692 research outputs found
Surface-electronic structure of La(0001) and Lu(0001)
Most spectroscopic methods for studying the electronic structure of metal
surfaces have the disadvantage that either only occupied or only unoccupied
states can be probed, and the signal is cut at the Fermi edge. This leads to
significant uncertainties, when states are very close to the Fermi level. By
performing low-temperature scanning tunneling spectroscopy and ab initio
calculations, we study the surface-electronic structure of La(0001) and
Lu(0001), and demonstrate that in this way detailed information on the
surface-electronic structure very close to the Fermi energy can be derived with
high accuracy.Comment: 6 pages, 4 figures, 1 table submitted to PR
Temperature-induced reversal of magnetic interlayer exchange coupling
For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange
coupled through a non-magnetic Y spacer layer, element-specific hysteresis
loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth
thresholds. This allowed us to quantitatively determine the strength of
interlayer exchange coupling (IEC). In addition to the expected oscillatory
behavior as a function of spacer-layer thickness , a temperature-induced
sign reversal of IEC was observed for constant , arising from
magnetization-dependent electron reflectivities at the magnetic interfaces.Comment: 4 pages, 4 figures; accepted version; minor changes and new Figs. 2
and 4 containing more dat
Clean and Dirty Superconductivity in Pure, Al doped, and Neutron Irradiated MgB2: a Far-Infrared Study
The effects of Al substitution and neutron irradiation on the conduction
regime (clean or dirty) of the - and -band of MgB have been
investigated by means of far-infrared spectroscopy. The intensity reflected by
well characterized polycrystalline samples was measured up to 100 cm in
both normal and superconducting state. The analysis of the superconducting to
normal reflectivity ratios shows that only the effect of the opening of the
small gap in the dirty -band can be clearly observed in pure MgB,
consistently with previous results. In Al-doped samples the dirty character of
the -band is increased, while no definitive conclusion on the conduction
regime of the -band can be drawn. On the contrary, results obtained
for the irradiated sample show that the irradiation-induced disorder drives the
-band in the dirty regime, making the large gap in -band
observable for the first time in far-infrared measurements.Comment: 11 pages, 1 figur
Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases
We report a comprehensive experimental study and detailed model analysis of
the terahertz dielectric response and density kinetics of excitons and unbound
electron-hole pairs in GaAs quantum wells. A compact expression is given, in
absolute units, for the complex-valued terahertz dielectric function of
intra-excitonic transitions between the 1s and higher-energy exciton and
continuum levels. It closely describes the terahertz spectra of resonantly
generated excitons. Exciton ionization and formation are further explored,
where the terahertz response exhibits both intra-excitonic and Drude features.
Utilizing a two-component dielectric function, we derive the underlying exciton
and unbound pair densities. In the ionized state, excellent agreement is found
with the Saha thermodynamic equilibrium, which provides experimental
verification of the two-component analysis and density scaling. During exciton
formation, in turn, the pair kinetics is quantitatively described by a Saha
equilibrium that follows the carrier cooling dynamics. The terahertz-derived
kinetics is, moreover, consistent with time-resolved luminescence measured for
comparison. Our study establishes a basis for tracking pair densities via
transient terahertz spectroscopy of photoexcited quasi-two-dimensional
electron-hole gases.Comment: 14 pages, 8 figures, final versio
Broadband electromagnetic response and ultrafast dynamics of few-layer epitaxial graphene
We study the broadband optical conductivity and ultrafast carrier dynamics of
epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally
buffer, monolayer, and multilayer graphene exhibit significant terahertz and
near-infrared absorption, consistent with a model of intra- and interband
transitions in a dense Dirac electron plasma. Non-equilibrium terahertz
transmission changes after photoexcitation are shown to be dominated by excess
hole carriers, with a 1.2-ps mono-exponential decay that reflects the
minority-carrier recombination time.Comment: 4 pages, 3 figures, final versio
Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the Kondo system Ce2Rh3Al9
We have used temperature dependent x-ray absorption at the Ce-L3 edge to
investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic
changes of the spectral lineshape with decreasing temperature are analyzed and
found to be related to a change in the occupation number, n_f, as the
system undergoes a transition into a Kondo state. The temperature dependence of
indicates a characteristic temperature of 150K, which is clearly related
with the high temperature anomaly observed in the magnetic susceptibility of
the same system. The further anomaly observed in the resistivity of this system
at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo
origin.Comment: 7 pages, three figures, submitted to PR
Autosomal Recessive Primary Microcephaly: Not Just a Small Brain
Microcephaly or reduced head circumference results from a multitude of abnormal developmental processes affecting brain growth and/or leading to brain atrophy. Autosomal recessive primary microcephaly (MCPH) is the prototype of isolated primary (congenital) microcephaly, affecting predominantly the cerebral cortex. For MCPH, an accelerating number of mutated genes emerge annually, and they are involved in crucial steps of neurogenesis. In this review article, we provide a deeper look into the microcephalic MCPH brain. We explore cytoarchitecture focusing on the cerebral cortex and discuss diverse processes occurring at the level of neural progenitors, early generated and mature neurons, and glial cells. We aim to thereby give an overview of current knowledge in MCPH phenotype and normal brain growth
Nodal quasiparticle meltdown in ultra-high resolution pump-probe angle-resolved photoemission
High- cuprate superconductors are characterized by a strong
momentum-dependent anisotropy between the low energy excitations along the
Brillouin zone diagonal (nodal direction) and those along the Brillouin zone
face (antinodal direction). Most obvious is the d-wave superconducting gap,
with the largest magnitude found in the antinodal direction and no gap in the
nodal direction. Additionally, while antinodal quasiparticle excitations appear
only below , superconductivity is thought to be indifferent to nodal
excitations as they are regarded robust and insensitive to . Here we
reveal an unexpected tie between nodal quasiparticles and superconductivity
using high resolution time- and angle-resolved photoemission on optimally doped
BiSrCaCuO. We observe a suppression of the nodal
quasiparticle spectral weight following pump laser excitation and measure its
recovery dynamics. This suppression is dramatically enhanced in the
superconducting state. These results reduce the nodal-antinodal dichotomy and
challenge the conventional view of nodal excitation neutrality in
superconductivity.Comment: 7 pages, 3 figure. To be published in Nature Physic
Assessment of myelination in infants and young children by T1 relaxation time measurements using the magnetization-prepared 2 rapid acquisition gradient echoes sequence
Background: Axonal myelination is an important maturation process in the developing brain. Increasing myelin content correlates with the longitudinal relaxation rate (R1=1/T1) in magnetic resonance imaging (MRI).
Objective: By using magnetization-prepared 2 rapid acquisition gradient echoes (MP2RAGE) on a 3-T MRI system, we provide R1 values and myelination rates for infants and young children.
Materials and methods: Average R1 values in white and grey matter regions in 94 children without pathological MRI findings (age range: 3 months to 6 years) were measured and fitted by a saturating-exponential growth model. For comparison, R1 values of 36 children with different brain pathologies are presented. The findings were related to a qualitative evaluation using T2, magnetization-prepared rapid acquisition gradient echo (MP-RAGE) and MP2RAGE.
Results: R1 changes rapidly in the first 16 months of life, then much slower thereafter. R1 is highest in pre-myelinated structures in the youngest subjects, such as the posterior limb of the internal capsule (0.74-0.76 +/- 0.04 s(-1)) and lowest for the corpus callosum (0.37-0.44 +/- 0.03 s(-1)). The myelination rate is fastest in the corpus callosum and slowest in the deep grey matter. R1 is decreased in hypo- and dysmyelination disorders. Myelin maturation is clearly visible on MP2RAGE, especially in the first year of life.
Conclusion: MP2RAGE permits a quantitative R1 mapping method with an examination time of approximately 6 min. The age-dependent R1 values for children without MRI-identified brain pathologies are well described by a saturating-exponential function with time constants depending on the investigated brain region. This model can serve as a reference for this age group and to search for indications of subtle pathologies. Moreover, the MP2RAGE sequence can also be used for the qualitative assessment of myelinated structures
XANES study of rare-earth valency in LRu4P12 (L = Ce and Pr)
Valency of Ce and Pr in LRu4P12 (L = Ce and Pr) was studied by L2,3-edge
x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES
spectrum suggests that Ce is mainly trivalent, but the 4f state strongly
hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by
strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr
exists in trivalent state over a wide range in temperature, 20 < T < 300 K. We
find that the metal-insulator (MI) transition at TMI = 60 K in PrRu4P12 does
not originate from Pr valence fluctuation.Comment: 4 page
- …