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

本文<an><4n>用电导率、磁阻、热电势、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_ NaH、C_ KH_x和C_8RbH_x的电子性质。在钠化合物中，热电功率和c轴电导率表明石墨平面上的π-载流子浓度与同构钾化合物相比相当小。这些实验也证明了插层导电载流子的存在。根据这些事实，Na^+-H^--Na^+夹心三原子层插层具有Na/H=1的化学计量组成，它们插入石墨层之间，具有完整的离子晶格和小浓度的剩余电荷，使得从插层到石墨层的电荷转移速率显著降低。钾化合物的磁阻和霍尔电阻揭示了两种载流子的存在下，多数电子载流子的石墨π带和少数空穴载流子的氢1 s带。根据双载流子模型分析了磁电阻的行为。这两种载流子的迁移率都在-10^3cm ^2/V·s之间，但它们的温度依赖性不同。大的迁移率的garaphitic π-载流子与它们的轻质量范围-0.1m_e。在离子型K^+-H^--K^+夹层三原子层晶格上，氢空穴载流子的电子-声子耦合很小，约为-0.4eV，这导致了它们的迁移率很大。c轴电导率的压力依赖性表明，c轴导电机制主要由杂质辅助跳跃过程，其中石墨带和插层带之间的波函数的重叠起主导作用。

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.