Crystal and electronic structure of a quasi-two-dimensional semiconductor Mg_(3)Si_(2)Te_(6)

We report the synthesis and characterization of a Si-based ternary semiconductor Mg_(3)Si_(2)Te_(6),which exhibits a quasitwo-dimensional structure,where the trigonal Mg_(3)Si_(2)Te_(6)layers are separated by Mg ions.Ultraviolet-visible absorption spectroscopy and density functional theory calculations were performed to investigate the electronic structure.The experimentally determined direct band gap is 1.39 eV,consistent with the value of the density function theory calculations.Our results reveal that Mg_(3)Si_(2)Te_(6)is a direct gap semiconductor,which is a potential candidate for near-infrared optoelectronic devices.

Synthesis and properties of La_(1-x)Sr_(x)NiO_(3) and La_(1-x)Sr_(x)NiO_(2)

Superconductivity has been realized in films of La_(1-x)Sr_(x)NiO_(2). Here we report synthesis and characterization of polycrystalline samples of La_(1-x)Sr_(x)NiO_(3) and La_(1-x) Sr_(x)NiO_(2)(0 ≤ x ≤ 0.2). Magnetization and resistivity measurements reveal that La_(1-x)Sr_(x)NiO_(3) are paramagnetic metal and La_(1-x)Sr_(x)NiO_(2) exhibit an insulating behavior. Superconductivity is not detected in bulk samples of La_(1-x)Sr_(x)NiO_(2). The absence of superconductivity in bulk La_(1-x)Sr_(x)NiO_(2) may be due to the generation of hydroxide during reduction, a small amount of nickel impurity, or incomplete reduction of apical oxygen.The effect of interface in films of La_(1-x)Sr_(x)NiO_(2) may also play a role for superconductivity.

Evidence for charge and spin density waves in single crystals of La_(3)Ni_(2)O_(7) and La_(3)Ni_(2)O_(6)

Charge and spin orders are intimately related to superconductivity in copper oxide superconductors.Elucidation of the competing orders in various nickel oxide compounds is crucial,given the fact that superconductivity has been discovered in Nd_(0.8)Sr_(0.2)NiO_(2)films.Herein,we report structural,electronic transport,magnetic,and thermodynamic characterizations of single crystals of La_(3)Ni_(2)O_(7)and La_(3)Ni_(2)O_(6).La_(3)Ni_(2)O_(7)is metallic with mixed Ni^(2+)and Ni^(3+)valent states.Resistivity measurements yield two transition-like kinks at~110 and 153 K.The kink at 153 K is further revealed from magnetization and specific heat measurements,indicative of the formation of charge and spin density waves.La_(3)Ni_(2)O_(6)single crystals obtained from the topochemical reduction of La_(3)Ni_(2)O_(7)are insulating and show an anomaly at~176 K on magnetic susceptibility.The transition-like behaviors of La_(3)Ni_(2)O_(7)and La_(3)Ni_(2)O_(6)are analogous to those observed in La_(4)Ni_(3)O_(10) and La_(4)Ni_(3)O_(8),suggesting that charge and spin density waves are a common feature in the ternary La-Ni-O system with mixed-valent states of nickel.

Magnetism variation of the compressed antiferromagnetic topological insulator EuSn_(2)As_(2)

We report a comprehensive high-pressure study,up to 21.1 GPa,on the antiferromagnetic topological insulator EuSn_(2)As_(2) achieved through synchrotron X-ray diffraction,Raman scattering,electrical resistance,magnetic resistance,and Hall transport measurements in combination with first-principles calculations.The Néel temperatures determined from resistance are increased from(24±1)to(77±8)K under pressure,which is a result of enhanced magnetic exchange couplings between Eu^(2+) ions yielded by our first-principles calculations.The negative magnetoresistance of EuSn_(2)As_(2) persists to higher temperatures accordingly.However,the enhancement of the observed Néel temperatures deviates from the calculations above 10.0 GPa.In addition,the magnitude of the magnetoresistance,Hall coefficients,and charge carrier densities show abrupt changes between 6.9 and 10.0 GPa.The abrupt changes likely originate from a pressure-induced valence change of Eu ions from a divalent state to a divalent and trivalent mixed state or are related to the structural transition revealed by Raman scattering measurements.Our results provide insight into magnetism variation in EuSn_(2)As_(2) and similar antiferromagnetic topological insulators under pressure.

Structure and magnetic properties of the S=3/2 zigzag spin chain antiferromagnet BaCoTe_(2)O_(7)

We report a study of the structure and magnetic properties of the S=3/2 zigzag spin chain compound BaCoTe_(2)O_(7).Neutron diffraction measurements show that it crystallizes in the noncentrosymmetric space group Ama2 with a canted↑↑↓↓spin structure along the quasi-one-dimensional zigzag chain and a moment size of 1.89(2)μBat 2 K.Both magnetic susceptibility and specific heat measurements yield an antiferromagnetic phase transition at TN=6.2 K.A negative Curie-Weiss temperature,ΘCW=-74.7(2) K,and an empirical frustration parameter,f=|ΘCW|/TN≈12,are obtained by fitting the magnetic susceptibility,indicating antiferromagnetic interactions and strong magnetic frustration.From ultraviolet-visible absorption spectroscopy and first-principles calculations,an indirect band gap of 2.68(2) eV is determined.We propose that the canted zigzag spin chain of BaCoTe_(2)O_(7) may produce a change in the polarization via the exchange-striction mechanism.