arXiv subjects

Saurabh Maiti

Publications and source records attributed to Saurabh Maiti.

At least 19 recordsLinked to original sources

arXiv2026

Electronic Raman scattering from 2D metals with broken inversion symmetry

Lack of inversion symmetry in metals breaks SU(2) symmetry which results in spin-splitting of the electronic states at the Fermi level due to various types of spin-orbit coupling (SOC) such as Dresselhaus, Rashba, or Ising (also called valley-Zeeman). This splitting is known to enable both incoherent spin-flip excitations and coherent chiral-spin modes. Another effect of breaking of SU(2) is the introduction of a direct spin-photon interaction. We use this concept to formulate a theory of inelastic scattering of photons from the charge carriers of such a system [electronic Raman scattering (eRS)]. As a result of broken SU(2), we show that the eRS probe, unlike conventional theory of Raman scattering, couples to spin excitations even without tuning the laser to an internal resonance. We show that the spin dependent excitations induced by photon scattering are sensitive to the polarization geometries as well as to the spin structure of the Hilbert space of the low-energy states. As a concrete realization, we examine doped/gated graphene on substrates with strong SOC with various compositions of Rashba and valley-Zeeman SOC and compare their spectra with those for a model 2D electron gas (2DEG). The spectra are shown to have a resonant feature in select polarization geometries near the SOC-splitting energy and, importantly, is shown to be different in the two systems. The signal in graphene systems is shown to be stronger than that in a 2DEG by orders of magnitude owing to the large Dirac velocity. We also outline how the lineshapes from the spectra can be used to infer various components of SOC in the system.

cond-mat.mes-hall↗

arXiv2025

Resonant Edelstein and inverse-Edelstein effects, charge-to-spin conversion, and spin pumping from chiral-spin modes

Spin-orbit coupling in systems with broken inversion symmetry gives rise to the Edelstein effect, which is the spin polarization induced by an electric field or current, and the inverse-Edelstein effect (also known as the spin-galvanic effect), which is the electric current induced by an oscillatory magnetic field or spin polarization. At the same time, an interplay between spin-orbit coupling and electron-electron interaction leads to a special type of collective excitations -- chiral-spin modes -- which are oscillations of spin polarization in the absence of a magnetic field. As a result, both Edelstein and inverse-Edelstein effects exhibit resonances at the frequencies of chiral-spin collective modes. Here, we present a detailed study of the effect of electron correlation on the resonances in Edelstein and inverse-Edelstein effects in a single-valley two-dimensional electron gas and in a multi-valley Dirac system with proximity-induced spin-orbit coupling. While the chiral-spin modes involve both in-plane and out-of-plane oscillations of spins, we show that only the in-plane modes are responsible for the above resonances. In the multi-valley system, electron correlation splits the in-plane modes into two. We study the spectral weight distribution between the two modes over a large parameter space of intra- and inter-valley interactions. Finally, we demonstrate that using the chiral-spin modes one can get a resonant enhancement of charge-to-spin conversion and gain a directional control of the injected spins in the spin-pumping process, both of which are relevant to spintronics.

cond-mat.mes-hall↗

arXiv2025

A modified-residue prescription to calculate dynamical correlation functions

One of the challenges in using numerical methods to address many-body problems is the multi-dimensional integration over poles. More often that not, one needs such integrations to be evaluated as a function of an external variable. An example would be calculating dynamical correlations functions that are used to model response functions, where the external variable is the frequency. The standard numerical techniques rely on building an adaptive mesh, using special points in the Brillouin zone or using advanced smearing techniques. Most of these techniques, however, suffer when the grid is coarse. Here we propose that, if one knows the nature of the singularity in the integrand, one can define a residue and use it to faithfully estimate the integral and reproduce all the resulting singular features even with a coarse grid. We demonstrate the effectiveness of the method for different scenarios of calculating correlation functions with different resulting singular features, for calculating collective modes and densities of states. We also present a quantitative analysis of the error and show that this method can be widely applicable.

physics.comp-ph↗

arXiv2025

Effects of electron correlation on resonant Edelstein and inverse-Edelstein effects

Spin-orbit coupling in systems with broken inversion symmetry gives rise to the Edelstein effect, which is the induced spin polarization in response to an applied electric field or current, and the inverse Edelstein effect, which is the induced electric current in response to an oscillatory magnetic field or spin polarization. At the same time, an interplay between spin-orbit coupling and electron-electron interaction leads to a special type of collective excitations -- chiral-spin modes -- which are oscillations of spin polarization in the absence of a magnetic field. As a result, both Edelstein and inverse Edelstein effects exhibit resonances at the frequencies of spin-chiral collective modes. Here, we present a detailed study of the effect of electron correlation on the Edelstein and inverse Edelstein effects in a single-valley two-dimensional electron gas and a multi-valley Dirac system with proximity-induced spin-orbit coupling. While the chiral-spin modes involve both in-plane and out-of-plane oscillations of spins, we show that only the in-plane modes are responsible for the above resonances. In the multi-valley system, electron correlation splits the in-plane modes into two. We also study the spectral weight distribution between the two resonances over a large parameter space of intra- and inter-valley interactions.

cond-mat.str-el↗

arXiv2024

Universal nonanalytic features in response functions of anisotropic superconductors

Nonanalytic features are interesting in physics as they carry valuable information about the physical properties of the system. These properties manifest themselves in observables containing a one- or two-particle spectral function. In this work, we use a stationary-point analysis to deduce the nonanalytic features of spectral functions that appear while computing dynamical correlation functions. We focus on the correlation functions relevant to inelastic light scattering from anisotropic superconductors and show that nodal regions of the order parameters are, quite generally, associated with linear-in-frequency scaling at low frequencies, the minima points of the order parameters are associated with step jumps, while the maxima points are associated with $\ln$ singularities. Despite this general association, we show that depending on the anisotropy of the light-scattering vertex, these features can manifest themselves as various power laws, and even not remain singular at all. We demonstrate the conditions under which these happen. We are also able to demonstrate that the association of these nonanalytic features with the extrema and nodal points of the order parameter is, in fact, derived from a more universal behaviour of functions near parabolic-like and saddle-like stationary points. We provide a general prescription that maps the universal behaviour of systems near such stationary points to different scenarios. The approach is readily extendable to other types of spectral functions and we exemplify it by also analyzing the density of states on a square lattice.

cond-mat.supr-con↗

arXiv2024

Spin-orbit interaction enabled electronic Raman scattering from charge collective modes

Electronic Raman scattering in the fully symmetric channel couples to the charge excitations in the system, including the plasmons. However, the plasmon response has a spectral weight of $\sim q^2$, where $q$, the momentum transferred by light, is small. In this work, we show that in inversion symmetry broken systems where Rashba type spin-orbit coupling affects the states at the Fermi energy (which is a known low energy effect) as well as the transition elements to other states (a high energy effect), there is an additional coupling of the plasmons to the Raman vertex, even at zero momentum transfer, that results in a spectral weight that is proportional to the spin-orbit coupling. The high energy effect is due to the breaking of SU(2) spin invariance in the spin-flip transitions to the intermediate state. We present a theory for this coupling near the resonant regime of Raman scattering and show that in giant Rashba systems it can dominate over the conventional $q^2$ weighted coupling. We also provide experimental support along with a symmetry based justification for this spin-mediated coupling by identifying a prominent c-axis plasmon peak in the fully symmetric channel of the resonant Raman spectrum of the giant Rashba material BiTeI. This new coupling could lead to novel ways of manipulating coherent charge excitations in inversion-broken systems. This process is also relevant for spectroscopic studies in ultrafast spectroscopies, certain driven Floquet systems and topologically non-trivial phases of matter where strong inversion-breaking spin-orbit coupling plays a role.

cond-mat.mes-hall↗

arXiv2023

Many-body physics-induced selection rules: application to Raman spectroscopy

Spectroscopic measurements in quantum systems are subject to selection rules, usually based on space-time symmetries, that allow or disallow transitions between states. In many-body systems, in addition to the single-particle states, there emerge new ones due to collective excitations of the system. Here we demonstrate the existence of a "fragile" selection rule that emerges as a manifestation of many-body effects and outlines the conditions for collective excitations to couple to a given spectroscopic probe beyond the usual symmetry considerations. As an example, we apply the rule to Raman spectroscopy of multiband superconductors and settle some unresolved features in experiments.

cond-mat.str-el↗

arXiv2023

Electronic Raman response of a superconductor across a time reversal symmetry breaking phase transition

Polarization-resolved electronic Raman spectroscopy is an important experimental tool to investigate collective excitations in superconductors. In this work, we present a general theory that allows us to study the evolution of all Raman active collective modes in multiple symmetry channels across a time-reversal symmetry (TRS) breaking superconducting transition. This comprehensive approach reveals that multiple modes belonging to different symmetry channels show a tendency to soften, even when the interactions in the subleading channel are held constant. This indicates an increased competition induced by the proximity to the TRS breaking transition. The entry into the TRS broken phase is marked by the introduction of an additional mode into the gap in multiple symmetry channels. These new modes have a phase character complementary to the ones that are already present. Even though all the modes in the TRS broken phase acquire an amplitude character, we explicitly demonstrate that the coupling to the Raman probe is exclusively through the phase sector. We demonstrate that the Raman spectrum collected in lower symmetry channels shows a selective sensitivity to the sign of the ground state order parameters and the sign of the interband interactions. Finally, we demonstrate the applicability of an interaction induced selection rule that clearly explains the spectral weights of various modes in various irreps, including the possible of $``$dark$"$ Leggett and Bardasis-Schrieffer modes.

cond-mat.mes-hall↗

arXiv2023

Isolated flat bands in 2D lattices based on a novel path-exchange symmetry

The increased ability to engineer two-dimensional (2D) systems, either using materials, photonic lattices, or cold atoms, has led to the search for 2D structures with interesting properties. One such property is the presence of flat bands. Typically, the presence of these requires long-ranged hoppings, fine-tuning of nearest neighbor hoppings, or breaking time-reversal symmetry by using a staggered flux distribution in the unit cell. We provide a prescription based on carrying out projections from a parent system to generate different flat band systems. We identify the conditions for maintaining the flatness and identify a path-exchange symmetry in such systems that cause the flat band to be degenerate with the other dispersive ones. Breaking this symmetry leads to lifting the degeneracy while still preserving the flatness of the band. This technique does not require changing the topology nor breaking time-reversal symmetry as was suggested earlier in the literature. The prescription also eliminates the need for any fine-tuning. Moreover, it is shown that the subsequent projected systems inherit the precise fine-tuning conditions that were discussed in the literature for similar systems, in order to have and isolate a flat band. As examples, we demonstrate the use of our prescription to arrive at the flat band conditions for popular systems like the Kagome, the Lieb, and the Dice lattices. Finally, we are also able to show that a flat band exists in a recently proposed chiral spin-liquid state of the Kagome lattice only if it is associated with a gauge field that produces a flux modulation of the Chern-Simons type.

cond-mat.str-el↗

arXiv2022

Collective spin modes in Fermi liquids with spin-orbit coupling

A combination of spin-orbit coupling and electron-electron interaction gives rise to a new type of collective spin modes, which correspond to oscillations of magnetization even in the absence of the external magnetic field. We review recent progress in theoretical understanding and experimental observation of such modes, focusing on three examples of real-life systems: a two-dimensional electron gas with Rashba and/or Dresselhaus spin-orbit coupling, graphene with proximity-induced spin-orbit coupling, and the Dirac state on the surface of a three-dimensional topological insulator. This paper is dedicated to the 95th birthday of Professor Emmanuel I. Rashba.

cond-mat.str-el↗

arXiv2021

Zero-field spin resonance in graphene with proximity-induced spin-orbit coupling

We investigate collective spin excitations in graphene with proximity-induced spin-orbit coupling (SOC) of the Rashba and valley-Zeeman types, as it is the case, e.g., for graphene on transition- metal-dichalcogenide substrates. It is shown that, even in the absence of an external magnetic field, such a system supports collective modes, which correspond to coupled oscillations of the uniform and valley-staggered magnetizations. These modes can be detected via both zero-field electron spin resonance (ESR) and zero-field electric-dipole spin resonance (EDSR), with EDSR response coming solely from Rashba SOC. We analyze the effect of electron-electron interaction within the Fermi- liquid kinetic equation and show that the interaction splits both the ESR and EDSR peaks into two. The magnitude of splitting and the relative weights of the resonances can be used to extract the spin-orbit coupling constants and many-body interaction parameters that may not be accessible by other methods.

cond-mat.str-el↗

arXiv2019

Is the composite fermion state of Graphene a doped Chern insulator?

Graphene in the presence of a strong external magnetic field is a unique attraction for investigations of the fractional quantum Hall (fQH) states with odd and even denominators of the fraction. Most of the attempts to understand Graphene in the strong-field regime were made through exploiting the universal low-energy effective description of Dirac fermions emerging from the nearest neighbor hopping model of electrons on a honeycomb lattice. We highlight that accounting for the next-nearest-neighbor hopping terms in doped Graphene can lead to a unique redistribution of magnetic fluxes within the unit cell of the lattice. While this affects all the fQH states, it has a striking effect at a half-filled Landau-level state: it leads to a composite fermion state that is equivalent to the doped topological Chern insulator on a honeycomb lattice. At energies comparable to the Fermi energy, this state possesses a Haldane gap in the bulk proportional to the next-nearest-neighbor hopping and density of dopants. We argue that this microscopically derived energy gap survives the projection to the lowest band. We also conjecture that the gap should be present in a microscopic theory giving the recently proposed particle-hole symmetric Dirac composite fermion scenario of the half-filled Landau-level. The proposed gap is lower than the chemical potential, and is predicted to be parametrically separated from the Dirac point in the latter description. Finally we conclude by proposing experiments to detect this gap; the associated boundary mode; and encourage cold-atom setups to test other predictions of the theory.

cond-mat.str-el↗

arXiv2019

Fermionization of Bosons in a Flat Band

Strongly interacting bosons that live in a lattice with degeneracy in its lowest energy band experience frustration that can prevent the formation of a Bose-Einstein condensate. Such systems form an ideal playground to investigate spin-liquid behavior. We use the variational principle and the Chern-Simons technique of fermionization of hard-core bosons on Kagome lattice to find that below lattice filling fraction $ν=1/3$ the system favors a topologically ordered chiral spin-liquid state that is gapped in bulk, spontaneously breaks Time-Reversal Symmetry, and supports massless chiral bosonic edge mode. We construct the many-body variational wave function of the state and show that the corresponding energy coincides with the energy of the flat band. This result proves that the ground state of the system cannot stabilize a Bose condensate below $ν=1/3$. The fermionization and variational scheme we outline apply to any non-Bravais lattice. We distinguish between the roles played by the Chern-Simons gauge field in lattices with a flat band and those exhibiting a moat-like dispersion (which is degenerate along a closed contour in the reciprocal space). We also suggest experimental probes to differentiate the proposed ground state from a condensate.

cond-mat.str-el↗

arXiv2017

Microscopic pairing fingerprint of the iron-based superconductor ${\rm Ba_{1-x}K_xFe_2As_2}$

Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue for this quest by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor ${\rm Ba_{1-x}K_xFe_2As_2}$ for $0.22\le x \le 0.70$ in all symmetry channels, Raman spectroscopy allows us to distill the leading $s$-wave state. In addition, the spectra collected in the $B_{1g}$ symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet sub-dominant, pairing tendencies of $d_{x^2-y^2}$ symmetry type. A comprehensive functional Renormalization Group (fRG) and random-phase approximation (RPA) study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The synopsis of experimental evidence and theoretical modelling supports a spin-fluctuation mediated superconducting pairing mechanism.

cond-mat.supr-con↗

arXiv2017

Raman Scattering by a Two-Dimensional Fermi Liquid with Spin-Orbit Coupling

We present a microscopic theory of Raman scattering by a two-dimensional Fermi liquid (FL) with Rashba and Dresselhaus types of spin-orbit coupling, and subject to an in-plane magnetic field (B). In the long-wavelength limit, the Raman spectrum probes the collective modes of such a FL: the chiral spin waves. The characteristic features of these modes are a linear-in-q term in the dispersion and the dependence of the mode frequency on the directions of both q and B. All of these features have been observed in recent Raman experiments on CdTe quantum wells.

cond-mat.mes-hall↗

arXiv2017

Conservation laws, vertex corrections, and screening in Raman spectroscopy

We present a microscopic theory for the Raman response of a clean multiband superconductor accounting for the effects of vertex corrections and long-range Coulomb interaction. The measured Raman intensity, $R(Ω)$, is proportional to the imaginary part of the fully renormalized particle-hole correlator with Raman form-factors $γ(\vec k)$. In a BCS superconductor, a bare Raman bubble is non-zero for any $γ(\vec k)$ and diverges at $Ω= 2Δ+0$, where $Δ$ is the largest gap along the Fermi surface. However, for $γ(\vec k) =$ const, the full $R(Ω)$ is expected to vanish due to particle number conservation. It was long thought that this vanishing is due to the singular screening by long-range Coulomb interaction. We argue that this vanishing actually holds due to vertex corrections from the same short-range interaction that gives rise to superconductivity. We further argue that long-range Coulomb interaction does not affect the Raman signal for $any$ $γ(\vec k)$. We argue that vertex corrections eliminate the divergence at $2Δ$ and replace it with a maximum at a somewhat larger frequency. We also argue that vertex corrections give rise to sharp peaks in $R(Ω)$ at $Ω< 2Δ$, when $Ω$ coincides with the frequency of one of collective modes in a superconductor, e.g, Leggett mode, Bardasis-Schrieffer mode, or an excitonic mode.

cond-mat.supr-con↗

arXiv2016

Distinguishing between $s+id$ and $s+is$ pairing symmetries in multiband superconductors through spontaneous magnetization pattern induced by a defect

The symmetry of the pairing state in iron pnictide superconductor $\mathrm{Ba_{1-x}K_xFe_2As_2}$ is still controversial. At optimal doping ($x \approx 0.4$), it is very likely $s$-wave, but for $x=1$ there are experimental and theoretical arguments for both $s$-wave and $d$-wave. Depending on the choice for $x=1$, intermediate $s+is$ and $s+id$ states have been proposed for intermediate doping $ 0.4 < x < 1$. In both states, the time reversal symmetry is broken and a spontaneous magnetization is allowed. In this work we study a spontaneous magnetization induced by a nonmagnetic defect in the $s+is$ and $s+id$ states by using a perturbation theory and numerical calculations for the Ginzburg-Landau free energy functional. We show that the angular dependence of the magnetization is distinct in these two states due to the difference in symmetry properties of the order parameters. Our results indicate a possible way to distinguish between the $s+is$ and $s+id$ pairing symmetries in multi-band superconductors.

cond-mat.supr-con↗

arXiv2015

Electron Spin Resonance in a Two-Dimensional Fermi Liquid with Spin-Orbit Coupling

Electron spin resonance (ESR) is usually interpreted as a single-particle phenomenon protected from the effect of many-body correlations. We show that this is not the case in a two-dimensional Fermi liquid (FL) with spin-orbit coupling (SOC). Depending on whether the magnetic field is below or above some critical value, ESR in such a system probes up to three collective chiral-spin modes, augmented by the presence of the field, or the Larmor mode, augmented both by SOC and FL renormalizations. We argue that ESR can be used as a probe not only for SOC but also for many-body physics.

cond-mat.mes-hall↗