4,514 research outputs found
Electronic Structure and Charge Dynamics of Huesler Alloy Fe2TiSn Probed by Infrared and Optical Spectroscopy
We report on the electrodynamics of a Heusler alloy Fe2TiSn probed over four
decades in energy: from the far infrared to the ultraviolet. Our results do not
support the suggestion of Kondo-lattice behavior inferred from specific heat
measurements. Instead, we find a conventional Drude-like response of free
carriers, with two additional absorption bands centered at around 0.1 and 0.87
eV. The latter feature can be interpreted as excitations across a pseudogap, in
accord with band structure calculations.Comment: 3 pages, 4 figure
Pressure-tuning of the c-f hybridization in Yb metal detected by infrared spectroscopy up to 18 GPa
It has been known that the elemental Yb, a divalent metal at mbient pressure,
becomes a mixed-valent metal under external pressure, with its valence reaching
~2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the
evolution of microscopic electronic states associated with the valence
crossover in Yb at external pressures up to 18 GPa. The measured infrared
reflectivity spectrum R(w) of Yb has shown large variations with pressure. In
particular, R(w) develops a deep minimum in the mid-infrared, which shifts to
lower energy with increasing pressure. The dip is attributed to optical
absorption due to a conduction c-f electron hybridization state, similarly to
those previously observed for heavy fermion compounds. The red shift of the dip
indicates that the - hybridization decreases with pressure, which is
consistent with the increase of valence.Comment: 2 pages, to appear in J. Phys. Soc. Jpn. Supp
Scaling of the superfluid density in high-temperature superconductors
A scaling relation \rho_s \simeq 35\sigma_{dc}T_c has been observed in the
copper-oxide superconductors, where \rho_s is the strength of the
superconducting condensate, T_c is the critical temperature, and \sigma_{dc} is
the normal-state dc conductivity close to T_c. This scaling relation is
examined within the context of a clean and dirty-limit BCS superconductor.
These limits are well established for an isotropic BCS gap 2\Delta and a
normal-state scattering rate 1/\tau; in the clean limit 1/\tau \ll 2\Delta, and
in the dirty limit 1/\tau > 2\Delta. The dirty limit may also be defined
operationally as the regime where \rho_s varies with 1/\tau. It is shown that
the scaling relation \rho_s \propto \sigma_{dc}T_c is the hallmark of a BCS
system in the dirty-limit. While the gap in the copper-oxide superconductors is
considered to be d-wave with nodes and a gap maximum \Delta_0, if 1/\tau >
2\Delta_0 then the dirty-limit case is preserved. The scaling relation implies
that the copper-oxide superconductors are likely to be in the dirty limit, and
that as a result the energy scale associated with the formation of the
condensate is scaling linearly with T_c. The a-b planes and the c axis also
follow the same scaling relation. It is observed that the scaling behavior for
the dirty limit and the Josephson effect (assuming a BCS formalism) are
essentially identical, suggesting that in some regime these two effects may be
viewed as equivalent. This raises the possibility that electronic
inhomogeneities in the copper-oxygen planes may play an important role in the
nature of the superconductivity in the copper-oxide materials.Comment: 8 pages with 5 figures and 1 tabl
Lighting up the Electrochemiluminescence of Carbon Dots through Pre- and Post-Synthetic Design
Carbon dots (CDs), defined by their size of less than 10\ua0nm, are a class of photoluminescent (PL) and electrochemiluminescent (ECL) nanomaterials that include a variety of carbon-based nanoparticles. However, the control of their properties, especially ECL, remains elusive and afflicted by a series of problems. Here, the authors report CDs that display ECL in water via coreactant ECL, which is the dominant mechanism in biosensing applications. They take advantage of a multicomponent bottom-up approach for preparing and studying the luminescence properties of CDs doped with a dye acting as PL and ECL probe. The dependence of luminescence properties on the surface chemistry is further reported, by investigating the PL and ECL response of CDs with surfaces rich in primary, methylated, or propylated amino groups. While precursors that contribute to the core characterize the PL emission, the surface states influence the efficiency of the excitation-dependent PL emission. The ECL emission is influenced by surface states from the organic shell, but states of the core strongly interact with the surface, influencing the ECL efficiency. These findings offer a framework of pre- and post-synthetic design strategies to improve ECL emission properties, opening new opportunities for exploring biosensing applications of CDs
The Nature of Heavy Quasiparticles in Magnetically Ordered Heavy Fermions
The optical conductivity of the heavy fermions UPd2Al3 and UPt3 has been
measured in the frequency range from 10 GHz to 1.2 THz (0.04 meV to 5 meV) at
temperatures 1 K < T < 300 K. In both compounds a well pronounced pseudogap of
less than a meV develops in the optical response at low temperatures; we relate
this to the antiferromagnetic ordering. From the energy dependence of the
effective electronic mass and scattering rate we derive the energies essential
for the heavy quasiparticle. We find that the enhancement of the mass mainly
occurs below the energy which is related to magnetic correlations between the
local magnetic moments and the itinerant electrons. This implies that the
magnetic order in these compounds is the pre-requisite to the formation of the
heavy quasiparticle and eventually of superconductivity.Comment: RevTeX, 4 pages, 3 figures, email:
[email protected]
The Missing Link: Magnetism and Superconductivity
The effect of magnetic moments on superconductivity has long been a
controversial subject in condensed matter physics. While Matthias and
collaborators experimentally demonstrated the destruction of superconductivity
in La by the addition of magnetic moments (Gd), it has since been suggested
that magnetic fluctuations are in fact responsible for the development of
superconducting order in other systems. Currently this debate is focused on
several families of unconventional superconductors including high-Tc cuprates,
borocarbides as well as heavy fermion systems where magnetism and
superconductivity are known to coexist. Here we report a novel aspect of
competition and coexistence of these two competing orders in an interesting
class of heavy fermion compounds, namely the 1-1-5 series: CeTIn5 where T=Co,
Ir, or Rh. Our optical experiments indicate the existence of regions in
momentum space where local moments remain unscreened. The extent of these
regions in momentum space appears to control both the normal and
superconducting state properties in the 1-1-5 family of heavy fermion (HF)
superconductors.Comment: 6 pages, 2 figure
Optical investigation of the charge-density-wave phase transitions in
We have measured the optical reflectivity of the quasi
one-dimensional conductor from the far infrared up to the
ultraviolet between 10 and 300 using light polarized along and normal to
the chain axis. We find a depletion of the optical conductivity with decreasing
temperature for both polarizations in the mid to far-infrared region. This
leads to a redistribution of spectral weight from low to high energies due to
partial gapping of the Fermi surface below the charge-density-wave transitions
at 145 K and 59 K. We deduce the bulk magnitudes of the CDW gaps and discuss
the scattering of ungapped free charge carriers and the role of fluctuations
effects
Pseudogap Formation and Heavy Carrier Dynamics in Intermediate Valence YbAl3
Infrared optical conductivity [] of the intermediate valence
compound YbAl has been measured at temperatures 8 K 690 K to
study its microscopic electronic structures. Despite the highly metallic
characters of YbAl, exhibits a clear pseudogap (strong
depletion of spectral weight) of about 60 meV below 40 K. It also shows a
strong mid-infrared peak centered at 0.25 eV. Energy-dependent effective
mass and scattering rate of the carriers obtained from the data indicate the
formation of a heavy-mass Fermi liquid state. These characteristic results are
discussed in terms of the hybridization states between the Yb 4 and the
conduction electrons. It is argued, in particular, that the pseudogap and the
mid-infrared peak result from the indirect and the direct gaps, respectively,
within the hybridization state. band.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Conductivity of Strongly Coupled Striped Superconductor
We study the conductivity of a strongly coupled striped superconductor using
gauge/gravity duality (holography). The study is done analytically, in the
large modulation regime. We show that the optical conductivity is inhomogeneous
but isotropic at low temperatures. Near but below the critical temperature, we
calculate the conductivity analytically at small frequency \omega, and find it
to be both inhomogeneous and anisotropic. The anisotropy is imaginary and
scales like 1/\omega. We also calculate analytically the speed of the second
sound and the thermodynamic susceptibility.Comment: 32 page
Infrared probe of the anomalous magnetotransport of highly oriented pyrolytic graphite in the extreme quantum limit
We present a systematic investigation of the magnetoreflectance of highly
oriented pyrolytic graphite in magnetic field B up to 18 T . From these
measurements, we report the determination of lifetimes tau associated with the
lowest Landau levels in the quantum limit. We find a linear field dependence
for inverse lifetime 1/tau(B) of the lowest Landau levels, which is consistent
with the hypothesis of a three-dimensional (3D) to 1D crossover in an
anisotropic 3D metal in the quantum limit. This enigmatic result uncovers the
origin of the anomalous linear in-plane magnetoresistance observed both in bulk
graphite and recently in mesoscopic graphite samples
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