3,196 research outputs found
Forecasting using twinned principal curves and twinned self-organising maps
We extend the principal curves algorithm by creating twinned principal curves which extend through two related data sets simultaneously. The criteria for accepting a pair of data points as neighbours for any other pair of data points is that each of the relevant points must be close in the appropriate space. We illustrate the algorithm's predictive power on artificial data sets before using it to predict on a real financial time series. We compare the error from this twinning with that achieved by a related algorithm which twins self-organising maps
Magnetic phase diagram of the frustrated S=1/2 chain magnet LiCu_2O_2
We present the results of the magnetization and dielectric constant
measurements on untwinned single crystal samples of the frustrated S=1/2 chain
cuprate LiCu_2O_2. Novel magnetic phase transitions were observed. A spin flop
transition of the spiral spin plane was observed for the field orientations
H||a,b. The second magnetic transition was observed at H~15 T for all three
principal field directions. This high field magnetic phase is discussed as a
collinear spin-modulated phase which is expected for an S=1/2 nearest-neighbor
ferromagnetic and next-nearest-neighbor antiferromagnetic chain system
Quasiparticle thermal Hall angle and magnetoconductance in YBa_2Cu_3O_x
We present a way to extract the quasiparticle (qp) thermal conductivity
Kappa_e and mean-free-path in YBa_2Cu_3O_x, using the thermal Hall effect and
the magnetoconductance of Kappa_e. The results are very consistent with heat
capacity experiments. Moreover, we find a simple relation between the thermal
Hall angle Theta_Q and the H-dependence of Kappa_e, as well as numerical
equality between Theta_Q and the electrical Hall angle. The findings also
reveal an anomalously anisotropic scattering process in the normal state.Comment: 4 pages in Tex, 5 figures in EPS; replaced on 5/12/99, minor change
On the origin of non-monotonic doping dependence of the in-plane resistivity anisotropy in Ba(Fe)As, = Co, Ni and Cu
The in-plane resistivity anisotropy has been measured for detwinned single
crystals of Ba(FeNi)As and Ba(FeCu)As.
The data reveal a non-monotonic doping dependence, similar to previous
observations for Ba(FeCo)As. Magnetotransport measurements
of the parent compound reveal a non-linear Hall coefficient and a strong linear
term in the transverse magnetoresistance. Both effects are rapidly suppressed
with chemical substitution over a similar compositional range as the onset of
the large in-plane resistivity anisotropy. It is suggested that the relatively
small in-plane anisotropy of the parent compound in the spin density wave state
is due to the presence of an isotropic, high mobility pocket of reconstructed
Fermi surface. Progressive suppression of the contribution to the conductivity
arising from this isotropic pocket with chemical substitution eventually
reveals the underlying in-plane anisotropy associated with the remaining FS
pockets.Comment: 12 pages, 9 figure
Structural response to O*-O' and magnetic transitions in orthorhombic perovskites
We present a temperature dependent single crystal x-ray diffraction study of
twinned orthorhombic perovskites La1-xCaxMnO3, for x=0.16 and x=0.25. These
data show the evolution of the crystal structure from the ferromagnetic
insulating state to the ferromagnetic metallic state. The data are modelled in
space group Pnma with twin relations based on a distribution of the b axis over
three perpendicular cubic axes. The twin model allows full structure
determination in the presence of up to six twin fractions using the single
crystal x-ray diffraction data.Comment: 13 pages, including 13 figures and 2 table
Flux-Line Lattice Structures in Untwinned YBa2Cu3O
A small angle neutron scattering study of the flux-line lattice in a large
single crystal of untwinned YBa2Cu3O is presented. In fields parallel to the
c-axis, diffraction spots are observed corresponding to four orientations of a
hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy,
the penetration depth ratio, of 1.18(2) was obtained. The high quality of the
data is such that second order diffraction is observed, indicating a well
ordered FLL. With the field at 33 degrees to c a field dependent re-orientation
of the lattice is observed around 3T.Comment: 4 pages, 4 figure
Electrical resistivity of the Ti4O7 Magneli phase under high pressure
We have measured resistivity as a function of temperature and pressure of
Ti4O7 twinned crystals using different contact configurations. Pressures over
4kbar depress the localization of bipolarons and allow the study of the
electrical conduction of the bipolaronic phase down to low temperatures. For
pressures P > 40 kbar the bipolaron formation transition is suppressed and a
nearly pressure independent behavior is obtained for the resistivity. We
observed an anisotropic conduction. When current is injected parallel to the
principal axis, a metallic conduction with interacting carrier effects is
predominant. A superconducting state was not obtained down to 1.2 K, although
evidences of the proximity of a quantum critical point were noticed. While when
current is injected non-parallel to the crystal's principal axis, we obtained a
logarithmic divergence of the resistivity at low temperatures. For this case,
our results for the high pressure regime can be interpreted in the framework of
interacting carriers (polarons or bipolarons) scattered by Two Level Systems.Comment: 9 Revtex pages, 12 EPS figures included, submitted to The European
Physical Journal B. Contact author: C. Acha (e-mail address: [email protected]
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