A Tight - Binding

PACS. 71.20.-b – Electron density of states and band structure of crystalline solids. Abstract. – We perform an orthogonal basis tight binding fit to an LAPW calculation of paramagnetic NaxCoO2 for several dopings. The optimal position of the apical oxygen at each doping is resolved, revealing a non-trivial dependence of the band structure and Fermi surface on oxygen height. We find that the small e g ′ hole pockets are preserved throughout all investigated dopings and discuss some possible reasons for the lack of experimental evidence for these Fermi sheets. The compound Na x CoO 2 has been synthesized for a wide range of Na contents, 0.3 < x < 0.9, and exhibits a variety of unusual phenomena that are strongly dependent on doping level. Study of Na 0.5 CoO 2 for its surprisingly good thermoelectric properties began several years ago [1] and more recently, attention has been focused on the hydrated compound Na 0.35 CoO 2 ·1.3H 2 O which undergoes a superconducting phase transition at T ∼ 4.5K [2, 3]. The behavior and characteristics of Na x CoO 2 do not vary smoothly along the spectrum of Na contents, but instead split very generally into two separate metallic phases separated at x=0.5 by an insulating phase [4]. The low Na content region at x <0.5 is characterized by conventional metallic behavior [5] and a nearly temperature independent susceptibility [6, 8] indicating a Pauli paramagnetic precursor to the superconducting state. The linear specific heat coefficient γ is measured to be ∼ 12-16 mJ molK 2 which shows only light mass renormalization [6,7]. In contrast, the high Na region at x > 0.5, though also metallic, exhibits a Curie-Weiss-like susceptibility [9, 10, 12]. A higher (compared to the previous metallic phase) measured γ of 25-30 mJ molK 2 indicates that mass renormalization in this region is substantial [9, 12, 15]. Interestingly, the Kadawaki-Woods ratio, a comparison of γ to the quadratic coefficient of resisitivity, which is often used as a measure of electron-electron interaction, is reported to be the largest ever measured and considerably field dependent [16]. This may evidence a nearby quantum critical point and indeed, µsR and transport measurements show a magnetic transition in single crystals at T=22K for dopings of x= 0.7 and x= 0.75, usually interpreted as a spin density wave [10, 13, 14]. Strong ferromagnetic spin fluctuations have been predicted based on LDA calculations [11] …