1,032 research outputs found
Nucleation and growth of a quasicrystalline monolayer: Bi adsorption on the five-fold surface of i-Al70Pd21Mn9
Scanning tunnelling microscopy has been used to study the formation of a Bi
monolayer deposited on the five-fold surface of i-Al70Pd21Mn9. Upon deposition
of low sub-monolayer coverages, the nucleation of pentagonal clusters of Bi
adatoms of edge length 4.9 A is observed. The clusters have a common
orientation leading to a film with five-fold symmetry. By inspection of images
where both the underlying surface and the Bi atoms are resolved, the pentagonal
clusters are found to nucleate on pseudo-Mackay clusters truncated such that a
Mn atom lies centrally in the surface plane. The density of these sites is
sufficient to form a quasiperiodic framework, and subsequent adsorption of Bi
atoms ultimately leads to the formation of a quasicrystalline monolayer. The
initial nucleation site is different to that proposed on the basis of recent
density functional theory calculations.Comment: 6 pages, 5 figure
Magnetic excitations and anomalous spin wave broadening in multiferroic FeV2O4
We report on the different roles of two orbital-active Fe at the A
site and V at the B site in the magnetic excitations and on the
anomalous spin wave broadening in FeVO. FeVO exhibits
three structural transitions and successive paramagnetic (PM)-collinear
ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The
high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by
the appearance of an energy gap with a high magnitude in the magnetic
excitations due to strong spin-orbit coupling induced anisotropy at the
Fe site. While there is no measurable increase in the energy gap from
the orbital ordering of V at the orthorhombic/CFI-tetragonal/NCFI
transition, anomalous spin wave broadening is observed in the orthorhombic/CFI
state due to V spin fluctuations at the B site. The spin wave broadening
is also observed at the zone boundary without softening, which is discussed in
terms of magnon-phonon coupling.Comment: 4 pages, 3 figures, including one supplemental materia
Evolution of London penetration depth with scattering in single crystals of KNaFeAs
London penetration depth, , was measured in single crystals of
KNaFeAs, =0 and 0.07, down to temperatures of 50~mK,
. Isovalent substitution of Na for K significantly increases
impurity scattering, with rising from 0.2 to 2.2 cm,
and leads to a suppression of from 3.5~K to 2.8~K. At the same time, a
close to -linear in pure samples changes to almost
in the substituted samples. The behavior never becomes exponential as
expected for the accidental nodes, as opposed to dependence in
superconductors with symmetry imposed line nodes. The superfluid density in the
full temperature range follows a simple clean and dirty -wave dependence,
for pure and substituted samples, respectively. This result contradicts
suggestions of multi-band scenarios with strongly different gap structure on
four sheets of the Fermi surface
Reversible tuning of the surface state in a psuedo-binary Bi2(Te-Se)3 topological insulator
We use angle-resolved photoemission spectroscopy to study non-trivial surface
state in psuedobinary Bi2Se0.6Te2.3 topological insulator. We show that unlike
previously studied binaries, this is an intrinsic topological insulator with
conduction bulk band residing well above the chemical potential. Our data
indicates that under good vacuum condition there are no significant aging
effects for more then two weeks after cleaving. We also demonstrate that shift
of the Kramers point at low temperature is caused by UV assisted absorption of
molecular hydrogen. Our findings pave the way for applications of these
materials in devices and present an easy scheme to tune their properties.Comment: 4 pages, 4 figure
Fermi surface reconstruction in (BaK)FeAs (0.44 1) probed by thermoelectric power measurements
We report in-plane thermoelectric power measurements on single crystals of
(BaK)FeAs (0.44 1). We observe a minimum in
the S versus x at x ~ 0.55 that can be associated with the change
in the topology of the Fermi surface, a Lifshitz transition, related to the
electron pockets at the center of M point crossing the Fermi level. This
feature is clearly observable below ~ 75 K. Thermoelectric power also shows a
change in the x ~ 0.8 - 0.9 range, where maximum in the thermoelectric power
collapses into a plateau. This Lifshitz transition is most likely related to
the reconstruction of the Fermi surface associated with the transformation of
the hole pockets at the M point into four blades as observed by ARPES
measurements.Comment: Accepted for publication in Phys. Rev.
Surface-driven electronic structure in LaFeAsO studied by angle resolved photoemission spectroscopy
We measured the electronic structure of an iron arsenic parent compound
LaFeAsO using angle resolved photoemission spectroscopy (ARPES). By comparing
with a full-potential Linear Augmented PlaneWave calculation we show that the
extra large Gamma hole pocket measured via ARPES comes from electronic
structure at the sample surface. Based on this we discuss the strong
polarization dependence of the band structure and a temperature-dependent
hole-like band around the M point. The two phenomena give additional evidences
for the existence of the surface-driven electronic structure.Comment: 6 pages, 6 figure
X-ray diffuse scattering measurements of chemical short-range order and lattice strains in a highly magnetostrictive Fe0.813Ga0.187 alloy in an applied magnetic field
The rapid growth of the magnetostriction coefficient of ferromagnetic Fe1−xGax alloys that occurs at a composition range from
Dynamic Response in the Low-kHz Range and Delta-E Effect in Ferromagnetic Shape Memory Ni-Mn-Ga
Recent work on ferromagnetic shape memory nickel-manganese-gallium (Ni-Mn-Ga) has demonstrated several characteristics which make this material attractive as an active element for the next generation of intelligent transducers. Alloys of martensitic Ni-Mn-Ga can strain up to 6% as a result of the rotation of twin variants and associated twin boundary motion which occur in these materials in response to magnetic fields. The magnetic actuation holds promise in transducer design because it can lead to enhanced frequency response compared with shape memory alloys with comparable strains. In this paper, we report on experimental measurements collected from a Ni50 Mn28.7 Ga21.3 sample which has been tested in a solenoid transducer by means of a novel drive configuration consisting of a collinear uniaxial field-uniaxial stress pair. We have observed that the elastic modulus of a Ni-Mn-Ga sample driven in these conditions changes substantially in response to varying bias field. In this paper, we further investigate the dependence of the elastic modulus on ac field intensity and mechanical load as well as bias field. Quasistatic, white noise, and swept-sine excitations were employed to examine the behavior of Ni50 Mn28.7 Ga21.3 driven under various combinations of magnetic fields and mechanical loads. Mechanically free quasi-static tests demonstrate reversible strains of 6300 με which are consistent with prior measurements on samples with similar composition near the Heusler stoichiometry. Dynamic measurements reveal a significant stiffness increase, of up to 209%, with dc bias field. This frequency shift or ΔE effect is shown to originate in the Ni-Mn-Ga sample and is believed to stem from the reorientation of twin variants in response to varying dc field. These results might facilitate a new class of solenoid-based Ni-Mn-Ga transducers for tunable vibration absorber applications, and lay the ground work for developing methods and criteria for the implementation of broadband Ni-Mn-Ga transducer technologies
Magnetic Fluctuations, Precursor Phenomena and Phase Transition in MnSi under Magnetic Field
The reference chiral helimagnet MnSi is the first system where skyrmion
lattice correlations have been reported. At zero magnetic field the transition
at to the helimagnetic state is of first order. Above , in a region
dominated by precursor phenomena, neutron scattering shows the build up of
strong chiral fluctuating correlations over the surface of a sphere with radius
, where is the pitch of the helix. It has been suggested that
these fluctuating correlations drive the helical transition to first order
following a scenario proposed by Brazovskii for liquid crystals. We present a
comprehensive neutron scattering study under magnetic fields, which provides
evidence that this is not the case. The sharp first order transition persists
for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and
start to concentrate along the field direction already above 0.2 T. Our results
thus disconnect the first order nature of the transition from the precursor
fluctuating correlations. They also show no indication for a tricritical point,
where the first order transition crosses over to second order with increasing
magnetic field. In this light, the nature of the first order helical transition
and the precursor phenomena above , both of general relevance to chiral
magnetism, remain an open question
Energy Gap Evolution Across the Superconductivity Dome in Single Crystals of (BaK)FeAs
The mechanism of unconventional superconductivity in iron-based
superconductors (IBSs) is one of the most intriguing questions in current
materials research. Among non-oxide IBSs, (BaK)FeAs has
been intensively studied because of its high superconducting transition
temperature and fascinating evolution of the superconducting gap structure from
being fully isotropic at optimal doping (0.4) to becoming nodal at
0.8. Although this marked evolution was identified in several independent
experiments, there are no details of the gap evolution to date because of the
lack of high-quality single crystals covering the entire K-doping range of the
superconducting dome. We conducted a systematic study of the London penetration
depth, , across the full phase diagram for different
concentrations of point-like defects introduced by 2.5 MeV electron
irradiation. Fitting the low-temperature variation with the power law, , we find that the exponent is the highest and
suppression rate with disorder is the smallest at optimal doping, and they
evolve with doping being away from optimal, which is consistent with increasing
gap anisotropy, including an abrupt change around , indicating the
onset of nodal behavior. Our analysis using a self-consistent -matrix
approach suggests the ubiquitous and robust nature of s pairing in IBSs
and argues against a previously suggested transition to a wave state near
in this system
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