116 research outputs found
M\"obius molecules and fragile Mott insulators
Motivated by the concept of M\"obius aromatics in organic chemistry, we
extend the recently introduced concept of fragile Mott insulators (FMI) to
ring-shaped molecules with repulsive Hubbard interactions threaded by a
half-quantum of magnetic flux (). In this context, a FMI is the
insulating ground state of a finite-size molecule that cannot be adiabatically
connected to a single Slater determinant, i.e., to a band insulator, provided
that time-reversal and lattice translation symmetries are preserved. Based on
exact numerical diagonalization for finite Hubbard interaction strength and
existing Bethe-ansatz studies of the one-dimensional Hubbard model in the
large- limit, we establish a duality between Hubbard molecules with and
sites, with integer. A molecule with sites is an FMI in the
absence of flux but becomes a band insulator in the presence of a half-quantum
of flux, while a molecule with sites is a band insulator in the absence
of flux but becomes an FMI in the presence of a half-quantum of flux. Including
next-nearest-neighbor-hoppings gives rise to new FMI states that belong to
multidimensional irreducible representations of the molecular point group,
giving rise to a rich phase diagram
Emerging chiral edge states from the confinement of a magnetic Weyl semimetal in CoSnS
The quantum anomalous Hall effect (QAHE) and magnetic Weyl semimetals (WSMs)
are topological states induced by intrinsic magnetic moments and spin-orbit
coupling. Their similarity suggests the possibility of achieving the QAHE by
dimensional confinement of a magnetic WSM along one direction. In this study,
we investigate the emergence of the QAHE in the two-dimensional (2D) limit of
magnetic WSMs due to finite size effects in thin films and step-edges. We
demonstrate the feasibility of this approach with effective models and real
materials. To this end, we have chosen the layered magnetic WSM
CoSnS, which features a large anomalous Hall conductivity and
anomalous Hall angle in its 3D bulk, as our material candidate. In the 2D limit
of CoSnS two QAHE states exist depending on the stoichiometry of
the 2D layer. One is a semimetal with a Chern number of 6, and the other is an
insulator with a Chern number of 3. The latter has a band gap of 0.05 eV, which
is much larger than that in magnetically doped topological insulators. Our
findings naturally explain the existence of chiral states in step edges of bulk
CoSnS which habe been reported in a recent experiment at
and present a realistic avenue to realize QAH states in thin films of magnetic
WSMs.Comment: Revised 3rd version of the manuscrip
Towards a Topological Classification of Nonadiabaticity in Chemical Reactions
The application of topology, a branch of mathematics, to the study of
electronic states in crystalline materials has had a revolutionary impact on
the field of condensed matter physics. For example, the development of
topological band theory has delivered new approaches and tools to characterize
the electronic structure of materials, resulting in the discovery of new phases
of matter with exotic properties. In the framework of topological band theory,
the crossings between energy levels of electrons are characterized by
topological invariants, which predict the presence of topological boundary
states. Given the frequency of energy level crossings on the potential energy
surface in molecules, the applicability of these concepts to molecular systems
could be of great interest for our understanding of reaction dynamics. However,
challenges arise due to differing quantum mechanical descriptions of solids and
molecules. Out work aims to bridge the gap between topological band theory and
molecular chemistry. We propose that the Euler Class, a topological invariant,
can be used to categorize and analyse the distribution of nonadiabatic
couplings on the potential energy surface. To exemplify this connection, we
introduce a model system with two distinct regimes that are characterized by
different values of the Euler Class, yet identical potential energy surfaces.
Contrary to expectations set by the Born-Oppenheimer approximation, we propose
that these two regimes don't exhibit identical dynamics, due to a qualitatively
distinct distribution of nonadiabatic couplings
Prediction of a magnetic Weyl semimetal without spin-orbit coupling and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti2MnAl
We predict a magnetic Weyl semimetal in the inverse Heusler Ti2MnAl, a
compensated ferrimagnet with a vanishing net magnetic moment and a Curie
temperature of over 650 K. Despite the vanishing net magnetic moment, we
calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It
derives from the Berry curvature distribution of the Weyl points, which are
only 14 meV away from the Fermi level and isolated from trivial bands.
Different from antiferromagnets Mn3X (X= Ge, Sn, Ga, Ir, Rh, and Pt), where the
AHE originates from the non-collinear magnetic structure, the AHE in Ti2MnAl
stems directly from the Weyl points and is topologically protected. The large
anomalous Hall conductivity (AHC) together with a low charge carrier
concentration should give rise to a large anomalous Hall angle. In contrast to
the Co-based ferromagnetic Heusler compounds, the Weyl nodes in Ti2MnAl do not
derive from nodal lines due to the lack of mirror symmetries in the inverse
Heusler structure. Since the magnetic structure breaks spin-rotation symmetry,
the Weyl nodes are stable without SOC. Moreover, because of the large
separation between Weyl points of opposite topological charge, the Fermi arcs
extent up to 75% of the reciprocal lattice vectors in length. This makes
Ti2MnAl an excellent candidate for the comprehensive study of magnetic Weyl
semimetals. It is the first example of a material with Weyl points, large
anomalous Hall effect and angle despite a vanishing net magnetic moment.Comment: 6 pages, 4 figure
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