Soft X-ray Angle-resolved Photoemission Spectroscopy Research Articles

ARPES studies of the inverse perovskite Ca3PbO : Experimental confirmation of a candidate 3D Dirac fermion system

We investigate the band structure of the inverse perovskite ${\mathrm{Ca}}_{3}\mathrm{PbO}$, a candidate three-dimensional (3D) Dirac fermion material, through soft x-ray angle-resolved photoemission spectroscopy. Conelike band dispersions are observed for ${\mathrm{Ca}}_{3}\mathrm{PbO}$, in close agreement with the predictions of electronic structure calculations. We further demonstrate that chemical substitution of Bi for Pb is effective in tuning the Fermi level of ${\mathrm{Ca}}_{3}\mathrm{PbO}$ while leaving its electronic structure intact. Our study confirms that the inverse perovskite family provides a promising platform for the exploration of 3D Dirac fermion systems.

Electronic band structure of the buried SiO2/SiC interface investigated by soft x-ray ARPES

The electronic structure of the SiO2/SiC (0001) interface, buried below SiO2 layers with a thickness from 2 to 4 nm, was explored using soft X-ray angle-resolved photoemission spectroscopy with photon energies between 350 and 1000 eV. The measurements have detected the characteristic k-dispersive energy bands of bulk Silicon Carbide (SiC) below the SiO2 layer without any sign of additional dispersive states, up to an estimated instrumental sensitivity of ≈5 × 109 cm2 eV. This experimental result supports the physical picture that the large density of interface traps observed in macroscopic measurements results from dangling bonds randomized by the SiO2 rather than from Shockley-Tamm surface derived states extending into the bulk SiC.

Origin of robust nanoscale ferromagnetism in Fe-doped Ge revealed by angle-resolved photoemission spectroscopy and first-principles calculation

Ge$_{1-x}$Fe$_{x}$ (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K, and hence is a promising material for spintronic applications compatible with Si technology. We have studied its electronic structure by soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurements in order to elucidate the mechanism of the ferromagnetism. We observed finite Fe 3$d$ components in the states at the Fermi level ($E_{F}$) in a wide region in momentum space and $E_{F}$ was located above the valence-band maximum (VBM). First-principles supercell calculation also suggested that the $E_{F}$ is located above the VBM, within the narrow spin-down $d$($e$) band and within the spin-up impurity band of the deep acceptor-level origin derived from the strong $p$-$d$($t_{2}$) hybridization. We conclude that the narrow $d$($e$) band is responsible for the ferromagnetic coupling between Fe atoms while the acceptor-level-originated band is responsible for the transport properties of Ge:Fe.

Soft X-ray synchrotron radiation spectroscopy study of rare-earth chalcogenide charge-density wave compounds

The electronic structures of the layered rare-earth chalcogenide compounds of CeTe2, PrTe2, and PrTe3, which have the charge-density wave (CDW) transition and possibly the chiral transition, have been investigated by employing soft X-ray absorption spectroscopy (XAS) and angle-resolved photoemission spectroscopy (ARPES). R 3d XAS measurements show that the valence states of Ce and Pr ions are nearly trivalent in all the compounds. Similar band dispersions are observed in their measured ARPES data, but with the band positions in PrTe3 being shifted up in energy compared to those in CeTe2 and PrTe2. These findings suggest that their Te 5p band structures are determined mainly by the 2D interactions in the Te(2)/Te(3) sheets, but with a larger number of holes in the Te 5p bands in PrTe3 than in CeTe2 and PrTe2. The measured constant energy maps of CeTe2, PrTe2, and PrTe3 for high binding energies are similar to one another, reflecting the Te 5p band structures of the Te(2)/Te(3) square nets. In contrast, the Fermi surfaces (FSs) of CeTe2 and PrTe3 exhibit extra features, different from the FS of the ideal Te(2)/Te(3) square nets, which arise from the CDW-induced FS reconstruction in the Te(2)/Te(3) sheets.

Entanglement and manipulation of the magnetic and spin-orbit order in multiferroic Rashba semiconductors.

Entanglement of the spin–orbit and magnetic order in multiferroic materials bears a strong potential for engineering novel electronic and spintronic devices. Here, we explore the electron and spin structure of ferroelectric α-GeTe thin films doped with ferromagnetic Mn impurities to achieve its multiferroic functionality. We use bulk-sensitive soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to follow hybridization of the GeTe valence band with the Mn dopants. We observe a gradual opening of the Zeeman gap in the bulk Rashba bands around the Dirac point with increase of the Mn concentration, indicative of the ferromagnetic order, at persistent Rashba splitting. Furthermore, subtle details regarding the spin–orbit and magnetic order entanglement are deduced from spin-resolved ARPES measurements. We identify antiparallel orientation of the ferroelectric and ferromagnetic polarization, and altering of the Rashba-type spin helicity by magnetic switching. Our experimental results are supported by first-principles calculations of the electron and spin structure.

Fermi states and anisotropy of Brillouin zone scattering in the decagonal Al-Ni-Co quasicrystal.

Quasicrystals (QCs) are intermetallic alloys that have excellent long-range order but lack translational symmetry in at least one dimension. The valence band electronic structure near the Fermi energy EF in such materials is of special interest since it has a direct relation to their unusual physical properties. However, the Fermi surface (FS) topology as well as the mechanism of QC structure stabilization are still under debate. Here we report the first observation of the three-dimensional FS and valence band dispersions near EF in decagonal Al70Ni20Co10 (d-AlNiCo) QCs using soft X-ray angle-resolved photoemission spectroscopy. We show that the FS, formed by dispersive Al sp-states, has a multicomponent character due to a large contribution from high-order bands. Moreover, we discover that the magnitude of the gap at the FS related to the interaction with Brillouin zone boundary (Hume–Rothery gap) critically differs for the periodic and quasiperiodic directions.

Observation of Weyl nodes in TaAs

In 1929, H. Weyl proposed that the massless solution of Dirac equation represents a pair of new type particles, the so-called Weyl fermions [1]. However the existence of them in particle physics remains elusive for more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of Fermi energy (called as Weyl nodes), the low energy excitation behaves exactly the same as Weyl fermions. Here, by performing soft x-ray angle-resolved photoemission spectroscopy measurements which mainly probe bulk band structure, we directly observe the long-sought-after Weyl nodes for the first time in TaAs, whose projected locations on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence of existence of Weyl fermion quasiparticles in TaAs.

Tuning electronic correlations in transition metal pnictides: Chemistry beyond the valence count

The effects of electron-electron correlations on the low-energy electronic structure and their relationship with unconventional superconductivity are central aspects in the research on the iron-based pnictide superconductors. Here we use soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study how electronic correlations evolve in different chemically substituted iron pnictides. We find that correlations are intrinsically related to the effective filling of the correlated orbitals, rather than to the filling obtained by valence counting. Combined density functional theory (DFT) and dynamical mean-field theory (DMFT) calculations capture these effects, reproducing the experimentally observed trend in the correlation strength. The occupation-driven trend in the electronic correlation reported in our work supports the recently proposed connection between cuprate and pnictides phase diagrams.

Soft x-ray angle-resolved and resonance photoemission study of CeCu2Ge2 and LaCu2Ge2

We have performed bulk-sensitive soft x-ray angle-resolved photoemission spectroscopy for CeCu2Ge2 isostructural to a heavy fermion superconductor CeCu2Si2, indicating the localized 4f character in ambient pressure. Resonance enhancement is seen at the La 3d5/2-edge for LaCu2Ge2 in both angle-resolved and integrated photoemission although the magnitude of enhancement is less than that in the Ce 3d5/2-edge resonance photoemission for CeCu2Ge2, which indicates that the rare-earth 5d contributions can also be enhanced in addition to the 4f contributions at the resonance conditions.

Unveiling the impurity band induced ferromagnetism in the magnetic semiconductor (Ga,Mn)As

(Ga,Mn)As is a paradigm of a diluted magnetic semiconductor which shows ferromagnetism induced by doped hole carriers. With a few controversial models emerging from numerous experimental and theoretical studies, the mechanism of the ferromagnetism in (Ga,Mn)As still remains a puzzling enigma. In this article, we use soft x-ray angle-resolved photoemission spectroscopy to positively identify the ferromagnetic Mn $3d$-derived impurity band (IB) in (Ga,Mn)As. The band appears dispersionless and hybridized with the light-hole band of the host GaAs. These findings conclude the picture of the valence-band structure of (Ga,Mn)As disputed for more than a decade. The nondispersive character of the IB and its location in vicinity of the valence-band maximum indicate that the Mn $3d$-derived IB is formed as a split-off Mn-impurity state predicted by the Anderson impurity model. Responsible for the ferromagnetism is predominantly the transport of hole carriers in the IB.

Direct Observation of the Bandwidth Control Mott Transition in theNiS2−xSexMultiband System

The bulk electronic structure of NiS$_{2-x}$Se$_x$ is studied across the bandwidth-control Mott transition (BCMT) by soft X-ray angle-resolved photoemission spectroscopy. The microscopic picture of the BCMT in this multiband non-half-filled system is revealed for the first time. We show that Se doping does not alter the Fermi surface volume. When approaching the insulating phase with decreasing Se concentration, we observed that the Fermi velocity continuously decreases. Meanwhile, the coherent quasiparticle weight continuously decreases and is transferred to higher binding energies, until it suddenly disappears across the Mott transition. In the insulating phase, there is still finite spectral weight at the Fermi energy, but it is incoherent and dispersionless due to strong correlations. Our results provide a direct observation of BCMT, and unveil its distinct characters in a multiband non-half-filled system.

Observation of bulk band dispersions of YbRh2Si2using soft x-ray angle-resolved photoemission spectroscopy

We have performed soft x-ray angle-resolved photoemission spectroscopy (ARPES) measurements on YbRh${}_{2}$Si${}_{2}$ and clarified its three-dimensional bulk valence-band structures. The ARPES spectra have not only Yb${}^{3+}$ multiplet peaks but also a finite contribution of Yb${}^{2+}$ peaks at 15 K, corresponding to the valence fluctuating behavior in this compound. This means that Yb 4$f$ electrons in this compound have itinerant character below the ${T}_{K}$. We have found that dispersions of the valence bands except the vicinity of the Yb 4$f$ bands agree better with those of the band-structure calculation of LuRh${}_{2}$Si${}_{2}$ than those of YbRh${}_{2}$Si${}_{2}$ within a local-density approximation. In addition, the Yb 3$d$-4$f$ resonant photoemission spectra of YbRh${}_{2}$Si${}_{2}$ strongly suggest the existence of Yb 5$d$ electrons in the valence band. We conclude that the charge transfer from the Yb 4$f$ state to the Yb 5$d$ state has an important role in the formation of the valence band of YbRh${}_{2}$Si${}_{2}$.

Three-dimensional bulk band dispersion in polar BiTeI with giant Rashba-type spin splitting

In layered polar semiconductor BiTeI, giant Rashba-type spin-split band dispersions show up due to the crystal structure asymmetry and the strong spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band structures of BiTeI using the bulk-sensitive $h\nu$-dependent soft x-ray angle resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is shown to be well reproducible by the first-principles calculations, with huge spin splittings of ${\sim}300$ meV at the conduction-band-minimum and valence-band-maximum located in the $k_z=\pi/c$ plane. It provides the first direct experimental evidence of the 3D Rashba-type spin splitting in a bulk compound.

Bulk Electronic Structure of SuperconductingLaRu2P2Single Crystals Measured by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy

We present a soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichiometric pnictide superconductor LaRu(2)P(2). The observed electronic structure is in good agreement with density functional theory (DFT) calculations. However, it is significantly different from its counterpart in high-temperature superconducting Fe pnictides. In particular, the bandwidth renormalization present in the Fe pnictides (~2-3) is negligible in LaRu(2)P(2) even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu(2) P(2) has a different origin with respect to the iron pnictides. Finally, we demonstrate that the increased probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is essential in determining the bulk electronic structure in the experiment.

Pseudogap in the chain states of YBa2Cu3O6.6

As established by scanning tunneling microscopy (STM), cleaved surfaces of the high-temperature superconductor YBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$ develop charge-density wave (CDW) modulations in the one-dimensional (1D) CuO chains. At the same time, no signatures of the CDW have been reported in the spectral function of the chain band previously studied by photoemission. We use soft x-ray angle-resolved photoemission spectroscopy to detect a chain-derived surface band that had not been detected in previous work. The $2{k}_{\mathrm{F}}$ for the new surface band is found to be 0.55 \AA{}${}^{\ensuremath{-}1}$, which matches the wave vector of the CDW observed in direct space by STM. This reveals the relevance of the Fermi-surface nesting for the formation of CDWs in the CuO chains in YBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$. In agreement with the short-range nature of the CDW order the newly detected surface band exhibits a pseudogap whose energy scale also corresponds to that observed by STM.

Itinerant U 5f Nature in Antiferromagnet U(Ru0.97Rh0.03)2Si2: Soft X-ray Angle-Resolved Photoemission Spectroscopy

We have carried out soft X-ray angle-resolved photoemission spectroscopy experiments on U(Ru 0.97 Rh 0.03 ) 2 Si 2 , which shows antiferromagnetic (AFM) ordering at low temperatures. We have revealed that U 5 f electrons contribute to the formation of the energy band as well as Fermi surface. This suggests that the AFM ordering is caused by a spin density wave of itinerant U 5 f electrons. The observed band dispersion and Fermi surface are nearly identical to those of URu 2 Si 2 . Therefore, the AFM ordering vector Q =(1,0,0) can be a possible ordering vector of the hidden order phase in URu 2 Si 2 .

Electronic structure of YbCu2Ge2studied by soft x-ray angle-resolved photoemission spectroscopy

We have measured three-dimensional band structure and Fermi surfaces (FSs) of YbCu${}_{2}$Ge${}_{2}$, which has recently become recognized as a nearly divalent Yb-based compound, by using soft x-ray angle-resolved photoemission spectroscopy. We clarified that the bands with finite Yb $4f$ contribution cross the Fermi level and are slightly involved in the formation of the FSs, in line with the valence-fluctuating state of YbCu${}_{2}$Ge${}_{2}$. The obtained valence-band dispersions and the FSs are well explained by a relativistic band-structure calculation based on a local-density-approximation (LDA). These results suggest that the LDA calculation is a good starting point for understanding of the electronic state of YbCu${}_{2}$Ge${}_{2}$ as well as nearly divalent Yb-based compounds.

Absence of nesting in the charge-density-wave system1T-VS2as seen by photoelectron spectroscopy

We report on the electronic structure and Fermi surfaces of the transition-metal dichalcogenide $1{\text{T-VS}}_{2}$ in the low-temperature charge-density-wave (CDW) ordered phase. Using soft x-ray angle-resolved photoemission spectroscopy (ARPES), we investigate the in-plane and out-of-plane vanadium- and sulfur-derived band dispersions and identify ${k}_{z}$ dispersions in this layered system. Core-level photoemission and x-ray absorption spectroscopy show that vanadium electrons are in the ${d}^{1}$ configuration while $2p\text{\ensuremath{-}}3d$ resonant ARPES shows only $3d$-derived dispersive bands near the Fermi level. Comparison of energy- and angle-dependent data with band-structure calculations reveals renormalization of the $3d$ bands, but no lower Hubbard band, a signature of the rather weak electron-electron correlations in ${\text{VS}}_{2}$. High-resolution temperature-dependent low-energy ARPES measurements show the opening of an energy gap at the Fermi level that is attributed to the condensation of the CDW phase. The results indicate a CDW transition in the absence of nesting for $1{\text{T-VS}}_{2}$

Fermi surface of Co(0001) and initial-state linewidths determined by soft x-ray angle-resolved photoemission spectroscopy

Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the band dispersion and Fermi surface of bulk ferromagnetic Co(0001). The spectral peak widths are narrower than previously observed in low-energy ARPES experiments. From very general considerations and in contrast to low-energy experiments, we show that the final-state broadening is significantly reduced in the experimentally measured linewidth. SX-ARPES yields the intrinsic initial-state lifetime ${\ensuremath{\Gamma}}_{i}=1/2{\ensuremath{\Sigma}}_{i}(\stackrel{P\vec}{k},E)$, the latter being the initial state self-energy, quantifying the correlations and indicating an energy and momentum dependence. Our results open a way to a direct measurement of the many-particle effects in solid-state physics, which does not require a modeling of the final state and provides a genuine description of the initial state.

Experimental observation of bulk band dispersions in the oxide semiconductor ZnO using soft x-ray angle-resolved photoemission spectroscopy

The electronic structure of the oxide semiconductor ZnO has been investigated using soft x-ray angle-resolved photoemission spectroscopy (ARPES). The obtained band dispersions within the kx−ky planes reflect the symmetry of the Brillouin zone and show no surface-state-derived flat bands. Band dispersions along the kz direction have also been observed. The obtained band dispersions qualitatively agree with band-structure calculations except for the bandwidth. The observations provide experimental evidence that soft x-ray ARPES enables us to study the bulk band structure of semiconductors.