Scanning tunneling spectroscopy on neutron irradiated MgB2 thin films

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Scanning tunneling spectroscopy on neutron irradiated MgB

2

thin ﬁlms

Roberto Di Capua

a,*

, Marco Salluzzo

a

, Ruggero Vaglio

a

, Carlo Ferdeghini

b

,

Valeria Ferrando

b,c

, Marina Putti

b

, Xiaoxing Xi

c

, Hans U. Aebersold

d a

University of Napoli and CNR-INFM/Coherentia, Via Cinthia, Napoli I-80126, Italy

b

CNR-INFM/LAMIA, Via Dodecaneso 33, Genova I-16146, Italy

c_{The Pennsylvania State University, University Park, PA 16802, USA} d_{Paul Scherrer Institut, Villigen CH-5232, Switzerland}

Available online 14 April 2007

Abstract

Neutron irradiation was performed on MgB2thin films grown by hybrid physical chemical vapor deposition. Samples irradiated with different neutron fluences, having different critical temperatures, were studied by scanning tunneling spectroscopy in order to investigate the effect of the introduced disorder on the superconducting and spectroscopic properties. A monotonic increase of the p gap with increasing disorder was found.

Keywords: MgB2; Eﬀect of disorder; STM; Tunneling measurements

1. Introduction

Because of its multiband nature, MgB2 has peculiar

behaviors as concerns the eﬀects of scattering. Non-mag-netic impurities can suppress superconductivity as magNon-mag-netic ones do in a conventional superconductor. For strong inter-band scattering, a complete isotropization of the Fermi sur-face and a merging of the two gaps Dpand Drare expected.

From theoretical works[1–5], it is not clear at which degree of disorder, i.e. at which critical temperature (Tc) value, the

gaps have to merge. Experiments have not yet clariﬁed this point, probably because of the poor control on the eﬀects on dirty samples. In general, disorder comes from Al[6,7]or C

[8,9]substitutions, neutron irradiation[10,11], or from the grown in case of thin ﬁlms[12].

We report on scanning tunneling spectroscopy (STS) measurements on MgB2 thin ﬁlms disordered by neutron

irradiation, which avoids charge doping and electronic or structural instabilities, and assures (on thin ﬁlms) a uni-form distribution of damage and disorder.

2. Experimental

The investigated MgB2 thin ﬁlms, 2000 A˚ thick and

c-axis oriented, were grown by hybrid physical chemical vapor deposition on 5· 5 mm2SiC substrates [13]at The Pennsylvania State University. Neutron irradiation was carried out at the spallation neutron source SINQ of Paul Sherrer Institut in Villigen (Zurich). Several samples were irradiated, achieving Tcs ranging from 41 K to 17 K.

Scanning tunneling spectroscopy (STS) experiments were performed at CNR-INFM/Coherentia in Naples. The ﬁlms were mounted on a scanning tunneling micro-scope (STM) in inert He atmosphere, with home-made PtIr tips. Reproducibility of tunnel spectra when changing the tunnel resistance assures a reduced role of contaminated surface layers.

3. Results and discussion

Fig. 1 reports STS conductance spectra on diﬀerently irradiated samples. Because of the c-axis orientation of the ﬁlms, the p band contribution largely dominates and only one gap spectra were observed on all samples. As Tc

*

Corresponding author. Tel.: +39 081 676267; fax: +39 081 676346. E-mail address:rdicapua@na.infn.it(R. Di Capua).

www.elsevier.com/locate/physc Physica C 460–462 (2007) 574–575

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decreases, with increasing neutron fluences, the zero-bias conductance increases and the coherence superconductivity peaks shift to higher voltages and appear less pronounced. Both effects reveal a broadening of the superconducting local density of states (LDOS) on the films as a conse-quence of the introduced disorder. Besides the smearing of LDOS, the shift of the peaks towards higher energies can be also due to an intrinsic change of the Dpvalue as

dis-order increases and Tcdecreases. This was proved by ﬁtting

the collected spectra with a BCS model with a broadening Dynes parameter C[14](in the c-axis tunneling regime, the two-band tunneling model reduces to a one gap BCS model). The results are shown in the inset toFig. 1, where the estimated Dpand C values are plotted as a function of

Tc.

As predicted by the two-band model for increasing interband scattering [1,5], Dp slightly increases with the

degree of disorder. In principle both intraband and inter-band events can contribute to the scattering processes, and they cannot be sorted out from the present data. How-ever, in the regime of small interband scattering, the inter-band scattering rate Cinter can be calculated by DTc/

Tc0= 0.2Cinter/kBTc0[15], Tc0being the critical temperature

of the clean ﬁlm and DTc= Tc Tc0. For the less

irradi-ated samples, a good agreement between this estimation of Cinterand C evaluated by STS is found. This agreement

probably reﬂects the fact that in a two-band superconduc-tor only the interband scattering acts as pair breaking, therefore aﬀecting the broadening parameter in the LDOS of p band. The evaluated Dpvalues as a function of Tcare

also consistent with data from speciﬁc heat on bulk irradi-ated samples when Tc/Tc0> 0.85 [10,11], while for lower

Tcs there is a poorer agreement between our results and

those in [10,11]. In both cases the theoretical calculation for Dpfrom[5], which is performed in the small interband

scattering limit, can be shown to provide a good ﬁt only when Tc/Tc0> 0.85, while it cannot ﬁt the data for the most

disordered samples. This can be ascribed to the occurrence of scattering channels that become not negligible for high disorder; furthermore, for increasing disorder the small interband scattering limit could not be achieved longer. As concerns the disagreement between tunnel and speciﬁc heat measurements of Dpfor low Tc, it must be noted that

when C becomes comparable with Dpthe reliability of the

simple adopted tunnel model in describing only the p band is seriously aﬀected, and we cannot exclude an interplay of the two bands occurring for such samples, as suggested by the results in[11]and by the 2Dp/kBTcvalues we found (3.9

when Tc= 17 K, relatively high for the p band but

reason-able for the r one). A more quantitative discussion on this issue can be found in[16].

Acknowledgments

This work is partially supported by Ministry of Italian Research by PRIN2004022024 project. The work at Penn State is supported in part by NSF under Grant No. DMR-0306746 and by ONR under Grant No. N00014-00-1-0294.

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Fig. 1. STS spectra on MgB2 thin ﬁlms with diﬀerent degree of

irradiation. In the inset, the best ﬁt values for Dpand C as a function of

the samples Tc.