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  1. Abstract Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr 3 Si 7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr 3 Si 7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300โ€‰mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the 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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  2. Abstract. Let ๐˜Ž be a real semisimple algebraic Lie group and ๐˜ a real reductive algebraic subgroup. We describe the pairs (๐˜Ž,๐˜) for which the representation of ๐˜Ž in Lยฒ(๐˜Ž/๐˜) is tempered. The proof gives the complete list of pairs (๐˜Ž,๐˜) for which Lยฒ(๐˜Ž/๐˜) is not tempered. When ๐˜Ž and ๐˜ are complex Lie groups, the temperedness condition is characterized by the fact that ๐˜ต๐˜ฉ๐˜ฆ ๐˜ด๐˜ต๐˜ข๐˜ฃ๐˜ช๐˜ญ๐˜ช๐˜ป๐˜ฆ๐˜ณ ๐˜ช๐˜ฏ ๐˜ ๐˜ฐ๐˜ง ๐˜ข ๐˜จ๐˜ฆ๐˜ฏ๐˜ฆ๐˜ณ๐˜ช๐˜ค ๐˜ฑ๐˜ฐ๐˜ช๐˜ฏ๐˜ต ๐˜ฐ๐˜ฏ ๐˜Ž/๐˜ ๐˜ช๐˜ด ๐˜ท๐˜ช๐˜ณ๐˜ต๐˜ถ๐˜ข๐˜ญ๐˜ญ๐˜บ ๐˜ข๐˜ฃ๐˜ฆ๐˜ญ๐˜ช๐˜ข๐˜ฏ. ๐˜”๐˜ข๐˜ต๐˜ฉ๐˜ฆ๐˜ฎ๐˜ข๐˜ต๐˜ช๐˜ค๐˜ด ๐˜š๐˜ถ๐˜ฃ๐˜ซ๐˜ฆ๐˜ค๐˜ต ๐˜Š๐˜ญ๐˜ข๐˜ด๐˜ด๐˜ช๐˜ง๐˜ช๐˜ค๐˜ข๐˜ต๐˜ช๐˜ฐ๐˜ฏ: Primary 22๐„46; secondary 43๐€85, 22๐…30. ๐˜’๐˜ฆ๐˜บ ๐˜ž๐˜ฐ๐˜ณ๐˜ฅ๐˜ด: Lie groups, homogeneous spaces, tempered representations, unitary representations, matrix coefficients, symmetric spaces. 
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