arXiv subjects

N. M. R. Peres

Publications and source records attributed to N. M. R. Peres.

At least 37 records · Page 2Linked to original sources

arXiv2022

Laser induced enhanced coupling between photons and squeezed magnons in antiferromagnets

In this paper we consider a honeycomb antiferromagnet subject to an external laser field. Obtaining a time-independent effective Hamiltonian, we find that the external laser renormalizes the exchange interaction between the in-plane components of the spin-operators, and induces a synthetic Dzyaloshinskii-Moria interaction (DMI) between second neighbors. The former allows the control of the magnon dispersion's bandwidth and the latter breaks time-reversal symmetry inducing non-reciprocity in momentum space. The eigen-excitations of the system correspond to squeezed magnons whose squeezing parameters depend on the properties of the laser. When studying how these spin excitations couple with cavity photons, we obtain a coupling strength which can be enhanced by an order of magnitude via careful tuning of the laser's intensity, when compared to the case where the laser is absent. The transmission plots through the cavity are presented, allowing the mapping of the magnons' dispersion relation.

cond-mat.mes-hall↗

arXiv2022

Theoretical methods for excitonic physics in two-dimensional materials

In this tutorial we introduce the reader to several theoretical methods of determining the exciton wave functions and the corresponding eigenenergies. The methods covered are either analytical, semi-analytical, or numeric. We make explicit all the details associated with the different methods, thus allowing newcomers to do research on their own, without experiencing a steep learning curve. The tutorial starts with a variational method and ends with a simple semi-analytical approach to solve the Bethe-Salpeter equation in two-dimensional (2D) gapped materials. For the first methods addressed in this tutorial, we focus on a single layer of hexagonal Boron Nitride (hBN) and of transition metal dichalcogenide (TMD), as these are exemplary materials in the field of 2D excitons. For explaining the Bethe- Salpeter method we choose the biased bilayer graphene, which presents a tunnable band gap. The system has the right amount of complexity (without being excessive). This allows the presentation of the solution of the Bethe-Salpeter equation in a context that can be easily generalized to more complex systems or to apply it to simpler models.

cond-mat.mes-hall↗

arXiv2022

Tunable Excitons in Rhombohedral Trilayer Graphene

Trilayer graphene is receiving an increasing level of attention due to its stacking--dependent magnetoelectric and optoelectric properties, and its more robust ferromagnetism relative to monolayer and bilayer variants. Additionally, rhombohedral stacked trilayer graphene presents the possibility of easily opening a gap via either an external electric field perpendicular to the layers, or via the application of external strain. In this paper, we consider an external electric field to open a bandgap in rhombohedral trilayer graphene and study the excitonic optical response of the system. This is done via the combination of a tight binding model with the Bethe--Salpeter equation, solved semi--analytically and requiring only a simple numerical quadrature. We then discuss the valley--dependent optical selection rules, followed by the computation of the excitonic linear optical conductivity for the case of a rhombohedral graphene trilayer encapsulated in hexagonal boron nitride. The tunability of the excitonic resonances via an external field is also discussed, together with the increasing localization of the excitonic states as the field increases.

cond-mat.mes-hall↗

arXiv2022

Two limit cases of twisted hBN bilayers and their excitonic response

In this paper we discuss the optical response due to the excitonic effect of two types of hBN bilayers: AB and AA'. Understanding the properties of these bilayers is of great utility to the study of twisted bilayers at arbitrary angles, since these two configurations correspond to the limit cases of 0 and 60 degree rotation. To obtain the excitonic response we present a method to solve a four-band Bethe-Salpeter equation, by casting it into a 1D problem, thus greatly reducing the numerical burden of the calculation when compared with strictly 2D methods. We find results in good agreement with ab initio calculations already published in the literature for the AA' bilayer, and predict the excitonic conductivity of the AB bilayer, which remains largely unstudied. The main difference in the conductivity of these two types of bilayers is the appearance of a small, yet well resolved, resonance between two larger ones in the AB configuration. This resonance is due to a mainly interlayer exciton, and is absent in the AA' bilayer. Also, the conductivity of the AB bilayer is due to both intralayer and interlayer excitons and is dominated by p-states, while intralayer states are the relevant ones for the AA' configuration, like in a monolayer. The effect of introducing a bias in the AA' bilayer is also discussed.

cond-mat.mes-hall↗

arXiv2021

Absorption and optical selection rules of tunable excitons in biased bilayer graphene

Biased bilayer graphene, with its easily tunable band gap, presents itself as the ideal system to explore the excitonic effect in graphene based systems. In this paper we study the excitonic optical response of such a system by combining a tight binding model with the solution of the Bethe-Salpeter equation, the latter being solved in a semi-analytical manner, requiring a single numerical quadrature, thus allowing for a transparent calculation. With our approach we start by analytically obtaining the optical selection rules, followed by the computation of the absorption spectrum for the case of a biased bilayer encapsulated in hexagonal boron nitride, a system which has been the subject of a recent experimental study. An excellent agreement is seen when we compare our theoretical prediction with the experimental data.

cond-mat.mes-hall↗

arXiv2021

Third-order polarizability of interlayer excitons in hetero-bilayers

In this paper, we employ a fully microscopic approach to the study of interlayer excitons in hetero-bilayers. We use Fowler's and Karplus' method to access the dynamical polarizability of non--interacting interlayer excitons in a $\mathrm{WSe}_{2}/\mathrm{WS}_{2}$--based van der Waals heterostructure. Following from the calculation of the linear polarizability, we consider Svendsen's variational method to the calculation of the dynamic third--order polarizability. With this variational method, we study both two--photon absorption and third--harmonic generation processes for interlayer excitons in a $\mathrm{WSe}_{2}/\mathrm{WS}_{2}$ hetero--bilayer, discussing the various selection rules of intra--excitonic energy level transitions.

cond-mat.mes-hall↗

arXiv2021

Enhancing the hybridization of plasmons in graphene with 2D superconductor collective modes

We explore ways in which the close proximity between graphene sheets and monolayers of 2D superconductors can lead to hybridization between their collective excitations. We consider heterostructures formed by combinations of graphene sheets and 2D superconductor monolayers. The broad range of energies in which the graphene plasmon can exist, together with its tunability, makes such heterostrucutres promising platforms for probing the many-body physics of superconductors. We show that the hybridization between the graphene plasmon and the Bardasis-Schrieffer mode of a 2D superconductor results in clear signatures on the near-field reflection coefficient of the heterostructure, which in principle can be observed in scanning near-field microscopy experiments.

cond-mat.mes-hall↗

arXiv2021

Plasmonics response of a nanorod in the vicinity of a metallic surface: local approach with analytical solution

In this paper we present an analytical solution for the eigenmodes and corresponding electric field of a composite system made of a nanorod in the vicinity of a plasmonic semi-infinite metallic system. To be specific, we choose Silver as the material for both the nanorod and the semi-infinite metal. The system is composed of two sub-systems with different symmetries: the rod has symmetry, while the interface has a rectangular one. Using a boundary integral method, proposed by Eyges, we are able to compute analytically the integrals that sew together the two systems. In the end, the problem is reduced to one of linear algebra, where all the terms in the system are known analytically. For large distances between the rod and the planar surface, only a few of those integrals are needed and a full analytical solution can be obtained. Our results are important to benchmark other numerical approaches and represent a starting point in the discussion of systems composed of nanorods and two-dimensional materials.

cond-mat.other↗

arXiv2021

Two-level Systems Coupled to Graphene plasmons: A Lindblad equation approach

In this paper we review the theory of open quantum systems and macroscopic quantum electrodynamics, providing a self-contained account of many aspects of these two theories. The former is presented in the context of a qubit coupled to a electromagnetic thermal bath, the latter is presented in the context of a quantization scheme for surface-plasmon polaritons (SPPs) in graphene based on Langevin noise currents. This includes a calculation of the dyadic Green's function (in the electrostatic limit) for a Graphene sheet between two semi-infinite linear dieletric media, and its subsequent application to the construction of SPP creation and annihilation operators. We then bring the two fields together and discuss the entanglement of two qubits in the vicinity of a graphene sheet which supports SPPs. The two qubits communicate with each other via the emission and absorption of SPPs. We find that a Schödinger cat state involving the two qubits can be partially protected from decoherence by taking advantage of the dissipative dynamics in graphene. A comparison is also drawn between the dynamics at zero temperature, obtained via Schrodinger's equation, and at finite temperature, obtained using the Lindblad equation.

cond-mat.mes-hall↗

arXiv2021

Quantum Surface-Response of Metals Revealed by Acoustic Graphene Plasmons

A quantitative understanding of the electromagnetic response of materials is essential for the precise engineering of maximal, versatile, and controllable light--matter interactions. Material surfaces, in particular, are prominent platforms for enhancing electromagnetic interactions and for tailoring chemical processes. However, at the deep nanoscale, the electromagnetic response of electron systems is significantly impacted by quantum surface-response at material interfaces, which is challenging to probe using standard optical techniques. Here, we show how ultra-confined acoustic graphene plasmons (AGPs) in graphene--dielectric--metal structures can be used to probe the quantum surface-response functions of nearby metals, here encoded through the so-called Feibelman $d$-parameters. Based on our theoretical formalism, we introduce a concrete proposal for experimentally inferring the low-frequency quantum response of metals from quantum shifts of the AGPs' dispersion, and demonstrate that the high field confinement of AGPs can resolve intrinsically quantum mechanical electronic length-scales with subnanometer resolution. Our findings reveal a promising scheme to probe the quantum response of metals, and further suggest the utilization of AGPs as plasmon rulers with ångström-scale accuracy.

cond-mat.mes-hall↗

arXiv2021

The polarizability of a confined atomic system: An application of Dalgarno-Lewis method

In this paper we give an application of Dalgarno-Lewis method, the latter not usually taught in quantum mechanics courses. This is very unfortunate since this method allows to bypass the sum over states appearing in the usual perturbation theory. In this context, and as an example, we study the effect of an external field, both static and frequency dependent, on a model-atom at fixed distance from a substrate. This can happen, for instance, when some organic molecule binds from one side to the substrate and from the other side to an atom or any other polarizable system. We model the polarizable atom by a short range potential, a Dirac$-δ$ and find that the existence of a bound state depends on the ratio of the effective "nuclear charge" to the distance of the atom to the substrate. Using an asymptotic analysis, previously developed in the context of a single $δ-$function potential in an infinite medium, we determine the ionization rate and the Stark shift of our system. Using Dalgarno-Lewis theory we find an exact expression for the static and dynamic polarizabilities of our system valid to all distances. We show that the polarizability is extremely sensitive to the distance to the substrate creating the possibility of using this quantity as a nanometric ruler. Furthermore, the line shape of the dynamic polarizability is also extremely sensitive to the distance to the substrate, thus providing another route to measure nanometric distances. The ditactic value of the $δ-$function potential is well accepted in teaching activities due to its simplicity, while keeping the essential ingredients of a given problem.

quant-ph↗

arXiv2021

Analytical description of the 1s exciton linewidth temperature-dependence in transition metal dichalcogenides

We obtain an analytical expression for the linewidth of the 1s-exciton as a function of temperature in transition metal dichalcogenides. The total linewidth, as function of temperature, is dominated by three contributions: (i) the radiative decay (essentially temperature independent); (ii) the phonon-induced intravalley scattering; (iii) the phonon-induced intervalley scattering. Our approach uses a variational \emph{Ansatz} to solve the Wannier equation allowing for an analytical treatment of the excitonic problem, including rates of the decay dynamics. Our results are in good agreement with experimental data already present in the literature and can be used to readily predict the value of the total linewidth at any temperature in the broad class of excitonic two-dimensional materials.

cond-mat.mes-hall↗

arXiv2021

Calculation of the nonlinear response functions of intra-exciton transitions in two-dimensional transition metal dichalcogenides

In this paper, we study the third-order nonlinear optical response due to transitions between excitonic levels in two-dimensional transition metal dichalcogeniedes. To accomplish this, we use methods not applied to the description of excitons in two-dimensional materials so far and combined with a variational approach to describe the $1s$ excitonic state. The aforementioned transitions allow to probe dark states which are not revealed in absorption experiments. We present general formulas capable of describing any third-order process. The specific case of two-photon absorption in WSe2 is studied. The case of the circular well is also studied as a benchmark of the theory.

cond-mat.mes-hall↗

arXiv2021

A perturbative approach to the polaron shift of excitons in transition metal dichalcogeniedes

In this paper we study the phonon's effect on the position of the 1s excitonic resonance of the fundamental absorption transition line in two-dimensional transition metal dichalcogenides. We apply our theory to WS$_{2}$a two-dimensional material where the shift in absorption peak position has been measured as a function of temperature. The theory is composed of two ingredients only: i) the effect of longitudinal optical phonons on the absorption peak position, which we describe with second order perturbation theory; ii) the effect of phonons on the value of the single particle energy gap, which we describe with the Huang Rhys model. Our results show an excellent agreement with the experimentally measured shift of the absorption peak with the temperature.

cond-mat.mes-hall↗

arXiv2021

Microscopic theory of the polarizability of transition metal dichalcogenides excitons: Application to WSe2

In this paper we develop a fully microscopic theory of the polarizability of excitons in transition metal dichalcogenides. We apply our method to the description of the excitation $2$p dark states. These states are not observable in absorption experiments but can be excited in a pump-probe experiment. As an example we consider $2$p dark states in WSe\textsubscript{2}. We find a good agreement between recent experimental measurements and our theoretical calculations.

cond-mat.mes-hall↗

arXiv2021

Harnessing Ultraconfined Graphene Plasmons to Probe the Electrodynamics of Superconductors

We show that the Higgs mode of a superconductor, which is usually challenging to observe by far-field optics, can be made clearly visible using near-field optics by harnessing ultraconfined graphene plasmons. As near-field sources we investigate two examples: graphene plasmons and quantum emitters. In both cases the coupling to the Higgs mode is clearly visible. In the case of the graphene plasmons, the coupling is signaled by a clear anticrossing stemming from the interaction of graphene plasmons with the Higgs mode of the superconductor. In the case of the quantum emitters, the Higgs mode is observable through the Purcell effect. When combining the superconductor, graphene, and the quantum emitters, a number of experimental knobs become available for unveiling and studying the electrodynamics of superconductors.

cond-mat.mes-hall↗

arXiv2020

A colloquium on the variational method applied to excitons in 2D materials

In this colloquium, we review the research on excitons in van der Waals heterostructures from the point of view of variational calculations. We first make a presentation of the current and past literature, followed by a discussion on the connections between experimental and theoretical results. In particular, we focus our review of the literature on the absorption spectrum and polarizability, as well as the Stark shift and the dissociation rate. Afterwards, we begin the discussion of the use of variational methods in the study of excitons. We initially model the electron-hole interaction as a soft-Coulomb potential, which can be used to describe interlayer excitons. Using an \emph{ansatz}, based on the solution for the two-dimensional quantum harmonic oscillator, we study the Rytova-Keldysh potential, which is appropriate to describe intralayer excitons in two-dimensional (2D) materials. These variational energies are then recalculated with a different \emph{ansatz}, based on the exact wavefunction of the 2D hydrogen atom, and the obtained energy curves are compared. Afterwards, we discuss the Wannier-Mott exciton model, reviewing it briefly before focusing on an application of this model to obtain both the exciton absorption spectrum and the binding energies for certain values of the physical parameters of the materials. Finally, we briefly discuss an approximation of the electron-hole interaction in interlayer excitons as an harmonic potential and the comparison of the obtained results with the existing values from both first--principles calculations and experimental measurements.

cond-mat.mes-hall↗

arXiv2020

Exciton-polariton mediated interaction between two nitrogen-vacancy color centers in diamond using two-dimensional transition metal dichalcogenides

In this paper, starting from a quantum master equation, we discuss the interaction between two negatively charged Nitrogen-vacancy color centers in diamond via exciton-polaritons propagating in a two-dimensional transition metal dichalcogenide layer in close proximity to a diamond crystal. We focus on the optical 1.945 eV transition and model the Nitrogen-vacancy color centers as two-level (artificial) atoms. We find that the interaction parameters and the energy levels renormalization constants are extremely sensitive to the distance of the Nitrogen-vacancy centers to the transition metal dichalcogenide layer. Analytical expressions are obtained for the spectrum of the exciton-polaritons and for the damping constants entering the Lindblad equation. The conditions for occurrence of exciton mediated superradiance are discussed.

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