265 research outputs found
Non-equilibrium properties of the S=1/2 Heisenberg model in a time-dependent magnetic field
The time-dependent behavior of the Heisenberg model in contact with a phonon
heat bath and in an external time-dependent magnetic field is studied by means
of a path integral approach. The action of the phonon heat bath is taken into
account up to the second order in the coupling to the heath bath. It is shown
that there is a minimal value of the magnetic field below which the average
magnetization of the system does not relax to equilibrium when the external
magnetic field is flipped. This result is in qualitative agreement with the
mean field results obtained within -theory.Comment: To be published in Physica
Hall conductivity as bulk signature of topological transitions in superconductors
Topological superconductors may undergo transitions between phases with
different topological numbers which, like the case of topological insulators,
are related to the presence of gapless (Majorana) edge states. In
topological insulators the charge Hall conductivity is quantized, being
proportional to the number of gapless states running at the edge. In a
superconductor, however, charge is not conserved and, therefore,
is not quantized, even in the case of a topological
superconductor. Here it is shown that while the evolves
continuously between different topological phases of a topological
superconductor, its derivatives display sharp features signaling the
topological transitions. We consider in detail the case of a triplet
superconductor with p-wave symmetry in the presence of Rashba spin-orbit (SO)
coupling and externally applied Zeeman spin splitting. Generalization to the
cases where the pairing vector is not aligned with that of the SO coupling is
given. We generalize also to the cases where the normal system is already
topologically non-trivial.Comment: 10 pages, 10 figure
implications for metabolic diseases
Funding: Joana F. Sacramento and Fatima O. Martins are funded by contracts from the Portuguese Foundation for Science and Technology with reference CEEC IND/02428/2018 and CEECIND/04266/2017, respectivelyNeuro-immune communication has gained enormous interest in recent years due to increasing knowledge of the way in which the brain coordinates functional alterations in inflammatory and autoimmune responses, and the mechanisms of neuron-immune cell interactions in the context of metabolic diseases such as obesity and type 2 diabetes. In this review, we will explain how this relationship between the nervous and immune system impacts the pro- and anti-inflammatory pathways with specific reference to the hypothalamus-pituitary-adrenal gland axis and the vagal reflex and will explore the possible involvement of the carotid body (CB) in the neural control of inflammation. We will also highlight the mechanisms of vagal anti-inflammatory reflex control of immunity and metabolism, and the consequences of functional disarrangement of this reflex in settlement and development of metabolic diseases, with special attention to obesity and type 2 diabetes. Additionally, the role of CB in the interplay between metabolism and immune responses will be discussed, with specific reference to the different stimuli that promote CB activation and the balance between sympathetic and parasympathetic in this context. In doing so, we clarify the multivarious neuronal reflexes that coordinate tissue-specific responses (gut, pancreas, adipose tissue and liver) critical to metabolic control, and metabolic disease settlement and development. In the final section, we will summarize how electrical modulation of the carotid sinus nerve may be utilized to adjust these reflex responses and thus control inflammation and metabolic diseases, envisioning new therapeutics horizons.publishersversionpublishe
Change of an insulator's topological properties by a Hubbard interaction
We introduce two dimensional fermionic band models with two orbitals per
lattice site, or one spinful orbital, and which have a non-zero topological
Chern number that can be changed by varying the ratio of hopping parameters. A
topologically non-trivial insulator is then realized if there is one fermion
per site. When interactions in the framework of the Hubbard model are
introduced, the effective hopping parameters are renormalized and the system's
topological number can change at a certain interaction strength, ,
smaller than that for the Mott transition. Two different situations may then
occur: either the anomalous Hall conductivity changes abruptly at
, as the system undergoes a transition from one topologically
non-trivial insulator to another, or the transition is through an anomalous
Hall metal, and changes smoothly between two different quantized
values as grows. Restoring time-reversal symmetry by adding spin to
spinless models, the half-filled system becomes a topological
insulator. The topological number then changes at a critical coupling
and the quantized spin Hall response changes abruptly.Comment: 5 pages, 3 figure
Fidelity Between Partial States as Signature of Quantum Phase Transitions
We introduce a partial state fidelity approach to quantum phase transitions.
We consider a superconducting lattice with a magnetic impurity inserted at its
centre, and look at the fidelity between partial (either one-site or two-site)
quantum states. In the vicinity of the point of the quantum phase transition,
we observe a sudden drop of the fidelity between two one-site partial states
corresponding to the impurity location and its close vicinity. In the case of
two-site states, the fidelity reveals the transition point as long as one of
the two electron sites is located at the impurity, while the other lies
elsewhere in the lattice. We also determine the Uhlmann mixed state geometric
phase, recently introduced in the study of the structural change of the system
state eigenvectors in the vicinity of the lines of thermal phase transitions,
and find it to be trivial, both for one- and two-site partial states, except
when an electron site is at the impurity. This means that the system partial
state eigenvectors do not contribute significantly to the enhanced state
distinguishability around the point of this quantum phase transition. Finally,
we use the fidelity to analyze the total amount of correlations contained
within a composite system, showing that, even for the smallest two-site states,
it features an abrupt quantitative change in the vicinity of the point of the
quantum phase transition.Comment: 11 pages, 5 figure
Anomalous Hall effect in superconductors with spin-orbit interaction
We calculate the anomalous Hall conductance of superconductors with
spin-orbit interaction and with either uniform or local magnetization. In the
first case we consider a uniform ferromagnetic ordering in a spin triplet
superconductor, while in the second case we consider a conventional s-wave spin
singlet superconductor with a magnetic impurity (or a diluted set of magnetic
impurities). In the latter case we show that the anomalous Hall conductance can
be used to track the quantum phase transition, that occurs when the spin
coupling between the impurity and electronic spin density exceeds a certain
critical value. In both cases we find that for large spin-orbit coupling the
superconductivity is destroyed and the Hall conductance oscillates strongly.Comment: 10 pages, 6 figure
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