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991.
Hämäläinen SK Sun Z Boneschanscher MP Uppstu A Ijäs M Harju A Vanmaekelbergh D Liljeroth P 《Physical review letters》2011,107(23):236803
Despite the enormous interest in the properties of graphene and the potential of graphene nanostructures in electronic applications, the study of quantum-confined states in atomically well-defined graphene nanostructures remains an experimental challenge. Here, we study graphene quantum dots (GQDs) with well-defined edges in the zigzag direction, grown by chemical vapor deposition on an Ir(111) substrate by low-temperature scanning tunneling microscopy and spectroscopy. We measure the atomic structure and local density of states of individual GQDs as a function of their size and shape in the range from a couple of nanometers up to ca. 20 nm. The results can be quantitatively modeled by a relativistic wave equation and atomistic tight-binding calculations. The observed states are analogous to the solutions of the textbook "particle-in-a-box" problem applied to relativistic massless fermions. 相似文献
992.
Huang J Wang AM Shetty A Maitz AH Yan D Doyle D Richey K Park S Pieper DR Chen PY Grills IS 《Magnetic resonance imaging》2011,29(7):993-1001
Objective
To determine the accuracy of magnetic resonance spectroscopy (MRS), perfusion MR imaging (MRP), or volume modeling in distinguishing tumor progression from radiation injury following radiotherapy for brain metastasis.Methods
Twenty-six patients with 33 intra-axial metastatic lesions who underwent MRS (n=41) with or without MRP (n=32) after cranial irradiation were retrospectively studied. The final diagnosis was based on histopathology (n=4) or magnetic resonance imaging (MRI) follow-up with clinical correlation (n=29). Cho/Cr (choline/creatinine), Cho/NAA (choline/N-acetylaspartate), Cho/nCho (choline/contralateral normal brain choline) ratios were retrospectively calculated for the multi-voxel MRS. Relative cerebral blood volume (rCBV), relative peak height (rPH) and percentage of signal-intensity recovery (PSR) were also retrospectively derived for the MRPs. Tumor volumes were determined using manual segmentation method and analyzed using different volume progression modeling. Different ratios or models were tested and plotted on the receiver operating characteristic curve (ROC), with their performances quantified as area under the ROC curve (AUC). MRI follow-up time was calculated from the date of initial radiotherapy until the last MRI or the last MRI before surgical diagnosis.Results
Median MRI follow-up was 16 months (range: 2-33). Thirty percent of lesions (n=10) were determined to be radiation injury; 70% (n=23) were determined to be tumor progression. For the MRS, Cho/nCho had the best performance (AUC of 0.612), and Cho/nCho >1.2 had 33% sensitivity and 100% specificity in predicting tumor progression. For the MRP, rCBV had the best performance (AUC of 0.802), and rCBV >2 had 56% sensitivity and 100% specificity. The best volume model was percent increase (AUC of 0.891); 65% tumor volume increase had 100% sensitivity and 80% specificity.Conclusion
Cho/nCho of MRS, rCBV of MRP, and percent increase of MRI volume modeling provide the best discrimination of intra-axial metastatic tumor progression from radiation injury for their respective modalities. Cho/nCho and rCBV appear to have high specificities but low sensitivities. In contrast, percent volume increase of 65% can be a highly sensitive and moderately specific predictor for tumor progression after radiotherapy. Future incorporation of 65% volume increase as a pretest selection criterion may compensate for the low sensitivities of MRS and MRP. 相似文献993.
M. H. Tang Y. G. Xiao B. Jiang J. W. Hou J. C. Li J. He 《Applied Physics A: Materials Science & Processing》2011,105(3):679-683
The effect of applied dc bias electric field on dielectric permittivity in bulk Y2NiMnO6 is investigated in this paper. It is found that a small bias field of 40 V/cm can greatly reduce the dielectric permittivity
around the room temperature, compared to the much larger electric field that is required for conventional ferroelectric materials.
The observed giant dielectric tunability is retained over a broad range of around room temperature and is most likely related
to the charge ordering of Ni2+ and Mn4+ ions. This may further confirm the existence of electronic ferroelectricity in Y2NiMnO6. 相似文献
994.
A dual-wavelength continuous-wave (CW) diode end-pumped gadolinium scandium aluminum garnet (Nd:GSAG) laser that generates
simultaneous laser action at the wavelengths 1061 and 942 nm is demonstrated. A total output power of 589 mW (476 mW at 1061
nm and 113 mW at 942 nm) for the dual-wave-length was achieved at the incident pump power of 18.2 W. The M
2 values for 942 and 1061 nm lights were found to be around 1.18 and 1.37, respectively. 相似文献
995.
We report for the first time a continuous-wave (CW) orange radiation at 598 nm by intracavity sum-frequency generation of
1341 nm Nd:GdVO4 laser and 1080 nm Nd:YAlO3 (Nd:YAP) laser. Orange laser is obtained by using a doubly cavity, type-II critical phase matching KTP crystal sum-frequency
mixing. With total pump power of 36 W, TEM00 mode orange laser at 598 nm of 268 mW is obtained. The orange power stability in 30 min is better than 3.8%. 相似文献
996.
A novel variable-sized block encoding with threshold control for colour image quantization (CIQ) is presented in this paper. In CIQ, the colour palette used has a great influence on the reconstructed image quality. Typically, a higher image quality and a larger storage cost are obtained when a larger-sized palette is used in CIQ. To cut down the storage cost while preserving quality of the reconstructed images, the threshold control policy for quadtree segmentation is used in this paper. Experimental results show that the proposed method adaptively provides desired bit rates while having better image qualities comparing to CIQ with the usage of multiple palettes of different sizes. 相似文献
997.
A. El Allati M. El Baz Y. Hassouni A. Kassou Ou Ali F. Saif 《Journal of Russian Laser Research》2011,32(2):177-187
We suggest a model where the influence of an environment on the atom interference is associated with a random phase. The model consists of sending a two-level state atom through two cavities, both containing a standing wave field in the Bragg regime and Raman–Nath regime, respectively. In view of this model, we can visualize the loss of interference fringes if the randomness of the phase increases, and the restoration of the pattern when it decreases, as which-path information. Then, the controllable random noise acts as a decoherence that would destroy the quantum features. 相似文献
998.
In this research we study lasing without population inversion from a three-level atom interacting with two laser fields, in
the ladder or cascade scheme. We investigate counterintuitive sequencing as well as regular sequencing of the time of laser
fields application. In a counterintuitive sequence scheme a short probe pulse is introduced prior to the application of the
coupling field, in contrast, to a standard sequence scheme, where both fields are introduced at the same time. The influence
of varying the probe pulse width and time delay between the initiation of probe and coupling fields on transient probe gain
is investigated. The calculations indicate the potential of producing gain without inversion via counterintuitive sequence
scheme. 相似文献
999.
Nishimoto S Drechsler SL Kuzian RO van den Brink J Richter J Lorenz WE Skourski Y Klingeler R Büchner B 《Physical review letters》2011,107(9):097201
A thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field H(s) (T=0) is proposed. Using modern theoretical methods we study how H(s) is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J(1)-J(2)-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li(2)CuO(2) and Ca(2)Y(2)Cu(5)O(10), we show that H(s) is solely determined by the IC and predict H(s)≈61 T for the latter. With H(s)≈55 T from magnetization data one reads out a weak IC for Li(2)CuO(2) close to that obtained from inelastic neutron scattering. 相似文献
1000.
Poem E Kenneth O Kodriano Y Benny Y Khatsevich S Avron JE Gershoni D 《Physical review letters》2011,107(8):087401
We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance. 相似文献