arXiv subjects
Publications and source records attributed to Wei Wu.
We derive a large deviations principle for the two-dimensional two-component plasma in a box. As a consequence, we obtain a variational representation for the free energy, and also show that the macroscopic empirical measure of either positive or negative charges converges to the uniform measure. An appendix, written by Wei Wu, discusses applications to the supercritical complex Gaussian multiplicative chaos.
This paper describes a new design of superconducting dipole magnet prototype by the use of tilted solenoids. The magnet prototype, which consists of four layers of superimposed tilted solenoids with operating current of 3708 A, will produce a 2.5 T magnetic field in an aperture of 50 mm diameter. The detailed magnetic field design by using two kinds of software is presented. And their results show a good agree in the magnetic fields. So far we have accomplished the prototype construction and expect a cryogenic test. The process of the magnet fabrication is also reported in detail.
A conduction-cooled superconducting magnet producing a transverse field of 4 Tesla has been designed for the new generation multi-field coupling measurement system, which was used to study the mechanical behavior of superconducting samples at cryogenic temperature and intense magnetic fields. Considering experimental costs and coordinating with system of strain measurements by contactless signals (nonlinear CCD optics system), the racetrack type for the coil winding was chosen in our design, and a compact cryostat with a two-stage GM cryocooler was designed and manufactured for the superconducting magnet. The magnet was composed of a pair of flat racetrack coils wound by NbTi/Cu superconducting composite wires, a copper and stainless steel combinational form and two Bi2Sr2CaCu2Oy superconducting current leads. All the coils were connected in series and can be powered with a single power supply. The maximum central magnetic field is 4 T. In order to support the high stress and uniform thermal distribution in the superconducting magnet, a detailed finite element (FE) analysis has been performed. The detailed design of superconducting racetrack magnet system is described in this paper.
Statistical analysis and inferences on spike trains are one of the central topics in neural coding. It is of great interest to understand the underlying distribution and geometric structure of given spike train data. However, a fundamental obstacle is that the space of all spike trains is not an Euclidean space, and non-Euclidean metrics have been commonly used in the literature to characterize the variability and pattern in neural observations. Over the past few years, two Euclidean-like metrics were independently developed to measure distance in the spike train space. An important benefit of these metrics is that the spike train space will be suitable for embedding in Euclidean spaces due to their Euclidean properties. In this paper, we systematically compare these two metrics on theory, properties, and applications. Because of its Euclidean properties, one of these metrics has been further used in defining summary statistics (i.e. mean and variance) and conducting statistical inferences in the spike train space. Here we provide equivalent definitions using the other metric and show that consistent statistical inferences can be conducted. We then apply both inference frameworks in a neural coding problem for a recording in geniculate ganglion stimulated by different tastes. It is found that both frameworks achieve desirable results and provide useful new tools in statistical inferences in neural spike train space.
When a simply connected domain $D\subset{\mathbb{R}}^d$ ($d\ge 2$) is approximated in a "good" way by embedded connected weighted graphs, we prove that the transfer current matrix (defined on the edges of the graph viewed as an electrical network) converges, up to a local weight factor, to the differential of Green's function on $D$. This observation implies that properly rescaled correlations of the spanning tree model and correlations of minimal subconfigurations in the abelian sandpile model have a universal and conformally covariant limit. We further show that, on a periodic approximation of the domain, all pattern fields of the spanning tree model, as well as the minimal-pattern (e.g. zero-height) fields of the sandpile, converge weakly in distribution to Gaussian white noise.
The electronic structure and magnetic properties of CaCr$\mathrm{O}_3$ have been calculated by two methods, including hybrid-exchange density-function theory and density-functional theory + $U$. The computed densities of states from both of these methods are in a qualitative agreement with the previous x-ray spectroscopy. On the other hand, the opening of the band gap separates them apart. hybrid-exchange density-functional theory always gives a finite band gap, down to $\sim 1.2$ eV from HSE06 functional, whereas by tuning the Hubbard-$U$ parameter down to $0.5$ eV, a conducting state with AFM-C (defined in the text) spin configuration can be achieved. From hybrid density-functional theory, the computed nearest-neighbouring exchange interaction along the $c$-axis and in the $ab$-plane are $\sim 4$ meV and $\sim 6$ meV (anti-ferromagnetic), respectively, which are qualitatively in agreement with the previous magnetic measurements. These anti-ferromagnetic exchange interaction, together with the in-plane anti-ferro-orbital ordering will induce a spin-orbital frustration, which could play a role for the abnormal electronic properties in CaCrO$_3$. In hybrid-exchange density-functional theory, an abrupt reduction ($\sim 0.2$ eV) of the majority-spin band gap of the ferromagnetic state between 60 K and 100 K has been found as lowering temperature, which shows a strong link to the previous optical conductivity measurements in [A. C. Komarek, et. al., Phys. Rev. B \textbf{84}, 125114 (2011)]. In sharp contrast, the density-functional theory + $U$ methods predicted AFM-C state as the lowest AFM state for the crystal structure measured below 90 K, above which AFM-A is however the lowest. The closely related concepts including electron-hole liquid and surface-plasmon-mediating spin-spin interactions have been discussed as well.
The High Intensity Heavy-Ion Accelerator Facility is a new project in the Institute of Modern Physics. The dipole magnets of all rings are conceived as fast cycled superconducting magnet with high magnetic field and large gap, the warm iron and superconducting coil structure (superferric) is adopted. The reasonable structure design of coil and cryostat is very important for reliable operation. Based on the finite element software ANSYS, the mechanical analysis of electromagnetic stress, the thermal stress in the cooling down and the stress in the pumping are showed in detail. According to the analysis result, the supporter structure is the key problem of coil system. With reasonable support's structure design, the stress and the deformation of coil structure can be reduced effectively, which ensure the stable operation of superconducting coil system.
The electronic structures of the cubic and tetragonal $\mathrm{MnV}_2\mathrm{O}_4$ have been studied by using hybrid-exchange density functional theory. The computed electronic structure of the tetragonal phase shows an anti-ferro orbital ordering on V sites and a ferrimagnetic ground state (the spins on V and Mn are anti-aligned). These results are in a good agreement with the previous theoretical result obtained from the local-density approximation+$U$ methods [S. Sarkar, et. al., Phys. Rev. Lett. 102, 216405 (2009)]. Moreover, the electronic structure, especially the projected density of states of the cubic phase has been predicted with a good agreement with the recent soft x-ray spectroscopy experiment. Similar to the tetragonal phase, the spins on V and Mn in the cubic structure favour a ferrimagnetic configuration. Most interesting is that the computed charge densities of the spin-carrying orbitals on V in the cubic phase show an exotic orbital ordering, i.e., a ferro-orbital ordering along [110] but an anti-ferro-orbital ordering along [$\overline{1}$10].
We investigate the metal-insulator transition (MIT) and phase diagram of the half-filled Fermi Hubbard model with Rashba-type spin-orbit coupling (SOC) on a square optical lattice. The interplay between the atomic interactions and SOC results in distinctive features of the MIT. Significantly, in addition to the diverse spin ordered phases, a nonmagnetic insulating phase emerges in a considerably large regime of parameters near the Mott transition. This phase has a finite single-particle gap but vanishing magnetization and spin correlation exhibits a power-law scaling, suggesting a potential algebraic spin-liquid ground state. These results are confirmed by the non-perturbative cluster dynamical mean-field theory.
Superconductivity in heavy-fermion materials can sometimes appear in the incoherent regime and in proximity to an antiferromagnetic quantum critical point. Here we study these phenomena using large scale determinant quantum Monte Carlo simulations and the dynamical cluster approximation with various impurity solvers for the periodic Anderson model with frustrated hybridization. We obtain solid evidence for a $d_{x^2-y^2}$ superconducting phase arising from an incoherent normal state in the vicinity of an antiferromagnetic quantum critical point. There is a coexistence region and the width of the superconducting dome increases with frustration. Through a study of the pairing dynamics we find that the retarded spin-fluctuations give the main contribution to the pairing glue. These results are relevant for unconventional superconductivity in the Ce-$115$ family of heavy-fermions.
The spin dynamics of radical-triplet system (RTS) has been calculated by using the Lindblad formalism within the theory of open quantum system. The single-radical-triplet system (SRTS) is considered here for single-qubit quantum gate operations while double-radical-triplet system (DRTS) for two-qubit operations. The environment effects taken into account include the spin-lattice relaxation of the triplet exciton and radical spin-$\frac{1}{2}$, the inter-system crossing process that induces the transition from singlet excited state to the triplet ground state, and the rather slow relaxation process from the triplet ground state back down to the singlet ground state. These calculations shown that the line shape broadening is strongly related to the exchange interaction between triplet and exciton, which can be understood as a spontaneous magnetic field created by the triplet renormalises the original spin-$\frac{1}{2}$ electron spin resonance spectra. This work will provide key information about the spin dynamics for building optically-controlled molecular quantum gate out of radical-bearing molecules. Moreover, this has generated the further theoretical question on how the mixture of fermion and boson behaves.
CrAs was firstly observed possessing the bulk superconductivity (Tc~2 K) under high pressure (0.8 GPa) in the very recent work (Wei Wu, et al. Nature Communications 5, 5508 (2014)). To explore the correlation between the structure and the superconductivity, the high-pressure structure evolution of CrAs was investigated using angle dispersive X-ray diffraction (XRD) method with small steps of ~0.1 GPa in a diamond anvil cell (DAC) up to 1.8 GPa. In the pressure range covered by our current experiment, the structure of CrAs keeps stable. However, the lattice parameters exhibit anomalous compression behaviors. With the pressure increasing, the lattice parameters a and c both show a process of first increasing and then decreasing, and the lattice parameter b goes through a quick contract at 0.35 GPa, which suggests a pressure-induced isostructural phase transition occurs in CrAs around this pressure point. Above the phase transition pressure, the axial compressibilities of CrAs present remarkable anisotropy. The compressibilities along the a- and c-axis are about an order of magnitude smaller than that along the b-axis, which is closely related to the different stacking modes in CrAs6 octahedron along different crystallographic axes. A schematic band model was used for addressing above anomalous compression behavior in CrAs.
We study superconductivity on the honeycomb lattice close to the Mott state at half-filling. Due to the sixfold lattice symmetry and disjoint Fermi surfaces at opposite momenta, we show that several different fully gapped superconducting states naturally exist on the honeycomb lattice, of which the chiral $d+id'$-wave state has previously been shown to appear when superconductivity appears close to the Mott state. Using renormalized mean-field theory to study the t-J model and quantum Monte Carlo calculations of the Hubbard-U model we show that the $d+id'$-wave state is the favored superconducting state for a wide range of on-site repulsion U, from the intermediate to the strong coupling regime. We also investigate the possibility of a mixed chirality d-wave state, where the overall chirality cancels. We find that a state with $d+id'$-wave symmetry in one valley but $d-id'$-wave symmetry in the other valley is not possible in the t-J model without reducing the translational symmetry, due to the zero-momentum and spin-singlet nature of the superconducting order parameter. Moreover, any extended unit cells result either in disjoint Dirac points, which cannot harbor this mixed chirality state, or the two valleys are degenerate at the zone center, where valley hybridization prevents different superconducting condensates. We also investigate extended unit cells where the overall chirality cancels in real space. For supercells containing up to eight sites, including the Kekulé distortion, we find no energetically favorable d-wave solution with an overall zero chirality within the restriction of the t-J model.
One of the common features of unconventional, magnetically mediated superconductivity as found in the heavy-fermions, high-transition-temperature (high-Tc) cuprates, and iron pnictides superconductors is that the superconductivity emerges in the vicinity of long-range antiferromagnetically ordered state.[1] In addition to doping charge carriers, the application of external physical pressure has been taken as an effective and clean approach to induce the unconventional superconductivity near a magnetic quantum critical point (QCP).[2,3] Superconductivity has been observed in a majority of 3d transition-metal compounds,[4-9] except for the Cr- and Mn-based compounds in the sense that the low-lying states near Fermi level are dominated by their 3d electrons. Herein, we report on the discovery of superconductivity on the verge of antiferromagnetic order in CrAs via the application of external high pressure. Bulk superconductivity with Tc ~ 2 K emerges at the critical pressure Pc ~ 8 kbar, where the first-order antiferromagnetic transition at TN = 265 K under ambient pressure is completely suppressed. Abnormal normal-state properties associated with a magnetic QCP have been observed nearby Pc. The close proximity of superconductivity to an antiferromagnetic order suggests an unconventional pairing mechanism for the superconducting state of CrAs. The present finding opens a new avenue for searching novel superconductors in the Cr and other transitional-metal based systems.
Triplet excitonic state in the organic molecule may arise from a singlet excitation and the following inter-system crossing. Especially for a spin-bearing molecule, an exchange interaction between the triplet exciton and the original spin on the molecule can be expected. In this paper, such exchange interaction in copper-phthalocyanine (CuPc, spin-$\frac{1}{2}$) was investigated from first-principles by using density-functional theory within a variety of approximations to the exchange correlation, ranging from local-density approximation to long-range corrected hybrid-exchange functional. The magnitude of the computed exchange interaction is in the order of meV with the minimum value (1.5 meV, ferromagnetic) given by the long-range corrected hybrid-exchange functional CAM-B3LYP. This exchange interaction can therefore give rise to a spin coherence with an oscillation period in the order of picoseconds, which is much shorter than the triplet lifetime in CuPc (typically tens of nanoseconds). This implies that it might be possible to manipulate the localised spin on Cu experimentally using optical excitation and inter-system crossing well before the triplet state disappears.
We study random two-component spanning forests ($2$SFs) of finite graphs, giving formulas for the first and second moments of the sizes of the components, vertex-inclusion probabilities for one or two vertices, and the probability that an edge separates the components. We compute the limit of these quantities when the graph tends to an infinite periodic graph in ${\mathbb R}^d$.
The Hubbard model and extended Hubbard model on the honeycomb lattice can be seen as prototype models of single layer graphene placed in a high dielectric constant environment that screens the Coulomb interaction. Taking advantage of the absence of a sign problem at half-filling, we study this problem with clusters up to 96 sites with the Determinant Quantum Monte Carlo Method as an impurity solver for the the Dynamical Cluster Approximation at finite temperatures. After determining the stability of the semi-metallic phase to interaction-induced spin-density wave (SDW), charge-density wave (CDW) and Mott insulating phases, we study the single particle dynamics of the Dirac fermions. We show that when spontaneous symmetry breaking is avoided, the semi-metallic phase is a stable Fermi liquid in the presence of repulsive interactions and that Kondo screening dominates the low temperature regime, even though there is a $ρ(ω) = |ω|$ type local density of states. We also investigate the impact of the correlation effects on the renormalization of the Fermi velocity $v_F$. We find that $v_F$ is not renormalized when only on-site repulsion $U$ is present, but that near-neighbor repulsion $V$ does renormalize $v_F$. This may explain the variations between different measurements of $v_F$ in graphene.
We construct a natural discrete random field on $\mathbb{Z}^{d}$, $d\geq 5$ that converges weakly to the bi-Laplacian Gaussian field in the scaling limit. The construction is based on assigning i.i.d. Bernoulli random variables on each component of the uniform spanning forest, thus defines an associated random function. To our knowledge, this is the first natural discrete model (besides the discrete bi-Laplacian Gaussian field) that converges to the bi-Laplacian Gaussian field.