arXiv subjects

Z. A. Ren

Publications and source records attributed to Z. A. Ren.

At least 37 records · Page 2Linked to original sources

arXiv2008

Superconductivity in heavily boron-doped silicon carbide

The discoveries of superconductivity in heavily boron-doped diamond (C:B) in 2004 and silicon (Si:B) in 2006 renew the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily-boron doped silicon carbide (SiC:B). The sample used for that study consists of cubic and hexagonal SiC phase fractions and hence this lead to the question which of them participates in the superconductivity. Here we focus on a sample which mainly consists of hexagonal SiC without any indication for the cubic modification by means of x-ray diffraction, resistivity, and ac susceptibility.

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arXiv2008

Combined microstructural and magneto optical study of current flow in polycrystalline forms of Nd and Sm Fe-oxypnictides

In order to understand why the inter- and intra-granular current densities of polycrystalline superconducting oxypnictides differ by three orders of magnitude, we have conducted combined magneto-optical and microstructural examinations of representative randomly oriented polycrystalline Nd and Sm single-layer oxypnictides. Magneto optical images show that the highest Jc values are observed within single grains oriented with their c axes perpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lower intergranular Jc is at least partially due to many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are found at many grain boundaries. However, some grain boundaries are structurally clean under high resolution transmission electron microscopy examination. Because the whole-sample global Jc(5K) values of the two samples examined are 1000-4000 A/cm2, some 10-40 times that found in random, polycrystalline YBa2Cu3O7-x, it appears that the dominant obstruction to intergranular current flow of many present samples is extrinsic, though some intrinsic limitation of current flow across grain boundaries cannot yet be ruled out.

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arXiv2008

Lattice and Magnetic structures of PrFeAsO, PrFeAsO0.85F0.15 and PrFeAsO0.85

We use powder neutron diffraction to study the spin and lattice structures of polycrystalline samples of nonsuperconducting PrFeAsO and superconducting PrFeAsO0.85F0.15 and PrFeAsO0.85. We find that PrFeAsO exhibits an abrupt structural phase transitions at 153 K, followed by static long range antiferromagnetic order at 127 K. Both the structural distortion and magnetic order are identical to other rare-earth oxypnictides. Electron-doping the system with either Fluorine or oxygen deficiency suppresses the structural distortion and static long range antiferromagnetic order, therefore placing these materials into the same class of FeAs-based superconductors.

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arXiv2008

Comparative High Field Magneto-Transport of Rare Earth Oxypnictides with Maximum Transition Temperatures

The recent discovery of a new class of superconducting oxypnictides with high transition temperatures may have profound implications for understanding unconventional high-temperature superconductivity. Like the cuprates, the oxypnictides seem to manifest an interleaving of charge donor and superconducting layers emerging upon doping of an antiferromagnetic parent semi-metal. Here we report magneto-transport measurements of three rare earth (Re = La, Nd, Sm) oxypnicide compounds with the transition temperatures near the maximum reported to date, in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Our resistivity, Hall coefficient and critical magnetic fields data suggest that these oxypnictide superconductors bridge the gap between MgB$_2$ and YBaCu$_3$O$_{7-x}$ as far as electromagnetic and vortex properties are concerned.

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arXiv2008

Structural properties, defects and structural phase transition in the ROFeM (R=La, Nd; M=As, P) materials

The structural properties of the ROFeM (R=La, Nd; M=As, P) materials have been analyzed by means of electron diffraction, high-resolution transmission-electron microscopy (TEM) and in-situ cooling TEM observations. The experimental results demonstrate that the layered ROFeM crystals often contain a variety of structural defects, such as stacking faults and small-angle boundaries. The in-situ TEM investigations reveal that, in association with the remarkable spin-density-wave (SDW) instability near 150 K, complex structural transitions can be clearly observed in both crystal symmetry and local microstructure features.

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arXiv2008

Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides

Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either wide-spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium rare-earth iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. Assuming that the highest current density loops are restricted to circulation only within grains leads to values of ~5 MA/cm2 at 5 K and self field, while whole-sample current densities, though two orders of magnitude lower are 1000-10000 A/cm2, some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain significant second phase, some of which wets the grain boundaries and produces evidences of SNS proximity effect in the whole sample critical current.

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arXiv2008

Common Features in Electronic Structure of the Fe-Based Layered Superconductors from Photoemission Spectroscopy

High resolution photoemission measurements have been carried out on non-superconducting LaOFeAs parent compound and various superconducting R(O1-xFx)FeAs (R=La, Ce and Pr) compounds. We found that the parent LaOFeAs compound shows a metallic character. Through extensive measurements, we have identified several common features in the electronic structure of these Fe-based compounds: (1). 0.2 eV feature in the valence band; (2). A universal 13~16 meV feature; (3). A clear Fermi cutoff showing zero leading-edge shift in the superconducting state;(4). Lack of superconducting coherence peak(s); (5). Near EF spectral weight suppression with decreasing temperature. These universal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials.

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arXiv2008

Spin-singlet superconductivity with multiple gaps in PrO0.89F0.11FeAs

Since the discovery of high transition-temperature (Tc) superconductivity in copper oxides two decades ago, continuous efforts have been devoted to searching for similar phenomenon in other compounds. With the exception of MgB2 (Tc =39 K), however, Tc is generally far lower than desired. Recently, breakthrough has been made in a new class of oxypnictide compounds. Following the initial discovery of superconductivity in LaO1-x FxFeAs (Tc =26 K), Tc onset has been raised to 55 K in ReO1-xFxFeAs (Re: Ce, Pr, Nd, Sm). Meanwhile, unravelling the nature of the energy associated with the formation of current-carrying pairs (Cooper pairs), referred to as the superconducting energy gap, is the first and vital step towards understanding why the superconductivity occurs at such high temperature and is also important for finding superconductors with still higher Tc. Here we show that, on the basis of the nuclear magnetic resonance (NMR) measurements in PrO0.89F0.11FeAs (Tc =45 K), the Cooper pair is in the spin-singlet state (two spins are anti-paralleled), with two energy gaps opening below Tc. The results strongly suggest the existence of nodes (zeros) in the gap. None of superconductors known to date has such unique gap features, although copper-oxides and MgB2 share part of them.

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arXiv2008

Can antiferromagnetism and superconductivity coexist in the high-field paramagnetic superconductor Nd(O,F)FeAs?

We present measurements of the temperature and field dependencies of the magnetization M(T,H) of Nd(O0.89F0.11)FeAs at fields up to 33T, which show that superconductivity with the critical temperature Tc ~ 51K cannot coexist with antiferromagnetic ordering. Although M(T,H) at 55 < T < 140K exhibits a clear Curie-Weiss temperature dependence corresponding to the Neel temperature TN ~ 11-12K, the behavior of M(T,H) below Tc is only consistent with either paramagnetism of weakly interacting magnetic moments or a spin glass state. We suggest that the anomalous magnetic behavior of an unusual high-field paramagnetic superconductor Nd(O1-xFx)FeAs is mostly determined by the magnetic Nd ions.

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arXiv2008

Superconducting Gap and Pseudogap in Sm(O$_{1-x}$F$_x$)FeAs Layered Superconductor from Photoemission Spectroscopy

High resolution photoemission measurements have been carried out on non-superconducting SmOFeAs parent compound and superconducting Sm(O$_{1-x}$F$_x$)FeAs (x=0.12, and 0.15) compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state which suggests the Fermi surface sheet(s) around the $Γ$ point may not be gapped in this multiband superconductors. A robust feature at 13 meV is identified in all these samples. Spectral weight suppression near E$_F$ with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.

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arXiv2008

Pressure Effect on the superconducting properties of LaO_{1-x}F_{x}FeAs(x=0.11) superconductor

Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing low T_c superconducting phase toward the main (optimal) superconducting phase.

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arXiv2005

Evidence for s-wave pairing from measurement on lower critical field in $MgCNi_3$

Magnetization measurements in the low field region have been carefully performed on a well-shaped cylindrical and an ellipsoidal sample of superconductor $MgCNi_3$. Data from both samples show almost the same results. The lower critical field $H_{c1}$ and the London penetration depth $λ$ are thus derived. It is found that the result of normalized superfluid density $λ^2(0)/λ^2(T)$ of $MgCNi_3$ can be well described by BCS prediction with the expectation for an isotropic s-wave superconductivity.

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arXiv2005

Competition between the BCS superconductivity and ferromagnetic spin fluctuations in MgCNi$_3$

The low temperature specific heat of the superconductor MgCNi$_3$ and a non-superconductor MgC$_{0.85}$Ni$_3$ is investigated in detail. An additional contribution is observed from the data of MgCNi$_3$ but absent in MgC$_{0.85}$Ni$_3$, which is demonstrated to be insensitive to the applied magnetic field even up to 12 Tesla. A detailed discussion on its origin is then presented. By subtracting this additional contribution, the zero field specific heat of MgCNi$_3$ can be well described by the BCS theory with the gap ratio ($Δ/k_BT_c$) determined by the previous tunneling measurements. The conventional s-wave pairing state is further proved by the magnetic field dependence of the specific heat at low temperatures and the behavior of the upper critical field.

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arXiv2003

$Fe_{0.5}Cu_{0.5}(Ba_{1-x}Sr_{x})_2YCu_2O_{7+δ}$ ($x$ = 0, 0.5 and 1) superconductors prepared by high pressure synthesis

In this paper, the $Fe$-containing superconductors $Fe_{0.5}Cu_{0.5}Ba_2YCu_2O_{7+δ}$, $Fe_{0.5}Cu_{0.5}BaSrYCu_2O_{7+δ}$ and $Fe_{0.5}Cu_{0.5}Sr_2YCu_2O_{7+δ}$ were successfully prepared by common solid-state reaction followed with a procedure of high pressure synthesis. The structural change and superconducting properties in $(Fe_xCu_{1-x})BaSrYCu_2O_{7+δ}$ ($x$ = 0 $\sim$ 1.0) systems were also investigated. Annealing experiments indicate that the occurrence of superconductivity in $Fe_{0.5}Cu_{0.5}(Ba_{1-x}Sr_{x})_2YCu_2O_{7+δ}$ ($x$ = 0, 0.5 and 1) systems is mainly induced by the procedure of high pressure synthesis, which causes the increase of oxygen content and the redistribution of $Fe$ atoms between $Cu(1)$ and $Cu(2)$ sites, but not from possible secondary phase of $YBa_2Cu_3O_{7-δ}$, $YBaSrCu_3O_{7-δ}$ or $YSr_2Cu_3O_{7-δ}$ superconductors.

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arXiv2003

The mechanical relaxation study of polycrystalline MgCNi3

The mechanical relaxation spectra of a superconducting and a non-superconducting MgCNi3 samples were measured from liquid nitrogen temperature to room temperature at frequency of kilohertz. There are two internal friction peaks (at 300 K labeled as P1 and 125 K as P2) for the superconducting sample. For the non-superconducting one, the position of P1 shifts to 250 K, while P2 is almost completely depressed. It is found that the peak position of P2 shifts towards higher temperature under higher measuring frequency. The calculated activation energy is 0.13eV. We propose an explanation relating P2 to the carbon atom jumping among the off-center positions. And further we expect that the behaviors of carbon atoms maybe correspond to the normal state crossovers around 150 K and 50 K observed by many other experiments.

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arXiv2003

Influence of Carbon Concentration on the Superconductivity in MgCxNi3

The influence of carbon concentration on the superconductivity (SC) in MgC$_{x}$Ni$_3$ has been investigated by measuring the low temperature specific heat combined with first principles electronic structure calculation. It is found that the specific heat coefficient $γ_n=C_{en}/T$ of the superconducting sample ($x\approx1$) in normal state is twice that of the non-superconducting one ($x\approx 0.85$). The comparison of measured $γ_n$ and the calculated electronic density of states (DOS) shows that the effective mass renormalization changes remarkably as the carbon concentration changes. The large mass renormalization for the superconducting sample and the low $T_{c}$(7K) indicate that more than one kind of boson mediated electron-electron interactions exist in MgC$_{x}$Ni$_3$.

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arXiv2002

Superconducting Properties of Atomic-Disordered Compound MgCNi_3

The effect of radiation-induced disordering in a nuclear reactor (fast neutrons fluence Φ= 5\cdot10^{19} cm^2, T_{\text{irr}} = 340 K) on resistivity ρ, superconducting transition temperature T_C and upper critical field H_{C_2} of polycrystalline MgCNi_3 samples was investigated. It was found that T_C decreases under irradiation from 6.5 to 2.9 K and completely recovers after annealing at 600 ^\circC. Temperature dependences ρ(T) are characteristic of compounds with strong electron-phonon interaction. The dH_{C_2}/dT behaviour testifies to a considerable decrease in density of electronic state at Fermi level N(E_F) in the course of disordering.

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arXiv2002

Superconducting properties and c-axis superstructure of Mg1-xAlxB2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover,transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is essentially related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2 by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy.

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