Effective field and the Bloch-Siegert shift at bichromatic excitation of multiphoton EPR

The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ω_mw and ω_rf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω₀ = ω_mw + rω_rf (ω₀ is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies.     

Nonlinear and multiquantum effects in dual-frequency EPR experiment: the intensities of sideband resonances

Two microwave frequencies ω₁ and ω₂ simultaneously exciting a paramagnetic spin probe near the resonance condition ω₁ = γB ₀ effect a pattern of resonances occurring at multiples of the difference frequency δ = ω₂ ? ω₁. Their intensities, measured in the absorption or dispersion mode in the magnetic field-sweep experiment, decrease rapidly as the distance from the centerband increases. Numerically solved Bloch equations are used to discuss the intensities of the transverse components of magnetization up to seven, harmonics of the frequency difference δ. In conclusion, it is suggested that the experimental investigation of these patterns can be used for the purpose of a continuous-wave monitoring the relaxation rates of spin probes.     

Resonance fluorescence spectrum of two atoms,coherently driven by a strong resonant laser field

In Lehmberg's approach, we consider the resonance fluorescence spectrum of two radiatively interacting atoms. In the strong field limit we have obtained analytical solutions for the spectrum of the symmetric and antisymmetric modes without decoupling approximation. Our solutions are valid for all values of the distance r₁₂ separating the atoms. The spectrum of the symmetric modes contains additional sidebands in 2Ω (Ω is the Rabi frequency) with amplitude dependent on (a/Ω)², where a is a parameter dependent on r₁₂. The antisymmetric part of the spectrum has no additional sidebands in 2Ω. For small distances r₁₂ (a=1) our results for the symmetric modes are identical with those of Agarwal et al. apart from the so-called scaling factor. For large distances r₁₂ (a=0) the spectra of the symmetric and antisymmetric modes are identical with the well-known one-atom spectrum.     

Magnetization reversal dynamics in La0.65Sr0.28Mn1.07O3 ceramics and NixZn1−x Fe2O4 nickel-zinc ferrite placed in complicated pulsating fields

The magnetization reversal dynamics in La_0.65Sr_0.28Mn_1.07O₃ ceramics and Ni_xZn_1?x Fe₂O₄ nickel-zinc ferrite placed in a magnetic field is studied experimentally. The field is a superposition of quasi-static field H, rf field h _ω cosωt, and low-frequency field h _ΩcosΩt with H ∥ h _ω ∥ h _Ω. The low-frequency response is found to be nonlinear in h _Ω. A procedure discriminating between the linear and nonlinear components of this response is proposed. Unlike the linear component, the nonlinear component is an essentially anharmonic function of time. The results are analyzed in detail using the high-frequency and low-frequency magnetization reversal curves of the materials.     

Antiferromagnetic Resonance in GdB<Subscript>6</Subscript>

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB₆ antiferromagnet (T_N = 15.5K) and in the antiferromagnetic state (T < T_N). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd³⁺ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ₀H₀ ≈ 1.9 T to μ₀H₀ ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω₀(H₀) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H ₀ ² +2H_AH_E)^1/2, where H_E is the exchange field and H_A is the anisotropy field. This provides an estimate for the anisotropy field, H_A ≈ 800 Oe. This value can result from the dipole?dipole interaction related to the mutual displacement of Gd³⁺ ions, which occurs at the antiferromagnetic transition.     

Relation between giant volume magnetostriction and colossal magnetoresistance near the Curie temperature of Sm0.55Sr0.45MnO3

The magnetic, transport, and elastic properties of Sm_0.55Sr_0.45MnO₃ have been established to be interrelated. At the Curie point, one observes a large volume compression ΔV/V≈0.1%, a sharp minimum in the temperature dependence of negative volume magnetostriction ω(T), and a maximum in the temperature dependence of the electrical resistivity. Giant negative volume magnetostriction ω=?5×10^?4 has been found in a magnetic field H=0.9 T, which is accompanied by a colossal negative magnetoresistance of 44% in the same field. The results obtained are discussed in terms of a model of electronic phase separation.     

An YBCO film as a Josephson medium near T c : Frequency and field dependences and scaling relationships

The diamagnetic response of a laser-deposited YBCO film was studied in weak exciting magnetic fields H _ac =H ₀sinωt (H ₀=2–500 mOe, ω/2π=1–30 kHz). An analysis of the experimentally measured diamagnetic response showed evidence of a ring character of the electric field flowing in the sample at small H ₀ values. The plots of T _m (the temperature of maximum dissipative losses χ″₁) versus H ₀ measured at various frequencies showed the existence of a certain critical field strength H*(ω) at which the (1?T _m /T _c ) versus H ₀ ^2/3 plots exhibit a break. A logarithmic dependence of T _m on the frequency ω of the exciting field H _ac is reliably valid only in the region of (ω/2π)>5 kHz. Verification of the scaling relationship showed that this relationship is valid (to within the experimental accuracy) for the frequencies ω above 5 kHz and the field amplitudes H ₀>H*(ω). However, the exponent n in the scaling relationship under consideration is not correlated with the exponent n in the power relationship describing the current-voltage characteristic.     

Quantum beats in two-photon resonance fluorescence

The two-photon resonance fluorescence spectrum of a three-level atom is shown to consist of the low frequency modes in addition to the high frequency ones in the limit of high photon densities. The spectral function for the low frequency modes consists of two lorentzian lines describing: the peak occuring at the renormalized beat frequency Δ₊ and that of the zero-photon excitation at the frequency Δ_-, where Δ_±=Δ-3Ω²/2ω_a±Ω²u/2ω_a, u²=1+(2Δω_a/Ω ²)². Here, 2Δ is the energy splitting between the two excited states, ω_a is the photon energy of the pump field and Ω is the Rabi frequency. The peak at the renormalized beat frequency Δ₊ occurs provided that the condition (2Δω_a/Ω²)² > 1 is satisfied. The two-photon laser spectroscopy is expected to be a useful tool for the observation of the low frequency modes in question.     

Determination of effective interaction between two plasmons

Using the Bohm-Pines hamiltonian the problem of the bound state of two plasmons is investigated following two different decoupling procedures where the energy transfer $\text{Ω = 0}$ or 2ω_pl. The effective interaction g₄ between the two plasmons is found to be repulsive for $\text{Ω = 2ω}{}_{\mathrm{pl}}$ and attractive when $\text{Ω = 0}$. The case of $\text{Ω= 2ω}{}_{\mathrm{pl}}$ is shown to be analogous to the situation under which the critical coupling g_4c is determined.     

Magnetic field contribution to soliton propagation and reflection in an inhomogeneous plasma

The propagation and reflection characteristics of an ion acoustic soliton are studied in an inhomogeneous plasma through two coupled equations of the KdV family, and the contribution of external static magnetic field is evaluated. The incident and reflected solitons behave oppositely with the angle ψ   between the directions of magnetic field and the wave propagation. Two cases of ω_pi>Ωi${\omega }_{pi}>{\Omega }_i$ and ω_pi<Ωi${\omega }_{pi}<{\Omega }_i$ are examined, where ω_pi${\omega }_{pi}$ is the ion plasma frequency and Ωi${\Omega }_i$ is the ion gyrofrequency. It is found that the soliton gets reflected more strongly when the condition ω_pi>Ωi${\omega }_{pi}>{\Omega }_i$ is achieved in the plasma. The effect of magnetic field is found to be more pronounced on the reflected soliton.     

Giant magnetocaloric effect near the curie temperature in the Sm0.6Sr0.4MnO3 manganite

A method is proposed for the calculation of the magnetocaloric effect from simultaneous measurements of thermal expansion and magnetostriction made in different regimes (adiabatic and isothermal). The magnitude of the magnetocaloric effect for Sm_0.6Sr_0.4MnO₃ is estimated. It is found that near the Curie temperature T _C it passes through a maximum to reach a giant value ΔT=4.6 K for ΔB=0.84 T. In addition, in the neighborhood of T _C, we observed colossal magnetoresistance Δρ/ρ = [ρ(H) ? ρ(0)]/ρ(0) = 72% in a weak magnetic field of 0.84 T, a giant negative volume magnetostriction ω=?5×10^?4 in a field of the same strength, and a large change in the sample volume ΔV/V ≈ 0.1%.     

The ground state of two-dimensional electrons in a nonuniform magnetic field

An exact solution to the Schrödinger equation for the ground state of two-dimensional Pauli electrons in a nonuniform transverse magnetic field H is presented for two cases. In the first case, the field H depends on a single variable, H = H(y), while in the second case, the field is axially symmetric, H = H(ρ), ρ²=x ²+y ². The electron density distributions n = n(y) and n = n(ρ) that correspond to a completely filled lower level are found. For quasiuniform fields of fixed sign, the functions n(y) and n(ρ) are locally related to the magnetic field: n(y) = H(y)/?₀ and n(ρ) = H(ρ)/?₀, where ?₀ = hc/|e| is a magnetic flux quantum. Magnetic fields are considered that are periodic, singular, and bounded in the plane xy. Finite electron objects in a nonuniform magnetic field are analyzed.     

Two-mode resonant polaron in the Hg0.72Cd0.28Te semiconductor

A two-mode polaron behavior is observed in the mixed crystal Hg_0.72Cd_0.28Te at magnetic fields corresponding to ω_c = ω_L1 and ω_c = ω_L2 resonances. Consistently including the small-gap structure of energy bands and a multi-mode dielectric formulation we can quantitatively describe our observations.     

Relaxationsphänomene in Mössbauerspektren von magnetisierten paramagnetischen Substanzen

The Mössbauer spectra of partly magnetized FeNH₄(SO₄)₂·12 H₂O show a broadening and a shift of the hyperfine structure lines, reflecting the paramagnetic spin fluctuations. These fluctuations and their influence on the γ-spectrum may be treated in a spin wave model without introducing phenomenological parameters. By means of a simple diagram technique we get a line broadening γ and line shift δ, proportional to second and third order polynomials of the magnetization and to ∫g ² dΩ and ∫g ³ dΩ, respectively.g(Ω) is the frequency spectrum of spin waves. The values of the two frequency integrals, as deduced from the measured Mössbauer data γ and δ of ferric alum, are in reasonable agreement with the results obtained from the calculated spin wave spectrum, assuming pure magnetic dipole-dipole coupling (long wave length approximation of Holstein-Primakoff). A small contribution of non-magnetic dipole-dipole interaction (van Vleck) cannot be excluded.     

Parametric Excitation of Plasma instabilities by two Monochromatic Pump waves

The dispersion characteristics of a plasma in a pump field ??(t) = ?? sub ω₀t + ??₁ sin ω₁t are considered. Firstly we assume, that the second wave is weak (|??₁| ? |??₀|) and the frequency ω₁ is near sω₀(ω₁ = sω₀ + Ω,Ω ? ω₀). We obtain the dispersion equation, describing the parametric coupling of the waves driven by the strong field ??₀ sin ω₀t under the resonance condition ω₀ ≈ ω_Le/P and derive the expressions for the growth rates (ω_Le is the electron LANGMUIR frequency; s, p are integers). In the second part it is shown, that a strong field ??₁ with a frequency ω₁ much larger than ω _Le(ω _Le ≈ pω₀) stabilizes the plasma; the growth rates are reduced and the frequency region of the parametric instability is contracted.     

Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa2Cu3O7 − δ high-temperature superconductor

The temperature dependences of the electrical resistivity ρ(T) _{H = const} have been measured in external magnetic fields H _ext (0 ≤ H _ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa₂Cu₃O_{7 ? δ} high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T _min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) _{H = 0} for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H _ext) _{T = const}. A comparative analysis of the specific features in the behavior of the curves ρ(H) _{T = const} for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H _c1g , H _c2g ) and Josephson weak links (H _c2J ) have been determined, and the H-T phase diagrams of granular YBa₂Cu₃O_{7 ? δ} HTSCs have been recovered.     

Polarization spectroscopy of ICl: The D′, a and a′ states

Three previously-unanalyzed states of ICl are reported, an ion-pair state D′(Ω =2) which converges to the limit I⁺(P₂+ Cl^-(S_o), and two shallow states a(Ω = 1) and a′(Ω = 0⁺) both of which converge to the ground states of separated atoms I(P_{$\text{3}\text{2}$}) + Cl(P_{$\text{3}\text{2}$}). The a(0⁺) state is responsible for the well-known interruption of the B(0⁺) state above υ_B = 3. Spectroscopic constants are given for the D′ and a′ states.     

Resonant polaron coupling of the n = 1 Landau level and the 2p+ donor state in GaAs

The n = 0 → > n = 1 Landau level and 1s ?2p⁺ impurity transitions in GaAs were investigated up to energies above the optical phonon energy ?Ω_LO and d.c. magnetic fields up to 25 T. Pinning of both transitions to an energy slightly above and below ?Ω_LO was observed. At an energy very close to ?Ω_LO two additional impurity transitions are found. These features are attributed to the resonant polaron effect which leads to hybridization and dipole selection rule breakdown. Also the spin doublet splitting of both transitions were resolved showing a strong magnetic field dependence which can not be explained by nonparabolicity of the conduction band alone.     

Interplay between a homogeneous magnetic field and an antiferromagnetic field in one-dimensional superconductors

Mean-field theory applied to superconductors with one-dimensional band in the presence of both the homogeneous magnetic field H₀ and the antiferromagnetic field H_Q, the second-order phase transition temperature is investigated for the arbitrary angle θ between H₀ and H_Q. It is found that the remarkable superconducting region in the case of θ = 0 is retained only for small θ and that the spatially dependent order parameter coexists with the spatially uniform order parameter except for θ = τ/2.     

Effect of the Bloch-Siegert shift on the frequency responses of Rabi oscillations in the case of nutation resonance

The dynamics of a two-level spin system dressed by bichromatic radiation is studied under the conditions of double resonance when the frequency of one (microwave) field is equal to the Larmor frequency of the spin system and the frequency of the other (radio-frequency) field ω_rf is close to the Rabi frequency ω₁ in a microwave field. It is shown theoretically that Rabi oscillations between dressed-spin states with the frequency ? are accompanied by higher-frequency oscillations at frequencies nω_rf and nω_rf ± ?, where n = 1, 2,.... The most intense among these are the signals corresponding to n = 1. The counter-rotating (antiresonance) components of the RF field give rise to a shift of the dressed-state energy, i.e., to a frequency shift similar to the Bloch-Siegert shift. In particular, this shift is manifested as the dependence of the Rabi-oscillation frequency ? on the sign of the detuning ω₁ ? ω_rf from resonance. In the case of double resonance, the oscillation amplitude is asymmetric; i.e., the amplitude at the sum frequency ω_rf + ? increases, while the amplitude at the difference frequency ω_rf ? ? decreases. The predicted effects are confirmed by observations of the nutation signals of the electron paramagnetic resonance (EPR) of E′₁ centers in quartz and should be taken into account to realize qubits with a low Rabi frequency in solids.