@phdthesis{Joestingmeier2005, author = {J{\"o}stingmeier, Martin}, title = {On the competition of superconductivity, antiferromagnetism and charge order in the high-Tc compounds}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-13036}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {Diese Arbeit l{\"a}ßt sich in zwei grobe Abschnitte gliedern. Der erste Teil umfaßt die Kapitel 1-3, in denen drei verschiedene Konzepte beschrieben werden, die zum Verst{\"a}ndis stark korrelierter Vielteilchen-Systeme dienen. Dies sind zun{\"a}chst einmal die SO(5)-Theorie in Kapitel 3, die den allgemeinen Rahmen vorgibt und auf der numerischen Seite die Stochastische Reihen Entwicklung (SSE) in Kapitel 1 und der Contractor Renormierungsgruppen Ansatz (CORE), s.Kapitel 2). Die zentrale Idee dieser Dissertationsschrift besteht darin, diese verschiedenen Konzepte zu kombinieren, um ein besseres Verst{\"a}ndnis der Hochtemperatursupraleiter zu erhalten. Im zweiten Teil dieser Arbeit (Kap. 4 und Kap. 5) werden die so gewonnenen Ergebnisse dargestellt. Die zentrale Idee dieser Arbeit, d.h. die Kombination der SO(5)-Theorie mit den F{\"a}higkeiten bosonischer Quanten-Monte-Carlo Verfahren und den {\"u}berlegungen der Renormierungsgruppe, hat sich sich am Beispiel der Physik der Hochtemperatur-Supraleiter als sehr tragf{\"a}hig erwiesen. Die numerischen Simulationen reproduzieren bei den behandelten Modelle eine Reihe wichtiger experimenteller Daten. Die Grundlage f{\"u}r eine k{\"u}nftige weitere schrittweise Erweiterung des Modells wurde so geschaffen. Eine offene Frage ist z.B. die Restaurierung der SO(5)-Symmetrie an einem multi-kritischen Punkt, wenn die l{\"a}ngerreichweitigen Wechselwirkungen mit in das Modell einbezogen sind.}, subject = {Hochtemperatursupraleitung}, language = {en} } @article{OkadaRotenbergKevanetal.2013, author = {Okada, Michio and Rotenberg, Eli and Kevan, S. D. and Sch{\"a}fer, J. and Ujfalussy, Balazs and Stocks, G. Malcolm and Genatempo, B. and Bruno, E. and Plummer, E. W.}, title = {Evolution of the electronic structure in \(Mo_{1-x}Re_x\) alloys}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {093010}, issn = {1367-2630}, doi = {10.1088/1367-2630/15/9/093010}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-122993}, year = {2013}, abstract = {We report a detailed experimental and theoretical study of the electronic structure of \(Mo_{1-x}Re_x\) random alloys. We have measured electronic band dispersions for clean and hydrogen-covered \(Mo_{1-x}Re_x\) ( 110) with x = 0-0.25 using angle-resolved photoemission spectroscopy. Our results suggest that the bulk and most surface electronic bands shift relative to the Fermi level systematically and approximately rigidly with Re concentration. We distinguish and quantify two contributions to these shifts: a raise of the Fermi energy and an increase of the overall bandwidth. Alloy bands calculated using the first-principles Korringa-Kohn-Rostoker coherent-potential-approximation method accurately predict both of these effects. As derived from the rigid band model, the Fermi energy shift is inversely related to the bulk density of states in this energy region. Using our results, we also characterize an electronic topological transition of the bulk Fermi surface and relate this to bulk transport properties. Finally, we distinguish effects beyond the rigid band approximation: a highly surface-localized state and a composition-dependent impact of the spin-orbit interaction.}, language = {en} } @article{OostingaMaierSchueffelgenetal.2013, author = {Oostinga, Jeroen B. and Maier, Luis and Sch{\"u}ffelgen, Peter and Knott, Daniel and Ames, Christopher and Br{\"u}ne, Christoph and Tkachov, Grigory and Buhmann, Hartmut and Molenkamp, Laurens W.}, title = {Josephson Supercurrent through the Topological Surface States of Strained Bulk HgTe}, series = {Physical Review X}, volume = {3}, journal = {Physical Review X}, number = {021007}, doi = {10.1103/PhysRevX.3.021007}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129834}, year = {2013}, abstract = {Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (~ 30 000 cm\(^2\)=Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K, with critical supercurrents of several microamperes. Moreover, we observe a magnetic-field-induced Fraunhofer pattern of the critical supercurrent, indicating a dominant \(2\pi\)-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments.}, language = {en} } @article{BareilleFortunaRoedeletal.2014, author = {Bareille, C. and Fortuna, F. and R{\"o}del, T. C. and Bertran, F. and Gabay, M. and Hijano Cubelos, O. and Taleb-Ibrahimi, A. and Le F{\`e}vre, P. and Bibes, M. and Barthelemy, A. and Maroutian, T. and Lecoeur, P. and Rozenberg, M. J. and Santander-Syro, A. F.}, title = {Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3}, series = {Scientific Reports}, volume = {4}, journal = {Scientific Reports}, issn = {2045-2322}, doi = {10.1038/srep03586}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117703}, pages = {3586}, year = {2014}, abstract = {Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.}, language = {en} } @article{HansmannParraghToschietal.2014, author = {Hansmann, P. and Parragh, N. and Toschi, A. and Sangiovanni, G. and Held, K.}, title = {Importance of d-p Coulomb interaction for high T-C cuprates and other oxides}, series = {New Journal of Physics}, volume = {16}, journal = {New Journal of Physics}, number = {33009}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/3/033009}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117165}, year = {2014}, abstract = {Current theoretical studies of electronic correlations in transition metal oxides typically only account for the local repulsion between d-electrons even if oxygen ligand p-states are an explicit part of the effective Hamiltonian. Interatomic interactions such as U-pd between d- and (ligand) p-electrons, as well as the local interaction between p-electrons, are neglected. Often, the relative d-p orbital splitting has to be adjusted 'ad hoc' on the basis of the experimental evidence. By applying the merger of local density approximation and dynamical mean field theory to the prototypical case of the three-band Emery dp model for the cuprates, we demonstrate that, without any 'ad hoc' adjustment of the orbital splitting, the charge transfer insulating state is stabilized by the interatomic interaction U-pd. Our study hence shows how to improve realistic material calculations that explicitly include the p-orbitals.}, language = {en} } @phdthesis{Maier2015, author = {Maier, Luis}, title = {Induced superconductivity in the topological insulator mercury telluride}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119405}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {The combination of a topological insulator (TI) and a superconductor (S), which together form a TI/S interface, is expected to influence the possible surface states in the TI. It is of special interest, if the theoretical prediction of zero energy Majorana states in this system is verifiable. This thesis presents the experimental realization of such an interface between the TI strained bulk HgTe and the S Nb and studies if the afore mentioned expectations are met. As these types of interfaces were produced for the first time the initial step was to develop a new lithographic process. Optimization of the S deposition technique as well as the application of cleaning processes allowed for reproducible fabrication of structures. In parallel the measurement setup was upgraded to be able to execute the sensitive measurements at low energy. Furthermore several filters have been implemented into the system to reduce high frequency noise and the magnetic field control unit was additionally replaced to achieve the needed resolution in the μT range. Two kinds of basic geometries have been studied: Josephson junctions (JJs) and superconducting quantum interference devices (SQUIDs). A JJ consists of two Nb contacts with a small separation on a HgTe layer. These S/TI/S junctions are one of the most basic structures possible and are studied via transport measurements. The transport through this geometry is strongly influenced by the behavior at the two S/TI interfaces. In voltage dependent differential resistance measurements it was possible to detect multiple Andreev reflections in the JJ, indicating that electrons and holes are able to traverse the HgTe gap between both interfaces multiple times while keeping phase coherence. Additionally using BTK theory it was possible to extract the interface transparency of several junctions. This allowed iterative optimization for the highest transparency via lithographic improvements at these interfaces. The increased transparency and thus the increased coupling of the Nb's superconductivity to the HgTe results in a deeper penetration of the induced superconductivity into the HgTe. Due to this strong coupling it was possible to enter the regime, where a supercurrent is carried through the complete HgTe layer. For the first time the passing of an induced supercurrent through strained bulk HgTe was achieved and thus opened the area for detailed studies. The magnetic dependence of the supercurrent in the JJ was recorded, which is also known as a Fraunhofer pattern. The periodicity of this pattern in magnetic field compared to the JJ geometry allowed to conclude how the junction depends on the phase difference between both superconducting contacts. Theoretical calculations predicted a phase periodicity of 4p instead of 2p, if a TI is used as weak link material between the contacts, due to the presence of Majorana modes. It could clearly be shown that despite the usage of a TI the phase still was 2p periodic. By varying further influencing factors, like number of modes and phase coherence length in the junction, it might still be possible to reach the 4p regime with bound Majorana states in the future. A good candidate for further experiments was found in capped HgTe samples, but here the fabrication process still has to be developed to the same quality as for the uncapped HgTe samples. The second type of geometry studied in this thesis was a DC-SQUID, which consists of two parallel JJs and can also be described as an interference device between two JJs. The DC-SQUID devices were produced in two configurations: The symmetric SQUID, where both JJs were identical, and the asymmetric SQUID, where one JJ was not linear, but instead has a 90° bent. These configurations allow to test, if the predicted uniformity of the superconducting band gap for induced superconductivity in a TI is valid. While the phase of the symmetric SQUID is not influenced by the shape of the band gap, the asymmetric SQUID would be in phase with the symmetric SQUID in case of an uniform band gap and out of phase if p- or d-wave superconductivity is dominating the transport, due to the 90° junction. As both devices are measured one after another, the problem of drift in the coil used to create the magnetic field has to be overcome in order to decide if the oscillations of both types of SQUIDs are in phase. With an oscillation period of 0.5 mT and a drift rate in the range of 5.5 μT/h the measurements on both configurations have to be conducted in a few hours. Only then the total shift is small enough to compare them with each other. For this to be possible a novel measurement system based on a real time micro controller was programmed, which allows a much faster extraction of the critical current of a device. The measurement times were reduced from days to hours, circumventing the drift problems and enabling the wanted comparison. After the final system optimizations it has been shown that the comparison should now be possible. Initial measurements with the old system hinted that both types of SQUIDs are in phase and thus the expected uniform band gap is more likely. With all needed optimizations in place it is now up to the successors of this project to conclusively prove this last point. This thesis has proven that it is possible to induce superconductivity in strained bulk HgTe. It has thus realized the most basic sample geometry proposed by Fu and Kane in 2008 for the appearance of Majorana bound states. Based on this work it is now possible to further explore induced superconductivity in strained bulk HgTe to finally reach a regime, where the Majorana states are both stable and detectable.}, subject = {Quecksilbertellurid}, language = {en} } @article{ScharfBraggioStambinietal.2020, author = {Scharf, Benedikt and Braggio, Alessandro and Stambini, Elia and Giazotto, Francesco and Hankiewicz, Ewelina M.}, title = {Topological Josephson heat engine}, series = {Communications Physics}, volume = {3}, journal = {Communications Physics}, doi = {10.1038/s42005-020-00463-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-230603}, year = {2020}, abstract = {Topological superconductors represent a fruitful playing ground for fundamental research as well as for potential applications in fault-tolerant quantum computing. Especially Josephson junctions based on topological superconductors remain intensely studied, both theoretically and experimentally. The characteristic property of these junctions is their 4-periodic ground-state fermion parity in the superconducting phase difference. Using such topological Josephson junctions, we introduce the concept of a topological Josephson heat engine. We discuss how this engine can be implemented as a Josephson-Stirling cycle in topological superconductors, thereby illustrating the potential of the intriguing and fruitful marriage between topology and coherent thermodynamics. It is shown that the Josephson-Stirling cycle constitutes a highly versatile thermodynamic machine with different modes of operation controlled by the cycle temperatures. Finally, the thermodynamic cycle reflects the hallmark 4 pi -periodicity of topological Josephson junctions and could therefore be envisioned as a complementary approach to test topological superconductivity. Topological superconductors are expected to be a key component of quantum computing systems but reliably detecting their exotic properties is a challenge. Here, the authors propose a topological Josephson heat engine which uses thermodynamic effects to probe the 4 pi -periodic ground state of a topological superconductor.}, language = {en} }