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"Hartmann, R."
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Quasi-exact solutions for guided modes in two-dimensional materials with tilted Dirac cones
We show that if the solutions to the (2+1)-dimensional massless Dirac equation for a given one-dimensional (1D) potential are known, then they can be used to obtain the eigenvalues and eigenfunctions for the same potential, orientated at an arbitrary angle, in a 2D Dirac material possessing tilted, anisotropic Dirac cones. This simple set of transformations enables all the exact and quasi-exact solutions associated with 1D quantum wells in graphene to be applied to the confinement problem in tilted Dirac materials such as 8-
Pmmn
borophene. We also show that smooth electron waveguides in tilted Dirac materials can be used to manipulate the degree of valley polarization of quasiparticles travelling along a particular direction of the channel. We examine the particular case of the hyperbolic secant potential to model realistic top-gated structures for valleytronic applications.
Journal Article
Towards ultrafast dynamics with split-pulse X-ray photon correlation spectroscopy at free electron laser sources
One of the important challenges in condensed matter science is to understand ultrafast, atomic-scale fluctuations that dictate dynamic processes in equilibrium and non-equilibrium materials. Here, we report an important step towards reaching that goal by using a state-of-the-art perfect crystal based split-and-delay system, capable of splitting individual X-ray pulses and introducing femtosecond to nanosecond time delays. We show the results of an ultrafast hard X-ray photon correlation spectroscopy experiment at LCLS where split X-ray pulses were used to measure the dynamics of gold nanoparticles suspended in hexane. We show how reliable speckle contrast values can be extracted even from very low intensity free electron laser (FEL) speckle patterns by applying maximum likelihood fitting, thus demonstrating the potential of a split-and-delay approach for dynamics measurements at FEL sources. This will enable the characterization of equilibrium and, importantly also reversible non-equilibrium processes in atomically disordered materials.
X-ray photon correlation spectroscopy has been mainly used to measure slow dynamics using synchrotron sources. Here the authors demonstrate the split-and- delay pulse set-up to study nanosecond dynamics of gold nanoparticles using XPCS with free electron laser pulses.
Journal Article
Two-dimensional Dirac particles in a Pöschl-Teller waveguide
We obtain exact solutions to the two-dimensional (2D) Dirac equation for the one-dimensional Pöschl-Teller potential which contains an asymmetry term. The eigenfunctions are expressed in terms of Heun confluent functions, while the eigenvalues are determined via the solutions of a simple transcendental equation. For the symmetric case, the eigenfunctions of the supercritical states are expressed as spheroidal wave functions, and approximate analytical expressions are obtained for the corresponding eigenvalues. A universal condition for any square integrable symmetric potential is obtained for the minimum strength of the potential required to hold a bound state of zero energy. Applications for smooth electron waveguides in 2D Dirac-Weyl systems are discussed.
Journal Article
مدارس التحليل النفسي : التحليل النفسي في حركة مستمرة
ما هو دور الجمعية الدولية للتحليل النفسي؟ لماذا توجد مدارس واتجاهات متعددة تدعي اتباع تعاليم فرويد؟ هل إنكار الدور الأساسي للجنس الطفولي هو أصل كل الانشقاق؟ من المرجح أن تؤدي الاختلافات بين مدارس التحليل النفسي إلى إرباك القارئ الساذج في هذا المجال وتستمر في التسبب في مشاكل للمحللين النفسيين أنفسهم. في حين أنه من السهل إظهار كيف أن يونج أدلر. رايخ وغيره، منفصلين عن فرويد في النقاط النظرية الأساسية، وليس فقط في المسائل التفصيلية، فمن الصعب جدا الإجابة على هذا السؤال: هل هناك نظرية أفضل من غيرها، أم لا بد من أنه يعتقد أنه لا توجد نظرية في الوقت الحاضر.
Book
Unveiling unconventional magnetism at the surface of Sr2RuO4
Materials with strongly correlated electrons often exhibit interesting physical properties. An example of these materials is the layered oxide perovskite Sr
2
RuO
4
, which has been intensively investigated due to its unusual properties. Whilst the debate on the symmetry of the superconducting state in Sr
2
RuO
4
is still ongoing, a deeper understanding of the Sr
2
RuO
4
normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of surface magnetism in Sr
2
RuO
4
in its normal state. We detect static weak dipolar fields yet manifesting at an onset temperature higher than 50 K. We ascribe this unconventional magnetism to orbital loop currents forming at the reconstructed Sr
2
RuO
4
surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence electron pairing with broken time reversal symmetry.
Strontium Ruthenate, Sr
2
RuO
4
, displays a remarkable number of intriguing physical phenomena, from superconductivity, to strain-induced ferromagnetism. Here, using low-energy muon spectroscopy, Fittipaldi et al. demonstrate the existence of unconventional magnetism at the surface of Sr
2
RuO
4
in its normal state and without any applied strain.
Journal Article
Nanoscale spin reversal by non-local angular momentum transfer following ultrafast laser excitation in ferrimagnetic GdFeCo
The dynamics of thin magnetic films revealed by ultrafast laser techniques cannot be explained by standard equilibrium descriptions. Diffraction experiments using an X-ray laser now allow the spin dynamics of the separate magnetic constituents of ferromagnetic GdFeCo alloys to be spatially resolved.
Ultrafast laser techniques have revealed extraordinary spin dynamics in magnetic materials
1
,
2
,
3
,
4
,
5
,
6
that equilibrium descriptions of magnetism
7
cannot explain. Particularly important for future applications is understanding non-equilibrium spin dynamics following laser excitation on the nanoscale, yet the limited spatial resolution of optical laser techniques has impeded such nanoscale studies. Here we present ultrafast diffraction experiments with an X-ray laser that probes the nanoscale spin dynamics following optical laser excitation in the ferrimagnetic alloy GdFeCo, which exhibits macroscopic all-optical switching
4
,
8
,
9
. Our study reveals that GdFeCo displays nanoscale chemical and magnetic inhomogeneities that affect the spin dynamics. In particular, we observe Gd spin reversal in Gd-rich nanoregions within the first picosecond driven by the non-local transfer of angular momentum from larger adjacent Fe-rich nanoregions. These results suggest that a magnetic material’s microstructure can be engineered to control transient laser-excited spins, potentially allowing faster (~ 1 ps) spin reversal than in present technologies
10
,
11
,
12
.
Journal Article
High resolution cryo EM analysis of HPV16 identifies minor structural protein L2 and describes capsid flexibility
Human papillomavirus (HPV) is a significant health burden and leading cause of virus-induced cancers. HPV is epitheliotropic and its replication is tightly associated with terminal keratinocyte differentiation making production and purification of high titer virus preparations for research problematic, therefore alternative HPV production methods have been developed for virological and structural studies. In this study we use HPV16 quasivirus, composed of HPV16 L1/L2 capsid proteins with a packaged cottontail rabbit papillomavirus genome. We have achieved the first high resolution, 3.1 Å, structure of HPV16 by using a local subvolume refinement approach. The high resolution enabled us to build L1 unambiguously and identify L2 protein strands. The L2 density is incorporated adjacent to conserved L1 residues on the interior of the capsid. Further interpretation with our own software for Icosahedral Subvolume Extraction and Correlated Classification revealed flexibility, on the whole-particle level through diameter analysis and local movement with inter-capsomer analysis. Inter-capsomer expansion or contraction, governed by the connecting arms, showed no bias in the magnitude or direction of capsomer movement. We propose that papillomavirus capsids are dynamic and capsomers move as rigid bodies connected by flexible linkers. The resulting virus structure will provide a framework for continuing biochemical, genetic and biophysical research for papillomaviruses. Furthermore, our approach has allowed insight into the resolution barrier that has previously been a limitation in papillomavirus structural studies.
Journal Article