arXiv subjects

A. Pradhan

Publications and source records attributed to A. Pradhan.

13 recordsLinked to original sources

arXiv2026

Topological spin freezing in frustrated quantum materials

Competing interactions, non-trivial electronic band topology, quantum fluctuations, and the interplay between emergent degrees of freedom in frustrated quantum materials can give rise to a wide range of exotic phenomena. Glassy dynamics, originally studied in amorphous materials and biological systems, has recently attracted considerable interest in quantum condensed matter, particularly in relation to the collective behavior of spins, quasiparticle excitations, and topological spin textures. Here, we investigate the emergence of unconventional glassy spin dynamics in a broad class of frustrated quantum materials, where spin freezing exhibit distinct signatures in both thermodynamic and microscopic measurements. Using a comprehensive set of experimental probes, including thermodynamic, NMR, ($μ$SR), and neutron scattering, we identify characteristic signatures of topological spin-glass behavior and these complementary techniques reveal unconventional spin dynamics, short-range spin correlations, emergent low-energy excitations, and glassy behavior of topological origins, distinguishing these states from conventional spin glasses and disordered magnets. Furthermore, we discuss the role of hydrodynamic spin modes in governing glassy dynamics and the emergence of spin-jam states in frustrated lattices, providing a unified framework for understanding unconventional spin freezing of topological origin and bridging experimental observations with theoretical models. This review aims to advance our understanding of collective many-body phenomena arising from competing interactions, topological defects, collective excitations, quantum entanglement, and symmetry constraints. Such insights may facilitate the discovery and design of novel quantum materials and help address fundamental questions in contemporary condensed matter physics, with potential implications for future quantum technologies.

cond-mat.str-el↗

arXiv2026

Magnetic properties of a buckled honeycomb lattice antiferromagnet

The intriguing interplay between competing degrees of freedom in frustrated magnets can lead to non-trivial magnetic phenomena with exotic low-energy excitations that are highly relevant for addressing some of the fundamental questions in quantum condensed matter as well as potential technological applications. Herein, we report the synthesis and thermodynamic results on a frustrated magnet Co3ZnNb2O9. The Co2+ moments constitute buckled AB-type honeycomb layers in the ab-plane. The temperature-dependent magnetic susceptibility shows a sharp anomaly at 14 K, indicating the onset of long-range magnetic ordering. The Curie-Weiss fit of the magnetic susceptibility above 100 K, yields a Curie-Weiss temperature of -70 K, suggesting strong antiferromagnetic (AFM) interactions between the Co2+ spins and an effective magnetic moment of 5.54 muB, indicating the presence of unquenched orbital angular momentum. A field-induced spin-flop-like metamagnetic transition below the ordering temperature is characterized by a critical magnetic field of 1.2 T. The specific heat shows a lambda-type anomaly at 14 K, confirming the presence of long-range magnetic ordering, due to finite interlayer interaction. Interestingly, our study of the magnetocaloric effect near the transition temperature revealed an entropy change of 2.81 J/kg.K, which is ascribed to competing interactions, underlying anisotropy, and reduced net magnetization lead to relatively small isothermal entropy changes that suggest that frustrated honeycomb magnets are promising contenders for field-induced exotic phases and magnetocaloric response.

cond-mat.str-el↗

arXiv2025

Effective $\Lambda$CDM model emerging from $f(Q,T)$ under a special EOS limit in symmetric cosmology with Bayesian and ANN observational constraints

In this work, we investigate the cosmological consequences of an effective $\Lambda$CDM model emerging from the more general $f(Q,T)$ gravity theory under the special equation-of-state condition $\rho + p = 0$. Under this limit, the field equations yield the constraint $F(Q,T)H(t)=C$, implying that the function $F=f_Q$ becomes purely dependent on the non-metricity scalar $Q$, and the background evolution mimics that of the standard $\Lambda$CDM model. We derive the resulting functional forms of $f(Q)$, obtain the corresponding effective cosmological constant, and analyze the physical nature of this reduction. To test the model against observations, we perform a background-level parameter estimation for $H_{0}$ and $\Omega_{m}$, and evaluate the derived parameter $S_{8}$ (anchored to the Planck baseline $\sigma_8$) using cosmic chronometers (CC), baryon acoustic oscillations (BAO), and Pantheon+ SN Ia datasets. A dual-pipeline analysis is carried out using conventional Bayesian Markov Chain Monte Carlo (MCMC) sampling alongside a machine-learning based Artificial Neural Network (ANN) emulator deployed as a complementary consistency check. We demonstrate that the ANN approach successfully emulates the continuous parameter space with high computational efficiency, yielding results in close agreement with the standard MCMC likelihood framework. Because the background evolution strictly mimics the standard $\Lambda$CDM paradigm, the model successfully reproduces recent observational trends and recovers standard parameter constraints without dynamically resolving the baseline $H_{0}$ and $S_{8}$ tensions. Our results indicate that effective $\Lambda$CDM scenarios derived from $f(Q,T)$ gravity provide a viable and consistent with recent observational data.

gr-qc↗

arXiv2019

Interacting Tsallis holographic dark energy: Cosmic behaviour, statefinder analysis and $ω_D-ω'_D$ pair in the non-flat universe

The paper explores the interacting Tsallis holographic dark energy (THDE) model in a non-flat universe following an infrared cutoff as the apparent horizon. The equation of state (EoS) and the deceleration parameter of THDE model are determined to understand the cosmological evolution for interacting THDE model in the nonflat universe. By applying the statefinder $(r, s)$ parameter-pairs diagnostic and $ω_D-ω'_D$ pair dynamical analysis for the derived THDE model, we plot the evolutionary trajectories for different cases of Tsallis parameter $δ$ and interaction term $b^{2}$ and also, for spatial curvature $Ω_{k0}= 0, -0.0012$ and $0.0026$ corresponding to flat, open and closed universes, respectively, in the framework of Planck 2018 base cosmology results VI-LCDM observational data.

physics.gen-ph↗

arXiv2019

Evidence for an emerging disc wind and collimated outflow during an X-ray flare in the narrow-line Seyfert 1 galaxy Mrk 335

A triggered 140 ks XMM-Newton observation of the narrow-line Seyfert 1 (NLS1) Mrk 335 in December 2015 caught the active galaxy at its lowest X-ray flux since 2007. The NLS1 is relatively quiescent for the first ~120 ks of the observation before it flares in brightness by a factor of about five in the last 20 ks. Although only part of the flare is captured before the observation is terminated, the data reveal significant differences between the flare and quiescent phases. During the low-flux state, Mrk 335 demonstrates a reflection-dominated spectrum that results from a compact corona around a Kerr black hole. In addition to the rapid brightening, the flare is further described by spectral softening and a falling reflection fraction that are consistent with previous observations advocating at least part of the corona in Mrk 335 could be the base of an aborted jet. The spectrum during the flaring interval reveals several residuals between the 2-3 sigma level that could be attributed to absorption lines from a highly ionised plasma that is moving outward at v~0.12c. It could be that the increased luminosity during the flare enhances the radiation pressure sufficiently to launch a possible wind. If the wind is indeed responding to the change in corona luminosity then it must be located within ~80 Rg. The escape velocity at this distance is comparable to the estimated wind velocity. If confirmed, this is the first example of a radio-quiet AGN exhibiting behaviour consistent with both diffuse and collimated outflow.

astro-ph.HE↗

arXiv2016

Anisotropic String Cosmological Models in Heckmann-Suchuking Space-Time

In the present work we have searched the existence of the late time acceleration of the universe with string fluid as source of matter in anisotropic Heckmann-Suchking space-time by using 287 high red shift $(0.3 \leq z\leq 1.4)$ SN Ia data of observed absolute magnitude along with their possible error from Union 2.1 compilation. It is found that the best fit values for $(Ω_{m})_{0}$, $(Ω_Λ)_{0}$, $(Ω_σ)_{0}$ and $(q)_{0}$ are 0.2820, 0.7177, 0.0002 $\&$ -0.5793 respectively. Several physical aspects and geometrical properties of the model are discussed in detail.

gr-qc↗

arXiv2013

The rise of an ionized wind in the Narrow Line Seyfert 1 Galaxy Mrk 335 observed by XMM-Newton and HST

We present the discovery of an outflowing ionized wind in the Seyfert 1 Galaxy Mrk 335. Despite having been extensively observed by most of the largest X-ray observatories in the last decade, this bright source was not known to host warm absorber gas until recent XMM-Newton observations in combination with a long-term Swift monitoring program have shown extreme flux and spectral variability. High resolution spectra obtained by the XMM-Newton RGS detector reveal that the wind consists of three distinct ionization components, all outflowing at a velocity of 5000 km/s. This wind is clearly revealed when the source is observed at an intermediate flux state (2-5e-12 ergs cm^-2 s^-1). The analysis of multi-epoch RGS spectra allowed us to compare the absorber properties at three very different flux states of the source. No correlation between the warm absorber variability and the X-ray flux has been determined. The two higher ionization components of the gas may be consistent with photoionization equilibrium, but we can exclude this for the only ionization component that is consistently present in all flux states (log(xi)~1.8). We have included archival, non-simultaneous UV data from HST (FOS, STIS, COS) with the aim of searching for any signature of absorption in this source that so far was known for being absorption-free in the UV band. In the COS spectra obtained a few months after the X-ray observations we found broad absorption in CIV lines intrinsic to the AGN and blueshifted by a velocity roughly comparable to the X-ray outflow. The global behavior of the gas in both bands can be explained by variation of the covering factor and/or column density, possibly due to transverse motion of absorbing clouds moving out of the line of sight at Broad Line Region scale.

astro-ph.CO↗

arXiv2008

Bianchi Type-I Cosmological Models with Variable G and 4Λ$-Terms in General Relativity

Einstein's field equations with variable gravitational and cosmological ``constant'' are considered in presence of perfect fluid for Bianchi type-I spacetime. Consequences of the four cases of the phenomenological decay of $Λ$ have been discussed which are consistent with observations. The physical significance of the cosmological models have also been discussed.

gr-qc↗

arXiv2006

Cosmological Models of Universe with Variable Deceleration Parameter in Lyra's Manifold

FRW models of the universe have been studied in the cosmological theory based on Lyra's manifold. A new class of exact solutions has been obtained by considering a time dependent displacement field for variable deceleration parameter from which three models of the universe are derived (i) exponential (ii) polynomial and (iii) sinusoidal form respectively. The behaviour of these models of the universe are also discussed. Finally some possibilities of further problems and their investigations have been pointed out.

gr-qc↗

arXiv2004

Velocity-selective resonance dips in the probe absorption spectra of Rb D2 transitions induced by a pump laser

We report experimental observation of velocity-selective resonances in the Doppler-broadened probe absorption spectra of 85Rb and 87Rb D2 transitions in the presence of a strong copropagating pump laser locked to a frequency within the Doppler profile of the transition. The set of three dips having the separation of allowed hyperfine transitions can be moved along the Doppler profile by tuning the pump laser frequency indicating a resonance between the pump laser frequency and the velocity shifted probe laser frequency.

physics.class-ph↗

arXiv2002

Bulk Viscous Cosmological Models in Barber's Second Self Creation Theory

Barber's second self creation theory with bulk viscous fluid source for an LRS Bianchi type-I metric is considered by using deceleration parameter to be constant where the metric potentials are taken as function of $x$ and $t$. The coefficient of bulk viscosity is assumed to be a power function of the mass density. Some physical and geometrical features of the models are discussed

gr-qc↗

arXiv2000

Extinction transition in bacterial colonies under forced convection

We report the spatio-temporal response of {\it Bacillus subtilis} growing on a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover temperature, the bacteria are confined to regions that are shielded from UV radiation. A forced convection of the population is effected by rotating a UV radiation shield relative to the petri dish. The extinction speed at which the bacterial colony lags behind the shield is found to be qualitatively similar to the front velocity of the colony growing in the absence of the hostile environment as predicted by the model of Dahmen, Nelson and Shnerb. A quantitative comparison is not possible without considering the slow dynamics and the time-dependent interaction of the population with the hostile environment.

cond-mat↗

arXiv1999

Size Segregation of Granular Matter in Silo Discharges

We present an experimental study of segregation of granular matter in a quasi-two dimensional silo emptying out of an orifice. Size separation is observed when multi-sized particles are used with the larger particles found in the center of the silo in the region of fastest flow. We use imaging to study the flow inside the silo and quantitatively measure the concentration profiles of bi-disperse beads as a function of position and time. The angle of the surface is given by the angle of repose of the particles, and the flow occurs in a few layers only near the top of this inclined surface. The flowing region becomes deeper near the center of the silo and is confined to a parabolic region centered at the orifice which is approximately described by the kinematic model. The experimental evidence suggests that the segregation occurs on the surface and not in the flow deep inside the silo where velocity gradients also are present. We report the time development of the concentrations of the bi-disperse particles as a function of size ratios, flow rate, and the ratio of initial mixture. The qualitative aspects of the observed phenomena may be explained by a void filling model of segregation.

cond-mat.stat-mech↗