Biomedical subjects

Michael Galperin

Publications and source records attributed to Michael Galperin.

8 recordsLinked to original sources

PubMed2006

Optical properties of current carrying molecular wires.

We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied molecular orbitals, and the analysis involves the simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads, and the incident and/or emitted photon flux. Using a unified theoretical approach based on the nonequilibrium Green's function method we derive expressions for the absorption line shape (not an observable but a useful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We also consider conditions under which resonance radiation can induce electronic current in an unbiased junction. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge-transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias is distributed on the junction.

Journal Article↗

PubMed2006

Molecular transport junctions: current from electronic excitations in the leads.

Using a model comprising a two-level bridge connecting free electron reservoirs we show that coupling of a molecular bridge to electron-hole excitations in the leads can markedly effect the source-drain current through a molecular junction. In some cases, e.g., molecules that exhibit strong charge transfer transitions, the contribution from electron-hole excitations can exceed the Landauer elastic current and dominate the observed conduction.

Journal Article↗

PubMed2005

Current-induced light emission and light-induced current in molecular-tunneling junctions.

The interaction of metal-molecule-metal junctions with light is considered within a simple generic model. We show, for the first time, that light-induced current in unbiased junctions can take place when the bridging molecule is characterized by a strong charge-transfer transition. The same model shows current-induced light emission under potential bias that exceeds the molecular excitation energy. Results based on realistic estimates of molecule-lead coupling and molecule-radiation field interaction suggest that both effects should be observable.

Journal Article↗

PubMed2005

Molecular transport junctions: asymmetry in inelastic tunneling processes.

Inelastic electron tunneling spectroscopy (IETS) measurements are usually carried out in the low-voltage ("Ohmic", i.e., linear) regime where the elastic conduction/voltage characteristic is symmetric to voltage inversion. Inelastic features, normally observed in the second derivative d(2)I/dV(2) are also symmetric (in fact antisymmetric) in many cases, but asymmetry is sometimes observed. We show that such asymmetry can occur because of different energy dependences of the two contact self-energies. This may be attributed to differences in contact density of states (different contact material) or different energy dependence of the coupling (STM-like geometry or asymmetric positioning of molecular vibrational modes in the junction). The asymmetry scales with the difference between the energy dependence of these self-energies and disappears when this dependence is the same for the two contacts. Our nonequilibrium Green function approach goes beyond proposed WKB scattering theory in properly accounting for Pauli exclusion, as well as providing a path to generalizations, including consideration of phonon dynamics and higher-order perturbation theory.

Journal Article↗

PubMed2005

Hysteresis, switching, and negative differential resistance in molecular junctions: a polaron model.

Within a simple mean-field model (self-consistent Hartree approximation) we discuss the possibility of polaron formation on a molecular wire as a mechanism for negative differential resistance (NDR), switching, and/or hysteresis in the I-V characteristic of molecular junctions. This mechanism differs from earlier proposed mechanisms of charging and conformational change. The polaron model captures the essential physics and provides qualitative correspondence with experimental data. The importance of active redox centers in the molecule is indicated.

Journal Article↗

PubMed2004

Inelastic electron tunneling spectroscopy in molecular junctions: peaks and dips.

We study inelastic electron tunneling through a molecular junction using the nonequilibrium Green's function formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage Phi when plotted against Phi. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative dI/dPhi and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed.

Journal Article↗

PubMed2003

An integrated analysis of the genome of the hyperthermophilic archaeon Pyrococcus abyssi.

The hyperthermophilic euryarchaeon Pyrococcus abyssi and the related species Pyrococcus furiosus and Pyrococcus horikoshii, whose genomes have been completely sequenced, are presently used as model organisms in different laboratories to study archaeal DNA replication and gene expression and to develop genetic tools for hyperthermophiles. We have performed an extensive re-annotation of the genome of P. abyssi to obtain an integrated view of its phylogeny, molecular biology and physiology. Many new functions are predicted for both informational and operational proteins. Moreover, several candidate genes have been identified that might encode missing links in key metabolic pathways, some of which have unique biochemical features. The great majority of Pyrococcus proteins are typical archaeal proteins and their phylogenetic pattern agrees with its position near the root of the archaeal tree. However, proteins probably from bacterial origin, including some from mesophilic bacteria, are also present in the P. abyssi genome.

Adaptation, Physiological↗

PubMed2003

NEGF-HF method in molecular junction property calculations.

Electron-electron interaction is an essential issue in predicting the properties of a molecular conduction junction. An accurate treatment requires taking proper account of the potential distribution across the junction as well as for the change in electronic structure under the external voltage drop. Another important point is the necessity to treat molecules as open quantum systems. In this paper we address these issues within the framework of the non-equilibrium Green's function formalism at the Hartree-Fock level (NEGF-HF), which permits a self-consistent treatment of the problem. We apply the method to study the experimentally observed asymmetry in the I-V curves with respect to polarity of voltage bias in Hg-Au junctions containing bilayers of alkanethiols of various chain lengths. The origin of the effect is suggested to be the asymmetric behavior of the character of the highest occupied molecular orbital (HOMO) of the junction at opposite biases, which leads to different effective barriers for electron transfer across the junction at opposite signs of the voltage drop. The calculated potential profile shows the capacitor-like nature for the junction with the weak link.

Algorithms↗