Tuning the chemical potential of a solid to the vicinity of a van Hove singularity (vHS) is a well-established route to discovering emergent quantum phases. In monolayer graphene, the use of electron-donating metal layers has recently emerged as a method to dope the chemical potential to the nearest vHS, as evidenced by Angle-Resolved Photoemission Spectroscopy (ARPES) measurements. In this work, we study the spatial uniformity of the doping from this process using spectroscopic imaging scanning tunneling microscopy (SI-STM). Using molecular beam epitaxy (MBE), we achieve electron doping of graphene on SiC using Ytterbium (Yb-Graphene). We show using in-situ ARPES that the chemical potential is shifted to within 250 meV of the vHS. Using in-situ SI-STM, we establish that there exists significant inhomogeneity in the vHS position in overdoped graphene. We find two separate reasons for this. First, the spatial inhomogeneity of the intercalated Yb leads to local variations in the doping, with a length scale of inhomogeneity set by the screening length of 3 nm. Second, we observe the presence of substitutional Yb dopants in the graphene basal plane. These Yb dopants cause a strong local shift of the doping, along with a renormalization of the quasiparticle amplitude. Theoretical calculations confirm that the Yb impurities effectively change the local potential, thus energetically shifting the position of the van Hove singularity. Our results point to the importance of considering the spatial structure of doping and its inextricable link to electronic structure.

The many-body electron-electron interaction in cuprates causes broadening of the electronic bands in k space, leading to a deviation from the standard Fermi liquid. While a k-dependent anisotropic electronic scattering (k-DAES) has been assessed by photoemission, its fingerprint in Q space has been scarcely considered. Here, we explore the Q-dependent electron dynamics in optimally doped Bi2Sr2CaCu2O8+delta through the evolution of low-energy charge excitations as measured by resonant inelastic x-ray scattering (RIXS). In the normal state, the RIXS spectra display a continuum of excitations down to 0 meV, while the superconducting state features a spectral weight suppression below 80 meV without any enhancement at higher energies. To interpret the energy and Q evolution of our data, we introduce a phenomenological expression of the charge susceptibility by including the k-DAES. We show that only the charge susceptibility with k-DAES captures the RIXS data, highlighting the importance of k-DAES when describing the Q dependence of charge excitations from 0 to a few eV scale. Furthermore, we also find that the inclusion of k-DAES is essential when quantitative parameters such as the electronic energy gap are extracted from RIXS data.

Determining the doping level in high-temperature cuprate superconductors is crucial for understanding the origin of superconductivity in these materials and for unlocking their full potential. However, accurately determining the doping level remains a significant challenge due to a complex interplay of factors and limitations in various measurement techniques. In particular, in Bi_2Sr_2CuO_6+δ and Bi_2Sr_2CaCu_2O_8+δ, where the mobile carriers are introduced by non-stoichiometric oxygen δ, the determination has been extremely problematic. Here, we study the doping dependence of the electronic structure of these materials in angle-resolved photoemission and find that both the doping level, p, and the superconducting transition temeprature, T_c can be precisely determined from the binding energy of the Bi 5d core-levels. The measurements can be performed at room temperature, enabling the determination of p and T_c without cooling the samples. This should be very helpful for further studies of these materials.

Determining the doping level in high-temperature cuprate superconductors is crucial for understanding the origin of superconductivity in these materials and for unlocking their full potential. However, accurately determining the doping level remains a significant challenge due to a complex interplay of factors and limitations in various measurement techniques. In particular, in Bi2Sr2CuO6+8 and Bi2Sr2CaCu2O8+8, where the mobile carriers are introduced by nonstoichiometric oxygen 8, the determination has been extremely problematic. Here, we study the doping dependence of the electronic structure of these materials in angle-resolved photoemission and find that both the doping level p and the superconducting transition temperature Tc can be precisely determined from the binding energy of the Bi 5d core levels. The measurements can be performed at room temperature, enabling the determination of p and Tc without cooling the samples. This should be very helpful for further studies of these materials.

Determining the doping level in high-temperature cuprate superconductors is crucial for understanding the origin of superconductivity in these materials and for unlocking their full potential. However, accurately determining the doping level remains a significant challenge due to a complex interplay of factors and limitations in various measurement techniques. In particular, in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$, where the mobile carriers are introduced by nonstoichiometric oxygen $\ensuremath{\delta}$, the determination has been extremely problematic. Here, we study the doping dependence of the electronic structure of these materials in angle-resolved photoemission and find that both the doping level $p$ and the superconducting transition temperature ${T}_{c}$ can be precisely determined from the binding energy of the Bi $5d$ core levels. The measurements can be performed at room temperature, enabling the determination of $p$ and ${T}_{c}$ without cooling the samples. This should be very helpful for further studies of these materials.

In this work, using first-principles calculations and Monte Carlo simulations, we investigate the critical (Curie or Neel) temperature Tc and magnetic coupling in two-dimensional multilayer Cr2Sn2Te6 (CST). The results indicate that the value of Tc is strongly dependent on thickness. We find that the Tc value of CST increases up to 133 K (bulk CST) from 48 K (monolayer CST), which is significantly greater than that of the homogeneous compounds Cr2Si2Te6 and Cr2Ge2Te6. Then, from the perspective of magnetic exchange coupling, it is found that the intralayer ferromagnetic coupling in CST is mostly associated with the competition between direct and superexchange interactions. In particular, the interlayer magnetic coupling is primarily attributed to the supersuperexchange interactions (Cr-Te-Te-Cr) between antiferromagnetically and ferromagnetically coupled multi-nearest neighbors within the layers. In addition, these findings demonstrate that manipulation of the interlayer distance using z-directional strain is an efficient way to enhance the Tc value of CST. When the interlayer Cr-Cr distance is decreased to 6.17 angstrom, the transition temperature of bulk CST increases to 429 K, and an interlayer magnetic coupling transition is observed in which the ferromagnetic layers are coupled antiferromagnetically.

Using first-principles calculations and micro-magnetic simulations, we propose a Janus MnSbBiSe2Te2 (MSBST) monolayer derived from the MnBi2Te4 (MBT) ferromagnet and investigate the influence of biaxial strain on the electronic structures, topological characteristics and spin textures. Different from pristine MBT with an out-of-plane easy axis, the anisotropy of MSBST prefers an in-plane direction. Intriguingly, switching the easy axis direction of MSBST will significantly modify the band structure. Topological phase transition can be achieved by applying a compressive strain, making MSBST become a topological insulator with . Moreover, due to the inherent inversion asymmetry of Janus MSBST, a large Dzyaloshinskii-Moriya interaction (DMI) is induced for generating and stabilizing skyrmions. By micro-magnetic simulations, the results of spin textures show that the skyrmions phase can be achieved in MSBST with an external magnetic field of 0.8 T. Our findings provide guidelines for the development and application of spintronic devices with nontrivial topological properties and a large DMI.

Zintl compounds have been extensively studied for their outstanding thermoelectric properties, but their electronic structure remains largely unexplored. Here, we present a detailed investigation of the electronic structure of the isostructural thermopower materials YbMg 2 Bi 2 and CaMg 2 Bi 2 using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). The ARPES results show a significantly smaller Fermi surface and Fermi velocity in CaMg 2 Bi 2 than in YbMg 2 Bi 2 . Our ARPES results also reveal that in the case of YbMg 2 Bi 2 , Yb-4 f states reside well below the Fermi level and likely have a negligible impact on transport properties. To properly model the position of 4 f -states, as well as the overall electronic structure, a Hubbard U at the Yb sites and spin-orbit coupling (SOC) have to be included in the DFT calculations. The theoretical results reveal that both materials belong to a Z 2 topological class and host topological surface states around E F . Due to the intrinsic hole doping, the topological states reside above the Fermi level, inaccessible by ARPES. Our results also suggest that in addition to SOC, vacancies and the resulting hole doping play an important role in the transport properties of these materials.

We report a CrISe monolayer as a room temperature ferromagnetic (FM) semiconductor with the Curie temperature (TC), magnetic anisotropy energy (MAE), and bandgap being 322 K, 113 μeV, and 1.76 eV, respectively. The TC and MAE can be further enhanced up to 385 K and 313 μeV by a tensile strain. Interestingly, the magnetic easy axis can be switched between off-plane and in-plane by compressive strain. Particularly, due to the broken inversion symmetry and strong spin–orbital coupling of Se atoms, a large Dzyaloshinskii–Moriya interaction (DMI) of 2.40 meV is obtained. More importantly, by micromagnetic simulations, stable skyrmions with sub-10 nm radius are stabilized by the large DMI above room temperature in a wide range of strain from −2% to 6%. Our work demonstrates CrISe as a promising candidate for next-generation skyrmion-based information storage devices and provides guidance for the research of DMI and skyrmions in room temperature FM semiconductors.

Synthesis of vertical heterostructures that include atomically thin layers of materials with topologically nontrivial energy bands is desirable for exploring exotic quantum states. Here, the authors report on atomic-layer-by-layer deposition of magnesium on copper(111) surface by molecular beam epitaxy, monitored in situ by low-energy electron microscopy and diffraction, and modeled by ab initio theory. It is found that a 2D MgCu2 intermetallic compound forms during initial Mg deposition and persists till a full monolayer of Mg is formed. Deposition of additional Mg triggers a phase transition from the commensurate MgCu2 to an incommensurate Mg2Cu layer and enables growth of the second layer of Mg. Ab initio calculations indicate non-trivial topology of the electron bands in the interfacial Mg2Cu layer and the existence of Dirac nodal lines near the Fermi level. The new 2D Mg2Cu material emerges as a promising platform to study new topological states of matter.

By first-principles calculations and Monte Carlo simulations, we investigate the influence of biaxial strain on the band structures, magnetic characteristics, and Curie temperature (T-C) of MnBi2Te4 (MBT)/CrI3 heterojunction. Different from bilayer MBT or CrI3 with antiferromagnetic ordering, the interlayer magnetic coupling of MBT/CrI3 prefers ferromagnetic ordering. By applying biaxial strain, a phase transition from semiconductor to metal can be found and a band inversion is observed under a strain of -5%, suggesting the existence of nontrivial topological phase transition. Moreover, magnetic anisotropy energy (MAE) and T-C are sensitive to compressive strain rather than tensile strain. The magnitude of MAE is increased ten times in the strain range -5 to 5%. Meanwhile, the compressive strain enhances the ferromagnetism, leading to a boost of T-C about 13.8% up to 91.1 K. Our findings can provide beneficial guidance for designing the spintronic which hosts robust ferromagnetism and improved T-C.

In recent years, great effort has been made in the study of two-dimensional (2D) van der Waals ferromagnets that can stabilize peculiar chiral spin textures, such as magnetic skyrmions and merons. Here, by first-principles calculations and micromagnetic simulations, we systematically investigate the in-plane magnetic anisotropy, Dzyaloshinskii-Moriya interaction (DMI) and magnetic merons in a Mn2I3Br3 monolayer. Mn2I3Br3 exhibits half-metallic behavior with a large band gap (∼2.7 eV) for spin-down electrons, but is gapless for spin-up ones. In addition, unlike most 2D ferromagnets with an off-plane magnetic easy axis and negligible DMI, the magnetic easy axis of Mn2I3Br3 is in-plane, with a large magnetic anisotropy energy of -13.2 meV and a strong DMI of 4.6 meV, which are mainly induced by the strong spin-orbital coupling of I atoms, microscopically. In particular, spontaneous magnetic merons, stabilized by the DMI, can exist in a wide magnetic field range (0-6 T). Our work not only provides important guidelines for the investigation of the DMI and merons in half-metallic materials, but also demonstrates the Mn2I3Br3 monolayer as an ideal platform to explore the deep physics of magnetic merons and as a promising candidate for magnetic storage devices, as well as spin filters.

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at $T_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at $T_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to $T_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

Recently, great effort has been devoted to the search for two-dimensional (2D) ferromagnetic materials with inherent strong Dzyaloshinskii--Moriya interaction (DMI). Here, through a first-principles approach, we systematically investigate the effect of biaxial strain on the DMI, the Heisenberg exchange interaction, and the magnetic anisotropy energy (MAE) of Janus Cr$_{2}$X$_{3}$Y$_{3}$ (X, Y = Cl, Br, I, X $\neq$ Y) monolayers. Both DMI and MAE can be significantly enhanced by tensile strain, while a reversal of the chirality of DMI in Cr$_{2}$Cl$_{3}$Br$_{3}$ and a switch of MAE from off-plane to in-plane in Cr$_{2}$I$_{3}$Cl$_{3}$ are induced by a compressive strain of $2\%$. Microscopically, DMI and MAE are associated mainly with the large spin--orbit coupling of the heavy nonmagnetic halogen atoms rather than that of the magnetic Cr atoms. In particular, the peculiar magnetic transition of Cr$_{2}$I$_{3}$Cl$_{3}$ is explained by competition between direct exchange and superexchange interactions. Micromagnetic simulations show that a small external magnetic field of 65~mT stabilizes a skyrmion with a diameter of 9.8~nm in the Cr$_2$I$_3$Cl$_3$ monolayer. Our results will provide guidance for further research on DMI and skyrmions in 2D Janus materials, as well as a basis for the potential applications in spintronic devices.

Theoretical prediction of Curie temperature (TC) is of vital importance for designing spintronic devices in two-dimensional (2D) ferromagnetic materials. Herein, based on the extensive investigation of Monte Carlo simulations, we summarize and propose an improved method to estimate TC more precisely, which includes the different contributions of multiple near-neighbor interactions. Using monolayer CrI3 as an example, the trends of TC with biaxial strain are investigated via Monte Carlo simulations, mean-field formulas, and our method. Our method is not only accurate and convenient to predict the TC in 2D ferromagnetic honeycomb lattice CrI3, but it can also be extended to predicting the TC of other 2D lattices. Our work paves the way to accelerate the prediction and discovery of novel 2D ferromagnets for spintronic applications.

We report a Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) study of a DyBa2Cu3O7-δ (DBCO) thin film (Tc ~ 79 K) synthesized by the molecular beam epitaxy (MBE). We observed an unusual transfer of spectral weight in the local density of states (LDOS) spectra occurring only within the superconducting gap. By a systematic control of the tip-sample distance and the junction resistance, we demonstrate that the spectral weight transfer can be switched at a nano-meter length scale. These results suggest that an interaction between the STM tip and the sample alters the electronic configurations in the film. This probably originates from a combination of an intrinsic band bending at the interface between the surface and the bulk, and a tip-induced band bending. These results may open a new avenue for band engineering and applications of thin films of high-Tc cuprates.

The pristine anti-ferromagnetic interlayer coupling of even layers MnBi2Te4 greatly restricts the possibility of realizing quantum anomalous Hall (QAH) effect. In this work, based on first-principles calculations, we find that the interlayer coupling can be transformed into ferromagnetic order from anti-ferromagnetic by substituting cations with V atoms. Furthermore, when applying biaxial strain, a strong band inversion occurs around Fermi level, achieving quantum anomalous Hall effect. A gapless surface states can further confirm it. Meanwhile, the magnetic moment of Mn and V atoms increases with the increase of strain. We attribute this to the electron hopping to magnetic atoms from p orbitals of nonmagnetic Bi and Te atoms. Our work extends the range for designing and applying for spintronic devices with QAH effect.