Publications by authors named "Turan Birol"

14 Publications

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Dopant Segregation Inside and Outside Dislocation Cores in Perovskite BaSnO and Reconstruction of the Local Atomic and Electronic Structures.

Nano Lett 2021 May 11;21(10):4357-4364. Epub 2021 May 11.

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.

Distinct dopant behaviors inside and outside dislocation cores are identified by atomic-resolution electron microscopy in perovskite BaSnO with considerable consequences on local atomic and electronic structures. Driven by elastic strain, when A-site designated La dopants segregate near a dislocation core, the dopant atoms accumulate at the Ba sites in compressively strained regions. This triggers formation of Ba vacancies adjacent to the core atomic sites resulting in reconstruction of the core. Notwithstanding the presence of extremely large tensile strain fields, when La atoms segregate inside the dislocation core, they become B-site dopants, replacing Sn atoms and compensating the positive charge of the core oxygen vacancies. Electron energy-loss spectroscopy shows that the local electronic structure of these dislocations changes dramatically due to segregation of the dopants inside and around the core ranging from formation of strong La-O hybridized electronic states near the conduction band minimum to insulator-to-metal transition.
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http://dx.doi.org/10.1021/acs.nanolett.1c00966DOI Listing
May 2021

Metallic line defect in wide-bandgap transparent perovskite BaSnO.

Sci Adv 2021 Jan 15;7(3). Epub 2021 Jan 15.

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA.

A line defect with metallic characteristics has been found in optically transparent BaSnO perovskite thin films. The distinct atomic structure of the defect core, composed of Sn and O atoms, was visualized by atomic-resolution scanning transmission electron microscopy (STEM). When doped with La, dopants that replace Ba atoms preferentially segregate to specific crystallographic sites adjacent to the line defect. The electronic structure of the line defect probed in STEM with electron energy-loss spectroscopy was supported by ab initio theory, which indicates the presence of Fermi level-crossing electronic bands that originate from defect core atoms. These metallic line defects also act as electron sinks attracting additional negative charges in these wide-bandgap BaSnO films.
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http://dx.doi.org/10.1126/sciadv.abd4449DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7810381PMC
January 2021

Voltage-induced ferromagnetism in a diamagnet.

Sci Adv 2020 Jul 29;6(31):eabb7721. Epub 2020 Jul 29.

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA.

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that these devices either electrically tune a known ferromagnet or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we demonstrate that ferromagnetism can be voltage-induced even from a diamagnetic (zero-spin) state suggesting that useful magnetic phases could be electrically induced in "nonmagnetic" materials. We use ionic liquid-gated diamagnetic FeS as a model system, showing that as little as 1 V induces a reversible insulator-metal transition by electrostatic surface inversion. Anomalous Hall measurements then reveal electrically tunable surface ferromagnetism at up to 25 K. Density functional theory-based modeling explains this in terms of Stoner ferromagnetism induced via filling of a narrow band.
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http://dx.doi.org/10.1126/sciadv.abb7721DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7439324PMC
July 2020

Sputtered SrNbO as a UV-Transparent Conducting Film.

ACS Appl Mater Interfaces 2020 Jul 25;12(27):30520-30529. Epub 2020 Jun 25.

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

Expanding the application space of transparent electrodes toward the ultraviolet range has been found challenging when utilizing the conventional approach to degenerately dope semiconductors with band gaps larger than ZnO or InO. Here, it is shown that the correlated metal SrNbO with < 1 is ideally suited as a UV-transparent electrode material, enabling UV light-emitting diodes for a wide range of applications from water disinfection to polymer curing. It is demonstrated that SrNbO thin films can be grown by radio frequency (RF) sputtering and that they remain in the perovskite phase despite a sizeable Sr deficiency. The electrical and optical properties are characterized and compared to those of commonly used indium tin oxide (ITO) and Sn-doped GaO transparent conductor standards. SrNbO films were found to have sheet resistances as low as 30 Ω sq with optical transmission at a wavelength of 280 nm up to 86%, marking a two-order-of-magnitude increase over the performance of traditional UV-transparent conductors. The compatibility of SrNbO with a physical vapor deposition technique that is widely employed in the transparent conductor coating industry along with the robustness of the highly electrically conducting and optically transparent perovskite phase makes it an ideal transparent electrode for applications in the UV spectrum.
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http://dx.doi.org/10.1021/acsami.0c04854DOI Listing
July 2020

Spin-lattice Coupling and the Emergence of the Trimerized Phase in the S=1 Kagome Antiferromagnet Na_{2}Ti_{3}Cl_{8}.

Phys Rev Lett 2020 Apr;124(16):167203

Department of Physics, Florida State University, Tallahassee, Florida 32306, USA.

Spin-1 antiferromagnets are abundant in nature, but few theories exist to understand their properties and behavior when geometric frustration is present. Here we study the S=1 kagome compound Na_{2}Ti_{3}Cl_{8} using a combination of density functional theory, exact diagonalization, and density matrix renormalization group approaches to achieve a first principles supported explanation of its exotic magnetic phases. We find that the effective magnetic Hamiltonian includes essential non-Heisenberg terms that do not stem from spin-orbit coupling, and both trimerized and spin-nematic magnetic phases are relevant. The experimentally observed structural transition to a breathing kagome phase is driven by spin-lattice coupling, which favors the trimerized magnetic phase against the quadrupolar one. We thus show that lattice effects can be necessary to understand the magnetism in frustrated magnetic compounds and surmise that Na_{2}Ti_{3}Cl_{8} is a compound that cannot be understood from only electronic or only lattice Hamiltonians, very much like VO_{2}.
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http://dx.doi.org/10.1103/PhysRevLett.124.167203DOI Listing
April 2020

Engineering SrSnO Phases and Electron Mobility at Room Temperature Using Epitaxial Strain.

ACS Appl Mater Interfaces 2018 Dec 4;10(50):43802-43808. Epub 2018 Dec 4.

National Synchrotron Radiation Laboratory , University of Science and Technology of China , Hefei , Anhui 230026 , China.

High-speed electronics require epitaxial films with exceptionally high carrier mobility at room temperature (RT). Alkaline-earth stannates with high RT mobility show outstanding prospects for oxide electronics operating at ambient temperatures. However, despite significant progress over the last few years, mobility in stannate films has been limited by dislocations because of the inability to grow fully coherent films. Here, we demonstrate the growth of coherent, strain-engineered phases of epitaxial SrSnO (SSO) films using a radical-based molecular beam epitaxy approach. Compressive strain stabilized the high-symmetry tetragonal phase of SSO at RT, which, in bulk, exists only at temperatures between 1062 and 1295 K. We achieved a mobility enhancement of over 300% in doped films compared with the low-temperature orthorhombic polymorph. Using comprehensive temperature-dependent synchrotron-based X-ray measurements, electronic transport, and first principles calculations, crystal and electronic structures of SSO films were investigated as a function of strain. We argue that strain-engineered films of stannate will enable high mobility oxide electronics operating at RT with the added advantage of being optically transparent.
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http://dx.doi.org/10.1021/acsami.8b16592DOI Listing
December 2018

Phonon Softening due to Melting of the Ferromagnetic Order in Elemental Iron.

Phys Rev Lett 2018 May;120(18):187203

Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019, USA.

We study the fundamental question of the lattice dynamics of a metallic ferromagnet in the regime where the static long-range magnetic order is replaced by the fluctuating local moments embedded in a metallic host. We use the ab initio density functional theory + embedded dynamical mean-field theory functional approach to address the dynamic stability of iron polymorphs and the phonon softening with an increased temperature. We show that the nonharmonic and inhomogeneous phonon softening measured in iron is a result of the melting of the long-range ferromagnetic order and is unrelated to the first-order structural transition from the bcc to the fcc phase, as is usually assumed. We predict that the bcc structure is dynamically stable at all temperatures at normal pressure and is thermodynamically unstable only between the bcc-α and the bcc-δ phases of iron.
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http://dx.doi.org/10.1103/PhysRevLett.120.187203DOI Listing
May 2018

Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO.

Nat Commun 2016 11 24;7:13547. Epub 2016 Nov 24.

Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, 5232 Villigen, Switzerland.

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross-section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system contains not only the phonon dispersion but also the so far undetected magnetic correlation function. Here we report the observation of strong magnon-phonon coupling in LiCrO that enables the measurement of magnetic correlations throughout the Brillouin zone via IXS. We find electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and heavily damped electromagnons in the paramagnetic phase of LiCrO. We predict that several (frustrated) magnets with dominant direct exchange and non-collinear magnetism show surprisingly large IXS cross-section for magnons and multi-magnon processes.
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http://dx.doi.org/10.1038/ncomms13547DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5123047PMC
November 2016

Atomic scale imaging of competing polar states in a Ruddlesden-Popper layered oxide.

Nat Commun 2016 08 31;7:12572. Epub 2016 Aug 31.

Department of Materials Science and Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA.

Layered complex oxides offer an unusually rich materials platform for emergent phenomena through many built-in design knobs such as varied topologies, chemical ordering schemes and geometric tuning of the structure. A multitude of polar phases are predicted to compete in Ruddlesden-Popper (RP), An+1BnO3n+1, thin films by tuning layer dimension (n) and strain; however, direct atomic-scale evidence for such competing states is currently absent. Using aberration-corrected scanning transmission electron microscopy with sub-Ångstrom resolution in Srn+1TinO3n+1 thin films, we demonstrate the coexistence of antiferroelectric, ferroelectric and new ordered and low-symmetry phases. We also directly image the atomic rumpling of the rock salt layer, a critical feature in RP structures that is responsible for the competing phases; exceptional quantitative agreement between electron microscopy and density functional theory is demonstrated. The study shows that layered topologies can enable multifunctionality through highly competitive phases exhibiting diverse phenomena in a single structure.
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http://dx.doi.org/10.1038/ncomms12572DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5013660PMC
August 2016

Free Energy from Stationary Implementation of the DFT+DMFT Functional.

Phys Rev Lett 2015 Dec 16;115(25):256402. Epub 2015 Dec 16.

Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey, USA.

The stationary functional of the density functional plus embedded dynamical mean field theory 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 good accuracy. In cerium, we show that our method predicts the isostructural transition between the α and γ phases, and resolve the long-standing controversy in the driving mechanism of this transition.
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http://dx.doi.org/10.1103/PhysRevLett.115.256402DOI Listing
December 2015

J(eff)=1/2 Mott-insulating state in Rh and Ir fluorides.

Phys Rev Lett 2015 Mar 5;114(9):096403. Epub 2015 Mar 5.

Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA.

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.
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http://dx.doi.org/10.1103/PhysRevLett.114.096403DOI Listing
March 2015

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics.

Nature 2013 Oct 16;502(7472):532-6. Epub 2013 Oct 16.

1] Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA [2] Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA [3].

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.
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http://dx.doi.org/10.1038/nature12582DOI Listing
October 2013

Interface control of emergent ferroic order in Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1).

Phys Rev Lett 2011 Dec 13;107(25):257602. Epub 2011 Dec 13.

School of Applied Engineering Physics, Cornell University, Ithaca, New York 14853, USA.

We discovered from first principles an unusual polar state in the low n Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO(3). These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.
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http://dx.doi.org/10.1103/PhysRevLett.107.257602DOI Listing
December 2011