Session A27: Focus Session: Advances in Scanned Probe Microscopy I: Low Temperatures

Development of an Ultra Low Temperature Scanning Tunneling Microscope

In this talk we give an update on the next generation of ultra low temperature, high magnetic field (15T) scanning tunneling microscope (STM). With this system, we plan to extend the capability of STM to include higher energy resolution ($\sim $1$\mu $eV) for scanning tunneling spectroscopy (STS) with operation at 20 mK. To realize this energy resolution in STS, we constructed an ultra high vacuum dilution refrigerator (DR) for STM applications. It operates with two independent modes of He3-He4 mixture gas condensation: a traditional 1K pot condenser, or a Joule-Thomson condenser for possible lower noise operation. This eliminates potential vibration problems during operation of the DR. To match the very low limit of thermal noise in this system, our new system includes extensive vibration isolation and RF shielding. Our STM sample holder has five isolated electrical contacts. This allows four-probe macroscopic electrical measurements to be performed simultaneously with microscopic STM measurements. The current progress and performance of this new system will be discussed.   

Design and construction of a millikelvin scanning tunneling microscopy system

We are developing a scanning tunneling microscopy and spectroscopy system for work at millikelvin temperatures, intended for studies of superconductor, semiconductor, and other materials and systems of interest to quantum computing research. Our approach incorporates recent advances in this field as well as original insights and innovations to help achieve low noise and low effective operating temperatures.   

Direct evidence of the surface state contribution to the Kondo resonance

We performed low temperature scanning tunneling microscopy/spectroscopy on the isolated single 5, 10, 15, 20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecules adsorbed on the Si(111)- $\sqrt 3 \times \sqrt 3 $ Ag substrate. On this substrate, all the TBrPP-Co molecules show a square shape, indicating a planar conformation with a spin-active Co atom caged at its center. As the substrate supports a two-dimensional surface state and does not have bulk state near the Fermi level, the observed Fano-shaped peak near the Fermi level taken above the single molecule is a direct evidence of the contribution of the surface state electrons to the Kondo resonance. The long decay length ($\sim $ 1.4 nm) of the resonance also support for the surface state contribution. [1] Q. Li, S. Yamazaki, T. Eguchi, Y. Hasegawa, H. Kim, S.-J. Kahng, J. F. Jia, and Q. K. Xue, Nanotechnology 19, 465707 (2008).   

Kondo Effect in a Co-Porphyrin on Au(111) probed by Scanning Tunneling Spectroscopy

Kondo effect is a core topic in condensed matter physics, exhibiting a localized state raised by the interaction between a single magnetic impurity and Fermi electrons in metals. We have studied Kondo effect in a Co-porphyrin on Au(111) using low-temperature scanning tunneling spectroscopy. A localized state is observed at Fermi level from the spectra measured above the Co atom. The spectra were fitted by Fano line shape, revealing the Kondo temperature of the system $\sim $ 400K. By taking spectra at points along some symmetry directions, decaying behavior of the Kondo effect could be analyzed. With the help of simulated d-electron orbital, the observed decaying behavior is accounted for. Our study implies that lattice reconstruction in a system can induce d-electron orbital distortion, resulting in magnetic asymmetries.   

Imaging the Quantum Berry Phase

Geometric phase operations are attractive to quantum information technology because they are time-independent and relatively insensitive to topological perturbations. However, in most coherent devices where these operations could be performed, electron wave functions are inaccessible to local probes. Here, we demonstrate Berry phase rotations on two-dimensional electron wave functions by using atomic manipulation to adiabatically alter their confinement potential. By consecutively changing the boundary of a quantum corral, we traverse a closed circuit in deformation space that engenders a net $\pi$ phase shift in two electron eigenstates. With scanning tunneling microscopy, we trace both the energetic and spatial evolution of these states and directly track their accrual of geometric phase, revealing information that would be obscured in other two-dimensional electron devices. This enables the determination of the two-point transconductance through the device, thus making contact to other nanostructures such as semiconductor quantum dots, where this promising technique for phase control can be implemented using only voltages controlling appropriately patterned gates.   

Evolution of Single-Molecule Vibrational Modes from Tunneling to Quantum Point Contact

A detailed understanding of how molecular junctions form and evolve is vital for emerging fields such as molecular electronics. We present high-precision scanning tunneling microscopy studies tracing the evolution of molecular junctions from the tunneling regime to quantum point contact. We employ a model system of CO molecules on Cu(111) and are able to extend inelastic spectroscopy into the point contact regime, thus following the energy shifts of specific vibrational modes as the molecular contact is formed. We observe surprising non- monotonic shifts, confirmed by simultaneous noise measurements traceable to molecular motion. In point contact, we also observe a novel ``nucleonic gating'' effect in which the carbon nucleus controls a measurable dc molecular conductance shift. This shows that the electrical properties of molecular wires can be profoundly altered by their isotopic makeup. We extend these measurements to geometries where the three-dimensional approach vector of the tip relative to the target molecule is finely controlled, a technique not possible in break junction measurements.   

Low Temperature Scanning Tunneling Microscopy of High Temperature Superconductors: What We Gain By Taking a Closer Look

Scanning tunneling microscopy (STM) and spectroscopy have been applied to a wide variety of experimental systems. In this talk I will focus on one which was discovered at nearly the same time as STM -- high temperature superconductors. After two decades of intense research these materials still hold many mysteries, mainly due to the rich variety of states of matter that may coexist, cooperate, or compete with superconductivity. I will present the unique perspective that STM is capable of bringing to our study of these materials through atomic-scale temperature dependent mapping of the density of states. After describing widely observed spatial ``checkerboard'' patterns which we have found to have a distinct doping dependence suggestive of charge density wave order, I will demonstrate how local variations of this order can help us understand nanoscale inhomogeneity in these materials. Taken together, these results not only show the power of STM to untangle complex nanoscale phenomena but also suggest a new path towards understanding high temperature superconductivity.   

Vortex excitation in nano-sized Pb island structures using low temperature scanning tunneling microscopy

Vortex behaviors in nano-size superconductors have attracted a lot of attention since there are various novel phenomena due to the size and shape effects. Using scanning tunneling microscopy/spectroscopy (STM/S) at low temperature ($<$2 K) we have visualized vortex phases on atomically-flat nano-sized Pb islands formed on the Si(111)-7x7 substrate and measured the critical magnetic fields for vortex penetration and expulsion [1]. In this study we demonstrate the excitation of a vortex with additional pulsed tunneling current from an STM probe tip. We found that probability of the excitation depends on the amount of the tunneling current, the pulse duration and a tip position in the island. These dependences suggest that the formation of normal state region below the tip due to the excess tunneling current induces the vortex penetration. Experimental details and theoretical results will be explained in the presentation. [1] T. Nishio \textit{et al}., PRL \textbf{101}, 167001(2008).   

Spatial and temperature-evolved tunneling spectroscopic studies of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$(LCMO) films and LCMO/organic-semiconductor heterostructures with spin-polarized scanning tunneling microscopy (SP-STM)

We report studies of spatially resolved tunneling spectra (TS) of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO) (T$_{c}$ = 260K) epitaxial films and related heterostructures of tris(8-hydroxyquinoline) aluminum (Alq3)/(LCMO) using a UHV, variable temperature STM. At 77K with a Pt/Ir tip we observe sharp spatial transitions between two cluster types with disparate normalized conductance. The majority type region exhibits high conductance peaks at high bias (+/- 2V) and a low energy gap, consistent with band structure calculations. The minority type region reveals moderate conductance over the entire bias range, from -3V to +3V. In contrast, spin-polarized tunneling spectra taken with Cr-coated STM tips show a spatially varying low bias gap in all regions. Further experiments using SP-STM on LCMO under varying temperatures and applied magnetic fields and on Alq3/LCMO structures to study the spin transport length in Alq3 will be reported.   

Degeneracy lifting of zero energy (Majorana) modes in a chiral p-wave superconductor due to the tunneling between vortices.

We study lifting of the degeneracy of the zero energy (Majorana) modes in a chiral $p_x+ip_y$ superconductor caused by tunneling between states localized in two different vortex cores. Using Bogoliubov-de Gennes equations, we analytically calculate the energy splitting of the Majorana modes as a function of the distance between two vortices. Our result may have applications in testing Majorana state by tunneling spectroscopy and the realization of topological quantum computation in chiral p-wave superconductors.   

The role of magnetic anisotropy in the Kondo effect

The Kondo effect is a fascinating many-body phenomenon, the origin of which is often unclear. Using a Scanning Tunneling Microscope operating at 0.5~K, we study inelastic spin excitations on individual atoms bound atop a thin insulating Cu$_{2}$N layer. We find that, unlike previously studied Fe and Mn atoms, the spins of Co and Ti atoms are Kondo screened in this environment. By applying strong magnetic fields in various directions we are able to precisely analyze the magneto- crystalline anisotropy experienced by the spins, and consequently their orientations relative to the surface. We show that the anisotropy plays a major role in determining whether or not a spin becomes Kondo screened, and how the Kondo effect is influenced by a magnetic field.   

Tunneling through a single magnetic atom: spin-dependent elastic and inelastic processes

Recent low-temperature scanning tunneling microscopy and spectroscopy studies have used inelastic electron tunneling to probe the spin excitations of magnetic atoms, molecules, and bulk surfaces. Here we describe the mechanisms that drive these spin excitations using a combination of resonant two-step and three-step virtual processes, with the latter including a simple exchange coupling between the tunneling electron and the electrons that comprise the atomic spin. Our description predicts the existence of a sum rule that includes a previously unnoticed type of spin-dependent elastic scattering, and evidence of both are seen in the observed spectra. We discuss the key factors that determine the relative strength of the inelastic tunneling, providing insight on when such processes can be observed and potentially how they might be enhanced.   

Implementation of a cryogenic scanning microwave impedance microscope

We have implemented a near-field scanning microwave impedance microscope in a variable temperature (2-300K) cryostat equipped with 9T magnet. Reflected microwave signals at 1GHz from a shielded cantilever probe were detected using room-temperature electronics. During the tip-sample approach, a small oscillating voltage was applied to the z-piezo and the modulated microwave signals were monitored to locate the sample surface. The approaching curve toward bulk dielectric materials can be quantitatively simulated by finite-element analysis. We have obtained the first low-T and high-B microwave images on a patterned silicon wafer with ion-implanted stripes. The results show clear impedance contrast in both the capacitive and loss channels. In particular, high-loss regions were seen between the heavily doped areas and the insulating substrate, allowing us to visualize the local conductivity variation. With this novel instrument, we expect to study electronic inhomogeneity in complex materials and explore local properties during phase transitions.