In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.

Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T(c) approximately 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.

When a pure material is tuned to the point where a continuous phase-transition line is crossed at zero temperature, known as a quantum critical point (QCP), completely new correlated quantum ordered states can form 1-7 . These phases include exotic forms of superconductivity. However, as superconductivity is generally suppressed by a magnetic field, the formation of superconductivity ought not to be possible at extremely high field 8 . Here, we report that as we tune the ferromagnet, URhGe, towards a QCP by applying a component of magnetic field in the material's easy magnetic plane, superconductivity survives in progressively higher fields applied simultaneously along the material's magnetic hard axis. Thus, although superconductivity never occurs above a temperature of 0.5 K, we find that it can survive in extremely high magnetic fields, exceeding 28 T. (refs 5,6). Theoretically, on tuning a material towards a QCP by application of pressure or magnetic field, the strength of the magnetic fluctuations that potentially bring about superconductivity increases. On approaching a ferromagnetic QCP, longitudinal magnetic fluctuations promote the formation of unconventional spin-triplet superconductivity 9-11 . Theories predict d-wave superconductivity close to QCPs involving antiferromagnetic states. The evolution of the superconductivity as a material is tuned more closely to the field or pressure of the underlying QCP depends on the balance between the weights of fluctuations that are pair forming and pair breaking 12 . The critical temperature for superconductivity, T s , is predicted to saturate 9 or may even decrease 10 . Experimentally, superconducting states in antiferromagnetic systems have been more extensively studied than in ferromagnets and measurements show T s to have a dome-shaped pressure dependence crossing the underlying QCP 2,5 . However, recent work indicates that the pressure-temperature phase diagrams of antiferromagnetic systems might be more complex than previously thought 5,6,13 . As it is difficult to vary pressure continuously at low temperature, other properties of the superconducting state, such as the critical field to suppress superconductivity in CePd 2 Si 2 (ref. 14), lack measurements at a sufficient number of pressures to discern how they vary approaching a QCP. For URhGe, the QCP is ferromagnetic rather than antiferromagnetic and can be approached by applying a magnetic field, which can be swept continuously.In zero magnetic field, URhGe undergoes a ferromagnetic transition at T Curie = 9.5 K. Below this temperature, the ordered magnetic moment is aligned parallel or antiparallel to the crystallographic c axis 15 . Magnetic fields that correspond to opposite directions of the c-axis moment are separated in low magnetic field by a plane of first-order transitions that is crossed when the c-axis component of the magnetic field changes sign. The temperature above which this first-order transition plane ceases defines a line along which the phase transition is continuous. The schemati...

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We present magnetic torque measurements on the Shastry-Sutherland quantum spin system SrCu2(BO3)2 in fields up to 31 T and temperatures down to 50 mK. A new quantum phase is observed in a 1 T field range above the 1/8 plateau, in agreement with recent NMR results. Since the presence of the DM coupling precludes the existence of a true Bose-Einstein condensation and the formation of a supersolid phase in SrCu2(BO3)2, the exact nature of the new phase in the vicinity of the plateau remains to be explained. Comparison between magnetization and torque data reveals a huge contribution of the Dzyaloshinskii-Moriya interaction to the torque response. Finally, our measurements demonstrate the existence of a supercooling due to adiabatic magnetocaloric effects in pulsed field experiments.

Uniaxial pressure applied in the b crystallographic direction perpendicular to spontaneous mag-netization in heavy fermion ferromagnet URhGe strongly stimulates superconductivity in this compound. The phenomenological approach allows point out two mechanisms of superconducting temperature raising. They originates from stimulation by the uniaxial stress both intraband and inter-band amplitudes of triplet Cooper pairing. The phenomenon of reentrant superconductivity under magnetic field along b-axis is also strongly sensitive to the uniaxial stress in the same direction. The uniaxial stress accelerates suppression the Curie temperature by the transversal magnetic field. The emergence of the first order transition to the paramagnetic state occurs at much lower field than in the absence of uniaxial stress.

The application of pressure to elemental bismuth reduces its conduction-valence band overlap, and results in a semimetal-semiconductor (SMSC) transition around 25 kbar. This transition is nominally of the topological ''Lifshitz'' Fermi surface variety, but there are open questions about the role of interactions at low charge densities. Using a novel pressure cell with optical access, we have performed an extensive study of bismuth's infrared conductivity under pressure. In contrast to the expected pure band behavior we find signatures of enhanced interaction effects, including strongly coupled charge-plasmon (plasmaron) features and a plasma frequency that remains finite up to the transition. These effect are inconsistent with a pure Lifshitz bandlike transition. We postulate that interactions play a central role in driving the transition.

As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of their coexistence in UGe(2), URhGe, UIr and UCoGe has attracted a lot of interest. The mechanism to explain such a state has, however, not yet been fully elucidated. In these compounds superconductivity may be unconventional: Cooper pairs could be formed by electrons with parallel spins and magnetic fluctuations might be involved in the pairing mechanism. URhGe becomes ferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to the c-axis. For temperatures below 260 mK and fields lower than 2 T, superconductivity was first observed in 2001. Recently, we discovered a second pocket of superconductivity. This new pocket of superconductivity appears at higher fields applied close to the b-axis, enveloping a sudden magnetic moment rotation transition at H(R) = 12 T. Detailed studies of the field induced metamagnetic transition and superconductivity are presented. The possibility that magnetic fluctuations emerging from a quantum critical point provide the pairing mechanism for superconductivity is discussed.