arXiv subjects

W. Zhou

Publications and source records attributed to W. Zhou.

46 records · Page 3Linked to original sources

arXiv2009

Empires and Percolation: Stochastic Merging of Adjacent Regions

We introduce a stochastic model in which adjacent planar regions $A, B$ merge stochastically at some rate $λ(A,B)$, and observe analogies with the well-studied topics of mean-field coagulation and of bond percolation. Do infinite regions appear in finite time? We give a simple condition on $λ$ for this {\em hegemony} property to hold, and another simple condition for it to not hold, but there is a large gap between these conditions, which includes the case $λ(A,B) \equiv 1$. For this case, a non-rigorous analytic argument and simulations suggest hegemony.

math.PR↗

arXiv2009

Neutron powder diffraction atomic pair distribution function analysis using the ARCS chopper spectrometer at the Spallation Neutron Source

The frst neutron powder diffraction based atomic pair distribution functions (PDFs) are reported from the new wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source at Oak Ridge National Laboratory. The spectrometer was run in white-beam mode, with no Fermi chopper. The PDF patterns of Si, Ni, and Al2O3 were refined using the PDFfit method and the results compared to data collected at the NPDF diffractometer at Los Alamos National Laboratory. The resulting fits are of high quality demonstrating that quantitatively reliable powder diffraction data can be obtained from ARCS when operated in this configuration.

cond-mat.mtrl-sci↗

arXiv2008

Central limit theorem for signal-to-interference ratio of reduced rank linear receiver

Let $\mathbf{s}_k=\frac{1}{\sqrt{N}}(v_{1k},...,v_{Nk})^T,$ with $\{v_{ik},i,k=1,...\}$ independent and identically distributed complex random variables. Write $\mathbf{S}_k=(\mathbf{s}_1,...,\mathbf {s}_{k-1},\mathbf{s}_{k+1},... ,\mathbf{s}_K),$ $\mathbf{P}_k=\operatorname {diag}(p_1,...,p_{k-1},p_{k+1},...,p_K)$, $\mathbf{R}_k=(\mathbf{S}_k\mathbf{P}_k\mathbf{S}_k^*+σ^2\mathbf{I})$ and $\mathbf{A}_{km}=[\mathbf{s}_k,\mathbf{R}_k\mathbf{s}_k,... ,\mathbf{R}_k^{m-1}\mathbf{s}_k]$. Define $β_{km}=p_k\mathbf{s}_k^*\mathbf{A}_{km}(\mathbf {A}_{km}^*\times\ mathbf{R}_k\mathbf{A}_{km})^{-1}\mathbf{A}_{km}^*\mathbf{s}_k$, referred to as the signal-to-interference ratio (SIR) of user $k$ under the multistage Wiener (MSW) receiver in a wireless communication system. It is proved that the output SIR under the MSW and the mutual information statistic under the matched filter (MF) are both asymptotic Gaussian when $N/K\to c>0$. Moreover, we provide a central limit theorem for linear spectral statistics of eigenvalues and eigenvectors of sample covariance matrices, which is a supplement of Theorem 2 in Bai, Miao and Pan [Ann. Probab. 35 (2007) 1532--1572]. And we also improve Theorem 1.1 in Bai and Silverstein [Ann. Probab. 32 (2004) 553--605].

math.PR↗

arXiv2007

Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study

Rhenium diboride is a recently recognized ultra-incompressible superhard material. Here we report the electronic (e), phonon (p), e-p coupling and thermal properties of ReB$_2$ from first-principles density-functional theory (DFT) calculations and neutron scattering measurements. Our calculated elastic constants ($c_{11}$ = 641 GPa, $c_{12}$ = 159 GPa, $c_{13}$ = 128 GPa, $c_{33}$ = 1037 GPa, and $c_{44}$ = 271 GPa), bulk modulus ($B$ $\approx$ 350 GPa) and hardness ($H$ $\approx$ 46 GPa) are in good agreement with the reported experimental data. The calculated phonon density of states (DOS) agrees very well with our neutron vibrational spectroscopy result. Electronic and phonon analysis indicates that the strong covalent B-B and Re-B bonding is the main reason for the super incompressibility and hardness of ReB$_2$. The thermal expansion coefficients, calculated within the quasi-harmonic approximation and measured by neutron powder diffraction, are found to be nearly isotropic in $a$ and $c$ directions and only slightly larger than that of diamond in terms of magnitude. The excellent agreement found between calculations and experimental measurements indicate that first-principles calculations capture the main interactions in this class of superhard materials, and thus can be used to search, predict, and design new materials with desired properties.

cond-mat.mtrl-sci↗

arXiv2007

Hydrogen Absorption Properties of Metal-Ethylene Complexes

Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a single ethylene molecule can form stable complexes with light transition metals (TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and 14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles molecular dynamics (MD) simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to 500 K. The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties.

cond-mat.mtrl-sci↗

arXiv2006

Transition Metal-Ethylene Complexes as High-Capacity Hydrogen Storage Media

From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of 14 wt%. Dimerization, polymerizations and incorporation of the TM-ethylene complexes in nanoporous carbon materials have been also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen storage materials discovery.

cond-mat.other↗

arXiv2006

Lattice Dynamics of Metal-Organic Frameworks (MOFs): Neutron Inelastic Scattering and First-Principles Calculations

By combining neutron inelastic scattering (NIS) and first-principles calculations, we have investigated the lattice dynamics of MOF5. The structural stability of MOF5 was evaluated by calculating the three cubic elastic constants. We find that the shear modulus, c44 = 1.16 GPA, is unusually small, while two other moduli are relatively large (i.e. c11 = 29.42 GPa and c12 = 12.56 GPa). We predict that MOF5 is very close to structural instability and may yield interesting new phases under high pressure and strain. The phonon dispersion curves and phonon density of states were directly calculated and our simulated NIS spectrum agrees very well with our experimental data. Several interesting phonon modes are discussed, including the softest twisting modes of the organic linker.

cond-mat.mtrl-sci↗

arXiv2006

Transport in the Laughlin quasiparticle interferometer: Evidence for topological protection in an anyonic qubit

We report experiments on temperature and Hall voltage bias dependence of the superperiodic conductance oscillations in the novel Laughlin quasiparticle interferometer, where quasiparticles of the 1/3 fractional quantum Hall fluid execute a closed path around an island of the 2/5 fluid. The amplitude of the oscillations fits well the quantum-coherent thermal dephasing dependence predicted for a two point-contact chiral edge channel interferometer in the full experimental temperature range 10.2<T<141 mK. The temperature dependence observed in the interferometer is clearly distinct from the behavior in single-particle resonant tunneling and Coulomb blockade devices. The 5h/e flux superperiod, originating in the anyonic statistical interaction of Laughlin quasiparticles, persists to a relatively high T~140 mK. This temperature is only an order of magnitude less than the 2/5 quantum Hall gap. Such protection of quantum logic by the topological order of fractional quantum Hall fluids is expected to facilitate fault-tolerant quantum computation with anyons.

cond-mat.mes-hall↗

arXiv2005

Thermal Dephasing in the Laughlin Quasiparticle Interferometer: Chiral Luttinger Liquid Behavior

We report experimental temperature dependence of the amplitude of Aharonov-Bohm oscillations in the Laughlin quasiparticle interferometer. The results fit very well the thermal dephasing dependence predicted for a g = 1/3 chiral Luttinger liquid interferometer, and are clearly distinct from the activated behavior observed in single-particle resonant tunneling and Coulomb blockade devices. The small deviation from the zero-bias theory seen below 20 mK indicates yet unrecognized source of experimental decoherence, not included in theory.

cond-mat.mes-hall↗

arXiv2005

Aharonov-Bohm electron interferometer in the integer quantum Hall regime

We report experiments on a quantum electron interferometer fabricated from high mobility, low density GaAs/AlGaAs heterostructure material. In this device, a nearly circular electron island is defined by four front gates deposited in etched trenches. The island is separated from the 2D electron bulk by two nearly open constrictions. In the quantum Hall regime, two counterpropagating edge channels are coupled by tunneling in the constrictions, thus forming a closed electron interference path.For several fixed front gate voltages, we observe periodic Aharonov-Bohm interference oscillations in four-terminal resistance as a function of the enclosed flux. The oscillation period DeltaB gives the area of the interference path S via quantization condition S=h/eDeltaB. We experimentally determine the dependence of S on the front gate voltage, and find that the Aharonov-Bohm quantization condition does not require significant corrections due to the confining potential. These results can be interpreted as a constant integrated compressibility of the island with respect to the front gates. We also analyze experimental results using two classical electrostatics models: one modeling the 2D electron density due to depletion from an etch trench, and another modeling the gate voltage dependence of the electron density profile in the island.

cond-mat.mes-hall↗