All Set for Majoranas When put in the proximity of a superconductor, topological insulators (TIs) are expected to support Majorana fermions, exotic particles that are their own antiparticles. For this to be realized, the interface between the TI and superconductor layers has to be atomically sharp but electronically transparent. Wang et al. (p. 52, published online 15 March) fabricated this heterostructure by growing a film of the TI material Bi2Se3 on the superconductor NbSe2 covered with a Bi bilayer. Scanning tunneling spectroscopy revealed a superconducting gap on the TI surface of the heterostructure with varying thickness of the Bi2Se3 film. This coexistence of superconductivity and topological order should now allow observation of exotic phenomena such as Majorana fermions. A thin layer of a topological insulator grown on the surface of a superconductor is shown to acquire a superconducting gap. Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi2Se3) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi2Se3 surface in the regime of Bi2Se3 film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

/pdf/the-coexistence-of-superconductivity-and-topological-order-59btue84zw.pdf

The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films

Borophene, an atomically thin covalently bonded boron sheet, has attracted great attention as a novel quantum material because of its structural tunability and potential utilization in flexible and transparent electronics. So far, borophene has been synthesized on silver or copper single crystals, but these substrates are small, very expensive, and unsuitable for study of transport properties or electronics applications. Here, we report synthesis of borophene on nanometer-scale thick Cu(111) films grown on sapphire. We have developed a process of enlarging faceted borophene islands, by repeated submersion of boron into copper at high temperature and resurfacing and re-crystallization at lower temperature. This discovery was enabled by real-time feedback from low-energy electron microscopy and diffraction. We demonstrate synthesis of borophene as faceted micrometer-size monocrystal islands or as full-monolayer sheets. The process is scalable to wafer size; moreover, Cu films could be sacrificed and sapphire reused. Our work opens the door for new experiments and brings applications one step closer.

/pdf/large-area-borophene-sheets-on-sacrificial-cu-111-films-98r3mcdfes.pdf

Large-area borophene sheets on sacrificial Cu(111) films promoted by recrystallization from subsurface boron

We clarify the pairing mechanism of high-$T_c$ superconductivity in bilayer La$_3$Ni$_2$O$_7$ under high pressure by employing the static auxiliary field Monte Carlo approach to simulate a minimal effective model that contains local $d_{z^2}$ interlayer spin singlets and metallic $d_{x^2-y^2}$ bands. Superconductivity is induced when the local spin singlet pairs are mobilized and attain long-distance phase coherence by hybridization with the metallic bands. When projected onto realistic Fermi surfaces, it yields a nodeless $s$-wave gap on the $\gamma$ Fermi surface, and extended $s$-wave gaps of the same (opposite) sign on the $\alpha$ ($\beta$) Fermi surface due to its bonding (antibonding) character, with nodes or gap minima along the diagonal direction of the two-dimensional Brillouin zone. We find a dual role of the hybridization that not only induces global phase coherence but also competes with the spin singlet formation. This lead to a tentative phase diagram where $T_c$ varies nonmonotonically with the hybridization, in good correspondence with experimental observations. A roughly linear relation is obtained for realistic hopping and hybridization parameters: $T_c\approx 0.04-0.05 J$, where $J$ is the interlayer superexchange interaction. We emphasize the peculiar tunability of the bilayer structure and propose that $T_c$ may be further enhanced by hole doping or applying uniaxial pressure along the $c$ axis on superconducting La$_3$Ni$_2$O$_7$. Our work provides reliable numerical evidence for the pairing mechanism of high-$T_c$ superconductivity in La$_3$Ni$_2$O$_7$ and points out a potential route to achieve even higher $T_c$.

pdf/high-t-c-superconductivity-by-mobilizing-local-spin-singlets-35p13ah73t.pdf

High-$T_c$ superconductivity by mobilizing local spin singlets and possible route to higher $T_c$ in pressurized La$_3$Ni$_2$O$_7$

In this work, we report on the design of a wide-band digital lock-in amplifier (DLIA) of up to 65 MHz and its application for electrical impedance measurements in microfluidic devices. The DLIA is comprised of several dedicated technologies. First, it features a fully differential analog circuit, which includes a preamplifier with a low input noise of 4.4 nV/√Hz, a programmable-gain amplifier with a gain of 52 dB, and an anti-aliasing, fully differential low-pass filter with -76 dB stop-band attenuation. Second, the DLIA has an all-digital phase lock loop, which features a phase deviation of less than 0.02° throughout the frequency range. The phase lock loop utilizes an equally accurate period-frequency measurement, with a sub-ppm precision of frequency detection. Third, a modified clock link is implemented in the DLIA to improve the signal-to-noise ratio of the analog-to-digital converter affected by clock jitter of up to 20 dBc. A series of measurements were performed to characterize the DLIA, and the results showed an accurate performance. Additionally, impedance measurements of standard-size microparticles were performed by frequency sweep from 300 kHz to 30 MHz, using the DLIA in a microfluidic device. Different diameters of microparticle could be accurately distinguished according to the relative impedance at 2.5 MHz. The results confirm the promising applications of the DLIA in microfluidic electrical impedance measurements.

https://www.mdpi.com/1424-8220/19/16/3519/pdf

A Wide-Band Digital Lock-In Amplifier and Its Application in Microfluidic Impedance Measurement

Intrigued by the discovery of high-temperature superconductivity in a single unit-cell layer of FeSe film on SrTiO_{3}, researchers recently found large superconductinglike energy gaps in K-adsorbed multilayer FeSe films by angle-resolved photoemission and scanning tunneling spectroscopy. However, the existence and nature of the high-temperature superconductivity inferred by the spectroscopic studies has not been investigated by measurements of zero resistance or the Meissner effect due to the fragility of K atoms in air. Using a self-developed multifunctional scanning tunneling microscope, we succeed in observing the diamagnetic response of K-adsorbed multilayer FeSe films, and thus find a dome-shaped relation between the critical temperature (T_{c}) and K coverage. Intriguingly, T_{c} exhibits an approximately linear dependence on the superfluid density in the whole K adsorbed region. Moreover, the quadratic low-temperature variation in the London penetration depth indicates a sign-reversal order parameter. These results provide compelling information towards further understanding of the high-temperature superconductivity in FeSe-derived superconductors.

Diamagnetic Response of Potassium-Adsorbed Multilayer FeSe Film.

How does one detect the Meissner effect in ultrathin superconducting films that are usually extremely sensitive to air? Here, the authors succeed in observing the diamagnetic response of a superconducting surface superstructure by using an $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ two-coil mutual inductance technique embedded in a specially designed scanning tunneling microscope. The diamagnetic measurement allows for the calculation of the penetration depth, which provides invaluable information about the nature of superconductivity and the symmetry of the underlying order parameter.

Diamagnetic response of a superconducting surface superstructure: Si(111)- √ 7 × √ 3 -In

To uncover the critical effect of disorder on superconductivity, the ideal method is to visualize the microscopic crystalline deficiencies in real space while measuring the macroscopic superconducting properties. By using a self-developed multifunctional scanning tunneling microscope, we investigated the correlation between controllably introduced disorder and superconductivity in the Si(111)- 7× 3-In surface reconstruction. It is revealed that not only the density but also the spatial distribution of surface vacancies makes a significant influence on the diamagnetic response of the superconducting surface reconstruction. The higher density of vacancies uniformly dispersed on terraces results in a lower critical temperature and critical magnetic field, while the presence of grooves formed by aggregation of vacancies at step edges reduces the critical supercurrent and thus weakens the diamagnetic shielding effect remarkably.

Influence of disorder on superconductivity in the Si(111)- 7× 3-In surface

Superconducting thin films have been a focal point for intensive research efforts since their reduced dimension allows for a wide variety of quantum phenomena. Many of these films, fabricated in UHV chambers, are highly vulnerable to air exposure, making it difficult to measure intrinsic superconducting properties such as zero resistance and perfect diamagnetism with ex situ experimental techniques. Previously, we developed a multifunctional scanning tunneling microscope (MSTM) containing in situ four-point probe (4PP) electrical transport measurement capability in addition to the usual STM capabilities [Ge et al., Rev. Sci. Instrum. 86, 053903 (2015)]. Here we improve this MSTM via development of both transmission and reflection two-coil mutual inductance techniques for in situ measurement of the diamagnetic response of a superconductor. This addition does not alter the original STM and 4PP functions of the MSTM. We demonstrate the performance of the two-coil mutual inductance setup on a 10-nm-thick NbN thin film grown on a Nb-doped SrTiO3(111) substrate.

Development of in situ two-coil mutual inductance technique in a multifunctional scanning tunneling microscope.

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Diamagnetic Response of Potassium-Adsorbed Multilayer FeSe Film.

Diamagnetic response of a superconducting surface superstructure: Si(111)- √ 7 × √ 3 -In

Influence of disorder on superconductivity in the Si(111)- 7× 3-In surface

Development of in situ two-coil mutual inductance technique in a multifunctional scanning tunneling microscope.