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Abstract
Spin ice systems display a variety of very nontrivial properties, the most striking being the existence in them of magnetic monopoles. Such monopole states can also have nontrivial electric properties: there exist electric dipoles attached to each monopole. A novel situation is encountered in the moment fragmentation (MF) state, in which monopoles and antimonopoles are perfectly ordered, whereas spins themselves remain disordered. We show that such partial ordering strongly modifies the electric activity of such systems: the electric dipoles, which are usually random and dynamic, become paired in the MF state in (d, -d) pairs, thus strongly reducing their electric activity. The electric currents existing in systems with noncoplanar spins are also strongly influenced by MF. We also consider modifications in dipole and current patterns in magnetic textures (domain walls, local defects) and at excitations with nontrivial dynamics in a MF state, which show very rich behaviour and which could in principle allow to control them by electric field.
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Affiliation(s)
- D I Khomskii
- II. Physikalisches Institut, Universität zu Köln, Köln, Germany.
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2
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Guo H, Li ZW, Chang CF, Hu Z, Kuo CY, Perring TG, Schmidt W, Piovano A, Schmalzl K, Walker HC, Lin HJ, Chen CT, Blanco-Canosa S, Schlappa J, Schüßler-Langeheine C, Hansmann P, Khomskii DI, Tjeng LH, Komarek AC. Charge disproportionation and nano phase separation in [Formula: see text]. Sci Rep 2020; 10:18012. [PMID: 33093480 PMCID: PMC7582202 DOI: 10.1038/s41598-020-74884-2] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2020] [Accepted: 09/04/2020] [Indexed: 11/29/2022] Open
Abstract
We have successfully grown centimeter-sized layered [Formula: see text] single crystals under high oxygen pressures of 120-150 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter-integer magnetic peaks below [Formula: see text] that are accompanied by steep upwards dispersing spin excitations. Within the high-frequency Ni-O bond stretching phonon dispersion, a softening at the propagation vector for a checkerboard modulation can be observed. We were able to simulate the magnetic excitation spectra using a model that includes two essential ingredients, namely checkerboard charge disproportionation and nano phase separation. The results thus suggest that charge disproportionation is preferred instead of a Jahn-Teller distortion even for this layered [Formula: see text] system.
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Affiliation(s)
- H. Guo
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - Z. W. Li
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
- Institute of Applied Magnetics, Key Lab for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou, 730000 People’s Republic of China
| | - C. F. Chang
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - Z. Hu
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - C.-Y. Kuo
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
- National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu, 30076 Taiwan
| | - T. G. Perring
- ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX UK
| | - W. Schmidt
- Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science at ILL, 71 avenue des Martyrs, 38000 Grenoble, France
| | - A. Piovano
- Institut Laue-Langevin, 71 avenue des Martyrs, 38000 Grenoble, France
| | - K. Schmalzl
- Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science at ILL, 71 avenue des Martyrs, 38000 Grenoble, France
| | - H. C. Walker
- ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX UK
| | - H. J. Lin
- National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu, 30076 Taiwan
| | - C. T. Chen
- National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu, 30076 Taiwan
| | - S. Blanco-Canosa
- IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Basque Country Spain
- Donostia International Physics Center, DIPC, 20018 Donostia-San Sebastian, Basque Country, Spain
| | - J. Schlappa
- European X-ray Free Electron Laser Facility GmbH, Holzkoppel 4, 22869 Schenefeld, Germany
| | - C. Schüßler-Langeheine
- Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany
| | - P. Hansmann
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - D. I. Khomskii
- Physics Institute II, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany
| | - L. H. Tjeng
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - A. C. Komarek
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
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Jenni K, Kunkemöller S, Brüning D, Lorenz T, Sidis Y, Schneidewind A, Nugroho AA, Rosch A, Khomskii DI, Braden M. Interplay of Electronic and Spin Degrees in Ferromagnetic SrRuO_{3}: Anomalous Softening of the Magnon Gap and Stiffness. Phys Rev Lett 2019; 123:017202. [PMID: 31386396 DOI: 10.1103/physrevlett.123.017202] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/11/2019] [Indexed: 06/10/2023]
Abstract
The magnon dispersion of ferromagnetic SrRuO_{3} was studied by inelastic neutron scattering experiments on single crystals as a function of temperature. Even at low temperature the magnon modes exhibit substantial broadening pointing to strong interaction with charge carriers. We find an anomalous temperature dependence of both the magnon gap and the magnon stiffness, which soften upon cooling in the ferromagnetic phase. Both effects trace the temperature dependence of the anomalous Hall effect and can be attributed to the impact of Weyl points, which results in the same relative renormalization in the spin stiffness and magnon gap.
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Affiliation(s)
- K Jenni
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
| | - S Kunkemöller
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
| | - D Brüning
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
| | - T Lorenz
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
| | - Y Sidis
- Laboratoire Léon Brillouin, C.E.A./C.N.R.S., F-91191 Gif-sur-Yvette CEDEX, France
| | - A Schneidewind
- Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstraße 1, 85748 Garching, Germany
| | - A A Nugroho
- Faculty of Mathematics and Natural Science, Institut Teknologi Bandung, Jalan Ganesha 10, 40132 Bandung, Indonesia
| | - A Rosch
- Institut für Theoretische Physik, Universität zu Köln, Zülpicher Str. 77a, D-50937 Köln, Germany
| | - D I Khomskii
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
| | - M Braden
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
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Ushakov AV, Streltsov SV, Khomskii DI. Structural transition in AuAgTe 4 under pressure. J Phys Condens Matter 2019; 31:235601. [PMID: 30818287 DOI: 10.1088/1361-648x/ab0b8a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Gold is inert and forms very few compounds. One of the most interesting of those is calaverite AuTe2, which has incommensurate structure and which becomes superconducting when doped or under pressure. There exist a 'sibling' of AuTe2, the mineral sylvanite AuAgTe4, which properties are almost unknown. In sylvanite Au and Ag ions are ordered in stripes, and Te6 octahedra around metals are distorted in such a way that Ag becomes linearly coordinated, what is typical for Ag1+ , whereas Au is square coordinated-it is typical for d 8 configurations, i.e. one can assign to Au the valence 3+. Our theoretical study shows that at pressure [Formula: see text] GPa there should occur in it a structural transition such that above this critical pressure Te6 octahedra around Au and Ag become regular and practically identical. Simultaneously Te-Te dimers, existing at P = 0 GPa, disappear, and material from a bad metal becomes a usual metal with predominantly Te 5p states at the Fermi energy. We expect that, similar to AuTe2, AuAgTe4 should become superconducting above [Formula: see text].
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Affiliation(s)
- A V Ushakov
- M.N. Miheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences, 620108 Ekaterinburg, Russia
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5
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Revelli A, Moretti Sala M, Monaco G, Becker P, Bohatý L, Hermanns M, Koethe TC, Fröhlich T, Warzanowski P, Lorenz T, Streltsov SV, van Loosdrecht PHM, Khomskii DI, van den Brink J, Grüninger M. Resonant inelastic x-ray incarnation of Young's double-slit experiment. Sci Adv 2019; 5:eaav4020. [PMID: 30746479 PMCID: PMC6357738 DOI: 10.1126/sciadv.aav4020] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/12/2018] [Accepted: 12/14/2018] [Indexed: 06/09/2023]
Abstract
Young's archetypal double-slit experiment forms the basis for modern diffraction techniques: The elastic scattering of waves yields an interference pattern that captures the real-space structure. Here, we report on an inelastic incarnation of Young's experiment and demonstrate that resonant inelastic x-ray scattering (RIXS) measures interference patterns, which reveal the symmetry and character of electronic excited states in the same way as elastic scattering does for the ground state. A prototypical example is provided by the quasi-molecular electronic structure of insulating Ba3CeIr2O9 with structural Ir dimers and strong spin-orbit coupling. The double "slits" in this resonant experiment are the highly localized core levels of the two Ir atoms within a dimer. The clear double-slit-type sinusoidal interference patterns that we observe allow us to characterize the electronic excitations, demonstrating the power of RIXS interferometry to unravel the electronic structure of solids containing, e.g., dimers, trimers, ladders, or other superstructures.
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Affiliation(s)
- A. Revelli
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - M. Moretti Sala
- European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France
- Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
| | - G. Monaco
- Dipartimento di Fisica, Università di Trento, via Sommarive 14, 38123 Povo (TN), Italy
| | - P. Becker
- Abteilung Kristallographie, Institut für Geologie und Mineralogie, Zülpicher Strasse 49b, D-50674 Köln, Germany
| | - L. Bohatý
- Abteilung Kristallographie, Institut für Geologie und Mineralogie, Zülpicher Strasse 49b, D-50674 Köln, Germany
| | - M. Hermanns
- Institut für Theoretische Physik, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
- Department of Physics, University of Gothenburg, SE-412 96 Gothenburg, Sweden
- Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden
- Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden
| | - T. C. Koethe
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - T. Fröhlich
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - P. Warzanowski
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - T. Lorenz
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - S. V. Streltsov
- M.N. Mikheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, 620137 Ekaterinburg, Russia
- Ural Federal University, Mira Street 19, 620002 Ekaterinburg, Russia
| | - P. H. M. van Loosdrecht
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - D. I. Khomskii
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
| | - J. van den Brink
- Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany
| | - M. Grüninger
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
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6
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Kobayashi H, Ikeda S, Yoda Y, Hirao N, Ohishi Y, Alonso JA, Martinez-Lope MJ, Lengsdorf R, Khomskii DI, Abd-Elmeguid MM. Pressure-induced unusual metallic state in EuNiO3. Phys Rev B 2015; 91:195148. [DOI: 10.1103/physrevb.91.195148] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/02/2023]
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Abstract
The modelling of magnetic monopoles in solids is a hot topic nowadays. Here, I propose that in solids with the linear magnetoelectric effect there should exist, close to electric charges, magnetic textures of magnetic monopole type. Their existence can lead to rather striking consequences, such as (magneto)electric Hall effect, magnetophotovoltaic effect and so on, which can be observed experimentally. In addition, in ordinary magnetoelectric materials not only magnetic monopoles can accompany the charge, but also more complicated local magnetic objects can be created, for example, local toroics, which can also lead to unusual effects in transport and other properties of such systems.
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Affiliation(s)
- D I Khomskii
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
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Abstract
Contrary to previous studies that classify Na(2)IrO(3) as a realization of the Heisenberg-Kitaev model with a dominant spin-orbit coupling, we show that this system represents a highly unusual case in which the electronic structure is dominated by the formation of quasimolecular orbitals (QMOs), with substantial quenching of the orbital moments. The QMOs consist of six atomic orbitals on an Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of such QMOs in solids invokes very different physics compared to the models considered previously. Employing density functional theory calculations and model considerations we find that both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na(2)IrO(3) naturally follow from the QMO model.
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Affiliation(s)
- I I Mazin
- Code 6393, Naval Research Laboratory, Washington, DC 20375, USA
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Abstract
The close connection of electricity and magnetism is one of the cornerstones of modern physics. This connection has a crucial role from a fundamental point of view and in practical applications, including spintronics and multiferroic materials. A breakthrough was a recent proposal that in magnetic materials called spin ice the elementary excitations have a magnetic charge and behave as magnetic monopoles. I show that, besides magnetic charge, there should be an electric dipole attached to each magnetic monopole. This opens new possibilities to study and control such monopoles using an electric field. Thus, the electric-magnetic analogy goes even further than usually assumed: whereas electrons have electric charge and magnetic dipole (spin), magnetic monopoles in spin ice, while having magnetic charge, also have an electric dipole.
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Affiliation(s)
- D I Khomskii
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany.
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10
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Toriyama T, Nakao A, Yamaki Y, Nakao H, Murakami Y, Hasegawa K, Isobe M, Ueda Y, Ushakov AV, Khomskii DI, Streltsov SV, Konishi T, Ohta Y. Peierls mechanism of the metal-insulator transition in ferromagnetic hollandite K2Cr8O16. Phys Rev Lett 2011; 107:266402. [PMID: 22243170 DOI: 10.1103/physrevlett.107.266402] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/28/2010] [Revised: 09/27/2011] [Indexed: 05/31/2023]
Abstract
Synchrotron x-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite K(2)Cr(8)O(16) is accompanied by a structural distortion from the tetragonal I4/m to monoclinic P112(1)/a phase with a √2×√2×1 supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the c direction, leading to the formation of tetramers of Cr ions below the transition temperature. This provides a rare example of the Peierls transition of fully spin-polarized electron systems.
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Affiliation(s)
- T Toriyama
- Department of Physics, Chiba University, Chiba 263-8522, Japan
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Ushakov AV, Streltsov SV, Khomskii DI. Crystal field splitting in correlated systems with negative charge-transfer gap. J Phys Condens Matter 2011; 23:445601. [PMID: 22005022 DOI: 10.1088/0953-8984/23/44/445601] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Special features of the crystal field splitting of d-levels in the transition metal compounds with small or negative charge-transfer gaps Δ(CT) are considered. We show that in this case the Coulomb term and the covalent contribution to the t(2g)-e(g) splitting have different signs. In order to check theoretical predictions we carried out ab initio band structure calculations for Cs(2)Au(2)Cl(6), in which the charge-transfer gap is negative, so that the d-electrons predominantly occupy low-lying bonding states. For these states the e(g)-levels lie below the t(2g) ones, which demonstrates that at least in this case the influence of the p-d covalency on the total value of the crystal field splitting is stronger than the Coulomb interaction (which would lead to the opposite level order). We also show that the states in the conduction band are made predominantly of p-states of ligands (Cl), with a small admixture of d-states of Au.
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Affiliation(s)
- A V Ushakov
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany.
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Forthaus MK, Hearne GR, Manyala N, Heyer O, Brand RA, Khomskii DI, Lorenz T, Abd-Elmeguid MM. Pressure-induced quantum phase transition in Fe 1−xCo xSi ( x=0.1,0.2). Phys Rev B 2011; 83:085101. [DOI: 10.1103/physrevb.83.085101] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/02/2023]
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Forthaus MK, Sengupta K, Heyer O, Christensen NE, Svane A, Syassen K, Khomskii DI, Lorenz T, Abd-Elmeguid MM. Superconductivity in SnO: a nonmagnetic analog to Fe-based superconductors? Phys Rev Lett 2010; 105:157001. [PMID: 21230928 DOI: 10.1103/physrevlett.105.157001] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2010] [Indexed: 05/30/2023]
Abstract
We discovered that under pressure SnO with α-PbO structure, the same structure as in many Fe-based superconductors, e.g., β-FeSe, undergoes a transition to a superconducting state for p≳6 GPa with a maximum Tc of 1.4 K at p=9.3 GPa. The pressure dependence of Tc reveals a domelike shape and superconductivity disappears for p≳16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of Tc as a function of pressure.
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Affiliation(s)
- M K Forthaus
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, D-50937 Köln, Germany
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Khomskii DI. Spin chirality and nontrivial charge dynamics in frustrated Mott insulators: spontaneous currents and charge redistribution. J Phys Condens Matter 2010; 22:164209. [PMID: 21386415 DOI: 10.1088/0953-8984/22/16/164209] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
The standard point of view is that at low energies Mott insulators exhibit only magnetic properties, while charge degrees of freedom are frozen out because electrons are localized. We demonstrate (Bulaevskii et al 2008 Phys. Rev. B 78 024402) that in general this is not true: for certain spin textures there exist quite nontrivial charge effects in the ground and lowest excited states. We show that in frustrated systems spontaneous orbital currents may exist in the ground state, proportional to the scalar spin chirality. For other spin structures spontaneous charge redistribution may exist, so that the average charge at a site is different from 1. This can lead to the appearance of dipole moments and possibly of the net spontaneous polarization. This is a novel, purely electronic mechanism of multiferroic behaviour. We also discuss some dynamic consequences, such as dipole-active 'ESR' transitions. Also, the possibility of using chirality instead of spin in memory applications is briefly discussed.
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Affiliation(s)
- D I Khomskii
- II Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
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16
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Blanco-Canosa S, Rivadulla F, Piñeiro A, Pardo V, Baldomir D, Khomskii DI, Abd-Elmeguid MM, López-Quintela MA, Rivas J. Enhanced dimerization of TiOCl under pressure: spin-Peierls to Peierls transition. Phys Rev Lett 2009; 102:056406. [PMID: 19257532 DOI: 10.1103/physrevlett.102.056406] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/02/2008] [Indexed: 05/27/2023]
Abstract
We report x-ray diffraction and magnetization measurements under pressure combined with ab initio calculations to show that high-pressure TiOCl corresponds to an enhanced Ti3+-Ti3+ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between Ti3+ ions along the b axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.
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Affiliation(s)
- S Blanco-Canosa
- Departamento de Química-Física, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
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Wu H, Burnus T, Hu Z, Martin C, Maignan A, Cezar JC, Tanaka A, Brookes NB, Khomskii DI, Tjeng LH. Ising magnetism and ferroelectricity in Ca3CoMnO6. Phys Rev Lett 2009; 102:026404. [PMID: 19257300 DOI: 10.1103/physrevlett.102.026404] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/09/2008] [Indexed: 05/27/2023]
Abstract
The origin of both the Ising chain magnetism and ferroelectricity in Ca3CoMnO6 is studied by ab initio electronic structure calculations and x-ray absorption spectroscopy. We find that Ca3CoMnO6 has alternate trigonal prismatic Co2+ and octahedral Mn4+ sites in the spin chain. Both the Co2+ and Mn4+ are in the high-spin state. In addition, the Co2+ has a huge orbital moment of 1.7micro_{B} which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed upward arrow upward arrow downward arrow downward arrow antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.
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Affiliation(s)
- Hua Wu
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
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Pardo V, Blanco-Canosa S, Rivadulla F, Khomskii DI, Baldomir D, Wu H, Rivas J. Homopolar bond formation in ZnV2O4 close to a metal-insulator transition. Phys Rev Lett 2008; 101:256403. [PMID: 19113730 DOI: 10.1103/physrevlett.101.256403] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/08/2008] [Indexed: 05/27/2023]
Abstract
Electronic structure calculations for spinel vanadate ZnV2O4 show that partial electronic delocalization in this system leads to a structural instability, with the formation of V-V dimers along the [011] and [101] directions, and readily accounts for the intriguing magnetic structure of this material. The formation of V-V bonds is a consequence of the proximity to the itinerant-electron boundary and is not related to orbital ordering. We discuss how this mechanism naturally couples charge and lattice degrees of freedom in magnetic insulators close to such a crossover.
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Affiliation(s)
- V Pardo
- Departamento de Física Aplicada, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
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Podlesnyak A, Russina M, Furrer A, Alfonsov A, Vavilova E, Kataev V, Büchner B, Strässle T, Pomjakushina E, Conder K, Khomskii DI. Spin-state polarons in lightly-hole-doped LaCoO3. Phys Rev Lett 2008; 101:247603. [PMID: 19113665 DOI: 10.1103/physrevlett.101.247603] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/18/2008] [Indexed: 05/27/2023]
Abstract
Inelastic neutron scattering (INS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly-hole-doped La1-xSrxCoO3, x approximately 0.002. Both INS and ESR low temperature spectra show intense excitations with large effective g factors approximately 10-18. NMR data indicate the creation of extended magnetic clusters. From the Q dependence of the INS magnetic intensity, we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions. The present INS, ESR, and NMR data give evidence for two regimes in the lightly-hole-doped samples: (i) T<35 K dominated by spin polarons; (ii) T>35 K dominated by thermally activated magnetic Co3+ ions.
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Affiliation(s)
- A Podlesnyak
- Hahn-Meitner-Institut, Glienicker Strasse 100, Berlin 14109, Germany.
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20
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Komarek AC, Streltsov SV, Isobe M, Möller T, Hoelzel M, Senyshyn A, Trots D, Fernández-Díaz MT, Hansen T, Gotou H, Yagi T, Ueda Y, Anisimov VI, Grüninger M, Khomskii DI, Braden M. CaCrO3: an anomalous antiferromagnetic metallic oxide. Phys Rev Lett 2008; 101:167204. [PMID: 18999709 DOI: 10.1103/physrevlett.101.167204] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/16/2008] [Revised: 05/07/2008] [Indexed: 05/27/2023]
Abstract
Combining infrared reflectivity, transport, susceptibility, and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transition-metal oxide with a three-dimensional electronic structure. Local spin density approximation calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizable pd hybridization remarkably strong next-nearest-neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magnetoelastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.
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Affiliation(s)
- A C Komarek
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, Köln, Germany
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21
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Jackeli G, Khomskii DI. Classical dimers and dimerized superstructure in an orbitally degenerate honeycomb antiferromagnet. Phys Rev Lett 2008; 100:147203. [PMID: 18518068 DOI: 10.1103/physrevlett.100.147203] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/19/2008] [Indexed: 05/26/2023]
Abstract
We discuss the ground state of the spin-orbital model for spin-one ions with partially filled t_{2g} levels on a honeycomb lattice. We find that the orbital degrees of freedom induce a spontaneous dimerization of spins and drive them into nonmagnetic manifold spanned by hard-core dimer (spin-singlet) coverings of the lattice. The cooperative "dimer Jahn-Teller" effect is introduced through a magnetoelastic coupling and is shown to lift the orientational degeneracy of dimers leading to a peculiar valence bond crystal pattern. The present theory provides a theoretical explanation of nonmagnetic dimerized superstructure experimentally seen in Li2RuO3 compound at low temperatures.
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Affiliation(s)
- G Jackeli
- Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
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22
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Podlesnyak A, Streule S, Mesot J, Medarde M, Pomjakushina E, Conder K, Tanaka A, Haverkort MW, Khomskii DI. Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states. Phys Rev Lett 2006; 97:247208. [PMID: 17280319 DOI: 10.1103/physrevlett.97.247208] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/13/2005] [Indexed: 05/13/2023]
Abstract
A gradual spin-state transition occurs in LaCoO3 around T approximately 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at approximately 0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal-field interaction and spin-orbit coupling, we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g approximately 3, the second interpretation is definitely favored.
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Affiliation(s)
- A Podlesnyak
- Laboratory for Neutron Scattering, ETH Zürich Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
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23
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Wu H, Hu Z, Burnus T, Denlinger JD, Khalifah PG, Mandrus DG, Jang LY, Hsieh HH, Tanaka A, Liang KS, Allen JW, Cava RJ, Khomskii DI, Tjeng LH. Orbitally driven spin-singlet dimerization in S=1 La4Ru2O10. Phys Rev Lett 2006; 96:256402. [PMID: 16907328 DOI: 10.1103/physrevlett.96.256402] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/24/2006] [Indexed: 05/11/2023]
Abstract
Using x-ray absorption spectroscopy at the Ru-L2,3 edge we reveal that the Ru4+ ions remain in the S=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U band structure calculations that the crystal fields in the low-temperature phase are not strong enough to stabilize the S=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La4Ru2O10 appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi-two-dimensional S=1 system.
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Affiliation(s)
- Hua Wu
- II. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany
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24
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Streltsov SV, Popova OA, Khomskii DI. Comment on "sodium pyroxene NaTiSi2O6: possible haldane spin-1 chain system". Phys Rev Lett 2006; 96:249701; author reply 249702. [PMID: 16907287 DOI: 10.1103/physrevlett.96.249701] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2005] [Indexed: 05/11/2023]
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25
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Haverkort MW, Hu Z, Tanaka A, Reichelt W, Streltsov SV, Korotin MA, Anisimov VI, Hsieh HH, Lin HJ, Chen CT, Khomskii DI, Tjeng LH. Orbital-assisted metal-insulator transition in VO2. Phys Rev Lett 2005; 95:196404. [PMID: 16384003 DOI: 10.1103/physrevlett.95.196404] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/06/2005] [Indexed: 05/05/2023]
Abstract
We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.
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Affiliation(s)
- M W Haverkort
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
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26
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Abstract
We find using local spin density approximation + Hubbard U band structure calculations that the novel one-dimensional cobaltate Ca3Co2O6 is not a ferromagnetic half-metal but a Mott insulator. Both the octahedral and the trigonal Co ions are formally trivalent, with the octahedral being in the low-spin and the trigonal in the high-spin state. The inclusion of the spin-orbit coupling leads to the occupation of the minority-spin d2 orbital for the unusually coordinated trigonal Co, producing a giant orbital moment (1.57 microB). It also results in an anomalously large magnetocrystalline anisotropy (of order 70 meV), elucidating why the magnetism is highly Ising-like. The role of the oxygen holes, carrying an induced magnetic moment of 0.13 microB per oxygen, for the exchange interactions is discussed.
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Affiliation(s)
- Hua Wu
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
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27
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Abstract
We consider the superstructures, which can be formed in spinels containing on B sites the transition-metal ions with partially filled t(2g) levels. We show that, when such systems are close to the itinerant state (e.g., have an insulator-metal transition), there may appear in them an orbitally driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that a similar phenomenon should be observed in NaTiO2 and possibly in some other systems.
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Affiliation(s)
- D I Khomskii
- II Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany
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28
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Haverkort MW, Hu Z, Tanaka A, Ghiringhelli G, Roth H, Cwik M, Lorenz T, Schüssler-Langeheine C, Streltsov SV, Mylnikova AS, Anisimov VI, de Nadai C, Brookes NB, Hsieh HH, Lin HJ, Chen CT, Mizokawa T, Taguchi Y, Tokura Y, Khomskii DI, Tjeng LH. Determination of the orbital moment and crystal-field splitting in LaTiO3. Phys Rev Lett 2005; 94:056401. [PMID: 15783666 DOI: 10.1103/physrevlett.94.056401] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/23/2004] [Indexed: 05/24/2023]
Abstract
Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Ne el temperature. Using Ti L2,3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t2g subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.
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Affiliation(s)
- M W Haverkort
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
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29
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Abd-Elmeguid MM, Ni B, Khomskii DI, Pocha R, Johrendt D, Wang X, Syassen K. Transition from Mott insulator to superconductor in GaNb4Se8 and GaTa4Se8 under high pressure. Phys Rev Lett 2004; 93:126403. [PMID: 15447289 DOI: 10.1103/physrevlett.93.126403] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2004] [Indexed: 05/05/2023]
Abstract
Electronic conduction in GaM4Se8 (M=Nb,Ta) compounds with the fcc GaMo4S8-type structure originates from hopping of localized unpaired electrons (S=1 / 2) among widely separated tetrahedral M4 metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T(C)=2.9 and 5.8 K at 13 and 11.5 GPa for GaNb4Se8 and GaTa4Se8, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe6 octahedral distortion and simultaneous softening of the phonon associated with M-Se bonds.
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Affiliation(s)
- M M Abd-Elmeguid
- II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
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30
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Abstract
We discuss a new mechanism of orbital ordering, which in charge transfer insulators is more important than the usual exchange interactions and which can make the very type of the ground state of a charge transfer insulator, i.e., its orbital and magnetic ordering, different from that of a Mott-Hubbard insulator. This purely electronic mechanism allows us to explain why orbitals in Jahn-Teller materials typically order at higher temperatures than spins, and to understand the type of orbital ordering in a number of materials, e.g., K2CuF4, without invoking the electron-lattice interaction.
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Affiliation(s)
- M V Mostovoy
- Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
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32
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Abstract
We show that one-dimensional topological objects (kinks) are natural degrees of freedom for an antiferromagnetic Ising model on a triangular lattice. Its ground states and the coexistence of spin ordering with an extensive zero-temperature entropy can easily be understood in terms of kinks forming a hard-sphere liquid. Using this picture we explain effects of quantum spin dynamics on that frustrated model, which we also study numerically.
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Affiliation(s)
- M V Mostovoy
- Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
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33
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Abstract
We show that superexchange interactions in frustrated Jahn-Teller systems with transition metal ions connected by the 90 degrees metal-oxygen-metal bonds (e.g., NaNiO2, LiNiO2, and ZnMn2O4) are much different from those in materials with the 180 degrees bonds. In the 90 degrees -exchange systems spins and orbitals are decoupled: the spin exchange is much weaker than the orbital one and it is ferromagnetic for all orbital states. Though the mean-field orbital ground state is strongly degenerate, quantum orbital fluctuations select particular ferro-orbital states. We explain the orbital and magnetic ordering observed in NaNiO2 and show that LiNiO2 is not a spin-orbital liquid.
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Affiliation(s)
- M V Mostovoy
- Materials Science Center Rijksuniversiteit Groningen, Nijenborgh 4, The Netherlands
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Starykh OA, Zhitomirsky ME, Khomskii DI, Singh RR, Ueda K. Origin of Spin Gap in CaV4O9: Effects of Frustration and Lattice Distortions. Phys Rev Lett 1996; 77:2558-2561. [PMID: 10061984 DOI: 10.1103/physrevlett.77.2558] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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35
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Korotin MA, Ezhov SY, Solovyev IV, Anisimov VI, Khomskii DI, Sawatzky GA. Intermediate-spin state and properties of LaCoO3. Phys Rev B Condens Matter 1996; 54:5309-5316. [PMID: 9986488 DOI: 10.1103/physrevb.54.5309] [Citation(s) in RCA: 295] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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37
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Khomskii DI, Eliashberg GM. Long-range Coulomb forces and the behavior of the chemical potential of electrons in metals at a second-order phase transition. Phys Rev B Condens Matter 1994; 50:16594-16597. [PMID: 9976047 DOI: 10.1103/physrevb.50.16594] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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38
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Khomskii DI, Kusmartsev FV. Charge redistribution and properties of high-temperature superconductors. Phys Rev B Condens Matter 1992; 46:14245-14248. [PMID: 10003508 DOI: 10.1103/physrevb.46.14245] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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39
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Khomskii DI. THE ELECTRONIC STRUCTURE AND PROPERTIES OF HIGH-TEMPERATURE SUPERCONDUCTIVITY AT HIGH DOPING LEVELS. Ann N Y Acad Sci 1990. [DOI: 10.1111/j.1749-6632.1990.tb22694.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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