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1.
The influence of an external axial electric field on the absorption spectrum of a Cr 3+−Cr 2+ mixed-valence pair center coupled by the double-exchange mechanism in a KZnF 3 crystal is investigated. It is shown experimentally that the Cr 3+−Cr 2+ pair has an electric dipole moment. The migration of an electron is accompanied by local lattice strain. At the minima of
the adiabatic potential, the intermediate fluorine atom is displaced from the lattice site.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 147–149 (25 July 1998) 相似文献
2.
The paper reports on a comprehensive study of magnetic properties of CuFe 2−x
Cr xO 4 ferrites ( x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0). The curves of magnetization versus temperature, σ
s( T), of the ferrites with x=1.0, 1.4, and 1.6 have anomalous shapes: the magnetization begins to fall off at lower temperatures than the Curie point
TC. The experimental results and analysis of exchange interactions suggest that in ferrites with high Cr 3+ content, magnetic transitions to either a frustrated magnetic structure or a clustered spin glass can take place.
Zh. éksp. Teor. Fiz. 111, 1732–1737 (May 1997) 相似文献
3.
The effect of an external electric field on the R absorption lines of LiNbO 3:Cr 3+ crystals has been studied by a high-sensitivity differential technique at 77 K. Linear Stark effect has been observed in
a field directed along the trigonal crystal axis. No effect of the field was observed when directed perpendicular to this
axis. These characteristics of the Stark effect provide a convincing argument for the electric dipole moments of Cr 3+ centers being oriented along the trigonal C
3 axis of the crystal and indicate that the chromium centers in LiNbO 3 have C
3 symmetry.
Fiz. Tverd. Tela (St. Petersburg) 39, 2053–2056 (November 1997) 相似文献
4.
Low-temperature luminescence spectra of stoichiometric Cr:LiNbO 3, congruent Cr:LiNbO 3 and congruent Cr,Mg:LiNbO 3 were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr 3+ impurity ions that preferentially occupy Li + sites (Cr Li) in the Cr:LiNbO 3 crystals. Low-field centers related to Cr 3+ substitution of Nb 5+ (Cr Nb) occur in addition to Cr Li in co-doped Cr,Mg:LiNbO 3 samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr 3+ centers from low- to high-field type due to strong pressure-induced blue shift of the 4
T
2 state, resulting in its crossing with the 2
E state of Cr 3+. This level-crossing effect was observed for the dominant Cr 3+
Li and Cr 3+
Nb centers at pressures that correlate well with estimations based on the 4
T
2- 2
Eenergy gap (230 cm -1 and 1160 cm -1) and on the rate of their pressure-induced change (14.35 and 11.4 cm -1/kbar, respectively). We also studied inhomogeneous broadeningof the 2
E? 4
A
2transitions at ambient pressure for the minor high-field “defect” Cr 3+
Li centers in congruent LiNbO 3. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis
of a microscopic hierarchic model for perturbed Cr 3+ ions in the LiNbO 3 lattice.
Received: 25 June 2001 / Published online: 2 November 2001 相似文献
5.
The local lattice structure and EPR, optical spectra for Cr 3+ doped in RbCdF 3 crystal have been studied by diagonalizing the complete energy matrices. The results show that the local structure of the Cr 3+ ions in RbCdF 3 exhibits a compressed distortion at the trigonal and tetragonal sites. The compressed distortion can be ascribed to the fact that the radius of Cr 3+ ion is smaller than that of Cd 2+ ion, and therefore Cr 3+ ion will draw the fluorin ligands inwards. The variational ranges of the local structural parameters for Cr 3+ doped in RbCdF 3 crystal R =?1.9491 Å ~?1.9814 Å, θ?= 55.234° ~?55.286° at the trigonal site and R 1 =?1.8617 Å ~?1.8928 Å, R 2 =?1.9527 Å ~?1.9851 Å at tetragonal site are obtained respectively, and the EPR and optical spectra agree well with the experimental results. 相似文献
6.
Anisotropy of the nonlinear absorption of Co 2+ ions in MgAl 2O 4 single crystal at the wavelengths of 1.35 and 1.54 μm has been experimentally demonstrated. The experimental data are analyzed
in the framework of a phenomenological model when the Co 2+ ions are described as three sets of linear dipoles oriented along the crystallographic axes. Ground-state and excited state
absorption cross-sections at 1.35 and 1.54 μm are evaluated to be σ gsa=(4.0±0.3)×10 -19, σ esa=(3.6±0.4)×10 -20 cm 2 and σ gsa=(5.1±0.3)×10 -19, σ esa=(4.6±0.4)×10 -20 cm 2, respectively.
PACS 42.55.Rz; 71.20.Be 相似文献
7.
A Cr 3+ EPR study of lithium heptagermanate crystals, Li 2Ge 7O 15 (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG
have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the
vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T
C
as a result of emergence of two structurally nonequivalent positions for Cr 3+ ions.
Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998) 相似文献
8.
Electron spectra of optical absorption and EPR of Cr 3+ ions in a LaBeAl 11O 19 crystal are investigated. It is shown that the Cr 3+ ions occupy, three different octahedral positions of Al 3+ in the LaBeAl 11O 19 structure, namely, 12k, 2a, and 4f 2; the ratio of their intensitites is 1∶2∶30, respectively. Parameters of the Cr 3+ centers are determined and its is shown that the optical absorption spectra in the visible region are practically determined
by the Cr 3+ (III) occupying the 4f 2-positions.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 275–277, March–April, 1999. 相似文献
9.
A study is reported of the dependence of magnetoresistance Δ ρ/ρ on the square of magnetization σ
2 of the semiconducting spinelide Cu 0.625Ga 0.375Cr 2Se 4, which exhibits a low-temperature transition from long-range magnetic order (LRMO) to the spin glass (SG) state in strong
magnetic fields. It is shown that at the freezing temperature T
f the Δ ρ/ρ( σ
2) relations change their slope, and that below T
f this slope is about one half that for T> T
f. This finding, together with the earlier observation that the freezing temperature does not depend on the frequency of the
ac magnetic field in which it was measured, suggests that the spin-glass phase consists of spins of individual Cr 3+ ions, and that the SG-LRMO crossover is a phase transition.
Fiz. Tverd. Tela (St. Petersburg) 40, 315–317 (February 1998) 相似文献
10.
EPR and the method of dielectric losses have been used to investigate Fe 3+ centers of axial and orthorhombic symmetry in KTaO 3 single crystals. The EPR spectrum of orthorhombic-symmetry Fe 3+ obtained in the 8-mm wavelength range at T=77 K is described by the spin-Hamiltonian
with parameters g
x=1.98, g
y=2.01, g
z=2.00, D=0.43 cm −1, and E=5.87×10 t-2 cm −1. From the dielectric measurement data we have obtained the following parameters of the relaxation of the orthorhombic Fe 3+ centers in KTaO 3: characteristic relaxation frequency τ
0
−1
=2.33×10 12 Hz and activation energy E
a=0.044 eV. A model of the orthorhombic Fe 3+ center in KTaO 3 is discussed within the framework of the kinetic parameters obtained.
Fiz. Tverd. Tela (St. Petersburg) 39, 861–864 (May 1997) 相似文献
11.
This paper discusses the symmetry of the wave field that lies to the right and left of a two-sided accelerated mirror in 1+1
space and satisfies a single condition on it. The symmetry is embodied in the Bogolyubov matrix coefficients α and β that connect the two complete sets of solutions of the wave equations. The amplitudes of the quantum processes in the right
and left half-spaces are expressed in terms of α and β and are related to each other by the transformation (12). The coefficient β
ω′ω
*
plays the role of the source amplitude of a pair of oppositely directed particles with frequencies ω and ω′ of which one is in the left half-space and the other is in the right half-space because one of them has undergone reflection.
Such an interpretation makes β
ω′ω
*
observable and explains why, as shown by Eq. (1) and found earlier by Nikishov and Ritus [Zh. éksp. Teor. Fiz. 108, 1121 (1995); transl. JETP 81, 615 (1995)] and by Ritus [Zh. éksp. Teor. Fiz. 110, 526 (1996); transl. JETP 83, 282 (1996)], the emission spectra of a mirror in 1+1 space coincide with those of charges in 3+1 space. The reason is that
the angular momentum of the pair emitted by the mirror coincides with the angular momentum of the single particle emitted
by the charge.
Zh. éksp. Teor. Fiz. 114, 46–62 (July 1998) 相似文献
12.
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 Cr 3+ ion Δ s = 66.7 GHz and between spin sublevels of the Cr 3+-Cr 3+ 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 Cr 3+ ions with an Mg 2+ vacancy between them replacing three Mg 2+ 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. 相似文献
13.
Electrical resistance measurements are reported on the binary liquid mixtures CS 2 + CH 3CN and CS 2 + CH 3NO 2 with special reference to the critical region. Impurity conduction seems to be the dominant mechanism for charge transport.
For the liquid mixture filled at the critical composition, the resistance of the system above T
c follows the relation R= R
c− A( T− T
c)
b
with b=0·6±0·1. Below T
c the conductivities of the two phases obey a relation σ 2−σ 1= B( T
c− T) β with β=0·34±0·02, the exponent of the transport coefficient being the same as the exponent of the order parameter, an equilibrium
property. 相似文献
15.
Employing the quasi-classical trajectory method and the potential energy surface of Panda and Sathyamurhy [Panda A N and Sathyamurthy N 2004 J. Chem. Phys. 121 9343], the effect of the reagent vibration on vector correlation of the ion-molecule reactions D - + H 2 and H - + D 2 is studied at a collision energy of 35.7 kcal/mol. Four generalized polarization-dependent differential cross sections (2 π/ σ)(d σ00/d ωt),(2 π/ σ)(d σ20/d ωt),(2 π/ σ)(d σ22/d ωt), and (2 π/ σ)(d σ20/d ωt) are presented in the centre-of-mass reference frame, separately. At the same time, the effects on the product angular distributions P(θr), P(φr) and P(θr,φr) of the title reactions are also analysed. The calculated results show that the scattering tendencies of the product HD, the alignment and the orientation of j' sensitively depend on reagent molecule vibration. 相似文献
16.
Yb 3+:GdAl 3(BO 3) 4 (hereafter Yb 3+:GAB) crystals with large sizes and good optical quality have been grown by the top-seed solution growth (TSSG) method. The polarized absorption and emission spectra have been investigated at room temperature. For the σ-polarization, the intensities of both absorption and emission spectra are stronger than those for the π-polarization, the σ-absorption cross section of Yb 3+ in GAB being 3.43×10 -20 cm 2 at 977 nm, and the σ-emission cross section being 0.98×10 -20 cm 2 at 1045 nm. The fluorescence lifetime of the 2
F
5/2→ 2
F
7/2 transition was measured to be 800 μs in the 5% doped sample used for our laser experiments, 993 μs in a 10% doped sample and 569 μs in a 0.5% doped sample. The laser parameters were estimated as: β min=0.022, I sat=10.4 kW/cm 2 and I min=0.23 kW/cm 2. About 0.4 W laseroutput at the wavelength of 1043 nm was achieved when the Yb 3+:GAB crystal was pumped by a 974 nm laser diode, with 27.4% slope efficiency. PACS 42.55.-f; 42.70.Hj; 78.20.-e; 81.10.Dn 相似文献
17.
We have grown crystals Na 0.4Y 0.6F 2.2:Ho 3+ (NYF:Ho 3+) by the Bridgman-Stockbarger method. The optical spectra and luminescence kinetics of NYF:Ho 3+ crystals have been studied. Based on the analysis of low-temperature absorption spectra, we determine the structure of the
Stark splitting of holmium levels in NYF:Ho 3+ crystals. From absorption spectra examined at T = 300 K, we calculate absorption cross-section spectra and oscillator strengths of transitions from the ground state of holmium
to excited multiplets. We show that the absorption spectra of NYF:Ho 3+ crystals consist of broad bands that lie in the UV, visible, and near-IR ranges. The most intense bands are observed in the
visible range, they correspond to transitions 5
I
8 → ( 5
F
1, 5
G
6) and 5
I
8 → ( 5
F
4, 5
S
2), and their maximal absorption cross sections are σ absmax (λ = 450.3 nm) = 1.16 × 10 −20 cm 2 and σ absmax (λ = 535.1 nm) = 0.9 × 10 −20 cm 2. The intensity parameters Ω
t
have been calculated by the Judd-Ofelt method taking into account 10, 12, and 20 transitions from the 5
I
8 ground state to excited multiplets. We show that, with an increasing number of transitions taken into account in the calculation,
the parameters Ω
t
somewhat increase. For 20 transitions, we have obtained the following intensity parameters: Ω 2 = 0.97 × 10 −20, Ω 4 = 1.74 × 10 −20, and Ω 6 = 1.15 × 10 −20 cm 2. With these parameters, we have calculated the probabilities of radiative transitions, the radiative lifetimes, and the branching
ratios. The rates of multiphoton nonradiative transitions have been estimated. The luminescence decay kinetics from excited
holmium levels 5
F
3 ( 5
F
4, 5
S
2) and 5
F
5 have been studied upon selective excitation in the range of 490 nm, and the lifetimes of these levels have been experimentally
determined. We find that the calculated and experimental rates of radiative and nonradiative relaxation from excited holmium
levels agree well with each other. We show that, upon pumping in the range of 490 nm, the multiplet ( 5
F
4, 5
S
2) is populated as a result of the radiative and nonradiative excitation relaxation from the 5
F
3 level, while the lower-lying 5
F
5 level is populated due to direct radiative transitions 5
F
3, 2 → 5
F
5, obviating the cascade scheme 5
F
3 → ( 5
F
4, 5
S
2) ↝ 5
F
5. We conclude that NYF:Ho 3+ crystals are processable; admit doping by holmium in high concentrations (up to 100%); and, with respect to all their radiative
characteristics, can be considered as potential active media for solid-state continuously tunable lasers in the IR and visible
ranges. 相似文献
18.
The results from experimental and theoretical studies of the total and partial cross sections of photoneutron reactions on
the 197Au isotope were analyzed. The cross sections for reactions σ(γ, n X) = σ(γ, n) + σ(γ, np) + … + σ(γ, 2n X) = σ(γ, 2n) + σ(γ, 2np) + … were evaluated in the energy range 7 ≤ E
γ ≤ 30 MeV using an approach free of the shortcomings of experimental photoneutron multiplicity sorting methods. The total
photoneutron reaction cross sections σ exp(γ, xn) = σ exp(γ, n X) + 2σ exp(γ, 2n X) + … = σ exp(γ, n) + σ exp(γ, np) + 2σ exp(γ, 2n) + 2σ exp(γ, 2np) + … were used as the initial experimental data. The contributions from the cross sections σ(γ, n X) and σ(γ, 2n X) to the cross sections σ exp(γ, xn) were separated using the multiplicity transition functions F
1
theor = σ theor(γ, 1n X)/σ theor(γ, xn) and F
2
theor = σ theor(γ, 2n X)/σ theor(γ, xn), calculated within an updated version of the pre-equilibrium model of photonuclear reactions. New evaluated data for both
partial reaction cross sections, i.e., σ eval (γ, 1n X) = F
1
theorσ exp(γ, xn) and σ eval(γ, 2n X) = F
2
theorσ exp(γ, xn), were obtained. The cross sections σeval(γ, n X) and σ eval.(γ, 2n X) evaluated using the theoretically calculated functions F
1,2theor are consistent with the Livermore data, but substantially contradict the Saclay data. 相似文献
19.
The fundamental optical absorption of films of the solid electrolyte RbAg 4I 5 electrolyte films decreases (by approximately 25%) after vacuum evaporation of Sm films onto them, and a broad strong-absorption
band with a maximum at 2.4 eV appears within the bad gap. The films bleach after 5–10 days in dry air. The observed phenomena
are attributed to a high concentration (∼3×10 20 cm −3) of point defects, including F-centers, in nonstoichiometric RbAg 4I 5:Sm, and also to the oxidation of Sm. In colored films the ionic conductivity is σ⋍0.9 σ
0, and in bleached films it is close to the initial value σ
0.
Fiz. Tverd. Tela (St. Petersburg) 39, 1544–1547 (September 1997) 相似文献
20.
Sensitization of the fluorescence of Er3+ in fluoride phosphate glass containing up to 20 mol% phosphates by codoping with Cr3+ and Yb3+ is shown. The low order of ligand field strength for Cr3+ (Dq/B=2.04) results in broad Cr3+ fluorescence overlapping the Yb3+ absorption. The electronic energy transfer efficiency approaches 100%. Deviations of donor decay from the Förster law are interpreted in terms of the inhomogeneously acceptor distribution. The electronic energy transfer efficiency of Yb3+ → Er3+ reaches a maximum value of 75% for glasses containing 20 mol% phosphates. The transfer is shown to be migrationally accelerated by means of GAF-LAF-FB theory. From Judd-Ofelt parameters a stimulated emission cross-section for the transition4I13/2 →4I15/2 of Er3+ of 6.2×10−20 cm2 is derived. The c.w. laser action of Er3+ by Cr3+ excitation and double-step energy transfer is shown. The output is tuned continuously from 1536 to 1596 nm. Flashlamp pumping is also shown. 相似文献
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