arXiv subjects

Zengwei Zhu

Publications and source records attributed to Zengwei Zhu.

99 records · Page 6Linked to original sources

arXiv2010

Phase diagram of bismuth in the extreme quantum limit

Elemental bismuth provides a rare opportunity to explore the fate of a three-dimensional gas of highly mobile electrons confined to their lowest Landau level. Coulomb interaction, neglected in the band picture, is expected to become significant in this extreme quantum limit with poorly understood consequences. Here, we present a study of the angular-dependent Nernst effect in bismuth, which establishes the existence of ultraquantum field scales on top of its complex single-particle spectrum. Each time a Landau level crosses the Fermi level, the Nernst response sharply peaks. All such peaks are resolved by the experiment and their complex angular-dependence is in very good agreement with the theory. Beyond the quantum limit, we resolve additional Nernst peaks signaling a cascade of additional Landau sub-levels caused by electron interaction.

cond-mat.str-el↗

arXiv2010

Comparative study on the thermoelectric effect of parent oxypnictides La$T$AsO ($T$ = Fe, Ni)

The thermopower and Nernst effect were investigated for undoped parent compounds LaFeAsO and LaNiAsO. Both thermopower and Nernst signal in iron-based LaFeAsO are significantly larger than those in nickel-based LaNiAsO. Furthermore, abrupt changes in both thermopower and Nernst effect are observed below the structural phase transition temperature and spin-density wave (SDW) type antiferromagnetic (AFM) order temperature in Fe-based LaFeAsO. On the other hand, Nernst effect is very small in the Ni-based LaNiAsO and it is weakly temperature-dependent, reminiscent of the case in normal metals. We suggest that the effect of SDW order on the spin scattering rate should play an important role in the anomalous temperature dependence of Hall effect and Nernst effect in LaFeAsO. The contrast behavior between the LaFeAsO and LaNiAsO systems implies that the LaFeAsO system is fundamentally different from the LaNiAsO system and this may provide clues to the mechanism of high $T_c$ superconductivity in the Fe-based systems.

cond-mat.supr-con↗

arXiv2009

Nernst effect and dimensionality in the quantum limit

Nernst effect, the transverse voltage generated by a longitudinal thermal gradient in presence of magnetic field has recently emerged as a very sensitive, yet poorly understood, probe of electron organization in solids. Here we report on an experiment on graphite, a macroscopic stack of graphene layers, which establishes a fundamental link between dimensionality of an electronic system and its Nernst response. In sharp contrast with single-layer graphene, the Nernst signal sharply peaks whenever a Landau level meets the Fermi level. This points to the degrees of freedom provided by finite interlayer coupling as a source of enhanced thermoelectric response in the vicinity of the quantum limit. Since Landau quantization slices a three-dimensional Fermi surface, each intersection of a Landau level with the Fermi level modifies the Fermi surface topology. According to our results, the most prominent signature of such a topological phase transition emerges in the transverse thermoelectric response.

cond-mat.str-el↗

arXiv2009

Anisotropic inelastic scattering and its interplay with superconductivity in URu$_{2}$Si$_{2}$

In contrast to almost all anisotropic superconductors, the upper critical field of URu$_{2}$Si$_{2}$ is larger when the field is oriented along the less conducting direction. We present a study of resistivity and Seebeck coefficient extended down to sub-Kelvin temperature range uncovering a singular case of anisotropy. When the current is injected along the c-axis URu$_{2}$Si$_{2}$ behaves as a low-density Fermi liquid. When it flows along the a-axis, even in presence of a large field, resistivity remains T-linear down to T$_{c}$ and the Seebeck coefficient undergoes a sign change at very low temperatures. We conclude that the characteristic energy scale is anisotropic and vanishingly small in the basal plane.

cond-mat.str-el↗

arXiv2009

Suppression of spin-density-wave transition and emergence of ferromagnetic ordering of Eu$^{2+}$ moments in EuFe$_{2-x}$Ni$_{x}$As$_{2}$

We present a systematic study on the physical properties of EuFe$_{2-x}$Ni$_{x}$As$_{2}$ (0$\leq$\emph{x}$\leq$0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe$_{2}$As$_{2}$ undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu$^{2+}$ moments. For $x\geq$0.06, the magnetic ordering of Eu$^{2+}$ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for $x\geq$0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe$_{2-x}$Ni$_{x}$As$_{2}$ system are discussed.

cond-mat.supr-con↗

arXiv2008

Metamagnetic transition in EuFe$_2$As$_2$ single crystals

We report the measurements of anisotropic magnetization and magnetoresistance on single crystals of EuFe$_2$As$_2$, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin-density-wave transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu$^{2+}$ spins ($J=S=7/2$). The latter AFM state exhibits metamagnetic transition under magnetic fields. Upon applying magnetic field with $H\parallel c$ at 2 K, the magnetization increases linearly to 7.0 $μ_{B}$/f.u. at $μ_{0}H$=1.7 T, then keeps at this value of saturated Eu$^{2+}$ moments under higher fields. In the case of $H\parallel ab$, the magnetization increases step-like to 6.6 $μ_{B}$/f.u. with small magnetic hysteresis. A metamagnetic phase was identified with the saturated moments of 4.4 $μ_{B}$/f.u. The metamagnetic transition accompanies with negative in-plane magnetoresistance, reflecting the influence of Eu$^{2+}$ moments ordering on the electrical conduction of FeAs layers. The results were explained in terms of spin-reorientation and spin-reversal based on an $A$-type AFM structure for Eu$^{2+}$ spins. The magnetic phase diagram has been established.

cond-mat.supr-con↗

arXiv2008

Antiferromagnetic transition in EuFe$_2$As$_2$: A possible parent compound for superconductors

Ternary iron arsenide EuFe$_2$As$_2$ with ThCr$_2$Si$_2$-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall and specific heat measurements. The compound undergoes two magnetic phase transitions at about 200 K and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an A-type antiferromagnetic ordering of Eu$^{2+}$ moments. Comparing with the physical properties of the iso-structural compounds BaFe$_2$As$_2$ and SrFe$_2$As$_2$, we expect that superconductivity could be induced in EuFe$_2$As$_2$ through appropriate doping.

cond-mat.supr-con↗

arXiv2008

Superconductivity induced by cobalt doping in iron-based oxyarsenides

Chemical doping has recently become a very important strategy to induce superconductivity especially in complex compounds. Distinguished examples include Ba-doped La$_2$CuO$_4$ (the first high temperature superconductor), K-doped BaBiO$_3$, K-doped C$_{60}$ and Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O. The most recent example is F-doped LaFeAsO, which leads to a new class of high temperature superconductors. One notes that all the above dopants are non-magnetic, because magnetic atoms generally break superconducting Cooper pairs. In addition, the doping site was out of the (super)conducting structural unit (layer or framework). Here we report that superconductivity was realized by doping magnetic element cobalt into the (super)conducting-active Fe$_2$As$_2$ layers in LaFe$_{1-x}$Co$_{x}$AsO. At surprisingly small Co-doping level of $x$=0.025, the antiferromagnetic spin-density-wave transition in the parent compound is completely suppressed, and superconductivity with $T_c\sim $ 10 K emerges. With increasing Co content, $T_c$ shows a maximum of 13 K at $x\sim 0.075$, and then drops to below 2 K at $x$=0.15. This result suggests essential differences between previous cuprate superconductor and the present iron-based arsenide one.

cond-mat.supr-con↗

arXiv2008

Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO

Following the discovery of superconductivity in an iron-based arsenide LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc was pushed up surprisingly to above 40 K by either applying pressure[2] or replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to 55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands for the lanthanide elements) at an apparently optimal doping level. However, the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+ in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice mismatch, hence induces the high temperature superconductivity.

cond-mat.supr-con↗