$\rm Sr_2IrO_4$ is an archetypal spin-orbit-coupled Mott insulator and has been extensively studied in part because of a wide range of predicted novel states. Limited experimental characterization of these states thus far brings to light the extraordinary susceptibility of the physical properties to the lattice, particularly, the Ir-O-Ir bond angle. Here, we report a newly observed microscopic rotation of the IrO$_6$ octahedra below 50~K measured by single crystal neutron diffraction. This sharp lattice anomaly provides keys to understanding the anomalous low-temperature physics and a direct confirmation of a crucial role that the Ir-O-Ir bond angle plays in determining the ground state. Indeed, as also demonstrated in this study, applied electric current readily weakens the antiferromagnetic order via the straightening of the Ir-O-Ir bond angle, highlighting that even slight change in the local structure can disproportionately affect the physical properties in the spin-orbit-coupled system.
The similarities to cuprates make iridates an interesting potential platform for investigating superconductivity. Equally attractive are their puzzling complex intrinsic interactions. Here, we report an ultrafast optical spectroscopy investigation of a coherent phonon mode in Sr3Ir2O7, a bilayer Ruddlesden-Popper perovskite iridate. An anomaly in the A1g optical phonon (ν = 4.4 THz) is unambiguously observed below the Néel temperature (TN), which we attribute to pseudospin-lattice coupling (PLC). Significantly, we find that PLC is the dominant interaction at low temperature, and we directly me…
Iridates are characterized by a fascinating interplay of spin-orbit and electron-electron interactions. The honeycomb-lattice materials A2IrO3 (A=Na,Li) have been proposed to realize pseudospin-1/2 Mott insulating states with strongly anisotropic exchange interactions, described by the Heisenberg-Kitaev model, but other scenarios involving longer-range exchange interactions or more delocalized electrons have been put forward as well. Here we study the influence of non-magnetic doping, i.e., depleted moments, on the magnetic properties of experimentally relevant variants of the Heisenberg-Kitae…
Hyperkagome iridate, Na$_4$Ir$_3$O$_8$, has been regarded as a promising candidate material for a three-dimensional quantum spin liquid. Here the three-dimensional network of corner-sharing triangles forms the hyperkagome lattice of Ir$^{4+}$ ions. Due to strong spin-orbit coupling, the local moments of Ir$^{4+}$ ions are described by the pseudospin $j_{\rm eff} = 1/2$ Kramers doublet. The Heisenberg model on this lattice is highly frustrated and quantum/classical versions have been studied in earlier literature. In this work, we derive a generic local-moment model beyond the Heisenberg limit …
Here, we report an investigation of structural, magnetic and electronic properties in Y-doped layered iridate (Sr$_{1-x}$Y$_x$)$_2$IrO$_4$ ($x$ $\leq$ 0.1). The parent Sr$_2$IrO$_4$ is a well-studied spin-orbit coupling (SOC) induced insulator with an antiferromagnetic ground state. The Y-doping here equivalently acts for electron doping without altering the vital parameters such as, SOC and electron correlation. Experimental results show a minute change in structural parameters and an equivalent charge conversion from Ir$^{4+}$ to Ir$^{3+}$. Unlike similarly other electron-doped system, the l…
We have used Raman scattering to investigate the magnetic excitations and lattice dynamics in the prototypical spin-orbit Mott insulators Sr2IrO4 and Sr3Ir2O7. Both compounds exhibit pronounced two-magnon Raman scattering features with different energies, lineshapes, and temperature dependencies, which in part reflect the different influence of long-range frustrating exchange interactions. Additionally, we find strong Fano asymmetries in the lineshapes of low-energy phonon modes in both compounds, which disappear upon cooling below the antiferromagnetic ordering temperatures. These unusual pho…
Iridates provide a fertile ground to investigate correlated electrons in the presence of strong spin-orbit coupling. Bringing these systems to the proximity of a metal-insulator quantum phase transition is a challenge that must be met to access quantum critical fluctuations with charge and spin-orbital degrees of freedom. Here, electrical transport and Raman scattering measurements provide evidence that a metal-insulator quantum critical point is effectively reached in 5 % Co-doped Sr$_2$IrO$_4$ with high structural quality. The dc-electrical conductivity shows a linear temperature dependence …
We report results of our study of a newly synthesized honeycomb iridate NaxIrO3 (0.60 < x < 0.80). Single-crystal NaxIrO3 adopts a honeycomb lattice noticeably without distortions and stacking disorder inherently existent in its sister compound Na2IrO3. The oxidation state of the Ir ion is a mixed valence state resulting from a majority Ir5+(5d4) ion and a minority Ir6+(5d3) ion. NaxIrO3 is a Mott insulator likely with a predominant pseudospin = 1 state. It exhibits an effective moment of 1.1 Bohr Magneton/Ir and a Curie-Weiss temperature of -19 K but with no discernable long-range order above…
We have studied the magnetic excitations of electron-doped Sr$_{2-x}$La$_x$IrO$_4$ ($0 \leq x \leq 0.10$) using resonant inelastic x-ray scattering (RIXS) at the Ir L$_3$-edge. The long range magnetic order is rapidly lost with increasing $x$, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interact…
The goal of this work is studying the evolution of thermoelectric transport across the members of the Ruddlesden-Popper series iridates Srn+1IrnO3n+1, where a metal-insulator transition driven by bandwidth change occurs, from the strongly insulating Sr2IrO4 to the metallic non Fermi liquid behavior of SrIrO3. Sr2IrO4 (n=1), Sr3Ir2O7 (n=2) and SrIrO3 (n=inf.) polycrystals are synthesized at high pressure and characterized by structural, magnetic, electric and thermoelectric transport analyses. We find a complex thermoelectric phenomenology in the three compounds. Thermal diffusion of charge car…
Effects of spin-orbit interactions in condensed matter are an important and rapidly evolving topic. Strong competition between spin-orbit, on-site Coulomb and crystalline electric field interactions in iridates drives exotic quantum states that are unique to this group of materials. In particular, the Jeff = 1/2 Mott state served as an early signal that the combined effect of strong spin-orbit and Coulomb interactions in iridates has unique, intriguing consequences. In this Key Issues Review, we survey some current experimental studies of iridates. In essence, these materials tend to defy conv…
A burning question in the emerging field of spin-orbit driven insulating iridates, such as Na2IrO3 and Li2IrO3 is whether the observed insulating state should be classified as a Mott-Hubbard insulator derived from a half-filled relativistic j_eff=1/2 band or as a band insulator where the gap is assisted by spin-orbit interaction, or Coulomb correlations, or both. The difference between these two interpretations is that only for the former, strong spin-orbit coupling (lambda >~ W, where W is the band width) is essential. We have synthesized the isostructural and isoelectronic Li2RhO3 and report…
The high brilliance of the modern synchrotron radiation sources facilitates experiments with high energy x-rays. In this Letter we report on Nuclear Resonance Scattering at the 73 keV nuclear level in 193Ir. The transitions between the hyperfine split levels show an exceptionally large E2/M1 multi-polarity mixing ratio combined with an increased sensitivity to certain changes in the hyperfine field direction compared to non-mixing transitions. The method opens a new way for probing local magnetic and electronic properties of correlated materials containing iridium and provides novel insights i…
Pyrochlore iridates have recently attracted growing interest in condensed matter physics because of their potential for realizing new topological states. In order to achieve such quantum states, it is essential to understand themagnetic properties of these compounds, as their electronic structures are strongly coupled with theirmagnetic ground states. In this work, we report a systematic study of the magnetic properties of pyrochlore Y2Ir2O7 and its hole-doped compounds by performing magnetic, electron spin resonance, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurement…
Spin-orbit entangled pseudospins hold promise for a wide array of exotic magnetism ranging from a Heisenberg antiferromagnet to a Kitaev spin liquid depending on the lattice and bonding geometry, but many of the host materials suffer from lattice distortions and deviate from idealized models in part due to inherent strong pseudospin-lattice coupling. Here, we report on the synthesis of a magnetic superlattice comprising the single ($n$=1) and the double ($n$=2) layer members of the Ruddlesden-Popper series iridates Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ alternating along the $c$-axis, and provide a comp…
Frustration in magnetic systems is fertile ground for complex behaviour, including unconventional ground states with emergent symmetries, topological properties, and exotic excitations. A canonical example is the emergence of magnetic-charge-carrying quasiparticles in spin-ice compounds. Despite extensive work, a reliable experimental indicator of the density of these magnetic monopoles in spin-ice systems is yet to be found. Here, using measurements on single crystals of Ho$_{2}$Ir$_{2}$O$_{7}$ in combination with dipolar Monte Carlo simulations, we show that the magnetoresistance is highly s…
The hidden ordered state of the frustrated pyrochlore oxide Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ is possibly one of the two electric multipolar, or quadrupolar, states of the effective pseudospin-1/2 Hamiltonian derived from crystal-field ground state doublets of non-Kramers Tb$^{3+}$ ions. These long-range orders are antiparallel or parallel alignments of transverse pseudospin components representing electric quadrupole moments, which cannot be observed as magnetic Bragg reflections by neutron scattering. However pseudospin waves of these states are composite waves of the magnetic-dipole and electri…
We have investigated the effect of Cu substitution in Eu2(Ir1-xCux)2O7 with the help of magnetic and transport property measurements. XPS measurement reveals that each Cu2+ converts Ir4+ to double amount of Ir5+ ions. The metal-insulator transition temperature (T_MI) is obtained around 120 K. In the insulating phase, at lower temperature below 50 K, the temperature dependent resistivity follows a power law dependence and the magnitude of the exponent increases with Cu concentrations. The temperature dependent thermopower is observed to follow the electrical resistivity down to 50 K, except for…
Spin-orbit coupling (SOC) plays a crucial role in magnetic and electronic properties of 5$d$ iridates. In this paper we have experimentally investigated the structural and physical properties of a series of Ir-based double perovskite compounds Pr$_{2-x}$Sr$_x$MgIrO$_6$ ($x$ = 0, 0.5, 1; hereafter abbreviated as PMIO, PSMIO1505, and PSMIO). Interestingly, these compounds have recently been proposed to undergo a transition from the spin-orbit-coupled Mott insulating phase at $x$ = 0 to the elusive half-metallic antiferromagnetic (HMAFM) state with Sr doping at $x$ = 1. However, our detailed magn…
We report the structure, magnetic, and thermal property measurements on single crystalline and polycrystalline samples of Ru substituted honeycomb lattice iridate Na$_2$Ir$_x$Ru$_{1-x}$O$_3$ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5). The evolution of magnetism in Na$_2$Ir$_x$Ru$_{1-x}$O$_3$ has been studied using dc and ac magnetic susceptibility, and heat capacity measurements. The parent compound Na$_2$IrO$_3$ is a spin-orbit driven Mott insulator with magnetic order of reduced moments below $T_N = 15$~K\@. In the Ru substituted samples the antiferromagnetic long range state is replaced by a s…
We have measured the doping and temperature dependence of lattice vibrations and magnetic excitations in the prototypical doped spin-orbit Mott insulator Sr$_{2-x}$La$_x$IrO$_4$ (x=0, 0.015, and 0.10). Our findings show that the pseudospin-lattice coupling -- responsible for the renormalization of several low energy phonon modes -- is preserved even when long-range magnetic order is suppressed by doping. In our most highly doped sample, the single magnon (Gamma-point) excitation disappears while the two-magnon mode softens and becomes heavily damped. Doping induced electron-phonon coupling is …
Tuning of spin-orbit coupling and electron correlation effects in pyrochlore iridates is considered for many interesting phenomena. We have investigated the temperature evolution of structural, magnetic and electronic properties in doped Y$_2$(Ir$_{1-x}$Ti$_{x}$)$_2$O$_7$ ($x$ = 0.0, 0.02, 0.05, 0.10 and 0.15) where the substitution of nonmagnetic Ti$^{4+}$ (3$d^0$) for Ir$^{4+}$ (5$d^5$) amounts to dilution of magnetic network and tuning of these parameters in opposite way. The system retains its original structural symmetry but local structural parameters show an evolution with Ti content. W…
Giant Magnetocrystalline Anisotropy in Honeycomb Iridate NiIrO3 with Large Coercive Field Exceeding 17 T
School of Chemistry, Sun Yat-Sen University（中山大学化学学院）；School of Chemistry and Environment, Jiaying University（嘉应学院化学与环境学院）；School of Physics, Sun Yat-Sen University（中山大学物理学院）；School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；School of Chemistry and Chemical Engineering, Hainan University（海南大学化学化工学院）
A rare combination of strong spin-orbit coupling and electron-electron correlations makes the iridate Mott insulator Sr$_2$IrO$_4$ a promising host for novel electronic phases of matter. The resemblance of its crystallographic, magnetic and electronic structures to La$_2$CuO$_4$, as well as the emergence upon doping of a pseudogap region and a low temperature $d$-wave gap, has particularly strengthened analogies to cuprate high-$T_c$ superconductors. However, unlike the cuprate phase diagram that features a plethora of broken symmetry phases in a pseudogap region that include charge density wa…
The ground states of the frustrated pyrochlore oxide Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ have been studied by inelastic neutron scattering experiments. Three single-crystal samples are investigated; one shows no phase transition ($x=-0.007<x_{\text{c}}\sim -0.0025$), being a putative quantum spin-liquid (QSL), and the other two ($x=0.000, 0.003$) show electric quadrupole ordering (QO) below $T_{\text{c}} \sim 0.5$ K. The QSL sample shows continuum excitation spectra with an energy scale 0.1 meV as well as energy-resolution limited (nominally) elastic scattering. As $x$ is increased, pseudospin wave …
Optical conductivity of the metallic pyrochlore iridate Pr$_2$Ir$_2$O$_7$: Influence of spin-orbit coupling and electronic correlations on the electronic structure
I propose the emergence of the spin-orbital-coupled vector chirality in a non-frustrated Mott insulator with the strong spin-orbit coupling due to $ab$-plane's inversion-symmetry (IS) breaking. I derive the superexchange interactions for a $t_{2g}$-orbital Hubbard model on a square lattice with the strong spin-orbit coupling and the IS-breaking-induced hopping integrals, and explain the microscopic origins of the Dzyaloshinsky-Moriya (DM) -type and the Kitaev-type interactions. Then, by adopting the mean-field approximation to a minimal model including only the Heisenberg-type and the DM-type …
We report a systematical structural, transport and magnetic study of Ca or Ba doped Sr2IrO4single crystals. Isoelectronically substituting Ca2+ (up to 15%) or Ba2+ (up to 4%) ion for the Sr2+ ion provides no additional charge carriers but effectively changes the lattice parameters in Sr2IrO4. In particular, 15% Ca doping considerably reduces the c-axis and the unit cell by nearly 0.45% and 1.00 %, respectively. These significant, anisotropic compressions in the lattice parameters conspicuously cause no change in the Néel temperature which remains at 240 K, but drastically reduces the electrica…
We propose moiré bilayer as a platform where exotic quantum phases can be stabilized and electrically detected. Moiré bilayer consists of two separate moiré superlattice layers coupled through the inter-layer Coulomb repulsion. In the small distance limit, an SU(4) spin can be formed by combining layer pseudospin and the real spin. As a concrete example, we study an SU(4) spin model on triangular lattice in the fundamental representation. By tuning a three-site ring exchange term $K\sim \frac{t^3}{U^2}$, we find SU(4) symmetric crystallized phase and an SU(4)$_1$ chiral spin liquid (CSL) at th…
Twisted van der Waals bilayers provide an ideal platform to study the electron correlation in solids. Of particular interest is the 30 degree twisted bilayer honeycomb lattice system, which possesses an incommensurate moire pattern and uncommon electronic behaviors may appear due to the absence of phase coherence. Such system is extremely sensitive to further twist and many intriguing phenomena will occur. In this work, based on first-principles calculations we show that, for further twist near 30 degree, there could induce dramatically different dielectric behaviors of electron between left a…
The Kitaev model is a remarkable spin model with gapped and gapless spin liquid phases, which are potentially realized in iridates and $α$-RuCl$_3$. In the recent experiment of $α$-RuCl$_3$, the signature of a nematic transition to the gapped toric code phase, which breaks the $C_3$ symmetry of the system, has been observed through the angle dependence of the heat capacity. We here propose a mechanism by which the nematic transition can be detected electrically. This is seemingly impossible because $J_\textrm{eff}=1/2$ spins do not have an electric quadrupole moment (EQM). However, in the seco…
We report a new peculiar effect of the interaction between a sublattice of frustrated quantum spin-1/2 chains and a sublattice of pseudospin-1/2 centers (quantum electric dipoles) uniquely co-existing in the complex oxide Li2ZrCuO4. 7Li nuclear magnetic-, Cu2+ electron spin resonance and a complex dielectric constant data reveal that the sublattice of Li+-derived electric dipoles orders glass like at Tg ~ 70 K yielding a spin site nonequivalency in the CuO2 chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom might strongly influence the properties…
Iridate oxides display exotic physical properties that arise from the interplay between a large spin-orbit coupling and electron correlations. Here, we present a comprehensive study of the effects of hydrostatic pressure on the electronic transport properties of SrIrO3 (SIO), a system that has recently attracted a lot of attention as potential correlated Dirac semimetal. Our investigations on untwinned thin films of SIO reveal that the electrical resistivity of this material is intrinsically anisotropic and controlled by the orthorhombic distortion of the perovskite unit cell. These effects pr…
Motivated by recent terahertz (THz) spectroscopy measurements in $α$-RuCl$_3$, we develop a theory for magnetoelectric (ME) effects in Mott insulators of $d^5$ transition metal ions in an octahedral crystal field. For $4d$ and $5d$ compounds, the relatively wide-spread orbitals favor charge fluctuations of localized electrons to neighboring ions and a significant ME effect from electronic mechanisms is expected. From a three-orbital Hubbard model with strong spin-orbit coupling, we derive the mechanisms for the electric polarization originating from virtual hopping of the localized holes carry…
Sr2IrO4 is a magnetic insulator driven by spin-orbit interaction (SOI) whereas the isoelectronic and isostructural Sr2RhO4 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. Our investigation of structural, transport, magnetic and thermal properties reveals that substituting 4d Rh4+ (4d5) ions for 5d Ir4+(5d5) ions in Sr2IrO4 directly reduces the SOI and rebalances the competing energies so profoundly that it generates a rich phase diagram for Sr2Ir1-xRhxO4 featuring two major effects: (1) Light Rh doping (0…
Frustration, spin correlations and interplay between competing degrees of freedom are some of the key ingredients that underlie exotic states with fractional excitations in quantum materials. Rare-earth based two dimensional magnetic lattice wherein crystal electric field, spin-orbit coupling, anisotropy and electron correlation between rare-earth moments offer a new paradigm in this context. Herein, we present crystal structure, magnetic susceptibility and specific heat accompanied by crystal electric field calculations on the polycrystalline sample of Ba3RB9O18 (R = Yb, Er) in which R3+ ions…
Spin orbit assisted Mott insulators such as sodium iridate (Na$_2$IrO$_3$) have been an important subject of study in the recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of Na$_2$IrO$_3$ has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic tra…
