111 research outputs found
Superconducting and ferromagnetic phases induced by lattice distortions in SrFe2As2
Single crystals of SrFe2As2 grown using a self-flux solution method were
characterized via x-ray, transport and magnetization studies, revealing a
superconducting phase below T_c = 21 K characterized by a full electrical
resistivity transition and partial diamagnetic screening. The reversible
destruction and reinstatement of this phase by heat treatment and mechanical
deformation studies, along with single-crystal X-ray diffraction measurements,
indicate that internal crystallographic strain originating from c-axis-oriented
planar defects plays a central role in promoting the appearance of
superconductivity under ambient pressure conditions in ~90% of as-grown
crystals. The appearance of a ferromagnetic moment with magnitude proportional
to the tunable superconducting volume fraction suggests that these phenomena
are both stabilized by lattice distortion.Comment: 4 pages, 4 figure
Tunable electronic anisotropy in single-crystal A2Cr3As3 (A = K, Rb) quasi-one-dimensional superconductors
Single crystals of A2Cr3As3 (A = K, Rb) were successfully grown using a
self-flux method and studied via structural, transport and thermodynamic
measurement techniques. The superconducting state properties between the two
species are similar, with critical temperatures of 6.1 K and 4.8 K in K2Cr3As3
and Rb2Cr3As3, respectively. However, the emergence of a strong normal state
electronic anisotropy in Rb2Cr3As3 suggests a unique electronic tuning
parameter is coupled to the inter-chain spacing in the A2Cr3As3 structure,
which increases with alkali metal ionic size while the one-dimensional
[(Cr3As3)^{2-}]_{\infty} chain structure itself remains essentially unchanged.
Together with dramatic enhancements in both conductivity and magnetoresistance
(MR), the appearance of a strong anisotropy in the MR of Rb2Cr3As3 is
consistent with the proposed quasi-one-dimensional character of band structure
and its evolution with alkali metal species in this new family of
superconductors.Comment: 6 pages, 8 figures; to appear in Phys. Rev.
Superconductivity at 23 K in Pt doped BaFe2As2 single crystals
We report superconductivity in single crystals of the new iron-pnictide
system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized
via x-ray, transport, magnetic and thermodynamic measurements. The magnetic
ordering associated with a structural transition at 140 K present in BaFe2As2
is completely suppressed by substitution of 5% Fe with Pt and superconductivity
is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the
magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk
nature of the superconducting phase. All properties of the superconducting
state including transition temperature Tc, the lower critical field Hc1=200 mT,
upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to
the those found in other transition-metal-substituted BaFe2As2 series,
indicating the robust nature of superconductivity induced by substitution of
Group VIII elements.Comment: 6 pgs, 4 figs, and 1 tbl, slightly revised, updated reference
Anomalous symmetry breaking in Weyl semimetal CeAlGe
CeAlGe, a proposed type-II Weyl semimetal, orders antiferromagnetically below
5 K. At 2 K, spin-flop and a spin-flip transitions to less than 1 /Ce
are observed in the data below 30 kOe, ( and ,
and 4.3 kOe, , respectively, indicating a four-fold
symmetry of the data along the principal directions in the tetragonal
plane with set of easy directions. However,
anomalously robust and complex twofold symmetry is observed in the angular
dependence of resistivity and magnetic torque data in the magnetically ordered
state once the field is swept in the plane. This twofold symmetry is
independent of temperature and field hystereses and suggests a magnetic phase
transition that separates two different magnetic structures in the
plane. The boundary of this magnetic phase transition and possibly the type of
low-field magnetic structure can be tuned by an Al deficiency
Single-Crystal Investigation of the Proposed Type-II Weyl Semimetal CeAlGe
We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in a noncentrosymmetric I41md space group, in line with predictions of nontrivial topology. Magnetization, specific heat, and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of Δ = 9.11 K from heat capacity and hole-like carrier density of 1.4 x 1020 cm-3 from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal ab plane below 7 K. A spin-flop transition to less than 1 μB/Ce is observed to occur below 30 kOe at 1.8 K in the M(H) (H||a) data. Small magnetic fields of 3 and 30 kOe are sufficient to suppress magnetic order when applied along the a and c axes, respectively, resulting in a complex T-H phase diagram for H||a and a simpler one for H||c
Single crystal investigation of proposed type-II Weyl semimetal CeAlGe
We present details of materials synthesis, crystal structure, and anisotropic
magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl
semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in
noncentrosymmetric I4md space group, in line with predictions of
non-trivial topology. Magnetization, specific heat and electrical transport
measurements were used to confirm antiferromagnetic order below 5 K, with an
estimated magnon excitation gap of = 9.11 K from heat capacity and
hole-like carrier density of 1.44 10 cm from Hall effect
measurements. The easy magnetic axis is along the [100] crystallographic
direction, indicating that the moment lies in the tetragonal -plane
below 7 K. A spin-flop transition to less than 1 /Ce is observed to
occur below 30 kOe at 1.8 K in the () data. Small
magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order
when applied along the - and -axes, respectively, resulting in
a complex phase diagram for and a simpler one for
Structural collapse and superconductivity in rare earth-doped CaFe2As2
Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2
single-crystals is used to fine-tune structural, magnetic and electronic
properties of this iron-based superconducting system. Neutron and single
crystal x-ray scattering experiments indicate that an isostructural collapse of
the tetragonal unit cell can be controllably induced at ambient pressures by
choice of substituent ion size. This instability is driven by the interlayer
As-As anion separation, resulting in an unprecedented thermal expansion
coefficient of K. Electrical transport and magnetic
susceptibility measurements reveal abrupt changes in the physical properties
through the collapse as a function of temperature, including a reconstruction
of the electronic structure. Superconductivity with onset transition
temperatures as high as 47 K is stabilized by the suppression of
antiferromagnetic order via chemical pressure, electron doping or a combination
of both. Extensive investigations are performed to understand the observations
of partial volume-fraction diamagnetic screening, ruling out extrinsic sources
such as strain mechanisms, surface states or foreign phases as the cause of
this superconducting phase that appears to be stable in both collapsed and
uncollapsed structures.Comment: 15 pages, 18 figure
Temperature and pressure evolution of the crystal structure of Ax(Fe1-ySe)2 (A = Cs, Rb, K) studied by synchrotron powder diffraction
Temperature-dependent synchrotron powder diffraction on Cs0.83(Fe0.86Se)2
revealed first order I4/m to I4/mmm structural transformation around 216{\deg}C
associated with the disorder of the Fe vacancies. Irreversibility observed
during the transition is likely associated with a mobility of intercalated
Alkali atoms. Pressure-dependent synchrotron powder diffraction on
Cs0.83(Fe1-ySe)2, Rb0.85(Fe1-ySe)2 and K0.8(Fe1-ySe)2 (y ~ 0.14) indicated that
the I4/m superstructure reflections are present up to pressures of 120 kbar.
This may indicate that the ordering of the Fe vacancies is present in both
superconducting and non-superconductive states.Comment: 11 pages, 5 figures, 1 tabl
Electronic and Magnetic Structures of Chain Structured Iron Selenide Compounds
Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2
(2212\ast) and BaFe2Se3(123\ast) are studied by the first-principles
calculations. We find that while all these compounds are composed of
one-dimensional (1D) Fe chain (or ladder) structures, their electronic
structures are not close to be quasi-1D. The magnetic exchange couplings
between two nearest-neighbor (NN) chains in 2212\ast and between two NN
two-leg-ladders in 123\ast are both antiferromagnetic (AFM), which is
consistent with the presence of significant third NN AFM coupling, a common
feature shared in other iron-chalcogenides, FeTe (11\ast) and KyFe2-xSe2
(122\ast). In magnetic ground states, each Fe chain of 2212\ast is
ferromagnetic and each two-leg ladder of 123\ast form a block-AFM structure. We
suggest that all magnetic structures in iron-selenide compounds can be unified
into an extended J1-J2-J3 model. Spin-wave excitations of the model are
calculated and can be tested by future experiments on these two systems.Comment: 6 pages, 6 figures, 2 table
Phase Separation and Magnetic Order in K-doped Iron Selenide Superconductor
Alkali-doped iron selenide is the latest member of high Tc superconductor
family, and its peculiar characters have immediately attracted extensive
attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2)
thin films by molecular beam epitaxy and unambiguously demonstrated the
existence of phase separation, which is currently under debate, in this
material using scanning tunneling microscopy and spectroscopy. The
stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while
the insulating phase has a \surd5\times\surd5 vacancy order. The iron vacancies
are shown always destructive to superconductivity in KFe2Se2. Our study on the
subgap bound states induced by the iron vacancies further reveals a
magnetically-related bipartite order in the superconducting phase. These
findings not only solve the existing controversies in the atomic and electronic
structures in KxFe2-ySe2, but also provide valuable information on
understanding the superconductivity and its interplay with magnetism in
iron-based superconductors
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