Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics

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Visualizing Encapsulated Graphene, its Defects and its Charge Environment by Sub-Micrometer Resolution Electrical Imaging (1811.05912v2)

Michael A. Altvater, Tianhui Zhu, Guohong Li, Kenji Watanabe, Takashi Taniguchi, Eva Y. Andrei

2018-11-14

Devices made from two-dimensional (2D) materials such as graphene or transition metal dichalcogenides possess interesting electronic properties that can become accessible to experimental probes when the samples are protected from deleterious environmental effects by encapsulating them between hexagonal boron nitride (hBN) layers. While the encapsulated flakes can be detected through post-processing of optical images or confocal Raman mapping, these techniques lack the sub-micrometer scale resolution to identify tears, structural defects or impurities, which is crucial for the fabrication of high-quality devices. Here we demonstrate a simple method to visualize such buried flakes with sub-micrometer resolution, by combining Kelvin force probe microscopy (KPFM) with electrostatic force microscopy (EFM). KPFM, which measures surface potential fluctuations, is extremely effective in spotting charged contaminants within and on top of the heterostructure, making it possible to distinguish contaminated regions in the buried flake. When applying a tip bias larger than the surface potential fluctuations, EFM becomes extremely efficient in highlighting encapsulated flakes and their sub-micron structural defects. We show that these imaging modes, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for locating encapsulated conductors, for visualizing nanometer scale defects and bubbles, and for characterizing their local charge environment.

Maxwell plates and phonon fractionalization (1907.06620v1)

Kai Sun, Xiaoming Mao

2019-07-15

In the past a few years, topologically protected mechanical phenomena have been extensively studied in discrete lattices and networks, leading to a rich set of discoveries such as topological boundary/interface floppy modes and states of self stress, as well as one-way edge acoustic waves. In contrast, topological states in continuum elasticity without repeating unit cells remain largely unexplored, but offer wonderful opportunities for both new theories and broad applications in technologies, due to their great convenience of fabrication. In this paper we examine continuous elastic media on the verge of mechanical instability, extend Maxwell-Calladine index theorem to continua in the nonlinear regime, classify elastic media based on whether stress can be fully released, and identify two types of elastic media with topological states. The first type, which we name ``Maxwell plates'', are in strong analogy with Maxwell lattices, and exhibit a sub-extensive number of holographic floppy modes. The second type, which arise in thin plates with a small bending stiffness and a negative Gaussian curvature, exhibit fractional excitations and topological degeneracy, in strong analogy to spin liquids and dimerized spin chains.

Superinjection in diamond p-i-n diodes: bright single-photon electroluminescence of color centers beyond the doping limit (1804.01066v2)

Igor A. Khramtsov, Dmitry Yu. Fedyanin

2018-04-03

Efficient generation of single photons on demand at a high repetition rate is a key to the practical realization of quantum-communication networks and optical quantum computations. Color centers in diamond are considered to be the most promising platform for building such single-photon sources owing to the outstanding emission properties of color centers at room temperature. However, their efficient electrical excitation remains a challenge due to the inability to create a high density of free electrons in diamond. Here, we show that using the self-gating effect in a diamond p-i-n diode, one can overcome the doping problem and inject four orders of magnitude more carriers into the i-region of the diamond diode than the doping of the n-region allows. This high density of free electrons can be efficiently used to boost the single-photon electroluminescence process and enhance the brightness of the single-photon source by more than three orders of magnitude. Moreover, we show that such a high single-photon emission rate can be achieved at exceptionally low injection current densities of only 0.001 A/mm, which creates the backbone for the development of low-power and cost-efficient diamond quantum optoelectronic devices for quantum information technologies.

Exact nonequilibrium transport in the topological Kondo effect (1610.03064v2)

B. Béri

2016-10-10

A leading candidate for experimental confirmation of the non-local quantum dynamics of Majorana fermions is the topological Kondo effect, predicted for mesoscopic superconducting islands connected to metallic leads. We identify an anisotropic, Toulouse-like, limit of the topological Kondo problem where the full nonequilibrium conductance and shot noise can be calculated exactly. Near the Kondo fixed point, we find novel asymptotic features including a universal conductance scaling function, and fractional charge quantisation observable via the Fano factor. In the universal regime, our results apply for generic anisotropy and even away from the Kondo limit as long as the system supports an emergent topological Kondo fixed point. Our approach thus provides key new qualitative insights and exact expressions for quantitative comparisons to future experimental data.

Coherent states in magnetized anisotropic 2D-Dirac materials (1907.06551v1)

Erik Díaz-Bautista, Maurice Oliva-Leyva, Yajaira Concha-Sánchez, Alfredo Raya

2019-07-15

In this work, we construct coherent states for electrons in anisotropic 2D-Dirac materials immersed in a uniform magnetic field perpendicularly oriented to the sample. In order to describe the bidimensional effects on electron dynamics in a semiclassical approach, we adopt the symmetric gauge vector potential to describe the external magnetic field through a vector potential. By solving a Dirac-like equation with an anisotropic Fermi velocity, we identify two sets of scalar ladder operators that allow us to define generalized annihilation operators, which are generators of either the Heisenberg-Weyl or su(1,1) algebra. We construct both bidimensional and su(1,1) coherent states as eigenstates of such annihilation operators with complex eigenvalues. In order to illustrate the effects of the anisotropy on these states, we obtain their probability density and mean energy value. Depending upon the anisotropy, expressed by the ration between the Fermi velocities along the - and -axes, the shape of the probability density is modified on the -plane with respect to the isotropic case and according to the classical dynamics.

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