Angle-resolved Photoemission Results Research Articles

Crystal growth, characterization and electronic band structure of TiSeS

Layered semimetallic van der Waals material $\mathrm{Ti}{\mathrm{Se}}_{2}$ has attracted a lot of attention because of interplay of a charge density wave (CDW) state and superconductivity. Its sister compound ${\mathrm{TiS}}_{2}$, being isovalent to $\mathrm{Ti}{\mathrm{Se}}_{2}$ and having the same crystal structure, shows a semiconducting behavior. The natural question rises---what happens at the transition point in $\mathrm{Ti}{\mathrm{Se}}_{2\ensuremath{-}x}{\mathrm{S}}_{x}$, which is expected for x close to one. Here we report the growth and characterization of TiSeS single crystals and the study of the electronic structure using density functional theory (DFT) and angle-resolved photoemission (ARPES). We show that TiSeS single crystals have the same morphology as $\mathrm{Ti}{\mathrm{Se}}_{2}$. Transport measurements reveal a metallic state; no evidence of CDW was found. DFT calculations suggest that the electronic band structure in TiSeS is similar to that of $\mathrm{Ti}{\mathrm{Se}}_{2}$, but the electron and hole pockets in TiSeS are much smaller. The ARPES results are in good agreement with the calculations.

Electronic structure of the intermediate-valence compound EuNi2P2 studied by soft x-ray photoemission spectroscopy

We carried out angle-resolved photoemission (ARPES) experiments using soft x rays to investigate the electronic structure of the intermediate-valence compound ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$. Both the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ components arising from the $4{f}^{6}$ and $4{f}^{5}$ final states were observed in the valence spectra, directly confirming an intermediate-valence character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and the ARPES results were compared with calculations based on the density-functional theory for the non-4$f$ reference compounds ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ and ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. We found that the ARPES spectra up to just below the Fermi level are better reproduced by the calculation of ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ rather than that of ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. The heavy-fermion bands in ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$ are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$, and the ${\mathrm{Eu}}^{2+}$ components located at the very vicinity of the Fermi level.

The band shifts in MoS2(0001) and WSe2(0001) induced by palladium adsorption

The band structures of the transition metal dichalcogenides (TMD's) 2H-MoS2(0001) and 2H-WSe2(0001), before and after palladium adsorption, were investigated through angle-resolved photoemission. Palladium adsorption on 2H-MoS2(0001) is seen to result in very different band shifts than seen for palladium on 2H-WSe2(0001). The angle resolved photoemission results of palladium adsorbed on WSe2(0001) indicate that palladium accepts electron density from substrate. The resulting band shift will lead to a decrease in the barriers to the hole injection. The opposite band shifts occur upon palladium adsorption between 2H-MoS2(0001). The overall trend is consistent with the deposition of other metals deposited on TMD's, except that for palladium adsorption on MoS2(0001), there is an increase in the MoS2(0001) substrate band gap with palladium adsorption, as is evident from the combination of photoemission and inverse photoemission.

Absence of Y-pocket in 1-Fe Brillouin zone and reversed orbital occupation imbalance in FeSe

The FeSe nematic phase has been the focus of recent research on iron-based superconductors (IBSs) due to its unusual properties, which are distinct from those of the pnictides. A series of theoretical/experimental studies were performed to determine the origin of the nematic phase. However, they yielded conflicting results and caused additional controversies. Here, we report the results of angle-resolved photoemission and X-ray absorption spectroscopy studies on FeSe detwinned by a piezo stack. We fully resolved band dispersions with orbital characters near the Brillouin zone (BZ) corner, and revealed an absence of any Fermi pocket at the Y point in the 1-Fe BZ. In addition, the occupation imbalance between {d}_{{xz}} and {d}_{{yz}} orbitals was the opposite of that of iron pnictides, consistent with the identified band characters. These results resolve issues associated with the FeSe nematic phase and shed light on the origin of the nematic phase in IBSs.

Role of the Shockley State in Doping of Organic Molecule Monolayer

We present angle-resolved photoemission results of the Shockley state behavior in NTCDA (1,4,5,8-naphthalene tetracarboxylic dianhydride) monolayer covering Na atoms preadsorbed on (111) noble metal surfaces. It is well-know that deposition of low amount of alkali metal atoms on such surfaces induces a shift of the surface state to high binding energies. This shift originates from an electron transfer occurring at the interface. Furthermore, the Shockley state is sensitive to the presence of organic films on the surface. NTCDA monolayer on the (111) surface of noble metals leads to upward displacement of the Shockley state, much stronger in the case of Ag and Cu than Au. Here we observe that in the case of combining both adsorbates on Au(111); deposition of Na atoms followed by growth of NTCDA monolayer, the Shockley state exhibits similar properties as that on pristine Au(111) surface. This behavior is also observed on the (111) surfaces of Cu and Ag and does not depend on the preadsorbed Na amount. We d...

Disorder-driven metal-insulator-transition assisted by interband Coulomb repulsion in a surface transfer doped electron system

The two-dimensional conducting properties of the Si(111) surface doped by the charge surface transfer mechanism have been calculated in the frame of a semiclassical Drude-Boltzmann model considering donor scattering mechanisms. To perform these calculations, the required values of the carrier effective mass were extracted from reported angle-resolved photoemission results. The calculated doping dependence of the surface conductance reproduces experimental results reported and reveals an intricate metallization process driven by disorder and assisted by interband interactions. The system should behave as an insulator even at relatively low doping due to disorder. However, when doping increases, the system achieves to attenuate the inherent localization effects introduced by disorder and to conduct by percolation. The mechanism found by the system to conduct appears to be connected with the increasing of the carrier effective mass observed with doping, which seems to be caused by interband interactions involving the conducting band and deeper ones. This mass enhancement reduces the donor Bohr radius and, consequently, promotes the screening ability of the donor potential by the electron gas.

Orbital Degeneracy, Jahn–Teller Effect, and Superconductivity in Transition-Metal Chalcogenides

We have studied the electronic structure of FeSe1−xTex and Ir1−xPtxTe2 using photoemission spectroscopy. For FeSe1−xTex, angle-resolved photoemission results indicate that the Fe 3d yz/zx orbital degeneracy at Γ point and orbitally induced Peierls effect in the tetragonal lattice play important roles for the superconductivity. It is suggested that the Jahn-Teller instability of the yz/zx states couples with local lattice distortion derived from the Te substitution for Se and provides an inhomogeneous electronic state. Photoemission results of IrTe2 with triangular lattice are also consistent with the orbitally induced Peierls scenario. The Pt substitution for Ir suppresses the static band Jahn–Teller effect and induces an inhomogeneous electronic state in which orbital (or bond or nematic) fluctuations may help superconductivity through the Peierls effect.

Scaling analysis of normal-state properties of high-temperature superconductors

We propose a model-independent scaling method to study the physical properties of high-temperature superconductors in the normal state. We have analyzed the experimental data of the $c$-axis resistivity, the in-plane resistivity, the Hall coefficient, the magnetic susceptibility, the spin-lattice relaxation rate, and the thermoelectric power using this method. It is shown that all these physical quantities exhibit good scaling behaviors, controlled purely by the pseudogap energy scale in the normal state. The doping dependence of the pseudogap obtained from this scaling analysis agrees with the experimental results of angle-resolved photoemission and other measurements. It sheds light on the understanding of the basic electronic structure of high-${T}_{c}$ oxides.

Non-fermi liquid angle resolved photoemission line shapes of li0. 9Mo6O17

A recent letter by Xue et al. (PRL v.83, 1235 ('99)) reports a Fermi-Liquid (FL) angle resolved photoemission (ARPES) lineshape for quasi one-dimensional Li0.9Mo6O17, contradicting our report (PRL v.82, 2540 ('99)) of a non-FL lineshape in this material. Xue et al. attributed the difference to the improved angle resolution. In this comment, we point out that this reasoning is flawed. Rather, we find that their data have fundamental differences from other ARPES results and also band theory.

Far-infrared ellipsometric study of the spectral gap in thec-axis conductivity ofY1−xCaxBa2Cu3O7−δcrystals

The temperature (T) and doping dependence of the spectral gap in the far-infrared c-axis conductivity of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ has been studied by ellipsometry. For underdoped crystals a T-independent spectral gap is observed which persists almost unchanged into the normal state. The normal-state gap disappears at optimum doping whereas a conventional T dependence of the spectral gap is found for overdoped samples. The analogy to the results of angle-resolved photoemission suggests that the c-axis conductivity may be dominated by the carriers at the X point of the two-dimensional Brillouin zone.

Electronic structure of SrNbO 3.45: angle-resolved photoemission results

The electronic structure of the layered perovskite related compound SrNbO 3.45 has been investigated by angle-resolved photoemission spectroscopy. Two bands near the Fermi level with very different behavior were observed: along Λ-X, one band exhibits no discernible dispersion, whereas the other disperses strongly and crosses the Fermi level. No dispersion appears along the Λ-Y direction.

Evolution of the electronic structure with doping in Bi2Sr2CaCu2O8+δ - angle resolved photoemission results

We present experimental results showing that the electronic structure of the high Tc cuprate superconductors evolves from having large pieces of Fermi surface (FS) at high doping levels to small pieces of FS centered at (π/2,π/2) at low doping levels. The dramatic change in the FS occurs with only subtle changes in the electronic structure, which occur mostly near the (π,0) point of the Brillouin zone. We suggest that this evolution is intimately related to the evolution of the antiferromagnetic (AF) order with doping.

Evolution of the electronic structure with doping in Bi 2Sr 2CaCu 2O 8+δ - Angle resolved photoemission results

We present experimental results showing that the electronic structure of the high Tc cuprate superconductors evolves from having large pieces of Fermi surface (FS) at high doping levels to small pieces of FS centered at (π/2,π/2) at low doping levels. The dramatic change in the FS occurs with only subtle changes in the electronic structure, which occur mostly near the (π,0) point of the Brillouin zone. We suggest that this evolution is intimately related to the evolution of the antiferromagnetic (AF) order with doping.

Quasiparticle dispersion of Sr2CuO2Cl2

Recent angle resolved photoemission (ARPES) measurements for the insulating cuprate Sr2CuO2Cl2 have provided the first experimental data which can be directly compared to the (theoretically) well-studied problem of a single hole propagating in an antiferromagnet. Some discrepancies withthe familiar 2D t— J model were observed. Here we discuss a comparison between the ARPES results and the quasiparticle dispersion of both (i) the extended t— t′— J Hamiltonian and (ii) the three-band Hubbard model.

Electronic Structure and Renormalized Fermi Surface of La2-xSrxCuO4

We have derived the effective one-electron-type band structure, the density of states and the Fermi surface structure for a carrier system, by renormalizing the effects of the supper-exchange coupling between localized spins, the exchange integral between the spins of a carrier hole and a localized spin, and the Hubbard U -like parameter of a carrier hole into the electronic state of the carriers. The obtained energy bands and Fermi surface structure are consistent with the recent experimental results of angle-resolved photoemission.

Geometry and electronic band structure of an ordered monolayer deposition of Bi on III-V(110) semiconductor surfaces.

We present calculational studies of the electronic and geometric structure of an ordered monolayer deposition of Bi on III-V(110) surfaces. The technique which we have applied to these systems relies on the complete self-consistent solution of the Kohn-Sham equations for both the electronic and ionic degrees of freedom. An ab initio pseudopotential method, within the local-density approximation, is used in a supercell approach. From a given initial set of atomic positions, a conjugate-gradient technique is used to achieve the equilibrium geometry by moving along the Born-Oppenheimer subspace. The calculated relaxed geometries of the clean and Bi-covered (110) surface of GaAs, InP, and InAs compare well with available low-energy electron-diffraction and x-ray standing-wave studies, and the electronic band structures agree with angle-resolved photoemission results. The orbital nature of states that might participate in Schottky-barrier formation at Bi-covered surfaces is also discussed.

Real Space Observation of Standing Waves at Metal Surfaces and the Determination of Surface State Dispersion with the Scanning Tunneling Microscope

Standing waves produced by the scattering and interference of surface state electrons by surface steps and adsorbates are observed in real space using scanning tunneling spectroscopy (STS). From the structure of the standing waves, the wave number of the surface electrons can be obtained as a function of the electron energy. Here we compare the thus obtained dispersion with the results of angle-resolved photoemission and the predictions of a simple theoretical model. Based on these comparisons we determine the best way of performing the STS experiment and analyze the results to obtain a reliable measure of the dispersion of surface states.

Low temperature photoemission study of La 2CuO 4+x single crystals

New angle-resolved photoemission results obtained using synchrotron radiation of 15–40 eV from cleaved La 2CuO 4+ x single crystals at a temperature of about 30 K are presented. We have observed spectral enhancements at photon energies around 17 eV that are consistent with autoionization resonance excitations of the hybridized La 5p-O 2s states to O 2p empty states. The crystal surface was found to be unstable. A broad feature around 5 eV binding energy that could be caused by surface reconstruction was observed.

Fermiology of YBa 2Cu 4O 8

We present angle-resolved photoemission results of YBa 2Cu 4O 8 (124). The energy distribution curves are very similar to those observed in the related compound YBa 2Cu 3O 69 (123). The structure of the 124 compound is very similar to that of the 123, except that the 124 possesses two chain bands instead of one crossing the Fermi energy. This has an effect of lowering the density of states at E F and making the 124 compound more stable, to oxygen content. Otherwise, their observed electronic structures are very similar, including the sharp peaks at E F at the X and Y symmetry points, and the very sharp state at 1 eV binding energy at the S point The states at E F along the Γ-S line exhibit the same symmetry as those in the 123, i.e. d 2 x 2 −y , while the states at X and Y exhibit d xy symmetry

Angle-resolved inverse photoemission study of Bi 2Sr 2CaCu 2O 8: Existence of two CuO 2 bands across the Fermi level

Angle-resolved inverse photoemission spectroscopy was performed on a Bi 2Sr 2CaCu 2O 8 single crystal to study the electronic structure in the vicinity of the Fermi level. Inverse photoemission intensity shows two maxima at about 0.21 ΓX and 0.6ΓX from the Γ point along the ΓX direction, suggesting that there are two bands crossing the Fermi level. The present experimental result was compared with angle-resolved photoemission results and band structure calculations.