Incommensurate spin-density wave order in electron-doped BaFe2 As2 superconductors

Neutron diffraction studies of Ba(Fe(1-x)Co(x))(2)As)(2) reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, ± ε, 0) from the commensurate antiferromagnetic propagation vector Q(AFM) = (1,0,1) (in orthorhombic notation) where ε ≈ 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.     

Carr-Purcell sequences with composite pulses

We present novel Carr-Purcell-like sequences using composite pulses that exhibit improved performance in strongly inhomogeneous fields. The sequences are designed to retain the intrinsic error correction of the standard Carr-Purcell-Meiboom-Gill (CPMG) sequence. This is achieved by matching the excitation pulse with the refocusing cycle such that the initial transverse magnetization lies along the axis n(Beta) characterizing the overall rotation of the refocusing cycle. Such sequences are suitable for relaxation measurements. It is shown that in sufficiently inhomogeneous fields, the echo amplitudes have an initial transient modulation that is limited to the first few echoes and then decay with the intrinsic relaxation time of the sample. We show different examples of such sequences that are constructed from simple composite pulses. Sequences of the form 90 degrees (0)-(90 degrees (90-theta/2)-theta(180-theta/2)-90 degrees (90-theta/2))(n) with theta approximately 90 degrees and 270 degrees generate signal over a bandwidth larger than that of the conventional CPMG sequence, resulting in an improved signal-to-noise ratio in inhomogeneous fields. The new sequence 127 degrees (x,y)-(127 degrees (x)-127 degrees (-x))(n) only excites signal off-resonance with a spectrum that is bimodal, peaking at Delta omega(0)=+/-omega(1). Depending on the phase and exact timing of the first pulse, symmetric or antisymmetric excitation is obtained. We also demonstrate several new sequences with improved dependence on the RF field strength. The sequence (22.5 degrees (67.5)-90 degrees (-22.5))-(90 degrees (67.5)-45 degrees (157.5)-90 degrees (67.5))(n) has the property that the phase of the signal depends on B(1), allowing coarse B(1) imaging in a one-dimensional experiment.     

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.     

(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.     

Evidence for negative cross correlations in vibrational dephasing in liquids: isotropic raman-line shift and width phenomena in isotopic mixtures of N2 and O2

To experimentally confirm the hypothesis of negative cross terms between different dephasing mechanisms, potentially leading to a broadening of the isotropic Raman line upon isotopic dilution, we present a detailed study on the concentration dependencies of all of the line shifts and widths in the four isotopic mixtures of type (15N14N)(x)-(14N2)(1-x), (15N2)(x)-(14N2)(1-x), (15N2)(x)-(15N14N)(1-x), and (16O2)(x)-(18O2)(1-x) near the normal boiling point of nitrogen at T = 77.35 K. A quite disparate behavior of the nitrogen oscillators compared to the oxygen oscillators was observed in respect to a change of their isotopic baths.     

Distribution of NMR relaxations in a random Heisenberg chain

NMR measurements of the (29)Si spin-lattice relaxation time T(1) were used to probe the spin-1/2 random Heisenberg chain compound BaCu(2)(Si(1-x)Ge(x))(2)O(7). Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.     

Bifurcations and traveling waves in a delayed partial differential equation

Here cell population dynamics in which there is simultaneous proliferation and maturation is considered. The resulting mathematical model is a nonlinear first-order partial differential equation for the cell density u(t,x) in which there is retardation in both temporal (t) and maturation variables (x), and contains three parameters. The solution behavior depends on the initial function varphi(x) and a three component parameter vector P=(delta,lambda,r). For strictly positive initial functions, varphi(0) greater, similar 0, there are three homogeneous solutions of biological (i.e., non-negative) importance: a trivial solution u(t) identical with 0, a positive stationary solution u(st), and a time periodic solution u(p)(t). For varphi(0)=0 there are a number of different solution types depending on P: the trivial solution u(t), a spatially inhomogeneous stationary solution u(nh)(x), a spatially homogeneous singular solution u(s), a traveling wave solution u(tw)(t,x), slow traveling waves u(stw)(t,x), and slow traveling chaotic waves u(scw)(t,x). The regions of parameter space in which these solutions exist and are locally stable are delineated and studied.     

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.     

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.     

Anomalous peak in the superconducting condensate density of cuprate high- T(c) superconductors at a unique doping state

The doping dependence of the ratio of the superconducting condensate density to the effective mass, n(o)(s)/m(*)(ab), was studied in detail by muon-spin rotation for Y(0.8)Ca(0.2)Ba(2)(Cu(1-z)Zn(z))(3)O(7-delta) and Tl(0.5-y)Pb(0.5+y)Sr(2)Ca(1-x)Y(x)Cu(2)O(7). Our data show that n(o)(s)/m(*)(ab) exhibits a peak at a unique doping state in the overdoped regime. Its position coincides with the critical doping state, where the normal state pseudogap was reported to appear and to deplete the electronic density of states. This finding implies that the pseudogap primarily arises from a change in the electronic ground state rather than from thermal fluctuations.     

A change from second- to first-order transition in (La(1-x)Eu(x))0.67Ca0.33MnO3 (0 ≤ x ≤ 0.2)

A detailed investigation of the paramagnetic to ferromagnetic transition in (La(1-x)Eu(x))(0.67)Ca(0.33)MnO(3) having small Eu(3+)-content (0 ≤ x ≤ 0.2) has been carried out through resistivity and magnetization measurements. X-ray diffraction patterns of the compounds reveal a single phase (La(1-x)Eu(x))(0.67)Ca(0.33)MnO(3) (0 ≤ x ≤ 0.2) of an orthorhombic crystal structure after annealing the precursor at 800 °C for 2 h in air. With increasing Eu(3+)-content, the second-order transition (at x = 0 and 0.1) changes to first-order at x = 0.2. The experimental results demonstrate thermomagnetic irreversibility of the transition for x = 0.2 composition. This arises between the supercooling and superheating regimes where both the ferromagnetic and paramagnetic phases coexist.     

First experimental measurements of the plasma potential throughout the presheath and sheath at a boundary in a weakly collisional plasma

Experimental data obtained with emissive probes and Langmuir probes show that the plasma potential profile in the presheath scales as -ephi /T(e)= sqrt[(x(0) -x)/lambda ], consistent with ion flux conservation, and that the sheath consists of a transition region and an electron-free collisionless sheath with thicknesses scaling as lambda( 1/5)lambda (4/5 )(D) and lambda(D )(ephi/ T(e))(3 /4), respectively, where lambda is the ion-neutral collision length. Results support Rieman's presheath and transitional region model [Phys. Plasmas 4, 4158 (1997)]]. The potential drop over the presheath and transition sheath region were the order of T(e) /e and 2T(e )/3e, respectively, increasing with increasing pressure.     

Magnetic excitations in EuCu(2)(Si(x)Ge(1-x))(2): from mixed valence towards magnetism

We report the inelastic neutron scattering study of spin dynamics in EuCu(2)(Si(x)Ge(1-x))(2) (x?=?1, 0.9, 0.75, 0.6), performed in a wide temperature range. At x?=?1 the magnetic excitation spectrum was found to be represented by the double-peak structure well below the energy range of the Eu(3+) spin-orbit (SO) excitation (7)F(0)→(7)F(1), so that at least the high-energy spectral component can be assigned to the renormalized SO transition. Change of the Eu valence towards 2?+ with increased temperature and/or Ge concentration results in further renormalization (lowering the energy) and gradual suppression of both inelastic peaks in the spectrum, along with developing sizeable quasielastic signal. The origin of the spectral structure and its evolution is discussed in terms of excitonic model for the mixed valence state.     

Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered spin-liquid antiferromagnet with non-magnetic substitution. The parent compound Ba(3)Mn(2)O(8) is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic V(5+) for the S=1 Mn(5+) ions, Ba(3)(Mn(1-x)V (x))(2)O(8), and find that the singlet-triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, up to x=0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values.     

无箔二极管电流电压关系研究

本文给出了无箔二极管物理特性的数值模拟研究结果。研究得到了二极管电流与二极管结构参数、二极管电压以及外加磁场强度等参数的变化关系特性，并且应用这些关系特性得到了在外加磁场强度足够大情况下二极管电流的近似表达为Ib=(7.5/x)(x+(0.81-x)/(1+0.7Ld2/δr))(r2/3-1)3/2，其中Ld为阴阳极间距；Rc为阴极发射端面的外半径；Rp为漂移管管壁半径；x=ln(Rp/Rc)，δr =Rp-Rc。这一表达式在大量二极管结构参数和二极管电压参数情况下与数值模拟结果在10％误差范围内相一致，能够比较准确的表达无箔二极管的电流电压关系。     

Study of a spin-3/2 system by a quadrupolar-echo sequence: suppression of spurious signals.

The density matrix describing the evolution of a spin-3/2 system excited by a quadrupolar-echo sequence consisting of two rf pulses in quadrature phase, (X)-tau 2-(Y)-tau 4-[acquisition(y)], is calculated from the equilibrium state to the acquisition period. The interactions involved are the first-order quadrupolar interaction throughout the experiment and a local heteronuclear magnetic dipolar term between the two pulses and during the acquisition period. Three echoes, one due to a satellite transition at tau 4 = tau 2 and two due to the central transition at tau 4 = tau 2 and tau 4 = 3 tau 2, are predicted. They have similar expressions than those obtained with two rf pulses of the same phase, (X)-tau 2-(X)-tau 4-[acquisition(y)], except the signs. Moreover, it is shown experimentally that a combination of these two sequences, namely: (X)-tau 2-(X)-tau 4-[acquisition(y)]-recycle delay-(X)-tau 2-(-X)-tau 4-[acquisition(y)]-recycle delay-(X)-tau 2-(Y)-tau 4-[acquisition(-y)]-recycle delay-(X)-tau 2-(-Y)-tau 4-[acquisition(-y)]-recycle delay, cancels the spurious piezo-electric signals when studying a ferro-electric material in the single-crystal form.     

Noncritically phase-matched frequency conversion in Gd(x)Y(1-x)Ca(4)O(BO(3))(3) crystal

A nonlinear optical crystal YCa(4)O(BO(3))(3) (YCOB) is phase matchable for third-harmonic generation (THG) of a Nd:YAG laser by type I mixing between 1.064 and 0.532mum . By partial substitution of Gd for Y in YCOB, a solid solution Gd(x)Y(1-x)Ca(4)O(BO(3))(3) gradually changes the phase-matching angles of THG to (theta, ?)=(90( degrees ), 90 ( degrees )) . We present the properties of noncritically phase-matched THG in Gd(x) Y(1-x)Ca(4)O(BO(3))(3) (x=0.28) .     

Suppression of ferromagnetic double exchange by vibronic phase segregation

From Raman spectroscopy, magnetization, and thermal expansion on the system La(2/3)(Ca(1-x)Sr(x))(1/3)MnO3, we have been able to provide a quantitative basis for the heterogeneous electronic model for manganites exhibiting colossal magnetoresistance (CMR). We construct a mean-field model that accounts quantitatively for the measured deviation of T(C)(x) from the T(C) predicted by de Gennes double-exchange in the adiabatic approximation and predicts the occurrence of a first-order transition for a strong coupling regime, in accordance with the experiments. The existence of a temperature interval T(C) < T < T*, where CMR may be found, is discussed in connection with the occurrence of an idealized Griffiths phase.     

Test of cosmic spatial isotropy for polarized electrons using a rotatable torsion balance

To test the cosmic spatial isotropy, we use a rotatable torsion balance carrying a transversely spin-polarized ferrimagnetic Dy6Fe23 mass. With a rotation period of 1 h, the period of anisotropy signal is reduced from one sidereal day by about 24 times, and hence the 1/f noise is reduced. Our present experimental results constrain the cosmic anisotropy Hamiltonian H=C(1)sigma(1)+C2sigma(2)+C3sigma(3) (sigma(3) is in the axis of earth rotation) to (C(2)(1)+C(2)(2))(1/2)=(1.1+/-2.0)x10(-20) eV and /C(3)/=(1.1+/-6.0)x10(-19) eV. This improves the previous limits on (C(2)(1)+C(2)(2))(1/2) by 27 times.     

Isomorphous substitution in bimetallic oxide clusters

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V(2)O(5))(n-1)(VTiO(5))(-) (n=2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V(2)O(5))(n) clusters. Upon systematic exchange of V by Ti atoms in V(4-n)Ti(n)O(-)(10) (n=1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti(4)O(-)(10).