The European Physical Journal B最新文献

In this paper, we present the solutions of the Dirac–Weyl equation for graphene under a constant magnetic field. The resulting spectrum is used to determine the partition function, a key quantity in the study of thermodynamic properties. From this function, we analyze the mean energy, specific heat, entropy, and free energy in two different frameworks: the canonical ensemble and the superstatistical approach. The study confirms the relativistic nature of electron transport in graphene under a magnetic field. It also reveals that fluctuations introduce additional disorder in the system. The obtained results are in good agreement with those already reported in the literature.

This study investigates the performance of a nonlinear piezoelectric energy harvester subjected to (alpha )-stable Lévy noise, which features infinite variance and long-tail distributions. Using numerical simulations, we analyze how the harvester’s nonlinear restoring potential interacts with these non-Gaussian fluctuations to enhance both average power output and efficiency. Our findings show that as the stability parameter (alpha ) decreases from 2 (Gaussian noise), the system exploits the large jumps characteristic of Lévy processes, achieving significant improvements in energy harvesting, even under weak noise conditions. We also observe distinct efficiency behaviors for different load times, including a valley-shaped trend for high values of (alpha ). This work builds upon previous studies—J. I. Deza, R. R. Deza, and H. S. Wio. Epl. 100:38001, 2012.—that explored harvesters driven by noise within the framework of Tsallis entropy, allowing a direct comparison between Lévy and Tsallis processes. By extending the analysis to the infinite variance regime, this study highlights the broader potential of nonlinear systems to harvest energy from realistic, broadband environmental fluctuations.

The critical and magnetic properties of a ferromagnetic (hbox {CrI}_3) bilayer were studied using the mean-field approximation within the spin-3/2 Ising model. The impacts of key system parameters, notably the interlayer exchange interaction (J_{int}) and the crystal field D, on the magnetization and susceptibility of the system were outlined and discussed. In two different planes ((J_{int}), temperature) and (D, temperature), two phase diagrams were plotted and analyzed. Interesting findings were reported in this study, namely, first- and second-order phase transitions and critical end-points. Under specific physical conditions, we assessed how the blocking temperature is influenced by the crystal field and the external magnetic field.

This paper shows that the singlet-bond (SB) superconductivity theory can explain the phase diagram of cuprate high-temperature superconductor and presents the Ginzburg-Landau (GL) free energy formulation of the SB theory together with the London penetration depth (lambda ) and the GL coherence length (xi ). The experimental temperature-doping (T-delta ) phase diagrams covering the underdoped and overdoped regimes are shown to result from various physical phenomena in the SB superconductivity theory. There is the monotonically increasing doping-dependent coherence onset temperature (T_{coh}) between the superconducting transition temperature (T_{c}) and the monotonically decreasing pseudogap onset temperature (T^{*}), ((T_{c}< T_{coh} < T^{*})), becoming the condensation onset temperature (T_{0}), ((T_{coh} = T_{0})), in the underdoped regime. When (T_{coh}) reaches (T^{*}), the (T_{0}) temperature switches over from (T_{coh}) to (T^{*}) temperature, becoming highest (also (T_{c}) becoming highest) at the intersection at the optimal doping (delta _{m} (sim 0.15)), and then follows (T^{*}) temperature in the overdoped regime. (T_{c}) value is scaled by (T_{0}) value in the same expression throughout the two doing regimes. At the end (delta _{pg} (sim 0.2)) of the pseudogap phase the overdoped strong-coupling superconductivity crossovers to the weak-coupling BCS superconductivity. The pseudogap phase PG+ below (T_{0}) is different from that PG above (T_{0}) because in the background of the PG+ phase movable SB-pairs exist and easily lead, with a small kinetic energy gain, to a variety of collective states such as CDW order. The GL free energy formulation is more suitable for the SB superconductivity than for the BCS superconductivity since it can be applied for the whole temperature range (0< T < T_{0}) due to the local nature of SB order-parameter. The GL free energy of the SB theory predicts the linear temperature dependence of (frac{1}{lambda ^{2}(T)} = frac{1}{lambda ^{2}(0)} (1 - a T)) with (a sim 6.4 times 10^{-3} K^{-1}) and (lambda (0) sim 10^{3} A) for (delta = 0.15) and the T-independent GL coherence length (xi sim 1 A), both of which are in good agreement with experiments.

We explore the emergence of magnetic order in geometrically frustrated quasiperiodic systems, focusing on the interplay between local tile symmetry and frustration-induced constraints. In particular, we study the (J_1)-(J_2) Ising model on the two-dimensional Ammann–Beenker quasicrystal. Through large-scale Monte Carlo simulations and general arguments, we map the phase diagram of the model. For small (J_2), a Néel phase appears, whereas a stripe phase is stable for dominant antiferromagnetic (J_2), despite the system’s lack of periodicity. Although long-range stripe order emerges below a critical temperature, unlike in random systems, it is softened by the nucleation of competing stripe domains pinned at specific quasiperiodic sites. This behavior reveals a unique mechanism of symmetry breaking in quasiperiodic lattices, where geometric frustration and local environment effects compete to determine the magnetic ground state. Our results show how long-range order adapts to non-periodic structures, with implications for understanding nematic phases and other broken-symmetry states in quasicrystals.

Recent advances have highlighted the rich low-temperature kinetics of the long-range Ising model (LRIM). This study investigates domain growth in an LRIM with quenched disorder, following a deep low-temperature quench. Specifically, we consider an Ising model with interactions that decay as (J(r) sim r^{-(D+sigma )}), where D is the spatial dimension and (sigma > 0) is the power-law exponent. The quenched disorder is introduced via random pinning fields at each lattice site. For nearest-neighbor models, we expect that domain growth during activated dynamics is logarithmic in nature: (R(t) sim (ln t)^{alpha }), with growth exponent (alpha >0). Here, we examine how long-range interactions influence domain growth with disorder in dimensions (D = 1) and (D = 2). In (D = 1), logarithmic growth is found to persist for various (sigma > 0). However, in (D = 2), the dynamics is more complex due to the non-trivial interplay between extended interactions, disorder, and thermal fluctuations.

This study investigates the effect of organic spacer layer thickness on spin transport in magnetic tunnel junctions (MTJs) of the form (hbox {Fe}_{3}hbox {O}_{4})/x/Co (x=Rubrene, (hbox {C}_{60})) with Rubrene and (hbox {C}_{60}) as organic spacer layers. The simulation uses a nonequilibrium Green’s function, assuming spin precession at defect states at the ferromagnet/organic semiconductor interface. Parallel and antiparallel resistances have been observed to be thickness-independent at low thicknesses due to excellent magnetic coupling and minimal interfacial imperfections that eventually increased at high thicknesses. Drastic reduction of parallel and antiparallel currents at high-thickness regime has been attributed to the trapping of spins in deeper pinning wells with strong pinning strengthNotably, the rise in tunnel magnetoresistance with thickness is high in (hbox {Fe}_{3})O(_{4})/C(_{60})/Co device compared to the (hbox {Fe}_{3})O(_{4})/Rubrene/Co device and has been attributed to the change in defect state depth with thickness and electron-predominant nature of (hbox {Fe}_{3})O(_{4})/C(_{60})/Co device. Therefore, engineering of spacer layer thickness-dependent spin transport in MTJs resulted in successful implementation of organic spacer MTJs for high-performance spintronic memory applications.