Fermi Cutoff Research Articles

Role of electron correlation and disorder on the electronic structure of layered nickelate (La0.5Sr0.5)2NiO4

We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate ${({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5})}_{2}{\mathrm{NiO}}_{4}$ using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean-field theory (DMFT) calculations. Sr $3d$ and La $4d$ core level spectra exhibit multiple features associated with photoemission final-state effects. An increase of unscreened features in the Sr $3d$ and La $4d$ core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, ${E}_{F}$. Valence band spectra collected using different photon energies reveal finite intensity at ${E}_{F}$ and overall spectra are well captured by $\mathrm{DFT}+\mathrm{DMFT}$. Strong renormalization of partially filled ${e}_{g}$ bands in $\mathrm{DFT}+\mathrm{DMFT}$ result indicates strong correlation in this system. Mass enhancement factor, ${m}^{*}/{m}_{\text{DFT}}\ensuremath{\sim}$ 3, agrees well with values obtained from specific heat measurements. High-resolution spectra in the vicinity of ${E}_{F}$ show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cutoff at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. $|E\ensuremath{-}{E}_{F}{|}^{1/2}$ dependence of the spectral DOS and square-root temperature dependence of spectral DOS at ${E}_{F}$ evidences the role of disorder in the electronic structure of ${({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5})}_{2}{\mathrm{NiO}}_{4}$.

Ultrafast probes of photovoltage and two-dimensional electron gas in the topological insulator Bi2Te3 by angle-resolved photoemission and terahertz spectroscopy

Photoexcited carriers in three-dimensional topological insulators (3D TIs) decay rapidly through the electron-electron and electron-phonon scattering. While most studies focus on such fast dynamics, recent experiments find the slow photovoltage (PV) dynamics arising from the band-bending potentials, in which the optical transitions in two-dimensional electron gas (2DEG) are effective. Although early investigations speculated the existence of multiple band-bending structures from the TI surface to the TI bulk, how PV and 2DEG are correlated in the presence of such multiple band bendings has been less explored. Here, we employ the combination of time- and angle-resolved photoemission spectroscopy (tr-ARPES) and ultrafast time-resolved terahertz (tr-THz) spectroscopy to investigate the PV and 2DEG dynamics in the prototypical topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. Our tr-ARPES analysis identifies two spatially separated PV dynamics associated with two types of band bending: one is the well-known surface PV, and another PV is formed deep within the bulk, which we call ``internal bulk PV.'' For the surface PV, our tr-THz spectra substantiate that the $\ensuremath{\mu}\mathrm{s}$-long transient signal arises from the surface-PV-induced increase of the TSS and 2DEG carrier density, which appears as a transient blueshift of a Fermi cutoff and an increased ARPES intensity in the tr-ARPES measurements. In contrast, the effect of the internal bulk PV shows only marginal changes in the 2DEG and TSS carrier densities but shifts the entire binding energy of the near-surface bands.

Fermi Gamma-Ray Pulsars: Understanding the High-energy Emission from Dissipative Magnetospheres

Abstract Based on the Fermi observational data, we reveal meaningful constraints for the dependence of the macroscopic conductivity (σ) of dissipative pulsar magnetosphere models on the corresponding spin-down rate, . Our models are refinements of the FIDO (Force-free Inside, Dissipative Outside) models, which have dissipative regions that are restricted on the equatorial current sheet outside of the the light-cylinder. Taking into account the observed cutoff energies of all of the Fermi pulsars and assuming that (a) the corresponding γ-ray pulsed emission is due to curvature radiation at the radiation-reaction-limit regime, and (b) this emission is produced at the equatorial current sheet near the light cylinder, we show that the Fermi data provide clear indications about the corresponding accelerating electric-field components. A direct comparison between the Fermi cutoff energies and the model ones reveals that σ increases with for high -values, while it saturates for low ones. This comparison indicates also that the corresponding gap width increases toward low -values. Assuming the Goldreich–Julian flux for the emitting particles, we calculate the total γ-ray luminosity (L γ ). A comparison between the dependence of the Fermi L γ -values and the model ones on indicates an increase of the emitting particle multiplicity with . Our modeling, guided by the Fermi data alone, enhances our understanding of the physical mechanisms behind the high-energy emission in pulsar magnetospheres.

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system.

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively, the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm2.

Bilayer graphene with parallel magnetic field and twisting: Phases and phase transitions in a highly tunable Dirac system

The effective theory for bilayer graphene (BLG), subject to parallel/in-plane magnetic fields, is derived. With a sizable magnetic field the trigonal warping becomes irrelevant, and one ends up with two Dirac points in the vicinity of each valley in the low-energy limit, similar to the twisted BLG. Combining twisting and parallel field thus gives rise to a Dirac system with tunable Fermi velocity and cutoff. If the interactions are sufficiently strong, several fully gapped states can be realized in these systems, in addition to the ones in a pristine setup. Transformations of the order parameters under various symmetry operations are analyzed. The quantum critical behavior of various phase transitions driven by the twisting and the magnetic field is reported. The effects of an additional perpendicular field and possible ways to realize the new massive phases are highlighted.

Size estimation of embedded Cu nanoprecipitates in Fe by using affinitively trapped positrons

By utilizing a unique characteristic of the affinitively trapped positrons, here we demonstrate a powerful method to estimate the sizes of the embedded Cu nanoprecipitates (and subnanoprecipitates) (ECNPs) in Fe. The positron-electron momentum density distributions in the ECNPs with various sizes in a thermally aged Fe-Cu dilute alloy were measured by using the two-dimensional angular correlation of positron annihilation radiation technique and the momentum smearing around the Fermi cutoff was clearly observed. It is clarified that this smearing effect is proportional to the inverse of the average radii of the ECNPs and thus it can be employed to estimate the ECNPs' sizes. Moreover, the ECNPs' sizes obtained by the present method are compared with those obtained by the three-dimensional atom probe technique and good agreement is observed, justifying the size estimation using the smearing effect. It is noteworthy that the present technique can detect the smearing due to the ECNPs with their radii even below about 0.5 nm and thus gives an important trend in the smearing reflecting the average size of the ECNPs in this range.

Electronic excitation and isentropic coefficients of high temperature planetary atmosphere plasmas

In this paper, we have discussed the effects of electronically excited states of atomic species in affecting the isentropic coefficients of plasmas, focusing on mixtures representing the atmospheres of Jupiter, Mars, and Earth. General behaviors have been rationalized on the basis of simplified approaches. The contribution of the electronically excited states has been evidenced by comparing results obtained considering only the ground state and those obtained using either Fermi or Griem cutoff criteria.

Sheet Dependence on Superconducting Gap in Oxygen-Deficient Iron-Based Oxypnictide Superconductors NdFeAsO0.85

Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO 0.85 ( T c =52 K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level ( E F ). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Γ(0,0) point but on the electron-like sheets around M(π,π) point.

Pseudogap and Superconducting Gap in Sm FeAs(O1-xFx) Superconductor from Photoemission Spectroscopy

High resolution photoemission measurements are carried out on non-superconducting SmOFeAs parent compound and superconducting SmFeAs(O1-xFx) (x = 0.12, and 0.15) compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13meV is identified in all these samples. Spectral weight suppression near Ef with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.

Temperature-dependent photoemission and x-ray absorption studies of the metal-insulator transition inBi1−xLaxNiO3

Perovskite-type $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic, and for $0.05\ensuremath{\le}x\ensuremath{\le}0.1$ a broad metal-insulator transition (MIT) occurs as a function of temperature. We have measured the temperature dependence of the photoemission and x-ray absorption (XAS) spectra of ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Ni}{\mathrm{O}}_{3}$ to investigate how the electronic structure changes across the MIT. From the $\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}2p$ XAS spectra of $x=0.05$, we found almost no change in the valence of Ni across the MIT. In the valence-band photoemission spectra, the Fermi cutoff disappeared for $x=0.05$ at a low temperature, whereas for $x=0.1$ and 0.2, it remained at all temperatures but the intensity at the Fermi level decreased gradually with decreasing temperature. Our experimental results suggest that the MIT is caused by the localization of holes in the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ band and that the ``insulating'' phase below the MIT is indeed a mixture of insulating and metallic regions.

TU-D-224C-05: Validation of a Linear Accelerator Source Model and Commissioning Process for Routine Clinical Monte Carlo Calculations

Purpose: Many source models for Monte Carlotreatment planning include parameters which are difficult or ambiguous to determine. We developed and tested a straightforward source model and commissioning process for clinical Monte Carlo calculations. Method and Materials: Our commissioning process of fitting treatment planning system data includes fitting the photon spectrum, electron contamination, penumbra fluence blurring, jaw leakage, and the flattening filter effect. The energy spectrum is fit using a modified Fatigue Life Distribution multiplied by a Fermi. Electron contamination is modeled separately an exponential function, as suggested by Fippel. Penumbral blurring is modeled using a Gaussian filter. Leakage radiation is modeled as a low‐intensity wide‐field monoenergetic source. The flattening filter effect is modeled by multiplying the optimized fluence by a Gaussian reduction. The penumbra and the flattening filter are applied to the fluence map. We tested our methodology on doses produced by a Varian accelerator for 6 MV and 18 MV photons and 5×5, 10×10, and 20×20 cm2field sizes.Results: We found that nine published photon spectra of Varian, Eleckta, and Siemens linear accelerators, ranging in energy from 4 MV to 25 MV could be modeled by the Fatigue Life Distribution with a Fermi cutoff. The agreement between the TPS doses and the commissioned MCdoses were within 2%. Off‐axis energy spectrum softening was unneeded. Conclusion: We have developed a straightforward, yet flexible source modeling system. The commissioning process affords a high‐degree of automation with an unambiguous determination of the relevant parameters. Commissioning of clinical Monte Carlotreatment planning systems is facilitated by using a source model which is only as complicated as necessary to accurately simulates dose distributions. Conflict of Interest: This research was partially supported by NIH grant R01 CA90445 and a grant from Sun Nuclear, Corp.

Valence changes associated with the metal-insulator transition inBi1−xLaxNiO3

Perovskite-type $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray-absorption (XAS) spectra of ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Ni}{\mathrm{O}}_{3}$ to investigate how the electronic structure changes with La doping. From Ni $2p$ XAS, we observed an increase of the valence of Ni from $2+$ toward $3+$. Combined with the core-level photoemission study, it was found that the average valence of Bi remains $\ensuremath{\sim}4+$ and that the Ni valence behaves as $\ensuremath{\sim}(2+x)+$, that is, La substitution results in a hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for $x>0$, consistent with the metallic behavior of the La-doped compounds. The Ni $2p$ XAS, Ni $2p$ core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual ${\mathrm{Ni}}^{2+}\ensuremath{\rightarrow}{\mathrm{Ni}}^{3+}$ valence change.

Metal–insulator transition in V 6O 13 probed by photoemission and optical studies

The metal–insulator (MI) transition in quasi-one-dimensional compound V 6O 13 has been investigated by angle-resolved photoemission spectroscopy (ARPES) at the photon energy of h ν = 10 eV using synchrotron radiation and optical study below 2 eV. Along the b ‖ direction, a clear Fermi cutoff and a band dispersion near E F are observed in the ARPES spectra. The energy distribution curves (EDC) show an opening of a gap ∼ 0.2 eV across the transition temperature T MI = 150 K. The ARPES at h ν = 10 eV is confirmed to be a bulk-sensitive spectroscopy with high resolutions in energy and momenta.

Doppler broadening of positron annihilation radiation: fitting the coincidence spectrum

The profile of the annihilation radiation in aluminum using positrons from a 22 Na source was observed in a two-HPGe-detectors arrangement. The coincidence photon energy spectrum was fitted using a model function, accounting for both Doppler broadening and detector system response. Intensities of the thermalized positron annihilation with band, 2p, 2s, and 1s electrons, and in-flight positron annihilation were fitted. The binding energies of the 1s, 2s, and 2p electrons and the Fermi cutoff parameters of the band electrons were also fitted.

In situ Mn 2p–3d resonant photoemission study on La 0.6Sr 0.4MnO 3 epitaxial thin films grown by laser MBE

We have performed an in situ resonant photoemission (RPES) study on well-ordered surfaces of La 0.6Sr 0.4MnO 3 thin films grown by laser molecular beam epitaxy. In sharp contrast with the previous photoemission results, the RPES spectra exhibit clear evidence of a Fermi cutoff. This result suggests that the importance of in situ photoemission measurement on well-ordered surfaces of transition metal oxides for revealing their intrinsic electronic structures.

Evidence of electron fractionalization from photoemission spectra in the high temperature superconductors.

In the normal state of the high temperature superconductors Bi(2)Sr(2)CaCu(2)O(8+delta) and La2(-x)Sr(x)CuO4, and in the related "stripe ordered" material, La(1.25)Nd(0.6)Sr(0.15)CuO4, there is sharp structure in the measured single hole spectral function, A<(k-->,omega), considered as a function of k--> at fixed small binding energy omega. At the same time, as a function of omega at fixed k--> on much of the putative Fermi surface, any structure in A<(k-->,omega), other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one-dimensional electron gas.

High-resolution Compton scattering study of the electron momentum density in Al

We report high-resolution Compton profiles (CP's) of Al along the three principal symmetry directions at a photon energy of 59.38 keV, together with corresponding highly accurate theoretical profiles obtained within the local-density approximation (LDA) based band-theory framework. A good accord between theory and experiment is found with respect to the overall shapes of the CP's, their first and second derivatives, as well as the anisotropies in the CP's defined as differences between pairs of various CP's. There are however discrepancies in that, in comparison to the LDA predictions, the measured profiles are lower at low momenta, show a Fermi cutoff which is broader, and display a tail which is higher at momenta above the Fermi momentum. A number of simple model calculations are carried out in order to gain insight into the nature of the underlying 3D momentum density in Al, and the role of the Fermi surface in inducing fine structure in the CP's. The present results when compared with those on Li show clearly that the size of discrepancies between theoretical and experimental CP's is markedly smaller in Al than in Li. This indicates that, with increasing electron density, the conventional picture of the electron gas becomes more representative of the momentum density and that shortcomings of the LDA framework in describing the electron correlation effects become less important.

Electronic states of charge-orderingNd0.5Sr0.5MnO3probed by photoemission

Electronic structures of the charge-ordering manganite ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}(x\ensuremath{\sim}0.5)$ have been studied by resonant and high-resolution photoemission. The Mn $2p$ (Nd $3d)$ resonant photoemission confirmed the Mn $3d$ (Nd $4f)$ states hybridized with the O $2p$ states. The high-resolution Mn $3p$ resonance spectra show a clear transition from the metal to the charge-ordered insulator for $x=0.5$ upon cooling while a Fermi cutoff is seen for $x=0.47$ at all temperatures, suggesting that the transition is induced by a subtle competition between the charge-ordering instability and the double-exchange interaction. Temperature and filling dependence of the spectra implies that the electronic states change not gradually but suddenly at the phase boundary.

Semimetallic behavior inFe2VAl:NMR evidence

We report the results of a ${}^{27}\mathrm{Al}$ and ${}^{51}\mathrm{V}$ nuclear magnetic resonance study of ${\mathrm{Fe}}_{2}\mathrm{VAl}$ at temperatures between 4 and 550 K. This material has been a subject of current interest due to indications of possible heavy fermion behavior. The low-temperature NMR relaxation rate follows a Korringa law, indicating a small density of carriers at the Fermi level. At elevated temperatures, the shifts and relaxation rates go over to a thermally activated response, a semiconductorlike behavior, consistent with separate low-lying bands removed from the Fermi-level. These results are consistent with recent electronic structure calculations, and can explain both the reported activated resistivity as well as the Fermi cutoff exhibited in photoemission studies. While we observe nonstoichiometric samples of $({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{V}}_{x}{)}_{3}\mathrm{Al}$ to be magnetic, the $x=0.33$ composition is nonmagnetic, with narrow NMR linewidths.

A potassium doped μm thick C60 film: direct observations of threshold doping and phases evolution

The evolution of the valence band spectra of K doped μm thick C60 film is presented. The data shows directly that there exists a threshold concentration before the first stable phase, K3C60, starts to appear. Below the threshold concentration, no Fermi emission is seen and the K atoms distributed randomly in the bulk retain their 4s electron whose binding energy is at 0.9eV below the Fermi level. This finding differs from all the previous experiments, which reported the immediate appearance of the K3C60 phase with a distinct Fermi cutoff upon K doping. A second stable phase, K6C60, evolves upon further doping with two characteristic states at 0.5 and 2.0eV and coexists with the K3C60 phase. Our work also demonstrates for the first time that the superfulleride phase which arises from the filling of the t1g band can be obtained at high doping. This phase is found on the surface.