arXiv subjects

Sanjeev Kumar

Publications and source records attributed to Sanjeev Kumar.

At least 145 records · Page 8Linked to original sources

arXiv2012

Correlation and isospin dynamics of participant-spectator matter in neutron-rich colliding nuclei at 50 MeV/nucleon

The sensitivities of isospin asymmetry and collision geometry dependencies of participant (overlapping region)- spectator (quasiprojectile and quasitarget region) matter towards the symmetry energy using the isospin quantum molecular dynamical model are explored. Particularly, the difference of the number of nucleons in the overlapping zone to the quasi-projectile-target matter is found to be quite sensitive to the symmetry energy at semiperipheral geometries compared to the individual yield. It gives us a clue that this quantity can be used as a measure of isospin migration. Further, the yield of neutrons (charge of the second-largest fragment) is provided as a tool for overlapping region (quasi-projectile-target) matter to check the sensitivity of the above-mentioned observable towards the symmetry energy experimentally.

nucl-th↗

arXiv2012

Quark deconfinement phase transition in nuclear matter for improved quark mass density-dependent model

The improved quark mass density-dependent (IQMDD) model, which has been successfully used to describe the properties of both infinite nuclear matter and finite nuclei, is applied to investigate the properties of quark deconfinement phase transition. By using the finite-temperature quantum field theory, we calculate the finite temperature effective potential and extend the IQMDD model to finite temperature and finite nuclear matter density. The critical temperature and the critical density of nuclear matter are given and the QCD phase diagram is addressed. It is shown that this model can not only describe the saturation properties of nuclear matter, but also explain the quark deconfinement phase transition successfully.

nucl-th↗

arXiv2012

Four Zero Texture Fermion Mass Matrices in SO(10) GUT

We attempt the integration of the phenomenologically successful four zero texture of fermion mass matrices with the renormalizable SO(10) GUT. The resulting scenario is found to be highly predictive. Firstly, we examine the phenomenological implications of a class of the lepton mass matrices with parallel texture structures and obtain interesting constraints on the parameters of the charged lepton and the neutrino mass matrices. We combine these phenomenological constraints with the constraints obtained from SO(10) GUT to reduce the number of the free parameters and to further constrain the allowed ranges of the free parameters. The solar/atmospheric mixing angles obtained in this analysis are in fairly good agreement with the data.

hep-ph↗

arXiv2012

Rapidity distribution as a probe for elliptical flow at intermediate energies

Interplay between the spectator and participant matter in heavy-ion collisions is investigated within isospin dependent quantum molecular dynamics (IQMD) model in term of rapidity distribution of light charged particles. The effect of different types and size rapidity distributions is studied in elliptical flow. The elliptical flow patterns show important role of the nearby spectator matter on the participant zone. This role is further explained on the basis of passing time of the spectator and expansion time of the participant zone. The transition from the in-plane to out-of-plane is observed only when the mid-rapidity region is included in the rapidity bin, otherwise no transition occurs. The transition energy is found to be highly sensitive towards the size of the rapidity bin, while weakly on the type of the rapidity distribution. The theoretical results are also compared with the experimental findings and are found in good agreement.

nucl-th↗

arXiv2012

Observation of Andreev bound state and multiple energy gaps in the non-centrosymmetric superconductor BiPd

We report directional point contact Andreev reflection (PCAR) measurements on high-quality single crystals of the non-centrosymmetric superconductor, BiPd. The PCAR spectra measured on different crystallographic faces of the single crystal clearly show the presence of multiple superconducting energy gaps. For point contacts with low resistance, in addition to the superconducting gap feature, a pronounced zero bias conductance peak is observed. These observations provide strong evidence of the presence of unconventional order parameter in this material.

cond-mat.supr-con↗

arXiv2012

Switchable Quantum Anomalous Hall state in a strongly frustrated lattice magnet

We establish that the interplay of itinerant fermions with localized magnetic moments on a checkerboard lattice leads to magnetic flux-phases. For weak itineracy the flux-phase is coplanar and the electronic dispersion takes the shape of graphene-like Dirac fermions. Stronger itineracy drives the formation of a non-coplanar, chiral flux-phase, in which the Dirac fermions acquire a topological mass that is proportional to a ferromagnetic spin polarization. Consequently the system self-organizes into a ferromagnetic Quantum Anomalous Hall state in which the direction of its dissipationless edge-currents can be switched by an applied magnetic field.

cond-mat.str-el↗

arXiv2012

Microscopic origin of large negative magneto-electric coupling in Sr$_{1/2}$Ba$_{1/2}$MnO$_{3}$

With a combined ab initio density functional and model Hamiltonian approach we establish that in the recently discovered multiferroic phase of the manganite Sr$_{1/2}$Ba$_{1/2}$MnO$_{3}$ the polar distortion of Mn and O ions is stabilized via enhanced in-plane Mn-O hybridizations. The magnetic superexchange interaction is very sensitive to the polar bond-bending distortion, and we find that this dependence directly causes a strong magnetoelectric coupling. This novel mechanism for multiferroicity is consistent with the experimentally observed reduced ferroelectric polarization upon the onset of magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2011

Phase diagram of a strongly disordered s-wave superconductor, NbN, close to the metal-insulator transition

We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the system enters the critical disorder for the destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and tunneling spectroscopy measurements. Our studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4), the system follows the mean field Bardeen-Cooper-Schrieffer behavior where the superconducting energy gap vanishes at the temperature where electrical resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state emerges where a gap in the electronic spectrum persists up to temperatures much higher than Tc, suggesting that Cooper pairs continue to exist in the system even after the zero resistance state is destroyed. Finally, very strongly disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at low temperatures, suggesting that localized Cooper pairs continue to survive in the system even after the global superconducting ground state is completely destroyed.

cond-mat.supr-con↗

arXiv2011

Sensitivity of elliptical flow in intermediate energy heavy-ion collisions

The detailed analysis of excitation function of elliptical flow by taking into account the nature of equation of state, nucleon-nucleon cross sections as well as momentum dependent interactions is performed within Quantum Molecular Dynamics (QMD)model. All the observables are found to sensitive towards the elliptical flow. The transition energy is found to be decrease with increase in the size of the fragment in the presence of static equation of state as well as with large cross sections, while, trend gets opposite when momentum dependent interactions are included. At last, the experimental data is verified with static hard equation of state in the presence of large cross sections.

nucl-th↗

arXiv2011

Probing the density dependence of symmetry energy via multifragmentation at sub-saturation densities

Symmetry energy for asymmetric nuclear matter at subsaturation densitieswas investigated in the framework of an isospin-dependent quantum molecular dynamics model.Asingle ratio of neutrons and protons is comparedwith the experimental data of Famiano et al. [Phys. Rev. Lett. 97, 052701 (2006)]. We have also performed a comparison for the double ratio with experimental as well as different theoretical results of Boltzmann-Uehling-Uhlenbeck in 1997, Isospin-dependent Boltzmann-Uehling-Uhlenbeck in 2004, Boltzmann-Nordeim-Vlasov, and Improved QuantumMolecular Dynamics models. It is found that the double ratio predicts the softness of symmetry energy, which is a little underestimated in the single ratio. Furthermore, the study of the single ratio is extended for different kinds of fragments, while the double ratio is extended for different neutron-rich isotopes of Sn.

nucl-th↗

arXiv2011

Implications of a class of neutrino mass matrices with texture zeros for non-zero $θ_{13}$

A class of neutrino mass matrices with texture zeros realizable using the group $Z_3$ within the framework of type (I+II) seesaw mechanism naturally admits a non-zero $θ_{13}$ and allows for deviations from maximal mixing. The phenomenology of this model is reexamined in the light of recent hints for non-zero $θ_{13}$.

hep-ph↗

arXiv2011

Macroscopic Degeneracy and Emergent Frustration in a Honeycomb Lattice Magnet

Using a hybrid method based on fermionic diagonalization and classical Monte Carlo, we investigate the interplay between itinerant and localized spins, with competing double- and super-exchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightly larger superexchange a dimerized groundstate is stable that has macroscopic degeneracy. The presence of these states on a non-frustrated honeycomb lattice highlights a novel phenomenon in this itinerant electron system: emergent geometrical frustration and degeneracy.

cond-mat.str-el↗

arXiv2011

Role of the vortex-core energy on the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN

We analyze the occurrence of the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below $T_{BKT}$ and of the resistivity above $T_{BKT}$. By direct comparison between the experimental data and the theory we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor.

cond-mat.supr-con↗

arXiv2011

Magnetism and Charge ordering in TMTTF$_2$-PF$_6$ organic crystals

Using a combination of Density Functional Theory, mean-field analysis and exact diagonalization calculations we reveal the emergence of a dimerized charge ordered state in TMTTF$_2$-PF$_6$ organic crystal. The interplay between charge and spin order leads to a rich phase diagram. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase, which has been observed experimentally. The tendency to the dimerization is magnetically driven revealing TMTTF$_2$-PF$_6$ as a multiferroic material.

cond-mat.str-el↗

arXiv2010

Frustration-induced insulating chiral spin state in itinerant triangular-lattice magnets

We study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte-Carlo methods. We find that in between the expected itinerant ferromagnetic and $120^{\circ}$ Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet - scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets R$_2$Mo$_{2}$O$_7$.

cond-mat.str-el↗

arXiv2010

Phase fluctuations in a strongly disordered s-wave superconductor close to the metal-insulator transition

We explore the role of phase fluctuations in a 3-dimensional s-wave superconductor, NbN, as we approach the critical disorder for the destruction of the superconducting state. Scanning tunneling spectroscopy measurements reveal that in the presence of strong disorder, a finite gap in the electronic spectrum continues to persist at temperatures above Tc. With increase in disorder, the superfluid density is strongly suppressed at low temperatures and evolves towards a linear-T variation at higher temperatures. These observations provide strong evidence that phase fluctuation leads to the formation of a pseudogap state in a disordered conventional superconductor.

cond-mat.supr-con↗

arXiv2010

Energy of vanishing flow in heavy-ion collisions: Role of Coulomb interactions and asymmetry of a reaction

We aim to understand the role of Coulomb interactions as well as of different equations of state on the disappearance of transverse flow for various asymmetric reactions leading to same total mass. For the present study, the total mass of the system is kept constant (ATOT = 152) and asymmetry of the reaction is varied between 0.2 and 0.7. We find that the contribution of mean-field at low incident energies is more for nearly symmetric systems, while the trend is opposite at higher incident energies. The Coulomb interactions as well as different equations of state are found to affect the balance energy significantly for larger asymmetric reactions.

nucl-th↗

arXiv2010

L1 Projections with Box Constraints

We study the L1 minimization problem with additional box constraints. We motivate the problem with two different views of optimality considerations. We look into imposing such constraints in projected gradient techniques and propose a worst case linear time algorithm to perform such projections. We demonstrate the merits and effectiveness of our algorithms on synthetic as well as real experiments.

cs.DS↗