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We report measurements of in-plane electrical and thermal transport properties in the limit T → 0 near the unconventional quantum critical point in the heavy-fermion metal β-YbAlB4. The high Kondo temperature TK 200 K in this material allows us to probe transport extremely close to the critical point, at unusually small values of T /TK < 5×10 −4 . Here we find that the Wiedemann-Franz law is obeyed at the lowest temperatures, implying that the Landau quasiparticles remain intact in the critical region. At finite temperatures we observe a non-Fermi liquid T-linear dependence of inelastic scattering processes to energies lower than those previously accessed. These processes have a weaker temperature dependence than in comparable heavy fermion quantum critical systems, and suggest a new temperature scale of T ∼ 0.3K which signals a sudden change in character of the inelastic scattering.PACS numbers: 71.27.+a,72.15.Eb, 75.30.Mb The effect of quantum fluctuations on the properties of matter has become an important area of research motivated by the potential for technological advances through harnessing and manipulating quantum mechanical properties. A quantum critical point (QCP) arises when a continuous transition between competing order occurs at zero temperature. Here, strong quantum fluctuations can drive the formation of new phases of matter [1], and may lead to the breakdown of normal Fermiliquid behaviour in metals. Much of our understanding of QCPs comes from the study of heavy-fermion compounds, which are canonical systems for investigating antiferromagnetic quantum criticality. An important open question in these materials is whether multiple types of QCPs exist, differentiated by their microscopic behaviour in the critical regime.The standard model of quantum criticality in metals is the Hertz-Moriya-Millis framework [2], where the suppression of itinerant antiferromagnetic order results in a paramagnetic state with heavy quasiparticles formed as a result of the Kondo screening of f -electron moments by electrons in the conduction band. While this works very well in describing many materials, the presence of localized moments, Fermi surface reconstruction, and diverging effective masses at the magnetic transition in YbRh 2 Si 2 has been interpreted as evidence for a more exotic type of quantum criticality [3]. Alternate frameworks have been proposed where a new energy scale, the 'effective Kondo temperature', collapses at the QCP leading to the breakdown of the heavy electron metal [4]. At the so-called Kondo-breakdown QCP, the Kondo effect is destroyed and the entire Fermi surface is destabilized.Distinguishing between these pictures is a challenging experimental task. One promising approach is to search for the breakdown of the Landau quasiparticle picture via thermal transport measurements, which would provide compelling evidence for the existence of less conventional classes of QCPs. In the Kondo-breakdown model for instance, a fractionalised Fermi liquid [5] emerges and the presence of additional e...

Thermal transport measurements have been made on the spin-ice material Ho(2)Ti(2)O(7) in an applied magnetic field with both the heat current and the field parallel to the [111] direction for temperatures from 50 mK to 1.2 K. A large magnetic field >6 T is applied to suppress the magnetic contribution to the thermal conductivity in order to extract the lattice conductivity. The low field thermal conductivity thus reveals a magnetic field dependent contribution to the conductivity which both transfers heat and scatters phonons. We interpret these magnetic excitations as monopolelike excitations and describe their behavior via existing Debye-Hückel theory.

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We have measured the thermal conductivity of the iron pnictide superconductor LaFePO down to temperatures as low as T =60mK and in magnetic fields up to 5 T. The data shows a large residual contribution that is linear in temperature, consistent with the presence of low energy electronic quasiparticles. We interpret the magnitude of the linear term, as well as the field and temperature dependence of thermal transport in several pairing scenarios. The presence of an unusual supralinear temperature dependence of the electronic thermal conductivity in zero magnetic field, and a high scattering rate with minimal Tc suppression argues for a sign-changing nodal s± state.

We present a compact mechanically robust thermal conductivity measurement apparatus for measurements at low temperatures (<1 K) and high magnetic fields on small high-purity single crystal samples. A high-conductivity copper box is used to enclose the sample and all the components. The box provides protection for the thermometers, heater, and most importantly the sample increasing the portability of the mount. In addition to physical protection, the copper box is also effective at shielding radio frequency electromagnetic interference and thermal radiation, which is essential for low temperature measurements. A printed circuit board in conjunction with a braided ribbon cable is used to organize the delicate wiring and provide mechanical robustness.

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Low temperature thermal conductivity measurements have been conducted on an oxygen annealed single crystal of Yb2Ti2O7 from 60 mK to 50 K and in magnetic fields up to 8 T applied in the [111] crystallographic direction. The temperature dependence of the conductivity in zero field shows a significant peak in thermal conductivity at T ∼ 13 K and a sharp anomaly at Tc ∼ 0.2 K suggesting that the sample's behaviour is representative of the high-purity limit, with low levels of disorder. The magnetic field dependence of the thermal conductivity close to Tc reveals a re-entrant magnetic phase for a field in the [111] direction. With this new information, analysis of the very low magnetic field behaviour of the thermal conductivity suggests the presence of significant fluctuations close to the phase line.