Quantum Fluctuations Of Charge Research Articles

Electron self-energy from quantum charge fluctuations in the layered t−J model with long-range Coulomb interaction

Employing a large-N scheme of the layered t-J model with the long-range Coulomb interaction, which captures fine details of the charge excitation spectra recently observed in cuprate superconductors, we explore the role of the charge fluctuations on the electron self-energy. We fix temperature at zero and focus on quantum charge fluctuations. We find a pronounced asymmetry of the imaginary part of the self-energy Im$\Sigma({\bf k}, \omega)$ with respect to $\omega = 0$, which is driven by strong electron correlation effects. The quasiparticle weight is reduced dramatically, which occurs almost isotropically along the Fermi surface. Concomitantly an incoherent band and a sharp side band are newly generated and acquire sizable spectral weight. All these features are driven by usual on-site charge fluctuations, which are realized in a rather high-energy region and yield plasmon excitations. On the other hand, the low-energy region with the scale of the superexchange interaction J is dominated by bond-charge fluctuations. Surprisingly, compared with the effect of the on-site charge fluctuations, their effect on the electron self-energy is much weaker even if the system approaches close to bond-charge instabilities. Furthermore, quantum charge dynamics does not produce a clear kink nor a pseudogap in the electron dispersion.

Investigating time evolution and quantum effects of mesoscopic complicated RLC circuit involving inductance coupling

The quantization of mesoscopic complicated RLC circuits involving inductance coupling is proposed. The time evolution is investigated by employing multiple unitary transformations. The quantum fluctuations of charge and current of each loop are calculated in vacuum state and evolving state. It is shown that the quantum fluctuations of charge and current in evolving state are independent of sources and only rely on inherent parameters of the circuit. And the uncertainty relations do not satisfy the uncertainty principle, except when the resistances become zero.

Stability of the Coexistence Phase of Chiral Superconductivity and Noncollinear Spin Ordering with a Nontrivial Topology and Strong Electron Correlations

We show that the quantum charge and spin fluctuations, while sufficiently renormalizing the magnetic order parameter, do not destroy the coexistence phase of chiral d+id superconductivity and 120-degree spin ordering in a strongly correlated 2D system with a triangular lattice. The nontrivial topology characterized by the topological invariant N3 is also preserved. It is shown that the Majorana mode exist among edge states in the topologically nontrivial phase. The spatial structure of such mode is determined. The spin and charge fluctuations shift the critical values of electron density at which quantum topological transitions occur. Increasing intersite Coulomb repulsion leads to decrease in the number of the topological transitions.

Quantum charge fluctuations of a proximitized nanowire

Motivated by recent experiment, we consider charging of a nanowire which is proximitized by a superconductor and connected to a normal-state lead by a single-channel junction. The charge $Q$ of the nanowire is controlled by gate voltage $e{\cal N}_g/C$. A finite conductance of the contact allows for quantum charge fluctuations, making the function $Q(\mathcal{N}_g)$ continuous. It depends on the relation between the superconducting gap $\Delta$ and the effective charging energy $E^*_C$. The latter is determined by the junction conductance, in addition to the geometrical capacitance of the proximitized nanowire. We investigate $Q(\mathcal{N}_g)$ at zero magnetic field $B$, and at fields exceeding the critical value $B_c$ corresponding to the topological phase transition. Unlike the case of $\Delta = 0$, the function $Q(\mathcal{N}_g)$ is analytic even in the limit of negligible level spacing in the nanowire. At $B=0$ and $\Delta>E^*_C$, the maxima of $dQ/d\mathcal{N}_g$ are smeared by $2e$-fluctuations described by a single-channel "charge Kondo" physics, while the $B=0$, $\Delta<E^*_C$ case is described by a crossover between the Kondo and mixed-valence regimes of the Anderson impurity model. In the topological phase, $Q(\mathcal{N}_g)$ is analytic function of the gate voltage with $e$-periodic steps. In the weak tunneling limit, $dQ/d\mathcal{N}_g$ has peaks corresponding to Breit-Wigner resonances, whereas in the strong tunneling limit (i.e., small reflection amplitude $r$ ) these resonances are broadened, and $dQ/d\mathcal{N}_g-e \propto r\cos(2\pi \mathcal{N}_g)$.

Hall effect in quantum critical charge-cluster glass

Upon doping, cuprates undergo a quantum phase transition from an insulator to a d-wave superconductor. The nature of this transition and of the insulating state is vividly debated. Here, we study the Hall effect in La2-xSrxCuO4(LSCO) samples doped near the quantum critical point atx∼ 0.06. Dramatic fluctuations in the Hall resistance appear belowTCG∼ 1.5 K and increase as the sample is cooled down further, signaling quantum critical behavior. We explore the doping dependence of this effect in detail, by studying a combinatorial LSCO library in which the Sr content is varied in extremely fine steps,Δx∼ 0.00008. We observe that quantum charge fluctuations wash out when superconductivity emerges but can be restored when the latter is suppressed by applying a magnetic field, showing that the two instabilities compete for the ground state.

Correlation between quantum charge fluctuations and magnetic ordering in multiferroic LuFe2O4

We examine the interplay between quantum charge fluctuations and magnetic ordering in multiferroic LuFe2O4 and show that this can couple spin and charge degrees of freedom in a LuFe2O4 bilayer below the Neel temperature TN. Our analysis supports the idea that the double exchange mechanism normally used in metallic systems can be applied to charge-ordered insulators. This causes ferrimagnetic spin order to reduce the transfer integrals between Fe2+ and Fe3+ in LuFe2O4, decreasing charge fluctuations and increasing the polarization in this system below TN. This work thus provides a more detailed understanding of the mechanism for spin-charge coupling in LuFe2O4.

Probing the Interplay between Quantum Charge Fluctuations and Magnetic Ordering in LuFe2O4

The mechanisms producing strong coupling between electric and magnetic order in multiferroics are not always well understood, since their microscopic origins can be quite different. Hence, gaining a deeper understanding of magnetoelectric coupling in these materials is the key to their rational design. Here, we use ultrafast optical spectroscopy to show that the influence of magnetic ordering on quantum charge fluctuations via the double-exchange mechanism can govern the interplay between electric polarization and magnetism in the charge-ordered multiferroic LuFe2O4.

Light Emission Probing Quantum Shot Noise and Charge Fluctuations at a Biased Molecular Junction

The emission of plasmonic light from a single C(60) molecule on Cu(111) is probed in a scanning tunneling microscope from the weak-coupling, tunneling range to strong coupling of the molecule to the electrodes at contact. At positive sample voltage the photon yield decreases owing to shot-noise suppression in an increasingly transparent quantum contact. At reversed bias an unexpected nonlinear increase occurs. First-principles transport calculations reveal that ultrafast charge fluctuations on the molecule give rise to additional noise at optical frequencies beyond the shot noise of the current that is injected to the tip.

Detecting entanglement of two-electron spin qubits with witness operators

We propose a scheme for detecting entanglement between two electron spin qubits in a double quantum dot using an entanglement witness operator. We first calculate the optimal configuration of the two electron spins, defined as the position in the energy level spectrum where, averaged over the nuclear spin distribution, 1) the probability to have two separated electrons, and 2) the degree of entanglement of the quantum state quantified by the concurrence are both large. Using a density matrix approach, we then calculate the evolution of the expectation value of the witness operator for the two-spin singlet state, taking into account the effect of decoherence due to quantum charge fluctuations modeled as a boson bath. We find that, for large interdot coupling, it is possible to obtain a highly entangled and robust ground state.

Out-of-equilibrium admittance of single electron box under strong Coulomb blockade

We study admittance and energy dissipation in an out-of-equlibrium single electron box. The system consists of a small metallic island coupled to a massive reservoir via single tunneling junction. The potential of electrons in the island is controlled by an additional gate electrode. The energy dissipation is caused by an AC gate voltage. The case of a strong Coulomb blockade is considered. We focus on the regime when electron coherence can be neglected but quantum fluctuations of charge are strong due to Coulomb interaction. We obtain the admittance under the specified conditions. It turns out that the energy dissipation rate can be expressed via charge relaxation resistance and renormalized gate capacitance even out of equilibrium. We suggest the admittance as a tool for a measurement of the bosonic distribution corresponding collective excitations in the system.

Relaxation dynamics of the electron distribution in the Coulomb-blockade problem

We study the relaxation dynamics of electron distribution function on the island of a single-electron transistor. We focus on the regime of not very low temperatures in which an electron coherence can be neglected but quantum fluctuations of charge are strong due to Coulomb interaction. The quantum kinetic equation governing evolution of the electron distribution function due to escape of electrons to the reservoirs is derived. Analytical solutions for time dependence of the electron distribution are obtained in the regimes of weak and strong Coulomb blockade. We find that usual exponential in time relaxation is strongly modified due to the presence of Coulomb interaction.

QUANTUM FLUCTUATIONS OF A DISSIPATIVE MESOSCOPIC CAPACITANCE–RESISTANCE–INDUCTANCE COUPLED CIRCUIT

Starting from Kirchhoff's equation for each mesoscopic coupled circuit, we investigated the quantum fluctuations of a dissipative mesoscopic capacitance–resistance–inductance coupled circuit using canonical transformation and unitary transformation. It is shown that the quantum fluctuations of charges and currents are not only related to the parameters of components in the self-circuit and coupled part, but also influenced by the parameters of other circuit. The uncertainty relations of the charges and currents are analyzed briefly.

Implementation of the quantum-walk step operator in lateral quantum dots

We propose a physical implementation of the step operator of the discrete quantum walk for an electron in a one-dimensional chain of quantum dots. The operating principle of the step operator is based on locally enhanced Zeeman splitting and the role of the quantum coin is played by the spin of the electron. We calculate the probability of successful transfer of the electron in the presence of decoherence due to quantum charge fluctuations, modeled as a bosonic bath. We then analyze two mechanisms for creating locally enhanced Zeeman splitting based on, respectively, locally applied electric and magnetic fields and slanting magnetic fields. Our results imply that a success probability of > 90% is feasible under realistic experimental conditions.

Charge relaxation resistance in the Coulomb blockade problem

We study the dissipation in a system consisting of a small metallic island coupled to a gate electrode and to a massive reservoir via single tunneling junction. The dissipation of energy is caused by a slowly oscillating gate voltage. We compute it in the regimes of weak and strong Coulomb blockade. We focus on the regime of not very low temperatures when electron coherence can be neglected but quantum fluctuations of charge are strong due to Coulomb interaction. The answers assume a particularly transparent form while expressed in terms of specially chosen physical observables. We discovered that the dissipation rate is given by a universal expression in both limiting cases.

Site dilution in the half-filled one-band Hubbard model: Ring exchange, charge fluctuations, and application toLa2Cu1−x(Mg/Zn)xO4

We study the ground-state quantum spin fluctuations around the N\'eel ordered state for the one-band $(t,U)$ Hubbard model on a site-diluted square lattice. An effective spin Hamiltonian ${H}_{s}^{(4)}$ is generated using the canonical transformation method, expanding to order $t{(t/U)}^{3}$. ${H}_{s}^{(4)}$ contains four-spin ring exchange terms as well as second- and third-neighbor bilinear spin-spin interactions. Transverse spin fluctuations are calculated to order $1/S$ using a numerical real-space algorithm first introduced by Walker and Walstedt [Phys. Rev. B 22, 3816 (1980)]. Additional quantum charge fluctuations appear to this order in $t/U$, coming from electronic hopping and virtual excitations to doubly occupied sites. The ground-state staggered magnetization on the percolating cluster decreases with site dilution $x$, vanishing very close to the percolation threshold. We compare our results in the Heisenberg limit, $t/U\ensuremath{\rightarrow}0$, with quantum Monte Carlo (QMC) results on the same model and confirm the existence of a systematic $x$-dependent difference between $1/S$ and QMC results away from $x=0$. For finite $t/U$, we show that the effects of both the ring exchange and charge fluctuations die away rapidly with increasing $t/U$. We use our finite $t/U$ results to make a comparison with results from experiments on ${\text{La}}_{2}{\text{Cu}}_{1\ensuremath{-}x}{(\text{Mg}/\text{Zn})}_{x}{\text{O}}_{4}$.

Dynamical behavior of a mesoscopic circuit in phonons bath derived by virtue of the IWOP technique

Taking into account the interactions between electrons and phonons, we study the dynamic behavior of a dissipative mesoscopic circuit for pure initial coherent state of phonon bath modes by virtue of the IWOP technique. It shows that if the bath modes are initially in coherent states, some phenomena like Brownian behavior will appear in mean charge and current of the mesoscopic circuit. The quantum fluctuations of charge and current are constant and irrelevant to the coupled coefficients between electrons and phonons.

The energy and thermal effects of mesoscopic capacitance coupling LC circuit at finite temperature

For the mesoscopic capacitance coupling LC circuit, the Hamiltonian is given by the canonical quantization. The Hamiltonian operator is diagonalized by a unitary transformation. The ensemble average energy and its fluctuations are derived by ensemble theory. In virtue of the generalized Hellmann-Feynman theorem, the quantum fluctuations of charge and self-conductance magnetic flux for the system at finite temperature are investigated. It is found that, the quantum fluctuations of charge and self-conductance magnetic flux depend on the temperature as well as the parameters of the circuit components.

Quantum fluctuations of mesoscopic damped double resonance RLC circuit with mutual capacitance–inductance coupling in thermal excitation state

Based on the scheme of damped harmonic oscillator quantization and thermo-field dynamics (TFD), the quantization of mesoscopic damped double resonance RLC circuit with mutual capacitance–inductance coupling is proposed. The quantum fluctuations of charge and current of each loop in a squeezed vacuum state are studied in the thermal excitation case. It is shown that the fluctuations not only depend on circuit inherent parameters, but also rely on excitation quantum number and squeezing parameter. Moreover, due to the finite environmental temperature and damped resistance, the fluctuations increase with the temperature rising, and decay with time.

Effect of quantum fluctuations on even-odd energy difference in a Cooper-pair box

We study the effect of quantum charge fluctuations on the discrete spectrum of charge states of a small superconducting island (Cooper-pair box) connected to a large finite-size superconductor by a tunnel junction. In particular, we calculate the reduction of the even-odd energy difference $\ensuremath{\delta}E$ due to virtual tunneling of electrons across the junction. We show that the renormalization effects are important for understanding the quasiparticle ``poisoning'' effect because $\ensuremath{\delta}E$ determines the activation energy of a trapped quasiparticle in the Cooper-pair box. We find that renormalization of the activation energy depends on the dimensionless normal-state conductance of the junction ${g}_{T}$ and becomes strong at ${g}_{T}⪢1$.

Quantum fluctuations of mesoscopic RLC circuit involving complicated coupling in thermal squeezed state

Starting from the Kirchhoff's equation for electric circuits and in reference of damped harmonic oscillator quantization and thermo-field dynamics (TFD), the quantization of damped double-resonance mesoscopic RLC circuit involving complicated coupling is proposed. The quantum fluctuations of charge and current of each loop are calculated in thermal squeezed state, thermal coherent state and thermal vacuum state, respectively. The results not only depend on the circuit proper parameters and coupled magnitude, but also rely on the squeezing coefficients, environmental temperature and damped resistance. The fluctuations increase with temperature rising and decay with time.