† Contributed equally to this project. Magnetic monopoles 1 -3 are hypothetical elementary particles exhibiting quantized magnetic charge = ±( ) ⁄ and quantized magnetic flux = ± / . In principle, such a magnetic charge can be detected by the quantized jump in magnetic flux it generates upon passage through a superconducting quantum interference device (SQUID) 4 . Naturally, with the theoretical discovery that a plasma of emergent magnetic charges should exist in several lanthanide-pyrochlore magnetic insulators 5,6 including Dy2Ti2O7, this SQUID technique was proposed for their direct detection 6 . Experimentally, this has proven challenging because of the high number density, and the generation-recombination (GR) fluctuations, of the monopole plasma. Recently, however, theoretical advances have allowed the spectral density of magnetic-flux noise ( , ) due to GR fluctuations of ± * magnetic charge pairs to be predicted 7 , 8 . Here we report development of a SQUID based flux-noise spectrometer, and consequent measurements of the frequency and temperature dependence of ( , ) for Dy2Ti2O7 samples. Virtually all the elements of ( , )predicted for a magnetic monopole plasma, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence,

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The synthesis of the first 4d transition metal oxide-hydride, LaSr NiRuO H , is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO sheets present in the superconducting cuprates. DFT calculations confirm the presence of S=1/2  Ni and S=0, Ru centers, but neutron diffraction and μSR data show no evidence for long-range magnetic order between the Ni centers down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3dz2 and H 1s in the super-exchange pathways.

The synthesis of the first 4d transition metal oxidehydride,LaSr 3 NiRuO 4 H 4 ,isprepared via topochemical anion exchange.Neutron diffraction data showthat the hydride ions occupyt he equatorial anion sites in the host lattice and as aresult the Ru and Ni cations are located in aplane containing only hydride ligands,au nique structural feature with obvious parallels to the CuO 2 sheets present in the superconducting cuprates.DFT calculations confirm the presence of S = 1 = 2 Ni + and S = 0, Ru 2+ centers,b ut neutron diffraction and mSR data show no evidence for long-range magnetic order between the Ni centers down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3d z 2 and H1si nthe super-exchange pathways.Complex transition-metal oxides continue to be the subject of extensive study because they exhibit aw ide variety of interesting physical and chemical properties.T hese range from magnetoresistance,high-temperature superconductivity, and collective magnetism, to ferroelectricity,ionic conductivity,a nd unusual catalytic and photocatalytic behavior. [1] Ty pically the properties of complex oxides are tuned via cation substitutions,b ut modifications to the anion lattice, either by the introduction of anion vacancies or by substituting non-oxide heteroanions,c an also be used to modify the chemical and physical behavior of oxides.F or example,anion doping allows metal oxidation states to be adjusted, the onsite electronic configuration of transition metal centers to be modified (through ligand-field interactions) and inter-cation couplings to be tuned (though cation-anion-cation linkages), providing access to novel electronic states.Thecontrasting features of oxide (O 2À )and hydride (H À ) anions offer many attractive opportunities to modify the electronic properties of host oxide phases by anion substitution. Themost obvious difference between oxide and hydride ions is their charge,a saresult of which hydride-for-oxide substitution necessarily involves reduction, allowing access to unusually low transition-metal oxidation states.T he conversion of insulating A II TiO 3 oxides into metallic A II TiO 3Àx H y oxide-hydrides is aclassic example of the use of hydride-foroxide substitution to modify materials properties. [2] Thelower electronegativity of hydride compared to oxide also implies ah igher degree of covalency and orbital mixing in M À H bonds compared to M À Oanalogues,and as aresult the band structures of oxide-hydrides will be qualitatively different from their parent oxides.F urthermore,m agnetic coupling strengths can be strongly enhanced in oxide-hydride phases, resulting in the high magnetic ordering temperatures observed for LaSrCoO 3 H 0.7 ,S rVO 2 H, and SrCrO 2 H. [3] A final, more subtle difference between oxide and hydride ions is the absence of p-symmetry valence orbitals on the H À anion. Theo rbital connectivity of ap hase can therefore be altered dramatically,e specially if the oxide and hydride anions adopt an ordered arrangement....

We report the results of a muon-spin rotation (μSR) experiment to determine the superconducting ground state of the iron-based superconductor CsCa 2 Fe 4 As 4 F 2 with T c ≈ 28.3 K. This compound is related to the fully gapped superconductor CaCsFe 4 As 4 , but here the Ca-containing spacer layer is replaced with one containing Ca 2 F 2. The temperature evolution of the penetration depth strongly suggests the presence of line nodes and is best modeled by a system consisting of both an sand a d-wave gap. We also find a potentially magnetic phase which appears below ≈10 K but does not appear to compete with the superconductivity. This compound contains the largest alkali atom in this family of superconductors, and our results yield a value for the in-plane penetration depth of λ ab (T = 0) = 244(3) nm.