141 research outputs found
Attempts to test an alternative electrodynamic theory of superconductors by low-temperature scanning tunneling and atomic force microscopy
We perform an experiment to test between two theories of the electrodynamics
of superconductors: the standard London theory and an alternative proposed by
J. E. Hirsch [Phys. Rev. B 69, 214515 (2004)]. The two alternatives give
different predictions with respect to the screening of an electric field by a
superconductor, and we try to detect this effect using atomic force microscopy
on a niobium sample. We also perform the reverse experiment, where we
demonstrate a superconductive tip mounted on a qPlus force sensor. Due to
limited accuracy, we are able neither to prove nor to disprove Hirsch's
hypothesis. Within our accuracy of 0.17 N/m, the superconductive transition
does not alter the atomic-scale interaction between tip and sample.Comment: 7 pages, 5 figures. Minor amendment
Localization of the phantom force induced by the tunneling current
The phantom force is an apparently repulsive force, which can dominate the atomic contrast of an AFM image when a tunneling current is present. We described this effect with a simple resistive model, in which the tunneling current causes a voltage drop at the sample area underneath the probe tip. Because tunneling is a highly local process, the areal current density is quite high, which leads to an appreciable local voltage drop that in turn changes the electrostatic attraction between tip and sample. However, Si(111)-7×7 has a metallic surface state and it might be proposed that electrons should instead propagate along the surface state, as through a thin metal film on a semiconducting surface, before propagating into the bulk. In this paper, we first measure the phantom force on a sample that displays a metallic surface state [here, Si(111)-7×7] using tips with various radii. If the metallic surface state would lead to a constant electrostatic potential on the surface, we would expect a direct dependence of the phantom force with tip radius. In a second set of experiments, we study H/Si(100), a surface that does not have a metallic surface state. We conclude that a metallic surface state does not suppress the phantom force, but that the local resistance Rs has a strong effect on the magnitude of the phantom force
Investigating Atomic Details of the CaF(111) Surface with a qPlus Sensor
The (111) surface of CaF has been intensively studied with
large-amplitude frequency-modulation atomic force microscopy and atomic
contrast formation is now well understood. It has been shown that the apparent
contrast patterns obtained with a polar tip strongly depend on the tip
terminating ion and three sub-lattices of anions and cations can be imaged.
Here, we study the details of atomic contrast formation on CaF(111) with
small-amplitude force microscopy utilizing the qPlus sensor that has been shown
to provide utmost resolution at high scanning stability. Step edges resulting
from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp
structures and on flat terraces, the atomic corrugation is seen in high clarity
even for large area scans. The atomic structure is also not lost when scanning
across triple layer step edges. High resolution scans of small surface areas
yield contrast features of anion- and cation sub-lattices with unprecedented
resolution. These contrast patterns are related to previously reported
theoretical results.Comment: 18 pages, 9 Figures, presented at 7th Int Conf Noncontact AFM
Seattle, USA Sep 12-15 2004, accepted for publication in Nanotechnology,
http://www.iop.or
Application of the equipartition theorem to the thermal excitation of quartz tuning forks
The deflection signal of a thermally excited force sensor of an atomic force
microscope can be analyzed to gain important information about the detector
noise and about the validity of the equipartion theorem of thermodynamics.
Here, we measured the temperature dependence of the thermal amplitude of a
tuning fork and compared it to the expected values based on the equipartition
theorem. In doing so, we prove the validity of these assumptions in the
temperature range from 140K to 300K. Furthermore, the application of the
equipartition theorem to quartz tuning forks at liquid helium temperatures is
discussed.Comment: 8 pages, 3 figures, published in Applied Physics Letter
Spin Resolution and Evidence for Superexchange on NiO(001) observed by Force Microscopy
The spin order of the nickel oxide (001) surface is resolved, employing
non-contact atomic force microscopy at 4.4 K using bulk Fe- and SmCo-tips
mounted on a qPlus sensor that oscillates at sub-50 pm amplitudes. The
spin-dependent signal is hardly detectable with Fe-tips. In contrast, SmCo-tips
yield a height contrast of 1.35 pm for Ni ions with opposite spins. SmCo tips
even show a small height contrast on the O atoms of 0.5 pm within the 2x1 spin
unit cell, pointing to the observation of superexchange. We attribute this to
the increased magnetocrystalline anisotropy energy of SmCo, which stabilizes
the magnetic moment at the apex. Atomic force spectroscopy on the Ni up, Ni
down and O lattice site reveals a magnitude of the exchange energy of merely 1
meV at the closest accessible distance with an exponential decay length of
\lambda_exc = 18 pm.Comment: 5 pages, 3 figure
Seeing the Reaction
What happens as a molecule goes through a chemical reaction? Model studies have provided important insights into these processes, but it remains extremely difficult to follow all the atomic rearrangements of a chemical reaction experimentally. In many cases, a reaction cannot be observed directly in real space, for example, because the reactants are in the gas state, zooming around at the speed of sound. On page 1434 of this issue, de Oteyza et al. (1) report atomically resolved imaging of a complex molecule as it undergoes a chemical reaction on a metal surface
The qPlus sensor, a powerful core for the atomic force microscope
Atomic force microscopy (AFM) was introduced in 1986 and has since made its way into surface science, nanoscience, chemistry, biology, and material science as an imaging and manipulating tool with a rising number of applications. AFM can be employed in ambient and liquid environments as well as in vacuum and at low and ultralow temperatures. The technique is an offspring of scanning tunneling microscopy (STM), where the tunneling tip of the STM is replaced by using a force sensor with an attached tip. Measuring the tiny chemical forces that act between the tip and the sample is more difficult than measuring the tunneling current in STM. Therefore, even 30 years after the introduction of AFM, progress in instrumentation is substantial. Here, we focus on the core of the AFM, the force sensor with its tip and detection mechanism. Initially, force sensors were mainly micro-machined silicon cantilevers, mainly using optical methods to detect their deflection. The qPlus sensor, originally based on a quartz tuning fork and now custom built from quartz, is self-sensing by utilizing the piezoelectricity of quartz. The qPlus sensor allows us to perform STM and AFM in parallel, and the spatial resolution of its AFM channel has reached the subatomic level, exceeding the resolution of STM. Frequency modulation AFM (FM-AFM), where the frequency of an oscillating cantilever is altered by the gradient of the force that acts between the tip and the sample, has emerged over the years as the method that provides atomic and subatomic spatial resolution as well as force spectroscopy with sub-piconewton sensitivity. FM-AFM is precise; because of all physical observables, time and frequency can be measured by far with the greatest accuracy. By design, FM-AFM clearly separates conservative and dissipative interactions where conservative forces induce a frequency shift and dissipative interactions alter the power needed to maintain a constant oscillation amplitude of the cantilever. As it operates in a noncontact mode, it enables simultaneous AFM and STM measurements. The frequency stability of quartz and the small oscillation amplitudes that are possible with stiff quartz sensors optimize the signal to noise ratio. Here, we discuss the operating principles, the assembly of qPlus sensors, amplifiers, limiting factors, and applications. Applications encompass unprecedented subatomic spatial resolution, the measurement of forces that act in atomic manipulation, imaging and spectroscopy of spin-dependent forces, and atomic resolution of organic molecules, graphite, graphene, and oxides
A comparsion of force sensors for atomic force microscopy based on quartz tuning forks and length extensional resonators
The force sensor is key to the performance of atomic force microscopy (AFM).
Nowadays, most AFMs use micro-machined force sensors made from silicon, but
piezoelectric quartz sensors are applied at an increasing rate, mainly in
vacuum. These self sensing force sensors allow a relatively easy upgrade of a
scanning tunneling microscope to a combined scanning tunneling/atomic force
microscope. Two fundamentally different types of quartz sensors have achieved
atomic resolution: the 'needle sensor' that is based on a length extensional
resonator and the 'qPlus sensor' that is based on a tuning fork. Here, we
calculate and measure the noise characteristics of these sensors. We find four
noise sources: deflection detector noise, thermal noise, oscillator noise and
thermal drift noise. We calculate the effect of these noise sources as a factor
of sensor stiffness, bandwidth and oscillation amplitude. We find that for self
sensing quartz sensors, the deflection detector noise is independent of sensor
stiffness, while the remaining three noise sources increase strongly with
sensor stiffness. Deflection detector noise increases with bandwidth to the
power of 1.5, while thermal noise and oscillator noise are proportional to the
square root of the bandwidth. Thermal drift noise, however, is inversely
proportional to bandwidth. The first three noise sources are inversely
proportional to amplitude while thermal drift noise is independent of the
amplitude. Thus, we show that the earlier finding that quoted optimal
signal-to-noise ratio for oscillation amplitudes similar to the range of the
forces is still correct when considering all four frequency noise
contributions. Finally, we suggest how the signal-to-noise ratio of the sensors
can be further improved and briefly discuss the challenges of mounting tips.Comment: 40 pages, 14 figure
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