arXiv subjects

Pritam Bhattacharyya

Publications and source records attributed to Pritam Bhattacharyya.

At least 19 recordsLinked to original sources

arXiv2026

Near itinerancy and slow singlet formation in the triangular lattice NaRuO2

NaRuO$_2$ forms a delafossite-like structure that contains triangular sublattices of edge-sharing RuO$_6$ octahedra. It shows no evidence of magnetic order down to 100 mK and persistent spin fluctuations, suggestive of a quantum disordered magnetic ground state. In order to characterize the physical regime from which this disordered state arises, we use resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) at the Ru-$L_{2,3}$-edge, along with pulsed high-field magnetization to characterize both the local electronic structure and the magnetic interactions. Despite significant spin-orbit coupling inferred from XAS measurements, a spin-orbit exciton, characteristic of a spin-orbit assisted Mott insulator, was not observed with RIXS due to the presence of damped intraorbital excitations, which are characteristic of a metal. Corroborated by models of the high-field magnetization to a random singlet model, we propose a picture of a nearly itinerant system with strong magnetic and charge fluctuations that destabilize long-range magnetic order.

cond-mat.str-el↗

arXiv2026

Relevance of on-site and intersite Coulomb interactions in the Kitaev-Heisenberg magnet Na$_3$Co$_2$SbO$_6$

The detection of considerable spin frustration in honeycomb cobalt oxide compounds indicates the presence of sizable Kitaev interactions in these systems, enlarging the pool of Kitaev spin liquid candidates. Several key questions remain to be answered, as basic as the mechanisms behind Kitaev couplings in Co$^{2+}$ $t_{2g}^5e_g^2$ magnets. Analyzing the quantum chemistry of interacting magnetic moments in Na$_3$Co$_2$SbO$_6$, a representative $LS$-coupled $t_{2g}^5e_g^2$ oxide, we find that the Kitaev and off-diagonal $Γ$ interactions are substantial and antiferromagnetic but somewhat weaker than the Heisenberg contribution. All nearest-neighbor couplings feature massive contributions from direct Coulomb exchange and/or on-site multiconfigurational dressing, mechanisms not considered so far in descriptive models of Kitaev-Heisenberg magnetism. These findings call for systematic wave-function quantum chemical studies in order to understand direct-indirect exchange synergies in Kitaev-Heisenberg magnets and how to possibly tune intersite couplings towards the Kitaev spin liquid ground state.

cond-mat.str-el↗

arXiv2025

Physical properties of new delafossite triangular-lattice compounds TlErSe$_2$ and TlTmSe$_2$

Delafossite compounds containing rare-earth ions have been proven to be an ideal platform to investigate frustrated magnetic ground states. Here, we discuss two triangular-lattice antiferromagnets, TlErSe$_2$ and TlTmSe$_2$, as potential candidates for hosting exotic quantum states. Powder X-ray diffraction data analysis of the black-color polycrystalline Tl$RE$Se$_2$ ($RE$: Er and Tm) samples confirms the phase purity. Both materials crystallize in the trigonal $α$-NaFeO$_2$ structure ($R\overline{3}m$) with lattice parameters $a$ = 4.1070(4) Å and $c$ = 23.1472(1) Å for the erbium compound and $a$ = 4.0916(1) Å and $c$ = 23.1483(2) Å for the thulium compound. Magnetic susceptibility measurements show an effective moment of $μ_{\text{eff}} = 9.6(2) μ_B$/f.u. ($7.5(1) μ_B$/f.u.) for TlErSe$_2$ (TlTmSe$_2$) for temperatures above 200 K. While $^3$He specific-heat measurements reveal long-range magnetic order below $T_N = 0.42 $K for TlErSe$_2$, no sign of long-range magnetic order was observed for TlTmSe$_2$. Based on our results, we map out the T-H phase diagram for polycrystalline TlErSe$_2$ and discuss the striking difference in the magnetic behavior of TlTmSe$_2$ based on our ab initio quantum chemical calculations.

cond-mat.str-el↗

arXiv2025

More bridging ligands activate direct exchange: the case of anisotropic Kitaev effective magnetic interactions

A magnet is a collection of magnetic moments. How those interact is determined by what lies in between. In transition-metal and rare-earth magnetic compounds, the configuration of the ligands around each magnetic center and the connectivity of the ligand cages are therefore pivotal -- for example, the mutual interaction of magnetic species connected through one single ligand is qualitatively different from the case of two bridging anions. Two bridging ligands are encountered in Kitaev magnets. The latter represent one of the revelations of the 21st century in magnetism research: they feature highly anisotropic intersite couplings with seemingly counterintuitive directional dependence for adjacent pairs of magnetic sites and unique quantum spin-liquid ground states that can be described analytically. Current scenarios for the occurrence of pair-dependent magnetic interactions as proposed by Kitaev rely on $indirect$ exchange mechanisms based on intersite electron hopping. Analyzing the wavefunctions of Kitaev magnetic bonds at both single- and multi-configuration levels, we find however that $direct$, Coulomb exchange may be at least as important, in 5$d$ and 4$d$ $t_{2g}^5$, 3$d$ $t_{2g}^5e_g^2$, and even rare-earth 4$f^1$ Kitaev-Heisenberg magnets. Our study provides concept clarification in Kitaev magnetism research and the essential reference points for reliable computational investigation of how novel magnetic ground states can be engineered in Kitaev, Kitaev-Heisenberg, and Heisenberg edge-sharing systems.

cond-mat.str-el↗

arXiv2023

Spin-orbit-lattice entangled state in A$_2$MgReO$_6$ (A = Ca, Sr, Ba) revealed by resonant inelastic X-ray scattering

The $5d^1$ ordered double perovskites present an exotic playground for studying novel multi-polar physics due to large spin-orbit coupling. We present Re L3 edge resonant inelastic X-ray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A$_2$MgReO$_6$ (A = Ca, Sr, Ba) family of $5d^1$ double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric lineshape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbit-lattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibronic coupling for a complete description of the ground state physics of $5d^1$ double perovskites. Usage: Secondary publications and information retrieval purposes.

cond-mat.str-el↗

arXiv2023

Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels

The valence electronic structure of magnetic centers is one of the factors that determines the characteristics of a magnet. It may refer to orbital degeneracy, as for $j_\text{eff}=1/2$ Kitaev magnets, or near-degeneracy, e.g. involving the third and fourth shells in cuprate superconductors. Here we explore the inner structure of magnetic moments in group-5 lacunar spinels, fascinating materials featuring multisite magnetic units in the form of tetrahedral tetramers. Our quantum chemical analysis reveals a very colorful landscape, much richer than the single-electron, single-configuration description applied so far to all group-5 Ga$M_4X_8$ chalcogenides, and clarifies the basic multiorbital correlations on $M_4$ tetrahedral clusters: while for V strong correlations yield a wave-function that can be well described in terms of four V$^{4+}$V$^{3+}$V$^{3+}$V$^{3+}$ resonant valence structures, for Nb and Ta a picture of dressed molecular-orbital-like $j_\text{eff}=3/2$ entities is more appropriate. These internal degrees of freedom likely shape vibronic couplings, phase transitions, and magneto-electric properties in each of these systems.

cond-mat.str-el↗

arXiv2023

NaRuO$_2$: Kitaev-Heisenberg exchange in triangular-lattice setting

Kitaev exchange, a new paradigm in quantum magnetism research, occurs for 90$^{\circ}$ metal-ligand-metal links, $t_{2g}^5$ transition ions, and sizable spin-orbit coupling. It is being studied in honeycomb compounds but also on triangular lattices. While for the former it is known by now that the Kitaev intersite couplings are ferromagnetic, for the latter the situation is unclear. Here we pin down the exchange mechanisms and determine the effective coupling constants in the $t_{2g}^5$ triangular-lattice material NaRuO$_2$, recently found to host a quantum spin liquid ground state. We show that, compared to honeycomb compounds, the characteristic triangular-lattice cation surroundings dramatically affect exchange paths and effective coupling parameters, changing the Kitaev interactions to antiferromagnetic. The quantum chemical analysis and subsequent effective spin model computations provide perspective onto the nature of the experimentally observed quantum spin liquid -- it seemingly implies finite longer-range exchange, and the atypical proximity to ferromagnetic order is related to sizable ferromagnetic Heisenberg nearest-neighbor couplings.

cond-mat.str-el↗

arXiv2023

Anisotropic Coulomb exchange as source of Kitaev and off-diagonal symmetric anisotropic couplings

Exchange underpins the magnetic properties of quantum matter. In its most basic form, it occurs through the interplay of Pauli's exclusion principle and Coulomb repulsion, being referred to as Coulomb exchange. Pauli's exclusion principle combined with inter-atomic electron hopping additionally leads to kinetic exchange and superexchange. Here we disentangle the different exchange channels in anisotropic Kitaev-Heisenberg context. By quantum chemical computations, we show that anisotropic Coulomb exchange, completely neglected so far in the field, may be as large as (or even larger than) other contributions -- kinetic exchange and superexchange. This opens new perspectives onto anisotropic exchange mechanisms and sets the proper conceptual framework for further research on tuning Kitaev-Heisenberg magnetism.

cond-mat.str-el↗

arXiv2023

Sweet spot in the RuCl$_3$ magnetic system: nearly ideal $j_{\mathrm{eff}}\!=\!1/2$ moments and maximized $K/J$ ratio under pressure

Maximizing the ratio between Kitaev and residual Heisenberg interactions is a major goal in nowadays research on Kitaev-Heisenberg quantum magnets. Here we investigate Kitaev-Heisenberg exchange in a recently discovered crystalline phase of RuCl$_3$ under presure -- it displays unusually high symmetry, with only one type of Ru-Ru links, and uniform Ru-Cl-Ru bond angles of $\approx$93$^{\circ}$. By quantum chemical calculations in this particular honeycomb-lattice setting we find a very small $J$, which yields a $K/J$ ratio as large as $\sim$100. Interestingly, we also find that this is associated with vanishingly small $d$-shell trigonal splittings, i.\,e., minimal departure from ideal $j_{\mathrm{eff}}\!=\!1/2$ moments. This reconfirms RuCl$_3$ as a most promising platform for materializing the much sought-after Kitaev spin-liquid phase and stimulates further experiments under strain and pressure.

cond-mat.str-el↗

arXiv2023

Benchmarking Gaussian Basis Sets in Quantum-Chemical Calculations of Photoabsorption Spectra of Light Atomic Clusters

The choice of Gaussian basis functions for computing the ground-state properties of molecules, and clusters, employing wave-function-based electron-correlated approaches, is a well-studied subject. However, the same cannot be said when it comes to the excited-state properties of such systems, in general, and optical properties, in particular. The aim of the present study is to understand how the choice of basis functions affects the calculations of linear optical absorption in clusters, qualitatively, and quantitatively. For this purpose, we have calculated linear optical absorption spectra of several small charged and neutral clusters, namely, Li$_{2}$, Li$_{3}$, Li$_{4}$, B$_{2}^{+}$, B$_{3}^{+}$, Be$_{2}^{+}$, and Be$_{3}^{+}$, using a variety of Gaussian basis sets. The calculations were performed within the frozen-core approximation, and a rigorous account of electron correlation effects in the valence sector was taken by employing various levels of configuration interaction (CI) approach both for the ground and excited states. Our results on the peak locations in the absorption spectra of Li$_{3}$ and Li$_{4}$ are in very good agreement with the experiments. Our general recommendation is that for excited-state calculations, it is very important to utilize those basis sets which contain augmented functions. Relatively smaller aug-cc-pVDZ basis sets also yield high-quality results for photoabsorption spectra, and are recommended for such calculations if the computational resources are limited.

physics.chem-ph↗

arXiv2022

Electronic and structural properties of RbCeX$_2$ (X$_2$: O$_2$, S$_2$, SeS, Se$_2$, TeSe, Te$_2$)

Triangular lattice delafossite compounds built from magnetic lanthanide ions are a topic of recent interest due to their frustrated magnetism and realization of quantum disordered magnetic ground states. Here we report the evolution of the structure and electronic ground states of RbCe$X_2$ compounds, built from a triangular lattice of Ce$^{3+}$ ions, upon varying their anion character ($X_2$= O$_2$, S$_2$, SeS, Se$_2$, TeSe, Te$_2$). This includes the discovery of a new member of this series, RbCeO$_2$, that potentially realizes a quantum disordered ground state analogous to NaYbO$_2$. Magnetization and susceptibility measurements reveal that all compounds manifest mean-field antiferromagnetic interactions and, with the exception of the oxide, possess signatures of magnetic correlations onset below 1 K. The crystalline electric field level scheme is explored via neutron scattering and \textit{ab initio} calculations in order to model the intramultiplet splitting of the $J=5/2$ multiplet. In addition to the two excited doublets expected within the $J=5/2$ manifold, we observe one extra, local mode present across the sample series. This added mode shifts downward in energy with increasing anion mass and decreasing crystal field strength, suggesting a long-lived anomalous mode endemic to anion motion about the Ce$^{3+}$ sites.

cond-mat.str-el↗

arXiv2022

Crystal-field effects competing with spin-orbit interactions in NaCeO$_2$

Ce compounds feature a remarkable diversity of electronic properties, which motivated extensive investigations over the last decades. Inelastic neutron scattering represents an important tool for understanding their underlying electronic structures but in certain cases a straightforward interpretation of the measured spectra is hampered by the presence of strong vibronic couplings. The latter may give rise to extra spectral features, which complicates the mapping of experimental data onto standard multiplet diagrams. To benchmark the performance of embedded-cluster quantum chemical computational schemes for the case of $4f$ systems, we here address the Ce 4$f^1$ multiplet structure of NaCeO$_2$, an antiferromagnet with $D_{2d}$ magnetic-site symmetry for which neutron scattering measurements indicate only weak vibronic effects. Very good agreement with the experimental results is found in the computations, which validates our computational approach and confirms NaCeO$_2$ as a 4$f$ magnet in the intermediate coupling regime with equally strong 4$f$-shell spin-orbit and crystal-field interactions.

cond-mat.str-el↗

arXiv2022

Yb$^{3+}$ $f$-$f$ excitations in NaYbSe$_2$: benchmarking embedded-cluster quantum chemical schemes for 4$f$ insulators

$\tilde{S}\!=\!1/2$ triangular-lattice $f$-electron materials define a dynamic research area in condensed matter magnetism. In various Yb 4$f^{13}$ triangular-lattice compounds, for example, spin-liquid ground states seem to be realized. Using {\it ab initio} quantum chemical methods, we here investigate how correlation effects involving the 4$f$ electrons affect the on-site $f$-$f$ excitation spectrum in NaYbSe$_2$. The system is well suited for such a study since unambiguous inelastic neutron scattering data are available for the Yb$^{3+}$ $f$-$f$ transitions. The excitation energies obtained by configuration-interaction calculations with single and double substitutions agree within 3-4 meV with experimental values, which provides a not so expensive alternative to fitting experimental data at the model-Hamiltonian level in order to analyze $f$-center multiplet structures.

cond-mat.str-el↗

arXiv2022

Crystal Growth, Exfoliation and Magnetic Properties of Quaternary Quasi-Two-Dimensional CuCrP$_2$S$_6$

We report optimized crystal growth conditions for the quaternary compound CuCrP$_2$S$_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive X-ray spectroscopy and powder X-ray diffraction. CuCrP$_2$S$_6$ is structurally closely related to the $M_2$P$_2$S$_6$ family ($M$: transition metal), which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus. We present successful exfoliation of such as-grown crystals down to thicknesses of 2.8 nm corresponding to 4 layers. CuCrP$_2$S$_6$ crystallizes in the monoclinic space group $C2/c$. Magnetization measurements reveal an antiferromagnetic ground state with $T_\textrm{N} \approx 30$ K and a positive Curie-Weiss temperature in agreement with dominant ferromagnetic intralayer coupling. Specific heat measurements confirm this magnetic phase transition and the magnetic order is suppressed in an external magnetic field of about 6 T (8 T) applied parallel (perpendicular) to the $ab$ plane. At higher temperatures between 140-200 K additional broad anomalies associated with structural changes accompanying antiferroelectric ordering are detected in our specific heat studies.

cond-mat.str-el↗

arXiv2021

Why does B$_{12}$H$_{12}$-icosahedron need two electrons to be stable: A first-principles electron-correlated investigation of B$_{12}$H$_{n}$ ($n=$6,12) clusters

In this work, we present large-scale electron-correlated computations on various conformers of B$_{12}$H$_{12}$ and B$_{12}$H$_{6}$ clusters, to understand the reasons behind the high stability of di-anion icosahedron ($I_{h}$) and cage-like B$_{12}$H$_{6}$ geometries. Although the B$_{12}$-icosahedron is the basic building block in some structures of bulk boron, it is unstable in its free form. Furthermore, its H-passivated entity, i.e., B$_{12}$H$_{12}$ icosahedron is also unstable in free form. However, dianion B$_{12}$H$_{12}$ has been predicted to be stable as a perfect icosahedron in the free-standing form. In order to capture the correct picture for the stability of B$_{12}$H$_{12}^{-2}$ and B$_{12}$H$_{6}$ clusters, we optimized these structures by employing the coupled-cluster singles-doubles (CCSD) approach and cc-pVDZ basis set. We also performed vibrational frequency analysis of the isomers of these clusters, using the same level of theory to ensure the stability of the structures. For all the stable geometries obtained from the vibrational frequency analysis, we additionally computed their optical absorption spectra using the time-dependent density functional theory (TDDFT) approach, at the the B3LYP/6-31G{*} level of theory. Our calculated absorption spectra could be probed in future experiments on these clusters.

physics.chem-ph↗

arXiv2021

Molecular Switching Operation in Gate Constricted Interface of MoS$_2$ and hBN Heterostructure

Combined diverse two-dimensional (2D) materials for semiconductor interfaces are attractive for electrically controllable carrier confinement to enable excellent electrostatic control. We investigated the transport characteristic in heterointerface of multilayer molybdenum disulfide and hexagonal boron nitride (MoS$_2$/h-BN) to reveal that the charge transfer switching (CTS) is highly dependent on both the local gate constriction and bias in the channel. Notably, the CTS is shown to be controlled at a molecular level through electrotunable gated constriction. The resulting significant change in conductance due to exposing 100 parts-per-billion of nitrogen dioxide gas led to a high on/off ratio of 10 2 for completely switching off the channel thus, acting as a molecular switch. First-principle calculations further explained the mechanism of molecular CTS in the device. The molecular tunability of CTS has not been previously reported in any of the van der Waals semiconductor interfaces. Our finding opens avenues to exploit various atomically thin heterostructures for the mesoscopic transport phenomena towards molecular switching operation at room temperature.

cond-mat.mes-hall↗

arXiv2021

Role of Defect Induced Interfacial States in Molecular Sensing: Ultrahigh- Sensitive Region for Molecular Interaction

The defect induced interfacial states are created in an atomically thin two-dimensional molybdenum disulfide channel by underlying a narrow pattern of a graphene layer in a field effect transistor. Nondestructive method for the generation of charge-state allowed a highly sensitive molecular interaction with the sensitivity of nearly three-order of magnitude at room temperature. The presence of interfacial states in the channel lead to a conductance fluctuation and its magnitude is modulated using the nitrogen dioxide gas molecules in the subthreshold region. The study provides a systematic approach to establish a correlation between modulated conductance fluctuation and the molecular concentration upto parts-per-billion. First-principles density functional theory further explains the role of unique interfacial configuration on conductance fluctuation. Therefore, our study demonstrates an experimental approach to induce charge-state for the modulation of carrier concentration and exploits the role of defect induced interfacial states in atomically thin interfaces for the molecular interaction.

cond-mat.mes-hall↗

arXiv2020

Pariser-Parr-Pople Model based Configuration-Interaction Study of Linear Optical Absorption in Lower-Symmetry Polycyclic Aromatic Hydrocarbon Molecules

The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo[ghi]perylene (C$_{22}$H$_{12}$), benzo[a]coronene (C$_{28}$H$_{14}$), naphtho[2,3a]coronene (C$_{32}$H$_{16}$), anthra[2,3a]coronene (C$_{36}$H$_{18}$), and naphtho[8,1,2-abc]coronene (C$_{30}$H$_{14}$) were investigated. For the purpose, we performed electron-correlated calculations using screened, and standard parameters in the $π$-electron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using the multi-reference singles-doubles configuration-interaction (MRSDCI) methodology. PPP model Hamiltonian includes long-range Coulomb interactions, which increase the accuracy of our calculations. The results of our calculations predict that, with the increasing sizes of the coronene derivatives, optical spectra are red shifted, and the optical gaps decrease. In each spectrum, the first peak representing the optical gap is of moderate intensity, while the more intense peaks appear at higher energies. Our computed spectra are in good agreement with the available experimental data. For the purpose of comparison, we also performed first-principles time-dependent density-functional theory (TDDFT) calculations of the optical gaps of these molecules using Gaussian basis functions, and found that they yielded values lower than our CI results.

physics.chem-ph↗