The European Physical Journal B最新文献

We study a quantum annealing approach for estimating the ground state energy of the Sherrington–Kirpatrick mean field spin glass model using the Suzuki–Kubo–deGennes dynamics applied for individual local magnetization components. The solutions of the coupled differential equations, in discretized state, give a fast annealing algorithm (cost (N^3)) in estimating the ground state of the model: classical ((E^0= -0.7629 pm 0.0002)), quantum ((E^0=-0.7623 pm 0.0001)), and mixed ((E^0=-0.7626 pm 0.0001)), all of which are to be compared with the best known estimate (E^0= -0.763166726 dots ) . We infer that the continuous nature of the magnetization variable used in the dynamics here is the reason for reaching close to the ground state quickly and also the reason for not observing the de-Almeida–Thouless line in this approach.

We present some considerations about the parallel implementations of the kinetic (Monte Carlo) version of the Ising model. In some cases, the equilibrium distribution of the parallel version does not present the symmetry breaking phenomenon in the low-temperature phase, i.e., the stochastic trajectory originated by the Monte Carlo simulation can jump between the distributions corresponding to both kinds of magnetization, or the lattice can break into two disjoint sublattices, each of which goes into a different asymptotic distribution (phase). In this latter case, by introducing a small asynchronism (dilution), we can have a transition between the homogeneous and the checkerboard phases, with metastable transients.

In this work, we consider a large number of water molecules placed on a lattice far apart, so that they are very weakly interacting with each other and are in contact with a heat bath at temperature T. A strong static electric field, (E_{0}), is applied to these molecules along the z-axis, causing a three-level energy splitting. A weak alternating electric field, applied in the (xy-)plane for a finite-time (tau ), induces transitions between these three levels. This weak alternating field acts as a time-dependent protocol (zeta (t)), which is switched on at (t=0) and off at (t=tau ). The same cyclic process is repeated for a large number of times. The data available for this finite-time non-equilibrium process allow us to extract equilibrium thermodynamic quantities, such as the difference in free energy between the final and initial states of the system. We analytically obtain the work distributions and analyze the average work of the three-level system as a function of (omega ) and time around the optimum frequency, where (omega ) is the frequency of the alternating electric field. Furthermore, our theoretical framework is further validated through Monte Carlo simulations, which confirm the fidelity of extracting equilibrium free-energy differences and the consistency with the Jarzynski equality.

Quantum anomalies impose profound constraints on our fundamental descriptions of nature, traditionally forbidding the existence of isolated Weyl fermions due to gauge inconsistencies. Here, we introduce a novel quantisation method based on Becchi–Rouet–Stora–Tyutin (BRST) symmetry and cohomological field theory, explicitly demonstrating that a single Weyl fermion doublet under an SU(2) gauge symmetry can be consistently quantised without anomalies. This approach introduces auxiliary BRST-exact fields that shift the anomalous fermionic measure into a trivial cohomological class, explicitly removing the anomaly while leaving no residual physical degrees of freedom. By bridging topological field theory and quantum consistency, this method not only overturns long-standing theoretical constraints but also opens experimental pathways to detecting standalone Weyl fermions. Our results significantly broaden the landscape of quantum field theories and highlight a deep interplay between topology, anomalies, and quantum gauge invariance.

We study quasiparticle dynamics in two-dimensional (2D) integrable Kitaev honeycomb model, both without and in the presence of an external periodic drive. We identify light cones in wavefunction propagation as a signature of quantum caustics, i.e., bright structures formed during quantum dynamics analogous to that of imperfect focusing in geometrical optics. We show that this dynamics follows an angle in spatial direction and it is anisotropic with respect to model parameters. Using coalescence of critical points, we provide an exact solution to the envelope of caustics, which corresponds to the Lieb–Robinson bound in 2D. Further, considering the system to be periodically driven, we point out that the caustics structure completely changes in the presence of external time-dependent drive.

Altermagnets provide promising platforms for novel magnetism, which could empower a new generation of spintronic devices. While various bulk altermagnets have been discovered, altermagnetism in infinite-layer van der Waals (vdW) semiconductors remains elusive. Motivated thereby, here we perform a theory study for the electronic structure reconstruction within the magnetically ordered phase of CrSBr vdW crystal. Based on the Green’s function formalism combined with density functional plus dynamical mean-field theory calculations, we unveil the role played by interlayer hybridization and local many-particle interactions on the electronic state of infinite-layer CrSBr, using its ferromagnetic monolayer spin–orbital state as the building block toward c-axis altermagnetism. This work is a step forward in describing the manifestation of the electronic state of antiferromagnetic CrSBr and its anisotropic evolution induced by the interplay between orbital- and spin-selective electronic structure and layer-by-layer magnetic ordering. Our study identifies CrSBr as a prototypical infinite-layer vdW material featuring altermagnetic order.

Using density functional theory (DFT), we investigated the mechanical, optoelectronic, and thermoelectric properties of the double perovskites (Sr_{2}NbXO_{6} (X = Al, In, text {and}~Tl)). Calculations of the formation energy, cohesion energy, tolerance factor ((tau )), as well as the analysis of the Born–Huang approximation stability criteria, confirm the thermodynamic and mechanical stability of these compounds. Electronic properties show that the compounds (Sr_{2}NbInO_{6}) and (Sr_{2}NbTlO_{6}) have an indirect band gap with values of 3.43 eV and 2.60 eV,  respectively. In contrast, (Sr_{2}NbAlO_{6}) has a direct band gap of 3.69 eV. An analysis of the optical properties, including the real and imaginary parts of the dielectric function ((varepsilon _1, varepsilon _2)), the absorption coefficient (alpha (omega )), the refractive index (n(omega )), the optical conductivity (sigma (omega )), and the reflectivity (R(omega )), was also performed. The results reveal significant absorption of incident light in the ultraviolet (UV) region, indicating that these oxides have strong potential for use in UV optical sensors as well as in various other UV-based optoelectronic devices. Furthermore, the BoltzTraP2 code reveals promising values for the Seebeck coefficient, electrical and thermal conductivity, power factor, and figure of merit (ZT). These findings suggest that (Sr_2NbXO_6) compounds are suitable candidates for sustainable energy technologies, particularly in thermoelectric and UV sensor devices.

Motivated by modelling in physics and other disciplines, such as sociology and psychology, we derive the mean field of the general-spin Ising model from the variational principle of the Gibbs free energy. The general-spin Ising model has (2k+1) spin values, generated by (-(k-j)/k), with (j=0,1,2ldots ,2k), such that for (k=1) we obtain (-1,0,1), for example; the Hamiltonian is identical to that of the standard Ising model. The general-spin Ising model exhibits spontaneous magnetisation, similar to the standard Ising model, but with the location translated by a factor depending on the number of categories (2k+1). We also show how the accuracy of the mean field depends on both the number of nodes and node degree, and that the hysteresis effect decreases and saturates with the number of categories (2k+1). Monte Carlo simulations confirm the theoretical results.