Kashiwaya H. Anomalous Magnetic Field Tunneling of YBa2Cu3O 7 Junctions Possible Detection of Non Fermi Liquid States 2004

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Anomalous magnetic-field tunneling of YBa2Cu3O7 junctions: Possible detection of

non-Fermi-liquid states

H. Kashiwaya,1,2 S. Kashiwaya,1 B. Prijamboedi,1 A. Sawa,3 I. Kurosawa,2 Y. Tanaka,4 and I. Iguchi51Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, Tsukuba,

Ibaraki 305-8568, Japan2Japan Womens University, 2-8-1 Mejirodai, Bunkyou-ku, Tokyo 112-8681, Japan3Correlated Electron Research Center of AIST, Tsukuba, Ibaraki, 305-8562, Japan

4Department of Applied Physics, Nagoya University, 464-8603, Nagoya, Japan5Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan

(Received 18 June 2004; published 8 September 2004)

Magnetic-field tunneling spectroscopy was applied to optimally doped YBa2Cu3O7 (YBCO) to detect the

quasiparticle properties in the superconducting phase. Three types of epitaxial YBCO/La 0.67Sr0.33MnO3 junc-

tions with different orientations were fabricated, and the responses of conductance spectra to an applied

parallel magnetic field were measured at below 1 K. The measured conductance spectra showed neither

splitting nor broadening with V resolution. This result apparently contradicts the model based on the weak

coupling theory. One possibility for this contradiction is the breakdown of the superconducting Fermi-liquid in

optimally doped YBCO.

DOI: 10.1103/PhysRevB.70.094501 PACS number(s): 74.50.r, 74.25.Ha, 74.45.c, 74.72.h

The mechanism and the phase diagram of high-Tc super-conductors (HTSCs) are controversial issues in solid-statephysics. Because the superconductivity in HTSCs isstrongly influenced by magnetism originated from strongCoulomb interaction,13 various experimental techniquessuch as NMR, neutron diffraction, and magnetization mea-surements have been applied to elucidate magnetic propertiesof HTSCs. On the other hand, tunneling spectroscopy hasbeen accepted as one of the highest energy-resolution probesthat can determine quasiparticle states and thus has signifi-cantly contributed to the identification of the d-wave symme-

try as well as the pseudogap. Especially, the phase sensitivityof tunneling conductance spectra has been established by nu-merous studies on HTSCs.46 However, it should be stressedthat most of the previous experimental works on the tunnel-ing spectroscopy of HTSCs focused on the charge informa-tion of quasiparticles, and the spin information has not beenwell clarified.

Here we clarify an anomaly related to the quasiparticlespin states detected by magnetic-field tunneling spectroscopy(MFTS) of HTSCs. Conductance spectra of a ferromagnet/insulator/superconductor (FIS) tunneling junction exhibitspin-dependent splitting in the field that is applied parallel tothe plane of the junction.7,8 In the present experimental con-

ditions (so-called Zeeman magnetic field), the influence ofthe Doppler shift can be neglected. Assuming no spin-orbitand no spin-flip scattering, the conductance spectra of the

junctions V in the applied field is simply represented by

V = PN V+ /2 + PN V /2 , 1

where P, is the polarization of the normal side for

spin and N is the conductance spectrum in the absence ofthe applied field. The amplitude of splitting is proportionalto N /N , where N and N are the densities of

states obtained from Pauli susceptibility and normal-state

electronic specific heat, respectively.9 In general, N andN of superconductors decrease due to the gap formationbelow their Tcs and their temperature dependence is domi-nated by Yoshida function f T if they are measured indepen-dently. The advantage of the MFTS is that the ratio ofN to N is directly measured and that the effect of f Tis canceled, and thus the quasiparticle ground state can bemeasured with high accuracy irrelevant to the superconduct-ing gap formation. In fact, detailed information on quasipar-ticle properties in a superconductor, such as the renormaliza-tion effect corresponding to the Fermi-liquid parameter G0,

have been successfully determined for s-wave supercon-ductor Al from the field-induced splitting of the BCS coher-ent peaks.9,10

According to the generic phase diagram of HTSCs (Fig.1 in Ref. 3), Fermi-liquid phase (region III) transits topseudogap phase (region II) across the quantum critical point(QCP) as the doping rate is decreased from overdoped tounderdoped. This phase transition is masked by the super-conducting dome, however, thus making it difficult to di-rectly identify the breakdown of Fermi liquid at thepseudogap phase. Here, to detect the possible phase transi-tion beneath the superconducting dome of HTSCs, we mea-sured the detailed quasiparticle properties by using MFTS.Due to its high sensitivity to quasiparticle states, MFTSshould be able to detect this phase transition. Although ac-cording to the Bogoliubov de-Gennes equation11 the straight-forward application of MFTS to d-wave superconductorsshould be possible, experimentally observing the splitting inreal HTSC junctions is difficult. The difficulty is mainly be-cause the conductance spectra of HTSC significantly broad-ens possibly due to the random scattering near junctioninterfaces.1214 Therefore, high quality junctions are the es-sential for successful application of MFTS to HTSCs.

In this study, three different types of YBa2Cu3O7/ La0.67Sr0.33MnO3 YBCO/LSMO spin-

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polarized tunneling junctions were fabricated: (100) and

(110) edge junctions and overlayer junctions. All junctionswere epitaxially grown by pulsed laser deposition on SrTiO3(STO) substrates by using both in situ and ex situ processes,and their interfaces at the atomic scale were well identified inour previous work.15 Figure 1 shows a micrograph and sche-matics of the YBCO/ LSMO junctions, which had crossstrip-line structure and whose size was typically 3040 m2. The tunneling current of edge junctions was re-stricted to either (110) or (100) orientation by inserting aninsulating layer STO between YBCO and LSMO [Figs. 1(b)and 1(c)]. The orientation was controlled by the relative

angle between the strip-line and the crystal orientation ofSTO substrates as shown in Fig. 1(c). Although the interfaceof the overlayer junction was composed of in-plane andc-axis components, its interface quality was higher than thatfor edge junctions because of its simple fabrication process.The spin polarization of LSMO might help to identify theorigin of the magnetic-field-induced splitting, because spin-dependent effects and spin-independent effects can easily be

distinguished based on the height of the split peak.11

For alljunctions, YBCO films were optimally doped for a Tc ofabout 90 K and a thickness of about 100150 nm, whichwas much smaller than the penetration depth for the c-axisdirection. The conductance spectra were measured using aconventional four-terminal setup with a lock-in amplifier ineither a helium 3 or a dilution refrigerator.

For all YBCO/LSMO junctions, the conductance spectrahad parabolic or V-shaped backgrounds and the supercon-ducting gap or zero-bias peak (ZBP) was distinct below5060 K. Figures 2(a), 2(b), 3(a), and 3(b) show the re-sponse of the conductance spectra to an applied parallel mag-netic field obtained at 0.51 K for the (100) and (110) junc-tions, respectively. Consistent with dominant dx2y2-wave

symmetry of YBCO, the (100) junction showed gap struc-ture, whereas the (110) junction showed ZBP. Figures 2(b)and 3(b) show the enlarged-scale plot of magnetic field re-sponse. Except for the shift in background conductance ob-served for all of the YBCO/LSMO junctions, the gap andZBP features were relatively insensitive to the applied field.Figures 2(c) and 3(c) show the respective theoretical conduc-tance spectra calculated using Eq. (1) and assuming g =2 andP, =0.5. Inconsistent with these theoretical calculation, the

measured conductance spectra showed neither splitting norbroadening, independent of junction orientation.

FIG. 1. (Color online) Micrograph and schematics of

YBCO/ LSMO junctions. (a) Top-view micrograph of the cross-

strip line structure junctions. (b) Schematic of intersectional view of

(100) and (110) junctions. Current component in the c-axis direc-

tion is blocked by the STO layer. (c) Orientation of an edge junction

controlled by rotating the relative angle of the cross-strip line and

STO substrate. (d) Schematic of intersectional view of an overlayer

junction.

FIG. 2. Conductance spectra of (100) junc-

tions measured at 1 K. (a) Magnetic-field re-

sponse in a relatively wide voltage range shows

that the gap structure was almost invariant for the

applied field except for the slight background

shift and the reduction of the gap amplitude. (b)

Enlarged-scale plot of magnetic field response

near zero-bias level in order to see detailed fea-

tures clearly. All curves fall onto a single curve ifthey are normalized by the background. (c) The-

oretical conductance spectra calculated using Eq.

(1) and assuming = 2BHa.

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To further clarify this lack of splitting and broadening, wemeasured the conductance spectra of overlayer junctions in adilution refrigerator at 0.1 K, as shown in Fig. 4. Comparingwith the ZBP of nonepitaxial YBCO/Ag edge junctions fab-ricated by an Ar ion-milling process,16 the sharpness of thepeak for the YBCO/LSMO junction is quite evident.17 Ac-cording to the calculation based on quasiclassical theory and

lattice models for d-wave superconductors with short coher-ence length, the specularity of interface has a fatal influenceon the sharpness.1214 The sharpness of the peak for theYBCO/LSMO junction is consistent with these theories be-cause epitaxial growth at the interface is realized inYBCO/LSMO as shown in a TEM image in Ref. 15, al-though lattice mismatch as well as interface roughness dur-

FIG. 3. Conductance spectra of (110) junc-tions measured at 0.5 K. Reflecting the (110) ori-

entation of junction interface, the spectra show

ZBP structures. (a) The magnetic-field response

of the junction in a relatively wide voltage range

shows that the ZBP is almost invariant for the

applied field except for a slight background shift.

(b) Enlarged-scale plot of magnetic-field response

near the zero-bias level. (c) Theoretical conduc-

tance spectra calculated using Eq. (1) and assum-

ing = 2BHa.

FIG. 4. Conductance spectra of the overlayer

junction. (a) Temperature dependence of the con-

ductance spectra. (b) Enlarged-scale plot of the

temperature dependence near the zero-bias level.

The broad peak of nonepitaxial YBCO/Ag junc-

tions measured at 0.1 K suggests the presence of

intrinsic broadening effects other than the thermaleffect. (c) Magnetic-field response of the over-

layer junction near the zero-bias level. Peak split-

ting and broadening (measured at high accuracy

by MFTS) are completely absent.

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ing the ion-milling etching process cannot be avoided inYBCO/Ag ramp-edge junctions. The appearance of a sharppeak in the conductance spectrum of the YBCO/ LSMO

junctions enabled MFTS to measure the spectra with highaccuracy. To avoid heat generation during the measurement,only a small voltage range 300 V was measured, andthe magnetic field range was from 07 T [Fig. 4(c)]. Sur-prisingly, no noticeable splitting was observed up to 7 T,

verifying the complete disappearance of the splitting withV accuracy.18

The conductance spectra yield crucial information aboutthe pairing symmetry of YBCO. According to the conduc-tance formula for anisotropic superconductors, the peakstructure directly reflects the phase difference between thea-axis and the b-axis directions.4,5 In the case of subdomi-nant imaginary (i.e., broken time-reversal symmetry) s waveand dxy wave are induced near the interface of the (110)orientation, the amplitude of the peak splitting approximatelycorresponding to the subdominant components can then bemeasured.19,20 Based on the experimental results, the singlepeak center was located just at the zero-bias level, suggestingthat optimally doped YBCO has a pure dx2y2-wave state with

the accuracy better than 0.1%. Furthermore, the presentmagnetic-field response indicates that the symmetry was in-variant even in a parallel applied field.

Details about the spin-dependent splitting of FIS junctionshave been analyzed based on the weak coupling theory forconventional superconductors.10,21 The presence of spin-orbitscattering is shown to obscure observation of the clear split-ting. In general, although the influence of the spin-orbit scat-tering is significant for heavier atoms, in hole-doped HTSCsthe doped carriers mainly occupy the O2p, thus the spin-orbitscattering of these HTSCs should be smaller than that of Al.Actually, the Elliot estimate based on residual resistivitiesgives a spin-orbit scattering time so of the order of 10

10 s

and gives a characteristic energy

/so that is far smallerthan the pair potential amplitude of YBCO.22 Even if weassume non-negligible spin-orbit scattering, the conductancespectra should exhibit significant broadening when a mag-netic field is applied. The measured spectra, however, did notexhibit such broadening.

If spin-orbit scattering is negligible, Eq. (1) gives conduc-tance spectra in the applied field. The splitting amplitude isgiven by 2BHT, where HT=Ha +Hint, Ha is the applied field,HT is the total field, Hint is the internal field, and B is theBohr magneton. For a superconducting Fermi-liquid such asAl, is 2BHa/ 1 + G

0 at the region close to the phase

boundaries, but converges to 2BHa at the zero temperatureand low magnetic-field limit due to the disappearance ofscattering rates of the quasiparticles.9,10 Our results show that

the splitting is absent in the 0.1 K, evidently contradicting

this theory. In other experimental work, ESR studies report ag factor (of conducting electrons above Tc) of about 2 formost HTSCs.23 Although magnetization and specific-heatmeasurements are reportedly influenced by the formation ofa pseudogap,24,25 no significant change in the Wilson ratiohas been detected. Also above Tc, the closing of thepseudogap by the Zeeman term has been reported by using

c-axis tunneling, which is also consistent with a g factor of2.26 Based on these data, no serious anomaly exists above Tcexcept the gradual opening of pseudogap in N and N .

A possible explanation for the present result is the signifi-cant influence of the QCP as discussed by several models.27

Actually, anomalous behaviors of electronic specific heat andPauli susceptibility near the QCP have been reported forheavy fermion systems, and the origin of these behaviors hasbeen explained based on the spin fluctuation model.28 A simi-lar anomaly should exist in HTSCs due to the enhancedquantum fluctuation as approaching the QCP. However, thefluctuation effect does not account for the complete disap-pearance of . Therefore, the most plausible explanation is

that the quasiparticles of optimally doped YBCO beneath thesuperconducting dome are in a spin-gapped metal statewhere N /f T is zero, while N /f T is finite at the low-

temperature limit. Because the spin-gapped metal cannot becontinuously connected to the Fermi-liquid phase, the super-conducting Fermi-liquid picture is broken already at opti-mally doped YBCO. It is likely that the pseudogap phase isprescribed by finite spin gap because the experimental obser-vation of the spin-gap formation at the underdoped regionhas widely been reported. It should be noted that the spin-gapped quasiparticle ground states detected in the presentexperiment have nothing to do with the singlet formation dueto the superconducting fluctuation. Although the consistency

between theoretical models of the HTSCs and the presentresults is not clear at present, clarifying the present propertiesat higher temperature and in the whole doping rate is impor-tant to identify the phase diagram of the HTSCs.

In summary, precise measurements of the conductancespectra of epitaxial LSMO/ YBCO junctions at very lowtemperature reveal the pure d-wave state of optimally dopedYBCO. The tunneling conductance showed no noticeablesplitting when a magnetic field was applied parallel to the

junctions and perpendicular to the c axis. The lack of spin-dependent splitting cannot be explained by the conventionalmodel based on the weak coupling theory. The most plau-sible explanation is that the quasiparticle of the optimallydoped YBCO are in the spin-gapped metal states, i.e., non-Fermi-liquid.

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and the references therein.7 R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173 (1994).

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8 When the thickness of film is comparable to the penetration depth

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Kashiwaya H. Anomalous Magnetic Field Tunneling of YBa2Cu3O 7 Junctions Possible Detection of Non Fermi Liquid States 2004