Magnetic phase transitions in the Nd(Mn0.9Me0.1)O3 (Me=Al, Fe, Cr, Zn) perovskites

It is shown that perovskite NdMnO₃ is a weak ferromagnet with an anomalous magnetization behavior due to Nd sublattice contribution. Ferromagnetic component drastically increases whereas T_N slightly decreases when a part of manganese ions is replaced with Cr, Al, Fe, Zn. It is suggested that the Mn³⁺–O–Mn³⁺ superexchange interaction changes a sign in the microdomains enriched with Me=Cr, Al, Fe, Zn ions due to removing static Jahn–Teller distortions. All these substituted perovskites show a sharp drop of the magnetization as temperature decreases. A large temperature hysteresis indicates first-order phase transition. Below this transition neodymium magnetic moments orient opposite to a moment of manganese magnetic sublattice. It is supposed that this phase transition results from a change of the ground state of Nd ions.     

Jahn–Teller effect in the 4T2g excited state of Cr3+ ion in Cs2NaYF6 crystal

Calculations of the fine structure of Cr³⁺ energy levels in Cs₂NaYF₆ accompanied by estimations of the Jahn–Teller (JT) stabilization energy in the first excited ⁴T_2g state of Cr³⁺ ion are presented. Two independent approaches—effective second-order spin–orbit Hamiltonian and analysis of the potential energy surfaces—are used. The JT energy was estimated to be 216 and 257 cm^–1 in the first and the second models, respectively. It is shown that the octahedral [CrF₆]^3? complex undergoes an equatorial expansion by 0.09 Å and an axial elongation by 0.02 Å due to the combined effect of the a_1g and e_g normal modes.     

Study on visible luminescence of the Tm3 +: 1D2 → 3 F4 emission state in lead borate titanate aluminumfluoride glasses

This paper reports the visible luminescence properties of ¹D₂ state of Tm^3 + -doped lead borate titanate aluminumfluoride (LBTAFTm) glasses. The absorption and luminescence was analyzed within the frame work of Judd-Ofelt model. The reliability of J-O intensity parameters obtained from the experimental oscillator strengths have satisfactorily been correlated with the calculated oscillator strengths with small r.m.s deviation of ± 0.12 × 10^-6 by the least square fit analysis. Upon 359 nm excitation, the luminescence spectra show only one emission band at 458 nm (blue) corresponding to the ¹D₂ → ³ F₄ transition in the spectral region 400–500 nm. No luminescence quenching has been observed with the increase of Tm^3 + concentration. The decay profiles of the ¹D₂ level have shown single-exponential nature for all the concentrations and the decay times were found to decrease with the increase of concentration. The stimulated emission cross-section (σ_e) for the observed emission transition has also been computed. The large quantum efficiency (η) of the ¹D₂ level suggests the utility of LBTAFTm glass as a potential host for optical device applications at 458 nm emission wavelength.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Synthesis and photoluminescence properties of YVO4:Eu3+, Al3+ phosphor

The Y_0.95?xAl_xVO₄:5%Eu³⁺ (0≤x≤0.1) phosphors were successfully synthesized by solid state reaction at 900 °C for 6 h, and their luminescence properties were investigated under UV and VUV excitation. Monitoring at 619 nm, a strong broad absorption was enhanced by co-doping of Al³⁺ into the YVO₄:Eu³⁺ lattices at 256 nm under UV excitation. The VUV excitation spectra also showed the enhanced excitation bands at about 156 and 200 nm. Under 254 or 147 nm excitation, it was found that Y_0.95?xAl_xVO₄:Eu³⁺(0≤x≤0.1) phosphors showed strong red emission at about 619 nm corresponding to the electric dipole ⁵D₀^–7F₂ transition of Eu³⁺. The improvement of luminescence intensity of YVO₄:Eu³⁺ was also observed after partial substituting Y³⁺ by Al³⁺ and the optimal luminescence intensity appeared with incorporation of 2.5 mol% Al³⁺.     

Photoluminescence characterization of a novel red-emitting phosphor In2(MoO4)3:Eu3+ for white light emitting diodes

The red-emitting phosphor In₂(MoO₄)₃:Eu³⁺ with cubic crystal structure was synthesized by a conventional solid-state reaction technique and its photoluminescence properties were investigated. The prepared phosphor can be efficiently excited by ultraviolet (395 nm) and blue (466 nm) light. The emission spectra of the phosphor manifest intensive red-emitting lines at 612 nm due to the electric dipole ⁵D₀→⁷F₂ transitions of Eu³⁺. The chromaticity coordinates of x=0.63, y=0.35 (λ_ex=395 nm) and x=0.60, y=0.38 (λ_ex=466 nm) are close to the standard of National Television Standard Committee values (NTSC) values. The concentration quenching of In₂(MoO₄)₃:Eu³⁺ is 40 mol% and the concentration self-quenching mechanism under 466 nm excitation was the dd intereaction. As a result of the strong emission intensity and good excitation, the phosphor In₂(MoO₄)₃:Eu³⁺ is regarded as a promising red-emitting conversion material for white LEDs.     

Ultrafast photoisomerization and its single-shot pump pulse efficiency of trans-azobenzene derivative: Compound for photosensitive DNA

The femtosecond photoisomerization processes of trans (T) 4-carboxy-2′,6′-dimethylazobenzen, which has been employed recently as an efficient photoregulator of DNA hybridization, were clarified by the rate equation analysis of measured transient absorbance changes with (350 nm) and without (380 nm) ground-state absorption of both the reactant (T) and photoproduct (cis: C) isomers under S₂^T-band excitation (360 nm, 150 fs pump): after excitation to the S₂^T state with a 450-fs lifetime, ~ 1.5% of the T-molecules in the S₂^T state are isomerized to the C-form within ~ 6 ps through the intermediate state (so called bottleneck state), but most of those return back to the T ground-state S₂^T via the internal conversion processes with an ultrafast kinetic rate of 2.2 × 10¹² s^? 1. Moreover, the rate equation analysis enables us to determine the T-to-C photoisomerization rate η^T,C per pump pulse to be 0.0011 at the pump energy of 80 nJ from the amplitude A_3,350 of the offset component in the 350-nm probe signal, and to obtain the photoisomerization quantum yield Φ^T,C = 0.094. The latter value is slightly lower than that of T-azobenzene, and well agrees with that (Φ^T,C = 0.097) measured by the conventional CW irradiation method using a photostationary state.     

Monitoring of laser heating temperature in laser spectroscopic measurements

During laser spectroscopic measurement, a part of laser energy will be converted into heat in the processes of excitation and light emission. Temperature monitoring can help to evaluate such nonradiative process. Upconversion luminescence of phosphor Y₂O₃:Er,Yb under laser excitation at 980 nm was investigated in this work. Point temperature of the phosphor was monitored using the fluorescent intensity ratio (FIR) technology. Laser induced temperature rising was identified by comparison with a theoretically ideal temperature calibration function: lnR = 3.1738–1167/T. The monitored temperature of laser heating rises monotonically with increasing laser power. Circumstances around heating point will modify the calibration function, but the linear slope of lnR ~ 1/T is constant.     

Luminescence properties of green emission of SiO2/Zn2SiO4:Mn nanocomposite prepared by sol–gel method

Green light emitting Mn²⁺ doped Zn₂SiO₄ particles embedded in SiO₂ host matrix were synthesized by a sol–gel method. After the incorporation of ZnO:Mn nanoparticles in a silica monolith using sol–gel method with supercritical drying of ethyl alcohol in two steps, it was heat treated in air at 1200 °C for 2 h in order to obtain the SiO₂/α-Zn₂SiO₄:Mn nanocomposites. The microstructure of phosphor crystals was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). XRD results indicate that the pure phase α-Zn₂SiO₄ with rhombohedral structure was obtained after thermal treatment at 1200 °C. The SiO₂/α-Zn₂SiO₄:Mn nanocomposites with a Mn doping concentration of 1.5 at% exhibit two broadband emissions in the visible range: a strong green emission at around 525 nm and a second one in the range between 560 and 608 nm. This nanocomposite with a Mn doping concentration of 0.05 shows the highest relative emission intensity. Upon 255 nm excitation, the luminescence decay time of the green emission of Zn₂SiO₄:Mn around 525 nm is 11 ms. The luminescence spectra at 525 nm (⁴T₁–⁶A₁) and lifetime of the excited state of Mn²⁺ ions-doped Zn₂SiO₄ nanocrystals are investigated.     

Two-color polarization spectroscopy of rubidium

We report on two-color, two-photon polarization spectroscopy in a room-temperature rubidium vapor. We use two separate lasers, a strong pump at 780 nm to induce an anisotropy in the atomic polarization and a weak probe at 776 nm to interrogate this anisotropy. The lasers are resonant with the 5²S_1/2 → 5²P_3/2 and 5²P_3/2 → 5²D_5/2 transitions in rubidium, respectively. Finally, we have used our polarization spectroscopy signal as an error signal to lock the 776 nm laser. This modulation-free locking scheme allows us to detune the lock point of the second laser by adjusting the detuning of the laser used for the first transition.     

Synthesis and photophysical behavior of a novel zinc phthalocyanine containing a single carboxylic acid and three phenylthio substituents

Zinc 2, (3)-tri-(phenylthio)-2, (3)-carboxy phthalocyanine (ZnPc(COOH)(SPh)₃), zinc 2, (3)-tetra-(phenylthio) phthalocyanine (ZnPc(SPh)₄) and 2, (3)-tetra-(phenylthio) phthalocyanine (H₂Pc(SPh)₄) were synthesized and their photophysical behavior were compared with those of a number of zinc phthalocyanine (ZnPc) derivatives. ZnPc(COOH)(SPh)₃ and ZnPc(SPh)₄ had similar fluorescence (Φ_F=0.14) and triplet state (Φ_T=0.65) quantum yields in dimethylsulfoxide, hence showing no effects of the replacement of one of the phenylthio groups with a carboxylic acid group. ZnPc(COOH)(SPh)₃ displayed a slightly shorter triplet lifetime (τ_T=331 μs) than ZnPc (τ_T=350 μs) in DMSO, but within the range of ZnPc derivatives. The triplet lifetime for ZnPc(COOH)(SPh)₃ is much longer than for the symmetrical derivative (ZnPc(SPh)₄) with τ_T=149 μs in DMSO.     

High-frequency electron paramagnetic resonance investigation of Mn3+ centers in SrTiO3

A high-frequency (49 and 70 GHz) electron paramagnetic resonance (EPR) study is reported on SrTiO₃ single crystals doped with 0.5-at% of manganese. EPR spectra originating from the S=2 ground state of Mn³⁺ ions are shown to belong to the different types of Jahn–Teller centers that arise as a result of varied ordering of the oxygen vacancies due to the reducing process. The present results indicate the strong covalent reduction of the density of unpaired spins (Fermi term) at the manganese nucleus for the various types of centers.     

On the origin of blue light emission from Ge-nanocrystals containing a-SiOxfilms

We have studied the photoluminescence of a-Si_xGe_yO_{1 − x − y}films with average Ge-nanocrystal sizes ranging from over 100 nm down to 2 nm. No systematic peak shift of the luminescence bands at 3.0 eV and 2.0 eV with the diameter of the nanocrystals is observed. Comparision with a simplified confinement model shows that quantum size effects cannot explain the blue luminescence. We propose the Ge²₀defect as a likely source for this band, based on considerations about the crystallization process.     

Influence of growth morphology on the Néel temperature of CrRu thin films and heterostructures

Dimensionality effects on epitaxial and polycrystalline Cr_1?xRu_x alloy thin films and in Cr/Cr–Ru heterostructures are reported. X-ray analysis on Cr_0.9965Ru_0.0035 epitaxial films indicates an increase in the coherence length in growth directions (1 0 0) and (1 1 0) with increasing thickness (d), in the range 20≤d≤300 nm. Atomic force microscopy studies on these films shows pronounced vertical growth for d>50 nm, resulting in the formation of columnar structures. The Néel temperatures (T_N) of the Cr_0.9965Ru_0.0035 films show anomalous behaviour as a function of d at thickness d≈50 nm. It is interesting to note that this thickness corresponds to that for which a change in film morphology occurs. Experiments on epitaxial Cr_1?xRu_x thin films, with 0≤x≤0.013 and d=50 nm, give T_N–x curves that correspond well with that of bulk Cr_1?xRu_x alloys. Studies on Cr/Cr_0.9965Ru_0.0035 superlattices prepared on MgO(1 0 0), with the Cr layer thickness varied between 10 and 50 nm, keeping the Cr_0.9965Ru_0.0035 thickness constant at 10 nm, indicate a sharp decrease in T_N as the Cr separation layers reaches a thickness of 30 nm; ascribed to spin density wave pinning in the Cr layers for d<30 nm by the adjacent CrRu layers.     

Synthesis and luminescence properties of Gd6MoO12:Yb3+, Er3+ phosphor with enhanced photoluminescence by Li+ doping

Yb³⁺/Er³⁺ co-doped Gd₆MoO₁₂ and Yb³⁺/Er³⁺/Li⁺ tri-doped Gd₆MoO₁₂ phosphors were prepared by adjusting the annealing temperature via the high temperature solid-state method. Under the excitation of 980 nm semiconductor, the upconversion luminescence properties were investigated and discussed. In the experimental process, we get the optimum Yb³⁺ concentration and the concentration quench effect will happen while the concentration extends the given region. According to the Yb³⁺ concentration quenching effects, the critical distance between Yb³⁺ ions had been calculated. The measured UC luminescence exhibited a strong red emission near 660 nm and green emission at 530 nm and 550 nm, which are due to the transitions of Er³⁺(⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺(⁴I_15/2). Then the effect of excitation power density in different regions on the upconversion mechanisms was investigated and the calculated results demonstrate that the green and red upconversion is a two-photon process. A possible mechanism was discussed. After Li⁺ ions mixing, the upconversion emission enhanced largely, and the optimum Li⁺ concentration was obtained while fixed the Yb³⁺ and Er³⁺ on the above optimum concentration. This enhancement owns to the decrease of the local symmetry around Er³⁺ after Li⁺ ions doping into the system. This result indicates that Li⁺ is a promising candidate for improving luminescence in some case.     

Structural,electrical, magnetic,elastic, and internal friction studies of Nd1−xCaxMnO3 (x=0.2, 0.33, 0.4, and 0.5) manganites

A series of Nd_1?xCa_xMnO₃ (x=0.2, 0.33, 0.4, and 0.5) manganites was prepared by sol–gel route by sintering at 1300 °C, mainly to understand the correlation between electron, spin, and phonon couplings. The internal friction and longitudinal modulus along with electrical and magnetic properties have been measured. All the samples are found to exhibit anomalies at T_C, T_N, and T_CO transition temperatures. The anomalies in longitudinal modulus and the internal friction peak at T_CO are attributed to Jahn–Teller effect. A strong correlation between the temperature dependent elastic, anelastic, resistivity, and ac susceptibility properties has been observed and an effort has been made to explain the observed anomalous behavior by a qualitative model.     

High-peak-power,short-pulse-width,LD end-pumped,passively Q-switched Nd:YAG 946 nm laser

High-peak-power, short-pulse-width diode pumped 946 nm Nd:YAG laser in passively Q-switching operation with Cr⁴⁺:YAG is reported. The highest average output power reaches 3.4 W using the Cr⁴⁺:YAG with initial transmissivity T₀=95%. When the T₀=90% Cr⁴⁺:YAG is employed, the maximum peak power of 31.4 kW with a pulse width of 8.3 ns at 946 nm is generated.     

Magnetotransport properties of ferromagnetic LaMnO3+δ nano-sized crystals

Transport and magnetic properties of LaMnO_3+δ nanoparticles with average size of 18 nm have been investigated. The ensemble of nanoparticles exhibits a paramagnetic to ferromagnetic (FM) transition at T_C～246 K, while the spontaneous magnetization disappears at T≈270 K. It was found that the blocking temperature lies slightly below T_C. The temperature dependence of the resistivity shows a metal–insulator transition at T≈192 K and low-temperature upturn at T<50 K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. The low temperature I–V characteristics are well described by indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.