Spin fluctuations in the light-induced high spin state of Cobalt valence tautomers

We present a study of the static magnetic properties and spin dynamics in Cobalt valence tautomers (VT), molecules where a low-spin (LS) to high-spin (HS) crossover driven by an intramolecular electron transfer can be controlled by the temperature, by the external pressure or by light irradiation. In the investigated complex, a LS-Co(III) ion bound to a dinegative organic ligand can be reversibly converted into the HS-Co(II) bound to a mononegative one. By combining magnetization measurements with Nuclear Magnetic Resonance (NMR) and Muon Spin Relaxation ({\mu}SR), we have investigated the static magnetic properties and the spin dynamics as a function of the temperature. Moreover, the effect of the external pressure as well as of the infrared light irradiation have been explored through magnetometry and NMR measurements to determine the spin dynamics of the HS state. The photoinduced HS state, which can have a lifetime of several hours below 30 K, is characterized by spin dynamics in the MHz range, which persist at least down to 10 K. The application of an external pressure causes a progressive increase of the LS-HS crossover, which reaches room temperature for pressures around 10 kbar

Orthorhombic fulleride (CH3NH2)K3C60 close to Mott-Hubbard instability: Ab initio study

We study the electronic structure and magnetic interactions in methylamine-intercalated orthorhombic alkali-doped fullerene (CH3NH2)K3C60 within the density functional theory. As in the simpler ammonia intercalated compound (NH3)K3C60, the orthorhombic crystal-field anisotropy \Delta lifts the t1u triple degeneracy at the \Gamma point and drives the system deep into the Mott-insulating phase. However, the computed \Delta and conduction electron bandwidth W cannot alone account for the abnormally low experimental N\'eel temperature, T_N = 11 K of the methylamine compound, compared to the much higher value T_N = 40 K of the ammonia one. Significant interactions between CH3NH2 and C60^{3-} are responsible for the stabilization of particular pseudo-Jahn-Teller fullerene-cage distortions and the ensuing low-spin S = 1/2 state. These interactions also seem to affect the magnetic properties, as interfullerene exchange interactions depend on the relative orientation of pseudo-Jahn-Teller distortions of neighboring C60^{3-} molecules. For the ferro-orientational order of CH3NH2-K^+ groups we find an apparent reduced dimensionality in magnetic exchange interactions, which may explain the suppressed N\'eel temperature. The disorder in exchange interactions caused by orientational disorder of CH3NH2-K^+ groups could further contribute to this suppression.Comment: 8 pages, 5 figures, 1 tabl

Processing-property relationship for solid-state synthesized CuAlO2 ceramic

Copper aluminate (CuAlO2) is the most extensively studied cuprous delafossite (general formula: A+B3+O2, group: R-3m) p-type semiconductor, whose exceptional electrical, optical, magnetic and catalytic properties found many applications in catalysis and (opto)electronic technology, e.g. as field electron emitters, light-emitting diodes, functional windows or solar cells. While the processing-property correlations have been well explored for many materials with perovskite or spinel structure, the investigations of the mentioned relationship for delafosittes are still eliciting the attention of scientists. In order to attain the optimized CuAlO2 material for a specific application, for example, as ceramic targets for further physical vapour deposition of thin films, the preparation routes should be precisely designed. The adequate synthesis parameters should allow the formation of pure CuAlO2 and disable its disproportionation to CuO and detrimental spinel CuAl2O4. For the spinel CuAl2O4 phase is known that can notably influence the final product properties, since it is an n-type material and insulator. Concerning the synthesis of CuAlO2 sputtering targets, our approach to process single-phase delafossite CuAlO2 has been to enhance the solid-state synthesis by milling of nano-boehmite AlOOH.xH2O and pre-milled Cu2O powders mixture, and to control the atmosphere both during the synthesis and sintering steps. The pure delafossite CuAlO2 powder was prepared by the double heating of the above mentioned powder mixture for 10 h at 1100 oC in argon atmosphere. A short sintering time of 2 h at 1100 oC in air atmosphere was essential to obtain single phase and dense CuAlO2 ceramic with 86 % of theoretical density. In contrast to X-ray analysis, which confirmed the single phase composition of the sintered sample, the BEI/EDXS analysis revealed the presence of traces of Cu-rich impurities at the surface of the pellet, while the bulk of the sample was single phase delafossite with uniformly distributed porosity. For further use, the sintered samples were cut and polished to remove the CuO from the surface. Beside the strong modes at 418 cm-1 and 767 cm-1, typical for delafossite structured CuAlO2, the low intensity features were also observed in the Raman spectrum of the sintered sample and could be mainly related to the leaning delafossite or 2H-CuAlO2 hexagonal polytype. Nominally pure CuAlO2 sample was analysed by electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopy to get microscopic insight into composition of the sintered sample, i.e. presence of impurities within the bulk network. The semiconducting nature of the sintered sample was confirmed by the apparent increase of both dielectric permittivity and dielectric loss factor with increasing temperature and decrease with increasing frequency. Furthermore, the temperature dependence of the Hall parameters (resistivity-ρ, carrier concentration-ND, mobility-µ) of the CuAlO2 ceramic was analysed. The resistivity of the samples decreased with increasing temperature from 830 Ωcm at 230 K to 11 Ωcm at 350 K, due to the increase in both the number of charge carriers and their mobility. Also, room temperature Hall measurements revealed a positive Hall coefficient RH = 33.8 cm-3C-1, which confirmed the p-type conduction of the material. Within this contribution the relationship between processing, composition and properties of the CuAlO2 ceramic is discussed

The pairing symmetry in quasi-one-dimensional superconductor Rb2Mo3As3

Quasi-one-dimensional electron systems display intrinsic instability towards long-range ordered phases at sufficiently low temperatures. The superconducting orders are of particular interest as they can possess either singlet or triplet pairing symmetry and frequently compete with magnetism. Here we report on muon spin rotation and relaxation ($\mathrm{\mu}$SR) study of Rb$_2$Mo$_3$As$_3$ characterised by one of the highest critical temperatures $T_{\rm c}=10.4\ \mathrm{K}$among quasi-one-dimensional superconductors. The transverse-field$\mathrm{\mu}$SR signal shows enhanced damping below$T_{\rm c}$due to the formation of vortex lattice. Comparison of vortex lattice broadening against single gap$s-$,$p-$and$d-$wave models shows the best agreement for the$s-$wave scenario but with the anomalously small superconducting gap,$\Delta_0$, to$T_{\rm c}$ratio of$2\Delta_0/k_{\rm B}T_{\rm c}=2.74(1)$. The alternative nodal$p-$wave or$d-$wave scenarios with marginally worse goodness of fit would yield more realistic$2\Delta_0/k_{\rm B}T_{\rm c}=3.50(2)$and$2\Delta_0/k_{\rm B}T_{\rm c}=4.08(1)$, respectively, and thus they cannot be ruled out when accounting for the superconducting state in Rb$_2$Mo$_3$As$_3$.Comment: 6 page

Influence of magnetic interaction between impurity and impurity-liberated spins on the magnetism in the doped Haldane chain compounds PbNi{}_{2-x{A}x{}_{x}V2{}_{2}O8{}_{8} (A = Mg, Co)

A comprehensive study of impurity-induced magnetism in nonmagnetically (Mg${}^{2+}$) and magnetically (Co${}^{2+}$) doped PbNi${}_{2}$V${}_{2}$O${}_{8}$ compounds is given, using both macroscopic dc susceptibility and local-probe electron spin resonance (ESR) techniques. Magnetic coupling between impurity-liberated spins is estimated from a linewidth of low-temperature ESR signal in Mg-doped samples. In addition, in the case of magnetic cobalt dopants the impurity-host magnetic exchange is evaluated from the Co-induced contribution to the linewidth in the paramagnetic phase. The experimentally observed severe broadening of the ESR lines in the magnetically doped compounds with respect to nonmagnetic doping is attributed to a rapid spin-lattice relaxation of the Co${}^{2+}$ ions, which results in a bottleneck-type of temperature dependence of the induced linewidth. The exchange parameters obtained from the ESR analysis offer a satisfactory explanation of the observed low-temperature magnetization in doped samples.Comment: 8 pages, 6 figure