arXiv subjects

Ravi Shankar Singh

Publications and source records attributed to Ravi Shankar Singh.

At least 19 recordsLinked to original sources

arXiv2026

Room-Temperature Terahertz Photoconductivity Polarity Switching in High Entropy Nickelates with Implications for Photonic Synapses

High entropy oxides (HEO) hold the potential to revolutionize the conventional material paradigms by leveraging high order of chemical disorder that induces highly desirable exotic phases for advanced applications. Here, we devise a methodology to enhance the efficiency of an artificial photonic synapse using a high entropy rare earth nickelate. Combined with epitaxial strain, we show that high entropy can further manipulate the phase of these locally disordered materials. Using time-averaged and time resolved Terahertz (THz) spectroscopy as dynamic probe, for the first time we show a rare combination of i) crystal axis dependent insulator to metal THz electronic phase transition and ii) coexistence of negative and positive THz photoconductivity at room temperature. Detailed analysis within theoretical models, including density functional theory (DFT)-based band structure calculations, suggest origin of these properties as disproportionate ordering of oxygen vacancies. Based on these findings, a conceptual THz-based artificial photonic synapse is proposed. This work underlines the pivotal role of HEO in advancing diverse THz functionalities, representing a critical step toward futuristic applications like THz-based high-speed computing and communication with an emphasis in THz frequency domain.

cond-mat.mtrl-sci↗

arXiv2025

Spectroscopic evidence of Kondo resonance in 3$d$ van der Waals ferromagnets

Two-dimensional van der Waals (vdW) ferromagnets drive the advancement in spintronic applications and enable the exploration of exotic magnetism in low-dimensional systems. The entanglement of dual $-$ localized and itinerant $-$ nature of electrons lies at the heart of the correlated electron systems giving rise to exotic ground state properties such as complex magnetism, heavy fermionic behavior, Kondo lattice formation, \textit{etc}. Through temperature-dependent electronic structure of vdW ferromagnets, (Co$_{x}$Fe$_{1-x}$)$_{3}$GeTe$_{2}$, probed using high-resolution photoemission spectroscopy and density functional theory combined with dynamical mean field theory (DFT+DMFT), we provide direct evidence of the emergence of Kondo resonance peak driven by complex interplay between localized and itinerant electrons. In overall agreement with experimental electronic structure and magnetic properties, DFT+DMFT also reveals finite spin band splitting well beyond $T_{C}$. Core levels, valence band photoemission spectra together with DFT+DMFT spectral functions reveal insignificant change across $T_{C}$ indicating non-Stoner magnetism in (Co$_{x}$Fe$_{1-x}$)$_{3}$GeTe$_{2}$. Our results provide a way forward to the understanding of complex interplay between electronic structure, exotic magnetism and heavy fermionic behavior leading to Kondo scenerio in 3$d$ vdW ferromagnets.

cond-mat.str-el↗

arXiv2025

Unraveling the role of disorder in the electronic structure of high entropy alloys

Disorder in high entropy alloys, arising from the random distribution of multiple elements, plays a crucial role in their novel properties desirable for various advanced engineering applications. We investigate the role of compositional and structural disorder on the electronic structure of osmium-based superconducting high entropy alloys, (Ru/Re)$_{0.35}$Os$_{0.35}$Mo$_{0.10}$W$_{0.10}$Zr$_{0.10}$, using photoemission spectroscopy and density functional theory (DFT). Elemental and cumulative core level shifts are found to be commensurate with elemental electronegativities and valence electron counts (VEC), respectively. Valence band spectra together with DFT results indicate that the crystal structure plays an important role in deciding the electronic structure of these high entropy alloys. Through temperature dependent high-resolution spectra, we unveil strongly suppressed spectral density of states (SDOS) in the close vicinity of Fermi level. Energy and temperature dependence of the SDOS in accordance with Altshuler-Aronov theory confirms localization of charge carriers in the presence of strong intrinsic disorder. Computed electron-phonon coupling strength and superconducting transition temperature aligning reasonably well with experiments further shed light on phonon-mediated pairing mechanism and role of disorder in these systems. Our results provide a way forward to the understanding of superconducting high entropy alloys through strategic control of disorder, VEC and crystal structure.

cond-mat.mtrl-sci↗

arXiv2025

Strongly correlated topological surface states in type-II Dirac semimetal NiTe$_{2}$

Nontrivial topology in type-II Dirac semimetal NiTe$_2$ leading to topologically protected surface states give rise to fascinating phenomena holding great promise for next-generation electronic and spintronic devices. Key parameters $-$ such as lattice parameter, disorder, vacancies, and electron correlation $-$ significantly influence the electronic structure and, subsequently, the physical properties. To resolve the discrepancy between the theoretical description and experimentally observed topological surface states, we comprehensively investigate the electronic structure of NiTe$_2$ using angle-resolved photoemission spectroscopy and density functional theory. Although the bulk electronic structure is found to be well-described within mean field approaches, an accurate description of topological surface states is obtained only by incorporating surface electronic correlation. We reveal that the strongly correlated surface states forming Dirac-like conical crossing much below Fermi level have hybridized Ni 3$d$ and Te 5$p$ character. These findings underscore the intricate interplay between electron correlation and band topology, broadening our understanding of many-body correlation effects on the topological surface states in quantum materials.

cond-mat.str-el↗

arXiv2025

Ising superconductivity in bulk layered non-centrosymmetric 4H-NbSe2

Transition metal dichalcogenides exhibit multiple polymorphs that enable the exploration of diverse quantum states, including valley-selective spin polarization, the valley Hall effect, Ising superconductivity, and nontrivial topology. Monolayer 2$H$-NbSe$_2$ is a promising candidate for realizing Ising superconductivity due to its spin-split, out-of-plane spin-polarized states arising from inversion symmetry breaking and strong spin-orbit coupling. In contrast, bulk 2$H$-NbSe$_2$ retains inversion symmetry and lacks spin splitting, limiting its suitability for hosting Ising superconductivity. Here, we report the growth of high-quality single crystals of the acentric bulk superconducting polymorph, 4$H$-NbSe$_2$, which intrinsically breaks the inversion symmetry and supports valley-selective spin-polarized states. Magnetization and resistivity measurements reveal anisotropic superconductivity, with the in-plane upper critical field exceeding the Pauli limit, while out-of-plane fields suppress superconductivity more rapidly, before reaching the Pauli limit, which strongly suggests the presence of Ising pairing. First-principles calculations and symmetry analysis confirm significant valley-selective spin splitting with out-of-plane spin polarization, further supporting the emergence of Ising superconductivity in 4$H$-NbSe$_2$. These results establish 4$H$-NbSe$_2$ as a robust bulk platform to investigate Ising superconductivity and valley-selective phenomena in transition-metal dichalcogenides.

cond-mat.supr-con↗

arXiv2025

Origin of dimensional crossover in quasi-one-dimensional hollandite K$_{2}$Ru$_{8}$O$_{16}$

Intriguing phenomenon of dimensional crossover is comprehensively studied by experimental and theoretical investigation of electronic structure in quasi-one-dimensional hollandite K$_{2}$Ru$_{8}$O$_{16}$. Valence band photoemission spectra in conjunction with density functional theory within local density approximation combined with dynamical mean field theory (LDA+DMFT) reveal moderately correlated electronic structure. Anomalous temperature dependence of high-resolution spectra in the vicinity of Fermi level suggests Tomonaga-Luttinger liquid state down to 150 K, below which it undergoes a dimensional crossover from one-dimensional to three-dimensional electronic behaviour. Monotonously decreasing spectral intensity at the Fermi level along with Fermi cut-off at low temperature suggests non-Fermi liquid like behaviour. Many body effects captured within LDA+DMFT reveal increased warping of the Fermi surface with lowering temperature. A simple analysis suggests that the warping dominates the thermal energy induced momentum broadening at low temperature, leading to the 3D electronic behaviour. Our results offer valuable insight in understanding the interplay of dimensionality, electron correlation and thermal energy governing various exotic phenomena in quasi-one-dimensional systems.

cond-mat.str-el↗

arXiv2024

High-temperature observation of intralayer, interlayer, and Rydberg excitons in bulk van der Waals alloy single crystals

Transition metal dichalcogenides (TMDs) exhibit remarkable optical properties due to the diverse number of strongly bound excitons, which can be fine-tuned by alloying. Despite a flurry of research activity in characterizing these excitons, a comprehensive and profound understanding of their behavior with temperature is lacking. Here, we report the rich spectrum of excitonic features within bulk van der Waals alloy Mo$_{0.5}$W$_{0.5}$S$_2$ and Mo$_{0.5}$W$_{0.5}$Se$_2$ single crystals through temperature-dependent reflectance spectroscopy and first-principle calculations. We observed Rydberg excitons and interlayer excitons in both the single crystals. Notably, we provide the first experimental evidence of highly energetic A$^\prime$ and B$^\prime$ excitons in Mo$_{0.5}$W$_{0.5}$S$_2$ at room temperature. The strong carrier-phonon scattering significantly broadens the A$^\prime$, B$^\prime$ and interlayer excitons at room temperature in bulk Mo$_{0.5}$W$_{0.5}$S$_2$ single crystal compared to its selenide. Our findings, supported by density functional theory and Bethe-Salpeter equation calculations, signify the crucial role of carrier-phonon interactions. These results open pathways for next-generation optoelectronic devices and quantum technologies operating at high temperature.

cond-mat.mtrl-sci↗

arXiv2024

Influence of anti-ferromagnetic ordering and electron correlation on the electronic structure of MnTiO$_3$

Electron correlation and long-range magnetic ordering have a significant impact on the electronic structure and physical properties of solids. Here, we investigate the electronic structure of ilmenite MnTiO$_{3}$ using room temperature photoemission spectroscopy and theoretical approaches within density functional theory (DFT), DFT+$U$ and DFT+dynamical mean field theory (DMFT). Mn 2$p$ (Ti 2$p$) core level photoemission spectra, confirming Mn$^{2+}$ (Ti$^{4+}$) oxidation state, exhibit multiple satellites which are very similar to that of MnO (TiO$_{2}$), suggesting similar strength of various interactions in this system. Valence band spectra collected at different photon energies suggest dominant Mn 3$d$ character in the highest occupied band with a wide insulating gap. DFT(+$U$) correctly predicts the experimentally observed anti-ferromagnetic (AFM) insulating ground state for MnTiO$_3$ where the requirement of a large $U$ to reproduce the experimental values of magnetic moment and band gap signifies the importance of electron correlation. Magnetically disordered paramagnetic (PM) phase could be well captured within DFT+DMFT, which provides an excellent agreement for the experimental band gap, paramagnetic moment, valence band spectra as well as dominant Mn 3$d$ character in the highest occupied band. The calculated spectral function remains largely unaffected and exhibits sharper features in the magnetically ordered AFM phase. We show that the electronic structure of MnTiO$_{3}$ in both the PM and AFM phases can be accurately described within DFT+DMFT.

cond-mat.str-el↗

arXiv2024

Manifestation of incoherent-coherent crossover and non-Stoner magnetism in the electronic structure of Fe$_3$GeTe$_2$

Two-dimensional (2D) van der Waals ferromagnets have potential applications as next-generation spintronic devices and provide a platform to explore the fundamental physics behind 2D magnetism. The dual nature (localized and itinerant) of electrons adds further complexity to the understanding of correlated magnetic materials. Here, we present the temperature evolution of electronic structure in 2D van der Waals ferromagnet, Fe$_{3}$GeTe$_{2}$, using photoemission spectroscopy in conjunction with density functional theory (DFT) plus dynamical mean field theory (DMFT). With the appearance of quasiparticle peak and its evolution in the vicinity of Fermi energy, we unveil empirical evidences of incoherent-coherent crossover at around 125 K. DFT+DMFT results show that the quasiparticle lifetime surpasses thermal energy for temperature below 150 K, confirming incoherent-coherent crossover in the system. No appreciable change in the Fe 2$p$ core level, overall valence band spectra across the magnetic transition, and temperature dependent ferromagnetic DFT+DMFT results, provide substantial evidence for non-stoner magnetism in Fe$_{3}$GeTe$_{2}$. We elucidate the temperature dependent intimate relation between magnetism and electronic structure in Fe$_{3}$GeTe$_{2}$. Sommerfeld coefficient of $\sim$ 104 mJ mol$^{-1}$ K$^{-2}$ obtained in the low temperature limit from DFT+DMFT calculations resolve the long standing issue of large Sommerfeld coefficient ($\sim$ 110 mJ mol$^{-1}$ K$^{-2}$) obtained from specific heat measurements.

cond-mat.str-el↗

arXiv2023

Role of electron correlation and disorder on the electronic structure of layered nickelate (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$

We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$ using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean field theory (DMFT) calculations. Sr 3$d$ and La 4$d$ core level spectra exhibit multiple features associated with photoemission final state effects. An increase of unscreened features in the Sr 3$d$ and La 4$d$ core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, $E_F$. Valence band spectra collected using different photon energies reveal finite intensity at $E_F$ and overall spectra are well captured by DFT+DMFT. Strong renormalization of partially filled $e_g$ bands in DFT+DMFT result indicates strong correlation in this system. Mass enhancement factor, $m^*/m_{\text{DFT}} \sim$ 3, agrees well with values obtained from specific heat measurements. High resolution spectra in the vicinity of $E_F$ show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cut-off at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. $|E-E_F|^{1/2}$ dependence of the spectral DOS and square root temperature dependence of spectral DOS at $E_F$ evidences the role of disorder in the electronic structure of (La$_{0.5}$Sr$_{0.5}$)$_2$NiO$_4$.

cond-mat.str-el↗

arXiv2023

Topological phase transition in MoTe$_2$: A Review

Transition metal dichalcogenides (TMDs) are a branch of two-dimensional materials which in addition to having an easy-to-exfoliate layered structure, also host semiconducting, metallic, superconducting, and topological properties in various polymorphs with potential applications. MoTe$_2$ is an example of such a TMD, which shows semiconducting (in 2H phase), metallic (in 1T' phase), topological Weyl semimetallic and superconducting behavior (in Td phase). Consequently, an extensive amount of research has been done on MoTe$_2$, particularly on the topological phase transition between the metallic-type 1T' phase and the topological Td phase. This phase transition has been reviewed and its association with the crystal structure, charge transport, and electronic band structure is elaborately discussed. Also, the effect of various stimuli like reduced dimensionality, pressure, charge doping, and chemical substitution, which affect the structural transition as well as the superconducting transition temperatures is reviewed; thereby, suggesting certain correlations between the apparently unrelated structural and superconducting phase transitions. The review also brings out some open questions which are likely to interest the community to address the physics associated with the phase transition and its potential applications.

cond-mat.supr-con↗

arXiv2023

Evidence of electron correlation and weak bulk plasmon in SrMoO$_{3}$

We investigate the electronic structure of highly conducting perovskite SrMoO$_{3}$ using valence band photoemission spectroscopy and electronic structure calculations. Large intensity corresponding to coherent feature close to Fermi level is captured by density functional theory (DFT) calculation. An additional satellite at $\sim$ 3 eV binding energy remains absent in DFT, hybrid functional (DFT-hybrid) and dynamical mean field theory (DFT + DMFT) calculations. Mo 4$d$ spectra obtained with different surface sensitive photoemission spectroscopy suggest different surface and bulk electronic structures. DFT + DMFT spectral function is in excellent agreement with the coherent feature in the bulk Mo 4$d$ spectra, revealing moderate electron correlation strength. A large plasmon satellite and signature of strong electron correlation are observed in the surface spectra, while the bulk spectra exhibits a $weak$ plasmon satellite.

cond-mat.str-el↗

arXiv2023

Revelation of Mott insulating state in layered honeycomb lattice Li$_2$RuO$_3$

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li$_2$RuO$_3$ using photoemission spectroscopy and band structure calculations. Monoclinic Li$_2$RuO$_3$ having Ru network as honeycomb lattice undergoes magneto-structural transition at T$_c$ $\sim$ 540 K from high temperature phase $C2/m$ to low temperature dimerized phase $P2_1/m$. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level ($E_F$). Ru 4$d$ band extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation (GGA) leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerate $d_{zx}$/$d_{yz}$ orbital, whereas $d_{xy}$ orbital is already gapped. Ru 3$d$ core level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4$d$ electrons which is also manifested by $|E-E_F|^2$ dependence of spectral density of states (DOS) in the vicinity of $E_F$ in the high-resolution spectra, establishing Li$_2$RuO$_3$ as Mott insulator.

cond-mat.str-el↗

arXiv2023

Role of disorder and strong 5$d$ electron correlation in the electronic structure of Sr2TiIrO6

Transport and magnetic properties along with high resolution valence band photoemission study of disordered double perovskite Sr$_{2}$TiIrO$_{6}$ has been investigated. Insulator to insulator transition along with a magnetic transition concurrently occurs at 240 K. Comparison of valence band photoemission with band structure calculations suggests that the spin orbit coupling as well as electron correlation are necessary to capture the line shape and width of the Ir 5$d$ band. Room temperature valence band photoemission spectra show negligibly small intensity at Fermi energy, $E_{F}$. Fermi cut-off is observed at low temperatures employing high resolution. The spectral density of states at room temperature exhibits $|E-E_{F}|^{2}$ energy dependence signifying the role of electron-electron interaction. This energy dependence changes to $|E-E_{F}|^{3/2}$ below the magnetic transition evidencing the role of electron-magnon coupling in magnetically ordered state. The evolution of pseudogap ($\pm$12 meV) explains the sudden increase in resistivity ($ρ$) below 50 K in this disordered system. The temperature dependent spectral density of states at $E_{F}$ exhibiting $T^{1/2}$ behaviour verifies Altshuler-Aronov theory for correlated disordered systems.

cond-mat.str-el↗

arXiv2022

Evidence of Lattice Strain as a Precursor to Superconductivity in BaPb$_{0.75}$Bi$_{0.25}$O$_3$

In this work, we have investigated the precursor effects to superconductivity in BaPb$_{0.75}$Bi$_{0.25}$O$_3$ using temperature dependent resistivity, x-ray diffraction technique and photoemission spectroscopy. The present compound exhibits superconductivity around 11 K ($T_C$). The synthesis procedure adopted is much simpler as compared to the procedure available in the literature. In the temperature range (10 K-25 K) i.e. above $T_C$, our results show an increase in both the orthorhombic and tetragonal strain. The well screened features observed in Bi and Pb 4$f_{7/2}$ core levels are indicative of the metallic nature of the sample. The compound exhibits finite intensity at the Fermi level at 300 K and this intensity decreases with decrease in temperature and develops into a pseudogap; the energy dependence of the spectral density of states suggests disordered metallic state. Furthermore, our band structure calculations reveal that the structural transition upon Pb doping results in the closing of the band gap at the Fermi level.

cond-mat.supr-con↗

arXiv2020

Electronic structure of ternary palladates and effect of hole doping: A valence band photoemission spectroscopic study

We investigate the electronic structure of ternary palladates $A$Pd$_3$O$_4$ ($A$ = Sr, Ca) using valence band photoemission spectroscopy and band structure calculations. Overall width of the valence band and energy positions of various features in experimental valence band spectra are well captured by band structure calculations using hybrid functional. Band structure calculations within local density approximations lead to metallic ground state while the calculations using hybrid functional provide band gap of 0.25 eV and 0.22 eV for CaPd$_3$O$_4$ and SrPd$_3$O$_4$ respectively, suggesting moderated to strong electron correlation strength in these narrow band gap semiconducting palladates. High resolution spectra reveals negligibly small intensity at Fermi level for parent compounds while hole doped SrPd$_3$O$_4$ (by 15\% Li substitution at Sr site) exhibits a Fermi cut-off suggesting metallic character in contrast to semiconducting transport. These observations reveal the importance of localization of electrons in case where the Fermi edge falls in the mobility edge.

cond-mat.str-el↗

arXiv2014

Complex electronic structure of Ca(1-x)Sr(x)RuO(3)

We investigate the core level spectra of Ca(1-x)Sr(x)RuO(3) employing high resolution photoemission spectroscopy. Sample surface appears to be dominated by the contributions from Ru-O layers. Sr 3p core level spectra are sharp and asymmetric in SrRuO(3) as expected in a metallic system, and exhibit multiple features for the intermediate compositions that can be attributed to the difference in Ca-O and Sr-O covalency. The Ru core level spectra exhibit distinct signature of satellite features due to the finite electron correlations strength among Ru 4d electrons. The intensity of the satellite feature is weaker in the surface spectra compared to the bulk. The low temperature spectra exhibit enhancement of satellite intensity in the spectra corresponding to ferromagnetic compositions due to the inter-site exchange coupling induced depletion of the intensity at the Fermi level. The increase in x leads to a decrease in satellite intensity that has been attributed to the increase in hopping interaction strength due to the enhancement of the Ru-O-Ru bond angle. Evidently, the complex electronic properties of these materials are derived from the interplay between the electron correlation and hopping interaction strengths.

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↗