The density of the transmission eigenvalues of Pb nano-contacts has been estimated recently in mechanically controllable break-junction experiments. Motivated by these experimental analyses, here we study the evolution of the density of the transmission eigenvalues with the disorder strength and the number of channels supported by the ballistic constriction of a quantum point contact in the framework of the Dorokhov-Mello-Pereyra-Kumar equation. We find that the transmission density evolves rapidly into the density in the diffusive metallic regime as the number of channels $N_c$ of the constriction increase. Therefore, the transmission density distribution for a few $N_c$ channels comes close to the known bimodal density distribution in the metallic limit. This is in agreement with the experimental statistical-studies in Pb nano-contacts. For the two analyzed cases, we show that the experimental densities are seen to be well described by the corresponding theoretical results.
The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation for transmission eigenvalues is derived for metallic carbon nanotubes with several conducting channels when the potential range of scatterers is larger than the lattice constant. With increasing system length L, the system approaches a fixed point, where only one channel is perfectly conducting and other channels are completely closed. The asymptotic behavior of the conductance in the long-L regime is investigated on the basis of the DMPK equation. It is shown that the length scale for the exponential decay of the typical conductance is reduced …
Poor screening of the long-range Coulomb interaction in one-dimensional carbon nanotubes results in a peculiar picture of contact phenomena. Being brought to a contact with a metal, conducting nanotube accumulates electric charge whose density decays slowly with the distance from the contact. This should be contrasted to a conventional metal-metal contact where the charge density decreases exponentially at atomic distances. Implications for experiments are discussed.
We use numerical simulations to analyze recent experimental measurements of short-channel carbon nanotube field-effect transistors with palladium contacts. We show that the gate strongly modulates the contact properties, an effect that is distinct from that observed in Schottky barrier carbon nanotube transistors. This modulation of the contacts by the gate allows for the realization of superior subthreshold swings for short channels, and improved scaling behavior. These results further elucidate the behavior of carbon nanotube-metal contacts, and should be useful in the optimization of high-p…
At reduced dimensionality, Coulomb interactions play a crucial role in determining device properties. While such interactions within the same carbon nanotube have been shown to have unexpected properties, device integration and multi-nanotube devices require the consideration of inter-nanotube interactions. We present calculations of the characteristics of planar carbon nanotube transistors including interactions between semiconducting nanotubes and between semiconducting and metallic nanotubes. The results indicate that inter-tube interactions affect both the channel behavior and the contacts…
Electrical contacts between nano-engineered systems are expected to constitute the basic building blocks of future nano-scale electronics. However, the accurate characterization and understanding of electrical contacts at the nano-scale is an experimentally challenging task. Here we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nano-contacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The diff…
We theoretically analyzed a magnetic wall confined in a nano-contact spin valve paying special attention to the penetration of the magnetic wall into thin ferromagnetic electrodes. We showed that, compared with the Bloch wall, the penetration of the Neel wall is suppressed by increases of the demagnetization energy. We found the optimal conditions of the radius and height of the nano-contact to maximize the power of the current-induced oscillation of the magnetic wall. We also found that the thermal stability of the Bloch wall increases when the nano-contact's radius increases or height decrea…
We report on a technique enabling electrical control of the contact silicidation process in silicon nanowire devices. Undoped silicon nanowires were contacted by pairs of nickel electrodes and each contact was selectively silicided by means of the Joule effect. By a realtime monitoring of the nanowire electrical resistance during the contact silicidation process we were able to fabricate nickel-silicide/silicon/nickel- silicide devices with controlled silicon channel length down to 8 nm.
Current induced spin wave excitations in spin transfer torque nano-contacts are known as a promising way to generate exchange-dominated spin waves at the nano-scale. It has been shown that when these systems are magnetized in the film plane, broken spatial symmetry of the field around the nano-contact induced by the Oersted field opens the possibility for spin wave mode co-existence including a non-linear self-localized spin-wave bullet and a propagating mode. By means of micromagnetic simulations, here we show that in systems with strong perpendicular magnetic anisotropy (PMA) in the free lay…
In quasiballistic semiconductor nanostructures, carrier exchange between the active region and dissipative contacts is the mechanism that governs relaxation. In this paper, we present a theoretical treatment of transient quantum transport in quasiballistic semiconductor nanostructures, which is based on the open system theory and valid on timescales much longer than the characteristic relaxation time in the contacts. The approach relies on a model interaction between the current-limiting active region and the contacts, given in the scattering-state basis. We derive a non-Markovian master equat…
The modeling of carbon nanotube-metal contacts is important from both basic and applied view points. For many applications, it is important to design contacts such that the transmission is dictated by intrinsic properties of the nanotube rather than by details of the contact. In this paper, we calculate the electron transmission probability from a nanotube to a free electron metal, which is side-contacted. If the metal-nanotube interface is sufficiently ordered, we find that k-vector conservation plays an important role in determining the coupling, with the physics depending on the area of con…
We present an experimental determination of the conduction channel distribution in lead nanoscale contacts with total conductances ranging from 1 to 15 G0, where G0=2e2/h. It is found that even for contacts having a cross section much smaller than the mean free path the distribution tends to be remarkably close to the universal diffusive limit. With the help of theoretical calculations we show that this behavior can be associated with the specific band structure of lead which produces a significant contribution of partially open channels even in the absence of atomic disorder. Published in Eur…
We investigate the shot noise of an impurity-free graphene flake as a function of the chemical potential. For large width to length ratios, this noise has been predicted and observed to exhibit universal characteristics at the Dirac point. Furthermore, a sharp decrease of the shot noise with increasing carrier density has been predicted. This decrease has also been observed in experiments, but with much smaller slope than predicted. We reconcile this discrepancy between theory and experiment by including the effects of the contacts to the graphene ribbon.
A scanning tunnelling microscope has been used to determine the conductance of single molecular wires with the configuration X-bridge-X, X-bridge-Y and Y-bridge-Y (X = thiol terminus and Y = COOH). We find that for molecular wires with mixed functional groups (X-bridge-Y) the single molecule conductance decreases with respect to the comparable symmetric molecules. These differences are confirmed by theoretical computations based on a combination of density functional theory and the non-equilibrium Green functions formalism. This study demonstrates that the apparent contact resistance, as well …
We report on the observation of anisotropic magnetoresistance (AMR) in vertical asymmetric nano-contacts (NCs) made through AlO$_x$ nano-oxide layer (NOL) formed by ion-assisted oxidation method in the film stack of Co/AlO$_x$-NOL/Pt. Analysis of NC formation was based on \emph{in situ} conductive atomic force microscopy and transmission electron microscopy. Depending on the purity of NCs from Al contamination, we observed up to 29% AMR ratio at room temperature.
Electronic current in a nanometer-size rod is theoretically investigated by an eigen-channel decomposition method in nonequilibrium Green's function formalism. Physical properties, such as the local density of electrons and local current, are decomposed into contributions of eigen-channels. We observe that the evanescent modes and nonlinear conductance are enhanced in d-orbital systems, and the structure of the transmission function, local current density, and penetration depth are discussed. The two effects of the ill-contact at electrodes in d-orbital systems, evanescent modes and the nonlin…
We present detailed Raman studies of graphene deposited on gallium nitride nanowires with different variations in height. Our results show that different density and height of nanowires being in contact with graphene impact graphene properties like roughness, strain and carrier concentration as well as density and type of induced defects. Detailed analysis of Raman spectra of graphene deposited on different nanowire substrates shows that bigger differences in nanowires height increase graphene strain, while higher number of nanowires in contact with graphene locally reduce the strain. Moreover…
Short wave-length exchange-dominated propagating spin waves will enable magnonic devices to operate at higher frequencies and higher data transmission rates.1 While GMR based magnetic nano-contacts are highly efficient injectors of propagating spin waves2,3, the generated wave lengths are 2.6 times the nano-contact diameter4, and the electrical signal strength remains much too weak for practical applications. Here we demonstrate nano-contact based spin wave generation in magnetic tunnel junction stacks, and observe large discrete frequency steps consistent with the hitherto ignored possibility…
The effects of contact geometry and ideality on InGaAs and Si nano-scale n-channel FinFET performance are studied using a quantum-corrected semi-classical Monte Carlo method. Illustrative end, saddle/slot, and raised source/drain contacts were modeled, and with ideal transmissivity and reduced transmissivity more consistent with experimental contact resistivities. Far-from-equilibrium degenerate statistics, quantum-confinement effects on carrier distributions in real-space and among energy valleys, quasi-ballistic transport inaccessible through drift-diffusion and hydrodynamic simulations, and…
We derive Bardeen-like expressions for the transmission probabilities between two multi-probe mesoscopic conductors coupled by a weak tunneling contact. We emphasize especially the dual role of a weak coupling contact as a current source and sink and analyze the magnetic field symmetry. In the limit of a point-like tunneling contact the transmission probability becomes a product of local, partial density of states of the two mesoscopic conductors. We present expressions for the partial density of states in terms of functional derivatives of the scattering matrix with respect to the local poten…
The crossed Andreev reflection in a hybrid nanostructure consisting of a d-wave superconductor and two quantum wires is theoretically studied. When the (110) oriented surface of the superconductor is in contact with the wires parallel and placed close to each other, the Andreev bound state is formed by the crossed Andreev reflection. The conductance has two resonance peaks well below the gap structure in the case of tunnel contacts. These peaks originate from the bonding and antibonding Andreev bound states of hole wave functions.
Bulk electrical dissipation caused by charge-density-wave (CDW) depinning and sliding is a classic subject. We present a novel local, nanoscale mechanism describing the occurrence of mechanical dissipation peaks in the dynamics of an atomic force microscope tip oscillating above the surface of a CDW material. Local surface 2$π$ slips of the CDW phase are predicted to take place giving rise to mechanical hysteresis and large dissipation at discrete tip surface distances. The results of our static and dynamic numerical simulations are believed to be relevant to recent experiments on NbSe$_2$; ot…
Nano-electromechanical (NEM) contact switches have been extensively studied to suppress the limitations of conventional complementary metal-oxide-semiconductor (CMOS) transistors. The attributes of NEM contact switches includes reduced power consumption, reduced off-state leakage current, increased on-state current and sub-thermal switching. However, unacceptably high pull-in voltage and low contact lifetime posed a significant challenge for the use of NEM contact switches in energy efficient CMOS applications. Here, we demonstrate a graphene-based electro-statically actuated NEM contact switc…
Contact resistance and current crowding are important to nanoscale electrical contacts. In this paper, we present a self-consistent model to characterize partially overlapped parallel contacts with varying specific contact resistivity along the contact length. For parallel tunneling contacts formed between contacting members separated by a thin insulating gap, we examine the local voltage-dependent variation of potential barrier height and tunneling current along the contact length, by solving the lumped circuit transmission line model (TLM) equations coupled with the tunneling current self co…
We study the current controlled modulation of a nano-contact spin torque oscillator. Three principally different cases of frequency non-linearity ($d^{2}f/dI^{2}_{dc}$ being zero, positive, and negative) are investigated. Standard non-linear frequency modulation theory is able to accurately describe the frequency shifts during modulation. However, the power of the modulated sidebands only agrees with calculations based on a recent theory of combined non-linear frequency and amplitude modulation.
The physics of dynamic friction on water molecule contaminated surfaces is still poorly understood. In line with the growing interest in hydrophobic contact for industrial applications, this paper focuses on friction mechanisms in such interfaces. As a commonly used material, contact with graphite is considered in a twin-fold approach based on experimental investigations using the circular mode atomic force microscopy technique combined with molecular dynamic simulations. We demonstrate that an intuitive paradigm, which asserts that water molecules are squeezed out of a hydrophobic contact, sh…
Here we report on the translocation of folded polymers through nano-pores using molecular dynamic simulations. Two cases are studied; one in which a folded molecule unfolds upon passage and one in which the folding remains intact as the molecule passes through the nano-pore. The topology of a folded polymer chain is defined as the arrangement of the intramolecular contacts, known as circuit topology. In the case where intramolecular contacts remain intact, we show that the dynamics of passage through a nano-pore varies for molecules with differing topologies: a phenomenon that can be exploited…
