Vibrational assignments and line shapes in inelastic tunnelling spectroscopy: H on Cu(100)

We have carried out a computational study of the inelastic electron tunneling spectrum (IETS) of the two vibrational modes of a single hydrogen atom on a Cu(100) surface in a scanning tunneling microscopy (STM) junction. This study addresses key issues about vibrational assignment and line shape of observed peaks in IETS within the framework of density functional theory calculations and the Lorente-Persson theory for STM-IETS. We argue that the observation of only a single, broad peak in the STM-IETS [L.J. Lauhon and W. Ho, Phys. Rev. Lett. 85, 4566 (2000)] is not caused by any symmetry restrictions or any cancellation between inelastic and elastic vibrational contributions for one of the two modes but is due to strongly overlapping superposition of the contributions from the two modes caused by the rather large instrumental broadening and the narrow vibrational energy separation between the modes. In particular, we find that this broadening and the large asymmetry of the vibrational line shapes gives rise to substantial apparent vibrational energy shifts of the two modes and decrease their apparent energy separation

Detection of exchange interaction in STM measurements through Fanolike interference effects

We address Fano-like interference effects in scanning tunneling microscopy (STM) measurements of nanoscale systems, e.g. two-level systems. Common for these systems is that second order tunneling contributions give rise to interference effects that cause suppressed transmission through the system for certain energies. The suppressed transmission is measurable either in the differential conductance or in the bias voltage derivative thereof.Comment: 9 pages, 3 figures, submitted: in addition to the results published in Phys. Rev. B, 75, 153309 (2007), this paper contains a more thorough discussion on the used transport formalism, studies of asymmetric couplings to the substrate, and discussion of non-resonant levels. The non-resonant case is related to spin-dependent tunnelin

Inelastic effects in electron transport studied with wave packet propagation

A time-dependent approach is used to explore inelastic effects during electron transport through few-level systems. We study a tight-binding chain with one and two sites connected to vibrations. This simple but transparent model gives insight about inelastic effects, their meaning and the approximations currently used to treat them. Our time-dependent approach allows us to trace back the time sequence of vibrational excitation and electronic interference, the ibrationally introduced time delay and the electronic phase shift. We explore a full range of parameters going from weak to strong electron-vibration coupling, from tunneling to contact, from one-vibration description to the need of including all vibrations for a correct description of inelastic effects in transport. We explore the validity of single-site resonant models as well as its extension to more sites via molecular orbitals and the conditions under which multi-orbital, multi-vibrational descriptions cannot be simplified. We explain the physical meaning of the spectral features in the second derivative of the electron current with respect to the bias voltage. This permits us to nuance the meaning of the energy value of dips and peaks. Finally, we show that finite-band effects lead to electron back-scattering off the molecular vibrations in the regime of high-conductance, although the drop in conductance at the vibrational threshold is rather due to the rapid variation of the vibronic density of states.Comment: 38 pages, 14 figure

Inelastic transport theory from first-principles: methodology and applications for nanoscale devices

We describe a first-principles method for calculating electronic structure, vibrational modes and frequencies, electron-phonon couplings, and inelastic electron transport properties of an atomic-scale device bridging two metallic contacts under nonequilibrium conditions. The method extends the density-functional codes SIESTA and TranSIESTA that use atomic basis sets. The inelastic conductance characteristics are calculated using the nonequilibrium Green's function formalism, and the electron-phonon interaction is addressed with perturbation theory up to the level of the self-consistent Born approximation. While these calculations often are computationally demanding, we show how they can be approximated by a simple and efficient lowest order expansion. Our method also addresses effects of energy dissipation and local heating of the junction via detailed calculations of the power flow. We demonstrate the developed procedures by considering inelastic transport through atomic gold wires of various lengths, thereby extending the results presented in [Frederiksen et al., Phys. Rev. Lett. 93, 256601 (2004)]. To illustrate that the method applies more generally to molecular devices, we also calculate the inelastic current through different hydrocarbon molecules between gold electrodes. Both for the wires and the molecules our theory is in quantitative agreement with experiments, and characterizes the system-specific mode selectivity and local heating.Comment: 24 pages, 17 figure

Relation between inelastic electron tunneling and vibrational excitation of single adsorbates on metal surfaces

We analyse theoretically a relation between the vibrational generation rate of a single adsorbate by tunneling electrons and the inelastic tunneling (IET) current in scanning tunneling microscope, and the influence of the vibrational excitations on the rate of adsorbate motions. Special attention is paid to the effects of finite lifetime of the vibrational excitations. We show that in the vicinity and below the IET threshold the rate of adsorbate motion deviates from a simple power-law dependence on the bias voltage due to the effects of bath temperature and adsorbate vibrational lifetime broadenings. The temperature broadening appears to be confined near the threshold voltage within a narrow region of several $k_B T$, whereas the lifetime broadening manifests itself in a much wider region of applied voltages below the IET threshold.Comment: 8 pages including 4 figure

Nonequilibrium resonant spectroscopy of molecular vibrons

Quantum transport through single molecules is essentially affected by molecular vibrations. We investigate the behavior of the prototype single-level model with intermediate electron-vibron coupling and arbitrary coupling to the leads. We have developed a theory which allows to explore this regime via the nonequilibrium Green function formalism. We show that the nonequilibrium resonant spectroscopy is able to determine the energies of molecular orbitals and the spectrum of molecular vibrations. Our results are relevant to scanning tunneling spectroscopy experiments, and demonstrate the importance of the systematic and self-consistent investigation of the effects of the vibronic dynamics onto the transport through single molecules.Comment: 4 pages, 5 figures, submitte

Identifying and addressing barriers to treatment for child sexual abuse survivors and their non-offending caregivers

Mental health treatment is a critical part of an effective and compassionate response to the disclosure of child sexual abuse (CSA). Given the vast negative consequences for children and families following CSA, engagement in treatment can benefit youth and their non-offending caregivers. Yet, these families face unique barriers to treatment initiation, adherence, and effectiveness. The identification of these barriers allows clinicians, researchers, and policy makers to increase treatment utilization, engagement, and value. The current review and its recommendations derive from the existing literature combined with knowledge gained from a clinical research team with more than 20 years of experience offering a treatment program for CSA survivors and their non-offending family members. The review organizes barriers around factors related to individual characteristics of children and caregivers, perceptions and beliefs commonly held following CSA, and challenging family interactions in the context of individual and group treatment for CSA. Finally, barriers related to systemic and societal factors are examined given the importance of understanding the legal and cultural context in which families seek and engage in treatment. Recommendations for further research, suggestions for clinicians, and considerations for policy change to decrease the identified treatment barriers for families impacted by CSA are provided

Perturbation Study of the Conductance through an Interacting Region Connected to Multi-Mode Leads

We study the effects of electron correlation on transport through an interacting region connected to multi-mode leads based on the perturbation expansion with respect to the inter-electron interaction. At zero temperature the conductance defined in the Kubo formalism can be written in terms of a single-particle Green's function at the Fermi energy, and it can be mapped onto a transmission coefficient of the free quasiparticles described by an effective Hamiltonian. We apply this formulation to a two-dimensional Hubbard model of finite size connected to two noninteracting leads. We calculate the conductance in the electron-hole symmetric case using the order $U^2$ self-energy. The conductance shows several maximums in the $U$ dependence in some parameter regions of $t_y/t_x$, where $t_x$ ($t_y$) is the hopping matrix element in the $x$- ($y$-) directions. This is caused by the resonance occurring in some of the subbands, and is related with the $U$ dependence of the eigenvalues of the effective Hamiltonian.Comment: 17 pages, 12 figures, to be published in J.Phys.Soc.Jpn. 71(2002)No.

Nonlinear climatic sensitivity to greenhouse gases over past 4 glacial/interglacial cycles.

The paleoclimatic sensitivity to atmospheric greenhouse gases (GHGs) has recently been suggested to be nonlinear, however a GHG threshold value associated with deglaciation remains uncertain. Here, we combine a new sea surface temperature record spanning the last 360,000 years from the southern Western Pacific Warm Pool with records from five previous studies in the equatorial Pacific to document the nonlinear relationship between climatic sensitivity and GHG levels over the past four glacial/interglacial cycles. The sensitivity of the responses to GHG concentrations rises dramatically by a factor of 2-4 at atmospheric CO2 levels of >220 ppm. Our results suggest that the equatorial Pacific acts as a nonlinear amplifier that allows global climate to transition from deglacial to full interglacial conditions once atmospheric CO2 levels reach threshold levels

Evolution of supermassive black holes

Supermassive black holes (SMBHs) are nowadays believed to reside in most local galaxies, and the available data show an empirical correlation between bulge luminosity - or stellar velocity dispersion - and black hole mass, suggesting a single mechanism for assembling black holes and forming spheroids in galaxy halos. The evidence is therefore in favour of a co-evolution between galaxies, black holes and quasars. In cold dark matter cosmogonies, small-mass subgalactic systems form first to merge later into larger and larger structures. In this paradigm galaxy halos experience multiple mergers during their lifetime. If every galaxy with a bulge hosts a SMBH in its center, and a local galaxy has been made up by multiple mergers, then a black hole binary is a natural evolutionary stage. The evolution of the supermassive black hole population clearly has to be investigated taking into account both the cosmological framework and the dynamical evolution of SMBHs and their hosts. The seeds of SMBHs have to be looked for in the early Universe, as very luminous quasars are detected up to redshift higher than z=6. These black holes evolve then in a hierarchical fashion, following the merger hierarchy of their host halos. Accretion of gas, traced by quasar activity, plays a fundamental role in determining the two parameters defining a black hole: mass and spin. A particularly intriguing epoch is the initial phase of SMBH growth. It is very challenging to meet the observational constraints at z=6 if BHs are not fed at very high rates in their infancy.Comment: Extended version of the invited paper to appear in the Proceedings of the Conference "Relativistic Astrophysics and Cosmology - Einstein's Legacy