Fractional Josephson vortices
I investigate experimentally and theoretically fractional vortices in
long Josephson junctions... I guess that for majority of you this
sounds like a Greek. Anyway that is what I am doing at work. A bit of
patience and I will try to explain it.
Long Josephson junction (LJJ) is formed by two superconducting films
with very small (about 1nm) gap between them. This gap is so small
that the quantum mechanical tunneling of electrons (and Cooper pairs)
from one film to the other takes place. In particular, if the current
is not very large, there is no voltage drop across this contact and
the current which flows through the gap is called superconducting
current or supercurrent. This is so-called Josephson effect and the
system of two superconductors separated by a barrier or gap is called
a Josephson junction. If you increases the current through the
junction above some critical value, the voltage across the junction
appears. The maximum supercurrent is called a critical current.
I study Josephson junctions which are long in one direction much more
than in the others. This is so-called Long Josephson junction (LJJ).
The characteristic lengths in this business is the so-called
Josephson length which is typically about 1..30 microns. Junction is
called long, if its length at least in one direction is longer than
Josephson length.
In LJJ the vortices of supercurrent may exist. Each such vortex is
similar to a small coil with circulating current, and creates
magnetic field. Such vortex carries one quantum of magnetic flux and
therefore is called fluxon.
It is found that the fluxons may move along a Josephson junction i.e.
between these two films. Their motion depends on applied external
magnetic field H and current I which is applied through
the junction from one superconductor to the other.
Recently it was discovered that in Josephson junctions made of
cuprate superconductors or in Josephson junctions with ferromagnetic
barrier the critical current may be negative. They are called pi-junctions.
One can even fabricate the junction one part of which behaves as
usual (0-junction) and another part as pi-junction. At the boundary
where 0 and pi parts meet each other, a new type of vortex can be
formed. This vortex carries only half of the magnetic flux quantum
and called semifluxon. The properties of semifluxon are very
different from the the properties of fluxons:
Semifluxons are fluxons interact, e.g., the fluxon of positive
polarity may "annihilate" with the semifluxon of negative
polarity. The result of such an "annihilation" is a
semifluxon of positive polarity.
One can go even further and study arbitrary fractional
vortices, rather than only semi-integer, experimentally. This is the
topic of ongoing research...
Stack of coupled long Josephson junctions
The subject of my postgraduate studies was "The fluxon dynamics
in stack of coupled long Josephson junctions" i.e. the stack of
superconducting films with many tunnel barriers. Fluxons moving in
one layer interacts with the fluxon in the other layer and this
results in very complicated but interesting fluxon dynamics. Some
unexpected phenomena, for example the emission of Cherenkov radiation
by fast moving vortex was discovered. Such a stack of coupled
Josephson junctions closely resembles the structure of cuprate
superconductors such as BSCCO which is under intensive investigation
since the discovery of cuprate family of high temperature superconductors.
Josephson vortex ratchets
To produce the energy our of thermal fluctuation or noise was the
dream of mankind since the time when the Brownian motion was
recognized. Unfortunately one cannot produce work out of equilibrium
thermal fluctuations, because otherwise one should violate the second
law of thermodynamics. But... one can extract a useful work out of
the non-equilibrium thermal fluctuations using the devices
with broken reflection symmetry, the so-called ratchets.
I am interested in the ratchets based vortex motion in long Josephson
junctions. In this systems there are several ways to create an
asymmetric potential for a vortex. Directed (in average) motion of
the vortex results in dc voltage across the junction. Josephson
junctions are fast devices and therefore may rectify noise in the
wide frequency range from 0 up to 100 GHz. |