Evidence for Multiple Underlying Fermi Surface and Isotropic Energy Gap in the Cuprate Parent Compound Ca2CuO2Cl2

The parent compounds of the high-temperature cuprate superconductors are Mott insulators.It has been generally agreed that understanding the physics of the doped Mott insulators is essential to understanding the mechanism of high temperature superconductivity.A natural starting point is to elucidate the basic electronic structure of the parent compound.Here we report comprehensive high resolution angle-resolved photoemission measurements on Ca_2CuO_2Cl_2,a Mott insulator and a prototypical parent compound of the cuprates.Multiple underl.ying Fermi surface sheets are revealed for the first time.The high energy waterfall-like band dispersions exhibit different behaviors near the nodal and antinodal regions.Two distinct energy scales are identified：a d-wave-like low energy peak dispersion and a nearly isotropic lower Hubbard band gap.These observations provide new information of the electronic structure of the cuprate parent compound,which is important for understanding the anomalous physical properties and superconductivity mechanism of the high temperature cuprate superconductors.

Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe2

The Mott transition is one of the fundamental issues in condensed matter physics,especially in the system with antiferromagnetic long-range order.However,such a transition is rare in quantum spin liquid(QSL)systems without long-range order.Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe2 with a triangular lattice of 4 f Yb^3+ions.Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity.An irreversible structure phase transition occurs around 11 GPa,which is revealed by the x-ray diffraction.These results shed light on the Mott transition in the QSL systems.

Synthesis, structure, and properties of Ba9Co3Se(15) with one-dimensional spin chains

A new compound with one-dimensional spin chains, Ba9Co3Se(15), was synthesized under high pressure and high temperature conditions and systematically characterized via structural, transport and magnetic measurements. Ba9Co3Se(15) crystallizes in a hexagonal structure with the space group P-6c2(No. 188) and lattice constants of a = b = 9.6765 ? and c = 18.9562 ?. The structure consists of trimeric face-sharing octahedral CoSe6 chains, which are arranged in a triangular lattice in the ab-plane and separated by Ba atoms. The distance of the nearest neighbor of CoSe6 chains is very large, given by the lattice constant a = 9.6765 ?. The Weiss temperature Tθ associated with the intra-chain coupling strength is about -346 K. However, no long-range magnetic order but a spin glass transition at ~ 3 K has been observed. Our results indicate that the spin glass behavior in Ba9Co3Se(15) mainly arises from the magnetic frustration due to the geometrically frustrated triangular lattice.

Superconductivity in Topological Semimetal θ-TaN at High Pressure

Recently,θ-TaN was proposed to be a topological semimetal with a new type of triply degenerate nodal points.Here,we report studies of pressure dependence of transport,Raman spectroscopy and synchrotron x-ray diffraction on θ-TaN up to 61 GPa.We find that θ-TaN becomes superconductive above 24.6 GPa with Tc at 3.1 K.The θ-TaN is of n-type carrier nature with carrier density about 1.1 × 1020/cm3 at 1.2 GPa and 20 K, while the carrier density increases with the pressure and saturates at about 40 GPa in the measured range.However,there is no crystal structure transition with pressure up to 39 GPa,suggesting the topological nature of the pressure induced superconductivity.

Doping effect on the structure and physical properties of quasi-one-dimensional compounds Ba_(9)Co_(3)(Se_(1−x)S_(x))_(15)(x=0-0.2)

A series of samples of Ba_(9)Co_(3)(Se_(1−x)S_(x))_(15)(x=0,0.05,0.1,0.15,0.2)with quasi-one-dimensional(1D)structure were successfully synthesized under high-temperature and high-pressure conditions.The influence of partial substitution of S for Se on the structure,electronic transport,and magnetic properties of Ba_(9)Co_(3)(Se_(1−x)S_(x))_(15) has been investigated in detail.The x-ray diffraction data shows that the lattice constant decreases linearly with increasing S-doping level,which follows the Vegrad’s law.The doped S atoms preferentially occupy the site of Se atoms in CoSe6 octahedron.Physical properties measurements indicate that all the samples of Ba_(9)Co_(3)(Se_(1−x)S_(x))_(15) are semiconducting and display spin glass behavior.As the replacement of Se by smaller size S,although the inter-chain distance decreases,the electronic hopping between CoSe/S6 chains is weakened and leads to an increase of band gap from 0.75 eV to 0.86 eV,since the S-3p electrons are more localized than Se-4p ones.Ba_(9)Co_(3)(Se_(1−x)S_(x))_(15) exhibits 1D conducting chain characteristic.

Structure-Spin-Transport Anomaly in Quasi-One-Dimensional Ba9Fe3Te15 under High Pressure

Recently,a series of novel compounds Ba3MX5(M=Fe,Ti,V;X=Se,Te)with hexagonal crystal structures composed of quasi-1-dimensional(1D)magnetic chains has been synthesized by our research team using high-pressure and high-temperature methods.The initial hexagonal phases persist to the maximum achievable pressure,while spin configurations and magnetic interactions may change dramatically as a result of considerable reductions in interchain separations upon pressurization.These compounds therefore offer unique possibilities for studying the evolution of intrinsic electronic structures in quasi-1D magnetic systems.Here we present a systematic investigation of Ba9Fe3Te15,in which the interchain separations between trimerized 1D chains(~10.2Å)can be effectively modulated by external high pressure.The crystal structure especially along the 1D chains exhibits an abnormal expansion at^GPa,which accompanies trimerization entangled anomalous mixed-high-low spin transition.An insulator-metal transition has been observed under high pressure as a result of charge-transfer gap closing.Pressure-induced superconductivity emerges at 26 GPa,where the charge-transfer gap fully closes,3D electronic configuration forms and local spin fully collapses.