Quantum oscillations in an optically-illuminated two-dimensional electron system at the LaAlO3/SrTiO3interface.

We have investigated the illumination effect on the magnetotransport properties of a two-dimensional electron system at the LaAlO3/SrTiO3interface. The illumination significantly reduces the zero-field sheet resistance, eliminates the Kondo effect at low-temperature, and switches the negative magnetoresistance into the positive one. A large increase in the density of high-mobility carriers after illumination leads to quantum oscillations in the magnetoresistance originating from the Landau quantization. The carrier density (∼2 × 1012 cm-2) and effective mass (∼1.7me) estimated from the oscillations suggest that the high-mobility electrons occupy thedxz/yzsubbands of Ti:t2gorbital extending deep within the conducting sheet of SrTiO3. Our results demonstrate that the illumination which induces additional carriers at the interface can pave the way to control the Kondo-like scattering and study the quantum transport in the complex oxide heterostructures.

Magnetoquantum Oscillations at THz Frequencies in InSb.

The ac magnetoconductance of bulk InSb at THz frequencies in high magnetic fields, as measured by the transmission of THz radiation, shows a field-induced transmission, which at high temperatures (≈100 K) is well explained with classical magnetoplasma effects (helicon waves). However, at low temperatures (4 K), the transmitted radiation intensity shows magnetoquantum oscillations that represent the Shubnikov-de Haas effect at THz frequencies. At frequencies above 0.9 THz, when the radiation period is shorter than the Drude scattering time, an anomalously high transmission is observed in the magnetic quantum limit that can be interpreted as carrier localization at high frequencies.

A low-temperature scanning tunneling microscope capable of microscopy and spectroscopy in a Bitter magnet at up to 34 T.

We present the design and performance of a cryogenic scanning tunneling microscope (STM) which operates inside a water-cooled Bitter magnet, which can attain a magnetic field of up to 38 T. Due to the high vibration environment generated by the magnet cooling water, a uniquely designed STM and a vibration damping system are required. The STM scan head is designed to be as compact and rigid as possible, to minimize the effect of vibrational noise as well as fit the size constraints of the Bitter magnet. The STM uses a differential screw mechanism for coarse tip-sample approach, and operates in helium exchange gas at cryogenic temperatures. The reliability and performance of the STM are demonstrated through topographic imaging and scanning tunneling spectroscopy on highly oriented pyrolytic graphite at T = 4.2 K and in magnetic fields up to 34 T.

Ultrafast Magnetism of a Ferrimagnet across the Spin-Flop Transition in High Magnetic Fields.

We show that applying magnetic fields up to 30 T has a dramatic effect on the ultrafast spin dynamics in ferrimagnetic GdFeCo. Upon increasing the field beyond a critical value, the dynamics induced by a femtosecond laser excitation strongly increases in amplitude and slows down significantly. Such a change in spin response is explained by different dynamics of the Gd and FeCo magnetic sublattices following a spin-flop phase transition from a collinear to a noncollinear spin state.

Linear Magnetoresistance in a Quasifree Two-Dimensional Electron Gas in an Ultrahigh Mobility GaAs Quantum Well.

We report a high-field magnetotransport study of an ultrahigh mobility (µ[over ¯]≈25×10^{6} cm^{2} V^{-1} s^{-1}) n-type GaAs quantum well. We observe a strikingly large linear magnetoresistance (LMR) up to 33 T with a magnitude of order 10^{5}% onto which quantum oscillations become superimposed in the quantum Hall regime at low temperature. LMR is very often invoked as evidence for exotic quasiparticles in new materials such as the topological semimetals, though its origin remains controversial. The observation of such a LMR in the "simplest system"-with a free electronlike band structure and a nearly defect-free environment-excludes most of the possible exotic explanations for the appearance of a LMR and rather points to density fluctuations as the primary origin of the phenomenon. Both, the featureless LMR at high T and the quantum oscillations at low T follow the empirical resistance rule which states that the longitudinal conductance is directly related to the derivative of the transversal (Hall) conductance multiplied by the magnetic field and a constant factor α that remains unchanged over the entire temperature range. Only at low temperatures, small deviations from this resistance rule are observed beyond ν=1 that likely originate from a different transport mechanism for the composite fermions.

Shaping polymersomes into predictable morphologies via out-of-equilibrium self-assembly.

Polymersomes are bilayer vesicles, self-assembled from amphiphilic block copolymers. They are versatile nanocapsules with adjustable properties, such as flexibility, permeability, size and functionality. However, so far no methodological approach to control their shape exists. Here we demonstrate a mechanistically fully understood procedure to precisely control polymersome shape via an out-of-equilibrium process. Carefully selecting osmotic pressure and permeability initiates controlled deflation, resulting in transient capsule shapes, followed by reinflation of the polymersomes. The shape transformation towards stomatocytes, bowl-shaped vesicles, was probed with magnetic birefringence, permitting us to stop the process at any intermediate shape in the phase diagram. Quantitative electron microscopy analysis of the different morphologies reveals that this shape transformation proceeds via a long-predicted hysteretic deflation-inflation trajectory, which can be understood in terms of bending energy. Because of the high degree of controllability and predictability, this study provides the design rules for accessing polymersomes with all possible different shapes.

Polymersome magneto-valves for reversible capture and release of nanoparticles.

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.

Magneto-optical properties of wurtzite-phase InP nanowires.

The possibility to grow in zincblende (ZB) and/or wurtzite (WZ) crystal phase widens the potential applications of semiconductor nanowires (NWs). This is particularly true in technologically relevant III-V compounds, such as GaAs, InAs, and InP, for which WZ is not available in bulk form. The WZ band structure of many III-V NWs has been widely studied. Yet, transport (that is, carrier effective mass) and spin (that is, carrier g-factor) properties are almost experimentally unknown. We address these issues in a well-characterized material: WZ indium phosphide. The value and anisotropy of the reduced mass (µ exc) and g-factor (g exc) of the band gap exciton are determined by photoluminescence measurements under intense magnetic fields (B, up to 28 T) applied along different crystallographic directions. µ exc is 14% greater in WZ NWs than in a ZB bulk reference and it is 6% greater in a plane containing the WZ c axis than in a plane orthogonal to c. The Zeeman splitting is markedly anisotropic with g exc = |ge| = 1.4 for B⊥c (where ge is the electron g-factor) and g exc = |ge - gh,//| = 3.5 for B//c (where gh,// is the hole g-factor). A noticeable B-induced circular dichroism of the emitted photons is found only for B//c, as expected in WZ-phase materials.

Giant magnetic susceptibility of gold nanorods detected by magnetic alignment.

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 (Δχ(V)). Δχ(V) increases with decreasing rod size and increasing aspect ratio and corresponds to an average volume susceptibility (χ(V)), which is drastically enhanced relative to bulk Au. This high value of χ(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 χ(V) is the result of orbital magnetism due to mesoscopic electron trajectories within the nanorods.

Doubly periodic instability pattern in a smectic-A liquid crystal.

We report the observation of a doubly periodic surface defect pattern in the liquid crystal 8CB, formed during the nematic-smectic-A phase transition. The pattern results from the antagonistic alignment of the 8CB molecules, which is homeotropic at the surface and planar in the bulk of the sample cell. Within the continuum Landau-de Gennes theory of smectic liquid crystals, we find that the long period (≈10 µm) of the pattern is given by the balance between the surface anchoring and the elastic energy of curvature wall defects. The short period (≈1 µm) we attribute to a saddle-splay distortion, leading to a nonzero Gaussian curvature and causing the curvature walls to break up.

Selection of supramolecular chirality by application of rotational and magnetic forces.

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.

High sensitivity magnetometer for measuring the isotropic and anisotropic magnetisation of small samples.

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.

Aggregation kinetics of macrocycles detected by magnetic birefringence.

We have used magnetic-field-induced birefringence as a new sensitive technique to probe the aggregation kinetics of macrocyclic molecules in solution. We have found three consecutive aggregation stages: disordered objects, ordered fibers, and a network. The transition from disordered objects to ordered fibers is found to be slow, taking days or weeks to complete. We attribute this to the molecular tails of the macrocycles, which hamper fiber formation. We anticipate that linking aggregation kinetics to molecular properties will lead to a better understanding of the mechanisms by which molecules self-assemble, allowing for a more rational design of the molecular building blocks.

Molecular organization of cylindrical sexithiophene aggregates measured by X-ray scattering and magnetic alignment.

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with pi-pi-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.

Using magnetic birefringence to determine the molecular arrangement of supramolecular nanostructures.

Supramolecular aggregates can be aligned in solution using a magnetic field. Because of the optical anisotropy of the molecular building blocks, the alignment results in an anisotropic refractive index of the solution parallel and perpendicular to the magnetic field. We present a model for calculating the magnetic birefringence, using solely the magnetic susceptibilities and optical polarizabilities of the molecules, for any molecular arrangement. We demonstrate that magnetic birefringence is a very sensitive tool for determining the molecular organization within supramolecular aggregates.

Oscillatory persistent currents in self-assembled quantum rings.

We report the direct measurement of the persistent current carried by a single electron by means of magnetization experiments on self-assembled InAs/GaAs quantum rings. We measured the first Aharonov-Bohm oscillation at a field of 14 T, in perfect agreement with our model based on the structural properties determined by cross-sectional scanning tunneling microscopy measurements. The observed oscillation magnitude of the magnetic moment per electron is remarkably large for the topology of our nanostructures, which are singly connected and exhibit a pronounced shape asymmetry.

Magnetic effects at the interface between non-magnetic oxides.

The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. Here we show how, in analogy to this remarkable interface-induced conductivity, magnetism can be induced at the interface between the otherwise non-magnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida interactions, the Kondo effect and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise non-magnetic materials.

Anharmonic magnetic deformation of self-assembled molecular nanocapsules.

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bolaamphiphilic sexithiophene molecules. At low fields the deformation--measured through linear birefringence-scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction [W. Helfrich, Phys. Lett. A 43, 409 (1973)10.1016/0375-9601(73)90396-4], and permit the determination of the bending rigidity of the capsules as (2.6+/-0.8) x 10(-21) J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behavior, and allows discussion of the formation and stability of nanocapsules in solution.

Room-temperature quantum Hall effect in graphene.

The quantum Hall effect (QHE), one example of a quantum phenomenon that occurs on a truly macroscopic scale, has attracted intense interest since its discovery in 1980 and has helped elucidate many important aspects of quantum physics. It has also led to the establishment of a new metrological standard, the resistance quantum. Disappointingly, however, the QHE has been observed only at liquid-helium temperatures. We show that in graphene, in a single atomic layer of carbon, the QHE can be measured reliably even at room temperature, which makes possible QHE resistance standards becoming available to a broader community, outside a few national institutions.

Quantum-Hall activation gaps in graphene.

We have measured the quantum-Hall activation gaps in graphene at filling factors nu=2 and nu=6 for magnetic fields up to 32 T and temperatures from 4 to 300 K. The nu=6 gap can be described by thermal excitation to broadened Landau levels with a width of 400 K. In contrast, the gap measured at nu=2 is strongly temperature and field dependent and approaches the expected value for sharp Landau levels for fields B>20 T and temperatures T>100 K. We explain this surprising behavior by a narrowing of the lowest Landau level.