Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T6: Focus Session: Iridates and Osmates |
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Sponsoring Units: DMP Chair: Saicharan Aswartham, University of Kentucky Room: 006A |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T6.00001: A tale of three double perovskites: Ba$_{2}$XOsO$_{6}$ (X$=$Na,Ca,Y) Shruba Gangopadhyay, Warren Pickett High valent Os based double perovskites are one center of current interest because they display extreme interplay of large spin orbit coupling and strong electronic correlation. We present electronic and magnetic structures of three cubic Os based double perovskites with Os$^{+7}$~(d$^{1})$, Os$^{+6}$(d$^{2})$, Os$^{+5}$(d$^{3})$. For these first principles based calculation we used an onsite hybrid exchange only on Os(5d), as ~implemented in Wien2k. While Ba$_{2}$NaOsO$_{6}$ is a ferromagnetic Mott insulator, the other two show antiferromagnetic ordering.~For comparison purposes we have investigated only the ferromagnetic ordered phase of these three materials. A metal-insulator transition by changing spin orbit coupling direction is found in all three materials, however each double perovskite is metallic for different magnetic directions. Surprises from looking at the radial charge densities will be discussed. We provide a resolution to the riddle: why, despite d1~configuration, does Ba$_{2}$NaOsO$_{6}$~remain cubic. This material introduces a new class of J$=$ 3/2 Mott insulator. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T6.00002: Trigonal crystal field effect in spin-orbit Mott insulating Sr$_3$NiIrO$_6$ Jun Okamoto, Wen-Bin Wu, Hirofumi Ishii, Kang-Li Yu, Nozomu Hiraoka, Ru-Pan Wang, Deng-Ming Juo, Jiunn Chen, Viveka Singh, Guang-Yu Guo, Qinghui Jiang, Sang-Wook Cheong, Ku-Ding Tsuei, Di-Jing Huang Sr$_3$NiIrO$_6$ is a member of $5d$ transition-metal Ir oxides with a long chain alternating Ni$^{2+}$ in trigonal prismatic coordination and Ir$^{4+}$ in octahedral coordination along $c$ axis. Sr$_3$NiIrO$_6$ is considered to be a spin-orbit Mott insulator like Sr$_2$IrO$_4$. Since the Ir $5d$ spin-orbit coupling (SOC) and crystal field (CF) effect in $t_{2g}$ orbitals from trigonal distortion are comparable, however, competition between the SOC and CF effect makes the Ir $5d$ electronic structures different from the so-called $j_{eff}$ = 1/2 ground state. The modified Ir $5d$ electronic structures under the trigonal crystal field effect are still unclear. We have studied the Ir $5d$ electronic structures of Sr$_3$NiIrO$_6$ by using Ir $L_3$-edge resonant inelastic x-ray scattering and O $K$-edge x-ray absorption spectroscopy measurements. Through the analyses of the low-energy $d$-$d$ excitations and the unoccupied Ir $5d$ states, we discuss the influence of the competition of SOC and CF effects on the Ir $5d$ electronic structures of Sr$_3$NiIrO$_6$. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T6.00003: Magnetism and anisotropy of Ir$^{5+}$ based double perovskites Sr$_{2}$CoIrO$_{6}$ and Sr$_{2}$FeIrO$_{6}$ Jasminka Terzic, S.J. Yuan, W.H. Song, S. Aswartham, G. Cao We report on structural, thermodynamic and transport study of single-crystal double perovskites Sr$_{2}$CoIrO$_{6}$ and Sr$_{2}$FeIrO$_{6}$. The isostructural Sr$_{2}$CoIrO$_{6}$ and Sr$_{2}$FeIrO$_{6}$ feature a cubic crystal structure with pentavalent Ir$^{5+}$(5d$^{4})$ which are anticipated to have J$=$0 singlet ground states in the strong spin-orbit coupling limit. Here we observe magnetic coupling between 5d and 3d (Co, Fe) elements, which result in antiferromagnetic order at high temperatures in both double perovskites. Of the two, Sr$_{2}$CoIrO$_{6}$ displays antiferromagnetic metallic behavior with a pronounced magnetic anisotropy; in sharp contrast, the isostructural Sr$_{2}$FeIrO$_{6}$ exhibits an antiferroamagnetic, insulating ground state without discernible magnetic anisotropy. The data will be discussed and presented with comparisons drawn with similar systems. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:27PM |
T6.00004: Spin-orbit coupling, magnetic anisotropy and hard magnetism in Sr$_{3}$NiIrO$_{6}$ Invited Speaker: Vivien Zapf Strong spin-orbit coupling is a pre-requisite for hard magnetism with high coercive magnetic fields. Magnetic oxides containing 5d ions such as Ir$^{4+}$ should show significant spin-orbit coupling due to the high Z value. Furthermore, in 5d ions, the comparable energy scales of crystal-electric field splitting, Coulomb repulsion and spin-orbit interactions create unusual electronic ground states that can entangle spins and orbits, mix t$_{2g}$ and e$_{g}$ levels and drive magnetic exchange anisotropy. Another set of interesting electronic ground states can arise when 5d orbitals overlap 3d orbitals. In the compound Sr$_{3}$NiIrO$_{6}$, electronic structure calculations predict that the 3d orbitals of the Ni$^{2+}$ ion directly overlap 5d orbitals of the Ir$^{4+}$ ion. In addition to a ``Jeff $=$ 1/2'' Ir$^{4+}$ ground state that mixes t$_{2g}$ and e$_{g}$ levels, the Ni$^{2+}$ should show strong single-ion anisotropy [1-3]. We present magnetization measurements of Sr$_{3}$NiIrO$_{6}$ to high magnetic fields. [4] We demonstrate magnetic hysteresis with a record 55 Tesla coercive magnetic field and long stability over time in some crystals. More generally, the A$_{3}$BB'O$_{6}$ family of compounds shows hard magnetism as B' ion goes from 3d to 4d to 5d. Further complexities to do with evolving magnetic order and magnetic frustration also present in this family. \\[4pt] [1] G. R. Zhang, X.L. Zhang, T. Jia, Z. Zeng, and H. Q. Lin J. Appl. Phys. 107 09E120 (2010)\\[0pt] [2] S. Sarkar, S. Kanungo and T. Saha-Dasgupta, Phys. Rev. B 82, 235122 (2010)\\[0pt] [3] X. Ou and H. Wu, arXiv:1312.7411\\[0pt] [4] J. Singleton, J. W. Kim, C. V. Topping, A. Hansen, E.-D. Mun, S. Ghannadzadeh, P. Goddard, X. Luo, Y. S. Oh, S.-W. Cheong and V. S. Zapf, arXiv:1408.0758v1 [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T6.00005: Novel Magnetic and Charge Orders in Dimer-Chain Iridate Ba$_{5}$AlIr$_{2}$O$_{11}$ Feng Ye, J. Terzic, J.C. Wang, W.H. Song, S.J. Yuan, S. Aswartham, G. Cao We report a novel magnetic state coexisting with a charge ordering state in a dimer-chain system Ba$_{5}$AlIr$_{2}$O$_{11}$. This newly synthesized single-crystal iridate features both tetravalent Ir$^{4+}$ and pentavalent Ir$^{5+}$ ions in each of dimers that are only linked via AlO$_{4}$-tetrahedra along the b-axis. Despite the evident one-dimensional characteristic, the dimer-chains undergo an unexpected long-rang order at T$_{M} =$ 4.5 K with a large magnetic anisotropy. The magnetic transition is unusually resilient to magnetic field up to 14 T but more susceptible to even modest hydrostatic pressure up to 10 kbar. Furthermore, a subtle structural change discerned at T$_{S}=$200 K marks a charge ordering that accompanies a huge enhancement in the dielectric constant and changes in the electrical resistivity. It is evident that the strong SOC imposes a j$=$1/2 (Ir$^{4+})$ and singlet j$=$0 (Ir$^{5+})$ states in each dimer, which critically hinges on the orbital and lattice degrees of freedom. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T6.00006: Giant spin-phonon coupling in a 5d NaOsO3 Stuart Calder, Jun Hee Lee, Mathew Stone, Mark Lumsden, Jonathan Lang, Mikhail Feygenson, Youguo Shi, Ying Sun, Yoshihiro Tsugimoto, Kazunari Yamaura, Andrew Christianson The coupling of distinct properties offers avenues to multifunctional materials. A limiting factor, however, is the degree that one parameter has to be modified to sufficiently alter the coupled property. Through a neutron scattering and first-principles density functional theory study of the 5d perovskite NaOsO$_3$ we reveal that from only a 0.1\% lattice change an unprecedentedly large coupling emerges. The manifestation is a ``giant'' spin-phonon coupled mode shift of $\Delta \omega$=40 cm$^{-1}$, the largest observed in any material. By identifying the dominant phonon as the octahedral breathing mode we show isosymmetric ordering and cooperation between the lattice and the exotic magnetically driven Slater metal-insulator transition in this material. The occurrence of the dramatic spin-phonon-electronic coupling in NaOsO$_3$ is due to a property common to all 5d materials: the large spatial extent of the 5d ion. Consequently examining 5d materials in a new light offers novel routes for multifunctional devices with enhanced coupled phenomena. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:27PM |
T6.00007: Metal-insulator transitions in effective J=1/2 insulating iridates Invited Speaker: Hongbin Zhang The competition between spin-orbit coupling, crystal field splitting and electron correlations with comparable magnitude gives rise to many interesting phenomena. For instance, the so-called effective $J\!=\!1/2$ state has been observed in many iridates compounds, {\it e.g.}, Ruddlesden-Popper (RP) Sr$_{n+1}$Ir$_n$O$_{3n+1}$ and pyrochlore RE$_2$Ir$_2$O$_7$ (RE=Bi, Pr, Nd, Sm, Eu, Y) iridates, where metal-insulator transitions occur driven by the interplay of electron correlations with magnetic ordering. Using first-principles methods, for correlated solids based on density functional theory and dynamical mean field theory (DFT+DMFT), we have investigated the metal-insulator transitions in both classes of iridates. We explore the robustness of the effective $J\!=\!1/2$ state against band effects due to itineracy, structural distortion, and strain. We show how single-particle spectra, optical conductivities, and orbital and spin moments change with strain, and we demonstrate that the ground state can be well characterized in terms of an effective energy-dependent $J\!=\!1/2$ state. For RP compounds, we demonstrate that the crystal field splittings induced by local structural distortions and hybridization are critical to understand previous experimental results. For pyrochlore compounds, the total energies obtained using charge self-consistent DFT+DMFT method reveal that the all-in-all-out magnetic ordering is stable at low temperature in late rare earth pyrochlores, while a bad metallic state is found in early rare earth pyrochlores, in agreement with experiments. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T6.00008: Unquenched $e_g^1$ orbital moment in the Mott insulating antiferromagnet KOsO$_4$ Kwan-Woo Lee, Young-Joon Song, Kyo-Hoon Ahn, Warren E. Pickett In condensed matter physics, spin-orbit coupling (SOC) has many important consequences, including some recent hot topics such as topological insulators, unconventional metal-insulator transitions, so-called relativistic Mott insulators, large orbital moments, and of course magnetic anisotropy. Whereas SOC in a $t_{2g}$ manifold has been intensively discussed recently, SOC in an $e_g$ manifold has rarely been considered due to the conventional (usually correct) wisdom that an orbital moment is completely quenched in the $e_g$ subshell. In this presentation, using correlated band theory including SOC, we will address effects of SOC in an $e_g^1$ of KOsO$_4$, which contains OsO$_4$ tetrahedra such that the $e_g$ subshell is partially occupied. In contrast to the conventional wisdom, our results show very definite magnetocrystalline anisotropy and unquenched orbital moments of substantial size in KOsO$_4$ (half that of the Os spin moment). We have analyzed and interpreted the origin of the orbital moment on the basis of a small crystal splitting and symmetry breaking (crystalline, and additionally due to SOC itself). [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T6.00009: Hierarchical stripe phases in IrTe$_2$ driven by competition between Ir dimerization and Te bonding Weida Wu, Jixia Dai, Kristjan Haule, Junjie Yang, Yoon Seok Oh, Sang Wook Cheong Iridium di-telluride (IrTe$_2$) belongs to the family of transition metal dichalcogenides (TMD), but it distinguishes from the traditional TMDs due to the existence of multi-step single-q charge-density wave like phase transitions. Despite of intensive studies, there is still no consensus on the physical origin of the stripe phases or even the ground state modulation for this 5d material. In this study, we present atomic-resolving images and spectroscopic measurements from scanning tunneling microscopy and spectroscopy (STM/STS). We show that the ground state of IrTe$_2$ is a q=1/6 stripe phase, identical to that of the Se-doped compound. Furthermore, our data suggest that the multi-step transitions and the stripe phases are driven by the intralayer Ir-Ir dimerization that competes against the interlayer Te-Te bonding. This competition results in a unified phase diagram with a series of hierarchical modulated stripe phases. [Preview Abstract] |
(Author Not Attending)
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T6.00010: Optical evidence for bonding-antibonding splitting in IrTe$_{2}$ Dipanjan Mazumdar, Kristjan Haule, J.J Yang, G.L. Pascut, B.S. Holinsworth, K.R. O'Neal, Valery Kiryukhin, Sang-Wook Cheong, J.L. Musfeldt We combined optical spectroscopy with first principles calculations to reveal the electronic signatures of Ir dimer formation in the 1/5th phase of IrTe2. Our measurements uncover two interband transitions into the unoccupied dxy anti-bonding orbital, one from mixed Iridium/Tellurium bands, the other from the dxy bonding orbital of the dimerized Ir centers. The bonding-antibonding splitting demonstrates that Iridium, not Tellurium, plays the dominant role in stabilizing the low temperature phase of IrTe2 through localized bonding orbital formation. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T6.00011: Spectroscopic evidence for negative electronic compressibility in a quasi-three-dimensional spin-orbit correlated metal Junfeng He, T. Hogan, Thomas Mion, H. Hafiz, Y. He, J.D. Denlinger, S.-K. Mo, C. Dhital, X. Chen, Qisen Lin, Y. Zhang, M. Hashimoto, H. Pan, D.H. Lu, M. Arita, K. Shimada, R.S. Markiewicz, Z. Wang, K. Kempa, M.J. Naughton, A. Bansil, S.D. Wilson, Rui-Hua He In quantum materials consisting of multiple mutually-coupled subsystems, the effective compressibility of one subsystem can be negative when it is countered by the positive compressibility of other subsystems. Manifestations of such negative compressibility in quantum materials have so far been limited to low-dimensional dilute electron systems. Their origins have been commonly attributed to a dominance of the exchange energy over kinetic energy of electrons. Here we present evidence from ARPES for negative electronic compressibility in a quasi 3D spin-orbit correlated metal, (Sr$_{\mathrm{1-x}}$La$_{\mathrm{x}}$)$_{3}$Ir$_{2}$O$_{7}$, which is driven by a dominance of the correlation energy of electrons at a relatively high density. Increased electron filling results in both an expansion of the electron Fermi pockets and an anomalous decrease of the chemical potential. This anomaly, suggestive of negative electronic compressibility, is made possible by a concomitant rapid lowering in energy of the correlated conduction band on which the chemical potential is defined, unveiling a new band picture of doping Mott insulators. [Preview Abstract] |
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