Fermi Surface Mapping Research Articles

Wafer-scale 30° twisted bilayer graphene epitaxially grown on Cu0.75Ni0.25 (111)

Twisted bilayer graphene (TBG) has been extensively studied because of its novel physical properties and potential application in electronic devices. Here we report the synthesis and characterization of 30° TBG naturally grown on Cu0.75Ni0.25 (111) film and investigate the electronic structure by angle-resolved photoemission spectroscopy. Compared with other substrates, our TBG with a wafer scale is acquired with a shorter growth time. The Fermi velocity and energy gap of Dirac cones of TBG are comparable with those of a monolayer on Cu0.85Ni0.15 (111). The signature of moiré lattices has not been observed in either the low-energy electron diffraction patterns or the Fermi surface map within experimental resolution, possibly due to different Cu and Ni contents in the substrates enhancing the different couplings between the substrate and the first/second layers and hindering the formation of a quasiperiodic structure.

In-plane Fermi surface mapping of ZrSiS and HfSiS by de Haas-van Alphen oscillations

We have conducted a comprehensive experimental investigation of the in-plane de Haas van Alphen quantum oscillations in ZrSiS and HfSiS and compared the results to the ab-initio calculations. We found a moderately good agreement for a set of frequencies located between 100 T and 200 T. These could be associated with extremal orbitals of the electron pockets located around the k_z=\pm\pi/2ckz=±π/2c planes at the edge of the 1st Brillouin zone. In contrast, the experimentally detected low-frequency set of oscillations found around 20 T could not be reproduced by the calculations. Further refinement of the ab-initio calculation has to be conducted to fully capture all the observed features of the Fermi surface in ZrSiS and HfSiS.

Geometry-Induced Spin Filtering in Photoemission Maps from WTe_{2} Surface States.

We demonstrate that an important quantum material WTe_{2} exhibits a new type of geometry-induced spin filtering effect in photoemission, stemming from low symmetry that is responsible for its exotic transport properties. Through the laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping, we showcase highly asymmetric spin textures of electrons photoemitted from the surface states of WTe_{2}. Such asymmetries are not present in the initial state spin textures, which are bound by the time-reversal and crystal lattice mirror plane symmetries. The findings are reproduced qualitatively by theoretical modeling within the one-step model photoemission formalism. The effect could be understood within the free-electron final state model as an interference due to emission from different atomic sites. The observed effect is a manifestation of time-reversal symmetry breaking of the initial state in the photoemission process, and as such it cannot be eliminated, but only its magnitude influenced, by special experimental geometries.

Multispectral time-resolved energy-momentum microscopy using high-harmonic extreme ultraviolet radiation.

A 790-nm-driven high-harmonic generation source with a repetition rate of 6kHz is combined with a toroidal-grating monochromator and a high-detection-efficiency photoelectron time-of-flight momentum microscope to enable time- and momentum-resolved photoemission spectroscopy over a spectral range of 23.6-45.5eV with sub-100fs time resolution. Three-dimensional (3D) Fermi surface mapping is demonstrated on graphene-covered Ir(111) with energy and momentum resolutions of ≲100 meV and ≲0.1 Å-1, respectively. The tabletop experiment sets the stage for measuring the kz-dependent ultrafast dynamics of 3D electronic structure, including band structure, Fermi surface, and carrier dynamics in 3D materials as well as 3D orbital dynamics in molecular layers.

Fermi surface tomography

Fermi surfaces are essential for predicting, characterizing and controlling the properties of crystalline metals and semiconductors. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (kF) from energy- and angular distribution of photoelectrons dislodged by monochromatic light. Existing apparatus is able to determine a number of kF -vectors simultaneously, but direct high-resolution 3D Fermi surface mapping remains problematic. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces. Here we show that using a simpler instrumentation it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface as well as other experimental results featuring advantages of our technique. In combination with various light sources our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences.

Janus Layers in the TiSe2–TiS2 System

Our experimental results indicate the formation of extended ordered S–Ti–Se layers (Janus layers) in the Ti(Se1–xSx)2 system. The presence of these layers is observed as a folding of the typical Fermi surface of TiS2 and TiSe2. High-resolution transmission electron microscopy (HRTEM) allows us to observe these layers by a typical rotation of the moiré fringe images. Analysis of the dependence of the electromotive force of the electrochemical cell on the S:Se ratio, angle-resolved photoelectron spectroscopy, resonant photoelectron spectroscopy, Fermi surface mapping, and HRTEM data allows us to conclude that the structure of the solid solution consists of S–Ti–Se and TiCh2 (Ch = S or Se) Janus layers.

Electronic structures of CO adsorbed Pt-skin surface on Pt–Co and Pt–Ni alloys

By using angle resolved photoemission spectroscopy, we investigate the electronic structures of Pt-skin layer of Pt–Co and Pt–Ni alloys with CO molecules on the surface. Measured Fermi surface maps and band dispersions reflect the signatures of chemical bonding between Pt-skin layer and CO molecules. Furthermore, the degree of chemical bonding strength of CO molecules, estimated from the energy shift of the participating bands, is found to be reduced on both Pt bimetallic alloys. Our results show how the surface band structure of Pt bimetallic alloys is modified with molecular orbitals of CO molecules on the surface, revealing the important role of the electronic structure in the determination of chemical properties of bimetallic alloys.

New insight on the role of localisation in the electronic structure of the Si(111)(7\u2009\xd7\u20097) surfaces

New angle-resolved photoelectron spectroscopy (ARPES) data, recorded at several different photon energies from the Si(111)(7 × 7) surface, show that the well-known S1 and S2 surface states that lie in the bulk band gap are localised at specific (adatom and rest atom) sites on the reconstructed surface. The variations in the photoemission intensity from these states as a function of polar and azimuthal emission angle, and incident photon energy, are not consistent with Fermi surface mapping but are well-described by calculations of the multiple elastic scattering in the final state. This localisation of the most shallowly bound S1 state is consistent with the lack of significant dispersion, with no evidence of Fermi surface crossing, implying that the surface is not, as has been previously proposed, metallic in character. Our findings highlight the importance of final state scattering in interpreting ARPES data, an aspect that is routinely ignored and can lead to misleading conclusions.

Fermi surface of the flat-band intermetallics APd3 (A=Pb,Sn)

The intermetallic phases $A\mathrm{P}{\mathrm{d}}_{3}$ $(A=\mathrm{Pb},\phantom{\rule{0.28em}{0ex}}\mathrm{Sn})$ were recently predicted to host an unconventional combination of unique electronic structure features, namely, flat bands near the Fermi energy coexisting with topologically protected surface states at the \ensuremath{\Gamma} point. These features each could independently produce alternative electronic states, including electronically or magnetically ordered states coexisting with unconventional edge dominated transport and a significantly large thermopower coexisting with topological characteristics. To investigate these expectations, we report the synthesis, structural/chemical characterization, electrical and thermal transport properties, magnetic torque (up to 45 T), and Fermi surface mapping for single crystals produced using the Czochralski technique. X-ray diffraction and scanning transmission electron microscope measurements establish the absence of defects, while small measured values of the thermopower indicate that the Fermi level is located away from the flat-band region. The electronic properties are further clarified by the topography of the Fermi surfaces, measured through the de Haas--van Alphen effect. We find that the Fermi levels are placed at higher energy values than the original ones resulting from the density functional theory calculations, 54 meV higher for ${\mathrm{PbPd}}_{3}$ and 68 meV higher for ${\mathrm{SnPd}}_{3}$. The molten flux method was also used to synthesize ${\mathrm{PbPd}}_{3}$, yielding nearly identical Fermi surfaces between the specimens grown using different synthesis techniques, indicating the robustness of the Fermi level position. According to the density functional theory calculations, the flat band is mainly formed by the $4d$ bands of Pd. Therefore, we propose monovalent doping on the Pb/Sn site as a viable approach to accessing the flat band while maintaining the unique band structure features of these compounds.

Fermi surface map of large-scale single-orientation graphene on SiO2

Large scale tetraoctylammonium-assisted electrochemical transfer of graphene grown on single-crystalline Ir(1 1 1) films by chemical vapour deposition is reported. The transferred samples are characterized in air with optical microscopy, Raman spectroscopy and four point transport measurements, providing the sheet resistance and the Hall carrier concentration. In vacuum we apply low energy electron diffraction and photoelectron spectroscopy that indicate transferred large-scale single orientation graphene. Angular resolved photoemission reveals a Fermi surface and a Dirac point energy which are consistent with charge neutral graphene.

Electronic Structure of EuAl4 Studied by Photoelectron Spectroscopy

The electronic structure of the divalent Eu compound EuAl4, which shows a charge density wave transition at TCDW = 140 K, was studied by hard X-ray angle-integrated photoelectron spectroscopy (HAXPES) and soft X-ray angle-resolved photoelectron spectroscopy (ARPES). The valence band and core-level spectra obtained by HAXPES are consistent with the divalent nature of Eu atoms in EuAl4. From the ARPES results, the Fermi surface as well as band structure in the vicinity of the Fermi energy (EF) of EuAl4 are very similar to those of its isostructural divalent Sr compound SrAl4, which has no 4f electrons. This suggests that the Eu atoms are divalent in EuAl4, and the 4f electrons are localized below 1.8 eV with the Eu 4f7 electronic configuration in the ground state. The ARPES spectra measured along the Γ–(Σ)–Z high-symmetry line did not show significant temperature dependences above and below TCDW within the energy resolution of 80–90 meV. Moreover, the Fermi surface mapping along the kz direction showed that...

Evidence for Anionic Excess Electrons in a Quasi-Two-Dimensional Ca2N Electride by Angle-Resolved Photoemission Spectroscopy.

Angle-resolved photoemission spectroscopy (ARPES) study of a layered electride Ca2N was carried out to reveal its quasi-two-dimensional electronic structure. The band dispersions and the Fermi-surface map are consistent with the density functional theory results except for a chemical potential shift that may originate from the high reactivity of surface excess electrons. Thus, the existence of anionic excess electrons in the interlayer region of Ca2N is strongly supported by ARPES.

レーザー光励起スピン・角度分解光電子分光装置の開発

Spin- and angle-resolved photoelectron spectrometer using a laser light of 6.994 eV as a photon source has been developed at Laser and Synchrotron Research Center in Institute for Solid State Physics. The spectrometer consists of a high-energy resolution photoelectron analyzer and two high-efficient spin detectors associating very low energy electron diffraction, which allows us to analyze the three-dimensional spin polarization of electrons with high-energy and -angular resolutions. Fermi surface mapping and spin- and angle-resolved photoelectron spectroscopy of a surface state of Bi(111) have been performed to demonstrate the performance of the new spectrometer.

Electronic structure of self-assembled Ag nanowires on Si(557): spectroscopic evidence for dimensionality

We investigate the electronic structure of silver nanowires at 1 monolayer (ML) Ag coverage on a vicinal Si(557) substrate by angle-resolved photoemission spectroscopy. Fermi surface mapping reveals two-dimensional (2D) Ag-induced electron pockets, which shift to higher binding energies by n-type doping of the step edges. The electronic structure of the 1 ML Ag/Si(557) system exhibits additional 2D metallic states near the Fermi level, which may originate from substrate induced periodicities. By considering the confining potential and electronic coherence length we reconcile the seemingly conflicting views of the dimensionality in this system probed by photoemission spectroscopy, and the dispersion of collective excitations as detected by electron energy loss spectroscopy .

Liquid-source growth of graphene on Ag(001)

The epitaxial growth of graphene on the Ag(001) surface via chemical vapor deposition (CVD) of propene and via liquid precursor deposition (LPD) by ex situ rinsing with acetone is investigated by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and Fermi surface mapping (FSM). In case of LPD, the growth of graphene is quite similar to that previously obtained for h-BN monolayers on the same surface. The weak interaction between the graphene layer and the Ag(001) surface prevents a preferred alignment of the graphene lattice, leading to graphene domains with statistically distributed azimuthal orientations. In contrast to the LPD method, it is not possible to provide any carbon via the standard CVD technique. The present results give evidence that LPD represents a promising alternative for the growth of graphene, especially on weakly interacting surfaces.

Electronic band structure and Fermi surfaces of the quasi–two-dimensional monophosphate tungsten bronze, P4W12O44

The electronic structure of quasi-two-dimensional monophosphate tungsten bronze, P4W12O44, has been investigated by high-resolution angle-resolved photoemission spectroscopy and density functional theoretical calculations. Experimental electron-like bands around point and Fermi surfaces have similar shapes as predicted by calculations. Fermi surface mapping at different temperatures shows a depletion of density of states at low temperature in certain flat portions of the Fermi surfaces. These flat portions of the Fermi surfaces satisfy the partial nesting condition with incommensurate nesting vectors and , which leads to the formation of charge density waves in this phosphate tungsten bronzes. The setting up of charge density wave in these bronzes can well explain the anomaly observed in its transport properties.

Yb valence change in Ce1−xYbxCoIn5from spectroscopy and bulk properties

The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of photoemission, x-ray absorption and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x>0.3 were confirmed. One new result of this study is that the Yb valence for x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of about 50% cross sectional area reductions of the \alpha- and \beta-sheets for x=1 compared to x=0, and a smaller, essentially proportionate, size change of the \alpha-sheet for x=0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.

Time-resolved Fermi surface mapping of the charge density wave material DyTe3

The femtosecond dynamics of the Fermi surface of DyTe3 and its band structure are investigated by time- and angle-resolved photoemission spectroscopy. We directly monitor the ultrafast collapse of the charge density wave gap within 200 fs.

Electronic structure of reconstructed Au(100): Two-dimensional and one-dimensional surface states

The clean surface of Au(100) presents a complex reconstruction characterized by a hexagonal topmost layer. We report an angle-resolved photoemission study of the electronic structure of this surface, including an analysis of the Fermi surface, combined with structural information from low-energy electron diffraction and scanning tunneling microscopy. In the complex Fermi surface map found, we identify different contributions from the bulk bands, from interface states located below the hexagonal topmost layer, and from the hexagonal topmost layer itself. The electronic states related to this layer exhibit quasi-one-dimensional character, in agreement with the chain aspect of the reconstructed layer, as demonstrated by their dispersion, periodicity, and reciprocal space location.

Full characterization and optimization of a femtosecond ultraviolet laser source for time and angle-resolved photoemission on solid surfaces

A novel experimental apparatus for time and angle-resolved photoemission on solid surfaces is presented. A 6.28 eV laser source operating at 250 kHz repetition rate is obtained by frequency mixing in nonlinear beta barium borate crystals. This UV light source has a high photon flux of 10(13) photons/s with relatively low number of photons/pulse so that Fermi surface mapping over a wide region of the Brillouin zone is possible while mitigating space charge effects. The UV source has been fully characterized spatially, spectrally, and temporally. Its potential for time and angle-resolved photoemission is demonstrated through Fermi surface mapping and photoexcited electron dynamics in Bismuth. True femtosecond time resolution <65 fs is obtained while the energy resolution of 70 meV appears to be mainly limited by the laser bandwidth.