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Register a new userAngular dependence of the upper critical field in the high-pressure $1T$ phase of MoTe$_2$
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Superconductivity in the type-II Weyl semimetal candidate MoTe$_2$ has attracted much attention due to the possible realization of topological superconductivity. Under applied pressure, the superconducting transition temperature is significantly enhanced, while the structural transition from the high-temperature 1$T$ phase to the low-temperature $T_d$ phase is suppressed. Hence, applying pressure allows us to investigate the dimensionality of superconductivity in 1$T$-MoTe$_2$. We have performed a detailed study of the magnetotransport properties and upper critical field $H_{c2}$ of MoTe$_2$ under pressure. The magnetoresistance (MR) and Hall coefficient of MoTe$_2$ are found to be decreasing with increasing pressure. In addition, the Kohlers scalings for the MR data above $sim$11 kbar show a change of exponent whereas the data at lower pressure can be well scaled with a single exponent. These results are suggestive of a Fermi surface reconstruction when the structure changes from the $T_d$ to 1$T$ phase. The $H_{c2}$-temperature phase diagram constructed at 15 kbar, with $Hparallel ab$ and $Hperp ab$, can be satisfactorily described by the Werthamer-Helfand-Hohenberg model with the Maki parameters $alpha sim$ 0.77 and 0.45, respectively. The relatively large $alpha$ may stem from a small Fermi surface and a large effective mass of semimetallic MoTe$_2$. The angular dependence of $H_{c2}$ at 15 kbar can be well fitted by the Tinkham model, suggesting the two-dimensional nature of superconductivity in the high-pressure 1$T$ phase.
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We have carried out high-field resistivity measurements up to 27,T in EuFe$_2$As$_2$ at $P$,=,2.5,GPa, a virtually optimal pressure for the $P$-induced superconductivity, where $T_mathrm{c}$,=,30,K. The $B_mathrm{c2}-T_mathrm{c}$ phase diagram has been constructed in a wide temperature range with a minimum temperature of 1.6 K ($approx 0.05 times T_mathrm{c}$), for both $B parallel ab$ ($B_mathrm{c2}^mathrm{ab}$) and $B parallel c$ ($B_mathrm{c2}^mathrm{c}$). The upper critical fields $B_mathrm{c2}^mathrm{ab}$(0) and $B_mathrm{c2}^mathrm{c}$(0), determined by the onset of resistive transitions, are 25 T and 22 T, respectively, which are significantly smaller than those of other Fe-based superconductors with similar values of $T_mathrm{c}$. The small $B_mathrm{c2}(0)$ values and the $B_mathrm{c2}(T)$ curves with positive curvature around 20 K can be explained by a multiple pair-breaking model that includes the exchange field due to the magnetic Eu$^{2+}$ moments. The anisotropy parameter, $Gamma=B_mathrm{c2}^{ab}/B_mathrm{c2}^{c}$, in EuFe$_2$As$_2$ at low temperatures is comparable to that of other 122 Fe-based systems.
We present measurements of the superconducting critical temperature Tc and upper critical field Hc2 as a function of pressure in the transition metal dichalcogenide 2H-NbS2 up to 20 GPa. We observe that Tc increases smoothly from 6K at ambient pressure to about 8.9K at 20GPa. This range of increase is comparable to the one found previously in 2H-NbSe2. The temperature dependence of the upper critical field Hc2(T) of 2H-NbS2 varies considerably when increasing the pressure. At low pressures, Hc2(0) decreases, and at higher pressures both Tc and Hc2(0) increase simultaneously. This points out that there are pressure induced changes of the Fermi surface, which we analyze in terms of a simplified two band approach.
We studied $ab$-plane transport properties in single crystals of the superconductor $beta$-FeSe up to 16 T. In the normal state, below 90 K, the crystals present a strongly anisotropic positive magnetoresistance that becomes negligible above that temperature. In the superconducting state (T$_c$=8.87(5) K) the upper critical field anisotropy $H$$_{c2}$$parallel$$ab$ / $H$$_{c2}$$parallel$$c$ changes with temperature and the angular dependence of the dissipation for fixed temperatures and fields reflects a strongly anisotropic behavior. Our results make evident that multiband effects are needed to describe the measured transport properties. We model the magnetoresistance and upper critical field behavior with a two-band model showing that the diffusivities ratio parameter remains unchanged going from the normal to the superconducting state.
We measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criterion-dependence Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2/dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than mu_0H_LLL=8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc. In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound.
Detailed measurements of the in-plane resistivity were performed in a high-quality Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x=0.065$) single crystal, in magnetic fields up to 9 T and with different orientations $theta$ relative to the crystal $c$ axis. A significant $rho(T)_{H,theta}$ rounding is observed just above the superconducting critical temperature $T_c$ due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us to carry out a criterion-independent determination of the angular dependence of the upper critical field, $H_{c2}(theta)$. In spite of the relatively small anisotropy of this compound, it is found that $H_{c2}(theta)$ presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large $theta$ (typically above $sim60^o$). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar $H_{c2}(theta)$ anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near $T_c$, a result that differs from the ones obtained by using a single-band model.
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