arXiv subjects

H. Zheng

Publications and source records attributed to H. Zheng.

At least 127 records · Page 7Linked to original sources

arXiv2009

Strain engineering and one-dimensional organization of metal-insulator domains in single-crystal VO2 beams

Spatial phase inhomogeneity at the nano- to microscale is widely observed in strongly-correlated electron materials. The underlying mechanism and possibility of artificially controlling the phase inhomogeneity are still open questions of critical importance for both the phase transition physics and device applications. Lattice strain has been shown to cause the coexistence of metallic and insulating phases in the Mott insulator VO2. By continuously tuning strain over a wide range in single-crystal VO2 micro- and nanobeams, here we demonstrate the nucleation and manipulation of one-dimensionally ordered metal-insulator domain arrays along the beams. Mott transition is achieved in these beams at room temperature by active control of strain. The ability to engineer phase inhomogeneity with strain lends insight into correlated electron materials in general, and opens opportunities for designing and controlling the phase inhomogeneity of correlated electron materials for micro- and nanoscale device applications.

cond-mat.str-el↗

arXiv2009

Search For the Origin of the Away-side Double-humped Structure of jets in Heavy Ion Collisions

A model is constructed for the origin of the double-humped structure found in the di-hahron azimuthal correlation on the away-side in heavy ion collisions. The parameters in the model are determined by fitting $Δϕ$ azimuthal distribution in central Au+Au collisions at RHIC given by STAR collaboration when the trigger momentum is in $3-4\rm {GeV/c}$ and the associated particle momentum is in $1.3-1.8\rm{GeV/c}$. Then we apply our model to the semi-central and peripheral Au+Au collisions at RHIC and the responding $\triangle ϕ$ azimuthal distribution are reproduced respectively. Our results show that the transverse flow effect plays an essential role in the experimentally observed di-hadron azimuthal distribution structure. The spread of the hard parton creation point and initial moving direction is also necessary for the double-humped structure.

nucl-th↗

arXiv2009

Magnetic correlations in YBaCo4O7 probed by single-crystal neutron scattering

We have studied the frustrated system YBaCo4O7 generally described as an alternating stacking of Kagome and triangular layers of magnetic ions on a trigonal lattice, by single crystal neutron diffraction experiments above the Neel ordering transition. Experimental data reveals pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte-Carlo simulations. Long-range magnetic correlations are found along the c-axis, due to the presence of corner-sharing bipyramids, creating quasi one-dimensional order at finite temperature. In contrast, in the Kagome layers ab-plane, the spin-spin correlation function -displaying a short-range 120 degrees configuration- decays rapidly as typically found in spin-liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead act as an external "field".

cond-mat.mtrl-sci↗

arXiv2009

Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies

The combined local structure techniques, extended x-ray absorption fine structure (EXAFS) and neutron pair distribution function analysis, have been used for temperatures 4 <= T <= 330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co sites (x <= 0.35), indicating few, if any, JT-active, singly occupied e_g Co sites exist.

cond-mat.str-el↗

arXiv2008

Quantum Zeno and Anti-Zeno Effect without Rotating Wave Approximation

The effect of the anti-rotating terms on the short-time evolution and the quantum Zeno (QZE) and anti-Zeno (AQZE) effects is studied for a two-level system coupled to a bosonic environment. A unitary transformation and perturbation theory are used to obtain the electron self-energy, energy shift and the enhanced QZE or the AQZE, simultaneously. The calculated Zeno time depends on the atomic transition frequency sensitively. When the atomic transition frequency is smaller than the central frequency of the spectrum of boson environment, the Zeno time is prolonged and the anti-rotating terms enhance the QZE; when it is larger than that the Zeno time is reduced and the anti-rotating terms enhance the AQZE.

quant-ph↗

arXiv2008

Scaling Behavior of Transverse Kinetic Energy Distributions in Au+Au Collisions at $\sqrt{s_{\rm NN}}=200$ GeV

With the experimental data from STAR on the centrality dependence of transverse momentum $p_T$ spectra of pions and protons in Au+Au collisions at $\sqrt{s_{NN}}=200 {\rm GeV}$, we investigate the scaling properties of transverse energy $E_T$ distributions at different centralities. In the framework of cluster formation and decay mechanism for particle production, the universal transverse energy distributions for pion and proton can be described separately but not simultaneously.

nucl-th↗

arXiv2007

Polaron Coherence Condensation as the Mechanism for Colossal Magnetoresistance in Layered Manganites

Angle-resolved photoemission spectroscopy data for the bilayer manganite La1.2Sr1.8Mn2O7 show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La1.2Sr1.8Mn2O7 can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.

cond-mat.str-el↗

arXiv2007

Quantum dynamics of a qubit coupled with structured bath

The dynamics of an unbiased spin-boson model with Lorentzian spectral density is investigated theoretically in terms of the perturbation theory based on a unitary transformation. The non-equilibrium correlation function $P(t)$ and susceptibility $χ^{\prime\prime}(ω)$ are calculated for both the off-resonance case $Δ\lesssim 0.5Ω$ and the on-resonance case $Δ\sim Ω$. The approach is checked by the Shiba's relation and the sum rule. Besides, the coherent-incoherent transition point $α_c$ can be determined, which has not been demonstrated for the structured bath by previous authors up to our knowledge.

cond-mat.dis-nn↗

arXiv2007

Feature selection for high dimensional data in astronomy

With an exponentially increasing amount of astronomical data, the complexity and dimension of astronomical data are likewise growing rapidly. Extracting information from such data becomes a critical and challenging problem. For example, some algorithms can only be employed in the low-dimensional spaces, so feature selection and feature extraction become important topics. Here we describe the difference between feature selection and feature extraction methods, and introduce the taxonomy of feature selection methods as well as the characteristics of each method. We present a case study comparing the performance and computational cost of different feature selection methods. For the filter method, ReliefF and fisher filter are adopted; for the wrapper method, improved CHAID, linear discriminant analysis (LDA), Naive Bayes (NB) and C4.5 are taken as learners. Applied on the sample, the result indicates that from the viewpoints of computational cost the filter method is superior to the wrapper method. Moreover, different learning algorithms combined with appropriate feature selection methods may arrive at better performance.

astro-ph↗

arXiv2007

Influence of finite baryon density on hadronization in nucleus-nucleus collisions via recombination

In this paper is investigated the influence of net baryon density on baryon and meson yields in relativistic nucleus-nucleus collisions, based on the recombination model for hadronization. Unitarity condition is used as a constraint on the model. Three cases with different assumptions on the expansion of partonic system are considered and the baryon to meson ratio is calculated for those situations.

nucl-th↗

arXiv2007

Characteristic Energy of the Coulomb Interactions and the Pileup of States

Tunneling data on $\mathrm{La_{1.28}Sr_{1.72}Mn_2O_7}$ crystals confirm Coulomb interaction effects through the $\sqrt{\mathrm{E}}$ dependence of the density of states. Importantly, the data and analysis at high energy, E, show a pileup of states: most of the states removed from near the Fermi level are found between ~40 and 130 meV, from which we infer the possibility of universal behavior. The agreement of our tunneling data with recent photoemission results further confirms our analysis.

cond-mat.str-el↗

arXiv2007

Surface effects on the orbital order in the single layered manganite La0.5Sr1.5MnO4

We report the first observation of `orbital truncation rods' -- the scattering arising from the termination of bulk orbital order at the surface of a crystal. The x-ray measurements, performed on a cleaved, single-layered perovskite, La0.5Sr1.5MnO4, reveal that while the crystallographic surface is atomically smooth, the orbital `surface' is much rougher, with an r.m.s. deviation from the average `surface' of ~0.7nm. The temperature dependence of this scattering shows evidence of a surface-induced second order transition.

cond-mat.str-el↗

arXiv2007

Reentrant Orbital Order and the True Ground State of LaSr2Mn2O7

Contrary to conventional wisdom, our purified La2-2xSr1+2xMn2O7 crystals exhibit CE-type orbital and charge order as the low-temperature ground state for a hole doping level h = 0.5. For small deviations from h = 0.5, the high temperature CE phase is replaced at low temperatures by an A-type antiferromagnet without coexistence. Larger deviations result in a lack of CE order at any temperature. Thus, small inhomogeneities in cation or oxygen composition could explain why others commonly see this reentrance with coexistence.

cond-mat.str-el↗

arXiv2006

A local metallic state in globally insulating $La_{1.24}Sr_{1.76}Mn_2O_7$ well above the metal-insulator transition

Angle-resolved photoemission spectroscopy was used to investigate the evolution of the electronic structure across the metal-insulator transition in bi-layer manganite $La_{1.24}Sr_{1.76}Mn_2O_7$. We found that this system is a metal for $T T^*$. These results indicate the critical role of electronic phase separation and percolation effects for the metal-insulator transition in $La_{1.24}Sr_{1.76}Mn_2O_7$.

cond-mat.str-el↗

arXiv2006

Direct Observation of Phase Oscillation Modes in Josephson-coupled Layered Superconductors

We have proposed to employ the microwave S parameters method to measure phase oscillation modes (POMs) in Josephson coupled-layered superconductors. For the first time, in SmBa2Cu3O7 (SmBCO) and YBa2Cu3O7-b (YBCO) at 77 K we have directly observed some POMs far below the Josephson plasma frequency. A series of discrete modes have been observed and discussed qualitatively in the POMs. The method may be a powerful and flexible tool in this field.

cond-mat.supr-con↗

arXiv2006

Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites

What happens to ferromagnetism at the surfaces and interfaces of manganites? With the competition between charge, spin, and orbital degrees of freedom, it is not surprising that the surface behavior may be profoundly different than that of the bulk. Using a powerful combination of two surface probes, tunneling and polarized x-ray interactions, this paper reviews our work on the nature of the electronic and magnetic states at manganite surfaces and interfaces. The general observation is that ferromagnetism is not the lowest energy state at the surface or interface, which results in a suppression or even loss of ferromagnetic order at the surface. Two cases will be discussed ranging from the surface of the quasi-2D bilayer manganite (La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$) to the 3D Perovskite (La$_{2/3}$Sr$_{1/3}$MnO$_3$)/SrTiO$_3$ interface. For the bilayer manganite, that is, ferromagnetic and conducting in the bulk, these probes present clear evidence for an intrinsic insulating non-ferromagnetic surface layer atop adjacent subsurface layers that display the full bulk magnetization. This abrupt intrinsic magnetic interface is attributed to the weak inter-bilayer coupling native to these quasi-two-dimensional materials. This is in marked contrast to the non-layered manganite system (La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrTiO$_3$), whose magnetization near the interface is less than half the bulk value at low temperatures and decreases with increasing temperature at a faster rate than the bulk.

cond-mat.str-el↗

arXiv2006

Local Dielectric Measurements of BaTiO3-CoFe2O4 Nano-composites Through Microwave Microscopy

We report on linear and non-linear dielectric property measurements of BaTiO3 - CoFe2O4 (BTO-CFO) ferroelectro-magnetic nano-composites and pure BaTiO3 and CoFe2O4 samples with Scanning Near Field Microwave Microscopy. The permittivity scanning image with spatial resolution on the micro-meter scale shows that the nano-composites have very uniform quality with an effective dielectric constant ε_r = 140 +/- 6.4 at 3.8 GHz and room temperature. The temperature dependence of dielectric permittivity shows that the Curie temperature of pure BTO was shifted by the clamping effect of the MgO substrate, whereas the Curie temperature shift of the BTO ferroelectric phase in BTO-CFO composites is less pronounced, and if it exists at all, would be mainly caused by the CFO. Non-linear dielectric measurements of BTO-CFO show good ferroelectric properties from BTO.

cond-mat.mtrl-sci↗

arXiv2006

Competing magnetic interactions in the extended Kagome system YBaCo4O7

YBaCo4O7 belongs to a new class of geometrically frustrated magnets like the pyrochlores, in which Co-spins occupy corners of tetrahedra. The structure can be viewed as an alternating stacking of Kagome and triangular layers. Exactly half of the triangular units of the Kagome plane are capped by Co ions to form columns running perpendicular to the Kagome sheets. Neutron powder diffraction reveals a broad temperature range of diffuse magnetic scattering, followed by long range magnetic ordering below 110K. A unique low-temperature magnetic structure simultaneously satisfies an S=0 arrangement in the uncapped triangular units and antiferromagnetic coupling along the columns. A spin reorientation above 30K tracks the relative strengths of the in-plane and out-of-plane interactions.

cond-mat.mtrl-sci↗