Kondo effect in the normal state of T′-Ln2−xCexCuO4 (Ln=La, Pr, Nd)

Journal of Physics and Chemistry of Solids 63 (2002) 1089±1092

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Kondo effect in the normal state of T 0 -Ln22xCexCuO4 (Ln ˆ La, Pr, Nd) Tsuyoshi Sekitani a,*, Michio Naito b, Noboru Miura a, Kazuhito Uchida a a

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

Abstract We have measured low-temperature magnetotransport in the normal state of the electron-doped superconductors, Nd22xCexCuO4, Pr22xCexCuO4, and La22xCexCuO4, by suppressing the superconductivity with high magnetic ®elds. The normal state r ±T curve shows an up-turn at low temperatures, which has a log T dependence with saturation at lowest temperatures. The up-turn is gradually suppressed by increasing the magnetic ®eld, resulting in negative magnetoresistance. We discuss these ®ndings on the basis of the Kondo scattering originating from the magnetic moments of Cu 21 ions. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Superconductors; A. Thin ®lms; D. Transport properties; D. Superconductivity

1. Introduction The electron-doped cuprate superconductors with the T 0 structure [1,2], such as Nd22xCexCuO4 (NCCO) and Pr22xCexCuO4 (PCCO), have attracted much attention due to the possible difference in the superconducting nature from hole-doped superconductors (s versus d) [3±5]. 1 Furthermore, regarding the normal state, it has been claimed, though not established, that there may be a signi®cant difference between them; normal or Fermi liquid behavior for electron-doped cuprates in contrast with abnormal or non-Fermi liquid behavior for hole-doped cuprates (electron±hole doping asymmetry). Hence, it is important to study the normal state of the electron-doped superconductors at low temperatures by suppressing superconductivity under external magnetic ®elds, and to make a comparison between the electron- and hole-doped superconductors. The low-temperature normal state induced by high magnetic ®elds has been studied in many cuprates, which includes recent extensive studies on hole-doped La22xSrxCuO4 (LSCO) and Bi-2201 by Boebinger et al. and Ando et al. [10,11]. Recently, Fournier et al. [12] investigated the * Corresponding author. Tel.: 181-471-36-3338; fax: 181-47136-338. E-mail address: [email protected] (T. Sekitani). 1 Some very recent results indicate d for electron-doped superconductors [6±9].

normal state of electron-doped PCCO in detail, and pointed out many similarities to hole-doped LSCO in spite of the `apparent' electron±hole doping asymmetry as mentioned earlier. Cuprates, regardless the hole- or electron-doped, commonly show an insulator±metal crossover as a function of doping level. In the insulating regime, the resistivity shows an up-turn …dr=dT , 0† with log T dependence at low temperatures in many cases. There has been much controversy on the origin of this log T up-turn, but with no clear explanation yet. We have undertaken a systematic magnetotransport experiment on high-quality NCCO, PCCO, and La22xCexCuO4 (LCCO) ®lms, in order to unveil the low-temperature normal state, especially the nature of the log T up-turn. In this article we discuss our experimental ®ndings on the basis of the Kondo scattering originating from the magnetic moments of Cu 21 ions. 2. Experiment Magnetoresistance was measured in pulsed high magnetic ®elds up to 50 T applied in the direction of c-axis at low temperatures from 0.5 to 300 K. Pulsed high ®elds were produced by pulse magnet, which are energized by a capacitor bank of 900 kJ (5 or 10 kV). The resistivity was measured by the standard four-probe method with electrodes formed by Ag or Au evaporation. The dc current in the order of 1±10 mA was supplied to the in-plane. The

0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(02)00097-5

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Table 1 Important parameters for Ln22xCexCuO4 (Ln ˆ Nd, Pr, La) ®lms in this work: Ce content (x), the critical temperatures (Tc), and residual resistivities (r 0) Sample

x

Tc (K)

r 0 (mV cm)

Nd22xCexCuO4 ®lm

0.131; under-dope 0.146; optimum-dope 0.166; slight over-dope 0.185; heavy over-dope

13.6 23.0 20.5 13.5

81 27 21 10

Pr22xCexCuO4 ®lm La22xCexCuO4 ®lm

0.150; optimum-dope 0.136; over-dope

24.0 22.0

17 14

measurements were performed on six c-axis oriented ®lms, which were grown by MBE on SrTiO3 (001) substrates [13]. Ê . Table 1 sumThe thickness of the ®lms were ,1000 A marizes important ®lm parameters. For LCCO, Ce contents of x , 0:10 gives the highest Tc, then we regard LCCO of x ˆ 0:136 as over-doped ®lm [14]. The resistivities for all of the ®lms are low, indicating very high quality.

3. Results In the absence of magnetic ®elds, the in-plane resistivity of all ®lms showed a roughly T 2 dependence at high temperatures above ,50 K. This seems to indicate that electron±electron scattering dominates in this region. Below 50 K, however, the resistivity versus T (r ±T ) curves observed depend on the doping: semiconducting behavior in under-doped samples and anomalous T-linear dependence

Fig. 1. Resistivity versus B curves at different temperatures for the optimal-doped NCCO ®lm …x ˆ 0:146†: The upper panel shows raw data, and the lower shows an enlarged view.

in over-doped samples [12]. The residual resistivity decreases with increasing Ce doping. Fig. 1 shows the resistivity as a function of magnetic ®eld at various temperatures for the optimal-doped NCCO ®lm …x ˆ 0:146†: Negative magnetoresistance can be seen at low temperatures above the upper critical ®eld (Hc2) of superconductivity, in addition to positive magnetoresistance as a background. The background positive magnetoresistance is probably due to an orbital effect, which we do not discuss in this article. Here we focus on the negative magnetoresistance component. The negative magnetoresistance component becomes more prominent with decreasing temperature, and it seems to saturate at high magnetic ®elds. Fig. 2 shows r ±T curves of under-doped …x ˆ 0:131†; optimal-doped …x ˆ 0:146†; slightly over-doped …x ˆ 0:166†; and heavily over-doped …x ˆ 0:185† NCCO ®lms. This demonstrates a systematic evolution of the normalstate resistivity with doping. Except for the heavily overdoped ®lm that is metallic down to the lowest temperature, all the ®lms show a low-temperature up-turn at low magnetic ®elds (but above Hc2). The up-turn follows a log T dependence below 50 K in the under-doped ®lm, below 10 K in the optimal-doped ®lm, and below 3 K in the slightly over-doped ®lm. In the under-doped ®lm, this dependence is already seen in zero-magnetic ®elds above Tc. In the slightly over-doped ®lm, however, the up-turn is very weak, and can be seen only at very low temperatures. Correspondingly, the resistivity minimum shifts to the lower temperature with increasing Ce-doping. Furthermore, in the lowest temperature region, it should be noted that the resistivity for all ®lms tends to deviate from a simple log T dependence and saturate toward T ˆ 0 K. The up-turn in resistivity occurs at an anomalously small value for the CuO2 plane sheet resistance …Rsheet ; rab =dc †; corresponding to a large value of kF lab …ˆ hdc =rab e2 † . 17 …x ˆ 0:131† and .70 …x ˆ 0:166† in our experiments. Here lab is the in-plane mean free path and dc the interlayer distance. Thus variable range hopping (VHR) is discarded because it should be observed only when kF lab , 1 [12]. With increasing magnetic ®eld, the log T dependent up-turn is suppressed for all the ®lms. In the under-doped ®lm, the up-turn behavior persists up to the highest magnetic ®eld studied even with noticeable suppression. In the

T. Sekitani et al. / Journal of Physics and Chemistry of Solids 63 (2002) 1089±1092

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Fig. 2. Resistivity versus T plots under various magnetic ®elds for NCCO ®lms with various x values. The insets show resistivity versus log T plots.

optimal-doped ®lm, the resistivity shows metallic behavior at high magnetic ®elds, except for very low temperatures, where small up-turn remains. In the slightly over-doped ®lm, the low-temperature resistivity turns to be completely metal-like in high magnetic ®elds. Namely, in the optimaldoped and slightly over-doped ®lms, insulator±metal transition occurs by applying magnetic ®elds. Another notable ®nding is that the negative magnetoresistance is almost independent of the direction of magnetic ®eld, namely isotropic [15]. 4. Discussion To understand the observed anomalies in the present experiment, especially the log T dependent up-turn and the related phenomena, we can think of two candidates as its origin: the Kondo effect and the twodimensional weak localization including electron± electron correlation. Here from the discussion below, we believe that the Kondo effect is responsible for the observed anomalies. The low-temperature up-turn observed in the present experiments on NCCO has the following features: 1. It has a log T dependent up-turn with saturation at lowest temperatures. 2. It is suppressed by high magnetic ®elds resulting in negative magnetoresistance. 3. The negative magnetoresistance is isotropic.

4. With increasing Ce-doping, the anomalous properties are decreased. These features are qualitatively consistent with the Kondo scattering. The log T dependent up-turn of resistivity in Kondo systems results from the second-order perturbation of the spin inversion process in the scattering of conduction electron by local magnetic impurities. Below the Kondo temperature (TK), the AF coupling between a conduction electron spin and a local spin predominates, forming a singlet state. At 0 K, the resistivity has a ®nite maximum value (unitarity limit) without the divergence to in®nity. High magnetic ®elds act to suppress the spin inversion, and this effect is expected to give isotropic negative magnetoresistance with a log B dependence. The overall features observed in the present study agree with such theoretical behavior. Furthermore, the negative magnetoresistance observed in the present experiment looks very similar to that for the typical Kondo material (La,Ce)B6 [16]. Next we discuss the origin of the Kondo scatterer. In the case of NCCO, we have to pay attention to the presence of the paramagnetic spin moment of Nd 31. However, we can exclude this possibility. It is because, as demonstrated in Fig. 3, essentially the same magnetotransport behavior is observed also in PCCO, and LCCO, where Pr 31 and La 31 have no spin moment. Therefore we have to resort to the other possibility, namely Cu 21 spins in the CuO2 plane. We conclude that the low-temperature up-turn observed in our present experiments is due to the Kondo scattering caused by the Cu 21 spins in the CuO2 plane. The magnetic phase

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cuprates. In these ®lms, the low-temperature up-turn has a log T dependence and saturates toward 0 K. We observed that the up-turn is suppressed by applying high magnetic ®elds, resulting in negative magnetoresistance, which was found to be independent of the direction of magnetic ®eld. On the basis of these results, we conclude that the lowtemperature up-turn is due to Kondo scattering. Furthermore the observed behavior is essentially independent among NCCO, PCCO, and LCCO, where Nd 31 has a magnetic moment but Pr 31 and La 31 do not. Therefore, the Kondo scatterers seem to be Cu 21 spins in the CuO2 planes.

References

Fig. 3. Resistivity versus T plots under various magnetic ®elds for Pr22xCexCuO4 (upper) and La22xCexCuO4 (lower) ®lms. Both ®lms show behavior qualitatively similar to NCCO.

diagram for the CuO2 planes in the T 0 electron-doped superconductors has not been well established partly because of the dif®culty in sample preparation (incomplete removal of apical oxygen), and may differ from that in the T hole-doped superconductors (LSCO or LBCO). However, Cu 21 spins seem to exist in a wide range of doping, and their density or magnitude and also antiferromagnetic correlation seem to decrease with doping, as is qualitatively similar to LSCO or LBCO [17]. Our magnetotransport results support this view. Two-dimensional weak localization included in electron± electron correlation may be another possible explanation for the observed log T dependence [18,19]. However, this possibility is unlikely because it cannot explain the observed isotropic negative magnetoresistance. Moreover, in the twodimensional weak localization, it is expected that the coef®cient of the log T dependence of the conductivity per sheet should be always a common universal value irrespective of the Ce-doping. However, we found that the values of coef®cient vary by almost one order of magnitude for different dopings. 5. Summary In conclusion, we have performed systematic magnetotransport measurements on high quality electron-doped T 0 ®lms with various Ce doping in order to unveil the nature of the low-temperature up-turn frequently seen in high-Tc

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