Many essential biological molecules exist only in one of two possible mirror-image structures, either because they possess a chiral unit or through their structure (helices, for example, are intrinsically chiral), but so far the origin of this homochirality has not been unraveled. Here we demonstrate that the handedness of helical supramolecular aggregates formed by achiral molecules can be directed by applying rotational, gravitational and orienting forces during the self-assembly process. In this system, supramolecular chirality is determined by the relative directions of rotation and magnetically tuned effective gravity, but the magnetic orientation of the aggregates is also essential. Applying these external forces only during the nucleation step of the aggregation is sufficient to achieve chiral selection. This result shows that an almost instantaneous chiral perturbation can be transferred and amplified in growing supramolecular self-assemblies, and provides evidence that a falsely chiral influence is able to induce absolute enantioselection.

Birefringence measurements at high magnetic field strength of up to 33 T were used to detect magnetically induced alignment of bicelles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), cholesterol, and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-diethylenetriaminepentaacetate (DMPE-DTPA) with complexed lanthanide ions. These birefringence measurements together with a small-angle neutron scattering (SANS) analysis in a magnetic field showed parallel alignment of the bicelles if the lanthanide was thulium (Tm(3+)), and perpendicular alignment with dysprosium (Dy(3+)). With the birefringence measurements, the order parameter S can be determined as a function of the magnetic field strength, if the magnetic alignment reaches saturation. Additional structural information can be obtained if the maximum induced birefringence is considered. The degree of alignment of the studied bicelles increased with decreasing temperature from 40 to 5 °C and showed a new bicellar structure comprising a transient hole formation at intermediate temperatures (20 °C) during heating from 5 to 40 °C.

We have determined the magnetic properties of single-crystalline Au nanorods in solution using an optically detected magnetic alignment technique. The rods exhibit a large anisotropy in the magnetic volume susceptibility (Á V ). Á V increases with decreasing rod size and increasing aspect ratio and corresponds to an average volume susceptibility ( V ), which is drastically enhanced relative to bulk Au. This high value of V is confirmed by SQUID magnetometry and is temperature independent (between 5 and 300 K). Given this peculiar size, shape, and temperature dependence, we speculate that the enhanced V is the result of orbital magnetism due to mesoscopic electron trajectories within the nanorods. DOI: 10.1103/PhysRevLett.111.127202 PACS numbers: 75.75.Àc, 73.22.Àf, 75.20.En, 78.67.Qa Bulk Au is a diamagnetic material, i.e., one with a negative volume magnetic susceptibility Au . Recently, it was reported that Au nanoparticles (NPs), with functionalized surfaces, show a broad range of magnetic behavior, ranging from (enhanced) diamagnetic [1,2] to (super)paramagnetic [3][4][5] and even ferromagnetic up to room temperature [6,7]. The NP size and the type of capping molecules, strongly binding to or weakly interacting with Au, appear to influence the magnetic response. Several explanations were suggested, such as competing magnetic contributions of the NP core and surface [3], the formation of a magnetic moment due to the exchange of charges at the Au-ligand interface [5,6,8], the creation of large orbital moments due to electron motion within surface clusters [9], and the occurrence of persistent currents in the Au core [2]. However, so far, the origin of this unexpected magnetism and why it differs strongly between different types of NPs is not yet understood [2,10,11].We employ a novel magnetic alignment technique to measure the magnetic properties of rod-shaped Au NPs in solution. We focus on relatively large NPs (all dimensions >7 nm) that are single crystalline. The degree of alignment is measured optically, through the magnetic field-induced linear dichroism and birefringence, across the Au surface plasmon resonance (SPR) that arises due to collective oscillation modes of the conduction electrons [12,13]. We find an enhanced (dia)magnetic behavior, which does not depend on temperature (in the range 5-300 K). We speculate that this enhanced magnetism is an orbital effect, resulting from mesoscopic electron trajectories within the NPs [2,14].The optically detected magnetic alignment technique relies on the anisotropy of both the optical and magnetic properties of the Au nanorods. Because of their shape, the rods exhibit an anisotropic optical response, determined by their longitudinal ( k ) and transverse ( ? ) polarizabilities [15]. Polarized light, therefore, provides a sensitive tool to determine the alignment of rods [16][17][18][19]. In this Letter, rod alignment is induced by a magnetic field (B) because of the difference in the magnetic susceptibility parallel ( k ) and perpendicular ( ? ) to the lo...

Stomatocytes are polymersomes with an infolded bowl-shaped architecture. This internal cavity is connected to the outside environment via a small ‘mouth’ region. Stomatocytes are assembled from diamagnetic amphiphilic block-copolymers with a highly anisotropic magnetic susceptibility, which permits to magnetically align and deform the polymeric self-assemblies. Here we show the reversible opening and closing of the mouth region of stomatocytes in homogeneous magnetic fields. The control over the size of the opening yields magneto-responsive supramolecular valves that are able to reversibly capture and release cargo. Furthermore, the increase in the size of the opening is gradual and starts at fields below 10 T, which opens the possibility of using these structures for delivery and nanoreactor applications.

We studied spontaneously self-assembled aggregates in a suspension of CdSe/CdS core/shell nanorods (NRs). The influence of the length and concentration of the NRs and the suspension temperature on the size of the aggregates was investigated using in situ small-angle X-ray scattering (SAXS) and linear dichroism (LD) measurements under high magnetic fields (up to 30 T). The SAXS patterns reveal the existence of crystalline 2-dimensional sheets of ordered NRs with an unusually large distance between the rods. The LD measurements show that the size of the sheets depends on the free-energy driving force for NR self-assembly. More precisely, the sheets are larger if the attraction between NRs is stronger, if the temperature is lower, or if the NR concentration is higher. We show that the formation of large NR sheets is a slow process that can take days. Our in situ results of the structures that spontaneously form in the bulk suspension could further our understanding of NR self-assembly into mono- or multilayer superlattices that occurs at the suspension/air interface upon evaporation of the solvent.

Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields, however, much less is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-sized single crystals of the triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5, 20, 28 and 39 T in fields applied along the easy axis, and we combine magnetic torque, specific heat and transport data to construct the field-temperature phase diagram. The results are discussed in the context of a frustrated easy-axis Heisenberg model for the localized moments where intermediate applied magnetic fields are predicted to stabilize a magnetic supersolid phase. Deviations in the measured phase diagram from this model predictions are attributed to the role played by the itinerant electrons.

We describe how the full, isotropic and anisotropic, magnetisation of samples as small as tens of micrometers in size can be sensitively measured using a piezoresistive microcantilever and a small, moveable ferromagnet. Depending on the position of the ferromagnet, a strong but highly local field gradient of up to ∼4200 T/m can be applied at the sample or removed completely during a single measurement. In this way, the magnetic force and torque on the sample can be independently determined without moving the sample or cycling the experimental system. The technique can be used from millikelvin temperatures to ∼85 K and in magnetic fields from 2 T to the highest fields available. We demonstrate its application in measurements of the semimagnetic semiconductor Hg 1−x Fe x Se, where we achieved a moment sensitivity of better than 2.5 × 10 −14 J/T for both isotropic and anisotropic components.