Yildirim, Taner
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Publication Charge and Spin Ordering in the Mixed-Valence Compound LuFe2O4(2010-04-15) Harris, A. Brooks; Yildirim, TanerLandau theory and symmetry considerations lead us to propose an explanation for several seemingly paradoxical behaviors of charge ordering (CO) and spin ordering (SO) in the mixed valence compound LuFe2O4. Both SO and CO are highly frustrated. We analyze a lattice gas model of CO within mean-field theory and determine the magnitude of several of the phenomenological interactions. We show that the assumption of a continuous phase transitions at which CO or SO develops implies that both CO and SO are incommensurate. To explain how ferroelectric fluctuations in the charge-disordered phase can be consistent with an antiferroelectric-ordered phase, we invoke an electron-phonon interaction in which a low-energy (20 meV) zone-center transverse phonon plays a key role. The energies of all the zone center phonons are calculated from first principles. We give a Landau analysis which explains SO and we discuss a model of interactions which stabilizes the SO state, if it is assumed commensurate. However, we suggest a high-resolution experimental determination to see whether this phase is really commensurate, as believed up to now. The applicability of representation analysis is discussed. A tentative explanation for the sensitivity of the CO state to an applied magnetic field in field-cooled experiments is given.Publication Frustration and Quantum Fluctuations in Heisenberg fcc Antiferromagnets(1998-08-01) Yildirim, Taner; Harris, A. Brooks; Shender, Eugene FWe consider the quantum Heisenberg antiferromagnet on a face-centered-cubic lattice in which J, the second-neighbor (intrasublattice) exchange constant, dominates J′, the first-neighbor (intersublattice) exchange constant. It is shown that the continuous degeneracy of the classical ground state with four decoupled (in a mean-field sense) simple cubic antiferromagnetic sublattices is removed so that at second order in J′/J the spins are collinear. Here we study the degeneracy between the two inequivalent collinear structures by analyzing the contribution to the spin-wave zero-point energy which is of the form Heff/J=C0+C4σ1σ2σ3σ4(J′/J)4+O(J′/J)5, where σi specifies the phase of the ith collinear sublattice, C0 depends on J′/J but not on the σ’s, and C4 is a positive constant. Thus the ground state is one in which the product of the σ’s is −1. This state, known as the second kind of type A, is stable in the range |J′|<2|J| for large S. Using interacting spin-wave theory, it is shown that the main effect of the zero-point fluctuations is at small wave vector and can be well modeled by an effective biquadratic interaction of the form ΔEQeff=−1/2Q∑i,j[S(i)⋅S(j)]2/S3. This interaction opens a spin gap by causing the extra classical zero-energy modes to have a nonzero energy of order J′√S. We also study the dependence of the zero-point spin reduction on J′/J and the sublattice magnetization on temperature. The resulting experimental consequences are discussed.Publication Neutron Vibrational Spectroscopy and First-Principles Calculations of the Ternary Hydrides Li4Si2H(D) and Li4Ge2H(D): Electronic Structure and Lattice Dynamics(2007-12-17) Wu, Hui; Zhou, Wei; Udovic, Terrence J.; Rush, John J.; Yildirim, Tanner; Hartman, Michael R.; Bowman, Robert C.; Vajo, John J.Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li4Tt2H (Tt=Si and Ge). In these compounds, all hydrogen atoms are located in a single type of Li6-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li6-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the Tt-Tt chain are bonded covalently. The electronic density of states reveals that both hydrides exhibit metallic behavior. The observed vibrational spectra of these hydrides are in good overall agreement with the calculated phonon modes. There is evidence of dispersion induced splitting in the optical phonon peaks that can be ascribed to the coupling of H vibrations within the Li6-octahedra layers.Publication Spin Dynamics of Trimers on a Distorted Kagome Lattice(2013-07-11) Harris, A. Brooks; Yildirim, TanerWe treat the ground state, elementary excitations, and neutron scattering cross section for a system of trimers consisting of three tightly bound spins 1/2 on a distorted kagome lattice, subject to isotropic nearest-neighbor (usually antiferromagnetic) Heisenberg interactions. The interactions between trimers are assumed to be weak compared to the intratrimer interactions. We compare the spin-wave excitation spectrum of trimers with that obtained from standard spin-wave theory and attribute the differences at low energy to the fact that the trimer formulation includes exactly the effects of intratrimer zero-point motion.Publication Lattice Dynamics of Solid C60(1992-09-15) Yildirim, Taner; Harris, A. BrooksThe lattice dynamics of C60 has been studied first by means of group theory and then by diagonalizing the dynamical matrix for two recently proposed intermolecular potentials. The libron and phonon energies are calculated as a function of momentum along various symmetry directions with and without phonon–libron interactions. The effects of these interactions on the density of states are also discussed. Explicit expressions for the energies of these modes at zero wave vector are given. It is found that both potential models have nearly the same phonon but a somewhat different libron spectrum. The calculated libron energies agree reasonably well with currently available experimental results.Publication Transition-Metal-Ethylene Complexes as High-Capacity Hydrogen-Storage Media(2006-11-30) Durgun, Engin; Ciraci, Salim; Zhou, Wei; Yildirim, Tanerwith two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of ~ 14 wt%. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.Publication Spin Structures of Tetragonal Lamellar Copper Oxides(1994-06-06) Yildirim, Taner; Harris, A. Brooks; Entin-Wohlman, Ora; Aharony, AmnonThe spin Hamiltonian of tetragonal lamellar antiferromagnets is shown to contain several novel anisotropies. Symmetry allows bond-dependent anisotropic exchange interactions, which lead to (a) interplane mean-field coupling and (b) an in-plane anisotropy which vanishes classically but arises from quantum zero point energy (QZPE). A similar QZPE involving the interplane isotropic interaction prefers collinear spins. Adding also diploar anisotropy, the competition between all these effects explains for the first time the spin structures of many cuprates.Publication Structure and Interstitial Deuterium site of ß-phase ZrNi Deuteride(2007-02-09) Wu, Hui; Zhou, Wei; Udovic, Terrence J.; Rush, John J.; Yildirim, Tanner; Huang, Qingzhen; Bowman, Robert C.ß-ZrNiD1-x (for x≈0.1, near the ß-γ phase boundary) was found to possess a triclinic P1(overline) structure as determined by high-resolution neutron power diffraction. This is very different from the widely accepted orthohombic and distorted orthorhombic Cmcm structures previously proposed. In contrast to the single type of D site associated with these latter structures, the true ß-ZrNiD1-x structure contains two crystallographically distinct interstitial D sites: "Zr4Ni2" octahedral sites and "Zr4" tetrahedral sites, alternately ordered along the a direction. From first-principles calculations, the total energy of the P1(overline) structure was found to be ≈0.24 eV per unit cell lower than Cmcm-symmetry ZrNiD and could be rationalized in terms of different D local-bonding configurations and metal-deuterium interactions. Resultant phonon calculations based on this structure were also consistnet with the measured neutron vibrational spectrum.Publication Charge and Spin Ordering in the Mixed-Valence Compound LuFe2O4(2010-04-15) Harris, A Brooks; Yildirim, TannerLandau theory and symmetry considerations lead us to propose an explanation for several seemingly paradoxical behaviors of charge ordering (CO) and spin ordering (SO) in the mixed valence compound LuFe2O4. Both SO and CO are highly frustrated. We analyze a lattice gas model of CO within mean-field theory and determine the magnitude of several of the phenomenological interactions. We show that the assumption of a continuous phase transitions at which CO or SO develops implies that both CO and SO are incommensurate. To explain how ferroelectric fluctuations in the charge-disordered phase can be consistent with an antiferroelectric-ordered phase, we invoke an electron-phonon interaction in which a low-energy (20 meV) zone-center transverse phonon plays a key role. The energies of all the zone center phonons are calculated from first principles. We give a Landau analysis which explains SO and we discuss a model of interactions which stabilizes the SO state, if it is assumed commensurate. However, we suggest a high-resolution experimental determination to see whether this phase is really commensurate, as believed up to now. The applicability of representation analysis is discussed. A tentative explanation for the sensitivity of the CO state to an applied magnetic field in field-cooled experiments is given.Publication Quantum Dynamics of a Hydrogen Molecule Confined in a Cylindrical Potential(2003-06-27) Yildirim, Taner; Harris, A. BrooksWe study the coupled rotation-vibration levels of a hydrogen molecule in a confining potential with cylindrical symmetry. We include the coupling between rotations and translations and show how this interaction is essential to obtain the correct degeneracies of the energy level scheme. We applied our formalism to study the dynamics of H2 molecules inside a “smooth” carbon nanotube as a function of tube radius. The results are obtained both by numerical solution of the (2J+1)-component radial Schrödinger equation and by developing an effective Hamiltonian to describe the splitting of a manifold of states of fixed angular momentum J and number of phonons N. For nanotube radius smaller than ≈3.5Å, the confining potential has a parabolic shape and the results can be understood in terms of a simple toy model. For larger radius, the potential has the “Mexican hat” shape and therefore the H2 molecule is off centered, yielding radial and tangential translational dynamics in addition to rotational dynamics of H2 molecule which we also describe by a simple model. Finally, we make several predictions for the the neutron scattering observation of various transitions between these levels.