arXiv subjects

Michal Jaščur

Publications and source records attributed to Michal Jaščur.

11 recordsLinked to original sources

arXiv2026

Cavity-QED enhancement of quantum entanglement and battery performance in double quantum dots

We theoretically investigate quantum correlations and energy storage in a single-electron silicon double quantum dot (eDQD) coupled to a single-mode microwave cavity. The charge and spin degrees of freedom are hybridized by Rashba spin-orbit coupling (SOC), while the cavity interacts with the eDQD through spin-photon and charge-photon couplings. Using the reduced thermal density matrix, we characterize spin-charge entanglement by concurrence and quantum coherence by the $l_1$ norm. At low temperature, the cavity strongly modifies the spin-charge correlations, producing distinct regimes of enhanced and suppressed entanglement. We identify a nonlinear crossover between an eDQD-dominated regime and a photon-dressed regime. This crossover is revealed independently by two signatures: a rapid change in the concurrence and the onset of finite photon occupation in the cavity. Its boundary exhibits a dominant $G_{\rm c}\propto\sqrtΩ$ dependence and is robust against cavity Hilbert-space truncation. Beyond the Rashba coupling studied in [Ferreira et al., Phys. Rev. A 107, 052408 (2023)], we show that the cavity provides an additional tunable means of controlling quantum correlations and energy storage. We further characterize the eDQD as a quantum battery under coherent charging and evaluate its stored energy and ergotropy. The ergotropy varies markedly across a cavity-dressing crossover resembling that identified from entanglement, connecting cavity-induced modification of the dressed states to extractable work. Optimization over the charge-photon coupling reveals parameter-dependent optimal charging regimes and enhanced maximum ergotropy with increasing Rashba coupling. Our results establish cavity coupling and SOC as complementary controls of quantum correlations and extractable energy in semiconductor eDQD-cavity systems.

cond-mat.mes-hall↗

arXiv2026

Reservoir-controlled electromagnetically induced gratings in a weakly driven two-level medium

We theoretically investigate the transmission and diffraction of a weak probe field from an electromagnetically induced grating formed in a weakly driven two-level medium coupled to engineered quantum reservoirs. Using a perturbative solution of the optical Bloch equations in the weak-driving regime, we analyze how normal-vacuum, thermal, and broadband squeezed-vacuum environments modify the probe susceptibility and consequently reshape both the spatial transmission function and the far-field diffraction patterns. We show that reservoir statistics have a pronounced impact on the diffraction response by altering the amplitude and phase of the induced grating. Thermal reservoirs enhance the transmission modulation and increase the intensity of the dominant diffraction orders, whereas squeezed-vacuum reservoirs generate strongly phase-sensitive modifications that selectively redistribute optical power among diffraction channels. We further demonstrate that the detuning between the squeezed reservoir and the driving field provides an efficient mechanism for controlling diffraction directionality, leading to substantial amplification of selected angular orders. In two-dimensional geometries, squeezed-vacuum correlations produce highly structured phase landscapes and strongly anisotropic diffraction patterns, enabling directional enhancement of specific diffraction channels while suppressing others. These results establish reservoir engineering as a versatile approach for controlling transmission, diffraction efficiency, and angular selectivity in minimal two-level systems, with potential applications in programmable photonic devices, beam steering, and quantum optical platforms.

physics.optics↗

arXiv2026

Quantum Mpemba Effect in a Four-Site Bose-Hubbard Model

We investigate relaxation-order inversion, known as the quantum Mpemba effect (QME), in a minimal open many-body system called a one-dimensional four-site Bose--Hubbard chain governed by Lindblad dynamics with local number dephasing. Families of thermal initial states are prepared at a fixed temperature and evolved under a common reference Liouvillian toward the same stationary state. Relaxation is characterized using four complementary diagnostics: trace distance, quantum relative entropy, symmetry-projected entropy imbalance (entanglement asymmetry), and the $\ell_{1}$-norm of coherence in the Fock basis. We find that QME emerges robustly in -the clean interacting regime, where on-site interactions redistribute the overlaps of initial states with slow Liouvillian decay modes, enabling states initially farther from equilibrium to converge faster at late times. In contrast, the noninteracting limit exhibits a monotonic relaxation hierarchy across all metrics. Introducing a linear Stark potential or random on-site disorder suppresses relaxation and eliminates QME signatures by inhibiting transport-assisted mixing and enhancing the dominance of slow modes. Within the explored parameter regime, the Stark field induces significantly stronger retardation than disorder. We further show that symmetry-projected entropy imbalance is particularly sensitive to charge-sector decoherence in reduced subsystems and provides a stringent probe of QME in bosonic platforms. Our results elucidate the essential role of interactions in enabling anomalous relaxation in open lattice systems and connect the suppression of QME under spatial inhomogeneity to localization phenomena in tilted and disordered Bose--Hubbard chains.

cond-mat.quant-gas↗

arXiv2025

Quantum Entanglement Generation in the Heterometallic Ni$^\text{2+}_4$Gd$_4^\text{3+}$ Complexes

We investigate various types of quantum entanglement in the octanuclear heterometallic $3d/4f$ complexes denoted as Ni$^{2+}_4$Gd$^{3+}_4$ under an external magnetic field, using the exact diagonalization approach. These molecular magnets, which can be effectively described by Heisenberg spin models, consist of two identical $\{\text{Ni}^{2+}_2\text{Gd}^{3+}_2\}$ cubane subunits bridged by acetate and hydroxide ligands. Our analysis reveals that their magnetization exhibits intermediate plateaus at low temperatures, indicating distinct ground states characteristic of Ni-containing compounds. Using negativity as a measure of quantum entanglement, we examine the influence of single-ion anisotropy and magnetic field on tetrapartite, bipartite, 1$-$3 tangle, and 2$-$2 tangle entanglements in two families of Ni$^{2+}_4$Gd$^{3+}_4$ complexes: $\boldsymbol{(1)}$ without anisotropy and $\boldsymbol{(2)}$ with anisotropy. Complex $\boldsymbol{(1)}$ exhibits strong bipartite entanglement between Ni ions, which persists up to $T \approx 3.0\,\text{K}$ and $B \approx 4.0\,\text{T}$, but shows significantly weaker tetrapartite entanglement and vanishing bipartite entanglement between Gd$\cdots$Gd and Ni$\cdots$Gd pairs. In contrast, complex $\boldsymbol{(2)}$ displays nonzero and sizable values for all types of entanglement considered. These findings emphasis the crucial role of single-ion anisotropy in generating and shaping the entanglement landscape of heterometallic Ni$^{2+}_4$Gd$^{3+}_4$ complexes. Notably, we find that the 1$-$3 tangle entanglement between a Ni ion and the remaining sites in a cubane unit serves as a reliable indicator of ground-state phase transitions, exhibiting distinct changes across phase boundaries irrespective of the presence of single-ion anisotropy.

cond-mat.mtrl-sci↗

arXiv2025

Jahn-Teller Effect for Controlling Quantum Correlations in Hexanuclear Fe$^{3+}$ Magnets

We investigate the low-temperature magnetic and quantum properties of hexanuclear Fe$^ {3+}_6$ complexes under an external magnetic field. We primarily study the impact of competing exchange interactions and their asymmetries induced by the Jahn-Teller distortion on the quantum properties of the complexes. The inequality in exchange interactions lifts the ground-state degeneracy that gives rise to complex quantum behavior. By constructing the ground-state phase diagram and analyzing magnetization, we identify key magnetic phases and critical phenomena. We further quantify quantum correlations using tripartite entanglement negativity and conditional von Neumann entropy to unveil how the Jahn-Teller effect enhances intra-triangle entanglement while modulating inter-triangle correlations. Our findings highlight the Fe$^{3+}_6$ complex as a promising molecular platform for tunable quantum correlations, with potential applications in quantum information processing and molecular qubits.

cond-mat.mtrl-sci↗

arXiv2022

Robust quantum entanglement and teleportation in the tetrapartite spin-1/2 square clusters: Theoretical study on the effect of a cyclic four-spin exchange

The whole entanglement measure so-called geometric $Π_4$ average of tangles and bipartite entanglement of the antiferromagnetic spin-1/2 XXX Heisenberg model on a tetranuclear square cluster with cyclic four-spin interaction are rigorously examined by the help of thermal negativities. The model comprises two nearest-neighbor exchange couplings $J_1$ and $J_2$ such that $J_1\gg J_2$. When the cyclic four-spin exchange is zero, the maximum value of whole entanglement $Π_4$ is achieved at low enough temperatures and relatively high magnetic fields $(B\approx J_1)$. Also, maximum bipartite entanglement between pair spins with exchange coupling $J_1$ is achievable at high temperature and high magnetic field. This quantity remains alive for sufficiently high temperature and high magnetic field values comparable with the relevant exchange coupling $J_1$. A nonzero value of the cyclic four-spin exchange notably enhances the degree of the whole entanglement, while it weakens the bipartite entanglement degree. We demonstrate that the whole entanglement reaches an unconventional minimum at a special parameter region of cyclic four-spin exchange almost ten order of magnitude smaller than $J_1$, where the system is in a quantum antiferromagnetic state. The real complex $[\text{Cu}_4\text{L}_4(\text{H}_2\text{O})_4](\text{ClO}_4)_4$ as a strong antiferromagnetic tetranuclear square compound provides us an experimental representative to estimate the strength of the whole and bipartite entanglements at high enough temperature. It is demonstrated that the entanglement negativities of this complex are yet depend on the considered cyclic four-spin interaction even though its magnitude is significantly smaller than $J_1$.

cond-mat.stat-mech↗

arXiv2017

Mixed spin-1/2 and 3/2 Ising model with multi-spin interactions on a decorated square lattice

A mixed spin-1/2 and spin-3/2 Ising model on a decorated square lattice with a nearest- neighbor interaction, next-nearest-neighbor bilinear interaction, three-site four-spin in- teraction and single-ion anisotropy is exactly investigated using a generalized decoration- iteration transformation, Callen-Suzuki identity and differential operator technique. The ground-state and finite-temperature phase boundaries are obtained by identifying all rel- evant phases corresponding to minimum internal or free energy of the system. The thermal dependencies of magnetization, correlation functions, entropy and specific heat are also calculated exactly and the most interesting cases are discussed in detail.

cond-mat.stat-mech↗

arXiv2016

Thermodynamic properties of a Hubbard model on a cubic cluster - exact diagonalization study at quarter filling

We study the thermodynamics of a zero-dimensional, cubic cluster described with a Hubbard Hamiltonian, focusing our interest on the magnetic properties. The range in which the studied cluster is paramagnetic is considered. The results are obtained by means of exact numerical diagonalization. Such thermodynamic quantities as entropy, specific heat, magnetic susceptibility, spin-spin correlations and double occupancy are discussed. Particular emphasis is put on the behaviour of local maxima of specific heat and susceptibility, which are analysed in terms of Schottky anomalies.

cond-mat.str-el↗

arXiv2014

A self-consistent thermodynamic model of metallic systems. Application for the description of gold

A self-consistent thermodynamic model of metallic system is presented. The expression for the Gibbs energy is derived, which incorporates elastic (static) energy, vibrational energy within the Debye model, and electronic part in Hartee-Fock approximation. The elastic energy is introduced by a volume-dependent anharmonic potential. From the Gibbs energy all thermodynamic quantities, as well as the equation of state, are self-consistently obtained. The model is applied for the description of bulk gold in temperature range $0 \leq T \lesssim 1300$K and external pressure up to 30 GPa. The calculated thermodynamic properties are illustrated in figures and show satisfactory agreement with experimental data. The advantages and opportunities for further development of the method are discussed.

cond-mat.stat-mech↗

arXiv2013

Monte Carlo Study of a Spin-3/2 Blume-Emery-Griffiths Model on a Honeycomb Lattice

We study the phase diagram of the spin-3/2 Blume-Emery-Griffiths model on a honeycomb lattice by Monte Carlo simulations in order to verify the presence of some peculiar features predicted by the effective field theory (EFT) with correlations. The locations of the order-disorder phase boundaries are estimated from thermal variations of the magnetic susceptibility curves. It is found that for positive values of the biquadratic interactions the critical boundary shows a discontinuous character as a function of the single-ion anisotropy strength, in line with the EFT expectations. However, for negative values of the biquadratic interactions the step-like variation of the critical frontier predicted by EFT was not reproduced.

cond-mat.stat-mech↗

arXiv2013

A simple thermodynamic description of the combined Einstein and elastic models

Simple application of the Einstein model combined with the elastic description of solid state is developed. The frequency of quantum oscillators has been assumed as volume dependent and, furthermore, elastic energy terms of static character have been included to complete the description. Such an extension enables to construct the complete thermodynamics. In particular, the model yields practical equation of state and describes the thermal expansion coefficient as well as the isothermal compressibility of solids. The thermodynamic properties resulting from the Gibbs free-energy analysis have been calculated and illustrated in figures. Some comparison of the theoretical results with experimental data for solid argon has been made.

cond-mat.stat-mech↗