243 research outputs found
Crystals, magnetic and electronic properties of a new ThCr2Si2-type BaMn2Bi2 and K-doped compositions
This is a report on the new 122 ternary transition-metal pnictide of
BaMn2Bi2, which is crystallized from bismuth flux. BaMn2Bi2 adopts
ThCr2Si2-type structure (I4/mmm) with a = 4.4902(3) {\AA} and c = 14.687(1)
{\AA}; it is antiferromagnetic with anisotropic magnetic susceptibility and
semiconducting with a band gap of Eg = 6 meV. Heat capacity result confirms the
insulating ground state in BaMn2Bi2 with the electronic residual Sommerfeld
coefficient of {\gamma} ~ 0. The high temperature magnetization results show
that magnetic ordering temperature is TN ~ 400 K. Hole-doping in BaMn2Bi2 via
potassium in Ba1-xKxMn2Bi2 results in metallic behavior for x = 0.10(1),
0.32(1) and 0.36(1). With K-doping, more magnetically anisotropic behavior is
observed. Although there is a downturn in electrical resistivity and low-field
magnetization data below 4 K in > 30%-doped crystals, there is no sign of zero
resistance or diamagnetism. This manuscript is a report on new materials of
BaMn2Bi2 and Ba1-xKxMn2Bi2 (0 < x < 0.4). Results from powder X-ray
diffraction, anisotropic temperature- and field-dependent magnetization,
temperature-and field-dependent electrical resistivity, and heat capacity are
presented
Local inhomogeneity and filamentary superconductivity in Pr-doped CaFeAs
We use multi-scale techniques to determine the extent of local inhomogeneity
and superconductivity in CaPrFeAs single crystal.
The inhomogeneity is manifested as a spatial variation of praseodymium
concentration, local density of states, and superconducting order parameter. We
show that the high- superconductivity emerges from clover-like defects
associated with Pr dopants. The highest is observed in both the
tetragonal and collapsed tetragonal phases, and its filamentary nature is a
consequence of non-uniform Pr distribution that develops localized, isolated
superconducting regions within the crystals.Comment: Accepted for publication in Phys. Rev. Lett. (January 6, 2014
Magnetic structure and spin excitations in BaMn2Bi2
We present a single crystal neutron scattering study of BaMn2Bi2, a recently
synthesized material with the same ThCr2Si2-type structure found in several
Fe-based unconventional superconducting materials. We show long range magnetic
order, in the form of a G-type antiferromagnetic structure, to exist up to 390
K with an indication of a structural transition at 100 K. Utilizing inelastic
neutron scattering we observe a spin-gap of 16meV, with spin-waves extending up
to 55 meV. We find these magnetic excitations to be well fit to a J1-J2-Jc
Heisenberg model and present values for the exchange interactions. The spin
wave spectrum appears to be unchanged by the 100 K structural phase transition
Fermi-Surface Reconstruction and Complex Phase Equilibria in CaFeAs
Fermi-surface topology governs the relationship between magnetism and
superconductivity in iron-based materials. Using low-temperature transport,
angle-resolved photoemission, and x-ray diffraction we show unambiguous
evidence of large Fermi surface reconstruction in CaFeAs at
magnetic spin-density-wave and nonmagnetic collapsed-tetragonal ()
transitions. For the transition, the change in the Fermi surface topology
has a different character with no contribution from the hole part of the Fermi
surface. In addition, the results suggest that the pressure effect in
CaFeAs is mainly leading to a rigid-band-like change of the valence
electronic structure. We discuss these results and their implications for
magnetism and superconductivity in this material.Comment: Accepted for publication in Phys. Rev. Lett. (April 3, 2014
Direct spectroscopic evidence for completely filled Cu shell in BaCuAs and -BaCuSb
We use angle-resolved photoemission spectroscopy to extract the band
dispersion and the Fermi surface of BaCuAs and -BaCuSb.
While the Cu bands in both materials are located around 3.5 eV below the
Fermi level, the low-energy photoemission intensity mainly comes from As
states, suggesting a completely filled Cu shell. The splitting of the As
core levels and the lack of pronounced three-dimensionality in the
measured band structure of BaCuAs indicate a surface state likely
induced by the cleavage of this material in the collapsed tetragonal phase,
which is consistent with our observation of a Cu oxydation state.
However, the observation of Cu states at similar energy in
-BaCuSb without the pnictide-pnictide interlayer bonding
characteristic of the collapsed tetragonal phase suggests that the short
interlayer distance in BaCuAs follows from the stability of the
Cu rather than the other way around. Our results confirm the prediction
that BaCuAs is an metal with weak electronic correlations.Comment: 6 pages, 4 figure
Temperature-composition Phase Diagrams for Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2
Single crystals of mixed alkaline earth metal iron arsenide materials of
Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2 are synthesized via the self-flux
method. Ba1-xSrxFe2As2 display spin-density wave features (TN) at temperatures
intermediate to the parent materials, x = 0 and 1, with TN(x) following an
approximately linear trend. Cobalt doping of the 1 to 1 Ba:Sr mixture,
Ba0.5Sr0.5(Fe1-yCoy)2As2, results in a superconducting dome with maximum
transition temperature of TC = 19 K at y = 0.092, close to the maximum
transition temperatures observed in unmixed A(Fe1-yCoy)2As2; however, an
annealed crystal with y = 0.141 showed a TC increase from 11 to 16 K with a
decrease in Sommerfeld coefficient from 2.58(2) to 0.63(2) mJ/(K2 mol atom).
For the underdoped y = 0.053, neutron diffraction results give evidence that TN
and structural transition (To) are linked at 78 K, with anomalies observed in
magnetization, resistivity and heat capacity data, while a superconducting
transition at TC ~ 6 K is seen in resistivity and heat capacity data. Scanning
tunneling microscopy measurements for y = 0.073 give Dynes broadening factor of
1.15 and a superconducting gap of 2.37 meV with evidence of surface
inhomogeneity.Comment: Submitted to PR
Absence of structural transition in TM0.5IrTe2 (TM=Mn, Fe, Co, Ni)
TM-doped IrTe2(TM=Mn, Fe, Co, Ni) compounds were synthesized by solid state
reaction. Single crystal x-ray diffraction experiments indicate that part of
the doped TM ions (TM=Fe, Co, and Ni) substitute for Ir, and the rest
intercalate into the octahedral interstitial sites located in between IrTe2
layers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited
solubility in IrTe2 lattice. The trigonal structure is stable in the whole
temperature range 1.80<T<300K for all doped compositions. No long range
magnetic order or superconductivity was observed in any doped compositions
above 1.80K. A spin glass behavior below 10K was observed in Fe-doped IrTe2
from the temperature dependence of magnetization, electrical resistivity, and
specific heat. The low temperature specific heat data suggest the electron
density of states is enhanced in Fe- and Co-doped compositions but reduced in
Ni-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice
parameters increases but the c-lattice parameter decreases. Detailed analysis
of the single crystal x-ray diffraction data shows that interlayer Te-Te
distance increases despite a reduced c-lattice. The importance of the Te-Te,
Te-Ir, and Ir-Ir bonding is discussed.Comment: 8 pages, 7 figure
Complex structures of different CaFeAs samples
The interplay between magnetism and crystal structures in three
CaFeAs samples is studied. For the nonmagnetic quenched crystals,
different crystalline domains with varying lattice parameters are found, and
three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist
between T = 95 K and 45 K. Annealing of the quenched crystals at
350{\deg}C leads to a strain relief through a large (~1.3 %) expansion of the
c-parameter and a small (~0.2 %) contraction of the a-parameter, and to local
~0.2 {\AA} displacements at the atomic-level. This annealing procedure results
in the most homogeneous crystals for which the antiferromagnetic and
orthorhombic phase transitions occur at T/T = 168(1) K. In the
700{\deg}C-annealed crystal, an intermediate strain regime takes place, with
tetragonal and orthorhombic structural phases coexisting between 80 to 120 K.
The origin of such strong shifts in the transition temperatures are tied to
structural parameters. Importantly, with annealing, an increase in the Fe-As
length leads to more localized Fe electrons and higher local magnetic moments
on Fe ions. Synergistic contribution of other structural parameters, including
a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter,
which enhances the Fermi surface nesting in CaFeAs, are also
discussed.Comment: 5 pages main text, 5 figures and 6 pages Supporting Informatio
Spin Glass and Semiconducting Behavior in 1D BaFe2-{\delta}Se3 Crystals
We investigate the physical properties and electronic structure of
BaFe2-{\delta}Se3 crystals, which were grown out of tellurium flux. The crystal
structure of the compound, an iron-deficient derivative of the ThCr2Si2-type,
is built upon edge-shared FeSe4 tetrahedra fused into double chains. The
semiconducting BaFe2-{\delta}Se3 with {\delta} \approx 0.2 ({\rho}295K = 0.18
{\Omega}\cdotcm and Eg = 0.30 eV) does not order magnetically, however there is
evidence for short-range magnetic correlations of spin glass type (Tf \approx
50 K) in magnetization, heat capacity and neutron diffraction results. A
one-third substitution of selenium with sulfur leads to a slightly higher
electrical conductivity ({\rho}295K = 0.11 {\Omega}\cdotcm and Eg = 0.22 eV)
and a lower spin glass freezing temperature (Tf \approx 15 K), corroborating
with higher electrical conductivity reported for BaFe2S3. According to the
electronic structure calculations, BaFe2Se3 can be considered as a
one-dimensional ladder structure with a weak interchain coupling.Comment: 17 pages, 9 figure
Crystal, magnetic, and electronic structures, and properties of new BaMnPnF (Pn = As, Sb, Bi)
New fluoropnictides BaMnPnF with Pn = As, Sb, Bi, are synthesized by
stoichiometric reaction of elements with BaF\_2. The compounds crystallize in
the tetragonal P4/nmm (No. 129, Z = 2) space group, with the ZrCuSiAs-type
structure, as indicated by single crystal and powder X-ray diffraction results.
Electrical resistivity results indicate that Pn = As, Sb, and Bi are
semiconductors with band gaps of E\_g = 0.73 eV, E\_g = 0.48 eV and E\_g =
0.003 eV, respectively. Powder neutron diffraction reveals a G-type
antiferromagnetic order below T\_N = 338(1) K for Pn = As, and below T\_N =
272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above
100 K for all the materials. Density functional calculations also find
semiconducting antiferromagnetic compounds with strong in-plane and weaker
out-of-plane exchange coupling that may result in non-Curie Weiss behavior
above T\_N. There is strong covalency between Mn and pnictogen elements. The
ordered magnetic moments are 3.65(5) {\mu}B/Mn for Pn = As, and 3.66(3)
{\mu}B/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.Comment: Accepted for publication in Scientific Report
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