Bernd Buechner

Magnetodielectric and magnetoelastic coupling in TbFe3(BO3)(4)

We have studied the magnetodielectric and magnetoelastic coupling in TbFe3(BO3)(4) single crystals by means of capacitance, magnetostriction, and Raman spectroscopy measurements. The data reveal strong magnetic field effects on the dielectric constant and on the macroscopic sample length which are associated to long-range magnetic ordering and a field-driven metamagnetic transition. We discuss the coupling of the dielectric, structural, and magnetic order parameters and attribute the origin of the magnetodielectric coupling to phonon mode shifts according to the Lyddane-Sachs-Teller relation.

Structural transformations of carbon chains inside nanotubes

In situ aberration-corrected high-resolution transmission electron microscopy is used to examine the structural transformations of carbon chains that occur in the interior region of carbon nanotubes. We find electron-beam irradiation leads to the formation of two-dimensional carbon structures that are freely mobile inside the nanotube. The inner diameter of the nanotube influences the structural transformations of the carbon chains. As the diameter of the nanotube increases, electron-beam irradiation leads to curling of the chains and eventually the formation of closed looped structures. The closed looped structures evolve into spherical fullerenelike structures that exhibit translational motion inside the nanotubes and also coalesce to form larger nanotube structures. These results demonstrate the use of carbon nanotubes as test tubes for growing small carbon nanotubes within the interior by using only electron-beam irradiation at 80 kV.

Tilting of the CuO{sub 6} octahedra in La{sub 1.83{minus}{ital x}}Eu{sub 0.17}Sr{sub {ital x}}CuO{sub 4} as seen by {sup 151}Eu M{umlt o}ssbauer spectroscopy

{sup 151}Eu M{umlt o}ssbauer studies of La{sub 1.83{minus}{ital x}}Eu{sub 0.17}Sr{sub {ital x}}CuO{sub 4} for 0{le}{ital x}{le}0.26 exhibit a quadrupole interaction at the Eu site whose magnitude {ital eQV}{sub {ital zz}} strongly depends on both the Sr content {ital x} and the temperature {ital T}. For a fixed temperature, a linear relationship is obtained between the quadrupole interaction and the averaged CuO{sub 6} octahedra tilting angle determined from diffraction studies. The unusually large temperature dependence of {ital eQV}{sub {ital zz}} strongly indicates a temperature dependence of the local structure. In contrast to recent studies of the local structure La{sub 2}CuO{sub 4}-type superconductors by means of x-ray-absorption and neutron-scattering techniques, our data give strong evidence that the local octahedra tilting in La{sub 1.83{minus}{ital x}}Eu{sub 0.17}Sr{sub {ital x}}CuO{sub 4} corresponds well to the average tilting obtained by diffraction techniques. {copyright}{ital 1996 The American Physical Society.}

CaCuMn6O12 vs. CaCu2Mn5O12: A comparative study

The ferrimagnetic compounds Ca(CuxMn3−x)Mn4O12 of the double distorted perovskites AC3B4O12 family exhibit a rapid increase of the ferromagnetic component in magnetization at partial substitution of square coordinated (Mn3+)C for (Cu2+)C. In the transport properties, this is seen as a change of the semiconducting type of resistivity for the metallic one. The evolution of magnetic properties of Ca(CuxMn3−x)Mn4O12 is driven by strong antiferromagnetic exchange interaction of (Cu2+)C with (Mn3+/Mn4+)B coordinated octahedra. The competing interactions of (Mn3+)C with (Mn3+/Mn4+)B lead to the formation of noncollinear magnetic structures that can be aligned by magnetic fields.

Suppression of scattering in quantum confined 2D helical Dirac systems

Transport properties of helical Dirac fermions in disordered quantum wires are investigated in the large energy limit. In the quasiballistic regime, the conductance and the Fano factor are sensitiv ...

Heat conductivity of the spin-Peierls compounds TiOCl and TiOBr

We report experimental results on the heat conductivity kappa of the S=1/2 spin chain compounds TiOBr and TiOCl for temperatures 5 K <T <300 K and magnetic fields up to 14 T. Surprisingly, we find no evidence of a significant magnetic contribution to kappa, which is in stark contrast to recent results on S=1/2 spin chain cuprates. Despite this unexpected result, the thus predominantly phononic heat conductivity of these spin-Peierls compounds exhibits a very unusual behavior. In particular, we observe strong anomalies at the phase transitions T(c1) and T(c2). Moreover, we find an overall but anisotropic suppression of kappa in the intermediate phase which extends even to temperatures higher than T(c2). An external magnetic field causes a slight downshift of the transition at T(c1) and enhances the suppression of kappa up to T(c2). We interpret our findings in terms of strong spin-phonon coupling and phonon scattering arising from spin-driven lattice distortions.

Symmetry disquisition on the TiOX phase diagram (X=Br,Cl)

The sequence of phase transitions and the symmetry of, in particular, the low temperature incommensurate and spin-Peierls phases of the quasi-one-dimensional inorganic spin-Peierls system TiOX (X=Br and Cl) have been studied using inelastic light scattering experiments. The anomalous first-order character of the transition to the spin-Peierls phase is found to be a consequence of the different symmetries of the incommensurate and spin-Peierls (P2(1)/m) phases. The pressure dependence of the lowest transition temperature strongly suggests that magnetic interchain interactions play an important role in the formation of the spin-Peierls and the incommensurate phases. Finally, a comparison of Raman data on VOCl to the TiOX spectra shows that the high energy scattering previously observed has a phononic origin.

Three-Dimensional Composition and Electric Potential Mapping of III–V Core–Multishell Nanowires by Correlative STEM and Holographic Tomography

The nondestructive characterization of nanoscale devices, such as those based on semiconductor nanowires, in terms of functional potentials is crucial for correlating device properties with their morphological/materials features, as well as for precisely tuning and optimizing their growth process. Electron holographic tomography (EHT) has been used in the past to reconstruct the total potential distribution in three-dimension but hitherto lacked a quantitative approach to separate potential variations due to chemical composition changes (mean inner potential, MIP) and space charges. In this Letter, we combine and correlate EHT and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography on an individual ⟨111⟩ oriented GaAs–AlGaAs core–multishell nanowire (NW). We obtain excellent agreement between both methods in terms of the determined Al concentration within the AlGaAs shell, as well as thickness variations of the few nanometer thin GaAs shell acting as quantum well tube. Subtracting the MIP determined from the STEM tomogram, enables us to observe functional potentials at the NW surfaces and at the Au–NW interface, both ascribed to surface/interface pinning of the semiconductor Fermi level.

Long-range magnetic order in Li{sub x}Na{sub 1-x}Cu{sub 2}O{sub 2}

Single crystals of LixNa1 − xCu2O2 solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x ≤ 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Néel temperature TN in these compounds increases with Li content.Single crystals of Li{sub x}Na{sub 1-x}Cu{sub 2}O{sub 2} solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x {<=} 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Neel temperature T{sub N} in these compounds increases with Li content.

Unidirectional Diagonal Order and 3D Stacking of Charge Stripes in Orthorhombic Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4

The interplay between crystal symmetry and charge stripe order in Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 has been studied by means of single crystal x-ray diffraction. In contrast to tetragonal La1.67Sr0.33NiO4, these crystals are orthorhombic. The corresponding distortion of the NiO2 planes is found to dictate the direction of the charge stripes, similar to the case of diagonal spin stripes in the insulating phase of La2-xSrxCuO4. In particular, diagonal stripes seem to always run along the short a-axis, which is the direction of the octahedral tilt axis. In contrast, no influence of the crystal symmetry on the charge stripe ordering temperature itself was observed, with T_CO 240K for La, Pr, and Nd. The coupling between lattice and stripe degrees of freedom allows one to produce macroscopic samples with unidirectional stripe order. In samples with stoichiometric oxygen content and a hole concentration of exactly 1/3, charge stripes exhibit a staggered stacking order with a period of three NiO2 layers, previously only observed with electron microscopy in domains of mesoscopic dimensions. Remarkably, this stacking order starts to melt about 40K below T_CO. The melting process can be described by mixing the ground state, which has a 3-layer stacking period, with an increasing volume fraction with a 2-layer stacking period.