National Institute Of Materials Physics - Romania

The time-dependent transport of a 2D quantum wire (QW) connected to source/drain leads and electrostatically coupled to a singly-clamped InAs cantilever is investigated. The latter is placed above the nanowire and acts as a nanoresonator (NR) in the quantum regime. The vibron dynamics and the transport properties of this nano-electromechanical system (NEMS) are described within a generalized master equation approach which is exact with respect to the electron-vibron coupling. A detailed description of the electron-vibron coupling by taking into account its dependence on the wavefunctions of the quantum nanowire is introduced. It is shown that the tunneling processes in the QW trigger periodic oscillations of the average vibron number even in the absence of a bias. The time-dependent filling of the vibronic states changes as the nanoresonator is swept along the quantum wire.

This paper presents an evaluation regarding the influence of substrate material characteristics and deposition parameters on the tribological behaviour of carbon-based coatings. Chromium nitride ceramic interlayers and carbon-based thin films were deposited by magnetron sputtering on hardened AISI 5115 (16MnCr5) case hardening steel. The physical vapour deposition (PVD) deposition was performed at three different temperatures: 180 degrees C, 200 degrees C and 250 degrees C. The chemical composition of the samples was assessed by Rutherford Backscattering Spectroscopy (RBS), the structure by X-ray Diffraction (XRD), and the surface morphology by Atomic Force Microscopy (AFM). The surface chemistry was analysed by X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The coatings are homogeneous, amorphous, with a smooth surface. The mechanical behaviour has been assessed on a pin-on disk rotational tribometer (wear characteristics), on a micro scratch tester (adhesion to the substrate), by ball-cratering (film thickness) and by nanoindentation (hardness and the modulus of elasticity). A strong correlation has been observed between the substrate characteristics and, more importantly, the deposition temperature, and the mechanical properties of the assembly. The fracture toughness is positively influenced by the presence of the ceramic chromium nitride interlayer. The modulus of elasticity and friction coefficient (both in dry and lubricated conditions) are decreased for higher deposition temperatures, however the higher deposition temperature negatively affects the mechanical characteristics of the steel substrate.

In this work, we theoretically model the time-dependent transport through an asymmetric double quantum dot etched in a two-dimensional wire embedded in a far-infrared (FIR) photon cavity. For the transient and the intermediate time regimes, the current and the average photon number are calculated by solving a Markovian master equation in the dressed-states picture, with the Coulomb interaction also taken into account. We predict that in the presence of a transverse magnetic field the interdot Rabi oscillations appearing in the intermediate and transient regime coexist with slower non-equilibrium fluctuations in the occupation of states for opposite spin orientation. The interdot Rabi oscillation induces charge oscillations across the system and a phase difference between the transient source and drain currents. We point out a difference between the steady-state correlation functions in the Coulomb blocking and the photon-assisted transport regimes.

We investigate the spectral and transport properties of a diatomic square lattice with hopping to the next-nearest-neighbors and broken time-reversal symmetry, which behaves as a Chern insulator. In a finite-size approach, the attention is paid to the formation of chiral edge states in the topological insulating phase, but also in the semimetallic one. The edge states are revealed in the ribbon and plaquette geometries by analytical and numerical methods, significant differences being produced by the specific atomic connectivity at the boundary. The Hall resistance R-H is calculated in the plaquette geometry using the Landauer-Mittiker approach. The chiral edge states located in the unique gap of the energy spectrum manifest themselves by quantized values R-H = +/- h/e(2) specific to the Chern insulator. The semimetallic system containing chiral edge states embedded in the quasicontinuum of bulk states shows a disorder-driven AQHE as a consequence of the Anderson localization process.

Herein, the spectral and transport properties in the finite phosphorene lattice are investigated using the tight-binding model in the presence of Anderson disorder potential. The focus is on the zig-zag edge states localization, provided by the numerically calculated inverse participation number. At low disorder, the zig-zag states undergo a localization process, keeping their 1D character, while further increasing the disorder leads to delocalization due to hybridization with the extended 2D states. The disorder-induced changes in the electronic conductance, from one zig-zag edge to the other, are also discussed.

We theoretically describe electric transport through an optically active quantum dot embedded in a single-mode cavity, and coupled to source-drain particle reservoirs. The populations of various many-body configurations (e.g., excitons, trions, biexciton) and the photon-number occupancies are calculated from a master equation which is derived in the basis of dressed states. These take into account both the Coulomb and the light-matter interaction. The former is essential in the description of the transport, while for the latter we identify situations in which it can be neglected in the expression of tunneling rates. The fermionic nature of the particle reservoirs plays an important role in the argument. The master equation is numerically solved for the s-shell many-body configurations of disk-shaped quantum dots. If the cavity is tuned to the biexciton-exciton transition, the most efficient optical processes take place in a three-level A system. The alternative exciton-ground-state route is inhibited as nonresonant due to the biexciton binding energy. The steady-state current is analyzed as a function of the photon frequency and the coupling to the leads. An unexpected feature appears in its dependence on the cavity loss rate, which turns out to be nonmonotonic.

The main goal of this paper is to put on solid mathematical grounds the so-called non-equilibrium Green's function transport formalism for open systems. In particular, we derive the Jauho-Meir-Wingreen formula for the time-dependent current through an interacting sample coupled to non-interacting leads. Our proof is non-perturbative and uses neither complex-time Keldysh contours nor Langreth rules of 'analytic continuation.' We also discuss other technical identities (Langreth, Keldysh) involving various many-body Green's functions. Finally, we study the Dyson equation for the advanced/retarded interacting Green's function and we rigorously construct its (irreducible) self-energy, using the theory of Volterra operators.

We study the validity of Hund's first rule for the spin multiplicity in circular molecules-made of real or artificial atoms such as quantum dots-by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction.

This paper reports on the aging of carbon nanowalls (CNW) and modification of their wettability by the storage time, growth conditions, and post-fabrication plasma treatments. The as-deposited CNW initially exhibit marked hydrophilic behavior (fresh CNW), but within a few days they become highly hydrophobic (aged CNW). Their final hydrophobicity is closely related to their topography which is controlled by the deposition parameters. In addition, subsequent fluorinated plasma treatments result in super-hydrophobic CNW layers, irrespective of the hydrophilic or hydrophobic character of the pre-treated samples. To explain this, we show that the CNW edges contain many defects initially, but such defects become passivated in time. As a result, the surfaces become highly hydrophobic after aging or fluorination, having inert stable terminations.

Using the kp theory and configuration interaction method, we analyze the effect of heavy hole-light hole (HH-LH) mixing in CdTe quantum dots (QDs) with a single manganese (Mn) ion. We find that the hole-Mn exchange switches the coupling between two excitons whose Luttinger spinors have both HH and LH components. If the magnetic dopant is off-centered and the QD is subjected to a single pulse the system periodically bounces between bright and dark mostly HH excitons with opposite HH spins. A pump-and-probe setup allows to estimate the efficiency of this HH spin unpinning from the biexciton response. The biexciton absorption spectrum is also discussed.