31 research outputs found
Experimental observation of Dirac-like surface states and topological phase transition in PbSnTe(111) films
The surface of a topological crystalline insulator (TCI) carries an even
number of Dirac cones protected by crystalline symmetry. We epitaxially grew
high quality PbSnTe(111) films and investigated the TCI phase by
in-situ angle-resolved photoemission spectroscopy. PbSnTe(111)
films undergo a topological phase transition from trivial insulator to TCI via
increasing the Sn/Pb ratio, accompanied by a crossover from n-type to p-type
doping. In addition, a hybridization gap is opened in the surface states when
the thickness of film is reduced to the two-dimensional limit. The work
demonstrates an approach to manipulating the topological properties of TCI,
which is of importance for future fundamental research and applications based
on TCI
Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of
unique physics properties and could be used in the development of electronics,
photonics, spintronics and quantum computing devices. The mechanical
exfoliation technique of micro-size TMD flakes has attracted particular
interest due to its simplicity and cost effectiveness. However, for most
applications, large area and high quality films are preferred. Furthermore,
when the thickness of crystalline films is down to the 2D limit (monolayer),
exotic properties can be expected due to the quantum confinement and symmetry
breaking. In this paper, we have successfully prepared macro-size atomically
flat monolayer NbSe2 films on bilayer graphene terminated surface of
6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films
exhibit an onset superconducting critical transition temperature above 6 K, 2
times higher than that of mechanical exfoliated NbSe2 flakes. Simultaneously,
the transport measurements at high magnetic fields reveal that the parallel
characteristic field Bc// is at least 4.5 times higher than the paramagnetic
limiting field, consistent with Zeeman-protected Ising superconductivity
mechanism. Besides, by ultralow temperature electrical transport measurements,
the monolayer NbSe2 film shows the signature of quantum Griffiths singularity
when approaching the zero-temperature quantum critical point
Detection of a superconducting phase in a two-atom layer of hexagonal Ga film grown on semiconducting GaN(0001)
The recent observation of superconducting state at atomic scale has motivated
the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we
report on a superconducting phase in two-monolayer crystalline Ga films
epitaxially grown on wide band-gap semiconductor GaN(0001). This phase exhibits
a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings
about a superconducting transition temperature Tc as high as 5.4 K, confirmed
by in situ scanning tunneling spectroscopy, and ex situ electrical
magneto-transport and magnetization measurements. The anisotropy of critical
magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed,
typical for the 2D superconductivity. Our results demonstrate a novel platform
for exploring atomic-scale 2D superconductor, with great potential for
understanding of the interface superconductivity
KFe_2Se_2 is the parent compound of K-doped iron selenide superconductors
We elucidate the existing controversies in the newly discovered K-doped iron
selenide (KxFe2-ySe2-z) superconductors. The stoichiometric KFe2Se2 with
\surd2\times\surd2 charge ordering was identified as the parent compound of
KxFe2-ySe2-z superconductor using scanning tunneling microscopy and
spectroscopy. The superconductivity is induced in KFe2Se2 by either Se
vacancies or interacting with the anti-ferromagnetic K2Fe4Se5 compound. Totally
four phases were found to exist in KxFe2-ySe2-z: parent compound KFe2Se2,
superconducting KFe2Se2 with \surd2\times\surd5 charge ordering,
superconducting KFe2Se2-z with Se vacancies and insulating K2Fe4Se5 with
\surd5\times\surd5 Fe vacancy order. The phase separation takes place at the
mesoscopic scale under standard molecular beam epitaxy condition