2Charge density wave (CDW) transitions are a frequent occurrence in transition metal chalcogenides due to their low structural dimensionality. Layered MX 2 compounds and chain-based MX 3 compounds, where M is a group 4 or 5 metal and X = S, Se, or Te, are the best known examples [1][2][3][4][5][6][7]. These transitions arise to allow electronic systems to minimize their energy by removing electronic states at the Fermi level. This is achieved by introducing a new structural periodicity at the Fermi wave vector, inducing a band gap. Superconductivity and the CDW state are two very different cooperative electronic phenomena, and yet both occur due to Fermi surface instabilities and electron-phonon coupling. A number of CDW-bearing materials are also superconducting [8][9][10][11][12][13], and the idea that superconductivity and CDW states are competing electronic states at low temperatures is one of the fundamental concepts of condensed matter physics. Surprisingly, no system has yet been reported in which the emergence of a superconducting state after a charge density wave state has been suppressed via doping has been studied in detail: a transition that implies a deep connection between the two states, i.e., that the same electrons are participating in both transitions. TiSe 2 was one of the first CDW-bearing compounds known, and is also one of the most frequently studied as the nature of its CDW transition has been controversial for decades. The CDW transition, at approximately 200 K, is to a state with a commensurate (2a,2a,2c) wavevector without an intermediate incommensurate phase [3,16,17]. The commensurate CDW wavevector and electronic structure calculations indicate that, unlike the case in most materials, the CDW in TiSe 2 is not driven by Fermi surface nesting. The normal state is presently believed to be either a semimetal or a semiconductor with a small indirect gap [3, 16, 18 -22] (Fig. 1a, inset). This results in a systematic expansion of the unit cell with Cu content in Cu x TiSe 2 , as evidenced by the lattice parameters shown in Fig. 1a. The expansion of the cell parameters is maintained up to x = 0.11. For higher Cu contents, both a and c remain unchanged from their value at x = 0.11. It can therefore be concluded that the solubility limit for Cu in TiSe 2 is x = 0.11 ± 0.01.Of particular interest is the evolution of the charge density wave with Cu doping.Electron and X-ray diffraction studies of pure TiSe 2 at low temperatures show the presence of reflections corresponding to the basic trigonal structure and also the 2a, 2c superstructure reflections associated with the CDW state [3,19]. increases with Cu content. This suggests that the Cu doping introduces carriers into the conduction band in TiSe 2 , increasing the electronic density of states and therefore the Pauli paramagnetism. This is further confirmed by specific heat measurements, described below. A drop in the susceptibility of pure TiSe 2 is seen as the temperature is lowered below the CDW transition at 200 K, consistent with th...

Although the microscopic origin of the superconductivity in high T c copper oxides remains the subject of active inquiry, several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics is the dependence of the superconducting transition temperature on the degree of electronic band filling. Since the discovery of cuprate superconductivity in 1986 (1), the search for other families of superconductors that might help shed light on the superconducting mechanism of the cuprates has been of great interest. The recent report of superconductivity near 4K in the triangular lattice, layered sodium cobalt oxyhydrate, Na 0.35 CoO 2 ⋅1.3H 2 O, suggests that superconductors related to the cuprates may be found (2). Here we show that the superconducting transition temperature of this compound displays the same kind of chemical-doping controlled behavior that is observed in the cuprates. Specifically, the optimal superconducting T c occurs in a narrow range of sodium concentrations, and therefore electron concentration, and decreases for both underdoped and overdoped materials, in analogy to the phase diagram of the cuprate superconductors. Our results suggest that detailed characterization of this new superconductor may help establish which of the many special characteristics of the cuprates is fundamental to their high T c superconductivity.Like the high T c superconductors, the Na x CoO 2 ·1.3H 2 O crystal structure (2) consists of electronically active planes (in this case, edge sharing CoO 6 octahedra) separated by layers (in this case, Na x ·1.3H 2 O) that act as spacers, to yield electronic twodimensionality, and also act as charge reservoirs. We have found that varying the Na content in Na x CoO 2 ·1.3H 2 O results in the same type of out-of-plane chemical doping control of in-plane electronic charge that is found for the cuprate superconductors. This is achieved by changing the Br concentration used in the deintercalation of the host material. (See caption of Fig. 1 for the synthesis procedure). Powder X-ray diffraction (XRD) patterns for the synthesized samples are shown in Fig. 1. The brominetreated samples made with substoichiometric (0.5X) and stoichiometric (1X) bromine solutions consist primarily of a partially deintercalated, anhydrous, non-superconducting Na x CoO 2 phase (c ≈ 11.2 Å). A small amount of the hydrated superconducting phase Na 1XFigure 1. Powder X-ray diffraction patterns (Cu Kα radiation) for Na x CoO 2 ·yH 2 O samples prepared using different concentrations of the bromine deintercalant. The inset shows an enlargement of the 006 reflections for each sample, highlighting the shift in the layer spacing as a function of sodium content. The Na x CoO 2 ·yH 2 O samples were prepared by chemically deintercalating sodium from Na 0.7 CoO 2 using bromine as an oxidizing agent (2,3). One-half gram of Na 0.7 CoO 2 was stirred in 20 mL of a Br 2 solutio...

We report pressure-induced superconductivity in a single crystal of CaFe 2 As 2 . At atmospheric pressure, this material is antiferromagnetic below 170 K but under an applied pressure of 0.69 GPa becomes superconducting, with a transition temperature T c exceeding 10 K. The rate of T c suppression with applied magnetic field is −0.7 K T −1 , giving an extrapolated zero-temperature upper critical field of 10-14 T.