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Improving the superconductivity of thin films, especially the critical current density (JC) and the critical field (HC), is crucial for the application of superconducting devices. Here, we apply a ...

Single crystalline Selenium (Se) nanoribbons with large length-diameter ratio was catalytically synthesized by laser ablation in liquids. In this study, the unique growth, pressure-dependent phase evolution and superconducting transition of Se nanoribbons are systematically investigated. Importantly, we first found that the semiconductor-metal transition pressure of such Se nanoribbons located at 11.1 GPa at room temperature, which was extremely close to the theoretically predicated value of 11.16 GPa of hexagonal Se. The structure evolution of Se nanoribbons revealed via in situ Raman spectrum indicated three subsequent phase transition stages. The temperature dependence of resistance measurements revealed the occurrence of superconducting state of monoclinic Se at 11.8 GPa, which is markedly lower than reported pressures to date. The phase transition barrier attenuation originating from the volumetric contraction of Se lattice is assumed to offset the influences of temperature and surface energy in low-dimensional Se nanoribbons with large length-diameter ratios, resulting in a transition pressure approaching the predicted value.

Copper monotelluride CuTe has a layered orthorhombic structure (space group Pmmn, No. 59) but exhibits a quasi-one-dimensional charge density wave (CDW) order. Here, we present a high-pressure study of CuTe in a diamond anvil cell at pressures up to 41.8--49.8 GPa, through a combination of electrical transport, synchrotron x-ray diffraction (XRD), and Raman spectroscopy measurements as well as theoretical calculations. Our transport experiments show that the CDW transition is manifested as a hump in resistivity; with the application of external pressure, its transition temperature ${T}_{\mathrm{CDW}}$ decreases gradually and the amplitude of the resistivity hump decays rapidly. The CDW gets suppressed completely above \ensuremath{\sim}10 GPa as estimated by a linear extrapolation of the ${T}_{\mathrm{CDW}}$ versus $P$ trend. Upon further compression, a sudden drop in resistivity is initially observed at \ensuremath{\sim}2.4 K and \ensuremath{\sim}20 GPa, and zero resistance is established above \ensuremath{\sim}37 GPa, suggesting the occurrence of superconductivity (SC). Combined XRD and Raman data as well as theoretical calculations together evidence that the emergence of SC at \ensuremath{\sim}20 GPa is accompanied by an orthorhombic to monoclinic (space group $\mathit{Cm}$, No. 8) structural transition.

Chalcopyrite II-IV-V2 semiconductors are promising materials in nonlinear optical, optoelectronic, and photovoltaic applications. In this work, pressure-tailored optical properties as well as pressure-driven emergent superconductivity in chalcopyrite ZnSiP2 are reported via photoluminescence (PL) spectroscopy and electrical transport experiments. During compression, the PL peak energy exhibits a plateau between 1.4 and 8.7 GPa, which is accompanied by a piezochromic transition and correlated with the progressive development of cation disorder. Upon further compression across a phase transition from tetragonal to cubic rock-salt structure, superconductivity with a critical temperature Tc ~ 8.2 K emerges immediately. Tc decreases in the range of 24.6–37.1 GPa but inversely increases at higher pressures, thereby exhibiting an unusual V-shaped superconducting phase diagram. These findings present vivid structure–property relationships, which not only offer important clues to optimize the optical and electronic properties, but also provide a new way to use compression to switch between different functionalities. A semiconductor that is compatible with silicon but has better optical properties has been fully characterized by researchers in China. Silicon is perhaps the best-known example of a semiconductor but is not good at emitting light. Scientists continue to identify and characterize alternatives that might be better suited for specific applications. Yifang Yuan from the High Magnetic Field Laboratory in Hefei and co-workers have investigated in detail the properties of semiconducting zinc-silicon-phosphorus. ZnSiP2 is a member of a class of materials known as chalcopyrites and is compatible with and can be grown on silicon but is better at generating light. The team studied how external pressure affects the semiconductor’s structural, optical and electronic properties. They showed that at a sufficiently high pressure, the semiconductor exhibits superconducting behavior. Pressure engineered optical properties as well as presssure induced emergent superconductivity are revealed in II-IV-V2 semiconductor ZnSiP2, which demonstrate vivid structure–property relationships. Especially, along with a structural phase transition from tetragonal to cubic phase under compression, ZnSiP2 evolves from a photovoltaic semiconductor into a superconductor.

It is commonly anticipated that an insulating state will collapse in favor of an emergent metallic state at high pressures: The average electron density must increase with pressure, while the electronic bandwidth is expected to broaden and fill the insulating energy band gap. Here we report an unusually stable insulating state that persists up to at least 185 GPa in $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$, the archetypical spin-orbit-driven ${J}_{\mathrm{eff}\phantom{\rule{0.28em}{0ex}}}=1/2$ insulator. This study shows that the electrical resistance R of single-crystal $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ initially decreases with applied pressure P, reaches a minimum in the range 32--38 GPa, then abruptly rises to recover the insulating state with increasing P up to 185 GPa. However, evidence of a saturation of R below 80 K for $P\ensuremath{\ge}124\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ GPa raises the possibility of a low-temperature exotic state. Our synchrotron x-ray diffraction and Raman scattering data show the emergence of the rapid increase in R is accompanied by a structural phase transition from the native tetragonal $I{4}_{1}/acd$ phase to an orthorhombic $Pbca$ phase (with much reduced symmetry) at 40.6 GPa. The clear correspondence of the onset pressures of these two anomalies is key to understanding the stability of the insulating state at megabar pressures: Pressure-induced, structural distortions prevent the expected onset of metallization, despite the sizable volume compression attained at the highest pressure accessed in this study.

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 (TaSe 4 ) 2 I is compressed and a system is created where the TaSe 4 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 (TaSe 4 ) 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.

We performed in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on topological semimetal TaSb2 in diamond anvil cells with pressures up to 60.0–63.0 GPa. No evident trace of structural phase transitions is observed from our XRD. A fit of the volume versus pressure data by using the third-order Birch–Murnaghan equation of state yields the bulk modulus B 0 = 131.2 ± 3.4 GPa and its first-order derivative B 0 ′ = 6 ± 0.3. From ambient pressure to 27.8 GPa, the low-temperature conduction behaviors of TaSb2 stay almost unchanged, exhibiting a power law with the exponent around 3; meanwhile, the magnetoresistance (MR) at 5 K follows a same relationship of MR∝Bm with m= 1.45 ± 0.10. These results imply that the topological semimetal state of TaSb2 may be robust subjected to pressure, at least to 27.8 GPa.

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 NbTe$_4$, 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 $H_{c2}$= 2 T. We also find large magnetoresistance (MR) up to 102\% at low temperatures, which is a distinct feature differentiating NbTe$_4$ from other conventional CDW materials., https://rdcu.be/LX8w

We report a systematic high-pressure study of magnetic topological semimetal ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ through measurements of synchrotron x-ray diffraction (XRD), magnetization, electrical, and Hall transports combined with first-principle theoretical calculations. No evident trace of structural phase transition is detected through synchrotron x-ray diffraction over the measured pressure range of 0.2--50.9 GPa. We find that the ferromagnetism and the anomalous Hall resistivity are monotonically suppressed as increasing pressure and almost vanish around 22 GPa. The anomalous Hall conductivity varies nonmonotonically against pressure at low temperatures, involving competition between original and emergent Weyl nodes. Combined with first-principle calculations, we reveal that the intrinsic mechanism due to the Berry curvature dominates the anomalous Hall effect under high pressure.

© 2019 American Physical Society. We report on the pressure-driven superconductor-insulator transition in heavily boron-doped nanodiamond films. By systematically increasing the pressure, we suppress the Josephson coupling between the superconducting nanodiamond grains. The diminished intergrain coupling gives rise to an overall insulating state in the films, which is interpreted in the framework of a parallel-series circuit model to be the result of bosonic insulators with preserved localized intragrain superconducting order parameters. Our investigation opens up perspectives for the application of high pressure in research on quantum confinement and coherence. Our data unveil the percolative nature of the electrical transport in nanodiamond films, and highlight the essential role of grain boundaries in determining the electronic properties of this material. ispartof: PHYSICAL REVIEW MATERIALS vol:3 issue:3 status: published

Layered lanthanum silver antimonide LaAgSb2 exhibits both charge density wave (CDW) order and Dirac-cone-like band structure at ambient pressure. Here, we systematically investigate the pressure evolution of structural and electronic properties of LaAgSb2 single crystal. We show that the CDW order is destabilized under compression, as evidenced by the gradual suppression of magnetoresistance. At P C ∼ 22 GPa, synchrotron x-ray diffraction and Raman scattering measurements reveal a structural modification at room-temperature. Meanwhile, the sign change of the Hall coefficient is observed at 5 K. Our results demonstrate the tunability of CDW order in the pressurized LaAgSb2 single crystal, which can be helpful for its potential applications in the next-generation devices.

The p-type CdCr2Se4 exhibits semiconducting conductivity while n-type CdCr2Se4 displays a semiconductor–metal transition and a concomitant colossal magnetoresistance near the Curie temperature. Here, we investigate the pressure effect on the conductivity for both types of compounds. We show that the resistance of p-type sample decreases continuously upon compression, while the semiconducting behavior dominates up to 27.9 GPa. For the n-type sample, the semiconductor-metal transition is suppressed gradually with the increase in pressure; meanwhile, the magnetoresistance becomes less and less pronounced and is negligible at 9.2 GPa. Combined with in situ high-pressure magnetization, x-ray diffraction, and Raman spectra investigations, a ferromagnetic ground state is deduced in the pressurized n-type CdCr2Se4 before the structural transition, which is in stark contrast to the pressure effect on n-type HgCr2Se4.

CdCr2S4 single crystal was reported by Hemberger et al. to be multiferroic with the evidences of relaxor ferroelectricity and colossal magnetocapacitance (CMC), but whether these effects are intrinsic is under debate. Recently, we reported a one-to-one correlation between CMC and colossal magnetoresistance (CMR) in CdCr2S4 polycrystalline samples, and argued that CMC could be explained by the superposition of the CMR and the Maxwell–Wagner effects. In this paper, we further examined the magnetic, dielectric, and electric transport properties of CdCr2S4 and CdCr1.8In0.2S4 single crystals before and after annealing in cadmium vapor. The CdCr2S4 single crystal sample has no relaxor ferroelectricity and CMC, and in contrast to the CdCr2S4 single crystal reported by Hemberger et al. , only the annealed CdCr1.8In0.2S4 displays CMC, but still does not exhibit the relaxor behavior. At the same time, it also shows CMR. All these results are in accordance with the results of our polycrystalline samples, and further confirm the resistive origin of the CMC in the CdCr2S4 system.

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.

We present a high-pressure study of van der Waals ferromagnetic metal Fe3GeTe2 through electrical transport and Raman scattering measurements in diamond anvil cells at pressures up to 22.4GPa. Upon compression, the ferromagnetic transition temperature T c manifested by a kink in resistance curve decreases monotonically and becomes undiscernable around P c = 10GPa, indicative of suppression of the itinerant ferromagnetism. Meanwhile, by fitting the low temperature resistance to the Fermi liquid behavior of R = R 0 + AT 2, we found that R 0 shows a cusp-like anomaly and the coefficient A diverges around P c. These transport anomalies imply a tricritical point as commonly observed in itinerant ferromagnets under pressure. Unexpectedly, the Raman-active E 2g and A 1g modes soften remarkably after an initial weak hardening and the peak widths of both modes broaden evidently on approaching P c, followed by complete disappearance of both modes above this critical pressure. A possible underlying mechanism for such anomalous lattice softening near P c is discussed.

Here we report a systematic investigation of Co-doping effects on the magnetism and transport properties of Fe 1+ y Te single crystals. It is found that the anti-ferromagnetism transition temperature decreases upon Co doping, and a spin-glass like behavior emerges at low temperature. With substituting more than x =0.09 Co for Fe, both the metallic behavior and the structural transition disappear, but the density of states at Fermi-level does not change significantly. These results suggest that Co doping does not act as carrier doping but is more likely to bring in random magnetic scatters. In addition, the analysis of R H measurements reveals that the Fermi-level reconstruction is not closely related to the long rang AFM transition, which further supports for the local moment picture with frustrated character in dominating the AFM ground state of Fe 1+ y Te.

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.

We have performed a joint study of magnetization and magnetoresistance in the antiferromagnetic (AFM) semimetal NdSb under pulsed high magnetic field up to 60 T. We show that NdSb displays extremely large magnetoresistance (XMR), reaching values of $1.77\ifmmode\times\else\texttimes\fi{}{10}^{6}%$ at 1.3 K and 60 T without any sign of saturation. The field-dependent magnetization at 4.2 K reveals a field-induced metamagnetic transition from the AFM to the ferromagnetic state around 9 T. During the metamagnetic transition, a steplike reduction of resistance occurs, resembling the giant magnetoresistance (GMR) behavior in metallic multilayers. The investigation of the Shubnikov--de Haas effect in NdSb suggests that a particular orbital texture might contribute to the XMR effect in addition to the perfect electron-hole compensation.

We present in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on quasi-one-dimensional single-crystal $\mathrm{Ti}{\mathrm{S}}_{3}$ up to 29.9--39.0 GPa in diamond-anvil cells, coupled with first-principles calculations. Counterintuitively, the conductive behavior of semiconductor $\mathrm{Ti}{\mathrm{S}}_{3}$ becomes increasingly insulating with pressure until ${P}_{C1}\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, where extremes in all three axial ratios are observed. Upon further compression to ${P}_{C2}\ensuremath{\sim}22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, the XRD data evidence a structural phase transition. Based on our theoretical calculations, this structural transition is determined to be isosymmetric, i.e., without change of the structural symmetry ($P{2}_{1}/m$), mainly resulting from rearrangement of the dangling ${\mathrm{S}}_{2}$ pair along the $a$ axis.

Unsaturated magnetoresistance (MR) has been reported in WTe2, and remains irrepressible up to very high field. Intense optimization of the crystalline quality causes a squarely-increasing MR, as interpreted by perfect compensation of opposite carriers. Herein we report our observation of linear MR (LMR) in WTe2 crystals, the onset of which is first identified by constructing the mobility spectra of the MR at low fields. The LMR further intensifies and predominates at fields higher than 20 Tesla while the parabolic MR gradually decays. The LMR remains unsaturated up to a high field of 60 Tesla and persists, even at a high pressure of 6.2 GPa. Assisted by density functional theory calculations and detailed mobility spectra, we find the LMR to be robust against the applications of high field, broken carrier balance, and mobility suppression. Angle-resolved photoemission spectroscopy reveals a unique quasilinear energy dispersion near the Fermi level. Our results suggest that the robust LMR is the low bound of the unsaturated MR in WTe2.

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.

Among the family of TMDs, ReS2 takes a special position, which crystalizes in a unique distorted low-symmetry structure at ambient conditions. The interlayer interaction in ReS2 is rather weak, thus its bulk properties are similar to that of monolayer. However, how does compression change its structure and electronic properties is unknown so far. Here using ab initio crystal structure searching techniques, we explore the high-pressure phase transitions of ReS2 extensively and predict two new high-pressure phases. The ambient pressure phase transforms to a "distorted-1T" structure at very low pressure and then to a tetragonal I41/amd structure at around 90 GPa. The "distorted-1T" structure undergoes a semiconductor-metal transition (SMT) at around 70 GPa with a band overlap mechanism. Electron-phonon calculations suggest that the I41/amd structure is superconducting and has a critical superconducting temperature of about 2 K at 100 GPa. We further perform high-pressure electrical resistance measurements up to 102 GPa. Our experiments confirm the SMT and the superconducting phase transition of ReS2 under high pressure. These experimental results are in good agreement with our theoretical predictions., 25 pages, 6 figures, accepted by npj Quantum Materials

We conducted in-situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on Dirac-like semimetal PdSn4 in diamond anvil cells with quasi-hydrostatic pressure condition up to 44.5 GPa–52.0 GPa. The XRD data show that the ambient orthorhombic phase (Ccca) is stable with pressures to 44.5 GPa, and the lattice parameters and unit-cell volume decrease monotonously upon compression. The temperature dependence of the resistance exhibits a metallic conduction and follows a Fermi-liquid behavior below 50 K, both of which keep unchanged upon compression to 52.0 GPa. The magnetoresistance curve at 5 K maintains a linear feature in a magnetic field range of 2.5 T–7 T with increasing pressure to 20.0 GPa. Our results may provide pressure-transport constraints on the robustness of the Dirac fermions.

The insulator-metal transition triggered by pressure in charge transfer insulator NiS2 is investigated by combining high-pressure electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements up to 40–50 GPa. Upon compression, we show that the metallization firstly appears in the low temperature region at ∼3.2 GPa and then extends to room temperature at ∼8.0 GPa. During the insulator-metal transition, the bond length of S–S dimer extracted from the synchrotron x-ray diffraction increases with pressure, which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa. Considering the decreasing bonding-antibonding splitting due to the expansion of S–S dimer, the charge gap between the S-ppπ* band and the upper Hubbard band of Ni-3d e g state is remarkably decreased. These results consistently indicate that the elongated S–S dimer plays a predominant role in the insulator-metal transition under high pressure, even though the p-d hybridization is enhanced simultaneously, in accordance with a scenario of charge-gap-controlled type.

In this work, BiFeO3 (BFO) thin films were prepared on metallic Ni (200) tapes with and without a La0.5Sr0.5TiO3 (LSTO) buffer layer at different temperatures by chemical solution deposition. The effects of the annealing temperature as well as the LSTO buffer layer on the dielectric, leakage and ferroelectric properties have been studied in detail. The crystallite size, dielectric constant and leakage current density increase, while the coercive field decreases with increasing annealing temperature. The BFO thin films deposited directly on the Ni tapes are prone to wrinkling, while the wrinkles are smoothed by introducing a thin LSTO buffer layer. Decreased compressive microstrain as well as improved ferroelectric and leakage properties are observed in the BFO thin films deposited on the LSTO buffered Ni tapes. The results will provide an instructive route to optimize BiFeO3-based thin films on metallic tapes by chemical solution deposition methods.

The magnetism of the colossal magnetoresistance material FeCr2S4 was studied through substitution by nonmagnetic element Cd. With the increase of Cd content, the high-field magnetization measured at 5 K increases, demonstrating a ferromagnetic Cr–Cr interaction in CdCr2S4. As comparing with the anti-ferromagnetic material ZnCr2S4, it is deduced that the ferromagnetic interaction in CdCr2S4 is favored by its larger Cr–Cr distance. On the other hand, due to Cd substitution, the low-field magnetization irreversibility between zero-field-cooling and field-cooling magnetization weakens with the increase of Cd content and disappears at last when x = 1 in the Fe1−x Cd x Cr2S4 (0 ≤ x ≤ 1.0) system. By taking account of the single-ion anisotropy of the ferrous ion on the tetrahedral site, the picture of irreversible magnetic behavior induced by magnetic anisotropy is examined.

Y 1− x Gd x Ba 2 Cu 3 O 7− δ (0 ⩽ x ⩽ 0.5) thin films were prepared on LaAlO 3 (0 0 1) substrates by trifluoroacetate metal organic deposition (TFA-MOD). It was observed that the critical current density under self-field of Y 0.9 Gd 0.1 Ba 2 Cu 3 O 7− δ thin film was about two times higher than that of YBCO thin film within the range of 5–85 K. Then, codoped Y 0.9− y Eu y Gd 0.1 BCO thin films (0 ⩽ y ⩽ 0.2) were fabricated by TFA-MOD. It was observed that the critical current density under magnetic fields was enhanced largely due to codoping. The flux pinning type was not changed with Gd doping and codoping, which could be attributed to the normal surface pins.

(Y, Gd)Ba2Cu3Oy and (Y, Eu, Gd)Ba2Cu3Oy films with and without Zr addition are prepared. The effect of Zr addition on critical current density Jc is different for the two series. Compared to (Y, Gd)Ba2Cu3Oy films without Zr addition, Jc(B) is clearly enhanced by adding Zr, especially in high field. But for (Y, Eu, Gd)Ba2Cu3Oy films, the addition of Zr is disadvantageous to Jc. Analysis indicates that the flux pinning mechanism is altered for (Y, Eu, Gd)Ba2Cu3Oy films, whereas it remains the same for (Y, Gd)Ba2Cu3Oy films after adding Zr.

Terahertz (THz) time domain spectroscopy (THz-TDS) of a CoCr 2 O 4 and ZnCr 2 Se 4 single crystals have been performed under magnetic fields up to 10 Tesla and low temperatures. We observed the magnetic resonances originated from the spin dynamics of inter-sublattice exchange and screw spin structure in THz frequency region, which show pronouncedly anisotropic effect. The phase and polarization sensitive detection in THz-TDS allow us to reveal the large circular absorption dichroism and Faraday rotations under magnetic field in these materials.

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.

We systematically investigated the various properties of MnTi2O4. The compounds are found to be conducted by small polarons with different activation energies across the temperature T1 and deviate from the Curie–Weiss law for the magnetic susceptibility at T1. Also at T1 do the specific heat undergo a crossover. As indicated by the X-ray diffraction peaks, the series of phenomena near the characteristic temperature T1 are attributed to the happening of the micro-structural distortion, and the distortion implies the short-range orbital ordering state below T1. This short-range oribtal ordering state is supported by the little upturn of the thermal conductance at T1. As the temperature decreases, the systems enter a long-ranged collinear ferrimagnetic state at a lower temperature TN. Moreover, the compounds decreases in thermal-electric power with the decrease of temperature and undergoes a p-type to n-type transition at the temperature Tpn. Through the quantitative theoretical analysis, both the decrease of the thermal power and the p-type to n-type transition are cooperatively induced by spin–orbit (SO) coupling and the Jahn–Teller (JT) effect.

Y 1− x Ho x Ba 2 Cu 3 O 7− δ ( x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) thin films were prepared on LaAlO 3 (0 0 1) substrates by trifluoroacetate metal organic deposition (TFA-MOD) without change of the processing parameters. The highest J c was attributed to the sample of Y 0.6 Ho 0.4 Ba 2 Cu 3 O 7− δ thin film, whose critical current density is about 1.6 times as compared to that of YBa 2 Cu 3 O 7− δ thin film at 77 K and self field. The flux pinning type was not varied with Ho substitution and can be attributed to δl pinning model, which is attributed to the close ionic radius between the Y 3+ and Ho 3+ ions. The improvement of J c by Ho substitution without change of the processing parameters will provide an effective route to enhance the J c of YBCO-based thin films using TFA-MOD method.

In this article, ZnO:Cd films were successfully deposited on glass substrates by a sol–gel technique. The influence of doping concentration and annealing temperature effects was carefully investigated. All films exhibited c -axis preferential orientation and optical transparency with visible transmittance >80%. The minimum room temperature resistivity of 0.0341 Ω cm was obtained with 10 at.% Cd doping under 600 °C annealing temperature. The optical band gap of ZnO:Cd film was reduced as Cd doping concentration increased, while the band gap increased with the increase of annealing temperature.

ZnFe2O4 films have been prepared using a sol–gel route. The influences of annealing temperature as well as annealing atmosphere on the structural, morphological, magnetic, and optical properties were investigated. The TEM and SEM images show that ZnFe2O4 films are polycrystalline in nature and homogeneous with densely packed grains. ZnFe2O4 films exhibit ferromagnetic behavior at 300 K and high transparency (≥80%) above the absorption band edge. With decreasing annealing temperature and annealing oxygen partial pressure, the saturated magnetism at 300 K is improved and the optical band gap is decreased. The reasons are discussed, and the results show that semitransparent ZnFe2O4 films with room- temperature ferromagnetism can be successfully prepared using a sol–gel method.

ZnO:Al thin films were prepared on n-type (1 0 0)-oriented Si and glass slide substrates by chemical solution deposition method. The effects of Al content, the annealing temperature in air, the annealing temperature in reducing atmosphere and the solution concentration on the structural, morphological, electrical and optical characteristics have been investigated systematically. The results show that the processing parameters play an important role in the microstructures as well as the properties. The lowest resistivity value (0.091 Ω cm) was observed by optimization of the processing parameters.

The structure, magnetic properties, and magnetocaloric effect of La0.7Ca0.3MnO3 ceramics with different particle sizes have been investigated. It is found that the Curie temperature increases first, and then decreases as particle size decreases and the type of magnetic phase transition changes from first-order to second-order, which may be attributed to surface pressure effects. The maximum magnetic entropy change − Δ S M max and relative cooling power (RCP) show non-monotonic behaviors with decreasing the particle size. However, for the 3400 nm sample, the magnetic entropy change −ΔSM reaches the maximum values of 6.41 and 8.63 J/kg K for the field changes of 2.0 and 4.5 T, respectively. Furthermore, the estimated large RCP values under lower magnetic fields in La0.7Ca0.3MnO3 are comparable with those of typical magnetic refrigerant materials in the corresponding temperature range, suggesting those compounds might be promising candidates for magnetic refrigeration.

PtS2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressure-dependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ~ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to 26.8 GPa, which indicates a stable lattice structure of PtS2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.

Critical behavior of Ti-doped La 0.7 Ca 0.3 MnO 3 ceramics are studied using magnetization methods. The results show that the paramagnetic–ferromagnetic transition is first order for the undoped sample, then for low-doping samples the mean-field model is suitable, and finally a 3D Heisenberg model is satisfied for higher doping samples. These findings demonstrate that the critical behavior of the magnetic transition for manganites is sensitive to Mn-site doping.

CoFe 2 O 4 ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group Fd 3 m - O h 7 . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe 2 O 4 . The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co 2+ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe 2 O 4 samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.

Metallic and transparent La0.5Sr0.5TiO3+x/2 films were prepared by the chemical solution deposition (CSD) method using topotactic reduction processing. The use of Si powder as the reducing agent was facile and allowed easy manipulation. It was observed that metallic (resistivity at 300 K ∼2.43 mΩ cm) and transparent (∼80% transmittance at visible light) La0.5Sr0.5TiO3+x/2 films could be obtained with an annealing temperature of 900°C, which was significantly lower than the hydrogen reduction temperature (∼1400°C). The successful preparation of metallic and transparent La0.5Sr0.5TiO3+x/2 films using CSD has provided a feasible route for depositing other perovskite-structured functional layers on La0.5Sr0.5TiO3+x/2 films using this low-cost all CSD method.

M-type strontium hexaferrite (SrFe12O19) particles had been prepared by a modified chemical co-precipitation route. Structural and magnetic properties were systematically investigated. Rietveld refinement of X-ray powder diffraction results showed that the sample was single-phase with the space group of P63/mmc and cell parameter values of a=5.8751 A and c=23.0395 A. The results of field-emission scanning electronic microscopy showed that the grains were regular hexagonal platelets with sizes from 2 to 4 μm. The composition determined by energy dispersive spectroscopy is the stoichiometry of SrFe12O19. The ferrimagnetic to paramagnetic transition was sharp with Curie temperature TC=737 K, which further confirmed that the samples were single phase. However, it was found that the coercivity, saturation magnetization and the squareness ratio of the synthesized SrFe12O19 samples were lower than the theoretical values, which could be explained by the multi-domain structure and the increase of the demagnetizing factor.

La 2 Zr 2 O 7 /NiW buffer layers as very promising buffer layers for coated conductors were fabricated by chemical solution deposition using cheap precursors, La-acetate and Zr-(IV) n -propoxide (70% w/w in n -propanol). The effects on the orientation and microstructure of La 2 Zr 2 O 7 buffer layers, including seed layer, thickness and solution concentration, were investigated. The results showed that insertion of a seed layer could obviously improve the crystallization and orientation, and decrease of single layer thickness could also obviously improve the in-plane orientation. The results of all metalorganic depositions derived YBCO/CeO 2 /La 2 Zr 2 O 7 /NiW showed that the prepared La 2 Zr 2 O 7 buffer layers were suitable for coated conductors using all metalorganic depositions.

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.

We review the progress of nonmagnetic impurity doping study on the Fe-based superconductors. On the theoretical side, two highly potential candidates for the pairing symmetry order parameter, i.e. the multi-gap s++ and s+- wave models, have been proposed but continuously debated. The debate arises because of the complex gap structure, exceptive magnetic and metallic behaviors of Fe-based superconductors which may make the influences of nonmagnetic defects varied in chemical potential, impurity disorder, inter- and intra-band scattering strength, and electron localization, and hence difficulty in directly obtaining the most important information for understanding the symmetry order parameter. Experimentally, the nonmagnetic impurity substitution study has been widely carried out, which has provided very useful insights. We review herein the various nonmagnetic impurity doping experiments, including the controlled defects within the superconducting Fe2X2 planes through samples quality improvement, a single impurity effects on the electronic state and local moment, the magnetic response of the Fe2X2 planes both on macroscopic scale as antiferromagnetic state and local scale of moment, as well as the significant effect in modifying the transport properties. The experiments enable us to qualitatively analyze the nonmagnetic impurities effects on the superconducting state for many Fe-based superconductors. We also propose herein some strategies for the nonmagnetic impurity doping study. As an important model for explaining the nonmagnetic impurities doping effects, the pair-breaking model is compared with various theoretical approaches via analyzing the pair-breaking rates of various Fe-superconductors., 82 pages, 44 figures, topical review article to be published in Superconductor Science and Technology