45 research outputs found
The two colors of MgB2
We present the anisotropic optical conductivity of MgB between 0.1 and
3.7 eV at room temperature obtained on single crystals of different purity by
the spectroscopic ellipsometry and reflectance measurements. The bare
(unscreened) plasma frequency is almost isotropic and equal to 6.3
eV, which contrasts some earlier reports of a very small value of .
The data suggests that the -bands are characterized by a stronger
electron-phonon coupling but smaller impurity scattering
, compared to the -bands. The optical response along the
boron planes is marked by an intense interband transition at 2.6 eV, due to
which the reflectivity plasma edges along the a- and c-axes are shifted with
respect to each other. As a result, the sample spectacularly changes color from
a blueish-silver to the yellow as the polarization is rotated from the in-plane
direction towards the c-axis. The optical spectra are in good agreement with
the published {\it ab initio} calculations. The remaining discrepancies can be
explained by the relative shift of -bands and -bands by about 0.2
eV compared to the theoretical band structure, in agreement with the de
Haas-van Alphen experiments. The widths of the Drude and the interband peaks
are both very sensitive to the sample purity.Comment: 11 pages, 13 figure
Optical study of archetypical valence-fluctuating Eu-systems
We have investigated the optical conductivity of the prominent valence
fluctuating compounds EuIr2Si2 and EuNi2P2 in the infrared energy range to get
new insights into the electronic properties of valence fluctuating systems. For
both compounds we observe upon cooling the formation of a renormalized Drude
response, a partial suppression of the optical conductivity below 100 meV and
the appearance of a mid-infrared peak at 0.15 eV for EuIr2Si2 and at 0.13 eV
for EuNi2P2. Most remarkably, our results show a strong similarity with the
optical spectra reported for many Ce- or Yb-based heavy fermion metals and
intermediate valence systems, although the phase diagrams and the temperature
dependence of the valence differ strongly between Eu- and Ce-/Yb-systems. This
suggests that the hybridization between 4f- and conduction electrons, which is
responsible for the properties of Ce- and Yb-systems, plays an important role
in valence fluctuating Eu-systems
Investigate the effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors
The effect of anisotropic order parameter to specific heat of anisotropic
two-band superconductors in BCS weak-coupling limit are investigated. An
analytical specific heat jump and the numerical specific heat are shown by
using anisotropic order parameters, and the electron-phonon interaction and
non-electron-phonon interaction. The two model of anisotropic order parameters
are used for numerical calculation that we find little effect to the numerical
results . The specific heat jump of, and superconductors can fit well with the
both of them. By comparing to experimental data overall range of temperature,
the best fit is ,, and superconductors, respectively.Comment: 10 pages, 3 fingure
Optical evidence for heavy charge carriers in FeGe
The optical spectrum of the cubic helimagnetic metal FeGe has been
investigated in the frequency range from 0.01 - 3.1 eV for different
temperatures from 30 K to 296 K. The optical conductivity shows the evolution
of a low energy (0.22 eV) interband transition and the development of a narrow
free carrier response with a strong energy and temperature dependence. The
frequency dependent effective mass and scattering rate derived from the optical
data indicate the formation of dressed quasi-particles with a mass
renormalization factor of 12. Similar to FeSi the spectral weight in FeGe is
not recovered over a broad frequency range, an effect usually attributed to the
influence of the on-site Coulomb interaction.Comment: 5 pages, 5 figure
Anisotropic optical conductivity of the putative Kondo insulator CeRuSn
Kondo insulators and in particular their non-cubic representatives have
remained poorly understood. Here we report on the development of an anisotropic
energy pseudogap in the tetragonal compound CeRuSn employing optical
reflectivity measurements in broad frequency and temperature ranges, and local
density approximation plus dynamical mean field theory calculations. The
calculations provide evidence for a Kondo insulator-like response within the
plane and a more metallic response along the c axis and qualitatively
reproduce the experimental observations, helping to identify their origin
Optical spectra of the heavy fermion uniaxial ferromagnet UGe
We report a detailed study of UGe single crystals using infrared
reflectivity and spectroscopic ellipsometry. The optical conductivity suggests
the presence of a low frequency interband transition and a narrow free-carrier
response with strong frequency dependence of the scattering rate and effective
mass. We observe sharp changes in the low frequency mass and scattering rate
below the upper ferromagnetic transition . The characteristic
changes are exhibited most strongly at an energy scale of around 12 meV (100
cm). They recover their unrenormalized value above and for 40 meV. In contrast no sign of an anomaly is seen at the lower transition
temperature of unknown nature 30 K, observed in transport and
thermodynamic experiments. In the ferromagnetic state we find signatures of a
strong coupling to the longitudinal magnetic excitations that have been
proposed to mediate unconventional superconductivity in this compound
Optical conductivity and superconductivity in LaSb
We have measured the resistivity, optical conductivity, and magnetic
susceptibility of LaSb to search for clues as to the cause of the
extraordinarily large linear magnetoresistance and to explore the properties of
the superconducting state. We find no evidence in the optical conductivity for
the formation of a charge density wave state above 20 K despite the highly
layered crystal structure. In addition, only small changes to the optical
reflectivity with magnetic field are observed indicating that the MR is due to
scattering rate, not charge density, variations with field. Although a
superconducting ground state was previously reported below a critical
temperature of 0.4 K, we observe, at ambient pressure, a fragile
superconducting transition with an onset at 2.5 K. In crystalline samples, we
find a high degree of variability with a minority of samples displaying a full
Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The
application of pressure stabilizes the superconducting transition and reduces
the anisotropy of the superconducting phase.Comment: 4 pages with 4 figure
Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy
Electronic Raman scattering studies on MgB2 single crystals as a function of
excitation and polarization have revealed three distinct superconducting
features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78
cm-1 which we identify as the superconducting gaps in \pi- and \sigma-bands and
as the Leggett's collective mode arising from the fluctuation in the relative
phase between two superconducting condensates residing on corresponding bands.
The temperature and field dependencies of the superconducting features have
been established. A phononic Raman scattering study of the E2g boron stretching
mode anharmonicity and of superconductivity induced self-energy effects is
presented. We show that anharmonic two phonon decay is mainly responsible for
the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of
the E2g phonon frequency upon cooling into the superconducting state and
estimate the electron-phonon coupling strength associated with this
renormalization.Comment: Invited review in a special issue "Superconductivity in MgB_2:
Physics and Applications