Editing Fractional quantum Hall effect

{{short description|Electromagnetic effect in physics}}
The '''fractional quantum Hall effect''' (fractional QHE or '''FQHE''') is the observation of precisely quantized plateaus in the [[Quantum Hall effect|Hall conductance]] of [[Two-dimensional electron gas|2-dimensional (2D) electrons]] at [[Decimals|fractional values]] of <math>e^2/h</math>, where ''e'' is the [[electron charge]] and ''h'' is the [[Planck constant]]. 
At the same time, longitudinal resistance drops to zero (for low enough temperatures) as for the [[Quantum Hall Effect|integer QHE]].
It is a property of a collective state in which electrons bind magnetic flux lines to make new [[quasiparticle]]s, and [[excited state|excitations]] have a [[fractionalization|fractional]] [[elementary charge]] and possibly also fractional statistics. The 1998 [[Nobel Prize in Physics]] was awarded to [[Robert Laughlin]], [[Horst Störmer]], and [[Daniel Tsui]] "for their discovery of a new form of quantum fluid with fractionally charged excitations".<ref>{{Cite web|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1998/|title=The Nobel Prize in Physics 1998|website=www.nobelprize.org|access-date=2018-03-28}}</ref><ref name=ptoday1998>{{cite journal|last=Schwarzschild|first=Bertram|title=Physics Nobel Prize Goes to Tsui, Stormer and Laughlin for the Fractional Quantum Hall Effect|journal=Physics Today|year=1998|volume=51|issue=12|doi=10.1063/1.882480|url=http://www.physicstoday.org/resource/1/phtoad/v51/i12/p17_s1|archive-url=https://archive.today/20130415185712/http://www.physicstoday.org/resource/1/phtoad/v51/i12/p17_s1|url-status=dead|archive-date=15 April 2013|access-date=20 April 2012|bibcode=1998PhT....51l..17S|pages=17–19}}</ref> 
The microscopic origin of the FQHE is a major research topic in [[condensed matter physics]].

== Descriptions==
{{unsolved|physics|2=What mechanism explains the existence of the ''&nu;''=5/2 state in the fractional quantum Hall effect?}}
The fractional quantum Hall effect (FQHE) is a collective behavior in a 2D system of electrons. In particular magnetic fields, the [[2DEG|electron gas]] condenses into a remarkable liquid state, which is very delicate, requiring high quality material with a low [[charge carrier|carrier]] concentration, and extremely low temperatures. As in the integer [[quantum Hall effect]], the Hall resistance undergoes certain [[quantum Hall transitions]] to form a series of plateaus. Each particular value of the magnetic field corresponds to a filling factor (the ratio of number of electrons to [[magnetic flux quanta]] corresponding to given area)

:<math>\nu = p/q,\ </math>

where ''p'' and ''q'' are integers with no common factors. Here ''q'' turns out to be an odd number with the exception of filling factor 5/2<ref>{{cite journal |last1=Rezayi |first1=Edward H. |title=Landau Level Mixing and the Ground State of the ν = 5 / 2 Quantum Hall Effect |journal=Physical Review Letters |date=14 July 2017 |volume=119 |issue=2 |page=026801 |doi=10.1103/PhysRevLett.119.026801|arxiv=1704.03026 }}</ref>
and few others (7/2 or 2+3/8). The principal series of such fractions are

:<math>{1\over 3}, {2\over 5}, {3\over 7}, \mbox{etc.,} </math>

and their particle-hole conjugates

:<math>{2\over3}, {3\over 5}, {4\over 7}, \mbox{etc.} </math>

Depending on the fraction, both spin-polarised and zero-spin fractional QHE states may exist.<ref>{{cite journal |last1=Výborný |first1=K. |title=Spin in fractional quantum Hall systems |journal=Annalen der Physik |date=16 February 2007 |volume=16 |issue=2 |page=30 |doi=10.1002/andp.200610228|arxiv=cond-mat/0603451 }}</ref>
Fractionally charged quasiparticles are neither [[boson]]s nor [[fermion]]s and exhibit [[anyon]]ic statistics. The fractional quantum Hall effect continues to be influential in theories about [[topological order]]. Certain fractional quantum Hall phases appear to have the right properties for building a [[topological quantum computer]].

== History and developments ==
The FQHE was experimentally discovered in 1982 by [[Daniel Chee Tsui|Daniel Tsui]] and [[Horst Ludwig Störmer|Horst Störmer]], in experiments performed on  [[heterostructure]]s made out of [[gallium arsenide]] developed by [[Arthur Gossard]].

There were several major steps in the theory of the FQHE.

*'''Laughlin states and fractionally-charged [[quasiparticle]]s''': this theory, proposed by [[Robert B. Laughlin]], is based on [[Laughlin wavefunction|accurate trial wave functions]] for the [[ground state]] at fraction <math>1/q</math> as well as its quasiparticle and quasihole excitations. The excitations have fractional charge of magnitude <math>e^*={e\over q}</math>.
*'''Fractional exchange statistics of quasiparticles''': [[Bertrand Halperin]] conjectured, and Daniel Arovas, [[John Robert Schrieffer]], and [[Frank Wilczek]] demonstrated, that the fractionally charged quasiparticle excitations of the Laughlin states are [[anyon]]s with fractional statistical angle <math>\theta = {\pi \over q}</math>; the wave function acquires phase factor of <math> e^{i \theta}</math> (together with an [[Aharonov-Bohm phase factor]]) when identical quasiparticles are exchanged in a counterclockwise sense. A recent experiment seems to give a clear demonstration of this effect.<ref>{{Cite arXiv |last1=An |first1=Sanghun |last2=Jiang |first2=P. |last3=Choi |first3=H. |last4=Kang |first4=W. |last5=Simon |first5=S. H. |last6=Pfeiffer |first6=L. N. |last7=West |first7=K. W. |last8=Baldwin |first8=K. W. |date=2011 |title=Braiding of Abelian and Non-Abelian Anyons in the Fractional Quantum Hall Effect |eprint=1112.3400 |class=cond-mat.mes-hall }}</ref>
*'''Hierarchy states''': this theory was proposed by [[Duncan Haldane]], and further clarified by [[Bertrand Halperin]], to explain the observed filling fractions not occurring at the Laughlin states' <math>\nu = 1/q</math>. Starting with the Laughlin states, new states at different fillings can be formed by condensing quasiparticles into their own Laughlin states. The new states and their fillings are constrained by the fractional statistics of the quasiparticles, producing e.g. <math>\nu = 2/5</math> and <math>2/7</math> states from the Laughlin <math>\nu = 1/3</math> state. Similarly constructing another set of new states by condensing quasiparticles of the first set of new states, and so on, produces a hierarchy of states covering all the odd-denominator filling fractions. This idea has been validated quantitatively,<ref>{{cite journal|first=M.|year=1994|title=Microscopic formulation of the hierarchy of quantized Hall states|journal=[[Phys. Lett. B|Physics Letters B]]|volume=336|issue=1|pages=48–53|arxiv=cond-mat/9311062|bibcode=1994PhLB..336...48G|doi=10.1016/0370-2693(94)00957-0|last=Greiter|s2cid=119433766}}</ref> and brings out the observed fractions in a natural order. Laughlin's original plasma model was extended to the hierarchy states by [[Allan H. MacDonald]] and others.<ref>{{cite journal|first1=A.H.|first2=G.C.|first3=M.W.C.|year=1985|title=Hierarchy of plasmas for fractional quantum Hall states|journal=[[Phys. Rev. B|Physical Review B]]|volume=31|issue=8|pages=5529–5532|bibcode=1985PhRvB..31.5529M|doi=10.1103/PhysRevB.31.5529|last1=MacDonald|last2=Aers|last3=Dharma-wardana|pmid=9936538}}</ref> Using methods introduced by [[Greg Moore (physicist)|Greg Moore]] and [[Nicholas Read]],<ref>{{cite journal |last1=Moore |first1=G. |last2=Read |first2=N. |title=Nonabelions in the fractional quantum Hall effect |journal=Nucl. Phys. |date=1990 |volume=B360 |issue=2 |page=362|bibcode=1991NuPhB.360..362M |doi=10.1016/0550-3213(91)90407-O |doi-access=free }}</ref> based on [[conformal field theory]] explicit wave functions can be constructed for all hierarchy states.<ref>{{cite journal |last1=Hansson |first1=T.H. |last2=Hermanns |first2=M. |last3=Simon |first3=S.H. |last4=Viefers|first4=S.F.|author4-link= Susanne Viefers |title=Quantum Hall physics: Hierarchies and conformal field theory techniques |journal=Rev. Mod. Phys. |date=2017 |volume=89 |issue=2 |page=025005 |doi=10.1103/RevModPhys.89.025005|arxiv=1601.01697 |bibcode=2017RvMP...89b5005H |s2cid=118614055 }}</ref>
*'''[[Composite fermions]]''': this theory was proposed by [[Jainendra K. Jain]], and further extended by Halperin, [[Patrick A. Lee]] and Read. The basic idea of this theory is that as a result of the repulsive interactions, two (or, in general, an even number of) vortices are captured by each electron, forming integer-charged quasiparticles called composite fermions. The fractional states of the electrons are understood as the integer [[quantum Hall effect|QHE]] of composite fermions. For example, this makes electrons at filling factors 1/3, 2/5, 3/7, etc. behave in the same way as at filling factor 1, 2, 3, etc. Composite fermions have been observed, and the theory has been verified by experiment and computer calculations. Composite fermions are valid even beyond the fractional quantum Hall effect; for example, the filling factor 1/2 corresponds to zero magnetic field for composite fermions, resulting in their Fermi sea.
Tsui, Störmer, and [[Robert B. Laughlin]] were awarded the 1998 [[Nobel Prize in Physics]] for their work.

Jain, [[James P. Eisenstein]], and [[Mordehai Heiblum]] won the 2025 [[Wolf Prize in Physics]] "for advancing our understanding of the surprising properties of two-dimensional electron systems in strong magnetic fields".<ref>{{Cite web |last=מיכל |date=2025-03-10 |title=James P. Eisenstein |url=https://wolffund.org.il/james-p-eisenstein/ |access-date=2025-03-17 |website=Wolf Foundation |language=en-US}}</ref>

== Evidence for fractionally-charged quasiparticles ==

Experiments have reported results that specifically support the understanding that there are fractionally-charged quasiparticles in an electron gas under FQHE conditions.

In 1995, the fractional charge of Laughlin quasiparticles was measured directly in a quantum antidot electrometer at [[State University of New York at Stony Brook|Stony Brook University]], [[New York (state)|New York]].<ref>{{cite journal|first1=V.J.|first2=B.|year=1995|title=Resonant Tunneling in the Quantum Hall Regime: Measurement of Fractional Charge|journal=[[Science (journal)|Science]]|volume=267|issue=5200|pages=1010–2|bibcode=1995Sci...267.1010G|doi=10.1126/science.267.5200.1010|last1=Goldman|last2=Su|pmid=17811442|s2cid=45371551}}
*{{cite web |date=2003 |title=Direct Observation of Fractional Charge |website=Stony Brook University |url=http://quantum.physics.sunysb.edu/index.html|archive-url=https://web.archive.org/web/20031007040231/http://quantum.physics.sunysb.edu/index.html |archive-date=2003-10-07 }}</ref> In 1997, two groups of physicists at the [[Weizmann Institute of Science]] in [[Rehovot]], Israel, and at the [[Commissariat à l'énergie atomique]] laboratory near [[Paris]],<ref>
{{cite journal
 |author=L. Saminadayar |author2=D. C. Glattli |author3=Y. Jin |author4=B. Etienne
 |year=1997
 |title=Observation of the e/3 fractionally charged Laughlin quasiparticle
 |journal=[[Physical Review Letters]]
 |volume=79|issue=13
 |pages=2526–2529
 |doi=10.1103/PhysRevLett.79.2526
|arxiv=cond-mat/9706307
 |bibcode=1997PhRvL..79.2526S
 |s2cid=119425609
 }}</ref> detected such quasiparticles carrying an [[electric current]], through measuring quantum [[shot noise]]<ref>
{{cite web|date=24 October 1997|title=Fractional charge carriers discovered|url=https://physicsworld.com/a/fractional-charge-carriers-discovered/ |access-date=2010-02-08|work=[[Physics World]]}}</ref><ref>
{{cite journal
 |author1=R. de-Picciotto |author2=M. Reznikov |author3=M. Heiblum |author4=V. Umansky |author5=G. Bunin |author6=D. Mahalu |year=1997
 |title=Direct observation of a fractional charge
 |journal=[[Nature (journal)|Nature]]
 |volume=389 |pages=162
 |doi=10.1038/38241
|bibcode = 1997Natur.389..162D
 |issue=6647|arxiv=cond-mat/9707289|s2cid=4310360 }}</ref>
Both of these experiments have been confirmed with certainty.{{citation needed|date=March 2024}}

A more recent experiment,<ref>
{{cite journal
|doi=10.1126/science.1099950
|author=J. Martin
|author2=S. Ilani
|author3=B. Verdene
|author4= J. Smet
|author5=V. Umansky
|author6=D. Mahalu
|author7=D. Schuh
|author8=G. Abstreiter
|author9=A. Yacoby 
|title =Localization of Fractionally Charged Quasi Particles 
|journal= [[Science (journal)|Science]] 
|volume=305 |pages=980–3
|year=2004 
|bibcode = 2004Sci...305..980M
 |pmid=15310895
|issue=5686|s2cid=2859577
}}</ref>  measures the quasiparticle charge. In 2020, interferometry experiments conducted by two different groups, at Paris<ref>{{Cite journal |last=Bartolomei |first=H. |last2=Kumar |first2=M. |last3=Bisognin |first3=R. |last4=Marguerite |first4=A. |last5=Berroir |first5=J.-M. |last6=Bocquillon |first6=E. |last7=Plaçais |first7=B. |last8=Cavanna |first8=A. |last9=Dong |first9=Q. |last10=Gennser |first10=U. |last11=Jin |first11=Y. |last12=Fève |first12=G. |date=2020-04-10 |title=Fractional statistics in anyon collisions |url=https://www.science.org/doi/10.1126/science.aaz5601 |journal=Science |volume=368 |issue=6487 |pages=173–177 |doi=10.1126/science.aaz5601|arxiv=2006.13157 }}</ref> and Purdue,<ref>{{Cite journal |last=Nakamura |first=J. |last2=Liang |first2=S. |last3=Gardner |first3=G. C. |last4=Manfra |first4=M. J. |date=September 2020 |title=Direct observation of anyonic braiding statistics |url=https://www.nature.com/articles/s41567-020-1019-1 |journal=Nature Physics |language=en |volume=16 |issue=9 |pages=931–936 |doi=10.1038/s41567-020-1019-1 |issn=1745-2481|arxiv=2006.14115 }}</ref> were both able to probe and confirm the braiding statistics of anyons.

== Impact ==
{{expand section|date=April 2019}}
The FQH effect shows the limits of [[Lev Landau|Landau]]'s [[symmetry breaking]] theory. Previously it was held that the symmetry breaking theory could explain all the important concepts and properties of forms of matter. According to this view, the only thing to be done was to apply the [[symmetry breaking]] theory to all different kinds of phases and [[phase transition]]s.<ref>{{cite journal |vauthors=Rychkov VS, Borlenghi S, Jaffres H, Fert A, Waintal X |title=Spin torque and waviness in magnetic multilayers: a bridge between Valet-Fert theory and quantum approaches |journal=Phys. Rev. Lett. |volume=103 |issue=6 |pages=066602 |date=August 2009 |pmid=19792592|doi=10.1103/PhysRevLett.103.066602|bibcode=2009PhRvL.103f6602R|arxiv = 0902.4360 |s2cid=209013 }}</ref> From this perspective, the importance of the FQHE discovered by
Tsui, Stormer, and Gossard is notable for contesting old perspectives.

The existence of FQH liquids suggests that there is much more to discover beyond the present [[symmetry breaking]] paradigm in condensed matter physics.
Different FQH states all have the same symmetry
and cannot be described by symmetry breaking theory.
The associated [[fractional charge]], [[fractional statistics]], [[Non-abelian gauge theory|non-Abelian]] statistics,
[[chiral]] edge states, etc. demonstrate the power and the fascination of [[emergence]] in many-body systems.
Thus FQH states represent new states of matter that contain a
completely new kind of order—[[topological order]].
For example, properties once deemed isotropic for all materials may be anisotropic in 2D planes.
The new type of orders represented by FQH states greatly enrich our
understanding of quantum phases and [[quantum phase transitions]].<ref>{{cite journal |author=Callaway DJE |author-link=David J E Callaway |title=Random matrices, fractional statistics, and the quantum Hall effect |journal=Phys. Rev. B |volume=43 |issue=10 |pages=8641–8643 |date=April 1991 |pmid=9996505 |doi=10.1103/PhysRevB.43.8641|bibcode = 1991PhRvB..43.8641C }}</ref><ref>{{Cite journal|last1=Selby|first1=N. S.|last2=Crawford|first2=M.|last3=Tracy|first3=L.|last4=Reno|first4=J. L.|last5=Pan|first5=W.|date=2014-09-01|title=In situ biaxial rotation at low-temperatures in high magnetic fields|journal=Review of Scientific Instruments|volume=85|issue=9|pages=095116|doi=10.1063/1.4896100|pmid=25273781|issn=0034-6748|bibcode=2014RScI...85i5116S|url=https://zenodo.org/record/1232073|doi-access=free}}</ref>

== See also ==
* [[Hall probe]]
* [[Laughlin wavefunction]]
* [[Macroscopic quantum phenomena]]
* [[Quantum anomalous Hall effect]]
* [[Quantum Hall Effect]]
* [[Quantum spin Hall effect]]
* [[Topological order]]
* [[Fractional Chern insulator]]

==Notes==
{{reflist}}

==References==
* {{cite journal
 |author1=D.C. Tsui |author2=H.L. Stormer |author3=A.C. Gossard |year=1982
 |title=Two-Dimensional Magnetotransport in the Extreme Quantum Limit
 |journal=[[Physical Review Letters]]
 |volume=48 |pages=1559
 |doi=10.1103/PhysRevLett.48.1559
|bibcode=1982PhRvL..48.1559T
 |issue=22
|doi-access=free}}
* {{cite journal
 |author=H.L. Stormer
 |year=1999
 |title=Nobel Lecture: The fractional quantum Hall effect
 |journal=[[Reviews of Modern Physics]]
 |volume=71 |pages=875–889
 |doi=10.1103/RevModPhys.71.875
|bibcode=1999RvMP...71..875S
 |issue=4
|doi-access=free
 }}
* {{cite journal
 |author=R.B. Laughlin
 |year=1983
 |title=Anomalous Quantum Hall Effect: An Incompressible Quantum Fluid with Fractionally Charged Excitations
 |journal=[[Physical Review Letters]]
 |volume=50 |pages=1395–1398
 |doi=10.1103/PhysRevLett.50.1395
|bibcode=1983PhRvL..50.1395L
 |issue=18
}}

{{DEFAULTSORT:Fractional Quantum Hall Effect}}
[[Category:Hall effect]]
[[Category:Correlated electrons]]
[[Category:Quantum phases]]
[[Category:Mesoscopic physics]]
[[Category:Unsolved problems in physics]]
[[Category:Unexplained phenomena]]