超伝導

Superconductivity — Grokipedia Fact-checked by Grok 3 months ago Superconductivity Ara Eve Leo Sal 1x Superconductivity is a quantum mechanical phenomenon observed in certain materials that, when cooled below a characteristic critical temperature ( T c ), exhibit zero electrical resistance to the flow of direct current (DC) electricity . These materials also demonstrate the complete expulsion of magnetic fields from their interior, a property known as the Meissner effect . [1] This behavior allows for the conduction of electrical current without energy loss, fundamentally distinguishing superconductors from ordinary conductors. [1] The discovery of superconductivity occurred in 1911 when Dutch physicist Heike Kamerlingh Onnes observed the sudden drop in electrical resistance to zero in mercury cooled to approximately 4.2 K using liquid helium . This finding earned him the 1913 Nobel Prize in Physics . [1] Initially empirical, the phenomenon puzzled scientists until 1933, when Walther Meissner and Robert Ochsenfeld identified the magnetic field expulsion, solidifying its unique quantum nature. [2] In 1957, John Bardeen , Leon Cooper , and John Robert Schrieffer developed the BCS theory , which explains superconductivity in conventional materials through the formation of Cooper pairs—bound electrons mediated by lattice vibrations (phonons). This leads to a coherent quantum state that enables resistance-free flow. This work was recognized with the 1972 Nobel Prize in Physics . [1] Superconductors are classified into two main types: low-temperature (conventional) superconductors and high-temperature (unconventional) superconductors. Low-temperature superconductors require cooling to near absolute zero . For example, niobium-titanium alloys have a T c around 9-10 K. High-temperature superconductors were first discovered in 1986 by J. Georg Bednorz and K. Alex Müller in a copper-oxide ceramic with T c of 30 K. [2] Developments advanced to over 90 K in yttrium barium copper oxide (YBCO) by 1987. The current record for cuprates reaches about 134 K in mercury-based compounds, operable with liquid nitrogen cooling. [2] The mechanism for high- T c materials often involves d-wave pairing and doping effects in cuprates and other unconventional families such as iron-based and nickelates. It remains an active area of research beyond the phonon-based BCS framework. [2] [3] Superconductivity is also limited by critical parameters: a critical magnetic field ( H c ) beyond which the Meissner effect fails, and a critical current density ( J c ) above which resistance reappears. [1] Practical applications of superconductivity leverage these properties for efficient energy use and advanced technologies, including superconducting magnets in magnetic resonance imaging (MRI) machines, particle accelerators like the Large Hadron Collider, and nuclear magnetic resonance (NMR) spectrometers. Zero resistance enables strong, stable fields without power loss. [1] Emerging uses span power transmission lines to reduce energy dissipation in grids. They also include magnetically levitated (maglev) trains for frictionless high-speed transport and quantum computing components exploiting the quantum coherence of superconducting states. [1] Five Nobel Prizes have been awarded for superconductivity-related discoveries (1913, 1972, 1973, 1987, 2003). [1] This underscores its profound impact on physics and engineering. Discovery and Basic Properties Historical Discovery In 1911, Dutch physicist Heike Kamerlingh Onnes and his team at the University of Leiden discovered superconductivity while investigating the electrical resistivity of pure mercury at cryogenic temperatures. On April 8, 1911, they observed an abrupt drop in resistance to apparently zero as the temperature reached approximately 4.2 K, achieved using liquid helium that Onnes had pioneered the liquefaction of in 1908. [4] [5] This unexpected result, detailed in Onnes's seminal paper "The Resistance of Pure Mercury at Helium Temperatures," marked the first identification of a material exhibiting zero electrical resistance below a critical temperature. [6] The breakthrough relied heavily on prior advancements in low-temperature physics, particularly the development of the Dewar flask by Scottish physicist James Dewar in 1892. This vacuum-insulated vessel allowed for the efficient storage and transfer of liquefied gases like air and hydrogen, enabling Onnes to pre-cool helium gas effectively before its liquefaction and maintain the ultra-low temperatures required for his experiments. [7] Without such innovations, reaching and sustaining temperatures near absolute zero would have been impractical, underscoring how incremental progress in cryogenics facilitated the discovery. [4] Onnes initially viewed the phenomenon as a novel state of matter , coining the term "supraconductivity" in 1913 (later standardized as "superconductivity") to describe the complete disappearance of resistance. He speculated it might align with contemporary theories of electron behavior in metals, such as reduced scattering at low temperatures, but subsequent checks revealed no dissipation even after prolonged current flow, defying existing models and prompting recognition of it as a distinct physical effect. [8] [4] Building on the mercury findings, Onnes's group extended observations in 1912 to other pure metals, noting zero resistance in lead at about 7.2 K and in tin at roughly 3.7 K , establishing superconductivity as a property shared by multiple elements under similar cryogenic conditions. [4] These early experiments highlighted the need for high-purity samples, as impurities had initially suggested gradual resistance decreases rather than the sharp transition observed in refined materials. [5] Zero Electrical Resistance One of the defining characteristics of superconductivity is the complete disappearance of electrical resistance below a critical temperature , enabling infinite DC conductivity. This phenomenon was first observed in 1911 by Heike Kamerlingh Onnes , who measured the resistivity of mercury and found it to drop abruptly to zero at approximately 4.2 K when cooled using liquid helium . [1] Subsequent experiments confirmed this zero-resistivity state in other materials, such as lead and tin, establishing it as a universal property of superconductors under appropriate conditions. The absence of resistance permits persistent current s to circulate indefinitely in closed superconducting loops without energy dissipation. These currents arise from induced electromagnetic forces and maintain themselves due to the lack of ohmic losses, allowing practical applications like superconducting magnets. Experimental verification includes observations of such currents in lead cylinders, where a persistent current persisted for over two years without measurable decay, limited only by an external interruption in cooling. [9] Unlike an ideal classical conductor, which might sustain persistent currents through inertia but permit magnetic field penetration, a superconductor achieves zero resistance while expelling internal magnetic field s entirely—a complementary property known as the Meissner effect . In the phenomenological framework of the London theory, this zero-resistance behavior for DC fields is captured by the implication of the first London equation in steady state : E = 0 \mathbf{E} = 0 E = 0 inside the superconductor, where E \mathbf{E} E is the electric field , ensuring no voltage drop and constant current flow. [10] Meissner Effect The Meissner effect is the expulsion of a magnetic field from the interior of a superconductor upon cooling below its critical temperature $ T_c $, resulting in perfect diamagnetism . This phenomenon was discovered in 1933 by German physicists Walther Meissner and Robert Ochsenfeld through experiments on lead and tin samples. Their work revealed that, unlike the normal state where magnetic field s penetrate materials, the superconducting state actively excludes internal fields, distinguishing it from mere zero electrical resistance observed earlier. In their experimental setup, Meissner and Ochsenfeld used cylindrical samples of polycrystalline lead and single-crystal tin, approximately 140 mm long and 3 mm in diameter, placed parallel and separated by 1.5 mm, within a uniform external magnetic field of about 5 gauss generated by an electromagnet . They measured magnetic flux changes using a small search coil, roughly 10 mm long, connected to a ballistic galvanometer , positioned either between the cylinders or inside a hollow lead tube (130 mm long, 3 mm outer diameter, 2 mm inner diameter) for internal field assessment. Upon cooling the samples below $ T_c $ (around 7.2 K for lead and 3.7 K for tin) via liquid helium , the galvanometer registered deflections indicating a sudden expulsion of flux from the interior, with the external field lines compressing around the sample surfaces as if the material had zero permeability. If the field was applied after achieving the superconducting state, no penetration occurred, confirming the effect's thermodynamic nature. [11] This discovery had profound implications, establishing superconductivity as a true thermodynamic phase transition to an equilibrium state rather than a metastable condition tied solely to dissipationless current flow. Prior understanding of zero resistance, found in mercury by Heike Kamerlingh Onnes in 1911, suggested persistent currents but not field expulsion; the Meissner effect clarified that the superconducting state minimizes magnetic energy through flux exclusion, enabling thermodynamic treatments like specific heat measurements and phase diagrams. Theoretically, the effect is captured by the condition that the magnetic induction $ \mathbf{B} = 0 $ inside the superconductor in the absence of currents, arising from Maxwell's equations $ \nabla \cdot \mathbf{B} = 0 $ and $ \nabla \times \mathbf{H} = \mathbf{J}