Ulrike Stockert

Thermal and electrical transport across a magnetic quantum critical point

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 YbRh2Si2 (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.

Introducing a Magnetic Guest to a Tetrel-Free Clathrate: Synthesis, Structure, and Properties of EuxBa8–xCu16P30 (0 ≤ x ≤ 1.5)

The europium-containing clathrate-I Eu(x)Ba(8-x)Cu(16)P(30) was synthesized from the elements. Powder X-ray diffraction in combination with energy dispersive X-ray absorption spectroscopy (EDXS) and metallographic studies showed the homogeneity range with x ≤ 1.5. Determination of the crystal structure confirmed the presence of an orthorhombic superstructure of clathrate-I and revealed that Eu atoms exclusively resided in small pentagonal-dodecahedral cages. Magnetic measurements together with X-ray absorption spectroscopy are consistent with a 4f(7) (Eu(2+)) ground state for Eu(x)Ba(8-x)Cu(16)P(30). Below 3 K the Eu moments order antiferromagnetically. Resistivity measurements revealed metallic behavior of the investigated clathrate, in line with the composition deviating from the Zintl counting scheme. Local vibrations of the guest atoms inside the cages are analyzed with the help of specific heat investigations.

Specific heat and angle-resolved photoemission spectroscopy study of the superconducting gaps in LiFeAs

U. Stockert, 2, ∗ M. Abdel-Hafiez, D. V. Evtushinsky, V. B. Zabolotnyy, A. U. B. Wolter, S. Wurmehl, I. Morozov, 1 R. Klingeler, S. V. Borisenko, and B. Büchner Institute for Solid State Research, IFW Dresden, D-01171 Dresden, Germany MPI for Chemical Physics of Solids, D-01187 Dresden, Germany Moscow State University, Moscow 119991, Russia Kirchhoff Institute for Physics, Heidelberg University, D-69120 Heidelberg (Dated: November 19, 2010)

Interplay between Kondo suppression and Lifshitz transitions in YbRh2Si2 at high magnetic fields.

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.

Single-ion Kondo Scaling of the Coherent Fermi Liquid Regime in Ce1 xLaxNi2Ge2

Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi(2)Ge(2) were studied in a wide temperature range. The Ce-rich alloys Ce(1-x)La(x)Ni(2)Ge(2) were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions (0.7≤x≤0.9), non-Fermi liquid properties have been observed, followed by the local Fermi liquid behavior in the dilute limit. The 4f-electron contribution to the specific heat was found to follow the predictions of the Kondo-impurity model in both the local as well as the coherent regimes, with the characteristic Kondo temperature decreasing rapidly from about 30 K for the parent compound CeNi(2)Ge(2) to about 1 K in the most dilute samples. The specific heat does not show any evidence for the emergence of a new characteristic energy scale related to the formation of the coherent Kondo lattice.

Pr magnetism and its interplay with the Fe spin-density wave in PrFeAsO1−xFx (x=0,0.15)

We have studied the magnetism of the Pr

Calorimetric study of the superconducting and normal state properties of Ca(Fe1−xCox)2As2

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Superconducting gap structure of the skutterudite LaPt4Ge12 probed by specific heat and thermal transport

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Local antiferromagnetic correlations in the iron pnictide superconductors LaFeAsO 1 − x F x and Ca ( Fe 1 − x Co x ) 2 As 2 as seen via normal-state susceptibility

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Evidence of a Kondo Destroying Quantum Critical Point in YbRh2Si2

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