Enhancing Thermopower and Nernst Signal of High-Mobility Dirac Carriers by Fermi Level Tuning in the Layered Magnet EuMnBi2

Dirac/Weyl semimetals hosting linearly dispersing bands have received recent attention for potential thermoelectric applications, since their ultrahigh-mobility carriers could generate large thermoelectric and Nernst power factors. To optimize these efficiencies, the Fermi energy needs to be chemically controlled in a wide range, which is generally difficult in bulk materials because of disorder effects from the substituted ions. Here it is shown that the Fermi energy is tunable across the Dirac point for layered magnet EuMnBi₂ by partially substituting Gd³⁺ for Eu²⁺ in the insulating block layer, which dopes electrons into the Dirac fermion layer without degrading the mobility. Clear quantum oscillation observed even in the doped samples allows to quantitatively estimate the Fermi energy shift and optimize the power factor (exceeding 100 µ W K⁻² cm⁻¹ at low temperatures) in combination with the first-principles calculation. Furthermore, it is shown that Nernst signal steeply increases with decreasing carrier density beyond a simple theoretical prediction, which likely originates from the field-induced gap reduction of the Dirac band due to the exchange interaction with the Eu moments. Thus, the magnetic block layer provides high controllability for the Dirac fermions in EuMnBi₂, which would make this series of materials an appealing platform for novel transport phenomena.     

Field-Effect Chiral Anomaly Devices with Dirac Semimetal

Charge-based field-effect transistors (FETs) greatly suffer from unavoidable carrier scattering and heat dissipation. Analogous to valley degree of freedom in semiconductors, chiral anomaly current in Weyl/Dirac semimetals is theoretically predicted to be nearly nondissipative over long distances, but still lacks experimental ways to efficiently control its transport. Here, field-effect chirality devices are demonstrated with Dirac semimetal PtSe₂, in which its Fermi level is close to the Dirac point in the conduction band owing to intrinsic defects. The chiral anomaly is further corroborated by the planar Hall effect and nonlocal valley transport measurement, which can also be effectively modulated by external fields, showing robust nonlocal valley transport with micrometer diffusion length. Similar to charge-based FETs, the chiral conductivity in PtSe₂ devices can be modulated by electrostatic gating with an ON/OFF ratio of more than 10³. Basic logic functions in the devices are also demonstrated with electric and magnetic fields as input signals.     

Topological Surface States of Multicomponent Thermoelectrics Based on Bismuth Telluride

Semiconductors - The surface states of Dirac fermions in p-Bi2Te3, p-Bi2 –xSbxTe3 –ySey, and p-Bi2 –x–ySnxGeyTe3 thermoelectrics were studied and its topological...     

Transport properties of heteroepitaxial films based on bismuth telluride in strong magnetic fields

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi₂Te₃ heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.     

Surface Instability and Chemical Reactivity of ZrSiS and ZrSiSe Nodal‐Line Semimetals

Materials exhibiting nodal‐line fermions promise superb impact on technology for the prospect of dissipationless spintronic devices. Among nodal‐line semimetals, the ZrSiX (X = S, Se, Te) class is the most suitable candidate for such applications. However, the surface chemical reactivity of ZrSiS and ZrSiSe has not been explored yet. Here, by combining different surface‐science tools and density functional theory, it is demonstrated that the formation of ZrSiS and ZrSiSe surfaces by cleavage is accompanied by the washing up of the exotic topological bands, giving rise to the nodal line. Moreover, while the ZrSiS has a termination layer with both Zr and S atoms, in the ZrSiSe surface, reconstruction occurs with the appearance of Si surface atoms, which is particularly prone to oxidation. It is demonstrated that the chemical activity of ZrSiX compounds is mostly determined by the interaction of the Si layer with the ZrX sublayer. A suitable encapsulation for ZrSiX should not only preserve their surfaces from interaction with oxidative species, but also provide a saturation of dangling bonds with minimal distortion of the surface.     

On the morphology of the interlayer surface and micro-Raman spectra of layered films in topological insulators based on bismuth telluride

Resonant micro-Raman spectra and the morphology of the interlayer Van der Waals surface are studied for layered thin films of n-Bi₂Te₃ and solid solutions based on Bi2Te3. It is found that the composition, thickness, surface morphology, and the method of obtaining films affect the relative intensity of Raman phonons, which are sensitive to the topological surface states of Dirac fermions.     

Ultrasensitive Room‐Temperature Terahertz Direct Detection Based on a Bismuth Selenide Topological Insulator

Despite their huge application capabilities, millimeter‐ and terahertz‐wave photodetectors still face challenges in the detection scheme. Topological insulators (TIs) are predicted to be promising candidates for long‐wavelength photodetection, due to the presence of Dirac fermions in their topologically protected surface states. However, photodetection based on TIs is usually hindered by the large dark current, originating from the mixing of bulk states with topological surface states (TSSs) in most realistic samples of TIs. Here millimeter and terahertz detectors based on a subwavelength metal–TI–metal (MTM) heterostructure are demonstrated. The achieved photoresponse stems from the asymmetric scattering of TSS, driven by the localized surface plasmon‐induced terahertz field, which ultimately produces direct photocarriers beyond the interband limit. The device enables high responsivity in both the self‐powered and bias modes even at room temperature. The achieved responsivity is over 75 A/W, with response time shorter than 60 ms in the self‐powered mode. Remarkably, the responsivity increases by several orders of magnitude in the biased configuration, with the noise‐equivalent power (NEP) of 3.6 × 10^?13 W Hz^?1/2 and a detectivity of 2.17 × 10¹¹ cm Hz^?1/2 W^?1 at room temperature. The detection performances open a way toward realistic exploitation of TIs for large‐area, real‐time imaging within long‐wavelength optoelectronics.     

狄拉克光子晶体在面发射激光器中的应用

光子晶体由于具有可设计、可调谐以及对光的超 常调控等优异性能,近年已成为重 要的光学拓扑态研究平台。狄拉克(Dirac)锥型线性色散的奇异特性可实现丰富的物理现 象,诸如Dirac振荡、拓扑边缘态、零折射率等,更是凝聚态拓扑现象的物理根源。本文重 点对近年Dirac光子晶体在面发射激光器中的应用进行了详细介绍,指出将Dirac光子晶 体引入到半导体激光器中,可 实现大面积超低阈值、高亮度、单纵模和单横模的拓扑腔面发射激光器(topological cavity surface emitting lasers,TCSELs),同时对基于Dirac光子晶体原理发展出的TCSELs进行了总结与展望。     

基于狄拉克半金属的太赫兹探测器研究进展简介

太赫兹波具有安全性好、透射性强、指纹特性等特点，在无损探测、雷达成像、空间通信等领域展现出巨大的应用前景，在现阶段的科学研究中热度不减。狄拉克半金属具有量子反常霍尔效应、零带隙受拓扑保护和超高迁移率等特性，在太赫兹探测领域展现出非凡特性，为探索实现室温太赫兹探测提供了新思路。本文介绍了基于狄拉克半金属材料太赫兹光电探测器的研究现状，讨论了器件性能与探测机理，并对其在太赫兹光电探测领域的发展前景进行了展望。     

Topological Surface States of Dirac Fermions in n-Bi2Te3 –ySey Thermoelectrics

Semiconductors - In n-Bi2Te3 and n-Bi2Te3 –ySey thermoelectrics, the surface states of Dirac fermions of the interlayer Van der Waals surface (0001) are studied by scanning tunneling...     

Cyclotron resonance of dirac fermions in InAs/GaSb/InAs quantum wells

The band structure of three-layer symmetric InAs/GaSb/InAs quantum wells confined between AlSb barriers is analyzed theoretically. It is shown that, depending on the thicknesses of the InAs and GaSb layers, a normal band structure, a gapless state with a Dirac cone at the center of the Brillouin zone, or inverted band structure (two-dimensional topological insulator) can be realized in this system. Measurements of the cyclotron resonance in structures with gapless band spectra carried out for different electron concentrations confirm the existence of massless Dirac fermions in InAs/GaSb/InAs quantum wells.     

HBT中基区内建电场的物理机制及其理论分析

计算了具有线性Ｇｅ 分布的ＳｉＧｅ 基区价带有效态密度和空穴浓度的分布，分析了基区内建电场的物理机制，分别采用Ｂｏｌｔｚｍ ａｎｎ 统计和Ｆｅｒｍ ｉ Ｄｉｒａｃ统计给出了内建电场分布的表达式并进行了计算，最后讨论了基区重掺杂对电场的影响     

Doping efficiency and energy-level scheme in C60F48-doped zinc–tetraphenylporphyrin films

High resolution synchrotron-based core level spectroscopy was used to examine the energy level alignment at the interface of zinc–tetraphenylporphyrin films doped by the surface acceptor C₆₀F₄₈. Two distinct fluorofullerene charge states were identified, corresponding to ionized and neutral molecules, and their relative concentration as a function of coverage was used to evaluate the probability of occupation of the acceptor lowest unoccupied molecular orbital (LUMO). From an initial acceptor energy of ?0.25 eV, the C₆₀F₄₈ LUMO shifts upwards with coverage due to a doping-induced interfacial dipole potential, and stabilization of the LUMO at an energy 0.45 eV above the Fermi energy was obtained. While the energy difference upon saturation is consistent with the results obtained for other donor–acceptor systems that have been interpreted as Fermi level pinning, the present work shows that the energy offset is a direct consequence of the interplay between Fermi–Dirac statistics in combination with the interfacial dipole potential.     

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> solid solution

It is shown that the Seebeck coefficient α, the power factor α²σ, and the density-of-states effective mass m/m ₀ in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α²σ, and m/m ₀ is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.     

Dirac Fermions in Blue Phosphorene Monolayer

2D materials beyond graphene and in particular 2D semiconductors have raised interest due to their unprecedented electronic properties, such as high carrier mobility or tunable bandgap. Blue phosphorene is an allotrope of black phosphorene that resembles graphene as it presents a honeycomb structure. However, it is known to have semiconductor character and the crucial point is to determine whether this hexagonal phase of phosphorene presents Dirac fermions as in graphene. Here, the first compelling experimental evidence of Dirac fermions in blue phosphorene layer grown on Cu(111) surface is presented. The results highlight the formation of a highly ordered blue phosphorene sheet with a clear Dirac cone at the high symmetry points of the Brillouin Zone. The charge carriers behave as massless relativistic particles. Therefore, all the expectations held for graphene, such as high-speed electronic devices based on ballistic transport at room temperature, may also be applied to blue phosphorene.     

Large Damping Enhancement in Dirac-Semimetal–Ferromagnetic-Metal Layered Structures Caused by Topological Surface States

This article reports damping enhancement in a ferromagnetic NiFe thin film due to an adjacent α-Sn thin film. Ferromagnetic resonance studies show that an α-Sn film separated from a NiFe film by an ultrathin Ag spacer can cause an extra damping in the NiFe film that is three times bigger than the intrinsic damping of the NiFe film. Such an extra damping is absent in structures where the α-Sn film interfaces directly with a NiFe film, or is replaced by a β-Sn film. The data suggest that the extra damping is associated with topologically nontrivial surface states in the topological Dirac semimetal phase of the α-Sn film. This work suggests that, like topological insulators, topological Dirac semimetal α-Sn may have promising applications in spintronics.     

Optimization of graphene-MoS2 barristor by 3-aminopropyltriethoxysilane (APTES)

We theoretically and experimentally investigated the influence of the Fermi level position of graphene relative to the Dirac point on the performance of a graphene/MoS₂ heterojunction barristor. A large Fermi level modulation (ΔE_F = 0.28 eV) of graphene, when the V_GS is changed between −20 V and +20 V, was theoretically predicted when the Fermi level is located at the Dirac point. For reference, ΔE_F = 0.11 eV when the Fermi level is far from the Dirac point. This prediction was experimentally proven using two kinds of barristors with pristine (strongly p-type) and 2.4% APTES-treated (intrinsic) graphene. The on/off-current ratio was improved by a factor of 32 (a 2.1-fold increase in the on-current density and a 15-fold increase in the off-current density) in the APTES-treated device, as compared to the control. Using a temperature-dependent current-voltage measurement, we quantitatively confirmed the larger modulation of the barrier height in the APTES-treated barristor (ΔE_F = 0.27 eV) compared to that of the control device (ΔE_F = 0.14 eV). This study can be used to guide the design and optimization of graphene-based heterojunction devices.     

Quantum state depressions in thin metal films with an indented surface

Modification of properties of metal films caused by indents on their surface are studied. It is shown that indents on a film surface lead to quantum state depression (QSD), i.e., a decrease in the density of quantum states of a free electron. The density of the wave vector in the k-space decreases throughout the Fermi sphere. At the same time, the total number of electrons is conserved, since the metal remains electrically neutral. According to the Pauli Exclusion Principle, some electrons will occupy states with higher wave numbers. The Fermi vector and the Fermi energy of the thin metal films increase and, therefore, the work function decreases. Experiments have demonstrated a decrease in the work function of the thin indented films of Au, Nb, Cr, and SiO₂. Experimental results are qualitatively consistent with the theoretical predictions.     

A strongly correlated electron state at one-dimensional quantum points

The method of exact diagnoalization is applied to find the distribution of electrons over single-particle states at a 1D quantum point and the Fourier spectrum of the electron density. It is shown that the distribution function contains an unexpected signularity at the Fermi level. It is found that (i) the singularity results from the Wigner electron ordering and (ii) the Wigner ordering involves suppression of spatial Fourier harmonics of the electron density observed when the interaction potential grows over the entire range of wave vectors except for the double Fermi wave vector. It is demonstrated that the distribution function calculated within the Luttinger model also has a singularity at the Fermi level and this singularity is of an irregular shape.     

Features of the Damping and Amplification of Terahertz Plasmon Eigenmodes in Graphene Taking the Spatial Dispersion into Account

Semiconductors - Abstract—The effect of charge-carrier drift on the plasmon excitation modes (plasmons) in a Dirac electron liquid in graphene with a displaced Fermi level is considered. The...