arXiv subjects

Hechang Lei

Publications and source records attributed to Hechang Lei.

At least 181 records · Page 10Linked to original sources

arXiv2012

Superconducting state in the metastable binary bismuthide Rh3Bi14 single crystals

We report detailed magnetic, transport and thermodynamic properties of metastable Rh3Bi14 single crystals in superconducting and normal state. We show that Rh3Bi14 is nearly isotropic, weak to intermediately coupled BCS superconductor, whereas the electronic resistivity above superconducting Tc = 2.94 K is dominated by the phonon scattering in the large unit cell with pores filled by Bi atoms. Superconductivity is strongly influenced by the nature of atoms that fill the voids in the crystal structure.

cond-mat.supr-con↗

arXiv2012

Surface-induced Magnetism Fluctuations in Single Crystal of NiBi3 Superconductor

We report anistropy in superconducting and normal state of NiBi3 single crystals with Tc = 4.06 K. The magnetoresistance results indicate the absence of scattering usually associated with ferromagnetic metals, suggesting the absence of bulk long range magnetic order below 300 K. However, the electron spin resonance results demonstrate that ferromagnetism fluctuations exist on the surface of the crystal below 150K.

cond-mat.supr-con↗

arXiv2012

Local structural disorder and superconductivity in KxFe2-ySe2

We report significantly enhanced magnetic moment in K0.69(2)Fe1.45(1)Se2.00(1) single crystals with sharp Tc and bulk superconductivity obtained by postannealing and quenching process. There are two Fe sites in the K0.69(2)Fe1.45(1)Se2.00(1) unit cell: Fe1, which has higher symmetry with longer average Fe-Se bond length, and Fe2, which has lower symmetry with shorter average Fe-Se bond length. Temperature-dependent x-ray absorption fine-structure (XAFS) analysis results on quenched and as-grown K0.69(2)Fe1.45(1)Se2.00(1) crystals show that quenched K0.69(2)Fe1.45(1)Se2.00(1) have increased average Fe-Se bond length and decreased static disorder. Our results indicate that nonzero population of Fe1 sites is the key structural parameter that governs the bulk superconductivity. We also show clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.

cond-mat.supr-con↗

arXiv2012

Structure and physical properties of new layered iron oxychalcogenide BaFe2OSe2

We have successfully synthesized a new layered iron oxychalcogenide BaFe2OSe2 single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the c-axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the b-axis and are bridged by oxygen along the a-axis. Physical property measurements indicate that BaFe2OSe2 is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long range antiferromagnetic (AFM) transition at 240 K, corresponding to the peak in specific heat measurement and two glassy transitions at 115 K and 43 K. The magnetic entropy up to 300 K is much smaller than the expected value for Fe2+ in tetrahedral crystal fields and Mosbauer spectrum indicates that long range magnetic order is unlikely at 294 K. Both results suggest that a short range magnetic correlations exist above the room temperature.

cond-mat.supr-con↗

arXiv2012

Multiband effects on beta-FeSe single crystals

We present the upper critical fields Hc2(T) and Hall effect in beta-FeSe single crystals. The Hc2(T) increases as the temperature is lowered for field applied parallel and perpendicular to (101), the natural growth facet of the crystal. The Hc2(T) for both field directions and the anisotropy at low temperature increase under pressure. Hole carriers are dominant at high magnetic fields. However, the contribution of electron-type carriers is significant at low fields and low temperature. Our results show that multiband effects dominate Hc2(T) and electronic transport in the normal state.

cond-mat.supr-con↗

arXiv2012

Two dimensional Dirac fermions and quantum magnetoresistance in CaMnBi$_2$

We report two dimensional Dirac fermions and quantum magnetoresistance in single crystals of CaMnBi$_2$. The non-zero Berry's phase, small cyclotron resonant mass and first-principle band structure suggest the existence of the Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^*$ from semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear magnetoresistance ($\sim 120%$ in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The temperature dependence of $B^*$ satisfies quadratic behavior, which is attributed to the splitting of linear energy dispersion in high field. Our results demonstrate the existence of two dimensional Dirac fermions in CaMnBi$_2$ with Bi square nets.

cond-mat.str-el↗

arXiv2012

Quantum transport of two-dimensional Dirac fermions in SrMnBi2

We report two-dimensional quantum transport in SrMnBi$_2$ single crystals. The linear energy dispersion leads to the unusual nonsaturated linear magnetoresistance since all Dirac fermions occupy the lowest Landau level in the quantum limit. The transverse magnetoresistance exhibits a crossover at a critical field $B^*$ from semiclassical weak-field $B^2$ dependence to the high-field linear-field dependence. With increase in the temperature, the critical field $B^*$ increases and the temperature dependence of $B^*$ satisfies quadratic behavior which is attributed to the Landau level splitting of the linear energy dispersion. The effective magnetoresistant mobility $μ_{MR}\sim 3400$ cm$^2$/Vs is derived. Angular dependent magnetoresistance and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces. Our results illustrate the dominant 2D Dirac fermion states in SrMnBi$_2$ and imply that bulk crystals with Bi square nets can be used to study low dimensional electronic transport commonly found in 2D materials like graphene.

cond-mat.str-el↗

arXiv2012

Giant increase in critical current density of KxFe2-ySe2 single crystals

By using post-annealing and quenching technique, we show that the critical current density Jc,ab of KxFe2-ySe2 single crystals can be enhanced more than one order of magnitude up to ~ 2.1 \times 10^4 A/cm^2 at 1.8 K. The scaling law between normalized pinning force and reduced field for all measured temperatures was observed, reflecting the presence of only one dominant pinning mechanism. Analysis indicates presence of 3D point-like normal core pinning sources in quenched KxFe2-ySe2 samples whereas dominant vortex interaction with pinning centers is via spatial variations in Tc ("deltaTc pinning").

cond-mat.supr-con↗

arXiv2011

Anisotropy in BaFe2Se3 single crystals with double chains of FeSe tetrahedra

We studied the anisotropy in physical properties of Ba1.00(4)Fe1.9(1)Se3.1(1) single crystals. BaFe2Se3 is a semiconductor below 300 K. Magnetization measurements show that there is a crossover from short-range antiferromagnetic (AFM) correlations at room temperature to a long-range AFM order with TN = 255 K. The anisotropy of magnetization is consistent with the previous neutron results. This crossover is supported by the heat-capacity measurement, where the phase-transition peak is absent at 255 K. The superconducting transition at about 10 K is likely due to the small amount of beta-FeSe impurities.

cond-mat.supr-con↗

arXiv2011

Upper critical fields and superconducting anisotropy of K0.70Fe1.55Se1.01S0.99 and K0.76Fe1.61Se0.96S1.04 single crystals

We have investigated temperature and angular dependence of resistivity of K0.70(7)Fe1.55(7)Se1.01(2)S0.99(2) and K0.76(5)Fe1.61(5)Se0.96(4)S1.04(5) single crystals. The upper critical fields Hc2(T) for both field directions decrease with the increase in S content. On the other hand, the angle-dependent magnetoresistivity for both compounds can be scaled onto one curve using the anisotropic Ginzburg-Landau theory. The obtained anisotropy of Hc2(T) increases with S content, implying that S doping might decrease the dimensionality of certain Fermi surface parts, leading to stronger two dimensional character.

cond-mat.supr-con↗

arXiv2011

Anisotropy of upper critical fields and thermally-activated flux flow of quenched KxFe2-ySe2 single crystals

We report the anisotropy of the upper critical fields mu0Hc2(T) and thermally-activated flux flow (TAFF) behavior of quenched KxFe2-ySe2. Even though the post-annealing and quenching process enhances the superconducting volume fraction, it has a minor effect on the upper critical fields for H//c and H//ab. Analysis of the angular-dependence of resistivity rho_ab(theta,H) indicates that it follows the scaling law based on the anisotropic Ginzburg-Landau (GL) theory and the anisotropy Gamma(T) increases with decreasing temperature with Gamma(T) ~ 3.6 at 27 K. The resistivity of quenched sample exhibits an Arrhenius TAFF behavior for both field directions. Field dependence of thermally activated energy U0(H) implies that the collective flux creep is dominant in high fields and point defects are the main pinning source in this regime.

cond-mat.supr-con↗

arXiv2011

Phase Diagram of KxFe2-ySe2-zSz and the Suppression of its Superconducting State by an Fe2-Se/S Tetrahedron Distortion

We report structurally tuned superconductivity in KxFe2-ySe2-zSz (0<=z<=2) phase diagram. Superconducting Tc is suppressed as S is incorporated into the lattice, eventually vanishing at 80% of S. The magnetic and conductivity properties can be related to stoichiometry on poorly occupied Fe1 site and the local environment of nearly fully occupied Fe2 site. The decreasing Tc coincides with the increasing Fe1 occupancy and the overall increase in Fe stoichiometry from z = 0 to z = 2. Our results indicate that the irregularity of Fe2-Se/S tetrahedron is an important controlling parameter that can be used to tune the ground state in the new superconductor family.

cond-mat.supr-con↗

arXiv2011

Critical current density and mechanism of vortex pinning in KxFe2-ySe2 doped with S

We report the critical current density Jc in KxFe2-ySe2-zSz crystals. The Jc can be enhanced significantly with optimal S doping (z=0.99). For K0.70(7)Fe1.55(7)Se1.01(2)S0.99(2), the weak fishtail effect is found for H||c. The normalized vortex pinning forces follow the scaling law with a maximum position at 0.41 of the reduced magnetic field. These results demonstrate that the small size normal point defects dominate the vortex pinning mechanism.

cond-mat.supr-con↗

arXiv2011

Antiferromagnetism in semiconducting KFe0.85Ag1.15Te2 single crystals

We have synthesized single crystals of K1.00(3)Fe0.85(2)Ag1.15(2)Te2.0(1). The materials crystallizes in the ThCr2Si2 structure with I4/mmm symmetry and without K and Fe/Ag deficiencies, unlike in KxFe2-ySe2 and KxFe2-yS2. In contrast to theoretical prediction for higher Tc in KFe2Te2, KFe0.85Ag1.15Te2 is a semiconductor with long-range antiferromagnetic transition at TN = 35 K.

cond-mat.supr-con↗

arXiv2011

Critical fields, thermally-activated transport and critical current density of beta-FeSe single crystals

We present critical fields, thermally-activated flux flow (TAFF) and critical current density of tetragonal phase beta-FeSe single crystals. The upper critical fields Hc2(T) for H||(101) and H\bot(101) are nearly isotropic and are likely governed by Pauli limiting process. The obtained large Ginzburg-Landau parameter k \sim 72.3(2) indicates that beta-FeSe is a type-II superconductor with smaller penetration depth than in Fe(Te,Se). The resistivity below Tc follows Arrhenius TAFF behavior. For both field directions below 30 kOe single vortex pinning is dominant whereas collective creep becomes important above 30 kOe. The critical current density Jc from M-H loops for H||(101) is about five times larger than for H\bot(101), yet much smaller than in other iron-based superconductors.

cond-mat.supr-con↗

arXiv2011

Raising Tc in charge density wave superconductor ZrTe3 by Ni intercalation

We report discovery of bulk superconductivity in Ni0.05ZrTe3 at Tc = 3.1 K, obtained through Ni intercalation. Superconductivity coexists with charge density wave (CDW) state with TCDW = 41 K. When compared to parent material ZrTe3, filamentary superconducting transition is substantially increased whereas TCDW was suppressed. The analysis of superconducting state indicates that Ni0.05ZrTe3 is an intermediately coupled superconductor.

cond-mat.supr-con↗

arXiv2011

Synthesis, crystal structure and magnetism of beta-Fe1.00(2)Se1.00(3) single crystals

Understanding iron based superconductors requires high quality impurity free single crystals. So far they have been elusive for beta-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic impurity phases. Herein we report synchrotron - clean beta-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect induced weak ferromagnetism that coexists with superconductivity below Tc. In contrast to Fe1+yTe - based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).

cond-mat.supr-con↗

arXiv2011

Coexistence of bulk superconductivity and charge density wave in CuxZrTe3

We report coexistence of bulk superconductivity and charge density wave (CDW) with superconducting critical temperature Tc = 3.8 K in Cu intercalated quasi-two-dimensional crystals of ZrTe3. The Cu intercalation results in the expansion of the unit cell orthogonal to the Zr-Zr metal chains (b - axis) and partial filling of CDW energy bandgap without obvious shift of CDW transition temperature. b - axis resistivity ?Rhob is not related to CDW, and its dominant scattering mechanism for both ZrTe3 and Cu0.05ZrTe3 is the electron - electron Umklapp scattering.

cond-mat.supr-con↗