Sergey L. Bud'ko

Thermodynamic and Transport Properties of Superconducting Mg 10 B 2

Transport and thermodynamic properties of a sintered pellet of the newly discovered MgB2 superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, Hc2, with magnetization data giving dHc2/dT = 0.44 T/K at the transition temperature of Tc = 40.2 K. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of 1 T. The value of dHc/dT at Tc is estimated to be about 12 mT/K, a value similar to classical superconductors like Sn. Hence, the Ginzburg-Landau parameter kappa approximately 26. Estimates of the critical supercurrent density, Jc, using hysteresis loops and the Bean model, give critical current densities on the order of 10(5) A/cm2. Hence the supercurrent coupling through the grain boundaries is comparable to intermetallics like Nb3Sn.

Pressure induced superconductivity in CaFe2As2.

CaFe2As2 has been found to be exceptionally sensitive to the application of hydrostatic pressure and can be tuned to reveal all the salient features associated with FeAs superconductivity without introducing any disorder. The ambient pressure, 170 K, structural/magnetic, first-order phase transition is suppressed to 128 K by 3.5 kbar. At 5.5 kbar a new transition is detected at 104 K, increasing to above 300 K by 19 kbar. A low temperature, superconducting dome (T(c) approximately 12 K) is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity too dramatically.

Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1–xCox)2As2

Nematic Electronic Order in Iron Superconductors The properties of many high-temperature superconductors vary strongly as the composition of a doping element changes, and at sufficient under- or overdoping, other phases with different types of electronic ordering can form. Chuang et al. (p. 181; see the Perspective by Fradkin and Kivelson) use scanning tunneling microscopy techniques to probe the electronic structure of an underdoped compound in the iron superconductor family, Ca(Fe1−xCox)2As2. They observed periodic nanostructures oriented along Fe–Fe bonds that exhibit an electronic ordering related to ordering seen in nematic liquid crystals. Unusual electronic ordering was observed in surface measurements of an underdoped phase of a ferropnictide superconductor. The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging–scanning tunneling microscopy to study the electronic structure of a representative compound CaFe1.94Co0.06As2 in the “parent” state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter–iron-atom distance aFe-Fe and aligned along the crystal a axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b axis only and are consistent with a nematic α2 band with an apparent band folding having wave vector q≅±22π/8aFe-Fe along the a axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.

Classical and Quantum Magnetism in Giant Keplerate Magnetic Molecules

Complementary theoretical modeling methods are presented for the classical and quantum Heisenberg model to explain the magnetic properties of nanometer-sized magnetic molecules. Excellent quantitative agreement is achieved between our experimental data down to 0.1 K and for fields up to 60 Tesla and our theoretical results for the giant Keplerate species {Mo72Fe30}, by far the largest paramagnetic molecule synthesized to date.

FeAs-Based Superconductivity: A Case Study of the Effects of Transition Metal Doping on BaFe2As2

The recently discovered FeAs-based superconductors are a new, promising set of materials for technological and basic research. They offer transition temperatures as high as 55 K as well as essentially isotropic, and extremely large, upper, superconducting critical fields, in excess of 40 T at 20 K. In addition, they may well provide insight into exotic superconductivity that extends beyond just FeAs-based superconductivity, perhaps even shedding light on the still-perplexing CuO-based high-Tc materials. Whereas superconductivity can be induced in the RFeAsO (R = rare earth) and AEFe2As2 (AE = Ba, Sr, Ca) families by several means, transition metal (TM) doping of BaFe2As2 [e.g., Ba(Fe1–xTMx)2As2] offers the easiest experimental access to a wide set of materials and states. In this review, we present an overview and summary of the effect of TM-doping (TM = Co, Ni, Cu, Pd, and Rh) on BaFe2As2. The resulting phase diagrams reveal the nature of the interaction between the structural, magnetic, and superconduct...

Anisotropy and large magnetoresistance in the narrow-gap semiconductor FeSb2

A study of the anisotropy in magnetic, transport, and magnetotransport properties of

Possible co-existence of superconductivity and weak ferromagnetism in ErNi2B2C

{\mathrm{FeSb}}_{2}

Superconducting MgB2 thin films by pulsed laser deposition

has been made on large single crystals grown from Sb flux. Magnetic susceptibility of

Anomalous temperature-dependent transport in YbNi2B2C and its correlation to microstructural features

{\mathrm{FeSb}}_{2}

Structural transition and anisotropic properties of single-crystalline SrFe2As2

shows diamagnetic to paramagnetic crossover around 100 K. Electrical transport along two axes is semiconducting, whereas the third axis exhibits a metal-semiconductor crossover at temperature