73 research outputs found
Origin of correlated isolated flat bands in copper-substituted lead phosphate apatite
A recent report of room temperature superconductivity at ambient pressure in
Cu-substituted apatite (`LK99') has invigorated interest in the understanding
of what materials and mechanisms can allow for high-temperature
superconductivity. Here I perform density functional theory calculations on
Cu-substituted lead phosphate apatite, identifying correlated isolated flat
bands at the Fermi level, a common signature of high transition temperatures in
already established families of superconductors. I elucidate the origins of
these isolated bands as arising from a structural distortion induced by the Cu
ions and a chiral charge density wave from the Pb lone pairs. These results
suggest that a minimal two-band model can encompass much of the low-energy
physics in this system. Finally, I discuss the implications of my results on
possible superconductivity in Cu-doped apatit
Emergence of topological electronic phases in elemental lithium under pressure
Lithium, a prototypical simple metal under ambient conditions, has a
surprisingly rich phase diagram under pressure, taking up several structures
with reduced symmetry, low coordination numbers, and even semiconducting
character with increasing density. Using first-principles calculations, we
demonstrate that some predicted high-pressure phases of elemental Li also host
topological electronic structures. Beginning at 80 GPa and coincident with a
transition to the Pbca phase, we find Li to be a Dirac nodal line semimetal. We
further calculate that Li retains linearly-dispersive energy bands in
subsequent predicted higher pressure phases, and that it exhibits a Lifshitz
transition between two Cmca phases at 220 GPa. The Fd-3m phase at 500 GPa forms
buckled honeycomb layers that give rise to a Dirac crossing 1 eV below the
Fermi energy. The well-isolated topological nodes near the Fermi level in these
phases result from increasing p-orbital character with density at the Fermi
level, itself a consequence of rising 1s core wavefunction overlap, and a
preference for nonsymmorphic symmetries in the crystal structures favored at
these pressures. Our results provide evidence that under pressure, bulk 3D
materials with light elements, or even pure elemental systems, can undergo
topological phase transitions hosting nontrivial topological properties near
the Fermi level with measurable consequences; and that, through pressure, we
can access these novel phases in elemental lithium.Comment: 5 pages, 5 figures, accepted for publicatio
Ab initio amorphous spin Hamiltonian for the description of topological spin textures in FeGe
Topological spin textures in magnetic materials such as skyrmions and
hopfions are interesting manifestations of geometric structures in real
materials, concurrently having potential applications as information carriers.
In the crystalline systems, the formation of these topological spin textures is
well understood as a result of the competition between interactions due to
symmetry breaking and frustration. However, in systems without translation
symmetry such as amorphous materials, a fundamental understanding of the
driving mechanisms of non-trivial spin structures is lacking owing to the
structural and interaction complexity in these systems. In this work, we use a
suite of first-principles-based calculations to propose an ab initio spin
Hamiltonian that accurately represents the diversity of structural and magnetic
properties in the exemplar amorphous FeGe. Monte Carlo simulations of our
amorphous Hamiltonian find emergent skyrmions that are driven by frustrated
geometric and magnetic exchange, consistent with those observed in experiment.
Moreover, we find that the diversity of local structural motifs results in a
large range of exchange interactions, far beyond those found in crystalline
materials. Finally, we observe the formation of large-scale emergent structures
in amorphous materials, far beyond the relevant interaction length-scale in the
systems, suggesting a new route to emergent correlated phases beyond the
crystalline limit
Multi-channel direct detection of light dark matter: theoretical framework
We present a unified theoretical framework for computing spin-independent direct detection rates via various channels relevant for sub-GeV dark matter — nuclear re- coils, electron transitions and single phonon excitations. Despite the very different physics involved, in each case the rate factorizes into the particle-level matrix element squared, and an integral over a target material- and channel-specific dynamic structure factor. We show how the dynamic structure factor can be derived in all three cases following the same procedure, and extend previous results in the literature in several aspects. For electron transitions, we incorporate directional dependence and point out anisotropic target materials with strong daily modulation in the scattering rate. For single phonon excitations, we present a new derivation of the rate formula from first principles for generic spin-independent couplings, and include the first calculation of phonon excitation through electron couplings. We also discuss the interplay between single phonon excitations and nuclear recoils, and clarify the role of Umklapp processes, which can dominate the single phonon production rate for dark matter heavier than an MeV. Our results highlight the complementarity between various search channels in probing different kinematic regimes of dark matter scattering, and provide a common reference to connect dark matter theories with ongoing and future direct detection experiments
Prediction of Tunable Spin-Orbit Gapped Materials for Dark Matter Detection
New ideas for low-mass dark matter direct detection suggest that narrow band
gap materials, such as Dirac semiconductors, are sensitive to the absorption of
meV dark matter or the scattering of keV dark matter. Here we propose
spin-orbit semiconductors - materials whose band gap arises due to spin-orbit
coupling - as low-mass dark matter targets owing to their ~10 meV band gaps. We
present three material families that are predicted to be spin-orbit
semiconductors using Density Functional Theory (DFT), assess their electronic
and topological features, and evaluate their use as low-mass dark matter
targets. In particular, we find that that the tin pnictide compounds are
especially suitable having a tunable range of meV-scale band gaps with
anisotropic Fermi velocities allowing directional detection. Finally, we
address the pitfalls in the DFT methods that must be considered in the ab
initio prediction of narrow-gapped materials, including those close to the
topological critical point.Comment: 10 pages, 7 figures + SI 6 pages, 5 figure
Controlling Topology through Targeted Symmetry Manipulation in Magnetic Systems
The possibility of selecting magnetic space groups by orienting the
magnetization direction or tuning magnetic orders offers a vast playground for
engineering symmetry protected topological phases in magnetic materials. In
this work, we study how selective tuning of symmetry and magnetism can
influence and control the resulting topology in a 2D magnetic system, and
illustrate such procedure in the ferromagnetic monolayer MnPSe. Density
functional theory calculations reveals a symmetry-protected accidental
semimetalic (SM) phase for out-of-plane magnetization which becomes an
insulator when the magnetization is tilted in-plane, reaching band gap values
close to meV. We identify an order-two composite antiunitary symmetry and
threefold rotational symmetry that induce the band crossing and classify the
possible topological phases using symmetry analysis, which we support with
tight-binding and models. Breaking of inversion
symmetry opens a gap in the SM phase, giving rise to a Chern insulator. We
demonstrate this explicitly in the isostructural Janus compound
MnPSSe, which naturally exhibits Rashba spin-orbit coupling
that breaks inversion symmetry. Our results map out the phase space of
topological properties of ferromagnetic transition metal phosphorus
trichalcogenides and demonstrate the potential of the magnetization-dependent
metal-to-insulator transition as a spin switch in integrated two-dimensional
electronics
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