Non-adiabatic electronic relaxation of tetracene from its brightest singlet excited state.

The ultrafast relaxation dynamics of tetracene following UV excitation to the bright singlet state S6 has been studied with time-resolved photoelectron spectroscopy. With the help of high-level ab initio multireference perturbation theory calculations, we assign photoelectron signals to intermediate dark electronic states S3, S4, and S5 as well as to a low-lying electronic state S2. The energetic structure of these dark states has not been determined experimentally previously. The time-dependent photoelectron yields assigned to the states S6, S5, and S4 have been analyzed and reveal the depopulation of S6 within 60 fs, while S5 and S4 are populated with delays of about 50 and 80 fs. The dynamics of the lower-lying states S3 and S2 seem to agree with a delayed population coinciding with the depopulation of the higher-lying states S4-S6 but could not be elucidated in full detail due to the low signal levels of the corresponding two-photon ionization probe processes.

Efficient Steady-State Solver for the Hierarchical Equations of Motion Approach: Formulation and Application to Charge Transport through Nanosystems.

An iterative approach is introduced, which allows the efficient solution of the hierarchical equations of motion (HEOM) for the steady-state of open quantum systems. The approach combines the method of matrix equations with an efficient preconditioning technique to reduce the numerical effort of solving the HEOM. Illustrative applications to simulate nonequilibrium charge transport in single-molecule junctions demonstrate the performance of the method.

Hierarchical quantum master equation approach to vibronic reaction dynamics at metal surfaces.

A novel quantum dynamical method to simulate vibronic reaction dynamics in molecules at metal surfaces is proposed. The method is based on the hierarchical quantum master equation approach and uses a discrete variable representation of the nuclear degrees of freedom in combination with complex absorbing potentials and an auxiliary source term. It provides numerically exact results for a range of models. By taking the coupling to the continuum of electronic states of the surface properly into account, nonadiabatic processes can be described and the effect of electronic friction is included in a nonperturbative and non-Markovian way. Illustrative applications to models for desorption of a molecule at a surface and the current-induced bond rupture in single-molecule junctions demonstrate the performance and versatility of the method.

Extending the hierarchical quantum master equation approach to low temperatures and realistic band structures.

The hierarchical quantum master equation (HQME) approach is an accurate method to describe quantum transport in interacting nanosystems. It generalizes perturbative master equation approaches by including higher-order contributions as well as non-Markovian memory and allows for the systematic convergence to the numerically exact result. As the HQME method relies on a decomposition of the bath correlation function in terms of exponentials, however, its application to systems at low temperatures coupled to baths with complexer band structures has been a challenge. In this publication, we outline an extension of the HQME approach, which uses re-summation over poles and can be applied to calculate transient currents at a numerical cost that is independent of temperature and band structure of the baths. We demonstrate the performance of the extended HQME approach for noninteracting tight-binding model systems of increasing complexity as well as for the spinless Anderson-Holstein model.

Dynamical simulation of electron transfer processes in self-assembled monolayers at metal surfaces using a density matrix approach.

A single-particle density matrix approach is introduced to simulate the dynamics of heterogeneous electron transfer (ET) processes at interfaces. The characterization of the systems is based on a model Hamiltonian parametrized by electronic structure calculations and a partitioning method. The method is applied to investigate ET in a series of nitrile-substituted (poly)(p-phenylene)thiolate self-assembled monolayers adsorbed at the Au(111) surface. The results show a significant dependence of the ET on the orbital symmetry of the donor state and on the molecular and electronic structure of the spacer.

Controlling the Conductance of a Graphene-Molecule Nanojunction by Proton Transfer.

The possibility of using single molecule junctions as components of nanoelectronic devices has motivated intensive experimental and theoretical research on the underlying transport mechanism in these systems. In this Letter, we investigate from a theoretical perspective intramolecular proton transfer reactions as a mechanism for controlling the conductance state of graphene-based molecular junctions. Employing a methodology that combines first-principles electronic structure methods with transport approaches, we show that the proton transfer reaction proceeds via a stepwise mechanism, giving rise to several tautomers with different conductance states. The analysis reveals that the relative stability of the tautomers as well as the energy barrier for their interconversion can be controlled by means of an external electrostatic field, which provides a mechanism for switching the nanojunction.

Does multiple paternity influence offspring disease resistance?

It has been suggested that polyandry allows females to increase offspring genetic diversity and reduce the prevalence and susceptibility of their offspring to infectious diseases. We tested this hypothesis in wild-derived house mice (Mus musculus) by experimentally infecting the offspring from 15 single- and 15 multiple-sired litters with two different strains of a mouse pathogen (Salmonella Typhimurium) and compared their ability to control infection. We found a high variation in individual infection resistance (measured with pathogen loads) and significant differences among families, suggesting genetic effects on Salmonella resistance, but we found no difference in prevalence or infection resistance between single- vs. multiple-sired litters. We found a significant sex difference in infection resistance, but surprisingly, males were more resistant to infection than females. Also, infection resistance was correlated with weight loss during infection, although only for females, indicating that susceptibility to infection had more harmful health consequences for females than for males. To our knowledge, our findings provide the first evidence for sex-dependent resistance to Salmonella infection in house mice. Our results do not support the hypothesis that multiple-sired litters are more likely to survive infection than single-sired litters; however, as we explain, additional studies are required before ruling out this hypothesis.

Modeling charge transport in C60-based self-assembled monolayers for applications in field-effect transistors.

We have investigated the conductance properties of C60-containing self-assembled monolayers (SAMs), which are used in organic field-effect transistors, employing a combination of molecular-dynamics simulations, semiempirical electronic structure calculations, and Landauer transport theory. The results reveal the close relation between the transport characteristics and the structural and electronic properties of the SAM. Furthermore, both local pathways of charge transport in the SAMs and the influence of structural fluctuations are analyzed.

Major histocompatibility complex heterozygosity enhances reproductive success.

We investigated how heterozygosity at the major histocompatibility complex (MHC) affects fitness in wild-derived (F2) house mice (Mus musculus musculus). To compare and control for potential confounding effects from close inbreeding and genome-wide heterozygosity, we used mice that were systematically outbred. We assessed how heterozygosity at MHC and background loci (using 15 microsatellite markers on 11 different chromosomes) affects individual survival and reproductive success (RS) in large, semi-natural population enclosures. We found that overall heterozygosity significantly increased RS, and this correlation was entirely explained by heterozygosity at two MHC loci. Moreover, we found that the effects of MHC heterozygosity depend on the level of background heterozygosity, and the benefits of maximal MHC heterozygosity show a curvilinear effect with increasing background heterozygosity. The enhanced RS from MHC heterozygosity was not because of increased survival, and although MHC heterozygosity was correlated with body mass, body mass did not correlate with RS when heterozygosity is controlled. Breeders were more MHC heterozygous than nonbreeders for both sexes, indicating that MHC heterozygosity enhanced fecundity, mating success or both. Our results show that (i) MHC heterozygosity enhances fitness among wild, outbred as well as congenic laboratory mice; (ii) heterozygosity-fitness correlations can potentially be explained by a few loci, such as MHC; (iii) MHC heterozygosity can increase fitness, even without affecting survival, by increasing mating and RS; and (iv) MHC effects depend on background genes, and maximal MHC heterozygosity is most beneficial at intermediate or optimal levels of background heterozygosity.

Quantum interference and decoherence in single-molecule junctions: how vibrations induce electrical current.

Quantum interference and decoherence in single-molecule junctions is analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasidegenerate states of a molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. This decoherence mechanism may cause significantly larger electrical currents and is particularly pronounced if the junction is vibrationally highly excited, e.g., due to inelastic processes in the resonant transport regime.

Molecular Transistor Controlled through Proton Transfer.

The potential of proton transfer reactions as a fundamental mechanism to realize a nanoscale molecular transistor is investigated. Employing density functional theory and the nonequilibrium Green's function formalism, we identify molecule-graphene nanojunctions, which exhibit high- and low-conducting states depending on the specific location of protons in the molecular bridge. In addition, we show that an electrostatic gate field can control the proton transfer process and thus allow specific conductance states to be selected. In this way, the current in the junction can be switched on and off as in a field-effect transistor. The underlying mechanism is analyzed in detail.

Regulation of volatile and non-volatile pheromone attractants depends upon male social status.

We investigated the regulation of chemical signals of house mice living in seminatural social conditions. We found that male mice more than doubled the excretion of major urinary proteins (MUPs) after they acquired a territory and become socially dominant. MUPs bind and stabilize the release of volatile pheromone ligands, and some MUPs exhibit pheromonal properties themselves. We conducted olfactory assays and found that female mice were more attracted to the scent of dominant than subordinate males when they were in estrus. Yet, when male status was controlled, females were not attracted to urine with high MUP concentration, despite being comparable to levels of dominant males. To determine which compounds influence female attraction, we conducted additional analyses and found that dominant males differentially upregulated the excretion of particular MUPs, including the pheromone MUP20 (darcin), and a volatile pheromone that influences female reproductive physiology and behavior. Our findings show that once male house mice become territorial and socially dominant, they upregulate the amount and types of excreted MUPs, which increases the intensities of volatiles and the attractiveness of their urinary scent to sexually receptive females.

Author Correction: Regulation of volatile and non-volatile pheromone attractants depends upon male social status.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Major urinary protein (MUP) profiles show dynamic changes rather than individual 'barcode' signatures.

House mice (Mus musculus) produce a variable number of major urinary proteins (MUPs), and studies suggest that each individual produces a unique MUP profile that provides a distinctive odor signature controlling individual and kin recognition. This 'barcode hypothesis' requires that MUP urinary profiles show high individual variability within populations and also high individual consistency over time, but tests of these assumptions are lacking. We analyzed urinary MUP profiles of 66 wild-caught house mice from eight populations using isoelectric focusing. We found that MUP profiles of wild male house mice are not individually unique, and though they were highly variable, closer inspection revealed that the variation strongly depended on MUP band type. The prominent ('major) bands were surprisingly homogenous (and hence most MUPs are not polymorphic), but we also found inconspicuous ('minor') bands that were highly variable and therefore potential candidates for individual fingerprints. We also examined changes in urinary MUP profiles of 58 males over time (from 6 to 24 weeks of age), and found that individual MUP profiles and MUP concentration were surprisingly dynamic, and showed significant changes after puberty and during adulthood. Contrary to what we expected, however, the minor bands were the most variable over time, thus no good candidates for individual fingerprints. Although MUP profiles do not provide individual fingerprints, we found that MUP profiles were more similar among siblings than non-kin despite considerable fluctuation. Our findings show that MUP profiles are not highly stable over time, they do not show strong individual clustering, and thus challenge the barcode hypothesis. Within-individual dynamics of MUP profiles indicate a different function of MUPs in individual recognition than previously assumed and advocate an alternative hypothesis ('dynamic changes' hypothesis).

Vibrational nonequilibrium effects in the conductance of single molecules with multiple electronic states.

Vibrational nonequilibrium effects in charge transport through single-molecule junctions are investigated. Focusing on molecular bridges with multiple electronic states, it is shown that electronic-vibrational coupling triggers a variety of vibronic emission and absorption processes, which influence the conductance properties and mechanical stability of single-molecule junctions profoundly. Employing a master equation and a nonequilibrium Green's function approach, these processes are analyzed in detail for a generic model of a molecular junction and for benzenedibutanethiolate bound to gold electrodes.

The light sensitivity of the human visual system depends on the direction of view.

The near-stable North-South orientation of the natural geomagnetic field provides an ideal basis for navigation. Sailors have used it since ancient times, animals for much longer. Various mechanisms have developed for this purpose. Experiments have pointed to a connection between orientation in the geomagnetic field and light perception. Such observations are supported by theoretical considerations. The underlying interaction should also modulate the light sensitivity of the visual system. Recently we demonstrated the effect of an oscillating field. Here we report the existence of a weak influence of the static field on visual sensitivity in man. By comparison with control experiments, if the directions of view line and field vector coincide the perception threshold of a light stimulus is slightly but significantly increased. This significance is lost if the view line deviates by 10 degrees from the field direction.

Periodic inversion of the vertical component of the earth's magnetic field influences fluctuations of visual sensitivity in humans.

According to theoretical considerations, the magnetic field of the earth could influence the first steps of light-induced changes in ocular photopigment, an effect that is thought to underlie the magnetic orientation of some animals. To find out whether man could be influenced in this way, we have tested the effect of an artificial fluctuation in the direction of the earth's magnetic field on oscillations of the visual sensitivity in 27 healthy subjects. The resultant spectra show a significant influence of the field fluctuations, indicating that man is sensitive to changes in the direction of the earth's magnetic field.

The magnetic field sensitivity of the human visual system shows resonance and compass characteristic.

Orientation in the geomagnetic field is essential for many animal species. As yet, the interaction mechanisms of this weak field with the organisms are understood only incompletely. One mechanism in question is the interaction with the photochemical reaction in the retina. We show that the visual sensitivity of man is influenced by periodic sinusoidal inversion of the vertical component of the geomagnetic field. This effect indicates visual fixation in north-south direction and shows a pronounced resonance at a period duration of 110 s. These findings should be helpful in identifying in detail the mechanisms which are influenced by the geomagnetic field.