Observation of unconventional chiral fermions with long Fermi arcs in CoSi.

Chirality-the geometric property of objects that do not coincide with their mirror image-is found in nature, for example, in molecules, crystals, galaxies and life forms. In quantum field theory, the chirality of a massless particle is defined by whether the directions of its spin and motion are parallel or antiparallel. Although massless chiral fermions-Weyl fermions-were predicted 90 years ago, their existence as fundamental particles has not been experimentally confirmed. However, their analogues have been observed as quasiparticles in condensed matter systems. In addition to Weyl fermions1-4, theorists have proposed a number of unconventional (that is, beyond the standard model) chiral fermions in condensed matter systems5-8, but direct experimental evidence of their existence is still lacking. Here, by using angle-resolved photoemission spectroscopy, we reveal two types of unconventional chiral fermion-spin-1 and charge-2 fermions-at the band-crossing points near the Fermi level in CoSi. The projections of these chiral fermions on the (001) surface are connected by giant Fermi arcs traversing the entire surface Brillouin zone. These chiral fermions are enforced at the centre or corner of the bulk Brillouin zone by the crystal symmetries, making CoSi a system with only one pair of chiral nodes with large separation in momentum space and extremely long surface Fermi arcs, in sharp contrast to Weyl semimetals, which have multiple pairs of Weyl nodes with small separation. Our results confirm the existence of unconventional chiral fermions and provide a platform for exploring the physical properties associated with chiral fermions.

Robust Crystal Phase Separation with Distinct Charge, Orbital, and Spin Orders in AgMn7O12.

An AA'3B4O12-type A-site-ordered quadruple perovskite oxide AgMn7O12 was prepared by high-pressure and high-temperature methods. At room temperature, the compound crystallizes into a cubic Im3̅ symmetry with a charge distribution of AgMn33+Mn43.5+O12. With the temperature decreasing to TCO,OO ≈ 180 K, the compound undergoes a structural phase transition toward a monoclinic C2/m symmetry, giving rise to a B-site charge- and orbital-ordered AgMn33+Mn23+Mn24+O12 phase. Moreover, this charge-/orbital-ordered main phase coexists with the initial cubic AgMn33+Mn43.5+O12 phase in the wide temperature range we measured. The charge-/orbital-ordered phase shows two antiferromagnetic phase transitions near 125 and 90 K, respectively. Short-range ferromagnetic correlations are found to occur for the initial B-site mixed cubic phase around 35 K. Because of the robust phase separation, considerable magnetoresistance effects are observed below TCO,OO in AgMn7O12.

High-Pressure Synthesis of Quadruple Perovskite Oxide CaCu3Cr2Re2O12 with a High Ferrimagnetic Curie Temperature.

An AA'3B2B'2O12-type quadruple perovskite oxide of CaCu3Cr2Re2O12 was synthesized at 18 GPa and 1373 K. Both an A- and B-site ordered quadruple perovskite crystal structure was observed, with the space group Pn-3. The valence states are verified to be CaCu32+Cr23+Re25+O12 by bond valence sum calculations and synchrotron X-ray absorption spectroscopy. The spin interaction among Cu2+, Cr3+, and Re5+ generates a ferrimagnetic transition with the Curie temperature (TC) at about 360 K. Moreover, electric transport properties and specific heat data suggest the presence of a half-metallic feature for this compound. The present study provides a promising quadruple perovskite oxide with above-room-temperature ferrimagnetism and possible half-metallic properties, which shows potential in the usage of spintronic devices.

CaCu3Mn2Te2O12: An Intrinsic Ferrimagnetic Insulator Prepared Under High Pressure.

CaCu3Mn2Te2O12 was synthesized using high-temperature and high-pressure conditions. The compound possesses an A- and B site ordered quadruple perovskite structure in Pn3̅ symmetry with the charge combination of CaCu32+Mn22+Te26+O12. A ferrimagnetic phase transition originating from the antiferromagnetic interaction between A' site Cu2+ and B site Mn2+ ions is found to occur at TC ≈ 100 K. CaCu3Mn2Te2O12 also shows insulating electric conductivity. Optical measurement demonstrates the energy bandgap to be about 1.9 eV, in agreement with the high B site degree of chemical order between Mn2+ and Te6+. The first-principles theoretical calculations confirm the Cu2+(↓)-Mn2+(↑) ferrimagnetic coupling as well as the insulating nature with an up-spin direct bandgap. The current CaCu3Mn2Te2O12 provides an intriguing example of an intrinsic ferrimagnetic insulator with promising applications in advanced spintronic devices.

Magnetoelectric and Magnetostrictive Effects in Scheelite-Type HoCrO4.

Scheelite-type HoCrO4 was prepared by treating the ambient-pressure zircon-type precursor phase under 8 GPa and 700 K. A long-range antiferromagnetic phase transition is found to occur at TN ≈ 23 K due to the spin order of Ho3+ and Cr5+ magnetic ions. However, the antiferromagnetic ground state is sensitive to an external magnetic field and a moderate field of about 1.1 T can induce a metamagnetic transition, giving rise to the presence of a large magnetization up to 8.5 µB/f.u. at 2 K and 7 T. Considerable linear magnetoelectric effect is observed in the antiferromagnetic state, while the induced electric polarization experiences a sharp increase near the critical field of the metamagnetic transition. Ferromagnetism and ferroelectricity thus rarely coexist under higher magnetic fields in scheelite-type HoCrO4. Moreover, a magnetic field also plays an important role in the longitudinal constriction of HoCrO4, and a significant magnetostrictive effect with a value of up to 300 ppm is observed at 2 K and 9 T, which can be attributed to the strong anisotropy of the rare-earth Ho3+ ion. Possible coupling between magnetoelectric and magnetoelastic effects is discussed.

Comparative Study on the Magnetic and Transport Properties of B-Site Ordered and Disordered CaCu3Fe2Os2O12.

The B-site Fe/Os ordered and disordered quadruple perovskite oxides CaCu3Fe2Os2O12 were synthesized under different high-pressure and high-temperature conditions. The B-site ordered CaCu3Fe2Os2O12 is a system with a very high ferrimagnetic ordering temperature of 580 K having the Cu2+(↑)Fe3+(↑)Os5+(↓) charge and spin arrangement. In comparison, the highly disordered CaCu3Fe2Os2O12 has a reduced magnetic transition temperature of about 350 K. The Cu2+Fe3+Os5+ charge combination remains the same without any sign of changes in the valence state of the constituent ions. Although the average net moments of each sublattice are reduced, the average ferrimagnetic spin arrangement is unaltered. The robustness of the basic magnetic properties of CaCu3Fe2Os2O12 against site disorder may be taken as an indication of the tendency to maintain the short-range order of the atomic constituents.

High-Pressure Synthesis and Magnetism of the 4H-BaMnO3 Single Crystal and Its 6H-Type Polymorph.

A 4H-type BaMnO3 single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P63/mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO6 octahedral dimers connect with each other by corner O atoms along the c-axis direction, forming an -A-B-A-C-type 4H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at TN ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P63/mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c-axis MnO6 dimers are separated by MnO6 octahedral layers in the ab plane, forming an -A-B-C-A-C-B-type 6H structure. The 6H phase exhibits two long-range AFM orderings at TN1 ≈ 220 K and TN2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4H- and 6H-BaMnO3.

Os Doping Suppressed Cu-Fe Charge Transfer and Induced Structural and Magnetic Phase Transitions in LaCu3Fe4-xOsxO12 (x = 1 and 2).

B-site Os-doped quadruple perovskite oxides LaCu3Fe4-xOsxO12 (x = 1 and 2) were prepared under high-pressure and high-temperature conditions. Although parent compound LaCu3Fe4O12 experiences Cu-Fe intermetallic charge transfer that changes the Cu3+/Fe3+ charge combination to Cu2+/Fe3.75+ at 393 K, in the Os-doped samples, the Cu and Fe charge states are found to be constant 2+ and 3+, respectively, indicating the complete suppression of charge transfer. Correspondingly, Os6+ and mixed Os4.5+ valence states are determined by X-ray absorption spectroscopy for x = 1 and x = 2 compositions, respectively. The x = 1 sample crystallizes in an Fe/Os disordered structure with the Im3̅ space group. It experiences a spin-glass transition around 480 K. With further Os substitution up to x = 2, the crystal symmetry changes to Pn3̅, where Fe and Os are orderly distributed in a rocksalt-type fashion at the B site. Moreover, this composition shows a long-range Cu2+(↑)Fe3+(↑)Os4.5+(↓) ferrimagnetic ordering near 520 K. This work provides a rare example for 5d substitution-suppressed intermetallic charge transfer as well as induced structural and magnetic phase transitions with high spin ordering temperature.

Tunable Room-Temperature Ferromagnetism in Two-Dimensional Cr2Te3.

The manipulation of magnetism provides a unique opportunity for the development of data storage and spintronic applications. Until now, electrical control, pressure tuning, stacking structure dependence, and nanoscale engineering have been realized. However, as the dimensions are decreased, the decrease of the ferromagnetism phase transition temperature (Tc) is a universal trend in ferromagnets. Here, we make a breakthrough to realize the synthesis of 1 and 2 unit cell (UC) Cr2Te3 and discover a room-temperature ferromagnetism in two-dimensional Cr2Te3. The newly observed Tc increases strongly from 160 K in the thick flake (40.3 nm) to 280 K in 6 UC Cr2Te3 (7.1 nm). The magnetization and anomalous Hall effect measurements provided unambiguous evidence for the existence of spontaneous magnetization at room temperature. The theoretical model revealed that the reconstruction of Cr2Te3 could result in anomalous thickness-dependent Tc. This dimension tuning method opens up a new avenue for manipulation of ferromagnetism.

Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3.

Spin state transitions and intermetallic charge transfers can essentially change material structural and physical properties while excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO3 with a simple ABO3 composition under high pressure. PbCoO3 possesses a peculiar A- and B-site ordered charge distribution Pb2+Pb4+3Co2+2Co3+2O12 with insulating behavior at ambient conditions. The high spin Co2+ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to an anomalous increase of resistance magnitude. Between 15 and 30 GPa, the intermetallic charge transfer occurs between Pb4+ and Co2+ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ∼20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.

High-Pressure Synthesis of a B-site Co2+/Mn4+ Disordered Quadruple Perovskite LaMn3Co2Mn2O12.

A new oxide, LaMn3Co2Mn2O12, was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA'3B4O12-type quadruple perovskite structure with space group Im3̅. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn3+3Co2+2Mn4+2O12, where the La3+ and Mn3+ are 1:3 ordered respectively at the A and A' sites, whereas the Co2+ and Mn4+ are disorderly distributed at the B site. This is in sharp contrast to R2Co2+Mn4+O6 (R = La and rare earth) double perovskites, in which the Co2+ and Mn4+ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn3Co2Mn2O12 displays spin glassy magnetic properties due to the random Co2+ and Mn4+ distribution, as demonstrated by dc and ac magnetic susceptibility as well as specific heat measurements. Possible factors that affect the B-site degree of order in perovskite structures are discussed.

High-Temperature Ferrimagnetic Half Metallicity with Wide Spin-up Energy Gap in NaCu3Fe2Os2O12.

A new oxide NaCu3Fe2Os2O12 is synthesized using high pressure and temperature conditions. The Rietveld structural analysis shows that the compound possesses both A- and B-site ordered quadruple perovskite structure in Pn3̅ symmetry. The valence states of transition metals are confirmed to be Cu2+/Fe3+/Os5.5+. The three transition metals all take part in magnetic interactions and generate strong Cu2+(↑)Fe3+(↑)Os5.5+(↓) ferrimagnetic superexchange interactions with a high Curie temperature about 380 K. Electrical transport measurements suggest its half-metallic properties. The first-principles theoretical calculations demonstrate that the compound has a spin-down conducting band and a spin-up insulating band with a wide energy gap.

Near-Room-Temperature Ferrimagnetic Ordering in a B-Site-Disordered 3d-5d-Hybridized Quadruple Perovskite Oxide, CaCu3Mn2Os2O12.

A new 3d-5d hybridization oxide, CaCu3Mn2Os2O12 (CCMOO), was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes to an A-site-ordered but B-site-disordered quadruple perovskite structure with a space group of Im3̅ (No. 204). The charge states of the transition metals are determined to be Cu2+/Mn3.5+/Os4.5+ by X-ray absorption spectroscopy. Although most B-site-disordered perovskites possess lower spin-ordering temperatures or even nonmagnetic transitions, the current CCMOO displays a long-range ferrimagnetic phase transition with a critical temperature as high as ∼280 K. Moreover, a large saturated magnetic moment is found to occur [7.8 µB/formula units (f.u.) at 2 K]. X-ray magnetic circular dichroism shows a Cu2+(↑)Mn3.5+(↑)Os4.5+(↓) ferrimagnetic coupling. The corner-sharing Mn/OsO6 octahedra with mixed Mn and Os charge states make the compound metallic in electrical transport, in agreement with a specific heat fitting at low temperature. This work provides a rare example with high spin-ordering temperature and a large magnetic moment in B-site-disordered 3d-5d hybridization perovskite oxides.

Splash-Resistant and Light-Weight Silk-Sheathed Wires for Textile Electronics.

Silk has outstanding mechanical properties and biocompatibility. It has been used to fabricate traditional textiles for thousands of years and can be produced in large scale. Silk materials are potentially attractive in modern textile electronics. However, silk is not electrically conductive, thus limiting its applications in electronics. Moreover, regenerated silk is generally rigid and brittle, which hinder post processing. Here we report the fabrication of conductive silk wire in which carbon nanotube (CNT) yarns are wrapped with fluffy and flexible silk nanofiber films. The silk nanofiber film was prepared by electrospinning and then wrapped around a rotating CNT yarn in situ. The obtained silk-sheathed CNT (CNT@Silk) wire has an insulating sheath, which protects the body against electrical shock. In addition, the fabricated wires exhibit a high electrical conductivity (3.1 × 104 S/m), good mechanical strength (16 cN/tex), excellent flexibility, and high durability. More importantly, the wires have an extremely low density (2.0-7.8 × 104 g/m3), which is 2 orders of magnitude lower than that of the traditional metal wire (for example, Cu). Moreover, the wires display a good resistance to humidity, and a simple post treatment can make the wires splash-resistant, thereby expanding its applications. On the basis of these features, we demonstrate the use of the lightweight CNT@Silk wires in smart clothes, including electrochromism and near-field communication.

Formation of ZnO4 Tetrahedra and ZnO6 Octahedra in TeZnO3 Synthesized under High Pressure.

A new TeZnO3 phase was synthesized by high-pressure techniques. Different from the ambient-pressure orthorhombic phase composed of ZnO5 units, the current high-pressure one crystallizes to a monoclinic structure with space group P21/ n. Moreover, both ZnO4 tetrahedral and ZnO6 octahedral polyhedra are found to occur in this new phase, providing a unique Zn-based material system that simultaneously possesses two distinct coordinated units. Because the outermost orbitals are fully occupied for both Zn2+ and Te4+, the compound exhibits diamagnetism and strong insulating behavior with a wide bandgap as large as 6.0 eV. Dielectric constant and specific heat measurements show a broad anomaly around 240 K. Low-temperature synchrotron X-ray diffraction reveals an isostructural phase transition at this temperature.

High-Pressure Synthesis of the Cobalt Pyrochlore Oxide Pb2Co2O7 with Large Cation Mixed Occupancy.

The novel A2B2O7-type compound Pb2Co2O7 was synthesized at 8 GPa and 1673 K. Synchrotron X-ray diffraction shows a cubic pyrochlore structure with space group Fd3̅m. Rietveld structural analysis reveals a large cation mixed occupancy at both A and B sites by about 40%, the greatest value found in the pyrochlore family. In combination with the X-ray absorption spectroscopy results, the specific chemical composition and charge states are determined to be (Co0.6Pb0.4)3+2(Pb0.6Co0.4)4+2O7, in which both the A-site Co3+ and the B-site Co4+ are low-spin. Due to the tetrahedral geometric frustration effects as well as the random Co4+ and Pb4+ distribution at the B site, spin glassy behavior is well observed following the conventional critical slowing down feature in Pb2Co2O7.

Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3.

The rare physical property of negative thermal expansion (NTE) is intriguing because materials with a large NTE over a wide temperature range can serve as high-performance thermal expansion compensators. However, the applications of NTE are hindered by the fact that most of the available NTE materials show small magnitudes of NTE, and/or NTE occurs only in a narrow temperature range. Herein, for the first time, we investigated the effect of anion substitution instead of general Pb/Ti-site substitutions on the thermal expansion properties of a typical ferroelectric NTE material, PbTiO3. Intriguingly, the substitution of S for O in PbTiO3 further increases the tetragonality of PbTiO3. Consequently, an unusually enhanced NTE with an average volumetric coefficient of thermal expansion of V = -2.50 × 10-5 K-1 was achieved over a wide temperature range (300-790 K), which is in contrast to that of pristine PbTiO3 (V = -1.99 × 10-5 K-1, RT-763 K). The intensified NTE is attributed to the enhanced hybridization between Pb/Ti and O/S atoms by the substitution of S, as evidenced by our theoretical investigations. We therefore demonstrate a new technique for introducing mixed anions to achieve a large NTE over a wide temperature range in PbTiO3-based ferroelectrics.

High-pressure single crystal growth and magnetoelectric properties of CdMn7O12.

The concurrent presence of large electric polarization and strong magnetoelectric coupling is quite desirable for potential applications of multiferroics. In this paper, we report the growth of CdMn7O12single crystals by flux method under a high pressure of 8 GPa for the first time. An antiferromagnetic (AFM) order with a polar magnetic point group is found to occur at the onset temperature ofTN1= 88 K (AFM1 phase). As a consequence, the pyroelectric current emerges atTN1and gradually increases and reaches its maximum atTset= 63 K, at which the AFM1 phase finally settles down. BelowTset, CdMn7O12single crystal exhibits a large ferroelectric polarization up to 2640µC m-2. Moreover, the spin-induced electric polarization can be readily tuned by applying magnetic fields, giving rise to considerable magnetoelectric coupling effects. Thus, the current CdMn7O12single crystal acts as a rare multiferroic system where both large polarization and strong magnetoelectric coupling merge concurrently.

Emergent physical properties of perovskite-type oxides prepared under high pressure.

The perovskite ABO3 family demonstrates a wide variety of structural evolutions and physical properties and is arguably the most important family of complex oxides. Chemical substitutions of the A- and/or B-site and modulation of oxygen content can effectively regulate their electronic behaviors and multifunctional performances. In general, the BO6 octahedron represents the main unit controlling the electronic and magnetic properties while the A-site ion is often not involved. However, a series of unconventional perovskite materials have been recently synthesized under high pressure, such as the s-d level controlled Pb-based perovskite family and quadruple perovskite oxides containing transition metal ions at the A-site. In these compounds, the intersite A-B correlations play an important role in electronic behaviors and further induce many emergent physical properties.

Fluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen under multiple excitation channels.

The development of organic solid-state luminescent materials, especially those sensitive to aggregation microenvironment, is critical for their applications in devices such as pressure-sensitive elements, sensors, and photoelectric devices. However, it still faces certain challenges and a deep understanding of the corresponding internal mechanisms is required. Here, we put forward an unconventional strategy to explore the pressure-induced evolution of the aggregation microenvironment, involving changes in molecular conformation, stacking mode, and intermolecular interaction, by monitoring the emission under multiple excitation channels based on a luminogen with aggregation-induced emission characteristics of di(p-methoxylphenyl)dibenzofulvene. Under three excitation wavelengths, the distinct emission behaviors have been interestingly observed to reveal the pressure-induced structural evolution, well consistent with the results from ultraviolet-visible absorption, high-pressure angle-dispersive X-ray diffraction, and infrared studies, which have rarely been reported before. This finding provides important insights into the design of organic solid luminescent materials and greatly promotes the development of stimulus-responsive luminescent materials.