Effective Dynamics of Translationally Invariant Magnetic Schrödinger Equations in the High Field Limit

We study the large field limit in Schr\"odinger equations with magnetic vector potentials describing translationally invariant $B$-fields with respect to the $z$-axis. Using analytic perturbation theory, we derive an approximate description of the solution, provided the initial data is compactly supported in the Fourier-variable dual to $z\in \mathbb{R}$. The effective dynamics is thereby seen to produce high-frequency oscillations and large magnetic drifts. In a second step we show that this asymptotic description is stable under a fairly general class of singular perturbations by using the theory of almost invariant subspaces. To this end, we also give an elementary proof of the exponential decay of the resolvent and the eigenfunctions of confining Schr\"odinger operators.

On the semiclassical regularity of thermal equilibria

We study the regularity properties of fermionic equilibrium states at finite positive temperature and show that they satisfy certain semiclassical bounds. As a corollary, we identify explicitly a class of positive temperature states satisfying the regularity assumptions of [J.J. Chong, L. Lafleche, C. Saffirio: arXiv:2103.10946 (2021)].

Conservation laws of damped nonlinear wave equations

All energy-momentum type conservation laws are found for a general class of damped nonlinear wave equations in one dimension. The classification shows that conservation laws exist only for linear damping if the wave equation is non-singular. In the simplest cases, the conservation laws generalize ordinary momentum and energy, and also null-energies. An explanation of the existence of these conservation laws is provided by a point transformation that maps the damped nonlinear wave equation to an undamped wave equation with a time-dependent nonlinearity. Specializations to damped nonlinear wave equations that are invariant under time and space translations are summarized. In the general case, the conservation laws describe a further generalization of momentum, energy, and null-energies, as well as a generalized energy-momentum. The latter two types of conservation laws are shown to arise from a mapping to a wave equation that has a null-form damping (namely, a combination of spatial and temporal damping tied to the null lines of the linear wave operator). This represents a certain type of null structure which is closely related to the null-form nonlinearities studied in analysis of nonlinear wave equations.

Dirac field in $\mathrm{AdS}_2$ and representations of $\widetilde{\mathrm{SL}}(2,\mathbb{R})$

We study the solutions to the Dirac equation for the massive spinor field in the universal covering space of two-dimensional anti-de Sitter space. For certain values of the mass parameter, we impose a suitable set of boundary conditions which make the spatial component of the Dirac operator self-adjoint. Then, we use the transformation properties of the spinor field under the isometry group of the theory, namely, the universal covering group of $\mathrm{SL}(2,\mathbb{R})$, in order to determine which self-adjoint boundary conditions are invariant under this group. We identify the corresponding solution spaces with unitary irreducible representations of this group using the classification given by Pukanzki, and determine which of these correspond to invariant positive- and negative-frequency subspaces and, hence, in a vacuum state invariant under the isometry group. Finally, we examine the cases where the self-adjoint boundary condition leads to an invariant theory with non-invariant vacuum state and determine the unitary representation to which the vacuum state belongs.

Schwarzian derivative in higher-order Riccati equations

The Sturm-Liouville equation represents the linearized form of the first-order Riccati equation. This provides an evidence for the connection between Schwarzian derivative and this first-order nonlinear differential equation. Similar connection is not obvious for higher-order equations in the Riccati chain because the corresponding linear equations are of order greater than two. With special attention to the second- and third-order Riccati equations we demonstrate that Schwarzian derivative has a natural space in higher Riccati equations .

A Systematic Eigenspectral Investigation of Spatially Varying Effective Mass Schrödinger Equation for Squared Class Trigonometric Potentials

In the present paper, one particular attempt is to acquire explicit and analytical solutions of the position-dependent effective mass (PDEM) Schr\"odinger equation for various types of the squared trigonometric potentials. The algebraic procedure entitled as the point canonical transformation (PCT) is implemented in the process. Three different spatially dependent mass configurations are taken into account the establishment of the target system. In the final step, performing the computational tasks lead to the explicit determination of both discrete energy spectra and their corresponding wavefunctions.

McKay quivers and decomposition

When a quantum field theory in $d$-spacetime dimensions possesses a global $(d-1)$-form symmetry, it can decompose into disjoint unions of other theories. This is reflected in the physical quantities of the theory and can be used to study properties of the constituent theories. In this note we highlight the equivalence between the decomposition of orbifold $\sigma$-models and disconnected McKay quivers. Specifically, we show in numerous examples that each component of a McKay quiver can be given definitive geometric meaning through the decomposition formulae. In addition, we give a purely group and representation theoretic derivation of the quivers for the cases where the trivially acting part of the orbifold group is central. As expected, the resulting quivers are compatible with the case of $\sigma$-models on `banded' gerbes.

Existence and Stability of $α-$ harmonic Maps

In this paper, we first study the $\alpha-$energy functional, Euler-Lagrange operator and $\alpha$-stress energy tensor. Second, it is shown that the critical points of $\alpha-$ energy functional are explicitly related to harmonic maps through conformal deformation. In addition, an $\alpha-$harmonic map is constructed from any smooth map between Riemannian manifolds under certain assumptions. Next, we determine the conditions under which the fibers of horizontally conformal $\alpha-$ harmonic maps are minimal submanifolds. Then, the stability of any $\alpha-$harmonic map from a Riemannian manifold to a Riemannian manifold with non-positive Riemannian curvature is demonstrated. Finally, the instability of $\alpha-$harmonic maps from a compact manifold to a standard unit sphere is investigated.

On Wronskians and $qq$-systems

We discuss the $qq$-systems, the functional form of the Bethe ansatz equations for the twisted Gaudin model from a new geometric point of view. We use a concept of $G$-Wronskians, which are certain meromorphic sections of principal $G$-bundles on the projective line. In this context, the $qq$-system, similar to its difference analog, is realized as the relation between generalized minors of the $G$-Wronskian. We explain the link between $G$-Wronskians and twisted $G$-oper connections, which are the traditional source for the $qq$-systems.

On the long-time asymptotics of the Camassa-Holm equation in solitonic regions

We study the long time asymptotic behavior for the Cauchy problem of the Camassa-Holm (CH) equation with the appropriate Sobolev initial data. in the solitonic regions. Our main technical tool is the $\bar\partial$ generalization of Deift-Zhou steepest descent method. Through introducing a new scale $(y,t)$ and constructing the RH problem,we derive different long time asymptotic expansion of the solution $u(y,t)$ in different space-time solitonic regions of $\xi=y/t$. We divide the half-plane $\{ (y,t): -\infty <y<\infty, \ t> 0\}$ into two asymptotic regions: 1. phase point absent region: $\xi \in(-\infty,-1/4)\cup(2,+\infty)$,corresponding asymptotic approximations can be characterized with an $N(\Lambda)$-solitons with diverse residual error order $\mathcal{O}(t^{-1+2\rho})$; 2. phase points region: $\xi \in(0,2)$ and $\xi \in(-1/4,0)$.The corresponding asymptotic approximations can be characterized with an $N(\Lambda)$-soliton and an interaction term between soliton solutions and the dispersion term with diverse residual error order $\mathcal{O}(t^{-3/4})$.Our results also confirm the soliton resolution conjecture and asymptotically stability of the N-soliton solutions for the CH equation.

The Zoo of Opers and Dualities

We investigate various spaces of $SL(r+1)$-opers and their deformations. For each type of such opers, we study the quantum/classical duality, which relates quantum integrable spin chains with classical solvable many body systems. In this context, quantum/classical dualities serve as an interplay between two different coordinate systems on the space of opers. We also establish correspondences between the underlying oper spaces, which recently had multiple incarnations in symplectic duality and bispectral duality.

Flat $GL(1|1)$-connections and fatgraphs

We study the moduli space of flat $GL(1|1)$-connections on a punctured surface from the point of view of graph connections. To each fatgraph, a system of coordinates is assigned, which involves two bosonic and two fermionic variables per edge, subject to certain relations. In the case of trivalent graphs, we provide a closed explicit formula for the Whitehead moves. In addition, we discuss the invariant Poisson bracket.

Zeta-regularized Lattice Field Theory with Lorentzian background metrics

Lattice field theory is a very powerful tool to study Feynman's path integral non-perturbatively. However, it usually requires Euclidean background metrics to be well-defined. On the other hand, a recently developed regularization scheme based on Fourier integral operator $\zeta$-functions can treat Feynman's path integral non-pertubatively in Lorentzian background metrics. In this article, we formally $\zeta$-regularize lattice theories with Lorentzian backgrounds and identify conditions for the Fourier integral operator $\zeta$-function regularization to be applicable. Furthermore, we show that the classical limit of the $\zeta$-regularized theory is independent of the regularization. Finally, we consider the harmonic oscillator as an explicit example. We discuss multiple options for the regularization and analytically show that they all reproduce the correct ground state energy on the lattice and in the continuum limit. Additionally, we solve the harmonic oscillator on the lattice in Minkowski background numerically.

The grammar of the Ising model: A new complexity hierarchy

How complex is an Ising model? Usually, this is measured by the computational complexity of its ground state energy problem. Yet, this complexity measure only distinguishes between planar and non-planar interaction graphs, and thus fails to capture properties such as the average node degree, the number of long range interactions, or the dimensionality of the lattice. Herein, we introduce a new complexity measure for Ising models and thoroughly classify Ising models with respect to it. Specifically, given an Ising model we consider the decision problem corresponding to the function graph of its Hamiltonian, and classify this problem in the Chomsky hierarchy. We prove that the language of this decision problem is (i) regular if and only if the Ising model is finite, (ii) constructive context free if and only if the Ising model is linear and its edge language is regular, (iii) constructive context sensitive if and only if the edge language of the Ising model is context sensitive, and (iv) decidable if and only if the edge language of the Ising model is decidable. We apply this theorem to show that the 1d Ising model and the Ising model on layerwise complete graphs are constructive context free, and the 2d Ising model, the all-to-all Ising model, and the Ising model on perfect binary trees are constructive context sensitive. We also provide a grammar for the 1d and 2d Ising model. This work is a first step in the characterisation of physical interactions in terms of grammars.

Mixing times and cutoff for the TASEP in the high and low density phase

We study the totally asymmetric simple exclusion process with open boundaries in the high density and the low density phase. In the bulk of the two phases, we show that the process on a segment of length $N$ exhibits cutoff at order $N$, while in the intersection of the phases, the coexistence line, the mixing time is of order $N^2$, and no cutoff occurs. In particular, we determine the $\varepsilon$-mixing time in the coexistence line up to constant factors, which do not dependent on $\varepsilon$. Combined with previous results on the maximal current phase, this completes the picture on mixing times for the TASEP with open boundaries.

Blow-up of 2D attractive Bose-Einstein condensates at the crittical rotational speed

We study the ground states of a 2D focusing non-linear Schr\"odinger equation with rotation and harmonic trapping. When the strength of the interaction approaches a critical value from below, the system collapses to a profile obtained from the optimizer of a Gagliardo--Nirenberg interpolation inequality. This was established before in the case of fixed rotation frequency. We extend the result to rotation frequencies approaching, or even equal to, the critical frequency at which the centrifugal force compensates the trap. We prove that the blow-up scenario is to leading order unaffected by such a strong deconfinement mechanism. In particular the blow-up profile remains independent of the rotation frequency.