Session Y20: Focus Session: Electron, Ion, Exciton Transport in Nanostructures: Quantum Dots and Low-dimension Structures

Flux-dependent effects in degenerate and symmetric double dot Aharonov-Bohm interferometer with and without interactions

We study the steady-state characteristics and the transient behaviour of the non equilibrium double-dot Aharonov-Bohm interferometer using analytical tools and numerical simulations. Our simple setup includes non-interacting degenerate quantum dots that are coupled to two biased metallic leads at the same strength. A magnetic flux $\Phi$ is piercing the set-up perpendicularly. As we tune the degenerate dots energies away from the symmetric point we observe four non trivial magnetic flux control effects: (i) flux dependency of the dots occupation, (ii) magnetic flux induced occupation difference between the dots, at degeneracy, (iii) the effect of ``phase-localization" of the dots coherence holds only at symmetric point, while in general both real and imaginary parts are non-zero, and (iv) coherent evolution survives even when the dephasing strength, introduced into our model using Buttiker probe, is large and comparable to the dots energies and the bias voltage. Moreover, not only finite dephasing strength does not destroy the coherence features, it can provide a new type of coherent oscillation. These four phenomena take place when the dot energies are gated, away from the symmetric point,demonstrating the delicate controllability over the dot occupation and coherence.   

Optical phonon lasing in transport through semiconductor double quantum dots

We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller. Both effects disappear by an effective thermalization induced by the Franck-Condon effect in a DQD fabricated in a suspended carbon nanotube with strong electron-phonon coupling. $^1$ \newline $^1$ R.\ Okuyama, M.\ Eto, and T.\ Brandes, arXiv:1205.6955 (2012).   

Quantum Confined Silicon Clathrate Quantum Dots

Silicon (Si) allotropes can be synthesized in such a way that tetrahedrally bonded atoms form cage-like structures with bulk mechanical and opto-electronic properties distinct from those of diamond silicon (dSi). We use DFT, supplemented with many-body Green function analysis, to explore the structural stability of clathrate Si quantum dots (QDs) and to characterize their confinement as a function of crystal symmetry and size. Our results show that that there is a simple relationship between the confinement character of the QDs and the effective mass of the associated bulk crystals. Clathrate QDs and dSiQDs of the same size can exhibit differences of gap energies by as much as 2 eV. This offers the potential of synthesizing Si dots on the order of 1 nm that have optical gaps in the visible range but that do not rely on high-pressure routes such as those explored for the metastable BC8 and R8 phases. These results prompt the question as to how minimal quantum confinement can be in dots composed of Si. More broadly, clathrate QDs can in principle be synthesized for a wide range of semiconductors, and the design space can be further enriched via doping.   

Excess current noise in electrically conductive, crack-free, nanopatterned films of semiconductor nanocrystals

We present the first electrical measurements of semiconductor nanocrystal films that have nanoscale dimensions and are crack-free. These films make it possible to study the electrical properties intrinsic to the nanocrystals unimpeded by defects such as cracking and clustering that typically exist in larger-scale films. The films' dimensions are as small as 30 nm and are positioned on a surface with 30 nm precision. The electrical conductivity of the crack-free nanoscale films is 180 times higher than that of drop-cast, microscopic films made of the same type of nanocrystal. In the nanoscopic patterns, we find excess noise in the current that is thermally activated. This noise is unusual in that it is of a comparable order of magnitude to the average current, and both the average current and the noise fluctuate by several orders of magnitude in time. The noise increases with an applied field and with a gate. The inability to explain these effects by commonly known origins of electrical noise suggests that we are observing a novel effect in the nanocrystals.   

Energy spectrometry of electrons ejected from dynamic quantum dots driven up a potential slope by a surface acoustic wave

Surface acoustic waves (SAWs) in a GaAs/AlGaAs heterostructure generate an electrostatic wave which propagates at the sound velocity. This potential wave is capable of collecting electrons from a 2D electron gas (2DEG) and transporting them through a depleted channel. The SAW minima form a continuous series of dynamic quantum dots, each transporting a controllable number of electrons along the channel. The confinement of the electrons in each dot increases as the potential rises along the channel, ejecting electrons one-by-one back into the 2DEG above the Fermi energy. These electrons can travel several microns before thermalising. We measure their energy spectrum using a variable potential barrier upstream as the channel is squeezed by split gates, and correlate this with the SAW-driven current along the channel.   

Measurement of Valley Kondo Effect in a Si/SiGe Quantum Dot

The Kondo effect in Si/SiGe QDs can be enriched by the valley degree of freedom in Si. We have observed resonances showing temperature dependence characteristic of the Kondo effect in two consecutive Coulomb diamonds. These resonances exhibit unusual magnetic field dependence that we interpret as arising from Kondo screening of the valley degree of freedom. In one diamond two Kondo peaks due to screening of the valley index exist at zero magnetic field, revealing a zero-field valley splitting of $\Delta \approx$ 0.28 meV. In a non-zero magnetic field the peaks broaden and coalesce due to Zeeman splitting. In the other diamond, a single resonance at zero bias persists without Zeeman splitting for non-zero magnetic field, a phenomenon characteristic of valley non-conservation in tunneling.   

The impact of finite-area inhomogeneities on resistive and Hall measurement

I derive an iterative expression for the electric potential in an otherwise homogeneous thin specimen as the result of a finite-area inhomogeneity in either the direct conductance, the Hall conductance, or both. This expression extends to the finite-area regime the calculation of the effect of such inhomogeneities on the measurement error in the sheet resistance and Hall sheet resistance. I then test these results on the exactly-solvable case of a circular inhomogeneity equally distant from the four electrodes of either a square four-point-probe array on an infinitely large conducting specimen or a circular van der Pauw specimen with symmetrically-placed electrodes.   

Quasibound States and Evidence for a Spin 1 Kondo Effect in Asymmetric Quantum Point Contacts

Linear conductance below $2e^2/h$ shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even odd effect, observable in the longer channels, is consistent with the formation of quasi-localized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin one Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple-peak provides the first evidence suggestive of a spin singlet triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons.   

Enhancement of the excition emission in ZnO nanowires

The ZnO nanowires were grown by the chemical vapor transport method using a thin gold film as a catalyst. Their light emission in the visible and near UV spectral range was excited by continuous wave and pulsed UV light and by electrons within an SEM. The emission spectrum consists typically of the exciton emission band and a band in the green spectral range related to structural defects. We have followed the evolution of the ratio between the exciton and green band between our samples. The highly localized excitation by the electron beam allowed the profiling of the emission spectrum across the thickness of nanowire samples. We demonstrate that the tips of the nanowires show substantially higher exciton emission. Depth of excitation was varied independently by the electron accelerating voltage. The results have been interpreted within a model accounting for the surface effects and associated band banding at the surface.   

BEC-BCS crossover of a dipolariton condensate in a semiconductor microcavity

We study the electron-tunnel-coupling condensation of dipolar exciton-polariton (dipolariton) at the BEC-BCS crossover. An exciton-polariton (EP) is an extremely light bosonic quasiparticle composed of excitons and photons and can condense a temperatures as high as room temperature. Electron tunneling between nearby quantum wells can coupled spatially direct and indirect excitons and therefore also the corresponding exciton-polaritons; the indirect EPs in particular carry the interesting dipolar nature. We use a fermionic mean-field theory to examine the influence of this coupling on EP condensates from the dilute BEC to the dense BCS limits. A wide variety of distinct states are found, including mixed direct and indirect EP condensates, and metallic condensates, depending on particle-densities and on the relative positions of the quantum well states in different wells. Possible experimental manifestations of these phenomena will be discussed.   

Strain-controlled band engineering and Self-doping in Ultrathin LaNiO3 films

We discover a unique self-doping carrier transition by strain-induced in LaNiO$_3$ ultra thin film. Transport properties evolving from compressive to tensile strains are similar to those of different hole-doping superconducting cuprates. DFT calculations show the changes in low-energy electronic band structure account for the charge transfer between O p and Ni d states. The results indicate that ultrathin films can be used to change the carrier concentration transition metal oxides without resorting to chemical substitution.   

Monte Carlo simulations of electron transport for electron beam-induced deposition of nanostructures

Tungsten hexacarbonyl, W(CO)$_6$, is a particularly interesting precursor molecule for electron beam-induced deposition of nanoparticles, since it yields deposits whose electronic properties can be tuned from metallic to insulating. However, the growth of tungsten nanostructures poses experimental difficulties: the metal content of the nanostructure is variable. Furthermore, fluctuations in the tungsten content of the deposits seem to trigger the growth of the nanostructure. Monte Carlo simulations of electron transport have been carried out with the radiation-transport code Penelope in order to study the charge and energy deposition of the electron beam in the deposit and in the substrate. These simulations allow us to examine the conditions under which nanostructure growth takes place and to highlight the relevant parameters in the process.   

Monte Carlo simulations of neon versus helium ion beam induced deposition, sputtering and etching

The ion beam induced nanoscale synthesis of PtCx (where x $\sim$ 5) using the trimethyl (methylcyclopentadienyl)platinum(IV) (MeCpPt$^{\mathrm{IV}}$Me$_3)$ precursor is investigated by performing Monte Carlo simulations of helium and neon ions. The helium beam leads to more lateral growth relative to the neon beam because of its larger interaction volume. The lateral growth of the nanopillars is dominated by molecules deposited via secondary electrons in the both simulations. Notably, the helium pillars are dominated by SE-I electrons whereas the neon pillars by SE-II electrons. Using a low precursor residence time of 70$\mu $s resulting in an equilibrium coverage of $\sim$ 4{\%}, the neon simulation has a lower deposition efficiency (3.5{\%}) compared to that of the helium simulation (6.5{\%}). At larger residence time (10ms) and consequently larger equilibrium coverage (85{\%}) the deposition efficiencies of helium and neon increased to 49{\%} and 21{\%}, respectively; which is dominated by increased lateral growth rates leading to broader pillars. The nanoscale growth is further studied by varying the ion beam diameter at 10 ms precursor residence time. The study shows that total SE yield decreases with increasing beam diameters for the both ion types. However, the helium has the larger SE yield as compared to that of neon in the both low and high precursor residence time, and thus pillars are wider in all the simulations studied.   

Electron field emission from freestanding Diamond nanomembranes and Application to time-of-flight mass spectrometry

We introduce a prototype of a freestanding diamond nanomembrane for large protein detection in time-of-flight mass spectrometry. Doped diamond as a material for mass spectroscopy is extremely interesting due to its mechanical and electrical properties. The freestanding diamond nanomembranes we are able to fabricate have lateral extensions of $400\,\mu m\,\times \,400\,\mu m$ with a thickness of 100nm. We employ optical lithography and a Buffered Oxide Etch (BOE) of SiO$_{\mathrm{2}}$ followed by anisotropic etching of the substrate silicon using TMAH solution and finally removing SiO$_{\mathrm{2}}$. The electron field emission from the surface of the membrane is traced in the \textit{IV} characteristics at room temperature. The membrane is then applied for detection of the large ionized proteins using time-of-flight mass spectrometry. Ion detection is demonstrated in our nanomembrane MALDI-TOF analysis of Insulin (5,735 Da). That is when the ions with a large kinetic energy bombard the nanomembrane, their energy is thermalized upon impact into phonons. The phonons give a thermal energy to the electrons with the membrane, which are then excited to higher energetic states. Given an extraction voltage this leads to electron field emission from the membrane which we labeled phonon-assisted field emission (PAFE). In other words, the MALDI mass spectra are obtained by exploiting ballistic phonon propagation and quasi-diffusive phonon propagation.   

A Density Functional Study of the Redox Properties of H2TPP Porphyrin

Properties of the ground state of free base meso-tetraphenylporphyrin (H2TPP) have been calculated with various charges using the B3LYP functional and the 6-31$+$G basis set. The porphyrin skeleton was rippled and saddled and the meso-phenyl rings were twisted to yield the C$_{\mathrm{2h}}$, C$_{\mathrm{2v}}$, D$_{2}$, D$_{\mathrm{2h}}$ planar and D$_{\mathrm{2h}}$ non-planar symmetries and the structures optimized. The ground state structure was found to be of C$_{\mathrm{2v}}$ symmetry although the C$_{\mathrm{2h}}$, D$_{\mathrm{2}}$ and D$_{\mathrm{2h}}$ non-planar structures were basically degenerate, a conclusion not supported by experimental data. The C$_{\mathrm{2v}}$ structure indicated a nonzero net dipole moment for all levels of charge studied. Increase in negative charge increased the distortion of the H2TPP structure. The Raman spectra was calculated and compared with experimental data.\footnote{J.E. Parker, R. J. Thomas, D. R. Morrison, L. Brancaleon, \textit{J. Phys. Chem. B}, 2012, 116 (36), pp 11032--11040.} In addition, the results were used to select the most likely binding configuration among a set of solutions yielded by computational docking algorithms. Calculations using higher basis sets will also be presented.