Tag: quantum materials

Disorder begins at the surface of quantum materials

Disorder begins at the surface of quantum materials

A new study reveals that the response of quantum materials to light is more complex than previously assumed. Using ultrafast X-ray pulses at the X-ray free electron laser SwissFEL, researchers found that the surface of a layered manganese oxide reacts differently than the bulk when its orbital order is disturbed. These results challenge the idea that light-induced changes happen uniformly and suggest that the path from order to disorder is shaped by local differences inside the material. 

In certain materials, the electrons arrange themselves in a well-defined, ordered pattern. This internal order can influence everything from how the material conducts electricity to how it responds to magnetic fields. One example is the layered manganese oxide and quantum material La0.5Sr1.5MnO4, in which electrons of manganese atoms arrange themselves into a regular pattern – known as orbital ordering – leading to distinctive electronic and magnetic behaviour.

Researchers are increasingly interested in how light can be used to understand and control the orbital state of these materials. With the right kind of light pulse, it may be possible to switch or reshape their properties at incredible speeds. Therefore, understanding how these materials switch is an important step to making devices.

In many devices, surfaces of and interfaces between materials are known to play a major role in the device properties. Yet until now, it has not been possible to measure how quantum materials change at the surface when switched at high speeds by light. Previous studies have only captured the average response over the whole crystal. 

In this study, a team of scientists led by Aarhus University asked if the average response measured to date accurately captures the processes that occur at the surface, which will be relevant for any device. Remarkably, they found that they did not. 

Read more on the PSI website

Image: Using ultrafast X-rays from SwissFEL, scientists have revealed unexpected light responses in quantum materials

Credit: © AdobeStock

Stabilising fleeting quantum states with light

Stabilising fleeting quantum states with light

Quantum materials exhibit remarkable emergent properties when they are excited by external sources. However, these excited states decay rapidly once the excitation is removed, limiting their practical applications. A team of researchers from Harvard University and the Paul Scherrer Institute PSI have now demonstrated an approach to stabilise these fleeting states and probe their quantum behaviour using bright X-ray flashes from the X-ray free electron laser SwissFEL at PSI. The findings are published in the journal Nature Materials.

Some materials exhibit fascinating quantum properties that can lead to transformative technologies, from lossless electronics to high-capacity batteries. However, when these materials are in their natural state, these properties remain hidden, and scientists need to gently ask for them to pop up. One way they can do this is by using ultrashort pulses of light to alter the microscopic structure and electronic interactions in these materials so that these functional properties emerge. But good things do not last forever – these light-induced states are transient, typically persisting only a few picoseconds, making them difficult to harness in practical applications. In rare cases, light-induced states become long-lived. Yet our understanding of these phenomena remains limited, and no general framework exists for designing excited states that last.

A team of scientists from Harvard University together with PSI colleagues overcame this challenge by manipulating the symmetry of electronic states in a copper oxide compound. Using the X-ray free electron laser SwissFEL at PSI, they demonstrated that tailored optical excitation can induce a ‘metastable’ non-equilibrium electronic state persisting for several nanoseconds – about a thousand times longer than they usually last for. 

Steering electrons with light

The compound under study, Sr14Cu24O41 – a so-called cuprate ladder – is nearly one-dimensional. It is composed of two distinct structural units, the ladders and chains, representing the shape in which copper and oxygen atoms organise. This one-dimensional structure offers a simplified platform to understand complex physical phenomena that also show up in higher-dimensional systems. “This material is like our fruit fly. It is the idealised platform that we can use to study general quantum phenomena,” comments experimental condensed matter physicist Matteo Mitrano from Harvard University, who lead the study. 

One way to achieve a long-lived (‘metastable’) non-equilibrium state is to trap it in an energy well from which it does not have enough energy to escape. However, this technique risks inducing structural phase transitions that change the material’s molecular arrangement, and that is something Mitrano and his team wanted to avoid. “We wanted to figure out whether there was another way to lock the material in a non-equilibrium state through purely electronic methods,” explains Mitrano. For that reason, an alternative approach was proposed.

In this compound, the chain units hold a high density of electronic charge, while the ladders are relatively empty. At equilibrium, the symmetry of the electronic states prevents any movement of charges between the two units. A precisely engineered laser pulse breaks this symmetry, allowing charges to quantum tunnel from the chains to the ladders. “It’s like switching on and off a valve,” explains Mitrano. Once the laser excitation is turned off, the tunnel connecting ladders and chains shuts down, cutting off the communication between these two units and trapping the system in a new long-lived state for some time that allows scientists to measure its properties.

Cutting-edge fast X-ray probes

The ultra-bright femtosecond X-ray pulses generated at the SwissFEL allowed the ultrafast electronic processes governing the formation and subsequent stabilisation of the metastable state to be caught in action. Using a technique known as time-resolved Resonant Inelastic X-ray scattering (tr-RIXS) at the SwissFEL Furka endstation, researchers can gain unique insight into magnetic, electric, and orbital excitations – and their evolution over time – revealing properties that often remain hidden to other probes. 

“We can specifically target those atoms that determine the physical properties of the system,” comments Elia Razzoli, group leader of the Furka endstation and responsible for the experimental setup. 

This capability was key to dissecting the light-induced electronic motion that gave rise to the metastable state. “With this technique, we could observe how the electrons moved at their intrinsic ultrafast timescale and hence reveal electronic metastability,” adds Hari Padma, postdoctoral scholar at Harvard and lead author of the paper.

Read more on PSI website

Image: Laser pulses trigger electronic changes in a cuprate ladder, creating long-lived quantum states that persist for about a thousand times longer than usual.

Credit: Brad Baxley/Part to Whole

New AI-driven tool streamlines experiments

New AI-driven tool streamlines experiments

Researchers at the Department of Energy’s SLAC National Accelerator Laboratory have demonstrated a new approach to peer deeper into the complex behavior of materials. The team harnessed the power of machine learning to interpret coherent excitations, collective swinging of atomic spins within a system. 

This groundbreaking research, published recently in Nature Communications, could make experiments more efficient, providing real-time guidance to researchers during data collection, and is part of a DOE-funded project led by Howard University including researchers at SLAC and Northeastern University to use machine learning to accelerate research in materials. 

The team created this new data-driven tool using “neural implicit representations,” a machine learning development used in computer vision and across different scientific fields such as medical imaging, particle physics and cryo-electron microscopy. This tool can swiftly and accurately derive unknown parameters from experimental data, automating a procedure that, until now, required significant human intervention.

Peculiar behaviors

Collective excitations help scientists understand the rules of systems, such as magnetic materials, with many parts. When seen at the smallest scales, certain materials show peculiar behaviors, like tiny changes in the patterns of atomic spins. These properties are key for many new technologies, such as advanced spintronics devices that could change how we transfer and store data. 

To study collective excitations, scientists use techniques such as inelastic neutron or X-ray scattering. However, these methods are not only intricate, but also resource-intensive given, for example, the limited availability of neutron sources. 

Machine learning offers a way to address these challenges, although even then there are limitations. Past experiments used machine learning techniques to enhance the accuracy of X-ray and neutron scattering data interpretation. These efforts relied on traditional image-based data representations. But the team’s new approach, using neural implicit representations, takes a different route. 

Read more on SLAC website

Meet Xiaoqian Chen, NSLS-II Beamline Scientist

Meet Xiaoqian Chen, NSLS-II Beamline Scientist

Xiao leads the quantum materials program at NSLS-II’s Coherent Hard X-Ray Scattering beamline

You’ve already spent a few years at Brookhaven, starting in 2016. What brought you back to the Laboratory?

My history with the Laboratory goes back to when I was a user at the original National Synchrotron Light Source (NSLS). I was in graduate school, and my group had our own beamline at NSLS, X1B. I was the person maintaining that beamline, so I came to know many people here.

When I became a postdoc, I decided to join the x-ray scattering group in Brookhaven’s Condensed Matter Physics & Materials Science Department because we shared the same research interests and coherent x-ray science was emerging. I joined when the Coherent Soft X-ray Scattering (CSX) beamline at NSLS-II had just achieved first light. I was able to participate in the first experiments there, and that was my postdoc work.

I then took a postdoc position at DOE’s Lawrence Berkeley National Laboratory and worked more on coherent x-ray science. After three years, I was offered a position at CHX to lead the quantum materials program.

What does your role at CHX entail? What kind of research do you do?

My research, in a broad sense, is in the field of condensed matter physics, looking at quantum materials. These are materials that have behaviors that are very heavily guided by quantum mechanical effects. For example, some have superconducting properties, and some have interesting magnetism. With help from my postdoc, I am looking into discovering new quantum effects in materials, and our work is supported by a Laboratory Directed Research and Development (LDRD) grant. Apart from my own research, I prepare an environment at CHX for scientists in our community to perform coherent x-ray experiments and study dynamical properties in their materials.

At CHX, we use coherent x-ray photons to track the dynamics inside materials. We can detect electron dynamics as fast as the nanosecond time scale with our recent detector installation. I’m currently trying to detect and study dynamics in materials that take place at that time scale. The ultimate goal is to find a signature of quantum entanglement. This property is a specific type of interaction between electrons; they show very different behavior from electrons in classical materials.

There are some other new capabilities that we are excited for users to experience. We have added a resonance scattering capability to look at element-specific information. For example, we can look at electrons in specific orbitals. We also added low temperature capability with a new cryostat that can chill samples to 4K. That’s very cold, and it will be very useful for studying quantum materials.

Read more on the NSLS-II website

Image: Xiaoqian Chen is a physicist and beamline scientist with the complex scattering program at the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Brookhaven National Laboratory. At NSLS-II’s Coherent Hard X-ray Scattering (CHX) beamline, she studies materials with quantum behaviors and guides users in their investigations into quantum materials.

LCLS-II ushers in a new era of science

LCLS-II ushers in a new era of science

SLAC fires up the world’s most powerful X-ray laser

With up to a million X-ray flashes per second, 8,000 times more than its predecessor, it transforms the ability of scientists to explore atomic-scale, ultrafast phenomena that are key to a broad range of applications, from quantum materials to clean energy technologies and medicine.

The newly upgraded Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL) at the Department of Energy’s SLAC National Accelerator Laboratory successfully produced its first X-rays, and researchers around the world are already lined up to kick off an ambitious science program. 

The upgrade, called LCLS-II, creates unparalleled capabilities that will usher in a new era in research with X-rays. Scientists will be able to examine the details of quantum materials with unprecedented resolution to drive new forms of computing and communications; reveal unpredictable and fleeting chemical events to teach us how to create more sustainable industries and clean energy technologies; study how biological molecules carry out life’s functions to develop new types of pharmaceuticals; and study the world on the fastest timescales to open up entirely new fields of scientific investigation. 

“This achievement marks the culmination of over a decade of work,” said LCLS-II Project Director Greg Hays. “It shows that all the different elements of LCLS-II are working in harmony to produce X-ray laser light in an entirely new mode of operation.”  

Reaching “first light” is the result of a series of key milestones that started in 2010 with the vision of upgrading the original LCLS and blossomed into a multi-year ($1.1 billion) upgrade project involving thousands of scientists, engineers, and technicians across DOE, as well as numerous institutional partners. 

“For more than 60 years, SLAC has built and operated powerful tools that help scientists answer fundamental questions about the world around us. This milestone ensures our leadership in the field of X-ray science and propels us forward to future innovations,” said Stephen Streiffer, SLAC’s interim laboratory director. “It’s all thanks to the amazing efforts of all parts of our laboratory in collaboration with the wider project team.”

Read more on the SLAC website

Image: The newly upgraded Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL) at the Department of Energy’s SLAC National Accelerator Laboratory successfully produced its first X-rays. The upgrade, called LCLS-II, creates unparalleled capabilities that will usher in a new era in research with X-rays.

Credit: Greg Stewart/SLAC National Accelerator Laboratory

Researchers make a new type of quantum material with a dramatic distortion pattern

Researchers make a new type of quantum material with a dramatic distortion pattern

Created by an electronic tug-of-war between the material’s atomic layers, this ‘beautiful’ herringbone-like pattern could give rise to unique features that scientists are just starting to explore.

Researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have created a new type of quantum material whose atomic scaffolding, or lattice,  has been dramatically warped into a herringbone pattern.

The resulting distortions are “huge” compared to those achieved in other materials, said Woo Jin Kim, a postdoctoral researcher at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC who led the study. 

“This is a very fundamental result, so it’s hard to make predictions about what may or may not come out of it, but the possibilities are exciting,” said SLAC/Stanford Professor and SIMES Director Harold Hwang. 

“Based on theoretical modeling from members of our team, it looks like the new material has intriguing magnetic, orbital and charge order properties that we plan to investigate further,” he said. Those are some of the very properties that scientists think give quantum materials their surprising characteristics. 

The research team described their work in a paper published in Nature today.

Read more on the SLAC website

Image: This illustration depicts a herringbone-like pattern in the atomic lattice of a quantum material created by researchers at SLAC and Stanford. An electronic tug-of-war between its layers has dramatically warped the lattice. Researchers are just staring to explore how this ‘huge’ distortion affects the material’s properties. 

Credit: Greg Stewart/SLAC National Accelerator Laboratory

Spin keeps electrons in line in iron-based superconductor

Spin keeps electrons in line in iron-based superconductor

Researchers from PSI’s Spectroscopy of Quantum Materials group together with scientists from Beijing Normal University have solved a puzzle at the forefront of research into iron-based superconductors: the origin of FeSe’s electronic nematicity. Using Resonant inelastic X-ray scattering (RIXS) at the Swiss Light Source (SLS), they discovered that, surprisingly, this electronic phenomenon is primarily spin driven. Electronic nematicity is believed to be an important ingredient in high-temperature superconductivity, but whether it helps or hinders it is still unknown. Their findings are published in Nature Physics.

Near PSI, where the Swiss forest is ever present in people’s lives, you often see log piles: incredibly neat log piles. Wedge shaped logs for firewood are stacked carefully lengthways but with little thought to their rotation. When particles in a material spontaneously line up, like the logs in these log piles, such that they break rotational symmetry but preserve translational symmetry, a material is said to be in a nematic state. In a liquid crystal, this means that the rod shaped molecules are able to flow like a liquid in the direction of their alignment, but not in other directions. Electronic nematicity occurs when the electron orbitals in a material align in this way. Typically, this electronic nematicity manifests itself as anisotropic electronic properties: for example, resistivity or conductivity exhibiting vastly different magnitudes when measured along different axes.

Since their discovery in 2008, the past decade has seen enormous interest in the family of iron based superconductors. Alongside the well-studied cuprate superconductors, these materials exhibit the mysterious phenomenon of high temperature superconductivity. The electronic nematic state is a ubiquitous feature of iron-based superconductors. Yet, until now, the physical origin of this electronic nematicity is a puzzle; in fact, arguably one of the most important puzzles in the study of iron-based superconductors.

Read more on the PSI website

Image: Resonant inelastic x-ray scattering reveals high-energy nematic spin correlations in the nematic state of the iron-based superconductor, FeSe

Credit: Beijing Normal University/Qi Tang and Xingye Lu

Rich electronic features of a kagome superconductor

Rich electronic features of a kagome superconductor

The recently discovered layered kagome metals AV3Sb5 (A=K, Rb, Cs) exhibit diverse correlated phenomena, thought to be linked to so-called Van Hove singularities (VHSs) and flat bands in the material. Using a combination of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory, researchers led by Professor Ming Shi at the Paul Scherrer Institute directly revealed the sublattice properties of 3d-orbital VHSs and flat bands, as well as topologically non-trivial surface states in CsV3Sb5. The research reveals important insights into the material’s electronic structure and provides a basis for understanding correlation phenomena in the metals.

So-called kagome metals, named after the Japanese woven bamboo pattern their structure resembles, feature symmetrical patterns of interlaced, corner-sharing triangles. This unusual lattice geometry and its inherent features lead, in turn, to curious quantum phenomena such as unconventional, or high-temperature, superconductivity.

The potential for devices that might transport electricity without dissipation at room temperature—as well as a thirst for fundamental theoretical understanding—have led researchers to investigate this new class of quantum materials and try to figure out how electrons interact with the kagome lattice to generate such remarkable features.

A recently discovered class of AV3Sb5 kagome metals, where A can be =K, Rb or Cs, was shown, for instance, to feature bulk superconductivity in single crystals at a maximum Tc of 2.5 K at ambient pressure. Researchers suspect that this is a case of unconventional superconductivity, driven by some mechanism other than the phonon exchange that characterizes bonding in the electron-phonon coupled superconducting electron-pairs of conventional superconductivity.

This, as well as other exotic properties observed in the metal, are thought to be connected to its multiple “Van Hove singularities” (VHSs) near the Fermi level. VHSs, associated with the density of states (DOS), or set of different states that electrons may occupy at a particular energy level, can enhance correlation effects when a material is close to or reaches this energy level. If the Fermi level lies in the vicinity of a Van Hove point, the singular DOS determines the physical behavior due to the large number of available low-energy states. In particular, interaction effects get amplified not only in the particle-particle, but also in the particle-hole channels, leading to the notion of competing orders.

Read more on the PSI website

Image: Yong Hu, first author, and Nicholas Clark Plumb, who made the experimental station, at the Surface/Interface Spectroscopy (SIS) beamline of the Swiss Light Source (SLS) (L to R)

Credit: Paul Scherrer Institut / Mahir Dzambegovic