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ABSTRACT: 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 energy resolution is about 100 μeV.
Review of Scientific Instruments 10/2004; 75(11):4871-4879. · 1.37 Impact Factor
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ABSTRACT: The local density of states (DOS) of n-InAs(110) is measured in the extreme quantum limit by low-temperature scanning tunneling microscopy. Based on the general trends published previously [Phys. Rev. Lett. 86, 1582 (2001)] we analyze the data in more detail. First the influence of the tip induced quantum dot is studied. It turns out that the observed serpentine structures are not correlated with the properties of the quantum dot, which lead us to the conclusion that they are caused by the three-dimensional (3D) DOS of the semiconductor. The serpentine structures move with increasing energy similar to equipotential lines of a two-dimensional potential landscape. With increasing magnetic field additional serpentines appear until a complete network penetrates the visible area. Moreover, an uncorrugated DOS coexists with the serpentine structures up to the highest magnetic field of 6 T. The results lead us to the conclusion that an increasing part of the DOS is localized in the direction parallel to the magnetic field, effectively acting as a two-dimensional DOS and thus exhibiting edgelike states. This implies that the steplike feature of the Hall resistance in the extreme quantum limit is a precursor of a quantized Hall step.
Phys. Rev. B. 10/2001; 64(20).
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ABSTRACT: The electronic structure of the narrow gap semiconductor InAs is investigated by scanning tunneling spectroscopy and magnetotransport measurements in the extreme quantum limit. The well-known oscillations of the Hall coefficient are reproduced and the last, most pronounced oscillation is shown to be correlated with the appearance of corrugations in the local density of states. While the increasing part of the Hall constant corresponds to the existence of isolated patterns indicating magnetic field induced localization, the decreasing part correlates with the development of a network which most likely consists of one-dimensional channels. We conclude that the decrease of the Hall constant in the extreme quantum limit is caused by a transition from a purely three-dimensional to a partly one-dimensional transport regime.
Physical Review Letters 03/2001; 86(8):1582-5. · 7.37 Impact Factor
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ABSTRACT: The magnetic field induced oscillations in dI/dV curves recorded with a low-temperature scanning tunneling microscope on n-InAs(110) are analyzed in detail. It is found that the previous interpretation of the oscillations as due to the Landau quantization of the bulk conduction band of InAs has to be reconsidered. While the distance between the maxima of the oscillation corresponds to the effective mass of the InAs conduction band, the energetic positions of the maxima depend on the individual tip and can only be understood if the tip induced quantum dot is taken into account. A comparison of measured quantities (spatial fluctuations of the Landau level energies and spin splittings) with Hartree-Fock calculations of the tip induced quantum dot reveals quantitative correspondence. From this comparison, we conclude that the tunneling experiment detects the (m=0) states of different Landau and spin levels of the quantum dot, which are only marginally influenced by their resonant coupling to the bulk conduction band.
Phys. Rev. B. 09/2000; 62(11).
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ABSTRACT: We present the design of a new ultrahigh vacuum scanning tunneling microscope (STM) which operates at T<20 K inside the bore of a 2.5 T superconducting split-coil magnet. The tip/sample region can easily be controlled visually, thus allowing safe and fast exchange of samples and tips while the microscope stays at low temperatures. A newly developed rotary motion stepper motor is presented which allows rotation of the sample by >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 instrument is illustrated on Au(111)/mica, on Tb(0001)/W(110), and on Gd(0001)/W(110). © 2000 American Institute of Physics.
Review of Scientific Instruments 01/2000; 71(2):424-430. · 1.37 Impact Factor
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ABSTRACT: Scanning tunneling spectroscopy performed at 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 . It is found that the 3DES at can be described by Bloch states weakly interacting with the potential disorder. The 2DES at 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.
Physica E Low-dimensional Systems and Nanostructures 16(1):121-128. · 1.53 Impact Factor
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ABSTRACT: Scanning tunneling spectroscopy is applied to a three-dimensional electron system in the extreme quantum limit (EQL). A parabolic pseudogap of a width of ±2.5meV is found at EF and interpreted as the Coulomb gap predicted by Efros and co-workers [J. Phys. C 8, L49 (1975); Sov. Phys. Semicond. 14, 487 (1980)] for localized particles. The fact that the depth of the gap is markedly above zero and fluctuates in space is interpreted as evidence for partial localization supporting our previous assumption that a distinct electron phase exists in the magnetic-field range between the transition to the EQL and the magnetic freeze-out.
Phys. Rev. B. 66(12).
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ABSTRACT: The coverage dependence of the Fe-induced Fermi-level shift on p- and n-InAs(110) was investigated by angle-resolved photoelectron spectroscopy at 300 K. The Fermi-level position was found to be coverage dependent, exhibiting a maximum at 300 meV above the conduction-band minimum. The coverage dependence is explained by the surface doping model, if inhomogeneities in the Fe-adatom distribution and the resulting ionization probabilities are taken into account. The Fe-adatom distribution is determined by scanning tunneling microscopy. Photoemission spectra provided direct evidence of a two-dimensional electron gas at the Fe-covered surface.
Phys. Rev. B. 61(20).
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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 dI/dU-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 dI/dU-curves exhibit peaks attributed to the Landau quantization and the spin splitting of the quantum dot.
Journal of Electron Spectroscopy and Related Phenomena.
