Fe-spin reorientation in PrFeAsO : Evidences from resistivity and specific heat studies

We report the magnetic field dependence of resistivity ($\rho$) and specific heat ($C$) for the non-superconducting PrFeAsO compound. Our study shows a hitherto unobserved anomaly at $T_{SR}$ in the resistivity and specific heat data which arises as a result of the interplay of antiferromagnetic (AFM) Pr and Fe sublattices. Below the AFM transition temperature ($T_N^{\rm{Pr}}$), Pr moment orders along the crystallographic c axis and its effect on the iron subsystem causes a reorientation of the ordered inplane Fe moments in a direction out of the $ab$ plane. Application of magnetic field introduces disorder in the AFM Pr sublattice, which, in turn, reduces the out-of-plane Pr-Fe exchange interaction responsible for Fe spin reorientation. Both in $\rho$($T$) and $d(C/T)/dT$ curves, the peak at $T_{SR}$ broadens with the increase of $H$ due to the introduction of the disorder in the AFM Pr sublattice by magnetic field. In $\rho$($T$) curve, the peak shifts towards lower temperature with $H$ and disappears above 6 T while in $d(C/T)/dT$ curve the peak remains visible up to 14 T. The broadening of the anomaly at $T_N^{\rm{Pr}}$ in $C(T)$ with increasing $H$ further confirms that magnetic field induces disorder in the AFM Pr sublattice.Comment: 8 pages, 10 Figure

Quantm Magnetoresistance of the PrFeAsO oxypnictides

We report the observation of an unusual $B$ dependence of transverse magnetoresistance (MR) in the PrFeAsO, one of the parent compound of pnictide superconductors. Below the spin density wave transition, MR is large, positive and increases with decreasing temperature. At low temperatures, MR increases linearly with $B$ up to 14 T. For $T$$\geq$40 K, MR vs $B$ curve develops a weak curvature in the low-field region which indicates a crossover from $B$ linear to $B^2$ dependence as $B$$\rightarrow$0. The $B$ linear MR originates from the Dirac cone states and has been explained by the quantum mechanical model proposed by Abrikosov.Comment: accepted for publication in Appl. Phys. Let

Superconductivity in 2-2-3 system Y2Ba2Cu2O(8+delta)

Researchers synthesized a new high T(sub c) 2-2-3 superconductor Y2Ba2Cu3O(8+delta) by a special preparation technique and characterized it by ac-susceptibility measurements. Diamagnetism and Meissner effect sets in at low fields and superconducting transition onsets at 90 K. The systematic investigation of the real and imaginary components of ac-susceptibility as a function of temperature and applied ac magnetic field reveals that the magnetic behavior is that of a granular type superconductor

Scaling of Circulation in Buoyancy Generated Vortices

The temporal evolution of the fluid circulation generated by a buoyancy force when two-dimensional (2D) arrays of 2D thermals are released into a quiescent incompressible fluid is studied through the results of numerous lattice Boltzmann simulations. It is observed that the circulation magnitude grows to a maximum value in a finite time. When both the maximum circulation and the time at which it occurs are non-dimensionalised by appropriately defined characteristic scales, it is shown that two simple Prandtl number (Pr) dependent scaling relations can be devised that fit these data very well over nine decades of Pr spanning the viscous and diffusive regimes and six decades of Rayleigh number (Ra) in the low Ra regime. Also, obtained analytically is the exact result that circulation magnitude continues to grow in time for a single buoyant vortex ring in an infinite unbounded fluid.Comment: 4 pages, 3 figures, submitted to Physical Review Letter

Effective interactions and charges in <SUP>58</SUP>Ni

The structure of the low-lying states of 58Ni has been calculated in shell model by assuming an inert 56Ni core plus two valence nucleons in the p3/2, f5/2 and p1/2 orbitals. The two-body matrix elements are first expressed in terms of seven radial matrix elements and these are then parametrized to give best fit between the computed and the observed energies of the levels below 4 MeV. The wave-functions obtained using these two-body matrix elements are used to study the concept of effective charges. It is found that a single effective charge is not sufficient to predict the B(E2) rates equally well for the thirteen known transitions for which experimental values are available. Assumption of state-dependent effective charges gives a far better agreement. An analysis using wave functions obtained with Kuo's two-body matrix elements also gives a similar result

Structure of nuclei in the region A=70

The structure of the selenium nuclei in the region A=70 is studied using our deformed configuration mixing (DCM) shell model based on Hartree-Fock states. An effective interaction given by Kuo and modified by Bhatt is used. An attempt is made to understand the coexistence of shapes in selenium nuclei

Anisotropic magnetic properties and giant magnetocaloric effect in antiferromagnetic RRMnO3_3 crystals (RR=Dy, Tb, Ho and Yb)

We have systematically investigated the magnetic properties and magnetocaloric effect (MCE) in $R$MnO$_3$ ($R$$=$Dy, Tb, Ho and Yb) single crystals. Above a critical value of applied field ($H_c$), $R$MnO$_3$ undergo a first-order antiferromagnetic (AFM) to ferromagnetic (FM) transition below the ordering temperature ($T_{N}^{R}$) of $R^{3+}$ moment and a second-order FM to paramagnetic (PM) transition above $T_{N}^{R}$. Both $H$ and $T$ dependence of $M$ shows that the system is highly anisotropic in the FM as well as PM states and, as a result, the magnetic entropy change ($\Delta S_{M}$) is extremely sensitive to the direction of applied field and can be negative (normal MCE) or positive (inverse MCE). For hexagonal HoMnO$_3$ and YbMnO$_3$ systems, a very small inverse MCE is observed only for $H$ parallel to c axis and it decreases with increasing $H$ and crosses over to normal one above $H_c$. On the other hand, for orthorhombic DyMnO$_3$ and TbMnO$_3$, though the inverse MCE disappears above $H_c$ along easy-axis of magnetization, it increases rapidly with $H$ along hard-axis of magnetization for $T$$\ll$$T_{N}^{R}$. Except for YbMnO$_3$, the values of $\Delta S_{M}$, relative cooling power and adiabatic temperature change along easy-axis of magnetization are quite large in the field-induced FM state for a moderate field strength. The large values of these parameters, together with negligible hysteresis, suggest that the multiferroic manganites could be potential materials for magnetic refrigeration in the low-temperature region.Comment: 10 pages, 11 figure