EPR Spectra of Cr3+ Ions in Sr3Ga2Ge4O14:Cr Single Crystals

The results of investigations of chromiumdoped Sr₃Ga₂Ge₄O₁₄ single crystals by the EPRspectroscopy method are presented. It is shown that activating chromium ions form Cr³⁺ Ga³⁺ (Ge⁴⁺) substitution centers in the 1aoctahedral positions of the lattice of Sr₃Ga₂Ge₄O₁₄. Depending on the combination of occupation of the 3ftetrahedral positions of the first cationic coordination sphere by Ga³⁺ and Ge⁴⁺ ions, Cr³⁺ centers of two types are formed. Their individual magnetic spectra are characterized by axial and rhombic symmetry. The magnetic multiplicity of the axialsymmetry spectrum is equal to unity. There exist rhombicsymmetry centers of two types differing in the orientation of the principal magnetic axes and the value of the spinHamiltonian parameter E. The magnetic multiplicity of the individual magnetic spectra of rhombic centers of each type is equal to three. The detected EPR spectra of Cr³⁺ ions have been described by the spin Hamiltonian of rhombic symmetry. Its parameters and their spread have been determined.     

Continuous wave 1.6 μm laser action in Er doped garnets at room temperature⋆

We report on room-temperature cw laser action of Er³⁺: Yttrium-aluminum-garnet (YAG) and Er³⁺: Yttrium-gallium-garnet (YGG) crystals at 1.64 m. The laser operates from the metastable⁴ I _13/2 manifold into an upper Stark level of the⁴ I _15/2 ground-state manifold of Er³⁺. Due to reabsorption losses, the Er³⁺ concentration of the laser crystals must be low. Laser pumping at a wavelength of 647.1 nm yields lowest thresholds around 30 mW and slope efficiencies up to 12.7% for Er:YAG. Laser operation in Er:YGG is achieved with higher thresholds of about 200 mW and smaller slope efficiencies of 0.9%. The effective emission cross section in YAG is estimated to be _e5×10^–21 cm².Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Strange and singlet form factors of the nucleon: Predictions for G0, A4, and HAPPEX II experiments

We investigate the strange and flavor-singlet electric and magnetic form factors of the nucleon within the framework of the SU(3) chiral quark-soliton model. Isospin symmetry is assumed and the symmetry-conserving SU(3) quantization is employed, rotational and strange-quark mass corrections being included. For the experiments G0, A4, and HAPPEX II we predict the quantities G⁰_E + G⁰_M and G^s_E + G^s_M. The dependence of the results on the parameters of the model and the treatment of the Yukawa asymptotic behavior of the soliton are investigated.     

NMR self-diffusion study of organic glasses: COANP,MBANP, PNP,NPP

A pulsed field gradient proton spin-echo NMR self-diffusion study of organic glasses COANP, MBANP, PNP and NPP in their liquid and weakly supercooled states was performed. The NMR phase diagrams, based on the proton NMR transverse relaxation time (T ₂) temperature hysteresis data of these materials, clearly give evidence of the onset of a glass phase on cooling the isotropic liquids below their respective melting temperatures. The self-diffusion data exhibit in the supercooled glassy state a non-Arrhenius behaviour and can be described in terms of the Vogel-Fulcher modification of the Arrhenius law,D=D exp{–E _a /[k _B (T–T _VF )]}. The activation energiesE _a and Vogel-Fulcher temperaturesT _VF are 83.2 meV and 239 K for COANP, 66 meV and 249 K for MBANP, and 85 meV and 245 K for PNP, respectively. The flow viscosity data obtained for COANP in the same temperature region as well conform to the Vogel-Fulcher behaviour, exp{E _a ⁽⁾ /[k _B (T–T _VF )]}, withE _a ⁽⁾ =80.4 meV andT _VF =239 K. In case of COANPD was found to increase with decreasing diffusion time in the supercooled (glassy) melt just belowT _M whereas no such behaviour was found aboveT _M .     

Excitation of Intra-4f Shell Luminescence of Rare Earth Ions (Er3+ and Yb3+) by the Energy Transfer from Si Nanocrystals

Photoluminescence (PL) properties of SiO₂ films containing Si nanocrystals (nc-Si) and Er³⁺ (Yb³⁺) were studied. PL peaks attributable to the recombination of electron–hole pairs in nc-Si (1.5eV) and the intra-4f shell transition of Er³⁺ (0.81eV) (Yb³⁺ (1.26eV)) were observed simultaneously at room temperature. Correlation of the two peaks was studied as a function of nanocrystalline size. It was found that the intensity of the Er³⁺-related (Yb³⁺-related) peak increases drastically as the size of nc-Si decreases. Temperature dependence of PL spectra was studied. In the case of Yb-doped samples, temperature quenching of the PL became small as the size decreased, while in the case of Er-doped samples, no remarkable temperature dependence was observed. Two major features of the quantum size effects of nc-Si, i.e., the band-gap widening and the increase in the PL efficiency with decreasing the size, are thought to contribute to the improvement of room temperature PL efficiency of Er³⁺ (Yb³⁺).     

On the dependence of the coercive field of BaTiO3 single crystals on their thickness

The paper gives the results of measuring the coercive field of single-domain single crystals of BaTiO₃, the thickness of which was lowered by successive etching. The initial decrease in the thicknessd is accompanied by a sudden increase in the coercive fieldE _c (E _c /d –1·5 × × 10⁵ V/cm²); after etching off layers larger than 10^–3 ÷ 2×10^–3cm the coercive field grows much more slowly (E _c /d –7×10³ V/cm²). The high initial growth ofE _c is interpreted by means of Schottky exhaustion layers with non-zero gradient of the electric potential.In conclusion, the authors would like to thank H. Arend, P. Coufová and J. Jarý for providing high-quality single crystals and for much valuable advice during the work and V. Dvoák and K. Pátek for remarks on this paper.     

Abnormal Raman spectra in Er‐doped BaTiO3 ceramics

Greatly enhanced and abnormal Raman spectra were discovered in the nominal (Ba_{1 − x}Er_x)Ti_{1 − x/4}O₃ (x = 0.01) (BET) ceramic for the first time and investigated in relation to the site occupations of Er³⁺ ions. BaTiO₃ doped with Ti‐site Er³⁺ mainly exhibited the common Raman phonon modes of the tetragonal BaTiO₃. Er³⁺ ions substituted for Ba sites are responsible for the abnormal Raman spectra, but the formation of defect complexes will decrease spectral intensity. A large increase in intensity showed a hundredfold selectivity for Ba‐site Er³⁺ ions over Ti‐site Er³⁺ ions. A strong EPR signal at g = 1.974 associated with ionized Ba vacancy defects appeared in BET, and the defect chemistry study indicated that the real formula of BET is expressed by (Ba_{1 − x}Er_3x/4)(Ti_{1 − x/4}Er_x/4)O₃. These abnormal Raman signals were verified to originate from a fluorescent effect corresponding to ⁴S_3/2→⁴I_15/2 transition of Ba‐site Er³⁺ ions. The fluorescent signals were so intense that they overwhelmed the traditional Raman spectra of BaTiO₃. The significance is that the abnormal Raman spectra may act as a probe for the Ba‐site Er³⁺ occupation in BaTiO₃ co‐doped with Er³⁺ and other dopants. A new broad EPR signal at g = 2.23 was discovered, which originated from Er³⁺ Kramers ions. Copyright © 2014 John Wiley & Sons, Ltd.     

Structure and annealing of (n, γ)-Induced point defects in CaF2

Neutron activated²⁰F nuclei (T _1/2=11 s) in CaF₂ serve for two purposes: first they produce well defined point defects by their (n, )-production mechanism, second, they act as probes to detect the properties of these defects in a- and-ray detected nuclear magnetic resonance experiment. Below about 70 K an interstitial defect is observed due to a quadrupolar splitting of the²⁰F-NMR line. The defect induced electric field gradient is oriented along the 111-axis of the crystal and causes a quadrupole coupling constante ² qQ/h=–2.0(1) MHz at the²⁰F site. The defect anneals above 70 K with an Arrhenius like temperature dependence corresponding to an activation energyE _M =0.17(4) eV.     

Acoustomagnetoelectric effects at meso-ultrasonic frequencies in anisotropic semiconductors in arbitrary (classical) magnetic fields

Anisotropic acoustomagnetoelectric (AME) effects at meso-ultrasonic frequencies are calculated analytically in semiconductors with an anisotropic mobility () in arbitrary classical magnetic fields. For Bq(q is the ultrasonic wave vector) and an arbitrary direction of q two transverse components of the AME field (E ^B q E _y ^B ) occur in the crystal, and the longitudinal acoustoelectric field changes under the action of a longitudinal magnetic field (E _q ^B =E _q ^B -E _q ⁰ ),E ^B is even, and E ^B is odd in B; for B 1 the component E _y ^B E ^B /B, andE ^B and E _q ^B are independent of B and can be commensurate with the zero-field acoustoelectric field E _q ⁰ if the anisotropy of is large (hexagonal ZnS and ZnO or n-Ge highly compressed along [111]). The transverse AME field E _st ^B is calculated in the configuration E _st ^B qBE _st ^B (standard AMEeffect). For B >> 1 the field B 1E _st ^B B ^–3, so thatE ^B , E _y ^B , and _q ^B can be greater than E _st ^B here. The acoustoelectric analog of the Grabner effect (E _G ^B ), i.e., the component of the AME field along a transverse magnetic field (E _G ^B Bq) is also calculated. For pB > 1 the componentE _G ^B B ^–3.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 93–97, June, 1989.     

Internal Lifschitz singularities of disordered finite-difference Schrödinger operators

The integrated density of states has C-like singularities, ln|k(E)–k(E _c )|=–|E–E _c |^–v/2 _c (E), with _c >0, a milder function at the edges of the spectral gaps which appear when the distribution function of the potentiald has a sufficiently large gap. The behaviour of _c nearE _c is determined by the local continuity properties ofd near the relevant edge: _c (E)=O(1) ifd has an atom and =O(ln|E–E _c |) if is (absolutely) continuous and power bounded.     

On Strong Superadditivity of the Entanglement of Formation

We employ a basic formalism from convex analysis to show a simple relation between the entanglement of formation E_F and the conjugate function E^* of the entanglement function E()=S(Tr_A). We then consider the conjectured strong superadditivity of the entanglement of formation E_F()E_F(_I)+E_F(_II), where _I and _II are the reductions of to the different Hilbert space copies, and prove that it is equivalent with subadditivity of E^*. Furthermore, we show that strong superadditivity would follow from multiplicativity of the maximal channel output purity for quantum filtering operations, when purity is measured by Schatten p-norms for p tending to 1.     

Study of the Influence of the Excitation Laser Power Density on the Intermediate-Field Electric Modulation Component of the GaAs Photoreflection E 0-Signal

The influence of the excitation laser power density L on the amplitude A and characteristic time constant of the intermediate-field electric modulation component of photoreflection spectra measured in the region of the E ₀ fundamental transition of GaAs is studied. Crystalline samples with the charge carrier concentration n 10¹⁶ cm^–3 are investigated for L = 100 W/cm²–1 W/cm². A logarithmic dependence of the electric modulation signal on the excitation laser power density has been established for all examined samples. It is demonstrated that the observed change in the characteristic time constant does not have any noticeable effect on the dependence A(L).     

Three-dimensional Gross-Neveu model at nonzero temperature and in the presence of an external electromagnetic field

The phase structure of the (2+1)-dimensional Gross-Neveu model is considered at a nonzero temperatureT and in the presence of an external electromagnetic field. The major results are as follows: 1. AtT=0,H0 (a magnetic field), the original symmetry of the model breaks for anyH values. 2. AtT=0,E0 (an electric field) anE _c is shown to exist, such that forEE _c the original symmetry is broken, but forE>E _c the vacuum of the model is symmetric. 3. AtT0,H0, and at a fixedT value such anH _c (T) is shown to exist that forH>H _c (T) the chiral invariance of the model is spontancously broken. For any fixedH value there exists such anT _c (H) that forT>T _c (H) the symmetry restores. The phase portrait of the model is drawn.     

Diffusion of positive mouns in nickel and iron

The depolarization rate for spin polarized ⁺ particles implanted into high purity polycrystalline nickel and iron samples were studied as a function of temperature in zero external field. In nickel a rapid decrease of the depolarization rate above 300 K was found. Assuming an Arrhenius behaviour, an activation energy of E_a=173 meV was determined. In iron it was not possible to fit the data with a simple Arrhenius law. At least two different slopes can be observed in an Arrhenius plot with E_a=(124±35) meV and E_a=(19±2) meV, respectively.     

Explaining μ+ spin-lattice relaxation in MnF2, belowT N,by scattering function

A relation between the spin-lattice relaxation rate (1/T ₁) of positive muons and the scattering functionS ^T (q,) is derived for the antiferromagnet MnF₂. We find good agreement between previousSR measurements of 1/T ₁ and our calculation using neutron scattering measurements ofS ^T (q,).     

High-frequency expansion of relativistic classical plasma dielectric tensor

A high-frequency sum-rule expansion is derived for the transverse elements of the relativistic classical plasma dielectric tensor in an isotropic system. The relativistic results are different from the nonrelativistic ones by a factor of ^–1(1 – ²/3c ²) for ₂ (k) and longitudinal plasmon and transverse photon frequencies, and by ^–2(1–2 ²/3c ²) for ₂ (k).     

High energy spin dynamics of La2CuO4 and La1.9Sr0.1CuO4

The magnetic excitations in La₂CuO₄ and La_1.9Sr_0.1CuO₄ were studied by inelastic neutron scattering up to energies of 76 meV. For the pure sample, the results forE15 meV can be well described by conventional spin wave theory with a spin wave velocity c=0.89±0.07 eVÅ. For lower energies, the observed intensities were somewhat higher than expected from spin wave theory and did not follow the Bose-Einstein factor. For the doped sample, the linewidth in constant-E scans at smallE shows a very short correlation length of =7.5 Å only, which is considerably below the value expected from the concentration dependence reported by other authors. An increase of the linewidth for largeE indicates a reduced spin-wave stiffness when compared to the undoped material. However, the use of a spin-wave picture may not be appropriate as the standard expression for damped spin waves in a paramagnetic material is in serious conflict with experiment.     

The temperature dependence of the methyl rotational potential in methyl iodide under pressure

Inelastic neutron scattering experiments have been performed on CH₃I under pressure. The tunnel splitting of the methyl librational groundstate decreases almost exponentially with increasing hydrostatic pressure: _t (p)=2.48 eV·exp (–0.138·p[kbar]), the librational energy increases asE ₀₁ (p)=13.2 meV (1+0.050p[kbar]). Both relations can be explained by a rotational potentialV which depends on the interatomic distancer asVr ^–9.6 . This large exponent shows the importance of valence forces. The shape of the rotational potential is almost unaffected by pressure. The coupling of the methyl group to phonons via a shaking and a breathing term which was observed at atmospheric pressure is also found at high pressure. The effect appears at higher temperature as expected for the stronger potential.     

Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

Fluorescence and attenuation properties of Er3+ -doped phosphate-glass fibers and efficient infrared-to-visible up-conversion

Fluorescence spectra of phosphate glasses with different erbium doping have been measured. The flourescence intensity reaches its maximum at an Er³⁺-doping concentration of 0.75 mol%. When the Er³⁺ doping exceeds 0.75 mol%, the fluorescence intensity decreases due to concentration quenching. The attenuation at 1.53 µm of the fiber is 12.8 db/m. The fluorescence up-conversion of 1.064 µm Nd:YAG laser pulses into intense green 547 and 667 nm light in the fiber has been measured. The fluorescence output power of green (547 nm) and red (667 nm) light is 178 and 42 W, respectively with an excitation power of 1 W. The two signals are referred to as⁴ S _3/2 ⁴ I _{1 5/2} and⁴ F _9/2 ⁴ I _{1 5/2} transitions through two-photon absorption fluorescence.