Low-temperature specific-heat and thermal-expansion measurements on a single crystal of the heavyfermion superconductor UBe 13 reveal an anomaly in the superconducting state. The evolution of this feature is studied as a function of Th doping on several U 12x Th x Be 13 polycrystals (x # 0.03), giving rise to a new "line of anomalies," T L ͑x͒, in the T -x diagram. The anomaly at T L ͑x͒ marks the precursor of the lower of the two phase transitions at x . x cr ഠ 0.019. Implications for the interpretation of the phase diagram of superconducting U 12x Th x Be 13 are addressed. [S0031-9007(98)

Kondo insulator materials--such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3 and SmB6--are 3d, 4f and 5f intermetallic compounds that have attracted considerable interest in recent years. At high temperatures, they behave like metals. But as temperature is reduced, an energy gap opens in the conduction band at the Fermi energy and the materials become insulating. This contrasts with other f-electron compounds, which are metallic at all temperatures. The formation of the gap in Kondo insulators has been proposed to be a consequence of hybridization between the conduction band and the f-electron levels, giving a 'spin' gap. If this is indeed the case, metallic behaviour should be recovered when the gap is closed by changing external parameters, such as magnetic field or pressure. Some experimental evidence suggests that the gap can be closed in SmB6 (refs 5, 8) and YbB12 (ref. 9). Here we present specific-heat measurements of Ce3Bi4Pt3 in d.c. and pulsed magnetic fields up to 60 tesla. Numerical results and the analysis of our data using the Coqblin-Schrieffer model demonstrate unambiguously a field-induced insulator-to-metal transition.

The measurement of low-temperature specific heat (LTSH) (0.1 K<T<60 K) has seen a number of breakthroughs both in design concepts and instrumentation in the last 15 years—particularly in small sample calorimetry. This review attempts to provide an overview of both large and small sample calorimetry techniques at temperatures below 60 K, with sufficient references to enable more detailed study. A comprehensive review is made of the most reliable measurements of the LTSH of 84 of the elements to illustrate briefly some of the problems of measurements and analysis, as well as to provide additional references. More detail is devoted to three special areas of low-temperature calorimetry that have seen rapid development recently—(1) measurement of the specific heat of highly radioactive samples, (2) measurement of the specific heat of materials in high magnetic fields (18 T), and (3) measurement of the specific heat of very small (100 μg) samples. The review ends with a brief discussion of the frontier research currently underway on microcalorimetry for nanogram sample weights.

Abstract:We report specific heat measurements on the Fe-based superconductor BaFe 2 (As 0.7 P 0.3 ) 2 , a material on which previous penetration depth, NMR, and thermal conductivity measurements have observed a high density of low-energy excitations, which have been interpreted in terms of order parameter nodes. Within the resolution of our measurements, the low temperature limiting C/T is found to be linear in field, i.e. we find no evidence for a Volovik effect associated with nodal quasiparticles in either the clean or dirty limit. We discuss possible reasons for this apparent contradiction.

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics. High-Tc cuprates crystallize into a layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high-Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside. This scenario gives rise to 2D characteristics in high-Tc cuprates that favor d-wave pairing symmetry. Here, we report superconductivity in a cuprate Ba2CuO4-y, wherein the local octahedron is in a very exceptional compressed version. The Ba2CuO4-y compound was synthesized at high pressure at high temperatures and shows bulk superconductivity with critical temperature (Tc) above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron, the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant 3D nature in addition to the conventional 3dx2-y2 orbital. This work sheds important light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials.

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms. In memoriam, to Neil Ashcroft, who inspired us all.

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We report the successful growth of high quality single crystals of LiFeAs with lateral sizes up to 5