Osamu Sakai

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

Kondo Lattice Effects of YbAl3 Suggested by Temperature Dependence of High-Accuracy High-Energy Photoelectron Spectroscopy

2e^{2}/h

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

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.

Optical Properties of Magnetic Exciton States in Europium-Chalcogenides

By use of high-resolution soft X-rays, intrinsic temperature dependences of bulk Yb 4f photoelectron spectra were obtained for fractured surfaces of high-purity YbAl 3 single crystals. The spectra ...

Observation of Bose-Einstein Condensation of Triplons in Quasi 1D Spin-Gap System Pb2V3O9

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.

Electronic Structure of Mixed Valence State

Various optical properties of magnetic exciton states in Eu-chalcogenides are investigated theoretically on the basis of detailed electronic band structures. The excited 5d(t 2g ) state extends from the central Eu ton onto the neighbouring ones by the amount of 40∼10%. This requires the treatment based on the detailed atomic model of the 5d and 4f electrons. The calculated results agree satisfactory with experimental data, such as the intensity, structures, the kinetic shift, the anomalous magnetic red shift and magneto-optical effects, for the broad band at the low energy end of the absorption spectrum. The calculated properties are not sensitive to the energy position of the exciton state relative to those of 6s band of Eu and p bands of chalcogen.

Magnetic Properties in the Mixed State of Ferromagnetic Superconductors

We observed a field-induced magnetic ordering in the quasi one-dimensional antiferromagnetic alternating chain system Pb 2 V 3 O 9 for the first time. The characteristic temperature dependence of t...

Resonant Light Scattering and Luminescence in Exciton-Phonon System. I

A model for the mixed valence state is presented. The basic lattice is composed of N trivalent rare earth ions. Extra N electrons enter either the localized f level at each ion site or the conduction band. When an electron occupies the f level of rare earth ion, this ion becomes divalent ion. The electron can jump from the f level to the conduction band and vice versa through the hybridization between these states. The effect of the strong intra-atomic Coulomb and the exchange interactions between the f electrons on the hybridized states is studied by applying the coherent potential approximation. The gap in the density of states and/or the mobility gap appear around the f level, and the Fermi level is located within the gap. The energy of the paramagnetic state is shown to be lower than that of the ferromagnetic state.

Monte Carlo Simulation for the Photo-induced Low-spin to High-Spin State Transitions –Difference and Similarity with the Thermal Transition–

The lower and the upper critical fields H c1 and H c2 , and the slope of magnetization at H c2 , (∂ M /∂ H ) c2 , are calculated in ferromagnetic superconductors. The theory takes account of the electromagnetic and the s-f exchange interactions between conduction electrons and localized f-electrons. When temperature decreases, (∂ M /∂ H ) c2 in the mixed state increases and diverges at a certain temperature. This indicates that below this temperature, the transition at H c2 is of a first order. Near the spin ordering temperature, H c2 decreases more rapidly than H c1 with decreasing temperature. The theoretical results are compared with experimental results in single crystals ErRh 4 B 4 in detail.

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

The resonant light scattering of exciton is studied, with particular attention to the coexistence of cascade LO phonon scattering lines, LA phonon scattering structures and luminescence, by using a model system consisting of one exciton band and LO and LA phonons. Including all the higher order exciton-phonon interactions within the ladder approximation combined with the real process approximation, an integral equation for exciton population is derived and solved numerically to give the spectral shape of the emitted light at low temperatures. It is shown how the real and virtual intermediate states contribute to the cascade LO phonon scattering spectrum, and how the broad luminescence band is formed through the accumulation of multiple scatterings of exciton by LA phonons. The effect of the nonradiative decay of exciton on the relative intensity between the Raman and luminescence spectra is also studied.