Symmetry (physics)

About: Symmetry (physics) is a research topic. Over the lifetime, 26435 publications have been published within this topic receiving 500189 citations. The topic is also known as: symmetry (physics) & physical symmetry.

Symmetry and the macroscopic dynamics of magnetic materials

Abstract: A systematic exposition is given of a recently developed theory of the low-frequency spin dynamics of any magnetic material in which the magnetic order is caused by exchange forces that considerably exceed the relativistic interactions. The proposed approach does not use any model representations of the state of the magnet (localized spins, sublattices, etc.). A fundamental concept in the theory is that of approximate exchange symmetry.

Symmetry-protected topological orders for interacting fermions: Fermionic topological nonlinear σ models and a special group supercohomology theory

Abstract: Symmetry-protected topological (SPT) phases are gapped quantum phases with a symmetry, which can be smoothly connected to the trivial product states only if we break the symmetry. For a given symmetry, we can have many different SPT phases. But how to describe/construct those different SPT phases that can not be distinguished by their symmetry? It has been shown that different bosonic SPT phases in any dimensions and for any symmetry groups can be described/constructed using group cohomology theory of the symmetry group. In this paper, we introduce a group super-cohomology theory which is a generalization of the standard group cohomology theory. Using the group super-cohomology theory, we can describe/construct different interacting fermionic SPT phases, in any dimensions and for symmetry groups where the fermions form 1D representations of the symmetry group. Just like the boson case, our systematic construction is based on constructing discrete fermionic topological non-linear σ-models from the group super-cohomology theory. Our discrete fermionic topological non-linear σ-model, when defined on a space-time with boundary, can be viewed as a “non-local” boundary effective Lagrangian, which is a fermionic and discrete generalization of the bosonic continuous Wess-Zumino-Witten term. Thus we believe that the boundary excitations of a non-trivial SPT phase are gapless if the symmetry is not broken. As an application of this general result, we construct three non-trivial SPT phases in 3D, for interacting fermionic superconductors with coplanar spin order (which have T 2 = 1 time-reversal Z 2 and fermion-number-parity Z 2 symmetries described by a full symmetry group Z T 2 × Z 2 ). We also construct three interacting fermionic SPT phases in 2D with a full symmetry group Z2×Z 2 . Those 2D fermionic SPT phases all have central-charge c = 1 gapless edge excitations, if the symmetry is not broken.

Using the Noether symmetry approach to probe the nature of dark energy

Abstract: We propose to use a model-independent criterion based on first integrals of motion, due to Noether symmetries of the equations of motion, in order to classify the dark energy models in the context of scalar field (quintessence or phantom) Friedmann-Lemaitre-Robertson-Walker cosmologies. In general, the Noether symmetries play an important role in physics because they can be used to simplify a given system of differential equations as well as to determine the integrability of the system. The Noether symmetries are computed for nine distinct accelerating cosmological scenarios that contain a homogeneous scalar field associated with different types of potentials. We verify that all the scalar field potentials, presented here, admit the trivial first integral, namely, energy conservation, as they should. We also find that the exponential potential inspired from scalar field cosmology, as well as some types of hyperbolic potentials, include extra Noether symmetries. This feature suggests that these potentials should be preferred along the hierarchy of scalar field potentials. Finally, using the latter potentials, in the framework of either quintessence or phantom scalar field cosmologies that contain also a nonrelativistic matter (dark matter) component, we find that the main cosmological functions, such as the scale factor of the Universe, themore » scalar field, the Hubble expansion rate, and the metric of the Friedmann-Lemaitre-Robertson-Walker space-time, are computed analytically. Interestingly, under specific circumstances the predictions of the exponential and hyperbolic scalar field models are equivalent to those of the {Lambda}CDM model, as far as the global dynamics and the evolution of the scalar field are concerned. The present analysis suggests that our technique appears to be very competitive to other independent tests used to probe the functional form of a given potential and thus the associated nature of dark energy.« less