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S. Y. Li

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

86 records · Page 5Linked to original sources
arXiv2008

Phase Diagram and Quantum Critical Point in Newly Discovered Superconductors: SmO_{1-x}F_xFeAs

The magnetic fluctuations associated with a quantum critical point (QCP) are widely believed to cause the non-Fermi liquid behaviors and unconventional superconductivities, for example, in heavy fermion systems and high temperature cuprate superconductors. Recently, superconductivity has been discovered in iron-based layered compound $LaO_{1-x}F_xFeAs$ with $T_c$=26 K\cite{yoichi}, and it competes with spin-density-wave (SDW) order\cite{dong}. Neutron diffraction shows a long-rang SDW-type antiferromagnetic (AF) order at $\sim 134$ K in LaOFeAs\cite{cruz,mcguire}. Therefore, a possible QCP and its role in this system are of great interests. Here we report the detailed phase diagram and anomalous transport properties of the new high-Tc superconductors $SmO_{1-x}F_xFeAs$ discovered by us\cite{chenxh}. It is found that superconductivity emerges at $x\sim$0.07, and optimal doping takes place in the $x\sim$0.20 sample with highest $T_c \sim $54 K. While $T_c$ increases monotonically with doping, the SDW order is rapidly suppressed, suggesting a QCP around $x \sim$0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the $x<0.14$ regime, but at low temperatures just above $T_c$ in the $x>0.14$ regime; a drop in carrier density evidenced by a pronounced rise in Hall coefficient are observed, which mimic the high-$T_c$ cuprates. The simultaneous occurrence of order, carrier density change and criticality makes a compelling case for a quantum critical point in this system.

cond-mat.supr-con↗
arXiv2007

Doubly Heavy Baryon Production at γγCollider

The inclusive production of doubly heavy baryons $Ξ_{cc}$ and $Ξ_{bb}$ at $γγ$ collider is investigated. It is found that the contribution from the heavy quark pair $QQ$ in color triplet and color sextet are important.

hep-ph↗
arXiv2007

Low-temperature phonon thermal conductivity of cuprate single crystals

The effect of sample size and surface roughness on the phonon thermal conductivity $κ_p$ of Nd$_2$CuO$_4$ single crystals was studied down to 50 mK. At 0.5 K, $κ_p$ is proportional to $\sqrt{A}$, where $A$ is the cross-sectional area of the sample. This demonstrates that $κ_p$ is dominated by boundary scattering below 0.5 K or so. However, the expected $T^3$ dependence of $κ_p$ is not observed down to 50 mK. Upon roughing the surfaces, the $T^3$ dependence is restored, showing that departures from $T^3$ are due to specular reflection of phonons off the mirror-like sample surfaces. We propose an empirical power law fit, to $κ_p \sim T^α$ (where $α< 3$) in cuprate single crystals. Using this method, we show that recent thermal conductivity studies of Zn doping in YBa$_2$Cu$_3$O$_y$ re-affirm the universal heat conductivity of d-wave quasiparticles at $T \to 0$.

cond-mat.supr-con↗
arXiv2007

Doping dependence of the superconducting gap in Tl_2Ba_2CuO_{6+delta} from heat transport

We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of 0.1 hole/Cu. As hole concentration is decreased, the gap-to-Tc ratio increases, showing that the suppression of the superconducting transition temperature Tc (relative to the gap) begins in the overdoped regime.

cond-mat.str-el↗
arXiv2007

Single-gap s-wave superconductivity in Cu$_x$TiSe$_2$

The in-plane thermal conductivity $κ$ of the layered superconductor Cu$_x$TiSe$_2$ was measured down to temperatures as low as $T_c$/40, at $x$ = 0.06 near where the CDW order vanishes. The absence of a residual linear term at $ T \to 0$ is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe$_2$, where the superconducting gap is three times larger on the Nb 4$d$ band than on the Se 4$p$ band, suggests that in Cu$_{0.06}$TiSe$_2$ the Se 4$p$ band is below the Fermi level and the Ti 3$d$ band is alone responsible for the superconductivity.

cond-mat.supr-con↗
arXiv2006

Onset of a boson mode at superconducting critical point of underdoped YBa2Cu3Oy

The thermal conductivity $κ$ of underdoped \Y was measured in the $T \to 0$ limit as a function of hole concentration $p$ across the superconducting critical point at $p_{SC}$ = 5.0%. ``Time doping'' was used to resolve the evolution of bosonic and fermionic contributions with high accuracy. For $p \leqslant p_{SC}$, we observe an additional $T^3$ contribution to $κ$ which we attribute to the boson excitations of a phase with long-range spin or charge order. Fermionic transport, manifest as a linear term in $κ$, is seen to persist unaltered through $p_{SC}$, showing that the state just below $p_{SC}$ is a thermal metal. In this state, the electrical resistivity varies as log$(1/T)$ and the Wiedemann-Franz law is violated.

cond-mat.supr-con↗
arXiv2006

Hybrid gap structure in the heavy-fermion superconductor CeIrIn$_5$

The thermal conductivity $κ$ of the heavy-fermion superconductor CeIrIn$_5$ was measured as a function of temperature down to $T_c$/8, for current directions perpendicular ($J \parallel a$) and parallel ($J \parallel c$) to the tetragonal c axis. For $J \parallel a$, a sizable residual linear term $κ_0 / T$ is observed, as previously, which confirms the presence of line nodes in the superconducting gap. For $J \parallel c$, on the other hand, $κ/ T \to 0$ as $T \to 0$. The resulting precipitous decline in the anisotropy ratio $κ_c / κ_a$ at low temperature rules out a gap structure with line nodes running along the c-axis, such as the d-wave state favoured for CeCoIn$_5$, and instead points to a hybrid gap of $E_g$ symmetry. It therefore appears that two distinct superconducting states are realized in the Ce$M$In$_5$ family.

cond-mat.str-el↗
arXiv2005

Delocalized fermions in underdoped cuprates

Low temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly-doped cuprate YBa$_{2}$Cu$_{3}$O$_{6.33}$. Samples were measured on either side of the superconducting phase boundary, in both zero and applied magnetic field. We report the observation of delocalized fermionic excitations at zero energy in the non-superconducting state, which shows that the ground state of underdoped cuprates is metallic. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e. nodal. The insulating ground state observed in underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ is attributed to the competing spin-density-wave order present in that system.

cond-mat.supr-con↗
arXiv2005

Ballistic magnon transport and phonon scattering in the antiferromagnet Nd$_2$CuO$_4$

The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured down to 50 mK. Using the spin-flop transition to switch on and off the acoustic Nd magnons, we can reliably separate the magnon and phonon contributions to heat transport. We find that magnons travel ballistically below 0.5 K, with a thermal conductivity growing as $T^3$, from which we extract their velocity. We show that the rate of scattering of acoustic magnons by phonons grows as $T^3$, and the scattering of phonons by magnons peaks at twice the average Nd magnon frequency.

cond-mat.supr-con↗
arXiv2004

Giant electron-electron scattering in the Fermi-liquid state of Na_0.7CoO_2

The in-plane resistivity, rho, and thermal conductivity, kappa, of a single crystal of Na_0.7CoO_2 were measured down to 40 mK. Verification of the Wiedemann-Franz law, kappa/T = L_0/rho as T -> 0, and observation of a T^2 dependence of rho at low temperature, rho = rho_0 + AT^2, establish the existence of a well-defined Fermi-liquid state. The measured value of coefficient A reveals enormous electron-electron scattering, characterized by the largest Kadowaki-Woods ratio, A/gamma^2, encountered in any material. The rapid suppression of A with magnetic field suggests a possible proximity to a magnetic quantum critical point. We also speculate on the possible role of magnetic frustration and proximity to a Mott insulator.

cond-mat.str-el↗
arXiv2001

Thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$

The thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$ ($T_c \approx$ 8 K) have been studied. The thermopower is negative from room temperature to 10 K. Combining with the negative Hall coefficient reported previously, the negative thermopower definetly indicates that the carrier in $MgCNi_3$ is electron-type. The nonlinear temperature dependence of thermopower below 150 K is explained by the electron-phonon interaction renormalization effects. The thermal conductivity is of the order for intermetallics, larger than that of borocarbides and smaller than $MgB_2$. In the normal state, the electronic contribution to the total thermal conductivity is slightly larger than the lattice contribution. The transverse magnetoresistance of $MgCNi_3$ is also measured. It is found that the classical Kohler's rule is valid above 50 K. An electronic crossover occures at $T^* \sim 50 K$, resulting in the abnormal behavior of resistivity, thermopower, and magnetoresistance below 50 K.

cond-mat.supr-con↗
arXiv2001

Normal state resistivity, upper critical field and Hall effect in superconducting perovskite $MgCNi_3$

The normal state resistivtity, upper critical field $H_{c2}$ and Hall coefficient $R_H$ in superconducting perovskite $MgCNi_3$ ($T_c \approx 8 K$) have been studied. Above 70 K, $ρ(T)$ fits well curve predicted by Bloch-Grüneisen theory consistently with electron-phonon scattering. $H_{c2}(0)$ was estimated to be about 15.0 Tesla within the weak-coupling BCS theory, and the superconducting coherence length $ξ(0)$ is approximately 47 Å. $R_H$ of $MgCNi_3$ is negative for the whole temperature range which definitely indicates that the carrier in $MgCNi_3$ is electron-type. $R_H$ is temperature independent between $T_c$ and $\sim$ 140 K. Above $\sim$ 140 K, the magnitude of $R_H$ decreases as temperature rises. At T = 100 K, the carrier density is $1.0 \times 10^{22}/cm^3$, which is comparable with that in perovskite $(Ba,K)BiO_3$, and less than that of the metallic binary $MgB_2$.

cond-mat.supr-con↗
arXiv1999

Structure and properties of a novel fulleride Sm6C60

A novel fulleride Sm6C60 has been synthesized using high temperature solid state reaction. The Rietveld refinement on high resolution synchrotron X-ray powder diffraction data shows that Sm6C60 is isostructural with body-centered cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60, and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak T-linear behavior above 180 K, the transport at low temperature is mainly dominated by granular-metal theory.

cond-mat.mtrl-sci↗