Effects of a collective spin resonance mode on the scanning tunneling microscopy spectra of d-wave superconductors

A high-energy spin resonance mode is known to exist in many high-temperature superconductors. Motivated by recent scanning tunneling microscopy experiments in superconducting Bi(2)Sr(2)CaCu(2)O(8+delta), we study the effects of this resonance mode on the local density of states (LDOS). The coupling between the electrons in a d-wave superconductor and the resonance mode produces high-energy peaks in the LDOS, which displays a two-unit-cell periodic modulation around a nonmagnetic impurity. This suggests a new means to not only detect the dynamical spin collective mode but also study its coupling to electronic excitations.     

Fractionalization noise in edge channels of integer quantum Hall states

A theoretical calculation is presented of current noise which is due charge fractionalization, in two interacting edge channels in the integer quantum Hall state at filling factor ν=2. Because of the capacitive coupling between the channels, a tunneling event, in which an electron is transferred from a biased source lead to one of the two channels, generates propagating plasma mode excitations which carry fractional charges on the other edge channel. When these excitations impinge on a quantum point contact, they induce low-frequency current fluctuations with no net average current. A perturbative treatment in the weak tunneling regime yields analytical integral expressions for the noise as a function of the bias on the source. Asymptotic expressions of the noise in the limits of high and low bias are found.     

Effects of tunneling coupling on plasmon modes in asymmetric double-quantum-well structures

The collective charge density excitations in asymmetric double-quantum-well (DQW) structures with different tunneling strengths are systematically studied. In particular, the damping properties of the plasmon modes in various tunneling strengths are investigated in detail. It is shown that plasmon modes in asymmetric DQW structures are quite different from those in symmetric DQW systems. In weak tunneling regime, an intra-subband mode ω _- with an acoustic-like dispersion relation which is damped in symmetric DQW structures arises and coexists with the optical-like mode ω ₊ while the inter-subband mode ω ₁₀ is highly damped. With the tunneling strength being increased, the ω ₁₀ branch gradually becomes undamped and emerges out of the (1-0) single-particle continuum, whereas the ω _- branch gradually approaches the (0-0) single-particle continuum. In intermediate coupling regime, these three branches of modes coexist undamped. In strong tunneling regime, ω _- enters the (0-0) single-particle continuum and becomes damped. Consequently, only the ω ₊ and ω ₁₀ modes exist in this regime. Received 10 July 2001 and Received in final form 17 September 2001     

Coherent ultrafast dynamics in the quantum Hall effect regime

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in the quantum Hall effect regime. We find that the time and frequency profile of the four-wave-mixing non-linear optical spectrum is strongly influenced by Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG collective excitations. We discuss experimental and theoretical results showing non-Markovian memory effects in the polarization dephasing, and an optically induced time-dependent coupling between the two lowest Landau level magnetoexcitons.     

Spin waves in a one-dimensional spinor bose gas

We study a one-dimensional (iso)spin 1/2 Bose gas with repulsive delta-function interaction by the Bethe Ansatz method and discuss the excitations above the polarized ground state. In addition to phonons the system features spin waves with a quadratic dispersion. We compute analytically and numerically the effective mass of the spin wave and show that the spin transport is greatly suppressed in the strong coupling regime, where the isospin-density (or "spin charge") separation is maximal. Using a hydrodynamic approach, we study spin excitations in a harmonically trapped system and discuss prospects for future studies of two-component ultracold atomic gases.     

Classical behavior of strongly correlated electron systems of solids near a quantum critical point

The spectra of collective excitations of electron systems situated near a Fermi-liquid quantum critical point are analyzed. Phonon-like soft and weakly damped branches of the transverse zero-sound mode are found that have extremely low effective Debye temperatures. Their contributions to the collision integral are shown to drive electron transport in the vicinity of the critical point toward a classical regime.     

Enhanced photon emission by field emission resonances and local surface plasmon in tunneling junction

We investigated the photon emission spectra on Ag (111) surface excited by tunneling electrons using a low temperature scanning tunneling microscope in ultrahigh vacuum. Characteristic plasmon modes were illustrated as a function of the bias voltage. The one electron excitation process was revealed by the linear relationship between the luminescence intensity and the tunneling current. Luminescence enhancement is observed in the tunneling regime for the relatively high bias voltages, as well as at the field emission resonance with bias voltage increased up to 9 V. Presence of a silver (Ag) nanoparticle in the tunneling junction results in an abnormally strong photon emission at the high field emission resonances, which is explained by the further enhancement due to coupling between the localized surface plasmon and the vacuum. The results are of potential value for applications where ultimate enhancement of photon emission is desired.     

Interband collective electronic excitations in resonant Raman scattering of two-subband quantum wires

Using the bosonization technique, a theory for the collective excitations of the interacting electrons in quantum wires with two subbands occupied is developed. The dispersion relations for the inter-subband charge and spin density excitations are determined. The results are used to interpret the features observed in recent measurements of the Raman spectra of AlGaAs/GaAs quantum wires, particularly for photon energies near band gap resonance. It is shown that peaks previously identified as “single particle excitations” are signatures of higher order collective spin density excitations. Predictions about the observability of the interband modes are made. Received 8 February 1999     

Transport properties of quantum dots

Linear and nonlinear transport through a quantum dot that is weakly coupled to ideal quantum leads is investigated in the parameter regime where charging and geometrical quantization effects coexist. The exact eigenstates and spins of a finite number of correlated electrons confined within the dot are combined with a rate equation. The current is calculated in the regime of sequential tunneling. The analytic solution for an Anderson impurity is given. The phenomenological charging model is compared with the quantum mechanical model for interacting electrons. The current-voltage characteristics show Coulomb blockade. The excited states lead to additional fine-structure in the current voltage characteristics. Asymmetry in the coupling between the quantum dot and the leads causes asymmetry in the conductance peaks which is reversed with the bias voltage. The spin selection rules can cause a ‘spin blockade’ which decreases the current when certain excited states become involved in the transport. In two-dimensional dots, peaks in the linear conductance can be suppressed at low temperatures, when the total spins of the corresponding ground states differ by more than 1/2. In a magnetic field, an electron number parity effect due to the different spins of the many-electron ground states is predicted in addition to the vanishing of the spin blockade effect. All of the predicted features are consistent with recent experiments.     

Spin valve effect in a magnetic nanoelectromechanical shuttle

We study spin-dependent shuttle phenomena in a nanoelectromechanical single electron transistor (NEM-SET) with magnetic leads by considering the coupling between the transport of spin-polarized electrons and mechanical oscillations of the nanometer quantum dot. It is shown that there are two different bias-voltage thresholds for the shuttle instability in electronic transport through the NEM-SET, respectively, corresponding to parallel (P) and antiparallel (AP) magnetization alignments. In between the two thresholds, the electronic transport is in the shuttling regime for the P alignment but in the tunneling regime for the AP one, resulting in a very large spin valve effect.     

ac Josephson effect and resonant Cooper pair tunneling emission of a single Cooper pair transistor

We measure the high-frequency emission of a single Cooper pair transistor (SCPT) in the regime where transport is only due to tunneling of Cooper pairs. This is achieved by coupling on chip the SCPT to a superconductor-insulator-superconductor junction and by measuring the photon assisted tunneling current of quasiparticles across the junction. This technique allows a direct detection of the ac Josephson effect of the SCPT and provides evidence of Landau-Zener transitions for proper gate voltage. The emission in the regime of resonant Cooper pair tunneling is also investigated. It is interpreted in terms of transitions between charge states coupled by the Josephson effect.     

Effect of frequency dependent electron-electron interaction on resonant tunneling

In electron resonant tunneling through a double barrier structure, we show that dynamical electron-electron interactions in the resonant well can give rise to additional tunneling satellites due to collective electronic excitations. We present a first principle treatment for frequency-dependent electron-electron interactions in the resonant tunneling problem. The result confirms the previously proposed plasmon assisted resonant tunneling mechanism. We also find that the particle-hole excitation has very little effect on resonant tunneling. Our result can be applied to study the effects of various electronic excitations on the resonant tunneling of electrons.     

Mesoscopic stoner instability in metallic nanoparticles revealed by shot noise

We study sequential tunneling through a metallic nanoparticle close to the Stoner instability coupled to parallel magnetized electrodes. Increasing the bias voltage successively opens transport channels associated with excitations of the nanoparticle's total spin. For the current this leads just to a steplike increase. The Fano factor, in contrast, shows oscillations between large super-Poissonian and sub-Poissonian values as a function of bias voltage. We explain the enhanced Fano factor in terms of generalized random-telegraph noise and propose the shot noise as a convenient tool to probe the mesoscopic Stoner instability.     

Model for dissipative conductance in fractional quantum Hall states

We present a model of dissipative transport in the fractional quantum Hall regime. Our model takes account of tunneling through saddle points in the effective potential for excitations created by impurities. We predict the temperature range over which activated behavior is observed and explain why this range nearly always corresponds to around a factor two in temperature in both integer quantum Hall and fractional quantum Hall systems. We identify the ratio of the gap observed in the activated behavior and the temperature of the inflection point in the Arrhenius plot as an important diagnostic for determining the importance of tunneling in real samples.     

Strongly correlated fractional quantum Hall line junctions

We have studied a clean finite-length line junction between interacting counterpropagating single-branch fractional quantum Hall edge channels. Exact solutions for low-lying excitations and transport properties are obtained when the two edges belong to quantum Hall systems with different filling factors and interact via the long-range Coulomb interaction. Charging effects due to the coupling to external edge-channel leads are fully taken into account. Conductances and power laws in the current-voltage characteristics of tunneling are strongly affected by interedge correlations.     

Supersymmetry and electron-hole excitations in semiconductors at finite temperature

The fermionic and bosonic electron-hole low-lying excitations in a semiconductor are analyzed at finite temperature in a unified way following Nambu's quasi-supersymmetric approach for the BCS model of superconductivity. The effective lagrangian for the fermionic modes and for the bosonic low-lying collective excitations in the semiconductor is no longer supersymmetric in a conventional finite-temperature treatment. However, the bosonic excitations do not couple directly to the heat bath and as a result, quasi-supersymmetry is restored to the effective lagrangian when a redefinition of the coupling constant associated with the collective excitations is performed. Our result shows that although the mass and coupling parameters are now temperature dependent, the fermion and boson excited states pair together and can still be transmuted into one another.     

Low-energy monopole modes of a trapped atomic Fermi gas

We consider the low energy collective monopole modes of a trapped weakly interacting atomic Fermi gas in the collisionless regime. The spectrum is calculated for varying coupling strength and chemical potential. Using an effective Hamiltonian, we derive analytical results that agree well with numerical calculations in various regimes. The onset of superfluidity is shown to lead to effects such as the vanishing of the energy required to create a Cooper molecule at a critical coupling strength and to the emergence of pair vibration excitations. Our analysis suggests ways to experimentally detect the presence of the superfluid phase in trapped atomic Fermi gases.     

A computational study of the optical response of strongly coupled metal nanoparticle chains

We study the optical response of strongly coupled metal nanoparticle chains using rigorous multiple scattering calculations. The collective resonant frequency of silver nanosphere chains and the coupling between chains are considered. The coupling between silver nanoparticle chains are understood by the transmission and reflection calculations of 2D periodic arrays of nanospheres. The results are in agreement with recent experiments. The splitting of plasmon resonance modes for different polarizations of the incident light are explored. Our results show that the transverse mode resonant wavelength is very sensitive to the inter-chain distance. Results on the effect of disorder are also presented.