The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (T c ). However, the absence of hole pockets near the Fermi level of the iron-selenide (FeSe) derived high-T c superconductors raises a fundamental question whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-T c phase induced by pressure in bulk FeSe from magneto-transport measurements and first-principles calculations. With increasing pressure, the low-T c superconducting phase transforms into high-T c phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in contrast to other FeSe-derived high-T c systems. Moreover, the Hall coefficient is enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-T c phase. These results in FeSe highlight similarities with high-T c phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity. 2The Fermi surface (FS) topology and its interplay with magnetism have been considered key ingredients in understanding the mechanism of the iron-based superconductors.1, 2 For the FeAsbased superconductors, the FS typically consists of hole-and electron-like pockets near the Brillouin zone center (Γ point) and corners (M point), respectively. As such, an interband scattering between the hole and electron pockets has been proposed as an important mechanism for electron pairing in the iron-based superconductors, leading to an s± pairing state favored by the antiferromagnetic fluctuations. carriers, yet can still enhance T c of bulk FeSe up to ~40 K near 6 GPa. 16,17 More importantly, our recent high-pressure study has shown explicitly that the optimal T c is achieved when the longrange antiferromagnetic order just vanishes, 18 Fig. 1, reminiscent of the situations seen frequently in the FeAs-based superconductors. However, to make this connection, it is important to have information about the evolution of FS under high pressure -a regime in which ARPES experiments are impractical, and where quantum oscillation measurements are challenging.Here we report Hall resistivity ρ xy measurements under hydrostatic pressures up to 8.8 GPa in order to gain insights into the electronic structure evolution of FeSe at high pressure. Our results demonstrate that the electrical transport properties of FeSe at high pressures with T c max = 38.3 K are dominated by the hole carriers, Fig. 1, which is in contrast with the known FeSe-derived high-T c superconductors that are usually heavily electron doped. In addition, we observed an enhancement of Hall coefficient R H near the critical pressure where the optimal T c is realized with a simultaneous suppression of the long-range magnetic order. This implies a strong reconstruction of the Fermi surface due to antifer...

We report a giant resistance drop induced by dc electrical currents in La 0.67 Ca 0.33 MnO 3 epitaxial thin films. Resistance of the patterned thin films decreases exponentially with increasing current and a maximum drop shows at the temperature of resistance peak T p . Variation of resistance with current densities can be scaled below and above T p , respectively. This work can be useful for the future applications of electroresistance.

In this paper, we present an experimental study of the hexagonal compound Na 0.70 MnO 2 . Zero-field-cooled and field-cooled susceptibilities display divergences at low temperatures, and the magnetic measurements of frequency dependence of ac susceptibility, hysteresis effect, and long-time relaxation are performed, indicating that Na 0.70 MnO 2 undergoes a spin-glass transition at T f = 39 K. The spin-glass order parameter q͑T͒ determined from the dc spin susceptibility exhibits the relation q͑T͒ ϰ ͑1−T / T f ͒, in agreement with the prediction of conventional spin-glass theory. Spin dynamics in the spin-glass state is carefully examined, and the time decay of the thermoremanent magnetization can be well scaled with a reduced effective waiting time / t w . The magnetic entropy extracted from the specific heat implies that the spin degrees of freedom of Mn 3+ /Mn 4+ ions are completely frozen at low temperatures, and the origin of this spin-glass behavior is attributed to the mixture of Mn 3+ /Mn 4+ ions and geometrical frustrations on the triangular lattices. Comparisons with the magnetic properties of Na 0.70 CoO 2 are also made.

In order to elucidate pressure-induced second superconducting phase (SC-II) in AxFe2−ySe2 (A = K, Rb, Cs, and Tl) having an intrinsic phase separation, we perform a detailed high-pressure magnetotransport study on the isoelectronic, phase-pure (Li1−xFex)OHFe1−ySe single crystals. Here we show that its ambient-pressure superconducting phase (SC-I) with a critical temperature Tc ≈ 40 K is suppressed gradually to below 2 K and an SC-II phase emerges above Pc ≈ 5 GPa with Tc increasing progressively to above 50 K up to 12.5 GPa. Our high-precision resistivity data uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-II phase. In addition, the reemergence of high-Tc SC-II is found to accompany with a concurrent enhancement of electron carrier density. Without structural transition below 10 GPa, the observed SC-II with enhanced carrier density should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction.

Here we report the successful synthesis of a spin-& charge-decoupled diluted magnetic semiconductor (Ca,Na)(Zn,Mn) 2 As 2 , crystallizing into the hexagonal CaAl 2 Si 2 structure. The compound shows a ferromagnetic transition with a Curie temperature up to 33 K with 10% Na doping, which gives rise to carrier density of n p~1 0 20 cm -3 . The new DMS is a soft magnetic material with H C <400 Oe. The anomalous Hall effect is observed below the ferromagnetic ordering temperature.With increasing Mn doping, ferromagnetic order is accompanied by an interaction between the local spin and mobile charge, giving rise to a minimum in resistivity at low temperatures and localizing the conduction electrons. The system provides an ideal platform for studying the interaction of the local spins and conduction electrons. PACS number(s): 75.50. Pp, 75.30.Kz, 76.75.+i 2 Diluted magnetic semiconductors (DMS) have received much attention due to their potential for application in the field of spintronics [1][2][3][4][5] . In typical III-V DMS systems, such as (Ga,Mn)As, (In,Mn)As and (Ga,Mn)N, substitution of divalent Mn atoms into trivalent Ga or In sites leads to severely limited chemical solubility, resulting in metastable specimens only available as epitaxial thin films. Moreover, the hetero-valence substitution, which simultaneously dopes both charge and spin, makes it difficult to individually control each quantum degree of freedom.Recently, a new type of DMS, Li(Zn,Mn)As (termed "111" following the three chemical composition ratios), was discovered by Deng et al, The "122" DMS (Ba,K)(Zn,Mn) 2 As 2 shares the tetragonal ThCr 2 Si 2 structure with the "122" iron pnictide superconductor (Ba,K)Fe 2 As 2 and the antiferromagnet BaMn 2 As 2 9 , each with a lattice mismatch of less than 2%, giving rise to the unprecedented possibility of designing various interface-based devices among DMS, superconductors and magnets. Moreover, an increase in the carrier density could enhance the Curie temperature further to 230K in this 122 system. 10The idea of decoupling spin and charge has since been extended, resulting in the discovery of two 3 additional DMS system, "111" type Li(Zn,Mn)P 11 and "1111" types (La,Ca)(Zn,Mn)SbO & (La,Ca)(Zn,Mn)AsO 12,13 Depending on the composition, "122" compounds can crystallize into the tetragonal ThCr 2 Si 2 structure, as in the case of (Ba,K)(Zn,Mn) 2 As 2 , or the hexagonal CaAl 2 Si 2 structure. Interestingly, the hexagonal compounds CaMn 2 As 2 and CaMn 2 Sb 2 are fully frustrated classical magnetic systems with a honeycomb lattice of Mn atoms, exhibiting a complex magnetic phase diagram. 14 Here we report the synthesis of bulk specimens of a new DMS (Ca,Na)(Zn,Mn) 2 As 2 with the hexagonal CaAl 2 Si 2 structure, as shown in Fig. 1(a). ExperimentalPolycrystalline samples of (Ca,Na)(Zn,Mn) 2 As 2 were synthesized via the solid-state reaction method. The synthesis of starting materials CaAs and Na 3 As was described in an earlier paper. 8These starting materials were mixed with high-purity Zn, M...