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Daniel C Fredrickson - One of the best experts on this subject based on the ideXlab platform.
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progress in visualizing Atomic Size effects with dft chemical pressure analysis from isolated atoms to trends in ab5 intermetallics
Journal of Chemical Theory and Computation, 2014Co-Authors: Veronica M Berns, Joshua Engelkemier, Brandon J Kilduff, Daniel C FredricksonAbstract:The notion of Atomic Size poses an important challenge to chemical theory: empirical evidence has long established that atoms have spatial requirements, which are summarized in tables of covalent, ionic, metallic, and van der Waals radii. Considerations based on these radii play a central role in the design and interpretation of experiments, but few methods are available to directly support arguments based on Atomic Size using electronic structure methods. Recently, we described an approach to elucidating Atomic Size effects using theoretical calculations: the DFT-Chemical Pressure analysis, which visualizes the local pressures arising in crystal structures from the interactions of Atomic Size and electronic effects. Using this approach, a variety of structural phenomena in intermetallic phases have already been understood in terms that provide guidance to new synthetic experiments. However, the applicability of the DFT-CP method to the broad range of the structures encountered in the solid state is limit...
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first principles elucidation of Atomic Size effects using dft chemical pressure analysis origins of ca36sn23 s long period superstructure
Journal of Chemical Theory and Computation, 2013Co-Authors: Joshua Engelkemier, Veronica M Berns, Daniel C FredricksonAbstract:The space requirements of atoms are empirically known to play key roles in determining structure and reactivity across compounds ranging from simple molecules to extended solid state phases. Despite the importance of this concept, the effects of Atomic Size on stability remain difficult to extract from quantum mechanical calculations. Recently, we outlined a quantitative yet visual and intuitive approach to the theoretical analysis of Atomic Size in periodic structures: the DFT-Chemical Pressure (DFT-CP) analysis. In this Article, we describe the methodological details of this DFT-CP procedure, with a particular emphasis on refinements of the method to make it useful for a wider variety of systems. A central improvement is a new integration scheme with broader applicability than our earlier Voronoi cell method: contact volume space-partitioning. In this approach, we make explicit our assumption that the pressure at each voxel is most strongly influenced by its two closest atoms. The unit cell is divided i...
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dft chemical pressure analysis visualizing the role of Atomic Size in shaping the structures of inorganic materials
Journal of the American Chemical Society, 2012Co-Authors: Daniel C FredricksonAbstract:Atomic Size effects have long played a role in our empirical understanding of inorganic crystal structures. At the level of electronic structure calculations, however, the contribution of Atomic Size remains difficult to analyze, both alone and relative to other influences. In this paper, we extend the concepts outlined in a recent communication to develop a theoretical method for revealing the impact of the space requirements of atoms: the density functional theory-chemical pressure (DFT-CP) analysis. The influence of Atomic Size is most pronounced when the optimization of bonding contacts is impeded by steric repulsion at other contacts, resulting in nonideal interAtomic distances. Such contacts are associated with chemical pressures (CPs) acting upon the atoms involved. The DFT-CP analysis allows for the calculation and interpretation of the CP distributions within crystal structures using DFT results. The method is demonstrated using the stability of the Ca2Ag7 structure over the simpler CaCu5-type al...
J. Cuevas - One of the best experts on this subject based on the ideXlab platform.
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thermal conductance of metallic Atomic Size contacts phonon transport and wiedemann franz law
Physical Review B, 2017Co-Authors: Jan C Klockner, Manuel Matt, Peter Nielaba, Fabian Pauly, J. CuevasAbstract:Motivated by recent experiments [Science 355, 6330 (2017); Nat. Nanotechnol. 12, 430 (2017)], we present here an extensive theoretical analysis of the thermal conductance of Atomic-Size contacts made of three different metals, namely gold (Au), platinum (Pt) and aluminum (Al).
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orbital origin of the electrical conduction in ferromagnetic Atomic Size contacts insights from shot noise measurements and theoretical simulations
Physical Review B, 2016Co-Authors: Ran Vardimon, Peter Nielaba, Manuel Matt, J. CuevasAbstract:With the goal to elucidate the nature of spin-dependent electronic transport in ferromagnetic Atomic contacts, we present here a combined experimental and theoretical study of the conductance and shot noise of metallic Atomic contacts made of the 3d ferromagnetic materials Fe, Co, and Ni. For comparison, we also present the corresponding results for the noble metal Cu. Conductance and shot noise measurements, performed using a low-temperature break junction setup, show that in these ferromagnetic nanowires: (i) there is no conductance quantization of any kind, (ii) transport is dominated by several partially-open conduction channels, even in the case of single-atom contacts, and (iii) the Fano factor of large contacts saturates to values that clearly differs from those of monovalent (nonmagnetic) metals. We rationalize these observations with the help of a theoretical approach that combines molecular dynamics simulations to describe the junction formation with nonequilibrium Green's function techniques to compute the transport properties within the Landauer-Buettiker framework. Our theoretical approach successfully reproduces all the basic experimental results and it shows that all the observations can be traced back to the fact that the d bands of the minority-spin electrons play a fundamental role in the transport through ferromagnetic Atomic-Size contacts. These d bands give rise to partially open conduction channels for any contact Size, which in turn lead naturally to the different observations described above. Thus, the transport picture for these nanoscale ferromagnetic wires that emerges from the ensemble of our results is clearly at variance with the well established conduction mechanism that governs the transport in macroscopic ferromagnetic wires, where the d bands are responsible for the magnetism but do not take part in the charge flow.
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quantum thermopower of metallic Atomic Size contacts at room temperature
Nano Letters, 2015Co-Authors: Charalambos Evangeli, J. Cuevas, Manuel Matt, Peter Nielaba, Fabian Pauly, Laura Rincongarcia, Gabino Rubiobollinger, Nicolás AgraïtAbstract:We report conductance and thermopower measurements of metallic Atomic-Size contacts, namely gold and platinum, using a scanning tunneling microscope (STM) at room temperature. We find that few-atom gold contacts have an average negative thermopower, whereas platinum contacts present a positive thermopower, showing that for both metals, the sign of the thermopower in the nanoscale differs from that of bulk wires. We also find that the magnitude of the thermopower exhibits minima at the maxima of the conductance histogram in the case of gold nanocontacts while for platinum it presents large fluctuations. Tight-binding calculations and Green’s function techniques, together with molecular dynamics simulations, show that these observations can be understood in the context of the Landauer–Buttiker picture of coherent transport in Atomic-scale wires. In particular, we show that the differences in the thermopower between these two metals are due to the fact that the elastic transport is dominated by the 6s orbita...
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Transport properties of normal and ferromagnetic Atomic-Size constrictions with superconducting electrodes
Physica C-superconductivity and Its Applications, 2001Co-Authors: A. Martín-rodero, A. Levy Yeyati, J. CuevasAbstract:Abstract We present a theoretical approach to the transport properties of normal (N) and ferromagnetic (F) Atomic-Size contacts between superconducting (S) electrodes. This approach is based on non-equilibrium Green function techniques and a Hamiltonian description of the electron transfer processes in the constriction region. For a normal Atomic-Size contact the theory allows to relate the conduction channels to the Atomic orbital structure. In the ferromagnetic case spin polarization gives rise to a spin dependent transmission distribution. For the case of S–N–S constrictions this theory predicts a d.c. current and shot noise in remarkable agreement with recent experimental results. We also analyze S–F and S–F–S constrictions. In this last case the current–voltage characteristic exhibits a very peculiar subgap structure associated to the competition between the multiple Andreev reflection mechanism and the ferromagnetic spin polarization.
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microscopic origin of conducting channels in metallic Atomic Size contacts
Physical Review Letters, 1998Co-Authors: J. Cuevas, Levy A Yeyati, A MartinroderoAbstract: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.
J. M. Van Ruitenbeek - One of the best experts on this subject based on the ideXlab platform.
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Atomic Size oscillations in conductance histograms for gold nanowires and the influence of work hardening
Physical Review Letters, 2005Co-Authors: I K Yanson, O I Shklyarevskii, Sz Csonka, H Van Kempen, S Speller, A I Yanson, J. M. Van RuitenbeekAbstract:Nanowires of different nature have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for gold wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials and we discover a new set of periodic structures, which we interpret as due to the Atomic discreteness of the contact Size for the three principal crystal orientations.
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conductance fluctuations as a tool for investigating the quantum modes in Atomic Size metallic contacts
Physical Review B, 2000Co-Authors: B. Ludoph, J. M. Van RuitenbeekAbstract:Recently it has been observed that the conductance fluctuations of Atomic-Size gold contacts are suppressed when the conductance is equal to an integer multiple of the conductance quantum. The fact that these contacts tend to consist exclusively of fully open or closed modes has been argued to be the origin for this suppression. Here the experiments have been extended to a wide range of metallic elements with different chemical valences, and they provide information about the relation between the mode composition and statistically preferred conductance values observed in conductance histograms.
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THERMOPOWER OF Atomic-Size METALLIC CONTACTS
Physical Review B, 1999Co-Authors: B. Ludoph, J. M. Van RuitenbeekAbstract: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.
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evidence for saturation of channel transmission from conductance fluctuations in Atomic Size point contacts
Physical Review Letters, 1999Co-Authors: B. Ludoph, M H Devoret, D Esteve, C Urbina, J. M. Van RuitenbeekAbstract:Metallic contacts consisting of only a few atoms can be obtained using scanning tunneling microscopy or mechanically controllable break junction [1] techniques. The electrical conductance through such contacts is described in terms of electronic wave modes by the Landauer-Buttiker formalism [2]. Each of the N modes forms a channel for the conductance, with a transmission probability Tn between 0 and 1. The total conductance is given by the sum over these channels G › P N›1 TnG0, where G0 › 2e 2 yh is the quantum of conductance. By recording histograms of conductance values [3] for contacts of simple metals (Na, Au), a statistical preference was observed for conductances near integer values. This statistical preference was interpreted as an indication that transmitted modes in the most probable contacts are completely opened (Tn › 1, i.e., saturation of channel transmission), in analogy with the conductance quantization observed in 2D electron gas devices [4]. Here, we test this interpretation by performing a new type of measurement giving access to the second moment of the distribution of the Tn’s. The Atomic contacts are formed by breaking a gold wire at low temperatures, and then finely adjusting the Size of the contact between the fresh fracture surfaces using a piezoelectric element [1]. Figure 1 shows the differential conductance, ›Iy›V measured as a function of bias voltage for three Atomic-Size contacts with different conductance values, using a modulation voltage eV ? kBu (with u the temperature). For each contact, both of the curves for increasing and decreasing bias voltage are given. Measurements such as those of Fig. 1 suggest that the fluctuation pattern changes randomly between contact configurations and that the amplitude of the fluctuations is suppressed for conductance values near G0. In order to establish such a relation, it is necessary to statistically average over a large number of contacts. We do this by measuring the voltage dependence of the conductance s›Gy›V › › 2 Iy›V 2 d and
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adjustable nanofabricated Atomic Size contacts
Review of Scientific Instruments, 1996Co-Authors: J. M. Van Ruitenbeek, P. Joyez, A Alvarez, I Pineyro, C Grahmann, M H Devoret, D Esteve, C UrbinaAbstract:Metallic point contacts and tunnel junctions with a small and adjustable number of conduction channels have been obtained in the last few years using scanning tunneling microscope and break junction techniques. For conventional break junctions, the reported drift of the interelectrode spacing in the tunnel regime is typically of the order of 0.5 pm/min (1 pm=10−12 m). We have nanofabricated break junctions which display a drift smaller than 0.2 pm/h. The improvement results from the scaling down by two orders of magnitude of the device dimensions. We describe the nanofabrication process, which can be adapted to most metals. We have performed measurements on Al, Cu, and Nb devices. The results illustrate the ability of the technique to explore phenomenalike conductance quantization and two level fluctuations. These new adjustable Atomic Size contacts and tunnel junctions can be integrated in complex circuits.
Elke Scheer - One of the best experts on this subject based on the ideXlab platform.
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Magnetotransport in Atomic-Size bismuth contacts
Journal of Physics: Condensed Matter, 2014Co-Authors: Hans-fridtjof Pernau, Torsten Pietsch, Elke ScheerAbstract:We report low-temperature transport experiments on Atomic-Size contacts of bismuth that are fabricated using the mechanically controlled break-junction technique at low temperatures. We observe stable contacts with conductance values at fractions of one conductance quantum G0 = 2e 2 /h, as is expected for systems with long Fermi wavelength. We defer two preferred conductance scales: the lower one is in the order of 0.015 G0 and can be attributed to singleatom Bi contact, while the higher one amounts to 0.15 G0, as indicated by the appearance of multiples of this value in the conductance histogram. Rich magneto-transport behaviour with significant changes in the magneto-conductance is found in the whole conductance range. Although for the pristine samples and large contacts with G > 5 G0, indications for Shubnikov-de Haas oscillations are present, the smallest contacts show pronounced conductance fluctuations that decay rapidly when a magnetic field is applied. Moreover, large variations are observed when a finite bias voltage is applied. These findings are interpreted as the transition from the diffusive to the ballistic and the ultra-quantum regime when lowering the contact Size.
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Transmission of surface plasmon polaritons through Atomic-Size constrictions
New Journal of Physics, 2013Co-Authors: Daniel Benner, Johannes Boneberg, Philipp Nürnberger, Golaleh Ghafoori, Paul Leiderer, Elke ScheerAbstract:We study the excitation and propagation of surface plasmon polaritons (SPPs) on a micron-Sized thin gold stripe. Grating couplers are embedded for both excitation of SPPs and detection after propagation. The experimental setup allows measuring of the decay length of the SPPs on the gold stripe excited in the near infrared part of the electromagnetic spectrum. We show that SPPs are transmitted with a surprisingly high probability across a tapered constriction, with smallest lateral dimensions of Atomic Size.
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magnetoresistance of Atomic Size contacts realized with mechanically controllable break junctions
Physical Review B, 2010Co-Authors: Stefan Egle, Hans-fridtjof Pernau, Cecile Bacca, Magdalena Huefner, Denise Hinzke, Ulrich Nowak, Elke ScheerAbstract:We present a comprehensive study of the conductance behavior of Atomic-Size contacts made of ferromagnetic metals (Co) or noble metals (Au) with ferromagnetic electrodes (Co). In order to separate the influence of the large electrodes from the influence of the contacts themselves, we used different sample geometries. These include combinations of nonmagnetic electrodes connected to magnetic bridges and vice versa as well as different orientations of the magnetic field. The magnetoresistance (MR) curves show very rich behavior with strong MR ratios (MRR). In all geometries the MRR values are of comparable Size, reaching up to a few thousand percent in the tunneling regime. We study the possible influence of the micromagnetic order of the domains in the vicinity of the contact as well as ballistic MR, giant MR, tunnel MR, Atomically enhanced anisotropic MR (AAMR), and magnetostriction. We conclude that AAMR is the most important origin for the MR at high magnetic fields $(|B|g2\text{ }\text{T})$, while magnetostriction, tunnel MR, and giant MR govern the low-field regime $(|B|l2\text{ }\text{T})$.
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influence of laser light on electronic transport through Atomic Size contacts
Physical Review Letters, 2007Co-Authors: Daniel C Guhr, Johannes Boneberg, Paul Leiderer, Dennis Rettinger, Artur Erbe, Elke ScheerAbstract:This Letter reports on the influence of laser irradiation onto the electrical conductance of gold nanocontacts established with the mechanically controllable break-junction technique. We concentrate on the study of reversible conductance changes which can be as high as 200%. We investigate the dependence on the initial conductance of the contacts, and on the wavelength, the intensity, and the position of the laser spot with respect to the sample. Under most conditions an enhancement of the conductance is observed. Several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification, plasmon excitation, and photon-assisted transport are discussed, among which the two latter ones are most likely the dominating ones.
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conduction channel transmissions of Atomic Size aluminum contacts
Physical Review Letters, 1997Co-Authors: Elke Scheer, P. Joyez, D Esteve, C Urbina, M H DevoretAbstract:The isV , td curves present a series of sharp current steps at voltage values V › 2Dyne, where n is a positive integer and D is the superconducting gap. The well-known process of single quasiparticle transport corresponds to n › 1. The common phenomenon behind the other steps is multiple Andreev reflection (MAR) of quasiparticles between the two superconducting banks [14]. The order n › 2, 3, . . . , of a step corresponds to the number of electronic charges transferred in the underlying MAR process. As the transmission of the channel rises from 0 to 1, the higher order processes grow stronger and the subgap current increases progressively. This so-called “subharmonic gap structure” has already been observed in superconducting weak links and tunnel junctions with a very large number of channels [15,16]. Measurements [17] of the current-voltage characteristic ( IV ) of Nb and Pb single channel tunnel junctions with an adjustable transmission t1 have shown that the height of the successive current steps is proportional to increasing powers of t1 ,i n FIG. 1. Measured current-voltage characteristics (symbols) of four different configurations of sample #1 at 30 mK and best numerical fits (lines). The individual channel transmissions and total transmission T obtained from the fits are (a) t1 › 0.997, t2 › 0.46, t3 › 0.29, T › 1.747; (b) t1 › 0.74, t2 › 0.11, T › 0.85; (c) t1 › 0.46, t2 › 0.35, t3 › 0.07, T › 0.88; and (d) T › t1 › 0.025. Voltage and current are in reduced units. The measured superconducting gap was Dye › 182.5 6 2.0 mV. Left inset: Theoretical IVs for a single channel superconducting contact for different values of its transmission coefficient t (from bottom to top: 0.1, 0.4, 0.7, 0.9, 0.99, 1) after [12]. Right inset: Typical total transmission traces measured at V $ 5 Dye, while opening the contact at around 6 pmys, for samples #1 and #2. The bar indicates the distance scale.
D. Ugarte - One of the best experts on this subject based on the ideXlab platform.
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Correlation between Quantum Conductance and Atomic Arrangement of Silver Atomic-Size Nanowires
arXiv: Materials Science, 2012Co-Authors: Miguel Lagos, Douglas S Galvao, Pedro Alves Da Silva Autreto, D. UgarteAbstract:We have studied the effect of thermal effects on the structural and transport response of Ag Atomic-Size nanowires generated by mechanical elongation. Our study involves both time-resolved Atomic resolution transmission electron microscopy imaging and quantum conductance measurement using an ultra-high-vacuum mechanically controllable break junction. We have observed drastic changes in conductance and structural properties of Ag nanowires generated at different temperatures (150 and 300 K). By combining electron microscopy images, electronic transport measurements and quantum transport calculations, we have been able to obtain a consistent correlation between the conductance and structural properties of Ag NWs. In particular, our study has revealed the formation of metastable rectangular rod-like Ag wire (3/3) along the (001) crystallographic direction, whose formation is enhanced. These results illustrate the high complexity of analyzing structural and quantum conductance behaviour of metal Atomic-Size wires; also, they reveal that it is extremely difficult to compare NW conductance experiments performed at different temperatures due to the fundamental modifications of the mechanical behavior.
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Low temperature (LN2) and UHV mechanically controllable break junction setup to study quantum electrical transport of Atomic-Size metal nanowire
Journal of Physics: Conference Series, 2008Co-Authors: Vinicius Rodrigues, Miguel Lagos, D. UgarteAbstract:Reliable metal nanowire studies requires experimental stringent conditions, as clean samples and environment. In this sense, we have designed and built a dedicated instrument to study electrical transport properties of Atomic-Size metal contacts based on the mechanically controlled break junction technique, operating at ultra-high-vacuum conditions. Here we describe the chosen setup, its implementation and performance. © 2008 IOP Publishing Ltd.
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Elongation of Atomic-Size Wires: Atomistic Aspects and Quantum Conductance Studies
EMC 2008 14th European Microscopy Congress 1–5 September 2008 Aachen Germany, 2008Co-Authors: Miguel Lagos, Varlei Rodrigues, D. UgarteAbstract:The study of Atomic-Size metal nanowires (NW’s) is attracting a great interest due to occurrence a novel physical and chemical phenomena. Among these new phenomena, we can mention conductance quantization that will certainly influence the design of nanodevices. NW’s are usually generated by mechanical deformation and the conductance is measured during the wire elongation. The interpretation of the results is troublesome, because conductance is measured during the modification of the Atomic structure. This kind of experimental study has been performed by many research groups and, a quite wide range of temperatures (4 – 300 K) and vacuum conditions have been used (from ambient to UHV). In fact, the results display significant variation, what has generated several controversial interpretations. It must be emphaSized that many models have been derived without taking into account that the NW structural deformation should be significantly dependent on temperature.
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Real-time Atomic resolution study of metal nanowires
Applied Physics A, 2005Co-Authors: J Bettini, J C Gonzalez, V. Rodrigues, D. UgarteAbstract:The conductance of Atomic-Size wires generated by mechanical stretching is determined by the preferred Atomic structure, and atomistic descriptions are essential to interpret the quantum transport behavior of metal nanostructures. Here, we present a thorough analysis of real-time in situ high resolution transmission electron microscopy studies of Atomic-Size metal nanowires (NWs). By associating careful image processing with image simulation, we have been able to derive detailed information of the Atomic arrangement evolution of rod-like NWs. This shows that the Atomic resolution imaging can provide quantitative information of the Atomic-Size NW structure.
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indication of unusual pentagonal structures in Atomic Size cu nanowires
Physical Review Letters, 2004Co-Authors: J C Gonzalez, Varlei Rodrigues, J Bettini, Luis G C Rego, A R Rocha, P Z Coura, S O Dantas, Fernando Sato, Douglas S Galvao, D. UgarteAbstract:We present a study of the structural and quantum conductance properties of Atomic-Size copper nanowires generated by mechanical stretching. The atomistic evolution was derived from time-resolved electron microscopy observations and molecular dynamics simulations. We have analyzed the quantum transport behavior by means of conductance measurements and theoretical calculations. The results suggest the formation of an unusual and highly stable pentagonal Cu nanowire with a diameter of $\ensuremath{\sim}0.45\text{ }\text{ }\mathrm{n}\mathrm{m}$ and $\ensuremath{\sim}4.5$ conductance quanta.