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Register a new userCoexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements
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Pabitra Kumar Biswas
Publication date
2019
fields
Physics
and research's language is
English
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Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.
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The application of the muon-spin rotation/relaxation ($mu$SR) technique for studying type-I superconductivity is discussed. In the intermediate state, i.e. when a type-I superconducting sample with non-zero demagnetization factor $N$ is separated into normal state and Meissner state (superconducting) domains, the $mu$SR technique allows to determine with very high precision the value of the thermodynamic critical field $B_{rm c}$, as well as the volume of the sample in the normal and the superconducting state. Due to the microscopic nature of $mu$SR technique, the $B_{rm c}$ values are determined directly via measurements of the internal field inside the normal state domains. No assumptions or introduction of any type of measurement criteria are needed. Experiments performed on a classical type-I superconductor, a cylindrically shaped $beta-$Sn sample, allowed to reconstruct the full $B-T$ phase diagram. The zero-temperature value of the thermodynamic critical field $B_{rm c}(0)=30.578(6)$ mT and the transition temperature $T_{rm c}=3.717(3)$ K were determined and found to be in a good agreement with the literature data. An experimentally obtained demagnetization factor is in very good agreement with theoretical calculations of the demagnetization factor of a finite cylinder. The analysis of $B_{rm c}(T)$ dependence within the framework of the phenomenological $alpha-$model allow to obtain the value of the superconducting energy gap $Delta=0.59(1)$ meV, of the electronic specific heat $gamma_e=1.781(3)$ ${rm mJ}/{rm mol}; {rm K}^2$ and of the jump in the heat capacity ${Delta C(T_c)}/{gamma T_{rm c}}=1.55(2)$.
The type-II Dirac semimetal PdTe2 was recently reported to be a type-I superconductor with a superconducting transition temperature Tc = 1.65 K. However, the recent results from tunneling and point contact spectroscopy suggested the unusual state of a mixture of type-I and type-II superconductivity. These contradictory results mean that there is no clear picture of the superconducting phase diagram and warrants a detailed investigation of the superconducting phase. We report here the muon spin rotation and relaxation ($mu$SR) measurements on the superconducting state of the topological Dirac semimetal PdTe2. From $mu$SR measurements, we find that PdTe2 exhibits mixed type-I/type-II superconductivity. Using these results a phase diagram has been determined. In contrast to previous results where local type-II superconductivity persists up to Hc2 = 600 G, we observed that bulk superconductivity is destroyed above 225 G.
The recent observation of superconductivity with critical temperatures up to 55 K in the FeAs based pnictide compounds marks the first discovery of a non copper-oxide based layered high-Tc superconductor (HTSC) [1-3]. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprates, since both exhibit superconductivity following charge doping of a magnetic parent material. Here we present a muon spin rotation study on SmFeAsO1-xFx (x=0-0.30), which shows that static magnetism persists well into the superconducting regime. The analogy with the cuprates is quite surprising since the parent compounds appear to have different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings suggest that proximity to magnetic order and associated soft magnetic fluctuations, rather than the strong electronic correlations in the vicinity of a Mott-Hubbard-metal-to-insulator transition, may be the key ingredients of HTSC.
We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7/PrBa2Cu3O7 trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures we show that a finite superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7 layers when juxtaposed to YBa2Cu3O7 electrodes while the intrinsic antiferromagnetic order is unaffected.
The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T-structured cuprate, T-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_csimeq21$ K) in its {sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the parent compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.
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