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A self-driving laboratory autonomously explores high concentration water-in-salt electrolyte blends containing three or four anions.

An Arthur M. Sackler Colloquium titled “Status and Challenges in Science for Decarbonizing our Energy Landscape” was held at the Arnold and Mabel Beckman Center in Irvine, California in October 2018. The papers that follow in this issue of PNAS (1⇓⇓⇓⇓⇓–7) stem from that activity, which addressed a topic of compelling interest and importance to our community from a perspective often not addressed. It is evermore clear, based on incontrovertible climate evidence, that the way we produce and use energy must transition rapidly from what we have done in the past. Population growth, urbanization, and the need for both energy and materials to support this evolution has led to environmental alteration that can only be explained by human activity. As the Intergovernmental Panel on Climate Change and others have emphasized over the last several decades, the average annual temperature of planet Earth has been increasing with greater annual increments. The origin of the increase and its consequences are the greater amounts of CO2 in the atmosphere from approximately 284 parts per million at the start of the industrial revolution to 316 parts per million in 1956, when detailed records were initiated, to 415 parts per million in 2018 (8⇓⇓⇓–12). The well-known “greenhouse” gas effect of CO2 for trapping heat has been described in detail and extends to other polyatomic molecules, such as methane, that are present in much smaller amounts (although their respective greenhouse gas effects may be greater on a molecular basis). The … [↵][1]1To whom correspondence may be addressed. Email: eisenberg{at}chem.rochester.edu. [1]: #xref-corresp-1-1

This century will witness a major transformation in how energy is acquired, stored, and utilized globally. The impetus for this change comes from the deep impacts that both developed and developing societies have had on our planet’s environment during the past century, and the projections going forward of what will happen if we do not act transformatively within the next 2 decades. This paper describes the basis for a meeting held in October 2018 on the need for decarbonization in our energy landscape, and specifically the status and challenges of the science that provides the foundation for such technology. Within the realm of decarbonization in energy generation lies the science of solar energy conversion using new or improved photovoltaic materials and artificial photosynthesis for water splitting and other energy-storing reactions. The intimately related issue of renewable energy storage is being addressed with new strategies, materials, and approaches under current investigation and development. The need to improve the interactions between scientists working on these connected but separately considered challenges and on the transition of scientific achievement to practical application was also addressed, with specific efforts enumerated.

Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

A future electric transportation market will depend on battery innovation

The paper presents a comprehensive overview of electrical and thermal energy storage technologies but will focus on mid-size energy storage technologies for demand charge avoidance in commercial and industrial applications. Utilities bill customers not only on energy use but peak power use since transmission costs are a function of power and not energy. Energy storage (ES) can deliver value to utility customers by leveling building demand and reducing demand charges. With increasing distributed energy generation and greater building demand variability, utilities have raised demand charges and are even including them in residential electricity bills. This article will present a comprehensive overview of electrical and thermal energy storage technologies but will focus on mid-size energy storage technologies for demand charge avoidance in commercial and industrial applications. Of the ES technologies surveyed, lithium ion batteries deliver the highest value for demand charge reduction especially with systems that have larger power to energy ratios. Current lithium ion ES systems have payback periods below 5 years when deployed in markets with high demand charges.

Applying science to the current art of producing engineering and research knowledge has proven difficult, in large part because of its seeming complexity. We posit that the microscopic processes underlying research are not so complex, but instead are iterative and interacting cycles of divergent (generation of ideas) and convergent (testing and selecting of ideas) thinking processes. This reductionist framework coherently organizes a wide range of previously disparate microscopic mechanisms which inhibit these processes. We give examples of such inhibitory mechanisms and discuss how deeper scientific understanding of these mechanisms might lead to dis-inhibitory interventions for individuals, networks and institutional levels.

The motion of Abrikosov vortices in type-II superconductors results in a finite resistance in the presence of an applied electric current. Elimination or reduction of the resistance via immobilization of vortices is the "holy grail" of superconductivity research. Common wisdom dictates that an increase in the magnetic field escalates the loss of energy since the number of vortices increases. Here we show that this is no longer true if the magnetic field and the current are applied parallel to each other.Our experimental studies on the resistive behavior of a superconducting Mo$_{0.79}$Ge$_{0.21}$ nanostrip reveal the emergence of a dissipative state with increasing magnetic field, followed by a pronounced resistance drop, signifying a reentrance to the superconducting state. Large-scale simulations of the 3D time-dependent Ginzburg-Landau model indicate that the intermediate resistive state is due to an unwinding of twisted vortices. When the magnetic field increases, this instability is suppressed due to a better accommodation of the vortex lattice to the pinning configuration. Our findings show that magnetic field and geometrical confinement can suppress the dissipation induced by vortex motion and thus radically improve the performance of superconducting materials., Comment: 10 pages, 4 figures

Materials play a critical enabling role in many energy technologies, but their development and commercialization often follow an unpredictable and circuitous path. In this article, we illustrate this concept with the history of lithium-ion (Li-ion) batteries, which have enabled unprecedented personalization of our lifestyles through portable information and communication technology. These remarkable batteries enable the widespread use of laptop and tablet computers, access to entertainment on portable devices such as hand-held music players and video game consoles, and enhanced communication and networking on personal devices such as cellular telephones and watches. A similar transformation of transportation to electric cars and of the electricity grid to widespread deployment of variable renewable solar and wind generation, effortless time-shifting of energy generation and demand, and a transition from central to distributed energy services requires next-generation energy storage that delivers much higher performance at lower cost. The path to these next-generation batteries is likely to be as circuitous and unpredictable as the path to today’s Li-ion batteries. We analyze the performance and cost improvements needed to transform transportation and the electricity grid, and we evaluate the outlook for meeting these needs with next-generation beyond Li-ion batteries.

We report investigations on the magnetotransport in LaSb, which exhibits extremely large magnetoresistance (XMR). Foremost, we demonstrate that the resistivity plateau can be explained without invoking topological protection. We then determine the Fermi surface from Shubnikov - de Haas (SdH) quantum oscillation measurements and find good agreement with the bulk Fermi pockets derived from first principle calculations. Using a semiclassical theory and the experimentally determined Fermi pocket anisotropies, we quantitatively describe the orbital magnetoresistance, including its angle dependence. We show that the origin of XMR in LaSb lies in its high mobility with diminishing Hall effect, where the high mobility leads to a strong magnetic field dependence of the longitudinal magnetoconductance. Unlike a one-band material, when a system has two or more bands (Fermi pockets) with electron and hole carriers, the added conductance arising from the Hall effect is reduced, hence revealing the latent XMR enabled by the longitudinal magnetoconductance. With diminishing Hall effect, the magnetoresistivity is simply the inverse of the longitudinal magnetoconductivity, enabling the differentiation of the electron and hole contributions to the XMR, which varies with the strength and orientation of the magnetic field. This work demonstrates a convenient way to separate the dynamics of the charge carriers and to uncover the origin of XMR in multi-band materials with anisotropic Fermi surfaces. Our approach can be readily applied to other XMR materials., 34 pages, 8 figures

A vibrant, interactive, and rapidly advancing global society needs an adequate, low cost, predictable and diverse supply of energy; a stable climate; and an international market for energy that mediates across countries, regions, and energy carriers. The science discoveries needed to achieve these energy and societal outcomes are analyzed.

What is mesoscale science? The modifier “meso” can mean different things to different communities. In many areas of science, “mesoscale” generally refers to a middle-ground domain of length, energy, or time where theories accurate at both lower and higher scales fail. In materials science, for example, mesoscale behavior often rises when quantum behavior begins to fade, collective effects become important, or statistical variation and defects appear, often at length scales larger than a few nm. However, for atmospheric scientists and ecologists, mesoscale means miles. For meterologists, mesoscale means hundreds to thousands of miles. The mesoscale arena for cosmologists is many light-years across.

Two-dimensional heterostructures with strong spin-orbit coupling have direct relevance to topological quantum materials and potential applications in spin-orbitronics. In this work, we report on novel quantum phenomena in [Pb2BiS3][AuTe2], a new 2D strong spin-orbit coupling heterostructure system. Transport measurements reveal the spin-related carrier scattering is at odds with the Abrikosov-Gorkov model due to strong spin-orbit coupling. This is consistent with our band structure calculations which reveal a large spin-orbit coupling gap of εso = 0.21 eV. The band structure is also characterized by helical-like spin textures which are mainly induced by strong spin-orbit coupling and the inversion symmetry breaking in the heterostructure system.

Vortex cutting and reconnection is an intriguing and still-unsolved problem central to many areas of classical and quantum physics, including hydrodynamics, astrophysics, and superconductivity. Here, we describe a comprehensive investigation of the crossing of magnetic vortices in superconductors using time dependent Ginsburg-Landau modeling. Within a macroscopic volume, we simulate initial magnetization of an anisotropic high temperature superconductor followed by subsequent remagnetization with perpendicular magnetic fields, creating the crossing of the initial and newly generated vortices. The time resolved evolution of vortex lines as they approach each other, contort, locally conjoin, and detach, elucidates the fine details of the vortex-crossing scenario under practical situations with many interacting vortices in the presence of weak pinning. Our simulations also reveal left-handed helical vortex instabilities that accompany the remagnetization process and participate in the vortex crossing events.

We present the new paradigm of critical current by design. Analogous to materials by design, it aims at predicting the optimal defect landscape in a superconductor for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To highlight this approach, we demonstrate the synergistic combination of critical current measurements on commercial high-temperature superconductors containing self-assembled and irradiation tailored correlated defects by using large-scale time-dependent Ginzburg-Landau simulations for vortex dynamics., Comment: 12 pages, 6 figures; extended version

The key ingredient of high critical currents in a type-II superconductor is defect sites that 'pin' vortices. Contrary to earlier understanding on nano-patterned artificial pinning, here we show unequivocally the advantages of a random pinscape over an ordered array in a wide magnetic field range. We reveal that the better performance of a random pinscape is due to the variation of its local-density-of-pinning-sites (LDOPS), which mitigates the motion of vortices. This is confirmed by achieving even higher enhancement of the critical current through a conformally mapped random pinscape, where the distribution of the LDOPS is further enlarged. The demonstrated key role of LDOPS in enhancing superconducting critical currents gets at the heart of random versus commensurate pinning. Our findings highlight the importance of random pinscapes in enhancing the superconducting critical currents of applied superconductors., Comment: To appear in Physical Review B

Artificial ices enable the study of geometrical frustration by design and through direct observation. However, it has proven difficult to achieve tailored long-range ordering of their diverse configurations, limiting both fundamental and applied research directions. We designed an artificial spin structure that produces a magnetic charge ice with tunable long-range ordering of eight different configurations. We also developed a technique to precisely manipulate the local magnetic charge states and demonstrate write-read-erase multifunctionality at room temperature. This globally reconfigurable and locally writable magnetic charge ice could provide a setting for designing magnetic monopole defects, tailoring magnonics, and controlling the properties of other two-dimensional materials.

The regime of mesoscale science, where the granularity of atoms and quantization of energy gives way to apparently continuous and infinitely divisible matter and energy, yields strikingly complex architectures, phenomena, and functionalities that control macroscopic material behavior. Research in mesoscale materials and chemical science is an opportunity space for next-generation discovery, science, technology, and innovation, with promise of new solutions for societal problems such as energy, environment, climate, advanced manufacturing, and economic growth.

Our understanding and control of sustainable energy technologies is in its infancy. Many sustainable energy phenomena depend on the exchange of photons and electrons among quantized energy levels of semiconductors, molecules, and metals at nanoscale spatial scales and at fast or ultrafast time scales. Improving the performance of sustainable energy technologies to make them competitive with fossil technologies requires probing and understanding these quantum phenomena with advanced scientific techniques. This understanding must then be translated into control of the functionality and performance of the materials and chemistry that govern sustainable energy technologies. The review begins by contrasting the foundations of fossil fuel technology based on combustion, heat, and classical thermodynamics with the foundations of sustainable energy technology based on quantum exchange of energy among photons, chemical bonds, and electrons without conversion to heat. Two sets of tools that are essential to observe, understand, and control the quantum phenomena of sustainable energy are described: in situ and time-resolved experiments and theory, and numerical modeling of the functionality of large assemblies of atoms. Finally, the challenges and opportunities for understanding and ultimately controlling sustainable energy phenomena are presented for catalysis, solar water splitting, and superconductivity.

The study of materials under harsh conditions is essential to meet a range of energy challenges—from creating better turbines, reactors, and batteries to developing future energy systems in dense plasmas.

The oil shock of the 1970s triggered worldwide awareness of oil dependency and launched a search for alternative sources of energy. But three decades on, these efforts have barely had an impact: oil still accounts for almost 40% of global energy use, and fossil fuels make up 85%. The US, for example, imported 20% of its oil in 1970; today the figure is 60%, and other countries import even larger fractions of the oil they consume. The problem of oil dependency is compounded by cost. Before the current recession, the price of oil peaked at $140 a barrel – five times its price in 2002 and 10 times its price in 1976 – rewriting the economics of transportation, food, manufacturing and trade that underlie the operation of society. In addition to dependency and cost, energy security is a pervasive threat. The concentration of oil production in a few regions of the world makes the supply of oil vulnerable to unpredictable events such as weather, terrorism, and geopolitical manoeuvring. Because oil provides so much of our energy, severe reductions in its flow would dramatically change the way we live.

Combined experimental and theoretical studies are reported of the Fermi surface, band structure, generalized magnetic susceptibility, electron-phonon enhancement factor λ and superconducting transition temperature TT of LaB6. Whereas the unusually large λ values, ranging from 1.0 to 2.5, are expected to result in high TT values, TT is observed to be only 0.122°K. These results further emphasize the need for appropriate theoretical formulations for these systems.

Making major gains in energy efficiency is one of the most economical and effective ways our nation can wean itself off its dependence on foreign oil and reduce its emissions of greenhouse gases. Transportation and buildings, which account for two thirds of American energy usage, consume far more than they need to, but even though there are many affordable energy efficient technologies that can save consumers money, market imperfections inhibit their adoption. To overcome the barriers, the federal government must adopt policies that will transform the investments into economic and societal benefit. And the federal government must invest in research and development programs that target energy efficiency. Energy efficiency is one of America's great hidden energy reserves. We should begin tapping it now. Whether you want the United States to achieve greater energy security by weaning itself off foreign oil, sustain strong economic growth in the face of worldwide competition or reduce global warming by decreasing carbon emissions, energy efficiency is where you need to start. Thirty-five years ago the U.S. adopted national strategies, implemented policies and developed technologies that significantly improved energy efficiency. More than three decades have passed since then, and science and technology have progressed considerably, butmore » U.S. energy policy has not. It is time to revisit the issue. In this report we examine the scientific and technological opportunities and policy actions that can make the United States more energy efficient, increase its security and reduce its impact on global warming. We believe the findings and recommendations will help Congress and the next administration to realize these goals. Our focus is on the transportation and buildings sectors of the economy. The opportunities are huge and the costs are small.« less

Extremely large magnetoresistance (XMR) was recently discovered in WTe$_2$, triggering extensive research on this material regarding the XMR origin. Since WTe$_2$ is a layered compound with metal layers sandwiched between adjacent insulating chalcogenide layers, this material has been considered to be electronically two-dimensional (2D). Here we report two new findings on WTe$_2$: (1) WTe$_2$ is electronically 3D with a mass anisotropy as low as $2$, as revealed by the 3D scaling behavior of the resistance $R(H,\theta)=R(\varepsilon_\theta H)$ with $\varepsilon_\theta =(\cos^2 \theta + \gamma^{-2}\sin^2 \theta)^{1/2}$, $\theta$ being the magnetic field angle with respect to c-axis of the crystal and $\gamma$ being the mass anisotropy; (2) the mass anisotropy $\gamma$ varies with temperature and follows the magnetoresistance behavior of the Fermi liquid state. Our results not only provide a general scaling approach for the anisotropic magnetoresistance but also are crucial for correctly understanding the electronic properties of WTe$_2$, including the origin of the remarkable 'turn-on' behavior in the resistance versus temperature curve, which has been widely observed in many materials and assumed to be a metal-insulator transition.

Depending on the Ginzburg-Landau parameter kappa, superconductors can either be fully diamagnetic if kappa < 1/root 2 (type I superconductors) or allow magnetic flux to penetrate through Abrikosov vortices if kappa > 1/root 2 (type II superconductors; refs 1,2). At the Bogomolny critical point, kappa = kappa(c) = 1/root 2, a state that is infinitely degenerate with respect to vortex spatial configurations arises(3,4). Despite in-depth investigations of conventional type I and type II superconductors, a thorough understanding of the magnetic behaviour in the near-Bogomolny critical regime at kappa similar to kappa(c) remains lacking. Here we report that in confined systems the critical regime expands over a finite interval of kappa forming a critical superconducting state. We show that in this state, in a sample with dimensions comparable to the vortex core size, vortices merge into a multi-quanta droplet, which undergoes Rayleigh instability(5) on increasing kappa and decays by emitting single vortices. Superconducting vortices realize Nielsen-Olesen singular solutions of the Abelian Higgs model, which is pervasive in phenomena ranging from quantum electrodynamics to cosmology(6-9). Our study of the transient dynamics of Abrikosov-Nielsen-Olesen vortices in systems with boundaries promises access to non-trivial effects in quantum field theory by means of bench-top laboratory experiments.

In high-temperature superconductors, magnetic field lines penetrate the samples through vortices arranged in an Abrikosov vortex lattice. In a magnetic field Hm(T ) below the upper critical field Hc2(T ) that destroys bulk superconductivity, the vortex lattice melts to a liquid vortex state, in which each vortex line must be ‘pinned’ individually to prevent dissipation. Linear and planar defects have been found to be effective for pinning the vortex liquid because they trap an entire vortex within a single extended defect. However, up to now it is not known how far into the liquid state this pinning process can be effective. Here, we show that there is a universal magnetic field line Hl(T ) between Hm(T ) and Hc2(T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field Hl(T ) sets an upper limit to the irreversibility line Hirr(T ) marking the onset of dissipation. For that reason it determines a new magnetic-field–temperature region in which a superconductor can remain useful.

Columnar defects are shown to produce anisotropic pinning in the vortex liquid state of YBCO. We interpret the disappearance of anisotropic pinning as signaling the loss of longitudinal correlation of vortices as thermal fluctuations dominate their line tension.

The following article is based on the plenary presentation given by Mildred S. Dresselhaus of the Massachusetts Institute of Technology on November 29, 2004, at the Materials Research Society Fall Meeting in Boston. Advanced materials offer new promise for addressing some of the grand societal challenges of our future, including that of global energy. This article will review opportunities that have opened up at the nanoscale, with materials of reduced dimensionality and enhanced surface-to-volume ratio. Some examples of research accomplishments and opportunities at the nanoscale will be described, with special attention given to the potential for advanced materials and nanoscience to have an impact on the grand challenges related to a sustainable energy supply for the 21st century and beyond.

A very peculiar feature of the recently discovered superconductor MgB2 is the multigap nature of the superconducting state, which is now commonly accepted in the scientific community and confirmed by a large number of experiments. We report a systematic scanning tunnelling spectroscopy (STS) study performed on high quality thin films of MgB2. Electron microscopy images and STM topography together with the STS investigation allow a direct correlation between tunnelling direction and the observed tunnelling spectra, confirming that the two-gap state is intrinsic to MgB2.

The superconductivity in MgB 2 has a two-band character with the dominating band having a 2D character and the second band being isotropic in the three dimensions. We use tunneling microscopy and spectroscopy to reveal the two distinct energy gaps at Δ 1 =2.3 meV and Δ 2 =7.1 meV. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multi-band system. The results are consistent with the existence of two-band superconductivity in the presence of strong interband superconducting pair interaction and quasiparticle scattering. The temperature evolution of the tunneling spectra shows both gaps vanishing at the bulk T c .

With a superconducting transition temperature of 40 K and two superconducting gaps, MgB2 is full of surprises for both experimentalists and theorists.

We report specific-heat measurements on two samples of Mg 11 B 2 , one powder and one sintered, that give essentially identical results. Both samples are of exceptionally high quality: At the critical temperature the discontinuity in specific heat is higher than that of other samples, the transition is sharper than for most samples, and the signature feature of the small, non-BCS energy gap is particularly pronounced. The results are compared with a phenomenological model for a multi-gap superconductor, with band-structure calculations, and with spectroscopic determinations of the energy gaps.

Using magnetization, magneto-transport and single-crystal specific heat measurements we have determined the superconducting phase diagram of MgB 2 . The superconducting anisotropy γ changes monotonously from a value of around 2 near T c to above 4.5 at 22 K. For H||c a pronounced peak effect in the critical current occurs at the upper critical field. We present a strong evidence for a surface superconducting state for H||c which might account for the wide spread in reported values of the superconducting anisotropy γ.

We have studied the anisotropic superconductor MgB2 using a combination of scanning electron microscopy and scanning tunnelling spectroscopy. Tunnelling spectroscopy performed on thin films and pellets reveals two distinct energy gaps at ?1 = 2.3 meV and ?2 = 7.1 meV. On different crystallites within the polycrystalline sample different spectral weights of the partial densities of states (PDOS) were observed. They reflect different tunnelling directions with respect to the crystallographic orientation of the grain in a multiband system. Indeed when tunnelling in the c-axis films only one superconducting gap is observed, which is associated with the 3D band in this system. Temperature evolution of the tunnelling spectra reveals that both gaps close simultaneously near the bulk critical temperature. Our experimental findings are consistent with the two-band superconductivity scenario in the presence of strong pair interaction between the two bands [1].

Porous alumina membranes are commercially available and have been widely used in recent nanoscale research, for example, as templates in nanowire fabrication through electrodeposition. In this report, we present a new use for porous alumina membranes in the fabrication of alumina nanotubes/nanowires desired in electrochemical devices and catalytic applications. A high yield of alumina nanotubes/nanowires is obtained by etching porous alumina membranes in an aqueous sodium hydroxide solution. We studied the effects of etching time and solution concentration and characterized the alumina nanotubes/nanowires using a scanning electron microscope (SEM). A discussion of the possible mechanism for the formation of nanotubes/nanowires is given. Our results also imply that in nanowire fabrication through the template approach where alumina membranes are removed with sodium hydroxide solution to release the nanowires special attention is needed in characterizing the nanowires with the SEM because alumina nanotubes/nanowires can be easily mistaken for electrodeposited nanowires.

Columnar defects induced by high-energy heavy ion irradiation constitute one of the most effective vortex pinning sites in high temperature superconductors. We demonstrate a novel method of effectively patterning vortex pinning sites in single crystals of the high temperature superconductors YBa 2 Cu 3 O 7-d and Bi 2 Sr 2 CaCu 2 O x using heavy ion irradiation. These patterns include linear channels, a lattice of squares, and asymmetric pinning gradient sites where vortices can either flow unimpeded or are trapped in spatially periodic arrangements.

New melt textured REBa2Cu3O7/RE ′ 2 BaCuO5 composites, have been obtained by top seeding melt-texturing growth. Two different starting mixtures of REBa2Cu3O7 superconducting powders and insulating Y2BaCuO5 phase were used. On one hand RE is a natural mixture of heavy rare earth elements (Y, Yb, Lu, Er, Dy, Tm, Ho) extracted from the Brazilian mineral “Xenotime”, and on the other hand, RE is thulium. In both cases melt textured REBa2Cu3O7/RE ′ 2 BaCuO5 composites have been obtained where RE and RE ′ are different mixtures of heavy rare earth/yttrium and Tm/yttrium. The composition analysis shows different areas within the RE ′ 2 BaCuO5 as a consequence of an inhomogeneous RE distribution, due to the differential solubility of each rare earth in the high temperature semisolid state. During the crystallization process a profound inversion of the rare earth composition between the superconducting matrix and the insulating precipitates occurs. Yttrium is selectively located in the 123 matrix and RE in the 211 particles. Heavy RE ions can substitute yttrium in MTG REBCO without degradation of the superconducting properties. A model for the crystallization process is proposed.

Effects of bend strains on monocore Bi-2223 tapes were studied by transport measurements, magneto-optical imaging and electron microscopy. The strain tolerance of tapes that were sintered for 100 (uniform and well-textured microstructure with high Ic) and 50 h (nonuniform microstructure with low Ic) was evaluated. The strain tolerance of the 100-h sintered tape was superior to that of the 50-h sintered tape. We have shown that the observed strain tolerance is largely determined by the interplay of nonuniform strains and microstructural variation across the tape cross section, leading to nonuniform Ic values. An explanation that accounts for an observed concentration of the critical-current density at the Ag/BSCCO interface was presented for the measured strain dependence of the retained Ic.

A new welding procedure for bulk melt-textured YBa2Cu3O7 (YBCO) superconducting tiles has been developed leading to high quality joints. The welding agent consists of a YBCO–Ag composite, which has a peritectic temperature 40°C lower than YBCO. It is shown that through a proper selection of thermal treatments, the effect of Ag additives can be confined to the immediate welding zone, thus allowing a self-seeded growth process of the YBCO/Ag composite initiated at the adjacent solid YBCO crystals. Local magneto-optical observations, as well as trapped field measurements produced by circulating currents in ring samples, reveal that the critical current across the weld joint is as high as that of bulk melt-textured YBCO.

Magnetic structures of SmCo/Fe bilayers during remagnetization and after multiple field cycling are studied magneto-optically (MO). Rotation of the remanent magnetization in the soft Fe layer away from the easy axis is found after large enough remagnetizing fields. This is indicative of the appearance of biquadratic coupling between the layers. It is explained by the fluctuation exchange mechanism due to the field induced magnetic inhomogeneity of the hard layer. Such an inhomogeneity revealed in the MO patterns also explains the degradation of the bias field in exchange springs.

A magnetic field penetrates a superconductor through an array of ‘vortices’, each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex ‘matter’. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a ‘solid’, in which the vortices are pinned, but the solid can ‘melt’ into a ‘liquid’ phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram2.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes3,4,5,6,7,8,9,10. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition—namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory11—is in accord with theoretical predictions12,13,14 of a transition to a second vortex-liquid phase.

The critical current densities in bulk melt-textured YBa2Cu3Ox and across superconducting “weld” joints are measured using scanning Hall probe measurements of the trapped magnetic field in ring samples. With this method, critical current densities are obtained without the use of electrical contacts. Large persistent currents are induced in ring samples at 77 K, after cooling in a 3 kG field. These currents can be determined from the magnetic field they produce. At 77 K a supercurrent exceeding 2000 A (about 104 A/cm2) was induced in a 2 cm diameter ring; this current produces a magnetic field exceeding 1.5 kG in the bore of the ring. We demonstrate that when a ring is cut, and the cut is repaired by a superconducting weld, the weld joint can transmit the same high supercurrent as the bulk.

We report measurements of both the specific heat ( C ) and the magnetization ( M ) of a naturally untwinned YBa 2 Cu 3 O 7−δ (YBCO) single crystal in magnetic field ( H ) up to 26 Tesla. The specific heat data determine the thermodynamic nature of the melting and show its evolution with increasing H within a low— H region (0.1 ≤ H ≤ 10.5 Tesla) in which it is first order, with a discontinuity in C and a hysteretic latent heat, through a multicritical point, and into a high— H region (10.5 ≤ H ≤ 26 Tesla) in which it is second order, with only a discontinuity in C . The qualitative change in melting in that latter region seems to reflect not a change in the nature of the solid but rather that of the (reversible) liquid. The M data show the relation of irreversibility to melting and allow to distinguish between the thermodynamic features at melting to disorder-dependent properties of the solid.

The upper and lower critical points are investigated in untwinned YBa 2 Cu 3 O 7−δ single crystals with dilute columnar defects. Dilute columnar defects raise the upper critical point, indicating that the transition near the upper critical point is a vortex entanglement transition. The lower critical point is very sensitive to columnar defect disorder and its position can be described by a Lindemann-like criterion similar to that for melting. Dilute columnar defects induce non-linear behavior in the I-V curves of the vortex liquid state above the lower critical point, which we interpret as a vestige of the critical region associated with the Bose glass transition below the lower critical point.

The velocity profile of vortices driven by a non-uniform Lorentz force contains information on the dynamic correlation of the moving liquid, lattice, and glassy phases. We describe transport experiments using a disk geometry that allow the velocity profile to be measured and analyzed in terms of the hydrodynamic motion of the liquid, and the plastic and elastic motion of the solid. Application to the glassy states bordering the upper and lower critical points is discussed.

We report on magnetoresistance measurements of the effects of 9 MeV proton irradiation on a clean, untwinned single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. For the first time, evidence for a vortex glass transition is detected in an untwinned single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with induced pointlike disorder, in which the first order vortex melting transition is completely suppressed after proton irradiation. Our results suggest that a sufficiently high pinning disorder is required in order for the vortex glass phase to be observed.