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A series of high quality x%Pr3+:BaF2 (x = 0.6, 1, 2); (1%Pr3+, 2%Na+):BaF2; (1%Pr3+, y%Ca2+):BaF2 (y = 3, 5) and (0.6%Pr3+, 10%R3+):BaF2 (R = Y, Lu, Gd) crystals were grown via a temperature gradient technique (TGT).

High quality 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 single crystals were grown by the temperature gradient technique (TGT). The absorption spectra, fluorescence spectra, fluorescence decay curves and energy transfer process are measured and analyzed at room temperature. By co-doping Yb3+ ions as a sensitizer of Pr3+ ions, the broad near-infrared band emission from 1G4 energy level at 1.3 µm is observed in the Pr3+/Yb3+: BaF2 crystal with low maximum phonon energy for the first time. In addition, the visible fluorescence emission can be detected and the luminescence components are mainly blue-green transition. The emission cross sections of 1G4→3H5 and 3P0→3H4 transitions are calculated to be 0.21 × 10−20 cm2 and 1.12 × 10−20 cm2 with the larger FWHM of 103.8 nm and 4.2 nm in 0.6 at.% Pr, 3 at.% Yb:BaF2 crystal, respectively. The luminescence lifetime of 1G4 and 3P0 energy levels are fitted to be 259.6 µs and 126.1 µs in 0.3 at.% Pr, 2.5 at.% Yb:BaF2 crystal, which is much increased by Yb3+ ions co-doping. The energy transfer efficiency in BaF2 crystal from Yb3+ to Pr3+ ions is calculated to be 60.3% and 70.5%, respectively. All results show that the Pr3+/Yb3+:BaF2 crystal is a promising candidate of laser gain medium for near-IR laser at 1.3 µm and blue laser operation.

Pairs on the edge Many materials with topologically nontrivial band structures are characterized by currents that run around the edge of the sample and typically consist of single excitations. It is an open question whether a topological material that is also a superconductor can support an edge current that consists of paired excitations. W. Wang et al. studied the Weyl superconductor molybdenum ditelluride and, using systematic transport measurements in the presence of a magnetic field, revealed the signatures of an edge supercurrent. Science , this issue p. 534

Pr3+-doped GdScO3 single crystal was successfully grown by the Czochralski (Cz) method. Polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves were measured at room temperature. The polarized spectroscopic property of Pr:GdScO3 crystal for new-wavelength transition in the near infrared (NIR) region was discussed for the first time. The absorption cross-sections at 600 nm were calculated to be 0.39×10−20 cm2 (E∥a), 0.32×10−20 cm2 (E∥b) and 0.4×10−20 cm2 (E∥c). The emission cross sections at 1053 nm of 1D2 energy level were calculated to be 0.86×10−20 cm2 (E∥a), 0.79×10−20 cm2 (E∥b) and 0.78×10−20 cm2 (E∥c), with FWHM of 57.54 nm, 57.96 nm and 58.96 nm, respectively. The fluorescence lifetime and radiative lifetime of the 1D2 energy levels of Pr3+ ions were 45.46 µs and 393.94 µs, respectively. Those results show that the Pr:GdScO3 crystal should be the potential near infrared laser gain medium originated from the 1D2 transition.

Sr2Mg3P4O15:R (R = Bi3+, Mn2+, and Bi3+/Mn2+) phosphors are synthesized by a solid-state reaction method in air. Sr2Mg3P4O15:Bi3+ phosphor with excitation 380 nm shows blue light and its emission band peaking at ∼445 nm is observed. Under excitation at 355 and 416 nm, Sr2Mg3P4O15:Mn2+ phosphor shows red emission with a single broad emission band peaking at ∼630 nm in the range of 500–800 nm owing to the 4T1(G) → 6A1 transition of the Mn2+ ion. The optimal Mn2+ ion content in Sr2Mg3P4O15:Mn2+ phosphor is ∼0.05 and the lifetime of Sr2Mg2.95P4O15:0.05Mn2+ phosphor is ∼6.17 ms. After Bi3+ ion is co-doped, emission intensity of Sr2Mg3P4O15:Mn2+ phosphor can be enhanced obviously due to energy transfer (ET) from Bi3+ to Mn2+, which is confirmed by the luminescence properties and lifetimes of Sr2Mg3-xP4O15:Bi3+, Mn2+ phosphor. Luminous mechanism and ET process of Sr2Mg3-xP4O15:Bi3+, Mn2+ phosphor are explained by the simple energy level diagram of Bi3+ and Mn2+ ions.

ZrTe$_5$ has been of recent interest as a potential Dirac/Weyl semimetal material. Here, we report the results of experiments performed via in-situ 3D double-axis rotation to extract the full $4\pi$ solid angular dependence of the transport properties. A clear anomalous Hall effect (AHE) was detected for every sample, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Interestingly, the AHE takes large values when the magnetic field is rotated in-plane, with the values vanishing above $\sim 60$ K where the negative longitudinal magnetoresistance (LMR) also disappears. This suggests a close relation in their origins, which we attribute to Berry curvature generated by the Weyl nodes., Comment: 7 pages, 4 figures

NaY(MoO 4 ) 2 :R (R = Sm 3+ /Bi 3+ , Tb 3+ /Bi 3+ , Sm 3+ /Tb 3+ ) phosphors are synthesized by solid-state reaction method in air. The crystal structures, excitation and emission spectra, luminescence properties, the lifetimes, and the dopant concentration influences are investigated. The emission intensity of activators Sm 3+ and Tb 3+ can be enhanced ∼3.04 and 1.64 times due to energy transfer (ET) from Bi 3+ to Sm 3+ /Tb 3+ . NaY(MoO 4 ) 2 :Sm 3+ , Tb 3+ phosphor with excitation 300 nm shows a tunable emission from red to yellow with change Sm 3+ or Tb 3+ ion concentration. Emission intensity of NaY(MoO 4 ) 2 :Sm 3+ phosphor can be enhanced ∼2.24 times via co-doping Tb 3+ ion owing to ET from Tb 3+ to Sm 3+ ions. The results indicate that ET between sensitizer and activator (Bi 3+ → Tb 3+ , Bi 3+ → Sm 3+ , and Tb 3+ → Sm 3+ ) is beneficial to improve the luminescence properties of activator (Sm 3+ and Tb 3+ ). ET and luminous meachanisms are explained by energy level diagrams of Bi 3+ , Tb 3+ , and Sm 3+ ions.

A series of 0.6 at% Pr:CaF2-RF3 (R = Y3+, Gd3+, Lu3+) crystals have been successfully grown by the temperature gradient technique(TGT). The absorption spectra, fluorescence spectra, and fluorescence decay curves were measured and analyzed at room temperature. When the R3+ concentration is higher than a certain value, the diffraction peaks of Pr:CaF2-RF3 crystals were well indexed to the (Ca,Y)F2 phase. By co-doping R3+ ion (R=Y3+, Gd3+, Lu3+), the spectral performance of the crystals could be significantly improved and the function effect is also different. For 0.6 at% Pr, 28.5 at% Y:CaF2 crystal, the fluorescence lifetime of 3P0 energy level was fitted to be 45.46 µs, and the emission cross sections of 3P0→3H6 (605 nm), and 3P0→3F2 (641 nm) were calculated to be 1.78 × 10−20 cm2, and 4.61 × 10−20 cm2 with the full width at half (FWHM) of 20.1 nm and 6.8 nm, respectively. The 0.6 at% Pr, 28.5 at% Y:CaF2 crystal could be favorble laser medium for ultra-fast laser opreration at 605 nm and 641 nm. For 0.6 at% Pr, 18 at% Gd:CaF2 and 0.6 at% Pr, 10 at% Lu:CaF2 crystals, the spectral quality factors σem•τf of 3P0→3F2 transition (642 nm) were calculated to be 614.18 × 10−20 cm2•µs and 506.36 × 10−20 cm2•µs with the FWHM of 3.7 nm and 5.3 nm. All the results show that 0.6 at% Pr, 18 at% Gd:CaF2 and 0.6 at% Pr, 10 at% Lu:CaF2 crystals are promising gain media for high-efficiency visible laser output at 642 nm, which would compete with commercial Pr: YLF crystal in the future.

Ho3+-doped GdScO3 crystal was successfully grown by the edge-defined film-fed growth (EFG) method. The spectroscopic properties of Ho:GdScO3 crystal were investigated at room temperature. The peak absorption cross-sections were 0.33 × 10−20 cm2 and 1.24 × 10−20 cm2 at 1160 nm and 653 nm with full widths at half maximum of 171.7 nm and 11.5 nm. The peak emission cross-sections of 5I7→5I8 and 5I6→5I7 transitions for the crystal were 0.49 × 10−20 cm2 and 1.24 × 10−20 cm2, respectively. The fluorescence lifetimes of the 5I6 and 5I7 multiplets were measured to be 1.12 ms and 6.13 ms, respectively. The results show that Ho:GdScO3 crystal is a very promising material for 3 μm laser operation.

Ho3+, Pr3+: Sc2O3 crystal was successfully grown by the TGT method. The spectroscopic properties of the crystal were analyzed. The absorption cross section of Ho, Pr: Sc2O3 at 636 nm was 1.58×10–20 cm2. J-O theory was applied to analyze the fluorescence properties of the grown crystal at 2.9 μm. The intensity parameters Ωt (t=2,4,6), radiative transition rates, branching ratios and radiative lifetime were calculated. Ho, Pr: Sc2O3 exhibited strong emission at 2110 nm and 2864 nm with a stimulated emission cross section of 2.4 × 10–21 cm2 and 4.2 × 10–21 cm2, respectively. The fluorescence lifetime of the Ho3+ 5I6 and 5I7 levels were measured to be 1.64 ms and 5.74 ms. These results show that the Ho, Pr: Sc2O3 crystal would be a potential laser material at the 2.9 μm region.

A series of CaZn2P2O8:Sm3+, Mn2+ phosphors are synthesized by a solid-state reaction method in air. The crystal structures and luminescence properties are investigated. CaZn2P2O8:Sm3+ phosphor with excitation 402 nm emits red light in the range of 550–750 nm owing to the 4G5/2 → (6H5/2, 6H7/2, 6H9/2, and 6H11/2) transitions of Sm3+ ion. CaZn2P2O8:Mn2+ phosphor with excitation 400 nm emits yellow-green light in the range of 500–650 nm due to the 4T1(G) → 6A1 transition of the Mn2+ ion. CaZn2P2O8:Sm3+, Mn2+ phosphor with excitation 400 nm shows a tunable emission in the range of 500–750 nm with changing Sm3+ or Mn2+ ion content. The optimal Sm3+ and Mn2+ ion concentrations are ∼4 mol% and 3 mol%, respectively.

Novel Li 2 SnO 3 :Mn 4+ red-emitting phosphor is synthesized by high-temperature solid-state reaction method in air. Emission band of Li 2 SnO 3 :Mn 4+ phosphor within the range 620–760 nm is attributed to the 2 E → 4 A 2 electron transition of Mn 4+ ion. Excitation bands peaking at 330, 395, and 480 nm are attributed to O 2- → Mn 4+ charge transfer and 4 A 2 → ( 4 T 1 , 2 T 1 , and 4 T 2 ) transitions of Mn 4+ ion, respectively. The optimal Mn 4+ concentration is ∼0.4 mol% and the corresponding quantum efficiency and lifetime are ∼42.3% and 16.3 μs. The influences of temperature and Mn 4+ concentration to emission intensity are discussed and analysized. Lifetime data and time resolved spectra confirm that there is only a single type of Mn 4+ ion luminescent center in Li 2 SnO 3 :Mn 4+ phosphor. The influences of the crystal field and the “Mn 4+ -ligand” bonding to luminescence properties of Mn 4+ ion are summaried and analysized. The luminous mechanism is explained via Tanabe-Sugano energy level diagram and the configuration coordinate diagram of Mn 4+ in the octahedron. The results indicate that Li 2 SnO 3 :Mn 4+ phosphor has a hope to be used as red phosphor candidate for solid-state lighting.

Novel deep red-emitting Ca3Al4ZnO10:Mn4+ (CAZO:Mn4+) phosphor is synthesized by high-temperature solid-state reaction method in air. The crystal structures and optical characterizations of the phosphors are described. X-ray diffraction patterns indicate that all samples are a pure phase. CAZO:Mn4+ phosphor with excitation 330 and 465 nm exhibits strong deep red emission in the range of 650–790 nm. CAZO:Mn4+ phosphor can be efficiently excited by near-ultraviolet and blue light from 300 to 500 nm. The optimal Mn4+ ion concentration in CAZO:Mn4+ phosphor is ∼0.6 mol%. Lifetimes of CAZO:xMn4+ (0.3 ≤ x ≤ 1.8 mol%) phosphors decrease from 1.95 to 1.56 ms with increasing Mn4+ ion concentration in the range of 0.3–1.8 mol%. The paper content is help to develop other Mn4+ ion doped luminescence materials.

Li2Mg3SnO6:Mn4+ deep red phosphor is synthesized by high-temperature solid-state reaction method in air. With excitation 330 and 480 nm, the emission band peaking at ∼670 nm within the range 600–790 nm is observed owing to the 2E → 4A2 transition of Mn4+ ion. Excitation bands peaking at ∼330, 395, and 480 nm in the range of 220–600 nm indicate that Li2Mg3SnO6:Mn4+ phosphor can be effectively excited by (near) ultraviolet and blue light-emitting diode chips. The optimal Mn4+ ion concentration is ∼0.4 mol%. Fluorescence lifetime and quantum efficiency of Li2Mg3SnO6:0.4%Mn4+ phosphor with excitation 330 nm are ∼266.67 μs and 36.3%, respectively. The luminous mechanism is explained by the Tanabe-Sugano energy level diagram of Mn4+ in the octahedron.

CaSnO3 :Bi3+ blue-emitting phosphor was synthesized using a high-temperature solid-state reaction method in air. The crystal structures and luminescence properties were investigated. A broad emission band peaking at ~448 nm upon excitation at 262 and 308 nm was observed in the range 330-680 nm at room temperature due to 3 P1 → 1 S0 transition of the Bi3+ ion. The chromaticity coordinate was (0.1786, 0.1665). The optimal Bi3+ ion concentration was ~0.6 mol% in CaSnO3 :Bi3+ phosphor. The emission spectrum of CaSnO3 :Bi3+ phosphor showed a blue-shift with increasing temperature from 50 to 300 K due to the influence of temperature on the electron transition of the Bi3+ ion. The emission intensity of CaSnO3 :Bi3+ phosphor may be increased ~1.45 times by co-doping Li+ ions as a charge compensator and fluxing agent. The luminescence mechanism is explained by a configurational coordinate diagram of Bi3+ ion in CaSnO3 :Bi3+ phosphor.

Edge supercurrents in superconductors have long been an elusive target. Interest in them has reappeared in the context of topological superconductivity. We report evidence for the existence of a robust edge supercurrent in the Weyl superconductor molybdenum ditelluride (MoTe

Yb,Pr:CaF2 crystals have been grown successfully by the temperature gradient technique(TGT). The absorption spectra, fluorescence spectra, fluorescence decay lifetime of Pr3+ ions in CaF2 crystal sensitized by different Yb3+ ions concentration have been measured. The effective J-O intensity parameters were fitted to be Ω2=0.36 × 10−20cm2, Ω4=2.89 × 10−20cm2, Ω6=6.69 × 10−20cm2 for 0.3 at.%Pr,1.5 at.%Yb:CaF2. And the emission cross sections of 3P1→3H5 (535 nm), and 3P0→3F4 (750 nm) are calculated to be 2.02 × 10−20cm2, and 2.50 × 10−20cm2 with the FWHM of 17.2 nm, and 10.5 nm, respectively. The fluorescence lifetime of 3P0 has achieved great increasing by sensitizing of Yb3+ ion. Not only the up-conversion luminescence can be achieved, but also the broad band emission from 1D2 energy level can be observed. The results show that Yb,Pr:CaF2 could be a potential laser gain medium for visible and near-IR laser operation.

CaZr(PO4)2:Mn2+ and CaZr(PO4)2:Mn2+, Sm3+ phosphors are synthesized by high-temperature solid state reaction method in air. CaZr(PO4)2:Mn2+ phosphor shows green emission and its broad emission band peaking at ~545 nm within the range 470–650 nm is attributed to the 4T1(G) → 6A1(6S) transition of Mn2+ ion. The optimal Mn2+ doping concentration is ~5 mol% in CaZr(PO4)2:Mn2+ phosphor. CaZr(PO4)2:Mn2+, Sm3+ phosphor with excitation 399 nm emits reddish-orange light, and it shows a systematically varied hue from green to yellow with excitation 405 nm by changing Sm3+ and Mn2+ ions concentration. The luminous mechanism of CaZr(PO4)2:Mn2+, Sm3+ phosphor is analyzed and explained by the simplified energy level diagrams of Mn2+ and Sm3+ ions in CaZr(PO4)2:Mn2+, Sm3+ phosphor. The experimental results are helpful to develop new Mn2+/Sm3+-doped materials.

Breaking chiral symmetry in a solid Dirac semimetals have graphene-like electronic structure, albeit in three rather than two dimensions. In a magnetic field, their Dirac cones split into two halves, one supporting left-handed and the other right-handed fermions. If an electric field is applied parallel to the magnetic field, this “chiral” symmetry may break: a phenomenon called the chiral anomaly. Xiong et al. observed this anomaly in the Dirac semimetal Na 3 Bi (see the Perspective by Burkov). Transport measurements lead to the detection of the predicted large negative magnetoresistance, which appeared only when the two fields were nearly parallel to each other. Science , this issue p. 413 , see also p. 378

CaSnO

A striking prediction in topological insulators is the appearance of the quantized Hall resistance when the surface states are magnetized. The surface Dirac states become gapped everywhere on the surface, but chiral edge states remain on the edges. In an applied current, the edge states produce a quantized Hall resistance that equals the Chern number C = +1 or -1 (in natural units), even in zero magnetic field. This quantum anomalous Hall effect was observed by Chang et al. With reversal of the magnetic field, the system is trapped in a metastable state because of magnetic anisotropy. Here we investigate how the system escapes the metastable state at low temperatures (10-200 mK). When the dissipation (measured by the longitudinal resistance) is ultralow, we find that the system escapes by making a few, very rapid transitions, as detected by large jumps in the Hall and longitudinal resistances. Using the field at which the initial jump occurs to estimate the escape rate, we find that raising the temperature strongly suppresses the rate. From a detailed map of the resistance vs. gate voltage and temperature, we show that dissipation strongly affects the escape rate. We compare the observations with dissipative quantum tunneling predictions. In the ultralow dissipation regime, there exist two temperature scales T1 = 70 mK and T2 = 145 mK between which jumps can be observed. The jumps display a spatial correlation that extends over a large fraction of the sample., 18 pages, 9 figures

A long-standing issue in topological insulator research has been to find a bulk single crystal material that provides a high-quality platform for characterizing topological surface states without interference from bulk electronic states. This material would ideally be a bulk insulator, have a surface state Dirac point energy well isolated from the bulk valence and conduction bands, display quantum oscillations from the surface state electrons and be growable as large, high-quality bulk single crystals. Here we show that this material obstacle is overcome by bulk crystals of lightly Sn-doped Bi1.1Sb0.9Te2S grown by the vertical Bridgman method. We characterize Sn-BSTS via angle-resolved photoemission spectroscopy, scanning tunnelling microscopy, transport studies, X-ray diffraction and Raman scattering. We present this material as a high-quality topological insulator that can be reliably grown as bulk single crystals and thus studied by many researchers interested in topological surface states., An ideal topological insulator possesses an insulating bulk and a unique conducting surface however such behaviour is typically inhibited by bulk conduction due to defects. Here, the authors show that Sn-doped Bi1.1Sb0.9Te2S grown by the vertical Bridgman technique might overcome this hurdle.