arXiv subjects

Masashi Arita

Publications and source records attributed to Masashi Arita.

32 records · Page 2Linked to original sources

arXiv2017

Direct evidence of hidden local spin polarization in a centrosymmetric superconductor LaO$_{0.55}$F$_{0.45}$BiS$_2$

Conventional Rashba spin polarization is caused by the combination of strong spin-orbit interaction (SOI) and spatial inversion asymmetry. However, Rashba- and Dresselhaus-type spin-split states are predicted in LaOBiS$_2$ system by recent theory even though the crystal structure is centrosymmetric, which stem from the local inversion asymmetry of active BiS$_2$ layer. By performing high-resolution spin- and angle-resolved photoemission spectroscopy, we have investigated the electronic band structure and spin texture of superconductor LaO$_{0.55}$F$_{0.45}$BiS$_2$. Our studies present direct spectroscopic evidence for the local spin polarization in the vicinity of X point of both valence band and conduction band. Especially the coexistence of Rashba-like and Dresselhaus-like spin textures has been observed in the conduction band for the first time. The finding is of key importance for fabrication of proposed dual-gated spin-field effect transistor (SFET). Moreover, the spin-split band leads to a spin-momentum locking Fermi surface from which novel superconductivity emerges. Our demonstration not only expands the scope of spintronic materials but also enhances the understanding of SOI related superconductivity.

cond-mat.mtrl-sci↗

arXiv2017

Experimental evidence of type-II Dirac fermions in PtSe2

Topological semimetals have attracted extensive research interests for realizing condensed matter physics counterparts of three-dimensional Dirac and Weyl fermions, which were originally introduced in high energy physics. Recently it has been proposed that type-II Dirac semimetal can host a new type of Dirac fermions which break Lorentz invariance and therefore does not have counterpart in high energy physics. Here we report the electronic structure of high quality PtSe$_2$ crystals to provide direct evidence for the existence of three-dimensional type-II Dirac fermions. A comparison of the crystal, vibrational and electronic structure to a sister compound PtTe$_2$ is also discussed. Our work provides an important platform for exploring the novel quantum phenomena in the PtSe$_2$ class of type-II Dirac semimetals.

cond-mat.mtrl-sci↗

arXiv2016

Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe$_2$

Topological semimetals have recently attracted extensive research interests as host materials to condensed matter physics counterparts of Dirac and Weyl fermions originally proposed in high energy physics. These fermions with linear dispersions near the Dirac or Weyl points obey Lorentz invariance, and the chiral anomaly leads to novel quantum phenomena such as negative magnetoresistance. The Lorentz invariance is, however, not necessarily respected in condensed matter physics, and thus Lorentz-violating type-II Dirac fermions with strongly tilted cones can be realized in topological semimetals. Here, we report the first experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe$_2$ single crystal. Angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the $Γ$-A direction under the symmetry protection, confirming PtTe$_2$ as a type-II Dirac semimetal. The realization of type-II Dirac fermions opens a new door for exotic physical properties distinguished from type-I Dirac fermions in condensed matter materials.

cond-mat.mtrl-sci↗

arXiv2016

Observation of a Hidden Hole-Like Band Approaching the Fermi Level in K-Doped Iron Selenide Superconductor

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2014

Realization of bulk insulating property and carrier manipulation in reversible spin current regime of the ideal topological insulator TlBiSe2

The surfaces of three-dimensional topological insulators (TIs) characterized by a spin-helical Dirac fermion provide a fertile ground for realizing exotic phenomena as well as having potential for wide-ranging applications. To realize most of their special properties, the Dirac point (DP) is required to be located near the Fermi energy with a bulk insulating property while it is hardly achieved in most of the discovered TIs. It has been recently found that TlBiSe2 features an in-gap DP, where upper and lower parts of surface Dirac cone are both utilized. Nevertheless, investigations of the surface transport properties of this material are limited due to the lack of bulk insulating characteristics. Here, we present the first realization of bulk insulating property by tuning the composition of Tl1-xBi1+xSe2-d without introducing guest atoms that can bring the novel properties into the reality. This result promises to shed light on new exotic topological phenomena on the surface.

cond-mat.mes-hall↗

arXiv2012

Anomalous momentum dependence of the multiband electronic structure of FeSe_1-xTe_x superconductors induced by atomic disorder

When periodicity of crystal is disturbed by atomic disorder, its electronic state becomes inhomogeneous and band dispersion is obscured. In case of Fe-based superconductors, disorder of chalcogen/pnictogen height causes disorder of Fe 3d level splitting. Here, we report an angle-resolved photoemission spectroscopy study on FeSe_1-xTe_x with the chalcogen height disorder, showing that the disorder affects the Fe 3d band dispersions in an orbital-selective way instead of simple obscuring effect. The reverse of the Fe 3d level splitting due to the chalcogen height difference causes the splitting of the hole band with Fe 3d x^2-y^2 character around the Gamma point.

cond-mat.supr-con↗

arXiv2011

Observation of warping effects in the band and angular momentum structures of topological insulator Bi2Te3

We performed angle resolved photoemission (ARPES) experiments on Bi2Te3 with circularly polarized light. ARPES data show very strong circular dichroism, indicating existence of orbital angular momentum (OAM). Moreover, the alignment of OAM is found to have a strong binding energy dependence. Such energy dependence comes from a relatively strong band warping effect in Bi2Te3 compared to Bi2Se3. OAM close to Dirac point has an ideal chiral structure (sin ?) without out-of-plane component. Warping effect comes in as the binding energy decreases and circular dichroism along a constant energy contour can no longer be explained by a simple sin? function but requires a sin3? term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism gains an additional sin6? term. Such behavior is found to be compatible with the theoretically predicted OAM structure.

cond-mat.mtrl-sci↗

arXiv2011

Spin and Orbital Angular Momentum Structures of Cu(111) and Au(111) Surface States

We performed angle resolved photoemission (ARPES) studies on Cu(111) and Au(111) surface states with circularly polarized light. Existence of local orbital angular momentum (OAM) is confirmed as has been predicted to be broadly present in a system with an inversion symmetry breaking (ISB). The single band of Cu(111) surface states is found to have chiral OAM in spite of very small spin-orbit coupling (SOC) in Cu, which is consistent with theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first principles calculation and the results are found to be consistent with the experimental results. Moreover, majority of OAM is found to be from d-orbitals and a small contribution has p-orbital origin which is anti-aligned to the spins. We derive an effective Hamiltonian that incorporates the role of OAM and used it to extract the OAM and spin structures of surface states with various SOC strength. We discuss the evolution of angular momentum structures from pure OAM case to a strongly spin-orbit entangled state. We predict that the transition occurs through reversal of OAM direction at a k-point in the inner band if the system has a proper SOC strength.

cond-mat.mtrl-sci↗

arXiv2011

Emergence of Orbital Angular Momentum by Inversion Symmetry Breaking and Its Detection by ARPES

Rashba-split surface band is characterized by a one-to-one correspondence between the electron's momentum k and its spin orientation. Here we show that a similar correspondence between momentum and orbital angular momentum (OAM) must exist on surface bands once the inversion symmetry is broken. The correspondence is valid even when there is no spin-orbit interaction. Tight-binding and first-principles calculations are presented to support our claim. As a method to detect such OAM-momentum correspondence, we propose the circular dichroism (CD) experiment using the angle-resolved photoemission (ARPES) setup. CD-ARPES experiment performed on Cu surface confirms the existence of chiral OAM. A new concept of "orbital Galvanic effect" is proposed.

cond-mat.mtrl-sci↗

arXiv2011

Chiral orbital-angular-momentum in the surface states of Bi2Se3

Locking of the spin of a quasi-particle to its momentum in split bands of on the surfaces of metals and topological insulators (TIs) is understood in terms of Rashba effect where a free electron in the surface states feels an effective magnetic field. On the other hand, the orbital part of the angular momentum (OAM) is usually neglected. We performed angle resolved photoemission experiments with circularly polarized lights and first principles density functional calculation with spin-orbit coupling on a TI, Bi2Se3, to study the local OAM of the surface states. We show from the results that OAM in the surface states of Bi2Se3 is significant and locked to the electron momentum in opposite direction to the spin, forming chiral OAM states. Our finding opens a new possibility to have strong light-induced spin-polarized current in the surface states.

cond-mat.mtrl-sci↗

arXiv2011

Electronic structure of BaNi$_2$As$_2$

BaNi$_2$As$_2$, with a first order phase transition around 131 K, is studied by the angle-resolved photoemission spectroscopy. The measured electronic structure is compared to the local density approximation calculations, revealing similar large electronlike bands around $\bar{M}$ and differences along $\barΓ$-$\bar{X}$. We further show that the electronic structure of BaNi$_2$As$_2$ is distinct from that of the sibling iron pnictides. Particularly, there is no signature of band folding, indicating no collinear SDW related magnetic ordering. Moreover, across the strong first order phase transition, the band shift exhibits a hysteresis, which is directly related to the significant lattice distortion in BaNi$_2$As$_2$.

cond-mat.supr-con↗

arXiv2010

High-resolution angle-resolved photoemission spectroscopy study of the electronic structure of EuFe2As2

We report the high-resolution angle-resolved photoemission spectroscopy studies of electronic structure of EuFe2As2. The paramagnetic state data are found to be consistent with density-functional calculations. In the antiferromagnetic ordering state of Fe, our results show that the band splitting, folding, and hybridization evolve with temperature, which cannot be explained by a simple folding picture. Detailed measurements reveal that a tiny electron Fermi pocket and a tiny hole pocket are formed near (pi,pi) in the (0,0)-(pi,pi) direction, which qualitatively agree with the results of quantum oscillations, considering kz variation of Fermi surface. Furthermore, no noticeable change within the energy resolution is observed across the antiferromagnetic transition of Eu2+ ordering, suggesting weak coupling between Eu sublattice and FeAs sublattice.

cond-mat.str-el↗

arXiv2010

Electronic structure of Fe1.04(Te0.66Se0.34)

We report the electronic structure of the iron-chalcogenide superconductor, Fe1.04(Te0.66Se0.34), obtained with high resolution angle-resolved photoemission spectroscopy and density functional calculations. In photoemission measurements, various photon energies and polarizations are exploited to study the Fermi surface topology and symmetry properties of the bands. The measured band structure and their symmetry characters qualitatively agree with our density function theory calculations of Fe(Te0.66Se0.34), although the band structure is renormalized by about a factor of three. We find that the electronic structures of this iron-chalcogenides and the iron-pnictides have many aspects in common, however, significant differences exist near the Gamma-point. For Fe1.04(Te0.66Se0.34), there are clearly separated three bands with distinct even or odd symmetry that cross the Fermi energy (EF) near the zone center, which contribute to three hole-like Fermi surfaces. Especially, both experiments and calculations show a hole-like elliptical Fermi surface at the zone center. Moreover, no sign of spin density wave was observed in the electronic structure and susceptibility measurements of this compound.

cond-mat.str-el↗

arXiv2008

Sheet Dependence on Superconducting Gap in Oxygen-Deficient Iron-based Oxypnictide Superconductors NdFeAs0.85

Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85 (Tc=52K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level (EF). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Gamma (0,0) point but on the electron-like sheets around M(pi,pi) point.

cond-mat.supr-con↗