Physical realization of a quantum spin liquid based on a novel frustration mechanism

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are "frustrated" by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca$_{10}$Cr$_7$O$_{28}$. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism.Comment: Modified version accepted in Nature Physic

Entwicklung kompakter, gepulster Elektro-Dipolmagnete für die laserbasierte Protonentherapie

Hintergrund Die strahlentherapeutische Behandlung von Krebserkrankungen erfolgt zurzeit hauptsächlich durch eine Bestrahlung mit hochenergetischen Photonen und Elektronen aus kompakten Therapie-Linearbeschleunigern. Seltener werden auch Protonenstrahlen eingesetzt. Diese besitzen gegenüber Photonen und Elektronen vorteilhaftere physikalische und strahlenbiologische Eigenschaften, die besonders bei der Bestrahlung von tiefliegenden Tumoren in der Nähe von lebenswichtigen, strahlenempfindlichen Organen von Bedeutung sind. Die Behandlung mit Protonen erfordert jedoch sehr große und teure Bestrahlungsanlagen, weshalb es weltweit bisher nur ca. 50 solcher Anlagen an großen Zentren gibt. In den letzten Jahren wurde das völlig neuartige Prinzip der Teilchenbeschleunigung durch Hochleistungslaser soweit entwickelt, dass eine medizinische Anwendung zur Krebstherapie vorstellbar ist. Die laserbasierte Teilchenbeschleunigung verspricht deutlich kompaktere und kostengünstigere Protonenbeschleuniger, erzeugt jedoch im Unterschied zu herkömmlichen Beschleunigern sehr kurze (~ps) hochintensive Protonenpulse mit großer Strahldivergenz und breitem Energiespektrum. Im Rahmen des Verbundprojektes onCOOPtics wird die klinische Anwendbarkeit derartiger laserbeschleunigter Protonenstrahlen untersucht, was nicht nur die Entwicklung des notwendigen Laser-Teilchen-Beschleunigers, sondern auch die Entwicklung eines geeigneten Strahlführungssystems beinhaltet

Entwicklung kompakter, gepulster Elektro-Dipolmagnete für die laserbasierte Protonentherapie

Hintergrund Die strahlentherapeutische Behandlung von Krebserkrankungen erfolgt zurzeit hauptsächlich durch eine Bestrahlung mit hochenergetischen Photonen und Elektronen aus kompakten Therapie-Linearbeschleunigern. Seltener werden auch Protonenstrahlen eingesetzt. Diese besitzen gegenüber Photonen und Elektronen vorteilhaftere physikalische und strahlenbiologische Eigenschaften, die besonders bei der Bestrahlung von tiefliegenden Tumoren in der Nähe von lebenswichtigen, strahlenempfindlichen Organen von Bedeutung sind. Die Behandlung mit Protonen erfordert jedoch sehr große und teure Bestrahlungsanlagen, weshalb es weltweit bisher nur ca. 50 solcher Anlagen an großen Zentren gibt. In den letzten Jahren wurde das völlig neuartige Prinzip der Teilchenbeschleunigung durch Hochleistungslaser soweit entwickelt, dass eine medizinische Anwendung zur Krebstherapie vorstellbar ist. Die laserbasierte Teilchenbeschleunigung verspricht deutlich kompaktere und kostengünstigere Protonenbeschleuniger, erzeugt jedoch im Unterschied zu herkömmlichen Beschleunigern sehr kurze (~ps) hochintensive Protonenpulse mit großer Strahldivergenz und breitem Energiespektrum. Im Rahmen des Verbundprojektes onCOOPtics wird die klinische Anwendbarkeit derartiger laserbeschleunigter Protonenstrahlen untersucht, was nicht nur die Entwicklung des notwendigen Laser-Teilchen-Beschleunigers, sondern auch die Entwicklung eines geeigneten Strahlführungssystems beinhaltet

Inter- and Intragranular Effects in Superconducting Compacted Platinum Powders

Compacted platinum powders exhibit a sharp onset of diamagnetic screening at $T \simeq 1.9$ mK in zero magnetic field in all samples investigated. This sharp onset is interpreted in terms of the intragranular transition into the superconducting state. At lower temperatures, the magnetic ac susceptibility strongly depends on the ac field amplitude and reflects the small intergranular critical current density $j_{c}$. This critical current density shows a strong dependence on the packing fraction f of the granular samples. Surprisingly, $j_{c}$ increases significantly with decreasing f ($j_{c}(B=0, T=0) \simeq 0.07$ A/cm$^{2}$ for f = 0.67 and $j_{c}(B=0, T=0) \simeq 0.8$ A/cm$^{2}$ for f = 0.50). The temperature dependence of $j_{c}$ shows strong positive curvature over a wide temperature range for both samples. The phase diagrams of inter- and intragranular superconductivity for different samples indicate that the granular structure might play the key role for an understanding of the origin of superconductivity in the platinum compacts.Comment: 11 pages including 9 figures. To appear in Phys. Rev. B in Nov. 0

Superconducting properties of RuSr2GdCu2O8 studied by SQUID magnetometry

For polycrystalline RuSr2GdCu2O8 (Ru-1212), distinct peaks have been reported in d.c. magnetization in the superconducting state of the sample. Sr2GdRuO6 (Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in the same temperature regime. Based on measurements performed on both bulk and powdered samples of Ru-1212 and Sr-2116, we exclude the possibility, that the observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities. The effect is related to the superconductivity of Ru-1212, but it is not an intrinsic property of this compound. We provide evidence that the observation of magnetization peaks in the superconducting state of Ru-1212 is due to flux motion generated by the movement of the sample in an inhomogeneous field, during the measurement in the SQUID magnetometer. We propose several tests, that help to decide, whether the features observed in a SQUID magnetization measurement of Ru-1212 represent a property of the compound or not.Comment: 22 pages, 9 figure

Sensitivity reduction of the CL-20/HMX cocrystal via advanced crystallization process

Being able to produce high explosive particles in a desired particle size, morphology, and crystal quality is a key factor in producing high quality polymer bonded explosives. In this poster we present our advances in the sensitivity reduction of the 2:1 CL-20/HMX cocrystal produced by solvent based crystallization via reduction of internal crystal defects. This progress was achieved by thorough control of the process parameters, such as supersaturation, stoichiometry, seeding procedure, temperature, and more. Optical microscopy with crystals immersed in refractivity matched liquid was utilized to visualize the internal defects and, thereby differentiate between the crystal qualities. In addition we present the application of post treatment procedures to further reduce the sensitivity of the obtained crystals, by modifying the crystal morphology and crystal surface

Automated highly multiplexed super-resolution imaging of protein nano-architecture in cells and tissues

Understanding the nano-architecture of protein machines in diverse subcellular compartments remains a challenge despite rapid progress in super-resolution microscopy. While single-molecule localization microscopy techniques allow the visualization and identification of cellular structures with near-molecular resolution, multiplex-labeling of tens of target proteins within the same sample has not yet been achieved routinely. However, single sample multiplexing is essential to detect patterns that threaten to get lost in multi-sample averaging. Here, we report maS3TORM (multiplexed automated serial staining stochastic optical reconstruction microscopy), a microscopy approach capable of fully automated 3D direct STORM (dSTORM) imaging and solution exchange employing a re-staining protocol to achieve highly multiplexed protein localization within individual biological samples. We demonstrate 3D super-resolution images of 15 targets in single cultured cells and 16 targets in individual neuronal tissue samples with <10 nm localization precision, allowing us to define distinct nano-architectural features of protein distribution within the presynaptic nerve terminal

Automated highly multiplexed super-resolution imaging of protein nano-architecture in cells and tissues

Understanding the nano-architecture of protein machines in diverse sub-cellular compartments remains a challenge despite rapid progress in super-resolution microscopy. While singlemolecule localization microscopy techniques allow the visualization and identification of cellular structures with near-molecular resolution, multiplex-labeling of tens of target proteins within the same sample has not yet been achieved routinely. However, single sample multiplexing is essential to detect patterns that threaten to get lost in multi-sample averaging. Here, we report maS3TORM (multiplexed automated serial staining stochastic optical reconstruction microscopy), a microscopy approach capable of fully automated 3D dSTORM imaging and solution exchange employing a re-staining protocol to achieve highly multiplexed protein localization within individual biological samples. We demonstrate 3D super-resolution images of 15 target proteins in single cultured cells and 16 targets in individual neuronal tissue samples with <10 nm localization precision. This allowed us to define novel nano-architectural features of protein distribution within the presynaptic nerve terminal

Enhanced labeling density and whole-cell 3D dSTORM imaging by repetitive labeling of target proteins

With continuing advances in the resolving power of super-resolution microscopy, the inefficient labeling of proteins with suitable fluorophores becomes a limiting factor. For example, the low labeling density achieved with antibodies or small molecule tags limits attempts to reveal local protein nano-architecture of cellular compartments. On the other hand, high laser intensities cause photobleaching within and nearby an imaged region, thereby further reducing labeling density and impairing multi-plane whole-cell 3D super-resolution imaging. Here, we show that both labeling density and photobleaching can be addressed by repetitive application of trisNTA-fluorophore conjugates reversibly binding to a histidine-tagged protein by a novel approach called single-epitope repetitive imaging (SERI). For single-plane super-resolution microscopy, we demonstrate that, after multiple rounds of labeling and imaging, the signal density is increased. Using the same approach of repetitive imaging, washing and re-labeling, we demonstrate whole-cell 3D super-resolution imaging compensated for photobleaching above or below the imaging plane. This proof-of-principle study demonstrates that repetitive labeling of histidine-tagged proteins provides a versatile solution to break the 'labeling barrier' and to bypass photobleaching in multi-plane, whole-cell 3D experiments