Sr Atoms Research Articles

Dual-Mode Operation of an Optical Lattice Clock Using Strontium and Ytterbium Atoms.

We have developed an optical lattice clock that can operate in dual modes: a strontium (Sr) clock mode and an ytterbium (Yb) clock mode. Dual-mode operation of the Sr-Yb optical lattice clock is achieved by alternately cooling and trapping 87Sr and 171Yb atoms inside the vacuum chamber of the clock. Optical lattices for Sr and Yb atoms were arranged with horizontal and vertical configurations, respectively, resulting in a small distance of the order of between the trapped Sr and Yb atoms. The 1S0-3P0 clock transitions in the trapped atoms were interrogated in turn and the clock lasers were stabilized to the transitions. We demonstrated the frequency ratio measurement of the Sr and Yb clock transitions by using the dual-mode operation of the Sr-Yb optical lattice clock. The dual-mode operation can reduce the uncertainty of the blackbody radiation shift in the frequency ratio measurement, because both Sr and Yb atoms share the same blackbody radiation.

M-SrFe12O19 and ferrihydrite-like ultrathin nanoplatelets as building blocks for permanent magnets: HAADF-STEM study and magnetic properties

Mixtures of M-type strontium hexaferrite (M-SrFe12O19) and ferrihydrite-like particles were prepared by a microwave-assisted hydrothermal process at 200 °C with heating rates in the range 40–50 °C min−1. The particles exhibited a platelet shape with a diameter comprised between 20 and 200 nm and a thickness between 2 and 5 nm. HAADF-STEM observations and EDS analysis were carried out for a better understanding of nucleation and growth process. EDS showed that most of the particles contained Sr and HAADF-STEM revealed that very thin particles with a hexaferrite core extending over less than a unit cell and with surface disorder crystallized along with well crystallized hexaferrite and defect free ferrihydrite particles. The symmetric multilayer structures (SRS) of the ultrathin particles suggested that the nucleation step of the hexaferrite particles involved clusters containing Sr atoms. In comparison with the M-SrFe12O19 micrometer sized platelets prepared with heating rate of 25 °C min−1, the mixtures of ultrathin hexaferrite- and ferrihydrite-like particles combined after annealing a higher coercivity reaching 465 kA m−1 thanks to the smaller initial particle size and a high magnetization reaching 65 A m2 kg−1 thanks to a limited amount of hematite.

Frequency Measurements of Superradiance from the Strontium Clock Transition

We present the first characterization of the spectral properties of superradiant light emitted from the ultra-narrow, 1 mHz linewidth optical clock transition in an ensemble of cold $^{87}$Sr atoms. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. By comparing the frequency of our superradiant source to that of a state-of-the-art cavity-stabilized laser and optical lattice clock, we observe a fractional Allan deviation of $6.7(1)\times 10^{-16}$ at 1 second of averaging, establish absolute accuracy at the 2 Hz ($4\times 10^{-15}$ fractional frequency) level, and demonstrate insensitivity to key environmental perturbations.

Rydberg-Dressed Magneto-optical Trap.

We propose and demonstrate the laser cooling and trapping of Rydberg-dressed Sr atoms. By off-resonantly coupling the excited state of a narrow (7kHz) cooling transition to a high-lying Rydberg state, we transfer Rydberg properties such as enhanced electric polarizability to a stable magneto-optical trap operating at <1 μK. Simulations show that it is possible to reach a regime where the long-range interaction between Rydberg-dressed atoms becomes comparable to the kinetic energy, opening a route to combining laser cooling with tunable long-range interactions.

Influence of metal work function and incorporation of Sr atom on WO3 thin films for MIS and MIM structured SBDs

In this work, two different structure of Cu/Sr-WO3/p-Si metal-insulator-semiconductor (MIS) and Cu/Sr-WO3/FTO metal-insulator-metal (MIM) Schottky barrier diodes (SBDs) fabricated with an insulating layer of pure tungsten trioxide (WO3) and Sr-WO3 thin films have been reported. The Sr-WO3 layer was coated separately, with different concentrations (0, 4, 8 and 12 wt %) of strontium (Sr) via jet nebulizer spray pyrolysis technique (JNSP) on the p-type silica wafer (p-Si) and fluorine doped tin oxide (FTO) substrates which are been optimized at 400 °C. The XRD analysis reveals the multiphase crystalline structures for 12 wt % of Sr-WO3 film with higher average crystallite size. FE-SEM images show the randomly oriented sub-microsized slab and seashell like structures. Higher surface roughness with improved grain size for 12 wt % of Sr-WO3 film. The presence of W, O and Sr atoms was confirmed by EDX spectra. In optical studies, Maximum absorption with minimum optical band gap was observed for 12 wt % of Sr-WO3 composite film. There was a linear increase in the electrical conductivity of the films with higher wt. % of Sr. Evidently the activation energy decreased with Sr concentration which is in accordance with the bandgap values. The fitting results of the measured I-V, reveal that MIS (SBDs) under illumination condition have minimum ideality factor (n = 2.39) and maximum barrier height (Φb = 0.57) values for higher concentration (12 wt %) of Sr film compared to MIM SBDs.

The activation of reactants and intermediates promotes the selective photocatalytic NO conversion on electron-localized Sr-intercalated g-C3N4

Photocatalysis technology has been widely adapted to address air pollution. However, the photocatalysis efficiency and selectivity should be optimized to achieve efficient and safe air purification. Take the g-C3N4 (CN) as a case study, in order to promote the photocatalytic performance and the selectivity of g-C3N4 in oxidizing NO into target products of NO2− and NO3−, the electron localization has been built in Sr-intercalated g-C3N4 (CN-Sr) to promote the activation of reactants and intermediates, as well as the charge separation and transfer. The intercalated Sr atom causes the uneven electron distribution on the plane of CN. The O2 could capture the localized excess electrons and be activated producing O2− radicals. The NO and the reaction intermediates could deplete the electrons more easily and be activated on the surface of CN-Sr. The activated species possess longer bond length and have higher reactivity during photocatalysis, making them easier to be destroyed by active radicals and transforming to target products of NO2− and NO3− rather than other toxic byproducts. With the pivotal effect of localized electrons in CN-Sr, the photocatalytic activity and selectivity can be simultaneously promoted. With the in situ DRIFTS investigation and theoretical calculation, the present work specified the transportation and transformation of photogenerated carriers and revealed the mechanism of photocatalytic NO oxidation. This work could provide a new approach to enhance the photocatalytic activity and selectivity for efficient and safe air pollution control.

Ab-initio investigations on physisorption of alkaline earth metal atoms on monolayer hexagonal boron nitride (h-BN)

First-principles density functional theory (DFT) calculations are performed on the structural, electronic, magnetic and optical properties of alkaline earth metal (AEM) atoms-adsorbed hexagonal boron nitride (h-BN) structures. Different AEM atoms were adsorbed on the hollow (H) and bridge sites of monolayer h-BN and their effects on aforementioned properties were then investigated. Calculated adsorption energies indicate that, the physisorption of AEM atoms on h-BN layer is thermodynamically favorable. It is also observed that, the charge transfer occurs from AEM atoms to the h-BN layer. When AEM atoms were adsorbed at H-site of h-BN layer, only Mg and Sr atom adsorption introduced significant magnetic moments of 2.0 μB and 1.0 μB, respectively. During AEM atoms adsorption at bridge site, Ca, Mg and Sr atoms adsorption induced 1.92 μB, 1.98 μB and 0.712 μB magnetic moments in h-BN layer, respectively. Through electronic structure calculations, it is observed that, AEM atom adsorption on h-BN significantly modifies its band structure, by converting wide bandgap h-BN semiconductor to the half metal and/or semimetal. Through density of states (DOS) plots, it is revealed that, s and p orbitals of AEM atoms are mainly responsible for arising of magnetic moments in monolayer h-BN. Finally, the optical parameters specifically, absorption coefficient and reflectivity plots for AEM atoms adsorbed h-BN structures were obtained using DFT within random phase approximation (RPA). Pure h-BN layer presents zero absorption coefficient and low static reflectivity in low lying energy range. However, AEM atom adsorption on h-BN layer produces an increment in absorption coefficient quantity in 0–4 eV energy interval. Similarly, the higher reflectivity parameter is obtained in lower energy range. These obtained results predict that, the AEM atom adsorption on monolayer h-BN carries potential applications for nanoelectronics, spintronic and optoelectronic device applications.

Varying the Composition of Ferroelectric Films during Ion-Plasma Sputtering: Simulation and Experiment

Varying the elemental composition of ferroelectric thin films of barium strontium titanate, barium zirconate titanate, and barium stannate titanate solid solutions obtained by ion-plasma sputtering of ceramic targets at different pressures of the working gas is studied through simulation and experimentally. The experimental results are interpreted based on statistical simulation of the transport processes for fluxes of sputtered Ba, Sr, Sn, Zr, and Ti atoms in a gas medium. It is shown that the simulation results describe variations in the elemental composition satisfactorily (within 5%) in a wide range of working gas pressures. Theoretical and experimental studies enable us to choose the optimum values of technological parameters for obtaining ferroelectric thin films with specified elemental compositions that determine their electrophysical properties.

Quantitative Analysis of the Incorporation Behaviors of Sr and Ti Atoms During the Atomic Layer Deposition of SrTiO3 Thin Films.

The atomic layer deposition (ALD) of multication oxide films is complicated because the deposition behaviors of the component oxides are not independent of one another. In this study, the Ti and Sr atom incorporation behaviors during the ALD of SrTiO3 films were quantitatively examined via the carefully designed ALD process sequences. H2O and O3 were adopted as the oxygen sources of the SrO subcycles, whereas only O3 was used for the TiO2 ALD subcycles. Apart from the general conjecture on the roles of the different types of oxygen sources, the oxygen source that was adopted for the subcycles of the other component oxide had almost complete control of the metal atom incorporation behaviors. This means that the first half-cycle of ALD played a dominant role in determining the metal incorporation rate, which revealed the critical role of the steric hindrance effect during the metal precursor injection for the ALD rate. O3 had almost doubled its reactivity toward the Ti and Sr precursors compared with H2O. Although these are the expected results from the common knowledge on ALD, the quantitative analysis of the incorporation behaviors of each metal atom provided insightful viewpoints for the ALD process of this technically important oxide material. Furthermore, the SrTiO3 films with a bulk dielectric constant as high as 236 were obtained by the Ru-SrTiO3-RuO2 capacitor structure.

Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp.

We report on the demonstration of Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp (HCL). We employed a custom Sr HCL, which is filled with a mixture of 0.5 Torr Ne and 0.5 Torr Xe as a buffer gas to suppress velocity changing collisions and increase the populations in all of the (5s5p)3PJ(J=0,1,2) metastable states. We performed frequency modulation spectroscopy for the (5s5p)3P0-(5s6s)3S1, (5s5p)3P1-(5s6s)3S1, (5s5p)3P2-(5s5d)3D2, and (5s5p)3P2-(5s5d)3D3 transitions with sufficient signal-to-noise ratios for laser frequency stabilization. We also observed the hyperfine transitions of (5s5p)3P2-(5s5d)3D3 of Sr87. This method would greatly facilitate laser cooling of Sr.

Crystal structure and electronic properties of Sr-substituted barium disilicide Ba1-xSrxSi2 for solar cells: Computational and experimental studies

The effects of Sr substitution in BaSi2, a promising photoabsorber for thin-film solar cells, were examined computationally and experimentally. The calculations showed that Ba0.5Sr0.5Si2 with Sr atoms at the A1 site (A1-Sr-BSS) is more stable than that with Sr atoms at the A2 site (A2-Sr-BSS). Both materials are indirect bandgap semiconductors. The indirect and direct bandgaps of A1-Sr-BSS are larger than those of BaSi2, while those of A2-Sr-BSS are smaller. The indirect bandgap of A1-Sr-BSS is 0.01 eV higher than that of BaSi2, while that of A2-Sr-BSS is 0.06 eV lower. This trend is attributable to the difference in the electronic properties around the conduction band minimum, which is determined by the type of atoms (Ba or Sr) that occupy the A1 site. Single-crystal X-ray diffraction and diffuse reflectance measurements of Ba1-xSrxSi2 (0.0 ≤ x ≤ 0.8) revealed that the Ba atoms at the crystallographic site A1 are preferentially substituted by Sr atoms and that the bandgap, Eg, decreases with x (1.24 eV at x = 0 and 1.15 eV at x = 0.65). The preferential occupation of the A1 site by Sr atoms is consistent with the computational result that A1-Sr-BSS is more stable than A2-Sr-BSS. However, the experimentally observed decrease in Eg with x is closer to the decrease of −0.06 eV in the indirect bandgap of A2-Sr-BSS than to the 0.01 eV increase in the case of A1-Sr-BSS. The reason for this discrepancy is discussed.

Tuning the electronic properties of LaAlO3 / SrTiO3 interfaces by irradiating the LaAlO3 surface with low-energy cluster ion beams

We have investigated the effects of low-energy ion beam irradiations using argon clusters on the chemical and electronic properties of LaAlO3/SrTiO3 (LAO/STO) heterointerfaces by combining X-ray photoelectron spectroscopy (XPS) and electrical transport measurements. Due to its unique features, we show that a short-time cluster ion irradiation of the LAO surface induces indirect modifications in the chemical properties of the buried STO substrate, with (1) a lowering of Ti atoms oxidation states (from Ti4+ to Ti3+ and Ti2+) correlated to the formation of oxygen vacancies at the LAO surface and (2) the creation of new surface states for Sr atoms. Contrary to what is observed by using higher energy ion beam techniques, this leads to an increase of the electrical conductivity at the LAO/STO interface. Our XPS data clearly reveal the existence of dynamical processes on the titanium and strontium atoms, which compete with the effect of the cluster ion beam irradiation. These relaxation effects are in part attributed to the diffusion of the ion-induced oxygen vacancies in the entire heterostructure, since an increase of the interfacial metallicity is also evidenced far from the irradiated area. These results demonstrate that a local perturbation of the LAO surface can induce new properties at the interface and in the entire heterostructure. This study highlights the possibility of tuning the electronic properties of LAO/STO interfaces by surface engineering, confirming experimentally the intimate connection between LAO surface chemistry and electronic properties of LAO/STO interfaces.

New, Efficient, and Reliable Air Electrode Material for Proton-Conducting Reversible Solid Oxide Cells.

Driven by the demand to minimize fluctuation in common renewable energies, reversible solid oxide cells (RSOCs) have drawn increasing attention for they can operate either as fuel cells to produce electricity or as electrolysis cells to store electricity. Unfortunately, development of proton-conducting RSOCs (P-RSOCs) faces a major challenge of poor reliability because of the high content of steam involved in air electrode reactions, which could seriously decay the lifetime of air electrode materials. In this work, a very stable and efficient air electrode, SrEu2Fe1.8Co0.2O7-δ (SEFC) with layer structure, is designed and deployed in P-RSOCs. X-ray diffraction analysis and High-angle annular dark-filed scanning transmission electron microscopy images of SEFC reveal that Sr atoms occupy the center of perovskite slabs, whereas Eu atoms arrange orderly in the rock-salt layer. Such a special structure of SEFC largely depresses its Lewis basicity and therefore its reactivity with steam. Applying the SEFC air electrode, our button switches smoothly between both fuel cell and electrolysis cell (EC) modes with no obvious degradation over a 135 h long-term test under wet H2 (∼3% H2O) and 10% H2O-air atmospheres. A record of over 230 h is achieved in the long-term stability test in the EC mode, doubling the longest test that had been previously reported. Besides good stability, SEFC demonstrates great catalytic activity toward air electrode reactions when compared with traditional La0.6Sr0.4Co0.2Fe0.8O3-δ air electrodes. This research highlights the potential of stable and efficient P-RSOCs as an important part in a sustainable new energy power system.

Possible structural origin of superconductivity in Sr-doped Bi2Se3

Doping bismuth selenide (Bi2Se3) with elements such as copper and strontium (Sr) can induce superconductivity, making the doped materials interesting candidates to explore potential topological superconducting behaviors. It was thought that the superconductivity of doped Bi2Se3 was induced by dopant atoms intercalated in van der Waals gaps. However, several experiments have shown that the intercalation of dopant atoms may not necessarily make doped Bi2Se3 superconducting. Thus, the structural origin of superconductivity in doped Bi2Se3 remains an open question. Herein, we combined material synthesis and characterization, high-resolution transmission electron microscopy, and first-principles calculations to study the doping structure of Sr-doped Bi2Se3. We found that the emergence of superconductivity is strongly related with n-type dopant atoms. Atomic-level energy-dispersive X-ray mapping revealed various n-type Sr dopants that occupy intercalated and interstitial positions. First-principles calculations showed that the formation energy of a specific interstitial Sr doping position depends strongly on Sr doping level. This site changes from a metastable position at low Sr doping level to a stable position at high Sr doping level. The calculation results explain why quenching is necessary to obtain superconducting samples when the Sr doping level is low and also why slow furnace cooling can yield superconducting samples when the Sr doping level is high. Our findings suggest that Sr atoms doped at interstitial locations, instead of those intercalated in van der Waals gaps, are most likely to be responsible for the emergence of superconductivity in Sr-doped Bi2Se3.

Excited-state population distributions of alkaline-earth metal in a hollow cathode lamp

The intensities of fluorescence spectral lines of Ca atoms and Sr atoms in two different hollow cathode lamps (HCLs) are measured by element-balance-detection technology. In the wavelength range of 350–750 nm in the visible spectral region, using the individual strongest line (Ca 422.67 nm, Sr 460.73 nm) as the bench mark, the population ratios between the excited states of Ca atoms and Sr atoms are calculated by rate equations and the spontaneous transition probabilities. The HCLs with populations at excited states can be used to realize the frequency stabilization reference of the laser frequency standard.

The RbSr 2Σ+ ground state investigated via spectroscopy of hot and ultracold molecules.

We report on spectroscopic studies of hot and ultracold RbSr molecules, and combine the results in an analysis that allows us to fit a potential energy curve (PEC) for the X(1)2Σ+ ground state bridging the short-to-long-range domains. The ultracold RbSr molecules are created in a μK sample of Rb and Sr atoms and probed by two-colour photoassociation spectroscopy. The data yield the long-range dispersion coefficients C6 and C8, along with the total number of supported bound levels. The hot RbSr molecules are created in a 1000 K gas mixture of Rb and Sr in a heat-pipe oven and probed by thermoluminescence and laser-induced fluorescence spectroscopy. We compare the hot molecule data with spectra we simulated using previously published PECs determined by three different ab initio theoretical methods. We identify several band heads corresponding to radiative decay from the B(2)2Σ+ state to the deepest bound levels of X(1)2Σ+. We determine a mass-scaled high-precision model for X(1)2Σ+ by fitting all data using a single fit procedure. The corresponding PEC is consistent with all data, thus spanning short-to-long internuclear distances and bridging an energy gap of about 75% of the potential well depth, still uncharted by any experiment. We benchmark previous ab initio PECs against our results, and give the PEC fit parameters for both X(1)2Σ+ and B(2)2Σ+ states. As first outcomes of our analysis, we calculate the s-wave scattering properties for all stable isotopic combinations and corroborate the locations of Fano-Feshbach resonances between alkali Rb and closed-shell Sr atoms recently observed [V. Barbéet al., Nat. Phys., 2018, 14, 881]. These results and more generally our strategy should greatly contribute to the generation of ultracold alkali-alkaline-earth dimers, whose applications range from quantum simulation to state-controlled quantum chemistry.

Simulation Calculation and Analysis of Electronic Structure and Electrical Properties of Metal‐Doped SnO2

SnO2 is an almost insulated semiconductor material, which increases the contact resistance of the AgSnO2 electrical contact material. Therefore, by improving the electrical performance of SnO2, the electrical properties of the AgSnO2 can be optimized. The first principle method based on density functional theory is used to calculate the electronic structure, formation energy, band structure, density of states, and differential charge density of SnO2 doped with the metals Ti, Sr, Ge, Sb, and Ga. The results show that metal‐doped SnO2 materials are still direct bandgap semiconductor materials, and the effect of the electronic states of the metallic elements enhances the localization of the energy band, decreases the bandgap, increases the carrier concentration at the Fermi level, and enhances the electrical performance of the materials, and the bandgap of Ga‐doped SnO2 is the smallest, 0.041 eV. And the charge transfer between Sb, Sr, Ga, Ti, and Ge metal atoms and O atoms increases, especially between Ga atom and O atom; that is, the electrical performance of Ga doping is better.

Efficient enhancement of thermoelectric performance of CdTe via dilute hole doping together with heavy isoelectronic doping

Thermoelectric performances of p type and isoelectronically doped CdTe are investigated using first principles method and Boltzmman transport theory. For the supercell Cd32Te32, when one N atom is doped into it, the maximal S and ZeT are only 156 μV/K and 0.76, respectively, but the ZeT values are much smaller at 300–700 K for P, As and Sb dopings. However, if eight Sr atoms are further doped into Cd32Te31M (M= pnicogen) to substitute Cd atoms, all the four maximal S values exceed 242 μV/K, and the ZeT reaches 1.5 at 1000 K for N doping, but for P, As and Sb dopings the values do not exceed 1.0. The large S arises from the steep but low DOS at EF. The enhancement in ZeT results from the S dominating over σ/κe. The number and distribution of Sr atoms have an impact on the ZeT values. Adding one or two O atoms further can hoist the ZeT at 700 K and 800 K significantly. When one Cl atom and one Na atom are doped into Cd24Sr8Te31N, the ZeT reaches 1.7 at 1000 K. The results show that hole doping combined with heavily isoelectronic doping is efficient on improving the electronic figures of merit of semiconductors if appropriate doped elements are chosen. The way of doping assumed here should be applicable for other semiconductor systems.

Tip-induced band bending on Sr/Si(100)-2×3 reconstructed surface

Using Vary-temperature Scanning Tunnelling Microscopy (VT-STM), the geometric and electronic structures of technological important Sr/Si(100)-2×3 surface were investigated. Two important findings explained in this research. First, a phenomenon similar to quantum corral observed in the empty state STM image that near the bottom of the conduction band. This is aroused from the surface vacancies and phase boundary in the Sr/Si(100)-2×3 surface. And a new Sr/Si(100)-2×6 reconstructed structure coexist with Sr/Si(100)-2×3 surface has been prepared by accurately controlling the annealing temperature, both surface’s geometric structures can be described by dimer vacancy model. Second, tip-induced band bending phenomenon was observed in the Sr/Si(100)-2×3 surface at substrate temperature range from 76K to 300K. Experimental LDOS results from n and p-type silicon substrate confirms 0.3eV up the motion of the valence band minimum compare with bare Si(100)-2×1 surface. Buckled and unbuckled silicon dimer coexisting in the Sr/Si(100)-2×3 reconstructed surface contribute to the tip-induced band bending and energy band gap widen phenomenon. Which confirms that Sr atoms transfer electrons to the nearby silicon dimers, make the first layer silicon dimers kept at unbuckled state.

Exploration of the magnetic-field-induced 3P0–5s2 1S0 forbidden transition in bosonic Sr atom

We present experimental and theoretical studies of the effect of an external magnetic field on the 3P0–5s2 1S0 forbidden transition in the bosonic Sr atom. In our ultra-cold atomic system, the excitation fraction of the forbidden transition was measured under the circumstance of the different magnetic field strengths by using the normalized detection method. Based on the perturbation theory, we calculated the magnetic-field-induced 3P0–5s2 1S0 transition rate as a function of the magnetic field strength. Electronic matrix elements entering the calculation of the transition rate were also used to derive the theoretical excitation fraction as a function of the magnetic field strength. A good agreement was found between the experimental measurements and the calculations. This study should be helpful in evaluating the magnetic field effects on the forbidden transition rate with higher accuracy. Moreover, it can help to understand the ultra-cold atomic interaction in the external magnetic field.