28 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
Admixture of an s-wave component to the d-wave gap symmetry in high-temperature superconductors
Neutron crystal-field spectroscopy experiments in the Y- and La-type
high-temperature superconductors HoBa2Cu3O6.56, HoBa2Cu4O8, and
La1.81Sr0.15Ho0.04CuO4 are reviewed. By this bulk-sensitive technique,
information on the gap function is obtained from the relaxation behavior of
crystal-field transitions associated with the Ho3+ ions which sit as local
probes close to the superconducting copper-oxide planes. The relaxation data
exhibit a peculiar change from a convex to a concave shape between the
superconducting transition temperature Tc and the pseudogap temperature T*
which can only be modelled satisfactorily if the gap function of predominantly
d-wave symmetry includes an s-wave component of the order of 20-25%,
independent of the doping level. Moreover, our results are compatible with an
unusual temperature dependence of the gap function in the pseudogap region
(Tc<T<T*), i.e., a breakup of the Fermi surface into disconnected arcs.Comment: 14 pages, 3 figures, 1 table; accepted for publication in J.
Supercond. Nov. Mag
Dynamical charge inhomogeneity and crystal-field fluctuations for 4f ions in high-Tc cuprates
The main relaxation mechanism of crystal-field excitations in rare-earth ions
in cuprates is believed to be provided by the fluctuations of crystalline
electric field induced by a dynamic charge inhomogeneity generic for the doped
cuprates. We address the generalized granular model as one of the model
scenario for such an ingomogeneity where the cuprate charge subsystem remind
that of Wigner crystal with the melting transition and phonon-like positional
excitation modes. Formal description of R-ion relaxation coincides with that of
recently suggested magnetoelastic mechanism.Comment: 4 page
Spin-phonon interaction and band effects in the high-T_C superconductor HgBa_2CuO_4
Band calculations show that a stripe-like anti-ferromagnetic spin wave is
enforced by a 'half-breathing' phonon distortion within the CuO plane of
HgBa_2CuO_4. This spin-phonon coupling is increased further by shear distortion
and by increased distance between Cu and apical oxygens. The effects from
spin-phonon coupling are consistent with many observations in high-T_C
materials. Spin-phonon coupling can be important for the mechanism of spin
fluctuations and superconductivity, although the effects are quantitatively
weak when using the local density potential.Comment: 4 pages, 1 figur
Pressure Studies on a High- Superconductor Pseudogap and Critical Temperatures
We report simultaneous hydrostatic pressure studies on the critical
temperature and on the pseudogap temperature performed through
resistivity measurements on an optimally doped high- oxide
. The resistivity is measured as
function of the temperature for several different applied pressure below 1GPa.
We find that both and increases linearly with the pressure. This
result demonstrate that the well known intrinsic pressure effect on is
also present at and both temperatures are originated by the same
superconducting mechanism.Comment: 4 pages and 2 figures in eps, final versio
Poor screening and nonadiabatic superconductivity in correlated systems
In this paper we investigate the role of the electronic correlation on the
hole doping dependence of electron-phonon and superconducting properties of
cuprates. We introduce a simple analytical expression for the one-particle
Green's function in the presence of electronic correlation and we evaluate the
reduction of the screening properties as the electronic correlation increases
by approaching half-filling. The poor screening properties play an important
role within the context of the nonadiabatic theory of superconductivity. We
show that a consistent inclusion of the reduced screening properties in the
nonadiabatic theory can account in a natural way for the - phase
diagram of cuprates. Experimental evidences are also discussed.Comment: 12 Pages, 6 Figures, Accepted on Physical Review
Pseudogap behavior of nuclear spin relaxation in high Tc superconductors in terms of phase separation
We analyze anew experiments on the NMR in cuprates and find an important
information on their phase separation and its stripe character hidden in the
dependence of on degree of doping. In a broad class of materials
is the sum of two terms: the temperature independent one
attributed to ``incommensurate'' stripes that occur at external doping, and an
``universal'' temperature dependent term ascribed to moving metallic and AF
sub-phases. We argue that the frustrated first order phase transition in a
broad temperature interval bears a dynamical character.Comment: 5 pages, 3 figures; some comments and references added; accepted for
publication in JETP Letter
A Theory for High- Superconductors Considering Inhomogeneous Charge Distribution
We propose a general theory for the critical and pseudogap
temperature dependence on the doping concentration for high- oxides,
taking into account the charge inhomogeneities in the planes. The well
measured experimental inhomogeneous charge density in a given compound is
assumed to produce a spatial distribution of local . These differences
in the local charge concentration is assumed to yield insulator and metallic
regions, possibly in a stripe morphology. In the metallic region, the
inhomogeneous charge density yields also spatial distributions of
superconducting critical temperatures and zero temperature gap
. For a given sample, the measured onset of vanishing gap
temperature is identified as the pseudogap temperature, that is, , which
is the maximum of all . Below , due to the distribution of
's, there are some superconducting regions surrounded by insulator or
metallic medium. The transition to a superconducting state corresponds to the
percolation threshold among the superconducting regions with different
's. To model the charge inhomogeneities we use a double branched
Poisson-Gaussian distribution. To make definite calculations and compare with
the experimental results, we derive phase diagrams for the BSCO, LSCO and YBCO
families, with a mean field theory for superconductivity using an extended
Hubbard Hamiltonian. We show also that this novel approach provides new
insights on several experimental features of high- oxides.Comment: 7 pages, 5 eps figures, corrected typo
Upper critical field for underdoped high-T_c superconductors. Pseudogap and stripe--phase
We investigate the upper critical field in a stripe--phase and in the
presence of a phenomenological pseudogap. Our results indicate that the
formation of stripes affects the Landau orbits and results in an enhancement of
. On the other hand, phenomenologically introduced pseudogap leads to a
reduction of the upper critical field. This effect is of particular importance
when the magnitude of the gap is of the order of the superconducting transition
temperature. We have found that a suppression of the upper critical field takes
place also for the gap that originates from the charge--density waves.Comment: 7 pages, 5 figure
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