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Register a new userMultiple phase transitions in single-crystalline Na$_{1-delta}$FeAs
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Specific heat, resistivity, susceptibility and Hall coefficient measurements were performed on high-quality single crystalline Na$_{1-delta}$FeAs. This compound is found to undergo three successive phase transitions at around 52, 41, and 23 K, which correspond to structural, magnetic and superconducting transitions, respectively. The Hall effect result indicates the development of energy gap at low temperature due to the occurrence of spin-density-wave instability. Our results provide direct experimental evidence of the magnetic ordering in the nearly stoichiometric NaFeAs.
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We report an optical investigation on the in-plane charge dynamics for Na$_{1-delta}$FeAs single crystal. A clear optical evidence for the spin-density wave (SDW) gap is observed. As the structural/magnetic transitions are separated in the Na$_{1-delta}$FeAs case, we find the SDW gap opens in accordance with the magnetic transition. Comparing with the optical response of other FeAs-based parent compounds, both the gap value 2$Delta$ and the energy scale for the gap-induced spectral weight redistribution are smaller in Na$_{1-delta}$FeAs. Our findings support the itinerant origin of the antiferromagnetic transition in the FeAs-based system.
A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films epitaxially grown on cubic substrates is developed using the Landau-Devonshire approach. The eighth-order thermodynamic potential for BT single crystal and modified fourth-order potential for ST single crystal were used as starting potentials for the end-members of the solid solution with the aim to develop potential of BST-$x$ solid solution valid at high temperatures. Several coefficients of these potentials for BT were changed to obtain reasonable agreement between theory and experimental phase diagram for BST-x (x > 0.2) solid solutions. For low Ba content we constructed the specific phase diagram where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and all transitions are of the second order. The concentration-misfit strain phase diagrams for BST-x thin films at room temperature and temperature-misfit strain phase diagrams for particular concentrations are constructed and discussed. Near T_N2 coupling between polarization and structural order parameter in the epitaxial film is modified considerably and large number of new phases not present in the bulk materials appear on the phase diagram.
Specific heat, magnetic torque, and magnetization studies of LiCoPO4 olivine are presented. They show that an unique set of physical properties of LiCoPO4 leads to the appearance of features characteristic of 2D Ising systems near the Neel temperature, T_N =21.6 K, and to the appearance of an uncommon effect of influence of magnetic field on the magnetocrystalline anisotropy. The latter effect manifests itself as a first-order transition, discovered at ~9 K, induced by magnetic field of 8 T. Physical nature of this transition was explained and a model describing experimental dependences satisfactorily was proposed.
Single crystals of Na$_{1-delta}$Fe$_{1-x}$T$_x$As with T = Co, Rh have been grown using a self-flux technique. The crystals were thoroughly characterized by powder X-ray diffraction, magnetic susceptibility and electronic transport with particular focus on the Rh-doped samples. Measurements of the specific heat and ARPES were conducted exemplarily for the optimally doped compositions. The spin-density wave transition (SDW) observed for samples with low Rh concentration ($0,leq,x,leq,0.013$) is fully suppressed in the optimally doped sample. The superconducting transition temperature ($T_c$) is enhanced from $10$~K in Na$_{1-delta}$FeAs to $21$~K in the optimally doped sample ($x$ = 0.019) of the Na$_{1-delta}$Fe$_{1-x}$Rh$_x$As series and decreases for the overdoped compounds, revealing a typical shape for the superconducting part of the electronic phase diagram. Remarkably, the phase diagram is almost identical to that of Co-doped Na$_{1-delta}$FeAs, suggesting a generic phase diagram for both dopants.
Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na$_{1-delta}$Fe$_{1-x}$Co$_x$As for electrical current directions along, $rho_a (T)$, and transverse, $rho_c (T)$, to the Fe-As layers. Doping with Co increases stability of this compound to reaction with the environment and suppresses numerous features in both $rho_a(T)$ and $rho_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $rho_a (T)$ with $x$ follows a universal trend observed in other pnictide superconductors, exhibiting a $T$-linear temperature dependence close to the optimal doping and development of $T^2$ dependence upon further doping. $rho_c (T)$ in parent compound shows a non - monotonic behavior with a crossover from non-metallic resistivity increase on cooling from room temperature down to $sim$ 80 K to a metallic decrease below this temperature. Both $rho_a (T)$ and $rho_c (T)$ show several correlated crossover - like features at $T>$ 80 K. Despite a general trend towards more metallic behavior of inter - plane resistivity in Co-doped samples, the temperature of the crossover from insulating to metallic behavior (80 K) does not change much with doping.
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