Low-Scaling GW Algorithm Applied to Twisted Transition-Metal Dichalcogenide Heterobilayers.

The GW method is widely used for calculating the electronic band structure of materials. The high computational cost of GW algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling, and highly efficient GW algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables G0W0 calculations on a MoSe2/WS2 bilayer with 984 atoms per unit cell, in 42 h using 1536 cores. This is 4 orders of magnitude faster than a plane-wave G0W0 algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale.

Charge Transfer and Asymmetric Coupling of MoSe2 Valleys to the Magnetic Order of CrSBr.

van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in nonmagnetic TMDs. Here, we report magneto photoluminescence (PL) investigations of monolayer (ML) MoSe2 on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe2, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley g-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first-principles calculations suggest that MoSe2/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.

Strain-Induced Exciton Hybridization in WS_{2} Monolayers Unveiled by Zeeman-Splitting Measurements.

Mechanical deformations and ensuing strain are routinely exploited to tune the band gap energy and to enhance the functionalities of two-dimensional crystals. In this Letter, we show that strain leads also to a strong modification of the exciton magnetic moment in WS_{2} monolayers. Zeeman-splitting measurements under magnetic fields up to 28.5 T were performed on single, one-layer-thick WS_{2} microbubbles. The strain of the bubbles causes a hybridization of k-space direct and indirect excitons resulting in a sizable decrease in the modulus of the g factor of the ground-state exciton. These findings indicate that strain may have major effects on the way the valley number of excitons can be used to process binary information in two-dimensional crystals.

Engineering Proximity Exchange by Twisting: Reversal of Ferromagnetic and Emergence of Antiferromagnetic Dirac Bands in Graphene/Cr_{2}Ge_{2}Te_{6}.

We investigate the twist-angle and gate dependence of the proximity exchange coupling in twisted graphene on monolayer Cr_{2}Ge_{2}Te_{6} from first principles. The proximitized Dirac band dispersions of graphene are fitted to a model Hamiltonian, yielding effective sublattice-resolved proximity-induced exchange parameters (λ_{ex}^{A} and λ_{ex}^{B}) for a series of twist angles between 0° and 30°. For aligned layers (0° twist angle), the exchange coupling of graphene is the same on both sublattices, λ_{ex}^{A}≈λ_{ex}^{B}≈4 meV, while the coupling is reversed at 30° (with λ_{ex}^{A}≈λ_{ex}^{B}≈-4 meV). Remarkably, at 19.1° the induced exchange coupling becomes antiferromagnetic: λ_{ex}^{A}<0, λ_{ex}^{B}>0. Further tuning is provided by a transverse electric field and the interlayer distance. The predicted proximity magnetization reversal and emergence of an antiferromagnetic Dirac dispersion make twisted graphene/Cr_{2}Ge_{2}Te_{6} bilayers a versatile platform for realizing topological phases and for spintronics applications.

Swapping Exchange and Spin-Orbit Coupling in 2D van der Waals Heterostructures.

The concept of swapping the two most important spin interactions-exchange and spin-orbit coupling-is proposed based on two-dimensional multilayer van der Waals heterostructures. Specifically, we show by performing realistic ab initio simulations, that a single device consisting of a bilayer graphene sandwiched by a 2D ferromagnet Cr_{2}Ge_{2}Te_{6} (CGT) and a monolayer WS_{2}, is able not only to generate, but also to swap the two interactions. The highly efficient swapping is enabled by the interplay of gate-dependent layer polarization in bilayer graphene and short-range spin-orbit and exchange proximity effects affecting only the layers in contact with the sandwiching materials. We call these structures ex-so-tic, for supplying either exchange (ex) or spin-orbit (so) coupling in a single device, by gating. Such bifunctional devices demonstrate the potential of van der Waals spintronics engineering using 2D crystal multilayers.

Quantum Anomalous Hall Effects in Graphene from Proximity-Induced Uniform and Staggered Spin-Orbit and Exchange Coupling.

We investigate an effective model of proximity modified graphene (or symmetrylike materials) with broken time-reversal symmetry. We predict the appearance of quantum anomalous Hall phases by computing bulk band gap and Chern numbers for benchmark combinations of system parameters. Allowing for staggered exchange field enables quantum anomalous Hall effect in flat graphene with Chern number C=1. We explicitly show edge states in zigzag and armchair nanoribbons and explore their localization behavior. Remarkably, the combination of staggered intrinsic spin-orbit and uniform exchange coupling gives topologically protected (unlike in time-reversal systems) pseudohelical states, whose spin is opposite in opposite zigzag edges. Rotating the magnetization from out of plane to in plane makes the system trivial, allowing us to control topological phase transitions. We also propose, using density functional theory, a material platform-graphene on Ising antiferromagnet MnPSe_{3}-to realize staggered exchange (pseudospin Zeeman) coupling.

Proximity Spin-Orbit Torque on a Two-Dimensional Magnet within van der Waals Heterostructure: Current-Driven Antiferromagnet-to-Ferromagnet Reversible Nonequilibrium Phase Transition in Bilayer CrI3.

The recently discovered two-dimensional magnetic insulator CrI3 is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk but an antiferromagnet in bilayer form, with its magnetic ordering amenable to external manipulations. Using the first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of the bilayer-CrI3/monolayer-TaSe2 van der Waals (vdW) heterostructure will induce spin-orbit torque and thereby drive the dynamics of magnetization on the first monolayer of CrI3 in direct contact with TaSe2. By combining the calculated complex angular dependence of spin-orbit torque with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI3 from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI3 carries current due to evanescent wave functions injected by metallic transition metal dichalcogenide TaSe2, while concurrently acquiring strong spin-orbit coupling via such a proximity effect, whereas the second monolayer of CrI3 remains insulating. The transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by a bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find a tunneling magnetoresistance of ≃240%.

Probability of success of conservative treatment of infertility: exact timing as an important prognostic factor.

OBJECTIVE: To find out which forms of conservative infertility therapy led to the highest pregnancy rate and which factors influence the occurrence of a pregnancy. STUDY DESIGN: A total of 433 conservative treatment cycles were retrospectively evaluated. Ovarian stimulation was performed with clomiphene citrate (CC) or gonadotropins (follicle stimulating hormone [FSH] or human menopausal gonadotropin) with ovulation induction and luteal phase support in some cases. Patients then received intrauterine insemination (IUI) or had timed intercourse (TI). RESULTS: The pregnancy rates were 2.7% in CC/TI cycles, 8.2% in FSH/TI cycles, 10.3% in CC/IUI cycles, and 15.5% in FSH/IUI cycles. A cycle regulation by means of an ovulation induction and a luteal phase-supporting medication resulted in significantly higher pregnancy rates. When the TI and/or the IUI were carried out postovulatorily and in the case of already beginning endogenous ovulation (increase of the luteinizing hormone value shown in the last control), the success rate was significantly lower. CONCLUSION: Not only the choice of the optimal form of treatment but also a sufficient supportive medication in terms of an ovulation induction and a luteal phase support as well as exact timing are vital for the treatment success in conservative infertility treatment.

Good fertilization results associated with high IL-1beta concentrations in follicular fluid of IVF patients.

OBJECTIVE: To determine the levels of interleukin-1 beta (IL-1beta) in follicular fluid and embryo culture fluid after controlled ovarian hyperstimulation and to assess the association of this cytokine with the outcome of in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) treatment and embryo transfer. STUDY DESIGN: A total of 256 couples undergoing the IVF/ICSI program were included in this prospective study. Zygote quality, embryo and blastocyst morphology were evaluated, and embryo transfer was performed 5 days after oocyte recovery. IL-1beta concentrations were measured in follicular fluid and embryo culture fluid of the third and fifth culture days. RESULTS: Embryo replacement was performed with a median of 2 embryos per cycle. In all, 44 clinical pregnancies were achieved in 256 assisted reproductive technology (ART) cycles (pregnancy rate: 19.8% per transfer). Follicular fluid concentrations of IL-1P were not significantly different in pregnant (2.1 pg/mL) and nonpregnant women (2.7 pg/mL). Follicular fluid of lVF, but not ICSI, patients with good fertilization rates (> 90%) contained significantly higher levels of IL-1beta (3.3 pg/ mL) than did follicular fluid of women with fertilization rates < or = 90% (2.0 pg/mL, p < 0.05). No correlation was found between intrafollicular IL-1beta and zygote morphology, day 3 and day 5 embryo morphology. There was no relationship between IL-1beta in culture fluid supernatants and embryonic development. CONCLUSION: In IVF patients high levels of intrafollicular IL-1beta were associated with good fertilization rates. There seems to be no correlation between IL-1beta concentrations in follicular fluid or embryo culture fluid and embryo morphology or pregnancy outcome of ART cycles.

3D-Endometrial volume and outcome of cryopreserved embryo replacement cycles.

PURPOSE: The success of artificial reproductive techniques not only depends on the quality of oocytes and spermatozoa but also on the receptivity of the endometrium. The aim of this study was to assess the value of measurement of endometrial volume by three-dimensional (3D) in comparison to 2D-ultrasound in the prediction of implantation in women having transfer of cryopreserved embryos. METHODS: One hundred and eight couples were included in this prospective study. All patients underwent the IVF or ICSI program and had transfer of cryopreserved embryos. Sixty-eight transfers were done in a spontaneous cycle and 40 in an artificial cycle. Endometrial thickness, pattern and three-dimensional volume were measured immediately before embryo transfer. RESULTS: Twenty clinical pregnancies were achieved (PR 18.5 % per transfer), the PR being similar in spontaneous (22.1 %) and artificial (12.5 %, ns) cycles. Three to five days after ovulation (spontaneous cycles) or after the endometrium reached a thickness of at least 8 mm (artificial cycles), a median of three embryos were replaced. In spontaneous cycles, there were no significant differences in endometrial thickness or volume between pregnant (11.9 mm, 2.9 ml) and non-pregnant women (10.7 mm, 3.4 ml). In artificial cycles, the endometrial volume (3.9 vs. 2.5 ml, p < 0.05), but not endometrial thickness (10.7 vs. 10.2 mm, ns) was significantly higher in pregnant than in non-pregnant women. CONCLUSIONS: In artificial cycles, a low endometrial volume is associated with a poor likelihood of implantation. Endometrial volume measured by 3D-ultrasound is an objective parameter to predict endometrial receptivity.

Comparison of two media for sequential culture after IVF and ICSI shows no differences in pregnancy rates: a randomized trial.

BACKGROUND: Optimal culture conditions are crucial for embryonic development and blastocyst formation after in vitro fertilization (IVF) or Intracytoplasmic sperm injection (ICSI). A prospective randomized trial was carried out to compare the effects of two different sequential media (Vitrolife and MediCult) on embryo development and implantation. MATERIAL/METHODS: 176 couples were included in this prospective randomized study. All couples undergoing the IVF (n=84) or ICSI (n=92) program at the University of Würzburg were randomly assigned to either Vitrolife (n=87) or MediCult (n=89) sequential culture media. RESULTS: The overall median fertilization rate (FR) was 84.0% and was similar in the Vitrolife (FR=87.5%) and MediCult group (FR=80.0%). A mean of 2.7 embryos were replaced into the uterus, not significantly different between both groups. The pronuclear development was significantly enhanced in embryos cultured in Vitrolife than in those cultured in MediCult media (p=0.035). The embryonic development on day 3 was also superior in the Vitrolife group, but only in ICSI cycles (p<0.01). After 5 days of culturing, the blastocyst development was equivalent in both groups. The clinical pregnancy rate was 31.0% per transfer in the Vitrolife group and 28.1% in the MediCult group (not significant). CONCLUSIONS: The results demonstrate that implantation and clinical pregnancy rates with both media are comparable. Embryonic development until day three seemed to be enhanced when the embryos were cultured in Vitrolife media.

Pronuclear scoring. Time for international standardization.

Zygote scoring is an efficient tool for embryo selection not only in countries where embryo selection is not permitted. Several different scoring systems have been published so far, making comparisons of assessments between investigators and laboratories extremely difficult. Pronuclear evaluation should be standardized in a manner analogous to the standardization of cleavage stage embryo scoring or of semen evaluation by the World Health Organization. The ideal score should be clear and easily applicable. The items that have the greatest influence on embryonic development seem to be alignment and size of pronuclei, alignment and number of nucleoli, halo effect and appearance of vacuoles. These morphologic parameters can be observed in different features and can be summarized as a zygote score.