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BiSbTe2S doped with Sn is believed to be the “perfect” topological insulator [1] with the Fermi energy located in the energy gap. In our samples, we observed well-pronounced quantum oscillations (QOs) that survive up to 30 K. Measuring the QOs at different temperatures, we observed the shift in the frequency to lower values as the temperature is increased. Up to our knowledge, this is the first experimental evidence of this kind of behavior. The surface-sensitive ARPES measurement didn’t observe any change in the position of the Fermi level as a function of temperature, Fig1. Indicating that the oscillations are coming from the bulk and not the Dirac surface states. Future experiments using ion-liquid gating, which allows tuning the Fermi level, have to be carried on to unambiguously find out the origin of the observed QOs.

Chalcogenide phase-change materials show strikingly contrasting optical and electrical properties, which has led to their extensive implementation in various memory devices. By performing spin-, time-, and angle-resolved photoemission spectroscopy combined with the first-principles calculation, we report the experimental results that the crystalline phase of GeSb

Spintronics devices utilizing a magnetic domain-wall motion have attracted increasing attention, and ferrimagnetic materials with almost-compensated magnetic moments are highly required to realize the fast magnetic domain-wall motion. Here, we report a key function for this purpose in the antiperovskite $\mathrm{C}{\mathrm{o}}_{x}\mathrm{M}{\mathrm{n}}_{4\ensuremath{-}x}\mathrm{N}$ film. We have grown $\mathrm{C}{\mathrm{o}}_{x}\mathrm{M}{\mathrm{n}}_{4\ensuremath{-}x}\mathrm{N}$ films with various Co/Mn ratios on $\mathrm{SrTi}{\mathrm{O}}_{3}(001)$ by molecular-beam epitaxy. High-quality growth is confirmed and a perpendicular magnetization emerges at $x=0$, 0.2, 0.5, and 0.8, whereas it turns into in plane for $x\ensuremath{\ge}1.1$. The saturation magnetization ${M}_{S}$ decreases as $x$ increases and reaches a minimum value of $15\phantom{\rule{0.16em}{0ex}}\mathrm{emu}/\mathrm{c}{\mathrm{m}}^{3}$ at $x=0.8$. Then, it increases with $x$ when $0.8\ensuremath{\le}x\ensuremath{\le}3.6$ and saturates. These results indicate that ${M}_{S}$ and magnetic anisotropy of $\mathrm{C}{\mathrm{o}}_{x}\mathrm{M}{\mathrm{n}}_{4\ensuremath{-}x}\mathrm{N}$ films can be manipulated by the Co composition. X-ray absorption spectroscopy and magnetic circular dichroism measurements revealed that Co atoms tend to occupy the I site in the antiperovskite lattice and reasonably explains the origin of minimum ${M}_{S}$ near $x=0.8$, where a compensation of magnetic moments occurs among different atomic sites. We consider that the nearly compensated ferrimagnetic $\mathrm{C}{\mathrm{o}}_{0.8}\mathrm{M}{\mathrm{n}}_{3.2}\mathrm{N}$ is suitable for application to current-induced domain-wall motion devices.

Chalcogenide phase-change materials show strikingly contrasting optical and electrical properties, which has led to their extensive implementation in various memory devices. By performing spin-, time-, and angle-resolved photoemission spectroscopy combined with the first-principles calculation, we report the experimental results that the crystalline phase of GeSb2Te4 is topologically nontrivial in the vicinity of the Dirac semimetal phase. The resulting linearly dispersive bulk Dirac-like bands that cross the Fermi level and are thus responsible for conductivity in the stable crystalline phase of GeSb2Te4 can be viewed as a 3D analogue of graphene. Our finding provides us with the possibility of realizing inertia-free Dirac currents in phase-change materials., The ARPES experiments were performed with the approval of the Proposal Assessing Committee of HSRC (Proposal No. 18BG039). The TARPES measurements were jointly carried out by the Laser and Synchrotron Research Center of the Institute for Solid State Physics at the University of Tokyo. This work was financially supported by KAKENHI (Grant No. 17H06138, No. 17H06152, No. 18H03683, No. 18H01148). This work was also supported by the ‘‘Tomsk State University competitiveness improvement programme’’ (Project No. 8.1.01.2018), by Saint Petersburg State University (Project ID 51126254), and by Russian Science Foundation No. 17-12-01047 (in the crystal growth part). T.V.M. and I.P.R. acknowledge support from Ministry of Education and Science of the Russian Federation (State Task No. 0721-2020-0033) (study of intermixed crystalline phase of GST-124). Calculations were performed at the SKIF-Cyberia supercomputer of Tomsk State University (Russian Federation). K.A.K. acknowledges RFBR Grant 17-08-00955 and a state contract of IGM SB RAS.

Controlled extraction of spin-polarized currents from the surface of topological insulators (TIs) would be an important step to use TIs as spin-electronic device materials. One way is to utilize the surface photovoltage (SPV) effect, by which the surface current may flow upon irradiation of light. To date, unipolar SPV has been observed on TIs, while the realization of ambipolar SPV is crucial for taking control over the direction of the flow. By using time-resolved photoemission, we demonstrate the ambipolar SPV realized on the TI ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The topological surface states showed downward and upward photovoltaic shifts for the $n$- and $p$-type samples, respectively. We also discerned the photogenerated carriers accumulated in the surface states for $g4\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. We provide the keys besides the in-gap Fermi level to engineer the SPV on TIs.

Ferromagnetic topological insulators have emerged as a promising platform for quantum anomalous Hall (QAH) effect with a dissipationless edge transport. However, the observation of QAH effect has so far been restricted to extremely low temperatures. We investigate the microscopic origin of ferromagnetism coupled with topological insulators in vanadium-doped (Sb, Bi)(2)Te-3 employing the x-ray magnetic circular dichroism, angle-resolved two-photon photoemission spectroscopy, combined with first-principles calculations. We found a negative spin polarization, and thus an antiparallel magnetic moment at the Te site with respect to that of the vanadium dopants, which plays the key role in the ferromagnetic order. We ascribe it to the hybridization between Te p and V d majority spin states at the Fermi energy (E-F), being supported by a Zener-type p-d exchange interaction scenario. The substitution of Bi at the Sb site suppresses the bulk ferromagnetism by introducing extra electron carriers in the majority spin channel of Te p states that compensates the antiparallel magnetic moment on the Te site. Our findings reveal important clues to designing magnetic topological insulators with higher Curie temperature that work under ambient conditions.

Maintaining a population inversion in electron distributions is the first step towards lasing. There is a strong interest in realizing the inversion in a Dirac conical band structure, because broad-band lasing may then be realized owing to the zero-gap nature of the Dirac cone. Here we show that the population inversion can be elongated to $g7$ ps at 8 K and $g10$ ps at 300 K on the surface of a $p$-type topological insulator ${({\mathrm{Sb}}_{0.73}{\mathrm{Bi}}_{0.27})}_{2}{\mathrm{Te}}_{3}$. Time- and angle-resolved photoemission spectroscopy gives us the direct evidence for the elongated duration of the inversion in the topological surface states. We hereby provide a guideline to prolong the population inversion at finite temperatures. Our study strengthens the route toward the Dirac materials to be a lasing medium.

Transient electron-hole pairs generated in semiconductors can exhibit unconventional excitonic condensation. Anisotropy in the carrier mass is considered as the key to elongate the life time of the pairs, and hence to stabilize the condensation. Here we employ time- and angle-resolved photoemission spectroscopy to explore the dynamics of photo-generated carriers in black phosphorus. The electronic structure above the Fermi level has been successfully observed, and a massive-and-anisotropic Dirac-type dispersions are confirmed; more importantly, we directly observe that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps. Our finding confirms that black phosphorus is a suitable platform for excitonic condensations, and also open an avenue for future applications in broadband mid-infrared BP-based optoelectronic devices., Comment: 13 pages, 5 figures

Quantum anomalous Hall effect (QAHE) is a key phenomenon for low power-consumption device applications owing to its dissipationless spin-polarized and quantized current in the absence of an external magnetic field. However, the recorded working temperature of the QAHE is still very low. Here we systematically investigate the magnetic dopants induced modifications from the view points of magnetic, structural and electronic properties and the ultrafast carrier dynamics in a series of V-doped Sb2Te3 samples of composition Sb2−x V x Te3 with x = 0, 0.015 and 0.03. Element specific x-ray magnetic circular dichroism signifies that the ferromagnetism of V-doped Sb2Te3 is governed by the p–d hybridization between the host carrier and the magnetic dopant. Time- and angle-resolved photoemission spectroscopy excited with mid-infrared pulses 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 the 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 that would work toward the robust QAHE. Our findings shed light on the material design for low-energy-consuming device applications.

An electronic structure of a Au atomic chain on Ni(110), which was investigated decades ago, has been reinvestigated by high-resolution spin- and angle-resolved photoemission spectroscopy. Clear evidence of a Rashba spin split, i.e., spin-polarization reversal between positive and negative wave vectors with respect to the symmetry point, has been observed in the electronic bands possessing quasi-one-dimensional character. The observed spin-split state caused by spin-orbit interaction (SOI) was overlooked in the previous spin-resolved photoemission study probably due to the dominant effect of exchange splitting of the Ni substrate than the Rashba effect and poorer resolution of the previous measurement. The system containing both the Rashba-type SOI and the exchange interaction with one-dimensional character will provide a platform to investigate the effect of exchange interaction on the spin-split states caused by SOI with low dimensionality.

Magnetically doped topological insulators, possessing an energy gap created at the Dirac point through time-reversal-symmetry breaking, are predicted to exhibit exotic phenomena including the quantized anomalous Hall effect and a dissipationless transport, which facilitate the development of low-power-consumption devices using electron spins. Although several candidates of magnetically doped topological insulators were demonstrated to show long-range magnetic order, the realization of the quantized anomalous Hall effect is so far restricted to the Cr-doped (Sb,Bi)2Te3 system at extremely low temperature; however, the microscopic origin of its ferromagnetism is poorly understood. Here we present an element-resolved study for Cr-doped (Sb,Bi)2Te3 using X-ray magnetic circular dichroism to unambiguously show that the long-range magnetic order is mediated by the p-hole carriers of the host lattice, and the interaction between the Sb(Te) p and Cr d states is crucial. Our results are important for material engineering in realizing the quantized anomalous Hall effect at higher temperatures., Magnetically doped topological insulators may exhibit exotic transport phenomena such as the quantum anomalous Hall effect, however the underlying mechanisms of ferromagnetic order are currently debated. Here, the authors reveal stabilized ferromagnetism in Cr-doped (Sb,Bi)2Te3 mediated by Te and Sb p-hole carriers.

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence., We have clarified that a topological insulator, Bi2Te 2Se, shows two surface states with gigantic Rashba-type spin-splitting located at a binding energy deeper than the topological surface state. The magnitude of the Rashba parameter, as well as the momentum splitting, is found to be large enough to realize a number of nanometer-sized spintronic devices. This novel finding paves the way to studies of gigantic Rashba systems that are suitable for future spintronic applications. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft., A portion of this work has been done under the Japan–Russia Bilateral Joint Research Project (JSPS). This work was financially supported by KAKENHI (grant nos. 23340105, 23244066, 25800179), Grant-in-Aid for Scientific Research (A), (B) and for Young Scientists (B) of JSPS. KAK and OET acknowledge financial support by the RFBR (grant nos. 13-02-92105 and 12-02-00226), and by the Ministry of Education and Science of the Russian Federation.

The surface termination of the promising topological insulator TlBiSe${}_{2}$ has been studied by surface- and bulk-sensitive probes. Our scanning tunneling microscopy has unmasked for the first time the unusual surface morphology of TlBiSe${}_{2}$ obtained by cleaving, where islands are formed by residual atoms on the cleaved plane. The chemical condition of these islands was identified using core-level spectroscopy. We observed thallium core-level spectra that are strongly deformed by a surface component in sharp contrast to the other elements. We propose a simple explanation for this behavior by assuming that the sample cleaving breaks the bonding between thallium and selenium atoms, leaving the thallium layer partially covering the selenium layer. These findings will assist the interpretation of future experimental and theoretical studies of this surface.

Bias voltage dependent scattering of the topological surface state is studied by scanning tunneling microscopy/spectroscopy for a clean surface of the topological insulator Bi${}_{2}$Te${}_{2}$Se. A strong warping of constant energy contours in the unoccupied part of the spectrum is found to lead to a spin-selective scattering. The topological surface state persists to higher energies in the unoccupied range far beyond the Dirac point, where it coexists with the bulk conduction band. This finding sheds light on the spin and charge dynamics over a wide energy range and opens a way to designing optospintronic devices.