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]

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy (ARPES) with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe2As2 without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.

Physical properties of polycrystalline CeCrGe3 and LaCrGe3 have been investigated by x-ray absorption spectroscopy, magnetic susceptibility χ(T ), isothermal magnetization M(H), electrical resistivity ρ(T ), specific heat C(T ) and thermoelectric power S(T ) measurements. These compounds are found to crystallize in the hexagonal perovskite structure (space group P63/mmc), as previously reported. The ρ(T ), χ(T ) and C(T ) data confirm the bulk ferromagnetic ordering of itinerant Cr moments in LaCrGe3 and CeCrGe3 with TC = 90 K and 70 K respectively. In addition a weak anomaly is also observed near 3 K in the C(T ) data of CeCrGe3. The T dependences of ρ and finite values of Sommerfeld coefficient γ obtained from the specific heat measurements confirm that both the compounds are of metallic character. Further, the T dependence of ρ of CeCrGe3 reflects a Kondo lattice behavior. An enhanced γ of 130 mJ/mol K 2 together with the Kondo lattice behavior inferred from the ρ(T ) establish CeCrGe3 as a moderate heavy fermion compound with a quasiparticle mass renormalization factor of ∼ 45.

We have measured the thermopower across the metamagnetic transition of the heavy fermion compound CeRu2Si2 at temperatures down to 0.1K and magnetic fields up to 11.5T. We find a large negative enhancement of the thermopower on crossing the metamagnetic field, as well as a sudden change in slope. We argue that this is consistent with the Zeeman-driven deformation of the Fermi surface through a topological transition. The field dependence of the thermopower highlights the discrepancy between thermodynamic and transport properties across the metamagnetic transition.Comment: 5 pages, 4 figure

We study the band structure of twinned and detwinned BaFe2As2 using angle-resolved photoemission spectroscopy (ARPES). The combination of measurements in the ordered and normal state along four high-symmetry momentum directions Γ/Z-X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the dxz and dyz orbitals as well as folding due to magnetic order with a wave vector of (π, π.π). We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe2As2.