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Ashutosh Kumar

Publications and source records attributed to Ashutosh Kumar.

At least 55 records · Page 3Linked to original sources
arXiv2023

Quantum self-consistent equation-of-motion method for computing molecular excitation energies, ionization potentials, and electron affinities on a quantum computer

Near-term quantum computers are expected to facilitate material and chemical research through accurate molecular simulations. Several developments have already shown that accurate ground-state energies for small molecules can be evaluated on present-day quantum devices. Although electronically excited states play a vital role in chemical processes and applications, the search for a reliable and practical approach for routine excited-state calculations on near-term quantum devices is ongoing. Inspired by excited-state methods developed for the unitary coupled-cluster theory in quantum chemistry, we present an equation-of-motion-based method to compute excitation energies following the variational quantum eigensolver algorithm for ground-state calculations on a quantum computer. We perform numerical simulations on H$_2$, H$_4$, H$_2$O, and LiH molecules to test our quantum self-consistent equation-of-motion (q-sc-EOM) method and compare it to other current state-of-the-art methods. q-sc-EOM makes use of self-consistent operators to satisfy the vacuum annihilation condition, a critical property for accurate calculations. It provides real and size-intensive energy differences corresponding to vertical excitation energies, ionization potentials and electron affinities. We also find that q-sc-EOM is more suitable for implementation on NISQ devices as it is expected to be more resilient to noise compared with the currently available methods.

quant-ph↗
arXiv2023

Effect of crystal field engineering and Fermi level optimization on thermoelectric properties of Ge$_{1.01}$Te: Experimental investigation and theoretical insight

This study shows a method of enhancing the thermoelectric properties of GeTe-based materials by Ti and Bi co-doping on cation sites along with self-doping with Ge via simultaneous optimization of electronic (via crystal field engineering, and precise Fermi level optimization) and thermal (via point-defect scattering) transport properties. The pristine GeTe possesses high carrier concentration ($n$) due to intrinsic Ge vacancies, low Seebeck coefficient ($α$), and high thermal conductivity ($κ$). The Ge vacancy optimization and crystal field engineering results in an enhanced $α$ via excess Ge and Ti doping, which is further improved by band structure engineering through Bi doping. As a result of improved $α$ and optimized Fermi level (carrier concentration), an enhanced power factor ($α^2σ$) is obtained for Ti--Bi co-doped Ge$_{1.01}$Te. These experimental results are also evidenced by theoretical calculations of band structure, and thermoelectric parameters using density functional theory and Boltztrap calculations, respectively. A significant reduction in the phonon thermal conductivity ($κ_{ph}$) from $\sim$ 3.5 W.m$^{-1}$.K$^{-1}$ to $\sim$ 1.06 W.m$^{-1}$.K$^{-1}$ at 300\,K for Ti--Bi co-doping in GeTe, attributed to point-defect scattering due to mass and strain field fluctuation, in line with the Debye-Callaway model. The phonon dispersion calculations show a decreasing group velocity in Ti--Bi co-doped GeTe, supporting the obtained reduced $κ_{ph}$. The strategies used in the present study can significantly increase the effective mass, optimize the carrier concentration, and decrease phonon thermal conductivity while achieving an impressive maximum zT value of 1.75 at 773\,K and average zT (zT$_{av}$) of 1.03 for Ge$_{0.91}$Ti$_{0.02}$Bi$_{0.08}$Te over a temperature range of 300-773\,K.

cond-mat.mtrl-sci↗
arXiv2023

Quantum simulation of molecular response properties

Accurate modeling of the response of molecular systems to an external electromagnetic field is challenging on classical computers, especially in the regime of strong electronic correlation. In this paper, we develop a quantum linear response (qLR) theory to calculate molecular response properties on near-term quantum computers. Inspired by the recently developed variants of the quantum counterpart of equation of motion (qEOM) theory, the qLR formalism employs "killer condition" satisfying excitation operator manifolds that offers a number of theoretical advantages along with reduced quantum resource requirements. We also used the qEOM framework in this work to calculate state-specific response properties. Further, through noise-less quantum simulations, we show that response properties calculated using the qLR approach are more accurate than the ones obtained from the classical coupled-cluster based linear response models due to the improved quality of the ground-state wavefunction obtained using the ADAPT-VQE algorithm.

quant-ph↗
arXiv2022

Thermoelectric properties of Sb doped AlFe$_2$B$_2$

In this work, thermoelectric properties of Al$_{1.2}$Fe$_2$B$_2$ compound were investigated over a temperature range from 300\,K to 773\,K. Al$_{1.2}$Fe$_2$B$_2$ compound was produced by vacuum arc melting of Al, Fe, and B followed by annealing at 1323 K under argon atmosphere. The annealed ingots were subsequently crushed into powder and hot pressed at 1273 K under vacuum. The hot-pressed alloy predominantly contained Al$_{1.2}$Fe$_2$B$_2$ phase with a small fraction of FeB, which decreases further upon 0.1 \% Sb doping in Al$_{1.2}$Fe$_2$B$_2$. The pristine Al$_{1.2}$Fe$_2$B$_2$ exhibits n-type conductivity with a maximum figure of merit (zT) of 0.03 at 773\,K. The Sb doping improves the Seebeck coefficient at high temperatures and also reduces the phonon thermal conductivity across the temperature range studied. The decrease in phonon thermal conductivity is attributed to the point-defect phonon scattering due to mass fluctuation between the Fe and Sb atoms. The 0.1 at\% Sb doping at the Fe site results in improved zT of 0.056 at 773\,K in spite of its limited dissolution in Al$_{1.2}$Fe$_2$B$_2$ and forms FeSb$_2$ secondary phase.

cond-mat.mtrl-sci↗
arXiv2022

Magnetic and Electrical Properties of high-entropy rare-earth manganites

Detailed investigations of structural, magnetic and electronic transport properties of hole-doped high-entropy rare-earth manganites are presented. The high-entropy samples (LaNdPrSmEu)$_{1-x}$Sr$_x$MnO$_3$ (0$\leq$\textit{x}$\leq$0.5), synthesized using the solid-state technique, show a change in the crystal structure from \textit{Pbnm} to \textit{R-3c} with increasing Sr substitution, attributed to the change in the tolerance factor. Prominent ferromagnetic ordering is observed in the sample with a rhombohedral structure (\textit{x}$\geq$0.3), originating from the dominant double exchange mechanism mediated by itinerant electrons. Further, the Curie temperature is smaller for the high-entropy sample with \textit{x}=0.3, as compared to La$_{0.7}$Sr$_{0.3}$MnO$_3$, suggesting a strong relation between the Curie temperature and the Mn-O-Mn bond angle associated with the reduced ionic radii at the rare-earth site. The electrical resistivity of the high-entropy samples is larger than those of La$_{1-x}$Sr$_x$MnO$_3$, which can be ascribed to the reduced bandwidth due to the enhanced structural distortion. A concomitant rise in magnetoresistance is observed for high-entropy samples with the increase in Sr concentration. These findings considering the configurational complexity of different rare-earths advance the understanding of high-entropy rare earth manganites.

cond-mat.str-el↗
arXiv2022

Is there evidence for exponential quantum advantage in quantum chemistry?

The idea to use quantum mechanical devices to simulate other quantum systems is commonly ascribed to Feynman. Since the original suggestion, concrete proposals have appeared for simulating molecular and materials chemistry through quantum computation, as a potential ``killer application''. Indications of potential exponential quantum advantage in artificial tasks have increased interest in this application, thus, it is critical to understand the basis for potential exponential quantum advantage in quantum chemistry. Here we gather the evidence for this case in the most common task in quantum chemistry, namely, ground-state energy estimation. We conclude that evidence for such an exponential advantage across chemical space has yet to be found. While quantum computers may still prove useful for quantum chemistry, it may be prudent to assume exponential speedups are not generically available for this problem.

physics.chem-ph↗
arXiv2022

NL-Augmenter: A Framework for Task-Sensitive Natural Language Augmentation

Data augmentation is an important component in the robustness evaluation of models in natural language processing (NLP) and in enhancing the diversity of the data they are trained on. In this paper, we present NL-Augmenter, a new participatory Python-based natural language augmentation framework which supports the creation of both transformations (modifications to the data) and filters (data splits according to specific features). We describe the framework and an initial set of 117 transformations and 23 filters for a variety of natural language tasks. We demonstrate the efficacy of NL-Augmenter by using several of its transformations to analyze the robustness of popular natural language models. The infrastructure, datacards and robustness analysis results are available publicly on the NL-Augmenter repository (https://github.com/GEM-benchmark/NL-Augmenter).

cs.CL↗
arXiv2022

Road Rutting Detection using Deep Learning on Images

Road rutting is a severe road distress that can cause premature failure of road incurring early and costly maintenance costs. Research on road damage detection using image processing techniques and deep learning are being actively conducted in the past few years. However, these researches are mostly focused on detection of cracks, potholes, and their variants. Very few research has been done on the detection of road rutting. This paper proposes a novel road rutting dataset comprising of 949 images and provides both object level and pixel level annotations. Object detection models and semantic segmentation models were deployed to detect road rutting on the proposed dataset, and quantitative and qualitative analysis of model predictions were done to evaluate model performance and identify challenges faced in the detection of road rutting using the proposed method. Object detection model YOLOX-s achieves mAP@IoU=0.5 of 61.6% and semantic segmentation model PSPNet (Resnet-50) achieves IoU of 54.69 and accuracy of 72.67, thus providing a benchmark accuracy for similar work in future. The proposed road rutting dataset and the results of our research study will help accelerate the research on detection of road rutting using deep learning.

cs.CV↗
arXiv2022

Thermodynamics of one and two-qubit nonequilibrium heat engines running between squeezed thermal reservoirs

Quantum heat engines form an active field of research due to their potential applications. There are several phenomena that are unique to the quantum regime, some of which are known to give these engines an edge over their classical counterparts. In this work, we focus on the study of one and two-qubit finite-time Otto engines interacting with squeezed thermal baths, and discuss their important distinctions as well as the advantage of using the two-qubit engine. In particular, the two-qubit engine offers an interesting study of the interplay between the degree of squeezing and that of the coherence between the two qubits. We find that the two-qubit engine generally yields higher power than its one-qubit counterpart. The effective temperature of the squeezed baths can be calculated both for the one and two-qubit engines, and they tend to show an exponential growth with increase in squeezing parameters $r_h$ and $r_c$. It is also observed that by tuning the squeezing parameters, the machine can be made to work either in the engine or in the refrigerator mode. Additional effects due to the change in the inter-qubit separation have been studied.

cond-mat.stat-mech↗
arXiv2022

Thermoelectric properties of high-entropy rare-earth cobaltates

High-entropy concept introduced with a promising paradigm to obtain exotic physical properties has motivated us to explore the thermoelectric properties of Sr-substituted high-entropy rare-earth cobaltates i.e., (LaNdPrSmEu)$_{1-x}$Sr$_x$CoO3 (0 \leq x \leq 0.10). The structural analysis of the samples synthesized using the standard solid-state route, confirms the orthorhombic structure with the Pbnm space group. The Seebeck coefficient and electrical resistivity decrease with rising Sr concentration as well as with an increase in temperature. The multiple A-site ions in high-entropy rare-earth cobaltates result in an improved Seebeck coefficient (α) compared to La$_{0.95}$Sr$_{0.05}$CoO$_3$, associated with a decrease in the Co-O-Co bond angle, which further enhances the power factor. The random distribution of cations at the rare-earth site results in a significant lowering of phonon thermal conductivity. As a result, a maximum figure of merit (zT) of 0.23 is obtained at 350K for (LaNdPrSmEu)$_{0.95}$Sr$_{0.05}$CoO$_3$, which is one of the highest values of zT reported at this temperature for oxide materials. This study shows promise to decouple thermoelectric parameters using the high-entropy concept in several materials.

cond-mat.mtrl-sci↗
arXiv2022

Striking a Balance: Alleviating Inconsistency in Pre-trained Models for Symmetric Classification Tasks

While fine-tuning pre-trained models for downstream classification is the conventional paradigm in NLP, often task-specific nuances may not get captured in the resultant models. Specifically, for tasks that take two inputs and require the output to be invariant of the order of the inputs, inconsistency is often observed in the predicted labels or confidence scores. We highlight this model shortcoming and apply a consistency loss function to alleviate inconsistency in symmetric classification. Our results show an improved consistency in predictions for three paraphrase detection datasets without a significant drop in the accuracy scores. We examine the classification performance of six datasets (both symmetric and non-symmetric) to showcase the strengths and limitations of our approach.

cs.CL↗
arXiv2022

Accurate quantum simulation of molecular ground and excited states with a transcorrelated Hamiltonian

NISQ era devices suffer from a number of challenges like limited qubit connectivity, short coherence times and sizable gate error rates. Thus, quantum algorithms are desired that require shallow circuit depths and low qubit counts to take advantage of these devices. We attempt to realize this with the help of classical quantum chemical theories of canonical transformation and explicit correlation. In this work, compact ab initio Hamiltonians are generated classically through an approximate similarity transformation of the Hamiltonian with a) an explicitly correlated two-body unitary operator with generalized pair excitations that remove the Coulombic electron-electron singularities from the Hamiltonian and b) a unitary one-body operator to efficiently capture the orbital relaxation effects required for accurate description of the excited states. The resulting transcorelated Hamiltonians are able to describe both ground and excited states of molecular systems in a balanced manner. Using the fermionic-ADAPT-VQE method based on the unitary coupled cluster with singles and doubles (UCCSD) ansatz and only a minimal basis set (ANO-RCC-MB), we demonstrate that the transcorrelated Hamiltonians can produce ground state energies comparable to the much larger cc-pVTZ basis. This leads to a potential reduction in the number of required CNOT gates by more than three orders of magnitude for the chemical species studied in this work. Furthermore, using the qEOM formalism in conjunction with the transcorrelated Hamiltonian, we reduce the errors in excitation energies by an order of magnitude. The transcorrelated Hamiltonians developed here are Hermitian and contain only one- and two-body interaction terms and thus can be easily combined with any quantum algorithm for accurate electronic structure simulations.

physics.chem-ph↗
arXiv2021

Quantum simulation of electronic structure with a transcorrelated Hamiltonian: improved accuracy with a smaller footprint on the quantum computer

Quantum simulations of electronic structure with a transformed Hamiltonian that includes some electron correlation effects are demonstrated. The transcorrelated Hamiltonian used in this work is efficiently constructed classically, at polynomial cost, by an approximate similarity transformation with an explicitly correlated two-body unitary operator. This Hamiltonian is Hermitian, includes no more than two-particle interactions, and is free of electron-electron singularities. We investigate the effect of such a transformed Hamiltonian on the accuracy and computational cost of quantum simulations by focusing on a widely used solver for the Schrodinger equation, namely the variational quantum eigensolver method, based on the unitary coupled cluster with singles and doubles (q-UCCSD) Ansatz. Nevertheless, the formalism presented here translates straightforwardly to other quantum algorithms for chemistry. Our results demonstrate that a transcorrelated Hamiltonian, paired with extremely compact bases, produces explicitly correlated energies comparable to those from much larger bases. For the chemical species studied here, explicitly correlated energies based on an underlying 6-31G basis had cc-pVTZ quality. The use of the very compact transcorrelated Hamiltonian reduces the number of CNOT gates required to achieve cc-pVTZ quality by up to two orders of magnitude, and the number of qubits by a factor of three.

quant-ph↗
arXiv2021

Synergistic effect of workfunction and acoustic impedance mismatch for improved thermoelectric performance in GeTe/WC composite

The preparation of composite materials is promising for concurrent optimization of electrical and thermal transport properties to realize an improved thermoelectric (TE) performance. We report the effect of work function and acoustic impedance mismatch (AIM) on the TE properties of (1-z)Ge0.87Mn0.05Sb0.08Te/(z)WC composite. In particular, a composite consisting of Mn and Sb co-doped GeTe as a matrix and WC as a dispersed phase is prepared, and its structural and TE properties are investigated. The simultaneous increase in electrical conductivity (σ) and Seebeck coefficient (α) with WC volume fraction (z) results in an enhanced power factor (α^2σ) in the composite. The rise in σ is attributed to increased carrier mobility in the composite. This is further established from the work function measurement using the Kelvin probe force microscopy (KPFM) technique and is also supported by the density functional theory (DFT) calculations. The difference in elastic properties (sound velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM that leads to a large interface thermal resistance (Rint) between the phases. The correlation between Rint and the Kapitza radius results in reduced phonon thermal conductivity (κ_ph) of the composite and is discussed using the Bruggeman asymmetrical model. The decrease in κ_{ph} is further established using phonon dispersion calculations that indicates the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced α^2σ and reduced κ_ph results in a maximum figure of merit (zT) of 1.93 at 773K for (1-z)Ge0.87Mn0.05Sb0.08Te/(z)WC composite having z=0.010. This study shows promise to achieve higher zTav across a wide range of composite materials having similar electronic structure and different elastic properties.

physics.app-ph↗
arXiv2021

Electrical and Thermal transport studies of Sr and Mn co-substituted NdCoO3

Oxide thermoelectrics are exciting due to their chemical and thermal stability at high temperatures. However, the efficacy of these materials are limited by poor figure of merit (zT). In this study, the role of Sr and Mn co-substitution on the thermoelectric properties of NdCoO3 (Nd_{1-x}Sr_xCo_{1-y}Mn_yO_3; 0.00 \leq x \leq 0.10; 0.00 \leq y \leq 0.10) is investigated. The Seebeck coefficient decreases with single Sr substitution at Nd site; however, the Sr and Mn co-substitution enhances the Seebeck coefficient compared to single Sr substitution and is attributed to the localization effect. Sr substitution at La site creates hole in the system and results in enhanced electrical conductivity (σ); however, σ reduces with Mn substitution at Co site in NdCoO_3. A reduced thermal conductivity for the co-substituted samples is observed and attributed to decrease in phonon thermal conductivity. Simultaneous optimization of TE parameters results in improved zT \sim 0.038 for Nd_{0.95}Sr_{0.05}Co_{0.95}Mn_{0.05}O_3 at 540 K.

cond-mat.mtrl-sci↗
arXiv2021

Improved Thermoelectric Properties in (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 Composite

A high Seebeck coefficient (S), large electrical conductivity (σ), and reduced thermal conductivity (κ) are required to achieve a high figure-of-merit (zT) in an ideal thermoelectric (TE) system, which is challenging in a single system due to the interdependence of TE parameters. Composite approach is promising to manipulate the TE parameters. In this study, TE properties of (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 (0.00 \leq x \leq 0.05) composite is discussed. The structural analysis confirms individual phases in the composite, which is further supported by electron microscopy analysis. The x-ray photoelectron analysis indicates that oxygen vacancies (VO) are present in the parent LaCoO3 system and increase with the addition of La0.7Sr0.3CoO3 (LSCO) in the composite. The increase in VO raises the degenerate states of cobalt and hence improves S in the composites. Temperature variation in S and σ are consistent with the spin-state transition and shows the correlation between these two parameters. The reduction in κ and σ with the addition of ball-milled La0.7Sr0.3CoO3 in the composite is attributed to the enhanced phonon-phonon and charge carrier scattering, respectively. A synergistic effect of enhanced S and reduced κ} result in five times improvement in zT of the composite compared to the parent LaCoO3 system at 800 K. This approach also improves the operating temperature for LaCoO3 based systems.

cond-mat.mtrl-sci↗
arXiv2021

Vertical GaN Devices: Process and Reliability

This paper reviews recent progress and key challenges in process and reliability for high-performance vertical GaN transistors and diodes, focusing on the 200 mm CMOS-compatible technology. We particularly demonstrated the potential of using 200 mm diameter CTE matched substrates for vertical power transistors, and gate module optimizations for device robustness. An alternative technology path based on coalescence epitaxy of GaN-on-Silicon is also introduced, which could enable thick drift layers of very low dislocation density.

physics.app-ph↗