Hector K. Vivanco

Bio: Hector K. Vivanco is an academic researcher from Johns Hopkins University. The author has contributed to research in topics: Magnetization & van der Waals force. The author has an hindex of 7, co-authored 10 publications receiving 176 citations. Previous affiliations of Hector K. Vivanco include University of Maryland, College Park.

Metastable Layered Cobalt Chalcogenides from Topochemical Deintercalation

Abstract: We present a general strategy to synthesize metastable layered materials via topochemical deintercalation of thermodynamically stable phases. Through kinetic control of the deintercalation reaction, we have prepared two hypothesized metastable compounds, CoSe and CoS, with the anti-PbO type structure from the starting compounds KCo2Se2 and KCo2S2, respectively. Thermal stability, crystal structure from X-ray and neutron diffraction, magnetic susceptibility, magnetization, and electrical resistivity are studied for these new layered chalcogenides; both CoSe and CoS are found to be weak itinerant ferromagnets with Curie temperatures close to 10 K. Due to the weak van der Waals forces between the layers, CoSe is found to be a suitable host for further intercalation of guest species such as Li-ethylenediamine. From first-principles calculations, we explain why the Co chalcogenides are ferromagnets instead of superconductors as in their iron analogues. Bonding analysis of the calculated electronic density of s...

Superconductivity and magnetism in iron sulfides intercalated by metal hydroxides.

Abstract: Inspired by naturally occurring sulfide minerals, we present a new family of iron-based superconductors. A metastable form of FeS known as the mineral mackinawite forms two-dimensional sheets that can be readily intercalated by various cationic guest species. Under hydrothermal conditions using alkali metal hydroxides, we prepare three different cation and metal hydroxide-intercalated FeS phases including (Li1−xFexOH)FeS, [(Na1−xFex)(OH)2]FeS, and KxFe2−yS2. Upon successful intercalation of the FeS layer, the superconducting critical temperature Tc of mackinawite is enhanced from 5 K to 8 K for the (Li1−xFexOH)δ+ intercalate. Layered heterostructures of [(Na1−xFex)(OH)2]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na1−xFex)(OH)2]FeS displays long-range magnetic ordering near 15 K, KxFe2−yS2 displays short range antiferromagnetism.

Frustrated magnetism in the tetragonal CoSe analog of superconducting FeSe

Abstract: Recently synthesized metastable tetragonal CoSe, isostructural to the FeSe superconductor, offers a new avenue for investigating systems in close proximity to the iron-based superconductors. We present magnetic and transport property measurements on powders and single crystals of CoSe. High field magnetic susceptibility measurements indicate a suppression of the previously reported 10 K ferromagnetic transition with the magnetic susceptibility, exhibiting time dependence below the proposed transition. Dynamic scaling analysis of the time dependence yields a critical relaxation time of ${\ensuremath{\tau}}^{*}=0.064\ifmmode\pm\else\textpm\fi{}0.008$ s which in turn yields activation energy ${E}_{a}^{*}=14.84\ifmmode\pm\else\textpm\fi{}0.59$ K and an ideal glass temperature ${T}_{0}^{*}=8.91\ifmmode\pm\else\textpm\fi{}0.09$ K from Vogel-Fulcher analysis. No transition is observed in resistivity and specific heat measurements, but both measurements indicate that CoSe is metallic. These results are interpreted on the basis of CoSe exhibiting frustrated magnetic ordering arising from competing magnetic interactions. Arrott analysis of single crystal magnetic susceptibility has indicated the transition temperature occurs in close proximity to previous reports and that the magnetic moment lies solely in the $ab$ plane. The results have implications for understanding the relationship between magnetism and transport properties in the iron chalcogenide superconductors.

Competing antiferromagnetic-ferromagnetic states in a d 7 Kitaev honeycomb magnet

Abstract: The Kitaev model is a rare example of an analytically solvable and physically instantiable Hamiltonian yielding a topological quantum spin liquid ground state. Here we report signatures of Kitaev spin liquid physics in the honeycomb magnet ${\mathrm{Li}}_{3}{\mathrm{Co}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$, built of high-spin ${d}^{7}$ (${\mathrm{Co}}^{2+}$) ions, in contrast to the more typical low-spin ${d}^{5}$ electron configurations in the presence of large spin-orbit coupling. Neutron powder diffraction measurements, heat capacity, and magnetization studies support the development of a long-range antiferromagnetic order space group of ${C}_{C}2/m$, below ${T}_{N}=11\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ${\ensuremath{\mu}}_{0}H=0\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The magnetic entropy recovered between $T=2$ and 50 K is estimated to be $0.6R\mathrm{ln}2$, in good agreement with the value expected for systems close to a Kitaev quantum spin liquid state. The temperature-dependent magnetic order parameter demonstrates a \ensuremath{\beta} value of 0.19(3), consistent with XY anisotropy and in-plane ordering, with Ising-like interactions between layers. Further, we observe a spin-flop-driven crossover to ferromagnetic order with space group of $C2/m$ under an applied magnetic field of ${\ensuremath{\mu}}_{0}H\ensuremath{\approx}0.7\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ at $T=2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Magnetic structure analysis demonstrates these magnetic states are competing at finite applied magnetic fields even below the spin-flop transition. Both the ${d}^{7}$ compass model, a quantitative comparison of the specific heat of ${\mathrm{Li}}_{3}{\mathrm{Co}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$, and related honeycomb cobaltates to the anisotropic Kitaev model further support proximity to a Kitaev spin liquid state. This material demonstrates the rich playground of high-spin ${d}^{7}$ systems for spin liquid candidates and complements known ${d}^{5}$ Ir- and Ru-based materials.

Coupled Vacancy Pairs in Ni-Doped CoSe for Improved Electrocatalytic Hydrogen Production Through Topochemical Deintercalation

Abstract: Vacancy engineering plays vital role in the design of high-performance electrocatalysts. Here, we introduced coupled cation-vacancy pairs in Ni-doped CoSe to achieve boosted hydrogen evolution reaction (HER) activity through a facile topochemical intercalation approach. Adjacent Co vacancy pairs and heteroatom Ni doping contribute together for the upshift of the Se 4pz orbital, which induces larger overlap between the Se 4p and H 1s orbitals. As a result, the free energy of H adsorption can be lowered significantly. With an advanced HER activity of 185.7 mV at 10 mA cm-2 , this work provides new direction and guidance for the design of novel electrocatalysts.