参考文献:
1. A. Inui et al., PRL 124, 166602 (2020).
2. K. Shiota et al., PRL 127, 126602 (2021).
3. K. Ishito et al., Nat. Phys. 19, 35 (2023).
4. K. Ohe et al., PRL 132, 056302 (2024).
5. Y. Togawa et al., JPSJ 92, 081006 (2023).
6. 戸川欣彦ら, 数理科学 693, 9-15 (2021): 日本物理学会誌 76, 646 (2021).
7. 固体物理 2024年11月 特集号「物質科学におけるカイラリティ」.
There has been a growing interest in exploring two-dimensional (2D) materials beyond graphene. Starting from 2017, new platforms have been discovered with which magnetism at low dimensions is explored. The introduction of a variety of atomically thin magnetic crystals like transition metal dihalides (TMHs) has inspired efforts to not only understand the nature of magnetism but also to investigate the growth mechanism in these magnetic crystals. In this regard, nickel bromide and manganese iodide monolayer islands were grown on metallic substrates. I will show how Low Temperature Multimodal Scanning Probe Microscopy (SPM) imaging combined with Kelvin Probe Force Microscopy (KPFM) and Magnetic Force Microscopy (MFM) can reveal a ferromagnetic ground state persisting even in the monolayer regime. Occasionally, various phases have been formed giving rise to a reach variety of electronic structures as revealed by KPFM. These van der Waals materials are expected to open a wide range of possibilities for quantum applications.
Complexes containing rare-earth ions attract great attention for their technological applications ranging from spintronic devices to quantum information science. While charged rare-earth coordination complexes are ubiquitous in solution, they are challenging to form on material surfaces that would allow investigations for potential solid-state applications. In this talk, we will present the formation and atomically precise manipulation of rare-earth complexes on solid surfaces [1,2]. Atomic scale manipulations and characterization of individual rare-earth complexes are performed with scanning tunneling microscopy and synchrotron X-ray scanning tunneling microscopy. I will also present our breakthrough research, where X-ray spectroscopy has been successfully performed for the first time to simultaneously characterize the elemental, chemical, and magnetic properties of just one rare earth atom [3].
[1] D. Trainer, A. T. Lee, S. Sarkar, V. Singh, X. Cheng, N. K. Dandu, K. Z. Latt, S. Wang, T. M. Ajayi, S. Premarathna, D. Facemyer, L. A. Curtiss, S. E. Ulloa, A. T. Ngo, E. Masson, & S. W. Hla. Adv. Sci. 2308813 (2024).
[2] T.M. Ajayi, V. Singh, K.Z. Latt, S. Sarkar, X. Cheng, S. Premarathna, N.K. Dandu, S. Wang, F. Movahedifar, S. Wieghold, N. Shirato, V. Rose, L.A. Curtiss, A.T. Ngo, E. Masson, and S.-W. Hla. Nat. Commun. 13, 6305 (2022).
[3] T.M. Ajayi, N. Shirato, T. Rojas, S. Wieghold, X. Cheng, K. Z. Latt, D. J. Trainer, N. K. Dandu, Y. Li, S. Premarathna, S. Sarkar, D. Rosenmann, Y. Liu, N. Kyritsakas, S. Wang, E. Masson, V. Rose, X. Li, A. T. Ngo, & S.-W. Hla. Nature 618, 69-73 (2023).
第2種超伝導体に磁場を印加すると，内部に侵入した磁束線は量子化磁束の単位で束ねられ，ナノ粒子として振る舞う。この粒子は現実の物質と同様に固体や液体などの相を形成する[1]。さらに温度，粒子密度，駆動力などを簡単に制御できることから，多粒子系の秩序形成や非平衡ダイナミクスを調べる舞台として用いられている[2,3]。本セミナーでは，最近我々が取り組んだ2つの研究について紹介する。
① 2次元超伝導体はゆらぎの影響を強く受けるため，厚い超伝導体とは大きく異なる性質を示す。ゆらぎには，⾼温で顕著になる熱的なゆらぎと，極低温で重要となる量⼦的なゆらぎがあり，後者は様々な興味深い現象を引き起こすと予想されている。しかし，これまでの多くの実験は電気抵抗測定に限られていたため，完全な実証には至っていない。我々は，極低温・高磁場域で予想されている，絶対零度でも凍結しない磁束の量子液体の存在を，熱電効果測定を用いて実証した[4-6]。
② 超伝導体内の磁束は，温度や磁場の減少により液体相から固体相へと秩序化する。この秩序化は，比熱測定によって熱的(古典的)な相転移であることが示されている。一方で，外部電流で磁束への駆動力を増加させた場合には，液体状フローから格子状フローへと動的な構造の秩序化が生じることが知られている。この秩序化が駆動力をパラメータとした非平衡相転移であることを，フロー方向に垂直な電圧応答の測定[7]に加え，電流駆動中の熱電効果測定[8]により実証した。
参考文献:
[1] G. Blatter, et al., Rev. Mod. Phys., 66, 1125 (1994).
[2] C. Reichhardt and C. J. Olson Reichhardt, Rep. Prog. Phys. 80, 026501 (2017).
[3] 大熊哲, 固体物理 51, 547 (2016).
[4] K. Ienaga, et al., Phys. Rev. Lett. 125, 257001 (2020).
[5] 家永紘一郎, 大熊哲, 固体物理 55, 723 (2020).
[6] K. Ienaga, et al., Nature Commun. 15, 2388 (2024).
[7] S. Maegochi, K. Ienaga, and S. Okuma, Sci. Rep. 14, 1232 (2024).
[8] 家永紘一郎 他, 日本物理学会2024年 春季大会 19aF1-4.
参考文献
[1] D. M. Eigler and E. K. Schweizer, Nature 344 (1990) 524.
[2] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl, A. J. Heinrich. Science 319 (2008) 1066.
[3] N. Okabayashi, A. Gustafsson, A. Peronio, M. Paulsson, T. Arai, and F. J. Giessibl, Phys. Rev. B 93 (2016) 165415
[4] N. Okabayashi, A. Peronio, M. Paulsson, T. Arai, and F. J. Giessibl, PNAS 115 (2018) 4571
[5] 岡林則夫、日本物理学会誌 75 （2020） 279
[6] N. Okabayashi, T. Frederiksen, A. Liebig, F. J. Giessibl, Phys. Rev. Lett. 131 (2023) 148001
[7] N. Okabayashi, T. Frederiksen, A. Liebig, F. J. Giessibl, Phys. Rev. B 108 (2023) 165401
近藤格子物質と呼ばれる希土類元素から構成された化合物では、遍歴的な伝導電子と局在したf電子の混成(c-f混成)によって、低温において近藤効果、価数揺動状態、重い電子状態などの様々な量子現象が発現する[1]。これらの量子現象の発現機構などを理解するためには、フェルミエネルギー近傍電子状態を調べることが重要であり、近年、走査トンネル分光法 (STS)を用いた近藤格子物質の電子状態測定が試みられている[2]。一方、探針と試料が直接接触していないSTS実験では、局在したf軌道への電子のトンネル確率が小さいため、f電子の電子状態を直接観測することが困難であることが指摘されている[3]。これらの背景から我々は、探針と試料を直接接触した状態で電子状態測定を行う点接合分光法(PCS)が、f電子の電子状態測定に有効であると考え、PCS法を用いた近藤格子物質の電子状態測定を行っている。
本セミナーでは、はじめに、近藤格子物質で発現する重い電子状態などの量子現象について概説する。その後、近藤格子物質の電子状態についての理論・実験研究について紹介する。最後に、我々が行った近藤格子物質のPCS実験[4-7]について説明する予定である。
[1] A. C. Hewson, The Kondo Problem to Heavy Fermions (Cambridge University Press, Cambridge, 1993).
[2] S. Ernst, et al., Nature (London) 474, 362 (2011).
[3] W. K. Park, et al., Phys. Rev. Lett. 108, 246403 (2012).
[4] M. Shiga, et al., Phys. Rev. B 100, 245117 (2019).
[5] M. Shiga, et al., Phys. Rev. B 103, L041113 (2021).
[6] M. Shiga, et al., Phys. Rev. B 108, 195130 (2023).
[7] T. Takahashi, M. Shiga, et al., J. Phys. Soc. Jpn. 93, 023704, (2024).
Since graphene was discovered as a 2D materials, the study of this system is gradually increased. The relative twist angle between layers creates the superlattice with Moire-pattern influenced the electronic band structure that give rise to van Hove singularities. Large-scale density functional theory (DFT) has become a powerful tool to study the electronic and structural properties of heterostructures composed of graphene on top of hexagonal nitride (hBN). We observe that the large corrugation occurs when the twisted angle becomes small. The number of hBN layer effect to the electronic structure near the Fermi level. Additionally, we discuss the role of van der Waals interactions and the importance of including this effect in large-scale DFT studies to accurate capture the behavior of 2D materials heterostructures in the multilayers systems.
References:
1. K. Uchida, S. Furuya, J-I. Iwata, A. Oshiyama, PRB 90, 155451 (2014).
2. F. Haddadi, QS. Wu, A. J. Kruchkov, O. V. Yazyev, Nano Lett., 20, 2410-2415 (2020).
3. M. Long et. al, npj Com. Mat., 8, 73 (2022).
Qubits implemented using solid-state spins are easy to manipulate, but such architectures remain tricky to scale up. Addressability to individual atoms and atom-by-atom position control using a scanning tunneling microscope (STM) [1] opens the bottom-up design of functional quantum devices. As an extension of such potential to atomic/molecular spins, STM can provide a platform of solid-state qubits, which is unique in the sense of qubit platform design at a scale of ~1 nm, with an advantage of atom precision control of its structure and inter-qubit couplings. In this talk, I first introduce a recent advance of STM by combining conventional electron spin resonance (ESR), which picks up the advantages of the two techniques, high spatial resolution of STM and high energy resolution of ESR, enabling to drive and detect spin resonance of individual atoms on surfaces [2,3]. Then, I continue a successful demonstration of a qubit platform using atoms on a surface, where fast single-, two-, and three-qubit operations were performed in an all-electrical fashion, realized by atom-by-atom construction, coherent operations, and readout of coupled electron-spins in a STM [4–7].
References:
1. D. M. Eigler, E. K. Schweizer, Nature 344, 524–526 (1990).
2. S. Baumann et al. Science 350, 417-420 (2015).
3. K. Yang et al. Science 366, 509-512 (2019).
4. Y. Wang et al. npj Quantum Info. 9, 48 (2023).
5. S. Phark et al. Adv. Sci. 10, 2302033 (2023).
6. S. Phark et al. ACS Nano 17, 14144 (2023).
7. Y. Wang et al. Science 382, 87-92 (2023).
物質の対称性と機能性は密接に関係している。対称性を自発的に破る代表的な秩序としては、強磁性や強誘電性が挙げられるが、電子の軌道・スピンさらに構造の自由度までを考えると、強的な秩序の可能性はさらに広がる。強磁性や強誘電性とは異なるタイプの秩序として、近年脚光を浴びているのがトロイダルの秩序である。
本セミナーでは、トロイダル秩序に関する話題として、時間・空間反転対称性は破らず
鏡映対称性のみを破るフェロアキシャル秩序[1]と時間反転だけを破るフェロトロイダル
モノポール秩序[2]について紹介する。前者においては、軸性の秩序変数がベクトル量を回転させる役割を担うことから、特に電場方向に平行なスピン流を生成できる可能性があり、新しい量子伝導現象を生み出す起源になる得る。一方、後者は時間反転のみを破る秩序変数により、物理量の時間反転の性質だけを反転させる応答を生み出すことができる。これにより、電場による反強磁性の制御や電場・磁場の複合場によるカイラリティ制御などを実現できる可能性がある。これらの新しい交差相関応答について紹介したい。
[1] S. Hayami, R. Oiwa, H. Kusunose, J. Phys. Soc. Jpn. 91, 113702 (2022)
[2] S. Hayami, H. Kusunose, Phys. Rev. B 108, L140409 (2023)
参考文献:
[1] Y.A. Bychkov and E.J. Rashba, JETP Lett. 39, 78 (1984).
[2] K. Sakamoto et al., Phys. Rev. Lett. 102, 096805 (2009), Phys. Rev. Lett. 103, 156801 (2009), Nat. Commun. 4, 2073 (2013).
[3] E. Annese et al., Phys. Rev. Lett. 117, 016803 (2016).
[4] K. Kobayashi et al., Phys. Rev. Lett. 125, 176401 (2020).
[5] K. Kobayashi et al., Nano Lett. 23, 7675 (2023).
[6] S. Yoshizawa et al., Nano Lett. 17, 2287 (2017).
[7] S. Inagaki et al., Phys. Rev. Mater. 7, 024805 (2023).
Optical reflected second harmonic generation (RSHG) has been proven to be a sensitive tool for obtaining information regarding the structure of semiconductor surfaces and interfaces. Rotational anisotropy RSHG (RA–RSHG) is used to analyze the structural symmetry of crystals, especially the surface region of centrosymmetric materials. Due to RSHG with the advantage of analyzing planar dipole symmetry, RSHG method has been widely used to study 2D materials, nano surface and interface.
Non-destructive examination of dopant concentration is essential in advanced semiconductor fabrication. One of the main issues to be tackled by the silicon device industry for miniaturization is the production of ultra-shallow doped layers, currently a key process in the silicon technology. The symmetry of the second optical susceptibility that governs the process of RSHG is directly related to the lattice symmetry and dopant situation. Therefore, RSHG has proven to be an efficient and powerful non-destructive tool for investigating the structural and electronic properties of material implanted on the surface layer, which reveal the change of dipole structure due to varied dopant concentration in RA-RSHG spectrum. However, RA-RSHG method to inspect symmetrical dipole contribution is not suitable for doped Si thin film (DSTF) since DSTF is grown by CVD method with in-situ doping and has less crystalline property. To tackle this issue, we suggest a revised time-dependent second harmonic generation (TD-SHG) for quantifying the phosphorus (P) concentration in DSTF. The correlation between the development of electric field-induced second harmonic generation (EFISHG) and dopant concentration forms the foundation of this approach. The technique is based on analyzing the evolution of the internal photoemission induced charge trapping and the concomitant electric field induced SHG. We further demonstrate a strategy to estimate the dopant concentration by considering the Fermi-Dirac distribution and the tunneling probability, without involving the crystallinity of DSUTF. The dopant concentration between 1017 to 1020 (atom/cm3) is unambiguously evaluated by this method. The unprecedented approach of using in-situ method to reveal dopant concentration of DSUTF via time-dependent SHG constitutes an important step towards in-line monitoring and optimizing the fabrication conditions.
参考文献:
H. Hosono, A. Yamamoto, H. Hiramatsu, Y. Ma,
Recent advances in iron-based superconductors superconductors toward applications,
Materials today, 21, 278-302 (2008).
Electron wave function in solids can change electron’s trajectory in
a non-trivial way. The chief example is the Berry curvature that
leads to anomalous electron velocity. In this talk, we will explore
several phenomena of such effects in optoelectronics and transport.
Firstly, spatial gap inversion induces topological boundary states in
bilayer graphene. We investigate the collective motion (plasmons) of
the topological states [1]. Next, we show non-trivial optical
responses of anomalous Hall materials such as cyclotron motion
without magnetic field and dynamical Hall currents without breaking
time reversal symmetry [2,3]. Finally, I will talk about the effect
of wave functions in electron hydrodynamics [4,5].
References
[1] EHH & JCW Song, Long-lived domain wall plasmons in gapped bilayer
graphene, Nano Letters 17, 7252 (2017).
[2] EHH, AJ Frenzel, JCW Song, Cyclotron motion without magnetic
field, New Journal of Physics 21, 083026 (2019).
[3]JM Adhidewata, RWM Komalig, MS Ukhtary, ART Nugraha, BE Gunara,
EHH, Trigonal warping effects on optical properties of anomalous Hall
materials, Physical Review B 107, 155415 (2023).
[4]EHH, J Ekström, EG Idrisov, TL Schmidt, Electron hydrodynamics of
two-dimensional anomalous Hall materials Physical Review B 103,
125106 (2021).
[5]EG Idrisov, EHH, BN Radhakrishnan, TL Schmidt, Hydrodynamic
Navier-Stokes equations in two-dimensional systems with Rashba
spin-orbit coupling, arXiv preprint arXiv:2307.07408.
This seminar series was initiated in May 2023 by several condensed matter scientists in Kanazawa University. Through this seminar series, we hope that both domestic and international researchers at the forefront and young researchers within the university will discuss the latest research topics and that collaborative research and various personal exchanges will emerge from such discussions. Anyone who is interested in the seminar is welcome to attend. We also welcome those who are involved as seminar organizers.
