Latest Papers in Condensed Matter Physics # Mesoscale And Nanoscale Physics

### [Coexistence of Majorana and topologically nontrivial Andreev bound states in 1D superconductors](http://arxiv.org/abs/1907.05416v1) (1907.05416v1) Pasquale Marra, Muneto Nitta 2019-07-11 > The unambiguous detection of Majorana bound states in nanowires and in magnetic atom chains is hindered by the possible presence of near-zero-energy Andreev bound states which have similar experimental signatures. These near-zero energy states are expected to be topologically trivial. Here, we report the theoretical prediction of topologically nontrivial Andreev bound states in one-dimensional superconductors with spatially varying magnetic fields. These states correspond to a novel topological invariant defined in a synthetic two-dimensional space, the particle-hole Chern number, which is an analogue of the spin Chern number in quantum spin Hall systems. Topologically nontrivial Andreev bound states and Majorana bound states have distinct features and are topologically nonequivalent. Yet they can coexist in the same system, have similar spectral signatures, and materialize with the concomitant opening of the particle-hole gap. Consequently, the simultaneous observation of a zero-bias peak and the closing and reopening of the gap cannot be considered an exclusive fingerprint of Majorana bound states. In contrast to Majorana states, which appear simultaneously at both edges and at zero energy, nontrivial Andreev states may appear with different energies at the opposite edges of the system. ### [Superconductor to insulator transition in wafer-scale NbSe2](http://arxiv.org/abs/1907.05392v1) (1907.05392v1) Cliff Chen, Protik Das, Ece Aytan, Weimin Zhou, Justin Horowitz, Alexander A. Balandin, Roger Lake, Peng Wei 2019-07-11 > Superconducting tunneling devices, such as Josephson junctions etc., that are made of triplet or topological superconductors may enable new functionalities compared to those conventional ones made of s-wave superconductors. The tunnel barrier, an insulating or normal material layer separating two superconductors, is an important component for the Josephson tunneling devices. Thin layers of NbSe2 have been shown to manifest Ising superconductivity behavior, which can give rise to topological superconductivity driven by large magnetic exchange field. Here we demonstrate the control between superconducting and insulating phases of NbSe2 in epitaxially grown samples with wafer-scale uniformity. We provide the first electrical transport and Raman spectroscopy characterizations of the insulating phase, and show that the energy shift of the Raman modes is closely tied to the phase transition. Our observation paves the way for high quality heterojunction tunnel barriers to be seamlessly built into NbSe2 itself, thereby enabling highly scalable tunneling devices for superconductor-based quantum electronics. ### [Topological properties and helical edge states in elastic phononic waveguides with Kekulé distortion](http://arxiv.org/abs/1907.05389v1) (1907.05389v1) Ting-Wei Liu, Fabio Semperlotti 2019-07-11 > This study investigates the topological behavior of an elastic phononic structure characterized by Kekul\'e distortion. The truss-like elastic waveguide consists of a hexagonal unit cell whose geometric dimensions are intentionally perturbed according to a Kekul\'e scheme. The resulting structure exhibits an effective Hamiltonian that resembles a quantum spin Hall system, hence suggesting that the waveguide can support helical topological edge states. This study also reveals the existence of finer structures including frequency splitting of pseudospin states in Kekul\'e distorted lattices as well as their 6-lobe alternating pattern. Important insights are also provided concerning the, so far considered, topologically indistinguishable states in Kekul\'e lattices and their connection to the existence of gapless edge states. ### [Non-Hermitian Majorana modes protect degenerate steady states](http://arxiv.org/abs/1904.07481v2) (1904.07481v2) Simon Lieu 2019-04-16 > We introduce non-Hermitian generalizations of Majorana zero modes (MZMs) which appear in the topological phase of a weakly dissipative Kitaev chain coupled to a Markovian bath. Notably, the presence of MZMs ensures that the steady state in the absence of decoherence events is two-fold degenerate. Within a stochastic wavefunction approach, the effective Hamiltonian governing the coherent, non-unitary dynamics retains BDI classification of the closed limit, but belongs to one of four non-Hermitian "flavors" of the ten-fold way. We argue for the stability of MZMs due to a generalization of particle-hole symmetry, and uncover the resulting topological phase diagram. Qualitative features of our study generalize to two-dimensional chiral superconductors. The dissipative superconducting chain can be mapped to an Ising model in a complex transverse field, and we discuss potential signatures of the degeneracy. ### [Magnetization Dynamics in 1D Chains of Ferromagnetic Nanoparticles Coupled with Dipolar Interactions: Blocking Temperature](http://arxiv.org/abs/1907.05382v1) (1907.05382v1) F. Vernay, H. Kachkachi 2019-07-11 > There is so far no clear-cut experimental analysis that can determine whether dipole-dipole interactions enhance or reduce the blocking temperature ![](http://latex2png.com/output//latex_c7f03202317581a283adc93648a207fb.png) of nanoparticle assemblies. It seems that the samples play a central role in the problem and therefore, their geometry should most likely be the key factor in this issue. Yet, in a previous work, J\"onsson and Garcia-Palacios did investigate theoretically this problem in a weak-interaction limit and without the presence of an external DC field. Based on symmetry arguments they reached the conclusion that the variation of the relaxation rate is monotonous. In the presence of an external magnetic field we show that these arguments may no longer hold depending on the experimental geometry. Therefore, the aim of this paper is to evaluate the variation of ![](http://latex2png.com/output//latex_c7f03202317581a283adc93648a207fb.png) for a model system consisting of a chain of ferromagnetic nanoparticles coupled with long-range dipolar interaction with two different geometries. Rather than addressing a quantitative analysis, we focus on the qualitative variation of ![](http://latex2png.com/output//latex_c7f03202317581a283adc93648a207fb.png) as a function of the interparticle distance a and of the external field ![](http://latex2png.com/output//latex_8d4a18b217e0f4bffc4a2e5fa8a7a196.png). The two following situations are investigated: a linear chain with a longitudinal axial anisotropy in a longitudinal DC field and a linear chain with a longitudinal axial anisotropy in a transverse field.

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