Session H46: 4d/5d Transition Metal Systems -- Pyrochlores

Unconventional spin-phonon coupling in 4d and 5d transition metal oxides

The spin-phonon (SP) coupling has long been a central theme in condensed matter physics. To date, most of these phenomena have been understood by considering the Heisenberg interaction between spins. However, recent studies on 4d/5d transition metal oxides (TMOs) have revealed that strong spin-orbit coupling (SOC) plays the crucial role in their magnetism. Since the orbitals are generically coupled to the lattice, the strong SOC present in 4d/5d orbitals creates strong and inevitable interactions between spin and lattice.
Using optical spectroscopy, we carried out comparative studies on the 5d TMOs. From infrared (IR) phonon spectra, we discovered unconventional SP coupling in Cd₂Os₂O₇ and Y₂Ir₂O₇. For Cd₂Os₂O₇, we observed large phonon frequency shifts below T_N. This is due to the SP coupling arising from single-ion anisotropy [1,2]. We also identified frequency shifts of Y₂Ir₂O₇ near T_N. However, in this case, the Dzyaloshinskii-Moriya interaction plays a crucial role in the phonon renormalization [3]. Using near-IR pump-probe technique, we investigated coherent phonon oscillations of 4d TMO. Recently, Ca₂RuO₄ has been of particular interest due to its exotic magnetic ground state driven by strong SOC. We observed that a coherent A_g phonon oscillation changes abruptly its amplitude and phase as the spin order develops. Density functional theory calculations revealed that the structural deformation by the magnetic order can induce such variations in coherent phonons, which we suggest as a unique signature of SP coupling [4].
Our comprehensive studies on unconventional SP coupling may lead to scientific advancement in 4d/5d TMOs, and possibly endow opening new research fields come across.

[1] C. H. Sohn et al., Phys. Rev. Lett. 118, 117201 (2017).
[2] T. M. H. Nguyen et al., Nat. Commun. 8, 251 (2017).
[3] J. Son et al., in preparation.
[4] M.-C. Lee et al., Phys. Rev. B 98, 161115 (2018).   

AC magnetic susceptibility and heat capacity studies of the geometrically frustrated pyrochlores Ce2Zr2O7 and Ce2Hf2O7, as possible quantum spin liquids (QSL’s)

QSL’s are of great interest due to their unusual ground states; they do not exhibit spin freezing even at 0K, have no conventional order parameter associated with a broken symmetry, and can emerge in insulators containing localized spin degrees of freedom. In this work, single crystals of the low-spin geometrically frustrated 2D lattice pyrochlore compounds Ce₂Zr₂O₇ and Ce₂Hf₂O, prepared by the floating zone method, were investigated for a possible QSL state. Measurements of AC magnetic susceptibility from 100 Hz to 10 kHz and heat capacity up to 9T were performed down to 100 mK. Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between Ce³⁺ ions in Ce₂Zr₂O₇ and Ce₂Hf₂O₇, no evidence for a transition to a long-range ordered state was found, possibly due to strong quantum fluctuations that prevent spin freezing and lead to QSL ground state. With increasing magnetic field, the Schottky heat capacity anomaly moves to higher temperatures, indicating the presence of a gap in the excitation spectrum, which increases monotonically with increasing field. The behavior for a two-level system with an excited level at energy Δ above the ground state is compared with the heat capacity data.   

Time-resolved optical study of the metal-to-insulator transition in Cd2Os2O7

The 5d transition metal oxide Cd₂Os₂O₇ undergoes a continuous metal-to-insulator transition (MIT) with a concomitant magnetic transition into an all-in-all-out (AIAO) antiferromagnetic state below a critical temperature T_MIT = 227 K. Here, we investigate the effects of this transition on its photo-carrier dynamics by performing temperature and wavelength dependent ultrafast optical reflectivity measurements. We show that the opening of a charge gap at T_MIT causes a divergence like behavior of the photo-carrier relaxation time consistent with a Rothwarf-Taylor model. Our efforts to perform these experiments under high pressurize environments will also be discussed, which is motivated by recent resonant x-ray diffraction results on Cd₂Os₂O₇ showing a monotonic suppression of AIAO order with pressure.   

Tuning the ground state of a candidate Weyl semimetal Eu2Ir2O7 by Bi doping

The interplay of spin-orbit coupling (λ) and onsite coulomb interactions (U) in the pyrochlore iridates A₂Ir₂O₇ has been predicted to give rise to a plethora of non-trivial topological phases. Here we investigate the pyrochlore series (Eu_1-xBi_x)₂Ir₂O₇ whose end members are located in two different regions of the λ-U phase space. Eu₂Ir₂O₇ (EIO) shows a metal to insulator (MI) transition at 120 K concomitant with an antiferromagnetic ordering where the Ir moments order in an all-in/all-out(AIAO) structure. EIO is predicted to be a Weyl semimetal; whereas Bi₂Ir₂O₇ (BIO) is a non-Fermi liquid metal with at least two magnetic transitions below 2 K. In this presentation, we show that Bi substitution as high as 3.5% dopes electrons in the system without altering the electronic structure and tunes the Fermi energy with little change in MI/AIAO transitions. However at higher doping levels both these transitions are very strongly suppressed. The sample prepared with 10% Bi doping showed an highly unconventional metallic behaviour and no signs of MI/AIAO ordering down to at least 2 K. For higher doping, the behaviour gradually evolved to that of BIO. Our study exemplifies the appearance of unusual behaviours at the boundary between Weyl semimetal and non-Fermi liquid of the λ-U phase space.   

Magnetic torque in spin-orbit coupled metal Cd2Re2O7

Spin-orbit (SO) interactions lead to peculiar symmetry-broken phases in highly correlated systems. Among various SO coupled metals, a metallic pyrochlore Cd₂Re₂O₇ is known to show unique multipole orders associated with the inversion symmetry breaking below ~200 K, whose order parameter has E_u symmetry. We have measured the magnetic torque of Cd₂Re₂O₇ in a wide temperature and field range. We find that a four-fold symmetry of the magnetic torque signal as well as a two-fold symmetry is significantly enhanced at low temperatures below ~200 K. The symmetry analysis of the torque signal strongly suggests the presence of the primary order parameter (OP) with the even parity E_g symmetry, which induces the four-fold term, whereas the two-fold arises from the odd parity E_u OP. It is very likely that the E_g symmetry OP is purely of electronic origin and the coexistence with the odd-parity E_u OP provides important insights into the origin of the multipole orders induced by the SO coupling.
  

Electronic and magnetic ground states of the Kitaev material candidates $A_2$CeIrO$_6$ ($A$~$=$~Ba,Sr)

Quantum magnets with significant bond-directional Ising interactions, so-called Kitaev materials, have attracted tremendous attention recently in the search for exotic spin liquid states. Here we present a comprehensive set of measurements that enables us to elucidate the crystal structures, Ir$^{4+}$ single ion properties, and magnetic ground states of the double perovskite iridates $A_2$CeIrO$_6$ ($A$~$=$~Ba,Sr). The crystal structures of Ba$_2$CeIrO$_6$ and Sr$_2$CeIrO$_6$ are found to be cubic and monoclinic respectively, with a large nearest neighbor distance $>$~5~\AA~between Ir$^{4+}$ ions. X-ray absorption spectroscopy and resonant inelastic x-ray scattering were used to establish $J_{eff}$~$=$~1/2 spin-orbit-assisted Mott insulating states, while bulk characterization and neutron powder diffraction identify A-type antiferromagnetic order. These electronic and magnetic ground states are both consistent with expectations for face-centered-cubic magnets with dominant Kitaev exchange, which indicates that spacing magnetic ions far apart may be a promising design principle for uncovering new Kitaev materials.   

Global Metal-Insulator Transition in the Pyrochlore Iridates (Nd1-xCax)2Ir2O7

We report on the suppression of the metal-insulator transition (MIT) in polycrystalline samples of Nd₂Ir₂O₇ under substitution of Nd for Ca. Each calcium ion effectively dopes one hole on an Ir⁴⁺ (5d⁴, j_eff=1/2) site, mediating a filling-controlled Mott-like transition. Ca substitution also changes bandwidth by modulating Ir-O bond lengths and angles, without altering site symmetry. Local structure and absorption spectroscopy confirm that Ca substitution does not result in phase separation or large changes to Ir electronic configuration. The MIT has a coincident antiferromagnetic transition on the Ir sublattice (T_N = T_MIT) for all measured samples, which both decrease with Ca content. Weak low-temperature upturns in susceptibility and resistivity for samples with high Ca content suggest that Nd sublattice magnetism couples to carriers in the metallic regime.   

Enhanced Thermopower in Hole-doped Pyrochlore Iridates: Correlated Metal with Quadratic Band Touching

Quadratic band touching (QBT) is a band structure in which the parabolic valence and conduction band touch at a point, protected by spatial and time inversion symmetry. The possibility that QBT becomes the source of exotic electronic phases such as Weyl semimetal has been discussed. The existence of QBT has been confirmed in Pr₂Ir₂O₇ (PIO), which is a member of pyrochlore-type iridate R₂Ir₂O₇ (RIO, R=Rare earth). Since QBT in PIO is stabilized by the crystal symmetry, we can consider the possibility of the universal existence of QBT in the paramagnetic metallic phase of RIO. Therefore, we made hole-doped samples of insulator Eu₂Ir₂O₇ to compare its electronic properties with PIO.
Using the charge transport measurement and ab-initio calculation, we have explored the thermopower and electronic structure of correlated metal with QBT in hole-doped pyrochlore iridates. We found that the hole-doped QBT yields a thermopower peak, which can be enhanced exceeding 40 V/K below 50 K by tuning the band-filling and lattice distortion. The thermopower peak is observed even in the doping induced metallic phase created from Mott insulator, indicating that the QBT is robust against the strong electron correlation as well as the lattice distortion while keeping the cubic symmetry.   

Stoichiometry Control, Electronic and Transport Studies of Pyrochlore Iridate Thin Films

Pyrochlore iridate thin films are predicted to host a variety of exotic magnetic and topological states. Experimental probing of the emergent quantum states strongly relies on the synthesis of high-quality thin film samples. In this work, we demonstrate the epitaxial growth and stoichiometry control of pyrochlore Bi₂Ir₂O₇ thin films via pulsed laser deposition followed by a post annealing process. The as-deposited films form a bilayer-like structure consisting of an Ir-deficient top-surface and an Ir metal layer at the bottom. Post annealing in iridium oxide atmosphere significantly improves the stoichiometry and homogeneity throughout the film thickness. Density functional theory calculation shows a fourfold degenerate Dirac node slightly below the Fermi energy at the X point, coexisting with some trivial bands around the Γ point. Transport studies revealed a transition from a weakly-metallic to weakly-insulating behavior at around 150 K, accompanied by positive magnetoresistance at low temperatures.   

Mapping the magnetic and electronic structure of (Eu1-xCa1-x)2Ir2O7 across a global metal-insulator transition

Motivated by the prediction of quantum critical behavior with the suppression of the temperature dependent metal-insulator transition in Ln₂Ir₂O₇ (Ln = Y, lanthanide), including the formation of a topological semi-metal phase^1,2, we present characterization of polycrystalline samples for the series (Eu_1-xCa_1-x)₂Ir₂O₇. Doping completely suppresses the MIT with only a weak decrease in the magnetic transition, leading to a fully metallic state that retains the bulk magnetism of the x = 0 compound. XRD and XAS suggest that the change of T_MIT occurs due to an interplay between bandwidth tuning via a change in the Ir-O-Ir bond angle as well as filling control. X-ray PDF shows no changes in short-range order associated with either the MIT or the magnetic transition, additionally confirming that Ca-doping does not result in significant phase separation. Preliminary XMCD data probing the magnetic state across the MIT will be discussed.”_.
1. L. Savary et al. Phys. Rev. X 4, (2014)
2. X. Wan et al. Phys. Rev. B 83, (2011)