Publisher Correction: A weak topological insulator state in quasi-one-dimensional bismuth iodide.

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A weak topological insulator state in quasi-one-dimensional bismuth iodide.

The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z2-type topological insulator-a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either 'strong' or 'weak'1,2, and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions3-5. By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals6-10. Here we provide experimental evidence for the WTI state in a bismuth iodide, ß-Bi4I4. Notably, the crystal has naturally cleavable top and side planes-stacked via van der Waals forces-which have long been desirable for the experimental realization of the WTI state11,12. As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the ß-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase-viewed as quantum spin Hall insulators stacked three-dimensionally13,14-will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.

Multipole polaron in the devil's staircase of CeSb.

Rare-earth intermetallic compounds exhibit rich phenomena induced by the interplay between localized f orbitals and conduction electrons. However, since the energy scale of the crystal-electric-field splitting is only a few millielectronvolts, the nature of the mobile electrons accompanied by collective crystal-electric-field excitations has not been unveiled. Here, we examine the low-energy electronic structures of CeSb through the anomalous magnetostructural transitions below the Néel temperature, ~17 K, termed the 'devil's staircase', using laser angle-resolved photoemission, Raman and neutron scattering spectroscopies. We report another type of electron-boson coupling between mobile electrons and quadrupole crystal-electric-field excitations of the 4f orbitals, which renormalizes the Sb 5p band prominently, yielding a kink at a very low energy (~7 meV). This coupling strength is strong and exhibits anomalous step-like enhancement during the devil's staircase transition, unveiling a new type of quasiparticle, named the 'multipole polaron', comprising a mobile electron dressed with a cloud of the quadrupole crystal-electric-field polarization.

Topological Kagome Magnet Co3Sn2S2 Thin Flakes with High Electron Mobility and Large Anomalous Hall Effect.

Magnetic Weyl semimetals attract considerable interest not only for their topological quantum phenomena but also as an emerging materials class for realizing quantum anomalous Hall effect in the two-dimensional limit. A shandite compound Co3Sn2S2 with layered kagome-lattices is one such material, where vigorous efforts have been devoted to synthesize the two-dimensional crystal. Here, we report a synthesis of Co3Sn2S2 thin flakes with a thickness of 250 nm by chemical vapor transport method. We find that this facile bottom-up approach allows the formation of large-sized Co3Sn2S2 thin flakes of high-quality, where we identify the largest electron mobility (∼2600 cm2 V-1 s-1) among magnetic topological semimetals, as well as the large anomalous Hall conductivity (∼1400 Ω-1 cm-1) and anomalous Hall angle (∼32%) arising from the Berry curvature. Our study provides a viable platform for studying high-quality thin flakes of magnetic Weyl semimetal and stimulate further research on unexplored topological phenomena in the two-dimensional limit.

Large Variation of Dirac Semimetal State in Perovskite CaIrO_{3} with Pressure-Tuning of Electron Correlation.

The impact of electron correlation on the Dirac semimetal state is investigated for perovskite CaIrO_{3} in terms of the magnetotransport properties under varying pressures. The reduction of electron correlation with a pressure of 1 GPa enhances the Fermi velocity as much as 40%, but it reduces the mobility by an order of magnitude by detuning the Dirac node from the Fermi energy. Moreover, the giant magnetoresistance at the quantum limit due to the one-dimensional confinement of Dirac electrons is critically suppressed under pressure. These results indicate that the electron correlation is a crucial knob for controlling the transport of a correlated Dirac semimetal.

Evidence for magnetic Weyl fermions in a correlated metal.

Weyl fermions have been observed as three-dimensional, gapless topological excitations in weakly correlated, inversion-symmetry-breaking semimetals. However, their realization in spontaneously time-reversal-symmetry-breaking phases of strongly correlated materials has so far remained hypothetical. Here, we report experimental evidence for magnetic Weyl fermions in Mn3Sn, a non-collinear antiferromagnet that exhibits a large anomalous Hall effect, even at room temperature. Detailed comparison between angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations reveals significant bandwidth renormalization and damping effects due to the strong correlation among Mn 3d electrons. Magnetotransport measurements provide strong evidence for the chiral anomaly of Weyl fermions-namely, the emergence of positive magnetoconductance only in the presence of parallel electric and magnetic fields. Since weak magnetic fields (approximately 10 mT) are adequate to control the distribution of Weyl points and the large fictitious fields (equivalent to approximately a few hundred T) produced by them in momentum space, our discovery lays the foundation for a new field of science and technology involving the magnetic Weyl excitations of strongly correlated electron systems such as Mn3Sn.

Experimental Determination of the Topological Phase Diagram in Cerium Monopnictides.

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.

Orbital-Dependent Band Narrowing Revealed in an Extremely Correlated Hund's Metal Emerging on the Topmost Layer of Sr_{2}RuO_{4}.

We use a surface-selective angle-resolved photoemission spectroscopy and unveil the electronic nature on the topmost layer of Sr_{2}RuO_{4} crystal, consisting of slightly rotated RuO_{6} octahedrons. The γ band derived from the 4d_{xy} orbital is found to be about three times narrower than that for the bulk. This strongly contrasts with a subtle variation seen in the α and ß bands derived from the one-dimensional 4d_{xz/yz}. This anomaly is reproduced by the dynamical mean-field theory calculations, introducing not only the on-site Hubbard interaction but also the significant Hund's coupling. We detect a coherence-to-incoherence crossover theoretically predicted for Hund's metals, which has been recognized only recently. The crossover temperature in the surface is about half that of the bulk, indicating that the naturally generated monolayer of reconstructed Sr_{2}RuO_{4} is extremely correlated and well isolated from the underlying crystal.

Extremely high electron mobility in a phonon-glass semimetal.

The electron mobility is one of the key parameters that characterize the charge-carrier transport properties of materials, as exemplified by the quantum Hall effect as well as high-efficiency thermoelectric and solar energy conversions. For thermoelectric applications, introduction of chemical disorder is an important strategy for reducing the phonon-mediated thermal conduction, but is usually accompanied by mobility degradation. Here, we show a multilayered semimetal ß-CuAgSe overcoming such a trade-off between disorder and mobility. The polycrystalline ingot shows a giant positive magnetoresistance and Shubnikov de Haas oscillations, indicative of a high-mobility small electron pocket derived from the Ag s-electron band. Ni doping, which introduces chemical and lattice disorder, further enhances the electron mobility up to 90,000 cm(2) V(-1) s(-1) at 10 K, leading not only to a larger magnetoresistance but also a better thermoelectric figure of merit. This Ag-based layered semimetal with a glassy lattice is a new type of promising thermoelectric material suitable for chemical engineering.

Dependence of carrier doping on the impurity potential in transition-metal-substituted FeAs-based superconductors.

In order to examine to what extent the rigid-band-like electron doping scenario is applicable to the transition metal-substituted Fe-based superconductors, we have performed angle-resolved photoemission spectroscopy studies of Ba(Fe(1-x)Ni(x))(2)As(2) (Ni-122) and Ba(Fe(1-x)Cu(x))(2)As(2) (Cu-122), and compared the results with Ba(Fe(1-x)Co(x))(2)As(2) (Co-122). We find that Ni 3d-derived features are formed below the Fe 3d band and that Cu 3d-derived ones further below it. The electron and hole Fermi surface (FS) volumes are found to increase and decrease with substitution, respectively, qualitatively consistent with the rigid-band model. However, the total extra electron number estimated from the FS volumes (the total electron FS volume minus the total hole FS volume) is found to decrease in going from Co-, Ni-, to Cu-122 for a fixed nominal extra electron number, that is, the number of electrons that participate in the formation of FS decreases with increasing impurity potential. We find that the Néel temperature T(N) and the critical temperature T(c) maximum are determined by the FS volumes rather than the nominal extra electron concentration or the substituted atom concentration.

Strongly spin-orbit coupled two-dimensional electron gas emerging near the surface of polar semiconductors.

We investigate the two-dimensional highly spin-polarized electron accumulation layers commonly appearing near the surface of n-type polar semiconductors BiTeX (X=I, Br, and Cl) by angular-resolved photoemission spectroscopy. Because of the polarity and the strong spin-orbit interaction built in the bulk atomic configurations, the quantized conduction-band subbands show giant Rashba-type spin splitting. The characteristic 2D confinement effect is clearly observed also in the valence bands down to the binding energy of 4 eV. The X-dependent Rashba spin-orbit coupling is directly estimated from the observed spin-split subbands, which roughly scales with the inverse of the band-gap size in BiTeX.

Giant Rashba-type spin splitting in bulk BiTeI.

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.

Ab initio studies on the interplay between spin-orbit interaction and Coulomb correlation in Sr2IrO4 and Ba2IrO4.

Ab initio analyses of A(2)IrO(4) (A=Sr,Ba) are presented. Effective Hubbard-type models for Ir 5d t(2g) manifolds downfolded from the global band structure are solved based on the dynamical mean-field theory. The results for A=Sr and Ba correctly reproduce paramagnetic metals undergoing continuous transitions to insulators below the Néel temperature T(N). These compounds are classified not into Mott insulators but into Slater insulators. However, the insulating gap opens by a synergy of the Néel order and significant band renormalization, which is also manifested by a 2D bad metallic behavior in the paramagnetic phase near the quantum criticality.

Mechanisms of enhanced orbital dia- and paramagnetism: application to the Rashba semiconductor BiTeI.

We study the magnetic susceptibility of a layered semiconductor BiTeI with giant Rashba spin splitting both theoretically and experimentally to explore its orbital magnetism. Apart from the core contributions, a large temperature-dependent diamagnetic susceptibility is observed when the Fermi energy E(F) is near the crossing point of the Rashba spin-split conduction bands at the time-reversal symmetry point A. On the other hand, when E(F) is below this band crossing, the susceptibility turns to be paramagnetic. These features are consistent with first-principles calculations, which also predict an enhanced orbital magnetic susceptibility with both positive and negative signs as a function of E(F) due to band (anti)crossings. Based on these observations, we propose two mechanisms for the enhanced paramagnetic orbital susceptibility.

Enhanced infrared magneto-optical response of the nonmagnetic semiconductor BiTeI driven by bulk Rashba splitting.

We study the magneto-optical (MO) response of the polar semiconductor BiTeI with giant bulk Rashba spin splitting at various carrier densities. Despite being nonmagnetic, the material is found to yield a huge MO activity in the infrared region under moderate magnetic fields (up to 3 T). Our first-principles calculations show that the enhanced MO response of BiTeI comes mainly from the intraband transitions between the Rashba-split bulk conduction bands. These transitions connecting electronic states with opposite spin directions become active due to the presence of strong spin-orbit interaction and give rise to distinct features in the MO spectra with a systematic doping dependence. We predict an even more pronounced enhancement in the low-energy MO response and dc Hall effect near the crossing (Dirac) point of the conduction bands.

Magnetic structures and electronic properties of cubic-pyrochlore ruthenates from first principles.

The magnetic ground states ofR2Ru2O7andA2Ru2O7withR= Pr, Gd, Ho, and Er, as well asA= Ca, Cd are predicted devising a combination of the cluster-multipole (CMP) theory and spin-density-functional theory (SDFT). The strong electronic correlation effects are estimated by the constrained-random-phase approximation (cRPA) and taken into account within the dynamical-mean-field theory (DMFT). The target compounds feature d-orbital magnetism on Ru4+and Ru5+ions forRandA, respectively, as well as f-orbital magnetism on theRsite, which leads to an intriguing interplay of magnetic interactions in a strongly correlated system. We find CMP + SDFT is capable of describing the magnetic ground states in these compounds. The cRPA captures a difference in the screening strength betweenR2Ru2O7andA2Ru2O7compounds, which leads to a qualitative and quantitative understanding of the electronic properties within DMFT.

Optical response of relativistic electrons in the polar BiTeI semiconductor.

The transitions between the spin-split bands by spin-orbit interaction are relevant to many novel phenomena such as the resonant dynamical magnetoelectric effect and the spin Hall effect. We perform optical spectroscopy measurements combined with first-principles calculations to study these transitions in the recently discovered giant bulk Rashba spin-splitting system BiTeI. Several novel features are observed in the optical spectra of the material including a sharp edge singularity due to the reduced dimensionality of the joint density of states and a systematic doping dependence of the intraband transitions between the Rashba-split branches. These confirm the bulk nature of the Rashba-type splitting in BiTeI and manifest the relativistic nature of the electron dynamics in a solid.

Pseudogap of metallic layered nickelate R(2-x)Sr(x)NiO4 (R = Nd, Eu) crystals measured using angle-resolved photoemission spectroscopy.

We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates, R(2-x)Sr(x)NiO4 (R = Nd, Eu), isostructural to La(2-x)Sr(x)CuO4. Angle-resolved photoemission spectroscopy on the barely metallic Eu(0.9)Sr(1.1)NiO4 (R = Eu, x = 1.1) has revealed a large hole surface of x2-y2 character with a high-energy pseudogap of the same symmetry and comparable magnitude with those of underdoped (x<0.1) cuprates, although the antiferromagnetic interactions are 1 order of magnitude smaller. This finding strongly indicates that the momentum-dependent pseudogap feature in the layered nickelate arises from the real-space charge correlation.

Two-dimensional and three-dimensional Fermi surfaces of superconducting BaFe2(As(1-x)P(x))2 and their nesting properties revealed by angle-resolved photoemission spectroscopy.

We have studied the three-dimensional shapes of the Fermi surfaces (FSs) of BaFe(2)(As(1-x)P(x))(2) (x=0.38), where superconductivity is induced by isovalent P substitution and by angle-resolved photoemission spectroscopy. Moderately strong electron mass enhancement has been identified for both the electron and hole FSs. Among two observed hole FSs, the nearly two-dimensional one shows good nesting with the outer two-dimensional electron FS, but its orbital character is different from the outer electron FS. The three-dimensional hole FS shows poor nesting with the electron FSs. The present results suggest that the three dimensionality and the difference in the orbital character weaken FS nesting while partial nesting among the outer electron FSs of d(xy) character and/or that within the three-dimensional hole FS becomes dominant, which may lead to the nodal superconductivity.

Magneto-optical spectroscopy on Weyl nodes for anomalous and topological Hall effects in chiral MnGe.

Physics of Weyl electrons has been attracting considerable interests and further accelerated by recent discoveries of giant anomalous Hall effect (AHE) and topological Hall effect (THE) in several magnetic systems including non-coplanar magnets with spin chirality or small-size skyrmions. These AHEs/THEs are often attributed to the intense Berry curvature generated around the Weyl nodes accompanied by band anti-crossings, yet the direct experimental evidence still remains elusive. Here, we demonstrate an essential role of the band anti-crossing for the giant AHE and THE in MnGe thin film by using the terahertz magneto-optical spectroscopy. The low-energy resonance structures around ~ 1.2 meV in the optical Hall conductivity show the enhanced AHE and THE, indicating the emergence of at least two distinct anti-crossings near the Fermi level. The theoretical analysis demonstrates that the competition of these resonances with opposite signs is a cause of the strong temperature and magnetic-field dependences of observed DC Hall conductivity. These results lead to the comprehensive understanding of the interplay among the transport phenomena, optical responses and electronic/spin structures.