arXiv subjects

G. Sharma

Publications and source records attributed to G. Sharma.

30 records · Page 2Linked to original sources

arXiv2021

Experimental study on controlled production of two-electron temperature plasma

A two-electron temperature plasma is produced by the method of diffusion of two different plasmas with distinct temperatures and densities. The method is simple and provides an adequate control over the plasma parameters. The study reveals that the temperature and density of both the electron groups can be effectively controlled by just changing the discharge currents of both the plasmas. An ion-acoustic (IA) wave is excited in the plasma and is detected using a planar Langmuir probe. The damped amplitude of the wave is measured and is used as a diagnostic tool for establishing the presence of two-electron components. This production method can be helpful in controlling the hot electron density and temperature in plasma processing industries.

physics.plasm-ph↗

arXiv2019

Effect of two temperature electrons in a collisional magnetized plasma sheath

A collisional magnetized plasma consisting of two temperature electrons has been investigated numerically to study the sheath structure and the ion energy flux to the wall. The low-temperature electrons are described by Maxwellian distribution, and the high-temperature electrons are described by truncated Maxwellian distribution. It has been observed that high-temperature electrons play a major role in the sheath potential as well as the ion energy flux to the wall. The presence of collision in the sheath has a significant effect on the properties of the sheath. The study of such a system can help in understanding of plasma surface interaction.

physics.plasm-ph↗

arXiv2019

Gauge phonon dominated resistivity in twisted bilayer graphene near magic angle

Recent experiments on twisted bilayer graphene (tBG) close to magic angle show that a small relative rotation in a van der Waals heterostructure greatly alters its electronic properties. We consider various scattering mechanisms and show that the carrier transport in tBG is dominated by a combination of charged impurities and acoustic gauge phonons. Charged impurities still dominate at low temperature and densities because of the inability of Dirac fermions to screen long-range Coulomb potentials at charge neutrality; however, the gauge phonons dominate for most of the experimental regime because although they couple to current, they do not induce charge and are therefore unscreened by the large density of states close to magic angle. We show that the resistivity has a strong monotonically decreasing carrier density dependence at low temperature due to charged impurity scattering, and weak density dependence at high temperature due to gauge phonons. Away from charge neutrality, the resistivity increases with temperature, while it does the opposite close to the Dirac point. A non-monotonic temperature dependence observed only at low temperature and carrier density is a signature of our theory that can be tested in experimentally available samples.

cond-mat.mes-hall↗

arXiv2016

Transport studies of LPA electron beam towards the FEL amplification at COXINEL

Laser Plasma Acceleration (LPA) [1] is an emerging concept enabling to generate electron beams with high energy, high peak current and small transverse emittance within a very short distance. The use of LPA can be applied to the Free Electron Laser (FEL) [2] case in order to investigate whether it is suitable for the light amplification in the undulator. However, capturing and guiding of such beams to the undulator is very challenging, because of the large divergence and high energy spread of the electron beams at the plasma exit, leading to large chromatic emittances. A specific beam manipulation scheme was recently proposed for the COXINEL (Coherent X-ray source inferred from electrons accelerated by laser) setup, which makes an advantage from the intrinsically large chromatic emittance of such beams [3]. The electron beam transport is studied using two simulation codes: a SOLEIL in-house one and ASTRA [4]. The influence of the collective effects on the electron beam performance is also examined.

physics.acc-ph↗

arXiv2015

Field induced spin reorientation in [Fe/Cr]n multilayers studied by nuclear resonance reflectivity

We present the depth-resolved nuclear resonance reflectivity (NRR) studies of the magnetization evolution in [57Fe(3 nm)/Cr(1.2 nm)]*10 multilayer under the applied external field. The measurements have been performed at the station BL09XU of SPring-8 at different values of the external field (from 0 up to 1500 Oe). We apply the joint fit of the delayed reflectivity curves and the time spectra of the nuclear resonance reflectivity measured at different grazing angles for enhancement of the depth resolution and reliability of the results. For the first time we show that the azimuth angle, which is used in all papers devoted to the magnetization profile determination, has more complicated physical sense due to the partially coherent averaging of the scattering amplitudes from magnetic lateral domains. We describe the way how to select the true azimuth angle from the determined "effective azimuth angle". Finally we obtain the noncollinear twisted magnetization depth-profiles where the spin flop state appears sequentially in different 57Fe layers at increasing applied field.

cond-mat.mes-hall↗

arXiv2014

Improper Ferroelectricity in Helicoidal Antiferromagnet Cu3Nb2O8

Cu3Nb2O8 is an unusual multiferroic compound that undergoes a series of magnetic ordering at low temperatures. Concurrent development of electric polarization has been reported at TN1 ~25 K corresponding to a non-collinear helicoidal ordering (Phys. Rev. Lett. 107, 137205 (2011)). But questions remain on the microscopic origin of such phenomena. In this communication, we report a detailed study of induced polar ordering in Cu3Nb2O8 by performing polarization, magnetization, dielectric constant and heat capacity measurements over a broad range of temperature and magnetic field. The dielectric constant shows suppression in the magnetic field but it does not shift to lower temperature with the application of external field. The appearance of magneto-dielectric effect signifies the contribution of q-dependent magnetic correlation function with enhanced weightage in the presence of magnetic field. This magnetic correlation function is associated with the ferro-axial vector and our overall results support this mechanism for the observation of muliferroicity in Cu3Nb2O8.

cond-mat.str-el↗

arXiv2014

Magnetic entropy change and critical exponents in double perovskite Y2NiMnO6

We report on the magnetic entropy change and the critical exponents in the double perovskite manganite Y2NiMnO6 with a ferromagnetic to paramagnetic transition TC=85K. For a magnetic field change of 80kOe, a maximum magnetic entropy change -6.57 J /kg K is recorded around TC. The corresponding relative cooling power (134 J / kg) is appreciable towards potential application as a magnetic refrigerant. The critical exponents, beta and gamma satisfy well to values derived for a 3D Heisenberg ferromagnet.

cond-mat.mtrl-sci↗

arXiv2013

Evidence for Multiferroic Characteristics in NdCrTiO5

We report NdCrTiO5 to be an unusual multiferroic material with large magnetic field dependent electric polarization. While magneto-electric coupling in this two magnetic sub-lattice oxide is well established, the purpose of this study is to look for spontaneous symmetry breaking at the magnetic transition. The conclusions are based on extensive magnetization, dielectric and polarization measurements around its antiferromagnetic ordering temperature of 18K. Room temperature X-ray diffraction pattern of NdCrTiO5 reveals that the sample is single phase with an orthorhombic crystal structure that allows linear magneto-electric coupling. DC magnetization measurement shows magnetization downturn at 11K together with a small kink corresponding to the Co+3 sub-lattice ordering at ~18K. An anomaly in dielectric constant is observed around the magnetic ordering temperature that increases substantially with increasing magnetic field. Through detailed pyroelectric current measurements at zero magnetic field, particularly as a function of thermal cycling, we establish that NdCrTiO5 is a genuine multiferroic material that is possibly driven by collinear magneto-striction.

cond-mat.mtrl-sci↗

arXiv2013

Spontaneous electric polarization and Magneto-dielectric coupling in ceramic multiferroic Co3TeO6

We report observation of magneto-electric and magneto-dielectric couplings in ceramic Co3TeO6. Temperature dependent DC magnetization and dielectric constant measurements together indicate coupling between magnetic order and electronic polarization. Strong anomaly in dielectric constant at ~ 18K in zero magnetic field indicates presence of spontaneous polarization. Observations like weak ferromagnetic order at lower temperature, field and temperature dependences of the ferroelectric transition provide experimental verification of the recent theoretical proposal by P. Toledano et al., Phys. Rev. B 85, 214439 (2012). We provide direct evidence of spin-phonon coupling as possible origin of magnetic order.

cond-mat.mtrl-sci↗

arXiv2013

Magnetism driven ferroelectricity above liquid nitrogen temperature in Y2CoMnO6

We report multiferroic behavior in double perovskite Y2CoMnO6 with ferroelectric transition temperature Tc = 80K. The origin of ferroelectricity is associated with magnetic ordering of Co2+ and Mn4+ moments in a up-up-down-down arrangement. The saturation polarization and magnetization are estimated to be 65 uC/m2 and 6.2 Bohr magneton/f.u. respectively. The magnetoelectric coupling parameter, on the other hand, is small as a 5 Tesla field suppresses the electric polarization by only ~8%. This is corroborated with observed hysteretic behaviour at 5K that remains unsaturated even upto 7 Tesla. A model based on exchange-striction is proposed to explain the observed high temperature ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2012

Bulk Superconductivity in Bismuth-oxy-sulfide Bi4O4S3

Very recent report [1] on observation of superconductivity in Bi4O4S3 could potentially reignite the search for superconductivity in a broad range of layered sulphides. We report here synthesis of Bi4O4S3 at 5000C by vacuum encapsulation technique and basic characterizations. Detailed structural, magnetization, and electrical transport results are reported. Bi4O4S3 is contaminated by small amounts of Bi2S3 and Bi impurities. The majority phase is tetragonal I4/mmm space group with lattice parameters a = 3.9697(2)Å, c = 41.3520(1)Å. Both AC and DC magnetization measurements confirmed that Bi4O4S3 is a bulk superconductor with superconducting transition temperature (Tc) of 4.4K. Isothermal magnetization (MH) measurements indicated closed loops with clear signatures of flux pinning and irreversible behavior. The lower critical field (Hc1) at 2K, of the new superconductor is found to be ~39 Oe. The magneto-transport R(T, H) measurements showed a resistive broadening and decrease in Tc (R=0) to lower temperatures with increasing magnetic field. The extrapolated upper critical field Hc2(0) is ~ 310kOe with a corresponding Ginzburg-Landau coherence length of ~100Å . In the normal state the ρ ~ T2 is not indicated. Our magnetization and electrical transport measurements substantiate the appearance of bulk superconductivity in as synthesized Bi4O4S3. On the other hand same temperature heat treated Bi is not superconducting, thus excluding possibility of impurity driven superconductivity in the newly discovered Bi4O4S3 superconductor.

cond-mat.supr-con↗

arXiv2010

Anisotropic ultrafast electron dynamics induced by high-field terahertz pulses in n-doped InGaAs

The anisotropic effective mass of electrons is directly measured using time-resolved THz- pump/THz-probe techniques in a n-doped InGaAs semiconductor thin film. A microscopic theory is used to attribute this anisotropy in the THz probe transmission to the nonparabolicity of the conduction band. Self-consistent light-matter coupling is shown to contribute significantly to the THz response.

cond-mat.mtrl-sci↗