Hiroyuki Yabu

Collective ferromagnetism in two-component Fermi-degenerate gas trapped in a finite potential

Spin asymmetry of ground states is studied in trapped spin-degenerate (two-component) gases of fermionic atoms with repulsive interaction between different components; and, for a large particle number, the asymmetric (collective ferromagnetic) states are shown to be stable because it can be energetically favorable to increase the Fermi energy of one component rather than increase the interaction energy between up-down components. We formulate the Thomas-Fermi equations and give algebraic methods to solve them. From the Thomas-Fermi solutions, we find three kinds of ground states in the finite system: (1) paramagnetic (spin-symmetric), (2) ferromagnetic (equilibrium), and (3) ferromagnetic (nonequilibrium) states. We present the density profiles and the critical atom numbers for these states obtained analytically, and, in ferromagnetic states, the spin asymmetries are shown to occur in the central region of the trapped gas, and increase with increasing particle number. Based on the obtained results, we discuss the experimental conditions and current difficulties in realizing the ferromagnetic states of the trapped atom gas, which should be overcome.

Induced instability for boson-fermion mixed condensates of alkali-metal atoms due to the attractive boson-fermion interaction

Instabilities for boson-fermion mixed condensates of trapped alkali-metal atoms due to the boson-fermion attractive interaction are studied using a variational method. A stable, metastable, and unstable region as a function of the boson-fermion interaction exists. The stability condition is obtained analytically from the asymptotic expansion of the variational total energy. The lifetime of metastable states is discussed for tunneling decay and is estimated to be very long. It suggests that, except near the instability border, metastable mixed condensate should be almost stable against tunneling decay. The critical border between metastable and unstable phases is calculated numerically and is shown to be consistent with the M\o{}lmer scaling condition.

Sum-rule approach to collective oscillations of a boson-fermion mixed condensate of alkali-metal atoms

The behavior of collective oscillations of a trapped boson-fermion mixed condensate is studied in the sum-rule approach. Mixing angle of bosonic and fermionic multipole operators is introduced so that the mixing characters of the low-lying collective modes are studied as functions of the boson-fermion interaction strength. For an attractive boson-fermion interaction, the low-lying monopole mode becomes a coherent oscillation of bosons and fermions and shows a rapid decrease in the excitation energy towards the instability point of the ground state. In contrast, the low-lying quadrupole mode keeps a bosonic character over a wide range of the interaction strengths. For the dipole mode the boson-fermion in-phase oscillation remains to be the eigenmode under the external oscillator potential. For weak repulsive values of the boson-fermion interaction strengths we found that an average energy of the out-of-phase dipole mode stays lower than the in-phase oscillation. Physical origin of the behavior of the multipole modes against boson-fermion interaction strength is discussed in some detail.

Boson-fermion pairing in a boson-fermion environment

Propagation of a Boson-Fermion (BF) pair in a BF environment is considered. The possibility of formation of stable strongly correlated BF pairs, embedded in the continuum, is pointed out. The Fermi gas of correlated BF pairs shows a strongly modified Fermi surface. The interaction between like particles is neglected in this exploratory study. Various physical situations where our pairing mechanism could be of importance are invoked.

Random-phase approximation study of collective excitations in the Bose-Fermi mixed condensate of alkali-metal gases

We perform a random-phase approximation study of collective excitations in a Bose-Fermi mixed degenerate gas of alkali-metal atoms at

Study on laser cooling of ortho-positronium

T=0.

Monopole oscillations and dampings in a boson and fermion mixture in the time-dependent Gross-Pitaevskii and Vlasov equations

The calculation is done by diagonalization in a model space composed of particle-hole type excitations from the ground state, the latter being obtained from the coupled Gross-Pitaevskii and Thomas-Fermi equations. We investigate strength distributions for different combinations of Bose and Fermi multipole (L) operators with

Static properties of the trapped Bose-Fermi mixed condensate of alkali atoms

L=0,1,2,3.

Peierls instability, periodic Bose-Einstein condensates, and density waves in quasi-one-dimensional boson-fermion mixtures of atomic gases

Transition densities and dynamical structure factors are calculated for collective excitations. A comparison with the sum rule prediction for the collective frequency is given. The time dependent behavior of the system after an external impulse is studied.

Atomic Bose–Fermi mixed condensates with boson–fermion quasi-bound cluster states

Abstract We have been theoretically and experimentally studying details of laser cooling of ortho-positronium. Experimental apparatus consists of a pulsed positron beam generator, a long pulse wide bandwidth ultraviolet laser and a time-of-flight system to measure kinetic energy of positronium atoms is constructed and tested. Using this apparatus, production of thermally activated positronium is confirmed. Its production rate is larger as the target temperature increases and ratio of thermally activated ortho-positronium to all γ-ray annihilation events is estimated to be 32.3±6.7% for the target temperature 1000 K.