Spin dynamics of layered triangular antiferromagnets with uniaxial anisotropy

The spin dynamics of the semiclassical Heisenberg model with uniaxial anisotropy, on the layered triangular lattice with antiferromagnetic coupling for both intralayer nearest neighbor interaction and interlayer interaction is studied both in the ordered phase and in the paramagnetic phase, using the Monte Carlo-molecular dynamics technique. The important quantities calculated are the full dynamic structure function S(q,ω)$S(\mathbf{q},\omega )$, the chiral dynamic structure function _Schi(ω)$S_{\mathrm{chi}}(\omega )$, the static order parameter and some thermodynamic quantities. Our results show the existence of propagating modes corresponding to both S(q,ω)$S(\mathbf{q},\omega )$ and _Schi(ω)$S_{\mathrm{chi}}(\omega )$ in the ordered phase, supporting the recent conjectures. Our results for the static properties show the magnetic ordering in each layer to be of coplanar 3-sublattice type deviating from 120^°${120}^{°}$ structure. In the presence of magnetic trimerization, however, we find the 3-sublattice structure to be weakened along with the tendency towards non-coplanarity of the spins, supporting the experimental conjecture. Our results for the spin dynamics are in qualitative agreement with those from the inelastic neutron scattering experiments performed recently.     

On DBI Textures with generalized Hopf fibration

In this Letter we show numerical existence of O(4)$O(4)$ Dirac–Born–Infeld (DBI) Textures living in (N+1)$(N+1)$ dimensional spacetime. These defects are characterized by S^N→S³$S^N\to S^3$ mapping, generalizing the well-known Hopf fibration into _πN(S³)${\pi }_N(S^3)$, for all N>3$N>3$. The nonlinear nature of DBI kinetic term provides stability against size perturbation and thus renders the defects having natural scale.     

Matrix regularization of embedded 4-manifolds

We consider products of two 2-manifolds such as S²×S²$S^2\times S^2$, embedded in Euclidean space and show that the corresponding 4-volume preserving diffeomorphism algebra can be approximated by a tensor product SU(N)⊗SU(N)$\mathit{SU}(N)\otimes \mathit{SU}(N)$ i.e. functions on a manifold are approximated by the Kronecker product of two SU(N)$\mathit{SU}(N)$ matrices.     

Angular dependence of tunneling magnetoresistance in La0.7Sr0.3MnO3 step-edge junctions

La_0.7Sr_0.3MnO₃ (LSMO) tunneling magnetoresistance (TMR) junctions have been fabricated on step-edge (0 0 1) SrTiO₃ substrates with a high step-edge angle. In the measurement of magnetoresistance (MR) ratio versus external magnetic field H, butterfly-like MR curves are clearly observed. The MR(H  ) curves vary with θ$\theta$, the angle between the applied magnetic field and the current direction in the substrate plane, showing anisotropic MR properties. A much broader MR(H) response is observed for the configuration of H perpendicular to the substrate plane. Additionally, the maxima-MR field H_p almost coincides with the coercive field H_c for θ<60°$\theta <60°$ but obeys a different form from _Hc(θ)$H_{\mathrm{c}}(\theta )$. The high-field junction resistance shows an intrinsic sin²θ${\sin }^2\theta$ angular dependence, while the low-field resistance shows an extrinsic cos(4θ)$\cos (4\theta )$ angular dependence. The distinctive features are mainly due to the induced magnetization anisotropy in the artificial steps of grain boundaries.     

Bulk and local magnetic susceptibility in CeAl2 and CeB6

A comprehensive and high-precision magnetoresistance (MR) Δρ/ρ(H,T)$\mathrm{\Delta }\rho /\rho (H,T)$ and magnetization M(H,T) measurements have been carried out for two well known and archetypal magnetic strongly correlated electron systems—CeAl₂ and CeB₆. It was shown that the main Brillouin-type component of MR in these magnetic heavy fermion compounds can be consistently interpreted in the frameworks of a simple relation between resistivity and magnetization—Δρ/ρ∼M²$\mathrm{\Delta }\rho /\rho \sim M^2$ obtained by Yosida [Phys. Rev. 107 (1957) 396]. A local magnetic susceptibility _χloc(T,H)=(1/H^*(d(Δρ/ρ)/dH))^1/2${\chi }_{\mathrm{loc}}(T,H)=(1/H^{*}(\mathrm{d}(\mathrm{\Delta }\rho /\rho )/\mathrm{d}H){)}^{1/2}$ was deduced directly from this part of MR and compared in details with the data of bulk susceptibility χ(T,H) measurements. Two additional contributions to MR have been also deduced for CeAl₂ ((i) linear (∼H) and (ii) nanoscale ferromagnetic components) and applied for a characterization of spin polarons in this magnetic material. The dependencies χ_loc(T,H) and χ(T,H) obtained in this study for CeB₆ and CeAl₂ allow us to analyze the H–T magnetic phase diagram in these magnetic heavy fermion compounds.     

Transition between SU(4) and SU(2) Kondo effect

Motivated by experiments in nanoscopic systems, we study a generalized Anderson, which consist of two spin degenerate doublets hybridized to a singlet by the promotion of an electron to two conduction bands, as a function of the energy separation δ$\delta$ between both doublets. For δ=0$\delta =0$ or very large, the model is equivalent to a one-level SU(N$N$) Anderson model, with N=4$N=4$ and 2 respectively. We study the evolution of the spectral density for both doublets (ρ_1σ(ω)${\rho }_{1\sigma }(\omega )$ and ρ_2σ(ω)${\rho }_{2\sigma }(\omega )$) and their width in the Kondo limit as δ$\delta$ is varied, using the non-crossing approximation (NCA). As δ$\delta$ increases, the peak at the Fermi energy in the spectral density (Kondo peak) splits and the density of the doublet of higher energy ρ_2σ(ω)${\rho }_{2\sigma }(\omega )$ shifts above the Ferrmi energy. The Kondo temperature T_K (determined by the half-width at half maximum of the Kondo peak in density of the doublet of lower energy ρ_1σ(ω)${\rho }_{1\sigma }(\omega )$) decreases dramatically. The variation of T_K with δ$\delta$ is reproduced by a simple variational calculation.     

Superluminality and UV completion

The idea that the existence of a consistent UV completion satisfying the fundamental axioms of local quantum field theory or string theory may impose positivity constraints on the couplings of the leading irrelevant operators in a low-energy effective field theory is critically discussed. Violation of these constraints implies superluminal propagation, in the sense that the low-frequency limit of the phase velocity vph(0)$v_{\mathrm{ph}}(0)$ exceeds c  . It is explained why causality is related not to vph(0)$v_{\mathrm{ph}}(0)$ but to the high-frequency limit vph(∞)$v_{\mathrm{ph}}(\infty )$ and how these are related by the Kramers–Kronig dispersion relation, depending on the sign of the imaginary part of the refractive index Imn(ω)$\mathrm{Im}n(\omega )$ which is normally assumed positive. Superluminal propagation and its relation to UV completion is investigated in detail in three theories: QED in a background electromagnetic field, where the full dispersion relation for n(ω)$n(\omega )$ is evaluated numerically and the role of the null energy condition T_μνkμkν?0$T_{\mu \nu }{k}^{\mu }{k}^{\nu }?0$ is highlighted; QED in a background gravitational field, where examples of superluminal low-frequency phase velocities arise in violation of the positivity constraints; and light propagation in coupled laser–atom Λ  -systems exhibiting Raman gain lines with Imn(ω)<0$\mathrm{Im}n(\omega )<0$. The possibility that a negative Imn(ω)$\mathrm{Im}n(\omega )$ must occur in quantum field theories involving gravity to avoid causality violation, and the implications for the relation of IR effective field theories to their UV completion, are carefully analysed.     

Coalescence of BnNn fullerenes: A new pathway to produce boron nitride nanotubes with small diameter

Using density functional theory calculations, we predict that single-walled hemispherical-caped boron nitride (BN) nanotubes with small diameters can be produced via the coalescence of stable nanoclusters. Specifically, the assembly of B_nN_n (n=12,24$n=12,24$) clusters exhibiting particularly high stability and leading to armchair (3,3)$(3,3)$ and (4,4)$(4,4)$ BN nanotubes, respectively, are considered. The formed finite-length BN nanotubes have semiconducting properties with wide band gaps attractive to nano-device applications.     

Two-dimensional gauge theory and matrix model

We study a matrix model obtained by dimensionally reducing Chern–Simons theory on S³$S^3$. We find that the matrix integration is decomposed into sectors classified by the representation of SU(2)$\mathit{SU}(2)$. We show that the N  -block sectors reproduce SU(N)$\mathit{SU}(N)$ Yang–Mills theory on S²$S^2$ as the matrix size goes to infinity.     

Tunable dielectric response of transition metals dichalcogenides MX2 (M=Mo,W; X=S,Se, Te): Effect of quantum confinement

The first principle calculations have been performed to study the influence of number of layers on the dielectric properties of dichalcogenides of Mo and W for in-plan (E⊥c)$(E\perp c)$ as well as out-of-plan polarization (E∥c)$(E\parallel c)$. We have taken bulk, mono, bi, four and 6-layer setup for this study. The EELS shows significant red shift in the energies of π$\pi$ plasmons, while prominent red shift has been found for the energies of (π+σ)$(\pi +\sigma )$ plasmons of all the studied materials by reducing the number of layers from bulk to monolayer limit. The ?_s${?}_s$ has been found to red shifted by 62.5% (66.3%), 48.5% (62.1%), 52.7% (66.2%), 61.7% (64.6%), 61.5% (66.7%) and 62.5% (70.5%) from bulk values of MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, WTe₂ respectively for E⊥c$E\perp c$(E∥c)$(E\parallel c)$ as one goes from bulk to monolayer of these materials. The interband transitions are found to remain independent of the number of layers, however their intensity decreases with decrease in the number of layers. The dielectric functions are highly anisotropic in low energy range and becomes isotropic in high energy range.     

Lifshitz-point correlation length exponents from the large-n expansion

The large-n expansion is applied to the calculation of thermal critical exponents describing the critical behavior of spatially anisotropic d-dimensional systems at m  -axial Lifshitz points. We derive the leading non-trivial 1/n$1/n$ correction for the perpendicular correlation-length exponent _νL2${\nu }_{L2}$ and hence several related thermal exponents to order O(1/n)$O(1/n)$. The results are consistent with known large-n expansions for d  -dimensional critical points and isotropic Lifshitz points, as well as with the second-order epsilon expansion about the upper critical dimension d^?=4+m/2$d^{?}=4+m/2$ for generic m∈[0,d]$m\in [0,d]$. Analytical results are given for the special case d=4$d=4$, m=1$m=1$. For uniaxial Lifshitz points in three dimensions, 1/n$1/n$ coefficients are calculated numerically. The estimates of critical exponents at d=3$d=3$, m=1$m=1$ and n=3$n=3$ are discussed.     

Quantum information entropies for a squared tangent potential well

The particle in a symmetrical squared tangent potential well is studied by examining its Shannon information entropy and standard deviations. The position and momentum information entropy densities _ρs(x)${\rho }_s(x)$, _ρs(p)${\rho }_s(p)$ and probability densities ρ(x)$\rho (x)$, ρ(p)$\rho (p)$ are illustrated with different potential range L and potential depth U  . We present analytical position information entropies _Sx$S_x$ for the lowest two states. We observe that the sum of position and momentum entropies _Sx$S_x$ and _Sp$S_p$ expressed by Bialynicki-Birula–Mycielski (BBM) inequality is satisfied. Some eigenstates exhibit entropy squeezing in the position. The entropy squeezing in position will be compensated by an increase in momentum entropy. We also note that the _Sx$S_x$ increases with the potential range L, while decreases with the potential depth U  . The variation of _Sp$S_p$ is contrary to that of _Sx$S_x$.