5 research outputs found
Kitaev anisotropy induces mesoscopic Z2 vortex crystals in frustrated hexagonal antiferromagnets
The triangular-lattice Heisenberg antiferromagnet (HAF) is known to carry topological Z2 vortex excitations which form a gas at finite temperatures. Here we show that the spin-orbit interaction, introduced via a Kitaev term in the exchange Hamiltonian, condenses these vortices into a triangular Z2 vortex crystal at zero temperature. The cores of the Z2 vortices show abrupt, soliton-like magnetization modulations and arise by a special intertwining of three honeycomb superstructures of ferromagnetic domains, one for each of the three sublattices of the 120 state of the pure HAF. This is an example of a nucleation transition, analogous to the spontaneous formation of magnetic domains, Abrikosov vortices in type-II superconductors, blue phases in cholesteric liquid crystals, and skyrmions in chiral helimagnets. As the mechanism relies on the interplay of geometric frustration and spin-orbital anisotropies, such vortex mesophases can materialize as a ground state property in spin-orbit coupled correlated systems with nearly hexagonal topology, as in triangular or strongly frustrated honeycomb iridates
Strong magnetic frustration and anti-site disorder causing spin-glass behavior in honeycomb Li2RhO3
With large spin-orbit coupling, the electron configuration in d-metal oxides is prone to highly anisotropic exchange interactions and exotic magnetic properties. In 5d5 iridates, given the existing variety of crystal structures, the magnetic anisotropy can be tuned from antisymmetric to symmetric Kitaev-type, with interaction strengths that outsize the isotropic terms. By many-body electronic-structure calculations we here address the nature of the magnetic exchange and the intriguing spin-glass behavior of Li2RhO3, a 4d5 honeycomb oxide. For pristine crystals without Rh-Li site inversion, we predict a dimerized ground state as in the isostructural 5d5 iridate Li2IrO3, with triplet spin dimers effectively placed on a frustrated triangular lattice. With Rh-Li anti-site disorder, we explain the observed spin-glass phase as a superposition of different, nearly degenerate symmetry-broken configurations
The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3
The Skyrme-particle, the skyrmion, was introduced over half a century ago in the context of dense nuclear matter. But with skyrmions being mathematical objects -special types of topological solitons -they can emerge in much broader contexts. Recently skyrmions were observed in helimagnets, forming nanoscale spin-textures. Extending over length scales much larger than the interatomic spacing, they behave as large, classical objects, yet deep inside they are of quantum nature. Penetrating into their microscopic roots requires a multi-scale approach, spanning the full quantum to classical domain. Here, we achieve this for the first time in the skyrmionic Mott insulator Cu 2 OSeO 3. We show that its magnetic building blocks are strongly fluctuating Cu 4 tetrahedra, spawning a continuum theory that culminates in 51a ā°nm large skyrmions, in striking agreement with experiment. One of the further predictions that ensues is the temperature-dependent decay of skyrmions into half-skyrmions
Polymeric Frameworks as Organic Semiconductors with Controlled Electronic Properties
The rational assembly of monomers,
in principle, enables the design
of a specific periodicity of polymeric frameworks, leading to a tailored
set of electronic structure properties in these solid-state materials.
The further development of these emerging systems requires a combination
of both experimental and theoretical studies. Here, we investigated
the electronic structures of two-dimensional polymeric frameworks
based on triazine and benzene rings by means of electrochemical techniques.
The experimental density of states was obtained from quasi-open-circuit
voltage measurements through a galvanostatic intermittent titration
technique, which we show to be in excellent agreement with first-principles
calculations performed for two- and three-dimensional structures of
these polymeric frameworks. These findings suggest that the electronic
properties depend not only on the number of stacked layers but also
on the ratio of the different aromatic rings
BerezinskiiāKosterlitzāThouless Transition in the TypeāI Weyl Semimetal PtBi<sub>2</sub>
Symmetry breaking in topological matter has become in
recent years
a key concept in condensed matter physics to unveil novel electronic
states. In this work, we predict that broken inversion symmetry and
strong spināorbit coupling in trigonal PtBi2 lead
to a type-I Weyl semimetal band structure. Transport measurements
show an unusually robust low dimensional superconductivity in thin
exfoliated flakes up to 126 nm in thickness (with Tc ā¼ 275ā400 mK), which constitutes the first
report and study of unambiguous superconductivity in a type-I Weyl
semimetal. Remarkably, a Berezinskii-Kosterlitz-Thouless transition
with TBKT ā¼ 310 mK is revealed
in up to 60 nm thick flakes, which is nearly an order of magnitude
thicker than the rare examples of two-dimensional superconductors
exhibiting such a transition. This makes PtBi2 an ideal
platform to study low dimensional and unconventional superconductivity
in topological semimetals