Hiromasa Ohnishi

Direct determination of exciton wavefunction amplitudes by the momentum-resolved photo-electron emission experiment

We study conceptional problems of a photo-electron emission (PEE) process from a free exciton in insulating crystals. In this PEE process, only the electron constituting the exciton is suddenly emi...

AN ATTEMPT AT A RESONATING MEAN-FIELD THEORETICAL DESCRIPTION OF THERMAL BEHAVIOR OF TWO-GAP SUPERCONDUCTIVITY

The resonating mean-field theory (Res-MFT) has been applied and shown to effectively describe two-gap superconductivity (SC). Particularly at T = 0 using a suitable chemical potential, the two-gap SC in MgB2 has been well described by the Res-Hartree-Bogoliubov theory (Res-HBT). The Res-HB ground state generated with HB wavefunctions almost exhausts the ground-state correlation energy in all the correlation regimes. In this paper we make an attempt at a Res-MF theoretical description of thermal behavior of the two-gap SC. In an equal energy-gap case we find a new formula leading to a higher critical temperature Tc than the Tc of the usual HB formula.

Group-theoretical deduction of a dyadic Tamm–Dancoff equation by using a matrix-valued generator coordinate

The traditional Tamm–Dancoff (TD) method is one of the standard procedures for solving the Schrodinger equation of fermion many-body systems. However, it meets a serious difficulty when an instability occurs in the symmetry-adapted ground state of the independent particle approximation (IPA) and when the stable IPA ground state becomes of broken symmetry. If one uses the stable but broken symmetry IPA ground state as the starting approximation, TD wave functions also become of broken symmetry. On the contrary, if we start from a symmetry-adapted but unstable wave function, the convergence of the TD expansion becomes bad. Thus, the requirements of symmetry and rapid convergence are not in general compatible in the conventional TD expansion of the systems with strong collective correlations. Along the same line as Fukutomes, we give a group-theoretical deduction of a U(n) dyadic TD equation by using a matrix-valued generator coordinate.

Thermal resonating Hartree–Bogoliubov theory based on the projection method

We propose a rigorous thermal resonating mean-field theory (Res-MFT). A state is approximated by superposition of multiple MF wavefunctions (WFs) composed of non-orthogonal Hartree–Bogoliubov (HB) WFs. We adopt a Res-HB subspace spanned by Res-HB ground and excited states. A partition function (PF) in a SO(2N) coherent state representation (CS Reps) |g 〉(N: Number of single-particle states) is expressed as Tr(e-βH) = 2N-1∫〈g|e-βH|g〉dg (β = 1/kBT). Introducing a projection operator P to the Res-HB subspace, the PF in the Res-HB subspace is given as Tr(Pe-βH), which is calculated within the Res-HB subspace by using the Laplace transform of e-βH and the projection method. The variation of the Res-HB free energy is made, which leads to a thermal HB density matrix expressed in terms of a thermal Res-FB operator as . A calculation of the PF by an infinite matrix continued fraction (IMCF) is cumbersome and a procedure of tractable optimization is too complicated. Instead, we seek for another possible and more practical way of computing the PF and the Res-HB free energy within the Res-MFT.