arXiv subjects

Sumit Sarkar

Publications and source records attributed to Sumit Sarkar.

14 recordsLinked to original sources

arXiv2026

Unraveling the temperature-responsive charge-disproportionation in BaBiO$_3$

This study shows that the charge disproportionation at the Bi site in BaBiO$_3$ alters as a function of temperature. Decreasing the temperature from 300K down to 160K leads to a significant modification of the density of states corresponding to the Bi-O hybridized band near the Fermi level (E$_\text{F}$). This modification indicates reduction of Bi 6$sp$ - O 2$p$ hybridization and O 2$p$ spectral weight near E$_\text{F}$. The strong decrement of covalency at lower temperatures is accompanied by a decrement in O 2$p$ hole density due to possible charge transfer from Bi 6$s$ to the O 2$p$ band. Bi-charge state analysis from Bi-4$f$ core-level spectra showed that at 300K, $\delta$ (charge difference between alternate Bi sites) value in 4$\pm\delta$ is much less than at 160K, which reveals the transition towards the ionic nature of CD or static CD in BBO at low temperature. On the other hand, O 1$s$ core-level spectra displayed an asymmetric shape, and temperature-dependent modifications of the asymmetric shape and intensity have been observed. This highlights the significant influence of the O 2$p$ band hole on the dynamical CD at the Bi site.

cond-mat.mtrl-sci↗

arXiv2026

Role of Disorder in Governing the Magnetic Properties of Cu2IrO3

Cu$_2$IrO$_3$ is a honeycomb iridate which has been studied recently as a candidate Kitaev quantum spin liquid. Its magnetic ground state however, has been reported to be quantum disordered, spin glassy, or magnetically ordered depending on synthesis details. We have prepared a Cu$_2$IrO$_3$ sample with large antisite disorder and studied in detail its structure (global and local), charge states, and thermodynamic properties to try to quantify and characterize the disorder and its connection to the magnetic ground state. X-ray diffraction, Extended x-ray absorption fine structure(EXAFS) and X-ray pair distribution function analysis revealed a large site disorder ($\sim$25\%), while XPS and XANES reveal mixed valence of Cu and Ir following Cu$^{1+}$ + Ir$^{4+}$ $\rightarrow$ Cu$^{2+}$ + Ir$^{3+}$. This combination of site disorder and charge redistribution generates competing antiferromagnetic interactions and magnetic frustration, resulting in dynamically fluctuating AFM clusters near 80K that freeze below 29K. These results demonstrate the crucial role of synthesis dependent disorder in determining the magnetic ground state of Cu$_2$IrO$_3$.

cond-mat.str-el↗

arXiv2024

A Quantum Algorithm Based Heuristic to Hide Sensitive Itemsets

Quantum devices use qubits to represent information, which allows them to exploit important properties from quantum physics, specifically superposition and entanglement. As a result, quantum computers have the potential to outperform the most advanced classical computers. In recent years, quantum algorithms have shown hints of this promise, and many algorithms have been proposed for the quantum domain. There are two key hurdles to solving difficult real-world problems on quantum computers. The first is on the hardware front -- the number of qubits in the most advanced quantum systems is too small to make the solution of large problems practical. The second involves the algorithms themselves -- as quantum computers use qubits, the algorithms that work there are fundamentally different from those that work on traditional computers. As a result of these constraints, research has focused on developing approaches to solve small versions of problems as proofs of concept -- recognizing that it would be possible to scale these up once quantum devices with enough qubits become available. Our objective in this paper is along the same lines. We present a quantum approach to solve a well-studied problem in the context of data sharing. This heuristic uses the well-known Quantum Approximate Optimization Algorithm (QAOA). We present results on experiments involving small datasets to illustrate how the problem could be solved using quantum algorithms. The results show that the method has potential and provide answers close to optimal. At the same time, we realize there are opportunities for improving the method further.

quant-ph↗

arXiv2023

Competing s-p and p-p fluctuations in charge-disproportionation of BaBiO3

Here we investigate the mechanism of charge-disproportionation (CD) in BaBiO3 (BBO) using density functional theory under different crystal symmetries and by employing strain as an external perturbation. The competition between Bi 6sp-O 2p (s-p) and O 2p-O 2p (p-p) charge-fluctuations decides the electronic ground state, charge-disproportionation and bond-disproportionation (BD) in BBO. An extended Hubbard Hamiltonian involving onsite (U) and long-range (V) coulomb repulsions is also employed to ascertain the microscopic conditions for the formation of the lone-pair on bismuth site. A strong tensile strain increases p-p fluctuation and enhances negative-charge transfer (CT) character, while strong compressive strain favors s-p fluctuation leading to more positive-CT character. Bulk BBO is at the verge of positive and negative-CT regimes.

cond-mat.mtrl-sci↗

arXiv2022

Role of local structural distortions on the origin of j=1/2 pseudo-spin state in sodium iridate

Na2IrO3 (NIO) is known to be a spin-orbit (SO) driven j=1/2 pseudo-spin Mott-Hubbard (M-H) insulator. However, the microscopic origin of the pseudo-spin state and the role of local structural distortions have not been clearly understood. Using a combination of theoretical calculations and x-ray spectroscopy, we show that the energetics in the vicinity of Fermi level (EF) is governed by SO interactions, electron correlation and local octahedral distortions. Contrary to the earlier understanding, here we show that the j=3/2 and 1/2 pseudo-spin states have admixture of both t2g and eg characters due to local structural distortion. Reduction of local octahedral symmetry also enables Ir 5d- O2p hybridization around the EF resulting in a M-H insulator with enhanced charge transfer character. The possibility of Slater insulator phase is also ruled out by a combination of absence of room temperature DoS in valence band spectra, calculated moments and temperature dependent magnetization measurements.

cond-mat.mtrl-sci↗

arXiv2022

Separating the output ports of a Bragg interferometer via velocity selective transport

We report on the study of a detection scheme based on a Bloch separator in two-photon Bragg interferometers. We increase the spatial separation between the two output ports of the interferometer by selectively imparting 30 Bloch oscillations to one of them before their detection via time of flight. This method allows increasing the duration of the interferometer by reducing the time for discriminating the ports of the interferometer at detection. We study in detail the impact of this separator on the performance of a dual gravity sensor, and in particular on its measurement sensitivities to gravity acceleration and gravity gradients.

physics.atom-ph↗

arXiv2021

Role of electronic excitation on the anomalous magnetism of elemental Copper

Magnetic susceptibility of elemental copper (Cu) shows an anomalous rise at low temperatures superimposed on the expected atypical diamagnetic response. Such temperature dependent susceptibility, which is also known as the Curie tail, can not be explained on the basis of Larmor diamagnetic and Pauli paramagnetic contributions expected in Cu. Using valence band resonant photoemission spectroscopy results and density functional theory calculations, we show the magnetic anomaly appears due to presence of holes in Cu 3d band, which originates from thermally excited electronic configuration. Our study therefore highlights that the Curie tail, which is generally overlooked presuming it either due to paramagnetic impurities or defects, can in fact be intrinsic to a material, and even simple systems like elemental Cu is susceptible to electronic excitations giving rise to anomalous magnetic state.

cond-mat.mtrl-sci↗

arXiv2019

Atom interferometry using temporal Talbot effect on a Bose-Einstein condensate

We experimentally investigate a uniform pulse sequence in which atom interference is realized using the temporal matter-wave Talbot effect in an atom-optic kicked rotor system. Multi-path interference is obtained in asymmetric configuration with momentum differences up to $\pm$14 $\hbar k$, by virtue of Talbot resonance. We experimentally confirm the theoretical limit placed on the performance of this interferometer by the finite momentum distribution of the initial ensemble consisting of a Bose-Einstein condensate (BEC). This limitation on sensitivity, occurring due to the degradation of resonant dynamics is also important in the realization of a one-dimensional continuous-time quantum walk in the implementation of quantum search algorithms.

physics.atom-ph↗

arXiv2019

Non-monotonic diffusion rates in atom-optics L\'{e}vy kicked rotor

The dynamics of chaotic Hamiltonian systems such as the kicked rotor continues to guide our understanding of transport and localization processes. The localized states of the quantum kicked rotor decay due to decoherence effects if subjected to stationary noise. The associated quantum diffusion increases monotonically as a function of a parameter characterising the noise distribution. In this work, for the Levy kicked atom-optics rotor, it is experimentally shown that by tuning a parameter characterizing the Levy distribution, quantum diffusion displays non-monotonic behaviour. The parameters for optimal diffusion rates are analytically obtained and they reveal a good agreement with the cold atom experiments and numerics. The non-monotonicity is shown to be a quantum effect that vanishes in the classical limit.

nlin.CD↗

arXiv2018

A simple atomic beam oven with a metal thermal break

We report the design and construction of a simple, easy to machine high-temperature oven for generating an atomic beam in laser cooling experiments. This design eliminates the problem of thermal isolation of the oven region from the rest of the vacuum system without using a glass or ceramic thermal break. This design simplifies the construction and operation of high-temperature ovens for elements having low vapor pressure. We demonstrate the functionality of such a source for Strontium (Sr) atoms. We generate a high flux of Sr atoms for use in laser cooling and trapping experiments. The optimization of the design of the metal thermal break is done using a finite element analysis.

physics.atom-ph↗

arXiv2018

Diffraction of a CW atom laser in the Raman-Nath regime

Atom interferometry is the most successful technique for precision metrology. However, current interferometers using ultracold atoms allows one to probe the interference pattern only momentarily and has finite duty cycle, resulting in an aliasing effect and a low-bandwidth measurement -- also known as Dick effect. Interferometry with a continuous-wave atom laser shows promise in overcoming these limitations due a continuous monitoring of the interference pattern. In this work, we demonstrate a key step towards such an interferometry by demonstrating a diffraction of an `atom laser' in the Raman-Nath regime. We outcouple a continuous beam of coherent atoms from a reservoir of $^{87}$Rb Bose-Einstein condensate (BEC) upto 400 ms. The `atom laser' interacts with a grating formed by a standing wave of a far detuned laser light. The atom laser diffracts into several orders going up to 9$^{th}$ order or up to momenta of $\pm 18\ \hbar k$. We have characterized the diffraction of atom laser for different conditions and the results match with numerical simulations. Such atom laser will allow for construction of an atom-interferometer to probe physics phenomenon continuously up to a time of the order of few hundred millisecond.

physics.atom-ph↗

arXiv2016

Non-exponential decoherence and subdiffusion in atom-optics kicked rotor

Quantum systems lose coherence upon interaction with the environment and tend towards classical states. Quantum coherence is known to exponentially decay in time so that macroscopic quantum superpositions are generally unsustainable. In this work, slower than exponential decay of coherences is experimentally realized in an atom-optics kicked rotor system subjected to non-stationary L\'{e}vy noise in the applied kick sequence. The slower coherence decay manifests in the form of quantum subdiffusion that can be controlled through the L\'{e}vy exponent. The experimental results are in good agreement with the analytical estimates and numerical simulations for the mean energy growth and momentum profiles of atom-optics kicked rotor.

quant-ph↗

arXiv2014

Bose-Einstein Condensation in an electro-pneumatically transformed quadrupole-Ioffe magnetic trap

We report a novel approach for preparing a Bose-Einstein condensate (BEC) of $^{87}$Rb atoms using electro-pneumatically driven transfer of atoms into a Quadrupole-Ioffe magnetic trap (QUIC Trap). More than 5$\times$$10^{8}$ atoms from a Magneto-optical trap are loaded into a spherical quadrupole trap and then these atoms are transferred into an Ioffe trap by moving the Ioffe coil towards the center of the quadrupole coil, thereby, changing the distance between quadrupole trap center and the Ioffe coil. The transfer efficiency is more than 80 \%. This approach is different from a conventional approach of loading the atoms into a QUIC trap wherein the spherical quadrupole trap is transformed into a QUIC trap by changing the currents in the quadrupole and the Ioffe coils. The phase space density is then increased by forced rf evaporative cooling to achieve the Bose-Einstein condensation having more than $10^{5}$ atoms.

physics.atom-ph↗

arXiv2013

Using Tree-Decomposable Structures to Approximate Belief Networks

Tree structures have been shown to provide an efficient framework for propagating beliefs [Pearl,1986]. This paper studies the problem of finding an optimal approximating tree. The star decomposition scheme for sets of three binary variables [Lazarsfeld,1966; Pearl,1986] is shown to enhance the class of probability distributions that can support tree structures; such structures are called tree-decomposable structures. The logarithm scoring rule is found to be an appropriate optimality criterion to evaluate different tree-decomposable structures. Characteristics of such structures closest to the actual belief network are identified using the logarithm rule, and greedy and exact techniques are developed to find the optimal approximation.

cs.AI↗