We present magnetotransport studies on a series of BaFe_2-xNi_xAs₂ (0.03≤ x ≤ 0.10) single crystals. In the underdoped (x = 0.03) non-superconducting sample, the temperature-dependent resistivity exhibits a peak at 22 K, which is associated with the onset of filamentary superconductivity (FLSC). FLSC is suppressed by an external magnetic field in a manner similar to the suppression of bulk superconductivity in an optimally-doped (x=0.10) compound, suggesting the same possible origin as the bulk superconductivity. Our magnetoresistivity measurements reveal that FLSC persists up to the optimal doping and disappears in the overdoped regime where the long-range antiferromagnetic order is completely suppressed, pointing to a close relation between FLSC and the magnetic order.

We have studied the angular magnetoresistance of iron pnictides BaFe_2-xNi_xAs₂, which shows clear 180 degree periodicity as fitted by a cosine function. In the x = 0.065 sample, the phase of the two-fold symmetry changes 90 degrees above the tetragonal-to-orthorhombic structural transition temperature T_s. Since the phase at low temperature is associated with the rotation of orthorhombic domains by magnetic field, we show that even vacuum grease can push the presence of orthorhombic domains at temperatures much higher than T_s. Our results suggest that residual stress may have significant effects in studying the nematic orders and its fluctuations in iron pnictides.

In this article, the Sm-doping single crystals Ca_1-xSm_xFe₂As₂ (x=0～0.2) were prepared by the CaAs flux method, and followed by a rapid quenching treatment after the high temperature growth. The samples were characterized by structural, resistive, and magnetic measurements. The successful Sm-substitution was revealed by the reduction of the lattice parameter c, due to the smaller ionic radius of Sm³⁺ than Ca²⁺. Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping. The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples. Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x >0.10. The doping dependences of the c-axis length and onset T_c were summarized. The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.

In this work, Raman scattering measurements have been performed on the collapsed phase CaCo₂As₂ crystals. At least 8 Raman modes were observed at room temperature though CaCo₂As₂ is structurally similar to other 122 compounds like BaFe₂As₂. Two Raman modes are assigned to the intrinsic A_1g and B_1g of this material system respectively. The other ones are considered to originate from the local vibrations relevant to cobalt vacancies. Careful polarized measurements allow us to clearly resolve the four-fold symmetry of the B_1g mode, which put strong constraints on possible point group symmetries of the system with Co vacancies. The temperature-dependent measurements demonstrate that the anomalies in both frequency and width of the B_1g mode occur around Neel temperature T_N. The anomalies are considered to be related to the gap opening near the magnetic transition. The study may shed light on the structural and magnetic changes and their correlations with superconductivity in 122 systems.

We propose two possible new compounds, Ba₂CuO₂Fe₂As₂ and K₂CuO₂Fe₂Se₂, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO₂ layers and antifluorite-type Fe₂X₂ (X=As, Se) layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high T_c superconductivity.

It was found that selenium doping can suppress the charge-density-wave (CDW) order and induce bulk superconductivity in ZrTe₃. The observed superconducting dome suggests the existence of a CDW quantum critical point (QCP) in ZrTe_3-xSe_x near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe_3-xSe_x single crystals (x = 0.044 and 0.051) down to 80 mK. For both samples, the residual linear term κ₀/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ₀/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe_3-xSe_x, which indicates conventional superconductivity despite of the existence of a CDW QCP.

Large superconducting FeSe crystals of (001) orientation have been prepared via a hydrothermal ion release/introduction route for the first time. The hydrothermally derived FeSe crystals are up to 10 mm×5 mm×0.3 mm in dimension. The pure tetragonal FeSe phase has been confirmed by x-ray diffraction (XRD) and the composition determined by both inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive x-ray spectroscopy (EDX). The superconducting transition of the FeSe samples has been characterized by magnetic and transport measurements. The zero-temperature upper critical field H_c2 is calculated to be 13.2-16.7 T from a two-band model. The normal-state cooperative paramagnetism is found to be predominated by strong spin frustrations below the characteristic temperature T_sn, where the Ising spin nematicity has been discerned in the FeSe superconductor crystals as reported elsewhere.

We report the success in introducing Mn into (Li_1-xFe_x)OHFe_1-ySe superconducting crystals by applying two different hydrothermal routes, ion exchange (1-step) and ion release/introduction (2-step). The micro-region x-ray diffraction and energy dispersive x-ray spectroscopy analyses indicate that Mn has been doped into the lattice, and its content in the 1-step fabricated sample is higher than that in the 2-step one. Magnetic susceptibility and electric transport properties reveal that Mn doping influences little on the superconducting transition, regardless of 1-step or 2-step routes. By contrast, the characteristic temperature T^*, at which the negative Hall coefficient reaches its minimum, is significantly reduced by Mn doping. This implies that the hole carriers contribution is obviously modified, and hence the hole band might have no direct relationship with the superconductivity in (Li_1-xFe_x)OHFe_1-ySe superconductors. Our present hydrothermal methods of ion exchange and ion release/introduction provide an efficient way for elements substitution/doping into (Li_1-xFe_x)OHFe_1-ySe superconductors, which will promote the in-depth investigations on the role of multiple electron and hole bands and their interplay with the high-temperature superconductivity in the FeSe-based superconductors.

Using scanning tunneling spectroscopy, we studied the transition from tunneling regime to local point contact on the iron-based superconductor Ba_0.6K_0.4Fe₂As₂. By gradually reducing the junction resistance, a series of spectra were obtained with the characteristics evolving from single-particle tunneling into Andreev reflection. The spectra can be well fitted to the modified Blonder-Tinkham-Klapwijk (BTK) model and exhibit significant changes of both spectral broadening and orbital selection due to the formation of point contact. The spatial resolution of the point contact was estimated to be several nanometers, providing a unique way to study the inhomogeneity of unconventional superconductors on such a scale.

We have carried out high-resolution angle-resolved photoemission measurements on the Ce-based heavy fermion compound CePt₂In₇ that exhibits stronger two-dimensional character than the prototypical heavy fermion system CeCoIn₅. Multiple Fermi surface sheets and a complex band structure are clearly resolved. We have also performed detailed band structure calculations on CePt₂In₇. The good agreement found between our measurements and the calculations suggests that the band renormalization effect is rather weak in CePt₂In₇. A comparison of the common features of the electronic structure of CePt₂In₇ and CeCoIn₅ indicates that CeCoIn₅ shows a much stronger band renormalization effect than CePt₂In₇. These results provide new information for understanding the heavy fermion behaviors and unconventional superconductivity in Ce-based heavy fermion systems.

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF₂ substrate by pulsed laser deposition (PLD) method. Measurements on FeSe/CaF₂ samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140–160 K, much higher than~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF₂ films by PLD method with the FeSe/SrTiO₃ films by molecular beam epitaxy (MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF₂ is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF₂ films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO₃ films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.