# Mathematical Physics

###### Featured Researches

## 3D Ising Model: a view from the Conformal Bootstrap Island

We explain how the axioms of Conformal Field Theory are used to make predictions about critical exponents of continuous phase transitions in three dimensions, via a procedure called the conformal bootstrap. The method assumes conformal invariance of correlation functions, and imposes some relations between correlation functions of different orders. Numerical analysis shows that these conditions are incompatible unless the critical exponents take particular values, or more precisely that they must belong to a small island in the parameter space.

Read more## 3D Topological Models and Heegaard Splitting II: Pontryagin duality and Observables

In a previous article, a construction of the smooth Deligne-Beilinson cohomology groups H p D (M) on a closed 3 -manifold M represented by a Heegaard splitting X L ∪ f X R was presented. Then, a determination of the partition functions of the U(1) Chern-Simons and BF Quantum Field theories was deduced from this construction. In this second and concluding article we stay in the context of a Heegaard spitting of M to define Deligne-Beilinson 1 -currents whose equivalent classes form the elements of H 1 D (M ) ⋆ , the Pontryagin dual of H 1 D (M) . Finally, we use singular fields to first recover the partition functions of the U(1) Chern-Simons and BF quantum field theories, and next to determine the link invariants defined by these theories. The difference between the use of smooth and singular fields is also discussed.

Read more## A H 3 (G,T) -valued index of symmetry protected topological phases with on-site finite group symmetry for two-dimensional quantum spin systems

We consider SPT-phases with on-site finite group G symmetry β for two-dimensional quantum spin systems. We show that they have H 3 (G,T) -valued invariant.

Read more## A Z 2 -Topological Index for Free-Fermion Systems in Disordered Media

We use infinite dimensional self--dual CAR C ∗ -algebras to study a Z 2 -index, which classifies free--fermion systems embedded on Z d -index disordered lattices. Combes-Thomas estimates are pivotal to show that the Z 2 -index is uniform with respect to the size of the system. We additionally deal with the set of ground states to completely describe the mathematical structure of the underlying system. Furthermore, the weak ∗ --topology of the set of linear functionals is used to analyze paths connecting different sets of ground states.

Read more## A Coordinate Free Formulation of Effective Diffusion on Channels

We study diffusion processes in regions generated by sliding a cross section by the phase flow of vector filed on curved spaces of arbitrary dimension. We do this by studying the effective diffusion coefficient D that arises when trying to reduce the n-dimensional diffusion equation to a 1-dimensional diffusion equation by means of a projection method. We use the mathematical language of exterior calculus to derive a coordinate free formula for this coefficient in both infinite and finite transversal diffusion rate cases. The use of these techniques leads to a formula for D which provides a deeper understanding of effective diffusion than when using a coordinate dependent approach.

Read more## A Hamiltonian Approach for Obtaining Irreducible Projective Representations and the k?�p Perturbation for Anti-unitary Symmetry Groups

As is known, the irreducible projective representations (Reps) of anti-unitary groups contain three different situations, namely, the real, the complex and quaternion types with torsion number 1,2,4 respectively. This subtlety increases the complexity in obtaining irreducible projective Reps of anti-unitary groups. In the present work, a physical approach is introduced to derive the condition of irreducibility for projective Reps of anti-unitary groups. Then a practical procedure is provided to reduce an arbitrary projective Rep into direct sum of irreducible ones. The central idea is to construct a hermitian Hamiltonian matrix which commutes with the representation of every group element g?�G , such that each of its eigenspaces forms an irreducible representation space of the group G . Thus the Rep is completely reduced in the eigenspaces of the Hamiltonian. This approach is applied in the k?�p effective theory at the high symmetry points (HSPs) of the Brillouin zone for quasi-particle excitations in magnetic materials. After giving the criterion to judge the power of single-particle dispersion around a HSP, we then provide a systematic procedure to construct the k?�p effective model.

Read more## A Jacobi Algorithm in Phase Space: Diagonalizing (skew-) Hamiltonian and Symplectic Matrices with Dirac-Majorana Matrices

Jacobi's method is a well-known algorithm in linear algebra to diagonalize symmetric matrices by successive elementary rotations. We report about the generalization of these elementary rotations towards canonical transformations acting in Hamiltonian phase spaces. This generalization allows to use Jacobi's method in order to compute eigenvalues and eigenvectors of Hamiltonian (and skew-Hamiltonian) matrices with either purely real or purely imaginary eigenvalues by successive elementary symplectic "decoupling"-transformations.

Read more## A Matrix Basis Formulation For The Green's Functions Of Maxwell's Equations And The Elastic Wave Equations In Layered Media

A matrix basis formulation is introduced to represent the 3 x 3 dyadic Green's functions in the frequency domain for the Maxwell's equations and the elastic wave equation in layered media. The formulation can be used to decompose the Maxwell's Green's functions into independent TE and TM components, each satisfying a Helmholtz equation, and decompose the elastic wave Green's function into the S-wave and the P-wave components. In addition, a derived vector basis formulation is applied to the case for acoustic wave sources from a non-viscous fluid layer.

Read more## A Microlocal Approach to Renormalization in Stochastic PDEs

We present a novel framework for the study of a large class of non-linear stochastic PDEs, which is inspired by the algebraic approach to quantum field theory. The main merit is that, by realizing random fields within a suitable algebra of functional-valued distributions, we are able to use techniques proper of microlocal analysis which allow us to discuss renormalization and its associated freedomw without resorting to any regularization scheme and to the subtraction of infinities. As an example of the effectiveness of the approach we apply it to the perturbative analysis of the stochastic Φ 3 d model.

Read more## A Neumann series of Bessel functions representation for solutions of the radial Dirac system

A new representation for a regular solution of the radial Dirac system of a special form is obtained. The solution is represented as a Neumann series of Bessel functions uniformly convergent with respect to the spectral parameter. For the coefficients of the series convenient for numerical computation recurrent integration formulas are given. Numerical examples are presented.

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