Absence of magnetically induced fractional quantization in atomic contacts

Abstract

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 Ni͒ and nonmagnetic ͑Pt͒ 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 ͑fractional͒ 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. DOI: 10.1103/PhysRevB.69.081401 PACS number͑s͒: 73.63.Rt, 75.75.ϩa When a metallic wire is stretched its conductance becomes smaller as a result of the decrease of its cross section. This process continues until the breaking of the wire, and just before this event takes place, the contact is formed by just one atom. In this way atomic-sized contacts between two metallic electrodes can be formed and studied. The instruments that have made these studies possible are the mechanically controllable break junctions and the scanning tunneling microscope. In both techniques the relative displacement of two electrodes is controlled with a resolution of a few picometers by the use of a piezoelectric element which allows us to monitor the formation and breaking of the contact between the two electrodes.Properties of such atomic-sized contacts have been extensively studied during the past decade 1 for many different metals both magnetic and nonmagnetic. The conductance of these contacts can be described by the Landauer formulawhere the summation is extended to all the available channels for the electrons traversing the contact, T i is a number between 0 and 1 for the transmission of the ith channel, and G 0 ϭ2e 2 /h the quantum of conductance ͑assuming degeneracy of spin͒ in terms of the electron charge e and Planck's constant h. In the case in which the degeneracy of spin would be removed the channels would have to be redefined for each spin and each of these would carry up to 1 2 G 0 . The number of channels available in a one-atom contact is determined by the valence of the metal, 2 and the transmission of each channel is influenced by other parameters such as the number of neighbors or the bond distance. 3,4 For special cases (s-type metals such as Au or Na͒, electronic transport through a single atom will be due to a single channel with a transmission close to unity, but this will not be true for other metals where all kinds of combinations of channels with different transmissions will add up to produce the total conductance of the atom. This is the case for transition metals with partial occupation of the d orbitals and therefore they are not expected to have a one-atom conductance of 1 2 G 0 or 1 G 0 for magnetic or nonmagnetic metals, r...

Anisotropic magnetoresistance in nanocontacts

“…As a consequence, T͑E͒, shown in Fig. 2͑a͒, is not quantized anymore, as expected, 22,23 and yet the BAMR ͓Fig. 2͑c͔͒ is close to that of the ideal case for values of d / a ഛ 1.4.…”

“…21 According to the scattering-free theory, the conductance of atomic contacts of Ni, in units of e 2 / h, would be 6 or 7 for Ni, 14 in quantitative disagreement with the measured 22 conductance of Ni nanocontacts around 3e 2 / h. The same applies to Fe, Co, and Pt. Scattering definitely affects d bands, which suffer the so-called orbital blocking.…”

Effect of electron correlations in Pd, Ni, and Co monowires

“…10), 1.4 in Ni (Ref. 10), and 1.7 in Pd (Ref. 30), it does leave the fractional conductance steps G ∼ 0.5G 0 reported in STM experiments at room temperature completely unexplained.…”

“…4 It was recently predicted that quantum confinement of the electronic states in metallic atomic chains should lead to a magnetic ground state, and to a superparamagnetic state at finite temperatures, in Ru, Rh, Pd, Os and Pt. 5,6,7,8 In the present paper, we build on those results, also stimulated by some recent intriguing experimental results indicating unexpectedly low fractional conductance through Co, Pd and Pt nanocontacts and nanowires observed at room temperature 9,10,11 (note however that at low temperature, the uncontaminated break junctions fail to show fractional conductance and even nanowires in the case group III and IV transition metals. 10 ) Assuming that, as in the room temperature experiments, short nanowires could easily form at the nanocontact, we wish to explore the possible effects of strong correlations on the electronic structure of the wires, and if and in what way strong correlations might relate to these recent experimental results.…”

Electron Transport in Magnetic Quantum Point Contacts

“…For example, there are numerous reports demonstrating the absence of G 0 /2 conductance steps in magnetic junctions, both experimentally [51,52] and theoretically [53][54][55][56][57][58][59]. Moreover, half-integer conductance quantization has also been found as a result of contaminations of the point contact with H 2 and CO [60]. Therefore, the interpretation of the conductance in magnetic point contacts is not always straightforward.…”