arXiv subjects

Zhiwei Wang

Publications and source records attributed to Zhiwei Wang.

294 records · Page 17Linked to original sources

arXiv2015

Revealing puddles of electrons and holes in compensated topological insulators

Three-dimensional topological insulators harbour metallic surface states with exotic properties. In transport or optics, these properties are typically masked by defect-induced bulk carriers. Compensation of donors and acceptors reduces the carrier density, but the bulk resistivity remains disappointingly small. We show that measurements of the optical conductivity in BiSbTeSe$_2$ pinpoint the presence of electron-hole puddles in the bulk at low temperatures, which is essential for understanding DC bulk transport. The puddles arise from large fluctuations of the Coulomb potential of donors and acceptors, even in the case of full compensation. Surprisingly, the number of carriers appearing within puddles drops rapidly with increasing temperature and almost vanishes around 40 K. Monte Carlo simulations show that a highly non-linear screening effect arising from thermally activated carriers destroys the puddles at a temperature scale set by the Coulomb interaction between neighbouring dopants, explaining the experimental observation semi-quantitatively. This mechanism remains valid if donors and acceptors do not compensate perfectly.

cond-mat.mes-hall↗

arXiv2015

Superconducting dome and microstructure properties of Rb0.8Fe1.6+xSe2 superconductors

High quality Rb0.8Fe1.6+xSe2 single crystals are grown by a one step melting growth method. Superconductivity has been observed in all samples with x more than 0.05 and the maximum critical temperature Tc~32 K has been obtained in samples with x from 0.1 to 0.2 from electronic resistivity measurement. A superconducting dome in the electronic phase diagram of Tc as a function of Fe content has been observed for the first time in this AxFeySe2 (A=K,Rb) superconducting system. Two coexisting phases with structural modulation vectors of q1=1/5 [3a*+b*] and q2=1/2[a*+b*] have been observed in all superconducting samples by TEM observations. Moreover, phase separation in the superconducting crystals, which can be recognized as micro-stripes in the ab plane, are found to be highly dependent on the Fe content.

cond-mat.supr-con↗

arXiv2015

Ferromagnetism in Cr-doped topological insulator TlSbTe2

We have synthesized a new ferromagnetic topological insulator by doping Cr to the ternary topological-insulator material TlSbTe2. Single crystals of Tl_{1-x}Cr_{x}SbTe2 were grown by a melting method and it was found that Cr can be incorporated into the TlSbTe2 matrix only within the solubility limit of about 1%. The Curie temperature θ_c was found to increase with the Cr content but remained relatively low, with the maximum value of about 4 K. The easy axis was identified to be the c-axis and the saturation moment was 2.8 μ_B (Bohr magneton) at 1.8 K. The in-plane resistivity of all the samples studied showed metallic behavior with p-type carriers. Shubnikov-de Hass (SdH) oscillations were observed in samples with the Cr-doping level of up to 0.76%. We also tried to induce ferromagnetism in TlBiTe2 by doping Cr, but no ferromagnetism was observed in Cr-doped TlBiTe2 crystals within the solubility limit of Cr which turned out to be also about 1%.

cond-mat.mtrl-sci↗

arXiv2014

Bergman kernels and eigenvalue estimate of $\bar{\partial}$-laplacian

Let $(X,ω)$ be a compact Kähler manifold. Let $(L,h)$ be a hermitian holomorphic line bundle over $X$, such that $Θ_{L,h}\geq -\varepsilonω$ for a small $\varepsilon>0$, $E$ be a holomorphic line bundle over $X$. For $k\in \mathbb{N}_+$, denote by $X_k:=(X,ω^k)$ the Kähler manifold $X$ with new scaled metric $ω^k=kω$. Estimates of the number of eigenvalues smaller than $λ$ of the $\debar$-Laplacian on forms on $X_k$ with values in $L^k\otimes E$ are presented for $0\leq λ<k$. In particular, when $λ=0$, we get a numeric bound for the cohomology groups.

math.CV↗

arXiv2014

Strong Coupling of the Iron-Quadrupole and Anion-Dipole Polarizations in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We use a quantitative convergent beam electron diffraction (CBED) based method to image the valence electron density distribution in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We show a remarkable increase in both the charge quadrupole of the Fe cations and the charge dipole of the arsenic anions upon Co doping from $x=0$ ($T_c=0$ K) to $x=0.1$ ($T_c=22.5$ K). Our data suggest that an unexpected electronic correlation effect, namely strong coupling of Fe orbital fluctuation and anion electronic polarization, is present in iron-based superconductors.

cond-mat.supr-con↗

arXiv2010

Free-standing Graphene as Gold Standard

We demonstrate using graphene sheets as a novel mass standard in the scanning transmission electron microscopy (STEM) based mass spectrometry. Here, free-standing graphene sheets are investigated by STEM. The discrete number of graphene layers enables an accurate calibration of STEM intensity, as performed over an extended thickness and with single atomic layer sensitivity. This leads to the direct and accurate measurement of the electron mean free path. As an application, we gain the insight of carbon nanoparticles of complex structure.

cond-mat.mes-hall↗