arXiv subjects

Atanu Samanta

Publications and source records attributed to Atanu Samanta.

9 recordsLinked to original sources

arXiv2026

Soft Collinear Effective Theory for Heavy QCD Axions

We develop a soft-collinear effective theory (SCET) framework for heavy QCD axion, considering two low-energy realizations and taking $B\to Ka$ as a benchmark mode. In the first realization, $aG\widetilde G$ is assumed to be the only independent axion interaction at the scale $\mu\sim m_b$. We show that eliminating the redundant flavor-changing derivative-gluon operator in the weak effective theory generates a new dimension-seven axion operator identified as $\mathrm{O}_{\partial ag}$. We match this operator onto SCET and derive the corresponding leading-power soft and spectator-scattering contributions to $B\to Ka$. We obtain a factorized expression for the spectator contribution in terms of perturbative hard kernels and the $B$- and $K$-meson light-cone distribution amplitudes. The spectator contribution arises at the same order in the power expansion as the soft-overlap term and amounts to approximately $25\%$ of the soft contribution. In the second realization, the Wilson coefficient of $aG\widetilde G$ is assumed to be present above the electroweak scale. Renormalization-group evolution and matching then induce a direct $b\to sa$ operator, which subsequently results in a dominant soft form-factor contribution, whereas the gluonic spectator term turns out to be numerically subleading ($\sim 6-7\%$). We thus identify the conditions under which spectator scattering becomes relevant for heavy-axion production in rare $B$-meson decays. Finally, we derive the corresponding bounds on the axion decay constant $f_a$ for both realizations and compare their phenomenological implications.

hep-ph↗

arXiv2026

Mechanochemical Nano-Writing of an Atomically Thin Metal

Mechanical energy accelerates many physicochemical processes, including materials syntheses that are hard to produce with thermal energy alone. However, physical understanding connecting applied mechanical forces with internal stresses and ensuing reaction mechanisms is lacking. Here we demonstrate mechanical force-enabled synthesis and nanoscale patterning to metallize a two-dimensional (2D) material, producing an atomically-thin superconducting material. Localized force applied by atomic force microscope tips to van der Waals (vdW) encapsulated stacks of 2D bilayer MoTe2 and adjacent source Pd guides 2D Pd7MoTe2 growth with 50 nm lateral resolution. Force accelerates reaction kinetics exponentially per Eyring's stress-assisted thermal activation model, reducing synthesis temperatures from ~200 {\deg}C to near-room temperature. Finite element simulations, density functional theory, and ab-initio grand canonical Monte Carlo calculations show that tip-induced compression facilitates Pd chemisorption to tensile-strained MoTe2 that converts to uniform Pd7MoTe2. This demonstrates a new, generalizable paradigm for nanoscale synthesis of quantum materials, and high-precision engineering of superconductivity.

cond-mat.mes-hall↗

arXiv2026

Axion EFT in the BMHV Scheme: Flavor Currents, Evanescent Operators and Ward Identities

We present a systematic analysis of axion effective field theory within the Breitenlohner-Maison-`t Hooft-Veltman (BMHV) scheme, focusing on the renormalization of fermionic dimension-five operators and the associated chiral flavor currents. In this framework, the non-anticommuting nature of $\gamma_5$ in $d \neq 4$ dimensions leads to violations of naive Ward identities through the emergence of evanescent operators. We derive the bare and renormalized Ward identities for chiral currents, explicitly identifying the equation-of-motion and evanescent operator contributions. Using diagrammatic calculations, we verify the validity of these identities up to two-loop order $\mathcal{O}(\alpha_s^2)$, including both pole and finite terms. We demonstrate how evanescent operators mix into physical operators and determine the finite renormalization required to restore four-dimensional Ward identities, recovering the expected structure of axial current renormalization and the anomaly. Our results provide a consistent and transparent framework for multi-loop computations in axion EFT and highlight the essential role of evanescent operators in maintaining scheme consistency.

hep-ph↗

arXiv2024

A comprehensive study of ALPs from $B$-decays

We present a comprehensive study of axion-like particles (ALPs) through flavor changing neutral current processes, such as $B\to K a$ followed by $a\to\text{hadronic}, \gamma\gamma,\mu^+\mu^-$ channels. Our generic framework encompasses different ultraviolet scenarios similar to KSVZ, DFSZ and flavorful axions etc. Starting from the effective Lagrangian written at the high scale, we compute the anomalous dimension matrix, taking into account all one-loop and relevant two-loop contributions. The latter is most important for the KSVZ and heavy QCD axion scenarios. We recognized that such two-loop diagrams can have both ultraviolet (UV) and infrared (IR) divergences. We show explicitly that UV divergences cancel by inserting appropriate counterterms, which are new operators involving the axion field and required to be present at the UV itself, to renormalize the theory. On the other hand, the cancellation of IR divergences is subtle and demonstrated through matching with the effective theory at the electroweak scale. We also utilize chiral perturbation theory and vector meson dominance framework to compute the decay and branching fractions of the ALP pertaining to our framework. We find that for KSVZ-like scenario, axion decay constant, $f_a \lesssim 1$ TeV can be ruled out. The bound becomes stronger for the DFSZ and Flaxion-like models, reaching upto $10^4$ TeV and $10^6$ TeV, respectively. We also provide projections on the parameter space based on 3 ab$^{-1}$ data from Belle II and 300 fb$^{-1}$ data from LHCb.

hep-ph↗

arXiv2024

Ferroelectric Fractals: Switching Mechanism of Wurtzite AlN

The advent of wurtzite ferroelectrics is enabling a new generation of ferroelectric devices for computer memory that has the potential to bypass the von Neumann bottleneck, due to their robust polarization and silicon compatibility. However, the microscopic switching mechanism of wurtzites is still undetermined due to the limitations of density functional theory simulation size and experimental temporal and spatial resolution. Thus, physics-informed materials engineering to reduce coercive field and breakdown in these devices has been limited. Here, the atomistic mechanism of domain wall migration and domain growth in wurtzites is uncovered using molecular dynamics and Monte Carlo simulations of aluminum nitride. We reveal the anomalous switching mechanism of fast 1D single columns of atoms propagating from a slow-moving 2D fractal-like domain wall. We find that the critical nucleus in wurtzites is a single aluminum ion that breaks its bond with one nitrogen and bonds to another nitrogen; this creates a cascade that only flips atoms directly in the same column, due to the extreme locality (sharpness) of the domain walls in wurtzites. We further show how the fractal shape of the domain wall in the 2D plane breaks assumptions in the KAI model and leads to the anomalously fast switching in wurtzite structured ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2022

Strong Bulk Photovoltaic Effect in Planar Barium Titanate Thin Films

The bulk photovoltaic effect (BPE) leads to the generation of a photocurrent from an asymmetric material. Despite drawing much attention due to its ability to generate photovoltages above the band gap ($E_g$), it is considered a weak effect due to the low generated photocurrents. Here, we show that a remarkably high photoresponse can be achieved by exploiting the BPE in simple planar BaTiO$_3$ (BTO) films, solely by tuning their fundamental ferroelectric properties via strain and growth orientation induced by epitaxial growth on different substrates. We find a non-monotonic dependence of the responsivity ($R_{\rm SC}$) on the ferroelectric polarization ($P$) and obtain a remarkably high BPE coefficient ($\beta$) of $\approx$10$^{-2}$ 1/V, which to the best of our knowledge is the highest reported to date for standard planar BTO thin films. We show that the standard first-principles-based descriptions of BPE in bulk materials cannot account for the photocurrent trends observed for our films and therefore propose a novel mechanism that elucidates the fundamental relationship between $P$ and responsivity in ferroelectric thin films. Our results suggest that practical applications of ferroelectric photovoltaics in standard planar film geometries can be achieved through careful joint optimization of the bulk structure, light absorption, and electrode-absorber interface properties.

cond-mat.mtrl-sci↗

arXiv2021

Advancing from phenomenological to predictive theory of ferroelectric oxide solution properties through consideration of domain walls

Prediction of properties from composition is a fundamental goal of materials science and can greatly accelerate development of functional materials. It is particularly relevant for ferroelectric perovskite solid solutions where compositional variation is a primary tool for materials design. To advance beyond the commonly used Landau-Ginzburg-Devonshire and density functional theory methods that despite their power are not predictive, we elucidate the key interactions that govern ferroelectrics using 5-atom bulk unit cells and non-ground-state defect-like ferroelectric domain walls as a simple as possible but not simpler model systems. We also develop a theory relating properties at several different length scales that provides a unified framework for the prediction of ferroelectric, antiferroelectric and ferroelectric phase stabilities and the key transition temperature, coercive field and polarization properties from composition. The elucidated physically meaningful relationships enable rapid identification of promising piezoelectric and dielectric materials.

cond-mat.mtrl-sci↗

arXiv2014

Semiconductor to metal transition in bilayer phosphorene under normal compressive strain

Phosphorene, a two-dimensional (2D) analog of black phosphorous, has been a subject of immense interest recently, due to its high carrier mobilities and a tunable bandgap. So far, tunability has been predicted to be obtained with very high compressive/tensile in-plane strains, and vertical electric field, which are difficult to achieve experimentally. Here, we show using density functional theory based calculations the possibility of tuning electronic properties by applying normal compressive strain in bilayer phosphorene. A complete and fully reversible semiconductor to metal transition has been observed at $\sim13.35\%$ strain, which can be easily realized experimentally. Furthermore, a direct to indirect bandgap transition has also been observed at $\sim3\%$ strain, which is a signature of unique band-gap modulation pattern in this material. The absence of negative frequencies in phonon spectra as a function of strain demonstrates the structural integrity of the sheets at relatively higher strain range. The carrier mobilities and effective masses also do not change significantly as a function of strain, keeping the transport properties nearly unchanged. This inherent ease of tunability of electronic properties without affecting the excellent transport properties of phosphorene sheets is expected to pave way for further fundamental research leading to phosphorene-based multi-physics devices.

cond-mat.mtrl-sci↗

arXiv2014

Strain-induced electronic phase transition and strong enhancement of thermopower of TiS2

Using first principles density functional theory calculations, we show a semimetal to semiconducting electronic phase transition for bulk TiS 2 by applying uniform biaxial tensile strain. This electronic phase transition is triggered by charge transfer from Ti to S, which eventually reduces the overlap between Ti-(d) and S-(p) orbitals. The electronic transport calculations show a large anisotropy in electrical conductivity and thermopower, which is due to the difference in the effective masses along the in-plane and out of plane directions. Strain induced opening of band gap together with changes in dispersion of bands lead to three-fold enhancement in thermopower for both p- and n-type TiS2 . We further demonstrate that the uniform tensile strain, which enhances the thermoelectric performance, can be achieved by doping TiS2 with larger iso-electronic elements such as Zr or Hf at Ti sites.

cond-mat.mtrl-sci↗