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Fractal and Fractional

Quantum Dynamics in a Comb Geometry: Green Function Solutions with Nonlocal and Fractional Potentials

Enrique Chipicoski Gabrick, Ervin Kaminski Lenzi, Antonio S. M. de Castro, José Trobia, Antonio Marcos Batista

Published Jul 4, 2025• 0 citations
Publisher

Abstract

We investigate a generalized quantum Schrödinger equation in a comb-like structure that imposes geometric constraints on spatial variables. The model is extended by the introduction of nonlocal and fractional potentials to capture memory effects in both space and time. We consider four distinct scenarios: (i) a time-dependent nonlocal potential, (ii) a spatially nonlocal potential, (iii) a combined space–time nonlocal interaction with memory kernels, and (iv) a fractional spatial derivative, which is related to distributions asymptotically governed by power laws and to a position-dependent effective mass. For each scenario, we propose solutions based on the Green’s function for arbitrary initial conditions and analyze the resulting quantum dynamics. Our results reveal distinct spreading regimes, depending on the type of non-locality and the fractional operator applied to the spatial variable. These findings contribute to the broader generalization of comb models and open new questions for exploring quantum dynamics in backbone-like structures.

DOI
10.3390/fractalfract9070446

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We investigate a generalized quantum Schrödinger equation in a comb-like structure that imposes geometric constraints on spatial variables. The model is extended by the introduction of nonlocal and fractional potentials to capture memory effects in both space and time. We consider four distinct scenarios: (i) a time-dependent nonlocal potential, (ii) a spatially nonlocal potential, (iii) a combined space–time nonlocal interaction with memory kernels, and (iv) a fractional spatial derivative, which is related to distributions asymptotically governed by power laws and to a position-dependent effective mass. For each scenario, we propose solutions based on the Green’s function for arbitrary initial conditions and analyze the resulting quantum dynamics. Our results reveal distinct spreading regimes, depending on the type of non-locality and the fractional operator applied to the spatial variable. These findings contribute to the broader generalization of comb models and open new questions for exploring quantum dynamics in backbone-like structures.
We consider a quantum motion governed by the time-dependent Schrödinger equation on a three dimensional comb structure. We derive the corresponding fractional Schrödinger equations for the reduced probability density functions by projection of the three dimensional comb dynamics in the two- and one-dimensional configuration space. This represents another physical example of a system where fractional calculus emerges. We give closed-form solutions of the corresponding equations for the reduced probability density functions in terms of the Fox H-function, by using the Green’s function approach.
We propose and investigate a generalized Schrödinger equation by introducing a fractional Riesz derivative to account for anomalous transport and a memory kernel to describe temporal nonlocal effects. Additionally, we include a long-range interactions term, modeled by an integral operator, which captures spatially extended interactions. Using the Green function approach, we derive analytical solutions and explore their implications in the time-space domain. Our findings reveal novel quantum phenomena arising from the interplay of fractional dynamics, nonlocal potentials, and memory effects, including the emergence of new local maxima in the evolution of Green's functions and distinct localization behaviors.
An example of non-Markovian quantum dynamics is considered in the framework of a geometrical (topological) subordination approach. The specific property of the model is that it coincides exactly with the fractional diffusion equation, which describes the geometric Brownian motion on combs. Both classical diffusion and quantum dynamics are described using the dilatation operator xddx. Two examples of geometrical subordinators are considered. The first one is the Gaussian function, which is due to the comb geometry. For the quantum consideration with a specific choice of the initial conditions, it corresponds to the integral representation of the Mittag–Leffler function by means of the subordination integral. The second subordinator is the Dirac delta function, which results from the memory kernels that define the fractional time derivatives in the fractional diffusion equation.
We investigate numerically the time-dependent Schrödinger equation on a two-dimensional comb in the presence of a localized Dirac delta potential on the backbone. The geometric constraint and the point interaction are both regularized via narrow Gaussian approximations, and the resulting equation is discretized with second-order finite differences and integrated with a fourth-order Runge–Kutta scheme on a finite domain. Two initial states are considered—a Gaussian wave packet and a comb-like wave function—and two barrier configurations are studied: a potential centered at the origin and one displaced along the backbone. In both cases, the Dirac delta interaction suppresses the probability density at the barrier position and reorganizes the wave packet into a persistent split-peak profile, in contrast to the single spreading maximum observed in the free case. When the barrier is displaced, due to reflection from the barrier, the left–right symmetry is broken and the density remains asymmetrically concentrated near the interaction site. To elucidate the results further, we also analyze the second moment, which undoubtedly confirms that the anomalous dynamics features, such as the subdiffusive behavior on comb-like structures in classical systems are retained in the quantum case as well.
Quantum dynamics of a particle on a two-dimensional comb structure is considered. This dynamics of a Hamiltonian system with a topologically constrained geometry leads to the non-Markovian behavior. In the framework of a rigorous analytical consideration, it is shown how a fractional time derivative appears for the relevant description of this non-Markovian quantum mechanics in the framework of fractional time Schrödinger equations. Analytical solutions for the Green functions are obtained for both conservative and periodically driven in time Hamiltonian systems.