arXiv subjects

Ashok K. Verma

Publications and source records attributed to Ashok K. Verma.

12 recordsLinked to original sources

arXiv2025

Realizing high-temperature superconductivity in compressed molecular-hydrogen through Li doping

In this study, we explore lithium-doped stable molecular hydrogen structures by performing first-principles crystal structure searches across varying compositions in the Li-H system under high pressure. Our search reveals a cubic phase of LiH12, which shows promise as a high-temperature superconductor. Our Bader charge analysis suggests that electron transfer from Li to H atoms tunes the intra- and inter-molecular H-H distances, which are critical for the metallization of molecular hydrogen. This modulation alters the interaction between bonding and anti-bonding 1s states of hydrogen molecules. Furthermore, Li ions serve as stabilizers for the distorted H2 molecular network through ionic interactions. Numerical solutions to the fully anisotropic Migdal-Eliashberg equations reveals that this phase could exhibit superconductivity above 300 K at a pressure of 250 GPa, a pressure value that is typically achievable using a diamond anvil cell. Detailed analysis of species-specific phonons and the Eliashberg function shows that low- and intermediate-energy phonons are crucial in promoting strong electron-phonon coupling. Thus, our study establishes lithium doping as a promising approach to induce high-temperature superconductivity in compressed molecular hydrogen without causing molecular dissociation.

cond-mat.supr-con↗

arXiv2023

Pressure induced electride phase formation in calcium: A key to its strange high-pressure behavior

Elemental calcium (Ca), a simple metal at ambient conditions, has attracted huge interest because of its unusual high-pressure behavior in structural, electrical, and melting properties whose origin remain unsolved. Here, using a theoretical framework appropriate for describing electride phase formation, i.e., the presence of anionic electrons, we establish electride formation in Ca at a pressure as low as 8 GPa. Our analysis shows that under pressure the valence electrons of Ca localize at octahedral holes and exhibit anionic character which is responsible for its strange pressure behavior. Our calculated enthalpy and electrical resistance indicate that Ca will directly transform from an FCC-electride phase to an SC-electride phase near 30 GPa thereby avoiding the intermediate BCC phase. These findings are not limited to Ca but might hold a key to the understanding of host-guest type structures which occur in other elemental solids though at much higher pressures.

cond-mat.mtrl-sci↗

arXiv2021

Prediction of an unusual trigonal phase of superconducting LaH$_{\bf 10}$ stable from 250 to 425 GPa pressure

Based on evolutionary crystal structure searches in combination with ab initio calculations, we predict an unusual structural phase of the superconducting LaH$_{10}$ that is stable from about 250 GPa to 425 GPa pressure. This new phase belongs to a trigonal $R\bar{3}m$ crystal lattice with an atypical cell angle, $α_{rhom}$ $\sim$ 24.56$^{\circ}$. We find that the new structure contains three units of LaH$_{10}$ in its primitive cell, unlike the previously known trigonal phase, where primitive cell contains only one LaH$_{10}$ unit. In this phase, a 32-H atoms cage encapsulates La atoms, analogous to the lower pressure face centred cubic phase. However, the hydrogen cages of the trigonal phase consist of quadrilaterals and hexagons, in contrast to the cubic phase, that exhibits squares and regular hexagons. Surprisingly, the shortest H-H distance in the new phase is shorter than that of the lower pressure cubic phase and of atomic hydrogen metal. We find a structural phase transition from trigonal to hexagonal at 425 GPa, where the hexagonal crystal lattice coincides with earlier predictions. Solving the anisotropic Migdal-Eliashberg equations we obtain that the predicted trigonal phase (for standard values of the Coulomb pseudopotential) is expected to become superconducting at a critical temperature of about 175 K, which is less than $T_c \sim$250 K measured for cubic LaH$_{10}$.

cond-mat.supr-con↗

arXiv2020

Phonon-mode specific contributions to room-temperature superconductivity in atomic hydrogen at high pressures

We investigate the role of specific phonon mode symmetries for the room temperature superconductivity in atomic hydrogen under large pressure. Using anisotropic Migdal-Eliashberg theory with ab initio input from density functional theory, we show that the $E_u$ phonon modes are the dominant driving force for obtaining such high critical temperatures. When going from 400 to 600 GPa, we find an increased transition temperature, however, the total electron-phonon coupling strength is counterintuitively reduced. Our analysis reveals that this is due to an enhanced contribution to the coupling strength by the $E_u$ phonon mode.

cond-mat.supr-con↗

arXiv2020

Yarkovsky Drift Detections for 247 Near-Earth Asteroids

The Yarkovsky effect is a thermal process acting upon the orbits of small celestial bodies, which can cause these orbits to slowly expand or contract with time. The effect is subtle (da/dt ~ 10^-4 au/My for a 1 km diameter object) and is thus generally difficult to measure. We analyzed both optical and radar astrometry for 600 near-Earth asteroids (NEAs) for the purpose of detecting and quantifying the Yarkovsky effect. We present 247 NEAs with measured drift rates, which is the largest published set of Yarkovsky detections. This large sample size provides an opportunity to examine the Yarkovsky effect in a statistical manner. In particular, we describe two independent population-based tests that verify the measurement of Yarkovsky orbital drift. First, we provide observational confirmation for the Yarkovsky effect's theoretical size dependence of 1/D, where D is diameter. Second, we find that the observed ratio of negative to positive drift rates in our sample is 2.34, which, accounting for bias and sampling uncertainty, implies an actual ratio of $2.7^{+0.3}_{-0.7}$. This ratio has a vanishingly small probability of occurring due to chance or statistical noise. The observed ratio of retrograde to prograde rotators is two times lower than the ratio expected from numerical predictions from NEA population studies and traditional assumptions about the sense of rotation of NEAs originating from various main belt escape routes. We also examine the efficiency with which solar energy is converted into orbital energy and find a median efficiency in our sample of 12%. We interpret this efficiency in terms of NEA spin and thermal properties.

astro-ph.EP↗

arXiv2018

Expected Precision of Europa Clipper Gravity Measurements

The primary gravity science objective of NASA's Clipper mission to Europa is to confirm the presence or absence of a global subsurface ocean beneath Europa's Icy crust. Gravity field measurements obtained with a radio science investigation can reveal much about Europa's interior structure. Here, we conduct extensive simulations of the radio science measurements with the anticipated spacecraft trajectory and attitude (17F12v2) and assets on the spacecraft and the ground, including antenna orientations and beam patterns, transmitter characteristics, and receiver noise figures. In addition to two-way Doppler measurements, we also include radar altimeter crossover range measurements. We concentrate on +/-2 hour intervals centered on the closest approach of each of the 46 flybys. Our covariance analyses reveal the precision with which the tidal Love number k2, second-degree gravity coefficients C20 and C22, and higher-order gravity coefficients can be determined. The results depend on the Deep Space Network (DSN) assets that are deployed to track the spacecraft. We find that some DSN allocations are sufficient to conclusively confirm the presence or absence of a global ocean. Given adequate crossover range performance, it is also possible to evaluate whether the ice shell is hydrostatic.

astro-ph.EP↗

arXiv2017

Prospects of dynamical determination of General Relativity parameter beta and solar quadrupole moment J2 with asteroid radar astronomy

We evaluated the prospects of quantifying the parameterized post-Newtonian parameter beta and solar quadrupole moment J2 with observations of near-Earth asteroids with large orbital precession rates (9 to 27 arcsec century$^{-1}$). We considered existing optical and radar astrometry, as well as radar astrometry that can realistically be obtained with the Arecibo planetary radar in the next five years. Our sensitivity calculations relied on a traditional covariance analysis and Monte Carlo simulations. We found that independent estimates of beta and J2 can be obtained with precisions of $6\times10^{-4}$ and $3\times10^{-8}$, respectively. Because we assumed rather conservative observational uncertainties, as is the usual practice when reporting radar astrometry, it is likely that the actual precision will be closer to $2\times10^{-4}$ and $10^{-8}$, respectively. A purely dynamical determination of solar oblateness with asteroid radar astronomy may therefore rival the helioseismology determination.

astro-ph.EP↗

arXiv2017

Asteroid 1566 Icarus's size, shape, orbit, and Yarkovsky drift from radar observations

Near-Earth asteroid (NEA) 1566 Icarus ($a=1.08$ au, $e=0.83$, $i=22.8^{\circ}$) made a close approach to Earth in June 2015 at 22 lunar distances (LD). Its detection during the 1968 approach (16 LD) was the first in the history of asteroid radar astronomy. A subsequent approach in 1996 (40 LD) did not yield radar images. We describe analyses of our 2015 radar observations of Icarus obtained at the Arecibo Observatory and the DSS-14 antenna at Goldstone. These data show that the asteroid is a moderately flattened spheroid with an equivalent diameter of 1.44 km with 18% uncertainties, resolving long-standing questions about the asteroid size. We also solve for Icarus' spin axis orientation ($λ=270^{\circ}\pm10^{\circ}, β=-81^{\circ}\pm10^{\circ}$), which is not consistent with the estimates based on the 1968 lightcurve observations. Icarus has a strongly specular scattering behavior, among the highest ever measured in asteroid radar observations, and a radar albedo of $\sim$2\%, among the lowest ever measured in asteroid radar observations. The low cross-section suggests a high-porosity surface, presumably related to Icarus' cratering, spin, and thermal histories. Finally, we present the first use of our orbit determination software for the generation of observational ephemerides, and we demonstrate its ability to determine subtle perturbations on NEA orbits by measuring Icarus' orbit-averaged drift in semi-major axis ($(-4.62\pm0.48) \times 10^{-4}$ au/My, or $\sim$60 m per revolution). Our Yarkovsky rate measurement resolves a discrepancy between two published rates that did not include the 2015 radar astrometry.

astro-ph.EP↗

arXiv2016

Electronic topological transitions in Nb$_3$X (X = Al, Ga, In, Ge and Sn) under compression investigated by first principles calculations

First principles electronic structure calculations of A-15 type Nb$_3$X (X = Al, Ga, In, Ge and Sn) compounds are performed at ambient and high pressures. Mechanical stability is confirmed in all the compounds both at ambient as well as under compression from the calculated elastic constants. We have observed four holes and two electron Fermi surfaces (FS) for all the compounds studied and FS nesting feature is observed at M and along X - $Γ$ in all the compounds. A continuous change in the FS topology is observed under pressure in all the compounds which is also reflected in the calculated elastic constants and density of states under pressure indicating the Electronic topological transitions (ETT). The ETT observed at around 21.5 GPa, 17.5 GPa in Nb$_3$Al and Nb$_3$Ga are in good agreement with the anomalies observed by the experiments.

cond-mat.mtrl-sci↗

arXiv2015

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure

We report here the evidences of hydrogen bond symmetrization in the simplest amino acid- carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The protonation of the semioxalate units through dynamic proton movement results in infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen bonded supramolecular assemblies held together by these chains.

cond-mat.mtrl-sci↗

arXiv2013

Structural, Vibrational, Elastic and Topological Properties of PaN Under Pressure

Electronic, structural, vibrational and elastic properties of PaN have been studied both at ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. Under pressure PaN is found to undergo a structural transition from NaCl to the R-3m structure near 58 GPa. The high pressure behavior of the acoustic phonon branch along the (1,0,0) and (1,1,0) directions, and the C44 elastic constant are anomalous, which signals the structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). Therefore, this constitutes an example of GGA and LDA leading to qualitatively different predictions, and it is of great interest to examine experimentally whether this topological transition occurs.

cond-mat.mtrl-sci↗

arXiv2007

Phonon instability and structural phase transition in Vanadium under high pressure

Results of the first-principles calculations are presented for the group-VB metals V, Nb and Ta up to couple of megabar pressure. An unique structural phase transition sequence BCC-->(at 60 GPa) rhombohedral (angle=110.5 degree)-->(at ~ 160 GPa)rhombohedral(angle=108.5 degree)--> (at ~ 430 GPa) BCC is predicted in V. We also find that BCC-V becomes mechanically and vibrationally unstable at around 112 GPa pressure. Similar transitions are absent in Nb and Ta.

cond-mat.mtrl-sci↗