arXiv subjects

Swagata Acharya

Publications and source records attributed to Swagata Acharya.

At least 37 records · Page 2Linked to original sources

arXiv2024

Magnon-mediated exciton-exciton interaction in a van der Waals antiferromagnet

Excitons are fundamental excitations that govern the optical properties of semiconductors. Interacting excitons can lead to various emergent phases of matter and large nonlinear optical responses. In most semiconductors, excitons interact via exchange interaction or phase space filling. Correlated materials that host excitons coupled to other degrees of freedom offer hitherto unexplored pathways for controlling these interactions. Here, we demonstrate magnon-mediated excitonic interactions in CrSBr, an antiferromagnetic semiconductor. This interaction manifests as the dependence of exciton energy on exciton density via a magnonic adjustment of the spin canting angle. Our study demonstrates the emergence of quasiparticle-mediated interactions in correlated quantum materials, leading to large nonlinear optical responses and potential device concepts such as magnon-mediated quantum transducers.

cond-mat.mtrl-sci↗

arXiv2024

Giant exchange splitting in the electronic structure of A-type 2D antiferromagnet CrSBr

We present the evolution of the electronic structure of CrSBr from its antiferromagnetic ground state to the paramagnetic phase above T_N=132 K, in both experiment and theory. Low temperature angle-resolved photoemission spectroscopy (ARPES) results are obtained using a novel method to overcome sample charging issues, revealing quasi-2D valence bands in the ground state. The results are very well reproduced by our QSGŴ calculations, which further identify certain bands at the X points to be exchange-split pairs of states with mainly Br and S character. By tracing band positions as a function of temperature, we show the splitting disappears above T_N. The energy splitting is interpreted as an effective exchange splitting in individual layers in which the Cr moments all align, within the so-called A-type antiferromagnetic arrangement. Our results lay firm foundations for the interpretation of the many other intriguing physical and optical properties of CrSBr.

cond-mat.str-el↗

arXiv2024

Quasiparticle band structure and excitonic optical response in V2O5 bulk and monolayer

The electronic band structure of V$_2$O$_5$ is calculated using an all-electron quasiparticle self-consistent (QS) $GW$ method, including electron-hole ladder diagrams in the screening of $W$. The optical dielectric function calculated with the Bethe-Salpeter equation exhibits excitons with large binding energy, consistent with spectroscopic ellipsometry data and other recent calculations. Sharp peaks in the direction perpendicular to the layers at high energy are found to be an artifact of the truncation of the numbers of bands included in the BSE calculation of the macroscopic dielectric function. The $\varepsilon_1(ω=0)$ gives indices of refraction in good agreement with experiment. The excitons are charge transfer excitons with the hole primarily on oxygen and electrons on vanadium, but depending on which exciton, the distribution over different oxygens changes. The exciton wave functions have a spread of about 5-15Å, with asymmetric character for the electron distribution around the hole depending on which oxygen the hole is fixed at. The monolayer quasiparticle gap increases inversely proportional to interlayer distance once the initial interlayer covalent couplings are removed which is thanks to the long-range nature of the self-energy and the reduced screening in a 2D system. The optical gap on the other hand is relatively independent of interlayer spacing because of the compensation between the self-energy gap shift and the exciton binding energy, both of which are proportional to the screened Coulomb interaction $\hat{W}$. Recent experimental results on very thin layer V$_2$O$_5$ obtained by chemical exfoliation provide experimental support for an increase in gap.

cond-mat.mtrl-sci↗

arXiv2024

Good plasmons in a bad metal

Correlated materials may exhibit unusually high resistivity increasing linearly in temperature, breaking through the Mott-Ioffe-Regel bound, above which coherent quasiparticles are destroyed. The fate of collective charge excitations, or plasmons, in these systems is a subject of debate. Several studies suggest plasmons are overdamped while others detect unrenormalized plasmons. Here, we present direct optical images of low-loss hyperbolic plasmon polaritons (HPPs) in the correlated van der Waals metal MoOCl2. HPPs are plasmon-photon modes that waveguide through extremely anisotropic media and are remarkably long-lived in MoOCl2. Many-body theory supported by photoemission results reveals that MoOCl2 is in an orbital-selective and highly incoherent Peierls phase. Different orbitals acquire markedly different bonding-antibonding character, producing a highly-anisotropic, isolated Fermi surface. The Fermi surface is further reconstructed and made partly incoherent by electronic interactions, renormalizing the plasma frequency. HPPs remain long-lived in spite of this, allowing us to uncover previously unseen imprints of electronic correlations on plasmonic collective modes.

cond-mat.str-el↗

arXiv2024

Lattice fluctuations, not excitonic correlations, mediated electronic localization in TiSe$_2$

TiSe$_2$ is thought to be an insulator with a bandgap of ~0.1eV. It has attracted a much interest because, among of a rich array of unique properties, many have thought TiSe$_2$ is a rare realisation of an excitonic insulator. Below 200 K, TiSe$_2$ undergoes a transition from a high-symmetry ({P-3m1}) phase to a low-symmetry ({P-3c1}) phase. Here we establish that TiSe$_2$ is indeed an insulator in both {P-3m1} and {P-3c1} phases. However, the insulating state is driven not by excitonic effects but by symmetry-breaking of the {P-3m1} phase. In the CDW phase the symmetry breaking is static. At high temperature, thermally driven instantaneous deviations from {P-3m1} break the symmetry on the characteristic time scale of a phonon. Even while the time-averaged \emph{lattice} structure assumes {P-3m1} symmetry, the time-averaged \emph{energy band} structure is closer to the CDW phase -- a rare instance of a metal-insulator transition induced by dynamical symmetry breaking. We establish these conclusions from a high-fidelity, self-consistent form of many body perturbation theory, in combination with molecular dynamics simulations to capture the effects of thermal disorder. The many-body theory includes explicitly ladder diagrams in the polarizability, which incorporates excitonic effects in an \emph{ab initio} manner. The excitonic modification to the potential is slight, ruling out the possibility that TiSe$_2$ is an excitonic insulator. Charge self-consistency is essential distinguish the metallic from insulating state.

cond-mat.mtrl-sci↗

arXiv2023

Pb-apatite framework as a generator of novel flat-band CuO based physics

Based on DFT calculations, we present the basic electronic structure of CuPb9(PO4)6O (Cu-doped lead apatite, LK-99), in two scenarios: (1) where the structure is constrained to the P3 symmetry and (2) where no symmetry is imposed. At the DFT level, the former is predicted to be metallic while the latter is found to be a charge-transfer insulator. In both cases the filling of these states is nominally d9, consistent with the Cu2+ valence state, and Cu with a local magnetic moment ~0.7mB. In the metallic case we find these states to be unusually flat (0.2 eV dispersion), giving high DOS at EF that we argue can be a host for novel electronic physics, including potentially high temperature superconductivity. The flatness of the bands is the likely origin of symmetry-lowering gapping possibilities that would remove the spectral weight from EF. Since some experimental observations show metallic/semiconducting behavior, we propose that disorder is responsible for closing the gap. We consider a variety of possibilities that could possibly close the gap, but limit consideration to kinds of disorder that preserve electron count. For all possibilities we considered (spin disorder, O on vacancy sites, Cu on different Pb sites), the local Cu moment, and consequently the gap remains robust. We conclude that disorder responsible for metallic behavior entails some kind of doping where the electron count changes. We claim that the emergence of the flat bands should be due to weak wave function overlap between the Cu and O orbitals, owing to the directional character of the constituent orbitals. So, finding an appropriate host structure for minimizing hybridization between Cu and O while allowing them to still weakly interact should be a promising route for generating flat bands at EF which can lead to interesting electronic phenomena, regardless of whether LK-99 is a room-temperature superconductor.

cond-mat.supr-con↗

arXiv2023

Multiple Slater determinants and strong spin-fluctuations as key ingredients of the electronic structure of electron- and hole-doped Pb$_{10-x}$Cu$_x$(PO4)$_6$O

LK-99, with chemical formula Pb$_{10-x}$Cu$_x$(PO4)$_6$O, was recently reported to be a room-temperature superconductor. While this claim has met with little support in a flurry of ensuing work, a variety of calculations (mostly based on density-functional theory) have demonstrated that the system possesses some unusual characteristics in the electronic structure, in particular flat bands. We have established previously that within DFT, the system is insulating with many characteristics resembling the classic cuprates, provided the structure is not constrained to the $P$3(143) symmetry nominally assigned to it. Here we describe the basic electronic structure of LK-99 within self-consistent many-body perturbative approach, quasiparticle self-consistent GW (QSGW) approximation and their diagrammatic extensions. QSGW predicts that pristine LK-99 is indeed a Mott/charge transfer insulator, with a bandgap gap in excess of 3eV, whether or not constrained to the $P$3(143) symmetry. The highest valence bands occur as a pair, and look similar to DFT bands. The lowest conduction band is an almost dispersionless state of largely Cu $d$ character. When Pb$_9$Cu(PO$_4$)$_6$O} is hole-doped, the valence bands modify only slightly, and a hole pocket appears. However, two solutions emerge: a high-moment solution with the Cu local moment aligned parallel to neighbors, and a low-moment solution with Cu aligned antiparallel to its environment. In the electron-doped case the conduction band structure changes significantly: states of mostly Pb character merge with the formerly dispersionless Cu $d$ state, and high-spin and low spin solutions once again appear. Thus we conclude that with suitable doping, the ground state of the system is not adequately described by a band picture, and that strong correlations are likely.

cond-mat.supr-con↗

arXiv2023

One-particle and excitonic band structure in cubic Boron Arsenide

Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Here we report predictions from an advanced self-consistent many body perturbative theory and show that with respect to one-particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses . With respect to two-particle properties, significant differences with Si appear. We report the excitonic spectrum for both q=0 and finite q, and show that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about $\sim$1.5 eV, which may play a crucial role in application of BAs in optoelectronics.

cond-mat.mtrl-sci↗

arXiv2023

Vertex dominated superconductivity in intercalated FeSe

Bulk FeSe becomes superconducting below 9\,K, but the critical temperature (T$_{c}$) is enhanced almost universally by a factor of $\sim$4-5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why T$_{c}$ changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain $\bf q$ vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate ${1}/{(T_{1}T)}$ may not reflect this enhancement. Intercalation modifies electronic screening both in the plane and also between layers. We disentangle quantitatively how superconducting pairing vertex gains from each such changes in electronic screening. Intercalated FeSe provides an archetypal example of superconductivity where information derived from the single-particle electronic structure appears to be insufficient to account for the origins of superconductivity, even when they are computed including correlation effects. We show that the five-fold enhancement in T$_{c}$ on intercalation is not sensitive to the exact position of the d$_{xy}$ at $Γ$ point, as long as it stays close to E$_{F}$. Finally we show that intercalation also significantly softens the collective charge excitations, suggesting the electron-phonon interaction could play some role in intercalated FeSe.

cond-mat.supr-con↗

arXiv2023

A Theory for Colors of Strongly Correlated Electronic Systems

Many strongly correlated transition metal insulators are colored, even though they have large fundamental band gaps and no quasi-particle excitations in the visible range. Why such insulators possess the colors they do poses a serious challenge for any many-body theory to reliably pick up the interactions responsible for the color. We pick two archetypal cases as examples: NiO with green color and MnF\textsubscript{2} with pink color. The body of literature around the collective charge transitions (excitons) that are responsible for the color in these and other strongly correlated systems, often fail to disentangle two important factors: what makes them form and what makes them optically bright. An adequate answer requires a theoretical approach able to compute such excitations in periodic crystals, reliably and without free parameters -- a formidable challenge. We employ two kinds of advanced \emph{ab initio} many body Green's function theories to investigate both optical and spin susceptibilities. The first, a perturbative theory based on low-order extensions of the $GW$ approximation, is able to explain the color in NiO, and indeed well describe the dielectric response over the entire frequency spectrum, while the same theory is unable to explain why MnF\textsubscript{2} is pink. We show its color originates from higher order spin-flip transitions that modify the optical response. This phenomenon is not captured by low-order perturbation theory, but it is contained in dynamical mean-field theory (DMFT), which has a dynamical spin-flip vertex that contributes to the charge susceptibility. We show that symmetry lowering mechanisms, such as spin-orbit coupling, odd-parity phonons and Jan-Teller distortions, determine how `bright' these excitons are, but are not fundamental to their existence.

cond-mat.str-el↗

arXiv2023

Paramagnetic Electronic Structure of CrSBr: Comparison between Ab Initio GW Theory and Angle-Resolved Photoemission Spectroscopy

We explore the electronic structure of paramagnetic CrSBr by comparative first principles calculations and angle-resolved photoemission spectroscopy. We theoretically approximate the paramagnetic phase using a supercell hosting spin configurations with broken long-range order and applying quasiparticle self-consistent $GW$ theory, without and with the inclusion of excitonic vertex corrections to the screened Coulomb interaction (QS$GW$ and QS$G\hat{W}$, respectively). Comparing the quasi-particle band structure calculations to angle-resolved photoemission data collected at 200 K results in excellent agreement. This allows us to qualitatively explain the significant broadening of some bands as arising from the broken magnetic long-range order and/or electronic dispersion perpendicular to the quasi two-dimensional layers of the crystal structure. The experimental band gap at 200 K is found to be at least 1.51 eV at 200 K. At lower temperature, no photoemission data can be collected as a result of charging effects, pointing towards a significantly larger gap, which is consistent with the calculated band gap of $\approx$ 2.1 eV.

cond-mat.str-el↗

arXiv2023

Possible realization of hyperbolic plasmons in a few-layered rhenium disulfide

The in-plane structural anisotropy in low-symmetric layered compound rhenium disulfide ($\text{ReS}_2$) makes it a candidate to host and tune electromagnetic phenomena specific for anisotropic media. In particular, optical anisotropy may lead to the appearance of hyperbolic plasmons, a highly desired property in optoelectronics. The necessary condition is a strong anisotropy of the principal components of the dielectric function, such that at some frequency range, one component is negative and the other is positive, i.e., one component is metallic, and the other one is dielectric. Here, we study the effect of anisotropy in $\text{ReS}_2$ and show that it can be a natural material to host hyperbolic plasmons in the ultraviolet frequency range. The operating frequency range of the hyperbolic plasmons can be tuned with the number of $\text{ReS}_2$ layers.

cond-mat.mtrl-sci↗

arXiv2022

Electron-beam induced emergence of mesoscopic ordering in layered MnPS$_{3}$

Ordered mesoscale structures in 2D materials induced by small misorientations have opened pathways for a wide variety of novel electronic, ferroelectric, and quantum phenomena. Until now, the only mechanism to induce this periodic ordering was via mechanical rotations between the layers, with the periodicity of the resulting moiré pattern being directly related to twist angle. Here we report a fundamentally new mechanism for emergence of mesoscopic periodic patterns in multilayer sulfur-containing metal phosphorous trichalcogenide, MnPS$_{3}$, induced by the electron beam. The formation under the beam of periodic hexagonal patterns with several characteristic length scales, nucleation and transitions between the phases, and local dynamics are demonstrated. The associated mechanisms are attributed to the relative contraction of the layers caused by beam-induced sulphur vacancy formation with subsequent ordering and lattice parameter change. As a result, the plasmonic response of the system is locally altered, suggesting an element of control over plasmon resonances by electron beam patterning. We pose that harnessing this phenomenon provides both insight into fundamental physics of quantum materials and opens a pathway towards device applications by enabling controlled periodic potentials on the atomic scale.

cond-mat.mtrl-sci↗

arXiv2022

Electronic and optical properties of crystalline nitrogen versus black phosphorus: A comparative first-principles study

Crystalline black nitrogen (BN) is an allotrope of nitrogen with the black phosphorus (BP) structure recently synthesized at high pressure by two independent research groups [Ji et al., Sci. Adv. 6, eaba9206 (2020); Laniel et al., Phys. Rev. Lett. 124, 216001 (2020)]. Here, we present a systematic study of the electronic and optical properties of BN focusing on its comparison with BP. To this end, we use the state-of-the-art quasiparticle self-consistent $GW$ approach with vertex corrections in both the electronic and optical channels. Despite many similarities, the properties of BN are found to be considerably different. Unlike BP, BN exhibits a larger optical gap (2.5 vs 0.26 eV), making BN transparent in the visible spectral region with a highly anisotropic optical response. This difference can be primarily attributed to a considerably reduced dielectric screening in BN, leading to enhancement of the effective Coulomb interaction. Despite relatively strong Coulomb interaction, exciton formation is largely suppressed in both materials. Our analysis of the elastic properties shows exceptionally high stiffness of BN, comparable to that of diamond.

cond-mat.mtrl-sci↗

arXiv2022

Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe

FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90\,K, and becomes a superconductor below 8\,K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in $x$ and $y$. How nematicity affects superconductivity has recently become a matter of intense speculation. Here we employ an advanced \emph{ab-initio} Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same T$_{c}$ as the nematic phase. The mechanism driving the observed nematicity is not yet understood. Since the present theory underestimates nematicity, we simulate the full nematic asymmetry by artificially enhancing the orthorhombic distortion. For benchmarking, we compare theoretical spin susceptibilities against experimentally observed data over all energies and relevant momenta. When the orthorhombic distortion is adjusted to correlate with observed nematicity in spin susceptibility, the enhanced nematicity causes spectral weight redistribution in the Fe-3d$_{xz}$ and d$_{yz}$ orbitals, but it leads to at most 10-15$\%$ increment in T$_{c}$. This is because the d$_{xy}$ orbital always remains the most strongly correlated and provides most of the source of the superconducting glue. Nematicity suppresses the density of states at Fermi level; nevertheless T$_{c}$ increases, in contradiction to both BCS and BEC theories. We show how the increase is connected to the structure of the particle-particle vertex. Our results suggest while nematicity may be intrinsic property of bulk FeSe, is not the primary force driving the superconducting pairing.

cond-mat.str-el↗

arXiv2021

Excitons in Bulk and Layered Chromium Tri-Halides: From Frenkel to the Wannier-Mott Limit

Excitons with large binding energies $\sim$2-3 eV in CrX$_{3}$ are historically characterized as being localized (Frenkel) excitons that emerge from the atomic $d{-}d$ transitions between the Cr-3$d$-$t_{2g}$ and $e_{g}$ orbitals. The argument has gathered strength in recent years as the excitons in recently made monolayers are found at almost the same energies as the bulk. The Laporte rule, which restricts such parity forbidden atomic transitions, can relax if, at least, one element is present: spin-orbit coupling, odd-parity phonons or Jahn-Teller distortion. While what can be classified as a purely Frenkel exciton is a matter of definition, we show using an advanced first principles parameter-free approach that these excitons in CrX$_{3}$, in both its bulk and monolayer variants, have band-origin and do not require the relaxation of Laporte rule as a fundamental principle. We show that, the character of these excitons is mostly determined by the Cr-$d$ orbital manifold, nevertheless, they appear only as a consequence of X-p states hybridizing with the Cr-$d$. The hybridization enhances as the halogen atom becomes heavier, bringing the X-$p$ states closer to the Cr-$d$ states in the sequence Cl{\textrightarrow}Br{\textrightarrow}I, with an attendant increase in exciton intensity and decrease in binding energy. By applying a range of different kinds of perturbations, we show that, moderate changes to the two-particle Hamiltonian that essentially modifies the Cr-$d$-X-$p$ hybridization, can alter both the intensities and positions of the exciton peaks. A detailed analysis of several deep lying excitons, with and without strain, reveals that the exciton is most Frenkel like in CrCl$_{3}$ and acquires mixed Frenkel-Wannier character in CrI$_{3}$.

cond-mat.mtrl-sci↗

arXiv2021

First principles vs second principles: Role of charge self-consistency in strongly correlated systems

First principles approaches have been successful in solving many-body Hamiltonians for real materials to an extent when correlations are weak or moderate. As the electronic correlations become stronger often embedding methods based on first principles approaches are used to better treat the correlations by solving a suitably chosen many-body Hamiltonian with a higher level theory. Such combined methods are often referred to as second principles approaches. At such level of the theory the self energy, i.e. the functional that embodies the stronger electronic correlations, is either a function of energy or momentum or both. The success of such theories is commonly measured by the quality of the self energy functional. However, self-consistency in the self-energy should, in principle, also change the real space charge distribution in a correlated material and be able to modify the electronic eigenfunctions, which is often undermined in second principles approaches. Here we study the impact of charge self-consistency within two example cases: TiSe$_{2}$, a three-dimensional charge-density-wave candidate material, and CrBr$_{3}$, a two-dimensional ferromagnet, and show how real space charge re-distribution due to correlation effects taken into account within a first principles Green's function based many-body perturbative approach is key in driving qualitative changes to the final electronic structure of these materials.

cond-mat.str-el↗

arXiv2021

Electronic Structure of Chromium Trihalides beyond Density Functional Theory

We explore the electronic band structure of free standing monolayers of chromium trihalides, CrX\textsubscript{3}{, X= Cl, Br, I}, within an advanced \emph{ab-initio} theoretical approach based in the use of Green's function functionals. We compare the local density approximation with the quasi-particle self-consistent \emph{GW} approximation (QS\emph{GW}) and its self-consistent extension (QS$G\widehat{W}$) by solving the particle-hole ladder Bethe-Salpeter equations to improve the effective interaction \emph{W}. We show that at all levels of theory, the valence band consistently changes shape in the sequence Cl{\textrightarrow}Br{\textrightarrow}I, and the valence band maximum shifts from the M point to the $Γ$ point. However, the details of the transition, the one-particle bandgap, and the eigenfunctions change considerably going up the ladder to higher levels of theory. The eigenfunctions become more directional, and at the M point there is a strong anisotropy in the effective mass. Also the dynamic and momentum dependent self energy shows that QS$G\widehat{W}$ adds to the localization of the systems in comparison to the QS\emph{GW} thereby leading to a narrower band and reduced amount of halogens in the valence band manifold.

cond-mat.str-el↗