arXiv subjects

K. V. Germash

Publications and source records attributed to K. V. Germash.

5 recordsLinked to original sources

arXiv2019

Anderson-Bogoliubov and Carlson-Goldman modes in counterflow superconductors: Case study of a double monolayer graphene

The impact of electron-hole pairing on the spectrum of plasma excitations in double layer systems is investigated. The theory is developed with reference to a double monolayer graphene. Taking into account the coupling of scalar potential oscillations with oscillations of the order parameter $Δ$, we show that the spectrum of antisymmetric (acoustic) plasma excitations contains two modes: a weakly damped mode below the gap $2Δ$ and a strongly damped mode above the gap. The lower mode can be interpreted as an analog of the Carlson-Goldman mode. This mode has an acoustic dispersion relation at small wave vectors and it saturates at the level $2Δ$ at large wave vectors. Its velocity is larger than the velocity of the Anderson-Bogoliubov mode $v_{AB}=v_F$/$\sqrt{2}$, and it can be smaller than the Fermi velocity $v_F$. The damping rate of this mode strongly increases under increase of temperature. Out-of-phase oscillations of two order parameters in two spin subsystems are also considered. This part of the spectrum contains two more modes. One of them is interpreted as an analog of the Anderson-Bogoliubov (phase) mode and the other, as an analog of the Schmid (amplitude) mode. With minor modifications the theory can be extended to describe collective modes in a double bilayer graphene as well.

cond-mat.mes-hall↗

arXiv2017

Strong enhancement of third harmonic generation in a double layer graphene system caused by electron-hole pairing

A manifestation of electron-hole pairing in nonlinear electromagnetic response of a double layer graphene system is studied. It is shown that the pairing causes the appearance of a number of peaks in the frequency dependence of the intensity of the third harmonic generation (THG). The highest peak corresponds to \hbarω= (2/3)Δ, where ω is the incident wave frequency, and Δ is the order parameter of the electron-hole pairing. The absolute value of the THG intensity in the systems with electron-hole pairing is in several orders of magnitude greater than the THG intensity in the unpaired state. It is shown that huge enhancement of the THG intensity occurs both in the double monolayer and double bilayer graphene systems.

cond-mat.str-el↗

arXiv2016

Electromagnetic properties of a double layer graphene system with electron-hole pairing

We study electromagnetic properties of a double layer graphene system in which electrons from one layer are coupled with holes from the other layer. The gauge invariant linear response functions are obtained. The frequency dependences of the transmission, reflection and absorption coefficients are computed. We predict a peak in the reflection and absorption at the frequency equals to the gap in the quasiparticle spectrum. It is shown that the electron-hole pairing results in an essential modification of the spectrum of surface TM plasmons. We find that the optical TM mode splits into a low frequency undamped branch and a high frequency damped branch. At zero temperature the lower branch disappears. It is established that the pairing does not influence the acoustic TM mode. It is also shown that the pairing opens the frequency window in the subgap range for the surface TE wave.

cond-mat.mes-hall↗

arXiv2015

Diamagnetism and suppression of screening as hallmarks of electron-hole pairing in a double layer graphene system

We study how the electron-hole pairing reveals itself in the response of a double layer graphene system to the vector and scalar potentials. Electron-hole pairing results in a rigid (London)relation between the current and the difference of vector potentials in two adjacent layers. The diamagnetic effect can be observed in multiple connected systems in the magnetic field parallel to the graphene layers. Such an effect would be considered as a hallmark of the electron-hole pairing, but the value of the effect is extremely small. Electron-hole pairing significantly changes the response to the scalar potential, as well. It results in a complete (at zero temperature) or partial (at finite temperature) suppression of screening of the electric field of a test charge situated at some distance to the double layer system. A strong increase of the electric field induced by the test charge under decrease in temperature can be considered as a spectacular hallmark of the electron-hole pairing.

cond-mat.mes-hall↗

arXiv2013

Electron-hole pairing in topological insulator heterostructures in the quantum Hall state

A thin film of a topological insulator (TI) on a dielectric substrate and a bulk TI - dielectric film - bulk TI structure are considered as natural double-well heterostructures suitable for realizing the counterflow superconductivity. The effect is connected with pairing of electrons and holes belonging to different surfaces of TI and the transition of a gas of electron-hole pairs into a superfluid state. The case of TI heterostructures subjected to a strong perpendicular magnetic field is considered. It is shown that such systems are characterized by two critical temperatures - a mean-field temperature of pairing and a much smaller temperature of the superfluid transition. The dependence of the critical temperatures on the magnetic field is computed. The advantages of TI based structures in comparison with GaAs heterostructures as well as graphene based heterostructures are discussed.

cond-mat.mes-hall↗