Yuh Tomio

Excitonic BCS-BEC crossover at finite temperature: Effects of repulsion and electron-hole mass difference

The BCS to Bose-Einstein condensation (BEC) crossover of electron-hole (e-h) pairs in optically excited semiconductors is studied using the two-band Hubbard model with both repulsive and attractive interactions. Applying the self-consistent t-matrix approximation combined with a local approximation, we examine the properties of a normal phase and an excitonic instability. The transition temperature from the normal phase to an e-h pair condensed one is studied to clarify the crossover from an e-h BCS-like state to an excitonic Bose-Einstein condensation, which takes place on increasing the e-h attraction strength. To investigate effects of the repulsive interaction and the e-h mass difference, we calculate the transition temperature for various parameters of the interaction strengths, the e-h particle density, and the mass difference. While the transition temperature in the e-h BCS regime is sufficiently suppressed by the repulsive interaction, that of the excitonic BEC is largely insensitive to it. We also show quantitatively that in the whole regime the mass difference leads to large suppression of the transition temperature.

Phase diagram for the exciton Mott transition in infinite-dimensional electron–hole systems

Abstract To understand the essence of the exciton Mott transition in three-dimensional electron–hole systems, the metal–insulator transition is studied for a two-band Hubbard model in infinite dimensions with interactions of electron–electron (hole–hole) repulsion U and electron–hole attraction - U ′ . By using the dynamical mean-field theory, the phase diagram in the U– U ′ plane is obtained (which is exact in infinite dimensions) assuming that electron–hole pairs do not condense. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott–Hubbard insulator for U > U ′ and the biexciton-like insulator for U U ′ . Even when away from half-filling, we find the phase transition between the exciton- or biexciton-like insulator and a metallic state. This transition can be assigned to the exciton Mott transition, whereas the Mott–Hubbard transition is absent.

Tunable electronic correlation effects in nanotube-light interactions

Electronic many-body correlation effects in one-dimensional (1D) systems such as carbon nanotubes have been predicted to modify strongly the nature of photoexcited states. Here we directly probe this effect using broadband elastic light scattering from individual suspended carbon nanotubes under electrostatic gating conditions. We observe significant shifts in optical transition energies, as well as line broadening, as the carrier density is increased. The results demonstrate the differing role of screening of many-body electronic interactions on the macroscopic and microscopic length scales, a feature inherent to quasi-1D systems. Our findings further demonstrate the possibility of electrical tuning of optical transitions and provide a basis for understanding of various optical phenomena in carbon nanotubes and other quasi-1D systems in the presence of charge carrier doping.

Novel Collective Mode in Quarter-Filled SDW States with Next-Nearest-Neighbor Coulomb Interaction

The collective modes representing density fluctuation in a spin density wave (SDW) have been examined for a one-dimensional electron system of a quarter-filled extended Hubbard model with the next-nearest-neighbor interaction ( V 2 ). Within the random phase approximation (RPA), we obtain a new collective mode, which appears with increasing V 2 . The mode describes the relative motion between density wave with up spin and that with down spin. The gap in the excitation spectrum vanishes at a critical value of V 2 corresponding to the onset of the coexistence of 2 k F -SDW, 2 k F -charge density wave and 4 k F -SDW in the ground state ( k F being the Fermi wave vector).

Coulomb Enhancement and Suppression of Peak Gain in Quantum Wire Lasers

The gain characteristics in quantum-wire-laser devices in the presence of many-body Coulomb interactions are systematically investigated by using the screened Hartree–Fock theory. We quantitatively analyze the peak gain, the transparency density, and the differential gain based on numerical calculations of the optical gain spectra in a model quantum-wire laser. We found that the transparency density is almost unaffected by the Coulomb interactions, and that the Coulomb enhancement of peak gain or differential gain takes place near the transparency density. At high density, however, a significant suppression of peak gain occurs, which is a peculiar feature in quantum-wire lasers.

Dynamical mean-field theory for the exciton Mott transition in electron?hole systems

In electron–hole (e–h) systems in photoexcited insulators, the exciton Mott transition may take place, which is a typical photoinduced phase transition. To understand how the exciton Mott transition depends on the e–h carrier density and the Coulomb correlation, we study an e–h two-band Hubbard model by means of the dynamical mean-field theory assuming that electron–hole pairs do not condense. The phase diagram on the plane of interactions at zero temperature is obtained. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott–Hubbard insulator and the biexciton-like insulator. Away from half-filling we find another insulating phase, the exciton-like insulator, which is followed by the formation of incoherent (not condensed) e–h pairs, while the Mott–Hubbard insulator phase disappears. The exciton Mott transition is found to be the first-order transition. Linear optical susceptibilities are also discussed.

Exciton Mott Transition and Pair Condensation in Electron‐Hole Systems: Dynamical Mean‐Field Theory

The exciton Mott transition and electron‐hole (e‐h) pair condensation are studied using a two‐band Hubbard model with both repulsion and attraction. With the use of the dynamical mean‐field theory, we clarify the phase diagram for the phase transitions at zero and finite temperatures. Away from half‐filling we find an exciton insulating phase, which is followed by the formation of incoherent e‐h pairs, and the first‐order transition between a metallic phase and the exciton insulating phase. We also examine the transition temperature to an e‐h pair condensed phase where BEC‐BCS crossover takes place, and discuss characteristic features of the finite‐temperature phase diagram.

Aharonov-Bohm effect on impurity-bound excitons in semiconducting carbon nanotubes

We study the Aharonov-Bohm effect on exciton absorption spectra in semiconducting carbon nanotubes with an impurity in an effective-mass theory. In the presence of an impurity potential, there are two impurity-bound exciton states, one is dark and the other is bright. The dark impurity-bound exciton becomes brightened with increasing a magnetic flux through the cross section. The energy level of the bright impurity-bound exciton shows a blueshift in the magnetic flux dependence for a long-range impurity; in contrast, it shows a redshift for a short-range impurity, due to an enhancement of the coupling between intra- and inter-valley excitons. This suggests that the magnetic flux dependence can distinguish the range of the impurity potential and the strength of the inter-valley coupling of excitons.

Stability of exciton states and screening effects in doped carbon nanotubes

Excitonic effects and the absorption spectra of doped semiconducting nanotubes are studied in an effective-mass theory by taking into consideration static and dynamic screening. The dynamic screening effects give rise to the enhancement of the exciton binding energy and makes the excitons in carbon nanotubes more stable than static screening approximation.

Chirality dependence of exciton energies in double-wall carbon nanotubes

Exciton energies in double-wall carbon nanotubes are theoretically studied based on an effective-mass approximation. Effects of tube chirality responsible for family pattern of the exciton energies are included as higher-order corrections describing trigonal warping, curvature, and lattice distortion. The redshift of the exciton energy due to interwall screening exhibits a family pattern with small energy spread. The shift and its family spread of the inner tube are almost determined by the interwall distance and by whether the outer tube is semiconducting or metallic regardless of the detailed structure.