Spin Density Modulations Research Articles

Direct observation of a one-dimensional static spin modulation in insulatingLa1.95Sr0.05CuO4

We report the results of an extensive elastic neutron scattering study of the incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an insulating spin glass at low temperatures. The present neutron scattering experiments on the same x=0.05 crystal employ a narrower instrumental Q-resolution and thereby have revealed that the crystal has only two orthorhombic twins at low temperatures with relative populations of 2:1. We find that, in a single twin, only two satellites are observed at (1, +/-0.064, L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only along the orthorhombic b*-axis. This demonstrates unambiguously that La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low temperatures, consistent with certain stripe models. We have also reexamined the x=0.04 crystal that previously was reported to show a single commensurate peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the x=0.04 sample in fact has the same IC structure as the $x=0.05$ sample. The incommensurability parameter d for x=0.04 and 0.05, where d is the distance from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear relation d=x. These results demonstrate that the insulator to superconductor transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is coincident with a transition from diagonal to collinear static stripes at low temperatures thereby evincing the intimate coupling between the one dimensional spin density modulation and the superconductivity.

Neutron scattering study on electron–hole doping symmetry of high- Tc superconductivity

We review recent results of neutron scattering experiment on spin correlations for both hole-doped La 2− x Sr x CuO 4+ y and electron-doped Nd 1.85Ce 0.15CuO 4+ y superconductors. For the former, hole-doping induces an incommensurate spin density modulation along the Cu–O bonds. For the latter, we observed well-defined spin fluctuations in the superconducting phase ( T c≈18 K) for the first time. In contrast to the hole-doping, the electron-doping induces no well-defined incommensurate spin modulation; the magnetic signal appears at (π, π) with a q-width broader than in the as-grown antiferromagnetic phase. For both systems a static magnetic order was observed in the superconducting state.

Raman spectroscopy of quantum wires and dots: magnetoplasmons and edge-spin-density modes

We review recent magneto-Raman experiments on electronic excitations in GaAs–AlGaAs quantum wires and dots. We observe simultaneously collective spin-density and charge-density excitations (SDEs and CDEs), and also single-particle-like excitations. In a magnetic field, the CDEs (magnetoplasmons) exhibit a complex anticrossing behavior, which, in quantum wires, strongly depends on the direction of lateral wave-vector transfer. In quantum dots, we observe a splitting of the lowest spin-density mode in a magnetic field, very similar to the splitting of the magnetoplasmons. The lower branch can be interpreted as an edge-spin-density mode, where the electrons move on skipping orbits around the edge of the dot. In a microscopic model, we can identify transitions between electronic shells, which contribute to the observed low-energy SDEs of the quasiatomic system.

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid.

Edge spin-density modes in quantum dots in a magnetic field

We investigate electronic excitations in GaAs–AlGaAs quantum dots by means of resonant inelastic light scattering in the presence of an external magnetic field B. At magnetic field B=0, we observe a series of discrete electronic excitations, namely, charge-density excitations (CDE i ) and spin-density excitations (SDE i ), in one and the same quantum dot sample. We find at finite magnetic field, additionally to the well-known splitting of the CDE i modes (plasmons), a quite similar splitting of the lowest SDE mode whose energy is very close to the single-particle level spacing in the quantum dot. In analogy to the edge magnetoplasmon, we call this excitation an edge spin-density mode.

Coupled spin and charge collective excitations in a spin-polarized electron gas

The charge and longitudinal spin responses induced in a spin-polarized quantum well by a weak electromagnetic field are investigated within the framework of the linear response theory. We evaluate the excitation frequencies for the intra- and inter-subband transitions of the collective charge and longitudinal spin density oscillations including many-body corrections beyond the random-phase approximation through the spin-dependent local field factors, G σ ±( q, ω) . An equation-of-motion method was used to obtain these corrections in the limit of long wavelengths, and the results are given in terms of the equilibrium pair correlation function. The finite degree of spin polarization is shown to introduce coupling between the charge and spin density modes, in contrast with the result for an unpolarized system.

Incommensurate stripe order in La2-xSrxNiO4 with x=0.225.

In recent studies of La{sub 2{minus}{ital x}}Sr{sub {ital x}}NiO{sub 4} it has been suggested that ordering of the dopant-induced holes occurs only commensurately at special values of {ital x}, such as 1/3 and 1/2. Commensurate order of both charge and spin densities has also been found in a crystal with {ital x}=0.20. The present neutron scattering study of an {ital x}=0.225 crystal demonstrates that the spin and charge order can also be incommensurate. In fact, the incommensurability is temperature dependent, as observed previously in La{sub 2}NiO{sub 4.125}, indicating that such behavior is intrinsic to the doped NiO{sub 2} layers and not dependent on ordering of the dopant ions. A careful analysis of the unusual variation of peak widths as a function of momentum transfer perpendicular to the planes shows that the charge- and spin-density modulations are tied to the lattice, with the shift in phase of the charge modulations from one layer to the next equal to exactly one in-plane lattice spacing. A comparison of results for a number of samples shows that the charge and spin ordering temperatures vary linearly with hole concentration, with charge order always occurring at higher temperature, clearly indicating that the ordering is driven bymore » the charge. {copyright} {ital 1996 The American Physical Society.}« less

Coulomb effects on 13C-NMR and polarons in (CH) x and a bound state of solitons

Using the PPP Hamiltonian model of (CH) x , we study the effects of the Coulomb interaction on the 13C-NMR of doped (CH) x and the electronic and lattice structures of polarons. The broad asymmetric 13C-NMR signal is explained by the Coulomb-induced charge density modulation around charged solitons. Polarons accompany the Coulomb-induced charge and spin density modulations which have a dip at the centre and different spreadings, the former being wider. The lattice dimerization in polarons is not only relaxed but also reversed in a central region. A polaron accompanies four midgap levels. With a suitably screened Coulomb interaction, we can get polaron and soliton levels in good agreement with the data of electrochemical doping voltage spectroscopy. The positively and negatively charged solitons make a breather-like bound state, which may explain the self-pinning of photogenerated solitons.

Inelastic scattering of polarized low-energy electrons by plasmons in ferromagnetic metals

In a ferromagnetic metal a plasmon involves a charge polarization together with a spin-density modulation. Therefore it can be excited by an external electron through both the Coulomb and exchange coupling. The electron mean free path for plasmon excitation through the Coulomb interaction was studied by Pines and Ferrell. Here we extend those results to incorporate exchange effects which lead to a different mean free path for electrons with spin parallel and antiparallel to the magnetization. The electron scattering techniques using spin-modulated beams should allow the measurement of these differential effects.

Longitudinal susceptibility and the question of zero magnons in low-temperature magnetic insulators

The longitudinal susceptibility of the low-temperature Heisenberg ferromagnet is treated in random-phase approximation (RPA) in order to discuss whether a zero magnon (spin-density mode in the collisionless regime) can exist. In the unrenormalized RPA, this mode is found at large wave vectors in systems which have an exchange anisotropy which enhances the repulsion between spin-density fluctuations. It is shown, however, that (with the possible exception of certain two-dimensional systems) spin-wave damping suppresses this mode. A similar approach is applied to the low-temperature Heisenberg antiferromagnet. For systems with non-negligible anisotropy, it is shown that, with the possible exception of certain two-dimensional systems, spin-wave damping suppresses the zero-magnon mode. An argument is then given which suggests that the mode does not exist in the isotropic antiferromagnet either.

Untersuchung des Einflusses von Leitsalz auf die ESR-Spektren neutraler und geladener Radikale in flüssigen Kristallen

Abstract In the course of an investigation of electrolytically generated radical ions in the nematic mesophase of liquid crystals, we studied the influence of supporting electrolyte on the degree of molecular ordering. In the case of the radical anion of tetracyanoethylene (TCNE-) electrolytic generation results in a lower degree of ordering than chemical generation with sodium iodide. By using the neutral radical perinaphthenyl as a probe it was assured that the degree of ordering of the solvent is not disturbed by adding 0.1-m. tetrapropylammoniumperchlorate. A pronounced linewidth variation is observed for the chemically generated TCNE- which is attributed to spin density modulation by dynamic interaction with the Na+ counter ion.