46 research outputs found
Dynamical and Topological Properties of the Kitaev Model in a [111] Magnetic Field
The Kitaev model exhibits a Quantum Spin Liquid hosting emergent
fractionalized excitations. We study the Kitaev model on the honeycomb lattice
coupled to a magnetic field along the [111] axis. Utilizing large scale matrix
product based numerical models, we confirm three phases with transitions at
different field strengths depending on the sign of the Kitaev exchange: a
non-abelian topological phase at low fields, an enigmatic intermediate regime
only present for antiferromagnetic Kitaev exchange, and a field-polarized
phase. For the topological phase, we numerically observe the expected cubic
scaling of the gap and extract the quantum dimension of the non-Abelian anyons.
Furthermore, we investigate dynamical signatures of the topological and the
field-polarized phase using a matrix product operator based time evolution
method.Comment: Changed convention to be in accordance with published articl
Dynamics of the Kitaev-Heisenberg Model
We introduce a matrix-product state based method to efficiently obtain
dynamical response functions for two-dimensional microscopic Hamiltonians,
which we apply to different phases of the Kitaev-Heisenberg model. We find
significant broad high energy features beyond spin-wave theory even in the
ordered phases proximate to spin liquids. This includes the phase with zig-zag
order of the type observed in -RuCl, where we find high energy
features like those seen in inelastic neutron scattering experiments. Our
results provide an example of a natural path for proximate spin liquid features
to arise at high energies above a conventionally ordered state, as the diffuse
remnants of spin-wave bands intersect to yield a broad peak at the Brillouin
zone center.Comment: 7 pages, 8 figure
Quantum spin liquid signatures in Kitaev-like frustrated magnets
Motivated by recent experiments on -RuCl, we investigate a
possible quantum spin liquid ground state of the honeycomb-lattice spin model
with bond-dependent interactions. We consider the model, where
and represent the Kitaev and symmetric-anisotropic interactions
between spin-1/2 moments on the honeycomb lattice. Using the infinite density
matrix renormalization group (iDMRG), we provide compelling evidence for the
existence of quantum spin liquid phases in an extended region of the phase
diagram. In particular, we use transfer matrix spectra to show the evolution of
two-particle excitations with well-defined two-dimensional dispersion, which is
a strong signature of quantum spin liquid. These results are compared with
predictions from Majorana mean-field theory and used to infer the quasiparticle
excitation spectra. Further, we compute the dynamical structure factor using
finite size cluster computations and show that the results resemble the
scattering continuum seen in neutron scattering experiments on
-RuCl. We discuss these results in light of recent and future
experiments.Comment: Modified manuscript: 17 pages, 21 figure
Thermal pure matrix product state in two dimensions: tracking thermal equilibrium from paramagnet down to the Kitaev honeycomb spin liquid state
We show that the matrix product state (MPS) provides a thermal quantum pure
state (TPQ) representation in equilibrium in two spatial dimensions over the
whole temperature range. We use the Kitaev honeycomb model as a prominent
example hosting a quantum spin liquid (QSL) ground state to target the two
specific-heat peaks previously solved nearly exactly using the free Majorana
fermionic description. Starting from the high-temperature random state, our
TPQ-MPS wrapping the cylinder precisely reproduces these peaks, showing that
the quantum many-body description based on spins can still capture the emergent
itinerant Majorana fermions in a gauge field. The truncation
process efficiently discards the high-energy states, eventually reaching the
long-range entangled topological state.Comment: 13 pages, 4 figures, submission to SciPos
Polarization Plateaus in Hexagonal Water Ice
The protons in water ice are subject to so called \emph{ice rules} resulting
in an extensive ground state degeneracy. We study how an external electric
field reduces this ground state degeneracy in hexagonal water ice I within
a minimal model. We observe polarization plateaus when the field is aligned
along the and directions. In each case, one plateau occurs at
intermediate polarization with reduced but still extensive degeneracy. The
remaining ground states can be mapped to dimer models on the honeycomb and the
square lattice, respectively. Upon tilting the external field, we observe an
order-disorder transition of Kasteleyn type into a plateau at saturated
polarization and vanishing entropy. This transition is investigated
analytically using the Kasteleyn matrix and numerically using a modified
directed-loop Monte Carlo simulation. The protons in both cases exhibit
algebraically decaying correlations. Moreover, the features of the static
structure factor are discussed.Comment: 11 pages, 11 figure
Generic Field-Driven Phenomena in Kitaev Spin Liquids: Canted Magnetism and Proximate Spin Liquid Physics
Topological spin liquids in two spatial dimensions are stable phases in the
presence of a small magnetic field, but may give way to field-induced phenomena
at intermediate field strengths. Sandwiched between the low-field spin liquid
physics and the high-field spin-polarized phase, the exploration of magnetic
phenomena in this intermediate regime however often remains elusive to
controlled analytical approaches. Here we numerically study such
intermediate-field magnetic phenomena for two representative Kitaev models (on
the square-octagon and decorated honeycomb lattice) that exhibit either Abelian
or non-Abelian topological order in the low-field limit. Using a combination of
exact diagonalization and density matrix renormalization group techniques, as
well as linear spin-wave theory, we establish the generic features of Kitaev
spin liquids in an external magnetic field. While ferromagnetic models
typically exhibit a direct transition to the polarized state at a relatively
low field strength, antiferromagnetic couplings not only substantially
stabilizes the topological spin liquid phase, but generically lead to the
emergence of a distinct field-induced intermediate regime, separated by a
crossover from the high-field polarized regime. Our results suggest that, for
most lattice geometries, this regime generically exhibits significant spin
canting, antiferromagnetic spin-spin correlations, and an extended proximate
spin liquid regime at finite temperatures. Notably, we identify a symmetry
obstruction in the original honeycomb Kitaev model that prevents, at least for
certain field directions, the formation of such canted magnetism without
breaking symmetries -- consistent with the recent numerical observation of an
extended gapless spin liquid in this case.Comment: 13 pages, 16 figures (Appendix: 7 pages, 7 figures
Emergence of a nematic paramagnet via quantum order-by-disorder and pseudo-Goldstone modes in Kitaev magnets
The appearance of nontrivial phases in Kitaev materials exposed to an
external magnetic field has recently been a subject of intensive studies. Here,
we elucidate the relation between the field-induced ground states of the
classical and quantum spin models proposed for such materials, by using the
infinite density matrix renormalization group (iDMRG) and the linear spin wave
theory (LSWT). We consider the model, where and
are off-diagonal spin exchanges on top of the dominant Kitaev
interaction . Focusing on the magnetic field along the direction, we
explain the origin of the nematic paramagnet, which breaks the
lattice-rotational symmetry and exists in an extended window of magnetic field,
in the quantum model. This phenomenon can be understood as the effect of
quantum order-by-disorder in the frustrated ferromagnet with a continuous
manifold of degenerate ground states discovered in the corresponding classical
model. We compute the dynamical spin structure factors using a matrix operator
based time evolution and compare them with the predictions from LSWT. We, thus,
provide predictions for future inelastic neutron scattering experiments on
Kitaev materials in an external magnetic field along the direction. In
particular, the nematic paramagnet exhibits a characteristic pseudo-Goldstone
mode which results from the lifting of a continuous degeneracy via quantum
fluctuations.Comment: 14 pages, 9 figur
Spurious Symmetry Enhancement and Interaction-Induced Topology in Magnons
Linear spin wave theory (LSWT) is the standard technique to compute the
spectra of magnetic excitations in quantum materials. In this paper, we show
that LSWT, even under ordinary circumstances, may fail to implement the
symmetries of the underlying ordered magnetic Hamiltonian leading to spurious
degeneracies. In common with pseudo-Goldstone modes in cases of quantum
order-by-disorder these degeneracies tend to be lifted by magnon-magnon
interactions. We show how, instead, the correct symmetries may be restored at
the level of LSWT. In the process we give examples, supported by
nonperturbative matrix product based time evolution calculations, where
symmetries dictate that there should be a topological magnon gap but where LSWT
fails to open up this gap. We also comment on possible spin split magnons in
MnF and similar rutiles by analogy to recently proposed altermagnets
Emergence of nematic paramagnet via quantum order-by-disorder and pseudo-Goldstone modes in Kitaev magnets
The appearance of nontrivial phases in Kitaev materials exposed to an external magnetic field has recently been a subject of intensive studies. Here, we elucidate the relation between the field-induced ground states of the classical and quantum spin models proposed for such materials, by using the infinite density matrix renormalization group (iDMRG) and the linear spin wave theory (LSWT). We consider the KΓΓ′ model, where Γ and Γ′ are off-diagonal spin exchanges on top of the dominant Kitaev interaction K. Focusing on the magnetic field along the [111] direction, we explain the origin of the nematic paramagnet, which breaks the lattice-rotational symmetry and exists in an extended window of magnetic field, in the quantum model. This phenomenon can be understood as the effect of quantum order-by-disorder in the frustrated ferromagnet with a continuous manifold of degenerate ground states discovered in the corresponding classical model. We compute the dynamical spin structure factors using a matrix operator based time evolution and compare them with the predictions from LSWT. We, thus, provide predictions for future inelastic neutron scattering experiments on Kitaev materials in an external magnetic field along the [111] direction. In particular, the nematic paramagnet exhibits a characteristic pseudo-Goldstone mode, which results from the lifting of a continuous degeneracy via quantum fluctuations
Fast automated placement of polar hydrogen atoms in protein-ligand complexes
<p>Abstract</p> <p>Background</p> <p>Hydrogen bonds play a major role in the stabilization of protein-ligand complexes. The ability of a functional group to form them depends on the position of its hydrogen atoms. An accurate knowledge of the positions of hydrogen atoms in proteins is therefore important to correctly identify hydrogen bonds and their properties. The high mobility of hydrogen atoms introduces several degrees of freedom: Tautomeric states, where a hydrogen atom alters its binding partner, torsional changes where the position of the hydrogen atom is rotated around the last heavy-atom bond in a residue, and protonation states, where the number of hydrogen atoms at a functional group may change. Also, side-chain flips in glutamine and asparagine and histidine residues, which are common crystallographic ambiguities must be identified before structure-based calculations can be conducted.</p> <p>Results</p> <p>We have implemented a method to determine the most probable hydrogen atom positions in a given protein-ligand complex. Optimality of hydrogen bond geometries is determined by an empirical scoring function which is used in molecular docking. This allows to evaluate protein-ligand interactions with an established model. Also, our method allows to resolve common crystallographic ambiguities such as as flipped amide groups and histidine residues. To ensure high speed, we make use of a dynamic programming approach.</p> <p>Conclusion</p> <p>Our results were checked against selected high-resolution structures from an external dataset, for which the positions of the hydrogen atoms have been validated manually. The quality of our results is comparable to that of other programs, with the advantage of being fast enough to be applied on-the-fly for interactive usage or during score evaluation.</p