Slope Of Critical Field Research Articles

From weak to strong-coupling superconductivity tuned by substrate in TiN films

Abstract The interplay between substrates and superconducting thin films has attracted increasing attention. Here, we report an in-depth investigation on superconducting properties of the epitaxial TiN thin films grown on three different substrates by dc reactive magnetron sputtering. The TiN films grown on (0001) sapphire exhibit (111) crystal orientation, while that grown on (100) Si and MgO substrates exhibit (100) orientation. Moreover, the samples grown on Si reveal a relatively lower level of disorder, accompanied by the higher critical transition temperature Tc and smaller magnitude of upper critical field slope near Tc. Remarkably, we uncovered a rather high value of superconducting gap (with Δ0/kBTc = 3.05) in TiN film on Si indicating a very strong coupling superconductivity, in sharp contrast to the case using sapphires and MgO as the substrate which reveals a weak-coupling feature. The comprehensive analysis considering the scenarios of the three substrate shows that the grain size of the thin films may be an important factor influencing the superconductivity.

Pressure-induced linear enhancement of the superconducting transition in Nd0.8Sr0.2NiO2 thin films

We report the pressure (P) effect on the superconducting transition temperature T c and the upper critical field μ 0 H c2 of infinite-layer Nd0.8Sr0.2NiO2 thin films by measuring the electrical transport properties under various hydrostatic pressures to 4.6 GPa. At ambient pressure, it shows the clear superconducting transition with T c ∼ 10 K. Based on the evolution of resistance R(T), we found that the T c is monotonically enhanced to ∼14 K upon increasing pressure to 2.9 GPa. The constructed temperature–pressure phase diagram indicates that the calculated slope dT c/dP is about 1.14 K GPa−1 and the superconducting T c shows no signatures of saturation with pressure. It thus gives the possibility to further enhance T c by employing higher pressures or heterostructure engineering. In addition, the normalized slope of upper critical field μ 0 H c2(0) implies that the electron correlations are gradually decreasing with pressure, which exhibits an opposite evolution with superconducting T c. Our work further confirms the positive pressure effects in nickelate superconductors and gives more insight to further enhance its superconducting transition temperature.

Spectroscopic evidence for the superconductivity of elemental metal Y under pressure

Very high applied pressure induces superconductivity with the transition temperature (Tc) exceeding 19 K in elemental yttrium, but relatively little is known about the nature of that superconductivity. From point-contact spectroscopy (PCS) measurements in a diamond anvil cell (DAC), a strong enhancement in the differential conductance is revealed near the zero-biased voltage owing to Andreev reflection, a hallmark of the superconducting (SC) phase. Analysis of the PCS spectra based on the extended Blonder-Tinkham-Klapwijk (BTK) model indicates two SC gaps at 48.6 GPa, where the large gap ΔL is 3.63 meV and the small gap ΔS is 0.46 meV. When scaled against a reduced temperature, both small and large SC gaps collapse on a single curve that follows the prediction from BCS theory. The SC gap-to-Tc ratio is 8.2 for the larger gap, and the initial slope of the upper critical field is −1.9 T/K, indicating that Y belongs to a family of strongly coupled BCS superconductors. The successful application of PCS to Y in DAC environments demonstrates its utility for future research on other pressure-induced high-Tc superconductors.

Electron correlations in the of Fe y SeS x (0.10 0.24, 0.9)

We report on the evolution of the electronic correlation strength across the nematic critical point in Fe y SeS x (), inferred from the measurements of the slope of the upper critical field . Superconducting transition T c and quasiparticle mass exhibit similar reduction with sulfur content x. Our results indicate that electronic correlations from all Fermi surface pockets show strong interconnection with pairing strength and are not governed by the change of nematic fluctuations. The upper critical field anisotropy γ H = / exhibits a minimum in the critical region and stronger temperature dependence away from it.

Slope stability assessment and landslide vulnerability mapping of the Institut Teknologi Kalimantan area

Slope stability analysis was carried out in the Institut Teknologi Kalimantan which is an area with educational facilities that have a high risk of soil movement. The research area consists of slopes that often reported landslides, especially during the rains time. Evaluation is carried out to the important factors that affect slope stability such as slope geometry consist of 23 zones, soil parameters consist of 7 drilling holes, and past landslide occurrence. Slope stability was evaluated using back analysis based on soil parameters under critical conditions and field investigation and slope geometry using SLOPE/W software to obtain the safety factor (SF) of a certain slope. Landslide vulnerability maps are created based on the risk and potential landslide or landslide occurrence. Based on the results of the stability analysis, there are 6 zones with unstable conditions and a high potential for landslides and 4 zones in critical conditions with moderate landslide potential. The modeling also shows that in the same soil parameter value with a steep slope, the value of the safety factor is getting smaller. This means that the potential for soil movement is greater. Besides, the loading at the top of the slope also causes a reduced safety factor.

Two Qualitatively Different Superconducting Phases under High Pressure in Single-Crystalline CeNiGe3

We have measured the temperature dependence of resistivity in single-crystalline CeNiGe$_{3}$ under hydrostatic pressure in order to establish the characteristic pressure-temperature phase diagram. The transition temperature to AFM-I phase $T_{\rm N1}$ = 5.5 K at ambient pressure initially increases with increasing pressure and has a maximum at $\sim$ 3.0 GPa. Above 2.3 GPa, a clear zero-resistivity is observed (SC-I phase) and this superconducting (SC) state coexists with AFM-I phase. The SC-I phase suddenly disappears at 3.7 GPa simultaneously with the appearance of an additional kink anomaly corresponding to the phase transition to AFM-II phase. The AFM-II phase is continuously suppressed with further increasing pressure and disappears at $\sim$ 6.5 GPa. In the narrow range near the critical pressure, an SC phase reappears (SC-II phase). A large initial slope of upper critical field $\mu_0H_{\rm c2}$ and non-Fermi liquid behavior indicate that the SC-II phase is mediated by antiferromagnetic fluctuations. On the other hand, the robust coexistence of the SC-I phase and AFM-I phase is unusual on the contrary to superconductivity near a quantum critical point on most of heavy-fermion compounds.

Magnetic and superconducting anisotropy in Ni-doped RbEuFe4As4 single crystals

We investigate the effect of Ni doping on the Fe-site in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific heat, resistivity and magnetization measurements. Upon Ni-doping, the resistivity curves shift up in a parallel fashion indicating a strong increase of the residual resistivity due to scattering by charged dopand atoms while the shape of the curve and thus the electronic structure appears largely unchanged. The observed step $\Delta C/T_c$ at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multi-band superconductivity and strong inter-band pairing. The upper critical field slopes are reduced upon Ni doping for in- as well as out-of-plane fields leading to a small reduction in the superconducting anisotropy. The specific heat measurements of the magnetic transition reveal the same BKT behavior close to the transition temperature $T_m$ for all doping levels. The transition temperature is essentially unchanged upon doping. The in to out-of-plane anisotropy of Eu-magnetism observed at small magnetic fields is unaltered as compared to the undoped compound. All of these observations indicate a decoupling of the Eu magnetism from superconductivity and essentially no influence of Ni doping on the Eu magnetism in this compound.

Critical-field slope reduction and upward curvature of the phase-transition lines of thin disordered superconducting [formula omitted] films in strong magnetic fields

We report the effect of disorder on superconducting phase transition of YBa2Cu3O7−x epitaxial thin films in external magnetic fields. The disorder was produced by several successive acts of oxygen ion implantation. Controlling a total accumulated dose of implanted ions nD, we carried out transport measurements in ab-plane for temperatures T below 91 K and external magnetic fields H up to 11 T. Temperature-field dependencies of in-plane resistivity allow us to analyze H−T phase diagrams for primary compound as well as for disordered structure. Considering the upper critical field Hc2 as the magnetic field which corresponds to a local resistivity drop at the onset of superconducting transition, we found out the following results. By gradual increasing of nD, the phase-transition line Hc2(T) suffers an unconventional critical-field slope reduction, while larger defect concentrations usually enhance the upper critical field in the vicinity of Tc0. Besides, for rather large nD, the curvature of Hc2(T) becomes upward for temperatures close to Tc0. Theoretical interpretation of the experimental data is developed in the framework of linearized Ginzburg-Landau theory with an inhomogeneous superconducting coherence length. A simple expression for the critical temperature Tc is obtained: Tc=Tc0(1−h+αh3/2), where h is the dimensionless magnetic field and α is a constant which describes the defects in a specimen. The formula nicely fits our experimental results.

Phase Diagrams of Thin Disordered Films Based on HTSC YBa2Cu3O7 – x in External Magnetic Fields

An unusual decrease in the slope of the upper critical field near Tc0 at a gradual increase in the ion implantation dose has been experimentally observed in narrow bridges formed on the base of thin HTSC YBa2Cu3O7 – x films, while an increase in the defect concentration usually leads to an increase in the local slope of the phase transition line Hc2(T). In addition, it has been found that the temperature dependence of the upper critical field has a positive curvature near Tc0. A possible theoretical interpretation of the results is proposed. It is based on the modified Ginzburg–Landau theory with a nonuniform length of superconducting correlations.

Effect of Hydrostatic Pressure on Superconductivity of FeSe Thin Films

The electronic transport in superconducting epitaxial c-axis-oriented FeSe thin films grown on (001)-oriented MgO substrates was investigated. To this end, the in-plane resistivity was measured in dependence on temperature, pressure, and magnetic field. The temperature ranged from 1.2 to 35 K; static magnetic fields with strengths up to 14 T were applied normal to the film surface, i.e. parallel to the FeSe c-axis; and hydrostatic pressure was applied from 0 to 2.7 GPa. Concerning the role of the MgO substrate in the pressure experiments, it is suggested that the substrate mainly reduces the in-plane compressibility of the film in comparison to bulk. The transition to superconductivity shifted to higher temperatures with increasing pressure. The onset critical temperature raised from 11.5 K at zero applied pressure with an initial rate of 2.5 K/GPa to 18.2 K at 2.7 GPa. The pressure-induced increase of the critical temperature was accompanied by a twofold broadening of the transition width. As a counterpart of pressure, the magnetic field shifted the superconducting transition to lower temperature. In addition to pressure, the field also induced a noticeable broadening of the superconductive transition rather than a parallel shift. The positive magnetoresistance at 20 K increased with enhanced pressure and reached 24% at the highest pressure and field. For each applied pressure, the magnetoresistance could be fitted by a Lorentzian function, i.e. it originates from classical Lorentz scattering. The resulting charge carrier mobility increased under pressure suggesting a decreasing collision rate. The upper critical field raised with higher pressure. Its temperature dependence could be fitted by conventional Werthamer-Hohenberg-Helfand theory under the assumption of the Pauli paramagnetic effect that became more pronounced under pressure. The anomalous behaviour of the normalized negative slope of the upper critical field at the critical temperature suggested a change of the Fermi surface above a critical pressure of 2 GPa.

Fermi-surface selective determination of the g -factor anisotropy in URu2Si2

The $g$-factor anisotropy of the heavy quasiparticles in the hidden order state of URu$_2$Si$_2$ has been determined from the superconducting upper critical field and microscopically from Shubnikov-de Haas (SdH) oscillations. We present a detailed analysis of the $g$-factor for the $\alpha$, $\beta$ and $\gamma$ Fermi-surface pockets. Our results suggest a strong $g$-factor anisotropy between the $c$ axis and the basal plane for all observed Fermi surface pockets. The observed anisotropy of the $g$-factor from the quantum oscillations is in good agreement with the anisotropy of the superconducting upper critical field at low temperatures, which is strongly limited by the paramagnetic pair breaking along the easy magnetization axis $c$. However, the anisotropy of the initial slope of the upper critical field near $T_c$ cannot be explained by the anisotropy of the effective masses and Fermi velocities derived from quantum oscillations.

The effects of Ge occupation and hydrostatic pressure on the metamagnetic phase transition and magnetocaloric effect in Mn2Sb alloy

The effects of Ge occupations at Sb and Mn sites in Mn2Sb system had been re-examined along with the influence of hydrostatic pressure. The metamagnetic phase transition from ferrimagnetic (FRI) to antiferromagnetic (AFM) state was observed in Mn2Sb1-xGex (x = 0.05, 0.1) and (Mn1-yGey)2Sb (y = 0.025, 0.04) alloys. The phase transition temperature, Tt, was gradually increased with increased Ge substitution for both Sb and Mn, but slightly decreased under increased hydrostatic pressure in Mn2Sb0.95Ge0.05 alloy. Meanwhile, the slope of critical field as the function of temperature was reduced with increased hydrostatic pressure and Ge substitution amount in (Mn1-yGey)2Sb (y = 0.025, 0.04) alloys. For a field change of 7 T, the maxima of magnetic entropy changes of 6 Jkg-1K-1 and 7.1 Jkg-1K-1 have been achieved in Mn2Sb0.95Ge0.05 and Mn1.95Ge0.05Sb alloys, which are gradually decreased by more Ge substitution and increased hydrostatic pressure.

Отличительные особенности фазовых диаграмм тонких неупорядоченных пленок на основе ВТСП YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-x-=/SUB=- во внешних магнитных полях

AbstractAn unusual decrease in the slope of the upper critical field near T _ c 0 at a gradual increase in the ion implantation dose has been experimentally observed in narrow bridges formed on the base of thin HTSC YBa_2Cu_3O_7 – _ x films, while an increase in the defect concentration usually leads to an increase in the local slope of the phase transition line H _ c 2( T ). In addition, it has been found that the temperature dependence of the upper critical field has a positive curvature near T _ c 0. A possible theoretical interpretation of the results is proposed. It is based on the modified Ginzburg–Landau theory with a nonuniform length of superconducting correlations.

Anisotropic superconductivity and magnetism in single-crystal RbEuFe4As4

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe$_4$As$_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of $T_m$ = 15 K and a superconducting transition temperature of $T_c$ = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $\Gamma$ = 1.7. Ginzburg-Landau parameters of $\kappa_c \sim 67$ and $\kappa_{ab} \sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $<$ 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.

Interstitial effects of B and Li on the magnetic phase transition and magnetocaloric effects in Gd2In alloy

Interstitial effects of B and Li on the phase transition and magnetocaloric effect in Gd2In alloys had been studied. The antiferromagnetic (AFM) - ferromagnetic (FM) phase transition was found to be of first-order nature while ferromagnetic - paramagnetic (PM) phase transition was of second-order nature in B- or Li-doped Gd2In alloys. AFM-FM phase transition temperature was increased while FM-PM phase transition was decreased with more doping concentrations. During AFM-FM phase transition, the slope of temperature-dependent critical field (μ0Hcr) was increased by increased doping amounts. The magnetic entropy changes under small field change were enhanced by B and Li addition, which showed the beneficial effects of B and Li additions.

Temperature Dependence of the Upper Critical Field in Disordered Hubbard Model with Attraction

We study disorder effects upon the temperature behavior of the upper critical magnetic field in attractive Hubbard model within the generalized $DMFT+\Sigma$ approach. We consider the wide range of attraction potentials $U$ - from the weak coupling limit, where superconductivity is described by BCS model, up to the strong coupling limit, where superconducting transition is related to Bose - Einstein condensation (BEC) of compact Cooper pairs, formed at temperatures significantly higher than superconducting transition temperature, as well as the wide range of disorder - from weak to strong, when the system is in the vicinity of Anderson transition. The growth of coupling strength leads to the rapid growth of $H_{c2}(T)$, especially at low temperatures. In BEC limit and in the region of BCS - BEC crossover $H_{c2}(T)$ dependence becomes practically linear. Disordering also leads to the general growth of $H_{c2}(T)$. In BCS limit of weak coupling increasing disorder lead both to the growth of the slope of the upper critical field in the vicinity of transition point and to the increase of $H_{c2}(T)$ in low temperature region. In the limit of strong disorder in the vicinity of the Anderson transition localization corrections lead to the additional growth of $H_{c2}(T)$ at low temperatures, so that the $H_{c2}(T)$ dependence becomes concave. In BCS - BEC crossover region and in BEC limit disorder only slightly influences the slope of the upper critical field close to $T_{c}$. However, in the low temperature region $H_{c2}(T)$ may significantly grow with disorder in the vicinity of the Anderson transition, where localization corrections notably increase $H_{c2}(T=0)$ also making $H_{c2}(T)$ dependence concave.

Effect of hydrostatic pressure on the superconducting properties of quasi-1D superconductor K2Cr3As3

K2Cr3As3 is a newly discovered quasi-1D superconductor with a Tc = 6.1 K and an upper critical field µ0Hc2(0) ≈ 40 T three times larger than the Pauli paramagnetic limit µ0Hp that is suggestive of a spin-triplet Cooper pairing. In this paper, we have investigated the effects of hydrostatic pressure on its Tc and µ0Hc2 by measuring the ac magnetic susceptibility χ'(T) under magnetic fields at various hydrostatic pressures up to 7.5 GPa. The major findings include: (1) Tc is suppressed gradually to below 2 K at 7.5 GPa; (2) the estimated µ0Hc2(0) decreases dramatically to below µ0Hp above ~2 GPa and becomes slight lower than the orbital limiting field estimated from the initial slope of upper critical field via = −0.73TcdHc2/ in the clean limit; (3) the estimated Maki parameter α = √2/Hp drops from 4 at ambient pressure to well below 1 at P > 2 GPa, suggesting the crossover from Pauli paramagnetic limiting to orbital limiting in the pair breaking process upon increasing pressure. These observations suggested that the application of hydrostatic pressure could drive K2Cr3As3 away from the ferromagnetic instability and lead to a breakdown of the spin-triplet pairing channel. We have also made a side-by-side comparison and discussed the distinct effects of chemical and physical pressures on the superconducting properties of K2Cr3As3.

Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure

The pressure dependence of the structural, magnetic and superconducting transitions and of the superconducting upper critical field were studied in sulfur-substituted Fe(Se$_{1-x}$S$_{x}$). Resistance measurements were performed on single crystals with three substitution levels ($x$=0.043, 0.096, 0.12) under hydrostatic pressures up to 1.8 GPa and in magnetic fields up to 9 T, and compared to data on pure FeSe. Our results illustrate the effects of chemical and physical pressure on Fe(Se$_{1-x}$S$_{x}$). On increasing sulfur content, magnetic order in the low-pressure range is strongly suppressed to a small dome-like region in the phase diagrams. However, $T_s$ is much less suppressed by sulfur substitution and $T_c$ of Fe(Se$_{1-x}$S$_{x}$) exhibits similar non-monotonic pressure dependence with a local maximum and a local minimum present in the low pressure range for all $x$. The local maximum in $T_c$ coincides with the emergence of the magnetic order above $T_c$. At this pressure the slope of the upper critical field decreases abruptly. The minimum of $T_c$ correlates with a broad maximum of the upper critical field slope normalized by $T_c$.

Ginzburg–Landau expansion in strongly disordered attractive Anderson–Hubbard model

We have studied disordering effects on the coefficients of Ginzburg - Landau expansion in powers of superconducting order - parameter in attractive Anderson - Hubbard model within the generalized $DMFT+\Sigma$ approximation. We consider the wide region of attractive potentials $U$ from the weak coupling region, where superconductivity is described by BCS model, to the strong coupling region, where superconducting transition is related with Bose - Einstein condensation (BEC) of compact Cooper pairs formed at temperatures essentially larger than the temperature of superconducting transition, and the wide range of disorder - from weak to strong, where the system is in the vicinity of Anderson transition. In case of semi - elliptic bare density of states disorder influence upon the coefficients $A$ and $B$ before the square and the fourth power of the order - parameter is universal for any value of electron correlation and is related only to the general disorder widening of the bare band (generalized Anderson theorem). Such universality is absent for the gradient term expansion coefficient $C$. In the usual theory of "dirty" superconductors the $C$ coefficient drops with the growth of disorder. In the limit of strong disorder in BCS limit the coefficient $C$ is very sensitive to the effects of Anderson localization, which lead to its further drop with disorder growth up to the region of Anderson insulator. In the region of BCS - BEC crossover and in BEC limit the coefficient $C$ and all related physical properties are weakly dependent on disorder. In particular, this leads to relatively weak disorder dependence of both penetration depth and coherence lengths, as well as of related slope of the upper critical magnetic field at superconducting transition, in the region of very strong coupling.

Superconductivity at 35 K by self doping in RbGd2Fe4As4O2

We report synthesis, crystal structure and physical properties of a novel quinary compound RbGd2Fe4As4O2. The new iron oxyarsenide is isostructural to the fluo-arsenide KCa2Fe4As4F2, both of which contain separate double Fe2As2 layers that are self hole-doped in the stoichiometric composition. Bulk superconductivity at K is demonstrated by the measurements of electrical resistivity, dc magnetic susceptibility and heat capacity. An exceptionally high value of the initial slope of the upper critical field (d/d T K−1) is measured for the polycrystalline sample.