arXiv subjects

X. Wang

Publications and source records attributed to X. Wang.

At least 325 records · Page 18Linked to original sources

arXiv2013

Dynamical Mean Field Theory of Nickelate Superlattices

Dynamical mean field methods are used to calculate the phase diagram, many-body density of states, relative orbital occupancy and Fermi surface shape for a realistic model of $LaNiO_3$-based superlattices. The model is derived from density functional band calculations and includes oxygen orbitals. The combination of the on-site Hunds interaction and charge-transfer between the transition metal and the oxygen orbitals is found to reduce the orbital polarization far below the levels predicted either by band structure calculations or by many-body analyses of Hubbard-type models which do not explicitly include the oxygen orbitals. The findings indicate that heterostructuring is unlikely to produce one band model physics and demonstrate the fundamental inadequacy of modeling the physics of late transition metal oxides with Hubbard-like models.

cond-mat.str-el↗

arXiv2013

Is hexagonal InMnO3 ferroelectric?

The presence of ferroelectricity in hexagonal (h-)InMnO3 has been highly under debate. The results of our comprehensive experiments of low-temperature (T) polarization, TEM and HAADF-STEM on well-controlled h-InMnO3 reveal that the ground state is ferroelectric with P6_3cm symmetry, but a non-ferroelectric P-3c1 state exists at high T, and can be quenched to room T. We also found that the ferroelectric P6_3cm state of h-InMnO3 exhibits the domain configuration of topological vortices, as has been observed in h-REMnO3 (RE=rare earths).

cond-mat.mtrl-sci↗

arXiv2013

Quasiconformal maps with bilipschitz or identity boundary values in Banach spaces

Suppose that $E$ and $E'$ denote real Banach spaces with dimension at least 2 and that $D\varsubsetneq E$ and $D'\varsubsetneq E'$ are uniform domains with homogeneously dense boundaries. We consider the class of all $φ$-FQC (freely $φ$-quasiconformal) maps of $D$ onto $D'$ with bilipschitz boundary values. We show that the maps of this class are $η$-quasisymmetric. As an application, we show that if $D$ is bounded, then maps of this class satisfy a two sided Hölder condition. Moreover, replacing the class $φ$-FQC by the smaller class of $M$-QH maps, we show that $M$-QH maps with bilipschitz boundary values are bilipschitz. Finally, we show that if $f$ is a $φ$-FQC map which maps $D$ onto itself with identity boundary values, then there is a constant $C\,,$ depending only on the function $φ\,,$ such that for all $x\in D$, the quasihyperbolic distance satisfies $k_D(x,f(x))\leq C$.

math.MG↗

arXiv2013

Tailoring the electronic properties of SrRuO3 films in SrRuO3/LaAlO3 superlattices

The electronic properties of SrRuO3/LaAlO3 (SRO/LAO) superlattices with different interlayer thicknesses of SRO layers were studied. As the thickness of SRO layers is reduced, the superlattices exhibit a metal-insulator transition implying transformation into a more localized state from its original bulk metallic state. The strain effect on the metal-insulator transition was also examined. The origin of the metal-insulator transition in ultrathin SRO film is discussed. All the superlattices, even those with SRO layers as thin as 2 unit cells, are ferromagnetic at low temperatures. Moreover, we demonstrate field effect devices based on such multilayer superlattice structures.

cond-mat.mtrl-sci↗

arXiv2013

Evolution of variable range hopping in strongly localized two dimensional electron gas at NdAlO3/SrTiO3 (100) heterointerfaces

We report evolution of the two-dimensional electron gas behavior at the NdAlO3/SrTiO3 heterointerfaces with varying thicknesses of the NdAlO3 overlayer. The samples with a thicker NdAlO3 show strong localizations at low temperatures and the degree of localization is found to increase with the NdAlO3 thickness. The T -1/3 temperature dependence of the sheet resistance at low temperatures and the magnetoresistance study reveal that the conduction is governed by a two-dimensional variable range hopping mechanism in this strong localized regime. We attribute this thickness dependence of the transport properties of the NdAlO3/SrTiO3 interfaces to the interface strain induced by the overlayers.

cond-mat.mes-hall↗

arXiv2013

Type Iax Supernovae: A New Class of Stellar Explosion

We describe observed properties of the Type Iax class of supernovae (SNe Iax), consisting of SNe observationally similar to its prototypical member, SN 2002cx. The class currently has 25 members, and we present optical photometry and/or optical spectroscopy for most of them. SNe Iax are spectroscopically similar to SNe Ia, but have lower maximum-light velocities (2000 < |v| < 8000 km/s), typically lower peak magnitudes (-14.2 > M_V,peak > -18.9 mag), and most have hot photospheres. Relative to SNe Ia, SNe Iax have low luminosities for their light-curve shape. There is a correlation between luminosity and light-curve shape, similar to that of SNe Ia, but offset from that of SNe Ia and with larger scatter. Despite a host-galaxy morphology distribution that is highly skewed to late-type galaxies without any SNe Iax discovered in elliptical galaxies, there are several indications that the progenitor stars are white dwarfs (WDs): evidence of C/O burning in their maximum-light spectra, low ejecta masses, strong Fe lines in their late-time spectra, a lack of X-ray detections, and deep limits on massive stars and star formation at the SN sites. However, two SNe Iax show strong He lines in their spectra. The progenitor system and explosion model that best fits all of the data is a binary system of a C/O WD that accretes matter from a He star and has a significant deflagration. At least some of the time, this explosion will not disrupt the WD. We estimate that in a given volume there are 31^+17_-13 SNe Iax for every 100 SNe Ia, and for every 1 M_sun of iron generated by SNe Ia at z = 0, SNe Iax generate 0.052^+0.017_-0.014 M_sun. Being the largest class of peculiar SNe, thousands of SNe Iax will be discovered by LSST. Future detailed observations of SNe Iax should further our understanding of both their progenitor systems and explosions as well as those of SNe Ia.

astro-ph.SR↗

arXiv2012

Lipschitz spaces and bounded mean oscillation of harmonic mappings

In this paper, we first study the bounded mean oscillation of planar harmonic mappings, then a relationship between Lipschitz-type spaces and equivalent modulus of real harmonic mappings is established. At last, we obtain sharp estimates on Lipschitz number of planar harmonic mappings in terms of bounded mean oscillation norm, which shows that the harmonic Bloch space is isomorphic to $BMO_{2}$ as a Banach space..

math.CV↗

arXiv2012

On relation between geometric momentum and annihilation operators on a two-dimensional sphere

With a recently introduced geometric momentum that depends on the extrinsic curvature and offers a proper description of momentum on two-dimensional sphere, we show that the annihilation operators whose eigenstates are coherent states on the sphere take the expected form αx+iβp, where α and β are two operators that depend on the angular momentum and x and p are the position and the geometric momentum, respectively. Since the geometric momentum is manifestly a consequence of embedding the two-dimensional sphere in the three-dimensional flat space, the coherent states reflects some aspects beyond the intrinsic geometry of the surfaces.

quant-ph↗

arXiv2012

R x B Drift Momentum Spectrometer

We propose a new type of momentum spectrometer, which uses the RxB drift effect to disperse the charged particles in a uniformly curved magnetic field. This kind of RxB spectrometer is designed for the momentum analyses of the decay electrons and protons in the PERC (Proton and Electron Radiation Channel) beam station, which provides a strong magnetic field to guide the charged particles in the instrument. Instead of eliminating the guiding field, the RxB spectrometer evolves the field gradually to the analysing field, and the charged particles can be adiabatically transported during the dispersion and detection. The drifts of the particles have similar properties as their dispersion in the normal magnetic spectrometer. Besides, the RxB spectrometer is especially ideal for the measurements of particles with low momenta and relative large incident angles. We present a design of the RxB spectrometer, which can be used in PERC. The resolution of the momentum spectra can reach 14.4 keV/c, if the particle position measurements have a resolution of 1 mm.

physics.ins-det↗

arXiv2012

Unexpected Anisotropic Two Dimensional Electron Gas at the LaAlO3/SrTiO3 (110) Interface

The observation of a two dimensional electron gas (2DEG) (1, 2), superconductivity (3, 4), magnetic effects (5) and electronic phase separation (6-8) at the interfaces of insulating oxides, especially LaAlO3/SrTiO3, has further enhanced the potential of complex oxides for novel electronics. The occurrence of the 2DEG is strongly believed to be driven by the polarization discontinuity (9) at the interface between the two oxides. In this scenario, the crystal orientation plays an important role and no conductivity would be expected for e.g., the interface between LaAlO3 and (110)-oriented SrTiO3, which should not have a polarization discontinuity (10, 11). Here, we report the observation of unexpected conductivity at the LaAlO3/SrTiO3 interface prepared on (110)-oriented SrTiO3. The conductivity was further found to be strongly anisotropic, with the ratio of the conductance along the different directions parallel to the substrate surface showing a remarkable dependence on the oxygen pressure during deposition. The conductance and its anisotropy are discussed based on the atomic structure at the interface, as revealed by Scanning Transmission Electron Microscopy (STEM) and further supported by density functional theory (DFT) calculations.

cond-mat.mtrl-sci↗

arXiv2012

Sequential Tunneling vs. Electron Correlation in Multiple Photo-ionization

We take advantage of the information provided by use of elliptical polarization in a recent two-electron release time experiment [A.N. Pfeiffer et al., Nature Physics 7, 428 (2011)]. This allows a comparative test of the currently dominant conjectures regarding independent-electron tunneling theory vs. fully electron-correlated classical release theory to describe electron release in strong-field double ionization.

physics.atom-ph↗

arXiv2012

Strong electronic correlation and strain effects at the interfaces between polar and nonpolar complex oxides

The interface between the polar LaAlO$_3$ and nonpolar SrTiO$_3$ layers has been shown to exhibit various electronic and magnetic phases such as two dimensional electron gas, superconductivity, magnetism and electronic phase separation. These rich phases are expected due to the strong interplay between charge, spin and orbital degree of freedom at the interface between these complex oxides, leading to the electronic reconstruction in this system. However, until now all of these new properties have been studied extensively based on the interfaces which involve a polar LaAlO$_3$ layer. To investigate the role of the A and B cationic sites of the ABO$_3$ polar layer, here we study various combinations of polar/nonpolar oxide (NdAlO$_3$/SrTiO$_3$, PrAlO$_3$/SrTiO$_3$ and NdGaO$_3$/SrTiO$_3$) interfaces which are similar in nature to LaAlO$_3$/SrTiO$_3$ interface. Our results show that all of these new interfaces can also produce 2DEG at their interfaces, supporting the idea that the electronic reconstruction is the driving mechanism for the creation of the 2DEG at these oxide interfaces. Furthermore, the electrical properties of these interfaces are shown to be strongly governed by the interface strain and strong correlation effects provided by the polar layers. Our observations may provide a novel approach to further tune the properties of the 2DEG at the selected polar/nonpolar oxide interfaces.

cond-mat.str-el↗

arXiv2012

Metallic state in La-doped YBa$_2$Cu$_3$O$_y$ thin films with $n$-type charge carriers

We report hole and electron doping in La-doped YBa$_2$Cu$_3$O$_y$(YBCO) thin films synthesized by pulsed laser deposition technique and subsequent \emph{in-situ} postannealing in oxygen ambient and vaccum. The $n$-type samples show a metallic behavior below the Mott limit and a high carrier density of $\sim2.8$ $\times$ 10$^{21}$ cm$^{-3}$ at room temperature (\emph{T}) at the optimally reduced condition. The in-plane resistivity ($ρ$$_{ab}$) of the $n$-type samples exhibits a quadratic \emph{T} dependence in the moderate-\emph{T} range and shows an anomaly at a relatively higher \emph{T} probably related to pseudogap formation analogous to underdoped Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO). Furthermore, $ρ$$_{ab}$(T), \emph{T}$_c$ and \emph{T} with minimum resistivity (\emph{T}$_{min}$) were investigated in both $p$- and $n$-side. The present results reveal the $n$-$p$ asymmetry (symmetry) within the metallic-state region in an underdoped cuprate and suggest the potential toward ambipolar superconductivity in a single YBCO system.

cond-mat.supr-con↗

arXiv2012

Giant Tunable Faraday Effect in a Semiconductor Magneto-plasma for Broadband Terahertz Polarization Optics

We report on a giant Faraday effect in an electron plasma in n-InSb probed via polarization-resolved terahertz (THz) time-domain spectroscopy. Polarization rotation angles and ellipticities reach as large as π/2 and 1, respectively, over a wide frequency range (0.3-2.5 THz) at magnetic fields of a few Tesla. The experimental results together with theoretical simulations show its promising ability to construct broadband and tunable THz polarization optics, such as a circular polarizer, half-wave plate, and polarization modulators.

physics.optics↗

arXiv2012

Absolute Dynamical Limit to Cooling Weakly-Coupled Quantum Systems

Cooling of a quantum system is limited by the size of the control forces that are available (the "speed" of control). We consider the most general cooling process, albeit restricted to the regime in which the thermodynamics of the system is preserved (weak coupling). Within this regime, we further focus on the most useful control regime, in which a large cooling factor, and good ground-state cooling can be achieved. We present a control protocol for cooling, and give clear structural arguments, as well as strong numerical evidence, that this protocol is globally optimal. From this we obtain simple expressions for the limit to cooling that is imposed by the speed of control.

quant-ph↗

arXiv2012

Atomically flat interface between a single-terminated LaAlO3 substrate and SrTiO3 thin film is insulating

The surface termination of (100)-oriented LaAlO3 (LAO) single crystals was examined by atomic force microscopy and optimized to produce a single-terminated atomically flat surface by annealing. Then the atomically flat STO film was achieved on a single-terminated LAO substrate, which is expected to be similar to the n-type interface of two-dimensional electron gas (2DEG), i.e., (LaO)-(TiO2). Particularly, that can serve as a mirror structure for the typical 2DEG heterostructure to further clarify the origin of 2DEG. This newly developed interface was determined to be highly insulating. Additionally, this study demonstrates an approach to achieve atomically flat film growth based on LAO substrates.

cond-mat.mtrl-sci↗

arXiv2012

Weighted Lipschitz continuity, Schwarz-Pick's Lemma and Landau-Bloch's theorem for hyperbolic-harmonic mappings in $\mathbb{C}^{n}$

In this paper, we discuss some properties on hyperbolic-harmonic mappings in the unit ball of $\mathbb{C}^{n}$. First, we investigate the relationship between the weighted Lipschitz functions and the hyperbolic-harmonic Bloch spaces. Then we establish the Schwarz-Pick type theorem for hyperbolic-harmonic mappings and apply it to prove the existence of Landau-Bloch constant for mappings in $α$-Bloch spaces.

math.CV↗

arXiv2012

On some properties of solutions of the $p$-harmonic equation

A $2p$-times continuously differentiable complex-valued function $f=u+iv$ in a simply connected domain $Ω\subseteq\mathbb{C}$ is \textit{p-harmonic} if $f$ satisfies the $p$-harmonic equation $Δ^pf=0.$ In this paper, we investigate the properties of $p$-harmonic mappings in the unit disk $|z|<1$. First, we discuss the convexity, the starlikeness and the region of variability of some classes of $p$-harmonic mappings. Then we prove the existence of Landau constant for the class of functions of the form $Df=zf_{z}-\barzf_{\barz}$, where $f$ is $p$-harmonic in $|z|<1$. Also, we discuss the region of variability for certain $p$-harmonic mappings. At the end, as a consequence of the earlier results of the authors, we present explicit upper estimates for Bloch norm for bi- and tri-harmonic mappings.

math.CV↗