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تسجيل مستخدم جديدKondo exhaustion and conductive surface states in antiferromagnetic YbIr$_3$Si$_7$
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The interplay of Kondo screening and magnetic ordering in strongly correlated materials containing local moments is a subtle problem.[1] Usually the number of conduction electrons matches or exceeds the number of moments, and a Kondo-screened heavy Fermi liquid develops at low temperatures.[2] Changing the pressure, magnetic field, or chemical doping can displace this heavy Fermi liquid in favor of a magnetically ordered state.[3,4] Here we report the discovery of a version of such a `Kondo lattice material, YbIr$_3$Si$_7$, in which the number of free charge carriers is much less than the number of local moments. This leads to `Kondo exhaustion:[5] the electrical conductivity tends to zero at low temperatures as all the free carriers are consumed in the formation of Kondo singlets. This effect coexists with antiferromagnetic long-range order, with a Neel temperature $Trm_N = 4.1,{rm K}$. Furthermore, the material shows conductive surface states with potential topological nature, and thus presents an exciting topic for future investigations.
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Materials where localized magnetic moments are coupled to itinerant electrons, the so-called Kondo lattice materials, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, including unconventi
onal superconductivity, strange metals, and correlated topological phases of matter. Here, we report what appears to be electron fractionalization in insulating Kondo lattice material YbIr$_3$Si$_7$, with emergent neutral excitations that carry heat but not electric current and contribute to metal-like specific heat. We show that these neutral particles change their properties as the material undergoes a transformation between two antiferromagnetic phases in an applied magnetic field. In the low-field AF-I phase, we find that the low temperature linear specific heat coefficient $gamma$ and the residual linear term in the thermal conductivity $kappa/T(Trightarrow 0)$ are finite, demonstrating itinerant gapless excitations. These results, along with a spectacular violation of the Wiedemann-Franz law, directly indicate that YbIr$_3$Si$_7$ is a charge insulator but a thermal metal. Nuclear magnetic resonance spectrum reveals a spin-flop transition to a high field AF-II phase. Near the transition field, $gamma$ is significantly enhanced. Most surprisingly, inside the AF-II phase, $kappa/T$ exhibits a sharp drop below $sim300$ mK, indicating either opening of a tiny gap or a linearly vanishing density of states. This finding demonstrates a transition from a thermal metal into an insulator/semimetal driven by the spin-flop magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.
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