arXiv subjects

Hector K. Vivanco

Publications and source records attributed to Hector K. Vivanco.

6 recordsLinked to original sources

arXiv2023

Chemical tuning of a honeycomb magnet through a critical point

BaCo2(AsO4)2 (BCAO) has seen extensive study since its initial identification as a proximate Kitaev quantum spin liquid candidate. Thought to be described by the highly anisotropic XXZ-J_1-J_3 model, the ease with which magnetic order is suppressed in the system indicates proximity to a spin liquid phase. Upon chemical tuning via partial arsenic substitution with vanadium, we show an initial suppression of long-range incommensurate order in the BCAO system to T = 3.0 K, followed by increased spin freezing at higher substitution levels. Between these two regions, at around 10% substitution, the system is shown to pass through a critical point where the competing J_1/J_3 exchange interactions become more balanced, producing a more complex magnetic ground state, likely stabilized by quantum fluctuations. This state shows how slight compositional change in magnetically-frustrated systems may be leveraged to tune ground state degeneracies and potentially realize a quantum spin liquid state.

cond-mat.str-el↗

arXiv2021

Twisting of 2D kagomé sheets in layered intermetallics

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ~ 300 K to 100 K, no evidence for a phase transition was found in physical properties measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modelling of the displacive disorder in the crystal structure shows the presence of second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits inter-layer twisting, but with a different pattern, consistent with the change in electron count due to removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.

cond-mat.mtrl-sci↗

arXiv2020

Competing Antiferromagnetic-Ferromagnetic States in $\it{d^7}$ Kitaev Honeycomb Magnet

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 $Li_3Co_2SbO_6$, built of high-spin $\it{d^7}$ ($Co^{2+}$) ions, in contrast to the more typical low-spin $\it{d^5}$ 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 $\it{C_C}2/\it{m}$, below $\it{T_N}$ = 11 K at $\it{μ_0H}$ = 0 T. The magnetic entropy recovered between $\it{T}$ = 2 K and 50 K is estimated to be 0.6Rln2, 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 $β$ 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 $\it{C}2/\it{m}$ under an applied magnetic field of $\it{μ_0H}$ $\approx$ 0.7 T at $\it{T}$ = 2 K. Magnetic structure analysis demonstrates these magnetic states are competing at finite applied magnetic fields even below the spin-flop transition. Both the $\it{d^7}$ compass model, a quantitative comparison of the specific heat of $Li_3Co_2SbO_6$, 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 $\it{d^7}$ systems for spin liquid candidates, and complements known $\it{d^5}$ Ir- and Ru-based materials.

cond-mat.str-el↗

arXiv2020

Introduction of spin centers in single crystals of Ba$_2$CaWO$_{6-δ}$

Developing the field of quantum information science (QIS) hinges upon designing viable qubits, the smallest unit in quantum computing. One approach to creating qubits is introducing paramagnetic defects into semiconductors or insulators. This class of qubits has seen success in the form of nitrogen-vacancy centers in diamond, divacancy defects in SiC, and P doped into Si. These materials feature paramagnetic defects in a low nuclear spin environment to reduce the impact of nuclear spin on electronic spin coherence. In this work, we report single crystal growth of Ba$_2$CaWO$_{6-δ}$, and the coherence properties of controllably introduced W$^{5+}$ spin centers generated by oxygen vacancies. Ba$_2$CaWO$_{6-δ}$ ($δ$ = 0) is a B-site ordered double perovskite with a temperature-dependent octahedral tilting wherein oxygen vacancies generate W$^{5+}$ (d$^1$), $S = \frac{1}{2}, I$ = 0, centers. We characterized these defects by measuring the spin-lattice ($T_1$) and spin-spin relaxation ($T_2$) times from T = 5 to 150 K. At T = 5 K, $T_1$ = 310 ms and $T_2$ = 4 $μ$s, establishing the viability of these qubit candidates. With increasing temperature, $T_2$ remains constant up to T = 60 K and then decreases to $T_2$ $\approx$ 1 $μ$s at T = 90 K, and remains roughly constant until T = 150 K, demonstrating the remarkable stability of $T_2$ with increasing temperature. Together, these results demonstrate that controlled defect generation in double perovskite structures can generate viable paramagnetic point centers for quantum applications and expand the field of potential materials for QIS.

cond-mat.mtrl-sci↗

arXiv2017

Superconductivity and magnetism in Iron Sulfides Intercalated by Metal Hydroxides

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 (Li$_{1-x}$Fe$_x$OH)FeS, [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS, and K$_x$Fe$_{2-y}$S$_2$. Upon successful intercalation of the FeS layer, the superconducting critical temperature $T_c$ of mackinawite is enhanced from 5 K to 8 K for the (Li$_{1-x}$Fe$_x$OH)$^{δ+}$ intercalate. Layered heterostructures of [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS displays long-range magnetic ordering near 15 K, K$_x$Fe$_{2-y}$S$_2$ displays short range antiferromagnetism.

cond-mat.supr-con↗

arXiv2016

The intercalation chemistry of layered iron chalcogenide superconductors

The iron chalcogenides FeSe and FeS are superconductors composed of two-dimensional sheets held together by van der Waals interactions, which makes them prime candidates for the intercalation of various guest species. We review the intercalation chemistry of FeSe and FeS superconductors and discuss their synthesis, structure, and physical properties. Before we review the latest work in this area, we provide a brief background on the intercalation chemistry of other inorganic materials that exhibit enhanced superconducting properties upon intercalation, which include the transition metal dichalcogenides, fullerenes, and layered cobalt oxides. From past studies of these intercalated superconductors, we discuss the role of the intercalates in terms of charge doping, structural distortions, and Fermi surface reconstruction. We also briefly review the physical and chemical properties of the host materials---mackinawite-type FeS and $β$-FeSe. The three types of intercalates for the iron chalcogenides can be placed in three categories: 1.) alkali and alkaline earth cations intercalated through the liquid ammonia technique; 2.) cations intercalated with organic amines such as ethylenediamine; and 3.) layered hydroxides intercalated during hydrothermal conditions. A recurring theme in these studies is the role of the intercalated guest in electron doping the chalcogenide host and in enhancing the two-dimensionality of the electronic structure by spacing the FeSe layers apart. We end this review discussing possible new avenues in the intercalation chemistry of transition metal monochalcogenides, and the promise of these materials as a unique set of new inorganic two-dimensional systems.

cond-mat.supr-con↗