81 research outputs found
Ordered low-temperature structure in K4C60 detected by infrared spectroscopy
Infrared spectra of a K4C60 single-phase thin film have been measured between
room temperature and 20 K. At low temperatures, the two high-frequency T1u
modes appear as triplets, indicating a static D2h crystal-field stabilized
Jahn-Teller distortion of the (C60)4- anions. The T1u(4) mode changes into the
known doublet above 250 K, a pattern which could have three origins: a dynamic
Jahn-Teller effect, static disorder between "staggered" anions, or a phase
transition from an orientationally-ordered phase to one where molecular motion
is significant.Comment: 4 pages, 2 figures submitted to Phys. Rev.
Optimized unconventional superconductivity in a molecular Jahn-Teller metal
Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above Tc is a major challenge for all unconventional superconductors. The molecular A3C60 fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C603– electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller–active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of Tc with interfulleride separation, demonstrating molecular electronic structure control of superconductivity
Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films
Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser
deposition, of thicknesses down to 40 nm, were studied using infrared
transmission and reflection spectroscopy. The complex dielectric responses of
polar phonon modes, particularly ferroelectric soft mode, in the films were
determined quantitatively. The compressed epitaxial STO films on (100)
La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened
phonon responses, whereas the soft mode in polycrystalline film on (0001)
sapphire substrate shows a strong broadening due to grain boundaries and/or
other inhomogeneities and defects. The stiffened soft mode is responsible for a
much lower static permittivity in the plane of the compressed film than in the
bulk samples.Comment: 11 page
Mott localization in the correlated superconductor Cs3C60 resulting from the molecular Jahn-Teller effect
Cs3C60 is a correlated superconductor under pressure, but an insulator under ambient conditions. The mechanism causing this insulating behavior is the combination of Mott localization and the dynamic Jahn-Teller effect. We show evidence from infrared spectroscopy for the dynamic Jahn-Teller distortion. The continuous change with temperature of the splitting of infrared lines is typical Jahn-Teller behavior, reflecting the change in population of solid-state conformers. We conclude that the electronic and magnetic solid-state properties of the insulating state are controlled by molecular phenomena. We estimate the time scale of the dynamic Jahn-Teller effect to be above 10^(-11) s and the energy difference between the conformers less than 20 cm-1
Measurement of the Far Infrared Magneto-Conductivity Tensor of Superconducting YBaCuO Thin Films
We report measurements of the far infrared transmission of superconducting
YBaCuO thin films from 5 cm to 200 cm in
fields up to 14. A Kramers-Kronig analysis of the magneto-transmission
spectrum yields the magneto-conductivity tensor. The result shows that the
magneto-conductivity of YBaCuO is dominated by three
terms: a London term, a low frequency Lorentzian ( 3 cm) of width 10 cm and a finite frequency Lorentzian of
width 17 cm at 24 cm in the hole
cyclotron resonance active mode of circular polarization.\\Comment: Revised LaTex file (12 pages) + 4 Postscript figures, uuencoded. In
response to referees' comments, we refined the paper a lot; we encourage you
to download this revised versio
Infrared conductivity of a d_{x^2-y^2}-wave superconductor with impurity and spin-fluctuation scattering
Calculations are presented of the in-plane far-infrared conductivity of a
d_{x^2-y^2}-wave superconductor, incorporating elastic scattering due to
impurities and inelastic scattering due to spin fluctuations. The impurity
scattering is modeled by short-range potential scattering with arbitrary phase
shift, while scattering due to spin fluctuations is calculated within a
weak-coupling Hubbard model picture. The conductivity is characterized by a
low-temperature residual Drude feature whose height and weight are controlled
by impurity scattering, as well as a broad peak centered at 4 Delta_0 arising
from clean-limit inelastic processes. Results are in qualitative agreement with
experiment despite missing spectral weight at high energies.Comment: 29 pages (11 tar-compressed-uuencoded Postscript figures), REVTeX 3.0
with epsf macro
Infrared Hall effect in high Tc superconductors: Evidence for non-Fermi liquid Hall scattering
Infrared (20-120 cm-1 and 900-1100 cm-1) Faraday rotation and circular
dichroism are measured in high Tc superconductors using sensitive polarization
modulation techniques. Optimally doped YBCO thin films are studied at
temperatures down to 15 K and magnetic fields up to 8 T. At 1000 cm-1 the Hall
conductivity varies strongly with temperature in contrast to the longitudinal
conductivity which is nearly independent of temperature. The Hall scattering
rate has a T^2 temperature dependence but, unlike a Fermi liquid, depends only
weakly on frequency. The experiment puts severe constraints on theories of
transport in the normal state of high Tc superconductors.Comment: 8 pages, 3 figure
Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60
We present an NMR study of Na2C60 and K4C60, two compounds that are related
by electron-hole symmetry in the C60 triply degenerate conduction band. In both
systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements
detect a gap in the electronic structure, most likely related to
singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or
C60^{4-}. However, the extended temperature range of the measurements presented
here (10 K to 700 K) allows to reveal deviations with respect to this general
trend, both at high and low temperatures. Above room temperature, 1/T1 deviates
from the activated law that one would expect from the presence of the gap and
saturates. In the same temperature range, a lowering of symmetry is detected in
Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60,
modifications of the 13C spectra lineshapes also indicate a structural
modification. We discuss this high temperature deviation in terms of a coupling
between JTD and local symmetry. At low temperatures, 1/TT tends to a
constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual
metallic character, which emphasizes the proximity of metallic and insulting
behaviors in alkali fullerides.Comment: 12 pages, 13 figure
Reversible Pressure-Induced Amorphization in Solid C70 : Raman and Photoluminescence Study
We have studied single crystals of by Raman scattering and
photoluminescence in the pressure range from 0 to 31.1 GPa. The Raman spectrum
at 31.1 GPa shows only a broad band similar to that of the amorphous carbon
without any trace of the Raman lines of . After releasing the pressure
from 31.1 GPa, the Raman and the photoluminescence spectra of the recovered
sample are that of the starting crystal. These results indicate that
the molecules are stable upto 31.1 GPa and the amorphous carbon high
pressure phase is reversible, in sharp contrast to the results on solid
. A qualitative explaination is suggested in terms of inter- versus
intra-molecular interactions.Comment: To appear in Phys. Rev. Lett., 12 pages, RevTeX (preprint format), 3
figures available upon reques
Mapping of functionalized regions on carbon nanotubes by scanning tunneling microscopy
Scanning tunneling microscopy (STM) gives us the opportunity to map the
surface of functionalized carbon nanotubes in an energy resolved manner and
with atomic precision. But this potential is largely untapped, mainly due to
sample stability issues which inhibit reliable measurements. Here we present a
simple and straightforward solution that makes away with this difficulty, by
incorporating the functionalized multiwalled carbon nanotubes (MWCNT) into a
few layer graphene - nanotube composite. This enabled us to measure energy
resolved tunneling conductance maps on the nanotubes, which shed light on the
level of doping, charge transfer between tube and functional groups and the
dependence of defect creation or functionalization on crystallographic
orientation.Comment: Keywords: functionalization, carbon nanotubes, few layer graphene,
STM, CITS, ST
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