Quantum Speed Limit for Dynamics of Central Spin Model with Initial System-Bath Correlations

We investigate the quantum speed limit (QSL) time of an electronic spin coupled to a bath of nuclear spins. We consider three types of initial states with different correlations between the system and bath, i.e., quantum correlation, classical correlation, and no any correlation. Interestingly, we show that the QSL times of the central spin for these three types of initial correlations are identical when the couplings are homogeneous. However, it is remarkable different for inhomogenous couplings. The QSL time of the central spin is sensitive to the initial states, the average coupling strength, the distribution of the couplings between the system and bath and the number of the nuclear spins in the bath. Furthermore, we find that the coherence in the initial state has significant influences on the QSL time of the system, and can lead to the increase of QSL time for homogeneous couplings.     

Preserving coherence in Rydberg quantum bits

The effectiveness of decoherence suppression schemes is explored using quantum bits (qubits) stored in Li np Rydberg states. Following laser excitation, pulsed electric fields coherently control the electronic spin-orbit coupling, facilitating qubit creation, manipulation, and measurement. Spin-orbit coupling creates an approximate decoherence-free subspace for extending qubit storage times. However, sequences of fast NOT operations are found to be substantially more effective for preserving coherence.     

Angle-resolved photoemission study for double Ag nanofilm structures

We have carried out an angle-resolved photoemission study for Ag/Cu/Ag/Cu(1 1 1) system in order to investigate the electronic coupling between the two quantum-well (QW) states in the double Ag nanofilm structures. It is found that the outer nanofilm thickness dependence of QW state in double Ag nanofilm structures can be explained as the electronic coupling through the thin Cu barrier layer between the QW states in the inner and outer Ag nanofilms. It is also found that the coupling strength depends on the Cu barrier thickness. From these results, we discuss the electronic coupling between the two QW states in the double Ag nanofilm structures.     

Coherent electronic coupling versus localization in individual molecular dimers

We have investigated electronic excitation transfer in individual molecular dimers by time and spectrally resolved confocal fluorescence microscopy. The single molecule measurements allow for directly probing the distribution of the electronic coupling strengths due to static disorder in the polymer host. We find dimers where the excitation is delocalized (superradiant emission) while for others emission originates from a localized state. Transitions between delocalized and localized states as observed for a given dimer are attributed to structural fluctuations of the guest-host system.     

Double resonance mechanism of ferromagnetism and magnetotransport in (Ga-Mn)As

We calculate the electronic states of the Mn-doped semiconductors and show that resonant states are formed at the top of the down spin valence band due to magnetic impurities and that they give rise to a strong and long-ranged ferromagnetic coupling between Mn moments. We propose that the coupling of the resonant states, in addition to the intra-atomic exchange interaction between the resonant and nonbonding states, is the origin of the ferromagnetism of (Ga-Mn)As. The mechanism is thus called "double resonance." The resonant states bring about the spin-dependent resistivity to produce magnetoresistive properties in (Ga-Mn)As and their junctions.     

Electronic states on the surface of graphite

Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip-sample distance beyond which the effect of sample-tip interaction is negligible.     

Anharmonic <Emphasis Type="Italic">T</Emphasis> ⊗ ɛ Jahn-Teller coupling in LiCaAlF<Subscript>6</Subscript>: Cr<Superscript>3+</Superscript>

For an octahedral system, we analyzed the coupling between the triple degenerate electronic states of a transition metal ion and the double degenerate vibration of the ligands of the host matrix. The vibrations of the ligands of the lattice are described by new anharmonic coherent states of the Morse potential. For the linear coupling between electronic states and anharmonic vibrations, we built the matrix elements from the interaction Hamiltonian and corresponding energy levels.     

Inter-state coupling and definitions of bright and dark states

Contrary to a standard definition of diabatic states (i.e., those without momentum-dependent coupling), based on the construction from adiabatic ones, we defined diabatic states as bright and dark states of a given experiment. Namely, they are defined as states providing maximum, respectively, zero value of electronic transition dipole moments projected to a given polarization vector. Second, the state from (or to) which the optical transition is performed is not from the space of investigated electronic excited state manifold, but it is chosen by the observer. It is shown, for this case, that the inter-state coupling is a general function of vibrational coordinates. The explicit dependence of the inter-state coupling on vibrational coordinates is particularly important for system with strong Stokes shift. The role of exact definitions of bright and dark states as well as the inter-state coupling is discussed with respect to the coherent structure of electronic population observed in optical spectroscopy.     

Chiroptical spectra of molecules with nearly degenerate electronic states

The influence of vibronic interactions on the chiroptical spectra associated with pairs of nearly degenerate electronic transitions in chiral systems is examined on a formal theoretical model. We consider the special case in which two nearly degenerate electronic states are coupled by a single non-totally symmetric vibrational mode. Formal expressions are developed for the rotatory strengths of individual vibronic transitions in this coupled system. Calculations based on these expressions are carried out for a large number of parameter sets appropriate for various energy spacings between the unperturbed electronic states, vibronic coupling strengths, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. The calculated results demonstrate the profound influence of vibronic interactions on the sign patterns and intensity distributions within the rotatory strength spectrum associated with the two coupled electronic states. The implications of these results for interpretations of circular dichroism spectra are discussed.     

Experimental evidence of synchronization of time-varying dynamical network

We investigate synchronization of time varying networks and stability conditions. We derive interesting relations between the critical coupling constants for synchronization and switching times for time-varying and time average networks. The relations are based on the additive property of Lyapunov exponents and are verified experimentally in electronic circuit.     

Mechanisms of ultrafast fluorescence depletion spectroscopy and applications to measure slovation dynamics of coummarin 153 in methanol

Subpicosecond fluorescence depletion spectroscopy (FDS) was used to measure the solvation dynamics of coumarin 153 (C153) in methanol. The FDS mechanisms were discussed. A quasi-continuous model was used to describe the solvational relaxation of excited states. The perturbations of the probe pulse on the excited sample system, including up-conversion and stimulated emission, were sufficiently discussed. For a probe molecule used in the FDS experiment, ensuring that the up-conversion perturbation can be negligible is important. FDS was found to be a good technique for measuring the solvation dynamics of C153 in methanol.     

Two electronic states and state exchange time control in spherical nanolayer

Two electronic states in impenetrable spherical quantum nanolayer are discussed. The Coulomb interaction between the electrons is considered as perturbation. The problem is discussed within the frameworks of Russell-Saunders coupling scheme, that is, the spin-orbit interaction is neglected. For this system the analogue of helium atom theory is represented. The dependence of the Coulomb interaction energy of the two electronic system is obtained both upon inner and outer radiuses of the studied nanolayer. The exchange interaction in the spherical nanolayer helium atom and its dependence upon the geometrical parameters of the nanolayer are investigated. It is shown that the exchange time of two electron states could be controlled via changing the geometrical parameters of the nanolayer.     

Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon

We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time.     

Highly asymmetric atom-lead coupling in nanowires for resonance regime

Concerning quantum transport at resonant states in a one-atom nanowire (nanoconstriction), for the first time, a theoretical study is presented upon strongly asymmetric coupling between the atom at the nanoconstriction and the involved leads. Under these conditions, an approximate relationship for the conductance at resonance in a single-atom wire is considered and an expression for the number of resonant states is derived and discussed. In addition, the density of resonant electronic states is determined under a quantum-box approach by which the conduction electrons are assumed to behave as quantum harmonic oscillators confined in a potential well transverse to the wire.     

Dephasing of excitons in quantum well structures studied by picosecond time-resolved resonant Rayleigh scattering

Electronic states in solids with disorder give rise to an elastic (Rayleigh) contribution to the scattering spectrum which becomes resonantly enhanced for excitation in the electronic transition. It is shown theoretically that from this resonant Rayleigh process, if temporally resolved, the coherence time of the electronic states may be deduced. Experimentally this is demonstrated for the first time by studying the n = 1 heavy-hole exciton in GaAs/AlGaAs quantum well structures. Employing picosecond time-resolved spectroscopy and analyzing the data within the developed theory, coherence times are found between 5 and 30 ps in agreement with earlier results obtained by non-linear optical techniques.     

Investigation of electronic coupling in semiconductor double quantum wells using coherent optical two-dimensional Fourier transform spectroscopy

We investigate electronic coupling in asymmetric semiconductor double quantum wells using a new spectroscopy method, optical two-dimensional Fourier transform (2D-FT) spectroscopy. Measurements on two samples with different barrier thicknesses show drastically different 2D-FT spectra. We compare these measurements to conventional one-dimensional four-wave-mixing measurements, highlighting the unique advantages of the 2D-FT spectroscopy. An oscillatory behavior in the intensity of the cross peaks as a function of the mixing time is observed. This oscillation is attributed to interference between different quantum mechanical pathways, and its features are determined by the non-radiative Raman coherence between dipole-forbidden states.     

Controlled modification of individual adsorbate electronic structure

Modification of the electronic structure of a single Mn adsorbate placed within a geometrical array of adatoms on Ag(111) is observed using local spectroscopy with the scanning tunneling microscope. The changes result from coupling between the adsorbate level and surface electronic states of the substrate. These surface states are scattered coherently within the adatom array, mediating the presence and shape of the array to the adsorbate within. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.     

Electronic states and tunneling times in coupled self-assembled quantum dots

Electron energy levels in single dots, and energy splitting and tunneling times in stacked quantum dots are calculated as functions of structure parameters. An effective mass approach is used to solve the Schrödinger equation for cylindrical dots with finite confinement potentials. Strong confinement due to small sizes produces quantized energy levels in single dots and strong interactions of the wavefunctions with adjacent dots. This electronic coupling induces significant energy splittings and short tunneling times for characteristic structures used in experiments. This coupling may even yield coherent artificial molecular states with different optical properties.