The phase diagram of non-hydrated NaxCoO2 has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x = 1 2 ) to a 'Curie-Weiss metal' (around 0.70), and finally to a weak-moment magnetically ordered state (x > 0.75). The unusual properties of the state at 1 2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.Research on oxide conductors has uncovered many interesting electronic states characterized by strong interaction, which include unconventional superconductivity, and charge-or spin-ordered states [1,2]. Recently, attention has focussed on the layered cobaltate Na x CoO 2 . At the doping x ∼ 2 3 , Na x CoO 2 exhibits an unusually large thermopower [3]. Although the resistivity is metallic, the magnetic susceptibility displays a surprising Curie-Weiss profile [4], with a magnitude consistent with antiferromagnetically coupled spin-1 2 local moments equal in number to the hole carriers [5]. The thermopower at 2.5 K is observed to be suppressed by an in-plane magnetic field [5]. This implies that the enhanced thermopower is largely due to spin entropy carried by strongly correlated holes (Co 4+ sites) hopping on the triangular lattice. When intercalated with water, Na x CoO 2 ·yH 2 O becomes superconducting at or below 4 K [6] for 1 4 < x < 1 3 [7,8,9]. These experiments raise many questions. Is the Curie-Weiss state at 2 3 continuous with the 1 3 state surrounding superconductivity? Are commensurability and charge-ordering effects important? To address these questions, we have completed a study of the phase diagram of non-hydrated Na x CoO 2 . As x increases from 0.3 to 0.75, we observe a series of electronic states, the most interesting of which is an insulating state at x = 1 2 that involves charge ordering of the holes together with the Na ions. We identify details specific to the triangular lattice, especially in the metallic state from which the superconducting composition evolves, and comment on recent theories.Starting with powder or single-crystal samples with x ∼ 0.75, we vary x by specific chemical deintercalation of Na (Fig. 1, caption). Powders of Na 0.77 CoO 2 were made by solid-state reaction of stoichiometric amounts of Na 2 CO 3 and Co 3 O 4 in oxygen at 800 C. Sodium deintercalation was then carried out by treatment of samples in solutions obtained by dissolving I 2 (0.2 M, 0.04 M) or Br 2 (1.0 M) in acetonitrile. After magnetic stirring for five days at ambient temperature, they were washed with copious amounts of acetonitrile and multiple samples were tested by the ICP-AES method to determine Na content. Unit-cell parameters were determined by powder X-ray diffraction (XRD) with internal Si standards. For the transport studies, we first grew a boule (with x = 0.75) in an optical furnace by the floating-zone technique. Crystals cleaved from the boul...

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate Na(x)CoO2 exhibits several interesting electronic phases as the Na content x is varied, including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, Na(x)CoO2 has moderately large thermopower S and conductivity sigma. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.60.75, S undergoes an even steeper enhancement. At the critical doping x(p) approximately 0.85, Z (at 80 K) reaches values approximately 40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.

Received Hc2-n1.tex)We present the resistively-determined upper critical field H ρ c2 (T ) and the irreversibility lines H ρ irr (T ) of various high-Tc cuprates, deduced from measurements in 61-T pulsed magnetic fields applied parallel to the c axis. The shape of both H ρ c2 (T ) and H ρ irr (T ) depends monotonically on the anisotropy of the material and none of the samples show saturation of H ρ (T ) at low temperatures. The anomalous positive curvature, d 2 H ρ /dT 2 > 0, is the strongest in materials with the largest normal state anisotropy, regardless of whether anisotropy is varied by changing the carrier concentration or by comparing a variety of optimally-doped compounds.

In a ferromagnet, an applied electric field E invariably produces an anomalous Hall current JH that flows perpendicular to the plane defined by E and M (the magnetization). For decades, the question whether JH is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr2Se4−xBrx, the resistivity ρ (at low temperature) may be increased 1000 fold by varying x(Br), without degrading the M. We show that JH /E (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples.A major unsettled question in the study of electron transport in a ferromagnet is whether the anomalous Hall current is dissipationless. In non-magnetic metals, the familiar Hall current arises when electrons moving in crossed electric (E) and magnetic (H) fields are deflected by the Lorentz force. However, in a ferromagnet subject to E alone, a large, spontaneous (anomalous) Hall current J H appears transverse to E (in practice, a weak H serves to align the magnetic domains) (1,2). Questions regarding the origin of J H , and whether it is dissipationless, have been keenly debated for decades. They have emerged anew because of fresh theoretical insights and strong interest in spin currents for spin-based electronics. Here we report measurements in the ferromagnet CuCr 2 Se 4−x Br x which establish that, despite a 100-fold increase in the scattering rate from impurities, J H (per carrier) remains constant, implying that it is indeed dissipationless.In 1954, Karplus and Luttinger (KL)(3,4) proposed a purely quantum-mechanical origin for J H . An electron in the conduction band of a crystal lattice spends part of its time in nearby bands because of admixing caused by the (intracell) position operator X. In the process, it acquires a spin-dependent 'anomalous velocity' (5). KL predicted that the Hall current is dissipationless: J H remains constant even as the longitudinal current (J||E) is degraded by scattering from added impurities. A conventional mechanism was later proposed (6) whereby the anomalous Hall effect (AHE) is caused instead by asymmetric scattering of electrons by impurities (skew scattering). Several authors (7,8,9) investigated the theoretical ramifications of these competing models. The role of impurities in the anomalous-velocity theory was clarified by Berger's side-jump model (7 AHE in a semiconductor has been given by Nozières and Lewiner (NL) who derive X = λk × S, with λ the enhanced spin-orbit parameter, k the carrier wavevector and S its spin (9). In the dc limit, NL obtain the AHE currentwhere n is the carrier density and e the charge. As noted, J H is linear in S but independent of the electron lifetime τ . In modern terms, the anomalous velocity term of KL is related to the Berry phase (10), and has been applied (11) to explain the AHE in Mn-doped GaAs (12). The close connection of the AH...