Hyunsoo Kim

Anisotropic London penetration depth and superfluid density in single crystals of iron-based pnictide superconductors

Abstract In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. The “122” system was represented by electron-doped Ba(Fe 1− x Co x ) 2 As 2 with the doping through the whole phase diagram with x xa0≈xa00.038, 0.047, 0.058, 0.074 and 0.10 ( T c ranged from 13 to 24xa0K) and by hole-doped (Ba 1− x K x )Fe 2 As 2 with doping close to optimal, with measured x xa0≈xa00.45 ( T c xa0≈xa028xa0K) and an underdoped sample with x xa0≈xa00.15 ( T c xa0≈xa019xa0K). The “1111” system was represented by single crystals of NdFeAs(O 1− x F x ) with nominal x xa0=xa00.1 ( T c xa0≈xa043xa0K). All studied samples of both 122 systems show a robust power-law behavior, λ ( T ) ∝ T n , with the sample-dependent exponent n xa0=xa02–2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a S ± state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of the results is complicated by magnetism of the rare-earth ions. For all three systems, the anisotropy ratio, γ λ ≡ λ c / λ ab , was found to decrease with increasing temperature, whereas the anisotropy of the coherence lengths, γ ξ ≡ ξ ab / ξ c = H c 2 ⊥ c / H c 2 ‖ c , has been found to increase (both opposite to the trend in two-band MgB 2 ). The overall anisotropy of the pnictide superconductors is small, in fact much smaller than that of the cuprates (except YBa 2 Cu 3 O 7− x (YBCO)). The 1111 system is about two times more anisotropic than the 122 system. Our data and analysis suggest that the iron-based pnictides are complex superconductors in which a multiband three-dimensional electronic structure and strong magnetic fluctuations play important roles.

From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and the pairing mechanism. We also compare KFe2As2 with the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

Nodal to nodeless superconducting energy-gap structure change concomitant with fermi-surface reconstruction in the heavy-fermion compound CeCoIn(5).

The London penetration depth λ(T) was measured in single crystals of Ce_{1-x}R_{x}CoIn_{5}, R=La, Nd, and Yb down to T_{min}≈50u2009u2009mK (T_{c}/T_{min}∼50) using a tunnel-diode resonator. In the cleanest samples Δλ(T) is best described by the power law Δλ(T)∝T^{n}, with n∼1, consistent with the existence of line nodes in the superconducting gap. Substitutions of Ce with La, Nd, and Yb lead to similar monotonic suppressions of T_{c}; however, the effects on Δλ(T) differ. While La and Nd substitution leads to an increase in the exponent n and saturation at n∼2, as expected for a dirty nodal superconductor, Yb substitution leads to n>3, suggesting a change from nodal to nodeless superconductivity. This superconducting gap structure change happens in the same doping range where changes of the Fermi-surface topology were reported, implying that the nodal structure and Fermi-surface topology are closely linked.

Anisotropic magnetism, resistivity, London penetration depth and magneto-optical imaging of superconducting K0.80Fe1.76Se2 single crystals

Single crystals of K0.80Fe1.76Se2 were successfully grown from a ternary solution. We show that, although crystals form when cooling a near-stoichiometric melt, crystals are actually growing out of a ternary solution that remains liquid to at least 850??C. We investigated their chemical composition, anisotropic magnetic susceptibility and resistivity, specific heat, thermoelectric power, London penetration depth and flux penetration via magneto-optical imaging. Whereas the samples appear to be homogeneously superconducting at low temperatures, there appears to be a broadened transition range close to Tc ~ 30?K that may be associated with small variations in stoichiometry.

Spin glass and glass-like lattice behaviour in HoB66at low temperatures

The low-temperature specific heat, magnetic susceptibility, and thermal expansion are studied experimentally in the fcc boride HoB66 and compared to similar quantities in non-magnetic LuB66. The anomalous behaviour observed in HoB66 is explained within a glass-like picture together with magnetic subsystem ordering processes. The character of the observed anomalies suggests the existence of a spin-glass phase transition in HoB66 below a characteristic temperature of T s u2009≈u20090.98u2009K.

Crystal growth and annealing study of fragile, non-bulk superconductivity in YFe2Ge2

We investigated the occurrence and nature of superconductivity in single crystals of grown out of Sn flux by employing X-ray diffraction, electrical resistivity and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as , by decanting the crystals from the molten Sn at and/or by annealing at temperatures between 550 and . We found that the samples with RRR 34 showed resistive signatures of superconductivity with the onset of the superconducting transition . RRR values vary between 35 and 65 with, on average, no systematic change in value, indicating that the systematic changes in RRR do not lead to comparable changes in . Specific heat measurements on samples that showed the clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain-stabilized superconductivity associated with small amounts of stressed (perhaps at twin boundaries or dislocations) or is a second crystallographic phase that is present at level below detection capability of conventional powder X-ray techniques.

Upper critical field of high-quality single crystals of KFe2As2

S. Yeninas, 2 M. A. Tanatar, 2 J. Murphy, 2 C. P. Strehlow, 2 O.E. Ayala-Valenzuela, R.D. McDonald, U. Welp, W. K. Kwok, T. Kobayashi, S. Miyasaka, 6 S. Tajima, 6 and R. Prozorov 2, ∗ The Ames Laboratory, Ames, IA 50011, USA Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA NHMFL, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan JST, Transformative Research-Project on Iron Pnictides (TRIP), Tokyo 102-0075, Japan (Dated: 25 January 2013)Measurements of temperature-dependent in-plane resistivity,

Ferromagnetic quantum critical point avoided by the appearance of another magnetic phase in LaCrGe3 under pressure

rho(T)

Tunable electronic anisotropy in single-crystal A2Cr3As3 (A=K, Rb) quasi-one-dimensional superconductors

, were used to determine the upper critical field and its anisotropy in high quality single crystals of stoichiometric iron arsenide superconductor KFe

Resolution of the discrepancy between the variation of the physical properties of Ce1-xYbxCoIn5 single crystals and thin films with Yb composition

_2