Center for Correlated Electron Systems

Selected Publication

  • Quantum distance and anomalous Landau levels of flat bands

  • Nature 584, 59-63 (2020) PDF
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  • Semiclassical quantization of electronic states under a magnetic field, as proposed by Onsager, describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field1,2,3,4,5. Even in graphene with relativistic energy dispersion, Onsager’s rule correctly describes the π Berry phase, as well as the unusual Landau level spectrum of Dirac particles6,7. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. Here we show that the semiclassical quantization rule breaks down for a class of dispersionless flat bands called ‘singular flat bands’8. The singular flat band has a band crossing with another dispersive band that is enforced by the band-flatness condition, and shows anomalous magnetic responses. The Landau levels of a singular flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field, and exhibit an unusual 1/n dependence on the Landau level index n, which results in diverging orbital magnetic susceptibility. The total energy spread of the Landau levels of a singular flat band is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert?Schmidt quantum distance. We show that there is a universal and simple relationship between the total Landau level spread of a flat band and the maximum Hilbert?Schmidt quantum distance, which can be verified in various candidate materials. The results indicate that the anomalous Landau level spectrum of flat bands is promising for the direct measurement of the quantum geometry of wavefunctions in condensed matter.
Large anomalous Hall current induced by topological nodal lines in a ferromagnetic van der Waals semimetal
  • Large anomalous Hall current induced by topological nodal lines in a ferromagnetic van der Waals semimetal

  • Nature Materials 17, 194-799 (2018) PDF
  • Kyoo Kim, Junho Seo, Eunwoo Lee, K.-T. Ko, B. S. Kim, Bo Gyu Jang, Jong Mok Ok, Jinwon Lee, Youn Jung Jo, Woun Kang, Ji Hoon Shim, C. Kim, Han Woong Yeom, Byung Il Min, Bohm-Jung Yang & Jun Sung Kim
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  • Topological semimetals host electronic structures with several band-contact points or lines and are generally expected to exhibit strong topological responses. Up to now, most work has been limited to non-magnetic materials and the interplay between topology and magnetism in this class of quantum materials has been largely unexplored. Here we utilize theoretical calculations, magnetotransport and angle-resolved photoemission spectroscopy to propose Fe3GeTe2, a van der Waals material, as a candidate ferromagnetic (FM) nodal line semimetal. We find that the spin degree of freedom is fully quenched by the large FM polarization, but the line degeneracy is protected by crystalline symmetries that connect two orbitals in adjacent layers. This orbital-driven nodal line is tunable by spin orientation due to spin?orbit coupling and produces a large Berry curvature, which leads to a large anomalous Hall current, angle and factor. These results demonstrate that FM topological semimetals hold significant potential for spin- and orbital-dependent electronic functionalities.
Experimental Observation of Hidden Berry Curvature in Inversion-Symmetric Bulk 2H?WSe2
  • Experimental Observation of Hidden Berry Curvature in Inversion-Symmetric Bulk 2H?WSe2

  • Physical Review Letters 121, 186401 (2018) PDF
  • Soohyun Cho, Jin-Hong Park, Jisook Hong, Jongkeun Jung, Beom Seo Kim, Garam Han, Wonshik Kyung, Yeongkwan Kim, S.-K. Mo, J.?D. Denlinger, Ji Hoon Shim, Jung Hoon Han, Changyoung Kim, and Seung Ryong Park
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  • We investigate the hidden Berry curvature in bulk 2H?WSe2 by utilizing the surface sensitivity of angle resolved photoemission (ARPES). The symmetry in the electronic structure of transition metal dichalcogenides is used to uniquely determine the local orbital angular momentum (OAM) contribution to the circular dichroism (CD) in ARPES. The extracted CD signals for the K and K' valleys are almost identical, but their signs, which should be determined by the valley index, are opposite. In addition, the sign is found to be the same for the two spin-split bands, indicating that it is independent of spin state. These observed CD behaviors are what are expected from Berry curvature of a monolayer of WSe2. In order to see if CD-ARPES is indeed representative of hidden Berry curvature within a layer, we use tight binding analysis as well as density functional calculation to calculate the Berry curvature and local OAM of a monolayer WSe2. We find that measured CD-ARPES is approximately proportional to the calculated Berry curvature as well as local OAM, further supporting our interpretation.
Magnetism in two-dimensional van der Waals materials
  • Magnetism in two-dimensional van der Waals materials

  • Nature 563, 47-52 (2018) PDF
  • Kenneth S. Burch, David Mandrus & Je-Geun Park
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  • The discovery of materials has often introduced new physical paradigms and enabled the development of novel devices. Two-dimensional magnetism, which is associated with strong intrinsic spin fluctuations, has long been the focus of fundamental questions in condensed matter physics regarding our understanding and control of new phases. Here we discuss magnetic van der Waals materials: two-dimensional atomic crystals that contain magnetic elements and thus exhibit intrinsic magnetic properties. These cleavable materials provide the ideal platform for exploring magnetism in the two-dimensional limit, where new physical phenomena are expected, and represent a substantial shift in our ability to control and investigate nanoscale phases. We present the theoretical background and motivation for investigating this class of crystals, describe the material landscape and the current experimental status of measurement techniques as well as devices, and discuss promising future directions for the study of magnetic van der Waals materials.
Selective control of multiple ferroelectric switching pathways using a trailing flexoelectric field
  • Selective control of multiple ferroelectric switching pathways using a trailing flexoelectric field

  • Nature Nanotechnology 13, 366-370 (2018) PDF
  • Sung Min Park, Bo Wang, Saikat Das, Seung Chul Chae, Jin-Seok Chung, Jong-Gul Yoon, Long-Qing Chen, Sang Mo Yang & Tae Won Noh
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  • Flexoelectricity is an electromechanical coupling between electrical polarization and a strain gradient1 that enables mechanical manipulation of polarization without applying an electrical bias2,3. Recently, flexoelectricity was directly demonstrated by mechanically switching the out-of-plane polarization of a uniaxial system with a scanning probe microscope tip3,4. However, the successful application of flexoelectricity in low-symmetry multiaxial ferroelectrics and therefore active manipulation of multiple domains via flexoelectricity have not yet been achieved. Here, we demonstrate that the symmetry-breaking flexoelectricity offers a powerful route for the selective control of multiple domain switching pathways in multiaxial ferroelectric materials. Specifically, we use a trailing flexoelectric field that is created by the motion of a mechanically loaded scanning probe microscope tip. By controlling the SPM scan direction, we can deterministically select either stable 71° ferroelastic switching or 180° ferroelectric switching in a multiferroic magnetoelectric BiFeO3 thin film. Phase-field simulations reveal that the amplified in-plane trailing flexoelectric field is essential for this domain engineering. Moreover, we show that mechanically switched domains have a good retention property. This work opens a new avenue for the deterministic selection of nanoscale ferroelectric domains in low-symmetry materials for non-volatile magnetoelectric devices and multilevel data storage.
The low-temperature highly correlated quantum phase in the charge-density-wave 1T-TaS2 compound
  • The low-temperature highly correlated quantum phase in the charge-density-wave 1T-TaS2 compound

  • npj Quantum Materials 2, 42 (2017) PDF
  • Marie Kratochvilova, Adrian D. Hillier, Andrew R. Wildes, Lihai Wang, Sang-Wook Cheong & Je-Geun Park
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  • A prototypical quasi-2D metallic compound, 1T-TaS2 has been extensively studied due to an intricate interplay between a Mott-insulating ground state and a charge-density-wave order. In the low-temperature phase, 12 out of 13 Ta4+ 5d-electrons form molecular orbitals in hexagonal star-of-David patterns, leaving one 5d-electron with S??=?½ spin free. This orphan quantum spin with a large spin-orbit interaction is expected to form a highly correlated phase of its own. And it is most likely that they will form some kind of a short-range order out of a strongly spin-orbit coupled Hilbert space. In order to investigate the low-temperature magnetic properties, we performed a series of measurements including neutron scattering and muon experiments. The obtained data clearly indicate the presence of the short-ranged phase and put the upper bound on ~0.4?μB for the size of the magnetic moment, consistent with the orphan-spin scenario.
Jahn-Teller distortion driven magnetic polarons in magnetite
  • Jahn-Teller distortion driven magnetic polarons in magnetite

  • Nature Communication8, 15929 (2017) PDF
  • H. Y. Huang , Z. Y. Chen , R. -P. Wang , F. M. F. de Groot , W. B. Wu , J. Okamoto , A. Chainani , A. Singh , Z. -Y. Li , J. -S. Zhou , H. -T. Jeng , G. Y. Guo , Je-Geun Park , L. H. Tjeng , C. T. Chen & D. J. Huang
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  • The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120?K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin?orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350?K but persist at least up to 550?K, are distinct from optical excitations and are best explained as magnetic polarons.
Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3Van der Waals Crystals
  • Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3Van der Waals Crystals

  • Scientific Reports6, 20904 (2016) PDF
  • Cheng-Tai Kuo, Michael Neumann, Karuppannan Balamurugan, Hyun Ju Park, Soonmin Kang, Hung Wei Shiu, Jin Hyoun Kang, Byung Hee Hong, Moonsup Han, Tae Won Noh & Je-Geun Park
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  • The range of mechanically cleavable Van der Waals crystals covers materials with diverse physical and chemical properties. However, very few of these materials exhibit magnetism or magnetic order, and thus the provision of cleavable magnetic compounds would supply invaluable building blocks for the design of heterostructures assembled from Van der Waals crystals. Here we report the first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation. This material belongs to the class of transition metal phosphorus trisulfides (MPS3), several of which exhibit antiferromagnetic order at low temperature, and which have not been reported in the form of ultrathin sheets so far. We establish layer numbers by optical bright field microscopy and atomic force microscopy, and perform a detailed Raman spectroscopic characterization of bilayer and thicker NiPS3 flakes. Raman spectral features are strong functions of excitation wavelength and sample thickness, highlighting the important role of interlayer coupling. Furthermore, our observations provide a spectral fingerprint for distinct layer numbers, allowing us to establish a sensitive and convenient means for layer number determination.
  • Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li2RuO3

  • Scientific Reports6, 25238 (2016)
  • Junghwan Park, Teck-Yee Tan, D. T. Adroja, A. Daoud-Aladine, Seongil Choi, Duck-Yong Cho, SangHyun Lee, Jiyeon Kim, Hasung Sim, T. Morioka, H. Nojiri, V. V. Krishnamurthy, P. Manuel, M. R. Lees, S.V. Streltsov, D.I. Khomskii, and Je-Geun Park
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  • When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.
  • Weyl Fermions in Spin Dynamics of Metallic Ferromagnet SrRuO3

  • Nature Communication7, 11788 (2016) PDF
  • Shinichi Itoh, Yasuo Endoh, Tetsuya Yokoo, Je-Geun Park, Yoshio Kaneko, Yoshinori Tokura, and Naoto Nagaosa
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  • Weyl fermions that emerge at band crossings in momentum space caused by the spin?orbit interaction act as magnetic monopoles of the Berry curvature and contribute to a variety of novel transport phenomena such as anomalous Hall effect and magnetoresistance. However, their roles in other physical properties remain mostly unexplored. Here, we provide evidence by neutron Brillouin scattering that the spin dynamics of the metallic ferromagnet SrRuO3 in the very low energy range of milli-electron volts is closely relevant to Weyl fermions near Fermi energy. Although the observed spin wave dispersion is well described by the quadratic momentum dependence, the temperature dependence of the spin wave gap shows a nonmonotonous behaviour, which can be related to that of the anomalous Hall conductivity. This shows that the spin dynamics directly reflects the crucial role of Weyl fermions in the metallic ferromagnet.
Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3
  • Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3

  • Nature Communications7, 13146 (2016) PDF
  • Joosung Oh, Manh Duc Le, Ho-Hyun Nahm, Hasung Sim, Jaehong Jeong, T. G. Perring, Hyungje Woo, Kenji Nakajima, Seiko Ohira-Kawamura, Zahra Yamani, Y. Yoshida, H. Eisaki, S.-W. Cheong, A. L. Chernyshev, and Je-Geun Park
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  • Magnons and phonons are fundamental quasiparticles in a solid and can be coupled together to form a hybrid quasi-particle. However, detailed experimental studies on the underlying Hamiltonian of this particle are rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored, although it is essential for a proper understanding of their diverse thermodynamic behaviour and intrinsic zero-temperature decay. Here we show that in non-collinear antiferromagnets, a strong magnon?phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon?phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it.
  • Superconductivity below 20K in heavily electron-doped surface layer of FeSe bulk crystal

  • Nature Communication7, 11116 (2016) PDF
  • J. J. Seo, B. Y. Kim, B. S. Kim, J. K. Jeong, J. M. Ok, Jun Sung Kim, J. D. Denlinger, S. -K. Mo, C. Kim & Y. K. Kim
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  • A superconducting transition temperature (Tc) as high as 100?K was recently discovered in one monolayer FeSe grown on SrTiO3. The discovery ignited efforts to identify the mechanism for the markedly enhanced Tc from its bulk value of 8?K. There are two main views about the origin of the Tc enhancement: interfacial effects and/or excess electrons with strong electron correlation. Here, we report the observation of superconductivity below 20?K in surface electron-doped bulk FeSe. The doped surface layer possesses all the key spectroscopic aspects of the monolayer FeSe on SrTiO3. Without interfacial effects, the surface layer state has a moderate Tc of 20?K with a smaller gap opening of 4.2?meV. Our results show that excess electrons with strong correlation cannot induce the maximum Tc, which in turn reveals the need for interfacial effects to achieve the highest Tc in one monolayer FeSe on SrTiO3.
Detection of a Cooper-pair density wave in Bi2Sr2CaCu2O8+x
  • Detection of a Cooper-pair density wave in Bi2Sr2CaCu2O8+x

  • Nature532, 343 (2016)
  • M. H. Hamidian, S. D. Edkins, Sang Hyun Joo, A. Kostin, H. Eisaki, S. Uchida, M. J. Lawler, E.-A. Kim, A. P. Mackenzie, K. Fujita, Jinho Lee & J. C. Seamus Davis
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  • The quantum condensate of Cooper pairs forming a superconductor was originally conceived as being translationally invariant. In theory, however, pairs can exist with finite momentum Q, thus generating a state with a spatially modulated Cooper-pair density1,2. Such a state has been created in ultracold 6Li gas3 but never observed directly in any superconductor. It is now widely hypothesized that the pseudogap phase4 of the copper oxide superconductors contains such a ‘pair density wave’ state5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21. Here we report the use of nanometre-resolution scanned Josephson tunnelling microscopy22,23,24 to image Cooper pair tunnelling from a d-wave superconducting microscope tip to the condensate of the superconductor Bi2Sr2CaCu2O8+x. We demonstrate condensate visualization capabilities directly by using the Cooper-pair density variations surrounding zinc impurity atoms25 and at the Bi2Sr2CaCu2O8+x crystal supermodulation26. Then, by using Fourier analysis of scanned Josephson tunnelling images, we discover the direct signature of a Cooper-pair density modulation at wavevectors QP ? (0.25, 0)2π/a0 and (0, 0.25)2π/a0 in Bi2Sr2CaCu2O8+x. The amplitude of these modulations is about five per cent of the background condensate density and their form factor exhibits primarily s or s′ symmetry. This phenomenology is consistent with Ginzburg?Landau theory5,13,14 when a charge density wave5,27 with d-symmetry form factor28,29,30 and wavevector QC?=?QP coexists with a d-symmetry superconductor; it is also predicted by several contemporary microscopic theories for the pseudogap phase18,19,20,21.
Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3Composite Barriers
  • Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3Composite Barriers

  • Nano Letters16, 3911-3918 (2016)
  • Lingfei Wang, Myung Rae Cho, Yeong Jae Shin, Jeong Rae Kim, Saikat Das, Jong-Gul Yoon, Jin-Seok Chung, and Tae Won Noh
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  • Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for nonvolatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs) and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities.
Ising-Type Magnetic Ordering in Atomically Thin FePS3
  • Ising-Type Magnetic Ordering in Atomically Thin FePS3

  • Nano Letters16, 7433-7438 (2016)
  • Jae-Ung Lee, Sungmin Lee, Ji Hoon Ryoo, Soonmin Kang, Tae Yun Kim, Pilkwang Kim, Cheol-Hwan Park, Je-Geun Park, and Hyeonsik Cheong
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  • Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order?disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with TN ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.
Enhanced superconductivity in surface-electron-doped iron pnictide Ba(Fe1.94Co0.06)2As2
  • Enhanced superconductivity in surface-electron-doped iron pnictide Ba(Fe1.94Co0.06)2As2

  • Nature Materials15, 1233-1236 (2016)
  • W. S. Kyung, S. S. Huh, Y. Y. Koh, K.-Y. Choi, M. Nakajima, H. Eisaki, J. D. Denlinger, S.-K. Mo, C. Kim & Y. K. Kim
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  • The superconducting transition temperature (TC) in a FeSe monolayer on SrTiO3 is enhanced up to 100?K (refs 1,2,3,4). High TC is also found in bulk iron chalcogenides with similar electronic structure5,6,7 to that of monolayer FeSe, which suggests that higher TC may be achieved through electron doping, pushing the Fermi surface (FS) topology towards leaving only electron pockets. Such an observation, however, has been limited to chalcogenides, and is in contrast to the iron pnictides, for which the maximum TC is achieved with both hole and electron pockets forming considerable FS nesting instability8,9,10,11. Here, we report angle-resolved photoemission characterization revealing a monotonic increase of TC from 24 to 41.5?K upon surface doping on optimally doped Ba(Fe1?xCox)2As2. The doping changes the overall FS topology towards that of chalcogenides through a rigid downward band shift. Our findings suggest that higher electron doping and concomitant changes in FS topology are favourable conditions for the superconductivity, not only for iron chalcogenides, but also for iron pnictides.
Size Dependence of Metal?Insulator Transition in Stoichiometric Fe3O4Nanocrystals
  • Size Dependence of Metal?Insulator Transition in Stoichiometric Fe3O4Nanocrystals

  • Nano Letters15, 4337 (2015)
  • Jisoo Lee, Soon Gu Kwon, Je-Geun Park, Taeghwan Hyeon
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  • Magnetite (Fe3O4) is one of the most actively studied materials with a famous metal?insulator transition (MIT), so-called the Verwey transition at around 123 K. Despite the recent progress in synthesis and characterization of Fe3O4 nanocrystals (NCs), it is still an open question how the Verwey transition changes on a nanometer scale. We herein report the systematic studies on size dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey transition when they are characterized by conductance, magnetization, cryo-XRD, and heat capacity measurements. The Verwey transition is weakly size-dependent and becomes suppressed in NCs smaller than 20 nm before disappearing completely for less than 6 nm, which is a clear, yet highly interesting indication of a size effect of this well-known phenomena. Our current work will shed new light on this ages-old problem of Verwey transition.
Dimensionality Control of d-orbital Occupation in Oxide Superlattices
  • Dimensionality Control of d-orbital Occupation in Oxide Superlattices

  • Scientific Reports4, 6124 (2014) PDF
  • Da Woon Jeong, Woo Seok Choi, Satoshi Okamoto, Jae?Young Kim, Kyung Wan Kim, Soon Jae Moon, Deok?Yong Cho, Ho Nyung Lee and Tae Won Noh
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  • Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t2g and eg multiplets in quasi-two-dimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3 and a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t2g and eg orbitals are partially filled, to a band insulator by completely filling (emptying) the t2g (eg) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter.
Flexoelectric Control of Defect Formation in Ferroelectric Epitaxial Thin Films
  • Flexoelectric Control of Defect Formation in Ferroelectric Epitaxial Thin Films

  • Advanced Materials26, 5005 (2014)
  • Daesu Lee, Byung Chul Jeon, Aram Yoon, Yeong Jae Shin, Myang Hwan Lee, Tae Kwon Song, Sang Don Bu, Miyoung Kim, Jin-Seok Chung, Jong-Gul Yoon, and Tae Won Noh
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  • Flexoelectric control of defect formation and associated electronic function is demonstrated in ferroelectric BiFeO3 thin films. An intriguing, so far never demonstrated, effect of internal electric field (Eint) on defect formation is explored by a means of flexoelectricity. Our study provides novel insight into defect engineering, as well as allows a pathway to design defect configuration and associated electronic function.
Temperature-Dependent Interplay of Dzyaloshinskii-Moriya Interaction and Single-Ion Anisotropy in Multiferroic BiFeO3
  • Temperature-Dependent Interplay of Dzyaloshinskii-Moriya Interaction and Single-Ion Anisotropy in Multiferroic BiFeO3

  • Physical Review Letters113, 107202 (2014)
  • Jaehong Jeong, Manh Duc Le, P. Bourges, S. Petit, S. Furukawa, Shin-Ae Kim, Seongsu Lee, S-W. Cheong, and Je-Geun Park
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  • Low-energy magnon excitations in multiferroic BiFeO3 were measured in detail as a function of temperature around several Brillouin zone centers by inelastic neutron scattering experiments on single crystals. Unique features around 1 meV are directly associated with the interplay of the Dzyaloshinskii-Moriya interaction and a small single-ion anisotropy. The temperature dependence of these and the exchange interactions were determined by fitting the measured magnon dispersion with spin-wave calculations. The spectra best fit an easy-axis type magnetic anisotropy and the deduced exchange and anisotropy parameters enable us to determine the anharmonicity of the magnetic cycloid. We then draw a direct connection between the changes in the parameters of spin Hamiltonian with temperature and the physical properties and structural deformations of BiFeO3.