arXiv subjects

A. Chroneos

Publications and source records attributed to A. Chroneos.

7 recordsLinked to original sources

arXiv2025

Influence of strong electron irradiation on fluctuation conductivity and pseudogap in YBa$_2$Cu$_3$O$_7$--$\delta$ single crystals

The effect of high-energy electron irradiation on the temperature dependences of the resistivity $\rho(T)$, fluctuation conductivity (FLC), and pseudogap (PG) $\Delta^{*}(T)$ of YBa$_2$Cu$_3$O$_7$--$\delta$ (YBCO) single crystals without twins was studied. Irradiation causes a linear increase in $\rho(T)$ and a decrease in the superconducting transition temperature $T_c$ with dose $\phi$. For small $\phi$, the reduction of $T_c$ follows the Abrikosov--Gorkov (AG) pair-breaking theory, while for large $\phi$ it is described by the Emery--Kivelson (EK) model, where quantum phase fluctuations dominate. At $\phi_3 = 2.5 \times 10^{19}$ e/cm$^2$, which corresponds to the AG--EK crossover, the spacing between CuO$_2$ planes $d_{01}$, the coherence length $\xi_c(0)$, and the fluctuation region $T_{\mathrm{fl}}$ increase sharply, and the two-dimensional Maki--Thompson (2D--MT) contribution is replaced by the Aslamazov--Larkin (2D--AL) term. Surprisingly, no signatures of the crossover appear in $\rho(\phi)$ or $T_c(\phi)$. At the same $\phi_3$, a sharp rise in the pseudogap opening temperature $T^{*}$ and in $\Delta^{*}$ indicates a possible reduction of the density of states. With further increase in $\phi$, both PG parameters and their energy scale decrease markedly, and $\Delta^{*}(T)$ acquires an unusual form. However, at $\phi_5 = 5.6 \times 10^{19}$ e/cm$^2$, the temperature dependences of FLC and PG again show behavior typical of well-structured YBCO, regardless of defect density.

cond-mat.supr-con↗

arXiv2022

Influence of electron irradiation on fluctuation conductivity and pseudogap in $YBa_2Cu_3O_{7-δ}$ single crystals

The effect of electron irradiation with the energy of 2.5 MeV on the temperature dependences of the resistivity of an optimally doped $YBa_2Cu_3O_{7-δ}$ single crystal has been studied. The temperature dependences of both fluctuation conductivity $σ^\prime(T)$ and the pseudogap $Δ^*(T)$ on irradiation dose $ϕ$ have been calculated within the local pair model. Here we show that with an increase in $ϕ$, the value of $ρ(300 K)$ increases linearly, while $T_c$ decreases linearly. Concurrently, the value of $ρ{100 K}$ increases nonlinearly, demonstrating a feature for $ϕ_3$ = 4.3 10$^{18}$ e/$cm^2$, which is also observed in the number of other dose-dependent parameters.Regardless of the irradiation dose, in the temperature range from $T_c$ up to $T_{01}$, $σ^\prime(T)$ obeys the classical fluctuation theories of Aslamazov$-$Larkin and Maki$-$Thompson, demonstrating 3D$-$2D crossover with increasing temperature. The crossover temperature $T_0$ makes it possible to determine the coherence length along the $c$-axis, $ξ_c(T)$, which increases by $\approx 3$ times under irradiation. Furthermore, the range of superconducting fluctuations above $T_c$ also noticeably increases.At $ϕ$ = 0, the dependence $Δ^*(T)$ typical for single crystals containing pronounced twin boundaries is observed with a maximum at $T_{pair}$ ~ 120 K and a distinct minimum at $T = T_{01}$. It was determined for the first time that at $ϕ_3$ = 4.3 10$^{18}$ the shape of $Δ^*(T)$ changes strongly and becomes the same as in optimally doped $YBa_2Cu_3O_{7-δ}$ single crystals with a very low pseudogap opening temperature $T^*$ and noticeably reduced $T_{pair}$, while at $T_c(ϕ)$ there are no singularities.

cond-mat.supr-con↗

arXiv2019

Structural, elastic, thermal and lattice dynamic properties of new 321 MAX phases

A new series of MAX family designated as 321 phases are recently reported with Nb3As2C, V3As2C, Nb3P2C and Ta3P2C. Most of the physical properties of these new MAX phase compounds are unexplored and the present study aims to investigate their structural, elastic, thermal, and lattice dynamical properties. Their mechanical and dynamical stabilities are examined. All the phases are elastically anisotropic and brittle in nature. The Debye temperatures and lattice thermal conductivities have been calculated. Lattice dynamical features are investigated in detail and infrared and Raman modes are identified. High melting temperature of these compounds are favorable for their applications at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2019

Engineering Transport in Manganites by Tuning Local Non-Stoichiometry in Grain Boundaries

Interface-dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In particular, it has been proved that strain-induced defects in grain boundaries of manganites deeply impact their functional properties by boosting their oxygen mass transport while abating their electronic and magnetic order. In this work, the origin of these dramatic changes is correlated for the first time with strong modifications of the anionic and cationic composition in the vicinity of strained grain boundary regions. We are also able to alter the grain boundary composition by tuning the overall cationic content in the films, which represents a new and powerful tool, beyond the classical space charge layer effect, for engineering electronic and mass transport properties of metal oxide thin films useful for a collection of relevant solid state devices.

cond-mat.mtrl-sci↗

arXiv2017

Mechanical behavior, bonding nature, and defect processes of Mo2ScAlC2: a new ordered MAX phase

In the present study we employed density functional theory calculations to investigate the mechanical behavior, bonding nature and defect processes of the new ordered MAX phase Mo2ScAlC2. The mechanical stability of the compound is verified with its single crystal elastic constants. The new phase Mo2ScAlC2 is anticipated to be prone to shear along the crystallographic b and c axes, when a rational force is applied to the crystallographic a axis. The compressibility along the <001> direction under uniaxial stress is expected to be easier in Mo2ScAlC2. Additionally, the volume deformation should be easier in Mo2ScAlC2 than in the isostructural Mo2TiAlC2. Mo2ScAlC2 is predicted to behave in a brittle manner. Due to its higher Debye temperature, Mo2ScAlC2 is expected to be thermally more conductive than Mo2TiAlC2. The cross-slip pinning procedure should be significantly easier in Mo2ScAlC2 as compared to Mo2TiAlC2. The new ordered MAX phase Mo2ScAlC2 has a mixed character of strong covalent and metallic bonding with limited ionic nature. Both Mo-C and Mo-Al bonds are expected to be more covalent in Mo2ScAlC2 than those of Mo2TiAlC2. the level of covalency of Sc-C bond is somewhat low compared to a similar bond Ti-C in Mo2TiAlC2. Due to its reduced hardness, Mo2ScAlC2 should be softer and more easily machinable compared to Mo2TiAlC2. Fermi surface topology of the new compound is formed mainly due to the low-dispersive Mo 4d-like bands. The intrinsic defect processes reveal that the level of radiation tolerance in Mo2ScAlC2 is not as high as in other MAX phases such as Ti3AlC2.

cond-mat.mtrl-sci↗

arXiv2014

The thermodynamics of hydride precipitation: the importance of entropy, enthalpy and disorder

The thermodynamics of H/α-Zr solid solution and zirconium hydride phases were studied using density functional theory. Disorder in ζ, γ and δ hydrides and solid solutions were modelled using a statistically significant number of randomly generated structures in combination with special quasi-random structures and solid solutions with a range of concentrations. This is used in conjunction with a calculation of thermodynamic parameters of the system, including the temperature dependent sensible enthalpy, configurational entropy and vibrational entropy of the different crystals in the system, developed from phonon density of states. It was found that precipitation of hydrides is not thermodynamically favourable for Zr-H solid solutions containing less than 300 ppm H, suggesting that a mechanism must cause local concentration of H atoms to a greater amount than found globally in experimental samples containing hydrides. Temperature drives the reaction in the direction of solution, primarily due to entropic effects. Generally, γ hydride is the most stable phase, although it is very close in energy to the δ-phase. The sensible enthalpy of precipitation assists in stabilising HCP hydrides, and the configurational entropy change during precipitation favours FCC hydrides. None of the thermodynamic contributions are found to be negligible in driving precipitation.

cond-mat.mtrl-sci↗

arXiv2010

Evolution of the fishtail-effect in pure and Ag-doped MG-YBCO

We report on magnetic measurements carried out in a textured YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$(Cu$_{1-x}$Ag$_x$)$_3$O$_{7-δ}$ (at $x \approx$ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4~$<T/T_c<$~0.8 for $\textbf{H}\parallel c$. The origin of the FE arises for the competition between surface barrier and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate $R$. The value $H_{max}$ for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.

cond-mat.supr-con↗