Ph.D Student | Kapon Itzik |
---|---|

Subject | Searching for 2D Superconductivity in La2-xSrxCuO4 Single Crystals |

Department | Department of Physics |

Supervisor | Professor Amit Keren |

Full Thesis text |

This thesis consists of two parts. In
the first one, we present a new method we have developed to measure the
superconducting stiffness tensor r_{s},
critical current density J_{c}, and coherence length x without subjecting the sample to
magnetic field or attaching leads. The method is based on the London equation **J**=-r_{s}**A**, where **J** is the current density and **A**
is the vector potential. Using rotor free **A** and measuring **J** via
the magnetic moment of superconducting rings, we extract r_{s} at TàT_{c}. By increasing **A** until the London
equation does not hold anymore we determine J_{c} and x. The technique, named
Stiffnessometer, is sensitive to very small stiffness, which translates to
penetration depth on the order of a few millimeters. Naturally, the method does
not suffer from demagnetization factor complications, the presence of vortices,
or out-of-equilibrium conditions. Therefore, the absolute values of the
different parameters can be determined.

We apply this method to two different
La_{2-x}Sr_{x}CuO_{4} (LSCO) rings: one with the
current running only in the CuO_{2} planes, and another where the
current must cross between them. We find different transition temperatures for
the two rings, namely, there is a temperature range with two-dimensional
stiffness. The Stiffnessometer results are accompanied by Low Energy mSR measurements on the same sample to
determine the stiffness anisotropy at T < T_{c}.

In the second part of the thesis, we investigated whether the spin or charge degrees of freedom were responsible for the nodal gap in underdoped cuprates by performing inelastic neutron scattering and x-ray diffraction measurements on LSCO x=0.0192. We found that fluctuating incommensurate spin-density-wave (SDW) with a bottom part of an hourglass dispersion exists even in this magnetic sample. The strongest component of these fluctuations diminishes at the same temperature where the nodal gap opens. X-ray scattering data from the same crystal show no signature of charge-density-wave (CDW). Therefore, we suggest that the nodal gap in the electronic band of this cuprate opens due to fluctuating SDW with no contribution from CDW.