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Register a new userGiant isotropic Nernst effect in an anisotropic Kondo semimetal
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Added by
Silke Buehler-Paschen
Publication date
2016
fields
Physics
and research's language is
English
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The failed Kondo insulator CeNiSn has long been suspected to be a nodal metal, with a node in the hybridization matrix elements. Here we carry out a series of Nernst effect experiments to delineate whether the severely anisotropic magnetotransport coefficients do indeed derive from a nodal metal or can simply be explained by a highly anisotropic Fermi surface. Our experiments reveal that despite an almost 20-fold anisotropy in the Hall conductivity, the large Nernst signal is isotropic. Taken in conjunction with the magnetotransport anisotropy, these results provide strong support for an isotropic Fermi surface with a large anisotropy in quasiparticle mass derived from a nodal hybridization.
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Nontrivial topology in condensed matter systems enriches quantum states of matter, to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry broken phases by Landaus order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal Ce3Bi4Pd3 that is noncentrosymmetric but preserves time reversal symmetry. We attribute this finding to Weyl nodes - singularities of the Berry curvature - that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields, as well as its robust bulk nature may aid the exploitation in topological quantum devices.
Negative thermal expansion is an unusual phenomenon appearing in only a handful of materials, but pursuit and mastery of the phenomenon holds great promise for applications across disciplines and industries. Here we report use of X-ray spectroscopy and diffraction to investigate the 4f-electronic properties in Y-doped SmS and employ the Kondo volume collapse model to interpret the results. Our measurements reveal an unparalleled decrease of the bulk Sm valence by over 20% at low temperatures in the mixed-valent golden phase, which we show is caused by a strong coupling between an emergent Kondo lattice state and a large isotropic volume change. The amplitude and temperature range of the negative thermal expansion appear strongly dependent on the Y concentration and the associated chemical disorder, providing control over the observed effect. This finding opens new avenues for the design of Kondo lattice materials with tunable, giant and isotropic negative thermal expansion.
In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$_2$MnGa, reaching $S_{yx}sim -6$ $mu$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $alpha_{yx}$ shows a crossover between $T$-linear and $-T log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $alpha_{yx}$. The $T$ dependence of $alpha_{yx}$ in experiments and numerical calculations can be understood in terms of a quantum critical scaling function predicted by the low energy effective theory over more than a decade of temperatures. Moreover, the observation of chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.
Zero-field muSR experiments in the heavy-fermion alloys Ce_{1-x}La_xAl_3, x = 0 and 0.2, examine a recent proposal that the system exhibits a strong anisotropic Kondo effect. We resolve a damped oscillatory component for both La concentrations, indicative of disordered antiferromagnetism. For x = 0.2 the oscillation frequency decreases smoothly with increasing temperature, and vanishes at the specific heat anomaly temperature T* approx 2.2 K. Our results are consistent with the view that T* is due to a magnetic transition rather than anisotropic Kondo behavior.
The recently proposed novel materials class called Weyl-Kondo semimetal (WKSM) is a time reversal invariant but inversion symmetry broken Kondo semimetal in which Weyl nodes are pushed to the Fermi level by the Kondo interaction. Here we explore whether CeCu$_{1+x}$As$_2$ may be a new WKSM candidate. We report on its single-crystal growth, structure determination and physical properties investigation. Previously published studies on polycrystalline samples suggest that it is indeed a Kondo semimetal, which is confirmed by our investigations on single crystals. X-ray diffraction reveals that CeCu$_{1+x}$As$_2$ crystallizes in a tetragonal centrosymmetric structure, although the inversion symmetry could still be broken locally due to partially occupied Cu sites. Chemical analysis results in an average occupation $x$ = 0.11(1). The electrical resistivity increases logarithmically with decreasing temperature, and saturates below 10 K. A Kondo temperature $T_{mathrm{K}}$ $approx$ 4 K is extracted from entropy, estimated from the specific heat measurements. From Hall effect experiments, a charge carrier density of $8.8 times 10^{20}$ cm$^{-3}$ is extracted, a value characteristic of a semimetal. The magnetization shows pronounced anisotropy, with no evidence of magnetic ordering down to 0.4 K. We thus classify CeCu$_{1.11}$As$_2$ as a tetragonal Kondo semimetal with anisotropic magnetic properties, with a possibly broken inversion symmetry, thus fulfilling the necessary conditions for a WKSM state.
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