A unit cell represents the smallest repeating structure in solid-state physics and serves as the fundamental building block of a material. In iron-based superconductors, each unit cell contains two iron atoms, which form two sublattices in the two-dimensional iron layers. Under normal circumstances, these sublattices are expected to have identical physical properties due to space inversion symmetry. However, we discover that this sublattice structure can introduce a novel degree of freedom for probing unconventional pairing mechanisms in iron-based superconductors. We observe distinct dual tunneling spectra within the pairing gap energy corresponding to the two sublattices in the monolayer FeSe with atomically homogeneous (1×1) structures on SrTiO_{3}(001) substrates-a phenomenon we term sublattice dichotomy. This dichotomy can be quantitatively explained by a parity-breaking superconducting state, characterized by the coexistence of conventional pairing and interband odd-parity pairing. The interband singlet pairing arises due to the lack of inversion symmetry, which is naturally broken from the interface coupling between the FeSe and TiO_{2} layer.

Nickelates represent an emerging class of superconductors that demand innovative approaches for structural and electronic phase modulations. Continuous control over superconductor-insulator transition (SIT) in nickelates remains challenging, hindering both fundamental understanding and potential applications. Here, we demonstrate SIT in infinite-layer nickelate superconductors utilizing multiple techniques, including an operando monitored reduction (OMR) method. OMR enables ultrawide-range continuous modulation of the Ni 3d orbital electron occupancy from ∼3d7 to ∼3d9. The 3d occupancy is calibrated through systematic synchrotron X-ray absorption (XAS), combined with scanning transmission electron microscopy (STEM) annular bright field (ABF) analysis of oxygen atoms. SIT is further modulated via ionic liquid gating and magnetic field. Strikingly different from cuprates, our Nernst effect measurements show that pairing initiates at the onset of the resistive drop. The subsequent emergence of the Meissner effect at zero resistance marks the establishment of global phase coherence. Angle-dependent magnetotransport within the transition temperature regime indicates a mixture of two-dimensional (2D) and three-dimensional (3D) superconducting characters, suggesting the observed SIT deviates from the canonical 2D model. Our results provide a unique perspective on the interplay of structural and electronic phase transitions in the infinite-layer nickelates across the oxygen content-magnetic field-temperature parameter space.

Ruddlesden-Popper bilayer nickelates provide an emerging platform for studying high-temperature superconductivity, yet the superconducting pairing symmetry remains under debate. Here, we use atomic-resolution scanning tunnelling microscopy and spectroscopy to investigate superconducting 1.5-unit-cell (La,Pr)3Ni2O7 films grown on SrLaAlO4. A cryogenic ultrahigh-vacuum (UHV) sample transfer preserves an ordered sqrt(2) * sqrt(2) surface and yields reproducible U-shaped spectra with two gap scales of 14 and 20 meV and extended flat zero-conductance bottoms. By contrast, samples exposed for a longer time in UHV without cooling during transfer show V-shaped spectra despite retaining the surface reconstruction and a transport superconducting transition onset above 40 K. Wide-energy-range spectra indicate that oxygen loss can mix density-wave-related spectral weight. Our measurements provide an atomic-scale observation of the intrinsic nodeless superconducting gap in bilayer nickelate ultrathin films.

High-temperature superconductivity has been successively realized in cuprates, iron pnictides/chalcogenides, and nickelates, whereas the cobalt-based high-temperature superconductors remain to be explored. We report the discovery of superconductivity with an onset temperature of 27 K in tetragonal CoSe monolayers synthesized on redox-engineered SrTiO3(001) substrates. Using molecular beam epitaxy, we prepared atomically thin CoSe films on FeOx overlayers that expand the in-plane lattice and reduce the surface work function relative to TiOx-terminated surfaces. In-situ scanning tunneling spectroscopy reveals a symmetric superconducting gap of 14 meV with well-defined coherence peaks. Ex-situ transport measurements confirm a sharp resistance drop with zero resistance at 11 K and a Meissner diamagnetic signal at 10 K. In contrast, CoSe monolayers on TiOx-terminated surfaces show no superconducting transition down to 4 K. These results demonstrate that interface engineering via metal oxide functionalization enables tunable lattice strain and charge doping, establishing CoSe as a new platform for exploring cobalt-based high-temperature superconductivity.

Interface-enhanced superconductivity in single-unit-cell FeSe on SrTiO_{3} has been extensively pursued recently. The interfacial electron-phonon coupling (EPC) is widely proposed to enhance pairing, yet to be directly verified. Herein, using ultrafast pump-probe spectroscopy, we discover a coherent 4.2 THz optical phonon mode that emerges only upon photoexcitation in FeTe/FeSe/SrTiO_{3} or FeTe/SrTiO_{3} heterostructures, but is absent in bare Nb-doped SrTiO_{3} under identical subgap pumping. Atomic-scale electron energy loss spectroscopy in scanning transmission electron microscopy identifies this mode as out-of-plane oxygen vibrations localized at double TiO_{x} terminated interface. Crucially, the phonon amplitude in FeTe/FeSe/SrTiO_{3} is more than twice as large as that in FeTe/SrTiO_{3}, which can be attributed to higher electron doping that strengthens dipole moments penetrating the FeSe layer, thereby resulting in an enhanced interfacial EPC. Temperature-dependent dynamics further highlight the unique electronic nature of monolayer FeSe in sustaining this robust coupling. These findings provide direct evidence for dipole-mediated interfacial EPC as a critical mechanism of interfacial superconductivity in FeSe/SrTiO_{3}.

The interface-enhanced superconductivity in monolayer iron selenide (FeSe) films on SrTiO3 has been actively pursued in the past decade. Although a synergistic effect between interfacial charge transfer and interfacial electron-phonon coupling (EPC) is proposed to be responsible for the mechanism, the microscopic nature of the interfacial EPC in the enhancement of superconductivity remains highly controversial. Herein we experimentally reveal that a coherent optical phonon mode at 4.2 THz from the SrTiO3 substrate couples to FeSe electrons and modulates the quasiparticle relaxations using ultrafast pump-probe spectroscopy. This mode originates from the antiferrodistortive (AFD) transition in SrTiO3 and is significantly stronger in the presence of monolayer FeSe than that in purely FeTe-capped Nb-doped SrTiO3. Pump fluence and temperature-dependent spectroscopy measurements suggest that SrTiO3 substrate facilitates the stabilization of FeSe structure and possibly prevents the occurrence of nematic phase transition, supporting that SrTiO3 substrate modifies the electronic structure of monolayer FeSe through a strong interfacial EPC strength as large as 0.77. Our results provide unprecedented direct evidence that the strong coupling of SrTiO3 coherent phonon to FeSe electrons is indeed responsible for the high-temperature superconductivity in monolayer FeSe and SrTiO3 heterostructure.

We perform systematic first-principles calculations on the electronic structure of n-type magnetic semiconductor Ba(Zn1-x Co-x)(2)As-2 with the facilitation of HSE06 hybrid functional. Supercells are used to consider the doping of Co atoms, and the first-principles band structures are unfolded for clarity. Based on the calculation results, magnetic states are preferred by individual Co atoms doped in Ba(Zn1-x Co-x)(2)As-2 at diluted limit, and carriers are originated mainly from situations where only one Co atom exists in the nearest neighbor Zn sites out of certain doped Co atoms. The origination of carriers can be explained by the density of states and the unfolded band structure, where it is found that the scattering effects from single Co atom is small but quite large when more Co atoms are located at adjacent Zn sites. The large scattering effects of two adjacent Co atoms will alter the band structures near the Fermi-level. Carriers in Ba(Zn1-x Co-x)(2)As-2 mainly originate from the As-4p orbitals, with partial contributions from the Co-3d orbitals. Our work provides new insights into the origin of the n-type carriers in magnetic semiconductors and will inspire the development of new magnetic semiconducting systems.

The intrinsic magnetic order, large topological-magnetic gap and rich topological phases make MnBi2Te4 a wonderful platform to study exotic topological quantum states such as axion insulator and Chern insulator. To realize and manipulate these topological phases in a MnBi2Te4 thin film, precise manipulation of the electric field across the film is essential, which requires a dual-gate structure. In this work, we achieve dual-gate tuning of MnBi2Te4 thin films grown with molecular beam epitaxy on SrTiO3(111) substrates by applying the substrate and an AlOx layer as the gate dielectrics of bottom and top gates, respectively. Under magnetic field of 9T and temperature of 20 mK, the Hall and longitudinal resistivities of the films show inversed gate-voltage dependence, for both top- and bottom-gates, signifying the existence of the dissipationless edge state contributed by Chern insulator phase in the ferromagnetic configuration. The maximum of the Hall resistivity only reaches 0.8 h/e2, even with dual-gate tuning, probably due to the high density of bulk carriers introduced by secondary phases. In the antiferromagnetic state under zero magnetic field, the films show normal insulator behavior. The dual-gated MnBi2Te4 thin films lay the foundation for developing devices based on electrically tunable topological quantum states.

We perform systematic first-principles calculations on the electronic structure of n -type magnetic semiconductor Ba(Zn $_{1-x}$ Co _x ) _2 As _2 with the facilitation of HSE06 hybrid functional. Supercells are used to consider the doping of Co atoms, and the first-principles band structures are unfolded for clarity. Based on the calculation results, magnetic states are preferred by individual Co atoms doped in Ba(Zn $_{1-x}$ Co _x ) _2 As _2 at diluted limit, and carriers are originated mainly from situations where only one Co atom exists in the nearest neighbor Zn sites out of certain doped Co atoms. The origination of carriers can be explained by the density of states and the unfolded band structure, where it is found that the scattering effects from single Co atom is small but quite large when more Co atoms are located at adjacent Zn sites. The large scattering effects of two adjacent Co atoms will alter the band structures near the Fermi-level. Carriers in Ba(Zn $_{1-x}$ Co _x ) _2 As _2 mainly originate from the As-4p orbitals, with partial contributions from the Co-3d orbitals. Our work provides new insights into the origin of the n -type carriers in magnetic semiconductors and will inspire the development of new magnetic semiconducting systems.

The pairing mechanism behind the monolayer FeSe is one essential question for iron-based superconductors. In this work, we show the sublattice degree of freedoms of monolayer FeSe plays a special role in its pairing properties, namely the sublattice dichotomy. The high-quality monolayer FeSe samples with atomic flat 1×1 topography on the SrTiO_3(001) substrates are grown by molecular beam epitaxy. By comparing the tunneling spectra at α and β Fe sublattices, we find the coherence peak of α-Fe at the inner gap +V_i is higher than β-Fe while the coherence peak of β-Fe at -V_i is higher than α-Fe with a similar amount. We also observed a reversed effect at the outer gap ± V_o. We propose the η-pairing mechanism between k and -k+Q is the key mechanism for this unconventional sublattice dichotomy effect.

The discovery of high-mobility two-dimensional electron gas and low carrier density superconductivity in multiple SrTiO3-based heterostructures has stimulated intense interest in the surface properties of SrTiO3. The recent discovery of high-Tc superconductivity in the monolayer FeSe/SrTiO3 led to the upsurge and underscored the atomic precision probe of the surface structure. By performing atomically resolved cryogenic scanning tunneling microscopy/spectroscopy characterization on dual-TiO2-δ-terminated SrTiO3(001) surfaces with (√13 × √13), c(4 × 2), mixed (2 × 1), and (2 × 2) reconstructions, we disclosed universally broken rotational symmetry and contrasting bias- and temperature-dependent electronic states for apical and equatorial oxygen sites. With the sequentially evolved surface reconstructions and simultaneously increasing equatorial oxygen vacancies, the surface anisotropy reduces and the work function lowers. Intriguingly, unidirectional stripe orders appear on the c(4 × 2) surface, whereas local (4 × 4) order emerges and eventually forms long-range unidirectional c(4 × 4) charge order on the (2 × 2) surface. This work reveals robust unidirectional charge orders induced by oxygen vacancies due to strong and delicate electronic-lattice interaction under broken rotational symmetry, providing insights into understanding the complex behaviors in perovskite oxide-based heterostructures.

The interfacial FeSe/TiO2-δ coupling induces high-temperature superconductivity in monolayer FeSe films. Using cryogenic atomically resolved scanning tunneling microscopy/spectroscopy, we obtained atomic-site dependent surface density of states, work function, and the pairing gap in the monolayer FeSe on the SrTiO3(001)-(√13 × √13)-R33.7° surface. Our results disclosed the out-of-plane Se-Fe-Se triple layer gradient variation, switched DOS for Fe sites on and off TiO5□, and inequivalent Fe sublattices, which gives global spatial modulation of pairing gap contaminants with the (√13 × √13) pattern. Moreover, the coherent lattice coupling induces strong inversion asymmetry and in-plane anisotropy in the monolayer FeSe, which is demonstrated to correlate with the particle-hole asymmetry in coherence peaks. These results disclose delicate atomic-scale correlations between pairing and lattice-electronic coupling in the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation crossover regime, providing insights into understanding the pairing mechanism of multiorbital superconductivity.

In designing material functionalities for transition metal oxides, lattice structure and d-orbital occupancy are key determinants. However, the modulation of these two factors is inherently limited by the need to balance thermodynamic stability, growth kinetics and stoichiometry precision, particularly for metastable phases. We introduce a methodology, namely gigantic-oxidative atomic-layer-by-layer epitaxy (GOALL-Epitaxy), to enhance oxidation power by three to four orders of magnitude beyond conventional pulsed laser deposition and oxide molecular beam epitaxy, while ensuring atomic-layer-by-layer growth of the designed complex structures. Thermodynamic stability is markedly augmented with stronger oxidation at elevated temperatures, whereas growth kinetics is sustained by using laser ablation at lower temperatures. We demonstrate the accurate growth of complex nickelates and cuprates-especially an artificially designed structure with alternating single and double NiO2 layers that possess distinct nominal d-orbital occupancy, as a parent of the high-temperature superconductor. GOALL-Epitaxy enables material discovery within the vastly broadened growth parameter space.

ABSTRACT Superconductivity transition temperature (Tc) marks the inception of a macroscopic quantum phase-coherent paired state in fermionic systems. For 2D superconductivity, the paired electrons condense into a coherent superfluid state at Tc, which is usually lower than the pairing temperature, between which intrinsic physics including Berezinskii–Kosterlitz–Thouless transition and pseudogap state are hotly debated. In the case of monolayer FeSe superconducting films on SrTiO3(001), although the pairing temperature (Tp) is revealed to be 65–83 K by using spectroscopy characterization, the measured zero-resistance temperature (${{T}}_{{\rm c}}^0$) is limited to 20 K. Here, we report significantly enhanced superconductivity in monolayer FeSe films by δ-doping of Eu or Al on SrTiO3(001) surface, in which ${{T}}_{{\rm c}}^0$ is enhanced by 12 K with a narrowed transition width ΔTc ∼ 8 K, compared with non-doped samples. Using scanning tunneling microscopy/spectroscopy measurements, we demonstrate lowered work function of the δ-doped SrTiO3(001) surface and enlarged superconducting gaps in the monolayer FeSe with improved morphology/electronic homogeneity. Our work provides a practical route to enhance 2D superconductivity by using interface engineering.

Determining the pairing symmetry of single-layer FeSe on SrTiO 3 is the key to understanding the enhanced pairing mechanism. It also guides the search for superconductors with high transition temperatures. Despite considerable efforts, it remains controversial whether the symmetry is the sign-preserving s - or the sign-changing s ± -wave. Here, we investigate the pairing symmetry of single-layer FeSe from a topological point of view. Using low-temperature scanning tunneling microscopy/spectroscopy, we systematically characterize the superconducting states at edges and corners of single-layer FeSe. The tunneling spectra collected at edges and corners show a full energy gap and a substantial dip, respectively, suggesting the absence of topologically non-trivial edge and corner modes. According to our theoretical calculations, these spectroscopic features can be considered as strong evidence for the sign-preserving s -wave pairing in single-layer FeSe.

AbstractWe report the ferromagnetism in a new bulk form Cu-based magnetic semiconductor (La,Ba)(Cu,Mn)SO, which is iso-structural to the prototypical iron-based 1111-type superconductor LaFeAsO. Starting from the parent compound LaCuSO, carriers are introduced via the substitutions of La for Ba while spins are introduced via the substitutions of Cu for Mn. Spins are mediated by carriers, which develops into the long range ferromagnetic ordering. The maximum Curie temperature $$T_{C}$$ T C reaches up to $$\sim$$ ∼ 170 K with the doping levels of 10% Ba and 5% Mn. By comparing to the (La,Sr)(Cu,Mn)SO where Sr and Mn are co-doped into LaCuSO, we demonstrate that negative chemical pressure would suppress the ferromagnetic ordering.

The superconductivity in bulk FeSe depends sensitively on the stoichiometry, ditto for the monolayer FeSe films grown on SrTiO3-delta and other substrates. To tune the stoichiometry, we deposit Fe onto monolayer FeSe films after epitaxial growth. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we find that the postdeposited Fe atoms incorporate into monolayer films and combine with excess Se to form FeSe, with an upper coverage limit of similar to 0.4 unit-cell. Consequently, the superconducting gap concentrates in 10 to 12 meV. The work demonstrates a simple method for stoichiometry tuning and improving the spatial uniformity of monolayer FeSe films. Compared with the gap of similar to 15 to 20 meV for the annealed films, the smaller superconducting gap indicates weakened FeSe-TiO2 coupling due to interfacial Se atom substitution.

Spatially uniform high-temperature superconducting films are highly desirable for exploring novel properties and popularizing applications. To improve the uniformity, we fabricate monolayer FeSe x Te 1− x (0 < x ≤ 1) films on SrTiO 3 (001) by topotactic reaction of monolayer FeTe films with selenium. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we demonstrate atomic-level uniformity of element distribution and well-defined superconducting gaps of ∼ 15 meV in FeSe x Te 1− x films. In particular, the monolayer FeSe films exhibit fewer line defects and higher superfluid density as evidenced by sharper coherence peaks than those prepared by the co-evaporation method. Our results provide a promising way to optimize sample quality and lay a foundation for studying new physics and drawing reliable conclusions.

Atomic characterization on tetragonal FeAs layer and engineering FeAs superlattices is highly desirable to get deep insight into the multi-band superconductivity in iron-pnictides. We fabricate the tetragonal FeAs layer by topotactic reaction of FeTe films with arsenic and then obtain KxFe2As2 upon potassium intercalation using molecular beam epitaxy. The in-situ low-temperature scanning tunneling microscopy/spectroscopy investigations demonstrate characteristic √(2)×√(2) reconstruction of the FeAs layer and stripe pattern of KxFe2As2, accompanied by the development of a superconducting-like gap. The ex-situ transport measurement with FeTe capping layers shows a superconducting transition with an onset temperature of 10 K. This work provides a promising way to characterize the FeAs layer directly and explore rich emergent physics with epitaxial superlattice design.

Ultrathin films of intrinsic magnetic topological insulator MnBi2Te4 exhibit fascinating quantum properties such as the quantum anomalous Hall effect and the axion insulator state. In this work, we systematically investigate the evolution of the electronic structure of MnBi2Te4 thin films. With increasing film thickness, the electronic structure changes from an insulator type with a large energy gap to one with in-gap topological surface states, which is, however, still in drastic contrast to the bulk material. By surface doping of alkali-metal atoms, a Rashba split band gradually emerges and hybridizes with topological surface states, which not only reconciles the puzzling difference between the electronic structures of the bulk and thin-film MnBi2Te4 but also provides an interesting platform to establish Rashba ferromagnet that is attractive for (quantum) anomalous Hall effect. Our results provide important insights into the understanding and engineering of the intriguing quantum properties of MnBi2Te4 thin films.