3D-2D Crossover Research Articles

3D–2D Crossover and Phase Shift of Beats of Quantum Oscillations of Interlayer Magnetoresistance in Quasi-2D Metals

3D–2D Crossover and Phase Shift of Beats of Quantum Oscillations of Interlayer Magnetoresistance in Quasi-2D Metals

Effects of annealing on the fluctuation conductivity and pseudogap in slightly doped HoBa2Cu3O7–δ single crystals

The effect of annealing at room temperature on the fluctuation conductivity (FLC) σ′(T) and pseudogap (PG) Δ*(7) in the basal ab plane of ReBa2Cu3O7–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 Tc can be described by the Aslamazov-Larkin and Maki-Thompson fluctuation theories, demonstrating a 3D-2D crossover with increasing temperature. The crossover temperature T0 was used to determine the coherence length along the c axis, ξc(0) = (2.82 ± 0.2) Å. At the intermediate stage of annealing, an anomalous increase in 2D FLC was revealed, which is associated with the influence of uncompensated magnetic moments in HoBa2Cu3O7–5 (HoBCO): μeff, Ho = 9.7μΒ. 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. But the slope does not change (sample S3). 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. The comparison of the pseudogap parameter Δ∗(T)/Δmax∗ near Tc with the Peters-Bauer theory revealed a slight increase in the density of local pairs < n↑n↓>, which should explain the observed increase in Tc by 9 K during annealing.

Analysis of fluctuation conductivity in Y-=SUB=-0.5-=/SUB=-Cd-=SUB=-0.5-=/SUB=-Ba-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=-

A study was made of the influence of substitution of up to 50% of yttrium for cadmium in YBa2Cu3O7-delta polycrystals on the mechanism of formation of excess conductivity. It has been established that such a substitution led to a significant increase in the resistivity of Y0.5Cd0.5Ba2Cu3O7-delta, and the value of the critical transition temperature Tc to the superconducting state decreases. The mechanism of formation of fluctuation conductivity (T) near Tc is considered within the framework of the Aslamazov-Larkin theory. The Ginzburg temperature, the critical temperature in the mean field approximation, and the 3D-2D crossover temperature were determined. It is shown that the doping of YBa YBa2Cu3O7-delta with cadmium leads to an increase in the coherence length along the c axis by a factor of 3.2. An analysis of the excess conductivity of the Y0.5Cd0.5Ba2Cu3O7-delta sample within the framework of the local pair model made it possible to determine the temperature dependences of the pseudogap and its maximum value. Keywords: superconductivity, fluctuation conductivity, pseudo-gap, coherence length.

Анализ флуктуационной проводимости в Y-=SUB=-0.5-=/SUB=-Cd-=SUB=-0.5-=/SUB=-Ba-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=-

A study was made of the influence of substitution of up to 50% of yttrium for cadmium in YBa 2 Cu 3 O 7-δ polycrystals on the mechanism of formation of excess conductivity. It has been established that such a substitution led to a significant increase in the resistivity of Y 0,5 Cd 0,5 Ba 2 Cu 3 O 7-δ , and the value of the critical transition temperature Tc to the superconducting state decreases. The mechanism of formation of fluctuation conductivity (T) near Tc is considered within the framework of the Aslamazov-Larkin theory. The Ginzburg temperature, the critical temperature in the mean field approximation, and the 3D-2D crossover temperature were determined. It is shown that the doping of YBa 2 Cu 3 O 7-δ with cadmium leads to an increase in the coherence length along the c axis by a factor of 3.2. An analysis of the excess conductivity of the Y 0,5 Cd 0,5 Ba 2 Cu 3 O 7-δ sample within the framework of the local pair model made it possible to determine the temperature dependences of the pseudogap and its maximum value.

Influence of electron irradiation on fluctuation conductivity and pseudogap in YBa2Cu3O7−δ single crystals

The effect of electron irradiation with the energy of 2.5 MeV on the temperature dependences of the resistivity ρ(T) of an optimally doped YBa2Cu3O7−δ single crystal has been studied. The temperature dependences of both fluctuation conductivity σ′ (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 Tc decreases linearly. Concurrently, the value of ρ(100 K) increases nonlinearly, demonstrating a feature for φ3 = 4.3⋅1018 e/cm2, which is also observed in the number of other dose-dependent parameters. Regardless of the irradiation dose, in the temperature range from Tc up to T01, σ′(T) obeys the classical fluctuation theories of Aslamazov-Larkin (3D-AL) and Maki-Thompson (2D-MT), demonstrating 3D-2D crossover with increasing temperature. The crossover temperature T0 makes it possible to determine the coherence length along the c axis, ξc(0), which increases by ∼3 times under irradiation. Furthermore, the range of superconducting fluctuations above Tc also noticeably increases. At φ1 = 0, the dependence Δ*(T) typical for single crystals containing pronounced twin boundaries is observed with a maximum at Tpair ∼120 K and a distinct minimum at T = T01. It was determined for the first time that at φ3 = 4.3⋅1018 e/cm2 the shape of Δ*(T) changes strongly and becomes the same as in optimally doped YBa2Cu3O7−δ single crystals with a very low pseudogap opening temperature T* and noticeably reduced Tpair, while at Tc(φ) there are no singularities. With an increase in the irradiation dose up to φ4 = 8.81018 e/cm2, the shape of Δ*(T) is restored and becomes the same as in well-structured YBa2Cu3O7−δ films and untwined single crystals. Moreover, in this case, Tpair and T* increase noticeably.

Comparative analysis of the temperature dependences of the resistivity, pseudogap, and thermoelectric power in polycrystals YBa2Cu3O7–δ with a decrease in the density of charge carriers

Comparative analysis of the temperature dependences of resistivity ρ(T), excess conductivity σ′(T), pseudogap (PG) Δ*(T), and thermoelectric power S(T), measured on textured YBa2Cu3O7–δ (YBCO) polycrystals with different charge carrier density nf, depending on the level of doping with oxygen, modified by annealing, has been carried out. It is shown that for an optimally doped (OD) sample with Tc = 90 K (sample S1), σ′(T) near Tc is well described by the Aslamazov–Larkin (AL−3D) and Maki–Thompson (MT-2D) fluctuation theories, demonstrating 3D–2D crossover with increasing temperature. The crossover temperature T0 was used to determine the coherence length along the с axis, ξс(0). With a decrease in nf (samples S2 with Tc = 84 K and S3 with Tc = 80 K), the MT contribution is suppressed, and the σ′(T) dependence obeys the Lawrence–Doniach model, which is typical for samples with defects. The dependence Δ*(T) obtained for S1 has a form typical for OD single crystals of YBCO with a maximum at Tpair ∼114 K and a linear section descending to T01 ∼94 K, which limits the region of superconducting fluctuations above Tc. As nf decreases, the shape of Δ*(T) noticeably changes and becomes typical for YBCO films with a symmetric maximum at Tpair, which is the BEC–BCS transition temperature in high-Tc superconductors. As nf decreases, the slope S(T) changes from positive to negative, demonstrating a feature at the PG opening temperature T*. Accordingly, the dependence of S(T)/T on log T changes from linear to nonlinear, which indicates a change in the nature of interactions in the YBCO electronic subsystem with decreasing nf, since S/T ∼1/nf.

Theory of the thickness dependence of the charge density wave transition in 1 T-TiTe2

Most metallic transition metal dichalcogenides undergo charge density wave (CDW) instabilities with similar or identical ordering vectors in bulk and in single layer, albeit with different critical temperatures. Metallic 1 T-TiTe2 is a remarkable exception as it shows no evidence of charge density wave formation in bulk, but it displays a stable 2 × 2 reconstruction in single-layer form. The mechanism for this 3D-2D crossover of the transition is still unclear, although strain from the substrate and the exchange interaction have been pointed out as possible formation mechanisms. Here, by performing non-perturbative anharmonic calculations with gradient corrected and hybrid functionals, we explain the thickness behaviour of the transition in 1 T-TiTe. We demonstrate that the CDW in single-layer TiTe2 occurs from the interplay of non-perturbative anharmonicity and an exchange enhancement of the electron-phonon interaction, larger in the single layer than in the bulk. Finally, we study the electronic and structural properties of the single-layer CDW phase and provide a complete description of its electronic structure, phonon dispersion as well as infrared and Raman active phonon modes.

Analysis of fluctuation conductivity in Y1−хCdxBa2Cu3O7−δ (x = 0−0.4)

The effect that the partial substitution of Cd for Y has on the mechanism of excess conductivity formation in polycrystalline Y1−хCdxBa2Cu3O7−δ with x = 0 (Y1), 0.1 (Y2), 0.3 (Y3), and 0.4 (Y4) is investigated. The resistivity ρ of the samples increases markedly with increasing x, and the critical temperature of the superconducting (SC) state transition, Tc, decreases. The mechanism responsible for the formation of fluctuation conductivity, σ'(T), is considered within the framework of the Aslamazov–Larkin theory near Tc. The Ginzburg temperature (TG), the critical temperature in the mean-field approximation (Tcmf), the temperature of the 3D–2D crossover (T0), and T01, which limits the region of the SC fluctuations from above, are determined. It is shown that doping with Cd at x = 0−0.4 increases the coherence length along the c axis, ξc(0), by 2.7 times, and the distance between the CuO2 planes, d01, by 2.2 times. The temperature dependences of the pseudogap (PG), Δ*(T), are determined by analyzing the excess conductivity within the framework of the local pair model. It is found that with an increase in substitution, the maximum value of the PG Δ*(Tpair) decreases from 250.2 to 215.7 K, while the real value of the PG, measured at TG,Δ*(TG), increases from 217.4 to 224.2 K.

Features of Excess Conductivity Behavior in a Magnetic Superconductor Dy0.6Y0.4Rh3.85Ru0.15B4

The temperature dependencies of the excess conductivity σ′(T) and possible pseudogap (PG), Δ*(T), in a Dy0.6Y0.4Rh3.85Ru0.15B4 polycrystal were studied for the first time. It was shown that σ′(T) near Tc is well described by the Aslamazov–Larkin (AL) fluctuation theory, demonstrating a 3D–2D crossover with increasing temperature. Using the crossover temperature T0, the coherence length along the c axis, ξc(0), was determined. Above the level of T2D > T0, an unusual dependence σ′(T) was found, which is not described by the fluctuation theories in the range from T0 to TFM, at which a ferromagnetic transition occurs. The range in which superconducting fluctuations exist is apparently quite narrow and amounts to ΔTfl ≈ 2.8 K. The resulting temperature dependence of the PG parameter Δ*(T) has the form typical of magnetic superconductors with features at Tmax ≈ 154 K and the temperature of a possible structural transition at Ts ∼ 95 K. Below Ts, dependence Δ*(T) has a shape typical for PG in cuprates, which suggests that the PG state can be realized in Dy0.6Y0.4Rh3.85Ru0.15B4 in this temperature range. Comparison of Δ*(T) with the Peters–Bauer theory made it possible to determine the density of local pairs near Tc, 〈n↑n↓〉(TG) ≈ 0.35, which is 1.17 times greater than in optimally doped YBa2Cu3O7–δ single crystals.

Effect of annealing on a pseudogap state in untwinned YBa2Cu3O7\u2212\u03b4 single crystals

The effect of annealing both in the oxygen atmosphere and at room temperatures on physical properties such as the pseudogap (Δ*(T)) and excess conductivity (σ′(T)) of untwined YBa2Cu3O7−δ (YBCO) single crystal with a small deviation from oxygen stoichiometry is studied. It was revealed that as the charge carrier density, nf, increases, Тс also slightly increases, whereas the temperature of the pseudogap opening, T*, decreases noticeably, which is consistent with the phase diagram (PD) of cuprates. The excess conductivity in the vicinity of Tc is represented by the Aslamazov-Larkin and Hikami-Larkin fluctuation theories, illustrating the three-dimensional to two-dimensional (i.e. 3D-2D) crossover with an increase in temperature. The crossover temperature T0 determines the coherence length along the c axis is ξc(0) = 0.86 Å, that is 2.6 times larger than for optimally doped YBCO single crystals with defects. Taking into account the short coherence length in high-temperature superconductors, in the model of free charge carriers the phase relaxation time of fluctuating Cooper pairs is determined, τφ (100 K) = (4.55 ± 0.4) · 10−13 s, which is slightly (1.2 times) larger than in well-structured YBCO films, and as in films, does not depend on nf. It is shown that Δ*(T) at different annealing stages practically does not change its shape. As in the well-structured YBCO films, Δ*(T) demonstrates maximum at Tpair~124 K which depends weakly on nf. However, the maximum value of Δ*(Tpair) increases with increasing nf, as it follows from the PD of cuprates. Comparing the experimental data with the Peters-Bauer theory we estimated the density of local pairs <n↑n↓> ≈ 0.3 near Tc that is a common value for high-temperature superconductors.

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D–2D crossover in monolayer VSe2

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials, as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene. Transition-metal dichalcogenides (TMDs) provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change, since they exhibit a variety of electronic/magnetic states owing to quantum confinement. Here we report an anomalous metal-insulator transition induced by three-dimensional (3D)–two-dimensional (2D) crossover in monolayer 1T-VSe2 grown on bilayer graphene. We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures, in sharp contrast to metallic nature of bulk. More surprisingly, monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature, showing a close resemblance to high-temperature cuprates. This similarity suggests a common underlying physics between two apparently different systems, pointing to the importance of charge/spin fluctuations to create the novel electronic states, such as pseudogap and Fermi arc, in these materials.

Pseudogap and fluctuation conductivity in Y1-xPrxBa2Cu3O7-δ single crystals with different concentrations of praseodymium

The effects of praseodymium doping on the excess conductivity σ′(T) and pseudogap (PG) Δ*(T) in Y1–xPrxBa2Cu3O7–δ single crystals with a change in the Pr concentration from x = 0 to x = 0.43 are presented. It is found that as x increases the resistance of the samples increases, whereas the critical temperature Tc drops. At the same time the shape of the ρ(T) curves changes from metallic (x ≤ 0.34) to one that is typical for weakly doped YBa2Cu3O7–δ single crystals with a characteristic thermally activated deflection (x &amp;gt; 0.34). Regardless of the value of x, close to Tc, σ′(T) is well described by the Aslamazov-Larkin and Maki-Thompson theories, demonstrating a 3D-2D crossover with increasing temperature. The crossover temperature T0 makes it possible to determine the coherence length along the c axis, ξc(0). At x = 0, Δ*(T) displays a maximum at Tpair ≈ 110 К that is typical for YBCO films. As x increases the maximum at Tpair is washed out, but a pronounced maximum Δ*(T) in the high temperature region appears, followed by a linear section with a positive slope. Such a dependence Δ*(T) is normal for magnetic superconductors and is, most likely, caused by the influence of the intrinsic magnetic moment of Pr (μPr ≈ 4μB).

Reentrant Resistive Behavior and Dimensional Crossover in Disordered Superconducting TiN Films

A reentrant temperature dependence of the normal state resistance often referred to as the N-shaped temperature dependence, is omnipresent in disordered superconductors – ranging from high-temperature cuprates to ultrathin superconducting films – that experience superconductor-to-insulator transition. Yet, despite the ubiquity of this phenomenon its origin still remains a subject of debate. Here we investigate strongly disordered superconducting TiN films and demonstrate universality of the reentrant behavior. We offer a quantitative description of the N-shaped resistance curve. We show that upon cooling down the resistance first decreases linearly with temperature and then passes through the minimum that marks the 3D–2D crossover in the system. In the 2D temperature range the resistance first grows with decreasing temperature due to quantum contributions and eventually drops to zero as the system falls into a superconducting state. Our findings demonstrate the prime importance of disorder in dimensional crossover effects.

Fluctuation-induced magnetoconductivity in pristine and proton-irradiated Ca8.5La1.5(Pt3As8)(Fe2As2)5 single crystals

The influence of the proton irradiation on the fluctuation-induced conductivity in Ca8.5La1.5(Pt3As8)(Fe2As2)5 single crystal was investigated. The in-plane magnetoconductivity was measured up to μ0H = 13 T. It is observed that the Tc was suppressed up to 30.3 K from 32.5 K as a result of proton irradiation whereas the amplitude of the superconducting fluctuations is almost the same in both pristine and irradiated samples. The magnetoconductivity results analyzed by the Ullah and Dorsey scaling approaches showed that in the pristine sample, the 3D–2D crossover is situated near the Tc. Furthermore, once the 3D–2D crossover occurs, it is found that there is a regime simultaneously described by 2D and 3D fluctuation behaviors in our Ca8.5La1.5(Pt3As8)(Fe2As2)5 sample. Meanwhile, the proton-irradiated sample showed the 3D fluctuation behavior.

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

The influence of the hydrostatic pressure P up to 0.95 GPa on the excess conductivity σ′(T) and the pseudogap Δ∗(T) in optimally doped YBa2Cu3O7−δ single crystals (Tc ≃ 91.1 K 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∗(Tc)/kBTc ≈ 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 Tc is minor: dTc/dP ≈ +0.73 K GPa−1, whereas dlnρ/dP ≈ (−17 ± 0.2)% GPa−1 is comparable with that in lightly doped YBCO single crystals. This suggests that the mechanisms of the pressure effect on ρ(T) and Tc are noticeably different. Independently of pressure, near Tc, σ′(T) 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 Tc < T < T01, in which σ′(T) obeys the classical fluctuation theories, is exceptionally narrow (≈1.16 K). Nevertheless, a peculiarity at the temperature T01, up to which the wave function phase stiffness in the superconductor is maintained, is clearly observed in the dependence Δ∗(T). Below T01 a fast growth of Δ∗(T) is revealed for the first time. It can be associated with a sudden increase of the superfluid density, ns, that is the density of fluctuating Cooper pairs (short-range phase correlations) forming in the sample when T approaches Tc.

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

The influence of hydrostatic pressure P up to 1.05GPa on resistivity ρ, excess conductivity σ′(T) and pseudogap Δ⁎(T) is investigated in slightly doped single crystals of YBa2Cu3O7−δ (Tc(P=0)≈49.2K and δ≈0.5). The critical temperature Tc is found to increase with increasing pressure at a rate dTc/dP=+5.1KGPa−1, while ρ(300K) decreases at a rate dlnρ/dP=(−19±0.2)%GPa−1. Near Tc, independently on pressure, 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 T0 determines the coherence length along the c-axis ξc(0)≃(3.43±0.01)A˚ at P=0, which is found to decrease with increasing P. At the same time, Δ⁎ and the BCS ratio 2Δ⁎/kBTc both increase with increasing hydrostatic pressure at a rate dlnΔ⁎/dP≈0.36GPa−1, implying an increase of the coupling strength with increasing P. At low temperatures below Tpair, the shape of the Δ⁎(T) curve is found to be almost independent of 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.

Atomically flat superconducting nanofilms: multiband properties and mean-field theory

Recent progress in materials synthesis enabled fabrication of superconducting atomically flat single-crystalline metallic nanofilms with thicknesses down to a few monolayers. Interest in such nano-thin systems is attracted by the dimensional 3D–2D crossover in their coherent properties which occurs with decreasing the film thickness. The first fundamental aspect of this crossover is dictated by the Mermin–Wagner–Hohenberg theorem and concerns frustration of the long-range order due to superconductive fluctuations and the possibility to track its impact with an unprecedented level of control. The second important aspect is related to the Fabri–Pérot modes of the electronic motion strongly bound in the direction perpendicular to the nanofilm. The formation of such modes results in a pronounced multiband structure that changes with the nanofilm thickness and affects both the mean-field behavior and superconductive fluctuations. Though the subject is very rich in physics, it is scarcely investigated to date. The main obstacle is that there are no manageable models to study a complex magnetic response in this case. Full microscopic consideration is rather time consuming, if practicable at all, while the standard Ginzburg–Landau theory is not applicable. In the present work we review the main achievements in the subject to date, and construct and justify an efficient multiband mean-field formalism which allows for numerical and even analytical treatment of nano-thin superconductors in applied magnetic fields.

Fluctuation conductivity and pseudogap in [formula omitted] single crystals under pressure with transport current flowing under an angle 45° to the twin boundaries

The influence of hydrostatic pressure up to 0.48GPa on the fluctuation conductivity σ′(T) and pseudogap (PG) Δ∗(T) of slightly doped HoBa2Cu3O7-δ single crystals with Tc≈ 62K and δ≈0.35 is studied with current passing under an angle 45° to the twin boundaries. It is shown that near Tc the conductivity σ′(T) is well described by the Aslamasov–Larkin and Hikami–Larkin fluctuation theories demonstrating 3D–2D crossover with the increase of temperature. Δ∗(T) displays two representative maxima at Tmax1≈219K and Tmax2≈241K likely caused by the phase stratification of the single crystal. Pressure leads to disappearance of these maxima and linear Δ∗(T) with a positive gradient at high temperatures. Essentially, with the removal of pressure the maxima are restored. The comparison of our results with those obtained for YBa2Cu3O7-δ sheds more light on the role of magnetic subsystem in the high-Tc superconductors.

Aging Effect on Electrical Conductivity of Pure and Al-Doped YBa2Cu3O7−δSingle Crystals with a Given Topology of Planar Defects

The conducting properties in the basalabplane of pure and Al-dopedYBa2Cu3O7-δsingle crystals before and after long-time exposure in air atmosphere are investigated. It is shown that prolonged aging leads to an increase of the density of effective scattering centers for the normal carriers. The aluminum doping has been revealed to partially slowdown the degradation of the conducting properties in process of aging. The excess conductivity,Δσ(T), has been found to obey exponential dependence in the broad temperature rangeTc&lt;T&lt;T*. In the pseudogap regime, the mean-field transition temperatureT*and the 3D-2D crossover point in the excess conductivity have been quantified. Near the critical temperature,Δσ(T)is described well within the Aslamazov-Larkin theoretical model. Herewith, both aluminum doping and prolonged aging have been found to essentially expand the temperature interval of implementation of the pseudogap state, thus narrowing the linear section in the dependenceρab(T).

Peculiarities of fluctuation conductivity and pseudogap behavior in slightly doped HoBa2Cu3O7−δ single crystals under pressure

The effect of hydrostatic pressure up to 5 kbar on fluctuation conductivity σ′(T) and pseudogap Δ*(T) of slightly doped high-Tc single crystals (HTSC) of HoBa2Cu3O7−δ was investigated. It is shown that the dependence σ′(T) near Tc is well described by the Aslamazov-Larkin and Hikami-Larkin fluctuation theories, demonstrating the 3D–2D crossover with increasing temperature. Δ*(T) exhibits two unexpected maxima at Tm1 ≈ 195 K and Tm2 ≈ 210 K, possibly due to the two-phase state of the single crystal. Pressure-induced redistribution of labile oxygen leads to the disappearance of these peaks and a linear dependence Δ*(T) with a positive slope at high temperatures. Notably, when the pressure is removed, peaks are restored. In general, dependencies of σ′(T) and Δ*(T) are significantly different from those obtained for YBCO films with varying oxygen content. They resemble dependencies of Fe-based conductor SmFeAsO0.85. This can be explained by the influence of paramagetism in HoBa2Cu3O7−δ.