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For three-dimensional (3D) topological insulators that have a layered structure, strain was used to control critical physical properties. Here, we show that tensile strain decreases bulk carrier density while accentuating transport of topological surface state using temperature-dependent resistance and magneto-resistance measurements, terahertz-time domain spectroscopy and density functional theory calculations. The induced strain was confirmed by transmittance X-ray scattering measurements. The results show the possibility of reversible topological surface state device control using structural deformation.

In a three-dimensional topological insulator Bi2Se3, a stress control for band gap manipulation was predicted but no systematic investigation has been performed yet due to the requirement of large external stress. We report herein on the strain-dependent results for Bi2Se3 films of various thicknesses that are grown via a self-organized ordering process. Using small angle X-ray scattering and Raman spectroscopy, the changes of d-spacings in the crystal structure and phonon vibration shifts resulted from stress are clearly observed when the film thickness is below ten quintuple layers. From the UV photoemission/inverse photoemission spectroscopy (UPS/IPES) results and ab initio calculations, significant changes of the Fermi level and band gap were observed. The deformed band structure also exhibits a Van Hove singularity at specific energies in the UV absorption experiment and ab initio calculations. Our results, including the synthesis of a strained ultrathin topological insulator, suggest a new direction for electronic and spintronic applications for the future.

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Multilayer films composed of alternating layers of Bi and Se[Bi(4.55 Å)/Se(6.82 Å)] n (Bi4Se6), [Bi(6.13 Å)/Se(12.26) Å] n (Bi6Se12), and [Bi(4.86 Å)/Se(18.46 Å)] n (Bi4Se18)were fabricated by controlling the layer thickness at the atomic scale using thermal evaporation techniques. After annealing treatment, the Bi4Se18 alternately layered film shows a single phase of Bi2Se3 rhombohedral crystalline structure with the characteristic density of single crystal Bi2Se3, whereas the Bi6Se12 and Bi4Se6 films show locally disordered Bi2Se3 crystalline structure. The effectively controlled layered structure in the as-grown Bi4Se18 film enhances the Bi–Se chemical bonding state. The formation of a layered crystalline structure during the annealing process increased as the thickness of Se increased. After interdiffusion and the crystallization process, alternately layered Bi4Se18 films become stable Bi2Se3 single crystals with a continuous and uniform layered structure. Finally, in the Bi–Se system, atomically controlled multilayers with an optimized ratio of each unit layer can be transformed to a perfect single-crystalline structure on oxidized Si with an amorphous phase through a self-organized ordering process.

Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.

Electrons and phonons in chalcogenide-based materials are important factors in the performance of optical data-storage media and thermoelectric devices. However, the fundamental kinetics of carriers in chalcogenide materials remains controversial, and active debate continues over the mechanism responsible for carrier relaxation. In this study, we used optical-pump terahertz-probe spectroscopy, which permits the relationship between structural phase transition and optical property transitions to be examined, to investigate the ultrafast carrier dynamics in a multilayered [Sb(3 Å)/Te(9 Å)] thin film during the transition from the disordered to crystalline phase. Using terahertz time-domain spectroscopy and a contact-free optical technique, we demonstrated that the optical conductance and carrier concentration vary as functions of annealing temperature. Moreover, we observed that the topological surface state (TSS) affects the enhancement of the carrier lifetime, which is closely related to the degree of spin-orbit coupling (SOC). The combination of the optical technique and proposed carrier relaxation mechanism provides a powerful tool for monitoring TSS and SOC. Consequently, it was determined that the response of the disordered phase is dominated by an electron-phonon coupling effect, while that of the crystalline structure is controlled by a Dirac surface state and SOC effects. These results are important for understanding the fundamental physics of phase change materials and for optimizing and designing materials with better performance in optoelectronic devices.

151 Background: Pancreatic cancer is known to be most frequently associated with venous thromboembolism (VTE), with the incidence of 10~20% in Western countries. As for the East Asian ethnic groups, just one small retrospective study showed lower incidence (5.3%) than other ethnic groups. However, there have been no large retrospective cohort studies of VTE in pancreatic cancer of East Asian ethnic group. Methods: We retrospectively reviewed the medical records of patients diagnosed with pancreatic adenocarcinoma in 2005~2010 at Severance Hospital, Seoul, Korea. Principal outcomes were incident VTE events and mortality. Cox proportional hazards models were used to analyze associations between specific risk factors and principal outcomes. Results: We investigated 1334 patients with pancreatic adenocarcinoma and 218 (16.3%) patients were excluded due to incomplete medical records and loss to follow-up. Among 1116 eligible patients, the overall and 1-year cumulative VTE incidence were 13.9% and 11.2% respectively. The incidence rate during the half year, 1-year and 2-years were 23.4, 17.7, 15.6 events per 100 person-years, respectively. Among total of 155 VTE patients, abdominal VTE was 52.9%, deep vein thrombosis was 21.3%, pulmonary thromboembolism was 19.4%, and head and neck VTE was 18.7%. In multivariable analyses, significant predictors of developing VTE included advanced metastatic stage (HR=2.08, 95% CI 1.37 to 3.17) and treatment of chemotherapy or radiotherapy, including CCRT (HR=1.52, 95% CI 16 to 1.99). VTE was a significant risk factor of 1-year and overall mortality (HR=1.44, 95% CI 1.15 to 1.79 and HR=1.45, 95% CI 1.21 to 1.73). Among 155 VTE patients, head and neck (H&N) VTE revealed highest risk of overall mortality (HR=2.05, 95% CI 1.27 to 3.33, versus non-H&N VTE). Conclusions: Approximately 13.9% of pancreatic adenocarcinoma patients developed VTE in East Asian ethnic group and this incidence was not significantly different from other ethnic groups. Advanced metastatic stage was the strongest predictor of VTE which would be a significant risk factor of 1-year mortality. Among VTE patients, head and neck VTE was 18.7% and showed the highest risk of overall death.