arXiv subjects

E. F. Talantsev

Publications and source records attributed to E. F. Talantsev.

At least 19 recordsLinked to original sources

arXiv2026

Universal temperature-dependent electrical resistivity in actinides

Temperature-dependent electrical resistivity $ρ(T)$ is one of the most common types of experimental data analyzed in condensed matter physics. For one group of pure metals, the actinides, experimental $ρ(T)$ curves differ radically from one another to the point that there is no unified theoretical approach to understanding and fitting $ρ(T)$ data in these elements. First-principles calculations result in $ρ(T)$ curves that differ from experimental data, even qualitatively. In an attempt to unravel this long-standing problem, here I propose a simple model that accurately fits the $ρ(T)$ data for nine phases of elemental actinides (from thorium (Th) to curium (Cm)) for which experimental data are publicly available to date. The model is based on the concept of two parallel conduction channels: one is described by the Bloch-Grüneisen equation, which is associated with the classical electron-phonon dissipation mechanism, and the other by the Arrhenius equation, which is associated with the nearest-neighbor hopping (NNH) conductivity. Debye temperatures $Θ_D$ obtained by applying the model to the $ρ(T)$ data for nine elemental actinide phases are in good agreement with published values deduced from heat capacity measurements. For neptunium (Np) a maximum Arrhenius activation energy (among all actinides) of $E_a=15.9$ $meV$ was derived. The model was also successfully applied to $ρ(T)$ data measured on $δ$-phase plutonium-based alloys Pu-Ce and Pu-Ce-Ga.

cond-mat.mtrl-sci↗

arXiv2025

Size-strain characteristics of lead and gold under fast ramp compression

Phase transitions in materials under fast ramp compression are an ongoing research topic, which is part of several global projects like inertial fusion. Currently, X-ray diffraction (XRD) examination of samples under fast ramp compression is limited to the determination of the sample phase state and the unit cell lattice parameters. Here, we propose to extend this examination route by introducing the Williamson-Hall analysis of the XRD data measured in samples under fast ramp rate conditions. To demonstrate the applicability of the method, we performed an analysis for ramp compressed lead $(P = 200 GPa)$ and gold $(P = 1003 GPa)$, which both exhibit a transition from the face-centred cubic (fcc) lattice to the body-centred cubic (bcc) lattice at the studied pressures. The analysis showed that lead under fast ramp compression has a nanocrystalline structure with a crystalline size of $D = (4 \pm 1) nm$ and lattice strain $\varepsilon = 0.006 \pm 0.002 $. The effect of extreme hardening of bcc-Pb under fast ramp compression can be explained as the formation of an ultrafine grain structure in this metal. Elemental gold exhibits average crystalline size $D > 12 nm $ and unprecedentedly high, for a pure metallic element, lattice strain $\varepsilon = 0.014 \pm 0.001 $ under fast ramp compression.

cond-mat.mtrl-sci↗

arXiv2025

Einstein and Debye temperatures, electron-phonon coupling constant and a probable mechanism for ambient-pressure room-temperature superconductivity in intercalated graphite

Recently, Ksenofontov et al (arXiv:2510.03256) observed ambient pressure room-temperature superconductivity in graphite intercalated with lithium-based alloys with transition temperature (according to magnetization measurements) $T_c=330$ $K$. Here, I analyzed the reported temperature dependent resistivity data $ρ(T)$ in these graphite-intercalated samples and found that $ρ(T)$ is well described by the model of two series resistors, where each resistor is described as either an Einstein conductor or a Bloch-Grüneisen conductor. Deduced Einstein and Debye temperatures are $Θ_{E,1} \approx 250$ $K$ and $Θ_{E,2} \approx 1,600$ $K$, and $Θ_{D,1} \approx 300$ $K$ and $Θ_{D,2} \approx 2,200$ $K$, respectively. Following the McMillan formalism, from the deduced $Θ_{E,2}$ and $Θ_{D,2}$, the electron-phonon coupling constant $λ_{e-ph} = 2.2 - 2.6$ was obtained. This value of $λ_{e-ph}$ is approximately equal to the value of $λ_{e-ph}$ in highly compressed superconducting hydrides. Based on this, I can propose that the observed room-temperature superconductivity in intercalated graphite is localized in nanoscale Sr-Ca-Li metallic flakes/particles, which adopt the phonon spectrum from the surrounding bulk graphite matrix, and as a result, conventional electron-phonon superconductivity arises in these nano-flakes/particles at room temperature. Experimental data reported by Ksenofontov et al (arXiv:2510.03256) on trapped magnetic flux decay in intercalated graphite samples supports the proposition.

physics.gen-ph↗

arXiv2025

The compliance of the molecular hydride superconductor $BiH_4$ with the Migdal's theorem

The discovery of near-room-temperature superconductivity in H3S sparked experimental and theoretical studies of highly compressed hydrides with the aim of obtaining room-temperature superconductivity. There are two dominant hydride classes where the search is ongoing: the first class is the covalently bonded hydrides (which is represented by H3S), and the second class is the clathrate-type hydrides (which is represented by LaH10, YH6, CaH6). Recently, the third class of superconducting hydrides, where the hydrogen remains its molecular form, has been discovered. This class is represented by BaH12 and BiH4. Here, we analyzed experimental data for the BaH12 and BiH4. We found that the BaH12 exhibits grains of an average size of 26 nm and a low level of microstrain 0.1%, in the range of 126 GPa < P < 160 GPa. We also derived the Debye $Θ_D$ and Einstein $Θ_E$ temperatures, and the electron-phonon coupling constant $Λ_{e-ph} $ in BaH12 and BiH4. The $Λ_{e-ph} $ in BiH4 significantly differs from the values obtained by first-principles calculations. The derived Fermi temperature $T_F = 20,000 K$ for BiH4 positions this molecular hydride between the unconventional and conventional superconductors bands in the Uemura plot. This position is outside of the band where covalently bonded and clathrate hydrides are located. The ratio $Θ_D / T_F = 0.026 $ of BiH4 is typical for pure metals and A-15 alloys. This implies that the BiH4, is the first hydride superconductor which complains with the Migdal's theorem.

cond-mat.supr-con↗

arXiv2024

Debye temperature, electron-phonon coupling constant, and three-dome shape of crystalline strain as a function of pressure in highly compressed La$_3$Ni$_2$O$_{7-δ}$

Besides ongoing studies of phase structural transitions, pairing mechanism, and physical properties of recently discovered highly compressed high-temperature superconductor La$_3$Ni$_2$O$_{7-δ}$, here we explored a possibility for the electron-phonon pairing mechanism as an origin of the superconducting state and determined the microcrystalline strain, $ε(P)$, in high pressure $Fmmm$-phase, and low-pressure $Ammm$-phase of this nickelate. To do this, we analyzed temperature dependent resistance and extracted pressure dependent Debye temperature, $Θ_D(P)$, in La$_3$Ni$_2$O$_{7-δ}$ with an approximate value of $Θ_D(P = 25 GPa) = 550$ $K$. From this we established that the La$_3$Ni$_2$O$_{7-δ}$ is strong-coupled superconductor with the electron-phonon coupling constant $λ_{e-ph}(P=22.4 GPa) = 1.75$. This value is close to $λ_{e-ph} = 1.70$ of ambient pressure superconductors $Nb_3R (R = Sn, Al)$. To address ongoing discussion that the lattice strain can be the origin for the emergence of high-temperature superconductivity in the La$_3$Ni$_2$O$_{7-δ}$, we determined the microcrystalline strain, $0.011 < ε(P)$, in the high-pressure $Fmmm$-phase, and $ε(P) < 0.011$ of low-pressure $Fmmm$-phase. Our analysis showed that $ε(P)$ has three-dome shape in the pressure range of $1.6 GPa < P < 41.2 GPa$. One of these two $ε(P)$ deeps at $P \approx 15 GPa$ coincides with the pressure at which the $Ammm$-phase into the $Fmmm$-phase phase transition occurs. Based on our analysis, we proposed probable condition to observe the zero-resistance state in La$_3$Ni$_2$O$_{7-δ}$.

cond-mat.supr-con↗

arXiv2024

Revaluation of the lower critical field in superconducting H$_3$S and LaH$_{10}$ (Nature Comm. 13, 3194, 2022)

In our paper [1], we studied the magnetic response of H$_3$S and LaH$_{10}$ superconductors to an applied magnetic field using Superconducting Quantum Interference Device (SQUID) magnetometry. Hirsch, in his comment [2], highlighted an inconsistency in the data averaging procedure while questioning whether high-Tc hydrides are superconductors at all. We accept the criticism regarding our method of extracting the penetration field HP from the original data. Our SQUID magnet becomes noisy at high magnetic fields, which necessitated the smoothing of a small portion of the data. To eliminate any data processing issues, we have performed an alternative data analysis that does not require data smoothing to estimate the penetration field Hp values. The formulation of the analysis is identical to the one widely used for determining critical currents in superconductors3. Recently, it has been shown to work effectively for extracting Hp and the lower critical field Hc1 from DC magnetization data4. The Hp values of the present analysis are consistent with those published in our original work1. We wish to emphasize very clearly that the criticism pertains to the secondary issue of determining Ginzburg-Landau parameters for these hydride superconductors and does not undermine the validity of the existence of hydride superconductivity. Indeed, as part of our paper1, we also published m(H) curves demonstrating the virgin curve (about which the analysis issues were raised) followed by magnetic hysteretic loops that have the classic form of the hysteresis curves of superconductors. Above Tc, in both H$_3$S and LaH$_{10}$, the hysteresis is absent. We make all the data available so that readers can judge for themselves.

cond-mat.supr-con↗

arXiv2024

A-15 type superconducting hydride $La_4H_{23}$: Nanograined structure with low strain, strong electron-phonon interaction, and moderate level of nonadiabaticity

For seven decades by A-15 superconductors we meant metallic $A_3B$ alloys (where A is a transition metal, and B is groups IIIB and IVB element) discovered by Hardy and Hulm (Phys. Rev. 89, 884 (1953)). Nb3Ge exhibited the highest superconducting transition temperature, $T_c = 23 K$, among these alloys. One of these alloys, $Nb_3Sn$, is primary material in modern applied superconductivity. Recently Guo et al (arXiv:2307.13067) extended the family of superconductors where the metallic ions arranged in the beta tungsten (A-15) sublattice by observation of $T_{c,zero} = 81 K$ in $La_4H_{23}$ phase compressed at $P = 118 GPa$. Despite the $La_4H_{23}$ has much lower $T_c$ in comparison with near-room-temperature superconducting $LaH_{10}$ phase ($T_{c,zero} = 250 K$ at $P = 200 GPa$) discovered by Drozdov et al (Nature 569, 531 (2019)), the $La_4H_{23}$ holds the record high $T_c$ within A-15 family. Cross et al (Phys. Rev. B 109, L020503 (2024)) confirmed the high-temperature superconductivity in the compressed $La_4H_{23}$. In this paper, we analyzed available experimental data measured in $La_4H_{23}$ and found that this superconductor exhibits nanograined structure, 5.5 nm < D < 35 nm, low crystalline strain < 0.003, high electron-phonon coupling constant, 1.5 < $λ_{e-ph}$ < 2.55, and moderate level of the nonadiabaticity $Θ_{D}/T_{F}$. We found that derived $Θ_{D}/T_{F}$ and $T_c/T_F$ values for the $La_4H_{23}$ phase are similar to the ones in cuprates, pnictides, and near-room-temperature superconductors $H_3S$ and $LaH_{10}$, which implies that the $La_4H_{23}$ phase falls to unconventional superconductors band in the Uemura plot.

cond-mat.supr-con↗

arXiv2024

In-Field Transport Critical Currents in Superhydride Superconductors: Highly-Compressed CeH$_9$

Experimental discovery of near-room-temperature superconductivity in highly compressed hydrogen sulfate started a new era in superconductivity. To date, researchers have made the discovery of dozens of superconducting hydride phases with transition temperatures above LN2. While the primary focus of the research in this field is to determine fundamental superconducting parameters of these superconductors, here we revealed primary applied property of these superconductors which is the field dependence of transport critical current, $I_{c}(B,T)$. We analyzed reported $V(I)$ curves for a highly compressed CeH$_9$ sample with $T_c = 70$ $K$, and showed that this hydride exhibits practically identical $I_{c}(B,T)$ to HTS 1G-wire (Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_{11}$. Additionally, we demonstrated that the $n$-value in CeH$_9$ superhydride follows a nearly identical dependence on critical current, as observed in Nb$_3$Sn conductors.

cond-mat.supr-con↗

arXiv2023

Broadening of In-Field Superconducting Transitions in Hydrides

J. E. Hirsch and F. Marsiglio in their publication, Phys. Rev. B 103, 134505 (2021), assert that hydrogen-rich compounds do not exhibit superconductivity. Their argument hinges on the absence of broadening of superconducting transitions in applied magnetic fields. We argue, that this assertion is incorrect, as it relies on a flawed analysis and a selective and inaccurate report of published data, where data supporting the authors' perspective are highlighted while data demonstrating clear broadening are disregarded.

cond-mat.supr-con↗

arXiv2023

Is MgB$_2$ a superconductor?

Hirsch and Marsiglio, in their recent publication (J. Supercond. Nov. Mag. 35, 3141-3145 (2022)), assert that experimental data on the trapping of magnetic flux by hydrogen-rich compounds clearly demonstrate the absence of superconductivity in hydrides at high pressures. We argue that this assertion is incorrect, as it relies on the wrong model coupled with selective manipulations (hide/delete) of calculated datasets and ignores the reference measurements after the release of pressure. A critical examination of the authors' claim of having performed fitting of experimental data to the model reveals that, in fact, the authors conducted simulations where all free parameters were fixed. Importantly, an application of the Hirsch-Marsiglio model to MgB$_2$ leads to the conclusion that it is not a superconductor.

cond-mat.supr-con↗

arXiv2023

Strain-Hardening Stages and Structure Evolution in Pure Niobium and Vanadium upon High Pressure Torsion

High pressure torsion (HPT) is one of the ways to form nanostructured materials with high strength properties. However, HPT hardening mechanisms vary from material to material and are poorly understood for some BCC metals, particularly niobium and vanadium. This work aims to identify strain hardening stages for Nb and V metals during HPT. Two approaches have been used to identify the deformation stages during high pressure torsion. The approaches are based on the application of a "piecewise" model, taking into account the different deformation mechanisms that determine the type of the forming structure, and on the analysis of the hardness vs. true strain dependence according to the $H$$-$${e}^{0.5}$ law. We compared the identified stages with the results of the electron microscopic study of the structure. Both models describe well the structural changes observed microscopically in HPT-deformed niobium. However, we have shown that only the piecewise model gives an adequate description of the stages of structure development in vanadium. We have provided an explanation for the observed difference in the behavior of niobium and vanadium upon HPT.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying interaction mechanism in infinite layer nickelate superconductors

The relationship between the long-range antiferromagnetic order in cuprates and the high-temperature superconductivity in these compounds represents unresolved, nearly four-decades long scientific problem. Because recently discovered nickelate superconductors are crystallographical counterparts of cuprates, many properties and difficulties into describing these compounds are common to both families. Recently, Fowlie et al (2022 Nature Physics 18 1043) aimed to detect the antiferromagnetic order in $R_{1-x}Sr_{x}NiO_{2}$ (R = Nd, Pr, La, x ~ 0.2) films by using the muon spin rotation (muSR) technique. This research group reported on the existence of short-range antiferromagnetic order in all studied nickelates. Here, we aimed to reveal the existence of this interaction in the same nickelate films by analyzing the temperature dependent resistivity, $ρ(T)$, reported by the same research group. Global $ρ(T)$ data fits to the advanced Bloch-Grüneisen model showed that each of R1-xSrxNiO2 compounds can be characterized by a unique power-law exponent, p (where p=2 for the electron-electron scattering, p=3 for the electron-magnon scattering, and p=5 for the electron-phonon scattering), and global characteristic temperature, $T_ω$ (which has the meaning of the Debye temperature at p=5). We found that p=2.0 in Nd- and Pr-based compounds, and p=1.3 for La-based compound. The latter value does not have any interpretation within established theoretical models. We also analyzed $ρ(T)$ data for $Nd_{1-x}Sr_{x}NiO_{2}$ (0.125 < x < 0.325) reported by Lee et al (2022 arXiv2203.02580). Because our analysis showed that p-values in nickelates are remarkably different from p=3, we call for the developent of a new theoretical model to describe $ρ(T)$ in materials exhibiting a short-range antiferromagnetic order.

cond-mat.supr-con↗

arXiv2023

Quantifying the nonadiabaticity strength constant in recently discovered highly-compressed superconductors

Superconductivity in highly-pressurized hydrides became primary direction for the exploration of fundamental upper limit for the superconducting transition temperature, Tc, after Drozdov et al (Nature 2015, 525, 73) discovered superconducting state with $T_c=203 K$ in highly-compressed sulphur hydride. To date several dozens of high-temperature superconducting polyhydrides have been discovered. In addition, recently, it was reported that highly-compressed titanium and scandium exhibit record-high $T_c$ (up to 36 K), which is by manifold exceeded $T_c=9.2 K$ of niobium, which is the record high-$T_c$ ambient pressure metallic superconductor. Here we analysed experimental data on for recently discovered high-pressure superconductors (which exhibit high transition temperatures within their classes): elemental titanium (Zhang et al, Nature Communications 2022; Liu et al, Phys. Rev. B 2022), $TaH_3$ (He et al, Chinese Phys. Lett. 2023), $LaBeH_8$ (Song et al, Phys. Rev. Lett. 2023), and black (Li et al, Proc. Natl. Acad. Sci. 2018) and violet (Wu et al, arXiv 2023) phosphorous, to reveal the nonadiabaticity strength constant, $T_θ/T_F$ (where $T_θ$ is the Debye temperature, and $T_F$ the Fermi temperature) in these superconductors. The analysis showed that $δ$-phase of titanium and black phosphorous exhibit the $T_θ/T_F$ which are nearly identical to ones associated in A15 superconductors, while studied hydrides and violet phosphorous exhibit the constants in the same ballpark with $H_3S$ and $LaH_{10}$.

cond-mat.supr-con↗

arXiv2023

Characteristic length for pinning force density in $Nb{_3}Sn$

The pinning force density $F{_p}(J{_c},B)=J{_c} \times B$ (where $J_c$ is the critical current density and $B$ is the magnetic field) is one of the main parameters that characterize the resilience of a superconductor to carry a dissipative-free transport current in an applied magnetic field. Kramer (1973 J.Appl.Phys. 44 1360), and Dew-Hughes (1974 Phil.Mag. 30 293) proposed a widely used scaling law for the pinning force density amplitude: $F{_p}(B)=F{_{p,max}}((p+q){^{(p+q)}}/({p^p}{q^q}))(B/B_{c2}){^p}(1-B/B{_{c2}})^q$, where $F{_{p,max}}$, $B{_{c2}}$, $p$, and $q$ are free-fitting parameters. Since the late 1970-s till now, several research groups have reported experimental data on the dependence of $F_{p,max}$ on the average grain size, $d$, in $Nb{_3}Sn$-based conductors. Godeke (2006 Superc.Sci.Techn. 19 R68) proposed that the dependence obeys the law $|F{_{p,max}}(d)|=A \times ln(1/d)+B $. However, this scaling law has several problems, for instance, the logarithm is taken from a non-dimensionless variable, and $|F{_{p,max}}(d)|< 0 $ for large grain sizes and $|F{_{p,max}}(d)|\rightarrow \infty $ for $d \rightarrow 0$. Here we reanalysed the full inventory of publicly available $|F{_{p,max}}(d)|$ data for $Nb{_3}Sn$ conductors and found that the dependence can be described by $F_{p,max}(d)= F_{p,max}(0)exp(-d/δ)$ law, where the characteristic length, $δ$, varies within a remarkably narrow range, that is, $δ=(175 \pm 13) nm$, for samples fabricated by different technologies. The interpretation of the result is based on the idea that the in-field supercurrent flows within a thin surface layer (thickness of $δ$) near the grain boundary surfaces. An alternative interpretation is that $δ$ represents characteristic length of the exponential decay flux pinning potential from the dominant defects in $Nb{_3}Sn$ superconductors, which are grain boundaries.

cond-mat.supr-con↗

arXiv2023

Intrinsic coherence length anisotropy in nickelate, and some pnictide, and chalcogenide superconductors

Nickelate superconductors, ${R_{1-x}}{A_x}Ni{O_2}$ (where R is a rare earth metal and A = Sr, Ca), experimentally discovered in 2019 exhibit many unexplained mysteries as the existence of a superconducting state with $T_c$ up to 18 K in thin films and its absence in bulk materials. Another unexplained mystery of nickelates is their temperature-dependent upper critical field, $B_{c2}(T)$, which can be nicely fitted to two-dimensional (2D) models; however the deduced film thickness, $d_{sc,GL}$, exceeds the physical film thickness, $d_{sc}$, by a manifold. To address the latter, it should be noted that 2D models assume that $d_{sc}$ is less than the in-plane, $ξ_{ab}(0)$, and out-of-plane, $ξ_{c}(0)$, ground state coherence lengths, respectively, and, in addition, that the inequality $ξ_{c}(0) < ξ_{ab}(0)$ satisfies. Analysis of the reported experimental $B_{c2}(T)$ data showed that at least one of these conditions does not satisfy for ${R_{1-x}}{A_x}Ni{O_2}$ films. This implies that nickelate films are not 2D superconductors, even despite though that the superconducting state is observed only in thin films. Based on this, here we proposed analytical three dimensional (3D) model for global data fit of in-plane and out-of-plane $B_{c2}(T)$ in nickelates. The model is based on a heuristic expression for temperature dependent coherence length anisotropy, $γ_ξ(T)$. The proposed expression for $γ_ξ(T)$, perhaps, has a much broader application because it has been successfully applied to some bulk pnictide and chalcogenide superconductors.

cond-mat.supr-con↗

arXiv2023

$d$-wave superconductivity as a model for diborides apart MgB$_2$

Recently, Pei et al. (arXiv2105.13250) reported that ambient pressure $β$-MoB$_2$ exhibits a phase transition to $α$-MoB$_2$ (space group: $P6/mmm$) at pressure P~70 GPa and this high-pressure phase is a high-temperature superconductor exhibited $T_c=32 K$ at P~110 GPa. Despite $α$-MoB$_2$ has the same crystalline structure as ambient pressure MgB$_2$2 and the $T_c$'s of $α$-MoB$_2$ and MgB$_2$ are very close, the first principles calculations showed that in $α$-MoB$_2$ the states near the Fermi level, $ε_F$, are dominated by the $d$-electrons of Mo atoms, while in MgB$_2$ the $p$-orbitals of boron atomic sheets dominantly contribute to the states near the $ε_F$. More recently, Hire et al. (arXiv2212.14869) reported that the $P6/mmm$-phase can be stabilized at ambient pressure in $Nb_{1-x}Mo_{x}B_{2}$ solid solutions, and these ternary alloys exhibit $T_c=8 K$. In addition, Pei et al. (Sci. China-Phys. Mech. Astron. 65, 287412 (2022)) showed that compressed WB$_2$ exhibits $T_c=15 K$ at P~121 GPa. Here, we analyzed experimental data reported for $P6/mmm$-phases of $Nb_{1-x}Mo_{x}B_{2}$ (x = 0.25; 1.0) and highly-compressed WB$_2$, and showed that these three phases exhibit $d$-wave superconductivity. We also deduced the gap-to-transition temperature ratio for these three phases. We found that $Nb_{0.75}Mo_{0.25}B_{2}$ exhibits high strength of nonadiabaticity, which is quantified by the ratio of $T_θ/T_F=3.5$, which is by one order of magnitude exceeds the ratio in MgB$_2$, $α$-MoB$_2$, WB$_2$, pnictides, cuprates, and highly-compressed hydrides.

cond-mat.supr-con↗

arXiv2022

Quantifying nonadiabaticity in major families of superconductors

The classical Bardeen-Cooper-Schrieffer and Eliashberg theories of the electron-phonon-mediated superconductivity are based on the Migdal theorem, which is an assumption that the energy of charge carriers, $k{_B}T{_F}$, significantly exceeds the phononic energy, $\hbar{ω{_D}} $, of the crystalline lattice. This assumption, which is also known as adiabatic approximation, implies that the superconductor exhibits fast charge carriers and slow phonons. This picture is valid for pure metals and metallic alloys because these superconductors exhibit $\hbar{ω{_D}}$/$k{_B}T{_F}<0.01$. However, n-type doped semiconducting $SrTiO_3$ was the first superconductor which beyond this adiabatic approximation, because this material exhibits $\hbar{ω{_D}}$/$k{_B}T{_F} $~$ 50$. There is growing number of newly discovered superconductors which also beyond the adiabatic approximation. Here, leaving apart pure theoretical aspects of nonadiabatic superconductors, we classified major classes of superconductors (including, elements, A-15 and Heusler alloys, Laves phases, intermetallics, noncentrosymmetric compounds, cuprates, pnictides, highly-compressed hydrides and oxygen, and magic-angle twisted bilayer graphene) by the strength of nonadiabaticity (for which the ratio of the Debye temperature to the Fermi temperature, $T{_θ}/T{_F}$, is used as a criterion for the nonadiabaticity) versus the superconducting transition temperature, $T{_c}$. The discussion of this classification scheme and its relation to other known classification counterparts is given.

cond-mat.supr-con↗

arXiv2022

Fermi-liquid nonadiabatic highly-compressed cesium iodide superconductor

Experimental discovery that compressed sulphur hydride exhibits superconducting transition temperature Tc=203 K (Drozdov et al 2015 Nature 525 73) sparked intensive studies of superconducting hydrides. However, this discovery was not a straight forward experimental examination of theoretically predicted phase, instead it was nearly five-decade long experimental quest for superconductivity in highly-compressed matters, which varied from pure elements (hydrogen, oxygen, sulphur, lithium), cuprates, and hydrides (SiH4, YH3, and AlH3), to semiconductors and ionic salts. One of these salts was cesium iodide, CsI, which converts into metallic state at P=115 GPa and at P=180 GPa this compound exhibits the onset of the superconducting transition temperature Tc~2 K (Eremets et al 1998 Science 281 1333). Detailed first principles calculations (Xu et al 2009 Phys Rev B 79 144110) showed that within Eliashberg theory of superconductivity, the CsI should exhibits Tc=0.03 K at pressure P=180 GPa, which is by two orders of magnitude lower than the observed value. In attempt to understand the nature of this discrepancy, here we analyzed temperature dependent resistance in compressed CsI and found that this compound is perfect Fermi liquid metal which exhibits extremely high (~ 17) ratio of the Debye temperature, Td, to the Fermi energy, Tf. This implies that direct utilization of the Eliashberg theory is incorrect for this compound, because the theory valid for the ratio Td/Tf << 1. We also showed that highly-compressed CsI exhibits the ratio of Tc/Tf = 0.04-0.07 and it falls in unconventional superconductors band in the Uemura plot.

cond-mat.supr-con↗