We present a detailed powder and single crystal neutron diffraction study of the spin chain compound Ca3Co2O6. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order co-exists with a shorter range order with a correlation length scale of ∼ 180Å in the ab plane. Remarkably, on cooling, the volume of material exhibiting short range order increases at the expense of the long-range order.PACS numbers: 75.30. Gw, 75.25.+z, 75.30.Fv, 75.50.Ee The appearance of plateaux in the magnetization curves of low dimensional quantum spin systems (e.g. NH 4 CuCl 3 [1] and SrCu 2 (BO 3 ) 2 [2]) has generated considerable attention from both an experimental and a theoretical point of view. In this respect the spin chain system Ca 3 Co 2 O 6 is very interesting because at low temperatures several steps, equally spaced in magnetic field, appear in the magnetization [3,4,5], a behavior reminiscent of quantum tunneling of magnetization in molecular magnets [6]. The origin of this intriguing phenomenon is still an open question [7,8] and the magnetic properties of Ca 3 Co 2 O 6 have been intensely studied in the past decade using many techniques, including x-ray [9,10,11] and neutron [12,13,14] Ca 3 Co 2 O 6 is a rare example of a material where ferromagnetic (FM) 1D Ising spin chains are coupled through a much weaker antiferromagnetic (AF) exchange on a triangular lattice. The spin chains in Ca 3 Co 2 O 6 are made up of alternating face-sharing octahedral (CoI) and trigonal prismatic (CoII) CoO 6 polyhedra, running along the c axis and arranged in a triangular lattice in the ab plane [17]. The different Co environments leave the Co 3+ ions on the CoI sites in a low-spin (S=0) state, and those on the CoII sites in the high-spin (S=2) state [9,11,15]. Crystalline electric fields also lead to a very strong anisotropy with the moments preferentially aligned along the c axis [3,4]. The zero-field magnetic structure of Ca 3 Co 2 O 6 has yet to be described unambiguously. Most theoretical descriptions of the magnetic structure center around the two stable configurations for 1D Ising chains coupled antiferromagnetically on a 2D triangular lattice, the ferrimagnetic structure (M, M, −M ) and the partially disordered antiferromagnetic (PDA) structure (M, −M, 0) [18] (the labels in parenthesis indicate the relative magnetizations on the triangular lattice). The step-like magnetization has been described using the Ising model on a triangular lattice [7]. There are, however, experimental data which suggest a more complex magnetic structure for Ca 3 Co 2 O 6 . A pronounced drop in the intensity of the magnetic peaks on cooling below 18 K in zero field has been observed in powder [12,13] and single crystal diffraction studies [14]. Recent resonant x-ray scattering (RXS) studies [10] showed a small (∼ 0.01Å −1 ) incommensu...

We have studied the variation of superconducting critical temperature Tc as a function of charge density and lattice parameters in Mg1-xAlxB2 superconducting samples. The AB2 heterostucture of metallic boron layers (intercalated by A = magnesium, aluminum layers, playing the role of spacers) is made by direct chemical reaction. The spacing between boron layers and their charge density are controlled by chemical substitution of Mg by Al atoms. We show that high Tc superconductivity is realized by tuning the chemical potential at a `shape resonance' according with the patent for `high-temperature superconductors made by metal heterostructures at the atomic limit'. The energy width of the superconducting shape resonance is found to be about 400 meV.

The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.

We show the key role of the elastic local strain (or micro-strain) ε of the CuO 2 lattice in the phase diagram of cuprate superconductors. The superconducting critical temperature T c (δ, ε) is shown to be a function of two variables, the doping δ and the microstrain ε.