arXiv subjects

S. L. Li

Publications and source records attributed to S. L. Li.

At least 19 recordsLinked to original sources

arXiv2018

Photo-induced new metastable state with modulated Josephson coupling strengths in Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$

Photoexcitations on a superconductor using ultrafast nir-infrared (NIR) pulses, whose energy is much higher than the superconducting energy gap, are expected to suppress/destroy superconductivity by breaking Cooper pairs and excite quasiparticles from occupied state to unoccupied state far above the Fermi level. This appears to be true only for small pumping fluence. Here we show that the intense NIR pumping has different effect. We perform an intense NIR pump, c-axis terahertz probe measurement on an electron-doped cuprate superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ with T$_c$=22 K. The measurement indicates that, instead of destroying superconductivity or exciting quasiparticles, the intense NIR pump drives the system from an equilibrium superconducting state with uniform Josephson coupling strength to a new metastable superconducting phase with modulated Josephson coupling strengths below T$_c$.

cond-mat.supr-con↗

arXiv2014

Phase Separation, Competition, and Volume Fraction Control in NaFe$_{1-x}$Co$_x$As

We report a detailed nuclear magnetic resonance (NMR) study by combined $^{23}$Na and $^{75}$As measurements over a broad range of doping to map the phase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \le$ 0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order, which we denote the {\it s}-AFM phase, cohabiting with a phase of weak and possibly proximity-induced AFM order ({\it w}-AFM) whose volume fraction $V \simeq 8$\% is approximately constant. Near optimal doping, at $x = 0.0175$, we observe a phase separation between static antiferromagnetism related to the {\it s}-AFM phase and a paramagnetic (PM) phase related to {\it w}-AFM. The volume fraction of AFM phase increases upon cooling, but both the N{é}el temperature and the volume fraction can be suppressed systematically by applying a $c$-axis magnetic field. On cooling below $T_c$, superconductivity occupies the PM region and its volume fraction grows at the expense of the AFM phase, demonstrating a phase separation of the two types of order based on volume exclusion. At higher dopings, static antiferromagnetism and even critical AFM fluctuations are completely suppressed by superconductivity. Thus the phase diagram we establish contains two distinct types of phase separation and reflects a strong competition between AFM and superconducting phases both in real space and in momentum space. We suggest that both this strict mutual exclusion and the robustness of superconductivity against magnetism are consequences of the extreme two-dimensionality of NaFeAs.

cond-mat.supr-con↗

arXiv2014

PandaX: A Liquid Xenon Dark Matter Experiment at CJPL

PandaX is a large upgradable liquid-xenon detector system that can be used for both direct dark-matter detection and $^{136}$Xe double-beta decay search. It is located in the Jinping Deep-Underground Laboratory in Sichuan, China. The detector operates in dual-phase mode, allowing detection of both prompt scintillation, and ionization charge through proportional scintillation. The central time projection chamber will be staged, with the first stage accommodating a target mass of about 120\,kg. In stage II, the target mass will be increased to about 0.5\,ton. In the final stage, the detector can be upgraded to a multi-ton target mass. In this paper a detailed description of the stage-I detector design and performance results established during the commissioning phase is presented.

physics.ins-det↗

arXiv2013

Non-integer optical modes in a Möbius-ring resonator

In-plane polarized light experiences a non-trivial topological evolution as it propagates resonantly in a Möbius ring resonator. The resultant geometric phase varies continuously when changing the light ellipticity, which leads to constructive interference for a non-integer number of wavelengths, and therefore to the occurrence of an arbitrary fractional number of optical modes. The geometric phase in Möbius-ring resonators is topologically robust and implies excellent intrinsic fault-tolerance.

physics.optics↗

arXiv2011

Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting RbyFe(1.6+x)Se2

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb$_y$Fe$_{1.6+x}$Se$_2$. For the insulating Rb$_{y}$Fe$_{1.6+x}$Se$_2$, neutron polarization analysis and single crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\times\sqrt{5}$ block antiferromagnetic structure. For superconducting samples ($T_c=30$ K), we find that in addition to the tetragonal $\sqrt{5}\times\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\times \sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb$_{y}$Fe$_{1.6+x}$Se$_2$ is phase separated with coexisting $\sqrt{2}\times \sqrt{2}$ and $\sqrt{5}\times\sqrt{5}$ superlattice structures.

cond-mat.supr-con↗

arXiv2007

Distinct Pairing Symmetries in $Nd_{1.85}Ce_{0.15}CuO_{4-y}$ and $La_{1.89}Sr_{0.11}CuO_{4}$ Single Crystals: Evidence from Comparative Tunnelling Measurements

We used point-contact tunnelling spectroscopy to study the superconducting pairing symmetry of electron-doped $Nd_{1.85}Ce_{0.15}CuO_{4-y}$ (NCCO) and hole-doped $La_{1.89}Sr_{0.11}CuO_{4}$ (LSCO). Nearly identical spectra without zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast, LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave superconductor. Detailed analysis reveals an s-wave component in the pairing symmetry of the NCCO sample with $Δ/k_BT_c=1.66$, a value remarkable close to that of a weakly coupled BCS superconductor. We argue that this s-wave component is formed at the Fermi surface pockets centered at ($\pmπ$,0) and (0,$\pmπ$) although a d-wave component may also exist.

cond-mat.supr-con↗

arXiv2006

Universal quasiparticle decoherence in hole- and electron-doped high-Tc cuprates

We use angle-resolved photoemission to unravel the quasiparticle decoherence process in the high-$T_c$ cuprates. The coherent band is highly renormalized, and the incoherent part manifests itself as a nearly vertical ``dive'' in the $E$-$k$ intensity plot that approaches the bare band bottom. We find that the coherence-incoherence crossover energies in the hole- and electron-doped cuprates are quite different, but scale to their corresponding bare bandwidth. This rules out antiferromagnetic fluctuations as the main source for decoherence. We also observe the coherent band bottom at the zone center, whose intensity is strongly suppressed by the decoherence process. Consequently, the coherent band dispersion for both hole- and electron-doped cuprates is obtained, and is qualitatively consistent with the framework of Gutzwiller projection.

cond-mat.supr-con↗

arXiv2003

Hole Doping Dependence of the Coherence Length in $La_{2-x}Sr_xCuO_4$ Thin Films

By measuring the field and temperature dependence of magnetization on systematically doped $La_{2-x}Sr_xCuO_4$ thin films, the critical current density $j_c(0)$ and the collective pinning energy $U_p(0)$ are determined in single vortex creep regime. Together with the published data of superfluid density, condensation energy and anisotropy, for the first time we derive the doping dependence of the coherence length or vortex core size in wide doping regime directly from the low temperature data. It is found that the coherence length drops in the underdoped region and increases in the overdoped side with the increase of hole concentration. The result in underdoped region clearly deviates from what expected by the pre-formed pairing model if one simply associates the pseudogap with the upper-critical field.

cond-mat.supr-con↗

arXiv2003

Absence of a True Vortex-Glass Phase above the Bragg Glass Transition Line in Bi-2212

In magnetic measurements on Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi-2212) single crystals, a general peak with a dynamical feature on both $S-H$ and $S-T$ curves was found with S the magnetic relaxation rate. At higher fields, the characteristic exponent $μ$ becomes negative, together with the positive curvature of $logE$ vs. $ logj$ and the scaling based on the 2D vortex glass theory or plastic creep theory, we conclude that the vortex motion above the second peak is plastic when $j\to 0$ and there is no vortex glass phase at finite temperatures in Bi-2212. The peak of S is then explained as the crossover between different meta-stable vortex states.

cond-mat.supr-con↗

arXiv2003

Dimensional Crossover of Vortex Dynamics Induced by Gd Substitution on Bi2212 Single Crystals

The vortex dynamics of Bi$_2$Sr$_2$Ca$_{1-x}$Gd$_x$Cu$_2$O$_{8+δ}$ single crystals is investigated by magnetic relaxation and hysteresis measurements. By substituting $Ca$ with $Gd$, it is found that the interlayer Josephson coupling is weakened and the anisotropy is increased, which leads to the change of vortex dynamics from 3D elastic to 2D plastic vortex creep. Moreover, the second magnetization peak, which can be observed in samples near the optimal doping, is absent in the strongly underdoped (with 2D vortex) region.

cond-mat.supr-con↗

arXiv2001

Suppression of Superconducting Critical Current Density by Small Flux Jumps in $MgB_2$ Thin Films

By doing magnetization measurements during magnetic field sweeps on thin films of the new superconductor $MgB_2$, it is found that in a low temperature and low field region small flux jumps are taking place. This effect strongly suppresses the central magnetization peak leading to reduced nominal superconducting critical current density at low temperatures. A borderline for this effect to occur is determined on the field-temperature (H-T) phase diagram. It is suggested that the small size of the flux jumps in films is due to the higher density of small defects and the relatively easy thermal diffusion in thin films in comparison with bulk samples.

cond-mat.supr-con↗

arXiv2001

Possible Superconductivity at 37 K in Graphite-Sulfur Composite

Sulfur intercalated graphite composites with diamagnetic transitions at 6.7 K and 37 K are prepared. The magnetization hysteresis loops (MHL), Xray diffraction patterns, and resistance were measured. From the MHL, a slight superconducting like penetration process is observed at 15 K in low field region. The XRD shows no big difference from the mixture of graphite and sulfur indicating that the volume of the superconducting phase (if any) is very small. The temperature dependence of resistance shows a typical semiconducting behavior with a saturation in low temperature region. This saturation is either induced by the de-localization of conducting electrons or by possible superconductivity in this system.

cond-mat.supr-con↗

arXiv2001

No Ending Point in The Bragg-to-Vortex Glass Phase Transition Line at Low Temperatures

We have measured the magnetic hysteresis loops and the magnetic relaxation for $Bi_2Sr_2CaCu_2O_{8+δ}$ (Bi-2212) single crystals which exhibit the second magnetization peak effect. Although no second peak effect is observed below 20 K in the measurement with fast field sweeping rate, it is found that the second peak effect will appear again after long time relaxation or in a measurement with very slow field sweeping rate at 16 K. It is anticipated that the peak effect will appear at very low temperatures (approaching zero K) when the relaxation time is long enough. We attribute this phenomenon to the profile of the interior magnetic field and conclude that the phase transition line of Bragg glass to vortex glass has no ending point at low temperatures.

cond-mat.supr-con↗

arXiv2001

Magnetic relaxation and critical current density of $MgB_2$ thin films

Magnetic relaxation and critical current density have been measured on a $MgB_2$ thin film in a wide region of temperature with the magnetic field up to 8 T. The irreversibility line has also been determined. It is found that the relaxation rate has a very weak temperature dependence below $1/2 T_c$ showing a clear residual relaxation rate at zero K, which cannot be easily explained as due to thermally activated flux creep. Furthermore the relaxation rate has a strong field dependence. The flux dynamics of thin films are very similar to that of high pressure synthesized bulks although the relaxation rate in thin film is systematically higher than that of a bulk sample. All the results here together with those from the bulk samples suggest that the flux dynamics may be dominated by the quantum effects, such as quantum fluctuation and tunneling.

cond-mat.supr-con↗

arXiv2001

Preparation and Superconductivity of a MgB2 Superconducting Tape

A MgB2 superconducting tape has been successfully prepared by a new way of boron diffusion into Mg tape. This tape superconductor has a Tc of 39 K with a sharp transition width of 0.4 K, high bulk critical current density jc of 6.7*10^5 A/cm^2 in zero field at 6 K, higher irreversibility line comparing to that of a high pressure synthesized bulk sample and very small crystalline grain with size in the scale of 10-200 nm.

cond-mat.supr-con↗

arXiv2001

Intrinsic Percolative Superconductivity in Heavily Overdoped High Temperature Superconductors

Magnetic measurements on heavily overdoped $La_{2-x}Sr_xCuO_4$, $Tl_2Ba_2CuO_6$, $Bi_2Sr_2CuO_6$ and $Bi_2Sr_2CaCu_2O_8$ single crystals reveal a new type magnetization hysteresis loops characterized by the vanishing of usual central peak near zero field. Since this effect has been observed in various systems with very different structural details, it reflects probably a generic behavior for all high temperature superconductors. This easy penetration of magnetic flux can be understood in the picture of percolative superconductivity due to the inhomogeneous electronic state in heavily overdoped regime.

cond-mat.supr-con↗

arXiv2001

Flux dynamics and vortex phase diagram of the new superconductor $MgB_2$

Magnetic critical current density and relaxation rate have been measured on $MgB_2$ bulks from 1.6 K to $T_c$ at magnetic fields up to 8 Tesla. A vortex phase diagram is depicted based on these measurement. Two phase boundaries $H_{irr}^{bulk}(T)$ and $H_{irr}^{g}(T)$ characterizing different irreversible flux motions are found. The $H_{irr}^{bulk}(T)$ is characterized by the appearance of the linear resistivity and is attributed to quantum vortex melting induced by quantum fluctuation of vortices in the rather clean system. The second boundary $ H_{irr}^g(T) $ reflects the irreversible flux motion in some local regions due to either very strong pinning or the surface barrier on the tiny grains.

cond-mat.supr-con↗

arXiv2001

Strong quantum fluctuation of vortices in the new superconductor $MgB_2$

By using transport and magnetic measurement, the upper critical field $H_{c2}(T)$ and the irreversibility line $H_{irr}(T)$ has been determined. A big separation between $H_{c2}(0)$ and $H_{irr}(0)$ has been found showing the existence of a quantum vortex liquid state induced by quantum fluctuation of vortices in the new superconductor $MgB_2$. Further investigation on the magnetic relaxation shows that both the quantum tunneling and the thermally activated flux creep weakly depends on temperature. But when the melting field $H_{irr}$ is approached, a drastic rising of the relaxation rate is observed. This may imply that the melting of the vortex matter at a finite temperature is also induced by the quantum fluctuation of vortices.

cond-mat.supr-con↗