arXiv subjects

M. Nandi

Publications and source records attributed to M. Nandi.

7 recordsLinked to original sources

arXiv2026

Good Practice Guide for quantifying uncertainties for machine learning models applied to photoplethysmography signals

This Good Practice Guide presents work done in the QUMPHY project (Uncertainty quantification for machine learning models applied to photoplethysmography signals) that considered both machine learning and uncertainty quantification for problems which used photoplethysmography (PPG) signals from wearable devices as input. It provides high-level guidance on what types of machine learning model might be used and how different models compare when applied to both regression and classification tasks. It provides guidance on the implementation of different methods for uncertainty quantification, covering both model-dependent and model-independent techniques, and on the validation of the results provided by those methods. It also describes six benchmark problems together with pointers to different benchmark datasets for each problem. Software is described that can assist practitioners in implementing the methods described herein and there is a brief consideration of ethical issues. It concludes with a summary and recommendations.

cs.LG↗

arXiv2021

Quasi-one-dimensional magnetism in the spin-$\frac12$ antiferromagnet BaNa$_{2}$Cu(VO$_{4}$)$_{2}$

We report synthesis and magnetic properties of quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain compound BaNa$_2$Cu(VO$_4$)$_2$. This orthovanadate has a centrosymmetric crystal structure, $C2/c$, where the magnetic Cu$^{2+}$ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62$^0$ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the inter-chain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, ESR intensity, and NMR shift agree well with the uniform spin-$1/2$ Heisenberg chain model with an intrachain coupling of $J/k_{\rm B} \simeq 5.6$ K. The saturation field obtained from the magnetic isotherm measurement consistently reproduces the value of $J/k_{\rm B}$. Further, the $^{51}$V NMR spin-lattice relaxation rate mimics the 1D character in the intermediate temperature range, whereas magnetic long-range order sets in below $T_{\rm N} \simeq 0.25$ K. The effective interchain coupling is estimated to be $J_{\perp}/k_{\rm B} \simeq 0.1$ K. The theoretical estimation of exchange couplings using band-structure calculations reciprocate our experimental findings and unambiguously establish the 1D character of the compound. Finally, the spin lattice of BaNa$_2$Cu(VO$_4$)$_2$ is compared with the chemically similar but not isostructural compound BaAg$_2$Cu(VO$_4)_2$.

cond-mat.mtrl-sci↗

arXiv2017

NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure

We present $^{77}$Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\rm s}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe.

cond-mat.str-el↗

arXiv2016

Magnetodielectric and spin-lattice coupling in quasi 1D Ising spin chain CoNb$_{2}$O$_{6}$

We have studied magnetodielectric and spin-lattice coupling in CoNb$_{2}$O$_{6}$ single crystals. Magnetostriction and magnetodielectric experiments are performed at temperatures in and above antifferomagnetic phase of quasi 1D Ising spin chain CoNb$_{2}$O$_{6}$. Field induced magnetic transitions are clearly reflected in magnetodielectric measurement as well as magnetostriction measurement also. Two sharp anomalies are found around the critical fields of antiferromagnetic to ferrimagnetic transition and ferrimagnetic to saturated paramagnetic transition in both magnetodielectric and magnetostriction experiments. High field anomaly is more pronounced for magnetodielectric response and magnetostriction also. So, in CoNb$_{2}$O$_{6}$, spins are strongly coupled with lattice as well as charges also.

cond-mat.str-el↗

arXiv2016

Magnetic and magnetocaloric properties of quasi-one-dimensional Ising spin chain CoV$_{2}$O$_{6}$

We have investigated the magnetic and magnetocaloric properties of antiferromagnetic Ising spin chain CoV$_{2}$O$_{6}$ by magnetization and heat capacity measurements. Both monoclinic $α$-CoV$_{2}$O$_{6}$ and triclinic $γ$-CoV$_{2}$O$_{6}$ exhibit field-induced metamagnetic transition from antiferromagnetic to ferromagnetic state via an intermediate ferrimagnetic state with 1/3 magnetization plateau. Due to this field-induced metamagnetic transition, these systems show large conventional as well as inverse magnetocaloric effects. In $α$-CoV$_{2}$O$_{6}$, we observe field-induced complex magnetic phases and multiple magnetization plateaux at low temperature when the field is applied along $c$ axis. Several critical temperatures and fields have been identified from the temperature and field dependence of magnetization, magnetic entropy change and heat capacity to construct the $H$-$T$ phase diagram. As compared to $α$-CoV$_{2}$O$_{6}$, $γ$-CoV$_{2}$O$_{6}$ displays a relatively simple magnetic phase diagram. Due to the large magnetic entropy change and adiabatic temperature change at low or moderate applied magnetic field, $γ$-CoV$_{2}$O$_{6}$ may be considered as a magnetic refrigerant in the low-temperature region.

cond-mat.str-el↗

arXiv2015

Field induced metamagnetic transitions in quasi-one-dimensional Ising spin chain CoV$_{2}$O$_{6}$

We have investigated the temperature and magnetic field dependence of magnetization ($M$), specific heat ($C_p$), and relative sample length change ($ΔL/L_0$) for understanding the field-induced metamagnetic transitions in quasi-one-dimensional monoclinic ($α$ phase) and triclinic ($γ$ phase) CoV$_2$O$_6$ spin chains. With the application of external magnetic field, a sharp peak emerges in $C_p$($T$) below $T_N$$=$15 K in $α$ phase and a hump-like feature appears above $T_N$$=$6.5 K in $γ$ phase due to the field-induced ferrimagnetic and ferromagnetic transitions, respectively. Strong field dependence of linear thermal expansion and large positive magnetostriction have been observed in $α$ phase. Though magnetization data indicate strong spin-orbit coupling for both phases, other measurements show that this effect is very weak for the $γ$ phase.

cond-mat.str-el↗

arXiv2014

Field-induced spin-structural transition and giant magnetostriction in Ising chain $α$-CoV$_2$O$_6$

We have investigated the temperature and magnetic field dependence of magnetization, specific heat ($C_p$), and relative sample length change ($ΔL/L_0$) for understanding the field-induced spin-structural change in quasi-one-dimensional spin chain $α$-CoV$_2$O$_6$ which undergoes antiferromagnetic (AFM) transition below $T_N$$=$15 K. Analysis of $C_p$($T$) shows that an effective $S$$=$1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above $T_N$. $C_p$ and the coefficient of linear thermal expansion ($α$) exhibit strong $H$ dependence in the AFM state. We also observe a huge positive magnetostriction [$ΔL$($H$)/$L_0$] below 20 K which does not show any tendency of saturation up to 9 T. With increasing field, a sharp and symmetric peak emerges below $T_N$ in both $C_p$($T$) and $α$($T$) due to field-induced first order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below $T_N$ suggests strong spin-lattice coupling in $α$-CoV$_2$O$_6$.

cond-mat.str-el↗