arXiv subjects

X. Lu

Publications and source records attributed to X. Lu.

At least 91 records · Page 5Linked to original sources

arXiv2017

Crossover from Collective to Incoherent Spin Excitations in Superconducting Cuprates Probed by Detuned Resonant Inelastic X-ray Scattering

Spin excitations in the overdoped high temperature superconductors Tl$_2$Ba$_2$CuO$_{6+δ}$ and (Bi,Pb)$_2$(Sr,La)$_{2}$CuO$_{6+δ}$ were investigated by resonant inelastic x-ray scattering (RIXS) as functions of doping and detuning of the incoming photon energy above the Cu-$L_3$ absorption peak. The RIXS spectra at optimal doping are dominated by a paramagnon feature with peak energy independent of photon energy, similar to prior results on underdoped cuprates. Beyond optimal doping, the RIXS data indicate a sharp crossover to a regime with a strong contribution from incoherent particle/hole excitations whose maximum shows a fluorescence-like shift upon detuning. The spectra of both compound families are closely similar, and their salient features are reproduced by exact-diagonalization calculations of the single-band Hubbard model on a finite cluster. The results are discussed in the light of recent transport experiments indicating a quantum phase transition near optimal doping.

cond-mat.supr-con↗

arXiv2017

Dilatancy of Shear Transformations in a Colloidal Glass

Shear transformations, as fundamental rearrangement events operating in local regions, hold the key of plastic flow of amorphous solids. Despite their importance, the dynamic features of shear transformations are far from clear. Here, we use a colloidal glass under shear as the prototype to directly observe the shear transformation events in real space. By tracing the colloidal particle rearrangements, we quantitatively determine two basic properties of shear transformations: local shear strain and dilatation (or free volume). It is revealed that the local free volume undergoes a significantly temporary increase prior to shear transformations, eventually leading to a jump of local shear strain. We clearly demonstrate that shear transformations have no memory of the initial free volume of local regions. Instead, their emergence strongly depends on the dilatancy ability of these regions, i.e., the dynamic creation of free volume. More specifically, the particles processing the high dilatancy ability directly participate in subsequent shear transformations. These results experimentally support the Argon's statement about the dilatancy nature of shear transformations, and also shed insight into the structural origin of amorphous plasticity.

cond-mat.mtrl-sci↗

arXiv2017

Damped spin excitations in a doped cuprate superconductor with orbital hybridization

A resonant inelastic x-ray scattering (RIXS) study of overdamped spin-excitations in slightly underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) with $x=0.12$ and $0.145$ is presented. Three high-symmetry directions have been investigated: (1) the antinodal $(0,0)\rightarrow (1/2,0)$, (2) the nodal $(0,0)\rightarrow (1/4,1/4)$ and (3) the zone boundary direction $(1/2,0)\rightarrow (1/4,1/4)$ connecting these two. The overdamped excitations exhibit strong dispersions along (1) and (3), whereas a much more modest dispersion is found along (2). This is in strong contrast to the undoped compound La$_{2}$CuO$_4$ (LCO) for which the strongest dispersions are found along (1) and (2). The $t-t^{\prime}-t^{\prime\prime}-U$ Hubbard model used to explain the excitation spectrum of LCO predicts $-$ for constant $U/t$ $-$ that the dispersion along (3) scales with $(t^{\prime}/t)^2$. However, the diagonal hopping $t^{\prime}$ extracted on LSCO using single-band models is low ($t^{\prime}/t\sim-0.16$) and decreasing with doping. We therefore invoked a two-orbital ($d_{x^2-y^2}$ and $d_{z^2}$) model which implies that $t^{\prime}$ is enhanced. This effect acts to enhance the zone-boundary dispersion within the Hubbard model. We thus conclude that hybridization of $d_{x^2-y^2}$ and $d_{z^2}$ states has a significant impact on the zone-boundary dispersion in LSCO.

cond-mat.supr-con↗

arXiv2016

Superconductivity and multiple pressure-induced phases in BaPt$_2$As$_2$

The newly discovered BaPt$_2$As$_2$ shows a structural distortion at around 275~K, followed by the emergence of superconductivity at lower temperatures. Here we identify the presence of charge density wave (CDW) order at room temperature and ambient pressure using single crystal x-ray diffraction, with both a superlattice and an incommensurate modulation, where there is a change of the superlattice structure below $\simeq$ 275~K. Upon applying pressure, BaPt$_2$As$_2$ shows a rich temperature-pressure phase diagram with multiple pressure-induced transitions at high temperatures, the emergence or disappearance of which are correlated with sudden changes in the superconducting transition temperature $T_c$. These findings demonstrate that BaPt$_2$As$_2$ is a promising new system for studying competing interactions and the relationship between high-temperature electronic instabilities and superconductivity.

cond-mat.supr-con↗

arXiv2016

Study of Tomlinson-Harashima Precoding Strategies for Physical-Layer Security in Wireless Networks

In this paper, we propose novel non-linear precoders for the downlink of a multi-user MIMO system with the existence of multiple eavesdroppers. The proposed non-linear precoders are designed to improve the physical-layer secrecy rate. Specifically, we combine the non-linear successive optimization Tomlinson-Harashima precoding (SO-THP) with generalized matrix inversion (GMI) technique to maximize the physical-layer secrecy rate. For the purpose of comparison, we examine different traditional precoders with the proposed algorithm in terms of secrecy rate as well as BER performance. We also investigate simplified generalized matrix inversion (S-GMI) and lattice-reduction (LR) techniques in order to efficiently compute the parameters of the precoders. We further conduct computational complexity and secrecy rate analysis of the proposed and existing algorithms. In addition, in the scenario without knowledge of channel state information (CSI) to the eavesdroppers, a strategy of injecting artificial noise (AN) prior to the transmission is employed to enhance the physical-layer secrecy rate. Simulation results show that the proposed non-linear precoders outperform existing precoders in terms of BER and secrecy rate performance.

cs.IT↗

arXiv2016

Multiple quantum phase transitions and superconductivity in Ce-based heavy fermions

Heavy fermions have served as prototype examples of strongly-correlated electron systems. The occurrence of unconventional superconductivity in close proximity to the electronic instabilities associated with various degrees of freedom points to an intricate relationship between superconductivity and other electronic states, which is unique but also shares some common features with high temperature superconductivity. The magnetic order in heavy fermion compounds can be continuously suppressed by tuning external parameters to a quantum critical point, and the role of quantum criticality in determining the properties of heavy fermion systems is an important unresolved issue. Here we review the recent progress of studies on Ce based heavy fermion superconductors, with an emphasis on the superconductivity emerging on the edge of magnetic and charge instabilities as well as the quantum phase transitions which occur by tuning different parameters, such as pressure, magnetic field and doping. We discuss systems where multiple quantum critical points occur and whether they can be classified in a unified manner, in particular in terms of the evolution of the Fermi surface topology.

cond-mat.supr-con↗

arXiv2016

Domain Topology and Domain Switching Kinetics in a Hybrid Improper Ferroelectric

Charged polar interfaces such as charged ferroelectric domain walls or heterostructured interfaces of ZnO/(Zn,Mg)O and LaAlO3/SrTiO3, across which the normal component of electric polarization changes suddenly, can host large two-dimensional conduction. Charged ferroelectric walls, which are energetically unfavorable in general, were found to be mysteriously abundant in hybrid improper ferroelectric (Ca,Sr)3Ti2O7 crystals. From the exploration of antiphase boundaries in bilayer-perovskites, we discover that each of four polarization-direction states is degenerated with two antiphase domains, and these eight structural variants form a Z4xZ2 domain structure with Z3 vortices and five distinct types of domain walls, whose topology is directly relevant to the presence of abundant charged walls. We also discover a zipper-like nature of antiphase boundaries; they are the reversible creation/annihilation centers of pairs of two types of ferroelectric walls (and also Z3-vortex pairs) in 90 and 180 degree polarization switching. Our results demonstrate the unexpectedly rich nature of hybrid improper ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2015

Study of Opportunistic Cooperation Techniques using Jamming and Relays for Physical-Layer Security in Buffer-aided Relay Networks

In this paper, we investigate opportunistic relay and jammer cooperation schemes in multiple-input multiple-output (MIMO) buffer-aided relay networks. The network consists of one source, an arbitrary number of relay nodes, legitimate users and eavesdroppers, with the constraints of physical layer security. We propose an algorithm to select a set of relay nodes to enhance the legitimate users' transmission and another set of relay nodes to perform jamming of the eavesdroppers. With Inter-Relay interference (IRI) taken into account, interference cancellation can be implemented to assist the transmission of the legitimate users. Secondly, IRI can also be used to further increase the level of harm of the jamming signal to the eavesdroppers. By exploiting the fact that the jamming signal can be stored at the relay nodes, we also propose a hybrid algorithm to set a signal-to-interference and noise ratio (SINR) threshold at the node to determine the type of signal stored at the relay node. With this separation, the signals with high SINR are delivered to the users as conventional relay systems and the low SINR performance signals are stored as potential jamming signals. Simulation results show that the proposed techniques obtain a significant improvement in secrecy rate over previously reported algorithms.

cs.IT↗

arXiv2015

Nodeless superconductivity in noncentrosymmetric PbTaSe$_2$ single crystals

We report an investigation of the superconducting order parameter of the noncentrosymmetric compound PbTaSe$_2$, which is believed to have a topologically nontrivial band structure. Precise measurements of the London penetration depth $Δλ(T)$ obtained using a tunnel diode oscillator (TDO) based method show an exponential temperature dependence at $T\ll T_c$, suggesting a nodeless superconducting gap structure. A single band s-wave model well describes the corresponding normalized superfluid density, with a gap magnitude of $Δ(0)=1.85T_c$. This is very close to the value of $1.76T_c$ for weak-coupling BCS superconductors, indicating conventional fully-gapped superconductivity in PbTaSe$_2$.

cond-mat.supr-con↗

arXiv2015

Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb2Ni12(P,As)7

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses T_FL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.

cond-mat.str-el↗

arXiv2015

Economics of Internet of Things (IoT): An Information Market Approach

Internet of things (IoT) has been proposed to be a new paradigm of connecting devices and providing services to various applications, e.g., transportation, energy, smart city, and healthcare. In this paper, we focus on an important issue, i.e., economics of IoT, that can have a great impact to the success of IoT applications. In particular, we adopt and present the information economics approach with its applications in IoT. We first review existing economic models developed for IoT services. Then, we outline two important topics of information economics which are pertinent to IoT, i.e., the value of information and information good pricing. Finally, we propose a game theoretic model to study the price competition of IoT sensing services. Some outlooks on future research directions of applying information economics to IoT are discussed.

cs.NI↗

arXiv2015

Evidence for two distinct superconducting phases in EuBiS$_2$F under pressure

We present a pressure study of the electrical resistivity, AC magnetic susceptibility and powder x-ray diffraction (XRD) of the newly discovered BiS$_2$-based superconductor EuBiS$_2$F. At ambient pressure, EuBiS$_2$F shows an anomaly in the resistivity at around $T_0\approx 280$ K and a superconducting transition at $T_c\approx 0.3$ K. Upon applying hydrostatic pressure, there is little change in $T_0$ but the amplitude of the resistive anomaly is suppressed, whereas there is a dramatic enhancement of $T_c$ from 0.3 K to about 8.6 K at a critical pressure of $p_c$ $\approx{1.4}$ GPa. XRD measurements confirm that this enhancement of $T_c$ coincides with a structural phase transition from a tetragonal phase ($P4/nmm$) to a monoclinic phase ($P2_1$/m), which is similar to that observed in isostructural LaO$_{0.5}$F$_{0.5}$BiS$_2$. Our results suggest the presence of two different superconducting phases with distinct crystal structures in EuBiS$_2$F, which may be a general property of this family of BiS$_2$-based superconductors.

cond-mat.supr-con↗

arXiv2015

Pentaquark $Θ^+$ search at HERMES

The earlier search at HERMES for narrow baryon states excited in quasi-real photoproduction, decaying through the channel $pK_S^0\rightarrow pπ^+π^-$, has been extended with improved decay-particle reconstruction, more advanced particle identification, and increased event samples. The structure observed earlier at an invariant mass of 1528 MeV shifts to 1522 MeV and the statistical significance drops to about 2$σ$ for data taken with a deuterium target. The number of events above background is $68_{-31}^{+98}\text{(stat)}\pm13\text{(sys)}$. No such structure is observed in the hydrogen data set.

hep-ex↗

arXiv2014

Superconductivity and structural distortion in BaPt$_2$As$_2$

We successfully synthesized the BaPt$_2$As$_2$ single crystals and studied their structural and physical properties at low temperatures. BaPt$_2$As$_2$ crystallizes in the CaBe$_2$Ge$_2$-type tetragonal structure (P4/nmm) at room temperature and undergoes a first-order structural transition at $T_S\simeq 275$ K, which is likely associated with a charge-density-wave (CDW) instability. BCS-type superconductivity with two subsequent transitions at $T_{c1}=1.67$K and $T_{c2}$=1.33K are observed in this compound. Thus, BaPt$_2$As$_2$ may serve as a new system for studying the interplay of superconductivity and the CDW order.

cond-mat.supr-con↗

arXiv2014

How can one measure group cohesion? From individual organisms to their interaction

Measuring atomic and molecular interactions was one of the main objectives of physics during the past century. It was an essential step not only in itself but because most macroscopic properties can be derived once one knows interaction strengths. At the present time, except for systems that can be described as discrete networks (like the Internet network) our knowledge of social and biological ties still remains very limited. An important step is to develop experimental means for measuring social and biological interactions. In this talk there are two parts. Firstly, we describe experimental evidence of inter-individual attraction in populations of insects. Secondly, we focus on a specific system, namely populations of Euglena gracilis, a green, swimming unicellular organism, for which we try to determine individual and interaction properties.

physics.bio-ph↗

arXiv2014

Film structure of epitaxial graphene oxide on SiC: Insight on the relationship between interlayer spacing, water content, and intralayer structure

Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter-layer structure of these ultra-thin oxide films. X-ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter-layer registry, little amount (<10%) of intercalated water, and unexpectedly large interlayer separations of about 9.35 Å. Density functional theory calculations show that the apparent contradiction of "little water but large interlayer spacing in the graphene oxide films" can be explained by considering a multilayer film formed by carbon layers presenting, at the nanoscale, a non-homogenous oxidation, where non-oxidized and highly oxidized nano-domains coexist and where a few water molecules trapped between oxidized regions of the stacked layers are sufficient to account for the observed large inter-layer separations. This work sheds light on both the vertical and intra-layer structure of graphene oxide films grown on silicon carbide, and more in general, it provides novel insight on the relationship between inter-layer spacing, water content, and structure of graphene/graphite oxide materials.

cond-mat.mtrl-sci↗

arXiv2014

Anisotropic superconductivity in the noncentrosymmetric BiPd

We report measurements of London penetration depth $λ(T)$ for the noncentrosymmetric superconductor BiPd by using a tunnel diode oscillator. Pronounced anisotropic behavior is observed in the low-temperature penetration depth; the in-plane penetration depth $λ_{ac}(T)$ follows an exponential decrease, but the interplane penetration depth $λ_b(T)$ shows power-law-type behavior. The superfluid density $ρ_s(T)$, converted from the penetration depth $λ(T)$, is best fitted by an anisotropic two-band BCS model. We argue that such a complex order parameter is attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of antisymmetric spin-orbit coupling in BiPd.

cond-mat.supr-con↗

arXiv2013

CeIrIn5: Superconductivity on a Magnetic Instability

We report the doping-induced antiferromagnetic state and Fermi liquid state that are connected by a superconducting region in a series of CeIrIn$_{5-x}$Hg$_x$, CeIrIn$_{5-x}$Sn$_x$ and CeIr$_{1-x}$Pt$_x$In$_5$ single crystals. Measurements of the specific heat $C(T)$ and electrical resistivity $ρ(T)$ demonstrate that hole doping via Hg/In substitution gives rise to an antiferromagnetic ground state, but substitutions of In by Sn or Ir by Pt (electron doping) favor a paramagnetic Fermi liquid state. A cone-like non-Fermi liquid region is observed near CeIrIn$_5$, showing a diverging effective mass on the slightly Hg-doped side. The obtained temperature-doping phase diagram suggests that CeIrIn$_5$ is in proximity to an antiferromagnetic quantum critical point, and heavy fermion superconductivity in this compound is mediated by magnetic quantum fluctuations rather than by valence fluctuations.

cond-mat.str-el↗