45 research outputs found
Wave-vector dependent intensity variations of the Kondo peak in photoemission from CePd
Strong angle-dependent intensity variations of the Fermi-level feature are
observed in 4d - 4f resonant photoemission spectra of CePd(111), that
reveal the periodicity of the lattice and largest intensity close to the Gamma
points of the surface Brillouin zone. In the framework of a simplified periodic
Anderson model the phenomena may quantitatively be described by a wave-vector
dependence of the electron hopping matrix elements caused by Fermi-level
crossings of non-4f-derived energy bands
CeFePO: f-d hybridization and quenching of superconductivity
Being homologue to the new, Fe-based type of high-temperature
superconductors, CeFePO exhibits magnetism, Kondo and heavy-fermion phenomena.
We experimentally studied the electronic structure of CeFePO by means of
angle-resolved photoemission spectroscopy. In particular, contributions of the
Ce 4f-derived states and their hybridization to the Fe 3d bands were explored
using both symmetry selection rules for excitation and their photoionization
cross-section variations as a function of photon energy. It was experimentally
found - and later on confirmed by LDA as well as DMFT calculations - that the
Ce 4f states hybridize to the Fe 3d states of d_{3z^2-r^2} symmetry near the
Fermi level that discloses their participation in the occurring
electron-correlation phenomena and provides insight into mechanism of
superconductivity in oxopnictides.Comment: 5 pages, 3 figure
How chemical pressure affects the fundamental properties of rare-earth pnictides: an ARPES view
Angle-resolved photoelectron spectroscopy, supplemented by theoretical
calculations has been applied to study the electronic structure of
heavy-fermion material CeFePO, a homologue to the Fe-based high-temperature
superconductors, and CeFeAs_0.7P_0.3O, where the applied chemical pressure
results in a ferromagnetic order of the 4f moments. A comparative analysis
reveals characteristic differences in the Fe-derived band structure for these
materials, implying a rather different hybridization of valence electrons to
the localized 4f orbitals. In particular, our results suggest that the
ferromagnetism of Ce moments in CeFeAs_0.7P_0.3O is mediated mainly by Fe
3d_xz/yz orbitals, while the Kondo screening in CeFePO is instead due to a
strong interaction of Fe 3d_3z^2-r^2 orbitals.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. B (Rapid
Evolution of magnetism in Yb(Rh_(1-x)Co_x)2Si2
We present a study of the evolution of magnetism from the quantum critical
system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of
Yb(Rh_(1-x)Co_x)2Si2 were grown for 0 < x < 1 and studied by means of magnetic
susceptibility, electrical resistivity, and specific heat measurements, as well
as photoemission spectroscopy. The results evidence a complex magnetic phase
diagram, with a non-monotonic evolution of T_N and two successive transitions
for some compositions resulting in two tricritical points. The strong
similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co
substitution basically corresponds to the application of positive chemical
pressure. Analysis of the data proves a strong reduction of the Kondo
temperature T_K with increasing Co content, T_K becoming smaller than T_N for x
~ 0.5, implying a strong localization of the 4f electrons. Furthermore,
low-temperature susceptibility data confirm a competition between ferromagnetic
and antiferromagnetic exchange. The series Yb(Rh_(1-x)Co_x)2Si2 provides an
excellent experimental opportunity to gain a deeper understanding of the
magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the
antiferromagnetic phase disappears (T_N=>0).Comment: 11 pages, 9 figure
Band Calculations for Ce Compounds with AuCu-type Crystal Structure on the basis of Dynamical Mean Field Theory I. CePd and CeRh
Band calculations for Ce compounds with the AuCu-type crystal structure
were carried out on the basis of dynamical mean field theory (DMFT). The
auxiliary impurity problem was solved by a method named NCAvc
(noncrossing approximation including the state as a vertex correction).
The calculations take into account the crystal-field splitting, the spin-orbit
interaction, and the correct exchange process of the virtual excitation. These are necessary features in the
quantitative band theory for Ce compounds and in the calculation of their
excitation spectra. The results of applying the calculation to CePd and
CeRh are presented as the first in a series of papers. The experimental
results of the photoemission spectrum (PES), the inverse PES, the
angle-resolved PES, and the magnetic excitation spectra were reasonably
reproduced by the first-principles DMFT band calculation. At low temperatures,
the Fermi surface (FS) structure of CePd is similar to that of the band
obtained by the local density approximation. It gradually changes into a form
that is similar to the FS of LaPd as the temperature increases, since the
band shifts to the high-energy side and the lifetime broadening becomes
large.}Comment: 12 pasges, 13 figure
Divalent EuRh2Si2 as a reference for the Luttinger theorem and antiferromagnetism in trivalent heavy-fermion YbRh2Si2
Application of the Luttinger theorem to the Kondo lattice YbRh2Si2 suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local-moment antiferromagnet (AFM) EuRh2Si2 in its PM regime. Here we show by angle-resolved photoemission spectroscopy that paramagnetic EuRh2Si2 has a large FS essentially similar to the one seen in YbRh2Si2 down to 1 K. In EuRh2Si2 the onset of AFM order below 24.5 K induces an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Fermi-surface sheets. Our results on EuRh2Si2 indicate that the formation of the AFM state in YbRh2Si2 is very likely also connected with similar changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system
Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs
We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds
in relation to crystal structure. The two compounds crystallize in two
structural polymorphs, both tetragonal. The high temperature polymorph (HTP)
adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low
temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying
polycrystals prepared by arc melting we have observed that the rapidly cooled
samples retain the HTP even at room temperature (RT) and below. Annealing such
samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP
and LTP were subsequently studied with respect to magnetism and
superconductivity by electrical resistivity, magnetization, AC susceptibility
and specific heat measurements. The HTP and LTP of both compounds respectively,
behave as Pauli paramagnets. Superconductivity has been found exclusively in
the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in
LaIr2Si2). The relations of magnetism and superconductivity with the electronic
and crystal structure are discussed with comparing experimental data with the
results of first principles electronic structure calculations