12 research outputs found
Crossover between Weak Antilocalization and Weak Localization of Bulk States in Ultrathin Bi2Se3 Films
We report transport studies on the 5 nm thick Bi2Se3 topological insulator
films which are grown via molecular beam epitaxy technique. The angle-resolved
photoemission spectroscopy data show that the Fermi level of the system lies in
the bulk conduction band above the Dirac point, suggesting important
contribution of bulk states to the transport results. In particular, the
crossover from weak antilocalization to weak localization in the bulk states is
observed in the parallel magnetic field measurements up to 50 Tesla. The
measured magneto-resistance exhibits interesting anisotropy with respect to the
orientation of B// and I, signifying intrinsic spin-orbit coupling in the
Bi2Se3 films. Our work directly shows the crossover of quantum interference
effect in the bulk states from weak antilocalization to weak localization. It
presents an important step toward a better understanding of the existing
three-dimensional topological insulators and the potential applications of
nano-scale topological insulator devices
High temperature superconducting FeSe films on SrTiO3 substrates
Interface enhanced superconductivity at two dimensional limit has become one
of most intriguing research directions in condensed matter physics. Here, we
report the superconducting properties of ultra-thin FeSe films with the
thickness of one unit cell (1-UC) grown on conductive and insulating SrTiO3
(STO) substrates. For the 1-UC FeSe on conductive STO substrate (Nb-STO), the
magnetization versus temperature (M-T) measurement shows a diamagnetic signal
at 85 K, suggesting the possibility of superconductivity appears at this high
temperature. For the FeSe films on insulating STO substrate, systematic
transport measurements were carried out and the sheet resistance of FeSe films
exhibits Arrhenius TAFF behavior with a crossover from a single-vortex pinning
region to a collective creep region. More intriguing, sign reversal of Hall
resistance with temperature is observed, demonstrating a crossover from hole
conduction to electron conduction above Tc in 1-UC FeSe films
Direct observation of high temperature superconductivity in one-unit-cell FeSe films
Heterostructure based interface engineering has been proved an effective
method for finding new superconducting systems and raising superconductivity
transition temperature (TC). In previous work on one unit-cell (UC) thick FeSe
films on SrTiO3 (STO) substrate, a superconducting-like energy gap as large as
20 meV, was revealed by in situ scanning tunneling microscopy/spectroscopy
(STM/STS). Angle resolved photoemission spectroscopy (ARPES) further revealed a
nearly isotropic gap of above 15 meV, which closes at a temperature of ~ 65 K.
If this transition is indeed the superconducting transition, then the 1-UC FeSe
represents the thinnest high TC superconductor discovered so far. However, up
to date direct transport measurement of the 1-UC FeSe films has not been
reported, mainly because growth of large scale 1-UC FeSe films is challenging
and the 1-UC FeSe films are too thin to survive in atmosphere. In this work, we
successfully prepared 1-UC FeSe films on insulating STO substrates with
non-superconducting FeTe protection layers. By direct transport and magnetic
measurements, we provide definitive evidence for high temperature
superconductivity in the 1-UC FeSe films with an onset TC above 40 K and a
extremely large critical current density JC ~ 1.7*106 A/cm2 at 2 K. Our work
may pave the way to enhancing and tailoring superconductivity by interface
engineering
Magnetic anisotropy of epitaxial La2/3Sr1/3MnO3 thin films on SrTiO3 with different orientations
Epitaxial La2/3Sr1/3MnO3 thin films with different crystallographic orientations were fabricated on (001)-, (110)-, and (111)-oriented SrTiO3 substrates by pulsed laser deposition. Out-of-plane magnetic anisotropy was studied with the field angle fixed at 0°, 30°, 60°, and 90° relative to the film surface. The results show that there is a remarkable dependence of the magnetization on the magnetic field direction and crystallographic orientation. Furthermore, the (110)- and (111)-oriented thin films show stronger angular-dependent magnetic anisotropy than the (001) film, and the (110)-oriented one can reach the saturated magnetization more easily than the other two films. Such findings are correlated with the strain imposed on the films via substrates with different orientations. Our results have implications for the better understanding of magnetic anisotropy and the tunability of the magnetoelectric coupling coefficient involving multiferroic composite thin films