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論文

Ultrafast electron dynamics in a topological surface state observed in two-dimensional momentum space

Reimann, J.*; 角田 一樹; 鹿子木 将明*; Kokh, K. A.*; Tereshchenko, O. E.*; 木村 昭夫*; G$"u$dde, J.*; H$"o$fer, U.*

Scientific Reports (Internet), 13, p.5796_1 - 5796_8, 2023/04

 被引用回数:0 パーセンタイル:0(Multidisciplinary Sciences)

We study ultrafast population dynamics in the topological surface state of Sb$$_2$$Te$$_3$$ in two-dimensional momentum space with time- and angle-resolved two-photon photoemission spectroscopy. Linear polarized mid-infrared pump pulses are used to permit a direct optical excitation across the Dirac point. We show that this resonant excitation is strongly enhanced within the Dirac cone along three of the six $$bar{Gamma}$$-$$bar{M}$$ directions and results in a macroscopic photocurrent when the plane of incidence is aligned along a $$bar{Gamma}$$-$$bar{K}$$ direction. Our experimental approach makes it possible to disentangle the decay of transiently excited population and photocurent by elastic and inelastic electron scattering within the full Dirac cone in unprecedented detail. This is utilized to show that doping of Sb$$_2$$Te$$_3$$ by vanadium atoms strongly enhances inelastic electron scattering to lower energies, but only scarcely affects elastic scattering around the Dirac cone.

論文

Magnetic-impurity-induced modifications to ultrafast carrier dynamics in the ferromagnetic topological insulators Sb$$_{2-x}$$V$$_{x}$$Te$$_{3}$$

角田 一樹*; 鹿子木 将明*; Reimann, J.*; Nurmamat, M.*; 後藤 伸一*; 竹田 幸治; 斎藤 祐児; Kokh, K. A.*; Tereshchenko, O. E.*; G$"u$dde, J.*; et al.

New Journal of Physics (Internet), 21(9), p.093006_1 - 093006_8, 2019/09

 被引用回数:10 パーセンタイル:65.58(Physics, Multidisciplinary)

We systematically investigate the magnetic, structural and electronic properties and the ultrafast carrier dynamics in a series of V-doped Sb$$_{2}$$Te$$_{3}$$ samples of composition Sb$$_{2-x}$$V$$_{x}$$Te$$_{3}$$ with x = 0, 0.015 and 0.03. Element specific X-ray magnetic circular dichroism signifies that the ferromagnetism of V-doped Sb$$_{2}$$Te$$_{3}$$ is governed by the p-d hybridization between the host carrier and the magnetic dopant. Time- and angle-resolved photoemission spectroscopy has revealed that the V impurity induced states underlying the topological surface state (TSS) add scattering channels that significantly shorten the duration of transient surface electrons down to 100 fs scale. This is in a sharp contrast to the prolonged duration reported for pristine samples though the TSS is located inside the bulk energy gap of the host in either magnetic or non-magnetic cases. It implies the presence of a mobility gap in the bulk energy gap region of the host material.

論文

Dirac gap opening and Dirac-fermion-mediated magnetic coupling in antiferromagnetic Gd-doped topological insulators and their manipulation by synchrotron radiation

Shikin, A. M.*; Estyunin, D. A.*; Surnin, Yu. I.*; Koroleva, A. V.*; Shevchenko, E. V.*; Kokh, K. A.*; Tereshchenko, O. E.*; Kumar, S.*; Schwier, E. F.*; 島田 賢也*; et al.

Scientific Reports (Internet), 9(1), p.4813_1 - 4813_17, 2019/03

 被引用回数:19 パーセンタイル:71.98(Multidisciplinary Sciences)

A new kind of magnetically-doped antiferromagnetic (AFM) topological insulators (TIs), Bi$$_{1.09}$$Gd$$_{0.06}$$Sb$$_{0.85}$$Te$$_{3}$$, has been studied by angle-resolved photoemission, superconducting magnetometry (SQUID) and X-ray magnetic circular dichroism (XMCD). It has been shown that this TI is characterized by the Dirac gap at the Fermi level. In the paramagnetic phase, a surface magnetic layer is supposed to develop, where the coupling between the Gd magnetic moments is mediated by the topological surface states (TSSs). This assumption can be confirmed by opening a gap at the Dirac point indicated by the surface-sensitive ARPES, a weak hysteresis loop measured by SQUID, the XMCD showing a surface magnetic moment and the temperature dependence of electrical resistance demonstrating a mid-gap semiconducting behavior, which correlates with the temperature dependence of the surface magnetization and confirms the conclusion that only TSSs are located at the Fermi level.

口頭

可視光および中赤外光を用いた時間分解光電子分光によるトポロジカル絶縁体(Sb,Bi)$$_2$$Te$$_3$$の超高速ダイナミクス

角田 一樹; 鹿子木 将明*; Reimann, J.*; Guedde, J.*; Kokh, K. A.*; Tereshchenko, O. E.*; Hoefer, U.*; 木村 昭夫*

no journal, , 

グラフェンやトポロジカル絶縁体などに代表されるディラック電子系の高光強度下での光学特性に最近大きな注目が集まっている。ディラック電子系は、ディラックコーンと呼ばれる円錐型のバンド分散によって特徴づけられ、バンドギャップが存在しないため幅広い波長帯域の光を吸収することができる。また、バンド交差点(ディラック点)がボトルネックとして作用するため、光励起後の電子が反転分布を形成し、ディラック電子系物質を媒質とした広帯域レーザー発振に応用することも可能である。グラフェンとトポロジカル絶縁体に現れるディラックコーンには決定的な違いが存在しており、前者はスピン縮退しているが、後者はスピン分裂している。しかしながら、トポロジカル絶縁体のスピン偏極電子構造に関連した超高速ダイナミクスはこれまでほとんど議論されてこなかった。そこで我々は、可視光(2.5eV)および中赤外光(0.3eV)をポンプ光として用いた時間・角度分解光電子分光をトポロジカル絶縁体(Sb,Bi)$$_2$$Te$$_3$$に対して行い、トポロジカル表面状態における直接・間接励起および初期緩和の様子を系統的に調べた。

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