940 research outputs found
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Polarization control at the microscopic and electronic structure observatory
The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%
A universal high energy anomaly in angle resolved photoemission spectra of high temperature superconductors - possible evidence of spinon and holon branches
A universal high energy anomaly in the single particle spectral function is
reported in three different families of high temperature superconductors by
using angle-resolved photoemission spectroscopy. As we follow the dispersing
peak of the spectral function from the Fermi energy to the valence band
complex, we find dispersion anomalies marked by two distinctive high energy
scales, E_1=~ 0.38 eV and E_2=~0.8 eV. E_1 marks the energy above which the
dispersion splits into two branches. One is a continuation of the near
parabolic dispersion, albeit with reduced spectral weight, and reaches the
bottom of the band at the gamma point at ~0.5 eV. The other is given by a peak
in the momentum space, nearly independent of energy between E_1 and E_2. Above
E_2, a band-like dispersion re-emerges. We conjecture that these two energies
mark the disintegration of the low energy quasiparticles into a spinon and
holon branch in the high T_c cuprates.Comment: accepted for publication in Phys. Rev. Let
Instability of two dimensional graphene: Breaking sp2 bonds with soft X-rays
We study the stability of various kinds of graphene samples under soft X-ray
irradiation. Our results show that in single layer exfoliated graphene (a
closer analogue to two dimensional material), the in-plane carbon-carbon bonds
are unstable under X-ray irradiation, resulting in nanocrystalline structures.
As the interaction along the third dimension increases by increasing the number
of graphene layers or through the interaction with the substrate (epitaxial
graphene), the effect of X-ray irradiation decreases and eventually becomes
negligible for graphite and epitaxial graphene. Our results demonstrate the
importance of the interaction along the third dimension in stabilizing the long
range in-plane carbon-carbon bonding, and suggest the possibility of using
X-ray to pattern graphene nanostructures in exfoliated graphene.Comment: 4 pages, 3 figures, Phys. Rev. B rapid communication, in pres
Magnetic effects in sulfur-decorated graphene
The interaction between two different materials can present novel phenomena
that are quite different from the physical properties observed when each
material stands alone. Strong electronic correlations, such as magnetism and
superconductivity, can be produced as the result of enhanced Coulomb
interactions between electrons. Two-dimensional materials are powerful
candidates to search for the novel phenomena because of the easiness of
arranging them and modifying their properties accordingly. In this work, we
report magnetic effects of graphene, a prototypical non-magnetic
two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic
insulator. In contrast to the well-defined metallic behaviour of clean
graphene, an energy gap develops at the Fermi energy for the graphene/sulfur
compound with decreasing temperature. This is accompanied by a steep increase
of the resistance, a sign change of the slope in the magneto-resistance between
high and low fields, and magnetic hysteresis. A possible origin of the observed
electronic and magnetic responses is discussed in terms of the onset of
low-temperature magnetic ordering. These results provide intriguing insights on
the search for novel quantum phases in graphene-based compounds.Comment: 6 pages and 5 figure
Pnictogens Allotropy and Phase Transformation during van der Waals Growth
Pnictogens have multiple allotropic forms resulting from their ns2 np3
valence electronic configuration, making them the only elemental materials to
crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout
the group. Light group VA elements are found in the layered orthorhombic A17
phase such as black phosphorus, and can transition to the layered rhombohedral
A7 phase at high pressure. On the other hand, bulk heavier elements are only
stable in the A7 phase. Herein, we demonstrate that these two phases not only
co-exist during the vdW growth of antimony on weakly interacting surfaces, but
also undertake a spontaneous transformation from the A17 phase to the
thermodynamically stable A7 phase. This metastability of the A17 phase is
revealed by real-time studies unraveling its thickness-driven transition to the
A7 phase and the concomitant evolution of its electronic properties. At a
critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle
transition from AB to AA stacked alpha-antimonene followed by a gradual
relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of
this intermediate phase is found to be determined by surface self-passivation
and the associated competition between A7- and A17-like bonding in the bulk.
These results highlight the critical role of the atomic structure and
interfacial interactions in shaping the stability and electronic
characteristics of vdW layered materials, thus enabling a new degree of freedom
to engineer their properties using scalable processes
Bilayer splitting and c-axis coupling in bilayer manganites showing colossal magnetoresistance
By performing angle-resolved photoemission spectroscopy of the bilayer
colossal magnetoresistive (CMR) manganite, , we
provide the complete mapping of the Fermi level spectral weight topology. Clear
and unambiguous bilayer splitting of the in-plane 3d band, mapped
throughout the Brillouin zone, and the full mapping of the 3d band
are reported. Peculiar doping and temperature dependencies of these bands imply
that as transition from the ferromagnetic metallic phase approaches, either as
a function of doping or temperature, coherence along the c-axis between planes
within the bilayer is lost, resulting in reduced interplane coupling. These
results suggest that interplane coupling plays a large role in the CMR
transition.Comment: 8 pages, 6 figure
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MERLIN - A meV resolution beamline at the ALS
An ultra-high resolution beamline is being constructed at the Advanced Light Source (ALS) for the study of low energy excitations in strongly correlated systems with the use of high-resolution inelastic scattering and angle-resolved photoemission. This new beamline, given the acronym Merlin (for meV resolution line), will cover the energy range 10-150 eV. The monochromator has fixed entrance and exit slits and a plane mirror that can illuminate a spherical grating at the required angle of incidence (as in the SX-700 mechanism). The monochromator can be operated in two different modes. In the highest resolution mode, the energy scanning requires translating the monochromator chamber (total travel 1.1 m) as well as rotating the grating and the plane mirror in front of the grating. The resolution in this mode is practically determined by the slits width. In the second mode, the scanning requires rotating the grating and the plane mirror. This mode can be used to scan a few eV without a significant resolution loss. The source for the beamline is a 1.9 m long, 90 mm period quasi periodic EPU. The expected flux at the sample is higher than 10 photons/s at a resolving power of 5 × 10 in the energy range 16-130 eV. A second set of gratings can be used to obtain higher flux at the expense of resolution. © 2007 American Institute of Physics. 11
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