Showing papers by "Shik Shin published in 2018"
TL;DR: The researchers found that the surface of the iron superconductor FeTe0.55Se0.45 satisfies the required conditions for topological superconductivity, providing a simple and possibly high-temperature platform for realizing Majorana states.
Abstract: Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe 1– x Se x ( x = 0.45; superconducting transition temperature T c = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below T c . Our study shows that the surface states of FeTe 0.55 Se 0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.
359 citations
Journal Article•
TL;DR: In this paper, the surface of the iron-based superconductor FeTe0.55Se0.45 has been shown to be topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.
Abstract: A topological superconductor A promising path toward topological quantum computing involves exotic quasiparticles called the Majorana bound states (MBSs). MBSs have been observed in heterostructures that require careful nanofabrication, but the complexity of such systems makes further progress tricky. Zhang et al. identified a topological superconductor in which MBSs may be observed in a simpler way by looking into the cores of vortices induced by an external magnetic field. Using angle-resolved photoemission, the researchers found that the surface of the iron superconductor FeTe0.55Se0.45 satisfies the required conditions for topological superconductivity. Science, this issue p. 182 Angle-resolved photoemission spectroscopy indicates that FeTe0.55Se0.45 harbors Dirac-cone–type spin-helical surface states. Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe1–xSex (x = 0.45; superconducting transition temperature Tc = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below Tc. Our study shows that the surface states of FeTe0.55Se0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.
347 citations
TL;DR: Angle-resolved photoemission spectroscopy is used to characterize the surface states of bismuth iodide, providing experimental evidence of a weak topological insulator state that had previously been only theoretically predicted.
Abstract: The major breakthroughs in the understanding of topological materials over the past decade were all triggered by the discovery of the Z$_2$ topological insulator (TI) In three dimensions (3D), the TI is classified as either "strong" or "weak", and experimental confirmations of the strong topological insulator (STI) rapidly followed the theoretical predictions In contrast, the weak topological insulator has so far eluded experimental verification, since the topological surface states emerge only on particular side surfaces which are typically undetectable in real 3D crystals Here we provide experimental evidence for the WTI state in a bismuth iodide, $\beta$-Bi4I4 Significantly, the crystal has naturally cleavable top and side planes both stacked via van-der-Waals forces, which have long been desirable for the experimental realization of the WTI state As a definitive signature of it, we find quasi-1D Dirac TSS at the side-surface (100) while the top-surface (001) is topologically dark Furthermore, a crystal transition from the $\beta$- to $\alpha$-phase drives a topological phase transition from a nontrivial WTI to the trivial insulator around room temperature This topological phase, viewed as quantum spin Hall (QSH) insulators stacked three-dimensionally, and excellent functionality with on/off switching will lay a foundation for new technology benefiting from highly directional spin-currents with large density protected against backscattering
103 citations
TL;DR: By utilizing polarization-dependent laser-excited angle-resolved photoemission spectroscopy, a detailed momentum dependence of the gap is reported in single- and multi-domain regions of orthorhombic FeSe crystals, revealing an unusual node lifting of thegap structure in multi- domain regions.
Abstract: The structure of the superconducting gap in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions. In superconducting FeSe, there have been controversial results reporting nodal and nodeless gap structures, raising a fundamental issue of pairing mechanisms of iron-based superconductivity. Here, by utilizing polarization-dependent laser-excited angle-resolved photoemission spectroscopy, we report a detailed momentum dependence of the gap in single- and multi-domain regions of orthorhombic FeSe crystals. We confirm that the superconducting gap has a twofold in-plane anisotropy, associated with the nematicity due to orbital ordering. In twinned regions, we clearly find finite gap minima near the vertices of the major axis of the elliptical zone-centered Fermi surface, indicating a nodeless state. In contrast, the single-domain gap drops steeply to zero in a narrow angle range, evidencing for nascent nodes. Such unusual node lifting in multi-domain regions can be explained by the nematicity-induced time-reversal symmetry breaking near the twin boundaries.
69 citations
TL;DR: In this article, the topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level were resolved in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively.
Abstract: Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level ($E_{\text{F}}$) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to $E_{\text{F}}$ by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-$T_{\text{c}}$ topological superconductivity.
67 citations
TL;DR: In this paper, photo-induced insulator-to-metal transitions in Ta2Ni(Se1−xSx)5 including the excitonic insulator phase were realized using time and angle-resolved photoemission spectroscopy.
Abstract: Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realisation of photo-induced insulator-to-metal transitions in Ta2Ni(Se1−xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron–hole coupled systems. Ultrafast light pulses can manipulate and probe materials faster than relaxation timescales, leading to new electronic states and insights into equilibrium properties. Okazaki et al. use the properties of photo-induced metallic states to investigate unconventional correlated behaviour in Ta2NiSe5 and Ta2NiS5.
66 citations
TL;DR: This work investigates the bulk electronic structures of the series materials, Ce monopnictides, and unambiguously reveals the topological phase transition from a trivial to a nontrivial regime in going from CeP to CeBi induced by the band inversion.
Abstract: Experimental determinations of bulk band topology in the solid states have been so far restricted to only indirect investigation through the probing of surface states predicted by electronic structure calculations. We here present an alternative approach to determine the band topology by means of bulk-sensitive soft x-ray angle-resolved photoemission spectroscopy. We investigate the bulk electronic structures of the series materials, Ce monopnictides (CeP, CeAs, CeSb, and CeBi). By performing a paradigmatic study of the band structures as a function of their spin-orbit coupling, we draw the topological phase diagram and unambiguously reveal the topological phase transition from a trivial to a nontrivial regime in going from CeP to CeBi induced by the band inversion. The underlying mechanism of the phase transition is elucidated in terms of spin-orbit coupling in concert with their semimetallic band structures. Our comprehensive observations provide a new insight into the band topology hidden in the bulk states.
57 citations
TL;DR: The ARPES spectral map as a function of temperature and film thickness up to 10 QLs reveals key characteristics relevant to the mechanism of coupling between the topological surface states and the superconducting Nb substrate; the effective coupling length is found to be much larger than the decay length of the topology surface states.
Abstract: A topological insulator film coupled to a simple isotropic s-wave superconductor substrate can foster helical pairing of the Dirac fermions associated with the topological surface states. Experimental realization of such a system is exceedingly difficult, however using a novel "flip-chip" technique, we have prepared single-crystalline Bi2Se3 films with predetermined thicknesses in terms of quintuple layers (QLs) on top of Nb substrates fresh from in situ cleavage. Our angle-resolved photoemission spectroscopy (ARPES) measurements of the film surface disclose superconducting gaps and coherence peaks of similar magnitude for both the topological surface states and bulk states. The ARPES spectral map as a function of temperature and film thickness up to 10 QLs reveals key characteristics relevant to the mechanism of coupling between the topological surface states and the superconducting Nb substrate; the effective coupling length is found to be much larger than the decay length of the topological surface states.
37 citations
TL;DR: The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials.
Abstract: Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.
27 citations
TL;DR: The result suggests that the superconducting Nd_{2}CuO_{4} film is doped with electrons despite the absence of the Ce substitution, suggesting that superconductivity in cuprates with the T^{'}-type structure is induced by well-designed reduction annealing alone.
Abstract: In order to realize superconductivity in cuprates with the ${T}^{\ensuremath{'}}$-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the ${T}^{\ensuremath{'}}$-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped ${T}^{\ensuremath{'}}$-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ ($x=0$, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the ${\mathrm{CuO}}_{2}$ plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ film is doped with electrons despite the absence of the Ce substitution.
21 citations
TL;DR: Realisation of photo-induced insulator-to-metal transitions in Ta2Ni(Se1−xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy is reported.
Abstract: Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally-friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realization of photo-induced insulator-to-metal transitions in Ta2Ni(Se1-xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic-insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron-hole coupled systems.
TL;DR: The detailed study of the mixed valence compound α-YbAlB4 reveals that a small chemical substitution induces a sharp valence crossover, accompanied by a pronounced non–Fermi liquid behavior characterized by a divergent effective mass and unusual T/B scaling in the magnetization.
Abstract: A valence critical end point existing near the absolute zero provides a unique case for the study of a quantum version of the strong density fluctuation at the Widom line in the supercritical fluids. Although singular charge and orbital dynamics are suggested theoretically to alter the electronic structure significantly, breaking down the standard quasi-particle picture, this has never been confirmed experimentally to date. We provide the first empirical evidence that the proximity to quantum valence criticality leads to a clear breakdown of Fermi liquid behavior. Our detailed study of the mixed valence compound α-YbAlB4 reveals that a small chemical substitution induces a sharp valence crossover, accompanied by a pronounced non-Fermi liquid behavior characterized by a divergent effective mass and unusual T/B scaling in the magnetization.
TL;DR: In this paper, the authors investigated the band structure, nematic state, and superconducting gap structure of two selected FeSe single crystals containing different amounts of disorder and revealed that the reported controversy on the gap structure is due to the disorder-sensitive nodelike small gap.
Abstract: We investigate the band structure, nematic state, and superconducting gap structure of two selected FeSe single crystals containing different amounts of disorder. Transport and angle-resolved photoemission spectroscopy measurements show that the small amount of disorder has little effect on the band structure and the nematic state of FeSe. However, temperature and magnetic field dependencies of specific heat for the two samples are quite different. Wave-vector-dependent gap structure is obtained from the three-dimensional field-angle-resolved specific heat measurements. A small gap with two vertical-line nodes or gap minima along the ${k}_{z}$ direction is found only in the sample with higher quality. Such symmetry-unprotected nodes or gap minima are found to be smeared out by a small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive nodelike small gap.
TL;DR: In this article, the spin-polarized structure of the Bi/InAs(110)-$(2\ifmmode\times\else\texttimes\fi{}1) surface was revealed by angle-resolved photoelectron spectroscopy (ARPES).
Abstract: Electronic states of the Bi/InAs(110)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surface and its spin-polarized structure are revealed by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES, and density-functional-theory calculation. The surface electronic state showed quasi-one-dimensional (Q1D) dispersion curves and a nearly metallic character; the top of the holelike band is just below the Fermi level. The size of the Rashba parameter ${\ensuremath{\alpha}}_{\mathrm{R}}$ reached a much larger value ($\ensuremath{\sim}5.5$ eV\AA{}) than for previously reported 1D systems. The present result will provide a fertile playground for further studies of the exotic electronic phenomena in 1D or Q1D systems with the spin-split electronic states as well as for advanced spintronic devices.
TL;DR: It is shown that the existence of the analytic forms of f-1 is guaranteed in a variety of setups, and the mapping methodology is also usable in other spectroscopic methods such as momentum-resolved electron-energy loss spectroscopy.
Abstract: The distribution of photoelectrons acquired in angle-resolved photoemission spectroscopy can be mapped onto the energy-momentum space of the Bloch electrons in the crystal. The explicit forms of the mapping function f depend on the configuration of the apparatus as well as on the type of the photoelectron analyzer. We show that the existence of the analytic forms of f−1 is guaranteed in a variety of setups. The variety includes the case when the analyzer is equipped with a photoelectron deflector. Thereby, we provide a demonstrative mapping program implemented by an algorithm that utilizes both f and f−1. The mapping methodology is also usable in other spectroscopic methods such as momentum-resolved electron-energy loss spectroscopy.
TL;DR: In this paper, spin and angle-resolved photoelectron spectroscopy (SARPES) with a vacuum-ultraviolet laser was used to investigate spin textures of the Ag(111) and Cu(111), and the results were consistent with the theoretical calculations for the orbital resolved surface state.
Abstract: Spin-resolved band structures of $L$-gap surface states on Ag(111) and Cu(111) are investigated by spin- and angle-resolved photoelectron spectroscopy (SARPES) with a vacuum-ultraviolet laser. The observed spin textures of the Ag(111) and Cu(111) surface states agree with that expected by the conventional Rashba effect. The Rashba parameter of the Ag(111) surface state is estimated quantitatively and is $80%$ of that of Cu(111). The surface-state wave function is found to be predominantly of even mirror symmetry with negligible odd contribution by SARPES using a linearly polarized light. The results are consistent with our theoretical calculations for the orbital-resolved surface state.
TL;DR: In this paper, the authors used time and angle-resolved photoelectron spectroscopy on the surface of Bi2Te3 to demonstrate that the magnitude of the surface photovoltage is almost doubled in optically aged samples, i.e., samples whose surface has been exposed to intense infrared light illumination.
Abstract: The efficient generation of spin-polarized current is one of the keys to realizing spintronic devices with a low power consumption. Topological insulators are strong candidates for this purpose. A surface photovoltaic effect can be utilized on the surface of a topological insulator, where a surface spin-polarized current can flow upon illumination. Here, we used time- and angle-resolved photoelectron spectroscopy on the surface of Bi2Te3 to demonstrate that the magnitude of the surface photovoltage is almost doubled in optically aged samples, i.e., samples whose surface has been exposed to intense infrared light illumination. Our findings pave the way for optical control of the spin-polarized current by utilizing topological insulators.
TL;DR: In this article, the authors investigate the transient electronic structure of BaFe, a parent compound of iron-based superconductors, by time and angle-resolved photoemission spectroscopy.
Abstract: We investigate the transient electronic structure of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, a parent compound of iron-based superconductors, by time- and angle-resolved photoemission spectroscopy. In order to probe the entire Brillouin zone, we utilize extreme ultraviolet photons and observe photoemission intensity oscillation with the frequency of the ${A}_{1g}$ phonon which is antiphase between the zone-centered hole Fermi surfaces (FSs) and zone-cornered electron FSs. We attribute the antiphase behavior to the warping in one of the zone-centered hole FSs accompanying the displacement of the pnictogen height and find that this displacement is the same direction as that induced by substitution of P for As, where superconductivity is induced by a structural modification without carrier doping in this system.
TL;DR: In this paper, the authors employed time and angle-resolved photoemission spectroscopy to explore the dynamics of photo-generated carriers in black phosphorus and found that the photo-carriers generated across the direct band gap have the life time exceeding 400ps.
Abstract: 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.
TL;DR: Using high-resolution spin-resolved photoemission spectroscopy, a thermal spin depolarization is observed to which all spin-polarized electrons contribute and opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.
Abstract: Using high-resolution spin-resolved photoemission spectroscopy, we observe a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore, we observe a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristic of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.
TL;DR: In this article, an ultrabrilliant free electron laser (FEL) in the soft X-ray range was facilitated for the detection of transient signals of resonant MOKE from the ultrathin Fe film.
Abstract: Time-resolved magneto-optical Kerr effect (MOKE) measurement was demonstrated on a sample of the Au/Fe/Au heterostructure with the Fe layer of 0.35 nm thickness under Fe M-edge resonance condition. An ultrabrilliant free electron laser (FEL) in the soft X-ray range was facilitated for the detection of transient signals of resonant MOKE from the ultrathin Fe film. A variation in the Kerr rotation angle was successfully observed on the femtosecond timescale. This technique enables us to reveal the transient magnetization dynamics of such a-few-monolayer magnetic films, which promote the development of spintronic devices.
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TL;DR: In this paper, the authors employed time and angle-resolved photoemission spectroscopy to explore the dynamics of photo-generated carriers in black phosphorus and found that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps.
Abstract: 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.
TL;DR: The results show the effects of amino group protonation on the electronic state around the carboxyl group and suggest X-ray emission spectroscopy might be a tool to investigate intramolecular interactions between functional groups in a molecule.
Abstract: The valence electronic structures of the amino acid glycine in aqueous solution were investigated in detail through X-ray emission spectroscopy at O 1s excitation under selective excitation conditions of the C[double bond, length as m-dash]O site in the carboxyl group. The X-ray emission spectra of glycine were similar to that of acetic acid (CH3COOH), suggesting a resemblance between the molecular orbitals associated with the carboxyl groups in the two molecules. The changes of O 1s X-ray emission spectra as a function of pH were investigated in detail. In addition to spectral changes due to protonation/deprotonation of the carboxyl group for lower pH-values around the pKa value (∼2.3), the spectra of glycine exhibited further changes in the higher-pH region near the pKb value of glycine (dissociation constant of amino group ∼9.5). These results show the effects of amino group protonation on the electronic state around the carboxyl group. X-ray emission spectroscopy might be a tool to investigate intramolecular interactions between functional groups in a molecule.
TL;DR: The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics.
Abstract: The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics. Laser-SARPES achieves two great capabilities. Firstly, by examining orbital selection rule of linearly polarized lasers, orbital selective excitation can be carried out in SAPRES experiment. Secondly, the technique can show full information of a variation of the spin quantum axis as a function of the light polarization. To demonstrate the power of the collaboration of these capabilities in laser-SARPES, we apply this technique for the investigations of spin-orbit coupled surface states of Bi2Se3. This technique affords to decompose spin and orbital components from the spin-orbit coupled wavefunctions. Moreover, as a representative advantage of using the direct spin detection collaborated with the polarization-variable laser, the technique unambiguously visualizes the light polarization dependence of the spin quantum axis in three-dimension. Laser-SARPES dramatically increases a capability of photoemission technique.
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TL;DR: In this paper, the topological insulator (TI) phase and topological Dirac semimetal (TDS) phase near Fermi level were resolved in the iron-based superconductor Li(Fe,Co)As.
Abstract: Topological insulators/semimetals and unconventional iron-based superconductors have attracted major attentions in condensed matter physics in the past 10 years. However, there is little overlap between these two fields, although the combination of topological states and superconducting states will produce more exotic topologically superconducting states and Majorana bound states (MBS), a promising candidate for realizing topological quantum computations. With the progress in laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) with very high energy- and momentum-resolution, we directly resolved the topological insulator (TI) phase and topological Dirac semimetal (TDS) phase near Fermi level ($E_F$) in the iron-based superconductor Li(Fe,Co)As. The TI and TDS phases can be separately tuned to $E_F$ by Co doping, allowing a detailed study of different superconducting topological states in the same material. Together with the topological states in Fe(Te,Se), our study shows the ubiquitous coexistence of superconductivity and multiple topological phases in iron-based superconductors, and opens a new age for the study of high-Tc iron-based superconductors and topological superconductivity.
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TL;DR: In this paper, the existence of topological Dirac semimetals (TDSs) in Fe(Te,Se) has been observed in angle-resolved photoelectron spectroscopy (ARPES) and transport experiments.
Abstract: Topological Dirac semimetals (TDSs) exhibit bulk Dirac cones protected by time reversal and crystal symmetry, as well as surface states originating from non-trivial topology. While there is a manifold possible onset of superconducting order in such systems, few observations of intrinsic superconductivity have so far been reported for TDSs. We observe evidence for a TDS phase in FeTe$_{1-x}$Se$_x$ ($x$ = 0.45), one of the high transition temperature ($T_c$) iron-based superconductors. In angle-resolved photoelectron spectroscopy (ARPES) and transport experiments, we find spin-polarized states overlapping with the bulk states on the (001) surface, and linear magnetoresistance (MR) starting from 6 T. Combined, this strongly suggests the existence of a TDS phase, which is confirmed by theoretical calculations. In total, the topological electronic states in Fe(Te,Se) provide a promising high $T_c$ platform to realize multiple topological superconducting phases.
TL;DR: In this paper, an angle-resolved photoemission (ARPES) study was conducted on a quasiparticle peak (QP) emerging from hybridization, characterized by a binding energy and an onset of coherence both at about 4 meV, which conforms with a previously observed reduced Kondo scale at about 40 K.
Abstract: We report an angle-resolved photoemission (ARPES) study of $\ensuremath{\beta}\ensuremath{-}{\mathrm{YbAlB}}_{4}$, which is known to harbor unconventional quantum criticality (QC) without any tuning. We directly observe a quasiparticle peak (QP) emerging from hybridization, characterized by a binding energy and an onset of coherence both at about 4 meV. This value conforms with a previously observed reduced Kondo scale at about 40 K. Consistency with an earlier study of carriers in $\ensuremath{\beta}\ensuremath{-}{\mathrm{YbAlB}}_{4}$ via the Hall effect strongly suggests that this QP is responsible for the QC in $\ensuremath{\beta}\ensuremath{-}{\mathrm{YbAlB}}_{4}$. A comparison with the sister polymorph $\ensuremath{\alpha}\ensuremath{-}{\mathrm{YbAlB}}_{4}$, which is not quantum critical at ambient pressure, further supports this result. Indeed, within the limitation of our instrumental resolution, our ARPES measurements do not show tangible sign of hybridization in this locally isomorphic system, while the conduction band we observe is essentially the same as in $\ensuremath{\beta}\ensuremath{-}{\mathrm{YbAlB}}_{4}$. We therefore claim that we identified by ARPES the carriers responsible for the QC in $\ensuremath{\beta}\ensuremath{-}{\mathrm{YbAlB}}_{4}$. The observed dispersion and the underlying hybridization of this QP are discussed in the context of existing theoretical models.
TL;DR: Three electron pockets are found around the [Formula: see text] line, where the Fermi surfaces are closed in a surface Brillouin zone (SBZ), and the relation between SBZ and the direction of the atomic NW, and the symmetry of the surface state are clarified.
Abstract: The electronic states of Au-induced atomic nanowires on Ge(0 0 1) (Au/Ge(0 0 1) NWs) have been studied by angle-resolved photoelectron spectroscopy with linearly polarized light. We have found three electron pockets around the line, where the Fermi surfaces are closed in a surface Brillouin zone (SBZ). The results indicate 2D Fermi surfaces of Au/Ge(0 0 1) NWs whereas the atomic structure is 1D. On the basis of the polarization-dependent spectra, the relation between SBZ and the direction of the atomic NW, and the symmetry of the surface state are clarified. These are very useful for further studies on the atomic structure of NWs.
01 Jun 2018
TL;DR: In this paper, a contactless measurement of the picosecond dynamics of photovoltage in two GaAs solar cells, one with n-layer on the top and the other with p-layer, was performed using time-resolved photoemission spectroscopy.
Abstract: A contactless measurement of the picosecond dynamics of photovoltage in two GaAs solar cells, one with n-layer on the top and the other with p-layer, was performed using time-resolved photoemission spectroscopy. We found that the shift of photoelectron spectra occurs in opposite directions for the two samples. This agrees with the interpretation that the spectral shift is caused by photovoltage at the p-n junction. We also summarize the limiting factors on the present technique including the degradation of the temporal resolution caused by propagation effect, and the deformation of spectra originating from the work-function cutoff, and discuss the possible solutions.