Angle-resolved Resonant Photoemission Research Articles

Theory of circular dichroism in angle-resolved resonant photoemission from magnetic surfaces

A theoretical method is presented for angle-resolved photoemission at the transition metal $L$-edge resonance. It combines atomic multiplet calculations for the second-order resonant photoemission amplitude on the core-level site and a single scattering calculation of the photoelectron final state. The theory is applied to a magnetized Ni(111) surface excited with circularly polarized x rays at the Ni ${L}_{2,3}$-edge resonance with a focus on the circular dichroism (CD) signal. Good agreement with available experimental data is achieved. It is shown that the CD pattern is composed of a slowly varying magnetic signal induced by the atomic resonant process and a signal of fast angular modulations that are due to the interference of primary and scattered waves, known as the Daimon effect. The two types of CD signals are found to be nearly additive. At the Ni ${L}_{2}$-edge resonance, the angular dependence of the magnetic CD is well described by a simple expression known from x-ray magnetic CD. At the ${L}_{3}$ edge, however, the angular dependence is more complex and shows a pronounced final state multiplet dependence. With the present theory, it becomes possible to extract element- and site-selective magnetic information of surfaces from the CD in angle-resolved resonant photoemission data.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0401000, 2015CB921300, 2016YFA0300303, 2016YFA0401002 and 2017YFA0303103, the National Natural Science Foundation of China under Grant Nos 11674371, 11774401 and 11874330, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB07000000, and

We utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature , which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization. However, the Fermi vector and the Fermi surface volume have little change around Tcoh, which challenges the widely believed evolution from a high-temperature small Fermi surface to a low-temperature large Fermi surface. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Tracing crystal-field splittings in the rare-earth-based intermetallic CeIrIn5

Crystal electric field states in rare earth intermetallics show an intricate entanglement with the many-body physics that occurs in these systems and that is known to lead to a plethora of electronic phases. Here, we attempt to trace different contributions to the crystal electric field (CEF) splittings in CeIrIn$_5$, a heavy-fermion compound and member of the Ce$M$In$_5$ ($M$= Co, Rh, Ir) family. To this end, we utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) and present a spectroscopic study of the electronic structure of this unconventional superconductor over a wide temperature range. As a result, we show how ARPES can be used in combination with thermodynamic measurements or neutron scattering to disentangle different contributions to the CEF splitting in rare earth intermetallics. We also find that the hybridization is stronger in CeIrIn$_5$ than CeCoIn$_5$ and the effects of the hybridization on the Fermi volume increase is much smaller than predicted. By providing the first experimental evidence for $4f_{7/2}^{1}$ splittings which, in CeIrIn$_5$, split the octet into four doublets, we clearly demonstrate the many-body origin of the so-called $4f_{7/2}^{1}$ state.

Band Dependent Interlayer f-Electron Hybridization in CeRhIn_{5}.

A key issue in heavy fermion research is how subtle changes in the hybridization between the 4f (5f) and conduction electrons can result in fundamentally different ground states. CeRhIn_{5} stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn_{5}, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the 4f crystal field states and details on the 4f and conduction electron hybridization, which together determine the ground state. We directly observe weakly dispersive Kondo resonances of f electrons and identify two of the three Ce 4f_{5/2}^{1} crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce 4f states in the CeIn_{3} layer and two more three-dimensional bands composed of the Rh 4d and In 5p orbitals in the RhIn_{2} layer. Our results allow us to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound.

Covalent versus localized nature of 4f electrons in ceria: Resonant angle-resolved photoemission spectroscopy and density functional theory

We have conducted resonant angle-resolved photoemission spectroscopy of well-defined ${\mathrm{CeO}}_{2}$(111) and $c--{\mathrm{Ce}}_{2}{\mathrm{O}}_{3}$(111) model surfaces, revealing distinct $f$ contributions in the valence band of the two compounds. In conjunction with density functional theory calculations, we show that the $f$ contribution in ${\mathrm{CeO}}_{2}$ is of a covalent nature, arising from hybridization with the $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ bands. In contrast, $c--{\mathrm{Ce}}_{2}{\mathrm{O}}_{3}$ exhibits an almost nondispersive $f$ state at 1.3 eV, which is indicative of almost negligible c-f hybridization.

Valence instability in the bulk and at the surface of the antiferromagnet SmRh2Si2

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh2Si2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh2Si2 (RE = rare-earth elements) compounds are discussed.

Circular dichroism in resonant angle-resolved photoemission spectra of LaNi2Ge2

We report the soft X-ray angle-resolved photoemission study for LaNi2Ge2 to reveal the electronic structures derived from non-4f bands of the heavy fermion compound CeNi2Ge2. The photoemission spectra recorded at the La M4,5 absorption edges clearly show the enhancement of the La 5d components in the valence band spectra. The circular dichroism of photoemission spectra reveals the band-dependent dichroic response due to the orbital symmetry.

Fingerprints of entangled spin and orbital physics in itinerant ferromagnets via angle-resolved resonant photoemission

A method for mapping the local spin and orbital nature of the ground state of a system via corresponding flip excitations is proposed based on angle-resolved resonant photoemission and related diffraction patterns, obtained here via an ab initio modified one-step theory of photoemission. The analysis is done on the paradigmatic weak itinerant ferromagnet bcc Fe, whose magnetism, a correlation phenomenon given by the coexistence of localized moments and itinerant electrons, and the observed non-Fermi-Liquid behavior at extreme conditions both remain unclear. The combined analysis of energy spectra and diffraction patterns offers a mapping of local pure spin-flip, entangled spin-flip--orbital-flip excitations and chiral transitions with vortexlike wave fronts of photoelectrons, depending on the valence orbital symmetry and the direction of the local magnetic moment. Such effects, mediated by the hole polarization, make resonant photoemission a promising tool to perform a full tomography of the local magnetic properties even in itinerant ferromagnets or macroscopically nonmagnetic systems.

Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO3 embedded in GdTiO3

For certain conditions of layer thickness, the interface between GdTiO3 (GTO) and SrTiO3 (STO) in multilayer samples has been found to form a two-dimensional electron gas (2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7, with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening.

Fermi surface variation of Ce 4f-electrons in hybridization controlled heavy-fermion systems

Ce 3d–4f resonant angle-resolved photoemission measurements on CeCoGe1.2Si0.8 and CeCoSi2 have been performed to understand the Fermi surface topology as a function of hybridization strength between Ce 4f- and conduction electrons in heavy-fermion systems. We directly observe that the hole-like Ce 4f-Fermi surfaces of CeCoSi2 is smaller than that of CeCoGe1.2Si0.8, indicating the evolution of the Ce 4f-Fermi surface with the increase of the hybridization strength. In comparison with LDA calculation, the Fermi surface variation cannot be understood even though the overall electronic structure is roughly explained, indicating the importance of strong correlation effects.

Doping-dependent band structure of LaAlO3/SrTiO3interfaces by soft x-ray polarization-controlled resonant angle-resolved photoemission

Polarization-controlled synchrotron radiation was used to map the electronic structure of buried conducting interfaces of LaAlO$_3$/SrTiO$_3$ in a resonant angle-resolved photoemission experiment. A strong dependence on the light polarization of the Fermi surface and band dispersions is demonstrated, highlighting the distinct Ti 3d orbitals involved in 2D conduction. Samples with different 2D doping levels were prepared and measured by photoemission, revealing different band occupancies and Fermi surface shapes. A direct comparison between the photoemission measurements and advanced first-principle calculations carried out for different 3d-band fillings is presented in conjunction with the 2D carrier concentration obtained from transport measurements.

Real-space Green's function approach to angle-resolved resonant photoemission: Spin polarization and circular dichroism in itinerant magnets

A first principles approach, based on the real space multiple scattering Green's function method, is presented for spin- and angle-resolved resonant photoemission from magnetic surfaces. It is applied to the Fe(010) valence band photoemission excited with circularly polarized X-rays around the Fe L3 absorption edge. When the photon energy is swept through the Fe 2p-3d resonance, the valence band spectra are strongly modified in terms of absolute and relative peak intensities, degree of spin-polarization and light polarization dependence. New peaks in the spin-polarized spectra are identified as spin-flip transitions induced by exchange decay of spin-mixed core-holes. By comparison with single atom and band structure data, it is shown that both intra-atomic and multiple scattering effects strongly influence the spectra. We show how the different features linked to states of different orbital symmetry in the d band are differently enhanced by the resonant effect. The appearance and origin of circular dichroism and spin polarization are analyzed for different geometries of light incidence and electron emission direction, providing guidelines for future experiments.

First-principles calculations of angle-resolved and spin-resolved photoemission spectra of Cr(110) surfaces at the 2p-3d Cr resonance.

A first principles approach for spin- and angle-resolved resonant photoemission is developed within multiple scattering theory and applied to a Cr(110) surface at the 2p-3d resonance. The resonant photocurrent from this nonferromagnetic system is found to be strongly spin polarized by circularly polarized light, in agreement with experiments on antiferromagnetic and magnetically disordered systems. By comparing the antiferromagnetic and Pauli-paramagnetic phases of Cr, we explicitly show that the spin polarization of the photocurrent is independent of the existence of local magnetic moments, solving a long-standing debate on the origin of such polarization. New spin polarization effects are predicted for the paramagnetic phase even with unpolarized light, opening new directions for full mapping of spin interactions in macroscopically nonmagnetic or nanostructured systems.

Angle-resolved photoemission and quasiparticle calculation of ZnO: The need fordband shift in oxide semiconductors

ZnO is a prototypical semiconductor with occupied ${d}^{10}$ bands that interact with the anion $p$ states and is thus challenging for electronic structure theories. Within the context of these theories, incomplete cancellation of the self-interaction energy results in a Zn $d$ band that is too high in energy, resulting in upwards repulsion of the valence band maximum (VBM) states, and an unphysical reduction of the band gap. Methods such as GW should significantly reduce the self-interaction error, and in order to evaluate such calculations, we measured high-resolution and resonant angle-resolved photoemission spectroscopy (ARPES) and compared these to several electronic structure calculations. We find that, in a standard GW calculation, the $d$ bands remain too high in energy by more than 1 eV irrespective of the Hamiltonian used for generating the input wave functions, causing a slight underestimation of the band gap due to the $p\ensuremath{-}d$ repulsion. We show that a good agreement with the ARPES data over the full valence band spectrum is obtained, when the Zn-$d$ band energy is shifted down by applying an on-site potential ${V}_{d}$ for Zn-$d$ states during the GW calculations to match the measured $d$ band position. The magnitude of the GW quasiparticle energy shift relative to the initial density functional calculation is of importance for the prediction of charged defect formation energies, band-offsets, and ionization potentials.

First-Principles Local-Density Approximation Study of Electronic Structure in CeCoSi2

The first-principles band calculation of an itinerant cerium compound CeCoSi 2 is performed using a relativistic LAPW method with exchange and correlation potentials within a local-density approximation (LDA). The c– f hybridized bands with a large dispersion form two-dimensional Fermi surface (FS). The FS formation and the band dispersion including the contribution from each orbital obtained by the LDA calculation are in a qualitatively agreement with Ce 3 d –4 f resonant angle-resolved photoemission spectra. The LDA calculation is a good starting point for the investigation of the electronic structure of the heavy-fermion Ce compounds.

Direct Observation of Dispersive Kondo Resonance Peaks in a Heavy-Fermion System

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe1.2Si0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.

Hybridization effects inCeCoIn5observed by angle-resolved photoemission

We have investigated the low-energy electronic structure of the heavy fermion superconductor $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$ by angle-resolved photoemission. We focus on the dispersion and the peak width of the prominent quasi-two-dimensional Fermi surface sheet at the corner of the Brillouin zone as a function of temperature along certain $k$ directions with a photon energy of $h\ensuremath{\nu}=100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We find slight changes of the Fermi vector and an anomalous broadening of the peak width when the Fermi energy is approached. Additionally, we performed resonant angle-resolved photoemission spectroscopy experiments with $h\ensuremath{\nu}=121\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. A flat $f$-derived band is observed with a distinct temperature dependence and a $k$-dependent spectral weight. These results, including both off- and on-resonant measurements, fit qualitatively to a two-level mixing model derived from the periodic Anderson model.

K- and spin-dependent hybridization effects in Ce monolayer

Here we present applications of the periodic Anderson model (PAM) to consideration of wave vector (k)- and spin-dependent hybridization effects in Ce metal. It was shown that k-dependent splitting of the 4f ionization peak of Ce/W(1 10) are correctly described in the framework of the PAM (Coulomb repulsion between two f electrons localized on the same lattice site Uff→∞). Our results show that the wave vector is conserved upon hybridization. In case of the magnetically ordered Ce monolayer, spin- and angle-resolved resonant photoemission spectra reveal spin-dependent changes of the Fermi-level peak intensities (which reflect the hybridization strength). That indicate a spin-dependence of 4f hybridization and, thus, of 4f occupancy and local moment. The phenomenon was also described in the framework of PAM by 4f electron hopping into the exchange split Fe 3d derived bands that form a spin-gap at the Fermi energy around the T point of the surface Brillouin zone.

Spin-dependent hybridization and magnetic order of Ce/Fe(1 1 0) studied by spin-resolved resonant photoemission

Electronic and magnetic structures of the Ce/Fe(1 1 0) system were studied by means of spin- and angle-resolved resonant photoemission. Antiferromagnetic coupling of Fe 3d and Ce 4f spin magnetic moments was concluded from spin-resolved photoemission measurements supported by the electronic structure calculations. The relative intensity of the minority-spin component of hybridization feature at the Fermi level is larger than the majority-spin one, that can be assigned to spin-dependent hybridization between 4f and valence-band electrons induced by Fe substrate.

Wave-Vector Conservation upon Hybridization of4fand Valence-Band States Observed in Photoemission Spectra of a Ce Monolayer on W(110)

Angle-resolved resonant photoemission data for a hexagonally ordered monolayer of Ce on W(110) are presented. The spectra reveal a splitting of the 4f(0) ionization peak around a point in k space where a degeneracy with a valence-band state is expected. The phenomenon is described within a simple approach to the periodic Anderson model. It is found that the Ce 4f state forms a band and hybridization predominantly occurs between the 4f and the valence-band states at the same wave vector.