arXiv subjects

Kalobaran Maiti

Publications and source records attributed to Kalobaran Maiti.

At least 73 records · Page 4Linked to original sources

arXiv2009

Study of magnetic interactions in a spin liquid, Sr3NiPtO6 using density functional approach

We investigate the magnetic interactions in Sr3NiPtO6, characterized to be a spin liquid using ab initio calculations. The results reveal a novel metal to insulator transition due to finite exchange interaction strength; the magnetic solutions (independent of magnetic ordering) are large band gap insulators, while the non-magnetic solution is metallic. The Ni moment is found to be large and the coupling among intra-chain Ni moments is antiferromagnetic unlike other compounds in this family. These results, thus, reveal the importance of intra-chain antiferromagnetic interaction in addition to geometrical frustration to derive spin liquid phase.

cond-mat.str-el↗

arXiv2008

Comment on "Electronic structure and orbital ordering of SrRu$_{1-x}$Ti$_x$O$_3$: GGA+U investigations"

In the paper, PRB {\bf 77}, 085118 (2008), the authors conclude that the observation of Ti-doping induced half-metallicity in SrRu$_{1-x}$TI$_x$O$_3$ within the limit of local density approximations is not valid as the experimental results indicate insulating behavior. It was described that the metal-insulator transition (MIT) at x = 0.5 observed in this system appears due to the enhancement of on-site Coulomb repulsion strength, U with Ti substitutions, x. The MIT primarily depends on U and partially on x and/or disorder. All these conclusions are in sharp contrast to the experimental observations, which predicted Anderson insulating phase at x = 0.5 (finite localized density of states at the Fermi level). The hard gap due to electron correlation appears at much higher x (~ 0.8). In addition, it is well established that homovalent substitution has negligible influence on on-site $U$ (a local variable). These inconsistencies appear due to the fact that the calculated results representing the bulk electronic structure are compared with the experimental results dominated by surface contributions. The experimental bulk spectra, already available in the literature exhibit finite density of states at the Fermi level even for x >= 0.5 sample, which suggests that the conclusion of half metallic phase in an earlier study is reasonable. Thus, the insulator to metal transition in these systems is driven by bandwidth, W rather than U and disorder plays dominant role in the metal-insulator transition. One needs to consider the bulk spectra to reproduce numerically the bulk electronic structure.

cond-mat.str-el↗

arXiv2008

Electronic and magnetic properties of a quasi-one-dimensional spin chain system, Sr$_{3}$NiRhO$_{6}$

We investigate the electronic structure of Sr$_3$NiRhO$_6$, a quasi-one-dimensional spin chain system using \emph{ab initio} band structure calculations. Spin polarized calculations within GGA reveal that Ni and Rh have finite moments and they are antiferromagnetically coupled along the chain axis in the ground state. While these results obtained within the local spin density approximations provide remarkable representation of the magnetic phase, the experimentally observed insulating behavior could not be captured within this method. GGA+$U$ calculations show that opening up of an insulating gap requires on-site Coulomb interaction among the Rh 4$d$ electrons, $U_{dd}^{Rh}$ $\approx$ 2.5 eV and the correlation among Ni 3$d$ electrons, $U_{dd}^{Ni}$ $\approx$ 4.5 eV suggesting this system to be a Mott insulator. Electron correlation among $d$ electrons leads to significant enhancement of the O 2$p$ character in the energy bands in the vicinity of the Fermi level and the $d$ bands appear at lower energies. Energy gap in the up spin density of states appears to be significantly small ($\sim$ 0.12 eV) while it is $>$ 2 eV in the down spin density of states suggesting possibility of spin polarized conduction in the semiconducting phase.

cond-mat.str-el↗

arXiv2008

Role of vacancies and impurities in the ferromagnetism of semiconducting CaB6

We investigate the influence of impurities and vacancies in the formation of magnetic moment in CaB6 using full potential ab initio band structure calculations. CaB6 is found to be a band insulator with a band gap of about 0.2 eV. The calculated results indicate that carbon and oxygen substitution in boron sublattice, usually expected as impurity in low purity borons, do not play significant role in the local moment formation. Boron vacancy in the boron sublattice, on the other hand, leads to the formation of an impurity band in the vicinity of the Fermi level, which exhibit finite exchange splitting and magnetic moment. All these results provide remarkable representation of the experimental results. This finding of the role of vacancies in the sublattice responsible for electronic conduction is an important revelation in the understanding of ferromagnetism in the diluted electron system.

cond-mat.mtrl-sci↗

arXiv2007

Electronic and structural transition in $La_{0.2} Sr_{0.8} Mn O_3$

We investigate the interplay of the electronic and structural transition in La$_{0.2}$Sr$_{0.8}$MnO$_{3}$. The transport and specific heat measurements exhibit unusual evolutions and signature of a first order phase transition around 265 K. Mn K-edge extended $x$-ray absorption fine structure results reveal distortion in the MnO$_6$ octahedra even in the cubic phase and a remarkable evolution of the distortion across the phase transition. These results manifest the importance of fluctuations in Mn 3$d$ orbital occupancy and disorder in their electronic properties, which may help in understanding the orbital and spin ordering proposed in these systems.

cond-mat.str-el↗

arXiv2007

Electronic structure of PrCoO_3 and its temperature evolution

We investigate the detailed electronic structure of PrCoO$_3$ and its temperature evolution using state-of-the-art photoemission spectroscopy and ab initio band structure calculations. We observe that in addition to the correlation effect, spin-orbit interaction plays an important role in determining the electronic properties of this system. Pr 4$f$ states are found to be strongly hybridized with the O 2$p$ and Co 3$d$ valence electronic states, and thus influences the electronic properties significantly. The calculated results corresponding to the intermediate spin state of Co provide a good description of the experimental spectra at 300 K. The decrease in temperature from 300 K leads to a gradual enhancement of the low spin state contributions in the electronic structure. The temperature evolution of the band gap is found to be consistent with the transport data.

cond-mat.str-el↗

arXiv2007

Evidence for strong 5d electron correlations and electron-magnon coupling in a pyrochlore, Y2Ir2O7

We report the observation of an unusual behavior of highly extended 5d electrons in Y2Ir2O7 belonging to pyrochlore family of great current interest using high resolution photoemission spectroscopy. The experimental bulk spectra reveal an intense lower Hubbard band in addition to weak intensities in the vicinity of the Fermi level, e_F. This provides a direct evidence for strong electron correlation among the 5d electrons, despite their highly extended nature. The high resolution spectrum at room temperature exhibits a pseudogap at e_F and |e - e_F|^2 dependence demonstrating the importance of electron correlation in this system. Remarkably, in the magnetically ordered phase (T < 150 K), the spectral lineshape evolves to a |e - e_F|^1.5 dependence emphasizing the dominant role of electron-magnon coupling.

cond-mat.str-el↗

arXiv2007

Role of long range ferromagnetic order in the electronic structure of Sr$_{1-x}$Ca$_x$RuO$_3$

We investigate the role of long range ferromagnetic order in the electronic structure of Sr$_{1-x}$Ca$_x$RuO$_3$ using high resolution photoemission spectroscopy. SrRuO$_3$ is a ferromagnetic metal but isostructural, isoelectronic CaRuO$_3$ is an enhanced paramagnet. Surface spectra of CaRuO$_3$ exhibit temperature induced modifications. This is not significant in other compositions. This may be attributed to the structural changes observed in previous studies. Interestingly, the bulk spectra reveal unusual spectral changes exhibiting large decrease in the coherent feature intensity corresponding to only ferromagnetic samples, although the Ru moment is very similar in all the compositions.

cond-mat.str-el↗

arXiv2007

Revelation of the role of impurities and conduction electron density in the high resolution photoemission study of ferromagnetic hexaborides

We investigate the temperature evolution of the electronic structure of ferromagnetic CaB$_6$ using ultra-high resolution photoemission spectroscopy; electronic structure of paramagnetic LaB$_6$ is used as a reference. High resolution spectra of CaB$_6$ reveal finite density of states at the Fermi level, $ε_F$ at all the temperatures and evidence of impurity induced localized features in the vicinity of $ε_F$, which are absent in the spectra of LaB$_6$. Analysis of the high resolution spectra suggests that disorder in B-sublattice inducing partial localization in the mobile electrons and low electron density at $ε_F$ is important to achieve ferromagnetism in these systems.

cond-mat.mtrl-sci↗

arXiv2007

Fabrication of half metallicity in a ferromagnetic metal

We investigate the growth of half metallic phase in a ferromagnetic material using state-of-the-art full potential linearized augmented plane wave method. To address the issue, we have substituted Ti at the Ru-sites in SrRuO3, where SrRuO3 is a ferromagnetic material. Calculated results establish Ti4+ valence states (similar to SrTiO3), which was predicted experimentally. Thus, Ti substitution dilutes the Ru-O-Ru connectivity, which is manifested in the calculated results in the form of significant band narrowing leading to finite gap between t2g and eg bands. At 75% substitution, a large gap (> 2 eV) appears at the Fermi level, e_F in the up spin density of states, while the down spin states contributes at e_F characterizing the system a half-metallic ferromagnet. The t2g - eg gap can be tailored judiciously by tuning Ti concentrations to minimize thermal effects, which is often the major bottleneck to achieve high spin polarization at elevated temperatures in other materials. This study, thus, provides a novel but simple way to fabricate half-metallicity in ferromagnetic materials, which are potential candidates for spin-based technology.

cond-mat.str-el↗

arXiv2007

Evolution of a band insulating phase from a correlated metallic phase

We investigate the evolution of the electronic structure in SrRu_(1-x)Ti_xO_3 as a function of x using high resolution photoemission spectroscopy, where SrRuO3 is a weakly correlated metal and SrTiO3 is a band insulator. The surface spectra exhibit a metal-insulator transition at x = 0.5 by opening up a soft gap. A hard gap appears at higher x values consistent with the transport properties. In contrast, the bulk spectra reveal a pseudogap at the Fermi level, and unusual evolution exhibiting an apparent broadening of the coherent feature and subsequent decrease in intensity of the lower Hubbard band with the increase in x. Interestingly, the first principle approaches are found to be sufficient to capture anomalous evolutions at high energy scale. Analysis of the spectral lineshape indicates strong interplay between disorder and electron correlation in the electronic properties of this system.

cond-mat.str-el↗

arXiv2007

Observation of particle hole asymmetry and phonon excitations in non-Fermi liquid systems: A high-resolution photoemission study of ruthenates

We investigate the temperature evolution of the electronic states in the vicinity of the Fermi level of a non-Fermi liquid (NFL) system, CaRuO3 using ultra high-resolution photoemission spectroscopy; isostructural SrRuO3 exhibiting Fermi liquid behavior despite similar electron interaction parameters as that of CaRuO3, is used as a reference. High-energy resolution in this study helps to reveal particle-hole asymmetry in the excitation spectra of CaRuO3 in contrast to that in SrRuO3. In addition, we observe signature of phonon excitations in the photoemission spectra of CaRuO3 at finite temperatures while these are weak in SrRuO3.

cond-mat.str-el↗

arXiv2006

Manifestation of lattice distortions in the O 1s spectra in Ca1-xSrxRuO3

We investigate the effect of temperature on the electronic and crystal structures of Ca1-xSrxRuO3 via the evolution of O 1s core level spectra as a function of temperature and composition, x. O 1s spectra in SrRuO3 exhibit a dominant sharp feature at all the temperatures with a small trace of impurity feature at higher binding energies. The spectra in Ca doped samples, however, exhibit two distinct features. Analysis of the experimental spectral functions and the band structure results suggest that different Madelung potential at the two types of oxygen sites in the orthorhombically distorted structure leads to such splitting in the O 1s spectra. Interestingly, the energy separation of these two features becomes smaller at low temperatures in the Ca dominated samples concomitant to the observation of non-Fermi liquid behavior in their bulk properties. Such temperature evolution, thus, indicates a direct connection of the lattice degrees of freedom with the electronic properties of these compounds.

cond-mat.str-el↗

arXiv2006

Manifestation of ground state anomaly in the core level spectra of Ca and Sr in Ca(1-x)SrxRuO3

We investigate the evolution of Ca 2p and Sr 3d core level spectra in Ca(1-x)SrxRuO3 using photoemission spectroscopy. Core level spectra in this system exhibit multiple features and unusual evolution with the composition and temperatures. Analysis of the core level spectra in conjunction with the band structure results reveal novel final state effects due to different core hole screening channels. Changes in the core level spectra suggest significant modification in Ca-O covalency in Ca dominated samples, which gradually reduces with the increase in Sr content and becomes minimum in SrRuO3. This study thus, provides a direct evidence of the role of cation-oxygen covalency in the ground state properties of these novel materials.

cond-mat.str-el↗

arXiv2006

Electronic structure of BaIrO3: A first principle study using local spin-density approximations

We investigate the electronic structure of BaIrO3, an interesting compound exhibiting charge density wave transition in its insulating phase and ferromagnetic transition at the same temperature, using full potential linearized augmented plane wave method within the local spin density approximations. The ferromagnetic ground state could exactly be described in these calculations and the calculated spin magnetic moment is found to be small as observed in the magnetic measurements. Interestingly, no signature of exchange splitting is observed in the density of states corresponding to Ir 5d and/or any other electronic states. The small spin moment appears essentially due to unequal population of the up- and down-spin Ir 5d bands. Comparison of the valence band density of states with the experimental spectral functions suggests that a rigid shift of the Fermi level towards higher energies in the calculated density of states provides a good description of the experimental spectra. This indicates that the intrinsic oxygen non-stoichiometry leads to electron doping in the system and plays the primary role in determining the electronic structure rather than the electron correlation effects as often observed in other systems. The calculated results for Ba 5p core levels show that the Madelung potential of one of the three non-equivalent Ba atoms is different from that of other two as predicted in the recent experiments.

cond-mat.str-el↗

arXiv2006

Role of covalency in the ground state properties of perovskite ruthenates: A first principle study using local spin density approximations

We investigate the electronic structure of SrRuO3 and CaRuO3 using full potential linearized augmented plane wave method within the local spin density approximations. The ferromagnetic ground state in SrRuO3 could exactly be described in these calculations and the calculated spin magnetic moment is found to be close to the experimentally observed values. Interestingly, the spin polarized calculations for CaRuO3 exhibit large spin moment as observed in the experiments but the magnetic ground state has higher energy than that in the non-magnetic solution. Various calculations for different structural configurations indicate that Ca-O covalency plays the key role in determining the electronic structure and thereby the magnetic ground state in this system.

cond-mat.str-el↗

arXiv2005

Understanding the bulk electronic structure of Ca1-xSrxVO3

We investigate the electronic structure of Ca1-xSrxVO3 using careful state-of-the-art experiments and calculations. Photoemission spectra using synchrotron radiation reveal a hitherto unnoticed polarization dependence of the photoemission matrix elements for the surface component leading to a substantial suppression of its intensity. Bulk spectra extracted with the help of experimentally determined electron escape depth and estimated suppression of surface contributions resolve outstanding puzzles concerning the electronic structure in Ca1-xSrxVO3.

cond-mat.str-el↗

arXiv2005

Origin of charge density wave formation in insulators from a high resolution photoemission study of BaIrO3

We investigate the origin of charge density wave (CDW) formation in insulators by studying BaIrO3 using high resolution (1.4 meV) photoemission spectroscopy. The spectra reveal the existence of localized density of states at the Fermi level in the vicinity of room temperature. These localized states are found to vanish as the temperature is lowered thereby, opening a soft gap at the Fermi level, as a consequence of CDW transition. In addition, the energy dependence of the spectral density of states reveals the importance of magnetic interactions, rather than well-known Coulomb repulsion effect, in determining the electronic structure thereby implying a close relationship between ferromagnetism and CDW observed in this compound. Also, Ba core level spectra surprisingly exhibit an unusual behavior prior to CDW transition.

cond-mat.str-el↗