167 research outputs found
Electronic structure and Magnetism in BaMnAs and BaMnSb
We study the properties of ThCrSi structure BaMnAs and
BaMnSb using density functional calculations of the electronic and
magnetic as well experimental measurements on single crystal samples of
BaMnAs. These materials are local moment magnets with moderate band gap
antiferromagnetic semiconducting ground states. The electronic structures show
substantial Mn - pnictogen hybridization, which stabilizes an intermediate spin
configuration for the nominally Mn. The results are discussed in the
context of possible thermoelectric applications and the relationship with the
corresponding iron / cobalt / nickel compounds Ba(Fe,Co,Ni)As
Synthesis, Structure and Properties of Tetragonal Sr2M3As2O2 (M3 = Mn3, Mn2Cu and MnZn2) Compounds Containing Alternating CuO2-Type and FeAs-Type Layers
Polycrystalline samples of Sr2Mn2CuAs2O2, Sr2Mn3As2O2, and Sr2Zn2MnAs2O2 were
synthesized. Their temperature- and applied magnetic field-dependent
structural, transport, thermal, and magnetic properties were characterized by
means of x-ray and neutron diffraction, electrical resistivity rho, heat
capacity, magnetization and magnetic susceptibility measurements. These
compounds have a body-centered-tetragonal crystal structure (space group
I4/mmm) that consists of MO2 (M = Zn and/or Mn) oxide layers similar to the
CuO2 layers in high superconducting transition temperature Tc cuprate
superconductors, and intermetallic MAs (M = Cu and/or Mn) layers similar to the
FeAs layers in high-Tc pnictides. These two types of layers alternate along the
crystallographic c-axis and are separated by Sr atoms. The site occupancies of
Mn, Cu and Zn were studied using Rietveld refinements of x-ray and neutron
powder diffraction data. The temperature dependences of rho suggest metallic
character for Sr2Mn2CuAs2O2 and semiconducting character for Sr2Mn3As2O2 and
Sr2Zn2MnAs2O2. Sr2Mn2CuAs2O2 is inferred to be a ferrimagnet with a Curie
temperature TC = 95(1) K. Remarkably, we find that the magnetic ground state
structure changes from a G-type antiferromagnetic structure in Sr2Mn3As2O2 to
an A-type ferrimagnetic structure in Sr2Mn2CuAs2O2 in which the Mn ions in each
layer are ferromagnetically aligned, but are antiferromagnetically aligned
between layers.Comment: 18 pages, 16 figures, 6 tables; submitted to Phys. Rev.
Magnetic, Transport, and Thermal Properties of Single Crystals of the Layered Arsenide BaMn2As2
Growth of BaMn2As2 crystals using both MnAs and Sn fluxes is reported. Room
temperature crystallography, anisotropic isothermal magnetization M versus
field H and magnetic susceptibility chi versus temperature T, electrical
resistivity in the ab plane rho(T), and heat capacity C(T) measurements on the
crystals were carried out. The tetragonal ThCr2Si2-type structure of BaMn2As2
is confirmed. After correction for traces of ferromagnetic MnAs impurity phase
using M(H) isotherms, the inferred intrinsic chi(T) data of the crystals are
anisotropic with chi_{ab}/chi_{c} \approx 7.5 at T = 2 K. The temperature
dependences of the anisotropic chi data suggest that BaMn2As2 is a collinear
antiferromagnet at room temperature with the easy axis along the c axis, and
with an extrapolated Neel temperature T_N \sim 500 K. The rho(T) decreases with
decreasing T below 310 K but then increases below \sim 50 K, suggesting that
BaMn2As2 is a small band-gap semiconductor with an activation energy of order
0.03 eV. The C(T) data from 2 to 5 K are consistent with this insulating ground
state, exhibiting a low temperature Sommerfeld coefficient gamma = 0.0(4)
mJ/mol K^2. The Debye temperature is determined from these data to be theta_D =
246(4) K. BaMn2As2 is a potential parent compound for ThCr2Si2-type
superconductors.Comment: 7 pages, 6 figures; v2: typos corrected, additional data and
discussion, accepted for publication in Phys. Rev.
Comprehensive airborne characterization of aerosol from a major bovine source
We report an extensive airborne characterization of aerosol downwind of a massive bovine source in the San Joaquin Valley (California) on two flights during July 2007. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed chemical composition, particle size distribution, mixing state, sub- and supersaturated water uptake behavior, light scattering properties, and the interrelationship between these parameters and meteorology. Total PM_(1.0) levels and concentrations of organics. nitrate. and ammonium were enhanced in the plume from the source as compared to the background aerosol. Organics dominated the plume aerosol mass (~56-64%), followed either by sulfate or nitrate. and then ammonium. Particulate amines were detected in the plume aerosol by a particle-into-liquid sampler (PILS) and via mass spectral inarkers in the Aerodvne C-ToF-AMS. Amines were found to be a significant atmospheric base even in the presence of arnmonia; particulate amine concentrations are estimated as at least 14-23% of that of ammonium in the plume. Enhanced sub- and supersaturated water uptake and reduced refractive indices were coincident with lower organic mass fractions, higher nitrate mass fractions, and the detection of amines. The likelihood of suppressed droplet growth owing to kinetic limitations from hydrophobic organic material is explored. After removing effects associated with size distribution and mixing state, the normalized activated fraction of cloud condensation nuclei (CCN) increased as a function of the subsaturated hygroscopic growth factor, with the highest activated fractions being consistent with relatively lower organic mass fractions and higher nitrate mass fractions. Subsaturated hygroscopic growth factors for the organic fraction of the aerosol are estimated based on employing the Zdanovskii-Stokes Robinson (ZSR) mixing rule. Representative values for a parameterization treating particle water uptake in both the sub- and supersaturated regimes are reported for incorporation into atmospheric models
Two dimensional Dirac fermions and quantum magnetoresistance in CaMnBi
We report two dimensional Dirac fermions and quantum magnetoresistance in
single crystals of CaMnBi. The non-zero Berry's phase, small cyclotron
resonant mass and first-principle band structure suggest the existence of the
Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance
exhibits a crossover at a critical field from semiclassical weak-field
dependence to the high-field unsaturated linear magnetoresistance ( in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The
temperature dependence of satisfies quadratic behavior, which is
attributed to the splitting of linear energy dispersion in high field. Our
results demonstrate the existence of two dimensional Dirac fermions in
CaMnBi with Bi square nets.Comment: 5 pages, 4 figure
Characterisation and airborne deployment of a new counterflow virtual impactor inlet
A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (∼15 l min^(−1)) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter
Chemistry of layered d-metal pnictide oxides and their potential as candidates for new superconductors
Layered d-metal pnictide oxides are a unique class of compounds which
consists of characteristic d-metal pnictide layers and metal oxide layers. More
than 100 of these layered compounds, including the recently discovered Fe-based
superconducting pnictide oxides, can be classified into 9 structure types.
These structure types and the chemical and physical properties of the
characteristic d-metal pnictide layers and metal oxide layers of the layered
d-metal pnictide oxides are reviewed and discussed. Furthermore, possible
approaches to design new superconductors based on these layered d-metal
pnictide oxides are proposed.Comment: 29 pages including 6 tables and 2 figure
Ba{1-x}KxMn2As2: An Antiferromagnetic Local-Moment Metal
The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a
local-moment antiferromagnetic insulator with a Neel temperature TN = 625 K and
a large ordered moment mu = 3.9 mu_B/Mn. We demonstrate that this compound can
be driven metallic by partial substitution of Ba by K, while retaining the same
crystal and antiferromagnetic structures together with nearly the same high TN
and large mu. Ba_{1-x}K_xMn2As2 is thus the first metallic ThCr2Si2-type
MAs-based system containing local 3d transition metal M magnetic moments, with
consequences for the ongoing debate about the local moment versus itinerant
pictures of the FeAs-based superconductors and parent compounds. The
Ba_{1-x}K_xMn2As2 class of compounds also forms a bridge between the layered
iron pnictides and cuprates and may be useful to test theories of high Tc
superconductivity.Comment: 5 two-column typeset pages, 5 figures, 20 references; v2: minor
revisions, 4 new references, published versio
Magnetic and electrical properties and carrier doping effects on the Fe-based host compound Sr4Sc2Fe2As2O6
Additional charge carriers were introduced to the iron oxyarsenide
Sr4Sc2Fe2As2O6 under a high-pressure condition, followed by measurements of
electrical resistivity, Hall coefficient, and magnetic susceptibility. The host
compound Sr4Sc2Fe2As2O6 shows metallic conductivity down to ~200 K and turns to
show a semiconducting-like conductivity accompanied by a positive
magneto-resistance (22% at 70 kOe). Although the carrier density is comparable
at 300 K (5.9x1021 cm-3) with that of the other Fe-based superconductors, no
superconductivity appears down to 2 K. This is primarily because the net
carrier density decreases over 3 orders of magnitude on cooling and
additionally a possible magnetic order at ~120 K prevents carriers from
pairing. The properties were altered largely by introducing the additional
carriers.Comment: 22 pages, 9 figures, 1 table, 41 references, accepted Phys. Rev. B
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