4,459 research outputs found
A view from inside iron-based superconductors
Muon spin spectroscopy is one of the most powerful tools to investigate the
microscopic properties of superconductors. In this manuscript, an overview on
some of the main achievements obtained by this technique in the iron-based
superconductors (IBS) are presented. It is shown how the muons allow to probe
the whole phase diagram of IBS, from the magnetic to the superconducting phase,
and their sensitivity to unravel the modifications of the magnetic and the
superconducting order parameters, as the phase diagram is spanned either by
charge doping, by an external pressure or by introducing magnetic and
non-magnetic impurities. Moreover, it is highlighted that the muons are unique
probes for the study of the nanoscopic coexistence between magnetism and
superconductivity taking place at the crossover between the two ground-states.Comment: 28 pages, 18 figure
Evidence for impurity-induced frustration in La2CuO4
Zero-field muon spin rotation and magnetization measurements were performed
in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by
M=Mg2+ spinless impurity. It is shown that while the doping dependence of the
sublattice magnetization (M(x)) is nearly the same for both compounds, the
N\'eel temperature (T_N(x)) decreases unambiguously more rapidly in the
Zn-doped compound. This difference, not taken into account within a simple
dilution model, is associated with the frustration induced by the Zn2+ impurity
onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin
stiffness is derived and found to be reduced by Zn doping more significantly
than expected within a dilution model. The effect of the structural
modifications induced by doping on the exchange coupling is also discussed.Comment: 4 pages, 4 figure
The poisoning effect of Mn in LaFe(1-x)Mn(x)AsO(0.89)F(0.11): unveiling a quantum critical point in the phase diagram of iron-based superconductors
A superconducting-to-magnetic transition is reported for
LaFeMnAsOF where a per thousand amount of Mn
impurities is dispersed. By employing local spectroscopic techniques like muon
spin rotation (muSR) and nuclear quadrupole resonance (NQR) on compounds with
Mn contents ranging from x=0.025% to x=0.75%, we find that the electronic
properties are extremely sensitive to the Mn impurities. In fact, a small
amount of Mn as low as 0.2% suppresses superconductivity completely. Static
magnetism, involving the FeAs planes, is observed to arise for x > 0.1% and
becomes further enhanced upon increasing Mn substitution. Also a progressive
increase of low energy spin fluctuations, leading to an enhancement of the NQR
spin-lattice relaxation rate 1/T1, is observed upon Mn substitution. The
analysis of 1/T1 for the sample closest to the the crossover between
superconductivity and magnetism (x = 0.2%) points towards the presence of an
antiferromagnetic quantum critical point around that doping level.Comment: 11 pages, 10 figure
Orbital order and spin-orbit coupling in BaVS3
The correlated 3d sulphide BaVS3 undergoes a sequence of three symmetry
breaking transitions which are reflected in the temperature dependence of the
magnetic susceptibility, and its anisotropy. We introduce a microscopic model
based on the coexistence of wide band a(1g) and localized e(g) d-electrons, and
give the classification of the order parameters under the double space group
and time reversal symmetries. Allowing for the relativistic spin-orbit
coupling, the d-shell multipoles acquire a mixed spin-orbital character. It
follows that orbital ordering is accompanied by a change in the susceptibility
anisotropyComment: 2 pages, submitted to the SCES05 conference, uses elsart41.cl
Critical chain length and superconductivity emergence in oxygen-equalized pairs of YBa2Cu3O6.30
The oxygen-order dependent emergence of superconductivity in YBa2Cu3O6+x is
studied, for the first time in a comparative way, on pair samples having the
same oxygen content and thermal history, but different Cu(1)Ox chain
arrangements deriving from their intercalated and deintercalated nature.
Structural and electronic non-equivalence of pairs samples is detected in the
critical region and found to be related, on microscopic scale, to a different
average chain length, which, on being experimentally determined by nuclear
quadrupole resonance (NQR), sheds new light on the concept of critical chain
length for hole doping efficiency.Comment: 7 RevTex pages, 2 Postscript figures. Submitted to Phys. Rev.
Direct evaluation of the isotope effect within the framework of density functional theory for superconductors
Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, Tc, with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of Tc with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC6). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications
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