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Register a new userElectronic Transport in Gadolinium Atomic-Size Contacts
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We report on the fabrication, transport measurements, and density functional theory (DFT) calculations of atomic size contacts made out of gadolinium (Gd). Gd is known to have local moments mainly associated with $f$ electrons. These coexist with itinerant $s$ and $d$ bands that account for its metallic character. Here we explore whether and how the local moments influence electronic transport properties at the atomic scale. Using both Scanning Tunneling Microscope (STM) and lithographic Mechanically Controllable Break Junction (MCBJ) techniques under cryogenic conditions, we study the conductance of Gd when only few atoms form the junction between bulk electrodes made out of the very same material. Thousands of measurements shows that Gd has an average lowest conductance, attributed to an atom-size contact, below $frac{2e^2}{h}$. Our DFT calculations for monostrand chains anticipate that the $f$ bands are fully spin polarized and insulating, and that the conduction may be dominated by $s$, $p$, and $d$ bands. DFT quantum transport calculations quantitatively reproduce the experimental results for zero bias and reveal that, while $s-p$ bands are dominant for transport, $d$ orbitals seem to have a relevant contribution in some cases.
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Metallic atomic junctions pose the ultimate limit to the scaling of electrical contacts. They serve as model systems to probe electrical and thermal transport down to the atomic level as well as quantum effects occurring in one-dimensional systems. Charge transport in atomic junctions has been studied intensively in the last two decades. However, heat transport remains poorly characterized because of significant experimental challenges. Specifically the combination of high sensitivity to small heat fluxes and the formation of stable atomic contacts has been a major hurdle for the development of this field. Here we report on the realization of heat transfer measurements through atomic junctions and analyze the thermal conductance of single atomic gold contacts at room temperature. Simultaneous measurements of charge and heat transport reveal the proportionality of electrical and thermal conductance, quantized with the respective conductance quanta. This constitutes an atomic scale verification of the well-known Wiedemann-Franz law. We anticipate that our findings will be a major advance in enabling the investigation of heat transport properties in molecular junctions, with meaningful implications towards the manipulation of heat at the nanoscale
Point-contact spectroscopy is applied to study the energy dependence of paramagnetic impurities in noble metals. The samples are in the form of the so-called mechanically controllable break-junctions where the investigated piece of alloy makes a nanowire connecting two bulk electrodes.The lateral dimensions of the bridge are of the order of a few nm and can be continuously changed. Three qualitative dependences manifest the size effect while decreasing the contact diameter: 1) Intensity of Kondo peak due to Kondo scattering in point-contact spectra decreases slower than the nonlinearities due to phonon scatterings, 2) The width of the Kondo peak becomes broder, and 3) Due to the Zeeman energy, the splitting of Kondo peak in external and internal (for a spin glass) fields, is suppressed. Explanation of the phenomena is given in terms of the theory of Zarand and Udvardi where the local density of states (LDOS) of conduction electron fluctuates strongly inside a nanowire due to interference of electronic states. Since Kondo effect is a local probe of LDOS, the impurities located close to the maxima of LDOS show the increase of the Kondo temperature and give the primary contribution to the contact resistance.
Coherent control of quantum states has been demonstrated in a variety of superconducting devices. In all these devices, the variables that are manipulated are collective electromagnetic degrees of freedom: charge, superconducting phase, or flux. Here, we demonstrate the coherent manipulation of a quantum system based on Andreev bound states, which are microscopic quasiparticle states inherent to superconducting weak links. Using a circuit quantum electrodynamics setup we perform single-shot readout of this Andreev qubit. We determine its excited state lifetime and coherence time to be in the microsecond range. Quantum jumps and parity switchings are observed in continuous measurements. In addition to possible quantum information applications, such Andreev qubits are a testbed for the physics of single elementary excitations in superconductors.
Electrochemical methods have recently become an interesting tool for fabricating and characterizing nanostructures at room temperature. Simplicity, low cost and reversibility are some of the advantages of this technique that allows to work at the nanoscale without requiring sophisticated instrumentation. In our experimental setup, we measure the conductance across a nanocontact fabricated either by dissolving a macroscopic gold wire or by depositing gold in between two separated gold electrodes. We have achieved a high level of control on the electrochemical fabrication of atomic-sized contacts in gold. The use of electrochemistry as a reproducible technique to prepare nanocontacts will open several possibilities that are not feasible with other methodologies. It involves, also, the possibility of reproducing experiments that today are made by more expensive, complicated or irreversible methods. As example, we show here a comparison of the results when looking for shell effects in gold nanocontacts with those obtained by other techniques.
We study electron transport through a multichannel fractional quantum Hall edge in the presence of both interchannel interaction and random tunneling between channels, with emphasis on the role of contacts. The prime example in our discussion is the edge at filling factor 2/3 with two counterpropagating channels. Having established a general framework to describe contacts to a multichannel edge as thermal reservoirs, we particularly focus on the line-junction model for the contacts and investigate incoherent charge transport for an arbitrary strength of interchannel interaction beneath the contacts and, possibly different, outside them. We show that the conductance does not explicitly depend on the interaction strength either in or outside the contact regions (implicitly, it only depends through renormalization of the tunneling rates). Rather, a long line-junction contact is characterized by a single parameter which defines the modes that are at thermal equilibrium with the contact and is determined by the interplay of various types of scattering beneath the contact. This parameter -- playing the role of an effective interaction strength within an idealized model of thermal reservoirs -- is generically nonzero and affects the conductance. We formulate a framework of fractionalization-renormalized tunneling to describe the effect of disorder on transport in the presence of interchannel interaction. Within this framework, we give a detailed discussion of charge equilibration for arbitrarily strong interaction in the bulk of the edge and arbitrary effective interaction characterizing the line-junction contacts.
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