arXiv subjects

Danko Radić

Publications and source records attributed to Danko Radić.

12 recordsLinked to original sources

arXiv2025

Hall Coefficient of the Intercalated Graphite CaC$_6$ in the Uniaxial CDW Ground State

We evaluate the Hall coefficient characterising magnetotransport in an intercalated graphite CaC$_6$ with the Fermi surface reconstructed by an uniaxial charge density wave from closed pockets to open sheets. As the typical order parameter, corresponding to the pseudo-gap in electronic spectrum and consequently to spacing between electron trajectories in reciprocal space, is of the order of $10^2$K, magnetic breakdown in strong experimentally achievable fields of the order of 10T is inevitable. The classical expressions for the components of the magnetoconductivity tensor are strongly modified by magnetic field-assisted over-gap tunneling causing quantum interference. Due to magnetic breakdown, all magnetoconductivity components undergo strong quantum oscillations reflected in the Hall coefficient. In their nature, these are different than standard Shubnikov de Haas oscillations which would not appear in a system with an open Fermi surface.

cond-mat.mes-hall↗

arXiv2025

Magnetoconductivity and quantum oscillations in intercalated graphite CaC$_6$ with the Fermi surface reconstructed by the uniaxial charge density wave

We report a magnetoconductivity tensor $σ$ for the intercalated graphite CaC$_6$, in the ground state of the uniaxial charge density wave (CDW), under conditions of coherent magnetic breakdown due to strong external magnetic field $B$ perpendicular to the conducting plane. The uniaxial charge density wave reconstructs initially closed Fermi surface into an open one, accompanied with formation of a pseudo-gap in the electron density of states around the Fermi energy. The magnetoconductivity tensor is calculated within the quantum density matrix and semiclassical magnetic breakdown approach focused on modification of the main, so-called classical contribution to magnetoconductivity by magnetic breakdown, neglecting the higher order corrections. In the presence of magnetic breakdown, in spite of open Fermi surface configuration, all classical magnetoconductivity components, the one along the CDW apex $σ_{xx} \sim 1/B^2$, perpendicular to the CDW apex $σ_{yy} \sim const$, as well as the Hall conductivity $σ_{xy} \sim 1/B$, undergo strong quantum oscillations vs. inverse magnetic field. Those oscillations do not appear as a mere additive correction, but rather alter the classical result becoming an inherent part of it, turning it to essentially non-classical.

cond-mat.mes-hall↗

arXiv2024

Creation of two-dimensional circular motion of charge qubit

We suggest a nanoelectromechanical setup which generates a particular type of motion - the circular motion of mesoscopic superconducting grain, where motion is described by entangled nanomechanical coherent states. The setup is based on mesoscopic terminal utilizing the AC Josephson effect between the superconducting electrodes and the grain, operating in the regime of the Cooper pair box controlled by the gate voltage. The grain is placed on the free end of the suspended cantilever, performing controlled two-dimensional mechanical vibrations. Required functionality is achieved by operating two external parameters, bias voltage between the superconducting electrodes and voltage between gate electrodes, by which the nanomechanical coherent states are formed and organised in a pair of entangled cat-states in two perpendicular spatial directions which evolve in time in the way to provide a circular motion.

cond-mat.mes-hall↗

arXiv2024

Nanomechanical ancilla qubits generator for error correction algorithms in quantum computation

We suggest a nanoelectromechanical setup that generates properly entangled ancillary ("ancilla") qubits for error correction algorithms in quantum computing, demonstrated as an encoder for the three-qubit bit flip code. The setup is based on mesoscopic terminal utilizing the AC Josephson effect between voltage biased superconducting electrodes and mechanically vibrating mesoscopic superconducting grain in the regime of the Cooper pair box, controlled by the gate voltage. Required functionality is achieved by specifically tailored time-protocol of operating two external parameters: bias voltage and gate voltage. The superconducting grain is fixed on the free end of a cantilever, performing controlled in-plane mechanical vibrations, generating the nanomechanical coherent states organised in a pair of entangled cat-states in two perpendicular spatial directions. Cooper pair box and nanomechanical coherent states become three entangled qubits in a particular way: quantum information, initially encoded in superposition of the Cooper pair box states, is transduced into quantum superposition of two special 3-qubit entangled states, $\vert \uparrow + \, + \rangle$ and $\vert \downarrow - \, - \rangle$. It constitutes the basic input state for the three-qubit bit flip code, used in quantum computation mainly for error correction, "installed" on a single physical object in which the last two ancilla qubits are generated by the nanoelectromechanical setup.

cond-mat.mes-hall↗

arXiv2023

Nanomechanical manipulation of superconducting charge-qubit quantum networks

We suggest a nanoelectromechanical setup and corresponding time-protocol for controlling parameters in order to demonstrate nanomechanical manipulation of superconducting charge-qubit quantum network. We illustrate it on an example reflecting important task for quantum information processing - transmission of quantum information between two charge-qubits facilitated by nanomechanics. The setup is based on terminals utilizing the AC Josephson effect between bias voltage-controlled bulk superconductors and mechanically vibrating mesoscopic superconducting grain in the regime of the Cooper pair box, controlled by the gate voltage. The described manipulation of quantum network is achieved by transduction of quantum information between charge-qubits and intentionally built nanomechanical coherent states, which facilitate its transmission between qubits. This performance is achieved using quantum entanglement between electrical and mechanical states.

cond-mat.mes-hall↗

arXiv2021

DC and optical signatures of the topological reconstruction of the Fermi surface for electrons with parabolic band dispersion

We study the main intra-band and inter-band transport properties at zero temperature of free electron-like system undergoing a topological reconstruction of the Fermi surface for the two-dimensional and three-dimensional case. The calculated intra-band properties include the single-particle density of states, the total and the effective concentrations of electrons and the thermopower. As for the inter-band case, the real part of the conductivity has been calculated within the vanishing inter-band relaxation approximation as a function of the incident photon energy. Within this approach, it is shown that the optical conductivity has a nonvanishing component parallel to the reconstruction wave vector and the shape which depends on the value of the Fermi energy. Each dimensionality has its particular features in the transport quantities presented in the paper, which are discussed and compared with those in the free electron scenario. Finally, we identify the signature of the topological reconstruction of the Fermi surface in the intra-band and inter-band transport functions.

cond-mat.str-el↗

arXiv2021

Onset of pseudogap and density wave in a system with a closed Fermi surface

We study the influence of anisotropy, treated as a dimensional crossover between 1D and 3D system, on the topological instability induced by a (self-consistent) uniaxial periodic potential. The mechanism on which the instability is based involves the topological reconstruction of the Fermi surface, from initially closed pockets to the surface with open Fermi sheets, creating two peculiar points in the band dispersion - the saddle point and elliptical point, between which the pseudogap in electron density of states develops. The self-consistent periodic potential appears as a result of interactions, either electron-phonon, or electron electron, which, linked with the topological instability of the system, results in formation of a new ground state of the system - the density wave provided that the relevant coupling constant is larger than critical. Our analysis shows that the phase transition takes place along the whole continuous interval of a dimensional crossover between 1D and 3D, but that the critical coupling strength significantly increases with the dimensionality of the system. It is our intention to give an initial framework for understanding the nature of charge density waves experimentally observed in a number of materials, like high$-T_c$ cuprates or graphite intercalates, both being materials with a closed, rather isotropic Fermi surface far from the nesting regime.

cond-mat.mtrl-sci↗

arXiv2020

Frequency dependent dielectric response of ferroelectric-dielectric junction with negative electric capacitance

We calculated the frequency dependent dielectric response (electric susceptibility) of layered ferroelectric-dielectric junction, biased by the time-dependent harmonic voltage with single frequency $ω$. Working point is stabilized, by the charge boundary condition between the layers, in the region with negative electric capacitance. The static susceptibility $χ_0$ is negative and relative dielectric constant $ε_r$ smaller than one, clearly indicating the opposite direction of electric field and polarization in the ferroelectric layer due to the negative electric capacitance. At finite frequencies this sign is preserved in real part of susceptibility which gains the frequency dependence. Also, frequency dependent imaginary part arises due to the phase shift between electric field and polarization. The type of that frequency dependence in linear regime is so-called relaxation (Debye) response, i.e. $χ'(ω)=χ_0/(1+(τω)^2)$ and $χ"(ω)=χ_0τω/(1+(τω)^2)$, where $τ$ is polarization switching time characteristic to ferroelectric material. In particular, we modeled the junction of ferroelectric BaTiO$_3$ and dielectric Al$_2$O$_3$, taking the experimental values of material parameters, and addressed the role of nonlinearity with respect to result of the linear response theory.

cond-mat.mes-hall↗

arXiv2019

Density wave and topological reconstruction of an isotropic two-dimensional electron band in external magnetic field

We predict a mechanism of spontaneous stabilization of a uniaxial density wave in a two-dimensional metal with an isotropic Fermi surface in the presence of external magnetic field. The topological transformation of a closed Fermi surface into an open one decreases the electron band energy due to delocalization of electrons initially localized by magnetic field, additionally affected by the magnetic breakdown effect. The driving mechanism of such reconstruction is a periodic potential due to the self-consistently formed electron density wave. It is accompanied with quantum oscillations periodic in inverse magnetic field, similar to the standard de Haas - van Alphen effect, due to Landau level filling. The phase transition appears as a quantum one at T=0, provided the relevant coupling constant is above the critical one. This critical value rapidly decreases, and finally saturates toward zero on the scale of tens of Tesla. Thus, a strong enough magnetic field can induce the density wave in the system in which it was absent in zero field.

cond-mat.mes-hall↗

arXiv2018

Topological instability of two-dimensional conductors

In the present paper we propose a mechanism of the structural instability with a periodic charge ordering in two-dimensional isotropic conductors with a closed Fermi surface which completely excludes the conventional nesting mechanism. We show that the structural instability in such conductors may arise as a topological reconstruction under which the initially closed Fermi surface is transformed into an open one. We have found that the order parameter of the charge ordering ground state may exceed one hundredth of the Fermi energy. Furthermore, this charge ordering is a quantum phase transition with respect to the dimensionless coupling constant $λ$ related to the mechanism that drives the band reconstruction (e. g. electron-phonon coupling), with the critical value given by $λ_c= (1+2/π)^{-1}$. Preliminary estimations show that the suggested mechanism can be the origin of density waves observed in such materials as high$-T_c$ cuprates or graphite intercalates.

cond-mat.str-el↗

arXiv2018

The Quantum Hall Effect with Wilczek's charged magnetic flux tubes instead of electrons

Composites formed from charged particles and magnetic flux tubes, proposed by Wilczek, are one model for anyons - particles obeying fractional statistics. Here we propose a scheme for realizing charged flux tubes, in which a charged object with an intrinsic magnetic dipole moment is placed between two semi-infinite blocks of a high permeability ($μ_r$) material, and the images of the magnetic moment create an effective flux tube. We show that the scheme can lead to a realization of Wilczek's anyons, when a two-dimensional electron system, which exhibits the integer quantum Hall effect (IQHE), is sandwiched between two blocks of the high-$μ_r$ material with a temporally fast response (in the cyclotron and Larmor frequency range). The signature of Wilczek's anyons is a slight shift of the resistivity at the plateau of the IQHE. Thus, the quest for high-$μ_r$ materials at high frequencies, which is underway in the field of metamaterials, and the quest for anyons, are here found to be on the same avenue.

cond-mat.str-el↗

arXiv2011

Spin controlled nanomechanics induced by single-electron tunneling

We consider dc-electronic transport through a nanowire suspended between normal- and spin-polarized metal leads in the presence of an external magnetic field. We show that magnetomotive coupling between the electrical current through the nanowire and vibrations of the wire may result in self excitation of mechanical vibrations. The self-excitation mechanism is based on correlations between the occupancy of the quantized electronic energy levels inside the nanowire and the velocity of the nanowire. We derive conditions for the occurrence of the instability and find stable regimes of mechanical oscillations.

cond-mat.mes-hall↗