Superconductors Under Pressure

The zoo of superconductors expanded in 2025. Researchers directly measured how hydrogen-rich compounds achieve their remarkable properties, confirmed *unconventional superconductivity* in twisted graphene, discovered superconducting behavior in materials previously thought ordinary, and uncovered exotic quantum states in kagome metals. These are strange creatures: materials that conduct electricity without resistance, that break symmetries spontaneously, that exhibit quantum effects visible at human scales. The quest for room-temperature superconductivity advanced on multiple fronts, while artificial intelligence began closing the gap between computational prediction and experimental synthesis. ## The Hydrogen Sulfide Mystery Solved Hydrogen sulfide (H₃S) becomes superconducting at 203 Kelvin (–70°C) under extreme pressure, a record-setting temperature when first achieved in 2015. But how it worked remained unclear. Standard *BCS theory*[^1] predicts superconductivity from electron-phonon coupling, yet quantitative agreement with H₃S was always approximate. Reproducibility was established, but explanation lagged. This year, researchers directly measured the superconducting state of hydrogen sulfide using a novel tunneling technique adapted for *diamond anvil cells*. Measurements confirmed *phonon-mediated pairing* at high pressure. Light hydrogen atoms vibrate at high frequencies, enabling strong *electron-phonon coupling*. A dimensionless coupling constant $\lambda$ captures this interaction: $$\lambda = 2 \int_0^{\omega_{\max}} \frac{\alpha^2 F(\omega)}{\omega} d\omega$$ This integral sums contributions from phonons at all frequencies $\omega$, weighted by the *Eliashberg function* $\alpha^2 F(\omega)$ that describes how strongly electrons couple to each phonon mode. For H₃S, $\lambda \approx 2$, unusually large, and now confirmed to explain the high critical temperature. ## Magic-Angle Graphene Delivers The hydrogen sulfide result validates the theoretical framework guiding the search for room-temperature superconductors. Lanthanum decahydride reaches 250 Kelvin under crushing pressure, tantalizingly close to ambient conditions. The physics works. The engineering does not, yet: pressures exceeding 150 gigapascals deform even diamond anvils. But a different path emerged from 2D materials, not compressed by brute force but twisted by geometry. Magic-angle graphene offered a new vocabulary for superconductivity. MIT physicists reported key evidence of unconventional superconductivity in *magic-angle twisted trilayer graphene* (MATTG).[^2] Fabrication involves stacking three atomically thin sheets of graphene and rotating one by approximately 1.1 degrees, the *magic angle* that causes electronic bands to become nearly flat. At this angle, the *Fermi velocity* effectively vanishes, dramatically enhancing electron-electron interactions. Superconductivity emerges at low temperatures through a mechanism distinct from conventional BCS theory. MIT's observation provides evidence for specific *pairing symmetry*, with electron pairs exhibiting a momentum-dependent phase structure rather than the uniform phase of conventional superconductors. This observation does not resolve the microscopic mechanism. Whether this connects to the decades-old mystery of cuprate high-temperature superconductivity remains hotly debated. This finding represents "a crucial step in the global search for room-temperature superconductors." Whether *flat-band superconductivity* can reach higher temperatures remains open, but *moiré materials* provide unusual tunability for exploring the phase diagram. Twist the angle, tune the physics. It is a remarkable degree of control over quantum behavior. ## Kagome Metals Break Symmetry Twisted graphene is not the only geometry hosting exotic states. Kagome metals, named for the Japanese basket weave their atoms resemble, exhibit quantum behavior that challenges conventional understanding in different ways. The *kagome lattice* naturally hosts flat bands and *Dirac fermions*, creating a playground for correlated electron physics. Strange things happen in these geometric arrangements. CsV₃Sb₅, a prototypical kagome superconductor, revealed new surprises this year. Researchers observed *nonreciprocal* superconducting critical currents at zero applied magnetic field: current flows more easily in one direction than the opposite. This spontaneous asymmetry requires breaking both *time-reversal symmetry*[^3] and *inversion symmetry*. The material somehow knows which direction is "forward." Stranger still: the direction of asymmetry could be "trained" by applying a magnetic field above the superconducting transition, even above the *charge density wave* transition at higher temperature. This behavior indicates that the charge density wave state itself breaks time-reversal symmetry through loop currents circulating within the kagome lattice. The symmetry breaking persists into the superconducting state, creating a fundamentally new type of superconductor, one with memory, one with preference. Separate studies using *quasiparticle interference*[^4] revealed how in-plane and out-of-plane vanadium 3d orbitals cooperate to form the charge density wave, providing new clarity on the microscopic origins of symmetry breaking in kagome materials. ## A Topological Surprise Symmetry breaking in kagome materials creates superconductors with memory, with preference, with a sense of direction. These are strange enough. But 2025 revealed something stranger still: superconductivity appearing where least expected, in forms no one anticipated, in materials previously thought ordinary. Researchers at IFW Dresden discovered that platinum bismuthide (PtBi₂), an outwardly ordinary material, exhibits singular superconducting behavior. Electrons pair in ways unlike any known superconductor. More intriguingly, the edges show signatures consistent with *Majorana zero modes*,[^5] exotic quasiparticles that are their own antiparticles, though unambiguous demonstration of *non-Abelian statistics* remains elusive. Majorana modes are proposed as building blocks for fault-tolerant quantum bits because they encode information nonlocally, protecting it from the perturbations that plague conventional qubits. This discovery suggests *topological superconductivity* may appear in materials not specifically engineered for it, though replication and alternative explanations require further study. If confirmed, this would accelerate the development of topological quantum computers by providing more accessible platforms for studying Majorana physics. Exotic physics, it turns out, may lurk in ordinary compounds. ## High-Temperature Superconducting Diodes Platinum bismuthide hints that topological superconductivity may lurk in ordinary compounds, waiting to be recognized. Meanwhile, researchers demonstrated that known high-temperature superconductors can do something new: act as diodes, allowing current to flow in one direction while blocking the reverse. Qi et al. achieved a *superconducting diode* effect at record-high temperatures in the *cuprate superconductor* BSCCO (Bi₂Sr₂CaCu₂O₈₊δ).[^6] A superconducting diode allows current to flow without resistance in one direction while blocking it in the opposite, the superconducting analog of a semiconductor diode. Their devices operated up to 72 Kelvin with efficiency reaching 22% at 53 Kelvin, all without external magnetic field. The *diode effect coefficient* $\eta$ quantifies the asymmetry: $$\eta = \frac{I_c^+ - |I_c^-|}{I_c^+ + |I_c^-|}$$ Here $I_c^+$ and $I_c^-$ are critical currents in opposite directions. This coefficient measures how much more current can flow one way before superconductivity breaks down. Observed values of $\eta \approx 0.22$ represent substantial asymmetry. Previous superconducting diodes required complex structures, applied fields, or operated only at low temperatures. BSCCO changes that equation, operating at temperatures achievable with liquid nitrogen rather than liquid helium. ## Germanium Goes Superconducting Non-dissipative electronics using high-temperature superconductors become practical only if they can integrate with conventional semiconductors. Researchers achieved exactly this bridge by making germanium, a foundational semiconductor, superconduct for the first time. Using *molecular beam epitaxy*[^7] to embed gallium atoms precisely into the germanium lattice, they induced superconducting behavior in a semiconductor material. This matters because germanium-based superconductors could integrate with existing semiconductor manufacturing. Hybrid devices combining superconducting and semiconducting elements on a single chip become feasible, potentially enabling new architectures for quantum computing and low-power electronics. Critical temperature remains low (below 1 Kelvin), but the demonstration opens new materials space. ## 2D Ferroelectrics for Memory Germanium-based superconductors could integrate with existing semiconductor manufacturing, enabling hybrid devices on single chips. Information storage demands similar integration. Ferroelectric materials that maintain switchable polarization are essential for non-volatile memory, and scaling them to two dimensions enables new device architectures. Researchers demonstrated *ferroelectric amplitude switching* in compositionally graded barium strontium titanate heterostructures.[^8] Unlike conventional ferroelectrics that switch between two discrete polarization states, these structures support continuous modulation of polarization magnitude without altering its direction. This enables *analog memory* with many distinguishable states on a single device. Separately, *sliding ferroelectricity*[^9] in hexagonal boron nitride, where polarization switches through in-plane atomic sliding rather than ionic displacement, was optically detected using an adjacent WSe₂ monolayer. Coupling between a 2D ferroelectric and a 2D semiconductor provides a non-destructive probe of ferroelectric switching, offering a path toward compact non-volatile memory devices. Wurtzite ferroelectrics like aluminum nitride revealed an anomalous switching mechanism in which fast 1D chains of atoms propagate from a slow-moving 2D fractal-like domain wall. Understanding this microscopic mechanism may enable engineering of lower *coercive fields*,[^10] reducing the energy required to switch memory states. ## Programmable Metamaterials Understanding such microscopic details drives the broader project of engineering materials for specific functions. Metamaterials, artificial structures with properties not found in nature, represent the extreme of that ambition: materials designed atom by atom for capabilities no natural substance provides. Software-defined metamaterial arrays can now reconfigure their electromagnetic properties in real time, enabling dynamic *beam steering*[^11] for wireless power transfer and communications. A 2-bit programmable transmit metamaterial array demonstrated 60° beam scanning with 90.7% beamforming accuracy, enabling tracking of mobile users with minimal hardware complexity. At terahertz frequencies,[^12] similar reconfigurable arrays achieved beam steering from -138° to +138°, supporting single, dual, and multibeam modes. Programming material properties through software, rather than fabricating new structures, inverts the traditional order. Applications range from 6G communications to industrial quality control to medical imaging. ## AI Accelerates Discovery This flexibility requires knowing what to design. In 2025, AI systems advanced from predicting material properties to guiding what should be synthesized next. The question shifted from whether computation could help to whether laboratories could keep pace. Google DeepMind's *GNoME* (Graph Networks for Materials Exploration), introduced in 2023, continued generating predictions that experimentalists could verify. But the challenge has shifted from prediction to synthesis. Computational screening identifies thousands of candidates. Laboratory validation fails more often than it succeeds.[^13] Predicted stability does not guarantee synthesizability, and simulated properties often degrade under real-world conditions. A gap between virtual materials and tangible ones remains wide. Nevertheless, the feedback loop between computation and experiment is accelerating. Materials databases now include millions of predicted compounds, prioritized by stability, synthesizability, and target properties. *Autonomous laboratories* that combine AI prediction with robotic synthesis are beginning to close the loop entirely. Computers design experiments that robots run, with results feeding back into improved predictions. *High-entropy alloys*, materials containing five or more elements in roughly equal proportions, exemplify the challenge. Combinatorial explosion of possible compositions makes systematic experimental exploration impossible. AI-guided screening can narrow the search space, but validating predictions still requires human expertise and laboratory resources. The bottleneck has moved, but it has not disappeared. ## From Laboratory to Industry Even with these constraints, 2025 moved discoveries closer to applications than any year before. Direct measurement of hydrogen sulfide's superconducting mechanism validates the theoretical framework guiding the search for room-temperature superconductors. We now understand the physics. A path forward is clearer, even if the destination remains distant. Kagome metal and topological superconductor discoveries expand the space of known superconducting phenomena. Some of these materials may prove more practical than compressed hydrogen compounds, operating at accessible pressures even if at lower temperatures. The zoo grows stranger and more useful. High-temperature superconducting diodes demonstrate that superconductor-based electronics are feasible with existing high-Tc materials. Commercial applications may arrive before room-temperature superconductors themselves, a reminder that breakthroughs sometimes come sideways. 2D ferroelectric advances bring memory and computing applications closer to integration with 2D electronics ecosystems. Analog memory with continuous states could enable new computing paradigms beyond binary logic. Computing's future need not be digital. ## The Synthesis Gap Hydrogen-rich superconductors may one day work at ambient pressure, but current record-holders require millions of atmospheres. Diamond anvils themselves deform under such force. Understanding has advanced while implementation lags, and the gap between them may not close in our lifetimes. Magic-angle graphene and cuprate superconductors both involve flat bands and strong correlations, but a unified theoretical framework remains elusive. We may be glimpsing different faces of the same phenomenon, or two entirely different beasts. Either answer would reshape the search for room-temperature superconductivity. AI systems now predict millions of potential materials. Laboratories can validate only a handful per year. Autonomous systems that combine prediction with robotic synthesis are beginning to close this loop, but the bottleneck has moved from computation to fabrication. What can be imagined far exceeds what can be built. Tomorrow's materials are taking shape in laboratories today. Whether they reach applications in years or decades depends on answers not yet known, and on hands not yet trained. --- **Citations**: [1] "Scientists unlocked a superconductor mystery under crushing pressure." ScienceDaily, December 19, 2025. [2] "MIT physicists observe key evidence of unconventional superconductivity in magic-angle graphene." MIT News, November 6, 2025. [3] Ge, J., et al. "Nonreciprocal superconducting critical currents with normal state field trainability in kagome superconductor CsV₃Sb₅." arXiv:2506.04601, June 2025. [4] Huang, Z., et al. "Revealing the orbital origins of exotic electronic states with Ti substitution in kagome superconductor CsV₃Sb₅." arXiv:2502.02923, February 2025. [5] "A new superconductor breaks rules physicists thought were fixed." ScienceDaily, December 26, 2025. [6] Qi, S., et al. "High-temperature field-free superconducting diode effect in high-Tc cuprates." arXiv:2501.02425, January 2025. [7] "Scientists unveil breakthrough that could transform energy transmission." EurekAlert, 2025. [8] Kim, G.-H., et al. "Ferroelectric amplitude switching and continuous memory." arXiv:2510.14491, October 2025. [9] Roux, S., et al. "Optical detection of the sliding ferroelectric switching in hBN with a WSe₂ monolayer." arXiv:2412.12703, December 2024. [10] Behrendt, D., et al. "Ferroelectric Fractals: Switching Mechanism of Wurtzite AlN." arXiv:2410.18816, October 2024. [11] Ahamed, E., et al. "Software-defined Programmable Metamaterial Lens System for Dynamic Wireless Power Transfer Applications." arXiv:2408.15485, August 2024. [12] Ahamed, E., et al. "Terahertz Digital Reconfigurable Metamaterial Array for Dynamic Beamforming Applications." arXiv:2407.07743, July 2024. [13] "AI materials discovery now needs to move into the real world." MIT Technology Review, December 15, 2025. **Footnotes**: [^1]: BCS theory (Bardeen-Cooper-Schrieffer) explains conventional superconductivity through the pairing of electrons into "Cooper pairs" mediated by lattice vibrations (phonons). The coupling constant $\lambda$ quantifies the strength of this interaction. [^2]: The "magic angle" of approximately 1.1 degrees causes the electronic bands to become nearly flat, drastically increasing the density of states and enhancing electron-electron interactions. [^3]: Time-reversal symmetry breaking in a superconductor indicates the presence of spontaneous currents or magnetic moments, distinguishing it from conventional superconductors. [^4]: Quasiparticle interference (QPI) uses scanning tunneling microscopy to map the momentum-space electronic structure of a material by analyzing interference patterns in the local density of states. [^5]: Majorana zero modes are quasiparticle excitations that are their own antiparticles. In condensed matter systems, they arise at defects or edges of topological superconductors and have potential applications in topological quantum computing. [^6]: BSCCO is a bismuth strontium calcium copper oxide, one of the first high-temperature superconductors discovered in the 1980s. It superconducts below approximately 110 Kelvin. [^7]: Molecular beam epitaxy is a technique for growing crystalline materials one atomic layer at a time, enabling precise control over composition and structure. [^8]: Conventional ferroelectric memories store binary states (up polarization or down polarization). Analog ferroelectric memories with continuous states could enable neuromorphic computing architectures. [^9]: Sliding ferroelectricity occurs when atoms shift in-plane between two stable stacking configurations. This is distinct from conventional ferroelectricity where ions move perpendicular to the polarization direction. [^10]: The coercive field is the electric field required to switch a ferroelectric. Lower coercive fields enable lower operating voltages and reduced energy consumption in memory devices. [^11]: Beam steering allows an antenna array to direct electromagnetic radiation in different directions without physically moving the antenna, essential for 5G/6G communications and radar. [^12]: Terahertz frequencies (0.1-10 THz) lie between microwaves and infrared light. Applications include imaging, spectroscopy, and next-generation wireless communications. [^13]: The synthesis gap refers to the discrepancy between computationally predicted material properties and experimentally realized performance. Bridging this gap requires advances in both prediction accuracy and synthesis techniques.