Physics Today

Thermoelectrics is an old field. In 1823, Thomas Seebeck discovered that a voltage drop appears across a sample that has a temperature gradient. This phenomenon provided the basis for thermocouples used for measuring temperature and for thermoelectric power generators. In 1838, Heinrich Lenz placed a drop of water on the junction of metal wires made of bismuth and antimony. Passing an electric current through the junction in one direction caused the water to freeze, and reversing the current caused the ice to quickly melt; thus thermoelectric refrigeration was demonstrated (figure 1).

Kỹ thuật cấu trúc băng tần đã dẫn đến sự phát triển của một loại laser hoàn toàn mới với các ứng dụng từ phân tích khí vết cực kỳ nhạy đến truyền thông.

Carbon 60 is a fascinating and arrestingly beautiful molecule. With 12 pentagonal and 20 hexagonal faces symmetrically arrayed in a soccer-ball-like structure that belongs to the icosahedral point group Ih, its high symmetry alone invites special attention. The publication in September 1990 of a simple technique for manufacturing and concentrating macroscopic amounts of this new form of carbon (see Donald R. Huffman's article in PHYSICS TODAY, November 1991, page 22) announced to the scientific community that enabling technology had arrived. Macroscopic amounts of C60 (and the higher fullerenes, such as C70 and C84) can now be made with anapparatus as simple as an arc furnace powered with an arc welding supply. Accordingly, chemists, physicists and materials scientists have joined forces in an explosion of effort to explore the properties of this unusual molecular building block.

Photosynthesis, the process by which plants convert solar energy into chemical energy, results in about 10 billion tons of carbon entering the biosphere annually as carbohydrate—equivalent to about eight times mankind's energy consumption in 1990. The apparatus used by plants to perform this conversion is both complex and highly efficient. Two initial steps of photosynthesis—energy transfer and electron transfer—are essential to its efficiency: Molecules of the light-harvesting system transfer electronic excitation energy to special chlorophyll molecules, whose role is to initiate the directional transfer of electrons across a biological membrane; the electron transfer, which takes place in a pigment-protein complex called the reaction center, then creates a potential difference that drives the subsequent biochemical reactions that store the energy. (Higher plants use two different reaction centers, called photosystems I and II, while purple bacteria make do with a single reaction center. The difference is that the bacteria do not generate oxygen in the photosynthetic process.) Both the elementary energy transfer and the primary electron transfer are ultrafast (occurring between 10−13 and 10−12 seconds), leading to the trapping of excitation energy at the reaction center (on a 100-picosecond timescale) and subsequent electron transfer in about 3 picoseconds with almost 100% quantum yield.

Few superconducting materials have presented us with the structural elegance and complexity displayed by the recently discovered high-Tc copper oxides. The structures of these materials, consisting of metal-oxygen layers stacked in a variety of sequences, with the metal atoms often in unusual coordinations, are interesting in their own right. More importantly, our present understanding of the properties of the oxide superconductors depends heavily on a knowledge of their structures.

While physical sciences deal with the interactions of matter and energy, economics can be said to deal with the production and exchange of goods and services. Because goods and services incorporate matter and energy, the physical sciences are clearly relevant to economics. In particular, one can expect the laws of thermodynamics to impose constraints on economic processes as they do on physical processes (figure 1). It is clear that the laws of conservation—of matter and energy, for example—have implications for the use of resources and for the generation and treatment of wastes. The law of the increase of entropy—the second law of thermodynamics—constrains economic processes to those that reduce available work, increasing the entropy of the Universe.

Conduction electrons in real crystalline solids behave very much like electrons in free space, moving in straight lines between collisions when subject to an electric field. But in an ideal (although cold) world, free from scattering by impurities, imperfections and thermal vibrations of the lattice, how would conduction electrons behave? That question, answered in principle long ago in light of the then newly developed quantum mechanics, was purely academic until recently.