No abstract

Since the discovery of superconductivity, there has been a drive to understand the mechanisms by which it occurs. The BCS (Bardeen-Cooper-Schrieffer) model successfully treats the electrons in conventional superconductors as pairs coupled by phonons (vibrational modes of oscillation) moving through the material, but there is as yet no accepted model for high-transition-temperature, organic or 'heavy fermion' superconductivity. Experiments that reveal unusual properties of those superconductors could therefore point the way to a deeper understanding of the underlying physics. In particular, the response of a material to a magnetic field can be revealing, because this usually reduces or quenches superconductivity. Here we report measurements of the heat capacity and magnetization that show that, for particular orientations of an external magnetic field, superconductivity in the heavy-fermion material CeCoIn(5) is enhanced through the magnetic moments (spins) of individual electrons. This enhancement occurs by fundamentally altering how the superconducting state forms, resulting in regions of superconductivity alternating with walls of spin-polarized unpaired electrons; this configuration lowers the free energy and allows superconductivity to remain stable. The large magnetic susceptibility of this material leads to an unusually strong coupling of the field to the electron spins, which dominates over the coupling to the electron orbits.

Microscopic, structural, transport, and thermodynamic measurements of single crystalline Ba͑Fe 1−x TM x ͒ 2 As 2 ͑TM= Ni and Cu͒ series, as well as two mixed TM= Cu/ Co series, are reported. In addition, high-magnetic field, anisotropic H c2 ͑T͒ data were measured up to 33 T for the optimally Ni-doped BaFe 2 As 2 sample. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure BaFe 2 As 2 are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the Ba͑Fe 1−x Ni x ͒ 2 As 2 ͑x Յ 0.072͒, superconductivity, with T c up to 19 K, is stabilized for 0.024Յ x Յ 0.072. In the Ba͑Fe 1−x Cu x ͒ 2 As 2 ͑x Յ 0.356͒ series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the x = 0.044 samples. In the Ba͑Fe 1−x−y Co x Cu y ͒ 2 As 2 series, superconductivity, with T c values up to 12 K ͑x ϳ 0.022 series͒ and 20 K ͑x ϳ 0.047 series͒, is stabilized. Quantitative analysis of the detailed temperature-dopant concentration ͑T − x͒ and temperature-extra electrons ͑T − e͒ phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further re-enforces the conclusion that suppression of the structural/magnetic phase transition temperature enhances T c on the underdoped side, but for the overdoped side T C max is determined by e. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines ͑structural and magnetic phase transitions͒ and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped BaFe 2 As 2 .

Heavy fermion compounds represent one of the most strongly correlated forms of electronic matter and give rise to low temperature states that range from small moment ordering to exotic superconductivity, both of which are often in close proximity to quantum critical points. These strong electronic correlations are associated with the transfer of entropy from the local moment degrees of freedom to the conduction electrons, and, as such, are intimately related to the low temperature degeneracy of the (originally) moment bearing ion. Here we report the discovery of six closely related Yb-based heavy fermion compounds, YbT 2Zn20, that are members of the larger family of dilute rare earth bearing compounds: RT 2Zn20 (T ‫؍‬ Fe, Co, Ru, Rh, Os, Ir). This discovery doubles the total number of Yb-based heavy fermion materials. Given these compounds' dilute nature, systematic changes in T only weakly perturb the Yb site and allow for insight into the effects of degeneracy on the thermodynamic and transport properties of these model correlated electron systems. correlated electron ͉ intermetallic compound H eavy fermion compounds have been recognized as one of the premier examples of strongly correlated electron behavior for several decades. Ce-and U-based heavy fermion compounds have been well studied, and in recent years a small number of Yb-based heavy fermions have been identified as well (1-3). Unfortunately, in part due to the somewhat unpredictable nature of 4f ion hybridization with the conduction electrons, it has been difficult to find closely related (e.g., structurally) heavy fermion compounds, other than of the ThCr 2 Si 2 structure, especially Yb-based ones, that allow for systematic studies of the Yb ion degeneracy. Part of this difficulty is associated with the fact that the 4f hybridization depends so strongly on the local environment of the rare earth ion.Dilute, rare earth (R) bearing, intermetallic compounds are ordered materials with Ͻ5 atomic percent rare earth fully occupying a unique crystallographic site. Such materials offer the possibility of investigating the interaction between conduction electrons and 4f electrons in fully ordered compounds for relatively low concentrations of rare earths. For the case of R ϭ Yb or Ce, these materials offer the possibility of preserving low temperature, coherent effects while more closely approximating the single ion Kondo impurity limit. A very promising example of such compounds is derived from the family of RT 2 Zn 20 (4) (T ϭ transition metal), which has recently been shown to allow for the tuning of the nonmagnetic R ϭ Y and Lu members to exceedingly close to the Stoner limit as well as allowing for the study of the effects of such a highly polarizable background on local moment magnetic ordering for R ϭ Gd (5). DiscoveryHere, we present thermodynamic and transport data on six strongly correlated Yb-based intermetallic compounds found in the RT 2 Zn 20 family for T ϭ Fe, Co, Ru, Rh, Os, and Ir, effectively doubling the number of known Yb-based heavy fermi...

The upper critical field, H(c2), of Mg(B1-xCx)(2) has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped MgB2 filaments were prepared, and for carbon levels below 4% the transition temperatures are depressed by about 1 K/% C and H(c2)(T=0) rises by about 5 T/% C. This means that 3.8% C substitution will depress T(c) from 39.2 to 36.2 K and raise H(c2)(T=0) from 16.0 to 32.5 T. These rises in H(c2) are accompanied by a rise in resistivity at 40 K from about 0.5 to about 10 microOmega cm.