arXiv subjects

K. Kummer

Publications and source records attributed to K. Kummer.

40 records · Page 3Linked to original sources

arXiv2013

Interplay of Dirac fermions and heavy quasiparticles in solids

Many-body interactions in crystalline solids can be conveniently described in terms of quasiparticles with strongly renormalized masses as compared to those of non-interacting particles. Examples of extreme mass renormalization are on the one hand graphene, where the charge carriers obey the linear dispersion relation of massless Dirac fermions, and on the other hand heavy-fermion materials where the effective electron mass approaches the mass of a proton. Here we show that both extremes, Dirac fermions like they are found in graphene and extremely-heavy quasiparticles characteristic for Kondo materials, may not only coexist in a solid but can undergo strong mutual interactions. Using the example of EuRh2Si2 we explicitly demonstrate that these interactions can take place at the surface and in the bulk. The presence of the linear dispersion is imposed solely by the crystal symmetry while the existence of heavy quasiparticles is caused by the localized nature of the 4f states.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.supr-con↗

arXiv2011

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).

cond-mat.str-el↗

arXiv2010

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.

cond-mat.supr-con↗