Good day Steemit community. Today, here are some interesting papers in the Superconductivity.

### [Grooved Dayem nanobridges as building blocks of high-performance YBa![](http://latex2png.com/output//latex_4223ec05912b994c2067ff9b6a5e0098.png)Cu![](http://latex2png.com/output//latex_ceb601cfa46671c9d5f89605cccc2ec9.png)O![](http://latex2png.com/output//latex_012e385e3820a374488ef65b9cfdab56.png) SQUID magnetometers](http://arxiv.org/abs/1903.07511v1) (1903.07511v1) Edoardo Trabaldo, Christoph Pfeiffer, Eric Andersson, Riccardo Arpaia, Alexei Kalaboukhov, Dag Winkler, Floriana Lombardi, Thilo Bauch 2019-03-18 > We present noise measurements performed on a YBa![](http://latex2png.com/output//latex_4223ec05912b994c2067ff9b6a5e0098.png)Cu![](http://latex2png.com/output//latex_ceb601cfa46671c9d5f89605cccc2ec9.png)O![](http://latex2png.com/output//latex_d70f5a240653832869ec61cc59b92295.png) nanoscale weak-link-based magnetometer consisting of a Superconducting QUantum Interference Device (SQUID) galvanically coupled to a ![](http://latex2png.com/output//latex_c7fd758c18631411bad8ffed84f98b1c.png)mm![](http://latex2png.com/output//latex_c325d54e893a48d5fd764007d924c553.png) pick-up loop, reaching white flux noise levels and magnetic noise levels as low as ![](http://latex2png.com/output//latex_761e0654f5c61f612cba7569c0e7f821.png) and ![](http://latex2png.com/output//latex_4c35d3877096e3bc4af3dab040b268f8.png)~fT/![](http://latex2png.com/output//latex_b7780a3eb287a9eb6803d1873d003743.png) at ![](http://latex2png.com/output//latex_15313091d54395034881c0de262adea9.png)~K, respectively. The low noise is achieved by introducing Grooved Dayem Bridges (GDBs), a new concept of weak-link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise. ### [HTSC cuprates and ferropnictides as self-doping CT-excitonic insulators](http://arxiv.org/abs/1903.07423v1) (1903.07423v1) Kirill Mitsen, Olga Ivanenko 2019-03-18 > We have proposed a generalized model for the electronic structure of HTSC cuprates and ferropnictides, which treats these materials as charge transfer (CT) excitonic insulators (CT phase) with an intrinsic self-doping mechanism that provides the generation of free carriers. This mechanism is not directly related to external doping, but is due to the interaction of band electrons with so called Heitler-London (HL) centres inherently existing in the CT phase and representing pairs of adjacent CuO4 or AsFe4 plaquettes in the CuO2 or FeAs basal planes. Material in this state has been shown to represent a medium, where the mechanism of excitonic superconductivity can be realized. The role of doping according to the model consists in only a local change in the electronic structures of cuprates and ferropnictides, leading to the formation of a percolation cluster of the CT phase in material. The ranges of dopant concentrations conforming to the existence of percolation clusters of the CT phase have been determined; these ranges have been shown to coincide with the positions of the superconducting domes on the phase diagrams of these compounds. This coincidence may serve to prove the validity of the proposed model for the basic state of cuprate and ferropnictide HTSCs. ### [Effects of Vanadium doping on BaFe2As2](http://arxiv.org/abs/1903.07233v1) (1903.07233v1) Xing-Guang Li, Jie-Ming Sheng, Cong-Kuan Tian, Yi-Yan Wang, Tian-Long Xia, Le Wang, Feng Ye, Wei Tian, Jin-Chen Wang, Juan-Juan Liu, Hong-Xia Zhang, Wei Bao, Peng Cheng 2019-03-18 > We report an investigation of the structural, magnetic and electronic properties of Ba(Fe(1-x)V(x))2As2 using x-ray, transport, magnetic susceptibility and neutron scattering measurements. The vanadium substitutions in Fe sites are possible up to 40\%. Hall effect measurements indicate strong hole-doping effect through V doping, while no superconductivity is observed in all samples down to 2K. The antiferromagnetic and structural transition temperature of BaFe2As2 is gradually suppressed to finite temperature then vanishes at x=0.245 with the emergence of spin glass behavior, suggesting an avoided quantum critical point (QCP). Our results demonstrate that the avoided QCP and spin glass state which were previously reported in the superconducting phase of Co/Ni-doped BaFe2As2 can also be realized in non-superconducting Ba(Fe(1-x)V(x))2As2. ### [Exactly Solvable Kondo Lattice Model in Anisotropic Limit](http://arxiv.org/abs/1903.05295v2) (1903.05295v2) Yin Zhong, Wei-Wei Yang, Ji-Ze Zhao, Hong-Gang Luo 2019-03-13 > In this paper we introduce an exactly solvable Kondo lattice model without any fine-tuning local gauge symmetry. This model describes itinerant electrons interplaying with localized moment via only longitude Kondo exchange. Its solvability results from the conservation of the local moment at each site and is valid for arbitrary lattice geometry and electron filling. A case study on square lattice shows that the ground state is a N\'{e}el antiferromagnetic insulator at half-filling. At finite temperature, paramagnetic phases including Mott insulator and correlated metal are found. The former is a melting antiferromagnetic insulator with strong short-range magnetic fluctuation while the latter corresponds to a Fermi liquid-like metal. Numerical Monte Carlo simulation and field theoretical analysis confirm that the transition from paramagnetic phases into the antiferromagnetic insulator is a continuous ![](http://latex2png.com/output//latex_6ffe575c6e907a9cafecc5e1254486a0.png) Ising transition. Away from half-filling, patterns of spin stripes (inhomogeneous magnetic order) at weak coupling, and phase separation at strong coupling are predicted. The spin stripe found here may be relevant to novel quantum liquid crystal order in hidden order compound URu![](http://latex2png.com/output//latex_57f60f4c0872f38de5c4e8563958e649.png)Si![](http://latex2png.com/output//latex_57f60f4c0872f38de5c4e8563958e649.png). ### [Unconventional Scaling of the Superfluid Density with the Critical Temperature in Transition Metal Dichalcogenides](http://arxiv.org/abs/1903.05292v2) (1903.05292v2) F. O. von Rohr, J. -C. Orain, R. Khasanov, C. Witteveen, Z. Shermadini, A. Nikitin, J. Chang, A. R. Wieteska, A. N. Pasupathy, M. Z. Hasan, A. Amato, H. Luetkens, Y. J. Uemura, Z. Guguchia 2019-03-13 > We report on muon spin rotation experiments probing the temperature-dependent magnetic penetration depth in 2H-NbSe_2 under high-pressure (p_max = 2.1 GPa) and in 4H-NbSe_2 at ambient pressure. The current results for ambient pressure along with our earlier findings on 1T'-MoTe_2 (Z. Guguchia et. al., Nature Communications 8, 1082 (2017)) demonstrates that the superfluid density n_s scale linearly with T_c in the three transition metal dichalcogenide superconductors 2H-NbSe_2, 4H-NbSe_2, and 1T'-MoTe_2. Upon increasing pressure, we observe a substantial increase of the superfluid density n_s in 2H-NbSe_2, which we find to correlate with T_c. However, the n_s vs T_c dependence shows a deviation from the Uemura linear relation. A similar deviation from the Uemura line was also observed in previous pressure studies of optimally doped cuprates. The correlation between the superfluid density and the critical temperature is considered a hallmark feature of unconventional superconductivity, especially in high-temperature cuprate superconductors. Here, we show that this correlation is also an intrinsic property of the superconductivity in transition metal dichalcogenides, whereas the ratio T_c/T_F is approximately a factor of 20 lower than the ratio observed in hole-doped cuprates. We, furthermore, find that the values of the superconducting gaps are insensitive to the suppression of the quasi-two-dimensional CDW state, indicating that the CDW ordering and the superconductivity in 2H-NbSe_2 are independent of each other. ### [Fraunhofer patterns in magnetic Josephson junctions with non-uniform magnetic susceptibility](http://arxiv.org/abs/1902.01709v2) (1902.01709v2) Bence Börcsök, Sachio Komori, Alexander Buzdin, Jason Robinson 2019-02-05 > The development of superconducting memory and logic based on magnetic Josephson junctions relies on an understanding of junction properties and, in particular, the dependence of critical current on external magnetic flux (i.e. Fraunhofer patterns). With the rapid development of Josephson junctions with various forms of inhomogeneous barrier magnetism, Fraunhofer patterns are increasingly complex. In this paper we model Fraunhofer patterns for magnetic Josephson junctions in which the barrier magnetic susceptibility is position- and external magnetic field dependent. The model predicts anomalous Fraunhofer patterns in which local minima in the Josephson critical current can be nonzero and non-periodic with external magnetic flux due to an interference effect between highly magnetised and demagnetised regions. ### [Realizing Majorana zero mode with a stripe of \emph{ionized} and \emph{isolated} magnetic atoms on a \emph{layered} superconductor](http://arxiv.org/abs/1804.10198v3) (1804.10198v3) Wenjie Ji, Xiao-Gang Wen 2018-04-26 > It has been proposed that a line junction between spiral magnet and superconductor or between ferromagnet and superconductor with Rashba spin-orbital coupling can produce Majorana zero mode (MZM) at the ends of the line. However, a strong magnetic exchange coupling between the magnetic atoms and the superconductor (about half of the bandwidth of the superconductor) is needed to obtain MZM. Here, we design devices to reduce the needed magnetic exchange coupling. In the first proposal, we cover a very narrow ![](http://latex2png.com/output//latex_3175b4b6e88ed8bf5654d9c2d064c835.png)-wave superconducting wire formed by a monolayer film or a layered material (such as FeSe or FeSe monolayer on SrTiO![](http://latex2png.com/output//latex_ceb601cfa46671c9d5f89605cccc2ec9.png)) with \emph{isolated} magnetic atoms. In our second proposal, we place a line of \emph{isolated} magnetic \emph{ions} on a monolayer superconductor or a \emph{layered} superconductor (such as FeSe). We show that, in the above devices, a spiral magnetic order will develop spontaneously to produce a 1D ![](http://latex2png.com/output//latex_fcf34992423b3238c306f6d41c8f78d8.png)-wave topological superconductor with a sizable gap (above 1/2 of the parent superconducting gap), even for a weak magnetic exchange coupling (less than 1/10 of the bandwidth). The topological superconductor has MZM at ends of the wire. ### [Observation of superconductivity with Tc onset at 12K in electrically tunable twisted double bilayer graphene](http://arxiv.org/abs/1903.06952v1) (1903.06952v1) Cheng Shen, Na Li, Shuopei Wang, Yanchong Zhao, Jian Tang, Jieying Liu, Jinpeng Tian, Yanbang Chu, Kenji Watanabe, Takashi Taniguchi, Rong Yang, Zi Yang Meng, Dongxia Shi, Guangyu Zhang 2019-03-16 > Electron-electron interactions play an important role in graphene and related systems to induce exotic quantum states, especially in a stacked bilayer with a small twist angle. Under a magic twist angle in a bilayer graphene, flat band and strong many-body effects would lead to Mott-like insulating states and emergent superconductivity. Different from monolayer graphene, the band structure of AB-stacked bilayer graphene can be further tuned by displacement fields, providing an extra knob to realize the flat band in addition to the very sensitive twist angle. Here, we report the discovery and characterization of such displacement-field tunable twisted double bilayer graphene. Insulating states at half-filling for a wide range of twist angle and superconductivity with onset Tc at 12K, much higher than those observed in any other graphene heterostructures, are observed. Furthermore, the resistance gap in the Mott insulator increases with respect to the in-plane magnetic fields and the as-measured g factor of ~2 suggests possible ferromagnetic fluctuations in the Mott phase which might mediate the corresponding unconventional pairing mechanism in the superconductivity phase observed in our system. These results establish the twisted double bilayer graphene as easily tunable platform for exploring new paradigm of quantum many-body states. ### [Determination of effective Cooper pair mass by electrostatic doping of fluxoids of superconductor surface. Theory](http://arxiv.org/abs/1903.06940v1) (1903.06940v1) T. M. Mishonov, A. M. Varonov, V. I. Danchev 2019-03-16 > We propose an experimental setup for the determination of the effective Cooper pair mass ![](http://latex2png.com/output//latex_431fe37b7753dde05a27ea9694f3db1b.png) by electrostatic doping of Abrikosov vortices of a superconductor surface. To that end, a superconductor is placed in a constant external magnetic field, perpendicular to the superconducting planes. One of the electrodes is connected to the superconductor, while another one forms a capacitor with the superconductor itself. The nature of the insulator is irrelevant to the experiment. In some sense our experiment aims to measure is a new effect. After applying alternating voltage to the capacitor, electrostatic induction leads to oscillations of the magnetic moment. Those oscillations are detected by electromotive force inducted in a detector coil. The electric charges accumulated on the superconducting surface are partially superfluid and contribute to the magnetic field of the vortices. In addition to ![](http://latex2png.com/output//latex_431fe37b7753dde05a27ea9694f3db1b.png), this experiment would determine the sign of the Cooper pair charge. ### [Can the dynamical Lamb effect be observed in a superconducting circuit?](http://arxiv.org/abs/1903.06880v1) (1903.06880v1) Mirko Amico, Oleg L. Berman, Roman Ya. Kezerashvili 2019-03-16 > The dynamical Lamb effect is predicted to arise in superconducting circuits when the coupling of a superconducting qubit with a resonator is periodically switched "on" and "off" nonadiabatically. We show that by using a superconducting circuit which allows to switch between longitudinal and transverse coupling of a qubit to a resonator, it is possible of to observe the dynamical Lamb effect. {The switching between longitudinal and transverse coupling can be achieved by modulating the magnetic flux through the circuit loops.} By solving the Schr\"{o}dinger equation for a qubit coupled to a resonator, we calculate the time evolution of the probability of excitation of the qubit and the creation of ![](http://latex2png.com/output//latex_a78da909d65184636aa635647d3ad1fd.png) photons in the resonator due to the dynamical Lamb effect. The probability is maximum when the coupling is periodically switched between longitudinal and transverse using a square-wave or sinusoidal modulation of the magnetic flux with frequency equal to the sum of the average qubit and photon transition frequencies.

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