In-gap density of states in hole- and electron-doped CuO2 planes probed by scanning tunneling spectroscopy

The low-energy electronic structure of a c-axis Sr_xA_yCuO₂ (A is alkaline earth cation, x+y≦1, hole- and electron-doped infinite layer) thin film, grown by laser-molecular-beam epitaxy on a SrTiO₃ (001) substrate, has been studied using ultrahigh-vacuum scanning tunneling microscopy/spectroscopy. Images have been obtained for co-deposited Sr_xA_yCuO₂ thin films, which show the surface consisting of flat terraces separated by steps that are unit cell high. Tunneling spectra of undoped Sr_0.3Ca_0.7CuO₂ indicate a wide band gap of 1.8 eV which is consistent with the charge transfer gap. Hole-doped Sr_0.85CuO₂ shows in-gap states appearing at both the valence and conduction band edges. In contrast, for the electron-doped Sr_0.9La_0.1CuO₂, in-gap states appear predominantly above the Fermi level, and the spectral shape becomes asymmetric around the Fermi level. When these two systems are compared, barrier-height measurements reveal that there is no apparent shift of the Fermi level measured from the vacuum level. This suggests that the framework of the rigid-band picture might break down implying a strongly correlated electron system. Received: 20 August 1998 / Accepted: 15 February 1999 / Published online: 5 May 1999     

Investigation of the spectrum of surface states in bismuth by scanning tunneling spectroscopy

Scanning tunneling spectroscopy of trigonal (0001) and “quasitrigonal” surfaces of a twin interlayer on a cleaved face of bismuth is performed. It is found that both surfaces are characterized by surface electron states with spectra exhibiting clearly defined singularities, namely, relatively narrow maxima and minima of the density of states in the energy range of ?1 eV. An analysis of the behavior of the current-voltage characteristics at low (of the order of tens of millivolts) voltages has revealed the existence on the bismuth surface of a two-dimensional layer, in which the density of states of electrons, unlike its anomalously small value in the bulk of bismuth, is of the order of magnitude typical of metals.     

Real-space observation of dipolar antiferromagnetism in magnetic nanowires by spin-polarized scanning tunneling spectroscopy

We have performed spin-polarized scanning tunneling spectroscopy of dipolar antiferromagnetically coupled Fe nanowires with a height of two atomic layers and an average separation of 8 nm grown on stepped W(110). Domain walls within the nanowires exhibit a significantly reduced width when pinned at structural constrictions. The lateral spin reorientation in the direction perpendicular to the wires has been studied with subnanometer spatial resolution. It is found that the spin canting in the Fe nanowires monotonously increases towards the step edges.     

Spatial resolution of imaging contaminations on the GaAs surface by scanning tunneling microscope-cathodoluminescence spectroscopy

We obtained the luminescence image of the GaAs (1 1 0) surface by scanning tunneling microscope-cathodoluminescence (STM-CL) spectroscopy, where low-energy (∼100 eV) electrons field emitted from the STM tip were used as a bright excitation source. The STM-CL image with high photon signal (1.25 × 10⁴ cps) showed the dark image corresponding to the surface contamination in the STM image working as the nonradiative recombination centers of carriers. This dark image demonstrated the spatial resolution of about 100 nm in STM-CL spectroscopy of the GaAs (1 1 0) surface, which was determined by the field-emitted electron beam diameter.     

Detecting electronic states at stacking faults in magnetic thin films by tunneling spectroscopy

Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated with the stacking fault are visualized by tunneling spectroscopy, and its origin is identified by band structure calculations.     

Chemical fingerprinting at the atomic level with scanning tunneling spectroscopy

Scanning tunneling spectroscopy (STS) based on scanning tunneling microscopy (STM) makes it possible to map the local electronic density of states for clean surfaces and for those with adsorbates. We have developed a protocol that allows us to obtain the spectral fingerprints of halogen atoms on Si(0 0 1), and we use those fingerprints to distinguish between adatom species for surfaces with Cl and Br mixed adsorbates. The key to the process is the energy distribution of the antibonding states that depend on the halogen species.     

Theory of the density of states of pure type-II superconductors in high magnetic fields

This paper deals with the theory of the density of states of pure type-II superconductors in high magnetic fields. An approximate expression of theGreen's function is derived by using the fact that it has the periodicity of the lattice of flux lines in the center-of-mass coordinates. In comparison to the normal stateGreen's function a correction term proportional to the absolute square of the order parameter arises. This is seen to act as a self-energy part depending strongly on the polar angleΘ of the “quasiparticle” momentum relative to the direction of the field. The corresponding density of states, as a function of the excitation energy, is found to vary from the gapless to the BCS type as the parameterΘ changes fromπ/2 to zero (orπ). The averaged state density varies gradually from a uniform to a BCS-like state density as the external field decreases sufficiently far below the upper critical field. The results of the present theory for the averaged density of states agree fairly well with the results due to a conjecture byMaki if the field is very close to the upper critical field.     

Special features of wide-band EPR spectroscopy of singlet states in weak magnetic fields

A resonance change in microwave absorption in zero magnetic field, which is not due to magnetic-field dependence of the energy levels of the spin systems, is observed for a number of non-Kramers paramagnetic ions (Cr²⁺, Ni²⁺, Fe²⁺) in synthetic forsterite. It is shown that these signals could be due to narrowing of the homogeneous spin packet of an inhomogeneously broadened EPR line in zero magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 370–375 (10 September 1998)     

Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy

Using low-temperature scanning tunneling spectroscopy applied to the Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe electron-electron interaction effects in the quantum Hall regime. The 2DES is decoupled from bulk states and exhibits spreading resistance within the insulating quantum Hall phases. In quantitative agreement with calculations we find an exchange enhancement of the spin splitting. Moreover, we observe that both the spatially averaged as well as the local density of states feature a characteristic Coulomb gap at the Fermi level. These results show that electron-electron interaction can be probed down to a resolution below all relevant length scales.     

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy

Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.     

Discretization of electronic states in large InAsP/InP multilevel quantum dots probed by scanning tunneling spectroscopy

The topography and the electronic structure of InAsP/InP quantum dots are probed by cross-sectional scanning tunneling microscopy and spectroscopy. The study of the local density of states in such large quantum dots confirms the discrete nature of the electronic levels whose wave functions are measured by differential conductivity mapping. Because of their large dimensions, the energy separation between the discrete electronic levels is low, allowing for quantization in both the lateral and growth directions as well as the observation of the harmonicity of the dot lateral potential.     

Plasmon resonance in sputtered gold films observed in scanning tunneling microscopy

Gold films grown on BaTiO₃ substrates, illuminated by a monochromatic beam in the visible range, were investigated using scanning tunneling microscopy (STM). Irregular variations in the tip displacements versus photon energy are observed at energies of the surface plasmon resonances (2.0 ± 0.1 and 2.7 ± 0.1 eV) of the rough gold film.     

Enhanced fluctuations of the tunneling density of states near the bottom of a landau band measured by a local spectrometer

We have found that the local density of state fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to Shubnikov-de Haas and de Haas-van Alphen effects) oscillations in strong magnetic fields, omega(c)tau>/=1. We attribute this to the dominant role of the states near the bottom of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate fields the LDOSF increase with field B in accordance with the results obtained in the diffusion approximation.