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In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin 1/2, spin 3/2, etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks and leptons and all composite particles made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose–Einstein statistics.
In physics, the Kondo effect describes the scattering of conduction electrons in a metal due to magnetic impurities, resulting in a characteristic change i.e. a minimum in electrical resistivity with temperature. The cause of the effect was first explained by Jun Kondo, who applied third-order perturbation theory to the problem to account for scattering of s-orbital conduction electrons off d-orbital electrons localized at impurities. Kondo's calculation predicted that the scattering rate and the resulting part of the resistivity should increase logarithmically as the temperature approaches 0 K. Experiments in the 1960s by Myriam Sarachik at Bell Laboratories provided the first data that confirmed the Kondo effect. Extended to a lattice of magnetic impurities, the Kondo effect likely explains the formation of heavy fermions and Kondo insulators in intermetallic compounds, especially those involving rare earth elements such as cerium, praseodymium, and ytterbium, and actinide elements such as uranium. The Kondo effect has also been observed in quantum dot systems.
In condensed matter physics, the Fermi surface is the surface in reciprocal space which separates occupied from unoccupied electron states at zero temperature. The shape of the Fermi surface is derived from the periodicity and symmetry of the crystalline lattice and from the occupation of electronic energy bands. The existence of a Fermi surface is a direct consequence of the Pauli exclusion principle, which allows a maximum of one electron per quantum state. The study of the Fermi surfaces of materials is called fermiology.
An intermetallic is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds.
Strongly correlated materials are a wide class of compounds that include insulators and electronic materials, and show unusual electronic and magnetic properties, such as metal-insulator transitions, heavy fermion behavior, half-metallicity, and spin-charge separation. The essential feature that defines these materials is that the behavior of their electrons or spinons cannot be described effectively in terms of non-interacting entities. Theoretical models of the electronic (fermionic) structure of strongly correlated materials must include electronic (fermionic) correlation to be accurate. As of recently, the label quantum materials is also used to refer to strongly correlated materials, among others.
A Majorana fermion, also referred to as a Majorana particle, is a fermion that is its own antiparticle. They were hypothesised by Ettore Majorana in 1937. The term is sometimes used in opposition to a Dirac fermion, which describes fermions that are not their own antiparticles.
The De Haas–Van Alphen effect, often abbreviated to DHVA, is a quantum mechanical effect in which the magnetic susceptibility of a pure metal crystal oscillates as the intensity of the magnetic field B is increased. It can be used to determine the Fermi surface of a material. Other quantities also oscillate, such as the electrical resistivity, specific heat, and sound attenuation and speed. It is named after Wander Johannes de Haas and his student Pieter M. van Alphen. The DHVA effect comes from the orbital motion of itinerant electrons in the material. An equivalent phenomenon at low magnetic fields is known as Landau diamagnetism.
In solid-state physics, heavy fermion materials are a specific type of intermetallic compound, containing elements with 4f or 5f electrons in unfilled electron bands. Electrons are one type of fermion, and when they are found in such materials, they are sometimes referred to as heavy electrons. Heavy fermion materials have a low-temperature specific heat whose linear term is up to 1000 times larger than the value expected from the free electron model. The properties of the heavy fermion compounds often derive from the partly filled f-orbitals of rare-earth or actinide ions, which behave like localized magnetic moments. The name "heavy fermion" comes from the fact that the fermion behaves as if it has an effective mass greater than its rest mass. In the case of electrons, below a characteristic temperature (typically 10 K), the conduction electrons in these metallic compounds behave as if they had an effective mass up to 1000 times the free particle mass. This large effective mass is also reflected in a large contribution to the resistivity from electron-electron scattering via the Kadowaki–Woods ratio. Heavy fermion behavior has been found in a broad variety of states including metallic, superconducting, insulating and magnetic states. Characteristic examples are CeCu6, CeAl3, CeCu2Si2, YbAl3, UBe13 and UPt3.
The quantum spin Hall state is a state of matter proposed to exist in special, two-dimensional semiconductors that have a quantized spin-Hall conductance and a vanishing charge-Hall conductance. The quantum spin Hall state of matter is the cousin of the integer quantum Hall state, and that does not require the application of a large magnetic field. The quantum spin Hall state does not break charge conservation symmetry and spin- conservation symmetry.
A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, meaning that electrons can only move along the surface of the material.
In condensed matter physics, a quantum spin liquid is a phase of matter that can be formed by interacting quantum spins in certain magnetic materials. Quantum spin liquids (QSL) are generally characterized by their long-range quantum entanglement, fractionalized excitations, and absence of ordinary magnetic order.
Spin engineering describes the control and manipulation of quantum spin systems to develop devices and materials. This includes the use of the spin degrees of freedom as a probe for spin based phenomena. Because of the basic importance of quantum spin for physical and chemical processes, spin engineering is relevant for a wide range of scientific and technological applications. Current examples range from Bose–Einstein condensation to spin-based data storage and reading in state-of-the-art hard disk drives, as well as from powerful analytical tools like nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy to the development of magnetic molecules as qubits and magnetic nanoparticles. In addition, spin engineering exploits the functionality of spin to design materials with novel properties as well as to provide a better understanding and advanced applications of conventional material systems. Many chemical reactions are devised to create bulk materials or single molecules with well defined spin properties, such as a single-molecule magnet. The aim of this article is to provide an outline of fields of research and development where the focus is on the properties and applications of quantum spin.
In condensed matter physics, quantum oscillations describes a series of related experimental techniques used to map the Fermi surface of a metal in the presence of a strong magnetic field. These techniques are based on the principle of Landau quantization of Fermions moving in a magnetic field. For a gas of free fermions in a strong magnetic field, the energy levels are quantized into bands, called the Landau levels, whose separation is proportional to the strength of the magnetic field. In a quantum oscillation experiment, the external magnetic field is varied, which causes the Landau levels to pass over the Fermi surface, which in turn results in oscillations of the electronic density of states at the Fermi level; this produces oscillations in the many material properties which depend on this, including resistance, Hall resistance, and magnetic susceptibility. Observation of quantum oscillations in a material is considered a signature of Fermi liquid behaviour.
Dirac cones, named after Paul Dirac, are features that occur in some electronic band structures that describe unusual electron transport properties of materials like graphene and topological insulators. In these materials, at energies near the Fermi level, the valence band and conduction band take the shape of the upper and lower halves of a conical surface, meeting at what are called Dirac points.
YbBiPt is an intermetallic material which at low temperatures exhibits an extremely high value of specific heat, which is a characteristic of heavy-fermion behavior. YbBiPt has a noncentrosymmetric cubic crystal structure; in particular it belongs to the ternary half-Heusler compounds.
The term Dirac matter refers to a class of condensed matter systems which can be effectively described by the Dirac equation. Even though the Dirac equation itself was formulated for fermions, the quasi-particles present within Dirac matter can be of any statistics. As a consequence, Dirac matter can be distinguished in fermionic, bosonic or anyonic Dirac matter. Prominent examples of Dirac matter are Graphene, topological insulators, Dirac semimetals, Weyl semimetals, various high-temperature superconductors with -wave pairing and liquid Helium-3. The effective theory of such systems is classified by a specific choice of the Dirac mass, the Dirac velocity, the Dirac matrices and the space-time curvature. The universal treatment of the class of Dirac matter in terms of an effective theory leads to a common features with respect to the density of states, the heat capacity and impurity scattering.
In solid-state physics, the kagome metal or kagome magnet is a type of ferromagnetic quantum material. The atomic lattice in a kagome magnet has layered overlapping triangles and large hexagonal voids, akin to the kagome pattern in traditional Japanese basket-weaving. This geometry induces a flat electronic band structure with Dirac crossings, in which the low-energy electron dynamics correlate strongly.
Iron germanide (FeGe) is an intermetallic compound, a germanide of iron. At ambient conditions it crystallizes in three polymorphs with monoclinic, hexagonal and cubic structures. The cubic polymorph has no inversion center, it is therefore helical, with right-hand and left-handed chiralities.
Manganese monosilicide (MnSi) is an intermetallic compound, a silicide of manganese. It occurs in cosmic dust as the mineral brownleeite. MnSi has a cubic crystal lattice with no inversion center; therefore its crystal structure is helical, with right-hand and left-hand chiralities.
References
- 1 2 Ye, Linda; Kang Mingu; Liu Junwei; von Cube, Felix; Wicker, Christina R.; Suzuki Takehito; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Bell, David C.; Fu Liang (19 March 2018). "Massive Dirac fermions in a ferromagnetic kagome metal". Nature. 555 (7698): 638–642. arXiv: 1709.10007 . Bibcode:2018Natur.555..638Y. doi:10.1038/nature25987. ISSN 1476-4687. PMID 29555992. S2CID 4470420.
- ↑ Aristos Georgiou (March 20, 2018). "Kagome metal: new exotic quantum material developed by scientists". Newsweek.
- ↑ Chu, Jennifer (March 19, 2018). "Physicists discover new quantum electronic material". MIT News. Massachusetts Institute of Technology.
- ↑ "The Electronic Structure of a 'Kagome' Material". ALS . Lawrence Berkeley National Lab. 2018-06-15. Retrieved 2020-04-17.
- ↑ Ye, Linda; Chan Mun K.; McDonald, Ross D.; Graf, David; Kang Mingu; Liu Junwei; Suzuki Takehito; Comin, Riccardo; Fu Liang; Checkelsky, Joseph G. (2019-10-25). "De Haas-van Alphen effect of correlated Dirac states in kagome metal Fe3Sn2". Nature Communications. 10 (1): 4870. arXiv: 1809.11159 . Bibcode:2019NatCo..10.4870Y. doi: 10.1038/s41467-019-12822-1 . ISSN 2041-1723. PMC 6814717 . PMID 31653866.
- ↑ Wang Binbin; Wu Po-kuan; Bagués Salguero, Núria; Zheng Qiang; Yan Jiaqiang; Randeria, Mohit; McComb, David W. (2021-08-24). "Stimulated Nucleation of Skyrmions in a Centrosymmetric Magnet". ACS Nano. 15 (8): 13495–13503. doi:10.1021/acsnano.1c04053. ISSN 1936-0851. OSTI 1819517 . PMID 34374281. S2CID 236967261.