Novel two-dimensional metallic hydrogen in alkali-metal-hydrogen-graphite intercalation compounds.

用电导率、磁阻、热电动势、电子比热、ESR和磁化率等方法研究了氢化碱金属石墨层间化合物C_&lt；An&gt；NaH，C_&lt；4N&gt；KH_x和C_8RbH_x的电学性质。在钠化合物中，温差电势和c轴电导表明，与等结构的钾化合物相比，在石墨化平面上的pi载流子浓度相当小。这些实验也证明了插层导电载流子的缺失。根据这些事实，化学计量比为Na/H=1的Na^-H^-Na^夹心三原子层插入石墨层之间，具有完整的离子晶格，剩余电荷浓度很小，从而大大降低了从夹层到石墨层的电荷转移速率。化合物的磁阻和霍尔电阻表明存在两种载流子：大部分电子载流子位于石墨化pi带，少数空穴载流子位于氢1s带。用双载流子模型分析了磁阻的行为。这两种载流子的迁移率估计范围为-10^3 cm~2/V.s，但与温度的关系不同。钙磷质pi载流子的大迁移率与它们的质量范围-0.1Me有关，离子K^-H^-K^夹心三原子层晶格上的氢空穴载流子的小电声耦合-0.4 eV导致了它们的大迁移率。C轴电阻率与压力的关系表明，c轴导电机制主要由杂质辅助的跳跃过程控制，其中石墨化带和插层带之间的波函数重叠起主导作用。

We investigated electronic properties of hydrogenated-alkali-metal-graphite intercalation compounds, C_<an>NaH, C_<4n>KH_x and C_8RbH_x, by means of electrical conductivities, magnetoresistances, thermoelectric powers, electronic specific heats, ESR, and magnetic susceptibilities. In sodium compounds, thermoelectric powers and c-axis conductivities suggest that the pi-carrier concentration on graphitic planes is considerably small in comparison with that of the isostructural potassium compounds. The absence of intercalate conduction carriers is also demonstrated by these experiments. According to these facts, Na^+-H^--Na^+ sandwiched triple atomic layer intercalates with a stoichiometric composition of Na/H=1, which are inserted between graphitic layers, have a complete ionic lattice with a small concentration of remaining charges, so that the charge transfer rate from the intercalates to graphitic layers is considerably reduced. The magnetoresistances and Hall resistances of the potassium compounds revealed the presence of two kinds of carriers; majority electron carriers on the graphitic pi-band and minority hole carriers on the hydrogen 1s band. The behaviors of the magnetoresistances are analyzed in terms of the two-carrier model. The estimated mobilities of these two kinds of carriers range -10^3cm^2/V.s with different temperature dependence from each other. The large mobilities of the garaphitic pi-carriers are associated with their light mass ranging -0.1m_e. The small electron-phonon coupling -0.4eV for the hydrogen hole carriers on the ionic K^+-H^--K^+sandwiched triple atomic layer lattice causes their large mobilities. The pressure dependence of c-axis resistivities suggests that the c-axis conduction mechanism is governed mainly by the impurity-assisted hopping process, where the overlap of wave functions between the graphitic band and the intercalate band play a dominant role.