A new proposal for Galactic dark matter: Effect of f(T) gravity

It is still a challenging problem to the theoretical physicists to know the exact nature of the galactic dark matter which causes the galactic rotational velocity to be more or less a constant. We have proposed that the dark matter as an effect of f(T) gravity. Assuming the flat rotation curves as input we have shown that f(T) gravity can explain galactic dynamics. Here, we don' have to introduce dark matter. Spacetime metric inspired by f(T) gravity describes the region up to which the tangential velocity of the test particle is constant. This inherent property appears to be enough to produce stable circular orbits as well as attractive gravity.Comment: 7 pages and 1 figure. Minor corrections are made. Accepted for publication in Int.J.Theor.Phy

Parameterizing Intensity of 4f\u3csup\u3e2\u3c/sup\u3e→4f\u3csup\u3e2\u3c/sup\u3e Electric-Dipole Transitions in Pr\u3csup\u3e3+\u3c/sup\u3e Doped LiYF\u3csub\u3e4\u3c/sub\u3e

A parametrization method is reported to calculate the electric-dipole transition intensities within the 4f2configuration in LiYF4:Pr3+ by considering the main perturbing component of 4f5d. On the basis of calculating the energy level of the 4f5d configuration, the main 4f5d components that can mix with the4f2 transitional states are determined. We exhibit the calculated densities of the 4f5d states which can mix with the initial and the final states of 3P0 energy level in Pr3+ doped LiYF4. Whereafter a new set of phenomenological intensity parameters, Tkq, is put forward. In addition to the explicit static-coupling mechanism, the approach is extended by the non-explicit dynamic mechanism. Detailed description of the calculated method is brought out. The results of present fitting are compared with that obtained by the traditional parametrization scheme and obvious improvements are exhibited

Cooperative Down-Conversion Luminescence in Tb3+/Yb3+ Co-Doped LiYF4 Single Crystals

Cooperative down-conversion (DC) with emission of two near-infrared photons for each blue photon absorbed was achieved in Tb3+/Yb3+ co-doped yttrium lithium fluoride single crystals grown by an improved Bridgman method. The luminescent properties of the crystals were investigated through photoluminescence excitation, emission spectra, and decay curves. With the excitation of Tb3+ ion by a 486 nm light, emission between 980 and 1030 nm from the Yb3+ : F-2(5/2) -> F-2(7/2) transition was observed, and this emission originated from the DC between Tb3+ and Yb3+ ions. The energy transfer processes are studied based on the Inokuti-Hirayama's model, and the interaction between Tb3+ and Yb3+ is confirmed to be electric dipole-dipole. The large quantum cutting efficiency approaches up to 166.7% for 0.32 mol% of Tb3+ and 7.98 mol% of Yb3+ co-doped LiYF4, which is potentially used as a DC layer in silicon-based solar cells

Efficient Quantum Cutting in Tb3+/Yb3+ Codoped alpha-NaYF4 Single Crystals Grown by Bridgman Method Using KF Flux for Solar Photovoltaic

Tb3+/Yb3+ codoped alpha-NaYF4 single crystals with various Yb3+ concentrations and similar to 0.40 mol% Tb3+ are grown by Bridgman method using KF (Potassium fluoride) as flux with a temperature gradient of 70 degrees C/cm-90 degrees C/cm across the solid-liquid interface. The effect and mechanisms of KF in the growing process are studied. The crystal structure is characterized by means of XRD. The high transmission from 300 to 7350 nm and maximum phonon energy about 390 cm(-1) for the alpha-NaYF4 single crystal are obtained from the measured transmission and Raman spectra, respectively. The luminescent properties of the crystals are investigated through excitation, emission spectra, and decay curves. Downconversion with emission of two near-infrared photons about 1000 nm for each blue photon at 486-nm absorption is obtained in Tb3+/Yb3+ codoped alpha-NaYF4 single crystals. Moreover, the energy transfer processes is studied based on the Inokuti-Hirayama model from the measured luminescent decay curves, and the results indicated that the interaction between Tb3+ and Yb3+ is electric dipole-dipole. The maximum quantum cutting efficiency approached 174.6% in alpha-NaYF4 single crystal with 0.42 mol% Tb3+ and 9.98 mol% Yb3+, making it a potential candidate for applications in silicon-based solar cells

White Light Emission From Tb3+/Sm3+ Codoped LiYF4 Single Crystal Excited by UV Light

LiYF4 single crystals codoped with Tb3+/Sm3+ ions were synthesized by the vertical Bridgman method. When excited by 374-nm ultraviolet light, the crystals emit violet-blue, blue, green, yellow, orange, and red light. The white light as a result of a combination of these emissions can be achieved from Tb3+ (1.25 mol%) and Sm3+ (2.22 mol%) codoped LiYF4 crystal with chromaticity coordinates of x = 0.3143, y = 0.3484, and color temperature T-c = 6322 K by 374-nm excitation. Both chromaticity coordinates and photoluminescence intensity vary with the concentration of rare earth dopants and the excitation wavelengths. This crystal may be a potential candidate for the UV-light excited white LED device

Concentration effect of Nd3+ ion on the spectroscopic properties of Er3+/Nd3+ co-doped LiYF4 single crystal

High quality Er3+/Nd3+:LiYF4 single crystals were grown by a Bridgman method. Their spectroscopic properties were studied to understand the Nd3+ concentration effect upon excitation of an 800 nm laser diode. The intensest 2.7 mu m emission was observed in the LiYF4 crystal codoped with 0.99 mol% Er3+ and 0.62 mol% Nd3+. Meanwhile, the emission intensity for the green up-conversion and 1.5 mu m downconversion of Er3+ decreased with increasing of the Nd3+ concentration. The modified Inokuti-Hirayama model was used to analyze the decay curves of the 1.06 (Nd3+) and 1.5 (Er3+) gm emissions. The results indicated that the energy transfer process (Er3+:I-4(13/2) + Nd3+:I-4(9/2) -> Er3+:I-4(15/2) + Nd3+:I-4(15/2)) is mainly due to the electric dipole-dipole interaction. The energy transfer efficiencies between Nd3+ and Er3+ ions were calculated. All results suggested that the Er3+/Nd3+:LiYF4 single crystals may have potential applications in mid-infrared lasers. (C) 2014 Elsevier B.V. All rights reserved