arXiv subjects

T. -L. Xia

Publications and source records attributed to T. -L. Xia.

13 recordsLinked to original sources

arXiv2019

Quantum oscillation of thermal conductivity and violation of Weidemann-Franz law in TaAs$_2$ and NbAs$_2$

We report a study of thermal conductivity and resistivity at ultra-low temperatures and in high magnetic fields for semi-metal materials TaAs$_2$ and NbAs$_2$ by using single crystal samples. The thermal conductivity is strongly suppressed in magnetic fields, having good correspondence with the large positive magnetoresistance, which indicates a dominant electronic contribution to thermal conductivity. In addition, not only the resistivity but also the thermal conductivity display clear quantum oscillations behavior at subKelvin temperatures and in magnetic fields up to 14 T. The most striking phenomenon is that the thermal conductivity show a $T^4$ behavior at very low temperatures, while the resistivity show a $T$-independent behavior. This indicates a strong violation of the Weidemann-Franz law and points to a non-Feimi liquid state of these materials.

cond-mat.mtrl-sci↗

arXiv2012

NMR study of superconductivity and spin fluctuations in hole-doped superconductor Ca1-xNaxFe2As2 (Tc =32 K)

We report both 23Na and 75As NMR studies on hole-doped Ca1-xNaxFe2As2 superconducting single crystals (x\approx 0.67) with Tc =32 K. Singlet superconductivity is suggested by a sharp drop of the Knight shift 75K below Tc. The spin-lattice relaxation rate 1/T1 does not show the Slichter-Hebel coherence peak, which suggests an unconventional pairing. The penetration depth is estimated to be 0.24 μm at T=2 K. 1/75T1T shows an anisotropic behavior and a prominent low-temperature upturn, which indicates strong low-energy antiferromagnetic spin fluctuations and supports a magnetic origin of superconductivity.

cond-mat.supr-con↗

arXiv2011

Unconventional superconducting gap in NaFe$_{0.95}$Co$_{0.05}$As observed by angle-resolved photoemission spectroscopy

We have performed high resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe$_{0.95}$Co$_{0.05}$As ($T_{c}\sim$18 K). We observed a hole-like Fermi surface around the zone center and two electron-like Fermi surfaces around the M point which can be connected by the $Q=(π, π)$ wavevector, suggesting that scattering over the near-nested Fermi surfaces is important to the superconductivity of this "111" pnicitide. Nearly isotropic superconducting gaps with sharp coherent peaks are observed below $T_c$ on all three Fermi surfaces. Upon increasing temperature through $T_c$, the gap size shows little change while the coherence vanishes. Large ratios of $2Δ/k_{B}T_{c}\sim8$ are observed for all the bands, indicating a strong coupling in this system. These results are not expected from a classical phonon-mediated pairing mechanism.

cond-mat.supr-con↗

arXiv2011

23Na and 75As NMR Study of Antiferromagnetism and Spin Fluctuations on NaFeAs Single Crystals

We report the $^{23}$Na and $^{75}$As NMR studies on NaFeAs single crystals. The structure transition temperature $T_S$ (55 K) and the spin density wave (SDW) transition temperature $T_{SDW}$ (40.5 K) are determined by the NMR line splits. The spin-lattice relaxation rates indicate that the spin fluctuations are strongly enhanced just below $T_S$ and drive a second order SDW transition. A fluctuating feature of the SDW ordering is also seen below the $T_{SDW}$. We further performed high-pressure NMR studies on NaFeAs, and found that the $T_{SDW}$ increases by $\sim$7 K and the magnetic moment increases by 30% under 2.5 GPa pressure.

cond-mat.supr-con↗

arXiv2011

77Se NMR study of pairing symmetry and spin dynamics in KyFe2-xSe2

We present a 77Se NMR study of the newly discovered iron selenide superconductor KyFe2-xSe2, in which Tc = 32 K. Below Tc, the Knight shift 77K drops sharply with temperature, providing strong evidence for singlet pairing. Above Tc, Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-Tc superconductivity in this iron selenide system.

cond-mat.supr-con↗

arXiv2011

The effect of varying Fe-content on transport properties of K intercalated iron selenide KxFe2-ySe2

We report the successful growth of high-quality single crystals of potassium intercalated iron selenide KxFe2-ySe2 by Bridgeman method. The effect of iron vacancies on transport properties was investigated by electrical resistivity and magnetic susceptibility measurements. With varying iron content, the system passes from semiconducting/insulating to superconducting state. Comparing with superconductivity, the anomalous "hump" effect in the normal state resistivity is much more sensitive to the iron deficiency. The electrical resistivity exhibits a perfect metallic behavior (R300K/R35K=42) for the sample with little iron vacancies. Our results suggest that the anomalous "hump" effect in the normal state resistivity may be due to the ordering process of the cation vacancies in this non-stoichiometric compound rather than magnetic/structure transition. A trace of superconductivity extending up to near 44 K was also detected in some crystals of KxFe2-ySe2, which has the highest Tc of the reported iron selenides.

cond-mat.supr-con↗

arXiv2010

23Na and 75As NMR Studies on NaFeAs0.8P0.2 Single Crystals

We report our $^{23}$Na and $^{75}$As NMR studies on isovalent phosphorus-doped NaFeAs$_{0.8}$P$_{0.2}$ ($T_c=$ 33 K) single crystals. Our data suggest a dramatic enhancement of the electron density of states on the Fermi surface, and a large residual electron density of state below $T_c$. However, evidence of antiferromagnetic spin fluctuations is shown by the spin-lattice relaxation rate. The penetration depth, revealed from $^{23}$Na NMR linewidth, follows the universal Uemura relation in iron pnictides. These observations suggest that the correlation effects are still important for the superconductivity in the phosphorus doped sample, although the Fermi surface is probably significantly changed upon doping.

cond-mat.supr-con↗

arXiv2010

Unusual Electronic Structure and Observation of Dispersion Kink in CeFeAsO Parent Compound of FeAs-Based Superconductors

We report the first comprehensive high-resolution angle-resolved photoemission measurements on CeFeAsO, a parent compound of FeAs-based high temperature superconductors with a mangetic/structural transition at $\sim$150 K. In the magnetic ordering state, four hole-like Fermi surface sheets are observed near $Γ$(0,0) and the Fermi surface near M(+/-$π$,+/-$π$) shows a tiny electron-like pocket at M surrounded by four Dirac cone-like strong spots. The unusual Fermi surface topology deviates strongly from the band structure calculations. The electronic signature of the magnetic/structural transition shows up in the dramatic change of the quasiparticle scattering rate. A dispersion kink at $\sim$ 25meV is for the first time observed in the parent compound of Fe-based superconductors.

cond-mat.supr-con↗

arXiv2010

Site-dependent NMR Spin-lattice Relaxation in the Superconducting State of an Iron Pnictide Superconductor

In a conventional superconductor, the spin-lattice relaxation rate on all nuclei should have the same temperature dependence below Tc. We performed $^{23}$Na, $^{75}$As, and $^{59}$Co NMR studies on single crystals of NaFe0.95Co0.05As, and found that spin-lattice relaxation rates show very different temperature dependent power-law behavior on three sites. We propose that such site-dependent behavior is due to the facts that the superconductor has two gaps of very different sizes. The power-law exponent of each nucleus is affected by the strength of the hyperfine coupling to the small gap. We also found that the large superconducting gap on the cobalt site is smaller than on other two sites. It suggests a local suppression of the superconducting gap on the dopant site.

cond-mat.supr-con↗

arXiv2010

Phase diagram of the NdFe1-xRhxAsO superconductor

Polycrystalline samples with a nominal composition of NdFe1-xRhxAsO (0 <= x <= 0.25) were synthesized using a solid state reaction method. Bulk superconductivity with a maximum Tc = 13 K is observed in the x = 0.1 sample. A temperature-composition phase diagram is established for the NdFe1-xRhxAsO system based on the electrical resistivity and magnetization measurements. A first-order-like transition from an antiferromagnetic state to the superconducting state at a critical Rh-doping point xc = 0.045 is observed in the present phase diagram. The value for the upper critical field Hc2(0) is estimated to be about 26 T for the x = 0.10 sample by using the WHH theory.

cond-mat.supr-con↗

arXiv2010

Superconductivity at 33 K in "111" single crystals at ambient pressure

We have successfully grown single crystalline NaFeAs with cobalt or phosphor doping. Bulk superconductivity occurs in NaFe$_{0.95}$Co$_{0.05}$As at 19 K while much higher transition temperature is observed in NaFeAs$_{0.8}$P$_{0.2}$, in which the Tc of 33 K is even higher than the highest value realized by applying pressure in NaFeAs and all other isoelectronic element substituted samples. We discuss these behaviours by comparison with 122 and 1111 systems. We hope that our findings will help improve our understanding of iron-based superconductivity.

cond-mat.supr-con↗

arXiv2009

Possible high temperature superconductivity in Ti-doped A-Sc-Fe-As-O (A= Ca, Sr) system

We report a systematic study on the effect of partial substitution of Sc$^{3+}$ by Ti$^{4+}$ in Sr$_{2}$ScFeAsO$_{3}$, Ca$_{2}$ScFeAsO$_{3}$ and Sr$_{3}$Sc$_{2}$Fe$_{2}$As$_{2}$O$_{5}$ on their electrical properties. High level of doping results in an increased carrier concentration and leads to the appearance of superconductivity with the onset of T$_{c}$ up to 45 K.

cond-mat.supr-con↗

arXiv2009

Structural and magnetic phase transitions in Na$_{1-δ}$FeAs

We use neutron scattering to study the spin and lattice structures of single crystal and powder samples of Na$_{1-δ}$FeAs ($T_c = 23$ K). On cooling from room temperature, the system goes through a series of phase transitions: first changing the crystal symmetry from tetragonal to orthorhombic at 49 K, then ordering antiferromagnetically with a spin structure similar to that of LaFeAsO and a small moment (0.09$\pm$0.04 $μ_B$), and finally becoming superconducting below about 23 K. These results confirm that antiferromagnetic order is ubiquitous for the parent compounds of the iron arsenide superconductors, and suggest that the separated structural and magnetic phase transition temperatures are due to the reduction in the c-axis exchange coupling of the system.

cond-mat.supr-con↗