Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet

Abstract

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.

X-ray magnetic diffraction under high pressure

“…Alternatively, the FWHM of the diffraction lines indicate a lattice coherence length evolving from resolution-limited (>1700 Å ) at 0 GPa to $350 Å at 41 GPa. Similar diffraction line profile evolutions of (1, 1, 1) and (2, 2, 0) are shown in the work by Wang et al (2018). These single-crystal diffraction line profiles provide a critical evaluation of the sample pressure condition and are much more sensitive than other evaluation methods using polycrystalline materials, such as ruby line shapes and Ag diffraction profiles (Feng et al, 2010).…”

“…Thinning the rear of both anvils with wide-angle conical (2 = 60 ) perforations [Fig. 2(b)] provides a further reduction in the total diamond thickness in the X-ray path to 0.9-1.0 mm (Feng et al, 2014(Feng et al, , 2015aWang et al, 2016Wang et al, , 2018.…”

“…Since the first demonstration of resonant X-ray magnetic diffraction three decades ago (Gibbs et al, 1988), this technique has illustrated numerous antiferromagnetic structures at ambient pressure, where neutron magnetic diffraction was difficult because of either strong neutron absorption without isotope enrichment or small single-crystal size. The all-in-allout (AIAO) type of spin order in Os-or Ir-based pyrochlore compounds (such as Cd 2 Os 2 O 7 and R 2 Ir 2 O 7 with R = Eu, Sm and Nd) embodies both difficulties and represents by far one of the most sophisticated spin structures that are studied predominantly by resonant X-ray magnetic diffraction techniques (Yamaura et al, 2012;Sagayama et al, 2013;Donnerer et al, 2016;Wang et al, 2018). Residing locally on the atomic sites of a pyrochlore lattice, the AIAO spin order has all four spins at the corner of each tetrahedron pointing along the local h1, 1, 1i axis either all towards the tetrahedron center or away from it, creating a highly symmetric three-dimensional spin structure that is fully compatible with the space group of the underlying pyrochlore lattice (Bramwell & Harris, 1998).…”

“…X-ray magnetic diffraction at high pressure has become accessible over the past 15 years, with examples of both nonresonant and resonant diffraction. These include studies of antiferromagnetic Cr (Feng et al, 2007(Feng et al, , 2015a, CeFe 2 (Wang et al, 2012), GdSi (Feng et al, 2014), MnP (Wang et al, 2016), CeFe 2 at the Ce L 3 edge (E = 5.720 keV) (Kernavanois et al, 2005;Braithwaite et al, 2006;Paolasini et al, 2007), -Li 2 IrO 3 (albeit without polarization analysis by Breznay et al, 2017) and Sm 2 Ir 2 O 7 (Wang et al, 2019) at the Ir L 3 edge (E = 11.214 keV) and Cd 2 Os 2 O 7 at the Os L 2 edge (E = 12.387 keV) (Wang et al, 2018). Recently, we have extended these techniques to samples pressurized to 40 GPa and beyond at temperatures down to $3.5 K. With fourthgeneration synchrotron radiation sources on the horizon offering enhanced brilliance and total flux (Hettel, 2014), we hope that a detailed description of the techniques of highpressure X-ray magnetic diffraction will promote broader interest and understanding in the community.…”

Magnetic field-temperature phase diagram of multiferroic (NH4)2FeCl5·H2O

“…1,2 These transitions are controlled by external stimuli such as voltage, magnetic field, pressure, and light -quite different from traditional phase transitions that rely upon thermal fluctuations. [3][4][5][6][7][8][9] Molecule-based multiferroics offer several advantages over their oxide counterparts for the study of quantum phase transitions. These include overall low-energy scales, flexible molecular architectures, and ease of chemical substitution.…”