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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Magnetic structures and interplay between rare-earth Ce and Fe magnetism in single-crystal CeFeAsO

Neutron and synchrotron resonant x-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in a CeFeAsO single crystal. The RXMS of magnetic reflections at the Ce L II edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at T Ce ≈ 4 K with short-range Ce ordering above T Ce , whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at T*≈ 12 K with an unusual order parameter. Detailed order-parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K that we associate with the Ce ordering. The successive transitions at T Ce and T* can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at T*≈ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the ab plane. The Fe spin reorientation transition and short-range Ce ordering above T Ce reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through T* and T Ce is proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microscopic parameters from high-resolution specific heat measurements on superoptimally substituted BaFe 2 ( As 1 - x P x ) 2 single crystals

We investigate the electronic specific heat of superoptimally substituted BaFe 2 (As 1-x P x ) 2 single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Δ, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump ΔC ∝ to T$3\atop{c}$ , found in many iron-based superconductors, in this system originates from a T c -dependent ratio Delta/k B T c in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n s yields a penetration depth lambda that decreases with increasing T c without sharp divergence at the quantum critical point. Uemura scaling indicates that T c is governed by the Fermi temperature T F for this multiband system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Superconductivity between standard types: Multiband versus single-band materials

In the nearest vicinity of the critical temperature, types I and II of conventional single-band superconductors interchange at the Ginzburg-Landau parameter κ = 1/√2. At lower temperatures this point unfolds into a narrow but finite interval of κ’s, shaping an intertype (transitional) domain in the (κ,T ) plane. In the present work, based on the extended Ginzburg-Landau formalism, we show that the same picture of the two standard types with the transitional domain in between applies also to multiband superconductors. However, the intertype domain notably widens in the presence of multiple bands and can become extremely large when the system has a significant disparity between the band parameters. It is concluded that many multiband superconductors, such as recently discovered borides and iron-based materials, can belong to the intertype regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Effective Control of the Charge and Magnetic States of Transition-Metal Atoms on Single-Layer Boron Nitride

Developing approaches to effectively control the charge and magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we suggest that the magnetic and charge states of transition-metal (TM) doped single-layer boron-nitride (SLBN) systems can be easily controlled by the (internal) defect engineering and (external) electric fields (E{sub ext}). The relative positions and symmetries of the in-gap levels induced by defect engineering and the TM d-orbital energy levels effectively determine the charge states and magnetic properties of the TM/SLBN system. Remarkably, the application of an E{sub ext} can easily control the size of the crystal field splitting of the TM d orbitals and thus, leading to the spin crossover in TM/SLBN, which could be used as E{sub ext}-driven nonvolatile memory devices. Our conclusion obtained from TM/SLBN is valid generally in other TM adsorbed layered semiconductors.

36 MATERIALS SCIENCE↗