Critical spin dynamics of the 2D quantum Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2

We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional S = 1/2 square lattice Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2. The characteristic energy scale, omega(0)(T/J), is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ( 0.2 less similarT/J less similar0.7). The obtained values for omega(0)(T/J) agree quantitatively between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, omega approximately xi(-z), over the entire temperature range with z = 1.0(1), in agreement with dynamic scaling theory.     

Systematic (63)Cu NQR and (89)Y NMR study of spin dynamics in Y(1-z)Ca(z)Ba(2)Cu(3)O(y) across the superconductor-insulator boundary

We demonstrate that the spin dynamics in underdoped Y(1-z)Ca(z)Ba(2)Cu(3)O(y) for y approximately equal to 6.0 exhibit qualitatively the same behavior to underdoped La(2-x)Sr(x)CuO(4) for an equal amount of hole concentration p = z/2 = x< or =0.11. However, a spin gap appears as more holes are doped into the CuO(2) plane by increasing the oxygen concentration to y approximately equal to 6.5 for a fixed value of Ca concentration z. Our results also suggest that Ca doping causes disorder effects that enhance the low frequency spin fluctuations.     

Novel in-gap spin state in Zn-doped La1.85Sr0.15CuO4

Low-energy spin excitations of La(1.85)Sr(0.15)Cu(1-y)Zn(y)O4 were studied by neutron scattering. In y=0.004, the incommensurate magnetic peaks show a well-defined "spin gap" below T(c). The magnetic signals at omega=3 meV decrease below T(c)=27 K for y=0.008, also suggesting the gap opening. At lower temperatures, however, the signal increases again, implying a novel in-gap spin state. In y=0.017, the spin gap vanishes and elastic magnetic peaks appear. These results clarify that doped Zn impurities induce the novel in-gap state, which becomes larger and more static with increasing Zn.     

Incommensurate magnetic ordering in Sr2Ru(1-x)TixO4

In Sr2RuO4 the spin excitation spectrum is dominated by incommensurate fluctuations at q = (0.3 0.3q(z)), which arise from Fermi-surface nesting. We show that upon Ti substitution, known to suppress superconductivity, a short range magnetic order develops with a propagation vector (0.307 0.307 1). In Sr2Ru0.91Ti0.09O4, the ordered moment points along the c direction. This finding shows that superconducting Sr2RuO4 is extremely close to an incommensurate spin density wave instability.     

Hidden itinerant-spin phase in heavily overdoped La(2-x)Sr(x)CuO4 superconductors revealed by dilute Fe doping: a combined neutron scattering and angle-resolved photoemission study

We demonstrated experimentally a direct way to probe a hidden propensity to the formation of a spin-density wave in a nonmagnetic metal with strong Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an incommensurate magnetic order below 20 K in heavily overdoped La(2-x)Sr(x)CuO(4). Elastic neutron scattering suggested that this order cannot be ascribed to the localized spins on Cu or doped Fe. Angle-resolved photoemission revealed a strong Fermi surface nesting inherent in the pristine La(2-x)Sr(x)CuO(4) that likely drives this order. Our finding presents the first example of the long-sought "itinerant-spin extreme" of cuprates, where the spins of itinerant doped holes define the magnetic ordering ground state; it complements the current picture of cuprate spin physics that highlights the predominant role of localized spins at lower dopings.     

Electronic confinement and ordering instabilities in colossal magnetoresistive bilayer manganites

We present angle-resolved photoemission studies of (La{1-z}Pr{z}){2-2x}Sr{1+2x}Mn{2}O{7} with x=0.4 and z=0.1, 0.2, and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO{2} layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ～400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, causes the observed renormalization of the spectral features.     

The magnetoelectric properties of A- or B-site-doped PbVO3 films： A first-principles study

We employ first-principles calculations to study the magnetic and ferroelectric properties of PbVO3 with A （XA = Ca, Sr, Bi, Ba, and La） or B （XB = Ti, Cr, Mn, Fe, Co, Ni, and Cu） site dopants, with the aim of ascertaining a large ferroelectric polarization and a long magnetic order, or even a macro fen＇i/ferromagnetism, which is critical to their potential applications in magnetoelectronic devices. It is found that PbTXAVsO24 （XA =Ca, Sr, and Ba,） are inclined to maintain the spin glass and large ferroelectric polarization. The degenerated G- and C-antiferromagnetic （AFM） couplings in the ideal PbVO3 are broken up, accompanied by the loss of ferroelectric properties, when La or Bi is doped at the A site. In contrast, the above-mentioned 3d transition elements doped at the B site of PbVO3 could induce remnant magnetic moments and preserve the large ferroelectric polarization, except for Ni and Cu. The Fe or Cr at the B site clearly remove the degenerated G- and C-AFM coupling, but the nonmagnetic Ti cannot do so. For the Mn, Co, Ni, or Cu doped at the B sites, even the two-dimensional AFM ordering in PbVO3 is destabilized. The various doping effects are further discussed with inner strain and charge transfer.     

Triangular to square flux lattice phase transition in YBa2Cu3O7

We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in a YBa2Cu3O7 single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular coordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories but is 45 degrees from that recently observed in La(1.83)Sr(0.17)CuO(4+delta).     

High-energy spin dynamics in La1.69Sr0.31NiO4

To test the prediction that the dispersion of the magnetic resonance in superconducting YBa2Cu3O(6+x) is similar to magnons in an incommensurate antiferromagnet, we have mapped out the spin dynamics in a stripe-ordered nickelate, La(2-x)SrxNiO4, with x approximately equal to 0.31, using inelastic neutron scattering. We observe spin-wave excitations up to 80 meV emerging from the incommensurate magnetic peaks with a surprisingly large and almost isotropic spin velocity: variant Planck's over 2 pi c(s) approximately 0.32 eV A. A comparison indicates that the inferred spin-excitation spectrum is not, by itself, an adequate model for the magnetic resonance feature of the superconductor.     

Muon spin relaxation studies of magnetic-field-induced effects in high-Tc superconductors

Muon spin relaxation measurements in high transverse magnetic fields [FORMULA: SEE TEXT] revealed strong field-induced quasistatic magnetism in the underdoped and Eu-doped (La,Sr)2CuO4 and La1.875Ba0.125CuO4, existing well above Tc and TN. The susceptibility counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards T approximately 25 K. No field-induced magnetism was detected in overdoped La1.81Sr0.19CuO4, optimally doped Bi2212, and Zn-doped YBa2Cu3O7.     

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.     

Accurate prediction of large antiferromagnetic interactions in high- T(c) HgBa2Ca(n-1)Cu(n)O(2n+2+delta) ( n = 2,3) superconductor parent compounds

The in-plane nearest-neighbor Heisenberg magnetic coupling constant, J, of La2CuO4, Nd2CuO4, Sr2CuO2Cl2, YBa2Cu3O6, and undoped HgBa(2)Ca(n-1)Cu(n)O(2n+2+delta) ( n = 1,2,3) is calculated from accurate ab initio configuration interaction calculations. For the first four compounds, the theoretical J values are in quantitative agreement with experiment. For the Hg-based compounds the predicted values are -135 meV ( n = 1) and approximately -160 meV ( n = 2,3), the latter being much larger than in previous cases and, for n = 3, increasing with pressure. Nevertheless, the physics governing J in all these layered cuprates appears to be the same. Moreover, calculations suggest a possible relationship between J and T(c).     

Bi2Sr2CaCu2O8的晶体结构与超导性能

本文研究了Bi-Sr-Ca-Cu-O体系中理想成份为Bi₂Sr₂CaCu₂O₈化合物的超导性能和晶体结构。名义成份为BiSrCaCu₂O_5.5零电阻超导转变温度T_c(0)=81.5K。用X射线粉末衍射方法测定了Bi₂Sr₂CaCu₂O₈的晶体结构,其基本结构属体心四方晶系,空间群为D_4h¹⁷-l4/mmm,点阵常数a=3.825?,c=30.82?。每单胞化合式单位为2.2Ca占据2(a)等效点系,4Sr,4Bi和4Cu占据三组4(e)位置,其原子参数z分别为0.110,0.302和0.445,16O分别占据8(g),z=0.445和二组4(e),z=0.210和0.380。Bi₂Sr₂CaCu₂O₈晶体结构可认为是阳离子沿z轴的(00z)和((1/2)(1/2)z)交错排列,由Aurivillius相导生出来的。讨论了在Bi-Sr-Ca-Cu-O体系中可能存在的其它沿z轴不同堆垛层数的超导相。 关键词：     

Dispersive excitations in the high-temperature superconductor La2-xSrxCuO4

High-resolution neutron scattering experiments on optimally doped La2-xSrxCuO4 (x=0.16) reveal that the magnetic excitations are dispersive. The dispersion is the same as in YBa2Cu3O6.85, and is quantitatively related to that observed with charge sensitive probes. The associated velocity in La2-xSrxCuO4 is only weakly dependent on doping with a value close to the spin-wave velocity of the insulating (x=0) parent compound. In contrast with the insulator, the excitations broaden rapidly with increasing energy, forming a continuum at higher energy and bear a remarkable resemblance to multiparticle excitations observed in 1D S=1/2 antiferromagnets. The magnetic correlations are 2D, and so rule out the simplest scenarios where the copper oxide planes are subdivided into weakly interacting 1D magnets.     

钙钛矿锰氧化物La0.7Sr0.3MxMn1-xO3(M=Cr,Fe)的巨磁电阻效应与磁性

研究了溶胶 -凝胶法制备氧化物巨磁电阻材料的工艺 ,制备了La0 .7Sr0 .3 CrxMn1-xO3 (x =0 ,0 .10 ,0 .15 )和La0 .7Sr0 .3 FexMn1-xO3 (x =0 .0 5 ,0 .10 ,0 .16 )两系列的单相钙钛矿锰氧化物多晶样品 ,并研究了Cr ,Fe替代La0 .7Sr0 .3 MnO3 中部分Mn后对其结构、磁性和巨磁电阻性质的影响 .观察到La0 .7Sr0 .3 Cr0 .15Mn0 .85O3 和La0 .7Sr0 .3 Fe0 .0 5Mn0 .95O3 两个样品的电阻 温度曲线都出现了双峰 .定性讨论了可能产生双峰的机制 .随Cr(或Fe)替代量的增加 ,材料的居里温度很快下降 ,铁磁性减弱 ,导电性降低 ,巨磁电阻效应增强 .但与Fe掺杂相比 ,相同数量的Cr掺杂对材料的影响要小 .     

Field-induced ferromagnetic metallic state of bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7: a polarized neutron diffraction study

Unpolarized and polarized neutron diffraction measurements have been carried out on the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7 which undergoes simultaneous semiconductor-metal paraferromagnetic transitions under magnetic field. Maximum entropy magnetization density reconstruction and multipole refinement on flipping ratios evidence the existence of two distinct field-induced states. The field-induced ferromagnetic state where the field is parallel to the c axis is characterized by the presence of magnetic moment on the Sr site of 0.48(2)mu(B), due to the Pr substitution. It also shows a high population of the d3(z(2)-r(2)) orbitals of Mn3+. For the field-induced state where the field is parallel to the a or b axes no magnetization density was found at the Sr site and the dx(2)(-y(2)) orbital is slightly more populated than the d3(z(2)-r(2)) one.     

Nature of e(g) electron order in La(1-x)Sr(1+x)MnO(4)

X-ray scattering measurements of the low-temperature structure of La(1-x)Sr(1+x)MnO(4) ( 0.33< or =x< or =0.67) indicate the existence of three distinct regions: a disordered phase (x<0.4), a charge-ordered phase (x> or =0.5), and a mixed phase (0.4< or =x<0.5). For x>0.5, the modulation vector associated with the charge order is incommensurate with the lattice and depends linearly on the concentration of e(g) electrons. The primary superlattice reflections are strongly suppressed along the modulation direction and the higher harmonics are weak, implying the existence of a largely transverse and nearly sinusoidal structural distortion, consistent with a charge-density wave of the e(g) electrons.     

Persistence of high-frequency spin fluctuations in overdoped superconducting La2-xSrxCuO4 (x=0.22)

We report a detailed inelastic neutron scattering study of the collective magnetic excitations of overdoped superconducting La(1.78)Sr(0.22)CuO(4) for the energy range 0-160 meV. Our measurements show that overdoping suppresses the strong response present for optimally doped La(2-x)Sr(x)CuO(4) which is peaked near 50 meV. The remaining response is peaked at incommensurate wave vectors for all energies investigated. We observe a strong high-frequency magnetic response for E approximately >80 meV suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram.     

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.     

Angle-resolved photoemission experiments on Bi2Sr2CaCu2O8+δ(001)

Core-level X-ray photoelectron-diffraction patterns have been measured from cleaved Bi₂Sr₂CaCu₂O₈₊ (001) surfaces for all elements present in this compound. The incommensurate modulation alongb ([010]) leads to a strong inequivalence ofa- andb-directions for Bi, Sr and Cu photoelectrons, while Ca and O emission show less effect. Ultraviolet-photoemission experiments recording the emission intensity at the Fermi energy over a large solid angle are also presented, providing a direct mapping of the Fermi surface. Ac(2×2) superstructure is observed on the Fermi surface suggesting antiferromagnetic correlations within the Cu–O planes. The effects of the lattice modulation are clearly observable at the Fermi energy, and they are enhanced for binding energies higher than a few tens of meV.