Functional renormalization group study of parallel double quantum dots: Effects of asymmetric dot-lead couplings

We explore the effects of asymmetry of hopping parameters between double parallel quantum dots and the leads on the conductance and a possibility of local magnetic moment formation in this system using functional renormalization group approach with the counterterm. We demonstrate a possibility of a quantum phase transition to a local moment regime (so called singular Fermi liquid (SFL) state) for various types of hopping asymmetries and discuss respective gate voltage dependences of the conductance. It is shown, that depending on the type of the asymmetry, the system can demonstrate either a first order quantum phase transition to SFL state, accompanied by a discontinuous change of the conductance, similarly to the symmetric case, or the second order quantum phase transition, in which the conductance is continuous and exhibits Fano-type asymmetric resonance near the transition point. A semi-analytical explanation of these different types of conductance behavior is presented.Comment: 11 pages, 9 figure

Local magnetic moments and electronic transport in closed loop quantum dot systems: a case of quadruple quantum dot ring at and away from equilibrium

We apply the non-equilibrium functional renormalization group approach treating flow of the electronic self-energies, to describe local magnetic moments formation and electronic transport in a quadruple quantum dot (QQD) ring, coupled to leads, with moderate Coulomb interaction on the quantum dots. We find that at zero temperature depending on parameters of the QQD system the regimes with zero, one, or two almost local magnetic moments in the ring can be realized, and the results of the considered approach in equilibrium agree qualitatively with those of more sophisticated fRG approach treating also flow of the vertices. It is shown that the almost formed local magnetic moments, which exist in the equilibrium, remain stable in a wide range of bias voltages near equilibrium. The destruction of the local magnetic moments with increasing bias voltage is realized in one or two stages, depending on the parameters of the system; for two-stage process the intermediate phase possesses fractional magnetic moment. We present zero-temperature results for current-voltage dependences and differential conductances of the system, which exhibit sharp features at the transition points between different magnetic states. The occurrence of interaction induced negative differential conductance phenomenon is demonstrated and discussed. For one local moment in the ring and finite hopping between the opposite quantum dots, connected to the leads, we find suppression of the conductance for one of the spin projections in infinitesimally small magnetic field, which occurs due to destructive interference of different electron propagation paths and can be used in spintronic devices.Comment: 19 pages, 19 figure

Interaction-induced local moments in parallel quantum dots within the functional renormalization group approach

We propose a version of the functional renormalization-group (fRG) approach, which is, due to including Litim-type cutoff and switching off (or reducing) the magnetic field during fRG flow, capable of describing a singular Fermi-liquid (SFL) phase, formed due to the presence of local moments in quantum dot structures. The proposed scheme allows one to describe the first-order quantum phase transition from the "singular" to the "regular" paramagnetic phase with applied gate voltage to parallel quantum dots, symmetrically coupled to leads, and shows sizable spin splitting of electronic states in the SFL phase in the limit of vanishing magnetic field H→0; the calculated conductance shows good agreement with the results of the numerical renormalization group. Using the proposed fRG approach with the counterterm, we also show that for asymmetric coupling of the leads to the dots the SFL behavior similar to that for the symmetric case persists, but with occupation numbers, effective energy levels, and conductance changing continuously through the quantum phase transition into the SFL phase. © 2016 American Physical Society

Dipole Response of Spaser on an External Optical Wave

We find the conditions upon the amplitude and frequency of an external electromagnetic field at which the dipole moment of a Bergman-Stockman spaser oscillates in antiphase with the field. For these values of the amplitude and frequency the losses in metal nanoparticles is exactly compensated of by gain. This shows that spasers may be used as inclusions in designing lossless metamaterials

Magnetic, Charge, and Transport Properties of Graphene Nanoflakes

We investigate magnetic, charge, and transport properties of hexagonal graphene nanoflakes (GNFs) connected to two metallic leads by using the functional renormalization group method. The interplay between the on-site and long-range interactions leads to a competition of semimetal (SM), spin-density-wave (SDW), and charge-density-wave (CDW) phases. The ground-state phase diagrams are presented for the GNF systems with screened realistic long-range electron interaction [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)10.1103/PhysRevLett.106.236805], as well as uniformly screened long-range Coulomb potential ∝1/r. We demonstrate that the realistic screening of Coulomb interaction by σ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential. This enhancement gives rise to a wide region of stability of the SM phase for realistic interaction, such that freely suspended GNFs are far from both SM-SDW and SM-CDW phase-transition boundaries and correspond to the SM phase. Close relation between the linear conductance and the magnetic or charge states of the systems is discussed. A comparison of the results with those of other studies on GNF systems and infinite graphene sheets is presented. ©2021 American Physical Society.The authors are grateful to A. Valli and M. Capone for stimulating discussions. The work was performed within the state assignment from the Ministry of Science and Higher Education of Russia (theme “Quant” AAAA-A18-118020190095-4) and partly supported by RFBR Grant No. 20-02-00252a. A.A.K. also acknowledges the financial support from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2021-606). The calculations were performed on the Uran supercomputer at the IMM UB RAS

Формирование и внедрение новых компонентов развития, которые повышают конкурентоспособность предприятия

У статті розглядаються основні напрями та нові компоненти підвищення конкурентоспроможності підприємств, які залежать від їх потенційних можливостей, умов реалізації стратегічних напрямів, розвитку і використання ресурсного забезпечення.The paper examines the main trends and new components enhance the competitiveness of enterprises, depending on the capabilities, the conditions of strategic direction, development and utilization of resources. The criteria of competitiveness evaluation, methods and mechanisms for their implementation, their uses are described advantages and disadvantages of each. Detailed analysis of approaches to the selection of the parameters that characterize the individual components of multifactorial concept of «competitiveness». The recommendations on the implementation process of comparing totals that determine the competitiveness of enterprises. measured. The ways of implementing the strategy of innovative development, as the basic conditions for the formation of the intellectual potential of the company and society.В статье рассматриваются основные направления и новые компоненты повышения конкурентоспособности предприятий, которые зависят от их потенциальных возможностей, условий реализации стратегических направлений, развития и использования ресурсного обеспечения

Conditional quantum logic using two atomic qubits

In this paper we propose and analyze a feasible scheme where the detection of a single scattered photon from two trapped atoms or ions performs a conditional unitary operation on two qubits. As examples we consider the preparation of all four Bell states, the reverse operation that is a Bell measurement, and a CNOT gate. We study the effect of atomic motion and multiple scattering, by evaluating Bell inequalities violations, and by calculating the CNOT gate fidelity.Comment: 23 pages, 8 figures in 11 file