Melissa Gooch

Unusual superconducting state at 49 K in electron-doped CaFe2As2 at ambient pressure

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at 49 K, which has a low critical field < 4 Oe, and the other at 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr, or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K phase detected will be discussed.

Ba1–xNaxTi2Sb2O (0.0 ≤ x ≤ 0.33): A Layered Titanium-Based Pnictide Oxide Superconductor

A new layered Ti-based pnictide oxide superconductor, Ba(1-x)Na(x)Ti(2)Sb(2)O (0.0 ≤ x ≤ 0.33), is reported. X-ray studies revealed that it crystallizes in the tetragonal CeCr(2)Si(2)C structure. The undoped parent compound, BaTi(2)Sb(2)O [P4/mmm; a = 4.1196(1) Å; c = 8.0951(2) Å], exhibits a charge density wave (CDW)/spin density wave (SDW) transition at 54 K. Upon chemical doping with Na, the CDW/SDW transition is systematically suppressed, and superconductivity arises with the critical temperature (T(c)) increasing to 5.5 K. Bulk superconductivity was confirmed by resistivity, magnetic, and heat capacity measurements. Like the high-T(c) cuprates and the iron pnictides, the superconductivity in BaTi(2)Sb(2)O arises from an ordered state. Similarities and differences between BaTi(2)Sb(2)O and the cuprate and iron pnictide superconductors are discussed.

Pressure shift of the superconducting Tc of LiFeAs

The effect of hydrostatic pressure on the superconductivity in LiFeAs is investigated up to 1.8 GPa. The superconducting transition temperature, Tc, decreases linearly with pressure at a rate of 1.5 K/GPa. The negative pressure coefficient of Tc and the high ambient pressure Tc indicate that LiFeAs is the high-pressure analogue of the isoelectronic SrFe2As2 and BaFe2As2.

Pressure-induced shift of T c in K x Sr 1 − x Fe 2 As 2 ( x = 0.2 , 0.4 , 0.7 ) : Analogy to the high- T c cuprate superconductors

The systematic pressure shifts of T_c were investigated in the whole phase diagram of the FeAs-based superconducting compound K_xSr_{1-x}Fe_2As_2. Different regions, arising from corresponding responses of T

The superconductor KxSr1- xFe2As2: normal state and superconducting properties

_c

Evidence of quantum criticality in the phase diagram of K x Sr 1−x Fe 2 As 2 from measurements of transport and thermoelectricity

to pressure (dT_c/dp>0, =0, or <0), can be clearly distinguished. This reveals an interesting similarity of the FeAs superconductors and the high-T_c cuprates. This behavior is a manifestation of the layered structure of the FeAs compounds and the pressure-induced charge transfer between the (Fe_2As_2) and (K/Sr) layers. The coexistence of superconductivity and spin-density wave behavior were also observed, and the pressure effects on the latter is explored.

Weak coupling BCS-like superconductivity in the pnictide oxide Ba1−xNaxTi2Sb2O (x=0and 0.15)

The normal state and superconducting properties are investigated in the phase diagram of KxSr1- xFe2As2 for 0≤x≤1. The ground state upper critical field, Hc2(0), is extrapolated from magnetic field-dependent resistivity measurements. Hc2(0) scales with the critical temperature, Tc, of the superconducting transition. In the normal state, the Seebeck coefficient is shown to experience a dramatic change near a critical substitution of x0.3. 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.

High pressure study of the normal and superconducting states of the layered pnictide oxide Ba1−xNaxTi2Sb2O with x = 0, 0.10, and 0.15

The electrical transport and thermoelectric properties of

Pressure effects on strained FeSe0.5Te0.5 thin films

{\text{K}}_{x}{\text{Sr}}_{1\ensuremath{-}x}{\text{Fe}}_{2}{\text{As}}_{2}

Synthesis, Structure, and Superconductivity in the New-Structure-Type Compound: SrPt6P2

are investigated for