Ab initio study on the magneto-structural properties of MnAs

The magnetic and structural properties of MnAs are studied with ab initio methods, and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is an antiferromagnetic component in the hexagonal plane. For the paramagnetic state the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above which it remains hexagonal. This leads to the second order structural phase transition between paramagnetic states at about 400 K, where the lattice parameter increases above this critical value with rising temperature due to the thermal expansion. For the paramagnetic state an analytic approximation for the magnitude of the orthorhombic distortion as a function of the lattice constant is given. Within the mean field approximation the dependence of the Curie temperature on the volume and on the orthorhombic distortion is calculated. For orthorhombically distorted cells the Curie temperature is much smaller than for hexagonal cells. This is mainly due to the fact that some of the exchange coupling constants in the hexagonal plane become negative for distorted cells. With these results a description of the susceptibility as function of temperature is given

CrAs: heat capacity, enthalpy increments, thermodynamic properties from 5 to 1280 K, and transitions

The heat capacity of chromium arsenide has been measured by adiabatic calorimetry from 5 to 1050 K and enthalpy increments have been taken over the range 875 to 1280 K with respect to 298.15 K by drop calorimetry. The heat capacity shows a distinct bell-shaped transition with a peak at 259.9 K related to the disappearance of antiferromagnetic helical ordering on heating. The enthalpy and entropy of this transition are 177 calth mol-1 and 0.69 calth K-1 mol-1, respectively. At 1170 K another transition is observed related to the phase change from the MnP- to the NiAs-type structure. The enthalpy and entropy of the latter gradual transition are 280 calth mol-1 and 0.22 calth K-1 mol-1, respectively. Thermodynamic functions have been evaluated and the values of Cp, {So(T)-So(0)}, -{o(T)-Ho(0)}/T at 298.15 K are 12.501, 15.40, and 6.990 calth K-1 mol-1 and 16.09, 32.53, and 19.86 calth K-1 mol-1 at 1000 K.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22750/1/0000305.pd

Probing the Ground State Properties of Iron-based Superconducting Pnictides and Related Systems by Muon-Spin Spectroscopy

In this short review, we attempt to give a comprehensive discussion of studies performed to date by muon-spin spectroscopy (more precisely the relaxation and rotation technique, also know as \muSR technique) on the recently discovered layered iron-based superconductors. On one side, \muSR has been used to characterized the magnetic state of different families of layered iron-based systems. Similarly the subtle interplay of the magnetic state and the structural transition present in some families has been investigated. We will also discuss the information provided by this technique on the interaction between the magnetic state and the superconducting phase. Finally the \muSR technique has been used to investigate the magnetic penetration depth of the superconducting ground state. The study of its absolute value, temperature and magnetic field dependence provides crucial tests for investigating possible unconventional superconducting states in such systems.Comment: 10 pages, 5 figures. Physica C Special Edition on Superconducting Pnictides. Version 2: Fig. 3 modified due to data corrections in Ref. 6, typos correcte

75As NMR Study of Hole-Doped Superconductor Ba1-xKxFe2As2 (Tc = 38K)

We report the 75As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba1-xKxFe2As2 with different lattice parameters and different superconducting volume fractions (Tc = 38K). 75As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/T1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/T1 in the superconducting state down to the lowest measurement temperature T varies close to T^3. In addition, it exhibits no coherence peak just below Tc. This shows a T dependence similar to those of other iron pnictides.Comment: 5 pages, 4 figres, to be published in J. Phys. Soc. Jpn. 78 (2009) No.3 issu

A theoretical study on the formation of iodine oxide aggregates and monohydrates

Biotic and abiotic emissions of molecular iodine and iodocarbons from the sea or the ice surface and the intertidal zone to the coastal/polar marine boundary layer lead to the formation of iodine oxides, which subsequently nucleate forming iodine oxide particles (IOPs). Although the link between coastal iodine emissions and ultrafine aerosol bursts is well established, the details of the nucleation mechanism have not yet been elucidated. In this paper, results of a theoretical study of a range of potentially relevant aggregation reactions of different iodine oxides, as well as complexation with water molecules, are reported. Thermochemical properties of these reactions are obtained from high level ab initio correlated calculations including spin-orbit corrections. The results show that the nucleation path most likely proceeds through dimerisation of I2O4. It is also shown that water can hinder gas-to-particle conversion to some extent, although complexation with key iodine oxides does not remove enough of these to stop IOP formation. A consistent picture of this process emerges from the theoretical study presented here and the findings of a new laboratory study reported in the accompanying paper (Gomez Martin et al., 2013)