טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKapon Itzik
SubjectSearching for 2D Superconductivity in La2-xSrxCuO4 Single
Crystals
DepartmentDepartment of Physics
Supervisor Professor Amit Keren
Full Thesis textFull thesis text - English Version


Abstract

This thesis consists of two parts. In the first one, we present a new method we have developed to measure the superconducting stiffness tensor rs, critical current density Jc, and coherence length x without subjecting the sample to magnetic field or attaching leads. The method is based on the London equation J=-rsA, 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 rs at TàTc. By increasing A until the London equation does not hold anymore we determine Jc 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 La2-xSrxCuO4 (LSCO) rings: one with the current running only in the CuO2 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 < Tc.


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.