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Pascale F, Mustapha S, D’Arco P, Dovesi R. The d Orbital Multi Pattern Occupancy in a Partially Filled d Shell: The KFeF 3 Perovskite as a Test Case. MATERIALS (BASEL, SWITZERLAND) 2023; 16:1532. [PMID: 36837162 PMCID: PMC9962142 DOI: 10.3390/ma16041532] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/25/2022] [Revised: 02/03/2023] [Accepted: 02/09/2023] [Indexed: 06/18/2023]
Abstract
The occupancy of the d shell in KFeF3 is t2g4eg2, with five α and one β electrons. The Jahn-Teller lift of degeneracy in the t2g sub-shell produces a tetragonal relaxation of the unit cell (4.09 vs. 4.22 Å, B3LYP result) not observed experimentally. In order to understand the origin of this apparent contradiction, we explored, with a 2 × 2 × 2 supercell (40 atoms per cell), all possible local structures in which contiguous Fe atoms have a different occupancy of the t2g orbitals with the minority spin electron. A total of 6561 configurations (with occupancies from (8,0,0) to (3,2,2) of the 3 t2g orbitals of the 8 Fe atoms) have been explored, with energies in many cases lower (by up to 1550 μEh per 2 Fe atoms) than the one of the fully ordered case, both for the ferromagnetic and the anti-ferromagnetic solutions. The results confirm that the orientation of the β d electron of Fe influences the electrostatics (more efficient relative orientation of the Fe quadrupoles of the d shell) of the system, but not the magnetic interactions. Three hybrid functionals, B3LYP, PBE0, and HSE06, provide very similar results.
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Affiliation(s)
- Fabien Pascale
- Laboratoire de Physique et Chimie Théoriques, Université de Lorraine, CNRS, UMR 7019, F-54506 Vandoeuvre-lès-Nancy, France
| | - Sami Mustapha
- Institut de Mathématiques de Jussieu, Sorbonne Université, UMR 7586, F-75005 Paris, France
| | - Philippe D’Arco
- Institut des Sciences de la Terre, Sorbonne Université, CNRS-INSU, ISTeP UMR 7193, F-75005 Paris, France
| | - Roberto Dovesi
- Dipartimento di Chimica, Università di Torino, Via P. Giuria 5, 10125 Torino, Italy
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2
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Volkova LM, Marinin DV. Crystal chemistry criteria of the existence of spin liquids on the kagome lattice. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2021; 33:415801. [PMID: 34261046 DOI: 10.1088/1361-648x/ac145e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/29/2020] [Accepted: 07/14/2021] [Indexed: 06/13/2023]
Abstract
The structural-magnetic models of 25 antiferromagnetic kagome cuprates similar to herbertsmithite (ZnCu3(OH)6Cl2)-a perspective spin liquid-have been calculated and analyzed. Main correlations between the structure and magnetic properties of these compounds were revealed. It has been demonstrated that, in all AFM kagome cuprates, including herbertsmithite, there exists the competition between the exchange interaction and the antisymmetric anisotropic exchange one (the Dzyaloshinskii-Moriya interaction), as magnetic ions are not linked to the center of inversion in the kagome lattice. This competition is strengthened in all the kagome AFM, except herbertsmithite, by one more type of the anisotropy (duality) of the third in lengthJ3 magnetic couplings (strongJ3(J12) next-to-nearest-neighbor couplings in linear chains along the triangle edges and very weak FM or AFMJ3(Jd) couplings along the hexagon diagonals). The above couplings are crystallographically identical, but are divided to two types of different in strength magnetic interactions. The existence of duality ofJ3 couplings originated from the structure of the kagome lattice itself. Only combined contributions of dualJ3 couplings with anisotropic Dzyaloshinskii-Moriya interactions are capable to suppress frustration of kagome antiferromagnetics. It has been demonstrated that the possibility of elimination of such a duality in herbertsmithite, which made it a spin liquid, constitutes a rare lucky event in the kagome system. Three crystal chemistry criteria of the existence of spin liquids on the kagome lattice have been identified: first, the presence of frustrated kagome lattices with strong dominant antiferromagnetic nearest-neighborJ1 couplings competing only with each other in small triangles; second, magnetic isolation of these frustrated kagome lattices; and third, the absence of duality of the third in lengthJ3 magnetic couplings.
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Affiliation(s)
- L M Volkova
- Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, 690022 Vladivostok, Russia
| | - D V Marinin
- Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, 690022 Vladivostok, Russia
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Li J, Xu L, Garcia-Fernandez M, Nag A, Robarts HC, Walters AC, Liu X, Zhou J, Wohlfeld K, van den Brink J, Ding H, Zhou KJ. Unraveling the Orbital Physics in a Canonical Orbital System KCuF_{3}. PHYSICAL REVIEW LETTERS 2021; 126:106401. [PMID: 33784112 DOI: 10.1103/physrevlett.126.106401] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/08/2020] [Revised: 12/16/2020] [Accepted: 01/21/2021] [Indexed: 06/12/2023]
Abstract
We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF_{3} using the Cu L_{3}-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the Cu^{2+} dd orbital excitations. These high-energy modes display good agreement with the ab initio quantum chemistry calculation, indicating that the dd excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized dd excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in KCuF_{3}, which predominantly drives orbital ordering.
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Affiliation(s)
- Jiemin Li
- Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
- Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
| | - Lei Xu
- Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany
| | | | - Abhishek Nag
- Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
| | - H C Robarts
- Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
- H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom
| | - A C Walters
- Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
| | - X Liu
- School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
| | - Jianshi Zhou
- The Materials Science and Engineering Program, Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA
| | - Krzysztof Wohlfeld
- Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, PL-02093 Warsaw, Poland
| | - Jeroen van den Brink
- Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany
- Institute for Theoretical Physics and Würzburg-Dresden Cluster of Excellence ct.qmat, TU Dresden, 01069 Dresden, Germany
| | - Hong Ding
- Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
| | - Ke-Jin Zhou
- Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
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Abstract
Quantum spin liquids are an exciting playground for exotic physical phenomena and emergent many-body quantum states. The realization and discovery of quantum spin liquid candidate materials and associated phenomena lie at the intersection of solid-state chemistry, condensed matter physics, and materials science and engineering. In this review, we provide the current status of the crystal chemistry, synthetic techniques, physical properties, and research methods in the field of quantum spin liquids. We highlight a number of specific quantum spin liquid candidate materials and their structure-property relationships, elucidating their fascinating behavior and connecting it to the intricacies of their structures. Furthermore, we share our thoughts on defects and their inevitable presence in materials, of which quantum spin liquids are no exception, which can complicate the interpretation of characterization of these materials, and urge the community to extend their attention to materials preparation and data analysis, cognizant of the impact of defects. This review was written with the intention of providing guidance on improving the materials design and growth of quantum spin liquids, and to paint a picture of the beauty of the underlying chemistry of this exciting class of materials.
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Affiliation(s)
- Juan R Chamorro
- Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States.,Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Tyrel M McQueen
- Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States.,Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, United States.,Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Thao T Tran
- Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States
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Li J, Strand HUR, Werner P, Eckstein M. Theory of photoinduced ultrafast switching to a spin-orbital ordered hidden phase. Nat Commun 2018; 9:4581. [PMID: 30389918 PMCID: PMC6214932 DOI: 10.1038/s41467-018-07051-x] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2018] [Accepted: 10/06/2018] [Indexed: 11/09/2022] Open
Abstract
Photo-induced hidden phases are often observed in materials with intertwined orders. Understanding the formation of these non-thermal phases is challenging and requires a resolution of the cooperative interplay between different orders on the ultra-short timescale. In this work, we demonstrate that non-equilibrium photo-excitations can induce a state with spin-orbital orders entirely different from the equilibrium state in the three-quarter-filled two-band Hubbard model. We identify a general mechanism governing the transition to the hidden state, which relies on a non-thermal partial melting of the intertwined orders mediated by photoinduced charge excitations in the presence of strong spin-orbital exchange interactions. Our study theoretically confirms the crucial role played by orbital degrees of freedom in the light-induced dynamics of strongly correlated materials and it shows that the switching to hidden states can be controlled already on the fs timescale of the electron dynamics. Ultrafast excitation of materials can cause the formation of hidden phases that are not accessible in thermal equilibrium. Li et al. identify and investigate theoretically a hidden phase that can be accessed in systems with intertwined spin and orbital-ordering such as KCuF3.
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Affiliation(s)
- Jiajun Li
- Department of Physics, University Erlangen-Nürnberg, 91058, Erlangen, Germany.
| | - Hugo U R Strand
- Center for Computational Quantum Physics, Flatiron Institute, 162 Fifth Avenue, New York, NY, 10010, USA.,Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211, Geneva 4, Switzerland.,Department of Physics, University of Fribourg, 1700, Fribourg, Switzerland
| | - Philipp Werner
- Department of Physics, University of Fribourg, 1700, Fribourg, Switzerland
| | - Martin Eckstein
- Department of Physics, University Erlangen-Nürnberg, 91058, Erlangen, Germany
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Volkova LM, Marinin DV. Antiferromagnetic spin-frustrated layers of corner-sharing Cu 4 tetrahedra on the kagome lattice in volcanic minerals Cu 5O 2(VO 4) 2(CuCl), NaCu 5O 2(SeO 3) 2Cl 3, and K 2Cu 5Cl 8(OH) 4·2H 2O. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:425801. [PMID: 30166500 DOI: 10.1088/1361-648x/aade0b] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The objective of the present work was to analyze the possibility of realization of quantum spin liquids in three volcanic minerals-averievite (Cu5O2(VO4)2(CuCl)), ilinskite (NaCu5O2(SeO3)2Cl3), and avdononite (K2Cu5Cl8(OH)4·2H2O)-from the crystal chemistry point of view. Based on the structural data, the sign and strength of magnetic interactions have been calculated and the geometric frustrations serving as the main reason of the existence of spin liquids have been investigated. According to our calculations, the magnetic structures of averievite and ilinskite are composed of antiferromagnetic (AFM) spin-frustrated layers of corner-sharing Cu4 tetrahedra on the kagome lattice. However, the direction of nonshared corners of tetrahedra is different in them. The oxygen ions centering the OCu4 tetrahedra in averievite and ilinskite provide the main contribution to the formation of AFM interactions along the tetrahedra edges. The local electric polarization in averievite and the possibility of spin configuration fluctuations due to vibrations of tetrahedra-centering oxygen ions have been discussed. The existence of structural phase transitions accompanied with magnetic transitions was assumed in ilinskite because of the effect of a lone electron pair by Se4+ ions. As was demonstrated through comparison of averievite and avdoninite, at the removal of centering oxygen ions from tetrahedra, the magnetic structure of the pyrochlore layer present in averievite transformed into an openwork curled net with large cells woven from corner-sharing open AFM spin-frustrated tetrahedra ('butterflies') in avdoninite.
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Affiliation(s)
- L M Volkova
- Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, 690022 Vladivostok, Russia
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FeCr₂S₄ in magnetic fields: possible evidence for a multiferroic ground state. Sci Rep 2014; 4:6079. [PMID: 25123960 PMCID: PMC4133713 DOI: 10.1038/srep06079] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2014] [Accepted: 07/23/2014] [Indexed: 11/08/2022] Open
Abstract
We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K.
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Fink J, Schierle E, Weschke E, Geck J. Resonant elastic soft x-ray scattering. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2013; 76:056502. [PMID: 23563216 DOI: 10.1088/0034-4885/76/5/056502] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Resonant (elastic) soft x-ray scattering (RSXS) offers a unique element, site and valence specific probe to study spatial modulations of charge, spin and orbital degrees of freedom in solids on the nanoscopic length scale. It is not only used to investigate single-crystalline materials. This method also enables one to examine electronic ordering phenomena in thin films and to zoom into electronic properties emerging at buried interfaces in artificial heterostructures. During the last 20 years, this technique, which combines x-ray scattering with x-ray absorption spectroscopy, has developed into a powerful probe to study electronic ordering phenomena in complex materials and furthermore delivers important information on the electronic structure of condensed matter. This review provides an introduction to the technique, covers the progress in experimental equipment, and gives a survey on recent RSXS studies of ordering in correlated electron systems and at interfaces.
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Affiliation(s)
- J Fink
- Leibniz-Institute for Solid State and Materials Research Dresden, PO Box 270116, D-01171 Dresden, Germany.
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Kovaleva NN, Kusmartseva OE, Kugel KI, Maksimov AA, Nuzhnyy D, Balbashov AM, Demikhov EI, Dejneka A, Trepakov VA, Kusmartsev FV, Stoneham AM. Anomalous multi-order Raman scattering in LaMnO3: a signature of quantum lattice effects in a Jahn-Teller crystal. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2013; 25:155602. [PMID: 23507775 DOI: 10.1088/0953-8984/25/15/155602] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The multi-order Raman scattering is studied up to fourth order for a detwinned LaMnO3 crystal. Based on a comprehensive data analysis of the polarization-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm(-1). We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn-Teller Mn(3+) ion due to the lattice anharmonicity and that the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.
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Affiliation(s)
- N N Kovaleva
- Lebedev Institute of Physics, Russian Academy of Sciences, Moscow, Leninsky Prospekt 53, 119991, Russia.
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Yuan S, Kim M, Seeley JT, Lee JCT, Lal S, Abbamonte P, Cooper SL. Inelastic light scattering measurements of a pressure-induced quantum liquid in KCuF3. PHYSICAL REVIEW LETTERS 2012; 109:217402. [PMID: 23215614 DOI: 10.1103/physrevlett.109.217402] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/22/2011] [Revised: 06/12/2012] [Indexed: 06/01/2023]
Abstract
Pressure-dependent, low-temperature inelastic light (Raman) scattering measurements of KCuF(3) show that applied pressure above P* ~ 7 kbar suppresses a previously observed structural phase transition temperature to zero temperature in KCuF(3), resulting in the development of a fluctuational (quasielastic) response near T ~ 0 K. This pressure-induced fluctuational response--which we associate with slow fluctuations of the CuF(6) octahedral orientation--is temperature independent and exhibits a characteristic fluctuation rate that is much larger than the temperature, consistent with quantum fluctuations of the CuF(6) octahedra. A model of pseudospin-phonon coupling provides a qualitative description of both the temperature- and pressure-dependent evolution of the Raman spectra of KCuF(3).
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Affiliation(s)
- S Yuan
- Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA
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11
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Zhou JS, Alonso J, Han J, Fernández-Díaz M, Cheng JG, Goodenough J. Jahn–Teller distortion in perovskite KCuF3 under high pressure. J Fluor Chem 2011. [DOI: 10.1016/j.jfluchem.2011.06.047] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
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12
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Pavarini E, Koch E, Lichtenstein AI. Mechanism for orbital ordering in KCuF3. PHYSICAL REVIEW LETTERS 2008; 101:266405. [PMID: 19437658 DOI: 10.1103/physrevlett.101.266405] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
The Mott insulating perovskite KCuF3 is considered the archetype of an orbitally ordered system. By using the local-density approximation+dynamical mean-field theory method, we investigate the mechanism for orbital ordering in this material. We show that the purely electronic Kugel-Khomskii super-exchange mechanism alone leads to a remarkably large transition temperature of T(KK) to approximately 350 K. However, orbital order is experimentally believed to persist to at least 800 K. Thus, Jahn-Teller distortions are essential for stabilizing orbital order at such high temperatures.
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Affiliation(s)
- E Pavarini
- Institut für Festkörperforschung and Institute for Advanced Simulation, Forschungzentrum Jülich, 52425 Jülich, Germany
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Deisenhofer J, Leonov I, Eremin MV, Kant C, Ghigna P, Mayr F, Iglamov VV, Anisimov VI, van der Marel D. Optical evidence for symmetry changes above the Néel temperature of KCuF3. PHYSICAL REVIEW LETTERS 2008; 101:157406. [PMID: 18999642 DOI: 10.1103/physrevlett.101.157406] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/13/2007] [Revised: 08/05/2008] [Indexed: 05/27/2023]
Abstract
We report on optical measurements of the 1D Heisenberg antiferromagnet KCuF3. The crystal-field excitations of the Cu2+ ions have been observed and their temperature dependence can be understood in terms of magnetic and exchange-induced dipole mechanisms and vibronic interactions. Above TN we observe a new temperature scale TS characterized by the emergence of narrow absorption features that correlate with changes of the orbital ordering as observed by Paolasini et al. [Phys. Rev. Lett. 88, 106403 (2002)]. The appearance of these optical transitions provides evidence for a symmetry change above the Néel temperature that affects the orbital ordering and paves the way for the antiferromagnetic ordering.
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Affiliation(s)
- J Deisenhofer
- Département de Physique de la Matière Condensée, Université de Genève, CH-1211 Genève 4, Switzerland
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Oliva C, Scavini M, Cappelli S, Bottalo C, Mazzoli C, Ghigna P. Melting of Orbital Ordering in KMgxCu1-xF3 Solid Solution. J Phys Chem B 2007; 111:5976-83. [PMID: 17489623 DOI: 10.1021/jp067539p] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The long-range and short-range structures of KMgxCu1-xF3 (0 < x < 1) have been investigated by means of XRPD and EPR. Two different solid solutions are present, based on the structure of KMgF3 (for x > 0.42) and of KCuF3 (for x < 0.26), respectively, and they are separated by a biphasic zone. Positional disorder is induced by doping due to the different Cu and Mg environments. In fact, the EPR measurements have shown that the Cu environment is isotropic for x > 0.8. It shows axial symmetry for 0.45 < x < 0.70 and orthorhombic symmetry for x = 0.43. For x > 0.42, the crystallographic structure is cubic, and in absence of local disorder, a fully isotropic octahedral undistorted environment is expected for Cu. In the tetragonal structure, collective magnetic interactions arise, and a progressive EPR signal symmetrization is observed due to anisotropic exchange and to Dzialoshinsky-Moriya antisymmetric exchange processes. The mixing of triplet and singlet states induced by the above exchange mechanisms leads to the conclusion that the orbital order is melt in the x = 0.1 sample, for which the cooperative Jahn-Teller distortion is still active and the 3D magnetic order is still antiferromagnetic, as in KCuF3.
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Affiliation(s)
- Cesare Oliva
- Dipartimento di Chimica Fisica ed Elettrochimica, Università di Milano, Milano, Italy
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