Energy Level Separation Research Articles

Control of vibrational states by spin-polarized transport in a carbon nanotube resonator

We study spin-dependent transport in a suspended carbon nanotube quantum dot in contact with two ferromagnetic leads and with the dot's spin coupled to the flexural mechanical modes. The spin-vibration interaction induces spin-flip processes between the two energy levels of the dot. This interaction arises from the spin-orbit coupling or a magnetic field gradient. The inelastic vibration-assisted spin flips give rise to a mechanical damping and, for an applied bias voltage, to a steady nonequilibrium occupation of the harmonic oscillator. We analyze these effects as function of the energy-level separation of the dot and the magnetic polarization of the leads. Depending on the magnetic configuration and the bias-voltage polarity, we can strongly cool a single mode or pump energy into it. In the latter case, we find that within our approximation, the system approaches eventually a regime of mechanical instability. Furthermore, owing to the sensitivity of the electron transport to the spin orientation, we find signatures of the nanomechanical motion in the current-voltage characteristic. Hence, the vibrational state can be read out in transport measurements.

Optical properties in symmetric and asymmetric Pöschl–Teller potentials under intense laser field

The effect of a high-frequency laser field on both an asymmetric and a symmetric Pöschl–Teller potentials is studied by the laser dressing parameter which depends on intensity and frequency of the laser field. The asymmetry in the symmetric Pöschl–Teller potential is introduced through an applied electric field. The parameters in the potentials and the electric field strength in the symmetric potential are carefully adjusted to obtain similar energy level separations in both the wells. Both the laser dressed potentials have infinite barrier heights. In these wells the linear susceptibility, the second-order susceptibilities such as optical rectification and second harmonic generation and the third-order nonlinear susceptibility as well as the susceptibility for the third harmonic generation are calculated using the density matrix method. The absorption coefficients and the changes in refractive index are obtained from linear and third-order nonlinear susceptibilities. When the laser dressing parameter increases, the optical properties are blue shifted and reduced in magnitudes due to higher energy separations and lower dipole matrix elements, respectively. The optical properties in the symmetric potential are higher than those in the asymmetric potential due to stronger dipole matrix elements in the symmetric potential.

Motional averaging in a superconducting qubit

Superconducting circuits with Josephson junctions are promising candidates for developing future quantum technologies. Of particular interest is to use these circuits to study effects that typically occur in complex condensed-matter systems. Here we employ a superconducting quantum bit--a transmon--to perform an analogue simulation of motional averaging, a phenomenon initially observed in nuclear magnetic resonance spectroscopy. By modulating the flux bias of a transmon with controllable pseudo-random telegraph noise we create a stochastic jump of its energy level separation between two discrete values. When the jumping is faster than a dynamical threshold set by the frequency displacement of the levels, the initially separate spectral lines merge into a single, narrow, motional-averaged line. With sinusoidal modulation a complex pattern of additional sidebands is observed. We show that the modulated system remains quantum coherent, with modified transition frequencies, Rabi couplings, and dephasing rates. These results represent the first steps towards more advanced quantum simulations using artificial atoms.

Adiabatic quantum optimization with qudits

Most realistic solid state devices considered as qubits are not true two-state systems. If the energy separation of the upper energy levels from the lowest two levels is not large, then these upper states may affect the evolution of the ground state over time and therefore cannot be neglected. In this work, we study the effect of energy levels beyond the lowest two energy levels on adiabatic quantum optimization in a device with a double-well potential as the basic logical element. We show that the extra levels can be modeled by adding additional ancilla qubits coupled to the original logical qubits, and that the presence of upper levels has no effect on the final ground state. We also study the influence of upper energy levels on the minimum gap for a set of 8-qubit spin glass instances.

Ultrastrong Coupling Regime and Plasmon Polaritons in Parabolic Semiconductor Quantum Wells

Ultrastrong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohn's theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance frequency as well as a polariton gap of width 2π × 670 GHz are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultrastrong coupling regime.

Zero-field splitting of the ground state and local lattice distortions for Fe3+ ions at the tetragonal FeF5O cluster center in Fe3+:KMgF3 crystals

The relations between the zero-field splitting (ZFS) parameters and the structural parameters of the tetragonal FeF5O cluster center in Fe3+:KMgF3 crystals have been established by means of the microscopic spin Hamiltonian theory and the superposition model (SPM). On the basis of this, the local structure distortions, the second-order ZFS parameter D, the fourth-order one (a+2F/3), and the energy level separations Δ1 and Δ2 of the ground spin state for Fe3+ ion doped in Fe3+:KMgF3 crystals are theoretically investigated. We use complete diagonalization method (CDM) and take into account the electronic magnetic interactions, i.e. the spin–spin (SS), the spin-other-orbit (SOO), and the orbit–orbit (OO) interactions, besides the well-known spin–orbit (SO) interaction. This investigation reveals that the replacement of O2− for F− and the induced lattice relaxation ΔR2(O), combined with an inward relaxation of the nearest five fluorine ions ΔR1(F) give rise to a strong tetragonal crystal field, which yields the large ZFS of the ground state. The theoretically calculated parameters D, (a+2F/3), Δ1, and Δ2 for Fe3+:KMgF3 crystals are in good agreement with experimental ones when the five F− ions move toward Fe3+ by |ΔR1(F)|=6.40×10−4nm and the O2− ion toward Fe3+ by |ΔR2(O)|=10.55×10−3nm.

Magnetic phase diagram of Eu1−xGdxS

We propose a model to investigate the magnetic properties of Eu${}_{1\ensuremath{-}x}$Gd${}_{x}$S consisting of an s-f term with chemical disorder due to the separation of the atomic energy levels of the $s$ electrons $\ensuremath{\Delta}V$ and a disordered Heisenberg term. The model is solved by using a self-consistent modified Rudermann-Kittel-Kasuya-Yosida technique. Disorder is accounted for by the coherent potential approximation, and the electronic self-energy is obtained by using a suitable alloy analogy. By comparing with experimental data for the red-shift of the absorption edge at $x=0.01$, and considering the fact that an insulator-metal transition takes place at the same concentration, we are able to ascertain $\ensuremath{\Delta}V$ at about $0.58$ eV. We compute the critical temperatures in dependence on $x$ and obtain a transition from ferromagnetism to AFM-II type antiferromagnetism at $x\ensuremath{\approx}0.4$. The spin-glass phase is briefly discussed within the mean-field approximation. We also compute the critical temperatures for the pure s-f model including antiferromagnetism on the simple cubic and the face-centered-cubic lattices in dependence on the band occupation $n$, and obtain the following sequences of phases: ferromagnetism (FM) $\ensuremath{\rightarrow}$ AFM-A $\ensuremath{\rightarrow}$ AFM-C $\ensuremath{\rightarrow}$ AFM-G for the former, and FM $\ensuremath{\rightarrow}$ AFM-II $\ensuremath{\rightarrow}$ AFM-III $\ensuremath{\rightarrow}$ AFM-I for the latter.

Low-lying electronic states of the ferrous high-spin ( S = 2) heme in deoxy-Mb and deoxy-Hb studied by highly-sensitive multi-frequency EPR

The low-lying electronic states of the ferrous high-spin heme in deoxy-myoglobin (deoxy-Mb) and deoxy-hemoglobin (deoxy-Hb) were probed by multi-frequency electron paramagnetic resonance (MFEPR) spectroscopy. An unexpected broad EPR signal was measured at the zero magnetic field using cavity resonators at 34–122 GHz that could not be simulated using any parameter sets for the S = 2 spin Hamiltonian assuming spin quintet states in the 5 B 2 ground state. Furthermore, we have observed novel, broad EPR signals measured at 70–220 GHz and 1.5 K using a single pass transmission probe. These signals are attributed to the ferrous high-spin heme in deoxy-Mb and deoxy-Hb. The resonant peaks shifted to a higher magnetic field with increasing frequency. The energy level separation between the ground singlet and the first excited state at the zero magnetic field was directly estimated to be 3.5 cm − 1 for deoxy-Hb. For deoxy-Mb, the first two excited singlet states are separated by 3.3 cm − 1 and 6.5 cm − 1 , respectively, from the ground state. The energy gap at the zero magnetic field is directly derived from our MFEPR for deoxy-Mb and deoxy-Hb and strongly supports the theoretical analyses based on the Mössbauer and magnetic circular dichroism experiments.

Redshift and discrete energy level separation of self-assembled quantum dots induced by strain-reducing layer

We theoretically studied the role of a InGaAs and InAlAs strain-reducing layer (SRL) with different thicknesses and indium compositions covered on InAs/GaAs self-assembled quantum dots (QDs). The ground-state transition wavelength increases as the thickness and indium composition of the SRL increase. The energy separation between ground state and excited state can achieve the maximum by a proper design. The redshift is due to (1) the strain reducing in QD, (2) the potential barrier, and (3) effective mass reducing in SRL, but the latter two tend to cancel each other and the energy level separation is mainly determined by (2) and (3).

Energy bands, conductance, and thermoelectric power for ballistic electrons in a nanowire with spin-orbit interaction

We calculated the effects of spin-orbit interaction on the energy bands, ballistic conductance (G), and the electron-diffusion thermoelectric power (Sd) of a nanowire by varying the temperature, electron density, and width of the wire. The potential barriers at the edges of the wire are assumed to be very high. A consequence of the boundary conditions used in this model is determined by the energy band structure, resulting in wider plateaus when the electron density is increased due to larger energy-level separation as the higher subbands are occupied by electrons. The nonlinear dependence of the transverse confinement on position with respect to the well center excludes the “polelike feature” in G which is obtained when a harmonic potential is employed for confinement. At low temperature, Sd increases linearly with T but deviates from the linear behavior for large values of T.

Energy level scheme ofInAs/InxGa1−xAs/GaAsquantum-dots-in-a-well infrared photodetector structures

A thorough investigation of quantum-dots-in-a-well structures for infrared photodetector applications has been performed employing different experimental techniques. The electronic structure of self-assembled InAs quantum dots embedded in an In0.15Ga0.85As/GaAs quantum well (QW) was deduced from photoluminescence (PL) and PL excitation (PLE) spectroscopy. From polarization-dependent PL it was revealed that the quantum dots hold two electron energy levels and two heavy-hole levels. Tunnel capacitance spectroscopy confirmed an electron energy level separation of about 50 meV, and additionally, that the conduction-band ground state and excited state of the dots are twofold and fourfold degenerates, respectively. Intersubband photocurrent spectroscopy, combined with simultaneous interband pumping of the dots, revealed a dominant transition at 150 meV (8.5 mu m) between the ground state of the quantum dots and the excited state of the QW. Results from detailed full three-dimensional calculations of the electronic structure, including effects of composition intermixing and interdot interactions, confirm the experimentally unravelled energy level scheme of the dots and well.

Thermoelectric and thermal rectification properties of quantum dot junctions

The electrical conductance, thermal conductance, thermal power and figure of merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green function technique. In the linear response regime the ZT values are still very impressive in the small tunneling rates case, although the effect of electron Coulomb interaction on ZT is significant. In the nonlinear response regime, we have demonstrated that the thermal rectification behavior can be observed for the coupled QDs system, where the very strong asymmetrical coupling between the dots and electrodes, large energy level separation between dots and strong interdot Coulomb interactions are required.

Tuning the Optoelectronic Properties of Carbazole/Oxadiazole Hybrids through Linkage Modes: Hosts for Highly Efficient Green Electrophosphorescence

AbstractA series of bipolar transport host materials: 2,5‐bis(2‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (o‐CzOXD) (1), 2,5‐bis(4‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (p‐CzOXD) (2), 2,5‐bis(3‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (m‐CzOXD) (3) and 2‐(2‐(9H‐carbazol‐9‐yl)phenyl)‐5‐(4‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (op‐CzOXD) (4) are synthesized through simple aromatic nucleophilic substitution reactions. The incorporation of the oxadiazole moiety greatly improves their morphological stability, with Td and Tg in the range of 428–464 °C and 97–133 °C, respectively. The ortho and meta positions of the 2,5‐diphenyl‐1,3,4‐oxadiazole linked hybrids (1 and 3) show less intramolecular charge transfer and a higher triplet energy compared to the para‐position linked analogue (2). The four compounds exhibit similar LUMO levels (2.55–2.59 eV) to other oxadiazole derivatives, whereas the HOMO levels vary in a range from 5.55 eV to 5.69 eV, depending on the linkage modes. DFT‐calculation results indicate that 1, 3, and 4 have almost complete separation of their HOMO and LUMO levels at the hole‐ and electron‐transporting moieties, while 2 exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Phosphorescent organic light‐emitting devices fabricated using 1–4 as hosts and a green emitter, Ir(ppy)3 or (ppy)2Ir(acac), as the guest exhibit good to excellent performance. Devices hosted by o‐CzOXD (1) achieve maximum current efficiencies (ηc) as high as 77.9 cd A−1 for Ir(ppy)3 and 64.2 cd A−1 for (ppy)2Ir(acac). The excellent device performance may be attributed to the well‐matched energy levels between the host and hole‐transport layers, the high triplet energy of the host and the complete spatial separation of HOMO and LUMO energy levels.

Electrically controlled Fano lines in double quantum dot system with intra-dot Coulomb interaction

Theoretical study of quantum interference processes which take place during electron transport through artificial molecule formed by double quantum dot system is presented. The non-equilibrium Green function formalism based on the equation of motion method within Hartree–Fock approximation is used to calculate the conductance spectrum. Interplay between intra-dot Coulomb correlations described by parameter U and the separation of energy levels in both dots is discussed. For small U multiple Fano resonances described by Fano parameters of opposite signs and correlated with steps in electron accumulation on the dots can be observed. Positions of Fano lines as well as their shapes can be easily controlled with use of the gate voltage.

Modeling and fabrication of electrically tunable quantum dot intersubband devices

We propose an idea of forming quantum dot intersubband transition devices based on lateral electrical confinement on quantum wells. Numerical simulations show that the energy level separation in the structure can be as large as about 50 meV, and with different electric field, the energy levels can be tuned. We also demonstrate the fabrication of a large number of field-induced quantum dots by our super lens lithography technique. We achieved uniform arrays of contacts that are about 200 nm using a conventional UV source of λ∼400 nm.

NMR imaging analogue of the individual qubit operations in superconducting flux-qubit chains

A solid-state quantum computer conventionally requires the control lines for each qubit to operate individually. This makes quantum circuits complicated, especially in multi-qubit systems. Here we propose a simple single-qubit operation in a superconducting flux-qubit chain without control lines in an analogy of the Nuclear Magnetic Resonance (NMR) imaging technique. A superconducting flux qubit can be regarded as an artificial atom with spin 1/2 that can be manipulated by an external magnetic field. To identify the location of each qubit in the qubit chain, a static field gradient is introduced along the qubit chain. This magnetic field gradient causes differences in the Zeeman frequencies of each separate qubit. This allows a large number of distinct qubits to be individually addressed. Single-qubit operations are then possible when the external electromagnetic frequency is adjusted to the resulting energy-level separation of the target qubit.

The two-Josephson-junction flux qubit with large tunneling amplitude

In this paper we discuss solid-state nanoelectronic realizations of Josephson flux qubits with large tunneling amplitude between the two macroscopic states. The latter can be controlled via the height and form of the potential barrier, which is determined by quantum-state engineering of the flux qubit circuit. The simplest circuit of the flux qubit is a superconducting loop interrupted by a Josephson nanoscale tunnel junction. The tunneling amplitude between two macroscopically different states can be increased substantially by engineering of the qubit circuit if the tunnel junction is replaced by a ScS contact. However, only Josephson tunnel junctions are particularly suitable for large-scale integration circuits and quantum detectors with present-day technology. To overcome this difficulty we consider here a flux qubit with high energy-level separation between the “ground” and “excited” states, consisting of a superconducting loop with two low-capacitance Josephson tunnel junctions in series. We demonstrate that for real parameters of resonant superposition between the two macroscopic states the tunneling amplitude can reach values greater than 1K. Analytical results for the tunneling amplitude obtained within the semiclassical approximation by the instanton technique show good correlation with a numerical solution.

Consistency of Ground State and Spectroscopic Measurements on Flux Qubits

We compare the results of ground state and spectroscopic measurements carried out on superconducting flux qubits which are effective two-level quantum systems. For a single qubit and for two coupled qubits we show excellent agreement between the parameters of the pseudospin Hamiltonian found using both methods. We argue that by making use of the ground state measurements the Hamiltonian of N coupled flux qubits can be reconstructed as well at temperatures smaller than the energy level separation. Such a reconstruction of a many-qubit Hamiltonian can be useful for future quantum information processing devices.

Electric quadrupole transitions of K-like ions (Z=22—42)

A fully relativistic multiconfiguration Dirac-Fock method with Breit and QED corrections is used to calculate the 4s 2S─3d 2D(Z=22—42)transition energy level separations, transition probabilities and oscillator strengths for the K-like ions. In the calculation, we have considered the significant Breit and QED corrections. The results are in good agreements with recent experimental data and other theoretical values. The results show that the electric quadrupole transition probabilities are in correspondence with these of E1 transitions and can not be ignored in the laser plasma of high temperature in ICF and MCF Fusion.

Simulation of quantum wells with ‘spikes’ and ‘dips’

The use of thin high-bandgap ‘spikes’ or thin low-bandgap ‘dips’ inside conventional rectangular quantum wells (QWs) gives supplementary flexibility in engineering intra- and inter-band energy level separation. The paper presents simulation and experimental studies on the effects of ‘spikes’ and ‘dips’ on the fundamental quantum well properties.