摘要:

Scanning tunneling spectroscopy is used to study the energy dependence of wavevectors of the local‐density‐of‐states (LDOS) modulations in near‐optimal Bi2Sr2CaCu2O8+&dgr;. Atomic‐resolution energy‐resolved spectroscopic images are acquired in a 650 Å field of view on the BiO cleave surface of these crystals at 4.2K. Fourier transformations are then used to determine the wavevectors of any spatial modulation in theLDOS. At sub‐gap energies, up to 16 inequivalent sets of dispersive wavevectors are observed. When analyzed within a model of quasiparticle scattering‐induced interference between a characteristic ‘octet’ of states in momentum‐space, they yield an estimate of the Fermi‐surface location and the energy gap |&Dgr;(k)| in good agreement with ARPES. At energies approaching the gap‐maximum, theLDOSmodulations become intense, commensurate with the crystal, and localized by the apparent nanoscale domains. This may indicate that the lifetimes of thek&vec;= (&pgr; /a0,0) quasiparticles, which are central to high‐Tcsuperconductivity, are determined by nanoscale disorder scattering. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639671

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Scanning tunneling spectroscopy (STS) is used to investigate the local density of states of the paradigmatic electron system located in the quasi‐parabolic conduction band of InAs. The n‐InAs(110) surface is used to investigate the system in all dimensionalities (0D‐3D) at low temperatureT= 6 K and in magnetic fields up toB= 6 T. Depending on the dimensionality and theB‐field, we identify different types of electron phases. In particular, we find Bloch‐like wave functions scattered at dopants in 3D atB= 0 T, more complex wave functions reminiscent of weak localization in 2D atB= 0 T and drift states in 2D and 3D atB= 6 T. In 1D, we find surprisingly that the results are explained by a single‐particle calculation, although the energy scales are dominated by electron‐electron interaction. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639672

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We have used scanning tunneling spectroscopy to spatially map the energy‐resolved local density of states of C60molecules in the monomer and monolayer regimes on the Ag(001) surface. Spectral maps were obtained for molecular states derived from the C60HOMO, LUMO, and LUMO+1 orbitals. An unexpected density inversion is observed between the spectral maps of the LUMO and LUMO+1 states. The observed spatial inhomogeneities and density inversion are explained usingab initiopseudopotential density functional calculations. These calculations demonstrate the need for explicitly including the STM tip trajectory when interpreting STM images of electronically inhomogeneous systems. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639673

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Semiconductor probe tips are explored as a tool for spectroscopic scanning tunneling microscopy (STM). Sharp tips capable of atomic‐resolution STM are obtained by cleaving (001) oriented InAs wafers. Tunneling spectra measured with InAs tips on graphite show clear signatures of the bulk band structure of the tip. The influence of the tip electronic structure on the tunneling probability can be used to achieve energy‐filtered imaging in filled‐state constant‐current STM with cleaved [111]‐oriented InAs tips. Energy gaps in the projected bulk band structure of the tip suppress tunneling, so that only sample states not aligned with a gap contribute significantly to the tunneling current. On Si(111)‐(7×7), such energy filtering enables state‐selective imaging. Dangling bonds localized on different near‐surface Si atoms — adatoms and rest atoms — are mapped selectively at different tip‐sample bias. Our findings suggest an important role of the bulk band structure of the probe tip in determining the tunneling current in STM, which may be used to develop novel paradigms of spectroscopic imaging at the atomic scale. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639674

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We present experimental and theoretical results for light emission from a scanning tunneling microscope probing the metallic quantum well system consisting of a Na overlayer on Cu(111). The experimental results can be summarized as follows: (i) At low bias voltages a broad, plasmon‐mediated spectrum of emitted light is seen. (ii) Once the bias voltageUis large enough that electrons can be injected from the tip into the second QWS, the spectrum becomes sharper and is centered at a photon energyhvcorresponding to the energy difference between the second and first QWS,E2−E1. (iii) Finally, and most strikingly, photons with this energy (E2−E1) are observed, albeit with lower intensity, also wheneU<hv. In this work we employ a one‐dimensional model potential to describe the electronic structure of the overlayer system, and combine this with a calculation of the electromagnetic “plasmon‐mediated” enhancement of the spontaneous emission. Furthermore the calculations show that two‐electron processes, either an Auger‐like mechanism or a hot‐electron‐hole cascade in the tip, can cause light emission in the case whenhv>eUwith quantum yield and power‐law variation of the intensity with the tunnel current in good agreement with experiment. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639675

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Magnetic resonance force microscopy is a technique aimed at achieving three‐dimensional sub‐surface imaging with atomic resolution. We discuss recent progress in using ultrasensitive force detection techniques to detect small ensembles of electron spins in SiO2. Using mass‐loaded cantilevers that suppress thermal vibration noise in the upper modes, we have achieved a sensitivity of equivalent to about 2 electron spins in the natural bandwidth of the measurement (0.12 Hz). © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639676

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The electronic structure of thin Co nanoislands on Cu(111) has been investigated by scanning tunneling spectroscopy at low temperature. Two surface related electronic states are found. An energetically localized state in the spectrum at 0.31 eV below the Fermi level (EF) and a mainly unoccupied dispersive state, giving rise to quantum interference patterns of standing electron waves on the Co surface.Ab initiocalculations reveal that both electronic states are spin‐polarized, originating fromd3z2−r2‐minority andsp‐majority bands, respectively. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639677

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

When the atomic force microscope (AFM) was introduced in 1986, its potential to resolve surfaces with true atomic resolution was already proposed. However, substantial problems had to be overcome before atomic resolution became possible by AFM. Today, true atomic resolution by AFM is standard practice. This article discusses the influence of the cantilever stiffness and — amplitude on noise and short‐range force sensitivity and introduces a sensor operating at near optimal conditions (qPlus sensor). The data achieved with this optimized sensing technology show substructures within single atom images, attributed to atomic orbitals. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639678

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A method is presented that utilizes the frequency modulation technique in ultra‐high vacuum to measure the tip‐sample force field in all three dimensions with atomic resolution. It is based on a systematic procedure to record frequency shift versus distance curves. After their conversion into the tip‐surface potential landscape the complete force field in all three dimensions can be calculated. Experimental results obtained in the non‐contact regime on NiO(001) with an iron‐coated silicon tip are presented to demonstrate that interatomic vertical and lateral forces can be determined and assigned to specific sites within the surface unit cell. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639679

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The facility of scanning probe microscopy techniques to interact with and manipulate objects with sub‐angstrom precision has made these the basic tools for nanotechnology. However, existing methods are very slow and restrict the range of possible applications. In this paper we present a novel approach that increases the rate at which experiments at the nanometre length scale can be performed by approximately 10,000 times. We have developed a micro scanner that utilises its own internal mechanical resonance to address a surface with nanometre resolution and is capable of imaging at over 100 frames/sec, the fastest scanning probe microscope to‐date. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639680

出版商:AIP    

年代:1903

数据来源: AIP