Magnonic superradiant phase transition

磁石の中で自然と現れる「止まった波」 --超放射相転移が起こる磁石を発見--. 京都大学プレスリリース. 2022-01-12.In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. However, such an equilibrium SRPT is forbidden in the case of charge-based light–matter coupling, known as no-go theorems. Here, we show that the low-temperature phase transition of ErFeO₃ at a critical temperature of approximately 4 K is an equilibrium SRPT achieved through coupling between Fe³⁺ magnons and Er³⁺ spins. By verifying the efficacy of our spin model using realistic parameters evaluated via terahertz magnetospectroscopy and magnetization experiments, we demonstrate that the cooperative, ultrastrong magnon–spin coupling causes the phase transition. In contrast to prior studies on laser-driven non-equilibrium SRPTs in atomic systems, the magnonic SRPT in ErFeO₃ occurs in thermal equilibrium in accordance with the originally envisioned SRPT, thereby yielding a unique ground state of a hybrid system in the ultrastrong coupling regime

Perfect Intrinsic Squeezing at the Superradiant Phase Transition Critical Point

Some of the most exotic properties of the quantum vacuum are predicted in ultrastrongly coupled photon–atom systems; one such property is quantum squeezing leading to suppressed quantum fluctuations of photons and atoms. This squeezing is unique because (1) it is realized in the ground state of the system and does not require external driving, and (2) the squeezing can be perfect in the sense that quantum fluctuations of certain observables are completely suppressed. Specifically, we investigate the ground state of the Dicke model, which describes atoms collectively coupled to a single photonic mode, and we found that the photon–atom fluctuation vanishes at the onset of the superradiant phase transition in the thermodynamic limit of an infinite number of atoms. Moreover, when a finite number of atoms is considered, the variance of the fluctuation around the critical point asymptotically converges to zero, as the number of atoms is increased. In contrast to the squeezed states of flying photons obtained using standard generation protocols with external driving, the squeezing obtained in the ground state of the ultrastrongly coupled photon–atom systems is resilient against unpredictable noise

Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons

Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~ 14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems

ニホン ビジュツ ノ スジョウ

日本美術の美しさはシンプリシティーに求めることができる。まず初めに、その理由を材質と形式の点から考え、次にシンフ゜リシティーの内容について、直観的、心象的、装飾的、詩歌的、諧謔的の五つに分けて論じたわけである。日本絵画の特質は、一言でいってシンプリシティーである。そのもとになった中国絵画と比較することによって、はっきりと理解することができる。しかし、このような日本美術のシンプリシティーは、同時に圧倒的迫力やモニュメンタリティーに弱いという欠点を生むこともないではなかった。明治時代の小説家二葉亭四迷が、日本人の大和魂というものは純粋であるけれども、徹底性を嫌うという欠点をも生んだと指摘していることは、きわめて興味深く感じられる。ところで、実をいうと、シンプリシティーは美術だけの特質ではなく、衣食住をはじめ技術・学問・文学・芸能・音楽・宗教など、日本文化全般にわたる特徴でもあるといってよいであろう。これらの場合も、単に簡潔純粋というだけではなく、複雑なものをそのように見せる意識が働いていたことを見逃してはならない。それでは、なぜ日本文化全体がシンプリシティーによって特徴づけられるのか、今後はこの問題を考えてみたいと思っている。その際ヒントになりそうなのは、近代の文豪島崎藤村が、日本に和歌や俳句の如く、ごく単純な詩形が発達してきた理由は、われらの心に経験することがあまりに複雑であるからではないかと指摘している点かもしれない。One can find the source of the beauty of Japanese art in its simplicity. I have attempted to thinkthrough the reasons for this first in terms of materials and format, and next in terms of the content of thesimplicity, dividing it into five categories, those of intuitive, internal, decorative, poetic, and humorous.In one word, the special characteristic of Japanese painting is simplicity. If compared with the Chinesepainting from which it originated, this can be seen quite clearly. However, this type of simplicity inJapanese art had the disadvantage of making the overpowering monumentality of the tradition weak.The Meiji novelist Futabatei Shimei stated that the "Yamato spirit" was pure, but was flawed in that ithated thoroughness, a characteristic of art, but also of clothing technique, academia, literature, the performingarts, music, religion, and so forth, a characteristic that defines Japanese culture as a whole. Onecannot overlook the fact that in these cases, it is not just concision and purity, but the desire to makecomplex things appear simple. In the future I would like to consider the problem of why Japanese cultureas a whole can be characterized by simplicity. When doing so, a hint is provided by the distinguishedmodern writer Shimazaki Toson, who speculated that the reason why a very simple poetic formdeveloped in Japan was that in the hearts and minds fo the Japanese, the lived and felt experiences arein fact quite complex

ニホン ブンジンガ ト チュウゴク ドウケイ

日本文人画誕生の秘密を解くためには、岡倉天心が『東洋の理想--特に日本美術について--』において指摘したような日本文化の本質的な性格を知らなければならない。その表層だけを眺めてはならない。これは日本文化について考察する際、つねに忘れてはならないことだが、日本文人画の場合には、とくに心に留めておく必要がある。それは中国絵画との関係がきわめて密接だからである。その基本的様式が中国絵画によって規定されているからである。そして日本文人画家の中国憧憬が誰よりも強かったからである。本論は『芥子園画伝』など中国画譜の学習と、荻生徂徠が重視した唐話学を改めて取り上げることによって、日本文人画誕生における中国憧憬の重要性を指摘したものである。In order to understand the secret of birth on Japanese literati paintings, we must get to know the essential character of Japanese culture Okakura Tenshin pointed out in "The Ideals of the East with Special Reference to the Art of Japan". We must not look at the surface. It is extremely significant point whenever we think about Japanese culture. In the case of Japanese literati paintings, it is much more important point. It is because Japanese literati paintings are much connected with Chinese paintings and the fundamental style is influenced by Chinese paintings. And it is because Japanese literati painters excessively longed for Chinese culture. I emphasized the significance of longing for Chinese culture when Japanese literati paintings came out from the viewpoint of master of Chinese illustrated book like "The Mustard Seeds Garden" and Chinese phonetic study Ogyu Sorai made much of