David Goldhaber-Gordon, Ph.D. - US grants

An outstanding challenge in correlated electron systems is to build an experimental realization of an arbitrary interesting Hamiltonian. Materials physicists traditionally tailor complex materials to achieve desired electronic properties. The present project's approach is complementary: instead of developing new and exotic materials, work with materials whose properties are simple and well-established, such as 2-dimensional electron gases in GaAs-based semiconductor heterostructures. Then use nanolithography
to carve them into arbitrary structures of coupled electron droplets, designed to match a Hamiltonian of interest. Finally, voltages on local gate electrodes can be used to tune every important parameter. This project will engineer nanostructures to display novel many-body ground states, notably a two-channel Kondo system built from a pair of coupled electron droplets. Undergraduates will be substantively involved in every stage of this research, preparing them for graduate school or a broad range of industrial jobs. In addition, a new course will be created to educate students about the physics of electrons at the nanoscale.
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Materials physicists often design complex materials to achieve desired electronic properties. The present project's goal is similar, but its approach is complementary: instead of developing new and exotic materials,
work with materials whose properties are simple and well-established. Then use tools borrowed from the semiconductor industry to carve these simple materials into complex patterns which can confine electrons. Multiple ultrafine metal wires (twenty nanometers in diameter, or thousands of times narrower than a human hair) can then be used to shift the confined electrons around and hence to tune the electrical behavior of the system. Despite the simple starting materials, the novel geometries will result in never-before-seen electronic properties. Undergraduates will be substantively involved in every stage of this research, preparing them for graduate school or a broad range of industrial jobs. In addition, a new course will be created to educate students about the physics of electrons at the nanoscale.

The Center for Probing the Nanoscale (CPN) addresses five overriding goals:

1.To develop novel probes that dramatically improve our capability to observe, manipulate, and control nanoscale objects and phenomena.

2.To educate the next generation of scientists and engineers regarding the theory and practice of these probes.

3. To apply these novel probes to answer fundamental questions and to shed light on technologically relevant issues.

4. To disseminate knowledge and to transfer technology so that other research scientists and engineers can make use of the advances, and so that corporations can manufacture and market the new novel probes.

5. To inspire thousands of middle school students by training their teachers at a Summer Institute.

The CPN includes thirteen core faculty-level Investigators (including three IBM Investigators who also serve as Consulting Professors at Stanford), twenty-eight affiliated faculty members and dozens of students and postdocs in eight academic departments: Applied Physics, Art and Art History, Electrical Engineering, Chemistry, Materials Science and Engineering, Mechanical Engineering, and Physics.

Research at the Center is focused on five Theme Groups:

Plasmonic Scanning Tunneling Microscopy. The plasmonic scanning tunneling microscope will perform spatially localized combined electronic and optical imaging and spectroscopy. The pivotal goal is to combine the exquisite spatial resolution inherent in high-resolution scanning tunneling microscopy and spectroscopy with the detailed information on high-frequency excitations of materials yielded by optical channels.

Nanoscale Electrical Imaging. Lithographically-patterned structures such as transistors now commonly have electronic properties that vary on scales of nanometers to tens of nanometers. The goal is to develop and apply a suite of techniques to measure electronic properties such as dielectric constant, conductivity, and carrier density of materials at the 10 nm scale, with sensitivity to variations deep beneath a sample surface.

Individual Nanomagnet Characterization. The volume, shape, and structure of magnetic nanoparticles lead to a number of present and proposed applications in biology and medicine. This theme will develop and advance a variety of nanoprobes with the spatial resolution and magnetic sensitivity to detect and characterize individual nanomagnets for nanobiotechnology applications

Nanoscale Magnetic Resonance. Recent work sponsored by CPN has proven that magnetic resonance force microscopy (MRFM) can achieve three dimensional imaging with spatial resolution below 10 nm. Research in the second five years will focus on extending MRFM resolution to below 1 nm - or roughly an order of magnitude improvement over today's capability. If sub-nanometer can be achieved, then MRFM?s true 3D imaging capability, lack of radiation damage and elementally specific contrast could make it a powerful tool for structural biologists.

BioProbes: Nanoscale Cantilevers. The objective of the BioProbes theme is to measure the forces, mechanical stiffness, electrostatics, and sequence of biological processes on cell membranes. Our theme will adapt unique cantilever designs for aqueous solution, based on recently invented torsional and dual-cantilever AFM cantilevers that have extremely high force sensitivity together with kHz sampling frequencies, ideal for biological samples.

Nanoprobes are arguably the enabling technology for all of the scientists and engineers working to realize the vast potential of nanotechnology. CPN research and outreach activities combine to broadly impact science, technology and education. The programs span graduate education, K-12 teacher development, academic courses, ethics for scientists and public scientific literacy. CPN intellectual contributions are widely disseminated through publications, on-line resources, conferences, workshops, and institutes.

The following activities constitute the educational and outreach programs:

Graduate and Postdoctoral Fellowships. Research students learn nanotechnology and precision measurement techniques as well as the fundamentals of their field. CPN Fellows are strongly engaged in Center events and outreach. The interdisciplinary Center provides a broader and deeper education for students than traditional graduate study.

CPN Prize Fellowships Program. CPN Graduate Prize Fellowships are competitively awarded on the basis of advisor recommendations and student-written proposals, and provide graduate students with a full stipend to initiate a novel nanoprobe research project.

Summer Institute for Middle School Teachers (SIMST). Middle school teachers participate in an intensive week-long program of content lectures, hands-on activities and curriculum development exercises. Teachers apply their new knowledge to create nanoscience lessons that are shared among participants for use during the school year and will be compiled for broader dissemination through workshops and online education resources.

CPN Annual Nanoprobe Workshop. The Annual Workshop provides an excellent opportunity for academic and industrial scientists to exchange knowledge and ideas and to broaden the horizons of the students.
Industrial Affiliates Program. The CPN Industrial Affiliates Program builds partnerships between CPN researchers and companies that have a strategic interest in nanoprobe development.

Collaborations. Collaborations with local institutions greatly enhance the impact and diversity of programs such as the Summer Institute for Middle School Teachers. Partners such as SRI International and the Exploratorium contribute curriculum and teaching best-practices, while collaborations with the National Hispanic University and the Latino College Preparatory Academy (LCPA) allow the Center to engage a diverse group of under-served students and teachers.

Curriculum. Probing the Nanoscale (AP275) is a graduate survey of scanning probes that draws on the expertise of CPN Investigators. Lecture videos are publicly available online as a rich source of information that has been accessed by people throughout the world.

Workshop on Professional Ethics for Students in Science and Engineering. A Workshop on Professional Ethics for Graduate Students in Science and Engineering was over-enrolled and reviewed very positively by the participants. This workshop will be offered annually during the renewal period.

This project aims to synthesize metal-organic semiconducting molecule-metal structures with nanoscale metallic contacts pre-assembled or templated by DNAs, or directly connected to the molecule as chemisorbed gold nanoparticles/nanowires. The metallic contacts will form ohmic contacts to molecular devices for circuits from DC to microwave frequencies. Precisely fabricated, ultrasmall gaps are not needed since the overall hybrid structure will be much longer than the organic molecule of interest. At optical frequencies, the metallic contacts will form an electromagnetic cavity around the molecular device, enhancing optical fields to be utilized in single-molecule spectroscopic measurements. Self-assembly of these new nanoscale objects will be investigated both theoretically and experimentally. Electrical devices will be fabricated to study charge transport through single molecules. New electrical, optical and physical phenomenon may arise from these unique nanoscale structures. The open planar geometry formed in this work is expected to allow electrostatic modification of electronic states using a nearby strongly-coupled gate electrode, and will reduce fluorescence quenching by nearby metallic electrodes. Single-molecule based transistors and light-emitting diodes may be generated from the proposed structures. The methods developed will lay the groundwork for developing molecular electronic and optical devices and integrating them into complex circuits.
Intellectual Merit. Fundamental advances across disciplines are essential to the advancement of nanoscale devices and to understanding their behavior. Molecular synthesis, self-assembly, and charge transport are essential components for realizing nanoscale devices with organic molecules. A coordinated team attack on such issues can advance the state of single-molecule devices. This project will be carried out by a team of two chemists, one solid-state physicist, one spectroscopist, and one theorist together with collaborators from industry, national labs, and foreign universities with expertise in polymer synthesis, surface chemistry, biochemistry, DNA self-assembly, DNA metallization, spectroscopy, charge transport, fluid dynamics simulation, and device fabrication.
Broader Impacts. This project may result in a new approach to make electrical contacts to single molecules, which will allow study of charge transport through single molecules with different chemical functionalities and length as well as measurement of unique optical properties arising from a single molecule confined in a nanogap. The proposed work will not only answer fundamental questions of intramolecular charge transport mechanisms in molecules with length scale of 5-100 nm, it will also provide answers to technological questions of whether organic molecules have sufficient performance for nanoelectronics and whether the mobility of molecular devices will be dramatically increased by alignment of organic semiconducting molecules between electrodes. This project also utilizes methods to self- assemble DNA-polymer-DNA and nanoparticle-molecule-nanoparticle structures using electrophoresis and dielectrophoresis to allow electrical connections to be made to single organic semiconducting molecules. The PIs will work closely with existing NSF centers and the Stanford Office of Science Outreach to reach a broad population ranging from K-12, community college, undergraduate, and graduate students as well as to prepare teachers of tomorrow for new areas of science and technology. Two internship positions every year for minority and/or women community college students are integral to the project. One research position per year will also be provided to a middle school or high school teacher during the summer; PIs will continue to work with them to develop their education plans after their summer research. PIs will also reach out to the general public through a public website and participation in various community events. The graduate students and postdoc involved in the project will actively interact with each other and have the opportunity to interact with researchers from industry, national labs, and international collaborators. They will be well equipped with a combination of technical engineering skills, basic scientific understanding, and communication skills, and poised to contribute to nanoscience and nanotechnology.

Objective:
The goal of this research program is to design, fabricate and characterize novel probes for high-resolution scanning gate microscopy, a technique providing information on electron distribution and transport. Probes that can produce a highly-localized potential perturbation are needed to apply scanning gate microscopy to semiconductor nanostructures. The proposed probes consist of cantilevers with integrated coaxial tips to generate a confined dipolar electric field at the underlying sample. Deflection of the cantilever resulting from tip-surface interactions will be detected by modulation of a piezoresistor located at the root. Initially, the probe will be used to map the probability densities of electrons in quantum dots and other semiconductor nanostructures, which to-date is a highly-desirable but unrealized goal.

Intellectual Merit:
The intellectual merit includes fundamental understanding of electron organization in semiconductor nanostructures, explaining properties of these low-dimensional systems, and designing novel devices (spintronic, single-electron, etc.) exploiting these properties. Beyond scanning gate microscopy, applications include the investigation of biological specimens, failure analysis of integrated circuits, and the study of noise in semiconductor devices. The coaxial tip may also improve the resolution and contrast of scanning electrochemical microscopy and scanning near-field microscopy.

Broader Impacts:
The resulting knowledge will have broad social, economic, and educational impact. Improved understanding of cellular voltage-gated ion channels may lead to more effective drugs while greater insight into quantum dots may yield higher-performance replacements for transistors. Results will be disseminated through educational outreach programs. By addressing the broad problem of generating and detecting localized electromagnetic fields, the probes will influence research throughout the basic sciences and engineering.

****NON-TECHNICAL ABSTRACT****
Modern technology relies on the ability to understand the electronic properties of materials. Toward this end, physicists develop theories in which individual components, such as a site where electrons are bound, interact with each other. Fully testing the predictions of a theory can be difficult, because material parameters are not individually tunable. This award supports a project that follows an alternative approach. Well-understood semiconductor materials and advanced patterning techniques are used to build nanoscale sites or ?quantum dots? that trap electrons. The great advantage to this approach is that parameters of the theory, such as the ability for the trapped electron in the dot to interact with the reservoir of electrons in the material, can be controlled electronically. The project uses this control to drive a drastic change in the electrons? behavior. Usually, one thinks of electrons acting independently, as they do when flowing through a copper wire. In contrast, the quantum dot structure can be tuned so that the electrons behave as a collective entity. This change is analogous to the transition from water to ice, except that it is not caused by temperature but by quantum fluctuations. The effect of quantum fluctuations is much less understood than their classical counterpart, temperature fluctuations. The goal of this project is to observe this quantum phase transition and quantitatively test current theories that seek to explain these phenomena. This project will train students and post-docs in advanced physical theories, semiconductor processing skills, and precision measurement techniques that will prepare them for cutting-edge careers in academia or industry.

****TECHNICAL ABSTRACT****
A crucial challenge in the field of correlated electron physics is to find an experimental system that corresponds to an interacting many-body Hamiltonian and allows fine control over the parameters of the Hamiltonian. This award supports a project that will meet the challenge by using well-understood AlGaAs/GaAs heterostructures and advanced patterning techniques to fabricate gated nano-structures. In these structures, small droplets of electrons called quantum dots are isolated from the remaining electron reservoirs. The two Hamiltonians of interest are the two-channel Kondo and the two-impurity Kondo Hamiltonians. In the two-channel Kondo system two independent reservoirs of delocalized electrons compete to screen an electron spin bound to the localized site (a quantum dot). In the two-impurity Kondo Hamiltonian an electron reservoir and a spin on a localized site compete to screen another spin on a second localized site. The advantage of using gated quantum dots is that parameters such as inter-dot interactions and dot-reservoir tunneling rates can be precisely measured and electrostatically tuned. This project will utilize this control to drive a quantum phase transition between a Fermi liquid and a non-Fermi liquid state. The goal is to tune to the very sensitive quantum critical point and quantitatively test theoretical predictions of how the highly-correlated non-Fermi liquid state evolves into the more conventional Fermi liquid state under the influence of relevant perturbations such as magnetic field and exchange coupling. This project will provide valuable training to students and post-docs in advanced many-body theories, semiconductor processing skills, and precision measurement techniques that will prepare them for cutting-edge careers in academia or industry.

Non-Technical Abstract
Materials at the focus of modern physics research often feature competition among several ways electrons can organize. This is both a blessing and a curse, opening rich possibilities but also hindering basic understanding of these materials. What if one could build an experimental system which displays the key features believed to drive behavior of a material of interest, while discarding other effects which might obscure the fundamental mechanism?
Modern technology such as transistors and lasers depends on the fact that adding even a small amount of impurities to a material can drastically alter its properties. Another example of such sensitivity is the Kondo effect: a few parts per million of magnetic impurities added to a metal drastically change its low-temperature electrical behavior. Such an impurity creates a "cloud" of electrons around it, which shields the magnetic moment of the impurity. This project aims to nanofabricate a device to measure the spatial extent of this cloud, and to study the competition between impurity-cloud and impurity-impurity interactions when two such impurities are brought together. This will shed light on a class of materials known as heavy fermion metals, in which exactly this competition is suspected to be key, by realizing the competition in a simplified and highly-controllable system.
In addition to the research component, both the principal investigator and the students have a track record and plans for active involvement in outreach regarding nanoscience, including short programs for middle school teachers, and involving high school and college students in research.

Technical Abstract
This project aims to determine the spatial extent of the Kondo screening cloud and study the Kondo-RKKY competition using a carefully-engineered model system based on gate-defined quantum dots in a GaAs/AlGaAs heterostructure. While the Kondo effect has been seen and studied in many different systems, a clear measurement of its spatial extent is still lacking. In the proposed device, the competition between the inter-impurity interaction and the impurity-bath interaction, suspected to be key to the behavior of several heavy-fermion compounds, is gate-tunable. Realizing the effect in a much more controllable nanostructure may elucidate the rich behavior occurring in those materials, including a quantum phase transition which could play a key role.
As a complement to precision transport studies, a scanning gate experiment, where a metallic tip is scanned over the surface of a nanopatterned bilayer heterostructure, can perturb the Kondo cloud at a controlled location, providing a direct mapping of the spatial extent of a quantum many-body state.

Engineering electronic materials with atomic precision will enable construction of tailored quantum devices exhibiting unique electronic, magnetic and optical properties, with the potential to impact fields ranging from quantum information science to next-generation computing and clean energy technologies. Atomically precise vertical structures can be formed by stacking different atomically-thin materials with diverse characteristics. The properties of individual layers, and their emergent interactions, show great promise for fundamental science and future technologies. To date, this process has been explored at the artisanal scale and has generated a wealth of scientific discoveries. However, the scalable construction of such heterostructures has remained a challenge, with individual stacks often built up though painstakingly manual processes with significant interfacial contamination. This project overcomes these challenges and enables scalable production of atomically-resolved heterostructures by developing an automated robotic platform to assemble layer materials under ultra-high vacuum conditions, resulting in a tool for production of materials with unprecedented complexity and interfacial cleanliness. Once developed, this system will be housed adjacent to the Stanford Nanofabrication Facility, where it will serve as a shared tool for Stanford researchers across a diverse interdisciplinary community. Additionally, this project will enhance undergraduate and graduate education through advanced coursework and research opportunities.

This project supports development of a novel instrument to automate the fabrication of designer quantum materials in the form of layered van der Waals (vdW) heterostructures. Existing processes which exclusively utilize exfoliated materials are limited by the stochastic nature of the samples, resulting in low throughput and geometric constraints on the resulting heterostructure. In contrast, this project utilizes both exfoliated and chemical vapor deposition (CVD)/molecular beam epitaxy (MBE)-grown source materials for enhanced throughput and improved sample size. The instrument employs interconnected ultra-high vacuum (UHV) chambers to maintain atomically pristine surfaces, and uses precision nanopositioning stages, microstructured adhesive effectors, an optical microscope, and computer vision algorithms to enable user-friendly, high-throughput fabrication and deposition onto arbitrary substrates. The UHV sample preparation chamber and vacuum suitcase facilitate inert sample processing, enabling study of air-sensitive 2D materials. Integrated in-situ UHV-CVD growth of large area, single-crystal graphene and hBN provide access to pristine samples which are critical to the investigation of vdW heterostructures with controlled rotational alignment. This instrument enables the study of diverse topics in condensed matter physics and materials science, including engineered strongly correlated phases in twisted many-layer structures, emergent topological superconductivity at heterointerfaces, and precisely localized studies of single-defect quantum devices. The five-year development project for this instrument enables a diverse team of Stanford researchers to explore these phenomena, and provides a nucleation point for collaboration across academia and industry.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.