Superconductivity has fascinated engineers and physicists for more than a century. Imagine a world in which power grids transmit energy with zero loss, electricity flows without obstruction, and cutting-edge technologies function with unmatched efficiency. The realization of the room-temperature superconductor, a scientific holy grail, is essential to this seemingly idealistic vision. One of the most significant superconductivity physics problems of our time is the long-standing difficulty of achieving superconductivity at room temperature; if this scientific breakthrough were to be made, it would revolutionize the field.
The Elusive Dream of Zero Resistance
Fundamentally, superconductivity is a remarkable quantum mechanical phenomenon in which some materials exhibit zero electrical resistance and the expulsion of magnetic fields (also referred to as the Meissner effect) when cooled below a critical temperature (Tc). Once started, an electrical current in a superconductor can continue indefinitely without the need for an external power source. This contrasts sharply with traditional conductors, such as copper, which, because of resistance, always release some energy as heat. A significant reorganization of electrons within the material is indicated by the Meissner effect, which makes a superconductor levitate above a magnet. This phenomenon is not just a result of zero resistance; rather, it is an independent signature of the superconducting state.
The enormous practical limitations of existing superconducting materials are the driving force behind the search for room-temperature superconductors. The majority of known superconductors only show their unique characteristics at very low temperatures, frequently necessitating cooling with pricy and cumbersome cryogens such as liquid nitrogen (77 Kelvin or -196°C) or liquid helium (4.2 Kelvin or -269°C). These cryogenic conditions are expensive and energy-intensive to maintain, which limits their widespread use to specialized industries like certain research accelerators and high-field magnets for MRI machines. These cooling requirements would be removed by a superconductor operating at or close to room temperature (about 20–30°C), opening the door to a wave of ground-breaking technologies.
The Foundations: A Century of Discovery
The discovery by Dutch physicist Heike Kamerlingh Ones in 1911 that mercury’s electrical resistance abruptly disappeared when cooled below 4.2 Kelvin marked the beginning of the story of superconductivity. A new area of condensed matter physics was made possible by this revolutionary discovery. Through the discovery of new metallic alloys and compounds, scientists gradually raised the critical temperature over the course of decades, frequently by a few Kelvin at a time. The highest known Tc in niobium-germanium Nb3Ge was 23 K by the middle of the 1970s.
In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer developed the Bardeen-Cooper-Schrieffer (BCS) theory, which provided a theoretical explanation for this phenomenon. Conventional superconductivity was elegantly explained by BCS theory: electrons, which normally repel one another, form weakly bound pairs known as “Cooper pairs” below a certain temperature. Phonons, which are vibrations in the material’s crystal lattice, mediate this pairing. Zero resistance results from these Cooper pairs’ ability to pass through the material without scattering off flaws or vibrations. BCS theory offered a framework for comprehending the microscopic origins of this macroscopic quantum state and effectively described the behavior of numerous low-temperature superconductors. It did, however, also forecast a maximum temperature for Tc, indicating that it would be extremely challenging, if not impossible, to reach much higher temperatures within its parameters.
The Dawn of High-Temperature Superconductors (High-Tc)
The discovery of cuprite superconductors by Georg Bednorz and K. Alex Müller at IBM in 1986 significantly altered the field of superconductivity. They discovered that lanthanum barium copper oxide (LaBaCuO), a ceramic material, became superconducting at 30 K, a temperature that many people working in the BCS framework had previously thought was impossible. A worldwide scientific race to discover new materials with even higher critical temperatures was sparked by this scientific discovery, which broke the perceived BCS limit. Yttrium Barium Copper Oxide YBa2Cu3O7, commonly referred to as YBCO, was found to be superconducting above 90 K in a matter of months. This was a huge accomplishment because liquid nitrogen, a much more affordable and accessible cryogen than liquid helium, has a boiling point of 77K, which is higher than 90K.
Conventional BCS superconductors are fundamentally different from cuprate superconductors. These are ceramics, and they frequently have intricate layered crystal structures with planes of copper and oxygen. Since their superconductivity is regarded as “unconventional,” electron-phonon interactions by themselves cannot adequately explain the electron pairing mechanism. Rather, magnetic fluctuations and strong electron correlations are thought to be important. One of the biggest problems in superconductivity physics is still comprehending these intricate processes. Despite their higher critical temperatures, their widespread practical application has been severely hampered by their anisotropic (direction-dependent) nature, fragility, and challenges in fabricating them into long, flexible wires.
Iron-based superconductors, a different class of high-Tc superconductors, were discovered in 2008 after the cuprates. Their discovery highlighted the complexity of unconventional superconductivity by challenging established theories and opening up new research avenues for high Tc superconductors, despite the fact that their critical temperatures are typically lower than those of the highest cuprates.
Pushing the Limits: Extreme Conditions and Novel Materials
The pursuit of greater Tc persisted, prompting scientists to investigate materials in harsh environments. The discovery of hydride superconductors under extreme pressures has been a major breakthrough in the study of high Tc superconductors. Since 2015, sulfur hydride H2S has been demonstrated to be superconducting at an astounding 203K (−70°C) when exposed to pressures exceeding 150 gigapascals (GPa), or roughly 1.5 million times atmospheric pressure. This was further raised by later discoveries: yttrium hydroxide YH 10 reached 262 K (-11°C) at 182 GPa, and lanthanum hydroxide LaH 10 demonstrated superconductivity at 250 K (−23°C) at 170 GPa. The drawback of these “warm” superconductors is the high pressure needed to operate at temperatures that are close to room temperature. Since it is experimentally difficult to achieve and sustain such pressures, these materials are not suitable for the majority of real-world applications outside of specialized labs. These findings are significant, however, because they show that superconductivity is theoretically possible at temperatures close to room temperature. This could lead to the discovery of new mechanisms or classes of materials that could have comparable qualities at lower pressures.
In July 2023, a South Korean team claimed to have synthesized a material called LK-99 and claimed it exhibited superconductivity at room temperature and ambient pressure. This was possibly the most intriguing room temperature superconductor news of recent years. The announcement made headlines around the world and rocked the scientific community. Evidence of partial levitation (a characteristic of the Meissner effect) and zero resistance were included in the claims. Rapid independent replication attempts around the world, however, mostly failed to replicate true superconductivity at room temperature, despite initially displaying some peculiar magnetic or resistive properties. The scientific community swiftly came to the conclusion that LK-99, as it was presented, was a complex material with intriguing but non-superconducting properties rather than a room-temperature superconductor. The LK-99 controversy brought to light the scientific community’s rigorous, if occasionally disorganized, verification process as well as the great desire for this discovery.
Beyond Conventional: Emerging Frontiers
High Tc superconductor research investigates a number of intriguing areas outside of the well-established classes of high-Tc materials and high-pressure hydrides, including:
- The 2016 Nobel Prize in Physics was given to the field of topological superconductors, which are materials that combine superconductivity and topological states of matter. Because of their intrinsic resilience to local perturbations, they are expected to harbor exotic particles known as Majorana fermions, which are their own antiparticles and could be essential components of fault-tolerant quantum computing.
- Carbon-based substances that are superconducting, usually at extremely low temperatures, are known as organic superconductors. Despite having low T$_{c}$ values, they are an intriguing subject for study because of their adaptability, chemical synthesis tunability, and potential for integration with organic electronics. Their sensitivity to impurities and fragility present difficulties, though.
- Nickelates: Because of their similar electronic structures to cuprates, rare-earth nickelates have recently surfaced as a new class of possible high-Tc superconductors. Although they are still in the early stages of study, they provide an additional way to investigate unusual pairing mechanisms.
- Twisted Bilayer Graphene: “Magic-angle” twisted bilayer graphene has demonstrated superconductivity at extremely low temperatures, bridging the gap between quantum materials and nanotechnology. This finding demonstrates how adjusting quantum interactions in two-dimensional materials can produce surprising results.
To predict and create new materials with improved superconducting qualities, these varied strategies—along with developments in computational materials design and artificial intelligence—are essential for negotiating the challenging terrain of superconductivity physics problems.
