A central question in the high temperature cuprate superconductors is the fate of the parent Mott insulator upon charge doping. Here we use scanning tunneling microscopy to investigate the local electronic structure of lightly doped cuprate in the antiferromagnetic insulating regime. We show that the doped charge induces a spectral weight transfer from the high energy Hubbard bands to the low energy in-gap states. With increasing doping, a V-shaped density of state suppression occurs at the Fermi level, which is accompanied by the emergence of checkerboard charge order.The new STM perspective revealed here is the cuprates first become a charge ordered insulator upon doping. Subsequently, with further doping, Fermi surface and high temperature superconductivity grow out of it.High temperature superconductivity in the cuprates is widely believed to originate from adding charge carriers into an antiferromagnetic (AF) Mott insulator (1). Elucidating the properties of the doped Mott insulator is among the most crucial issues concerning the mechanism of superconductivity. From the electronic structure point of view, the key question is how the large Mott-Hubbard gap, or more precisely the charge transfer gap, evolves into the d-wave superconducting (SC) gap upon charge doping. This has turned out to be a formidable challenge, both theoretically and experimentally, due to the presence of strong AF fluctuations and electron correlations. A major obstacle lying between the parent Mott insulator and the optimally doped cuprate is the pseudogap phase, which exhibits a normal state gap as revealed by early spectroscopic studies (2-4). More recently, imaging and diffraction techniques show that electrons in the pseudogap phase have strong propensity towards charge (5-16) or spin order (5,(17)(18)(19). Currently neither the gap-like density of state (DOS) suppression nor the charge/spin density wave is well understood (4).Most previous experiments on the pseudogap phase focused on the underdoped regime with finite transition temperature (T c ), and aimed to address its relationship with the SC phase by probing the strength of the two orders across T c (4). Recent scanning tunneling microscopy (STM) and x-ray spectroscopy experiments provide increasing evidence that the charge order associated with the pseudogap compete with superconductivity because its feature gets suppressed as the sample enters the SC phase below T c (4,11,16). However, due to the lack of spectroscopic data spanning the energy range of both the pseudogap and charge transfer gap, less is known about lightly doped, non-SC regime standing next to the parent To address these questions, here we carry out STM investigations on lightly doped Bi 2 Sr 2-x La x CuO 6+ (La-Bi2201) in the AF insulating regime. La-Bi2201 is an ideal cuprate system which not only has a well-cleaved surface, but also can be doped towards the Mott insulating limit by varying the . Figure 1A 1C displays spatially resolved tunneling spectra dI/dV(r, V), which is roughly proportional to ...

The layered transition-metal dichalcogenide 1T-TaS 2 has been recently found to undergo a Mottinsulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. By combining scanning tunneling microscopy measurements and first-principles calculations, we investigate the atomic scale electronic structure of the 1T-TaS 2 Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital texturesone localized and the other extended-and demonstrate that the interplay between them is the key factor that determines the electronic structure. In particular, we show that the continuous evolution of the charge gap visualized by scanning tunneling microscopy is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of Mottness collapse revealed here suggests an interesting route for creating novel electronic states and designing future electronic devices.

Exotic quantum phenomena have been demonstrated in recently discovered intrinsic magnetic topological insulator MnBi 2 Te 4 . At its two-dimensional limit, quantum anomalous Hall (QAH) effect and axion insulator state are observed in odd and even layers of MnBi 2 Te 4 , respectively. The measured band structures exhibit intriguing and complex properties. Here we employ low-temperature scanning tunneling microscopy to study its surface states and magnetic response. The quasiparticle interference patterns indicate that the electronic structures on the topmost layer of MnBi 2 Te 4 is different from that of the expected out-of-plane A-type antiferromagnetic phase. The topological surface states may be embedded in deeper layers beneath the topmost surface. Such novel electronic structure presumably related to the modification of crystalline structure during sample cleaving and re-orientation of magnetic moment of Mn atoms near the surface. Mn dopants substituted at the Bi site on the second atomic layer are observed. The ratio of Mn/Bi substitutions is 5%. The electronic structures are fluctuating at atomic scale on the surface, which can affect the magnetism of MnBi 2 Te 4 . Our findings shed new lights on the magnetic property of MnBi 2 Te 4 and thus the design of magnetic topological insulators.

Implementing power-e cient reservoir computing hardware systems is of great interest to the eld of neuromorphic computing. More and more studies attempt to use analog devices or components, such as memristors and spintronic oscillators, to partially replace fully digital systems to boost the power e ciency. However, a reservoir computing system operating real-time in fully analog fashion has not been demonstrated yet. In this work, a fully analog reservoir computing system was implemented using two types of memristors, where dynamic memristors were used to construct the reservoir while nonvolatile memristor arrays were used as the readout layer. The key features, such as threshold and window, extracted from the dynamic memristor-based physical nodes were found to have a signi cant impact on the system performance. By adjusting the features to the appropriate range, our system can e ciently process spatiotemporal signals in real time with extremely low power consumption, more than three orders of magnitudes lower than digital counterparts. Both temporal task of arrhythm detection and spatiotemporal task of dynamic gesture recognition were demonstrated, where high detection accuracy of 96.6% and recognition accuracy 97.9% were achieved respectively. Our work demonstrates that such memristor-based fully analog reservoir computing system could be attractive for spatial and temporal edge computing with extremely low power and hardware cost.

Abstract:One of the biggest puzzles concerning the cuprate high temperature superconductors is what determines the maximum transition temperature (T c,max ), which varies from less than 30 K to above 130 K in different compounds. Despite this dramatic variation, a robust trend is that within each family, the double-layer compound always has higher T c,max than the single-layer counterpart. Here we use scanning tunneling microscopy to investigate the electronic structure of four cuprate parent compounds belonging to two different families. We find that within each family, the double layer compound has a much smaller charge transfer gap size (∆ CT ), indicating a clear anticorrelation between ∆ CT and T c,max . These results suggest that the charge transfer gap plays a key role in the superconducting physics of cuprates, which shed important new light on the high T c mechanism from doped Mott insulator perspective.