Yildirim, Taner
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Publication Complex Magnetic Order in the Kagomé Staircase Compound Co3V2O8(2006-07-25) Chen, Ying; Lynn, Jeffrey W; Huang, Qingzhen; Yildirim, Taner; Woodward, F. Matthew; Lawes, Gavin J; Ramirez, Arthur P; Rogado, Nyrissa S; Cava, Robert J; Aharony, Amnon; Harris, A. Brooks; Entin-Wohlman, OraCo3V2O8 (CVO) has a different type of geometrically frustrated magnetic lattice, a kagomé staircase, where the full frustration of a conventional kagomé lattice is partially relieved. The crystal structure consists of two inequivalent (magnetic) Co sites, one-dimensional chains of Co(2) spine sites, linked by Co(1) cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a rich variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3K into an incommensurate, transversely polarized, spin density wave state, with wave vector k=(0,δ,0) with δ=0.55 and the spin direction along the a axis. δ is found to decrease monotonically with decreasing temperature and then locks into a commensurate antiferromagnetic structure with δ=1/2 for 6.9<8.6K. In this phase, there is a ferromagnetic layer where the spine site and cross-tie sites have ordered moments of 1.39μB and 1.17μB, respectively, and an antiferromagnetic layer where the spine-site has an ordered moment of 2.55μB, while the cross-tie sites are fully frustrated and have no observable ordered moment. Below 6.9K, the magnetic structure becomes incommensurate again, and the presence of higher-order satellite peaks indicates that the magnetic structure deviates from a simple sinusoid. δ continues to decrease with decreasing temperature and locks in again at δ=1/3 over a narrow temperature range (6.2<6.5K). The system then undergoes a strongly first-order transition to the ferromagnetic ground state (δ=0) at Tc=6.2K. The ferromagnetism partially relieves the cross-tie site frustration, with ordered moments on the spine-site and cross-tie sites of 2.73μB and 1.54μB, respectively. The spin direction for all spins is along the a axis (Ising-like behavior). A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2K, demonstrating that there is significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.Publication Orientational Phases for M3C60(1993-10-15) Yildirim, Taner; Harris, A. Brooks; Mele, Eugene J; Hong, SuklyunThe mechanism of the orientational ordering of C60 in alkali-metal-doped fullerenes M3C60 is studied. Since the M-C60 (M=K,Rb) interactions cause the C60 molecules to assume one of two standard orientations, this model is equivalent to a generalized Ising model on a fcc lattice. The Ising interactions depend on two type of energies: (1) the direct interaction, i.e., the orientationally dependent part of interactions between nearest-neighboring C60 molecules (each carrying charge -3e), and (2) the band energy of the electrons transferred from M+ ions to the C603- ions. It is shown that the contribution to the pairwise interaction from the direct orientational interaction is ferromagnetic and dominantly nearest neighbor. However, contributions from the band (kinetic) energy of the conduction electrons are found to be antiferromagnetic for first- and third-nearest neighbors, ferromagnetic for second- and fourth-nearest neighbors, and negligible for further neighbors. The total first-neighbor interaction is probably antiferromagnetic. a non-negligible four-spin interaction is also obtained. The implication of these results for the orientational structure is discussed.Publication Origin of the Exceptional Negative Thermal Expansion in Metal-Organic Framework-5 Zn4O(1,4-benzenedicarboxylate)3(2008-08-20) Zhou, Wei; Wu, Hui; Yildirim, Tanner; Simpson, Jeffrey R; Hight Walker, Angela R.Metal organic framework-5 (MOF-5)was recently suggested to possess an exceptionally large negative thermal-expansion coefficient. Our direct experimental measurement of the thermal expansion of MOF-5 using neutron powder diffraction, in the temperature range of 4 to 600 K, shows that the linear thermal-expansion coefficient is ≈−16×10−6 K−1. To understand the origin of this large negative thermal-expansion behavior, we performed first-principles lattice dynamics calculations. The calculated thermal-expansion coefficients within quasiharmonic approximation agree well with the experimental data. We found that almost all lowfrequency lattice vibrational modes (below ∼23 meV) involve the motion of the benzene rings and the ZnO4 clusters as rigid units and the carboxyl groups as bridges. These so-called “rigid-unit modes” exhibit various degrees of phonon softening (i.e., the vibrational energy decreases with contracting crystal lattice) and thus are directly responsible for the large negative thermal expansion in MOF-5. Initial efforts were made to observe the phonon softening experimentally.Publication SpinWaves in the Frustrated Kagomé Lattice Antiferromagnet KFe3(OH)6(SO4)2(2006-06-19) Matan, Kittiwit; Grohol, Daniel; Yildirim, Taner; Nocera, Daniel G.; Harris, A. Brooks; Lee, Seunghun H.; Nagler, Stephen E.; Lee, Young SThe spin wave excitations of the S = 5/2 kagomé lattice antiferromagnet KFe3(OH)6(SO4)2 have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted ‘‘zero energy mode,’’ verifying a fundamental prediction for the kagomé lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin wave data. The antisymmetric Dzyaloshinskii-Moriya interaction is the primary source of anisotropy and explains the low-temperature magnetization and spin structure.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 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 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 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 Symmetry Analysis of the 2a Phase of C60(1994-07-15) Harris, A. Brooks; Yildirim, Taner; Sachidanandam, RaviA symmetry analysis of the 2a phase recently observed in some samples of C60 is presented. This phase is described by a unit cell with eight molecules in inequivalent orientations. We first show that if this structure is assumed to be exactly cubic, there are only three allowed space groups, none of which corresponds to the Pa3¯ arrangement of threefold axes previously established for C60 by several groups. Our calculated powder diffraction spectra for these space groups are not consistent with existing experimental data. Second, if the symmetry of the Pa3¯ structure is lowered by a doubling of the unit cell, we show that the resulting structure is trigonal, space group R3¯. We calculate powder diffraction spectra for this scenario and thereby place upper limits on both the angular distortion and the trigonal lattice distortion. Third, since the microscopic origin of this distortion probably involves defects of some presently unknown type, we consider a phenomenological scenario for the origin of this trigonal distortion. Within this scenario, we study the symmetry of the interactions needed to explain this structure. We start by giving an analysis of the structural distortion within harmonic lattice dynamics. However, to obtain the correct (R3¯) symmetry structure we were forced to study the cubic coupling between zone-corner librons and macroscopic strains. In this way we relate the development of R3¯ symmetry from the Pa3¯ structure in terms of a phenomenological model of lattice dynamics. Fourth, we extend the above arguments to construct a Landau theory for the hypothesized Pa3¯→R3¯ phase transition, which occurs as a function of the concentration of the presumed defects. The resulting free energy has no cubic terms (so the transition can be continuous) but has five fourth-order invariants.