An ultrahigh-vacuum cryostat for simultaneous scanning tunneling microscopy and magneto-transport measurements down to 400mK

We present the design and calibration measurements of a scanning tunneling microscope setup in a 3He ultrahigh-vacuum cryostat operating at 400 mK with a hold time of 10 days. With 2.70 m in height and 4.70 m free space needed for assembly, the cryostat fits in a one-story lab building. The microscope features optical access, an xy table, in situ tip and sample exchange, and enough contacts to facilitate atomic force microscopy in tuning fork operation and simultaneous magneto-transport measurements on the sample. Hence, it enables scanning tunneling spectroscopy on microstructured samples which are tuned into preselected transport regimes. A superconducting magnet provides a perpendicular field of up to 14 T. The vertical noise of the scanning tunneling microscope amounts to 1 pmrms within a 700 Hz bandwidth. Tunneling spectroscopy using one superconducting electrode revealed an energy resolution of 120 mueV. Data on tip-sample Josephson contacts yield an even smaller feature size of 60 mueV, implying that the system operates close to the physical noise limit.Comment: 12 pages, 11 figure

Design and calibration of a vacuum compatible scanning tunneling microscope

A vacuum compatible scanning tunneling microscope was designed and built, capable of imaging solid surfaces with atomic resolution. The single piezoelectric tube design is compact, and makes use of sample mounting stubs standard to a commercially available surface analysis system. Image collection and display is computer controlled, allowing storage of images for further analysis. Calibration results from atomic scale images are presented

Infrared light emission from atomic point contacts

Gold atomic point contacts are prototype systems to evidence ballistic electron transport. The typical dimension of the nanojunction being smaller than the electron-phonon interaction length, even at room temperature, electrons transfer their excess energy to the lattice only far from the contact. At the contact however, favored by huge current densities, electron-electron interactions result in a nano hot electron gas acting as a source of photons. Using a home built Mechanically Controlled Break Junction, it is reported here, for the first time, that this hot electron gas also radiates in the infrared range (0.2eV to 1.2eV). Moreover, in agreement with the pioneering work of Tomchuk, we show that this radiation is compatible with a blackbody like spectrum emitted from an electron gas at temperatures of several thousands of Kelvin given by $(kB.Te)^2 = \alpha. I.V$ where $\alpha$, $I$ and $V$ are respectively a fitting parameter, the current flowing and the applied bias.Comment: 13 pages, 5 figure

STM study of self-assembled phthalocyanine derivatives and their hosting properties

Molecular self-assembly, as a most studied case of self-assembly, is one of the few practical strategies for making ensembles of nano- and micro structures. As an essential aspect of the “bottom-up” approach, it is attractive for both scientific research and technological applications. Therefore a detailed understanding of the molecule-substrate and intermolecular interactions involved in the self-assembly process is of great interest. In the first part of the thesis, the influence of the phenoxy substituents on the self-assembly of Pcs on (111)-oriented noble metal surfaces is described. The rotational degrees of freedom, characteristic for these substituents enable the formation of various stable and transient phases and allow the substituents to be arranged above the plane of the Pc core, forming a bowl-like structure, which in turn enables the interaction of the Pc core with the metal substrate. The proximity of the Pc core to the metal substrate together with the steric entanglement between neighboring substituents causes significant retardation of the thermodynamic optimization of the conformations. This accounts for the coexistence of some of the phases. In the second part, the influence of replacing two adjacent phenoxy substituents by a rigid tetraazatriphenylene substituent on the self-assembly of Pcs is analyzed and compared to the self-assembly of the above mentioned phenoxy substituted Pcs. The rigid substituent enhances the rotational degrees of freedom of the neighboring phenoxy substituents, hence facilitates their conformational optimization. As a result, novel interactions between the Pc derivatives are enabled and the formation of ordered phases with higher surface densities compared to the previous study is observed. In the third part, the hosting properties of a close-packed layer of phenoxy substituted Pc derivatives adsorbed on Ag(111) are investigated for the adsorption of C60 molecules. The C60 molecules bind to two clearly distinguishable sites, either to the underlying metal substrate in between two adjacent Pc derivatives or to the core of a Pc derivative. In the first case, the C60 exhibit morphologic and electronic properties analogous to those of a C60 adsorbed on clean Ag(111), whereas in the second case the electronic properties indicate a strong interaction between C60 and the phthalocyanine core

A scanning gate microscope for cold atomic gases

We present a scanning probe microscopy technique for spatially resolving transport in cold atomic gases, in close analogy with scanning gate microscopy in semiconductor physics. The conductance of a quantum point contact connected to two atomic reservoirs is measured in the presence of a tightly focused laser beam acting as a local perturbation that can be precisely positioned in space. By scanning its position and recording the subsequent variations of conductance, we retrieve a high-resolution map of transport through a quantum point contact. We demonstrate a spatial resolution comparable to the extent of the transverse wave function of the atoms inside the channel, and a position sensitivity below 10nm. Our measurements agree well with an analytical model and ab-initio numerical simulations, allowing us to identify a regime in transport where tunneling dominates over thermal effects. Our technique opens new perspectives for the high-resolution observation and manipulation of cold atomic gases.Comment: 5 + 6 pages, 4 + 5 figure

Calibration of piezoelectric positioning actuators using a reference voltage-to-displacement transducer based on quartz tuning forks

We use a piezoelectric quartz tuning fork to calibrate the displacement of ceramic piezoelectric scanners which are widely employed in scanning probe microscopy. We measure the static piezoelectric response of a quartz tuning fork and find it to be highly linear, non-hysteretic and with negligible creep. These performance characteristics, close to those of an ideal transducer, make quartz transducers superior to ceramic piezoelectric actuators. Furthermore, quartz actuators in the form of a tuning fork have the advantage of yielding static displacements comparable to those of local probe microscope scanners. We use the static displacement of a quartz tuning fork as a reference to calibrate the three axis displacement of a ceramic piezoelectric scanner. Although this calibration technique is a non-traceable method, it can be more versatile than using calibration grids because it enables to characterize the linear and non-linear response of a piezoelectric scanner in a broad range of displacements, spanning from a fraction of a nanometer to hundreds of nanometers. In addition, the creep and the speed dependent piezoelectric response of ceramic scanners can be studied in detail.Comment: 9 pages, 3 figure

What is the orientation of the tip in a scanning tunneling microscope?

We introduce a statistical correlation analysis method to obtain information on the local geometry and orientation of the tip used in scanning tunneling microscopy (STM) experiments based on large scale simulations. The key quantity is the relative brightness correlation of constant-current topographs between experimental and simulated data. This correlation can be analyzed statistically for a large number of modeled tip orientations and geometries. Assuming a stable tip during the STM scans and based on the correlation distribution, it is possible to determine the tip orientations that are most likely present in an STM experiment, and exclude other orientations. This is especially important for substrates such as highly oriented pyrolytic graphite (HOPG) since its STM contrast is strongly tip dependent, which makes interpretation and comparison of STM images very challenging. We illustrate the applicability of our method considering the HOPG surface in combination with tungsten tip models of two different apex geometries and 18144 different orientations. We calculate constant-current profiles along the $$ direction of the HOPG(0001) surface in the $|V|\le 1$ V bias voltage range, and compare them with experimental data. We find that a blunt tip model provides better correlation with the experiment for a wider range of tip orientations and bias voltages than a sharp tip model. Such a combination of experiments and large scale simulations opens up the way for obtaining more detailed information on the structure of the tip apex and more reliable interpretation of STM data in the view of local tip geometry effects.Comment: Progress in Surface Science, accepted for publication, 25 pages manuscript, 9 figures, abstract shortene