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Role of electronic structure in photoassisted transport through atomic-sized contacts  [PDF]
J. K. Viljas,J. C. Cuevas
Physics , 2006, DOI: 10.1103/PhysRevB.75.075406
Abstract: We study theoretically quantum transport through laser-irradiated metallic atomic-sized contacts. The radiation field is treated classically, assuming its effect to be the generation of an ac voltage over the contact. We derive an expression for the dc current and compute the linear conductance in one-atom thick contacts as a function of the ac frequency, concentrating on the role played by electronic structure. In particular, we present results for three materials (Al, Pt, and Au) with very different electronic structures. It is shown that, depending on the frequency and the metal, the radiation can either enhance or diminish the conductance. This can be intuitively understood in terms of the energy dependence of the transmission of the contacts in the absence of radiation.
Structure and conductance histogram of atomic-sized Au contacts  [PDF]
M. Dreher,F. Pauly,J. Heurich,J. C. Cuevas,E. Scheer,P. Nielaba
Physics , 2004, DOI: 10.1103/PhysRevB.72.075435
Abstract: Many experiments have shown that the conductance histograms of metallic atomic-sized contacts exhibit a peak structure, which is characteristic of the corresponding material. The origin of these peaks still remains as an open problem. In order to shed some light on this issue, we present a theoretical analysis of the conductance histograms of Au atomic contacts. We have combined classical molecular dynamics simulations of the breaking of nanocontacts with conductance calculations based on a tight-binding model. This combination gives us access to crucial information such as contact geometries, forces, minimum cross-section, total conductance and transmission coefficients of the individual conduction channels. The ensemble of our results suggests that the low temperature Au conductance histograms are a consequence of a subtle interplay between mechanical and electrical properties of these nanocontacts. At variance with other suggestions in the literature, our results indicate that the peaks in the Au conductance histograms are not a simple consequence of conductance quantization or the existence of exceptionally stable radii. We show that the main peak in the histogram close to one quantum of conductance is due to the formation of single-atom contacts and chains of gold atoms. Moreover, we present a detailed comparison with experimental results on Au atomic contacts where the individual channel transmissions have been determined.
Magnetoresistance of atomic-sized contacts: an ab-initio study  [PDF]
Alexei Bagrets,Nikos Papanikolaou,Ingrid Mertig
Physics , 2003, DOI: 10.1103/PhysRevB.70.064410
Abstract: The magnetoresistance (MR) effect in metallic atomic-sized contacts is studied theoretically by means of first-principle electronic structure calculations. We consider three-atom chains formed from Co, Cu, Si, and Al atoms suspended between semi-infinite Co leads. We employ the screened Korringa-Kohn-Rostoker Green's function method for the electronic structure calculation and evaluate the conductance in the ballistic limit using the Landauer approach. The conductance through the constrictions reflects the spin-splitting of the Co bands and causes high MR ratios, up to 50%. The influence of the structural changes on the conductance is studied by considering different geometrical arrangements of atoms forming the chains. Our results show that the conductance through s-like states is robust against geometrical changes, whereas the transmission is strongly influenced by the atomic arrangement if p or d states contribute to the current.
Thermopower of atomic-size metallic contacts  [PDF]
B. Ludoph,J. M. van Ruitenbeek
Physics , 1998, DOI: 10.1103/PhysRevB.59.12290
Abstract: The thermopower and conductance of atomic-size metallic contacts have been simultaneously measured using a mechanically controllable break junction. For contacts approaching atomic dimensions, abrupt steps in the thermopower are observed which coincide with jumps in the conductance. The measured thermopower for a large number of atomic-size contacts is randomly distributed around the value for large contacts and can be either positive or negative in sign. However, it is suppressed at the quantum value of the conductance G_0 = 2e^2/h. We derive an expression that describes these results in terms of quantum interference of electrons backscattered in the banks.
Formation of Atomic-Sized Contacts by Electrochemical Methods  [PDF]
M. R. Calvo,A. I. Mares,V. Climent,J. M. van Ruitenbeek,C. Untiedt
Physics , 2006, DOI: 10.1002/pssa.200675319
Abstract: 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.
Conductance of Atomic-Sized Lead Contacts in an Electrochemical Environment  [PDF]
F. -Q. Xie,F. Hüser,F. Pauly,Ch. Obermair,G. Sch?n,Th. Schimmel
Physics , 2010, DOI: 10.1103/PhysRevB.82.075417
Abstract: Atomic-sized lead (Pb) contacts are deposited and dissolved in an electrochemical environment, and their transport properties are measured. Due to the electrochemical fabrication process, we obtain mechanically unstrained contacts and conductance histograms with sharply resolved, individual peaks. Charge transport calculations based on density functional theory (DFT) for various ideal Pb contact geometries are in good agreement with the experimental results. Depending on the atomic configuration, single-atom-wide contacts of one and the same metal yield very different conductance values.
Anisotropic magnetoresistance in ferromagnetic atomic-sized metal contacts  [PDF]
M. H?fner,J. K. Viljas,J. C. Cuevas
Physics , 2008, DOI: 10.1103/PhysRevB.79.140410
Abstract: Recent experiments in ferromagnetic atomic-sized contacts have shown that the anisotropic magnetoresistance (AMR) is greatly enhanced and has an asymmetric angular dependence as compared with that of bulk samples. The origin of these effects is still under debate. In this work we present a theoretical analysis of the AMR in atomic contacts of the 3d ferromagnetic materials. Our results strongly suggest that the anomalous AMR stems from the reduced symmetry of the atomic contact geometries. We also present calculations supporting the idea that the pronounced voltage- and temperature dependence in some experiments can be attributed to impurities near the constrictions.
Analysis of the Kondo effect in ferromagnetic atomic-sized contacts  [PDF]
M. Reyes Calvo,David Jacob,Carlos Untiedt
Physics , 2012, DOI: 10.1103/PhysRevB.86.075447
Abstract: Atomic contacts made of ferromagnetic metals present zero-bias anomalies in the differential conductance due to the Kondo effect. These systems provide a unique opportunity to perform a statistical analysis of the Kondo parameters in nanostructures since a large number of contacts can be easily fabricated using break-junction techniques. The details of the atomic structure differ from one contact to another so a large number of different configurations can be statistically analyzed. Here we present such a statistical analysis of the Kondo effect in atomic contacts made from the ferromagnetic transition metals Ni, Co and Fe. Our analysis shows clear differences between materials that can be understood by fundamental theoretical considerations. This combination of experiments and theory allow us to extract information about the origin and nature of the Kondo effect in these systems and to explore the influence of geometry and valence in the Kondo screening of atomic-sized nanostructures.
Magnetism in Atomic-Sized Palladium Contacts and Nanowires  [PDF]
A. Delin,E. Tosatti,R. A. Weht
Physics , 2003, DOI: 10.1103/PhysRevLett.92.057201
Abstract: We have investigated Pd nanowires theoretically, and found that, unlike either metallic or free atomic Pd, they exhibit Hund's rule magnetism. In long, monoatomic wires, we find a spin moment of 0.7 Bohr magnetons per atom, whereas for short, monoatomic wires between bulk leads, the predicted moment is about 0.3 Bohr magnetons per wire atom. In contrast, a coaxial (6,1) wire was found to be nonmagnetic. The origin of the wire magnetism is analyzed.
Microscopic origin of the conducting channels in metallic atomic-size contacts  [PDF]
J. C. Cuevas,A. Levy Yeyati,A. Martin-Rodero
Physics , 1997, DOI: 10.1103/PhysRevLett.80.1066
Abstract: We present a theoretical approach which allows to determine the number and orbital character of the conducting channels in metallic atomic contacts. We show how the conducting channels arise from the atomic orbitals having a significant contribution to the bands around the Fermi level. Our theory predicts that the number of conducting channels with non negligible transmission is 3 for Al and 5 for Nb one-atom contacts, in agreement with recent experiments. These results are shown to be robust with respect to disorder. The experimental values of the channels transmissions lie within the calculated distributions.
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