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The addition of artificial pinning centers has led to an impressive increase in critical current density ($J_{\rm c}$) in a superconductor, enabling record-breaking all-superconducting magnets and other applications. $J_{\rm c}$ has reached $\sim 0.2$-$0.3$ $J_{\rm d}$, where $J_{\rm d}$ is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying $\lambda$ and/or $\xi$, the penetration depth and coherence length, respectively, we can increase $J_{\rm d}$. For (Y$_{0.77}$Gd$_{0.23}$)Ba$_2$Cu$_3$O$_y$ ((Y,Gd)123) we achieve this by controlling the carrier density, which is related to $\lambda$ and $\xi$. We also tune $\lambda$ and $\xi$ by controlling the chemical pressure in the Fe-based superconductors, BaFe$_2$(As$_{1-x}$P$_x$)$_2$ films. The variation of $\lambda$ and $\xi$ leads to an intrinsic improvement of $J_{\rm c}$, via $J_{\rm d}$, obtaining extremely high values of $J_{\rm c}$ of $130$ MA/cm$^2$ and $8.0$ MA/cm$^2$ at $4.2$ K, consistent with an enhancement of $J_{\rm d}$ of a factor of $2$ for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe$_2$(As$_{1-x}$P$_x$)$_2$ films, showing that this new material design is useful to achieving high critical current densities for a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached $\sim 3.17$ TN/m$^3$ at $4.2$ K and 18 T (${\bf H}\parallel c$), the highest values ever reported in any superconductor., Comment: 35 pages, 7 figures

Este trabajo forma parte de los resultados de una investigación que tiene como objetivo comprender las influencias del turismo después de la implementación del Parque Estatal Serra do Brigadeiro (PESB) en el paisaje de Araponga - MG., utilizado como marco de tiempo desde 1996 hasta 2020. Discute el papel del paisaje en la discusión sobre el patrimonio cultural en una pequeña ciudad de Minas Gerais, considerando que recibe influencia del turismo proveniente del PESB. Al acercarnos al "paisaje" consideramos aspectos morfológicos y simbólicos de este término polisémico, se toma que está ligado a la forma en que las personas experimentan y se manifiestan en el espacio, por lo tanto, se trata del interés de comprender la percepción de la comunidad que utiliza el paisaje, ya sea como vivienda, trabajo o fuente de alimento. El Parque Estatal Serra do Brigadeiro fue inaugurado en medio de conflictos, tales conflictos permitieron el reconocimiento de una comunidad unida dispuesta a luchar por el uso de su espacio, siendo Araponga uno de los protagonistas en este contexto. Hubo, más tarde, la búsqueda del equilibrio entre la mirada externa, del especialista en patrimonio cultural o ambiental, y la mirada de la comunidad local que percibía y utilizaba ese espacio. El estudio del paisaje exige comprender aportes que vayan más allá de lo visto, por lo tanto, es necesario profundizar en las relaciones sociales presentes en la memoria e historia de un grupo determinado para comprender los cambios que ocurrieron y sus significados, así como conciliar perspectivas externas e internas al municipio.

The concept of cultural landscape, historically associated with geographical science, was interpreted as a typology of the World Heritage site and incorporated by the UNESCO Committee in 1992. The article aims to understand this concept and the typologies derived from it and operated by UNESCO, in the two cases that occurred in Brazil: "Rio de Janeiro: carioca landscapes between the sea and the mountain" and "Conjunto Moderno da Pampulha". After analyzing the dossiers of application for the title of World Heritage it was possible to identify the uses of memory and narrative constructions derived from the confluence of the concept of cultural landscape and public policies of the fields of Brazilian and world heritage. The result is the discourses that consecrated these two Brazilian sites as exceptional cultural landscapes, recognized worldwide., El concepto de paisaje cultural, históricamente asociado con la ciencia geográfica, fue interpretado como una tipología del Patrimonio mundial e incorporado por el Comité de la UNESCO en 1992. El artículo tiene como objetivo entender este concepto y las tipologías derivadas de él y operadas por la UNESCO, en los dos casos ocurridos en Brasil: "Río de Janeiro: paisajes cariocas entre el mar y la montaña" y "Conjunto Moderno da Pampulha". Tras analizar los expedientes de solicitud para el título de Patrimonio Mundial fue posible identificar los usos de la memoria y las construcciones narrativas derivadas de la confluencia del concepto de paisaje cultural y políticas públicas de los campos del patrimonio brasileño y mundial. El resultado son los discursos que consagraron estos dos sitios brasileños como paisajes culturales excepcionales, reconocidos en todo el mundo., O conceito de paisagem cultural, historicamente associado à ciência geográfica, foi interpretado como uma tipologia do Patrimônio Mundial e incorporado pelo Comitê da UNESCO em 1992. O artigo objetiva a compreensão deste conceito e das tipologias dele derivadas e operadas pela UNESCO, nos dois casos ocorridos no Brasil: “Rio de Janeiro: paisagens cariocas entre o mar e a montanha” e “Conjunto Moderno da Pampulha”. Após a análise dos dossiês de candidatura ao título de Patrimônio Mundial foi possível identificar os usos da memória e as construções narrativas derivados da confluência do conceito de paisagem cultural e das políticas públicas dos campos do patrimônio brasileiro e mundial. O resultado são os discursos que consagraram esses dois sítios brasileiros como paisagens culturais excepcionais, mundialmente reconhecidas.

One of the most promising routes for achieving unprecedentedly high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce $T_c$. Consequently, an optimal balance must be achieved between the nanoparticle density $n_p$ and size $d$. Determining this balance requires garnering a better understanding of vortex-nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y$_{0.77}$,Gd$_{0.23}$)Ba$_2$Cu$_3$O$_{7-\delta}$ films in magnetic fields up to 35 T, and compare the trends to recent results from time-dependent Ginzburg-Landau simulations. We identify consistencies between the field-dependent critical current $J_c(B)$ and expectations from strong pinning theory. Specifically, we find that that $J_c \propto B^{-\alpha}$, where $\alpha$ decreases from $0.66$ to $0.2$ with increasing density of nanoparticles and increases roughly linearly with nanoparticle size $d/\xi$ (normalized to the coherence length). At high fields, the critical current decays faster ($\sim B^{-1}$), suggestive that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in $J_c(B)$. Due to a spread in defect sizes, this novel peak effect remains unresolved here. Lastly, we reveal that the dependence of the vortex creep rate $S$ on nanoparticle size and density roughly mirrors that of $\alpha$, and compare our results to low-$T$ nonlinearities in $S(T)$ that are predicted by strong pinning theory.

Fascination with glassy states has persisted since Fisher introduced the vortex-glass as a new thermodynamic phase that is a true superconductor that lacks conventional long-range order. Though Fisher’s original model considered point disorder, it was later predicted that columnar defects (CDs) could also induce glassiness — specifically, a Bose-glass phase. In YBa2Cu3O7−x (YBCO), glassy states can cause distinct behavior in the temperature (T ) dependent rate of thermally activated vortex motion (S). The vortex-glass state produces a plateau in S(T ) whereas a Bose-glass can transition into a state hosting vortex excitations called double-kinks that can expand, creating a large peak in S(T ). Although glass phases have been well-studied in YBCO, few studies exist of other materials containing CDs that could contribute to distinguishing universal behavior. Here, we report on the effectiveness of CDs tilted ~30° from the c-axis in reducing S in a NbSe2 crystal. The magnetization is 5 times higher and S is minimized when the field is parallel to the defects versus aligned with the c-axis. We see signatures of glassiness in both field orientations, but do not observe a peak in S(T ) nor a plateau at values observed in YBCO. Finally, we discuss the possibility that competing disorder induces a field-orientation-driven transition from a Bose-glass to an anisotropic glass involving both point and columnar disorder.

We report a detailed study of the magnetization modulus as a function of temperature and applied magnetic field under varying angle in Sr$_{4}$Ru$_{3}$O$_{10}$ close to the metamagnetic transition at $H_{c}\backsimeq 2.5\,$T for $H \perp c$. We confirm that the double-feature at $H_{c}$ is robust without further splitting for temperatures below 1.8 K down to 0.48 K. The metamagnetism in Sr$_{4}$Ru$_{3}$O$_{10}$ is accompanied by a reduction of the magnetic moment in the plane of rotation and large field-hysteretic behavior. The double anomaly shifts to higher fields by rotating the field from $H\,\perp \,c$ to $H\,\parallel\,c$. We compare our experimental findings with numerical simulations based on spin reorientation models caused by intrinsic magnetocrystalline anisotropy and Zeeman effect. Crystal anisotropy is able to explain a metamagnetic transition in the ferromagnetic ordered system Sr$_{4}$Ru$_{3}$O$_{10}$, but a Dzyaloshinskii-Moriya term is crucial to account for a reduction of the magnetic moment as discovered in the experiments., Comment: 7 pages, 8 figures

O presente artigo apresenta uma análise sobre o processo de transformação da paisagem urbana provocado pelas políticas patrimoniais que visam à preservação de alguns bens, alçados à condição de patrimônios da cidade. Para tanto, o artigo está divido em duas partes: na primeira, de caráter mais teórico, apresentamos uma análise do conceito de paisagem urbana e do papel das políticas patrimoniais nas transformações dessa paisagem. Na segunda parte, apresentamos dados empíricos relacionados, a uma pesquisa cujo objeto de estudo foi a política patrimonial desenvolvida na cidade de Viçosa (MG) na virada do século XX para o século XXI. O foco das investigações do estudo vem sendo os usos da memória e de narrativas memorialísticas ou históricas por determinados grupos da população, bem como, pelas autoridades constituídas, como normativas para as políticas públicas de conservação e preservação do patrimônio e, consequentemente, de requalificação da paisagem urbana. Do ponto de vista teórico, o que o estudo em questão evidenciou foi que, a despeito da atuação de especialistas em usos da memória ou em dinâmicas do espaço urbano, as política públicas de patrimônio interpretaram e trabalharam a chamada consciência histórica tendo por base o senso comum. Neste sentido, a experiência histórica de introdução de políticas públicas de patrimônio investigada na cidade de Viçosa, responde a uma concepção de tempo como ideia de progresso que, de maneira direta ou indireta, orientou as políticas públicas de conservação e preservação do patrimônio histórico e cultural e a construção de paisagens urbanas, no Brasil, durante todo o século XX.

We present magnetization measurements on Sr 4 Ru 3 O 10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr 4 Ru 3 O 10 below the ferromagnetic transition T C as a double step in the magnetization at two critical fields H c 1 and H c 2 . We map the H − T phase diagram with special focus on the temperature range 50 K ≤ T ≤ T C . We find that the critical field H c 1 ( T ) connects the field and temperature axes of the phase diagram, whereas the H c 2 boundary starts at 2.8 T for the lowest temperatures and ends in a critical endpoint at (1 T; 80 K). We conclude from the temperature dependence of the ratio Hc 1 Hc 2 ( T ) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

Room‐temperature magnetoelectric (ME) coupling is developed in artificial multilayers and nanocomposites composed of magnetostrictive and electrostrictive materials. While the coupling mechanisms and strengths in multilayers are widely studied, they are largely unexplored in vertically aligned nanocomposites (VANs), even though theory has predicted that VANs exhibit much larger ME coupling coefficients than multilayer structures. Here, strong transverse and longitudinal ME coupling in epitaxial BaTiO3:CoFe2O4 VANs measured by both optical second harmonic generation and piezoresponse force microscopy under magnetic fields is reported. Phase field simulations have shown that the ME coupling strength strongly depends on the vertical interfacial area which is ultimately controlled by pillar size. The ME coupling in VANs is determined by the competition between the vertical interface coupling effect and the bulk volume conservation effect. The revealed mechanisms shed light on the physical insights of vertical interface coupling in VANs in general, which can be applied to a variety of nanocomposites with different functionalities beyond the studied ME coupling effect.

We present an extensive study of vortex dynamics in a high-quality single crystal of HgBa2CuO4+δ, a highly anisotropic superconductor that is a model system for studying the effects of anisotropy. From magnetization M measurements over a wide range of temperatures T and fields H, we construct a detailed vortex phase diagram. We find that the temperature-dependent vortex penetration field Hp(T), second magnetization peak Hsmp(T), and irreversibility field Hirr(T) all decay exponentially at low temperatures and exhibit an abrupt change in behavior at high temperatures T/Tc >~ 0.5. By measuring the rates of thermally activated vortex motion (creep) S(T, H) = |dlnM(T, H)/dlnt|, we reveal glassy behavior involving collective creep of bundles of 2D pancake vortices as well as temperature- and time-tuned crossovers from elastic (collective) dynamics to plastic flow. Based on the creep results, we show that the second magnetization peak coincides with the elastic-to-plastic crossover at low T, yet the mechanism changes at higher temperatures.

Non-linear electrical transport studies at high-pulsed magnetic fields, above the range accessible by DC magnets, are of direct fundamental relevance to the physics of superconductors, domain-wall, charge-density waves, and topological semi-metal. All-superconducting very-high field magnets also make it technologically relevant to study vortex matter in this regime. However, pulsed magnetic fields reaching 100 T in milliseconds impose technical and fundamental challenges that have prevented the realization of these studies. Here, we present a technique for sub-microsecond, smart, current-voltage measurements, which enables determining the superconducting critical current in pulsed magnetic fields, beyond the reach of any DC magnet. We demonstrate the excellent agreement of this technique with low DC field measurements on Y$_{0.77}$Gd$_{0.23}$Ba$_2$Cu$_3$O$_7$ coated conductors with and without BaHfO$_3$ nanoparticles. Exploring the uncharted high magnetic field region, we discover a characteristic influence of the magnetic field rate of change ($dH/dt$) on the current-voltage curves in a superconductor. We fully capture this unexplored vortex physics through a theoretical model based on the asymmetry of the vortex velocity profile produced by the applied current., Comment: 9 pages, 7 figures

The specific heat [Formula: see text] of metallic SrB6 single crystals shows an anomalous behavior for [Formula: see text] K which varies strongly with an applied magnetic field. This is consistent with a two-level Schottky system. We ascribe the excess of [Formula: see text] in this temperature range to localized magnetic moments. In addition, features that are attributable to a partial ferromagnetic polarization of a conduction electron gas are observed. These results are supported by magnetization measurements and are compatible with the transport properties reported previously (Stankiewicz 2016 Phys. Rev. B 94 125141)., We acknowledge support from grant MAT2015-73914-JIN, from the Ministerio de Economía y Competividad of Spain. P F S R acknowledges support from the Laboratory Directed Research and Development program of Los Alamos National Laboratory under project number 20160085DR. Work by L.C. was supported by the U.S. DOE, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

The specific heat [Formula: see text] of metallic SrB

O artigo visa compreender o processo de constituicao do saber sobre a historia da cidade do Rio de Janeiro, levando em consideracao nao apenas as inclinacoes politicas de Delgado de Caralho e suas relacoes com eminentes figuras da cidade, mas tambem as estrategias narrativas do autor. Nao buscamos, assim, integrar o pensador em uma categoria mais ampla de intelectuais no pensamento social brasileiro, nem tampouco tecer consideracoes sobre o periodo historico em que o livro foi publicado. Ainda que estas questoes nao sejam negligenciadas, o artigo aborda a maneira pela qual o autor escreve a historia da cidade, enfocando nao so aspectos teoricos e metodologicos, mas tambem sua apropriacao do saber historico e geografico de sua epoca sobre o lugar e o tempo a fim de se inserir na vida social, politica e cultural da cidade do Rio de Janeiro. O artigo visa compreender como se construiu uma versao da historia do Rio que se confundia com os icones da cultura moderna da decada de 1920. A formacao intelectual de Delgado de Carvalho, os objetivos do livro, a concepcao de como fazer historia e geografia e a relacao com figuras politicas do periodo ajudam a compreender a interpretacao do autor sobre a historia de um lugar: a cidade do Rio de Janeiro.

We demonstrate a twofold increase in the in-field critical current of AMSC's standard 2G coil wire by irradiation with 18-MeV Au ions. The optimum pinning enhancement is achieved with a dose of $6\times 10^{11}\ \text{Au}\ \text{ions/cm}^{2}$ . Although the 77 K, self-field critical current is reduced by about 35%, the in-field critical current (H//c) shows a significant enhancement between 4 and 50 K in fields > 1 T. The process was used for the roll-to-roll irradiation of AMSC's standard 46-mm-wide production coated conductor strips, which were further processed into standard copper laminated coil wire. The long-length wires show the same enhancement as attained with short static irradiated samples. The roll-to-roll irradiation process can be incorporated in the standard 2G wire manufacturing, with no modifications to the current process. The enhanced performance of the wire will benefit rotating machine and magnet applications.

YBa2Cu3O7-based coated conductors (CCs) achieve the highest critical current densities (J c) of any known superconductor and are a key technology for applications such as rotatory machines, high-field magnets and power transmission. Incorporation of nano-sized non-superconducting second phases as additional vortex pinning centers has been considered the most amenable route to further enhance J c at an industrial scale, and has been successfully used in commercial CCs. The resulting pinning landscape is quite complex, with both synergistic and competing interactions among the various types of defects. Particle irradiation, on the other hand, allows for a controlled post-processing incorporation of a well-defined defect morphology. We have previously shown that irradiation with protons and other light ions can further enhance the in-field J c in commercial state-of-the-art CCs. Here we develop a combined irradiation process that increases J c above values previously achieved by irradiating with only one species. Our new approach involves sequentially irradiating with 250 MeV Au ions and 4 MeV protons. For example, at T∼ 27 K (liquid neon) and µ 0 H∼ 4 T, a region of interest for rotatory machines applications, we obtain J c ∼ 5 MA cm−2, which is about 40% higher than the values produced by the individual irradiations. Finally, we conclude that this is due to the synergistic pinning effects of the introduced splayed, non-uniform columnar defects and small clusters.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Disorder can have remarkably disparate consequences in superconductors, driving superconductor–insulator transitions in ultrathin films by localizing electron pairs and boosting the supercurrent carrying capacity of thick films by localizing vortices (magnetic flux lines). Though the electronic 3D-to-2D crossover at material thicknesses d ~ ξ (coherence length) is well studied, a similarly consequential magnetic crossover at d ~ Lc (pinning length) that should drastically alter material properties remains largely underexamined. According to collective pinning theory, vortex segments of length Lc bend to adjust to energy wells provided by point defects. Consequently, if d truncates Lc, a change from elastic to rigid vortex dynamics should increase the rate of thermally activated vortex motion S. Here, we characterize the dependence of S on sample thickness in Nb and cuprate films. The results for Nb are consistent with collective pinning theory, whereas creep in the cuprate is strongly influenced by sparse large precipitates. We leverage the sensitivity of S to d to determine the generally unknown scale Lc, establishing a new route for extracting pinning lengths in heterogeneously disordered materials.

Em um mundo marcado pela velocidade de circulacao de mercadorias, pessoas e, sobretudo do capital, nao ha como escapar de uma sensacao de um tempo presente que se eterniza. No entanto, a despeito da unificacao de mercados e da construcao de uma economia mundial, presenciamos no periodo historico que vai do final do seculo XX ao inicio do seculo XXI, a importância, cada vez maior, da memoria, da historia, do passado e das recordacoes individuais e coletivas. Essa forca historicista tem desejado e orientado a conservacao e transmissao de um patrimonio cultural comum de cunho material ou imaterial. Deste modo, podemos identificar por parte de autoridades governamentais, pela da acao politica de camadas de intelectuais, ou atraves da pressao de grupos identitarios, uma especie de obsessao memorialista, cujo objetivo consciente ou nao, e a preservacao da identidade de uma determinada comunidade. O desejo de conservacao e preservacao teria aumentado na medida em os diferentes grupos de pressao e as camadas intelectuais desconfiariam dos projetos de futuro, das promessas do desenvolvimento economico e das dadivas dos diferentes modelos de utopias comunitarias. No entanto, na discussao sobre o que se deve conservar e preservar, nao e a memoria e a identidade historica dos grupos subalternizados, mas sim aquelas de grupos dominantes com forte capacidade de exercer pressao social e coletiva. O presente trabalho se debruca sobre um exemplo concreto, a revitalizacao de uma praca no coracao do centro historico da cidade do Rio de Janeiro e procura refletir sobre o papel fundamental da preservacao do patrimonio cultural material ou intangivel como um instrumento de preservacao da identidade historica e cultural dos diferentes grupos que dividem o espaco urbano. O trabalho pretende assim revelar como os interesses politicos e culturais de determinados grupos podem iluminar alguns objetos de valor cultural e afetivos, mas, ao mesmo tempo produzir sombras outros tantos que sao fundamentais para a memoria coletiva de grupos com menor poder de pressao e representacao social. Ao se ocupar do exemplo recente do processo de revitalizacao do centro da cidade do Rio de Janeiro, o artigo tema intencao de destacar a importância da discussao do patrimonio cultural como politica publica. Palavras-chaves : paisagem cultural, patrimonio, espaco, memoria, identidade Abstract In spite of the unification of markets and the construction of a worldwide economy, we witnessed, during the historical period between the end of the 20th and the beginning of the 21st centuries, the ever-growing importance of the memory, the history, the past, and of the individual or collective reminiscences. This historical force has desired and oriented the conservation and transmission of a common cultural heritage, be it material or immaterial. Therefore, we are able to identify, on the part of governmental authorities, by means of the political actions of the intellectual segments or through the pressure by identity groups, a sort of memoirist obsession, whose objective, is to preserve the identity of a given community. However, during the discussion regarding what should be conserved and preserved, the conclusions reached was that it was not the memoirs and the historical identity of subordinate groups, but those of dominant groups, capable of exerting social and collective pressure. This paper examines a concrete example: the revitalization of a plaza in the historical center of Rio de Janeiro, Brazil, seeking to reflect on the fundamental role of preservation to our cultural heritage. This study aims, therefore, to reveal how the public and cultural interests behind given groups may highlight some objects of cultural or affective value, but, at the same time, undermine many others that are also fundamental for the memory of collective groups of lesser power and social representation. Keywords : cultural landscape, cultural heritage, space, memory, identity

We report the influence of two-band superconductivity on the flux creep and the critical current densities of a MgB2 thin film. The small magnetic penetration depth of λ=50±10 nm at T=4 K is related to a clean π-band. We find a high self-field critical current density Jc, which is strongly reduced with applied magnetic field, and attribute this to suppression of the superconductivity in the π-band. The temperature dependence of the creep rate S (T) at low magnetic field can be explained by a simple Anderson–Kim mechanism. The system shows high pinning energies at low field that are strongly suppressed by high field.

We report the intrinsic superconducting parameters in a heavily overdoped Ba(Fe1-xCox)2As2 (x=0.14) single crystal and their influence in the resulting vortex dynamics. We find a bulk superconducting critical temperature of 9.8 K, magnetic penetration depth λab (0)=660 ± 50 nm, coherence length ξab (0)=6.4 ± 0.2 nm, and the upper critical field anisotropy γT→Tc ≈ 3.7. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. The intrinsic pinning energy plays an important role in the resulting vortex dynamics as compared with similar pinning landscape and comparable intrinsic thermal fluctuations. Fil: Kim, Jeehoon. Pohang University Of Science And Technology; Corea del Sur Fil: Haberkorn, Nestor Fabian. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Gofryk, K.. Oak Ridge National Laboratory; Estados Unidos Fil: Graf, M.J.. Oak Ridge National Laboratory; Estados Unidos Fil: Ronning, F.. Oak Ridge National Laboratory; Estados Unidos Fil: Sefat, A.S.. Oak Ridge National Laboratory; Estados Unidos Fil: Movshovich, R.. Oak Ridge National Laboratory; Estados Unidos Fil: Civale, L.. Oak Ridge National Laboratory; Estados Unidos

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu2Ga2B. We demonstrate a feasible control both globally and locally of domain shapes and sizes by the external field as well as a smooth transition from single stripe to bubble domains, which opens the door to future applications based on magnetic domain tailoring.

The Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition. For disordered metals the BKV theory predicts that the second-order nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive. Our low-temperature magnetization study identifies the ferromagnetic QPT in the disordered metal UCo_{1-x}Fe_{x}Ge as the first clear example that exhibits the associated critical exponents predicted by the BKV theory.

Bulk niobium Superconducting Radio-Frequency cavities are a leading accelerator technology. Their performance is limited by the cavity loss and maximum acceleration gradient, which are negatively affected by vortex penetration into the superconductor when the peak magnetic field at the cavity wall surface exceeds the vortex penetration field (Hvp). It has been proposed that coating the inner wall of an SRF cavity with superconducting thin films increases Hvp. In this work, we utilized Nb ellipsoid to simulate an inverse SRF cavity and investigate the effect of coating it with magnesium diboride layer on the vortex penetration field. A significant enhancement of Hvp was observed. At 2.8 K, Hvp increased from 2100 Oe for an uncoated Nb ellipsoid to 2700 Oe for a Nb ellipsoid coated with ~200 nm thick MgB2 thin film. This finding creates a new route towards achieving higher acceleration gradient in SRF cavity accelerator beyond the theoretical limit of bulk Nb.

Superconductors are excellent testbeds for studying vortices, topological excitations that also appear in superfluids, liquid crystals, and Bose-Einstein condensates. Vortex motion can be disruptive; it can cause phase transitions, glitches in pulsars, and losses in superconducting microwave circuits, and it limits the current carrying capacity of superconductors. Understanding vortex dynamics is therefore of fundamental and technological importance, and the competition between the effects of thermal energy and energy barriers defined by material disorder is not completely understood. In particular, early measurements of thermally-activated vortex motion (creep) in iron-based superconductors unveiled fast rates (S) comparable to measurements of YBa2Cu3O7 (YBCO). This was puzzling because S is thought to somehow positively correlate with the Ginzburg number (Gi), and Gi is orders of magnitude lower in most iron-based superconductors than in YBCO. Here, we report very slow creep in BaFe2(As0.67P0.33)2 films, and propose that there is a universal minimum realizable (where Tc is the superconducting transition temperature) that has been achieved in our films, few other materials, and violated by none. This limitation provides new clues on how to design materials with slow creep and helps elucidate open questions regarding the interplay between system-specific length scales and vortex dynamics.

We report the synthesis, crystal structure, and characterization by means of single-crystal x-ray diffraction, neutron powder diffraction, and magnetic, thermal, and transport measurements of the new heavy-fermion compounds ${\mathrm{Ce}}_{2}{M\mathrm{Al}}_{7}{\mathrm{Ge}}_{4}$ $(M=\text{Co},\text{Ir},\text{Ni},\text{Pd})$. These compounds crystallize in a noncentrosymmetric tetragonal space group $\mathit{P}\overline{4}{2}_{1}m$, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. ${\mathrm{Ce}}_{2}{\mathrm{CoAl}}_{7}{\mathrm{Ge}}_{4},{\mathrm{Ce}}_{2}{\mathrm{IrAl}}_{7}{\mathrm{Ge}}_{4}$, and ${\mathrm{Ce}}_{2}{\mathrm{NiAl}}_{7}{\mathrm{Ge}}_{4}$ order magnetically below ${T}_{M}=1.8$, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in ${\mathrm{Ce}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ down to 0.4 K. The small amount of entropy released in the magnetic state of ${\mathrm{Ce}}_{2}M{\mathrm{Al}}_{7}{\mathrm{Ge}}_{4}$ $(M$ = Co, Ir, Ni) and the reduced specific heat jump at ${T}_{M}$ suggest a strong Kondo interaction in these materials. ${\mathrm{Ce}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point.

We show a simple and effective way to improve the vortex irreversibility line up to very high magnetic fields (60T) by increasing the density of second phase BaZrO3 nanoparticles. (Y0.77,Gd0.23)Ba2Cu3Oy films were grown on metal substrates with different concentration of BaZrO3 nanoparticles by the metal organic deposition method. We find that upon increase of the BaZrO3 concentration, the nanoparticle size remains constant but the twin-boundary density increases. Up to the highest nanoparticle concentration (n ~ 1.3 × 1022/m3), the irreversibility field (Hirr) continues to increase with no sign of saturation up to 60 T, although the vortices vastly outnumber pinning centers. We find extremely high Hirr, namely Hirr = 30 T (H||45°) and 24 T (H||c) at 65 K and 58 T (H||45°) and 45 T (H||c) at 50K. The difference in pinning landscape shifts the vortex solid-liquid transition upwards, increasing the vortex region useful for power applications, while keeping the upper critical field, critical temperature and electronic mass anisotropy unchanged.

Self-assembled epitaxial nanocomposite CoFe2O4:SrTiO3 and CoFe2O4:MgO thin films were grown on LaAlO3 substrates by a chemical solution approach of polymer-assisted deposition. X-ray diffraction and transmission electron microscopy analyses indicated that both phases in the composites were epitaxial with respect to the major axes of the substrate. Compared to the single-phase ferrimagnetic spinel CoFe2O4 films and the bilayer CoFe2O4/SrTiO3 and CoFe2O4/MgO films, the composite films exhibited reduced coercivity and enhanced magnetic anisotropy.

Second-generation (2G) HTS wires are now being produced routinely in kilometer lengths using Metal Organic Chemical Vapor Deposition (MOCVD) process with critical currents of 300 A/cm. While this achievement is enabling several prototype devices, in order to reach a substantial commercial market, the cost-performance metrics of 2G HTS wires need to be significantly improved in device operating conditions. Zr-doping has been found to be an effective approach to improve in-field critical current performance of MOCVD-based HTS wires. In this work, we have explored modifications to the Zr-doped precursor compositions to achieve three and two-fold increase in deposition rate in research and production MOCVD systems respectively. Production wires made with modified Zr-doped compositions exhibit a self-field critical current density of 50 MA/cm2 at 4.2 K and a 55 to 65% higher performance than our previous wires with Zr-doping, over magnetic field range of 0 to 30 T. We have also developed an alternate, low-cost technique, namely electrodeposition, to deposit silver overlayer on superconducting film. Wires made with electrodeposited silver are able to sustain the same level of overcurrent as sputtered silver layers. This process has been successfully scaled up to 100 m lengths.

Numerous experimental results have suggested that the J c of YBa 2 Cu 3 O 7 (YBCO) films is significantly higher near the film–substrate interface than in the remainder of the film. We previously proposed that this effect is due to interfacial pinning enhancement caused by stress and the resulting misfit dislocations at the heteroepitaxial interface. To test this hypothesis we have used a non-superconducting PrBa 2 Cu 3 O 7− δ (PrBCO) buffer layer to minimize the lattice mismatch with YBCO. We find that the PrBCO layers lower J c of the 0.4 μm YBCO films in a predictable way, and that, if sufficiently thick (∼0.5 μm), they eliminate interfacial enhancement altogether. Our interpretation of this result is that the defects responsible for interfacial enhancement of flux pinning originate at the bottom of the non-superconducting PrBCO layer, which screens the pinning centers from vortices in YBCO. This result demonstrates that the pinning enhancement arises from stress at the film–substrate interface.

In this work we describe the effects of adding different amount of single-wall (sw) carbon nanotubes (CNT) on the superconducting properties of MgB 2. We observed that the amount of swCNT that partially dilute into the MgB 2 matrix is lower than for double-wall (dw) CNT samples for the same nominal content. This C incorporation leads to an increase of the upper critical field (Hc2), with a maximum Hc2(0) extrapolated value of 33.5 T, expected for a two-gap superconductor in the dirty limit. At the same time, the fraction of swCNT that has not been incorporated into MgB 2 lattice may retain their structural integrity acting as strong vortex pinning centers. In this case the optimum nominal swCNT content for increasing critical current density (Jc) is in the range 7.5-10%at.

A composite layer has been successfully implemented in high temperature superconducting (HTS) coated conductors (CCs) based on ion beam assisted deposition (IBAD) MgO technology. This layer serves a dual function: a nucleation layer for the growth of high quality biaxially aligned MgO and a diffusion barrier to inhibit interdiffusion between the Ni-alloy substrate and the superconductor. The composition of the composite Y2O3-Al2O3 influences the texture and superconducting properties of YBCO films grown on STO buffered MgO/Y2O3-Al2O3/Ni-alloy. Our experimental results demonstrate that the superconducting properties of YBCO grown on STO buffered MgO/Y2O3-Al2O3/Ni-alloy are comparable with those of YBCO on standard architecture. The use of a single composite Y2O3-Al2O3 layer instead of individual layers of Y2O3 and Al2O3 for the fabrication of HTS CCs based on IBAD MgO provides advantages such as simplified architecture and potentially reduced cost due to the reduced number of fabrication steps.

Retaining a dissipation-free state while carrying large electrical currents is a challenge that needs to be solved to enable commercial applications of high-temperature superconductivity. Here, we show that the controlled combination of two effective pinning centres (randomly distributed nanoparticles and self-assembled columnar defects) is possible and effective. By simply changing the temperature or growth rate during pulsed-laser deposition of BaZrO(3)-doped YBa(2)Cu(3)O(7) films, we can vary the ratio of these defects, tuning the field and angular critical-current (Ic) performance to maximize Ic. We show that the defects' microstructure is governed by the growth kinetics and that the best results are obtained with a mixture of splayed columnar defects and random nanoparticles. The very high Ic arises from a complex vortex pinning landscape where columnar defects provide large pinning energy, while splay and nanoparticles inhibit flux creep. This knowledge is used to produce thick films with remarkable Ic(H) and nearly isotropic angle dependence.

In thin films of rare-earth manganites, RE1−xMxMnO3 (RE=rare earth, M=Ca, Sr, Ba), with mixed-valence perovskite structure, the transport properties highly depend on the deposition technique, processing conditions, and the substrate used. Chemical solution deposition techniques provide many advantages, such as low cost, easy setup, and coating of large areas. However, the crystalline quality, uniformity, and reproducibility of the film depend on the reactivity of the chemical used for solution preparation. In this paper, we report a novel process to grow rare-earth manganite films using low-cost polymer-assisted deposition technique. In this process, polymer controls the viscosity and binds metal ions, resulting in a homogeneous distribution of metal precursors in the solution and a uniform film. These solutions were stable, and crack-free films were obtained using these solutions. In this paper, thin films of La1−xSrxMnO3, La1−xCaxMnO3, and Pr0.5Ca0.5MnO3 were grown on (001) LaAlO3 substrates. The La1−xSrxMnO3 and La1−xCaxMnO3 films were highly c-axis oriented and epitaxial, and showed high magnetoresistance near their Curie temperature. Charge ordering was observed in the Pr0.5Ca0.5MnO3 film at 220 K and high magnetoresistance of nearly −100% was obtained at low temperatures. The structural, transport, and magnetic properties of these films were similar to those obtained for films grown by physical vapor deposition.

High critical current densities (J{sub c}) in thick films of the Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO, {Tc}{approx}92 K) superconductor directly depend upon the types of nanoscale defects and their densities within the films. A major challenge for developing a viable wire technology is to introduce nanoscale defect structures into the YBCO grains of the thick film suitable for flux pinning and the tailoring of the superconducting properties to specific, application-dependent, temperature and magnetic field conditions. Concurrently, the YBCO film needs to be integrated into a macroscopically defect-free conductor in which the grain-to-grain connectivity maintains levels of inter-grain J{sub c} that are comparable to the intra-grain J{sub c}. That is, high critical current (I{sub c}) YBCO coated conductors must contain engineered in homogeneities on the nanoscale, while being homogeneous on the macroscale. An analysis is presented of the advances in high-performance YBCO coated-conductors using chemical solution deposition (CSD) based on metal trifluoroacetates and the subsequent processing to nano-engineer the microstructure for tunable superconducting wires. Multi-scale structural, chemical, and electrical investigations of the CSD film processes, thick film development, key microstructural features, and wire properties are presented. Prospects for further development of much higher I{sub c} wires for large-scale, commercial applicationmore » are discussed within the context of these recent advances.« less

Twenty years ago in a series of amazing discoveries it was found that a large family of ceramic cuprate materials exhibited superconductivity at temperatures above, and in some cases well above, that of liquid nitrogen. Imaginations were energized by the thought of applications for zero-resistance conductors cooled with an inexpensive and readily available cryogen. Early optimism, however, was soon tempered by the hard realities of these new materials: brittle ceramics are not easily formed into long flexible conductors; high current levels require near-perfect crystallinity; and — the downside of high transition temperature — performance drops rapidly in a magnetic field. Despite these formidable obstacles, thousands of kilometres of high-temperature superconducting wire have now been manufactured for demonstrations of transmission cables, motors and other electrical power components. The question is whether the advantages of superconducting wire, such as efficiency and compactness, can outweigh the disadvantage: cost. The remaining task for materials scientists is to return to the fundamentals and squeeze as much performance as possible from these wonderful and difficult materials.

The influence of film thickness and growth rate on the vortex pinning in hybrid liquid phase epitaxy (HLPE) films was explored. Film growth rates as high as 12 nm s−1 (0.7 µm min−1) produced high Jc films. Weak or no thickness dependence was found in films of thickness ranging from 0.4 to 3 µm. Field and angular measurements of the critical current density (Jc) and the power-law exponent (N) of the current–voltage curves were used to determine the nature of pinning. Films thinner than 0.6 µm showed a higher density of correlated defects parallel to the ab plane than thicker films. Using HLPE, it was possible to achieve very strong pinning in films ~3 µm thick, yielding critical currents over 300 A cm−1 width at self-field, and as high as 35 A cm−1 width at μ0H = 3 T at T = 75.5 K. Decreasing the deposition rate allowed improving the high field performance, opening up the possibility to engineer the pinning landscape of the HLPE films.