Wataru Izumida

Two-impurity Kondo effect in double-quantum-dot systems: Effect of interdot kinetic exchange coupling

Tunneling conductance through two quantum dots, which are connected in series to left and right leads, is calculated by using the numerical renormalization group method. As the hopping between the dots increases from very small value, the following states continuously appear; (i) Kondo singlet state of each dot with its adjacent-site lead, (ii) singlet state between the local spins on the dots, and (iii) double occupancy in the bonding orbital of the two dots. The conductance shows peaks at the transition regions between these states. Especially, the peak at the boundary between (i) and (ii) has the unitarity limit value of

Effect of domain boundaries on the Raman spectra of mechanically strained graphene

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Kondo Effect in Single Quantum Dot Systems – Study with Numerical Renormalization Group Method –

because of coherent connection through the lead-dot-dot-lead. For the strongly correlated cases, the characteristic energy scale of the coherent peak shows anomalous decrease relating to the quantum critical transition known for the two-impurity Kondo effect. The two dots systems give the new realization of the two-impurity Kondo problem.

Spin–Orbit Interaction in Single Wall Carbon Nanotubes: Symmetry Adapted Tight-Binding Calculation and Effective Model Analysis

We investigate the effect of mechanical strain on graphene synthesized by chemical vapor deposition (CVD) transferred onto flexible polymer substrates by observing the change in the Raman spectrum and then compare this to the behavior of exfoliated graphene. Previous studies into the effect of strain on graphene have focused on mechanically exfoliated graphene, which consists of large single domains. However, for wide scale applications CVD produced films are more applicable, and these differ in morphology, instead consisting of a patchwork of smaller domains separated by domain boundaries. We find that under strain the Raman spectra of CVD graphene transferred onto a silicone elastomer exhibits unusual behavior, with the G and 2D band frequencies decreasing and increasing respectively with applied strain. This unusual Raman behavior is attributed to the presence of domain boundaries in polycrystalline graphene causing unexpected shifts in the electronic structure. This was confirmed by the lack of such behavior in mechanically exfoliated large domain graphene and also in large single-crystal graphene domains grown by CVD. Theoretical calculation of G band for a given large shear strain may explain the unexpected shifts while the shift of the Dirac points from the K point explain the conventional behavior of a 2D band under the strain.

Many Body Effects on Electron Tunneling through Quantum Dots in an Aharonov-Bohm Circuit

The tunneling conductance is calculated as a function of the gate voltage in wide temperature range for the single quantum dot systems with Coulomb interaction. We assume that two orbitals are active for the tunneling process. We show that the Kondo temperature for each orbital channel can be largely different. The tunneling through the Kondo resonance almost fully develops in the region \(T \lesssim 0.1 T_{\rm K}^{*} \sim 0.2 T_{\rm K}^{*}\), where T K * is the lowest Kondo temperature when the gate voltage is varied. At high temperatures the conductance changes to the usual Coulomb oscillations type. In the intermediate temperature region, the degree of the coherency of each orbital channel is different, so strange behaviors of the conductance can appear. For example, the conductance once increases and then decreases with temperature decreasing when it is suppressed at T =0 by the interference cancellation between different channels. The interaction effects in the quantum dot systems lead the sensitivit...

Kondo Effect in Electron Tunneling through Quantum Dots - Study with Quantum Monte Carlo Method -

Energy band for single wall carbon nanotubes with spin–orbit interaction is calculated using non-orthogonal tight-binding method. A Bloch function with spin degree of freedom is introduced to adapt the screw symmetry of nanotubes. The energy gap opened by spin–orbit interaction for armchair nanotubes, and the energy band splitting for chiral and zigzag nanotubes are evaluated quantitatively. Spin polarization direction for each split band is shown to be parallel to the nanotube axis. The energy gap and the energy splitting depend on the diameter and chirality in an energy scale of sub-milli-electron volt. An effective model for reproducing the low energy band structure shows that the two mechanism of the band modification, shift of the energy band in two dimensional reciprocal lattice space, and, effective Zeeman energy shift, are relevant. The effective model explains well the energy gap and splitting for more than 300 nanotubes within the diameter between 0.7 to 2.5 nm.

Kondo Effect in Tunneling through a Quantum Dot.

Tunneling conductance of an Aharonov-Bohm circuit including two quantum dots is calculated based on the general expression of the conductance in the linear response regime of the bias voltage. The calculation is performed in a wide temperature range by using numerical renormalization group method. Various types of AB oscillations appear depending on the temperature and the potential depth of the dots. Especially, AB oscillations have strong higher harmonics components as a function of the magnetic flux when the potential of the dots is deep. This is related to the crossover of the spin state due to the Kondo effect on quantum dots. When the temperature rises up, the amplitude of the AB oscillations becomes smaller reflecting the breaking of the coherency.

Angular momentum and topology in semiconducting single-wall carbon nanotubes

Based on the Quantum Monte Carlo (QMC) technique, we developed a method to calculate the tunneling conductance of the quantum dot systems for which the Kondo effect becomes important. The method computes the conductance directly from the current correlation function, and is applicable to the cases in which the calculation is not reduced to the computation of single particle Greens function. We compare the calculated conductance with experiments, and found good qualitative agreement. The temperature in experiments seems to be not low enough compared with the Kondo temperature in the mid region of the two paired Coulomb oscillation peaks. We studied the Zeeman field effect, and showed that the conductance is strongly suppressed when it has been enhanced by the Kondo effect. The conductance of an Aharonov-Bhom circuit including dots was also studied.

Valley coupling in finite-length metallic single-wall carbon nanotubes

Systematic calculation of tunneling conductance through a quantum dot, which has a single localized orbital, is carried out by using the numerical renormalization group method. In strongly correlated case, two paired Coulomb oscillation peaks grow without large increase of their width as the temperature decreases. At the same time the peak positions shift to their central side. This behavior corresponds well with the recent experimental data, and then suggests that the Kondo effect takes place at low temperatures in the experiment. Theoretically the conductance in valley region between the two peaks gradually increases, and the peaks merge into a broad single peak as the temperature further decreases. This behavior has not been observed in the experiment. The estimated Kondo temperature for the valley region is still much less than the lowest temperature in the experiment.

Quantum energy teleportation in a quantum Hall system

Semiconducting single-wall carbon nanotubes are classified into two types by means of orbital angular momentum of valley state, which is useful to study their low energy electronic properties in finite-length. The classification is given by an integer