By combining PARADIM’s new design of electron microscope pixel array detector (EMPAD), which has the dynamic range to record the complete distribution of transmitted electrons at every beam position and a phase retrieval algorithm to process the data, the research team has increased the spatial resolution well beyond the traditional lens limitations, setting a world record in 2018 for the highest resolution microscope.
Today, sub-angstrom resolution scanning trans-mission electron microscopy (STEM) imaging is routinely achieved. In addition to resolving individual atomic columns in crystals, STEM performed at room temperature can be used to determine their positions with picometer precision which allows us to directly map local properties.
PARADIM’s in-house research team is exploiting the world-leading tools of its Platform to provide a new modality of materials discovery for artificial quantum materials. This occurs through the unique combination of thin film growth with in-situ spectroscopy enabling scientists to directly see the impact of changes in structure on how the electrons move in these materials.
PARADIM theorists have recognized that lightly hole-doped transition metal dichalcogenides (TMDs) are natural candidates for the long sought odd-parity topological superconductor vital for ground-state topological quantum computing.
One Materials-by-design commonly starts with first principles theory to identify materials with desired properties. The significant energetic contribution of electron-electron interactions, however, makes it difficult for theory to accurately predict quantum materials. Experimental tools like ARPES that measure electronic band structure directly used in combination with synthesis tools like MBE make it possible to enter the materials-by-design loop from a different on-ramp and navigate quantum materials to achieve desired properties.
High entropy compounds are an emerging class of functional materials in which short range order enables superior combinations of properties not present in traditional pure or doped structures.
An invited perspective highlighting research in the high pressure domain at PARADIM was recently published in the Journal of Solid State Chemistry as part of a 50th anniversary special edition.
Materials Innovation Platforms (MIPs): are user facilities dedicated to accelerating materials discovery by providing a community of practitioners with access to cutting edge tools in theory, synthesis, and characterization and the ability to share knowledge effortlessly. MIPs are funded by the National Science Foundation (NSF) and are freely available to users from universities and national laboratories in the United States—from scientists just beginning their careers to seasoned experts—from all institutions.:
Substrates traditionally play a supporting role for the thin films grown upon them by providing purely structural support. Nonetheless for monolayer-thick transition metal dichalcogenides , the substrate plays an active role by providing or removing charge from the TMD layer in which it is in intimate contact.
The PARADIM research team has developed a system to transfer sensitive samples from ultrahigh vacuum directly into an inert environment glovebox for transport to the Advance Light Source for beamline ARPES measurements.
Members of PARADIM’s in-house research team have employed a combination of angle-resolved photoemission spectroscopy and in situ resistivity measurements to simultaneously probe both the electronic states and superconducting behavior of pristine monolayer FeSe/SrTiO3.
Sr2RuO4 is the most disorder-sensitive superconductor known. It has also been a leading candidate for a novel type of quantum computer that would enable calculations to occur over much longer time scales before suffering decoherence than is the case for today’s superconductor-based quantum computers.
The Materials-by-Design approach relies on strong theoretical capabilities to predict the properties of novel materials. Unfortunately, traditional ab initio techniques for calculating the electronic structure of materials are powerless when the lattice mismatch between two crystals leads to the absence of periodicity, as observed between many of the interface quantum materials that are the focus of PARADIM’s in-house research.
This highlight provides an example of the precise interface quantum materials that can be created by MBE and characterized by high-resolution STEM and elemental analysis in PARADIM facilities.
PARADIM has deployed the first deep learning model of crystal synthesis to assist facility users as they optimize synthesis conditions and accelerate the realization of their materials design plans.
Through the use of “remote epitaxy”—where graphene separates a growing oxide thin film from its substrate—superb crystallinity is achieved together with the ability to peel off oxide layers and create stacks of multiple high-quality layers assembled in a user-defined sequence. The approach heralds a new platform for the limitless stacking of any material into any heterostructure.
A new world record for highest pressure of a floating zone crystal growth, at 300 atmospheres, twice the previous record, was achieved by PARADIM staff in the United States in collaboration with our industry partner SciDre. This work was done at Johns Hopkins University by Tyrel M. McQueen and W. Adam Phelan.
Building on their growing expertise with interface materials, PARADIM researchers discovered a new mechanism for reorienting magnetization that is 10 times more energy efficient than prior techniques. An electrical current is passed through a low symmetry valleytronic material containing heavy atoms that is positioned under a magnetic layer.
PARADIM researchers have created superlattice valleytronic materials—still just a single monolayer thick—by modulating the sequence in which the gas precursors are supplied during growth.
Since the discovery of high-temperature superconductivity in copper-based oxides (cuprates), there has been a sustained effort to understand its origin and to discover new superconductors based on similar building principles. Indeed, superconductivity has recently been discovered in the doped rare-earth nickelate Nd:0.8:Sr:0.2:NiO:2:. Undoped NdNiO:2: is the infinite-layer end member of a larger family of layered nickelates, which can be explored by molecular-beam epitaxy (MBE).
The properties of any material are largely governed by its constituting elements and the arrangement of those atoms. Different structures can result in vastly different behavior, even for the same chemical composition. Thus, to achieve a desired structure—the one with the desired properties—it is vital to be able to navigate synthesis pathways.:Here, users from the Naval Research Lab wanted to grow ScFeO3, where five different ways are known in which the same atoms can be arranged. In the past, different substrates have been used to select between the various polymorphs by altering the energetics of materials growth through a process known as epitaxial stabilization.
The ultimate performance of some of the most powerful characterization tools including x-ray free electron lasers, ultrafast electron microscopes, and particle accelerators are determined by the ability of their electron sources to emit electrons. This small, yet vital element of these multimillion to multibillion dollar systems, has the potential to be improved greatly; the performance of commonly used electron sources pales in comparison to the theoretical limit due to roughness, disorder, and polycrystallinity. The path to maximally efficient electron sources is thus believed to lie with single-crystal films, where the smoothness, homogeneity, and termination can be controlled at the atomic level. Unfortunately, the most desired materials for electron sources contain highly reactive species like cesium, which has stymied the preparation of single-crystal films of these desired electron sources—until now.
The observation of replica bands by angle-resolved photoemission spectroscopy enables the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO3. Theoretical work suggests that the electrons in the ultra-thin FeSe layer couple to optical phonons in the SrTiO3 substrate that thereby contributes to the enhanced superconducting pairing temperature. So far, the inherent fragility of such single-layer thick materials and the weak intensity of replica features has limited the quantitative evaluation of their nature.
Charge density waves (CDW) are an emergent periodic modulation of the electron density that permeates crystals with strong electron-lattice coupling. TaS:2: and TaSe:x:S:2-x: host several charge density wave states that spontaneously break crystal symmetries, mediate metal-insulator transitions, and compete with superconductivity. These quantum states are promising candidates for novel devices, efficient ultrafast nonvolatile switching, and suggest elusive chiral superconductivity.
The ability to produce pristine atomic interfaces by growing the structures atom-by-atom :via: molecular beam epitaxy has opened the door to an entirely new class of emergent phenomena in the form of interface quantum materials. One remarkable example of such a material system is the interface between monolayer iron selenide (FeSe) films and SrTiO:3:, where superconductivity can be observed at dramatically higher temperatures compared to 8 K of bulk FeSe. So far, the inherent fragility of such ultra-thin layers has prevented the direct study of superconductivity :via: direct probes like electrical resistivity.
Electron microscopy is a widespread and often essential tool for structural and chemical analysis at the atomic level. Image resolution is dominated by the energy (or wavelength) of the electron beam and the quality of the lens. By combining our new design of electron microscope pixel array detector (EMPAD), which has the dynamic range to record the complete distribution of transmitted electrons at every beam position and a phase retrieval algorithm to process the data, PARADIM’s in-house research team has increased the spatial resolution well beyond the traditional lens limitations, setting a world record in 2018 for the highest resolution microscope (0.39 Å Abbe resolution ) at the same dose and imaging conditions where conventional imaging modes reach only 0.98 Å. The EMPAD is the culmination of over a decade of detector development at Cornell, supported by NSF (through CHESS, CCMR), DOE, the WM Keck Foundation, and the Kavli Institute, and has been commercially licensed by ThermoFisher Scientific and is now manufactured and sold at scale.
The ever-growing demand for electric vehicles and portable electronic devices continues to drive the craving for lightweight, high-energy-density, and long-lifespan batteries. Currently used lithium-ion batteries are limited by the capacity of the electrode and the scarcity of resources (lithium). The search for next-generation materials not only seeks to replace lithium with sodium but to provide suitable anode materials. Metal selenides (:M:Se, :M := Sn, Fe, Ni, Cu) offer the desired conductivity, stability, cost-effectiveness, and higher theoretical capacity compared to commercial graphite.
Currently, electronic device technology is based mainly on semiconductors. It first emerged in the middle of the 20:th: century and has improved ever since. Further technological advances including energy efficiency and information security might profit from exploiting quantum mechanical properties that are present in superconductors. The challenge is how to combine the two states and to make sure to get the best of both electrical worlds. A collaboration of researchers from Cornell and the Paul Scherrer Institute (PSI) in Switzerland grew thin films of the superconductor niobium nitride (NbN) on top of gallium nitride (GaN), a semiconductor and vital component in many optical and power electronics. The team measured the electronic properties of the two materials directly at their interface using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES).
H. Paik:, :S. Xie:, H. Shin, C. Choi, J.H. Le, :C. Dong:, :J.A. Robinson (2DCC):, :J.-H. Lee:, J.-H. Ahn, G.Y. Yeom, :D.G. Schlom (PARADIM):, J. Kim (MIT):Project Summary::Remote epitaxy has drawn attention as it offers epitaxy of functional materials that can be released from the substrates with atomic precision, thus enabling production and heterointegration of flexible, transferrable, and stackable freestanding single-crystalline membranes. In this highlight, 2DCC and PARADIM team up to work with the inventor of remote epitaxy, Prof. Kim (MIT), to unveil the respective roles and impacts of the substrate material, graphene, substrate–graphene interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single-crystal epitaxy in the overlying film.
The work was recognized as the highest resolution microscope (of any type) by Guinness World Records in July 2018.
Many electronic devices, like non-volatile high-density memories, ultra-fast switches, and thin film capacitors, rely on ferroelectric materials—a class of materials with spontaneous electrical polarization which can be reproducibly switched. In two-dimensional (2D) and quasi-2D ferroelectric materials the size of ferroelectric domains can be small which may enable the miniaturization of devices. Understanding of ferroelectric domain structure at the atomic scale is, however, limited, hindering the development of functional device units at the microscopic level.
For nearly 50 years, Terfenol-D (Tb:x:Dy:1-:x:Fe:2:) has reigned as the material for which an applied magnetic field results in the greatest change in shape, a property known as magnetostriction. A distant second to Terfenol-D is Galfenol (Fe:1–:x:Ga:x:), the best magnetostrictor free of rare-earth elements.
PARADIM’s in-house team worked with a collaborator at Penn State to develop a new variant of MBE that we call “suboxide MBE.” In contrast to conventional MBE where the molecular beams are elemental, in suboxide MBE the molecular beams are pre-oxidized. Achieving the desired suboxide beams relies on extensive thermodynamic calculations made for the entire periodic table, as shown in Figure 1. 
In virtually all implementations of a quantum computer, one of the states is an excited state that must, of necessity, decay spontaneously into its ground state. A fascinating alternative form of quantum computing has been proposed: braiding pairs of ground-state non-abelian anyons in two dimensions.
Quantum fabrics offer novel electronic, magnetic, or topological textures with functionalities that do not exist in bulk and could play an important role in future quantum technologies. Quantum fabrics are created by weaving together "threads" with different properties, such as superconductivity or magnetism. One method to make them is the atomically precise assembly of layered two-dimensional Van der Waals (vdW) materials. This assembly has traditionally been accomplished using artisan methods from micromechanical exfoliated flakes, but such fabrication is not compatible with scalable and rapid manufacturing.
The principal challenges in current thermoelectric power generation modules are the availability of stable, diffusion-resistant, lossless electrical and thermal metal–semiconductor contacts that do not degrade at the hot end nor cause reductions in device efficiency. Transverse thermoelectric devices, in which a thermal gradient in a single material induces a perpendicular voltage, promise to overcome these problems.
Sr:2:RuO:4: is the most disorder-sensitive superconductor known. It has also been a leading candidate for a novel type of quantum computer that would enable calculations to occur over much longer time scales before suffering decoherence than is the case for today’s superconductor-based quantum computers. Establishing whether Sr:2:RuO:4: is viable for such applications would be aided by the ability to make structures containing superconducting Sr:2:RuO:4: films.
Important applications like ultrashort pulsed lasers, sensors, laser amplifiers, and digital optical information processing, depend on materials with nonlinear optical properties—that is the material’s nonlinear interaction with electromagnetic waves, :e.g.:, light.
In quantum materials, electrons can interact strongly with each other and with the atomic lattice, giving rise to novel electronic states with functionalities not achievable in conventional materials. At the atomic scale, the local crystal symmetries are governed by variations in the charge distribution and subtle atomic displacements, dramatically affecting the material’s properties.
Atomic-resolution cryogenic STEM provides a path to probe the microscopic nature of low-temperature phases in quantum materials. To date, successful high-resolution cryo-experiments have been limited to few and fixed temperatures, dictated by the choice of cryogen, leaving most of the phase space of materials unexplored.
Probing the structure of layered materials offers a gateway to better understand and potentially manipulate their electronic properties. While interactions within the individual layers are dominant, the proximity of neighboring layers significantly impacts the properties of such quasi-2D systems. Transition-metal dichalcogenides containing tellurium are especially noteworthy for their modulated structures and prominent interlayer contributions.
One way to see atoms is to focus a beam of high-energy electrons to a tiny spot smaller than an atom and scan it around. The clarity of the resulting image is a tradeoff between collecting sufficient electrons to get a good signal, but too many electrons and the sample will be damaged.
Building on their growing expertise with valleytronic materials, PARADIM researchers, Jiwoong Park (University of Chicago) and David Muller (Cornell University) have discovered a way to assemble multi-layer stacks of monolayers of transition metal dichalcogenide (TMD) materials (MoS2, MoSe2, and WS2) with centimeter-scale dimensions in a user-defined sequence. The assembly is achieved without the use of etchants or solvents by exploiting the stronger binding that occurs between TMD layers than between the TMD layer and the underlying substrate.
Ultrathin quantum materials present a unique platform for the control of electronic, magnetic, and topological properties. A commonly observed phenomenon in many ultrathin quantum materials is that an undesired crossover from a metallic to insulating state occurs below a critical thickness. This presents a potential challenge for realizing ultrathin heterostructures of quantum materials when metallic properties are desired.
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