arXiv subjects

Kefeng Wang

Publications and source records attributed to Kefeng Wang.

53 records · Page 3Linked to original sources

arXiv2015

Breakdown of compensation and persistence of non-saturating magnetoresistance in WTe2 thin flakes

We present a detailed study of magnetoresistance \r{ho}xx(H), Hall effect \r{ho}xy(H), and electrolyte gating effect in thin (<100 nm) exfoliated crystals of WTe2. We observe quantum oscillations in H of both \r{ho}xx(H) and \r{ho}xy(H), and identify four oscillation frequencies consistent with previous reports in thick crystals. \r{ho}xy(H) is linear in H at low H consistent with near-perfect electron-hole compensation, however becomes nonlinear and changes sign with increasing H, implying a breakdown of compensation. A field-dependent ratio of carrier concentrations p/n can consistently explain \r{ho}xx(H) and \r{ho}xy(H) within a two-fluid model. We also employ an electrolytic gate to highly electron-dope WTe2 with Li. The non-saturating \r{ho}xx(H) persists to H = 14 T with magnetoresistance ratio exceeding 2 x 104 %, even with significant deviation from perfect electron-hole compensation (p/n = 0.84), where the two-fluid model predicts a saturating \r{ho}xx(H). Our results suggest electron-hole compensation is not the mechanism for extremely large magnetoresistance in WTe2, other alternative explanations need to be considered.

cond-mat.mtrl-sci↗

arXiv2015

Topological $R$PdBi half-Heusler semimetals: a new family of non-centrosymmetric magnetic superconductors

We report superconductivity and magnetism in a new family of topological semimetals, the ternary half Heusler compounds $R$PdBi ($R$ : rare earth). In this series, tuning of the rare earth $f$-electron component allows for simultaneous control of both lattice density via lanthanide contraction, as well as the strength of magnetic interaction via de Gennes scaling, allowing for a unique tuning of both the normal state band inversion strength, superconducting pairing and magnetically ordered ground states. Antiferromagnetism with ordering vector (0.5,0.5,0.5) occurs below a Neéel temperature that scales with de Gennes factor $dG$, while a superconducting transition is simultaneously linearly suppressed. With superconductivity appearing in a system with non-centrosymmetric crystallographic symmetry, the possibility of spin-triplet Cooper pairing with non-trivial topology analogous to that predicted for the normal state electronic structure provides a unique and rich opportunity to realize both predicted and new exotic excitations in topological materials.

cond-mat.supr-con↗

arXiv2014

Anisotropic giant magnetoresistance in NbSb2

The extremely large transverse magnetoreistance (the magnetoresistant ratio $\sim 1.3\times10^5\%$ in 2 K and 9 T field, and $4.3\times 10^6\%$ in 0.4 K and 32 T field, without saturation), and the metal-semiconductor crossover induced by magnetic field, are reported in NbSb$_2$ single crystal with electric current parallel to the $b$-axis. The metal-semiconductor crossover is preserved when the current is along the $ac$-plane but the magnetoresistant ratio is significantly suppressed. The sign reversal of the Hall resistivity in the field close to the crossover point, and the electronic structure calculation reveals the coexistence of a small number of holes with very high mobility and a large number of electrons with low mobility. These effects are attributed to the change of the Fermi surface induced by the magnetic field.

cond-mat.mtrl-sci↗

arXiv2014

Nonmetallic Low-Temperature Normal State of K0.70Fe1.46Se1.85Te0.15

The normal-state in-plane resistivity below the zero-field superconducting transition temperature $T_c$ and the upper critical field Hc2 were measured by suppressing superconductivity in pulsed magnetic fields for K0.70Fe1.46Se1.85Te0.15. The normal-state resistivity $ρ_{ab}$ is found to increase logarithmically with decrasing temperature as $\frac{T}{T_c}\rightarrow 0$. Similar to granular metals, our results suggest that a superconductor - insulator transition below zero-field T$_{c}$ may be induced in high magnetic fields. This is related to the intrinsic real-space phase-separated states common to all inhomogeneous superconductors.

cond-mat.supr-con↗

arXiv2014

$^{119}$Sn-NMR investigations on superconducting Ca$_3$Ir$_4$Sn$_{13}$: Evidence for multigap superconductivity

We report bulk superconductivity (SC) in Ca$_3$Ir$_4$Sn$_{13}$ by means of $^{119}$Sn nuclear magnetic resonance (NMR) experiments. Two classical signatures of BCS superconductivity in spin-lattice relaxation rate ($1/T_1$), namely the Hebel-Slichter coherence peak just below the $T_c$ and the exponential decay in the superconducting phase, are evident. The noticeable decrease of $^{119}$Sn Knight shift below $T_c$ indicates spin-singlet superconductivity. The temperature dependence of the spin-lattice relaxation rate ($1/T_1$) is convincingly described by the multigap isotropic superconducting gap. Present NMR experiments do not witness any sign of enhanced spin fluctuations in the normal state.

cond-mat.supr-con↗

arXiv2014

Superconducting and magnetic properties of Sr3Ir4Sn13

Magnetization and muon spin relaxation or rotation (muSR) measurements have been performed to study the superconducting and magnetic properties of Sr3Ir4Sn13. From magnetization measurements the lower and upper critical fields of Sr3Ir4Sn13 are found to be 81(1) Oe and 14.4(2) kOe, respectively. Zero-field muSR data show no sign of any magnetic ordering or weak magnetism in Sr3Ir4Sn13. Transverse-field muSR measurements in the vortex state provided the temperature dependence of the magnetic penetration depth. The dependence of penetration depth with temperature is consistent with the existence of single s-wave energy gap in the superconducting state of Sr3Ir4Sn13 with a gap value of 0.82(2) meV at absolute zero temperature. The magnetic penetration depth at zero temperature is 291(3) nm. The gap to Tc ratio is 2.1(1), indicates that Sr3Ir4Sn13 should be considered as a strong-coupling superconductor.

cond-mat.supr-con↗

arXiv2013

New layered fluorosulfide SrFBiS2

We have synthesized a new layered BiS$_{2}$-based compound SrFBiS2. This compound has similar structure to BiS2. It is built up by stacking up SrF layers and NaCl-type BiS2 layers alternatively along the c axis. Electric transport measurement indicates that SrFBiS2 is a semiconductor. Thermal transport measurement shows that SrFBiS2 has a small thermal conductivity and large Seebeck coefficient. First principle calculations are in agreement with experimental results and show that SrFBiS2 is very similar to LaOBiS2 which becomes superconductor with F doping. Therefore, SrFBiS2 may be a parent compound of new superconductors.

cond-mat.supr-con↗

arXiv2013

Superconductivity in Ir1-xRhxTe2 (0<=x<=0.3)

We report superconductivity, physical and structural properties of Ir1-xRhxTe2 (0<=x<=0.3). Superconducting transition with maximum Tc~2.6 K appears when the doping content of Rh x is in between 0.15 and 0.3. Further increasing the content of Rh will suppress the superconductivity. On the other hand, the high-temperature structural transition is suppressed gradually as Rh is incorporated into the lattice, eventually vanishing at x=0.2. These results imply competing relationship between structural transition and superconductivity. The isovalent ionic substitution of Rh into Ir has different effects on physical properties when compared to the isovalent anionic substitution of Se into Te, in which the structural transition is enhanced with Se substitution. It indicates the changes of structural parameters such as ionic size and electronegativity of elements will also have important effects on the evolution of physical properties in IrTe2.

cond-mat.supr-con↗

arXiv2012

Physical properties of KxNi2-ySe2 single crystals

We have synthesized K0.95(1)Ni1.86(2)Se2 single crystals. The single crystals contain K and Ni deficiencies not observed in KNi2Se2 polycrystals. Unlike KNi2Se2 polycrystals, the superconductivity is absent in single crystals. The detailed physical property study indicates that the K0.95Ni1.86Se2 single crystals exhibit Fermi liquid behavior with heavy-fermion-like characteristics. Transition from the high temperature local charge density wave state to heavy fermion state below T ~ 30 K results in an enhancement of electron-like carrier density whereas magnetic susceptibility shows little anisotropy and suggests presence of both itinerant and localized Ni orbitals.

cond-mat.supr-con↗

arXiv2012

Large magnetothermopower effect in Dirac materials (Sr/Ca)MnBi2

We report temperature and magnetic field dependence of the thermal transport properties in single crystals of (Sr/Ca)MnBi$_2$ with linear energy dispersion. In SrMnBi$_2$ thermopower is positive, indicating hole-type carriers and the magnetic field enhances the thermopower significantly. The maximum change of thermopower is about 1600% in 9 T field and at 10 K. A negative thermopower is observed in CaMnBi$_2$ with dominant electron-type carriers and, in contrast, the magnetic field suppresses the absolute value of thermopower. First-principle band structure shows that the chemical potential is close to the Dirac-cone-like points in linear bands. The magnetic field suppresses the apparent Hall carrier density of CaMnBi$_2$ below 50 K. The large magnetothermopower effect in (Sr/Ca)MnBi$_2$ is attributed to the magnetic field shift of chemical potential

cond-mat.mtrl-sci↗

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

Anharmonic Phonons and Magnons in BiFeO3

The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO$_3$ were measured for temperatures $200 \leq T \leq 750\,$K, using inelastic neutron scattering (INS). Our results indicate that the magnetic spectrum of BiFeO$_3$ closely resembles that of similar Fe perovskites, such as LaFeO$_3$, despite the cycloid modulation in BiFeO$_3$. We do not find any evidence for a spin gap. A strong $T$-dependence of the phonon DOS was found, with a marked broadening of the whole spectrum, providing evidence of strong anharmonicity. This anharmonicity is corroborated by large-amplitude motions of Bi and O ions observed with neutron diffraction. These results highlight the importance of spin-phonon coupling in this material.

cond-mat.mtrl-sci↗

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

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

Evolution of correlation strength in KxFe(2-y)Se2 superconductor doped with S

We report the evolution of thermal transport properties of iron-based superconductor K$_x$Fe$_{2-y}$Se$_2$ with sulfur substitution at Se sites. Sulfur doping suppresses the superconducting $T_c$ as well as the Seebeck coefficient. The Seebeck coefficient of all crystals in the low temperature range can be described very well by diffusive thermoelectric response model. The zero-temperature extrapolated value of Seebeck coefficient divided by temperature $S/T$ gradually decreases from $-0.48 μV/K^2$ to a very small value $\sim$ 0.03 $μ$V/K$^2$ where $T_c$ is completely suppressed. The normal state electron Sommerfeld term ($γ_n$) of specific heat also decreases with the increase of sulfur content. The dcrease of $S/T$ and $γ_n$ reflects a suppression of the density of states at the Fermi energy, or a change in the Fermi surface that would induce the suppression of correlation strength.

cond-mat.supr-con↗

arXiv2011

Thermoelectric studies of KxFe2-ySe2: weakly correlated superconductor

We report thermal transport measurement of KxFe2-ySe2 superconducting single crystal. Significant peak anomaly in thermal conductivity is observed at nearly TC/2 indicating a large phonon mean-free-path in the superconducting state. The zero-temperature extrapolated thermoelectric power is smaller than the value in typical strongly correlated superconductors, implying large normalized Fermi temperature. In contrast to other iron superconductors, thermoelectric power in our sample does not exhibit significant anomalies. These findings indicate that KxFe2-ySe2 is a weakly or intermediately correlated superconductor without significant Fermi surface nesting.

cond-mat.supr-con↗

arXiv2011

Significant suppression of thermal conductivity in FeSb2 by Te doping

Kondo insulator like material FeSb2 was found to exhibit colossal Seebeck coefficient. It would have had huge potential in thermoelectric applications in cryogenic temperature range if it had not been for the large thermal conductivity. Here we studied the influence of Te doping at Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. Surprisingly, only 5% Te doping suppresses thermal conductivity by two orders of magnitude, which may be attributed to the substitution disorder. Te doping also results in transition from an semiconductor to a metal. Consequently thermoelectric figure of merit (ZT ? 0:05) in Fe(Sb0:9Te0:1)2 at ? 100K was enhanced by about one order of magnitude when compared to ZT < 0:005 in undoped FeSb2.

cond-mat.mtrl-sci↗