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.

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.

In superconducting materials, electron clouds can align into a specific order termed nematicity, a word taken from a root meaning string-like and previously used for alignment of molecules in liquid crystal displays (LCDs). Most iron-based high temperature superconductors (FeSCs) exhibit nematic order and magnetic order in conjunction with superconducting behavior. Iron selenide (FeSe) is a type of FeSC material that obtains nematic but not magnetic alignment prior to reaching the superconducting state. This provides an excellent opportunity to disentangle the contribution of these two orders that usually emerge simultaneously. Studies of FeSe have faced the challenge that FeSe crystals break into orthogonally-oriented domains at the onset of nematic order, a process called twinning. A team of researchers has found a way to detwin FeSe crystals to examine the nematic state to gain a deeper understanding of how it affects superconductivity. 

Materials that act as superconductors at higher temperatures (as high as -70°C) are a subject of intense research, due to their use in magnets and quantum devices, including advanced medical and scientific instruments. Interactions of many quantum-level variables in superconducting materials make these systems difficult to model. The Hubbard Model, proposed in 1963 to explain the behavior of correlated electrons in solid materials and 20 years later applied to high-temperature superconducting materials, has been favored due to its relative simplicity along with limited experimental verification. This model focuses on the exclusively electronic variables for superconductivity, completely neglecting atomic-scale vibrations (termed phonons) of the lattice of the material. A team of scientists has challenged the assumption that phonons do not impact high temperature superconductivity through studying a cuprate material.

A superconductor can carry an electrical current with no resistance, so no energy is lost. This quantum mechanical effect was first discovered in certain materials when cooled to very low temperatures, with the highest record at -250°C. In 1986, a class of high temperature superconductor (HTSC) materials was discovered called cuprates, which show superconducting properties at temperatures as high as -135°C. More recently, superconductivity was found in some iron-containing compounds known as iron-based superconductors (FeSCs).

For years, scientists have chased after the promise of high-temperature superconductors – materials that carry current through a material with 100% efficiency. Yet the closest they have come to creating such a material still requires temperatures more than 100 degrees Celsius below freezing.

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.

In solids, Fermi surfaces are the boundaries between occupied and unoccupied electron levels, as defined in momentum space. Their properties dictate that each Fermi surface should form a single unbroken loop. To the surprise of physicists, disconnected segments of the Fermi surface – Fermi arcs – were discovered in cuprate superconductors in 1998.

In a study recently published in Nature Physics, researchers from the University of Colorado have used angle-resolved photoemission spectroscopy (ARPES) at SSRL Beam Line 5-4 to determine the origin of these Fermi arcs in the cuprates.

High-temperature superconductors (HTSCs) operate in mysterious ways, but scientists are starting to understand their peculiarities by using a state-of-the-art spectroscopy system at SSRL. One of the biggest mysteries is how a material that starts as an insulator-which does not conduct electricity-can become a high-temperature superconductor after being doped with electric carriers. Researchers Kyle Shen and Donghui Lu (both SSRL), working in Zhi-xun Shen's group at SSRL and Stanford, looked at the evolution from insulator to superconductor by studying an HTSC material at different doping concentrations.

Using SSRL's beam line 5-4, researchers from Fudan University in Shanghai and SSRL have worked out the mechanism behind the formation of charge density waves in 2H-structured transition metal dichalcogenides (2H-TMD's). The results were published in the November 21, 2007 edition of Physical Review Letters.

One of the strangest consequences of quantum mechanics is the seemingly instantaneous communication of subatomic particles over long distances. Known as quantum entanglement, pairs or groups of particles can become linked so that any changes made to one will cause the others to respond quicker than the time it takes for light to travel between them.

However, the superconducting gap distributions in iron-based superconductors do not fall neatly into either of these two symmetries. Nodeless gap distributions, such as are associated with s-wave pairing symmetry have been directly observed in some members of the iron-based family of high-temperature superconductors, and the signatures of nodal superconducting gaps have been reported in others.

The nature of the pseudogap, which exists above the superconducting transition temperature (T_c) of high-T_c cuprate superconductors, is one of the most important unsolved problems in condensed matter physics. Many possible origins for the pseudogap, such as fluctuating superconductivity and competing order, have been proposed, however, since its discovery two decades ago, there has not been a conclusive experiment.

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.