Turan Birol

Rutgers, The State University of New Jersey, Нью-Брансуик, New Jersey, United States

Are you Turan Birol?

Claim your profile

Publications (36)196.88 Total impact

  • Source
    Nicole A. Benedek · Turan Birol
    [Show abstract] [Hide abstract]
    ABSTRACT: The recent observation of a ferroelectric-like structural transition in metallic LiOsO$_3$ has generated a flurry of interest in the properties of polar metals. Such materials are thought to be rare because free electrons screen out the long-range electrostatic forces that favor a polar structure with a dipole moment in every unit cell. In this work, we question whether long-range electrostatic forces are always the most important ingredient in driving polar distortions. We use crystal chemical models, in combination with first-principles Density Functional Theory calculations, to explore the mechanisms of inversion-symmetry breaking in LiOsO$_3$ and both insulating and electron-doped ATiO$_3$ perovskites, A = Ba, Sr, Ca. Although electrostatic forces do play a significant role in driving the polar instability of BaTiO$_3$ (which is suppressed under electron doping), the polar phases of CaTiO$_3$ and LiOsO$_3$ emerge through a mechanism driven by local bonding preferences and this mechanism is `resistant' to the presence of charge carriers. Hence, our results suggest that there is no fundamental incompatibility between metallicity and polar distortions. We use the insights gained from our calculations to suggest design principles for new polar metals and promising avenues for further research.
    Preview · Article · Nov 2015 · Journal of Materials Chemistry C
  • Source
    Aleksander L. Wysocki · Turan Birol
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the magnetically-induced phonon splitting in cubic ACr$_2$O$_4$ (A=Mg, Zn, Cd, Hg) spinels from first principles, and demonstrate that the sign of the splitting, which is experimentally observed to be opposite in CdCr$_2$O$_4$ compared to ZnCr$_2$O$_4$ and MgCr$_2$O$_4$, is determined solely by the particular magnetic ordering pattern observed in these compounds. We further show that this interaction between magnetism and phonon frequencies can be fully described by the previously proposed spin-phonon coupling model that includes only the nearest neighbor exchange. Finally, using this model with materials specific parameters calculated from first principles, we provide additional insights into the physics of spin-phonon coupling in this intriguing family of compounds.
    Full-text · Article · Aug 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a comprehensive study of single crystals of the newly discovered 112 iron pnictide superconductors (FBS). In Ca$_{0.73}$La$_{0.27}$FeAs$_2$, we unraveled a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which a distinct magnetic structure appears with the spins ordering 45$^\circ$ away from the stripe direction. Both phase transitions can be suppressed upon Co substitution on Fe sites and bulk superconductivity is stabilized up to 20 K. Furthermore, we demonstrate, as the structural and chemical consequences of the As chains in the spacer layers, this magnetic FBS is naturally structurally detwinned at ambient pressure with the formation of spin rotation walls (S-walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in FBS, angle-resolved photoemission (ARPES) measurements resolve a novel Fermiology where an extra electron pocket exists at the Brillouin zone edge with the As chain character. These unique features open a new avenue to clarify the role of electronic nematicity and metallic spacer layer in affecting the superconductivity.
    Full-text · Article · May 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using Raman spectroscopy, we investigate the lattice phonons, magnetic excitations, and magnetoelastic coupling in the distorted triangular-lattice Heisenberg antiferromagnet α-SrCr2O4, which develops helical magnetic order below 43 K. Temperature-dependent phonon spectra are compared to predictions from density functional theory calculations which allows us to assign the observed modes and identify weak effects arising from coupled lattice and magnetic degrees of freedom. Raman scattering associated with two-magnon excitations is observed at 20 and 40 meV. These energies are in general agreement with our ab initio calculations of exchange interactions and earlier theoretical predictions of the two-magnon Raman response of triangular-lattice antiferromagnets. The temperature dependence of the two-magnon excitations indicates that spin correlations persist well above the Néel temperature.
    No preview · Article · Apr 2015 · Physical Review B
  • Turan Birol · Kristjan Haule
    [Show abstract] [Hide abstract]
    ABSTRACT: Discovery of new transition metal compounds with large spin orbit coupling coexisting with strong electron-electron correlation among the d electrons is essential for understanding the physics that emerges from the interplay of these two effects. In this study, we predict a novel class of J_{eff}=1/2 Mott insulators in a family of fluoride compounds that are previously synthesized, but not characterized extensively. First principles calculations in the level of all electron density functional theory+dynamical mean field theory indicate that these compounds have large Mott gaps and some of them exhibit unprecedented proximity to the ideal, SU(2) symmetric J_{eff}=1/2 limit.
    No preview · Article · Mar 2015 · Physical Review Letters
  • Source
    Chuck-Hou Yee · Turan Birol · Gabriel Kotliar
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a framework for designing novel materials, combining modern first-principles electronic structure tools, materials databases, and evolutionary algorithms capable of exploring large configurational spaces. Guided by the chemical principles introduced by Antipov, \emph{et. al.}, for the design and synthesis of the Hg-based high-temperature superconductors, we apply our framework to design a new layered copper oxysulfide, Hg(CaS)$_2$CuO$_2$. We evaluate the prospects of superconductivity in this oxysulfide using theories based on charge-transfer energies, orbital distillation and uniaxial strain.
    Preview · Article · Mar 2015 · EPL (Europhysics Letters)
  • Turan Birol · Kristjan Haule
    [Show abstract] [Hide abstract]
    ABSTRACT: Discovery of new transition metal compounds with large spin orbit coupling coexisting with strong electron-electron correlation among the d electrons is essential for understanding the physics that emerges from the interplay of these two effects. In this study, we predict a novel class of Jeff=1/2 Mott insulators in a family of fluoride compounds that are previously synthesized, but not characterized extensively. First principles calculations in the level of all electron density functional theory+dynamical mean field theory indicate that these compounds have large Mott gaps and some of them exhibit unprecedented proximity to the ideal, SU(2) symmetric Jeff=1/2 limit.
    No preview · Article · Mar 2015 · Physical Review Letters
  • Source
    Kristjan Haule · Turan Birol
    [Show abstract] [Hide abstract]
    ABSTRACT: The stationary functional of the all-electron density functional plus dynamical mean field theory (DFT+DMFT) formalism to perform free energy calculations and structural relaxations is implemented for the first time. Here, the first order error in the density leads to a much smaller, second order error in the free energy. The method is applied to several well known correlated materials; metallic SrVO$_3$, Mott insulating FeO, and elemental Cerium, to show that it predicts the lattice constants with very high accuracy. In Cerium, we show that our method predicts the iso-structural transition between the $\alpha$ and $\gamma$ phases, and resolve the long standing controversy in the driving mechanism of this transition.
    Preview · Article · Jan 2015 · Physical Review Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: A series of states with different densities of stripes of Ir dimers is investigated using x-ray diffraction and density functional theory in layered nonmagnetic metal IrTe2. With decreasing temperature, structures with and without inversion symmetry alternate. In noncentrosymmetric states, spin-orbit coupling splits the electronic energy bands into spin-polarized pairs. Factors affecting the stability of the observed dimerized states are established, and it is conjectured that an infinite series of alternating states with and without polarized bands is realized in IrTe2. Switching dimerized states with different symmetries by changing temperature or strain enables control of band polarization, adding a new tool for spintronics and valleytronics research.
    No preview · Article · Nov 2014 · Physical Review B
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Recently, Lee et al. [Nature (London) 502, 532 (2013)] used ∼1% tensile strain to induce a ferroelectric instability in thin films of Srn+1TinO3n+1(n=1−6) phases. They showed that the Curie temperature TC gradually increased with n, reaching 180K for Sr7Ti6O19(n=6). The permittivity of this (n=6) sample could also be tuned significantly by the application of an electric field with exceptionally low dielectric loss at 300 K, rivaling all known tunable microwave dielectrics. Here, we present microwave (MW), terahertz, and infrared spectra of strained Srn+1TinO3n+1 thin films deposited on (110) DyScO3. Near the ferroelectric phase transitions, we observe the splitting and shifting of phonon and central mode frequencies, demonstrating the change of crystal symmetry below TC. Moreover, our spectra reveal that the central mode contribution dominates MW loss. In the Sr7Ti6O19 thin film, the central mode vanishes at 300 K, explaining its low MW loss. Finally, we discuss the origin and general conditions for the appearance of central modes near ferroelectric phase transitions.
    Full-text · Article · Nov 2014 · Physical Review B
  • Source
    Turan Birol · Kristjan Haule
    [Show abstract] [Hide abstract]
    ABSTRACT: Discovery of new transition metal compounds with large spin orbit coupling (SOC) coexisting with strong electron-electron correlation among the $d$ electrons is essential for understanding the physics that emerges from the interplay of these two effects. In this study, we predict a novel class of $J_{eff}=1/2$ Mott insulators in a family of fluoride compounds that are previously synthesized, but not characterized extensively. First principles calculations in the level of all electron Density Functional Theory + Dynamical Mean Field Theory (DFT+DMFT) indicate that these compounds have large Mott gaps and some of them exhibit unprecedented proximity to the ideal, $SU(2)$ symmetric $J_{eff}=1/2$ limit.
    Preview · Article · Aug 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using Raman spectroscopy, we investigate the lattice vibrations, magnetic excitations, and magneto-elastic coupling in the distorted triangular-lattice Heisenberg antiferromagnet alpha-SrCr2O4, which develops helical magnetic order below 43 K. Temperature dependent phonon spectra are compared to predictions from density functional theory calculations which allows us to assign the observed modes and identify weak effects arising from coupled lattice and magnetic degrees of freedom. Raman scattering associated with two-magnon excitations is observed at 15 meV and 38 meV. These energies are in general agreement with our ab-initio calculations of exchange interactions and earlier theoretical predictions of the two-magnon Raman response of triangular-lattice antiferromagnets. The temperature dependence of the two-magnon excitations indicates two-dimensional spin correlations persist well above the N\'eel temperature.
    Full-text · Article · Apr 2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: The crystal structure of layered metal IrTe 2 is determined using single-crystal x-ray diffraction. At T=220 K , it exhibits Ir and Te dimers forming a valence-bond crystal. Electronic structure calculations reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the dimers exhibiting a signature of covalent bonding character development. Evidence for significant charge disproportionation among the dimerized and nondimerized Ir (charge order) is presented. We argue that the structural transition is driven by the Ir dimerization and bonding, while electronic correlations (dynamical mean field theory corrections to density functional theory) and spin orbit coupling play a secondary role.
    No preview · Article · Feb 2014 · Physical Review Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the discovery of surface states in the perovskite superconductor [Tl_{4}]TlTe_{3} (Tl_{5}Te_{3}) and its nonsuperconducting tin-doped derivative [Tl_{4}](Tl_{0.4}Sn_{0.6})Te_{3} as observed by angle-resolved photoemission spectroscopy. Density functional theory calculations predict that the surface states are protected by a Z_{2} topology of the bulk band structure. Specific heat and magnetization measurements show that Tl_{5}Te_{3} has a superconducting volume fraction in excess of 95%. Thus Tl_{5}Te_{3} is an ideal material in which to study the interplay of bulk band topology and superconductivity.
    Full-text · Article · Jan 2014 · Physical Review Letters
  • Source

    Full-text · Dataset · Oct 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The miniaturization and integration of frequency-agile microwave circuits-relevant to electronically tunable filters, antennas, resonators and phase shifters-with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss-Srn+1TinO3n+1 phases-in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics-doping or strain-in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.
    Full-text · Article · Oct 2013 · Nature
  • Source
    Kristjan Haule · Turan Birol · Gabriel Kotliar
    [Show abstract] [Hide abstract]
    ABSTRACT: A combination of dynamical mean field theory and density functional theory, as implemented in Phys. Rev. B 81, 195107 (2010), is applied to both the early and late transition metal oxides. For fixed value of the local Coulomb repulsion U=10eV, without fine tuning, we obtain the main features of these series, such as the metallic character of SrVO$_3$ and the the insulating gaps of LaVO$_3$, LaTiO$_3$ and La$_2$CO$_4$ which are in good agreement with experiment. The study highlights the importance of local physics and high energy hybridization in the screening of the Hubbard interaction and how different low energy behaviors can emerge from the a unified treatment of the transition metal series.
    Preview · Article · Oct 2013 · Physical Review B
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Octahedral distortion plays a key role in engineering the physical properties of heterostructures composed of perovskite oxides. We observe a strong in-plane uniaxial magnetic anisotropy in a strain-enabled multiferroic EuTiO3 thin film epitaxially grown on a (110)o DyScO3 substrate. First principles calculations show that the magnetic anisotropy is closely correlated with the uniaxial TiO6 octahedral tilting and the ferroelectric polarization of the film, indicating potential strong magnetoelectric coupling in the strain-engineered multiferroic system.
    Full-text · Article · Sep 2013 · Physical Review B
  • Source
    Turan Birol · Craig J. Fennie
    [Show abstract] [Hide abstract]
    ABSTRACT: We elucidate the microscopic mechanism that causes a suppression of ferroelectricity and an enhancement of octahedral rotations in EuTiO3 from first principles. We find that the hybridization of the rare-earth Eu 4f states with the B-site Ti cation drives the system away from ferroelectricity. We also show that the magnetic order dependence of this hybridization is the dominant source of spin-phonon coupling in this material. Our results underline the importance of rare-earth f electrons on the lattice dynamics and stability of these transition metal oxides.
    Preview · Article · May 2013 · Physical review. B, Condensed matter
  • [Show abstract] [Hide abstract]
    ABSTRACT: Materials with strong spin-orbit coupling have attracted attention following the prediction and subsequent discovery of strong two- and three-dimensional topological insulators in which a topological property of the bulk band structure of an insulator results in metallic surface states with Dirac-like dispersion. Here we report the discovery of Dirac-like surface states in the perovskite superconductor [Tl$_4$]TlTe$_3$ (Tl5Te3) and its non-superconducting tin-doped derivative, [Tl4](Tl$_{0.4}$Sn$_{0.6}$)Te$_3$, as observed by angle-resolved photoemission spectroscopy (ARPES). Density functional theory (DFT) calculations predict a single spin-orbit driven band parity inversion at the $Z$ point above the Fermi level of Tl5Te3, suggesting the surface states are protected by Z$_2$ topology. Calculations on [Tl$_4$]SnTe$_3$ show no parity inversions, implying that a topological transition from non-trivial to trivial must occur upon doping with tin, i.e., [Tl$_4$](Tl$_{1-x}$Sn$_{x}$)Te$_3$. Thus [Tl$_4$]{\it M}Te$_3$ perovskites are a possible new, non-trigonal class of Z$_2$ topological compounds. Additionally, as Tl5Te3 is a stoichiometric bulk superconductor, these perovskites are ideal materials in which to study the interplay between surface states and bulk superconductivity.
    No preview · Article · Mar 2013

Publication Stats

196 Citations
196.88 Total Impact Points

Institutions

  • 2013-2015
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      Нью-Брансуик, New Jersey, United States
  • 2011-2015
    • Cornell University
      • School of Applied and Engineering Physics
      Итак, New York, United States