Measurements of the absolute value of the penetration depth in high-Tc T_c superconductors using a tunnel diode resonator

A method is presented to measure the absolute value of the London penetration depth, $\lambda$, from the frequency shift of a resonator. The technique involves coating a high-$T_c$ superconductor (HTSC) with film of low - Tc material of known thickness and penetration depth. The method is applied to measure London penetration depth in YBa2Cu3O{7-\delta} (YBCO) Bi2Sr2CaCu2O{8+\delta} (BSCCO) and Pr{1.85}Ce{0.15}CuO{4-\delta}$(PCCO). For YBCO and BSCCO, the values of$\lambda (0)$are in agreement with the literature values. For PCCO$\lambda \approx 2790$ \AA, reported for the first time.Comment: RevTex 4 (beta 4). 4 pages, 4 EPS figures. Submitted to Appl. Phys. Let

Dimorphic aggregation behavior of a fusion polypeptide incorporating a stable protein domain (EGFP) with an amyloidogenic sequence (retroCspA)

AbstractWe describe the behavior of a polypeptide consisting of the genetic fusion of a structurally stable single-domain protein, EGFP (an analog of the green fluorescent protein) with an amyloidogenic sequence, retroCspA (known to readily form amyloid fibrils). Refolding of the fusion protein through single-step removal of denaturant and salt results in precipitation into amyloid aggregates displaying fibrillar morphology, thioflavin T binding as well as green fluorescence. Refolding through step-wise reduction of denaturant concentration in the presence of salt yields a soluble aggregate containing a folded, thermally-stable, non-fluorescent EGFP domain. Together, these results indicate that retroCspA forces the fusion protein to aggregate; however, the EGFP domain remains folded in a native-like structural format in both soluble aggregates and precipitates

New method for fast computation of gravity and magnetic anomalies from arbitrary polyhedra

We show that at any point the gravity field from a solid body bounded by plane surfaces and having uniform density can be computed as a field from a fictitious distribution of surface mass-density on the same body. The surface mass density at every surface element is equal to the product of the volume density of the body and the scalar product of (1) the unit outward vector normal to that surface element and (2) the position vector of the surface element with respect to the point of observation. Accordingly, the contribution to the gravity field from any plane surface of the body vanishes if the observation point lies in the plane of that surface. As a result, we can compute the gravity field everywhere, including points inside, on the surface, on an edge, or at a corner of the body where more than two surfaces meet. This new result lets us compute the gravity field using exactly the same simple procedure as for the magnetic field of a uniformly magnetized object, computed from an equivalent surface distribution of magnetic pole density. To get the gravity field while computing the magnetic field, one simply uses the product of this surface mass density and the universal gravitational constant instead of the surface magnetic pole density. Therefore, the same computer program can be used to compute the gravity, the magnetic field, or both simultaneously. This simple and novel approach makes the numerical computations much faster than all other previously published schemes

Considerations for the Control Design of Augmentative Robots

Robotic systems that are intended to augment human capabilities commonly require the use of semi-autonomous control and artificial sensing, while at the same time aiming to empower the user to make decisions and take actions. This work identifies principles and techniques from the literature that can help to resolve this apparent contradiction. It is postulated that augmentative robots must function as tools that have partial agency, as collaborative agents that provide conditional transparency, and ideally, serve as extensions of the human body.Comment: 7 pages. Presented at the IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2021) Workshop on Building and Evaluating Ethical Robotic Systems, Prague, Czech Republic, 28-30 September 202

Sharp Raman Anomalies and Broken Adiabaticity at a Pressure Induced Transition from Band to Topological Insulator in Sb2Se3

The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E2 g phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb2Se3 crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator

Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study

We report measurements of the magnetic penetration depth \lambda_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.Comment: M2S-HTSC-V

Discovery of highly spin-polarized conducting surface states in the strong spin-orbit coupling semiconductor Sb2_2Se3_3

Majority of the A$_2$B$_3$ type chalcogenide systems with strong spin-orbit coupling, like Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ etc., are topological insulators. One important exception is Sb$_2$Se$_3$, where a topological non-trivial phase was argued to be possible under ambient conditions, but such a phase could be detected to exist only under pressure. In this Letter, we show that like Bi$_2$Se$_3$, Sb$_2$Se$_3$, displays generation of highly spin-polarized current under mesoscopic superconducting point contacts as measured by point contact Andreev reflection spectroscopy. In addition, we observe a large negative and anisotropic magnetoresistance in Sb$_2$Se$_3$, when the field is rotated in the basal plane. However, unlike in Bi$_2$Se$_3$, in case of Sb$_2$Se$_3$ a prominent quasiparticle interference (QPI) pattern around the defects could be obtained in STM conductance imaging. Thus, our experiments indicate that Sb$_2$Se$_3$ is a regular band insulator under ambient conditions, but due to it's high spin-orbit coupling, non-trivial spin-texture exists on the surface and the system could be on the verge of a topological insulator phase.Comment: 5 pages, 4 figures, supplemental material not include

Evolution of Magnetic and Superconducting Fluctuations with Doping of High-Tc Superconductors (An electronic Raman scattering study)

For YBa_2Cu_3O_{6+\delta} and Bi_2Sr_2CaCu_2O_8 superconductors, electronic Raman scattering from high- and low-energy excitations has been studied in relation to the hole doping level, temperature, and energy of the incident photons. For underdoped superconductors, it is concluded that short range antiferromagnetic (AF) correlations persist with hole doping and doped single holes are incoherent in the AF environment. Above the superconducting (SC) transition temperature T_c the system exhibits a sharp Raman resonance of B_1g symmetry and about 75 meV energy and a pseudogap for electron-hole excitations below 75 meV, a manifestation of a partially coherent state forming from doped incoherent quasi-particles. The occupancy of the coherent state increases with cooling until phase ordering at T_c produces a global SC state.Comment: 5 pages, 4 EPS figures; SNS'97 Proceedings to appear in J. Phys. Chem. Solid