Materials with strong correlations are prone to spin and charge instabilities, driven by Coulomb, magnetic, and lattice interactions. In materials that have significant localized and itinerant spins, it is not obvious which will induce order. We combine electrical transport, X-ray magnetic diffraction, and photoemission studies with band structure calculations to characterize successive antiferromagnetic transitions in GdSi. GdSi has both sizable local moments and a partially nested Fermi surface, without confounding contributions from orbital effects. We identify a route to incommensurate order where neither type of moment dominates, but is rooted in cooperative feedback between them. The nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, whereas the ordered local moments in turn provide the necessary coupling for a spindensity wave to form among the itinerant electrons. This mechanism echoes the cooperative interactions between electrons and ions in charge-density-wave materials, and should be germane across a spectrum of transition-metal and rare-earth intermetallic compounds.itinerant magnetism | RKKY interaction | asymmetric line shape I ncommensurate density waves emerge in a wide variety of correlated electron systems. They are a common aspect in cuprate superconductors (1, 2), itinerant transition-metal magnets (3, 4), and rare-earth compounds (5-7), as well as low-dimensional charge-ordered materials (8, 9) and perovskite manganites (10, 11). In contrast with commensurate density waves, the incommensurate states are often electronically soft (11), and many spin and charge orders in metals are continuously tunable (9, 12). Furthermore, the incommensurate structures are often only weakly coupled to other degrees of freedom in the underlying lattice, giving rise to the rare possibility of direct theoretical modeling of a variety of experimentally accessible material systems, spanning from functional materials of technological importance (10, 11) to fundamental topics of emergent states in quantum critical phenomena (9, 12).Spin states with long-range incommensurate magnetic order may be stabilized by itinerant electrons through two distinct mechanisms. If the Fermi surface has well-nested regions, the itinerant electrons themselves typically become unstable toward the formation of a spin-density wave (SDW) (13). A prominent example of this class of materials is elemental chromium (4,13,14). Alternatively, in the presence of local magnetic moments, the itinerant electrons may form screening clouds which mediate magnetic interactions between the local moments and cause them to order through the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction (5, 15).To form a nesting-driven SDW or charge-density wave (CDW) (13), it is necessary to have a nonzero coupling between itinerant electron states on opposing portions of the nested Fermi surface. In the presence of a perfectly nested Fermi surface, the required coupling strength may be inf...

Pressure can transform a transparent material into an opaque one, quench the moments in a magnet and force solids to flow like liquids. At 15 GPa, the pressure found 500 km below the earth's surface, the semiconductors silicon and germanium superconduct. Yet, at this same pressure, we show here that the magnetism in metallic GdSi remains completely robust even as it shrinks by one-seventh of its volume. Non-resonant X-ray magnetic diffraction in a specially designed diamond anvil cell, combined with band structure calculations, reveal the stability of the incommensurate spin density wave, which can be traced to a persistently nested portion of the Fermi surface that becomes increasingly onedimensional under pressure. A cooperative interaction between nested, itinerant spins and local magnetic moments provides the organizing principle for the modulated magnetic order, salient both for its insights into the role of topology in ordered states and its potential functionality.

Dimensionality and symmetry play deterministic roles in the laws of Nature. They are important tools to characterize and understand quantum phase transitions, especially in the limit of strong correlations between spin, orbit, charge, and structural degrees of freedom. Here, using newly-developed, high-pressure resonant X-ray magnetic and charge diffraction techniques, we have discovered a quantum critical point in Cd2Os2O7 as the all-in-all-out antiferromagnetic order is continuously suppressed to zero temperature and, concomitantly, the cubic lattice structure continuously changes from space group Fd-3m to F-43m. Surrounded by three phases of different time reversal and spatial inversion symmetries, the quantum critical region anchors two phase lines of opposite curvature, with striking departures from a mean-field form at high pressure. As spin fluctuations, lattice breathing modes, and quasiparticle excitations interact in the quantum critical region, we argue that they present the necessary components for strongly-coupled quantum criticality in this three-dimensional compound.

Abstract:The nature of a material's Fermi surface is crucial to understanding its electronic, magnetic, optical, and thermal characteristics. Traditional measurements such as angle resolved photoemission spectroscopy and, de Haas-van Alphen quantum oscillations can be difficult to perform in the vicinity of a pressure-driven quantum phase transition, although the evolution of the Fermi surface may be tied to the emergence of exotic phenomena. We demonstrate here that magnetic x-ray diffraction in combination with Hall effect measurements in a diamond anvil cell can provide valuable insight into the Fermi surface evolution in spin-and charge-density-wave systems near quantum phase transitions. In particular, we track the gradual evolution of the Fermi surface in elemental chromium and delineate the critical pressure and absence of Fermi surface reconstruction at the spin-flip transition.

GdSi exhibits spin-density-wave (SDW) order arising from the cooperative interplay of sizeable local moments and a partially nested Fermi sea of itinerant electrons. Using magnetotransport, magnetization, and nonresonant magnetic x-ray diffraction techniques, we determine the H-T phase diagrams of GdSi for magnetic fields up to 21 T, where antiferromagnetic order is no longer stable, and field directions along each of the three major crystal axes. While the incommensurate magnetic ordering vector that characterizes the SDW is robust under magnetic field, the multiple spin structures of this compound are highly flexible and rotate relative to the applied field via either canting or spin-flop processes. The antiferromagnetic spin densities always arrange themselves transverse to the applied magnetic field direction. The phase diagrams are delineated by two types of phase boundaries: one separates a collinear from a planar spin structure associated with a lattice structural transition, and the other defines a spin flop transition that is only weakly temperature dependent. The major features of the phase diagrams along each of the crystal axes can be explained by the combination of local moment and global Fermi surface physics at play.