arXiv subjects

Daniela Dragoman

Publications and source records attributed to Daniela Dragoman.

At least 19 recordsLinked to original sources

arXiv2019

A ballistic transport model for an artificial neuron

We introduce a model for an artificial neuron which is based on ballistic transport in a multi-terminal device. Unlike standard configurations, the proposed design embeds the synaptic weights into the active region, thus significantly reducing the complexity of the input terminals. This is achieved by defining the basic elements of the ballistic artificial neuron as follows: the input values are set by the incoming wavefunctions amplitudes, while the weights correspond to the scattering matrix elements. Furthermore, the output value of the activation function of the artificial neuron is given by the transmission function. By tuning the gate voltage, the scattering potential and, consequently, the weights are changed so that the value of the transmission function gets closer to the target output, which is essential in the training process of artificial neural networks. Thus, we provide here the modus operandi of a ballistic artificial neuron.

physics.app-ph↗

arXiv2019

Graphene bandgap induced by ferroelectric Pca2$_1$ HfO$_2$ substrate: a first-principles study

The electronic properties of graphene on top of ferroelectric HfO$_2$ substrates in orthorhombic phase with space group Pca2$_1$ are investigated using density functional theory calculations. The space group Pca2$_1$ was recently identified as one of the two potential candidates for ferroelectricity in hafnia, with the polarization direction oriented along [001] direction. Our results indicate the appearance of sizable energy gaps in graphene induced by the HfO$_2$ substrate, as a consequence of orbital hybridization and locally deformed graphene structure. The gap sizes depend on the type of the HfO$_2$ terminations interacting with graphene, showing larger gaps for oxygen terminated slabs compared to hafnium terminated ones. These observations may prove to be highly significant for the development of graphene based field effect transistors using high-k dielectrics.

cond-mat.mes-hall↗

arXiv2018

Electric and thermoelectric properties of graphene bilayers with extrinsic impurities under applied electric field

In contrast to monolayer graphene, in bilayer graphene (BLG) one can induce a tunable bandgap by applying an external electric field, which makes it suitable for field effect applications. Extrinsic doping of BLGs enriches the electronic properties of the graphene-based family, as their behavior can be switched from an intrinsic small-gap semiconductor to a degenerate semiconductor. In the framework of density functional theory (DFT) calculations, we investigate the electronic and thermoelectric properties of BLGs doped with extrinsic impurities from groups III (B, Al, Ga), IV (Si, Ge) and V (N, P, As), in the context of applied external electric fields. Doping one monolayer of the BLG with p- or n-type dopants results in a degenerate semiconductor, where the Fermi energy depends on the type of the impurity, but also on the magnitude and orientation of the electric field, which modifies the effective doping concentration. Doping one layer with isoelectronic species like Si and Ge opens a gap, which may be closed upon applying an electric field, in contrast to the pristine BLG. Furthermore, dual doping by III-V elements, in a way that the BLG system is formed by one n-type and one p-type graphene monolayer, leads to intrinsic semiconductor properties with relatively large energy gaps. Si-Si and Ge-Ge substitutions render a metallic like behavior at zero field similar to the standard BLG, however with an asymmetric density of states in the vicinity of the Fermi energy. We analyze the suitability of the highly doped BLG materials for thermoelectric applications, exploiting the large asymmetries of the density of states. In addition, a sign change in the Seebeck coefficient is observed by tuning the electric field as a signature of narrow bands near the Fermi level.

cond-mat.mes-hall↗

arXiv2018

Reconfigurable quantum logic gates using Rashba controlled spin polarized currents

A reconfigurable logic gate is proposed in a two-dimensional double quantum wire system with a coupling window enabled by a Rashba field. Manipulating the spin states of incoming electrons several quantum logic gates (OR, AND, XOR, CNOT) can be implemented. The logic gate functionality can be switched by tuning the Rashba parameter only. In this context, we investigate suitable configurations of the device region by embedding a quantum point contact located in the coupling region to obtain all four logic gates. The ballistic spin polarized transmission functions are calculated using an effective mass scattering formalism in the framework of a multi-channel, multi-terminal system. Owing its versatility, the proposed logic gate can be integrated in programmable architectures, able to implement both classical and quantum algorithms.

cond-mat.mes-hall↗

arXiv2018

Electric field effect in boron and nitrogen doped graphene bilayers

Unlike single layer graphene, in the case of $AB$-stacked bilayer graphene (BLG) one can induce a non-zero energy gap by breaking the inversion symmetry between the two layers using a perpendicular electric field. This is an essential requirement in field-effect applications, particularly since the induced gap in BLG systems can be further tuned by the magnitude of the external electric field. Doping is another way to modify the electronic properties of graphene based systems. We investigate here BLG systems doped with boron and nitrogen in the presence of external electric field, in the framework of density functional theory (DFT) calculations. Highly doped BLG systems are known to behave as degenerate semiconductors, where the Fermi energy depends on the doping concentration but, in addition, we show that the electronic properties drastically depend also on the applied electric field. By changing the magnitude and the orientation of the electric field, the gap size and position relative to the Fermi level may be tuned, essentially controlling the effect of the extrinsic doping. In this context, we discuss in how far the external electric field may suitably adjust the effective doping and, implicitly, the conduction properties of doped BLG systems.

cond-mat.mes-hall↗

arXiv2018

Wafer-scale fabrication and room-temperature experiments on graphene-based gates for quantum computation

We have fabricated at wafer scale graphene-based configurations suitable for implementing at room temperature one-qubit quantum gates and a modified Deutsch-Jozsa algorithm. Our measurements confirmed the (quasi-)ballistic nature of charge carrier propagation through both types of devices, which have dimensions smaller than the room-temperature mean-free-path in graphene. As such, both graphene-based configurations were found to be suitable for quantum computation. These results are encouraging for demonstrating a miniaturized, room-temperature quantum computer based on graphene.

cond-mat.mes-hall↗

arXiv2017

Solving the graphene electronics conundrum: high mobility and high on-off ratio in graphene nanopatterned transistors

Tens of graphene transistors with nanoperforated channels and different channel lengths were fabricated at the wafer scale. The nanoholes have a central diameter of 20 nm and a period of 100 nm, the lengths of the channel being of 1, 2, 4 or 8 micrometers. We have found that the mobility in these 2 micrometer-wide transistors varies from about 10400 cm2/Vs for a channel length of 1 micrometer to about 550 cm2/Vs for a channel length of 8 micrometer. Irrespective of the mobility value, in all transistors the on-off ratio is in the range 10^3-10^4 at drain and gate voltages less than 2 V. The channel length-dependent mobility and conductance values indicate the onset of strong localization of charge carriers, whereas the high on-off ratio is due to bandgap opening by nanoperforations.

cond-mat.mes-hall↗

arXiv2016

Room temperature on-wafer ballistic graphene field-effect-transistor with oblique double-gate

We have fabricated and measured ballistic graphene transistors with two oblique gates that can be independently biased. The gate lengths are about 38 nm and are separated by a distance of 30 nm, the tilting angle being of 45o with respect to source and drain electrodes distanced at 190 nm. Electric measurements reveal specific properties of ballistic carrier transport, i.e. nonlinear drain voltage-drain current dependence, showing a saturation region, and negative differential resistance at certain bias voltages, which cannot be explained without physical mechanisms related to ballistic transport. Tens of ballistic transistors, with very large transconductances, were fabricated on a chip cut from a 4 inch graphene wafer. Such double-gate transistor configurations can be used also as extremely efficient, state-of-the-art photodetectors.

cond-mat.mes-hall↗

arXiv2016

Memristive GaN ultrathin suspended membrane array

In this paper, we show that ultrathin GaN membranes having a thickness of 15 nm and planar dimensions of 12x184 microns act as memristive devices. This fact is due to the migration of the negatively charged deep traps, which form in the volume of the membrane during the fabrication process, towards the unoccupied surface states of the suspended membranes. The time constant of the migration process is of the order of tens of second and varies with the current or voltage sweep.

cond-mat.mes-hall↗

arXiv2016

Room temperature nanostructured graphene transistors with high on-off ratio

We report the batch fabrication of graphene field-effect-transistors (GFETs) with nanoperforated graphene as channel. The transistors were cut and encapsulated. The encapsulated GFETs display saturation regions that can be tuned by modifying the top gate voltage, and have on/off ratios of up to 10^8 at room temperature. In addition, the nanoperforated GFETs display orders of magnitude higher photoresponses than any room-temperature graphene detector configurations that do not involve heterostructures with bandgap materials.

cond-mat.mes-hall↗

arXiv2015

Ballistic electron transport in wrinkled superlattices

Inspired by the problem of elastic wave scattering on wrinkled interfaces, we studied the scattering of ballistic electrons on a wrinkled potential energy region. The electron transmission coefficient depends on both wrinkle amplitude and periodicity, having different behaviors for positive and negative scattering potential energies. For scattering on potential barriers, minibands appear in electron transmission, as in superlattices, whereas for scattering on periodic potential wells the transmission coefficient has a more complex form. Besides suggesting that tuning of electron transmission is possible by modifying the scattering potential via voltages on wrinkled gate electrodes, our results emphasize the analogies between ballistic electrons and elastic waves even in scattering problems on non-typical configurations.

cond-mat.mes-hall↗

arXiv2014

Two-and three-qubit room-temperature graphene quantum gates

Proposed configurations for the implementation of graphene-based CNOT and Toffoli gates working at room temperature are presented. These two logic gates, essential for any quantum computing algorithm, involve ballistic Y junctions for qubit implementation, quantum interference for qubit interaction and oblique gates for optimizing the output, and can be fabricated using existing nanolitographical techniques. The proposed configurations of CNOT and Toffoli quantum logic gates are based on the very large mean-free-paths of carriers in graphene at room temperature.

cond-mat.mes-hall↗

arXiv2014

Enhanced architectures for room-temperature reversible logic gates in graphene

We show that reversible two- and three-input logic gates, among which we mention the universal Toffoli gate, can be implemented with three tilted gating electrodes patterned on a monolayer graphene flake. These low-dissipation gates are based on the unique properties of ballistic charge carriers in graphene, which induce the appearance of bandgaps in transmission for properly potential barriers. The enhanced architectures for reversible logic gate implementations proposed in this paper offer a remarkable design simplification in comparison to standard approaches based on field-effect transistor circuits and a potential high-frequency operation.

cond-mat.mes-hall↗

arXiv2014

Complementary memristive device based on a thin film of MoS2 monolayers

In this manuscript we demonstrate experimentally that a thin film of MoS2 monolayers formed by drop-casting on a gold interdigitated electrode on Si/SiO2 behaves like a complementary memristive device, which is a key device of future crossbar memories. The hysteretic behavior of this device is modulated by light in a spectral range expanding from UV up to IR. In contrast to previous complementary memristive devices based on complicated oxides heterostructures, this device has a simple fabrication procedure and can be controlled by light, in addition to electrical signals.

cond-mat.mes-hall↗

arXiv2014

Negative differential resistance in graphene-based ballistic field effect transistor with oblique top gate

Negative differential resistance (NDR) with room temperature peak-valley-ratio of 8 has been observed in a ballistic field-effect-transistor (FET) based on graphene, having an oblique top gate. Graphene FETs with a top gate inclination angle of 45 degrees and a drain-source distance of 400 nm were fabricated on a chip cut from a 4 inch graphene wafer grown by CVD. From the 60 measured devices, NDR was observed only in the regions where the CVD graphene displays the Raman signature of defectless monolayers. In other specific positions on the wafer, where graphene quality was not high enough and the Raman signature indicated the presence of defects, the ballistic character of transport is lost and the graphene FETs display nonlinear drain-voltage dependences tuned by the top and back gate voltage.

cond-mat.mes-hall↗

arXiv2013

Microwave and millimeterwave electrical permittivity of graphene monolayer

The effective electrical permittivity of a graphene monolayer is experimentally investigated in the 5-40 GHz range, which encompasses the microwave and the lower part of millimeterwave spectrum. The measurements were carried out using a coupled coplanar waveguide placed over a graphene monolayer flake, which is deposited on Si/SiO2. In contrast to some initial predictions, the effective permittivity of the graphene monolayer is slightly decreasing in the above-mentioned frequency range and has an average value of 3.3.

cond-mat.mes-hall↗

arXiv2013

Finite oscillator obtained through finite frame quantization

The Hamiltonian of the harmonic oscillator is usually defined as a differential operator, but an integral representation can be obtained by using the coherent state quantization. The finite frame quantization is a finite counterpart of the coherent state quantization and it allows us to define a finite oscillator by starting from the integral representation of the harmonic oscillator. Our purpose is to investigate the oscillator obtained in this way, and to present a possible application to the discrete fractional Fourier transform.

math-ph↗

arXiv2013

Dirac-Schrodinger transformations in contacted graphene structures

At an interface between contacts and graphene, the mathematical equation that governs the propagation of electrons transforms from the Schrodinger to the Dirac equation. The condition of current probability conservation at such an interface does not determine uniquely the boundary conditions for the quantum wavefunction. We discuss the possible form of boundary conditions, determine its influence on the transmission coefficient of a contacted graphene structure and suggest that optical experiments on photonic crystals with Dirac points can help identifying, under certain circumstances, the proper boundary condition at graphene- electrode interfaces.

cond-mat.mes-hall↗