Discovery Of Superconductivity Research Articles

Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity.

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Superconductivity in Quasi-One-Dimensional Ferromagnet CrSbSe3 under High Pressure.

Nearly a decade has passed since the discovery of superconductivity in CrAs, but until now, the discovered structure types of chromium-based superconductors are still scanty. It is urgent to expand this family to decipher the interplay between magnetism and superconductivity penetratingly. Here, we report the observation of superconductivity in ferromagnet CrSbSe3 with a quasi-one-dimensional structure under high pressure. Under compression, CrSbSe3 undergoes an insulator-to-metal transition and sequential isostructural phase transitions accompanied by volume collapse. Superconductivity emerges at 32.8 GPa concomitant with metallization in CrSbSe3. A maximum superconducting transition temperature Tc of 7.7 K is achieved at 57.9 GPa benefiting from both the phonon softening and the enhanced p-d hybridization between Se and Cr in CrSbSe3. The discovery of superconductivity in CrSbSe3 expands the existing chromium-based superconductor family and sheds light on the search for concealed superconductivity in low-dimensional van der Waals materials.

Superconductivity in a van der Waals layered quasicrystal

Van der Waals layered transition-metal chalcogenides are drawing significant attention owing to their intriguing physical properties. This group of materials consists of abundant members with various elements, having a variety of different structures. However, they are all crystalline materials, and the physical properties of van der Waals layered quasicrystals have never been studied to date. Here, we report on the discovery of superconductivity in a van der Waals layered quasicrystal of Ta1.6Te. The electrical resistivity, magnetic susceptibility, and specific heat of the quasicrystal unambiguously validate the occurrence of bulk superconductivity at a transition temperature of ~1 K. This discovery can promote new research on assessing the physical properties of novel van der Waals layered quasicrystals as well as two-dimensional quasicrystals; moreover, it paves the way toward new frontiers of superconductivity in thermodynamically stable quasicrystals.

Development of novel superconductivity with higher Tc via the suppression of magnetism in quasi-two-dimensional electride Y2C under high pressures.

Discovery of superconductivity in electride materials has been a topic of interest as their intrinsic electron-rich properties might suggest a considerable electron-phonon interaction.Layered Y2C is a ferromagnetic quasi-two-dimensional electride with polarized anionic electrons confined in the interlayer space. In this theoretical study, we report Y2C undergoes a series of structural phase transitions into two
superconducting phases with estimated Tc of 9.2 and 21.0 K at 19 and 80 GPa, respectively, via the suppression of magnetism. Our extensive first-principles swarm structure searches identify that these two high-pressure superconducting phases possess an orthorhombic Pnma and a tetragonal I4/m structures, respectively, where the Pnma phase is found to be a one-dimensional electride
characterized by electron confinements in channel spaces of the crystal lattice, while the electride property in I 4/m phase has been completely destroyed. We attribute the development of an unprecedentedly high Tc superconductivity in Y-C system to the destructions of magnetism and the delocalization of interlayered anionic electrons under pressures. This work provides a unique example of pressure-induced collapse of magnetism at the onset of superconductivity in electride materials, along with the dramatic changes of electron-confinement topology in crystal lattices.

Pressure-driven superconductivity in layered isostructural germanium phosphides

The discovery of superconductivity and its modulation are long-standing cutting-edge research topics in condensed matter physics. As a powerful tool, the high-pressure technique can be used to achieve novel superconductors and tune their physical properties. One typical example is binary germanium phosphides with different stoichiometries, which exhibit abundant physical properties with layered lattice structures similar to blue phosphorus. The detailed phase diagrams of the Ge–P systems are important for understanding the influence of stoichiometry on pressure-driven superconductivity, but it remains unexplored. Here, we measured and compared the detailed superconducting phase diagrams of the Ge–P systems of layered isostructural germanium phosphides GeP3 and GeP5 under pressure. Even though these two binary phosphides exhibit obviously different atomic occupations in the crystal structure due to their distinct stoichiometric ratios, the onset superconducting transition temperatures Tc of GeP3 and GeP5 both show dramatic enhancements from ∼2.5 K at 12.0 GPa to the maximum values of ∼9.0 K at 28.0 GPa, which are higher than those of other binary metal phosphides. Such pressure-enhanced superconductivity therein is accompanied by significant pressure-induced phonon mode softening, which is confirmed via in situ high-pressure Raman measurements. Our observations deepen the physical understanding of pressure-driven superconductivity in phosphorous-rich layered compounds and pave the way for potential applications in microsuperconducting devices.

Evidence for d-wave superconductivity of infinite-layer nickelates from low-energy electrodynamics.

The discovery of superconductivity in infinite-layer nickelates established another category of unconventional superconductors that shares structural and electronic similarities with cuprates. However, key issues of the superconducting pairing symmetry, gap amplitude and superconducting fluctuations are yet to be addressed. Here we utilize static and ultrafast terahertz spectroscopy to address these. We demonstrate that the equilibrium terahertz conductivity and non-equilibrium terahertz responses of an optimally Sr-doped nickelate film (superconducting transition temperature of Tc = 17 K) are in line with the electrodynamics of d-wave superconductivity in the dirty limit. The gap-to-Tc ratio (2Δ/kBTc) is found to be 3.4, indicating that the superconductivity falls in the weak coupling regime. In addition, we observed substantial superconducting fluctuations near Tc that do not extend into the deep normal state as the optimally hole-doped cuprates do. Our results support a d-wave system that closely resembles the electron-doped cuprates.

Discovery of superconductivity in technetium borides at moderate pressures.

Advances in theoretical calculations have boosted the search for high-temperature superconductors, such as sulfur hydrides and rare-earth polyhydrides. However, the required extremely high pressures for stabilizing these superconductors has handicapped further implementation. Based upon thorough structural searches, we identified a series of unprecedented superconducting technetium borides at moderate pressures, including TcB (P63/mmc) with a superconducting transition temperature of Tc = 20.2 K at ambient pressure and TcB2 (P6/mmm) with Tc = 23.1 K at 20 GPa. Superconductivity in these technetium borides mainly originates from the coupling between the low-frequency vibrations of technetium atoms and the dominant technetium-4d electrons at the Fermi level. Our work therefore presents a fresh group in the family of superconducting borides, whose diversified crystal structures suggest rich possibilities in the discovery of other superconducting transition-metal borides.

Effects of pressure and doping on Ruddlesden-Popper phases Lan+1NinO3n+1

Recently, the discovery of superconductivity with a critical temperature Tc up to 80 K in Ruddlesden–Popper phases Lan+1NinO3n+1 (n = 2) under pressure has garnered considerable attention. Up to now, the superconductivity was only observed in La3Ni2O7 single crystal grown with the optical-image floating zone furnace under oxygen pressure. It remains to be understood the effect of chemical doping on superconducting La3Ni2O7 as well as other Ruddlesden–Popper phases. Here, we systematically investigate the effect of external pressure and chemical doping on polycrystalline Ruddlesden–Popper phases. Our results demonstrate that the application of pressure and doping effectively tunes the transport properties of Ruddlesden–Popper phases. We find pressure-induced superconductivity up to 86 K in La3Ni2O7 polycrystalline sample, while no signatures of superconductivity are observed in La2NiO4 and La4Ni3O10 polycrystalline samples under high pressure up to 50 GPa. Our study sheds light on the exploration of high-Tc superconductivity in nickelates.

Metal–insulator–superconductor transition in nickelate-based heterostructures driven by topotactic reduction

The discovery of superconductivity in doped infinite-layer nickelates has attracted great interest recently. Here, a metal–insulator–superconductor transition is demonstrated by engineering the process of topotactic reduction. By employing topotactic reduction, a superconducting Nd0.8Sr0.2NiO2 layer is obtained from high-quality Nd0.8Sr0.2NiO3, which is characterized by layer-by-layer growth and low room-temperature resistivity, rather than the low-quality Nd0.8Sr0.2NiO3 with mixed phases. Moreover, an insulating intermediate state is uncovered within the transition from metallic Nd0.8Sr0.2NiO3 to superconducting Nd0.8Sr0.2NiO2, corresponding to a non-monotonic modulation of resistivity driven by topotactic reduction. In the incompletely reduced Nd0.8Sr0.2NiO2+δ, residual oxygen atoms in the Nd/Sr plane disrupt the long-range order of the infinite-layer structure, resulting in suppressed superconductivity with a low transition temperature and non-zero residual resistivity. On the other hand, the superconductivity is optimized in the fully reduced sample, where a sign change in the Hall coefficient is always observed at low temperatures. Our study highlights the diverse electronic states achievable by controlling topotactic reduction, providing valuable insight into the understanding and manipulating of superconductivity in infinite-layer nickelates.

Insights into the polaron physics of high temperature superconductors from optical and time-domain spectroscopy

Polarons and percolation were mentioned as guiding concepts in the paper by Bednorz and Müller in their report on the discovery of superconductivity in Ba–La–Cu–O. Both of these concepts seem to have survived, but their role in high-temperature superconductivity is somewhat different than originally envisaged. Optical and time-resolved experiments have been extremely useful for understanding the roles and dynamical interplay of polaronic and itinerant charge carriers in the different states of matter that appear in the cuprate phase diagram. The present article outlines some of the connections between the original concepts, systematics and apparent dichotomies, and discusses them in the light of more recent experimental observations, leading to some important theoretical considerations on the mechanism for superconductivity.

Moiré-Assisted Realization of Octahedral MoTe2 Monolayer.

A current key challenge in 2D materials is the realization of emergent quantum phenomena in hetero structures via controlling the moiré potential created by the periodicity mismatch between adjacent layers, as highlighted by the discovery of superconductivity in twisted bilayer graphene. Generally, the lattice structure of the original host material remains unchanged even after the moiré superlattice is formed. However, much less attention is paid for the possibility that the moiré potential can also modify the original crystal structure itself. Here, it is demonstrated that octahedral MoTe2 which is unstable in bulk is stabilized in a commensurate MoTe2 /graphene hetero-bilayer due to the moiré potential created between the two layers. It is found that the reconstruction of electronic states via the moiré potential is responsible for this stabilization, as evidenced by the energy-gap opening at the Fermi level observed by angle-resolved photoemission and scanning tunneling spectroscopies. The present results provide a fresh approach to realize novel 2D quantum phases by utilizing the moiré potential.

Thoughts on the influence of Alex Mueller on high magnetic field technology

I briefly summarize some of the impacts that Alex Mueller's drive to discover superconductivity in low carrier density oxides has had on high magnetic field technology, especially on the desire to use the cuprate superconductors to generate magnetic fields now almost twice those possible with the Nb-based low temperature superconductors. There were some important lessons for applications of any new exotic temperature superconductor that had to be learned before the Bednorz and Mueller discovery could really impact applications. Above all is the lesson that a high transition temperature is no guarantee of applications unless conductors with high current density can be manufactured. Recent reports and hopes for an ambient pressure room temperature superconductor remind us that the hopes for pervasive superconducting technologies germinated with the discovery of superconductivity in LaBaCuO and went truly ballistic following the discovery of YBa2Cu3O7-δ (YBCO) with a 92 K transition temperature. Now that magnets made out of rare earth alloys of YBCO (REBCO) have entered routine service in NMR labs and prototype magnets made with REBCO appear ready to enable compact fusion reactors, we can finally say that Bednorz and Mueller's magnificent scientific achievement is having real world applications impact too.

High-temperature superconductivity and strange metallicity: Simple observations with (possibly) profound implications

Bednorz and Müller's discovery of superconductivity in the cuprate perovskite La2-xBaxCuO4 gave the modern condensed matter community arguably two of its greatest gifts – high-temperature superconductivity and strange metallicity; certainly two of its most profound and challenging problem. While it was the multi-fold increase in Tc that rightly catapulted cuprates to the summit of global media attention and scientific curiosity, it is the nature of their normal (non-superconducting) state that has sustained interest over the intervening years. This in turn has inspired condensed-matter experimentalists and theoreticians to revolutionise their respective fields. Yet despite such boundary-pushing endeavors, the origin of both phenomena remains elusive. In this brief note, I offer a personal reflection on the link between strange metallicity and high-Tc superconductivity, highlighting a number of simple observations that may hold profound implications for their ultimate resolution.

Electron–phonon driven unconventional superconductivity: The role of small Fermi energies and of nonadiabatic processes

The very concept of high-Tc superconductivity has originated from the discovery of superconductivity in copper oxides by Bednorz and Müller in 1986. Soon after their discovery, cuprates were recognized as undoubtedly complex and radically unconventional superconducting materials, and research to understand the origin of superconducting phase has since then mainly focused on the strong-correlation aspects of these compounds, whereas the role of the electron–phonon coupling has been mostly ignored by much of the scientific community. Nowadays, however, thanks also to the steady later research of K.A. Müller, the presence of a relevant role of the electron–phonon coupling, in a unconventional scenario, has been assessed. Due to the small carrier concentration, and hence to the small Fermi energies, one of the concepts that needs to be revised in these compounds is the assumption of adiabaticity. In this contribution we summarize the main directions followed in this field for defining a microscopic theory of superconductivity in the nonadiabatic regime. The differences of such analysis with respect to a polaronic scenario are also discussed, as are some complementary paths of investigation of the complex many-body electron–phonon problem in different physical regimes.

A light scatterer’s vistas on copper–oxygen superconductors

This review summarizes studies of the energy gap and selected electronic properties of CuO2 superconductors. The main focus is placed on results of inelastic light scattering experiments starting as early as 1987 just a year after the discovery of superconductivity in copper–oxygen compounds by Georg Bednorz and Alex Muller. I add a few historical and personal remarks on superconductivity in general and on the contribution of the eminent scientist Karl Alex Muller who passed away in January 2023.

Incommensurate Magnetic Order in Hole‐Doped Infinite‐Layer Nickelate Superconductors due to Competing Magnetic Interactions

AbstractThe discovery of superconductivity in hole‐doped infinite‐layer nickelates has fueled intense research to identify the critical factor responsible for high‐Tc superconductivity. Magnetism and superconductivity are closely entangled, and elucidating the magnetic interactions in hole‐doped nickelates is critical for understanding the pairing mechanism. Here, these calculations based on the generalized Bloch theorem (GBT) and magnetic force theorem (MFT) consistently reveal that hole doping stabilizes an incommensurate (IC) spin state and increases the IC wave vector continuously, in a way strikingly similar to hole‐doped cuprates. Going further, a nonlinear Heisenberg model including first‐neighbor and third‐neighbor in‐plane magnetic interactions is developed. The analytical solutions successfully reproduce GBT and MFT results and reveal that the competition between the two magnetic interactions is the decisive factor for the IC magnetic transition. Eventually, by analyzing the doping‐controlled spin splitting of band and orbital‐contributed exchange interactions, direct links between hole doping, magnetization, exchange constants, and magnetic order are established. This discovery of the IC spin state, new understanding of its electronic origin, and establishment of direct connection with the paring electrons radically change the current understanding of the magnetic properties in hole‐doped NdNiO2 and open new perspectives for the superconducting mechanism in nickelates superconductors.

Tuning the lattice thermal conductivity of bilayer penta-graphene by interlayer twisting

Penta-graphene (PG) is the well-known prototype of the extensively studied 2D pentagon-based materials in recent years, and twisting strategy has been widely used to tune the properties of 2D sheets since the discovery of superconductivity in twisted bilayer graphene. However, no study is reported on how twisting affects the phonon properties of such penta-sheets. In this work, for the first time, we study the effect of twisting on the phonon transport properties of PG by using density functional theory and the phonon Boltzmann transport equation combined with two-channel transport theory and machine learning method. It is found that the in-plane lattice thermal conductivity of the twisted bilayer PG with a twist angle of 36.87° is only 96.72 W/mK at 300 K, reduced by 77.16% as compared to that of untwisted bilayer PG, and the four-phonon interaction further reduces the thermal conductivity by nearly 36%, while the coherence contribution to the thermal conductivity is enhanced to 9.69 W/mK from 0.13 W/mK by twisting. These results indicate that interlayer twisting not only can enhance three- and four-phonon scattering but also can uplift phonon coherence due to the resulted complex atomic configuration and high anharmonicity in bilayer PG.

Discovery of the High‐Entropy Carbide Ceramic Topological Superconductor Candidate (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C

AbstractHigh‐entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near‐equal atomic ratios of multi‐principal elements. Material design and property tailoring possibilities emerge from this new class of materials. Herein, the discovery of superconductivity 2.35 K and topological properties in the (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C high‐entropy carbide ceramic (HECC) is reported, which is not observed before in any of the investigated HECC. Density functional theory calculations show that six type‐II Dirac points exist in (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C, which mainly contributed from the t2g orbitals of transition metals and the p orbitals of C. Due to the stability of the structure, robust superconductivity (SC) under pressure in this HEC superconductor is also observed. This study expands the physical properties of HECs, which may become a new material platform for SC research, especially for studying the coupling between SC and topological physics.

Electronic structure of cuprate–nickelate infinite-layer heterostructure

The discovery of superconductivity in Sr/Ca-doped infinite-layer nickelates Nd(La)NiO2 thin films inspired extensive experimental and theoretical research. However, research on the possibilities of enhanced critical temperature by interface heterostructure is still lacking. Due to the similarities of the crystal structure and band structure of infinite-layer nickelate LaNiO2 and cuprate CaCuO2, we investigate the crystal, electronic and magnetic properties of LaNiO2:CaCuO2 heterostructure using density functional theory and dynamical mean-field theory. Our theoretical results demonstrate that, even a very weak inter-layer z-direction bond is formed, an intrinsic charge transfer between Cu-3d x 2–y 2 and Ni-3d x 2–y 2 orbitals is obtained. The weak interlayer hopping between Cu and Ni leaves a parallel band contributed by Ni/Cu-3d x 2–y 2 orbitals near the Fermi energy. Such an infinite-layer heterostructure with negligible interlayer interaction and robust charge transfer opens a new way for interface engineering and nickelate superconductors.

Colloquium : Room temperature superconductivity: The roles of theory and materials design

For half a century after the discovery of superconductivity, materials exploration for better superconductors proceeded without knowledge of the underlying mechanism. The 1957 BCS theory cleared that up: the superconducting state occurs due to pairing of electrons over the Fermi surface. Over the following half century higher critical temperature T$_c$ was achieved only serendipitously as new materials were synthesized. Meanwhile the formal theory of phonon-coupled superconductivity at the material-dependent level became highly developed: given a known compound, its value of T$_c$, the superconducting gap function, and several other properties of the superconducting state became available independent of further experimental input. More recently, density functional theory based computational materials design has progressed to a predictive level -- new materials can be predicted on the basis of various numerical algorithms. Taken together, these capabilities enable theoretical prediction of new superconductors. Here the process that resulted in three new highest temperature superconductors, predicted numerically, confirmed experimentally -- SH$_3$, LaH$_{10}$, and YH$_9$ -- is recounted. These hydrides have T$_c$ in the 200-280K range at megabar pressures, and here the development will be chronicled. Current activities and challenges are discussed, together with Regularities in compressed hydrides that can guide further exploration.