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Faculty of Arts and Sciences
University of
Pennsylvania
October 1979
A Supplement to Almanac
Physics: From Quarks to Solar Cells
Even people who have trouble recalling the parts of the atom
can't help but leave David Rittenhouse Lab or the Laboratory
for Research on the Structure of Matter with enthusiasm for
the work of Pennsylvania's physicists.
Nobel Laureate J. Robert Schrieffer's codiscovery of the
theory of superconductivity is just the beginning
of a long list
of fascinating contributions by the 43 physicists in this
department. Their work ranges from developing a new
standard for the volt to synthesizing a material that may
provide inexpensive solar cells for powering our homes. It
includes catching collapsing stars and helping to discover the
fourth quark,
one of the basic particles of the universe.
This one department,
which attracts $4.7 million to the
University
each year
in federal funds, contributes one third of
the Faculty of Arts and Sciences' research budget. In addition
to the 43 members of the teaching faculty, the department
has 20 research faculty members and a professional
staff. They work in three major areas: particle
physics, nuclear physics and condensed matter (solid state)
physics.
In each of these areas, there are both theoretical
physicists and experimental physicists.
Research, according
to Department Chairman Walter D.
Wales, is coupled with a heavy emphasis on teaching.
There
are 90 graduate students and 12 to 15 majors
in each un-
dergraduate
class. Each term 1200 to 1300 undergraduates
are enrolled in physics courses. All of the faculty
teach in-
troductory courses and each term half of the faculty
members
conduct undergraduate laboratory sections.
Outside the classroom, these physicists are working on
problems that may at first seem far removed from our day-to-
day world, but often turn out to have some fairly clear
practical implications. The work of E. Ward Plummer
exemplifies this kind of research. He is studying catalytic
reactions-how molecules react on the surface of certain
metals. His work concerns such problems as what happens to
the electrons in these reactions and where the atoms sit on
the surface of the metal. Working
with theoretical physicists
J.
Robert Schrieffer and Paul Soven, he has found out that a
carbon monoxide molecule stands up straight on the surface
when it is binding to nickel, whereas nitric oxide on nickel
cants over at a 25 degree angle.
In such seemingly
small differences in molecular posture,
there is information potentially
worth millions in the chemical
and energy
industries. These catalytic
reactions, in which the
metal surface channels the reaction, allow oil processors,
plastic
manufacturers and others to speed up
the desired
reaction by
as much as a million times while slowing
down
undesirable reactions that could occur. Thus Plummer's work
is quite likely
to provide
information that engineers
can use to
improve these reactions, thereby improving
the way
we
process
oil or make plastics.
In the following pages
are other examples
of physics
research, which may
lead to solutions to our energy problems
or a better understanding
of how the universe was created.
A Supplement to Almanac
Toward Life's Most Basic Elements
Top view of an earlier version of the experiment, designed to study
the interactions of neutrinos with matter. The experiment involved a
collaboration of physicists from the Fermi National Accelerator
Laboratory, Harvard University, the University of Pennsylvania and the
University of Wisconsin.
In one of man's basic quests in physics, he looks deeper and
deeper into the atom to understand just how it is composed
and what binds it together. This search has led scientists to
two major discoveries in the past decade, and Alfred K.
Mann's group at
Pennsylvania has been
instrumental in both.
Their experiments helped to turn up the missing
pieces in a
long sought theory to unify two of the four basic forces in the
universe. They also helped to identify the fourth quark, one of
the basic particles of matter. These discoveries took place in
experiments at a high energy particle accelerator in Batavia,
Illinois, where protons with an energy of 400,000,000,
electron volts split atoms into what scientists believe may be
their most fundamental parts.
Less than a century ago scientists thought that atoms
were the smallest particles in the universe. Today they believe
that an atom is composed of electrons and two kinds of
quarks-an up quark and a down quark, which combine to
form protons and neutrons. A number of other fundamental
particles have also been identified in cosmic
rays striking the
earth from outer space and in reactions produced at high
energy accelerators; among these are neutrinos,
particles that
have no mass or charge and are released when neutrons
decay into protons and electrons.
The interactions among these particles of the universe
are controlled by four basic forces: the gravitational force, the
electromagnetic force, the strong force and the weak force.
The gravitational force is comparatively feeble and has little
impact on the particles in the atom. The electromagnetic
force both draws objects of different electrical charges
together and holds the negatively charged electrons to the
positively charged nucleus to form the atom.
The strong or nuclear interaction holds the nucleus of an
atom together.
It is several hundred times stronger than the
electromagnetic force, but operates within a very short
distance-usually only with particles next to it in the nucleus.
The weak force is at work in the interaction between
electrons, neutrinos and muons, particles which behave like
an electron and frequently occur in cosmic rays. This force is
responsible for normal radioactivity.
To shatter the atom in such a way that the fundamental
particles are freed and the forces can be studied, Mann and
other particle experimentalists go to one of a half dozen high
energy accelerators, such as the one at Fermilab in
Batavia,
Illinois. Here they spin a beam of protons one millimeter wide
around a circle that is one and a quarter miles in diameter.
They then shoot this proton beam down a straight mile-long
run to hit a target one foot long and one inch in diameter. As
you can well imagine, a rather powerful reaction takes place.
While many particles come out of this reaction, many decay
almost instantly, and still others are trapped in earth banks
beyond the target. The particles necessary for the ex-
periments continue another mile until they strike the detector.
Particle physicists since the early 20th century have
been trying to form one unified theory that would explain the
weak and the electromagnetic forces together.
By the late
'50s particle theorists had proposed some plausible
possibilities. These theories, however, were contingent on
something that had not thus far been observed, the weak
neutral current.
Mann and his colleagues began looking for this neutral
Creating
a New Standard for the Volt
When you find the notation 120 V on the bottom of your
toaster, thank Donald N. Langenberg and his colleagues for
their series of experiments that established a new standard
measurement for the volt, which was adopted by the United
States in
This practical discovery was one of a number of new
measurements that came out of experiments in an area of
superconductivity called the Josephson effects. Super-
conductivity is a phenomenon in which many metals lose all
electrical resistance at temperatures near absolute zero and
thus become perfect conductors of electricity. In 1962 a
British graduate student named Brian Josephson developed a
theory that predicted how superconducting electron pairs
could move from one superconductor to another, passing
through a layer of insulation only five or ten atoms thick.
Scientists would have expected this
insulator to stop the flow
of such electron pairs almost completely. Josephson
predicted that if direct current was applied to this sandwich
composed of two superconductors separated by a thin in-
sulator, it would produce an alternating current through the
insulator. Today the sandwich of superconductors filled with
an insulator is called a Josephson junction. Josephson
developed an equation to describe this effect.
He said that the
frequency of the alternating current would equal twice the
charge on an electron divided by Planck's constant times the
dc voltage applied to the system, or as physicists say,
f=2eV/h.
A
Supplement
to Almanac
The Star Catchers
Kenneth Lande and his colleagues are catching stars-or,
more accurately,
little pieces of them-at their underground
detector in a South Dakota gold mine.
He and four colleagues have developed a neutrino
detector, a water-filled chamber that generates electronic
signals when it is struck by a neutrino or another cosmic
particle called a muon. The detector has three main functions.
First it is designed to settle a controversy over whether a
proton
lives on forever or can indeed decay. Second it detects
neutrinos from collapsing stars and thus provides an early
warning for astronomers as to the whereabouts of collapsing
stars in our galaxy.
Finally, the detector identifies the sources
and measures the composition of the very high energy cosmic
rays striking the earth's atmosphere.
Lande's neutrino detector, located in the Homestake
Gold Mine in Lead, South Dakota, is housed one mile un-
derground, deep enough so that all cosmic particles are
stopped by interactions with the earth-with the exception of
neutrinos and muons. Neutrinos are particles that have no
mass or charge and are produced when neutrons are created
or decay.
Muons are particles that behave like electrons, but
are over 200 times more massive and are created when
cosmic
rays strike our upper atmosphere. These two particles
create a reaction in the detector developed by Lande and his
colleagues, William
Frati, Richard Steinberg, C.K. Lee and
Marianne Deakyne.
This detector consists of 1,000 tons of water, which is
divided by sheets of plastic into a grid of 6 by 6 by 4-foot
cubes. Within each cube are four photomultipliers, devices
that turn the light flashes created by a reaction of a proton
decay, a neutrino interaction or a muon traversal into an
electrical impulse. The detector, coupled with elaborate
computer circuitry,
records what reactions are taking place
and where and when they are occurring.
At the moment, Lande has the only detector in the world
that might answer the question of whether a proton will
always stay a proton or will
instead decay into other particles.
If it does decay, it doesn't do it too fast. Present theories
suggest that protons might have a life expectancy of about
1,000,000,000,000,000,000,000,000,000,000 (10°) years!
Since Lande has over 1032
protons in his detector, he should
be able to explore the expected lifetime range.
If a proton
decays after 10° years, he could expect about ten protons a
month to decay in his detector, one a month if protons live for
years.
A decaying proton,
Lande expects, would cause two
particles to shoot out suddenly in equal and opposite direc-
tions, producing photomultiplier signals with paths beginning
inside the detector rather than from its edges.
The Homestake neutrino detector is also designed to
spot collapsing stars for astronomers. The current astro-
physical theory holds that as the center of a star gets ex-
tremely hot, a proton and an electron come together to form a
neutron and a neutrino. Since the neutron takes up much less
space than the entire atom with its swirl of electrons, there
is
a lot of space between these newly formed neutrons.
Gravitational forces then eliminate this space by shrinking the
Kenneth Lande servicing
the top
section of the Homestake Neutrino
Detector.
star from perhaps
a million miles to about ten miles across.
During this process of shrinking, millions and millions of
neutrinos ( to be more precise) come streaming
out of the
star. Soon afterward the matter left on the surface of the star
is blown away creating a light as bright as the daytime
sun.
This explosion is called the supernova phenomenon. Next the
star begins
to emit pulses of radio waves and becomes a
source of extremely high energy cosmic rays. This occurs in
our galaxy about once every
three years.
Lande is anxious to detect the burst of neutrinos from
these stars. He expects such a burst to trigger many signals
simultaneously in the Homestake detector as well as in other
neutrino detectors located in Ohio, Switzerland and the Soviet
Union. By the angles
at which neutrinos enter these detectors
and by the relative time of their arrival, he hopes
to plot
the
exact origin of the neutrino burst.
'This neutrino early warning system could then alert
radio and optical astronomers. In effect, it says, 'Hey, go
look
over there. Tomorrow you
will see the supernova glow
and in
three weeks you will see the radio pulsations,"
Kenneth
Lande explains. Without this warning system,
it would be easy
to miss these phenomena, since there is such a small chance
that the telescopes
would be aimed in the right direction.
While astronomers are picking up
the star's radio waves,
Lande's detector is set to study
the cosmic rays these stars
give off simultaneously.
He is finding out about these rays by
detecting
the muons the rays
create when they
hit our upper
atmosphere.
Lande's group
traces the muon particles
backwards to pinpoint the spot
in the sky where they were
created. In so doing, they
can determine where cosmic rays
are coming
from.
Lande and his colleagues
have already
made progress
in
catching
bits of collapsing
stars since their detector went into
operation
in January,
Hopefully,
their work will help
answer some basic questions
about these stars and thus
about the nature of the universe.
A Cosmic
Recipe for the Universe
Why
is there matter in the universe? Why isn't it just a vast
light-filled emptiness? To answer this question theoretical
particle physicist Anthony Zee has developed a theory with
colleagues
at Princeton.
"We give
a scenario, a cosmic recipe, of how the
universe came about," he explains.
Zee's theories rest on the big bang theory of cosmology,
the most widely accepted notion of the creation of the
universe. According to this theory,
the universe was originally
very
small, compact and hot-so hot in fact that only energy
could exist. Even the nuclei of atoms had melted apart. Our
10-billion-year-old universe was created by
an enormous
explosion,
called the big bang, which created the universe as
we know it, leaving
us with an expanding system that is now
cooling
down.
There are some phenomena that are not explained in this
theory,
and it is towards an understanding
of two of these
phenomena
that Zee has addressed his calculations. First of
all, he asks, where did the protons and other fundamental
particles come from? From all that has been observed, the
proton does not decay into other particles. Recently,
however,
there has been speculation by theoretical physicists that the
proton does indeed decay. Second, the universe should
consist of equal amounts of matter and antimatter according
to the laws of physics. Antimatter, however, has been iden-
tified only in laboratories, not in nature.
Zee and his colleagues
have developed calculations that
trace the cooling
down of the universe and explain how the
cooler temperatures
led to the creation of quarks, protons and
other fundamental particles
and to the destruction of anti-
matter. In the beginning, they postulate, the universe was so
hot that it consisted only
of energy. As the temperature
dropped,
however, a series of physical reactions took place
that created quarks
and antiquarks.
A small difference in
behavior between matter and antimatter known as the CP
violation became important
at these high temperatures. More
quarks
than antiquarks were produced, and ultimately the few
antiquarks
that were produced were annihilated. The quarks
lived on, and as the temperature
of the universe cooled down
even further, these fundamental particles coagulated to form
protons and other basic particles. Today the universe is just
too cold to create quarks, protons
and other particles.
Now Zee has turned his attention to another equally
baffling problem: why
does nature seem to repeat itself un-
necessarily?
Atoms are composed
of three fundamental
particles: up quarks and down quarks (which combine to form
protons and neutrons), and electrons. Nature, however, has
produced a duplicate
set of these particles, which are exactly
alike except
that they
are heavier. It now appears that there
may
be a third set of these basic particles even heavier than
the second. Zee plans
to use a branch of mathematics called
group theory,
which deals with principles of symmetry to get
to the bottom of this. Maybe, he suggests, three sets are
more beautiful than one!
Testing Laser-Proof Materials
Can LRSM's new organic
solids help
beam information to
satellites in outer space? Are these organic solids the future
switching gear that will replace
tons of copper cable in our
communications system? Or will they
allow engineers to write
computer circuits small enough
to fit in a wrist-watch?
All of these are real possibilities with a group of organic
solids now being
tested by Anthony
F. Garito and 15 scientists
at the Laboratory for Research on the Structure of Matter
(LRSM). These organic
solids may
be particularly useful in
laser technology because they
behave differently from other
materials under this high intensity light.
Laser light, unlike
normal light, is composed
of only one frequency and moves in
only one direction.
The organic solids under study can withstand extremely
high intensity lasers. While the inorganic counterparts of
these substances shatter at levels of laser light as low as 100
kilowatts, physicists
have not yet invented a laser beam
strong enough to shatter these organic solids. Physicists
are
even more interested in the fact that these organic
sub-
stances actually change the light that strikes them. When
most substances are hit by
a beam of light, they either reflect
it or absorb it. When the substances under study at LRSM are
hit by a laser beam, they
can modulate the light, change
its
amplitude
or filter it. For example, by frequency doubling they
can turn a beam of red light into a beam of green light.
Garito and his group are anxious to identify
as many
of
these changes
or electro-optical effects as they
can and to
understand how they occur. Once these materials are better
understood they
can be put to use in the rapidly developing
field of laser technology.
Laser beams are important in both communications and
computers
because they travel at the speed of light
and thus
can move a signal
from one place to another almost instan-
taneously.
In addition, the fibers for conducting
these light
impulses
are much smaller than electrical circuits. Tons and
tons of copper
cable, for example, can be replaced by
a
bundle of optical fibers. To make the best use of laser tech-
nology, industry needs improved materials for getting light
signals
in and out of the systems that transmit high intensity
laser beams to distant satellites and route beams of laser
light
in various directions.
Garito's group has already happened upon one in-
teresting application for their organic
solids. While they were
looking
at the electro-optical changes in these materials, they
saw that the photons from the laser beam created an ex-
tremely efficient chemical reaction that changed the chemical
identity
of solids. This process, the physicists realized, could
be used to make computer circuits as small as one-half a
micron, about one hundredth the diameter of a human hair
and smaller than any circuits thus far manufactured.
Garito's group is now working
to create a thin film of this
organic polymer that will be the most useful for this process,
which changes the material's chemical identity. They have
already modified their process to use the more effective
X-rays rather than laser beams for delineating
the circuits.
At the same time Garito's group
is fabricating and testing
over 300 different organic solids with these electro-optic
properties. Their federal grants
for this work range from
$500,000 to $ million a year.
In addition to their work on why
and how these materials change
laser light and withstand high
intensity
beams, they
are also considering the general issue
of how these materials behave in a high intensity electrical
field. This understanding is critical to the next generation of
computers, where the dimensions will be small, the electrical
field will be of very high intensity, and materials may well
behave in new ways.
'To maintain high technology, we must understand what
happens
to condensed matter systems
in the presence of an
electric field. That's the very beginning of physics, and it's
still with us today," concludes Garito.
Mathematical Links
Between Magnets and
Coffee
A
primary goal
of theoretical physics is to explain seemingly
disparate phenomena with a single unifying theory. Such
unifying
theories have led to significant advances in our
understanding
of how nature works. In the 19th century, for
example, James Clerk Maxwell explained the once distinct
phenomena of electricity
and magnetism with a single theory
that predicted altogether new phenomena such as elec-
tromagnetic
waves. In the past decades this electromagnetic
force was combined with a weak force into a single unifying
theory (which Penn physicist A.K. Mann helped to prove
with
his neutrino experiments).
"In condensed matter physics, the fundamental inter-
actions among particles are known. The interest and vitality of
this field comes from the enormous variety
of phenomena
that
can be produced by the 100,000,000,000,000,000,000,
(1023) interacting particles," explains
Tom Lubensky.
Professor Lubensky has been studying
the universal
properties
of phase transitions, that is, the change
of a
material from one state to another. As water evaporates into
a gas,
for example, it is undergoing
a phase transition.
Recently physicists have studied the liquid-gas transition
with the same universal theory
that describes the transition
from a magnetic
to a non-magnetic
state in an iron bar or the
transition from a normal to a superfluid state in helium. These
studies have been possible largely because of a sophisticated
mathematical tool called the renormalization group, developed
by Kenneth Wilson of Cornell. Wilson's work showed that all
of these transitions share a common mathematical property
in
the way they develop order.
Lubensky has demonstrated how Wilson's universal
theory
also applies to a variety
of "percolative" transitions.
Percolation occurs whenever fluids flow through random
networks. One example of this process
is the random path
that water takes down through
the spaces between the coffee
grounds
in a drip coffee maker. Other examples are the flow
of oil in porous rock and water moving through sandy soil.
Mathematically the process of percolation is very similar to
the formation of a gel (such as jello)
or the vulcanization of
rubber. In the cases of coffee and oil in porous rock, the
network filling
the container is the fluid passing
from one end
of the system to the other. In the case of a gel, a large
number of small molecules react with each other to form
larger molecules as time progresses.
At a critical time, a
container filling
network forms and gives the gel its charac-
teristic rigidity. Thus this single theory shows that such seem-
ingly unrelated phenomena
as evaporation of water, extracting
oil from porous rock and making jello
have much in common.
Polyacetylene:
A Metallic Chameleon
Polyacetylene
makes Penn's physicists
and chemists sound
like magicians. They can turn this material, which looks like
metallic saran wrap, from an insulator that stops the flow of
electricity to a metal that permits electricity to flow freely. Out
of it they can create solar cells for powering our homes with
the light
of the sun or rectifying junctions that change
alternating current into direct current.
Even to chemists and physicists polyacetylene seems
remarkable, for this organic polymer appears to conduct
electricity
in an entirely new way.
It is thus causing physicists
to introduce new concepts to the basic solid state physics
of
the last 40 years
and is leading
to the development of a whole
new class of materials.
Alan J. Heeger, Professor of Physics and Director of the
Laboratory
for Research on the Structure of Matter, and Alan
MacDiarmid, Professor of Chemistry, decided to study
polyacetylene because they wanted to combine the
techniques
of physics
and chemistry
to develop an organic
material that might
have electronic or magnetic properties.
They
were attracted to polyacetylene
because it is about the
simplest organic polymer;
it consists only
of carbon and
hydrogen
molecules linked together
in a herringbone
chain.
Initial solid state experiments on the substance were so in-
teresting
that Heeger
and MacDiarmid brought
in other
scientists, including
J. Robert Schrieffer, to explain
some of
its remarkable properties.
These scientists found that polyacetylene
could respond
to an electrical current in extremely
different ways depending
on how they
made this material. By adding
a small amount of
chemical impurities to polyacetylene,
a process
called doping,
Heeger
and MacDiarmid have literally
turned this substance
into an insulator, a semiconductor and a metal depending
upon
the chemicals they
add. This gives polyacetylene
the
distinction of having
the largest range
of electrical con-
ductivity yet
discovered. As physicists put
it, its electrical
conductivity
can be controlled over 1,000 billion times-from
1010 to 10+3.
The Penn scientists have also made progress
toward
developing some practical
devices out of polyacetylene. They
have successfully
constructed rectifying junctions,
which are
used to change alternating
current into direct current. They
have also made polyacetylene
into solar cells, which transfer
the light of the sun into electrical current. Because
polyacetylene
can be made fairly cheaply,
there is some hope
that it might be used for converting the sun's energy into
electrical power
that could supply electricity
to homes or
offices.
What really
excites the physicists,
however, is the fact
that polyacetylene may be a one-dimensional metal. This
means that it conducts electricity primarily
in the direction of
the polymer chain unlike other known metals, which conduct
electricity
in all three dimensions. Theoretical physicists
have
developed many concepts
based on one-dimensional models
of electrical conductivity, but only with the advent of
polyacetylene and related organic metals did they
have a
chance to test and indeed validate these ideas.
"We now believe that the conduction mechanism, the
mechanism that conducts charge along polyacetylene is
totally
different from anything people
have seen before,"
explains
J. Robert Schrieffer, who is now working
on this with
Scanning electron microscope picture of as-grown polyacetylene.
Scanning electron microscope picture of oriented polyacetylene.
The fibril diameter is approximately
200 Angstroms (1 Angstrom=
1 08cm).
a graduate student, Wu Pei Su.
Schrieffer, Heeger
and others believe that at low doping
levels, the properties of polyacetylene are dominated by
polymer
excitations called solitons, which are kinks moving
along
the polymer
chain. The moving
solitons are believed to
be carrying electrical current. Thus polyacetylene is the first
known system in which solitons may be playing a fundamental
role in carrying
an electrical charge.
Schrieffer and Wu Pei Su have come up with a theory of
solitons in polyacetylene, which they describe as very simple.
Their theory accounts for most of the experimental effects,
some of which completely disagree with the findings from the
last 40 years of solid state physics. These theoretical
physicists have predicted certain phenomena and are waiting
for the experimentalists to see if their predictions are ac-
curate.
Polyacetylene is already leading to the development of
more new materials, now that physicists understand the
physics and chemistry for making metallic or semiconducting
polymers.
"It's fairly clear that it's going to be a growing and
maybe very large field. Its ultimate importance will depend on
what underlying
science is brought out and what technology
comes from it," concludes Heeger.
FAS
Reports:
A
Supplement
to Almanac
Editor Michele Steege
Assistant Editor Jan Brodie
Design
E.R.
Landesberg
