arXiv subjects

M. S. Reis

Publications and source records attributed to M. S. Reis.

At least 19 recordsLinked to original sources

arXiv2021

Quantum battery based on quantum discord at room temperature

The study of advanced quantum devices for energy storage has attracted the attention of the scientific community in the past few years. Although several theoretical progresses have been achieved recently, experimental proposals of platforms operating as quantum batteries under ambient conditions are still lacking. In this context, this work presents a feasible realization of a quantum battery in a carboxylate-based metal complex, which can store a finite amount of extractable work under the form of quantum discord at room temperature, and recharge by thermalization with a reservoir. Moreover, the stored work can be evaluated through non-destructive measurements of the compound's magnetic susceptibility. These results pave the way for the development of enhanced energy storage platforms through material engineering.

quant-ph↗

arXiv2016

Influence of the external pressure on the quantum correlations of molecular magnets

The study of quantum correlations in solid state systems is a large avenue for research and their detection and manipulation are an actual challenge to overcome. In this context, we show by using first-principles calculations on the prototype material KNaCuSi$_{4}$O$_{10}$ that the degree of quantum correlations in this spin cluster system can be managed by external hydrostatic pressure. Our results open the doors for research in detection and manipulation of quantum correlations in magnetic systems with promising applications in quantum information science.

quant-ph↗

arXiv2016

Graphene infrared light emitting diode (GILED)

The present Letter proposes a device based on graphene for infrared light emission. It is based on a n- and p-doped monolayer graphene (MGs), with Fermi energies $E_F$ and -$E_F$, respectively, sandwiching a bilayer graphene (BG) with bandgap $Λ=2|eV_g-Δ|\geq 2E_F$, where $V_g$ is the gate voltage across the BG and $Δ$ the sub-lattice energy difference into each layer of the BG. This device works as simple as tuning the gate voltage to decrease the BG bandgap down to $2E_F$; and, once this condition is fulfilled, a current flows from the n-doped MG to the p-doped MG. However, when electrons achieve the other side of the device, i.e., into the p-doped MG, their energies ($E_F$) are much bigger than the holes energies ($-E_F$), and thus these electrons decay emitting infrared photons.

cond-mat.mes-hall↗

arXiv2016

Carboxylate based molecular magnet: one path toward achieving stable quantum correlations at room temperature

The control of quantum correlations in solid state systems by means of material engineering is a broad avenue to be explored, since it makes possible steps toward the limits of quantum mechanics and the design of novel materials with applications on emerging quantum technologies. In this context, this Letter explores the potential of molecular magnets to be prototypes of materials for quantum information technology. More precisely, we engineered a material and from its geometric quantum discord we found significant quantum correlations up to 9540 K (even without entanglement); and, in addition, a pure singlet state occupied up to around 80 K (above liquid nitrogen temperature). These results could only be achieved due to the carboxylate group promoting a metal-to-metal huge magnetic interaction.

quant-ph↗

arXiv2015

Quantum capacitance oscillations in graphene under crossed magnetic and electric fields

Quantum oscillations of metallic systems at low temperatures is one of the key rules to experimentally access their electronic properties, such as energy spectrum, scattering mechanisms, geometry of Fermi surface and many other features. The importance of these knowledge is enormous, since from these a thorough understanding of anomalous Hall effect, thermopower and Nernst coefficients, just to name a few, is possible; and from those knowledge, a plenty of applications arise as emerging technologies. In this direction, the present contribution focus on a complete description of quantum capacitance oscillations of monolayer and bilayer graphenes under crossed electric and magnetic fields. We found a closed theoretical expression for the quantum capacitance and highlight their amplitude, period and phase - important parameters to access the electronic properties of graphenes. These results open doors for further experimental studies.

cond-mat.mes-hall↗

arXiv2015

Magneto-oscillations on specific heat of graphene monolayer

Measurement of magnetic oscillations on thermodynamic quantities (like magnetization and specific heat), is one of the experimental methods to access the density of states of electronic systems. In the present paper we therefore theoretically explore the oscillatory phenomena on the specific heat of graphenes considering gapped and gapless cases in a quantized magnetic field. Further situations is also considered, as the influence of impurities, Coulomb interaction and phonons. We could then map the magnetic oscillations on the specific heat of graphenes under these constraints and the obtained results are a good starting point and guide for further experimental works.

cond-mat.mes-hall↗

arXiv2015

Magnetocaloric functional properties of $Sm_{0.6}Sr_{0.4}MnO_3$ manganite due to advanced nanostructured morphology

The magnetocaloric effect (MCE) is the key concept to produce new, advanced, freon-like free, low cost and environmental friendly magnetic refrigerators. Among several potential materials, $Sm_{0.6}Sr_{0.4}MnO_3$ manganite presents one of the highest MCE value in comparison to all other known manganites; however, its studied was only concentrated on the bulk material. To overcame this lack of the information we successfully produced advanced nanostructures, namely nanoparticles and nanotubes of that highlighted manganite by using a sol-gel modified method. High resolution transmission electron microscopy revealed nanoparticle and nanotube diameters of 29 nm and 200 nm, respectively; and, in addition, this technique also showed that the wall of the nanotube is formed by the nanoparticles with 25 nm of diameter. The magnetocaloric potentials, $ΔS_M$ versus T curves, of the nanostructures were obtained and they are broader than the their bulk counterpart. This increases the useful temperature range of a magnetic refrigerator. But also an undesired M-shape profile for the nanotube sample was observed, due to the rising of a superparamagnetic behavior. These results also evidenced the existence of a nanoparticle size threshold below which the advantage to make the transition wider is no longer valid.

cond-mat.mtrl-sci↗

arXiv2015

Multifunctional Heusler alloy: experimental evidences of enhanced magnetocaloric properties at room temperature and half-metallicity

Heusler alloys are widely studied due to their interesting structural and magnetic properties, like magnetic memory shape ability, coupled magneto-structural phase transitions and half-metallicity; ruled, for many cases, by the valence electrons number ($N_v$). The present work focuses on the magnetocaloric potentials of half-metals, exploring the effect of $N_v$ on the magnetic entropy change, preserving half-metallicity. The test bench is the Si-rich side of the half-metallic series Fe$_2$MnSi$_{1-x}$Ga$_x$. From the obtained experimental results it was possible to obtain $|ΔS|_{max}=ΔH^{0.8}(α+βN_v)$, i.e., the maximum magnetic entropy change depends in a linear fashion on $N_v$, weighted by a power law on the magnetic field change $ΔH$ ($α$ and $β$ are constants experimentally determined). In addition, it was also possible to predict a new multifunctional Heusler alloy, with enhanced magnetocaloric effect, Curie temperature close to 300 K and half-metallicity.

cond-mat.mtrl-sci↗

arXiv2015

Diamagnetic magnetocaloric effect due to a transversal oscillating magnetic field

The present Letter describes the magnetocaloric effect of a diamagnetic material with a magnetic field $B_\parallel$ along the $z$ axis and a transversal and oscillating field $B_\perp (\ll B_\parallel)$ parallel to the $x-y$ plane. We show that the magnetocaloric potentials due to a change in $B_\parallel$ is the same as those due to a change in the frequency of $B_\perp$. These results raise the possibility of building magnetocaloric devices without moving parts, since changing frequency is a simple electronic issue, while changing the field from permanent magnets depends on mechanical aspects.

cond-mat.mtrl-sci↗

arXiv2015

Influence of longitudinal electric field on the oscillating magnetocaloric effect of graphenes

The present effort explores the influence of a longitudinal applied electric field on the magnetocaloric properties of graphenes. The magnetic entropy change $ΔS(T,ΔB,E)$ has two contributions: $S_{cos}(T,B,E)$ with an oscillating character on $m$, inversely proportional to the magnetic field; and $S_{per}(T,B,E)$ with a periodic character also on $m$. In comparison to the case without electric field, the maximum value of the magnetic entropy change either increases or decreases due to an applied electric field, depending on the value of $m$; and, in addition, the temperature in which the maximum entropy change occurs decreases due to the electric field.

cond-mat.mtrl-sci↗

arXiv2015

Oscillating magnetocaloric effect of a 2D non-relativistic diamagnetic material

Among the magnetic materials, those with ferromagnetic character are, by far, the most studied in what concerns applications of the magnetocaloric effect. However, recently, diamagnetic materials received due attention never received before, and an oscillatory behavior, analogous to the de Haas-van Alphen effect, has been found. The present effort describes in details the magnetocaloric properties of a 2D non-relativistic material (a Gold thin film, for instance), where oscillations, depending on the reciprocal magnetic field $1/B$, are found. A comparison of the magnetic entropy change per electron for some cases is presented and we found $\approx10^{-1}$ k$_{\text{B}}$ (@109.3 K) for graphenes, $\approx10^{-5}$ k$_{\text{B}}$ (@0.7 K) for 2D Gold and $\approx 10^{-7}$ k$_{\text{B}}$ (@0.7 K) for 3D Gold.

cond-mat.mtrl-sci↗

arXiv2015

Step-like features on caloric effects of graphenes

We considered a graphene nano-ribbon with a longitudinal electric field (along $x$ direction) and a transversal magnetic field (along $z$ direction), and then observe (i) the electrocaloric effect ruled by an applied magnetic field and (ii) the magnetocaloric effect ruled by an applied electric field. We focused our attention to the limit of low temperatures, and then observed interesting step-like features. For each filled Landau level $n$, created by the applied magnetic field, both caloric effects increase proportionally to $n+1/2$; and this step measures either important graphene properties (like Fermi velocity) or quantum fundamental quantities (like Planck constant and magnetic flux quantum).

cond-mat.mes-hall↗

arXiv2013

Witnessing spin-orbit thermal entanglement in rare-earth ions

We explore spin-orbit thermal entanglement in rare-earth ions, based on a witness obtained from mean energies. The entanglement temperature $T_{E}$, below which entanglement emerges, is found to be thousands of kelvin above room temperature for all light rare earths. This demonstrate the robustness to environmental fluctuations of entanglement between internal degrees of freedom of a single ion.

quant-ph↗

arXiv2012

Evidence for entanglement at high temperatures in an engineered molecular magnet

The molecular compound [Fe$_{2}$($μ_{2}$-oxo)(C$_{3}$H$_{4}$N$_{2}$)$_{6}$(C$_{2}$O$_{4}$)$_{2}$] was designed and synthesized for the first time and its structure was determined using single-crystal X-ray diffraction. The magnetic susceptibility of this compound was measured from 2 to 300 K. The analysis of the susceptibility data using protocols developed for other spin singlet ground-state systems indicates that the quantum entanglement would remain at temperatures up to 732 K, significantly above the highest entanglement temperature reported to date. The large gap between the ground state and the first-excited state (282 K) suggests that the spin system may be somewhat immune to decohering mechanisms. Our measurements strongly suggest that molecular magnets are promising candidate platforms for quantum information processing.

quant-ph↗

arXiv2011

Writing electronic ferromagnetic states in a high-temperature paramagnetic nuclear spin system

In this paper we use the Nuclear Magnetic Resonance (NMR) to write eletronic states of a ferromagnetic system into a high-temperature paramagnetic nuclear spins. Through the control of phase and duration of radiofrequency pulses we set the NMR density matrix populations, and apply the technique of quantum state tomography to experimentally obtain the matrix elements of the system, from which we calculate the temperature dependence of magnetization for different magnetic fields. The effects of the variation of temperature and magnetic field over the populations can be mapped in the angles of spins rotations, carried out by the RF pulses. The experimental results are compared to the Brillouin functions of ferromagnetic ordered systems in the mean field approximation for two cases: the mean field is given by (i) $B=B_0+λM$ and (ii) $B=B_0+λM + λ^\prime M^3$, where $B_0$ is the external magnetic field, and $λ, λ^\prime$ are mean field parameters. The first case exhibits second order transition, whereas the second case has first order transition with temperature hysteresis. The NMR simulations are in good agreement with the magnetic predictions.

quant-ph↗

arXiv2009

Finite size analysis of a two-dimensional Ising model within a nonextensive approach

In this work we present a thorough analysis of the phase transitions that occur in a ferromagnetic 2D Ising model, with only nearest-neighbors interactions, in the framework of the Tsallis nonextensive statistics. We performed Monte Carlo simulations on square lattices with linear sizes L ranging from 32 up to 512. The statistical weight of the Metropolis algorithm was changed according to the nonextensive statistics. Discontinuities in the m(T) curve are observed for $q\leq 0.5$. However, we have verified only one peak on the energy histograms at the critical temperatures, indicating the occurrence of continuous phase transitions. For the $0.5<q\leq 1.0$ regime, we have found continuous phase transitions between the ordered and the disordered phases, and determined the critical exponents via finite-size scaling. We verified that the critical exponents $α$, $β$ and $γ$ depend on the entropic index $q$ in the range $0.5<q\leq 1.0$ in the form $α(q)=(10 q^{2}-33 q+23)/20$, $β(q)=(2 q-1)/8$ and $γ(q)=(q^{2}-q+7)/4$. On the other hand, the critical exponent $ν$ does not depend on $q$. This suggests a violation of the scaling relations $2 β+γ=d ν$ and $α+2 β+γ=2$ and a nonuniversality of the critical exponents along the ferro-paramagnetic frontier.

cond-mat.stat-mech↗

arXiv2009

New Consideration on Composed Nonextensive Magnetic Systems

In this paper a composed A+B magnetic system, with spins J_A=2 and J_B=3/2, is considered within the mean-field approximation, in the framework of Tsallis nonextensive statistics. Our motivation is twofold: (1) to approach the existing experimental data of manganese oxides (manganites), where Mn^{3+} and Mn^{4+} form two magnetic sublattices, and (2) to investigate the structure of nonextensive density matrices of composed systems. By imposing that thermodynamic quantities, such as the magnetization of sublattices A and B, must be invariant weather the calculation is taken over the total Hilbert space or over partial subspaces, we found that the expression for the nonextensive entropy must be adapted. Our argument is supported by calculation of sublattices magnetization M_A and M_B, internal energy, U_A and U_B, and magnetic specific heat, CA and CB. It is shown that only with the modified entropy the two methods of calculation agree to each other. Internal energy and magnetization are additive, but no clear relationship was found between S_A, S_B and the total entropy S_{A+B} for q \neq 1. It is shown that the reason for the failure of the standard way of calculation is the assumption of statistical independence between the two subsystems, which however does not affect the density matrix in the full Hilbert space.

cond-mat.stat-mech↗

arXiv2009

Entanglement temperature in molecular magnets composed of S-spin dimers

In the present work, we investigate the quantum thermal entanglement in molecular magnets composed of dimers of spin $S$, using an Entanglement Witness built from measurements of magnetic susceptibility. An entanglement temperature, $T_{e}$, is then obtained for some values of spin $S$. From this, it is shown that $T_{e}$ is proportional to the intradimer exchange interaction $J$ and that entanglement appears only for antiferromagnetic coupling. The results are compared to experiments carried on three isostructural materials: KNaMSi$_{4}$O$_{10}$ (M$=$Mn, Fe or Cu).

quant-ph↗