Spin-orbit controlled quantum capacitance of a polar heterostructure

Oxide heterostructures with polar films display special electronic properties, such as the electronic reconstruction at their internal interfaces with the formation of two-dimensional metallic states. Moreover, the electrical field from the polar layers is inversion-symmetry breaking and generates a Rashba spin-orbit coupling (RSOC) in the interfacial electronic system. We investigate the quantum capacitance of a heterostructure in which a sizeable RSOC at a metallic interface is controlled by the electric field of a surface electrode. Such a structure is, for example, given by a LaAlO_3 film on a SrTiO_3 substrate which is gated by a top electrode. Such heterostructures can exhibit a strong enhancement of their capacitance [Li et al., Science 332, 825 (2011)]. The capacitance is related to the electronic compressibility of the heterostructure, but the two quantities are not equivalent. In fact, the transfer of charge to the interface controls the relation between capacitance and compressibility. We find that due to a strong RSOC, the quantum capacitance can be larger than the classical geometric value. However, in contrast to the results of recent investigations [Caprara et al., Phys. Rev. Lett. 109, 196401 (2012); Bucheli et al., Phys. Rev. B 89, 195448 (2014); Seibold et al., Europhys. Lett. 109, 17006 (2015)] the compressibility does not become negative for realistic parameter values for LaAlO_3/SrTiO_3 and, therefore, we find that no phase-separated state is induced by the strong RSOC at these interfaces

Flux-Periodicity Crossover from hc/e in Normal Metallic to hc/2e in Superconducting Loops

The periodic response of a metallic or a superconducting ring to an external magnetic flux is one of the most evident manifestations of quantum mechanics. It is generally understood that the oscillation period hc/2e in the superconducting state is half the period hc/e in the metallic state, because the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple interpretation is valid and when a more careful analysis is needed. In fact, the knowledge of the oscillation period of the current in the ring provides information on the electron interactions. In particular, we analyze the crossover from the hc/e periodic normal current to the hc/2e periodic supercurrent upon turning on a pairing interaction in a metal ring. Further, we elaborate on the periodicity crossover when cooling a metallic loop through the superconducting transition temperature Tc.Comment: To be bublished in "Superconductors", InTech (Rijeka), 2012 (ISBN 979-953-307-798-6

Superconductivity with Finite-Momentum Pairing in Zero Magnetic Field

In the BCS theory of superconductivity, one assumes that all Cooper pairs have the same center of mass momentum. This is indeed enforced by self consistency, if the pairing interaction is momentum independent. Here, we show that for an attractive nearest neighbor interaction, this is different. In this case, stable solutions with pairs with momenta q and -q coexist and, for a sufficiently strong interaction, one of these states becomes the groundstate of the superconductor. This finite-momentum pairing state is accompanied by a charge order with wave vector 2q. For a weak pairing interaction, the groundstate is a d-wave superconductor

Fractional Flux Quantization in Loops of Unconventional Superconductors

The magnetic flux threading a conventional superconducting ring is typically quantized in units of $\Phi_0=hc/2e$. The factor 2 in the denominator of $\Phi_0$ originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of $\Phi_0$. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular at multiples of $\Phi_0/2$. In such states condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where $hc/4e$ flux periodicities as well as uniaxially modulated superconducting states were observed.Comment: 5 pages, 3 figure

Macrocyclization and Fatty Acid Modification during the Synthesis of Nonribosomal Peptides

Nonribosomal peptides (NRPs) constitute a large and diverse class of pharmacologically important natural products that find useful therapeutic application as immunosuppressants, antibiotics, or anticancer agents. The biological activity of many of these compounds relies on the macrocyclic structure of their peptide backbone and the incorporation of a wide assortment of building blocks including proteinogenic and nonproteinogenic amino acids as well as modified fatty acid moieties. Particularly, these structural features are key determinants of nonribosomal lipopeptide antibiotics that are in the focus of this thesis. To provide rapid access to these structurally demanding compounds, a chemoenzymatic approach towards the synthesis of the lipopeptide antibiotics daptomycin and A54145 was developed, based on the combined utilization of powerful solid phase peptide synthesis and the recombinant daptomycin and A54145 thioesterase (TE) domains. In vitro studies with these so-called peptide cyclases revealed their ability to catalyze the macrocyclization of linear peptidyl-thiophenol substrates with relaxed specificity for the cyclization nucleophile and electrophile. Ten lipopeptide variants were synthesized in order to explore the relatively sparse known acidic lipopeptide structure-activity relationship. Remarkably, this small library included a lipopeptide hybrid with a minimal inhibition concentration close to that of chemoenzymatic derived daptomycin as well as a bioactive macrolactam variant of A54145. Thus, single amino acid residues within the daptomycin and A54145 peptide sequences could be identified that are crucial for their antimicrobial potency. Additionally, a unique and hitherto unknown type of imine macrocyclization as found for the cyanobacterial nostocyclopeptide (ncp) was investigated during the course of these studies. Experiments with ncp-CoA substrate mimics showed that a reductase (R) domain located at the C-terminal end of the ncp nonribosomal peptide synthetase (NRPS) is responsible for the reductive release of a reactive peptide aldehyde. Subsequently, imine macrocyclization occurs enzyme-independent under physiological pH conditions as proven with synthetic analogs of the ncp peptide aldehyde. An alanine scan experiment elucidated structural elements within the linear heptapeptide precursor that are essential for imine macrocyclization. Further, the biochemical characterization of ncp R also revealed its broad tolerance towards the C- and N-terminal amino acids of ncp substrate mimics. In the third part of this work, the tailoring enzymes HxcO and HcmO from the calcium dependent antibiotic (CDA) trans 2,3 epoxyhexanoic acid biosynthetic pathway were chosen as a model system to investigate fatty acid modification during nonribosomal lipopeptide synthesis. While HxcO was characterized as a novel type of enzyme with dual function as an FAD-dependent fatty acid oxidase paired with intrinsic epoxidase activity, HcmO could be identified as a second epoxidase acting on 2,3-unsaturated fatty acids. Experiments with acyl-CoAs, acyl CoAs loaded onto an acyl carrier protein (ACP), and chemoenzymatically synthesized CDA variants revealed that both enzymes only accept ACP-bound substrates. To compare these ACP-bound HxcO and HcmO reaction products with synthetic standards a novel experimental approach had to be developed. Based on the thermodynamic activation inherent to thioester derivatives, the enzymatic products were cleaved from the ACP under mild conditions utilizing an amide ligation reaction and directly transformed into derivatives of smaller size suitable for HPLC-MS analysis. By the application of this versatile method the trans 2,3 epoxyhexanoic acid products of HxcO and HcmO were ascertained to have opposite absolute configuration, namely (2R,3S) and (2S,3R), respectively. In general, the established experimental approach holds great potential for the detailed analysis of all biochemical systems involving carrier protein-bound intermediates. These include integrated enzymes from NRPS and polyketide synthase (PKS) assembly lines or in trans acting tailoring enzyme

miR-200c sensitizes breast cancer cells to doxorubicin treatment by decreasing TrkB and Bmi1 expression.

Acquired resistance to classical chemotherapeutics is a major obstacle in cancer treatment. Doxorubicin is frequently used in breast cancer therapy either as single-agent or in combination with other drugs like docetaxel and cyclophosphamide. All these chemotherapies have in common that they are administered sequentially and often result in chemoresistance. Here, we mimicked this pulse therapy of breast cancer patients in an in vitro cell culture model, where the epithelial breast cancer cell line BT474 was sequentially treated with doxorubicin for several treatment cycles. In consequence, we obtained chemoresistant cells displaying a mesenchymal-like phenotype with decreased levels of miR-200c. To investigate the involvement of miR-200c in resistance formation, we inhibited and overexpressed miR-200c in different cell lines. Thereby, the cells were rendered more resistant or susceptible to doxorubicin treatment. Moreover, the receptor tyrosine kinase TrkB and the transcriptional repressor Bmi1 were identified as miR-200c targets mediating the drug resistance. Hence, we provide a mechanism of acquired resistance to doxorubicin that is caused by the loss of miR-200c. Along with this, our study demonstrates the complex network of microRNA mediated chemoresistance highlighting the challenges in cancer therapy and the importance of novel microRNA-modulating anticancer agents

Momentum-Space Spin Texture in a Topological Superconductor

A conventional superconductor with spin-orbit coupling turns into a topological superconductor beyond a critical strength of the Zeeman energy. The spin-expectation values $\mathbf{S}(\mathbf{k})$ in momentum space trace this transition via a characteristic change in the topological character of the spin texture within the Brillouin zone. At the transition the skyrmion counting number switches from 0 to 1/2 identifying the topological superconductor via its meron-like spin texture. The change in the skyrmion counting number is crucially controlled by singular points of the map $\mathbf{S}(\mathbf{k})/|\mathbf{S}(\mathbf{k})|$ from the Brillouin zone, i.e. a torus, to the unit sphere. The complexity of this spin-map is discussed at zero temperature as well as for the extension to finite temperatures.Comment: 16 pages, 9 figure

Towards an Economic Analysis of Routing in Payment Channel Networks

Payment channel networks are supposed to overcome technical scalability limitations of blockchain infrastructure by employing a special overlay network with fast payment confirmation and only sporadic settlement of netted transactions on the blockchain. However, they introduce economic routing constraints that limit decentralized scalability and are currently not well understood. In this paper, we model the economic incentives for participants in payment channel networks. We provide the first formal model of payment channel economics and analyze how the cheapest path can be found. Additionally, our simulation assesses the long-term evolution of a payment channel network. We find that even for small routing fees, sometimes it is cheaper to settle the transaction directly on the blockchain.Comment: 6 pages, 3 figures, SERIAL '17 Worksho