265 research outputs found
The Role of Crystal Symmetry in the Magnetic Instabilities of -YbAlB and -YbAlB
Density functional theory methods are applied to investigate the properties
of the new superconductor -YbAlB and its polymorph
-YbAlB. We utilize the generalized gradient approximation + Hubbard
U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of
the strong correlations due to the open shell of Yb. We examine closely
the differences in crystal bonding and symmetry of -YbAlB and
-YbAlB. The in-plane bonding structure amongst the dominant
itinerant electrons in the boron sheets is shown to differ significantly. Our
calculations indicate that, in both polymorphs, the localized 4 electrons
hybridize strongly with the conduction sea when compared to the related
materials YbRhSi and YbB. Comparing -YbAlB to the
electronic structure of related crystal structures indicates a key role of the
7-member boron coordination of the Yb ion in -YbAlB in producing its
enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend
strongly on the angle between the B neighbors and the Yb ion, relative to the
plane, which relates some of the physical behavior to structural
characteristics.Comment: 9 pages, 9 figures, 2 table
Doping driven magnetic instabilities and quantum criticality of NbFe
Using density functional theory we investigate the evolution of the magnetic
ground state of NbFe due to doping by Nb-excess and Fe-excess. We find
that non-rigid-band effects, due to the contribution of Fe-\textit{d} states to
the density of states at the Fermi level are crucial to the evolution of the
magnetic phase diagram. Furthermore, the influence of disorder is important to
the development of ferromagnetism upon Nb doping. These findings give a
framework in which to understand the evolution of the magnetic ground state in
the temperature-doping phase diagram. We investigate the magnetic instabilities
in NbFe. We find that explicit calculation of the Lindhard function,
, indicates that the primary instability is to finite
antiferromagnetism driven by Fermi surface nesting. Total energy
calculations indicate that antiferromagnetism is the ground
state. We discuss the influence of competing and finite
instabilities on the presence of the non-Fermi liquid behavior in
this material.Comment: 8 pages, 7 figure
Pressure-induced unconventional superconductivity near a quantum critical point in CaFe2As2
75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole
resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P =
4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice
relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak
just below Tc(P) and decrease with decreasing temperature. The
superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple
gaps at P = 10.8 kbar. We find that the superconductivity appears near a
quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar.
Both electron correlation and superconductivity disappear in the collapsed
tetragonal phase. A systematic study under pressure indicates that electron
correlations play a vital role in forming Cooper pairs in this compound.Comment: 5pages, 5figure
Modern microwave methods in solid state inorganic materials chemistry: from fundamentals to manufacturing
No abstract available
Synthesis and properties of La-doped CaFe2As2 single crystals with Tc = 42.7 K
Large single crystals of La-doped CaFe2As2 were successfully synthesized by
the FeAs self-flux method. The x-ray diffraction patterns suggest high
crystalline quality and c-axis orientation. By substitution of trivalent La3+
ions for divalent Ca2+, the resistivity anomaly in the parent compound CaFe2As2
is completely suppressed and a superconducting transition reaches the value of
42.7 K, which is higher than that of about 30 K reported in Saha S. R. et al.,
arXiv:1105.4798v1. The upper critical field has been determined with the
magnetic field along ab-plane and c-axis, yielding an anisotropy of about 3.3.Comment: 13 pages, 5 figure
Structural properties and superconductivity of SrFe2As2-xPx and CaFe2As2-yPy
The SrFe2As2-xPx and CaFe2As2-yPy materials were prepared by a solid state
reaction method. X-ray diffraction measurements indicate the single-phase
samples can be successfully obtained for SrFe2As2-xPx and CaFe2As2-yPy samples.
Clear contraction of the lattice parameters are clearly determined due to the
relatively smaller P ions substation for As. The SDW instability associated
with tetragonal to orthorhombic phase transition is suppressed visibly in both
systems following with the increase of P contents. The highest superconducting
transitions are respectively observed at about 27 K in SrFe2As1.3P0.7 and at
about 13 K in CaFe2As1.7P0.3.Comment: 11 pages, 5 figures, 2 table
- …