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Register a new userIntrinsic non-linear conduction in the super-insulating state of thin TiN films
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We investigate experimentally the electric transport at the insulating side of the superconductor to insulator transition in thin TiN-films. At temperatures T > 50 mK we observe an Arrhenius-type conductance, with an activation energy depending logarithmically on the sample size. At high bias the current voltage (I-V) characteristics display a large current jump into an electron heating dominated regime. For the largest samples, and below 50 mK we observe a low-bias power law I ~ V^alpha characteristics with an exponent alpha > 1 rapidly growing with decreasing temperature, which is expected for a binding-unbinding crossover of the charge-Berezinskii-Kosterlitz-Thouless type.
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We establish quasi-two-dimensional thin films of iron-based superconductors (FeSCs) as a new high-temperature platform for hosting intrinsic time-reversal-invariant helical topological superconductivity (TSC). Based on the combination of Dirac surface state and bulk extended $s$-wave pairing, our theory should be directly applicable to a large class of experimentally established FeSCs, opening a new TSC paradigm. In particular, an applied electric field serves as a topological switch for helical Majorana edge modes in FeSC thin films, allowing for an experimentally feasible design of gate-controlled helical Majorana circuits. Applying an in-plane magnetic field drives the helical TSC phase into a higher-order TSC carrying corner-localized Majorana zero modes. Our proposal should enable the experimental realization of helical Majorana fermions.
The effects of microwave radiation on the transport properties of atomically thin $La_{2-x}Sr_xCuO_4$ films were studied in the 0.1-13 GHz frequency range. Resistance changes induced by microwaves were investigated at different temperatures near the superconducting transition. The nonlinear response decreases by several orders of magnitude within a few GHz of a cutoff frequency $ u_{cut} approx$ 2 GHz. Numerical simulations that assume an ac response to follow the dc V-I characteristics of the films reproduce well the low frequency behavior, but fail above $ u_{cut}$. The results indicate that two-dimensional superconductivity is resilient against high-frequency microwave radiation, because vortex-antivortex dissociation is dramatically suppressed in two-dimensional superconducting condensates oscillating at high frequencies.
In two-dimensional (2D) superconductors an insulating state can be induced either by applying a magnetic field, $H$, or by increasing disorder. Many scenarios have been put forth to explain the superconductor to insulator transition (SIT): dominating fermionic physics after the breaking of Cooper pairs, loss of phase coherence between superconducting islands embedded in a metallic or insulating matrix and localization of Cooper pairs with concomitant condensation of vortex-type excitations. The difficulty in characterizing the insulating state and its origin stems from the lack of a continuous mapping of the superconducting to insulating phase diagram in a single sample. Here we use the two-dimensional (2D) electron liquid formed at the interface between the two insulators (111) SrTiO$_3$ and LaAlO$_3$ to study the superconductor to insulator transition. This crystalline interface surprisingly exhibits very strong features previously observed only in amorphous systems. By use of electrostatic gating and magnetic fields, the sample is tuned from the metallic region, where supeconductivity is fully manifested, deep into the insulating state. Through examination of the field dependence of the sheet resistance and comparison of the response to fields in different orientations we identify a new magnetic field scale, H$_{pairing}$, where superconducting fluctuations are muted. Our findings show that vortex fluctuations excitations and Cooper pair localization are responsible for the observed SIT and that these excitations surprisingly persist deep into the insulating state.
We show how intrinsic material properties modify light transmission through subwavelength hole arrays on thin metallic films in the THz regime. We compare the temperature-dependent transmittance of Au films and MgB$_{2}$ films. The experimental data is consistent with analytical calculations, and is attributed to the temperature change of the conductivity of both films. The transmission versus conductivity is interpreted within the open resonator model when taking the skin depth into consideration. We also show that the efficiency of this temperature control depends on the ratio of the transmission peak frequency to the superconducting energy gap in MgB$_{2}$ films.
We report the first experimental observation of the quenching of the superconducting state in current-voltage characteristics of an iron-based superconductor, namely, in FeSeTe thin films. Based on available theoretical models, our analysis suggests the presence of an intrinsic flux-flow electronic instability along with non-negligible extrinsic thermal effects. The coexistence and competition of these two mechanisms classify the observed instability as halfway between those of low-temperature and of high-temperature superconductors, where thermal effects are respectively largely negligible or predominant.
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