arXiv subjects

H. Chen

Publications and source records attributed to H. Chen.

At least 235 records · Page 13Linked to original sources

arXiv2009

Magnetism and superconductivity in single crystals $Eu_{1-x}Sr_xFe_{2-y}Co_{y}As_2$

We systematically studied the transport properties of single crystals of $Eu_{1-x}Sr_xFe_{2-y}$Co$_{y}As_2$. Co doping can suppress the spin-density wave (SDW) ordering and induces a superconducting transition, but a resistivity reentrance due to the antiferromagnetic ordering of $Eu^{2+}$ spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity. It is striking that the resistivity reentrance can be completely suppressed by external magnetic field (H) because a metamagnetic transition from antiferromagnetism to ferromagnetism for $Eu^{2+}$ spins is induced by magnetic field. Superconductivity without resistivity reentrance shows up by partial substitution of Eu$^{2+}$ with non-magnetic Sr$^{2+}$ to completely destroy the AFM ordering of $Eu^{2+}$ spins. These results suggest that the antiferromagnetism destroys the superconductivity, while the ferromagnetism can coexist with the superconductivity in the iron-based high-$T_c$ superconductors.

cond-mat.supr-con↗

arXiv2009

Magnetotransport properties in purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ Single Crystal

We have measured resistivity along the a, b and c axes of Li$_{0.9}$Mo$_6$O$_{17}$ single crystal. The anisotropy $ρ$$_c$ / $ρ$$_a$ and $ρ$$_c$ / $ρ$$_b$ is given, confirming the quasi-one-dimensionality of the compound. The sharp decrease in the anisotropy below a certain temperature (T$_M$) indicates dimensional crossover. Superconductivity occurs at 1.8 K well below $T_M$. Negative MRs are observed with magnetic field (H) applied along b axis. This could be ascribed to suppression of energy gap associated with CDW state. While large positive MR is observed with H $\parallel$ c-axis. The MR data can be well fitted by a modified two-band model which has been used in CDW compounds such as quasi-two-dimensional purple bronzes A$_{0.9}$Mo$_6$O$_{17}$ (A = K, Na, Tl) and quasi-one-dimensional conductor NbSe$_3$. The behavior of MR provides a strong evidence for the existence of CDW instability in $Li_{0.9}Mo_6O_{17}$.

cond-mat.str-el↗

arXiv2009

From Shock Breakout to Peak and Beyond: Extensive Panchromatic Observations of the Type Ib Supernova 2008D associated with Swift X-ray Transient 080109

We present extensive early photometric (ultraviolet through near-infrared) and spectroscopic (optical and near-infrared) data on supernova (SN) 2008D as well as X-ray data analysis on the associated Swift/X-ray transient (XRT) 080109. Our data span a time range of 5 hours before the detection of the X-ray transient to 150 days after its detection, and detailed analysis allowed us to derive constraints on the nature of the SN and its progenitor; throughout we draw comparisons with results presented in the literature and find several key aspects that differ. We show that the X-ray spectrum of XRT 080109 can be fit equally well by an absorbed power law or a superposition of about equal parts of both power law and blackbody. Our data first established that SN 2008D is a spectroscopically normal SN Ib (i.e., showing conspicuous He lines), and show that SN 2008D had a relatively long rise time of 18 days and a modest optical peak luminosity. The early-time light curves of the SN are dominated by a cooling stellar envelope (for Δt~0.1- 4 day, most pronounced in the blue bands) followed by 56^Ni decay. We construct a reliable measurement of the bolometric output for this stripped-envelope SN, and, combined with estimates of E_K and M_ej from the literature, estimate the stellar radius R_star of its probable Wolf-Rayet progenitor. According to the model of Waxman et al. and of Chevalier & Fransson, we derive R_star^{W07}= 1.2+/-0.7 R_sun and R_star^{CF08}= 12+/-7 R_sun, respectively; the latter being more in line with typical WN stars. Spectra obtained at 3 and 4 months after maximum light show double-peaked oxygen lines that we associate with departures from spherical symmetry, as has been suggested for the inner ejecta of a number of SN Ib cores.

astro-ph↗

arXiv2009

Structure and Physical properties of New layered oxypnictides $Sr_4Sc_2O_6M_2As_2$ (M=Fe and Co)

We have successfully prepared the new layered oxypnictides $Sr_4Sc_2O_6M_2As_2$ (M=Fe and Co). They adopt the tetragonal structure, being the same as that of $Sr_4Sc_2O_6Fe_2P_2$. The lattice constants are a=0.4045 nm and c= 0.5802 nm for M=Fe, and a=0.4045 nm and c=1.5695 nm for M=Co, respectively. Their transport and magnetic properties have been systematically studied. The temperature dependence of Hall coefficient and thermoelectric powder for $Sr_4Sc_2O_6Fe_2As_2$ compound show complicated behavior, similar to that of iron-based parent compounds LnOFeAs and $BaFe_2As_2$. It suggests that the $Sr_4Sc_2O_6Fe_2As_2$ could be considered a new parent compound as iron-based superconductors.

cond-mat.supr-con↗

arXiv2009

A Large Iron Isotope Effect in SmFeAsO1-xFx and Ba1-xKxFe2As2

The recent discovery of superconductivity in oxypnictides with the critical temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer (BCS) theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, while strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. However, superconductivity and magnetism in pnictide superconductors show a strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW) transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to 0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows the same effect on TSDW as TCc These results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for understanding the mechanism of high-TC superconductivity.

cond-mat.supr-con↗

arXiv2009

Superconductivity at 56 K in Samarium-doped SrFeAsF

We synthesized the samples Sr$_{1-x}$Sm$_x$FFeAs with ZrCuSiAs-type structure. These samples were characterized by resistivity and susceptibility. It is found that substitution of rare earth metal for alkaline earth metal in this system suppresses the anomaly in resistivity and induces superconductivity. Superconductivity at 56 K in nominal composition Sr$_{0.5}$Sm$_{0.5}$FFeAs is realized, indicating that the superconducting transition temperatures in the iron arsenide fluorides can reach as high as that in oxypnictides with the same structure.

cond-mat.supr-con↗

arXiv2009

Anisotropy in the electrical resistivity and susceptibility of superconducting BaFe$_{2}$As$_{2}$ single crystals

Sizable single crystals of $BaFe_2As_2$ have been grown with self-flux method. The crystals are plate-like with c-axis perpendicular to the plane. The size can be as large as 3 x 5 x 0.2 $mm^3$. The resistivity anisotropy ($ρ_c/ρ_{ab}$) is as large as about 150, and independent of temperature. The transport in ab plane and along c-axis direction shares the same scattering mechanism. In contrast to the magnetic behavior of polycrystalline samples, no Curie-Weiss behavior are observed, a temperature linear dependent susceptibility occurs above spin-density-wave (SDW) transition. The susceptibility behavior is very similar to that of antiferromagnetic SDW chromium. Magnetic behavior of single crystal definitely gives evidence for existence of local moment except for the contribution to susceptibility from itinerant electrons. A resistivity minimum strongly dependent on magnetic field is observed. A log(1/T) divergency, similar to that of the underdoped cuprates, happens at low temperature. Here we first present intrinsic transport and magnetic properties, and their anisotropy from high quality single crystal.

cond-mat.supr-con↗

arXiv2009

High-field phase-diagram of Fe arsenide superconductors

Here, we report an overview of the phase diagram of single layered and double layered Fe arsenide superconductors at high magnetic fields. Our systematic magnetotransport measurements of polycrystalline SmFeAsO$_{1-x}$F$_x$ at different doping levels confirm the upward curvature of the upper critical magnetic field $H_{c2}(T)$ as a function of temperature $T$ defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single and double layered compounds. In all compounds explored by us the zero temperature upper critical field $H_{c2}(0)$, estimated either through the Ginzburg-Landau or the Werthamer-Helfand-Hohenberg single gap theories, strongly surpasses the weak coupling Pauli paramagnetic limiting field. This clearly indicates the strong coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses $γ= (m_c/m_{ab})^{1/2}$ for carriers moving along the c-axis and the ab planes, respectively, is relatively modest as compared to the high-$T_c$ cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field $H_m(T)$, separating the vortex-solid from the vortex-liquid phase in the single layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor.

cond-mat.supr-con↗

arXiv2009

New Views of EIT Wave and CME from STEREO

On 2007 December 7, a small filament located in a small active region AR 10977 erupted and led to a B1.4 flare. An EIT wave associated with this eruption was observed both by SOHO/EIT and by EUVI on board STEREO. According to the observations from SOHO/LASCO and STEREO/COR A, we found that there was no CME associated with the EIT wave. This seems to challenge the argument that the cause of EIT waves is CME. However the data from STEREO/COR B indicated that there was a narrow CME associated with the EIT wave. This suggests that studying CMEs by investigating observations made in one direction alone may not be able to guarantee the reliability of the results.

astro-ph.SR↗

arXiv2009

Continuum Free-Energy formulation for a class of Lattice Boltzmann multiphase models

It is shown that the Shan-Chen (SC) model for non-ideal lattice fluids can be made compliant with a pseudo free-energy principle by simple addition of a gradient force, whose expression is uniquely specified in terms of the fluid density. This additional term is numerically shown to provide fairly negligible effects on the system evolution during phase-separation. To the best of our knowledge, these important properties of the SC model were not noted before. The approach developed in the present work is based on a continuum analysis: further extensions, more in line with a discrete lattice theory (X. Shan, {\it Phys Rev E}, {\bf 77} 066702 (2008)) can be envisaged for the future.

physics.comp-ph↗

arXiv2009

Probing multi-band superconductivity and magnetism in SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $τ(θ)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $γ$ extracted from $τ(θ)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $γ$ depends on both $T$ and $H$, saturating at $γ\simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes.

cond-mat.supr-con↗

arXiv2009

Lattice Boltzmann method with self-consistent thermo-hydrodynamic equilibria

Lattice kinetic equations incorporating the effects of external/internal force fields via a shift of the local fields in the local equilibria, are placed within the framework of continuum kinetic theory. The mathematical treatment reveals that, in order to be consistent with the correct thermo-hydrodynamical description, temperature must also be shifted, besides momentum. New perspectives for the formulation of thermo-hydrodynamic lattice kinetic models of non-ideal fluids are then envisaged. It is also shown that on the lattice, the definition of the macroscopic temperature requires the inclusion of new terms directly related to discrete effects. The theoretical treatment is tested against a controlled case with a non ideal equation of state.

physics.flu-dyn↗

arXiv2008

Superconductivity induced by Ni doping in BaFe$_2$As$_2$

A series of 122 phase BaFe$_{2-x}$Ni$_x$As$_2$ ($x$ = 0, 0.055, 0.096, 0.18, 0.23) single crystals were grown by self flux method and a dome-like Ni doping dependence of superconducting transition temperature is discovered. The transition temperature $T_c^{on}$ reaches a maximum of 20.5 K at $x$ = 0.096, and it drops to below 4 K as $x$ $\geq$ 0.23. The negative thermopower in the normal state indicates that electron-like charge carrier indeed dominates in this system. This Ni-doped system provides another example of superconductivity induced by electron doping in the 122 phase.

cond-mat.supr-con↗

arXiv2008

Abnormal T-linear susceptibility and Phase diagram of BaFe$_{2-x}$Co$_x$As$_2$ single crystals

We study systematically transport, susceptibility and heat capacity for BaFe$_{2-x}$Co$_x$As$_2$ single crystals. In the underdoped region, spin density wave (SDW) transition is observed in both resistivity and susceptibility. The magnetic susceptibility shows unusual T-linear dependence above SDW transition up to 700 K. With Co doping, SDW ordering is gradually suppressed and superconductivity emerges with a dome-like shape. Electrical transport, specific heat and magnetic susceptibility indicate that SDW and superconductivity coexist in the sample BaFe$_{2-x}$Co$_x$As$_2$ around x = 0.17, being similar with (Ba,K)Fe$_2$As$_2$. When x$>$0.34, the superconductivity completely disappears. A crossover from non-Fermi-liquid state to Fermi-liquid state is observed with increasing Co doping. A detailed electronic phase diagram about evolution from SDW to superconducting state is given.

cond-mat.supr-con↗

arXiv2008

Transport properties and superconductivity in K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ (Ln=Sm, Nd and La) system

We prepared the samples K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ (Ln=Sm, Nd and La) with ThCr$_2$Si$_2$-type structure. These samples were characterized by X-ray diffraction, resistivity, susceptibility and thermoelectric power (TEP). Substitution of Ln (Ln=La, Nd and Sm) for K in K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system raises the superconducting transition temperature to 34-36 K. The TEP measurements indicate that the TEP of K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ is positive, being similar to the case of the Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system with p-type carrier. In the K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system, the superconducting $KFe_2As_2$ with $T_c\sim 3$ K is the parent compound, and no structural and spin-density wave instabilities exist in this system.

cond-mat.supr-con↗

arXiv2008

Evidence for Local Moment by Electron Spin Resonance Study on Polycrystalline LaFeAsO$_{1-x}$F$_x$ (x=0 and 0.13)

The temperature dependence of electron spin resonance (ESR) was studied in the oxypnictide superconductors LaFeAsO$_{1-x}$F$_x$ (x = 0.0 and 0.13). In the samples, the ESR signal indicates that the g factor and peak-to-peak linewidth strongly depend on temperature, especially at low temperatures. It indicates a strong coupling picture with existence of local moment. The dependence mentioned above gradually attenuates, and tends to saturation around room-temperature. This behavior could be ascribed to "bottleneck" effect due to coupling of local moment and itinerant electron. In addition, a Curie-Weiss like behavior is also observed in temperature dependent integral intensity for the two samples. Our results strongly support the existence of local moments in these materials while its origin is still unclear. The results also indicate strong magnetic frustration in this system, and magnetic fluctuation mechanism for superconductivity is suggested.

cond-mat.supr-con↗

arXiv2008

Doping-Driven Collapse of the SDW Correlation Gap in SmFeAsO$_{1-x}$F$_{x}$

We report the Hall resistivity, $ρ_{xy}$ of polycrystalline SmFeAsO$_{1-x}$F$_{x}$ for four different fluorine concentrations from the onset of superconductivity through the collapse of the structural phase transition. For the two more highly-doped samples, $ρ_{xy}$ is linear in magnetic field up to 50 T with only weak temperature dependence, reminiscent of a simple Fermi liquid. For the lightly-doped samples with $x<0.15$, we find a low temperature regime characterized $ρ_{xy}(H)$ being both non-linear in magnetic field and strongly temperature dependent even though the Hall angle is small. The onset temperature for this non-linear regime is in the vicinity of the structural phase (SPT)/spin density wave (SDW) transitions. The temperature dependence of the Hall resistivity is consistent with a thermal activation of carriers across an energy gap. The evolution of the energy gap with doping is reported.

cond-mat.supr-con↗

arXiv2008

Rearrangement of Sodium ordering and its effect on physical properties in $Na_xCoO_2$ system

We systematically study Raman spectroscopy of cleaved Na$_x$CoO$_2$ single crystals with 0.37 $\leq$ x $\leq$ 0.80. The Raman shift of A$_{1g}$ mode is found to be linearly dependent on Na content, while the Raman shift of E$_{1g}$ mode has an abnormal shift to high frequency around x = 0.5. The abnormal shift is ascribed to the occurrence of Na rearrangement in O1 structure. Temperature dependent Raman spectrum for x = 0.56 sample shows that Na rearrangement transition from O1 structure to H1 structure occurs around 240 K. Electronic transport and susceptibility for the sample with $x=0.56$ show a response to the Na rearrangement transition from O1 to H1 structure, and that different Na ordering pattern causes distinct physical properties. These results give a direct evidence to proved Na ordering effect on physical properties of Co-O plane.

cond-mat.str-el↗