11.2 Terahertz characterization of superconductors and quantum materials

Terahertz spectroscopy is a game-changer for studying superconductors and quantum materials. It lets scientists peek into the weird world of zero resistance and exotic quantum states, revealing secrets hidden in the low-energy realm.

This powerful tool matches the energy scales of superconducting gaps and quantum effects perfectly. It's like having x-ray vision for the quantum world, showing us how these materials tick and paving the way for mind-bending new technologies.

Superconductors and Quantum Materials in Terahertz

Unique Properties of Superconductors and Quantum Materials

• Superconductors exhibit zero electrical resistance and perfect diamagnetism below a critical temperature with energy gaps typically in the terahertz range
• Quantum materials possess exotic electronic states governed by quantum mechanical effects (topological insulators, Weyl semimetals)
• Terahertz frequency range (0.1-10 THz) corresponds to energies of 0.4-41 meV matching many characteristic energies in superconductors and quantum materials
• Terahertz spectroscopy probes low-energy excitations, collective modes, and quasiparticle dynamics with high temporal and spectral resolution
  • Reveals information about superconducting condensate and quasiparticle contributions through complex conductivity measurements
  • Allows investigation of temperature and magnetic field dependence of material properties

Significance of Terahertz Range for Material Characterization

• Terahertz energies align with superconducting gap energies enabling direct probing of superconducting properties
• Matches characteristic energies of quantum materials facilitating investigation of exotic quantum states
• Provides access to low-energy excitations and collective modes crucial for understanding material behavior
• Enables high-resolution spectroscopy for detecting subtle features in electronic structure
• Allows non-contact measurements suitable for sensitive samples and in-situ characterization

Terahertz Characterization Techniques for Superconductors

Time-Domain Terahertz Spectroscopy (TDTS)

• Measures both amplitude and phase of transmitted or reflected terahertz field
• Extracts optical conductivity to determine superconducting energy gap and its temperature dependence
• Enables analysis using Mattis-Bardeen theory describing frequency-dependent conductivity of superconductors
• Provides information on superfluid density temperature dependence revealing superconducting order parameter symmetry
• Can be combined with other techniques (magnetic fields, pressure) to study interplay between superconductivity and competing orders

Terahertz Pump-Probe Spectroscopy

• Studies non-equilibrium dynamics in superconductors
• Investigates pair-breaking and recombination processes
• Allows observation of ultrafast phenomena on picosecond timescales
• Provides insights into quasiparticle dynamics and relaxation mechanisms
• Enables study of non-equilibrium superconducting states

Specialized Terahertz Techniques

• Terahertz emission spectroscopy probes dynamics of Josephson plasma oscillations in layered superconductors
• Terahertz circular dichroism measurements reveal chirality of Weyl fermions and topological surface states
• Combines terahertz spectroscopy with varying temperatures and magnetic fields to study phase transitions
• Utilizes high-resolution spectroscopy to detect subtle features in electronic structure

Terahertz Response of Quantum Materials

Probing Exotic Electronic States

• Investigates Dirac or Weyl fermion response in topological materials revealing unique band structure and electron dynamics
• Measures optical conductivity providing information about charge carrier density, mobility, and effective mass
• Studies terahertz response of quantum spin liquids offering evidence for fractionalized excitations (spinons)
• Examines temperature and magnetic field dependence of terahertz properties to reveal phase transitions and emergent phenomena

Dynamic Investigations and Collective Modes

• Conducts terahertz pump-probe experiments to study ultrafast switching of topological states and non-equilibrium phases
• Probes collective modes crucial for understanding quantum material properties (magnons, phonons)
• Investigates ultrafast dynamics of charge carriers and quasiparticles
• Examines coherent control of quantum states using terahertz pulses

Advanced Characterization Methods

• Employs terahertz circular dichroism to study chiral properties of topological materials
• Utilizes terahertz magneto-optical measurements to investigate Berry curvature and topological invariants
• Combines terahertz spectroscopy with other techniques (angle-resolved photoemission spectroscopy, scanning tunneling microscopy) for comprehensive material characterization
• Develops novel terahertz techniques for probing specific quantum material properties (terahertz nonlinear spectroscopy, terahertz near-field microscopy)

Terahertz Characterization for Superconductors and Quantum Materials

Advancing Scientific Understanding

• Provides unique insights into low-energy excitations and dynamics often inaccessible by other experimental methods
• Plays crucial role in validating theoretical models and predictions for novel superconducting and topological states
• Contributes to fundamental understanding of quantum phenomena and emergent properties in complex materials
• Enables investigation of subtle effects and interactions in superconductors and quantum materials
• Facilitates discovery and characterization of new quantum phases and topological states

Technological Applications and Device Development

• Contributes to development of novel superconducting devices (mixers, detectors, quantum information processing elements)
• Aids in design and optimization of quantum devices based on topological materials
• Supports advancement of terahertz technology for communications, sensing, and imaging applications
• Enables characterization and quality control of superconducting and quantum material-based devices
• Facilitates development of new materials and heterostructures for quantum computing and sensing applications