過去のセミナー

＜12/2019＞
12月9日 (月), 13:00-14:30
Dr. Satoshi Ikegaya (Max-Planck-Institute for Solid State Research, Germany)
Anomalous Nonlocal Conductance as a Fingerprint of Chiral Majorana edge states

During the past two decades, chiral Majorana edge states appearing at the boundary a spin-triplet chiral $p$-wave superconductor has attracted much attention. The perovskite superconductor ${\rm Sr_2RuO_4}$ is the most promising candidate material for spin-triplet chiral $p$-wave superconductors [1]. At present, finding a conclusive signature of chiral Majorana edge states in this compound is an urgent issue in physics of topological phase of matter and that of spin-triplet superconductivity.
In this work, we demonstrate that the chiral nature of Majorana edge states is drastically manifested in nonlocal conductance in a junction consisting of a chiral $p$-wave superconductor and two ferromagnetic leads. The nonlocal conductance in the present junction is insensitive to the distance between the two leads and is sensitive to the chirality of the pair potential. These two drastic features enable us to identify the moving direction of the chiral Majorana edge states in the single experimental setup only by changing the lead wire to which the bias voltage is applied. We propose a smoking-gun experiment to identify the chiral $p$-wave superconductivity.

[1] Y. Maeno, S. Kittaka, T. Nomura, S. Yonezawa and K. Ishida, “Evaluation of Spin-Triplet Superconductivity in Sr2RuO4”, J. Phys. Soc. Jpn. 81, 011009 (2012).
[2] S. Ikegaya, Y. Asano, and D. Manske, “Anomalous Nonlocal Conductance as a Fingerprint of Chiral Majorana Edge States”, arXiv: 1901.07652


12月2日 (月), 13:00-
Tianyu Liu (Max Planck Institute, Dresden)
Topological matter beyond fermionic paradigm

Topological matter comprises a special type of quantum materials whose electronic Bloch wave function exhibits nontrivial topology protected by band gaps and/or symmetries. When substituting the Bloch wave for other waves ? the electromagnetic wave, the acoustic wave, the mechanical wave, and the spin wave ? various topological materials made of bosons and classical oscillators can be obtained.
In this talk, I will first briefly review the development in the studies of topological photonic crystals, topological phononic metamaterials, topological mechanical oscillator arrays, and topological electric circuits. Then I will introduce topological magnonic systems with two examples ? honeycomb ferromagnets and honeycomb antiferromagnets ? and show that the physics of magnonic systems is even richer than electronic systems. In the end, I will discuss the magnonic Weyl semimetals under electromagnetic fields and strain-induced pseudo-EM fields, paying close attention to the magnon spin/heat transport in the form of quantum anomalies under different combinations of EM and pseudo-EM fields.


＜10/2019＞
10月3日 (木), 14:40-16:10
Dr. Lev Levitin (Royal Holloway, Univ. of London)
Superconductivity in ${\rm YbRh_2Si_2}$  

Cooper pairing of electrons to form a superconductor can occur via multiple mechanisms [1]. The well-established phonon-mediated pairing in conventional superconductors, such as ${\rm Al}$ and ${\rm Nb}$, results in spin-singlet pair wavefunction that has uniform phase over the Fermi surface. Alternative mechanisms, such as superconductivity driven by spin fluctuations, can lead to both spin-singlet and spin-triplet pairs with complex structures in momentum space. Such unconventional pairing is understood to be required for high temperature superconductivity; moreover some of the spin-triplet superconductors are predicted to have exotic topological properties [2]. There are numerous unconventional superconductors among heavy-fermion metals, however the precise pairing state is rarely known unambiguously. The canonical heavy-fermion metal ${\rm YbRh_2Si_2}$ hosts both antiferromagnetic and ferromagnetic fluctuations [3]. It orders antiferromagnetically at 70 mK, but doping allows to tune the magnetism across two antiferromagnetic and one ferromagnetic phase [4]. Evidence for superconductivity and nuclear magnetism below 2 mK have been reported recently from a study of magnetic properties of ${\rm YbRh_2Si_2}$ [5]. Transport measurements in this temperature regime are challenging and I will report on novel SQUID-based techniques we have developed and applied to probe high-quality single crystals of ${\rm YbRh_2Si_2}$ and microstructures machined from such crystals using focussed ion beam. We observe unambiguous signatures of superconductivity, such as quantised persistent currents. A complex phase diagram emerges in magnetic field, demonstrating interplay between superconductivity and magnetism. The observed superconductivity beyond Pauli limit and distinct transport regimes, that potentially represent different superfluid phases, point towards unconventional spin-triplet superconductivity driven by the ferromagnetic fluctuations. This opens an intriguing possibility that ${\rm YbRh_2Si_2}$ is a crystalline topological superconductor, a sought-after material with promising applications in quantum information processing.

[1] P. Monthoux, D. Pines and G. G. Lonzarich, Nature 450, 1177 (2007).
[2] T. Mizushima et al., J. of Phys. Soc. Japan 85, 022001 (2016).
[3] C. Stock et al., Phys. Rev. Lett. 109, 127201 (2012).
[4] S. Hamann et al., Phys. Rev. Lett. 122, 077202 (2019).
[5] E. Schuberth et al., Science 351, 495 (2016).


＜6/2019＞
6月20日 (木), 13:30
瀧川大地 (大阪大学基礎工学研究科)
Impact of off-diagonal exchange interaction on Kitaev spin liquid state

The Kitaev model on a 2D honeycomb lattice is an ideal system for realizing a quantum spin liquid state, which is described by itinerant Majorana fermions coupled with $\mathbb{Z}_{2}$ gauge fields [1]. With application of a magnetic field, this system is changed to the gapped chiral spin liquid state, in which the thermal quantum Hall effect is realized. In this chiral spin liquid state, the energy gap of itinerant Majorana fermions is proportional to the cube of the magnetic field. In a recent experiment, half-quantized thermal Hall conductivity is observed for the candidate material, $\alpha{\rm -RuCl}_{3}$[2]. According to the experiment [2], the temperature scale at which the quantization appears and the energy scale of the applied magnetic field are almost in the same order. Meanwhile, however, it is expected the quantized Hall effect occurs when the temperature scale is sufficiently smaller than the energy gap generated by the magnetic field, which should not exceed the Zeeman energy scale for the ideal Kitaev model. Thus, the above-mentioned experimental observation implies that the energy gap is enhanced in the real material, compared to the ideal Kitaev spin system. To explore for a possible mechanism of the enhancement of the energy gap, we investigate effects of non-Kitaev interactions, which exist in real candidate materials, on the gapped chiral spin liquid. It is found that off-diagonal exchange interactions arising from edge-shared octahedra structures with strong spin-orbit couplings, enhance significantly the mass gap of Majorana fermions. This result provides a possible explanation for robust quantization of the thermal Hall conductivity observed in the above-mentioned experiment. Furthermore, we investigate effects of off-diagonal exchange interactions on physical properties of the spin liquid state from two viewpoints. Firstly, we demonstrate that if the quantum spin liquid state and the zigzag antiferromagnetic order coexist around the border of these two phases, the off-diagonal exchange interactions induce the Fermi surface of Majorana fermions, leading to a state similar to the $U(1)$ spin liquid. [3] Secondly, we found that the off-diagonal exchange interactions induce the topological phase transition accompanying the change of the Chern number.

[1] A. Kitaev, Annals of Physics 321, 2 (2006).

[2] Y. Kasahara, et al, Nature 559, 227 (2018)

[3] Y.Jiang, et al, arXiv:1809.09091(2018)


6月6日 (木), 13:00
三野巧 (大阪大学基礎工学研究科)
クラスDトポロジカル超伝導体におけるMajorana Braiding Dynamicsと非可換統計性

近年、量子細線、接合系やバルク（鉄系超伝導体${\rm FeSe}_{1-x}{\rm Te}_x$）などの実験結果によりマヨラナ準粒子を示唆する結果が相次いで報告されており、非可換統計性観測に拍車がかかっている[1]。これらに関する理論的研究は、「マヨラナ有効模型」と「断熱近似」に基づいて行われ、マヨラナ準粒子間の量子干渉によってエネルギーがゼロからシフトする内因的なノイズの存在が明らかとなった[2][3]。この内因的なノイズは、マヨラナ量子ビットの脆弱化や量子演算のエラーを引き起こす要因である。さらにマヨラナ準粒子を保有する超伝導渦は$\mathbb{Z}_2$トポロジカル数で保護されているが、これらが複数ある場合はトポロジカルに不安定となる。このため「マヨラナ有効模型」による理論が成り立っているかは自明でない。本セミナーでは、電子正孔対称性(PHS)のみを有するクラスDトポロジカル超伝導体に焦点を当てたマヨラナ準粒子ダイナミクスと非可換統計性の議論を行う。非可換統計性とは、粒子の交換操作によって別の縮退した基底状態に移る統計性のことを指す[4]。結果としてベリー接続行列を用いた解析的評価により、PHSで結ばれた状態への一次遷移は禁止されることが明らかになった。またマヨラナ準粒子の存在を仮定しない超伝導の微視的モデルに基づいた数値シミュレーションによる定量的評価により、(1)超伝導渦2本において交換後に量子状態が獲得する幾何学的位相($\pm \pi/2$)の定量的評価、(2)超伝導渦4本(1量子ビット)において非可換統計性を実証した。

[1] T. Machida, Y. Sun, S. Pyon, S. Takeda, Y. Kohsaka, T. Hanaguri, T. Sasagawa, and T. Tamegai: arXiv:1812.08995 (2019)

[2] Meng Cheng, Roman M. Lutchyn, Victor Galitski, and S. Das Sarma Phys. Rev. Lett. 103, 107001 (2009)

[3] T. Mizushima and K. Machida Phys. Rev. A 82, 023624 (2010)

[4] S. Das Sarma, M. Freedman, C. Nayak: Phys. Rev. Lett. 94, 166802 (2008)


＜5/2019＞
5月23日 (木), 13:00-      
川上拓人 (大阪大学理学研究科)
スピン3/2トポロジカル絶縁体の超伝導

電子が通常よりも大きなスピンを持つ場合に実現する, 新奇なトポロジカル超伝導を紹介する. 逆ペロブスカイト酸化物では, スピン3/2 電子がバンド反転を起こし, トポロジカル結晶絶縁体が 実現すると考えられている. そして近年, その一つである Sr3SnO の超伝導相転移が実験的に 観測された. 本発表では, このような物質群における可能な超伝導状態を議論する. そして, 大きなスピンを持つ電子の奇パリティ超伝導は, 大きな巻付き数を持つトポロジカル超伝導 となることを説明する.

＜4/2019＞

4月18日 (木), 13:30-      
塩崎謙 (京都大学基礎物理学研究所)
磁気点群対称性とディラック・ハミルトニアン

任意の磁気点群の対称性を満たすディラック・ハミルトニアンを完全に分類する手法がわかったので，それを解説する．波数空間における対称点近傍のBloch波動関数の構造の分類，超伝導ノードの分類，あるいは高次のトポロジカル絶縁体・超伝導体の分類など，様々な応用が期待できる．いくつかのトピックについて基本的な考え方と計算手法を紹介する．