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Abstract:

The magnetic properties of a family of molecular-based quasi-two- dimensional S=1/2 Heisenberg antiferromagnets are reported. Three compounds, [Cu (pz) 2 (ClO4) 2, Cu (pz) 2 (BF4) 2, and [Cu (pz) 2 (NO3)] (PF6)] contain similar planes of Cu2+ ions linked into magnetically square lattices by bridging pyrazine molecules (pz= C4 H4 N2). The anions provide charge balance as well as isolation between the layers. Single crystal measurements of susceptibility and magnetization, as well as muon-spin-relaxation studies, reveal low ratios of Néel temperatures to exchange strengths (4.25/17.5=0.243, 3.80/15.3=0.248, and 3.05/10.8=0.282, respectively) while the ratio of the anisotropy fields HA (kOe) to the saturation field HSAT (kOe) are small (2.6/490=5.3× 10-3, 2.4/430=5.5× 10-3, and 0.07/300=2.3× 10-4, respectively), demonstrating close approximations to a two-dimensional Heisenberg model. The susceptibilities of ClO4 and BF4 show evidence of an exchange-anisotropy crossover (Heisenberg to XY) at low temperatures; their ordering transitions are primarily driven by the XY behavior with the ultimate three-dimensional transition appearing parasitically. The PF6 compound remains Heisenberg type at all temperatures, with its transition to the Néel state due to the interlayer interactions. Effects of field-induced anisotropy have been observed. © 2009 The American Physical Society.

Abstract:

The recent observation of superconductivity with critical temperatures (Tc) up to 55 K in the pnictide RFeAsO(1-x)F(x), where R is a lanthanide, marks the first discovery of a non-copper-oxide-based layered high-Tc superconductor. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprate superconductors, as both families exhibit superconductivity following charge doping of a magnetic parent material. In this context, it is important to follow the evolution of the microscopic magnetic properties of the pnictides with doping and hence to determine whether magnetic correlations coexist with superconductivity. Here, we present a muon spin rotation study on SmFeAsO(1-x)F(x), with x=0-0.30 that shows that, as in the cuprates, static magnetism persists well into the superconducting regime. This analogy is quite surprising as the parent compounds of the two families have rather different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings therefore suggest that the proximity to magnetic order and associated soft magnetic fluctuations, rather than strong electronic correlations in the vicinity of a Mott-Hubbard transition, may be the key ingredients of high-Tc superconductors.

Abstract:

We review the implementation of quantum information processing using quantum spins and pulsed spin resonance techniques. Molecular magnets, nanoscale clusters of coupled transition metal ions, offer various potential advantages over other spin systems as the building blocks of a quantum computer. We describe the strategies which must be employed in order to implement quantum algorithms in such nanoscale magnets and explain why, when evaluating the suitability of any physical system for embodying a qubit, it is essential to determine the phase relaxation time appropriate for an individual molecular spin. Experiments utilising pulsed spin resonance techniques show that the phase relaxation times in at least some molecular magnets are long enough to permit multiple qubit operations to be performed. © The Royal Society of Chemistry 2009.

Abstract:

On the application of hydrostatic pressures of up to 1.3 GPa, the superconducting transition temperatures (T(c)) of samples of LiFeAs are lowered approximately monotonically at approximately -2 K GPa(-1). Measurements of the X-ray powder diffraction pattern at hydrostatic pressures of up to 17 GPa applied by a He gas pressure medium in a diamond anvil cell reveal a bulk modulus for LiFeAs of 57.3(6) GPa which is much smaller than that of other layered arsenide and oxyarsenide superconductors. LiFeAs also exhibits much more isotropic compression than other layered iron arsenide superconductors. The higher and more isotropic compressibility is presumably a consequence of the small size of the lithium ion. At ambient pressure the FeAs(4) tetrahedra are the most compressed in the basal plane of those in any of the superconducting iron arsenides. On increasing the pressure the Fe-Fe distance contracts more rapidly than the Fe-As distance so that the FeAs(4) tetrahedra become even more distorted from the ideal tetrahedral shape. The decrease in T(c) with applied pressure is therefore consistent with the observations that in the iron arsenides and related materials investigated thus far, T(c) is maximized for a particular electron count when the FeAs(4) tetrahedra are close to regular.