Session G22: Electronic Phenomena of Nanostructures

Fermi liquid nature of the ground state of multiple-quantum dots in parallel

We argue through a combination of 1/N diagrammatic expansion, slave boson mean field theory and the Bethe ansatz that the ground state of multiple quantum dots arranged in parallel is a singlet Fermi-liquid ground state. This conclusion is arrived at by showing the validity of Friedel Sum Rule (a fingerprint of Fermi-liquid physics) and finding that impurity entropy vanishes in the limit of zero temperature (singlet). Our conclusion is in contradiction to other studies that predict a non-Fermi liquid ground state. We discuss possible reasons for this discrepancy.   

Coupled collective modes in electronic systems of different dimensionalities

We consider electronic collective modes in coupled systems in which the individual components have different dimensionalities. Many-body diagramnatic techniques are used to derive formal results for the screened intra- and inter-system Coulomb interaction. We specifically investigate the case of a quasi-one-dimensional quantum wire in close proximity to a two-dimensional electron gas. We evaluate the screened intra- and inter-system Coulomb interaction within the random phase approximation, and find the existence of modes which have hybrid properties characteristic of both one- and two-dimensional systems. We also investigate the spatial dependence of the coupled 1-d + 2-d collective modes within the two-dimensional electron gas, and show that the coupled modes within that layer vary from being purely two-dimensional in character far away from the quantum wire to being strongly hybridized close to the wire.   

Influence of Rashba spin-orbit interactions on the Kondo effect

Recent studies [1] have pointed out that the thermodynamics of the Kondo effect are essentially unaltered by the presence of Rashba spin-orbit interactions in a host two-dimensional electron gas. However, it has also been proposed [2] that the presence of bulk Rashba interactions induces a coupling between a magnetic impurity and conduction electrons with nonzero orbital angular momentum about the impurity site. In this work we revist this problem using the numerical renormalization group. In agreement with previous studies, we find only minor changes in the Kondo temperature scale when the Rashba coupling is increased at fixed Fermi energy. However, for fixed band filling, increasing the spin-orbit coupling can move the Fermi energy near to a Van Hove singularity in the effective density of states, leading to an exponential enhancement of the Kondo scale. Static spin correlations confirm that the impurity couples to conduction channels of nonzero orbital angular momentum. We also explore the effects of a magnetic field applied in the plane of the host system.\\[4pt] [1] J. Malecki, J. Stat. Phys. 129, 741 (2007); R. \u{Z}itko and J. Bon\u{c}a, Phys. Rev. B 84, 193411 (2011). \newline [2] M. Zarea, S. Ulloa and N. Sandler, Phys. Rev. Lett. 108, 046601 (2012).   

Spin-polarized conductance in double quantum dots: Interplay of Kondo, Zeeman and orbital effects

We study the effect of an external magnetic field in the Kondo regime of a double-quantum-dot system in which a strongly correlated dot is coupled to a noninteracting dot that is also connected to external leads. In zero field, the spectral function of the hanging dot has previously been shown to exhibit a split-peak structure near the Fermi level due to ``Kondo resonance filtering'' by the noninteracting dot. We show, using the numerical renormalization group, that application of a magnetic field leads to a subtle interplay between electronic interference, Kondo physics, and Zeeman splitting with nontrivial consequences for the spectral and transport properties. The value of the correlated-dot spectral function at the Fermi level exhibits a nonuniversal field dependence that can be explained using a generalized Friedel sum rule for a Kondo system with energy-dependent hybridization. By tuning gate voltages and the magnetic field, one can achieve complete spin polarization of the linear conductance between the leads, raising the prospect of applications of the device as a highly tunable spin filter.   

Controlling entanglement and spin-correlations in double quantum dots with electrical currents in the non-equilibrium regime

We study the non-equilibrium dynamics in a parallel double-quantum dot structure induced by a large bias voltage. By applying both a magnetic flux and a voltage, it is possible to generate spin-spin-correlations between the two quantum dots. The sign and absolute value of these correlations can be controlled by changing the bias voltage. Using a canonical transformation we argue that the mechanism that drives the spin-spin correlations can be understood in terms of an effective Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction that is mediated by the current. Our study is based on the Anderson-impurity model and we use time-dependent density matrix renormalization group simulations to obtain currents and spin-correlations in the non-equilibrium regime. We also perform quench in the Hamiltonian to prove the stability of the entangled state.   

SU(4) Kondo effect in a double quantum dot

Lateral quantum dots are highly tunable experimental systems ideal for exploring the interplay of spin and charge correlations. We present studies of a parallel-coupled double quantum dot system in a GaAs/AlGaAs heterostructure. In the limit of negligible inter-dot tunneling, the conductance through both dots is enhanced at inter-dot orbital degeneracies, where the energy for an electron to be on either dot is the same. We show how at four-fold orbital and spin degeneracies, signatures in the zero-bias conductance, the temperature dependence, and the bias spectroscopy suggest an SU(4) Kondo effect may be realized, combining spin and pseudospin.   

What Is Measured in the Scanning Gate Microscopy of a Quantum Point Contact?

The conductance change due to a local perturbation in a phase-coherent nanostructure is calculated. The general expressions to first and second order in the perturbation are applied to the scanning gate microscopy of a two-dimensional electron gas containing a quantum point contact. The first-order correction depends on two scattering states with electrons incoming from opposite leads and is suppressed on a conductance plateau; it is significant in the step regions. On the plateaus, the dominant second-order term likewise depends on scattering states incoming from both sides. It is always negative, exhibits fringes, and has a spatial decay consistent with experiments.   

Understanding Nanocontacts with atomic precision

Measuring the variations of the conductance indentation experiments between two electrodes, we can obtain information on the changes in the atomic structure of the contact. We have analysed the Jump-to-Contact(JC) phenomenon which can observed as the first contact when the two metals approach each other. Moreover, we have studied the Jump-out-of-contact(JOC) phenomenon which is the last contact before breaking the two electrodes. Secondly, as we further approach the two electrodes and when the indentation depth is limited to a certain value of conductance, almost the exact behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. That is, the same sequence of atomic configurations was followed. Both processes are rationalized using MD simulations together with DFT transport calculations, which show: a) the most probable atomic configurations in the first atomic contact following the JC or JOC processes; b) that after repeated indentations the two metallic electrodes are shaped into tips of a reproducible structure formed through a mechanical annealing process. These results improve our understanding of atomic-sized contacts and the evolution of their structural characteristics.   

Emergent Localization from Many-Body Physics in Clean Quantum Point Contacts

Quantized conductance in quantum point contacts (QPCs) is the signature of control over electron transport at the nanoscale. As a function of channel width the conductance then increases in steps of $G_0=2e^2/h$. However, experiments often show an additional feature with a conductance plateau near $0.7G_0$, known as the 0.7 anomaly. This has been studied since 1995 but its full understanding is still an open problem. Spontaneous localization due to many-body effects in open QPCs, and the associated Kondo effect, has emerged as a promising theory for the 0.7 anomaly [1]. This theory work predicted that the many-body effects should for certain QPC geometries not give a single localized state but a pair of localized states, but signatures of this were till now not reported. For the first time, we have fabricated length-tunable QPCs in clean semiconductors and we discovered a periodic modulation of the 0.7 anomaly as a function of length. This modulation correlates with signatures for single and paired quasi-localized states, in the form of single- and two-impurity Kondo physics. Our results demonstrate that Friedel oscillations and emergent impurity states from many-body physics are fundamental to these phenomena. [1] T. Rejec and Y. Meir, Nature 442, 900 (2006).   

Electrolyte gating of gold point contacts

Gold point contacts are fabricated in-situ by electromigration in an ionic liquid bath. These contacts are shown to be stable at room temperature at conductances as small as 50 G$_0$. By electrolyte gating the contacts using a counter electrode in the ionic liquid, conductance changes of 25$\%$ are observed, corresponding to accumulation of more than one electron per gold surface atom. Double step chronocoulometry and x-ray reflectometry suggest that ion ordering in the ionic liquid near the gold interface are consistent with the observed changes in conductance.   

Electron energy spectra in two dimensional quantum rings consisting of two nanoelements

Electron spectrum and ground state properties in two dimensional confined quantum rings with R$_{1,2}$ radiuses consisting of the two different (materials) nanoelements divided by two sectored finite size quantum wells with various potentials and spanning angles, is studied in the presence of transverse magnetic field. The calculated wave function shows oscillations along the radial direction which are progressing by approaching to the internal radius of the ring, R$_2$. Situation here is similar to the problem of fall of the particle on the attractive center. However, these oscillations are interrupted on the internal ring boundary by providing the new ground state which is sensitive to the change of magnetic flux. For shallow energy levels some energy states are undergoing changes controlled by magnetic field accompanied with the persistent current and abrupt phase transitions. Magnetization and magnetic susceptibility show characteristic two dimensional anomalous behaviors different from one found in one dimensional ring.   

Lateral quantization of two-dimensional electron states by embedded Ag nanocrystals

We show that quantization of image-potential state (IS)electrons \emph{above} the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy the $n = 1$ IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons \emph{in} the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect induced scattering \emph{above} nanostructured surfaces.   

MoS$_2$ grain-boundary: First-principles investigations

We present results of our first-principles electronic structure investigations, using the spin-polarized density-functional-theory, of the electronic and geometric structures of various models of grain-boundaries formed between different MoS$_2$ domains when grown as a single layer. From analysis of electronic band structures, we find, in all considered models, that the grain-boundaries exhibit metallic behavior. More interestingly, we find signatures of magnetism in the grain-boundary formed between two sulfur edges with 0\% sulfur coverage. Details analysis of the geometric structures lead us to the conclusion that certain grain-boundaries undergo $(2\times 1)$ reconstructions. We provide full details of the electronic and spin density states and change redistribution at the domain boundaries. We make contact with recent experimental observations and discuess the modifications in the characteristics for MoS$_2$ grown on Cu(111) [1]. \\[4pt] [1] D. Kim \textit{et al}, Langmuir \textbf{27}, 11650 (2011) and unpublished results.   

Ab initio Simulations of charge transfer properties at Quantum Dot/TiO$_{2}$ Interface in Quantum Dot-Sensitized Solar Cells

Quantum dot-sensitized solar cells (QDSSCs) have emerged as a very promising solar architecture for next generation photovoltaics. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO$_{2}$ acceptor with size quantization properties that allows for the modulation of QD band gaps to control the photoresponse and photoconversion efficiency of QDSSCs. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs on the TiO$_{2}$ substrate were simulated using a rigorous ab initio method. In contrast to the plane wave approaches, this method capitalized on localized orbital basis set that is computationally less intensive, and provides excellent electronic structure of the constituent systems. We consider QDs grown on TiO$_{2\, }$without functional ligands passivating the QD surface. We find a remarkable set of electron bridging states between QDs and TiO$_{2}$ occurring via the strong bonding between the conduction bands of QDs and TiO$_{2}$. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO$_{2}$ acceptor, and may be a general feature for other strongly coupled donor/acceptor systems and nanostructured semiconductor interfaces.   

Effect of hydrogen passivation on the electronic structure of ionic semiconductor nanostructures

In theoretical studies of thin film and nanostructured semiconductors, pseudohydrogen (PH) is widely used to passivate the surface dangling bonds. Based on these calculations, it is often believed that nanostructured semiconductors, due to quantum confinement, have a larger band gap than their bulk counterparts. Using first- principles band structure theory calculation and comparing systematically the differences between PH-passivated and real-hydrogen--passivated (RH-passivated) semiconductor surfaces and nanocrystals, we show that, unlike PH passivation that always increases the band gap with respect to the bulk value, RH passivation of the nanostructured semiconductors can either increase or decrease the band gap, depending on the ionicity of the nanocompounds. The difference between PH and RH passivations decreases when the covalency of the semiconductor increases and can be explained using a band coupling model. This observation greatly increases the tunability of nanostructured semiconductor properties, especially for wide-gap ionic semiconductors.