arXiv subjects

Dinesh Kumar

Publications and source records attributed to Dinesh Kumar.

At least 91 records · Page 5Linked to original sources

arXiv2019

Synthesis, Structural and Optical Properties of (ALa)(FeMn)O6 (A = Ba and Sr) Double Perovskites

Here, we report structural and optical properties of (ALa)(FeMn)O6 (A = Ba and Sr) double perovskite synthesized via auto-combustion followed by a calcination process. Rietveld refinement of the structure using x-ray diffraction data reveals that (BaLa)(FeMn)O6 crystallizes into cubic crystal structure with space group Pm-3m while (SrLa)(FeMn)O6 crystallizes into rhombohedral crystal structure having space group R-3c. The absorption spectrum measurement using UV-Vis spectroscopy reveals that these samples are perfect insulator having an energy band gap between conduction and valence band of the order of 6 eV.

cond-mat.mtrl-sci↗

arXiv2019

X-ray Diffraction Analysis of Cu2+ Doped Zn1-xCuxFe2O4 Spinel Nanoparticles using Williamson-Hall Plot Method

The nanoparticles (NPs) of Zn1-xCuxFe2O4 (ZCFO) spinels with x = 0, 0.2, 0.4, 0.6 and 0.8 were synthesized by a sol-gel combustion method using acetate precursor. The NPs of ZCFO were prepared by following calcination process at 600C for 8hrs. The synthesized NPs of ZCFO were characterized by X-ray diffraction (XRD) analysis using Rietveld refinement. The Rietveld refinement of the XRD patterns revealed that the ZCFO spinels crystallize into single diamond cubic structure with Fd-3m space group. The lattice constant and unit cell volume for ZCFO NPs shrink with enhancing doping concentration of Cu2+ ion. The crystalline growth in the NPs of ZCFO was examined by peak broadening present in the XRD pattern. The Williamson-Hall (W-H) plot method were used to study the individual involvements of crystallite sizes and lattice strain on the peak broadening of the NPs of ZCFO spinels. Whereas, particle size of the ZCFO sample with x = 0.40 was estimated by high-resolution scanning electron microscopy micrographs

cond-mat.mtrl-sci↗

arXiv2018

Crystallite Size Effect on Lattice Strain and Crystal Structure of Ba1/4Sr3/4MnO3 Layered Perovskite Manganite

The single phase polycrystalline Ba1/4Sr3/4MnO3 layered perovskite manganite has been synthesized by combustion method having various crystallite sizes. The room temperature X-ray diffraction patterns reveal that Ba1/4Sr3/4MnO3 crystallizes into hexagonal crystal structure with space group P63/mmc as confirmed by Rietveld refinement. The scanning electron micrographs of Ba1/4Sr3/4MnO3 reveal uniform crystallite size of the samples. Effect of crystallite size on lattice strain and crystal structure has been studied using Rietveld refinement and Williamson-Hall plot, respectively. The structural lattice parameters decrease with increasing crystallite size. However, lattice strain increases with increasing crystallite-size.

cond-mat.mtrl-sci↗

arXiv2018

On Solution of Second Order Complex Differential Equation

In this paper, we establish transcendental entire function $A(z)$ and polynomial $B(z)$ such that the differential equation $f''+A(z)f'+B(z)f=0$, has all non-trivial solution of infinite order. We use the notion of \emph{critical rays} of the function $e^{P(z)}$, where $A(z)=d(z)e^{P(z)}$ with some restrictions.

math.CV↗

arXiv2018

New physics solutions for $R_D$ and $R_{D^*}$

Recent measurements of $R_{D^*}$ have reduced tension with the Standard Model prediction. Taking all the present data into account, we obtain the values of the Wilson coefficients of each new physics four-fermion operator of a given Lorentz structure. We find that the combined data rule out most of the solutions based on scalar/pseudoscalar operators. By studying the inter-relations between different solutions, we find that there are only four allowed solutions, which are based on operators with $(V-A)$, linear combination of $(V-A)$ and $(V+A)$, tensor and linear combination of scalar/pseudoscalar and tensor structure. We demonstrate that the need for new physics is driven by those measurement of $R_D$ and $R_{D^*}$ where the $τ$ lepton is not studied. Further, we show that new physics only in $b\rightarrow c\,μ\,\barν$ is not compatible with the full set of observables in the decays $B\rightarrow Dl\barν$ and $B\rightarrow D^*l\barν$.

hep-ph↗

arXiv2018

Results on escaping set of an entire function and its composition

Given two permutable entire functions $f$ and $g,$ we establish vital relationship between escaping sets of entire functions $f, g$ and their composition. We provide some families of transcendental entire functions for which Eremenko's conjecture holds. In addition, we investigate the dynamical properties of the mapping $f(z)=z+1+e^{-z}.$

math.DS↗

arXiv2018

Resolution of $R_D$/$R_{D^*}$ puzzle

One of the exciting results in flavor physics in recent times is the $R_D$/$R_{D^*}$ puzzle. The measurements of these flavor ratios performed by the B-factory experiments, BaBar and Belle, and the LHCb experiment are about $4σ$ away from the Standard Model expectation. These measurements indicate that the mechanism of $b\rightarrow cτ\barν$ decay is not identical to that of $b\rightarrow c(μ/e)\barν$. This charge lepton universality violation is particularly intriguing because these decays occur at tree level in the Standard Model. In particular, we expect a moderately large new physics contribution to $b\rightarrow cτ\barν$. The different types of new physics amplitudes, which can explain the $R_D$/$R_{D^*}$ puzzle, have been identified previously. In this letter, we show that the polarization fractions of $τ$ and $D^*$ and the angular asymmetries $A_{FB}$ and $A_{LT}$ in $B\rightarrow D^*τ\barν$ decay have the capability to uniquely identify the Lorentz structure of the new physics. A measurement of these four observables will lead to such an identification.

hep-ph↗

arXiv2018

Geometric phase and neutrino mass hierarchy problem

We study the geometric phase for neutrinos at various man-made facilities, such as the reactor and accelerator neutrino experiments. The analysis is done for the three flavor neutrino scenario, in the presence of matter and for general, noncyclic paths. The geometric phase is seen to be sensitive to the CP violating phase in the leptonic sector and the sign ambiguity in Delta_{31}. We find that for neutrino experimental facilities where the geometric phase can complete one cycle, all the phase curves corresponding to different values of CP violating phase, converge to a single point, called the cluster point. There are two distinct cluster points for positive and negative signs of Delta_{31}. Thus the geometric phase can contribute to our understanding of the neutrino mass hierarchy problem.

hep-ph↗

arXiv2018

Ecologically Sustainable Partitioning of a Metapopulations Network

A stable population network is hard to interrupt without any ecological consequences. A communication blockage between patches may destabilize the populations in the ecological network. This work deals with the construction of a safe cut passing through metapopulations habitat such that populations remain stable. We combine the dynamical system stability analysis with graph partitioning algorithms in our approach to the problem. It finds such a safe construction, when one exists, provided the algebraic connectivity of the graph components is stronger than all the spatially local instabilities in the respective components. The dynamics of the populations on the spatially discrete patches (graph nodes) and their spatial communication with other patches is modeled as a reaction-diffusion system. By reversing the Turing-instability idea the stability conditions of the partitioned system are found to depend on local dynamics of the metapopulations and the Fiedler value of the Laplacian matrix of the graph. This leads to the necessary and sufficient conditions for removal of the graph edges subject to the stability of the partitioned graph networks. An heuristic bisection graph partitioning algorithm has been proposed and examples illustrate the theoretical result.

math.DS↗

arXiv2018

Low temperature magnetic properties of Nd2Ru2O7

We present magnetic susceptibility, heat capacity, and neutron diffraction measurements of polycrystalline Nd2Ru2O7 down to 0.4 K. Three anomalies in the magnetic susceptibility measurements at 146, 21 and 1.8 K are associated with an antiferromagnetic ordering of the Ru4+ moments, a weak ferromagnetic signal attributed to a canting of the Ru4+ and Nd3+ moments, and a long-range-ordering of the Nd3+ moments, respectively. The long-range order of the Nd3+ moments was observed in all the measurements, indicating that the ground state of the compound is not a spin glass. The magnetic entropy of Rln2 accumulated up to 5 K, suggests the Nd3+ has a doublet ground state. Lattice distortions accompany the transitions, as revealed by neutron diffraction measurements, and in agreement with earlier synchrotron x-ray studies. The magnetic moment of the Nd3+ ion at 0.4 K is estimated to be 1.54(2)μB and the magnetic structure is all-in all-out as determined by our neutron diffraction measurements.

cond-mat.str-el↗

arXiv2017

Probing new physics through $B^*_s \rightarrow μ^+ μ^-$ decay

We perform a model independent analysis of new physics in $B^*_s \rightarrow μ^+ μ^-$ decay. We intend to identify new physics operator(s) which can provide large enhancement in the branching ratio of $B^*_s \rightarrow μ^+ μ^-$ above its standard model prediction. For this, we consider new physics in the form of vector, axial-vector, scalar and pseudoscalar operators. We find that scalar and pseudoscalar operators do not contribute to the branching ratio of $B^*_s \rightarrow μ^+ μ^-$. We perform a global fit to all relevant $b \to s μ^+ μ^-$ data for different new physics scenarios. For each of these scenarios, we predict $Br(B^*_s \rightarrow μ^+ μ^-)$. We find that a significant enhancement in $Br(B^*_s \rightarrow μ^+ μ^-)$ is not allowed by any of these new physics operators. In fact, for all new physics scenarios providing a good fit to the data, the branching ratio of $B^*_s \rightarrow μ^+ μ^-$ is suppressed as compared to the SM value. Hence the present $b \to s μ^+ μ^-$ data indicates that the future measurements of $Br(B^*_s \rightarrow μ^+ μ^-)$ is expected to be suppressed in comparison to the standard model prediction.

hep-ph↗

arXiv2017

New Physics in $b \to s μ^+ μ^-$ after the Measurement of $R_{K^*}$

The recent measurement of $R_{K^*}$ is yet another hint of new physics (NP), and supports the idea that it is present in $b\to sμ^+μ^-$ decays. We perform a combined model-independent and model-dependent analysis in order to deduce properties of this NP. Like others, we find that the NP must obey one of two scenarios: (I) $C_9^{μμ}({\rm NP}) < 0$ or (II) $C_9^{μμ}({\rm NP}) = - C_{10}^{μμ}({\rm NP}) < 0$. A third scenario, (III) $C_9^{μμ}({\rm NP}) = - C_{9}^{\prime μμ}({\rm NP})$, is rejected largely because it predicts $R_K = 1$, in disagreement with experiment. The simplest NP models involve the tree-level exchange of a leptoquark (LQ) or a $Z'$ boson. We show that scenario (II) can arise in LQ or $Z'$ models, but scenario (I) is only possible with a $Z'$. Fits to $Z'$ models must take into account the additional constraints from $B^0_s$-${\bar B}^0_s$ mixing and neutrino trident production. Although the LQs must be heavy, O(TeV), we find that the $Z'$ can be light, e.g., $M_{Z'} = 10$ GeV or 200 MeV.

hep-ph↗

arXiv2017

High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestanding superconducting ($T_{c}$ = 4.3\,K) boron doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30\,GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients and the grain boundary components undergo an irreversible phase change at 14\,GPa. We show that the blue shift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_{0}$=510$\pm$28\,GPa. The comparative analysis of high-pressure data clarified that the sp$^{2}$ carbons in the grain boundaries transform into hexagonal diamond.

cond-mat.supr-con↗

arXiv2017

D* polarization as a probe to discriminate new physics in B --> D* tau nubar

The confirmation of excess in $R_{D^*}$ at the LHCb is an indication of lepton flavor non-universality. Various different new physics operators and their coupling strengths, which provide a good fit to $R_D$, $R_{D^*}$ and $q^2$ spectra, were identfied previously. In this work, we try to find angular observables in $\bar{B} \to D^* τ\barν$ which enable us to distinguish between these new physics operators. We find that the $D^*$ polarization fraction $f_L(q^2)$ is a good discriminant of scalar and tensor new physics operators. The change in $\langle f_L(q^2) \rangle$, induced by scalar and tensor operators, is about three times larger than the expected uncertainty in the upcoming Belle measurement.

hep-ph↗

arXiv2017

New Physics in $b \rightarrow s μ^+ μ^-$: Distinguishing Models through CP-Violating Effects

At present, there are several measurements of $B$ decays that exhibit discrepancies with the predictions of the SM, and suggest the presence of new physics (NP) in $b \to s μ^+ μ^-$ transitions. Many NP models have been proposed as explanations. These involve the tree-level exchange of a leptoquark (LQ) or a flavor-changing $Z'$ boson. In this paper we examine whether it is possible to distinguish the various models via CP-violating effects in $B \to K^{(*)} μ^+ μ^-$. Using fits to the data, we find the following results. Of all possible LQ models, only three can explain the data, and these are all equivalent as far as $b \to s μ^+ μ^-$ processes are concerned. In this single LQ model, the weak phase of the coupling can be large, leading to some sizeable CP asymmetries in $B \to K^{(*)} μ^+ μ^-$. There is a spectrum of $Z'$ models; the key parameter is $g_L^{μμ}$, which describes the strength of the $Z'$ coupling to $μ^+μ^-$. If $g_L^{μμ}$ is small (large), the constraints from $B^0_s$-${\bar B}^0_s$ mixing are stringent (weak), leading to a small (large) value of the NP weak phase, and corresponding small (large) CP asymmetries. We therefore find that the measurement of CP-violating asymmetries in $B \to K^{(*)} μ^+ μ^-$ can indeed distinguish among NP $b \to s μ^+ μ^-$ models

hep-ph↗

arXiv2016

Coherent microwave generation by spintronic feedback oscillator

The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different effect, which can drive a nanomagnet into spontaneous oscillations without the need of external spin torque injection. For the demonstration of this effect, we use a nano-pillar of magnetic tunnel junction (MTJ) powered by a dc current and connected to a coplanar waveguide (CPW) lying above the free layer of the MTJ. Any fluctuation of the free layer magnetization is converted into oscillating voltage via the tunneling magneto-resistance effect and is fed back into the MTJ by the CPW through inductive coupling. As a result of this feedback, the magnetization of the free layer can be driven into a continual precession. The combination of MTJ and CPW behaves similar to a laser system and outputs a stable rf power with quality factor exceeding 10,000.

cond-mat.mes-hall↗

arXiv2016

Approaching unstructured search from function bilateral symmetry detection - A quantum algorithm

Detection of symmetry is vital to problem solving. Most of the problems of computer vision and computer graphics and machine intelligence in general, can be reduced to symmetry detection problem. Unstructured search problem can also be looked upon from symmetry detection point of view. Unstructured search can be thought as searching a binary string satisfying some search condition in an unsorted list of binary strings. In this paper unstructured search problem is reduced to function bilateral symmetry detection problem with polynomial overhead in terms of the size of the input. Keywords: Unstructured Search, Quantum algorithm, Function bilateral symmetry detection, Decision Problem, Quantum Black Box, Solving NP complete problems.

quant-ph↗

arXiv2016

Flavor signatures of isosinglet vector-like down quark model

We consider a model where the standard model is extended by the addition of a vector-like isosinglet down-type quark b'. We perform a chi square fit to the flavor physics data and obtain the preferred central values along with errors of all the elements of the measurable 3 X 4 quark mixing matrix. The fit indicates that all the new-physics parameters are consistent with zero and the mixing of the b' quark with the other three is constrained to be small. The current flavor physics data rules out possibility of detectable new physics signals in most of the flavor physics observables. We also investigate possible deviations in the standard model Wtb couplings and bottom quark coupling to Higgs boson. We find that these deviations are less than a percent level which is too small to be observed at the LHC with current precision.

hep-ph↗