Pressure-induced superconductivity in a three-dimensional topological material ZrTe5.

As a new type of topological materials, ZrTe5 shows many exotic properties under extreme conditions. Using resistance and ac magnetic susceptibility measurements under high pressure, while the resistance anomaly near 128 K is completely suppressed at 6.2 GPa, a fully superconducting transition emerges. The superconducting transition temperature Tc increases with applied pressure, and reaches a maximum of 4.0 K at 14.6 GPa, followed by a slight drop but remaining almost constant value up to 68.5 GPa. At pressures above 21.2 GPa, a second superconducting phase with the maximum Tc of about 6.0 K appears and coexists with the original one to the maximum pressure studied in this work. In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopy combined with theoretical calculations indicate the observed two-stage superconducting behavior is correlated to the structural phase transition from ambient Cmcm phase to high-pressure C2/m phase around 6 GPa, and to a mixture of two high-pressure phases of C2/m and P-1 above 20 GPa. The combination of structure, transport measurement, and theoretical calculations enable a complete understanding of the emerging exotic properties in 3D topological materials under extreme environments.

Superconductivity in Pristine 2H_{a}-MoS_{2} at Ultrahigh Pressure.

As a follow-up of our previous work on pressure-induced metallization of the 2H_{c}-MoS_{2} [Chi et al., Phys. Rev. Lett. 113, 036802 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.036802], here we extend pressure beyond the megabar range to seek after superconductivity via electrical transport measurements. We found that superconductivity emerges in the 2H_{a}-MoS_{2} with an onset critical temperature T_{c} of ca. 3 K at ca. 90 GPa. Upon further increasing the pressure, T_{c} is rapidly enhanced beyond 10 K and stabilized at ca. 12 K over a wide pressure range up to 220 GPa. Synchrotron x-ray diffraction measurements evidenced no further structural phase transition, decomposition, and amorphization up to 155 GPa, implying an intrinsic superconductivity in the 2H_{a}-MoS_{2}. DFT calculations suggest that the emergence of pressure-induced superconductivity is intimately linked to the emergence of a new flat Fermi pocket in the electronic structure. Our finding represents an alternative strategy for achieving superconductivity in 2H-MoS_{2} in addition to chemical intercalation and electrostatic gating.

Pressure-Induced New Topological Weyl Semimetal Phase in TaAs.

We report a new pressure-induced phase in TaAs with different Weyl fermions than the ambient structure with the aid of theoretical calculations, experimental transport and synchrotron structure investigations up to 53 GPa. We show that TaAs transforms from an ambient I4_{1}md phase (t-TaAs) to a high-pressure hexagonal P-6m2 (h-TaAs) phase at 14 GPa, along with changes of the electronic state from containing 24 Weyl nodes distributed at two energy levels to possessing 12 Weyl nodes at an isoenergy level, which substantially reduces the interference between the surface and bulk states. The new pressure-induced phase can be reserved upon releasing pressure to ambient condition, which allows one to study the exotic behavior of a single set of Weyl fermions, such as the interplay between surface states and other properties.

Rare-Earth Triangular Lattice Spin Liquid: A Single-Crystal Study of YbMgGaO4.

YbMgGaO4, a structurally perfect two-dimensional triangular lattice with an odd number of electrons per unit cell and spin-orbit entangled effective spin-1/2 local moments for the Yb(3+) ions, is likely to experimentally realize the quantum spin liquid ground state. We report the first experimental characterization of single-crystal YbMgGaO4 samples. Because of the spin-orbit entanglement, the interaction between the neighboring Yb(3+) moments depends on the bond orientations and is highly anisotropic in the spin space. We carry out thermodynamic and the electron spin resonance measurements to confirm the anisotropic nature of the spin interaction as well as to quantitatively determine the couplings. Our result is a first step towards the theoretical understanding of the possible quantum spin liquid ground state in this system and sheds new light on the search for quantum spin liquids in strong spin-orbit coupled insulators.

Study on the effect of different additives on the anaerobic digestion of hybrid Pennisetum: Comparison of nano-ZnO, nano-Fe2O3 and nano-Al2O3.

The effects of three nanomaterials (ZnO, Al2O3, and Fe2O3) on the wet and dry anaerobic digestion (AD) processes of hybrid Pennisetum were assessed over 33 days, and the microbial communities of dry AD systems were studied. The results demonstrated that biogas production improved by 72.2% and 33.6% when nanoporous Al2O3 (nano-Al2O3) and nano-Fe2O3 were added during dry AD, respectively. However, biogas production decreased by 39.4% with nano-ZnO. Kinetic analysis showed that the three nanomaterials could shorten the lag phase of the AD sludge, while the 16S rRNA gene amplicon sequencing results demonstrated that microbes such as Longilinea and Methanosarcina were enriched in the nano-Al2O3 reactors and methanogenic communities community such as Methanobacterium sp., Methanobrevibacter sp., and Methanothrix sp., which were enriched in the nano-Al2O3 and nano-Fe2O3 reactors. However, the microbial community and some methanogenic communities diversity and richness were inhibited by the addition of nano-ZnO.

Pressure evolution of electronic and structural properties in transition metal dichalcogenide 1T-Co1.06Te2.

Two-dimensional transition metal dichalcogenides (TMDs) are important materials for promising electronic devices because they usually exhibit excellent and highly tunable electronic properties. Here, we report on the pressure-driven electronic phase transition in a TMD 1T-Co1.06Te2. High-pressure transport measurements reveal a sign reversal of the Hall coefficients at a critical point ofPC∼ 32 GPa, evidencing a transition from hole band(s) dominated transport into one that is dominated by electron band(s). Synchrotron x-ray diffraction experiments demonstrate that no structural phase transition occurs below 46.3 GPa, indicating an electronic origin of the transition. Moreover, a kink anomaly of the lattice constant ratioc/ais also observed atP=PC. These results might indicate a Lifshitz transition which refers to a change of Fermi surface topology in absence of structural transition.

Non-hydrostatic pressure-dependent structural and transport properties of BiCuSeO and BiCuSO single crystals.

High-pressure experiments usually expect a hydrostatic condition, in which the physical properties of materials can be easily understood by theoretical simulations. Unfortunately, non-hydrostatic effect is inevitable in experiments due to the solidification of the pressure transmitting media under high pressure. Resultantly, non-hydrostaticity affects the accuracy of the experimental data and sometimes even leads to false phenomena. Since the non-hydrostatic effect is extrinsic, it is quite hard to analyze quantitatively. Here, we have conducted high pressure experiments on the layered BiCuXO (X = S and Se) single crystals and quantitatively analyzed their pronounced non-hydrostatic effect by high throughput first-principles calculations and experimental Raman spectra. Our experiments find that the BiCuXO single crystals sustain the tetragonal structure up to 55 GPa (maximum pressure in our experiment). However, their pressure-dependent Raman shift and electric resistance show anomalous behaviors. Through optimization of thousands of crystal structures in the high throughput first-principles calculations, we have obtained the evolution of the lattice constants under external pressures, which clearly substantiates the non-hydrostatical pressure exerted in BiCuXO crystals. Our work indicates that the high throughput first-principles calculations could be a handy method to investigate the non-hydrostatic effect on the structural and electronic properties of materials in high pressure experiments.

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal.

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe4 )2 I is compressed and a system is created where the TaSe4 atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe4 )2 I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.

Anisotropic intermediate coupling superconductivity in Cu(0.03)TaS(2).

The anisotropic superconducting state properties in Cu(0.03)TaS(2) have been investigated by magnetization, magnetoresistance and specific heat measurements. They clearly show that Cu(0.03)TaS(2) undergoes a superconducting transition at T(C) = 4.03 K. The obtained superconducting parameters demonstrate that Cu(0.03)TaS(2) is an anisotropic type-II superconductor. Combining specific heat jump ΔC/γ(n)T(C) = 1.6(4), gap ratio 2Δ/k(B)T(C) = 4.0(9) and the estimated electron-phonon coupling constant λ∼0.68, the superconductivity in Cu(0.03)TaS(2) is explained within the intermediate coupling BCS scenario. First-principles electronic structure calculations suggest that copper intercalation of 2H-TaS(2) causes a considerable increase of the Fermi surface volume and the carrier density, which suppresses the CDW fluctuation and favors the raise of T(C).

Structural, vibrational and electrical properties of type-II Dirac semimetal PtSe2 under high pressure.

We present a high-pressure study of type-II Dirac semimetal PtSe2 single crystals through synchrotron x-ray diffraction (XRD), electrical transport and Raman scattering measurements in diamond anvil cells with pressures up to 36.1-42.3 GPa, from which two critical pressure points associated with unusual electron-phonon coupling are unraveled. We show that both resistance and phonon linewidth of Raman modes display anomalies at the first critical pressure of P r ~ 10 GPa, in accordance with a scenario of pressure-induced disappearance/appearance of type-II/type-I Dirac points around P r predicted previously. The second critical pressure P c ~ 20 GPa may correspond to a structural crossover of PtSe2 from quasi-2D lattice to 3D network, which is revealed via detailed analysis of the structural parameters extracted from XRD refinement, Raman modes shifts as well as parameters from fitting of the low-temperature resistance. Our results demonstrate great tunability of PtSe2 via strain engineering, thanks to the single p-orbital manifold derived electronic states that are susceptible to out-of-plane and in-plane distances.

Pressure induced superconductivity bordering a charge-density-wave state in NbTe4 with strong spin-orbit coupling.

Transition-metal chalcogenides host various phases of matter, such as charge-density wave (CDW), superconductors, and topological insulators or semimetals. Superconductivity and its competition with CDW in low-dimensional compounds have attracted much interest and stimulated considerable research. Here we report pressure induced superconductivity in a strong spin-orbit (SO) coupled quasi-one-dimensional (1D) transition-metal chalcogenide NbTe4, which is a CDW material under ambient pressure. With increasing pressure, the CDW transition temperature is gradually suppressed, and superconducting transition, which is fingerprinted by a steep resistivity drop, emerges at pressures above 12.4 GPa. Under pressure p = 69 GPa, zero resistance is detected with a transition temperature T c = 2.2 K and an upper critical field µ0Hc2 = 2 T. We also find large magnetoresistance (MR) up to 102% at low temperatures, which is a distinct feature differentiating NbTe4 from other conventional CDW materials.

Pressure-induced iso-structural phase transition and metallization in WSe2.

We present in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman spectroscopy study, and electrical transport measurement of single crystal WSe2 in diamond anvil cells with pressures up to 54.0-62.8 GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5 GPa and not being completed up to around 60 GPa. The Raman data also reveals a dominant role of the in-plane strain over the out-of plane compression in helping achieve the transition. Consistently, the electrical transport experiments down to 1.8 K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2-61.7 GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low- and high-pressure structures.

Negative thermal expansion and magnetostriction in the frustrated spinel ZnCr2(Se1-x S x )4 (0 ≤ x ≤ 0.1).

The bond-frustrated ZnCr2Se4 displays strong spin-lattice coupling characterized by large magnetostriction and negative thermal expansion. Here, we report on systematic investigations on the magnetization, heat capacity, thermal expansion and magnetostriction of single crystalline ZnCr2(Se1-x S x )4 (0 ⩽ x ⩽ 0.1) to study the evolution of its spin-lattice coupling with sulfur substitution. We show that with increasing sulfur content, the antiferromagnetic ordering is gradually replaced by a spin-glass state, the temperature region of the negative thermal expansion expands, and the magnetostriction is gradually suppressed. These phenomena are explained qualitatively by taking into account the enhancement of the antiferromagnetic interactions and bond disorder introduced by sulfur substitution.

Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe12O19.

Many exotic physics anticipated in topological insulators require a gap to be opened for their topological surface states by breaking time reversal symmetry. The gap opening has been achieved by doping magnetic impurities, which however inevitably create extra carriers and disorder that undermine the electronic transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic insulator may improve the device quality, thus promises a better way to open the gap while minimizing the side-effects. Here, we grow thin single-crystal Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van der Waals epitaxy technique. The micro flakes show a negative magnetoresistance in weak perpendicular fields below 50 K, which can be quenched by increasing temperature. The signature implies the weak localization effect as its origin, which is absent in intrinsic topological insulators, unless a surface state gap is opened. The surface state gap is estimated to be 10 meV by using the theory of the gap-induced weak localization effect. These results indicate that the magnetic proximity effect may open the gap for the topological surface attached to BaM insulating ferrimagnet. This heterostructure may pave the way for the realization of new physical effects as well as the potential applications of spintronics devices.

Magnetically modulated critical current densities of Co/Nb hybrid.

By tuning morphology and size of magnetic subsystem, ferromagnet-superconductor (F/S) hybrid system provides an effective way to modulate superconductivity due to the interaction between superconducting and magnetic-order parameters at the mesoscopic length scale. In this work, we report on investigations of critical current density in a large-area Co/Nb hybrid via facile colloidal lithography. Here, Co hexagon shell array as a magnetic template build on Nb film to modulate the critical current density. A novel superconducting transition has been observed in I-V curve with two metastable transition states: double-transition and binary-oscillation-transition states. Importantly, such unusual behavior can be adjusted by temperature, magnetic field and contact area of F/S. Such hybrid film has important implications for understanding the role of magnetic subsystem modulating superconductivity, as well as applied to low-energy electronic devices such as superconducting current fault limiters.

Orbital ordering to orbital glass transition in spinel FeCr(2-x)Al(x)S4 (0 ⩽ x ⩽ 0.2).

The polycrystalline (PC) sample of FeCr2S4 displays orbital ordering around TOO ∼ 9 K, while single crystal sample shows orbital glass. In this paper, with the substitution of Al for Cr, a step by step transition from the orbital ordering to the orbital glass is reported in FeCr(2-x)Al(x)S4 (0 ⩽ x ⩽ 0.2). For PC FeCr2S4, the onset of long-range orbital order at TOO is evidenced by the appearance of a step-like transition in the temperature dependence of the magnetization (M(T)), a small kink at about 5.5 T below 9 K in the isotherms' magnetic field dependence of the magnetization (M(H)) curves as well as a λ-type anomaly in specific heat. With increasing Al content, the TOO decreases gradually. For the samples with x ⩾ 0.1, the orbital ordering is replaced by orbital glass, where the specific heat obeys a T(2)-dependence. The calculated residual orbital entropy consistently increases with x, implying the progressive freezing of the orbital moments and the coexistence of orbital ordering and orbital glass in the middle doping level.

Gapless quantum spin liquid ground state in the two-dimensional spin-1/2 triangular antiferromagnet YbMgGaO4.

Quantum spin liquid (QSL) is a novel state of matter which refuses the conventional spin freezing even at 0 K. Experimentally searching for the structurally perfect candidates is a big challenge in condensed matter physics. Here we report the successful synthesis of a new spin-1/2 triangular antiferromagnet YbMgGaO4 with symmetry. The compound with an ideal two-dimensional and spatial isotropic magnetic triangular-lattice has no site-mixing magnetic defects and no antisymmetric Dzyaloshinsky-Moriya (DM) interactions. No spin freezing down to 60 mK (despite θw ~ -4 K), the power-law temperature dependence of heat capacity and nonzero susceptibility at low temperatures suggest that YbMgGaO4 is a promising gapless (≤|θw|/100) QSL candidate. The residual spin entropy, which is accurately determined with a non-magnetic reference LuMgGaO4, approaches zero (<0.6%). This indicates that the possible QSL ground state (GS) of the frustrated spin system has been experimentally achieved at the lowest measurement temperatures.

Pressure-driven dome-shaped superconductivity and electronic structural evolution in tungsten ditelluride.

Tungsten ditelluride has attracted intense research interest due to the recent discovery of its large unsaturated magnetoresistance up to 60 T. Motivated by the presence of a small, sensitive Fermi surface of 5d electronic orbitals, we boost the electronic properties by applying a high pressure, and introduce superconductivity successfully. Superconductivity sharply appears at a pressure of 2.5 GPa, rapidly reaching a maximum critical temperature (Tc) of 7 K at around 16.8 GPa, followed by a monotonic decrease in Tc with increasing pressure, thereby exhibiting the typical dome-shaped superconducting phase. From theoretical calculations, we interpret the low-pressure region of the superconducting dome to an enrichment of the density of states at the Fermi level and attribute the high-pressure decrease in Tc to possible structural instability. Thus, tungsten ditelluride may provide a new platform for our understanding of superconductivity phenomena in transition metal dichalcogenides.

Transparent conducting p-type thin films of c-axis self-oriented Bi2Sr2Co2O(y) with high figure of merit.

Transparent conducting p-type Bi2Sr2Co2O(y) thin films have been first grown on SrTiO3 substrates by a chemical solution deposition, showing c-axis self-orientation. The figure of merit can reach as high as 800 MΩ(-1), which is the highest value for p-type transparent conducting thin films by solution methods.

Superconductivity at 44 K in K intercalated FeSe system with excess Fe.

We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 Å. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.