arXiv subjects

Ashkan Salamat

Publications and source records attributed to Ashkan Salamat.

18 recordsLinked to original sources

arXiv2024

Standard Superconductivity in Carbonaceous Sulfur Hydride

The recently discovered room temperature superconductivity (RTSC) in carbonaceous sulfur hydride reveals the promise of a new superconducting technological era. Since the publication of that report, there has been discussion and debate about this technology. In particular, Hirsch et al. argue that all the recently discovered superconducting hydrides are incorrect, incorrectly interpreted, or point to BCS theory being wrong. A recent publication regarding the RTSC discovery argues that more detailed information about the RTSC discovery is needed. This manuscript aims to respond to this concern by providing the raw data under question as well as providing discussion regarding other aspects of this new RTSC for the sake of the broader scientific community.

cond-mat.supr-con↗

arXiv2023

Ultra-fast yttrium hydride chemistry at high pressures via non-equilibrium states induced by x-ray free electron laser

Controlling the formation and stoichiometric content of desired phases of materials has become a central interest for the study of a variety of fields, notably high temperature superconductivity under extreme pressures. The further possibility of accessing metastable states by initiating reactions by x-ray triggered mechanisms over ultra-short timescales is enabled with the development of x-ray free electron lasers (XFEL). Utilizing the exceptionally high brilliance x-ray pulses from the EuXFEL, we report the synthesis of a previously unobserved yttrium hydride under high pressure, along with non-stoichiometric changes in hydrogen content as probed at a repetition rate of 4.5\,MHz using time-resolved x-ray diffraction. Exploiting non-equilibrium pathways we synthesize and characterize a hydride with yttrium cations in an \textit{A}15 structure type at 125\,GPa, predicted using crystal structure searches, with a hydrogen content between 4.0--5.75 hydrogens per cation, that is enthalpically metastable on the convex hull. We demonstrate a tailored approach to changing hydrogen content using changes in x-ray fluence that is not accessible using conventional synthesis methods, and reveals a new paradigm in metastable chemical physics.

cond-mat.mtrl-sci↗

arXiv2023

The Isotope Effect and Critical Magnetic Fields of Superconducting YH$_{6}$: A Migdal-Eliashberg Theory Approach

The emergence of near-ambient temperature superconductivity under pressure in the metal hydride systems has motivated a desire to further understand such remarkable properties, specifically critical magnetic fields. YH$_6$ is suggested to be a departure from conventional superconductivity, due to apparent anomalous behavior. Using density functional calculations in conjunction with Migdal-Eliashberg theory we show that in YH$_6$ the critical temperature and the isotope effect under pressure, as well as the high critical fields, are consistent with strong-coupling conventional superconductivity; a property anticipated to extend to other related systems. Furthermore, the strong-coupling corrections occur to the expected BCS values for the Ginzburg-Landau parameter ($κ_{1}(T)$), London penetration depth ($λ_{L}(T)$), electromagnetic coherence length ($ξ(T)$), and the energy gap ($Δ_{0}$).

cond-mat.supr-con↗

arXiv2023

Observation of Conventional Near Room Temperature Superconductivity in Carbonaceous Sulfur Hydride

The phenomenon of high temperature superconductivity, approaching room temperature, has been realized in a number of hydrogen-dominant alloy systems under high pressure conditions1-12. A significant discovery in reaching room temperature superconductivity is the photo-induced reaction of sulfur, hydrogen, and carbon that initially forms of van der Waals solids at sub-megabar pressures. Carbonaceous sulfur hydride has been demonstrated to be tunable with respect to carbon content, leading to different superconducting final states with different structural symmetries. A modulated AC susceptibility technique adapted for a diamond anvil cell confirms a Tc of 260 kelvin at 133 GPa in carbonaceous sulfur hydride. Furthermore, direct synchrotron infrared reflectivity measurements on the same sample under the same conditions reveal a superconducting gap of ~85 meV at 100 K in close agreement to the expected value from Bardeen-Cooper-Schrieffer (BCS) theory13-18. Additionally, x-ray diffraction in tandem with AC magnetic susceptibility measurements above and below the superconducting transition temperature, and as a function of pressure at 107-133 GPa, reveal the Pnma structure of the material is responsible for the close to room-temperature superconductivity at these pressures.

cond-mat.supr-con↗

arXiv2023

Machine learning using structural representations for discovery of high temperature superconductors

The expansiveness of compositional phase space is too vast to fully search using current theoretical tools for many emergent problems in condensed matter physics. The reliance on a deep chemical understanding is one method to identify local minima of relevance to investigate further, minimizing sample space. Utilizing machine learning methods can permit a deeper appreciation of correlations in higher order parameter space and be trained to behave as a predictive tool in the exploration of new materials. We have applied this approach in our search for new high temperature superconductors by incorporating models which can differentiate structural polymorphisms, in a pressure landscape, a critical component for understanding high temperature superconductivity. Our development of a representation for machine learning superconductivity with structural properties allows fast predictions of superconducting transition temperatures ($T_c$) providing a $r^2$ above 0.94.

cond-mat.supr-con↗

arXiv2022

Gearbox designs for the diamond anvil cell: Applications to hard-to-reach-places

The uniaxial compression design of the diamond anvil high pressure cell (DAC) necessitates the use of a soft pressure-transmitting medium (PTM) to minimize non-hydrostatic effects at significantly high pressures, and many such media are gaseous at ambient conditions. There now exist a number of commercially-available instruments - high-pressure gas loading apparatus - for the insertion of gases into the diamond anvil cell up to several kbar. These machines allow the use of gases as a PTM, as a reagent for high-pressure chemistry, or to be loaded as the sample material itself. We present the development of two highly adaptable gearbox designs to allow the controlled closure of the DAC within an atmosphere of gas at several kbar, contained within the walls of a pressure vessel. The first applies a torque directly to the pressure screws and is thus DAC-specific, while the second applies a load to the body of the cell and may be used with a wide variety of DAC designs.

physics.ins-det↗

arXiv2022

Reply to "Comment on Nature 586, 373 (2020) by E. Snider et al."

In this paper, we respond to a recent criticism of our work on CSH, (1,2) raised by Hirsch and van der Marel on the arXiv (3). We point out that their non-peer reviewed critique (3) stems from either a lack of scientific understanding or a failure to appropriately analyze raw data. We explain that the use of their "Unwrapped" method was inappropriately performed. (3) We present scientific evidence that this basic and fundamental failure used in their method results in the false representations set forth in Ref (3). We also note that the raw data of Refs (1,2) were made available to Hirsch before he and van der Marel posted their comments on the arXiv. (3) We stand by the findings and conclusions of Refs (1,2) and remain committed to the integrity in scientific research.

cond-mat.supr-con↗

arXiv2022

Understanding Hydrogen Rich Superconductors: Importance of Effective Mass and Dirty Limit

A class of hydrogen-rich (H-rich) systems, consistent with Type II superconductivity, are known to have very high superconducting transition temperatures Tc and upper critical magnetic field Bc2 values (up to 288 K and 222 T respectively). By looking at all the experimental H-rich superconductors reported to date most are in the dirty limit, with only the highest Tc values being in the crossover to the clean regime. In this framework, there is a clear understanding of some previous unexplained behaviors: (1) A maximum in Bc2 as a function of Tc; (2) a clear change in slope in Tc as a function of pressure in CSH; (3) in zero magnetic field the width of the superconducting transition decreasing with increasing Tc; and (4) in applied magnetic field the slope of the superconducting width versus field decreasing with increasing Tc. Ginzburg-Landau-Abrikosov-Gorkov (GLAG) theory is used to explain all four of these effects within a framework where increasing Tc is related to increases in both the electron effective mass and scattering length.

cond-mat.supr-con↗

arXiv2021

Lower pressure phases and metastable states of superconducting photo-induced carbonaceous sulfur hydride

Room-temperature superconductivity was recently discovered in carbonaceous sulfur hydride (C-S-H) close to 3\,Mbar. We report significant differences in the superconducting response of C-S-H, with a maximum $T_{C}$ of 191(1)\,K, below a 1\,Mbar. Variations in intensity of the C-H Raman modes reveal carbon content can vary between crystals synthesized with the same photo-induced method. Synchrotron single crystal x-ray diffraction identifies polymorphism with increasing degrees of covalency. These unique metastable states are highly sensitive to thermodynamic pathways.

cond-mat.supr-con↗

arXiv2021

Decoupling lattice and magnetic instabilities in frustrated CuMnO$_2$

The $A$MnO$_{2}$ delafossites ($A$=Na, Cu), are model frustrated antiferromagnets, with triangular layers of Mn$^{3+}$~spins. At low temperatures ($T_{N}$=65 K), a $C2/m \rightarrow P\overline{1}$ transition is found in CuMnO$_2$, which breaks frustration and establishes magnetic order. In contrast to this clean transition, $A$=Na only shows short-range distortions at $T_N$. Here we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO$_2$. We show that, even in stoichiometric samples, non-zero anisotropic Cu displacements co-exist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures acts to decouple these degrees of freedom. This manifests as an isostuctural phase transition at $\sim$10 GPa, with a reversible collapse of the $c$-axis. This is shown to be the high pressure analog of the $c$-axis negative thermal expansion seen at ambient pressure. DFT simulations confirm that dynamical instabilities of the Cu$^{+}$ cations and edge-shared MnO$_{6}$ layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted re-emergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO$_2$ are quantitatively different to non-magnetic Cu delafossites, and raise questions about the role of intrinsic inhomogeniety in frustrated antiferromagnets.

cond-mat.str-el↗

arXiv2021

Implications of an improved water equation of state for water-rich planets

Water (H$_{2}$O), in all forms, is an important constituent in planetary bodies, controlling habitability and influencing geological activity. Under conditions found in the interior of many planets, as the pressure increases, the H-bonds in water gradually weaken and are replaced by ionic bonds. Recent experimental measurements of the water equation of state (EOS) showed both a new phase of H-bonded water ice, ice-VII$_t$, and a relatively low transition pressure just above 30 GPa to ionic bonded ice-X, which has a bulk modulus 2.5 times larger. The higher bulk modulus of ice-X produces larger planets for a given mass, thereby either reducing the atmospheric contribution to the volume of many exoplanets or limiting their water content. We investigate the impact of the new EOS measurements on the planetary mass-radius relation and interior structure for water-rich planets. We find that the change in the planet mass-radius relation caused by the systematic differences between previous and new experimental EOS measurements are comparable to the observational uncertainties in some planet sizes -- an issue that will become more important as observations continue to improve.

astro-ph.EP↗

arXiv2020

Superconductivity to 262 kelvin via catalyzed hydrogenation of yttrium at high pressures

Room temperature superconductivity has been achieved under high pressure in an organically derived carbonaceous sulfur hydride with a critical superconducting transition temperature (Tc) of 288 kelvin. This development is part of a new class of dense, hydrogen rich materials with remarkably high critical temperatures. Metal superhydrides are a subclass of these materials that provide a different and potentially more promising route to very high Tc superconductivity. The most promising binary metal superhydrides contain alkaline or rare earth elements, and recent experimental observations of LaH10 have shown them capable of Tc s up to 250 to 260 kelvin. Predictions have shown yttrium superhydrides to be the most promising with an estimated Tc in excess of 300 kelvin for YH10. Here we report the synthesis of an yttrium superhydride that exhibits superconductivity at a critical temperature of 262 kelvin at 182 gigapascal. A palladium thin film assists the synthesis by protecting the sputtered yttrium from oxidation and promoting subsequent hydrogenation. Phonon mediated superconductivity is established by the observation of zero resistance, an isotope effect and the reduction of Tc under an external magnetic field. The upper critical magnetic field is 103 tesla at zero temperature. We suggest YH9 is the synthesized product based on comparison of the measured Raman spectra and Tc to calculated Raman results.

cond-mat.supr-con↗

arXiv2019

Bond strengthening in dense H2O and implications to planetary composition

H2O is an important constituent in planetary bodies, controlling habitability and, in geologically-active bodies, plate tectonics. At pressures within the interior of many planets, the H-bonds in H2O collapse into stronger, ionic bonds. Here we present agreement between X-ray diffraction and Raman spectroscopy for the transition from ice-VII to ice-X occurring at a pressure of approximately 30.9 GPa by means of combining grain normalizing heat treatment via direct laser heating with static compression. This is evidenced by the emergence of the characteristic Raman mode of cuprite-like ice-X and an abrupt 2.5-fold increase in bulk modulus, implying a significant increase in bond strength. This is preceded by a transition from cubic ice-VII to a structure of tetragonal symmetry, ice-VIIt at 5.1 GPa. Our results significantly shift the mass/radius relationship of water-rich planets and define a high-pressure limit for release of chemically-bound water within the Earth, making the deep mantle a potential long-term reservoir of ancient water.

cond-mat.mtrl-sci↗

arXiv2018

Post-aragonite phases of CaCO$_{3}$ at lower mantle pressures

The stability, structure and properties of carbonate minerals at lower mantle conditions has significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years, there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating of CaCO$_{3}$ using a CO$_{2}$ laser, we identify a crystalline phase of the material above 40 GPa $-$ corresponding to a lower mantle depth of around 1,000 km $-$ which has first been predicted by \textit{ab initio} structure predictions. The observed $sp^{2}$ carbon hybridized species at 40 GPa is monoclinic with $P2_{1}/c$ symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of \textit{ab initio} random structure search (AIRSS) and quasi-harmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of CaCO$_{3}$. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of CaCO$_{3}$ and we postulate that the mineral is capable of undergoing $sp^{2}$-$sp^{3}$ hybridization change purely in the $P2_{1}/c$ structure $-$ forgoing the accepted post-aragonite $Pmmn$ structure.

cond-mat.mtrl-sci↗

arXiv2016

Expanded Comment on: Optical Properties of Fluid Hydrogen at the Transition to a Conducting State

We comment on a recent paper published by McWilliams et al claiming that high-pressure/high-temperature hydrogen is a semi-conductor or semi-metal, in conflict with all earlier measurements on this system which show that it is metallic. We point out problems and inconsistencies and propose an alternate interpretation such that their results can be compatible with the observation of metallic hydrogen.

cond-mat.mtrl-sci↗

arXiv2016

Carbon Nitride Frameworks and Dense Crystalline Polymorphs

We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp2 and sp3-bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp3-bonded C atoms and three-fold coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decomposition into diamond and pyrite-structured CN2.

cond-mat.mtrl-sci↗

arXiv2015

A First-order Phase Transition to Metallic Hydrogen

The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from T=0 K to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen in the region of 1.1-1.7 Mbar and up to 2200 K. We observe a first-order phase transition accompanied by changes in transmittance and reflectance characteristic of a metal. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition.

cond-mat.mtrl-sci↗

arXiv2014

Giant pressure-induced volume collapse in the pyrite mineral MnS2

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral Hauerite undergoes an unprecedented 22 % collapse driven by a conceptually different magnetic mechanism. Using synchrotron x-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic groundstate using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S=1/2 moments are quenched by dimerisation. Our results show how the emergence of metal-metal bonding can stabilise giant spin-lattice coupling in Earth's minerals.

cond-mat.str-el↗