We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in a transverse dc electric field. When the twist angle is not very small, the electronic spectrum of ...the bilayer consists of four Dirac cones inherited from each graphene layer. An applied bias voltage leads to the appearance of two holelike and two electronlike Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to the exciton band-gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by spin-density-wave order. The gap depends on the twist angle and can be varied by a bias voltage. This result correlates well with recent transport measurements J.-B. Liu et al., Sci. Rep. 5, 15285 (2015)SRCEC32045-232210.1038/srep15285. Our proposal allows the coexistence of (i) an externally controlled semiconducting gap and (ii) a nontrivial multicomponent magnetic order. This is interesting for both fundamental research and applications.
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Deliveries of American weapons to Ukraine lead to an aggravation of tension between Russia and the United States. The uncontrolled use of defense products by Ukrainians threatens not only with the ...curtailment of previous agreements, but also with a high risk of a collision between the world’s leading nuclear powers. The U.S. Congress could require increased accountability for the use of weapons through the use of arms export control laws (H.R.13680, Leahy Law, S.3394). Nevertheless, under the conditions of polarization, the parties of Republicans and Democrats cannot agree even on issues of strategic importance. As shown in the article, from the very beginning of the conflict in the South-East of Ukraine in 2014, both parties have been actively supporting the supply of lethal weapons to Kyiv. Politicians reacted negatively to any attempt to reduce military aid or slow down its export. For this reason, they repeatedly made mutual accusations, accusing their colleagues of unwillingness to provide military support to Ukraine. The most striking episode of the impact of polarization on the problem of arms exports was the scandal with the alleged pressure of the U.S. President D. Trump on the Ukrainian leader V. A. Zelenskii, when the head of the White House was suspected of trying to diminish the former Vice-President J. Biden. Despite the failed impeachment attempt by the Democrats, D. Trump was forced to resume the supply of lethal weapons to Kyiv. The influence of polarization continued later, when conservative Republicans in the House of Representatives and the Senate spoke out against arms sales. In the article, it is shown that despite the attempts of individual politicians, the United States is unlikely to be able to increase control over the supply of weapons, since such a step will be perceived critically by the opposite side.
This brief review discusses electronic properties of mesoscopic graphene-based structures. These allow controlling the confinement and transport of charge and spin; thus, they are of interest not ...only for fundamental research, but also for applications. The graphene-related topics covered here are: edges, nanoribbons, quantum dots,
pn-junctions,
pnp-structures, and quantum barriers and waveguides. This review is partly intended as a short introduction to graphene mesoscopics.
Hysteresis underlies a large number of phase transitions in solids, giving rise to exotic metastable states that are otherwise inaccessible. Here, we report an unconventional hysteretic transition in ...a quasi-2D material, EuTe_{4}. By combining transport, photoemission, diffraction, and x-ray absorption measurements, we observe that the hysteresis loop has a temperature width of more than 400 K, setting a record among crystalline solids. The transition has an origin distinct from known mechanisms, lying entirely within the incommensurate charge density wave (CDW) phase of EuTe_{4} with no change in the CDW modulation periodicity. We interpret the hysteresis as an unusual switching of the relative CDW phases in different layers, a phenomenon unique to quasi-2D compounds that is not present in either purely 2D or strongly coupled 3D systems. Our findings challenge the established theories on metastable states in density wave systems, pushing the boundary of understanding hysteretic transitions in a broken-symmetry state.
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The prospect of using high-voltage GaAs drift step-recovery diodes for the formation of subnanosecond pulses is shown. The electrical circuit of a generator is described, which provides (with a total ...efficiency of at least 25%) the formation of pulses at a load of 50 Ω with an amplitude of up to 550 V, a voltage rise time of 0.43 ns, a full width at half maximum of 0.73 ns, and a repetition frequency of up to 200 kHz.
The results of numerical simulation of the current dependence of the efficiency of injection into the active area of a laser based on separate-confinement double heterostructures are reported. The ...feature of carrier transport through isotype
N
–
n
heterojunctions at the interface between the waveguide and active areas is demonstrated. Using the classic dependences of the Drude–Lorentz theory, the electron (σ
e
) and hole (σ
p
) scattering cross sections for a GaAs waveguide are estimated. Using the obtained values of σ
e
= 1.05 × 10
–18
cm
2
and σ
p
= 1.55 × 10
–19
cm
2
and the current dependences of the injection efficiency, the primary cause for confinement of the pulse power of the semiconductor lasers is determined. It is established that the internal optical loss is a minor fraction of the loss and the decisive contribution to saturation of the power–current (
P–I
) characteristics is made by the escape of holes to the waveguide.
The influence of an external electric field on the character of ion motion, diffusion coefficients, and diffusion activation energies in cubic ZrO2 doped with Y2O3 with varying dopant content, ...temperature, and electric field strength has been studied by the molecular dynamics method. The diffusion coefficient of oxygen vacancies for models with different Y2O3 contents in the temperature range 873–1273 K without an imposed field decreases with an increase in the molar fraction of yttrium oxide in the crystal, while the dependence of the diffusion coefficient of O2− ions has a maximum (2.4∙10−11 m2/s) with an yttrium oxide content of 3 mol%. The activation energy of ionic conductivity, estimated in the absence of a field on the basis of diffusion of O2− ions, increases from 0.45 eV at a Y2O3 content of 4.6 mol% to 0.61 eV at a dopant content of 15.7 mol%. An external electric field of 0.02–0.08 V/Å noticeably changes the frequency of jumps of oxygen ions. At the electric field strength of 0.08 V/Å, a dopant content of 4.6 mol%, and a temperature of 1073 K, the О2− diffusion coefficient increases to 1.41∙10−9 m2/s. The dependence of diffusion coefficients on the field strength is close to quadratic.
•Effect of external electric field on the diffusion coefficients and activation energies in YSZ was studied.•The intrinsic diffusion coefficients of oxygen ions and vacancies was calculated.•External electric field of 0.02–0.08 V/Å noticeably enhance oxygen ions diffusion.•The dependence of diffusion coefficients on the field strength is close to quadratic.
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