arXiv subjects

Anna G. Ivanova

Publications and source records attributed to Anna G. Ivanova.

9 recordsLinked to original sources

arXiv2025

Ternary superhydrides under pressure of Anderson's theorem: Near-record superconductivity in (La,Sc)H$_{12}$

Lanthanum-hydrogen system and its derivatives remain among the most promising for achieving room-temperature superconductivity. In this study, we examined the formation of ternary lanthanum-scandium superhydrides at pressures up to 220 GPa. The primary product of the LaSc alloy's reaction with hydrogen is newly discovered cubic (La,Sc)H$_{12}$, demonstrating clear superconducting transition in all six channels of the van der Pauw scheme below 244-248 K. In this compound with an unusually large unit cell volume, virtually no magnetoresistance was observed in fields up to 68 Tesla. Synthesized samples of (La,Sc)H$_{12}$ demonstrate pronounced superconducting diode and SQUID-like effects at a record high temperature of 233 K. Furthermore, our analysis revealed the formation of lower hexagonal polyhydrides (La,Sc)H$_{6-7}$, which could potentially account for the anomaly in electrical resistance observed near 274 K. This anomaly between 265-290 K also appears in the radio-frequency transmission measurements and may be of a superconducting nature.

cond-mat.supr-con↗

arXiv2023

Non-Fermi-Liquid Behavior of Superconducting SnH$_4$

We studied chemical interaction of Sn with H$_2$ by X-ray diffraction methods at pressures of 180-210 GPa. A previously unknown tetrahydride SnH$_4$ with a cubic structure (${fcc}$) exhibiting superconducting properties below ${T}$$_C$ = 72 K was obtained; the formation of a high molecular ${C2/m}$-SnH$_{14}$ superhydride and several lower hydrides, ${fcc}$ SnH$_2$ and ${C2}$-Sn$_{12}$H$_{18}$, was also detected. The temperature dependence of critical current density ${J}$$_C$(T) in SnH$_4$ yields the superconducting gap 2$Δ$(0) = 23 meV at 180 GPa. SnH$_4$ has unusual behavior in strong magnetic fields: ${B,T}$-linear dependences of magnetoresistance and the upper critical magnetic field ${B}$$_{C2}$(T) $\propto$ (${T}$$_C$ - ${T}$). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of ${fcc}$ SnH$_4$ in non-superconducting state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Grüneisen model, and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.

cond-mat.supr-con↗

arXiv2022

Structural peculiarities, mineral inclusions and point defects in yakutites -- a variety of impact-related diamond

An unusual variety of impact-related diamond from the Popigai impact structure - yakutites - is characterized by complementary methods including optical microscopy, X-ray diffraction, radiography and tomography, infra-red, Raman and luminescence spectroscopy providing structural information at widely different scales. It is shown that relatively large graphite aggregates may be transformed to diamond with preservation of many morphological features. Spectroscopic and X-ray diffraction data indicate that the yakutite matrix represents bulk nanocrystalline diamond. For the first time, features of two-phonon infra-red absorption spectra of bulk nanocrystalline diamond are interpreted in the framework of phonon dispersion curves. Luminescence spectra of yakutite are dominated by dislocation-related defects. Optical microscopy supported by X-ray diffraction reveals the presence of single crystal diamonds with sizes of up to several tens of microns embedded into nanodiamond matrix. The presence of single crystal grains in impact diamond may be explained by CVD-like growth in a transient cavity and/or a seconds-long compression stage of the impact process due to slow pressure release in a volatile-rich target. For the first time, protogenetic mineral inclusions in yakutites represented by mixed monoclinic and tetragonal ZrO2 are observed. This implies the presence of baddeleyite in target rocks responsible for yakutite formation.

cond-mat.mtrl-sci↗

arXiv2020

Novel Strongly Correlated Europium Superhydrides

We conducted a joint experimental-theoretical investigation of the high-pressure chemistry of europium polyhydrides at pressures of 86-130 GPa. We discovered several novel magnetic Eu superhydrides stabilized by anharmonic effects: cubic $EuH_{9}$, hexagonal $EuH_{9}$, and an unexpected cubic (Pm-3n) clathrate phase, $Eu_{8}H_{46}$. Monte Carlo simulations indicate that cubic $EuH_{9}$ has antiferromagnetic ordering with T(Neel) up to 24 K, whereas hexagonal $EuH_{9}$ and Pm-3n-$Eu_{8}H_{46}$ possess ferromagnetic ordering with T(Curie) = 137 and 336 K, respectively. The electron-phonon interaction is weak in all studied europium hydrides, and their magnetic ordering excludes s-wave superconductivity, except, perhaps, for distorted pseudohexagonal $EuH_{9}$. The equations of state predicted within the DFT+U approach (the Hubbard corrections were found within linear response theory) are in close agreement with the experimental data. This work shows the great influence of the atomic radius on symmetry-breaking distortions of the crystal structures of superhydrides and on their thermodynamic stability.

cond-mat.str-el↗

arXiv2020

Superconductivity at 253 K in lanthanum-yttrium ternary hydrides

Polyhydrides offer intriguing perspectives as high-temperature superconductors. Here we report the high-pressure synthesis of a series of lanthanum-yttrium ternary hydrides: cubic hexahydride $(La,Y)H_{6}$ with a critical temperature $T_{C}$ = 237 +/- 5 K and decahydrides $(La,Y)H_{10}$ with a maximum $T_{C}$ ~${253 K}$ and an extrapolated upper critical magnetic field $B_{C2(0)}$ up to ${135 T}$ at 183 GPa. This is one of the first examples of ternary high-$T_{C}$ superconducting hydrides. Our experiments show that a part of the atoms in the structures of recently discovered ${Im3m}$-$YH_{6}$ and ${Fm3m}$-$LaH_{10}$ can be replaced with lanthanum (~70 %) and yttrium (~25 %), respectively, with a formation of unique ternary superhydrides containing incorporated $La@H_{24}$ and $Y@H_{32}$ which are specific for ${Im3m}$-$LaH_{6}$ and ${Fm3m}$-$YH_{10}$. Ternary La-Y hydrides were obtained at pressures of 170-196 GPa via the laser heating of $P6_{3}$${/mmc}$ lanthanum-yttrium alloys in the ammonia borane medium at temperatures above 2000 K. A novel tetragonal $(La,Y)H_{4}$ was discovered as an impurity phase in synthesized cubic $(La,Y)H_{6}$. The current-voltage measurements show that the critical current density $J_{C}$ in $(La,Y)H_{10}$ may exceed $2500 A/mm^{2}$ at 4.2 K, which is comparable with that for commercial superconducting wires such as ${NbTi}$, $Nb_{3}$${Sn}$. Hydrides that are unstable in a pure form may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the same structure.

cond-mat.supr-con↗

arXiv2020

Crystal structure and phase transitions at high pressures in the superconductor FeSe0.89S0.11

We report on the structural phase transitions in the S doped FeSe superconductor by powder synchrotron X ray diffraction at high pressures up to 18.5 GPa under compression and decompression modes. In order to create high quasi hydrostatic pressures, diamond anvil cells filled with helium as a pressure transmitting medium were used. It was found that at ambient pressure and room temperature, S doped FeSe has a tetragonal structure. Under compression, in the region of 10 GPa, a phase transition from the tetragonal into the orthorhombic structure is observed, which persists up to 18.5 GPa. Our results strongly suggest that, at decompression, as the applied pressure decreases to 6 GPa and then is completely removed, most of the sample recrystallizes into the hexagonal phase of the structural type NiAs. However, the other part of the sample remains in the high pressure orthorhombic phase, while the tetragonal phase is not restored. These observations illustrate a strong hysteresis of the structural properties of S doped FeSe during a phase transition under pressure.

cond-mat.supr-con↗

arXiv2020

Anomalous high-temperature superconductivity in YH$_6$

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.

cond-mat.supr-con↗

arXiv2020

Superconductivity and Equation of State of Distorted fcc-Lanthanum above Megabar Pressures

Lanthanum (La) is the first member of the rare-earth series of elements that has recently raised considerable interest because of its unique high-Tc superhydride LaH10. Although several studies have found superconductivity and phase transitions in metallic La, there was a lack of experimental evidence for the equation of state (EoS) and superconductivity above one megabar pressure. Here, we extend the pressure range up to 140 GPa to study EoS and superconductivity of lanthanum via electrical transport and X-ray diffraction measurements. The experimental XRD patterns point to a phase transition sequences R3m-Fm3m-Fmmm above 78 GPa. All the experimental pressure-volume data were fitted by the 3rd order Birch-Murnaghan equation: V0 = 35.2 (4) A^3, B0 = 27 (1) GPa and B0' = 4. Superconducting critical temperature Tc(onset) of lanthanum is 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. The upper critical magnetic field Bc2(0) was found to be 0.32-0.43 T at 140 GPa. Ab initio calculations give predicted Tc(A-D)=2.2 K (mu*=0.195), dTc/dP = 0.11-0.13 K/GPa and Hc=0.4 T at 140 GPa.

cond-mat.supr-con↗

arXiv2019

Superconductivity at 161 K in Thorium Hydride $ThH_{10}$: Synthesis and Properties

Here we report targeted high-pressure synthesis of two novel high-$T_C$ hydride superconductors, $P6_3/mmc$-$ThH_9$ and $Fm\bar{3}m$-$ThH_{10}$, with the experimental critical temperatures ($T_C$) of 146 K and 159-161 K and upper critical magnetic fields ($μ$$H_C$) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of $T_C$ under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as $LaH_{10}$, $H_3S$ and $HgBa_2Ca_xCu_2O_{6+z}$. Our experiments show that $fcc$-$ThH_{10}$ has stabilization pressure of 85 GPa, making this material unique among all known high-$T_C$ metal polyhydrides. Two recently predicted Th-H compounds, $I4/mmm$-$ThH_4$ (> 86 GPa) and $Cmc2_1$-$ThH_6$ (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.

cond-mat.mtrl-sci↗