{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"3341b0412754e9d6d60b0b8f6c25c9d5d5200d699004e64f7250483e5e84a69b","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["66594023fc0f40066193d516f5abcf6647baaf01576d0e0e936fb54f460180bf","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"a04c5f4c9753cdd62142351f21d25b0d134af4ca8adf7022a1b705335d415629","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"yes-no","assisted_by":[],"external_id":"cm.heavy-fermions.ybrh2si2-fermi-jump","kind":"well-posed","literature_status":"contested","n":"2","parents":[],"plain":"Hall-effect data suggest that the number of electrons in the Fermi sea of $\\mathrm{YbRh_2Si_2}$ changes abruptly at a critical magnetic field when cooled to zero temperature.","posed_since":"2004","precise":"In $\\mathrm{YbRh_2Si_2}$ at $T \\to 0$ across the critical field $B_c \\sim 600\\,\\mathrm{G}$ (field in the ab plane), determine whether the Fermi volume changes discontinuously from large (including the 4f hole) to small, as inferred from the Hall crossover whose width scales with $T$. An answer is a $T = 0$ extrapolation of the crossover width of Fermi-volume-sensitive transport (Hall coefficient, thermopower) in crystals with residual resistivity ratio of order 100 or more, or a Fermi-surface measurement at fields close to $B_c$, for example on chemically substituted samples whose critical field lies in the range accessible to quantum oscillations.","problem_ref":"66594023fc0f40066193d516f5abcf6647baaf01576d0e0e936fb54f460180bf","references":"","settled_by":"Hall and thermopower crossover widths measured below 10 mK on the cleanest crystals and extrapolated to $T = 0$, together with Fermi-surface data on substituted samples whose critical field is high enough for quantum oscillations.","status_note":"ARPES (2015) found a large Fermi surface at zero field down to about 1 K, well above the crossover temperatures; no direct Fermi-surface measurement at $B_{\\mathrm{c}}$ exists.","title":"Does the Fermi volume of $\\mathrm{YbRh_2Si_2}$ jump at its critical 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