A quantum critical point (QCP) arises when a continuous transition between competing phases occurs at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi-liquid description, which is the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations that possess the quantum numbers of the non-interacting electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh(2)Si(2) (refs 2, 3). We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann-Franz law for magnetic fields above the critical field at which the QCP is attained. This is also expected for magnetic fields below the critical field, where weak antiferromagnetic order and a Fermi-liquid phase form below 0.07 K (at zero field). At the critical field, however, the low-temperature electrical conductivity exceeds the thermal conductivity by about 10 per cent, suggestive of a non-Fermi-liquid ground state. This apparent violation of the Wiedemann-Franz law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles no longer holds. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations--these insights could be relevant to understanding other deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.

Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition proved to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here we report a detailed study of the electronic structure of fully detwnned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven bandreconstruction involving dxz, dyz and dxy orbitals. The nematic energy scale between dxz and dyz bands reach a maximum of 50meV at the Brillouin zone corner. We are also able to track the dxz electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe.

We present a new Kondo-lattice system, YbNi 4 P 2 , which is a clean heavy-fermion metal with a severely reduced ferromagnetic ordering temperature at T C = 0.17 K, evidenced by distinct anomalies in susceptibility, specific-heat, and resistivity measurements. The ferromagnetic nature of the transition, with only a small ordered moment of ∼ 0.05 µ B , is established by a diverging susceptibility at T C with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi 4 P 2 is a stoichiometric system with a quasi-one-dimensional crystal and electronic structure and strong correlation effects which dominate the low temperature properties. This is reflected by a stronger-thanlogarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above T C which cannot be explained by any current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and makes this an interesting material for further in-depth investigations. arXiv:1108.4274v1 [cond-mat.str-el]