85 research outputs found
The barocaloric effect: A Spin-off of the Discovery of High-Temperature Superconductivity
Some key results obtained in joint research projects with Alex M\"uller are
summarized, concentrating on the invention of the barocaloric effect and its
application for cooling as well as on important findings in the field of
high-temperature superconductivity resulting from neutron scattering
experiments.Comment: 26 pages, 9 figure
Evidence for competition between the superconducting and the pseudogap state in (BiPb)_2(SrLa)_2CuO_{6+\delta} from muon-spin rotation experiments
The in-plane magnetic penetration depth \lambda_{ab} in optimally doped
(BiPb)_2(SrLa)_2CuO_{6+\delta} (OP Bi2201) was studied by means of muon-spin
rotation. The measurements of \lambda_{ab}^{-2}(T) are inconsistent with a
simple model of a d-wave order parameter and a uniform quasiparticle weight
around the Fermi surface. The data are well described assuming the angular gap
symmetry obtained in ARPES experiments [Phys. Rev. Lett {\bf 98}, 267004
(2007)], where it was shown that the superconducting gap in OP Bi2201 exists
only in segments of the Fermi surface near the nodes. We find that the
remaining parts of the Fermi surface, which are strongly affected by the
pseudogap state, do not contribute significantly to the superconducting
condensate. Our data provide evidence that high temperature superconductivity
and pseudogap behavior in cuprates are competing phenomena.Comment: 5 pages, 3 figure
Oxygen-isotope effect on the superconducting gap in the cuprate superconductor Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta}
The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the zero-temperature
superconducting energy gap \Delta_0 was studied for a series of
Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta} samples (0.0\leq x\leq0.45). The OIE on
\Delta_0 was found to scale with the one on the superconducting transition
temperature. These experimental results are in quantitative agreement with
predictions from a polaronic model for cuprate high-temperature superconductors
and rule out approaches based on purely electronic mechanisms.Comment: 5 pages, 3 figure
Zero-field superfluid density in d-wave superconductor evaluated from the results of muon-spin-rotation experiments in the mixed state
We report on measurements of the in-plane magnetic penetration \lambda_{ab}
in the optimally doped cuprate superconductor (BiPb)_2(SrLa)_2CuO_6+\delta (OP
Bi2201) by means of muon-spin rotation (\muSR). We show that in unconventional
wave superconductors (like OP Bi2201), \muSR experiments conducted in
various magnetic fields allow to evaluate the zero-field magnetic penetration
depth \lambda_0, which relates to the zero-field superfluid density in terms of
\rho_s\propto\lambda_0^-2.Comment: 4 pages, 5 figure
Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18
The low-temperature elementary spin excitations in the AFM molecular wheel
Fe18 were studied experimentally by inelastic neutron scattering and
theoretically by modern numerical methods, such as dynamical density matrix
renormalization group or quantum Monte Carlo techniques, and analytical
spin-wave theory calculations. Fe18 involves eighteen spin-5/2 Fe(III) ions
with a Hilbert space dimension of 10^14, constituting a physical system that is
situated in a region between microscopic and macroscopic. The combined
experimental and theoretical approach allowed us to characterize and discuss
the magnetic properties of Fe18 in great detail. It is demonstrated that
physical concepts such as the rotational-band or L&E-band concepts developed
for smaller rings are still applicable. In particular, the higher-lying
low-temperature elementary spin excitations in Fe18 or AFM wheels in general
are of discrete antiferromagnetic spin-wave character.Comment: 16 pages, 10 figure
Universal observation of multiple order parameters in cuprate superconductors
The temperature dependence of the London penetration depth \lambda was
measured for an untwined single crystal of YBa_2Cu_3O_{7-\delta} along the
three principal crystallographic directions (a, b, and c). Both in-plane
components (\lambda_a and \lambda_b) show an inflection point in their
temperature dependence which is absent in the component along the c-direction
(\lambda_c). The data provide convincing evidence that the in-plane
superconducting order parameter is a mixture of s+d-wave symmetry whereas it is
exclusively s-wave along the c-direction. In conjunction with previous results
it is concluded that coupled s+d-order parameters are universal and intrinsic
to cuprate superconductors.Comment: 5 pages, 3 figure
Superfluid Density and Angular Dependence of the Energy Gap inOptimally Doped (BiPb)2(SrLa)2CuO6+ δ
We present a muon-spin rotation study of the optimally doped cuprate superconductor (BiPb)2(SrLa)2 CuO6+δ . The measured magnetic field dependence of the in-plane magnetic penetration λ ab suggests superconductivity with a dominant d-wave order parameter. The comparison of the temperature dependence of λ ab with calculations, assuming the angular gap symmetry as obtained from photoemission measurements, is consistent with a partial suppression of the quasi-particle weight towards the anti-nodal region of the Fermi surface. This suggests that the superconducting and the pseudogap state are dominated by different parts of the Fermi surfac
Strong coupling between Eu2+ spins and Fe2As2 layers in EuFe1.9Co0.1As2 observed with NMR
A combination of x-ray diffraction, magnetization, and 75As nuclear magnetic
resonance (NMR) experiments were performed on single-crystal EuFe1.9Co0.1As2.
The strength of the hyperfine interaction between the 75As nuclei and the
Eu^(2+) 4f states suggests a strong coupling between the Eu^(2+) moments and
the Fe1.9Co0.1As2 layers. Such a strong interlayer coupling may be due to an
indirect exchange interaction between the localized Eu^(2+) 4f moments,
mediated by the Fe 3d conduction electrons. Magnetic susceptibility as well as
75As-NMR measurements reveal a decrease of the SDW transition temperature to
T_SDW = 120 K as a result of Co doping. A change of the slope in the
temperature dependence of the NMR frequency of the 75As lower-satellite line
was observed at 225 K. At the same temperature also a change of the satellite
line shape was found. These changes of the NMR spectra may be caused by the
formation of a nematic phase below 225 K in EuFe1.9Co0.1As2.Comment: 8 pages, 7 figure
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