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Non Fermi LiquidEdit

Non Fermi liquid behavior describes electronic states in which the conventional Landau Fermi liquid picture—where low-energy excitations are long-lived quasiparticles with a one-to-one correspondence to free electrons—breaks down. In these systems, interactions among electrons are so strong or so intricately entangled with other degrees of freedom that the standard quasiparticle description ceases to be reliable at low energies. The phenomenon has been observed in a range of materials, most prominently in certain heavy-fermion compounds and in some families of high-temperature superconductors, but it also appears in engineered two-dimensional systems and other correlated electron materials. For a broader context, see Fermi liquid and non-Fermi liquid.

In the traditional view, a metal behaves as a Fermi liquid at low temperatures: the resistivity grows as T^2 due to electron-electron scattering, the specific heat coefficient C/T tends to a constant, and the electronic spectrum features sharp quasiparticle peaks. Non Fermi liquid states, by contrast, display unconventional power laws, anomalous temperature dependences, and often a breakdown of sharp quasiparticle peaks in spectral measurements. A hallmark of many NFL systems is a resistivity that scales linearly with temperature over an extended range, rather than the quadratic temperature dependence expected from a Landau Fermi liquid. Other signatures include non-quadratic temperature dependences in the specific heat and susceptibility, and anomalous scaling of the electronic self-energy and spectral functions. See strange metal and quantum critical point for related ideas.

Overview

Non Fermi liquid behavior is not a single, monolithic phenomenon but a broad umbrella that covers several distinct mechanisms by which the quasiparticle concept can fail. In some materials, NFL behavior arises in the vicinity of a continuous phase transition at zero temperature, a quantum critical point, where critical fluctuations of an order parameter scatter electrons so strongly that coherent quasiparticles cannot form. In others, NFL-like physics emerges from the coupling of itinerant electrons to fluctuating gauge fields, or from competing electronic orders in multi-band systems. See quantum phase transition and Kondo effect as relevant starting points for particular mechanisms.

NFL behavior also appears in systems with strong disorder, where rare-region effects (Griffiths physics) or inhomogeneous electronic states can mimic non-quasiparticle dynamics over a broad temperature window. In some cuprate materials and related oxides, the so-called strange metal regime has been linked to a high level of electronic correlations and proximity to various competing orders. See cuprate superconductors and heavy fermion for well-studied examples.

Mechanisms and theoretical approaches

  • Quantum critical fluctuations: Near a quantum critical point, critical modes associated with the ordering tendency (e.g., spin or charge density waves) can scatter electrons efficiently, washing out quasiparticles. This framework is central to many discussions of NFL behavior and has been explored in the context of quantum critical point in heavy-fermion compounds such as YbRh2Si2 and in some cuprates. See Hertz-Millis-Moryia theory for a classic starting point, and note criticisms and extensions that address deviations found in experiments. See also Dynamical mean-field theory approaches that attempt to treat strong correlations more microscopically.

  • Kondo breakdown and local quantum criticality: In certain heavy-fermion systems, the breakdown of Kondo screening at a quantum critical point can lead to a collapse of a large Fermi surface and to NFL behavior. This scenario is often discussed under the umbrella of local quantum criticality.

  • Gauge-field and fractionalization pictures: The coupling of electrons to emergent gauge fields in two dimensions can prevent the formation of stable quasiparticles, yielding NFL dynamics. These ideas are explored in models where electrons effectively decouple into separate degrees of freedom, sometimes invoked to describe spin-liquid-like states or certain strange-metal regimes.

  • Marginal and Planckian scenarios: Some phenomenological descriptions invoke a marginal Fermi liquid picture to capture non-quadratic self-energy, while other approaches emphasize universal scattering rates (so-called Planckian dissipation) that saturate a bound set by fundamental constants. See marginal Fermi liquid and discussions of Planckian transport in NFL contexts.

  • Strongly interacting multi-band systems: When multiple electronic bands with differing characters interact strongly, the low-energy spectrum can fail to organize into well-defined quasiparticles, especially near points of accidental or symmetry-protected degeneracy. This is often analyzed with computational tools such as Dynamical mean-field theory and its cluster extensions.

  • Holographic and AdS/CFT-inspired models: In some theoretical developments, techniques borrowed from high-energy physics provide qualitative insight into NFL behavior by modeling strongly coupled systems with dual gravitational descriptions. See AdS/CFT for a general reference point, while recognizing that these models are often criticized for limited microscopic specificity when applied to real materials.

Signatures and measurements

  • Transport: Linear-in-T resistivity over a broad range, sometimes extending to quite low temperatures, is a common experimental fingerprint. Other transport properties may show unconventional scaling that does not fit the Fermi-liquid T^2 paradigm.

  • Thermodynamics: The specific heat coefficient and magnetic susceptibility can show non-Fermi-liquid scaling, such as a divergent or non-constant C/T as T → 0, or anomalies in the temperature dependence that do not align with Landau theory.

  • Spectroscopy: Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) often reveal broadened spectral features and the absence of sharp quasiparticle peaks where one would expect them in a Fermi liquid. See ARPES and STM for related experimental probes.

  • Magnetic and charge fluctuations: Neutron scattering and other probes detect persistent fluctuations that may be slow to order or compete with incipient phases, consistent with NFL pictures driven by critical dynamics.

Materials and systems

  • Heavy-fermion metals: Compounds such as CeCu6, YbRh2Si2, and related Ce- and Yb-based intermetallics have displayed NFL behavior in temperature and tuning parameter (pressure, doping, magnetic field) space, often linked to proximity to quantum criticality or to Kondo breakdown scenarios. See heavy fermion for broader context.

  • Cuprate and related oxides: Certain copper-oxide superconductors exhibit a strange-metal phase with anomalous transport and thermodynamics that challenge standard quasiparticle descriptions. See cuprate superconductors and strange metal for more.

  • Iron-based superconductors: Some members of the iron pnictide/chalcogenide families show NFL-like behavior in specific doping ranges, offering a platform to study how multiple orbitals and competing orders influence low-energy excitations. See iron-based superconductors.

  • Two-dimensional electron systems and twisted layers: In high-midelity 2D electron gases and in moiré superlattices such as twisted bilayer graphene, NFL-like transport can appear near correlated or near-flat-band regimes, highlighting the role of reduced dimensionality and strong interactions. See two-dimensional electron gas and twisted bilayer graphene.

Controversies and debates

  • Universality vs material specificity: A key debate centers on whether NFL behavior reflects universal critical dynamics or whether it is strongly material-specific, arising from disorder, inhomogeneity, or particular band structures. Critics caution against over-generalizing from a subset of materials to all NFLs.

  • Realism of quantum-critical explanations: While quantum criticality provides an attractive organizing principle, some experiments imply that NFL-like scaling can occur over broad ranges without a clearly identifiable quantum critical point, or that other intertwined orders complicate a clean interpretation.

  • Role of disorder and inhomogeneity: In some systems, disorder-induced effects can mimic NFL signatures or broaden crossover regions, making it difficult to disentangle intrinsic many-body physics from extrinsic sample quality. This has spurred calls for cleaner materials and controlled disorder studies.

  • The predictive value of various theoretical frameworks: Classic approaches such as [Hertz–Millis–Moriya theory] can capture certain aspects of quantum critical NFL behavior but struggle with others observed in real materials. Alternative frameworks (Kondo-breakdown scenarios, gauge-field theories, holographic models) have sparked lively debate over what is essential to explain NFL phenomenology and what is merely attractive mathematically.

  • Relation to high-temperature superconductivity: NFL behavior often accompanies the emergence of unconventional superconductivity in several families of materials. The extent to which NFL physics is a precursor, competitor, or consequence of superconductivity remains a subject of ongoing investigation and debate.

See also