Cherenkov effect as a probe of photonic nanostructures

Electron energy-loss spectroscopy (EELS) is shown to be an excellent source of information both on photonic crystal bands and on radiation modes of complex nanostructures. Good agreement is reported between measurements and parameter-free calculations of EELS in porous alumina films, where Cherenkov radiation is scattered by the pores to yield a strong 8.3-eV (7-eV) feature for 120-keV (200-keV) electrons. The latter is related to the bands of two-dimensional photonic crystals formed by air cylinders in an alumina matrix with similar near-range ordering. Finally, the band structure is proved to be directly mapped by angle-resolved EELS.     

Quantum dot nanostructures for multi-band infrared detection

A dual-band (two-color) tunneling-quantum dot infrared photodetector (T-QDIP) structure, which provides wavelength selectivity using bias voltage polarity, is reported. In this T-QDIP, photoexcitation takes place in InGaAs QDs and the excited carriers tunnel through an AlGaAs/InGaAs/AlGaAs double-barrier by means of resonant tunneling when the bias voltage required to line up the QD excited state and the double-barrier state is applied. Two double-barriers incorporated on the top and bottom sides of the QDs provide tunneling conditions for the second and the first excited state in the QDs (one double-barrier for each QD excited state) under forward and reverse bias, respectively. This field dependent tunneling for excited carriers in the T-QDIP is the basis for the operating wavelength selection. Experimental results showed that the T-QDIP exhibits three response peaks at ～4.5 (or 4.9), 9.5, and 16.9 μm and selection of either the 9.5 or the 16.9 μm peak is obtained by the bias polarity. The peak detectivity (at 9.5 and 16.9 μm) of this detector is in the range of 1.0–6.0 × 10¹² Jones at 50 K. This detector does not provide a zero spectral crosstalk due to the peak at 4.5 μm not being bias-selectable. To overcome this, a quantum dot super-lattice infrared photodetector (SL-QDIP), which provides complete bias-selectability of the response peaks, is presented. The active region consists of two quantum dot super-lattices separated by a graded barrier, enabling photocurrent generation only in one super-lattice for a given bias polarity. According to theoretical predictions, a combined response due to three peaks at 2.9, 3.7, and 4.2 μm is expected for reverse bias, while a combined response of three peaks at 5.1, 7.8, and 10.5 μm is expected for forward bias.     

The quantum compass chain in a transverse magnetic field

We study the magnetic behaviors of a spin-1/2 quantum compass chain (QCC) in a transverse magnetic field, by means of the analytical spinless fermion approach and numerical Lanczos method. In the absence of the magnetic field, the phase diagram is divided into four gapped regions. To determine what happens by applying a transverse magnetic field, using the spinless fermion approach, critical fields are obtained as a function of exchanges. Our analytical results show, the field-induced effects depend on in which one of the four regions the system is. In two regions of the phase diagram, the Ising-type phase transition happens in a finite field. In another region, we have identified two quantum phase transitions (QPT)s in the ground state magnetic phase diagram. These quantum phase transitions belong to the universality class of the commensurate-incommensurate phase transition. We also present a detailed numerical analysis of the low energy spectrum and the ground state magnetic phase diagram. In particular, we show that the intermediate state (h _c1 < h < h _c2) is gapful, describing the spin-flop phase.     

Superconducting vortex pinning with artificial magnetic nanostructures

This review is dedicated to summarizing the recent research on vortex dynamics and pinning effects in superconducting films with artificial magnetic structures. The fabrication of hybrid superconducting/magnetic systems is presented together with the wide variety of properties that arise from the interaction between the superconducting vortex lattice and the artificial magnetic nanostructures. Specifically, we review the role that the most important parameters in the vortex dynamics of films with regular array of dots play. In particular, we discuss the phenomena that appear when the symmetry of a regular dot array is distorted from regularity towards complete disorder including rectangular, asymmetric, and aperiodic arrays. The interesting phenomena that appear include vortex-lattice reconfigurations, anisotropic dynamics, channeling, and guided motion as well as ratchet effects. The different regimes are summarized in a phase diagram indicating the transitions that take place as the characteristic distances of the array are modified respect to the superconducting coherence length. Future directions are sketched out indicating the vast open area of research in this field.     

Sensitive chemical compass and quantum criticality at finite temperature

We investigate the influence of a flip operation of the central spin on the quantum criticality of a radical pair system by employing the spin echo and its product yield. It is found that with echo control on the central spin, the critical behavior can be described by the product yield at very high temperatures. Moreover, we also study the short and long time behavior of the spin echo, and show that the decay factor of the short time evolution scales linearly. The long time evolution shows different statistics for varying chain lengths, temperature and external parameters of the Hamiltonian.     

Interaction of a magnetic vortex with the probe field of a magnetic force microscope

The interaction of a magnetic vortex in a circular ferromagnetic nanoparticle with the probe field of a magnetic force microscope (MFM) is theoretically investigated. In the calculations, the probe field is approximated by the point dipole field. The rigid magnetic vortex model is used to describe the vortex state of magnetization. It is found that the effect of the probe field on the rigid magnetic vortex shell is similar to the effect of a uniform magnetic field parallel to the particle plane. The effect of the Z component of the probe field on the core of the vortex results in mutual probe-vortex attraction or repulsion. It is shown that the magnetization direction of the core of the vortex in the MFM probe field can be changed without a change in the shell vorticity direction.     

Midinfrared resonant magnetic nanostructures exhibiting a negative permeability

We experimentally demonstrate the first midinfrared (mid-IR) resonant magnetic nanostructures exhibiting a strong magnetic response corresponding to a negative permeability. This result is an important step toward the achievement of a negative refractive index in the IR. The possibility of extending negative permeability to higher frequencies is discussed; a structure with a negative effective permeability at a near-IR resonance frequency of 230 THz (1.3 microm) is proposed.     

Synthesis of ZnO compound nanostructures via a chemical route for photovoltaic applications

A facile, low-temperature, and low-cost chemical route has been developed to prepare ZnO nanowire and nanosphere compound structures. The morphology, structure, and composition of the yielded products have been examined by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction measurements. We have systematically investigated the optical properties of the ZnO nanostructures by micro-Raman, photoluminescence, and transmission spectroscopy. The results demonstrate that the yielded ZnO nanostructures possess good optical quality with high light absorption. We have further successfully employed the obtained ZnO compound nanostructures in dye-sensitized solar cells. The light-to-electricity conversion results show that the compound nanostructure exhibits a significant enhancement of short-circuit current density due to the increased surface area and light scattering in the compound nanostructures. The present chemical route provides a simple way to synthesize various compound nanostructures with high surface area for nanodevice applications.     

Plasma simulation of a Langmuir probe with normal magnetic field

The response of a probe with varying applied potential is studied by numerical simulation. Each particle is followed in turn in its path through the test volume and leaves a space charge in its track for the next overall iteration. The space charge is inversely proportional to its local speed [L.W. Parker and E.C. Whipple, Ann. Phys. 44 (1967) 126].

Preliminary results indicate that the effects of a magnetic field is strong and consistent with an anomalous diffusion, indicating that we may be simulating some process in real plasmas, in spite of the fact that the simulation method employed in principle gives no information on the temporal development of the plasma. On the other hand, effects of collisions seem to be slight.     

自旋-1/2量子罗盘链的量子相与相变

基于矩阵乘积态表述的无限时间演化块算法,研究了具有x,y,z三个自旋方向的轨道自由度和轨道序竞争的量子罗盘自旋链模型.为了刻画该模型的量子相和相变,计算了基态能量、局域序参量、弦关联序参量、临界指数、冯诺依曼熵、有限纠缠标度和中心荷.结果表明:该量子基态相图由条纹反铁磁相、反铁磁相、单调奇数Haldane相和振荡奇数Haldane相构成.从条纹反铁磁相到反铁磁相,以及从单调奇数Haldane相到振荡奇数Haldane相发生了非连续相变;从振荡奇数Haldane相到条纹反铁磁相,以及从反铁磁相到单调奇数Haldane相发生了连续相变;连续相变线和非连续相变线的交点是多临界点.此外,连续相变点处的临界指数β=1/8和中心荷c=1/2表明连续相变的普适类属于Ising类.由此揭示了该模型量子基态相图的本性,对今后研究更高自旋以及更为复杂轨道序竞争的量子罗盘链模型的量子相与相变具有一定借鉴与参考意义.     

Quantum wire network with magnetic flux

The charge transport and the noise of a quantum wire network, made of three semi-infinite external leads attached to a ring crossed by a magnetic flux, are investigated. The system is driven away from equilibrium by connecting the external leads to heat reservoirs with different temperatures and/or chemical potentials. The properties of the exact scattering matrix of this configuration as a function of the momentum, the magnetic flux and the transmission along the ring are explored. We derive the conductance and the noise, describing in detail the role of the magnetic flux. In the case of weak coupling between the ring and the reservoirs, a resonant tunneling effect is observed. We also discover that a non-zero magnetic flux has a strong impact on the usual Johnson–Nyquist law for the pure thermal noise at small temperatures.     

Electric field as a switching tool for magnetic states in atomic-scale nanostructures

One of the most promising candidates for the construction of ultrahigh-density storage media is low-dimensional atomic-scale magnetic nanostructures exhibiting magnetic bi- or multistability. Here we propose a novel route of locally controlling and switching magnetism in such nanostructures. Our ab initio studies reveal that externally applied electric field can be used for this purpose.     

Electron-transition stimulated plasma chemical reactions on surfaces with nanostructures

A mechanism is proposed for dissipating the energy of the heterogeneous recombination of atoms from low-temperature plasma. The mechanism considers accommodation of the energy of the reaction through an electron channel. The results from simulations using the Monte Carlo method for gas-surface systems with metal nanodots are reported. It is demonstrated that in wide-gap solids with metal nanodots, the electron channel can determine the rate of the plasma-chemical reaction and the epitaxial growth of a semiconductor or a nanowhisker.     

Dielectric response of cylindrical nanostructures in a magnetic field

We study the magnetic field dependence of the dielectric response of large cylindrical molecules such as nanotubes. When a field-induced level crossing takes place, an applied electric field has two effects: it may cause a linear instead of the usual quadratic Stark effect or the difference in the quadratic Stark coefficient of the two levels leads to a discontinuity in the polarization. Explicit calculations are performed for doped nanotubes and a rich structure in the real part of the low-frequency dielectric function is found when a magnetic field is applied along the cylinder axis. It is suggested that studies of can serve as a spectroscopic tool for the investigation of large ring-shaped or cylindrical molecules. Received 11 January 2000 and Received in final form 19 May 2000     

Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a Co Fe B/Mg O/Co Fe B magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect(TRMOKE) for both the parallel state(P state) and the antiparallel state(AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two Co Fe B layers via the tunneling of hot electrons through the Mg O barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.     

Direct and interwell excitons in magnetic nanostructures

The energies of direct and interwell excitons in superlattices based on europium and lead sulfides have been calculated. It is established that these excitons have higher oscillator strengths and binding energies due to the indirect exchange. This circumstance can be used in semiconductor devices operating on exciton transitions.     

Quantum anti-zeno acceleration of a chemical reaction

Evidence of acceleration of a chemical reaction in a condensed phase due to the quantum anti-Zeno effect is presented by a quantum-mechanical calculation. The acceleration is caused by electronic decoherence. The mechanism clearly indicates the anti-Zeno effect and involves both delocalization of the electronic dynamics and a feedback loop by coupling to vibrations. Believed to be the first established example of the quantum anti-Zeno effect in chemistry, the observed phenomenon suggests the possibility of quantum control of chemical reactivity by choice of solvent.     

Giant magnetic impedance of film nanostructures adapted for biodetection

A comparative study of the magnetic properties and giant magnetic impedance of Fe₁₉Ni₈₁/Su/Fe₁₉Ni₈₁ film structures fabricated by the method of ion-plasma evaporation is performed versus the geometry of an impedance element. The geometrical parameters of the Fe₁₉Ni₈₁/Сu/Fe₁₉Ni₈₁ structures are estimated from the viewpoint of obtaining strong magnetic impedance effect and possible application of these structures for detection of bioelements with magnetic markers. Widening of the structures necessary for increasing their active surface area weakens the magnetic impedance effect, and the length increase is limited by the optimal working biodetector frequencies and electromagnetic wave propagation processes.