XOR gate response in a mesoscopic ring with embedded quantum dots

We address XOR gate response in a mesoscopic ring threaded by a magnetic flux . The ring, composed of identical quantum dots, is symmetrically attached to two semi-infinite one-dimensional metallic electrodes and two gate voltages, viz, V_a and V_b, are applied, respectively, in each arm of the ring which are treated as the two inputs of the XOR gate. The calculations are based on the tight-binding model and the Green’s function method, which numerically compute the conductance–energy and current–voltage characteristics as functions of the ring-electrodes coupling strengths, magnetic flux and gate voltages. Quite interestingly it is observed that, for =₀/2 (₀=ch/e, the elementary flux-quantum) a high output current (1) (in the logical sense) appears if one, and only one, of the inputs to the gate is high (1), while if both inputs are low (0) or both are high (1), a low output current (0) appears. It clearly demonstrates the XOR behavior and this aspect may be utilized in designing the electronic logic gate.     

NOR gate response in a double quantum ring: An exact result

NOR gate response in a double quantum ring, where each ring is threaded by a magnetic flux , is investigated. The double quantum ring is sandwiched symmetrically between two semi-infinite one-dimensional metallic electrodes, and two gate voltages, namely, V_a and V_b, are applied, respectively, in lower arms of the two rings those are treated as the two inputs of the NOR gate. A simple tight-binding model is used to describe the system, and all the calculations are done through the Green’s function formalism. Here we calculate exactly the conductance–energy and current–voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our numerical study predicts that, for a typical value of the magnetic flux =₀/2 (₀=ch/e, the elementary flux-quantum), a high output current (1) (in the logical sense) appears if both the inputs to the gate are low (0), while if one or both are high (1), a low output current (0) results. It clearly demonstrates the NOR gate behavior, and this aspect may be utilized in designing an electronic logic gate.     

Quantum transport in a mesoscopic ring: Evidence of an OR gate

We explore the OR gate response in a mesoscopic ring threaded by a magnetic flux . The ring is symmetrically attached to two semi-infinite one-dimensional metallic electrodes, and two gate voltages, V_a and V_b, are applied in one arm of the ring; these are treated as the two inputs of the OR gate. All the calculations are based on the tight-binding model and the Green’s function method, which numerically compute the conductance–energy and current–voltage characteristics as functions of the gate voltages, ring-to-electrode coupling strengths and magnetic flux. Our theoretical study shows that, for =₀/2 (₀=ch/e, the elementary flux-quantum), a high output current (1) (in the logical sense) appears if one or both the inputs to the gate are high (1), while if neither input is high (1), a low output current (0) appears. It clearly demonstrates the OR gate behavior, and this aspect may be utilized in designing an electronic logic gate.     

Edge current switch of two-dimensional electron gas using carrier density control

We have investigated the effects of electron density discontinuity on the transports of edge currents of two-dimensional electron gas (2DEG). The electric field applied to a gate, which covers the 2DEG partially, gives rise to change in the carrier density and results in a density gradient, which deforms the edge currents. The transverse and longitudinal resistances were measured as functions of gate voltage V_G in the quantum Hall regime. The deviations of the longitudinal resistances from the normal quantum Hall resistances are attributed to the reflections of the edge currents under the influence of the abrupt density discontinuity. A switching behavior of the transverse resistance by controlling the gate voltage was observed when V_G=−2.2 and −2.0 V for magnetic field H=5 and 7.2 T, respectively.     

Flux-dependent anti-crossing of resonances in parallel non-coupled double quantum dots

We present novel resonant phenomena through parallel non-coupled double quantum dots (QDs) embedded in each arm of an Aharonov-Bohm (AB) ring with magnetic flux passing through its center. The electron transmission through this AB ring with each QD formed by two short-range potential barriers is calculated using a scattering matrix at each junction and a transfer matrix in each arm. We show that as the magnetic flux modulates, a distortion of the grid-like square transmission occurs and an anti-crossing of the resonances appears. Hence, the modulation of magnetic flux in this system can have an equivalent effect to the control of inter-dot coupling between the two QDs.     

Thermodynamic quantum critical behavior of the anisotropic Kondo necklace model

The Ising-like anisotropy parameter δ in the Kondo necklace model is analyzed using the bond-operator method at zero and finite temperatures for arbitrary d dimensions. A decoupling scheme on the double time Green's functions is used to find the dispersion relation for the excitations of the system. At zero temperature and in the paramagnetic side of the phase diagram, we determine the spin gap exponent νz≈0.5 in three dimensions and anisotropy between 0?δ?1, a result consistent with the dynamic exponent z=1 for the Gaussian character of the bond-operator treatment. On the other hand, in the antiferromagnetic phase at low but finite temperatures, the line of Neel transitions is calculated for δ?1. For d>2 it is only re-normalized by the anisotropy parameter and varies with the distance to the quantum critical point (QCP) |g| as, T_N∝|g|^ψ where the shift exponent ψ=1/(d-1). Nevertheless, in two dimensions, a long-range magnetic order occurs only at T=0 for any δ?1. In the paramagnetic phase, we also find a power law temperature dependence on the specific heat at the quantum critical trajectoryJ/t=(J/t)_c, T→0. It behaves as C_V∝T^d for δ?1 and ≈1, in concordance with the scaling theory for z=1.     

Neutral and charged excitons in a quantum tube

The optical transition energies of neutral and charged excitons in a quantum tube are calculated as a function of the Aharonov-Bohm magnetic flux Φ. The oscillation amplitude of the ground state energy of the electron-hole relative motion is shown to be larger in a quantum tube than a quantum ring with strong confinement in the axis direction. We find a double maxima structure in the optical transition energy for a quantum tube with radius R = 0.5 in units of the effective Bohr radius because of the difference in the Φ dependencies between the single electron energy and the relative-motion energy of a charged exciton state.     

Commensurate photon-irradiated Fano-Kondo tunneling through a quantum dot embedded Aharonov-Bohm interferometer

The commensurate photon-irradiated mesoscopic transport in a strongly correlated quantum dot (QD) embedded Aharonov-Bohm (AB) interferometer has been investigated. We focus our investigation on the dynamic Kondo and Fano cooperated effect affected by the double commensurate MWFs with q=ω₂/ω₁ being an arbitrary integer, where ω₁ and ω₂ are the two frequencies of the fields. The general tunneling current formula is derived by employing the nonequilibrium Green's function technique, and the different photon absorption and emission processes induced nonlinear properties have been studied to compare with the single-field system where q=0. Our numerical calculations are performed for the special cases with two commensurate fields possessing q=1,2. The Kondo peak can be suppressed to be a Kondo valley for the case where the commensurate number q=1, and the Fano asymmetric structure exhibits in the differential conductance quite evidently. Different commensurate number q contributes different photon absorption and emission effects. However, the conductance for the case of q=2 possesses more peaks and heavier asymmetric structure than the situations of q=0,1. The enhancement of satellite peaks behaves quite differently for the two cases with q=1, and q=2. The asymmetric peak-valley structure is adjusted by the gate voltage, commensurate MWFs, AB flux, source-drain bias, and non-resonant tunneling strength to form novel Fano and Kondo resonant tunneling.     

Asymmetry with respect to the magnetic field direction in the interaction between the quantum states of two coupled superconducting rings

The interaction between the quantum states of two circularly asymmetric superconducting aluminum rings forming figure eight, threaded by a magnetic flux and biased by an external sinusoidal ac current with zero dc component, has been investigated. Quantum oscillations in the dependence V _dc(B) of the rectified dc voltage on magnetic field for these structures have been measured at different external currents and temperatures close to critical. Fourier and wavelet analyses of the function V _dc(B) have revealed, along with the two fundamental ring frequencies, various combination frequencies; this fact is indicative of interaction in the structure. Deviation of the function V _dc(B) from oddness with respect to the magnetic field direction has been found for the first time.     

Characterization of soft-X-ray detectors fabricated with high-quality CVD diamond thin films

We have characterized the performance of soft-X-ray detectors fabricated with undoped and B-doped homoepitaxial diamond layers of high quality which were grown on a commercially available type Ib (1 0 0) substrate by means of a high-power microwave-plasma chemical-vapor-deposition (CVD) method. The signal currents of the diamond-based detectors with thin TiN electrodes formed vertically (along the homoepitaxial growth direction) were measured at room temperature as a function of the applied voltage, V_a, for irradiations of 500-1200 eV soft-X-ray beams ranging from ≈6 × 10⁹ to ≈1 × 10¹¹ photons/s. The deduced apparent quantum efficiencies increased with the increasing V_a and reached to 2.5 × 10³ at V_a = 60 V. As expected from the device structure, the detector performance depended only very slightly on the applied magnetic field up to 10 T. The excellently high sensitivities attained for soft-X-ray photons are discussed in relation to carrier amplification mechanisms which invested the above diamond detectors.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Nonlinear Hall effect in a quasi-two-dimensional electron system

The nonlinear electron transport in GaAs double quantum wells with two occupied size-quantization levels has been studied at a temperature of 4.2 K in the magnetic fields B < 1 T. It has been found that a sinusoidal electric current I _ac induces the generation of higher harmonics of both longitudinal V _xx (B) and Hall V _xy (B) voltages in the quasi-two-dimensional electron system under consideration. The Hall voltage oscillating in the magnetic field has been shown to appear in the electron system with two occupied size-quantization levels in the presence of microwave radiation and dc electric current I _dc. The experimental data indicate the independent contributions of the diagonal and off-diagonal components of the conductivity tensor to the nonlinear magnetotransport at high filling factors.     

Magnetic and optical properties of self-organized InMnAs quantum dots

Ten layers of self-assembled InMnAs quantum dots with InGaAs barrier were grown on high resistivity (1 0 0) p-type GaAs substrates by molecular beam epitaxy (MBE). The presence of ferromagnetic structure was confirmed in the InMnAs diluted magnetic quantum dots. The ten layers of self-assembled InMnAs quantum dots were found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T_C=80 K. It is likely that the ferromagnetic exchange coupling of sample with T_C=80 K is hole mediated resulting in Mn substituting In and is due to the bound magnetic polarons co-existing in the system. PL emission spectra of InMnAs samples grown at temperature of 275, 260 and 240 °C show that the interband transition peak centered at 1.31 eV coming from the InMnAs quantum dot blueshifts because of the strong confinement effects with increasing growth temperature.     

Surface modification of exchange-coupled Co/NiOx magnetic bilayer by bias sputtering

We have investigated the effect of bias voltage on sheet resistance, surface roughness and surface coverage of Co/NiO_x magnetic bilayer. In addition, interface topography and corrosion resistance of the Ta/Co/Cu/Co/NiO_x/Si(1 0 0) system have been studied for Co layers deposited at an optimum bias voltage. Atomic force microscopy (AFM) and four point probe sheet resistance (R_s) measurement have been used to determine surface and electrical properties of the sputtered Co layer at different bias voltages ranging from 0 to −80 V. The Co/NiO_x bilayer exhibits a minimum surface roughness and low sheet resistance value with a maximum surface coverage at V_b=−60 V resulted in a slight increase of magnetic resistance and its sensitivity for the Co/Cu/Co/NiO_x/Si(1 0 0) magnetic multilayers, as compared with the same magnetic multilayers containing unbiased Co layers. The presence of Ta protection layer improves the corrosion resistance of the multilayers by three orders of magnitude in a humid environment.     

Spin-polarization-dependent transport in a quantum dot array coupled with an Aharonov-Bohm ring

In this paper the quantum transport in a dot-array coupled with an Aharonov–Bohm (AB) ring is investigated via single-band tight-binding Hamiltonian. It is shown that the output spin current is a periodic function of the magnetic flux in the quantum unit Φ₀. The resonance positions of the total transmission probability do not depend on the size of the AB ring but the electronic spectrum. Moreover, the persistent currents in the AB ring is also spin-polarization dependent and different from the isolated AB ring where the persistent current is independent of spin polarization.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

AB效应对自旋多端输运的影响

利用紧束缚近似和格林函数方法,研究了AB效应和AB环对电子自旋输运的影响.计算表明,当在AB环的不同位置上连接相同或不同属性的输出端时,在一些能量范围内,由不同的输出端所输出的自旋流的方向是相反的;当固定入射电子的能量时,在同一磁通范围,从两个输出端输出的自旋流属性也是相反的.从而,可以通过控制AB环的结构和环内的磁通在输出端得到不同属性的自旋流. 关键词： 自旋极化输运 量子点 极化率 自旋流     

Output voltage calculations in double barrier magnetic tunnel junctions with asymmetric voltage behavior

In this paper we study the asymmetric voltage behavior (AVB) of the tunnel magnetoresistance (TMR) for single and double barrier magnetic tunnel junctions (MTJs) in range of a quasi-classical free electron model. Numerical calculations of the TMR–V curves, output voltages and I–V characteristics for negative and positive values of applied voltages were carried out using MTJs with CoFeB/MgO interfaces as an example. Asymmetry of the experimental TMR–V curves is explained by different values of the minority and majority Fermi wave vectors for the left and right sides of the tunnel barrier, which arises due to different annealing regimes. Electron tunneling in DMTJs was simulated in two ways: (i) Coherent tunneling, where the DMTJ is modeled as one tunnel system and (ii) consecutive tunneling, where the DMTJ is modeled by two single barrier junctions connected in series.     

Gate adjustable coherent three and four level mixing in a vertical quantum dot molecule

We study level mixing in the single-particle energy spectrum of one of the constituent quantum dots in a vertical double quantum dot by performing magneto-resonant-tunneling spectroscopy. The device used in this study differs from previous vertical double quantum dot devices in that the single side gate is now split into four separate gates. Because of the presence of natural perturbations caused by anharmonicity and anisotropy, applying different combinations of voltages to these gates allows us to alter the effective potential landscape of the two dots and hence influence the level mixing. We present here preliminary results from one three level crossing and one four level crossing high up in the energy spectrum of one of the probed quantum dots, and demonstrate that we are able to change significantly the energy dispersions with magnetic field in the vicinity of the crossing regions.     

Transport and magnetic properties of disordered LixVyO2 (x=0.8 and y=0.8)

The magnetic and electron transport properties of rhombohedral Li_xV_yO₂ (x=0.8 and y=0.8) are studied. The dc susceptibility of Li_xV_yO₂ can be well fitted to the modified Curie-Weiss law, which verified the paramagnetic ground state. The magnetic hysteresis and ac susceptibility also confirm this paramagnetism. The Li_xV_yO₂ exhibits semiconducting behavior, which is explained by thermal activated process at high temperature and variable-range hopping mechanism at low temperature. Anderson localization plays an important role in both the electron transport behavior and the magnetic behavior due to the site disorder between the Li⁺ ion and V⁴⁺ ion.