Fermionic condensate makes its debut


28
January
2004

Physicists
in
the
US
have
created
an
elusive
state
of

matter
known
as
a
“fermionic
condensate”
for
the
first

time.
Deborah
Jin,
Markus
Greiner
and
Cindy
Regal
at

the
JILA
laboratory
in
Boulder,
Colorado,
made
the

condensate
from
pairs
of
ultracold
fermionic
atoms
(C

Regal
et
al.
2004
Phys.
Rev.
Lett.
92
040403)

The
breakthrough
could
help
physicists
improve
their

understanding
of
superconductivity
and
superfluidity.

“The
strength
of
pairing
in
our
fermionic
condensate

would
correspond
to
a
room
temperature

superconductor
when
adjusted
for
mass
and
density,”

said
Jin
at
a
press
conference
today.
“This
makes
me

optimistic
that
the
fundamental
physics
we
learn

through
fermionic
condensates
will
eventually
help

others
design
more
practical
superconducting
materials.”

Atoms
behave
very
differently
at
temperatures
near

absolute
zero
depending
on
the
value
of
their
intrinsic

angular
momentum
or
“spin”.
Bosons
have
spins
with

integer
values
in
units
of
the
Planck
constant
divided
by

2π,
while
fermions
have
spins
of
1/2,
3/2,
5/2
and
so

on.
Fermions
obey
the
Pauli
exclusion
principle,
which

means
that
they
cannot
occupy
the
same
quantum

state.
However,
there
are
no
such
restrictions
on

bosons,
so
they
can
all
collapse
into
the
same
quantum
ground
state.
This
process,
known
as
Bose-Einstein

condensation,
is
at
the
heart
of
superconductivity

the

flow
of
electric
current
without
any
resistance.

Since
electrons
are
fermions
they
must
form
Cooper

pairs

named
after
Leon
Cooper
of
the

Bardeen-Cooper-Schrieffer
(BCS)
theory
of

superconductivity

before
they
can
form
a
Bose

condensate.
If
this
process
could
be
mimicked
in
a
gas
of
fermionic
atoms,
it
should
be
possible
to
learn
a
great

deal
more
about
superconductivity.

Late
last
year
the
Boulder
team
and,
independently,
a

team
at
Innsbruck
managed
to
form
a
molecular

condensate
from
a
gas
of
fermionic
atoms.
The
atoms
in
a
molecule
are
much
more
strongly
bound
than
those

in
a
Cooper
pair.
Now,
the
JILA
team
has
made
a

condensate
from
pairs
of
individual
fermionic
atoms
in
a

gas.
The
two
fermions
are
not
bound
into
a
molecule

but
simply
move
together
in
a
correlated
way.

Collectively
the
pair
acts
as
a
boson
and
can
therefore

undergo
condensation.

Jin
and
co-workers
started
with
a
gas
of
potassium-40

atoms,
which
are
fermions,
in
an
optical
trap
at
a

temperature
of
about
300
nanokelvin.
Next,
they

applied
a
magnetic
field
to
change
the
interactions

between
the
atoms
and
create
a
“Fesbach
resonance”
at
which
the
interaction
changes
from
being
highly

repulsive
to
become
highly
attractive.
If
the
value
of
the
magnetic
field
is
carefully
controlled,
the
atoms
will
form
Cooper
pairs
rather
than
molecules.

To
confirm
that
they
had
produced
a
condensate
from

pairs
of
atoms

and
not
a
molecular
condensate
as
in

the
earlier
work

Jin
and
co-workers
actually

transformed
the
pairs
into
molecules.
They
did
this
by

applying
a
second
magnetic
field

which
had
exactly
the

right
strength
to
create
molecules

and
opening
the

optical
trap
at
the
same
time.
This
allowed
them
to

observe
the
characteristic
shape
of
a
condensate
cloud

(see
figure).
According
to
the
JILA
team,
the
change
in

the
magnetic
field
can
cause
molecules
to
form,
but
the

changes
are
too
fast
to
create
a
molecular
condensate.

“We
expect
that
the
fermionic
condensates
we
have

observed
will
exhibit
superfluid
behaviour,”
said
Jin.

“They
represent
a
novel
phase
that
lies
in
the
crossover
between
superconductors
and
Bose-Einstein

condensates.
This
opens
up
the
very
exciting
potential

to
study
superconductivity
and
superfluid
phenomena

under
extreme
conditions
that
have
never
existed

before.”

Author
Peter
Rodgers
is
Editor
of
Physics
World.
Belle
Dumé
is

Science
Writer
at
PhysicsWeb

Advertisements

2 Comments

  1. 关于这个费米子的BEC凝聚问题近来的报道比较多,我感到也非常有意思,所以就转到这里了,当作备忘。有意思的东西,未来也许会有很大的作用。我最大的幸福就是能够见证这个过程。 by:gorilla

    回复

发表评论

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / 更改 )

Twitter picture

You are commenting using your Twitter account. Log Out / 更改 )

Facebook photo

You are commenting using your Facebook account. Log Out / 更改 )

Google+ photo

You are commenting using your Google+ account. Log Out / 更改 )

Connecting to %s