arXiv subjects

Z. Y. Li

Publications and source records attributed to Z. Y. Li.

26 records · Page 2Linked to original sources

arXiv2011

Crystal structure, physical properties and superconductivity in $A_{x}$Fe$_2$Se$_2$ single crystals

We studied the correlation among structure and transport properties and superconductivity in the different $A_x$Fe$_2$Se$_2$ single crystals ($A$ = K, Rb, and Cs). Two sets of (00$l$) reflections are observed in the X-ray single crystal diffraction patterns, and arise from the intrinsic inhomogeneous distribution of the intercalated alkali atoms. The occurrence of superconductivity is closely related to the {\sl c}-axis lattice constant, and the $A$ content is crucial to superconductivity. The hump observed in resistivity seems to be irrelevant to superconductivity. There exist many deficiencies within the FeSe layers in $A_x$Fe$_2$Se$_2$, while their $T_{\rm c}$ does not change so much. In this sense, superconductivity is robust to the Fe and Se vacancies. Very high resistivity in the normal state should arise from such defects in the conducting FeSe layers. $A_x$Fe$_2$Se$_2$ ($A$ = K, Rb, and Cs) single crystals show the same susceptibility behavior in the normal state, and no anomaly is observed in susceptibility at the hump temperature in resistivity. The clear jump in specific heat for Rb$_x$Fe$_2$Se$_2$ and K$_x$Fe$_2$Se$_2$ single crystals shows the good bulk superconductivity in these crystals.

cond-mat.supr-con↗

arXiv2011

Pressure effect on superconductivity of $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs)

We performed the high hydrostatic pressure resistivity measurements (up to 1.7 GPa) on the newly discovered superconductors $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs) single crystals. Two batches of single crystals $K_xFe_2Se_2$ with different transition temperatures ($T_c$) were used to study the effect of pressure. The $T_c$ of the first one gradually decreases with increasing pressure from 32.6 K at ambient pressure. While a dome-like behavior was observed for the crystal with $T_c=31.1$ K, and $T_c$ reaches its maximum value of 32.7 K at the pressure of 0.48 GPa. It indicates that there exists a optimal doping with maximum $T_c$ of 32.7K in $K_xFe_2Se_2$ system. The behavior of $T_c$ vs. pressure for $Cs_xFe_2Se_2$ also shows a dome-like behavior, and $T_c$ reaches its maximum value of 31.1 K at the pressure of 0.82 GPa. The hump observed in temperature dependence of resistivity for all the samples tends to shift to high temperature with increasing pressure. The resistivity hump could arise from the vacancy of Fe or Se.

cond-mat.supr-con↗

arXiv2010

Superconductivity at 32 K in single crystal Rb$_{0.78}$Fe$_2$Se$_{1.78}$}

We successfully grew the high-quality single crystal of Rb$_{0.78}$Fe$_2$Se$_{1.78}$, which shows sharp superconducting transition in magnetic susceptibility and electrical resistivity. Resistivity measurements show the onset superconducting transition ($T_{\rm c}$) at 32.1 K and zero resistivity at 30 K. From the low-temperature iso-magnetic-field magnetoresistance, large upper critical field $H_{\rm c2}$(0) has been estimated as high as 180 T for in-plane field and 59 T for out-of-plane field. The anisotropy $H^{ab}_{\rm c2}$(0)/$H^{c}_{\rm c2}$(0) is around 3.0, right lying between those observed in K$_x$Fe$_2$Se$_2$ and Cs$_x$Fe$_2$Se$_2$.

cond-mat.supr-con↗

arXiv2010

Density-functional studies of spin-orbit splitting in graphene on metals

Spin-orbit splitting in graphene on Ni, Au, or Ag (111) substrates was examined on the basis of density-functional theory. Graphene grown on the three metals was found to have Rashba splitting of a few or several tens of meV. The strong splitting obtained on Au or Ag substrates was mainly ascribed to effective hybridization of graphene $p_{z}$ state with Au or Ag $d_{z^{2}}$ states, rather than charge transfer as previously proposed. Our work provides theoretical understandings of the metal-induced Rashba effect in graphene.

cond-mat.mes-hall↗

arXiv2008

Chandra Observation of the Edge-on Spiral NGC 5775: Probing the Hot Galactic Disk/Halo Connection

We study the edge-on galaxy NGC 5775, utilizing a 58.2 ks {\sl Chandra} ACIS-S observation together with complementary {\sl HST} ACS, {\sl Spitzer} IRAC and other multi-wavelength data sets. This edge-on galaxy, with its disk-wide active star formation, is particularly well-suited for studying the disk/halo interaction on sub-galactic scales. We detect 27 discrete X-ray sources within the $D_{25}$ region of the galaxy, including an ultra-luminous source with a 0.3-7 keV luminosity of $\sim7\times10^{40}\rm ergs s^{-1}$. The source-removed diffuse X-ray emission shows several prominent extraplanar features, including a $\sim10\rm kpc$ diameter ``shell-like'' feature and a ``blob'' reaching a projected distance of $\sim25\rm kpc$ from the galactic disk. The bulk of the X-ray emission in the halo has a scale height of $\sim$1.5 kpc and can be characterized by a two-temperature optically thin thermal plasma with temperatures of $\sim$ 0.2 and 0.6 keV and a total 0.3-2 keV luminosity of $\sim3.5\times10^{39}\rm ergs s^{-1}$. The high-resolution, multi-wavelength data reveal the presence of several extraplanar features around the disk, which appear to be associated with the in-disk star formation. We suggest that hot gas produced with different levels of mass loading can have different temperatures, which may explain the characteristic temperatures of hot gas in the halo. We have obtained a sub-galactic scale X-ray-intensity-star formation relation, which is consistent with the integrated version in other star forming galaxies.

astro-ph↗

arXiv2003

Collapse of Magnetized Singular Isothermal Toroids: I. Non-Rotating Case

We study numerically the collapse of non-rotating, self-gravitating, magnetized, singular isothermal toroids characterized by sound speed, $a$, and level of magnetic to thermal support, $H_0$. In qualitative agreement with previous treatments by Galli & Shu and other workers, we find that the infalling material is deflected by the field lines towards the equatorial plane, creating a high-density, flattened structure -- a pseudodisk. The pseudodisk contracts dynamically in the radial direction, dragging the field lines threading it into a highly pinched configuration that resembles a split monopole. The oppositely directed field lines across the midplane and the large implied stresses may play a role in how magnetic flux is lost in the actual situation in the presence of finite resistivity or ambipolar diffusion. The infall rate into the central regions is given to 5% uncertainty by the formula, $\dot M = (1+H_0)a^3/G$, where $G$ is the universal gravitational constant, anticipated by semi-analytical studies of the self-similar gravitational collapses of the singular isothermal sphere and isopedically magnetized disks. The introduction of finite initial rotation results in a complex interplay between pseudodisk and true (Keplerian) disk formation that is examined in a companion paper.

astro-ph↗

arXiv2003

Collapse of Magnetized Singular Isothermal Toroids: II. Rotation and Magnetic Braking

We study numerically the collapse of rotating, magnetized molecular cloud cores, focusing on rotation and magnetic braking during the main accretion phase of isolated star formation. Motivated by previous numerical work and analytic considerations, we idealize the pre-collapse core as a magnetized singular isothermal toroid, with a constant rotational speed everywhere. The collapse starts from the center, and propagates outwards in an inside-out fashion, satisfying exact self-similarity in space and time. For rotation rates and field strengths typical of dense low-mass cores, the main feature remains the flattening of the mass distribution along field lines -- the formation of a pseudodisk, as in the nonrotating cases. The density distribution of the pseudodisk is little affected by rotation. On the other hand, the rotation rate is strongly modified by pseudodisk formation. Most of the centrally accreted material reaches the vicinity of the protostar through the pseudodisk. The specific angular momentum can be greatly reduced on the way, by an order of magnitude or more, even when the pre-collapse field strength is substantially below the critical value for dominant cloud support. The efficient magnetic braking is due to the pinched geometry of the magnetic field in the pseudodisk, which strengthens the magnetic field and lengthens the level arm for braking. Both effects enhance the magnetic transport of angular momentum from inside to outside. The excess angular momentum is carried away in a low-speed outflow that has, despite claims made by other workers, little in common with observed bipolar molecular outflows. We discuss the implications of our calculations for the formation of true disks that are supported against gravity by rotation.

astro-ph↗

arXiv1996

Unconventional Ferromagnetic Transition in La1-xCaxMnO3

Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La1-xCaxMnO3 in the ferromagnetic regime (0<=x<1/2). For x=1/3 (Tc=250K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (<0.04meV) dispersion relation E=Dq^2 and D(T=0)~170 meV-A-1. However, an anomalous strongly-field-dependent diffusive component develops above ~200K and dominates the fluctuation spectrum as T approaches Tc. This component is not present at lower x.

cond-mat↗