Fermi Surface Mapping Research Articles

Efficient spin resolved spectroscopy observation machine at Hiroshima Synchrotron Radiation Center

Highly efficient spin- and angle-resolved photoelectron spectrometer named ESPRESSO (Efficient SPin REsolved SpectroScopy Observation) machine has been developed at the beamline BL-9B in Hiroshima Synchrotron Radiation Center. Combination of high-resolution hemispherical electron analyzer and the high-efficient spin detector based on very low energy electron diffraction by the ferromagnetic target makes the high-energy resolution and angular resolution compatible with spin- and angle-resolved photoemission (SARPES) measurement. 7.5 meV in energy and ±0.18° in angular resolution have been achieved with spin resolution. The ESPRESSO machine, combination of quick energy-band dispersion measurement and Fermi surface mapping by two-dimensional electron detector for the spin integrated ARPES and the high-efficient spin analysis by the efficient spin detector realizes the comprehensive investigation of spin electronic structure of materials.

Epitaxial growth of graphene on Ir(111) by liquid precursor deposition

The epitaxial growth of graphene on the surface of an Ir/YSZ/Si(111) multilayer substrate via the deposition of a liquid carbon precursor (acetone) was investigated by x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, low-energy electron diffraction (LEED), and Fermi surface mapping. It is shown that the onset of graphene formation starts in a low temperature range around 600 K and that subsequent annealing up to 1000 K finally results in well-ordered graphene monolayers. Comparison of temperature-dependent LEED data with model calculations suggests that the growth of graphene takes place via a backbonelike growth by the formation of a hexagonal network connecting the hcp and fcc configuration sites within the $\ensuremath{\sim}$10 \ifmmode\times\else\texttimes\fi{} 10/9 \ifmmode\times\else\texttimes\fi{} 9 supercell. In LEED, the low intensities of the superstructure related satellite spots give evidence for only small corrugations of the graphene layer due to weak interaction with the Ir(111) surface, making graphene on Ir(111) similar to free-standing graphene with the Fermi surface providing distinct spots at the K points.

Evolution of electronic structure in Eu1−xLaxFe2As2

We report an angle-resolved photoemission spectroscopy study of electronic structures of Eu1−xLaxFe2As2 single crystals, in which the spin density wave transition is suppressed with La doping. In the paramagnetic state, the Fermi surface maps are similar for all dopings, with chemical potential shifts corresponding to the extra electrons introduced by the La doping. In the spin density wave state, we identify electronic structure signatures that relate to the spin density wave transition. Bands around M show that the energy of the system is saved by the band shifts towards high energies, and the shifts decrease with increasing doping, in agreement with the weakened magnetic order.

Electronically driven phase transitions in a quasi-one-dimensional adsorbate system

A quasi-1D system is prepared using the Pt(110) surface as a template. The electronic surface resonance structure is studied by angle-resolved photoemission spectroscopy for the clean surface as well as for different Bromine coverages. A Fermi surface mapping reveals saddle points at the Fermi level in the interior of the surface Brillouin zone. Correspondingly, a maximum in the static response function χ(q, 0) at the connecting vector q is expected. With 1/2Gx < q < 2/3Gx one observes indeed a 3-fold periodicity around defects and a 2-fold periodicity at low temperature for ΘBr = 0.5 ML. Cooling of a defect-free c(2×2)-Br/Pt(110) preparation counter-intuitively results in a loss of long-range order. Motivated by DFT calculations this is attributed to an anomalous order-order phase transition into the (2×1) phase accompanied by intense, strongly anisotropic fluctuations within a temperature range of ~200 K. The peculiar behaviour is rationalised in terms of a competition between inter-adsorbate repulsion and an adsorbate triggered 2kF interaction in the substrate.

Fermi surfaces of single layer dielectrics on transition metals

Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years is shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(1 1 1), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(1 1 1) and Rh(1 1 1) are compared. A h-BN layer on Rh(1 1 1) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface.

Origin of “Hot Spots” in the pseudogap regime of Nd1.85Ce0.15CuO4: An LDA + DMFT + Σk study

The material-specific electronic band structure of the electron-doped high- T c cuprate Nd1.85Ce0.15CuO4 (NCCO) is calculated in the pseudogap regime using the recently developed generalized LDA + DMFT + Σ k scheme. The LDA/DFT (density-functional theory within local density approximation) provides model parameters (hopping integral values and local Coulomb interaction strength) for the one-band Hubbard model, which is solved by the DMFT (dynamical mean-field theory). To take pseudogap fluctuations into account, the LDA + DMFT is supplied with an “external” k-dependent self-energy Σ k that describes interaction of correlated conducting electrons with nonlocal Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for the pseudogap formation. Within this LDA + DMFT + Σ k approach, we demonstrate the formation of pronounced hot spots on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi2Sr2CaCu2O8 − δ (Bi2212), which have produced a rather extended region of the FS “destruction.” There are several physical reasons for this fact: (i) the hot spots in NCCO are located closer to the Brillouin zone center; (ii) the correlation length ξ of AFM fluctuations is longer for NCCO; (iii) the pseudogap potential Δ is stronger than in Bi2212. Comparison of our theoretical data with recent bulk-sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison, an alternative explanation of the van Hove singularity at −0.3 eV is proposed. Optical conductivity for both Bi2212 and NCCO is also calculated within the LDA + DMFT + Δ k scheme and is compared with experimental results, demonstrating satisfactory agreement.

Angle-resolved photoemission of orderedPb∕Ni(111)surface phases

$\mathrm{Pb}∕\mathrm{Ni}(111)$ surface phases were investigated by angle-resolved photoemission spectroscopy. Annealing of the $\mathrm{Pb}∕\mathrm{Ni}(111)$ surface led to the formation of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction characterized by a topmost layer consisting of a substitutional alloy. The transformation to the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure was accompanied by a modification of the surface band structure confirming the formation of a new phase in the Ni(111) surface layer, characterized by a transformation from ferromagnetic to a nonmagnetic state. The modification was displayed as a disappearance of the $\mathbit{k}$-dependent exchange splitting in the topmost alloy layer that was observed on the clean Ni surface due to the energy difference between spin-up and spin-down states. Fermi surface mapping showed partial disappearance of splitting of bands crossing the Fermi level.

Charge modulation driven Fermi surface of Pb–Bi2201

It is well known that the (1×5) superstructure of Bi cuprate superconductors will be suppressed due to doping with Pb. Nevertheless, a Fermi surface map of Bi 2− y Pb y Sr 2− x La x CuO 6+δ ( y = 0.4 and x = 0.4 ) determined by angular resolved photoemission (ARPES) revealed additional Fermi surface features. Low-energy electron diffraction (LEED) and X-ray diffraction of this sample showed no sign of any superstructure. Scanning tunneling microscopy (STM), on the other hand, revealed two distinct modulations of the charge density, one of (1×32) and a second of (6×6) periodicity. The wave vectors of both modulations have been extracted and used to simulate the corresponding Fermi surface, which is compared with the experimental one. The origin of these modulations is discussed in terms of dopant ordering.

Fermi surface and band mapping of the cerium/palladium surface alloy

X-ray and UV excitation angle-resolved photoemission spectroscopy of ultra-thin films of cerium deposited on Pd(1 1 1) single-crystal surface has been carried out. Photoelectron diffraction pattern showed that deposition of 1 ML of Ce led to a formation of Ce–Pd substitutional alloy. Valence band spectra measured with high angular resolution permitted to plot valence band maps and Fermi surface scans and showed formation of surface alloy exhibiting d- and f-electron orbital hybridization. A shift of Pd 4d-derived states to higher binding energy in the Ce–Pd systems was observed.

コンプトン散乱を用いた重元素結晶のフェルミ面マッピング

We report on the Fermi surface mapping of heavy element crystals using Compton scattering, which has been possible by the recent advent of intense high energy synchrotron radiation sources. Fermi surface mapping provides us insights for long range distortions or charge density waves in crystals. Two examples are shown : The first one is Fermi surface mapping of the η-Mo4O11 low-dimensional conductor. A clear nesting vector is observed in the Fermi surfaces that have experimentally been obtained. The second example is Fermi surface study of Ba1-xKxBiO3. A clear feature of the Fermi surface nesting is observed across the transition between the distorted (insulating) and non-distorted (metallic) phases.

Fermi surfaces and kink in the energy dispersion of Sr2RuO4

We have performed a high-resolution angle-resolved photoemission spectroscopy on the monolayer strontium ruthenate Sr2RuO4, to elucidate the origin of the kink. We present the Fermi surface mapping and the detailed energy dispersions near the Fermi level. The kink was clearly observed for γ Fermi surface sheet derived from the in-plane Ru 4dxy orbital. The kink of γ Fermi surface sheet was still observed at points far from (0, π), indicating a rather weak momentum dependence of the kink of Sr2RuO4.

Comments on reconstruction of Fermi surface from high-resolution Compton data

A few comments are made on reconstruction of three-dimensional (3D) momentum density and Fermi surface mapping from high-resolution Compton data. Equivalence of LCW-folding and Schuelke's method in converting observed momentum density to occupation number density is explicitly shown.

Mapping disorder–order induced changes to the Fermi surface of Cu 3Au using a new toroidal electron energy analyser

A second-generation toroidal spectrometer has been used to investigate the Fermi surface topology of the binary alloy system Cu 3Au when prepared as an ordered or substitutionally disordered alloy. We demonstrate using two complementary Fermi surface mapping techniques and an empirical FS model for the disordered phase, that structures in the data arise from two different mechanisms. We observe distinct effects on the FS upon ordering, and compare the results with a recent bandstructure calculation.

Si(111)√21×√21-Ag超構造のフェルミ円と表面電気伝導の研究

Si(111)√21×√21-Ag超構造のフェルミ円と表面電気伝導の研究

Fermi-surface topology of rare-earth dihydrides

We report high-resolution angle-resolved photoemission experiments on epitaxial thin films of different rare-earth (RE) dihydrides $(\mathrm{RE}=\mathrm{Gd},\mathrm{La})$ and of $\mathrm{Y}{\mathrm{H}}_{2}$ and $\mathrm{Sc}{\mathrm{H}}_{2}$. It is found through ab initio calculations and confirmed by Fermi surface mapping that the electronic structure becomes very similar upon hydrogenation, rendering the studied dihydrides isoelectronic. We propose that the dihydride phase acts as a common precursor state for the formation of the insulating trihydride phase. For states with higher binding energies (which exhibit considerable H character) the agreement between calculation and measurement is less convincing. Independent of the difficulties to describe these hydrogen related states, we note in the comparison between experiment and calculation a very convincing description of the Fermi surface for the dihydrides. Therefore we trace the apparent inability of density, functional theory to describe the hygrogenation up to the trihydride phase to an insufficient description of hydrogen states in general and, in particular, involving octahedral sites.

Pseudogapped Fermi surfaces of1T−TaS2and1T−TaSe2:A charge density wave effect

We report room temperature angle-resolved photoemission experiments on $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ and $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ complemented by density-functional theory calculations. Fermi-surface mapping experiments in the charge-density wave (CDW) phase are similar for the two compounds and do not show symmetries due to the CDW-induced new Brillouin zones. However, the band structure a few eV below the Fermi level ${(E}_{F})$ displays a clear modulation that we relate, in both cases, to the CDW. At ${E}_{F},$ the spectral weight distribution reflects the band structure of the normal state, but no clear quasiparticle crossing is located. Near the zone center $\overline{\ensuremath{\Gamma}},$ CDW-split quasilocalized Ta ${\mathrm{dz}}^{2}$ subbands are observed in the vicinity of ${E}_{F}.$ For $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ and $1T\ensuremath{-}{\mathrm{TaSe}}_{2},$ they are thermally populated slightly above ${E}_{F}$ and close to ${E}_{F},$ respectively. The observed behavior can be understood in terms of the CDW reconstructed, spectral function weighted band structure.

Angle resolved photoemission studies on Sm 2− xCe xCuO 4: remnant Fermi surfaces and coupling to (π,π) scattering

We report high-resolution angle-resolved photoelectron spectroscopy studies on electron doped superconductors, Sm 2− x Ce x CuO 4 ( x=0 and 0.15). The results on the undoped ( x=0) samples show Cu–O anti-bonding states with an overall dispersion of about 0.3 eV which is similar to other insulating cases. The low energy dispersive feature quickly looses intensity before it reaches the Fermi energy near the anti-ferromagnetic zone boundary, showing a remnant behavior of the original Fermi surface (FS). In doped cases ( x=0.15), FS mapping shows suppressed intensities at the Fermi energy where the FS crosses the anti-ferromagnetic zone boundary. This is similar to what was observed on another electron doped superconductor Nd 1.85Ce 0.15CuO 4 which was ascribed being due to the formation of a high-energy pseudogap assisted by Q =( π,π) scattering. Similarities and differences in comparison to other compounds are discussed.

An undulator-based spherical grating monochromator beamline for angle-resolved photoemission spectroscopy

A new spherical grating undulator beamline has been commissioned at the synchrotron radiation source ASTRID in Aarhus. It covers an energy range from 12 to 130 eV with a resolving power of about 15.000. This beamline is combined with an end-station for angle-resolved photoemission. The spectrometer is a 75 mm mean radius hemispherical electron energy analyzer which is mounted on a goniometer within the ultra-high vacuum chamber. The motions of the analyzer and several settings of the beamline are motorized and computerized scans can be performed. The system is in particular well suited for measurements of the electronic structure along arbitrary lines in k → || -space, Fermi surface mapping and studying the polarization dependence of photoemission features.

Bulk Fermi surface mapping with high-energy angle-resolved photoemission

We have studied the Fermi surface of copper by angle-resolved photoemission for photonenergies between 95.37 and 259.6 eV. Fermi surface images were obtained by scanning the polaremission angle as well as the photon energy while monitoring the photoemission intensity atthe Fermi level. Such images are distorted because of the non-conservation of the perpendiculark component in photoemission. However, if the scans are performed such that thek steps parallel to the surface are identical for all photon energies, the distortion can becorrected for using appropriate final states, for example free-electron final states. We findreasonable agreement between our results and the well-known Fermi surface of copper.

Matrix element effects on the Fermi surface mapping by angle resolved photoemission from Bi2Sr2CaCu2O8+x superconductors

We report angle resolved photoemission spectra (ARPES) from the Fermi energy (EF) over a large area of the reciprocal space using polarized and unpolarized synchrotron radiation light, in two particular distinctive geometries of detection from a mono-domain single crystal of Bi2Sr2CaCu2O8+x (Bi-2212) optimally doped. The data display a wide-ranging level of accord with previously reported ARPES results and clarify how many apparently dissimilar Fermi surface (FS) maps emerge in ARPES, under different experimental conditions. The energy and polarization dependences of the ARPES matrix elements help disentangle primary contributions from the main FS bands to the spectrum from those arising from secondary effects. The remarkable influence of the matrix elements on the ARPES intensity allows us to unravel the principal FS bands, from those robust features associated to the incommensurate superstructure modulations, as well as provide us a reliable route for separating closely placed FS sheets in low dimensional materials.