arXiv subjects

V. Tiwari

Publications and source records attributed to V. Tiwari.

13 recordsLinked to original sources

arXiv2025

Narrowing RIFT: Focused simulation-based-inference for interpreting exceptional GW sources

The Rapid Iterative FiTting (RIFT) parameter inference algorithm provides a simulation-based inference approach to efficient, highly-parallelized parameter inference for GW sources. Previous editions of RIFT have conservatively optimized for robust inference about poorly constrained observations. In this paper, we summarize algorithm enhancements and operating point choices to enable inference for more exceptional compact binaries. Using the previously-reported RIFT/asimov interface to efficiently perform analyses on events with reproducible settings consistent with past work, we demonstrate that the latest version of RIFT can efficiently analyze events with multiple costly models including the effects of precession or eccentricity.

astro-ph.IM↗

arXiv2024

Enhanced optical properties of MoSe$_2$ grown by molecular beam epitaxy on hexagonal boron nitride

Transition metal dichalcogenides (TMD) like MoSe$_2$ exhibit remarkable optical properties such as intense photoluminescence (PL) in the monolayer form. To date, narrow-linewidth PL is only achieved in micrometer-sized exfoliated TMD flakes encapsulated in hexagonal boron nitride (hBN). In this work, we develop a growth strategy to prepare monolayer MoSe$_2$ on hBN flakes by molecular beam epitaxy in the van der Waals regime. It constitutes the first step towards the development of large area single crystalline TMDs encapsulated in hBN for potential integration in electronic or opto-electronic devices. For this purpose, we define a two-step growth strategy to achieve monolayer-thick MoSe$_2$ grains on hBN flakes. The high quality of MoSe$_2$ allows us to detect very narrow PL linewidth down to 5.5 meV at 13 K, comparable to the one of encapsulated exfoliated MoSe$_2$ flakes. Moreover, sizeable PL can be detected at room temperature as well as clear reflectivity signatures of A, B and charged excitons.

cond-mat.mtrl-sci↗

arXiv2022

Spin dynamics of positively charged excitons in Cr$^+$-doped quantum dots probed by resonant photoluminescence

We study the dynamics of the spin system that consist of a positively charged II-VI semiconductor quantum dot doped with a single Cr$^+$ ion. The resonant photoluminescence (PL) of the positively charged exciton coupled with the Cr$^+$ spin is used to analyze the main spin relaxation channels. The intensity of the resonant PL is reduced by an optical pumping of the spin of the resident hole-Cr$^+$ complex that can be seen as a nano-magnet. The spin memory can be partially erased by a non-resonant optical excitation. This leads to an increase of the resonant PL signal. The resonant PL is co-circularly polarized and corresponds to relaxation channels that conserve the Cr$^+$ spin $\vert S_z \vert$. The observation in the resonant-PL excitation spectra of optical transitions with a change of the Cr$^+$ spin permits to determine the magnetic anisotropy of the magnetic atom. Optical pumping, auto-correlation measurements and the power dependence of the PL intensity distribution show that the effective temperature of the hole-Cr$^+$ spin system is affected by the optical excitation through the local generation of phonons.

cond-mat.mes-hall↗

arXiv2021

A hole-Cr$^{+}$ nano-magnet in a semiconductor quantum dot

We study a new diluted magnetic semiconductor system based on the spin of the ionized acceptor Cr$^+$. We show that the negatively charged Cr$^+$ ion, an excited state of the Cr in II-VI semiconductor, can be stable when inserted in a CdTe quantum dot (QD). The Cr$^+$ attracts a heavy-hole in the QD and form a stable hole-Cr$^+$ complex. Optical probing of this system reveals a ferromagnetic coupling between heavy-holes and Cr$^+$ spins. At low temperature, the thermalization on the ground state of the hole-Cr$^+$ system with parallel spins prevents the optical recombination of the excess electron on the 3$d$ shell of the atom. We study the dynamics of the nano-magnet formed by the hole-Cr$^+$ exchange interaction. The ferromagnetic ground states with M$_z$=$\pm$4 can be controlled by resonant optical pumping and a spin relaxation time in the 20 $μ$s range is obtained at T=4.2 K. This spin memory at zero magnetic field is limited by the interaction with phonons.

cond-mat.mes-hall↗

arXiv2021

Coherent WaveBurst, a pipeline for unmodeled gravitational-wave data analysis

coherent WaveBurst (cWB) is a highly configurable pipeline designed to detect a broad range of gravitational-wave (GW) transients in the data of the worldwide network of GW detectors. The algorithmic core of cWB is a time-frequency analysis with the Wilson-Daubechies-Meyer wavelets aimed at the identification of GW events without prior knowledge of the signal waveform. cWB has been in active development since 2003 and it has been used to analyze all scientific data collected by the LIGO-Virgo detectors ever since. On September 14, 2015, the cWB low-latency search detected the first gravitational-wave event, GW150914, a merger of two black holes. In 2019, a public open-source version of cWB has been released with GPLv3 license.

gr-qc↗

arXiv2020

Radio-frequency stress-induced modulation of CdTe/ZnTe quantum dots

We demonstrate radio-frequency tuning of the energy of individual CdTe/ZnTe quantum dots (QDs) by Surface Acoustic Waves (SAWs). Despite the very weak piezoelectric coefficient of ZnTe, SAW in the GHz range can be launched on a ZnTe surface using interdigitated transducers deposited on a c-axis oriented ZnO layer grown on ZnTe containing CdTe QDs. The photoluminescence (PL) of individual QDs is used as a nanometer-scale sensor of the acoustic strain field. The energy of QDs is modulated by SAW in the GHz range and leads to characteristic broadening of time-integrated PL spectra. The dynamic modulation of the QD PL energy can also be detected in the time domain using phase-locked time domain spectroscopy. This technique is in particular used for monitoring complex local acoustic fields resulting from the superposition of two or more SAW pulses in a cavity. Under magnetic field, the dynamic spectral tuning of a single QD by SAW can be used to generate single photons with alternating circular polarization controlled in the GHz range.

cond-mat.mes-hall↗

arXiv2019

Optical control of an individual Cr spin in a semiconductor quantum dot

We demonstrate that the spin of a Cr atom in a quantum dot (QD) can be controlled optically and we discuss the main properties of this single spin system. The photoluminescence of individual Cr-doped QDs and their evolution in magnetic field reveal a large magnetic anisotropy of the Cr spin induced by local strain. This results in a splitting of the Cr spin states and in a thermalization on the lower energy states states S$_z$=0 and S$_z$=$\pm$1. The magneto-optical properties of Cr-doped QDs can be modelled by an effective spin Hamiltonian including the spin to strain coupling and the influence of the QD symmetry. We also show that a single Cr spin can be prepared by resonant optical pumping. Monitoring the intensity of the resonant fluorescence of the QD during this process permits to probe the dynamics of the optical initialization of the spin. Hole-Cr flip-flops induced by an interplay of the hole-Cr exchange interaction and the coupling with acoustic phonons are the main source of relaxation that explains the efficient resonant optical pumping. The Cr spin relaxation time is measured in the $μs$ range. We evidence that a Cr spin couples to non-equilibrium acoustic phonons generated during the optical excitation inside or near the QD). Finally we show that the energy of any spin state of an individual Cr atom can be independently tuned by a resonant single mode laser through the optical Stark effect. All these properties make Cr-doped QDs very promising for the development of hybrid spin-mechanical systems where a coherent mechanical driving of an individual spin in an oscillator is required.

cond-mat.mes-hall↗

arXiv2019

Influence of non-equilibrium phonons on the spin dynamics of a single Cr atom

We analyse the influence of optically generated non-equilibrium phonons on the spin relaxation and effective spin temperature of an individual Cr atom inserted in a quantum dot. Using a three pulses pump-probe technique, we show that the spin relaxation measured in resonant optical pumping experiments strongly depends on the optical excitation conditions. We observe for an isolated Cr in the dark a heating time shorter than a few hundreds $ns$ after an initial high power non-resonant excitation pulse. A cooling time larger than a few tens of $μs$, independent on the excitation, is obtained in the same experimental conditions. We show that a tunable spin-lattice coupling dependent on the density of non-equilibrium phonons can explain the observed dynamics. Low energy excitation conditions are found where the Cr spin states S$_z$=$\pm$1 can be efficiently populated by a non-resonant optical excitation, prepared and read-out by resonant optical pumping and conserved in the dark during a few $μ$s.

cond-mat.mes-hall↗

arXiv2019

Charge fluctuations of a Cr atom probed in the optical spectra of a quantum dot

We study the emission of individual quantum dots in CdTe/ZnTe samples doped with a low concentration of Cr. In addition to dots with a photoluminescence (PL) split by the exchange interaction with a magnetic Cr atom, we observe another type of dots with a complex PL structure composed of a minimum of six lines on the exciton and biexciton and three lines on the charged excitons. In these dots, the linear polarization dependence and the magnetic field dependence of the PL behave like three similar quantum dots emitting at slightly different energies. Cross-correlation intensity measurements show that these emission lines are not independent but exchange intensities in a time scale of a few hundred nanoseconds depending on the optical excitation power. We attribute this PL structure to charge fluctuations of a Cr atom located in the vicinity the CdTe dots in the ZnTe barrier. We present a model which confirms that the presence of a single charge fluctuating between -e (Cr$^{+}$), 0 (Cr$^{2+}$) and +e (Cr$^{3+}$) and located a few nm away from the dot explains the observation of three emission energies. We finally show that the interaction between the confined carriers and the nearby fluctuating localized charge can be modified by an applied static electric field which modulates the splitting of the emission lines.

cond-mat.mes-hall↗

arXiv2017

Enhancing the significance of gravitational wave bursts through signal classification

The quest to observe gravitational waves challenges our ability to discriminate signals from detector noise. This issue is especially relevant for transient gravitational waves searches with a robust eyes wide open approach, the so called all- sky burst searches. Here we show how signal classification methods inspired by broad astrophysical characteristics can be implemented in all-sky burst searches preserving their generality. In our case study, we apply a multivariate analyses based on artificial neural networks to classify waves emitted in compact binary coalescences. We enhance by orders of magnitude the significance of signals belonging to this broad astrophysical class against the noise background. Alternatively, at a given level of mis-classification of noise events, we can detect about 1/4 more of the total signal population. We also show that a more general strategy of signal classification can actually be performed, by testing the ability of artificial neural networks in discriminating different signal classes. The possible impact on future observations by the LIGO-Virgo network of detectors is discussed by analysing recoloured noise from previous LIGO-Virgo data with coherent WaveBurst, one of the flagship pipelines dedicated to all-sky searches for transient gravitational waves.

astro-ph.IM↗

arXiv2016

Method for detection and reconstruction of gravitational wave transients with networks of advanced detectors

We present a method for detection and reconstruction of the gravitational-wave (GW) transients with the networks of advanced detectors. Originally designed to search for transients with the initial GW detectors, it uses significantly improved algorithms, which enhances both the low-latency searches with rapid localization of GW events for the electro-magnetic followup and high confidence detection of a broad range of the transient GW sources. In the paper we present the analytic framework of the method. Following a short description of the core analysis algorithms, we introduce a novel approach to the reconstruction of the GW polarization from a pattern of detector responses to a GW signal. This polarization pattern is a unique signature of an arbitrary GW signal that can be measured independent from the other source parameters. The polarization measurements enable rapid reconstruction of the GW waveforms, sky localization and helps identification of the source origin.

gr-qc↗

arXiv2014

Prospects for intermediate mass black hole binary searches with advanced gravitational-wave detectors

We estimated the sensitivity of the upcoming advanced, ground-based gravitational-wave observatories (the upgraded LIGO and Virgo and the KAGRA interferometers) to coalescing intermediate mass black hole binaries (IMBHB). We added waveforms modeling the gravitational radiation emitted by IMBHBs to detectors' simulated data and searched for the injected signals with the coherent WaveBurst algorithm. The tested binary's parameter space covers non-spinning IMBHBs with source-frame total masses between 50 and 1050 $\text{M}_{\odot}$ and mass ratios between $1/6$ and 1$\,$. We found that advanced detectors could be sensitive to these systems up to a range of a few Gpc. A theoretical model was adopted to estimate the expected observation rates, yielding up to a few tens of events per year. Thus, our results indicate that advanced detectors will have a reasonable chance to collect the first direct evidence for intermediate mass black holes and open a new, intriguing channel for probing the Universe over cosmological scales.

gr-qc↗

arXiv2005

Determination of the Jet Energy Scale at the Collider Detector at Fermilab

A precise determination of the energy scale of jets at the Collider Detector at Fermilab at the Tevatron $p\bar{p}$ collider is described. Jets are used in many analyses to estimate the energies of partons resulting from the underlying physics process. Several correction factors are developed to estimate the original parton energy from the observed jet energy in the calorimeter. The jet energy response is compared between data and Monte Carlo simulation for various physics processes, and systematic uncertainties on the jet energy scale are determined. For jets with transverse momenta above 50 GeV the jet energy scale is determined with a 3% systematic uncertainty.

hep-ex↗