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Papers by tanmoy das

Physica B-condensed Matter, 2005

We study the systematics of the electronic structures of the carbides of the transition series Sc... more We study the systematics of the electronic structures of the carbides of the transition series Sc, Ti, V, Zr and Hf and the actinides U and Th using the tight-binding linear muffin-tin orbitals method (TB-LMTO). The densities of states, charge distribution and energy bands are studied in the lowest energy configuration. Changes in the bonding configurations and the equations of state are also compared as we go from 3d Ti to 5d–4f Hf. For the actinide carbides, the f-states have features rather different from the 3d–5d transition metal ...

Physical Review B, 2008

Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct ... more Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature TcT_cTc, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order.

We report first principles computations of the dynamical structure factor S(q,φ) in the electron ... more We report first principles computations of the dynamical structure factor S(q,φ) in the electron doped cuprates Nd2-xCexCuO2 as a function of energy φ and momentum q extended over several Brillouin zones. We show the efficacy of obtaining S(q,φ) through the use of simple products of real space Green functions and fast Fourier transform (FFT). We also calculate the susceptibility χ(q,φ) efficiently by recalling that S(q,φ) is proportional to the imaginary part of the susceptibility χ(q,φ). The present work is useful in going beyond the standard LDA-based modeling of various highly resolved spectroscopies.[1-4] We will provide some illustrative examples. Work supported by the US DOE.[1] R. S. Markiewicz et al., Phys. Rev. Lett. 96, 107005 (2006).[2] S. Sahrakorpi et al., Phys. Rev. Lett. 95, 157601 (2005).[3] G. Stutz et al., Phys. Rev. B 60, 7099 (1999).[4] J. Nieminen et al., Phys. Rev. Lett. 102, 037001 (2009).

Physical Review B, 2009

An anomalous “high-energy kink” HEK in dispersion, which gives the associated angle-resolved phot... more An anomalous “high-energy kink” HEK in dispersion, which gives the associated angle-resolved photoemission ARPES spectrum the appearance of a “waterfall” was first seen 1, 2 in Bi2Sr2CaCu2O8 Bi2212 cuprate superconductors. Such HEKs or waterfalls have now been established as being a universal feature in the cuprates, 1–3 and interpreted as providing evidence for interaction of the quasiparticles with some bosonic mode of the system. 4, 5 The high-energy scale of this boson 500 meV would then provide a tangible ...

Physical Review B, 2011

We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of ... more We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of the overdoped high-T_c superconductor La_2-xSr_xCuO_4 in the hole doping range x = 0.20-0.30. Our computations are based on a real-space Green's function approach in which strong-correlation effects are taken into account in terms of a doping-dependent self-energy. The predicted O K-edge XANES is found to be in good accord with the corresponding experimental results in this overdoped regime. We find that the low energy spectra are dominated by the contribution of O-atoms in the cuprate planes, with little contribution from apical O-atoms.

Journal of Physics and Chemistry of Solids, 2011

We discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cupr... more We discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cuprates where effects of matrix element, crystal structure, strong electron correlations, and superconductivity are included realistically in material-specific detail. A number of illustrative examples drawn from our recent work are presented. Specific issues in the cuprate physics considered are: (i) Origin of high-energy kink or the waterfall effect; (ii) Dichroic effects in angle-resolved photoemission spectrum; (iii) Asymmetry of the scanning tunneling spectrum between the processes of electron extraction and injection; (iv) Persistence of 'Mott' like high-energy features with doping in optical spectra; (v) Magnetic excitations in electron and hole doped cuprates.

We show that a paramagnetic self-energy correction [1] to the real-space Green's function code FE... more We show that a paramagnetic self-energy correction [1] to the real-space Green's function code FEFF9 [2] can provide a good description of the x-ray absorption spectra (XAS) of cuprate system such as La(2-x)Sr(x)CuO4 (LSCO). This self energy includes coupling to both charge and magnetic excitations. We also find good agreement with recent XAS results of Peet et al. [3] in the over-doped regime of LSCO. We have also investigated various prescriptions for including core-hole effects. We infer that at low doping, the system behaves as an anti-ferromagnetic insulator, while Fermi liquid physics is recovered at high doping. [1]Tanmoy Das, R.S. Markiewicz, and A. Bansil, Phys.Rev. B 77, 134516 (2008). [2] J. J. Rehr et al., Comptes Rendus Physique 10, 548 (2009).[3] D.C. Peets et al., Phys. Rev. Lett. 103, 087402 (2009).

Physical Review Letters, 2007

We have investigated the doping dependence of the penetration depth vs. temperature in electron d... more We have investigated the doping dependence of the penetration depth vs. temperature in electron doped Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a dx2−y2d_{x^2-y^2}dx2−y2 pairing gap in the underlying spectrum, we find nodeless behavior of the low-$T$ penetration depth in underdoped case, in accord with experimental results. As doping increases, a linear-in-$T$ behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.

We present a method for efficient, accurate first-principles calculations of the dynamical struct... more We present a method for efficient, accurate first-principles calculations of the dynamical structure factor S(q,φ) in periodic systems, using products of real space Green functions and fast Fourier transforms (FFT). We further invert S(q,φ) via Fourier transformation [1] to reconstruct the propagator of electron density X(x,t) in real space and time domain, thereby visualising spatially the dynamics of an electron doped cuprate system in real time. The present method is useful for many-body perturbation theories of excitations based on Density Function Theory (DFT) and modeling of various highly resolved spectroscopies going beyond the standard LDA [2-5]. Some illustrative examples will be presented. Work supported by the US DOE.[4pt] [1] P. Abbamonte et al., Phys. Rev. Lett. 92, 237401 (2004).[0pt] [2] Susmita Basak et al., Phys. Rev. B 80, 214520 (2009).[0pt] [3] J. Nieminen et al., Phys. Rev. B 80, 134509 (2009). [0pt] [4] R. S. Markiewicz et al., Phys. Rev. B 77, 094518 (2008).[0pt] [5] G. Stutz et al., Phys. Rev. B 60, 7099 (1999).

Physica B-condensed Matter, 2005

We study the systematics of the electronic structures of the carbides of the transition series Sc... more We study the systematics of the electronic structures of the carbides of the transition series Sc, Ti, V, Zr and Hf and the actinides U and Th using the tight-binding linear muffin-tin orbitals method (TB-LMTO). The densities of states, charge distribution and energy bands are studied in the lowest energy configuration. Changes in the bonding configurations and the equations of state are also compared as we go from 3d Ti to 5d–4f Hf. For the actinide carbides, the f-states have features rather different from the 3d–5d transition metal ...

Physical Review B, 2008

Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct ... more Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature TcT_cTc, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order.

We report first principles computations of the dynamical structure factor S(q,φ) in the electron ... more We report first principles computations of the dynamical structure factor S(q,φ) in the electron doped cuprates Nd2-xCexCuO2 as a function of energy φ and momentum q extended over several Brillouin zones. We show the efficacy of obtaining S(q,φ) through the use of simple products of real space Green functions and fast Fourier transform (FFT). We also calculate the susceptibility χ(q,φ) efficiently by recalling that S(q,φ) is proportional to the imaginary part of the susceptibility χ(q,φ). The present work is useful in going beyond the standard LDA-based modeling of various highly resolved spectroscopies.[1-4] We will provide some illustrative examples. Work supported by the US DOE.[1] R. S. Markiewicz et al., Phys. Rev. Lett. 96, 107005 (2006).[2] S. Sahrakorpi et al., Phys. Rev. Lett. 95, 157601 (2005).[3] G. Stutz et al., Phys. Rev. B 60, 7099 (1999).[4] J. Nieminen et al., Phys. Rev. Lett. 102, 037001 (2009).

Physical Review B, 2009

An anomalous “high-energy kink” HEK in dispersion, which gives the associated angle-resolved phot... more An anomalous “high-energy kink” HEK in dispersion, which gives the associated angle-resolved photoemission ARPES spectrum the appearance of a “waterfall” was first seen 1, 2 in Bi2Sr2CaCu2O8 Bi2212 cuprate superconductors. Such HEKs or waterfalls have now been established as being a universal feature in the cuprates, 1–3 and interpreted as providing evidence for interaction of the quasiparticles with some bosonic mode of the system. 4, 5 The high-energy scale of this boson 500 meV would then provide a tangible ...

Physical Review B, 2011

We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of ... more We present results for realistic modeling of the x-ray absorption near edge structure (XANES) of the overdoped high-T_c superconductor La_2-xSr_xCuO_4 in the hole doping range x = 0.20-0.30. Our computations are based on a real-space Green's function approach in which strong-correlation effects are taken into account in terms of a doping-dependent self-energy. The predicted O K-edge XANES is found to be in good accord with the corresponding experimental results in this overdoped regime. We find that the low energy spectra are dominated by the contribution of O-atoms in the cuprate planes, with little contribution from apical O-atoms.

Journal of Physics and Chemistry of Solids, 2011

We discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cupr... more We discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cuprates where effects of matrix element, crystal structure, strong electron correlations, and superconductivity are included realistically in material-specific detail. A number of illustrative examples drawn from our recent work are presented. Specific issues in the cuprate physics considered are: (i) Origin of high-energy kink or the waterfall effect; (ii) Dichroic effects in angle-resolved photoemission spectrum; (iii) Asymmetry of the scanning tunneling spectrum between the processes of electron extraction and injection; (iv) Persistence of 'Mott' like high-energy features with doping in optical spectra; (v) Magnetic excitations in electron and hole doped cuprates.

We show that a paramagnetic self-energy correction [1] to the real-space Green's function code FE... more We show that a paramagnetic self-energy correction [1] to the real-space Green's function code FEFF9 [2] can provide a good description of the x-ray absorption spectra (XAS) of cuprate system such as La(2-x)Sr(x)CuO4 (LSCO). This self energy includes coupling to both charge and magnetic excitations. We also find good agreement with recent XAS results of Peet et al. [3] in the over-doped regime of LSCO. We have also investigated various prescriptions for including core-hole effects. We infer that at low doping, the system behaves as an anti-ferromagnetic insulator, while Fermi liquid physics is recovered at high doping. [1]Tanmoy Das, R.S. Markiewicz, and A. Bansil, Phys.Rev. B 77, 134516 (2008). [2] J. J. Rehr et al., Comptes Rendus Physique 10, 548 (2009).[3] D.C. Peets et al., Phys. Rev. Lett. 103, 087402 (2009).

Physical Review Letters, 2007

We have investigated the doping dependence of the penetration depth vs. temperature in electron d... more We have investigated the doping dependence of the penetration depth vs. temperature in electron doped Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a dx2−y2d_{x^2-y^2}dx2−y2 pairing gap in the underlying spectrum, we find nodeless behavior of the low-$T$ penetration depth in underdoped case, in accord with experimental results. As doping increases, a linear-in-$T$ behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.

We present a method for efficient, accurate first-principles calculations of the dynamical struct... more We present a method for efficient, accurate first-principles calculations of the dynamical structure factor S(q,φ) in periodic systems, using products of real space Green functions and fast Fourier transforms (FFT). We further invert S(q,φ) via Fourier transformation [1] to reconstruct the propagator of electron density X(x,t) in real space and time domain, thereby visualising spatially the dynamics of an electron doped cuprate system in real time. The present method is useful for many-body perturbation theories of excitations based on Density Function Theory (DFT) and modeling of various highly resolved spectroscopies going beyond the standard LDA [2-5]. Some illustrative examples will be presented. Work supported by the US DOE.[4pt] [1] P. Abbamonte et al., Phys. Rev. Lett. 92, 237401 (2004).[0pt] [2] Susmita Basak et al., Phys. Rev. B 80, 214520 (2009).[0pt] [3] J. Nieminen et al., Phys. Rev. B 80, 134509 (2009). [0pt] [4] R. S. Markiewicz et al., Phys. Rev. B 77, 094518 (2008).[0pt] [5] G. Stutz et al., Phys. Rev. B 60, 7099 (1999).