Dependence of the specific heat of Na x CoO 2 ∙ y H 2 O ∕ D 2 <mml:mi …
2005; American Physical Society; Volume: 72; Issue: 6 Linguagem: Inglês
10.1103/physrevb.72.060512
ISSN1550-235X
AutoresRongying Jin, B. C. Sales, Shiliang Li, David Mandrus,
Tópico(s)Magnetic and transport properties of perovskites and related materials
ResumoWe report specific heat measurements down to 0.4 K on the layered oxide ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}∙y{\mathrm{H}}_{2}\mathrm{O}∕{\mathrm{D}}_{2}\mathrm{O}$ with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.74$ and $y=0$ and 1.4. For the nonhydrated system $(y=0)$, the electronic specific heat coefficient ${\ensuremath{\gamma}}_{N}$ and the Debye temperature ${\ensuremath{\Theta}}_{D}$ vary nonmonotonically with $x$, both displaying minima when $x$ is close to 0.5. This indicates a systematic change of the electronic and vibrational structures with Na content. For both hydrated and deuterated systems ($x=0.35$ and $y=1.4$), the specific heat reveals a sharp peak with $\ensuremath{\Delta}{C}_{p}\ensuremath{\sim}45.5\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}\mathrm{K}$ at ${T\phantom{\rule{0.1em}{0ex}}}_{c}^{\mathit{mid}}\ensuremath{\sim}4.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and an anomaly at ${T}_{x}\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. While the origin of the latter is not clear, the former corresponds to the superconducting transition. With the application of magnetic fields up to 14 Tesla, ${T}_{c}$ decreases gradually but ${T}_{x}$ remains more or less unchanged. The implication of these results is discussed.
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