The Physics of Materials: How Science Improves Our Lives (1997)

The processes that are used to produce industrial materials—casting alloys for jet engines or fabricating microscopically small features of computer chips—are all exercises in what we call “nonequilibrium physics,” the study of systems that are changing their shapes or properties as we exert forces on them, freeze them, or otherwise disturb their states of equilibrium. Predicting and controlling these processes with the precision that will be needed for applications requires fundamental understanding of the nonequilibrium phenomena underlying them and is a challenge for physicists.

For example, snowflakes form by a branching process that is called “dendritic crystal growth.” Research in this area has been driven not only by our natural curiosity about snow-flakes, but also by the need to understand and control metallurgical microstructures. The interior of a grain of a freshly solidified alloy, when viewed under a microscope, often looks like a collection of overly ambitious snowflakes. Each grain is formed by a dendritic mechanism in which a crystal of the primary composition grows out rapidly in a cascade of branches and side branches, leaving solute-rich melt to solidify more slowly in the interstices. The speed at which the dendrites grow and the regularity and spacing of their side branches determine the observed microstructure, which in turn governs many of the properties of the solidified material such as its mechanical strength and its response to heating and deformation. We cannot yet predict microstructures accurately, but much progress has been made in the last decade. Figure 3.1 shows one of the best new theoretical efforts in this direction.

Much of the most important recent progress in nonequilibrium physics has consisted simply of recognizing that fundamental questions remain unanswered in many familiar situations. The recent growth of interest in fracture and friction, for example, has led us to realize that we need to establish first-principles understanding of the difference between brittleness and ductility, especially in noncrystalline materials. We are learning about the dynamics of granular materials, systems that are like liquids in some respects, like solids in others, and unlike either in many of the most important ways. And we are just beginning to learn which questions to ask in a search for understanding the dynamics of fracture at crack tips, failure at interfaces between different solids, or rupture on earthquake faults.

Because of the astonishingly rapid advances in both hardware and software, the small workstations or PCs that sit on almost every scientist's desk these days have the power of machines that we called supercomputers little more than a decade ago. Today's supercomputers can simulate the behavior of hundreds of millions of interacting classical molecules or follow the transitions among comparable numbers of quantum states. This exponential growth in computational power will continue for at least another decade.

Computers play a central role in modern experiments, controlling apparatus, acquiring and storing data, and analyzing data. Theorists also find them essential for solving mathematical problems that once seemed intractable. But the computer is now emerging as much more than just a tool for assisting the work of scientists; it is making a qualitative change in the kinds of research that will be done in the near future. Consider just a few examples.

Starting from little more than the masses and charges of electrons and atomic nuclei, as well as the rules of quantum mechanics, we are approaching the point where we will be able to predict accurately the properties of molecules, of atoms at solid surfaces and interfaces, of defects in solids, and even of larger structures such as the recently discovered fullerenes (see page 21).

In situations that justify neglecting quantum effects, multimillion-molecule simulations are beginning to provide valuable information about complex solid-state phenomena such as fluid flow, fracture, friction, and deformation. The great advantage of such computational investigations is that they can tell us in detail about the behavior of individual molecules. Thus, computer-based studies of this kind have features of both experimental and theoretical research.

FIGURE 3.1 Computer simulation of dendritic growth in the solidification of a nickel-copper alloy. The colors indicate relative concentration of copper, from low (red) to high (blue). The orange and red regions are solid; the green and blue regions are liquid. (Courtesy of the National Institute of Standards and Technology.)

Although the atomic picture of the world was formed over a century ago, it is only in the last few decades that compelling visualization of atoms has become possible. Since there are only 92 stable elements, the diverse materials known to us derive their complexity for the most part from the patterns of their atoms' arrangement, in molecules and in solids. Direct atomic-scale visualization of these patterns allows us to develop new materials and better understand old ones. It has been used to look at the action of semiconductor devices, the workings of chemical reactions, and the structure of genes.

Many new visualization tools emerged from condensed-matter and materials physics. One such advance was the invention of scanning probe microscopy. In this class of techniques, a sharp needle-like tip is moved around (scanned) near a surface. A map of the surface is then constructed in real time by measuring some response of the surface to the tip —the force of the surface on the tip, for example, or the electric current that flows between the tip and the surface. Using precision actuators called piezoelectrics, a tip can be moved up and down or sideways by less than the size of an atom. Scanning the tip across the surface in this way while measuring the response allows an atomic-level image to be built up, much as a blind person can acquire a mental picture of an object by feeling its shape.

FIGURE 3.2 A scanning tunneling microscope image showing a snapshot of the growth of germanium on a silicon surface. Each bright spot is a single atom. (Courtesy of IBM Research.)

The first suggestion of a super-resolution microscope can be traced back as far as 1928 to the British scientist E.H. Synge, but the first working scanning probe microscope with atomic resolution was not invented until 1981, at IBM's research laboratory in Zurich, Switzerland.

In the decade and a half since then, a wide variety of related visualization techniques have been developed. Scanning probe techniques now exist that allow atomic-scale sensing and mapping of electrical, optical, and magnetic properties, surface forces, and other phenomena.

A particularly important recent innovation has been the possibility of using a scanning tip not just to study a sample but also to manipulate atoms actively. The “quantum corral” shown on the front cover and in Figure 1.3 was made in this way. This microscopic atom-by-atom “engraving”makes even microsurgery look like the metaphorical bull in the china shop.

The fractional quantum Hall effect is an example of beautiful and fundamental new physics made possible by technological advances in the fabrication of artificially structured materials. It takes place in a two-dimensional electron “gas” produced in a transistor-like device subjected to extreme conditions of high magnetic fields and low temperatures. Under these conditions, electron correlations become dominant. The basic observation is a precise quantization of the Hall conductance with the unusual property of being described by a quantum number that is fractional rather than an integer.

The application of a strong magnetic field at low temperature induces large numbers of vortices (“whirlpools”) that attach themselves to the electrons to form composite objects, which condense into a special quantum “fluid.” This fluid of composite particles has the bizarre property that the low energy excitations consist of a single vortex that binds a fraction of an electron charge. These objects have recently been observed through direct measurement of their fractional charge and by tunneling experiments in which an electron added to the system is seen to break up into three excitations, each with one-third of the charge. Theoretical work on this problem has led to profound and intellectually exciting new concepts and techniques with applications both in other areas of condensed-matter physics and in quantum field theories studied in elementary particle physics. We are familiar with the idea in high-energy physics that certain elementary particles such as protons are actually composite objects made up of fractionally charged quarks. These quarks can be observed in collisions at very high energies (or equivalently, high temperatures) carried out using particle accelerators. In condensed-matter physics, one does the reverse: the analog of the accelerator is the refrigerator. At sufficiently low temperatures, in a strong magnetic field, electrons added to a quantum Hall system break up into fractionally charged elementary vortex excitations. This, then, is a fundamentally new form of conduction in an artificially created, layered material.

FIGURE 3.3 A pictorial representation of the many-particle state that underlies the fractional quantum Hall effect. The height of the green landscape represents the amplitude of the quantum wave of one electron as it travels among its companions (gold balls). The arrows indicate the vortices induced by the magnetic field. These vortices attach themselves to the electrons to form composite particles. (Courtesy of Lucent Technologies Bell Laboratories.)

An important frontier of materials science is in the behavior of condensed matter under extreme conditions: heat and cold, high pressures, mechanical stresses, large magnetic and electric fields, and intense radiation environments. Experiments at this frontier will continue to have a major impact on engineering, where breakdown of component materials in various hostile environments is a significant concern. Topics of major national interest include jet engine technology and weapons. Progress in the physics of materials under extreme conditions has great significance for other areas of science—notably geophysics, where conditions of high heat and pressure are routine, and astrophysics, because extreme conditions routinely occur elsewhere in the universe. Extreme conditions sometimes also occur unexpectedly on the simple laboratory scale and yield novel phenomena such as sonoluminescence, in which flashes of light are emitted from collapsing air bubbles under the influence of sound waves.

Many of the most important breakthroughs in materials physics, ranging from the discovery of superconductivity at the beginning of the century to the discovery of the quantum Hall effect near its end, have occurred as the result of explorations whose goal was simply to find out what happened to known substances under more extreme conditions than they had been exposed to previously. Discoveries found at this frontier can be translated into practically useful technologies either as subsequent advances make the extreme conditions routinely achievable (as for the superconducting magnets in magnetic resonance imaging devices with medical applications), or by inspiring scientists to create new materials that display the newly discovered phenomena under less extreme circumstances. We suspect that throughout the rest of human history, the agenda of this most human of pursuits in materials research—namely to produce the world's highest magnetic fields and pressures or the lowest temperatures, and to be the first to observe what these conditions imply for materials from hydrogen to gallium arsenide —will remain the same, with equally productive outcomes.

FIGURE 3.4 A generator at the National High Magnetic Field Laboratory. This generator serves as a power supply for a high-performance magnet that will provide unprecedented insights into the behavior of matter subjected to extremely strong magnetic fields. (Courtesy of Los Alamos National Laboratory.)

By the mid-1970s it seemed that the limiting temperature for superconducting materials was near 25 K, a temperature still requiring expensive liquid helium cooling. Theoretical calculations based on the mechanism that controlled electron pairs in known superconductors indicated a maximum temperature for superconductivity of less than 40 K. In 1986 a totally unexpected, even shocking, discovery was made. A class of copper oxide ceramic materials was found to become superconducting at temperatures much higher than 25 K. We now have many complex materials that become superconducting at temperatures well above the boiling point of inexpensive liquid nitrogen, 77 K.

The occurrence of superconductivity in a totally unexpected class of materials, and the potential for its practical use above the temperature of liquid nitrogen, have motivated a wide range of research and development efforts over the past decade. Superconductivity has now been observed in specially prepared ceramics at temperatures as high as 135 K.

Despite extensive worldwide efforts, however, an understanding of the mechanism for superconductivity in the new oxide materials is still lacking. For example, physicists cannot reliably predict whether a material could exist that would superconduct at room temperature. These new materials are brittle ceramics, with properties completely different from those of the ordinary metals in which low-temperature superconductivity occurs. Paradoxically, in their pure state, these oxides do not conduct electricity at all—they are magnetic insulators. Even when charge carriers are introduced by chemical doping, they are poor conductors at room temperature.

FIGURE 3.5 This magnetic image shows four high-temperature superconducting rings, each 60 microns in diameter, fabricated on at a constant elevation above the substrate. The volume of each structure thus represents the magnitude of the magnetic flux trapped within that ring. The trapped flux in the center ring is exactly half the unconventional electron pairing. (Courtesy of IBM Research.)

The superconducting state itself appears to have pairs of electrons orbiting around each other in an unconventional manner. (See Figure 3.5.) The sensitivity of this state to magnetic fields presents technological challenges that must be overcome for certain practical applications; but on the positive side, it has also led to exciting new fundamental scientific ideas.

The discovery of high-temperature superconductivity has focused attention on the enormous variety of complex oxide materials. In addition to superconductivity, many of these materials exhibit other novel magnetic and electrical properties.

Artificially structured materials are structures not available in nature. Often the surfaces and interfaces of these materials dominate their properties. Artificially structured materials are critically dependent on enabling technologies for fabrication and characterization, tying progress in science to advances in relevant technologies. Although the field was born in the 1960s, there has been impressive progress in the last decade. New structures are possible because of increased cleanliness, extreme growth conditions, and substrate modification before growth. Our scientific understanding has increased through atomic-level elucidation of surface and interface structure and defects using the new scanning probe microscopies that have completely changed our thinking about how to study surface phenomena.

Artificially layered structures have enabled the realization of many new devices, including high electron-mobility transistors, semiconductor lasers, giant magnetoresistance materials, and x-ray optics. Some of these devices are now ubiquitous in such consumer electronics as cellular telephones and compact disk players. Others promise major advances in computing and communications. These technological advances have in turn enabled the structures and materials required for many of the accomplishments detailed in other sections of this report, such as the fractional quantum Hall effect.

FIGURE 3.6 A self-organized ordered array of InGaAs quantum dots grown in three regular layers on a GaAs substrate. The dots are the bumps on the surface and the light-colored internal structures beneath them. The layers are separated by about 65 nm, and the dots within each layer are about 250 nm apart. (Courtesy of NTT Optoelectronics Laboratories.)

Solid carbon is well known for its two stable crystalline forms, diamond and graphite. It is also known to exist in a number of other metastable forms, such as coke and glassy carbon. These different forms of carbon are among the most widely used materials because of their remarkable properties, such as the hardness of diamond and the lubricity of graphite. Until recently, no one would have suspected that another large class of carbon structures could be made, with yet more remarkable properties. Yet over the last decade that is exactly what has happened.

In 1985, while working with gaseous carbon like that found in interstellar space, scientists found that under certain conditions (laser ablation of graphite in an atmosphere with a controlled partial pressure of helium) molecular clusters could be made that contained only certain specific “magic numbers” of carbon atoms. The structural motif they proposed for this class of clusters had its inspiration in the geodesic dome designs of the architect Buckminster Fuller. The simplest of these designs is that of C₆₀,Which is made of 5- and 6-atom carbon rings fused together into a structure resembling a soccer ball.

Large amounts of fullerene carbon were soon synthesized, allowing a wide variety of experiments to be performed. The proposed structure was dramatically proven correct, and a number of fascinating properties were discovered. One of the most remarkable is that molecular clusters of C₆₀ can be doped with electrons by donors such as alkali metals. This makes them into superconductors with critical temperatures surpassed only by the recently discovered copper oxides.

FIGURE 3.7 A computer model of the interior of a carbon nanotube. Such tubes exist in various forms, including this spiral structure. (Courtesy of the University of California, Berkeley.)

The fullerene forms of carbon are now also known to include capped cylinders, sometimes called buckytubes. These cylinders appear to be one of the promising approaches to the development of nanoscale wires and other electronic components.

For their initiation of fullerene research, Richard Smalley, Harold Kroto, and Robert Curl were awarded the 1996 Nobel Prize for Chemistry.

Progress in molecular biology depends on using a technique called gel electrophoresis to analyze DNA. A sample is placed at one end of a slab of gel, and an electric field is applied. The field pulls DNA molecules of different sizes through the gel at different speeds, separating the components of the sample.

Despite its widespread use, little was known about how DNA molecules actually move through a gel when physicist Pierre-Gilles de Gennes began work on the problem in 1971. Gels are a water-swollen tangle of long chain-like molecules called macromolecules. De Gennes proposed that when other macromolecules such as DNA move through a gel, the tangles force them to slide along their own contours in a snake-like motion called reptation. This theoretical model remained controversial for twenty years, for until recently the motions of individual molecules could not be observed. Optical tweezers can now tug on a single molecule of DNA while its motion is observed through a microscope. The DNA moves along its own contour as predicted.

FIGURE 3.8 The motion of a single fluorescently labeled DNA molecule (60 microns in stretched length) in a concentrated solution of unlabeled DNA. In the first image (top left) the molecule has been formed into an R shape by pulling on a small attached sphere (red) with optical tweezers. Subsequent images, taken 8.3 seconds apart, show the molecule moving along its own contour as it unstretches. (Courtesy of Stanford University.)

Recent experiments with simulated gels made from etched silicon have further improved our understanding of the motion of macromolecules. For example, we now know that long molecular chains tend to get caught on post-like obstacles, looping around them like a rope hanging over a pulley. Varying the strength of the applied electric field helps to free the molecules from such obstacles. Theory and experiments in this area are discovering the optimum variation of the applied field for efficient separation of different lengths of DNA.

DNA is not the only important macromolecule; synthetic polymers like polyethylene and nylon are also noteworthy. Predicting the flow properties of molten polymers, whose motion is also dominated by the entanglement of the long molecules with each other, has stimulated new extensions of the reptation model. A better understanding of industrial processes for shaping polymers, such as extrusion and injection molding, should result.

In this way, reptation—a simple idea in condensed-matter and materials physics—has had a major impact on both molecular biology and polymer engineering. Many challenges remain for the future, such as efficiently simulating the motion of macromolecules by computer and following the motion of biological macromolecules on surfaces at high resolution using new scanning probe microscopies. But the reptation idea will continue to provide a starting point for understanding these more difficult problems.

Imagine a string that is a ten-millionth of an inch across and a ten-thousandth of an inch long. Suppose you wanted to test its strength, measure its length, or pick it up and move it. How would you hold it? When the string is really a molecule of DNA, you need a pair of molecule-size tweezers.

The “optical tweezer” was invented around 1980. It turns out that a tiny bead attached to a strand of DNA can be attracted to the spot of a laser's light. Pick one end of the strand, shine the light, and you can hold the molecule in place. Fix the other end, move the light, and you can stretch the molecule.

In 1995, scientists were able to pull straight the normally crumpled DNA molecule and measure the amount of work it took. The force required was only about a millionth the weight of a drop of water. The researchers showed that DNA first stretches by being straightened; but once it is straight, the “string” itself can stretch. By looking at just one molecule, they were able to test a theory of how DNA acts as a mechanical object.

The entire genetic code for a human being (the human genome) has 4 billion “base pairs” of molecular data. The full set is stored in duplicate in almost every one of the body's roughly 1013 cells. Altogether, this amounts to about 20,000,000,000 miles of DNA per human body, enough to stretch around the earth a million times. At the normal rate of cell reproduction, each of us is making new DNA at a rate faster than 10,000 miles per hour.

Only a small part of this DNA is actually used in any single cell. The rest contains the code for making other types of cells in the rest of the body. This means that each cell must find just the right little bit of the DNA crowded into the small space of the cell nucleus. It becomes a big problem to hold all that DNA, pick out the right bit, and open it up to read its message. The physics of DNA stiffness, twisting, and sticking becomes a major factor in understanding how this genetic material works.

FIGURE 3.9 Using “optical tweezers” to stretch a strand of DNA. One end of the strand is attached to a stationary glass surface. A tiny bead attached to the other end is then trapped using laser light. Adjusting the laser trap moves the bead and stretches the DNA molecule. (Courtesy of Princeton University.)