טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZaberchik Moran
SubjectMagnetic Field-Induced Surface Potential in High
Temperature Superconductors
DepartmentDepartment of Physics
Supervisor Professor Michael Reznikov
Full Thesis textFull thesis text - English Version


Abstract

The goal of this work was to search for vortex charge in high temperature superconductors.
This charge is thought to play a significant role in the anomalous Hall effect observed
in copper oxide based high temperature superconductors. Namely, the vortex charge
might give a simple explanation for the sign change of the Hall voltage as a function of temperature. This sign change is one of the important problems of the high temperature superconductor physics.
There are two known reasons for a vortex to be charged: the Bernoulli potential, and
the chemical potential difference between the normal and the superconductive states.
The Bernoulli potential is required to compensate for the Lorentz force acting on a
current flowing around a vortex in an external magnetic field. The chemical potential
changes at the superconductive-normal metal transition since the superconductive state is energetically favorable to the normal one. We analyzed the relative importance of these two contributions to the vortex charge in BCS superconductors, and came to the conclusion that the Bernoulli one should be dominant.
We decided to search for the vortex charge by measuring the surface potential
induced by an external magnetic field. For this we adopted the experimental setup
developed earlier for thermodynamic magnetization measurements. In this method
a recharging current between a sample and a metal plate adjacent to it is measured.
This current is generated in response to a change of an external magnetic field. The
accumulated charge can be converted to the surface potential using sample-to-plate
capacitance.
We measured the surface potential for various La1−xSrxCuO4 samples with different
doping, ranging from strongly underdoped to overdoped. We changed the external
magnetic field in two different ways: by modulating it at a low frequency around a
fixed value, and by sweeping it at a constant rate. The surface potential, measured
with magnetic field modulation, is quadratic in the magnetic field. It is positive in
the underdoped region, and changes sign at optimal doping. The surface potential
becomes unmeasurable above Tc, which is the strongest argument for it to be related to superconductivity. Magnetic field sweep-induced current was dominated by large peaks, which correspond to a large, sometimes of the order of a Volt in magnitude, steps in the surface potential.
The observed surface potential, both with magnetic field modulation and sweep, is about three orders of magnitude larger than anticipated to be induced by the vortex
charge. Therefore we were forced to search for an alternative explanation for the
experimental observations. We suggested that this large surface potential is due to
polarization of the sample surface in response to the magnetic field-induced deformation, through the phenomenon known as surface piezoelectricity and flexoelectricity.

In this picture the current peaks appear due to the avalanche motion of vortices, which leads to instantaneous (on the time scale of our measurements) change of the sample shape, and therefore of the surface potential.