Here we present a systematic study of the effects of pressure on the superconducting and spin/charge density wave (SDW/CDW) transitions of Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O (x = 0, 0.10, and 0.15) by means of resistivity measurements. For x = 0 and 0.10,
external pressure results in a decease of the SDW/CDW transition temperature T$_c$; however, no measurable change is observed for the x = 0.15. The pressure effect on the superconducting transition temperature is different for all three samples. For BaTi$_2$Sb$_2$O (x=0), T$_c$ increases significantly from 1.2 K at zero pressure to $sim$ 2.9 K at 16.1 kbars. The 10 % Na-doped sample shows an initial T$_c$ increase up to 4.2 K with pressure which saturates at higher pressure values. For higher Na concentrations (x=0.15), T$_c$ continuously decreases with increasing pressure.
Two single crystalline samples with the same nominal composition of Rb0.8Fe2Se2 prepared via slightly different precursor routes under the same thermal processing conditions were investigated at ambient and high pressures. One sample was found superc
onducting with a Tc of ~31 K without the previously reported resistivity-hump and the other was unexpectedly found to be a narrow-gap semiconductor. While the high pressure data can be understood in terms of pressure-induced variation in doping, the detailed doping effect on superconductivity is yet to be determined.
The normal state and superconducting properties are investigated in the phase diagram of K_xSr_{1-x}Fe_2As_2 for 0<x<1. The ground state upper critical field, H_{c2}(0), is extrapolated from magnetic field dependent resistivity measurements. H_{c2}(0
) scales with the critical temperature, T_c, of the superconducting transition. In the normal state the Seebeck coefficient is shown to experience a dramatic change near a critical substitution of x=0.3. This is associated with the formation of a spin density wave state above the superconducting transition temperature. The results provide strong evidence for the reconstruction of the Fermi surface with the onset of magnetic order.
The effect of hydrostatic pressure on the superconductivity in LiFeAs is investigated up to 1.8 GPa. The superconducting transition temperature, T_c, decreases linearly with pressure at a rate of 1.5 K/GPa. The negative pressure coefficient of T_c an
d the high ambient pressure T_c indicate that LiFeAs is the high-pressure analogue of the isoelectronic SrFe_2As_2 and BaFe_2As_2.
