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Strong geometrical frustration in magnets leads to exotic states such as spin liquids, spin supersolids, and complex magnetic textures. SrCu2(BO3)2, a spin-1/2 Heisenberg antiferromagnet in the archetypical Shastry–Sutherland lattice, exhibits a rich spectrum of magnetization plateaus and stripe-like magnetic textures in applied fields. The structure of these plateaus is still highly controversial due to the intrinsic complexity associated with frustration and competing length scales. We discover magnetic textures in SrCu2(BO3)2 via magnetostriction and magnetocaloric measurements in fields up to 100.75 T. In addition to observing low-field fine structure with unprecedented resolution, the data also reveal lattice responses at 73.6 T and at 82 T that we attribute, using a controlled density matrix renormalization group approach, to a unanticipated 2/5 plateau and to the long-predicted 1/2 plateau.

/pdf/magnetostriction-and-magnetic-texture-to-100-75-tesla-in-445qycqtdq.pdf

Magnetostriction and magnetic texture to 100.75 Tesla in frustrated SrCu$_2$(BO$_3$)$_2$

Realistic description of competing phases in complex quantum materials has proven extremely challenging. For example, much of the existing density-functional-theory-based first-principles framework fails in the cuprate superconductors. Various many-body approaches involve generic model Hamiltonians and do not account for the interplay between the spin, charge, and lattice degrees of freedom. Here, by deploying the recently constructed strongly constrained and appropriately normed (SCAN) density functional, we show how the landscape of competing stripe and magnetic phases can be addressed on a first-principles basis both in the parent insulator YBa2Cu3O6 and the near-optimally doped YBa2Cu3O7 as archetype cuprate compounds. In YBa2Cu3O7, we find many stripe phases that are nearly degenerate with the ground state and may give rise to the pseudogap state from which the high-temperature superconducting state emerges. We invoke no free parameters such as the Hubbard U, which has been the basis of much of the existing cuprate literature. Lattice degrees of freedom are found to be crucially important in stabilizing the various phases.

Competing stripe and magnetic phases in the cuprates from first principles

Even though superconductivity has been studied intensively for more than a century, the vast majority of superconductivity research today is carried out in nearly the same manner as decades ago. That is, each study tends to focus on only a single material or small subset of materials, and discoveries are made more or less serendipitously. Recent increases in computing power, novel machine learning algorithms, and improved experimental capabilities offer new opportunities to revolutionize superconductor discovery. These will enable the rapid prediction of structures and properties of novel materials in an automated, high-throughput fashion and the efficient experimental testing of these predictions. Here, we review efforts to use machine learning to attain this goal. 

The 2021 room-temperature superconductivity roadmap

Universal scaling laws can guide the understanding of new phenomena, and for cuprate high-temperature superconductivity the influential Uemura relation showed, early on, that the maximum critical temperature of superconductivity correlates with the density of the superfluid measured at low temperatures. Here we show that the charge content of the bonding orbitals of copper and oxygen in the ubiquitous CuO2 plane, measured with nuclear magnetic resonance, reproduces this scaling. The charge transfer of the nominal copper hole to planar oxygen sets the maximum critical temperature. A three-dimensional phase diagram in terms of the charge content at copper as well as oxygen is introduced, which has the different cuprate families sorted with respect to their maximum critical temperature. We suggest that the critical temperature could be raised substantially if one were able to synthesize materials that lead to an increased planar oxygen hole content at the expense of that of planar copper.

/pdf/perspective-on-the-phase-diagram-of-cuprate-high-temperature-39ct1x6i6h.pdf

Perspective on the phase diagram of cuprate high-temperature superconductors.

High-sensitivity NMR beyond 200,000 atmospheres of pressure.

Cuprate superconductors still hold many open questions, and recently, the role of symmetry breaking electronic charge ordering resurfaced in underdoped cuprates as a phenomenon that competes with superconductivity. Here, unambiguous nuclear magnetic resonance (NMR) proof is presented for the existence of local charge ordering in nearly optimally doped YBa2Cu3O6.9, even up to room temperature. Increasing pressure and decreasing temperature leads to the highest degree of order in the sense that the two oxygen atoms of the unit cell of the CuO2 plane develop a charge difference of about 0.02 holes, and order throughout the whole crystal. At ambient conditions, a slightly smaller charge difference and a decreased order is found. Evidence from literature data suggests that this charge ordering is ubiquitous to the CuO2 plane of all cuprates. Thus, the role of charge ordering in the cuprates must be reassessed.

https://www.mdpi.com/2410-3896/3/3/23/pdf

Bulk Charge Ordering in the CuO2 Plane of the Cuprate Superconductor YBa2Cu3O6.9 by High-Pressure NMR

NMR shift and relaxation measurements in pulsed high-field magnets up to 58 T

Charge inhomogeneities in the cuprates were reported early on and have been in the focus of much research recently. Nuclear magnetic resonance (NMR) is very sensitive to local charge symmetry through the electric quadrupole interaction that must detect any static charge density variation. Recent experiments in high magnetic fields that seem to induce charge density waves in some systems have rekindled the interest in static inhomogeneities. It has long been known that excessive NMR linewidths can be observed in all cuprates, but with the exception of a few materials. However, the relation of the quadrupolar linewidths with respect to variations of the charge density in the cuprates is not understood. Here, we investigate YBa2Cu3O7 and we find even in a moderate magnetic field that below about 200 K, i.e., well above T
 c, a temperature dependent NMR linewidth appears that must be related to incipient static charge density variations. We argue that this establishes field induced charge density variation as a more general phenomenon in the cuprates. In view of the very recent understanding of the relation between the hole distribution in the CuO2 plane and T
 c, it is argued that charge density variations are ubiquitous, but appear not related to the maximum T
 c.

Steven Reichardt

Papers

Perspective on the phase diagram of cuprate high-temperature superconductors.

High-sensitivity NMR beyond 200,000 atmospheres of pressure.

Bulk Charge Ordering in the CuO2 Plane of the Cuprate Superconductor YBa2Cu3O6.9 by High-Pressure NMR

NMR shift and relaxation measurements in pulsed high-field magnets up to 58 T

Charge Variations in Cuprate Superconductors from Nuclear Magnetic Resonance