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.
Electronic spectrum of twisted bilayer graphene Sboychakov, A. O.; Rakhmanov, A. L.; Rozhkov, A. V. ...
Physical review. B, Condensed matter and materials physics,
08/2015, Letnik:
92, Številka:
7
Journal Article
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We study the electronic properties of twisted bilayer graphene in the tight-binding approximation. The interlayer hopping amplitude is modeled by a function which depends not only on the distance ...between two carbon atoms, but also on the positions of neighboring atoms as well. Using the Lanczos algorithm for the numerical evaluation of eigenvalues of large sparse matrices, we calculate the bilayer single-electron spectrum for commensurate twist angles in the range 1degrees lap theta 2272; 30degrees. We show that at certain angles theta greater than theta sub(c) approximately 1.89degrees the electronic spectrum acquires a finite gap, whose value could be as large as 80 meV. However, in an infinitely large and perfectly clean sample the gap as a function of theta behaves nonmonotonously, demonstrating exponentially large jumps for very small variations of theta. This sensitivity to the angle makes it impossible to predict the gap value for a given sample, since in experiment theta is always known with certain error. To establish the connection with experiments, we demonstrate that for a system of finite size L the gap becomes a smooth function of the twist angle. If the sample is infinite, but disorder is present, we expect that the electron mean-free path plays the same role as L. In the regime of small angles theta < theta sub(c) the system is a metal with a well-defined Fermi surface which is reduced to Fermi points for some values of theta. The density of states in the metallic phase varies smoothly with theta.
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.
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.
Half-metallicity (full spin polarization of the Fermi surface) usually occurs in strongly correlated electron systems. We demonstrate that doping a spin-density wave insulator in the weak-coupling ...regime may also stabilize half-metallic states. In the absence of doping, the spin-density wave is formed by four nested bands i.e., each band is characterized by charge (electron or hole) and spin (up or down) labels. Of these four bands, only two accumulate the charge carriers introduced by doping, forming a half-metallic two-valley Fermi surface. Depending on the parameters, the spin polarizations of the electronlike and holelike valleys may be either (i) parallel or (ii) antiparallel. The Fermi surface of (i) is fully spin polarized (similar to usual half-metals). Case (ii), referred to as "a spin-valley half-metal," corresponds to complete polarization with respect to the spin-valley operator. The properties of these states are discussed.
It is well known that, generically, one-dimensional interacting fermions cannot be described in terms of a Fermi liquid. Instead, they present a different phenomenology, that of a Tomonaga-Luttinger ...liquid: the Landau quasiparticles are ill defined, and the fermion occupation number is continuous at the Fermi energy. We demonstrate that suitable fine tuning of the interaction between fermions can stabilize a peculiar state of one-dimensional matter, which is dissimilar to both Tomonaga-Luttinger and Fermi liquids. We propose to call this state a quasi-Fermi liquid. Technically speaking, such a liquid exists only when the fermion interaction is irrelevant (in the renormalization group sense). The quasi-Fermi liquid exhibits the properties of both a Tomonaga-Luttinger liquid and a Fermi liquid. Similar to a Tomonaga-Luttinger liquid, no finite-momentum quasiparticles are supported by the quasi-Fermi liquid; on the other hand, its fermion occupation number demonstrates a finite discontinuity at the Fermi energy, which is a hallmark feature of a Fermi liquid. A possible realization of the quasi-Fermi liquid with the help of cold atoms in an optical trap is discussed.
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.
The heat transport in a system of S=1/2 large-J Heisenberg spin chains, describing closely Sr(2)CuO(3) and SrCuO(2) cuprates, is studied theoretically at T≪J by considering interactions of the ...bosonized spin excitations with optical phonons and defects. Treating rigorously the multiboson processes, we derive a microscopic spin-phonon scattering rate that adheres to an intuitive picture of phonons acting as thermally populated defects for the fast spin excitations. The mean-free path of the latter exhibits a distinctive T dependence reflecting a critical nature of spin chains and gives a close description of experiments. By the naturalness criterion of realistically small spin-phonon interaction, our approach stands out from previous considerations that require large coupling constants to explain the data and thus imply a spin-Peierls transition, absent in real materials.
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