Journal of Experimental and Theoretical Physics最新文献

Pumping a high-Q optical microresonator by an external laser is inevitably associated with thermal effects. They have a significant impact on the dynamics of nonlinear processes in such structures, including the generation of optical frequency combs and dissipative solitons. The generation process and the properties of bright solitons in such heated microresonators with anomalous group velocity dispersion (GVD) have been well studied, and a number of methods have been developed to minimize the effect of thermal processes. However, for dark solitons or platicons excited at normal GVD, these issues have been studied significantly less. In this work, the properties of platicons in heated microresonators are analyzed, and it is shown that in the case of “positive” thermal effects, when the direction of the thermal shift of the resonance frequencies of a microresonator coincides with the direction of the nonlinear shift, the widest high-energy platicons with the duration close to the round trip time in the resonator are stable. In the case of “negative” thermal effects, narrow low-energy platicons remain stable. Moreover, in microresonators with “negative” thermal effects, the interaction between cubic nonlinear and thermal processes can ensure the generation of platicons without special techniques required in other cases.

The effect of Gaussian noise on the switching of a ZrO₂(Y) based memristor from the low resistance state (LRS) into the high resistance state (HRS) including transitions from the LRS into intermediate metastable states has been studied. The series of positive (with addition of the noise signal or without the one) and negative rectangular voltage pulses were used as the switching signals. The adding of noise to the switching signal initiated the switching of the memristor from the LRS into the HRS at smaller pulse magnitudes than in the case of switching by the rectangular pulses without adding the noise. A necessary (preset) HRS can be achieved passing the intermediate states by adding the noise with certain parameters to the rectangular switching pulses. The resistive switching is performed without application of adaptive switching protocols. The results of the present study can be applied in the development of innovative memristor switching protocols.

The phase transitions and thermodynamic properties of the clock model with q = 5 spin states on a triangular lattice have been investigated using the Wang–Landau Monte Carlo algorithm. The phase transitions have been analyzed with the histogram method and the fourth-order Binder cumulant method. Two Berezinskii–Kosterlitz–Thouless phase transitions are shown to be observed in the ferromagnetic clock model, while a second-order phase transition has been detected in the antiferromagnetic clock model.

The influence of the competition of single-ion anisotropy and easy-plane anisotropy on the magnetic properties of the multilayer structure Co/Cu/Co is investigated. The peculiarities of the influence of anisotropy effects are revealed both in the vicinity of critical temperature T_c and in the low-temperature range T ( ll ) T_c. The magnetic properties of the multilayer structure are numerically simulated using the anisotropic Heisenberg model. In the vicinity of T_c, easy-plane anisotropy is shown to exert a predominant influence on the magnetic properties of the structure as compared to the influence of single-ion anisotropy. In the low-temperature range, the switching of the magnetic state of the ferromagnetic film in an external field leads to the appearance of specific features in hysteresis effects due to the competition of two types of magnetic anisotropy. The magnetic structure exhibits a size-induced transition from the behavior caused by easy-plane anisotropy to the behavior caused by single-ion anisotropy. The revealed size-induced transition is accompanied by a spin-flop effect.

In a previous report, we presented experiments which suggested that ferromagnetic ordering of the spins of localized holes in GaAs/AlGaAs quantum wells could be observed when doped with shallow (Be) acceptors at impurity concentrations near the metal-insulator transition. The compensating impurity (Si) was introduced into a narrow region at the center of the barriers [4]. In this paper, we present results from magnetotransport experiments performed on similar structures, but without the compensating impurity (Si). In these samples, the compensation degree is expected to be controlled by the background defects located at the edges of the quantum wells and within the barriers. At low temperatures T ≤ 10 K, we observed isotropic, linear magnetoresistance, anomalous behavior of the Hall effect as a function of the magnetic field, and slow relaxation of resistance after the application of a magnetic field. We explain this anomalous magnetotransport as the manifestation of a ferromagnetic transition or spin glass, originating from indirect spin exchange between localized holes on impurities near the metal-insulator transition. However, we note that perfect disorder, including signs of interspin interactions, leads to unstable configurations. In what follows, we present a model in which we start with this perfect disorder, but apply a procedure to obtain a stable configuration. We show that the resulting spin structure, a “closely packed” structure of “droplets,” can reproduce the features observed in the experiment, particularly isotropic, linear magnetoresistance.

The possibility of obtaining an object image using a fiber-optic endoscope based on ghost imaging principle is demonstrated experimentally. The endoscope consists of a multimode fiber and includes a radiation source with thermal statistics, which is formed by means of random modulation of He–Ne laser radiation with the help of a phase spatial light modulator. It is shown that after the passage through the fiber, the field preserves the pseudo-thermal statistics. Radiation obtained in this way is used for ghost imaging in transmitted as well as scattered light.

We consider the Lifshitz topological transitions and the corresponding changes in the galvano-magnetic properties of a metal from the point of view of the general classification of open electron trajectories arising on Fermi surfaces of arbitrary complexity in the presence of magnetic field. The construction of such a classification is the content of the Novikov problem and is based on the division of non-closed electron trajectories into topologically regular and chaotic trajectories. The description of stable topologically regular trajectories gives a basis for a complete classification of non-closed trajectories on arbitrary Fermi surfaces and is connected with special topological structures on these surfaces. Using this description, we describe here the distinctive features of possible changes in the picture of electron trajectories during the Lifshitz transitions, as well as changes in the conductivity behavior in the presence of a strong magnetic field. As it turns out, the use of such an approach makes it possible to describe not only the changes associated with stable electron trajectories, but also the most general changes of the conductivity diagram in strong magnetic fields.

We consider fluid flow with a free boundary, which is defined as a function of height from coordinate x with two asymptotes at positive and negative infinities (hydraulic jump). The Boussinesq approximation is used to describe the phenomenon, and an additional force is introduced. The force is chosen to depend only on the height of the surface. The problem is solved analytically without using numerical schemes. This technique is used to determine the jump surface and the acting force depending on the wave propagation coordinate.

Abstract

We construct a nonlinear theory of electric resistance of chiral helimagnets, in which the shape changes and the magnetization spiral starts rotating during the passage of electric current due to the spin transfer torque effect. It is shown that the rotation of the spin spiral under the action of the passing current, the electric resistance of the helimagnet is always lower than the resistance of a helimagnet in which the spin spiral is stationary. It is found that the current–voltage characteristic of the helimagnet in the presence of the spin transfer torque from the conduction electron system to the system of localized electrons can be essentially nonlinear. The possibility of the spin electric bistability effect in helimagnets is predicted for the situation when the spin contribution to electric resistance of a helimagnet can take two different values for the same value of the current passing through it. The possibility of realization of states with a negative differential resistance in helimagnets is demonstrated.