The classical and quantum dynamics of molecular spins on graphene

PMCID: PMC4800001.-- et al.Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic and quantum computing devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics and electrical spin manipulation. However, the influence of the graphene environment on the spin systems has yet to be unravelled. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets on graphene. Whereas the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly developed model. Coupling to Dirac electrons introduces a dominant quantum relaxation channel that, by driving the spins over Villain's threshold, gives rise to fully coherent, resonant spin tunnelling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin manipulation in graphene nanodevices.Financial support from Italian MIUR, Spanish MINECO (MAT2012-38318-C03-01), BW-Stiftung (Kompetenznetz Funktionelle Nanostrukturen), ERC StG-338258 “OptoQMol”, the Royal Society (URF fellowship and grant) and the AvH Stiftung (Sofja Kovalevskaja award).Peer Reviewe

A Systematic Review of the Research on Vocabulary Instruction That Impacts Text Comprehension

Although numerous studies have identified a correlational relationship between vocabulary and comprehension, we know less about vocabulary interventions that impact reading comprehension. Therefore, this study is a systematic review of vocabulary interventions with comprehension outcomes. Analyses of 36 studies that met criteria are organized around (a) type of comprehension measure (i.e., comprehension of passages that included taught words or more generalized comprehension measures) and (b) type of intervention (i.e., direct teaching of word meanings or word‐learning strategies). The authors looked for patterns in characteristics of vocabulary instruction within these analyses. Their findings led to four major themes: (1) Teaching of word meanings supported comprehension of text containing the target words in almost all cases; (2) instruction that focused on some active processing was typically more impactful than a definition or a dictionary method for supporting comprehension of text containing the target words, but we do not know how much instruction is sufficient; (3) there is very limited evidence that direct teaching of word meanings, even long‐term, multifaceted interventions of large numbers of words, can improve generalized comprehension; and (4) there is currently no empirical evidence that instruction in one or two strategies for solving word meanings will impact generalized comprehension. However, studies that actively teach students to monitor their understanding of vocabulary and to use multiple, flexible strategies for solving word meanings are a promising area for future research. The authors discuss the implications of these themes, as well as critical avenues for future vocabulary research.Chinese虽然许多研究已经确定了词汇与阅读理解之间的相关关系,但在词汇干预教学对阅读理解的影响方面,我们所知道的则较少。因此,本研究针对词汇干预教学及其阅读理解学习成果,作一个系统性文献综述。符合标准的36个研究按其类别分两方面作分析:(a)阅读理解测验的类型(即,包含已教过的单字的文章阅读理解测验或较广义的阅读理解测验);(b)干预教学的类型(即,单字意义的直接教学或单字学习策略)。本文作者在这些分析中寻找词汇教学特点的模式,其分析结果带出四个主要主题︰(1)在几乎所有情况下,单字意义教学能支援含有目标单字的文本阅读理解;(2)就支援含有目标单字的文本阅读理解而言,侧重于一些主动性文本处理的教学,通常比单字定义教学或字典教学更有影响力,但本文作者不知道多少教学才足够; (3)单字意义的直接教学,甚至是长期性的,多方面的大量单字干预教学,均只有非常有限的证据证明可以提高广义的阅读理解; (4)目前还没有实证研究证据证明只教一两个解决单字义的学习策略就能对广义阅读理解产生影响。然而,在未来研究中,那些积极指导学生监控自己对词汇的理解和使用多样灵活的字义解决策略的研究,应是一个有希望的领域。本文作者最后讨论这四个主题的启示,以及未来词汇研究的关键途径。SpanishAunque numerosos estudios han mostrado una relación correlativa entre el vocabulario y la comprensión, no sabemos tanto sobre las intervenciones de vocabulario que impactan la comprensión lectora. Por ende, este estudio es una revisión sistemática de intervenciones de vocabulario que incluyen comprensión. Los análisis de 36 estudios que satisfacen dicho criterio fueron organizados de acuerdo a (a) la clase de comprensión medida (p. ej., la comprensión de pasajes que incluían palabras enseñadas o medidas de comprensión más generalizadas) y (b) la clase de intervención (p. ej., la enseñanza directa del significado de las palabras o estrategias para aprender palabras). Los autores buscaron patrones en las características de la instrucción de vocabulario dentro de estos análisis. Encontraron cuatro temas principales: (1) La enseñanza del significado de palabras apoya la comprensión de textos que incluyen dichas palabras en casi todos los casos; (2) la instrucción enfocada en algún procesamiento activo tiene típicamente un impacto mayor que una definición o el uso del diccionario para apoyar la comprensión del texto con las palabras indicadas, pero no sabemos cuánta instrucción se necesita; (3) hay poca evidencia de que la enseñanza directa del significado de las palabras, aún en intervenciones multifacéticas a largo plazo de muchas palabras, mejora la comprensión en general; y (4) actualmente no hay evidencia empírica alguna que la instrucción usando una o dos estrategias para resolver el significado de palabras impacta la comprensión general. Sin embargo, instrucción que les enseña activamente a los estudiantes a supervisar su propio entendimiento del vocabulario y a usar varias estrategias flexibles para encontrar el significado de las palabras prometen ser un área para investigaciones futuras. Los autores discuten las implicaciones de estos temas al igual que vías de vital importancia para futuras investigaciones sobre el vocabulario.Arabicعلى الرغم من أن العديد من الدراسات قد حددت العلاقة الارتباطية بين المفردات والفهم، لا تزال معرفتنا اقل بالنسبة لتدخلات المفردات التي تؤثر على القراءة والفهم. وبالتالي، فإن هذه الدراسة تراجع منهجية تدخل المفردات المؤثرة على الفهم. تم تنظيم التحاليل التي أجريت على 36 دراسة تلبي معايير (أ) نوع مقياس الفهم (أي: فهم المقاطع التي شملت الكلمات المدرسة أو تدابير الفهم الأكثر عمومية) (ب) نوع التدخل (أي التعليم المباشر لمعاني الكلمات أو استراتيجيات تعلم الكلمات). نظر المؤلفون عن أنماط خصائص تعليم المفردات ضمن هذه التحليلات. وأدت نتائجهم إلى أربعة محاور رئيسية هي: (1) تدريس معاني الكلمات التي تدعم فهم النص الذي يحتوي على الكلمات المستهدفة في جميع الحالات تقريبا، (2) التعليمات التي تركز على المعالجة الفعَّلة كانت عادة أكثر تأثيرا من تعريف الكلمة أو الأسلوب القاموسي لدعم فهم النص الذي يحتوي على الكلمات المستهدفة، لكننا لا نعرف ما هو الكافي من التعليمات. 3) هناك أدلة محدودة للغاية أن التعليم المباشر لمعاني الكلمات، وحتى على المدى الطويل، او التدخلات المتعددة الأوجه لأعداد كبيرة من الكلمات، يمكن أن يحسن الفهم العام. و (4) لا يوجد حاليا أي أدلة تجريبية أن التعليمات العملية في واحد أو اثنين من استراتيجيات تحليل معاني كلمة سيؤثر على الفهم العام. لكن، الدراسات التي تعلم الطلاب بنشاط مراقبة فهمهم للمفردات، واستخدام عدة استراتيجيات مرنة من أجل تحليل معاني الكلمات لها مبَشِّـرات للبحوث المستقبلية. يناقش المؤلفون الآثار المترتبة على هذه المحاور، وكذلك السبل الهامة في بحثوث المفردات المستقبلية.RussianМногочисленные исследования установили корреляцию между словарным запасом и пониманием текста, однако о лексических вмешательствах, которые дают положительный эффект для понимания прочитанного, известно немного. Поэтому был предпринят системный обзор подобных лексических интервенций. Анализ выбранных по определенному принципу 36 исследований сосредоточен на (a) критериях понимания (понимание абзацев, содержащих изученные слова, или критерии более общего характера) и (б) типе вмешательства (прямое обучение значениям слова или освоение стратегий работы с лексикой). В рамках данного обзора авторы пытались классифицировать полученные выводы. Выделены четыре лейтмотива: (1) почти всегда обучение значениям слова приводит к пониманию текста, содержащего целевую лексику; (2) обучение, связанное с активными лексическими упражнениями, как правило, более эффективно для понимания текста, содержащего целевую лексику, чем работа с определениями или словарями, но пока неясно, каков должен быть объем обучения; (3) существует совсем мало доказательств того, что прямое обучение значениям слова и даже долгосрочные, многогранные вмешательства, включающие большое количество лексики, улучшают общее понимание почитанного; (4) в настоящее время нет никаких эмпирических доказательств того, что обучение одной или двум стратегиям, позволяющим уловить смысл слова, положительно влияет на общее понимание прочитанного. Однако наиболее перспективным направлением для исследований представляется ситуация, когда школьников активно обучают самоконтролю при освоении лексики и дают им разнообразные гибкие стратегии для понимания смысла слов. В статье обсуждается значение полученных выводов, а также критически важные направления для будущих исследований.FrenchQuoique de nombreuses recherches aient mis en évidence une corrélation entre le vocabulaire et la compréhension, nous ne savons pas grand‐chose des interventions relatives au vocabulaire qui ont un impact sur la compréhension de la lecture. Cette étude est par conséquent une revue systématique des interventions en matière de vocabulaire et de leurs effets sur la compréhension. Les analyses des 36 études répondant aux critères sont organisées autour de (a) le type de mesure de la compréhension (i.e. la compréhension de passages qui incluent les mots enseignés ou des mesures plus générales de la compréhension) et (b) le type d’intervention (i.e. un enseignement direct de la signification des mots ou des stratégies d’apprentissage des mots). Les auteurs ont examiné les structures des caractéristiques de l’enseignement du vocabulaire au cours de ces analyses. Les résultats conduisent à quatre conclusions principales : (1) Enseigner la signification des mots est presque toujours bénéfique à la compréhension du texte contenant les mots cibles ; (2) un enseignement focalisé sur un processus actif a de façon typique un impact supérieur à une méthode qui donne la définition du vocabulaire pour améliorer la compréhension du texte contenant les mots cibles, mais sans pouvoir préciser quelle quantité d’enseignement est suffisante. (3) Il y a très peu de preuves qu’un enseignement direct de la signification des mots, et même que des interventions longues et multidimensionnelles d’un grand nombre de mots, puissent conduire à une compréhension générale ; et (4) on n’a actuellement pas de preuve empirique que l’une ou l’autre de ces deux stratégies de connaissance du sens des mots ait un impact sur la compréhension en général. Toutefois, les recherches qui enseignent aux élèves à diriger de façon active leur compréhension du vocabulaire et qui utilisent des stratégies multiples et souples de compréhension des mots sont un domaine prometteur pour les recherches à venir. Les auteurs discutent des implications de ces conclusions, ainsi que des voies pertinentes pour les futures recherches sur le vocabulaire.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136457/1/rrq163_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136457/2/rrq163.pd

Spin-Wechselwirkungen in Graphen/Einzelmolekülmagneten Hybridstrukturen

This thesis work employs magnetic molecules, belonging to the family of single molecule magnets (SMMs), as model systems to study the relaxation of mesoscopic spins on graphene, and their interaction with graphene charge carriers in real devices. Their large uniaxial anisotropy makes SMMs behave like giant spins, with relaxation times of years at low temperatures. Their spin dynamics combines a classical and a quantum relaxation mechanism, that can be selectively switched on and off by either applying an external magnetic Field or by varying the temperature. The work is organized as follows. The First part presents a thorough structural characterization of the SMMs-graphene hybrid materials via multiple techniques, including atomic force microscopy, x-ray photoelectrons spectroscopy, mass spectrometry, Raman spectroscopy, and electronic transport measurements on graphene-based Feld-effect transistors. The analysis of the dynamical arrangement of molecular adsorbates on graphene reveals new opportunities to control the supramolecular surface arrangement. A comprehensive study of the magnetization dynamics of SMMs on graphene is carried out by means of ac-susceptibility techniques in a broad temperature range (T = 4K - 13 mK). The details of the complex spin-graphene interaction are unraveled in the framework of a newly developed theoretical model that accounts for all the possible fundamental contributions and the two-dimensional nature of graphene. The focus of the second part is the design, fabrication and characterization of graphene-based spintronic devices. Diffrent strategies for the injection of spin-polarized carriers in graphene are implemented and tested down to very low temperatures (T = 300 mK). To conclude the first spin-transport and spin-relaxation measurements in SMMs-graphene devices are presented.Die Kopplung von Graphen zu solch mesoskopischen magnetischen Systemen bietet neue Wege Spin-Zustände elektronisch zu manipulieren. Gleichwohl stellt es eine ungeheure Herausforderung dar solche Spin-Systeme in reale Bauelemente zu integrieren, da die Wechselwirkungen der Spins mit in einer komplexen Umgebung nur schwer zu verstehen oder gar handzuhaben sind. Zwei wichtige Größen die Kopplungseigenschaften zu charakterisieren sind zum einen die Spin-Relaxationszeit T1 und die Spin-Dephasierungs Zeit T2. Erstere beschreibt die Lebenszeit eines "klassischen Bits", in welchem die Informationen in "up" und "down" Zuständen kodiert sind. Im Gegensatz dazu beschreibt T2 die kohärente Lebenszeit eines "Quantenbits" wobei die Informationen durch die Phase der Spin-Wellenfunktion verschlüsselt werden. Im Zuge dieser Arbeit werden magnetische Moleküle aus der Familie der "Single Molecule Magnets" als Modellsysteme verwendet, um die Relaxationsprozesse solch mesoskopischer Spins in Verbindung mit Graphen und ihren Einfluss auf die Ladungsträger innerhalb realer Bauelemente zu untersuchen. Dabei werden zwei unterschiedliche Ansätze verfolgt. Im ersteren werden die Single Molecule Magnets (SMMs) als Modellsysteme genutzt um die komplexen Wechselwirkungs-Mechanismen zwischen Spin und Umgebung zu entwirren. Dahingehend kombinieren wir Graphen mit dem SMM [Fe4(L)2(dpm)6]. Als Ligande L, fungiert triply deprotonated H3L=2-hydroxymethyl-2-(4-(pyren-1-yl)butoxy)methylpropane-1,3-diol, welcher eine Pyrengruppe enthält welche eine nichtkovalente Bindung an die Graphenoberfläche via pi-stacking ermöglicht. Die erzeugten Hybridsysteme werden dann durch mehrere Techniken charakterisiert. Der zweite Teil dieser Arbeit beschäftigt sich mit der Fabrikation und Optimierung von Graphen basierten Spinvalve-Bauelementen und deren Charakterisierung bei tiefen Temperaturen

On the generating functions of pattern-avoiding Motzkin paths

Using a recursive approach, we show that the generating function for sets of Motzkin paths avoiding a single (not necessarily consecutive) pattern is rational over $x$ and the Catalan generating function $C(x) = \frac{1-\sqrt{1-4x^2}}{2x^2}$, where $x$ keeps track of the length of the path. Moreover, an algorithm is provided for finding the generating function in the more general case of an arbitrary set of patterns. In addition, this algorithm allows us to find a combinatorial specification for pattern-avoiding Motzkin paths, which can be used not only for enumeration, but also for exhaustive and random generation

Magnetic dynamics of Fe4 molecular clusters in crystals and on graphene

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España).Graphene, a two-dimensional carbon layer, is expected to contribute to a new revolution in electronics. However, the fact that graphene is not magnetic, just as any other carbon derivative, hinders its application to spintronics, i.e. to magnetic data storage and processing. A way to circumvent this difficulty is by doping graphene with either point-like defects or with molecules having a localized spin, such as free-radical systems. The nature of the ensuing magnetism and the existence of a coupling to graphene’s Dirac electrons remains, however, a subject of controversy. Besides, very little is known about the dynamics of spins in graphene. In this work, we have studied a new hybrid material formed by anchoring Fe4 molecular clusters, with a net spin S = 4, to graphene layers. In crystalline form, these clusters behave as single-molecule magnets, i.e. they show magnetic memory effects and slow relaxation at very low temperatures. Below 1 K, the dominant relaxation process is mediated by pure spin tunneling events. Because of the compact structure of its magnetic core, Fe4 clusters retain their electronic and magnetic properties when they are deposited onto solid substrates. Therefore, they provide an close to ideal situation to study how the spin dynamics is affected by the interaction with graphene. We have performed ac susceptibility experiments, with frequencies ranging from 0.1 Hz up to 200 kHz, using a high-sensitivity micro-SQUID susceptometer installed in a 3He-4He dilution refrigerator that gives access to the close neighbourhood of absolute zero (T > 11 mK). Experiments have been performed on the Ge4@graphene hybrid and on single crystals of Fe4 molecular magnets having different ligands and concentrations. The results confirm that properties such as the net molecular spin and magnetic anisotropy are preserved in the former material. However, the spin dynamics is dramatically affected by the presence of the graphene layer. In particular, the tunneling rate, measured at very low temperature, is enhanced by six orders of magnitude. We argue that the effect is due to a very effective shielding of dipole-dipole interactions between the molecules and to the crystal field generated by the graphene layer, which introduces new terms in the molecular spin Hamiltonian that break spin tunneling selection rules. As a result, the molecular spins enter a new dynamical regime, in which tunneling proceeds coherently. These results show that graphene can provide a useful platform for the coherent control of quantum spins with electric fields.Peer reviewe

Sub-Terahertz Frequency-Domain Spectroscopy Reveals Single-Grain Mobility and Scatter Influence of Large-Area Graphene

The response of individual domains in wafer-sized chemical vapor deposition graphene is measured by contactless sub-terahertz interferometry, observing the intrinsic optical conductance and reaching very high mobility values. It is shown that charged scatterers limit the mobility, validating previous theoretical predictions, and sub-terahertz quality assessment is demonstrated, as necessary for large-scale applications in touchscreens, as well as wearable and optoelectronic devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

CCDC 803376: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

The Food Standards Agency A force for change

Presented to Parliament by the Minister of Agriculture, Fisheries and Food by command of Her MajestyAvailable from British Library Document Supply Centre-DSC:OP-CM/3830 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo