We present an extensive overview of the phase diagram, spin-wave excitations, and finite-temperature transitions of the anisotropic-exchange magnets on an ideal nearest-neighbor triangular lattice. ...We investigate transitions between five principal classical phases of the corresponding model: ferromagnetic, Néel, its dual, and the two stripe phases. Transitions are identified by the spin-wave instabilities and by the Luttinger-Tisza approach, and we highlight the benefits of the former while outlining the shortcomings of the latter. Some of the transitions are direct and others occur via intermediate phases with more complicated forms of ordering. The spin-wave spectrum in the Néel phase is obtained and is shown to be nonreciprocal,ϵα,k≠ϵα,−k, in the presence of anisotropic bond-dependent interactions. In a portion of the Néel phase, we find spin-wave instabilities to a long-range spiral-like state. This transition boundary is similar to that of the spin-liquid phase of theS=1/2model discovered in our prior work, suggesting a possible connection between the two. Further, in the stripe phases, quantum fluctuations are mostly negligible, leaving the ordered moment nearly saturated even for theS=1/2case. However, for a two-dimensional surface of the full 3D parameter space, the spin-wave spectrum in one of the stripe phases exhibits an enigmatic accidental degeneracy manifested by pseudo-Goldstone modes. As a result, despite the nearly classical ground state, the ordering transition temperature in a wide region of the phase diagram is significantly suppressed from the mean-field expectation. We identify this accidental degeneracy as due to an exact correspondence to an extended Kitaev-Heisenberg model with emergent symmetries that naturally lead to the pseudo-Goldstone modes. There are previously studied dualities within the Kitaev-Heisenberg model on the triangular lattice that are exposed here in a wider parameter space. One important implication of this correspondence for theS=1/2case is the existence of a region of the spin-liquid phase that is dual to the spin-liquid phase discovered recently by us. We complement our studies by the density-matrix renormalization group of theS=1/2model to confirm some of the duality relations and to verify the existence of the dual spin-liquid phase.
Spin systems with frustrated anisotropic interactions are of significant interest due to possible exotic ground states. We have explored their phase diagram on a nearest-neighbor triangular lattice ...using the density-matrix renormalization group and mapped out the topography of the region that can harbor a spin liquid. We find that this spin-liquid phase is continuously connected to a previously discovered spin-liquid phase of the isotropic J_{1}-J_{2} model. The two limits show nearly identical spin correlations, making the case that their respective spin liquids are isomorphic to each other.
The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic ...magnons-quantized harmonic fluctuations of the ordered spins. In striking contrast is the recent discovery that strongly spin-orbital-coupled magnets, such as α-RuCl
, may display a broad excitation continuum inconsistent with conventional magnons. Due to incomplete knowledge of the underlying interactions unraveling the nature of this continuum remains challenging. The most discussed explanation refers to a coherent continuum of fractional excitations analogous to the celebrated Kitaev spin liquid. Here, we present a more general scenario. We propose that the observed continuum represents incoherent excitations originating from strong magnetic anharmonicity that naturally occurs in such materials. This scenario fully explains the observed inelastic magnetic response of α-RuCl
and reveals the presence of nontrivial excitations in such materials extending well beyond the Kitaev state.
We demonstrate that interactions can substantially undermine the free-particle description of magnons in ferromagnets on geometrically frustrated lattices. The anharmonic coupling, facilitated by the ...Dzyaloshinskii-Moriya interaction, and a highly degenerate two-magnon continuum yield a strong, nonperturbative damping of the high-energy magnon modes. We provide a detailed account of the effect for the S=1/2 ferromagnet on the kagome lattice and propose further experiments.
Features of Rayleigh scattering by a solid particle at a small distance compared to the wavelength from an impenetrable plane boundary are revealed. The choice of the Green’s function in the integral ...representation of the Helmholtz equation makes it possible to reduce integration only over the particle surface and eliminate the contribution of the interface surface. When expanding over a small wave parameter, a well-known approach is used, making it possible to represent the solution of a given order as the sum of a potential function and a component expressed in terms of lower-order approximations. The potential component is found, expressed in terms of solid irregular harmonics centered on the particle and its mirror image. The vibrational velocity of the center of a particle and the scattering amplitude are determined. In the lowest order of the wavenumber, the scattering amplitude is expressed in terms of the monopole and dipole components.
Abstract
A wide variety of emission processes by electron wave packets with an orbital angular momentum
ℓℏ
, called the vortex electrons, can be influenced by a nonparaxial contribution due to their ...intrinsic electric quadrupole moment. We study Smith–Purcell radiation from a conducting grating generated by a vortex electron, described as a generalized Laguerre–Gaussian packet, which has an intrinsic magnetic dipole moment and an electric quadrupole moment. By using a multipole expansion of the electromagnetic field of such an electron, we employ a generalized surface-current method, applicable for a wide range of parameters. The radiated energy contains contributions from the charge, from the magnetic moment, and from the electric quadrupole moment, as well as from their interference. The quadrupole contribution grows as the packet spreads while propagating, and it is enhanced for large
ℓ
. In contrast to the linear growth of the radiation intensity from the charge with a number of strips
N
, the quadrupole contribution reveals an
N
3
dependence, which puts a limit on the maximal grating length for which the radiation losses stay small. We study spectral-angular distributions of the Smith–Purcell radiation both analytically and numerically and demonstrate that the electron’s vorticity can give rise to detectable effects for non-relativistic and moderately relativistic electrons. On a practical side, preparing the incoming electron’s state in a form of a non-Gaussian packet with a quadrupole moment—such as the vortex electron, an Airy beam, a Schrödinger cat state, and so on—one can achieve quantum enhancement of the radiation power compared to the classical linear regime. Such an enhancement would be a hallmark of a previously unexplored quantum regime of radiation, in which non-Gaussianity of the packet influences the radiation properties much stronger than the quantum recoil.
Experimental investigations of AlAs/(Al,Ga)As/GaAs vertical-cavity surface-emitting lasers in multimode generation regime are performed. A high degree of circular polarization (>70%) is obtained for ...different modes of generation measured with high spectral resolution. Detailed maps of the spatial and angular distribution of the intensity of laser radiation are plotted.
This paper considers various numerical functions that determine the degree of similarity between two finite sequences. These similarity measures are based on the concept of embedding for sequences, ...which we define here. A special case of this embedding is a subsequence. Other cases additionally require equal distances between adjacent symbols of a subsequence in both sequences. This is a generalization of the concept of the substring with unit distances. Moreover, equality of distances from the beginning of the sequences to the first embedded symbol or from the last embedded symbol to the end of the sequences may be required. In addition to the last two cases, an embedding can occur in the sequence more than once. In the literature, functions such as the number of common embeddings or the number of pairs of occurrences of embeddings in a sequence are used. We introduce three additional functions: the sum of lengths of common embeddings, the sum of the minimum numbers of occurrences of a common embedding in both sequences, and the similarity function based on the longest common embedding. In total, we consider 20 numerical functions; for 17 of these functions, algorithms (including new ones) of polynomial complexity are proposed; for two functions, algorithms of exponential complexity with a reduced exponent are proposed. In Conclusions, we briefly compare these embeddings and functions.
This paper deals with the Braess paradox in quantum transport. The scattering matrix formalism is used to consider a two-parameter family of mesoscopic conductors with the topology of the classical ...Braess transport network. The study investigates the mutual influence of the congestion and smoothness of the system on the Braess behavior. Both the Braess paradox and normal transport regimes coexist within the two-parametric landscape under the same congestion.