Incommensurate spin-density modulation in a copper oxide chain compound with commensurate charge order

Neutron diffraction has been used to determine the magnetic structure of Na8Cu5O10, a stoichiometric compound containing chains based on edge-sharing CuO4 plaquettes. The chains are doped with 2/5 hole per Cu site and exhibit long-range commensurate charge order with an onset well above room temperature. Below TN=23 K, the neutron data indicate long-range collinear magnetic order with a spin-density modulation whose propagation vector is commensurate along, and incommensurate perpendicular to, the chains. Competing interchain exchange interactions are discussed as a possible origin of the incommensurate magnetic order.     

Excimer laser deposited CuO and Cu<Subscript>2</Subscript>O films with third-order optical nonlinearities by femtosecond <Emphasis Type="Italic">z</Emphasis>-scan measurement

By controlling the oxygen pressure, single-phase CuO and Cu₂O thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV–VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ ⁽³⁾ and Im χ ⁽³⁾, of the films were determined. Both CuO and Cu₂O films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with Cu₂O films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and Cu₂O films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and Cu₂O materials.     

Low temperature phase transitions and magnetic structure of PrB6

Specific heat, thermal expansion and electrical resistivity measurements on PrB₆ single crystals show that there are two low temperature phase transitions at 6.9 K and 4.2 K, respectively, the latter temperature varying somewhat among different crystals. Neutron diffraction measurements were made on both single and polycrystalline samples of PrB₆. The neutron data indicate a spontaneous incommensurate magnetic ordering at 6.9 K with Q = (0.23, 0.23, 0.5) 2π/a_o. At 4.2 K a commensurate magnetic phase is seen with Q = (0.25, 0.25, 0.5) 2π/a_o coexisting with the incommensurate phase At 1.74 K, only the commensurate phase remains. A model is proposed for the commensurate antiferromagnetic structure and a profile analysis based on that model yields a magnetic moment of 1.77 Bohr magnetons per praseodymium ion at 1.74 K.     

Selective excitation and suppression of coherent anti-Stokes Raman scattering by shaping femtosecond pulses

Femtosecond coherent anti-Stokes Raman scattering (CARS) suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump (or Stokes) spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump (or Stokes) spectrum with the $\pi $ spectral phase step.     

Evolution of the commensurate and incommensurate magnetic phases of the S = 3/2 kagome staircase Co?V?O? in an applied field

Single crystal neutron diffraction studies have been performed on the S = 3/2 kagome staircase compound Co(3)V(2)O(8) with a magnetic field applied along the magnetization easy-axis ([Formula: see text]). Previous zero-field measurements (Chen Y et al 2006 Phys. Rev. B 74 014430) reported a rich variety of magnetic phases, with a ferromagnetic ground state as well as incommensurate, transversely polarized spin density wave (SDW) phases (with a propagation vector of [Formula: see text]) interspersed with multiple commensurate lock-in transitions. The magnetic phase diagram with [Formula: see text] adds further complexity. For small applied fields, μ(0)H ≈ 0.05 T, the commensurate lock-in phases are destabilized in favor of the incommensurate SDW ones, while slightly larger applied fields restore the commensurate lock-in phase with δ = 1/2 and yield a new commensurate phase with δ = 2/5. For measurements in an applied field, higher-order scattering is observed that corresponds to the second harmonic.     

Enhancement of magnetic and dielectric properties of low temperature sintered NiCuZn ferrite by Bi2O3-CuO additives

With a series of 1.0 wt%Bi$_{2}$O$_{3}$-$x$ wt% CuO ($x =0.0$, 0.2, 0.4, 0.6, and 0.8) serving as sintering additives, Ni$_{0.23}$Cu$_{0.32}$Zn$_{0.45}$Fe$_{2}$O$_{4}$ ferrites are successfully synthesized at a low temperature (900 $^\circ$C) by using the solid state reaction method. The effects of the additives on the phase formation, magnetic and dielectric properties as well as the structural and gyromagnetic properties are investigated. The x-ray diffraction (XRD) results indicate that the added Bi$_{2}$O$_{3}$-CuO can lower the synthesis temperature significantly without the appearing of the second phase. The scanning electron microscope (SEM) images confirm that Bi$_{2}$O$_{3}$ is an important factor that determines the sintering behaviors, while CuO affects the grain size and densification. With CuO content $x=0.4$ or 0.6, the sample shows high saturation magnetization, low coercivity, high real part of magnetic permeability, dielectric permittivity, and small ferromagnetic resonance linewidth ($\Delta H$). The NiCuZn ferrites are a promising new generation of high-performance microwave devices, such as phase shifters and isolators.     

Chiral magnetic soliton lattice on a chiral helimagnet

Using Lorenz microscopy and small-angle electron diffraction, we directly present that the chiral magnetic soliton lattice (CSL) continuously evolves from a chiral helimagnetic structure in small magnetic fields in Cr(1/3)NbS2. An incommensurate CSL undergoes a phase transition to a commensurate ferromagnetic state at the critical field strength. The period of a CSL, which exerts an effective potential for itinerant spins, is tuned by simply changing the field strength. Chiral magnetic orders observed do not exhibit any structural dislocation, indicating their high stability and robustness in Cr(1/3)NbS2.     

Intense THz-radiation from semiconductors under magnetic field irradiated with femtosecond laser pulses

The THz-radiation power from InAs reaches sub-mW level in a 1.7-T magnetic field irradiated with femtosecond laser pulses of 1.5-W average power. The THz-radiation power is related almost quadratically both to the magnetic field and to the excitation laser power. Furthermore, the THz-radiation spectrum is found to be controlled by the excitation pulsewidth, chirp direction of the excitation pulse, and the magnetic field.     

Modeling the Ultrafast Response of Two‐Magnon Raman Excitations in Antiferromagnets on the Femtosecond Timescale

Illuminating a magnetic material with femtosecond laser pulses induces complex ultrafast dynamical processes. The resulting optically detectable response usually contains contributions from both the optical properties and the magnetic degrees of freedom. Disentangling all the different components concurring to the generation of the total signal is a major challenge of contemporary experimental solid‐state physics. Here, this problem is tackled, addressing the purely optical, nonmagnetic artifacts on the time resolved two‐magnon stimulated Raman spectrum of an antiferromagnet, rationalizing the recent observation on the exchange energy modification upon photo‐excitation. It is demonstrated how the genuine dynamics of the magnetic eigenmode can be disentangled from the nonlinear optical effects, generated by cross phase modulation, on the femtosecond timescale. The introduced approach can be extended for the investigation of <100 fs dynamic processes by means of coherent Raman scattering.     

Magnetic field induced incommensurate resonance in cuprate superconductors

The influence of a uniform external magnetic field on the dynamical spin response of cuprate superconductors in the superconducting state is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic scattering around low and intermediate energies is dramatically changed with a modest external magnetic field. With increasing the external magnetic field, although the incommensurate magnetic scattering from both low and high energies is rather robust, the commensurate magnetic resonance scattering peak is broadened. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime. The theory also predicts that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying an external magnetic field large enough.     

Percolative Superconductivity in Mg1-xB2

Our results from various transport experiments on Mg1-xB2 indicate a surprising effect associated with the presence of a Mg deficiency in MgB2: the phase separation between Mg-vacancy rich and Mg-vacancy poor phases. The Mg-vacancy poor phase is superconducting, but the insulating nature of the Mg-vacancy rich phase probably originates from the Anderson (disorder-induced) localization of itinerant carriers. Furthermore, electron diffraction measurements indicate that within vacancy-rich regions these defects tend to order with intriguing patterns. This electronic phase separation in Mg1-xB2 shows similar, but also distinct characteristics compared with that observed in La(2)CuO(4+delta).     

基于旋转相干光谱的整形飞秒激光转动动力学调控

我们实验上发展了基于飞秒激光旋转相干光谱的整形飞秒激光转动动力学调控方法,通过脉冲整形技术调控激发脉冲的光谱相位,从而实现对飞秒激光作用下转动态相干激发过程中复原信号及转动布居数的调控. 研究了飞秒激光旋转相干光谱对激光频谱相位的响应机制,突出了飞秒激光频谱相位在气相分子转动态相干激发中的重要作用. 为飞秒激光作用下生物大分子和团簇鉴别及结构探测研究提供了新的参考.     

Measurement of the electronic Grüneisen constant using femtosecond electron diffraction

We report the first accurate measurement of the electronic Grüneisen constant gamma(e) using a novel method employing the new technique of femtosecond electron diffraction. The contributions of the conduction electrons and the lattice to thermal expansion are differentiated in the time domain through transiently heating the electronic temperature well above that of the lattice with femtosecond optical pulses. By directly probing the associated thermal expansion dynamics in real time using femtosecond electron diffraction, we are able to separate the contributions of hot electrons from that of lattice heating, and make an accurate measurement of gamma(e) of aluminum at room temperature. This new approach opens the possibility of distinguishing electronic from magnetic contributions to thermal expansion in magnetic materials at low temperature.     

Manipulating the magnetic structure with electric fields in multiferroic ErMn2O5

Based on measurements of soft x-ray magnetic diffraction under in situ applied electric field, we report on significant manipulation and exciting of commensurate magnetic order in multiferroic ErMn2O5. The induced magnetic scattering intensity arises at the commensurate magnetic Bragg position whereas the initial magnetic signal almost persists. We demonstrate the possibility to imprint a magnetic response function in ErMn2O5 by applying an electric field.     

The magnetic phases of NdCu2

The magnetic phase diagram of a NdCu₂ single crystal is investigated by means of specific-heat measurements and neutron diffraction as a function of temperature and external magnetic field applied along the crystallographic b-direction. In the low temperature region we observe three commensurate phases AF1, F1, and F2. Their magnetic structures all consist of ferromagnetic (bc)-planes with different stacking sequences along the a-direction comprising 5, 3 and 8 chemical unit cells, respectively. Above 2.8 T the system is in a ferromagnetically aligned state (F3). Furthermore, there is an incommensurate phase AF3 between the low temperature commensurate phases and the paramagnetic state and, in a very narrow temperature region of only 0.2 K, an intermediate phase AF2 between AF1 and AF3. The Nd-moments are oriented along the b-direction in all phases.     

Spin-ordering quantum transitions of superconductors in a magnetic field

We argue that recent neutron scattering measurements by Lake et al. [Science 291, 1759 (2001)] of the spin excitation spectrum of La(2-delta)Sr(delta)CuO4 in a magnetic field can be understood in terms of proximity to a phase with co-existing superconductivity and spin density wave order. We present a general theory for such quantum transitions, and argue that their low energy spin fluctuations are controlled by a singular correction from the superflow kinetic energy, acting in the region outside the vortex cores. We propose numerous experimental tests of our theory.     

Low-energy spin fluctuations in the ground states of electron-doped Pr1-x LaCexCuO4+delta cuprate superconductors

Low-energy spin fluctuations have been investigated in the electron-doped Pr{1-x}LaCe{x}CuO{4+delta} over a wide concentration range of 0.07相似文献   

Field-induced three- and two-dimensional freezing in a quantum spin liquid

Field-induced commensurate transverse magnetic ordering is observed in the Haldane-gap compound Ni(C(5)D(14)N(2))2N(3)(PF(6)) by means of neutron diffraction. Depending on the direction of applied field, the high-field phase is shown to be either a three-dimensional ordered Néel state or a short-range ordered state with dominant two-dimensional spin correlations. The structure of the high-field phase is determined, and properties of the observed quantum phase transition are discussed.     

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.     

Field-induced spin-flip and spin-flip transitions in PrGa2

A detailed neutron diffraction study of a single crystal of hexagonal PrGa₂ under an applied field is presented. The field is applied along the easy [100] direction of the hexagonal plane. The compound exhibits several field-induced magnetic structures and, in particular, an uncompensated long period commensurate antiphase structure defined by the propagation vector (4/27, 4/27, 0) at zero field and below T_t = 3.5 K. Concerning the intermediate magnetic structures below T_t, two types of magnetic phase transitions are observed: a spin-flip transition where one moment per magnetic cell turns over, and a spin-slip transition from the low field commensurate propagation vector (4/27, 4/27, 0) to another commensurate vector (1/7, 1/7, 0). Between T_t and T_N = 7.2 K, the structure becomes apmlitude-modulated with a propagation vector (0.148, 0.148, 0.023). Important and original magnetic domain effects are also observed due to the high degeneracy of the easy direction of the hexagonal plane in both T<T_t and T_t<T<T_N regions.