139 research outputs found
Coherent phonon dynamics at the martensitic phase transition of Ni_2MnGa
We use time-resolved optical reflectivity to study the laser stimulated
dynamics in the magnetic shape memory alloy Ni_2MnGa. We observe two coherent
optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the
pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a
heavily damped amplitudon respectively. In the martensite phase the martensitic
phase transition can be induced by a fs laser pulse on a timescale of a few ps.Comment: 3 figure
Temperature dependence of the superconducting gap anisotropy in BiSrCaCuO
We present the first detailed data of the momentum-resolved, temperature
dependence of the superconducting gap of ,
complemented by similar data on the intensity of the photoemission
superconducting condensate spectral area. The gap anisotropy between the
and directions increases markedly with increasing
temperature, contrary to what happens for conventional anisotropic-gap
superconductors such as lead. Specifically, the size of the superconducting gap
along the direction decreases to values indistinguishable from zero
at temperatures for which the gap retains virtually full value along the
direction.Comment: APS_REVTEX. 19 pages, including 8 figures, available upon request.
UW-Madison preprin
Localization Effects in Bi2Sr2Ca(Cu,Co)2O8+y High Temperature Superconductors
Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator transition.
We study correlations between changes in the electrical resistivity RHOab(T)
and the electronic bandstructure using identical single crystalline samples.
For undoped samples the resistivity is linear in temperature and has a
vanishing residual resistivity. In angle resolved photoemission these samples
show dispersing band-like states. Co-doping decreases TC and causes and
increase in the residual resistivity. Above a threshold Co-concentration the
resistivity is metallic (drab/dT >0) at room temperature, turns insulating
below a characteristic temperature Tmin and becomes super- conducting at even
lower temperature. These changes in the resistivity correlate with the
disappearance of the dispersing band-like states in angle resolved
photoemission. We show that Anderson localization caused by the impurity
potential of the doped Co-atoms provides a consistent explanation of all
experimental features. Therefore the TC reduction in 3d-metal doped high-
temperature superconductors is not caused by Abrikosov Gor'kov pair- breaking
but by spatial localization of the carriers. The observed suppression of TC
indicates that the system is in the homogenous limit of the
superconductor-insulator transition. The coexistance of insulating (dRHOab/dT
<0) normal state behavior and super- conductivity indicates that the
superconducting ground state is formed out of spatially almost localized
carriers.Comment: Postscript file 11 pages plus 4 figures available on reques
Observation of a van Hove Singularity in with Angle Resolved Photoemission
We have performed high energy resolution angle-resolved photoemission studies
of the normal state band structure of oxygen overdoped
. We find that there is an extended saddle
point singularity in the density of states along direction.
The data also indicate that there is an asymmetry in the Fermi surface for both
the and perpendicular directions.Comment: APS_Revtex. 28 pages, including 16 figures, available upon request.
UW-Madison preprint#
INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN Pr-SUBSTITUTED Bi(2)Sr(2)CaCu(2)O(8+y)
Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a
metal-insulator transition. Using resonant photoemission we study how this
chemical substitution affects the electronic structure of the material. For the
partial Cu-density of states at E_F and in the region of the valence band we
observe no significant difference between a pure superconducting sample and an
insulating sample with 60% Pr for Ca. This suggests that the states responsible
for superconductivity are predomi- nately O-states. The partial Pr-4f density
of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance.
It consists of a single peak at 1.36eV binding energy. The peak shows a
strongly assymetric Doniach-Sunjic line- shape indicating the presence of a
continuum of electronic states with sharp cut off at E_F even in this
insulating sample. This finding excludes a bandgap in the insulating sample and
supports the existance of a mobility gap caused by spatial localization of the
carriers. The presence of such carriers at the Pr-site, between the CuO_2
planes shows that the electronic structure is not purely 2-dimensional but that
there is a finite interlayer coupling. The resonance enhancement of the
photoemission cross section, at the Pr-4d threshold, was studied for the Pr-4f
and for Cu-states. Both the Pr-4f and the Cu-states show a Fano-like resonance.
This resonance of Cu-states with Pr-states is another indication of coupling
between the the Pr-states and those in the CuO_2 plane. Because of the
statistical distribution of the Pr-ions this coupling leads to a non-periodic
potential for the states in the CuO_2 plane which can lead to localization and
thus to the observed metal-insulator transition.Comment: Gziped uuencoded postscript file including 7 figures Scheduled for
publication in Physical Review B, May 1, 1995
Localized electronic states and photoemission superconducting condensate in BiSrCaCuO
We present the first detailed angle-resolved photoemission evidence that
there are two types of carriers that contribute to the photoemission
superconducting condensate in . Our data
indicate that both itinerant and somewhat localized normal state carriers can
contribute to the formation of Cooper pairs.Comment: APS_Revtex, 11 pages, including 3 figures, available upon request.
UW-Madison preprint#
Structural and Magnetic Dynamics in the Magnetic Shape Memory Alloy NiMnGa
Magnetic shape memory Heusler alloys are multiferroics stabilized by the
correlations between electronic, magnetic and structural order. To study these
correlations we use time resolved x-ray diffraction and magneto-optical Kerr
effect experiments to measure the laser induced dynamics in a Heusler alloy
NiMnGa film and reveal a set of timescales intrinsic to the system. We
observe a coherent phonon which we identify as the amplitudon of the modulated
structure and an ultrafast phase transition leading to a quenching of the
incommensurate modulation within 300~fs with a recovery time of a few ps. The
thermally driven martensitic transition to the high temperature cubic phase
proceeds via nucleation within a few ps and domain growth limited by the speed
of sound. The demagnetization time is 320~fs, which is comparable to the
quenching of the structural modulation.Comment: 5 pages, 3 figures. Supplementary materials 5 pages, 5 figure
Anomalous Normal-State Properties of High-T Superconductors -- Intrinsic Properties of Strongly Correlated Electron Systems?
A systematic study of optical and transport properties of the Hubbard model,
based on Metzner and Vollhardt's dynamical mean-field approximation, is
reviewed. This model shows interesting anomalous properties that are, in our
opinion, ubiquitous to single-band strongly correlated systems (for all spatial
dimensions greater than one), and also compare qualitatively with many
anomalous transport features of the high-T cuprates. This anomalous
behavior of the normal-state properties is traced to a ``collective single-band
Kondo effect,'' in which a quasiparticle resonance forms at the Fermi level as
the temperature is lowered, ultimately yielding a strongly renormalized Fermi
liquid at zero temperature.Comment: 27 pages, latex, 13 figures, Invited for publication in Advances in
Physic
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