arXiv subjects

X. H. Chen

Publications and source records attributed to X. H. Chen.

At least 55 records · Page 3Linked to original sources

arXiv2016

Background Evaluation for the Neutron Sources in the Daya Bay Experiment

We present an evaluation of the background induced by $^{241}$Am-$^{13}$C neutron calibration sources in the Daya Bay reactor neutrino experiment. As a significant background for electron-antineutrino detection at 0.26$\pm$0.12 per detector per day on average, it has been estimated by a Monte Carlo simulation that was benchmarked by a special calibration data set. This dedicated data set also provided the energy spectrum of the background.

physics.ins-det↗

arXiv2016

Asteroseismology of the $δ$ Scuti star HD 50844

Aims. We aim to probe the internal structure and investigate more detailed information of the $δ$ Scuti star HD 50844 with asteroseismology. Methods. We analyse the observed frequencies of the $δ$ Scuti star HD 50844 obtained by Balona (2014), and search for possible multiplets based on the rotational splitting law of g-mode. We tried to disentangle the frequency spectra of HD 50844 by means of the rotational splitting only. We then compare them with theoretical pulsation modes, which correspond to stellar evolutionary models with various sets of initial metallicity and stellar mass, to find the best-fitting model. Results. There are three multiplets including two complete triplets and one incomplete quintuplet, in which mode identifications for spherical harmonic degree $l$ and azimuthal number $m$ are unique. The corresponding rotational period of HD 50844 is found to be 2.44$^{+0.13}_{-0.08}$ days. The physical parameters of HD 50844 are well limited in a small region by three modes identified as nonradial ones ($f_{11}$, $f_{22}$, and $f_{29}$) and by the fundamental radial mode ($f_{4}$). Our results show that the three nonradial modes ($f_{11}$, $f_{22}$, and $f_{29}$) are all mixed modes, which mainly represent the property of the helium core. The fundamental radial mode ($f_{4}$) mainly represents the property of the stellar envelope. In order to fit these four pulsation modes, both the helium core and the stellar envelope must be matched to the actual structure of HD 50844. Finally, the mass of the helium core of HD 50844 is estimated to be 0.173 $\pm$ 0.004 $M_{\odot}$ for the first time. The physical parameters of HD 50844 are determined to be $M=$ 1.81 $\pm$ 0.01 $M_{\odot}$, $Z=$ 0.008 $\pm$ 0.001. $T_{\rm eff}=$ 7508 $\pm$ 125 K, log$g=$ 3.658 $\pm$ 0.004, $R=$ 3.300 $\pm$ 0.023 $R_{\odot}$, $L=$ 30.98 $\pm$ 2.39 $L_{\odot}$.

astro-ph.SR↗

arXiv2016

Bulk Fermi surface of charge-neutral excitations in SmB6 or not: a heat-transport study

Recently there have been increasingly hot debates on whether a bulk Fermi surface of charge-neutral excitations exists in the topological Kondo insulator SmB6. To unambiguously resolve this issue, we performed the low-temperature thermal conductivity measurements of a high-quality SmB6 single crystal down to 0.1 K and up to 14.5 T. Our experiments show that the residual linear term of thermal conductivity at zero field is zero, within the experimental accuracy. Furthermore, the thermal conductivity is insensitive to magnetic field up to 14.5 T. These results exclude the existence of fermionic charge-neutral excitations in bulk SmB6, such as scalar Majorana fermions or spinons, thus put a strong constraint on the explanation of the quantum oscillations observed in SmB6.

cond-mat.str-el↗

arXiv2016

The Detector System of The Daya Bay Reactor Neutrino Experiment

The Daya Bay experiment was the first to report simultaneous measurements of reactor antineutrinos at multiple baselines leading to the discovery of $\barν_e$ oscillations over km-baselines. Subsequent data has provided the world's most precise measurement of $\rm{sin}^22θ_{13}$ and the effective mass splitting $Δm_{ee}^2$. The experiment is located in Daya Bay, China where the cluster of six nuclear reactors is among the world's most prolific sources of electron antineutrinos. Multiple antineutrino detectors are deployed in three underground water pools at different distances from the reactor cores to search for deviations in the antineutrino rate and energy spectrum due to neutrino mixing. Instrumented with photomultiplier tubes (PMTs), the water pools serve as shielding against natural radioactivity from the surrounding rock and provide efficient muon tagging. Arrays of resistive plate chambers over the top of each pool provide additional muon detection. The antineutrino detectors were specifically designed for measurements of the antineutrino flux with minimal systematic uncertainty. Relative detector efficiencies between the near and far detectors are known to better than 0.2%. With the unblinding of the final two detectors' baselines and target masses, a complete description and comparison of the eight antineutrino detectors can now be presented. This paper describes the Daya Bay detector systems, consisting of eight antineutrino detectors in three instrumented water pools in three underground halls, and their operation through the first year of eight detector data-taking.

physics.ins-det↗

arXiv2015

Pressure-induced Lifshitz transition in black phosphorus

In a semimetal, both electron and hole carriers contribute to the density of states at the Fermi level. The small band overlaps and multi-band effects give rise to many novel electronic properties, such as relativistic Dirac fermions with linear dispersion, titanic magnetoresistance and unconventional superconductivity. Black phosphorus has recently emerged as an exceptional semiconductor with high carrier mobility and a direct, tunable bandgap. Of particular importance is the search for exotic electronic states in black phosphorus, which may amplify the material's potential beyond semiconductor devices. Here we show that a moderate hydrostatic pressure effectively suppresses the band gap and induces a Lifshitz transition from semiconductor to semimetal in black phosphorus; a colossal magnetoresistance is observed in the semimetallic phase. Quantum oscillations in high magnetic field reveal the complex Fermi surface topology of the semimetallic black phosphorus. In particular, a Dirac-like fermion emerges at around 1.2 GPa, which is continuously tuned by external pressure. The observed semi-metallic behavior greatly enriches black phosphorus's material property, and sets the stage for the exploration of novel electronic states in this material. Moreover, these interesting behaviors make phosphorene a good candidate for the realization of a new two-dimensional relativistic electron system, other than graphene.

cond-mat.mtrl-sci↗

arXiv2015

Emergent Kondo scaling in iron-based superconductors AFe2As2 (A = K, Rb, Cs)

Unconventional superconductivity from heavy fermion (HF) is always observed in f-electron systems, in which Kondo physics between localized f-electrons and itinerant electrons plays an essential role. Whether HF superconductivity could be achieved in other systems without f electrons, especially for d-electron systems, is still elusive. Here, we experimentally study the origin of d-electron HF behavior in iron-based superconductors (FeSCs) AFe2As2 (A = K, Rb, Cs). Nuclear magnetic resonance on 75As reveals a universal coherent-incoherent crossover with a characteristic temperature T*. Below T*, a so-called 'Knight shift anomaly' is first observed in FeSCs, which exhibits a scaling behavior similar to f-electron HF materials. Furthermore, the scaling rule also regulates the manifestation of magnetic fluctuation. These results undoubtedly support an emergent Kondo scenario for the d-electron HF behavior, which suggests the AFe2As2 (A = K, Rb, Cs) as the first material realization of d-electron HF superconductors.

cond-mat.supr-con↗

arXiv2015

Evolution of high-temperature superconductivity from low-Tc phase tuned by carrier concentration in FeSe thin flakes

In contrast to bulk FeSe superconductor, heavily electron-doped FeSe-derived superconductors show relatively high Tc without hole Fermi surfaces and nodal superconducting gap structure, which pose great challenges on pairing theories in the iron-based superconductors. In the heavily electron-doped FeSe-based superconductors, the dominant factors and the exact working mechanism that is responsible for the high Tc need to be clarified. In particular, a clean control of carrier concentration remains to be a challenge for revealing how superconductivity and Fermi surface topology evolves with carrier concentration in bulk FeSe. Here, we report the evolution of superconductivity in the FeSe thin flake with systematically regulated carrier concentrations by liquid-gating technique. High-temperature superconductivity at 48 K can be achieved only with electron doping tuned by gate voltage in FeSe thin flake with Tc less than 10 K. This is the first time to achieve such a high temperature superconductivity in FeSe without either epitaxial interface or external pressure. It definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with Tc as high as 48 K in bulk FeSe. Intriguingly, our data also indicates that the superconductivity is suddenly changed from low-Tc phase to high-Tc phase with a Lifshitz transition at certain carrier concentration. These results help us to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on further pursuit of higher Tc in these materials.

cond-mat.supr-con↗

arXiv2015

Doping evolution of the superconducting gap structure in heavily hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$: heat transport study

We performed systematic thermal conductivity measurements on heavily hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with 0.747 $\leq x \leq$ 0.974. At $x$ = 0.747, the $κ_0/T$ is negligible, indicating nodeless superconducting gap. A small residual linear term $κ_0/T$ ($\approx$ 0.035 mW/K$^2$ cm) appears at $x$ = 0.826, and it increases slowly up to $x$ = 0.974, followed by a drastic increase of more than 20 times to the pure KFe$_2$As$_2$ ($x$ = 1.0). This doping dependence of $κ_0/T$ clearly shows that the nodal gap appears near $x = 0.8$, likely associated with the change of Fermi surface topology. The small values of $κ_0/T$ from $x$ = 0.826 to 0.974 support a "$\curlyvee$"-shaped nodal $s$-wave gap recently revealed by angle-resolved photoemission spectroscopy experiments at $x$ = 0.9. Furthermore, the drastic increase of $κ_0/T$ from $x$ = 0.974 to 1.0 is inconsistent with a symmetry-imposed $d$-wave gap in KFe$_2$As$_2$, and the possible nodal gap structure in KFe$_2$As$_2$ is discussed.

cond-mat.supr-con↗

arXiv2015

Giant enhancement in critical current density, up to a hundredfold, in superconducting NaFe0.97Co0.03As single crystals under hydrostatic pressure

Tremendous efforts towards improvement in the critical current density (Jc) of iron based superconductors (FeSCs), especially at relatively low temperatures and magnetic fields, have been made so far through different methods, resulting in real progress. Jc at high temperatures in high fields still needs to be further improved, however, in order to meet the requirements of practical applications. Here, we demonstrate a simple approach to achieve this. Hydrostatic pressure can significantly enhance Jc in NaFe0.97Co0.03As single crystals by at least tenfold at low field and more than a hundredfold at high fields. Significant enhancement in the in-field performance of NaFe0.97Co0.03As single crystal in terms of pinning force density (Fp) is found at high pressures. At high fields, the Fp is over 20 and 80 times higher than under ambient pressure at12K and 14K, respectively, at P=1GPa. We believe that the Co-doped NaFeAs compounds are very exciting and deserve to be more intensively investigated. Finally, it is worthwhile to say that by using hydrostatic pressure, we can achieve more milestones in terms of high Jc values in different superconductors.

cond-mat.supr-con↗

arXiv2015

Ultralow-frequency collective compression mode and strong interlayer coupling in multilayer black phosphorus

The recent renaissance of black phosphorus (BP) as a two-dimensional 2D layered material has generated tremendous interest in its tunable electronic band gap and highly anisotropic transport properties that offer new opportunities for device applications. Many of these outstanding properties are attributed to its unique structural characters that still need elucidation. Here we show Raman measurements that reveal an ultralow-frequency collective compression mode (CCM), which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling in BP, which is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than those previously achieved by using high-frequency Raman modes. These results offer fundamental insights and practical tools for exploring multilayer BP in new device applications.

cond-mat.mes-hall↗

arXiv2015

Surface electronic structure and isotropic superconducting gap in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe

Using angle-resolved photoemission spectroscopy (ARPES), we revealed the surface electronic structure and superconducting gap of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe, an intercalated FeSe-derived superconductor without antiferromagnetic phase or Fe-vacancy order in the FeSe layers, and with a superconducting transition temperature ($T_c$) $\sim$ 40 K. We found that (Li$_{0.8}$Fe$_{0.2}$)OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe resembles that of Rb$_x$Fe$_{2-y}$Se$_2$ except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li$_{0.8}$Fe$_{0.2}$)OHFeSe and Rb$_x$Fe$_{2-y}$Se$_2$. Superconducting gap is clearly observed below $T_c$, with an isotropic distribution around the electron Fermi surface. Compared with $A_x$Fe$_{2-y}$Se$_2$ (\textit{A}=K, Rb, Cs, Tl/K), the higher $T_c$ in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe might be attributed to higher homogeneity of FeSe layers or to some unknown roles played by the (Li$_{0.8}$Fe$_{0.2}$)OH layers.

cond-mat.supr-con↗

arXiv2015

Black Phosphorus Nanoelectromechanical Resonators Vibrating at Very High Frequencies

We report on experimental demonstration of a new type of nanoelectromechanical resonators based on black phosphorus crystals. Facilitated by a highly efficient dry transfer technique, crystalline black phosphorus flakes are harnessed to enable drumhead resonators vibrating at high and very high frequencies (HF and VHF bands, up to ~100MHz). We investigate the resonant vibrational responses from the black phosphorus crystals by devising both electrical and optical excitation schemes, in addition to measuring the undriven thermomechanical motions in these suspended nanostructures. Flakes with thicknesses from ~200nm down to ~20nm clearly exhibit elastic characteristics transitioning from the plate to the membrane regime. Both frequency- and time-domain measurements of the nanomechanical resonances show that very thin black phosphorus crystals hold interesting promises for moveable and vibratory devices, and for semiconductor transducers where high-speed mechanical motions could be coupled to the attractive electronic and optoelectronic properties of black phosphorus.

cond-mat.mes-hall↗

arXiv2015

Magnetoresistance evidence on surface state and field-dependent bulk gap in Kondo insulator SmB6

Recently, the resistance saturation at low temperature in Kondo insulator SmB6, a long-standing puzzle in condensed matter physics, was proposed to originate from topological surface state. Here,we systematically studied the magnetoresistance of SmB6 at low temperature up to 55 Tesla. Both temperature- and angular-dependent magnetoresistances show a similar crossover behavior below 5 K. Furthermore, the angular-dependent magnetoresistance on different crystal face confirms a two-dimensional surface state as the origin of magnetoresistances crossover below 5K. Based on two-channels model consisting of both surface and bulk states, the field-dependence of bulk gap with critical magnetic field (Hc) of 196 T is extracted from our temperature-dependent resistance under different magnetic fields. Our results give a consistent picture to understand the low-temperature transport behavior in SmB6, consistent with topological Kondo insulator scenario.

cond-mat.str-el↗

arXiv2015

Gate-tuned Superconductor-Insulator transition in (Li,Fe)OHFeSe

The antiferromagnetic(AFM) insulator-superconductor transition has been always a center of interest in the underlying physics of unconventional superconductors. The quantum phase transition between Mott insulator with AFM and superconductor can be induced by doping charge carriers in high-Tc cuprate superconductors. For the best characterized organic superconductors of k-(BEDT-TTF)2X (X=anion), a first order transition between AFM insulator and superconductor can be tuned by applied external pressure or chemical pressure. Also, the superconducting state can be directly developed from AFM insulator by application of pressure in Cs3C60. The resemblance of these phase diagrams hints a universal mechanism governing the unconventional superconductivity in close proximity to AFM insulators. However, the superconductivity in iron-based high-Tc superconductors evolves from an AFM bad metal by doping charge carriers, and no superconductor-insulator transition has been observed so far. Here, we report a first-order transition from superconductor to insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal (Li,Fe)OHFeSe. The Tc is continuously enhanced with electron doping by ionic gating up to a maximum Tc of 43 K, and a striking superconductor-insulator transition occurs just at the verge of optimal doping with highest Tc. A novel phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out, indicating that the superconductor -insulator transition is a common feature for unconventional superconductivity. These results help to uncover the underlying physics of iron-based superconductivity as well as the universal mechanism of high-Tc superconductivity. Our finding also suggests that the gate-controlled strong charge doping makes it possible to explore novel states of matter in a way beyond traditional methods.

cond-mat.supr-con↗

arXiv2015

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductors KFe2As2, RbFe2As2, and CsFe2As2

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a d-wave state favored by high-Q scattering at low pressure changes to a state with s+- symmetry favored by low-Q scattering at high pressure.

cond-mat.supr-con↗

arXiv2015

Superconductivity and Phase Diagram in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe$_{1-x}$S$_x$

A series of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe$_{1-x}$S$_x$ (0 $\leq$ x $\leq$ 1) samples were successfully synthesized via hydrothermal reaction method and the phase diagram is established. Magnetic susceptibility suggests that an antiferromagnetism arising from (Li$_{0.8}$Fe$_{0.2}$)OH layers coexists with superconductivity, and the antiferromagnetic transition temperature nearly remains constant for various S doping levels. In addition, the lattice parameters of the both a and c axes decrease and the superconducting transition temperature T$_c$ is gradually suppressed with the substitution of S for Se, and eventually superconductivity vanishes at $x$ = 0.90. The decrease of T$_c$ could be attributed to the effect of chemical pressure induced by the smaller ionic size of S relative to that of Se, being consistent with the effect of hydrostatic pressure on (Li$_{0.8}$Fe$_{0.2}$)OHFeSe. But the detailed investigation on the relationships between $T_{\rm c}$ and the crystallographic facts suggests a very different dependence of $T_{\rm c}$ on anion height from the Fe2 layer or $Ch$-Fe2-$Ch$ angle from those in FeAs-based superconductors.

cond-mat.supr-con↗

arXiv2015

Heat transport in RbFe2As2 single crystal: evidence for nodal superconducting gap

The in-plane thermal conductivity of iron-based superconductor RbFe$_2$As$_2$ single crystal ($T_c \approx$ 2.1 K) was measured down to 100 mK. In zero field, the observation of a significant residual linear term $κ_0/T$ = 0.65 mW K$^{-2}$ cm$^{-1}$ provides clear evidence for nodal superdonducting gap. The field dependence of $κ_0/T$ is similar to that of its sister compound CsFe$_2$As$_2$ with comparable residual resistivity $ρ_0$, and lies between the dirty and clean KFe$_2$As$_2$. These results suggest that the (K,Rb,Cs)Fe$_2$As$_2$ serial superconductors have a common nodal gap structure.

cond-mat.supr-con↗

arXiv2014

Electronic structure in one-Fe Brillouin zone of iron-pnictide superconductor CsFe$_2$As$_2$

The multiband nature of iron-pnictide superconductors is one of the keys to the understanding of their intriguing behavior. The electronic and magnetic properties heavily rely on the multiband interactions between different electron and hole pockets near the Fermi level. At the fundamental level, though many theoretical models were constructed on the basis of the so-called 1-Fe Brillouin zone (BZ) with an emphasis of the basic square lattice of iron atoms, most electronic structure measurements were interpreted in the 2-Fe BZ. Whether the 1-Fe BZ is valid in a real system is still an open question. Using angle-resolved photoemission spectroscopy (ARPES), here we show in an extremely hole-doped iron-pnictide superconductor CsFe$_2$As$_2$ that the distribution of electronic spectral weight follows the 1-Fe BZ, and that the emerging band structure bears some features qualitatively different from theoretical band structures of the 1-Fe BZ. Our analysis suggests that the interlayer separation is an important tuning factor for the physics of FeAs layers, the increase of which can reduce the coupling between Fe and As and lead to the emergence of the electronic structure in accord with the 1-Fe symmetry of the Fe square lattice. Our finding puts strong constraints on the theoretical models constructed on the basis of the 1-Fe BZ.

cond-mat.supr-con↗