Condensed matter physics

Definition:The condensed state of matter includes liquids and solids. Laboratory-grown crystals dominate the field of condensed matter physics, and powerful tools for examining their atomic structures are available. Out of the periodic table’s more than one hundred elements, there are about 10⁸ potential compounds. Some combination crystals have properties such as superconductivity at high temperature, special magnetism, or thermoelectric capability. The binding and structure of these new materials falls generally into lattices, superlattices, nanowires, and quantum dots. One-dimensional matter has been put to practical use, and two-dimensional systems are being fabricated worldwide.

Basic Principles

From the time of Sir Isaac Newton until the second half of the nineteenth century, knowledge of condensed matter was complete on a macroscopic scale. Scientists understood elasticity, hydrodynamics, and thermodynamics as these concepts applied to elements found in nature. Then, in 1916, a Polish metallurgist and chemist, Jan Czochralski, made a discovery about crystals. He was melting some tin and taking notes on crystallization. He had at his elbow an inkwell and a crucible of melted tin for the experiment, and he mistakenly dipped his pen into the melted tin. Drawing out his pen, he saw that a thin thread of metal came with it. He found the thread was a single crystal. It marked the beginning of the process of growing crystals artificially. At the beginning of this process, elements from the periodic table were used, but scientists soon used combinations of those elements, leading to “designer” crystals of paired and grouped atomic elements. In the twenty-first century, scientists have begun to investigate crystalline solids not found in nature. Growing crystals from diverse elements has opened the door to the creation of new materials.

At the end of the twentieth and the beginning of the twenty-first centuries, quantum mechanics introduced new theories; discoveries and research have been primarily at the micro- and nanoscales. The nanoscale is enabled by advances in technology that allow physicists to observe and manipulate matter at meters as small as 10^-9 (a nanometer is one billionth of a meter). Since so much of modern condensed matter physics takes place at the nanoscale, specialized equipment is necessary for nanoscale research. Mathematics, especially statistics, is as essential in condensed matter physics as it is in any branch of physics. However, in addition to mathematics, modern probing equipment has allowed for an explosion of information about condensed matter.

Core Concepts

Solidity. “Solidity” once meant hardness under pressure. Now it is understood that certain metals are indeed solid but also soft. In addition, types of solid glass behave more like liquids in a frozen state, melting slowly over decades. These discoveries are important to condensed matter physics because crystals are atomically regular. Many crystalline structures in condensed matter physics are described in terms of lattices, from one to several atoms thick. Such crystals are studied by the “unit cell,” or single atomic structural unit, which can be measured and described in terms of vectors. Whether they occur in nature or are grown in the lab, crystalline structures are, by definition, atomically the same from point to point across an entire sample. Unlike in kinds of alkali or glass, in crystals, a given vector follows a lattice pattern through the same part of each atom across the sample, which is itself conceived of as two-dimensional. Nanowires and nanotubes are lab-grown, one or sometimes two atoms thick, and long enough to be measured in microns. Carbon is an especially interesting material for nanotubes, because its electrical conductivity could make nanostructures of carbon highly practical in electronic components.

Probing Technology—X-Ray.Modern physicists’ power to conduct research into condensed matter has depended to some degree on the x-ray microscope, which uses x-ray beams to make visible objects as small as 30 nanometers (room for about 150 atoms). An image is created based on the absorption of the x-rays; darker spots show more absorption, lighter ones less, creating something like an old-fashioned black-and-white photograph, but with 3-D information. X-rays cause electrons to dissociate from the absorbing atom, giving off another x-ray, and with high enough resolution, the patterns of the emitted x-rays allow scientists to also determine chemical features of the material under study.

Probing Technology—Light Scattering.On biological samples, the x-ray must be less penetrating than it can be on chemical samples because, at high resolutions, it can harm living samples. This problem is partly solved by the light-scattering microscope. Being a field of electromagnetic nature, light is a force, and it scatters when it impacts very small material samples. With light-scattering probes, scientists are able to get accurate measurements of atomic-level phenomena. Both size and mass of sample material can be ascertained noninvasively.

Scattering light is a twofold methodology: Static light scattering, or Rayleigh scattering, measures the intensity of scattered light, inferring through angles the characteristics of the material; dynamic light scattering, also known as photon correlation spectroscopy, tracks fluctuations in scattered light, bringing molecular and atomic motion to light.

Probing Technology—Neutron.For some time, scientists have aimed neutron beams at objects. However, early in the twenty-first century, they discovered that neutron beams going through an object could be manipulated to form an image. Neutrons scatter when they collide with an object. Using an array of textured plates, or detectors, scientists catch the neutrons and convert them into photons. Then, optical technology creates electronic information, which can be read on computers. Like the light-scattering device, the neutron microscope promises new advances in biotechnology, as it allows scientists to peer into increasingly small samples of living tissue.

Electricity.Electric conductivity, or insularity, is the property of either allowing or resisting electric flow and is of prime interest to condensed matter physicists. Solid state physics, a relatively old branch of condensed matter physics, studies transistors and applies condensed matter studies to semiconductors. An electron’s motion is associated with a corresponding “hole” in the solid. A semiconductor, which both conducts and resists electricity, can be described as a substance in which the holes and electrons are moving in a highly activated manner.

Some natural elements, such as silicon, diamond, and germanium, are semiconductors. Silicon is important because its semiconductivity allows it to perform the binary operations that are the basis of computing. In recent research, physicists were surprised to find that iron, not naturally a conductive metal, is superconductive at super high temperatures. Because iron is a magnetic material and magnetism is a field all its own, the fact that iron is superconductive has excited scientists and has become an emerging area of research. The long-range aim of most research is the search for overarching principles that can be applied universally, creating broad new platforms for further development, which is the case with iron conductivity.

Theory of Energy Bands. Electrons are important for condensed matter physics because condensed matter is explored at the atomic level. In a carbon nanowire, for example, electrons have one-dimensional movements. They can go only up or down the wire because it is so narrow. The motion of electrons, and the energy in play, has led to the discovery of energy bands, which are signature forces for different kinds of material. Energy bands could be likened to bar codes, the black stripes representing “allowed” energy and the white stripes “forbidden” energy. Energy bands can be measured either in a particle accelerator or by photoemission. The structure of bands is used to understand properties of physical material. Bands also have promising practical applications in nanotechnology and electromagnetism. Band engineering seeks to create band gaps in certain patterns; being able to do so would allow scientists to control the frequencies of electromagnetism radiating from the substance.

Phonons. The phonon, introduced by Igor Tamm, who won the 1958 Nobel Prize in Physics, is the phenomenon of excited ions vibrating together. Condensed matter physicists calculate the amplitude of phonons using quantum mechanics. Phonon frequencies are also measured by means of Newton’s laws of motion. The word “phonon” has the same Greek root as the English word “phone” because sound arises from phonons with long wavelengths. In acoustic phonons, waves are created by all the movement going in the same direction. When excited ions move in opposite directions, they are called optical phonons. They are called optical because infrared light rays impel the phonon.

Acoustic phonon vibrations transfer heat (unless the material is an insulating crystal), and, thus, are important to the thermal systems industry. Because they are used so much in computing and have issues with excess heat, semiconductors, in particular, stand to benefit from further phonon engineering. In an example of ongoing research, a company in North Carolina has tried to use a thin film’s phonons to transform excess heat from industrial activity into power-grid-type electricity. The same technology also handles unneeded heat in a process called thermoelectric cooling, which has refrigeration applicability. However, neither process is efficient enough for widespread use as of 2012.

Applications Past and Present

Nanofabrication. In 1925, Percy Bridgman discovered a way to grow large crystals by packing a tube with the desired material then moving the apparatus through a heated area very slowly, melting it directionally and allowing material to recrystallize. Bridgman’s method allowed scientists to grow a single crystal of significant size (as opposed to the thin wires discovered by Czochralski). In the early twenty-first century, research emphasis has been on nanocrystals. Growing crystals at the nanoscale was a practice first mastered at Cornell and has since spread to many other universities, including the University of Houston, Stanford, and Penn State. Students and professors explore new materials for valuable properties, including superconductivity, thermodynamic properties, magnetoresistance, and optical characteristics.

In the lab, it is common to grow superlattices, which are essentially crystalline lattices a few atoms thick stacked atop one another. Generally the electrons are sandwiched in one layer, so their movement is one-dimensional within that layer. At Louisiana State University, academics predict nanofabrication will become a trillion-dollar industry by around 2035. Such a vibrant industry could have applications in computers, vaccines, and new materials for everyday products. This growing industry will call for two million trained scientists to fabricate new materials. While the field is international, Germany has one of the larger footprints, with both academic and industrial ties to the United States.

Liquid Crystal Display. The liquid crystal display (LCD) became commercially relevant in the last decades of the twentieth century. Liquid crystals are unique in that their atoms are partly organized and go through two phases, between liquid and solid. Molecules in liquid crystals tend to be shaped in ways that allow them to line up. Molecular rods in a liquid crystal line up similarly to boats in a harbor tied at anchor, which will tend to all point the same way, depending on the current or tide. This phase of liquid crystals is called “smectic” and is close to the solid phase. Characteristically, the smectic phase traps free-floating crystal pieces in layers. In the nematic phase, liquid crystals arrange themselves in a twisted pattern. This phase is similar to the liquid phase, in which molecules float freely; however nematic-phase liquid crystal molecules still float in a pattern. In both phases, the order of the molecules can be mechanically manipulated, or controlled by magnetic forces or electricity.

At a certain point in the nematic phase, the molecules shine brightly in many colors, creating the basic element of the pixel, a tiny liquid crystal package that can emit light. Arrays of pixels made to form images first became commercially available in watches and clocks in the 1970s. In the 1980s, the pocket calculator with an LCD display became standard equipment for students. Computer and television screens are commonly made with LCD technology today.

Both liquid crystals and incommensurate crystals (also called quasicrystals) arrange themselves. In a crystal, atoms repeat their lattices precisely for as far as they go, an action known as “translational periodicity.” In gases, atoms float randomly, but in liquids they are arranged loosely; in quasicrystals, the usual crystalline lattice is present but holds its translational periodicity in all but one or perhaps a few directions (if it is a complex lattice). Therefore, a quasicrystal is perfectly precise except for a hidden wobble.

Graphene.Graphene is made by applying adhesive tape to a graphite crystal. When the tape is removed, it takes one or two atomic layers of graphite with it. After the tape is dissolved, the graphite’s hexagonal structure translates into a two-dimensional layer of single carbon atoms, producing promising properties. One such property is a high degree of responsiveness to electric current. Thus, graphene makes a good transistor, one that is more efficient and faster than one made of silicon. An electron beam can cut graphene into one-dimensional lengths. If an end is lopped off, a quantum dot (nanocrystal) remains. Graphene has excellent conductive capability for both electricity and heat. Electricity and heat are fundamental powers in the world; therefore, any substance able to conduct them attracts significant attention from scientists. Because a great deal of thermal force is needed to melt graphene, and because graphene is one of the strongest materials known, the material has practical technological applications. For example, thin foil sheets of graphene are used in iPhones.

Biomedicine.In the film The Fantastic Voyage (1966), a craft and crew are shrunk to the size of a cell and injected into a human being on a mission to zap a tumor with ray guns. The fantasy of being able to manipulate things inside the human body on a micro scale has become reality with biomedical applications of condensed matter physics, including nanoscience. Though still in their infancy, biomedical applications are highly promising. At Cornell, scientists began their biomedical experiments with condensed matter needing information from trauma sites in the soft tissue of the liver and abdomen. Biological substances already in place in the body were developed into a logic system that would read digitally: 0 for no injury, 1 for injury. The reactions of enzymes were filtered to provide an optical signal.

Inside human bodies, the magnetism of nanoparticles responds to magnetic forces from outside the body, such as those from magnetic resonance imaging (MRI) equipment. In addition, the nanoparticles can be directed inside the body by an outside magnetic field, much the way iron filings are drawn around by magnets in elementary experiments. Furthermore, certain manipulations of the external field will cause nanoparticles with magnetic properties to heat up inside the body. Thus, there is a potential for aiming such particles at tumors for thermal treatment.

Condensed matter studies apply to biological implants, too. New materials mean possible improvements in such things as artificial joints, which, like any manufactured item, show wear and tear as time passes. Diamond surfaces and other “superhard” coatings are the subject of current research. “Superthin” coatings are possible with condensed matter. Implants include more than artificial joints. Some implants are being built as diagnostic or transmission devices, while other implants include structures with porosity that could aid tissue regeneration.

National Security. The US Intelligence Community is a network of sixteen agencies with a collective aim to protect the United States while employing a high level of secrecy. The Intelligence Advanced Research Projects Activity (IARPA) places a strong emphasis on carbon nanotube technology (CNT). Because of their size, nanodevices are especially promising to the intelligence community, where avoiding detection is extremely important. CNT is the subject of IARPA research in electronics. CNT could become so integral to computer and other electronic devices that memory, logic, and transistors may be interchangeable to some extent with standard silicon materials.

Social Context and Future Prospects

While graphene is already in transistors, it has fascinating future possibilities. It can detect a solitary molecule of gas; thus, gas-sensing devices can be made from graphene. It is very strong, yet netlike in its structure, so it could be used as a support material for other substances under study in electron microscopy, much the way glass slides have been used in conventional microscopes for centuries. In the future, perhaps graphene can be the “slide” for nanoparticles, quantum dots, and microsized samples. Furthermore, in many kinds of nanotubes, those made of carbon are nothing more than rolled graphene, so the strength of graphene translates into tubular wire. This structural strength is complemented by extreme resistance to some strong chemicals, such as hydrofluoric acid. Eventually, graphene coatings a few atoms thick may be able to protect devices in harsh environments.

Many universities are invested in the future of condensed matter physics and its applications. Researchers at Cornell have studied the growth of nanowires on substrate material, a theoretical and practical area that will affect the evolution of computer chips. Cornell students and faculty have also been involved in the computational angle of this growing field. Computers are essential to studying condensed matter physics, and as better algorithms are developed and software becomes more sophisticated, researchers worldwide will benefit.

Academic institutions advance biotechnology also, through departmental research and education. At Wayne State University, for example, scientists are trying to load drugs onto nanoparticles, especially nanoparticles with magnetic properties. If they succeed, a new kind of drug therapy will be possible, because the magnetism of certain nanoparticles will provide physicians with an ability to direct medications to specific sites within the body. Researchers worldwide have held conferences on problems in soft condensed matter because the field is so promising. Scientists, academics, and industrialists envision a new paradigm for drug delivery with strong medical, chemical, and commercial potential.

Bibliography

Chaikin, P. M., and T. C. Lubensky. Principles of Condensed Matter Physics. New York: Cambridge UP, 2013. Print. Requires a basic understanding of statistical and quantum mechanics. Outlines the tenets of condensed matter physics, covering conservations laws and symmetries.

“Condensed Matter News.” Phys.org, 26 Sept. 2023, phys.org/physics-news/materials/. Accessed 27 Sept. 2023.

Marder, Michael P. Condensed Matter Physics. 2nd ed, Wiley, 2010. Modernization of a classic text by Neil W. Ashcroft and N. David Merman. Print. Solid-state physics, bands, magnetism, and quantum theory are among the older parts of the field still covered by Marder. Expands areas of soft condensed matter, surfaces, optics, and more.

McGurn, Arthur. An Introduction to Condensed Matter Physics for the Nanosciences. CRC Press, 2023. eBook Collection (EBSCOhost), search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=3567422&site=ehost-live&scope=site. Accessed 27 Sept. 2023.

About the Author

Amanda R. Jones has an MA from Virginia Tech and a PhD in English from the University of Virginia. She has written several articles for EBSCO Publishing and has published in the Children’s Literature Association Quarterly.