523 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
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
Absence of fine structure in the photoemission spectrum of the icosahedral Al-Pd-Mn quasicrystal
The results of the low-temperature ultrahigh-energy-resolution photoemission studies of a single-grain icosahedral alloy Al70Pd21.5Mn8.5 have been presented. The existence of the theoretically predicted pseudogap in the density of states at the Fermi energy has been confirmed. No evidence of the theoretically predicted spikiness of the density of states could be observed. It has been suggested that the failure to detect the predicted spikiness of the density of states in icosahedral quasicrystals is consistent with the results of Zijlstra and Janssen [Europhys. Lett. 52, 578 (2000)] who showed that the spikiness occurs only in lower-order periodic approximants to icosahedral quasicrystals, but does not survive in the quasiperiodic limit
Magnetostriction of iron-germanium single crystals
The addition of nonmagnetic Ga into body-centered cubic Fe enhances the magnetostriction constant λ100 over tenfold. Literature reports for substitution of Ge at low concentrations suggest that the addition of Ge also enhances the magnetostriction. In this work, the magnetostriction and microstructure of Fe–Ge were investigated to correlate magnetostriction with microstructure. The magnetostriction of Fe100−xGexsingle crystals with x between 0.05 and 0.18 varies with Ge concentration and correlates with phase changes. The value of (3/2)λ100 increases with Ge additions in the A2 single phase region (up to x ∼ 10), reaching a maximum of 94 ppm at the solubility limit of the disordered A2 phase. Further increases in Ge in the A2+D03 two-phase region (12\u3cx\u3c16)result in a decrease in magnetostriction which changes from positive to negative. For Ge contents with x\u3e16, magnetostriction remains negative with an absolute value of strain of129 ppm at 18 at. % Ge. This behavior is similar to that observed for Fe–Si alloys
Polarized Light Microscopy Study on the Reentrant Phase Transition in a (Ba1-xKx) Fe2As2 Single Crystal with x=0.24
A sequence of structural/magnetic transitions on cooling is reported in the literature for hole-doped iron-based superconductor (Ba1 − xKx)Fe2As2 with x = 0.24. By using polarized light microscopy, we directly observe the formation of orthorhombic domains in (Ba1 − xKx)Fe2As2 (x = 0.24) single crystal below a temperature of simultaneous structural/magnetic transition TN ~ 80 K. The structural domains vanish below ~30 K, but reappear below T = 15 K. Our results are consistent with reentrance transformation sequence from high-temperature tetragonal (HTT) to low temperature orthorhombic (LTO1) structure at TN ~ 80 K, LTO1 to low temperature tetragonal (LTT) structure at Tc ~ 25 K, and LTT to low temperature orthorhombic (LTO2) structure at T ~ 15 K
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
Elastic properties of MnSi, FeSi and CoSi
Measurements of sound velocities in single crystals of MnSi, FeSi, and CoSi were performed in the temperature range 2.5-300 K and elastic constants were calculated. The temperature dependence of the elastic constants reveal nontrivial features, reflecting specifics of the magnetic and electronic subsystems in these materials
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