arXiv subjects

Huaixin Yang

Publications and source records attributed to Huaixin Yang.

30 records · Page 2Linked to original sources

arXiv2019

Direct observations of chiral spin textures in van der Waals magnet Fe3GeTe2 nanolayers

In two-dimensional van der Waals (vdW) magnets, the presence of magnetic orders, strong spin-orbit coupling and asymmetry at interfaces is the key ingredient for hosting chiral spin textures. However, experimental evidences for chiral magnetism in vdW magnets remain elusive. Here we demonstrate unambiguously the formation of chiral spin textures in thin Fe3GeTe2 nanoflakes using advanced magnetic electron microscopy and first-principles calculations. Specifically, electron holography analyses reveal the spin configurations of Néel-type, zero-field-stabilized skyrmions in 20-nm-thick Fe3GeTe2 nanoflakes at cryogenic temperature. In situ Lorentz transmission electron microscopy measurements further provide detailed magnetic phase diagrams of chiral spin textures including spirals and skyrmions in Fe3GeTe2 as a function of temperature, applied magnetic field and specimen thickness. First-principles calculations unveil a finite interfacial Dzyaloshinskii-Moriya interaction in the Te/Fe3Ge/Te slabs that induces the spin chirality in Fe3GeTe2. Our discovery of spin chirality in the prototypical vdW Fe3GeTe2 opens up new opportunities for studying chiral magnetism in two-dimensional vdW magnets from both fundamental and applied perspectives.

cond-mat.mtrl-sci↗

arXiv2019

Evolution of Incommensurate Superstructure and Electronic Structure with Pb Substitution in (Bi$_{2-x}$Pb$_{x}$)Sr$_2$CaCu$_2$O$_{8+δ}$ Superconductors

High-quality Bi$_{2-x}$Pb$_{x}$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) single crystals have been successfully grown by the traveling solvent floating zone technique with a wide range of Pb substitution ($x=0-0.8$). The samples are characterized by transmission electron microscope (TEM) and measured by high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) with different photon energies. A systematic evolution of the electronic structure and superstructure with Pb substitution has been revealed for the first time. The superstructure shows a significant change with Pb substitution and the incommensurate modulation vector ($\textbf{Q}$) decreases with increasing Pb substitution. In the meantime, the superstructure intensity from ARPES measurements also decreases dramatically with increasing Pb concentration. The superstructure in Bi2212 can be effectively suppressed by Pb substitution and it nearly disappears with a Pb substitution of $x=0.8$. We also find that the superstructure bands in ARPES measurements depend sensitively on the photon energy of lasers used; they can become even stronger than the main band when using a laser photon energy of 10.897 eV. These results provide important information on the origin of the incommensurate superstructure and its control and suppression in bismuth-based high temperature superconductors.

cond-mat.supr-con↗

arXiv2019

High-throughput investigation of tunable superconductivity in FeSe films

There is an ongoing debate about the relative importance of structural change versus doping charge carriers on the mechanism of superconductivity in Fe-based materials. Elucidating this issue is a major challenge since it would require a large number of samples where structure properties or the carrier density is systematically varied. FeSe, with its structural simplicity, is an ideal platform for addressing this question. It has been demonstrated that the superconductivity in this material can be controlled through crystal lattice tuning, as well as electronic structure manipulation. Here, we apply a high-throughput methodology to FeSe to systematically delineate the interdependence of its structural and electronic properties. Using a dual-beam pulsed laser deposition, we have generated FeSe films with a marked gradient in the superconducting transition temperature (below 2 K < Tc < 12 K) across 1 cm width of the films. The Tc gradient films display ~ 1% continuous stretch and compression in the out-of-plane and in-plane lattice constants respectively, triggering the continuous enhancement of superconductivity. Combining transport and angular-resolved photoemission measurements on uniform FeSe films with tunable Tc from 3 K to 14 K, we find that the electron carrier density is intimately correlated with Tc, i.e., it increases by a factor of 6 and ultimately surpasses the almost constant hole concentration. Our transmission electron microscope and band structure calculations reveal that rather than by shifting the chemical potential, the enhanced superconductivity is linked to the selective adjustment of the dxy band dispersion across the Fermi level by means of reduced local lattice distortions. Therefore, such novel mechanism provides a key to understand discrete superconducting phases in FeSe.

cond-mat.supr-con↗

arXiv2017

An Anomalous Circular Photogalvanic Effect in the Weyl Semimetal TaAs

Weyl semimetal (WSM) is expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking. A photocurrent generation in noncentrosymmetric WSM was also predicted owing to its broken inversion symmetry and linear energy dispersion which are unique to Weyl systems. In our recent measurements, the circular photogalvanic effect (CPGE) has been demonstrated in WSM of TaAs. The CPGE voltage is proportional to the helicity of the incident light and reverses its direction on changing the radiation helicity from left handed to right handed, a periodical oscillation therefore appears following with the alteration of optical polarization. We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone, which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin selection rule.

cond-mat.str-el↗

arXiv2017

Built-in Homojunction Dominated Intrinsically Rectifying-Resistive Switching in NiO Nanodots for Selection Device-Free Memory Application

The intrinsically rectifying-resistive switching (IR-RS) has been regarded as an effective way to address the crosstalk issue, due to the Schottky diodes formed at the metal/oxide interfaces in the ON states to suppress the sneak current at reverse biases. In this letter, we report for the first time another type of IR-RS that is related to the built-in homojunction. The IR-RS study was usually limited to macroscopic samples with micron-order pad-type electrodes, while this work is on NiO nanodots fabricated with ultrathin anodic-aluminum-oxide templates and acting as nanoscaled analogs of real devices. The NiO nanodots show high storage density and high uniformity, and the IR-RS behaviors are of good device performances in terms of retention, endurance, switching ratio and rectification ratio. The feasibility of the IR-RS for selection device-free memory application has been demonstrated, by calculating the maximum crossbar array size under the worst-case scenario to be 3 Mbit.

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↗

arXiv2014

Phase diagram of (Li1-xFex)OHFeSe: a bridge between iron selenide and arsenide superconductors

Previous experimental results have shown important differences between iron selenide and arsenide superconductors, which seem to suggest that the high temperature superconductivity in these two subgroups of iron-based family may arise from different electronic ground states. Here, we report the complete phase diagram of a newly synthesized superconducting (SC) system (Li1-xFex)OHFeSe with a similar structure to FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ~127 K. This is the first example to demonstrate such an SDW phase in FeSe-based superconductor system. Transmission electron microscopy (TEM) shows that a well-known sqrr(5) x sqrr(5) iron vacancy ordered state, resulting in an AFM order at ~ 500K in AyFe2-xSe2 (A = metal ions) superconductor system, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q=1/2(1, 1, 0), identical to that seen in SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

cond-mat.supr-con↗

arXiv2014

Clocking the anisotropic lattice dynamics of multi-walled carbon nanotubes by four-dimensional ultrafast transmission electron microscopy

Recent advances in the four-dimensional ultrafast transmission electron microscope (4D-UTEM) with combined spatial and temporal resolutions have made it possible to directly visualize structural dynamics of materials at the atomic level. Herein, we report on our development on a 4D-UTEM which can be operated properly on either the photo-emission or the thermionic mode. We demonstrate its ability to obtain sequences of snapshots with high spatial and temporal resolutions in the study of lattice dynamics of the multi-walled carbon nanotubes (MWCNTs). This investigation provides an atomic level description of remarkable anisotropic lattice dynamics at the picosecond timescales. Moreover, our UTEM measurements clearly reveal that distinguishable lattice relaxations appear in intra-tubular sheets on an ultrafast timescale of a few picoseconds and after then an evident lattice expansion along the radical direction. These anisotropic behaviors in the MWCNTs are considered arising from the variety of chemical bonding, i.e. the weak van der Waals bonding between the tubular planes and the strong covalent sp2-hybridized bonds in the tubular sheets.

cond-mat.mes-hall↗

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↗

arXiv2011

Superconductivity at 33 K in potassium-doped 1,2:8,9-dibenzopentacene

We report the observation of superconductivity at 33 K in K-doped 1,2:8,9-dibenzopentacene (C30H18). This is higher than any Tc reported previously for an organic superconductor besides the alkali-metal doped C60. This finding provides an indication that superconductivity at much higher temperature may be possible in such system, if PAHs with longer benzene rings can be synthesized.

cond-mat.supr-con↗

arXiv2011

Aharonov-Casher effect in Bi$_{\rm 2}$Se$_{\rm 3}$ square-ring interferometers

Electrical control of spin dynamics in Bi$_{\rm 2}$Se$_{\rm 3}$ was investigated in ring-type interferometers. Aharonov-Bohm and Altshuler-Aronov-Spivak resistance oscillations against magnetic field, and Aharorov-Casher resistance oscillations against gate voltage were observed in the presence of a Berry phase of $π$. A very large tunability of spin precession angle by gate voltage has been obtained, indicating that Bi$_{\rm 2}$Se$_{\rm 3}$-related materials with strong spin-orbit coupling are promising candidates for constructing novel spintronic devices.

cond-mat.mes-hall↗

arXiv2008

Electron energy-loss spectroscopy and ab initio electronic structure of the LaOFeP superconductor

The electronic band structures of the LaOFeP superconductor have been calculated theoretically by the first principles method and measured experimentally by electron energy loss spectroscopy. The calculations indicate that the Fe atom in LaOFeP crystal shows a weak magnetic moment and does not form a long-range magnetic ordering. Band structure, Fermi surfaces and fluorine-doping effects are also analyzed based on the data of the density functional theory. The fine structures of the EELS data have been carefully examined in both the low loss energy region and the core losses region (O K, Fe L2,3, and La M4,5). A slight bump edge at 44 eV shows notable orientation-dependence: it can be observed in the low loss EELS spectra with q parallel to c, but becomes almost invisible in the q vertical to c spectra. Annealing experiments indicate that low oxygen pressure favors the appearance of superconductivity in LaOFeP, this fact is also confirmed by the changes of Fe L2,3 and O K excitation edges in the experimental EELS data.

cond-mat.supr-con↗