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52. Topological Electronic Structure and Intrinsic Magnetization in MnBi4Te7 : A Bi2Te3 Derivative with a Periodic Mn Sublattice

Author

Vidal, Raphael C., primary, Zeugner, Alexander, additional, Facio, Jorge I., additional, Ray, Rajyavardhan, additional, Haghighi, M. Hossein, additional, Wolter, Anja U. B., additional, Corredor Bohorquez, Laura T., additional, Caglieris, Federico, additional, Moser, Simon, additional, Figgemeier, Tim, additional, Peixoto, Thiago R. F., additional, Vasili, Hari Babu, additional, Valvidares, Manuel, additional, Jung, Sungwon, additional, Cacho, Cephise, additional, Alfonsov, Alexey, additional, Mehlawat, Kavita, additional, Kataev, Vladislav, additional, Hess, Christian, additional, Richter, Manuel, additional, Büchner, Bernd, additional, van den Brink, Jeroen, additional, Ruck, Michael, additional, Reinert, Friedrich, additional, Bentmann, Hendrik, additional, and Isaeva, Anna, additional

Published
2019
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53. Influence of lattice termination on the edge states of the quantum spin Hall insulator monolayer 1T′-WTe2

Author

Lau, A., Ray, Rajyavardhan, Varjas, D., and Akhmerov, A.R.

Abstract

We study the influence of sample termination on the electronic properties of the novel quantum spin Hall insulator monolayer 1T′-WTe2. For this purpose, we construct an accurate, minimal four-orbital tight-binding model with spin-orbit coupling by employing a combination of density-functional theory calculations, symmetry considerations, and fitting to experimental data. Based on this model, we compute energy bands and two-terminal conductance spectra for various ribbon geometries with different terminations, with and without a magnetic field. Because of the strong electron-hole asymmetry, we find that the edge Dirac point is buried in the bulk bands for most edge terminations. In the presence of a magnetic field, an in-gap edge Dirac point leads to exponential suppression of conductance as an edge Zeeman gap opens, whereas the conductance stays at the quantized value when the Dirac point is buried in the bulk bands. Finally, we find that disorder in the edge termination drastically changes this picture: the conductance of a sufficiently rough edge is uniformly suppressed for all energies in the bulk gap regardless of the orientation of the edge.

Published
2019

55. Influence of lattice termination on the edge states of the quantum spin Hall insulator monolayer 1T′-WTe2

Author

Lau, A. (author), Ray, Rajyavardhan (author), Varjas, D. (author), Akhmerov, A.R. (author), Lau, A. (author), Ray, Rajyavardhan (author), Varjas, D. (author), and Akhmerov, A.R. (author)

Abstract

We study the influence of sample termination on the electronic properties of the novel quantum spin Hall insulator monolayer 1T′-WTe2. For this purpose, we construct an accurate, minimal four-orbital tight-binding model with spin-orbit coupling by employing a combination of density-functional theory calculations, symmetry considerations, and fitting to experimental data. Based on this model, we compute energy bands and two-terminal conductance spectra for various ribbon geometries with different terminations, with and without a magnetic field. Because of the strong electron-hole asymmetry, we find that the edge Dirac point is buried in the bulk bands for most edge terminations. In the presence of a magnetic field, an in-gap edge Dirac point leads to exponential suppression of conductance as an edge Zeeman gap opens, whereas the conductance stays at the quantized value when the Dirac point is buried in the bulk bands. Finally, we find that disorder in the edge termination drastically changes this picture: the conductance of a sufficiently rough edge is uniformly suppressed for all energies in the bulk gap regardless of the orientation of the edge., QN/Akhmerov Group, QuTech, QRD/Kouwenhoven Lab

Published
2019
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56. Air-stable redox-active nanomagnets with lanthanide spins radical-bridged by a metal–metal bond

Author

Liu, Fupin; https://orcid.org/0000-0002-8454-726X, Velkos, Georgios, Krylov, Denis S, Spree, Lukas; https://orcid.org/0000-0002-0577-7770, Zalibera, Michal, Ray, Rajyavardhan, Samoylova, Nataliya A, Chen, Chia-Hsiang, Rosenkranz, Marco, Schiemenz, Sandra, Ziegs, Frank, Nenkov, Konstantin, Kostanyan, Aram, Greber, Thomas; https://orcid.org/0000-0002-5234-1937, Wolter, Anja U B, Richter, Manuel, Büchner, Bernd, Avdoshenko, Stanislav M, Popov, Alexey A; https://orcid.org/0000-0002-7596-0378, Liu, Fupin; https://orcid.org/0000-0002-8454-726X, Velkos, Georgios, Krylov, Denis S, Spree, Lukas; https://orcid.org/0000-0002-0577-7770, Zalibera, Michal, Ray, Rajyavardhan, Samoylova, Nataliya A, Chen, Chia-Hsiang, Rosenkranz, Marco, Schiemenz, Sandra, Ziegs, Frank, Nenkov, Konstantin, Kostanyan, Aram, Greber, Thomas; https://orcid.org/0000-0002-5234-1937, Wolter, Anja U B, Richter, Manuel, Büchner, Bernd, Avdoshenko, Stanislav M, and Popov, Alexey A; https://orcid.org/0000-0002-7596-0378

Abstract

Engineering intramolecular exchange interactions between magnetic metal atoms is a ubiquitous strategy for designing molecular magnets. For lanthanides, the localized nature of 4f electrons usually results in weak exchange coupling. Mediating magnetic interactions between lanthanide ions via radical bridges is a fruitful strategy towards stronger coupling. In this work we explore the limiting case when the role of a radical bridge is played by a single unpaired electron. We synthesize an array of air-stable Ln2@C80(CH2Ph) dimetallofullerenes (Ln2 = Y2, Gd2, Tb2, Dy2, Ho2, Er2, TbY, TbGd) featuring a covalent lanthanide-lanthanide bond. The lanthanide spins are glued together by very strong exchange interactions between 4f moments and a single electron residing on the metal–metal bonding orbital. Tb2@C80(CH2Ph) shows a gigantic coercivity of 8.2 Tesla at 5 K and a high 100-s blocking temperature of magnetization of 25.2 K. The Ln-Ln bonding orbital in Ln2@C80(CH2Ph) is redox active, enabling electrochemical tuning of the magnetism.

Published
2019

59. Discovery, Crystal Growth, and Characterization of Garnet Eu2PbSb2Zn3O12.

Author

Morrow, Ryan, Sturza, Mihai I., Ray, Rajyavardhan, Himcinschi, Cameliu, Kern, Jonas, Schlender, Philipp, Richter, Manuel, Wurmehl, Sabine, and Büchner, Bernd

Subjects
CRYSTAL growth, BAND gaps, RARE earth metals, X-ray crystallography, SOLID state chemistry, GARNET, RAMAN effect
Abstract

Single crystal specimens of previously unknown garnet Eu2PbSb2Zn3O12 were grown in a reactive PbO:PbF2 flux medium. The crystals were characterized by a combination of X‐ray crystallography, magnetization measurements, and the optical techniques of Raman, photoluminescence, and UV/Vis spectroscopy. The material exhibits Van Vleck paramagnetism associated with the J = 0 state of Eu3+, which was possible to accurately fit to a theoretical model. Band structure calculations were performed and compared to the experimental band gap of 1.98 eV. The crystals demonstrate photoluminescence associated with the 4f6 configuration of the Eu3+ ions sitting at the distorted 8‐coordinate garnet A site. The title compound represents a unique quinary contribution to a relatively unexplored area of rare earth bearing garnet crystal chemistry. [ABSTRACT FROM AUTHOR]

Published
2020
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72. Optical And Electronic Properties Of Double Perovskite Ba2ScSbO6.

Author

Ray, Rajyavardhan, Himanshu, A. K., Lahiri, J., Kumar, Uday, Sen, Pintu, Bandyopadhyay, S. K., and Sinha, T. P.

Subjects
*OPTICAL properties of barium compounds, *DENSITY functional theory, *SPACE groups, *PEROVSKITE, *ELECTRONIC structure, *SOLID state chemistry
Abstract

The ordered double perovskite Ba2ScSbO6 (BSS) has been synthesized in polycrystalline form by solid state reaction at 1400 C for 72 Hrs. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis. The crystal structure is cubic, with space group Fm-3m (No. 225) and lattice parameter, a = 8.20 Å. Optical band-gap has been calculated using UV-Vis Spectroscopy and Kubelka-Munk (KM) function, yielding 4.23 eV. A detailed Abinitio Density Functional Theory (DFT) study of the electronic properties has been carried out using the Full-Potential Linear Augmented Plane Wave (FP-LAPW) as implemented in WIEN2k. BSS is found to be a large band-gap insulator with potential technological applications. [ABSTRACT FROM AUTHOR]

Published
2016
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73. Optical And Electronic Properties Of Double Perovskite Ba2ScSbO6.

Author

Ray, Rajyavardhan, Himanshu, A. K., Lahiri, J., Kumar, Uday, Sen, Pintu, Bandyopadhyay, S. K., and Sinha, T. P.

Subjects
OPTICAL properties of barium compounds, DENSITY functional theory, SPACE groups, PEROVSKITE, ELECTRONIC structure, SOLID state chemistry
Abstract

The ordered double perovskite Ba2ScSbO6 (BSS) has been synthesized in polycrystalline form by solid state reaction at 1400 C for 72 Hrs. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis. The crystal structure is cubic, with space group Fm-3m (No. 225) and lattice parameter, a = 8.20 Å. Optical band-gap has been calculated using UV-Vis Spectroscopy and Kubelka-Munk (KM) function, yielding 4.23 eV. A detailed Abinitio Density Functional Theory (DFT) study of the electronic properties has been carried out using the Full-Potential Linear Augmented Plane Wave (FP-LAPW) as implemented in WIEN2k. BSS is found to be a large band-gap insulator with potential technological applications. [ABSTRACT FROM AUTHOR]

Published
2016
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74. Dielectric relaxation, structural and thermal studies of 95 MeV O ion irradiated conducting polymer polyaniline–polyvinyl alcohol.

Author

Himanshu, A. K., Ray, Rajyavardhan, El-Sayed, S., Hassen, A., Bandyopadhayay, S. K., and Sinha, T. P.

Subjects
*POLYANILINES synthesis, *POLYVINYL alcohol, *CALORIMETERS, *SCANNING electron microscopes, *X-ray diffraction
Abstract

Polyaniline (PANI) film prepared from using water-soluble support polymer polyvinyl alcohol (PVA) has been irradiated with 95 MeV oxygen (O6+) ions at the fluences from 1011to 1012 ions/cm2. The thermal, dielectric, structural and microstructural properties of the samples are investigated by differential scanning calorimeter and thermogravimetric analysis, LCR meter, X-ray diffraction and scanning electron microscope. The structural studies reveal that amorphization on irradiation which may be due to the scission of polymer bonds as there is a signature of columnar tracts and the appearance of voids in the sample increases with increasing O6+ions irradiation-dose. Frequency-dependent spectra of the imaginary electric modulus suggest that long-range conduction in pristine sample changes to short-range,i.e.localized conduction in 95 MeV O6+ion-irradiation PANI–PVA. The behaviour of the ac conductivity reveals that the conduction mechanism in both pristine and irradiated sample is correlated to the barrier hopping model. [ABSTRACT FROM PUBLISHER]

Published
2014
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75. Phase Transitions In Double Perovskite Sr2ScSbO6: An Ab-initio Study.

Author

Ray, Rajyavardhan, Himanshu, Ajay K., Brajesh, Kumar, Bandyopadhyay, S. K., Kumar, Uday, and Sinha, T. P.

Subjects
*PHASE transitions, *PEROVSKITE, *STRONTIUM compounds, *AB initio quantum chemistry methods, *DENSITY functional theory, *ELECTRONIC structure, *BAND gaps
Abstract

First Principles study of the electronic properties of recently synthesized double perovskite Sr2ScSbO6 have been performed using density functional theory. With increasing temperature, the Sr compound undergoes three structural phase transitions at 400K, 550K and 650K approximately, leading to the following sequence of phases: P21/n → I2/m → I4/m → Fm-3m. Starting from the monoclinic phase P21/n at room temperature, resulting from the Sc/Sb ordering, the electronic structure for the tetragonal I4/m at 613K and cubic Fm-3m for T≤ 660K has been studied in terms of the density of states and band-structure. Presence of large band gap, both direct and indirect, has been reported and analyzed. [ABSTRACT FROM AUTHOR]

Published
2014
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76. Electronic Structure of Ordered Double Perovskite Ba2CoWO6.

Author

Ray, Rajyavardhan, Himanshu, A. K., Brajesh, Kumar, Choudhary, B. K., Bandyopadhyay, S. K., Sen, Pintu, Kumar, Uday, and Sinha, T. P.

Subjects
*ELECTRONIC structure, *PEROVSKITE, *BARIUM compounds, *POLYCRYSTALS, *CHEMICAL synthesis, *SOLID state chemistry, *X-ray diffraction
Abstract

Ba2CoWO6 (BCoW) has been synthesized in polycrystalline form by solid state reaction at 1200 °C. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis of the XRD pattern. The crystal structure is cubic, space group Fm-3m (No. 225) with the lattice parameter, a = 8.210. Optical bandgap of the present system has been calculated using the UV-Vis Spectroscopy and Kubelka-Munk function, it's value being 2.45 eV. A detailed study of the electronic properties has also been carried out using the density functional theory (DFT) techniques implemented on WIEN2k. Importance of electron-electron interaction between the Co ions leading to half-metallic behavior, crystal and exchange splitting together with the hybridization between O and Co, W has been investigated using the total and partial density of states. [ABSTRACT FROM AUTHOR]

Published
2014
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77. Electronic Structure of Ordered Double Perovskite Ba2CoWO6.

Author

Ray, Rajyavardhan, Himanshu, A. K., Brajesh, Kumar, Choudhary, B. K., Bandyopadhyay, S. K., Sen, Pintu, Kumar, Uday, and Sinha, T. P.

Subjects
ELECTRONIC structure, PEROVSKITE, BARIUM compounds, POLYCRYSTALS, CHEMICAL synthesis, SOLID state chemistry, X-ray diffraction
Abstract

Ba2CoWO6 (BCoW) has been synthesized in polycrystalline form by solid state reaction at 1200 °C. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis of the XRD pattern. The crystal structure is cubic, space group Fm-3m (No. 225) with the lattice parameter, a = 8.210. Optical bandgap of the present system has been calculated using the UV-Vis Spectroscopy and Kubelka-Munk function, it's value being 2.45 eV. A detailed study of the electronic properties has also been carried out using the density functional theory (DFT) techniques implemented on WIEN2k. Importance of electron-electron interaction between the Co ions leading to half-metallic behavior, crystal and exchange splitting together with the hybridization between O and Co, W has been investigated using the total and partial density of states. [ABSTRACT FROM AUTHOR]

Published
2014
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78. Electronic structure calculation of Sr2CoWO6 double perovskite using DFT+U.

Author

Mandal, Golak, Jha, Dhiraj, Himanshu, A. K., Ray, Rajyavardhan, Mukherjee, P., Das, Nisith, Singh, B. K., Sreenivas, K., Singh, M. N., Sinha, A. K., Singh, Biswas, and Das

Subjects
SYNCHROTRONS, RAMAN spectroscopy, LATTICE constants, DENSITY functional theory, ELECTRIC insulators & insulation, SEMIMETALS, BAND gaps
Abstract

Using the synchrotron and Raman spectroscopy we measured the lattice parameter and Raman modes of the half-metallic (HM) Sr2CoWO6 (SCoW) synthesied by the solid state reaction technique.. The physical properties of SCoW are studies within the framework of density function theory (DFT) under the generalised gradient approximation (GGA) of Perdew, Bruke, and Ernzerhof both by itself and including a coulomb repulsion via the Hubbard approach or GGA+U. Our results states that Sr2CoWO6 material behaves as insulators for the spin-up orientation and spindown orientation as found for the half metallic systems and at U = 0.06eV the ground state of spin up channel being insulating with spin gap of 2.27eV comparable to the experimental Band gap (BG). [ABSTRACT FROM AUTHOR]

Published
2018
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79. Ab-initio study of double perovskite Ba2YSbO6.

Author

Mondal, Golak, Jha, D., Himanshu, A. K., Lahiri, J., Singh, B. K., Kumar, Uday, Ray, Rajyavardhan, Singh, Biswas, and Das

Subjects
DENSITY functional theory, ELECTRONIC structure, PEROVSKITE, POLYCRYSTALS, X-ray diffraction, LATTICE constants, PLANE wavefronts
Abstract

The density functional theory with generalized gradient approximation has been used to investigate the electronic structure of double perovskite oxide Ba2YSbO6 (BYS) synthesized in polycrystalline form by solid state reaction. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Riedvelt analysis of the XRD pattern. The crystal structure is cubic, space group being Fm-3m (No. 225) with the lattice parameter, a = 8.424 Ǻ. Optical band-gap of this system has been calculated using UV-Vis Spectroscopy and Kubelka-Munk (KM) function, having the value 4.56eV. A detailed study of the electronic properties has also been carried out using the Full-Potential Linear Augmented Plane Wave (FPLAPW) as implemented in WIEN2k. BYS is found to be a large band-gap insulator with potential technological applications, such as dielectric resonators and filters in microwave applications. [ABSTRACT FROM AUTHOR]

Published
2018
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80. Intercalant-mediated Kitaev exchange in Ag3LiIr2O6

Author

Yadav, Ravi, Reja, Sahinur, Ray, Rajyavardhan, van den Brink, Jeroen, Nishimoto, Satoshi, and Yazyev, Oleg, V

Subjects
liquid
Abstract

The recently synthesized Ag3LiIr2O6 has been proposed as a Kitaev magnet in proximity to the quantum spin liquid phase. We explore its microscopic Hamiltonian and magnetic ground state using many-body quantum chemistry methods and exact diagonalization techniques. Our calculations establish a dominant bond dependent ferromagnetic Kitaev exchange between Ir sites and find that the inclusion of Ag 4d orbitals in the configuration interaction calculations strikingly enhances the Kitaev exchange. Furthermore, using exact diagonalization of the nearest-neighbor fully anisotropic J-K-Gamma Hamiltonian, we obtain the magnetic phase diagram as a function of further neighbor couplings. We find that the antiferromagnetic off-diagonal coupling stabilizes long range order, but the structure factor calculations suggest that the material is very close to the quantum spin liquid phase and the ordered state can easily collapse into a liquid by small perturbations such as structural distortion or bond disorder.

81. Strong Effect of Hydrogen Order on Magnetic Kitaev Interactions in H3LiIr2O6.

Author

Yadav, Ravi, Ray, Rajyavardhan, Eldeeb, Mohamed S., Satoshi Nishimoto, Liviu Hozoi, and van den Brink, Jeroen

Subjects
*HYDROGEN, *QUANTUM chemistry, *QUANTUM liquids
Abstract

Very recently a quantum liquid was reported to form in H3LiIr2O6, an iridate proposed to be a close realization of the Kitaev honeycomb model. To test this assertion we perform detailed quantum chemistry calculations to determine the magnetic interactions between Ir moments. We find that weakly bond dependent ferromagnetic Kitaev exchange dominates over other couplings, but still is substantially lower than in Na2IrO3. This reduction is caused by the peculiar position of the interlayer species: removing hydrogen cations next to a Ir2O2 plaquette increases the Kitaev exchange by more than a factor of 3 on the corresponding Ir-Ir link. Consequently, any lack of hydrogen order will have a drastic effect on the magnetic interactions and strongly promote spin disordering. [ABSTRACT FROM AUTHOR]

Published
2018
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82. Discovery, Crystal Growth, and Characterization of Garnet Eu2PbSb2Zn3O12.

Author

Morrow, Ryan, Sturza, Mihai I., Ray, Rajyavardhan, Himcinschi, Cameliu, Kern, Jonas, Schlender, Philipp, Richter, Manuel, Wurmehl, Sabine, and Büchner, Bernd

Subjects
*CRYSTAL growth, *BAND gaps, *RARE earth metals, *X-ray crystallography, *SOLID state chemistry, *GARNET, *RAMAN effect
Abstract

Single crystal specimens of previously unknown garnet Eu2PbSb2Zn3O12 were grown in a reactive PbO:PbF2 flux medium. The crystals were characterized by a combination of X‐ray crystallography, magnetization measurements, and the optical techniques of Raman, photoluminescence, and UV/Vis spectroscopy. The material exhibits Van Vleck paramagnetism associated with the J = 0 state of Eu3+, which was possible to accurately fit to a theoretical model. Band structure calculations were performed and compared to the experimental band gap of 1.98 eV. The crystals demonstrate photoluminescence associated with the 4f6 configuration of the Eu3+ ions sitting at the distorted 8‐coordinate garnet A site. The title compound represents a unique quinary contribution to a relatively unexplored area of rare earth bearing garnet crystal chemistry. [ABSTRACT FROM AUTHOR]

Published
2020
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83. Ab-initio study of double perovskite Ba2YSbO6.

Author

Mondal, Golak, Jha, D., Himanshu, A. K., Lahiri, J., Singh, B. K., Kumar, Uday, Ray, Rajyavardhan, Singh, Biswas, and Das

Subjects
*DENSITY functional theory, *ELECTRONIC structure, *PEROVSKITE, *POLYCRYSTALS, *X-ray diffraction, *LATTICE constants, *PLANE wavefronts
Abstract

The density functional theory with generalized gradient approximation has been used to investigate the electronic structure of double perovskite oxide Ba2YSbO6 (BYS) synthesized in polycrystalline form by solid state reaction. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Riedvelt analysis of the XRD pattern. The crystal structure is cubic, space group being Fm-3m (No. 225) with the lattice parameter, a = 8.424 Ǻ. Optical band-gap of this system has been calculated using UV-Vis Spectroscopy and Kubelka-Munk (KM) function, having the value 4.56eV. A detailed study of the electronic properties has also been carried out using the Full-Potential Linear Augmented Plane Wave (FPLAPW) as implemented in WIEN2k. BYS is found to be a large band-gap insulator with potential technological applications, such as dielectric resonators and filters in microwave applications. [ABSTRACT FROM AUTHOR]

Published
2018
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84. Engineering Kitaev exchange in stacked iridate layers: impact of inter-layer species on in-plane magnetism.

Author

Yadav R, Eldeeb MS, Ray R, Aswartham S, Sturza MI, Nishimoto S, van den Brink J, and Hozoi L

Abstract

Novel functionalities may be achieved in oxide electronics by appropriate stacking of planar oxide layers of different metallic species, MO p and M'O q . The simplest mechanism allowing the tailoring of the electronic states and physical properties of such heterostructures is of electrostatic nature-charge imbalance between the M and M' cations. Here we clarify the effect of interlayer electrostatics on the anisotropic Kitaev exchange in H 3 LiIr 2 O 6 , a recently proposed realization of the Kitaev spin liquid. By quantum chemical calculations, we show that the precise position of H + cations between magnetically active [LiIr 2 O 6 ] 3- honeycomb-like layers has a strong impact on the magnitude of Kitaev interactions. In particular, it is found that stacking with straight interlayer O-H-O links is detrimental to in-plane Kitaev exchange since coordination by a single H-ion of the O ligand implies an axial Coulomb potential at the O site and unfavorable polarization of the O 2p orbitals mediating the Ir-Ir interactions. Our results therefore provide valuable guidelines for the rational design of Kitaev quantum magnets, indicating unprecedented Kitaev interactions of ≈40 meV if the linear interlayer linkage is removed.

Published
2018
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