arXiv subjects

A. L. Solovjov

Publications and source records attributed to A. L. Solovjov.

At least 19 recordsLinked to original sources

arXiv2026

Hydrostatic pressure effect on the pseudogap in Y0.77Pr0.23Ba2Cu3O7-δsingle crystals

We studied the changes of resistivity $ρ(T)$, superconducting transition temperature $T_c$, fluctuation conductivity $σ'(T)$, and pseudogap $Δ^*(T)$ of $Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}$ (YPrBCO) ($x = 0.23$) single crystal under hydrostatic pressure (HP) up to $\sim 1.1$ GPa. Defects created by magnetic inclusions of $PrBa_2Cu_3O_{7-δ}$ (PrBCO) were found to play a significant role in the behavior of the sample. The largest decrease in $ρ(T)$ was found depending on HP at a rate of $d\lnρ(100~K)/dP = -(29 \pm 0.2) \% \cdot GPa^{-1}$. This indicates that the mechanisms of the influence of HP on the $ρ(T)$ and $T_c$ of YPrBCO and $YBa_2Cu_3O_{7-δ}$ single crystals are different. From the analysis of the pseudogap, it was revealed that the mechanism of the interaction of charge carriers with magnetic PrBCO impurities changes three times with increasing HP. Relatively low HP, up to $\sim 0.5$ GPa, promotes the formation of defects caused by PrBCO magnetic inclusions. Starting from 0.6 GPa, the HP neutralizes the influence of magnetic impurities, and the magnetic maximum on $Δ^*(T)$ disappears. Above $\sim 1.0$ GPa, HP aligns the magnetic moments of PrBCO, and the magnetic maximum reappears on $Δ^*(T)$, more pronounced than at $P = 0$.

cond-mat.supr-con↗

arXiv2026

Specific features of the magnetic-field dependences of electrical resistivity in Bi--Mn solid solutions with low Mn content

For the first time, the field dependences of the magnetoresistance of a textured polycrystalline Bi$_{88.08}$Mn$_{11.92}$ sample were investigated in the $H \perp I$ and $H \parallel I$ configurations, and the results were compared with those previously obtained for the solid solution Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$ with a lower manganese concentration. It was demonstrated that the magnetoresistance field dependencies for Bi$_{88.08}$Mn$_{11.92}$ differ significantly from those of Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$ below 100 K, becoming nearly identical as the temperature approaches room temperature. It was established that the maximum relative magnetoresistance values for Bi$_{88.08}$Mn$_{11.92}$ are observed at a magnetic field of 90 kOe and a temperature of 100 K in both the $H \perp I$ and $H \parallel I$ configurations, amounting to 3170\% and 380\%, respectively. These values are significantly lower than those previously reported for the solid solution containing less manganese (Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$), which were 3877\% and 742\%, respectively. Analysis of the results indicates that differences in field dependence among the studied materials are associated with varying amounts of magnetic $α$-BiMn. The anomalous magnetoresistance behavior, as compared with that of pure bismuth, is attributed to the influence of internal magnetism on electrical transport within the bismuth matrix of the studied solid solutions.

cond-mat.mtrl-sci↗

arXiv2025

Study of fluctuation conductivity in YBa$_2$Cu$_3$O$_7$--$δ$ films in strong magnetic fields

We report the effect of the \emph{ab}-plane magnetic field $B$ up to $8\,\mathrm{T}$ on the resistivity $ρ(T)$ and fluctuation conductivity $σ'(T)$ in YBa$_2$Cu$_3$O$_{7-δ}$ thin films. As expected, up to $\sim 2.5\,\mathrm{T}$ the magnetic field monotonously increases $ρ$, the width of the resistive transition $ΔT_c$, and the coherence length along the $c$ axis, $ξ_c(0)$, but decreases both $T_c$ and the range of superconducting (SC) fluctuations $Δ_{\mathrm{fl}}$. The fluctuation conductivity exhibits a crossover at characteristic temperature $T_0$ from the 3D Aslamazov--Larkin (AL) theory near $T_c$ to the 2D fluctuation theory of Maki--Thompson (MT). However, at $B = 3\,\mathrm{T}$, the MT term is completely suppressed, and above $T_0$ $σ'(T)$ is unexpectedly described by the fluctuation contribution of 2D AL, suggesting the formation of a two-dimensional vortex lattice in the film under the action of a magnetic field. At the same time, $ΔT_{\mathrm{fl}}$ sharply increases by a factor of about 7, and $ξ_c(0)$ demonstrates a very unusual dependence on $T_c$ when $B$ increases above $3\,\mathrm{T}$. Our results demonstrate the possibility of the formation of a vortex state in YBCO and its evolution with increasing $B$.

cond-mat.supr-con↗

arXiv2025

Evolution of the pseudogap and excess conductivity of YBa$_2$Cu$_3$O$_{7-δ}$ single crystals in the course of long-term aging

The temperature dependences of both fluctuation conductivity (FLC) $σ^\prime(T)$ and pseudogap (PG) $Δ^*(T)$ derived from measurements of resistivity $ρ(T)$ of an optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystal subjected to long-term storage have been studied. The as-grown sample S1 exhibits characteristics typical of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystals containing twins and twin boundaries. Analysis of both FLC and PG showed an unexpected improvement in all characteristics of the sample after 6 years of storage (sample S2), indicating that the effect of twin boundaries is somehow limited. After 17 years of storage, all characteristics of the sample changed dramatically, which indicates a strong influence of internal defects formed during the aging process. For the first time, the temperature dependences of both FLC and PG were obtained after 17 years of storage.

cond-mat.supr-con↗

arXiv2025

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

The effect of high-energy electron irradiation on the temperature dependences of the resistivity $ρ(T)$, fluctuation conductivity (FLC), and pseudogap (PG) $Δ^{*}(T)$ of YBa$_2$Cu$_3$O$_7$--$δ$ (YBCO) single crystals without twins was studied. Irradiation causes a linear increase in $ρ(T)$ and a decrease in the superconducting transition temperature $T_c$ with dose $ϕ$. For small $ϕ$, the reduction of $T_c$ follows the Abrikosov--Gorkov (AG) pair-breaking theory, while for large $ϕ$ it is described by the Emery--Kivelson (EK) model, where quantum phase fluctuations dominate. At $ϕ_3 = 2.5 \times 10^{19}$ e/cm$^2$, which corresponds to the AG--EK crossover, the spacing between CuO$_2$ planes $d_{01}$, the coherence length $ξ_c(0)$, 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 $ρ(ϕ)$ or $T_c(ϕ)$. At the same $ϕ_3$, a sharp rise in the pseudogap opening temperature $T^{*}$ and in $Δ^{*}$ indicates a possible reduction of the density of states. With further increase in $ϕ$, both PG parameters and their energy scale decrease markedly, and $Δ^{*}(T)$ acquires an unusual form. However, at $ϕ_5 = 5.6 \times 10^{19}$ 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↗

arXiv2025

Effect of a magnetic field up to 9 T on the temperature dependence of the pseudogap in YBa$_2$Cu$_3$O$_{7-δ}$ films

The work analyzes the effect of a magnetic field $B$ directed along the $c$ axis ($B \parallel c$) up to 9~T on the resistivity $ρ(T)$, fluctuation conductivity (FLC) $σ'(T)$ and pseudogap $Δ^*(T)$ in thin films of YBa$_2$Cu$_3$O$_{7-δ}$ with a critical temperature of the superconducting transition $T_c = 88.8$~K. In contrast to previous work, where the magnetic field was directed along the $ab$ plane ($B \parallel ab$), the influence of the field on the sample is stronger due to the contribution of both spin--orbit and Zeeman effects. It was found that the BEC--BCS transition temperature, $T_{\mathrm{pair}}$, which corresponds to the maximum of the $Δ^*(T)$ dependence, shifts to the region of lower temperatures with increasing $B$, and the maximum value of $Δ^*(T_{\mathrm{pair}})$ decreases in fields $B > 5$~T. It was found that with increasing field, the low-temperature maximum near $T_0$ is smeared and disappears at $B > 1$~T. In addition, above the Ginzburg temperature $T_G$, for $B > 1$~T, a minimum appears on $Δ^*(T)$ at $T_{\min}$, which becomes very pronounced with a subsequent increase in $B$. As a result, the overall value of $Δ^*(T_G)$ decreases noticeably, most likely due to the pair-breaking effect. At the same time, $ΔT_{\mathrm{fl}}$ and $ξ_c(0)$ increase sharply by approximately 3 times with increasing $B$ above 1~T. Our results confirm the possibility of the formation of a vortex state in YBa$_2$Cu$_3$O$_{7-δ}$ by a magnetic field and its evolution with increasing $B$.

cond-mat.supr-con↗

arXiv2024

Evolution of the pseudogap temperature dependence in YBa$_2$Cu$_3$O$_{7-δ}$ films under the influence of a magnetic field

The evolution of the temperature dependence of pseudogap $Δ$*(T) in optimally doped (OD) YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films with $T_{c}$ = 88.7 K under the influence of a magnetic field $B$ up to 8 T has been studied in detail. It has been established that the shape of $Δ$*(T) for various $B$ over the entire range from the pseudogap opening temperature $T$* to $T_{01}$, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature $T_{pair}$, which is typical for OD films and untwinned YBCO single crystals. $T$* was shown to be independent on $B$, whereas $T_{pair}$ shifts to the low temperature region along with increase of $B$, while the maximum value of $Δ$*($T_{pair}$) remains practically constant regardless of $B$. It was revealed that as the field increases, the low-temperature maximum near the 3D-2D transition temperature $T_{0}$ is blurred and disappears at $B$ > 5 T. Moreover, above the Ginzburg temperature $T_{G}$, which limits superconducting fluctuations from below, at $B$ > 0.5 T, a minimum appears on $Δ$*(T) at $T_{min}$, which becomes very pronounced with a further increase in the field. As a result, the overall value of $Δ$*(T) decreases noticeably most likely due to pair-breaking affect of a magnetic field. A comparison of $Δ$*(T) near $T_{c}$ with the Peters-Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from < > $\approx$ 0.31 at $B$ = 0 to < > $\approx$ 0.28 in the field 8 T. The observed behavior of $Δ$*(T) around $T_{min}$ is assumed to be due to the influence of a two-dimensional vortex lattice created by the magnetic field, which prevents the formation of fluctuating Cooper pairs near $T_{c}$.

cond-mat.supr-con↗

arXiv2023

Effects of annealing on the fluctuation conductivity and pseudogap in slightly doped $HoBa_2Cu_3O_{7-δ}$ single crystals

The effect of annealing at room temperature on the fluctuation conductivity (FLC) $σ^\prime(T)$ and pseudogap (PG) $Δ^*(T)$ in the basal ab plane of $ReBa_2Cu_3O_{7-δ}$ (Re = Ho) single crystals with a lack of oxygen has been studied. It is shown that at all stages of annealing, the FLC near $T_c$ can be described by the Aslamazov$-$Larkin and Maki$-$Thompson fluctuation theories, demonstrating a 3D$-$2D crossover with increasing temperature. The crossover temperature $T_0$ was used to determine the coherence length along the $c$-axis, $ξ_c(0)$=2.82 A. At the inter-mediate stage of annealing, an anomalous increase in 2D FLC was revealed, which is associated with the influence of uncompensated magnetic moments in $HoBa_2Cu_3O_{7-δ}$ (HoBCO): $μ_{eff, Ho}$=9.7$μ_{B}$. For the quenched sample S1, the temperature dependence of the PG has a shape typical of single crystals with a large number of defects. However, $Δ^*(T)$ has two small additional maxima at high temperature, which is a feature of HoBCO single crystals with pronounced twins and indicates the two-phase nature of the sample. Upon annealing, the shape of $Δ^*(T)$ noticeably changes, very likely due to an increase in the magnetic interaction (sample S2). More important is the change in the slope of the data at high temperatures, which has become about 3.5 times steeper. The ordering of the oxygen distribution due to the diffusion process during annealing somewhat compensates for the influence of magnetic interaction. Interestingly, the slope turns out to be the same as for FeAs-based superconductors, suggesting the possibility of the existence of spin density waves in HoBCO in the PG state.

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↗

arXiv2022

Temperature dependence of upper critical fields and coherence lengths for optimally-doped $YBa_2Cu_3O_{7-δ}$ thin films

We report the comprehensive comparative analysis of the upper critical magnetic fields $μ_{0}H_{c2}(0)$ obtained within Ginzburg-Landau and Werthamer-Helfand-Hohenberg theories for optimally-doped $YBa_2Cu_3O_{7-δ}$ thin films. For different orientations of the magnetic field, our calculations give 638 and 153 T for $μ_{0}H_{c2}(0)$, H || ab and $μ_{0}H_{c2}(0)$, H || c, respectively, when using $H_{c2}(0)$. For the first time, the temperature dependences of coherence lengths $ξ_{ab}(T)$ and $ξ_{c}(T)$ within proposed theories were determined using 50 and 90% criteria of the normal state resistivity value $ρ_{N}$. The Ginzburg-Landau 0.9$ρ_{N}$ approach gives $ξ_{ab}(0)$=11.8 A, and $ξ_{c}(0)$=3.0 A which are in a good agreement with literature data. The implications of very short coherence lengths in HTSCs are discussed.

cond-mat.supr-con↗

arXiv2021

Features of Excess Conductivity Behavior in a Magnetic Superconductor Dy$_{0.6}$Y$_{0.4}$Rh$_{3.85}$Ru$_{0.15}$B$_4$

The temperature dependencies of the excess conductivity $σ'(T)$ and possible pseudogap (PG), in a Dy$_{0.6}$Y$_{0.4}$Rh$_{3.85}$Ru$_{0.15}$B$_4$ polycrystal were studied for the first time. It was shown that $σ'(T)$ near T$_{c}$ is well described by the Aslamazov Larkin fluctuation theory, demonstrating a crossover with increasing temperature. Using the crossover temperature $T_0$, the coherence length along the c axis $ξ_c(0)$, was determined. Above the level of $T_{2D}>T_{0}$, an unusual dependence $σ'(T)$ was found, which is not described by the fluctuation theories in the range from $T_{0}$ to $T_{FM}$, at which a ferromagnetic transition occurs. The range in which superconducting fluctuations exist is apparently quite narrow and amounts to $ΔT_{fl}=2.8 K$. The resulting temperature dependence of the PG parameter $Δ^*(T)$ has the form typical of magnetic superconductors with features at $T_{max}=154 K$ and the temperature of a possible structural transition at $T_{s}=95 K$. Below $T_{s}$, dependence $Δ^*{T}$ has a shape typical for PG in cuprates, which suggests that the PG state can be realized in Dy$_{0.6}$Y$_{0.4}$Rh$_{3.85}$Ru$_{0.15}$B$_4$ in this temperature range. Comparison of $Δ^*(T)$ with the Peters Bauer theory made it possible to determine the density of local pairs ~0.35, near T$_{c}$, which is 1.17 times greater than in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-δ}$ single crystals.

cond-mat.supr-con↗

arXiv2020

Fluctuating Cooper pairs in FeSe at temperatures exceeding double Tc

Temperature dependencies of excess conductivity, Sigma', have been studied in detail for three FeSe_{0.94} textured polycrystalline samples prepared by partial melting and solid state reaction. It was revealed that both Sigma' and its temperature dependence are extremely sensitive to the method of sample preparation. Then, it was shown that in the range from the superconducting transition temperature Tc ~ 9 K up to the characteristic temperature T_01 ~ 19 K, Sigma'(T) obeys the classical fluctuation theories of Aslamazov-Larkin (AL) and Hikami-Larkin (Maki-Thompson (MT) term) pointing to the existence of fluctuating Cooper pairs in FeSe at temperatures exceeding double Tc. Like in cuprates, AL-MT crossover at T_0 < T_{01} is observed, which means the appearance of 3D-2D dimensional transition at this temperature. This allows us to determine the coherence length along the c-axis, Xi_c(0) ~ 3 A, and a set of additional samples' parameters, including the phase relaxation time, Tau_{Phi}, of fluctuating Cooper pairs, within a simple two-dimensional free-carrier picture. It was shown that Tau_{Phi} in FeSe coincides with that found for YBa2Cu3O7 suggesting that the nature of superconducting fluctuations is very similar for these high-temperature superconductors of different types.

cond-mat.supr-con↗

arXiv2018

Specific temperature dependence of pseudogap in YBa$_2$Cu$_3$O$_{7-δ}$ nanolayers

The pseudogap (PG) derived from the analysis of the excess conductivity $σ'(T)$ in superlattices and double-layer films of $YBa_2Cu_3O_{7-δ}$-$PrBa_2Cu_3O_{7-δ}$(YBCO-PrBCO), prepared by pulsed laser deposition, is studied for the first time. The $σ'(T)$ analysis has been performed within the local-pair (LP) model based on the assumption of the paired fermion (LPs) formation in the cuprate high-$T_c$ superconductors (cuprates) below the representative temperature $T^*\gg T_c$ resulting in the PG opening. Within the model, the temperature dependencies of the PG, $Δ^*(T)$, for the samples with different number of the PrBCO layers ($N_{Pr}$) were analyzed in the whole temperature range from $T^*$ down to $T_c$. Near $T_c$, excess conductivity was found to be perfectly described by the Aslamasov-Larkin and Hikami-Larkin (Maki-Thompson term) fluctuation theories, suggesting the presence of superconducting fluctuations in a relatively large (up to 15 K) temperature range above $T_c$. All sample parameters were found to change with increase of $N_{Pr}$, finally resulting in the appearance of the pronounced maximum of $Δ^*(T)$ at high temperatures. The result is most likely due to increasing influence of the intrinsic magnetism of PrBCO ($μ_{Pr}\approx 4\,μ_B$) and suggests the possibility to search in that way the change of interplay between the superconductivity and magnetism in cuprates.

cond-mat.supr-con↗

arXiv2018

Fluctuation conductivity due to the preformed local pairs

We investigate the properties of a system where the itinerant electrons coexist and interact with the preformed local pairs. Using the nonperturbative continuous unitary transformation technique we show that Andreev-type scattering between these charge carriers gives rise to the enhanced diamagnetic response and is accompanied by appearance of the Drude peak inside the pseudogap regime ω<2Δ_{pg}. Both thes effects are caused above the transition temperature T_c by the short-range superconducting correlations. In fact, the residual diamagnetism has been detected by the torque magnetometry in the lanthanum and bismuth cuprate superconductors at temperatures up to ~ 1.5T_c. In this work we show how the superconducting correlations can be observed in the a.c. and d.c. conductivity.

cond-mat.supr-con↗

arXiv2018

Peculiarities in the pseudogap behavior in optimally doped YBa2Cu3O7-δ single crystals under pressure up to 1GPa

The influence of the hydrostatic pressure P up to 0.95 GPa on the excess conductivity and the pseudogap Δ*(T) in optimally doped YBCO single crystals (T_c = 91.1K at ambient pressure) is investigated by electrical resistivity measurements. A pronounced enhancement of the pseudogap under pressure of dlnΔ*/dP= 0.32, which is only a factor of 1.12 smaller than in slightly doped single crystals, is revealed for the first time. This implies a somewhat more moderate increase of the coupling strength in optimally doped cuprates with increasing pressure. Simultaneously, the ratio 2Δ*(T_c)/k_BT_c=5 at P=0 GPa, which is typical for high-temperature superconductors with strong coupling, increases by 16% with increasing pressure. At the same time, the pressure effect on T_c is minor: dT_c/dP=+0.73 K/GPa, whereas dlnρ/dP=-17% 1/GPa is comparable with that in lightly doped YBCO single crystals. This suggests that the mechanisms of the pressure effect on ρ(T) and T_c are noticeably different. Independently of pressure, near T_c, the excess conductivity is well described by the Aslamazov-Larkin (3D-AL) and 2D Hikami-Larkin fluctuation theories, exhibiting a 3D-2D crossover with increasing temperature. However, the temperature interval T_c<T<T_{01}, in which excess conductivity obeys the classical fluctuation theories, is exceptionally narrow (~1.16 K). Nevertheless, a peculiarity at the temperature T_{01}, up to which the wave function phase stiffness in the superconductor is maintained, is clearly observed in the dependence Δ*(T). Below T_{01} a fast growth of Δ*(T) is revealed for the first time. It can be associated with a sudden increase of the superfluid density, n_s, that is the density of fluctuating Cooper pairs (short-range phase correlations) forming in the sample when T approaches T_c.

cond-mat.supr-con↗

arXiv2017

Hydrostatic-pressure effects on the pseudogap in slightly doped YBa2Cu3O7-δ single crystals

The influence of hydrostatic pressure up to P=1.05 GPa on resistivity R, excess conductivity $σ'(T)$ and pseudogap $Δ*(T)$ is studied in slightly doped single crystals of YBa2Cu3O7-$δ (Tc=49.2 K, δ~ 0.5)$. The critical temperature Tc is found to increase with increasing pressure at a rate dTc/dP = 5.1 K/GPa, while R(300)K decreases at a rate d(ln R)/dP = 0.19% 1/GPa. Near Tc, independently on pressure, the $σ'(T)$ is well described by the Aslamasov-Larkin and Hikami-Larkin fluctuation theories, demonstrating a 3D-2D crossover with increase of temperature. The crossover temperature To determines the coherence length along the c-axis $ξc(0) \sim 3.43 A$ at P=0, which is found to decrease with increasing P. At the same time, both, $Δ*$ and the BCS ratio $2Δ*/kTc$ increase with increasing hydrostatic pressure at a rate $d(ln Δ*)/dP= 0.36 1/GPa$, implying an increase of the coupling strength with increasing P. At low temperatures below Tp, the shape of the $Δ*(T)$ curve is found to be almost independent on pressure. At high temperatures, the shape of the $Δ*(T)$ curve changes noticeably with increasing P, suggesting a strong influence of pressure on the lattice dynamics. This unusual behavior is observed for the first time.

cond-mat.supr-con↗

arXiv2015

Fluctuation conductivity and possible pseudogap state in FeAs-based superconductor EuFeAsO(0.85)F(0.15)

The study of excess conductivity $σ'(T)$ in the textured polycrystalline FeAs-based superconductor $EuFeAsO_{0.85}F_{0.15}$ ($T_c = 11\, K$) prepared by the solid state synthesis is reported. The $σ'(T)$ analysis has been performed within the Local Pair (LP) model based on the assumption of the LP formation in cuprate high-$T_c$ superconductors (cuprates) below the pseudogap (PG) temperature $T^*\gg T_c$. Similarly to the cuprates, near $T_c$ $σ'(T)$ is adequately described by the 3D term of the Aslamasov-Larkin (AL) theory but the range of the 3D-AL fluctuations, $ΔT_{3D}$, is relatively short. Above the crossover temperature $T_0 \approx 11.7\,K$ $σ'(T)$ is described by the 2D Maki-Thompson (MT) fluctuation term of the Hikami-Larkin (HL) theory. But enhanced 2D-MT fluctuation contribution, being typical for the magnetic superconductors, is observed. Within the LP model approach the PG parameter, $Δ^*(T)$, was determined for the first time. $Δ^*(T)$ shows the narrow maximum at $T_s \approx 160\,K$ followed by the linear drop down to $T_{SDW}=T_{NFe}\approx 133\,K$. Both small $ΔT_{3D}$ and enhanced $σ'(T)$, including linear $Δ^*(T)$ drop, are considered to be the evidence of the enhanced magnetic interaction in $EuFeAsO_{0.85}F_{0.15}$. Importantly, the slop of the linear $Δ^*(T)$ and its length are found to be the same as observed for $SmFeAsO_{0.85}$. The results suggest both the similarity of magnetic interaction processes in different Fe-pnictides and applicability of the LP model approach to the $σ'(T)$ analysis even in magnetic superconductors.

cond-mat.supr-con↗

arXiv2011

Pseudogap and local pairs in high-Tc cuprate superconductors

Analysis of the resistivity data recently reported by Kondo et al. [1] for (Bi,Pb)2(Sr,La)2CuO{6-delta} (Bi2201) single-crystals has been performed within our model developed to study pseudogap (PG) in high-Tc superconductors (HTS's). The model is based on an assumption of the existence of local pairs in HTS's at temperatures well above Tc. Comparative analysis of our results and results of ARPES experiments reported by Kondo et al. suggests the local pairs to be one of the possible reason of the PG formation.

cond-mat.supr-con↗