11.2 Mott insulators

Mott insulators are fascinating materials that defy conventional wisdom about electrical conductivity. Unlike typical insulators, they should conduct electricity based on band theory, but strong electron interactions keep them insulating. This unique behavior stems from partially filled d or f orbitals in transition metals or rare earth compounds.

Understanding Mott insulators is crucial for grasping how electron interactions shape material properties. These materials exhibit large energy gaps, unique magnetic and optical characteristics, and can transition between insulating and metallic states under certain conditions. Their study has led to breakthroughs in our understanding of strongly correlated electron systems.

Fundamentals of Mott insulators

• Mott insulators represent a unique class of materials in condensed matter physics exhibiting insulating behavior due to strong electron-electron interactions
• Understanding Mott insulators provides crucial insights into the interplay between electronic correlations and material properties in solid-state systems

Definition and basic properties

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• Materials that should conduct electricity according to band theory but instead behave as insulators due to strong electron-electron interactions
• Characterized by partially filled d or f orbitals in transition metal or rare earth compounds
• Exhibit a large energy gap in the electronic spectrum despite having an odd number of electrons per unit cell
• Display unique magnetic and optical properties stemming from localized electronic states

Historical context and discovery

• Named after Sir Nevill Mott who proposed the concept in the late 1930s to explain insulating behavior in transition metal oxides
• Challenged the conventional band theory of solids which failed to predict the insulating nature of certain materials
• Pioneering work on nickel oxide (NiO) led to the development of the Mott-Hubbard model
• Subsequent research expanded to various transition metal compounds (vanadium oxides, cuprates) and rare earth materials

Electronic structure of Mott insulators

• Electronic structure of Mott insulators differs significantly from conventional band insulators or metals
• Understanding the electronic structure involves considering strong correlations and many-body effects

Band theory limitations

• Conventional band theory predicts metallic behavior for materials with partially filled bands
• Fails to account for strong electron-electron interactions in Mott insulators
• Neglects the energy cost of double occupancy of lattice sites by electrons
• Inability to explain the insulating behavior of materials like nickel oxide (NiO) and vanadium oxide (V2O3)

Hubbard model

• Simplest model capturing the essential physics of Mott insulators
• Describes electrons moving on a lattice with on-site Coulomb repulsion U and hopping integral t
• Hamiltonian: $H = -t \sum_{<i,j>,\sigma} (c_{i\sigma}^\dagger c_{j\sigma} + h.c.) + U \sum_i n_{i\uparrow} n_{i\downarrow}$
• Predicts a metal-insulator transition when U/t exceeds a critical value

Mott-Hubbard transition

• Transition from a metallic to an insulating state as electron correlations increase
• Occurs when the on-site Coulomb repulsion U becomes larger than the bandwidth W
• Characterized by the opening of a gap in the density of states at the Fermi level
• Can be induced by pressure, temperature, or chemical doping in some materials (V2O3, organic salts)

Electron correlation effects

• Electron correlations play a crucial role in determining the properties of Mott insulators
• Understanding these effects is essential for accurately describing the behavior of strongly correlated electron systems

Coulomb repulsion

• Strong electrostatic repulsion between electrons occupying the same lattice site
• Leads to localization of electrons and suppression of charge fluctuations
• Magnitude of Coulomb repulsion U typically ranges from 1-10 eV in transition metal oxides
• Competes with the kinetic energy of electrons, determining the insulating or metallic nature of the material

Exchange interaction

• Arises from the Pauli exclusion principle and Coulomb interaction between electrons
• Favors parallel alignment of electron spins on neighboring sites
• Responsible for magnetic ordering in many Mott insulators
• Can be described by the Heisenberg model: $H = J \sum_{<i,j>} \mathbf{S}_i \cdot \mathbf{S}_j$

Spin-orbit coupling

• Interaction between an electron's spin and its orbital angular momentum
• Becomes significant in materials containing heavy elements (5d transition metals, rare earths)
• Can lead to exotic magnetic states and topological phases in Mott insulators
• Modifies the electronic structure and magnetic anisotropy of the material

Types of Mott insulators

• Mott insulators encompass a diverse range of materials with varying chemical compositions and physical properties
• Understanding different types of Mott insulators provides insights into the universality of electron correlation effects

Transition metal oxides

• Most common and well-studied class of Mott insulators
• Include compounds like NiO, CoO, and V2O3
• Characterized by partially filled 3d orbitals and strong electron correlations
• Often exhibit antiferromagnetic ordering at low temperatures
• Display rich phase diagrams with metal-insulator transitions and various magnetic states

Rare earth compounds

• Mott insulators containing lanthanide or actinide elements
• Feature partially filled 4f or 5f orbitals with strong local moments
• Examples include cerium compounds (CeO2) and samarium hexaboride (SmB6)
• Often exhibit complex magnetic structures and heavy fermion behavior
• Can host exotic quantum phases like topological Kondo insulators

Organic Mott insulators

• Molecular crystals composed of organic molecules with unpaired electrons
• Include materials like κ-(BEDT-TTF)2Cu[N(CN)2]Cl and κ-(BEDT-TTF)2Cu2(CN)3
• Characterized by low bandwidth and strong electron correlations
• Often display pressure-induced superconductivity and quantum spin liquid states
• Provide a platform for studying Mott physics in low-dimensional systems

Experimental techniques

• Various experimental methods are employed to probe the electronic and magnetic properties of Mott insulators
• These techniques provide complementary information about the electronic structure, charge dynamics, and magnetic ordering

Optical spectroscopy

• Measures the frequency-dependent optical conductivity of Mott insulators
• Reveals information about the charge gap and electronic excitations
• Techniques include infrared and visible spectroscopy, ellipsometry
• Can probe the temperature and pressure dependence of the Mott gap
• Useful for studying the evolution of electronic structure across metal-insulator transitions

Photoemission spectroscopy

• Directly probes the electronic structure and occupied states of Mott insulators
• Includes angle-resolved photoemission spectroscopy (ARPES) for momentum-resolved measurements
• Reveals the presence of lower and upper Hubbard bands characteristic of Mott insulators
• Can measure the evolution of spectral weight transfer across metal-insulator transitions
• Provides information about quasiparticle dynamics and many-body effects

X-ray absorption spectroscopy

• Probes unoccupied electronic states in Mott insulators
• Sensitive to the local electronic environment and oxidation state of transition metal ions
• Techniques include X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)
• Can provide information about the crystal field splitting and hybridization effects
• Useful for studying the electronic structure of doped Mott insulators and charge transfer insulators

Physical properties

• Mott insulators exhibit unique physical properties that distinguish them from conventional insulators and metals
• These properties arise from the interplay between charge, spin, and orbital degrees of freedom

Electrical conductivity

• Mott insulators display insulating behavior at low temperatures due to strong electron correlations
• Temperature-dependent resistivity often follows an activated behavior: $\rho(T) \propto \exp(E_g/2k_BT)$
• Some Mott insulators undergo metal-insulator transitions with changes in temperature, pressure, or doping
• Doped Mott insulators can exhibit unusual transport properties like non-Fermi liquid behavior

Magnetic ordering

• Many Mott insulators develop long-range magnetic order at low temperatures
• Common magnetic structures include antiferromagnetic, ferromagnetic, and more complex orderings
• Magnetic transition temperatures can range from a few Kelvin to several hundred Kelvin
• Magnetic excitations (magnons) can be probed using neutron scattering techniques
• Some Mott insulators host exotic magnetic states like quantum spin liquids

Thermal properties

• Specific heat of Mott insulators often shows contributions from both lattice and magnetic degrees of freedom
• Magnetic specific heat can exhibit anomalies at ordering temperatures
• Thermal conductivity influenced by both phonons and magnetic excitations
• Some Mott insulators display unusual thermoelectric properties due to strong electron correlations
• Thermal expansion can be sensitive to changes in magnetic ordering and electronic structure

Mott vs band insulators

• Mott insulators and band insulators represent two distinct classes of insulating materials
• Understanding their differences is crucial for correctly interpreting experimental results and designing new materials

Energy gap formation

• Band insulators: gap forms due to the periodic potential of the lattice
• Mott insulators: gap opens due to strong electron-electron interactions
• Band gap typically larger in band insulators (several eV) compared to Mott gap (0.1-2 eV)
• Mott gap can be tuned by pressure, chemical substitution, or external fields
• Spectral weight transfer across the gap distinguishes Mott insulators from band insulators

Temperature dependence

• Band insulators: gap relatively insensitive to temperature changes
• Mott insulators: gap can show strong temperature dependence
• Some Mott insulators undergo temperature-driven metal-insulator transitions
• Thermal excitations in Mott insulators can lead to the formation of in-gap states
• Temperature dependence of conductivity often more complex in Mott insulators

Doping effects

• Band insulators: doping introduces carriers into conduction or valence bands
• Mott insulators: doping can lead to collapse of the Mott gap and metallization
• Doped Mott insulators often exhibit unconventional metallic states
• Chemical substitution in Mott insulators can induce phase transitions and emergent phenomena
• Doping-induced superconductivity observed in some Mott insulators (cuprates)

Applications and technological relevance

• Mott insulators and related materials have potential applications in various technological domains
• Understanding and controlling Mott physics opens new avenues for device engineering and quantum technologies

High-temperature superconductivity

• Many high-temperature superconductors derived from doped Mott insulators (cuprates, iron-based superconductors)
• Understanding Mott physics crucial for unraveling the mechanism of high-Tc superconductivity
• Potential applications in power transmission, magnetic levitation, and sensitive detectors
• Challenges include improving critical temperatures and developing practical wire fabrication techniques

Spintronics devices

• Mott insulators with strong spin-orbit coupling can host topological states useful for spintronics
• Antiferromagnetic Mott insulators proposed as active elements in memory devices
• Potential for ultrafast switching and high-density information storage
• Spin injection and detection in Mott insulator heterostructures for spin-based logic devices
• Challenges include optimizing material properties and integrating with existing semiconductor technologies

Quantum materials

• Mott insulators serve as a platform for realizing exotic quantum states of matter
• Quantum spin liquids in organic and inorganic Mott insulators for quantum computation
• Topological Mott insulators for robust quantum information processing
• Mott insulators in optical lattices for quantum simulation of strongly correlated systems
• Potential applications in quantum sensing, metrology, and next-generation quantum technologies

Theoretical approaches

• Various theoretical methods have been developed to describe the complex behavior of Mott insulators
• These approaches aim to capture the interplay between charge, spin, and orbital degrees of freedom

Dynamical mean-field theory

• Non-perturbative approach for studying strongly correlated electron systems
• Maps the lattice problem onto a self-consistent impurity problem
• Captures local quantum fluctuations and Mott metal-insulator transition
• Can be combined with density functional theory for realistic materials calculations
• Limitations include neglecting non-local correlations and challenges in treating multi-orbital systems

Density functional theory

• Ab initio method for calculating electronic structure of materials
• Standard DFT often fails for strongly correlated systems like Mott insulators
• DFT+U method introduces an on-site Coulomb interaction to improve description of localized states
• Hybrid functionals and meta-GGA functionals can partially capture some correlation effects
• Combining DFT with many-body techniques (DFT+DMFT) improves accuracy for Mott insulators

Quantum Monte Carlo simulations

• Numerical technique for solving many-body quantum systems
• Can provide exact solutions for model Hamiltonians like the Hubbard model
• Methods include determinant quantum Monte Carlo and auxiliary-field quantum Monte Carlo
• Useful for studying finite-temperature properties and dynamical correlations
• Limitations include the fermionic sign problem for certain models and materials

Current research and challenges

• Mott insulators remain an active area of research in condensed matter physics
• Ongoing efforts focus on discovering new materials, understanding emergent phenomena, and developing novel applications

Novel Mott materials

• Search for new classes of Mott insulators with unique properties
• Exploration of 4d and 5d transition metal compounds with strong spin-orbit coupling
• Investigation of mixed-valence systems and charge-transfer insulators
• Design of artificial Mott insulators in engineered heterostructures and superlattices
• Challenges include synthesizing high-quality samples and controlling material properties

Mott physics in low dimensions

• Study of Mott insulators in reduced dimensions (2D materials, 1D chains)
• Investigation of metal-insulator transitions and quantum criticality in low-dimensional systems
• Exploration of exotic phases like spin liquids and topological Mott insulators
• Development of new theoretical tools for treating strong correlations in low dimensions
• Experimental challenges in isolating and characterizing low-dimensional Mott systems

Non-equilibrium dynamics

• Investigation of ultrafast dynamics and photo-induced phase transitions in Mott insulators
• Study of non-equilibrium states and metastable phases using pump-probe spectroscopy
• Exploration of light-induced superconductivity and other emergent phenomena
• Development of theoretical frameworks for describing non-equilibrium strongly correlated systems
• Challenges in interpreting complex time-dependent responses and separating electronic and lattice effects