Linear Band Structure Research Articles

Experimental and theoretical investigation of nanodiamond insertion on the interlayer interaction in multilayer stacking graphene

Multilayer stacked graphene (Gr) exhibits a wider range of novel properties than monolayer Gr, and it can be further applied to larger scales. Moreover, by modulating the interlayer structure and interactions, we can move beyond the energetically favorable AB stacking and explore unprecedented phenomena. In this study, nanodiamonds (ND) were introduced as nanospacers to manipulate the microstructure of multilayer Gr stacking. The presence of nanospacers increases the interlayer spacing and reduces interlayer interaction, which retrieves the linear band structure. The Gr and ND (Gr–ND) stacking structure was fabricated experimentally using a layer-by-layer transfer method. Raman spectroscopy confirms a decrease in interlayer interaction within the stacking structure, and atomic force microscopy reveals an expansion of the interlayer distance, contributing to the weakening of interlayer interactions. Theoretical analysis was conducted by combining molecular dynamic simulation and a continuum model to understand the experimentally observed change in microstructure caused by ND insertion. The ND density and size determine the interlayer distance, indicating the preference for configuration with lower system energy. The guideline for forming Gr–ND stacking structures obtained in this study can contribute to the property modulation and application of multilayer stacked Gr.

Realization of a two-dimensional Weyl semimetal and topological Fermi strings

A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials.

PtTe2 photodetectors with negative photoconductivity under different wavelength laser irradiation

Platinum telluride (PtTe2), as a newly emerged type II Dirac semimetal, has attracted much attention. Due to its excellent linear dispersive band structure and ultra-high carrier mobility, PtTe2 show great potential in the new quantum phenomena, topological phase transitions, unconventional superconductivity and other photodetecting fields. In this paper, we report the negative photoelectric effect of photodetectors based on the type II Dirac semimetal PtTe2 film under the laser irradiation of 405 nm, 532 nm and 808 nm. Under the condition of applying 1 V voltage, the R of the PtTe2 device under 405 nm laser irradiation was 20 A/W and the D* was 6.9×109 Jones; the R of the PtTe2 device under 532 nm laser irradiation was 1.3 A/W and the D* was 4.1×108 Jones; the R of the PtTe2 device under 808 nm laser irradiation was 0.6 A/W and the D* was 2.1×108 Jones. Under normal atmospheric pressure and room temperature conditions, we observed a negative photoconductivity phenomenon for the PtTe2 device, where the change in photoconductivity is attributed to the light-induced desorption of O or H2O molecules from the surface of the material. In addition, a single-site scanning imaging system based on a PtTe2 photodetector was designed to test the detection capability of the device. The analysis of the photocurrent effect produced by the PtTe2 device further deepens the understanding of the photoconductivity mechanism in the field of photodetection.

On a Nonlinear Locally Resonant Metamaterial With Resistance-Inductance Shunt

Abstract Numerous recent works have established the potential of various types of metamaterials for simultaneous vibration control and energy harvesting. In this paper, we investigate a weakly nonlinear metamaterial with electromechanical (EM) local resonators coupled to a resistance-inductance shunt circuit, a system with no previous examination in the literature. An analytical solution is developed for the system, using the perturbation method of multiple scales, and validated through direct numerical integration. The resulting linear and nonlinear band structures are used for parametric analysis of the system, focusing on the effect of resonator and shunt circuit parameters on band gap formation and vibration attenuation. This band structure analysis informs further study of the system through wavepacket excitation as well as spectro-spatial analysis. The voltage response of the system is studied through spatial profiles and spectrograms to observe the effects of shunt inductance, nonlinearity, and their interactions. Results describe the impact of adding a shunted inductor, including significant changes to the band structure; multiple methods of tuning band gaps and pass bands of the system; and changes to wave propagation and voltage response. The results demonstrate the flexibility of the proposed metamaterial and its potential for both vibration control and energy harvesting, specifically compared to a previously studied system with resistance-only shunt.

Energy-efficient sintering-free Chemically synthesized carbon nanofibers for high-performance supercapacitors

This work introduces a cost-effective method to synthesize carbon nanofibers (CNFs) at a lower temperature (∼100 °C), addressing the synthesis cost and precursor selection challenges associated with traditional methods requiring temperatures of ⁓3000 °C. Acid-etched banana fibers served as sustainable precursors, while KClO3 acted as a liquid carbonization medium. The resulting CNFs (referred MCCNFs), exhibited hollow structures with intertwined tubular morphology (OD ∼20–40 nm and ID∼ 5–10 nm). They possessed a hexagonal porous structure (P63/mmc) with a high specific surface area (152.09 m2g-1), significant pore volume (0.459 ccg−1), and impressive tensile strength exceeding 2.71 TPa. Smaller intensity ratio of D and G bands (0.396) confirmed their purity and structural integrity. MCCNFs displayed semi-metallic behaviour with superior electrical conductivity (0.0386 S-m−1), validated by ab initio electronic band structure calculations. UV–vis absorption spectra revealed a linear band structure. Notably, MCCNFs displayed a record-breaking capacitance of 1412.5 Fg-1 at 5 Ag-1, making them promising for supercapacitor electrode applications. Furthermore, these CNFs were used to fabricate a flexible all-solid-state supercapacitor, successfully powering 6 LEDs for 10 s. Hence, this work set a pave for low-cost, low-temperature synthesis of CNFs having superior physical and chemical properties and immense promise for advancing energy storage applications.

Modulations for Quantum Electronic Material Transports by Vacuum Annealing Methods

Gapless linear band structure quantum electronic materials play important roles in quantum computing developments due to their novel superb electronic transport properties and two-dimensional (2D) gateable device possibility. However, the sensitive surface of quantum electronic materials easily reacted with ambient air molecules so as to change their intrinsic transport properties. We review a series of our works to show the importance of surface reactions in quantum electronic materials, such as graphene. By using different electronic transport facilities and in situ vacuum annealing methods, we are able to gently clean the absorbed air molecules on graphene so as to change the carrier densities and the electronic transport properties, which is a great improvement for 2D-quantum material-based quantum chips and computing devices.

A new method of anterior talofibular ligament reconstruction: Arthroscopically artificial ligament reconstruction with tensional remnant-repair

PurposeTo investigate the clinical effects of arthroscopically artificial ligament reconstruction with tensional remnant-repair in patients who are obese, and/or with demand for highly intensive sports, and/or with poor-quality ligament remnants. MethodsA retrospective case series study was performed on patients treated by arthroscopically anterior talofibular ligament (ATFL) reconstruction with tensional remnant repair technique from January 2019 to August 2021. General data, including demographics, surgical time, and postoperative adverse events, were recorded. The American Orthopaedic Foot and Ankle Society score (AOFAS), foot and ankle ability measure (FAAM), visual analog scale (VAS), and anterior talar translation were measured preoperatively and at 6 weeks, 3 months, and 2 years postoperatively. Ultrasonography examination was performed preoperatively and 2 years postoperatively to evaluate the ATFL. Data were analyzed using SPSS 19.0. F test was used to analyze the pre- and postoperative VAS, FAAM, and AOFAS scores. The significance was set at p < 0.05. ResultsThere were 20 males and 10 females among the patients with a mean age of (30.71 ± 5.81) years. The average surgical time was (40.21 ± 8.59) min. No adverse events were observed after surgery. At 2 years postoperatively, the anterior talar translation test showed grade 0 laxity in all patients. VAS score significantly decreased from preoperatively to 6 weeks, 3 months, and 2 years postoperatively (p < 0.001). Improvement of FAAM score and the AOFAS score from preoperatively to 6 weeks, 3 months, and 2 years postoperatively was statistically significant (p < 0.001). At 3 months postoperatively, most patients (23/30) could return to their pre-injured activities of daily living status. At 2 years postoperatively, all patients were able to return to their pre-injured activities of daily living status, and almost every patient (18/19) who expected highly intensive sports returned to sports with only 1 obese patient failing to achieve the goal. The ultrasonography examination at 2 years postoperatively showed that there was a linear band structure of soft tissue on the tension-rich fiber tape image from the fibular to the talar attachment sits of ATFL. ConclusionThe novel arthroscopically artificial ligament reconstruction with tensional remnant-repair technique for ATFL achieved satisfactory clinical outcomes in the short and medium term after operation, and allowed early return to pre-injured activities, which could be a reliable option for patients with chronic lateral ankle instability.

Multiband flattening and linear Dirac band structure in graphene with impurities

Flat bands in the energy spectrum have attracted a lot of attention in recent years because of their unique properties and promising applications. Special arrangement of impurities on monolayer graphene are proposed to generate multiflat bands in the electronic band structure. In addition to the single midgap states in the spectrum of graphene with low hydrogen density, we found closely spaced bands around the Fermi level with increasing impurity density, which are similar to discrete lines in the spectrum of quantum dots, as well as the unusual Landau-level energy spectrum of graphene in the presence of a strong magnetic field. The presence of flat bands crucially depends on whether or not there are odd or even electrons of H(F) atoms bound to graphene. Interestingly, we found that a fully hydrogenated (fluoridated) of a hexagon of graphene sheet with six hydrogen (fluorine) atoms sitting on top and bottom in consecutive order exhibits Dirac cones in the electronic band structure with a $20%$ smaller Fermi velocity as compared to the pristine graphene. Functionalizing graphene introduces various C-C bond lengths resulting in nonuniform strains. Such a nonuniform strain may induce a giant pseudomagnetic field in the system, resulting in quantum Hall effect.

Discrete breathers in a mechanical metamaterial.

We consider a previously experimentally realized discrete model that describes a mechanical metamaterial consisting of a chain of pairs of rigid units connected by flexible hinges. Upon analyzing the linear band structure of the model, we identify parameter regimes in which this system may possess discrete breather solutions with frequencies inside the gap between optical and acoustic dispersion bands. We compute numerically exact solutions of this type for several different parameter regimes and investigate their properties and stability. Our findings demonstrate that upon appropriate parameter tuning within experimentally tractable ranges, the system exhibits a plethora of discrete breathers, with multiple branches of solutions that feature period-doubling and symmetry-breaking bifurcations, in addition to other mechanisms of stability change such as saddle-center and Hamiltonian Hopf bifurcations. The relevant stability analysis is corroborated by direct numerical computations examining the dynamical properties of the system and paving the way for potential further experimental exploration of this rich nonlinear dynamical lattice setting.

Terahertz cyclotron emission from two-dimensional Dirac fermions

Since the emergence of graphene, we have seen several proposals for the realization of Landau lasers tunable over the terahertz frequency range. The hope was that the non-equidistance of the Landau levels from Dirac fermions would suppress the harmful non-radiative Auger recombination. Unfortunately, even with this non-equidistance an unfavorable non-radiative process persists in Landau-quantized graphene, and so far no cyclotron emission from Dirac fermions has been reported. One way to eliminate this last non-radiative process is to sufficiently modify the dispersion of the Landau levels by opening a small gap in the linear band structure. A proven example of such gapped graphene-like materials are HgTe quantum wells close to the topological phase transition. In this work, we experimentally demonstrate Landau emission from Dirac fermions in such HgTe quantum wells, where the emission is tunable by both the magnetic field and the carrier concentration. Consequently, these results represent an advance in the realization of terahertz Landau lasers tunable by magnetic field and gate-voltage.

Electronic properties of graphene/CdX (X=S, Se, and Te) semiconductor heterostructure and a proposal of all-optical injection and detection of electron spins in graphene

Graphene was predicted to have a long spin relaxation time in the range of microseconds to milliseconds due to its weak spin-orbit coupling and hyperfine interaction. However, very short spin relaxation times were measured experimentally on the order of nanoseconds. Magnetic proximity effect from spin valves were considered strongly to influence the electronic and spin properties in graphene. Here, a graphene/nonmagnetic semiconductor CdX heterostructure is proposed to eliminate the magnetic effect and to fulfill optical injection of spin polarization. Based on the first-principles calculation within the density functional theory, we perform a systematic study on the structural, electronic properties and band structures of the heterostructures, and find that graphene on CdX (X = S, Se and Te) slabs preserves its linear Dirac band structure within the bandgap of CdX, and a built-in electric field occurs near the interface. The built-in electric field drives spin polarized electrons into graphene preferably. An optical detection scheme of spin relaxation time of graphene is proposed based on Faraday and Hanle effects. • A graphene/nonmagnetic semiconductor heterostructure constructed to eliminate strong magnetic effects on spin relaxation time of electrons in graphene. • First-principles calculations reveal Graphene on CdX (X = S, Se and Te) slabs preserves its linear Dirac band structure which locates within the bandgap of CdX. • A built-in electric field exists within interface, which favor spin polarized electrons transported into graphene. • A new scheme of all-optical injection and detection of electron spin is proposed to measure true long spin relaxation time of graphene.

Thermoelectric properties of two-dimensional materials with combination of linear and nonlinear band structures

We investigate thermoelectric (TE) properties of two-dimensional materials possessing two Dirac bands (a Dirac band) and a nonlinear band within the three-(two-)band model using linearized Boltzmann transport theory and relaxation time approximation. In the three-band model, we find that combinations of Dirac bands with a heavy nonlinear band, either a parabolic or a pudding-mold band, does not give much difference in their TE performance. The apparent difference only occurs in the position of the nonlinear band that leads to the maximum figure of merit (ZT). The optimum ZT of the three-band model consisting of a nonlinear band is found when the nonlinear band intersects the Dirac bands near the Fermi level. By removing the linear conduction band, or, in other words, transforming the three-band model to the two-band model, we find better TE performance (in terms of higher ZT values) in the two-band model than in the three-band model for various band gaps and effective masses of the nonlinear band.

Detection of high energy ionizing radiation using deeply depleted graphene–oxide–semiconductor junctions

Graphene’s linear band structure and two-dimensional density of states provide an implicit advantage for sensing charge. Here, these advantages are leveraged in a deeply depleted graphene–oxide–semiconductor (D2GOS) junction detector architecture to sense carriers created by ionizing radiation. Specifically, the room temperature response of a silicon-based D2GOS junction is analyzed during irradiation with 20 MeV Si4+ ions. Detection was demonstrated for doses ranging from 12 to 1200 ions with device functionality maintained with no substantive degradation. To understand the device response, D2GOS pixels were characterized post-irradiation via a combination of electrical characterization, Raman spectroscopy, and photocurrent mapping. This combined characterization methodology underscores the lack of discernible damage caused by irradiation to the graphene while highlighting the nature of interactions between the incident ions and the silicon absorber.

Conical diffraction modulation in fractional dimensions with a [formula omitted]-symmetric potential

This study analyzes conical diffraction in space-fractional parity-time (PT) symmetry, which arises in a fractional Schrödinger equation with a periodic PT-symmetric potential. The peculiar characteristics of the corresponding Floquet Bloch modes are also examined in detail. We find that the band structure in such a model is different from that in regular PT symmetry, especially the critical point, which becomes symmetric linear for the one-dimensional case, and cone-like for the two-dimensional case. During propagation, the linear band structure has peculiar properties, including splitting or diffraction-free propagation, preferential propagation, and unidirectional propagation. Whether the cone-like band structure leads to cone diffraction or nondiffracting propagation depends on which Floquet Bloch modes are excited by the input. Our results may have potential application in light modulation in PT symmetry and provide a new platform to study other exciting topics related to conical diffraction.

Harnessing rotational geometry to design reconfigurable dispersion and refractive index in nonlinear acoustic metamaterials

We investigate 1D and 2D monatomic lattice structures composed of in-plane rotators coupled by angled elastic linkages and explore their reconfigurable dispersion, negative refraction, amplitude-dependent dynamics, and acoustoelastic effect. At small amplitude, the linear band structure can be configured to be either acoustic or optical in nature by changing the connecting locations on the rotators, which corresponds to positive and negative refractive index. An interface problem between two rotator lattices with opposite dispersion type is modeled, and illustrates negative refraction in numerical simulations—experimental measurements are in progress. Its frequency–transmission relation matches the linear theory. At higher amplitude, the hardening nonlinearity shifts the dispersion and induces amplitude-dependent transmission. A novel nonlinear phenomenon–amplitude saturation (a constant far-field transmission independent of input amplitude) is observed when the transmitted wave falls in the nonlinear stopband and simultaneously the linear passband of the receiving lattice. We analyze the nonlinear effects via perturbation methods and propose a framework for evanescent-specific nonlinear waves to explain the saturation phenomenon. Additionally, we observe a strong acoustoelastic effect in chiral-patterned rotator lattices, where a static stretch can shift the equilibrium positions of the rotators and morph the band structure from acoustic to optical.

Thermoelectric Properties of Two-Dimensional Materials with Combination of Linear and Nonlinear Band Structures

Thermoelectric Properties of Two-Dimensional Materials with Combination of Linear and Nonlinear Band Structures

Enhanced Hot Carrier Up‐Conversion in Graphene By Quantum Dot Coating

AbstractGraphene with unique linear band structure and enhanced electron–electron interaction exhibits peculiar hot‐carrier generation. The thermalized hot carriers in graphene have potential to achieve high energy which is well above energy of the incident photons. Together with its outstanding optical and electrical properties, the exotic thermalization of hot carriers makes graphene a promising material for applications in optical up‐conversion devices and ultrafast photonics. However, the excitation efficiency of hot carriers is limited by its weak absorption and atomically thin light–matter interaction length. Here, the efficient hot‐carrier amplification is demonstrated in graphene by adjacent quantum dots (QD). With pump–probe experiments, this hot carrier amplification is found to stem from the charge collection of the two‐photon excited hot carriers in QD. Taking advantages of strong absorption of QD, the hot‐carrier excitation efficiency of graphene can be efficiently improved under near‐infrared sub‐bandgap excitation of QD. The transferred hot carriers are with energy higher than those originally excited in graphene, leading to raised hot‐carrier temperature and enhanced hot‐carrier up‐conversion. This research demonstrates QD's immense potential to realize high efficiency hot‐carrier injection in graphene‐based devices, which will promote their ability for hot‐carrier‐driven devices, optical up‐conversion devices and high‐frequency optoelectronics.

Possible dimensionality transition behavior in localized plasmon resonances of confinement-controlled graphene devices

We investigated the dimensionality transition behavior of graphene localized plasmon resonances in confinement-controlled graphene devices. We first demonstrated a possibility of dimensionality transition, based on the devices carrier-density dependence, from a two-dimensional plasmon resonance to a one-dimensional plasmon one. We fabricated optical transparent devices and electrical transport devices on the same optical transparent wafer. These devices allow detailed control and analysis between carrier density and plasmon resonance peak positions. The carrier density from (two-dimensional) to constant (one-dimensional) is consistent with the theoretical predictions based on the Dirac Fermion carriers in linear-band structure materials.

Linear and Nonlinear Optical Properties of Graphene: A Review

This article gives a brief overview of the linear and nonlinear optical response of graphene and its applications. Graphene exhibits unique physical properties. The charge carriers in graphene follow the relativistic massless Dirac equation; thus, they are known as the Dirac fermions. These Dirac fermions in graphene exhibit peculiar linear band structure at the Dirac point in the first Brillouin zone. These carriers exhibit novel linear and nonlinear optical properties in the presence of an intense electromagnetic field. The unique optical properties (linear and nonlinear) are a consequence of the unusual low-energy electronic band structure of graphene. The linear and nonlinear optical response of graphene can be tuned in the terahertz regime by the application of magnetic field. The uniqueness of graphene is shown by different findings in nonlinear optics such as four-wave mixing, saturation and reverse saturation absorption, two-photon absorption and optical limiting, among others. Therefore, the present review article describes the recent development in the field of linear and nonlinear optics of graphene. The different applications of graphene in the context of optics have been highlighted.

Anomalous linear magnetoresistance in high-quality crystalline lead thin films

Intriguing novel phenomena in lead films inspire new understanding of quantum physics, such as quantum size effect and quantum phase transitions etc. The improvement of the sample quality makes it even more promising to explore the intrinsic properties in two-dimensional system. In this paper, we show that the crystalline interfacial striped incommensurate layer can increase the quality of the lead films and significantly enhance the magnitude of magnetoresistance. By performing systematic transport measurement, a predominant anomalous linear magnetoresistance is revealed, and the widely used Parish-Littlewood model and Abrikosov's explanation fail to describe the observation. Instead, we propose a new model of linear magnetoresistance based on linear band structure, which shows a good agreement with the experimental results. Our studies reveal a novel origin of linear magnetoresistance which may also be helpful to understand the linear magnetoresistance in other materials with linear dispersion of electronic structure.