Study of the e(g) orbitals in the bilayer manganite La(2--2x)Sr(1+2x)Mn(2)O(7) by using magnetic Compton-profile measurement.

From the magnetic Compton-profile (MCP) measurement, we have directly differentiated for the first time the populations in two e(g)-type orbitals ( x(2) - y(2) and 3z(2) - r(2)) in a manganite. The experimental MCP's along the [001] direction for La(2--2x)Sr(1+2x)Mn(2)O(7) at x = 0.35 and 0.42 are fitted by the theoretical profiles obtained from the (MnO(6))(8-) ab initio calculations. The calculation confirms that the MCP clearly detects the oxygen hybridization in the e(g) orbitals. The e(g) state is dominated by the x(2) - y(2)-type orbital with almost constant population, while the population in the 3z(2) - r(2)-type orbital decreases with increasing the hole concentration x.     

Phase Manipulation between c(4 x 2) and p(2 x 2) on the Si(100) surface at 4.2 K

Phase manipulation between c(4x2) and p(2x2) on the Si(100) surface has been demonstrated at 4.2 K for the first time using a low-temperature scanning tunneling microscope. We have discovered that it is possible to change the c(4x2) surface into the p(2x2) surface, artificially, through a flip-flop motion of the buckling dimers by using a sample bias voltage control. Also, scanning at a negative bias voltage or applying a pulse voltage can restore the c(4x2) surface. The STM images as a function of bias voltage and tunneling current reveal the interesting dynamics of the buckling dimers on the long debated surface. Our results will show that energetic tunneling electrons are most likely responsible for the observed phase transition from c(4x2) to p(2x2).     

双掺杂铁基1111型超导体Nd1-xBaxFeAsO1-2xF2x的磁传输性能研究

采用两步固相反应法,在母相FeAs=1111结构的NdFeAsO中掺入BaF2,实现电子与空穴的双掺杂．该体系超导转变温度（Tc）在33-50K范围,取决于名义BaF2掺杂量x．在实验范围内,掺杂量越大,转变温度越高,当x=0．2时,Tc达到50K．磁电阻测量表明：高掺杂量样品（例如x=0．2）具有较高的上临界场（Hc...     

Topological Superconductivity in Cu(x)Bi(2)Se(3)

A topological superconductor (TSC) is characterized by the topologically protected gapless surface state that is essentially an Andreev bound state consisting of Majorana fermions. While a TSC has not yet been discovered, the doped topological insulator Cu(x)Bi(2)Se(3), which superconducts below ～3 K, has been predicted to possess a topological superconducting state. We report that the point-contact spectra on the cleaved surface of superconducting Cu(x)Bi(2)Se(3) present a zero-bias conductance peak (ZBCP) which signifies unconventional superconductivity. Theoretical considerations of all possible superconducting states help us conclude that this ZBCP is due to Majorana fermions and gives evidence for a topological superconductivity in Cu(x)Bi(2)Se(3). In addition, we found an unusual pseudogap that develops below ～20 K and coexists with the topological superconducting state.     

Rovibrational Intensities of the (00(0)3) <-- (10(0)0) Dyad Absorption Bands of (12)C(16)O(2)

Absolute line intensities of (12)C(16)O(2) are experimentally measured for the first time for the (00(0)3)(I) <-- (10(0)0)(II) band at 5687.17 cm(-1) and the (00(0)3)(I) <-- (10(0)0)(I) band at 5584.39 cm(-1). The spectra were obtained using a Bomem DA8 Fourier transform spectrometer and a 25-m base-path White cell at NASA-Ames Research Center. The rotationless bandstrengths at a temperature of 296 K and the Herman-Wallis parameters are S(0)(vib) = 6.68(30) x 10(-25) cm(-1)/(molecule/cm(2)); A(1) = 1.4(9) x 10(-4), and A(2) = -1.1(5) x 10(-5) for the (00(0)3)(I) <-- (10(0)0)(II) band and S(0)(vib) = 6.07(22) x 10(-25) cm(-1)/(molecule/cm(2)); A(1) = 5.2(1.5) x 10(-4) and A(2) = -4.0(7) x 10(-5) for the (00(0)3)(I) <-- (10(0)0)(I) band.     

(Ta_2O_5)_(1-x)(TiO_2)_x陶瓷结构相变研究

采用固相反应技术制备了x分别为 0 0 5、0 0 8和 0 1 1的 (Ta2 O5) 1-x(TiO2 ) x 陶瓷 ;在室温至 6 0 0℃范围内 ,测量了这些陶瓷样品的拉曼光谱随温度的变化。随着温度的升高 ,拉曼光谱中位于 35～38cm- 1的最低频移的声子模发生软化 ,并随之发生结构相变。拉曼光谱和实验结果都表明 :组分x分别为0 0 5、0 0 8和 0 1 1的 (Ta2 O5) 1-x(TiO2 ) x 陶瓷分别在 36 0、4 5 0和 5 4 0℃发生了由三斜至单斜相的结构相变。上述结论得到了 (Ta2 O5) 0 92 (TiO2 ) 0 0 8单晶热膨胀系数测量数据的支持。     

(75)As NMR study of antiferromagnetic fluctuations in Ba(Fe(1-x)Ru(x))(2)As(2)

The evolution of (75)As NMR parameters with composition and temperature was probed in the Ba(Fe(1-x)Ru(x))(2)As(2) system where Fe is replaced by isovalent Ru. While the Ru end member was found to be a conventional Fermi liquid, the composition (x = 0.5) corresponding to the highest T(c) (20 K) in this system shows an upturn in the (75)As [Formula: see text] below about 80 K, evidencing the presence of antiferromagnetic (AFM) fluctuations. These results are similar to those obtained in another system with isovalent substitution, BaFe(2)(As(1-x)P(x))(2) (Nakai et al 2010 Phys. Rev. Lett. 105 107003) and point to a possible role of AFM fluctuations in driving superconductivity.     

Nonmonotonic d(x(2)-y(2)) superconducting order parameter in Nd2-xCexCuO4

Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd2-x Ce x CuO4 ( x = 0.15, T(c) = 22 K) has revealed a nonmonotonic d(x(2)-y(2)) superconducting order parameter. It has a maximum gap of 4.4k(B)T(c) at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k(B)T(c) at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(4-y)

We present measurements of the magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-y) and La(2-x)Ce(x)CuO(4-y) films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

Ca~(2+)离子替代对Sr_4Al_(14)O_(25):Eu~(2+),Dy~(3+)结构和发光性能的影响

采用高温固相反应按化学式Sr4-xCaxAl14O25:Eu2+,Dy3+(x=0,0.8,1.6,2.4,3.2,4)配比原料,合成长余辉发光材料.X射线衍射分析表明当x2.4时,产物物相均为Sr4Al14O25正交结构;当x2.4时,产物物相转变为CaAl4O7单斜结构.对掺Ca量不同,但结构仍保持Sr4Al14O25的样品采用360nm激光照射,发射光谱表明样品发光均由以Eu2+为发光中心的电子4f65d→4f7跃迁所致,并且随着Ca掺入量的增加,样品发射光谱峰位逐渐蓝移.这是由于Ca2+取代Sr2+位置后,导致晶格收缩,影响Eu2+的5d能级劈裂情况,从而影响电子4f65d→4f7跃迁.余辉衰减检测和热释光谱分析发现,不同Ca掺入量的样品余辉衰减快慢不同,是由于其中存在的陷阱能级深度不同,且陷阱能级越深,其余辉时间越长.     

Electronic properties of (2 x 1) and c(4 x 2) domains on Ge(001) studied by scanning tunneling spectroscopy

The surface electronic structure of Ge(001) was studied by scanning tunneling spectroscopy. The measured surface densities of states unequivocally reveal the presence of a metallic state on the (2 x 1) domains, which is absent on the c(4 x 2) domains. This metallic state, so far observed only in integral measurements, is attributed to the flip-flopping dimers that constitute the (2 x 1) domains. Our data also reveal a set of previously unresolved surface states, in perfect agreement with published theoretical predictions.     

Epitaxial growth and band alignment of (Gd(x)La(1-x))(2)O(3) films on n-GaAs (001)

Herein we demonstrate the epitaxial stabilization of single-crystalline (Gd(x)La(1-x))(2)O(3) films on n-GaAs (001) with a controlled lattice match. (Gd(x)La(1-x))(2)O(3) films have an in-plane epitaxial relationship with a twofold rotation on GaAs (001). Spectroscopic characterization by photoemission and absorption confirms that the band gap of (Gd(x)La(1-x))(2)O(3) film is approximately approximately 5.8eV. However, the conduction band offset is increased by the unpinned Fermi level of the n-GaAs in the (Gd(x)La(1-x))(2)O(3) film (x=0.97). The correlation of the crystalline property and the interfacial band offset by the electrical properties, as probed by capacitance and leakage current measurements, is also discussed.     

From stable divalent to valence-fluctuating behaviour in Eu(Rh(1-x)Ir(x))2Si2 single crystals

We have succeeded in growing high-quality single crystals of the valence-fluctuating system EuIr(2)Si(2), the divalent Eu system EuRh(2)Si(2) and the substitutional alloy Eu(Rh(1-x)Ir(x))(2)Si(2) across the range 0 < x < 1, which we characterized by means of x-ray diffraction, energy-dispersive x-ray spectroscopy, specific heat, magnetization and resistivity measurements. On increasing x, the divalent Eu ground state subsists up to x = 0.25 with a slight increase in Néel temperature, while for 0.3≤x < 0.7 a sharp hysteretic change in susceptibility and resistivity marks the first-order valence transition. For x?0.7 the broad feature observed in the physical properties is characteristic of the continuous valence evolution beyond the critical end point of the valence transition line, and the resistivity is reminiscent of Kondo-like behaviour while the Sommerfeld coefficient indicates a mass renormalization of at least a factor of 8. The resulting phase diagram is similar to those reported for polycrystalline Eu(Pd(1-x)Au(x))(2)Si(2) and EuNi(2)(Si(1-x)Ge(x))(2), confirming its generic character for Eu systems, and markedly different to those of homologue Ce and Yb systems, which present a continuous suppression of the antiferromagnetism accompanied by a very smooth evolution of the valence. We discuss these differences and suggest them to be related to the large polarization energy of the Eu half-filled 4f shell. We further argue that the changes in the rare earth valence between RRh(2)Si(2) and RIr(2)Si(2) (R = Ce, Eu, Yb) are governed by a purely electronic effect and not by a volume effect.     

Structure of the Ba-induced Si(111)- (3 x 2) reconstruction

The Ba/Si(111) surface, previously known as a 3 x 1 phase, is found to have a 3 x 2 periodicity and a semiconducting band gap. The substrate reconstructs into the honeycomb chain-channel (HCC) structure with Ba atoms in the channel, as in the alkali-metal-induced Si(111)-(3 x 1). However, the metal coverage is determined to be 1/6 monolayers, half the alkali-metal coverage. We propose that the structure and the metal coverage determined for the Ba adsorbate is universal for other alkaline-earth-metal adsorbates. With the alkali-metal-induced 3 x 1 case, our results lead to a rule that one donated electron per 3 x 1 surface unit is necessary to stabilize the HCC reconstruction of Si.     

Structure and local-equilibrium thermodynamics of the c(2x2) reconstruction of rutile TiO2 (100)

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.     

Crossing the gap from p- to n-type doping: nature of the states near the chemical potential in La(2)-(x)Sr(x)CuO(4) and Nd(2-x)Ce(x)CuO(4-delta)

We report on an x-ray absorption and resonant photoemission study on single crystals of the high-T(c) cuprates La2-xSrxCuO4 and Nd(2-x)Ce(x)CuO(4-delta). Using an intrinsic energy reference, we find that the chemical potential of La2-xSrxCuO4 lies near the top of the La2CuO4 valence band whereas in Nd(2-x)Ce(x)CuO(4-delta) it is situated near the bottom of the Nd2CuO4 conduction band. The data clearly establish that the introduction of Ce in Nd2CuO4 results in electrons being doped into the CuO2 planes. We infer that the states closest to the chemical potential have a Cu 3d(10) singlet origin in Nd(2-x)Ce(x)CuO(4-delta) and a 3d(9)L singlet origin in La2-xSrxCuO4.     

Phase fluctuations and the pseudogap in YBa(2)Cu(3)O (x)

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa(2)Cu(3)O (x) single crystals and Monte Carlo simulations of the anisotropic 3D- XY model. We directly show that T(c) of underdoped YBa(2)Cu(3)O (x) is strongly suppressed from its mean-field value (T(MF)(c)) by phase fluctuations of the superconducting order parameter. For overdoped YBa(2)Cu(3)O (x) fluctuation effects are greatly reduced and T(c) approximately T(MF)(c). We find that T(MF)(c) exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.     

Bragg gratings in ternary SiO(2):SnO(2):Na(2)O optical glass fibers

Bragg gratings have been written in an optical fiber with a core made from ternary SiO(2): SnO(2): Na(2)O glass and a SiO(2) cladding. The presence of Na(2)O allows for higher concentrations of SnO>(2) , which are believed to be responsible for the photorefractive response of this composition. In these preliminary experiments significant refractive-index modulations, up to 6.2x10(-4) , have been achieved with a 248-nm excimer laser and a phase mask to write gratings for reflectivity at ~1.5microm . The induced refractive-index changes show enhanced temperature stability, and there is no sign of any erasure up to temperatures exceeding 600 degrees C.