arXiv subjects

S. Sharma

Publications and source records attributed to S. Sharma.

At least 73 records · Page 4Linked to original sources

arXiv2023

First Evidence of Axial Shape Asymmetry and Configuration Coexistence in $^{74}$Zn: Suggestion for a Northern Extension of the $N=40$ Island of Inversion

The excited states of $N=44$ $^{74}$Zn were investigated via $γ$-ray spectroscopy following $^{74}$Cu $β$ decay. By exploiting $γ$-$γ$ angular correlation analysis, the $2_2^+$, $3_1^+$, $0_2^+$ and $2_3^+$ states in $^{74}$Zn were firmly established. The $γ$-ray branching and $E2/M1$ mixing ratios for transitions de-exciting the $2_2^+$, $3_1^+$ and $2_3^+$ states were measured, allowing for the extraction of relative $B(E2)$ values. In particular, the $2_3^+ \to 0_2^+$ and $2_3^+ \to 4_1^+$ transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the $N=40$ gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize $^{74}$Zn in its ground state. Furthermore, an excited $K=0$ band with a significantly larger softness in its shape is identified. A shore of the $N=40$ ``island of inversion'' appears to manifest above $Z=26$, previously thought as its northern limit in the chart of the nuclides.

nucl-ex↗

arXiv2023

Production of hydrogen isotopes and charged pions in p (3.5 GeV) + Nb reactions

The double differential production cross sections, $d^2σ/dΩdE$, for hydrogen isotopes and charged pions in the reaction of p + Nb at 3.5 GeV proton beam energy have been measured by the High Acceptance DiElectron Spectrometer (HADES). Thanks to the high acceptance of HADES at forward emission angles and usage of its magnetic field, the measured energy range of hydrogen isotopes could be significantly extended in comparison to the relatively scarce experimental data available in the literature. The data provide information about the development of the intranuclear cascade in the proton-nucleus collisions. They can as well be utilized to study the rate of energy/momentum dissipation in the nuclear systems and the mechanism of elementary and composite particle production in excited nuclear matter at normal density. Data of this type are important also for technological and medical applications. Our results are compared to models developed to describe the processes relevant to nuclear spallation (INCL++) or oriented to probe either the elementary hadronic processes in nuclear matter or the behavior of compressed nuclear matter (GiBUU).

nucl-ex↗

arXiv2023

Quantum sensing of electric field distributions of liquid electrolytes with NV-centers in nanodiamonds

To use batteries as large-scale energy storage systems it is necessary to measure and understand their degradation \textit{in-situ} and \textit{in-operando}. As a battery's degradation is often the result of molecular processes inside the electrolyte, a sensing platform which allows to measure the ions with a high spatial resolution is needed. Primary candidates for such a platform are NV-centers in diamonds. We propose to use a single NV-center to deduce the electric field distribution generated by the ions inside the electrolyte through microwave pulse sequences. We show that the electric field can be reconstructed with great accuracy by using a protocol which includes different variations of the Free Induction Decay to obtain the mean electric field components and a modified Hahn-echo pulse sequence to measure the electric field's standard deviation $σ_E$. From a semi-analytical ansatz we find that for a lithium ion battery there is a direct relationship between $σ_E$ and the ionic concentration. Our results show that it is therefore possible to use NV-centers as sensors to measure both the electric field distribution and the local ionic concentration inside electrolytes.

quant-ph↗

arXiv2023

J-PET detection modules based on plastic scintillators for performing studies with positron and positronium beams

The J-PET detector, which consists of inexpensive plastic scintillators, has demonstrated its potential in the study of fundamental physics. In recent years, a prototype with 192 plastic scintillators arranged in 3 layers has been optimized for the study of positronium decays. This allows performing precision tests of discrete symmetries (C, P, T) in the decays of positronium atoms. Moreover, thanks to the possibility of measuring the polarization direction of the photon based on Compton scattering, the predicted entanglement between the linear polarization of annihilation photons in positronium decays can also be studied. Recently, a new J-PET prototype was commissioned, based on a modular design of detection units. Each module consists of 13 plastic scintillators and can be used as a stand-alone, compact and portable detection unit. In this paper, the main features of the J-PET detector, the modular prototype and their applications for possible studies with positron and positronium beams are discussed. Preliminary results of the first test experiment performed on two detection units in the continuous positron beam recently developed at the Antimatter Laboratory (AML) of Trento are also reported.

physics.ins-det↗

arXiv2022

Skyrmion Alignment and Pinning Effects in a Disordered Multi-Phase Skyrmion Material Co8Zn8Mn4

Underlying disorder in skyrmion materials may both inhibit and facilitate skyrmion reorientations and changes in topology. The identification of these disorder-induced topologically active regimes is critical to realizing robust skyrmion spintronic implementations, yet few studies exist for disordered bulk samples. Here, we employ small-angle neutron scattering (SANS) and micromagnetic simulations to examine the influence of skyrmion order on skyrmion lattice formation, transition, and reorientation dynamics across the phase space of a disordered polycrystalline Co$_{8}$Zn$_{8}$Mn$_{4}$ bulk sample. Our measurements reveal a new disordered-to-ordered skyrmion square lattice transition pathway characterized by the novel promotion of four-fold order in SANS and accompanied by a change in topology of the system, reinforced through micromagnetic simulations. Pinning responses are observed to dominate skyrmion dynamics in the metastable triangular lattice phase, enhancing skyrmion stabilization through a remarkable and previously undetected skyrmion memory effect which reproduces previous ordering processes and persists in zero field. These results uncover the cooperative interplay of anisotropy and disorder in skyrmion formation and restructuring dynamics, establishing new tunable pathways for skyrmion manipulation.

cond-mat.mtrl-sci↗

arXiv2022

Electronic origin of x-ray absorption peak shifts

Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetisation. Employing state of the art first principles dynamical simulations we show that the experimentally observed energy shift of the L3 XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upwards in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish a electronic origin for early time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.

cond-mat.mtrl-sci↗

arXiv2022

Terahertz photodetection in scalable single-layer-graphene and hexagonal boron nitride heterostructures

The unique optoelectronic properties of single layer graphene (SLG) are ideal for the development of photonic devices across a broad range of frequencies, from X-rays to microwaves. In the terahertz (THz) range (0.1-10 THz frequency) this has led to the development of optical modulators, non-linear sources, and photodetectors, with state-of-the-art performances. A key challenge is the integration of SLG-based active elements with pre-existing technological platforms in a scalable way, while maintaining performance level unperturbed. Here, we report on the development of room temperature THz detection in large-area SLG, grown by chemical vapor deposition (CVD), integrated in antenna-coupled field effect transistors. We selectively activate the photo-thermoelectric detection dynamics, and we employ different dielectric configurations on SLG on Al2O3 with and without large-area CVD hBN capping to investigate their effect on SLG thermoelectric properties underpinning photodetection. With these scalable architectures, response times ~5ns and noise equivalent powers ~1nWHz-1/2 are achieved under zero-bias operation. This shows the feasibility of scalable, large-area, layered materials heterostructures for THz detection.

cond-mat.mes-hall↗

arXiv2022

Superconducting properties of pseudobinary telluride Chevrel Phase Mo$_4$Re$_2$Te$_8$

Unconventional superconductivity in the Chevrel phase offers a wide structural aspect to understand the superconducting ground state. A detailed investigation on the superconducting properties of Re based pseudobinary telluride Chevrel phase Mo$_4$Re$_2$Te$_8$ is reported. It crystallizes in a trigonal structure with the space group $R\bar{3}H$ having superconducting transition temperature at $T_C$ = 3.26(3) K. Specific heat measurements suggests a fully gapped superconducting state; however, the proximity of upper critical field value from the Pauli limiting field can be attributed to unconventional nature.

cond-mat.supr-con↗

arXiv2022

Transient spin injection efficiencies at ferromagnet/metal interfaces

Spin injection across interfaces driven by ultrashort optical pulses on femtosecond timescales constitutes a new way to design spintronics applications. Targeted utilization of this phenomenon requires knowledge of the efficiency of non-equilibrium spin injection. From a quantitative comparison of ab-initio time-dependent density functional theory and interface-sensitive, time-resolved non-linear optical experiment, we determine the spin injection efficiencies (SIE) across ferromagnetic/metal interfaces and discuss their microscopic origin, i.e. the influence of spin-orbit coupling and the interface electronic structure. Moreover, we find that the SIE can be optimized through laser pulse and materials parameters, namely the fluence, pulse duration, and substrate material.

cond-mat.mtrl-sci↗

arXiv2022

Mapping the energy-time landscape of spins with helical X-rays

Unveiling the key mechanisms that determine optically driven spin dynamics is essential both to probe the fundamental nature of ultrafast light-matter interactions, but also to drive future technologies of smaller, faster, and more energy efficient devices. Essential to this task is the ability to use experimental spectroscopic tools to evidence the underlying energy- and spin-resolved dynamics of non-equilibrium electron occupations. In this joint theory and experimental work, we demonstrate that ultrafast helicity-dependent soft X-ray absorption spectroscopy (HXAS) allows access to spin-, time- and energy specific state occupation after optical excitation. We apply this method to the prototype transition metal ferromagnet cobalt and find convincing agreement between theory and experiment. The richly structured energy-resolved spin dynamics unveil the subtle interplay and characteristic time scales of optical excitation and spin-orbit induced spin-flip transitions in this material: the spin moment integrated in an energy window below the Fermi level first exhibits an ultrafast increase as minority carriers are excited by the laser pulse, before it is reduced as spin-flip process in highly localized, low energy states start to dominate. The results of this study demonstrate the power of element specific transient HXAS, placing it as a potential new tool for identifying and determining the role of fundamental processes in optically driven spin dynamics in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2022

Large Interferometer For Exoplanets (LIFE): I. Improved exoplanet detection yield estimates for a large mid-infrared space-interferometer mission

One of the long-term goals of exoplanet science is the atmospheric characterization of dozens of small exoplanets in order to understand their diversity and search for habitable worlds and potential biosignatures. Achieving this goal requires a space mission of sufficient scale. We seek to quantify the exoplanet detection performance of a space-based mid-infrared nulling interferometer that measures the thermal emission of exoplanets. For this, we have developed an instrument simulator that considers all major astrophysical noise sources and coupled it with Monte Carlo simulations of a synthetic exoplanet population around main-sequence stars within 20 pc. This allows us to quantify the number (and types) of exoplanets that our mission concept could detect over a certain time period. Two different scenarios to distribute the observing time among the stellar targets are discussed and different apertures sizes and wavelength ranges are considered. Within a 2.5-year initial search phase, an interferometer consisting of four 2 m apertures with a total instrument throughput of 5% covering a wavelength range between 4 and 18.5 $μ$m could detect up to ~550 exoplanets with radii between 0.5 and 6 R$_\oplus$ with an integrated SNR$\ge$7. At least ~160 of the detected exoplanets have radii $\le$1.5 R$_\oplus$. Depending on the observing scenario, ~25-45 rocky exoplanets (objects with radii between 0.5 and 1.5 $_{\oplus}$) orbiting within the empirical habitable zone (eHZ) of their host stars are among the detections. With an aperture size of 3.5 m, the total number of detections can increase to up to ~770, including ~60-80 rocky, eHZ planets. With 1 m aperture size, the maximum detection yield is ~315 exoplanets, including $\le$20 rocky, eHZ planets. In terms of predicted detection yield, such a mission can compete with large single-aperture reflected light missions. (abridged)

astro-ph.EP↗

arXiv2022

High-precision half-life determination of $^{14}$O via direct $β$ counting

The half-life of the superallowed Fermi $β^+$ emitter $^{14}$O was determined to high precision via a direct $β$ counting experiment performed at the Isotope Separator and Accelerator (ISAC) facility at TRIUMF. The result, $T_{1/2}$($^{14}$O) = 70619.2(76) ms, is consistent with, but is more precise than, the world average obtained from 11 previous measurements. Combining the $^{14}$O half-life deduced in the present work with the previous most precise measurements of this quantity leads to a reduction in the overall uncertainty, by nearly a factor of 2. The new world average is $T_{1/2}$($^{14}$O) = 70619.6(63) ms with a reduced $χ^2$ value of 0.87 obtained from 8 degrees of freedom.

nucl-ex↗

arXiv2022

Volkov-Pankratov states in a 2d material: excited states of a structural soliton

We show that partial dislocations, defects that naturally arise in bilayer graphene, host an analogue of the mass inversion Volkov-Pankratov (VP) states, the spectrum of excited states at a topologically non-trivial interface. In contrast to the dislocation states arising from the change in valley Chern index, such states (i) exist both with and without an interlayer bias, i.e. for metallic as well as insulating bulk and (ii) have distinct electron and hole bound states, whose number is proportional to the dislocation width. Recently discovered at heterojunctions of 3d topological insulators, their existence at the partial dislocations of a 2d material opens a rich structural playground for their investigation, and we demonstrate that the dislocation type, i.e. screw or edge, as well as the dislocation width, both play a decisive role in the creation of a topological spectrum of exited states.

cond-mat.mes-hall↗

arXiv2022

Making a case for femto- phono- magnetism with FePt

In the field of femtomagnetism magnetic matter is controlled by ultrafast laser pulses; here we show that coupling phonon excitations of the nuclei to spin and charge leads to femto-phono-magnetism, a powerful route to control magnetic order at ultrafast times. With state-of-the-art theoretical simulations of coupled spin-, charge-, and lattice-dynamics we identify strong non-adiabatic spin-phonon coupled modes that dominate early time spin dynamics. Activating these phonon modes we show leads to an additional (up to 40\% extra) loss of moment in FePt occurring within 40 femtoseconds of the pump laser pulse. Underpinning this enhanced ultrafast loss of spin moment we identify a physical mechanism in which minority spin-current drives an enhanced inter-site minority charge transfer, in turn promoting increased on-site spin flips. Our finding demonstrates that the nuclear system, often assumed to play only the role of an energy sink aiding long time re-magnetisation of the spin system, can play a profound role in controlling femtosecond spin-dynamics in materials.

cond-mat.mtrl-sci↗

arXiv2022

Valley control by linearly polarized laser pulses

Underpinning the field of "valleytronics" is the coupling of the helicity of circularly polarized light to the valley degree of freedom, and this remains the only known lightform to exhibit this remarkable effect. Here we show that on femtosecond time scales valley coupling is a much more general effect. We find that two time separated linearly polarized pulses allow almost complete control over valley excitation, with the pulse time difference and polarization vectors emerging as key parameters for valley control. In contrast to the Berry curvature that underpins the effect for circularly polarized light, we demonstrate that a different phase structure drives this effect, with excitations during each linear pulse acquiring a valley discriminating phase involving the polarisation angle of linear light. Unimportant in a single linear pulse, for pairs of pulses these can constructively and destructively interfere. Employing state-of-the-art time dependent density function theory, we show that the effect is robust to the complexities of charge dynamics in a real material with the example of a transitional metal dichalcogenide.

physics.optics↗

arXiv2022

Superconductivity in Bi based Bi2PdPt

We report synthesis and properties of superconducting Bi$_2$PdPt, a new member of Bi-Pd based compounds known for their simultaneous existence of topological surfaces states and superconductivity. It crystallizes in a hexagonal structure having space group $P6_3/mmc$. A detailed investigation of the properties via transport, magnetization, and specific heat measurements confirm bulk superconductivity with transition temperature, \textit{T}$_{C}$ = 4.0(1) K in moderate coupling limit.

cond-mat.supr-con↗

arXiv2022

Synthesis, physical and magnetic properties of CuAlCr$_4$S$_8$: a new Cr-based breathing pyrochlore

We present the synthesis and physical properties of a new breathing pyrochlore magnet CuAlCr$_4$S$_8$ with the help of synchrotron x-ray diffraction (XRD), magnetization under ambient and applied hydrostatic pressure, heat capacity, and muon spin relaxation/rotation ($μ$SR) measurements. CuAlCr$_4$S$_8$ exhibits positive thermal expansion with concave upward temperature dependence. We observed a sharp antiferromagnetic ordering transition of a purely magnetic nature at 20 K, which shifts by as much as 3.2 K on the application of 600 MPa pressure. The breathing factor (B$_f$ = $J'/J$) in breathing pyrochlores can be an important parameter to tune the magnetic ground states of the pyrochlore lattice. The breathing factor can be modulated through breathing ratio, the ratio of sizes of the two tetrahedra, by using different elements at A and A' sites in the breathing pyrochlore structure. We find that CuAlCr$_4$S$_8$ has a breathing ratio of 1.0663(8), which is comparable to other sulfur breathing pyrochlores.

cond-mat.str-el↗

arXiv2021

A Comprehensive Study of the Young Cluster IRAS 05100+3723: Properties, Surrounding Interstellar Matter, and Associated Star Formation

We present a comprehensive multiwavelength investigation of a likely massive young cluster `IRAS 05100+3723' and its environment with the aim to understand its formation history and feedback effects. We find that IRAS 05100+3723 is a distant ($\sim$3.2 kpc), moderate mass ($\sim$500 \msun), young ($\sim$3 Myr) cluster with its most massive star being an O8.5V-type. From spectral modeling, we estimate the effective temperature and log $g$ of the star as $\sim$33,000 K and $\sim$3.8, respectively. Our radio continuum observations reveal that the star has ionized its environment forming an HII region of size $\sim$2.7 pc, temperature $\sim$5,700 K, and electron density $\sim$165 cm$^{-3}$. However, our large-scale dust maps reveal that it has heated the dust up to several parsecs ($\sim$10 pc) in the range 17$-$28 K and the morphology of warm dust emission resembles a bipolar HII region. From dust and $^{13}$CO gas analyses, we find evidences that the formation of the HII region has occurred at the very end of a long filamentary cloud around 3 Myr ago, likely due to edge collapse of the filament. We show that the HII region is currently compressing a clump of mass $\sim$2700 \msun at its western outskirts, at the junction of the HII region and filament. We observe several 70 $μ$m point sources of intermediate-mass and class 0 nature within the clump. We attribute these sources as the second generation stars of the complex. We propose that the star formation in the clump is either induced or being facilitated by the compression of the expanding HII region onto the inflowing filamentary material.

astro-ph.SR↗