Anisotropy Of Upper Critical Field Research Articles

Anisotropic upper critical field, Seebeck and Nernst coefficient of Nb0.20Bi2Se3 superconductor

We present the magneto-transport and the thermoelectric (Seebeck and Nernst coefficient) studies of the Nb-doped Bi2Se3 superconductor. The angle-dependent magnetoresistance study highlights the anisotropy of upper critical field (Hc 2) for in-plane and out-of-plane magnetic field orientation with the anisotropy parameter Γ ∼ 1.3. The estimated value of the carrier concentration (∼1019 cm−3) for Nb0.2Bi2Se3 is one order larger than for Bi2Se3. Doping of Nb shows a significant decrease in the Seebeck coefficient value and the estimated Fermi temperature of the three-dimensional Fermi surface at the centre of Brillouin zone in the zero-temperature limit enhances by ∼4 times in comparison to pristine Bi2Se3. We have observed a large value (∼2.3 µV K−1T−1) of Nernst coefficient for Bi2Se3 at room temperature, which decreases with Nb doping (∼0.5 µV K−1 T−1).

Anisotropic superconductivity in the topological crystalline metal Pb1/3TaS2 with multiple Dirac fermions

Topological crystalline metals/semimetals (TCMs) have stimulated a great research interest, which broaden the classification of topological phases and provide a valuable platform to explore topological superconductivity. Here, we report the discovery of superconductivity and topological features in Pb-intercalated transition-metal dichalcogenide Pb$_{1/3}$TaS$_2$. Systematic measurements indicate that Pb$_{1/3}$TaS$_2$ is a quasi-two-dimensional (q-2D) type-II superconductor ({\em T}$_c \approx$ 2.8 K) with a significantly enhanced anisotropy of upper critical field ($\gamma_{H_{c2}}$ = $H_{c2}^{ab}/H_{c2}^{c}$ $\approx$ 17). In addition, first-principles calculations reveal that Pb$_{1/3}$TaS$_2$ hosts multiple topological Dirac fermions in the electronic band structure. We discover four groups of Dirac nodal lines on the $k_z = \pi$ plane and two sets of Dirac points on the rotation/screw axes, which are protected by crystalline symmetries and robust against spin-orbit coupling (SOC). Dirac-cone-like surface states emerge on the (001) surface because of band inversion. Our work shows that the TCM candidate Pb$_{1/3}$TaS$_2$ is a promising arena to study the interplay between superconductivity and topological Dirac fermions.

The 2D–3D crossover and anisotropy of upper critical fields in Nb and NbN superconducting thin films

The upper critical field (Bc2) of superconducting thin films of Nb (film thickness d = 40 nm) and NbN (d = 10, 50 and 100 nm) have been measured and analyzed as a function of temperature (T = 1.8–15 K) in both parallel and perpendicular magnetic field (B = 0–7 T) directions with respect to the film plane. Dimensional crossover from 2D to 3D is observed for all the thin films except for the 10 nm thick NbN film which exhibits only 2D behavior in the measured T and B range. Further, the upper critical fields were found to be anisotropic in case of both Nb and NbN films. However, the anisotropy shows different behavior in all the four samples. In case of Nb, below the 2D–3D crossover temperature, the Bc2∥(T) is higher than Bc2⊥(T) and both curves branch away further from each other. On the other hand, in case of NbN (50 and 100 nm), there is a reversal of anisotropy, i.e., in the 2D region Bc2∥(T) > Bc2⊥(T) and in the 3D region at some temperature Bc2⊥(T) > Bc2∥(T). In the NbN films with increasing thickness, the 2D region shrinks and 3D region expands, and the 2D–3D crossover and anisotropy reversal characteristic temperatures shift toward Tc. These observations are quantitatively explained using the Ginzburg–Landau theoretical approach after Schneider and Locquet (Physica C, 179 (1991) 125).

Dimensional Crossover and Its Interplay with In-Plane Anisotropy of Upper Critical Field in β-(BDA-TTP)2SbF6

Resistance measurements have been performed to investigate the dimensionality and the in-plane anisotropy of the upper critical field (Hc2) for β-(BDA-TTP)2SbF6 in fields H up to 15 T and at temperatures T from 1.5 to 7.5 K, where BDA-TTP stands for 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene. The upper critical fields parallel and perpendicular to the conduction layer are determined and dimensional crossover from anisotropic three-dimensional behavior to two-dimensional behavior is found at around 6 K. When the direction of H is varied within the conducting layer at 6.0 K, Hc2 shows twofold symmetry: Hc2 along the minimum Fermi wave vector (maximum Fermi velocity) is larger than that along the maximum Fermi wave vector (minimum Fermi velocity). The normal-state magnetoresistance has twofold symmetry similar to Hc2 and shows a maximum when the magnetic field is nearly parallel to the maximum Fermi wave vector. This tendency is consistent with the Fermi surface anisotropy. At 3.5 K, we found ...

Bulk Superconductivity Induced by In-Plane Chemical Pressure Effect in Eu0.5La0.5FBiS2−xSex

We have investigated Se substitution effect to superconductivity of an optimally-doped BiS2-based superconductor Eu0.5La0.5FBiS2. Eu0.5La0.5FBiS2-xSex samples with x = 0-1 were synthesized. With increasing x, in-plane chemical pressure is enhanced. For x = 0.6, 0.8, and 1, superconducting transitions with a large shielding volume fraction are observed in magnetic susceptibility measurements, and the highest Tc is 3.8 K for x = 0.8. From low-temperature electrical resistivity measurements, a zero-resistivity state is observed for all the samples, and the highest Tc is observed for x = 0.8. With increasing Se concentration, characteristics of electrical resistivity changes from semiconducting-like to metallic, suggesting that the emergence of bulk superconductivity is linked with the enhanced metallicity. A superconductivity phase diagram of the Eu0.5La0.5FBiS2-xSex superconductor is established. Temperature dependences of electrical resistivity show an anomalous two-step transition under high magnetic fields. Hence, the resistivity data are analyzed with assuming in-plane anisotropy of upper critical field.

Superconducting properties in single crystals of the topological nodal semimetalPbTaSe2

We report the electronic properties of superconductivity in the topological nodal-line semimetal PbTaSe$_2$. Angle-resolved photoemission measurements accompanied by band calculations confirmed the nodal-line band structure in the normal state of single crystalline PbTaSe$_2$. Resistivity, magnetic-susceptibility and specific heat measurements have also been performed on high-quality single crystals. We observed upward features and large anisotropy in upper critical field ($H_{c2}$) measured in-plane (H//\textbf{ab}) and out-plane (H//\textbf{c}), respectively. Especially, $H_{c2}$ measured in H//\textbf{ab} shows sudden upward features rather than a signal of saturation in ultralow temperatures. The specific heat measurements under magnetic field reveal a full superconducting gap with no gapless nodes. These behaviors in this clean noncentrosymmetric superconductor is possibly related to the underlying exotic physics, providing important clue for realization of topological superconductivity.

Topological characteristics of electron density and magnetic anisotropy in MgB2

Electron density of MgB2 and hypothetical clusters {Mg14B24} was calculated and studied with quantum chemical methods based on the Density Functional Theory. Topological characteristics of electron density distribution are shown to be related to the anisotropy of local induced magnetic fields in {Mg14B24}. Our data correlate with experimental anisotropy of upper critical fields in MgB2 in the vicinity of the phase transition into the superconducting state.

Specific Heat and Upper Critical Field of Sc5Ir4Si10Superconductor

Temperature and magnetic field dependent specific heat of textured Sc5Ir4Si10 superconductor was investigated in detail. Based on the fitting of zero-field electronic specific heat by different gap structures as well as the discussion on field-induced specific heat coefficient, Sc5Ir4Si10 is proved to be an anisotropic s-wave superconductor with a gap anisotropy of 0.53. The anisotropy of upper critical field suggests the weakly one-dimensional Fermi surface, and the value is consistent with the gap anisotropy result obtained from the specific heat data.

Anisotropies and Homogeneities of Superconducting Properties in Iron–Platinum–Arsenide Ca10(Pt3As8)(Fe1.79Pt0.21As2)5

We report a study on the anisotropy of superconducting properties in single crystalline Ca10(Pt4-{\delta}As8)(Fe1.79Pt0.21As2)5 with Tc ~13.6 K. Under a field of 5 Oe, the sample reaches fully-diamagnetic state at ~10 K for H//c, and ~8 K for H//ab, indicating the presence of slight inhomogeneities. The magnetization measurements reveal fish-tail effect in the hysteresis loop for both H//c and H//ab. Averaged critical current densities at low magnetic fields along the c-axis and ab-plane Jc(H//c) and Jc(H//ab) at 5 K are estimated to be 0.9*10^5 and 0.7*10^5 A/cm2, respectively. Resistive transitions under magnetic field show broadening, which is consistent with a relatively large anisotropy of upper critical field. Magneto-optical images reveal homogenous current flow within the ab-plane.

Upper Critical Fields and Anisotropy of BaFe1.9 Ni0.1As2 Single Crystals

Temperature dependence of the upper critical magnetic field (Hc2) near Tc of 20 K in a BaFe1.9Ni0.1As2 single crystal is determined via magneto-resistance measurements, for the out-plane (H⊥ab) and in-plane (H || ab) directions in magnetic fields of up to 8 T. The upper critical fields at zero temperature estimated by the Werthamer-Helfand-Hohenberg (WHH) formula are μ0H||c2(0) = 137 T and μ0H⊥c2(0) = 51 T, both exceeding the weak-coupling Pauli paramagnetic limit (μ0Hp = 1.84Tc). However, the WHH formula could overestimate the μ0H||c2(0) value. The anisotropy of upper critical fields is around 3 in the temperature range close to Tc. The result is very similar to the Co-doped 122 superconductor BaFe2–xCoxAs2, indicating that electron-doped 122 superconductors exhibit similar superconducting properties.

Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductors

In this article, we review our recent studies of microwave penetration depth, lower critical fields, and quasiparticle conductivity in the superconducting state of Fe-arsenide superconductors. High-sensitivity microwave surface impedance measurements of the in-plane penetration depth λ ab in single crystals of electron-doped PrFeAsO 1 - y ( y ∼ 0.1 ) and hole-doped Ba 1 - x K x Fe 2 As 2 ( x ≈ 0.55 ) are presented. In clean crystals of Ba 1 - x K x Fe 2 As 2 , as well as in PrFeAsO 1 - y crystals, the penetration depth shows flat temperature dependence at low temperatures, indicating that the superconducting gap opens all over the Fermi surface. The temperature dependence of superfluid density λ ab 2 ( 0 ) / λ ab 2 ( T ) in both systems is most consistent with the existence of two different gaps. In Ba 1 - x K x Fe 2 As 2 , we find that the superfluid density is sensitive to degrees of disorder inherent in the crystals, implying unconventional impurity effect. We also determine the lower critical field H c 1 in PrFeAsO 1 - y by using an array of micro-Hall probes. The temperature dependence of H c 1 saturates at low temperatures, fully consistent with the superfluid density determined by microwave measurements. The anisotropy of H c 1 has a weak temperature dependence with smaller values than the anisotropy of upper critical fields at low temperatures, which further supports the multi-gap superconductivity in Fe-arsenide systems. The quasiparticle conductivity shows an enhancement in the superconducting state, which suggests the reduction of quasiparticle scattering rate due to the gap formation below T c . From these results, we discuss the structure of the superconducting gap in these Fe-arsenides, in comparison with the high- T c cuprate superconductors.

Transport and anisotropy in single-crystallineSrFe2As2andA0.6K0.4Fe2As2(A=Sr, Ba) superconductors

We have successfully grown high-quality single crystals of ${\text{SrFe}}_{2}{\text{As}}_{2}$ and ${A}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$($A=\text{Sr}$, Ba) using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound ${\text{SrFe}}_{2}{\text{As}}_{2}$, an anisotropic resistivity with ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{ab}$ as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave (SDW) instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold symmetry with field rotating within $ab$ plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K-doped ${A}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$($A=\text{Sr}$, Ba), the SDW instability is suppressed and the superconductivity appears with ${T}_{c}$ above 35 K. The rather low anisotropy in upper critical field between $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$ indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.

Specific-Heat Evidence for Two-Gap Superconductivity in the Ternary-Iron SilicideLu2Fe3Si5

We report low-temperature specific-heat studies on the single-crystalline ternary-iron silicide superconductor Lu(2)Fe(3)Si(5) with T(c)=6.1 K down to approximately T(c)/20. We confirm a reduced normalized jump in specific heat at T(c), and find that the specific heat divided by temperature C/T shows a sudden drop at approximately T(c)/5 and goes to zero with further decreasing temperature. These results indicate the presence of two distinct superconducting gaps in Lu(2)Fe(3)Si(5), similar to the typical two-gap superconductor MgB(2). We also report Hall coefficients, band structure calculations, and the anisotropy of upper critical fields for Lu(2)Fe(3)Si(5), which support the anisotropic multiband nature and reinforce the existence of two superconducting gaps in Lu(2)Fe(3)Si(5).

Anisotropic upper critical field of chemically substituted MgB2 single crystals studied by torque magnetometry

The influence of Al, Mn, and Fe ion substitution for Mg and C ion substitution for B on the anisotropy of upper critical field (Hc2) of high quality MgB2 single crystals was studied. The torque investigations of Mg1−xAlxB2 crystals showed the slight increase of Hc2∥c for the samples with small x and significant reduction of Hc2 anisotropy, γHc2, at lower temperatures, as compared to the value for unsubstituted crystals. In Mg(B0.94C0.06)2 single crystals, Hc2∥c(0)≈85kOe is more than twice as large as that for unsubstituted MgB2. Anisotropy of Hc2 decreases to about 4 at low temperatures, i.e., to the value considerably lower than that for MgB2, and its temperature dependence is much less pronounced. The corresponding Hc2∥ab(0)≈340kOe is close to the maximum possible enhancement of Hc2 due to the chemical substitutions. In addition to the increased scattering of carriers, substitution of magnetic Mn ions for Mg, strongly suppresses Tc and Hc2; however, it is not the case for the substitution of Fe ions for Mg.

Irreversibility fields and upper critical fields in multifilamentary Nb-Ti composites having artificial pins with anisotropic microstructure

Anisotropy of upper critical fields B/sub C2/ and irreversibility fields B/sub i/ was investigated for multifilamentary Nb-Ti composites having artificial pinning centers in perpendicular and parallel magnetic fields to the filament axis. It was observed that the B/sub C2/spl par// exceeded the B/sub C2/spl perp// below a certain temperature T/sup */ and the temperature dependence of B/sub C2/spl par// was close to (1-T/T/sub c/)/sup 0.5/. These results suggest that dimensional crossover occurs at T/sup */, i.e., the magnetic flux system in the parallel field changes from 3D to 2D state as predicted by the Takahashi-Tachiki theory for multilayer superconductors. B/sub i/spl par// is also enhanced due to no restriction of the superconductor size, as explained by the flux creep theory. Consequently, the flux pinning strength in the parallel magnetic field to the filament axis increased at high fields. Furthermore, tape specimens with arranged plate-like pins fabricated by rolling process indicated expected enhancement of flux pinning.

Comparative study on the anisotropic properties of MgB2

The anisotropy of upper critical field of MgB2 has been studied on single crystals and poly-crystals by both transport and magnetic measurements. The angular dependence of Hc2 shows deviation from the anisotropic Ginzburg–Landau model at lower temperatures. The value of anisotropy parameter is temperature and sample dependent, and is about 3 and 4.5 for single and poly-crystals, respectively, at temperatures near Tc. However, Hc2c(T) are almost the same for all samples. These features could be an indication of anisotropic s-wave superconductivity with pancake-like energy gap or resulted from the different impurity levels in these samples. The anisotropy of Hc2 in ab-plane has also been measured and we set an upper bound of 1% for the in-plane anisotropy.

Anisotropy of Upper Critical Field and Pairing Symmetry

We calculate the angular dependence of the upper critical field ${H}_{c2}$, taking account of the effect of the gap anisotropy and nonlocal correction. If the Fermi surface is nearly isotropic in the $a\ensuremath{-}b$ plane, the results show that the angular dependence of the upper critical field for ${\mathrm{LaSrCuO}}_{4}$ in the $a\ensuremath{-}b$ plane is qualitatively explained by an order parameter $\ensuremath{\Delta}$ with the symmetry of the pairing state ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$, consistent with a recent experiment by Raman spectroscopy.

Superconductivity in UPt3

The theory of basal plane anisotropy of upper critical field in hexagonal heavy fermionic superconductors UPt3 is developed. It is shown, that the recent experimental discovery of the sign change of this anisotropy above the tetracritical point in ( H, T) plane proves to be the most important experimental argument in favour of the model of unconventional superconductivity mixing in the order parameters transforming according to different one-dimensional representation of the hexagonal group. The current model describing the superconducting phase transition splittig in UPt3 are critically overviewed in the light of mentioned observation.

Magnetic properties of the heavy-fermion superconductors UPt 3 and URu 2Si 2

Magnetization measurements have been performed in order to investigate further the temperature dependence of the lower critical field H c 1, the Meissner effect and the critical currents on single crystals of the anisotropic Heavy-Fermion Superconductors UPt 3 and URu 2Si 2. For the UPt 3 single crystals, no marked anisotropy in the lower critical field has been observed, but a change in slope could be identified at Δ T≈;65 mK below T c . Extrapolation of the lower critical field to zero temperature gives a value of 6.5 mT in both directions ( H∥ c, H⊥ c). The lower critical field of URu 2Si 2 roughly shows a linear temperature dependence in the range from T c to ∼ 150 mK. At T ≈; 150 mK H c 1 seems to pass through a maximum before eventually dropping down. The magnetization measurements have shown that H c 1 of URu 2Si 2 is isotropic for H∥ c and H ⊥ c with a value H c 1 (0) of 3.3 mT. This isotropy of the H c 1 contrasts strongly with the anisotropy of upper critical field H c 2( T). The measured hysteresis of the magnetization of URu 2Si 2 is more important than in the case of UPt 3 and exhibits magnetization jumps at low temperatures. Applying the Bean-model, critical current densities of 3.8 × 10 3 A cm −2 and 2.4 × 10 4 A cm −2 are deduced for UPt 3 and URu 2Si 2 respectively at T/ T c ≈;0.2 and μ 0 H i ≈;80 mT.

Anisotropy of upper critical field near TC and magnetic gap of superconducting URu2Si2 single crystal.

Studying thermal conductivity and STM spectra in the normal and magnetic phase of superconducting URu2Si2 single crystal, we found, that the magnetic gap, partially opened on the Fermi surface below Neel temperature TN = 17.5 K, is strongly anisotropic: gapped states mainly correspond to tetragonal ab plane.