Research Update: Structural and transport properties of (Ca,La)FeAs2 single crystal

Abstract

Structural and transport properties in the normal and superconducting states are investigated in a Ca0.8La0.2FeAs2 single crystal with Tc = 27 K, belonging to the newly discovered 112 family of iron based superconductors. The transport critical current density Jc for both field directions measured in a focused ion beam patterned microbridge reveals a weakly field dependent and low anisotropic behaviour with a low temperature value as high as Jc(B = 0) ∼ 105 A/cm2. This demonstrates not only bulk superconductivity but also the potential of 112 superconductors towards applications. Interestingly, this superconducting compound undergoes a structural transition below 100 K which is evidenced by temperature-dependent X-ray diffraction measurements. Data analysis of Hall resistance and magnetoresistivity indicate that magnetotransport properties are largely dominated by an electron band, with a change of regime observed in correspondence of the onset of a structural transition. In the low temperature regime, the contribution of a hole band to transport is suggested, possibly playing a role in determining the superconducting state

Superconductivity and Dirac fermions in 112‐phase pnictides

“…Additionally, the presence of two different crystallographic phases in different temperature regions were further confirmed from an optical 2nd harmonic generation study (), where no significant modification of the electronic structure was observed as a result of the phase transitions. A similar structural phase transition was recently reported in superconducting () around 100 K using temperature dependent X‐ray measurements (), while Kawasaki () reported an AFM ordering around for a sample with bulk . This suggests a correlation between the structural and AFM phase transition in occurring in similar temperature window and a weak coupling between the structural and magnetic order.…”

“…The critical current density J c for 112-Ca 1−x La x FeAs 2 is about ∼ 10 5 A/cm 2 (see Fig. 11), only weakly dependent on the direction and strength of the applied magnetic field [47,77]. However, in Sb-doped Ca 0.85 La 0.15 Fe(As 0.92 Sb 0.08 ) 2 , significant improvement in the J c value [101] was measured (∼ 2.2 × 10 6 A/cm 2 ), which upon high energy proton irradiation can be enhaced upto a value of 6.2 × 10 6 A/cm 2 .…”

“…The upper critical fields (H c2 ) at zerotemperature for LaPd x Bi 2 was 3 T [56] which is relatively large for a T c ∼ 2 K suggesting type-II superconductivity. For Ca 1−x La x FeAs 2 , critical fields near T = 0 are as high as H c c2 (0) = 39.4 T (for H c) and H ab c2 (0) = 166.2 T (for H ab) which corresponds to coherence lengths [47,77] of ξ c (0) = 6.9 Å and ξ ab (0) = 28.9 Å. The anisotropy γ(0) near T c is 2.8 which lies in between 1111 (5 < γ < 9.2) [99]-and 122 (1 < γ < 2)[100]type pnictides.…”

Persistent antiferromagnetic order in heavily overdoped Ca1−x La x FeAs2

“…Its sign corroborates with that of S at room temperature, confirming that the electron-band dominate the transport properties. RH shows a weak temperature dependence, similar to that of the less doped samples [20]. Its function of T 2 .…”

“…The negative Seebeck coefficient at high temperatures suggests a dominant contribution of electrons, in agreement with the Hall coefficient (see Fig 4. A) and previous reports for samples with low doping [20]. The high temperature part is linear, and by using the Mott's formula: ( ) = 2 3 ( ) ( ), we extract an effective Fermi temperature (TF) of 3080 K (0.27 eV).…”