982 research outputs found
Terahertz conductivity of the heavy-fermion compound UNi2Al3
We have studied the optical properties of the heavy-fermion compound UNi2Al3
at frequencies between 100 GHz and 1 THz (3 cm^-1 and 35 cm^-1), temperatures
between 2 K and 300 K, and magnetic fields up to 7 T. From the measured
transmission and phaseshift of radiation passing through a thin film of
UNi2Al3, we have directly determined the frequency dependence of the real and
imaginary parts of the optical conductivity (or permittivity, respectively). At
low temperatures the anisotropy of the optical conductivity along the a- and
c-axes is about 1.5. The frequency dependence of the real part of the optical
conductivity shows a maximum at low temperatures, around 3 cm^-1 for the a-axis
and around 4.5 cm^-1 for the c-axis. This feature is visible already at 30 K,
much higher than the Neel temperature of 4.6 K, and it does not depend on
external magnetic fields as high as 7 T. We conclude that this feature is
independent of the antiferromagnetic order for UNi2Al3, and this might also be
the case for UPd2Al3 and UPt3, where a similar maximum in the optical
conductivity was observed previously.Comment: 7 pages, 9 figure
Optical investigations of the chemical pressurized EuFe2(As1-xPx)2: an s-wave superconductor with strong interband interaction
Superconducting EuFe2(As0.82P0.18)2 single crystals are investigated by
infrared spectroscopy in a wide frequency range. Below Tc=28K a superconducting
gap forms at 2\Delta_{0} = 9.5 meV = 3.8 k_B T_c causing the reflectivity to
sharply rise to unity at low frequency. In the range of the gap the optical
conductivity can be perfectly described by BCS theory with an -wave gap and
no nodes. From our analysis of the temperature dependent conductivity and
spectral weight at T>T_c, we deduce an increased interband coupling between
hole- and electron-sheets on the Fermi surface when approaches T_c
Collective Spin-Density-Wave Response Perpendicular to the Chains of the Quasi One-Dimensional Conductor (TMTSF)2PF6
Microwave experiments along all three directions of the spin-density-wave
model compound (TMTSF)PF reveal that the pinned mode resonance is
present along the and axes. The collective transport is
considered to be the fingerprint of the condensate. In contrast to common quasi
one-dimensional models, the density wave also slides in the perpendicular
direction. The collective response is absent along the least
conducting direction.Comment: 3 pages, 4 figure
Optical properties of the iron-pnictide analog BaMn2As2
We have investigated the infrared and Raman optical properties of BaMn2As2 in
the ab-plane and along the c-axis. The most prominent features in the infrared
spectra are the Eu and A2u phonon modes which show clear TO-LO splitting from
the energy loss function analysis. All the phonon features we observed in
infrared and Raman spectra are consistent with the calculated values. Compared
to the iron-pnictide analog AFe2As2, this compound is much more two-dimensional
in its electronic properties. For E || c-axis, the overall infrared
reflectivity is insulating like. Within the ab-plane the material exhibits a
semiconducting behavior. An energy gap 2{\Delta}=48 meV can be clearly
identified below room temperature.Comment: 5 pages, 7 figure
Observing the anisotropic optical response of the heavy-fermion compound UNi2Al3
The optical conductivity of heavy fermions can reveal fundamental properties
of the charge carrier dynamics in these strongly correlated electron systems.
Here we extend the conventional techniques of infrared optics on heavy fermions
by measuring the transmission and phase shift of THz radiation that passes
through a thin film of UNi2Al3, a material with hexagonal crystal structure. We
deduce the optical conductivity in a previously not accessible frequency range,
and furthermore we resolve the anisotropy of the optical response (parallel and
perpendicular to the hexagonal planes). At frequencies around 7cm^-1, we find a
strongly temperature-dependent and anisotropic optical conductivity that -
surprisingly - roughly follows the dc behavior.Comment: 3 pages, 2 figures, accepted for proceedings of QCnP 200
Spin excitations of the correlated semiconductor FeSi probed by THz radiation
By direct measurements of the complex optical conductivity of
FeSi we have discovered a broad absorption peak centered at frequency
that develops at temperatures below 20 K.
This feature is caused by spin-polaronic states formed in the middle of the gap
in the electronic density of states. We observe the spin excitations between
the electronic levels split by the exchange field of . Spin
fluctuations are identified as the main factor determining the formation of the
spin polarons and the rich magnetic phase diagram of FeSi.Comment: 5 pages, 4 figure
Evidence for a Bulk Complex Order-Parameter in Y0.9Ca0.1Ba2Cu3O7-delta Thin Films
We have measured the penetration depth of overdoped Y0.9Ca0.1Ba2Cu3O7-delta
(Ca-YBCO) thin films using two different methods. The change of the penetration
depth as a function of temperature has been measured using the parallel plate
resonator (PPR), while its absolute value was obtained from a quasi-optical
transmission measurements. Both sets of measurements are compatible with an
order parameter of the form: Delta*dx2-y2+i*delta*dxy, with Delta=14.5 +- 1.5
meV and delta=1.8 meV, indicating a finite gap at low temperature. Below 15 K
the drop of the scattering rate of uncondensed carriers becomes steeper in
contrast to a flattening observed for optimally doped YBCO films. This decrease
supports our results on the penetration depth temperature dependence. The
findings are in agreement with tunneling measurements on similar Ca-YBCO thin
films.Comment: 11 pages, 4 figure
Mobility gap in intermediate valent TmSe
The infrared optical conductivity of intermediate valence compound TmSe
reveals clear signatures for hybridization of light - and heavy f-electronic
states with m* ~ 1.6 m_0 and m* ~ 16 m_0, respectively. At moderate and high
temperatures, the metal-like character of the heavy carriers dominate the
low-frequency response while at low temperatures (T_N < T < 100 K) a gap-like
feature is observed in the conductivity spectra below 10 meV which is assigned
to be a mobility gap due to localization of electrons on local Kondo singlets,
rather than a hybridization gap in the density of states
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
We calculate the low-frequency optical conductivity sigma(w) of clean metals
and superconductors at zero temperature neglecting the effects of impurities
and phonons. In general, the frequency and temperature dependences of sigma
have very little in common. For small Fermi surfaces in three dimensions (but
not in 2D) we find for example that Re sigma(w>0)=const. for low w which
corresponds to a scattering rate Gamma proportional to w^2 even in the absence
of Umklapp scattering when there is no T^2 contribution to Gamma. In the main
part of the paper we discuss in detail the optical conductivity of d-wave
superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest
frequencies and the Umklapp processes typically set in smoothly above a finite
threshold w_0 smaller than twice the maximal gap Delta. In cases where the
nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among
them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure
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