We have studied the electronic structure of cleaved (100) single crystals of UAsxSe1−x, UxTh1−xSb, UxY1−xSb, and USbxTe1−x with high resolution (ΔE<150 meV) photoemission using synchrotron radiation 10⩽hν⩽130 eV. In contrast to the general belief we find significant differences in the photoemission density of states near EF for these compounds. We conclude that these differences are mainly due to valence d electrons and that U has a quasi‐localized 5f3 configuration in resonance with itinerant 6d states. This explains the quite similar magnetic behavior of UxTh1−xSb and USbxTe1−x, both of which change from highly anisotropic 〈100〉 antiferromagnets (x≳0.85) to 〈111〉 ferromagnets (x≲0.85), i.e., with an increasing number of 6d electrons. Namely, the smaller ordered moment in UTe and the gradually decreasing moment in UxTh1−xSb for 1.0⩾x⩾0.3 are attributed to the 6d moments which are polarized antiparallel to the 5f moments.

Electronic structure and magnetism of actinide compounds: 6d versus 5f electrons (invited)

High pressure X-ray diffraction studies were performed on AnX compounds (An ≡ Np, Pu; X ≡ Sb, Te) in the pressure range up to about 57 GPa, at room temperature, using a diamond anvil cell in an energy-dispersive X-ray diffraction facility. These AnX compounds have an f.c.c., NaCl-type structure at ambient pressure and undergo first-order phase transitions under pressure. Whereas NpTe and PuTe transform to a primitive cubic cell of the CsCl type at 13 GPa and 15 GPa respectively, a tetragonal structure, space group P4/mmm, is observed for NpSb above 10 GPa. This tetragonal structure can be regarded as a “distorted” CsCl-type structure, the atomic positions being the same as in the cubic CsCl cell. In PuSb two transitions are detected: the CsCl-type structure is observed above 17 GPa; then PuSb transforms to the tetragonal P4/mmm structure at about 42 GPa. In addition, the compressibility of the low pressure phase was determined from the V(p) data for each compound.

Phase transformation of AnX compounds under high pressure (An ≡ Np, Pu; X ≡ Sb, Te)

In the present investigation the B1–B2 phase transition pressure in four binary 1:1 actinide-arsenide compounds has been predicted. The calculated compression curves and values of different high pressure properties for these compounds viz: ThAs, UAs, NpAs and PuAs are reported and compared with the experimental values. The calculated values of elastic properties show predominantly ionic nature of these compounds. The results achieved in the present study are found in good agreement with the available experimental data.

High-pressure behavior of some binary 1:1 actinide-arsenide compounds

Uranium monochalcogenides and monopnictides crystallize in the NaCl-type structure and exhibit ferromagnetic and antiferromagnetic order, respectively. These series reveal interesting properties such as Kondo behavior of UTe. However, such interesting properties are much sample dependent. We grew single crystals of USb and UTe with high purity using the Bridgman technique, and measured transport properties and specific heat.

Transport properties and specific heat of UTe and USb

Recent progress in studies on low carrier concentration systems of magnetically ordered and non-magnetic heavy Fermion metals are discussed. Successive steps in the high field magnetisation, the magnetic structure, and the influence of high pressure are reported and the relation with the observed Fermi surface for CeP is discussed. The origin of the low carrier concentration in USb is also discussed based on the first observation of dHvA oscillations. It is also unambiguously demonstrated that the unconventional heavy Fermion state of Yb4As3 is intrinsic and not due to impurities.

Anomalous physical properties of the low carrier concentration state in f-electron systems

Angular resolved photoemission on an in situ cleaved USb single crystal was undertaken using synchroton radiation from 20 to 130 eV. A good correspondence between the observed and calculated peaks is obtained. Very strong enhancement of uranium 5f photoemission is observed for hv = 98 eV. We interpret this effect as an interference between the two excitation channels 5f + hv → eg and 5d + hv → 5f giving rise to a Fano-type profile for the intensity of the U-5f line.

M. Belakhovsky

Papers

Electronic structure and fano-type resonance effects in uranium antimonide studied by photoemission with synchrotron radiation