arXiv subjects

Antti-Pekka Jauho

Publications and source records attributed to Antti-Pekka Jauho.

At least 91 records · Page 5Linked to original sources

arXiv2011

Clar Sextet Analysis of Triangular, Rectangular and Honeycomb Graphene Antidot Lattices

Pristine graphene is a semimetal and thus does not have a band gap. By making a nanometer scale periodic array of holes in the graphene sheet a band gap may form; the size of the gap is controllable by adjusting the parameters of the lattice. The hole diameter, hole geometry, lattice geometry and the separation of the holes are parameters that all play an important role in determining the size of the band gap, which, for technological applications, should be at least of the order of tenths of an eV. We investigate four different hole configurations: the rectangular, the triangular, the rotated triangular and the honeycomb lattice. It is found that the lattice geometry plays a crucial role for size of the band gap: the triangular arrangement displays always a sizable gap, while for the other types only particular hole separations lead to a large gap. This observation is explained using Clar sextet theory, and we find that a sufficient condition for a large gap is that the number of sextets exceeds one third of the total number of hexagons in the unit cell. Furthermore, we investigate non-isosceles triangular structures to probe the sensitivity of the gap in triangular lattices to small changes in geometry.

cond-mat.mes-hall↗

arXiv2010

Field enhancement at metallic interfaces due to quantum confinement

We point out an apparently overlooked consequence of the boundary conditions obeyed by the electric displacement vector at air-metal interfaces: the continuity of the normal component combined with the quantum mechanical penetration of the electron gas in the air implies the existence of a surface on which the dielectric function vanishes. This, in turn, leads to an enhancement of the normal component of the total electric field. We study this effect for a planar metal surface, with the inhomogenous electron density accounted for by a Jellium model. We also illustrate the effect for equilateral triangular nanoislands via numerical solutions of the appropriate Maxwell equations, and show that the field enhancement is several orders of magnitude larger than what the conventional theory predicts.

cond-mat.mes-hall↗

arXiv2010

Counting statistics of transport through Coulomb blockade nanostructures: High-order cumulants and non-Markovian effects

Recent experimental progress has made it possible to detect in real-time single electrons tunneling through Coulomb blockade nanostructures, thereby allowing for precise measurements of the statistical distribution of the number of transferred charges, the so-called full counting statistics. These experimental advances call for a solid theoretical platform for equally accurate calculations of distribution functions and their cumulants. Here we develop a general framework for calculating zero-frequency current cumulants of arbitrary orders for transport through nanostructures with strong Coulomb interactions. Our recursive method can treat systems with many states as well as non-Markovian dynamics. We illustrate our approach with three examples of current experimental relevance: bunching transport through a two-level quantum dot, transport through a nano-electromechanical system with dynamical Franck-Condon blockade, and transport through coherently coupled quantum dots embedded in a dissipative environment. We discuss properties of high-order cumulants as well as possible subtleties associated with non-Markovian dynamics.

cond-mat.mes-hall↗

arXiv2010

Atomic carbon chains as spin-transmitters: an \textit{Ab initio} transport study

An atomic carbon chain joining two graphene flakes was recently realized in a ground-breaking experiment by Jin {\it et al.}, Phys. Rev. Lett. {\bf 102}, 205501 (2009). We present {\it ab initio} results for the electron transport properties of such chains and demonstrate complete spin-polarization of the transmission in large energy ranges. The effect is due to the spin-polarized zig-zag edge terminating each graphene flake causing a spin-splitting of the graphene $π_z$ bands, and the chain states. Transmission occurs when the graphene $π$-states resonate with similar states in the strongly hybridized edges and chain. This effect should in general hold for any $π$-conjugated molecules bridging the zig-zag edges of graphene electrodes. The polarization of the transmission can be controlled by chemically or mechanically modifying the molecule, or by applying an electrical gate.

cond-mat.mes-hall↗

arXiv2010

Influence of confining potentials on the exchange coupling in double quantum dots

We report simple expressions for the exchange coupling in double quantum dots calculated within the Heitler-London and the Hund-Mulliken approximations using four different confining potentials. At large interdot distances and at large magnetic fields the exchange coupling does not depend significantly on the details of the potentials. In contrast, at low fields and short distances different behaviors of the exchange coupling can be attributed to particular features of the potentials. Our results may be useful as guidelines in numerical studies and in the modeling of experiments.

cond-mat.mes-hall↗

arXiv2009

Localized edge vibrations and edge reconstruction by Joule heating in graphene nanostructures

Control of the edge topology of graphene nanostructures is critical to graphene-based electronics. A means of producing atomically smooth zigzag edges using electronic current has recently been demonstrated in experiments [Jia et al., Science 323, 1701 (2009)]. We develop a microscopic theory for current-induced edge reconstruction using density functional theory. Our calculations provide evidence for localized vibrations at edge-interfaces involving unpassivated armchair edges. We demonstrate that these vibrations couple to the current, estimate their excitation by Joule heating, and argue that they are the likely cause of the reconstructions observed in the experiments.

cond-mat.mtrl-sci↗

arXiv2009

Electronic properties of graphene antidot lattices

Graphene antidot lattices constitute a novel class of nano-engineered graphene devices with controllable electronic and optical properties. An antidot lattice consists of a periodic array of holes which causes a band gap to open up around the Fermi level, turning graphene from a semimetal into a semiconductor. We calculate the electronic band structure of graphene antidot lattices using three numerical approaches with different levels of computational complexity, efficiency, and accuracy. Fast finite-element solutions of the Dirac equation capture qualitative features of the band structure, while full tight-binding calculations and density functional theory are necessary for more reliable predictions of the band structure. We compare the three computational approaches and investigate the role of hydrogen passivation within our density functional theory scheme.

cond-mat.mes-hall↗

arXiv2009

Atomistic theory for the damping of vibrational modes in mono-atomic gold chains

We develop a computational method for evaluating the damping of vibrational modes in mono-atomic metallic chains suspended between bulk crystals under external strain. The damping is due to the coupling between the chain and contact modes and the phonons in the bulk substrates. The geometry of the atoms forming the contact is taken into account. The dynamical matrix is computed with density functional theory in the atomic chain and the contacts using finite atomic displacements, while an empirical method is employed for the bulk substrate. As a specific example, we present results for the experimentally realized case of gold chains in two different crystallographic directions. The range of the computed damping rates confirm the estimates obtained by fits to experimental data [Frederiksen et al., Phys. Rev. B, 75, 205413(R)(2007)]. Our method indicates that an order-of-magnitude variation in the damping is possible even for relatively small changes in the strain. Such detailed insight is necessary for a quantitative analysis of damping in metallic atomic chains, and in explaining the rich phenomenology seen in the experiments.

cond-mat.mes-hall↗

arXiv2009

Surface decorated silicon nanowires: a route to high-ZT thermoelectrics

Based on atomistic calculations of electron and phonon transport, we propose to use surface decorated Silicon nanowires (SiNWs) for thermoelectric applications. Two examples of surface decorations are studied to illustrate the underlying deas: Nanotrees and alkyl functionalized SiNWs. For both systems we find, (i) that the phonon conductance is significantly reduced compared to the electronic conductance leading to high thermoelectric figure of merit, $ZT$, and (ii) for ultra-thin wires surface decoration leads to significantly better performance than surface disorder.

cond-mat.mes-hall↗

arXiv2009

DFT study of graphene antidot lattices: The roles of geometry relaxation and spin

Graphene sheets with regular perforations, dubbed as antidot lattices, have theoretically been predicted to have a number of interesting properties. Their recent experimental realization with lattice constants below 100 nanometers stresses the urgency of a thorough understanding of their electronic properties. In this work we perform calculations of the band structure for various hydrogen-passivated hole geometries using both spin-polarized density functional theory (DFT) and DFT based tight-binding (DFTB) and address the importance of relaxation of the structures using either method or a combination thereof. We find from DFT that all structures investigated have band gaps ranging from 0.2 eV to 1.5 eV. Band gap sizes and general trends are well captured by DFTB with band gaps agreeing within about 0.2 eV even for very small structures. A combination of the two methods is found to offer a good trade-off between computational cost and accuracy. Both methods predict non-degenerate midgap states for certain antidot hole symmetries. The inclusion of spin results in a spin-splitting of these states as well as magnetic moments obeying the Lieb theorem. The local spin texture of both magnetic and non-magnetic symmetries is addressed.

cond-mat.mes-hall↗

arXiv2009

Thermal rectification in nonlinear quantum circuits

We present a theoretical study of radiative heat transport in nonlinear solid-state quantum circuits. We give a detailed account of heat rectification effects, i.e. the asymmetry of heat current with respect to a reversal of the thermal gradient, in a system consisting of two reservoirs at finite temperatures coupled through a nonlinear resonator. We suggest an experimentally feasible superconducting circuit employing the Josephson nonlinearity to realize a controllable low temperature heat rectifier with a maximal asymmetry of the order of 10%. We also discover a parameter regime where the rectification changes sign as a function of temperature.

cond-mat.mes-hall↗

arXiv2009

Heat conductance is strongly anisotropic for pristine silicon nanowires

We compute atomistically the heat conductance for ultra-thin pristine silicon nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature thermal conductance is found to be highly anisotropic: wires oriented along the <110> direction have 50-75% larger conductance than wires oriented along the <100> and <111> directions. We show that the anisotropies can be qualitatively understood and reproduced from the bulk phonon band structure. Ab initio density functional theory (DFT) is used to study the thinnest wires, but becomes computationally prohibitive for larger diameters, where we instead use the Tersoff empirical potential model (TEP). For the smallest wires, the thermal conductances obtained from DFT- and TEP calculations agree within 10%. The presented results could be relevant for future phonon-engineering of nanowire devices.

cond-mat.mtrl-sci↗

arXiv2008

Electron- and phonon transport in silicon nanowires: an atomistic approach to thermoelectric properties

We compute both electron- and phonon transmissions in thin disordered silicon nanowires. Our atomistic approach is based on tight-binding and empirical potential descriptions of the electronic and phononic systems, respectively. Surface disorder is modeled by including surface silicon vacancies. It is shown that the average phonon- and electron transmissions through long SiNWs containing many vacancies can be accurately estimated from the scattering properties of the isolated vacancies using a recently proposed averaging method [Phys. Rev. Lett. 99, 076803 (2007)]. We apply this averaging method to surface disordered SiNWs in the diameter range 1-3 nm to compute the thermoelectric figure of merit, ZT. It is found that the phonon transmission is affected more by the vacancies than the electronic transmission leading to an increased thermoelectric performance of disordered wires, in qualitative agreement with recent experiments. The largest ZT>3 is found in strongly disordered <111> oriented wires with a diameter of 2 nm.

cond-mat.mes-hall↗

arXiv2008

Nanostructure design for surface-enhanced Raman spectroscopy - prospects and limits

Surface-enhanced Raman spectroscopy (SERS) allows single-molecule detection due to the strong field localization occurring at sharp bends or kinks of the metal-vacuum interface. An important question concerns the limits of the signal enhancement that can be achieved via a judicious design of the surface. By using a specific example of a technologically realizable nanopatterned surface, we demonstrate that while very high enhancement factors (~10^12) can be found for an ideal surface, these are unlikely to be achieved in laboratory samples, because even a minute, inevitable rounding-off strongly suppresses the enhancement, as well as shifts the optimal frequency. Our simulations indicate that the geometric enhancement factors are unlikely to exceed ~10^8 for real samples, and that it is necessary to consider the geometric uncertainty to reliably predict the frequency for maximum enhancement.

physics.optics↗

arXiv2008

Optical properties of graphene antidot lattices

Undoped graphene is semi-metallic and thus not suitable for many electronic and optoelectronic applications requiring gapped semiconductor materials. However, a periodic array of holes (antidot lattice) renders graphene semiconducting with a controllable band gap. Using atomistic modelling, we demonstrate that this artificial nanomaterial is a dipole-allowed direct gap semiconductor with a very pronounced optical absorption edge. Hence, optical infrared spectroscopy should be an ideal probe of the electronic structure. To address realistic experimental situations, we include effects due to disorder and the presence of a substrate in the analysis.

cond-mat.mes-hall↗

arXiv2008

Counting Statistics of Non-Markovian Quantum Stochastic Processes

We derive a general expression for the cumulant generating function (CGF) of non-Markovian quantum stochastic transport processes. The long-time limit of the CGF is determined by a single dominating pole of the resolvent of the memory kernel from which we extract the zero-frequency cumulants of the current using a recursive scheme. The finite-frequency noise is expressed not only in terms of the resolvent, but also initial system-environment correlations. As an illustrative example we consider electron transport through a dissipative double quantum dot for which we study the effects of dissipation on the zero-frequency cumulants of high orders and the finite-frequency noise.

cond-mat.mes-hall↗

arXiv2008

Graphene Antidot Lattices - Designed Defects and Spin Qubits

Antidot lattices, defined on a two-dimensional electron gas at a semiconductor heterostructure, are a well-studied class of man-made structures with intriguing physical properties. We point out that a closely related system, graphene sheets with regularly spaced holes ("antidots"), should display similar phenomenology, but within a much more favorable energy scale, a consequence of the Dirac fermion nature of the states around the Fermi level. Further, by leaving out some of the holes one can create defect states, or pairs of coupled defect states, which can function as hosts for electron spin qubits. We present a detailed study of the energetics of periodic graphene antidot lattices, analyze the level structure of a single defect, calculate the exchange coupling between a pair of spin qubits, and identify possible avenues for further developments.

cond-mat.mes-hall↗

arXiv2008

Ab initio study of spin-dependent transport in carbon nanotubes with iron and vanadium adatoms

We present an ab initio study of spin dependent transport in armchair carbon nanotubes with transition metal adsorbates, iron or vanadium. We neglect the effect of tube curvature and model the nanotube by graphene with periodic boundary conditions. A density functional theory based nonequilibrium Green's function method is used to compute the electronic structure and zero-bias conductance. The presence of the adsorbate causes a strong scattering of electrons of one spin type only. The scattering is shown to be due to coupling of the two armchair band states to the metal 3d orbitals with matching symmetry causing Fano resonances appearing as dips in the transmission function. The spin type (majority/minority) being scattered depends on the adsorbate and is explained in terms of d-state filling. The results are qualitatively reproduced using a simple tight-binding model, which is then used to investigate the dependence of the transmission on the nanotube width. We find a decrease in the width of the transmission dip as the tube-size increases.

cond-mat.mtrl-sci↗