摘要:

We report on the construction of an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) specially designed for operation in the entire range of sample temperatures between 10 and 400 K. The sample is cooled by means of a liquid helium continuous‐flow cryostat, while the supporting manipulator and the surrounding devices remain at room temperature. This allows rapid variation of the sample temperature. The standard instruments for surface preparation and analysis and the STM are contained in a single UHV chamber. By rotation of the manipulator the sample can be positioned in front of any of these instruments without changing the sample temperature. The performance of the microscope is demonstrated by two examples of images of xenon adsorbed on platinum(111) showing: (a) the evolution of the morphology of a submonolayer of xenon from adsorption at 17K up to desorption at about 90 K and (b) atomically resolved images of the hexagonal incommensurate rotated phase for xenon at monolayer completion.  

ISSN:0034-6748    

DOI:10.1063/1.1145219

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

To expand the capabilities of the microwave frequency alternating current scanning tunneling microscope to include the ability to study isolated adsorbates and highly reactive surfaces, we have developed a low temperature, ultrahigh vacuum alternating current scanning tunneling microscope. In this alternating current scanning tunneling microscope, we employ the reliable beetle‐style sample approach mechanism with a number of other components unique to a low temperature scanning tunneling microscope. These include the sample transfer, delivery, retrieval, storage, sputtering, and heating systems. This alternating current scanning tunneling microscope operates at 77 and 4 K.

ISSN:0034-6748    

DOI:10.1063/1.1144551

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We report on the development of a new two‐dimensional micropositioning device, or walker, which is capable of moving across very large distances (in principle unlimited) and with a very small step size (as small as 100 A˚/step) in both directions. Based on a unique tracking design, the motion is extremely orthogonal with very little crosstalk between the two directions. Additionally, there is no detectable backlash in either direction. The walker performance has been extensively tested by using a position‐sensitive proximitor probe. Tests have been done between 77 and 300 K. However, we project that the walker will be able to operate at temperatures as low as 4 K. This walker system has shown extremely reliable performance in a UHV environment for use with scanning tunneling microscopy and has been especially useful for cross‐sectional scanning tunneling microscopy and spectroscopy studies of semiconductor hetero‐ and homostructures. We show one example of results on the (AlGa)As/GaAs heterostructure system.

ISSN:0034-6748    

DOI:10.1063/1.1144552

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We present an analog ramp generator for use in fast scanning tunneling or scanning probe microscopes. The analog design achieves low noise with a relatively simple circuit layout. The circuit requires minimal computer overhead because theXandYramps are generated using just two clocks and one digital control line. The resultingXandYramps are automatically set equal in magnitude, yielding images which remain square despite changes in scan size, number of lines per image, and tip velocity. We have used the circuit atXramp frequencies above 10 kHz.  

ISSN:0034-6748    

DOI:10.1063/1.1144553

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We used a new concept of tip preparation for magnetic force microscopy (MFM) proposed recently based on coating electron beam deposited carbon needles with appropriate magnetic thin film materials. In combining the advantages of electron beam fabricated needles with those of already widely used thin film tips, high resolution MFM tips can be prepared routinely and reproducibly on all type of cantilevers. Due to the fabrication procedure, which is described in detail, the effective magnetic tip volume is formed by a homogeneous, magnetically isolated, high aspect ratio thin film element favoring a single domain tip behavior. To reinforce the inherent shape anisotropy an additional uniaxial anisotropy is induced along the tip axis by applying an external field during the deposition of the ferromagnetic alloy. Because of the parallel side walls and the rounded tip end, most of the stray field will emanate from the apex region, resulting in a high field density and only little influence on the magnetization within the sample of interest. By using a material with high saturation magnetization, the thickness of the magnetic layer can be drastically reduced, leading to an increased resolution. To demonstrate the potential of the new concept various MFM observations on different types of materials for magnetic storage technology have been performed without changing the presented basic preparation parameters.

ISSN:0034-6748    

DOI:10.1063/1.1144554

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new beamline for radiometric applications (e.g., detector calibration and reflectometry) has been installed and characterized at the laboratory of the Physikalisch‐Technische Bundesanstalt at the electron storage ring BESSY. The beamline is equipped with a SX700 type plane grating monochromator and a toroidal mirror behind the exit slit of the monochromator for collimating the radiation to allow angle resolved reflectometry. A divergence of 0.1 mrad has been achieved. The beamline has been optimized for high spectral purity of the radiation. Within the photon energy range from 40 to about 1500 eV the total amount of stray light and higher order radiation stays below 1%. A photon flux of up to 1011s−1has been measured in the focal point of the toroidal mirror. It corresponds to a radiant power of a few &mgr;W and thus allows use of a cryogenic radiometer for detector calibration. The details of the beamline layout and the results of the performance characterization are presented.  

ISSN:0034-6748    

DOI:10.1063/1.1144555

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A Bonse–Hart ultrasmall‐angle x‐ray scattering (USAXS) instrument, employing a synchrotron x‐ray source at an energy of 18.86 keV (wavelength &lgr;=0.06573 nm), has been designed, constructed, and tested. Solid as well as suspensions of polystyrene and poly(chlorostyrene) latex spheres were used as reference standards in order to demonstrate the range of this instrument. The USAXS results measured at &lgr;=0.06573 nm were also compared with those from a calibrated Bonse‐Hart instrument operating at &lgr;=0.154 nm, i.e., the standard CuK&agr;radiation. The higher‐energy synchrotron x rays permit us to study absorbing and/or opaque materials having long range inhomogeneities in the order of microns. Furthermore, for many polymer systems, the optimal sample thickness at &lgr;&bartil;0.066 nm is increased to ∼10 mm and thin‐walled glass windows become acceptable. Thus, a cylindrical light‐scattering cell with a wall thickness of 0.3 mm could be used for both USAXS and laser light scattering measurements. The main hurdle for performing simultaneous static and dynamic light scattering as well as USAXS experiments on polymer solutions and gels with the same sample has been resolved. This capability should open up new frontiers in structural and dynamical studies of systems involving a large range of length scales from angstroms to microns.

ISSN:0034-6748    

DOI:10.1063/1.1144556

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A high‐resolution spectrometer has been developed and used to measure simultaneously impurity ion temperatures and flow velocities in high temperature plasmas with 10 &mgr;s temporal resolution (limited by digitization rate). This device is actually a duo‐spectrometer: measurements from two different chordal views of the plasma can be made simultaneously via two separate quartz input fiber optic bundles coupled to the entrance slits which are tilted to compensate for line curvature. The dispersed spectra on the exit plane of the duo‐spectrometer are coupled via quartz fiber optics to two arrays of 16 photomultiplier tubes each. Measurements made by recording the Doppler broadened and shifted 227.091 nm emission from the C vimpurity ions in the Madison Symmetric Torus (MST) reversed‐field pinch (RFP) plasma have achieved precisions of <6 eV for temperatures of 150 eV and <0.7 km/s for flow velocities of 6 km/s. Representative results from the MST RFP indicate that the toroidal flow velocity drops and ion temperature increase during sawtooth events in MST.

ISSN:0034-6748    

DOI:10.1063/1.1144557

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Initial testing of a new, commercially available, thermoluminescent dosimetry system is reported. The radiation detectors consist of two‐dimensional arrays of 104CaF2:Mn dosimeters, deposited on flexible polyimide sheets. The spatial resolution is 3 mm, and the combined thickness of the dosimeters and the substrate is 185 &mgr;m. Exposed sheets are processed by laser heating. This system is compatible with intense, pulsed Bremsstrahlung fields and electronics hardness testing. A two‐step calibration procedure is described. In spatial uniformity tests at 66 Gy the detectors exhibited random fluctuations of 1%–3% superposed on broad spatial drifts of ∼±5%. Reproducibility for strips of 288 elements, cut from sheets and given absorbed doses of 100 Gy, was ∼2%. For absorbed doses in the 10 Gy–1.5 kGy range, response vs exposure curves show reasonable features of linearity, supralinearity, and saturation for CaF2:Mn phosphor. Small systematic effects (<7%) can be observed for sheets and strips stacked during exposure; these effects are explained by radiation transport calculations and impact the dosimetric technique.

ISSN:0034-6748    

DOI:10.1063/1.1144558

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Experimental measurements are made with a novel two wire thermocouple. Signals from two wires of unequal diameters are recorded from the thermocouple suspended in constant flow with a periodic temperature fluctuation. It is demonstrated that the reconstructed signal from the two wire thermocouple requires no compensation for &ohgr;≤2&ohgr;1where &ohgr;1is the natural frequency of the smaller wire. A compensation factor is recommended for larger frequencies &ohgr;≳2&ohgr;1.  

ISSN:0034-6748    

DOI:10.1063/1.1144559

出版商:AIP    

年代:1994

数据来源: AIP