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The excited state of many-body systems can often be considered to be composition of a basic excited state, i.e. ``elementary excitation". A greater number of physically interesting phenomena occur when many-body systems are not in the ground state but in the excited state described by the elementary excitation.
Our laboratory aims at solving theoretically various physical phenomena on elementary excitation using field theory, computer simulation, and so on.
Elementary excitation physics is an interdisciplinary field, from which the elucidative problems overflows without limit. Our major researching topics currently include the followings.

top Theory of excited state processes in condensed matter down

The electron-hole system in semiconductors excited by a high-intensity light field has been one of the main topics on excited-state processes in condensed matter. This system is expected to exhibit macroscopic quantum phenomena. In high-density regime, the electron-hole system is regarded as a two-component Fermi liquid in which the cooperative pairing of electrons and holes arises in momentum space at low temperature, quite similar to the BCS state in superconductors. While, in low-density regime, the electron hole pairs are regarded as well-defined excitons which are the bound states in the real space. In a dense gas of excitons where the excitons are still regarded as bosons, their Bose-Einstein condensatation is expected.
Superfluidity is intimately connected with the Bose-Einstein condensation. The experiments to observe the superfluidity of excitons have been carried out, but there are no direct evidence. Since their charge is neutral, the current carried by excitons cannot be detected directly. In order to overcome such difficulty, we propose an exciton system in a type-II quantum well.
To characterize the superfluidity, we should find phenomena in which the phase of a macroscopic wave function of the condensed state plays a crucial role. We showed that, in a certain condition, the spatial structure of the phase are ordered, which lead to the formation of vortex lattice. The observation of this behavior could be the crucial evidence of the superfluidity of exciton systems.

top Theory of condensed matter physics: quantum fluids and solids up | down

Liquid helium go through the superfluid transition at a temperature below the lambda-point. This causes the anomalous property of the caacit, thus this is a phase transition related to the Bose-Einstein condensation.
Vortices in superfluid state have characteristic nature without in classical vortices. For example, the quantized vortices have a circulation of quanta, and their core size is the atomic size.

As theoretical prediction of the quantized vortices by Onsager and Feynman in 1950's, the physics of quantized vortices yielded by the macroscopic quantum state in the Bose condensed system has been studied extensively as the topic of the origin of the quantum fluid. For example, when the superfluid velocity extends over a certain critical velocity, the persistent current decay. This fact would be caused by the superfluid turbulent state, thus the dynamics of the quantized vortex plays the important role to the stability of the persistent current (superfluidity). The calculation of three dimensional vortex dynamics was performed by Schwarz in 1980's.

There are two point of view to describe the motion of quantized vortices. If the mean separation of vortices or the curvature is sufficiently larger than the radius of a vortex core, the vortex line approximation which neglect the structure of the vortex core is valid. In this case, the velocity field around the vortex line is represented by the potential field which obeys the Biot-Savart's low. While, when we consider the microscopic phenomena such as the nucleation or the reconnection of quantized vortices, we use the Gross-Pitaevskii equation for the order parameter of Bose-Einstein condensate. We have treated this problem in both point of view described above.
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