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The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems., Comment: 18 pages, 5 figures

Electronic nematicity, in which rotational symmetry is spontaneously broken by electronic degrees of freedom, has been demonstrated as a ubiquitous phenomenon in correlated quantum fluids including high-temperature superconductors and quantum Hall systems

This study aimed at the problems of unequal speed transmission ratio mutual restriction and side gap accumulation of the transplanting mechanism with single-degree-of-freedom K-H-V non-circular planetary gear train, which leads to poor trajectory and attitude, and poor precision of movement. This study has proposed a simple structure of transplanting mechanism with differential internal engagement non-circular planetary gear trains, which reconstructs the complex transplanting trajectory and attitude of the planting arm through single-stage unequal speed transmission. The working principle of the transplanting mechanism was analysed, and the kinematic theoretical model of the transplanting mechanism was established. The optimal design software for the transplanting mechanism was developed based on the visual platform. The dimensions optimisation on the transplanting mechanism was completed considering the requirements with strong coupling, and multi-objective and a set of superior mechanism parameters were obtained. The design theory and method of the internal engagement non-circular gear pair were proposed based on the generating principle. The correctness and accuracy of the trajectory and attitude of the transplanting mechanism were verified through virtual simulation experiments. The experiments show that the designed transplanting mechanism with internal engagement non-circular planetary gear trains was compact in structure, the trajectory meets the requirements of multi-objective transplanting, and the trajectory and attitude can be accurately reproduced, which provides a new feasible solution for the innovative design of the transplanting mechanism.

The discovery of several electronic orders in kagome superconductors AV

Recently discovered kagome metals AV3Sb5 (A = K, Rb, and Cs) provide an ideal platform to study the correlation among nontrivial band topology, unconventional charge density wave (CDW), and superconductivity. The evolution of electronic structures associated with the change of lattice modulations is crucial for understanding of the CDW mechanism, with the combination of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory calculations, we investigate how band dispersions change with the increase of lattice distortions. In particular, we focus on the electronic states around M ¯ point, where the van Hove singularities are expected to play crucial roles in the CDW transition. Previous ARPES studies reported a spectral weight splitting of the van Hove singularity around M ¯ point, which is associated with the 3D lattice modulations. Our studies reveal that this “splitting” can be connected to the two van Hove singularities at kz = 0 and kz = π/c in the normal states. When the electronic system enters into the CDW state, both van Hove singularities move down. Such novel properties are important for understanding of the CDW transition.

While most of the rare earth metals readily form trihydrides, due to increased stability of the filled 4f electronic shell for Yb(II), only YbH$_{2.67}$, formally corresponding to Yb$^{\rm II}$(Yb$^{\rm III}$H$_4$)$_2$ or Yb$_3$H$_8$, remains the highest hydride of ytterbium. Utilizing diamond anvil cell methodology and synchrotron powder x-ray diffraction we have attempted to push this limit further via hydrogenation of metallic Yb (at room temperature and heated in situ) and of Yb$_3$H$_8$. Compression of the latter has also been investigated in a neutral pressure transmitting medium, PTM. While the in situ heating of Yb facilitates the formation of YbH$_2$ plus x hydride, we have not observed the clear qualitative differences between the systems compressed in H$_2$ and He or Ne PTM. In all these cases a sequence of phase transitions from the unit cells of P-31m symmetry to the I4/m and I4/mmm systems occurred within ca. 13 to 18 GPa and around 27 GPa, respectively. At the same time, the molecular volume of the systems compressed in H$_2$ PTM is ca. 1.5% larger than of those compressed in inert gases, suggesting a small hydrogen uptake. Nevertheless, hydrogenation towards YbH$_3$ is incomplete, and polyhydrides do not form up to the highest pressure studied here of ca. 75 GPa. As pointed out by our electronic transport measurements under compression in NaCl PTM, the mixed-valence Yb$_3$H$_8$ retains its semiconducting character up to at least 50 GPa, although the very low remnant activation energy of conduction, smaller than 5 meV, suggests that the metallization under further compression should be achievable. Finally, we provide a theoretical description of a hypothetical stoichiometric YbH$_3$., Comment: 19 pages, 7 figures, 2 tables, and electronic supplement

The kagome lattice is host to flat bands, topological electronic structures, Van Hove singularities, and diverse electronic instabilities, providing an ideal platform for realizing highly tunable electronic states. Here, we report soft and mechanical point-contact spectroscopy (SPCS and MPCS) studies of the kagome superconductors ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$ and $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$. Compared to the superconducting transition temperature ${T}_{\mathrm{c}}$ from specific heat and electrical resistance measurements, significantly enhanced values of ${T}_{\mathrm{c}}$ are observed via the zero-bias conductance of SPCS, which become further enhanced in MPCS measurements. While the differential conductance curves from SPCS can be described by a two-gap $s$-wave model, a single $s$-wave gap reasonably captures the MPCS data, likely due to a diminishing spectral weight of the other gap. The enhanced superconductivity probably arises from local strain caused by the point contact, which also leads to two-gap or single-gap behaviors observed in different point contacts. Our results demonstrate highly strain-sensitive superconductivity in kagome metals $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ and ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$, which may be harnessed in the manipulation of possible Majorana zero modes.

We report the synthesis, structural and physical properties of a new layered transition metal arsenide Na4Cu3TaAs4. This material adopts the space group I 4 ¯ 2 m , with lattice parameters of a =5.9101(3) A and c =13.8867(12) A. This structure contains two layers of Na sandwiched by anti-PbO-type (Cu/Ta)As layers, similar to the “111”-type iron-based superconductor NaFeAs. The transition metal sites are occupied by 75% Cu and 25% Ta, with Ta forming a well-defined 2 × 2 × 2 superstructure. Cu and Ta were determined to be +1 and +5 oxidation state respectively. The band structure of the Na4Cu3TaAs4 measured by angle resolved photoemission spectroscopy (ARPES) is in good agreement with the density functional theory (DFT) calculation. Both ARPES and resistivity measurement indicate that this material exhibits metallic behavior with p-type carriers. Magnetic susceptibility measurement shows that the material exhibits nearly T -independent diamagnetism. This new material extends the material system with anti-PbO-type layers and offers a good playground to investigate this material system further.

Magnetism in the two-dimensional limit has become an intriguing topic for exploring new physical phenomena and potential applications. Especially, the two-dimensional magnetism is often associated with novel intrinsic spin fluctuations and versatile electronic structures, which provides vast opportunities in 2D material research. However, it is still challenging to verify candidate materials hosting two-dimensional magnetism, since the prototype systems have to be realized by using mechanical exfoliation or atomic layer deposition. Here, an alternative manipulation of two-dimensional magnetic properties via electrochemical intercalation of organic molecules is reported. Using tetrabutyl ammonium (TBA+), we synthesized a (TBA)Cr2Ge2Te6 hybrid superlattice with metallic behavior, and the Curie temperature is significantly increased from 67 K in pristine Cr2Ge2Te6 to 208 K in (TBA)Cr2Ge2Te6. Moreover, the magnetic easy axis changes from the ⟨001⟩ direction in Cr2Ge2Te6 to the ab-plane in (TBA)Cr2Ge2Te6. Theoretical calculations indicate that the drastic increase of the Curie temperature can be attributed to the change of magnetic coupling from a weak superexchange interaction in pristine Cr2Ge2Te6 to a strong double-exchange interaction in (TBA)Cr2Ge2Te6. These findings are the first demonstration of manipulation of magnetism in magnetic van der Waals materials by means of intercalating organic ions, which can serve as a convenient and efficient approach to explore versatile magnetic and electronic properties in van der Waals crystals.

Two kinds of ternary thorium nitride compounds, ThNF and ThNCl, are synthesized. Via the refinement of X-ray diffraction patterns, the accurate crystal structure of the two compounds is solved. Although ThNF and ThNCl share a similar structure with MNX (M = Ti, Zr, Hf; X = Cl, Br) compounds, the interaction between adjacent ThNF and ThNCl layers is not a van der Waals gap. For ThNF, the strong electronegativity of F ions leads to the bonding of Th to the F both in the nearest neighbor layer and the next nearest neighbor layer, which results in the absence of a van der Waals gap between ThNF layers. However, for ThNCl, the reason for the absence of a van der Waals gap could be attributed to the large Th–Cl bond length due to the partially covalent Th–Cl bond as well as the flat ThN layer. It is the absence of van der Waals gap that results in the failure of intercalating cations into ThNF and ThNCl. Our result reveals the reason for unsuccessful intercalation in ThNF and ThNCl, thereby providing a deeper u...

Magnetic topological semimetals provide new opportunities for power generation and solid-state cooling based on thermoelectric (TE) effect. The interplay between magnetism and nontrivial band topology prompts the magnetic topological semimetals to yield strong transverse TE effect, while the longitudinal TE performance is usually poor. Herein, it is demonstrated that the magnetic Weyl semimetal TbPtBi has high value for both transverse and longitudinal thermopower with large power factor (PF). At 300 K and 13.5 Tesla, the transverse thermopower and PF reach up to 214 µV K

Understanding the normal electronic state is crucial for unveiling the mechanism of unconventional superconductivity (SC). In this paper, by applying a magnetic field of up to 37 T on FeSe single crystals, we could reveal the normal-state transport properties after SC was completely suppressed. The normal-state resistivity exhibited a Fermi liquid behavior at low temperatures. Large orbital magnetoresistance (MR) was observed in the nematic state with H//c, whereas MR was negligible with H//ab. The magnitude of the orbital MR showed an unusual reduction, and Kohler’s rule was severely violated below 10–25 K; these were attributable to spin fluctuations. The results indicated that spin fluctuations played a paramount role in the normalstate transport properties of FeSe albeit the Fermi liquid nature was at low temperature.

Recently, SnSe has been found to possess a remarkable thermoelectric performance. As an isostructural compound to SnSe, SnS contains a more environmentally compatible and earth abundant element, which enables SnS a promising thermoelectric material for commercial application. In this work, we have performed systematic studies of the crystal structures and the lattice vibrations by means of neutron total scattering, Raman scattering measurements, and first-principle calculations. A structural transition has been observed by both temperature dependent neutron diffraction and pressure dependent Raman scattering. The lattice anharmonicity has been revealed by the local distortion induced by the Sn $5{s}^{2}$ lone pair and is also evidenced from the temperature dependent atomic displacement parameter of the Sn atom. By separating the intrinsic anharmonicity and lattice thermal expansion effects, we found that the former plays the primary role in the softening of the Raman active modes. Such anharmonicity has also been observed experimentally by the linewidth broadening of the high-energy optical modes and confirmed by frozen phonon calculations. Furthermore, our frozen phonon calculation reveals the presence of the quartic anharmonicity potential of those high-energy optical modes vibrating along the $b$ axis. Our data will contribute to a better understanding of the thermal conduction in SnS, which will be beneficial for the enhancement of the thermoelectric performance of this material.

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr

Understanding the competition between superconductivity and other ordered states (such as antiferromagnetic or charge-density-wave (CDW) state) is a central issue in condensed matter physics. The recently discovered layered kagome metal AV3Sb5 (A = K, Rb, and Cs) provides us a new playground to study the interplay of superconductivity and CDW state by involving nontrivial topology of band structures. Here, we conduct high-pressure electrical transport and magnetic susceptibility measurements to study CsV3Sb5 with the highest Tc of 2.7 K in AV3Sb5 family. While the CDW transition is monotonically suppressed by pressure, superconductivity is enhanced with increasing pressure up to P1 ≈ 0.7 GPa, then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa, after that, Tc is enhanced with increasing pressure again. The CDW is completely suppressed at a critical pressure P2 ≈ 2 GPa together with a maximum Tc of about 8 K. In contrast to a common dome-like behavior, the pressure-dependent Tc shows an unexpected double-peak behavior. The unusual suppression of Tc at P1 is concomitant with the rapidly damping of quantum oscillations, sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance. Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome lattice., The recently discovered kagome metal AV3Sb5 is a new playground to study the interplay between superconductivity and charge-density-wave (CDW) state. Here, the authors report pressure-dependent evolution of CDW and superconductivity in CsV3Sb5, suggesting an unusual competition between the two phases.

Controlling the anomalous Hall effect (AHE) inspires potential applications of quantum materials in the next generation of electronics. The recently discovered quasi-2D kagome superconductor CsV3Sb5 exhibits large AHE accompanying with the charge-density-wave (CDW) order which provides us an ideal platform to study the interplay among nontrivial band topology, CDW, and unconventional superconductivity. Here, we systematically investigated the pressure effect of the AHE in CsV3Sb5. Our high-pressure transport measurements confirm the concurrence of AHE and CDW in the compressed CsV3Sb5. Remarkably, distinct from the negative AHE at ambient pressure, a positive anomalous Hall resistivity sets in below 35 K with pressure around 0.75 GPa, which can be attributed to the Fermi surface reconstruction and/or Fermi energy shift in the new CDW phase under pressure. Our work indicates that the anomalous Hall effect in CsV3Sb5 is tunable and highly related to the band structure.

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate. Recently, a dominant B1g-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe1–x Co x )2As2, suggesting a strong interplay between nematicity and superconductivity. Since the long-range spin order is absent in FeSe superconductor, whether a similar strain effect could be also observed or not is an interesting question. Here, by utilizing a flexible film as substrate, we successfully achieve a wide-range-strain tuning of FeSe thin flake, in which both the tensile and compressive strain could reach up to ∼0.7%, and systematically study the strain effect on both superconducting and nematic transition (T c and T s) in the FeSe thin flake. Our results reveal a predominant A1g-type strain effect on T c. Meanwhile, T s exhibits a monotonic anti-correlation with T c and the maximum T c reaches to 12 K when T s is strongly suppressed under the maximum compressive strain. Finally, in comparison with the results in the underdoped Ba(Fe1–x Co x )2As2, the absence of B1g-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity. In addition, our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.

Magnetism in the two-dimensional limit has become an intriguing topic for exploring new physical phenomena and potential applications. Especially, the two-dimensional magnetism is often associated with novel intrinsic spin fluctuations and versatile electronic structures, which provides vast opportunities in 2D material research. However, it is still challenging to verify candidate materials hosting two-dimensional magnetism, since the prototype systems have to be realized by using mechanical exfoliation or atomic layer deposition. Here, an alternative manipulation of two-dimensional magnetic properties via electrochemical intercalation of organic molecules is reported. Using tetrabutyl ammonium (TBA

$\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ was recently suggested as the first example of an antiferromagnetic topological insulator. However, lacking good quality of single crystals hindered its further investigation. Here, we report the detailed transport properties of several $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ thin flakes in which samples are more homogeneous as compared to the bulk single crystals. We found all the samples exhibit antiferromagnetic transition around 25 K and the same field-driven magnetic transitions; however, temperature dependence of resistivities shows either insulating or metallic behaviors. Such behavior is in contrast with the as-grown thick single crystals in which only metallic behavior was observed. The Hall coefficients indicate the gradual decrease of the carrier density with decreasing the temperature for the sample with insulating behavior. Such difference may relate to different impurity content (antisite defects and/or Mn vacancies). Our findings indicate the bulk carrier can be localized at low temperature by introducing disorder while the magnetic ordering keeps invariant, which is quite crucial for realizing the theoretical proposed quantum anomalous Hall effect and axion insulators in this material.

Thermoelectric transport properties of binary lithium-based compounds Li3Sb and Li3Bi are investigated. Although the present results show zTs (0.3 for Li3Sb and 0.06 for Li3Bi) are low for the p-type compounds in this family, the experimental data analysis based on the single parabolic band model assuming the dominance of acoustic phonon scattering for carriers suggests that both Li3Sb and Li3Bi process good electronic quality factor BE over 3, which gives rise to the predicted peak zT values above 0.7 and 1.2 at 800 K for Li3Sb and Li3Bi, respectively. In addition, the electronic band structure and phonon dispersion of Li3Sb are given by first principles calculation. This work offers insight into the crystal structure and transport properties of Li3Sb and Li3Bi, indicating the potential of binary lithium-based compounds as promising thermoelectrics.

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr2 Ge2 Te6 . The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out-of-plane direction to an in-plane direction by electric field in the few-layered Cr2 Ge2 Te6 . Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few-layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC-FET device configuration, making further heterostructure-engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.

We present the electrical resistivity data under application of pressures up to $\ensuremath{\sim}26$ GPa and down to 50 mK on ${\mathrm{YbFe}}_{2}{\mathrm{Zn}}_{20}$. We find a pressure induced magnetic phase transition with an onset at ${p}_{c}=18.2\ifmmode\pm\else\textpm\fi{}0.8$ GPa. At ambient pressure, ${\mathrm{YbFe}}_{2}{\mathrm{Zn}}_{20}$ manifests a heavy fermion, nonmagnetic ground state and the Fermi liquid behavior at low temperatures. As pressure is increased, the power law exponent in resistivity, $n$, deviates significantly from Fermi liquid behavior and tends to saturate with $n=1$ near ${p}_{c}$. A pronounced resistivity maximum ${T}_{\mathrm{max}}$, which scales with the Kondo temperature, is observed. ${T}_{\mathrm{max}}$ decreases with increasing pressure and flattened out near ${p}_{c}$ indicating the suppression of Kondo exchange interaction. For $pg{p}_{c},{T}_{\mathrm{max}}$ shows a sudden upward shift, most likely becoming associated with crystal electric field scattering. Application of magnetic field for $pg{p}_{c}$ broadens the transition and shifts it toward the higher temperature, which is a typical behavior of a transition towards a ferromagnetic state, or a state with a significant ferromagnetic component. The magnetic transition appears to abruptly develop above ${p}_{c}$, suggesting probable first-order (with changing pressure) nature of the transition; once stabilized, the ordering temperature does not depend on pressure up to $\ensuremath{\sim}26$ GPa. Taken as a whole, these data suggest that ${\mathrm{YbFe}}_{2}{\mathrm{Zn}}_{20}$ has a quantum phase transition at ${p}_{c}=18.2$ GPa associated with the avoided quantum criticality in metallic ferromagnets.

We predict two new nickel hydrides, $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ ($C2/m$) and $\mathrm{Ni}{\mathrm{H}}_{2}$ ($I4/mmm$), to be thermodynamically stable at 60 GPa by DFT calculations. The calculated structure of $\mathrm{Ni}{\mathrm{H}}_{2}$ is similar to the one observed earlier for $\mathrm{Fe}{\mathrm{H}}_{2}$. However, to the best of our knowledge, the monoclinic structure predicted for $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ was never reported in the other hydrides. We successfully synthesized the monoclinic $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ phase using a laser-heating technique in a diamond anvil cell at pressures around 60 GPa. The $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ phase can be retained down to 17 GPa, and DFT calculations predict it to be nonmagnetic and nonsuperconducting. However, we did not find in our experiments the theoretically predicted $\mathrm{Ni}{\mathrm{H}}_{2}$ phase. Higher pressures and/or different synthesis conditions might be needed to synthesize this and other polyhydrides of nickel. Our results show that the Ni-H system behaves differently in comparison to the Fe-H system: Ni extends its own chemical identity to pressures as high as 60--80 GPa. This finding may have significant implications for the high-pressure behavior of Fe-Ni alloys at the high-pressure conditions relevant for the Earth and planetary sciences.

Compelling efforts to improve the critical temperature $({T}_{c})$ of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important; however, much remains unclear. Here we studied ortho-III ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.73}$ (YBCO) using x-ray scattering under hydrostatic pressure (HP) up to $\ensuremath{\sim}6.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. We found the reinforced oxygen order of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge-transfer phenomenon between the $\mathrm{Cu}{\mathrm{O}}_{2}$ plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge-density-wave signature in $\mathrm{Cu}{\mathrm{O}}_{2}$ plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the $\mathrm{Cu}{\mathrm{O}}_{2}$ plane. Using these results, we appropriately explain why pressure condition can achieve higher ${T}_{c}$ compared with the optimal ${T}_{c}$ under ambient pressure in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$. As an implication of these results, finally we have discussed that the change in disorder could make it easier for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ to undergo a transition to the nematic order under an external magnetic field.

We performed high pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe$_2$) under compression. The obtained single-crystal XRD data indicate the formation of a $(1/3,1/3,1)$-type superlattice above 17 GPa. According to our density functional theory results, the pressure-induced transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect of strong Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}=(1/3,1/3,1)$. In contrast, similar PLD transitions associated with charge density wave (CDW) orderings in transition metal dichalcogenides (TMDs) do not involve significant Fermi surface nesting. The discovered pressure-induced PLD is quite remarkable, as pressure usually suppresses CDW phases in related materials. Our findings, therefore, provide new playgrounds to study the intricate mechanisms governing the emergence of PLD in TMD-related materials., Comment: 6 pages, 4 figures + Supplemental (12 pages, 14 figures)

High pressure resistance, susceptibility, and Fe $K\ensuremath{\beta}$ x-ray emission spectroscopy measurements were performed on Fe-ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$. Pressure-induced superconductivity was observed which is similar to the previously reported superconductivity in the ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ samples. The slope of local magnetic moment versus pressure shows an anomaly across the insulator-metal transition pressure in the ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ samples. The local magnetic moment is continuously decreasing with increasing pressure, and the superconductivity appears only when the local magnetic moment value is comparable to the one in the iron-pnictide superconductors. Our results indicate that the compressed ${\mathrm{BaFe}}_{2}C{h}_{3}$ ($Ch=\mathrm{S}$, Se) is a new family of iron-based superconductors. Despite the crystal structures completely different from the known iron-based superconducting materials, the magnetism in this Fe-ladder material plays a critical role in superconductivity. This behavior is similar to the other members of iron-based superconducting materials.

Measurements of the resistivity, Hall coefficient, and Raman spectroscopy are performed on a Rashba semiconductor BiTeCl single crystal at high pressures up to 50 GPa. We find that applying pressure first induces a theoretically predicted insulating state, followed by a superconducting phase with an insulating normal state. Upon heavy compression, another different superconducting phase is entered into with a metallic normal state. A domelike evolution of the superconducting transition temperature with pressure is obtained with a crossover from the electron to hole carriers across the boundary of the two superconducting phases. These findings imply the possible realization of a topological state of the insulating and superconducting phases in this material.

We synthesized the samples Ba1−x Sm x FFeAs and Eu1−x Sm x FFeAs with ZrCuSiAs-type structure. These samples were characterized by resistivity and susceptibility. It is found that the substitution of rare earth metal for alkaline earth metal in the two systems suppresses the anomaly in resistivity and induces superconductivity. Superconductivity at 54 K in nominal composition Ba0.5Sm0.5FFeAs and at 51 K for Eu0.5Sm0.5FFeAs is realized, indicating that the superconducting transition temperatures in the iron arsenide fluorides is the same as that in oxypnictides with the same structure.

The recent discovery of superconductivity in oxypnictides with the critical temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer (BCS) theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, while strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. However, superconductivity and magnetism in pnictide superconductors show a strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW) transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to 0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows the same effect on TSDW as TCc These results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for understanding the mechanism of high-TC superconductivity., Comment: 22 pages, 7 figure

The intercalated layered nitride $\beta$-HfNCl has attracted much attention due to the high superconducting transition temperature up to 25.5 K. Electrons can be introduced into $\beta$-$M$NCl ($M$=Zr and Hf) through alkali-metals intercalation to realize the superconductivity. Here, we report the observation of superconductivity in rare-earth metals cointercalated compounds Yb$_x$($Me$)$_y$HfNCl with $Me$ = NH$_3$ and tetrahydrofuran (THF), which were synthesized by the liquid ammonia method at room temperature. The superconducting transition temperature is about 23 K and 24.6 K for Yb$_{0.2}$(NH$_3$)$_y$HfNCl and Yb$_{0.3}$(NH$_3$)$_y$HfNCl, respectively. Replacing the NH$_3$ with a larger molecule THF, superconducting transition temperature increases to 25.2 K in Yb$_{0.2}$(THF)$_y$HfNCl, which is almost the same as the highest $T_{\rm c}$ reported in the alkali-metals intercalated HfNCl superconductors. The $T_{\rm c}$ of Yb$_{0.2}$(THF)$_y$HfNCl is apparently suppressed by pressure up to 0.5 GPa, while the pressure effect on $T_{\rm c}$ becomes very small above 0.5 GPa. The liquid ammonia method is proved to be an effective synthetic method to intercalate metal ions into HfNCl. Our results suggest that the superconductivity in these layered intercalated superconductors nearly does not rely on the intercalated metal ions, even magnetic ion.

We successfully synthesized La- and Sm-doped phenanthrene powder samples and discovered superconductivity at $T_{\rm c}$ around 6 K in them. The $T_{\rm c}$s are 6.1 K for LaPhenanthrene and 6.0 K for SmPhenanthrene, which are enhanced by about 1 K and 0.5 K compared to those in $A_3$Phenanthrene ($A$=K and Rb) and in $Ae_{1.5}$Phenanthrene ($Ae$ = Sr and Ba) superconductors respectively. The superconductive shielding fractions for LaPhenanthrene and SmPhenanthrene are 46.1%$ and 49.8$%$ at 2 K, respectively. The little effect of the doping of the magnetic ion Sm$^{3+}$ on $T_c$ and the positive pressure dependence coefficient on $T_{\rm c}$ strongly suggests unconventional superconductivity in the doped phenanthrene superconductors. The charge transfer to organic molecules from dopants of La and Sm induces a redshift of 7 cm$^{-1}$ per electron for the mode at 1441 cm$^{-1}$ in the Raman spectra, which is almost the same as those observed in $A_3$Phenanthrene ($A$=K and Rb) and $Ae_{1.5}$Phenanthrene ($Ae$ = Sr and Ba) superconductors., Comment: 5 pages, 5 figures

We report specific-heat studies on high-quality Ba-doped phenanthrene, one of the organic superconductors with fused benzene rings, with 100$%$ shielding fraction. The specific-heat data indicate that Ba-doped phenanthrene is a single-gap $s$-wave superconductor with moderate coupling, and this sets constraints on theoretical models for the understanding of superconductivity in these hydrocarbon superconductors.

We performed the high hydrostatic pressure resistivity measurements (up to 1.7 GPa) on the newly discovered superconductors $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs) single crystals. Two batches of single crystals $K_xFe_2Se_2$ with different transition temperatures ($T_c$) were used to study the effect of pressure. The $T_c$ of the first one gradually decreases with increasing pressure from 32.6 K at ambient pressure. While a dome-like behavior was observed for the crystal with $T_c=31.1$ K, and $T_c$ reaches its maximum value of 32.7 K at the pressure of 0.48 GPa. It indicates that there exists a optimal doping with maximum $T_c$ of 32.7K in $K_xFe_2Se_2$ system. The behavior of $T_c$ vs. pressure for $Cs_xFe_2Se_2$ also shows a dome-like behavior, and $T_c$ reaches its maximum value of 31.1 K at the pressure of 0.82 GPa. The hump observed in temperature dependence of resistivity for all the samples tends to shift to high temperature with increasing pressure. The resistivity hump could arise from the vacancy of Fe or Se.

We synthesized the samples Sr(1-x)Sm(x)FFeAs with a ZrCuSiAs-type structure. These samples were characterized by resistivity and susceptibility. It is found that substitution of rare earth metal for alkaline earth metal in this system suppresses the anomaly in resistivity and induces superconductivity. Superconductivity at 56 K in nominal composition Sr(0.5)Sm(0.5)FFeAs is realized, indicating that the superconducting transition temperatures in the iron arsenide fluorides can reach as high as that in oxypnictides with the same structure.

We synthesized the samples $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with $ThCr_2Si_2$-type structure. These samples were systematically characterized by resistivity, thermoelectic power (TEP) and Hall coefficient ($R_H$). $BaFe_2As_2$ shows an anomaly in resistivity at about 140 K. Substitution of La for Ba leads to a shift of the anomaly to low temperature, but no superconducting transition is observed. Potassium doping leads to suppression of the anomaly in resistivity and induces superconductivity at 38 K as reported by Rotter et al.\cite{rotter}. The Hall coefficient and TEP measurements indicate that the TEP is negative for $BaFe_2As_2$ and La-doped $BaFe_2As_2$, indicating n-type carrier; while potassium doping leads to change of the sign in $R_H$ and TEP. It definitely indicates p-type carrier in superconducting $Ba_{1-x}K_xFe_2As_2$ with double FeAs layers, being in contrast to the case of $LnO_{1-x}F_xFeAs$ with single FeAs layer. A similar superconductivity is also observed in the sample with nominal composition $Ba_{1-x}K_xOFe_2As_2$.