Pecularities of EPR spectra of the Gd impurity in the Sn-Rich Pb1−x Sn x Te(Gd) solid solutions

An electron paramagnetic resonance (EPR) experimental study of crystalline and powder p-Pb_1?x?y Sn _y Gd _y Te samples for various matrix compositionx and Gd contenty has been carried out. The study reveals that grinding the crystals into powder as well as their low-temperature annealing turns Gd impurity ions from the EPR-silent Gd²⁺ state to the EPR-active Gd³⁺ state, whereas high-temperature annealing in vacuum quenches EPR signals from Gd³⁺ ions. The experimental results are interpreted in terms of a model assuming that the trivalent charge state of the Gd impurity ions in lead and tin tellurides is a component part of the “substituting Gd impurity-Te vacancy” complex.     

Decay of rabi oscillations induced by magnetic dipole interactions in dilute paramagnetic solids

Decay of Rabi oscillations of equivalent spins diluted in diamagnetic solid matrix and coupled by magnetic dipole interactions is theoretically studied. It is shown that these interactions result in random shifts of spin transient nutation frequencies and thus lead to the decay of the transient signal. Averaging over random spatial distribution of spins within the solid and over their spectral positions within magnetic resonance line, we obtain analytical expressions for the decay of Rabi oscillations. The rate of the decay in the case when the half-width of magnetic resonance line exceeds Rabi frequency is found to depend on the intensity of resonant microwave field and on the spin concentration. The results are compared with the literature data for E′₁ centers in glassy silica and [AlO₄]⁰ centers in quartz.     

Electron paramagnetic resonance of Ce3+ ions in lead thiogallate single crystals

The results of electron paramagnetic resonance (EPR) studies of Ce³⁺ impurity ions in single crystals of lead thiogallate PbGa₂S₄ have been reported. The Ce³⁺ ions substitute for Pb²⁺ ions in the crystal lattice of PbGa₂S₄. A number of paramagnetic cerium centers in lead thiogallate have been observed. The spectra are described by the spin Hamiltonian of rhombic symmetry with the effective spin S = 1/2. The g factors of the main cerium centers have been determined. A large number of paramagnetic centers are due to both nonequivalent positions of lead and local charge compensation under the substitution Ce³⁺ ?? Pb²⁺.     

X-Ray Luminescence of Defects in Spinel Single Crystals

The flareup of x-ray luminescence in spinel single crystals (MgAl₂O₄) depending on the time of x-ray irradiation and the decay of fluorescence depending on the time elapsed after the termination of irradiation have been investigated. These dependences were measured at different powers of the irradiation dose (power of the x-ray tube) and at different temperatures of the samples. The experimental results suggest the existence of large-size complexes of defects, which include antisite defects and impurity ions, the exchange of charge carriers between which during and after irradiation leads to luminescence of the impurity ions. Transfer of charge carriers between isolated elements of the pairs of antisite defects (not interacting with the impurity ions) leads to the formation of a competing channel of recombination luminescence in the UV region of the spectra. The decay of fluorescence attributable to transitions in the Mn²⁺ and Cr³⁺ ions depending on the time elapsed after the termination of x-ray irradiation points to the existence of various combinations of antisite defects in the surroundings of these ions.     

Mechanism of “GSI oscillations” in electron capture by highly charged hydrogen-like atomic ions

We suggest a qualitative explanation of oscillations in electron capture decays of hydrogen-like ¹⁴⁰Pr and ¹⁴²Pm ions observed recently in an ion experimental storage ring (ESR) of Gesellschaft für Schwerionenforschung (GSI) mbH, Darmstadt, Germany. This explanation is based on the electron multiphoton Rabi oscillations between two Zeeman states of the hyperfine ground level with the total angular momentum F = 1/2. The Zeeman splitting is produced by a constant magnetic field in the ESR. Transitions between these states are produced by the second, sufficiently strong alternating magnetic field that approximates realistic fields in the GSI ESR. The Zeeman splitting amounts to only about 10^?5 eV. This allows explaining the observed quantum beats with the period 7 s.     

Local structure of titanium pair centers in SrF2 crystals: EPR and ENDOR spectroscopic studies

The local structure of titanium pair centers in SrF₂: Ti crystals is investigated using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. It is found that titanium pair centers with spin moment S=2 and tetragonal symmetry of the magnetic properties are formed in SrF₂: Ti cubic crystals under certain growth conditions and during annealing. The tensor components of the fine and ligand hyperfine structures in the EPR and ENDOR spectra are determined. A model of the Ti⁺-Ti³⁺ paramagnetic dimer is proposed. This model provides an adequate interpretation of both the ferromagnetic nature of the exchange interaction and the observed displacements of four ligands in the first coordination sphere of titanium impurity ions in directions perpendicular to the impurity ion-ligand bonds.     

Magneto-optical spectroscopy and optical detection of EPR spectra of Yb<Superscript>3+</Superscript> paramagnetic centers with cubic symmetry in single crystals <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Cd,Ca, Pb)

Cubic paramagnetic centers formed by Yb³⁺ impurity ions in fluorite-type crystals MeF₂ (Me = Cd, Ca, Pb) have been investigated using electron paramagnetic resonance, magnetic circular dichroism, magnetic circular polarization of luminescence, Zeeman splitting of optical absorption and luminescence lines, and optical detection of electron paramagnetic resonance. The g factors of the ²Γ₇ state in the excited multiplet ² F _5/2 of Yb³⁺ ions in Me F₂ crystals, the hyperfine interaction constant ¹⁷¹ A (¹⁷¹Yb) for the excited multiplet ² F _5/2 in the CaF₂ crystal, and the energies and symmetry properties of all energy levels of Yb³⁺ ions in MeF₂ crystals are determined. The crystal-field parameters for the crystals under investigation are calculated.     

Phase-locked light sources with low-phase noise for manipulating terahertz-separated metastable states in 40Ca+

We report the development of phase-locked light sources for manipulating terahertz-separated metastable states in ⁴⁰Ca⁺. Two Ti:sapphire lasers with frequencies separated by 1.82 THz are phase-locked using an optical comb generator. The obtained phase noise is 49.8 mrad when the phase-locked loop is closed. Using the developed light sources, we excite Rabi oscillations between the terahertz-separated 3²D_3/2 and 3²D_5/2 states in ⁴⁰Ca⁺. We discuss the phase noises of the light sources and their effect on excitation of Rabi oscillations.     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

Radiation defect formation in strontium and calcium fluoride crystals doped by divalent cadmium and zinc ions

Ionization radiation is shown to reduce impurity ions to the univalent state in strontium and calcium fluoride crystals doped by divalent cadmium and zinc ions. In this case, a univalent ion is surrounded by eight equivalent fluorine ions and exhibits cubic symmetry O _h . At room temperature, the symmetry of the center is revealed to be sequentially lowered to C _3v and then to C _2v owing to the addition to the nearest environment of the impurity univalent ion of one or two anion vacancies, respectively, which are intrinsic defects not forming in undoped strontium and calcium fluoride crystals. Stable intrinsic defects are assumed to form through the separation of anion vacancy-interstitial fluorine ion pairs in the electric field induced by the reduced impurity ions. This electric field lowers the energy barrier to thermal separation of charged intrinsic defects.     

EPR of Yb3+ ions in Ba1−xLaxF2+x mixed crystals

Electron paramagnetic resonance (EPR) spectra of impurity Yb³⁺ ions (about 0.1 at.%) in mixed crystals BaF₂(1-x) plus LaF₃(x) have been investigated for different values of the concentrationx at a frequency of about 9.5 GHz by both continuous-wave (CW) EPR and electron spin echo methods. A spectrum of trigonal symmetry with a complex hyperfine structure is observed in “pure” BaF₂:Yb³⁺ (x=0). Upon admixture of small amounts of LaF₃ (x=0.001), additional EPR lines arise with intensities increasing with the increase ofx up to 0.005. These lines are attributed to trigonal centers including two rare-earth ions and two compensating fluorine ions. A further increase ofx results in a decrease of the total EPR spectrum intensity, and atx≥0.05 the CW resonance becomes practically unobservable. This may be due to the formation of rare-earth ion clusters with paramagnetic Yb³⁺ ions occurring in domains with a disordered structure of surroundings resulting in very broad EPR lines, which cannot be registered by CW EPR. Indeed, very broad (not less than 1 KG) EPR lines were observed by the electron spin echo method for concentrationsx<-0.02.     

Electron paramagnetic resonance and luminescence of chromium in calcium germanate crystals

The luminescence of Ca₂GeO₄: Cr⁴⁺ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca₂GeO₄: Cr⁴⁺ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr⁴⁺ ions substituted for Ge⁴⁺ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr⁴⁺ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr⁴⁺ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect.     

Electron paramagnetic resonance and thermoluminescence study of Ag ions in Li2B4O7 crystals

Electron paramagnetic resonance (EPR) is used to investigate the effects of ionizing radiation on Ag-doped lithium tetraborate (Li₂B₄O₇) crystals. Two similar, yet distinct, trapped-hole centers (Ag²⁺ ions substituting for Li⁺ ions) are produced by 60 kV x rays. One Ag²⁺ ion, labeled Center A, has no nearby defects and the other Ag²⁺ ion, labeled Center B, has a neighboring impurity which is most likely a Ag⁺ ion substituting for a Li⁺ ion. The production and thermal decay properties of the two Ag²⁺ ions are described and their g matrices and ¹⁰⁷Ag and ¹⁰⁹Ag hyperfine matrices are obtained from the EPR angular dependences. The principal values of the g matrices are similar for the two centers, but the hyperfine principal values differ significantly (Center B has smaller values than Center A). There are also differences in the directions of the principal axes for the two centers. Together, these results imply (1) that the unpaired spin is less localized for Center B and (2) that the ground-state positions of the neighboring oxygen ions are different for Centers A and B. This explains why the peaks of the Ag²⁺ charge-transfer photoluminescence bands associated with Centers A and B occur at different wavelengths (502 and 725 nm, respectively). An isochronal pulsed thermal anneal shows that these radiation-induced Ag²⁺ ions serve as the recombination site for the intense thermoluminescence peak observed near 152 °C.     

Nuclear spin induced collapse and revival shape of Rabi oscillations of a single electron spin in diamond

A collapse and revival shape of Rabi oscillations in an electron spin of a single nitrogen-vacancy centre has been observed in diamond at room temperature. Because of hyperfine interaction between the host ¹⁴N nuclear spin and the nitrogen-vacancy centre electron spin, different orientations of the ¹⁴N nuclear spins lead to a triplet splitting of the transition between ground state (m_s =0) and excited state (m_s =1). The manipulation of the single electron spin of nitrogen-vacancy centre is achieved by using a combination of selective microwave excitation and optical pumping at 532 nm. Microwaves can excite three transitions equally to induce three independent nutations and the shape of Rabi oscillations is a combination of the three nutations.     

High-frequency tunable EPR spectroscopy of Cr3+ in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from ?0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr³⁺ ion Δ_s = 66.7 GHz and between spin sublevels of the Cr³⁺-Cr³⁺ dimer Δ_d1 = 71.5 GHz and Δ_d2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr³⁺ ions with an Mg²⁺ vacancy between them replacing three Mg²⁺ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters J_z = 0.47 GHz and J_t = 0.79 GHz.     

Charge-density oscillations at (1 1 1) noble metal surfaces

Induced charge-density oscillations at noble metal surfaces caused by an external static impurity are studied within linear response theory. The calculation takes into account such properties of realistic surface electronic structure as an energy gap for three-dimensional (3D) bulk electrons and a s − p_z surface state that forms two-dimensional (2D) electron system. It is demonstrated that the coexistence of these 2D and 3D electron systems has profound impact on the induced charge-density in the surface region. Thus, the oscillations with the 1/R² decay as a function of lateral distance, R, are established in both electron systems with characteristic chess-board-like structure. Additionally, the charge-density penetrates into the solid at a finite angle with respect to the surface normal in contrast to that in jellium model. The origin of both these findings is investigated.     

Electronic structure of bivalent copper off-center complexes in SrF<Subscript>2</Subscript> crystals from data of EPR and ENDOR spectroscopy

The electronic structure of bivalent copper off-center complexes in SrF₂ crystals is calculated from experimental data obtained earlier by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. The electronic structure parameters characterizing the unpaired-electron density in the vicinity of the nucleus of a copper impurity ion are determined, and the parameters of covalent bonds between an impurity copper ion and three groups of the fluorine ions nearest to this impurity are calculated. It is demonstrated that states of the ground electron configuration of the bivalent copper impurity complex involve an admixture of excited electron configurations due to electron transfer from the ligand to 4s and 4p unfilled shells of the copper ion.     

Time-dependent photon correlations for incoherently pumped quantum dot strongly coupled to the cavity mode

The time dependence of correlations between the photons emitted from a microcavity with an embedded quantum dot under incoherent pumping is studied theoretically. Analytic expressions for the second-order correlation function g ⁽²⁾(t) are presented in strong and weak coupling regimes. The qualitative difference between the incoherent and coherent pumping schemes in the strong coupling case is revealed: under incoherent pumping, the correlation function demonstrates pronounced Rabi oscillations, but in the resonant pumping case, these oscillations are suppressed. At high incoherent pumping, the correlations decay monoexponentially. The decay time nonmonotonically depends on the pumping value and has a maximum corresponding to the self-quenching transition.     

The influence of local distortions on the static and dynamic properties of copper and silver eightfold-coordinated Jahn-Teller complexes in mixed crystals with a fluorite-type structure

The influence of local distortions on the structure and properties of copper and silver impurity Jahn-Teller complexes in mixed crystals, namely, Ca_xSr_1?x F₂: Me ²⁺ and Sr_1?x Ba_xF₂: Me ²⁺ (0≤x≤1, Me ²⁺=Cu²⁺ or Ag²⁺), is investigated using electron paramagnetic resonance (EPR) spectroscopy at frequencies of 9.3 and 37 GHz in the temperature range 4.2–250 K. Local distortions of the tensile and compressive types are induced by Ca²⁺, Sr²⁺, and Ba²⁺ impurity ions incorporated into the first or second coordination sphere of the cationic environment of the Me ²⁺ impurity ion during crystal growth.     

Multi-range high-frequency EPR spectroscopy of LiYF<Subscript>4</Subscript> and LiLuF<Subscript>4</Subscript> crystals doped by rare-earth ions

EPR spectra of isostructural LiYF₄ and LiLuF₄ crystals doped by Dy³⁺, Er³⁺, and Ho³⁺ ions are measured at 4.2 K in the frequency range 40–800 GHz. The effects caused by isotopic disorder in the lithium sublattice, the random crystal field, and the interaction between paramagnetic impurity ions are detected and studied. The results of the measurements are used to determine the spectral characteristics of the compounds and the crystal field parameters. It is demonstrated that the formation of the isotope structure of the EPR signal is dominated by local deformations of the crystal lattice induced by mass defects.