, Z. Huanga, R. P. Vasquezb,

and S.

Tajimac

aDepment ofPhysics, California Institute ofTechnology, Pasadena, CA 91 125, USA

bJet

Propulsion

Laboratory, California Institute of Technology, Pasadena, CA 9 1 1 09, USA

cSuperconductivity Research Laboratory, International Superconductivity Technology Center,

Tokyo 135-0062, Japan

ABSTRACT

The pairing symmetiy and the superconducting gap in high-temperature superconducting cuprates are investigated as a

function of the hole doping level (x) and temperature (7), using directional scanning tunneling spectroscopy (STS). It is

found that the predominant pairing symmetry is

which is insensitive to the irariations in T and x. In contrast, the

maximum superconducting gap (zld) in YBa2Cu3O74 and La2SrCuO44 scales with the superconducting transition

temperature (Ta), &1d the ratio of (2A,/kBT) increases with decreasing doping level. The dominance of d22 pairing is

consistent with strong spatial variations in the local quasiparticle spectra near non-magnetic impurities such as Zn and Mg in

a (Zn,Mg)-doped YBa2Cu3O74 single crystal. To further elucidate the nature of the pairing state, the c-axis spin-polarized

quasiparticle transport in the superconducting state of YBa2Cu3O74 is investigated by studying the critical currents and STS

under the injection of electrical currents from the underlying ferromagnetic La7Sr03MnO3 layer in various ferromagnet-

insulator-superconductor (F-I-S) heterostructures. The temperature dependent spin diffusion length (5) and signatures of

nonequilibrium quasiparticle distribution under spin injection in d-wave superconductors are determined for the first time.

Keywords: pairing symmetry, scanning tunneling spectroscopy (STS), cuprate superconductors, zero-bias conductance peak

(ZBCP), spin-polarized quasiparticles, F-I-S heterostructures, spin diffusion length, nonequiibrium superconductivity.

1. INTRODUCTION

1.1. Pairing Symmetry in Cuprate Superconductors

The purity of pairing symmetry in cuprate superconductors has been an issue of intense investigation and heated debate in

recent years'9. Despite overwhelming experimental evidence supporting the d2,2 symmetiy as the predominant pairing

component', whether there may exist a secondary pairing component, particularly one that results in the breaking of time-

reversal symmetry"15, remains controversial. Theoretically, pairing mechanism based on the antiferromagnetic spin

fluctuations would require a d22 pairing symmetry°, whereas anion superconductivity would favor the (d22+i4) pairing

symmetiy', yielding time-reversal symmetry breaking. It has also been conjectured that the dominant d22 pairing

channel may be suppressed within approximately a coherence length of a surface that permits the formation of Andreev

bound states, yielding a local time-reversal symmetiy breaking and (d+is) pairing symmetiy even in the absence of any

magnetic field"2. This "surface-induced time reversal-symmetiy breaking" effect would result in the splitting of the zero-

bias conductance peak (ZBCP) as the result of Doppler shift of quasiparticle energies, and the magnitude of such splitting

would increase linearly with increasing magnetic field'2. However, the lack of universal experimental evidence for the time-

reversal symmetry breaking phenomena in both zero and finite magnetic fields in a vast quantity of experimental da&'5

casts concerns on the scenario of surface-induced time reversal symmetry breaking. Recent theoretical investigation'6 of

competing superconducting pairing channels based on a lattice model with a pairing kemal involving onsite repulsion and

nearest-neighbor attraction on a square lattice has suggested that the d-wave pairing channel generally prevails if the single

particle density of states is close to or more than half-filling'6. Furthermore, a number of theoretical calculations'7"8 have

demonstrated that surface impurities can significantly influence the local quasiparticle spectra of cuprate superconductors. A

recent comprehensive calculation'9 for the ground state of doped antiferromagnetic insulators suggests that while d22 is the

pairing symmetry for the ground state, the (d22+i4) pairing may be a short-lived meta-stable state that can occur through

Superconducting and Related Oxides: Physics and Nanoengineering IV, Davor Pavuna, Ivan Bozovic,

Editors, Proceedings of SPIE Vol. 4058 (2000) • 0277-786X1001$15.00

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either thennal excitations or existence of impurities. All these unsettled issues obvious require further experimental and

theoretical investigation.

1.2. Spin and Charge Channels in the Normal and Superconducting States of Cuprates

Another central issue of debates is the physical origin ofa "pseudoap" and the non-Fermi liquid behavior in the nonnal state

of underdoped and opti.maliy doped cuprate superconductors242b. 'fl j there exists an energy scale comparable to a

characteristic temperature T*, below which the density of states in the normal state is suppressed, indicating the opening of a

pseudogap. The pseudogap temperature T* increases with decreasing doping level (x) and is nearly independent of the

temperature (77. Further, experimental evidence suggests that T* is related to the occurrence of a spin gap in the cuprates, and

T* is significantly above the superconducting transition T for the underdoped cuprates. The general consensus is that the

non-Fermi liquid behavior of the cuprates for temperatures between T and T* is the result of different behavior in the spin

and charge channels, known as the spin-charge separation273' .

addition, the absence of longrange phase coherence32' in

these "doped Mott insulators" between T and T* is believed to contribute to the unconventional normal state properties. This

view also leads to the proposed dynamic and Josephson-coupled "stripe phases" in the cuprates that may be responsible for

the existence of a quasi one-dimensional Luttinger liquid, the latter is known to result in spin-charge separation phenomena

in lower-dimensional physical systems.

To further elucidate the issue of spincharge separation below T*, it is important to design experiments that can directly

assess the characteristic lengths and times for the spin and charge channe1sMI Our approach to this investigation is to

perform spin injection experiments on perovskite ferromagnet-insulator-superconductor (F4S) thin-film heterostructures3

36•

characterizing how pairbreaking effects due to spin-polarized quasiparticles depend on the temperature and the

thickness of the superconducting layer, we are able to deduce the spin diffusion length 5(7) along the caxis of YBa2Cu3O,

in its superconducting state. In addition, by considering the pair-breaking efficiency and transmission of spin-polarized

quasiparticles in YBa2Cu3O74, we are able to demonstrate signatures of nonequilibrium superconductivity in d-wave

superconductors for the first time.

2. QUASIPARTICLE TUNNELING SPECTRA OF CUPRATE SUPERCONDUCTORS

2.1. Sample Characterizations and Surface Preparations for STS Studies

The samples used for our STS studies include optimally doped and underdoped pure YBa2Cu3O7.8 single crystals with

superconducting transition temperatures of 91

1 K and 60 3 K, one nearly optimally doped YBa2Cu3O7 single crystal

with controlled 0.26% Zn and 0.4% Mg impurities, and underdoped a-axis oriented La2 SrCuO4 films with x 0.15,

0. 125, and 0. 10. The samples for the STS studies have been characterized by x-ray refraction (XRD). The crystalline axes of

the single crystals selected for the tunneling studies are determined by optical microscopy, and those of the oriented films are

based on the XRD information. The technique used in this investigation involves a low-temperature scanning tunneling

microscope (LT-STM) for studying the directional tunneling spectroscopy of cuprate superconductors along various

crystalline axes. The tunneling tip is made of Pt(85%)-Ir(]5%), and the highest voltage resolution at the lowest temperature

is 200 1LIV.

The

temperature range covered in this work is between 2.4 K and 10 K.

The surface preparation is very important for STS and STM studies, because STM is a surface sensitive probe with a depth

no more than 10 nm. Consequently, it is necessary to reduce any appreciable non-stoicliiometric surface layers in order to

obtain reasonable information for the generic properties of the materials to be investigated. Fo the single crystal samples

used in our experiments, they were first cut to reveal the desirable crystalline plane, polished to optical flatness, and then

annealed at proper temperatures under a gas flow of different oxygen partial pressure to yield the necessary doping level. The

sample was subsequently chemically etched with 1% Br2 in pure ethanol (or 0.5% Br2 in pure ethanol for thin film samples),

and then thoroughly rinsed in pure ethanol. This process has been demonstrated by XPS studies to yield optimal electronic

properties at the surface of various cuprate superconductors42'43, and to passivate the surface to prevent degradation in air

over an extended period of time (-

tens

of minutes). Hence, the sample could be properly transferred to the STM cryostat to

achieve high vacuum and low temperature condition to ensure preservation ofthe surface quality before experiments.

2.2. Optimally Doped and Underdoped YBa2Cu3O74 Single Crystals

Following our previous investigation, we employ the generalized BTK theory44 by Hu9, Kashiwaya and Tanaka'° to analyze

the directional tunneling spectra. The relevant physical quantities thus derived include the percentage of possible secondary

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