We use Superconducting QUantum Interference Device (SQUID) microscopy to characterize the current-phase relation (CPR) of Josephson Junctions from 3-dimentional topological insulator HgTe (3D-HgTe). We find clear skewness in the CPRs of HgTe junctions ranging in length from 200 nm to 600 nm. The skewness indicates that the Josephson current is predominantly carried by Andreev bound states with high transmittance, and the fact that the skewness persists in junctions that are longer than the mean free path suggests that the effect may be related to the helical nature of the Andreev bound states in the surface of HgTe.Topological insulators (TI) have a special band structure with important consequences for proximity-induced superconductivity. In 3-dimentional topological insulators (3D-TI), the inversion of the conduction and valence bands leads to conducting 2D surface states with energies that are linearly proportional to their momenta [1][2][3][4][5]. Spinmomentum locking protects the charge carriers at the surface against elastic backscattering [6,7]. These special properties are reflected in the superconducting proximity effect in an S/3D-TI bilayer or an S/TI/S junction, which may host Majorana fermions in a quasi-1D channel or vortex core [8][9][10]. Most previous works characterized current-voltage characteristics to determine the critical current's dependence on temperature, gate voltage, or magnetic field [11][12][13][14][15][16][17][18][19][20][21][22], while a few studies characterized the CPR [23,24].Here, we use a scanning SQUID microscope to perform contactless measurements of the diamagnetic response of Nb/HgTe bilayers and of the CPR of Nb/HgTe/Nb junctions. In contrast to previous CPR results [23,24], we find no evidence for bulk states, 2 and we observe that the CPRs of many junctions of different sizes consistently exhibit forward skewness.The CPR in an S/TI/S junction is a key diagnostic [8,[25][26][27][28][29][30][31][32]. Weak disorder in the TI far from the superconducting contacts theoretically does not affect the induced superconducting state [33,34]; therefore, Andreev bound states should form in hightransmittance surface channels [8,26,27,29,31]. A CPR with forward skewness -that is, a deviation from a perfect sinusoidal form -is a signature of such high-transmittance Andreev bound states [35][36][37].To our knowledge, there have not been direct observations of forward skewed CPRs in topological insulators [23,24], although the skewness has been indirectly inferred [24] from the Fraunhofer interference pattern. Previous CPR experiments in topological insulators [23,24] were complicated in part by bulk states, self-inductance effects, and bias voltage, factors that are eliminated in this work.Moreover, a skewed CPR can also result from ballistic transport [35]. Measurements in metallic break junctions showed that the CPR approaches the predictions for quantum point contacts in the ballistic limit [38]. In metallic atomic point contacts, the CPR was significantly skewed only in contacts wi...

Measurements on nanoscale structures constructed from high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. To date, loops made from these compounds have had sizes of the order of hundreds of nanometres(8-11). Here, we report the results of measurements on loops of La(1.84)Sr(0.16)CuO(4), a high-temperature superconductor that loses its resistance to electric currents when cooled below approximately 38 K, with dimensions down to tens of nanometres. We observe oscillations in the resistance of the loops as a function of the magnetic flux through the loops. The oscillations have a period of h/2e, and their amplitude is much larger than the amplitude of the resistance oscillations expected from the Little-Parks effect. Moreover, unlike Little-Parks oscillations, which are caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally excited moving vortices and the oscillating persistent current induced in the loops. However, despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of h/e, as recently predicted for nanoscale loops of superconductors with d-wave symmetry, or with a period of h/4e, as predicted for superconductors that exhibit stripes.

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Proximity to a superconductor is predicted to induce exotic quantum phases in topological insulators. Here, scanning superconducting quantum interference device (SQUID) microscopy reveals that aluminum superconducting rings with topologically insulating Bi2Se3 junctions exhibit a conventional, nearly sinusoidal 2π-periodic current-phase relations. Pearl vortices occur in longer junctions, indicating suppressed superconductivity in aluminum, probably due to a proximity effect. Our observations establish scanning SQUID as a general tool for characterizing proximity effects and for measuring current-phase relations in new materials systems.

Much of the focus of modern condensed matter physics concerns control of quantum phases with examples that include flat band superconductivity in graphene bilayers (1), the interplay of magnetism and ferroelectricity (2), and induction of topological transitions by strain (3). Here we report the first observation of a reproducible and strong enhancement of the superconducting critical temperature, Tc, in strontium titanate (SrTiO3) obtained through careful strain engineering of interacting superconducting phase and the polar quantum phase (quantum paraelectric). Our results show a nearly 50% increase in Tc with indications that the increase could become several hundred percent. We have thus discovered a means to control the interaction of two quantum phases through application of strain, which may be important for quantum information science. Further, our work elucidates the enigmatic pseudogap-like and preformed electron pairs phenomena in low dimensional strontium titanate (4, 5) as potentially resulting from the local strain of jammed tetragonal domains. Main text:Among the main goals of this work is to address the open question of the nature of the superconducting pairing mechanism in strontium titanate (STO) (6, 7) and to inspire searches for enhanced superconducting temperatures in materials not just with suppressed to zero Kelvin structural transitions, as in (CaxSr1−x)3Rh4Sn13 (8), MoTe2 (9) and Lu(Pt1−xPdx)2In ( 10), but with incipient quantum phase transitions, for example, ScF3 which has a structural quantum phase transition (11), and may become superconducting when doped (12). It has been predicted that superconducting doped strontium titanate with its peculiar phonon dynamics (13-17) is an example of a superconductivity arising near an incipient quantum polar (quantum ferroelectric) phase transition (4,7,(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), but this has not been fully demonstrated experimentally, in part, due to the fact that existing results on isotope effect and Ca substitution (25, 28) may be explained by non-uniformity in the chemical composition, and the absolute enhancement of the critical temperature values have not been found.It is also unusual to find a pseudogap-like behavior in superconductors that cannot be explained by compositional inhomogeneities, as is the case in cuprates (29). A pseudogap-like behaviors, such as a tunneling gap and a 2e charge transport, occur in STO at temperatures up to about 0.9 K, almost twice the bulk superconducting transition temperature (4, 5).