arXiv subjects

Nobuhiko Yokoshi

Publications and source records attributed to Nobuhiko Yokoshi.

25 records · Page 2Linked to original sources

arXiv2008

Fano effect in a Josephson junction with a quantum dot

We theoretically investigate the Fano effect in dc Josephson current at the absolute zero of temperature. The system under consideration is a double-path Josephson junction in which one path is through an insulating barrier and the other one is through a quantum dot (QD). Here the Kondo temperature is assumed to be much smaller than the superconducting gap, and the Coulomb interaction inside the QD is treated by the Hartree-Fock approximation. It is shown that the Josephson critical current exhibits an asymmetric resonance against the QD energy level. This behavior is caused by the interference between the two tunneling processes between the superconductors; the direct tunneling across the insulating barrier and the resonant one through the QD. Moreover, we find that the Josephson critical current changes its sign around the resonance when the Coulomb interaction is sufficiently strong. Our results suggest that 0-$π$ transition is induced by the cooperation of the Fano effect and the Coulomb interaction inside the QD.

cond-mat.mes-hall↗

arXiv2007

AC Josephson current and supercurrent noise through one-dimensional correlated electron systems

AC Josephson effect in one-dimensional Tomonaga-Luttinger liquid (TLL) adiabatically connected to superconducting electrodes is theoretically investigated. It is found that density fluctuations due to repulsive electron-electron interactions in TLL inhibit Josephson oscillations, whereas they do not affect time-independent current part. We also show that the fluctuations reduce supercurrent noise caused by multiple Andreev reflections. This indicates that the quantum fluctuations in TLL disturb the superconducting phase coherence spreading across the junction.

cond-mat.mes-hall↗

arXiv2007

Collective Modes and Stability of Bose-Fermi Mixtures with a BCS-BEC Crossover

We investigate an ultracold Bose-Fermi mixture with a Feshbach resonance between two hyperfine states of fermions. Using a functional integral method, we calculate collective modes associated with fermion-pairs and bosons in a superfluid phase. We derive a stability condition of the mixtures, which is valid in the entire region of the BCS-BEC crossover. This stability condition, which smoothly connects the well-known results obtained in both the BCS and BEC limits, shows that sufficiently strong FB interactions destabilize the mixtures. In order to investigate the consequence of this instability, we study the ground state energy of both the uniformly mixed and the phase-separated states. We find that the FB repulsion induces a phase separation, whereas the FB attraction should cause a collapse of the mixture. The possibilities for the experimental observation of such instabilities are also discussed.

cond-mat.other↗

arXiv2006

Phase Dependence of Phonon Tunneling in Bosonic Superfluid-Insulator-Superfluid Junctions

We consider the tunneling of phonon excitations across a potential barrier spatially separating two condensates with different macroscopic phases. We analyze the relation between the phase difference $ϕ$ of the two condensates and the transmission coefficient {\it T} by solving the Bogoliubov equations. It is found that {\it T} strongly depends on $ϕ$, and that the perfect transmission of low-energy excitations disappears when the phase difference reaches the critical value which gives the maximum supercurrent of the condensate. We also discuss the feasibility of observing the phase differences in experiments.

cond-mat.soft↗

arXiv2005

Collective Excitations of Bose-Einstein Condensates in a Double-Well Potential

We investigate collective excitations of Bose-Einstein condensates at absolute zero in a double-well trap. We solve the Bogoliubov equations with a double-well trap, and show that the crossover from the dipole mode to the Josephson plasma mode occurs in the lowest energy excitation. It is found that the anomalous tunneling property of low energy excitations is crucial to the crossover.

cond-mat.other↗

arXiv2005

Effects of fermion-boson interaction in neutral atomic systems

We investigate the collective excitations of $^3$He-$^4$He mixture films at zero temperature within random phase approximation and linear response theory. In low concentration region of $^3$He, a level repulsion between zero sound and third sound mode is derived, which opens the possibility to observe quantum mechanical coherence between $^3$He particle-hole pair and the condensate of $^4$He. We also investigate the $^3$He-$^4$He vertex corrections in ladder approximation and show that the third branch, the combined mode of fermionic particle-hole pair and the third sound quanta, provides a unique correction to Landau $f$ function. Some implications to the fermion-boson mixture of alkali atoms in a potential trap is discussed.

cond-mat.stat-mech↗

arXiv2005

Josephson $π$-state in superconductor-Luttinger liquid hybrid systems

Josephson current through a Luttinger liquid (LL) under a magnetic field is theoretically studied. We derive an analytical expression of Josephson current for clean interfaces, by using quasiclassical Green's function and functional bosonization procedure. We show that critical currents can be renormalized by electron-electron interactions at perfect transparency when LL is adiabatically connected with superconductors. We also find that a generation of $π$-state, due to spin-dependent energy shift in Andreev bound states (ABS), is prohibited even at zero temperature when the strength of repulsive interactions reaches some critical value. The suppression of $π$-state is caused by the low energy fluctuations propagating in LL, and making the Zeeman splitting in ABS blurred.

cond-mat.mes-hall↗