11 research outputs found
On the origin and elimination of cross coupling between tunneling current and excitation in scanning probe experiments that utilize the qPlus sensor
The qPlus sensor allows simultaneous measurements of scanning tunneling
microscopy (STM) and atomic force microscopy (AFM). Its design for use in
frequency modulation AFM (FM-AFM) involves separate electrodes, applied on an
oscillating quartz cantilever, for the detection of an electronic tunneling
current and the deflection signal. Cable resistance and capacities in the
electronic setup can induce cross talk phenomena. We report a tunneling
current-induced cross coupling observed in a combined STM/AFM setup which uses
the qPlus sensor. This cross coupling can induce a positive or negative change
of the amplitude excitation signal, thus acting as an apparent dissipation or
drive. The experimental data is explained well by tunneling current-induced
fluctuations of the electric potential on the sensor electrodes
Revealing a spatially inhomogeneous broadening effect in artificial quantum structures caused by electron-adsorbate scattering
What defines the lifetime of electronic states in artificial quantum
structures? We measured the spectral widths of resonant eigenstates in a
circular, CO-based quantum corral on a Cu(111) surface and found that the
widths are related to the size of the corral and that the line shape is
essentially Gaussian. A model linking the energy dependence with the movement
of single surface electrons shows that the observed behavior is consistent with
lifetime limitations due to interaction with the corral walls
Connecting Gas Dynamics and Star Formation Histories in Nearby Galaxies: The VLA-ANGST Survey
In recent years, HST revolutionized the field of star formation in nearby
galaxies. Due to its high angular resolution it has now become possible to
construct star formation histories of individual stellar populations on scales
of a few arcseconds spanning a range of up to
~600 Myr. This method will be applied to the ANGST galaxies, a large HST
volume limited survey to map galaxies up to distances of 3.5-4.0 Mpc (excluding
the Local Group). The ANGST sample is currently followed--up by high, ~6''
resolution VLA observations of neutral, atomic hydrogen (HI) in the context of
VLA-ANGST, an approved Large VLA Project. The VLA resolution is well matched to
that of the spatially resolved star formation history maps. The combination of
ANGST and VLA-ANGST data will provide a new, promising approach to study
essential fields of galaxy evolution such as the triggering of star formation,
the feedback of massive stars into the interstellar medium, and the structure
and dynamics of the interstellar medium.Comment: to appear in the proceedings to the conference: "The Evolution of
Galaxies through the Neutral Hydrogen Window", Arecibo, PR, US
The Formation of Kiloparsec-scale HI Holes in Dwarf Galaxies
The origin of kpc-scale holes in the atomic hydrogen (H i) distributions of some nearby dwarf irregular galaxies
presents an intriguing problem. Star formation histories (SFHs) derived from resolved stars give us the unique
opportunity to study past star-forming events that may have helped shape the currently visible Hi distribution. Our
sample of five nearby dwarf irregular galaxies spans over an order of magnitude in both total Hi mass and absolute
B-band magnitude and is at the low-mass end of previously studied systems. We use Very Large Array Hi line
data to estimate the energy required to create the centrally dominant hole in each galaxy. We compare this energy estimate to the past energy released by the underlying stellar populations computed from SFHs derived from data taken with the Hubble Space Telescope. The inferred integrated stellar energy released within the characteristic ages exceeds our energy estimates for creating the holes in all cases, assuming expected efficiencies. Therefore, it appears that stellar feedback provides sufficient energy to produce the observed holes. However, we find no obvious signature of single star-forming events responsible for the observed structures when comparing the global SFHs of each galaxy in our sample to each other or to those of dwarf irregular galaxies reported in the literature. We also fail to find evidence of a central star cluster in FUV or Hα imaging. We conclude that large Hi holes are likely formed from multiple generations of star formation and only under suitable interstellar medium conditions
VLA-ANGST: A high-resolution HI Survey of Nearby Dwarf Galaxies
We present the "Very Large Array survey of Advanced Camera for Surveys Nearby
Galaxy Survey Treasury galaxies (VLA-ANGST)." VLA-ANGST is a National Radio
Astronomy Observatory Large Program consisting of high spectral (0.6-2.6 km/s)
and spatial (~6") resolution observations of neutral, atomic hydrogen (HI)
emission toward 35 nearby dwarf galaxies from the ANGST survey. ANGST is a
systematic HST survey to establish a legacy of uniform multi-color photometry
of resolved stars for a volume-limited sample of nearby galaxies (D\lesssim4
Mpc). VLA-ANGST provides VLA HI observations of the sub-sample of ANGST
galaxies with recent star formation that are observable from the northern
hemisphere and that were not observed in the "The HI Nearby Galaxy Survey"
(THINGS). The overarching scientific goal of VLA-ANGST is to investigate
fundamental characteristics of the neutral interstellar medium (ISM) of dwarf
galaxies. Here we describe the VLA observations, the data reduction, and the
final VLA-ANGST data products. We present an atlas of the integrated HI maps,
the intensity-weighted velocity fields, the second moment maps as a measure for
the velocity dispersion of the HI, individual channel maps, and integrated HI
spectra for each VLA-ANGST galaxy. We closely follow the observational setup
and data reduction of THINGS to achieve comparable sensitivity and angular
resolution. A major difference, however, is the high velocity resolution of the
VLA-ANGST observations (0.65 and 1.3km/s for the majority of the galaxies). The
VLA-ANGST data products are made publicly available at:
https://science.nrao.edu/science/surveys/vla-angst. With available star
formation histories from resolved stellar populations and lower resolution
ancillary observations from the FIR to the UV, VLA-ANGST will enable detailed
studies of the relationship between the ISM and star formation in dwarf
galaxies on a ~100 pc scale.Comment: 64 figures, grouped into 32. 115 pages, accepted for publication in
the Astronomical Journa
Lateral manipulation of single iron adatoms by means of combined atomic force and scanning tunneling microscopy using CO-terminated tips
CO-terminated tips currently provide the best spatial resolution obtainable in atomic force microscopy. Due to their chemical inertness, they allow us to probe interactions dominated by Pauli repulsion. The small size and inertness of the oxygen front atom yields unprecedented resolution of organic molecules, metal clusters, and surfaces. We study the capability of CO-terminated tips to laterally manipulate single iron adatoms on the Cu(111) surface with combined atomic force and scanning tunneling microscopy at 7 K. We find that even a slight asymmetry of the tip results in a distortion of the lateral force field. In addition, the influence of the tilt of the CO tip on the lateral force field is reversed compared to the use of a monoatomic metal tip which we can attribute to the inverted dipole moment of a CO tip with respect to a metal tip. Moreover, we demonstrate atom-by-atom assembly of iron clusters with CO tips while using the high-resolution capability of the CO tips in between to determine the arrangement of the individual iron atoms within the cluster. In all conducted experiments using CO tips within this study, the CO was never changed or lost from the tip's apex
Very weak bonds to artificial atoms formed by quantum corrals
We explored the bonding properties of the quantum corral (a circle of 48 iron atoms placed on a copper surface) reported by Crommie et al. in 1993, along with variants, as an artificial atom using an atomic force microscope (AFM). The original corral geometry confines 102 electrons to 28 discrete energy states, and we found that these states can form a bond to the front atom of the AFM with an energy of about 5 millielectron volts. The measured forces are about 1/1000 of typical forces in atomically resolved AFM. The confined electrons showed covalent attraction to metal tips and Pauli repulsion to CO-terminated tips. The repulsion at close distance was evident from the response of corral states created by deliberately placing single iron atoms inside the corral. The forces scaled appropriately with a 24-atom corral