arXiv subjects

S. Singh

Publications and source records attributed to S. Singh.

At least 73 records · Page 4Linked to original sources

arXiv2020

Periodic activity from a fast radio burst source

Fast radio bursts (FRBs) are bright, millisecond-duration radio transients originating from extragalactic distances. Their origin is unknown. Some FRB sources emit repeat bursts, ruling out cataclysmic origins for those events. Despite searches for periodicity in repeat burst arrival times on time scales from milliseconds to many days, these bursts have hitherto been observed to appear sporadically, and though clustered, without a regular pattern. Here we report the detection of a $16.35\pm0.15$ day periodicity (or possibly a higher-frequency alias of that periodicity) from a repeating FRB 180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September 16th, 2018 through February 4th, 2020, we find that all bursts arrive in a 5-day phase window, and 50% of the bursts arrive in a 0.6-day phase window. Our results suggest a mechanism for periodic modulation either of the burst emission itself, or through external amplification or absorption, and disfavour models invoking purely sporadic processes.

astro-ph.HE↗

arXiv2020

A bright millisecond-duration radio burst from a Galactic magnetar

Magnetars are highly magnetized young neutron stars that occasionally produce enormous bursts and flares of X-rays and gamma-rays. Of the approximately thirty magnetars currently known in our Galaxy and Magellanic Clouds, five have exhibited transient radio pulsations. Fast radio bursts (FRBs) are millisecond-duration bursts of radio waves arriving from cosmological distances. Some have been seen to repeat. A leading model for repeating FRBs is that they are extragalactic magnetars, powered by their intense magnetic fields. However, a challenge to this model has been that FRBs must have radio luminosities many orders of magnitude larger than those seen from known Galactic magnetars. Here we report the detection of an extremely intense radio burst from the Galactic magnetar SGR 1935+2154 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project. The fluence of this two-component bright radio burst and the estimated distance to SGR 1935+2154 together imply a 400-800 MHz burst energy of $\sim 3 \times 10^{34}$ erg, which is three orders of magnitude brighter than those of any radio-emitting magnetar detected thus far. Such a burst coming from a nearby galaxy would be indistinguishable from a typical FRB. This event thus bridges a large fraction of the radio energy gap between the population of Galactic magnetars and FRBs, strongly supporting the notion that magnetars are the origin of at least some FRBs.

astro-ph.HE↗

arXiv2019

Implicit-explicit-compact methods for advection diffusion reaction equations

We provide a preliminary comparison of the dispersion properties, specifically the time-amplification factor, the scaled group velocity and the error in the phase speed of four spatiotemporal discretization schemes utilized for solving the one-dimensional (1D) linear advection diffusion reaction (ADR) equation: (a) An explicit (RK2) temporal integration combined with the Optimal Upwind Compact Scheme (or OUCS3) and the central difference scheme (CD2) for second order spatial discretization, (b) a fully implicit mid-point rule for time integration coupled with the OUCS3 and the Lele's compact scheme for first and second order spatial discretization, respectively, (c) An implicit (mid-point rule)-explicit (RK2) or IMEX time integration blended with OUCS3 and Lele's compact scheme (where the IMEX time integration follows the same ideology as introduced by Ascher et al.), and (d) the IMEX (mid-point/RK2) time integration melded with the New Combined Compact Difference scheme (or NCCD scheme). Analysis reveal the superior resolution features of the IMEX-NCCD scheme including an enhanced region of neutral stability (a region where the amplification factor is close to one), a diminished region of spurious propagation characteristics (or a region of negative group velocity) and a smaller region of nonzero phase speed error. The dispersion error of these numerical schemes through the role of q-waves is further investigated using the novel error propagation equation for the 1D linear ADR equation. Again, the in silico experiments divulge excellent Dispersion Relation Preservation (DRP) properties of the IMEX-NCCD scheme including minimal dissipation via implicit filtering and negligible unphysical oscillations (or Gibbs' phenomena) on coarser grids.

math.NA↗

arXiv2019

CHIME/FRB Detection of Eight New Repeating Fast Radio Burst Sources

We report on the discovery of eight repeating fast radio burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 103.5 to 1281 pc cm$^{-3}$. They display varying degrees of activity: six sources were detected twice, another three times, and one ten times. These eight repeating FRBs likely represent the bright and/or high-rate end of a distribution of infrequently repeating sources. For all sources, we determine sky coordinates with uncertainties of $\sim$10$^\prime$. FRB 180916.J0158+65 has a burst-averaged DM = $349.2 \pm 0.3$ pc cm$^{-3}$ and a low DM excess over the modelled Galactic maximum (as low as $\sim$20 pc cm$^{-3}$); this source also has a Faraday rotation measure (RM) of $-114.6 \pm 0.6$ rad m$^{-2}$, much lower than the RM measured for FRB 121102. FRB 181030.J1054+73 has the lowest DM for a repeater, $103.5 \pm 0.3$ pc cm$^{-3}$, with a DM excess of $\sim$ 70 pc cm$^{-3}$. Both sources are interesting targets for multi-wavelength follow-up due to their apparent proximity. The DM distribution of our repeater sample is statistically indistinguishable from that of the first 12 CHIME/FRB sources that have not repeated. We find, with 4$σ$ significance, that repeater bursts are generally wider than those of CHIME/FRB bursts that have not repeated, suggesting different emission mechanisms. Our repeater events show complex morphologies that are reminiscent of the first two discovered repeating FRBs. The repetitive behavior of these sources will enable interferometric localizations and subsequent host galaxy identifications.

astro-ph.HE↗

arXiv2019

Observation of dual electromagnetically induced transparencies in cold $^{87}Rb$ atoms

We have observed two dual electromagnetically induced transparency (EIT) spectra in probe transmission through cold $^{87}Rb$ atoms trapped in a magneto-optical trap (MOT). These dual EIT peaks are generated in two different N-systems formed by probe, coupling and control beams interacting with different levels in $D_{2}$ line transitions of $^{87}Rb$ atom. The dependence of spectral features of two N-systems on driving beams (\textit{i.e.} coupling and control beams) strength and detuning has been investigated. The results show that the dual EIT spectral features in both the N-systems can be manipulated by varying the strength and detuning of both the driving beams. The two N-systems have been modelled theoretically using a density matrix formalism and results have shown a reasonable agreement with the experimental observations.

physics.atom-ph↗

arXiv2018

Direct observation of multi-spinon excitations outside of the two-spinon continuum in the antiferromagnetic spin chain cuprate Sr2CuO3

One-dimensional (1D) magnetic insulators have attracted significant interest as a platform for studying emergent phenomena such as quasiparticle fractionalization and quantum criticality. The antiferromagnetic Heisenberg chain of spins-1/2 is an important reference system; its elementary excitations are spin-1/2 quasiparticles called spinons that are always created in pairs. However, while inelastic neutron scattering (INS) experiments routinely observe the excitation continuum associated with two-spinon states, the presence of more complex dynamics associated with four-spinon states has only been inferred from comparison with theory. Here, we show that resonant inelastic x-ray scattering (RIXS) is capable of accessing the four-spinon excitations directly, in a spectroscopic region separated from the two-spinon continuum. Our results provide the first direct measurement of four-spinon excitations, which is made possible by the fundamentally different correlation functions probed by RIXS compared to INS. This advance holds great promise as a tool in the search for novel quantum states, in particular quantum spin liquids.

cond-mat.str-el↗

arXiv2017

A low-temperature scanning tunneling microscope capable of microscopy and spectroscopy in a Bitter magnet at up to 34 T

We present the design and performance of a cryogenic scanning tunneling microscope (STM) which operates inside a water-cooled Bitter magnet, which can attain a magnetic field of up to 38 T. Due to the high vibration environment generated by the magnet cooling water, a uniquely designed STM and vibration damping system are required. The STM scan head is designed to be as compact and rigid as possible, to minimize the effect of vibrational noise as well as fit the size constraints of the Bitter magnet. The STM uses a differential screw mechanism for coarse tip - sample approach, and operates in helium exchange gas at cryogenic temperatures. The reliability and performance of the STM are demonstrated through topographic imaging and scanning tunneling spectroscopy (STS) on highly oriented pyrolytic graphite (HOPG) at T = 4.2 K and in magnetic fields up to 34 T.

physics.ins-det↗

arXiv2017

Distribution of current fluctuations in a bistable conductor

We measure the full distribution of current fluctuations in a single-electron transistor with a controllable bistability. The conductance switches randomly between two levels due to the tunneling of single electrons in a separate single-electron box. The electrical fluctuations are detected over a wide range of time scales and excellent agreement with theoretical predictions is found. For long integration times, the distribution of the time-averaged current obeys the large-deviation principle. We formulate and verify a fluctuation relation for the bistable region of the current distribution.

cond-mat.mes-hall↗

arXiv2016

Detecting continuous gravitational waves with superfluid $^4$He

Direct detection of gravitational waves is opening a new window onto our universe. Here, we study the sensitivity to continuous-wave strain fields of a kg-scale optomechanical system formed by the acoustic motion of superfluid helium-4 parametrically coupled to a superconducting microwave cavity. This narrowband detection scheme can operate at very high $Q$-factors, while the resonant frequency is tunable through pressurization of the helium in the 0.1-1.5 kHz range. The detector can therefore be tuned to a variety of astrophysical sources and can remain sensitive to a particular source over a long period of time. For reasonable experimental parameters, we find that strain fields on the order of $h\sim 10^{-23} /\sqrt{\rm Hz}$ are detectable. We show that the proposed system can significantly improve the limits on gravitational wave strain from nearby pulsars within a few months of integration time.

gr-qc↗

arXiv2016

Room temperature tetragonal noncollinear antiferromagnet Pt$_2$MnGa

Here we present the tetragonal stoichiometric Heusler compound Pt$_2$MnGa with the noncollinear AFM order stable up to 350 K. It is resolved by the neutron diffraction as a helical spiral propagating along the tetragonal axis. Ab-initio calculations suggest a pure exchange origin of the spiral and explain its helical character being stabilized by a large basal plane magnetocrystalline anisotropy (MCA). Together with the inversion-symmetric crystal structure, this provides a bi-stability of a spiral with respect to the right- and left-handed magnetic helices. Despite the large MCA, the long period of a helix might greatly facilitate the switch of the helicity by the precessional reorientation, suggesting Pt$_2$MnGa as a potential candidate for the vector-helicity based non-volatile magnetic memory.

cond-mat.mtrl-sci↗

arXiv2015

On-chip error counting for hybrid metallic single-electron turnstiles

We perform in-situ detection of individual electrons pumped through a single-electron turnstile based on ultrasmall normal metal - insulator - superconductor tunnel junctions. In our setup, limited by the detector bandwidth, at low repetition rates we observe errorless sequential transfer of up to several hundred electrons through the system. At faster pumping speeds up to 100 kHz, we show relative error rates down to 10^-3, comparable to typical values obtained from measurements of average pumped current in non-optimized individual turnstiles. The work constitutes an initial step towards a self-referenced current standard realized with metallic single-electron turnstiles, complementing approaches based on semiconductor quantum dot pumps. It is the first demonstration of on-chip pumping error detection at operation frequencies exceeding the detector bandwidth, in a configuration where the average pumped current can be simultaneously measured. The scheme in which electrons are counted from the superconducting lead of the turnstile, instead of direct probing of the normal metal island, also enables studies of fundamental higher-order tunneling processes in the hybrid structures, previously not in reach with simpler configurations.

cond-mat.mes-hall↗

arXiv2015

Atom-based coherent quantum-noise cancellation in optomechanics

We analyze a quantum force sensor that uses coherent quantum noise cancellation (CQNC) to beat the Standard Quantum Limit (SQL). This sensor, which allows for the continuous, broad-band detection of feeble forces, is a hybrid dual-cavity system comprised of a mesoscopic mechanical resonator optically coupled to an ensemble of ultracold atoms. In contrast to the stringent constraints on dissipation typically associated with purely optical schemes of CQNC, the dissipation rate of the mechanical resonator only needs to be matched to the decoherence rate of the atomic ensemble -- a condition that is experimentally achievable even for the technologically relevant regime of low frequency mechanical resonators with large quality factors. The modular nature of the system further allows the atomic ensemble to aid in the cooling of the mechanical resonator, thereby combining atom-mediated state preparation with sensing deep in the quantum regime.

quant-ph↗

arXiv2015

Electromagnetically induced absorption in metastable 83Kr atoms

We report electromagnetically induced absorption (EIA) resonances of sub-natural linewidth (FWHM) in metastable noble gas 83Kr* atoms using degenerate two level schemes (DTLSs). This is the first observation of EIA effect in a metastable noble gas atoms. Using these spectrally narrow EIA signals obtained corresponding to the closed hyperfine transition from 4p55s[3/2]2 to 4p55p[5/2]3 hyperfine manifolds of 83Kr* atoms, we have measured the Lande's g-factor (gF) for the lower level (F = 13/2) of the closed transition accurately with small applied magnetic fields of few Gauss.

physics.atom-ph↗

arXiv2015

Resolution of hyperfine transitions in metastable 83Kr using Electromagnetically Induced Transparency

Narrow linewidth signals of Electromagnetically Induced Transparency (EIT) in the metastable 83Kr have been observed for the first time. Various hyperfine transitions in 4p55s[3/2]2 to 4p55p[5/2]3 manifolds of 83Kr have been identified through the experimentally observed EIT signals. Some unresolved or poorly resolved hyperfine transitions in saturated absorption spectroscopy (SAS) are clearly resolved in the present work. Using the spectral separation of these EIT identified hyperfine transitions, the magnetic hyperfine constant (A) and the electric quadrupole hyperfine constant (B) are determined with improved accuracy for 4p55s[3/2]2 and 4p55p[5/2]3 manifolds.

physics.atom-ph↗

arXiv2015

Dynamical spin injection at a quasi-one-dimensional ferromagnet-graphene interface

We present a study of dynamical spin injection from a three-dimensional ferromagnet into two-dimensional single-layer graphene. Comparative ferromagnetic resonance (FMR) studies of ferromagnet/graphene strips buried underneath the central line of a coplanar waveguide show that the FMR linewidth broadening is the largest when the graphene layer protrudes laterally away from the ferromagnetic strip, indicating that the spin current is injected into the graphene areas away from the area directly underneath the ferromagnet being excited. Our results confirm that the observed damping is indeed a signature of dynamical spin injection, wherein a pure spin current is pumped into the single-layer graphene from the precessing magnetization of the ferromagnet. The observed spin pumping efficiency is difficult to reconcile with the expected backflow of spins according to the standard spin pumping theory and the characteristics of graphene, and constitutes an enigma for spin pumping in two-dimensional structures.

cond-mat.mes-hall↗

arXiv2014

Velocity selective bi-polarization spectroscopy for laser cooling of metastable Krypton atoms

We report a velocity selective bi-polarization spectroscopy (VS-BPS) technique to generate a background-free, dispersion-like reference signal which is tunable over a wide range of frequency. In this technique, a pair of linearly polarized weak probe beams passing through a gas cell of metastable Krypton (Kr*) atoms, overlaps with a pair of counter-propagating circularly polarized strong pump beams derived from an independently tunable control laser. The polarization spectroscopy signals from the two probe beams, after subtraction, result in VS-BPS signal. The spectral shifting in VS-BPS signal can be achieved by tuning the frequency of the control laser. The dependence of the amplitude and slope of the VS-BPS signal on the RF power used for excitation of Kr atoms in the gas cell and on the power of pump beams has been studied. The frequency stability of a diode laser locked with VS-BPS signal has been found to be better than the frequency stability of the laser locked with a saturated absorption spectroscopy (SAS) signal. The VS-BPS signal is finally used for stabilization and tuning of the cooling laser frequency for a magneto-optical trap (MOT) for Kr* atoms.

physics.atom-ph↗

arXiv2014

Hybrid optomechanical cooling by atomic $Λ$ systems

We investigate a hybrid quantum system consisting of a cavity optomechanical device optically coupled to an ultracold quantum gas. We show that the dispersive properties of the ultracold gas can be used to dramatically modify the optomechanical response of the mechanical resonator. We examine hybrid schemes wherein the mechanical resonator is coupled either to the motional or the spin degrees of freedom of the ultracold gas. In either case, we find an enhancement of more than two orders of magnitude in optomechanical cooling due to this hybrid interaction. Significantly, based on demonstrated parameters for the cavity optomechanical device, we identify regimes that enable the ground state cooling of the resonator from room temperature. In addition, the hybrid system considered here represents a powerful interface for the use of an ultracold quantum gas for state preparation, sensing and quantum manipulation of a mesoscopic mechanical resonator.

quant-ph↗

arXiv2014

Induced magnetization in La$_{0.7}$Sr$_{0.3}$MnO$_3$/BiFeO$_3$ superlattices

Using polarized neutron reflectometry (PNR), we observe an induced magnetization of 75$\pm$ 25 kA/m at 10 K in a La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO)/BiFeO$_3$ superlattice extending from the interface through several atomic layers of the BiFeO$_3$ (BFO). The induced magnetization in BFO is explained by density functional theory, where the size of bandgap of BFO plays an important role. Considering a classical exchange field between the LSMO and BFO layers, we further show that magnetization is expected to extend throughout the BFO, which provides a theoretical explanation for the results of the neutron scattering experiment.

cond-mat.str-el↗