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Fei Han

Publications and source records attributed to Fei Han.

106 records · Page 6Linked to original sources
arXiv2009

Structural and transport properties of Sr2VO{3-delta}FeAs superconductors with different oxygen deficiencies

Sr2VO{3-delta}FeAs superconductors with different oxygen deficiencies have been successfully fabricated. It is found that the superconducting transition temperature drops down monotonically with the increase of oxygen deficiency. The diminishing of superconductivity is accompanied by the enhancement of residual resistivity, indicating an unraveled scattering effect induced by the oxygen deficiency. The highest superconducting transition temperature at about 40 K is achieved near the stoichiometrical sample Sr$_2$VO$_{3}$FeAs. Surprisingly, the X-ray photoelectron spectroscopy (XPS) shows that the vanadium has a "5+" valence state in the samples. The Hall effect measurements reveal that the density of charge carriers (electron-like here) varies qualitatively with the increase of oxygen deficiency. Magnetotransport measurements show that the superconducting transition changes from one-step-like shape at low fields to two-step-like one at high fields, indicating a high anisotropy.

cond-mat.supr-con↗
arXiv2009

Superconductivity at 15.6 K in Calcium-doped Tb_{1-x}Ca_xFeAsO: the structure requirement for achieving superconductivity in the hole-doped 1111 phase

Superconductivity at about 15.6 K was achieved in Tb_{1-x}Ca_xFeAsO by partially substituting Tb^{3+} with Ca^{2+} in the nominal doping region x = 0.40 \sim 0.50. A detailed investigation was carried out in a typical sample with doping level of x = 0.44. The upper critical field of this sample was estimated to be 77 Tesla from the magnetic field dependent resistivity data. The domination of hole-like charge carriers in the low-temperature region was confirmed by Hall effect measurements. The comparison between the calcium-doped sample Pr_{1-x}Ca_xFeAsO (non-superconductive) and the Strontium-doped sample Pr_{1-x}Sr_xFeAsO (superconductive) suggests that a lager ion radius of the doped alkaline-earth element compared with that of the rare-earth element may be a necessary requirement for achieving superconductivity in the hole-doped 1111 phase.

cond-mat.supr-con↗
arXiv2009

Transition of stoichiometricSr2VO3FeAs to a superconducting state at 37.2 K

The superconductor Sr4V2O6Fe2As2 with transition temperature at 37.2 K has been fabricated. It has a layered structure with the space group of p4/nmm, and with the lattice constants a = 3.9296Aand c = 15.6732A. The observed large diamagnetization signal and zero-resistance demonstrated the bulk superconductivity. The broadening of resistive transition was measured under different magnetic fields leading to the discovery of a rather high upper critical field. The results also suggest a large vortex liquid region which reflects high anisotropy of the system. The Hall effect measurements revealed dominantly electron-like charge carriers in this material. The superconductivity in the present system may be induced by oxygen deficiency or the multiple valence states of vanadium.

cond-mat.supr-con↗
arXiv2009

Superconductivity in Ti-doped Iron-Arsenide Compound Sr4Cr0.8Ti1.2O6Fe2As2

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr4Cr0.8Ti1.2O6Fe2As2 (abbreviated as Cr-FeAs-42622). The x-ray diffraction measurement shows that this material has a layered structure with the space group of \emph{P4/nmm}, and with the lattice constants a = b = 3.9003 A and c = 15.8376 A. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase, or due to some other minority phase.

cond-mat.supr-con↗
arXiv2009

Superconductivity at 22.3 K in SrFe2-xIrxAs2

By substituting the Fe with the 5d-transition metal Ir in SrFe2As2, we have successfully synthesized the superconductor SrFe2-xIrxAs2 with Tc = 22.3 K at x = 0.5. X-ray diffraction indicates that the material has formed the ThCr2Si2-type structure with a space group I4/mmm. The temperature dependence of resistivity and dc magnetization both reveal sharp superconducting transitions at around 22 K. An estimate on the diamagnetization signal reveals a high Meissner shielding volume. Interestingly, the normal state resistivity exhibits a roughly linear behavior up to 300 K. The superconducting transitions at different magnetic fields were also measured yielding a slope of -dHc2/dT = 3.8 T/K near Tc. Using the Werthamer-Helfand-Hohenberg (WHH) formula, the upper critical field at zero K is found to be about 58 T. Counting the possible number of electrons doped into the system in SrFe2-xIrxAs2, we argue that the superconductivity in the Ir-doped system is different from the Co-doped case, which should add more ingredients to the underlying physics of the iron pnictide superconductors.

cond-mat.supr-con↗
arXiv2009

Synthesis, structural and transport properties of the hole-doped Superconductor Pr_{1-x}Sr_xFeAsO

Superconductivity was achieved in PrFeAsO by partially substituting Pr^{3+} with Sr^{2+}. The electrical transport properties and structure of this new superconductor Pr_{1-x}Sr_xFeAsO at different doping levels (x = 0.05$\sim$ 0.25) were investigated systematically. It was found that the lattice constants (a-axis and c-axis) increase monotonously with Sr or hole concentration. The superconducting transition temperature at about 16.3 K (95% $ρ_n$) was observed around the doping level of 0.20$\sim$ 0.25. A detailed investigation was carried out in the sample with doping level of x = 0.25. The domination of hole-like charge carriers in this material was confirmed by Hall effect measurements. The magnetoresistance (MR) behavior can be well described by a simple two-band model. The upper critical field of the sample with T_c = 16.3 K (x = 0.25) was estimated to be beyond 45 Tesla. Our results suggest that the hole-doped samples may have higher upper critical fields comparing to the electron-doped ones, due to the higher quasi-particle density of states at the Fermi level.

cond-mat.supr-con↗
arXiv2008

(Sr_3Sc_2O_5)Fe_2As_2 as a possible parent compound for FeAs-based superconductors

A new compound with the FeAs-layers, namely (Sr_3Sc_2O_5)Fe_2As_2 (abbreviated as FeAs-32522), was successfully fabricated. It has a layered structure with the space group of I4/mmm, and with the lattice constants a = 4.069 $Å$ and c = 26.876 $Å$. The in-plane Fe ions construct a square lattice which is close to that of other FeAs-based superconductors, such as REFeAsO (RE = rare earth elements) and (Ba,Sr)Fe_2As_2. However the inter FeAs-layer spacing in the new compound is greatly enlarged. The temperature dependence of resistivity exhibits a weak upturn in the low temperature region, but a metallic behavior was observed above about 60 K. The magnetic susceptibility shows also a non-monotonic behavior. Interestingly, the well-known resistivity anomaly which was discovered in all other parent compounds, such as REFeAsO, (Ba,Sr)Fe_2As_2 and (Sr,Ca,Eu)FeAsF and associated with the Spin-Density-Wave (SDW)/structural transition has not been found in the new system either on the resistivity data or the magnetization data. This could be induced by the large spacing distance between the FeAs-planes, therefore the antiferromagnetic correlation between the moments of Fe ions in neighboring FeAs-layers cannot be established. Alternatively it can also be attributed to the self-doping effect between Fe and Sc ions. The Hall coefficient R_H is negative but strongly temperature dependent in wide temperature region, which indicates the dominance of electrical conduction by electron-like charge carriers and probably a multi-band effect or a spin related scattering effect. It is found that the magnetoresistance cannot be described by the Kohler's rule, which gives further support to above arguments.

cond-mat.supr-con↗
arXiv2008

High-T_c superconductivity induced by doping rare earth elements into CaFeAsF

We have successfully synthesized the fluoride-arsenide compounds Ca$_{1-x}$RE$_x$FeAsF (RE=Nd, Pr; x=0, 0.6). The x-ray powder diffraction confirmed that the main phases of our samples are Ca$_{1-x}$RE$_x$FeAsF with the ZrCuSiAs structure. By measuring resistivity, superconductivity was observed at 57.4 K in Nd-doped and 52.8 K in Pr-doped samples with x=0.6. Bulk superconductivity was also proved by the DC magnetization measurements in both samples. Hall effect measurements revealed hole-like charge carriers in the parent compound CaFeAsF with a clear resistivity anomaly below 118 K, while the Hall coefficient $R_H$ in the normal state is negative for the superconducting samples Ca$_{0.4}$Nd$_{0.6}$FeAsF and Ca$_{0.4}$Pr$_{0.6}$FeAsF. This indicates that the rare earth element doping introduces electrons into CaFeAsF which induces the high temperature superconductivity.

cond-mat.supr-con↗
arXiv2008

Superconductivity in Fluorine-Arsenide Sr_{1-x}La_xFeAsF

Since the discovery of superconductivity\cite{1} at 26 K in oxy-pnictide $LaFeAsO_{1-x}F_x$, enormous interests have been stimulated in the fields of condensed matter physics and material sciences. Among the five different structures in this broad type of superconductors\cite{2,3,4,5,6}, the ZrCuSiAs structure has received special attention since the $T_c$ has been quickly promoted to 55-56 K\cite{7,8,9,10,11} in fluorine doped oxy-pnictides REFeAsO (RE = rare earth elements). The superconductivity can also be induced by applying a high pressure to the undoped samples\cite{12,13}. The mechanism of superconductivity in the FeAs-based system remains unclear yet, but it turns out to be clear that any change to the structure or the building blocks will lead to a change of the superconducting transition temperatures. In this Letter, we report the fabrication of the new family of compounds, namely fluorine-arsenides DvFeAsF (Dv = divalent metals) with the ZrCuSiAs structure and with the new building block DvF instead of the REO (both the layers DvF and REO have the combined cation state of "+1"). The undoped parent phase has a Spin-Density-Wave like transition at about 173 K for SrFeAsF, 118 K for CaFeAsF and 153 K for EuFeAsF. By doping electrons into the system the resistivity anomaly associated with this SDW transition is suppressed and superconductivity appears at 32 K in the fluorine-arsenide Sr$_{1-x}$La$_x$FeAsF (x = 0.4). Our discovery here initiates a new method to obtain superconductors in the FeAs-based system.

cond-mat.supr-con↗
arXiv2008

SrFeAsF as a parent compound for iron pnictide superconductors

We have successfully synthesized the fluo-arsenide SrFeAsF, a new parent phase with the ZrCuAsSi structure. The temperature dependence of resistivity and dc magnetization both reveal an anomaly at about T_{an} = 173 K, which may correspond to the structural and/or Spin-Density-Wave (SDW) transition. Strong Hall effect and magnetoresistance were observed below T_{an}. Interestingly, the Hall coefficient R_H is positive below T_{an}, which is opposite to the cases in the two parent phases of FeAs-based systems known so far, i.e., LnFeAsO (Ln = rare earth elements) and (Ba, Sr)Fe_2As_2 where the Hall coefficient R_H is negative. This strongly suggests that the gapping to the Fermi surfaces induced by the SDW order is more complex than we believed before that it removes the density of states on some Fermi pockets and leaves one of the electron pockets less-gapped or un-gapped. Our data clearly show that it is possible for the parent phase to have electron-like or hole-like charge carriers.

cond-mat.supr-con↗
arXiv2008

Supersymmetric QFT, Super Loop Spaces and Bismut-Chern Character

In this paper, we give a quantum interpretation of the Bismut-Chern character form (the loop space lifting of the Chern character form) as well as the Chern character form associated to a complex vector bundle with connection over a smooth manifold in the framework of supersymmetric quantum field theories developed by Stolz and Teichner \cite{ST07}. We show that the Bismut-Chern character form comes up via a loop-deloop process when one goes from $1|1$D theory over a manifold down to a $0|1$D theory over its free loop space. Based on our quantum interpretation of the Bismut-Chern character form and Chern character form, we construct Chern character type maps for SUSY QFTs.

math.DG↗
arXiv2007

Elliptic Genera, Transgression and Loop Space Chern-Simons Forms

We compute the Chern-Simons transgressed forms of some modularly invariant characteristic forms, which are related to the elliptic genera. We study the modularity properties of these secondary characteristic forms and the relations among them. We also compute the Chern-Simons forms of some vector bundles over free loop space.

math.DG↗
arXiv2007

Witten Genus and String Complete Intersections

In this note, we prove that the Witten genus of nonsingular string complete intersections in product of complex projective spaces vanishes. Our result generalizes a known result of Landweber and Stong (cf. [HBJ]).

math.DG↗
arXiv2006

Modular Invariance and Twisted Anomaly Cancellations of Characteristic Numbers

By studying modular invariance properties of some characteristic forms, we obtain twisted anomaly cancellation formulas. We apply these twisted cancellation formulas to study divisibilities on spin manifolds and congruences on spin$^c$ manifolds. Especially, we get twisted Rokhlin congruences for $8k+4$ dimensional spin$^c$ manifolds

math.DG↗
arXiv2003

Modular invariance, characteristic numbers and $η$ invariants

We generalize the "miraculous cancellation" formulas of Alvarez-Gaumé, Witten and Kefeng Liu to a twisted version where an extra complex line bundle is involved. We also apply our result to discuss intrinsic relations between the higher dimensional Rokhlin type congruence formulas of Ochanine [O1], Finashin [F] and Zhang [Z1, 2]. In particular, an analytic proof of the Finashin congruence formula is given.

math.DG↗