Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics

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Phase relations in superconductor-normal metal-superconductor tunnel junctions (1901.05426v2)

Yu. S. Barash

2019-01-16

The phase difference , between the superconducting terminals in superconductor-normal metal-superconductor tunnel junctions (SINIS), incorporates the phase differences across thin interfaces of constituent junctions and the phase incursion between the side faces of the central electrode of length . It is demonstrated here that pass through over their proximity-reduced domain twice, there and back, while changes over the single period. Two corresponding solutions, that describe the double-valued order-parameter dependence on , jointly form the single-valued dependence on , operating in two adjoining regions of . The phase incursion plays a crucial role in creating such a behavior. The current-phase relation is composed of the two solutions and, at a fixed small , is characterized by the phase-dependent effective transmission coefficient.

Thermopower and thermal conductance of a superconducting quantum point contact (1901.10065v3)

Sergey S. Pershoguba, Leonid I. Glazman

2019-01-29

We find the charge and heat currents caused by a temperature difference applied to a superconducting point contact or to a quantum point contact between a superconducting and normal conductors. The results are formulated in terms of the properties of the electron scattering matrix of the quantum point contact in its normal state, and are valid at any transmission coefficient. In the low-transmission limit, the new theory provides reliable results, setting the limits for the use of the popular method of tunnel Hamiltonian.

Colloquium: Atomic spin chains on surfaces (1904.09941v1)

Deung-Jang Choi, Nicolas Lorente, Jens Wiebe, Kirsten von Bergmann, Alexander F. Otte, Andreas J. Heinrich

2019-04-22

In the present Colloquium, we focus on the properties of 1-D magnetic systems on solid surfaces. From the emulation of 1-D quantum phases to the potential realization of Majorana edge states, spin chains are unique systems to study. The advent of scanning tunnelling microscope (STM) based techniques has permitted us to engineer spin chains in an atom-by-atom fashion via atom manipulation and to access their spin states on the ultimate atomic scale. Here, we present the current state of research on spin correlations and dynamics of atomic spin chains as studied by the STM. After a brief review of the main properties of spin chains on solid surfaces, we classify spin chains according to the coupling of their magnetic moments with the holding substrate. This classification scheme takes into account that the nature and lifetimes of the spin-chain excitation intrinsically depend on the holding substrate. We first show the interest of using insulating layers on metals, which generally results in an increase in the spin state's lifetimes such that their quantized nature gets evident and they are individually accessible. Next, we show that the use of semiconductor substrates promises additional control through the tunable electron density via doping. When the coupling to the substrate is increased for spin chains on metals, the substrate conduction electron mediated interactions can lead to emergent exotic phases of the coupled spin chain-substrate conduction electron system. A particularly interesting example is furnished by superconductors. Magnetic impurities induce states in the superconducting gap. Due to the extended nature of the spin chain, the in-gap states develop into bands that can lead to the emergence of 1-D topological superconductivity and, consequently to the appearance of Majorana edge states.

Braiding Majorana corner modes in a two-layer second-order topological insulator (1904.07822v2)

Tudor E. Pahomi, Manfred Sigrist, Alexey A. Soluyanov

2019-04-16

The recent advances in the field of topological materials have established a novel understanding of material physics. Besides theoretical achievements, a number of proposals for decoherence-protected topological quantum computation were provided. It is, however, a yet unanswered question, what material could be the most feasible candidate in engineering the building blocks of a quantum computer (qubits). Here we propose a possible answer by describing a device based on a two-dimensional second-order topological insulator with particle-hole symmetry (PHS). This material has one-dimensional boundaries, but exhibits two zero-dimensional Majorana quasiparticles localized at the corners of a square-shaped sample. The two states reside at zero energy as long as PHS is conserved, whereas their corner-localization can be adjusted by in-plane magnetic fields. We consider an adiabatic cycle performed on the degenerate ground-state manifold and show that it realizes the braiding of the two zero-energy corner modes. We find that each zero-mode accumulates a non-trivial statistical phase within a cycle, which confirms that, indeed, PHS ensures non-Abelian Majorana excitation braiding in the proposed device. The fractional statistics of the corner states opens the possibility to perform logical operations and, ultimately, might enable building robust qubits for large scale implementations. We also suggest possible paths for experimental realizations of this proposal.

Higher-Order Topological Insulators in Quasicrystals (1904.09932v1)

Rui Chen, Chui-Zhen Chen, Jin-Hua Gao, Bin Zhou, Dong-Hui Xu

2019-04-22

We propose the higher-order topological insulators with a quantized quadrupolar moment can be realized in quasicrystals. As a specific example, we consider a quasicrystalline Ammann-Beenker tiling with a square-shaped boundary. Most saliently, we find that topological corner states emerge when the site density exceeds a threshold value. Further, we theoretically design an electric circuit of the Ammann-Beenker tiling and show that the topological corner states in quasicrystals can be realized in classical electric circuits for the first time. We confirm that our findings of topological corner states are generic and can also be applied to the Penrose tiling and other aperiodic structures such as amorphous systems.

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