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Local study of the Mg 1x Al x B 2 single crystals by scanning tunneling spectroscopy in magnetic field up to 3 Tesla F. Giubileo a, * , F. Bobba a , A. Scarfato a , A.M. Cucolo a , A. Kohen b , D. Roditchev b , N.D. Zhigadlo c , J. Karpinski c a CNR-INFM Laboratorio Regionale SUPERMAT, Dipartimento di Fisica ‘‘E.R. Caianiello, Universita ` degli Studi di Salerno, via Salvador Allende, 84081 Baronissi (SA), Italy b Institut des Nanosciences de Paris, INSP, Universite ´ P. et M. Curie Paris 6, CNRS, UMR 75-88, Paris, France c Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland Accepted 30 November 2007 Available online 7 March 2008 Abstract We have performed local tunneling spectroscopy on high quality Mg 1x Al x B 2 single crystals by means of variable temperature scan- ning tunneling spectroscopy in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively mea- sured, probing different amplitudes of the three-dimensional D p as a function of Al content (i.e. as a function of the critical temperature T C ). Temperature and magnetic field dependences of the conductance spectra were studied in S–I–N configuration: the effect of the dop- ing resulted in a monotonous reduction of the locally measured T C down to 24 K for x = 0.2. The magnetic field dependence was studied in a local way: An estimation for upper critical field H c2 was inferred from the evolution of the tunneling spectra with the field perpen- dicular to the sample surface, for different doping levels. The high spatial resolution of the STS technique allowed us to evidence possible non-homogeneities of the superconducting properties on the sample surface with variation of in the same sample depending on different local levels of doping. The locally measured upper critical field resulted to vary for different dopings, and the maximum value H c2 3T was found for samples with T C ¼ 33 K. The evolution of the density of states (DOS) was found to be characterized by two distinct regimes separated by a crossover region. Our results indicate a rapid suppression of the intrinsic term in p-band superconductivity for 0 T < B < 0.5 T. At high fields (0.8 T < B < 3 T) the superconductivity in the p-band survives uniquely due to the coupling to the r-band. The shape of tunneling spectra suggests an important role played by the quasiparticle inter-band scattering. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.50+r; 74.70.Ad Keywords: MgB 2 ; Scanning tunneling spectroscopy 1. Introduction The existence of two distinct superconducting energy gaps in the simple binary compound MgB 2 with a relatively high T C of 40 K offers a unique opportunity to study the electrodynamics of a multigap superconductor. The effects of inter-band coupling and scattering on the superconduc- ting order parameter and T C were discussed since long time ago [1], but only after the discovery of superconductivity in magnesium diboride [2] there has been a real opportunity to experimentally study the phenomenon. Two-gap super- conductivity in MgB 2 is now evidenced by numerous exper- imental results [3–11]. Chemical substitution effect in MgB 2 is of particular interest since it allows to modify its superconducting prop- erties in a systematic way. Substitution of C for B-site and Al or Mn for Mg-site are possible [12–16]. In all cases the doping produces a suppression of T C . Despite theoretical calculations predict the two-gaps collapsing into one gap 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.11.079 * Corresponding author. E-mail address: [email protected] (F. Giubileo). www.elsevier.com/locate/physc Available online at www.sciencedirect.com Physica C 468 (2008) 828–831

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Page 1: Local study of the single crystals by scanning tunneling spectroscopy in magnetic field up to 3 Tesla

Available online at www.sciencedirect.com

www.elsevier.com/locate/physc

Physica C 468 (2008) 828–831

Local study of the Mg1�xAlxB2 single crystals by scanningtunneling spectroscopy in magnetic field up to 3 Tesla

F. Giubileo a,*, F. Bobba a, A. Scarfato a, A.M. Cucolo a, A. Kohen b, D. Roditchev b,N.D. Zhigadlo c, J. Karpinski c

a CNR-INFM Laboratorio Regionale SUPERMAT, Dipartimento di Fisica ‘‘E.R. Caianiello”, Universita degli Studi di Salerno, via Salvador Allende,

84081 Baronissi (SA), Italyb Institut des Nanosciences de Paris, INSP, Universite P. et M. Curie Paris 6, CNRS, UMR 75-88, Paris, France

c Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland

Accepted 30 November 2007Available online 7 March 2008

Abstract

We have performed local tunneling spectroscopy on high quality Mg1�xAlxB2 single crystals by means of variable temperature scan-ning tunneling spectroscopy in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively mea-sured, probing different amplitudes of the three-dimensional Dp as a function of Al content (i.e. as a function of the critical temperatureT C). Temperature and magnetic field dependences of the conductance spectra were studied in S–I–N configuration: the effect of the dop-ing resulted in a monotonous reduction of the locally measured T C down to 24 K for x = 0.2. The magnetic field dependence was studiedin a local way: An estimation for upper critical field H c2 was inferred from the evolution of the tunneling spectra with the field perpen-dicular to the sample surface, for different doping levels. The high spatial resolution of the STS technique allowed us to evidence possiblenon-homogeneities of the superconducting properties on the sample surface with variation of in the same sample depending on differentlocal levels of doping. The locally measured upper critical field resulted to vary for different dopings, and the maximum value H c2 ’ 3 Twas found for samples with T C ¼ 33 K. The evolution of the density of states (DOS) was found to be characterized by two distinctregimes separated by a crossover region. Our results indicate a rapid suppression of the intrinsic term in p-band superconductivityfor 0 T < B < 0.5 T. At high fields (0.8 T < B < 3 T) the superconductivity in the p-band survives uniquely due to the coupling to ther-band. The shape of tunneling spectra suggests an important role played by the quasiparticle inter-band scattering.� 2008 Elsevier B.V. All rights reserved.

PACS: 74.50+r; 74.70.Ad

Keywords: MgB2; Scanning tunneling spectroscopy

1. Introduction

The existence of two distinct superconducting energygaps in the simple binary compound MgB2 with a relativelyhigh T C of 40 K offers a unique opportunity to study theelectrodynamics of a multigap superconductor. The effectsof inter-band coupling and scattering on the superconduc-ting order parameter and T C were discussed since long time

0921-4534/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2007.11.079

* Corresponding author.E-mail address: [email protected] (F. Giubileo).

ago [1], but only after the discovery of superconductivity inmagnesium diboride [2] there has been a real opportunityto experimentally study the phenomenon. Two-gap super-conductivity in MgB2 is now evidenced by numerous exper-imental results [3–11].

Chemical substitution effect in MgB2 is of particularinterest since it allows to modify its superconducting prop-erties in a systematic way. Substitution of C for B-site andAl or Mn for Mg-site are possible [12–16]. In all cases thedoping produces a suppression of T C. Despite theoreticalcalculations predict the two-gaps collapsing into one gap

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F. Giubileo et al. / Physica C 468 (2008) 828–831 829

below T C ’ 25 K because of inter-band scattering intro-duced by doping, the experimental results show that thegaps exist separated even at lower temperatures [17,18].

Moreover, various studies revealed a peculiar magneticfield response of MgB2. The study of the vortex lattice byscanning tunneling microscopy/spectroscopy (STM/STS)[19] evidenced an unusually large size of the p-band vortexcores. This discovery led the authors to suggest that gap inthe p-band is induced by the intrinsically superconductingr band. Heat capacity measurements have shown an anisot-ropy for Hkc and Hkab [20]. Remarkably, this anisotropyappears only for H > 0:5 Tesla, with a large anisotropydeveloping at higher fields and leading finally to H c2 � 3 T(Hkc) and�20 T (Hkab). Experimentally, the magnetic fieldresponse of the two-bands has been studied in several pointcontact experiments [7,8,17,21], with a reasonable accuracyin the p-band gap estimation only up to 0.5 T. Theoreticalstudies on the mixed state in a two-band superconductor[22–24] have qualitatively explained the above findings as aresult of the inter-band coupling. Finally, though manyexperiments revealed a modification in the superconductingproperties at the magnetic field of around 0.5–0.8 T, none ofthe observed changes were unambiguously linked to the evo-lution of the principal superconducting parameters such asthe superconducting gap(s) and/or the inter-band coupling.

In this paper we report an extensive study of the quasi-particle state density of Mg1�xAlxB2 single crystals bymeans of scanning tunneling spectroscopy (STS). We havesucceeded in selectively measuring the behavior of the onlyDp energy gap, and due to the high spatial resolution of the

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Fig. 1. Low temperature (T = 6 K) conductance spectra measured for sample(scattered graphs) are compared to theoretical fittings (solid lines). The gap am

STS technique, we have been able to relate the local valuesof T C, Dp and H c2. We found also the p-band DOS to havea non-trivial field dynamics. We observed two distinctregimes: A rapid evolution at low fields B 6 0.5 T and aslow one at high fields B P 0.8 T, separated by a crossoverregion where the Dp amplitude remains almost unchangedat any doping.

2. Experiment

Experiment has been carried out on the ultra-high vac-uum variable temperature Cryogenic-SFM (by OmicronNanotechnology) at the Physics Department of the Univer-sity of Salerno. The tunneling junctions were achieved byapproaching mechanically cut Pt/Ir tips to the c-axis ori-ented surface of the crystals. Single crystals ofMg1�xAlxB2 were grown by high pressure method in acubic-anvil press [16].

We have measured samples with different dopings rang-ing between x = 0 and x = 0.2 resulting in a T C rangingbetween 39 K and 24 K [25,26]. In Fig. 1 we show thelow temperature (at T = 6 K) tunneling conductance spec-tra measured for four different samples, all of them reveal-ing a single gap feature. This is not surprising: due to thejunction geometry (tunneling current flowing mainly alongthe c-axis direction) and the particular electronic structureof MgB2, the probability for direct tunneling to the 3D p-band is much higher than the probability for tunneling tothe 2D r-band which has no states with a wavevector cor-responding to the c-axis.

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s having T C = (a) 39 K, (b) 35 K, (c) 33 K, (d) 24 K. Experimental dataplitudes resulted from numerical simulation are indicated in the figure.

Page 3: Local study of the single crystals by scanning tunneling spectroscopy in magnetic field up to 3 Tesla

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830 F. Giubileo et al. / Physica C 468 (2008) 828–831

In a first approximation the shape of the tunneling spec-tra may be reproduced by Dynes’ formula with isotropicBCS state density (solid lines in Fig. 1). Our data show amaximum value for Dpð0Þ ¼ 2:4 meV for the sample withT C ¼ 33 K. The superconducting critical temperatures(indicated in Fig. 1 for each sample), has been determinedlocally by measuring the evolution of the tunneling conduc-tance spectra as a function of temperature.

The magnetic field dependence of the dI/dV vs. V spec-tra was measured by positioning the STM tip in a fixedlocation and by continuously recording the current–voltagecharacteristic, while sweeping the magnetic field (fielddirection parallel to the c-axis). After cooling the samplein zero field, the field was slowly (sweep rate of 0.05 T/min) increased from zero up to the maximum value andreduced to zero, to evidence any hysteretic behavior. Sincethe reported spectra were averaged over many vorticespassing under the tip [27–29], the main effect of the mag-netic field resulted, as expected, in the progressive fillingof states inside the energy gap. Such measurement has beenperformed for all samples characterized at low temperaturein Fig. 1. In particular, we report as example in Fig. 2 theevolution of the tunneling spectra in magnetic field for thepure crystal (x = 0).

The field dynamics of the DOS at the Fermi level is rep-resented by the zero-bias conductance (ZBC) in the tunnel-ing spectra (Fig. 3). We observe that, at low fields, the ZBCrises rapidly up to 80% of the normal state ZBC already at0.5 T, while, at higher fields (for B > 0.8 T), shows a muchslower dynamics. At intermediate fields (0.5 < B < 0.8 T)there is a crossover region in which the ZBC remainsroughly constant. This crossover in ZBC evolution is evi-

Fig. 2. Evolution dI/dV tunneling spectra (in the magnetic field) from 0 Tto 2.2 T.

dent only for the pure crystal, for which we reported bothmeasurements (rising and decreasing field) showing hyster-esis effect. We notice that, for all other (substituted) sam-ples, a clear separation of different dynamic regimes, aswell as the hysteretic behavior, are not observed inFig. 3. However, the field dynamics of the DOS at theFermi level again shows a rapid rising of the ZBC forlow magnetic fields. Extrapolation of the data for highfields leads to H c2 ’ 3 T for sample having T C ¼ 35 K, cor-responding to a value 30% higher than that observed in thecase of pure MgB2. Similarly, for sample with T C ¼ 33 K,we obtain H c2 P 3 T, while for the last sample(T C ¼ 24 K) H c2 ’ 1:8 T. Moreover, by analyzing the mag-netic field effect on the energy gap Dp, we found (as shownin Fig. 4) that there is a severe reduction of the gap ampli-tude for fields up to 0.5 T followed by a region between0.5 T and 1.0 T, in which no significant variations occur.This observation suggests that around 0.5 T the contribu-tion to the superconductivity due to the phonon mediated

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electron–electron interactions in the p-band itself is not effi-cient anymore, while for higher fields the energy gap sur-vives due to both the phonon exchange with r-band and/or to the quasiparticle inter-band scattering.

We notice that, to extract the magnetic field dependenceof the energy gap Dp, we simulated tunneling data, as firstapproximation, by accounting the field-induced pair break-ing through the C term in the Dynes formula for the densityof states (DOS). Although not rigorous, this method is use-ful at least for a first insight on the magnetic field effect on theevolution of the energy gap using only one fitting parameter.Indeed, by this oversimplified formula, we have obtained asatisfactory fitting of experimental data even at rather highfields, so, giving an indication about the gap evolution.

3. Conclusions

We have reported a systematic study performed by scan-ning tunneling spectroscopy (STS) on high qualityMg1�xAlx B2 single crystals, for different aluminium dop-ings. Directional c-axis tunneling allowed us to probe theenergy gap in the p-band. We have been able to relatethe local values of T C, Dp and H c2. In agreement with recenttheoretical models, we have measured the largest gap value(Dp ¼ 2:4 meV, 20% larger than in pure MgB2) in sampleswith T C ¼ 33 K, for which we also measured the maximumH c2 value (>3 T). In addition to this, our results indicate arapid suppression of the intrinsic term in p-band supercon-ductivity for 0 T < B < 0:5 T. At high fields (B > 0:8 T) thesuperconductivity in the p-band survives uniquely due tothe coupling to the r-band.

References

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