Physica C 470 (2010) 922–925

Contents lists available at ScienceDirect

Physica C

journal homepage: www.elsevier .com/locate /physc

Study of the pairing symmetry in the electron-doped cuprate Pr1�xLaCexCuO4�y

by tunneling spectroscopy

F. Giubileo a,b,*, S. Piano b,c, A. Scarfato b, F. Bobba b,a, A. Di Bartolomeo b, A.M. Cucolo b,a

a CNR-SPIN, Via Ponte don Melillo, 84084 Fisciano (SA), Italyb Dipartimento di Fisica ‘‘E.R. Caianiello”, Università di Salerno, Via Allende, Baronissi (SA), Italyc School of Physics and Astr., University of Nottingham, Nottingham NG7 2RD, UK

a r t i c l e i n f o a b s t r a c t

Article history:Available online 3 March 2010

Keywords:Tunneling spectroscopyPoint contactsPairing symmetriesCuprates

0921-4534/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.physc.2010.02.038

* Corresponding author. Address: CNR-SPIN, ViaFisciano (SA), Italy.

E-mail address: giubileo@sa.infn.it (F. Giubileo).

We performed Point Contact Andreev Reflection Spectroscopy experiments on Pr1�xLaCexCuO4�y crystals.The variety of the measured spectra are all explained in terms of a modified BTK model considering a d-wave symmetry of the superconducting order parameter. We give an estimation of the energy gapD ¼ ð3:5� 0:2ÞmeV and of the ratio 2D=kBTC ’ 3:6� 0:3. We measured the temperature evolution ofthe dI=dV vs. V characteristics and we extracted a conventional BCS temperature dependence of thesuperconducting energy gap. Finally, in order to explain the conductance spectra showing higher voltagegap-like features we consider a model in which the formation of a Josephson junction in series with thePoint Contact junction is realized due to the tip pressure.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

The symmetry of the superconducting order parameter is a cru-cial input for theories on the mechanism of superconductivity incuprates, and one essential ingredient in the search for possiblemechanisms of this phenomenon is the quasiparticle density ofstates (DOS), whose detailed shape and temperature dependenceare constraining parameters to any theoretical model. In the caseof hole-doped cuprates, there is strong evidence that the orderparameter has unconventional d-wave pairing symmetry. How-ever, the symmetry of the superconducting pair state in the elec-tron-doped cuprates R2�xCexCuO4 (R = Nd, Pr, Sm, or Eu) remainscontroversial. Early experiments on Nd2�xCexCuO4�y (NCCO) sug-gested, that the superconducting order parameter has s-wave sym-metry like most conventional, low-TC superconductors. However,later experiments, Phase-sensitive [1], angle resolved photoemis-sion spectroscopy [2], and some penetration depth [3,4] measure-ments on nominally identical optimally doped Pr2�xCexCuO4�y

(PCCO) and NCCO samples suggest d-wave pairing. Other penetra-tion depth measurements [5,6] and the absence of a zero-bias con-ductance peak in tunneling measurements [7,8] have contrastedthis conclusion indicating s-wave superconductivity.

In this paper, we report Point Contact Spectroscopy experi-ments on the electron-doped superconductor Pr1�xLaCexCuO4�y

(x = 0.12, Tc = 25 K) to give an estimation of the superconducting

ll rights reserved.

Ponte don Melillo, 84084

energy gap, its temperature behavior and also indication aboutthe symmetry of the order parameter.

2. Theory and experiment

2.1. Modeling PCAR measurements

The Point Contact Andreev Reflection Spectroscopy (PCAR) con-sists in establishing a contact between a tip of a normal metal anda superconducting sample (NS junction) and it is a standard tech-nique to investigate amplitude and symmetry of the superconduc-ting energy gap. By varying the distance and/or the pressurebetween tip and sample it is possible to obtain different tunnelbarriers, that is, different conductance regimes. Indeed, quasiparti-cle tunnel regime is obtained for high barriers, while PCAR geom-etry is achieved in case of low barriers. Often in the experiments,intermediate regimes are realized, in which through the N/S con-tact both quasiparticle tunneling and Andreev reflection processesoccur. This transition has been theoretically modeled by Blonderet al. (BTK theory) [9] within a generalized semiconductor modelusing the Bogoliubov equations to treat the transmission andreflection of quasiparticles at the interface. The model describesballistic NS metallic junctions by accounting for the processes ofthe Andreev reflection, in which an electron coming from the nor-mal electrode with energy smaller than the energy gap is reflectedas a hole, injecting a pair 2e in the superconductor. For a conven-tional BCS superconductor, there are two parameters in the modelthat are varied to reproduce the conductance curves: the supercon-ducting energy gap D and a dimensionless parameter Z taking into

F. Giubileo et al. / Physica C 470 (2010) 922–925 923

account the barrier strength: varying Z one ranges from AndreevReflection regime (small Z) to the tunneling limit ðZ � 1Þ. Themodified BTK model by Tanaka and Kashiwaya [10] extends thetheory to anisotropic d-wave superconductors: in such a case,the electron-like and hole-like quasiparticles, incident at a NSinterface, experience a different sign of the order parameter, withthe consequent formation of bound states at the Fermi energy.These states, named Andreev bound states, are responsible for anincrease of the tunneling conductance at zero-bias, in some casehigher than two, the theoretical limit for conventional s-wavesuperconductors. Thus, at a given energy E, the transport currentdepends both on the incident angle u of the electrons at the N/Sinterface as well as on the orientation angle a, that is the angle be-tween the a-axis of the superconducting order parameter and thex-axis. Because in PCAR experiments there is no preferential direc-tion of the quasiparticle injection angle u into the superconductor,the transport current results by an integration over all directionsinside a semisphere weighted by the scattering probability termin the expression for the current. Consequently, the fitting param-eters to reproduce the experimental data are: the superconductinggap D, the barrier strength Z, the angle a of the order parameterand the smearing factor C.

Fig. 1. Tuning of the conductance regime (from tunneling to contact) by varying thetip pressure on the sample surface.

(a)

Fig. 2. Conductance spectra measured in the point contact regime at low temperature (Tfittings considering a d-wave symmetry of the order parameter in the modified BTK mo

2.2. PCAR experiment

The experiments have been performed on high-quality singlecrystals of Pr1�xLaCexCuO4�y, grown by traveling-solvent floating-zone technique. Superconducting samples have been obtained byannealing in pure Ar. We selected samples with x = 0.12 corre-sponding to a bulk critical temperature TC ’ 25 K as measured byac susceptibility.

The contacts were established by driving the metallic (Pt/Ir) tipinto the sample surface at low temperatures. The vertical move-ment of the tip, driven by a micrometric screw, allowed the tuningof the contact resistance from tunneling regime to metallic contact.In Fig. 1 we report an example for a contact N–S in which by vary-ing the tip pressure we obtain a tuning of the transport regimefrom the quasiparticle tunneling regime (corresponding to lowpressure, i.e. high Z) to the point contact regime (high pressure,i.e. high transparent barrier – low Z). All measurements were per-formed in the temperature range between 4.2 K and 30 K. Currentand dI/dV vs. V characteristics were measured by using a standardfour-probe method and a lock-in technique by superimposing asmall ac modulation to a slowly varying bias voltage. Each mea-surement comprises two successive cycles in order to check forthe absence of heating hysteresis effects.

2.2.1. Contact regimeIn Fig. 2 we report three different conductance spectra mea-

sured at low temperature (T = 4.2 K) in the contact regime, byestablishing different contacts on different regions of the samePLCCO sample. Experimental data (scattered curves) are comparedto theoretical calculated curves (solid lines) by considering a d-wave symmetry for the superconducting order parameter in themodified BTK model. We observe that all the reported spectraare characterized by a zero-bias conductance peak (ZBCP) appear-ing for each contact with different shapes, amplitudes and energywidths. The maximum conductance ratio GðV ¼ 0Þ=GðV ¼ 30 mVÞis less than 2 for the curves in Fig. 2a and b, howeverGðV ¼ 0Þ=GðV ¼ 30 mVÞ ’ 2:7 for the data in Fig. 2c. In additionto this, the energy width of the main structure is very differentfor the reported spectra. At a first qualitative analysis, these dataappear quite puzzling, suggesting different values for the super-conducting energy gap, as already observed in local tunnelingexperiments on e-doped Sm–CeCuo [11]. However, it is necessaryto go deeper in the quantitative analysis of such data, to betterunderstand their meaning. The spectrum of Fig. 2a is fitted bythe modified BTK modeled considering a value of the superconduc-ting energy gap D ¼ 3:6 meV, a smearing factor C ¼ 0:5 meV, anangle a ¼ 0:45 and a barrier strength Z ¼ 0:65. By studying thetemperature dependence of this spectrum we also obtained the

(b) (c)

= 4.2 K). Scattered graphs represent the experimental data; solid lines are the bestdel [10]. Inset in (a) represents the meaning of angle a in the modified BTK model.

Fig. 4. Temperature dependence of the superconducting energy gap as obtainedfrom the theoretical fittings for the various junctions.

Table 1Summary of the values of the order parameter ðDÞ, the critical temperature ðTCÞ andthe BCS ratio ð2D=KBTCÞ as inferred from the theoretical fittings of the experimentaldata.

Exp. Regime D ðmeVÞ TC ðKÞ 2D=KBTC

PC Contact regime 3.6 23 3.6PC Contact regime 2.0 14 3.5PC Contact regime 3.6 23 3.6PC Tunnel. regime 3.5 22 3.7STS Vacuum tunnel. 3.5 25 3.5

924 F. Giubileo et al. / Physica C 470 (2010) 922–925

information of the local critical temperature TC ’ 23 K and conse-quently a ratio 2D=KBTC ¼ 3:6. The spectrum of Fig. 2b is fitted byconsidering the following values of the parameters:D ¼ 2:0 meV; C ¼ 0:4 meV; a ¼ 0:54 and Z ¼ 0:9. The small valuefor the superconducting energy is due to the fact that we wereprobably probing an area with reduced superconductivity. This isconfirmed by the observation of the local TC ’ 14 K as obtainedby measuring the temperature evolution of the conductance spec-tra (reported elsewhere [12]). We also notice that the resulting ra-tio is 2D=KBTC ¼ 3:5. The spectrum reported in Fig. 2c appears witha larger ZBCP, with height greater than 2 and also with minima inthe conductance curve at the edges of the peak. The interpretationof such a feature is nowadays quite well accepted in terms of theformation of intergrain Josephson junction in series to the pointcontact one [12–17]. In such a case, the measured voltage corre-sponds to the sum of the two terms:

VmeasuredðIÞ ¼ VPCðIÞ þ VJðIÞ; ð1Þ

where VPC is the voltage drop at the N/S Point Contact junction andVJ is the voltage drop at the intergrain Josephson junction [18,19].By applying this simple model we satisfactory fitted the experimen-tal data reported in Fig. 1c. Remarkably, the best fitting is obtainedby considering D ¼ 3:6 meV. We observe that, in this model, twomore parameters are needed, namely the resistance ratioRJ=RPCð¼ 0:3Þ between the two series junctions and the critical cur-rent IJð¼ 2:4 lAÞ of the Josephson junction. For this contact, thetemperature evolution of the conductance spectra has indicatedTC ’ 23 K and thus 2D=KBTC ’ 3:6.

2.2.2. Tunneling regimeBy slightly releasing the tip pressure we can tune the junction

from the contact regime to the tunneling regime. In Fig. 3a we re-

Fig. 3. (a) Conductance spectrum measured in the tunneling regime at lowtemperature (T = 4.2 K). (b) Conductance spectrum measured during the STSexperiment reported in Ref. [12].

port a conductance spectrum measured at low temperature; It iscompared to the best fit (solid line) resulted by considering the fol-lowing parameters: D ¼ 3:5 meV; C ¼ 0:8 meV; a ¼ 0:33 andZ ¼ 2:6. Also in the case of tunneling regime we performed com-plete temperature dependence of the conductance spectra thatgave indication for TC ’ 22 K and 2D=KBTC ’ 3:7. For comparison,we report a low temperature conductance spectrum measuredduring a scanning tunneling spectroscopy experiment (the detailsof the experimental setup and procedure are reported elsewhere[12,20]). Also in that case we fitted the data by considering a d-wave symmetry of the order parameter and obtaining a supercon-ducting energy gap D ¼ 3:5 meV.

In order to account for the temperature evolution of the ampli-tude of the superconducting gap DðTÞ, we kept all fitting parame-ters, except of D, fixed as they were established at T = 4.2 K foreach studied contact. In this way we succeeded in reproducingthe experimental data in finer details. In Fig. 4 we present the ex-tracted DðTÞ dependence (dots). As the critical temperature TC wasfound to vary from one junction to another, we used the reducedcoordinates DðTÞ=Dð0Þ vs. T=TC . The data points in Fig. 4 followthe expected BCS dependence (solid line) thus evidencing for aconventional DðTÞ dependence. The absolute values for the super-conducting energy gap D and the local critical temperature TC aresummarized in Table 1.

3. Conclusions

We performed Point Contact Spectroscopy experiment on theelectron-doped Pr1�xLaCexCuO4�y cuprate. By varying the pressureof the Pt/Ir tip on the sample surface we were able to tune the junc-tion from the contact regime to the tunneling regime. All theexperimental data reported have been consistently interpreted in

F. Giubileo et al. / Physica C 470 (2010) 922–925 925

terms of the modified BTK model by considering a d-wave symme-try for the superconducting order parameter. The theoretical fit-tings infer a value for the energy gap D ’ ð3:5� 0:1ÞmeV. Wealso measured the temperature evolution of the gap that resultedto follow the conventional BCS behavior.

References

[1] C.C. Tsuei, J.R. Kirtley, Phys. Rev. Lett. 85 (2000) 182.[2] N.P. Armitage et al., Phys. Rev. Lett. 86 (2001) 1126.[3] J.D. Kokales et al., Phys. Rev. Lett. 85 (2000) 3696.[4] R. Prozorov et al., Phys. Rev. Lett. 85 (2000) 3700.[5] L. Alff et al., Phys. Rev. Lett. 83 (1999) 2644.[6] J.A. Skinta et al., Phys. Rev. Lett. 88 (2002) 207003.[7] S. Kashiwaya et al., Phys. Rev. B 57 (1998) 8680.[8] L. Alff et al., Phys. Rev. B 58 (11) (1998) 197.

[9] G.E. Blonder, M. Tinkham, T.M. Klapwijk, Phys. Rev. B 25 (1982) 4515.[10] S. Kashiwaya, Y. Tanaka, Rep. Prog. Phys. 63 (2000) 1641.[11] A. Zimmers et al., Phys. Rev. B 76 (2007) 132505.[12] F. Giubileo et al., J. Phys.: Condens. Matter 22 (2010) 045702.[13] L. Shan, H.J. Tao, H. Gao, Z.Z. Li, Z.A. Ren, G.C. Che, H.H. Wen, Phys. Rev. B 68

(2003) 144510.[14] F. Giubileo, M. Aprili, F. Bobba, S. Piano, A. Scarfato, A.M. Cucolo, Phys. Rev. B 72

(2005) 174518.[15] F. Giubileo, F. Bobba, A. Scarfato, S. Piano, M. Aprili, A.M. Cucolo, Physica C

460–462 (2007) 587.[16] S. Piano, F. Bobba, F. Giubileo, A.M. Cucolo, M. Gombos, A. Vecchione, Phys.

Rev. B 73 (2006) 064514.[17] S. Piano, F. Bobba, F. Giubileo, A. Vecchione, A.M. Cucolo, J. Phys. Chem. Solids

67 (2006) 384.[18] P.A. Lee, J. Appl. Phys. 42 (1971) 325.[19] T. Van Duzer, C.W. Turner, Principles of Superconductive Devices and Circuits,

Edward Arnold, London, 1981.[20] F. Giubileo et al., Phys. Rev. B 76 (2007) 024507.