NMR investigation of superconductivity and Antiferromagnetism in CaFe2As2 under pressure

Emergent electronic structure of CaFe2As2

CaFe₂As₂ exhibits collapsed tetragonal (cT) structure and varied exotic behaviour under pressure at low temperatures that led to debate on linking the structural changes to its exceptional electronic properties like superconductivity, magnetism, etc. Here, we investigate the electronic structure of CaFe₂As₂ forming in different structures employing density functional theory. The results indicate that the stability of the cT phase under pressure arises from the enhancement in hybridization induced effects and shift of the energy bands towards lower energies. The Fermi surface centered around Γ point gradually vanishes with the increase in pressure. Consequently, the nesting between the hole and electron Fermi surfaces associated to the spin density wave state disappears indicating a pathway to achieve the proximity to quantum fluctuations. The magnetic moment at the Fe sites diminishes in the cT phase consistent with the magnetic susceptibility results. Notably, the hybridization of Ca 4s states (Ca-layer may be treated as a charge reservoir layer akin to those in cuprate superconductors) is significantly enhanced in the cT phase revealing its relevance in its interesting electronic properties.

Robust antiferromagnetism preventing superconductivity in pressurized (Ba 0.61 K 0.39)Mn2Bi2

BaMn2Bi2 possesses an iso-structure of iron pnictide superconductors and similar antiferromagnetic (AFM) ground state to that of cuprates, therefore, it receives much more attention on its properties and is expected to be the parent compound of a new family of superconductors. When doped with potassium (K), BaMn2Bi2 undergoes a transition from an AFM insulator to an AFM metal. Consequently, it is of great interest to suppress the AFM order in the K-doped BaMn2Bi2 with the aim of exploring the potential superconductivity. Here, we report that external pressure up to 35.6 GPa cannot suppress the AFM order in the K-doped BaMn2Bi2 to develop superconductivity in the temperature range of 300 K-1.5 K, but induces a tetragonal (T) to an orthorhombic (OR) phase transition at ~20 GPa. Theoretical calculations for the T and OR phases, on basis of our high-pressure XRD data, indicate that the AFM order is robust in the pressurized Ba0.61K0.39Mn2Bi2. Both of our experimental and theoretical results suggest that the robust AFM order essentially prevents the emergence of superconductivity.

Pressure-induced change of the electronic state in the tetragonal phase of CaFe2As2

We have investigated the electronic states of single-crystal CaFe2As2 under hydrostatic pressure using (57)Fe Mössbauer spectroscopy and magnetization measurements. The center shift and the quadrupole splitting were refined from observed (57)Fe Mössbauer spectra using the single-crystalline sample under pressure at room temperature. A discontinuous decrease in the pressure dependence of the refined center shift was observed at 0.33 GPa without any anomaly in the pressure dependence of the refined quadrupole splitting, indicating a purely electronic state change in CaFe2As2 with a tetragonal structure. Such a change is shown to be reflected in the peak-like anomalies observed in the pressure dependences of the magnetic susceptibility at 0.26 GPa above 150 K. Our results reveal that this pressure-induced electronic state change suppresses the tetragonal-to-orthorhombic structural phase transition accompanied by an antiferromagnetic ordering. We further observed superconductivity in CaFe2As2 below ∼ 8 K around 0.33 GPa although our sample was not in a single phase at this pressure. These findings suggest that the electronic state change observed in CaFe2As2 with the tetragonal structure is relevant to the appearance of the pressure-induced superconductivity in AFe2As2.

Pressure-induced superconductivity and its scaling with doping-induced superconductivity in the iron pnictide with skutterudite intermediary layers

Pressure-induced superconductivity is oberserved in Ca10 (Pt3 As8 )(Fe2 As2 )5 by in situ high-pressure resistance and magnetic susceptibility measurements. Scaling of the pressure-induced and doping-induced superconductivity shows that the electronic phase diagrams of the pressurized and chemically doped 10-3-8 compound are similar in the moderate pressure and doping range but are disparate at higher pressure and heavy doping.

Microscopic Coexistence of Superconductivity and Antiferromagnetism in Underdoped Ba(Fe(1-x)Ru(x))2As2

We use (75)As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.23). We find two phase transitions: to antiferromagnetism at T(N) ≈ 60 K and to superconductivity at T(C) ≈ 15 K. Below T(N), our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4μ(B)/Fe. The spin-lattice relaxation rate 1/(75)T(1) is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T(C), 1/(75)T(1) on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

Interplay between Fe 3d and Ce 4f magnetism and Kondo interaction in CeFeAs(1-x)P(x)O probed by 75As and 31P NMR

A detailed (31)P (I = 1/2) and (75)As (I = 3/2) NMR study on polycrystalline CeFeAs(1-x)P(x)O alloys is presented. The magnetism of CeFeAsO changes drastically upon P substitution on the As site. CeFePO is a heavy fermion system without long-range order whereas CeFeAsO exhibits an Fe 3d SDW type of ordering accompanied by a structural transition from tetragonal (TT) to orthorhombic (OT) structure. Furthermore, Ce 4f(1) orders antiferromagnetically (AFM) at low temperature. At the critical concentration where the Fe magnetism is diminished the Ce-Ce interaction changes to a ferromagnetic (FM) type of ordering. Three representative samples of the CeFeAs(1-x)P(x)O (x = 0.05, 0.3 and 0.9) series are systematically investigated. (1) For the x = 0.05 alloy a drastic change of the linewidth at 130 K indicates the AFM-SDW type of ordering of Fe and the structural change from the TT to the OT phase. The linewidth roughly measures the internal field in the ordered state and the transition is most likely first order. The small and nearly constant shift from (31)P and (75)As NMR suggests the presence of competing hyperfine interactions between the nuclear spins and the 4f and 3d ions of Ce and Fe. (2) For the x = 0.3 alloy, the evolution of the Fe-SDW type of order takes place at around 70 K corroborating the results of bulk measurement and μSR. Here we found evidence for phase separation of paramagnetic and magnetic SDW phases. (3) In contrast to the heavy fermion CeFePO for the x = 0.9 alloy a phase transition is found at 2 K. The field-dependent NMR shift gives evidence of FM ordering. Above the ordering the spin-lattice relaxation rate (31)(1/T(1)) shows unconventional, non-Korringa-like behaviour which indicates a complex interplay of Kondo and FM fluctuations.

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.

77Se NMR study of the pairing symmetry and the spin dynamics in K(y)Fe(2-x)Se2

We present a 77Se NMR study of the newly discovered iron selenide superconductor K(y)Fe(2-x)Se2, in which T(c) = 32  K. Below T(c), the Knight shift 77K drops sharply with temperature, providing strong evidence for singlet pairing. Above T(c), Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-T(c) superconductivity in this iron selenide system.