Pressure effects on the structural and superconducting transitions in La₃Co₄Sn₁₃

La3Co4Sn13 is a superconducting material with transition temperature at Tc = 2.70 K, which presents a superlattice structural transition at T ∗ ≃ 150 K, a common feature for this class of compounds. However, for this material, it is not clear that at T ∗ the lattice distortions arise from a charge density wave (CDW) or from a distinct microscopic origin. Interestingly, it has been suggested in isostructural non-magnetic intermetallic compounds that T ∗ can be suppressed to zero temperature, by combining chemical and external pressure, and a quantum critical point is argued to be observed near these critical doping/pressure. Our study shows that application of pressure on single-crystalline La3Co4Sn13 enhances Tc and decreases T ∗ . We observe thermal hysteresis loops for cooling/heating cycles around T ∗ for P & 0.6 GPa, in electrical resistivity measurements, which are not seen in x-ray diffraction data. The hysteresis in electrical measurements may be due to the pinning of the CDW phase to impurities/defects, while the superlattice structural transition maintains its ambient pressure second-order transition nature under pressure. From our experiments we estimate that T ∗ vanishes at around 5.5 GPa, though no quantum critical behavior is observed up to 2.53 GPa

Pressure tuning of bond-directional exchange interactions and magnetic frustration in the hyperhoneycomb iridate β−Li₂IrO₃

We explore the response of Ir 5 d orbitals to pressure in β − Li 2 IrO 3 , a hyperhoneycomb iridate in proximity to a Kitaev quantum spin-liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit interaction in 5 d orbitals, pushes β − Li 2 IrO 3 further away from the pure J eff = 1 / 2 limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to the realization of a QSL state in this hyperhoneycomb iridate at high pressures

Pressure-induced amorphization and collapse of magnetic order in the type-I clathrate Eu8Ga16Ge30

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We investigate the low temperature structural and electronic properties of the type-I clathrate Eu8Ga16Ge30 under pressure using x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD) techniques. The XRD measurements reveal a transition to an amorphous phase above 18 GPa. Unlike previous reports on other clathrate compounds, no volume collapse is observed prior to the crystalline-amorphous phase transition which takes place when the unit cell volume is reduced to 81% of its ambient pressure value. Fits of the pressure-dependent relative volume to a Murnaghan equation of state yield a bulk modulus B-0 = 65 +/- 3 GPa and a pressure derivative B '(0) = 3.3 +/- 0.5. The Eu L2-edge XMCD data shows quenching of the magnetic order at a pressure coincident with the crystalline-amorphous phase transition. This information along with the persistence of an Eu2+ valence state observed in the XANES spectra up to the highest pressure point (22 GPa) indicates that the suppression of XMCD intensity is due to the loss of long range magnetic order. When compared with other clathrates, the results point to the importance of guest ion-cage interactions in determining the mechanical stability of the framework structure and the critical pressure for amorphization. Finally, the crystalline structure is not found to recover after pressure release, resulting in an amorphous material that is at least metastable at ambient pressure and temperature.8814Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC-02-06CH11357]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [2009/10264-0, 2011/24166-0, 2012/10675-2, 2012/17562-9]US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC-02-06CH11357

Different routes to pressure-induced volume collapse transitions in gadolinium and terbium metals

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The sudden decrease in molar volume exhibited by most lanthanides under high pressure is often attributed to changes in the degree of localization of their 4f electrons. We give evidence, based on electrical resistivity measurements of dilute Y(Gd) and Y(Tb) alloys to 120 GPa, that the volume collapse transitions in Gd and Tb metals have different origins, despite their being neighbors in the periodic table. Remarkably, the change under pressure in the magnetic state of isolated Pr or Tb impurity ions in the nonmagnetic Y host appears to closely mirror corresponding changes in pure Pr or Tb metals. The collapse in Tb appears to be driven by an enhanced negative exchange interaction between 4f and conduction electrons under pressure (Kondo resonance) which, in the case of Y(Tb), dramatically alters the superconducting properties of the Y host, much like previously found for Y(Pr). In Gd, our resistivity measurements suggest that a Kondo resonance is not the main driver for its volume collapse. X-ray absorption and emission spectroscopies clearly show that 4f local moments remain largely intact across both volume collapse transitions ruling out 4f band formation (delocalization) and valence transition models as possible drivers. The results highlight the richness of behavior behind the volume collapse transition in lanthanides and demonstrate the stability of the 4f level against band formation to extreme pressure.8824National Science Foundation [DMR-1104742]Carnegie/DOE Alliance Center (CDAC) through NNSA/DOE [DE-FC52-08NA28554]US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences [DE-AC-02-06CH11357]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)National Science Foundation [DMR-1104742]Carnegie/DOE Alliance Center (CDAC) through NNSA/DOE [DE-FC52-08NA28554]US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences [DE-AC-02-06CH11357]FAPESP [2011/24166-0

Possible quantum fluctuations in the vicinity of the quantum critical point of (Sr,Ca)3Ir4Sn13 revealed by high-energy x-ray diffraction

We explore the evolution of the structural phase transition of ( Sr , Ca ) 3 Ir 4 Sn 13 , a model system to study the interplay between structural quantum criticality and superconductivity, by means of high-energy x-ray diffraction measurements at high pressures and low temperatures. Our results confirm a rapid suppression of the superlattice transition temperature T ∗ against pressure, which extrapolates to zero at a critical pressure of ≈ 1.79 ( 4 ) GPa . The temperature evolution of the superlattice Bragg peak in Ca 3 Ir 4 Sn 13 reveals a drastic decrease of the intensity and an increase of the linewidth when T → 0 K and p → p c . Such anomaly is likely associated with the emergence of quantum fluctuations that disrupt the formation of long-range superlattice modulation. The revisited temperature-pressure phase diagram of ( Sr , Ca ) 3 Ir 4 Sn 13 thus highlights the intertwined nature of the distinct order parameters present in this system and demonstrates some similarities between this family and the unconventional superconductors

Physical properties and magnetic structure of the intermetallic CeCuBi2 compound

In this work we combine magnetization, pressure dependent electrical resistivity, heat capacity, Cu63 nuclear magnetic resonance (NMR), and x-ray resonant magnetic scattering experiments to investigate the physical properties of the intermetallic CeCuBi2 compound. Our single crystals show an antiferromagnetic ordering at TN≃16 K and the magnetic properties indicate that this compound is an Ising antiferromagnet. In particular, the low temperature magnetization data revealed a spin-flop transition at T=5 K when magnetic fields of about 5.5 T are applied along the c axis. Moreover, the x-ray magnetic diffraction data below TN revealed a commensurate antiferromagnetic structure with propagation wave vector (0012) with the Ce3+ moments oriented along the c axis. Furthermore, our heat capacity, pressure dependent resistivity, and temperature dependent Cu63 NMR data suggest that CeCuBi2 exhibits a weak heavy fermion behavior with strongly localized Ce3+ 4f electrons. We thus discuss a scenario in which both the anisotropic magnetic interactions between the Ce3+ ions and the tetragonal crystalline electric field effects are taking into account in CeCuBi2