Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Nonequilibrium states driven by both electric bias voltages $V$ and temperature differences $\Delta T$ (or thermal voltages $eV_T\equiv k_B\Delta T$) are unique probes of various systems. Whereas average currents $I(V,V_T)$ are traditionally measured in majority of experiments, an essential part of nonequilibrium dynamics, stored particularly in fluctuations, remains largely unexplored. Here we focus on Majorana quantum dot devices, specifically on their differential shot noise $\partial S^>(V,V_T)/\partial V$, and demonstrate that in contrast to the differential electric or thermoelectric conductance, $\partial I(V,V_T)/\partial V$ or $\partial I(V,V_T)/\partial V_T$, it reveals a crossover from thermoelectric to pure thermal nonequilibrium behavior. It is shown that this Majorana crossover in $\partial S^>(V,V_T)/\partial V$ is induced by an interplay of the electric and thermal driving, occurs at an energy scale determined by the Majorana tunneling amplitude, and exhibits a number of universal characteristics which may be accessed in solely noise experiments or in combination with measurements of average currents.

Abstract: Dynamically tunable nanoengineered structures for coloration show promising applications in sensing, displays, and communication. However, their potential challenge remains in having a scalable manufacturing process over large scales in tens of cm of area. For the first time, we report a novel approach for fabricating chromogenic nanostructures that respond to mechanical stimuli by utilizing the fluidic properties of polydimethylsiloxane (PDMS) as a substrate and the interfacial tension of liquid metal-based plasmonic nanoparticles. Relying on the PDMS tunable property and a physical deposition method, our approach is single-step, scalable, and does not rely on high carbon footprint lithographic processes. By tuning the oligomer content in PDMS, we show that varieties of structural colors covering a significant gamut in CIE coordinates are achieved. We develop a model which depicts the formation of Ga nanodroplets from the capillary interaction of oligomers in PDMS with Ga. We showcase the capabilities of our processing technique by presenting prototypes of reflective displays and sensors for monitoring body parts, smart bandages, and the capacity of the nanostructured film to map force in real time. These examples illustrate this technology's broad range of applications, such as large-area displays, devices for human-computer interactions, healthcare, and visual communication.

Abstract: Band topology of materials describes the extent Bloch wavefunctions are twisted in momentum space. Such descriptions rely on a set of topological invariants, generally referred to as topological charges, which form a characteristic class in the mathematical structure of fiber bundles associated with the Bloch wavefunctions. For example, the celebrated Chern number and its variants belong to the Chern class, characterizing topological charges for complex Bloch wavefunctions. Nevertheless, under the space-time inversion symmetry, Bloch wavefunctions can be purely real in the entire momentum space; consequently, their topological classification does not fall into the Chern class, but requires another characteristic class known as the Stiefel-Whitney class. Here, in a three-dimensional acoustic crystal, we demonstrate a topological nodal-line semimetal that is characterized by a doublet of topological charges, the first and second Stiefel-Whitney numbers, simultaneously. Such a doubly charged nodal line gives rise to a doubled bulk-boundary correspondence: while the first Stiefel-Whitney number induces ordinary drumhead states of the nodal line, the second Stiefel-Whitney number supports hinge Fermi arc states at odd inversion-related pairs of hinges. These results establish the Stiefel-Whitney topological charges as intrinsic topological invariants for topological materials, with their unique bulk-boundary correspondence beyond the conventional framework of topological band theory.

Abstract: The magic angle twisted bilayer systems give rise to many exotic phenomena in two-dimensional electronic or photonic platforms. Here, we study the twisted near-field energy radiation between graphene metasurfaces with nonequilibrium drifted Dirac electrons. Anomalously, we find unconventional radiative flux that directs heat from cold to hot. This far-from-equilibrium phenomenon leads to a heating-cooling transition beyond a thermal magic twist angle, facilitated by twist-induced photonic topological transitions. The underlying mechanism is related to the spectrum match and mismatch caused by the cooperation between the non-reciprocal nature of drifted plasmon polaritons and their topological features. Furthermore, we report the unintuitive distance dependence of radiative energy flux under large twist angles. The near-field radiation becomes thermal insulating when increasing to a critical distance, and subsequently reverses the radiative flux to increase the cooling power as the distance increases further. Our results indicate the promising future of nonequilibrium drifted and twisted devices and pave the way towards tunable radiative thermal management.

Abstract: We investigate squeezing of magnons in a conical spin spiral configuration. We find that while the energy of magnons propagating along the $\boldsymbol{k}$ and the $-\boldsymbol{k}$ directions can be different due to the non-reciprocal dispersion, these two modes are connected by the squeezing, hence can be described by the same squeezing parameter. The squeezing parameter diverges at the center of the Brillouin zone due to the translational Goldstone mode of the system, but the squeezing also vanishes for certain wave vectors. We discuss possible ways of detecting the squeezing.

Abstract: Spiking neural networks aim to emulate the brain's properties to achieve similar parallelism and high-processing power. A caveat of these neural networks is the high computational cost to emulate, while current proposals for analogue implementations are energy inefficient and not scalable. We propose a device based on a single magnetic tunnel junction to perform neuron firing for spiking neural networks without the need of any resetting procedure. We leverage two physics, magnetism and thermal effects, to obtain a bio-realistic spiking behavior analogous to the Huxley-Hodgkin model of the neuron. The device is also able to emulate the simpler Leaky-Integrate and Fire model. Numerical simulations using experimental-based parameters demonstrate firing frequency in the MHz to GHz range under constant input at room temperature. The compactness, scalability, low cost, CMOS-compatibility, and power efficiency of magnetic tunnel junctions advocate for their broad use in hardware implementations of spiking neural networks.

Abstract: Controlling the charge density in low-dimensional materials with an electrostatic potential is a powerful tool to explore and influence their electronic and optical properties. Conventional solid gates impose strict geometrical constraints to the devices and often absorb electromagnetic radiation in the infrared (IR) region. A powerful alternative is ionic liquid (IL) gating. This technique only needs a metallic electrode in contact with the IL and the highest achievable electric field is limited by the electrochemical interactions of the IL with the environment. Despite the excellent gating properties, a large number of ILs is hardly exploitable for optical experiments in the mid-IR region, because they typically suffer from low optical transparency and degradation in ambient conditions. Here, we report the realization of two electrolytes based on bromide ILs dissolved in polymethyl methacrylate (PMMA). We demonstrate that such electrolytes can induce state-of-the-art charge densities as high as $20\times10^{15}\ \mathrm{cm^{-2}}$. Thanks to the low water absorption of PMMA, they work both in vacuum and in ambient atmosphere after a simple vacuum curing. Furthermore, our electrolytes can be spin coated into flat thin films with optical transparency in the range from 600 cm$^{-1}$ to 4000 cm$^{-1}$. Thanks to these properties, the electrolytes are excellent candidates to fill the gap as versatile gating layers for electronic and mid-IR optoelectronic devices.

Abstract: What defines the lifetime of electronic states in artificial quantum structures? We measured the spectral widths of resonant eigenstates in a circular, CO-based quantum corral on a Cu(111) surface and found that the widths are related to the size of the corral and that the line shape is essentially Gaussian. A model linking the energy dependence with the movement of single surface electrons shows that the observed behavior is consistent with lifetime limitations due to interaction with the corral walls.

Abstract: The non-Hermitian skin effects are representative phenomena intrinsic to non-Hermitian systems: the energy spectra and eigenstates under the open boundary condition (OBC) drastically differ from those under the periodic boundary condition (PBC). Whereas a non-trivial topology under the PBC characterizes the non-Hermitian skin effects, their proper measure under the OBC has not been clarified yet. This paper reveals that topological enhancement of non-normality under the OBC accurately quantifies the non-Hermitian skin effects. Correspondingly to spectrum and state changes of the skin effects, we introduce two scalar measures of non-normality and argue that the non-Hermitian skin effects enhance both macroscopically under the OBC. We also show that the enhanced non-normality correctly describes phase transitions causing the non-Hermitian skin effects and reveals the absence of non-Hermitian skin effects protected by average symmetry. The topological enhancement of non-normality governs the perturbation sensitivity of the OBC spectra and the anomalous time-evolution dynamics through the Bauer-Fike theorem.

Abstract: Bilayers consisting of two-dimensional (2D) electron and hole gases separated by a 10 nm thick AlGaAs barrier are formed by charge accumulation in epitaxially grown GaAs. Both vertical and lateral electric transport are measured in the millikelvin temperature range. The conductivity between the layers shows a sharp tunnel resonance at a density of $1.1 \cdot 10^{10} \text{ cm}^{-2}$, which is consistent with a Josephson-like enhanced tunnel conductance. The tunnel resonance disappears with increasing densities and the two 2D charge gases start to show 2D-Fermi-gas behavior. Interlayer interactions persist causing a positive drag voltage that is very large at small densities. The transition from the Josephson-like tunnel resonance to the Fermi-gas behavior is interpreted as a phase transition from an exciton gas in the Bose-Einstein-condensate state to a degenerate electron-hole Fermi gas.