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 .

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

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The ab-plane resistivity of Ba(Fe1−xRux)2As2 (x = 0.00, 0.09, 0.16, 0.21, and 0.28) was studied under nearly hydrostatic pressures, up to 7.4 GPa, in order to explore the T − P phase diagram and to compare the combined effects of iso-electronic Ru substitution and pressure. The parent compound BaFe2As2 exhibits a structural/magnetic phase transition near 134 K. At ambient pressure, progressively increasing Ru concentration suppresses this phase transition to lower temperatures at the approximate rate of ∼ 5 K/% Ru and is correlated with the emergence of superconductivity. By applying pressure to this system, a similar behavior is seen for each concentration: the structural/magnetic phase transition is further suppressed and superconductivity induced and ultimately, for larger x Ru and P , suppressed. A detailed comparison of the T − P phase diagrams for all Ru concentrations shows that 3 GPa of pressure is roughly equivalent to 10% Ru substitution. Furthermore, due to the sensitivity of Ba(Fe1−xRux)2As2 to pressure conditions, the melting of the liquid media, 4 : 6 light mineral oil : n-pentane and 1 : 1 iso-pentane : n-pentane, used in this study could be readily seen in the resistivity measurements. This feature was used to determine the freezing curves for these media and to infer their room temperature, hydrostatic limits: 3.5 and 6.5 GPa, respectively.

We carried out a study of the pressure dependence of the solidification temperature in nine pressure transmitting media that are liquid at ambient temperature, under pressures up to 2.3 GPa. These fluids are: 1:1 isopentane/n-pentane, 4:6 light mineral oil/n-pentane, 1:1 isoamyl alcohol/n-pentane, 4:1 methanol/ethanol, 1:1 FC72/FC84 (Fluorinert), Daphne 7373, isopentane, and Dow Corning PMX silicone oils 200 and 60,000 cst. We relied on the sensitivity of the electrical resistivity of Ba(Fe1-xRux)2As2 single crystals to the freezing of the pressure media, and crosschecked with corresponding anomalies observed in the resistance of the manganin coil that served as the ambient temperature resistive manometer. In addition to establishing the Temperature-Pressure line separating the liquid (hydrostatic) and frozen (non-hydrostatic) phases, these data permit rough estimates of the freezing pressure of these media at ambient temperature. This pressure establishes the extreme limit for the medium to be considered hydrostatic. For higher applied pressures the medium has to be treated as non-hydrostatic.2

We investigate the in-plane resistivity of single crystalline samples of Ba(Fe 1−x Co x ) 2 As 2 (x = 0.038, 0.047, 0.074, 0.1 and 0.114), Ba(Fe 0.973 Cr 0.027 ) 2 As 2 and slightly tin-doped BaFe 2 As 2 under various pressures up to 7.5 GPa, in order to establish temperature-pressure, T (P), phase diagrams and to compare the influence of pressure and doping on superconductivity. At ambient pressure, cobalt doping is known to lead to a decrease in the combined magnetic and structural transition temperature T 0 . Likewise, an increase of pressure tends to have the same effect for Ba(Fe 1−x Co x ) 2 As 2 for the various values of x. As was seen in the T (P) phase diagram of BaFe 2 As 2 , a superconducting dome is observed for Ba(Fe 1−x Co x ) 2 As 2 samples with the dome shifted to lower temperatures and pressures with increased cobalt doping levels. A very different behaviour is noticed for Ba(Fe 0.973 Cr 0.027 ) 2 As 2 and the slightly tin-doped BaFe 2 As 2 with the decrease of T 0 being close to linear down to 2 K, and no obvious sign of superconductivity in the pressure range investigated.

We present a simple novel technique to adapt a standard Bridgman cell for the use of a liquid pressure transmitting medium. The technique has been implemented in a compact cell, able to fit in a commercial Quantum Design PPMS system, and would also be easily adaptable to extreme conditions of very low temperatures or high magnetic fields. Several media have been tested and a mix of fluorinert FC84:FC87 has been shown to produce a considerable improvement over the pressure conditions in the standard steatite solid medium, while allowing a relatively easy setup procedure. For optimized hydrostatic conditions, the success rate is about 80% and the maximum pressure achieved so far is 7.1 GPa. Results are shown for the heavy fermion system YbAl(3) and for NaV(6)O(15), an insulator showing charge order.

We report electrical transport and calorimetry studies on high quality single crystals of YbCu 2 Si 2 under pressure in diamond and Bridgman anvil pressure cells with liquid pressure-transmitting media. At ambient pressure, in the best samples we find residual resistivities of less than 0.5 ⍀ cm. We have also determined the anisotropy of the temperature dependence of the resistivity and shown that this can account for sampledependent differences of the resistivity. We confirm the previously reported transition to a magnetically ordered ground state with pressure and we have precisely determined the ͑p , T͒ phase diagram using pure samples, good hydrostatic pressure conditions, and calorimetry measurements. Above 8.8 GPa we see a clear signature of the transition in the specific heat. We discuss the low-temperature resistivity and specific-heat behaviors in respect to the universal Kadowaki Woods ratio, to other ytterbium-based strongly correlated systems, and to the cerium-based counterpart, CeCu 2 Si 2 .