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Exploring Quantum Spin Nematic State Using Advanced X-Ray Techniques

Ever since superconductivity was discovered in the early 1900s, it has both captivated and mystified scientists. Superconductors conduct electricity with virtually zero resistance, allowing for highly efficient transmission of electrical currents. Among other uses, they create the strong magnetic fields we depend on for medical imaging with MRI machines.

The first known superconductor, mercury, only works when the temperature dips just below -450°F. Copper-containing materials called cuprates were found in the '80s to become superconductors at warmer temperatures, though still inconveniently cold -; closer to -200°F. Understanding how these so-called high-temperature superconductors work could eventually lead to ones that can operate in less frigid conditions.

One potential hallmark of high-temperature superconductors has remained purely theoretical, until now. A team of scientists, including several from the U.S. Department of Energy's (DOE) Argonne National Laboratory, has observed an elusive state of matter called quantum spin nematic. The study, which was published in the journal Nature, used the Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne that also happens to use superconductors. The results lend insight on both high-temperature superconductivity and some of the physics involved in quantum computing.

Superconductivity is when charged particles known as electrons travel through a channel with zero resistance. This unencumbered flow of electrons happens when a superconductor exerts a force that causes the electrons to pair off. In a regular copper wire, some electrons will inevitably interact with the wire itself, creating resistance, which translates to lost energy. The electron pairing in superconductors creates an electron highway, allowing particles to flow unimpeded.

But exactly how does a high-temperature superconductor achieve this effect, and can a material be altered in a way that makes this effect persist even when the thermometer rises- To answer these questions, scientists have been studying the way electron and spin states are arranged in these crystalline materials and how they behave.

Quantum spin nematic, a phase thought to be associated with high temperature superconductivity, describes arrangements of entangled spin states. The word "nematic" describes a type of liquid crystal structure where a group of molecules have parallel axes angled more or less in the same direction. The liquid crystal display, or LCD, of your TV or computer monitor is driven by nematic liquid crystals interacting with electric current. Spin is a quantum property that describes not a rotating motion but an intrinsic angular momentum.

"You can think of quantum spin nematic as the quantum version of liquid crystals," said Jong-Woo Kim, an Argonne physicist and co-author of the study. "It had been proposed many years ago, but never observed directly." These crystal systems exhibit the quantum property of entanglement, where two atoms share the same quantum state without being physically linked. In a quantum computing system, entanglement might be used to encode and transmit data.

The team, led by South Korea's Institute for Basic Science and Pohang University of Science and Technology, used the powerful X-rays at Argonne's APS to seek evidence of this state in an iridate oxide sample. Iridates have been shown to share key quantum characteristics with high-temperature superconductors.

The quantum spin nematic phase had been suggested, but not experimentally confirmed, because it is challenging to detect with conventional magnetic measurements -; it lacks a clear magnetic signature that can be easily observed. "When you shine an X-ray beam through a sample, the atomic arrangement has a signature. The spin arrangement and quantum entangled state can also give out signals," said Yongseong Choi, an Argonne physicist and study co-author. "The atomic structure's signal is the strongest. The spin is weak, but still measurable. The quantum entangled scale is this tiny signal sitting on top of many bigger ones from other states."

To detect these phenomena at the APS, scientists used a technique called resonant X-ray diffraction at the 4-ID-D beamline, which is designed to examine magnetic materials. The process involves measuring the intensity of circularly polarized X-rays scattered from the material. Polarization refers to the orientation of the electric field in the X-ray light. Circularly polarized waves have a rotating electric field that resembles a drill bit of light that creates unique responses, a consequence of the interference in X-ray scattering from conventional spin arrangement and spin entanglement, both of which present in this unique material.

The research team also used polarization-resolved resonant inelastic X-ray scattering techniques at the 27-ID-B beamline at the APS. In that survey, the X-ray light waves are linear and interact with a particular spin direction. Making measurements of the beam direction, sample direction and magnetic field direction, the team could detect magnetic excitations -; a collective response of many spins -; in the quantum spin nematic phase that significantly deviated from those found in a traditional magnetic state.

The study's intense bursts of X-ray light need to be administered in an ultra-stable environment. "There are only a few instruments in the world that have this capability of delivering intense X-rays, changing polarization, doing diffraction and maintaining extreme stability, all at the same time. That's what makes the beamline special," said Gilberto Fabbris, an Argonne physicist and paper co-author.

While the study confirms the existence of the quantum spin nematic state, more work needs to be done to connect it with the mechanism of high-temperature superconductivity. The impediment of generating adequately sized, high-quality samples has been a persistent challenge in research. The APS upgrade, which is currently underway to increase the X-ray brightness up to 500 times, will overcome this. This enhancement will not only facilitate the exploration of considerably smaller samples than those studied previously but will also allow for a more intricate and detailed analysis of the phenomena.

Additional Argonne authors are Joerg Strempfer, Daniel Haskel and Jung Ho Kim. Other authors on the paper included scientists from Institute for Basic Science, Pohang University of Science and Technology, Seoul National University, the European Synchrotron Radiation Facility and the Istituto per lo Studio dei Materiali Nanostrutturati (Institute for Nanostructured Materials). In addition to the U.S. Department of Energy, the Institute for Basic Science; the Samsung Science and Technology Foundation; and the Ministry of Science and Information and Communication Technology of Korea supported this study.

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