Effect of an intense laser field on the magnon damping in free carrier magnetic semiconductors

The influence of an intense radiation fieldE₀ on the magnon damping in free carrier magnetic semiconductors is discussed. It is shown that the effect of the intense field is to give a drift velocity v₀ = eE₀/mΩ to the carriers such that when v₀ exceeds the phase velocity of the spin waves the electrons are more able to emit magnons than they absorb and as a consequence the magnon population grows with time.     

Longitudinal relaxation spectra of the transverse Ising model close to Tc

The correlative effects of the nature of the interaction and of the method of calculation on the shape of the longitudinal relaxation function (LRF) for the transverse Ising model are analysed. The LRF is calculated in two ways: (i) its continued fraction representation within the three pole approximation (TPA); and (ii) the resolution of kinetic equations derived for the correlation functions beyond the random phase approximation (RPA). The effects of the nature of the interaction on the LRF spectral characteristics are investigated using an interaction made of three variable contributions: uniaxial dipolar, isotropic infinite range and anisotropic nearest-neighbour interactions. Contrary to the TPA, the kinetic-equation-method (KEM) leads to LRF's exhibiting a three peak structure for every q-value except q = 0 (q = 0_⊥ if the interaction is of dipolar nature) whatever the interaction. The approximations underlying both methods are specified and discussed. Comments on recent neutron scattering experiments on Li Tb_pY_1-pF₄ by Youngblood et al. are made.     

Cubic helimagnets in magnetic field and at pressure

Cubic helimagnets with B20 structure display several unusual properties such as anisotropy of the spin-wave spectrum al small momenta q, rotation of the helix vector k in magnetic field and quantum phase transition at pressure. We demonstrate that first two phenomena are a result of umklapp processes mixing excitations with momenta q, q+k and q−k. At very low magnetic field perpendicular to k the helical structure remains stable due to spin-wave gap Δ. Its square is sum of two parts. The first one is a result of the magnon interaction and the second negative part stems from magneto-elastic interaction. It is suggested that competition between these parts leads to the quantum phase transition observed in MnSi and FeGe. For MnSi from rough estimations at ambient pressure was shown that both parts are comparable with the experimentally observed gap. The magneto-elastic interaction is also responsible for 2k modulation of the lattice and contributes to the magnetic anisotropy. Experimental observation by X-ray and neutron scattering of this lattice modulation allows to determine the strength of the magneto-elastic interaction responsible for above phenomena and the lattice helicity.     

Precision measurement of the rate of nuclear muon capture in the muonic hydrogen atom and the determination of the pseudoscalar form factor of the nucleon

The rate Λ _S of nuclear muon capture by a proton from the hyperfine singlet ground state of the µp atom has been measured using a new experimental method based on a time-projection chamber operating in an ultrapure hydrogen gas at a pressure of 10 atm. The capture rate has been determined from the difference between the measured lifetime of the negative muon in hydrogen and the world average lifetime of the positive muon. The analysis of 10% of the collected statistics (2 × 10¹⁰) of µe decays yields the muon capture rate Λ _S = 725.0 ± 17.4 s^?1, from which the pseudoscalar form factor of the nucleon, g _P (q _c ² = ?0.88m _µ ² ) = 7.3 ± 1.1, is determined. The further analysis of the collected experimental data should improve the precision of this measurement by a factor of 3.     

Study of non-Fermi-liquid-like state and pairing symmetry in iron pnictides based on the multiorbital Hubbard-Holstein model

We study the five-orbital Hubbard-Holstein model, taking account of the electron–phonon (e-ph) interaction due to the Fe-ion oscillation. In order to include the self-energy correction, we employ the fluctuation exchange (FLEX) approximation. It is revealed that the orbital fluctuations are enhanced by the e-ph interaction, which causes a strong attractive pairing interaction. The orbital fluctuations give rise to highly anisotropic quasiparticle lifetime (hot/cold spot structure) and non-Fermi-liquid-like transport phenomena. From the phase diagram obtained by the FLEX approximation, we find that (i) s-wave state without sign reversal is stable for the moderate e-ph coupling in the broad parameter region, which is consistent with the experimental non-magnetic impurity effect, (ii) the renormalization induced by the orbital fluctuation is not so strong, and (iii) superconducting (SC) state with node can appear in the broad parameter region in the presence of impurities.     

Thermoelectric power of antiferromagnetic semiconductors near the néel temperature

Thermoelectric power in antiferromagnetic semiconductors is calculated in the vicinity of the Néel temperature by assuming the presence of the well-defined propagating modes (magnon) and a temperature-dependent magnon velocity. Carriers are specified by the mobile carriers in a broad band with an isotropic effective mass. Magnons and carriers are coupled via the sd-interaction. Anomaly near T_N does not result from the diffusion term S_diff. and it is ascribed to the magnon-drag contribution S_drag which is obtained from

$S_{drag}=?{\displaystyle \sum _{\mathit{q}}\mathit{R}(\mathit{q}){\mathit{c}}_m\left(q\right)/3e}$

,where R(q) is the momentum transfer ratio between the magnon system and carriers, c_m(q) denotesthe specific heat of the magnon q. The assumed temperature dependence of the magnon velocity makesSdru divergent at T_N with T_N ? T → +0 and it might be removed in consideration of the magnon frequency spreading. Except this divergence the calculation provides a qualitative explanation for the anomalous feature in MnTe.     

Possible explanation of magnetic dependence of the bragg peak intensity in the modulated phase of HoNo6S8

The possible explanation for the observed, by Lynn et al. [1], existence of the maximum in the magnetic field dependence of the Bragg peak intensity I_Q(H) in the modulated phase of HoMo₆S₈ is given. It is based on the so-called EX-model - the dominance of the exchange interaction between superconducting electrons and localized moments in the formation and destruction of the coexistence phase. In this approach the maximum in I_Q(H) results from the (zero wave-vector) electronic spin susceptibility dependence, in superconducting state, on the spin-orbit scattering time τ_So. The in [1] observed anisotropic character of I_Q(H), for H|Q and H⊥Q, is also explained.     

A three-peak structure for the critical relaxation function in the transverse Ising model

The dynamical properties of the transverse Ising model with an interaction of uniaxial dipolar nature are shown to exhibit a marked q-anisotropy in the critical region. For any wave vector q strictly non perpendicular to the z-axis the longitudinal relaxation shape function F_q^zz exhibits a three-peak structure and consequently a finite width of its central peak.     

Electrodynamic features of anisotropic hard superconductors

The low-frequency electromagnetic response of superconducting plates characterized by strong anisotropy of current-carrying capacity in the plane of the sample is studied experimentally and theoretically. Measurements are made on polycrystalline textured plates of the Y-123 system with the c axis lying in the plane of the sample and on a single crystal with a single preferred direction of twinning boundaries. It is shown that the shape of the curves describing the dependence of the relative losses q on the ac field amplitude h ₀ is quite sensitive to the orientation of vector h ₀ in the sample plane. As in the case of isotropic samples, the q(h ₀) dependence is characterized by a single size peak if vector h ₀ is oriented along one of the principal symmetry directions of the anisotropic critical current density. If h ₀ deviates considerably from principal directions, two size peaks are observed on the q(h ₀) curve. A detailed analysis of the evolution of the q(h ₀) curves upon a rotation of vector h ₀ in the sample plane is carried out.     

White and coloured external noise and transition phenomena in nonlinear systems

The magnetic excitations of the dilute two-dimensional ferromagnetic Heisenberg system K₂Cu_1?x Zn _x F₄ have been studied by inelastic neutron scattering forx = 0, 0.08 and 0.22, atT = 2 K. The dispersion of the frequency and of the linewidth has been obtained for wavevectorsq in the direction of the strongest dispersion,q~(1, 1,0), within the plane containing the strong exchange interaction. In the dilute samples,x = 0.08 and 0.22, the effect of the nonmagnetic impurities on both the frequency and the linewidth has been observed. Excellent agreement is obtained between the experimental results and the predictions of a continued-fraction-computer-simulation of the lineshapes, for the sample withx = 0.08. Forx = 0.22, there is still good agreement, although the experimental frequencies are slightly higher than predicted.     

Effects of four-fermion contact interactions in the Bhabha process at linear electron-positron colliders

For the case of longitudinally polarized primary beams, effects are investigated that are induced by four-fermion contact interactions in the process e⁺e^? → e⁺e^? occurring at linear electron-positron colliders of energy in the range \(\sqrt s = 0.5 - 1 TeV\). Model-independent constraints on the parameters of four-fermion contact interactions are obtained. It is shown that the sensitivity of the Bhabha process to the parameter Λ_LR is considerably higher than the sensitivity of the leptonic process e⁺e^? → μ⁺μ^?; at the same time, the sensitivities of these processes to the parameters Λ_RR and Λ_LL are commensurate.     

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb₃Fe₅O₁₂, the exchange effective field can be expressed as H_eff=−λ·M_Fe=−λχ_eff·H_e=−γ·H_e where γ is named as the exchange field parameter and H_e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter γ, the properties of exchange interaction in ytterbium iron garnet (Yb₃Fe₅O₁₂) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter γ is the function of the temperatures under different external magnetic fields, and γ=a+b·T+c·T², where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb₃Fe₅O₁₂ are revealed. Also, excellent fits to the available experiments are obtained.     

The rotational spectrum and molecular parameters of BrO in the 2Π32 state

The ν = 0 and 1 rotational spectrum of BrO in the ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ state has been observed in selected regions between 60 and 400 GHz. The precision of previously measured parameters and the quality of the fit has been improved. In addition, the centrifugal distortion constant, D₀₂, the effective Λ-doubling constant, q^eff, the perpendicular quadrupole coupling constant, eqQ₂, and a linear combination of the magnetic hyperfine constant, b, and nuclear spin-rotation constant, C_rs, have been measured for the first time. The values of ω_e and ω_ex_e derived from the mechanical constants are in good agreement with very recent optical data.     

The magnetic anisotropy of MnF2 AT 0 K

The contributions that dipolar, single-ion and exchange anisotropy make to the perpendicular susceptibility χ?, the field H′_c for which ω↓ (k = 0) vanishes, and other properties of antiferromagnetic MnF₂ at T = 0 K are considered, first, i n the ionic approximation. A comparison of the calculated dipolar and single-ion anisotropy with the measured χ?, H′_c and magnon dispersion would seemingly require then the inclusion of an unreasonably large anisotropic exchange field H_AE = -0.9 kOe. However, an estimate of the effects that overlap and covalency have on the anisotropy reveals that much of the discrepancy between the results obtained from the point-dipolar, ionic approximation and the measured quantities is a consequence of the spatial redictribution of the cation magnetization onto the ligands.     

High-resolution spectroscopy on the c1Π ← a1Δ transition in NH

The spectrum of the c¹Π, v = 0 ← a¹Δ, v = 0 band of the NH molecule at λ = 324 nm has been investigated under high resolution by laser-induced fluorescence in a molecular beam. From an analysis of the spectra we obtained: the magnetic dipole interaction constants a_N,H and the electric quadrupole constants eQq_1,2 for both electronic states, the improved values for the Λ-doubling constants q_π, q_π^D, and q_π^H for the c¹Π state, and rotational constants for both electronic states up to a third-order centrifugal distortion. Also, the Λ-doubling in the a¹Δ state could be determined.     

The variation of the electronic transition moment, Re, in the intensity theory of diatomic molecules

The electronic transition moment functions, R_e(r_v′v″), have been compiled, and their variation with the r-centroid has been investigated. Connections between the Λ-quantum numbers, the multiplicities, Δr_e, and the variation of R_e(r_v′v″) were not found.     

Tsallis’ statistics in the variability of El Niño/Southern Oscillation during the Holocene epoch

Analysis of the Tsallis q-triplet for the variability of El Niño Southern Oscillation (ENSO) index during the Holocene epoch (last 11,000 years) is presented. Three periods are analyzed, 0-7000, 7000-9700, 9700-11,000 years before the present. During the first and the third periods, the q-index values have the expected usual relations between them (q_sens<1<q_stat<q_rel), and in the second one there is an inversion between q_stat and q_rel (q_stat>q_rel).     

Constraints on the behaviour of the e+e− hadron annihilation cross section in asymptotically free theories and in theories with anomalous dimensions

A new approach is proposed to the problem of the relationship between the e⁺e^? hadron annihilation cross section and the behaviour of the hadronic vacuum polarization tensor Π_μν(q) in the space-like region. By using this approach based on the principles of functional analysis, one can uniquely transform the information about the behaviour of Π_μν(q) at q² → ?∞, which is available in asymptotically free theories and in theories with anomalous dimensions, into constraints on the e⁺e^? total cross section.     

A new type of phonon instability in semiconductors exposed to a strong electric field

The damping of a phonon γ_q is calculated for the case when the electron acceleration influenced by a strong electric field for the time of the electron-phonon interaction is essential.     

Anisotropic intermolecular potential from nuclear spin-lattice relaxation in hexafluoride gases

The data on fluorine spin-lattice relaxation times per unit densityT _jσ in pure SF₆ and UF₆ gases can be analyzed to obtain information on the anisotropic part of the intermolecular potential in these systems. A new and more performant potential, Morse-Morse-Spline-van der Waals potential (J. Chem. Phys.94, 1034 (1991)) was used for the isotropic part of the intermolecular interaction. The analysis was made using the Bloom-Oppenheim theory, assuming, that the correlation time of the spin-rotation interaction can be approximated by the average lifetime of a molecule in a givenJ state. We have obtained the strengths of the repulsive and attractive terms in the anisotropic potential. From the strength of the attractive term, the hexadecapole moment of SF6 and UF6 were also obtained, being in good agreement with the values reported earlier, based on other potentials and techniques.