K.-W. Ng

Compact coarse approach mechanism for scanning tunneling microscope

We report on the design and fabrication of a coarse approach mechanism, called a piezotube walker, based on a piezoelectric tube moving inside a triangular prism shaped cavity. This walker walks like a six legged insect moving its legs one by one and then the belly following. The walker works in any orientation from horizontal to vertical and its motion is found to be linear with the applied voltage above a threshold voltage. A compact scanning tunneling microscope (STM) was fabricated using this approach mechanism. The scanner tube of the STM is mounted on the inside of the walker tube, reducing the size of the STM considerably. Topographical images with atomic resolution were obtained for layered materials like graphite and NbSe2.

Characterization of a complementary metal‐oxide semiconductor operational amplifier from 300 to 4.2 K

We report the first operation of a commercially available complementary metal‐oxide semiconductor operational amplifier, at liquid helium temperature. In addition, we have characterized several factors important to the practical application of such a circuit from room temperature down to 4.2 K. The temperature dependence and measurement techniques for open‐loop gain, input offset voltage, input referred noise voltage, and quiescent current are presented. We will discuss our observations of low temperature behavior of the opamp with respect to others’ previous results. This work represents an advancement over earlier studies which only reported opamp operation down to 77 or 30 K with measurements taken only at a limited number of temperatures instead of a broad range. Our data suggest that under special operating conditions the opamps can be effectively used with careful consideration of noise and gain performance. Input offset voltage levels and quiescent current (including power consumption) resemble nor...

Effects of lead doping on Tc and energy gap of Bi2Sr2CaCu2O8 by tunneling spectroscopy

Abstract Doping the high- T c superconductor Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) with lead causes a partial substitution of the Bi atoms between the conducting CuO 2 planes, and modifies some of the electronic and structural properties of the material. We have studied Pb-doped Bi-2212 single crystals using a tunneling technique. We find that while Pb doping decreases T c significantly, it has a minimal effect on the in-plane energy gap, leading to an increase in the reduced gap, 2 Δ / k T c . This result is consistent with other studies of the effects of in-plane dopants on Bi-2212, but is the first evidence of such an effect attributable to defects introduced out of the plane. Inspection of the physical evidence leads to the conclusion that the inter-layer coupling has been modified and indicates a connection between high- T c superconductivity and the inter-layer coupling between the CuO 2 planes.

Non-conservation of density of states in Bi2Sr2CaCu2Oy: Coexistence of pseudogap and superconducting gap

The tunneling spectra obtained within the (a,b)-plane of Bi2Sr2CaCu2Oy (Bi2212) for temperatures below and above the critical temperature (Tc) are analyzed. We find that the tunneling conductance spectra for the underdoped compound in the superconducting state do not follow the conservation of states rule. There is a consistent loss of states for the underdoped Bi2212 implying an underlying depression in the density of states (DOS) and hence the pseudogap near the Fermi energy (EF). Such an underlying depression can also explain the peak-dip-hump structure observed in the spectra. Furthermore, the conservation of states is recovered and the dip-hump structure disappears after normalizing the low-temperature spectra with that of the normal state. We argue that this is a direct evidence for the coexistence of a pseudogap with the superconducting gap.

Operation of an opamp at liquid helium temperature.

Abstract The stray capacitance between long wires in a cryogenics systems will slow down measurement rate, and also introduce unnecessary noise pick up. It is necessary to install the preamplifier as close to the signal source as possible to diminish the capacitive coupling effects. The most commonly used semiconducting device for this purpose is the MOSFET, which can function at liquid helium temperatures. Under special operation procedures, an all MOSFET operational amplifier can also be operated at liquid helium temperature. The use of opamp will simplify the construction of more complicated circuitry for low temperature applications.

OBSERVATION OF PSEUDOGAP IN UNDERDOPED BI2SR2CACU2O8+DELTA BY TUNNELING SPECTROSCOPY

We have performed tunneling measurements on underdoped and optimally doped single crystals of Bi2Sr2CaCu2O8+δ(Bi2212). The tunneling is in the ab-plane, in the maximum gap direction for all of our experiments. Measurement temperature ranges from 4.2 K to well above 100 K. The superconducting gap has been measured for T < Tc. 2Δ/kTc for underdoped Bi2212 (~ 13.4) is significantly larger than that of optimally doped Bi2212 (~ 8.8). As the temperature is raised above Tc, the gap vanishes for the optimally doped sample. In contrary, the gap like feature for the underdoped Bi2212 persists with larger gap value at higher temperatures. There are interesting differences in the nature of superconducting gap and pseudogap.

A piezoelectric-driven micropositioner with magnetic locking mechanism

A micropositioner has been constructed with a piezoelectric tube and walking pads locked by magnets. These magnets were arranged in a special geometry to prevent slipping and trapping. The magnetic force created was large enough to operate the walker in any orientation. Tolerance of surface roughness was much higher than other similar types of micropositioners.

Scanning tunneling microscope with two-dimensional translator

Since the invention of the scanning tunneling microscope (STM), it has been a powerful tool for probing the electronic properties of materials. Typically STM designs capable of obtaining resolution on the atomic scale are limited to a small area which can be probed. We have built an STM capable of coarse motion in two dimensions, the z- and x-directions which are, respectively, parallel and perpendicular to the tip. This allows us to image samples with very high resolution at sites separated by macroscopic distances. This device is a single unit with a compact design making it very stable. It can operate in either a horizontal or vertical configuration and at cryogenic temperatures.

a-b plane tunneling spectroscopy and in-plane gap anisotropy in Bi2Sr2CaCu2O8

Abstract The quality of the tunneling spectroscopy of Bi 2 Sr 2 CaCu 2 O 8 can be improved significantly by tunneling into the edge of a single crystal, as opposed to directly into a cleaved face. This junction geometry allows tunneling exclusively in the a-b plane, and we will discuss some features that can be reproduced consistently with this technique. This experimental approach also provides us a unique opportunity to observe the gap anisotropy within the a-b plane. We find that the energy gap (Δ ab ) varies from 20 meV to 40 meV, with the maximum value along the orthorhombic crystal axes and the minimum value along the CuO bond direction. No nodal line has been observed, possibly due to the limitation in angular resolution of the tunneling technique.

STM of quasiparticle scattering resonances in Bi2Sr2CaCu2O8+δ

High-resolution, low-temperature scanning tunneling microscopy and spectroscopy on Bi 2 Sr 2 CaCu 2 O 8‘d reveal the existence of large numbers of scattering centers in this material. The spatial and spectroscopic characteristics of these features are consistent with theories of quasiparticle scattering from atomic scale impurities in a d-wave superconductor. These characteristics include breaking of local particle}hole symmetry and an inverse square dependence of their local density-of-states (LDOS) on distance from the scattering center. Furthermore, these observations identify a source for the anomalously high levels of low-energy excitations in Bi 2 Sr 2 CaCu 2 O 8‘d at low-temperatures. ( 2000 Elsevier Science B.V. All rights reserved.