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T. Yildirim

Publications and source records attributed to T. Yildirim.

At least 19 recordsLinked to original sources
arXiv2019

Variation of the fundamental band gap nature in curved two-dimensional WS2

We report a strain-induced direct-to-indirect band gap transition in mechanically deformed WS2 monolayers (MLs). The necessary amount of strain is attained by proton irradiation of bulk WS2 and the ensuing formation of one-ML-thick, H2-filled domes. The electronic properties of the curved MLs are mapped by spatially- and time-resolved micro-photoluminescence revealing the mechanical stress conditions that trigger the variation of the band gap character. This general phenomenon, also observed in MoS2 and WSe2, further increases our understanding of the electronic structure of transition metal dichalcogenide MLs and holds a great relevance for their optoelectronic applications.

cond-mat.mtrl-sci↗
arXiv2013

Spin Dynamics of Trimers on a Distorted Kagomé Lattice

We 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 Kagomé lattice, subject to isotropic nearest neighbor (usually antiferromagnetic) Heisenberg interactions. The interactions between trimers are assumed to be weak compared to the intra trimer 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 intra-trimer zero point motion. Application to existing systems is briefly discussed.

cond-mat.str-el↗
arXiv2010

Charge and Spin Ordering in the Mixed Valence Compound LuFe2O4

Landau 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 transition 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 (20meV) 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.

cond-mat.str-el↗
arXiv2009

The Unprecedented Giant Coupling of Fe-spin State and the As-As Hybridization in Iron-Pnictide

From first principles calculations we unravel surprisingly strong interactions between arsenic ions in iron-pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant reduction in the c-axis. For CaFe$_2$As$_2$ system, this reduction is as large as 1.4 Å. Since the large c-reduction has been recently observed only under high-pressure\cite{cTphase}, our results suggest that the iron magnetic moment should be present in Fe-pnictides at all times at ambient pressure. Finally, the conventional electron-phonon coupling in the collapsed phase of CaFe$_2$As$_2$ gives a maximum $T_c$ of 0.6 K and can not explain the $\sim12$ K superconductivity observed in some experiments. Implications of these findings on the mechanism of superconductivity in iron-pnictides are discussed.

cond-mat.supr-con↗
arXiv2008

Charge and Spin Ordering in the Mixed Valence Compound LuFe2O4

Landau theory and symmetry considerations are invoked to give a unified treatment of charge and spin ordering in the mixed valence compound LuFe2O4. In particular, the unusual evolution of charge ordering correlations can be explained by invoking phonon mediated interactions.

cond-mat.str-el↗
arXiv2008

Structure and Magnetic Order in the NdFeAs(O,F) Superconductor System

The transition temperature Tc~26 K of the recently discovered superconductor LaFeAs(O,F) has been demonstrated to be extremely sensitive to the lanthanide ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is important to determine how the moment on the lanthanide affects the overall magnetism in these systems. Here we report a neutron diffraction study of the Nd oxypnictides. Long ranged antiferromagnetic order is apparent in NdFeAsO below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering are required to describe the observed data with the staggered moment 1.55(4) Bohr magneton per Nd and 0.9(1) Bohr magneton per Fe at 0.3 K. The other structural properties such as the tetragonal-orthorhombic distortion are found to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the structural distortion occur in the superconducting sample NdFeAsO0.80F0.20 at any temperatures down to 1.5 K.

cond-mat.supr-con↗
arXiv2008

External magnetic field effects on a distorted kagome antiferromagnet

We report bulk magnetization, and elastic and inelastic neutron scattering measurements under an external magnetic field, $H$, on the weakly coupled distorted kagome system, Cu_{2}(OD)_3Cl. Our results show that the ordered state below 6.7 K is a canted antiferromagnet and consists of large antiferromagnetic $ac$-components and smaller ferromagnetic $b$-components. By first-principle calculations and linear spin wave analysis, we present a simple spin hamiltonian with non-uniform nearest neighbor exchange interactions resulting in a system of coupled spin trimers with a single-ion anisotropy that can qualitatively reproduce the spin dynamics of Cu_{2}(OD)_3Cl.

cond-mat.str-el↗
arXiv2008

Neutron scattering study of the oxypnictide superconductor La(O,F)FeAs

The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnetic field of 9 T, consistent with the proposal that SDW and superconductivity are competing in the laminar materials. While our inelastic measurements offer no constraints on the spin dynamic response from d-wave pairing, an upper limit for the magnetic resonance peak predicted from an extended s-wave pairing mechanism is provided. Our measurements also support the energy scale of the calculated phonon spectrum which is used in electron-phonon coupling theory, and fails to produce the high observed Tc.

cond-mat.supr-con↗
arXiv2008

Origin of the ~150 K Anomaly in LaOFeAs; Competing Antiferromagnetic Superexchange Interactions, Frustration, and Structural Phase Transition

From first principles calculations we find that the nearest and next nearest neighbor superexchange interactions between Fe ions in LaOFeAs are large, antiferromagnetic (AF), and give rise to a frustrated magnetic ground state which consists of two interpenerating AF square sublattices with M(Fe)=0.48$μ_B$. The system lowers its energy further by removing the frustration via a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaOFeAs recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions and the frustrated ground state suggest that magnetism and superconductivity in doped LaOFeAs may be strongly coupled, much like in the high-T$_c$ cuprates.

cond-mat.supr-con↗
arXiv2007

Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study

Rhenium diboride is a recently recognized ultra-incompressible superhard material. Here we report the electronic (e), phonon (p), e-p coupling and thermal properties of ReB$_2$ from first-principles density-functional theory (DFT) calculations and neutron scattering measurements. Our calculated elastic constants ($c_{11}$ = 641 GPa, $c_{12}$ = 159 GPa, $c_{13}$ = 128 GPa, $c_{33}$ = 1037 GPa, and $c_{44}$ = 271 GPa), bulk modulus ($B$ $\approx$ 350 GPa) and hardness ($H$ $\approx$ 46 GPa) are in good agreement with the reported experimental data. The calculated phonon density of states (DOS) agrees very well with our neutron vibrational spectroscopy result. Electronic and phonon analysis indicates that the strong covalent B-B and Re-B bonding is the main reason for the super incompressibility and hardness of ReB$_2$. The thermal expansion coefficients, calculated within the quasi-harmonic approximation and measured by neutron powder diffraction, are found to be nearly isotropic in $a$ and $c$ directions and only slightly larger than that of diamond in terms of magnitude. The excellent agreement found between calculations and experimental measurements indicate that first-principles calculations capture the main interactions in this class of superhard materials, and thus can be used to search, predict, and design new materials with desired properties.

cond-mat.mtrl-sci↗
arXiv2007

Hydrogen Absorption Properties of Metal-Ethylene Complexes

Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a single ethylene molecule can form stable complexes with light transition metals (TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and 14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles molecular dynamics (MD) simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to 500 K. The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties.

cond-mat.mtrl-sci↗
arXiv2006

Lattice Dynamics of Metal-Organic Frameworks (MOFs): Neutron Inelastic Scattering and First-Principles Calculations

By combining neutron inelastic scattering (NIS) and first-principles calculations, we have investigated the lattice dynamics of MOF5. The structural stability of MOF5 was evaluated by calculating the three cubic elastic constants. We find that the shear modulus, c44 = 1.16 GPA, is unusually small, while two other moduli are relatively large (i.e. c11 = 29.42 GPa and c12 = 12.56 GPa). We predict that MOF5 is very close to structural instability and may yield interesting new phases under high pressure and strain. The phonon dispersion curves and phonon density of states were directly calculated and our simulated NIS spectrum agrees very well with our experimental data. Several interesting phonon modes are discussed, including the softest twisting modes of the organic linker.

cond-mat.mtrl-sci↗
arXiv2006

Complex magnetic order in the kagome staircase compound Co3V2O8

Co3V2O8 (CVO) has a geometrically frustrated magnetic lattice, a Kagome staircase. The crystal structure consists of two inequivalent 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 variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k=(0,delta,0) with delta=0.55 and the spin direction along the a axis. Delta is found to decrease monotonically with decreasing temperature, and then it locks into a commensurate antiferromagnetic structure with delta=0.5 for 6.9<T<8.6 K. Below 6.9 K the magnetic structure becomes incommensurate again. Delta continues to decrease with decreasing temperature, and locks-in again at delta=1/3 over a narrow temperature range (6.2<T<6.5 K). The system then undergoes a strongly first order transition to the ferromagnetic ground state (delta=0) at Tc=6.2 K. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating a 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.

cond-mat.str-el↗
arXiv2006

Magnetic Structure and Spin Waves in the Kagomé Jarosite compound ${\bf KFe_3(SO_4)_2(OH)_6}$

We present a detailed study of the magnetic structure and spin waves in the Fe jarosite compound ${\rm KFe_3(SO_4)_2(OH)_6}$ for the most general Hamiltonian involving one- and two-spin interactions which are allowed by symmetry. We compare the calculated spin-wave spectrum with the recent neutron scattering data of Matan {\it et al.} for various model Hamiltonians which include, in addition to isotropic Heisenberg exchange interactions between nearest ($J_1$) and next-nearest ($J_2$) neighbors, single ion anisotropy and Dzyaloshinskii-Moriya (DM) interactions. We concluded that DM interactions are the dominant anisotropic interaction, which not only fits all the splittings in the spin-wave spectrum but also reproduces the small canting of the spins out of the Kagomé plane. A brief discussion of how representation theory restricts the allowed magnetic structure is also given.

cond-mat.mtrl-sci↗
arXiv2006

Spin waves in the Frustrated Kagome Lattice Antiferromagnet KFe3(OH)6(SO4)2

The spin wave excitations of the ideal S=5/2 Kagome 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 Kagome 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.

cond-mat.str-el↗
arXiv2005

Towards a Microscopic Model of Magnetoelectric Interactions in Ni3V2O8

We develop a microscopic magnetoelectric coupling in Ni$_3$V$_2$O$_8$ (NVO) which gives rise to the trilinear phenomenological coupling used previously to explain the phase transition in which magnetic and ferroelectric order parameters appear simultaneously. Using combined neutron scattering measurements and first-principles calculations of the phonons in NVO, we determine eleven phonons which can induce the observed spontaneous polarization. Among these eleven phonons, we find that a few of them can actually induce a significant dipole moment. Using the calculated atomic charges, we find that the required distortion to induce the observed dipole moment is very small (~0.001 Å) and therefore it would be very difficult to observe the distortion by neutron-powder diffraction. Finally, we identify the derivatives of the exchange tensor with respect to atomic displacements which are needed for a microscopic model of a spin-phonon coupling in NVO and which we hope will be obtained from a fundamental quantum calculation such as LDA+U. We also analyze two toy models to illustrate that the Dzyaloskinskii-Moriya interaction is very important for coexisting of magnetic and ferroelectric order but it is not the only mechanism when the local site symmetry of the system is low enough.

cond-mat.mtrl-sci↗
arXiv2005

Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8

We present powder and single-crystal neutron diffraction and bulk measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to 8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This system contains two types of Ni ions, which we call "spine" and "cross-tie". Our neutron measurements can be described with the paramagnetic space group Cmca for T < 15K and each observed magnetically ordered phase is characterized by the appropriate irreducible representation(s). Our zero-field measurements show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally-modulated structure with the spine spins mainly parallel to the a-axis. Upon further cooling, a transition is induced at T_HL=6.3K to an elliptically polarized incommensurate structure with both spine and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a first-order phase transition, below which the magnetic structure is a commensurate antiferromagnet with the staggered magnetization primarily along the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat peak at T_CC'=2.3K indicates an additional transition, which we were however not able to relate to a change of the magnetic structure. Neutron, specific heat, and magnetization measurements produce a comprehensive temperature-field phase diagram. The symmetries of the two incommensurate magnetic phases are consistent with the observation that only one phase has a spontaneous ferroelectric polarization. All the observed magnetic structures are explained theoretically using a simplified model Hamiltonian, involving competing nearest- and next-nearest-neighbor exchange interactions, spin anisotropy, Dzyaloshinskii-Moriya and pseudo-dipolar interactions.

cond-mat.str-el↗