Publications (36)196.88 Total impact
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ABSTRACT: The recent observation of a ferroelectriclike 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 longrange electrostatic forces that favor a polar structure with a dipole moment in every unit cell. In this work, we question whether longrange electrostatic forces are always the most important ingredient in driving polar distortions. We use crystal chemical models, in combination with firstprinciples Density Functional Theory calculations, to explore the mechanisms of inversionsymmetry breaking in LiOsO$_3$ and both insulating and electrondoped 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.  [Show abstract] [Hide abstract]
ABSTRACT: We study the magneticallyinduced 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 spinphonon 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 spinphonon coupling in this intriguing family of compounds. 
Article: Naturally structurally detwinned Fe pnictide superconducting family with metallic spacer layers
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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 (Swalls). Finally, in addition to the centralhole and cornerelectron Fermi pockets usually appearing in FBS, angleresolved 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.  [Show abstract] [Hide abstract]
ABSTRACT: Using Raman spectroscopy, we investigate the lattice phonons, magnetic excitations, and magnetoelastic coupling in the distorted triangularlattice Heisenberg antiferromagnet αSrCr2O4, which develops helical magnetic order below 43 K. Temperaturedependent 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 twomagnon 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 twomagnon Raman response of triangularlattice antiferromagnets. The temperature dependence of the twomagnon excitations indicates that spin correlations persist well above the Néel temperature.  [Show abstract] [Hide abstract]
ABSTRACT: Discovery of new transition metal compounds with large spin orbit coupling coexisting with strong electronelectron 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.  [Show abstract] [Hide abstract]
ABSTRACT: We describe a framework for designing novel materials, combining modern firstprinciples 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 Hgbased hightemperature 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 chargetransfer energies, orbital distillation and uniaxial strain.  [Show abstract] [Hide abstract]
ABSTRACT: Discovery of new transition metal compounds with large spin orbit coupling coexisting with strong electronelectron 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.  [Show abstract] [Hide abstract]
ABSTRACT: The stationary functional of the allelectron 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 isostructural transition between the $\alpha$ and $\gamma$ phases, and resolve the long standing controversy in the driving mechanism of this transition.  [Show abstract] [Hide abstract]
ABSTRACT: A series of states with different densities of stripes of Ir dimers is investigated using xray diffraction and density functional theory in layered nonmagnetic metal IrTe2. With decreasing temperature, structures with and without inversion symmetry alternate. In noncentrosymmetric states, spinorbit coupling splits the electronic energy bands into spinpolarized 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.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: Discovery of new transition metal compounds with large spin orbit coupling (SOC) coexisting with strong electronelectron 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.  [Show abstract] [Hide abstract]
ABSTRACT: Using Raman spectroscopy, we investigate the lattice vibrations, magnetic excitations, and magnetoelastic coupling in the distorted triangularlattice Heisenberg antiferromagnet alphaSrCr2O4, 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 twomagnon excitations is observed at 15 meV and 38 meV. These energies are in general agreement with our abinitio calculations of exchange interactions and earlier theoretical predictions of the twomagnon Raman response of triangularlattice antiferromagnets. The temperature dependence of the twomagnon excitations indicates twodimensional spin correlations persist well above the N\'eel temperature.  [Show abstract] [Hide abstract]
ABSTRACT: The crystal structure of layered metal IrTe 2 is determined using singlecrystal xray diffraction. At T=220 K , it exhibits Ir and Te dimers forming a valencebond crystal. Electronic structure calculations reveal an intriguing quasitwodimensional 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.  [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 tindoped derivative [Tl_{4}](Tl_{0.4}Sn_{0.6})Te_{3} as observed by angleresolved 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. 
Dataset: PhysRevB.EToonLSAT
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ABSTRACT: The miniaturization and integration of frequencyagile microwave circuitsrelevant to electronically tunable filters, antennas, resonators and phase shifterswith microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasistatic electric field. Appropriate systems such as BaxSr1xTiO3 have a paraelectricferroelectric 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 lossSrn+1TinO3n+1 phasesin which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating nonstoichiometry. 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 ferroelectricsdoping or strainin 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.  [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.  [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 inplane uniaxial magnetic anisotropy in a strainenabled 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 strainengineered multiferroic system.  [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 rareearth Eu 4f states with the Bsite Ti cation drives the system away from ferroelectricity. We also show that the magnetic order dependence of this hybridization is the dominant source of spinphonon coupling in this material. Our results underline the importance of rareearth f electrons on the lattice dynamics and stability of these transition metal oxides. 
Article: Topological Surface States and Superconductivity in [Tl$_4$](Tl$_{1x}$Sn$_{x}$)Te$_3$ Perovskites
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ABSTRACT: Materials with strong spinorbit coupling have attracted attention following the prediction and subsequent discovery of strong two and threedimensional topological insulators in which a topological property of the bulk band structure of an insulator results in metallic surface states with Diraclike dispersion. Here we report the discovery of Diraclike surface states in the perovskite superconductor [Tl$_4$]TlTe$_3$ (Tl5Te3) and its nonsuperconducting tindoped derivative, [Tl4](Tl$_{0.4}$Sn$_{0.6}$)Te$_3$, as observed by angleresolved photoemission spectroscopy (ARPES). Density functional theory (DFT) calculations predict a single spinorbit 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 nontrivial to trivial must occur upon doping with tin, i.e., [Tl$_4$](Tl$_{1x}$Sn$_{x}$)Te$_3$. Thus [Tl$_4$]{\it M}Te$_3$ perovskites are a possible new, nontrigonal 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.
Publication Stats
196  Citations  
196.88  Total Impact Points  
Top Journals
 Physical Review B (6)
 Physical Review Letters (5)
 Physical Review Letters (3)
 Applied Physics Letters (1)
 Nature (1)
Institutions

20132015

Rutgers, The State University of New Jersey
 Department Physics and Astronomy
НьюБрансуик, New Jersey, United States


20112015

Cornell University
 School of Applied and Engineering Physics
Итак, New York, United States
