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Anisotropic transport properties have been assessed in a number of cuprate superconductors, providing evidence for a nematic state. Here, we analyze recent experimental data for ultrathin YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) films, where nematicity is induced via strain engineering, leading to a suppression of charge density wave scattering along the orthorhombic a-axis and a concomitant enhancement of strange metal behavior along the b-axis. It is shown that the anisotropic properties strongly depend on the substrate, which we characterize by atomic force microscopy (AFM). Based on the AFM data, we provide a microscopic model that can account for the absence (presence) of nematicity and the resulting transport properties in films grown on SrTiO$_3$ (MgO) substrates., 7 pages, 5 figures

The strange metal phase, which dominates large part of the phase diagram of the cuprates, has been found in many quantum materials, including pnictides, heavy fermions and magic angle double layer graphene. One suggested origin of this universality is that it stems from the presence of a quantum critical point (QCP), i.e., a phase transition at zero temperature ruled by quantum fluctuations, in contrast to ordinary phase transitions ruled by thermal fluctuations. Although in cuprates superconductivity hinders the direct observation of the QCP phenomenology at low temperatures, indirect evidence comes from the identification of fluctuations compatible with the strange metal phase at higher temperatures. Here we show that the recently discovered charge density fluctuations (CDF) possess the right properties to be associated to a quantum phase transition. By using resonant x-ray scattering, we have studied the CDF in two families of cuprate superconductors over a wide range of doping $p$, up to $p$ = 0.22. We show that at $p^*\approx$0.19, corresponding to the putative QCP, the CDF intensity is the strongest, and the characteristic energy $\Delta$ is the smallest, marking a wedge-shaped region in the phase diagram indicative of a quantum critical behavior, although with anomalous properties. These results support the leading role of charge order in driving the unconventional phenomenology of the strange metal and add new elements for the understanding of high critical temperature superconductivity., Comment: 35 pages, 4 figures, 8 supplementary figures. Revised version, including data on Ca-YBCO (p=0.22)

We report on measurements of the doping dependence of the upper critical field $H_{c,2}$ in 50 nm thick YBa$_2$Cu$_3$O$_{7-δ}$ films. The films are untwinned and are characterized by a small in-plane compressive strain. We find that the $H_{c,2}$ shows a strong decrease in the underdoped region of the phase diagram, in agreement with what has been measured in relaxed single crystals. The origin of the decrease of $H_{c,2}$ in the underdoped regime is discussed within a scenario where charge density wave order competes with superconductivity. This demonstrates the potential of using thin films for studying the phase diagram of high-$T_c$ materials under strain.

The normal state of optimally doped cuprates is dominated by the "strange metal" phase that shows a linear temperature ($T$) dependence of the resistivity persisting down to the lowest $T$. For underdoped cuprates this behavior is lost below the pseudogap temperature $T^*$, where Charge Density Waves (CDW) together with other intertwined local orders characterize the ground state. Here we show that the $T$-linear resistivity of highly strained, ultrathin and underdoped YBa$_2$Cu$_3$O$_{7-��}$ films is restored when the CDW amplitude, detected by Resonant Inelastic X-ray scattering, is suppressed. This observation points towards an intimate connection between the onset of CDW and the departure from $T$-linear resistivity in underdoped cuprates. Our results illustrate the potential of using strain control to manipulate the ground state of quantum materials.

The transport properties of a YBa$_2$Cu$_3$O$_{7-��}$ superconducting quantum interference device (SQUID) based on grooved Dayem bridge weak links are studied as a function of temperature: at high temperatures ($60~$K$

Graphene on silicon carbide (SiC) has proved to be highly successful in Hall conductance quantization for its homogeneity at the centimetre scale. Robust Josephson coupling has been measured in co-planar diffusive Al/monololayer graphene/Al junctions. Graphene on SiC substrates is a concrete candidate to provide scalability of hybrid Josephson graphene/superconductor devices, giving also promise of ballistic propagation.

In superconductor-topological insulator-superconductor hybrid junctions, the barrier edge states are expected to be protected against backscattering, to generate unconventional proximity effects, and, possibly, to signal the presence of Majorana fermions. The standards of proximity modes for these types of structures have to be settled for a neat identification of possible new entities. Through a systematic and complete set of measurements of the Josephson properties we find evidence of ballistic transport in coplanar Al-Bi2Se3-Al junctions that we attribute to a coherent transport through the topological edge state. The shunting effect of the bulk only influences the normal transport. This behavior, which can be considered to some extent universal, is fairly independent of the specific features of superconducting electrodes. A comparative study of Shubnikov - de Haas oscillations and Scanning Tunneling Spectroscopy gave an experimental signature compatible with a two dimensional electron transport channel with a Dirac dispersion relation. A reduction of the size of the Bi2Se3 flakes to the nanoscale is an unavoidable step to drive Josephson junctions in the proper regime to detect possible distinctive features of Majorana fermions., Comment: 11 pages, 14 figures

We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stuckelberg (LZS) interference structure of a longitudinally driven two-level system. For even stronger drives we observe a significant change in the LZS pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing., 5 pages, 4 figures

We study the influence of dissipation on the switching current statistics of moderately damped Josephson junctions. Different types of both low- and high- $T_c$ junctions with controlled damping are studied. The damping parameter of the junctions is tuned in a wide range by changing temperature, magnetic field, gate voltage, introducing a ferromagnetic layer or in-situ capacitive shunting. A paradoxical collapse of switching current fluctuations occurs with increasing $T$ in all studied junctions. The phenomenon critically depends on dissipation in the junction and is explained by interplay of two counteracting consequences of thermal fluctuations, which on the one hand assist in premature switching into the resistive state and on the other hand help in retrapping back to the superconducting state. This is one of the rare examples of anticorrelation between temperature and fluctuation amplitude of a physically measurable quantity., Comment: 17 pages, 20 figures

We study thermal fluctuation phenomena in small Bi-2212 intrinsic Josephson junctions. Being able to measure switching currents of a {\it single} intrinsic junction, we observe that it's statistics can be very well described by thermal activation from a periodic Josephson potential with the sinusoidal current-phase relation. This is a direct evidence for the dc-intrinsic Josephson effect and the first unambiguous confirmation of the tunnelling nature of interlayer transport in strongly anisotropic high temperature superconductors. Furthermore, the fluctuation-free critical current, extracted from the analysis of switching current statistics, exhibits a temperature dependence typical for superconductor- insulator- superconductor tunnel junctions., Comment: 5 pages, 4 figs

We study the quantization of the supercurrent and conductance of a superconducting quantum point contact (SQPC) in a superconductor-two dimensional electrongas-superconductor (S-2DEG-S) Josephson junction with a split gate. The supercurrent and the conductance values change stepwise as a function of the gate voltage. We observe the onset of the first transport mode contributing both to the supercurrent and the conductance of the SQPC. Furthermore the steps in the supercurrent and the conductance appear at the same gate voltage values. This shows, that each transport mode in the SQPC contributes both to the normal state conductance and to the supercurrent., 5 pages, 3 figures

We have used Bi$_2$Se$_3$ nanoribbons, grown by catalyst-free Physical Vapor Deposition to fabricate high quality Josephson junctions with Al superconducting electrodes. In our devices we observe a pronounced reduction of the Josephson critical current density $J_c$ by reducing the width of the junction, which in our case corresponds to the width of the nanoribbon. Because the topological surface states extend over the entire circumference of the nanoribbon, the superconducting transport associated to them is carried by modes on both the top and bottom surfaces of the nanoribbon. We show that the $J_c$ reduction as a function of the nanoribbons width can be accounted for by assuming that only the modes travelling on the top surface contribute to the Josephson transport as we derive by geometrical consideration. This finding is of a great relevance for topological quantum circuitry schemes, since it indicates that the Josephson current is mainly carried by the topological surface states., Comment: Accepted for publication in the Journal of Applied Physics

We report noise measurements performed on a SQUID magnetometer implementing Grooved Dayem nanobridge of YBCO as weak-links. The SQUID shows magnetic flux noise as low as 10 $\mu \Phi_0$/Hz$^{0.5}$. The magnetometer is realized by coupling the SQUID to a flux transformer with a two-level coupling scheme using a flip-chip approach. This improves the effective area of the SQUID and result in a magnetic field noise of 50 fT/Hz$^{0.5}$ at T=77 K., Comment: 4 pages, 4 figures

Superconducting hybrid junctions are revealing a variety of novel effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical Vapor Deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in presence of an incipient Berezinskii-Kosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean Critical State model (CSM). We show that the CSM, with appropriate account for the low dimensional geometry, can partly explain the odd features measured in these junctions., Comment: 13 pages, 9 figures