Excitonic ground states in phosphorene nanoflakes.

By using a configuration-interaction approach beyond the framework of independent multiexcitons, we predict that an excitonic ground state may exist in phosphorene nanoflakes when an in-plain electric field is applied. The ground state of the system is shown to undergo a transition from purely electronic to almost fully biexcitonic with the increasing strength of the electric field. As the field exceeds 0.25 V nm-1, a biexcitonic ground state is revealed to be energetically more favorable by a few hundred meV than the system without excitons. A similar transformation of the ground state is also found as the screening effect varies from strong to weak. The enhanced electron-hole correlation, mostly caused by the applied electric field as well as the lack of strong screening in low-dimensional nanostructures, is believed to account for such an extraordinary transition. Furthermore, the system with a biexcitonic ground state is found to exhibit an absorption spectrum where many transitions are polarized along the zigzag direction, which breaks the optical anisotropy well-known for bulk phosphorene.

Disintegration of excitons in π-conjugated molecules.

Utilizing the exact diagonalization (ED) method, we find that excitons cannot form in π-conjugated molecules such as anthracene, phenanthrene, and pyrene when the electron-electron interaction is governed by the Rytova-Keldysh (RK) potential. Within the Pariser-Parr-Pople (PPP) model, however, the excitons may survive only in the presence of a weak screening effect. Either way, the optical gap is seen to be insensitive to the dielectric environment owing to the opposite contributions from the excitonic effect and quasiparticle correction. Furthermore, the latter two are shown to exhibit almost the same behavior in all three molecules when the screening parameter varies.

Abnormal scaling of excitons in phosphorene quantum dots.

Excitonic states of a many-electron system in phosphorene quantum dots (PQDs) are investigated theoretically by using a configuration interaction approach. For a triangular PQD in various dielectric environments, its exciton is found to obey two distinct scaling rules. When there is a strong screening effect present in the nanodot, the exciton binding energy (Δex) is shown to be around -150 meV as the long-range Coulomb interactions are totally suppressed and it increases to about 100 meV when the effective dielectric constant (εr) decreases to 12.5. Over this range of εr, Δex is found to be well fitted into a quadratic form of εr-1, which scales neither linearly with εr-2 like the case of bulk three-dimensional semiconductors nor linearly with εr-1 like the case previously reported for graphene nanostructures. When εr is reduced below 10.0, however, Δex is shown to exhibit a perfect linear relationship with εr-1, which behaves just like that of a two-dimensional graphene sheet. On the other hand, with the reduced εr, the quasiparticle gap is found to decrease instead of increasing like in most of the semiconductor nanostructures. As a result, it is revealed that the relationship of Δex with the quasi-particle gap deviates largely from the linear one previously reported for graphene and many other two-dimensional materials.

Abnormal blueshift of the absorption edge in graphene nanodots.

In a conventional semiconductor, when the dielectric screening effect is suppressed, the exciton binding energy increases and the corresponding excitonic transition would exhibit a redshift in the spectrum. In this work, I study the optical properties of hexagonal graphene nanodots by using a configuration interaction approach and reveal that the edge of the absorption spectrum shows an abnormal blueshift as the environmental dielectric constant ϵr decreases. The two dominant many-body effects in the nanodot: the quasiparticle and excitonic effects are both found to scale almost linearly with ϵr-1. The former is shown to have a larger proportionality constant and thus accounts for the blueshift of the absorption edge. In contrast to the long-range Coulomb interaction, the on-site Coulomb energy is found to have a negative impact on the bright excitonic states. In the presence of a strong dielectric screening effect, a strong short-range Coulomb interaction is revealed to be responsible for the disintegration of the bright exciton.

Identification and characterization of long non-coding RNAs in subcutaneous adipose tissue from castrated and intact full-sib pair Huainan male pigs.

BACKGROUND: Long non-coding RNAs (lncRNAs) regulate adipose tissue metabolism, however, their function on testosterone deficiency related obesity in humans is less understood. For this research, intact and castrated male pigs are the best model animal because of their similar proportional organ sizes, cardiovascular systems and metabolic features. RESULTS: We identified lncRNAs in subcutaneous adipose tissue by deep RNA-sequencing using the intact and castrated Huainan male pigs. The results showed that castration reduced serum testosterone but increased body fatness-related traits (serum triglyceride levels, backfat thickness, intramuscular fat content, and adipocyte size). Meanwhile, 343 lncRNAs from subcutaneous adipose tissue were identified, including 223 intergenic lncRNAs (lincRNAs), 68 anti-sense lncRNAs, and 52 intronic lncRNAs. It was predicted that there were 416 recognition sites for C/EBPα in the 303 lncRNA promoter region, and 13 adipogenesis-promoting miRNAs and five adipogenesis-depressing miRNAs target these lncRNAs. Eighteen lncRNAs, including nine up- and nine down-regulated had more than 2-fold differential expression between the castrated and intact male pigs (q-value < 0.05). Functional analysis indicated that these 18 lncRNAs and their target genes were involved in fatty acid, insulin, and the adipocytokine signaling pathway. We further analyzed the features of a conserved mouse lncRNA gene ENSMUST00000189966 and found it mainly expressed in the cell nucleus and target the Nuclear Receptor Subfamily 2 Group F Member 2 (NR2F2) gene. In 3 T3-L1 cells, differentiation down-regulated their expression, but dihydrotestosterone (DHT) significantly up-regulated their expression in a concentration-dependent manner (P < 0.05). CONCLUSIONS: These results suggested that lncRNAs and their target genes might participated in the castration-induced fat deposition and provide a new therapeutic target for combatting testosterone deficiency-related obesity.

Dark excitons and tunable optical gap in graphene nanodots.

By using a configuration interaction approach with up to the fifth excitations taken into account, we study the excitonic effect in the optical absorption in graphene nanodots. While the many-electron states are either singlet or doublet in a triangular nanodot system, all the excited singlet states are found to be optically dark in the absorption. These dark excitons are shown to originate mainly from the geometric symmetry of the system and would remain inactive even when the electron-hole or sublattice symmetry is broken. The first excited state in most of the cases is found to be a dark singlet; however, the order of dark and bright excitonic states is shown to be quite sensitive to the strength of electron-electron interactions such as the dielectric screening from the substrate. All the double degeneracies in the excitonic spectrum are found to be lifted when the rotational symmetry is absent such as in the case of a trapezoidal nanodot; however, the first excited state is shown to still remain a dark exciton when there is a strong screening effect. In order that the optical gap of a graphene nanodot can be efficiently tuned by its dielectric environment, the geometric symmetry is revealed to be a crucial factor.

Excitonic absorption spectra in graphene nanoflakes: Tuning of exciton binding energy by dielectric environments.

By solving the Bethe-Salpeter equation within the Hartree-Fock formalism, we study the excitonic absorption spectra of graphene nanoflakes embedded in various dielectric environments. With the excitonic effects fully taken into account, the exciton binding energy as a function of the dielectric constant is found to be well described by a single scaling rule in which the scaling factor is found to vary slowly with the size of the nanoflakes. Furthermore, it is revealed that the exciton binding energy scales almost linearly with the on-site interaction energy and exhibits more sensitive dependence in smaller nanoflakes. Our results are found to agree well with the recent experiment.

Tuning the magnetic phase of a graphene nanodot using its dielectric environment.

The magnetic properties of a hexagonal graphene nanodot are investigated using a configuration-interaction approach. The phase diagram of the nanodot system is obtained for a wide range of Coulomb interaction strengths and dielectric environments, and shows that the magnetic phase transition can be effectively tailored by changing the dielectric environment. Selective magnetic phase between non-magnetic and ferromagnetic could therefore be achieved simply by changing to an appropriate substrate for the graphene nanodot.

Scaling of excitons in graphene nanodots.

The binding energy of an exciton in a semiconductor or an insulator is known to scale linearly with εr-2, where εr is its dielectric constant. In graphene however, since the kinetic energy scales linearly with the wave number instead of its square, the exciton binding energy is thus expected to scale with εr-1. In this work we make use of the configuration interaction approach to study the properties of excitons in graphene nanodots embedded in various dielectric environments. With tens of million configurations taken into account in the calculation, we find that the exciton binding energy can be well described by a single scaling rule in which the scaling factor is found to vary with the dimension of the nanodots as well as with the on-site interaction parameter, which agrees well with a recent experiment. The linear relation of the exciton binding energy found with the quasi-particle gap also agrees with the previous work on bulk graphene and other two-dimensional materials.

Bias voltage control of magnetic phase transitions in graphene nanojunctions.

The magnetic state of a spintronic device is usually controlled by magnetic contacts or a transverse electric field generated by side gates. In this work, we consider a graphene nanojunction in the presence of a bias voltage that leads to magnetic phase transitions in the system. Combining the non-equilibrium Green's function with the Hubbard model, our self consistent calculation reveals that an increasing bias voltage induces consecutive transitions among antiferromagnetic and ferromagnetic states. It is further shown that the graphene nanojunction is turned off in the antiferromagnetic state when the bias voltage is low and can then be switched on to the ferromagnetic state by a high bias voltage. We therefore demonstrate that the magnetic state of the graphene system can be controlled by the bias voltage without resorting to any transverse gates.

Communication: generalization of Koopmans' theorem to optical transitions in the Hubbard model of graphene nanodots.

Koopmans' theorem implies that the Hartree-Fock quasiparticle gap in a closed-shell system is equal to its single-particle energy gap. In this work, the theorem is generalized to optical transitions in the Hubbard model of graphene nanodots. Based on systematic configuration interaction calculations, it is proposed that the optical gap of a closed-shell graphene system within the Hubbard model is equal to its tight-binding single-particle energy gap in the absence of electron correlation. In these systems, the quasiparticle energy gap and exciton binding energy are found to be dominated by the long-range Coulomb interaction, and thus, both become small when only on-site Hubbard interactions are present. Moreover, the contributions of the quasiparticle and excitonic effects to the optical gap are revealed to nearly cancel each other, which results in an unexpected overlap of the optical and single-particle gaps of the graphene systems.

Highly Efficient and Unsupervised Framework for Moving Object Detection in Satellite Videos.

Moving object detection in satellite videos (SVMOD) is a challenging task due to the extremely dim and small target characteristics. Current learning-based methods extract spatio-temporal information from multi-frame dense representation with labor-intensive manual labels to tackle SVMOD, which needs high annotation costs and contains tremendous computational redundancy due to the severe imbalance between foreground and background regions. In this paper, we propose a highly efficient unsupervised framework for SVMOD. Specifically, we propose a generic unsupervised framework for SVMOD, in which pseudo labels generated by a traditional method can evolve with the training process to promote detection performance. Furthermore, we propose a highly efficient and effective sparse convolutional anchor-free detection network by sampling the dense multi-frame image form into a sparse spatio-temporal point cloud representation and skipping the redundant computation on background regions. Coping these two designs, we can achieve both high efficiency (label and computation efficiency) and effectiveness. Extensive experiments demonstrate that our method can not only process 98.8 frames per second on 1024 ×1024 images but also achieve state-of-the-art performance.

Tuning of excitons in phosphorene atomic chains.

An universal scaling between the exciton binding energy and quasiparticle (QP) band gap was first discovered in two-dimensional (2D) semiconductors such as graphene derivatives, various transition materials dichalcogenides, and black phosphorus (Choiet al2015Phys. Rev. Lett.115066403; Jianget al2017Phys. Rev. Lett.118266401), and later extended to quasi one-dimensional (1D) systems such as carbon nanotubes and graphene nanoribbons. In this work we study the excitonic states in phosphorene atomic chains by using the exact diagonalization method and show that the linear scaling between the exciton binding energy (Ex) and QP shift (Δqs) can be easily tuned by the dielectric environment. In the presence of weak screening,Exis seen to increase withΔqsand exhibits a similar scaling as those 2D materials. As the screening becomes stronger, however, the dependence is found to be reversed, i.e.Exnow decreases whenΔqsincreases. More interestingly, we also reveal thatExmay even become nearly constant, independent on the system dimension andΔqswhen the screening reaches a certain strength. These abnormal scaling relations are attributed to the complex nature of excitons in the strongly correlated 1D system.

Scaling of energy gaps in phosphorene nanoflakes.

Electronic structure of phosphorene nanoflakes which consist of hundreds of phosphorus atoms are studied in the framework of unrestricted Hartree-Fock approach. On the base of Pariser-Parr-Pople model for electron-electron interactions, a simplified Bethe-Salpeter formalism is established for the calculation of excitation states of the system. Taking into account the electron-hole interaction in various dielectric environments, the optical gap of a triangular phosphorene nanoflake is shown to increase as the screening effect becomes stronger while its graphene counterpart exhibits just the opposite dependence. After confirming an exponential dependence of the optical gap on the effective dielectric constant, the quasiparticle and optical gaps are also found to obey an exponential scaling rule against the total number of atoms in the nanoflakes, respectively. By extrapolating the dependence on the size of the system, one is able to estimate the exciton binding energy of a monolayer phosphorene sheet on a SiO2substrate to be 0.894 eV. The result is found to agree well with the previous experimental result of 0.9 eV.

Hubbard excitons in two-dimensional nanomaterials.

Excitons in two-dimensional nanomaterials are studied by solving the many-electron Hamiltonian with a configuration-interaction approach. It is shown that graphene or phosphorene nanoflakes can not accommodate any excitonic bound states if the long-range Coulomb interaction is suppressed when the systems are placed in a high-k dielectric environment or on a metal substrate. Hence it is revealed that an electron-hole pair created by an optical excitation does not always form an exciton even in a confined nanostructure. The negative exciton binding energy is found to exhibit distinct dependence on the strength of short-range Coulomb interaction as the system undergoes a phase transition from non-magnetic to anti-ferromagnetic. It is further shown that the electron-hole pair may form an exciton state only when the long-range Coulomb interaction is recovered in the nanoflakes.

Magnetic phase diagram of graphene nanorings in an electric field.

Magnetic properties of graphene nanorings are investigated in the presence of an electric field. Within the formalism of Hubbard model, the graphene nanorings of various geometric configurations are found to exhibit rich phase diagram. For a nanoring system which has degenerate states at the Fermi level, the system is shown to undergo an abrupt phase transition from the antiferromagnetic to a nonmagnetic state in an electric field applied cross its zigzag edges. However, the nanoring is found to always stay in the antiferromagnetic state when the electric field is applied cross its armchair edges. For the other nanoring system with a finite single-particle gap, the magnetic moments of its antiferromagnetic ground state is seen to decrease gradually to zero with the electric field applied cross the zigzag edges. When the electric field is applied cross the armchair edges, the nanoring is shown to undergo several magnetic phase transitions before settling itself in a nonmagnetic ordering.

Longitudinal wave function control in single quantum dots with an applied magnetic field.

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots.

Electron cotunneling through doubly occupied quantum dots: effect of spin configuration.

A microscopic theory is presented for electron cotunneling through doubly occupied quantum dots in the Coulomb blockade regime. Beyond the semiclassic framework of phenomenological models, a fully quantum mechanical solution for cotunneling of electrons through a one-dimensional quantum dot is obtained using a quantum transmitting boundary method without any fitting parameters. It is revealed that the cotunneling conductance exhibits strong dependence on the spin configuration of the electrons confined inside the dot. Especially for the triplet configuration, the conductance shows an obvious deviation from the well-known quadratic dependence on the applied bias voltage. Furthermore, it is found that the cotunneling conductance reveals more sensitive dependence on the barrier width than the height.

Anomalous quantum-confined Stark effects in stacked InAs/GaAs self-assembled quantum dots.

Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs) are predicted to exhibit strong hole localization even with vanishing separation between the dots, and a nonparabolic dependence of the interband transition energy on the electric field, which is not encountered in single SAD structures. Our study based on an eight-band strain-dependent k x p Hamiltonian indicates that this anomalous quantum confined Stark effect is caused by the three-dimensional strain field distribution which influences drastically the hole states in the stacked SAD structures.