arXiv subjects

G. L. Celardo

Publications and source records attributed to G. L. Celardo.

At least 19 recordsLinked to original sources

arXiv2024

Nonstandard Hubbard model and electron pairing

We present a non-standard Hubbard model applicable to arbitrary single-particle potential profiles and inter-particle interactions. Our approach involves a novel treatment of Wannier functions, free from the ambiguities of conventional methods and applicable to finite systems without periodicity constraints. To ensure the consistent evaluation of Wannier functions, we develop a perturbative approach, utilizing the barrier penetration coefficient as a perturbation parameter. With the newly defined Wannier functions as a basis, we derive the Hubbard Hamiltonian, revealing the emergence of density-induced and pair tunneling terms alongside standard contributions. Our investigation demonstrates that long-range inter-particle interactions can induce a novel mechanism for repulsive particle pairing. This mechanism relies on the effective suppression of single-particle tunneling due to density-induced tunneling. Contrary to expectations based on the standard Hubbard model, an increase in inter-particle interaction does not lead to an insulating state. Instead, our proposed mechanism implies the coherent motion of correlated electron pairs, similar to bound states within a multi-well system, resistant to decay from single-electron tunneling transitions. These findings carry significant implications for various phenomena, including the formation of flat bands, the emergence of superconductivity in twisted bilayer graphene, and the possibility of a novel metal-insulator transition.

cond-mat.str-el↗

arXiv2024

Universal stability of coherently diffusive 1D systems with respect to decoherence

Static disorder in a 3D crystal degrades the ideal ballistic dynamics until it produces a localized regime. This Metal-Insulator Transition is often preceded by coherent diffusion. By studying three paradigmatic 1D models, namely the Harper-Hofstadter-Aubry-André and Fibonacci tight-binding chains, along with the power-banded random matrix model, we show that whenever coherent diffusion is present, transport is exceptionally stable against decoherent noise. This is completely at odds with what happens for coherently ballistic and localized dynamics, where the diffusion coefficient strongly depends on the environmental decoherence. A universal dependence of the diffusion coefficient on the decoherence strength is analytically derived: the diffusion coefficient remains almost decoherence-independent until the coherence time becomes comparable with the mean elastic scattering time. Thus, systems with a quantum diffusive regime could be used to design robust quantum wires. Moreover our results might shed new light on the functionality of many biological systems, which often operate at the border between the ballistic and localized regimes.

quant-ph↗

arXiv2023

Ultraviolet superradiance from mega-networks of tryptophan in biological architectures

Networks of tryptophan -- an aromatic amino acid with strong fluorescent response -- are ubiquitous in biological systems, forming diverse architectures in transmembrane proteins, cytoskeletal filaments, sub-neuronal elements, photoreceptor complexes, virion capsids, and other cellular structures. We analyze the cooperative effects induced by ultraviolet (UV) excitation of several biologically relevant tryptophan mega-networks, thus giving insight into novel mechanisms for cellular signalling and control. Our theoretical analysis in the single-excitation manifold predicts the formation of strongly superradiant states due to collective interactions among organized arrangements of up to more than $10^5$ tryptophan UV-excited transition dipoles in microtubule architectures, which leads to an enhancement of the fluorescence quantum yield that is confirmed by our experiments. We demonstrate the observed consequences of this superradiant behavior in the fluorescence quantum yield for hierarchically organized tubulin structures, which increases in different geometric regimes at thermal equilibrium before saturation -- highlighting the effect's persistence in the presence of disorder.

quant-ph↗

arXiv2018

On the existence of superradiant excitonic states in microtubules

Microtubules are biological protein polymers with critical and diverse functions. Their structures share some similarities with photosynthetic antenna complexes, particularly in the ordered arrangement of photoactive molecules with large transition dipole moments. As the role of photoexcitations in microtubules remains an open question, here we analyze tryptophan molecules, the amino acid building block of microtubules with the largest transition dipole strength. By taking their positions and dipole orientations from realistic models capable of reproducing tubulin experimental spectra, and using a Hamiltonian widely employed in quantum optics to describe light-matter interactions, we show that such molecules arranged in their native microtubule configuration exhibit a superradiant ground state, which represents an excitation fully extended on the chromophore lattice. We also show that such a superradiant ground state emerges due to supertransfer coupling between the ground states of smaller blocks of the microtubule. In the dynamics we find that the spreading of excitation is ballistic in the absence of external sources of disorder and strongly dependent on initial conditions. The velocity of photoexcitation spreading is shown to be enhanced by the supertransfer effect with respect to the velocity one would expect from the strength of the nearest-neighbor coupling between tryptophan molecules in the microtubule. Finally, such structures are shown to have an enhanced robustness to static disorder when compared to geometries that include only short-range interactions. These cooperative effects (superradiance and supertransfer) may induce ultra-efficient photoexcitation absorption and could enhance excitonic energy transfer in microtubules over long distances under physiological conditions.

physics.bio-ph↗

arXiv2016

Shielding and localization in presence of long range hopping

We investigate a paradigmatic model for quantum transport with both nearest-neighbor and infinite range hopping coupling (independent of the position). Due to long range homogeneous hopping, a gap between the ground state and the excited states can be induced, which is mathematically equivalent to the superconducting gap. In the gapped regime, the dynamics within the excited states subspace is shielded from long range hopping, namely it occurs as if long range hopping would be absent. This is a cooperative phenomenon since shielding is effective over a time scale which diverges with the system size. We named this effect {\it Cooperative Shielding}. We also discuss the consequences of our findings on Anderson localization. Long range hopping is usually thought to destroy localization due to the fact that it induces an infinite number of resonances. Contrary to this common lore we show that the excited states display strong localized features when shielding is effective even in the regime of strong long range coupling. A brief discussion on the extension of our results to generic power-law decaying long range hopping is also given. Our preliminary results confirms that the effects found for the infinite range case are generic.

cond-mat.mes-hall↗

arXiv2016

Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions

The specific mechanisms which leads to the formation of fractal nanostructures by pulsed laser deposition remain elusive despite intense research efforts, motivated mainly by the technological interest in obtaining tailored nanostructures with simple and scalable production methods. Here we focus on fractal nanostructures of titanium dioxide, $TiO_2$, a strategic material for many applications, obtained by femtosecond laser ablation at ambient conditions. We model the fractal formation through extensive Monte Carlo simulations based on a set of minimal assumptions: irreversible sticking and size independent diffusion. Our model is able to reproduce the fractal dimensions and the area distributions of the nanostructures obtained in the experiments for different densities of the ablated material. The comparison of theory and experiment show that such fractal aggregates are formed after landing of the ablated material on the substrate surface by a diffusive mechanism. Finally we discuss the role of the thermal conductivity of the substrate and the laser fluence on the properties of the fractal nanostructures. Our results represent an advancement towards controlling the production of fractal nanostructures by pulsed laser deposition.

cond-mat.mes-hall↗

arXiv2014

Cooperative robustness to dephasing: single-exciton Superradiance in a nanoscale ring to model natural light-harvesting systems

We analyze a 1-d ring structure composed of many two-levels systems, in the limit where only one excitation is present. The two-levels systems are coupled to a common environment, where the excitation can be lost, which induces super and subradiant behavior. Moreover, each two-levels system is coupled to another independent environment, modeled by a classical white noise, simulating a dephasing bath and described by the Haken-Strobl master equation. Single exciton Superradiance, an example of cooperative quantum coherent effect, is destroyed at a critical dephasing strength proportional to the system size, showing robustness of cooperativity to the action of the dephasing environment. We also show that the coupling to a common decay channel contrasts the action of dephasing, driving the entanglement decay to slow down on increasing the system size. Moreover, after a projective measurement which finds the excitation in the system, the entanglement reaches a stationary value, independent of the initial conditions.

cond-mat.mes-hall↗

arXiv2014

Cooperative Robustness to Static Disorder: Superradiance and localization in a nanoscale ring to model natural light-harvesting systems

We analyze a 1-d ring structure composed of many two-level systems, in the limit where only one excitation is present. The two-level systems are coupled to a common environment, where the excitation can be lost, which induces super and subradiant behavior, an example of cooperative quantum coherent effect. We consider time-independent random fluctuations of the excitation energies. This static disorder, also called inhomogeneous broadening in literature, induces Anderson localization and is able to quench Superradiance. We identify two different regimes: $i)$ weak opening, in which Superradiance is quenched at the same critical disorder at which the states of the closed system localize; $ii)$ strong opening, with a critical disorder strength proportional to both the system size and the degree of opening, displaying robustness of cooperativity to disorder. Relevance to photosynthetic complexes is discussed.

cond-mat.mes-hall↗

arXiv2013

Subradiant hybrid states in the open 3D Anderson-Dicke model

Anderson localization is a paradigmatic coherence effect in disordered systems, often analyzed in the absence of dissipation. Here we consider the case of coherent dissipation, occurring for open system with coupling to a common decay channel. This dissipation induces cooperative Dicke super- and subradiance and an effective long range hopping, expected to destroy Anderson localization. We are thus in presence of two competing effects, i.e localization driven by disorder and delocalization driven by dissipative opening. Here we demonstrate the existence of a {\it subradiant hybrid regime}, emerging from the interplay of opening and disorder, in which subradiant states are {hybrid with both features of localized and extended states}, while superradiant states are extended. We also provide analytical predictions for this regime, confirmed by numerical simulations.

cond-mat.mes-hall↗

arXiv2013

A Quantum Biological Switch Based on Superradiance Transitions

A linear chain of connected electron sites with two asymmetric sinks, one attached to each end, is used as a simple model of quantum electron transfer in photosynthetic bio-complexes. For a symmetric initial population in the middle of the chain, it is expected that electron transfer is mainly directed towards the strongest coupled sink. However, we show that quantum effects radically change this intuitive "classical" mechanism, so that electron transfer can occur through the weaker coupled sink with maximal efficiency. Using this capability, we show how to design a quantum switch that can transfer an electron to the left or right branch of the chain, by changing the coupling to the sinks. The operational principles of this quantum device can be understood in terms of superradiance transitions and subradiant states. This switching, being a pure quantum effect, can be used as a witness of wave--like behaviour of excitations in molecular chains. When realistic data are used for the photosystem II reaction center, this quantum biological switch is shown to retain its reliability, even at room temperature.

quant-ph↗

arXiv2012

Focusing in Multiwell Potentials: Applications to Ion Channels

We investigate out of equilibrium stationary distributions induced by a stochastic dichotomous noise on double and multi-well models for ion channels. Ion-channel dynamics is analyzed both through over-damped Langevin equations and master equations. As a consequence of the external stochastic noise, we prove a non trivial focusing effect, namely the probability distribution is concentrated only on one state of the multi-well model. We also show that this focusing effect, which occurs at physiological conditions, cannot be predicted by a simple master equation approach.

cond-mat.mes-hall↗

arXiv2012

Superradiance Transition in Photosynthetic Light-Harvesting Complexes

We investigate the role of long-lasting quantum coherence in the efficiency of energy transport at room temperature in Fenna-Matthews-Olson photosynthetic complexes. The excitation energy transfer due to the coupling of the light harvesting complex to the reaction center ("sink") is analyzed using an effective non-Hermitian Hamiltonian. We show that, as the coupling to the reaction center is varied, maximal efficiency in energy transport is achieved in the vicinity of the superradiance transition, characterized by a segregation of the imaginary parts of the eigenvalues of the effective non-Hermitian Hamiltonian. Our results demonstrate that the presence of the sink (which provides a quasi--continuum in the energy spectrum) is the dominant effect in the energy transfer which takes place even in absence of a thermal bath. This approach allows one to study the effects of finite temperature and the effects of any coupling scheme to the reaction center. Moreover, taking into account a realistic electric dipole interaction, we show that the optimal distance from the reaction center to the Fenna-Matthews-Olson system occurs at the superradiance transition, and we show that this is consistent with available experimental data.

cond-mat.mes-hall↗

arXiv2012

From closed to open 1D Anderson model: Transport versus spectral statistics

Using the phenomenological expression for the level spacing distribution with only one parameter, $0 \leq β\leq \infty$, covering all regimes of chaos and complexity in a quantum system, we show that transport properties of the one-dimensional Anderson model of finite size can be expressed in terms of this parameter. Specifically, we demonstrate a strictly linear relation between $β$ and the normalized localization length for the whole transition from strongly localized to extended states. This result allows one to describe all transport properties in the open system entirely in terms of the parameter $β$ and strength of coupling to continuum. For non-perfect coupling, our data show a quite unusual interplay between the degree of internal chaos defined by $β$, and degree of openness of the model. The results can be experimentally tested in single-mode waveguides with either bulk or surface disorder.

cond-mat.dis-nn↗

arXiv2012

Coherent transport in multi-branch circuits

A multi-branch quantum circuit is considered from the viewpoint of coherent electron or wave transport. Starting with the closed system, we give analytical conditions for the appearance of two isolated localized states out of the energy band. In the open system, using the method of the effective non-Hermitian Hamiltonian, we study signal transmission through such a circuit. The super-radiant transition occurs when the average level width becomes comparable to the mean level spacing. We consider also the case with on-site disorder and find an analytical estimate, confirmed by numerical data, for the robustness of the isolated states and their role in transport processes.

cond-mat.mes-hall↗

arXiv2012

Interplay of superradiance and disorder in the Anderson Model

Using a non-Hermitian Hamiltonian approach to open systems, we study the interplay of disorder and superradiance in a one-dimensional Anderson model. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian, a transition to a superradiant regime is shown to occur. As an effect of openness the structure of eigenstates undergoes a strong change in the superradiant regime: we show that the sensitivity to disorder of the superradiant and the subradiant subspaces is very different; superradiant states remain delocalized as disorder increases, while subradiant states are sensitive to the degree of disorder.

cond-mat.mes-hall↗

arXiv2012

Enhancement of magnetic anisotropy barrier in long range interacting spin systems

Magnetic materials are usually characterized by anisotropy energy barriers which dictate the time scale of the magnetization decay and consequently the magnetic stability of the sample. Here we present a unified description, which includes coherent rotation and nucleation, for the magnetization decay in generic anisotropic spin systems. In particular, we show that, in presence of long range exchange interaction, the anisotropy energy barrier grows as the volume of the particle for on site anisotropy, while it grows even faster than the volume for exchange anisotropy, with an anisotropy energy barrier proportional to $V^{2-α/d}$, where $V$ is the particle volume, $α\leq d $ is the range of interaction and $d$ is the embedding dimension. These results shows a relevant enhancement of the anisotropy energy barrier w.r.t. the short range case, where the anisotropy energy barrier grows as the particle cross sectional area for large particle size or large particle aspect ratio.

cond-mat.stat-mech↗

arXiv2010

Distribution of Resonance Widths and Dynamics of Continuum Coupling

We analyze the statistics of resonance widths in a many-body Fermi system with open decay channels. Depending on the strength of continuum coupling, such a system reveals growing deviations from the standard chi-square (Porter-Thomas) width distribution. The deviations emerge from the process of increasing interaction of intrinsic states through common decay channels; in the limit of perfect coupling this process leads to the super-radiance phase transition. The width distribution depends also on the intrinsic dynamics (chaotic vs regular). The results presented here are important for understanding the recent experimental data concerning the width distribution for neutron resonances in nuclei.

cond-mat.mes-hall↗

arXiv2010

Transport Through Nanostructures with Asymmetric Coupling to the Leads

Using an approach to open quantum systems based on the effective non-Hermitian Hamiltonian, we fully describe transport properties for a paradigmatic model of a coherent quantum transmitter: a finite sequence of square potential barriers. We consider the general case of asymmetric external barriers and variable coupling strength to the environment. We demonstrate that transport properties are very sensitive to the degree of opening of the system and determine the parameters for maximum transmission at any given degree of asymmetry. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian, we show a double transition to a super-radiant regime where the transport properties and the structure of resonances undergo a strong change. We extend our analysis to the presence of disorder and to higher dimensions.

cond-mat.mes-hall↗