Akihisa Koizumi

With the breakdown of coherent Kondo scattering with rising temperature, the Fermi surface (FS) in rare-earth Kondo lattices is expected to transition from the large FS counting the 4f moments, which form entangled quasiparticles with strongly enhanced effective masses with the conduction states, to the small FS with decoupled and localized 4f moments. A direct observation of this transition with temperature has, however, remained elusive, because conventional probes of the FS in Kondo lattices require high magnetic fields, which might reconstruct the FS or cannot work reliably at elevated temperatures. Using high-resolution Compton scattering, we overcome these limitations and show that the FS topology in the prototypical Kondo lattice YbRh2Si2 undergoes pronounced changes between 14 and 300 K in zero magnetic field. In particular, we present clear evidence for a largely restored small FS at room temperature. Our results suggest the relevant energy scale of the complex Kondo crossover phenomenon in YbRh2Si2.

We have measured directional Compton profiles on the (0001) plane in the UPd2Al3 single crystal with hexagonal symmetry in order to investigate the change in electronic structure related to the crossover phenomenon between the itinerant and localized states of 5 f electrons. Two-dimensional electron occupation number densities (2D-EONDs), which are the projection of Fermi volumes on the view plane, are obtained at 20 and 100 K through electron momentum reconstruction and Lock–Crisp–West folding analysis. We have also derived theoretical 2D-EONDs from the results of band calculations with the linear muffin-tin orbital method for comparison. The experimental 2D-EONDs at 20 and 100 K are well described by the theoretical ones based on the itinerant and localized models of 5 f electrons, respectively. Therefore, the contributions of 5 f electrons to the Fermi surfaces in the itinerant state are verified around Γ (A) points in the first Brillouin zone through comparison.

We have measured directional Compton profiles on the (001) plane in URu2Si2 single crystal at several temperatures. Two-dimensional electron occupation number densities (2D-EONDs) were obtained from the profiles through electron momentum reconstruction and Lock–Crisp–West folding analyses. We have also performed band calculations based on 5 f-electron itinerant and localized models and derived theoretical 2D-EONDs for comparison. The experimental 2D-EOND at 300 K is well described by the localized model, and the 2D-EOND at 10 K is consistent with the theoretical one based on the itinerant model. The difference between 2D-EONDs at 30 and 100 K reflects a gradual change in the electronic structure, which reveals some of the crossover phenomena from localized to itinerant states. The change from localized to itinerant states is also reflected in a B(r) function, which is obtained in the reconstruction analysis and is an autocorrelation function of the wave function in the position space. The process by which the electronic structure in URu2Si2 changes is demonstrated through a series of experimental results.

A cryostat was constructed for high-resolution X-ray Compton scattering measurements at temperature down to 1.7 K, in order to investigate superfluid helium-4. Compton profiles of helium were measured using synchrotron X-rays for gas and liquid phases, respectively. In the measurement of the liquid phase, we succeeded in measuring the Compton profile of the superfluid helium at 1.7 K. Comparison of the results with theoretical calculation reveals importance of many-body effects beyond the mean-field treatment of electron systems.