Local density of states in zero-dimensional semiconductor structures

Scanning tunneling microscopic images and scanning tunneling spectra for coupled rectangular quantum corrals

Assuming that an electron confined by double δ-function barriers lies in a quasi-stationary state, we derived eigenstates and eigenenergies of the electron. Such an electron has a complex eigenenergy, and the imaginary part naturally leads to the lifetime of the electron associated with tunneling through barriers. We applied this point of view to the electron confined in a rectangular quantum corral (QC) on a noble metal surface, and obtained scanning tunneling microscopic images and a scanning tunneling spectrum consistent with experimental ones. We investigated the electron states confined in coupled QCs and obtained the coupled states constructed with bonding and anti-bonding states. Using those energy levels and wavefunctions we specified scanning tunneling microscope (STM) images and scanning tunneling spectra (STS) for the doubly and triply coupled QCs. In addition we pointed out the feature of resonant electron states associated with the same QCs at both ends of the triply coupled QCs.

Quasi-stationary states of an electron confined in a rectangular quantum corral and STM images

We elucidated quasi-stationary states of an electron confined in a two-dimensional quantum corral (QC) built up with double barriers having a finite width. It is shown that the Hamiltonian of the QC is not a Hermitian operator for an electron confined in the QC, which results in a complex eigenenergy and the inverse of the imaginary part yields the lifetime of the electron. Considering that the electron confined in a rectangular QC stays in a quasi-stationary state, we obtained expressions for STM images as an explicit function of both the two-dimensional position and the bias voltage. From those expressions, we clarified relations among the surface local density of states (LDOS), the probability density, topographical images and differential conductance (dI/dV) images. We pointed out the existence of a filtering function that connects the dI/dV image with the LDOS image for the first time. Our results reproduced all experimental STM images observed in topographical and dI/dV modes for a rectangular QC. Moreover, we specified the main components of electron states that characterize those STM images.

Conductance oscillations and charge waves in zigzag shaped quantum wires

Electron transport through a quantum wire (or coupled quantum dots) with time-dependent couplings between the nearest-neighbor and next-neighbor sites is studied by means of the evolution operator method and tight-binding Hamiltonian. Two geometries of a wire (linear and zigzag shaped) are considered in our calculations. Charge waves inside the wire and the conductance oscillation effect, i.e. the conductance as a function of the wire length, are analyzed. For a zigzag shaped wire with time-dependent couplings the conductance is characterized by a Fano-like resonance with many sideband peaks.

Construction of a versatile ultralow temperature scanning tunneling microscope

We constructed a dilution-refrigerator (DR)-based ultralow temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR-based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in an UHV environment to a STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 h. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance, and applications for low temperature physics.

Spatial imaging of two-dimensional electronic states in semiconductor quantum wells

We measure the spatial distribution of the local density of states (LDOS) at cleaved surfaces of InAs/GaSb isolated quantum wells and double quantum wells (DQWs) by low-temperature scanning tunneling spectroscopy. Distinct standing wave patterns of LDOS corresponding to subbands are observed. These LDOS patterns and subband energies agree remarkably well with simple calculations with tip-induced band bending. Furthermore, for the DQWs, coupling of electronic states between the quantum wells is also clearly observed.