Magnetic Inversion Symmetry Breaking and Ferroelectricity in TbMnO\u3csub\u3e3\u3c/sub\u3e

TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature TN\u3c41  K is accompanied by ferroelectric order for T\u3c28  K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3

Spiral Spin Structures and Origin of the Magnetoelectric Coupling in YMn\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e

By combining neutron four-circle diffraction and polarized neutron-diffraction techniques we have determined the complex spin structures of a multiferroic YMn2O5 that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low-temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magnetoelectric coupling, our results are consistent with both the spin-current mechanism and the magnetostriction mechanism. Our results also explain why the electric polarization changes at the low-temperature commensurate-to-incommensurate phase transition

Chemical-Pressure Tailoring of Low-Field, Room-Temperature Magnetoresistance in (Ca, Sr, Ba)Fe₀.₅Mo₀.₅O₃

Chemical pressure effects of low-field intergrain magnetoresistance (IMR) in (Ca,Sr,Ba)Fe0.5M00.5O3 were studied by doping either Ba or Ca into the Sr site (IMR) was found to be highly tunable. The Curie temperature and magnetic softness were changed due to the chemical pressure exerted by the dopant. The room temperature IMR was reported to be 3.5% in 100 Oe at optimal doping

Temperature Dependence of Doping-Induced Modes in Cu-O Planes

Measurements of the frequency-dependent conductivity confirm that unusual lattice vibrational modes are induced by doping of several materials containing Cu-O planes. We find that the modes broaden rapidly with increasing temperature, consistent with the presence of a large linear component. This broadening of these vibrational modes is similar to that of electronic modes found previously in these same materials and analyzed in attempts to understand high-temperature superconductivity

First-Order Transition in the Itinerant Ferromagnet CoS\u3csub\u3e1.9\u3c/sub\u3eSe₀.₁

Undoped CoS2 is an isotropic itinerant ferromagnet with a continuous or nearly continuous phase transition at TC = 122 K. In the doped CoS1.9Se0.1 system, the Curie temperature is lowered to TC = 90 K, and the transition becomes clearly first order in nature. In particular we find a discontinuous evolution of the spin dynamics as well as strong time relaxation in the ferromagnetic Bragg intensity and small-angle neutron scattering in the vicinity of the ferromagnetic transition. In the ordered state the long-wavelength spin excitations were found to be conventional ferromagnetic spin waves with negligible spin-wave gap (\u3c0.04 meV), indicating that this system is also an excellent isotropic (soft) ferromagnet. In a wide temperature range up to 0.9TC, the spin-wave stiffness D(T) follows the prediction of the two-magnon interaction theory, D(T) = D(0)(1 - AT5/2), with D(0) = 131.7 ± 2.8 meV Å 2. The stiffness, however, does not collapse as T → TC from below. Instead a quasielastic central peak abruptly develops in the excitation spectrum, quite similar to results found in the colossal magnetoresistance oxides such as (La-Ca)MnO3

Incommensurate Structural Correlations in the Disordered Spin-Dimer State Induced by X-Ray and Electron Irradiation in CuIr₂S₄

Irradiation with ~10keV x rays or medium-energy electrons destroys long-range order of Ir spin dimers in CuIr2S4 while preserving the dimers locally. We find that as the order is destroyed, a new type of incommensurate structural correlations appears. This represents an intriguing example of order from disorder phenomenon, in which a previously unknown incommensurate order appears in the radiation-induced disordered state. These results suggest that two competing instabilities, one of which can be suppressed by radiation, are present in the system. Otherwise unrealized structural or electronic states can, therefore, be revealed in correlated systems by x-ray or electron irradiation

Optical Excitations of a Few Charges in Cuprates

In studies of the optical spectrum of lightly doped semiconducting crystals containing CuO2 planes, we have observed absorption maxima that occur at the same energies as excitations in the superconducting phase of these materials. One structure occurs just above the phonon energies and near the antiferromagnetic exchange energy

Metal-Insulator Transition in CuIr2S4: XAS Results on the Electronic Structure

S K and Ir L3 x-ray absorption measurements across the temperature-induced metal (M) to insulator (I) transition in Culr2S4 are presented. Dramatic S K-edge changes reflect the Ir d-electronic state redistribution across this transition. These changes, along with a detailed consideration of the I-phase structure, motivate a model in which the I-phase stabilization involves an interplay of charge and d-orbital orientation ordering along Ir chains, a quadrupling of the Ir-chain repeat unit, and correlated dimer spin-singlet formation

Dichotomy in ultrafast atomic dynamics as direct evidence of polaron formation in manganites

npj Quantum Materials. Volume 1, 25 November 2016, Article number 16026.© The Author(s) 2016. Polaron transport, in which electron motion is strongly coupled to the underlying lattice deformation or phonons, is crucial for understanding electrical and optical conductivities in many solids. However, little is known experimentally about the dynamics of individual phonon modes during polaron motion. It remains elusive whether polarons have a key role in materials with strong electronic correlations. Here we report the use of a new experimental technique, ultrafast MeV-electron diffraction, to quantify the dynamics of both electronic and atomic motions in the correlated LaSr2Mn2O7. Using photoexcitation to set the electronic system in motion, we find that Jahn-Teller-like O, Mn4+ and La/Sr displacements dominate the lattice response and exhibit a dichotomy in behavior—overshoot-and-recovery for one sublattice versus normal behaviour for the other. This dichotomy, attributed to slow electronic relaxation, proves that polaron transport is a key process in doped manganites. Our technique promises to be applicable for specifying the nature of electron–phonon coupling in complex materials