Thermodynamic Critical Field Hc Research Articles

Equilibrium magnetic properties of the Fe-doped HTSC YBaCuO

For the polycrystalline high-temperature superconductor Y1 –xFexBa2Cu3Oy (0 ≤ x ≤ 0.05), where a significant increase in the intragrain critical current density has recently been found, the effect of iron doping on the equilibrium magnetic characteristics is studied. The field dependences of the magnetization are measured in the temperature range of 40–85 K and in the field range up to 60 kOe with increasing and decreasing fields. The temperature dependences of the London penetration depth λab, the superconducting carrier density ns, and the thermodynamic critical field Hc are determined from an analysis of the middle lines of the magnetic hysteresis loop. It is shown that the thermodynamic properties of Fe substituted samples do not change at x ≤ 0.03. At a higher Fe content, a slight decrease in Hc and the superconducting transition temperature is observed. However, changes in the condensation energy turn out to be much smaller than values expected due to an increase in the density of effective pinning centres created by substitutional atoms in both the yttrium and copper sublattices. It is concluded that the pinning regions in chain CuO planes near substituted Cu sites are two-dimensional in nature and do not affect the superconducting pairing in CuO2 planes. Another possible pinning mechanism, which does not affect Hc, is the interaction of vortices with the magnetic moments of Fe3+ ions.

Correlation between electron-phonon coupling and superconductivity of Sn2+ ion irradiated MgB2-thin films

This study investigates the impact of 2 MeV Tin ion (Sn2+) irradiation with doses from 2E12 to 7E13 on the Raman spectroscopy and superconductivity properties of MgB2-thin films samples. Raman spectroscopy has been used to examine the behavior of electron-phonon coupling (EPC). For the superconductivity properties, the temperature dependence resistivity, R(T), and the magnetic field-dependent magnetization, M(H), were measured. The import of EPC, which is related to the change in critical temperature (Tc), was calculated using the McMillan-formula modified by Allen–Dynes, using the phonon frequency of the middle peak center ω2(E2g). Raman spectroscopy showed that the value of EPC decreased from 1.113 (Pristine) to 0.969 (7E13). Although the critical temperature (Tc) decreased with increasing Sn2+ ion doses, the values of the in-field critical current density (Jc) and upper critical field (Hc2) increased and reached a maximum for the 5E13 sample. In addition, the magnetic penetration depths of the pristine and optimal dose (5E13) samples were measured using magnetic force microscopy (MFM), providing the thermodynamic critical field (Hc) in the MgB2 films.

Mixed superconducting state without applied magnetic field

A superconducting (SC) mixed state occurs in type-II superconductors where the upper critical field Hc2 is higher than the thermodynamic critical field Hc. When an applied field is in between these fields, the free energy depends weakly on the order parameter which therefore can be small (SC state) or zero (normal state) at different parts of the sample. In this paper we demonstrate how a normal state along a line traversing a superconductor can be turned on and off externally in zero field. The concept is based on a long, current-carrying excitation coil, piercing a ringshaped superconductor. The ring experiences zero field, but the vector potential produced by the coil generates a circular current that destroys superconductivity along a radial line starting at preexisting nucleation points in the sample. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil, and different cooldowns yield the same normal lines. We suggest potential applications of this magnetic-field-free mixed state.

Superconductivity on a Bi Square Net in LiBi.

We present the crystallographic analysis, superconducting characterization and theoretical modeling of LiBi, that contains the lightest and the heaviest nonradioactive metal. The compound crystallizes in a tetragonal (CuAu-type) crystal structure with Bi square nets separated by Li planes (parameters a = 3.3636(1) Å and c = 4.2459(2) Å, c/a = 1.26). Superconducting state was studied in detail by magnetic susceptibility and heat capacity measurements. The results reveal that LiBi is a moderately coupled type-I superconductor (λe-p = 0.66) with Tc = 2.48 K and a thermodynamic critical field Hc(0) = 157 Oe. Theoretical studies show that bismuth square net is responsible for superconductivity in this compound, but the coupling between the Li planes and Bi planes makes a significant contribution to the superconductivity.

Strain effect on thermodynamic properties of superconducting Nb2InC

Well-known from their electric and mechanical properties, the MAX phases have proven to be good candidates for replacement of commonly used materials such as steel, copper and tungsten. Together with industrial usefulness, some representatives of this large group manifest the ability of superconducting state occurrence. Above factors complemented with still unexplained coupling mechanism lead to the necessity of extensive investigations on MAX phases on this field. The subject of this research is focused on the thermodynamic properties of superconducting state of Nb2InC. Using the strong-coupling Eliashberg theory of superconductivity the values of the superconducting critical temperature (Tc) have been attained together with the order parameter function (Δ), the thermodynamic critical field (Hc) and the specific heat jump (ΔC) at the transition temperature for the range of tension applied to bulk material. Interesting results of strain strongly affecting the thermodynamic properties of superconducting state can be highlighted. Major enrichment has been obtained for the most extreme strain values with best improvement in case of the compressed system. Moreover, significant changes of critical temperature are noticeable with the constrict strain increment (from −4% to −8%) which results in great development of transition temperature from roughly 0 K to about 35 K, respectively. This leads to conclusion that the applying stress on the material may have significant impact on the thermodynamic properties of the superconducting state. Furthermore, great divergence between the free energy difference, entropy difference and specific heat difference of the extreme strains can be observed. Finally, above values allowed to calculate the BCS theory constants which have been collated.

Study of the critical current density and the thermodynamic critical field in deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor

ABSTRACTWe have studied the reversible and irreversible part of the hysteresis loops as a function of slow cooling rate through the order–disorder transformation near 80 K for the deuterated (κ-D8-Br) κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor. We estimated the critical current density JC and the thermodynamic critical field HC from the magnetic hysteresis loops. Temperature dependence of the critical current density derived from the irreversible part using Bean’s model. The thermodynamic critical field HC has been obtained from the reversible part of the hysteresis loops.

Thickness dependence of the superconducting properties of γ- Mo2N thin films on Si (001) grown by DC sputtering at room temperature

We study the crystalline structure and superconducting properties of γ-Mo2N thin films grown by reactive DC sputtering on AlN buffered Si (001) substrates. The films were grown at room temperature. The microstructure of the films, which was studied by X-ray diffraction and transmission electron microscopy, shows a single-phase with nanometric grains textured along the (200) direction. The films exhibit highly uniform thickness in areas larger than 20 × 20 μm2. The superconducting critical temperature Tc is suppressed from 6.6 K to ≈ 3.0 K when the thickness decreases from 40 nm to 5 nm. The residual-resistivity ratio is slightly smaller than 1 for all the films, which indicates very short electronic mean free path. The films are in the superconducting dirty limit with upper critical field Hc2 (0) ≈ 12 T for films with thickness of 40 nm, and 9 T for films with thickness of 10 nm. In addition, from the critical current densities Jc in the vortex-free state, we estimate a penetration depth λ(0) ≈ (800 ± 50) nm and a thermodynamic critical field Hc (0) = (500 ± 80 Oe).

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa2Cu3O6.93 high-T c superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature Tc have been measured for a series of fine-crystalline YBa2Cu3Oy samples having nearly optimum values of y = 6.93 ± 0.3 and Tc = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field Hc are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in Hc(T) and the jump of the specific heat ΔC/Tc. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-Tc superconductors.

Superconducting properties of BaBi3

We report the superconducting properties of single crystals of the intermetallic compound BaBi3, whose crystal structure is perovskite related. The superconducting transition temperature ( K) was obtained from heat capacity measurements. Using the measured values for the critical fields , and the specific heat C, we estimate the thermodynamic critical field Hc(0), coherence length ξ(0), Debye temperature and coupling constant λep. and λep suggest that BaBi3 is a weakly coupled superconductor. Electronic band structure calculations show a complex Fermi surface and a moderately high DOS at the Fermi level. Further analysis of the electronic specific heat shows that the superconducting properties are dominated by s-wave gap.

Superconductivity in oxygen deficient LaNiAsO1−δ (δ = 0–0.3)

LaNiAsO itself is a superconductor with a Tc of ∼2.7K. Oxygen deficiency could be introduced up to a level of δ=0.3 with retaining the host structure and increase the Tc to ∼4.1K at δ=0.3. The Tc of LaNiAsO1−δ monotonically increases with increasing δ and changes negligibly when δ>0.3. Using the measured values of the lower critical field Hc1, the upper critical field Hc2, and the specific heat Cp, we estimated the thermodynamic critical field Hc(0), coherence length ξ(0), penetration depth λ(0), the Ginzburg–Landau parameter κ(0), and the electron–phonon coupling constant λep for LaNiAsO0.7 with an optimal Tc. The specific heat data follow a BCS-like exponential decrease below Tc, indicating a fully gapped superconductivity. In addition, the replacement of La by Sm failed to enhance the Tc. The many differences distinguish LaNiAsO from the 1111-type Fe-based superconductors, thus highlighting the importance of Fermi surface topology for superconductivity.

Scaling Behavior in Aluminum Doped MgB2 Superconductor

Magnetization measurements have been performed on Mg1−xAlxB2 (0≤x≤0.1) at various temperatures below Tc and in fields up to 9 T. The reversible magnetization has been used to calculate the thermodynamic critical field (Hc) which is then used to obtain a new universal behavior for both the critical current density (Jc) and the pinning force (Pf). This approach is based on vortex core energy (\({\sim}H^{2}_{c}\)) and clearly exhibits the deviation from single vortex pinning with increasing temperature and higher concentration of Al-doping.

PdTe: a strongly coupled superconductor

We report the electrical transport, magnetic, and thermodynamic properties of polycrystalline PdTe which exhibits superconductivity below 4.5 K. Using the measured values for the lower (Hc1) and upper (Hc2) critical fields, and the specific heat Cp, we estimate the thermodynamic critical field Hc(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg–Landau parameter κ. Compared with band structure calculations, the density of states at the Fermi level is enhanced due to electron–phonon coupling with λep = 1.4. Furthermore, the large values of ΔCp/γnTc and 2Δ(0)/kBTc suggest that PdTe is a strongly coupled superconductor.

Improved interplane and intergranular coupling by Mg doping at Ca site in Cu0.5Tl0.5Ba2Ca2(Cu0.5Zn2.5)O10−δ superconductor

Improvement in the interplane and intergranular coupling achieved by Mg doping at Ca site in high temperature Cu0.5Tl0.5Ba2Ca2(Cu0.5Zn2.5)O10−δ superconductor is studied by infield magnetic measurements. The decrease of c-axis length and volume of the unit cell of Cu0.5Tl0.5Ba2(Ca2−yMgy) (Cu0.5Zn2.5)O10−δ (y=0, 0.5, 1.0, and 1.5) superconductor with the increase of Mg doping shows the improved interplane coupling among CuO2∕ZnO2 planes. The presence of Mg at the termination ends of the crystals of Cu0.5Tl0.5Ba2(Ca2−yMgy) (Cu0.5Zn2.5)O10−δ compound has been found to improve intergrain coupling, which is evident from enhanced critical current density (Jc) of these samples. The exponent of temperature variation of thermodynamic critical field Hc∼(1−TP∕Tc)n suggests that the weak links form superconductor-insulator-superconductor type of junctions in all the samples. The presence of insulating material at the intergrain sites possibly increases the area of shielding current flow, which promotes the enhancement in Jc of final compound.

Superconducting proprties of noncentrosymmetric Mg10±δIr19B16?δ

Two non-centrosymmetric superconductors Mg9.3Ir19B16.7 and Mg11Ir19B15 are synthesized. The normal state electronic coefficient of specific heat γn, the Debye temperature ΘD and the superconducting upper critical field Hc2 are found to be sample-dependent, while the electron-phonon interaction constant λ=0.58 and the normalized specific heat jump ΔC/γnTC=1.66 are sample-independent. The higher superconducting transition temperature (TC=5.7K) in Mg9.3Ir19B16.7 is found to be due to the higher density of states and higher Debye temperature ΘD compared with that of Mg11Ir19B15. We also obtained other characteristic parameters of both superconductors, including the Ginzburg-London coherence ξGL, penetration λGL, lower critical field HC1 and thermodynamic critical field HC.

Upper critical field of RbOs2O6 analyzed within Eliashberg theory

The upper critical field Hc2(T) of the β-pyrochlore superconductor RbOs2O6 is analyzed within the framework of s-wave Eliashberg theory including anisotropy and scattering effects, yielding a small electron–phonon coupling anisotropy parameter of about 〈a2〉=0.005, and a Fermi velocity anisotropy parameter 〈b2〉=0.3–0.35. Agreement between theory and experiment is achieved for these parameters and a model spectral function α2F(ω) consisting of two Einstein peaks and a Debye contribution. A renormalization of the Fermi velocity by an enhancement factor due to electron–electron correlations 〈vF〉ee=1 is considered in the analysis of the experimental Hc2(T)-data. Comparison between experiment and theory of thermodynamic properties such as the specific-heat difference between superconducting and normal states Cs–Cn, and the thermodynamic critical field Hc(T) support the results from the upper critical field analysis.

Properties of the superconducting state in a two-band model

Eliashberg theory is used to investigate the range of thermodynamic properties possible within a two-band model for s-wave superconductivity and to identify signatures of its two-band nature. We emphasize dimensionless BCS ratios (those for the energy gaps, the specific heat jump and the negative of its slope near Tc, the thermodynamic critical field Hc(0), and the normalized slopes of the critical field and the penetration depth near Tc), which are no longer universal even in weak coupling. We also give results for temperature-dependent quantities, such as the penetration depth and the energy gap. Results are presented both for microscopic parameters appropriate to MgB2 and for variations away from these. Strong coupling corrections are identified and found to be significant. Analytic formulas are provided which show the role played by the anisotropy in coupling in some special limits. Particular emphasis is placed on small interband coupling and on the opposite limit of no diagonal coupling. The effect of impurity scattering is considered, particularly for the interband case.

Complementary study of heat capacity and magnetization for intermetallic YNi2B2C single crystal

The superconducting and magnetic properties of YNi2B2C single crystal have been investigated by heat capacity and dc-magnetic methods, with magnetic field applied parallel to the c-axis, i.e., H kð 001 Þ-direction. In the framework of heat capacity and magnetization analyses, we obtain the thermodynamic critical field Hc from both heat capacity and magnetization data. The heat capacity data deviate from predictions for both weak- and strong-coupling superconductivity, but are described relatively well in a medium-coupling analysis. The precise t 3 -dependence of the electronic heat capacity Ces indicates the gap anisotropy with the presence of point nodes for YNi2B2C single crystal.

Superconductivity based on a model with three overlapping energy bands and a nonphonon mechanism

A study is made of the thermodynamic properties of a three-band superconductor with an electron (or hole) mechanism of superconductivity. Interband carrier interaction is considered as the main reason for the occurrence of superconducting pairs. Expressions are obtained for the order parameters Δ n (T) (n=1, 2, 3), the specific heatC S, and the thermodynamic critical fieldH c in the region of low temperatures and also the temperatureT c of the superconducting transition, the jump in the specific heat (C S−CN) at the pointT=T c; and 2Δ n (0)/T c . It is shown that a characteristic feature in the behavior ofT c, (C S−CN), and Δ n (0) as functions of the carrier concentrationn is the presence of a smeared step. It is also shown that (C S−CN)/C N≤1.43 and that this ratio depends strongly on the relationship between the densities of the electron states of the considered bands.

Critical current density in quenched oxygen deficient YBa2Cu3O7−δ

The critical current density Jc was determined magnetically in polycrystalline sintered specimens of YBa2Cu3O7−δ as a function of magnetic field and temperature for 0.02⩽δ⩽0.31. The critical temperature of these quenched specimens decreases almost linearly with increasing δ. The highest Jc values at all temperatures and fields are found in the specimen with δ≈0. However, when taking into account that the thermodynamic critical field Hc decreases quite strongly with increasing δ it is concluded that the density of pinning centers increases as the oxygen deficiency increases. Low temperature equilibrations (keeping constant stoichiometry) significantly affect Jc values.

The electronic specific heat of YBa2 (Cu1−xZnx)3O7 from 1.6 K to 300 K

Using a differential technique we have determined between 1.6 K and 300 K and for 0⩽x⩽0.1 the difference in electronic specific heat coefficients γ(x, T)− γ(0.07, T)n of YBa2(Cu1−xZnx)3O7 and a YBa2(Cu0.93Zn0.07) reference. The results are consistent with a superconducting Fermi liquid and cannot be readily described in terms of the condensation at Tc of bosons preformed above Tc. We deduce γ(x, T) from these results, and find γn = 1.6±0.2 mJ/(g-at.K2) (density of states N (EF) = 8.8 states/(eV.fu)) independent of x. Superconducting anomalies are seen for all samples, with Tc and the anomaly height decreasing and γ(x, 0) increasing with x consistent with magnetic pair breaking. The thermodynamic critical field Hc(x, T) is determined for 0⩽x⩽0.07. We observe a (nearly) universal temperature dependence for the low temperature magnetic anomalies in Zn-doped and undoped samples with broad specific heat maxima at or above 5 K. For undoped YBa2Cu3O7 we find θ(0) = 406 K and an electronic term γ(0, 0) = 0.15±0.15 mJ/(g-at. K2) substantially smaller than the generally quoted value.