Abstract:We herein present the graphical user interface (GUI) TmoleX for the quantum chemical program package TURBOMOLE. TmoleX allows users to execute the complete workflow of a quantum chemical investigation from the initial building of a structure to the visualization of the results in a user friendly graphical front end. The purpose of TmoleX is to make TURBOMOLE easy to use and to provide a high degree of flexibility. Hence, it should be a valuable tool for most users from beginners to experts. The program is developed in Java and runs on Linux, Windows, and Mac platforms. It can be used to run calculations on local desktops as well as on remote computers.
Interest in manipulating the magnetic order by ultrashort laser pulses has thrived since it was observed that such pulses can be used to alter the magnetization on a sub-picosecond timescale. Usually this involves demagnetization by laser heating or, in rare cases, a transient increase of magnetization. Here we demonstrate a mechanism that allows the magnetic order of a material to be enhanced or attenuated at will. This is possible in systems simultaneously possessing a low, tunable density of conduction band carriers and a high density of magnetic moments. In such systems, the thermalization time can be set such that adiabatic processes dominate the photoinduced change of the magnetic order-the three-temperature model for interacting thermalized electron, spin and lattice reservoirs is bypassed. In ferromagnetic Eu 1 À x Gd x O, we thereby demonstrate the strengthening as well as the weakening of the magnetic order by B10% and within r3 ps by optically controlling the magnetic exchange interaction.
Bilayer graphene hosts valley-chiral one dimensional modes at domain walls between regions of different interlayer potential or stacking order. When such a channel is brought into proximity to a superconductor, the two electrons of a Cooper pair which tunnel into it move in opposite directions because they belong to different valleys related by the time-reversal symmetry. This is a kinetic variant of Cooper pair splitting, which requires neither Coulomb repulsion nor energy filtering but is enforced by the robustness of the valley isospin in the absence of atomic-scale defects. We derive an effective model for the guided modes in proximity to an s-wave superconductor, calculate the conductance carried by split and spin-entangled electron pairs, and interpret it as a result of local Andreev reflection processes, whereas crossed Andreev reflection is absent.PACS numbers: 72.80. Vp,74.45.+c,03.65.Ud Creating mobile nonlocal spin-entangled electrons in a transport experiment with the help of superconductornormal junctions has attracted a lot of attention in theory [1][2][3][4][5][6][7][8] and experiment [9][10][11][12][13][14] because the spin degree of freedom of the electron could serve as a solid-state qubit [15]. In the existing experiments, the envisaged process where a Cooper pair is split over two normal leads is crossed Andreev reflection (CAR) [16,17], which is enhanced by the repulsive electron-electron interaction on two quantum dots weakly coupled to the superconductor [1] or by energy filtering [2,18]. The basic mechanism of these entanglers is not very sensitive to the specific material used, i.e., the underlying band structure. It has been shown that characteristic features of new materials exhibiting Dirac-cones like graphene or topological insulators can be useful for splitting Cooper pairs [7,[19][20][21][22]. In these proposals, the efficiency of the splitting process, in the absence of interactions, relies on non-protected resonance conditions or the split Cooper pair is not spin-entangled due to spin-helicity or spinpolarization of the leads. Helical edge states of the quantum spin Hall regime have, however, been proposed to detect spin entanglement [8,23].Here, we propose to exploit the valley degree of freedom in bilayer graphene (BG), where valley-chiral, spindegenerate one-dimensional (1D) channels are formed at domain walls. Such domain walls can be engineered by switching the sign of an interlayer voltage or by reversing the stacking order [24,25]. If brought into proximity to a superconductor, the pairs emitted into the channel are split, i.e., two electrons propagate in different directions but remain spin-entangled since, as required by time-reversal symmetry, the two electrons forming the Cooper pair in the superconductor are from different valleys [26]. As long as the valley degree of freedom is robust, the splitting efficiency is unity, independent of resonance conditions. The device extends the upcoming "valleytronics" in graphene [27] to nonlocal Einstein-Podolsky-Rosen pairs. A...
We study the critical Josephson current flowing through a double quantum dot weakly coupled to two superconducting leads. We use analytical as well as numerical methods to investigate this setup in the limit of small and large bandwidth leads in all possible charging states, where we account for on-site interactions exactly. Our results provide clear signatures of nonlocal spin-entangled pairs, which support interpretations of recent experiments [Deacon, R. S. et al., Nat. Commun. 6, 7446 (2015)]. In addition, we find that the ground state with one electron on each quantum dot can undergo a tunable singlet-triplet phase transition in the regime where the superconducting gap in the leads is not too large, which gives rise to an additional new signature of nonlocal Cooper pair transport.
We propose a tunable topological Josephson junction in silicene where electrostatic gates could switch between a trivial and a topological junction. These aspects are a consequence of a tunable phase transition of the topologically confined valley-chiral states from a spin-degenerate to a spin-helical regime. We calculate the Andreev bound states in such a junction analytically using a low-energy approximation to the tight-binding model of silicene in proximity to s-wave superconductors as well as numerically in the short-and long-junction regime and in the presence of intervalley scattering. Combining topologically trivial and non-trivial regions, we show how intervalley scattering can be effectively switched on and off within the Josephson junction. This constitutes a topological Josephson junction with an electrically tunable quasiparticle poisoning source. arXiv:1808.02809v1 [cond-mat.mes-hall]
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