12 research outputs found
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
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
Response of the topological surface state to surface disorder in TlBiSe
Through a combination of experimental techniques we show that the topmost
layer of the topo- logical insulator TlBiSe as prepared by cleavage is
formed by irregularly shaped Tl islands at cryogenic temperatures and by mobile
Tl atoms at room temperature. No trivial surface states are observed in
photoemission at low temperatures, which suggests that these islands can not be
re- garded as a clear surface termination. The topological surface state is,
however, clearly resolved in photoemission experiments. This is interpreted as
a direct evidence of its topological self-protection and shows the robust
nature of the Dirac cone like surface state. Our results can also help explain
the apparent mass acquisition in S-doped TlBiSe.Comment: 16 pages, 5 figure
Comparison of force sensors for atomic force microscopy based on quartz tuning forks and length-extensional resonators
Atomic force microscopy in the picometer regime - resolving spins and non-trivial surface terminations
Atomic Force MIcrosocpy (AFM) offers the unique possiblity to study conductive as well as insulating surfaces at the atomic scale. While achieving atomic or sub-molecular resolution with low temperature AFM is well established nowadays, the resolution of subatomic features and magnetic exchange interactions remains a challenge. The first part of this work deals with the characterization and improvement of the signal-to-noise ratio (SNR) of quartz-based force sensors, namely the qPlus and the needle sensor.
The second issue is the resolution of non-trivial surface terminations. Subatomic features on a copper (Cu) adatom on Cu(110) were resolved. Another example for a non-trivial surface termination is the cleaved TlBiSe2 surface which consists of disordered Tl islands. This directly explains the absence of trivial surface states in photoemission experiments.
The third main topic is the resolution of the antiferromagnetic structure of nickel oxide (001) at the atomic scale. Additionally, the exchange interaction for this system was quantitatively measured with dynamic force spectroscopy. The results with SmCo tips indicate that tip materials with a large magnetocrystalline anisotropy energy should be selected to detect magnetic exchange forces
Chemical and Crystallographic Characterization of the Tip Apex in Scanning Probe Microscopy
The apex atom of a W scanning probe tip reveals a nonspherical charge distribution as probed by a CO molecule bonded to a Cu(111) surface [Welker et al., Science 336, 444 (2012).]. Three high-symmetry images were observed and related to three low-index crystallographic directions of the W bcc crystal. Open questions remained, such as the detectability of a contamination of W tips by sample material (here Cu), and the applicability of the method to distinguish other atomic species. In this work, we investigate bulk Cu and Fe tips. In both cases, we can associate our data with the fcc (Cu) and bcc (Fe) crystal structures using a simple electrostatic model that is based on the partial filling of d orbitals
Response of the topological surface state to surface disorder in TlBiSe2
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.-- et al.Through a combination of experimental techniques we show that the topmost layer of the topological insulator TlBiSe2 as prepared by cleavage is formed by irregularly shaped Tl islands at cryogenic temperatures and by mobile Tl atoms at room temperature. No trivial surface states are observed in photoemission at low temperatures, which suggests that these islands cannot be regarded as a clear surface termination. The topological surface state is, however, clearly resolved in photoemission experiments. This is interpreted as direct evidence of its topological self-protection and shows the robust nature of the Dirac cone-like surface state. Our results can also help explain the apparent mass acquisition in S-doped TlBiSe2.This work is supported by the Swiss National Science Foundation (PP00P2_144742/1). FP and FJG acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) within SFB 689. JW acknowledges funding by the priority programme SPP1666 of the DFG and AAK acknowledges funding from the Emmy Noether program of the DFG (KH324/1-1).Peer Reviewe
Impact of thermal frequency drift on highest precision force microscopy using quartz-based force sensors at low temperatures
In frequency modulation atomic force microscopy (FM-AFM) the stability of the eigenfrequency of the force sensor is of key importance for highest precision force measurements. Here, we study the influence of temperature changes on the resonance frequency of force sensors made of quartz, in a temperature range from 4.8–48 K. The sensors are based on the qPlus and length extensional principle. The frequency variation with temperature T for all sensors is negative up to 30 K and on the order of 1 ppm/K, up to 13 K, where a distinct kink appears, it is linear. Furthermore, we characterize a new type of miniaturized qPlus sensor and confirm the theoretically predicted reduction in detector noise
Subatomic resolution force microscopy reveals internal structure and adsorption sites of small iron clusters
Clusters built from individual iron atoms adsorbed on surfaces (adatoms) were investigated by atomic force microscopy (AFM) with subatomic resolution. Single copper and iron adatoms appeared as toroidal structures and multiatom clusters as connected structures, showing each individual atom as a torus. For single adatoms, the toroidal shape of the AFM image depends on the bonding symmetry of the adatom to the underlying structure [twofold for copper on copper(110) and threefold for iron on copper(111)]. Density functional theory calculations support the experimental data. The findings correct our previous work, in which multiple minima in the AFM signal were interpreted as a reflection of the orientation of a single front atom, and suggest that dual and triple minima in the force signal are caused by dimer and trimer tips, respectively