By applying density functional theory calculations, we study the electronic and magnetic properties of KFe2As2, which exhibits two superconducting domes under pressure. We find there are competing magnetic instabilities from itinerant electrons in both superconducting phases. The lower Tc of the first superconducting phase at ambient pressure can be attributed to the competing instabilities at (n, 0, n), which favor the double stripe type or plaquette antiferromagnetic order. The second superconducting phase is further suppressed by the competing ferromagnetic and stripe antiferromagnetic instabilities at higher pressure. In addition, we find that applying pressure acts as self-doping, leading to variations in instabilities from the itinerant electrons. Our results imply that the instability of the Pauli susceptibility from itinerant electrons is a possible quantity that describes the various characters of KFe2As2 under pressure and have important implications for understanding the pressure manipulation of superconductivity in iron-based superconductors.

Using density functional theory calculations, we investigate the microscopic origins of the distinct ground states in monolayer T-VSe2 and H-VSe2. We reveal that T-VSe2 exhibits competing tendencies towards either ferromagnetic (FM) or charge density wave phases, governed by the Fermi surface nesting in the weak-coupling limit. In contrast, H-VSe2 stabilizes a robust FM ground state within the local moment picture of the strongcoupling limit. The dichotomy originates from contrasting correlation strengths due to distinct orbital degrees of freedom controlled by the crystal field splittings between T-VSe2 and H-VSe2. We argue similar physics can also be applied to VS2 and VTe2 in the T and H phases. While triply degenerate t2g orbitals with a larger bandwidth favor the itinerant scenario in the weak-coupling limit in the T phase, a nondegenerate dz2 orbital with a narrower bandwidth supports the local moment picture in the strong-coupling limit in the H phase. Furthermore, we demonstrate the tunability of these states via Se height, offering pathways to manipulate quantum phases in T-VSe2. This work provides a comprehensive understanding of various conflicting experimental findings and theoretical predictions regarding polymorphic monolayer VSe2.

We report the magnetic and optical properties of Fe-2(HPO3)(3)center dot 4H(2)O. By combined magnetic susceptibility and specific heat measurements, we find two antiferromagnetic (AFM) transitions (T-N1=9 K, T-N2=5 K) which originate from the AFM exchange interactions mediated by the (HPO3)(2-) anions. Compared with the non-hydrated compound Fe-2(HPO3)(3), the magnetic interaction strength and ordering temperature of Fe-2(HPO3)(3)center dot 4H(2)O are both reduced by a factor of similar to 2. Through density functional theory (DFT) analysis we find almost degenerate AFM states in Fe-2(HPO3)(3)center dot 4H(2)O which are close in energy. The competition of these AFM states might lead to the two successive AFM phase transitions at low temperature. The low ordering temperature as well as competing AFM states both imply frustrated magnetism in Fe-2(HPO3)(3)center dot 4H(2)O, probably originating from the complex competing AFM interactions. The optical properties of Fe-2(HPO3)(3)center dot 4H(2)O are also investigated by photoluminescence and infrared spectroscopies.

It was recently reported that molten sulfur exhibits a first-order liquid-liquid transition (LLT) with a critical point. The phase diagrams of selenium (Se) and sulfur are very similar, and experiments have suggested that LLT may also happen in molten Se. Here, complete ab initio molecular dynamics simulations (AIMD) and a combination of AIMD and machine learning are used to study the pressure-induced structural change of molten Se. Here the simulation density range is expanded, the number of atoms and simulation time are significantly increased, and the van der Waals correction (VDW) is considered. According to our findings, the liquid structure undergoes two changes along the 1000 K isotherm. Nevertheless, neither change is first order. The significance of the VDW interaction is demonstrated by the perfect agreement of the transition pressure with the experiments after considering the VDW correction. We contend that the increase in conductivity seen in the experiments is connected to the first structural change, which results from the effect of temperature on the weakly Peierls distorted structures. The maximum of the melting curve of crystalline Se-I is associated with the second structural change, which is the pressure-induced Peierls transition.

We report the synthesis and superconductivity of a new Ru-based alloy Ru3Sb1.75Sn5.25. The single crystal and polycrystal samples were synthesized by the Sn-flux method and solid state reaction, respectively. The Ru3Sb1.75Sn5.25 crystals were characterized by powder X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. We find Ru3Sb1.75Sn5.25 crystallize in the Ir3Ge7-type structure and exhibits superconductivity with onset transition temperature Tc = 4.2 K. Density functional theory calculation on Ru3Sb1.75Sn5.25 unveils multiple peaks of density of states around the Fermi level, which may explain the emergence of superconductivity. Our study provides an example compound to explore the fascinating interplay of superconductivity, electronic band structure and paramagnetism in Ru-based superconductors.

Based on the strongly constrained and appropriately normed (SCAN) functional, phase behavior of metastable water is studied by ab initio molecular dynamics simulations. A clear van der Waals loop or two-phase coexistence line is observed in the simulated isotherms from 230 to 380 K, providing ab initio evidence for a first-order phase transition. The stability limit line formed by connecting the calculated spinodal points is observed to be a continuous reentrant curve, and the liquid-liquid critical point appears to be hidden at its minimum. The line of maximum density also tends to end at this minimum. These results suggest that the phase behavior of metastable water favors a critical-point-free scenario. Due to the potential limitations of the SCAN functional itself and the failure to take nuclear quantum effects into account, it remains to be further investigated whether the phase behavior of real water remains as predicted in the present work.

The electronic structures of the UT and CT phases of both LaFe2As2 and the counterpart CaFe2As2 are studied by density functional theory and tight-binding models. We find the 4p states of As contribute substantially to the relative instability between (pi, pi, pi) and (0, 0, pi) of Pauli susceptibility x0, which may result in fluctuations mediating the pairing between electrons. This means that considering only the 3d states of Fe is not enough to capture the superconducting properties of iron-based superconductors. This finding is supported by the even higher relative instability at (pi, pi, pi) of interorbital Pauli susceptibility between the 3d orbital of Fe and 4p orbital of As compared with the total intraorbital and interorbital Pauli susceptibility from the 3d orbital of Fe, indicating that the 4p states of As affect x0 considerably. Moreover, while the 3d states of Ca are found to contribute positively to the relative instability at (pi, pi, pi), the 5d states of La contribute negatively to the relative instability at (pi, pi, pi), illustrating the the 3d states of Ca and 5d states of La play contrary contributions to superconductivity. Our results provide qualitative criteria for the superconducting properties of LaFe2As2 and CaFe2As2, and they reveal that both the 4p states of As and interlayer cation states should not be neglected in further studies on superconductivity of iron-based superconductors.

Based on the SCAN functional, we performed ab initio molecular dynamics simulations of metastable water. Our simulated isotherms show obvious van der Waals loop or two-phase coexistence line, which provides fully ab initio evidence for first-order liquid-liquid phase transition. However, the obtained stability limit line is a continuous reentrant curve, the liquid-liquid critical point seems to be hidden at its minimum, and the line of density maximum also tends to end at it. Based on these results, therefore the phase behavior of metastable water is inclined to a critical-point-free scenario. Due to the potential limitations of the SCAN functional itself and not considering the nuclear quantum effects, whether the phase behavior of real water is still as predicted in present work still needs to be further studied.

First-principles calculations are performed to study the hole doping effect in a series of iron arsenic superconductors, such as the undoped CaFe2As2, CaFeAsF, and the hole-doped CaKFe4As4, KCa2Fe4As4F2, and KFe2As2. Charge occupations in the Fe 3d orbitals are found to show opposite variations when the filling is changed. Compared with the undoped CaFe2As2 and CaFeAsF, charges of the and orbitals decrease while those of the and d xy orbitals increase in the hole-doped materials CaKFe4As4, KCa2Fe4As4F2 and KFe2As2. By further analyzing the Pauli susceptibilities of CaFe2As2 and CaFeAsF, instability is found at the wave vector of , which is responsible for the stripe type antiferromagnetic order. In contrast, the Pauli susceptibilities of CaKFe4As4 and KCa2Fe4As4F2 at are strongly suppressed while the instabilities remain around , which may be the origin of superconductivity. Combining with the calculated orbital-resolved Pauli susceptibility, we conclude that hole doping mainly results in a decrease of charge occupations in the and orbitals, and these three orbitals play dominant roles in controlling the magnetic and superconducting properties in these iron-based superconductors.

Based on the newly developed SCAN meta-GGA and the widely used PBE-GGA functionals, ab initio molecular dynamics are performed on water. It is proved that, although the SCAN meta-GGA is not as good as the TIP4P/2005 model potential in describing the equation of state of water, it is much better than the PBE-GGA, the ST2 model potential, and ab initio trained neural network potentials. Moreover, the SCAN meta-GGA predicts a first-order liquid-liquid transition from high- to low-density water at negative pressure, in which the structures are qualitatively consistent with experimental observations, and the spinodal point of high-density water is very close to Speedy's stability limit line.

Using Born-Oppenheimer ab initio molecular dynamics (BOAIMD) simulations, the high-density water (HDW) and low-density water (LDW) structures based on SCAN meta-GGA are compared with those based on PBE GGA. Compared with Car-Parrinello ab initio molecular dynamics (CPAIMD) simulations, BOAIMD simulations can produce more accurate results because no fictitious electron mass is introduced. At each state point, our simulations continue for 100 ps after the system reached equilibrium, which is the longest ab initio simulations of liquid water reported so far and can ensure an accurate statistical average. The influence of the size effect and nuclear quantum effect on structure is not considered in the present work, but only that of two different functionals on the structure is discussed. It is found that, in HDW, just as shown using CPAIMD simulations, the SCAN-based hydrogen-bonds (HBs) are more flexible than the PBE-based ones, which makes the structure based on SCAN obviously closer to the experimental results than that based on PBE. However, it is not the case in LDW, and the difference between the results based on these two functionals is very small.

A 2D Heisenberg ferromagnet with ultra-small interlayer coupling realized in a metal–organic crystal.

It was recently reported in experiments that at temperatures below 2500 K liquid nitrogen (N) remains molecular up to 120 GPa [ Phys.Rev. Lett.119,235701 (2017)], which contradicts a liquid-liquid transition at 88 GPa and 2000 K predicted by PBE-GGA density functional. To clarify this, we perform extensive first principles molecular dynamics using SCAN meta-GGA density functional, which captures the intermediate range part of the van der Waals interaction better than PBE-GGA. It is found that SCAN gives more accurate bond energy and length of an isolated N-2 molecule than PBE. SCAN, as well as PBE, is capable of reproducing the first-order molecular-to-polymeric phase transition, but in contrast to PBE, it predicts a wider stability range for fluid N-2. The boundary of this range is 15 GPa higher than the one predicted with PBE, which is in closer agreement with experiments. In addition, SCAN predicts a higher amount of threefold coordinated atoms in the polymeric phase than PBE, which is expected from experiments in amorphous N. These improvements indicate that SCAN is more accurate than PBE in predicting the phase transition from molecular to polymeric fluid N.

The relationship between structural order and water-like anomalies in tetrahedral liquids is still open. Here, first-principle molecular dynamics are performed to study it in metastable liquid Si. It is found that in T-P phase diagram, there indeed exists a structural anomaly region, which encloses density anomaly but not diffusivity anomaly. This is consistent with that of SW Si and BKS SiO2 but different from that of SPC/E water. Two-body excess entropy anomaly can neither capture the diffusivity, structural, and density anomalies, as it can in a two-scale potential fluid. In structural anomaly region, tetrahedrality order qtetra (measuring the extent to which an atom and its four nearest neighbours adopt tetrahedral arrangement) and translational order ttrans (measuring the tendency of two atoms to adopt preferential separation) are not perfectly correlated, which is different from that in SW Si and renders it impossible to use the isotaxis line to quantify the degree of structural order needed for water-like anomalies to occur. Along the isotherm of critical temperature Tc, ttrans/qtetra is approximately linear with pressure. With decreasing pressure along the isotherm below Tc, ttrans/qtetra departs downward from the line, while it is the opposite case above Tc.

Although the existence of liquid-liquid phase transition has become more and more convincing, whether it will terminate at a critical point and what is the order parameter are still open. To explore these questions, we revisit the fluid-liquid phase transition (FLPT) in phosphorus (P) and study its phase behavior by performing extensive first-principles molecular dynamics simulations. The FLPT observed in experiments is well reproduced, and a fluid-liquid critical point (FLCP) at T = 3000 ∼ 3500 K, P = 1.5-2.0 Kbar is found. With decreasing temperature from the FLCP along the transition line, the density difference (Δρ) between two coexisting phases first increases from zero and then anomalously decreases; however, the entropy difference (ΔS) continuously increases from zero. These features suggest that an order parameter containing contributions from both the density and the entropy is needed to describe the FLPT in P, and at least at low temperatures, the entropy, instead of the density, governs the FLPT.

Objective: To study the impact to operation safety of preoperative renal artery embolization for management of ≥10 cm renal cell carcinoma. Methods: The clinical data of 239 cases with ≥10 cm renal cell carcinoma which all had underwent operation in Department of Urology, Peking University First Hospital from January 2002 to December 2014 were retrospectively analyzed. Fifty-three patients underwent preoperative renal artery embolization (therapeutic group) and 186 patients did not (control group). The effect of embolization on operative time, transfusion requirements, hospitalization, ICU stay and perioperative complications were analyzed by comparing the two groups using rank sum test and χ(2) test or Fisher exact test. Results: Comparing the therapeutic group and control group, there was significant difference in tumor location (on the left or right). The mean age, sex, mean primary tumor size, and TNM stage were similar in both groups. Comparing the therapeutic group and control group, there were more open surgeries in therapeutic group (96.2% vs. 82.3%, χ(2)=6.438, P=0.013). There were no significant differences in mean operative time (238 (525) minutes vs. 208 (583) minutes, Z=-2.182, P=0.062). The mean blood transfusion (700 (1 900) ml vs. 925 (8 800) ml, Z=-1.064, P=0.006) had significant difference. The therapeutic group had a longer mean hospitalization (21 (50) days vs. 15 (79) days, Z=-4.322, P=0.000) and higher rate of intensive care unit stay (54.7% vs. 34.4%, χ(2)=6.103, P=0.027). There was no significant difference in perioperative complications between two groups (0 vs.3.2%, P=0.408). Conclusion: Preoperative renal artery embolization in ≥10 cm renal cell carcinoma patients undergoing operation provides benefit in increasing operation safety and reducing perioperative death.

Objective: To investigate the clinical features and prognosis of rare subtypes of renal cell carcinoma. Methods: This retrospective study collected the data of 52 rare subtypes of renal cell carcinoma of patients who underwent surgery from January 2002 to December 2014 at Department of Urology, Peking University First Hospital. There were 12 patients with collecting duct carcinoma, 5 patients with Xp11.2 translocation renal cell carcinoma, 5 patients with mucinous tubular and spindle cell carcinoma, 30 patients with unclassified renal cell carcinoma. The study group included 25 male and 27 female patients, with mean age of 52 years. The mean tumour size was (6.5±3.9) cm (range: 1.5 to 21.0 cm). The basic clinical features, gross appearance, Fuhrman nuclear grade, TNM staging and prognosis of rare subtypes of RCC were studied. The OS curves were obtained for rare subtypes of renal cell carcinoma using the Kaplan-Meier method and compared using a Log-rank test. Results: The rate of lymph node and distant metastasis were 34.6% (18/52) and 17.3% (9/52). Malignancies were screened and detected by color Doppler ultrasonography or CT scan, however, no case was diagnosed before operation or aspiration, all cases were confirmed by the pathological examination. The average period of postoperative follow-up process was 65 months, and the mean survival time was (34±23) months. Conclusion: The clinical features of rare subtypes of renal cell carcinoma are similar to those of clear cell renal cell carcinoma, while the imaging changes will be helpful for diagnosis before operation.

By applying density functional theory calculations to iron chalcogenides, we find that magnetic order in Fe 1+ y Te and magnetic instability at (π, π) in K y Fe 2 Se 2 are controlled by interstitial and interlayer cations, respectively. While in Fe 1+ y Te, magnetic phase transitions occur among collinear, exotic bicollinear and plaquette-ordered antiferronmagnetic states when the height of interstitial irons measured from iron plane or the concentration of interstitial irons is varied, the magnetic instability at (π, π) which is believed to be responsible for the Cooper pairing in iron pnictides is significantly enhanced when y is much smaller than 1 in K y Fe 2 Se 2 . Our results indicate that, similar to iron pnictides, itinerant electrons play important roles in iron chalcogenides, even though the fluctuating local moments become larger.

Extensive first-principle molecular dynamics simulations are performed to study the phase behavior of metastable liquid Si at negative pressure. Our results show that the high-density liquid (HDL) and HDL-vapor spinodals indeed form a continuous reentrant curve and the liquid-liquid critical point seems to just coincide with its minimum. The line of density maxima also has a strong tendency to pass through this minimum. The phase behaviour of metastable liquid Si therefore tends to be a critical-point-free scenario rather than a second-critical-point one based on SW potential.

We propose that the contrasting low-temperature behaviors observed experimentally among isostructural and isoelectronic materials, like non-superconducting and nonmagnetic MgFeGe, magnetically ordered NaFeAs, and superconducting LiFeAs, can be well understood from itinerant weak coupling limit. We find that stronger (π,π) instability appearing in the d_x^2-y^2 orbital of NaFeAs is responsible for the occurrence of weak magnetism while weaker but still prominent (π,π) instability in LiFeAs leads to a superconducting state. In contrast, multiple competing instabilities coexisting in orbital-resolved momentum-dependent susceptibilities, serving as magnetic frustrations from itinerant electrons, may account for the nonmagnetic state in MgFeGe, while poorer Fermi surface nesting leads to a non-superconducting state. Based on above findings, we predict a possible way to make MgFeGe a new Fe-based superconductor.