The topology of the electronic structure of a crystal is manifested in its surface states. Recently, a distinct topological state has been proposed in metals or semimetals whose spin-orbit band ...structure features three-dimensional Dirac quasiparticles. We used angle-resolved photoemission spectroscopy to experimentally observe a pair of spin-polarized Fermi arc surface states on the surface of the Dirac semimetal Na3Bi at its native chemical potential. Our systematic results collectively identify a topological phase in a gapless material. The observed Fermi arc surface states open research frontiers in fundamental physics and possibly in spintronics.
Organic–inorganic hybrid two-dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We ...report the growth of high-quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic–inorganic Ruddelsden–Popper perovskites (RPPs) of (BA)2(MA) n −1Pb n I3 n +1 (n = 1, 2, and 3) by a slow evaporation at a constant-temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity, and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic–inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media.
A pressure‐induced topological quantum phase transition has been theoretically predicted for the semiconductor bismuth tellurohalide BiTeI with giant Rashba spin splitting. In this work, evolution of ...the electrical transport properties in BiTeI and BiTeBr is investigated under high pressure. The pressure‐dependent resistivity in a wide temperature range passes through a minimum at around 3 GPa, indicating the predicted topological quantum phase transition in BiTeI. Superconductivity is observed in both BiTeI and BiTeBr, while resistivity at higher temperatures still exhibits semiconducting behavior. Theoretical calculations suggest that superconductivity may develop from the multivalley semiconductor phase. The superconducting transition temperature, Tc, increases with applied pressure and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa for BiTeBr), followed by a slow decrease. The results demonstrate that BiTeX (X = I, Br) compounds with nontrivial topology of electronic states display new ground states upon compression.
A topological quantum phase transition and superconductivity induced by pressure are observed for the first time in bismuth tellurohalide BiTeI, which hosts giant Rashba spin splitting in its bulk form. The evolution of electrical transport properties demonstrates that novel physical states, besides the Rashba spin splitting, and electronic states, with nontrivial topology, exist for bismuth tellurohalides under high pressure.
A highly reproducible sample preparation method for pure GeTe in a rhombohedral structure without converting to the cubic structure up to ∼500 °C is reported to show control of the Ge-vacancy level ...and the corresponding herringbone-structured microdomains. The thermoelectric figure-of-merit ( ZT ) for GeTe powder could be raised from ∼0.8 to 1.37 at high temperature (HT) near ∼500 °C by tuning the Ge-vacancy level through the applied reversible in situ route, which made it highly controllable and reproducible. The enhanced ZT of GeTe was found to be strongly correlated with both its significantly increased Seebeck coefficient (∼161 μV K −1 at 500 °C) and reduced thermal conductivity (∼2.62 W m −1 K −1 at 500 °C) for a sample with nearly vacancy-free thicker herringbone-structured microdomains in the suppressed rhombohedral-to-cubic structure phase transformation. The microdomain and crystal structures were identified with HR-TEM (high-resolution transmission electron microscopy) and powder X-ray diffraction (XRD), while electron probe micro-analysis (EPMA) was used to confirm the stoichiometry changes of Ge : Te. Theoretical calculations for GeTe with various Ge-vacancy levels suggested that the Fermi level shifts toward the valence band as a function of increasing the Ge-vacancy level, which is consistent with the increased hole-type carrier concentration ( n ) and effective mass ( m *) deduced from the Hall measurements. The uniquely prepared sample of a near-vacancy-free GeTe in a rhombohedral structure at high temperature favoured an enhanced Seebeck coefficient in view of the converging L- and Σ-bands of the heavy effective mass at the Fermi level, while the high density domain boundaries for the domain of low carrier density were shown to reduce the total thermal conductivity effectively.
This study examined the associations of boredom proneness with Internet addiction and activities as well as the moderators for such associations in adolescents with attention-deficit/hyperactivity ...disorder (ADHD). In total, 300 adolescents with ADHD participated in this study. Their Internet addiction, the scores for lack of external and internal stimulation on the Boredom Proneness Scale-short form (BPS-SF), ADHD, parental characteristics, and the types of Internet activities were examined. The associations of boredom proneness with Internet addiction and Internet activities and the moderators of the associations were examined using logistic regression analyses. Higher scores for lack of external stimulation on the BPS-SF were significantly associated with a higher risk of Internet addiction. Maternal occupational socioeconomic status moderated the association of lack of external stimulation with Internet addiction. Higher scores for lack of external stimulation were significantly associated with a high tendency to engage in online gaming, whereas higher scores for lack of internal stimulation were significantly associated with a low tendency to engage in online studies. Lack of external stimulation on the BPS-SF should be considered a target in prevention and intervention programs for Internet addiction among adolescents with ADHD.
In addition to the Ge-vacancy control of GeTe, the antimony (Sb) substitution of GeTe for the improvement of thermoelectric performance is explored for Ge 1−x Sb x Te with x = 0.08–0.12. The ...concomitant carrier concentration ( n ) and the aliovalent Sb ion substitution led to an optimal doping level of x = 0.10 to show ZT ∼ 2.35 near ∼800 K, which is significantly higher than those single- and multi-element substitution studies of the GeTe system reported in the literature. In addition, Ge 0.9 Sb 0.1 Te demonstrates an impressively high power factor of ∼36 μW cm −1 K −2 and a low thermal conductivity of ∼1.1 W m −1 K −1 at 800 K. The enhanced ZT level for Ge 0.9 Sb 0.1 Te is explained through a systematic investigation of micro-structural change and strain analysis from room temperature to 800 K. A significant reduction of lattice thermal conductivity ( κ lat ) is identified and explained by the Sb substitution-introduced strained and widened domain boundaries for the herringbone domain structure of Ge 0.9 Sb 0.1 Te. The Sb substitution created multiple forms of strain near the defect centre, the herringbone domain structure, and widened tensile/compressive domain boundaries to support phonon scattering that covers a wide frequency range of the phonon spectrum to reduce lattice thermal conductivity effectively.
The aims of this study were to evaluate the predicting effects of psychological inflexibility/experiential avoidance (PI/EA) and stress coping strategies for Internet addiction, significant ...depression and suicidality among college students during the follow-up period of one year. A total of 500 college students participated in this study. The level of PI/EA and stress coping strategies were evaluated initially. One year later, 324 participants were invited to complete the Chen Internet Addiction Scale, Beck Depression Inventory-II and the questionnaire for suicidality to evaluate depression symptoms and internet addiction and suicidality. The predicting effects of PI/EA and stress coping strategies were examined by using logistic regression analysis controlling for the effects of gender and age. The results indicated that PI/EA at the initial assessment increased the risk of Internet addiction (OR = 1.087, 95% CI: 1.042-1.135), significant depression (OR = 1.125, 95% CI: 1.081-1.170), and suicidality (OR = 1.099, 95% CI: 1.053-1.147) at the follow-up assessment. Less effective coping at the initial assessment also increased the risk of Internet addiction (OR = 1.074, 95% CI: 1.011-1.140), significant depression (OR = 1.091, 95% CI: 1.037-1.147), and suicidality (OR = 1.074, 95% CI: 1.014-1.138) at the follow-up assessment. Problem focused and emotion-focus coping at the initial assessment was not significantly associated with the risks of Internet addiction, significant depression, and suicidality at the follow-up assessment. College students who have high PI/EA or are accustomed to using less effective stress coping strategies should be the target of prevention programs for IA (internet addiction), depression, and suicidality.
Two-dimensional crystals with a wealth of exotic dimensional-dependent properties are promising candidates for next-generation ultrathin and flexible optoelectronic devices. For the first time, we ...demonstrate that few-layered InSe photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of conducting broadband photodetection from the visible to near-infrared region (450–785 nm) with high photoresponsivities of up to 12.3 AW–1 at 450 nm (on SiO2/Si) and 3.9 AW–1 at 633 nm (on PET). These photoresponsivities are superior to those of other recently reported two-dimensional (2D) crystal-based (graphene, MoS2, GaS, and GaSe) photodetectors. The InSe devices fabricated on rigid SiO2/Si substrates possess a response time of ∼50 ms and exhibit long-term stability in photoswitching. These InSe devices can also operate on a flexible substrate with or without bending and reveal comparable performance to those devices on SiO2/Si. With these excellent optoelectronic merits, we envision that the nanoscale InSe layers will not only find applications in flexible optoelectronics but also act as an active component to configure versatile 2D heterostructure devices.
Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, ...distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe2-class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe2, PdTe2, and PtTe2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here, we report the synthesis of a new type-II Dirac semimetal NiTe2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry’s phase associated with these Dirac fermions. Our first-principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe2 may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers.
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