Engineering topics

Lv, B.

Publications and source records attributed to Lv, B..

3 records

DOE OSTI2019

Interface-Induced and Interface-Enhanced Superconductivity

Among the various theoretical mechanisms proposed to reach a higher Tc, the interface mechanism in different forms has been the one most explored, and it provides continual inspiration and hope for realizing the ultimate goal for researchers in the superconductivity field—room-temperature superconductivity. Difficulties do exist, as most of the materials proposed to exhibit the interfacial mechanism are artificially formed heterostructures and are by nature delicate and easily disturbed by strain and change in the stoichiometry at the interface. The discoveries of superconductivity in naturally assembled single crystals of rare earth (R)-doped CaFe2As2 (Ca122) with a Tc up to 49 K and undoped CaFe2As2 with a Tc up to 25 K have opened an alternate route to tackle the problem. The experimental observation has provided the most direct evidence for interface-induced superconductivity in Ca122 to date and possibly in R-doped Ca122. Given the fact that neither chemical doping nor the application of physical pressure has ever induced a Tc higher than 40 K in bulk FeSe, the reports of Tc up to 45–109 K in FeSe/STO thin films demonstrate that FeSe/STO is an ideal system in which to explore interface-enhanced superconductivity. Here we review the above three specific examples of our effort toward superconductors of higher Tc to demonstrate that the interface mechanism may be a promising paradigm to achieve higher Tc.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Effects of Nickel Doping on the Multiferroic and Magnetic Phases of MnWO 4

There are various orders in multiferroic materials with a frustrated spiral spin modulation inducing a ferroelectric state are extremely sensitive to small perturbations such as magnetic and electric fields, external pressure, or chemical substitutions. A classical multiferroic, the mineral Hubnerite with chemical formula MnWO 4 , shows three different magnetic phases at low temperature. The intermediate phase between 7.5K < T < 12.7K is multiferroic and ferroelectricity is induced by an inversion symmetry breaking spiral Mn-spin order and strong spin-lattice interactions. Furthermore, the substitution of Ni 2+ (spin 1) for Mn 2+ (spin 5/2) in MnWO 4 and its effects on the magnetic and multiferroic phases are studied. The ferroelectric phase is stabilized for low Ni content (up to 10%). Upon further Ni doping, the polarization in the ferroelectric phase is quickly suppressed while a collinear and commensurate magnetic phase, characteristic of the magnetic structure in NiWO 4 , appears first at higher temperature, gradually extends to lower temperature, and becomes the ground state above 30% doping. Between 10% and 30%, the multiferroic phase coexists with the collinear commensurate phase. In this concentration region, the spin spiral plane is close to the a-b plane which explains the drop of the ferroelectric polarization. Finally, the phase diagram of Mn 1-x Ni x WO 4 is derived by a combination of magnetic susceptibility, specific heat, electric polarization, and neutron scattering measurements.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Observation of universal strong orbital-dependent correlation effects in iron chalcogenides

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe 0.56 Se 0.44 , monolayer FeSe grown on SrTiO 3 and K 0.76 Fe 1.72 Se 2 . We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. As a result, these observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗