Energy dissipation on magic angle twisted bilayer graphene.

Traditional Joule dissipation omnipresent in today's electronic devices is well understood while the energy loss of the strongly interacting electron systems remains largely unexplored. Twisted bilayer graphene (tBLG) is a host to interaction-driven correlated insulating phases, when the relative rotation is close to the magic angle (1.08∘). We report on low-temperature (5K) nanomechanical energy dissipation of tBLG measured by pendulum atomic force microscopy (p-AFM). The ultrasensitive cantilever tip acting as an oscillating gate over the quantum device shows dissipation peaks attributed to different fractional fillings of the flat energy bands. Local detection allows to determine the twist angle and spatially resolved dissipation images showed the existence of hundred-nanometer domains of different doping. Application of magnetic fields provoked strong oscillations of the dissipation signal at 3/4 band filling, identified in analogy to Aharonov-Bohm oscillations, a wavefunction interference present between domains of different doping and a signature of orbital ferromagnetism.

Suppression of electronic friction on Nb films in the superconducting state.

Investigations on the origins of friction are still scarce and controversial. In particular, the contributions of electronic and phononic excitations are poorly known. A direct way to distinguish between them is to work across the superconducting phase transition. Here, non-contact friction on a Nb film is studied across the critical temperature TC using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient Γ is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of Γ is found to be in good agreement with the Bardeen-Cooper-Schrieffer theory, meaning that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state. This is supported by the dependence of friction on the probe-sample distance d and on the bias voltage V. Γ is found to be proportional to d-1 and V2 in the metallic state, whereas Γ∼d-4 and Γ∼V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature.

Kelvin probe force microscopy for material characterization.

Kelvin probe force microscopy is a scanning probe method for imaging the surface potential by atomic force microscopy. The surface potential is one of the most important surface properties and is correlated to e.g. the work function, surface dipoles, localized surface charges and structural properties. It gives detailed information on the electrical properties and can be combined with optical and electrical excitation mechanisms providing additional properties like surface band bending and charge carrier mobilities. We will introduce the main concept and will briefly describe the major methods of operation. Based on the analysis of a Si superjunction device, structures dopant profiling and the concept of surface photovoltage measurements will be introduced. The influence of local charge accumulation on these devices will be presented and the effect on the measured contact potential values will be discussed.

Magnetic properties of nanomagnetic and biomagnetic systems analyzed using cantilever magnetometry.

Magnetic properties of nanomagnetic and biomagnetic systems are investigated using cantilever magnetometry. In the presence of a magnetic field, magnetic films or particles deposited at the free end of a cantilever give rise to a torque on the mechanical sensor, which leads to frequency shifts depending on the applied magnetic field. From the frequency response, the magnetic properties of a magnetic sample are obtained. The magnetic field dependences of paramagnetic and ferromagnetic thin films and particles are measured in a temperature range of 5-320 K at a pressure below 10(-6) mbar. We present magnetic properties of the ferromagnetic materials Fe, Co and Ni at room temperature and also for the rare earth elements Gd, Dy and Tb at various temperatures. In addition, the magnetic moments of magnetotactic bacteria are measured under vacuum conditions at room temperature. Cantilever magnetometry is a highly sensitive tool for characterizing systems with small magnetic moments. By reducing the cantilever dimensions the sensitivity can be increased by an order of magnitude.

Mechanical dissipation from charge and spin transitions in oxygen-deficient SrTiO3 surfaces.

Bodies in relative motion separated by a gap of a few nanometers can experience a tiny friction force. This non-contact dissipation can have various origins and can be successfully measured by a sensitive pendulum atomic force microscope tip oscillating laterally above the surface. Here, we report on the observation of dissipation peaks at selected voltage-dependent tip-surface distances for oxygen-deficient strontium titanate (SrTiO3) surface at low temperatures (T = 5 K). The observed dissipation peaks are attributed to tip-induced charge and spin state transitions in quantum-dot-like entities formed by single oxygen vacancies (and clusters thereof, possibly through a collective mechanism) at the SrTiO3 surface, which in view of technological and fundamental research relevance of the material opens important avenues for further studies and applications.

Large area scanning probe microscope in ultra-high vacuum demonstrated for electrostatic force measurements on high-voltage devices.

BACKGROUND: The resolution in electrostatic force microscopy (EFM), a descendant of atomic force microscopy (AFM), has reached nanometre dimensions, necessary to investigate integrated circuits in modern electronic devices. However, the characterization of conducting or semiconducting power devices with EFM methods requires an accurate and reliable technique from the nanometre up to the micrometre scale. For high force sensitivity it is indispensable to operate the microscope under high to ultra-high vacuum (UHV) conditions to suppress viscous damping of the sensor. Furthermore, UHV environment allows for the analysis of clean surfaces under controlled environmental conditions. Because of these requirements we built a large area scanning probe microscope operating under UHV conditions at room temperature allowing to perform various electrical measurements, such as Kelvin probe force microscopy, scanning capacitance force microscopy, scanning spreading resistance microscopy, and also electrostatic force microscopy at higher harmonics. The instrument incorporates beside a standard beam deflection detection system a closed loop scanner with a scan range of 100 µm in lateral and 25 µm in vertical direction as well as an additional fibre optics. This enables the illumination of the tip-sample interface for optically excited measurements such as local surface photo voltage detection. RESULTS: We present Kelvin probe force microscopy (KPFM) measurements before and after sputtering of a copper alloy with chromium grains used as electrical contact surface in ultra-high power switches. In addition, we discuss KPFM measurements on cross sections of cleaved silicon carbide structures: a calibration layer sample and a power rectifier. To demonstrate the benefit of surface photo voltage measurements, we analysed the contact potential difference of a silicon carbide p/n-junction under illumination.

Design and performance of a combined secondary ion mass spectrometry-scanning probe microscopy instrument for high sensitivity and high-resolution elemental three-dimensional analysis.

State-of-the-art secondary ion mass spectrometry (SIMS) instruments allow producing 3D chemical mappings with excellent sensitivity and spatial resolution. Several important artifacts however arise from the fact that SIMS 3D mapping does not take into account the surface topography of the sample. In order to correct these artifacts, we have integrated a specially developed scanning probe microscopy (SPM) system into a commercial Cameca NanoSIMS 50 instrument. This new SPM module, which was designed as a DN200CF flange-mounted bolt-on accessory, includes a new high-precision sample stage, a scanner with a range of 100 µm in x and y direction, and a dedicated SPM head which can be operated in the atomic force microscopy (AFM) and Kelvin probe force microscopy modes. Topographical information gained from AFM measurements taken before, during, and after SIMS analysis as well as the SIMS data are automatically compiled into an accurate 3D reconstruction using the software program "SARINA," which was developed for this first combined SIMS-SPM instrument. The achievable lateral resolutions are 6 nm in the SPM mode and 45 nm in the SIMS mode. Elemental 3D images obtained with our integrated SIMS-SPM instrument on Al/Cu and polystyrene/poly(methyl methacrylate) samples demonstrate the advantages of the combined SIMS-SPM approach.