arXiv subjects

Natalia Cortés

Publications and source records attributed to Natalia Cortés.

14 recordsLinked to original sources

arXiv2026

Reaching maximum efficiency in quantum Stirling engines using multilayer graphene

In this work, quantum Stirling engines based on monolayer, AB-stacked bilayer, and ABC-stacked trilayer graphene under perpendicular magnetic fields are analyzed. Performance maps of the useful work $(ηW)$ reveal a robust optimum at low magnetic fields and moderately low temperatures, with all stackings capable of reaching Carnot efficiency under suitable configurations. The AB bilayer achieves this across the broadest parameter window while sustaining finite work, the monolayer exhibits highly constrained regimes, and the trilayer shows smoother trends with sizable $ηW$. These results identify multilayer graphene, particularly the AB bilayer, as a promising platform for efficient Stirling engines, while also highlighting the versatility of the monolayer in realizing all four operational regimes of the Stirling cycle.

cond-mat.stat-mech↗

arXiv2026

Thermal Hofstadter Butterflies

Fractal electronic spectra arising from the competition between lattice periodicity and magnetic flux are a fundamental hallmark of two-dimensional quantum systems. While the spectral properties of Hofstadter butterflies are well documented, their thermodynamic response has remained remarkably unexplored. We present an original characterization of the electronic entropy $S_{e}$, and specific heat $C_{e}$, at half-filling, for square, honeycomb, and triangular lattices under a magnetic field. We demonstrate that these observables exhibit fast and slow magneto-thermo oscillations and pronounced magnetocaloric effects. We identify striking self-similarity in $S_e$ and $C_e$, tracing heart-shaped specific heat and tunnel-like entropy contours that repeat at specific lattice-dependent magnetic fluxes. Entropy minima at low temperatures play a remarkable role, acting as fingerprints for the butterfly spines, resolving the underlying fractal spectra. These findings may establish thermal measurements as high-resolution spectroscopic probes, providing a robust framework for recognizing fractal signatures through thermodynamics in diverse nanostructures.

cond-mat.mes-hall↗

arXiv2025

Entropy-Seebeck ratio as a tool for elementary charge determination

In this work, we investigate the relationship between the Seebeck coefficient $(S)$, and the differential entropy per particle (DEP, $s$), as a tool for characterizing charge carriers in two-dimensional systems. Using armchair silicene nanoribbons as a model platform, we analyze how both quantities and their ratio depend on chemical potential at room temperature. While the Seebeck coefficient captures transport properties through the energy dependence of the electronic transmission, the DEP is directly connected to the system's electronic entropy, offering a direct thermodynamic alternative for estimating $S$. We evaluate these transport-thermodynamic properties considering diverse ribbon widths, defining metallic and semiconducting regimes. We find both quantities $S$ and $s$, are highly interconnected within the ribbon's band gap energy region, and their ratio $s/S$ converges to the elementary charge $e$ across that energy window, fulfilling the Kelvin formula $S=s/e$. On the contrary, $s/S$ is undefined for gapless ribbons in the energy window of the first transmission channel. These results establish the ratio between the DEP and the Seebeck coefficient as a reliable and complementary probe for the determination of the elementary charge, and to identify the cleanness of electronic band gaps as $s/S$ matches with $e$.

cond-mat.mes-hall↗

arXiv2024

Magnetic-thermodynamic phase transition in strained phosphorous-doped graphene

We explore quantum-thermodynamic effects in a phosphorous (P)-doped graphene monolayer subjected to biaxial tensile strain. Introducing substitutional P atoms in the graphene lattice generates a tunable spin magnetic moment controlled by the strain control parameter $\varepsilon$. This leads to a magnetic quantum phase transition (MQPT) at zero temperature modulated by $\varepsilon$. The system transitions from a magnetic phase, characterized by an out-of-plane $sp^3$ type hybridization of the P-carbon (P-C) bonds, to a non-magnetic phase when these bonds switch to in-plane $sp^2$ hybridization. Employing a Fermi-Dirac statistical model, we calculate key thermodynamic quantities as the electronic entropy $S_e$ and electronic specific heat $C_e$. At finite temperatures, we find the MQPT is reflected in both $S_e$ and $C_e$, which display a distinctive $Λ$-shaped profile as a function of $\varepsilon$. These thermodynamic quantities sharply increase up to $\varepsilon = 5\% $ in the magnetic regime, followed by a sudden drop at $\varepsilon = 5.5\% $, transitioning to a linear dependence on $\varepsilon$ in the nonmagnetic regime. Notably, $S_e$ and $C_e$ capture the MQPT behavior for low and moderate temperature ranges, providing insights into the accessible electronic states in P-doped graphene. This controllable magnetic-to-nonmagnetic switch offers potential applications in electronic nanodevices operating at finite temperatures.

cond-mat.mes-hall↗

arXiv2024

Magnetocaloric effect for a $Q$-clock type system

In this work, we study the magnetocaloric effect (MCE) in a working substance corresponding to a square lattice of spins with $Q$ possible orientations, known as the ``$Q$-state clock model". When the $Q$-state clock model has $Q\geq 5$ possible configurations, it presents the famous Berezinskii Kosterlitz Thouless (BKT) phase associated with vortices states. We calculate thermodynamic quantities using Monte Carlo simulations for even $Q$ numbers, ranging from $Q=2$ to $Q=8$ spin orientations per site in a lattice. We use lattices of different sizes with $L\times L = 8^{2}, 16^{2}, 32^{2}, 64^{2}, \text{and}\ 128^{2}$ sites, considering free boundary conditions and an external magnetic field varying between $B = 0$ and $B=1$ in natural units of the system. By obtaining the entropy, it is possible to quantify the MCE through an isothermal process in which the external magnetic field on the spin system is varied. In particular, we find the values of $Q$ that maximize the MCE depending on the lattice size and the magnetic phase transitions linked with the process. Given the broader relevance of the $Q$-state clock model in areas such as percolation theory, neural networks, and biological systems, where multi-state interactions are essential, our study provides a robust framework in applied quantum mechanics, statistical mechanics and related fields.

cond-mat.stat-mech↗

arXiv2022

Multilayer Graphene as an Endoreversible Otto Engine

Graphene is perhaps the most prominent "Dirac material," a class of systems whose electronic structure gives rise to charge carriers that behave as relativistic fermions. In multilayer graphene several crystal sheets are stacked such that the honeycomb lattice of each layer is displaced along one of the lattice edges. When subject to an external magnetic field, the scaling of the multilayer energy spectrum with the magnetic field, and thus the system's thermodynamic behavior, depends strongly on the number of layers. With this in mind, we examine the performance of a finite-time endoreversible Otto cycle with multilayer graphene as its working medium. We show that there exists a simple relationship between the engine efficiency and the number of layers, and that the efficiency at maximum power can exceed that of a classical endoreversible Otto cycle.

cond-mat.stat-mech↗

arXiv2022

Entropy and Seebeck signals meet on the edges

We explore the electronic entropy per particle $s$ and Seebeck coefficient $\mathcal{S}$ in zigzag graphene ribbons. Pristine and edge-doped ribbons are considered using tight-binding models to inspect the role of edge states in the observed thermal transport properties. As a bandgap opens when the ribbons are doped at one or both edges, due to asymmetric edge potentials, we find that $s$ and $\mathcal{S}$ signals are closely related to each other: both develop sharp dip-peak lineshapes as the chemical potential lies in the gap, while the ratio $s/\mathcal{S}$ exhibits a near constant value equal to the elementary charge $e$ at low temperatures. This constant ratio suggests that $\mathcal{S}$ can be seen as the transport differential entropy per charge, as suggested by some authors. Our calculations also indicate that measurement of $s$ and $\mathcal{S}$ may be useful as a spectroscopic probe of different electronic energy scales involved in such quantities in gapped materials.

cond-mat.mes-hall↗

arXiv2021

Proximity-induced spin-polarized magnetocaloric effect in transition metal dichalcogenides

We explore proximity-induced magnetocaloric effect (MCE) on transition metal dichalcogenides, focusing on a two-dimensional (2D) MoTe$_2$ monolayer deposited on a ferromagnetic semiconductor EuO substrate connected to a heat source. We model this heterostructure using a tight-binding model, incorporating exchange and Rashba fields induced by proximity to EuO, and including temperature through Fermi statistics. The MCE is induced on the 2D MoTe$_2$ layer due to the EuO substrate, revealing large spin-polarized entropy changes for energies out of the band gap of the MoTe$_2$-EuO system. By gating the chemical potential, the MCE can be tuned to produce heating for spin up and cooling for spin down across the $K$ and $K'$ valley splitting in the valence band, whereas either heats or cools for both spins in the conduction band. The Rashba field enhances the MCE in the valence zone while decreasing it in the conduction bands. The exchange field-induced MCE could be useful to produce tunable spin-polarized thermal responses in magnetic proximitized 2D materials.

cond-mat.mes-hall↗

arXiv2021

Gate-tunable direct and inverse electrocaloric effect in trilayer graphene

The electrocaloric (EC) effect is the reversible change in temperature and/or entropy of a material when it is subjected to an adiabatic electric field change. Our tight-binding calculations linked to Fermi statistics, show that the EC effect is sensitive to the stacking arrangement in trilayer graphene (TLG) structures connected to a heat source, and is produced by changes of the electronic density of states (DOS) near the Fermi level when external gate fields are applied on the outer graphene layers. We demonstrate the AAA-stacked TLG presents an inverse EC response (cooling), whereas the EC effect in ABC-stacked TLG remains direct (heating) regardless of the applied gate field potential strength. We reveal otherwise the TLG with Bernal-ABA stacking geometry generates both the inverse and direct EC response in the same sample, associated with a gate-dependent electronic entropy transition at finite temperature. By varying the chemical potential to different Fermi levels, we find maxima and minima of the DOS are located near the extremes of the electronic entropy, which are correlated with sign changes in the differential entropy per particle, giving a particular experimentally measurable electronic entropy spectrum for each TLG geometry. The EC effect in quantum two-dimensional layered systems may bring a wide variety of prototype van der Waals materials that could be used as versatile platforms to controlling the temperature in nanoscale electronic devices required in modern portable on-chip technologies.

cond-mat.mes-hall↗

arXiv2020

Otto Engine: Classical and Quantum Approach

In this paper, we analyze the total work extracted and the efficiency of the magnetic Otto cycle in its classic and quantum versions. As a general result, we found that the work and efficiency of the classical engine is always greater than or equal to that of its quantum counterpart independent of the working substance. In the classical case, this is due to the fact that the working substance is always in thermodynamic equilibrium at each point of the cycle, maximizing the energy extracted in the adiabatic paths. We apply this analysis to the case of a two-level system, finding that the work and efficiency in Otto's quantum and classical cycle are identical, regardless of the working substance, and we obtain similar results for a multilevel system where a linear relationship between the spectrum of energies of the working substance and the external magnetic field is fulfilled. Finally, we show an example of a three-level system in which we compare two zones in the entropy, temperature and magnetic field diagram to find which is the most efficient when performing a thermodynamic cycle. This work provides a practical way to look for temperature and magnetic field zones in the entropy diagram that can maximize the power extracted from Otto's magnetic engine.

cond-mat.stat-mech↗

arXiv2020

Reversible edge spin currents in antiferromagnetically proximitized dichalcogenides

We explore proximity effects on transition metal dichalcogenide ribbons deposited on antiferromagnetic (AFM) insulating substrates. We model these hybrid heterostructures using a tight-binding model that incorporates exchange and Rashba fields induced by proximity to the AFM material. The robust edge states that disperse in the midgap of the dichalcogenide are strongly affected by induced exchange fields that reflect different AFM ordering in the substrate. This results in enhanced spin-orbit coupling effects and complex spin projection content for states on zigzag ribbon edges. Gated systems that shift the Fermi level in the midgap range are also shown to exhibit spin polarized currents on these edges. Antiparallel exchange fields along the edge results in spin currents that can reverse polarization with the applied field. The added functionality of these hybrid structures can provide spintronic devices and versatile platforms to further exploit proximity effects in diverse material systems.

cond-mat.mes-hall↗

arXiv2019

Tunable spin-polarized edge currents in proximitized transition metal dichalcogenides

We explore proximity-induced ferromagnetism on transition metal dichalcogenide (TMD), focusing on molybdenum ditelluride (MoTe$_{2}$) ribbons with zigzag edges, deposited on ferromagnetic europium oxide (EuO). A three-orbital tight-binding model incorporates the exchange and Rashba fields induced by proximity to the EuO substrate. For in-gap Fermi levels, electronic modes in the nanoribbon are strongly spin-polarized and localized along the edges, acting as one-dimensional (1D) conducting channels with tunable spin-polarized currents. Hybrid structures such as the MoTe$_{2}$/EuO configuration can serve as building blocks for spintronic devices, and provide versatile platforms to further understand proximity effects in diverse materials systems.

cond-mat.mes-hall↗

arXiv2017

Stacking change in MoS$_{2}$ bilayers induced by interstitial Mo impurities

We use a theoretical approach to reveal the electronic and structural properties of molybdenum impurities between MoS$_{2}$ bilayers. We find that interstitial Mo impurities are able to reverse the well-known stability order of the pristine bilayer, because the most stable form of stacking changes from AA' (undoped) into AB (doped). The occurrence of Mo impurities in different positions shows their split electronic levels in the energy gap, following octahedral and tetrahedral crystal fields. The energy stability is related to the accommodation of Mo impurities compacted in hollow sites between layers. Other less stable configurations for Mo dopants have larger interlayer distances and band gaps than those for the most stable stacking. Our findings suggest possible applications such as exciton trapping in layers around impurities, and the control of bilayer stacking by Mo impurities in the growth process.

cond-mat.mtrl-sci↗

arXiv2012

Dicke and Fano effects in single photon transport

The single photon transport through a one dimensional array of cavities is studied theoretically. Analytical expressions of the reflection and transmission are given. The transmission shows an energy spectrum with forbidden and allowed bands that depends on the detuning parameter of the system. We show that the allowed miniband is formed due to the indirect coupling between the atoms in each cavity. In addition, the band edges can be controlled by the degree of detuning of the level of the atoms. We discuss the analogy between this phenomenon with the Fano and Dicke effects.

quant-ph↗