冷却原子気体を用いることで実現される非自明な量子輸送現象を明らかにすべく研究を行なった。今年度は、磁性不純物に由来するメゾスコピック輸送、ボース・アインシュタイン凝縮体におけるジョセフソンカレント、光学スピン伝導率に関する研究で成果を得ることができた。磁性不純物に由来するメゾスコピック輸送の研究では、京都大学と東京工業大学のグループとの共同研究を行なった。そして、軌道自由度のあるYb原子系で、不純物由来のスピン緩和現象を観測し、それがLandauer公式によって記述されることも明らかとなった。ボース・アインシュタイン凝縮体におけるジョセフソンカレントの研究では、トンネルハミルトニアン法とケルディッシュ形式を用いることで、非線形応答領域まで適用可能なDCジョセフソンカレントの表式を得た。その結果、非線形応答領域において、超伝導体のDCジョセフソンカレントとは大きく異なる輸送特性が得られることを明らかにした。光学スピン伝導率に関する研究では、東京大学と理化学研究所のグループとの共同研究を行なった。我々は、現在投稿中の光学スピン伝導率の論文を1次元p波超流動体に適用した。我々はまず、このような系が弱結合領域においてクラスBDIで記述されるトポロジカル超流動体になることを示し、原子間相互作用の大きさを変化させることで、トポロジカル転移が生じることを示した。さらに、このトポロジカル転移が光学スピン伝導率測定によって特徴付けられることを明らかにした。

In recent years, quantum simulation research has seen great success through artificial quantum systems such as ultracold atomic gases, trapped ions, and superconducting circuits. Among these, ultracold atomic gases have attracted significant attention since the first realization of Bose-Einstein condensation in 1995. In this project, we have conducted theoretical studies on quantum condensed matter physics, with results that can be experimentally verified using ultracold atomic gases.One of the key achievements of this project, conducted in collaboration with researchers from RIKEN, the Waseda Institute for Advanced Study, the University of Tokyo, and the Chinese Academy of Sciences, is the study of magnon dynamics in ferromagnetic junctions realized with a two-component Bose gas on an optical lattice. Using the nonequilibrium Green’s function approach, we demonstrate a divergence in spin conductance and a slowdown in spin relaxation. Furthermore, quantum-enhanced conductance leads to the breakdown of the magnonic Wiedemann-Franz law. These anomalous spin dynamics arise from the magnonic critical point, where magnons become gapless due to spontaneous magnetization. The corresponding papers have been published in Physical Review Letters and Physical Review A.Additionally, we have also investigated open quantum systems realized with ultracold atomic gases, such as quantum transport systems and ultracold atomic gases inside an optical cavity. In quantum transport systems, we have examined how atom losses introduced in ultracold atomic gas experiments affect conduction properties. In the case of ultracold atomic gases inside an optical cavity, we have studied quench and ramp dynamics near the transition point between the superfluid and density wave phases. We are currently preparing papers on these topics.

In mesoscopic transport phenomena through small constrictions, quantum mechanical effects are known to be directly reflected in transport coefficients. One of the best known is the Landauer formula in which the two-terminal conductance of normal metals is quantized.In addition, states of matter in reservoirs play an important role in mesoscopic transport. The prototype example is a superconducting point contact where reservoirs consist of superconductors. In this case, it is known that the direct current does not obey Ohm’s law. The key ingredient there is multiple Andreev reflections where quasiparticles repeat Andreev reflections at the boundaries between superconductor and contact. As a result, the current-bias characteristics become highly nonlinear.In contrast, each constituent particle in detail is expected to be irrelevant in mesoscopic transport. This type of universality can nowadays be confirmed with ultracold atomic gases. Indeed, a two-terminal transport setup with a quantum point contact has been realized in experiments of ultracold Fermi gases that observed the conductance quantization and nonlinear current-bias characteristics.It must be noted that the presence or absence of charge may cause a difference in transport between electron and atomic systems. Especially, this difference may qualitatively be important for systems with Bose- Einstein condensation of Cooper pairs where Nambu-Goldstone (NG) modes emerge due to spontaneous symmetry breaking. In the case of electrons, the NG modes become gapped ones due to the Coulomb interaction. In the case of neutral atoms, however, these NG modes remain gapless, and the NG modes may play an important role in low-energy transport. At the same time, as the NG modes are a non-superfluid component, there is a view that the effects of such modes are negligible at low temperature. Since the NG mode in mesoscopic transport have yet to be incorporated in an explicit manner, it is not clear whether it is reasonable to neglect the effect of the gapless mode in experiments of ultracold atomic gases.By using the effective theory and tunneling Hamiltonian, we have discussed DC transport of the NG modes in the fermionic superfluid point contact. We have focused on the BCS regime and revealed the anomalous contribution in mass transport, which is the conversion process between the condensate and NG mode. We also discussed that the anomalous contribution is not present in heat transport, which gives rise to breakdown of the Wiedemann-Franz law and the absence of the bunching effect in current noise.

本研究では、２端子量子ポイントコンタクト系におけるボース原子気体超流動体の量子輸送現象を理論的に解析した。トンネル・ハミルトニアン、Bogoliubov理論、Keldysh形式を組み合わせることで、２端子間に化学ポテンシャル勾配や温度勾配がある場合のACおよびDCカレントの表式を明らかにし、数値解析も行なった。そして、超伝導ポイントコンタクト系とは全く異なる輸送メカニズム、低バイアス領域におけるオーム則、Wiedemann-Franz則の破れ、冷却原子気体の実験で観測される２端子間の粒子数差の緩和ダイナミクスを明らかにした。<!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-536870145 1107305727 0 0 415 0;}@font-face {font-family:游明朝; panose-1:2 2 4 0 0 0 0 0 0 0; mso-font-charset:128; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-2147482905 717749503 18 0 131231 0;}@font-face {font-family:"\@游明朝"; mso-font-charset:128; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-2147482905 717749503 18 0 131231 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin:0mm; margin-bottom:.0001pt; text-align:justify; text-justify:inter-ideograph; mso-pagination:none; font-size:10.5pt; mso-bidi-font-size:12.0pt; font-family:"游明朝",serif; mso-ascii-font-family:游明朝; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:游明朝; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:游明朝; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-font-kerning:1.0pt;}.MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; font-family:"游明朝",serif; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}size:612.0pt 792.0pt; margin:99.25pt 30.0mm 30.0mm 30.0mm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;}div.WordSection1 {page:WordSection1;}

本課題では非平衡下での量子多体系の性質を理論的に解明すべく、冷却原子気体に焦点を当てた研究を行なった。具体的には、冷却原子気体を用いて、ポーラロンや量子輸送、そしてスピン自由度を持ったボース・アインシュタイン凝縮の研究を行なった。ポーラロン問題では、有限温度のスペクトル関数の非自明な性質を明らかにした。量子輸送問題では、ボース超流動系のメゾスコピック伝導を記述する基礎理論の構築に取り組んだ。スピン自由度を持ったボース・アインシュタイン凝縮の研究では、フロケ・エンジニアリングを考えることで、系に非自明な性質が出現することを明らかにした。@font-face { font-family: "Cambria Math";}@font-face { font-family: 游明朝;}@font-face { font-family: "@游明朝";}p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0mm 0mm 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: "游明朝", serif; }.MsoChpDefault { font-family: "游明朝", serif; }div.WordSection1 { }