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Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin–Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductors such as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions. Transport and scanning tunnelling microscopy studies of freestanding monolayers of an unconventional layered copper oxide establish that the superconducting properties of copper oxides are not changed in the 2D limit.

Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi

Recent experiments have shown that the transition-metal dichalcogenide 1T-TaS${}_{2-x}$Se${}_{x}$ can transition from a Mott insulator to a superconductor by varying the S/Se ratio. New measurements with a scanning tunneling microscope along with first-principles calculations reveal the mechanism underlying this transition.

One of the biggest puzzles concerning the cuprate high temperature superconductors is what determines the maximum transition temperature (Tc,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 Tc,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 ($\Delta_{CT}$), indicating a clear anticorrelation between $\Delta_{CT}$ and Tc,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 Tc mechanism from doped Mott insulator perspective., Comment: 4 figures

IEEE Std. 1687, or IJTAG, defines flexible serial scan-based architectures for accessing embedded instruments efficiently. In this paper, we present a novel test architecture that employs IEEE Std. 1687 together with an efficient test controller to carry out 3D-IC testing autonomously. The test controller can deliver parallel test data for the IEEE Std. 1687 structures and the cores under test, and provide required control signals to control the whole test procedure. This design can achieve at-speed, autonomous and programmable testing in 3D-ICs. Experimental results show that the additional area and test cycle overhead of this architecture is small considering its autonomous test capability.

Electron–electron and electron–phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. In layered compound 1T-TaS2, the intricate interplay between the two generates a Mott-insulating ground state with a peculiar charge-density-wave (CDW) order. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses, and find that the new phase exhibits electronic structures entirely different from that of the original Mott ground state. The mosaic phase consists of nanometre-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layers underneath. We discover that the nature of the new phase is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in 1T-TaS2., In correlated materials, new phases emerge when the balance between many-body interactions is perturbed. Here, Ma et al. induce a mosaic charge-density-wave phase out of Mott insulating state in layered 1T-TaS2 by voltage pulses, which reveals a dominating role of interlayer stacking order.

One of the key issues in unraveling the mystery of high Tc superconductivity in the cuprates is to understand the normal state outside the superconducting dome. Here we perform scanning tunneling microscopy and spectroscopy measurements on a heavily overdoped, non-superconducting (Bi,Pb)2Sr2CuO6+x cuprate. Spectroscopic imaging reveals dispersive quasiparticle interferences and the Fourier transforms uncover the evolution of momentum space topology. More interestingly, we observe nanoscale patches of static charge order with sqrt(2)*sqrt(2) periodicity. Both the dispersive quasiparticle interference and static charge order can be qualitatively explained by theoretical calculations, which reveal the unique electronic structure of strongly overdoped cuprate., 16 pages, 4 figures

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., 4 figures, with supplementary materials

We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{\mathrm{depin}} \simeq 4.5$ pN, corresponding to a critical current up to $J_c \simeq 7 \times 10^5$ A/cm$^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, $\lambda_{ab}=320 \pm 60$ nm, which is larger than previous bulk measurements.

One of the major puzzles regarding unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. Here we report scanning tunneling microscopy studies of the local electronic structure of Cu doped NaFeAs across the superconductor to insulator transition. We find that in the highly insulating regime the electronic spectrum develops an energy gap with diminishing density of state at the Fermi level. The overall lineshape and strong spatial variations of the spectra are strikingly similar to that of lightly doped cuprates close to the parent Mott insulator. We propose that the suppression of itinerant electron state and strong impurity potential induced by Cu dopants lead to this insulating iron pnictide., Comment: 6 figures, to appear in Phys. Rev. X

The IrTe2 transition metal dichalcogenide undergoes a series of structural and electronic phase transitions when doped with Pt. The nature of each phase and the mechanism of the phase transitions have attracted much attention. In this paper, we report scanning tunneling microscopy and spectroscopy studies of Pt doped IrTe2 with varied Pt contents. In pure IrTe2, we find that the ground state has a 1/6 superstructure, and the electronic structure is inconsistent with Fermi surface nesting induced charge density wave order. Upon Pt doping, the crystal structure changes to a 1/5 superstructure and then to a quasi-periodic hexagonal phase. First principles calculations show that the superstructures and electronic structures are determined by the global chemical strain and local impurity states that can be tuned systematically by Pt doping., Comment: 6 figures

Anomalous transport behaviour, i.e. the dependence of the tunnel resistance on the injection current, has been discovered in Ta/Co/Al2O3/FeNi tunnel junctions. The zero-field voltage-current characteristic of the magnetic tunnelling junction obeys the transport principle of the normal tunnel junction at low injection current, but it exhibits a negative resistance behaviour when the injection current is raised to the break-over current level. The physics of the restorable electric breakdown has been initially studied.

Using the ion-beam-sputtering technique, we have fabricated Fe/Al2O3/Fe magnetic tunnelling junctions (MTJs). We have observed double-peaked shapes of curves, which have a level summit and a symmetrical feature, showing the magnetoresistance of the junction as a function of applied field. We have measured the tunnel conductance of MTJs which have insulating layers of different thicknesses. We have studied the dependence of the magnetoresistance of MTJs on tunnel conductance. The microstructures of hard- and soft-magnetic layers and interfaces of ferromagnets and insulators were probed. Analysing the influence of MJT microstructures, including those having clusters or/and granules in magnetic and non-magnetic films, a magnetization reversal mechanism (MRM) is proposed, which suggests that the MRM of tunnelling junctions may be explained by using a group-by-group reversal model of magnetic moments of the mesoscopical particles. We discuss the influence of MTJ microstructures, including those with clusters or/and granules in the ferromagnetic and non-magnetic films, on the MRM.

We use scanning tunneling microscopy to investigate the (001) surface of a cleaved ${\mathrm{SmB}}_{6}$ Kondo insulator. Variable temperature $dI/dV$ spectroscopy up to 60 K reveals a gaplike density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm $5d$ band and localized Sm $4f$ band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin with the presence of spin-orbital coupling and mixed valence fluctuations. These results shed new light on the electronic structure evolution and transport anomaly in ${\mathrm{SmB}}_{6}$.

We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in ${\mathrm{NaFe}}_{1\ensuremath{-}\mathrm{x}}{\mathrm{Co}}_{\mathrm{x}}\mathrm{As}$ iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where fourfold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always fourfold symmetric. We argue this implies small nematic susceptibility and, hence, insignificant nematic fluctuation in optimally doped iron pnictides. Since ${T}_{C}$ is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.

We use scanning tunneling microscopy to investigate the (001) surface of a cleaved SmB6 Kondo insulator. Variable temperature dI/dV spectroscopy up to 60 K reveals a gaplike density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5d band and localized Sm 4f band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin with the presence of spin-orbital coupling and mixed valence fluctuations. These results shed new light on the electronic structure evolution and transport anomaly in SmB6.

We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where fourfold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always fourfold symmetric. We argue this implies small nematic susceptibility and, hence, insignificant nematic fluctuation in optimally doped iron pnictides. Since TC is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.

Although the mechanism of superconductivity in the cuprates remains elusive, it is generally agreed that at the heart of the problem is the physics of doped Mott insulators. The cuprate parent compound has one unpaired electron per Cu site, and is predicted by band theory to be a half-filled metal. The strong onsite Coulomb repulsion, however, prohibits electron hopping between neighboring sites and leads to a Mott insulator ground state with antiferromagnetic (AF) ordering. Charge carriers doped into the CuO2 plane destroy the insulating phase and superconductivity emerges as the carrier density is sufficiently high. The natural starting point for tackling high Tc superconductivity is to elucidate the electronic structure of the parent Mott insulator and the behavior of a single doped charge. Here we use a scanning tunneling microscope to investigate the atomic scale electronic structure of the Ca2CuO2Cl2 parent Mott insulator of the cuprates. The full electronic spectrum across the Mott-Hubbard gap is uncovered for the first time, which reveals the particle-hole symmetric and spatially uniform Hubbard bands. A single electron donated by surface defect is found to create a broad in-gap electronic state that is strongly localized in space with spatial characteristics intimately related to the AF spin background. The unprecedented real space electronic structure of the parent cuprate sheds important new light on the origion of high Tc superconductivity from the doped Mott insulator perspective., 26 pages, 4 figures, supplementary information included

We report the doping, temperature, and spatial evolutions of the electronic structure of ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{As}$ studied by scanning tunneling microscopy. In the parent state we directly observe the spin density wave gap, which exhibits unconventional features that are incompatible with simple Fermi surface nesting. The optimally doped sample has a single superconducting gap, but in the overdoped regime a novel pseudogaplike feature emerges. The pseudogaplike phase coexists with superconductivity in the ground state, persists well into the normal state, and shows strong spatial variations. The characteristics of the three distinct electronic states revealed here shed important new lights on the microscopic models for the iron-based superconductors.

We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K${}_{0.73}$Fe${}_{1.67}$Se${}_{2}$ with ${T}_{\mathrm{C}}$ $=$ 32 K. On the atomically resolved FeSe surface, we observe a well-defined superconducting gap and the microscopic coexistence of a charge-density modulation with $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block-antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides.

We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. Topographic imaging reveals two types of atomically flat surfaces, corresponding to the exposed LaO layer and FeAs layer respectively. On one type of surface, we observe strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. The distribution of scattering wavevectors exhibits pronounced two-fold symmetry, consistent with the nematic electronic structure found in the Ca(Fe1-xCox)2As2 parent state., 13 pages, 4 figures

High-quality Bi2Te3 films can be grown on Si by the state-of-art molecular beam epitaxy technique. In situ angle-resolved photo-emission spectroscopy measurement reveals that the as-grown films are intrinsic topological insulators and the single-Dirac-cone surface state develops at a thickness of two quintuple layers. The work opens a new avenue for engineering of topological materials based on well-developed Si technology.

By using x-ray diffraction technique and transmission electron microscopy the in situ growth property of metal crystal ultrafine particles (CUFP) which are deposited on the silica-substrates have been studied experimentially. It is found that the diffraction intensity and the nucleus diameter of Ag CUFP are grown with time by power-law. Thus, the changes of crystal nucleus and the structure of Ag CUFP with time shows a crystallization trend, at the same time, the in situ growth of Ag CUFP displays a self-similarity character.

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ∼10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO2 thin film in high vacuum.

Using the solid_state diffusion method, a series of Sn1-xMnxO2(x=002—006) diluted magnetic semiconductors(DMSs) are fabricated.By u sing x_ray diffraction(XRD) measurements and Fouier transform infrared spectrosc opic(NEXUS870) analyses,a uniform Mn distribution in the SnO2 is conf irmed. An obvious ferromagnetism in Mn_doped SnO2 DMS at room temperature is f ound and the ferromagnetism is weak with the increase of Mn concentration.

The microscopic structures of the diamond films generated on Si (100) substrates by chemical vapor deposition have been studied by x-ray wide angle diffraction spectroscopy, grazing incidence x-ray scattering spectra, positrons annihilation lifetime spectroscopy, and qualitative software and Positronfit program. The di ffraction x-ray spectra show a structure difference between the near substrate p art (NSP) and near free-surface part (NFP) of the diamond film specimens, which reflect the weak ［111］texture in NSP and the strong［220］ texture in NFP. The positron annihilation lifetime spectroscopy of the specirnens and the analyzing data indicate that the majority of positrons annihilate around 200 ps and 400 p s. The vacancies, vacancy clusters, and voids were found in the film specimens, depending on the processes in which the chemical vapor deposition is generated.

Using ion-beam-sputtering technique, Fe/Al2O3/Fe magnetic tunneling junctions (MTJ) were fabricated. Tunneling magnetoresistance (TMR) eff ect of MTJ samples has been successfully studied. The chemical composition and t he microstructural characteristics of hard-and soft-magnetic layers, insulating layer, and interface of MTJ were analyzed by X-ray photoelectron spectroscopy an d Atomic force microscopy. The dependence of MR effect on microstructure, chemic al composition, conductance, and Ｉ-Ｖ characteristic of the samples are also di scussed.

Ag ultrafine crystal films (AgUFCPs) of about 18nm thickness were deposited on a (111) monocrystalline silicon surface,by means of the vacuo-sputtering technique.The microstructure of AgUFCP and the interface morphology between the film and substrate were revealed by using of grazing incidence X-ray scattering (GIXS) method.A semi-crystalline structure of AgUFCP was- investigated by wide angle X-ray diffraction (WXRD) and radial distribution function(RDF).The surface atomic structure of Ag ultrafine crystal particle were analysed by DSC scans.and the temperature dependence of the growth dynamic mechanism of particle surface layer was explored.

The crystal phase, surface composition, particle shape and thermodynamic chaxacter of Ag ultrafine particle (UFP), which were prepared by sputtering in Ar gas, have been analysed by means of X-ray diffraction, X-ray photo-electron spectrum, transmission electron microscope (TEM) and differential scanning calorimeter. The results show that the particles with size larger than 4.6nm are composed of Ag-nuclens and Ag2O-surface-layer, the particles with size less than 4.6nm consisted of Ag2O only, the shape of particles with size less than 10nm are spherical and that with size larger than l0nm are non-definte when the particle were as-prepared. It is shown by DSC that there are different thermal effect between UFP and solid Ag. Also, it is shown by TEM that a few particles which had two phase of Ag and Ag2O and deposited on the carbon film have been reduced to metal Ag UFP after 40 days.