A recently developed Floquet theory-based formalism for computing electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic ac radiation field is applied to an experimentally relevant system, namely a xylyl-dithiol molecule in contact at either end with gold electrodes. In this treatment, a nondissipative tight-binding model is assumed to describe the conduction of electric current. Net current through the wire is calculated for two configurations of the electrode-wire-electrode system. In one, symmetric, configuration, the electrodes are close (ϳ2 Å) and equidistant from the bridge molecule. In the other, asymmetric configuration, one electrode is farther away (ϳ5 Å), representing the tip of a scanning tunneling microscope located at this distance from the bridge molecule ͑the other end being chemisorbed to a gold substrate͒. For both configurations, electron current is calculated for a range of experimental inputs, including dc bias and the intensity and frequency of the laser. Via absorption/emission of photons, resonant conditions may be achieved under which electron transport is significantly enhanced compared to the unilluminated analog. Calculations show that this can be accomplished with experimentally accessible laser field strengths.

This paper considers electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic radiation field. Current flow through the wire is calculated within a nondissipative one-electron tight binding model of the quantum dynamics. Using Floquet theory, the field-driven molecular wire is mapped to an effective time-independent quantum system characterized by a tight-binding Hamiltonian with the same essential structure as the nondriven analog. Thus, Green’s Function methods for computing current flow through the wire, which have been profitably applied to the molecular wire problem in the absence of driving, can also be used to analyze the corresponding field-driven system. Illustrative numerical calculations on a simple model system are presented.

In this work, we study the effect of a dual luminescence for CdSe quantum dots (QDs) doped by a manganese impurity. In this effect, one line has fast relaxation and corresponds to light emission from CdSe conduction band, whereas the second is very slow in relaxation with the relaxation time of 0.1−1.0 ms. The second line disappears for quantum dots (QDs) with diameters D ≥ 3.3 nm, and therefore, the luminescence becomes tunable by a QD size. The problem is computationally challenging because of large size QDs and a high degeneracy of the energy states. To overcome this problem, we make four assumptions. These assumptions are the following: (1) a QD optical gap is independent of an Mn impurity for small concentrations; (2) we combine electronic structure calculations for medium size CdSe QDs with the effective mass calculations for medium and large QD sizes and match them in the medium size region, assuming that based on assumption 1, the optical spectrum is independent of Mn even for larger QDs; (3) we cut an MnSe 4 fragment out of the middle of Cd 23 Mn 1 Se 24 , Cd 31 Mn 1 Se 32 , and Cd 81 Mn 1 Se 82 QDs and check whether we can quantitatively explain the mechanism of the second, slow relaxation emission in these experiments using the ab initio SAC−CI multiconfiguration method; and (4) the slow luminescence line is independent of a QD size. We have proved theoretically all these assumptions and found that the critical size of a quantum dot, the size when the second luminescence line disappears, is 3.2 nm, and the slow luminescence energy is 2.3 eV in a tetrahedral ligand field. We also study the case for an Mn impurity placed at a QD surface. Then the symmetry of a fragment is C 3v , and the results of the calculations reveal that the slow luminescence energy is 2.47 eV with the critical size D = 2.7 nm, instead of 3.2 nm for a tetrahedral ligand field. We also predict how this energy depends on the length of an Mn−Se bond. The dependencies appear to be the opposite in these two cases. For the tetrahedral symmetry, the luminescence energy grows with the bond length, whereas for the pyramid, C 3v , symmetry (an Mn is at the surface), it goes down. Furthermore, we study a luminescence energy dependence on a Se−Mn−Se pyramid angle. We find that it is angle independent. These results could be useful for CdSe nanocrystal structures different than Wurtzite.

Tunneling of two particles in synchronous and asynchronous regimes is studied in the framework of dissipative quantum tunneling. The critical temperature T_c corresponding to a bifurcation of the underbarrier trajectory is determined. The effect of a heat bath local mode on the probability of two-dimensional tunneling transfer is also investigated. At certain values of the parameters, the degeneracy of antiparallel tunneling trajectories is important. Thus, four, six, twelve, etc., pairs of the trajectories should be taken into account (a cascade of bifurcations). For the parallel particle tunneling the bifurcation resembles phase transition of a first kind, while for the antiparallel transfer it behaves as second order phase transition. The proposed theory allows for the explanation of experimental data on quantum fluctuations in two-proton tunneling in porphyrins near the critical temperature.Comment: RevTeX4, twocolumn, 12 pages, 11 figures (17 eps-files). Revised version, to appear in Phys. Rev.

Scanning tunneling microscopy is utilized to investigate the local density of states of a CHNHPbICl perovskite in cross-sectional geometry. Two electronic phases, 10-20 nm in size, with different electronic properties inside the CHNHPbICl perovskite layer are observed by the dI/dV mapping and point spectra. A power law dependence of the dI/dV point spectra is revealed. In addition, the distinct electronic phases are found to have preferential orientations close to the normal direction of the film surface. Density functional theory calculations indicate that the observed electronic phases are associated with local deviation of I/Cl ratio, rather than different orientations of the electric dipole moments in the ferroelectric phases. By comparing the calculated results with experimental data we conclude that phase A (lower contrast in dI/dV mapping at -2.0 V bias) contains a lower I/Cl ratio than that in phase B (higher contrast in dI/dV).