Most Recent Highlights
2011 | |
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Precursor Phenomena at the Magnetic Ordering of the Cubic Helimagnet FeGeWe report on detailed magnetic measurements on the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at Tc=278.2(3) K. Precursor phenomena display a complex succession of temperature-driven crossovers and phase transitions in the vicinity of Tc. The A-phase region, present below Tc and fields H<0.5 kOe, is split in several pockets. The complexity of the magnetic phase diagram is theoretically explained by the confinement of solitonic kinklike or Skyrmionic units that develop an attractive and oscillatory intersoliton coupling owing to the longitudinal inhomogeneity of the magnetization.
H. Wilhelm, M. Baenitz, M. Schmidt, U. K. Rößler, A. A. Leonov, and A. N. Bogdanov, Physical Review Letters 107, 127203 (2011), pdf | |
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Delta Doping of Ferromagnetism in Antiferromagnetic Manganite SuperlatticesWe demonstrate that delta doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AFM) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Theoretical consideration of these additional carriers shows that they cause a local enhancement of ferromagnetic double exchange with respect to AFM superexchange, resulting in local canting of the AFM spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy, and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AFM state as was predicted by de Gennes [Phys. Rev. 118, 141 (1960)] but had remained elusive.
T. S. Santos, B. J. Kirby, S. Kumar, S. J. May, J. A. Borchers, B. B. Maranville, J. Zarestky, S. G. E. te Velthuis, J. van den Brink, and A. Bhattacharya, Physical Review Letters 107, 167202 (2011), pdf | |
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Intrinsic Coupling of Orbital Excitations to Spin Fluctuations in Mott InsulatorsWe show how the general and basic asymmetry between two fundamental degrees of freedom present in strongly correlated oxides, spin and orbital, has very profound repercussions on the elementary spin and orbital excitations. Whereas the magnons remain largely unaffected, orbitons become inherently coupled with spin fluctuations in spin-orbital models with antiferromagnetic and ferro-orbital ordered ground states. The composite orbiton-magnon modes that emerge fractionalize again in one dimension, giving rise to spin-orbital separation in the peculiar regime where spinons are faster than orbitons.
Krzysztof Wohlfeld, Maria Daghofer, Satoshi Nishimoto, Giniyat Khaliullin, and Jeroen van den Brink, Physical Review Letters 107, 147201 (2011), pdf
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Narrowing of Topological Bands due to Electronic Orbital Degrees of Freedom
The fractional quantum Hall effect has been predicted to occur in the absence of magnetic fields and at high temperature in lattice systems that have flat bands with a nonzero Chern number. We demonstrate that orbital degrees of freedom in frustrated lattice systems lead to a narrowing of topologically nontrivial bands. This robust effect does not rely on fine-tuned long-range hopping parameters and is directly relevant to a wide class of transition-metal compounds.
Jörn W. F. Venderbos, Maria Daghofer, Jeroen van den Brink, Physical Review Letters 107, 116401 (2011), pdf | |
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Saturation Field of Frustrated Chain Cuprates: Broad Regions of Predominant Interchain Coupling
A thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field Hs (T=0) is proposed. Using modern theoretical methods we study how Hs is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J1-J2-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li2CuO2 and Ca2Y2Cu5O10, we show that Hs is solely determined by the IC and predict Hs≈61 T for the latter. With Hs≈55 T from magnetization data one reads out a weak IC for Li2CuO2 close to that obtained from inelastic neutron scattering.
S. Nishimoto, S.-L. Drechsler, R. O. Kuzian, J. van den Brink, J. Richter, W. E. A. Lorenz, Y. Skourski, R. Klingeler, and B. Büchner, Physical Review Letters 107, 097201 (2011), pdf | |
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Ab Initio determination of Cu 3d orbital energies in layered copper oxidesIt has long been argued that the minimal model to describe the low-energy physics of the high Tc superconducting cuprates must include copper states of other symmetries besides the canonical 3dx2-y2 one, in particular the 3dz2 orbital. Experimental and theoretical estimates of the energy splitting of these states vary widely. With a novel ab initio quantum chemical computational scheme we determine these energies for a range of copper-oxides and -oxychlorides, determine trends with the apical Cu–ligand distances and find excellent agreement with recent Resonant Inelastic X-ray Scattering measurements, available for La2CuO4, Sr2CuO2Cl2, and CaCuO2.
Liviu Hozoi, Liudmila Siurakshina, Peter Fulde & Jeroen van den Brink, Nature Scientific Reports 1, 65 (2011), pdf | |
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Macroscopic Degeneracy and Emergent Frustration in a Honeycomb Lattice MagnetUsing a hybrid method based on fermionic diagonalization and classical Monte Carlo techniques, we investigate the interplay between itinerant and localized spins, with competing double- and superexchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightly larger superexchange a dimerized ground state is stable that has macroscopic degeneracy. The presence of these states on a nonfrustrated honeycomb lattice highlights novel phenomena in this itinerant electron system: emergent geometrical frustration and degeneracy related to a symmetry intermediate between local and global.
Jörn W. F. Venderbos, Maria Daghofer, Jeroen van den Brink, and Sanjeev Kumar, Physical Review Letters 107, 076405 (2011), pdf | |
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Proximity of iron pnictide superconductors to a quantum tricritical pointIn several materials, unconventional superconductivity appears nearby a quantum phase transition where long-range magnetic order vanishes as a function of a control parameter like charge doping, pressure or magnetic field. The nature of the quantum phase transition is of key relevance, because continuous transitions are expected to favour superconductivity, due to strong fluctuations. Discontinuous transitions, on the other hand, are not expected to have a similar role. Here we determine the nature of the magnetic quantum phase transition, which occurs as a function of doping, in the iron-based superconductor LaFeAsO1–xFx. We use constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The outcome is intriguing, as this material turns out to be remarkably close to a quantum tricritical point, where the transition changes from continuous to discontinuous, and several susceptibilities diverge simultaneously. We discuss the consequences for superconductivity and the phase diagram.
Gianluca Giovannetti, Carmine Ortix, Martijn Marsman, Massimo Capone, Jeroen van den Brink & José Lorenzana, Nature Communications 2, 398 (2011), pdf | |
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Magnetic field dependence of the maximum adiabatic temperature changeThe field dependence of the maximum magnetocaloric ΔT-effect in ferromagnets with second-order phase transitions is studied by way of direct measurements. All studied materials are found to follow the formula ΔTmax = A(H+H0)2/3-AH02/3, where A and H0 are constants and H is the internal magnetic field. It is essential to distinguish the latter from external field Hext. The dependence of ΔTmax on Hext is qualitatively distinct, the difference being particularly pronounced in the low-field region. In the field range relevant to applications (0.1–2 T), ΔTmax follows a linear dependence on H2/3. It is proposed to use the slope of this dependence as a figure of merit of magnetic refrigerants.
M. D. Kuz'min, K. P. Skokov, D. Yu. Karpenkov, J. D. Moore, M. Richter, and O. Gutfleisch, Applied Physics Letters 99, 012501 (2011) , pdf | |
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| Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr0.9Ca0.1CuO2We report a comparative study of 63Cu nuclear magnetic resonance spin lattice relaxation rates T1-1 on undoped SrCuO2 and Ca-doped Sr0.9Ca0.1CuO2 spin chain compounds. A temperature independent T1-1 is observed for SrCuO2 as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T1-1 for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J1-J2 Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J2-exchange coupling as a possible cause of the spin gap.
F. Hammerath, S. Nishimoto, H.-J. Grafe, A. U. B. Wolter, V. Kataev, P. Ribeiro, C. Hess, S.-L. Drechsler, and B. Büchner , Physical Review Letters 107, 017203 (2011), pdf |
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Resonant inelastic x-ray scattering studies of elementary excitationsIn the past decade, resonant inelastic x-ray scattering (RIXS) has made remarkable progress as a spectroscopic technique. This is a direct result of the availability of high-brilliance synchrotron x-ray radiation sources and of advanced photon detection instrumentation. The technique’s unique capability to probe elementary excitations in complex materials by measuring their energy, momentum, and polarization dependence has brought RIXS to the forefront of experimental photon science. Both the experimental and theoretical RIXS investigations of the past decade are reviewed, focusing on those determining the low-energy charge, spin, orbital, and lattice excitations of solids. The fundamentals of RIXS as an experimental method are presented and then the theoretical state of affairs, its recent developments, and the different (approximate) methods to compute the dynamical RIXS response are reviewed. The last decade’s body of experimental RIXS data and its interpretation is surveyed, with an emphasis on RIXS studies of correlated electron systems, especially transition-metal compounds. Finally, the promise that RIXS holds for the near future is discussed, particularly in view of the advent of x-ray laser photon sources.
Luuk J.P. Ament, Michel van Veenendaal, Thomas P. Devereaux, John P. Hill, and Jeroen van den Brink, Reviews of Modern Physics, 83, 705–767 (2011), pdf
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Metacinnabar (β-HgS): A Strong 3D Topological Insulator with Highly Anisotropic Surface StatesWe establish the presence of topologically protected edge states on the (001) surface of HgS in the zinc-blende structure using density-functional electronic structure calculations. The Dirac point of the edge state cone is very close to the bulk valence band maximum. The Dirac cone is extremely anisotropic with a very large electron velocity along one diagonal of the surface elementary cell x' and a nearly flat dispersion in the perpendicular direction y'. The strong anisotropy originates from a broken fourfold rotoinversion symmetry at the surface.
Francois Virot, Roland Hayn, Manuel Richter, and Jeroen van den Brink, Physical Review Letters 106, 236806 (2011), pdf | |
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Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictide SuperconductorsWe introduce and study an extended “t-U-J” two-orbital model for the pnictides that includes Heisenberg terms deduced from the strong coupling expansion. Including these J terms explicitly allows us to enhance the strength of the (π,0)-(0,π) spin order which favors the presence of tightly bound pairing states even in the small clusters that are here exactly diagonalized. The A1g and B2g pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying B1g states, suggesting that small changes in parameters may render any of the three channels stable.
Andrew Nicholson, Weihao Ge, Xiaotian Zhang, José Riera, Maria Daghofer, Andrzej M. Oleś, George B. Martins, Adriana Moreo, and Elbio Dagotto, Physical Review Letters 106, 217002 (2011), pdf | |
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Spin-Exchange Dynamical Structure Factor of the S=1/2 Heisenberg ChainWe determine the spin-exchange dynamical structure factor of the Heisenberg spin chain, as is measured by indirect resonant inelastic x-ray scattering (RIXS). We find that two-spin RIXS excitations nearly entirely fractionalize into two-spinon states. These share the same continuum lower bound as single-spin neutron scattering excitations, even if the relevant final states belong to orthogonal symmetry sectors. The RIXS spectral weight is mainly carried by higher-energy excitations, and is beyond the reach of the low-energy effective theories of Luttinger liquid type. Antoine Klauser, Jorn Mossel, Jean-Sébastien Caux, and Jeroen van den Brink, Physical Review Letters 106, 157205 (2011), pdf | |
 | Existence, Character, and Origin of Surface-Related Bands in the High Temperature Iron Pnictide Superconductor BaFe2-xCoxAs2Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe2-xCoxAs2 (x=0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe2-xCoxAs2 system for accurate determination of self-energies using ARPES.
E. van Heumen, J. Vuorinen, K. Koepernik, F. Massee, Y. Huang, M. Shi, J. Klei, J. Goedkoop, M. Lindroos, J. van den Brink and M.S. Golden, Physical Review Letters 106, 027002 (2011), pdf |
 | High-Tc Ferroelectricity Emerging from Magnetic Degeneracy in Cupric OxideCupric oxide is multiferroic at unusually high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric, and the higher-T one is ferroelectric with a size and direction of polarization in good agreement with experiments. By mapping the ab initio results on to an effective spin model, we show that the system has a manifold of almost degenerate ground states. In the high-T magnetic state noncollinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii-Moriya interaction. This leads to an unconventional mechanism for multiferroicity, with the particular property that nonmagnetic impurities enhance the effect.
G. Giovanetti, S. Kumar, A. Stroppa, J. van den Brink, S. Picozzi and J. Lorenzana, Physical Review Letters 106, 026401 (2011), pdf |
2010 | |
Cobalt-carbon complexes with magnetic anisotropies larger than 0.2 eVWe recently reported a density-functional study of the heteronuclear CoIr dimer adsorbed on benzene or graphene. In either case, CoIr prefers an upright position above the center of a carbon hexagon with the Co atom next to it. (The figure shows the ground state geometry and spin state of the CoIrC_6H_6 complex, distances are given in Angstrom.) The Ir atom stays away from the carbon ring and thus preserves its free-atom-like properties. This results in a very large magnetic anisotropy of more than 0.2 eV per dimer. So high a value might suffice for long-term data storage at the temperature of liquid nitrogen, using one single magnetic molecule for each bit. In comparison with our earlier prediction for homonuclear Co dimers (see highligths 2009), the lower estimate for the magnetic anisotropy becomes five times higher by replacing one Co atom by Ir.Ruijuan Xiao, Michael D. Kuz'min, Klaus Koepernik and Manuel Richter, Appl. Phys. Lett. 97 (2010) 232501. weblink | |
T>0 ensemble-state density functional theory via Legendre transformA logical foundation of equilibrium state density functional theory in a Kohn-Sham-type formulation is presented on the basis of Mermin’s treatment of the grand canonical state by exploiting functional Legendre transforms. It is simpler and more satisfactory compared to the usual derivation of the ground-state theory and free of most remaining open points of the latter. The existence of the functional derivative of the corresponding density functional F[n] at all densities of grand canonical equilibrium states is proved even in the spin-density matrix version of the theory. It may, in particular, be relevant with respect to cases of spontaneous symmetry breaking such as noncollinear magnetism and orbital order.H. Eschrig, Physical Review B 82, 205120 (2010), pdf Selected for Viewpoint in Physics: Viewpoint | |
Measurement of Magnetic Excitations in the Two-Dimensional Antiferromagnetic Sr2CuO2Cl2 Insulator Using Resonant X-Ray Scattering: Evidence for Extended InteractionsWe measured the momentum dependence of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr2CuO2Cl2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (π, 0) and (π/2, π/2). Employing an extended t-t′-t′′-U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (π, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates.M. Guarise, L.J.P. Ament, J. van den Brink et al., Physical Review Letters 105, 157006 (2010), pdf | |
Frustration-Induced Insulating Chiral Spin State in Itinerant Triangular-Lattice MagnetsWe study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte Carlo methods. We find that in between the expected itinerant ferromagnetic and 120° Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet-scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets R2Mo2O7.S. Kumar, J. van den Brink, Physical Review Letters 105, 216405 (2010), pdf | |
Metal-Insulator Transition of Fermions on a Kagome Lattice at 1/3 FillingWe discuss the metal-insulator transition of the spinless fermion model on a kagome lattice at 1/3 filling. The system is analyzed by using exact diagonalization, density-matrix renormalization group methods, and random-phase approximation. In the strong-coupling region, the charge-ordered ground state is consistent with the predictions of an effective model, i.e., plaquette order. We find that the qualitative properties of the metal-insulator transition are totally different depending on the sign of the hopping matrix elements, reflecting the difference in the band structure near the Fermi level.S. Nishimoto et al., Physical Review Letters 104, 196401 (2010), pdf | |
Spin-Spiral States in Undoped Manganites - Role of Finite Hund’s Rule CouplingThe experimental observation of multiferroic behavior in perovskite manganites with a spiral spin structure requires a clarification of the origin of these magnetic states and their relation to ferroelectricity. We show that spin-spiral phases with a diagonal wave vector and also an E-type phase exist for intermediate value of Hund’s rule and the Jahn-Teller coupling in the orbitally ordered and insulating state of the standard two-band model Hamiltonian for manganites. Our results support the spin-current mechanism for ferroelectricity and present an alternative view to earlier conclusions where frustrating superexchange couplings were crucial to obtaining spin-spiral states.S. Kumar, J. van den Brink, et al., Physical Review Letters 104, 017201 (2010). pdf | |
Magnetic Excitations and Phase Separation and Resonant Inelastic X-Ray ScatteringWe probe the collective magnetic modes of La2CuO4 and underdoped La2-xSrxCuO4 (LSCO) bymomentum resolved resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge. For the undoped antiferromagnetic sample, we show that the single magnon dispersion measured with RIXS coincides with the one determined by inelastic neutron scattering, thus demonstrating that x rays are an alternative to neutrons in this field. In the spin dynamics of LSCO, we find a branch dispersing up to ~ 400 meV coexisting with one at lower energy. The high-energy branch has never been seen before. It indicates that underdoped LSCO is in a dynamic inhomogeneous spin state. L. Braicovich, J. van den Brink, et al., Physical Review Letters 104, 077002 (2010). pdf | |
Graphene - What lies betweenOpening a gap in graphene is still a considerable challenge on the path towards applications. A clever solution to this problem is to exploit the preferential adsorption of hydrogen in patterns that leave narrow stretches of pure graphene in between.The electronic properties of graphene — a one-atom-thick layer of graphite — are attracting ever-increasing attention. These single sheets of carbon already show electron mobilities comparable to presentday semiconductors. But a significant hurdle to overcome is that graphene intrinsically behaves more like a metal than a semiconductor. In contrast to silicon for instance it lacks a bandgap, which is essential to guarantee controlled and reliable transistor operation. Jeroen van den Brink, Nature Materials 9, 291 (2010). pdf | |
2009 | |
 | Co Dimers on Hexagonal Carbon Rings Proposed as Subnanometer Magnetic Storage BitsIt is demonstrated by means of density functional and ab initio quantum chemical calculations, that transition-metal–carbon systems have the potential to enhance the presently available area density of magnetic recording by 3 orders of magnitude. As a model system, Co2 benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy of the order of 0.1 eV per molecule, large enough to store permanently 1 bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite.R. Xiao et al., Physical Review Letters 103, 187201 (2009). pdf |
Multiferroicity in Rare-Earth Nickelates RNiO3We show that charge ordered rare-earth nickelates of the type RNiO3 (R ¼ Ho, Lu, Pr and Nd) are multiferroic with very large magnetically-induced ferroelectric (FE) polarizations. This we determine from first principles electronic structure calculations. The emerging FE polarization is directly tied to the long-standing puzzle of which kind of magnetic ordering is present in this class of materials: its direction and size indicate the type of ground-state spin configuration that is realized. Vice versa, the small energy differences between the different magnetic orderings suggest that a chosen magnetic ordering can be stabilized by cooling the system in the presence of an electric field.G. Giovannetti et al., Physical Review Letters 103, 156401 (2009). pdf | |
| g | Multiferroicity in TTF-CA Organic Molecular Crystals Predicted through Ab Initio CalculationsWe show by means of ab initio calculations that the organic molecular crystal TTF-CA is multiferroic: it has an instability to develop spontaneously both ferroelectric and magnetic ordering. Ferroelectricity is driven by a Peierls transition of the TTF-CA in its ionic state. Subsequent antiferromagnetic ordering strongly enhances the opposing electronic contribution to the polarization. It is so large that it switches the direction of the total ferroelectric moment. Within an extended Hubbard model, we capture the essence of the electronic interactions in TTF-CA, confirm the presence of a multiferroic groundstate, and clarify howthis state develops microscopically. G. Giovannetti et al., Physical Review Letters 103, 266401 (2009). pdf |
