arXiv subjects

Shumpei Iwasaki

Publications and source records attributed to Shumpei Iwasaki.

3 recordsLinked to original sources

arXiv2026

Dimensionality of a strongly interacting 2D-3D Fermi-Fermi mixture from the perspective of superfluid instability and excitation properties

We theoretically investigate strong-coupling properties of an attractively interacting Fermi atomic gas, where the Cooper-pair formation occurs between atoms belonging to different dimensional bands. Including pairing fluctuations within the framework of the self-consistent $T$-matrix approximation (SCTMA), we examine how the BCS-type superfluid phase transition temperature $T_\mathrm{c}$ varies as one moves from the 3D-3D to the 2D-3D system, in the wide parameter region with respect to the strength of the pairing interaction. In the 2D-3D limit, we find that, while the mean-field BCS theory predicts $T_\mathrm{c}>0$ in the strong-coupling regime, $T_\mathrm{c}$ is remarkably suppressed down to zero by pairing fluctuations that are strongly enhanced by the mixed-dimensionality of the system. As the origin of this, we clarify that the lower-dimensional (2D) component dominates the superfluid instability, so that the vanishing $T_\mathrm{c}$ is the same phenomenon as that in the 2D-2D case. We also point out that this can already be seen in the mean-field level, when one examines the propagation of the Goldstone mode. On the other hand, we find that the pseudogap phenomenon, which is known as a precursor of Cooper-pair formation, exhibits a 3D character of the 2D-3D system. These results indicate that the dimensionality of a strongly interacting Fermi gas depends on what we observe.

cond-mat.quant-gas↗

arXiv2024

Nuclear Spin-Lattice Relaxation Rate in Odd-Frequency Superconductivity

We theoretically investigate the temperature dependence of nuclear spin-lattice relaxation rate $T_1^{-1}$ in bulk odd-frequency superconductivity. For a model odd-frequency pairing interaction, we first evaluate the superconducting order parameter, within the framework of the combined path-integral formalism with the saddle-point approximation. We then calculate $T_1^{-1}$ below the superconducting phase transition temperature $T_{\rm c}$, to see how the odd-frequency pairing affects this physical quantity. In the odd-frequency $p$-wave state, while the so-called coherence peak is suppressed as in the even-frequency $p$-wave case, $T_1^{-1}$ is found to exhibit the Korringa-law-like behavior ($T_1^{-1}\propto T$) except just below $T_{\rm c}$, even without impurity scatterings. In the odd-frequency $s$-wave case, the behavior of $T_{1}^{-1}$ is found to be sensitive to the detailed spin structure of the superconducting order parameter: In a case, $T_1^{-1}$ is enhanced far below $T_{\rm c}$, being in contrast to the conventional (even-frequency) $s$-wave BCS case, where the coherence peak appears just below $T_{\rm c}$. We also show that the calculated $T_1^{-1}$ in the odd-frequency $p$-wave case well explains the recent experiment on CeRh$_{0.5}$Ir$_{0.5}$In$_5$, where the possibility of odd-frequency $p$-wave superconductivity was recently suggested experimentally.

cond-mat.supr-con↗

arXiv2024

Pairing properties of an odd-frequency superfluid Fermi gas

We theoretically investigate strong-coupling properties of an odd-frequency Fermi superfluid. This pairing state has the unique property that Cooper pairs are formed between fermions, not at the same time, but at different times. To see whether or not such unequal-time pairs still exhibit bosonic behavior, we examine the space-time structure of the odd-frequency Cooper-pair wavefunction at $T=0$, by employing the combined path-integral formalism with the BCS-Eagles-Leggett-type superfluid theory. In the strong-coupling regime, the odd-frequency pair wavefunction still has different space-time structure from that in the ordinary even-frequency $s$-wave superfluid state, their $\textit{magnitudes}$ are found to become close to each other, except for the equal-time pairing component. In this regime, we also evaluate the superfluid phase transition temperature $T_{\rm c}$ within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink. The calculated $T_{\rm c}$ in the strong-coupling regime of the odd-frequency system is found to be well described by the Bose-Einstein condensation of tightly bound Bose molecules. Our results indicates that, in spite of vanishing equal-time pairing, odd-frequency Cooper pairs still behave like bosons in the strong-coupling regime, as in the even-frequency $s$-wave superfluid case.

cond-mat.quant-gas↗