high-magnetic-field study of high-tc cuprates

Physica B 319 (2002) 310–320

High-magnetic-field study of high-Tc cuprates

N. Miuraa,*, H. Nakagawaa, T. Sekitania, M. Naitob, H. Satob, Y. Enomotoc

a Institute for Solid States Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, 277-8581 Chiba, JapanbNTT Basic Research Laboratories, Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan

cSuperconductivity Research Laboratory, International Technology Center, Shinonome, Koto-ku, Tokyo 135-0062, Japan

Received 11 March 2002

Abstract

We present our recent studies of high-Tc superconductors in very high pulsed magnetic fields exceeding 100T

(megagauss fields). In YBa2Cu3O7d, the super to normal transition was observed in transport measurements for B8c

under short pulse fields up to 120T. A plot of the upper critical fieldHc2 as a function of temperature demonstrates that

the curve obeys the conventional relation well established in other type-II superconductors in the dirty limit. From the

resistivity versus temperature curve at high magnetic fields above the super to normal transition, it was found that the

hole system shows a metallic temperature dependence for B8c: In Nd2xCexCuO4 as electron-doped cuprate, a large

magneto-resistance was observed above the transition field. It was also found that the temperature dependence of the

resistivity above the critical field is insulator-like showing a prominent logarithmic up-turn at low temperatures. A

similar logarithmic up-turn was observed in several other cuprates such as La2xSrxCuO4 (LSCO), La2xCexCuO, and

Pr2xCexCuO4. The experimental results for thin-film samples of LSCO with different strains suggest that the Kondo

effect plays an important role in these materials. A new phase diagram of LSCO is proposed on the basis of a viewpoint

of the interplay between superconductivity and the Kondo effect. r 2002 Published by Elsevier Science B.V.

PACS: 72.15.Gd; 74.72.Bk; 74.72.Jt; 74.76.Bz

Keywords: High magnetic fields; High-Tc cuprates

1. Introduction

The recent advances in the magnet technologyhave enabled us to produce very high magneticfields up to several megagauss (>100T) that canbe applied to solid-state experiments [1]. At theInstitute for Solid State Physics (ISSP) of theUniversity of Tokyo, we have succeeded in

producing very high magnetic fields up to 622Tby electromagnetic flux compression and up to302T by the single-turn-coil technique [2,3]. Non-destructive long pulse fields up to about 60T werealso generated by conventional wire-wound sole-noids. These fields were applied in many differentexperiments such as magneto-optical spectroscopy[4,5], cyclotron resonance [6], magnetization [7], ormagneto-transport measurements [8]. A variety ofinteresting experiments can be performed in theextreme limit of high magnetic fields in diverseresearch areas. As an example of experiments in

*Corresponding author. Tel.: +81-471-36-33-41; fax: +81-

471-36-33-35.

E-mail address: [email protected] (N. Miura).

0921-4526/02/$ - see front matter r 2002 Published by Elsevier Science B.V.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 1 3 4 - 1

very high magnetic fields, we present here a studyof high-Tc cuprates in semi-destructive fieldsgenerated by the single-turn-coil technique andnon-destructive pulse fields generated by wire-wound magnets.Since its discovery [9], high-Tc superconductors

have been one of the most important researchsubjects in solid-state physics. It has been pointedout that the electron system in this class ofmaterials might have unusual properties unlikenormal Fermi liquids. In order to investigate theorigin of the unique properties of these materialsand the mechanism of the high-Tc superconduc-tivity, we need to investigate the normal-stateproperties at low temperatures. These materialshave high critical temperatures and hence theupper critical field is also quite high. Therefore, inorder to break their superconductivity at lowtemperatures, either chemical-doping or applica-tion of high magnetic fields above the uppercritical field is an essential means to exploit.We investigated the super–normal transition

and the normal-state conductivity by using veryhigh magnetic fields. Measurement of the electricalresistance in a short pulse of megagauss fields is adifficult task, because of the large inductionassociated with the high and short magnetic fieldpulse. Some pioneering work on the measurementsin very high fields has been reported onYBa2Cu3O7d (YBCO) [10,11]. In order to avoidthe problems arising from the induction in the leadwire loop, we have developed a contactlesstechnique to measure the AC field transmissionthrough samples [12]. We have also developed atechnique to measure magnetization hysteresis fordetermining the field of irreversible point as afunction of temperature [13,14]. As regards theresistivity measurements with contacts on samples,we have succeeded in measuring the resistivity ofhigh-Tc superconductors in megagauss fields byminimizing the loop involved in the sample andthe lead wire, and by use of band path filters[8,15,16]. By using this technique, we found manynew features in YBCO. Another technique is toutilize a strip line measuring the microwavetransmission with a frequency of 0.5–1.0GHz[17]. In collaboration with the group of NationalPulsed Magnet Laboratory of the University of the

New South Wales, the strip-line technique wassuccessfully applied for experiments with the sin-gle-turn-coil technique [18] and explosive-drivenflux compression [19]. In long pulse fields up to60T, the transport measurements can be morereadily made and very accurate measurements arepossible on 214-type high-Tc cuprates, such ashole-doped La2xSrxCuO4 (LSCO) and electron-doped La2xCexCuO4 (LCCO), Pr2xCexCuO4(PCCO), and Nd2xCexCuO4 (NCCO) [20–22].One of the important problems on high-Tc

superconductors is the transition from the super-conducting state (S) to the normal state (N) by amagnetic field. Previous studies of the S–Ntransition have been mostly limited to the vicinityof the critical temperature Tc; since we need veryhigh fields to extend the measurements to lowertemperatures. In over-doped Tl or Bi compoundswhich have Tc lower than 20K, the upper criticalfield determined from the resistivity measurementswere found to increase abruptly at low tempera-tures down to the mK range [23,24]. This behavioris completely different from that of conventionaltype-II superconductors which is well described bythe theory of Werthamer–Helfand–Hoenberg(WHH) [25]. In high-Tc cuprates, the fluctuationeffect is very large due to the short coherencelength and the two-dimensional character of theelectron system. As a result, it is generally believedthat Hc2 is not a phase transition point, but thecross-over point from the flux-liquid state tothe normal state [26]. Therefore, the implicationof Hc2 determined by resistivity measurements isnot so clear. However, in order to clarify thenature of the S–N transition, it would certainly bevery important to investigate the S–N transition atlow temperatures in detail.Another important problem is the temperature

dependence of the normal resistivity at lowtemperatures created by the S–N transition inhigh magnetic fields. In order to investigate thenormal resistivity of high-Tc superconductors atlow temperatures, we need to break the super-conductivity either by chemical-doping or byapplication of high magnetic fields. The latter ismuch more preferable, since it allows measure-ments over a wide range of composition includingthe vicinity of the optimal-doping. Boebinger,

N. Miura et al. / Physica B 319 (2002) 310–320 311

Ando and coworkers reported results of experi-ments on the normal resistivity in the ab-plane andalong the c-axis at low temperatures and highmagnetic fields. They found in the temperaturedependence of LSCO that, for nearly optimallydoped samples, the c-axis resistivity is insulatingwhilst the ab-plane resistivity is metallic, but bothbecome insulating at low temperatures [27,28]. InBi2Sr2xLaxCuOy, however, the metallic behaviorof the ab-plane resistivity is preserved down tolow temperatures (0.66K) keeping the insulatingbehavior of the c-axis resistivity [29]. It is ofimportance to study the temperature dependenceof the normal resistivity in a wide range ofmaterials.

2. Experimental procedure

The samples of YBCO thin film with a thicknessof 1000 (A were grown on a MgO substrate by thelaser ablation technique. It is a slightly under-doped sample with a critical temperature Tc of84K. The samples were processed to make theirshapes to either a meander-like or a simple bar-likeshape with a length of about 500 mm. Fourcontacts method was employed with a DC (squarepulse) and AC current. In the case of AC meas-urements, AC current of frequencies of 5–10MHzwas supplied to the sample.Fig. 1 shows the magneto-resistance of YBCO

thin film for B8c at different temperatures in longpulse fields up to 50T. Magneto-resistance curveswith excellent signal-to-noise ratio were obtainedwith both up-rising and down-falling field slopes.It was confirmed that there was no heating effectfrom eddy currents in our experiments, since thetwo data obtained by both up- and down-sweep ofthe pulsed field coincided almost perfectly.In short pulses with peak field higher than 100T

produced by the single-turn-coil technique, themeasurement of the resistivity involves muchmore difficult technical problems. Fig. 2 showsthe typical wave forms of the magnetic field andthe current of the single-turn-coil technique [2,3].In this case, a peak field of 263T was obtained in abore of 6mm. The maximum field depends on thebore of the coil. The duration of the magnetic field

is about 7 ms. In such a short pulse, the inducedvoltage in the lead wires becomes enormous. Ifthere is a loop with a radius of 2mm in the circuitformed by the sample and the lead wire, theinduced voltage reaches B800V in a field up to100T, which is much larger than the bias voltageapplied to the sample for measurements. Byminimizing the cross-section and by making thelead wires parallel to the field as much as possible,the induced voltage was greatly reduced. Optimiz-ing wirings, filters and compensation circuits, it

0

50

100

150

200

250

300

350

0

R (

Ω)

B (T)

40 K

90 K

10 20 30 40 50

Fig. 1. Magneto-resistance of YBa2Cu3O7d thin film for B8c

at different temperatures in long pulse fields up to 50T. The

temperatures are 40, 45, 50, 55, 60, 62.8, 65, 67, 69, 71, 73, 75,

77.5, 79, 80.4, 82, 84, 86, 88 and 90K from bottom to top.

0 5 10 15 20

0

1

2

3

B (

T)

I (M

A)

Time (µs)

0

100

200

300

IB

Fig. 2. Pulse shapes of the current (I) and magnetic field (B)

generated by the single-turn-coil technique. The bore and the

length of the coil are 6.0mm and its wall thickness is 3mm. The

maximum of the field 263T is reached earlier than that of the

current because of the deformation of the coil. The maximum

field increases with decreasing bore.

N. Miura et al. / Physica B 319 (2002) 310–320312

has become possible to obtain the magneto-resistance curve with a reasonable signal-to-noiseratio [8,15,16]. The sample was cooled by flowingHe liquid or gas around the sample, and thetemperature was measured by a gold–iron Chro-mel thermocouple.

3. Results and discussion

3.1. YBa2Cu3O7d

Fig. 3 shows the DC magneto-resistance ofYBCO for B8c-axis measured with the single-turn-coil technique at various temperatures. Anabrupt jump of the resistivity corresponding to thesuper–normal transition is clearly observed. Atthe start of the pulse, large noise is induced due tothe extraordinary large dB=dt and the triggernoise. Except for the region at the first part of thepulse in the up-sweep (below B40T), the signalwas much deteriorated by the noise, but at about40T, the signal was in agreement with the datataken by a long pulse up to 40T. In addition, theup-sweep trace and down-sweep trace taken in onepulse coincide with each other above 60T. Below60T, therefore, we obtained just down-traces. Inthe down-trace we obtained reproducible datadepending just on temperature.

As the field-induced transition is gradual withseveral deflection points, there are many ways ofthe definition of the critical field for the S–Ntransition. The onset of the transition Honset; thatcan be determined as a cross-point betweenthe straight lines from the low field range andthe transition range, the offset field Hhf where thenearly constant gradient of the resistance vs. fieldcurve in the high-field range starts increasing in thetransition at lower fields, the midpoint Hmidpoint

determined by the midpoint where the resistivity isa half of that for Hhf ; etc. If we take Hhf ; thegradient of the Hc22T curve in the vicinity of TcbecomesB2T/K which is in good agreement withthe data reported by Welp et al. [30]. From suchmeasurements, we can determine the phase dia-gram of YBCO. The diagram for Hhf is shown inFig. 4. The phase diagram for B8c is well describedby the WHH model of the dirty-limit super-conductor [25] by choosing certain parameters ofa and lSO: It implies that the system is featured bya phase diagram similar to those for conventionaltype-II superconductors for B8c: This is in sharpcontrast to the case for B8c [18], where the uppercritical field exceeds the paramagnetic limit sug-gesting the FFLO state [31,32].Another interesting problem of high-Tc super-

conductors is the normal-state conductivity atlow temperatures. Study of the low-temperature

0

5

10

15

20

25

30

35

40

− 40 − 20 0 20 40

R (

Ω)

B (T)

4.2 K

20 K60 K

50 K

40 K

B // c

60 80 100 120

Fig. 3. Raw data of the magneto-resistance of YBa2Cu3O7d at

different temperatures measured in short pulse fields generated

by the single-turn-coil technique.

0

20

40

60

80

100

120

0 20 40 60 80

µ0Hc2_h

µ0Hc2_h (DC)

µ0Hc2_h (AC)

µ 0 H

c2 (

T)

T (K)

(α,λ SO) = (1.055, 6)

(α, λSO) = (1.055, 3.3)

(α,λ SO) = (0, 0)

(α, λSO) = (1.055, 0)

Fig. 4. Plot of Hc22h as a function of temperature. Data

obtained with DC current and AC current are shown together

with the theoretical curve calculated based on the WHH theory.

Different values of (a; lSO) are assumed, as shown in the figure.


transport provides a rich source of information ofthe mechanism of the transport and the super-conductivity itself. However, in order to realize thenormal state at low temperatures, we have to dopeimpurities or apply high magnetic fields to destroythe superconductivity. By applying high magneticfields exceeding the upper critical field, weinvestigated the normal state of YBCO at lowtemperatures. Fig. 5 shows the temperature de-pendence of the normal-state resistivity at differentmagnetic fields. At sufficiently high magnetic fieldsabove 100T, samples are considered to be in thenormal state. It is clearly seen that the resistivityexhibits metallic behavior. Namely, the resistivitydecreases with decreasing temperature.Boebinger et al. reported that in LSCO bulk, the

in-plane resistivity shows metallic behavior nearthe optimum-doping but converts to insulatingbehavior when the doping is reduced [25]. In a veryunder-doped sample of YBCO (d ¼ 0:62), aninsulating properties were observed [33]. Thepresent sample is almost optimally doped butslightly under doped (dB0:1). Therefore, we canconclude that in YBCO, the metal–insulatortransition occurs at a more over-doped region incomparison to LSCO.

3.2. Nd2xCexCuO4 and La2xSrxCuO4

The normal resistivity at low temperatures canbe studied in detail at lower fields in the so-called‘‘214’’-type cuprate compounds, such as hole-

doped LSCO and electron-doped LCCO, PCCO,and NCCO. Recently, the NTT group succeededin growing high-quality film samples of thesecompounds whose residual resistance is signifi-cantly lower than in previous samples grown byMBE [34–37]. The c-axis-oriented films weregrown on SrTiO3 (0 0 1) substrates and LaSrAlO4(0 0 1) substrates. It was found that the propertiesof the films, such as the critical temperature, canbe controlled significantly by choosing the latticeconstant of the substrates. For LSCO, for in-stance, films on LaSrAlO4 (0 0 1) substrates, arecompressively strained in plane (a-axis) andexpansively strained out of plane (c-axis) by thePoisson effect, and have TcB44K, whereas filmson SrTiO3 (0 0 1) substrates are strained in theopposite way, and have TcB26K.In the absence of magnetic fields, the in-plane (ab)

resistivity of NCCO films on SrTiO3 (0 0 1) sub-strates showed roughly a T2 dependence at hightemperatures above B50K. This seems to indicatethat electron–electron scattering dominates inthis region. Below 50K, however, a very differentr T relation was observed depending on thedoping; T-linear dependence in over-doped samples,and insulating behavior in under-doped samples.Fig. 6 shows the resistivity as a function of

magnetic field at various temperatures for anoptimal-doped sample (x ¼ 0:146). In addition topositive magneto-resistance as a background,negative magneto-resistance appears at low tem-peratures just above the magnetic field (Hc2). Itbecomes more prominent with decreasing tem-perature, and diminishes at very high magneticfields. The under-doped sample (x ¼ 0:131) showsbehavior qualitatively similar to this optimal-doped sample (x ¼ 0:146). In over-doped samples(x ¼ 0:166), however, the negative magneto-resis-tance is discernible only at the lowest temperatureregion. Fig. 7 shows r2T curves for under-,optimal- and over-doped samples. All of themshow a low-temperature up-turn at low magneticfields. The up-turn follows a logT dependencebelow 50K in the under-doped sample, below 10Kin the optimal-doped sample, and below 2K in theover-doped sample. In the under-doped samples,this dependence appears already in zero magneticfield above Tc: In over-doped samples, the up-turn

0

10

20

30

40

50

60

0 20

0T

10T

ρ ab (

µΩcm

)

T (K)

B // c

20T

30T

40T

60T

80T

100T

110T

40 60 80 100 120 140

Fig. 5. Temperature dependence of the resistivity at different

magnetic fields for YBa2Cu3O7dU


behavior of the resistivity is very weak and can beseen only at very low temperatures. The resistivityminimum shifts to lower temperature with increas-ing Ce-doping. Furthermore, it should be notedthat in the lowest temperature region, the resistiv-ity of all films tends to deviate from a simple logT

dependence showing a tendency of saturation.This result is very similar to that for optimallydoped PCCO [38].With increasing magnetic field, the logT depen-

dent up-turn is suppressed except for under-dopedsamples, where the up-turn behavior persists up tothe highest magnetic field studied. In optimal-doped samples, the resistivity shows metallicbehavior in high magnetic fields, except for verylow temperatures, where the up-turn remains. Inover-doped samples, the low-temperature resistiv-ity turns to be completely metal-like in highmagnetic fields. In optimal- and over-doped

samples, an insulator–metal transition occurs byapplying the magnetic field. Fig. 8 shows themagneto-resistance curve at different directionsof magnetic field relative to the normal of the filmfor the optimal-doped sample. A striking finding isthat the field gradient of the negative magneto-resistance is almost independent of the direction ofthe magnetic field, i.e. the negative magneto-resistance component is isotropic.In the ‘‘low-Tc’’ LSCO film on a SrTiO3 (0 0 1)

substrate (TcB26K), similar tendency was

26

28

30

32

34

0 5 10 15 20 25 30 35 40

1.5K4.2K6.0K8.0K19.0K24.0K

B (T)

0

4

8

12

16

20

24

28

32

4.2K

8.0K

19.0K

24.0K

Res

istiv

ity (µ

Ωcm

)

x = 0.146

x = 0.146

Res

istiv

ity (µ

Ωcm

)

Fig. 6. Magneto-resistance as a function of magnetic field at

different temperatures for an optimally doped NCCO sample

(x ¼ 0:146). Resistivity-versus-B curves at different tempera-

tures for the optimal-doped NCCO film (x ¼ 0:146). The upperpanel shows raw data, and the lower shows an enlarged view.

75

80

85

90

95

x = 0.131

8.0T

20T

40T

0T

Temperature (K)

Res

isti

vit

y (

µΩ c

m)

72

76

80

84

88

92

96

1 100T(K)

ρ (µ

Ωcm

)

8.0 T

0 T

25

27

29

31

33

35

x = 0.146

10T

20T

30T

40T

0T28

30

32

34

T (K)

ρ (µ

Ωcm

)

10 T

20

21

22

23

x = 0.166

0T

7.0T

10T

15T

30T40T

19.5

20

20.5

21

21.5

T (K)

ρ (µ

Ω cm

)

7.0 T

0 10 20 30 40 50 60

Temperature (K)0 10 20 30 40 50

Temperature (K)

0 5 10 15 20 25 30 35

Res

isti

vit

y (

µΩ c

m)

Res

isti

vit

y (

µΩ c

m)

1 10

1 10

23

10

Fig. 7. r2T relation of samples at different values of x: Theinsets show the relation on the extended scales below 10T.


observed, whereas the ‘‘high-Tc’’ film on aLaSrAlO4 (0 0 1) substrate (TcB44K) showed noup-turn even at the lowest temperature. Thetwo LSCO films show quite different magneto-transport properties as shown Fig. 9 whichdisplays raw magneto-resistivity data of the‘‘high-Tc’’ and ‘‘low-Tc’’ films taken by sweepingmagnetic fields applied parallel to the c-axis atvarious temperatures. Even in these raw data, thecontrast between the two films is prominent. Withincreasing magnetic field, the superconductivity isgradually suppressed, and the low-temperaturenormal behavior is unveiled accordingly. Withdecreasing temperature, the normal-state resistiv-ity above Hc2 monotonically decreases in the‘‘high-Tc’’ whereas it increases in the ‘‘low-Tc’’film.Fig. 10 shows the temperature dependences of

the resistivity in magnetic fields. The normal-stateresistivity of the ‘‘high-Tc’’ film is rather normaland metallic down to the lowest temperature. Incontrast, the normal-state resistivity of the ‘‘low-Tc’’ film shows an anomalous semiconducting up-turn (dr=dTo0) with a resistivity minimum ataround 30K. A low-temperature semiconductingup-turn with a resistivity minimum has frequentlybeen observed in high-Tc cuprates [27,28,38–42],and they are common with those in electron-dopedsuperconductors (Nd,Ce)2CuO4, (Pr,Ce)2CuO4,and (La,Ce)2CuO4 [20,22].

31.0

x = 0.146

0 degree

30 degree

Res

isti

vit

y (

µΩcm

)

B (T)

5.20

5.24

5.28

0 10 20 30

x10-2

A (

µΩcm

/T)

degree

30.5

30.0

29.5

29.0

28.5

5 10 15 20 25 30 35

Fig. 8. Magneto-resistance curve for different directions of the

magnetic field relative to the normal-state of the film for an

optimal-doped sample of NCCO at 4.2K. The inset shows the

angular dependence of the magnetic-field gradient A:

0

20

40

60

80

100

120

140

160

Res

istiv

ity (

µΩcm

)

B (T)

50 K

1.5 K

0

10

20

30

40

50

60

70

80 60.0 K

4.2 K

0 10 20 30 40

B (T)0 10 20 30 40

Res

istiv

ity (

µΩcm

)

Fig. 9. Comparison of the normal-state resistivity in high

magnetic fields (B8c-axis) between the ‘‘high-Tc’’ and ‘‘low-

Tc’’ LSCO films. The ‘‘high-Tc’’ film is grown on a LaSrAlO4substrate (lower panel), and the ‘‘low-Tc’’ film is grown on a

SrTiO3 substrate (upper panel). The temperature for each graph

is 4.2, 6, 8, 10, 13, 15, 17, 21, 26, 30, 33, 36, 40, 43, 45, 50, 55 and

60K (from bottom to top) in the lower panel; 1.5, 4.2, 5, 6, 8,

10, 13, 15, 17, 19, 21, 23, 25, 27, 30, 36, 45 and 50K in the upper

panel. The normal-state resistivity of the ‘‘high-Tc’’ film is

small, showing metallic behavior down to the lowest tempera-

ture, whereas that of the ‘‘low-Tc’’ film is large, showing a semi-

conducting up-turn with a resistivity minimum around 30K.

0

40

80

120

160

0 10

0T5T10T15T20T25T30T35T40T45T

Res

istiv

ity (

µΩcm

)

Temperature (K)20 30 40 50 60

Fig. 10. Temperature dependence of the resistivity in magnetic

fields for two LSCO films.


One possible explanation of the observed logT

dependence is in terms of simple two-dimensionalweak localization [39]. However, this possibility isunlikely because it cannot explain the observedisotropic negative magneto-resistance as shown inFig. 8. Moreover, in the two-dimensional weaklocalization, it is expected that the coefficient ofthe logT dependence of the conductivity per sheetshould be always a common universal valueirrespective of the doping. However, we foundthat the values of the coefficient of the optimal-doped films are almost one order of magnitudelarger than that of under- and over-doped films.Therefore, the possibility of weak localization isdiscarded.Another candidate for the mechanism of the

logT up-turn observed in 214 compounds is theKondo scattering. As is well known, materials inwhich the Kondo effect predominates the trans-port exhibit the logT dependence of the up-turncaused by the localization effect due to singletformation. They also show saturation of theup-turn at lower temperatures (unitarity limit ofscattering), and suppression of the up-turn by highmagnetic fields (delocalization effect due to singletdissociation by magnetic field). The normal-stateresistivity of the NCCO (x ¼ 0:13120:166) and the‘‘low-Tc’’ LSCO film follows logT behavior andsaturates at lower temperatures. In addition, thenegative magneto-resistance observed in thesesamples looks very similar to that observed inthe typical Kondo material (La,Ce)B6 [43]. Re-cently, NMR experiments have revealed that inhigh-Tc cuprates, local magnetic moments areinduced in the CuO2 plane by doping spinlessimpurities such as Li, Zn or Al [44–50]. These localmoments can play a role as Kondo scatterers.The suppression of the up-turn by high magnetic

fields (negative magneto-resistance) has not beenobserved in ‘‘high-Tc’’ LSCO, most likely becausethe magnetic field of 50T is not sufficient todissociate the Kondo singlets, which seem to bemore strongly bounded than in electron-dopedsuperconductors. Experiments in higher magneticfields of up to B100T, which we are planning,may clarify this issue.Based on the Kondo-effect model, we give here

a rough estimate for the Kondo temperature (TK)

and the Kondo magnetic field (HK). Following theprocedure by Samwer and Winzer [43] to define TKas the temperature at the half-height of theKondo resistivity step, the TK of ‘‘low-Tc’’ LSCOis B20K. Then the scaling relation, kBTK ¼SgeffmBHK; gives m0HKB30T, assuming S ¼ 1

2

and geff ¼ 2: However, the actual value form0HK of ‘‘low-Tc’’ LSCO seems to be much largerthan B30T. This discrepancy may partly beexplained by supposing geffo2: However, it maybe better to postpone quantitative discussions tolater publications since a single-impurity-scatter-ing approximation apparently does not apply tothe present case.Given the Kondo effect as the origin for the low-

temperature up-turn, the question arises what isthe origin of the Kondo scatterer. In the case ofNCCO, we have to pay attention to the presenceof the Nd3+ paramagnetic spin moment. How-ever, we can exclude the possibility of the role ofthe Nd3+ spin moments as the Kondo scatterer,because similar phenomena are observed inLCCO, PCCO and LSCO which have no spinmoments of Pr3+ or La3+ even at low tempera-tures [20–22]. So we have to resort to anothermechanism, i.e. residual Cu2+ spins in the CuO2plane. Actually, in LSCO, the existence ofsignificant AF spin fluctuations is indicated bythe incommensurate magnetic peaks in inelastic-neutron-scattering experiments [51]. On the otherhand, the neutron-scattering results on NCCO arecontroversial, partly due to the difficulty inremoving interstitial apical oxygen homogeneouslyin single crystals to achieve good superconductiv-ity [52,53]. In the early experiment by Matsudaet al. no well-defined AF magnetic correlation ofCu2+ spins was observed in high-quality super-conducting samples, although random spins mayexist. So, we speculate that there may exist muchfewer or/and much weaker magnetic moments dueto Cu2+ in NCCO than in LSCO. This mayexplain the weaker up-turn at low temperatures inNCCO than in LSCO. Furthermore, the lessanomalous properties of the over-doped filmscan also be explained by the above scenario,assuming that the Cu-3d orbit becomes moreitinerant and then the Cu2+ spin moments arereduced with doping.


Our experimental observation of strained LSCOfilms indicates that the Kondo effect prevails in the‘‘low-Tc’’ film, while it is practically negligible inthe ‘‘high-Tc’’ film of LSCO. This seems toindicate that a stronger Kondo interaction lowerssuperconducting transition temperature Tc; or inother words, Kondo-singlet formation hindersCooper-pair formation. The interplay betweenthe Kondo effect and superconductivity for low-Tc superconductors has been extensively investi-gated theoretically and also experimentally [54].However, it has not yet been considered seriouslyfor high-Tc cuprates. Both the Kondo interactionHK and the superconducting pairing interactionHpair are involved in this problem. In the case thatHK dominates Hpair; conduction electrons pair upwith local spins, resulting in Kondo localization.In the other extreme where Hpair dominates,conduction electrons pair up with each other,resulting in superconductivity. In the present casefor LSCO with xB0:15; HK and Hpair are bothimportant and in competition, and thereby there isfrustration for a conduction electron to pair upeither with a local spin or with another conductionelectron. Epitaxial strain seems to have a signifi-cant influence on this delicate balance. We assumethat epitaxial strain dominantly affects the Kondointeraction HK; while leaving Hpair nearly un-changed.Epitaxial strain changes only the lattice para-

meters (a0 and c0) of LSCO, leaving the dopinglevel constant. The explicit dependence of HK onthese lattice parameters requires microscopiccalculations. Our experimental results indicatethat shorter a0 and longer c0 reduces either theKondo-coupling constant JK or the number andthe magnitude of local spins [51]. We speculatethat a longer Cu–Oapex distance reduces theKondo interaction, and thereby raises Tc: Thisspeculation is supported by the systematic Tcsuppression by the induced Cu2+ local spins in(La1xPrx)1.85Sr0.15CuO4 with smaller c0 (and alsosmaller a0) [55].Based on the above scenario, we propose a new

electronic phase diagram for LSCO as shown inFig. 11. The thin line represents a virtual Tc withno Kondo interaction, the bold line a real Tc; andthe broken line a Kondo cross-over temperature

(TK). The gradation of the shadow schematicallyrepresents the development of the Kondo effect.Without the Kondo interaction, the virtual Tc (T

c )

would keep increasing with decreasing x: TheKondo interaction, however, sets in below acertain x value and becomes stronger for lowerx: This suppresses the virtual Tc to the real Tc;with more significant reduction in Tc for lower x;and eventually leads to the disappearance ofsuperconductivity. As regards the epitaxial straineffect, in-plane compressive strain weakens theKondo interaction and thereby results in less Tcsuppression. In-plane expansive strain will act inan opposite way.With over-doping, the Cu-3d orbit becomes

more itinerant, resulting in reduction or eventuallyloss of spin moments. As a result, the Kondointeraction between a Cu2+ local spin and aconduction electron essentially diminishes. Never-theless, against the above scenario, Tc decreases byover-doping. We speculate that the reduction of Tcby over-doping is caused by the weakening of thepairing interaction (Hpair), although we have nomicroscopic explanation for this trend at present.

Fig. 11. New electronic phase diagram proposed for LSCO.

The thin line represents a virtual Tc with no Kondo interaction,

the bold line a real Tc; and the broken line a Kondo cross-overtemperature TK: The gradation of the shadow schematicallyrepresents the development of the Kondo effect. The edge of the

shadowed region is given roughly below the resistivity mini-

mum temperature. The Kondo interaction shifts the virtual

Tc to the real Tc: In-plane compressive epitaxial strain weakensthe Kondo interaction, resulting in a smaller shift. In-plane

expansive strain will act in an opposite way. The symbol em ¼ek represents a Cooper pair, and Sm ¼ ek a Kondo singlet.


4. Summary

In this paper, we present the study of high-Tccuprates in pulsed high magnetic fields. In YBCO,the phase diagram of Hc2 as a function oftemperature was determined for B8c: It was foundto be very similar to that for conventional type-IIsuperconductors. The resistivity vs. temperaturecurve demonstrates that the system is metallic atlow temperatures. This indicates that the metallicregion extends to the more under-doped region incomparison to LSCO.In LCCO, PCCO, NCCO and LSCO, we

studied the temperature dependence of the normalresistivity in detail. We observed negativemagneto-resistance in the normal state. In NCCO,the field gradient of the negative magneto-resistance was found to be independent of thedirection of the magnetic field. All the samplesshowed prominent cross-over from insulatingto metallic conduction by a magnetic field. Thetemperature dependence was found to exhibita logT dependence at low temperatures. ThelogT dependence is suppressed at low tempera-tures and in high magnetic fields. These resultswere explained in terms of manifestation of theKondo effect originating from the magneticmoments of Cu ions. Based on these results, weproposed a new scenario for the electronic phasediagram of LSCO:Kondo interaction suppressessuperconducting Tc in the under-doped regime.Removing or weakening the Kondo interactioncan raise the superconducting transition tempera-ture Tc:

Acknowledgements

One of the authors (N.M.) is thankfulto Professor J.J.M. Franse for his great contribu-tion to the community of high-magnetic-fieldphysics and personal companionship for manyyears.This work was partially supported by a Grant-

In-Aid for Scientific Research from the Ministryof Education, Science, Sports and Culture, Japanand the New Energy and Industrial TechnologyDevelopment Organization (NEDO).

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high-magnetic-field study of high-tc cuprates

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