Fact-checked by Grok 2 weeks ago

Composite fermion

A composite fermion is an emergent quasiparticle in two-dimensional electron systems subjected to strong perpendicular magnetic fields at low temperatures, formed by binding an electron to an even number (typically 2) of quantized magnetic flux quanta, or vortices, in the many-body wave function.^[1] This binding endows the composite fermion with a topological character, allowing it to experience a reduced effective magnetic field B^* = B - 2p \rho \phi_0, where B is the external field, p is an integer, \rho is the electron density, and \phi_0 = h/e is the flux quantum.^[1] Proposed by physicist Jainendra K. Jain in 1989, who co-received the Wolf Prize in Physics in 2025 for this contribution, the theory maps the strongly interacting electrons of the fractional quantum Hall effect (FQHE)—first observed experimentally in 1982 by Daniel Tsui, Horst Störmer, and Arthur Gossard at filling factor \nu = 1/3—onto a system of these weakly interacting composite fermions undergoing an integer quantum Hall effect (IQHE). The composite fermion paradigm provides a unified framework for understanding the FQHE, where the observed fractional Hall conductance plateaus at odd-denominator filling factors \nu = n / (2pn \pm 1) (with n a positive integer) arise as integer quantized states of composite fermions at effective filling \nu^* = n.^[1] Key theoretical constructs include the composite fermion wave function, \Psi_{\nu} = P_{\rm LLL} \prod_{j < k} (z_j - z_k)^{2p} \Phi_{\nu^*}, which projects the integer quantum Hall wave function \Phi_{\nu^*} (a Slater determinant) onto the lowest Landau level while attaching the vortex factors, yielding exact agreement with numerical diagonalization for small systems (e.g., energy gaps within 0.07% at \nu = 1/3). Experimentally, the theory has been corroborated through observations such as Shubnikov–de Haas oscillations indicating a Fermi sea of composite fermions at half-filled Landau level (\nu = 1/2) in 1994, cyclotron resonance measurements of their effective mass in 2002.^[1] Beyond the FQHE, composite fermions explain a range of phenomena, including spin polarization transitions tunable by Zeeman energy, where ground states shift from fully spin-polarized to partially polarized configurations at critical fields matching theoretical predictions; compressible states like the \nu = 1/2 Fermi liquid; and even exotic paired states at even-denominator fractions, such as the Moore-Read Pfaffian at \nu = 5/2, proposed as p-wave superfluids of composite fermions.^[1] The effective mass of composite fermions, observed to be 1–2 times the bare electron mass despite originating from Coulomb interactions (not lattice effects), highlights their non-perturbative nature and has been quantified via activation gaps and transport, remaining a puzzle resolved only through higher-order interaction effects in the theory.^[1] Jain's 2007 monograph further formalizes the theory, deriving its foundations from Chern-Simons gauge field transformations and predicting novel transport properties like anisotropic composite fermion liquids under tilted fields.^[2] Ongoing research extends composite fermions to bilayer systems, twisted bilayer graphene, and higher Landau levels, underscoring their role in modern condensed matter physics.^[1]

Introduction

Definition and Formation

In two-dimensional electron systems subjected to a strong perpendicular magnetic field, composite fermions emerge as bound states of electrons and an even number (2p, where p is a positive integer) of quantized magnetic flux quanta, also known as vortices.^[3] This attachment effectively transforms the strongly interacting electrons into a gas of weakly interacting fermionic quasiparticles that obey Fermi-Dirac statistics, while preserving the overall fermionic nature of the system.^[4] The composite fermion concept provides a topological description of these quasiparticles, where the bound flux quanta screen part of the external magnetic field, leading to novel quantum phases.^[3] These quasiparticles form in the context of a two-dimensional electron gas (2DEG), typically realized in semiconductor heterostructures like GaAs/AlGaAs, where electrons are confined to a thin layer at the interface, or in graphene, a single layer of carbon atoms enabling relativistic electron behavior.^[5] Under a perpendicular magnetic field, the kinetic energy of the electrons quantizes into discrete Landau levels, with the degeneracy of each level determined by the magnetic field strength.^[6] In such systems, the application of high magnetic fields at low temperatures reveals the quantum Hall effects, where the Hall conductance exhibits plateaus at rational multiples of e²/h.^[4] The primary motivation for introducing composite fermions is to resolve the fractional quantum Hall effect (FQHE), observed at fractional filling factors, by mapping the strongly correlated electron system to an equivalent problem of non-interacting composite fermions experiencing a reduced effective magnetic field.^[3] This mapping simplifies the theoretical treatment, as the FQHE states correspond to integer quantum Hall states of the composite fermions.^[4] The filling factor ν, defined as the ratio of electron density n to the degeneracy of a Landau level, ν = n h / (e B)—where h is Planck's constant, e is the electron charge, and B is the magnetic field—relates to the composite fermion filling factor ν* through ν* = ν / (1 - 2p ν).^[6]^[3] For the principal sequence with p=1, this relation predicts FQHE states at ν = 1/3, 2/5, 3/7, and so on, aligning with experimental observations and establishing the composite fermion as a fundamental construct for understanding correlated electron liquids in reduced dimensions.^[4]

Historical Development

The discovery of the quantum Hall effect in 1980 by Klaus von Klitzing marked the beginning of intense research into two-dimensional electron systems under strong magnetic fields, followed by the observation of fractional quantum Hall states at odd-denominator fillings by Daniel Tsui, Horst Störmer, and Arthur Gossard in 1982. Early theoretical progress included Robert Laughlin's 1983 proposal of a trial wave function describing the incompressible state at filling factor ν=1/3, which introduced fractionally charged quasiparticles but left the hierarchy of observed fractional states unexplained.^[7] In 1989, Jainendra K. Jain introduced the composite fermion concept in a seminal paper, proposing that electrons in the fractional quantum Hall effect bind to an even number of magnetic flux quanta to form composite fermions, which experience an effective magnetic field and map the hierarchy of fractional states onto integer quantum Hall effects of these quasiparticles.^[3] This framework provided a unified understanding of the observed sequence of states at fillings like ν=2/5 and 3/7. In 1993, Bertrand Halperin, Patrick A. Lee, and Nathan Read extended the idea to the compressible state at half-filled Landau levels (ν=1/2), describing it as a Fermi sea of composite fermions in zero effective field, incorporating gauge field fluctuations via Chern-Simons theory.^[8] Experimental confirmation followed rapidly: transport measurements in 1994 revealed a drastic enhancement of the composite fermion effective mass near ν=1/2, consistent with strong interactions, while Shubnikov-de Haas oscillations in 1995 provided direct evidence for the composite fermion Fermi sea through density-dependent periodicity.^[9] The theory's predictive power was further validated in 1999 by the observation of an even-denominator fractional quantum Hall state at ν=5/2 in high-mobility samples, interpreted as the second Landau level of composite fermions forming a paired p-wave state.^[10] Post-2010 developments have extended the composite fermion paradigm to multi-component systems, such as bilayers and spinful electrons, revealing richer phases like paired states and anisotropic liquids, as well as higher-flux attachments (e.g., four or six fluxes) that explain states at smaller fillings like ν=1/4 and enable new topological orders.^[11] These advances have deepened insights into non-Abelian statistics and potential applications in topological quantum computing. In recognition of these foundational contributions, Jain shared the 2025 Wolf Prize in Physics with James P. Eisenstein and Mordehai Heiblum for pioneering work on composite fermions and quantum Hall phenomena.^[12]

Theoretical Foundations

Composite Fermion Transformation

The composite fermion transformation provides a theoretical mapping that reinterprets the strongly interacting two-dimensional electron system in a perpendicular magnetic field as a system of non-interacting composite fermions, each formed by attaching an even number of magnetic flux quanta to an original electron.^[13] This flux attachment procedure involves binding 2p vortices of Chern-Simons gauge flux (where p is a positive integer and each flux quantum φ₀ = h/e) to each electron in the opposite direction to the external magnetic field B, effectively reducing the magnetic field experienced by the composite fermions.^[13] In this picture, the composite fermions inherit the fermionic statistics of the electrons while seeing a modified environment that accounts for the correlations induced by electron-electron interactions.^[13] The key mathematical relation arises in the mean-field approximation, where the dynamical nature of the attached flux is replaced by its average value, leading to an effective magnetic field B* experienced by the composite fermions given by

B^* = B - 2p \phi_0 \rho,

where ρ is the two-dimensional electron density.^[13] Here, the term 2p φ₀ ρ represents the average field from the attached flux quanta, which opposes the external field and cancels part of it. The corresponding composite fermion filling factor ν* is related to the electron filling factor ν = ρ h / (e B) by

\nu^* = \frac{\nu}{1 - 2p \nu}.

^[13] This transformation implies that B* = 0 when ν = 1/(2p), corresponding to a Fermi sea of composite fermions at zero effective field.^[13] This mapping unifies the fractional quantum Hall effect (FQHE) and the integer quantum Hall effect (IQHE) by showing that FQHE states at filling factors ν = ν* / (2p ν* + 1), with ν* an integer, correspond directly to IQHE states of the composite fermions at integer filling ν*.^[13] For example, with p = 1, the FQHE state at ν = 1/3 maps to the ν* = 1 IQHE of composite fermions, while ν = 2/5 corresponds to ν* = 2.^[13] The mean-field approximation simplifies the strongly correlated electron problem into a weakly interacting composite fermion gas in B*, providing a unified framework for both integer and fractional Hall plateaus observed in experiments.^[13] Trial wave functions serve as explicit variational realizations of this transformation for computing ground-state properties.^[13]

Trial Wave Functions

The trial wave function for composite fermion states at filling factor \nu is constructed by attaching $2p units of flux to each electron, transforming them into composite fermions that experience an effective magnetic field and form integer quantum Hall states at effective filling \nu^*. The explicit form is given by

\psi_\nu = P_{\mathrm{LLL}} \Phi_{\nu^*} \prod_{i<j} (z_i - z_j)^{2p},

where z_i are the complex coordinates of the electrons in the plane, \Phi_{\nu^*} is the Slater determinant wave function for non-interacting fermions filling \nu^* Landau levels in the effective field, and P_{\mathrm{LLL}} projects the result onto the lowest Landau level of the original strong magnetic field.^[3] This ansatz captures the flux attachment conceptually by the Jastrow-like factor \prod (z_i - z_j)^{2p}, which correlates electrons to mimic bound vortices, while the projection ensures compliance with the single-particle Hilbert space constraints of the lowest Landau level.^[3] For the simplest case of p=1 and \nu^*=1, the construction reduces to the Laughlin wave function at \nu=1/3, \psi_{1/3} = P_{\mathrm{LLL}} \prod_{i<j} (z_i - z_j)^3, which describes a gapped incompressible fluid with fractionally charged quasiparticles.^[3] More generally, it generates the principal sequences of fractional quantum Hall states at fillings \nu = n/(2pn \pm 1), where n is a positive integer denoting the number of filled effective Landau levels of composite fermions, and the \pm accounts for reverse flux attachment directions.^[3] These sequences, such as \nu=2/5 for n=2, p=1 (\nu^*=2), explain a majority of observed fractions without invoking further hierarchies beyond the initial transformation.^[3] Exact diagonalization studies of small electron systems (up to N \approx 10) confirm the accuracy of these trial wave functions, showing overlaps exceeding 90% with the true Coulomb ground states and reproducing excitation gaps within a few percent for states like \nu=1/3 and $2/5.^[3] The variational energies from the ansatz closely match those from full diagonalization of the interacting Hamiltonian, validating the neglect of higher-order correlations in the effective theory for these fillings.^[3] Extensions of the construction to bilayer systems incorporate interlayer correlations by modifying the Jastrow factors to include relative coordinates between layers, enabling descriptions of excitonic superfluids at total \nu=1 or paired states at \nu=1/2 + 1/2.^[14] For spin-polarized systems beyond full polarization, spin-dependent flux attachment or singlet pairings of composite fermions yield trial functions for states like the spin-singlet at \nu=1/2, with numerical overlaps again confirming their fidelity to exact ground states in finite systems.

Chern-Simons Gauge Theory

The Chern-Simons gauge theory provides a field-theoretic description of composite fermions by treating them as electrons bound to an even number of magnetic flux quanta, effectively coupling the fermionic degrees of freedom to a dynamical U(1) Chern-Simons gauge field. This approach, pioneered in the Halperin-Lee-Read (HLR) theory, models the system at filling factor \nu = 1/2 as a Fermi liquid of composite fermions in an average zero effective magnetic field. The flux attachment mechanism statistically transmutes the electrons into composite fermions, altering their response to the external magnetic field while preserving fermionic statistics. The core of the HLR theory is encapsulated in the effective Lagrangian density for non-relativistic composite fermions in (2+1) dimensions:

\mathcal{L} = \bar{\psi} \left( i \partial_\tau - \frac{\nabla^2}{2m} - \mathbf{A} - \mathbf{a} \right) \psi + \frac{i}{4\pi k} \epsilon^{\mu\nu\lambda} a_\mu \partial_\nu a_\lambda,

where \psi represents the composite fermion field, m is the bare band mass, \mathbf{A} is the external electromagnetic vector potential, \mathbf{a} (or a_\mu) is the emergent Chern-Simons gauge field, and k = 1/(2p) with $2p the number of attached fluxes (e.g., p=1 for \nu=1/2). The Chern-Simons term \frac{i}{4\pi k} \epsilon^{\mu\nu\lambda} a_\mu \partial_\nu a_\lambda enforces the flux attachment at the mean-field level, binding $2p units of flux to each composite fermion and canceling the external field on average at half-filling. Coulomb interactions and disorder can be incorporated perturbatively into this framework. This theory excels in describing compressible states, such as the \nu=1/2 Fermi liquid, where the composite fermions form a metallic Fermi sea with finite density of states at the Fermi energy. Transverse gauge fluctuations, arising from the singular nature of the Chern-Simons interaction, generate logarithmic corrections to the fermion self-energy and enhance the effective mass, diverging as \delta m \sim m_b |\ln b| under renormalization group (RG) scaling, where m_b is the bandwidth and b the RG scale factor. The path integral formulation facilitates RG analysis of these gauge-mediated interactions, revealing non-Fermi liquid behavior at long wavelengths, including anomalous dimensions for the fermion propagator and vertex corrections that modify scattering rates. Despite its successes, the HLR theory has notable limitations, including the neglect of Landau level mixing, which becomes significant at higher energies or weaker magnetic fields, and its failure to capture pairing instabilities that could lead to gapped states or superconductivity-like phases in the composite fermion system. These shortcomings arise from the mean-field treatment of the gauge field and the projection onto the lowest Landau level, which overlooks higher-order effects in the electron interaction.

Key Properties

Effective Magnetic Field

In the composite fermion theory, electrons in a two-dimensional electron gas subjected to a perpendicular magnetic field B are mapped to composite fermions through flux attachment, where each electron binds to an even number $2p (with p a positive integer) of magnetic flux quanta. This transformation results in an effective magnetic field experienced by the composite fermions given by B^* = B - 2p \phi_0 \rho, with \phi_0 = h/e the magnetic flux quantum and \rho the areal electron density. The reduction in the effective field arises from the opposing Chern-Simons flux attached to each composite fermion, which partially cancels the external field. A key prediction of this mean-field picture is the sign reversal of B^* relative to B. At the filling factor \nu = 1/2, corresponding to the density where the attached flux exactly cancels the external field, B^* = 0, and the composite fermions form a compressible Fermi sea without Landau level quantization. For \nu > 1/2, B^* points opposite to B, while for \nu < 1/2, it aligns with B but is reduced in magnitude. In the presence of nonzero B^*, composite fermions organize into quantized Landau levels, with the effective filling factor \nu^* determining the number of filled levels.^[15] This effective field framework provides a natural explanation for the hierarchy of fractional quantum Hall effect (FQHE) states observed at odd-denominator filling factors, which map to integer quantum Hall states of composite fermions at integer \nu^* in B^*. For instance, the principal Jain sequence at \nu = n / (2pn \pm 1) (with n a positive integer) corresponds to \nu^* = n, where the FQHE conductance plateaus of electrons emerge from the integer filling of composite fermion Landau levels. Deviations from these mean-field predictions occur in real systems, where disorder scatters composite fermions and residual electron interactions beyond the flux attachment approximation cause slight shifts in the positions and widths of observed Hall plateaus, typically on the order of a few percent in filling factor.^[15] The orbital motion of composite fermions in B^* is characterized by an effective cyclotron frequency \omega_c^* = e |B^*| / m^*, where m^* is the dynamically generated effective mass of the composite fermions, several times larger than the bare electron band mass due to strong interactions.^[15] This frequency sets the energy scale for excitations between composite fermion Landau levels, influencing the gaps at FQHE states and underscoring the topological nature of the effective field reduction.

Effective Mass

In the Halperin-Lee-Read (HLR) theory, the effective mass m^* of composite fermions arises primarily from electron-electron interactions in the half-filled Landau level, representing a substantial enhancement over the bare band mass. Gauge fluctuations in the Chern-Simons theory further amplify m^*, yielding a logarithmically enhanced mass near the Fermi surface.^[16] In gallium arsenide (GaAs) two-dimensional electron gases, these enhancements result in typical values of m^* \approx 1{-}2 \, m_e.^[9] The bare electron band mass in GaAs is m_b \approx 0.067 m_e, underscoring how interactions dominate the composite fermion dynamics, increasing the mass by more than an order of magnitude. This distinction between the band mass (from single-particle band structure) and the interaction-enhanced mass highlights the many-body nature of composite fermions, where the latter incorporates Coulomb effects projected into the lowest Landau level. Landau level mixing, arising from finite thickness or higher-order interactions, further modifies the effective mass by allowing admixture of excited Landau levels, which can either stabilize or alter the enhancement depending on the filling factor and material parameters.^[17] The enhanced effective mass has key implications for the cyclotron motion of composite fermions, with the effective cyclotron energy given by \hbar \omega_c^* = \hbar e B^* / m^*, where B^* is the reduced effective magnetic field experienced by the composite fermions after flux attachment. This energy scale governs the composite fermion response to the residual field, influencing the overall excitation spectrum in fractional quantum Hall states. The gauge field contributions from Chern-Simons theory, in particular, drive a logarithmic divergence in m^* near the Fermi surface, emphasizing the role of transverse gauge fluctuations in the mass renormalization.^[18]

Spin and Statistics

Composite fermions, constructed by attaching an even number (2p, where p is a positive integer) of fictitious flux quanta to electrons, preserve the fermionic statistics of the underlying electrons. This even attachment introduces an even number of phase factors upon particle exchange, maintaining the overall antisymmetric wavefunction characteristic of fermions and ensuring adherence to the Pauli exclusion principle for composite fermions with identical spin projections.^[19]^[15] Like electrons, composite fermions carry a spin-1/2 degree of freedom. The spin polarization of composite fermion states is governed by the interplay between the Zeeman energy and Coulomb repulsion. In high magnetic fields, the bare Zeeman energy g_b \mu_B B (with g_b \approx -0.44 for GaAs heterostructures) dominates over interaction effects, resulting in fully spin-polarized configurations where all spins align parallel to the field.^[15]^[20] At the filling factor \nu = 5/2, corresponding to a paired state of composite fermions, numerical studies indicate partial spin polarization, arising from the balance of enhanced spin splitting and residual interactions that favor unpolarized or mixed-spin ground states depending on the short-range repulsion strength in the Coulomb potential.^[21] The effective Landé g-factor g^* for composite fermions is renormalized by electron-electron interactions and given by g^* = g_b (m^*/m_b), where m^*/m_b is the band mass ratio (typically 1.5–3 near half-filling), yielding enhancement beyond the bare value; measured magnitudes range from approximately 0.2 near \nu = 3/2 to over 6 at lower fillings due to strong exchange contributions.^[20]^[22]^[23] In composite fermion liquids, such as those realizing the \nu = 1/3 Laughlin state, nontrivial spin textures emerge, including fractionally charged skyrmions formed by dressing a composite fermion quasiparticle or hole with a spin-flip exciton. These topologically stable structures carry charge \pm e/3 and exhibit binding energies on the order of $0.005–$0.01 e^2 / \epsilon l_B, observable in the low-energy excitation spectrum below the Zeeman energy.^[24]

Experimental Evidence

Quantum Hall Effects

The fractional quantum Hall effect (FQHE) was experimentally observed in two-dimensional electron systems confined in GaAs-AlGaAs heterostructures under strong perpendicular magnetic fields and low temperatures, manifesting as quantized plateaus in the Hall resistance \rho_{xy} = h / (\nu e^2) at fractional filling factors \nu = 1/3, 2/5, and others, accompanied by minima in the longitudinal resistance \rho_{xx}. These plateaus occur alongside the integer quantum Hall effect (IQHE) at integer \nu = 1, 2, etc., where similar quantization is seen but attributed to filled Landau levels of non-interacting electrons. Composite fermion theory unifies these phenomena by mapping the FQHE at filling factor \nu to the IQHE of composite fermions at an effective filling \nu^* = \nu / (1 - 2p\nu), where p is typically 1 for primary sequences, transforming strongly interacting electrons into weakly interacting composite fermions in an effective magnetic field B^*.^[3] For instance, the compressible state at \nu = 1/2 corresponds to \nu^* = \infty, equivalent to zero effective field B^* = 0, forming a Fermi sea of composite fermions.^[25] The theory predicts hierarchical sequences of FQHE states, such as Jain's series \nu = n / (2n \pm 1) for p=1 and integer n, explaining observed plateaus like \nu = 1/3 (n=1), $2/5 (n=2), and their conjugates.^[26] Particle-hole symmetry around \nu = 1/2 further emerges naturally, with states at \nu and $1 - \nu related by this mapping.^[27] Transport in these gapped FQHE states exhibits thermally activated behavior, with \rho_{xx} following \rho_{xx} \propto \exp(-\Delta / 2k_B T), where the energy gap \Delta \approx 0.1 - 0.5 meV is measured via temperature dependence, reflecting the excitation energy for composite fermion quasiparticles.^[28]^[29]

Magnetotransport Oscillations

Shubnikov-de Haas (SdH) oscillations in the longitudinal magnetoresistance \rho_{xx} provide direct evidence for the formation of a Fermi sea of composite fermions at the half-filled Landau level (\nu = 1/2). These oscillations manifest as periodic minima in \rho_{xx} corresponding to integer fillings \nu^* = n h / (e B^*) of composite fermion Landau levels in the effective magnetic field B^* = B - 2 n h / e, where n is the two-dimensional electron density. The periodicity of the oscillations, analyzed via Fourier transform, yields a frequency f = (e B^* / h) A_F, with A_F the cross-sectional area of the composite fermion Fermi surface, consistent with the expected A_F = 2 \pi n for a spin-polarized system. This behavior was first observed in high-mobility GaAs/AlGaAs heterostructures, confirming the metallic nature of the state at \nu = 1/2. The effective mass m^* of composite fermions is determined from the temperature-dependent damping of SdH oscillation amplitudes using the Lifshitz-Kosevich formula, which describes the thermal smearing of the density of states:

R_T = \frac{\pi X}{\sinh(\pi X)}, \quad X = \frac{2\pi^2 k_B T m^*}{\hbar e B^*},

where k_B is Boltzmann's constant and \hbar is the reduced Planck's constant. Fitting experimental data reveals m^* \approx 1.2 m_e (with m_e the free electron mass) at low effective fields, far exceeding the GaAs band mass of \sim 0.067 m_e and arising purely from electron interactions. This enhanced mass is nearly independent of the external magnetic field near \nu = 1/2 but increases with density. An analogous de Haas-van Alphen effect appears in the oscillatory magnetization M, reflecting the thermodynamic response to the oscillating composite fermion density of states. These magnetization oscillations, periodic in $1/B^*, probe the extremal orbits of the composite fermion Fermi surface and corroborate the SdH transport data, with amplitude modulated similarly by temperature and scattering.^[15] The amplitude of SdH oscillations also encodes information on phase coherence through the Dingle damping factor R_D = \exp(- \pi / (\omega_c^* \tau)), where \omega_c^* = e B^* / m^* is the composite fermion cyclotron frequency and \tau is the scattering time. Analysis yields a phase coherence length l_\phi \sim \sqrt{D \tau_\phi} (with D the diffusion constant and \tau_\phi the dephasing time) on the order of microns in high-mobility samples, limited by disorder and interactions.^[30]

Cyclotron Resonance

Cyclotron resonance experiments have provided compelling evidence for the existence of composite fermions through the observation of their circular motion in the effective magnetic field B^*. These studies typically employ optical and microwave absorption techniques to detect transitions between composite fermion Landau levels, revealing the dynamics of these quasiparticles in two-dimensional electron systems (2DES) in GaAs/AlGaAs heterostructures. The resonance frequency is given by \omega_c^* = e B^* / m^*, where m^* is the effective mass of the composite fermion, distinct from the bare electron cyclotron frequency \omega_c = e B / m_b with band mass m_b \approx 0.067 m_e.^[31] In microwave absorption experiments at low temperatures (around 0.4 K), enhanced absorption peaks have been observed in the composite fermion regime near filling factor \nu = 1/2, interpreted as resonances with the cyclotron motion of composite fermions. These peaks occur at frequencies of approximately 12–17 GHz, corresponding to energies on the order of 0.05 meV, and yield an effective mass m^* varying from 0.7 to 1.2 times the free electron mass m_e, increasing with carrier density from 0.6 × 10^{11} cm^{-2} to 1.2 × 10^{11} cm^{-2}. This mass enhancement arises from electron-electron interactions, contrasting with the much smaller band mass of bare electrons. The resonance position shifts with the effective field B^*, consistent with the composite fermion picture, and the mass dependence aligns with theoretical expectations of m^* \propto \sqrt{B} due to interaction effects.^[31]^[32] Far-infrared spectroscopy in 1990s experiments on high-mobility GaAs samples further supported these findings by detecting absorption peaks associated with composite fermion transitions at energies around 10–50 meV, though these are often compared to the higher-energy bare electron resonances in the same spectra. Tilted magnetic field studies in these setups revealed dependence on the perpendicular field component for orbital motion, while spin effects followed the total field, allowing identification of spin-flip transitions between composite fermion states at energies on the order of the Zeeman splitting.^[33]^[32] In higher Landau levels, multiple absorption branches have been observed, corresponding to inter-Landau level transitions of composite fermions, providing evidence for the quantized energy structure predicted by the theory. These branches appear as distinct peaks in the absorption spectra, confirming the formation of composite fermion Landau levels beyond the lowest levels.^[34]

Fermi Sea Formation

At filling factor \nu = 1/2, where the effective magnetic field experienced by composite fermions vanishes (B^* = 0), the two-dimensional electron system forms a compressible metallic state described by the Halperin-Lee-Read (HLR) theory as a Fermi sea of composite fermions. This state manifests as finite longitudinal Drude conductivity \sigma_{xx} \approx e^2/h, which remains largely temperature-independent over a wide range extending up to approximately 10 K, consistent with the robustness of the Fermi liquid phase against thermal excitations. Such behavior contrasts with the insulating or gapped states at nearby fractional fillings, highlighting the metallic compressibility unique to the half-filled Landau level.^[35] Thermodynamic measurements provide further evidence for this Fermi sea through the linear temperature dependence of the specific heat, C = \gamma T, where \gamma is the Sommerfeld coefficient. Experiments on high-mobility GaAs samples at \nu = 1/2 yield \gamma \approx 50-100 mJ/m² K², indicating a heavy effective mass for the composite fermions and aligning with the enhanced density of states expected in a Fermi liquid. This linear term persists down to temperatures as low as 0.14 K, supporting the formation of a well-defined Fermi surface despite interactions, with deviations at higher temperatures attributed to spin splitting effects on the order of 1-2 K. In 2024, scanning tunneling microscopy on graphene revealed fingerprints of composite fermion Lambda levels in fractional quantum Hall states, confirming their quasiparticle nature through characteristic peak structures in spectroscopy.^[36] Disorder in real samples can smear the sharp Fermi sea, leading to finite scattering rates that broaden the density of states and slightly suppress the conductivity at low temperatures. However, in high-quality systems with mean free paths exceeding the Fermi wavelength, the metallic character persists, as evidenced by the consistency of transport and thermodynamic data with a weakly interacting Fermi liquid model.^[35] This smearing effect underscores the role of sample purity in observing the pristine composite fermion sea.

Pairing Phenomena

The even-denominator fractional quantum Hall effect at filling factor \nu = 5/2 is a prominent example of pairing in composite fermion liquids, characterized by a gapped state with an energy gap \Delta \approx 0.15 meV that arises from the effective attraction between composite fermions in the second Landau level.^[37] This gap reflects the formation of a paired liquid, distinct from the compressible Fermi sea at \nu = 1/2. Transport experiments in Corbino geometry, which suppress edge contributions, have revealed nonzero downstream resistance at low bias, a signature interpreted as evidence for chiral p_x + i p_y pairing where counterpropagating neutral modes along the interface lead to incomplete Andreev reflection.^[38] These observations align with the superconductivity-like behavior expected from paired composite fermions, where the gap protects against thermal excitations and enables dissipationless transport in the bulk.^[39] Theoretical descriptions of the \nu = 5/2 state invoke either the Moore-Read Pfaffian state, featuring p-wave pairing of composite fermions into a chiral topological superconductor, or the anti-Pfaffian state, its particle-hole conjugate with opposite chirality for charged and neutral modes.^[40] Interferometry experiments conducted in the 2010s, utilizing Fabry-Pérot setups to enclose quasiparticles, have detected e/4-periodic interference patterns at low temperatures, with even-odd effects in the interference amplitude that favor non-Abelian braiding statistics characteristic of the Pfaffian or anti-Pfaffian over Abelian alternatives.^[41] These results, observed in high-mobility GaAs heterostructures, confirm the presence of non-Abelian anyons with fusion channels that depend on the pairing symmetry, providing direct experimental support for the paired nature of the ground state.^[42] Recent 2025 experiments observed a developing fractional quantum Hall state at \nu = 1/6 in wide quantum wells using magnetotransport measurements, suggesting possible f-wave pairing of six-flux composite fermions.^[43] In driven composite fermion systems, nonequilibrium conditions such as microwave irradiation or bias-driven transport induce proximity to superconductivity-like states, where enhanced pairing correlations suppress dissipation and yield zero-resistance regimes beyond equilibrium FQHE plateaus. Spin-singlet pairing emerges in unpolarized ground states at these fillings, stabilizing the gapped phases against Zeeman effects.^[44]

Recent Advances

Higher-Flux Composite Fermions

Higher-flux composite fermions extend the composite fermion paradigm by attaching four or six flux quanta (2p=4 or 2p=6) to electrons, enabling descriptions of fractional quantum Hall effect (FQHE) states at lower filling factors such as ν=1/4 and ν=1/6. This theoretical proposal, a generalization of Jain's framework, predicts new sequences of incompressible FQHE states following the Jain hierarchy ν = n/(2pn ± 1), where n is the integer filling of the effective composite fermion Landau levels. For 2p=6 (six-flux composite fermions), states like ν=1/7, 2/13, and particle-hole conjugates such as ν=9/11 emerge, offering a unified view of previously enigmatic low-filling phenomena through the formation of these emergent particles with enhanced flux binding.^[45] In 2024, transport experiments in ultrahigh-quality GaAs/AlGaAs two-dimensional hole systems provided direct evidence for six-flux composite fermions through the observation of the FQHE state at ν=9/11, manifesting as precise Hall quantization (R_xy = 11h/9e² with <0.15% deviation) and suppression of longitudinal resistance at low temperatures (7.6 mK). This state, interpreted as two filled effective Landau levels of six-flux composite fermions (ν*=2), exhibits topological protection via an activation energy gap of approximately 32 mK, consistent with edge-state mediated conduction and incompressibility. Complementary studies in similar systems have identified even-denominator FQHE states at ν=3/10, 3/8, and 3/4, attributed to pairing of six-flux (or effective four-flux) composite fermions in higher Λ levels, with transport signatures including deep R_xx minima and emerging R_xy plateaus at 25 mK, alongside pseudogaps of several hundred mK.^[45]^[46] Neutral excitations in these higher-flux states are described as fractional excitons, bound particle-hole pairs of composite fermions, which carry fractional charge and exhibit magneto-roton minima in their dispersion.^[47]^[48] These developments imply potential realizations of non-Abelian anyons with expanded fusion spaces, as pairing instabilities in six-flux composite fermion systems—analogous to the Moore-Read state at ν=5/2—could yield topological orders supporting richer quasiparticle fusion channels and enhanced fault-tolerant quantum computing applications.^[46]

Zero-Field Composite Fermion Systems

In twisted bilayer semiconductors, such as MoTe₂, moiré superlattices can host composite fermion liquids at zero external magnetic field through the attachment of emergent flux quanta arising from the nontrivial Chern number of the flat bands. This moiré-induced effective flux mimics the flux attachment mechanism in conventional composite fermion theory, enabling the formation of a Fermi liquid of composite fermions at half-filling of the moiré valence band (ν = -1/2). Theoretical predictions from exact diagonalization and density matrix renormalization group calculations indicate that these anomalous composite Fermi liquids exhibit non-Fermi liquid behavior, with a circular Fermi surface for the composite fermions and gapless metallic states centered around the Γ point in the [Brillouin zone](/page/Brillouin zone).^[49]^[50] Transport measurements in twisted MoTe₂ bilayers reveal metallic conductivity at half-filling, characterized by vanishing longitudinal resistivity in the clean limit and a large Hall angle approaching π/2, distinguishing the composite fermion phase from ordinary Fermi liquids. Upon doping away from half-filling, commensurability oscillations in resistivity are theoretically predicted to emerge, periodic in the inverse doping density (1/δρ_e), analogous to Shubnikov–de Haas oscillations but occurring in zero magnetic field due to the effective field felt by composite fermions proportional to the doping deviation (Δν = ν + 1/2). These signatures confirm the presence of a compressible composite fermion sea, with the effective mass enhanced logarithmically by gauge fluctuations, leading to specific heat coefficients s(T)/T ∼ log(1/T).^[51]^[52] Experimental evidence for the zero-field composite fermion sea in twisted MoTe₂ bilayers includes optical spectroscopy showing a sign reversal in the circular polarization of trion photoluminescence as a function of hole doping, occurring precisely at the critical doping where the composite fermion chemical potential crosses zero and time-reversal symmetry is spontaneously broken.^[53] This provides direct verification of the emergent composite fermion Fermi sea without magnetic field complications. The absence of external fields also opens possibilities for tunable superconducting pairing instabilities in these systems, potentially stabilized by proximity effects or intrinsic interactions in the moiré lattice. In 2025, further theoretical advances include studies on entanglement scaling and charge fluctuations in composite fermion Fermi liquids, confirming non-Fermi-liquid correlations through enhanced entanglement (Phys. Rev. B 111, 115119), and extensions of the dipole representation of composite fermions to graphene quantum Hall systems (Phys. Rev. B 111, 045132). Additionally, composite fermion theory has been applied to the stability of fractional Chern insulators, predicting robust topological phases (New J. Phys. 27, 043007). These developments underscore the expanding scope of composite fermion physics in zero-field and moiré systems as of November 2025.^[54]^[55]^[56]

References

[1]
https://arxiv.org/pdf/2011.13488.pdf
[2]
https://doi.org/10.1017/CBO9780511607561
[3]
Composite-fermion approach for the fractional quantum Hall effect
Jul 10, 1989 · In this paper a new possible approach for understanding the fractional quantum Hall effect is presented. ... J. K. Jain (to be published).Missing: original | Show results with:original
[4]
https://doi.org/10.1063/1.883035
[5]
Discovery of an insulating ferromagnetic phase of electrons in two ...
The two-dimensional electron gas (2DEG) realized in semiconductor heterostructures has been the focus of fruitful study for decades.
[6]
[PDF] The Quantum Hall Effect
As the title suggests, this book focuses on the composite fermion approach as a lens through which to view all aspects of the quantum Hall effect.
[7]
Anomalous Quantum Hall Effect: An Incompressible Quantum Fluid ...
May 2, 1983 · Anomalous Quantum Hall Effect: An Incompressible Quantum Fluid with Fractionally Charged Excitations. R. B. Laughlin.
[8]
Theory of the half-filled Landau level | Phys. Rev. B
Mar 15, 1993 · A two-dimensional electron system in an external magnetic field, with Landau-level filling factor ν=1/2, can be transformed to a mathematically equivalent ...Missing: composite | Show results with:composite
[9]
Composite-fermion effective masses | Phys. Rev. B
Oct 15, 1995 · Effective masses deduced from the temperature dependence of the Shubnikov–de Haas ... Kang, D. C. Tsui, L. N. Pfeiffer, K. W. Baldwin and ...
[10]
Exact Quantization of the Even-Denominator Fractional Quantum ...
Oct 25, 1999 · We report ultralow temperature experiments on the obscure fractional quantum Hall effect at Landau level filling factor ν=5/2 in a very high-mobility specimen.
[11]
[2011.13488] Thirty Years of Composite Fermions and Beyond - arXiv
Nov 26, 2020 · Access Paper: View a PDF of the paper titled Thirty Years of Composite Fermions and Beyond, by J. K. Jain. View PDF · TeX Source · Other Formats.Missing: original | Show results with:original<|control11|><|separator|>
[12]
Jainendra Jain named 2025 Wolf Prize laureate in physics
Mar 10, 2025 · Jainendra Jain named 2025 Wolf Prize laureate in physics. Penn State physicist pioneered research that predicted a new class of exotic particles ...
[13]
https://doi.org/10.1103/PhysRevLett.63.199
[14]
Paired Composite Fermion Phase of Quantum Hall Bilayers at
The generated trial wave function (for N / 2 electrons in each layer) occurs on the sphere with a monopole flux N ϕ = 2 ( N / 2 - 1 ) + M for the case of M - ...
[15]
[PDF] THE COMPOSITE FERMION: A QUANTUM PARTICLE AND ITS ...
This composite fermion experiences a magnetic field drastically different from the external one, and its kinetic energy manifests itself through a Fermi sea and ...Missing: 1994 | Show results with:1994
[16]
[cond-mat/9709093] Effective Mass of Composite Fermions ... - arXiv
We work with the standard Halperin-Lee-Read model but in the temporal gauge. The advantage of this gauge to be employed is that the finite effective mass could ...
[17]
Stability and Effective Masses of Composite Fermions in the First ...
Jun 19, 1995 · These show that, in the absence of strong Landau level (LL) mixing effects, the CF liquid cannot form for a half-filled second LL. We relate ...
[18]
Effective mass of composite fermions and fermionic Chern-Simons ...
Apr 15, 1998 · The definitions of the effective mass for the composite fermion are discussed in half filled lowest-Landau-level systems. In a recent work, ...
[19]
Composite fermions and bosons: An invitation to electron ... - PNAS
Aug 3, 1999 · Electrons having spin ½ are fermions. In our three-dimensional (3D) world, the distinction between fermions and bosons is strictly enforced.Composite Fermions And... · Abstract · Sign Up For Pnas Alerts<|control11|><|separator|>
[20]
[PDF] Measurements of the Composite Fermion masses from the spin ...
Measurements of the reflectivity of a 2-D electron gas are used to deduce the polarization of the. Composite Fermion ... 0.2. 0.3. 0.4. 0.5. 0.6. 0.7. 0.8. 0.9.
[21]
Spin polarization of the ν = 5 / 2 quantum Hall state
Mar 30, 2009 · We numerically study the spin polarization of the fractional quantum Hall state at filling factor ν = 5 / 2.
[22]
Direct Measurement of the g-Factor of Composite Fermions
Aug 6, 2025 · Effective composite-fermion filling factors up to ν ∗ = 4 are visible and higher order composite-fermion states (with four flux quanta ...
[23]
Direct Measurement of the $g$ Factor of Composite Fermions
Apr 16, 2004 · This feature allows a direct measurement of the 𝑔 factor of composite fermions which appears to be heavily renormalized by interactions and ...
[24]
Fractionally charged skyrmions in fractional quantum Hall effect - PMC
Nov 26, 2015 · This work is concerned with minimal fractional skyrmions, namely the skyrmions for which a composite fermion (CF) particle or hole is dressed by ...
[25]
Composite Fermions in the Quantum Hall Regime - Science
This system is characterized by the existence of a particle called the composite fermion, which manifests itself in several dramatic experimental observations.Composite Fermions In The... · Abstract · Current Issue<|control11|><|separator|>
[26]
Microscopic theory of the fractional quantum Hall effect
Jul 28, 2006 · This is a review of the composite fermion theory of the fractional quantum Hall effect (FQHE). This theory provides a microscopic ...<|control11|><|separator|>
[27]
Composite Fermion Theory of Exotic Fractional Quantum Hall Effect
The fractional quantum Hall effect (FQHE) arises from strong correlations between electrons when they are confined to two dimensions and exposed to a strong ...<|control11|><|separator|>
[28]
[PDF] Additional background material on the Nobel Prize in Physics 1998
If the filling factor is a fraction, f=1/3 for instance, there is no energy gap in the independent electron model that defines the Landau levels. The gap of a ...
[29]
Fractional Quantum Hall Effect Energy Gaps: Role of Electron Layer ...
Jul 28, 2021 · Energy gaps of the 1/3 fractional quantum Hall effect states in GaAs 2D electron systems decrease with increasing layer thickness.
[30]
[cond-mat/9604146] Dephasing time of composite fermions - arXiv
Apr 23, 1996 · This result is used to describe the suppression of Shubnikov-de Haas (SdH) oscillations of composite fermions (oscillations in the conductivity ...
[31]
https://doi.org/10.1038/415409a
[32]
https://doi.org/10.1103/PhysRevLett.75.2989
[33]
[PDF] An optically detected cyclotron resonance study of bulk GaAs
May 18, 2014 · Far-infrared (FIR) magnetospectroscopy employing transmission and absorption measurements has been ex- tensively used to study band ...Missing: composite fermion
[34]
[PDF] Optical properties of a two-dimensional electron gas in magnetic fields.
Oct 19, 2005 · The cyclotron resonance of composite fermions experiments showed that. CFs execute a cyclotron motion when the magnetic field is changed from.
[35]
Experimental evidence for composite fermions - IOPscience
Emphasized are surface acoustic wave experiments which were responsible for measuring the composite fermion Fermi wavevector and mean free path. Also reviewed ...Missing: confirmation | Show results with:confirmation
[36]
Intrinsic Gap of the Fractional Quantum Hall State | Phys. Rev. Lett.
The 𝜈 = 5 2 excitation gap at 2.63 T is measured to be 2 6 2 ± 1 5 m K , similar to values obtained in samples with twice the electronic density. Examining the ...<|control11|><|separator|>
[37]
Stability of the 5/2 Fractional Quantum Hall State in a Corbino Disc ...
Feb 4, 2018 · In this work we explore the influence of in-plane electric fields on the stability of the 5/2 fractional quantum Hall state realized in Corbino ...Missing: gap energy 0.15 meV downstream resistance
[38]
Andreev Reflection in the Fractional Quantum Hall State
Jun 14, 2022 · ... downstream resistance R d = h 2 q N 2 e 2 R_d=\frac{h}{2q_N^2e^2} measurable in a FQH-SC junction. For edge states staying in the KFP phase ...
[39]
Edge states and interferometers in the Pfaffian and anti-Pfaffian ...
We quantitatively compare two possible candidates for the state at 𝜈 = 5 2 : the Moore-Read Pfaffian state, and its particle-hole conjugate, the anti-Pfaffian.Missing: 2010s | Show results with:2010s
[40]
Measurement of filling factor 5/2 quasiparticle interference ... - PNAS
Oscillations consistent in detail with the Aharonov–Bohm effect are observed for integer quantum Hall and FQH states (filling factors ν = 2, 5/3, and 7/3) with ...
[41]
Interference Measurements of Non-Abelian &amp
Mar 1, 2023 · Fractional quantum Hall (FQH) states are the best established examples of topological phases with FQH states at filling fractions ν = 5 / 2 and ...
[42]
Recent experimental progress of fractional quantum Hall effect: 5/2 ...
Abstract. The phenomenon of fractional quantum Hall effect (FQHE) was first experimentally observed 33 years ago. FQHE involves strong Coulomb interactions.
[43]
Evidence for Topological Protection Derived from Six-Flux ... - Nature
Feb 17, 2024 · The composite fermion theory opened a new chapter in understanding many-body correlations through the formation of emergent particles.Missing: post- | Show results with:post-
[44]
Next-generation even-denominator fractional quantum Hall states of ...
This is generally attributed to the larger effective mass of 2D holes ( ≃ 0.5 m 0 ) compared to GaAs 2D electrons ( 0.067 m 0 ), and the ensuing severe LL ...Missing: m_e | Show results with:m_e
[45]
Collective excitations of composite fermions across multiple Λ levels
May 10, 2009 · Exact diagonalization studies of the FQHE state do not by themselves ... Experimental evidence for new particles in the fractional quantum Hall ...
[46]
Composite fermion theory of excitations in the fractional quantum ...
This paper reviews the status of our theoretical understanding of these excitations. Introduction. Transport experiments told us quite early on that there is a ...
[47]
Zero-Field Composite Fermi Liquid in Twisted Semiconductor Bilayers
Certain twisted semiconductor bilayers are predicted to host a Fermi liquid of composite fermions---remarkably, without an applied magnetic field.
[48]
Zero-field composite Fermi liquid in twisted semiconductor bilayers
Jun 5, 2023 · Here we argue that a composite fermion description, already a unifying framework for the phenomenology of 2d electron gases at high magnetic ...Missing: graphene | Show results with:graphene
[49]
Composite Fermi Liquid at Zero Magnetic Field in Twisted M o T e 2
Sep 27, 2023 · We discover a composite Fermi liquid (CFL) phase even at zero magnetic field that covers a large portion of the phase diagram near twist angle.
[50]
Composite Fermi Liquid at Zero Magnetic Field in Twisted MoTe$_2
We discover a composite Fermi liquid (CFL) phase even at zero magnetic field that covers a large portion of the phase diagram centered around twist angle.