Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

The upper critical field of multiband superconductors is an important quantity that can reveal details about the nature of the superconducting pairing. Here we experimentally map out the complete upper-critical-field phase diagram of a stoichiometric superconductor, CaKFe4As4, up to 90T for different orientations of the magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching values close to ∼3Tc at the lowest temperature, and the anisotropy decreases dramatically with temperature, leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the upper critical field can be well described by a two-band model in the clean limit with band-coupling parameters favoring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean superconductor

Further analysis of the quantum critical point of Ce1−x_{1-x}Lax_{x}Ru2_{2}Si2_{2}

New data on the spin dynamics and the magnetic order of Ce$_{1-x}$La$_{x}$Ru$_{2}$Si$_{2}$ are presented. The importance of the Kondo effect at the quantum critical point of this system is emphasized from the behaviour of the relaxation rate at high temperature and from the variation of the ordered moment with respect to the one of the N\'eel temperature for various $x$.Comment: Contribution for the Festschrift on the occasion of Hilbert von Loehneysen 60 th birthday. To be published as a special issue in the Journal of Low Temperature Physic

Thermodynamic phase diagram of Fe(Se0.5Te0.5) single crystals in fields up to 28 tesla

International audienceWe report on specific heat Cp , transport, Hall probe, and penetration depth measurements performed on Fe Se0.5Te0.5 single crystals Tc 14 K . The thermodynamic upper critical field Hc2 lines has been deduced from Cp measurements up to 28 T for both H c and H ab, and compared to the lines deduced from transport measurements up to 55 T in pulsed magnetic fields . We show that this thermodynamic Hc2 line presents a very strong downward curvature for T→Tc which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that Hc2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/Tc 0.99 for H ab which is the consequence of a very small value of the coherence length c 0 4 Å and ab 0 15 Å , confirming the strong renormalization of the effective mass as compared to DMFT calculations previously observed in ARPES measurements A. Tamai, A. Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M. J. Rosseinsky, and F. Baumberger, Phys. Rev. Lett. 104, 097002 2010 . Hc1 measurements lead to ab 0 =430 50 nm and c 0 =1600 200 nm and the corresponding anisotropy is approximatively temperature independent 4 , being close to the anisotropy of Hc2 for T→Tc. The temperature dependence of both T2 and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor, CaKFe4_4As4_4

The upper critical field of multiband superconductors is an important quantity that can reveal the details about the nature of the superconducting pairing. Here we experimentally map out the complete upper critical field phase diagram of a stoichiometric superconductor, CaKFe$_4$As$_4$, up to 90T for different orientations of the magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching values close to ~3$T_c$ at the lowest temperature, and the anisotropy decreases dramatically with temperature leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the upper critical field can be well described by a two-band model in the clean limit with band coupling parameters favouring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean superconductor.Comment: to appear in Physical Review B (2020); 13 pages, 9 figure

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.Comment: 7 pages, 5 figures, submitted to J. Phys. Soc. Jp

Quenched nematic criticality and two superconducting domes in an iron-based superconductor

The nematic electronic state and its associated critical fluctuations have emerged as a potential candidate for the superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with magnetically ordered phases poses a significant challenge in determining their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we access a nematic quantum phase transition isolated from any other competing magnetic phases. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure and detect a Lifshitz transition that separates two distinct superconducting regions. One emerges from the nematic phase with a small Fermi surface and strong electronic correlations, while the other one has a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically ordered phase at high pressures. The absence of mass divergence at the nematic quantum phase transition suggests that the nematic fluctuations could be quenched by the strong coupling to the lattice or local strain effects. A direct consequence is the weakening of superconductivity at the nematic quantum phase transition in the absence of magnetically driven fluctuations

Heavy fermions in high magnetic field

We give an overview on experimental studies performed in the last 25 years on heavy-fermion systems in high magnetic field. The properties of field-induced magnetic transitions in heavy-fermion materials close to a quantum antiferromagnetic-to-paramagnetic instability are presented. Effects of a high magnetic field to the Fermi surface, in particular the splitting of spin-up and spin-down bands, are also considered. Finally, we review on recent advances on the study of non-centrosymmetric compounds and ferromagnetic superconductors in a high magnetic field.Comment: 37 pages, 26 figures, Special Issue of the "Comptes Rendus de l'Acad\'emie des Sciences" on the Physics in High Magnetic Fiel

Critical international relations and the impact agenda

How should critical International Relations (IR) scholars approach the ‘impact agenda’? While most have been quite resistant to it, I argue in this essay that critical IR should instead embrace the challenge of impact – and that both IR as a field and the impact agenda more broadly would gain greatly from it doing so. I make this case through three steps. I show, firstly, that critical IR has till now been very much at the impact agenda’s margins, and that this situation contrasts strikingly with its well-established importance within IR teaching and research. I argue, secondly, that critical IR scholars both could and should do more impact work – that the current political conjuncture demands it, that many of the standard objections to doing so are misplaced, and indeed that ‘critical’ modes of research are in some regards better suited than ‘problem-solving’ ones to generating meaningful change – and offer a series of recommended principles for undertaking critically-oriented impact and engagement work. But I also argue, thirdly, that critical social science holds important lessons for the impact agenda, and that future impact assessments need to take these lessons on board – especially if critical IR scholarship is to embrace impact more fully. Critical IR, I submit, should embrace impact; but at the same time, research councils and assessments could do with modifying their approach to it, including by embracing a more critical and political understanding of what impact is and how it is achieved