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Sanjeev Kumar

Publications and source records attributed to Sanjeev Kumar.

At least 163 records · Page 9Linked to original sources
arXiv2010

Study of gold induced heavy ion collisions using isospin dependent QMD model

We have studied the fragment production mechanism in the set of four reactions 197Au79+12C6, 197Au79+26Al13, 197Au79+63Cu29 and 197Au79+208Pb82. The reactions are simulated at an energy 600 MeV/nucleon and collision geometry is varied from central to peripheral (= b/bmax = 0 to 1). A theoretical investigation has been carried out on the study of mass dependence of intermediate mass fragments (5\leqA\leqAtot/6) and other fragments. It is observed that multiplicity shows a good agreement for low Zbound, but it fails for high Zbound. This failure is due the method of analysis MST which we had used in our analysis, because MST method gives one heavy cluster at the time of high density. The discrepancy between theory and experiments can be removed by using reduced isospin dependent NN cross section and sophisticated clustrization algorithm SACA.

nucl-th↗
arXiv2010

Rapidity distribution dependence of the transition energy in heavy-ion collisions

For the present analysis, simulations are carried out for thousand of events for the reaction of 79Au197 + 79Au197 at semi-central geometry using a hard equation of state. The whole of the analysis is performed for light charged particles (LCP's). The transition from in-plane to out-of-plane is observed only when the mid-rapidity region is included in the rapidity bin otherwise no transition is observed. The transition energy is found to be strongly dependent on the size of the rapidity bin. The transition energy is parameterized with a straight line interpolation.

nucl-th↗
arXiv2010

Experimental balance energies and isospin-dependent nucleon-nucleon cross-sections

The effect of different isospin-dependent cross-section on directed flow is studied for variety of systems(for which experimental balance energies are available) using an isospin-dependent Quantum Molecular Dynamic (IQMD) model. We show that balance energies are sensitive towards isospin-dependent cross-sections for light systems, while nearly no effect exist for heavier nuclei. A reduced cross-section $σ= 0.9σ_{NN}$ with stiff equation of state is able to explain experimental balance energies in most of systems. A power law behaviour is also given for the mass dependence of balance energy, which also follow N/Z dependence.

nucl-th↗
arXiv2010

Role of Isospin Degree of freedom on the impact parameter dependence of balance energy

For this study we have simulated the 28Ni58+28Ni58 and 26Fe58+26Fe58 systems at incident energies ranging from 45 MeV/nucleon to 105 MeV/nucleon by using soft equation of state. The geometry of reactions chosen are = b/bmax = 0.2, 0.4, 0.5, 0.6, and 0.7.The Px/A is becoming more positive for neutron - rich system 26Fi58 as compared to neutron-poor system 28Ni58. This point towards the higher value of balance energy for 26Fi58 system. Further study in this direction is ongoing.

nucl-th↗
arXiv2010

Role of momentum dependent interactions in nuclear stopping

In the present analysis, thousand of events were simulated for the neutron rich reaction of 54Xe131 + 54Xe131, at different impact parameters using hard (HMD) and soft (SMD) momentum dependent equation of state. one can clearly see more stopping in the presence of HMD. Production of LCP's and free particles follows the same trend as stopping parameter R. This simply implies that the fragment production can act as a global indicator for the nuclear stopping.

nucl-th↗
arXiv2010

Phase diagram and upper critical field of homogenously disordered epitaxial 3-dimensional NbN films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal-insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder we observe that as kFl-->1 the superconducting transition temperature (Tc) and minimum conductivity (sigma_0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal-insulator transition and the superconductor-insulator transition occur at a single quantum critical point at kFl~1.

cond-mat.supr-con↗
arXiv2010

High-Tc ferrolectricity emerging from magnetic degeneracy in cupric oxide

Cupric oxide is multiferroic at unusual high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric and the higher-T one ferroelectric, with a size and direction of polarization in good agreement with experiment. By mapping of the ab initio results on an effective spin model we find with Monte Carlo that in the high-T magnetic state non-collinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii Moriya interaction. This leads to a novel mechanism for multiferroicity, with the particular property that non-magnetic impurities enhance the effect.

cond-mat.str-el↗
arXiv2010

Spin-spiral states in undoped manganites

The experimental observation of multiferroic behavior in perovskite manganites with a spiral spin structure demands to clarify the origin of these magnetic states and their relation to ferroelectricity. We show that spin-spiral phases with diagonal wavevector and also the E-type phase exist for intermediate values of the Hund's rule and the Jahn-Teller coupling in the orbitally ordered and insulating state of the standard two-band model Hamiltonian for manganites. Our results support the spin-current mechanism for ferroelectricity and present an alternative view to earlier conclusions where frustrating superexchange couplings were crucial to obtain spin-spiral states.

cond-mat.str-el↗
arXiv2010

Unitarity constraints on trimaximal mixing

When the neutrino mass eigenstate $ν_2$ is trimaximally mixed, the mixing matrix is called trimaximal. The middle column of trimaximal mixing matrix is identical to tri-bimaximal mixing and the other two columns are subject to unitarity constraints. This corresponds to a mixing matrix with four independent parameters in the most general case. Apart from the two Majorana phases, the mixing matrix has only one free parameter in the CP conserving limit. Trimaximality results into interesting interplay between mixing angles and CP violation. A notion of maximal CP violation naturally emerges here: CP violation is maximal for maximal 2-3 mixing. Similarly, there is a natural constraint on the deviation from maximal 2-3 mixing which takes its maximal value in the CP conserving limit.

hep-ph↗
arXiv2010

Effect of Phase Fluctuations on the Superconducting Properties of Strongly Disordered 3D NbN Thin Films

We present transport, Hall effect, electronic tunnelling and penetration depth studies in 3D homogeneously disordered epitaxial NbN thin films with disorder ranging from the moderately clean limit (k_Fl~10.12) to the very dirty limit (k_Fl~1.24). The superconducting transition temperature (T_c) decreases from ~17K to less than 350mK with increasing disorder. The T_c and conductivity at the lowest temperature both asymptotically approach zero as k_Fl tends to 1, indicating a coincidence of the metal-insulator transition (MIT) and the superconductor-insulator transition. Close to critical disorder there is spatial inhomogeneity in the superconducting density of states (DOS) and the superconducting state is governed by quantum phase fluctuations. This results in suppression of the superfluid density (n_s) and a pseudogap state where the resistance is no longer zero but the energy gap remains finite

cond-mat.supr-con↗
arXiv2010

Systematic study of the system size dependence of global stopping: Role of momentum dependent interactions and symmetry energy

Using the isospin-dependent quantum molecular dynamical (IQMD) model, we systematically study the role of momentum dependent interactions in global stopping and analyze the effect of symmetry energy in the presence of momentum dependent interactions. For this, we simulate the reactions by varying the total mass of the system from 80 to 394 at different beam energies from 30 to 1000 MeV/nucleon over central and semi-central geometries. The study is carried in the presence of momentum dependent interactions and symmetry energy by taking into account hard equation of state. The nuclear stopping is found to be sensitive towards the momentum dependent interactions and symmetry energy at low incident energies. The momentum dependent interactions are found to weaken the finite size effects in nuclear stopping.

nucl-th↗
arXiv2010

Effect of the symmetry energy on nuclear stopping and its relation to the production of light charged fragments

We present a complete systematics (excitation function, impact parameter, system size, isospin asymmetry, and equations of state dependences) of global stopping and fragment production for heavy-ion reactions in the energy range between 50 and 1000 MeV/nucleon in the presence of symmetry energy and an isospin-dependent cross section. It is observed that the degree of stopping depends weakly on the symmetry energy and strongly on the isospin-dependent cross section. However, the symmetry energy and isospin-dependent cross section has an effect of the order of more than 10% on the emission of light charged particles (LCP's). It means that nuclear stopping and LCP's can be used as a tool to get the information of an isospin-dependent cross section. Interestingly, the LCP's emission in the presence of symmetry energy is found to be highly correlated with the global stopping.

nucl-th↗
arXiv2010

Elliptical flow and isospin effects in heavy-ion collisions at intermediate energies

The elliptical flow of fragments is studied for different systems at incident energies between 50 and 1000 MeV/nucleon using the isospin-dependent quantum molecular dynamics (IQMD) model. Our findings reveal that elliptical flow shows a transition from positive (in-plane) to negative (out-of-plane) values in the midrapidity region at a certain incident energy known as the transition energy. This transition energy is found to depend on the model ingredients, size of the fragments, and composite mass of the reacting system as well as on the impact parameter of the reaction. A reasonable agreement is observed for the excitation function of elliptical flow between the data and our calculations. Interestingly, the transition energy is found to exhibit a power-law mass dependence.

nucl-th↗
arXiv2010

Medium mass fragments production due to momentum dependent interactions

The role of system size and momentum dependent effects are analyzed in multifragmenation by simulating symmetric reactions of Ca+Ca, Ni+Ni, Nb+Nb, Xe+Xe, Er+Er, Au+Au, and U+U at incident energies between 50 MeV/nucleon and 1000 MeV/nucleon and over full impact parameter zones. Our detailed study reveals that there exist a system size dependence when reaction is simulated with momentum dependent interactions. This dependence exhibits a mass power law behavior.

nucl-th↗
arXiv2009

Theoretical prediction of multiferroicity in double perovskite Y$_2$NiMnO$_6$

We put forward double perovskites of the R$_2$NiMnO$_6$ family (with $R$ a rare-earth atom) as a new class of multiferroics on the basis of {\it ab initio} density functional calculations. We show that changing $R$ from La to Y drives the ground-state from ferromagnetic to antiferromagnetic with $\uparrow\uparrow\downarrow\downarrow$ spin patterns. This E$^*$-type ordering breaks inversion symmetry and generates a ferroelectric polarization of few $μC/cm^2$. By analyzing a model Hamiltonian we understand the microscopic origin of this transition and show that an external electric field can be used to tune the transition, thus allowing electrical control of the magnetization.

cond-mat.str-el↗
arXiv2009

Evidence for multiferroicity in TTF-CA organic molecular crystals

We show by means of ab-initio calculations that the organic molecular crystal TTF-CA is multiferroic: it has an instability to develop spontaneously both ferroelectric and magnetic ordering. Ferroelectricity is driven by a Peierls transition of the TTF-CA in its ionic state. Subsequent antiferromagnetic ordering strongly enhances the opposing electronic contribution to the polarization: it is so large that it switches the direction of the total ferroelectric moment. Within an extended Hubbard model we capture the essence of the electronic interactions in TTF-CA, confirm the presence of a multiferroic groundstate and clarify how this state develops microscopically.

cond-mat.mtrl-sci↗
arXiv2009

Multiferroicity in rare-earth nickelates RNiO3

We show that charge ordered rare-earth nickelates of the type RNiO3 (R= Ho, Lu, Pr and Nd) are multiferroic with very large magnetically induced ferroelectric (FE) polarizations. This we determine from first principles electronic structure calculations. The emerging FE polarization is directly tied to the long-standing puzzle of which kind of magnetic ordering is present in this class of materials: its direction and size indicate the type of ground-state spin configuration that is realized. Vice versa, the small energy differences between the different magnetic orderings suggest that a chosen magnetic ordering can be stabilized by cooling the system in presence of an electric field.

cond-mat.str-el↗