{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"5907a1875ee753ff4e9d1a939aae4a09490e286934f2448021aea0e3362c9fe7","created":"2026-10-03T07:17:58Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"17b14ea49a19e3e7fa7c19400aaa331b067a83b9edb88e4677ddb20972e6bd12","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"cm.amorphous-low-temperature.universal-tunneling-strength","kind":"phenomenon","literature_status":"open","n":"1","parents":[],"plain":"In every glass tested, sound waves lose nearly the same small fraction of their energy per cycle at low temperature, within a factor of about twenty. The standard tunneling model fits this but gives no reason why the number should be universal.","posed_since":"","precise":"Below $\\sim 1\\,\\mathrm{K}$ amorphous solids show specific heat $C \\sim T$, thermal conductivity $\\kappa \\sim T^{2}$ and an internal-friction plateau $Q^{-1} = (\\pi/2) C_{\\mathrm{tun}}$, with tunneling strength $C_{\\mathrm{tun}} = P \\gamma^{2}/(\\rho v^{2})$ between $\\sim 1e-4$ and $1e-3$ across chemically different glasses ($P$ = density of states of tunneling two-level systems, $\\gamma$ = deformation potential, $\\rho$ = mass density, $v$ = sound velocity); equivalently the phonon mean free path is $\\sim 150$ wavelengths. Explain this universality, e.g. by interaction-induced renormalization of the defects (Yu and Leggett; Burin and Kagan) or by a generic soft-mode mechanism, and predict where it fails.","problem_ref":null,"references":"","settled_by":"A theory deriving $C_{\\mathrm{tun}} \\sim 1e-4 \\text{ to } 1e-3$ from generic properties of glasses, confirmed by a predicted exception or by simulations that compute $P$ and $\\gamma$ in several model glasses.","status_note":"","title":"Why is sound absorption by tunneling defects the same in all glasses?","topic_ref":"d5ba98a0b7fd03c8a459d017fd4e5e33d6f390cd71005b8a63b71f0973f4abf6"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"dc48db54751644a4b489cac4f7984f838ea0d878538ad9c93f9a62cc569827ef","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"f79a31b463ab9c72fa044356cac9e582f71d3fb212c810939d95e1b3dde90e47","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"u8BJIX5DP9mV7N_NoZlgSc9K_X2L_m9awfnqElbCKd7Q4vaIiJPxGPKp6Rsn7HdN7dSV5UZ5SsDNQACa3T3WCA"},"schema":"pubphys.envelope/1"},"record_hash":"dc48db54751644a4b489cac4f7984f838ea0d878538ad9c93f9a62cc569827ef","leaf_index":908}