557 research outputs found
Topological Heat Transport and Symmetry-Protected Boson Currents
The study of non-equilibrium properties in topological systems is of
practical and fundamental importance. Here, we analyze the stationary
properties of a two-dimensional bosonic Hofstadter lattice coupled to two
thermal baths in the quantum open-system formalism. Novel phenomena appear like
chiral edge heat currents that are the out-of-equilibrium counterparts of the
zero-temperature edge currents. They support a new concept of dissipative
symmetry-protection, where a set of discrete symmetries protects topological
heat currents, differing from the symmetry-protection devised in closed systems
and zero-temperature. Remarkably, one of these currents flows opposite to the
decreasing external temperature gradient. As the starting point, we consider
the case of a single external reservoir already showing prominent results like
thermal erasure effects and topological thermal currents. Our results are
experimentally accessible with platforms like photonics systems and optical
lattices.Comment: RevTeX4 file, color figure
Two-dimensional density-matrix topological fermionic phases: topological Uhlmann numbers
We construct a topological invariant that classifies density matrices of symmetry-protected topological orders in two-dimensional fermionic systems. As it is constructed out of the previously introduced Uhlmann phase, we refer to it as the topological Uhlmann number n_(U). With it, we study thermal topological phases in several two-dimensional models of topological insulators and superconductors, computing phase diagrams where the temperature T is on an equal footing with the coupling constants in the Hamiltonian. Moreover, we find novel thermal-topological transitions between two nontrivial phases in a model with high Chern numbers. At small temperatures we recover the standard topological phases as the Uhlmann number approaches to the Chern number
Magnetorotational Instability in Core-Collapse Supernovae
We discuss the relevance of the magnetorotational instability (MRI) in
core-collapse supernovae (CCSNe). Our recent numerical studies show that in
CCSNe, the MRI is terminated by parasitic instabilities of the Kelvin-Helmholtz
type. To determine whether the MRI can amplify initially weak magnetic fields
to dynamically relevant strengths in CCSNe, we performed three-dimensional
simulations of a region close to the surface of a differentially rotating
proto-neutron star in non-ideal magnetohydrodynamics with two different
numerical codes. We find that under the conditions prevailing in proto-neutron
stars, the MRI can amplify the magnetic field by (only) one order of magnitude.
This severely limits the role of MRI channel modes as an agent amplifying the
magnetic field in proto-neutron stars starting from small seed fields.Comment: Proceedings in Acta Physica Polonica B, Proceedings Supplement, Vol.
10, No. 2, p.361, 4 pages, 1 figur
Robust nonequilibrium edge currents with and without band topology
We study two-dimensional bosonic and fermionic lattice systems under
nonequilibrium conditions corresponding to a sharp gradient of temperature
imposed by two thermal baths. In particular, we consider a lattice model with
broken time-reversal symmetry that exhibits both topologically trivial and
nontrivial phases. Using a nonperturbative approach, we characterize the
nonequilibrium current distribution in different parameter regimes. For both
bosonic and fermionic systems weakly coupled to the reservoirs, we find chiral
edge currents that are robust against defects on the boundary or in the bulk.
This robustness not only originates from topological effects at zero
temperature but, remarkably, also persists as a result of dissipative
symmetries in regimes where band topology plays no role. Chirality of the edge
currents implies that energy locally flows against the temperature gradient
without any external work input. In the fermionic case, there is also a regime
with topologically protected boundary currents, which nonetheless do not
circulate around all system edges.Comment: 5+4 pages, 4+2 figures. Comments welcom
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