Vacuum Growth and Characterization of Thin Films of Zinc Tin Diphosphide.

A vacuum growth technique has been employed for the first time to grow thin films of ZnSnP\sb2. Near stoichiometric thin films of polycrystalline ZnSnP\sb2 have been grown on GaAs and quartz substrates. A reasonable degree of reproducibility of the film composition from one run to the next was achieved by using a quadrupole mass analyzer probe in conjunction with a digital feedback controller to maintain constant Zn flux. The growth parameters for the system including the estimates for the Zn/Sn flux ratio at the substrate and the optimum substrate temperature have been obtained for the near stoichiometric film growth. The most uniform films showed less than $\pm$0.75% variation in the atomic composition for each of the three elements across a 1 cm\sp2 sample. The quality of layers grown on GaAs substrates was superior to those on quartz substrates. Different analytical techniques were used to characterize the grown films. The relations between film surface morphology and film composition were obtained from SEM and EDS analyses. The film morphology was most sensitive to the Zn content in the film. An absorption coefficient of greater than 10\sp5 cm\sp{-1} was obtained without taking surface scattering into account. The energy gap value of 1.62 eV was observed from the optical transmission analysis. X-Ray diffraction analysis indicated that the grown films have chalcopyrite structure with lattice constant values in the range of 5.64-5.67 A. Silver and Indium were used to form electrical contacts to the grown films. Resistivity and Hall measurements were made in the temperature range of 300-625\sp\circK. All the films examined were of p-type conductivity and the film resistivity was in the range of 0.1-10 $\Omega$-cm. The carrier concentrations in these p-type films were in the range of 10\sp{16}-10\sp{18} cm\sp{-3} and the room temperature Hall mobility values were in the range of 35-47 cm\sp2/V-sec. A small decrease in the bulk resistivity value was observed at 77\sp\circK. Heterostructure diodes fabricated from the p-type ZnSnP\sb2 films grown on n-type GaAs substrates showed a small photoresponse demonstrating the feasibility of photovoltaic application of the grown material

Role of thermal friction in relaxation of turbulent Bose-Einstein condensates

In recent experiments, the relaxation dynamics of highly oblate, turbulent Bose-Einstein condensates (BECs) was investigated by measuring the vortex decay rates in various sample conditions [Phys. Rev. A $\bf 90$, 063627 (2014)] and, separately, the thermal friction coefficient $\alpha$ for vortex motion was measured from the long-time evolution of a corotating vortex pair in a BEC [Phys. Rev. A $\bf 92$, 051601(R) (2015)]. We present a comparative analysis of the experimental results, and find that the vortex decay rate $\Gamma$ is almost linearly proportional to $\alpha$. We perform numerical simulations of the time evolution of a turbulent BEC using a point-vortex model equipped with longitudinal friction and vortex-antivortex pair annihilation, and observe that the linear dependence of $\Gamma$ on $\alpha$ is quantitatively accounted for in the dissipative point-vortex model. The numerical simulations reveal that thermal friction in the experiment was too strong to allow for the emergence of a vortex-clustered state out of decaying turbulence.Comment: 7 pages, 5 figure

Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient

We investigate the stability of a hard-axis polar state in a spin-1 antiferromagnetic Bose-Einstein condensate under a magnetic field gradient, where the easy-plane spin anisotropy is controlled by a negative quadratic Zeeman energy $q<0$. In a uniform magnetic field, the axial polar state is dynamically unstable and relaxes into the planar polar ground state. However, under a field gradient $B'$, the excited spin state becomes metastable down to a certain threshold $q_{th}$ and as $q$ decreases below $q_{th}$, its intrinsic dynamical instability is rapidly recalled. The incipient spin excitations in the relaxation dynamics appear with stripe structures, indicating the rotational symmetry breaking by the field gradient. We measure the dependences of $q_{th}$ on $B'$ and the sample size, and we find that $q_{th}$ is highly sensitive to the field gradient in the vicinity of $B'=0$, exhibiting power-law behavior of $|q_{th}|\propto B'^{\alpha}$ with $\alpha \sim 0.5$. Our results demonstrate the significance of the field gradient effect in the quantum critical dynamics of spinor condensates.Comment: 8 pages, 7 figure

Observation of vortex-antivortex pairing in decaying 2D turbulence of a superfluid gas

In a two-dimensional (2D) classical fluid, a large-scale flow structure emerges out of turbulence, which is known as the inverse energy cascade where energy flows from small to large length scales. An interesting question is whether this phenomenon can occur in a superfluid, which is inviscid and irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly oblate geometry provide an experimental venue for studying 2D superfluid turbulence, but their full investigation has been hindered due to a lack of the circulation sign information of individual quantum vortices in a turbulent sample. Here, we demonstrate a vortex sign detection method by using Bragg scattering, and we investigate decaying turbulence in a highly oblate BEC at low temperatures, with our lowest being $\sim 0.5 T_c$, where $T_c$ is the superfluid critical temperature. We observe that weak spatial pairing between vortices and antivortices develops in the turbulent BEC, which corresponds to the vortex-dipole gas regime predicted for high dissipation. Our results provide a direct quantitative marker for the survey of various 2D turbulence regimes in the BEC system.Comment: 8 pages, 8 figure

Portfolio Selection with Subsistence Consumption Constraints and CARA Utility

We consider the optimal consumption and portfolio choice problem with constant absolute risk aversion (CARA) utility and a subsistence consumption constraint. A subsistence consumption constraint means there exists a positive constant minimum level for the agent&apos;s optimal consumption. We use the dynamic programming approach to solve the optimization problem and also give the verification theorem. We illustrate the effects of the subsistence consumption constraint on the optimal consumption and portfolio choice rules by the numerical results

Finite rings with identity having GL(2m) as the group of units

주어진 환에 있어서 가역원 전체의 집합은 곱셉군을 이룬다. 이 곱셈군이 순회인 유한환은 그 구조가 완전히 결정된다는 것은 이미 알려져 있다. 본 논문에서는 다음 정리를 증명한다. 정리 (3. 1) 가역원 전체로 이루어진 곱셉군이 일반선형군 GL₂(2m)과 동형인 유한환 R은 다음과 같다. 즉, R≅M₂(2m)⊕Z₂⊕……⊕Z₂

Evaluation of the brain activation induced by functional electrical stimulation and voluntary contraction using functional magnetic resonance imaging

BACKGROUND: To observe brain activation induced by functional electrical stimulation, voluntary contraction, and the combination of both using functional magnetic resonance imaging (fMRI). METHODS: Nineteen healthy young men were enrolled in the study. We employed a typical block design that consisted of three sessions: voluntary contraction only, functional electrical stimulation (FES)-induced wrist extension, and finally simultaneous voluntary and FES-induced movement. MRI acquisition was performed on a 3.0 T MR system. To investigate activation in each session, one-sample t-tests were performed after correcting for false discovery rate (FDR; p < 0.05). To compare FES-induced movement and combined contraction, a two-sample t-test was performed using a contrast map (p < 0.01). RESULTS: In the voluntary contraction alone condition, brain activation was observed in the contralateral primary motor cortex (MI), thalamus, bilateral supplementary motor area (SMA), primary sensory cortex (SI), secondary somatosensory motor cortex (SII), caudate, and cerebellum (mainly ipsilateral). During FES-induced wrist movement, brain activation was observed in the contralateral MI, SI, SMA, thalamus, ipsilateral SII, and cerebellum. During FES-induced movement combined with voluntary contraction, brain activation was found in the contralateral MI, anterior cingulate cortex (ACC), SMA, ipsilateral cerebellum, bilateral SII, and SI. The activated brain regions (number of voxels) of the MI, SI, cerebellum, and SMA were largest during voluntary contraction alone and smallest during FES alone. SII-activated brain regions were largest during voluntary contraction combined with FES and smallest during FES contraction alone. The brain activation extent (maximum t score) of the MI, SI, and SII was largest during voluntary contraction alone and smallest during FES alone. The brain activation extent of the cerebellum and SMA during voluntary contraction alone was similar during FES combined with voluntary contraction; however, cerebellum and SMA activation during FES movement alone was smaller than that of voluntary contraction alone or voluntary contraction combined with FES. Between FES movement alone and combined contraction, activated regions and extent due to combined contraction was significantly higher than that of FES movement alone in the ipsilateral cerebellum and the contralateral MI and SI. CONCLUSIONS: Voluntary contraction combined with FES may be more effective for brain activation than FES-only movements for rehabilitation therapy. In addition, voluntary effort is the most important factor in the therapeutic process

Optical properties of iron-based superconductor LiFeAs single crystal

We have measured the reflectivity spectra of the iron based superconductor LiFeAs (Tc = 17.6 K) in the temperature range from 4 to 300 K. In the superconducting state (T < Tc), the clear opening of the optical absorption gap was observed below 25 cm-1, indicating an isotropic full gap formation. In the normal state (T > Tc), the optical conductivity spectra display a typical metallic behavior with the Drude type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this superconductor. A theoretical analysis of the low temperature data (T=4K < Tc) also suggests that two superconducting gaps best fit the data and their values were estimated as {\Delta}1 = 1.59 meV and {\Delta}2 = 3.15 meV, respectively. Using the Ferrell-Glover-Tinkham (FGT) sum rule and dielectric function {\epsilon}1({\omega}), the superconducting plasma frequency ({\omega}ps) is consistently estimated to be 6,665 cm-1, implying that about 59 % of the free carriers in the normal state condenses into the SC condensate. To investigate the various interband transition processes (for {\omega} > 200 cm-1), we have also performed the local-density approximation (LDA) band calculation and calculated the optical spectra of the interband transitions. The theoretical results provided a qualitative agreement with the experimental data below 4000 cm-1Comment: 19 pages, 5 figures. This paper has been accepted for publication in New Journal of Physic