arXiv subjects

Saikat Nandi

Publications and source records attributed to Saikat Nandi.

17 recordsLinked to original sources

arXiv2026

Low-temperature magnetism and spin dynamics in the disordered triangular-lattice Yb$^{3+}$ compound LiCaYb$_5$(BO$_3$)$_6$

The interplay between geometric frustration, spin--orbit coupling, and structural disorder can give rise to unconventional magnetic ground states in rare-earth triangular-lattice magnets. We report low-temperature magnetic and $^7$Li NMR investigations of the disordered triangular-lattice compound LiCaYb$_5$(BO$_3$)$_6$ (LCYBO). Rietveld refinement confirms a hexagonal $P6_522$ structure with partial Ca/Yb antisite disorder and fractional Li occupancy. Magnetic susceptibility and magnetization measurements indicate a well-isolated effective $J_{\mathrm{eff}}=1/2$ Kramers doublet with weak antiferromagnetic interactions ($\theta_{\mathrm{CW}} \approx -0.54$~K). The specific heat reveals a weak anomaly near 0.43 K, suggesting the development of short-range correlated magnetism rather than conventional long-range order. $^7$Li NMR spectra broaden strongly upon cooling, consistent with increasingly inhomogeneous internal magnetic fields. The spin-lattice relaxation exhibits two relaxation components consistent with disorder-induced distributions of local magnetic environments arising from antisite disorder and competing exchange pathways. Our results establish LCYBO as a structurally disordered frustrated triangular-lattice magnet with correlated low-energy spin dynamics.

cond-mat.dis-nn↗

arXiv2026

Novel Quantum Spin Liquid States in the $S = {\frac{1}{2}}$ Three-Dimensional Compound Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$

The three-dimensional $S = {\frac{1}{2}}$ system Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$ consists of two inequivalent Cu$^{2+}$ sites, each forming an edge shared triangular lattice. Our magnetic susceptibility $χ(T)$, specific heat $C_p(T)$, $^{89}$Y nuclear magnetic resonance (NMR), muon spin relaxation ($\upmu\mathrm{SR}$), and electron spin resonance (ESR) measurements on this system confirm the absence of any long-range magnetic ordering and the persistence of spin dynamics down to 0.077 K. In $^{89}$Y NMR we find an anomaly at about 120 K which we suggest arises from a fraction of the spins condensing into a singlet (a valence bond solid VBS) state. A plateau in the muon relaxation rate is observed between 60 K and 10 K (signifying the VBS state from a fraction of the spins) followed by an increase and another plateau below about 1 K (presumably signifying the quantum spin liquid state from all the spins). Our density functional theory calculations find a dominant antiferromagnetic interaction along the body diagonal with inequivalent Cu(1) and Cu(2) ions alternately occupying the corners of the cube. All other near-neighbour interactions between the Cu ions are also found to be antiferromagnetic and are thought to drive the frustration.

cond-mat.str-el↗

arXiv2026

Observation of a gapped phase in the one-dimensional $S = {\frac{1}{2}}$ Heisenberg antiferromagnetic chain Cu(Ampy)ClBr

Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C$_6$H$_8$N$_2$ = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, $^1$H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation ($μ$SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu$^{2+}$ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu$^{2+}$ spins as reflected by a Curie-Weiss temperature of about $-9$ K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the $^1$H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field $μ$SR evidences lack of any static magnetism.

cond-mat.str-el↗

arXiv2026

Femtosecond Nonadiabatic Confinement of Molecular Dication Yield

Doubly charged molecular cations often carry signatures of electronic correlation and electron-nuclear entanglement present in the parent cation. Here, we produce ethylene dications using a combination of an extreme ultraviolet pump and near-infrared probe pulses, observing a peak in the dication yield at a pump-probe delay of approximately 15 fs. Ab-initio calculations, which explicitly take into account coupled electron-nuclear dynamics induced by the pump and the multiphoton nature of the probe-induced ionization step, reproduced the observed delay in the yield. It originates from resonant enhancement of the multiphoton ionization of the electronically excited ethylene cation as the carbon-carbon double bond expands. However, this effect is tempered by rapid nonadiabatic relaxation of the excited ionic states. Our results suggest a general mechanism whereby ultrafast nonadiabatic relaxation of a molecular ion can compete with its strong-field ionization rate, confining the dication yield to a narrow temporal window of a few femtoseconds.

physics.chem-ph↗

arXiv2025

Frustration-driven unconventional magnetism in the Mn$^{2+}$ ($S=\frac{5}{2}$) based two-dimensional triangular-lattice antiferromagnet Ba$_{3}$MnTa$_{2}$O$_{9}$

A triple perovskite oxide Ba$_{3}$MnTa$_{2}$O$_{9}$ has been synthesized and its magnetic properties have been investigated through dc and ac magnetization, specific heat, electron spin resonance (ESR) measurements, and density functional theory (DFT) calculations. Mn$^{2+}$ ($S$ = 5/2) ions are the only magnetic species present in the material. These Mn$^{2+}$ ions constitute a quasi-two-dimensional triangular network in the crystallographic $ab$-plane. Magnetization and specific heat measurements reveal the absence of any long-range magnetic order down to 0.5\,K despite the presence of antiferromagnetic correlations between the magnetic ions, suggesting the presence of geometric frustration in the material. The entropy release is lower than the expected theoretical value of $Rln(6)$, further suggesting the presence of frustration. First-principles calculations using density functional theory (DFT) and atomistic spin dynamics (ASD) simulations further support this lack of static magnetic order even at low temperatures and identify the competing magnetic interactions along with the quasi-2D magnetic dimensionality as the underlying origin of such an unconventional magnetic behavior.

cond-mat.str-el↗

arXiv2025

Ultrafast probing of isotope-induced explciit symmetry breaking in ethylene

Symmetry governs nature's law, yet many of the natural phenomena occur due to the breakdown of symmetry. Here, we show how isotope-induced inversion symmetry breaking influences ultrafast photoisomerization processes in ethylene. Using extreme ultraviolet pump-near infrared probe time-of-flight mass spectrometry, we find that replacing one of the carbon atoms in ethylene with a 13C isotope leads to twice-faster structural relaxation via ethylene-ethylidene isomerization in the photo-excited molecular cation. Advanced trajectory surface hopping calculations incorporating the nuclear symmetry of the molecular systems, reveal that it arises from the mixing of different normal modes in the isotope-substituted species, interactions otherwise forbidden by symmetry. Although the mixing does not alter the symmetry of the electronic Hamiltonian, it modifies that of the nuclear Hamiltonian, causing explicit symmetry breaking. This facilitates efficient intra-molecular vibrational energy redistribution, lowering the isomerization yield. Our findings offer opportunities to use isotope-induced nuclear symmetry breaking to control the outcome of light-molecule interactions across ultrafast timescales.

physics.chem-ph↗

arXiv2025

Tracking photophysical relaxation in spiropyran with simulated time-resolved X-ray absorption spectroscopy

Spiropyran is a prototype photoswitch that undergoes photoinduced ring-opening via C-O bond fission. The quantum yield for this photochemical reaction is low, with photophysical relaxation being the dominant process. Previous theoretical studies have suggested that photophysical relaxation proceeds via cleavage and subsequent reformation of a C-N bond. However, experimental evidence for this mechanism is lacking; most time-resolved studies have probed in the ultraviolet-visible domain, where C-N bond fission is unlikely to leave clear signatures. Here, we use non-adiabatic dynamics simulations in conjunction with X-ray absorption spectroscopic calculations to investigate the possibility of tracking the photophysical relaxation in spiropyran with time-resolved X-ray absorption spectroscopy at the nitrogen K-edge. The simulations predict that cleavage of the C-N bond results in a transient red-shift of the X-ray absorption. These results indicate a potential experimental route to gain a mechanistic understanding of the efficient photophysical relaxation that limits the efficiency of spiropyran-based photochromic systems.

physics.chem-ph↗

arXiv2024

Angular momentum dependence in multiphoton ionization and attosecond time delays

Attosecond ionization time-delays at photoelectron energies above typically 10 eV are usually interpreted using the so called asymptotic approximation as a sum of the atomic or molecular delays with a universal laser-induced contribution. Here, we employ a two-harmonic RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transitions) configuration to isolate the multiphoton pathways and measure the ionization time delays as a function of the dressing field intensity. We show that the validity of the asymptotic theory can be extended to the threshold or to higher-order contributions by rigorously treating the angular-momentum dependence of the continuum-continuum transitions into universal and easily computable partial-wave-specific correction factors. Our asymptotic treatment is also valid for higher-order interfering amplitudes while significantly simplifying their evaluation and providing a transparent physical interpretation. The validity of the method for atomic and molecular targets in the vicinity of resonances, ionization thresholds, and for both the emission-integrated and angularly resolved signal is confirmed by comparison to ab initio calculations over a wide energy range.

physics.atom-ph↗

arXiv2024

Dissociation and isomerization following ionization of ethylene: insights from non-adiabatic dynamics simulations

Photoionized and electronically excited ethylene \ce{C2H4+} can undergo \ce{H}-loss, \ce{H2}-loss, and ethylene-ethylidene isomerization, where the latter entails a hydrogen migration. Recent pioneering experiments with few-femtosecond extreme ultraviolet pulses and complementary theoretical studies have shed light on the photodynamics of this prototypical organic cation. However, no theoretical investigation based on dynamics simulations reported to date has described the mechanisms and time scales of dissociation and isomerization. Herein, we simulate the coupled electron-nuclear dynamics of ethylene following vertical ionization and electronic excitation to its four lowest-lying cationic states. The electronic structure is treated at the CASSCF level, with an active space large enough to describe bond breaking and formation. The simulations indicate that dissociation and isomerization take place mainly on the cationic ground state and allow the probing of previous hypotheses concerning the correlation between the photochemical outcome and the traversed conical intersections. The results, moreover, support the long-standing view that \ce{H2}-loss may occur from the ethylidene form. However, the ethylene-ethylidene isomerization time predicted by the simulations is considerably longer than those previously inferred from indirect experimental measurements.

physics.chem-ph↗

arXiv2024

Generation of entanglement using a short-wavelength seeded free-electron laser

Quantum entanglement between the degrees of freedom encountered in the classical world is challenging to observe due to the surrounding environment. To elucidate this issue, we investigate the entanglement generated over ultrafast timescales in a bipartite quantum system comprising two massive particles: a free-moving photoelectron, which expands to a mesoscopic length-scale, and a light-dressed atomic ion, which represents a hybrid state of light and matter. Although the photoelectron spectra are measured classically, the entanglement allows us to reveal information about the dressed-state dynamics of the ion and the femtosecond extreme ultraviolet pulses delivered by a seeded free-electron laser. The observed generation of entanglement is interpreted using the time-dependent von Neumann entropy. Our results unveil the potential for using short-wavelength coherent light pulses from free-electron lasers to generate entangled photoelectron and ion systems for studying spooky action at a distance.

quant-ph↗

arXiv2022

Studying ultrafast Rabi dynamics with a short-wavelength seeded free-electron laser

Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field. They are ubiquitous in physics with applications in different areas such as photonics, nano-electronics, electron microscopy, and quantum information. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light-matter interaction within the rotating wave approximation\cite. Although intense nanometer-wavelength light-sources have been available for more than a decade, Rabi dynamics at such short wavelengths have not been observed directly. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser can drive Rabi oscillations between the ground state and an excited state in helium atoms. The measured photoemission signal revealed an Autler-Townes doublet as well as an avoided crossing, phenomena that are both trademarks of quantum optics. Using theoretical analyses that go beyond the strong-field approximation, we found that the ultrafast build-up of the doublet structure follows from a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond and few-femtosecond extreme-ultraviolet pulses, our results offer opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.

quant-ph↗

arXiv2021

Probing electronic decoherence with high-resolution attosecond photoelectron interferometry

Quantum coherence plays a fundamental role in the study and control of ultrafast dynamics in matter. In the case of photoionization, entanglement of the photoelectron with the ion is a well known source of decoherence when only one of the particles is measured. Here we investigate decoherence due to entanglement of the radial and angular degrees of freedom of the photoelectron. We study two-photon ionization via the 2s2p autoionizing state in He using high spectral resolution photoelectron interferometry. Combining experiment and theory, we show that the strong dipole coupling of the 2s2p and 2p$^2$ states results in the entanglement of the angular and radial degrees of freedom. This translates, in angle integrated measurements, into a dynamic loss of coherence during autoionization.

physics.atom-ph↗

arXiv2021

Spin-ice dynamics of pyrochlore Dy2GaSbO7 with enhanced Pauling zero-point entropy

We report the low-temperature magnetothermal properties and spin dynamics of a mixed pyrochlore, Dy2GaSbO7 through the coordination of measurements of dc magnetization, ac susceptibility, and heat capacity, and CF computation. In Dy2GaSbO7, the spins freeze at temperature Tice = 3.1 K, corresponding to dynamical spin-freezing into a disordered state in compliance with the spin-ice rules, but with enhanced zero-point entropy, unlike cannonical dipolar spin-ice materials, Dy2Ti2O7 and Ho2Ti2O7, due to random disorder and varied chemical pressure of B-site (GaSb tetrahedra) ions.

cond-mat.mtrl-sci↗

arXiv2020

A high-repetition rate attosecond light source for time-resolved coincidencespectroscopy

Attosecond pulses, produced through high-order harmonic generation in gases, have been successfully used for observing ultrafast, sub-femtosecond electron dynamics in atoms, molecules and solid state systems. Today's typical attosecond sources, however, are often impaired by their low repetition rate and the resulting insufficient statistics, especially when the number of detectable events per shot is limited. This is the case for experiments where several reaction products must be detected in coincidence, and for surface science applications where space-charge effects compromise spectral and spatial resolution. In this work, we present an attosecond light source operating at 200 kHz, which opens up the exploration of phenomena previously inaccessible to attosecond interferometric and spectroscopic techniques. Key to our approach is the combination of a high repetition rate, few-cycle laser source, a specially designed gas target for efficient high harmonic generation, a passively and actively stabilized pump-probe interferometer and an advanced 3D photoelectron/ion momentum detector. While most experiments in the field of attosecond science so far have been performed with either single attosecond pulses or long trains of pulses, we explore the hitherto mostly overlooked intermediate regime with short trains consisting of only a few attosecond pulses.e also present the first coincidence measurement of single-photon double ionization of helium with full angular resolution, using an attosecond source. This opens up for future studies of the dynamic evolution of strongly correlated electrons.

physics.atom-ph↗

arXiv2019

Controlling the Photoelectric Effect in the Time Domain

When an atom or molecule absorbs a high-energy photon, an electron is emitted with a well-defined energy and a highly-symmetric angular distribution, ruled by energy quantization and parity conservation. These rules seemingly break down when small quantum systems are exposed to short and intense light pulses, which raise the question of their universality for the simplest case of the photoelectric effect. Here we investigate the photoionization of helium by a sequence of attosecond pulses in the presence of a weak infrared dressing field. We continuously control the energy and introduce an asymmetry in the emission direction of the photoelectrons, thus contradicting well established quantum-mechanical predictions. This control is possible due to an extreme temporal confinement of the light-matter interaction. Our work extends time-domain coherent control schemes to one of the fastest processes in nature, the photoelectric effect.

physics.atom-ph↗

arXiv2019

Fano's propensity rule in angle-resolved attosecond pump-probe photoionization

In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. Here we generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular-dependence of the photoionization time delays and the delay-dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40 eV range.

physics.atom-ph↗

arXiv2010

The quantum Gaussian well

Different features of a potential in the form of a Gaussian well have been discussed extensively. Although the details of the calculation are involved, the general approach uses a variational method and WKB approximation, techniques which should be familiar to advanced undergraduates. A numerical solution of the Schrödinger equation through diagonalization has been developed in a self-contained way, and physical applications of the potential are mentioned.

quant-ph↗