Image charge effects in single-molecule junctions: Breaking of symmetries and negative differential resistance in a benzene transistor

Both experiments and theoretical studies have demonstrated that the interaction between the current carrying electrons and the induced polarization charge in single-molecule junctions leads to a strong renormalization of molecular charging energies. However, the effect on electronic excitations and molecular symmetries remain unclear. Using a theoretical framework developed for semiconductor nanostructure based single-electron transistors (SETs), we demonstrate that the image charge interaction breaks the molecular symmetries in a benzene based single-molecule transistor operating in the Coulomb blockade regime. This results in the appearance of a so-called blocking state, which gives rise to negative differential resistance (NDR). We show that the appearance of NDR and its magnitude in the symmetry-broken benzene SET depends in a complicated way on the interplay between the many-body matrix elements, the lead tunnel coupling asymmetry, and the bias polarity. In particular, the current reducing property of the blocking state causing the NDR, is shown to vanish under strongly asymmetric tunnel couplings, when the molecule is coupled stronger to the drain electrode. The calculated IV characteristic may serve as an indicator for image charge broken molecular symmetries in experimental situations.Comment: Accepted version (Phys. Rev. B), 16 pages, 8 figure

Benchmarking GW against exact diagonalization for semi-empirical models

We calculate groundstate total energies and single-particle excitation energies of seven pi conjugated molecules described with the semi-empirical Pariser-Parr-Pople (PPP) model using self-consistent many-body perturbation theory at the GW level and exact diagonalization. For the total energies GW captures around 65% of the groundstate correlation energy. The lowest lying excitations are overscreened by GW leading to an underestimation of electron affinities and ionization potentials by approximately 0.15 eV corresponding to 2.5%. One-shot G_0W_0 calculations starting from Hartree-Fock reduce the screening and improve the low-lying excitation energies. The effect of the GW self-energy on the molecular excitation energies is shown to be similar to the inclusion of final state relaxations in Hartree-Fock theory. We discuss the break down of the GW approximation in systems with short range interactions (Hubbard models) where correlation effects dominate over screening/relaxation effects. Finally we illustrate the important role of the derivative discontinuity of the true exchange-correlation functional by computing the exact Kohn-Sham levels of benzene.Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev.

Bayesian Error Estimation in Density Functional Theory

We present a practical scheme for performing error estimates for Density Functional Theory calculations. The approach which is based on ideas from Bayesian statistics involves creating an ensemble of exchange-correlation functionals by comparing with an experimental database of binding energies for molecules and solids. Fluctuations within the ensemble can then be used to estimate errors relative to experiment on calculated quantities like binding energies, bond lengths, and vibrational frequencies. It is demonstrated that the error bars on energy differences may vary by orders of magnitude for different systems in good agreement with existing experience.Comment: 5 pages, 3 figure

Electrical manipulation of spin states in a single electrostatically gated transition-metal complex

We demonstrate an electrically controlled high-spin (S=5/2) to low-spin (S=1/2) transition in a three-terminal device incorporating a single Mn2+ ion coordinated by two terpyridine ligands. By adjusting the gate-voltage we reduce the terpyridine moiety and thereby strengthen the ligand-field on the Mn-atom. Adding a single electron thus stabilizes the low-spin configuration and the corresponding sequential tunnelling current is suppressed by spin-blockade. From low-temperature inelastic cotunneling spectroscopy, we infer the magnetic excitation spectrum of the molecule and uncover also a strongly gate-dependent singlet-triplet splitting on the low-spin side. The measured bias-spectroscopy is shown to be consistent with an exact diagonalization of the Mn-complex, and an interpretation of the data is given in terms of a simplified effective model.Comment: Will appear soon in Nanoletter

First-principles study of the phonon-limited mobility in n-type single-layer MoS2

In the present work we calculate the phonon-limited mobility in intrinsic n-type single-layer MoS2 as a function of carrier density and temperature for T > 100 K. Using a first-principles approach for the calculation of the electron-phonon interaction, the deformation potentials and Fr\"ohlich interaction in the isolated MoS2 layer are determined. We find that the calculated room-temperature mobility of ~410 cm^2 V^-1 s^-1 is dominated by optical phonon scattering via deformation potential couplings and the Fr\"ohlich interaction with the deformation potentials to the intravalley homopolar and intervalley longitudinal optical phonons given by 4.1 x 10^8 eV/cm and 2.6 x 10^8 eV/cm, respectively. The mobility is weakly dependent on the carrier density and follows a \mu ~ T^-1 temperature dependence with \gamma = 1.69 at room temperature. It is shown that a quenching of the characteristic homopolar mode which is likely to occur in top-gated samples, boosts the mobility with 70 cm^2 V^-1 s^-1 and can be observed as a decrease in the exponent to \gamma = 1.52. Our findings indicate that the intrinsic phonon-limited mobility is approached in samples where a high-kappa dielectric that effectively screens charge impurities is used as gate oxide.Comment: Submitted to Phys. Rev.

First-principles modelling of molecular single-electron transistors

We present a first-principles method for calculating the charging energy of a molecular single-electron transistor operating in the Coulomb blockade regime. The properties of the molecule are modeled using density-functional theory, the environment is described by a continuum model, and the interaction between the molecule and the environment are included through the Poisson equation. The model is used to calculate the charge stability diagrams of a benzene and C$_{60}$ molecular single-electron transistor

Temperature induced crossing in the optical bandgap of mono and bilayer MoS2 on SiO2

Photoluminescence measurements in mono- and bilayer-MoS2 on SiO2 were undertaken to determine the thermal effect of the MoS2/SiO2 interface on the optical bandgap. The energy and intensity of the photoluminescence from monolayer MoS2 were lower and weaker than those from bilayer MoS2 at low temperatures, whilst the opposite was true at high temperatures above 200 K. Density functional theory calculations suggest that the observed optical bandgap crossover is caused by a weaker substrate coupling to the bilayer than to the monolayer

Phonon-assisted carrier transport through a lattice-mismatched interface

We showed the distinctive unconventional junction effect of MoS2 junctions: a lattice mismatched MoS2. It is unique to observe the difference originated from the atomic interrelation at the interface. The results revealed the dominant scattering source at the conventional naturally stepwise junction, while the misorientationally stacked layer exhibited effectively decoupled behavior and a significantly smaller junction resistance via phonon assist carrier. Therefore, our finding in this paper clearly shows the different mechanisms in carrier transport at both junction interface of MoS2

Electron beam-formed ferromagnetic defects on MoS2 surface along 1T phase transition

1 T phase incorporation into 2H-MoS2 via an optimal electron irradiation leads to induce a weak ferromagnetic state at room temperature, together with the improved transport property. In addition to the 1T-like defects, the electron irradiation on the cleaved MoS2 surface forms the concentric circletype defects that are caused by the 2 H/1 T phase transition and the vacancies of the nearby S atoms of the Mo atoms. The electron irradiation-reduced bandgap is promising in vanishing the Schottky barrier to attaining spintronics device. The simple method to control and improve the magnetic and electrical properties on the MoS2 surface provides suitable ways for the low-dimensional device applications.ope