Multiband Systems Research Articles

Topological characterization of non-Hermitian multiband systems using Majorana's stellar representation

For topological characterization of non-Hermitian multiband systems, Majorana's stellar representation (MSR) is applied to 1D multiband models consisting of asymmetric nearest-neighbor hopping and imaginary on-site potentials. The number of edge states isolated from the continuous bulk bands in the complex energy plane is successfully linked with a topological invariant constructed from MSR. Specifically, the number of isolated edge states can be obtained from a winding number defined for the Majorana stars, which also allows for a geometric visualization of the topology related to the isolated edge modes. A remarkable success of our approach is that our winding number characterization remains valid even in the presence of exceptional points of the continuous bulk bands, where the Hamiltonian becomes non-diagonalizable and hence conventional topological invariants such as the Zak phase and the Chern number cannot be properly defined. Furthermore, cases with the so-called non-Hermitian skin effect are also studied, showing that the bulk-boundary correspondence between our defined winding numbers and isolated edge states can be restored. Of particular interest is a four-band example with an odd number of isolated edge states, where the Zak phase approach necessarily fails upon removing the skin effect, but our MSR-based characterization works equally well. For these reasons, our study is expected to be widely useful in topological studies of non-Hermitian multiband systems, regardless of the skin effect or the presence of the exceptional points in non-Hermitian systems.

Design and fabrication of a high-performance microstrip multiplexer using computational intelligence for multi-band RF wireless communications systems

In this paper, a mathematical model based on artificial neural network (ANN) is used to design, fabricate and measure a quad-channel microstrip multiplexer. Multi-layer perceptron (MLP) neural network trained with back-propagation algorithm is adopted to model a new microstrip bandpass filter (BPF) based on a novel stub-loaded open-loop resonator. Then, a design process is proposed to obtain four BPFs. Finally, using the obtained BPFs a high-performance microstrip multiplexer is designed. The designed multiplexer has the smallest size and the best insertion and return losses in comparison with the previous multiplexers. It operates at 2.2 GHz, 2.8 GHz, 3.5 GHz and 4 GHz, which are appropriate for multi-band RF wireless communications systems. To validate the ANN model and the simulation results, the proposed quad-channel microstrip multiplexer is fabricated and measured where a good agreement is achieved.

A 4.2-to-5.4 GHz stacked GaAs HBT power amplifier for C-band applications

PurposePower amplifiers (PAs) play an important role in wireless communications because they dominate system performance. High-linearity broadband PAs are of great value for potential use in multi-band system implementation. The purpose of this paper is to present a cascode power amplifier architecture to achieve high power and high efficiency requirements for 4.2∼5.4 GHz applications.Design/methodology/approachA common emitter (CE) configuration with a stacked common base configuration of heterojunction bipolar transistor (HBT) is used to achieve high power. T-type matching network is used as input matching network. To increase the bandwidth, the output matching networks are implemented using the two L-networks.FindingsBy using the proposed method, the stacked PA demonstrates a maximum saturated output power of 26.2 dBm, a compact chip size of 1.17 × 0.59 mm2 and a maximum power-added efficiency of 46.3 per cent. The PA shows a wideband small signal gain with less than 3 dB variation over working frequency. The saturated output power of the proposed PA is higher than 25 dBm between 4.2 and 5.4 GHz.Originality/valueThe technology adopted for the design of the 4.2-to-5.4 GHz stacked PA is the 2-µm gallium arsenide HBT process. Based on the proposed method, a better power performance of 3 dB improvement can be achieved as compared with the conventional CE or common-source amplifier because of high output stacking impedance.

Theoretical Possibilities for Flat Band Superconductivity

One novel arena for designing superconductors with high $T_C$ is the flat-band systems. A basic idea is that flat bands, arising from quantum mechanical interference, give unique opportunities for enhancing $T_C$ with (i) many pair-scattering channels between the dispersive and flat bands, and (ii) an even more interesting situation when the flat band is topological and highly entangled. Here we compare two routes, which comprise a multi-band system with a flat band coexisting with dispersive ones, and a one-band case with a portion of the band being flat. Superconductivity can be induced in both cases when the flat band or portion is "incipient" (close to, but away from, the Fermi energy). Differences are, for the multi-band case, we can exploit large entanglement associated with topological states, while for the one-band case a transition between different (d and p) wave pairings can arise. These hint at some future directions.

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1−xSx

Understanding superconductivity requires detailed knowledge of the normal electronic state from which it emerges. A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique scattering relevant for superconductivity. Here, we investigate the normal transport of superconducting FeSe$_{1-x}$S$_x$ across a nematic phase transition using high magnetic fields up to 69 T to establish the temperature and field-dependencies. We find that the nematic state is an anomalous non-Fermi liquid, dominated by a linear resistivity at low temperatures that can transform into a Fermi liquid, depending on the composition $x$ and the impurity level. Near the nematic end point, we find an extended temperature regime with $T^{1.5}$ resistivity. The transverse magnetoresistance inside the nematic phase has as a $H^{1.55}$ dependence over a large magnetic field range and it displays an unusual peak at low temperatures inside the nematic phase. Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder

Single-Layer Mode Composite Coplanar Waveguide Dual-Band Filter With Large Frequency Ratio

A dual-band filter of a large frequency ratio is proposed, studied, and developed in this article, which is based on mode composite coplanar waveguide (MCCPW). Both low and high bands of the dual-band filter can be designed independently with a high degree of design freedom. The filter can achieve more than ten times the frequency difference between the two bands, which is usually hard to accomplish by conventional dual-band filter design techniques. All harmonics between the two bands can be suppressed by a cascaded low-pass filter and impedance matching structure to form a qualified dual-band frequency response. The characteristic of the proposed MCCPW dual-band filter makes it a promising candidate for multiband systems, especially in 5G communication systems. The demonstrated MCCPW dual-band filter is fabricated with a single-layer substrate. Measured results are in good agreement with simulated results, which validate the feasibility and superiority of the proposed filtering technique.

Mutual coupling reduction of multiple antenna systems

A multi-band multi-antenna system has become an important trend in the development of mobile communication systems. However, strong mutual coupling tends to occur between antenna elements with a small space, distorting array antennas’ performance. Therefore, in the multiple-input multiple-output (MIMO) antenna system, high isolation based on miniaturization of the antenna array has been pursued. We study in depth the methods of decoupling between antenna elements. Reasons for the existence of mutual coupling and advantages of mutual coupling reduction are analyzed. Then the decoupling methods proposed in recent works are compared and analyzed. Finally, we propose a metasurface consisting of double-layer short wires, which can be applied to improve the port isolation of antennas arranged along the H-plane and E-plane. Results show that the proposed metasurface has good decoupling effect on a closely placed antenna array.

Robustness of unconventional s -wave superconducting states against disorder

We investigate the robustness against disorder of superconductivity in multiband systems where the fermions have four internal degrees of freedom. This permits unconventional $s$-wave pairing states, which may transform nontrivially under crystal symmetries. Using the self-consistent Born approximation, we develop a general theory for the effect of impurities on the critical temperature, and find that the presence of these novel $s$-wave channels significantly modifies the conclusions of single-band theories. We apply our theory to two candidate topological superconductors, YPtBi and Cu$_x$Bi$_2$Se$_3$, and show that the novel $s$-wave states display an enhanced resilience against disorder, which extends to momentum-dependent pairing states with the same crystal symmetry. The robustness of the $s$-wave states can be quantified in terms of their superconducting fitness, which can be readily evaluated for model systems.

Iterative Sub-Band and Power Allocation in Downlink Multiband NOMA

Nonorthogonal multiple access (NOMA) promises higher throughput than the conventional orthogonal access by multiplexing the signal of a weak user with that of a strong one. Identifying the right combination of users with an appropriate assignment of power appears to be the bottleneck in achieving optimal throughput in NOMA-driven systems. Despite the large volume of literature, it is still not well understood how the task of joint user selection and power allocation (PA) can be efficiently performed in multiband NOMA systems in the presence of a large number of users'. To address this task, we propose a variable PA (VPA) scheme across sub-bands and evaluate its performance. Contrary to previous studies, we find that the proposed VPA scheme significantly outperforms the previously known schemes when the number of users and variation in their associated signal-to-interference-plus-noise ratios (SINRs) are large. Our investigation also helps to visualize how the system throughput, the average number of selected users, and the system fairness change as functions of system parameters like SINRs and the number of system users.

Dirac points emerging from flat bands in Lieb-kagome lattices

The energy spectra for the tight-binding models on the Lieb and kagom\'e lattices both exhibit a flat band. We present a model which continuously interpolates between these two limits. The flat band located in the middle of the three-band spectrum for the Lieb lattice is distorted, generating two pairs of Dirac points. While the upper pair evolves into graphene-like Dirac cones in the kagom\'e limit, the low energy pair evolves until it merges producing the band-bottom flat band. The topological characterization of the Dirac points is achieved by projecting the Hamiltonian on the two relevant bands in order to obtain an effective Dirac Hamiltonian. The low energy pair of Dirac points is particularly interesting in this respect: when they emerge, they have opposite winding numbers, but as they merge, they have the same winding number. This apparent paradox is due to a continuous rotation of their states in pseudo-spin space, characterized by a winding vector. This simple, but quite rich model, suggests a way to a systematic characterization of two-band contact points in multiband systems.

Attosecond spectral singularities in solid-state high-harmonic generation

Strong-field-driven electric currents in condensed-matter systems are opening new frontiers in petahertz electronics. In this regime, new challenges are arising as the roles of band structure and coherent electron–hole dynamics have yet to be resolved. Here, by using high-harmonic generation spectroscopy, we reveal the underlying attosecond dynamics that dictates the temporal evolution of carriers in multi-band solid-state systems. We demonstrate that when the electron–hole relative velocity approaches zero, enhanced constructive interference leads to the appearance of spectral caustics in the high-harmonic generation spectrum. We introduce the role of the dynamical joint density of states and identify its mapping into the spectrum, which exhibits singularities at the spectral caustics. By studying these singularities, we probe the structure of multiple unpopulated high conduction bands. High-harmonic waves are generated from a MgO crystal under experimental conditions where the simple semi-classical analysis fails. High-harmonic generation spectroscopy directly probes the strong-field attosecond dynamics over multiple bands.

Tunneling and fluctuating electron-hole Cooper pairs in double bilayer graphene

A strong low-temperature enhancement of the tunneling conductance between graphene bilayers has been reported recently, and interpreted as a signature of equilibrium electron-hole pairing, first predicted in bilayers more than forty years ago but previously unobserved. Here we provide a detailed theory of conductance enhanced by fluctuating electron-hole Cooper pairs, which are a precursor to equilibrium pairing, that accounts for specific details of the multi-band double graphene bilayer system which supports several different pairing channels. Above the equilibrium condensation temperature, pairs have finite temporal coherence and do not support dissipationless tunneling. Instead, they strongly boost the tunneling conductivity via a fluctuational internal Josephson effect. Our theory makes predictions for the dependence of the zero bias peak in the differential tunneling conductance on temperature and electron-hole density imbalance that capture important aspects of the experimental observations. In our interpretation of the observations, cleaner samples with longer disorder scattering times would condense at temperatures $T_c$ up to $\sim 50 {\rm K}$, compared to the record $T_c \sim 1.5 $K achieved to date in the experiment.

The Role of Odd‐Frequency Pairing in Multiband Superconductors

AbstractRecent progress in the understanding of multiband superconductivity and its relationship to odd‐frequency pairing are reviewed herein. The discussion begins by reviewing the emergence of odd‐frequency pairing in a simple two‐band model, providing a brief pedagogical overview of the formalism. Several examples of multiband superconducting systems are examined, in each case describing both the origin of the band degree of freedom and the nature of the odd‐frequency pairing. Throughout, it is attempted to convey a unified picture of how odd‐frequency pairing emerges in these materials and propose that similar mechanisms are responsible for odd‐frequency pairing in several analogous systems: layered 2D heterostructures, double quantum dots, double nanowires, Josephson junctions, and systems described by isolated valleys in momentum space. In addition, experimental probes of odd‐frequency pairing in multiband systems are reviewed, focusing on hybridization gaps in the electronic density of states, paramagnetic Meissner effect, and Kerr effect.

Role of a higher-dimensional interaction in stabilizing charge density waves in quasi-one-dimensional NbSe3 revealed by angle-resolved photoemission spectroscopy

We revisit charge density wave (CDW) behavior in the archetypal quasi-one-dimensional (quasi-1D) material NbSe$_3$ by high-resolution angle-resolved photoemission spectroscopy measurements utilizing a microfocused laser with a photon energy of 6.3 eV. We present a detailed view of the electronic structure of this complex multiband system and unambiguously resolve CDW gaps at the Fermi level ($E_F$). By employing a tight-binding model, we argue that these gaps are the result of interband coupling between electronic states that reside predominantly on distinct 1D chains within the material. Two such localized states are found to couple to an electronic state that extends across multiple 1D chains, highlighting the importance of a higher-dimensional interaction in stabilizing the CDW ordering in this material. In addition, the temperature evolution of intrachain gaps caused by the CDW periodicities far below $E_F$ deviate from the behavior expected for a Peierls-type mechanism driven by nesting; the upper and lower bands of the renormalized CDW dispersions maintain a fixed peak-to-peak distance while the gaps are gradually removed at higher temperatures. This points toward a gradual loss of long-range phase coherence as the dominant effect in reducing the CDW order parameter, which may correspond to the loss of coherence between the coupled chains. Furthermore, one of the gaps is observed above the bulk and surface CDW transition temperatures, implying the persistence of short-range incoherent CDW order. The influence of such higher-dimensional interactions likely plays an important role in a range of low-dimensional systems.

Effective Floquet Hamiltonian in the low-frequency regime

We develop a theory to derive effective Floquet Hamiltonians in the weak drive and low-frequency regime. We construct the theory in analogy with band theory for electrons in a spatially-periodic and weak potential, such as occurs in some crystalline materials. As a prototypical example, we apply this theory to graphene driven by circularly polarized light of low intensity. We find an analytic expression for the effective Floquet Hamiltonian in the low-frequency regime which accurately predicts the quasienergy spectrum and the Floquet states. Furthermore, we identify self-consistency as the crucial feature effective Hamiltonians in this regime need to satisfy to achieve a high accuracy. The method is useful in providing a realistic description of off-resonant drives for multi-band solid state systems where light-induced topological band structure changes are sought.

Pairing and non-Fermi liquid behavior in partially flat-band systems: Beyond nesting physics

While multiband systems are usually considered for flat-band physics, here we study one-band models that have flat portions in the dispersion to explore correlation effects in the 2D repulsive Hubbard model in an intermediate coupling regime. The FLEX+DMFT~(the dynamical mean-field theory combined with the fluctuation exchange approximation) is used to show that we have a crossover from ferromagnetic to antiferromagnetic spin fluctuations as the band filling is varied, which triggers a crossover from triplet to singlet pairings with a peculiar filling dependence that is dominated by the size of the flat region in the dispersion. A curious manifestation of the flat part appears as larger numbers of nodal lines associated with pairs extended in real space. We further detect non-Fermi liquid behavior in the momentum distribution function, frequency dependence of the self-energy and spectral function. These indicate correlation physics peculiar to flat-band systems.

Multi-Band and High Gain Antenna Using AMC Ground Characterized With Four Zero-Phases of Reflection Coefficient

In this paper, a multi-band AMC-backed antenna system characterized with high gain and low-profile is proposed. It is constructed by placing a quad-band coplanar-waveguide (CPW) antenna over an AMC with four zero-phases in the reflection coefficient. The CPW antenna consists of an ornamental-pillar-shaped patch with the specially designed parasitic elements, and a defected ground structure (DGS), for the application in the frequency bands of 2.45, 3.5, 4.6 and 5.8 GHz. To improve the CPW antenna’s radiation performance, an artificial magnetic conductor (AMC) ground with four zero-phases of the reflection coefficient is designed and employed as the reflector. The AMC unit cell is composed of four metallic nested rings (FMNR), whose innermost ring is connected in series to four lumped capacitors. It is observed that the proposed quad-band AMC ground can not only increase the gains of the multi-band antenna greatly but also combine the two intermediate frequency bands of the antenna into a broadband one. The prototypes of the antenna and the AMC array with $5\times {5}$ unit cells were fabricated and measured. It is found that in comparison to the antenna without AMC, the AMC-backed antenna obtains a gain enhancement by amounts of 4.93, 5.92, 5.54 and 4.95 dB at the frequencies of 2.45, 3.5, 4.6 and 5.8 GHz. The 10-dB impedance bandwidths of the AMC-backed antenna include three bands of 2.13–2.87, 3.22–4.75, and 5.54–5.86 GHz, with the corresponding relative bandwidths of 14.8%, 38.4%, and 5.3%. The proposed antenna can be potentially applied to the applications in WLAN, WiMAX, and 5G mobile communication systems.

Recent Advancements in Defected Ground Structure-Based Near-Field Wireless Power Transfer Systems

The defected ground structure (DGS) technique enables miniaturization of the resonator which leads to the development of the compact near-field wireless power transfer (WPT) systems. In general, numerous challenges are inherent in the design of the DGS-based WPT systems and, hence, appropriate trade-offs for achieving optimal performance are required. Furthermore, the design advancements have led to the development of the DGS-based multi-band WPT systems to fulfill the needs of simultaneous data and power transfer. The innovations in the DGS-based WPT systems have also resulted in the definition of more commonly used figures-of-merit for the benchmarking of various performance metrics. The literature is replete with the design schemes to address one or more associated design challenges and successful WPT system realizations with enhanced performance. With this in mind, this paper touches upon the DGS-based WPTs developments and presents a concise report on the current state-of-the-art and future directions.

Tri-Band Band-Pass Filter Based on Multi-Mode Spoof Surface Plasmon Polaritons

This paper presents a novel tri-band bandpass filter based on multi-mode spoof surface plasmon polaritons (SSPPs) in microwave frequencies. The combination of multi-mode resonance and cutoff characteristic of SSPPs contributes to a tri-band bandpass response. The proposed SSPPs-based filter consists of two coplanar waveguides, two conversion parts and a SSPPs transmission structure made of five unit-cells. Each unit cell is a multi-mode resonator, which is constructed by a pair of oppositely oriented double-side folded strips, and the ends of folded strips are placed closely to form coupled-stubs. Two distinct passbands divided by a sharp stopband are produced under the cutoff frequency of SSPPs transmission structure. The third passband above the cutoff frequency is introduced by the resonant tunneling effect due to the self-resonant mode of proposed SSPPs unit cell. Dispersion relations, electric field distributions and proposed equivalent transmission line model give a physical insight into the filter operating mechanism. As a demonstration, a prototype of the proposed filter has been fabricated and measured. The measurement results demonstrate that the proposed filter has a tri-band bandpass performance with excellent frequency selectivity, which may be very useful in many applications, such as multi-band wireless systems.

Quantum oscillations and electronic structure in the large–Chern number semimetal RhSn

We report the magnetoresistance, Hall effect, de Haas-van Alphen (dHvA) oscillations and the electronic structures of single crystal RhSn, which is a typical material of CoSi family holding a large Chern number. The large unsaturated magnetoresistance is observed with B//[001]. The Hall resistivity curve indicates that RhSn is a multi-band system with high mobility. Evident quantum oscillations have been observed, from which the light effective masses are extracted. Ten fundamental frequencies are extracted after the fast Fourier transform analysis of the dHvA oscillations with B//[001] configuration. The two low frequencies F$_1$ and F$_2$ do not change obviously and the two high frequencies F$_9$ and F$_{10}$ evolve into four when B rotates from B//[001] to B//[110], which is consistent with the band structure in the first-principles calculations with spin-orbit coupling (SOC). The extracted Berry phases of the relative pockets show a good agreement with the Chern number $\pm4$ (with SOC) in the first-principles calculations. Above all, our studies indicate that RhSn is an ideal platform to study the unconventional chiral fermions and the surface states.