Non-superconducting Materials Research Articles

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films.

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two nonsuperconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi2Te4 and an antiferromagnetic iron chalcogenide FeTe. Our electrical transport measurements reveal interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi2Te4 layer, confirming the coexistence of superconductivity and antiferromagnetism in the MnBi2Te4 layer. Our findings will advance the fundamental inquiries into the topological superconducting phase in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi2Te4-based heterostructures.

(Invited) Direct Bonding and Its Interface for High-Density Integration of Superconducting Qubits

Superconducting qubits are one of the intensive approaches to fabricate quantum computing (QC). Qubits can be manufactured by the current Si-based semiconductor manufacturing processes. However, circuits should be superconducting material such as Nb (Tc=9.29 K), Al (Tc=1.18 K), Pb (Tc=7.2 K), In (Tc=3.4 K) , etc. In current technology, qubits are arranged in planar geometry in a chip, and each qubit should wire to control and read signal lines in the same plane. When a large number of qubits are integrated, these wires would be too constrained, and several problems would be actualized, such as wiring for qubits located at the chip center, crosstalk among wiring, and so on. The signal integrity for the QC devices is much better to have qubits at short distances from the control circuitry. Therefore, 3D wiring is required for a large-scale qubit integration. 3D integration of qubits with interposer chips by flip-chip bonding is the proof-of-concept of the large-scale qubit integration. The 3D integration approaches for qubit- based devices are the same as conventional electronic devices, including the through silicon via (TSV) process for interposers and micro bump flip-chip bonding. However, they use superconducting metals instead of copper and other non-superconducting materials. Most flip-chip microbump bonding processes require thermal compression at over 170 °C. Superconducting elements are easily degraded by heat. Consequently, we apply surface activated bonding (SAB) to a superconducting metal as a low-temperature bonding process for superconducting circuits.The test element group (TEG) for the current-voltage (I-V) measurement of the bonded interface was bonded by SAB method, and at the Nb SAB interface layer is a 2- to 3-nm-thick amorphous layer and the layer consist of Si-rich interfacial layer compared to the Nb bulk area. The bonding specimens were activated on jigs made of Si during the bonding process; then the Si sputtered by FAB was redeposited on the bonding surfaces. Furthermore, the bonded specimen was wired using Al wire to a chip carrier with a stage installed into a 0.3-K cryostat. As a result, the Tc of this path is 9.3 K, which is very similar to that of Nb bulk (9.2 K). However, a small step of the Tc at 9.1-9.4 K was also observed. (Fig.1) [1]In this study, we discuss the superconducting interface of direct bonded Nb-Nb by Tc, and also the interfacial structure measured by polarized neutron reflectometry (PNR). The results show clear spin asymmetry in the superconducting state, as splitting is observed in the fringes of the parallel neutron (neutron spins parallel to the external magnetic field 0.1 T) reflection R+ and the antiparallel (neutron spins antiparallel to it) reflection R−, where the latter has become larger. This well-known effect is related to the flux penetration in the superconducting state. Therefore, unique information about the superconducting state of the buried thin film can be measured by using the polarized neutron beam. For this project the major advantage of PNR will be to characterize the superconducting state at the deeply buried Nb interfaces, sandwiched between the two substrate wafers, which would be infeasible using alternate techniques.[1] M. Fujino, et. al., J. Appl. Phys., 133, 015301, 2023 Figure 1

Temperature Dependence of the Anomalous Hall Effect from Electron Interactions

We consider the impact of electron-electron interactions on the temperature dependence of the anomalous Hall effect in disordered conductors. The microscopic analysis is carried out within the diagrammatic approach of the linear response Kubo-Streda formula with an account of both extrinsic skew-scattering and side-jump mechanisms of the anomalous Hall effect arising in the presence of spin-orbit coupling. We demonstrate the importance of electron interactions in the Cooper channel even for nominally non-superconducting materials and find that the corresponding low-temperature dependence of the anomalous Hall conductivity is asymptotically of the form sqrt[T]/ln(T_{0}/T) in three dimensions and ln[ln(T_{0}/T)] in two dimensions, where the scale of T_{0} is parametrically of the order of Fermi energy. These results, in particular, may provide a possible explanation for the recently observed unconventional temperature dependence of the anomalous Hall effect in HgCr_{2}Se_{4}.

Experimental search for the origin of low-energy modes in topological materials

Point-contact spectroscopy of several non-superconducting topological materials reveals a low temperature phase transition that is characterized by a Bardeen-Cooper-Schrieffer-type of criticality. We find such a behavior of differential conductance for topological surfaces of non-magnetic and magnetic Pb$_{1-y-x}$Sn$_y$Mn$_x$Te. We examine a possible contribution from superconducting nanoparticles, and show to what extent our data are consistent with Brzezicki's et al. theory [arXiv:1812.02168], assigning the observations to a collective state adjacent to atomic steps at topological surfaces.

Field-Free Nucleation of Antivortices and Giant Vortices in Nonsuperconducting Materials.

Giant vortices with higher phase winding than 2π are usually energetically unfavorable, but geometric symmetry constraints on a superconductor in a magnetic field are known to stabilize such objects. Here, we show via microscopic calculations that giant vortices can appear in intrinsically nonsuperconducting materials, even without any applied magnetic field. The enabling mechanism is the proximity effect to a host superconductor where a current flows, and we also demonstrate that antivortices can appear in this setup. Our results open the possibility to study electrically controllable topological defects in unusual environments, which do not have to be exposed to magnetic fields or intrinsically superconducting, but instead display other types of order.

RETRACTED: Current-induced strong diamagnetism in the Mott insulator Ca2RuO4.

Mott insulators can host a surprisingly diverse set of quantum phenomena when their frozen electrons are perturbed by various stimuli. Superconductivity, metal-insulator transition, and colossal magnetoresistance induced by element substitution, pressure, and magnetic field are prominent examples. Here we report strong diamagnetism in the Mott insulator calcium ruthenate (Ca2RuO4) induced by dc electric current. The application of a current density of merely 1 ampere per centimeter squared induces diamagnetism stronger than that in other nonsuperconducting materials. This change is coincident with changes in the transport properties as the system becomes semimetallic. These findings suggest that dc current may be a means to control the properties of materials in the vicinity of a Mott insulating transition.

Pressure-induced reinforcement of interfacial superconductivity in a Bi2Te3/Fe1+yTe heterostructure

We investigate the hydrostatic pressure dependence of interfacial superconductivity occurring at the atomically sharp interface between two non-superconducting materials: the topological insulator (TI) Bi2Te3 and the parent compound Fe1+yTe of the chalcogenide iron-based superconductors. Under pressure, a significant increase in the superconducting transition temperature Tc is observed. We interpret our data in the context of a pressure-induced enhanced coupling of the Fe1+yTe interfacial layer with the Bi2Te3 surface state, which modifies the electronic properties of the interface layer in a way that superconductivity emerges and becomes further enhanced under pressure. This demonstrates the important role of the TI in the interfacial superconducting mechanism.

Study on Thermal-Quench Behaviors of GdBCO Coils Wound With Silicon Grease as an Insulation Material

This paper investigated quench initiation and propagation characteristics of grease insulation coils by conducting thermal-quench tests for two GdBCO single-pancake coils, namely a coil co-wound with silicon grease (termed as the SiG coil) and the other coil co-wound with Kapton tape (termed as the INS coil), as turn-to-turn insulation. The test results confirmed that the SiG coil exhibited better thermal and electrical stabilities when compared with the INS coil because the operating current could be bypassed in the radial direction through the turn-to-turn contacts when a local hot spot was generated. The SiG coil had superior thermal and electrical stabilities. However, a nonrecovering resistive zone could be generated because excessive Joule heat energy could be induced by radial current flow due to the existence of radial resistance that was mainly generated by silicon grease and nonsuperconducting materials including the substrate, stabilizer, and buffer layers within the high-temperature superconductor (HTS). Therefore, it is essential to consider critical Joule heat energy that is influenced by operating current and stored magnetic energy as well as the radial resistance to achieve self-protective 2G HTS coils.

Investigation on quench initiation and propagation characteristics of GdBCO coil co-wound with a stainless steel tape as turn-to-turn metallic insulation.

This paper investigates the quench initiation and propagation characteristics of a metallic insulation (MI) coil by conducting thermal quench tests for a GdBCO single-pancake coil co-wound with a stainless steel tape as the turn-to-turn MI. The test results confirmed that the MI coil exhibited superior thermal and electrical stabilities compared to the conventional coils co-wound with organic insulation material because the operating current could flow along the radial direction due to the existence of a turn-to-turn contact when a local hot spot was generated. The results of the quench test at a heater current (Ih) of 12, 13, and 14 A indicate that the MI coil possesses a self-protecting characteristic resulting from the "current bypass" through the turn-to-turn contact. However, the test coil was not self-protecting at Ih = 15 A because the Joule heat energy generated by the radial current flow was not completely dissipated due to the characteristic resistance of the metallic insulation tape and the non-superconducting materials, including the substrate, stabilizer, and buffer layers within the high-temperature superconductor (HTS) tape. Even though the MI coil possesses superior thermal and electrical stability relative to those of conventional HTS coils co-wound with an organic material as turn-to-turn insulation, it is essential to consider the critical role of the Joule heat energy resulting from the operating current and stored magnetic energy as well as the characteristic resistances in order to further develop self-protective 2G HTS magnets.

Fermionic boundary modes in two-dimensional noncentrosymmetric superconductors

We calculate the spectrum of the Andreev boundary modes in a two-dimensional superconductor formed at an interface between two different non-superconducting materials, e.g. insulating oxides. Inversion symmetry is absent in this system, and both the electron band structure and the superconducting pairing are strongly affected by the spin-orbit coupling of the Rashba type. We consider isotropic s-wave pairing states, both with and without time-reversal symmetry breaking, as well as various d-wave states. In all cases, there exist subgap Andreev boundary states, whose properties, in particular, the number and location of the zero-energy modes, qualitatively depend on the gap symmetry and the spin-orbit coupling strength.

Anomalous independence of interface superconductivity from carrier density

The recent discovery of superconductivity at the interface of two non-superconducting materials has received much attention. In cuprate bilayers, the critical temperature (Tc) can be significantly enhanced compared with single-phase samples. Several explanations have been proposed, invoking Sr interdiffusion, accumulation and depletion of mobile charge carriers, elongation of the copper-to-apical-oxygen bond length, or a beneficial crosstalk between a material with a high pairing energy and another with a large phase stiffness. From each of these models, one would predict Tc to depend strongly on the carrier density in the constituent materials. Here, we study combinatorial libraries of La(2-x)Sr(x)CuO4-La2CuO4 bilayer samples--an unprecedentedly large set of more than 800 different compositions. The doping level x spans a wide range, 0.15 < x < 0.47, and the measured Hall coefficient varies by one order of magnitude. Nevertheless, across the entire sample set, Tc stays essentially constant at about 40 K. We infer that doping up to the optimum level does not shift the chemical potential, unlike in ordinary Fermi liquids. This result poses a new challenge to theory--cuprate superconductors have not run out of surprises.

Studies on the transport properties of RuSr2RECu2Oy, RuSr2(RE)2Cu2Oy(RE = Gd, Nd) and RuSr2Gd1.4Ce0.6Cu2O10: Possibility of variable range hopping

We study synthesis, characterization and transport properties of several superconducting and nonsuperconducting ruthenocuprate materials by using different routes. RuSr2(RE)Cu2Oy (Ru-1212) and RuSr2(RE)2Cu2Oy (Ru-1222) have been synthesized using two different rare earths, RE=Gd or Nd. A similar route has been followed for the synthesis of all these four compounds. All ruthenocuprate samples are characterized by using X-ray diffraction (XRD), Scanning Electron Micrograph (SEM) and Energy Dispersive X-ray (EDX). The lattice constants have been extracted and so are the grain sizes. Stoichiometries of several ruthenocuprates have been determined by using EDX spectrum. The resistivities of ruthenocuprates annealed in different conditions have been compared. The resistivity in Gd based ruthenocuprate varies in order of magnitude in comparison to that of ruthenocuprates with Nd. We have studied how the annealing temperature also affects transport properties strongly. Using a different route we have synthesized few RuSr2Gd1.4Ce0.6Cu2O10 which shows insulating to superconductor transition. Three dimensional variable range hopping (3DVRH) may be considered as a possible conduction mechanism in the normal state of superconducting RuSr2Gd1.4Ce0.6Cu2O10.

Experimental and numerical analysis of quench propagation on MgB2 tapes and pancake coils

In the design and feasibility of electric power applications with MgB2 conductors, thermal conductivity plays an important role. In composite wires and tapes it is mainly determined by the amount of stabilizer (usually copper), while for coils the electric insulation and epoxy binder characteristics are added. In this paper we present results on quench development and propagation on isolated superconducting tapes and small single pancake coils cooled by thermal conduction, for three types of MgB2 tape with different stabilizations. The quench parameters at different temperatures, such as minimum quench energy (MQE), minimum propagation zone (MPZ) and quench propagation velocity (vp), are reported. A complete electrical and thermal characterization of the superconducting tapes and of non-superconducting materials used in the coil manufacture has been made. With these data, numerical computational models using finite element simulations have been performed for isolated wires and single pancake coils, and the results of the relevant quench parameters have been compared with the measured values.

Spherical agglomeration of superconducting and normal microparticles with and without applied electric field

It was reported by R. Tao and coworkers that in the presence of a strong electric field superconducting microparticles assemble into balls of macroscopic dimensions. Such a finding has potentially important implications for the understanding of the fundamental physics of superconductors. However, we report here the results of experimental studies showing that (i) ball formation also occurs in the absence of an applied electric field, (ii) the phenomenon also occurs at temperatures above the superconducting transition temperature, and (iii) it can also occur for non-superconducting materials. Possible origins of the phenomenon are discussed.

Mechanical Properties of Non-Superconducting Components in YBCO and $\hbox{Nb}_{3}\hbox{Sn}$ Composites

Non-superconducting materials are essential components in both YBCO and Nb3Sn composites for magnet applications. The surface of the composites is made of pure Cu that is referred as stabilizer. The surface finish is very important for the applications, particularly in the case where thin layers of insulations are required. The stabilizer usually occupies 50% to 70% of the volume fraction of the composites, thus making an important contribution to the mechanical properties of the composites. The other components, such as Cu-Sn matrix in Nb3Sn conductors and substrates in YBCO conductors also contribute to overall mechanical strength. Therefore, understanding the performance of the non-superconducting components under loading helps engineers to make effective use of the superconductor composites and to manufacture conductors meeting the requirements of the magnets, particularly when the magnetic stress reaches the limit of the mechanical strength of the conductors. This paper compares the mechanical properties of YBCO and Nb3Sn composites and discusses the mechanical properties of non-superconducting components in both composite conductors and to relate such properties to dislocation densities and various boundaries. This paper also relates the mechanical properties to other microstructure parameters.

An hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet

Superconductivity in layered copper oxide compounds emerges when charge carriers are added to antiferromagnetically ordered CuO(2) layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to superconductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum that is present in a wide range of superconducting and non-superconducting materials. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, and this idea is supported by measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4) (ref. 15). Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from fluctuating stripes.

Ultrathin oxide films and interfaces for electronics and spintronics

Oxides have become a key ingredient for new concepts of electronic devices. To a large extent, this is due to the profusion of new physics and novel functionalities arising from ultrathin oxide films and at oxide interfaces. We present here a perspective on selected topics within this vast field and focus on two main issues. The first part of this review is dedicated to the use of ultrathin films of insulating oxides as barriers for tunnel junctions. In addition to dielectric non-magnetic epitaxial barriers, which can produce tunneling magnetoresistances in excess of a few hundred percent, we pay special attention to the possibility of exploiting the multifunctional character of some oxides in order to realize ‘active’ tunnel barriers. In these, the conductance across the barrier is not only controlled by the bias voltage and/or the electrodes magnetic state, but also depends on the barrier ferroic state. Some examples include spin-filtering effects using ferro- and ferrimagnetic oxides, and the possibility of realizing hysteretic, multi-state junctions using ferroelectric barriers. The second part of this review is devoted to novel states appearing at oxide interfaces. Often completely different from those of the corresponding bulk materials, they bring about novel functionalities to be exploited in spintronics and electronics architectures. We review the main mechanisms responsible for these new properties (such as magnetic coupling, charge transfer and proximity effects) and summarize some of the most paradigmatic phenomena. These include the formation of high-mobility two-dimensional electron gases at the interface between insulators, the emergence of superconductivity (or ferromagnetism) at the interface between non-superconducting (or non-ferromagnetic) materials, the observation of magnetoelectric effects at magnetic/ferroelectric interfaces or the effects of the interplay and competing interactions at all-oxide ferromagnetic/superconducting interfaces. Finally, we link up the two reviewed research fields and emphasize that the tunneling geometry is particularly suited to probe novel interface effects at oxide barrier/electrode interfaces. We close by giving some directions toward tunneling devices exploiting novel oxide interfacial phenomena.

Magnetic and superconducting properties of FeAs-based high-T c superconductors with Gd

We report on successful synthesis under high pressure of a series of polycrystalline GdFeAs O_{1-x}F_x high-Tc superconductors with different oxygen deficiency x=0.12 - 0.16 and also with no fluorine. We have found that the high-pressure synthesis technique is crucial for obtaining almost single-phase superconducting materials: by synthesizing the same compounds with no pressure in ampoules we obtained non-superconducting materials with an admixture of incidental phases. Critical temperature for all the materials was in the range 40 to 53K. The temperature derivative of the critical field dHc2/dT is remarkably high, indicating potentially high value of the second critical field Hc2 ~ 130T.

Superconducting characterization of Ni/Bi2Sr2Ca2Cu3Oy superconductors

We deposited non-superconducting materials, Ni metal on the surface of Bi2Sr2Ca2Cu3Oy (BSCCO) superconductors. Critical temperature (Tc) and critical current (Ic) of Ni/BSCCO specimens decreased with increasing thickness of Ni films. This indicates that the Tc and the Ic of the BSCCO films are degraded with a thickness of Ni. This behavior is approximately equal to that of a Ni/YBa2Cu3Oy (YBCO) specimen, where the Tc and the Ic of it were dependent on rf power during Ni deposition. Therefore, the degradation of the Ni/BSCCO specimen may be due to the Ni diffusion into the BSCCO films.

A possible scenario for the mechanism of high- Tc superconductivity based on experimental data

The issue of the mechanism of high- T c superconductivity remains open. In this contribution, we propose a new scenario for the mechanism of superconductivity in cuprates based on analysis of experimental data, mainly tunneling, neutron scattering and muon-spin-relaxation data, made earlier (see e.g., A. Mourachkine, Mod. Phys. Lett. B 19 (2005) 743). A specific feature of this scenario is the mechanism of the establishment of long-range phase coherence among Cooper pairs, based on recent experimental data obtained in nonsuperconducting materials.