Stanford Synchrotron Radiation Lightsource

Topological flat bands in quantum materials represent a fascinating subject in condensed matter physics, often associated with numerous exotic phenomena, including superconductivity, magnetism, and charge density wave order. Flat bands are commonly found in quantum materials where the Coulomb interactions are comparable or larger than the electron kinetic energy. Searching for flat bands in real materials and uncovering the related intriguing phenomena as well as the underlying microscopic mechanisms are collectively referred to as flat band physics.

An unconventional new class of superconducting materials discovered 35 years ago was met with much excitement.  These materials, known as copper oxides or cuprates, conducted electricity with no resistance or loss when chilled below a certain point – but at much higher temperatures than scientists had thought possible. This raised hopes of getting them to work at close to room temperature for perfectly efficient power lines and other uses. Research quickly confirmed that they showed two more classic traits of the transition to a superconducting state: As superconductivity developed, the material expelled magnetic fields, so that a magnet placed on a chunk of the material would levitate above the surface. And its heat capacity – the amount of heat needed to raise their temperature by a given amount – showed a distinctive anomaly at the transition. Despite decades of effort with a variety of experimental tools, the fourth signature, which can be seen only on a microscopic scale, remained elusive.

Since the groundbreaking discovery of copper oxide (cuprate) high-temperature superconductors in 1986, the quest to raise the superconducting transition temperature (T_c) in these systems never stopped. However, since the realization of the 135K T_c in Hg-containing cuprates in 1993, the record has remained off-limits to ensuing efforts under ambient conditions. Understanding the mechanisms that limit the superconducting T_c in cuprates has become an imperative task in order to effectively engineer T_c.

Lithium ion batteries (LIBs), which are widely used in consumer electronics ranging from mobile phones to electric cars, have enabled our electronics to become smaller and last longer on a charge. However, their functionality is limited by environmental conditions.

Lithium ion battery technology has made possible our most-used personal electronics.  Improvements in lithium ion battery energy storage, which can lead to advancements in technologies like electric vehicles, depend largely on improvements to the cathode materials. Researchers value Ni-rich NMC (LiNi_xMn_yCo_zO₂; x+y+z ≈ 1, x ≥ y+z) layered oxide materials for their ability to achieve high energy density, but the performance can be limited due to aberrant surface reactions. Characterization these surface reactions and their relationship to the material’s structure will aid in improving NMC materials, but it is a difficult task, requiring new methods. A team of scientists have integrated new experimental tools for studying how the bulk microstructure and the surface chemistry the NMC cathode material are related and affect performance.

Because they are highly efficient, low maintenance, and light, lithium-ion batteries have grown in popularity. Their use has improved the functionality of many electronics, such as allowing our cell phones to be more portable and our electric cars to travel longer distances. However, some precious metal components of these batteries are in short supply, prompting researchers to develop “beyond lithium-ion” alternatives that use elements more abundant on Earth, yet have the qualities that make lithium-ion batteries so useful. Attention has turned to using common divalent metals, such as calcium, magnesium, and zinc, at the anode for a new type of battery.

Physicists have been interested in crystalline materials where the quantum mechanical behavior of electrons is governed by topology, so-called topological quantum matter. Recently the community has been particularly excited about crystals which additionally exhibit magnetism, i.e. topological quantum magnets. What new topological behavior might such magnets exhibit? Can we find examples of such exotic quantum magnets in nature? And could magnetic topological phases of matter lead to insights about fundamental questions in science or pave the way to technological applications?
 

The field of superconductivity was surprised by the discovery of a manganese-based superconductor, published in 2015.  Because the electrons in manganese do not form couplets called Cooper pairs, it was not thought possible that manganese could have traits of superconductivity. This discovery necessitates a revised explanation for superconductivity, one not requiring Cooper pairing. The unconventional pairing of electrons in the manganese superconductor MnP provides a novel system to understand the phenomenon of superconductivity.

Many new electronic devices replace traditional silicon chips with silicon carbide (SiC) semiconductor chips, which are able to handle more power, function with less power loss, and operate at higher temperatures. Because these chips generate more heat, new materials that bond the chip to the heat sink are needed. A promising choice is sintered silver (Ag). However, detailed and quantitative information about the pore structure and evolution during aging of sintered Ag have not been well studied. A team of researchers quantitatively analyzed the pore structure of sintered silver at high temperatures over time. 

The more widespread use of solar electricity is not currently limited by the technology for generating energy from sunlight but by the storage of that energy, so that it can be used when needed.  Converting water to O₂ and H₂ via the oxygen evolution reaction (OER) is a fossil fuel free way to store energy for later use; catalysts that improve the efficiency of OER are being sought. Manganese oxide (MnO₂) films are good catalysts of OER, with additional benefits of being acid-stable and earth abundant.

Local differences in a battery’s structure and chemistry can lead to problems with function, such local over-charging or under-charging, and can affect the ability to hold charge. Understanding these heterogeneities is important for engineering well-functioning batteries but they are difficult to measure and study.  Scientists usually use either an electrochemical process or a chemical process to prepare materials when studying lithium ion battery heterogeneity at different state of charge. Both of these have flaws: the electrochemical process is close to real-life behavior but experiments may be complicated by structural complexity, and the chemical delithiation process creates a simpler structure but may not properly reflect real-world applications.

CoCrMo-based metal-on-metal hip implants were introduced, particularly for younger patients, due to their superior wear resistance and theoretical mechanical advantages over other hip implant materials (especially the most commonly used metal-on-polyethylene).  However, these CoCrMo-based implants suffered an unexpectedly high failure rate¹ raising concerns over their safety, and leading to considerable attention in the literature on explaining the reasons behind their failure.

Since the discovery of unconventional high-temperature superconductivity (HTSC) in cuprates, one of the central questions in high T_c research is the nature of the “normal state” which develops into HTSC. As one of the pursuits of normal state properties, the recent observation of charge density wave (CDW) order is expected to shed light on the nature of the competing phases in high T_c cuprates.

Given our increasing dependence of rechargeable battery containing electronic devices, including electric cars, it is important to engineer these systems to mitigate potential for catastrophic battery failure. One possible source of lithium ion battery failure is over-discharge (over-lithiation) of the cathode, which can permanently damage the battery. Electronic battery management systems are programmed to prevent and identify such failures, but sometimes do not catch problems of over-lithiation when they occur. To better understand the characteristics of battery failure from over-discharging, a team of scientists studied the chemical and morphological changes that occur from over-lithiation of a lithium battery cathode.

Over the past three years a team of researchers has worked to understand the thermodynamic transitions in the antiferromagnetic ferroelectric BiFeO₃ with La substitutions in relation to a new strategy for finding the ultimate magnetoelectric single phase material. The researchers made the striking finding that structural, ferroelectric, and magnetic phases evolve due to strong spin-lattice coupling, thereby producing a multiferroic triple phase point where three competing multiferroic phases merge.

Rare earth magnetic materials have many applications, such as MRI scanners, Maglev trains, and electric vehicles. Scientists are researching improvements to these magnets through optimizing the component materials. Taking a different approach, a team of scientists have studied the effects of nano-scale heterogeneity in the chemistry and structure of Nd₂Fe₁₄B, a very strong and widely-used rare earth magnet. 

Most portable electronic devices depend on lithium ion batteries for energy storage. The current capabilities of lithium ion batteries are insufficient for the requirements of emerging and growing industries, like electric cars and renewable energy storage. These industries require batteries that are longer-lived, smaller, lighter, and cheaper. One way to improve lithium ion batteries is to increase the charging cutoff voltage, which increases the energy that can be stored in the battery, but it leads to shortened battery life, called capacity fade. A team of scientists has discovered a new mechanism for capacity fade.

Most solar panels use technology that employs a silver-silicon interface. Because silver is expensive and the lead used in the creation of this interface is toxic, researchers are interested searching for other materials that could work instead of these components. A team of scientists are working to understand the process involved in the silver-silicon contact formation so that alternatives that perform the same function can be found.

The electrocatalytic conversion of carbon dioxide and water into useful chemicals and fuels is a promising way of mitigating greenhouse gas emissions and of providing sources for renewable energy. Part of these processes is the oxidation of water into molecular oxygen, a reaction that requires a catalyst. Previously, heterogeneous catalysts have been used, but adoption of homogeneous catalysts allows more understanding and fine-tuning of the atomic-level processes.

The widespread adoption of renewable energy in many applications, such as electric cars, is dependant on the development of better batteries. A lithium ion battery can be made to have a higher capacity, better thermally stability, and lower cost by changing the cobalt component of the battery cathode (usually LiCoO₂) to a mixture of nickel, manganese, and cobalt. While providing great benefits, this material, known as NMC, also has a downside: increased reactivity at the cathode resulting in a shorter battery lifetime. To counteract this reactivity, scientists at the National Renewable Energy Lab in Colorado developed a coating for the NMC cathode.

The heat that builds up in the shuttling of current in electronics is an important obstacle to packing more computing power into ever-smaller devices: Excess heat can cause them to fail or sap their efficiency.

Angle-resolved photoemission spectroscopy (ARPES) measurements taken at Beam Line 5-4 at SSRL and at the Advanced Light Source have observed an exotic property that could warp the electronic structure of a material in a way that reduces heat buildup and improves performance in ever-smaller computer components.

Advanced permanent magnets with low cost and high energy density are important for next-generation technologies, and one promising type of advanced magnet is the exchange spring magnet. This type of nanocomposite comprises two phases of magnetic materials: “hard” magnets, which can remain uniformly magnetized under large fluctuations in magnetic fields, and are often made of rare earth elements, and “soft” magnets, which have a high energy density but their magnetic states can easily be disturbed by small magnetic fields.

Lithium-ion batteries, the mobile power source for most electronic devices, play an important role in everyday life. In the coming decades, they could play an even greater role, powering electric vehicles or storing electrical energy for the grid – if researchers can find ways to improve them.

In particular, the energy density of current batteries is limited by the capacity of the positive electrode, which in turn is determined by the properties and concentration of its active material. By better understanding this material and its limitations, researchers hope to design the highest capacity electrodes possible.

Graphene – a one-atom thick sheet of carbon – shows great promise for future electronics. With its desirable electrical properties, flexibility and strength, the material could enable powerful capacitors, high-quality protective coatings and flexible transparent electronics.

Responsible, eco-friendly and sustainable use of energy is one of the biggest challenges in today’s world. Current rates of energy consumption demand the development of efficient ways to store energy, for instance in safe and durable rechargeable batteries. However, repeated charge cycles degrade batteries over time, eventually leading to their failure. Researchers from the University of Science and Technology of China, SSRL and Oak Ridge National Laboratory have recently developed a new approach to visualize and quantify changes in battery materials during electrochemical cycling – providing crucial information for a better understanding of battery failure and potential improvements of energy storage materials.  

Rechargeable lithium-ion batteries are widely used in applications ranging from consumer electronics to electric vehicles. An important feature of a high-quality battery is a long lifetime, i.e. a large number of possible charge-discharge cycles. However, every cycle introduces changes in the battery’s electrode material, limiting its cyclability. A research collaboration has recently examined the occurring structural and chemical changes in the electrode material during cycling and linked them to the performance of lithium-ion batteries.

Carbon-based materials are extremely lightweight and have thermal, mechanical and electrical properties that are of great interest for use in functional devices. Carbon materials can be manufactured in virtually any shape and even with dimensions on the micro- and nanoscales. Recent research is now aimed at exploiting the spin and magnetism of carbon-based materials for data storage devices – a field called spintronics.

Topological insulators comprise a new state of quantum matter that has been predicted theoretically and realized experimentally in the past few years. Strong inversion asymmetry in topological insulators could lead to many interesting phenomena, such as pyroelectricity, intrinsic topological p-n junctions and topological magneto-electric effects.

Researchers using Beam Line 5-4 at SSRL and Beam Line 10.0.1 at the ALS have shown the compound BiTeCl to be the first topological insulator with a strong inversion asymmetric crystal structure.

Structural changes leading to disordering of the cation-water arrangement within the pores of zeolite natrolite while exchanging sodium (Na⁺) with potassium (K⁺) have been investigated using x-ray diffraction (XRD) and oxygen K-edge x-ray absorption spectroscopy (XAS).

Films of semiconducting organic polymers are major candidates for new materials, with industrial applications ranging from lighting equipment to solar cells to electronic devices. In order to fully exploit these materials, scientists must first understand how polymer films transport electric charge.

Perovskites are mineral oxides with unique properties of great interest to scientists. Many of these materials show remarkable transitions in their behavior. The perovskites lanthanum aluminium oxide (LAO) and strontium titanium oxide (STO), for instance, are insulators. However, when sandwiched together to an LAO/STO heterostructure, the material can conduct electricity at its interface. Researchers can tune conductivity and other emergent properties by doping the perovskites and hope to exploit heterostructures in future industrial applications such as new electronic devices.

An international collaboration of scientists, including several from SSRL, has taken advantage of the broad range of photon science capabilities available at the lab to investigate a proposal that adsorption and desorption of a molecule to a surface – both fundamental processes of interfacial chemistry – proceed through a transient “precursor” state in which the molecule is weakly bound to the surface.

Although the behavior of conventional superconductors has been explained via the BCS theory, the mechanism of superconductivity in the cuprate high temperature superconductors remains unresolved. One approach to this problem is to explore the phases next to superconductivity on the temperature-doping phase diagram. The pseudogap phase above T_c has been a particular stumbling block because it is not a Fermi liquid as with conventional superconductors.

There has been increasing evidence that the pseudogap phase is distinct from superconductivity and persists below T_c, and not simply  a precursor to superconductivity.  In a study recently published in PNAS, researchers at SSRL Beam Line 5-4 and Stanford explored the full doping, temperature, and momentum dependence of spectral gaps in the superconducting state of Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) with unprecedented precision and completeness.

Olefins are the basic building blocks for many products from the petrochemical industry and are currently produced by steam cracking of naphtha or ethane, but increasing oil and gas prices are driving the industry toward producing olefins from syngas derived from cheaper feedstocks via the Fischer-Tropsch process instead. A team of scientists used full-field Transmission hard X-ray Microscopy (TXM) and a special reactor designed and built at SSRL and installed on SSRL Beam Line 6-2 to learn more about the catalyst at the heart of the Fischer-Tropsch-to-Olefins (FTO) process.

The structural, electronic, and magnetic properties of U and Pu elements and intermetallics remain poorly understood despite decades of effort, and currently represent an important scientific frontier toward understanding matter. The last decade has seen great progress both due to the discovery of superconductivity in PuCoGa₅ and advances in theory that finally can explain fundamental ground state properties in elemental plutonium, such as the phonon dispersion curve, the non-magnetic ground state, and the volume difference between different phases of the pure element.

The ability to design and control the activities of transition metal catalysts, which are scarce in nature and thus expensive, has been of great importance to the development of economical industrial and energy-saving processes. Over the years several methods have been suggested, especially for processes using platinum, which is the most active metal catalyst for many important reactions, including the reduction of oxygen in fuel cells.

Structurally incorporated impurities have been shown to have systematic effects on the rate of the thermally driven transformations in titania nanoparticles. For example, the anatase-to-rutile transformation is slowed when anatase nanoparticles are doped with a cation of valence >+4, but favored when the valence < +4. Based on these observations, Y³⁺ dopants should promote the anatase-to-rutile transformation. However, prior studies showed that the transformation is actually inhibited by such impurities. So far these [1,2], observations have remained unexplained.

Since the discovery of high-temperature superconductor by Bednorz and Müller in 1986, this field has become one of the most important research topics in solid state physics. In the past 20 years many unconventional properties have been discovered in this new class of materials. These have challenged our conventional wisdom and driven the development of many novel theories.

New approaches to the fabrication of ferroelectric nanostructures onto substrates are critical for the development of competitive functional devices that successfully integrate at nanoscale ferroelectrics as alternative materials in the microelectronic industry. These approaches have to meet reliability and utilization requirements to realize a cost-effective production of an increasing demand for ultra-high-density memories or nanometric electromechanical systems. An important challenge in the fabrication of ferroelectric nanomaterials supported onto substrates is the ability to fabricate an organized arrangement of the nanostructures. This is a key point for the applications of ferroelectrics in nanoelectronic devices.

Electrocatalysis is the science of modifying the overall rates of electrochemical reactions so that selectivity, yield and efficiency are maximized. Studies in electrocatalysis have resulted in tools such as highly selective multicomponent gas mixture sensors and better electrocatalysts for the fuel cells. Markovic and Lucas have been very active in studying the mechanisms by which these catalysts operate and developing in-situ surface x-ray scattering (SXS) techniques for their studies. 

Panoramic images are captivating in any form, with their wide field of view and extremely high resolution. Now, SSRL scientists have demonstrated a new x-ray holographic technique for imaging wide areas of a nanoscale sample without losing resolution. The results were published in the November 2007 edition of the journal Optics Letters.

The recent discovery of superconductivity in iron-based layered compounds known as iron oxypnictides has renewed interest in high-temperature superconductivity. Now, SLAC and Stanford researchers, using SSRL's angle resolved photoemission spectrometer at Beam Line 5-4, have furthered the quest to understand this iron-based compound. In a recent paper published in Nature, SSRL scientist Donghui Lu, with colleagues at SSRL and Stanford, reported on the mechanism behind the superconductivity of a lanthanum-oxygen-iron-phosphorus (LaOFeP) compound, one of the new iron-based superconducting materials.

Nothing seems to move as fast as the field of consumer electronics. A browse through a technology store reveals the dizzying array of space-age -seeming products like flat screen TVs, touch screen phones, and mp3 players. A new development in electronics is on the horizon, one that may bring us roll-up flat screens and high-definition display clothing. These will be made possible using the thin and energy efficient organic light emitting diodes (OLEDs), which are based on organic semiconductor technology. Both a desire for less expensive, more convenient technologies and a concern for energy conservation have heightened interest in the field of organic semiconductors.

The arrangement of atoms in molecules and complexes that include atoms with many interacting electrons can be hard to predict. The Jahn-Teller (JT) effect sometimes predicts a geometric distortion of the oxygen octahedra surrounding transition metals such as Mn, Co, and Cu. In this model, for certain ground state configurations of the electrons on the metal atom, the total electronic energy can be reduced if the surrounding oxygen octahedra adopt a distorted structure that might seem unstable. The JT distortion is seen in many copper complexes, produces a metal-insulator transition in many manganites, and was predicted to affect the shape of the oxygen octahedra about Co (CoO₆) in La_1-xSr_xCoO₃.

High-temperature superconductors—which conduct electricity without energy loss at relatively high temperatures—are used in advanced technologies including MRI machines, yet their unusual properties are not well understood, preventing the realization of their full application potential.  Many of these unusual properties lie in high-temperature superconductors' normal state, the so-called “strange metal phase.”  One of the puzzling characteristics of this strange metal phase is an anomalous line shape measured by angle resolved photoelectron spectroscopy (ARPES).  ARPES—whether conducted with higher-energy synchrotron or lower-energy laser light—offers information about a material's underlying electronic structure by measuring the energy and trajectory of electrons ejected after the sample absorbs a photon. Yet the two photon sources yield two sets of data that, until now, could not both be described by a single theory.

Attempting to determine and describe the atomic arrangements in an amorphous material is a daunting prospect. A considerable advance has been made in the anomalous X-ray scattering approach to determining these arrangements in materials containing two atomic species.

In the well-known Greek legend the touch of King Midas would convert anything to metallic gold. Recently, a team working at SSRL lead by Professor Jorge Gardea-Torresdey from the University of Texas at El Paso have shown that ordinary alfalfa plants can accumulate very small particles (nanoparticles) of metallic gold (1). The best-known materials that contain nanoparticles of metallic gold are gold colloids. These lack the familiar metallic luster, but show bright colors which range from red, violet or blue, depending upon the size of the nanoparticles (2,3). 

Extensive research efforts to study the novel electronic properties of high-Tc superconductors and their related materials by angle-resolved photoemission spectroscopy at a recently commissioned Beam Line 5-4 (led by Z.-X. Shen) continue to be successful, producing many important results. These results, which are highlighted by five articles recently published in Physical Review Letters and one in Science, brought our understanding steps closer to solving the mystery of the high-T_c superconductivity.