The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

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The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa2Cu3Oy (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La1.8−xEu0.2SrxCuO4 (Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping. The fact that in Eu-LSCO this reconstruction coexists with spin and charge modulations that break translational symmetry shows that stripe order is the generic non-superconducting ground state of hole-doped cuprates.

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.

We present a study of the upper critical field of the newly discovered heavy fermion superconductor UTe 2 by magnetoresistivity measurements in pulsed magnetic fields up to 60 T and static magnetic fields up to 35 T. We show that superconductivity survives up to the metamagnetic transition at H m ≈ 35 T at low temperature. Above H m superconductivity is suppressed. At higher temperature superconductivity is enhanced under magnetic field leading to reentrance of superconductivity or an almost temperature independent increase of H c2 . By studying the angular dependence of the upper critical field close to the b axis (hard magnetization axis) we show that the maximum of the reentrant superconductivity temperature is depinned from the metamagnetic field. A key ingredient for the field-reinforcement of superconductivity on approaching H m appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction. 1 arXiv:1905.05181v1 [cond-mat.str-el]

We studied the influence of the disorder introduced in polycrystalline MgB 2 samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and 10 B we employed isotopically enriched 11 B which contains 40 times less 10 B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10 B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1 -2 ϫ 10 18 cm −2 . With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.

The Seebeck and Nernst coefficients S and ν of the cuprate superconductor YBa2Cu3Oy (YBCO) were measured in a single crystal with doping p = 0.12 in magnetic fields up to H = 28 T. Down to T = 9 K, ν becomes independent of field by H ≃ 30 T, showing that superconducting fluctuations have become negligible. In this field-induced normal state, S/T and ν/T are both large and negative in the T → 0 limit, with the magnitude and sign of S/T consistent with the small electron-like Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The change of sign in S(T ) at T ≃ 50 K is remarkably similar to that observed in La2−xBaxCuO4, La2−x−yNdySrxCuO4 and La2−x−yEuySrxCuO4, where it is clearly associated with the onset of stripe order. We propose that a similar density-wave mechanism causes the Fermi surface reconstruction in YBCO.PACS numbers: 74.25.Fy A major hurdle in understanding high-temperature superconductivity is the nature of the pseudogap phase. No consensus has yet been reached on whether this enigmatic phase is a precursor of superconductivity or a second ordered phase [1]. One way to shed light on this question is to study the ground state of the pseudogap phase in the absence of superconductivity, achieved by applying a strong enough magnetic field. This approach has recently revealed a qualitative change in the Fermi surface of cuprates measured via quantum oscillations, from a large hole-like cylinder in the overdoped regime outside the pseudogap phase [2] to a Fermi surface containing small electron-like pockets [3] in the underdoped regime inside the pseudogap phase [3][4][5][6][7]. Because the presence of an electron pocket in the Fermi surface of hole-doped cuprates almost certainly implies that the lattice translational symmetry is broken by some density-wave order [8], it is important to confirm the electron-like nature of the Fermi pocket detected in YBa 2 Cu 3 O y (YBCO), and elucidate the mechanism that causes it to emerge.In this Letter, we show that: 1) the low-temperature Nernst coefficient of YBCO at p = 0.12 is independent of field by H ≃ 30 T, proof that the vortex contribution is negligible by then, and the normal state has been reached; 2) the magnitude and negative sign of the thermopower at low temperature are consistent with the frequency and cyclotron mass of quantum oscillations only if these come from orbits around an electron pocket. From the fact that both the thermopower and the Hall coefficient of YBCO are very similar to those of three cuprate materials exhibiting 'stripe' order, a form of spin/charge density wave, we infer that the Fermi surface of underdoped YBCO also undergoes a reconstruction due to a similar form of spin and/or charge ordering.When a temperature difference ∆T is applied along the x-axis of a metallic sample, a longitudinal voltage V x develops across a length L, and the Seebeck coefficient (or thermopower) is defined as S ≡ V x /∆T . In the presence of a perpendicular magnetic field H (along the z-axis), a transverse voltage V ...

Heat-capacity measurements of a 39 microg MgB2 single crystal in fields up to 14 T and below 3 K allow the determination of the low-temperature linear term of the specific heat, its field dependence, and its anisotropy. Our results are compatible with two-band superconductivity, the band carrying the smaller gap being isotropic, that carrying the larger gap having an anisotropy of approximately 5. Three different upper critical fields are thus needed to describe the superconducting state of MgB2.