Quantum phase transition inside the superconducting dome of Ba(Fe1-xCox)2As2 from diamond-based optical magnetometry

Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature $T_c$ acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in $\lambda (x)$ near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity

Analysis of the London penetration depth in Ni-doped CaKFe4 As4

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe1−xNix)4As4 (CaK1144) for x=0, 0.017, and 0.034. To obtain the superfluid density ρ=[1+ΔλL(T)/λL(0)]−2, the temperature dependence of the London penetration depth ΔλL(T) was measured by using a tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of λL(T≪Tc)≈λL(0) was measured by using MWR, λL(5K)≈170±20 nm, which agreed well with the NV centers in diamond optical magnetometry that gave λL(5K)≈196±12 nm, which agreed well with the NV centers in diamond optical magnetometry that gave λL(5K)≈196±12 nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless s± order parameter and that upon Ni doping the interband interaction increases

Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe4As4

Measurements of the London penetration depth Δλ(T) and tunneling conductance in single crystals of the recently discovered stoichiometric iron-based superconductor CaKFe4As4 (CaK1144) show nodeless, two-effective-gap superconductivity with a larger gap of about 6-10 meV and a smaller gap of about 1-4 meV. Having a critical temperature Tc,onset≈35.8 K, this material behaves similar to slightly overdoped (Ba1-xKx)Fe2As2 (e.g., x=0.54,Tc≈34 K), a known multigap s± superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap s± superconductivity is an essential property of high-temperature superconductivity in iron-based superconductors, independent of the degree of substitutional disorderWe thank A. Gurevich, D. D. Johnson, A. Kaminski, V. G. Kogan, and Lin-Lin Wang for useful discussions. This work was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. Ames Laboratory is operated for the US DOE by Iowa State University under Contract DE-AC02-07CH11358. The work in Madrid was supported by the Spanish Ministry of Economy and Competitiveness (FIS2014-54498-R and MDM-2014-0377), by the Comunidad de Madrid through program Nanofrontmag-CM (S2013/MIT-2850) by Axa Research Fund, FP7-PEOPLE-2013-CIG 618321, and the European Research Council (Grant Agreement No. 679080). Madrid's group also acknowledges SEGAINVEX-UAM. W.R.M. was funded by the Gordon and Betty Moore Foundation's EPiQS Initiative through Grant GBMF441

DNA intercalation facilitates efficient DNA-targeted covalent binding of phenanthriplatin

Phenanthriplatin, a monofunctional anticancer agent derived from cisplatin, shows significantly more rapid DNA covalent-binding activity compared to its parent complex. To understand the underlying molecular mechanism, we used single-molecule studies with optical tweezers to probe the kinetics of DNA–phenanthriplatin binding as well as DNA binding to several control complexes. The time-dependent extensions of single λ-DNA molecules were monitored at constant applied forces and compound concentrations, followed by rinsing with a compound-free solution. DNA–phenanthriplatin association consisted of fast and reversible DNA lengthening with time constant τ ≈ 10 s, followed by slow and irreversible DNA elongation that reached equilibrium in ∼30 min. In contrast, only reversible fast DNA elongation occured for its stereoisomer trans-phenanthriplatin, suggesting that the distinct two-rate kinetics of phenanthriplatin is sensitive to the geometric conformation of the complex. Furthermore, no DNA unwinding was observed for pyriplatin, in which the phenanthridine ligand of phenanthriplatin is replaced by the smaller pyridine molecule, indicating that the size of the aromatic group is responsible for the rapid DNA elongation. These findings suggest that the mechanism of binding of phenanthriplatin to DNA involves rapid, partial intercalation of the phenanthridine ring followed by slower substitution of the adjacent chloride ligand by, most likely, the N7 atom of a purine base. The cis isomer affords the proper stereochemistry at the metal center to facilitate essentially irreversible DNA covalent binding, a geometric advantage not afforded by trans-phenanthriplatin. This study demonstrates that reversible DNA intercalation provides a robust transition state that is efficiently converted to an irreversible DNA-Pt bound state

Quantum phase transition inside the superconducting dome of Ba(Fe1-xCox)2As2 from diamond-based optical magnetometry

Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature $T_c$ acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in $\lambda (x)$ near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.</p

Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe4 As4

Measurements of the London penetration depth Δλ(T) and tunneling conductance in single crystals of the recently discovered stoichiometric iron-based superconductor CaKFe4As4 (CaK1144) show nodeless, two-effective-gap superconductivity with a larger gap of about 6–10 meV and a smaller gap of about 1–4 meV. Having a critical temperature Tc,onset≈35.8 K, this material behaves similar to slightly overdoped (Ba1−xKx)Fe2As2 (e.g., x=0.54,Tc≈34 K), a known multigap s± superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap s± superconductivity is an essential property of high-temperature superconductivity in iron-based superconductors, independent of the degree of substitutional disorder.This article is published as Cho, Kyuil, A. Fente, S. Teknowijoyo, M. A. Tanatar, K. R. Joshi, N. M. Nusran, T. Kong, W. R. Meier, U. Kaluarachchi, I. Guillamon, H. Suderow, S. L. Bud’ko, P. C. Canfield, and R. Prozorov. "Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe4 As4." Physical Review B 95, no. 10 (2017): 100502. DOI: 10.1103/PhysRevB.95.100502. Posted with permission.</p

Analysis of the London penetration depth in Ni-doped CaKFe4 As4

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe1-xNix)(4) As-4 (CaK1144) for x = 0, 0.017, and 0.034. To obtain the superfluid density rho = [1 + Delta lambda(L)(T)/lambda(K)(0)](-2), the temperature dependence of the London penetration depth Delta lambda(L)(T) was measured by using a tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of lambda(L)(T << T-c) approximate to lambda(L)(0) was measured by using MWR, lambda(L)(5 K) approximate to 170 +/- 20 nm, which agreed well with the NV centers in diamond optical magnetometry that gave lambda(L)(5 K) 196 +/- 12 nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless s(+/-) order parameter and that upon Ni doping the interband interaction increases.</p

Decision Making in Phagocytosis.

Dictyostelium cells are professional phagocytes that are capable of handling particles of variable shapes and sizes. Here we offer long bacteria that challenge the uptake mechanism to its limits and report on the responses of the phagocytes if they are unable to engulf the particle by closing the phagocytic cup. Reasons for failure may be a length of the particle much larger than the phagocyte's diameter, or competition with another phagocyte. A cell may simultaneously release a particle and engulf another one. The final phase of release can be fast, causing the phagosome membrane to turn inside-out and to form a bleb. Myosin-II may be involved in the release by generating tension at the plasma membrane, it does however not accumulate on the phagosome to act there directly in expelling the particle. Labeling with GFP-2FYVE indicates that processing of the phagosome with phosphatidylinositol 3-phosphate begins at the base of a long phagosome already before closure of the cup. The decision of releasing the particle can be made even at the stage of the processed phagosome