86 research outputs found
Specific heat of CaNaFeAs single crystals: unconventional s multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings
We report a low-temperature specific heat study of high-quality single
crystals of the heavily hole doped superconductor
CaNaFeAs. This compound exhibits bulk
superconductivity with a transition temperature \,K, which is
evident from the magnetization, transport, and specific heat measurements. The
zero field data manifests a significant electronic specific heat in the normal
state with a Sommerfeld coefficient mJ/mol K. Using a
multi-band Eliashberg analysis, we demonstrate that the dependence of the zero
field specific heat in the superconducting state is well described by a
three-band model with an unconventional s pairing symmetry and gap
magnitudes of approximately 2.35, 7.48, and -7.50 meV. Our analysis
indicates a non-negligible attractive intraband coupling,which contributes
significantly to the relatively high value of . The Fermi surface averaged
repulsive and attractive coupling strengths are of comparable size and outside
the strong coupling limit frequently adopted for describing high- iron
pnictide superconductors. We further infer a total mass renormalization of the
order of five, including the effects of correlations and electron-boson
interactions.Comment: 8 Figures, Submitted to PR
Specific heat and upper critical field in KFe2As2 single crystals
We report low-temperature specific heat measurements for high-quality single
crystalline KFe2As2 (T_c about 3.5 K). The investigated zero-field specific
heat data yields an unusually large nominal Sommerfeld coefficient gamma_n of
94(3) mJ/mol K^2 which is however significantly affected by extrinsic
contributions as evidenced by a sizable residual linear specific heat and
various theoretical considerations including also an analysis of Kadowaki-Woods
relations. Then KFe2As2 should be classified as a weak to intermediately strong
coupling superconductor with a total electron-boson coupling constant
lambda_tot near 1 (including a calculated weak electron-phonon coupling
constant of lambda_el-ph =0.17. From specific heat and ac susceptibility
studies in external magnetic fields the magnetic phase diagram has been
constructed. We confirm the high anisotropy of the upper critical fields
B_c2(T) ranging from a factor of 5 near T_c to a slightly reduced value
approaching T=0 for fields B || ab$ and || c and show that their ratio Gamma
slightly exceeds the mass anisotropy of 4.35 derived from our full-relativistic
LDA-band structure calculations. Its slight reduction when approaching T=0 is
not a consequence of Pauli-limiting as in less perfect samples but point likely
to a multiband effect. We also report irreversibility field data obtained from
ac susceptibility measurements. The double-maximum in the T-dependence of its
imaginary part for fields B || c indicates a peak-effect in the T-dependence of
critical currents.Comment: 12 pages, 8 fiqures (2 new figures) specific heat raw-data corrected,
estimates for a reduced electronic specific heat added, extended theoretical
analysis, 2 new authors include
Nematic Fluctuations in Iron-Oxychalcogenide Mott Insulators
Nematic fluctuations occur in a wide range of physical systems from liquid
crystals to biological molecules to solids such as exotic magnets, cuprates and
iron-based high- superconductors. Nematic fluctuations are thought to be
closely linked to the formation of Cooper-pairs in iron-based superconductors.
It is unclear whether the anisotropy inherent in this nematicity arises from
electronic spin or orbital degrees of freedom. We have studied the iron-based
Mott insulators LaOFeO = (S, Se) which are
structurally similar to the iron pnictide superconductors. They are also in
close electronic phase diagram proximity to the iron pnictides. Nuclear
magnetic resonance (NMR) revealed a critical slowing down of nematic
fluctuations as observed by the spin-lattice relaxation rate (). This is
complemented by the observation of a change of electrical field gradient over a
similar temperature range using M\"ossbauer spectroscopy. The neutron pair
distribution function technique applied to the nuclear structure reveals the
presence of local nematic fluctuations over a wide temperature range
while neutron diffraction indicates that global symmetry is preserved.
Theoretical modeling of a geometrically frustrated spin- Heisenberg model
with biquadratic and single-ion anisotropic terms provides the interpretation
of magnetic fluctuations in terms of hidden quadrupolar spin fluctuations.
Nematicity is closely linked to geometrically frustrated magnetism, which
emerges from orbital selectivity. The results highlight orbital order and spin
fluctuations in the emergence of nematicity in Fe-based oxychalcogenides. The
detection of nematic fluctuation within these Mott insulator expands the group
of iron-based materials that show short-range symmetry-breaking
Evidence of d-wave Superconductivity in K_(1-x)Na_xFe_2As_2 (x = 0, 0.1) Single Crystals from Low-Temperature Specific Heat Measurements
From the measurement and analysis of the specific heat of high-quality
K_(1-x)Na_xFe_2As_2 single crystals we establish the presence of large T^2
contributions with coefficients alpha_sc ~ 30 mJ/mol K^3 at low-T for both x=0
and 0.1. Together with the observed square root field behavior of the specific
heat in the superconducting state both findings evidence d-wave
superconductivity on almost all Fermi surface sheets with an average gap
amplitude of Delta_0 in the range of 0.4 - 0.8 meV. The derived Delta_0 and the
observed T_c agree well with the values calculated within the Eliashberg
theory, adopting a spin-fluctuation mediated pairing in the intermediate
coupling regime.Comment: 8 pages, 5 figures, field dependence of the specific heat added,
slightly changed title, changed sequence of authors, one author added,
accepted by Phys. Rev. B Rapid Communication
- …