arXiv subjects

Lu Li

Publications and source records attributed to Lu Li.

At least 217 records · Page 12Linked to original sources

arXiv2012

Quantum oscillations in topological superconductor candidate Cu$_{0.25}$Bi$_2$Se$_3$

Quantum oscillations are generally studied to resolve the electronic structure of topological insulators. In Cu$_{0.25}$Bi$_2$Se$_3$, the prime candidate of topological superconductors, quantum oscillations are still not observed in magnetotransport measurement. However, using torque magnetometry, quantum oscillations (the de Hass - van Alphen effect) were observed in Cu$_{0.25}$Bi$_2$Se$_3$ . The doping of Cu in Bi$_2$Se$_3$ increases the carrier density and the effective mass without increasing the scattering rate or decreasing the mean free path. In addition, the Fermi velocity remains the same in Cu$_{0.25}$Bi$_2$Se$_3$ as that in Bi$_2$Se$_3$. Our results imply that the insertion of Cu does not change the band structure of Bi$_2$Se$_3$.

cond-mat.str-el↗

arXiv2012

The Peregrine rogue waves induced by interaction between the continuous wave and soliton

Based on the soliton solution on a continuous wave background for an integrable Hirota equation, the reduction mechanism and the characteristics of the Peregrine rogue wave in the propagation of femtosecond pulses of optical fiber are discussed. The results show that there exist two processes of the formation of the Peregrine rogue wave: one is the localized process of the continuous wave background, and the other is the reduction process of the periodization of the bright soliton. The characteristics of the Peregrine rogue wave are exhibited by strong temporal and spatial localization. Also, various initial excitations of the Peregrine rogue wave are performed and the results show that the Peregrine rogue wave can be excited by a small localized (single peak) perturbation pulse of the continuous wave background, even for the nonintegrable case. The numerical simulations show that the Peregrine rogue wave is unstable. Finally, through a realistic example, the influence of the self-frequency shift to the dynamics of the Peregrine rogue wave is discussed. The results show that in the absence of the self-frequency shift, the Peregrine rogue wave can split into several subpuslses; however, when the self-frequency shift is considered, the Peregrine rogue wave no longer splits and exhibits mainly a peak changing and an increasing evolution property of the field amplitude.

physics.optics↗

arXiv2011

Thermal Hall Conductivity as a Probe of Gap Structure in Multi-band Superconductors: The Case of $\rm Ba_{1-x}K_xFe_2As_2$

The sign and profile of the thermal Hall conductivity $κ_{xy}$ gives important insights into the gap structure of multi-band superconductors. With this perspective, we have investigated $κ_{xy}$ and the thermal conductivity $κ_{xx}$ in $\rm Ba_{1-x}K_xFe_2As_2$ which display large peak anomalies in the superconducting state. The anomalies imply that a large hole-like quasiparticle (qp) population exists below the critical temperature $T_c$. We show that the qp mean-free-path inferred from $κ_{xx}$ reproduces the observed anomaly in $κ_{xy}$, providing a consistent estimate of a large qp population. Further, we demonstrate that the hole-like signal is consistent with a theoretical scenario where despite potentially large gap variations on the electron pockets, the minimal homogeneous gap of the superconducting phase resides at a hole pocket. Implications for probing the gap structure in the broader class of pnictide superconductors are discussed.

cond-mat.supr-con↗

arXiv2011

Unusual Nernst effect suggestive of time-reversal violation in the striped cuprate La$_{2-x}$Ba$_x$CuO$_4$

The striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ ($x=\frac18)$ undergoes several transitions below the charge-ordering temperature $T_{co}$ = 54 K. From Nernst experiments, we find that, below $T_{co}$, there exists a large, anomalous Nernst signal $e_{N,even}(H,T)$ that is symmetric in field $H$, and remains finite as $H\to 0$. The time-reversal violating signal suggests that, below $T_{co}$, vortices of one sign are spontaneously created to relieve interlayer phase frustration.

cond-mat.supr-con↗

arXiv2011

Symmetry breaking and manipulation of nonlinear optical modes in an asymmetric double-channel waveguide

We study light-beam propagation in a nonlinear coupler with an asymmetric double-channel waveguide and derive various analytical forms of optical modes. The results show that the symmetry-preserving modes in a symmetric double-channel waveguide are deformed due to the asymmetry of the two-channel waveguide, yet such a coupler supports the symmetry-breaking modes. The dispersion relations reveal that the system with self-focusing nonlinear response supports the degenerate modes, while for self-defocusingmedium the degenerate modes do not exist. Furthermore, nonlinear manipulation is investigated by launching optical modes supported in double-channel waveguide into a nonlinear uniform medium.

physics.optics↗

arXiv2011

Ring-Like Solitons in Plasmonic Fiber Waveguide Composed of Metal-Dielectric Multilayers

We design a plasmonic fiber waveguide (PFW) composed of coaxial cylindrical metal-dielectric multilayers in nanoscale, and constitute the corresponding dynamical equations describing the modes of propagation in the PFW with the Kerr nonlinearity in the dielectric layers. The physics is connected to the discrete matrix nonlinear Schrödinger equations, from which the highly confined ring-like solitons in scale of subwavelength are found both for the visible light and the near-infrared light in the self-defocusing condition. Moreover, the confinement could be further improved when increasing the intensity of the input light due to the cylindrical symmetry of the PFW, which means both the width and the radius of the ring are reduced.

cond-mat.mes-hall↗

arXiv2011

Matter rogue wave in Bose-Einstein condensates with attractive atomic interaction

We investigate the matter rogue wave in Bose-Einstein Condensates with attractive interatomic interaction analytically and numerically. Our results show that the formation of rogue wave is mainly due to the accumulation of energy and atoms toward to its central part; Rogue wave is unstable and the decay rate of the atomic number can be effectively controlled by modulating the trapping frequency of external potential. The numerical simulation demonstrate that even a small periodic perturbation with small modulation frequency can induce the generation of a near-ideal matter rogue wave. We also give an experimental protocol to observe this phenomenon in Bose-Einstein Condensates.

cond-mat.quant-gas↗

arXiv2011

Coexistence of Magnetic Order and Two-dimensional Superconductivity at LaAlO$_3$/SrTiO$_3$ Interfaces

A two dimensional electronic system with novel electronic properties forms at the interface between the insulators LaAlO$_3$ and SrTiO$_3$. Samples fabricated until now have been found to be either magnetic or superconducting, depending on growth conditions. We combine transport measurements with high-resolution magnetic torque magnetometry and report here evidence of magnetic ordering of the two-dimensional electron liquid at the interface. The magnetic ordering exists from well below the superconducting transition to up to 200 K, and is characterized by an in-plane magnetic moment. Our results suggest that there is either phase separation or coexistence between magnetic and superconducting states. The coexistence scenario would point to an unconventional superconducting phase in the ground state.

cond-mat.mtrl-sci↗

arXiv2010

Nonautonomous Bright and Dark Solitons of Bose-Einstein Condensates with Feshbach-Managed Time-Dependent Scattering Length

We present a family of nonautonomous bright and dark soliton solutions of Bose-Einstein condensates with the time-dependent scattering length in an expulsive parabolic potential. These solutions show that the amplitude, width, and velocity of soliton can be manipulated by adjusting the atomic scattering length via Feshbach resonance. For the cases of both attractive and repulsive interaction, the total particles number is a conservation quantity, but the peak (dip) density can be controlled by the Feshbach resonance parameter. Especially, we investigate the modulation instability process in uniform Bose-Einstein condensates with attractive interaction and nonvanishing background, and clarify that the procedure of pattern formation is in fact the superposition of the perturbed dark and bright solitary wave. At last, we give the analytical expressions of nonautonomous dark one- and two-soliton solutions for repulsive interaction, and investigate their properties analytically.

cond-mat.quant-gas↗

arXiv2010

Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO$_3$/SrTiO$_3$ interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.

cond-mat.mes-hall↗

arXiv2010

Exact exact solutions of the Gross-Pitaevskii equation for stable vortex modes

We construct exact solutions of the Gross-Pitaevskii equation for solitary vortices, and approximate ones for fundamental solitons, in 2D models of Bose-Einstein condensates with a spatially modulated nonlinearity of either sign and a harmonic trapping potential. The number of vortex-soliton (VS) modes is determined by the discrete energy spectrum of a related linear Schrödinger equation. The VS families in the system with the attractive and repulsive nonlinearity are mutually complementary. \emph{% Stable} VSs with vorticity $S\geq 2$ and those corresponding to higher-order radial states are reported for the first time, in the case of the attraction and repulsion, respectively.

cond-mat.quant-gas↗

arXiv2010

Vortices and ring dark solitons in nonlinear amplifying waveguides

We consider the generation and propagation of (2+1)-dimensional beams in a nonlinear waveguide with the linear gain. Simple self-similar evolution of the beams is achieved at the asymptotic stage, if the input beams represent the fundamental mode. On the contrary, if they carry vorticity or amplitude nodes (or phase slips), vortex tori and ring dark solitons (RDSs) are generated, featuring another type of the self-similar evolution, with an exponentially shrinking vortex core or notch of the RDS. Numerical and analytical considerations reveal that these self-similar structures are robust entities in amplifying waveguides, being \emph{stable} against azimuthal perturbations.

physics.optics↗

arXiv2009

Diamagnetism and Cooper pairing above $T_c$ in cuprates

In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at $T_c$ is caused by the loss of long-range phase coherence, rather than the vanishing of the pair condensate. Here we report a series of torque magnetization measurements on single crystals of $\mathrm{La_{2-x}Sr_xCuO_4}$ (LSCO), $\mathrm{Bi_2Sr_{2-y}La_yCuO_6}$ (Bi 2201), $\mathrm{Bi_2Sr_2CaCu_2O_{8+δ}}$ (Bi 2212) and optimal $\mathrm{YBa_2Cu_3O_7}$. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above $T_c$, we show that the diamagnetic term $M_d$ appears at an onset temperature $T^M_{onset}$ high above $T_c$. We construct the phase diagram of both LSCO and Bi 2201 and show that $T^M_{onset}$ agrees with the onset temperature of the vortex Nernst signal $T^ν_{onset}$. Our results provide thermodynamic evidence against a recent proposal that the high-temperature Nernst signal in LSCO arises from a quasiparticle contribution in a charge-ordered state.

cond-mat.supr-con↗

arXiv2009

Plans for PANDA Online Computing

The PANDA experiment will not use any hardware trigger, i.e. all raw data are streaming in the data acquisition with a bandwidth of ~280 GB/s. The PANDA Online System is designed to perform data reduction by a factor of ~800 by reconstruction algorithms programmed in VHDL (Very High Speed Integrated Circuit Hardware Description Language) on FPGAs (Field Programmable Gate Arrays).

physics.ins-det↗

arXiv2009

Divergent resistance at the Dirac point in graphene: Evidence for a transition in a high magnetic field

We have investigated the behavior of the resistance of graphene at the $n=0$ Landau Level in an intense magnetic field $H$. Employing a low-dissipation technique (with power $P<$3 fW), we find that, at low temperature $T$, the resistance at the Dirac point $R_0(H)$ undergoes a 1000-fold increase from $\sim$10 k$Ω$ to 40 M$Ω$ within a narrow interval of field. The abruptness of the increase suggests that a transition to an insulating, ordered state occurs at the critical field $H_c$. Results from 5 samples show that $H_c$ depends systematically on the disorder, as measured by the offset gate voltage $V_0$. Samples with small $V_0$ display a smaller critical field $H_c$. Empirically, the steep increase in $R_0$ fits acccurately a Kosterlitz-Thouless-type correlation length over 3 decades. The curves of $R_0$ vs. $T$ at fixed $H$ approach the thermal-activation form with a gap $Δ\sim$15 K as $H\to H_c^{-}$, consistent with a field-induced insulating state.

cond-mat.mes-hall↗

arXiv2007

The zero-energy state in graphene in a high magnetic field

The fate of the charge-neutral Dirac point in graphene in a high magnetic field $H$ has been investigated at low temperatures ($T\sim$ 0.3 K). In samples with small $V_0$ (the gate voltage needed to access the Dirac point), the resistance $R_0$ at the Dirac point diverges steeply with $H$, signalling a crossover to an insulating state in intense field. The approach to the insulating state is highly unusual. Despite the steep divergence in $R_0$, the profile of $R_0$ vs. $T$ in fixed $H$ saturates to a $T$-independent value below 2 K, consistent with charge carrying gapless excitations.

cond-mat.mes-hall↗

arXiv2007

Soliton Solution for the Spin Current in Ferromagnetic Nanowire

We investigate the interaction of a periodic solution and one-soliton solution for the spin-polarized current in a uniaxial ferromagnetic nanowire. The amplitude and wave number of the periodic solution for the spin current have the different contribution to the width, velocity, and the amplitude of soliton solution, respectively. Moreover, we found that the soliton can be trapped only in space with a proper condition. At last we analyze the modulation instability and discuss dark solitary wave propagation for the spin current on the background of a periodic solution. In some special cases the solution can be expressed by the linear combination of periodic solution and soliton solution.

cond-mat.other↗

arXiv2007

Crystal Structure and Physical Properties of Mg6Cu16Si7-type M6Ni16Si7, for M = Mg, Sc, Ti, Nb, and Ta

Five compounds were investigated for magnetic character and superconductivity, all with non-magnetic nickel and band structures containing flat bands and steep bands. The syntheses and crystal structures, refined by powder X-ray diffraction, are reported for M6Ni16Si7, where M = Mg, Sc, Ti, Nb, and Ta. All compounds form in the Mg6Cu16Si7 structure type. Resistance measurements are also reported on M6Ni16Si7 (M = Mg, Sc, Ti, and Nb) down to 0.3 K, with all four showing metallic conductivity. No superconductivity is observed. Magnetization measurements for all compounds reveal essentially temperature independent paramagnetism, with a tendency toward more enhanced low temperature paramagnetism for the cases of Mg6Ni16Si7 and Sc6Ni16Si7.

cond-mat.supr-con↗