arXiv subjects

S. Harshini Tekur

Publications and source records attributed to S. Harshini Tekur.

9 recordsLinked to original sources

arXiv2026

Decoding Crystallographic Surface Chirality from Real Space and Momentum Space Images by Machine Learning

Intrinsically chiral metal surfaces, where handedness arises from the asymmetric step-kink-terrace topology of high-Miller index planes, are model systems for enantiospecific catalysis, sensing, and spintronics. Yet no consistent method exists to classify their handedness directly from experimental observables, without prior knowledge of crystallographic indices. Here, we report a dual-domain machine learning framework that recognizes handedness in chiral metal surfaces from two independent image representations: atomic structure models in real space and simulated momentum-resolved photoemission maps of Fermi surface projections in reciprocal space. A ResNet18 model pretrained on general image data is fine-tuned for chirality classification independently on both image representations and achieves meaningful classification accuracy for both real space and reciprocal space images. However, the Fermi surface classifier significantly outperforms the real space classifier. Critically, the reciprocal space classifier, trained only on synthetic images, correctly identifies synchrotron-acquired experimental Angle-Resolved Photoemission Spectroscopy (ARPES) maps of Cu(643)$^R$ and Cu(643)$^S$. The stronger performance in momentum space, as well as the transfer to experimental data, shows that handedness is encoded more globally and robustly in the electronic structure than in the spatially localized kink-site geometry. These results establish ARPES as a quantitative readout of crystallographic surface chirality and suggest a scalable route to identify chiral metal surfaces relevant to spin-selective phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Probing the localization effects in Krylov basis

Krylov complexity (K-complexity) is a measure of quantum state complexity that minimizes wavefunction spreading across all the possible bases. It serves as a key indicator of operator growth and quantum chaos. In this work, K-complexity and Arnoldi coefficients are applied to probe a variety of localization phenomena in the quantum kicked rotor system. We analyze four distinct localization scenarios -- ranging from compact localization effect arising from quantum anti-resonance to a weaker form of power-law localization -- each one exhibiting distinct K-complexity signatures and Arnoldi coefficient variations. In general, K-complexity not only indicates the degree of localization, but surprisingly also of the nature of localization. In particular, the long-time behaviour of K-complexity and the wavefunction evolution on Krylov chain can distinguish various types of observed localization in QKR. In particular, the time-averaged K-complexity and scaling of the variance of Arnoldi coefficients with effective Planck's constant can distinguish the localization effects induced by the classical regular phase structures and the dynamical localization arising from quantum interferences. Further, the Arnoldi coefficient is shown to capture the transition from integrability to chaos as well. This work shows how localization dynamics manifests in the Krylov basis.

quant-ph↗

arXiv2025

Assessment of spectral phases of non-Hermitian quantum systems through complex and singular values

Chaotic behavior or lack thereof in non-Hermitian systems is often diagnosed via spectral analysis of associated complex eigenvalues. Very recently, singular values of the associated non-Hermitian systems have been proposed as an effective measure to study dissipative quantum chaos. Motivated by the rich properties of non-Hermitian power-law banded random matrices and its promise as a platform to study localized and delocalized phases in non-Hermitian systems, we make an in-depth study to assess different spectral phases of these matrices through the lens of both complex eigenvalues and singular values. Remarkably, the results from complex spectra and singular value analysis are seemingly different, thereby necessitating caution while identifying different phases. We also exemplify our findings by studying a non-Hermitian Hamiltonian with a complex on-site disorder. Our work indicates that systems, where disorder is present both in the Hermitian and non-Hermitian segments of a Hamiltonian, are sensitive to the specific diagnostic tool that needs to be employed to study quantum chaos.

cond-mat.stat-mech↗

arXiv2025

Dichotomy in the effect of chaos on ergotropy

The maximum unitarily extractable work from a quantum system -- ergotropy -- is the basic principle behind quantum batteries, a rapidly emerging field. This work studies ergotropy in two quantum chaotic systems, the quantum kicked top and the kicked Ising spin chain, to illustrate the effects of chaotic dynamics. In an ancilla-assisted scenario, chaos enhances ergotropy when the state is known, a consequence of large entanglement production in the chaotic regime. When the state is unknown, we need to at least partially characterize the state using coarse-grained measurements for useful extraction of work. In this case, chaos impedes ergotropy by suppressing information gained from coarse-grained measurements, while entanglement with an ancilla still facilitates ergotropy. In this scenario, we study the interplay between chaos and entanglement and find a sweet spot in the chaos parameter for optimal work. Our results point to the potential of quantum chaos-assisted batteries for better work extraction.

quant-ph↗

arXiv2024

Higher-order gap ratios of singular values in open quantum systems

Understanding open quantum systems using information encoded in its complex eigenvalues has been a subject of growing interest. In this paper, we study higher-order gap ratios of the singular values of generic open quantum systems. We show that $k$-th order gap ratio of the singular values of an open quantum system can be connected to the nearest-neighbor spacing ratio of positions of classical particles of a harmonically confined log-gas with inverse temperature $β'(k)$ where $β'(k)$ is an analytical function that depends on $k$ and the Dyson's index $β=1,2,$ and $4$ that characterizes the properties of the associated Hermitized matrix. Our findings are crucial not only for understanding long-range correlations between the eigenvalues but also provide an excellent way of distinguishing different symmetry classes in an open quantum system. To highlight the universality of our findings, we demonstrate the higher-order gap ratios using different platforms such as non-Hermitian random matrices, random dissipative Liouvillians, Hamiltonians coupled to a Markovian bath, and Hamiltonians with in-built non-Hermiticity.

cond-mat.stat-mech↗

arXiv2020

Symmetry deduction from spectral fluctuations in complex quantum systems

The spectral fluctuations of complex quantum systems, in appropriate limit, are known to be consistent with that obtained from random matrices. However, this relation between the spectral fluctuations of physical systems and random matrices is valid only if the spectra are desymmetrized. This implies that the fluctuation properties of the spectra are affected by the discrete symmetries of the system. In this work, it is shown that in the chaotic limit the fluctuation characteristics and symmetry structure for any arbitrary sequence of measured or computed levels can be inferred from its higher-order spectral statistics without desymmetrization. In particular, we consider a spectrum composed of $k>0$ independent level sequences with each sequence having the same level density. The $k$-th order spacing ratio distribution of such a composite spectrum is identical to its nearest neighbor counterpart with modified Dyson index $k$. This is demonstrated for the spectra obtained from random matrices, quantum billiards, spin chains and experimentally measured nuclear resonances with disparate symmetry features.

quant-ph↗

arXiv2018

Scaling in the eigenvalue fluctuations of the empirical correlation matrices

The spectra of empirical correlation matrices, constructed from multivariate data, are widely used in many areas of sciences, engineering and social sciences as a tool to understand the information contained in typically large datasets. In the last two decades, random matrix theory-based tools such as the nearest neighbour eigenvalue spacing and eigenvector distributions have been employed to extract the significant modes of variability present in such empirical correlations. In this work, we present an alternative analysis in terms of the recently introduced spacing ratios, which does not require the cumbersome unfolding process. It is shown that the higher order spacing ratio distributions for the Wishart ensemble of random matrices, characterized by the Dyson index $β$, is related to the first order spacing ratio distribution with a modified value of co-dimension $β'$. This scaling is demonstrated for Wishart ensemble and also for the spectra of empirical correlation matrices drawn from the observed stock market and atmospheric pressure data. Using a combination of analytical and numerics, such scalings in spacing distributions are also discussed.

physics.data-an↗

arXiv2018

Higher order spacing ratios in random matrix theory and complex quantum systems

The distribution of the ratios of nearest neighbor level spacings has become a popular indicator of spectral fluctuations in complex quantum systems like interacting many-body localized and thermalization phases, quantum chaotic systems, and also in atomic and nuclear physics. In contrast to the level spacing distribution, which requires the cumbersome and at times ambiguous unfolding procedure, the ratios of spacings do not require unfolding and are easier to compute. In this work, for the class of Wigner-Dyson random matrices with nearest neighbor spacing ratios $r$ distributed as $P_β(r)$ for the three ensembles indexed by $β=1,2, 4$, their $k-$th order spacing ratio distributions are shown to be identical to $P_{β'}(r)$, where $β'$, an integer, is a function of $β$ and $k$. This result is shown for Gaussian and circular ensembles of random matrix theory and for several physical systems such as spin chains, chaotic billiards, Floquet systems and measured nuclear resonances.

quant-ph↗

arXiv2018

Exact distribution of spacing ratios for random and localized states in quantum chaotic systems

Typical eigenstates of quantum systems, whose classical limit is chaotic, are well approximated as random states. Corresponding eigenvalue spectra is modeled through appropriate ensemble of random matrix theory. However, a small subset of states violate this principle and display eigenstate localization, a counter-intuitive feature known to arise due to purely quantum or semiclassical effects. In the spectrum of chaotic systems, the localized and random states interact with one another and modifies the spectral statistics. In this work, a $3 \times 3$ random matrix model is used to obtain exact result for the ratio of spacing between a generic and localized state. We consider time-reversal-invariant as well as non-invariant scenarios. These results agree with the spectra computed from realistic physical systems that display localized eigenmodes.

quant-ph↗