Physics for the 21st Century logo


antiferromagnetic order
An antiferromagnet is a magnet in which the microscopic magnetic moments inside the material line up in a grid on which neighboring moments point in opposite directions. The interaction energy between two magnetic moments in an antiferromagnet is lower when the two moments point in opposite directions. This can lead to a frustrated system with multiple ground states.
BCS theory
BCS theory is the theory of superconductivity put forward in 1957 by John Bardeen, Leon Cooper, and John Schreiffer, who received the 1972 Nobel Prize for their effort. The basic premise of BCS theory is that under the right conditions inside a conductor, electrons can form weakly bound pairs called "Cooper pairs" that form a condensate. Pairs in the condensate experience no resistance as they travel through the conductor.
In condensed matter physics, doping refers to the deliberate introduction of impurities into an extremely pure crystal. For example, a crystal of pure silicon might be doped with boron atoms that change the material's electrical properties, making it a more effective semiconductor.
emergent behavior
Emergent behavior is behavior of a complex system that is not easily predicted from a microscopic description of the system's constituent parts and the rules that govern them.
Fermi surface
According to the Pauli exclusion principle, it is not possible for identical fermions to occupy the same quantum state. In a system with many identical fermions, such as electrons in a metal, the fermions fill in the available quantum states in order of increasing energy. The energy of the highest occupied quantum state defines the energy of the Fermi surface, which is a surface of constant energy in momentum space.
A ferromagnet is a magnet in which the microscopic magnetic moments inside the material all point in the same direction. Most magnetic materials we encounter in daily life are ferromagnets.
inelastic neutron scattering
Inelastic neutron scattering is an experimental technique for studying various properties of materials. A beam of neutrons of a particular energy is shot at a sample at a particular angle with respect to the crystal lattice. The energy of neutrons scattered by the sample is recorded, and the experiment is repeated at different angles and beam energies. The scattered neutrons lose some of their energy to the sample, so the scattering is inelastic. The results of inelastic neutron scattering are readily interpreted in terms of the wave nature of particles. The incident neutron beam is a wave with a frequency proportional to the neutron energy. The crystal preferentially absorbs waves with frequencies that correspond to its natural modes of vibration. Note that the vibrations can be magnetic or acoustic. Thus, the modes of the sample can be inferred by mapping out how much energy is absorbed from the incident beam as a function of the incident beam energy. Inelastic neutron scattering has also been used to study acoustic oscillations and their corresponding quasiparticles in liquids.
In condensed matter physics, the term itinerant is used to describe particles (or quasiparticles) that travel essentially freely through a material and are not bound to particular sites on the crystal lattice.
Magnons are the quasiparticles associated with spin waves in a crystal lattice.
In physics, the term phase has two distinct meanings. The first is a property of waves. If we think of a wave as having peaks and valleys with a zero-crossing between them, the phase of the wave is defined as the distance between the first zero-crossing and the point in space defined as the origin. Two waves with the same frequency are "in phase" if they have the same phase and therefore line up everywhere. Waves with the same frequency but different phases are "out of phase." The term phase also refers to states of matter. For example, water can exist in liquid, solid, and gas phases. In each phase, the water molecules interact differently, and the aggregate of many molecules has distinct physical properties. Condensed matter systems can have interesting and exotic phases, such as superfluid, superconducting, and quantum critical phases. Quantum fields such as the Higgs field can also exist in different phases.
Phonons are the quasiparticles associated with acoustic waves, or vibrations, in a crystal lattice or other material.
A plasma is a gas of ionized (i.e., electrically charged) particles. It has distinctly different properties than a gas of neutral particles because it is electrically conductive, and responds strongly to electromagnetic fields. Plasmas are typically either very hot or very diffuse because in a cool, relatively dense gas the positively and negatively charged particles will bind into electrically neutral units. The early universe is thought to have passed through a stage in which it was a plasma of quarks and gluons, and then a stage in which it was a plasma of free protons and electrons. The electron gas inside a conductor is another example of a plasma. The intergalactic medium is an example of a cold, diffuse plasma. It is possible to create an ultracold plasma using the techniques of atom cooling and trapping.
Plasmons are the quasiparticle associated with oscillations of charge density in a plasma.
A pulsar is a spinning neutron star with a strong magnetic field that emits electromagnetic radiation along its magnetic axis. Because the star's rotation axis is not aligned with its magnetic axis, we observe pulses of radiation as the star's magnetic axis passes through our line of sight. The time between pulses ranges from a few milliseconds to a few seconds, and tends to slow down over time.
Just as particles can be described as waves through the wave-particle duality, waves can be described as particles. Quasiparticles are the quantized particles associated with various types of waves in condensed matter systems. They are similar to particles in that they have a well-defined set of quantum numbers and can be described using the same mathematical formalism as individual particles. They differ in that they are the result of the collective behavior of a physical system.
A superconducting quantum interference device, or SQUID, is a tool used in laboratories to measure extremely small magnetic fields. It consists of two half-circles of a superconducting material separated by a small gap. The quantum mechanical properties of the superconductor make this arrangement exquisitely sensitive to tiny changes in the local magnetic field. A typical SQUID is sensitive to magnetic fields hundreds of trillions of times weaker than that of a simple refrigerator magnet.