D. Haude

A 300 mK ultra-high vacuum scanning tunneling microscope for spin-resolved spectroscopy at high energy resolution

We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the 3He pot of an entirely UHV compatible 3He refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base temperature of 315 mK with a hold time of 30 h without any recondensing or refilling of cryogenics is achieved. The STM can be moved from the cryostat into a lower UHV-chamber system where STM-tips and -samples can be exchanged without breaking UHV. The chambers contain standard surface science tools for preparation and characterization of tips and samples in particular for spin-resolved scanning tunneling spectroscopy (STS). Test measurements using either superconducting tips or samples show that the system is adequate for performing STS with both high spatial and high energy resolution. The vertical stability of the tunnel junction is shown to be 5 pmpp and the ene...

A low-temperature ultrahigh vacuum scanning tunneling microscope with a split-coil magnet and a rotary motion stepper motor for high spatial resolution studies of surface magnetism

We present the design of a new ultrahigh vacuum scanning tunneling microscope (STM) which operates at T 270° about an axis perpendicular to the tip axis. This feature allows metal or molecular beam evaporation normal to the sample surface. Even more important, by means of this device tip and sample can be brought into a parallel or antiparallel magnetic configuration thus opening a novel approach to the study of magnetic phenomena on an atomic length scale. In addition, measurements of the magneto-optical Kerr effect can be carried out without removing the sample from the STM. Also a new tip exchange mechanism is described. The microscopic and spectroscopic performance of the new ins...

Low temperature scanning tunneling spectroscopy on InAs(110)

Abstract We review our recent work on low temperature scanning tunneling spectroscopy (STS) in magnetic field on InAs(110). First, we describe the influence of the tip on the sample. It results in band bending at the InAs-surface, more precisely in a so called tip induced quantum dot. STS of the quantum dot states is used to reconstruct the quantum dot potential, a major requirement for all further measurements. Second, we analyze the appearance of ionized dopants in constant current images within a simple model based on the local band bending approach. Third, we show scattering states of ionized dopants at different energies appearing in normalized d I /d U -images. Comparison with calculated scattering states in the Wentzel–Kramers–Brillouin (WKB)-approximation gives good correspondance and a good estimate of the depth of individual dopants beneath the surface. Finally, we discuss the energy quantization of the unoccupied states of the tip induced quantum dot in magnetic field. The corresponding d I /d U -curves exhibit peaks attributed to the Landau quantization and the spin splitting of the quantum dot.

Growth of thin Mn films on W(110) studied by means of in-situ scanning tunnelling microscopy

The growth of Mn films on W(110) substrates has been investigated at room temperature by means of in-situ scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED). Our results show that highly strained bcc δ-Mn can be stabilized on W(110) by pseudomorphic growth up to a local thickness of three atomic layers. Sequences of STM images measured at the same location of the sample reveal that islands of the first and second monolayer grow anisotropically along the [001] direction of the substrate. The nucleation of roughly rectangular islands with a local coverage of five monolayers starts when the total amount of deposited material exceeds 1.0 ML. Local coverages of four atomic layers do not occur. The onset of nucleation of fifth monolayer patches coincides with an increased Mn density, indicating the onset of strain relaxation. The growth of Mn/W(110) is compared to that of Fe on the same substrate. The evolution of the filling factor of different layers is fitted by a simple model.

Visualizing the Influence of Interactions on the Nanoscale: Simple Electron Systems

Scanning tunneling spectroscopy (STS) is used to investigate the local density of states of the paradigmatic electron system located in the quasi‐parabolic conduction band of InAs. The n‐InAs(110) surface is used to investigate the system in all dimensionalities (0D‐3D) at low temperature T = 6 K and in magnetic fields up to B = 6 T. Depending on the dimensionality and the B‐field, we identify different types of electron phases. In particular, we find Bloch‐like wave functions scattered at dopants in 3D at B = 0 T, more complex wave functions reminiscent of weak localization in 2D at B = 0 T and drift states in 2D and 3D at B = 6 T. In 1D, we find surprisingly that the results are explained by a single‐particle calculation, although the energy scales are dominated by electron‐electron interaction.

Comparing the local density of states of three- and two-dimensional electron systems by low-temperature scanning tunneling spectroscopy

Abstract Scanning tunneling spectroscopy performed at T=6 K is used to investigate the local density of states (LDOS) of electron systems in the bulk conduction band of InAs. In particular, the 3DES of the n-doped material and an adsorbate-induced 2DES located at the surface are investigated at B =0 and 6 T . It is found that the 3DES at B=0 T can be described by Bloch states weakly interacting with the potential disorder. The 2DES at B=0 T exhibits much stronger LDOS corrugations revealing the tendency of weak localization. In a magnetic field both systems show drift states, which are expected in 2D, but are surprising in 3D, where they point to a new electron phase consisting of droplets of quasi 2D-systems.