Guest excitation energy in dilute organic mixed crystals with chemically distinct guests

Philpott's theory of vibronic transitions of substitutional impurities in molecular crystals has been examined. The lowest energy guest transition in a number of mixed crystals has been calculated and compared with experiment. Satisfactory agreement is achieved. In some cases consideration of charge overlap of the guest and nearest host gives better agreement.     

First-principles study of defect-induced magnetism in carbon

We have studied the role of defects on the magnetic properties of carbon materials using first-principles density functional methods. We show that, while the total magnetization decreases both for diamond and graphite with increase in vacancy density, the magnetization decreases more rapidly for graphitic structures. The presence of nitrogen nearby a vacancy is shown to produce larger macroscopic magnetic signals as compared to a standalone carbon vacancy. The results indicate the possibility of tuning magnetization in carbon by controlled defect generation and doping.     

Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles

First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field.     

Enhancement of Jc by Hf-doping in the superconductor MgB2: A hyperfine interaction study

Measurements of the critical current density (J_c) by magnetisation and the upper critical field (H_c2) by magnetoresistance have been performed for hafnium-doped MgB₂. There has been a remarkable enhancement of J_c as compared to that by ion irradiation without any appreciable decrease in T_c, which is beneficial from the point of view of applications. The irreversibility line extracted from J_c shows an upward shift. In addition, there has been an increase in the upper critical field which indicates that Hf partially substitutes for Mg. Hyperfine interaction parameters obtained from time differential perturbed angular correlation (TDPAC) measurements revealed the formation of HfB and HfB₂ phases along with the substitution of Hf. A possible explanation is given for the role of these species in the enhancement of J_c in MgB₂ superconductor.     

Neon ion irradiation studies on MgB2 superconductor

160 MeV of neon ion irradiation has been carried out on MgB₂ polycrystalline pellets at various doses. There has not been any significant change in T_c except at the highest dose of 1×10¹⁵ ions/cm². Increase in resistivity has been noticed. Resistivity data have been fitted with Bloch-Grüneisen function to extract the values of Debye temperature, residual resistivity and temperature coefficient of resistivity for irradiated as well as unirradiated samples. There has not been any significant effect on electron-phonon coupling due to irradiation as evident from Debye temperature and the electron-phonon coupling constant.     

Polarized absorption spectra of Co2+ in Na2Cd3Cl8 single crystals

Summary The polarized optical absorption spectra of Na₂Cd₃Cl₈: Co²⁺ in the range of 15 000 to 40 000 cm⁻¹ down to 15 K are reported. The Co²⁺ ion is found to occupy the Cd²⁺ sites in octahedral geometry and the spectra are interpreted satisfactorily in terms of a cubic ligand field model including spin-orbit coupling. The observed crystal field spectra are well reproduced withB=745 cm⁻¹,C=3410 cm⁻¹,Dq=700 cm⁻¹ and ζ (spin-orbit interaction) =520 cm⁻¹. No spectral evidence for tetragonal distortion is observed.