Using the mechanically controlled break junction technique at low
temperatures and under cryogenic vacuum conditions we have studied atomic
contacts of several magnetic (Fe, Co and Ni) and non-magnetic (Pt) metals,
which recently were claimed to show fractional conductance quantization. In the
case of pure metals we see no quantization of the conductance nor
half-quantization, even when high magnetic fields are applied. On the other
hand, features in the conductance similar to (fractional) quantization are
observed when the contact is exposed to gas molecules. Furthermore, the absence
of fractional quantization when the contact is bridged by H_2 indicates the
current is never fully polarized for the metals studied here. Our results are
in agreement with recent model calculations.Comment: 4 pages, 3 figure

Dekker, D. M. T.

Djukic, D.

Untiedt, C.

van Ruitenbeek, J. M.

English

arXiv

Dekker, D.T.M.

Ruitenbeek, J.M. van

Leiden University Scholary Publications

RAPID COMMUNICATIONSPHYSICAL REVIEW B 69, 081401~R! ~2004!Absence of magnetically induced fractional quantization in atomic contactsC. Untiedt,1,2 D. M. T. Dekker,1 D. Djukic,1 and J. M. van Ruitenbeek11Kamerlingh Onnes Laboratorium, Universiteit Leiden, Postbus 9504, NL-2300 RA Leiden, The Netherlands2Departamento de Fı´sica Aplicada, Universidad de Alicante, Campus de San Vicente del Raspeig, E-03690 Alicante, Spain~Received 6 October 2003; revised manuscript received 18 November 2003; published 9 February 2004!Using the mechanically controlled break junction technique at low temperatures and under cryogenicvacuum conditions we have studied atomic contacts of several magnetic~F , Co, and Ni! and nonmagnetic~Pt!metals, which recently were claimed to show fractional conductance quantization. In the case of pure metalswe see no quantization of the conductance nor half quantization, even when high magnetic fields are applied.On the other hand, features in the conductance similar to~fractional! quantization are observed when thecontact is exposed to gas molecules. Furthermore, the absence of fractional quantization when the contact isbridged by H2 indicates the current is never fully polarized for the metals studied here. Our results are inagreement with recent model calculations.DOI: 10.1103/PhysRevB.69.081401 PACS number~s!: 73.63.Rt, 75.75.1abetinbtwtrhlinti-wbeteelaa-innectuneoreltaees,thesev-gerag-redTheancee intionsns-imersthisintheandani-aothofoftingm-KmicThetherforofthenewtoWhen a metallic wire is stretched its conductancecomes smaller as a result of the decrease of its cross secThis process continues until the breaking of the wire, ajust before this event takes place, the contact is formedjust one atom. In this way atomic-sized contacts betweenmetallic electrodes can be formed and studied. The insments that have made these studies possible are the meccally controllable break junctions and the scanning tunnemicroscope. In both techniques the relative displacementwo electrodes is controlled with a resolution of a few pcometers by the use of a piezoelectric element which allous to monitor the formation and breaking of the contacttween the two electrodes.Properties of such atomic-sized contacts have been exsively studied during the past decade1 for many differentmetals both magnetic and nonmagnetic. The conductancthese contacts can be described by the Landauer formuG5G0(iTi , ~1!where the summation is extended to all the available chnels for the electrons traversing the contact,Ti is a numberbetween 0 and 1 for the transmission of thei th channel, andG052e2/h the quantum of conductance~assuming degeneracy of spin! in terms of the electron chargee and Planck’sconstanth. In the case in which the degeneracy of spwould be removed the channels would have to be redefifor each spin and each of these would carry up to12 G0.The number of channels available in a one-atom contadetermined by the valence of the metal,2 and the transmis-sion of each channel is influenced by other parameters sas the number of neighbors or the bond distance.3,4 For spe-cial cases (s-type metals such as Au or Na!, electronic trans-port through a single atom will be due to a single chanwith a transmission close to unity, but this will not be true fother metals where all kinds of combinations of channwith different transmissions will add up to produce the toconductance of the atom. This is the case for transition mals with partial occupation of thed orbitals and thereforethey are not expected to have a one-atom conductanc0163-1829/2004/69~8!/081401~4!/$22.50 69 0814-on.dyou-ani-gofs-n-ofn-dischlslt-of12 G0 or 1 G0 for magnetic or nonmagnetic metalrespectively.5–7 This will make it difficult to establish justfrom the conductance whether the spin degeneracy ofconductance channels has been lifted or not. However,eral claims have appeared of the observation of half-inteconductance quantization for both magnetic and nonmnetic metals.8–13 Some related results have already appeaearlier and have been discussed in Sec. 5.4 of Ref. 1.claims are based on the observation of peaks in conducthistograms at half integers of the quantum of conductancexperiments made at room temperature. These observacannot be understood from the present knowledge of traport properties of atomic-sized contacts. Especially the claof the observation of values of conductance at half integof the quantum in the case of Pt and the interpretation ofphenomenon as a result of the lifting of the spdegeneracy13 seem to contradict previous theoretical6 ndexperimental14–16 work.To further investigate this problem we have studiedconductance of several magnetic metals, namely, Fe, Co,Ni and a nonmagnetic one, Pt. We have used the mechcally controllable break junction technique that usesnotched wire of the metal under study, which is glued to bsides of the notch on top of a bending beam. By bendingthe beam the wire is broken at the notch and with the usea piezoelement the relative displacement of the two resulelectrodes can be controlled with a precision of a few picoeters. The first breaking of the wire is done only at 4.2under cryogenic vacuum in order to assure that the atocontact is formed between clean electrode surfaces.break junction device remains under these conditions forfull duration of the experiment. The starting purity of ousamples was 99.998% for Pt, Fe, and Ni and of 99.99%Co.As part of the experiments we have recorded tracesconductance while pulling the contacts until breaking~seenas a sudden drop of conductance to well below 1G0). Oncebroken the electrodes were pushed together again untilcontact was big enough so as to assure a completelyatomic contact configuration. This is important in orderavoid repetitive evolution in the pulling process.17 The pro-©2004 The American Physical Society01-1aednaginarellleiledriesowelthurxtioex-0 TThexedav-canra-eri-d inered beis isthe-eresyo-ofmson--veryc-f aan-ennedesm-onomerentsr 1ucedirstcesalsri-searsgesthetool-con-tals,ont theic-on-ousryTaRAPID COMMUNICATIONSUNTIEDT, DEKKER, DJUKIC, AND VAN RUITENBEEK PHYSICAL REVIEW B69, 081401~R! ~2004!cess of contact making followed by controlled breaking wrepeated a few thousand times and the traces were usbuild histograms of conductance18,19 such as those shown iFig. 1. All traces are included, without any selection.We collected histograms of conductance for three mnetic metals~Co, Fe, and Ni, Fig. 1! and a nonmagneticmetal, Pt, which has been predicted to become magneticone-dimensional atomic wire configuration.6,20 In agreementwith previous experiments done under similconditions,14–16,21–23the histograms for the variousd metalsin this work show a prominent first peak with a value wabove 1G0 that is attributed to the conductance of singatom contacts. Pt is discussed below and looks very simwith a first peak at about 1.5G0. Below the first peak thecounts rapidly drop until we find a new rise mainly causby the tunneling of electrons through the vacuum barbetween the two electrodes. Note, in particular, that peak1G0 and12 G0 are absent.To test whether the relative orientation of magnetic dmains around the contacts plays any role in our resultshave repeated the same experiments in high magnetic fiPrior to this we measured the hysteresis curves forsample wires used in the experiments to identify the sattion field, which was for all the cases below 2 T. In the eperiments we used fields up to 5 T, well above the saturafields. The conductance histograms~thick curves in Fig. 1!FIG. 1. Conductance histograms for the various metals withmagnetic field~thin curve! and when a magnetic field of 5 T waapplied ~thick curve!. The same structure was observed for evehistogram recorded on different samples for the same metals.conductance was measured using a dc bias voltage of 20 mV,the histograms were constructed using 2500 traces~F !, 1500 ~Coand Ni at 5 T! or 3000 traces~Ni at 0 T!.08140sto-a-arrat-eds.ea--nshow no significant difference compared to the zero-fieldperiments. We even increased the magnetic field up to 1in the case of Co, and again no changes were observed.small changes observed in Ref. 24 when magnetizing a fiatomic contact do not appear to survive in an ensembleerage. Note that for a given contact magnetostrictionhave a significant effect on the contact size and configution, but by considering conductance histograms our expment is not affected by such spurious effects.The present results seem to contradict those obtainethe room-temperature experiments mentioned above.8–13Moreover, if the effects reported for room temperatures wa result of the metals being magnetic these effects shouleven be more pronounced at low temperatures. Since thnot the case we have looked for other explanations forobserved~fractional! quantum peaks. The fact that all roomtemperature experiments are performed under atmosphthat are considerably less pure than that provided by crgenic vacuum we are led to consider the possibilityatomic-scale contamination of the contact by foreign atoor molecules.For Pt it has recently been shown that a controlled ctamination of the break junction with H2 leads to the appearance of a peak in the conductance histograms situatedclose to 1G0.23 It was shown that this peak in the condutance histogram corresponds to stable configurations osingle hydrogen molecule having a single conductance chnel that is nearly perfectly transmitting. Since a hydrogmolecule only allows a single channel of conductance omay guess that when such a molecule bridges two electrothat are fully spin polarized the conductance would be liited to 12 G0. Considering that hydrogen is a rather commcontaminant it may have influenced the experiments at rotemperature and was acting as a ‘‘filter’’ limiting the numbof channels to one.In order to test this idea we have repeated the experimin the presence of a hydrogen atmosphere of'1026 mbar.For all the experiments the sample was cooled to 4.2 K foday in cryogenic vacuum, at this stage we could reprodthe histograms of Fig. 1. Then the H2 gas was introduced anthe histograms changed dramatically as shown in Fig. 2. Fwe note that the histograms show counts at conductanbelow the first peak~which we will refer to as the ‘‘back-ground’’! in an amount much higher than for the clean metin Fig. 1. Second, under similar conditions as for the expement on Pt~Ref. 23! the characteristic peaks for the varioumagnetic metals are suppressed while a new peak appnear 1G0 in the conductance histogram taken at bias voltaabove about 100 mV. As argued in Ref. 23 the role ofhigher bias voltage is to provide some local heatingevaporate away the weakly bound excess hydrogen mecules. The results suggest that the hydrogen-bridgedfiguration observed for Pt is a general feature in these mebut we have not yet attempted to confirm this by phonspectroscopy.A dominant conductance of 1G0 with a hydrogen mol-ecule between electrodes of Co, Ni, and Fe suggests thacurrent is at best partially spin polarized in these atomsized contacts. As argued in Refs. 5–7, when there is a chend1-2thugynnitcotedirotemslabeiswe-nockaeail-ireksbenoThe-meotofbyol-ectisb-spe-o,an-eri-ic-eldandtion,ex-e ofionaldean acalpo-alnlyockndenttaorNtactwoeol-oper-toRAPID COMMUNICATIONSABSENCE OF MAGNETICALLY INDUCED FRACTIONAL . . . PHYSICAL REVIEW B 69, 081401~R! ~2004!tribution of both spin components at the Fermi energyconductance is still carried by two spin channels even thothe spin subbands may be shifted considerably in energthe exchange interaction. The same is true for Pt. We caexclude the possibility of a magnetic moment developingPt atomic-sized contacts, as has been predicted.6,20 However,its existence cannot be concluded exclusively based onappearance of peaks at fractional conductance quanta inductance histograms, as we will illustrate next.We have searched for effects of other possible gas mecules that could be present in some quantities at roomperatures or in high vacuum. We performed these stuagain at low temperatures in a controlled atmosphere. Fmany experiments we have found that it is easy towhether or not some contaminants have reached the saby just looking at the conductance histograms. In those cawhere the contaminants reached the sample there is acontribution to the histogram for values of conductancelow 1G0. Above a certain quantity of contaminants the htograms have a large smooth background that decreasesconductance.~Note the difference for the backgrounds btween Fig. 1 and the thin lines in Fig. 2!. This background isreduced when increasing the applied bias voltage andmally when it is above 200 mV both the new peaks and baground disappear and the histograms for the bare metalsrecovered.When introducing small amounts of carbon monoxidCO, to Pt atomic contacts we observed two new peaksFIG. 2. Conductance histograms for the various magnetic mein the presence of a hydrogen atmosphere. The traces were recusing a bias voltage of 20 mV~thin line! and 150 mV~thick line!and the histograms were built from 2000 traces, except forwhere we used 1000~at 20 mV! and 3000~at 150 mV!.08140ehbyotnheon-l-m-esmllpleesrge--ithr--re,p-pearing in the conductance histogram, one near12 G0 and onenear 1G0 ~Fig. 3!. The gas was introduced through a caplary heated by sending a current through a resistive wrunning inside the full length of the capillary. These peabehave as those from other contaminants, such as H2, andthe positions are remarkably similar to those that wouldexpected for half-integer quantization. There is, however,evidence that could relate these peaks to magnetism.peak near12 G0 likely results from a stable molecular geometry of CO between the metal electrodes. We would welconumerical calculations to verify this. Although we cannclaim that the reported observations of half-integer quantaconductance are all due to contamination of the contactsCO, our result shows that at least there is one kind of mecule that would produce similar behavior. Since a dirrelation of the observed half-integer peaks to magnetismoften not provided, we prefer to tentatively attribute the oserved fractional quantum peaks to adsorbed molecularcies.In conclusion, in contrast to previous reports on Fe, CNi, and Pt we have not detected fractional conductance qutization at low temperatures for the same metals. Our expments show that the conductance of pure metallic atomized contacts for Fe, Co, Ni, and Pt have no significant fidependence in the histogram-averaged conductance,show no peaks that can be associated with pure quantizain agreement with recent numerical results.5–7At least part ofthe previously reported fractional quantization may beplained by the presence of foreign molecules at the surfacthe studied samples, as we have demonstrated by intentcontamination of our samples. Our results support the it at the magnetic state of the samples is not related isimple manner to its conductance. Although the electricurrent in atomic-sized conductors may be partially spinlarized this is not a sufficient condition to obtain a fractionquantum of conductance for single-atom contacts. Owhen the conductance is dominated by a singles channel andthe exchange energy is large enough to completely bltransport through one of the spin subchannels we will fithe sought-after half-integer conductance. At the preslsdedi,FIG. 3. Conductance histogram for a Pt atomic-sized conbefore~thin curve! and after~thick curve! intentional contaminationby CO. Although the positions of the peaks at about the first tmultiples of 12 G0 are suggestive of the lifting of spin degeneracy wargue that the peaks reflect the typical conductance of a CO mecule between Pt electrodes and are not related to magnetic prties. A bias voltage as high as 150 mV was applied in orderreduce the background.1-3relmanh-.g-RAPID COMMUNICATIONSUNTIEDT, DEKKER, DJUKIC, AND VAN RUITENBEEK PHYSICAL REVIEW B69, 081401~R! ~2004!time, the only experiment that probably fulfills these requiments is the one recently reported by Suderowet al.,25 wherethe atomic contact is between a gold tip and a thin gold fithat is deposited on top of a half-metallic ferromagnetic mganite..Chys,n.steret08140--This work is part of the research program of the ‘‘Sticting FOM,’’ which is financially supported by NWO. C.Uacknowledges support of the Spanish Ramo´n y Cajal pro-gram of the MCyT. We are grateful to T.G. Sorop for manetization measurements.F.ev.en,v. Br, J.gh,k,anJ.. B.ar-1N. Agraı̈t, A. Levy Yeyati, and J. van Ruitenbeek, Phys. Rep.377,81 ~2003!.2E. Scheer, N. Agraı¨t, J. Cuevas, A. Levy Yeyati, B. Ludoph, AMartı́n-Rodero, G. Rubio Bollinger, J. van Ruitenbeek, andUrbina, Nature~London! 394, 154 ~1998!.3E. Scheer, P. Joyez, D. Esteve, C. Urbina, and M. Devoret, PRev. Lett.78, 3535~1997!.4J. Cuevas, A. Levy Yeyati, A. Martı´n-Rodero, G. Rubio Bollinger,C. Untiedt, and N. Agraı¨t, Phys. Rev. Lett.81, 2990~1998!.5A. Martı́n-Rodero, A. Levy Yeyati, and J. Cuevas, Physica C352,67 ~2001!.6A. Delin and E. Tosatti, Phys. Rev. B68, 144434~2003!.7A. Bagrets, N. Papanikolaou, and I. Mertig, http://arXiv.org/abcond-mat/0303480.8F. Elhoussine, S. Ma´téfi2Tempfli, A. Encinas, and L. PirauxAppl. Phys. Lett.81, 1681~2002!.9M. Shimizu, E. Saitoh, H. Miyajima, and Y. Otani, J. MagMagn. Mater.239, 243 ~2002!.10D. Gillingham, I. Linington, and J. Bland, J. Phys.: CondenMatter 14, L567 ~2002!.11D. Gillingham, C. Müller, and J. Bland, J. Phys.: Condens. Mat15, L291 ~2003!.12D. Gillingham, I. Linington, C. Mu¨ller, and J. Bland, J. Appl.Phys.93, 7388~2003!.13V. Rodrigues, J. Bettini, P. Silva, and D. Ugarte, Phys. Rev. L91, 096801~2003!..s./.t.14C. Sirvent, J. Rodrigo, S. Vieira, L. Jurczyszyn, N. Mingo, andFlores, Phys. Rev. B53, 16 086~1996!.15R. Smit, C. Untiedt, A. Yanson, and J. van Ruitenbeek, Phys. RLett. 87, 266102~2001!.16S. Nielsen, Y. Noat, M. Brandbyge, R. Smit, K. Hansen, L. ChA. Yanson, F. Besenbacher, and J. van Ruitenbeek, Phys. Re67, 245411~2003!.17C. Untiedt, G. Rubio, S. Vieira, and N. Agraı¨t, Phys. Rev. B56,2154 ~1997!.18L. Olesen, E. Lægsgaard, I. Stensgaard, F. BesenbacheSchio”tz, P. Stoltze, K. Jacobsen, and J. No”rskov, Phys. Rev. Lett.74, 2147~1995!.19J. Krans, J. van Ruitenbeek, V. Fisun, I. Yanson, and L. de JonNature~London! 375, 767 ~1995!.20S. Bahn and K. Jacobsen, Phys. Rev. Lett.87, 266101~2001!.21B. Ludoph and J. van Ruitenbeek, Phys. Rev. B61, 2273~2000!.22D.J. Bakker, Y. Noat, A.I. Yanson, and J.M. van RuitenbeePhys. Rev. B65, 235416~2002!.23R. Smit, Y. Noat, C. Untiedt, N. Lang, M. van Hemert, and J. vRuitenbeek, Nature~London! 419, 906 ~2002!.24M. Viret, S. Berger, M. Gabureac, F. Ott, D. Olligs, I. Petej,Gregg, C. Fermon, G. Francinet, and G.L. Goff, Phys. Rev66, 220401~2002!.25H. Suderow, M. Crespo, S. Vieira, M. Vila, MGarcia2Hernandez, A. de Andres, C. Prieto, C. Ocal, J. Mtinez, and Y. Mukovskii, Physica E~Amsterdam! 18, 264~2003!.1-4

Absence of magnetically-induced fractional quantization in atomic contacts

Using the mechanically controlled break junction technique at low temperatures and under cryogenic vacuum conditions we have studied atomic contacts of several magnetic (Fe, Co, and Ni) and nonmagnetic (Pt) metals, which recently were claimed to show fractional conductance quantization. In the case of pure metals we see no quantization of the conductance nor half quantization, even when high magnetic fields are applied. On the other hand, features in the conductance similar to (fractional) quantization are observed when the contact is exposed to gas molecules. Furthermore, the absence of fractional quantization when the contact is bridged by H2 indicates the current is never fully polarized for the metals studied here. Our results are in agreement with recent model calculations.This work is part of the research program of the “Stichting FOM,” which is financially supported by NWO. C.U. acknowledges support of the Spanish Ramón y Cajal program of the MCyT

Untiedt, Carlos

Dekker, D.M.T.

Absence of magnetically induced fractional quantization in atomic contacts

RUa Reposity University of Alicante

RAPID COMMUNICATIONSPHYSICAL REVIEW B 69, 081401~R! ~2004!Absence of magnetically induced fractional quantization in atomic contactsC. Untiedt,1,2 D. M. T. Dekker,1 D. Djukic,1 and J. M. van Ruitenbeek11Kamerlingh Onnes Laboratorium, Universiteit Leiden, Postbus 9504, NL-2300 RA Leiden, The Netherlands2Departamento de Fı´sica Aplicada, Universidad de Alicante, Campus de San Vicente del Raspeig, E-03690 Alicante, Spain~Received 6 October 2003; revised manuscript received 18 November 2003; published 9 February 2004!Using the mechanically controlled break junction technique at low temperatures and under cryogenicvacuum conditions we have studied atomic contacts of several magnetic ~Fe, Co, and Ni! and nonmagnetic ~Pt!metals, which recently were claimed to show fractional conductance quantization. In the case of pure metalswe see no quantization of the conductance nor half quantization, even when high magnetic fields are applied.On the other hand, features in the conductance similar to ~fractional! quantization are observed when thecontact is exposed to gas molecules. Furthermore, the absence of fractional quantization when the contact isbridged by H2 indicates the current is never fully polarized for the metals studied here. Our results are inagreement with recent model calculations.DOI: 10.1103/PhysRevB.69.081401 PACS number~s!: 73.63.Rt, 75.75.1aWhen a metallic wire is stretched its conductance be-comes smaller as a result of the decrease of its cross section.This process continues until the breaking of the wire, andjust before this event takes place, the contact is formed byjust one atom. In this way atomic-sized contacts between twometallic electrodes can be formed and studied. The instru-ments that have made these studies possible are the mechani-cally controllable break junctions and the scanning tunnelingmicroscope. In both techniques the relative displacement oftwo electrodes is controlled with a resolution of a few pi-cometers by the use of a piezoelectric element which allowsus to monitor the formation and breaking of the contact be-tween the two electrodes.Properties of such atomic-sized contacts have been exten-sively studied during the past decade1 for many differentmetals both magnetic and nonmagnetic. The conductance ofthese contacts can be described by the Landauer formulaG5G0(iTi , ~1!where the summation is extended to all the available chan-nels for the electrons traversing the contact, Ti is a numberbetween 0 and 1 for the transmission of the ith channel, andG052e2/h the quantum of conductance ~assuming degen-eracy of spin! in terms of the electron charge e and Planck’sconstant h. In the case in which the degeneracy of spinwould be removed the channels would have to be redefinedfor each spin and each of these would carry up to 12 G0.The number of channels available in a one-atom contact isdetermined by the valence of the metal,2 and the transmis-sion of each channel is influenced by other parameters suchas the number of neighbors or the bond distance.3,4 For spe-cial cases (s-type metals such as Au or Na!, electronic trans-port through a single atom will be due to a single channelwith a transmission close to unity, but this will not be true forother metals where all kinds of combinations of channelswith different transmissions will add up to produce the totalconductance of the atom. This is the case for transition met-als with partial occupation of the d orbitals and thereforethey are not expected to have a one-atom conductance of0163-1829/2004/69~8!/081401~4!/$22.50 69 081412 G0 or 1 G0 for magnetic or nonmagnetic metals,respectively.5–7 This will make it difficult to establish justfrom the conductance whether the spin degeneracy of theconductance channels has been lifted or not. However, sev-eral claims have appeared of the observation of half-integerconductance quantization for both magnetic and nonmag-netic metals.8–13 Some related results have already appearedearlier and have been discussed in Sec. 5.4 of Ref. 1. Theclaims are based on the observation of peaks in conductancehistograms at half integers of the quantum of conductance inexperiments made at room temperature. These observationscannot be understood from the present knowledge of trans-port properties of atomic-sized contacts. Especially the claimof the observation of values of conductance at half integersof the quantum in the case of Pt and the interpretation of thisphenomenon as a result of the lifting of the spindegeneracy13 seem to contradict previous theoretical6 andexperimental14–16 work.To further investigate this problem we have studied theconductance of several magnetic metals, namely, Fe, Co, andNi and a nonmagnetic one, Pt. We have used the mechani-cally controllable break junction technique that uses anotched wire of the metal under study, which is glued to bothsides of the notch on top of a bending beam. By bending ofthe beam the wire is broken at the notch and with the use ofa piezoelement the relative displacement of the two resultingelectrodes can be controlled with a precision of a few picom-eters. The first breaking of the wire is done only at 4.2 Kunder cryogenic vacuum in order to assure that the atomiccontact is formed between clean electrode surfaces. Thebreak junction device remains under these conditions for thefull duration of the experiment. The starting purity of oursamples was 99.998% for Pt, Fe, and Ni and of 99.99% forCo.As part of the experiments we have recorded traces ofconductance while pulling the contacts until breaking ~seenas a sudden drop of conductance to well below 1G0). Oncebroken the electrodes were pushed together again until thecontact was big enough so as to assure a completely newatomic contact configuration. This is important in order toavoid repetitive evolution in the pulling process.17 The pro-©2004 The American Physical Society01-1RAPID COMMUNICATIONSUNTIEDT, DEKKER, DJUKIC, AND VAN RUITENBEEK PHYSICAL REVIEW B 69, 081401~R! ~2004!cess of contact making followed by controlled breaking wasrepeated a few thousand times and the traces were used tobuild histograms of conductance18,19 such as those shown inFig. 1. All traces are included, without any selection.We collected histograms of conductance for three mag-netic metals ~Co, Fe, and Ni, Fig. 1! and a nonmagneticmetal, Pt, which has been predicted to become magnetic in aone-dimensional atomic wire configuration.6,20 In agreementwith previous experiments done under similarconditions,14–16,21–23 the histograms for the various d metalsin this work show a prominent first peak with a value wellabove 1G0 that is attributed to the conductance of single-atom contacts. Pt is discussed below and looks very similarwith a first peak at about 1.5G0. Below the first peak thecounts rapidly drop until we find a new rise mainly causedby the tunneling of electrons through the vacuum barrierbetween the two electrodes. Note, in particular, that peaks at1G0 and 12 G0 are absent.To test whether the relative orientation of magnetic do-mains around the contacts plays any role in our results wehave repeated the same experiments in high magnetic fields.Prior to this we measured the hysteresis curves for thesample wires used in the experiments to identify the satura-tion field, which was for all the cases below 2 T. In the ex-periments we used fields up to 5 T, well above the saturationfields. The conductance histograms ~thick curves in Fig. 1!FIG. 1. Conductance histograms for the various metals withoutmagnetic field ~thin curve! and when a magnetic field of 5 T wasapplied ~thick curve!. The same structure was observed for everyhistogram recorded on different samples for the same metals. Theconductance was measured using a dc bias voltage of 20 mV, andthe histograms were constructed using 2500 traces ~Fe!, 1500 ~Coand Ni at 5 T! or 3000 traces ~Ni at 0 T!.08140show no significant difference compared to the zero-field ex-periments. We even increased the magnetic field up to 10 Tin the case of Co, and again no changes were observed. Thesmall changes observed in Ref. 24 when magnetizing a fixedatomic contact do not appear to survive in an ensemble av-erage. Note that for a given contact magnetostriction canhave a significant effect on the contact size and configura-tion, but by considering conductance histograms our experi-ment is not affected by such spurious effects.The present results seem to contradict those obtained inthe room-temperature experiments mentioned above.8–13Moreover, if the effects reported for room temperatures werea result of the metals being magnetic these effects should beeven be more pronounced at low temperatures. Since this isnot the case we have looked for other explanations for theobserved ~fractional! quantum peaks. The fact that all room-temperature experiments are performed under atmospheresthat are considerably less pure than that provided by cryo-genic vacuum we are led to consider the possibility ofatomic-scale contamination of the contact by foreign atomsor molecules.For Pt it has recently been shown that a controlled con-tamination of the break junction with H2 leads to the appear-ance of a peak in the conductance histograms situated veryclose to 1G0.23 It was shown that this peak in the conduc-tance histogram corresponds to stable configurations of asingle hydrogen molecule having a single conductance chan-nel that is nearly perfectly transmitting. Since a hydrogenmolecule only allows a single channel of conductance onemay guess that when such a molecule bridges two electrodesthat are fully spin polarized the conductance would be lim-ited to 12 G0. Considering that hydrogen is a rather commoncontaminant it may have influenced the experiments at roomtemperature and was acting as a ‘‘filter’’ limiting the numberof channels to one.In order to test this idea we have repeated the experimentsin the presence of a hydrogen atmosphere of ’1026 mbar.For all the experiments the sample was cooled to 4.2 K for 1day in cryogenic vacuum, at this stage we could reproducethe histograms of Fig. 1. Then the H2 gas was introduced andthe histograms changed dramatically as shown in Fig. 2. Firstwe note that the histograms show counts at conductancesbelow the first peak ~which we will refer to as the ‘‘back-ground’’! in an amount much higher than for the clean metalsin Fig. 1. Second, under similar conditions as for the experi-ment on Pt ~Ref. 23! the characteristic peaks for the variousmagnetic metals are suppressed while a new peak appearsnear 1G0 in the conductance histogram taken at bias voltagesabove about 100 mV. As argued in Ref. 23 the role of thehigher bias voltage is to provide some local heating toevaporate away the weakly bound excess hydrogen mol-ecules. The results suggest that the hydrogen-bridged con-figuration observed for Pt is a general feature in these metals,but we have not yet attempted to confirm this by phononspectroscopy.A dominant conductance of 1G0 with a hydrogen mol-ecule between electrodes of Co, Ni, and Fe suggests that thecurrent is at best partially spin polarized in these atomic-sized contacts. As argued in Refs. 5–7, when there is a con-1-2RAPID COMMUNICATIONSABSENCE OF MAGNETICALLY INDUCED FRACTIONAL . . . PHYSICAL REVIEW B 69, 081401~R! ~2004!tribution of both spin components at the Fermi energy theconductance is still carried by two spin channels even thoughthe spin subbands may be shifted considerably in energy bythe exchange interaction. The same is true for Pt. We cannotexclude the possibility of a magnetic moment developing inPt atomic-sized contacts, as has been predicted.6,20 However,its existence cannot be concluded exclusively based on theappearance of peaks at fractional conductance quanta in con-ductance histograms, as we will illustrate next.We have searched for effects of other possible gas mol-ecules that could be present in some quantities at room tem-peratures or in high vacuum. We performed these studiesagain at low temperatures in a controlled atmosphere. Frommany experiments we have found that it is easy to tellwhether or not some contaminants have reached the sampleby just looking at the conductance histograms. In those caseswhere the contaminants reached the sample there is a largecontribution to the histogram for values of conductance be-low 1G0. Above a certain quantity of contaminants the his-tograms have a large smooth background that decreases withconductance. ~Note the difference for the backgrounds be-tween Fig. 1 and the thin lines in Fig. 2!. This background isreduced when increasing the applied bias voltage and nor-mally when it is above 200 mV both the new peaks and back-ground disappear and the histograms for the bare metals arerecovered.When introducing small amounts of carbon monoxide,CO, to Pt atomic contacts we observed two new peaks ap-FIG. 2. Conductance histograms for the various magnetic metalsin the presence of a hydrogen atmosphere. The traces were recordedusing a bias voltage of 20 mV ~thin line! and 150 mV ~thick line!and the histograms were built from 2000 traces, except for Ni,where we used 1000 ~at 20 mV! and 3000 ~at 150 mV!.08140pearing in the conductance histogram, one near 12 G0 and onenear 1G0 ~Fig. 3!. The gas was introduced through a capil-lary heated by sending a current through a resistive wirerunning inside the full length of the capillary. These peaksbehave as those from other contaminants, such as H2, andthe positions are remarkably similar to those that would beexpected for half-integer quantization. There is, however, noevidence that could relate these peaks to magnetism. Thepeak near 12 G0 likely results from a stable molecular geom-etry of CO between the metal electrodes. We would welcomenumerical calculations to verify this. Although we cannotclaim that the reported observations of half-integer quanta ofconductance are all due to contamination of the contacts byCO, our result shows that at least there is one kind of mol-ecule that would produce similar behavior. Since a directrelation of the observed half-integer peaks to magnetism isoften not provided, we prefer to tentatively attribute the ob-served fractional quantum peaks to adsorbed molecular spe-cies.In conclusion, in contrast to previous reports on Fe, Co,Ni, and Pt we have not detected fractional conductance quan-tization at low temperatures for the same metals. Our experi-ments show that the conductance of pure metallic atomic-sized contacts for Fe, Co, Ni, and Pt have no significant fielddependence in the histogram-averaged conductance, andshow no peaks that can be associated with pure quantization,in agreement with recent numerical results.5–7 At least part ofthe previously reported fractional quantization may be ex-plained by the presence of foreign molecules at the surface ofthe studied samples, as we have demonstrated by intentionalcontamination of our samples. Our results support the ideathat the magnetic state of the samples is not related in asimple manner to its conductance. Although the electricalcurrent in atomic-sized conductors may be partially spin po-larized this is not a sufficient condition to obtain a fractionalquantum of conductance for single-atom contacts. Onlywhen the conductance is dominated by a single s channel andthe exchange energy is large enough to completely blocktransport through one of the spin subchannels we will findthe sought-after half-integer conductance. At the presentFIG. 3. Conductance histogram for a Pt atomic-sized contactbefore ~thin curve! and after ~thick curve! intentional contaminationby CO. Although the positions of the peaks at about the first twomultiples of 12 G0 are suggestive of the lifting of spin degeneracy weargue that the peaks reflect the typical conductance of a CO mol-ecule between Pt electrodes and are not related to magnetic proper-ties. 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Absence of magnetically-induced fractional quantization in atomic contacts

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