⚛️Solid State Physics Unit 9 – Superconductivity

Introduction to Superconductivity

• Superconductivity phenomenon where certain materials exhibit zero electrical resistance and expel magnetic fields below a characteristic critical temperature ($T_c$)
• Discovered by Dutch physicist Heike Kamerlingh Onnes in 1911 while studying the electrical properties of mercury at extremely low temperatures
• Requires cooling materials to very low temperatures, typically below 20 Kelvin (-253°C or -424°F) for conventional superconductors
• Characterized by the formation of Cooper pairs, where electrons with opposite spins and momenta become bound together, allowing them to move through the material without resistance
• Enables the flow of electrical current without energy dissipation, leading to potential applications in power transmission, energy storage, and high-performance electronics
• Exhibits the Meissner effect, where a superconductor expels magnetic fields from its interior, making it a perfect diamagnet
  • This effect allows superconductors to levitate above strong magnets, demonstrating their unique magnetic properties

Historical Background

• Superconductivity first observed in mercury by Heike Kamerlingh Onnes in 1911 at the University of Leiden, Netherlands
• Onnes was studying the electrical properties of pure metals at extremely low temperatures using liquid helium as a coolant
• Discovered that the electrical resistance of mercury abruptly dropped to zero at a temperature of 4.2 Kelvin (-269°C or -452°F)
• This marked the first observation of superconductivity and led to further research into the phenomenon
• In 1933, Walther Meissner and Robert Ochsenfeld discovered that superconductors expel magnetic fields from their interior, known as the Meissner effect
• The Meissner effect demonstrated that superconductivity was a new thermodynamic state of matter and not simply a state of perfect conductivity
• In 1957, John Bardeen, Leon Cooper, and John Robert Schrieffer proposed the BCS theory, which provided a microscopic explanation for superconductivity based on the formation of Cooper pairs
  • The BCS theory was a major breakthrough in understanding the underlying mechanisms of superconductivity and earned the three physicists the Nobel Prize in Physics in 1972

Basic Principles of Superconductivity

• Superconductivity characterized by two main properties: zero electrical resistance and the Meissner effect
• Zero electrical resistance allows superconductors to conduct electrical current without energy loss, enabling the flow of persistent currents
• The Meissner effect causes superconductors to expel magnetic fields from their interior, making them perfect diamagnets
• Superconductivity occurs below a critical temperature ($T_c$), which varies depending on the material
  • Conventional superconductors have $T_c$ values typically below 30 Kelvin, while high-temperature superconductors can have $T_c$ values above 77 Kelvin (the boiling point of liquid nitrogen)
• Superconductivity also limited by critical magnetic field ($H_c$) and critical current density ($J_c$)
  • If the applied magnetic field or current density exceeds these critical values, the material will revert to its normal, non-superconducting state
• The formation of Cooper pairs, bound states of two electrons with opposite spins and momenta, is crucial to the occurrence of superconductivity
  • Cooper pairs can move through the material without resistance, leading to the observed zero electrical resistance

Types of Superconductors

• Superconductors classified into two main categories: type-I and type-II superconductors
• Type-I superconductors exhibit a complete Meissner effect, expelling all magnetic fields from their interior until a critical field ($H_c$) is reached
  • Examples of type-I superconductors include pure metals such as mercury, lead, and tin
• Type-II superconductors exhibit a partial Meissner effect and allow magnetic fields to penetrate their interior in the form of quantized flux vortices
  • Type-II superconductors have two critical magnetic fields: lower critical field ($H_{c1}$) and upper critical field ($H_{c2}$)
  • Between $H_{c1}$ and $H_{c2}$, type-II superconductors are in a mixed state, where magnetic flux vortices coexist with superconducting regions
  • Examples of type-II superconductors include alloys and compounds such as niobium-titanium (NbTi) and yttrium barium copper oxide (YBCO)
• High-temperature superconductors are a subclass of type-II superconductors with $T_c$ values above 30 Kelvin
  • Discovered in 1986 by Georg Bednorz and Karl Müller in copper oxide compounds known as cuprates
  • Examples include lanthanum barium copper oxide (LBCO) and bismuth strontium calcium copper oxide (BSCCO)
• Other types of superconductors include heavy fermion superconductors, organic superconductors, and iron-based superconductors

Microscopic Theory of Superconductivity

• The microscopic theory of superconductivity, known as the BCS theory, was proposed by Bardeen, Cooper, and Schrieffer in 1957
• BCS theory explains superconductivity through the formation of Cooper pairs, bound states of two electrons with opposite spins and momenta
• Cooper pairs form due to an attractive interaction between electrons, mediated by the exchange of virtual phonons (quantized lattice vibrations)
  • This attractive interaction overcomes the Coulomb repulsion between electrons, allowing them to form bound pairs
• The binding energy of Cooper pairs is typically on the order of 1 meV, much smaller than the Fermi energy of the electrons
• The formation of Cooper pairs leads to the opening of an energy gap ($\Delta$) in the electronic density of states near the Fermi level
  • This energy gap prevents the scattering of electrons by lattice vibrations or impurities, resulting in zero electrical resistance
• The BCS theory successfully explains the observed properties of conventional superconductors, such as the dependence of $T_c$ on the isotopic mass and the existence of an energy gap
• Extensions of the BCS theory, such as the Eliashberg theory, account for strong-coupling effects and provide a more accurate description of superconductivity in some materials

Properties and Applications

• Superconductors exhibit unique properties that make them attractive for various applications
• Zero electrical resistance enables the development of high-efficiency power transmission lines, reducing energy losses over long distances
  • Superconducting power cables are being developed and tested for use in urban power grids and renewable energy integration
• The Meissner effect allows for the creation of strong, stable magnetic fields, which are essential for applications such as magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy
  • Superconducting magnets can generate fields up to 20 Tesla or more, enabling high-resolution imaging and spectroscopy
• Superconducting quantum interference devices (SQUIDs) are highly sensitive magnetometers that can detect extremely weak magnetic fields
  • SQUIDs are used in various applications, including geophysical exploration, medical diagnostics, and quantum computing
• Josephson junctions, consisting of two superconductors separated by a thin insulating layer, exhibit unique quantum effects and are used in high-speed, low-power electronic devices
  • Josephson junctions are the basis for superconducting qubits, the building blocks of quantum computers
• Superconducting bearings and flywheels can be used for energy storage and high-efficiency transportation systems, such as magnetically levitated trains
• Superconducting filters and cavities are used in high-performance radio frequency (RF) applications, such as particle accelerators and wireless communication systems

Experimental Techniques

• Various experimental techniques are used to study superconductivity and characterize superconducting materials
• Electrical transport measurements, such as resistivity and critical current density, are used to determine the superconducting transition temperature ($T_c$) and the quality of the superconducting state
  • Four-point probe method is commonly used to measure the resistivity of superconducting samples
• Magnetic measurements, such as susceptibility and magnetization, are used to study the Meissner effect and the response of superconductors to external magnetic fields
  • SQUID magnetometers and vibrating sample magnetometers (VSMs) are sensitive tools for measuring the magnetic properties of superconductors
• Specific heat measurements provide information about the electronic and phononic contributions to the heat capacity of superconductors, as well as the presence of an energy gap
• Tunneling spectroscopy, such as scanning tunneling microscopy (STM) and point-contact spectroscopy, is used to probe the electronic density of states and the energy gap in superconductors
• Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for studying the electronic band structure and the Fermi surface of superconductors
• Neutron scattering and X-ray diffraction are used to investigate the crystal structure and lattice dynamics of superconducting materials
• Muon spin rotation ($\mu$SR) is a sensitive probe of local magnetic fields and can be used to study the vortex dynamics in type-II superconductors

Current Research and Future Prospects

• Superconductivity remains an active area of research, with ongoing efforts to discover new superconducting materials, understand the underlying mechanisms, and develop practical applications
• The search for room-temperature superconductors is a major goal in the field, as it would revolutionize energy transmission, electronics, and transportation
  • Recent discoveries of high-temperature superconductivity in hydrogen-rich compounds under extreme pressures (e.g., LaH10 with $T_c$ ≈ 250 K) have renewed interest in this pursuit
• Unconventional superconductors, such as heavy fermion systems, organic superconductors, and iron-based superconductors, are being studied to gain insights into novel pairing mechanisms and the interplay between superconductivity and other quantum phenomena
• Topological superconductors, which exhibit exotic surface states and Majorana fermions, are a topic of intense research due to their potential applications in quantum computing and fault-tolerant information processing
• The integration of superconductors with other quantum materials, such as topological insulators and Weyl semimetals, is being explored to create novel hybrid devices with enhanced functionality
• Superconducting quantum computing is a rapidly advancing field, with the development of more robust and scalable superconducting qubit architectures
  • Superconducting qubits have demonstrated high fidelity gate operations and are among the leading candidates for building large-scale quantum computers
• The application of superconductors in high-performance electronics, such as superconducting nanowire single-photon detectors (SNSPDs) and superconducting nanowire memory devices, is an area of active research and development
• Advances in materials science, including the use of thin films, heterostructures, and nanostructured materials, are enabling the engineering of superconducting properties and the creation of novel devices
• Computational methods, such as density functional theory (DFT) and machine learning, are being employed to predict new superconducting materials and optimize their properties