A Majorana fermion (/ /), also referred to as a Majorana particle, is a fermion that is its own antiparticle. They were hypothesized 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.
All of the Standard Model fermions except the neutrino behave as Dirac fermions at low energy (after electroweak symmetry breaking), but the nature of the neutrino is not settled and it may be either Dirac or Majorana. In condensed matter physics, Majorana fermions exist as quasiparticle excitations in superconductors and can be used to form Majorana bound states governed by non-abelian statistics.
|Standard Model of particle physics|
The concept goes back to Majorana's suggestion in 1937 that neutral spin-1/2 particles can be described by a real wave equation (the Majorana equation), and would therefore be identical to their antiparticle (because the wave functions of particle and antiparticle are related by complex conjugation).
The difference between Majorana fermions and Dirac fermions can be expressed mathematically in terms of the creation and annihilation operators of second quantization. The creation operator creates a fermion in quantum state (described by a real wave function), whereas the annihilation operator annihilates it (or, equivalently, creates the corresponding antiparticle). For a Dirac fermion the operators and are distinct, whereas for a Majorana fermion they are identical.
Because particles and antiparticles have opposite conserved charges, only an uncharged particle can have a Majorana mass.[clarification needed] All of the elementary fermions of the Standard Model have gauge charges, so they cannot have fundamental Majorana masses. However, the right-handed sterile neutrinos introduced to explain neutrino oscillation could have Majorana masses. If they do, then at low energy (after electroweak symmetry breaking), by the seesaw mechanism, the neutrino fields would naturally behave as six Majorana fields, with three expected to have very high masses (comparable to the GUT scale) and the other three expected to have very low masses (comparable to 1 eV). If right-handed neutrinos exist but do not have a Majorana mass, the neutrinos would instead behave as three Dirac fermions and their antiparticles with masses coming directly from the Higgs interaction, like the other Standard Model fermions.
The seesaw mechanism is appealing because it would naturally explain why the observed neutrino masses are so small. However, if the neutrinos are Majorana then they violate the conservation of lepton number and even B − L.
Neutrinoless double beta decay, which can be viewed as two beta decay events with the produced antineutrinos immediately annihilating with one another, is only possible if neutrinos are their own antiparticles. Experiments are underway to search for this type of decay.
The high-energy analog of the neutrinoless double beta decay process is the production of same sign charged lepton pairs at hadron colliders; it is being searched for by both the ATLAS and CMS experiments at the Large Hadron Collider. In theories based on left–right symmetry, there is a deep connection between these processes. In the most accepted explanation of the smallness of neutrino mass, the seesaw mechanism, the neutrino is naturally a Majorana fermion.
Majorana fermions cannot possess intrinsic electric or magnetic moments, only toroidal moments. Such minimal interaction with electromagnetic fields makes them potential candidates for cold dark matter. The hypothetical neutralino of supersymmetric models is a Majorana fermion.
Majorana bound states
In superconducting materials, Majorana fermions can emerge as (non-fundamental) quasiparticles (which are more commonly referred as Bogoliubov quasiparticles in condensed matter.). This becomes possible because a quasiparticle in a superconductor is its own antiparticle. Majorana fermions (ie the Bogoliubov quasiparticles) in superconductors were observed by many experiments many years ago.
Mathematically, the superconductor imposes electron hole "symmetry" on the quasiparticle excitations, relating the creation operator at energy to the annihilation operator at energy . Majorana fermions can be bound to a defect at zero energy, and then the combined objects are called Majorana bound states or Majorana zero modes. This name is more appropriate than Majorana fermion (although the distinction is not always made in the literature), because the statistics of these objects is no longer fermionic. Instead, the Majorana bound states are an example of non-abelian anyons: interchanging them changes the state of the system in a way that depends only on the order in which the exchange was performed. The non-abelian statistics that Majorana bound states possess allows them to be used as a building block for a topological quantum computer.
A quantum vortex in certain superconductors or superfluids can trap midgap states, so this is one source of Majorana bound states. Shockley states at the end points of superconducting wires or line defects are an alternative, purely electrical, source. An altogether different source uses the fractional quantum Hall effect as a substitute for the superconductor.
Experiments in superconductivity
In 2008, Fu and Kane provided a groundbreaking development by theoretically predicting that Majorana bound states can appear at the interface between topological insulators and superconductors. Many proposals of a similar spirit soon followed, where it was shown that Majorana bound states can appear even without any topological insulator. An intense search to provide experimental evidence of Majorana bound states in superconductors first produced some positive results in 2012. A team from the Kavli Institute of Nanoscience at Delft University of Technology in the Netherlands reported an experiment involving indium antimonide nanowires connected to a circuit with a gold contact at one end and a slice of superconductor at the other. When exposed to a moderately strong magnetic field the apparatus showed a peak electrical conductance at zero voltage that is consistent with the formation of a pair of Majorana bound states, one at either end of the region of the nanowire in contact with the superconductor.
This experiment from Delft marks a possible verification of independent 2010 theoretical proposals from two groups predicting the solid state manifestation of Majorana bound states in semiconducting wires.
In 2014, evidence of Majorana bound states was observed for the first time using a low-temperature scanning tunneling microscope, by scientists at Princeton University. It was suggested that Majorana bound states appeared at the edges of a chain of iron atoms formed on the surface of superconducting lead. Physicist Jason Alicea of California Institute of Technology, not involved in the research, said the study offered "compelling evidence" for Majorana fermions but that "we should keep in mind possible alternative explanations—even if there are no immediately obvious candidates".
- "Quantum Computation possible with Majorana Fermions" on YouTube, uploaded 19 April 2013, retrieved 5 October 2014; and also based on the physicist's name's pronunciation.
- Majorana, Ettore (1937). "Teoria simmetrica dell'elettrone e del positrone" (link is to English translation). Nuovo Cimento (in Italian) 14 (4): 171. doi:10.1007/bf02961314.
- Schechter, J.; Valle, J.W.F. (1982). "Neutrinoless Double beta Decay in SU(2) x U(1) Theories". Physical Review D 25 (11): 2951. Bibcode:1982PhRvD..25.2951S. doi:10.1103/PhysRevD.25.2951. (subscription required (. ))
- Rodejohann, Werner (2011). "Neutrino-less Double Beta Decay and Particle Physics". International Journal of Modern Physics E20 (9): 1833. arXiv:1106.1334. Bibcode:2011IJMPE..20.1833R. doi:10.1142/S0218301311020186. (registration required (. ))
- Keung, Wai-Yee; Senjanović, Goran (1983). "Majorana Neutrinos and the Production of the Right-Handed Charged Gauge Boson". Physical Review Letters 50 (19): 1427. Bibcode:1983PhRvL..50.1427K. doi:10.1103/PhysRevLett.50.1427. (subscription required (. ))
- Tello, Vladimir; Nemevšek, Miha; Nesti, Fabrizio; Senjanović, Goran; Vissani, Francesco (2011). "Left-Right Symmetry: from LHC to Neutrinoless Double Beta Decay". Physical Review Letters 106 (15): 151801. arXiv:1011.3522. Bibcode:2011PhRvL.106o1801T. doi:10.1103/PhysRevLett.106.151801. (subscription required (. ))
- Kayser, Boris; Goldhaber, Alfred S. (1983). "CPT and CP properties of Majorana particles, and the consequences". Physical Review D 28 (9): 2341–2344. Bibcode:1983PhRvD..28.2341K. doi:10.1103/PhysRevD.28.2341. (subscription required (. ))
- Radescu, E. E. (1985). "On the electromagnetic properties of Majorana fermions". Physical Review D 32 (5): 1266–1268. Bibcode:1985PhRvD..32.1266R. doi:10.1103/PhysRevD.32.1266. (subscription required (. ))
- Boudjema, F.; Hamzaoui, C.; Rahal, V.; Ren, H. C. (1989). "Electromagnetic Properties of Generalized Majorana Particles". Physical Review Letters 62 (8): 852–854. Bibcode:1989PhRvL..62..852B. doi:10.1103/PhysRevLett.62.852. (subscription required (. ))
- Pospelov, Maxim; ter Veldhuis, Tonnis (2000). "Direct and indirect limits on the electro-magnetic form factors of WIMPs". Physics Letters B 480: 181–186. arXiv:hep-ph/0003010. Bibcode:2000PhLB..480..181P. doi:10.1016/S0370-2693(00)00358-0.
- Ho, Chiu Man; Scherrer, Robert J. (2013). "Anapole Dark Matter". Physics Letters B 722 (8): 341–346. arXiv:1211.0503. Bibcode:2013PhLB..722..341H. doi:10.1016/j.physletb.2013.04.039.
- Wilczek, Frank (2009). "Majorana returns". Nature Physics 5 (9): 614–618. Bibcode:2009NatPh...5..614W. doi:10.1038/nphys1380.
- Nayak, Chetan; Simon, Steven H.; Stern, Ady; Freedman, Michael; Das Sarma, Sankar (2008). "Non-Abelian anyons and topological quantum computation". Reviews of Modern Physics 80 (3): 1083. arXiv:0707.1889. Bibcode:2008RvMP...80.1083N. doi:10.1103/RevModPhys.80.1083.
- N.B. Kopnin; M.M. Salomaa (1991). "Mutual friction in superfluid 3He: Effects of bound states in the vortex core". Physical Review B 44 (17): 9667. Bibcode:1991PhRvB..44.9667K. doi:10.1103/PhysRevB.44.9667.
- Volovik, G. E. (1999). "Fermion zero modes on vortices in chiral superconductors". JETP Letters 70 (9): 609–614. arXiv:cond-mat/9909426. Bibcode:1999JETPL..70..609V. doi:10.1134/1.568223.
- Read, N.; Green, Dmitry (2000). "Paired states of fermions in two dimensions with breaking of parity and time-reversal symmetries and the fractional quantum Hall effect". Physical Review B 61 (15): 10267. arXiv:cond-mat/9906453. Bibcode:2000PhRvB..6110267R. doi:10.1103/PhysRevB.61.10267.
- Kitaev, A. Yu (2001). "Unpaired Majorana fermions in quantum wires". Physics-Uspekhi (supplement) 44 (131): 131. arXiv:cond-mat/0010440. Bibcode:2001PhyU...44..131K. doi:10.1070/1063-7869/44/10S/S29.
- Moore, Gregory; Read, Nicholas (August 1991). "Nonabelions in the fractional quantum Hall effect". Nuclear Physics B 360 (2–3): 362. Bibcode:1991NuPhB.360..362M. doi:10.1016/0550-3213(91)90407-O.
- Fu, Liang; Kane, Charles L. (2008). "Superconducting Proximity Effect and Majorana Fermions at the Surface of a Topological Insulator". Physical Review Letters 10 (9): 096407. arXiv:0707.1692. Bibcode:2008PhRvL.100i6407F. doi:10.1103/PhysRevLett.100.096407.
- Fu, Liang; Kane, Charles L. (2009). "Josephson current and noise at a superconductor/quantum-spin-Hall-insulator/superconductor junction". Physical Review B 79 (16): 161408. arXiv:0804.4469. Bibcode:2009PhRvB..79p1408F. doi:10.1103/PhysRevB.79.161408. (subscription required (. ))
- Alicea, Jason (2012). "New directions in the pursuit of Majorana fermions in solid state systems". Reports on Progress in Physics 75 (7): 076501. arXiv:1202.1293. Bibcode:2012RPPh...75g6501A. doi:10.1088/0034-4885/75/7/076501. PMID 22790778. (subscription required (. ))
- Beenakker, C. W. J. (April 2013). "Search for Majorana fermions in superconductors". Annual Review of Condensed Matter Physics 4 (113): 113–136. arXiv:1112.1950. Bibcode:2013ARCMP...4..113B. doi:10.1146/annurev-conmatphys-030212-184337. (subscription required (. ))
- Reich, Eugenie Samuel (28 February 2012). "Quest for quirky quantum particles may have struck gold". Nature News. doi:10.1038/nature.2012.10124.
- Amos, Jonathan (13 April 2012). "Majorana particle glimpsed in lab". BBC News. Retrieved 15 April 2012.
- Mourik, V.; Zuo, K.; Frolov, S. M.; Plissard, S. R.; Bakkers, E. P. A. M.; Kouwenhoven, L. P. (12 April 2012). "Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices". Science 336 (6084): 1003–1007. arXiv:1204.2792. Bibcode:2012Sci...336.1003M. doi:10.1126/science.1222360.
- Lutchyn, Roman M.; Sau, Jay D.; Das Sarma, S. (August 2010). "Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures". Physical Review Letters 105 (7): 077001. arXiv:1002.4033. Bibcode:2010PhRvL.105g7001L. doi:10.1103/PhysRevLett.105.077001.
- Oreg, Yuval; Refael, Gil; von Oppen, Felix (October 2010). "Helical Liquids and Majorana Bound States in Quantum Wires". Physical Review Letters 105 (17): 177002. arXiv:1003.1145. Bibcode:2010PhRvL.105q7002O. doi:10.1103/PhysRevLett.105.177002.
- Nadj-Perge, Stevan; Drozdov, Ilya K.; Li, Jian; Chen, Hua; Jeon, Sangjun; Seo, Jungpil; MacDonald, Allan H.; Bernevig, B. Andrei; Yazdani, Ali (2 October 2014). "Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor". Science. arXiv:1410.3453. doi:10.1126/science.1259327. (subscription required (. ))
- "Majorana fermion: Physicists observe elusive particle that is its own antiparticle". Phys.org. October 2, 2014. Retrieved 3 October 2014.
- "New Particle Is Both Matter and Antimatter". Scientific American. October 2, 2014. Retrieved 3 October 2014.
- Pal, Palash B. (2011) [12 October 2010]. "Dirac, Majorana and Weyl fermions". American Journal of Physics 79 (5): 485. arXiv:1006.1718. Bibcode:2011AmJPh..79..485P. doi:10.1119/1.3549729. (subscription required (. ))