J. Jung

The Role of Ni-Mn Hybridization on the Martensitic Phase Transitions in Mn-rich Heusler Alloys

Room temperature x-ray diffraction, dc magnetization, and ac susceptibility measurements have been performed on a series of Mn rich Ni50Mn37-xCrxSb13 and Ni50+xMn37-xSb13 Heusler alloys. Depending on the value of x, the room temperature crystal structures of the samples are either L21 cubic or orthorhombic. It is a commonly accepted idea that the martensitic transition temperatures in Ni-Mn-Z (Z = Ga, In, Sb, Sn) based Heusler alloys decrease (increase) with decreasing (increasing) valence electron concentration, e/a. However, the present work shows that regardless of the change in e/a, the martensitic transition temperature (TM) decreases with increasing Cr or Ni concentration. These results support the model where, in the case of Mn rich Heusler alloys, it is the hybridization between the Ni atoms and the Mn atoms in the Z sites that plays the dominant role in driving the martensitic transformation.

ANISOTROPIC MAGNETORESISTANCE IN PEROVSKITE MANGANITES

We have summarized some important results of the anisotropic transport properties of the prototypical manganite La1-xCaxMnO3 as well as a few others. The temperature dependence of anisotropic magnetoresistance in manganites exhibits a peak near the magnetic ordering temperature which differs dramatically from the ones in 3d ferromagnetic metals and alloys. Depending on the strain-driven orbital state, the AMR in manganites could be enhanced dramatically. The AMR in manganites is much larger than in ferromagnetic metals, where its magnitude is only of the order of a few percent. At low fields (below few kG) the intrinsic magnetocrystalline anisotropy becomes important and the dependence of resistance on angle changes to a more switching-like behavior. The magnitude of the switching is sufficiently large such that this phenomenon could be useful in spintronics for magnetic field sensing and nonvolatile memory applications. Moreover, at temperatures far below the metal insulator transition temperatures, the AMR in LCMO/LAO also flips and changes a sign in contrast to LCMO/STO films. Also sign flip takes place in charge and orbital ordered manganites exhibiting field induced ferromagnetism. Polycrystalline manganite samples also exhibit strong AMR. At lower temperatures the dependence of the AMR on temperature is dramatically different in polycrystalline samples than the epitaxial or single crystal samples. The behavior in the the polycrystalline samples could be governed by spin-polarized transport across the grain boundaries.

Enhancement of ferromagnetism by Cr doping in Ni-Mn-Cr-Sb Heusler alloys

A series of Mn rich Ni50Mn37−xCrxSb13 Heusler alloys have been investigated by dc magnetization and electrical resistivity measurements. Due to the weakening of the Ni-Mn hybridization, the martensitic transition shifts to lower temperatures with increasing Cr concentration, while the saturation magnetization at 5 K increases. The magnetoresistance and exchange bias properties are dramatically suppressed with increasing Cr concentration. The observed behaviors suggest that substitution of Cr for Mn in Ni50Mn37−xCrxSb13 Heusler alloys not only destabilizes the martensitic phase but also enhances ferromagnetism in the system. The possible mechanisms responsible for the observed behavior are discussed.

Nanodomain structure and function of high-temperature superconductors

Abstract The causes of high-temperature superconductivity are still mysterious, although more than 50 000 experiments have studied this subject. The most severe test of any microscopic theory is generally considered to be its ability to predict the results of future experiments. Here we examine recent (1999–2001) studies of Bi2Sr2CaCu2O8+δ films by scanning tunnelling microscopy; these have revealed a nanodomain structure on a scale of 3 nm, which is closely correlated with both superconductive gaps and pseudogaps. This structure and these correlations were predicted as part of a discrete filamentary model of high temperature superconductivity in 1990. The nanodomain diameter of 3 nm was identified in experiments on YBa2Cu3O7 in 1996. While none of the experiments can directly establish causes, in the predictive theoretical model it was proposed that the underlying forces generating the nanostructure are ferroelastic. It was also predicted that the strong correlations of the superconductive gap and pseudogap electronic structure with nanostructure are the result of dopant selforganization. Here we describe a new method of preparing boride alloys, and we predict that it may produce materials with T c ≈ 150 K or more.

The effect of a semiconductor-metal interface on localized terahertz plasmons

We demonstrate the terahertz frequency plasmonic characteristics of a metallic/semiconductor interface. The high-frequency transmission of terahertz radiation through dense ensembles of subwavelength sized Cu/CuxO microparticles via near-field coupling of localized plasmons is shown to increase when a CuxO/Au or CuxO/Pt interface is introduced. This finding introduces a previously undocumented characteristic of plasmonic interaction with material interfaces, and lays the physical groundwork for the integration of plasmonic circuits with traditional semiconductor electronics.

Electrical Transport and Oxygen Disorder in YBCO

We investigated the origin of a non-linear temperature dependence of resistivity ρab(T) in the a-b planes of YBCO, frequently observed in optimally doped twinned YBCO single crystals and thin films and in untwinned YBCO crystals along the chain direction. We performed the measurements of ρab(T) in c-axis oriented YBCO thin films that were subjected to low temperature annealing at 120–140°C (400–420 K) in argon. Our results have shown that redistribution of oxygen at a fixed oxygen content in the chain layers of YBCO with a non-linear ρab(T), leads to an increase of Tc, an increase of resistivity and a reduction of the non-linear component in ρab(T).

Zigzag filamentary theory of broken symmetry of neutron and infrared vibronic spectra of YBa2Cu3O6+x

Abstract Filamentary high-temperature superconductor (HTSC) theory differs fundamentally from continuous HTSC theories because it emphasizes self-organized discrete dopant networks and does not make the effective-medium approximation. Analysis of neutron and infrared (especially with c-axis polarization) vibrational spectra, primarily for YBa2Cu3O6+x, within the filamentary framework, shows that the observed vibronic anomalies near 400 cm−1 (50meV) are associated with curvilinear filamentary paths. These paths pass through the cuprate chains and planes, as well as resonant tunnelling centres in the BaO layers. The analysis and the data confirm earlier filamentary structural models containing ferroelastic domains of diameter 3–4 nm in the CuO2 planes; it is these nanodomains that are responsible for the discrete glassy nature of both electronic and vibrational properties. Chemical trends in vibronic energies and oscillator strengths, for both neutron and photon scattering, that were anomalous in continuum models, are readily explained by the filamentary model.

Observation of instabilities leading to side-branch formation in the dendritic growth of cubic (F.C.C.) 4He

Abstract Observation of the dendritic growth of f.c.c. 4He shows that side-branch formation is a two-step process. The first instability of a dendrite growing in the preferred direction takes the form of four thin lateral wings at 90° to each other. At low supercooling, no other instability develops. At larger supercooling, the lateral wings develop a cellular Mullins–Sekerka type instability. These cells, through further growth, develop into the dendritic side-branches. Side-branch formation has not been observed in dendrites that did not undergo the initial development of lateral wings. Dendrites growing at very low supercooling do not develop lateral wings, and show no side-branch formation.