455 research outputs found
Signatures of electron-boson coupling in half-metallic ferromagnet MnGe: study of electron self-energy obtained from infrared spectroscopy
We report results of our infrared and optical spectroscopy study of a
half-metallic ferromagnet MnGe. This compound is currently being
investigated as a potential injector of spin polarized currents into germanium.
Infrared measurements have been performed over a broad frequency (50 - 50000
cm) and temperature (10 - 300 K) range. From the complex optical
conductivity we extract the electron self-energy
. The calculation of is based on novel
numerical algorithms for solution of systems of non-linear equations. The
obtained self-energy provides a new insight into electron correlations in
MnGe. In particular, it reveals that charge carriers may be coupled to
bosonic modes, possibly of magnetic origin
Central star formation in S0 galaxies
As a class, S0 galaxies are characterized by a lack of resolved bright stars in the disk. However, several lines of evidence support the hypothesis that a high rate of star formation is occurring at the centers of some S0 galaxies. Many of the warmest, most powerful far infrared sources in nearby bright galaxies occur in S0 galaxies. (Dressel 1988, Ap. J., 329, L69). The ratios of radio continuum flux to far infrared flux for these S0 galaxies are comparable to the ratios found for spiral galaxy disks and for star-burst galaxies. Very Large Array (VLA) maps of some of these S0 galaxies show that the radio continuum emission originates in the central few kiloparsecs. It is diffuse or clumpy, unlike the radio sources in active S0 galaxies, which are either extremely compact or have jet-lobe structures. Imaging of some of these galaxies at 10.8 microns shows that the infrared emission is also centrally concentrated. Many of the infrared-powerful S0 galaxies are Markarian galaxies. In only one case in this sample is the powerful ultraviolet emission known to be generated by a Seyfert nucleus. Optical spectra of the central few kiloparsecs of these S0 galaxies generally show deep Balmer absorption lines characteristic of A stars, and H beta emission suggestive of gas heated by O stars. A key question to our understanding of these galaxies is whether they really are S0 galaxies, or at least would have been recognized as S0 galaxies before the episode of central star formation began. Some of Nilson's classifications (used here) have been confirmed by Sandage or de Vaucouleurs and collaborators from better plates; some of the galaxies may be misclassified Sa galaxies (the most frequent hosts of central star formation); some are apparently difficult to classify because of mixed characteristics, faint non-S0 features, or peculiarities. More optical imaging is needed to characterize the host galaxies and to study the evolution of their star-forming regions
Evidence for the band broadening across the ferromagnetic transition in CrNbSe
The electronic structure of CrNbSe is studied via optical
spectroscopy. We observe two low-energy interband transitions in the
paramagnetic phase, which split into four peaks as the compound enters the
ferromagnetic state. The band structure calculation indicates the four peaks
are interband transitions to the spin up Cr e states. We show that the peak
splitting below the Curie temperature is \emph{not} due to the exchange
splitting of spin up and down bands, but directly reflects a band broadening
effect in Cr-derived states upon the spontaneous ferromagnetic ordering.Comment: 6 pages, 5 figures, to be published in Phys. Rev.
Asymmetric Lineshape due to Inhomogeneous Broadening of the Crystal-Field Transitions in Mn12ac Single Crystals
The lineshape of crystal-field transitions in single crystals of Mn12ac
molecular magnets is determined by the magnetic history. The absorption lines
are symmetric and Gaussian for the non-magnetized state obtained by zero-field
cooling (zfc). In the magnetized state which is reached when the sample is
cooled in a magnetic field (fc), however, they are asymmetric even in the
absence of an external magnetic field. These observations are quantitatively
explained by inhomogeneous symmetrical (Gaussian) broadening of the
crystal-field transitions combined with a contribution of off-diagonal
components of the magnetic susceptibility to the effective magnetic
permeability.Comment: 4 pages, 3 figure
Correlation gap in the optical spectra of the two-dimensional organic metal (BEDT-TTF)_4[Ni(dto)_2]
Optical reflection measurements within the highly conducting (a,b)-plane of
the organic metal (BEDT-TTF)_4[Ni(dto)_2] reveal the gradual development of a
sharp feature at around 200 cm as the temperature is reduced below 150 K. Below
this frequency a narrow Drude-like response is observed which accounts for the
metallic behavior. Since de Haas-von Alphen oscillations at low temperatures
confirm band structure calculations of bands crossing the Fermi energy, we
assign the observed behavior to a two-dimensional metallic state in the
proximity of a correlation induced metal-insulator transition.Comment: 4 pages, 2 figure
Optical Properties of Heavy Fermion Systems with SDW Order
The dynamical conductivity , reflectivity , and
tunneling density of states of strongly correlated systems (like
heavy fermions) with a spin-density wave (SDW) magnetic order are studied as a
function of impurity scattering rate and temperature. The theory is generalized
to include strong coupling effects in the SDW order. The results are discussed
in the light of optical experiments on heavy-fermion SDW materials. With some
modifications the proposed theory is applicable also to heavy fermions with
localized antiferromagnetic (LAF) order.Comment: 9 pages, 10 figure
Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices
We have fabricated electric double-layer field-effect devices to
electrostatically dope our active materials, either =0.015
GaMnAs or =3.2 GaBeAs. The devices
are tailored for interrogation of electric field induced changes to the
frequency dependent conductivity in the accumulation or depletions layers of
the active material via infrared (IR) spectroscopy. The spectra of the
(Ga,Be)As-based device reveal electric field induced changes to the IR
conductivity consistent with an enhancement or reduction of the Drude response
in the accumulation and depletion polarities, respectively. The spectroscopic
features of this device are all indicative of metallic conduction within the
GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show
enhancement of the far-IR itinerant carrier response and broad mid-IR resonance
upon hole accumulation, with a decrease of these features in the depletion
polarity. These later spectral features demonstrate that conduction in
ferromagnetic (FM) GaMnAs is distinct from genuine metallic
behavior due to extended states in the host VB. Furthermore, these data support
the notion that a Mn-induced impurity band plays a vital role in the electron
dynamics of FM GaMnAs. We add, a sum-rule analysis of the spectra
of our devices suggests that the Mn or Be doping does not lead to a substantial
renormalization of the GaAs host VB
Photon-assisted tunneling in a Fe8 Single-Molecule Magnet
The low temperature spin dynamics of a Fe8 Single-Molecule Magnet was studied
under circularly polarized electromagnetic radiation allowing us to establish
clearly photon-assisted tunneling. This effect, while linear at low power,
becomes highly non-linear above a relatively low power threshold. This
non-linearity is attributed to the nature of the coupling of the sample to the
thermostat.These results are of great importance if such systems are to be used
as quantum computers.Comment: 4 pages, 4 figure
Tc for heavy Fermion superconductors linked with other physical properties at zero and applied pressure
The superconducting transition temperature Tc has earlier been correlated
with coherence length and effective mass for a series of heavy Fermion (HF)
materials at atmospheric pressure. Here, a further physical property, the dc
electrical conductivity sigma(Tc), is one focal point, another being the
pressure dependence of both Tc and sigma(Tc) for several HF materials. The
relaxation time tau(Tc) is also studied in relation to an Uncertainty Principle
limit, involving only the thermal energy kB Tc and Planck's constant.Comment: Supercond. Sci. Tech., to appea
On-chain electrodynamics of metallic (TMTSF)_2 X salts: Observation of Tomonaga-Luttinger liquid response
We have measured the electrodynamic response in the metallic state of three
highly anisotropic conductors, (TMTSF)_2 X, where X=PF_6, AsF_6, or ClO_4, and
TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three
cases we find dramatic deviations from a simple Drude response. The optical
conductivity has two features: a narrow mode at zero frequency, with a small
spectral weight, and a mode centered around 200 cm^{-1}, with nearly all of the
spectral weight expected for the relevant number of carriers and single
particle bandmass. We argue that these features are characteristic of a nearly
one-dimensional half- or quarter-filled band with Coulomb correlations, and
evaluate the finite energy mode in terms of a one-dimensional Mott insulator.
At high frequencies (\hbar\omega > t_\perp, the transfer integral perpendicular
to the chains), the frequency dependence of the optical conductivity
\sigma_1(\omega) is in agreement with calculations based on an interacting
Tomonaga-Luttinger liquid, and is different from what is expected for an
uncorrelated one-dimensional semiconductor. The zero frequency mode shows
deviations from a simple Drude response, and can be adequately described with a
frequency dependent mass and relaxation rate.Comment: 12 pages, 7 figures, RevTeX; minor corrections to text and
references; To be published in Phys. Rev. B, 15 July 199
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