Electron spin secluded inside a bottom-up assembled standing metal-molecule nanostructure

Artificial nanostructures, fabricated by placing building blocks such as atoms or molecules in well-defined positions, are a powerful platform in which quantum effects can be studied and exploited on the atomic scale. Here, we report a strategy to significantly reduce the electron-electron coupling between a large planar aromatic molecule and the underlying metallic substrate. To this end, we use the manipulation capabilities of a scanning tunneling microscope (STM) and lift the molecule into a metastable upright geometry on a pedestal of two metal atoms. Measurements at millikelvin temperatures and magnetic fields reveal that the bottom-up assembled standing metal-molecule nanostructure has an $S = \frac{1}{2}$ spin which is screened by the substrate electrons, resulting in a Kondo temperature of only $291 \pm 13$ mK. We extract the Land\'e $g$-factor of the molecule and the exchange coupling $J\rho$ to the substrate by modeling the differential conductance spectra using a third-order perturbation theory in the weak coupling and high-field regimes. Furthermore, we show that the interaction between the STM tip and the molecule can tune the exchange coupling to the substrate, which suggests that the bond between the standing metal-molecule nanostructure and the surface is mechanically soft

How cold is the junction of a millikelvin scanning tunnelling microscope?

We employ a scanning tunnelling microscope (STM) cooled to millikelvin temperatures by an adiabatic demagnetization refrigerator (ADR) to perform scanning tunnelling spectroscopy (STS) on an atomically clean surface of Al(100) in a superconducting state using normal-metal and superconducting STM tips. Varying the ADR temperatures between 30 mK and 1.2 K, we show that the temperature of the STM junction $T$ is decoupled from the temperature of the surrounding environment $T_{\mathrm{env}}$. Simulating the STS data with the $P(E)$ theory, we determine that $T_{\mathrm{env}} \approx 1.5$ K, while the fitting of the superconducting gap spectrum yields the lowest $T=77$ mK.Comment: 12 pages, 10 figure

A millikelvin scanning tunneling microscope in ultra-high vacuum with adiabatic demagnetization refrigeration

We present the design and performance of an ultra-high vacuum (UHV) scanning tunneling microscope (STM) that uses adiabatic demagnetization of electron magnetic moments for controlling its operating temperature in the range between 30 mK and 1 K with the accuracy of up to 7 $\mu$K. The time available for STM experiments at 50 mK is longer than 20 h, at 100 mK about 40 h. The single-shot adiabatic demagnetization refrigerator (ADR) can be regenerated automatically within 7 hours while keeping the STM temperature below 5 K. The whole setup is located in a vibrationally isolated, electromagnetically shielded laboratory with no mechanical pumping lines penetrating through its isolation walls. The 1K pot of the ADR cryostat can be operated silently for more than 20 days in a single-shot mode using a custom-built high-capacity cryopump. A high degree of vibrational decoupling together with the use of a specially-designed minimalistic STM head provides an outstanding mechanical stability, demonstrated by the tunneling current noise, STM imaging, and scanning tunneling spectroscopy measurements all performed on atomically clean Al(100) surface.Comment: 12 pages, 15 figure

Tailoring Molecular Magnetism

The invention of the modern computer in the 20th century has significantly changed our way of living and ushered in a new epoch of information technology – the Information age. When Konrad Zuse completed the first programmable, fully automatic and digital computer, the Z3, in Berlin in 1941 [1] it would have been impossible to imagine how computers would become part of our daily lives. Although the computational power of the first computers back then is actually comparable with modern pocket calculators, they were enormous and consumed a lot of power. For example the Z3, which was based on 2000 electromechanical relays, operated at a clock frequency of only 5 − 10 Hz and had a power consumption of 4000W [1]. The first electronic programmable computer, the ENIAC, was presented in the USA in 1946 and used vacuum tubes instead of electromechanical relays. It was one thousand times faster than the electromechanical computers at that time, but it also had a power consumption of 150 kW and needed a space of approximately 170m2 [2]. Nowadays personal computers have typically clock frequencies of about 2−3 GHz, fit easily in a backpack and have a power consumption of only several hundreds of Watts in spite of much larger computational power. These values impressively show the rapid development of the computer technology within the last decades. The corner stone for this rapid development was laid by the American physicists John Bardeen, Walter Brattain and William Shockley when they built the first transistor in 1947. The transistor rolled up the field of electronics and paved the way to smaller, more powerful, less power consuming and cheaper electronic devices. For their achievement they were awarded the Nobel Prize in Physics in 1956. The transistor found its way into computer design already a few years after its invention and replaced vacuum tubes. The first fully transistorized computer was built in the group of Kilburn at Manchester University in 1953 [3]. Ultimately, the invention of the integrated circuit (IC) by Jack Kilby in 1958 led to a breakthrough in the commercial and personal use of computers. The fabrication of ICs by photolithography allowed a huge number of tiny electronic circuits and components, e.g. transistors, to be embedded on a small plate. This offered the possibility of an easy and low cost mass production of personal computers [...

Patterning a hydrogen-bonded molecular monolayer with a hand-controlled scanning probe microscope

One of the paramount goals in nanotechnology is molecular-scale functional design, which includes arranging molecules into complex structures at will. The first steps towards this goal were made through the invention of the scanning probe microscope (SPM), which put single-atom and single-molecule manipulation into practice for the first time. Extending the controlled manipulation to larger molecules is expected to multiply the potential of engineered nanostructures. Here we report an enhancement of the SPM technique that makes the manipulation of large molecular adsorbates much more effective. By using a commercial motion tracking system, we couple the movements of an operator's hand to the sub-angstrom precise positioning of an SPM tip. Literally moving the tip by hand we write a nanoscale structure in a monolayer of large molecules, thereby showing that our method allows for the successful execution of complex manipulation protocols even when the potential energy surface that governs the interaction behaviour of the manipulated nanoscale object(s) is largely unknown

A standing molecule as a single-electron field emitter

Scanning probe microscopy makes it possible to image and spectroscopically characterize nanoscale objects, and to manipulate1,2,3 and excite4,5,6,7,8 them; even time-resolved experiments are now routinely achieved9,10. This combination of capabilities has enabled proof-of-principle demonstrations of nanoscale devices, including logic operations based on molecular cascades11, a single-atom transistor12, a single-atom magnetic memory cell13 and a kilobyte atomic memory14. However, a key challenge is fabricating device structures that can overcome their attraction to the underlying surface and thus protrude from the two-dimensional flatlands of the surface. Here we demonstrate the fabrication of such a structure: we use the tip of a scanning probe microscope to lift a large planar aromatic molecule (3,4,9,10-perylenetetracarboxylic-dianhydride) into an upright, standing geometry on a pedestal of two metal (silver) adatoms. This atypical and surprisingly stable upright orientation of the single molecule, which under all known circumstances adsorbs flat on metals15,16, enables the system to function as a coherent single-electron field emitter. We anticipate that other metastable adsorbate configurations might also be accessible, thereby opening up the third dimension for the design of functional nanostructures on surfaces

Electron spin secluded inside a bottom-up assembled standing metal-molecule nanostructure

Artificial nanostructures, fabricated by placing atoms or molecules as building blocks in well-defined positions, are a powerful platform in which quantum effects can be studied and exploited. In particular, they offer the opportunity to reduce the electronic interaction between large aromatic molecules and the underlying metallic substrate, if the manipulation capabilities of scanning tunneling microscopy to lift the molecule into an upright geometry on a pedestal of two metal atoms are used. Here, we report a strategy to study this interaction by investigating the Kondo effect. Measurements at millikelvin temperatures and in magnetic fields reveal that this bottom-up assembled standing metal-molecule nanostructure has an S=1/2 spin which is screened by substrate electrons, resulting in a Kondo temperature of only 291±13 mK. We extract its Landé g factor and its exchange coupling Jρ to the substrate, using a third-order perturbation theory in the weak-coupling and high-field regimes. We also show that the interaction between the scanning tunneling microscope tip and the molecule can tune the exchange coupling

Transfering spin into an extended π orbital of a large molecule

By means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), we have investigated the adsorption of single Au atoms on a PTCDA monolayer physisorbed on the Au(111) surface. A chemical reaction between the Au atom and the PTCDA molecule leads to the formation of a radical that has an unpaired electron in its highest occupied orbital. This orbital is a π orbital that extends over the whole Au-PTCDA complex. Because of the large Coulomb repulsion in this orbital, the unpaired electron generates a local moment when the molecule is adsorbed on the Au(111) surface. We demonstrate the formation of the radical and the existence of the local moment after adsorption by observing a zero-bias differential conductance peak that originates from the Kondo effect. By temperature dependent measurements of the zero-bias differential conductance, we determine the Kondo temperature to be TK=(38±8)K. For the theoretical description of the properties of the Au-PTCDA complex we use a hierarchy of methods, ranging from density functional theory (DFT) including a van der Waals correction to many-body perturbation theory (MBPT) and the numerical renormalization group (NRG) approach. Regarding the high-energy orbital spectrum, we obtain an excellent agreement with experiments by both spin-polarized DFT/MBPT and NRG. Moreover, the NRG provides an accurate description of the low-energy excitation spectrum of the spin degree of freedom, predicting a Kondo temperature very close to the experimental value. This is achieved by a detailed analysis of the universality of various definitions of TK and by taking into account the full energy dependence of the coupling function between the molecule-metal complex and the metallic substrate

Hand Controlled Manipulation of Single Molecules via a Scanning Probe Microscope with a 3D Virtual Reality Interface

Considering organic molecules as the functional building blocks of future nanoscale technology, the question of how to arrange and assemble such building blocks in a bottom-up approach is still open. The scanning probe microscope (SPM) could be a tool of choice; however, SPM-based manipulation was until recently limited to two dimensions (2D). Binding the SPM tip to a molecule at a well-defined position opens an opportunity of controlled manipulation in 3D space. Unfortunately, 3D manipulation is largely incompatible with the typical 2D-paradigm of viewing and generating SPM data on a computer. For intuitive and efficient manipulation we therefore couple a low-temperature non-contact atomic force/scanning tunneling microscope (LT NC-AFM/STM) to a motion capture system and fully immersive virtual reality goggles. This setup permits "hand controlled manipulation" (HCM), in which the SPM tip is moved according to the motion of the experimenter's hand, while the tip trajectories as well as the response of the SPM junction are visualized in 3D. HCM paves the way to the development of complex manipulation protocols, potentially leading to a better fundamental understanding of nanoscale interactions acting between molecules on surfaces. Here we describe the setup and the steps needed to achieve successful hand-controlled molecular manipulation within the virtual reality environment