51 research outputs found
Magnetism and superconductivity in the layered hexagonal transition metal pnictides
We investigate the electronic and magnetic structures of the 122
(AMB) hexagonal transition-metal pnictides with A=(Sr, Ca), M=(Cr, Mn,
Fe, Co, Ni) and B=(As, P, Sb). It is found that the family of materials share
critical similarities with those of tetragonal structures that include the
famous iron-based high temperature superconductors. In both families, the next
nearest neighbor(NNN) effective antiferromagnetic(AFM) exchange couplings reach
the maximum value in the iron-based materials. While the NNN couplings in the
latter are known to be responsible for the C-type AFM state and to result in
the extended s-wave superconducting state upon doping, they cause the former to
be extremely frustrated magnetic systems and can lead to an time reversal
symmetry broken superconducting state upon doping. The iron-based
compounds with the hexagonal structure, thus if synthesized, can help us to
determine the origin of high temperature superconductivity.Comment: 10 pages, 8 figure
Topological Characters in Fe(TeSe) thin films
We investigate topological properties in the Fe(Te,Se) thin films. We find
that the single layer FeTeSe has nontrivial topological
invariance which originates from the parity exchange at point of
Brillouin zone. The nontrivial topology is mainly controlled by the Te(Se)
height. Adjusting the height, which can be realized as function of in
FeTeSe, can drive a topological phase transition. In a bulk
material, the two dimensional topology invariance is extended to a strong
three-dimensional one. In a thin film, we predict that the topological
invariance oscillates with the number of layers. The results can also be
applied to iron-pnictides. Our research establishes FeTeSe as a
unique system to integrate high T superconductivity and topological
properties in a single electronic structure.Comment: 4.5 pages and 5 figure
Experimental consequences of -wave spin triplet superconductivity in ACrAs
The experimental observable properties of the triplet -wave pairing
state, proposed by Wu {\em et al.} [arXiv:1503.06707] in quasi-one dimensional
ACrAs materials, are theoretically investigated. This pairing state
is characterized by the line nodes on the plane on the Fermi surfaces.
Based on the three-band tight binding model, we obtain the specific heat,
superfluid density, Knight shift and spin relaxation rate and find that all
these properties at low temperature () show powerlaw behaviors and
are consistent available experiments. Particularly, the superfluid density
determined by the -wave pairing state in this quasi-one dimensional system
is anisotropic: the in-plane superfluid density varies as
but the out-plane one varies as
at low temperature. The anisotropic upper critical
field reported in experiment is consistent with the (i.e.,
) -wave pairing state. We also
suggest the phase-sensitive dc-SQUID measurements to pin down the triplet
-wave pairing state.Comment: 5 pages, 5 figures, + supplemental materials, Fig.3 is update
Theoretical studies of superconductivity in doped BaCoSO
We investigate superconductivity that may exist in the doped BaCoSO, a
multi-orbital Mott insulator with a strong antiferromagnetic ground state. The
superconductivity is studied in both t-J type and Hubbard type multi-orbital
models by mean field approach and random phase approximation (RPA) analysis.
Even if there is no C4 rotational symmetry, it is found that the system still
carries a d-wave like pairing symmetry state with gapless nodes and sign
changed superconducting order parameters on Fermi surfaces. The results are
largely doping insensitive. In this superconducting state, the three t2g
orbitals have very different superconducting form factors in momentum space. In
particular, the intra-orbital pairing of the dx2-y2 orbital has a s-wave like
pairing form factor. The two methods also predict very different pairing
strength on different parts of Fermi surfaces.These results suggest that BaCoSO
and related materials can be a new ground to test and establish fundamental
principles for unconventional high temperature superconductivity.Comment: 6 pages, 7 figure
CaFeAs: a Staggered Intercalation of Quantum Spin Hall and High Temperature Superconductivity
We predict that CaFeAs, a newly discovered iron-based high temperature
(T) superconductor, is a staggered intercalation compound that integrates
topological quantum spin hall (QSH) and superconductivity (SC). CaFeAs has
a structure with staggered CaAs and FeAs layers. While the FeAs layers are
known to be responsible for high T superconductivity, we show that with
spin orbital coupling each CaAs layer is a topologically nontrivial
two-dimensional QSH insulator and the bulk is a 3-dimensional weak topological
insulator. In the superconducting state, the edge states in the CaAs layer are
natural 1D topological superconductors. The staggered intercalation of QSH and
SC provides us an unique opportunity to realize and explore novel physics, such
as Majorana modes and Majorana Fermions chains.Comment: 4.5 pages, 5 figures + supplemental material,published versio
Three-dimensional Critical Dirac semimetal in KMgBi
We predicted that AMgBi (A=K,Rb Cs), which have the same lattice structures
as the 111 family of iron-based superconductors (Na/LiFeAs), are
symmetry-protected Dirac semimetals located near the boundary of type-I and
type-II Dirac semimetal phases. Doping Rb or Cs into KMgBi can drive the
transition between the two phases. The materials can also be turned into Weyl
semimetals and topological insulators by explicitly or spontaneously breaking
time-reversal symmetry and C lattice symmetry respectively.Comment: 5 pages, 4 figure
Topological Vortex Phase Transitions in Iron-Based Superconductors
We study topological vortex phases in iron-based superconductors. Besides the
previously known vortex end Majorana zero modes (MZMs) phase stemming from the
existence of a three dimensional (3D) strong topological insulator state, we
show that there is another topologically nontrivial phase as iron-based
superconductors can be doped superconducting 3D weak topological insulators
(WTIs). The vortex bound states in a superconducting 3D WTI exhibit two
different types of quantum states, a robust nodal superconducting phase with
pairs of bulk MZMs and a full-gap topologically nontrivial superconducting
phase which has single vortex end MZM in a certain range of doping level.
Moreover, we predict and summarize various topological phases in iron-based
superconductors, and find that carrier doping and interlayer coupling can drive
systems to have phase transitions between these different topological phases
The effect of As-Chain layers in CaFeAs
The new discovered iron-based superconductors have chain-like As layers.
These layers generate an additional 3-dimensional hole pocket and cone-like
electron pockets. The former is attributed to the Ca and As1 orbitals
and the latter are attributed to the anisotropic Dirac cone, contributed by As1
and orbitals. We find that large gaps on these pockets open in the
collinear antiferromagnetic ground state of CaFeAs, suggesting that the
chain-like As layers are strongly coupled to FeAs layers. Moreover due to the
low symmetry crystal induced by the As layers, the bands attributed to FeAs
layers in plane are two-fold degenerate but in plane are
lifted. This degeneracy is protected by a hidden symmetry
. Ignoring the electron cones, the materials
can be well described by a six-band model, including five Fe and As1
orbitals. We suggest that these new features may help us to identify the sign
change and pairing symmetry in iron based superconductors.Comment: 7 pages, 10 figure
Topological Critical Materials of Ternary Compounds
We review topological properties of two series of ternary compounds AMgBi
(A=K, RB, Cs) and ABC with a hexagonal ZrBeSi type structure. The first series
of materials AMgBi are predicted to be topological critical Dirac semimetals.
The second series of ternary compounds, such as KZnP, BaAgAs, NaAuTe and KHgSb,
can be used to realize various topological insulating states and semimetal
states. The states are highly tunable as the realization of these topological
states depends on the competition between several energy scales, including the
energy of atomic orbitals, the energy of crystal splitting, the energy
difference between bonding and antibonding states, and the strength of
spin-orbit coupling. The exotic surface states in these series of compounds are
predicted and are closely related to their unique crystal structures
-CuI: a Dirac semimetal without surface Fermi arcs
Anomalous surface states with Fermi arcs are commonly considered to be a
fingerprint of Dirac semimetals (DSMs). In contrast to Weyl semimetals,
however, Fermi arcs of DSMs are not topologically protected. Using
first-principles calculations, we predict that -CuI is a peculiar DSM
whose surface states form closed Fermi pockets instead of Fermi arcs. In such a
fermiological Dirac semimetal, the deformation mechanism from Fermi arcs to
Fermi pockets stems from a large cubic term preserving all crystal symmetries,
and the small energy difference between the surface and bulk Dirac points. The
cubic term in -CuI, usually negligible in prototypical DSMs, becomes
relevant because of the particular crystal structure. As such, we establish a
concrete material example manifesting the lack of topological protection for
surface Fermi arcs in DSMsComment: 6 pages, 4 figure
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