Discrete breathers for understanding reconstructive mineral processes at low temperatures

Reconstructive transformations in layered silicates need a high tem- perature in order to be observed. However, very recently, some systems have been found where transformation can be studied at temperatures 600 C below the lowest experimental results previously reported, including sol-gel methods. We explore the possible relation with the existence of intrinsic localized modes, known as discrete breathers. We construct a model for nonlinear vibrations within the cation layer, obtain their parameters and calculate them numerically, obtaining their energies. Their statistics shows that although there are far less breathers than phonons, there are much more above the activation energy, being therefore a good candidate to explain the reconstructive transformations at low temperature.Comment: 27 pages, 11 figure

Breathers and kinks in a simulated crystal experiment

We develop a simple 1D model for the scattering of an incoming particle hitting the surface of mica crystal, the transmission of energy through the crystal by a localized mode, and the ejection of atom(s) at the incident or distant face. This is the first attempt to model the experiment described in Russell and Eilbeck in 2007 (EPL, v. 78, 10004). Although very basic, the model shows many interesting features, for example a complicated energy dependent transition between breather modes and a kink mode, and multiple ejections at both incoming and distant surfaces. In addition, the effect of a heavier surface layer is modelled, which can lead to internal reflections of breathers or kinks at the crystal surface.Comment: 15 pages, 12 figures, based on a talk given at the conference "Localized Excitations in Nonlinear Complex Systems (LENCOS)", Sevilla (Spain) July 14-17, 200

Role of electronic structure in photoassisted transport through atomic-sized contacts

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.Comment: 7 pages, 7 figures; four new figures adde

Vibrational Instabilities in Resonant Electron Transport through Single-Molecule Junctions

We analyze various limits of vibrationally coupled resonant electron transport in single-molecule junctions. Based on a master equation approach, we discuss analytic and numerical results for junctions under a high bias voltage or weak electronic-vibrational coupling. It is shown that in these limits the vibrational excitation of the molecular bridge increases indefinitely, i.e. the junction exhibits a vibrational instability. Moreover, our analysis provides analytic results for the vibrational distribution function and reveals that these vibrational instabilities are related to electron-hole pair creation processes.Comment: 19 pages, 3 figure

Molecular dynamics study of the thermopower of Ag, Au, and Pt nanocontacts

Using molecular dynamics simulations of many junction stretching processes we analyze the thermopower of silver (Ag), gold (Au), and platinum (Pt) atomic contacts. In all cases we observe that the thermopower vanishes on average within the standard deviation and that its fluctuations increase for decreasing minimum cross-section of the junctions. However, we find a suppression of the fluctuations of the thermopower for the s-valent metals Ag and Au, when the conductance originates from a single, perfectly transmitting channel. Essential features of the experimental results for Au, Ag, and copper (Cu) of Ludoph and van Ruitenbeek [Phys. Rev. B 59, 12290 (1999)], as yet unaddressed by atomistic studies, can hence be explained by considering the atomic and electronic structure at the disordered narrowest constriction of the contacts. For the multivalent metal Pt our calculations predict the fluctuations of the thermopower to be larger by one order of magnitude as compared to Ag and Au, and suppressions of the fluctuations as a function of the conductance are absent.Comment: 13 pages, 10 figure

Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current

Quantum interference effects and decoherence mechanisms in single-molecule junctions are analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasi-degenerate states of a nanoscale molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. As a result, the electrical current can be significantly larger than without electronic-vibrational coupling. The analysis reveals that the quenching of quantum interference is particularly pronounced if the junction is vibrationally highly excited, e.g. due to current-induced nonequilibrium effects in the resonant transport regime.Comment: 11 pages, 4 figure

Effect of the Introduction of Impurities on the Stability Properties of Multibreathers at Low Coupling

sing a theorem dubbed the {\em Multibreather Stabiliy Theorem} [Physica D 180 (2003) 235-255] we have obtained the stability properties of multibreathers in systems of coupled oscillators with on-site potentials, with an inhomogeneity. Analytical results are obtained for 2-site, 3-site breathers, multibreathers, phonobreathers and dark breathers. The inhomogeneity is considered both at the on-site potential and at the coupling terms. All the results have been checked numerically with excellent agreement. The main conclusion is that the introduction of a impurity does not alter the stability properties.Comment: 20 pages, 9 figure

Thermal conductance of metallic atomic-size contacts: Phonon transport and Wiedemann-Franz law

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)