arXiv subjects

Kalobaran Maiti

Publications and source records attributed to Kalobaran Maiti.

At least 37 records · Page 2Linked to original sources

arXiv2021

Anomalies in the temperature evolution of the Dirac states in a topological crystalline insulator SnTe

Discovery of topologically protected surface states, believed to be immune to weak disorder and thermal effects, opened up a new avenue to reveal exotic fundamental science and advanced technology. While time-reversal symmetry plays the key role in most such materials, the bulk crystalline symmetries such as mirror symmetry preserve the topological properties of topological crystalline insulators (TCIs). It is apparent that any structural change may alter the topological properties of TCIs. To investigate this relatively unexplored landscape, we study the temperature evolution of the Dirac fermion states in an archetypical mirror-symmetry protected TCI, SnTe employing high-resolution angle-resolved photoemission spectroscopy and density functional theory studies. Experimental results reveal a perplexing scenario; the bulk bands observed at 22 K move nearer to the Fermi level at 60 K and again shift back to higher binding energies at 120 K. The slope of the surface Dirac bands at 22 K becomes smaller at 60 K and changes back to a larger value at 120 K. Our results from the first-principles calculations suggest that these anomalies can be attributed to the evolution of the hybridization physics with complex structural changes induced by temperature. In addition, we discover drastically reduced intensity of the Dirac states at the Fermi level at high temperatures may be due to complex evolution of anharmonicity, strain, etc. These results address robustness of the topologically protected surface states due to thermal effects and emphasize importance of covalency and anharmonicity in the topological properties of such emerging quantum materials.

cond-mat.mtrl-sci↗

arXiv2020

Exceptional Dirac states in a non-centrosymmetric superconductor, BiPd

Quantum materials having Dirac fermions in conjunction with superconductivity is believed to be the candidate materials to realize exotic physics as well as advanced technology. Angle resolved photoemission spectroscopy (ARPES), a direct probe of the electronic structure, has been extensively used to study these materials. However, experiments often exhibit conflicting results on dimensionality and momentum of the Dirac Fermions (e.g. Dirac states in BiPd, a novel non-centrosymmetric superconductor), which is crucial for the determination of the symmetry, time-reversal invariant momenta and other emerging properties. Employing high-resolution ARPES at varied conditions, we demonstrated a methodology to identify the location of the Dirac node accurately and discover that the deviation from two-dimensionality of the Dirac states in BiPd proposed earlier is not a material property. These results helped to reveal the topology of the anisotropy of the Dirac states accurately. We have constructed a model Hamiltonian considering higher-order spin-orbit terms and demonstrate that this model provides an excellent description of the observed anisotropy. Intriguing features of the Dirac states in a non-centrosymmetric superconductor revealed in this study expected to have significant implication in the properties of topological superconductors.

cond-mat.supr-con↗

arXiv2020

Evidence of non-trivial Berry phase and Kondo physics in SmBi

Realization of semimetals with non-trivial topologies such as Dirac and Weyl semimetals, have provided a boost in the study of these quantum materials. Presence of electron correlation makes the system even more exotic due to enhanced scattering of charge carriers, Kondo screening etc. Here, we studied the electronic properties of single crystalline, SmBi employing varied state of the art bulk measurements. Magnetization data reveals two magnetic transitions; an antiferromagnetic order with a Neel temperature of ~ 9 K and a second magnetic transition at a lower temperature (= 7 K). The electrical resistivity data shows an upturn typical of a Kondo system and the estimated Kondo temperature is found to be close to the Neel temperature. High quality of the crystal enabled us to discover signature of quantum oscillation in the magnetization data even at low magnetic field. Using a Landau level fan diagram analysis, a non-trivial Berry phase is identified for a Fermi pocket revealing the topological character in this material. These results demonstrate an unique example of the Fermiology in the antiferromagnetic state and opens up a new paradigm to explore the Dirac fermion physics in correlated topological metal via interplay of Kondo interaction, topological order and magnetism.

cond-mat.str-el↗

arXiv2020

Structural anomaly in superconductivity of CaFe2As2 class of materials

Quantum transitions in Fe-based systems are believed to involve spin, charge and nematic fluctuations. Complex structural phase diagram in these materials often emphasizes importance of covalency in their exotic properties, which is directly linked to the local structural network and barely understood. In order to address this outstanding issue, we investigate the evolution of structural parameters and their implication in unconventional superconductivity of 122 class of materials employing extended x-ray absorption fine structure studies. The spectral functions near the Fe K- and As K-absorption edge of CaFe2As2 and its superconducting composition, CaFe1.9Co0.1As2 (Tc = 12 K) exhibit evidence of enhancement of Fe contribution with Co-substitution near the Fermi level. As-Fe and Fe-Fe bondlengths derived from the experimental data exhibit interesting changes with temperature across the magneto-structural transition. Curiously, the evolution in Co-doped composition is similar to its parent compound despite absence of magneto-structural transition. In addition, we discover anomalous change of Ca-X (X = Fe, As) bondlengths with temperature in the vicinity of magneto-structural transition and disorder appears to be less important presumably due to screening by the charge reservoir layer. These results reveal evidence of doping induced evolution to the proximity to critical behavior presumably leading to superconductivity in the system.

cond-mat.supr-con↗

arXiv2020

Mixed ground state in Fe-Ni Invar alloys

We investigate the ground state properties of Invar alloys via detailed study of the electronic structure of Fe$_{1-x}$Ni$_x$ alloys ($x$ = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) employing $x$-ray photoelectron spectroscopy (XPS). While all the alloys exhibit soft ferromagnetic behavior with Curie temperature much higher than the room temperature, the results for invar alloy, Fe$_{0.6}$Ni$_{0.4}$ exhibit anomalous behavior. Moreover, the magneto-resistance of the Invar alloy becomes highly negative while the end members possess positive magneto-resistance. The core level spectra of the Invar alloy exhibit emergence of a distinct new feature below 20~K while all other Fe-Ni alloys exhibit no temperature dependence down to 10~K. Interestingly, the shallow core level spectra (3$s$, 3$p$) of Fe and Ni of the Invar alloy reveal stronger deviation at low temperatures compared to the deep core levels (2$s$, 2$p$) indicating crystal field effect. It appears that there is a large precipitation of antiferromagnetic $γ^\prime$ phase below 20 K possessing low magnetic moment (0.5$μ_B$) on Fe within the $α$ phase. The discovery of negative magneto-resistance, anomalous magnetization at low temperature and the emergence of unusual new features in the core levels at low temperature provide an evidence of mixed phase in the ground state of Invar alloys.

cond-mat.str-el↗

arXiv2020

Preparation, characterization and X-ray photoemission spectroscopy study of a correlated semimetal, SmBi

We report high quality single crystalline growth of SmBi using flux method. The compound crystallizes in the simple rock salt structure with space group Fm3m. The cubic structure of the single crystal was confirmed by Laue diffraction pattern. The magnetic susceptibility measurements reveal sharp antiferromagnetic order with Neel temperature of 9 K. The core level photoemission spectroscopy study of Sm 3d has been performed using monochromatic Al Kalpha source. We observe multiple features in the experimental spectra due to fonal state effect - a signature of hybridization between Sm 4f - Bi 6p states. Intense satellite features are also observed presumably due to mixed valency arising from Kondo coupling. No signature of surface-bulk difference is observed in the Sm 3d core level spectra.

cond-mat.str-el↗

arXiv2020

Unusual role of ligand states in the electronic properties of a parent Fe-based superconductor, CaFe2As2

We investigate the role of ligand states in the electronic properties of CaFe2As2 using high-resolution hard x-ray photoemission spectroscopy (HAXPES) at different sample temperatures. Experimental results indicate that the binding energy of Ca is close to that for 2+ charge state of Ca atoms and the other constituent elements, Fe and As possess electronic configuration close to that in elemental systems. No difference is observed in the As 3p core level spectra with the change of emission angle and/or the change in sample temperature. This is surprising as the Ca atoms at the cleaved sample surface reorganizes itself to form linear structures which is expected to influence Ca-As hybridization leading to significant difference in surface and bulk electronic structures. Moreover, CaFe2As2 undergoes structural and magnetic phase transition at 170 K, and strong Fe-As hybridization provides pathways for electron dynamics. Clearly, further studies are required to resolve these puzzling observations.

cond-mat.supr-con↗

arXiv2020

Anomalous spectral evolution with bulk sensitivity in BiPd

We investigate the electronic structure of a noncentrosymmetric superconductor, BiPd using photoemission spectroscopy with multiple photon energies ranging from ultraviolet to hard x-ray. Experimental data exhibit interesting difference in the surface and bulk electronic structures of this system. While the surface Bi core level peaks appear at lower binding energies, the surface valence band features are found at the higher binding energy side of the bulk valence band; valence band is primarily constituted by the Pd 4d states. These changes in the electronic structure cannot be explained by the change in ionicity of the constituent elements via charge transfer. Analysis of the experimental data indicates that the Bi-Pd hybridization physics plays the key role in deriving the anomalous spectral evolution and the electronic properties of this system.

cond-mat.mtrl-sci↗

arXiv2020

Preparation, Characterization and electronic structure of Ti-doped Bi$_2$Se$_3$

We report the preparation of high-quality single crystal of Bi$_2$Se$_3$, a well-known topological insulator and its Ti-doped compositions using Bridgeman technique. Prepared single crystals were characterized by x-ray diffraction (XRD) to check the crystalline structure and energy dispersive analysis of x-rays for composition analysis. The XRD data of Ti-doped compounds show a small shift with respect to normal Bi$_2$Se$_3$ indicating changes in the lattice parameters while the structure type remained unchanged; this also establishes that Ti goes to the intended substitution sites. All the above analysis establishes successful preparation of these crystals with high quality using Bridgman technique. We carried out x-ray photo-emission spectroscopy to study the composition via investigating the core level spectra. Bi$_2$Se$_3$ spectra exhibit sharp and distinct features for the core levels and absence of impurity features. The core level spectra of the Ti-doped sample exhibit distinct signal due to Ti core levels. The analysis of the spectral features reveal signature of plasmon excitation and final state satellites; a signature of finite electron correlation effect in the electronic structure.

cond-mat.mtrl-sci↗

arXiv2020

Selective spin and nematic excitations using polarized pump pulses

Quantum materials display exotic behaviours related to the interplay between temperature-driven phase transitions. Here, we study electron dynamics in one such material, CaFe2As2, a parent Fe-based superconductor, employing time and angle-resolved photoemission spectroscopy. CaFe2As2 exhibits concomitant transition to spin density wave state and nematic order below 170 K. We discover that magnetic excitations can be induced selectively, by using polarized pump pulses, significantly before the destruction of nematic order. More specifically, we observe that s-polarized light enhances electron temperature by several hundreds of degrees at a time scale of about 200 fs, while the temperature of the electrons participating in magnetic order can be enhanced by similar amount at a much faster time scale (50 fs) using p-polarized pump pulse. These results provide a pathway to achieve selective electron heating, which is not possible with other methods, as well as to disentangle phase transitions in quantum materials.

cond-mat.supr-con↗

arXiv2020

On-demand local modification of high-$T_\text{c}$ superconductivity in few unit-cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

High-temperature superconductors (HTS) are important for potential applications and for understanding the origin of strong correlations. Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, will be of immense interest for superconducting electronics. Here we report transport studies on few unit-cell thick BSCCO and modify its superconductivity locally by depositing metal on the surface. Deposition of chromium (Cr) on the surface over a selected area of BSCCO results in insulating behavior of the patterned region. Cr locally depletes oxygen in CuO$_2$ planes and disrupts the superconductivity in the layers below. Our technique of modifying superconductivity is suitable for making sub-micron superconducting wires and more complex superconducting electronic devices.

cond-mat.supr-con↗

arXiv2019

Complex hybridization physics and evidence of structural anomaly to be a bulk property in an exotic Fe-based compound, CaFe2As2

Surface of quantum materials often exhibits significantly different behavior than the bulk due to changed topologies and symmetry protections. The outstanding problem is to find out if the exoticity of a material is linked to the changed topology at the surface or it is a bulk property. Hard x-ray photoemission spectroscopy (HAXPES) is a significantly bulk sensitive technique (escape depth of valence electrons is about 40 Å for 6 keV photon energy) and the probing depth can be tuned by changing the electron emission angle. Therefore, HAXPES is often used to reveal the surface-bulk differences in a material. Here, we show that the delineation of surface-bulk differences in the valence band spectral functions using this method is highly non-trivial due to the complexity arising from linear dichroic effect in addition to the change in surface sensitivity. We show that core level spectra can be used to reveal the surface-bulk differences in the electronic structure. The Ca 2p spectra exhibit evidence of significant hybridization with the conduction electrons revealing their importance in the electronic properties of the system as also found for the charge reservoir layers in cuprate superconductors. The Fe 2p core level spectra as a function of bulk sensitivity and temperature reveals an unusual scenario; while the surface electronic structure corroborates well with the observed phase transitions of the system, the bulk spectra exhibit signature of additional structural phases providing a rare evidence of structural anomaly to be a bulk property.

cond-mat.str-el↗

arXiv2019

Electronic structure studies on single crystalline Nd2PdSi3, an exotic Nd-based intermetallic: Evidence for Nd 4f hybridization

In the series R2PdSi3, Nd2PdSi3 is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4f localization. Here, we have studied the electronic structure of single crystalline Nd2PdSi3 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Theoretical results obtained for the effective electron correlation strength of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4d-Nd 4f hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4f-Si 3p states. Nd 3d core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4$f$ states with the conduction electrons possibly responsible for the exoticity of this compound.

cond-mat.str-el↗

arXiv2019

Anomalies in the core level spectroscopy of a noncentrosymmetric solid, BiPd

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and topological phase of matter. Employing high resolution photoemission spectroscopy with photon energies ranging from hard x-ray to extreme ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derived the escape depths close to the extreme surface sensitivity and find that the photon energies used for high resolution measurements such as ARPES fall in the surface sensitive regime. In addition, we discover deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such paradigm shift in core level spectroscopy can be attributed to the absence of center of symmetry and spin-orbit interactions.

cond-mat.mtrl-sci↗

arXiv2019

Local excitons in silicon induced by SiGe quantum huts

Conversion of Si to a direct bandgap semiconductor for optoelectronic application is a great challenge for many decades. It is proposed that embedment of suitable sized quantum dots into silicon matrix may be exploited to convert silicon to a direct bandgap semiconductor. The other bottleneck to this outstanding issue is the identification of local excitons, a signature of direct bandgap property and their comportment within the dots that can be utilized in engineering optoelectronic devices, quantum communications, etc. We studied the core level spectra of Si/Ge quantum huts embedded Si employing high resolution photoemission spectroscopy. Inverted quantum huts (IQHs) of Ge (13.3nm x 6.6nm) were grown on a Si buffer layer deposited on Si(001) surface using molecular beam epitaxy method and the photoemission experiments were carried out at different locations of the IQH structures exposed via controlled sputtering and annealing processes. We discover distinct features in the Ge 3$d$ core level spectra at the lower binging energy side of the bulk 3$d$ peak in contrast to the scenario of core level satellites often observed due to photoemission final state effects. The energy of these features are found to be sensitive to the location of the IQH structure probed revealing different core hole screening by the excitons located at different parts of IQHs. These results reveal local character of the excitons in the IQHs necessary for type I photoluminescence and establish core level spectroscopy as a direct probe of local excitons. These finding are expected to help amalgamation of microelectronics and solid state photonics important for optoelectronic applications.

cond-mat.mes-hall↗

arXiv2019

Anomalous ground state properties of SmB6 -- a density functional theoretical study

We studied the electronic structure of SmB6 employing density functional theory (DFT) using different exchange potentials, spin-orbit coupling (SOC) and electron correlation (U = correlation strength). All the calculations carried out in this study converge to metallic ground state indicating bulk metallicity in SmB6, which is in line with the low temperature anomalies observed in this system. We show that while spin-orbit coupling and electron correlation is important to capture the ground state properties, generalized gradient approximation provides the best description of the Sm 4f multiplets observed in angle-resolved photoemission spectroscopy (ARPES) data. The Fermi surface plots exhibit electron and hole pockets having dominant Sm 4f character; the observation of these Fermi surfaces is consistent with the recent quantum oscillation measurements. In addition to primarily Sm 4f contributions observed at the Fermi level, we discover significantly large contribution from B 2p states compared to weak Sm 5d contributions. This suggests important role of B 2p - Sm 4f hybridization in the exotic physics of this system.

cond-mat.str-el↗

arXiv2018

Orbital-dependent electron dynamics in Fe-pnictide superconductors

We report on orbital-dependent quasiparticle dynamics in EuFe$_2$As$_2$, a parent compound of Fe-based superconductors and a novel way to experimentally identify this behavior, using time- and angle-resolved photoelectron spectroscopy across the spin density wave transition. We observe two different relaxation time scales for photo-excited d$_x$$_z$/d$_y$$_z$ and d$_x$$_y$ electrons. While d$_x$$_z$/d$_y$$_z$ electrons relax faster through the electron-electron scattering channel, showing an itinerant character, d$_x$$_y$ electrons form a quasi-equilibrium state with the lattice due to their localized character, and the state decays slowly. Our findings suggest that electron correlation in Fe-pnictides is an important property, which should be taken into careful account when describing the electronic properties of both parent and electron-doped compounds, and therefore establish a strong connection with cuprates.

cond-mat.supr-con↗

arXiv2017

Hidden phase in parent Fe-pnictide superconductors

We investigate the origin of exoticity in Fe-based systems via studying the Fermiology of CaFe2As2 employing Angle Resolved Photoemission spectroscopy (ARPES). While the Fermi surfaces (FSs) at 200 K and 31 K are observed to exhibit two dimensional (2D) and three dimensional (3D) topology, respectively, the FSs at intermediate temperatures reveal emergence of the 3D topology at much lower temperature than the structural & magnetic phase transition temperature (170 K, for the sample under scrutiny). This leads to the conclusion that the evolution of FS topology is not directly driven by the structural transition. In addition, we discover the existence in ambient conditions of energy bands related to the collapsed tetragonal (cT) phase. These bands are distinctly resolved in the high-photon energy spectra exhibiting strong Fe 3d character. They gradually move to higher binding energies due to thermal compression with cooling, leading to the emergence of 3D topology in the Fermi surface. These results reveal the so-far hidden existence of a cT phase in ambient conditions, which is argued to lead to quantum fluctuations responsible for the exotic electronic properties in Fe-pnictide superconductors.

cond-mat.supr-con↗