Qianhui Mao

Superconductivity at 32 K and anisotropy in Tl0.58Rb0.42Fe1.72Se2 crystals

Tl0.58Rb0.42Fe1.72Se2 single crystals were successfully synthesized with the Bridgeman method. The physical properties are characterized by electrical resistivity, magnetization and Hall resistivity. Tl0.58Rb0.42Fe1.72Se2 undergoes a superconducting transition at Tconset=32 K and Tczero=31.4 K. Extrapolation of the upper critical field Hc2 at Tc to zero temperature gives a large value of Hc2(0) (Hc2(0)=221 T for H∥ab and Hc2(0)=44.2 T for H∥c). The Hall coefficient changes sign upon cooling down, indicating a possible multi-band behavior. The normal-state magnetic susceptibility decreases with decreasing temperature, suggesting strong antiferromagnetic (AFM) spin fluctuations, which might be related to the superconductivity (SC).

Large unsaturated positive and negative magnetoresistance in Weyl semimetal TaP

After successfully growing single-crystal TaP, we measured its longitudinal resistivity (ρxx) and Hall resistivity (ρyx) at magnetic fields up to 9 T in the temperature range of 2-300 K. At 8 T, the magnetoresistance (MR) reached 3.28 × 105% at 2 K, 176% at 300 K. Neither value appeared saturated. We confirmed that TaP is a hole-electron compensated semimetal with a low carrier concentration and high hole mobility of μh=3.71 × 105 cm2/V s, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, because a magnetic field (H) was applied in parallel to the electric field (E), a negative MR due to a chiral anomaly was observed and reached -3000% at 9 T without any sign of saturation, either, which is in contrast to other Weyl semimetals (WSMs). The analysis of the Shubnikov-de Haas (SdH) oscillations superimposed on the MR revealed that a nontrivial Berry’s phase with a strong offset of 0.3958, which is the characteristic feature of charge carriers enclosing a Weyl node. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.

Multiband Superconductivity of Heavy Electrons in a TlNi2Se2 Single Crystal

We have made the first observation of superconductivity in TlNi2Se2 at T(C)=3.7  K, and it appears to involve heavy electrons with an effective mass m*=(14-20)m(b), as inferred from the normal-state electronic specific heat and the upper critical field, H(C2)(T). We found that the zero-field electronic specific-heat data, C(es)(T) (0.5  K≤T<3.7  K) in the superconducting state can be fitted with a two-gap BCS model, indicating that TlNi2Se2 seems to be a multiband superconductor, which is consistent with the band calculation for the isostructural KNi2S2. It is also found that the electronic specific-heat coefficient in the mixed state γN(H) exhibits a H(1/2) behavior, which is considered as a common feature of the d-wave superconductors. TlNi2Se2, as a d-electron system with heavy electron superconductivity, may be a bridge between cuprate- or iron-based and conventional heavy-fermion superconductors.

Magnetic properties in layered ACo(2)Se(2) (A = K, Rb, Cs) with the ThCr2Si2-type structure

The magnetic properties of ThCr_2Si_2-type single crystals ACo_2Se_2 (A = K, Rb and Cs) have been investigated by magnetic susceptibility and isothermal magnetization measurements at various temperatures. The ferromagnetic phase transition temperatures are estimated as

Superconductivity in a new layered nickel selenide CsNi2Se2

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Camelback-shaped band reconciles heavy-electron behavior with weak electronic Coulomb correlations in superconducting TlNi2Se2

74 K and 76 K and 62 K for A = K, Rb and Cs (in case of high magnetic field), respectively. The susceptibility data in the paramagnetic state obey the modified Curie-Weiss law quite well and the derived effective magnetic moments of the Co atom are about 2.21, 2.04 and 2.04 \mu_B/Co and the corresponding spontaneous moments derived at the ground state are 0.72, 0.59 and 0.52 \mu_B/Co as well as the generalized Rhodes-Wohlfarth ratios as 3.07, 3.42 and 3.96 for A= K, Rb and Cs, respectively. The magnetic moment aligns within ab-plane and a metamagnetism-like behavior occurs at a field of 3.5 T in CsCo_2Se_2 with H//ab-plane. The magnetic properties in this system were discussed within the frameworks of the self-consistent renormalization (SCR) and Takahashis theory of spin fluctuations.

Pressure induced superconductivity in the antiferromagnetic Dirac material BaMnBi 2

The physical properties of CsNi2Se2 were characterized by electrical resistivity, magnetization and specific heat measurements. We found that the stoichiometric CsNi2Se2 compound undergoes a superconducting transition at T c = 2.7 K. A large Sommerfeld coefficient (~77.90 mJ/mol K−2) was obtained from the normal state electronic specific heat. However, the Kadowaki–Woods ratio of CsNi2Se2 was estimated to be about 0.041 × 10cm(mol K2/mJ)2, indicating the absence of strong electron–electron correlations. In the superconducting state, we found that the zero-field electronic specific heat data, C es(T) (0.5 K T < 2.7 K), can be fitted well with a two-gap BCS model, indicating the multi-gap feature of CsNi2Se2. The comparison with the density functional theory (DFT) calculations suggested that the large in these nickel selenide superconductors may be related to the large density of states (DOS) at the Fermi surface.

Large low field magnetocaloric effect in first-order phase transition compound TlFe 3 Te 3 with low-level hysteresis

Combining photoemission spectroscopy, Raman spectroscopy, and first-principles calculations, we characterize superconducting TlNi2Se2 as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi2Se2 it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity. The resulting heavy band mass resolves the apparent puzzle of a large normal-state Sommerfeld coefficient [H. Wang et al., Phys. Rev. Lett 111, 207001 (2013)] in this weakly correlated compound.

Superconductivity and disorder effect in TlNi2Se(2-x)S(x) compounds.

The so-called Dirac materials such as graphene and topological insulators are a new class of matter different from conventional metals and (doped) semiconductors. Superconductivity induced by doing or applying pressure in these systems may be unconventional, or host mysterious Majorana fermions. Here, we report a successfully observation of pressure-induced superconductivity in an antiferromagnetic Dirac material BaMnBi2 with Tc of ~4 K at 2.6 GPa. Both the higher upper critical field, μ0Hc2(0) ~ 7 Tesla, and the measured current independent of Tc precludes that superconductivity is ascribed to the Bi impurity. The similarity in ρab(B) linear behavior at high magnetic fields measured at 2 K both at ambient pressure (non-superconductivity) and 2.6 GPa (superconductivity, but at the normal state), as well as the smooth and similar change of resistivity with pressure measured at 7 K and 300 K in zero field, suggests that there may be no structure transition occurred below 2.6 GPa, and superconductivity observed here may emerge in the same phase with Dirac fermions. Our findings imply that BaMnBi2 may provide another platform for studying SC mechanism in the system with Dirac fermions.

Ferromagnetic quantum critical behavior in heavy-fermion compounds CeTi1−x Ni x Ge3

Magnetic refrigeration based on the magnetocaloric effect (MCE) is an environment-friendly, high-efficiency technology. It has been believed that a large MCE can be realized in the materials with a first-order magnetic transition (FOMT). Here, we found that TlFe3Te3 is a ferromagnetic metal with a first-order magnetic transition occurring at Curie temperature TC = 220 K. The maximum values of magnetic entropy change (Δ) along the crystallographic c-axis, estimated from the magnetization data, reach to 5.9 J kg−1K−1 and 7.0 J kg−1 K−1 for the magnetic field changes, ΔH = 0–1 T and 0–2 T, respectively, which is significantly larger than that of MCE materials with a second-order magnetic transition (SOMT). Besides the large ΔSM, the low-level both thermal and field hysteresis make TlFe3Te3 compound an attractive candidate for magnetic refrigeration. Our findings should inspire the exploration of high performance new MCE materials.