Ziting Li

Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in Layered CeO 1 − x F x FeAs

A series of layered CeO1-xFxFeAs compounds with x=0 to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high T_{c} strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce 4f electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.

Origin of the Spin Density Wave Instability in AFe(2)As(2) (A=Ba,Sr) as Revealed by Optical Spectroscopy

We performed optical spectroscopy measurement on single crystals of BaFe2As2 and SrFe2As2, the parent compounds of FeAs-based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave state, the formation of partial energy gaps was clearly observed with the surprising presence of two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the spin-density-wave instability is more likely to be driven by the Fermi surface nesting of itinerant electrons than a local-exchange mechanism.

Probing the superconducting energy gap from infrared spectroscopy on a Ba(0.6)K(0.4)Fe(2)As(2) single crystal with T(c)=37 K

We performed optical spectroscopy measurement on a superconducting Ba0.6K0.4Fe2As2 single crystal with T{c}=37 K. Formation of the superconducting energy gaps in the far-infrared reflectance spectra below T{c} is clearly observed. A flat and close to unity reflectance is observed roughly below 150 cm;{-1} for T<<T{c}, following an s-wave pairing line shape. A more rapid decrease occurs near 200 cm;{-1}, leading to a peak in the ratio of the reflectance at T<<T{c} over that for T>or=T{c}. We determined the absolute value of the penetration depth at 10 K as lambda approximately 2000+/-80 A. A spectral weight analysis shows that the Ferrell-Glover-Tinkham sum rule is satisfied at low energy scale, less than 6Delta.

Population Redistribution Among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel-ionized nitrogen molecules at various driver wavelengths in the near- and midinfrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground N_{2}^{+}(X^{2}Σ_{g}^{+}) state and the excited N_{2}^{+}(A^{2}Π_{u}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states occur in strong laser fields. The couplings result in a population inversion between the N_{2}^{+}(X^{2}Σ_{g}^{+}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states at wavelengths near 800xa0nm, which is verified by our experimental observation of the amplification of a seed at ∼391u2009u2009nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.

Nernst effect of a new iron-based superconductor LaO1?xFxFeAs

We report the first Nernst effect measurement on a new iron-based superconductor LaO(1-x)F(x)FeAs (x = 0(1). In the normal state, the Nernst signal is negative and very small. Below T(c), a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to the conventional type-II superconductors. The sharp decrease in the Nernst signal at the depairing magnetic field H(c2) is not obvious even for temperatures close to T(c). Furthermore, a clear deviation of the Nernst signal from normal state background and an anomalous suppression of off-diagonal thermoelectric current in the normal state between T(c) and 50K are observed. We argue that this anomaly in the normal state Nernst effect could correlate with the spin-density wave (SDW) fluctuations although the contribution of superconducting fluctuations cannot be excluded.

Backward nitrogen lasing actions induced by femtosecond laser filamentation: influence of duration of gain

We experimentally investigate generation of backward 357 nm N2 laser in a gas mixture of N2/Ar using 800 nm femtosecond laser pulses, and examine the involved gain dynamics based on pump-probe measurements. Our findings show that a minimum duration of gain in the excited N2 molecules is required for generating intense backward nitrogen lasers, which is ~0.8 ns under our experimental conditions. The results shed new light on the mechanism for generating intense backward lasers from nitrogen molecules, which are highly in demand for high sensitivity remote atmospheric sensing application.

Paraconductivity of the K-doped SrFe2As2 superconductor

Paraconductivity of the optimally K-doped SrFe2As2 superconductor is investigated within existing fluctuation mechanisms. The in-plane excess conductivity has been measured in high-quality single crystals, with a sharp superconducting transition at Tcxa0=xa035.5xa0K and a transition width less than 0.3xa0K. The data have also been acquired in an external magnetic field up to 14xa0T. We show that the fluctuation conductivity data in zero field and for temperatures close to Tc can be explained within a three-dimensional (3D) Lawrence–Doniach theory, with a negligible Maki–Thompson contribution. In the presence of the magnetic field, it is shown that paraconductivity obeys the 3D Ullah–Dorsey scaling law, above 2xa0T and for H∥c. The estimated upper critical field and the coherence length nicely agree with the available experimental data.

Generation of elliptically polarized nitrogen ion laser fields using two-color femtosecond laser pulses.

We experimentally investigate generation of nitrogen molecular ion () lasers with two femtosecond laser pulses at different wavelengths. The first pulse serves as the pump which ionizes the nitrogen molecules and excites the molecular ions to excited electronic states. The second pulse serves as the probe which leads to stimulated emission from the excited molecular ions. We observe that changing the angle between the polarization directions of the two pulses gives rise to elliptically polarized laser fields, which is interpreted as a result of strong birefringence of the gain medium near the wavelengths of the laser.

Free-space air molecular lasing from highly excited vibrational states pumped by circularly-polarized femtosecond laser pulses

We experimentally investigate N2+ lasing from highly excited vibrational states with ultrashort, intense 800-nm laser pulses. We observe laser emissions at multiple wavelengths (353.3, 353.8, and 354.9 nm) corresponding to B2Σu+(v = 5, 4, 3) → X2Σg+(v = 4, 3, 2) transitions. Specifically, it is found that two strong signals corresponding to the (5–4) transition at 353.3 nm and the (4–3) transition at 353.8 nm, which can be achieved only at relatively low pressures, show a strong dependence on the ellipticity of the pump pulses. This observation suggests that the ions could be populated to high vibrational energy levels of the B2Σu+ state by electron impact excitation.

Symmetrical broken and nonlinear response of Weyl semimetal TaAs influenced by the topological surface states and Weyl nodes: The topological surface states and Weyl nodes

© 2017 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim A Weyl semimetal (WSM) features Weyl fermions in its bulk and topological surface states on surfaces, and is novel material hosting Weyl fermions, a kind of fundamental particles. The WSM was regarded as a three-dimensional version of “graphene” under the illusion. In order to explore its promising photoelectric properties and applications in photonics and photoelectronics, here, we study the anisotropic linear and nonlinear optical responses of a WSM TaAs, which are determined by the relationship and balance between its topological surface states and Weyl nodes. We demonstrate that topological surface states which break the bulk symmetry are responsible for the anisotropy of the mobility, and the anisotropic nonlinear response shows saturable characteristic with extremely large saturable intensity. We also find that the mobility is anisotropic with the magnitude of 104 cm2V−1s−1 at room temperature and can be accelerated by the optical field. By analyzing the symmetry, the nonlinear response is mainly contributed by the fermions close to the Weyl nodes, and is related to the Paulis blocking of fermions, electron-electron interaction. This work experimentally discovers the anisotropic ultrahigh mobility of WSMs in the optical field and may start the field for the applications of WSMs in photonics and photoelectronics. (Figure presented.).