From quantum matter to high-temperature superconductivity in copper oxides

Evidence for even parity unconventional superconductivity in Sr2RuO4

Unambiguous identification of the superconducting order parameter symmetry in [Formula: see text] has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid 3He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent 17O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of <10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates.

Double Superconducting Dome and Triple Enhancement of T_{c} in the Kagome Superconductor CsV_{3}Sb_{5} under High Pressure

CsV_{3}Sb_{5} is a newly discovered Z_{2} topological kagome metal showing the coexistence of a charge-density-wave (CDW)-like order at T^{*}=94  K and superconductivity (SC) at T_{c}=2.5  K at ambient pressure. Here, we study the interplay between CDW and SC in CsV_{3}Sb_{5} via measurements of resistivity, dc and ac magnetic susceptibility under various pressures up to 6.6 GPa. We find that the CDW transition decreases with pressure and experience a subtle modification at P_{c1}≈0.6-0.9  GPa before it vanishes completely at P_{c2}≈2  GPa. Correspondingly, T_{c}(P) displays an unusual M-shaped double dome with two maxima around P_{c1} and P_{c2}, respectively, leading to a tripled enhancement of T_{c} to about 8 K at 2 GPa. The obtained temperature-pressure phase diagram resembles those of unconventional superconductors, illustrating an intimated competition between CDW-like order and SC. The competition is found to be particularly strong for the intermediate pressure range P_{c1}≤P≤P_{c2} as evidenced by the broad superconducting transition and reduced superconducting volume fraction. The modification of CDW order around P_{c1} has been discussed based on the band structure calculations. This work not only demonstrates the potential to raise T_{c} of the V-based kagome superconductors, but also offers more insights into the rich physics related to the electron correlations in this novel family of topological kagome metals.

Magnetic field reveals vanishing Hall response in the normal state of stripe-ordered cuprates

The origin of the weak insulating behavior of the resistivity, i.e. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\rho }_{xx}\propto {\mathrm{ln}}\,(1/T)$$\end{document}ρxx∝ln(1/T), revealed when magnetic fields (H) suppress superconductivity in underdoped cuprates has been a longtime mystery. Surprisingly, the high-field behavior of the resistivity observed recently in charge- and spin-stripe-ordered La-214 cuprates suggests a metallic, as opposed to insulating, high-field normal state. Here we report the vanishing of the Hall coefficient in this field-revealed normal state for all \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T\ <\ (2-6){T}_{{\rm{c}}}^{0}$$\end{document}T<(2−6)Tc0, where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{{\rm{c}}}^{0}$$\end{document}Tc0 is the zero-field superconducting transition temperature. Our measurements demonstrate that this is a robust fundamental property of the normal state of cuprates with intertwined orders, exhibited in the previously unexplored regime of T and H. The behavior of the high-field Hall coefficient is fundamentally different from that in other cuprates such as YBa₂Cu₃O_6+x and YBa₂Cu₄O₈, and may imply an approximate particle-hole symmetry that is unique to stripe-ordered cuprates. Our results highlight the important role of the competing orders in determining the normal state of cuprates.

The Hall effect has been used as a probe of the normal state of cuprates, when superconductivity is supressed by a magnetic field. Here, the authors report the vanishing of the Hall coefficient at high magnetic field in cuprates with stripe order and interpret it as a signature of the stripe-ordered phase.

Unusual competition of superconductivity and charge-density-wave state in a compressed topological kagome metal

Understanding the competition between superconductivity and other ordered states (such as antiferromagnetic or charge-density-wave (CDW) state) is a central issue in condensed matter physics. The recently discovered layered kagome metal AV₃Sb₅ (A = K, Rb, and Cs) provides us a new playground to study the interplay of superconductivity and CDW state by involving nontrivial topology of band structures. Here, we conduct high-pressure electrical transport and magnetic susceptibility measurements to study CsV₃Sb₅ with the highest T_c of 2.7 K in AV₃Sb₅ family. While the CDW transition is monotonically suppressed by pressure, superconductivity is enhanced with increasing pressure up to P1 ≈ 0.7 GPa, then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa, after that, T_c is enhanced with increasing pressure again. The CDW is completely suppressed at a critical pressure P2 ≈ 2 GPa together with a maximum T_c of about 8 K. In contrast to a common dome-like behavior, the pressure-dependent T_c shows an unexpected double-peak behavior. The unusual suppression of T_c at P1 is concomitant with the rapidly damping of quantum oscillations, sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance. Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome lattice.

The recently discovered kagome metal AV₃Sb₅ is a new playground to study the interplay between superconductivity and charge-density-wave (CDW) state. Here, the authors report pressure-dependent evolution of CDW and superconductivity in CsV₃Sb₅, suggesting an unusual competition between the two phases.

Charge order lock-in by electron-phonon coupling in La1.675Eu0.2Sr0.125CuO4

Charge order is universal to all hole-doped cuprates. Yet, the driving interactions remain an unsolved problem. Electron-electron interaction is widely believed to be essential, whereas the role of electron-phonon interaction is unclear. We report an ultrahigh-resolution resonant inelastic x-ray scattering (RIXS) study of the in-plane bond-stretching phonon mode in stripe-ordered cuprate La1.675Eu0.2Sr0.125CuO4 Phonon softening and lifetime shortening are found around the charge ordering wave vector. In addition to these self-energy effects, the electron-phonon coupling is probed by its proportionality to the RIXS cross section. We find an enhancement of the electron-phonon coupling around the charge-stripe ordering wave vector upon cooling into the low-temperature tetragonal structure phase. These results suggest that, in addition to electronic correlations, electron-phonon coupling contributes substantially to the emergence of long-range charge-stripe order in cuprates.

Controlling unconventional superconductivity in artificially engineeredf-electron Kondo superlattices

Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials, including high-T_ccuprates, iron pnictides, and heavy-fermion compounds. Interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures provide a new approach to this most fundamental and hotly debated subject. We have been able to use a recent state-of-the-art molecular-beam-epitaxy technique to fabricate superlattices consisting of different heavy-fermion compounds with atomic thickness. These Kondo superlattices provide a unique opportunity to study the mutual interaction between unconventional superconductivity and magnetic order through the atomic interface. Here, we design and fabricate hybrid Kondo superlattices consisting of alternating layers of superconducting CeCoIn₅withd-wave pairing symmetry and nonmagnetic metal YbCoIn₅or antiferromagnetic heavy fermion metals such as CeRhIn₅and CeIn₃. In these Kondo superlattices, superconducting heavy electrons are confined within the two-dimensional CeCoIn₅block layers and interact with neighboring nonmagnetic or magnetic layers through the interface. Superconductivity is strongly influenced by local inversion symmetry breaking at the interface in CeCoIn₅/YbCoIn₅superlattices. The superconducting and antiferromagnetic states coexist in spatially separated layers in CeCoIn₅/CeRhIn₅and CeCoIn₅/CeIn₃superlattices, but their mutual coupling via the interface significantly modifies the superconducting and magnetic properties. The fabrication of a wide variety of hybrid superlattices paves a new way to study the relationship between unconventional superconductivity and magnetism in strongly correlated materials.

Evidence for Moiré Trions in Twisted MoSe2 Homobilayers

Moiré superlattices of van der Waals structures offer a powerful platform for engineering band structure and quantum states. For instance, Moiré superlattices in magic-angle twisted bilayer graphene, ABC trilayer graphene have been shown to harbor correlated insulating and superconducting states, while in transition metal dichalcogenide (TMD) twisted bilayers, Moiré excitons have been identified. Here we show that the effects of a Moiré superlattice on the band structure are general: In TMD twisted bilayers, excitons and exciton complexes can be trapped in the superlattice in a manner analogous to ultracold bosonic or Fermionic atoms in optical lattices. Using twisted MoSe₂ homobilayers as a model system, we present evidence for Moiré trions. Our results thus open possibilities for designer van der Waals structures hosting arrays of Fermionic or bosonic quasiparticles, which can be used to realize tunable many-body states crucial for quantum simulation and quantum information processing.

Synthesis and physical properties of perovskite Sm1-xSrxNiO3(x= 0, 0.2) and infinite-layer Sm0.8Sr0.2NiO2nickelates

Recently, superconductivity at about 9-15 K was discovered in Nd_1-xSr_xNiO₂(Nd-112,x≈ 0.125-0.25) infinite-layer thin films, which has stimulated enormous interests in related rare-earth nickelates. Usually, the first step to synthesize this 112 phase is to fabricate theRNiO₃(R-113,R: rare-earth element) phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni³⁺oxidation state. And the difficulty of preparation is enhanced as the ionic radius of rare-earth element decreases. In this work, we report the synthesis and investigation on multiple physical properties of polycrystalline perovskites Sm_1-xSr_xNiO₃(x= 0, 0.2) in which the ionic radius of Sm³⁺is smaller than that of Pr³⁺and Nd³⁺in related superconducting thin films. The structural and compositional analyses conducted by x-ray diffraction and energy dispersive x-ray spectrum reveal that the samples mainly contain the perovskite phase of Sm_1-xSr_xNiO₃with small amount of NiO impurities. Magnetization and resistivity measurements indicate that the parent phase SmNiO₃undergoes a paramagnetic-antiferromagnetic transition at about 224 K on a global insulating background. In contrast, the Sr-doped sample Sm_0.8Sr_0.2NiO₃shows a metallic behavior from 300 K down to about 12 K, while below 12 K the resistivity exhibits a slight logarithmic increase. Meanwhile, from the magnetization curves, we can see that a possible spin-glass state occurs below 12 K in Sm_0.8Sr_0.2NiO₃. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm_0.8Sr_0.2NiO₂with square NiO₂planes. The compound shows an insulating behavior which can be described by the three-dimensional variable-range-hopping model. And superconductivity is still absent in the polycrystalline Sm_0.8Sr_0.2NiO₂.

Observation of superfluidity in a strongly correlated two-dimensional Fermi gas

Understanding how strongly correlated two-dimensional (2D) systems can give rise to unconventional superconductivity with high critical temperatures is one of the major unsolved problems in condensed matter physics. Ultracold 2D Fermi gases have emerged as clean and controllable model systems to study the interplay of strong correlations and reduced dimensionality, but direct evidence of superfluidity in these systems has been missing. We demonstrate superfluidity in an ultracold 2D Fermi gas by moving a periodic potential through the system and observing no dissipation below a critical velocity v _c We measure v _c as a function of interaction strength and find a maximum in the crossover regime between bosonic and fermionic superfluidity. Our measurements enable systematic studies of the influence of reduced dimensionality on fermionic superfluidity.

Distinguishing a Mott Insulator from a Trivial Insulator with Atomic Adsorbates

In an electronic system with various interactions intertwined, revealing the origin of its many-body ground state is challenging and a direct experimental way to verify the correlated nature of an insulator has been lacking. Here we demonstrate a way to unambiguously distinguish a paradigmatic correlated insulator, a Mott insulator, from a trivial band insulator based on their distinct chemical behavior for a surface adsorbate using 1T-TaS_{2}, which has been debated between a spin-frustrated Mott insulator or a spin-singlet trivial insulator. We start from the observation of different sizes of spectral gaps on different surface terminations and show that potassium adatoms on these two surface layers behave in totally different ways. This can be straightforwardly understood from distinct properties of Mott and band insulators due to the fundamental difference of the half- and full-filled orbitals involved, respectively. This work not only solves an outstanding problem in this particularly interesting material but also provides a simple touchstone to identify the correlated ground state of electrons experimentally.

Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System

The local structure of NaTiSi_{2}O_{6} is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that, on warming, the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490 K. The ODL state is correlated over the length scale spanning ∼6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems.

Sudden Collapse of Magnetic Order in Oxygen-Deficient Nickelate Films

Antiferromagnetic order is a common and robust ground state in the parent (undoped) phase of several strongly correlated electron systems. The progressive weakening of antiferromagnetic correlations upon doping paves the way for a variety of emergent many-electron phenomena including unconventional superconductivity, colossal magnetoresistance, and collective charge-spin-orbital ordering. In this study, we explored the use of oxygen stoichiometry as an alternative pathway to modify the coupled magnetic and electronic ground state in the family of rare earth nickelates (RENiO_{3-x}). Using a combination of x-ray spectroscopy and resonant soft x-ray magnetic scattering, we find that, while oxygen vacancies rapidly alter the electronic configuration within the Ni and O orbital manifolds, antiferromagnetic order is remarkably robust to substantial levels of carrier doping, only to suddenly collapse beyond 0.21 e^{-}/Ni without an accompanying structural transition. Our work demonstrates that ordered magnetism in RENiO_{3-x} is mostly insensitive to carrier doping up to significant levels unseen in other transition-metal oxides. The sudden collapse of ordered magnetism upon oxygen removal may provide a new mechanism for solid-state magnetoionic switching and new applications in antiferromagnetic spintronics.

Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates

Copper-based (cuprate) oxides are not only the original but also one of the best-studied families of “high-temperature” superconductors. With nominally identical crystal structure and electron count, nickel-based (nickelate) compounds have been widely pursued for decades as a possible analog to the cuprates. The recent demonstration of superconductivity in nickelate thin films has provided an experimental platform to explore the possible connections between the copper- and nickel-based superconductors. Here, we perform highly localized spectroscopic measurements to reveal a number of key differences between the two systems, particularly with regard to the hybridization between the O and metal (Cu or Ni) orbitals.

The recent observation of superconductivity in Nd0.8Sr0.2NiO2 has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott–Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott–Hubbard character of Nd1−xSrxNiO2. Upon doping, we observe emergent hybridization reminiscent of the Zhang–Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.

Moiré Patterns in 2D Materials: A Review

Quantum materials have attracted much attention in recent years due to their exotic and incredible properties. Among them, van der Waals materials stand out due to their weak interlayer coupling, providing easy access to manipulating electrical and optical properties. Many fascinating electrical, optical, and magnetic properties have been reported in the moiré superlattices, such as unconventional superconductivity, photonic dispersion engineering, and ferromagnetism. In this review, we summarize the methods to prepare moiré superlattices in the van der Waals materials and focus on the current discoveries of moiré pattern-modified electrical properties, recent findings of atomic reconstruction, as well as some possible future directions in this field.

Two-Dimensional Superconducting Fluctuations Associated with Charge-Density-Wave Stripes in La_{1.87}Sr_{0.13}Cu_{0.99}Fe_{0.01}O_{4}

The presence of a small concentration of in-plane Fe dopants in La_{1.87}Sr_{0.13}Cu_{0.99}Fe_{0.01}O_{4} is known to enhance stripelike spin and charge density wave (SDW and CDW) order and suppress the superconducting T_{c}. Here, we show that it also induces highly two-dimensional superconducting correlations that have been argued to be the signatures of a new form of superconducting order, the so-called pair density wave (PDW) order. In addition, using resonant soft x-ray scattering, we find that the two-dimensional superconducting fluctuation is strongly associated with the CDW stripe. In particular, the PDW signature first appears when the correlation length of the CDW stripe grows over eight times the lattice unit (∼8a). These results provide critical conditions for the formation of the PDW order.

First-principles calculations of effects of pressure on paramagnetic, ferromagnetic, and antiferromagnetic spin-web Cu3TeO6

The structure, electronic, and magnetic properties have been investigated by the first-principles calculations for paramagnetic, ferromagnetic, and antiferromagnetic Cu₃TeO₆ under pressure from 0 to 100 GPa. The calculated lattice parameters at 0 GPa are in excellent agreement with the available calculated and experimental values. With increasing pressure, the lattice parameters and volume decrease, but Cu₃TeO₆ keeps a stable cubic structure. The electronic calculations show that paramagnetic and ferromagnetic Cu₃TeO₆ are metallic, and antiferromagnetic Cu₃TeO₆ is non-metallic with a direct band gap which decreases with the increasing pressure. Under the pressure, their non-locality of density of states enhances and the electrons become more active. Moreover, for antiferromagnetic Cu₃TeO₆, the spin moments of Cu atoms are affected obviously by pressures, and Te atoms show nonmagnetic performance. The total magnetic moment, which is mainly contributed by Cu, reaches the maximum at 20 GPa, and decreases with the increasing pressure. The knowledge of these properties will provide reference and guidance for the subsequent study of Cu₃TeO₆.

Room Temperature Syntheses of ZnO and Their Structures

ZnO has many technological applications which largely depend on its properties, which can be tuned by controlled synthesis. Ideally, the most convenient ZnO synthesis is carried out at room temperature in an aqueous solvent. However, the correct temperature values are often loosely defined. In the current paper, we performed the synthesis of ZnO in an aqueous solvent by varying the reaction and drying temperatures by 10 °C steps, and we monitored the synthesis products primarily by XRD). We found out that a simple direct synthesis of ZnO, without additional surfactant, pumping, or freezing, required both a reaction (TP) and a drying (TD) temperature of 40 °C. Higher temperatures also afforded ZnO, but lowering any of the TP or TD below the threshold value resulted either in the achievement of Zn(OH)2 or a mixture of Zn(OH)2/ZnO. A more detailed Rietveld analysis of the ZnO samples revealed a density variation of about 4% (5.44 to 5.68 gcm−3) with the synthesis temperature, and an increase of the nanoparticles’ average size, which was also verified by SEM images. The average size of the ZnO synthesized at TP = TD = 40 °C was 42 nm, as estimated by XRD, and 53 ± 10 nm, as estimated by SEM. For higher synthesis temperatures, they vary between 76 nm and 71 nm (XRD estimate) or 65 ± 12 nm and 69 ± 11 nm (SEM estimate) for TP = 50 °C, TD = 40 °C, or TP = TD = 60 °C, respectively. At TP = TD = 30 °C, micrometric structures aggregated in foils are obtained, which segregate nanoparticles of ZnO if TD is raised to 40 °C. The optical properties of ZnO obtained by UV-Vis reflectance spectroscopy indicate a red shift of the band gap by ~0.1 eV.

Fermi surface in La-based cuprate superconductors from Compton scattering imaging

Compton scattering provides invaluable information on the underlying Fermi surface (FS) and is a powerful tool complementary to angle-resolved photoemission spectroscopy and quantum oscillation measurements. Here we perform high-resolution Compton scattering measurements for La_2−xSr_xCuO₄ with x = 0.08 (T_c = 20 K) at 300 K and 150 K, and image the momentum distribution function in the two-dimensional Brillouin zone. We find that the observed images cannot be reconciled with the conventional hole-like FS believed so far. Instead, our data imply that the FS is strongly deformed by the underlying nematicity in each CuO₂ plane, but the bulk FSs recover the fourfold symmetry. We also find an unusually strong temperature dependence of the momentum distribution function, which may originate from the pseudogap formation in the presence of the reconstructed FSs due to the underlying nematicity. Additional measurements for x = 0.15 and 0.30 at 300 K suggest similar FS deformation with weaker nematicity, which nearly vanishes at x = 0.30.

Compton scattering provides information on the Fermi surface (FS) hence very useful to understand the electronic structure of high temperature superconductors. Here, Yamase et al. perform Compton scattering measurements on La_2−xSr_xCuO₄ samples and observe deformed FS in CuO₂ plane due to nematicity but recovering fourfold symmetry in bulk FS.

Unmasking the Origin of Kinks in the Photoemission Spectra of Cuprate Superconductors

The origin of a ubiquitous bosonic coupling feature in the photoemission spectra of high-T_{c} cuprates, an energy-momentum dispersion "kink" observed at ∼70  meV binding energy, remains a two-decade-old mystery. Understanding this phenomenon requires an accurate description of the coupling between the electron and some collective modes. We report here ab initio calculations based on GW perturbation theory and show that correlation-enhanced electron-phonon interaction in cuprates gives rise to the strong kinks, which not only explains quantitatively the observations but provides new understanding of experiments. Our results reveal it is the electron density of states being the predominant factor in determining the doping dependence of the kink size, manifesting the multiband nature of the cuprates, as opposed to the prevalent belief of it being a measure of the mode-coupling strength.

Gate-controlled BCS-BEC crossover in a two-dimensional superconductor

Bardeen-Cooper-Schrieffer (BCS) superfluidity and Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems. We report crossover behavior from the BCS limit to the BEC limit realized by varying carrier density in a two-dimensional superconductor, electron-doped zirconium nitride chloride. The phase diagram, established by simultaneous measurements of resistivity and tunneling spectra under ionic gating, demonstrates a pseudogap phase in the low-doping regime. The ratio of the superconducting transition temperature and Fermi temperature in the low-carrier density limit is consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. These results indicate that the gate-doped semiconductor provides an ideal platform for the two-dimensional BCS-BEC crossover without added complexities present in other solid-state systems.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3

PbMO₃ (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO₃ remain unclear. Herein, we reveal that PbFeO₃ crystallizes into an unusual 2a_p × 6a_p × 2a_p orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb²⁺_0.5Pb⁴⁺_0.5FeO₃ lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb²⁺ and Pb⁴⁺) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe³⁺ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Tuning electron correlation in magic-angle twisted bilayer graphene using Coulomb screening

Controlling the strength of interactions is essential for studying quantum phenomena emerging in systems of correlated fermions. We introduce a device geometry whereby magic-angle twisted bilayer graphene is placed in close proximity to a Bernal bilayer graphene, separated by a 3-nanometer-thick barrier. By using charge screening from the Bernal bilayer, the strength of electron-electron Coulomb interaction within the twisted bilayer can be continuously tuned. Transport measurements show that tuning Coulomb screening has opposite effects on the insulating and superconducting states: As Coulomb interaction is weakened by screening, the insulating states become less robust, whereas the stability of superconductivity at the optimal doping is enhanced. The results provide important constraints on theoretical models for understanding the mechanism of superconductivity in magic-angle twisted bilayer graphene.

Surface localized magnetism in transition metal doped alumina

Alumina is a structural ceramic that finds many uses in a broad range of applications. It is widely employed in the aerospace and biomedical sectors due to its stability at high temperatures and in harsh chemical environments. Here, we show that magnetism can be induced at alumina surfaces by doping with 3d transition metals. We analyze the electronic structure, spin magnetic moments, and spin density of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha $$\end{document}α-Al\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{2}$$\end{document}2O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{3}$$\end{document}3 as a function of both dopant species (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) and depth using first principles calculations. Our results show that all dopants, with the exception of Sc, produce magnetic moments that are concentrated to the surface of alumina with varying degrees of delocalization. It is seen that all of the dopants are at least meta-stable on the surface and must overcome an energy barrier of 0.19–1.14 eV in order to diffuse from the surface into the bulk. As a result of judiciously doping with select 3d transition metals the surface of alumina can be made magnetic. This could lead to novel applications in data storage, catalysis, and biomedical engineering through an added surface functionality.

Metal-Insulator Transition and Emergent Gapped Phase in the Surface-Doped 2D Semiconductor 2H-MoTe_{2}

Artificially created two-dimensional (2D) interfaces or structures are ideal for seeking exotic phase transitions due to their highly tunable carrier density and interfacially enhanced many-body interactions. Here, we report the discovery of a metal-insulator transition (MIT) and an emergent gapped phase in the metal-semiconductor interface that is created in 2H-MoTe_{2} via alkali-metal deposition. Using angle-resolved photoemission spectroscopy, we found that the electron-phonon coupling is strong at the interface as characterized by a clear observation of replica shake-off bands. Such strong electron-phonon coupling interplays with disorder scattering, leading to an Anderson localization of polarons which could explain the MIT. The domelike emergent gapped phase could then be attributed to a polaron extended state or phonon-mediated superconductivity. Our results demonstrate the capability of alkali-metal deposition as an effective method to enhance the many-body interactions in 2D semiconductors. The surface-doped 2H-MoTe_{2} is a promising candidate for realizing polaronic insulator and high-T_{c} superconductivity.

Tracing the ionic evolution during ILG induced phase transformation in strontium cobaltite thin films

Ionic liquid gating (ILG) that drives the ions incorporate into or extract from the crystal lattice, has emerged as a new pathway to design materials. Although many intriguing emergent phenomena, novel physical properties and functionalities have been obtained, the gating mechanism governing the ion and charge transport remains unexplored. Here, by using the model system of brownmillerite SrCoO_2.5 and the corresponding electric-field controlled tri-state phase transformation among the pristine SrCoO_2.5, hydrogenated HSrCoO_2.5 and oxidized perovskite SrCoO_3-δ through the dual ion switch, the ionic diffusion and electronic transport processes were carefully investigated. Through controlling gating experiment by design, we find out that the collaborative interaction between charge transport and ion diffusion plays an essential role to prompt the hydrogen or oxygen ions incorporate into the crystal lattice of SrCoO_2.5, and therefore leading to formation of new phases. At region closer to the electrode, the electron can shuttle more readily in (out) the material, correspondingly the incorporation of hydrogen (oxygen) ions and phase transformation is largely affiliated. With the compensated charge of electron as well as the reaction front gradually moving away from the electrode, the new phases would be developed successively across the entire thin film. This result unveils the underlying mechanism in the electric-field control of ionic incorporation and extraction, and therefore provides important strategy to achieve high efficient design of material functionalities in complex oxide materials.

Thermally driven state in a spin-1 model with competing interactions

We study a recently proposed spin-1 model with competing antiferromagnetic first-neighbor interaction and a third-neighbor coupling mediated by nonmagnetic states, which reproduces topological features of the phase diagrams of high-T_{c} superconductors [S. A. Cannas and D. A. Stariolo, Phys. Rev. E 99, 042137 (2019)2470-004510.1103/PhysRevE.99.042137]. We employ a cluster mean-field approach to investigate effects of crystal field anisotropy on the phase transitions hosted by this model. At low temperatures, the temperature-crystal field phase diagram exhibits superantiferromagnetic (SAF), antiferromagnetic (AF), and paramagnetic (PM) phases. In addition, we found a thermally driven state between SAF and PM phases. This thermally driven state and the SAF phase appears in the phase diagram as a domelike structure. Our calculations indicate that only second-order phase transitions occur in the PM-AF phase boundary, as suggested by previous Monte Carlo simulations.

Strongly correlated superconductivity in a copper-based metal-organic framework with a perfect kagome lattice

Metal-organic frameworks (MOFs), which are self-assemblies of metal ions and organic ligands, provide a tunable platform to search a new state of matter. A two-dimensional (2D) perfect kagome lattice, whose geometrical frustration is a key to realizing quantum spin liquids, has been formed in the π - d conjugated 2D MOF [Cu₃(C₆S₆)] _n (Cu-BHT). The recent discovery of its superconductivity with a critical temperature T _c of 0.25 kelvin raises fundamental questions about the nature of electron pairing. Here, we show that Cu-BHT is a strongly correlated unconventional superconductor with extremely low superfluid density. A nonexponential temperature dependence of superfluid density is observed, indicating the possible presence of superconducting gap nodes. The magnitude of superfluid density is much smaller than those in conventional superconductors and follows the Uemura's relation of strongly correlated superconductors. These results imply that the unconventional superconductivity in Cu-BHT originates from electron correlations related to spin fluctuations of kagome lattice.

The morphologic correlation between vortex transformation and upper critical field line in opal-based nanocomposites

In this study, we investigate metallic nanocomposites to elucidate the properties of nanostructured conventional superconductors. Liquid tin, indium, and mercury are loaded into opal matrices by high pressure up to 10 kbar. The opal templates preserve the 3D dendritic morphology of confined superconducting metals to model a dendritic second phase with particular grain shape in bulk superconductors observed by a DualBeam microscope. We carry out measurements of the dc and ac magnetizations to study the superconducting phase diagrams, vortex dynamics, and impact of grain morphology in the opal composites. Besides, we apply the small-angle neutron scattering (SANS) to deny a regular vortex structure. The phase diagrams reveal an enhanced upper critical field H_c2(0) and curvature crossover in the upper critical field line. We also calculate the vortex activation barriers U_a and observe a transformation in the vortex system. According to the field dependence of U_a, the vortex structure transformation highly correlates with the curvature crossover in the upper critical field line. Our observations suggest that the similarity in the normalized phase diagrams and field dependences of U_a in the three nanocomposites is owing to their particular morphology of confinement.

Anomalous vortex liquid in charge-ordered cuprate superconductors

The magnetic-field scale at which superconducting vortices persist in underdoped cuprate superconductors has remained a controversial subject. Here we present an electrical transport study on three distinctly different cuprate families, at temperatures down to 0.32 K and magnetic fields up to 45 T. We reveal the presence of an anomalous vortex liquid state with a highly nonohmic resistivity in all three materials, irrespective of the level of disorder or structural details. The doping and field regime over which this anomalous vortex state persists suggests its occurrence is tied to the presence of long-range charge order under high magnetic field. Our results demonstrate that the intricate interplay between charge order and superconductivity can lead to an exotic vortex state.

The interplay between charge order and d-wave superconductivity in high-Tc cuprates remains an open question. While mounting evidence from spectroscopic probes indicates that charge order competes with superconductivity, to date little is known about the impact of charge order on charge transport in the mixed state, when vortices are present. Here we study the low-temperature electrical resistivity of three distinctly different cuprate families under intense magnetic fields, over a broad range of hole doping and current excitations. We find that the electronic transport in the doping regime where long-range charge order is known to be present is characterized by a nonohmic resistivity, the identifying feature of an anomalous vortex liquid. The field and temperature range in which this nonohmic behavior occurs indicates that the presence of long-range charge order is closely related to the emergence of this anomalous vortex liquid, near a vortex solid boundary that is defined by the excitation current in the T→ 0 limit. Our findings further suggest that this anomalous vortex liquid, a manifestation of fragile superconductivity with a suppressed critical current density, is ubiquitous in the high-field state of charge-ordered cuprates.

Asymptotic freedom and noninteger dimensionality

This paper shows that below a critical value of dimensionality that lies between two and three, the potential between objects begins to fall as the energy levels increase. For dimensionality below two, the potential becomes constant irrespective of separation and the force between them disappears, which represents a new paradigm of asymptotic freedom. Since asymptotic freedom is at the basis of many applications such as those associated with strange metals, unconventional superconductors, and fractional quantum Hall states, the new paradigm can have novel applications. It also is of relevance to the study of anomalous mechanical effects that are important in metamaterials.

The charge-density-wave signature on the superfluid density of cuprate superconductors

The superfluid density or superconducting (SC) carrier concentrationn_scof cuprates has been the subject of intense investigations but there is not any single theory capable to explain all the available data. Here we show that the behavior ofn_scin under and overdoped cuprates are a consequence of an SC interaction based on charge fluctuations in the incommensurate charge-density-waves (CDW) domains. We have shown that this interaction scales with the CDW amplitude or the pseudogap (PG) energy, yielding local SC amplitudes and Josephson currents. The average Josephson energyEJis proportional to the phase stiffness or superfluid densityρ_sc∝n_sc. We find thatn_sc(p) increases almost linearly with dopingpin the underdoped region and in the charge abundant overdoped only a few fractions of the holes condense leading to two kinds of carriers, a recently confirmed feature. The calculations and theρ_scdata uncover how the PG-CDW-SC intertwined orders operate to yield cuprates properties.

Dominant Fifth-Order Correlations in Doped Quantum Antiferromagnets

Traditionally, one- and two-point correlation functions are used to characterize many-body systems. In strongly correlated quantum materials, such as the doped 2D Fermi-Hubbard system, these may no longer be sufficient, because higher-order correlations are crucial to understanding the character of the many-body system and can be numerically dominant. Experimentally, such higher-order correlations have recently become accessible in ultracold atom systems. Here, we reveal strong non-Gaussian correlations in doped quantum antiferromagnets and show that higher-order correlations dominate over lower-order terms. We study a single mobile hole in the t-J model using the density matrix renormalization group and reveal genuine fifth-order correlations which are directly related to the mobility of the dopant. We contrast our results to predictions using models based on doped quantum spin liquids which feature significantly reduced higher-order correlations. Our predictions can be tested at the lowest currently accessible temperatures in quantum simulators of the 2D Fermi-Hubbard model. Finally, we propose to experimentally study the same fifth-order spin-charge correlations as a function of doping. This will help to reveal the microscopic nature of charge carriers in the most debated regime of the Hubbard model, relevant for understanding high-T_{c} superconductivity.

Time-reversal symmetry breaking in the Fe-chalcogenide superconductors

Topological superconductivity has been sought in a variety of heterostructure systems, the interest being that a material displaying such a phenomenon could prove to be the ideal platform to support Majorana fermions, which in turn could be the basis for advanced qubit technologies. Recently, the high-T_c family of superconductors, FeTe_1-xSe_x, have been shown to exhibit the property of topological superconductivity and further, evidence has been found for the presence of Majorana fermions. We have studied the interplay of topology, magnetism, and superconductivity in the FeTe_1-x Se _x family using high-resolution laser-based photoemission. At the bulk superconducting transition, a gap opens at the chemical potential as expected. However, a second gap is observed to open at the Dirac point in the topological surface state. The associated mass acquisition in the topological state points to time-reversal symmetry breaking, probably associated with the formation of ferromagnetism in the surface layer. The presence of intrinsic ferromagnetism combined with strong spin-orbit coupling provides an ideal platform for a range of exotic topological phenomena.

Review on quasi-2D square planar nickelates

Quasi-2D square planar nickelates exhibit key ingredients of high-T_c superconducting cuprates. Whether bulk samples are superconducting remains an open question, single crystals are ideal platforms for addressing such fundamental questions.

High-energy magnetic excitations from heavy quasiparticles in CeCu2Si2

Magnetic fluctuations is the leading candidate for pairing in cuprate, iron-based, and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in C e C u 2 S i 2 , and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in superconducting (S-type) C e C u 2 S i 2 using inelastic neutron scattering, finding a strongly asymmetric dispersion for E ≲ 1.5 m e V , which at higher energies evolves into broad columnar magnetic excitations that extend to E ≳ 5 m e V . While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in C e C u 2 S i 2 are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in C e C u 2 S i 2 arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in C e C u 2 S i 2 , and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals.

Detection of Acoustic Plasmons in Hole-Doped Lanthanum and Bismuth Cuprate Superconductors Using Resonant Inelastic X-Ray Scattering

High T_{c} superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic x-ray scattering to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors (La_{1.84}Sr_{0.16}CuO_{4} and Bi_{2}Sr_{1.6}La_{0.4}CuO_{6+δ}), crucially completing the picture. Interestingly, in contrast to the quasistatic charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behavior of valence charges in hole-doped cuprates.

Doping and temperature evolutions of optical response of Sr3(Ir1-xRux)2O7

We report on optical spectroscopic study of the Sr₃(Ir_1-xRu_x)₂O₇ system over a wide doping regime. We find that the changes in the electronic structure occur in the limited range of the concentration of Ru ions where the insulator-metal transition occurs. In the insulating regime, the electronic structure associated with the effective total angular momentum J_eff = 1/2 Mott state remains robust against Ru doping, indicating the localization of the doped holes. Upon entering the metallic regime, the Mott gap collapses and the Drude-like peak with strange metallic character appears. The evolution of the electronic structure registered in the optical data can be explained in terms of a percolative insulator-metal transition. The phonon spectra display anomalous doping evolution of the lineshapes. While the phonon modes of the compounds deep in the insulating and metallic regimes are almost symmetric, those of the semiconducting compound with x = 0.34 in close proximity to the doping-driven insulator-metal transition show a pronounced asymmetry. The temperature evolution of the phonon modes of the x = 0.34 compound reveals the asymmetry is enhanced in the antiferromagnetic state. We discuss roles of the S = 1 spins of the Ru ions and charge excitations for the conspicuous lineshape asymmetry of the x = 0.34 compound.

Nonadiabatic coupling of the dynamical structure to the superconductivity in YSr2Cu2.75Mo0.25O7.54 and Sr2CuO3.3

The Cu extended X-ray absorption fine structure of YSr₂Cu_2.75Mo_0.25O_7.54 (with superconducting critical temperature, T_c, = 84 K) and Sr₂CuO_3.3 (T_c = 95 K) through their superconducting transitions demonstrates that the common factor in superconductivity in cuprates, including those prepared by high-pressure oxygenation, is an internal quantum tunneling polaron in its dynamical structure. In addition, Sr₂CuO_3.3 is the first material to show a concomitant transformation in this structure involving atom displacements >1 Å that would be expected to modify its Fermi surface, which would complicate the transition beyond a purely electronic one consisting of the pairing of electrons of opposite momentum across fixed electronic states.

A crucial issue in cuprates is the extent and mechanism of the coupling of the lattice to the electrons and the superconductivity. Here we report Cu K edge extended X-ray absorption fine structure measurements elucidating the internal quantum tunneling polaron (iqtp) component of the dynamical structure in two heavily overdoped superconducting cuprate compounds, tetragonal YSr₂Cu_2.75Mo_0.25O_7.54 with superconducting critical temperature, T_c = 84 K and hole density p = 0.3 to 0.5 per planar Cu, and the tetragonal phase of Sr₂CuO_3.3 with T_c = 95 K and p = 0.6. In YSr₂Cu_2.75Mo_0.25O_7.54 changes in the Cu-apical O two-site distribution reflect a sequential renormalization of the double-well potential of this site beginning at T_c, with the energy difference between the two minima increasing by ∼6 meV between T_c and 52 K. Sr₂CuO_3.3 undergoes a radically larger transformation at T_c, >1-Å displacements of the apical O atoms. The principal feature of the dynamical structure underlying these transformations is the strongly anharmonic oscillation of the apical O atoms in a double-well potential that results in the observation of two distinct O sites whose Cu–O distances indicate different bonding modes and valence-charge distributions. The coupling of the superconductivity to the iqtp that originates in this nonadiabatic coupling between the electrons and lattice demonstrates an important role for the dynamical structure whereby pairing occurs even in a system where displacements of the atoms that are part of the transition are sufficiently large to alter the Fermi surface. The synchronization and dynamic coherence of the iqtps resulting from the strong interactions within a crystal would be expected to influence this process.

Modeling Unconventional Superconductivity at the Crossover between Strong and Weak Electronic Interactions

High-temperature superconductivity emerges in many different quantum materials, often in regions of the phase diagram where the electronic kinetic energy is comparable to the electron-electron repulsion. Describing such intermediate-coupling regimes has proven challenging as standard perturbative approaches are inapplicable. Here, we employ quantum Monte Carlo methods to solve a multiband Hubbard model that does not suffer from the sign problem and in which only repulsive interband interactions are present. In contrast to previous sign-problem-free studies, we treat magnetic, superconducting, and charge degrees of freedom on an equal footing. We find an antiferromagnetic dome accompanied by a metal-to-insulator crossover line in the intermediate-coupling regime, with a smaller superconducting dome appearing in the metallic region. Across the antiferromagnetic dome, the magnetic fluctuations change from overdamped in the metallic region to propagating in the insulating region. Our findings shed new light on the intertwining between superconductivity, magnetism, and charge correlations in quantum materials.

Anomalous Doping Evolution of Superconductivity and Quasiparticle Interference in Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ} Trilayer Cuprates

We use scanning tunneling microscopy to investigate Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ} trilayer cuprates from the optimally doped to overdoped regime. We find that the two distinct superconducting gaps from the inner and outer CuO_{2} planes both decrease rapidly with doping, in sharp contrast to the nearly constant T_{C}. Spectroscopic imaging reveals the absence of quasiparticle interference in the antinodal region of overdoped samples, showing an opposite trend to that in single- and double-layer compounds. We propose that the existence of two types of inequivalent CuO_{2} planes and the intricate interaction between them are responsible for these anomalies in trilayer cuprates.

Approaching Two-Dimensional Superconductivity in Ultrathin DyBa_{2}Cu_{3}O_{7-δ}

The temperature dependence of the superfluid density ρ_{s}(T) has been measured for a series of ultrathin MBE-grown DyBa_{2}Cu_{3}O_{7-δ} superconducting (SC) films by submillimeter wave interferometry combined with time-domain terahertz spectroscopy and IR ellipsometry. We find that all films 10 u.c. and thicker show the same universal temperature dependence of ρ_{s}(T), which follows the critical behavior characteristic of single crystal YBa_{2}Cu_{3}O_{7-δ} as T approaches T_{c}. In 7 u.c. thick films, ρ_{s}(T) declines steeply upon approaching T_{c}, as expected for the Berezinskii-Kosterlitz-Thouless vortex unbinding transition. Our analysis provides evidence for a sharply defined 4 u.c. non-SC interfacial layer, leaving a quasi-2D SC layer on top. We propose that the SC state in this interfacial layer is suppressed by competing (possibly charge) order.

Graphene bilayers with a twist

Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena, magic-angle twisted bilayer graphene hosts superconductivity, interaction-induced insulating states, magnetism, electronic nematicity, linear-in-temperature low-temperature resistivity and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open and comment on the place of magic-angle twisted bilayer graphene in the strongly correlated quantum matter world.

DMFT Reveals the Non-Hermitian Topology and Fermi Arcs in Heavy-Fermion Systems

When a strongly correlated system supports well-defined quasiparticles, it allows for an elegant one-body effective description within the non-Hermitian topological theory. While the microscopic many-body Hamiltonian of a closed system remains Hermitian, the one-body quasiparticle Hamiltonian is non-Hermitian due to the finite quasiparticle lifetime. We use such a non-Hermitian description in the heavy-fermion two-dimensional systems with the momentum-dependent hybridization to reveal a fascinating phenomenon which can be directly probed by the spectroscopic measurements, the bulk "Fermi arcs." Starting from a simple two-band model, we first combine the phenomenological approach with the perturbation theory to show the existence of the Fermi arcs and reveal their connection to the topological exceptional points, special points in the Brillouin zone where the Hamiltonian is nondiagonalizable. The appearance of such points necessarily requires that the electrons belonging to different orbitals have different lifetimes. This requirement is naturally satisfied in the heavy-fermion systems, where the itinerant c electrons experience much weaker interaction than the localized f electrons. We then utilize the dynamical mean field theory to numerically calculate the spectral function and confirm our findings. We show that the concept of the exceptional points in the non-Hermitian quasiparticle Hamiltonians is a powerful tool for predicting new phenomena in strongly correlated electron systems.

Manipulating Intertwined Orders in Solids with Quantum Light

Intertwined orders exist ubiquitously in strongly correlated electronic systems and lead to intriguing phenomena in quantum materials. In this Letter, we explore the unique opportunity of manipulating intertwined orders through entangling electronic states with quantum light. Using a quantum Floquet formalism to study the cavity-mediated interaction, we show the vacuum fluctuations effectively enhance the charge-density-wave correlation, giving rise to a phase with entangled electronic order and photon coherence, with putative superradiant behaviors in the thermodynamic limit. Furthermore, upon injecting even one single photon in the cavity, different orders, including s-wave and η-paired superconductivity, can be selectively enhanced. Our study suggests a new and generalizable pathway to control intertwined orders and create light-matter entanglement in quantum materials. The mechanism and methodology can be readily generalized to more complicated scenarios.

Bose-Einstein condensation superconductivity induced by disappearance of the nematic state

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe_{1 - x} S _x from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.

A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gating

Manipulating the superconducting states of high transition temperature (high-T_c) cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics. Here, employing ionic liquid gating, a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr₂CuO_4±δ (PCO) films, based on two distinct mechanisms. Firstly, with positive electric fields, the film can be reversibly switched between superconducting and non-superconducting states, attributed to the carrier doping effect. Secondly, the film becomes more resistive by applying negative bias voltage up to - 4 V, but strikingly, a non-volatile superconductivity is achieved once the gate voltage is removed. Such phenomenon represents a distinctive route of manipulating superconductivity in PCO, resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unravelled by high-resolution scanning transmission electron microscope and in situ X-ray diffraction experiments. The effective manipulation of volatile/non-volatile superconductivity in the same parent cuprate brings more functionalities to superconducting electronics, as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-T_c superconductors.