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Others display more unconventional features. In a new class layered oxypnictide superconductors , for example LaOFeAs, were discovered that do not include copper. Some other iron-based superconductors do not contain oxygen. Recently, other unconventional superconductors, not based on cuprate structure, have been discovered. After more than twenty years of intensive research the origin of high-temperature superconductivity is still not clear, but it seems that instead of electron-phonon attraction mechanisms, as in conventional superconductivity , one is dealing with genuine electronic mechanisms e.

One goal of all this research is room-temperature superconductivity. The question of how superconductivity arises in high-temperature superconductors is one of the major unsolved problems of theoretical condensed matter physics as of [update]. The mechanism that causes the electrons in these crystals to form pairs is not known.

Despite intensive research and many promising leads, an explanation has so far eluded scientists. One reason for this is that the materials in question are generally very complex, multi-layered crystals for example, BSCCO , making theoretical modeling difficult. The most controversial topic in condensed matter physics has been the mechanism for high- T c superconductivity HTS. Thus, in order to solve this unsettled problem, there have been numerous experiments such as photoelectron spectroscopy, NMR, specific heat measurement, etc. Unfortunately, the results were ambiguous, where some reports supported the d symmetry for the HTS but others supported the s symmetry.

The symmetry of the HTS order parameter has been studied in nuclear magnetic resonance measurements and, more recently, by angle-resolved photoemission and measurements of the microwave penetration depth in a HTS crystal. NMR measurements probe the local magnetic field around an atom and hence reflect the susceptibility of the material.

They have been of special interest for the HTS materials because many researchers have wondered whether spin correlations might play a role in the mechanism of the HTS. NMR measurements of the resonance frequency on YBCO indicated that electrons in the copper oxide superconductors are paired in spin-singlet states. This indication came from the behavior of the Knight shift , the frequency shift that occurs when the internal field is different from the applied field: In a normal metal, the magnetic moments of the conduction electrons in the neighborhood of the ion being probed align with the applied field and create a larger internal field.

As these metals go superconducting, electrons with oppositely directed spins couple to form singlet states.

In the anisotropic HTS, perhaps NMR measurements have found that the relaxation rate for copper depends on the direction of the applied static magnetic field, with the rate being higher when the static field is parallel to one of the axes in the copper oxide plane. While this observation by some group supported the d symmetry of the HTS, other groups could not observe it. Also, by measuring the penetration depth , the symmetry of the HTS order parameter can be studied. The microwave penetration depth is determined by the superfluid density responsible for screening the external field.

In that case, the penetration depth also varies exponentially with temperature T. If there are nodes in the energy gap as in the d symmetry HTS, electron pair can more easily be broken, the superfluid density should have a stronger temperature dependence, and the penetration depth is expected to increase as a power of T at low temperatures. If the symmetry is specially d x 2 - y 2 then the penetration depth should vary linearly with T at low temperatures. This technique is increasingly being used to study superconductors and is limited in application largely by the quality of available single crystals.

Photoemission spectroscopy also could provide information on the HTS symmetry. By scattering photons off electrons in the crystal, one can sample the energy spectra of the electrons. Because the technique is sensitive to the angle of the emitted electrons one can determine the spectrum for different wave vectors on the Fermi surface. However, within the resolution of the angle-resolved photoemission spectroscopy ARPES , researchers could not tell whether the gap goes to zero or just gets very small.

Also, ARPES are sensitive only to the magnitude and not to the sign of the gap, so it could not tell if the gap goes negative at some point. There was a clever experimental design to overcome the muddy situation. Furthermore, the possibility that junction interfaces can be in the clean limit no defects or with maximum zig-zag disorder was taken into account in this tricrystal experiment. Geshkenbein, A. Discover the University. Students and money.

Unconventional superconductor - Wikipedia

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Unconventional superconductor

Organising an event on Belval Campus. All rights reserved. Share this event:. Wednesday, 08 February pm - pm. The most controversial topic in condensed matter physics has been the mechanism for high- T c superconductivity HTS. Thus, in order to solve this unsettled problem, there have been numerous experiments such as photoelectron spectroscopy, NMR, specific heat measurement, etc.

Unfortunately, the results were ambiguous, where some reports supported the d symmetry for the HTS but others supported the s symmetry. The symmetry of the HTS order parameter has been studied in nuclear magnetic resonance measurements and, more recently, by angle-resolved photoemission and measurements of the microwave penetration depth in a HTS crystal. NMR measurements probe the local magnetic field around an atom and hence reflect the susceptibility of the material. They have been of special interest for the HTS materials because many researchers have wondered whether spin correlations might play a role in the mechanism of the HTS.

NMR measurements of the resonance frequency on YBCO indicated that electrons in the copper oxide superconductors are paired in spin-singlet states.

This indication came from the behavior of the Knight shift , the frequency shift that occurs when the internal field is different from the applied field: In a normal metal, the magnetic moments of the conduction electrons in the neighborhood of the ion being probed align with the applied field and create a larger internal field. As these metals go superconducting, electrons with oppositely directed spins couple to form singlet states. In the anisotropic HTS, perhaps NMR measurements have found that the relaxation rate for copper depends on the direction of the applied static magnetic field, with the rate being higher when the static field is parallel to one of the axes in the copper oxide plane.

While this observation by some group supported the d symmetry of the HTS, other groups could not observe it. Also, by measuring the penetration depth , the symmetry of the HTS order parameter can be studied.

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The microwave penetration depth is determined by the superfluid density responsible for screening the external field. In that case, the penetration depth also varies exponentially with temperature T.

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If there are nodes in the energy gap as in the d symmetry HTS, electron pair can more easily be broken, the superfluid density should have a stronger temperature dependence, and the penetration depth is expected to increase as a power of T at low temperatures. If the symmetry is specially d x 2 - y 2 then the penetration depth should vary linearly with T at low temperatures. This technique is increasingly being used to study superconductors and is limited in application largely by the quality of available single crystals.

Photoemission spectroscopy also could provide information on the HTS symmetry.


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By scattering photons off electrons in the crystal, one can sample the energy spectra of the electrons. Because the technique is sensitive to the angle of the emitted electrons one can determine the spectrum for different wave vectors on the Fermi surface. However, within the resolution of the angle-resolved photoemission spectroscopy ARPES , researchers could not tell whether the gap goes to zero or just gets very small. Also, ARPES are sensitive only to the magnitude and not to the sign of the gap, so it could not tell if the gap goes negative at some point.

There was a clever experimental design to overcome the muddy situation. Furthermore, the possibility that junction interfaces can be in the clean limit no defects or with maximum zig-zag disorder was taken into account in this tricrystal experiment. Geshkenbein, A. Larkin and A. Barone in In the first tricrystal pairing symmetry experiment [25] , the spontaneous magnetization of half flux quantum was clearly observed in YBCO, which convincingly supported the d-wave symmetry of the order parameter in YBCO.

Because YBCO is orthorhombic , it might inherently have an admixture of s-wave symmetry. From Wikipedia, the free encyclopedia. Superconductive materials not explained by existing established theories. This article may be too technical for most readers to understand.


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