Recent developments have led to an explosion of activity on skyrmions in three-dimensional (3D) chiral magnets. Experiments have directly probed these topological spin textures, revealed their non-trivial properties, and led to suggestions for novel applications. However, in 3D the skyrmion crystal phase is observed only in a narrow region of the temperature-field phase diagram. We show here, using a general analysis based on symmetry, that skyrmions are much more readily stabilized in two-dimensional (2D) systems with Rashba spin-orbit coupling. This enhanced stability arises from the competition between field and easy-plane magnetic anisotropy, and results in a nontrivial structure in the topological charge density in the core of the skyrmions. We further show that, in a variety of microscopic models for magnetic exchange, the required easy-plane anisotropy naturally arises from the same spin-orbit coupling that is responsible for the chiral Dzyaloshinskii-Moriya interactions. Our results are of particular interest for 2D materials like thin films, surfaces and oxide interfaces, where broken surface inversion symmetry and Rashba spin-orbit coupling leads naturally to chiral exchange and easy-plane compass anisotropy. Our theory gives a clear direction for experimental studies of 2D magnetic materials to stabilize skyrmions over a large range of magnetic fields down to T = 0. arXiv:1402.7082v2 [cond-mat.str-el]

The electronic properties of the polar interface between insulating oxides is a subject of great interest 1-3 . An exciting development is the observation of robust magnetism 4-8 at the interface of two non-magnetic materials, LaAlO 3 (LAO) and SrTiO 3 (STO). Here we present a microscopic theory for the formation and interaction of local moments that depends on essential features of the LAO/STO interface. We show that correlation-induced moments arise owing to interfacial splitting of orbital degeneracy. We find that conduction electrons with a gate-tunable Rashba spin-orbit coupling mediate ferromagnetic exchange with a twist. We predict that the zero-field ground state is a long-wavelength spiral. Its evolution in an external field accounts semi-quantitatively for torque magnetometry data 5 and describes qualitative aspects of the scanning superconducting quantum interference device measurements 6 . We make several testable predictions for future experiments.Recent experiments on the LAO/STO interface have seen tantalizing magnetic signals 4-8 , often persisting up to high temperatures ∼100 K. A large magnetization of 0.3-0.4µ B per interface Ti was observed by torque measurements 5 in an external field. In contrast, scanning superconducting quantum interference device (SQUID) experiments 6 found an inhomogeneous state with a dense set of local moments with no net magnetization, only isolated micron-scale ferromagnetic patches. Our goal is to reconcile these seemingly contradictory observations and to gain insight into the itinerant versus local moment nature of the magnetism, the exchange mechanism and the ordered state.LAO and STO are both band insulators, but the TiO 2 layers at the interface are n-doped when LAO is terminated by a LaO + layer. The polar catastrophe 1 arising from a stack of charged LaO + and AlO − 2 layers grown on STO is averted by the transfer of 0.5 electrons per interface Ti. Oxygen vacancies are also known to provide additional electrons at the interface 1 .What is the fate of these electrons at the interface? Transport data suggest that only a small fraction of the electrons (5-10% of the 0.5 electrons per Ti) are mobile 2,9-11 . Interestingly, magnetotransport studies show a large, gate-tunable Rashba spin-orbit coupling (SOC) for these conduction electrons, arising from broken inversion at the interface 12 . Most of the electrons (comparable to 0.5 electrons per Ti) seem to behave like local moments in the magnetic measurements 5,6 discussed above.We propose a microscopic model of electrons in Ti t 2g states at the LAO/STO interface that leads to the following results: local moments form in the top TiO 2 layer owing to correlations, with interfacial splitting of t 2g degeneracy playing a critical role; conduction electrons mediate ferromagnetic double-exchange interactions between the moments; Rashba SOC for the conduction Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA. *e-mail: randeria@mps.ohio-state.edu electrons leads to a Dzyaloshinskii-Moriya inte...

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Nasal drug administration has been used as an alternative route for the systemic availability of drugs restricted to intravenous administration. This is due to the large surface area, porous endothelial membrane, high total blood flow, the avoidance of first-pass metabolism, and ready accessibility. The nasal administration of drugs, including numerous compound, peptide and protein drugs, for systemic medication has been widely investigated in recent years. Drugs are cleared rapidly from the nasal cavity after intranasal administration, resulting in rapid systemic drug absorption. Several approaches are here discussed for increasing the residence time of drug formulations in the nasal cavity, resulting in improved nasal drug absorption. The article highlights the importance and advantages of the drug delivery systems applied via the nasal route, which have bioadhesive properties. Bioadhesive, or more appropriately, mucoadhesive systems have been prepared for both oral and peroral administration in the past. The nasal mucosa presents an ideal site for bioadhesive drug delivery systems. In this review we discuss the effects of microspheres and other bioadhesive drug delivery systems on nasal drug absorption. Drug delivery systems, such as microspheres, liposomes and gels have been demonstrated to have good bioadhesive characteristics and that swell easily when in contact with the nasal mucosa. These drug delivery systems have the ability to control the rate of drug clearance from the nasal cavity as well as protect the drug from enzymatic degradation in nasal secretions. The mechanisms and effectiveness of these drug delivery systems are described in order to guide the development of specific and effective therapies for the future development of peptide preparations and other drugs that otherwise should be administered parenterally. As a consequence, bioavailability and residence time of the drugs that are administered via the nasal route can be increased by bioadhesive drug delivery systems. Although the majority of this work involving the use of microspheres, liposomes and gels is limited to the delivery of macromolecules (e.g., insulin and growth hormone), the general principles involved could be applied to other drug candidates. It must be emphasized that many drugs can be absorbed well if the contact time between formulation and the nasal mucosa is optimized.

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Recent quantum oscillation experiments on SmB6 pose a paradox, for while the angular dependence of the oscillation frequencies suggest a 3D bulk Fermi surface, SmB6 remains robustly insulating to very high magnetic fields. Moreover, a sudden low temperature upturn in the amplitude of the oscillations raises the possibility of quantum criticality. Here we discuss recently proposed mechanisms for this effect, contrasting bulk and surface scenarios. We argue that topological surface states permit us to reconcile the various data with bulk transport and spectroscopy measurements, interpreting the low temperature upturn in the quantum oscillation amplitudes as a result of surface Kondo breakdown and the high frequency oscillations as large topologically protected orbits around the X point. We discuss various predictions that can be used to test this theory.SmB 6 , discovered 50 years ago [1,2], has attracted recent interest due to its unusual surface transport properties: while its insulating gap develops around T K 50K, the resistivity saturates below a few Kelvin[3]. The renewed interest derives in part from from the possibility that SmB 6 is a topological Kondo insulator, developing topologically protected surface states at low temperatures [4][5][6][7]. Experiments [8][9][10] have confirmed that the plateau conductivity derives from surface states, and these states have been resolved by angle-resolved photoemission spectroscopy (ARPES) [11][12][13][14]. Furthermore, spin-ARPES experiments have revealed the spinmomentum locking of the surface quasiparticles expected from topologically protected Dirac cones [15].Yet despite this progress, some important experimental results are unresolved. In particular, quantum oscillation experiments on SmB 6 have given rise to two dramatically different interpretations [16,17]. Ref [16] observes low frequency (small Fermi surface) oscillations with the characteristic 1/ cos(φ) dependence on field orientation expected from 2D topological surface states. On the other hand Ref [17] detects a wide range of frequencies (both high and low frequency oscillations) which have been interpreted in terms of angularly isotropic three dimensional quasiparticle orbits, resembling a metallic hexaboride without a hybridization gap (such as LaB 6 ). A striking aspect of these measurements, is that the oscillations strongly deviate from a classic Lifshitz-Kosevich formula below ∼ 1K. Two recent theoretical proposals have been advanced to account for this bulk behavior, as a consequence of magnetic breakdown [18], of the formation of non-conducting Fermi surfaces [19].Another aspect of recent measurements, is the wide disparity in the reported effective masses of the carriers. The effective mass observed in both quantum oscillation experiments, m * /m ∼ 0.1 − 0.2 is an order of magnitude smaller than effective mass observed in ARPES [11][12][13][14] [17]. Error bars are the size of symbols which include both experimental errorbars and also errors from extracting data from a logarithmic plot. Given...

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Heavy fermion materials have recently attracted attention for their potential to combine topological protection with strongly correlated electron physics. To date, the ideas of topological protection have been restricted to the heavy fermion or "Kondo" insulators with the simplest point-group symmetries. Here we argue that the presence of nonsymmorphic crystal symmetries in many heavy fermion materials opens up a new family of topologically protected heavy electron systems. Re-examination of archival resistivity measurements in nonsymorphic heavy fermion insulators Ce 3 Bi 4 Pt 3 and CeNiSn reveals the presence of low temperature conductivity plateau, making them candidate members of the new class of material. We illustrate our ideas with a specific model for CeNiSn, showing how glide symmetries generate surface states with a novel Möbius braiding that can be detected by ARPES or non-local conductivity measurements. One of the interesting effects of strong correlation, is the development of partially localization or "Kondo breakdown" on the surfaces, which transforms Möbius surface states into quasi-one dimensional conductors, with the potential for novel electronic phase transitions.