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

Abstract: The reliability of analysis is becoming increasingly important as point-of-care diagnostics are transitioning from single analyte detection towards multiplexed multianalyte detection. Multianalyte detection benefits greatly from complementary metal-oxide semiconductor (CMOS) integrated sensing solutions, offering miniaturized multiplexed sensing arrays with integrated readout electronics and extremely large sensor counts. The development of CMOS back end of line integration compatible graphene field-effect transistor (GFET) based biosensing has been rapid during the last few years, both in terms of the fabrication scale-up and functionalization towards biorecognition from real sample matrices. The next steps in industrialization relate to improving reliability and require increased statistics. Regarding functionalization towards truly quantitative sensors and on-chip bioassays with improved statistics require sensor arrays with reduced variability in functionalization. Such multiplexed bioassays, whether based on graphene or on other sensitive nanomaterials, are among the most promising technologies for label-free electrical biosensing. As an important step towards that, we report wafer-scale fabrication of CMOS integrated GFET arrays with high yield and uniformity, designed especially for biosensing applications. We demonstrate the operation of the sensing platform array with 512 GFETs in simultaneous detection for sodium chloride concentration series. This platform offers a truly statistical approach on GFET based biosensing and further to quantitative and multi-analyte sensing. The reported techniques can also be applied to other fields relying on functionalized GFETs, such as gas or chemical sensing or infrared imaging.

Abstract: Wigner crystals are predicted as the crystallization of the dilute electron gas moving in a uniform background when the electron-electron Coulomb energy dominates the kinetic energy. The Wigner crystal has previously been observed in the ultraclean two-dimensional electron gas (2DEG) present on the surface of liquid helium and in semiconductor quantum wells at high magnetic field. More recently, Wigner crystals have also been reported in WS2/WSe2 moir\'e heterostructures. ABC-stacked trilayer graphene on boron nitride (ABC-TLG/hBN) moir\'e superlattices provide a unique tunable platform to explore Wigner crystal states where the electron correlation can be controlled by electric and magnetic field. Here we report the observation of magnetic field stabilized Wigner crystal states in a ABC-TLG/hBN moir\'e superlattice. We show that correlated insulating states emerge at multiple fractional and integer fillings corresponding to v = 1/3, 2/3, 1, 4/3, 5/3 and 2 electrons per moir\'e lattice site under a magnetic field. These correlated insulating states can be attributed to generalized Mott states for the integer fillings (v = 1, 2) and generalized Wigner crystal states for the fractional fillings (v = 1/3, 2/3, 4/3, 5/3). The generalized Wigner crystal states are stabilized by a vertical magnetic field, and they are strongest at one magnetic flux quantum per three moir\'e superlattices. The correlated insulating states at v = 2 persists up to 30 Tesla, which can be described by a Mott-Hofstadter transition at high magnetic field. The tunable Mott and Wigner crystal states in the ABC-TLG/hBN highlight the opportunities to discover new correlated quantum phases due to the interplay between the magnetic field and moir\'e flatbands.

Abstract: Topology-related ideas might lead to noise-resilient quantum computing. For example, it is expected that the slow spatial exchange (`braiding') of Majorana zero modes in superconductors yields quantum gates that are robust against disorder. Here, we report our numerical experiments, which describe the dynamics of a Majorana qubit built from quantum dots controlled by time-dependent gate voltages. Our protocol incorporates non-protected control, braiding-based protected control, and readout, of the Majorana qubit. We use the Kitaev chain model for the simulations, and focus on the case when the main source of errors is quasistatic charge noise affecting the hybridization energy splitting of the Majorana modes. We provide quantitative guidelines to suppress both diabatic errors and disorder-induced qubit dephasing, such that a fidelity plateau is observed as the hallmark of the topological quantum gate. Our simulations predict realistic features that are expected to be seen in future braiding experiments with Majorana zero modes and other topological qubit architectures.

Abstract: The bulk-edge correspondence is one of the most important ingredients in the theory of topological phase of matter. While the bulk-edge correspondence is applicable for Hermitian junction systems where two subsystems with independent topological invariants are connected to each other, it has not been discussed for junction systems with non-Hermitian point-gap topological phases. In this Letter, based on analytical results obtained by the extension of non-Bloch band theory to junction systems, we establish the bulk-edge correspondence for point-gap topological phases in junction systems. Considering the eigenstates, further, we find that the non-Hermitian junction systems exhibit unique proximity effects.

Abstract: We show that the tunable gate voltage in n-doped AlGaAs/GaAs QW (quantum well) is a key in designing an efficient and ultrafast MRAM (magnetoresistive random access memory). The Rashba spin-orbit coupling in such QWs can be tuned appropriately by the gate voltage to create an intense spin-Hall field which in turns interacts with the ferromagnetic layer of the MRAM through the mechanism of spin orbit torque. The strong spin-Hall field leads to an infinitesimally small switching time of the MRAM. Our proposed MRAM is thus a better alternative to the conventional ferromagnetic/spin-Hall effect bi-layers MRAM for the reason that the switching time can be varied with ease, which is unfeasible in the later. Concisely, not only that this work signals a possibility to design an ultra-fast MRAM, but it also suggests a model to fabricate a tunable switching time MRAM.

Abstract: It has been known that the bulk-boundary correspondence (BBC) of the non-Hermitian skin effect is characterized by the topology of the complex eigenvalue spectra, while the topology of the wave function gives rise to Hermitian BBC with conventional boundary modes. In this work, we go beyond the known description of the non-Hermitian topological phase by discovering a new type of BBC that appears in generalized boundary conditions. The generalized Brillouin zone (GBZ) possesses non-trivial topological structures in the intermediate boundary condition between open and periodic boundary conditions. Unlike the conventional BBC, the topological phase transition is characterized by the generalized momentum touching of GBZ, which manifests as exceptional points. As a realization of our proposal, we suggest the non-reciprocal Kuramoto oscillator lattice, where the phase slips accompany the exceptional points as a signature of such topological phase transition. Our work establishes an understanding of non-Hermitian topological matter by complementing the non-Hermitian BBC as a general foundation of the non-Hermitian topological systems.

Abstract: The nonequilibrium behavior of Co/Cu/Co and Pt/Co/Cu/Co/Pt multilayer structures was studied by the Monte Carlo method for various types of magnetic anisotropy. An analysis of the results of calculations of the two-time autocorrelation function was carried out, and the evolution of structures from various initial states was considered. An analysis of the results shows aging with a slowdown in the correlation characteristics with an increase in the waiting time. The dependence of the aging characteristics of the studied structures on the film thickness is considered. There is a difference in behavior from bulk systems, aging in multilayer structures occurs in a wide temperature range at $T \leq T_c$ and not only at the ordering temperature $T_c$. Investigation of transport properties makes it possible to reveal nontrivial aging in the two-time dependence of magnetoresistance, as well as the influence of anisotropy and initial states on its values.

Abstract: Magnonics, an emerging field of Magnetism, studies spin waves (SWs) in nano-structures, with an aim towards possible applications. As information may be eventually transmitted with efficiency stored in the phase and amplitude of spin waves, a topic of interest within Magnonics is the propagation of SW modes. Thus, understanding mechanisms that may influence SW propagation is of interest. Here the effect of localized surface geometric defects on magnetostatic surface modes propagation is studied in ferromagnetic films and semi-infinite media. Theoretical results are developed that allow to calculate the scattering of these surface or Damon-Eshbach (DE) modes. A Green-Extinction theorem is used to determine the scattering of incident surface modes, through the determination of phase shifts of associated modes that are symmetric and anti-symmetric under inversion in the same geometry with geometric defects. Choosing localized symmetric depressions as geometric defects, scattering transmission coefficients are determined that show perfect transmission at specific frequencies or wave-lengths, that we associate with resonances in the system. Interestingly the system shows the appearance of localized modes in the depression regions, with associated discrete frequencies immersed in the continuum spectrum of these surface DE modes. These localized modes have a short wave-length content, and appear similarly in semi-infinite surfaces with depressions. The latter indicates that these types of scattering effects should appear in all surfaces with roughness or more pronounced geometric defects.

Abstract: The possibility of the appearance of a dielectric electron-hole liquid (EHL) in monolayers of transition metal dichalcogenides and heterostructures based on them is considered. It is shown that the coherent pairing of electrons and holes in them leads to the formation of a dielectric EHL when the degree of circular polarization of the exciting light exceeds a certain threshold value. Below this value, a metallic EHL is realized. Some possible physical manifestations of the transition between these two types of EHL are noted.