Kevin S. Graham

Design of a variable temperature scanning force microscope.

We have developed the variable temperature scanning force microscope capable of performing both magnetic resonance force microscopy (MRFM) and magnetic force microscopy (MFM) measurements in the temperature range between 5 and 300 K. Modular design, large scanning area, and interferometric detection of the cantilever deflection make it a sensitive, easy to operate, and reliable instrument suitable for studies of the dynamic and static magnetization in various systems. We have verified the performance of the microscope by imaging vortices in a Nb thin film in the MFM mode of operation. MRFM spectra in a diphenyl-picryl-hydrazyl film were recorded to evaluate the MRFM mode of operation.

Observation of a kink in the dispersion of f-electrons

Strong interactions in correlated electron systems may result in the formation of heavy quasiparticles that exhibit kinks in their dispersion relation. Spectral weight is incoherently shifted away from the Fermi energy, but Luttingers theorem requires the Fermi volume to remain constant. Our angle-resolved photoemission study of USb2 reveals a kink in a noncrossing 5f band, representing the first experimental observation of a kink structure in f-electron systems. The kink energy scale of 21 meV and the ultra-small peak width of 3 meV are observed. We propose the novel mechanism of renormalization of a point-like Fermi surface, and that Luttingers theorem remains applicable.

He discharge lamp for photoemission experiments with radioactive materials

A simple He discharge lamp is described. The lamp is designed specially for photoemission measurements performed on radioactive materials, mainly Pu and Pu compounds. The lamp construction allows easy single-use assembly/disassembly of the discharge tube with minimum risk of radiological contamination, minimum amount of waste generated, air cooling capability, very good He II/He I ratio at low He pressures, and stable operation for at least 200 h. Thanks to the high brightness in the He II range we were able to observe the He IIβ, He IIγ, and even He IIe features in the PuIn3 photoemission spectrum. Total cost of parts used to produce the discharge tube is less than

Unusual quasiparticle renormalizations from angle resolved photoemission on USb2

300, so that the cost of operation is less than

Photoemission from YbInCu4 : testing the single impurity model

1.5/h.

Pu Electronic Structure and Photoelectron Spectroscopy

Angle-resolved photoemission experiments have been performed on USb2, and very narrow quasiparticle peaks have been observed in a band, which local spin-density approximation (LSDA) predicts to osculate the Fermi energy. The observed band is found to be depressed by 17 meV below the Fermi energy. Furthermore, the inferred quasiparticle dispersion relation for this band exhibits a kink at an energy of about 23 meV below the Fermi energy. The kink is not found in LSDA calculations and, therefore, is attributable to a change in the quasiparticle mass renormalization by a factor of approximately 2. The existence of a kink in the quasiparticle dispersion relation of a band that does not cross the Fermi energy is unprecedented. The kink in the quasiparticle dispersion relation is attributed to the effect of the interband self-energy, involving transitions from the osculating band into a band that does cross the Fermi energy.

Integrated experimental setup for angle resolved photoemission spectroscopy of transuranic materials

Abstract The electronic properties of single crystal YbInCu4 and YbAgCu4 have been investigated by means of high resolution photoelectron spectroscopy. A first order, isostructural phase transition for YbInCu4 at 42 K leads to changes in the Kondo temperature, of more than an order of magnitude. This phase transition and accompanying Kondo temperature change provide the most direct test of the single impurity model to date. The photoemission results are incongruous with the single impurity model predictions for temperature dependence, binding energy and 4f occupancy, encouraging a re-evaluation of the single impurity model applicability.

Band renormalization effects in correlated f-electron systems

The electronic structure of PuCoGa5, Pu metal, and PuO2 is explored using photoelectron spectroscopy. Ground state electronic properties are inferred from temperature dependent photoemission near the Fermi energy for Pu metal. Angle-resolved photoemission details the energy vs. crystal momentum landscape near the Fermi energy for PuCoGa5 which shows significant dispersion in the quasiparticle peak near the Fermi energy. For the Mott insulators AnO2(An = U, Pu) the photoemission results are compared against hybrid functional calculations and the model prediction of a crossover from ionic to covalent bonding is found to be reasonable.

Localized and Itinerant States in Actinide Materials

We have developed the Angle Resolved Photoemission Spectroscopy (ARPES) system for transuranic materials. The ARPES transuranic system is an endstation upgrade to the Laser Plasma Light Source (LPLS) at Los Alamos National Laboratory. The LPLS is a tunable light source for photoemission with a photon energy range covering the vacuum ultraviolet (VUV) and soft x-ray regions (27-140 eV). The LPLS was designed and developed for transuranic materials. Transuranic photoemission is currently not permitted at the public synchrotrons worldwide in the VUV energy range due to sample encapsulation requirements. With the addition of the ARPES capability to the LPLS system there is an excellent opportunity to explore new details centered on the electronic structure of actinide and transuranic materials.

Photoemission of surface oxides and hydrides of delta plutonium

Two band renormalization effects, novel in the context of f-electron systems, are presented here. First, the interband scattering mechanism leads to the point-like Fermi surface renormalization along kx/ky directions in the Brillouin zone. Second, in the normal direction the renormalization leads to reduction in the number of Fermi sheets and in creation of additional periodicity in band dispersion. It is shown that such renormalization effects can lead to significant differences between calculated and measured electronic structures, and need to be considered in f-electron systems.