254 research outputs found
Relativistic J-matrix method
The relativistic version of the J-matrix method for a scattering problem on
the potential vanishing faster than the Coulomb one is formulated. As in the
non-relativistic case it leads to a finite algebraic eigenvalue problem. The
derived expression for the tangent of phase shift is simply related to the
non-relativistic case formula and gives the latter as a limit case. It is due
to the fact that the used basis set satisfies the ``kinetic balance
condition''.Comment: 21 pages, RevTeX, accepted for publication in Phys. Rev.
Universal magnetic and structural behaviors in the iron arsenides
Commonalities among the order parameters of the ubiquitous antiferromagnetism
present in the parent compounds of the iron arsenide high temperature
superconductors are explored. Additionally, comparison is made between the well
established two-dimensional Heisenberg-Ising magnet, KNiF and iron
arsenide systems residing at a critical point whose structural and magnetic
phase transitions coincide. In particular, analysis is presented regarding two
distinct classes of phase transition behavior reflected in the development of
antiferromagnetic and structural order in the three main classes of iron
arsenide superconductors. Two distinct universality classes are mirrored in
their magnetic phase transitions which empirically are determined by the
proximity of the coupled structural and magnetic phase transitions in these
materials.Comment: 6 pages, 4 figure
NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBaCuO
Nuclear magnetic resonance (NMR) measurements of CuO chains of detwinned
Ortho-II YBaCuO (YBCO6.5) single crystals reveal unusual and
remarkable properties. The chain Cu resonance broadens significantly, but
gradually, on cooling from room temperature. The lineshape and its temperature
dependence are substantially different from that of a conventional spin/charge
density wave (S/CDW) phase transition. Instead, the line broadening is
attributed to small amplitude static spin and charge density oscillations with
spatially varying amplitudes connected with the ends of the finite length
chains. The influence of this CuO chain phenomenon is also clearly manifested
in the plane Cu NMR.Comment: 4 pages, 3 figures, refereed articl
Electron doping evolution of the magnetic excitations in BaFe2-xNixAs2
We use inelastic neutron scattering (INS) spectroscopy to study the magnetic
excitations spectra throughout the Brioullion zone in electron-doped iron
pnictide superconductors BaFeNiAs with .
While the sample is near optimal superconductivity with K
and has coexisting static incommensurate magnetic order, the
samples are electron-overdoped with reduced of 14 K and 8 K,
respectively, and have no static antiferromagnetic (AF) order. In previous INS
work on undoped () and electron optimally doped () samples, the
effect of electron-doping was found to modify spin waves in the parent compound
BaFeAs below 100 meV and induce a neutron spin resonance at the
commensurate AF ordering wave vector that couples with superconductivity. While
the new data collected on the sample confirms the overall features of
the earlier work, our careful temperature dependent study of the resonance
reveals that the resonance suddenly changes its -width below similar
to that of the optimally hole-doped iron pnictides
BaKFeAs. In addition, we establish the dispersion of
the resonance and find it to change from commensurate to transversely
incommensurate with increasing energy. Upon further electron-doping to
overdoped iron pnictides with and 0.18, the resonance becomes weaker
and transversely incommensurate at all energies, while spin excitations above
100 meV are still not much affected. Our absolute spin excitation
intensity measurements throughout the Brillouin zone for
confirm the notion that the low-energy spin excitation coupling with itinerant
electron is important for superconductivity in these materials, even though the
high-energy spin excitations are weakly doping dependent.Comment: 16 pages, 16 figure
Antiferromagnetic Critical Fluctuations in BaFeAs
Magnetic correlations near the magneto-structural phase transition in the
bilayer iron pnictide parent compound, BaFeAs, are measured. In close
proximity to the antiferromagnetic phase transition in BaFeAs, a
crossover to three dimensional critical behavior is anticipated and has been
preliminarily observed. Here we report complementary measurements of
two-dimensional magnetic fluctuations over a broad temperature range about
T. The potential role of two-dimensional critical fluctuations in the
magnetic phase behavior of BaFeAs and their evolution near the
anticipated crossover to three dimensional critical behavior and long-range
order are discussed.Comment: 6 pages, 4 figures; Accepted for publication in Physical Review
Magnetic structure of the antiferromagnetic half-Heusler compound NdBiPt
We present results of single crystal neutron diffraction experiments on the
rare-earth, half-Heusler antiferromagnet (AFM) NdBiPt. This compound exhibits
an AFM phase transition at ~K with an ordered moment of
~ per Nd atom. The magnetic moments are aligned along
the -direction, arranged in a type-I AFM structure with ferromagnetic
planes, alternating antiferromagnetically along a propagation vector of
. The BiPt (= Ce-Lu) family of materials has been proposed as
candidates of a new family of antiferromagnetic topological insulators (AFTI)
with magnetic space group that corresponds to a type-II AFM structure where
ferromagnetic sheets are stacked along the space diagonal. The resolved
structure makes it unlikely, that NdBiPt qualifies as an AFTI.Comment: As resubmitted to PRB, corrected typos and changed symbols in Fig.
Magnetic properties of LaFe1-xCrxO3 and Fe2-2xCr2xO3 mixed oxides
Mixed oxides with formula LaFe1-xCrxO3 and Fe2-2xCr2xO3, where 0≤x≤1, are studied. The samples have been prepared using solid state reaction technique in air. The X-ray diffraction spectra indicated that the samples crystallize in a corundum phase with space group ( R3c ) for Fe2-2xCr2xO3 and in the perovskite structure for LaFe1-xCrxO3. Many techniques have been used to explore the magnetic properties of the systems. High field, ZFC and FC magnetization vs. temperature, d.c. susceptibility and Mössbauer spectroscopy were carried out. High temperature magnetic susceptibility measurements and high field magnetic magnetization (H ≤ 20 T) show that the behavior of the susceptibility and the magnetization are complex. Mössbauer spectra of the solid solutions have been measured at 4.2 K and in the temperature range 77 K to 300 K. The shapes of spectra are unusual, showing strong relaxation phenomena in a wide temperature range as recently observed for many frustrated systems. The results are discussed by establishing the existence of various magnetic structures, inducing intermediate magnetic phases between the antiferromagnetic and the paramagnetic states. Preliminary magnetic phase diagrams of the systems have been established.Mixed oxides with formula LaFe1-xCrxO3 and Fe2-2xCr2xO3, where 0≤x≤1, are studied. The samples have been prepared using solid state reaction technique in air. The X-ray diffraction spectra indicated that the samples crystallize in a corundum phase with space group ( R3c ) for Fe2-2xCr2xO3 and in the perovskite structure for LaFe1-xCrxO3. Many techniques have been used to explore the magnetic properties of the systems. High field, ZFC and FC magnetization vs. temperature, d.c. susceptibility and Mössbauer spectroscopy were carried out. High temperature magnetic susceptibility measurements and high field magnetic magnetization (H ≤ 20 T) show that the behavior of the susceptibility and the magnetization are complex. Mössbauer spectra of the solid solutions have been measured at 4.2 K and in the temperature range 77 K to 300 K. The shapes of spectra are unusual, showing strong relaxation phenomena in a wide temperature range as recently observed for many frustrated systems. The results are discussed by establishing the existence of various magnetic structures, inducing intermediate magnetic phases between the antiferromagnetic and the paramagnetic states. Preliminary magnetic phase diagrams of the systems have been established
J-matrix method of scattering in any L2 basis
The restriction imposed on the J-matrix method of using specific L2 bases is
lifted without compromising any of the advantages that it offers. This opens
the door to a wider range of application of the method to physical problems
beyond the restrictive SO(2,1) dynamical symmetry. The numerical scheme
developed to achieve this objective projects the J-matrix formalism in terms of
the eigenvalues of a finite Hamiltonian matrix and its submatrices in any
convenient L2 basis. Numerical stability and convergence of the original
analytic J-matrix method is still maintained in the proposed scheme, which can
be applied to multi-channel nonrelativistic as well as relativistic scattering
problems
The nature of the magnetic and structural phase transitions in BaFeAs
We present the results of an investigation of both the magnetic and
structural phase transitions in a high quality single crystalline sample of the
undoped, iron pnictide compound BaFeAs. Both phase transitions are
characterized via neutron diffraction measurements which reveal simultaneous,
continuous magnetic and structural orderings with no evidence of hysteresis,
consistent with a single second order phase transition. The onset of long-range
antiferromagnetic order can be described by a simple power law dependence
with ; a
value near the expected for a two-dimensional Ising system.
Biquadratic coupling between the structural and magnetic order parameters is
also inferred along with evidence of three-dimensional critical scattering in
this system.Comment: New figure and discussion added. Length: 11 pages, 7 figure
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