139 research outputs found
Metamagnetic transitions and anomalous magnetoresistance in EuAgAs single crystal
In this paper, the magnetic and transport properties were systematically
studied for EuAgAs single crystals, crystallizing in a centrosymmetric
trigonal CaCuP type structure. It was confirmed that two magnetic
transitions occur at = 10 K and = 15 K,
respectively. With the increasing field, the two transitions are noticeably
driven to lower temperature. At low temperatures, applying a magnetic field in
the plane induces two successive metamagnetic transitions. For
both and
, EuAgAs shows a positive, unexpected large
magnetoresistance (up to 202\%) at low fields below 10 K, and a large negative
magnetoresistance (up to -78\%) at high fields/intermediate temperatures. Such
anomalous field dependence of magnetoresistance may have potential application
in the future magnetic sensors. Finally, the magnetic phase diagrams of
EuAgAs were constructed for both
and
Fe-based high temperature superconductivity with Tc=31K bordering an insulating antiferromagnet in (Tl,K)FexSe2 Crystals
Up to now, there have been two material families, the cuprates and the
iron-based compounds with high-temperature superconductivity (HTSC). An
essential open question is whether the two classes of materials share the same
essential physics. In both, superconductivity (SC) emerges when an
antiferromagnetical (AFM) ordered phase is suppressed. However, in cuprates,
the repulsive interaction among the electrons is so strong that the parent
compounds are "Mott insulators." By contrast, all iron-based parents are
metallic. One perspective is that the iron-based parents are weakly correlated
and that the AFM arises from a strong "nesting" of the Fermi surfaces. An
alternative view is that the electronic correlations in the parents are still
sufficiently strong to place the system close to the boundary between
itinerancy and electronic localization. A key strategy to differentiate theses
views is to explore whether the iron-based system can be tuned into a Mott
insulator. Here we identify an insulating AFM in (Tl,K)FexSe2 by introducing
Fe-vacancies and creating superconductivity in the Fe-planar. With the
increasing Fe-content, the AFM order is reduced. When the magnetism is
eliminated, a superconducting phase with Tc as high as 31K (and a Tc onset as
high as 40K) is induced. Our findings indicate that the correlation effect
plays a crucial role in the iron-based superconductors. (Tl,K)FexSe2,
therefore, represents the first Fe-based high temperature superconductor near
an insulating AFM.Comment: 20 pages, 8 figure
Spontaneous breaking and re-making of the RS-Au-SR staple in self-assembled ethylthiolate/Au(111) interface
The stability of
the self-assembled RS–Au–SR (R =
CH<sub>2</sub>CH<sub>3</sub>)/AuÂ(111) interface at room temperature
has been investigated using scanning tunneling microscopy (STM) in
conjunction with density functional theory (DFT) and MD calculations.
The RS–Au–SR staple, also known as Au-adatom-dithiolate,
assembles into staple rows along the [112Ì…] direction. STM imaging
reveals that while the staple rows are able to maintain a static global
structure, individual staples within the row are subjected to constant
breaking and remaking of the Au–SR bond. The C<sub>2</sub>S–Au–SC<sub>2</sub>/AuÂ(111) interface is under a dynamic equilibrium and it is
far from rigid. DFT/MD calculations show that a transient RS–Au–Au–SR
complex can be formed when a free Au atom is added to the RS–Au–SR
staple. The relatively high reactivity of the RS–Au–SR
staple at room temperature could explain the reactivity of thiolate-protected
Au nanoclusters, such as their ability to participate in ligand exchange
and intercluster reactions
Superconductivity at 32K and anisotropy in Tl0.58Rb0.42Fe1.72Se2 crystals
Single crystals of Tl0.58Rb0.42Fe1.72Se2 are successfully grown with the
superconducting transition temperatures Tconset=32K and Tczero=31.4K. The Hall
coefficient exhibits a multi-band behavior, which is very similar to that of
all other Fe-based superconductors. We found that the susceptibility at the
normal state decreases with decreasing the temperature, indicating a strong
antiferromagnetic (AFM) spin fluctuation at the normal state, which might be
related to the superconductivity (SC). We also determined the upper critical
fields in ab-plane and along c-axis. The anisotropy of the superconductivity
determined by the ratio of Hc2ab and Hc2c is estimated to 5.0, which is larger
than that in (Ba,K)Fe2As2 and BaFe2-xCoxAs2, but smaller than that in cuprate
superconductors.Comment: 4 pages, 4 figure
- …