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Register a new userInduced transparency in EXAFS as cuprate superconductors go through Tc
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Added by
Juana Acrivos Dr. Mrs.
Publication date
2005
fields
Physics
and research's language is
English
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The increased transmission, observed in the EXAFS region of their X-ray absorption spectra, as cuprate materials go through the superconducting transition temperature Tc is correlated with an increase in Abrikosov Vortex expulsion in zero magnetic field as the temperature T approaches Tc.
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Understanding the electron pairing in hole-doped cuprate superconductors has been a challenge, in particular because the normal state from which it evolves is unprecedented. Now, after three and a half decades of research, involving a wide range of experimental characterizations, it is possible to delineate a clear and consistent cuprate story. It starts with doping holes into a charge-transfer insulator, resulting in in-gap states. These states exhibit a pseudogap resulting from the competition between antiferromagnetic superexchange $J$ between nearest-neighbor Cu atoms (a real-space interaction) and the kinetic energy of the doped holes, which, in the absence of interactions, would lead to extended Bloch-wave states whose occupancy is characterized in reciprocal space. To develop some degree of coherence on cooling, the spin and charge correlations must self-organize in a cooperative fashion. A specific example of resulting emergent order is that of spin and charge stripes, as observed in La$_{2-x}$Ba$_x$CuO$_4$. While stripe order frustrates bulk superconductivity, it nevertheless develops pairing and superconducting order of an unusual character. The antiphase order of the spin stripes decouples them from the charge stripes, which can be viewed as hole-doped, two-leg, spin-$frac12$ ladders. To achieve superconducting order, the pair correlations in neighboring ladders must develop phase order. In the presence of spin stripe order, antiphase Josephson coupling can lead to pair-density-wave superconductivity. Alternatively, in-phase superconductivity requires that the spin stripes have an energy gap, which empirically limits the coherent superconducting gap. Hence, superconducting order in the cuprates involves a compromise between the pairing scale, which is maximized at $xsimfrac18$, and phase coherence, which is optimized at $xsim0.2$.
We investigate the electronic structure and the hole content in the copper-oxygen planes of Hg based high Tc cuprates for one to four CuO2 layers and hydrostatic pressures up to 15 GPa. We find that with the pressure-induced additional number of holes of the order of 0.05e the density of states at the Fermi level changes approximately by a factor of 2. At the same time the saddle point is moved to the Fermi level accompanied by an enhanced k_z dispersion. This finding explains the pressure behavior of Tc and leads to the conclusion that the applicability of the van Hove scenario is restricted. By comparison with experiment, we estimate the coupling constant to be of the order of 1, ruling out the weak coupling limit.
We predict a novel nonlinear electromagnetic phenomenon in layered superconducting slabs irradiated from one side by an electromagnetic plane wave. We show that the reflectance and transmittance of the slab can vary over a wide range, from nearly zero to one, when changing the incident wave amplitude. Thus changing the amplitude of the incident wave can induce either the total transmission or reflection of the incident wave. In addition, the dependence of the superconductor transmittance on the incident wave amplitude has an unusual hysteretic behavior with jumps. This remarkable nonlinear effect (self-induced transparency) can be observed even at small amplitudes, when the wave frequency $omega$ is close to the Josephson plasma frequency $omega_J$.
The influence of a uniform external magnetic field on the dynamical spin response of cuprate superconductors in the superconducting state is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic scattering around low and intermediate energies is dramatically changed with a modest external magnetic field. With increasing the external magnetic field, although the incommensurate magnetic scattering from both low and high energies is rather robust, the commensurate magnetic resonance scattering peak is broadened. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime. The theory also predicts that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying an external magnetic field large enough.
In a recent study Viskadourakis et al. discovered that extremely underdoped La_2CuO_(4+x) is a relaxor ferroelectric and a magnetoelectric material at low temperatures. It is further observed that the magnetoelectric response is anisotropic for different directions of electric polarization and applied magnetic field. By constructing an appropriate Landau theory, we show that a bi-quadratic magnetoelectric coupling can explain the experimentally observed polarization dependence on magnetic field. This coupling leads to several novel low-temperature effects including a feedback enhancement of the magnetization below the ferroelectric transition, and a predicted magnetocapacitive effect.
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