63 research outputs found
Spin-Excitations Anisotropy in the Bilayer Iron-Based Superconductor CaKFeAs
We use polarized inelastic neutron scattering to study the spin-excitations
anisotropy in the bilayer iron-based superconductor CaKFeAs ( = 35
K). In the superconducting state, both odd and even modulations of spin
resonance have been observed in our previous unpolarized neutron scattering
experiments (T. Xie {\it et al.} Phys. Rev. Lett. {\bf 120}, 267003 (2018)).
Here we find that the high-energy even mode ( meV) is isotropic in
spin space, but the low-energy odd modes consist of a axis polarized mode
around 9 meV along with another partially overlapped in-plane mode around 12
meV. We argue that such spin anisotropy is induced by the spin-orbit coupling
in the spin-vortex-type fluctuations of this unique compound. The spin
anisotropy is strongly affected by the superconductivity, where it is weak
below 6 meV in the normal state and then transferred to higher energy and
further enhanced in the odd mode of spin resonance below .Comment: 6 pages, 4 figures. Accepted by Physical Review Researc
Tracking the nematicity in cuprate superconductors: a resistivity study under uniaxial pressure
Overshadowing the superconducting dome in hole-doped cuprates, the pseudogap
state is still one of the mysteries that no consensus can be achieved. It has
been suggested that the rotational symmetry is broken in this state and may
result in a nematic phase transition, whose temperature seems to coincide with
the onset temperature of the pseudogap state around optimal doping level,
raising the question whether the pseudogap results from the establishment of
the nematic order. Here we report results of resistivity measurements under
uniaxial pressure on several hole-doped cuprates, where the normalized slope of
the elastoresistivity can be obtained as illustrated in iron-based
superconductors. The temperature dependence of along particular lattice
axis exhibits kink feature at and shows Curie-Weiss-like behavior above
it, which may suggest a spontaneous nematic transition. While seems to
be the same as around the optimal doping and in the overdoped region,
they become very different in underdoped LaSrCuO. Our results
suggest that the nematic order, if indeed existing, is an electronic phase
within the pseudogap state.Comment: 6 pages, 4 figure
Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFeNiAs
We use polarized inelastic neutron scattering to study low-energy spin
excitations and their spatial anisotropy in electron-overdoped superconducting
BaFeNiAs ( K). In the normal state, the
imaginary part of the dynamic susceptibility, ,
at the antiferromagnetic (AF) wave vector increases linearly
with energy for meV. Upon entering the superconducting state, a spin
gap opens below meV and a broad neutron spin resonance appears at
meV. Our careful neutron polarization analysis reveals that
is isotropic for the in-plane and out-of-plane
components in both the normal and superconducting states. A comparison of these
results with those of undoped BaFeAs and optimally electron-doped
BaFeNiAs ( K) suggests that the spin anisotropy
observed in BaFeNiAs is likely due to its proximity to
the undoped BaFeAs. Therefore, the neutron spin resonance is isotropic
in the overdoped regime, consistent with a singlet to triplet excitation
Superconducting Ti15Zr15Nb35Ta35 High-Entropy Alloy With Intermediate Electron-Phonon Coupling
The body-centered cubic (BCC) Ti15Zr15Nb35Ta35 high-entropy alloy showed superconducting behavior at around 8 K. The electronic specific heat coefficient γ and the lattice specific heat coefficient β were determined to be γ = 9.3 ± 0.1 mJ/mol K2 and β = 0.28 ± 0.01 mJ/mol K4, respectively. It was found that the electronic specific heat Ces does follow the exponential behavior of the Bardeen-Cooper-Schrieffer (BCS) theory. Nevertheless, the specific heat jump (ΔC/γTc) at the superconducting transition temperature which was determined to be 1.71 deviates appreciably from that for a weak electron-phonon coupling BCS superconductor. Within the framework of the strong-coupled theory, our analysis suggests that theTi15Zr15Nb35Ta35 HEA is an intermediate electron-phonon coupled BCS-type superconductor
Avoided Quantum Criticality and Magnetoelastic Coupling in BaFe<sub>2-</sub>xNi<sub>x</sub>As<sub>2</sub>
We study the structural and magnetic orders in electron-doped BaFe2-xNixAs2
by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x,
the nearly simultaneous tetragonal-to-orthorhombic structural T_s and
antiferromagnetic (T_N) phase transitions in BaFe2As2 are gradually suppressed
and separated, resulting in T_s>T_N with increasing x as was previously
observed. However, the temperature separation between T_s and T_N decreases
with increasing x for x> 0.065 K
above the superconducting dome around . Our results imply an
avoided quantum critical point, which is expected to strongly influence the
properties of both the normal and superconducting states.Comment: 7 pages; 5 figure
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