Zhao-Zhong Wang

Scanning tunneling microscopy in TTF-TCNQ: Phase and amplitude modulated charge density waves

Charge density waves (CDW) have been studied at the surface of a cleaved TTF-TCNQ single crystal using a low temperature scanning tunneling microscope (STM) under ultra high vacuum (UHV) conditions, between 300K and 33K with molecular resolution. All CDW phase transitions of TTF-TCNQ have been identifled. The measurement of the modulation wave vector along the a direction provides the flrst evidence for the existence of domains comprising single plane wave modulated structures in the temperature regime where the transverse wave vector of the CDW is temperature dependent, as hinted by the theory more than 20 years ago.

STM images of subsurface Mn atoms in GaAs: evidence of hybridization of surface and impurity states.

We show that scanning tunneling microscopy (STM) images of subsurface Mn atoms in GaAs are formed by hybridization of the impurity state with intrinsic surface states. They cannot be interpreted in terms of bulk-impurity wave-function imaging. Atomic-resolution images obtained using a low-temperature apparatus are compared with advanced, parameter-free tight-binding simulations accounting for both the buckled (110) surface and vacuum electronic properties. Splitting of the acceptor state due to buckling is shown to play a prominent role.

Atomic structure of tip apex for spin-polarized scanning tunneling microscopy

We present a high resolution transmission electron microscopy study of a Cr-coated W tip apex prepared for spin-polarized scanning tunneling microscopy (SP-STM). The characterization of the tip apex structure has been done with atomic resolution. We show that the Cr film is epitaxially grown on W and presents a monocrystalline phase. The surface analysis of the apex reveals roughness which gives rise to structures that can be considered as nanotips. In spite of the monocrystalline structure of these nanotips, we show that their spin arrangement and resulting magnetization direction cannot be controlled. SP-STM measurements on a Cr/MgO(001) sample confirm this conclusion.

Low temperature scanning tunneling microscopy wave-function imaging of InAs∕GaAs cleaved quantum dots with similar height

The authors report on cross-sectional scanning tunneling microscopy/scanning tunneling spectroscopy and subsequent electronic wave-function imaging at a low temperature (T=77K) on cleaved In(Ga)As∕GaAs quantum dots (QDs). The dI∕dV spectra exhibit a set of discrete and well-defined peaks in the QD conduction band. Differential conductance dI∕dV maps, obtained on a series of individual dots, display clearly the real space spatial variation at the nanometer scale of the envelope (amplitude) of the electron wave functions for the successive ground state and excited states. Wave-function symmetry of the first two excited states are identified in the STS measurements.

Surface charge density wave phase transition in NbSe3.

The two charge-density wave (CDW) transitions in NbSe3 were investigated by scanning tunneling microscopy (STM) on an in situ cleaved (b, c) plane. The temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, was measured between 5 and 140 K to extract the surface critical temperatures (T{s}). The low-T CDW transition occurs at T{2s}=70-75  K, more than 15 K above the bulk T{2b}=59  K while at exactly the same wave number. A plausible mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation. The regime of 2D fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of surface transition, expected for this incommensurate 2D CDW, by extracting the temperature dependence of the order parameter correlation functions.

Electronic structure of cleaved InAsP/InP(001) quantum dots measured by scanning tunneling spectroscopy

We investigate the structural and electronic properties of cleaved InAsP quantum dots grown by metal organic chemical vapor deposition on a (001) InP substrate by means of cross-sectional scanning tunneling microscopy and spectroscopy. We performed spatially and energetically resolved differential conductance measurements on several dots and thus mapped their electronic wave functions. Five distinct quantum dot energy levels are identified, all of them strongly confined inside the quantum dot. We further discuss the structural characteristics inferred from topographical images in the specific case of parallelogram-based InAsP/InP(001) quantum dots as a mean of investigating the size of a buried quantum dot.

The building up of terrace periodicity by MBE growth on (0 0 1) GaAs vicinal surfaces

Abstract The morphology of MBE grown GaAs vicinal surfaces is studied using ex situ AFM. Analysing the statistical distribution of terrace width, we succeed in explaining our measurements either by a thermodynamical equilibrium model taking into account the step by step interaction or by a 2D Monte Carlo simulation of the growth. In the latter case, an anisotropic Schwoebel barrier at the step edge has to be introduced in order to get a good agreement. We argue that growth is the efficient process to get a low disordered step array at the usual time scale and temperature range of MBE. In addition the effect of a GaAs AlAs buffer superlattice, or of pure AlAs layer, or of Si doping in GaAs is also discussed.

Analysis of scanning tunneling microscopy images of the charge-density-wave phase in quasi-one-dimensional Rb 0.3 Mo O 3

The experimental STM images for the CDW phase of the blue bronze RbMoO3 have been successfully explained on the basis of first-principles DFT calculations. Although the density of states near the Fermi level strongly concentrates in two of the three types of Mo atoms Mo-II and Mo-III, the STM measurement mostly probes the contribution of the uppermost O atoms of the surface, associated with the Mo-IO6 octahedra. In addition, it is found that the surface concentration of Rb atoms plays a key role in determining the surface nesting vector and hence the periodicity of the CDW modulation. Significant experimental inhomogeneities of the b* surface component of the wavevector of the modulation, probed by STM, are reported. The calculated changes in the surface nesting vector are consistent with the observed experimental inhomogeneities.

Low-temperature scanning tunneling spectroscopy study of two-dimensional electron systems confined in semiconductor heterostructures

Low-temperature scanning tunneling spectroscopy under ultra-high vacuum was employed to investigate the two-dimensional electron system at the epitaxial surface of Sidoped In0.53Ga0.47As/In0.52Al0.48As(111)A quantum-well structures. The electron density in the near-surface region of the quantum well could be controlled through modulation doping. Spectra of the electronic local density of states in the conduction band showed a clear step-like energy dependence that reveals the subband states. In spectra acquired at some areas of nanometer size, peaks were observed near subband minima, indicating the existence of bound states.

Organized growth of GaAsAlAs lateral superlattices on vicinal surfaces: where are the limits?

Abstract We report on three issues concerning the growth of GaAs AlAs lateral superlattices on vicinal surfaces to obtain quantum wires with negligible inter-wire tunnel coupling. We show that the ledge roughness increases with increasing terrace length and decreases for slower growth rates. The tilt angle of lateral superlattices is not the relevant parameter as long as electronic properties are concerned: one has to consider the product of the coverage error by the lateral superlattice thickness. The Ga flux stability is a critical parameter to be controlled. The use of dual-filament cells improves it by a factor two compared to single-filament ones.