Transport Through a Quantum Dot with Coulombic Dot-Lead Coupling

We investigate joint effects of the Coulombic dot-lead interaction and intralead electron interaction on current through a quantum dot weakly coupled to Luttinger liquid leads. A general formula of current is derived by applying the canonical transformation and the nonequilibrium Green function technique. At low temperature and weak intralead interaction, the current exhibits staircase behavior with the Coulombic dot-lead interaction. When temperature or the intralead interaction increases, the steps are rounded. For a weak or moderately strong interaction the differential conductance as a function of bias voltage demonstrates resonant behavior. The dot-lead exchange scattering processes can dominate electron transport for a certain region of interaction strength. This result implies the possibility of controlling the differential conductance of the transistor by tuning the Coulombic dot-lead coupling.     

Gate-tunable split Kondo effect in a carbon nanotube quantum dot

We show a detailed investigation of the split Kondo effect in a carbon nanotube quantum dot with multiple gate electrodes. Two conductance peaks, observed at finite bias in nonlinear transport measurements, are found to approach each other for increasing magnetic field, to result in a recovered zero bias Kondo resonance at finite magnetic field. Surprisingly, in the same charge state, but under different gate configurations, the splitting does not disappear for any value of the magnetic field, but we observe an avoided crossing. We think that our observations can be understood in terms of a two-impurity Kondo effect with two spins coupled antiferromagnetically. The exchange coupling between the two spins can be influenced by a local gate, and the non-recovery of the Kondo resonance for certain gate configurations is explained by the existence of a small antisymmetric contribution to the exchange interaction between the two spins.     

Transport properties of coupled semiconductor quantum dots

We have measured the electronic transport properties of the coupled quantum dot devices at low temperatures. The interplay between the strong many body spin interaction and the molecular states are probed in linear and non-linear transport regime. We observe the formation of strong coherent molecular states clearly visible in the double dot conductance phase diagram. In our study, the spin configuration in multiply coupled quantum dots could be identified using Kondo phenomenon. In addition, the characteristics of the spin dependent molecular states and phase dependant tunneling have been also observed using non-linear conductance measurement of the double dots. The results suggest the importance of the diverse spin related physical issues in artificial quantum dot devices.     

Kondo effect in electromigrated gold break junctions

We present gate-dependent transport measurements of Kondo impurities in bare gold break junctions, generated with high yield using an electromigration process that is actively controlled. Thirty percent of measured devices show zero-bias conductance peaks. Temperature dependence suggests Kondo temperatures approximately 7 K. The peak splitting in magnetic field is consistent with theoretical predictions for g = 2, though in many devices the splitting is offset from 2g mu(B)B by a fixed energy. The Kondo resonances observed here may be due to atomic-scale metallic grains formed during electromigration.     

Spin-Resolved STM for a Kondo Impurity

We investigate the bias dependence of the tunneling conductance between a spin-polarized (SP) tip of a scanning tunneling microscope (STM) and a metallic surface with a magnetic impurity. The Fano–Kondo interference between the conduction channels tip-host and tip-impurity is considered. We pay particular attention to the interplay between the lateral tip-impurity distance R and the ferromagnetism of the tip. We observe a strong dependence of the conductance with respect to both R and the tip magnetization degree. In particular, for small values of R, a conductance plateau around the Fermi energy is observed, due to the interplay between Kondo effect, quantum interference, and ferromagnetism of the tip. For full polarized tip, we find a shift of the Fano–Kondo profile toward negative bias voltages.     

The Influence of Device Geometry on Many-Body Effects in Quantum Point Contacts: Signatures of the 0.7 Anomaly,Exchange and Kondo

The conductance of a quantum point contact (QPC) shows several features that result from many-body electron interactions. The spin degeneracy in zero magnetic field appears to be spontaneously lifted due to the so-called 0.7 anomaly. Further, the g-factor for electrons in the QPC is enhanced, and a zero-bias peak in the conductance points to similarities with transport through a Kondo impurity. We report here how these many-body effects depend on QPC geometry. We find a clear relation between the enhanced g-factor and the subband spacing in our QPCs, and can relate this to the device geometry with electrostatic modeling of the QPC potential. We also measured the zero-field energy splitting related to the 0.7 anomaly, and studied how it evolves into a splitting that is the sum of the Zeeman effect, and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations and no clear dependence on QPC geometry, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting. Signatures of the Kondo effect also show no regular dependence on QPC geometry, but are possibly correlated with splitting of the 0.7 anomaly.     

The dynamical conductance of graphene tunnelling structures

The dynamical conductances of graphene tunnelling structures were numerically calculated using the scattering matrix method with the interaction effect included in a phenomenological approach. The overall single-barrier dynamical conductance is capacitative. Transmission resonances in the single-barrier structure lead to dips in the capacitative imaginary part of the response. This is different from the ac responses of typical semiconductor nanostructures, where transmission resonances usually lead to inductive peaks. The features of the dips depend on the Fermi energy. When the Fermi energy is below half of the barrier height, the dips are sharper. When the Fermi energy is higher than half of the barrier height, the dips are broader. Inductive behaviours can be observed in a double-barrier structure due to the resonances formed by reflection between the two barriers.     

Electronic transport through ruthenium-based redox-active molecules in metal-molecule-metal nanogap junctions

Electronic transport through ruthenium-based redox-active organometallic molecules is measured by self-assembling diruthenium(III) tetra(2-anilinopyridinate)-di(4-thiolphenylethynyl) (trans-Ru2(ap)4(C'CC6H4S-)2 (A) and trans- Ru2(ap)4((C'CC6H4)2S-)2 (B) molecules in nanogap molecular junctions. Voltage sweeps at a high scan rate show low bias current peaks (at +/-0.35 +/- 0.05 V for A and +/-0.27 +/- 0.05 V for B), which change to plateaus in slow bias scans and a second conductance peak at approximately +/-1.05 +/- 0.15 V. The peaks/plateaus are not observed in the return bias sweeps, possibly due to charge storage in the molecules. The energy states for the molecular orbitals of these molecules as estimated from the conductance peaks are in close agreement with the respective energy values from voltammetric measurements in solution.     

Out-of-Equilibrium Singlet-Triplet Kondo Effect in a Single C60 Quantum Dot

We have used an electromigration technique to fabricate a C₆₀ single-molecule transistor (SMT). Besides describing our electromigration procedure, we focus and present an experimental study of a single molecule quantum dot containing an even number of electrons, revealing, for two different samples, a clear out-of-equilibrium Kondo effect. Low temperature magneto-transport studies are provided, which demonstrates a Zeeman splitting of the finite bias anomaly.     

Thermoelectric Transport Properties of a Quantum Wire,Coupled to a Quantum Dot: Finite Band Effects

We study the thermoelectric transport properties through a quantum wire, modeled on a tight-binding linear chain, with an embedded gate-defined quantum dot. We obtain the thermopower, thermal conductance and electrical conductance with a lateral Fano resonance, linked to a many-body renormalized quantum dot resonant level at the edge of the conduction band strongly hybridized with the van Hove singularity of the one-dimensional density of states of the lead; this resonance appears above the Kondo temperature and is due to a quantum interference thermally activated. We discuss the possibility of practical application of the system to a mesoscopic cooling process and thermopower generators, based on the thermoelectric figure of merit and thermal conductance values. Our results for the thermal transport properties are consistent with those obtained previously for electronic transport.     

Spin filtering and quantum phase transition in double quantum dots attached to spin-polarized leads

We study the spin filtering and quantum phase transition (QPT) in double quantum dots attached to spin-polarized leads. For spin-independent leads, we observe a Kosterlitz-Thouless transition between the local triplet and doublet. For spin-polarized leads, the above QPT becomes first order, and Kondo splitting, gate-controlled spin reversal and a perfect spin filtering are observed. The breaking of spin-rotation SU(2) symmetry and the interdot transport mediated by the conduction electron are responsible for the fully spin-polarized conductance. Because spin-polarized leads suppress the Kondo effect, in order to obtain a large conductance with perfect spin filtering, one should choose leads with small spin polarization, such as Rashba spin-orbital coupling leads.     

Theory of High Bias Coulomb Blockade in Ultrashort Molecules

We point out that single electron charging effects such as coulomb blockade (CB) and high-bias staircases play a crucial role in transport through single ultrashort molecules. A treatment of CB through a prototypical molecule, benzene, is developed using a master-equation in its complete many-electron Fock space, evaluated through exact diagonalization or full configuration interaction (CI). This approach can explain a whole class of nontrivial experimental features including vanishing zero bias conductances, sharp current onsets followed by ohmic current rises, and gateable current levels and conductance structures, most of which cannot be captured even qualitatively within the traditional self-consistent field (SCF) approach coupled with perturbative transport theories. By comparing the two approaches, namely SCF and CB, in the limit of weak coupling to the electrode, we establish that the inclusion of strong correlations within the molecule becomes critical in addressing the above experiments. Our approach includes on-bridge correlations fully, and is therefore well-suited for describing transport through short molecules in the limit of weak coupling to electrodes.     

Electric field control of spin rotation in bilayer graphene

The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin field-effect transistors. An alternative solution can be found in magnetic doping of graphene or, as discussed in the present work, in exploiting the proximity effect between graphene and ferromagnetic oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction, that acts as an effective Zeeman field for electrons in graphene, inducing a spin precession around the magnetization axis of the FO. Here we show that in an appropriately designed double-gate field-effect transistor, with a bilayer graphene channel and FO used as a gate dielectric, spin-precession of carriers can be turned ON and OFF with the application of a differential voltage to the gates. This feature is directly probed in the spin-resolved conductance of the bilayer.     

Low-temperature electronic transport in single K(0.27)MnO(2)·0.5H(2)O nanowires: enhanced electron-electron interaction

The current-voltage (I-V) characteristics and electrical resistivity of isolated potassium manganese oxide (K(0.27)MnO(2)·0.5H(2)O) nanowires prepared by a simple hydrothermal method were investigated over a wide temperature range from 300 to 4?K. With lowering temperature, a transition from linear to nonlinear I-V curves was observed around 50?K, and a clear zero bias anomaly (i.e., Coulomb gap-like structure) appeared on the differential conductance (dI/dV) curves, possibly due to enhanced electron-electron interaction at low temperatures. The temperature dependence of resistivity, [Formula: see text], follows the Efros-Shklovskii (ES) law, as expected in the presence of a Coulomb gap. Here we note that both the ES law and Coulomb blockade can in principle lead to a reduced zero bias conductance at low temperatures; in this study we cannot exclude the possibility of Coulomb-blockade transport in the measured nanowires, especially in the low-temperature range. It is still an open question how to pin down the origin of the observed reduction to a Coulomb gap (ES law) or Coulomb blockade.     

Tuning the bipolar conductance of an alkali-doped C(60) layer sandwiched between two tunneling barriers

Resonant tunneling through a C(60) monolayer doped with single Na, K, Rb, and Cs atoms was measured between the tip of a scanning tunneling microscope and a NiAl(110) substrate. By supporting the monolayer on a thin aluminum oxide film grown on the substrate, a double barrier tunnel junction is formed, consisting of the vacuum and oxide. This geometry enables conductance through an electronic state of the alkali-C(60) complex at both positive and negative sample bias. The positions of the conductance peaks can be varied by tuning the vacuum barrier. An opposite variation is found for Na and K as compared to Rb and Cs, suggesting the influence of bonding on nanoscale transport.     

Spin Polarization Dependence of Zero Bias Conductance in Ferromagnetic Quantum Wire/ D-wave Superconductor Junctions

A relation between the zero-bias conductance and spin polarization in a ferromagnetic quantum wire/ d-wave superconductor junction at finite temperature is studied, theoretically. In contrast to conventional d-wave superconductors which zero bias conductance peaks appear in tunneling spectra, we have found a disappearance of zero bias peak in tunneling spectra, in a way that it will be zero for all of barriers and polarizations parameters. Also, different temperature dependencies of zero bias conductance will be discussed.     

Effects of strong correlations in single electron traps in field-effect transistors

We study effects of strong electron-electron and electron-phonon correlations in single electron traps in metal-oxide-semiconductor field effect transistors (FETs). In order to explain the strong suppression of single electron tunneling in the trap, we introduce a model in which the excess charge of the trap couples to a local lattice deformation. By using nonperturbative techniques, we derive an effective low-energy action for the system. The behavior of the system is characterized by simultaneous polaron tunneling (corresponding to the charging and discharging of the trap) and Kondo screening of the trap spin in the singly occupied state. Hence, the obtained state of the system is a hybrid between the Kondo regime, typically associated with single electron occupancy, and the mixed valence regime, associated with large charge fluctuations. In the presence of a strong magnetic field, we demonstrate that the system is equivalent to a two-level system coupled to an Ohmic bath, with a bias controlled by the applied magnetic field. Due to the Kondo screening, the effect of the magnetic field is significantly suppressed in the singly occupied state. We claim that this suppression can be responsible for the experimentally observed anomalous magnetic field dependence of the average trap occupancy in Si-Si0/sub 2/ FETs.     

Graphene nanomesh-based devices exhibiting a strong negative differential conductance effect

Using atomistic quantum simulation based on a tight binding model, we have investigated the transport characteristics of graphene nanomesh-based devices and evaluated the possibilities of observing negative differential conductance. It is shown that by taking advantage of bandgap opening in the graphene nanomesh lattice, a strong negative differential conductance effect can be achieved at room temperature in pn junctions and n-doped structures. Remarkably, the effect is improved very significantly (with a peak-to-valley current ratio of a few hundred) and appears to be weakly sensitive to the transition length in graphene nanomesh pn hetero-junctions when inserting a pristine (gapless) graphene section in the transition region between n and p zones. The study therefore suggests new design strategies for graphene electronic devices which may offer strong advantages in terms of performance and processing over the devices studied previously.     

Nonreciprocal effects in the double passage of light through a random diffuser and a birefringent crystal

We present a theoretical and experimental study of the scattering of light by double passage through a system that consists of a strong diffuser, a piece of birefringent crystal, and a plane mirror. We show that this arrangement can produce not only enhanced backscattering and satellite peaks but also satellite dips in the angular distribution of the mean intensity. The experiments are in agreement with theoretical results based on scalar diffraction theory in the paraxial approximation.     

On the Sarma-Type Phase Transitions in Ferromagnet/Superconductor/Ferromagnet Tunneling Junctions with Highly Spin-Polarized Ferromagnets

In the ferromagnet/superconductor/ferromagnet double tunneling junctions, the spin-polarized tunneling current in the antiparallel alignment of the magnetization induces spin imbalance in the superconductor, which has a pair breaking effect depressing superconductivity in the same way that the Zeeman effect does in the paramagnetic limit. In particular, it is shown that when the ferromagnets are highly spin polarized, the strong spin imbalance may lead to a first-order phase transition from the superconducting phase to the normal phase at low temperature and low bias voltage. This phase transition accompanies a large discontinuous drop in superconducting gap parameter bring in distinctive features in low energy transport.