23 research outputs found
Ground State Properties of Quantum Skyrmions described by Neural Network Quantum States
We investigate the ground state properties of quantum skyrmions in a
ferromagnet using variational Monte Carlo with the neural network quantum state
as variational ansatz. We study the ground states of a two-dimensional quantum
Heisenberg model in the presence of the Dzyaloshinskii-Moriya interaction
(DMI). We show that the ground state accommodates a quantum skyrmion for a
large range of parameters, especially at large DMI. The spins in these quantum
skyrmions are weakly entangled, and the entanglement increases with decreasing
DMI. We also find that the central spin is completely disentangled from the
rest of the lattice, establishing a non-destructive way of detecting this type
of skyrmion by local magnetization measurements. While neural networks are well
suited to detect weakly entangled skyrmions with large DMI, they struggle to
describe skyrmions in the small DMI regime due to nearly degenerate ground
states and strong entanglement. In this paper, we propose a method to identify
this regime and a technique to alleviate the problem. Finally, we analyze the
workings of the neural network and explore its limits by pruning. Our work
shows that neural network quantum states can be efficiently used to describe
the quantum magnetism of large systems exceeding the size manageable in exact
diagonalization by far.Comment: 12 pages, 6 figure
The quantum skyrmion Hall effect in f electron systems
The flow of electric current through a two-dimensional material in a magnetic
field gives rise to the family of Hall effects. The quantum versions of these
effects accommodate robust electronic edge channels and fractional charges.
Recently, the Hall effect of skyrmions, classical magnetic quasiparticles with
a quantized topological charge, has been theoretically and experimentally
reported, igniting ideas on a quantum version of this effect. To this end, we
perform dynamical mean field theory calculations on localized electrons
coupled to itinerant electrons in the presence of spin-orbit interaction
and a magnetic field. Our calculations reveal localized nano quantum skyrmions
that start moving transversally when a charge current in the itinerant
electrons is applied. The results show the time-transient build-up of the
quantum skyrmion Hall effect, accompanied by an Edelstein effect and a
magnetoelectric effect that rotate the spins. This work motivates studies about
the steady state of the quantum skyrmion Hall effect, looking for eventual
quantum skyrmion edge channels and their transport properties.Comment: 12 pages, 9 figure
Majorana decoherence by bath-induced potential fluctuations
Braiding Majorana zero-modes around each other is a promising route towards
topological quantum computing. Yet, two competing maximes emerge when
implementing Majorana braiding in real systems: On the one hand, perfect
braiding should be conducted adiabatically slowly to avoid non-topological
errors. On the other hand, braiding must be conducted fast such that
unavoidable decoherence effects introduced by the environment are negligible.
This competition results in an intermediate time scale for Majorana braiding
that is optimal, but generally not error-free. Here, we calculate this
intermediate time scale for a T-junction of short one-dimensional topological
superconductors coupled to a bosonic bath that generates fluctuations in the
local electric potential, which stem from, e.g., environmental photons or
phonons of the substrate. We thereby obtain boundaries for the speed of
Majorana braiding with a predetermined gate fidelity. Our results emphasize the
general susceptibility of Majorana-based information storage in finite-size
systems and can serve as a guide for determining the optimal braiding times in
future experiments.Comment: 8 pages, 4 figure
Testing the topological nature of end states in antiferromagnetic atomic chains on superconductors
Edge states forming at the boundaries of topologically non-trivial phases of
matter are promising candidates for future device applications because of their
stability against local perturbations. Magnetically ordered spin chains
proximitized by an s-wave superconductor are predicted to enter a topologically
non-trivial mini-gapped phase with zero-energy Majorana modes (MMs) localized
at their ends. However, the presence of non-topological end states mimicking MM
properties can spoil their unambiguous observation. Here, we report on a method
to experimentally decide on the MM nature of end states observed for the first
time in antiferromagnetic spin chains. Using scanning tunneling spectroscopy,
we find end states at either finite or near-zero energy in Mn chains on Nb(110)
or Ta(110), respectively, within a large minigap. By introducing a locally
perturbing defect on one end of the chain, the end state on this side splits
off from zero-energy while the one on the other side doesn't - ruling out their
MM origin. A minimal model shows that, while wide trivial minigaps hosting such
conventional end states are easily achieved in antiferromagnetic spin chains,
unrealistically large spin-orbit couplings are required to drive the system
into the topologically nontrivial phase with MMs. The methodology of perturbing
chains by local defects is a powerful tool to probe the stability of future
candidate topological edge modes against local disorder
Controlling Majorana hybridization in magnetic chain-superconductor systems
We propose controlling the hybridization between Majorana zero modes at the
ends of magnetic adatom chains on superconductors by an additional magnetic
adatom deposited close by. By tuning the additional adatom's magnetization,
position, and coupling to the superconductor, we can couple and decouple the
Majorana modes as well as control the ground state parity. The scheme is
independent of microscopic details in ferromagnetic and helical magnetic chains
on superconductors with and without spin-orbit coupling, which we show by
studying their full microscopic models and their common low-energy description.
Our results show that scanning tunneling microscopy and electron spin resonance
techniques are promising tools for controlling the Majorana hybridization in
magnetic adatoms-superconductor setups, providing a basis for Majorana parity
measurements, fusion, and braiding techniques.Comment: 10 pages, 4 figures + supplementary (6 pages
Quantum spin helices more stable than the ground state: onset of helical protection
Topological magnetic structures are promising candidates for resilient
information storage. An elementary example are spin helices in one-dimensional
easy-plane quantum magnets. To quantify their stability, we numerically
implement the stochastic Schr\"odinger equation and time-dependent perturbation
theory for spin chains with fluctuating local magnetic fields. We find two
classes of quantum spin helices that can reach and even exceed ground-state
stability: Spin-current-maximizing helices and, for fine-tuned boundary
conditions, the recently discovered "phantom helices". Beyond that, we show
that the helicity itself (left- or right-rotating) is even more stable. We
explain these findings by separated helical sectors and connect them to
topological sectors in continuous spin systems. The resulting helical
protection mechanism is a promising phenomenon towards stabilizing helical
quantum structures, e.g., in ultracold atoms and solid state systems. We also
identify an - up to our knowledge - previously unknown new type of phantom
helices.Comment: 6+4 pages, 3 figures; version 2: minor updates, additional reference