Materials Science and Engineering Colloquium
Lei Fang, from Northwestern University, will present a talk at the Materials Science and Engineering Colloquium.
High Mobility Charge Carriers in Two-dimensional Semimetal
[Pb2BiS3][AuTe2] and Huge Critical Current
Density in Iron-based Superconductor SmFeAsO0.8F0.15
Department of Chemistry, Northwestern University, Evanston,
Materials Science Division, Argonne National Laboratory,
Studying two-dimensional (2D) materials is a mainstream in material
science due to the unique properties, emerging physics, and great impacts on next-generation
microelectronic devices. Currently, materials discovery is a compelling mission
in 2D materials research. Inspired by the discovery of quantum spin Hall
insulator state in HgTe quantum wells, our research focuses on novel electronic
states in heterostructure-like materials with strong spin-orbit coupling. In
this talk, I will first introduce our recent research on layered material [Pb2BiS3][AuTe2] which is composed of insulating layer [Pb2BiS3] and
conductive sheet [AuTe2]. This naturally formed heterostructure
exhibits a multiband electronic structure featuring high hole mobility of 1360
cm2/Vs. The insulating layer [Pb2BiS3] and the
weak interlayer coupling lead to an extremely large anisotropy Г≈104,
comparable to that of benchmark 2D materials graphite and Bi2Sr2CaCu2O8.
The weak interlayer coupling also endows this material with a highly cleavable
property and enables nano crystals with a thickness smaller than 10 nm via
exfoliation. More strikingly, this material exhibits linear band dispersions
which coincide with the half branch Dirac cone of graphene. Ultrahigh Fermi
velocities 106 m/s and light effective mass 0.046 me of
carriers were found. Our results provide
a novel candidate for a monolayer platform to investigate emerging electronic
properties at the atomic sale .
second part of my talk focuses on the newly discovered iron-based
superconductors. Due to the relatively high superconducting transition
temperature (TC) and
critical current density (JC),
iron-based superconductors have prompted great enthusiasm for electricity
distribution and high superconducting magnet applications. SmFeAsO1-xFx
is of particular interest as it has the highest TC (57 K) among these materials. However, the knowledge
of application potential of SmFeAsO1-xFx remains limited
because of the lack of sizeable single crystals for study. To meet this
challenge, we fabricated micro Hall magnetometer array and micro calorimeter to
characterize micrometer-sized SmFeAsO0.8F0.15 crystals
which incorporate columnar nano defects via particle irradiation. A record high
JC of 2×107
A/cm2 was observed at 5 K and self-fields. We also observed a
notable reduction of the thermodynamic anisotropy, from 8 to 4 upon
irradiation. The huge JC,
low anisotropy and high TC
of SmFeAsO0.8F0.15 strongly suggest that this material
may be the best iron-based superconductor for application .
the end of my talk, I will briefly discuss my future directions on 2D
electronic materials with highlights on synergistic research between design,
theoretical calculations, synthesis and advanced characterizations.
Particularly, I will introduce my 2D materials database which contains more
than 1000 novel inorganic systems. This database can be used for structure
mining to pursue powerful electronic and energy materials.
References L. Fang, et. al. to appear in JACS.  L. Fang, et al. Nat. Commun. doi: 10.1038/ncomms 3655 (2013).
Host: Irving Herman
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