Quantum Fluctuations Of Charge Research Articles

Kondo effect in single-molecule spintronic devices

We study the Kondo effect in a quantum dot or a single molecule coupled to ferromagnetic leads. Spin-dependent quantum charge fluctuations in the dot induce the lifting of the spin degeneracy of the dot. It leads to the dot's level spin splitting observed in the nonequilibrium transport as a splitting of a zero-bias anomaly in the differential conductance. We discuss basic properties of this effect and its temperature dependence using numerical renormalization group technique. Recent experimental results fit well to our theoretical consideration.

Large magnetoresistance induced by quantum charge fluctuations in magnetic double dots

We study electron tunnelling through two small ferromagnetic dots. Quantum charge fluctuations and interdot coupling cause each Coulomb peak of conductance at zero interdot coupling to split. The interdot tunnel coupling depends on the relative orientation of magnetizations of the two dots, leading to different splitting energies of the Coulomb peaks in parallel and antiparallel magnetization alignments. As a result, a very large tunnelling magnetoresistance occurs near the Coulomb peaks, and its sign may be either positive or negative.

Quantum effect of dissipative mesoscopic capacitance coupled circuit

The dissipative mesoscopic coupled circuits are obtained by the method of damped harmonic oscillator quantization . The quantum fluctuations of charge and current in an arbitrary eigenstate of the system have also been given .The result shows that the quantum fluctuations of the charge and current exist in all states, and the fluctuations in the component circuits are corelated.

Quantum charge fluctuations in a superconducting grain

We consider charge quantization in a small superconducting grain that is contacted by a normal-metal electrode and is controlled by a capacitively coupled gate. At zero temperature and zero conductance $G$ between the grain and the electrode, the charge $Q$ as a function of the gate voltage $V_g$ changes in steps. The step height is $e$ if $\Delta<E_c$, where $\Delta$ and $E_c$ are, respectively, the superconducting gap and the charging energy of the grain. Quantum charge fluctuations at finite conductance remove the discontinuity in the dependence of $Q$ on $V_g$ and lead to a finite step width $\propto G^2\Delta$. The resulting shape of the Coulomb blockade staircase is of a novel type. The grain charge is a continuous function of $V_g$ while the differential capacitance, $dQ/dV_g$, has discontinuities at certain values of the gate voltage. We determine analytically the shape of the Coulomb blockade staircase also at non-zero temperatures.

Density Matrix for Mesoscopic Distributed Parameter Circuits

Under the Born–von-Karmann periodic boundary condition, wepropose a quantization scheme for non-dissipative distributed parametercircuits (i.e. a uniform periodic transmission line). We find the unitaryoperator for diagonalizing the Hamiltonian of the uniform periodictransmission line. The unitary operator is expressed in a coordinaterepresentation that brings convenience to deriving the density matrix ρ(q,q′,β). The quantum fluctuations of charge and currentat a definite temperature have been studied. It is shown that quantumfluctuations of distributed parameter circuits, which also have distributedproperties, are related to both the circuit parameters and the positions andthe mode of signals and temperature T. The higher the temperatureis, the stronger quantum noise the circuitexhibits.

Quantum fluctuation of mesoscopic distributed parameter circuits

Under the Born–von-Karmann periodic boundary condition, we propose a quantization scheme for a non-dissipative distributed parameter circuit (a uniform transmission line). Quantum fluctuations of charge and current in the vacuum state are investigated by adopting the canonical transformation and unitary transformation method. Our results indicate that in distributed parameter circuits, quantum fluctuations, which also have distributed properties, are related to both the circuit parameters and the positions and the mode of signals.

Sensitivity and linearity of superconducting radio-frequency single-electron transistors: effects of quantum charge fluctuations.

We have investigated the effects of quantum fluctuations of quasiparticles on the operation of superconducting radio-frequency single-electron transistors (rf-SETs) for large values of the quasiparticle cotunneling parameter alpha = 8EJ/Ec, where EJ and Ec are the Josephson and charging energies. We find that, for alpha > 1, subgap rf-SET operation is still feasible despite quantum fluctuations that wash out quasiparticle tunneling thresholds. Surprisingly, such rf-SETs show linearity and signal-to-noise ratio superior to those obtained when quantum fluctuations are weak, while still demonstrating excellent charge sensitivity.

Quantization of dissipative mesoscopic capacitance coupling circuit

By means of canonicalization transformation, we study the quantization of dissi pative mesoscopic capacitance coupling circuit, and discuss the quantum fluctuat ions of charges and generalized currents in the system.

Quantum charge fluctuations and the polarizability of the single-electron box.

We measure the average charge on the island of a single-electron box, with an accuracy of two thousandths of an electron. Thermal fluctuations alone cannot account for the dependence of the average charge on temperature, on external potential, or on the quasiparticle density of states in the metal from which the box is formed. In contrast, we find excellent agreement between these measurements and a theory that treats the quantum fluctuations of charge perturbatively.

Effects of classical and quantum charge fluctuations on sequential electron tunneling in multiple quantum wells

A previous theory [M. Ershov et al., Appl. Phys. Lett. 67, 3147 (1995)] for studying the distribution of nonuniform fields in multiple-quantum-well photodetectors under an ac voltage is generalized to include nonadiabatic space-charge-field effects. From numerical results calculated by the generalized theory, it is found that field-domain effects are only important at high temperatures or high voltages, where both injection and sequential-tunneling currents are expected to be large. On the other hand, field-domain effects become negligible at low temperatures and low voltages, but nonadiabatic effects included in this extended theory are enhanced for small sequential-tunneling currents. The time duration for nonadiabatic effects is determined by the quantum capacitance. By using the generalized theory, a differential capacitance is calculated for a non-steady state, and a negative conduction current is predicted under a positive voltage in this case due to charge accumulation around the collecting contact.

Noise of a single-electron transistor in the regime of large quantum fluctuations of island charge out of equilibrium

By using the drone-fermion representation and the Schwinger-Keldysh approach, we calculate the current noise and the charge noise for a single-electron transistor in the nonequilibrium state in the presence of large quantum fluctuation of island charge. Our result interpolates between those of the ``orthodox'' theory and the ``cotunneling theory.'' We find the following effects which are not treated by previous theories: (i) At zero temperature $T=0$ and at finite applied bias voltage $|\mathrm{eV}|\ensuremath{\gg}{T}_{\mathrm{K}},$ where ${T}_{\mathrm{K}}$ is the ``Kondo temperature,'' we find that the Fano factor is suppressed more than the suppression caused by Coulomb correlation both in the Coulomb blockade regime and in the sequential tunneling regime. (ii) For $T\ensuremath{\gg}|\mathrm{eV}|/2\ensuremath{\gg}{T}_{\mathrm{K}},$ the current noise in the presence of large charge fluctuation is modified and deviates from the prediction of the orthodox theory. However, the Fano factor coincides with that of the orthodox theory and is proportional to the temperature. (iii) For $eV,T\ensuremath{\lesssim}{T}_{\mathrm{K}},$ the charge noise is suppressed due to the renormalization of system parameters caused by quantum fluctuation of charge. We interpret it in terms of the modification of the ``unit'' for island charge.

Quantum fluctuations of charge and phase transitions of a large Coulomb-blockaded quantum dot

We analyze ground-state properties of a large gated quantum dot coupled via a quantum point contact to a reservoir of one-dimensional spinless electrons whose interactions are characterized by the Luttinger liquid parameter g. We find that the classical steplike dependence of the average number of electrons on the dot n as a function of the gate voltage ${n}_{x}$ is preserved under certain conditions, the presence of quantum fluctuations notwithstanding. We point out that in its low-energy limit the problem is dual to that of a single-junction superconducting quantum interference device with Caldeira-Leggett dissipative coupling, and analogous to the classical problem of multilayer adsorption, and so is related to the classical statistical-mechanical problem of a one-dimensional Ising model with exchange interactions decaying as the inverse square of distance. This Ising universality class further subdivides into what we term (i) the Kondo/Ising class and (ii) the tricritical class. (i) For systems of the Kondo/Ising class, the ${n(n}_{x})$ dependence is always continuous for $g&gt;~1,$ while for $g&lt;1$ (repulsive electrons in the constriction) the ${n(n}_{x})$ dependence is continuous for sufficiently open dots, while taking the form of a modified staircase for dots sufficiently isolated from the reservoir. At the phase transition between the two regimes the magnitude of the dot population jump is only determined by the properties of the reservoir and is given by ${g}^{1/2}.$ (ii) For systems in the tricritical class we find in addition an intermediate regime where the dot population jumps from near-integer value to a region of stable population centered about a half-integer value. In particular, this tricritical behavior (together with the two dependences already seen in the Kondo/Ising class) is realized for noninteracting electrons, $g=1.$

Electron transport in ferromagnetic small tunnel junctions

Both the single-electron charging effect and spin-dependent tunneling are important for electron transport in small tunnel junctions made of ferromagnetic metals. In this paper, we review our experiments on such systems. The hysteretic negative magnetoresistance at low magnetic fields, known as the tunnel magnetoresistance (TMR), is largely enhanced when the Coulomb blockade takes effect at low temperatures. Although such enhancement is expected to some extent by the quantum fluctuation of charge at strong tunneling regime, the quantitative agreement has not been obtained yet. The resistance of ferromagnetic single-electron transistors (SET) oscillates at high magnetic fields. A shift of the chemical potential in magnetic fields by the Zeeman effect causes the migration of electrons between the electrodes, which appears as resistance oscillations. Experimental results for various structures are consistent with such a model.

Quantum fluctuations of a non-dissipative mesoscopic inductance coupling circuit in a displaced squeezed Fock state

In this Letter, starting from the classical equation of motion for a mesoscopic inductance coupling circuit, the quantum fluctuations of charge and current of the mesoscopic inductance coupling circuit in a displaced squeezed Fock state are investigated. It is found that the quantum fluctuations of charge and current in each component circuit depend on the device of two circuits and squeezing parameters, while the fluctuations do not depend on displacement parameters.

Quantum Wavefunctions and Fluctuations of Mesoscopic RLC Circuit

The quantum wavefunctions and the corresponding energy levels of a RLC (Resistance-Inductance-Capacity) electric circuit are obtained by using canonical quantization method and unitary transformation from the classical equation of motion. The quantum fluctuations of charge and current in an arbitrary eigenstate of the system have also been given as well as the uncertainty relation. It is showed that even if at 0 K charge and current in the circuit exhibit quantum fluctuations, which originates from fluctuations of zero point vibrations of the system.

Quantum Charge Fluctuations in Quantum Dots

Influence of the quantum charge fluctuations in a quantum dot on its charging effect has been investigated by using a quantum-dot device having four quantum point contacts (QPC's) which connect the dot to the reservoirs. The charging effect was detectable at combined tunnel conductance of four QPC's up to near 8 e 2 / h beyond the intuitive criterion e 2 / h . We observed anomalously nonlinear dependences of the conductances at the tops ( G top ) and bottoms ( G bot ) of the Coulomb oscillation on the tunnel conductances of QPC's, G QPC 's. The result on G top indicates that a transition from Coulombically regulated tunnelings to non-correlated tunnelings occurs by increasing G QPC 's. The result on G bot shows the existence of leakage conductance beyond the 2nd-order cotunneling contribution. The offset voltage in the current-voltage characteristics of the device showed significant decrease with the increase of G QPC 's, indicating the shrinkage of the effective charging energy. The results on G top and ...

QUANTUM FLUCTUATIONS OF A MESOSCOPIC CAPACITANCE COUPLING CIRCUITS IN A DISPLACED SQUEEZED FOCK STATE

Starting from the classical equation of motion for a mesoscopic capacitance coupling circuits,the quantum fluctuations of charge and current of the circuits in a displaced squeezed Fock state are investigated.It is found that the quantum fluctuations of charge and current in each component circuit depend on the device of the two loop circuits and squeezing parameters,while the fluctuation does not depend on displacement parameters.

Quantum effects of a nondissipative mesoscopic capacitance coupling circuit in a displaced squeezed Fock state

Starting from the classical equation of motion for a mesoscopic capacitance coupling circuit, we study the quantum fluctuations of charge and current of the mesoscopic capacitance coupling circuit in a displaced squeezed Fock state. It is found that the quantum fluctuations of charge and current in each component circuit depend on the devices of two circuits and the squeezing parameters, while the fluctuations do not depend on displacement parameters.

Quantum Fluctuations in a Mesoscopic Inductance Coupling Circuit

Starting from the equation of motion of a mesoscopic inductance coupling circuit, the quantum fluctuations of charge and current in the circuit are investigated in both the eigenstates of the system and the squeezed vacuum state. The results show that there exist quantum fluctuations of the charge and current in both cases, and the fluctuations in each component circuit are connected.

Evolution of Coulomb blockade spectra in parallel coupled quantum dots

We report low-temperature conductance measurements in the Coulomb blockade regime on two nominally identical tunnel-coupled quantum dots in parallel defined electrostatically in the two-dimensional gas of a GaAs/AlGaAs heterostructure. We find that the Coulomb blockade spectra of such devices exhibit two distinct sets of peaks, each of which behaves differently with varying interdot tunnel conductance and with temperature. The results conform to recent theories regarding the role of interdot quantum charge fluctuations, and provide evidence for the possible role of inelastic cotunneling between dots at finite interdot conductances.