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W. Wang

Publications and source records attributed to W. Wang.

At least 109 records · Page 6Linked to original sources
arXiv2018

Electron mass enhancement and magnetic phase separation near the Mott transition in double layer ruthenates

We present a detailed investigation of the specific heat in Ca$_3$(Ru$_{1-x}M_x$)$_2$O$_7$ ($M$ = Ti, Fe, Mn) single crystals. With different dopants and doping levels, three distinct regions are present, including a quasi-2D metallic state with an antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-$b$), a Mott insulating state with G-type AFM order (G-AFM) and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn dopings. We observed not only an anomalous large mass enhancement but also an additional term in the specific heat i.e. $C\propto T^2$ in the localized region. The $C\propto T^2$ term is most likely due to the long-wavelength excitations with both FM and AFM components. A decrease of Debye temperature is observed in the G-type AFM region, indicating a lattice softening associated with the Mott transition.

cond-mat.str-el↗
arXiv2018

First search for short-baseline neutrino oscillations at HFIR with PROSPECT

This Letter reports the first scientific results from the observation of antineutrinos emitted by fission products of $^{235}$U at the High Flux Isotope Reactor. PROSPECT, the Precision Reactor Oscillation and Spectrum Experiment, consists of a segmented 4 ton $^6$Li-doped liquid scintillator detector covering a baseline range of 7-9 m from the reactor and operating under less than 1 meter water equivalent overburden. Data collected during 33 live-days of reactor operation at a nominal power of 85 MW yields a detection of 25461 $\pm$ 283 (stat.) inverse beta decays. Observation of reactor antineutrinos can be achieved in PROSPECT at 5$σ$ statistical significance within two hours of on-surface reactor-on data-taking. A reactor-model independent analysis of the inverse beta decay prompt energy spectrum as a function of baseline constrains significant portions of the previously allowed sterile neutrino oscillation parameter space at 95% confidence level and disfavors the best fit of the Reactor Antineutrino Anomaly at 2.2$σ$ confidence level.

hep-ex↗
arXiv2018

Bi monocrystal formation on InAs(111)A and B substrates

The growth of Bi films deposited on both A and B faces of InAs(111) has been investigated by low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d5/2 photoelectron signals allow to get a comprehensive picture of the Bi/InAs(1 1 1) interface. From the initial stages the Bi growth on the A face (In-terminated InAs) is epitaxial, contrary to that on the B face (As- terminated InAs) that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a $\approx 10$~BL deposit on the A face is identical to that of bulk Bi, while more than $\approx 30$ BL are needed for the B face. Both bulk and surface states are well accounted for by fully relativistic ab initio spin-resolved photoemission calculations.

cond-mat.mtrl-sci↗
arXiv2018

Zero-viscosity limit of the Navier-Stokes equations with the Navier friction boundary condition

In this paper, we consider the zero-viscosity limit of the Navier-Stokes equations in a half space with the Navier friction boundary condition $$ (βu^{\varepsilon}-\varepsilon^γ\partial_y u^{\varepsilon})|_{y=0}=0, $$ where $β$ is a constant and $γ\in (0,1]$. In the case of $γ=1$, the convergence to the Euler equations and the Prandtl equation with the Robin boundary condition is justified for the analytic data. In the case of $γ\in (0,1)$, the convergence to the Euler equations and the linearized Prandtl equation is justified for the data in the Gevrey class $\frac 1 γ$.

math.AP↗
arXiv2018

Distinctive Picosecond Spin Polarization Dynamics in Bulk Half-Metals

Femtosecond laser excitations in half-metal (HM) compounds are theoretically predicted to induce an exotic picosecond spin dynamics. In particular, conversely to what is observed in conventional metals and semiconductors, the thermalization process in HMs leads to a long living partially thermalized configuration characterized by three Fermi--Dirac distributions for the minority, majority conduction and majority valence electrons respectively. Remarkably, these distributions have the same temperature but different chemical potentials. This unusual thermodynamics state causes a persistent non-equilibrium spin polarization only well above the Fermi energy. Femtosecond spin dynamics experiments performed on Fe$_3$O$_4$ by time-, spin-, and angle-resolved photoelectron spectroscopy confirm our model. Furthermore, the spin polarization response proves to be very robust and it can be adopted to selectively test the bulk HM character in a wide range of compounds.

cond-mat.mtrl-sci↗
arXiv2018

Anti-adiabatic evolution in quantum-classical hybrid system

The adiabatic theorem is an important concept in quantum mechanics, it tells that a quantum system subjected to gradually changing external conditions remains to the same instantaneous eigenstate of its Hamiltonian as it initially in. In this paper, we study the another extreme circumstance where the external conditions vary rapidly such that the quantum system can not follow the change and remains in its initial state (or wavefunction). We call this type of evolution anit-adiabatic evolution. We examine the matter-wave pressure in this situation and derive the condition for such an evolution. The study is conducted by considering a quantum particle in an infinitely deep potential, the potential width $Q$ is assumed to be change rapidly. We show that the total energy of the quantum subsystem decreases as $Q$ increases, and this rapidly change exerts a force on the wall which plays the role of boundary of the potential. For $Q Q_{0}$, the force is positive. The condition for the anti-adiabatic evolution is given via a spin-$\frac 1 2$ in a rotating magnetic field.

quant-ph↗
arXiv2017

Investigation of the atomic and electronic structures of highly ordered two-dimensional germanium on Au(111)

Low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and photoelectron spectroscopy have been used to study an ordered structure formed by Ge atoms deposited onto the Au(111) surface. Based on a careful analysis of STM images and LEED patterns, we propose a new unit cell for the atomic structure of the Ge layer. Core level data indicate that some Ge atoms diffuse into the Au(111) crystal during annealing after deposition at room temperature. This is further corroborated by angle resolved photoelectron spectroscopy measured for different amounts of Ge remaining after sputtering and annealing. The results of the ARPES study clearly exclude the interpretation, in the literature, of a parabolic band as part of a Dirac cone of germanene.

cond-mat.mtrl-sci↗
arXiv2017

Coexistence of strongly buckled germanene phases on Al(111)

We report a study of structural and electronic properties of a germanium layer on Al(111) using scanning tunneling microscopy (STM), low energy electron diffraction and core-level photoelectron spectroscopy. Experimental results show that a germanium layer can be formed at a relatively high substrate temperature showing either (3$\times$3) or ($\sqrt{7}$$\times$$\sqrt{7}$)R$\pm$19.1° reconstructions. First-principles calculations based on density functional theory suggest an atomic model consisting of a strongly buckled (2$\times$2) germanene layer, which is stable in two different orientations on Al(111). Simulated STM of both orientations fit nicely with experimental STM images and the Ge 3d core-level data decomposed into four components is consistent with the suggested model.

cond-mat.mtrl-sci↗
arXiv2017

Experimental and theoretical determination of σ-bands on ("2$\sqrt{3}$x2$\sqrt{3}$") silicene grown on Ag(111)

Silicene, the two-dimensional (2D) allotrope of silicon has very recently attracted a lot of attention. It has a structure that is similar to graphene and it is theoretically predicted to show the same kind of electronic properties which has put graphene into the focus of large research and development projects world-wide. In particular, a 2D structure made from Si is of high interest because of the application potential in Si-based electronic devices. However, so far there is not much known about the silicene band structure from experimental studies. A comprehensive study is here presented of the atomic and electronic structure of the silicene phase on Ag(111) denoted as (2$\sqrt{3}$x2$\sqrt{3}$)R30° in the literature. Low energy electron diffraction (LEED) shows an unconventional rotated ("2$\sqrt{3}$x2$\sqrt{3}$") pattern with a complicated set of split diffraction spots. Scanning tunneling microscopy (STM) results reveal a Ag(111) surface that is homogeneously covered by the ("2$\sqrt{3}$x2$\sqrt{3}$") silicene which exhibits an additional quasi-periodic long range ordered superstructure. The complex structure, revealed by STM, has been investigated in detail and we present a consistent picture of the silicene structure based on both STM and LEED. The homogeneous coverage by the ("2$\sqrt{3}$x2$\sqrt{3}$") silicene facilitated an angle-resolved photoelectron spectroscopy study which reveals a silicene band structure of unprecedented detail. Here, we report four silicene bands which are compared to calculated dispersions based on a relaxed (2$\sqrt{3}$x2$\sqrt{3}$) model. We find good qualitative agreement between the experimentally observed bands and calculated silicene bands of σ character.

cond-mat.mtrl-sci↗
arXiv2017

Band structure of hydrogenated silicene on Ag(111): Evidence for silicane

In the case of graphene, hydrogenation removes the conductivity due to the bands forming the Dirac cone by opening up a band gap. This type of chemical functionalization is of utmost importance for electronic applications. As predicted by theoretical studies, a similar change of the band structure is expected for silicene, the closest analogue to graphene. We here report a study of the atomic and electronic structures of hydrogenated silicene, so called silicane. The ($\small 2\sqrt{3} \times 2\sqrt{3}$) phase of silicene on Ag(111) was used in this study since it can be formed homogeneously across the entire surface of the Ag substrate. Low energy electron diffraction and scanning tunneling microscopy data clearly show that hydrogenation changes the structure of silicene on Ag(111) resulting in a (1 $\times$ 1) periodicity with respect to the silicene lattice. The hydrogenated silicene also exhibits a quasi-regular ($\small 2\sqrt{3} \times 2\sqrt{3}$)-like arrangement of vacancies. Angle resolved photoelectron spectroscopy revealed two dispersive bands which can be unambiguously assigned to silicane. The common top of these bands is located at $\small \backsim$ 0.9 eV below the Fermi level. We find that the experimental bands are closely reproduced by the theoretical band structure of free standing silicane with H adsorbed on the upper hexagonal sub-lattice of silicene.

cond-mat.mtrl-sci↗
arXiv2017

Generalized Floquet theory for open quantum systems

For a periodically driven open quantum system, the Floquet theorem states that the time evolution operator $Λ(t,0)$ of the system can be factorized as $Λ(t,0)=\mathcal{D}(t)e^{\mathcal{L}_{eff}t}$ with micro-motion operator $\mathcal{D}(t)$ possessing the same period as the external driving, and time-independent operator $\mathcal{L}_{eff}$. In this work, we extend this theorem to open systems that follow a modulated periodic evolution, in which the fast part is periodic while the slow part breaks the periodicity. We derive a factorization for the time evolution operator that separates the long time dynamics and the micro-motion for the open quantum system. High-frequency expansions for the effective evolution operator control the long time dynamics, and the micro-motion operator is also given and discussed. It may find applications in quantum engineering with open systems following modulated periodic evolution.

quant-ph↗
arXiv2017

Simulating Molecular Spectroscopy with Circuit Quantum Electrodynamics

Spectroscopy is a crucial laboratory technique for understanding quantum systems through their interactions with electromagnetic radiation. Particularly, spectroscopy is capable of revealing the physical structure of molecules, leading to the development of the maser - the forerunner of the laser. However, real-world applications of molecular spectroscopy are mostly confined to equilibrium states, due to computational and technological constraints; a potential breakthrough can be achieved by utilizing the emerging technology of quantum simulation. Here we experimentally demonstrate that a superconducting quantum simulator is capable of generating molecular spectra for both equilibrium and non-equilibrium states, reliably producing the vibronic structure of the molecules. Furthermore, our quantum simulator is applicable not only to molecules with a wide range of electronic-vibronic coupling strength characterized by the Huang-Rhys parameter, but also to molecular spectra not readily accessible under normal laboratory conditions. These results point to a new direction for predicting and understanding molecular spectroscopy, exploiting the power of quantum simulation.

quant-ph↗
arXiv2017

Converting quasiclassical states into arbitrary Fock state superpositions in a superconducting cavity

We propose and experimentally demonstrate a new method to generate arbitrary Fock-state superpositions in a superconducting quantum circuit, where a qubit is dispersively coupled to a microwave cavity mode without the need of fine-frequency tuning. Here the qubit is used to conditionally modulate the probability amplitudes of the Fock state components of a coherent state to those of the desired superposition state, instead of pumping photons one by one into the cavity as in previous schemes. Our method does not require the adjustment of the qubit frequency during the cavity state preparation, and is more robust to noise and accumulation of experimental errors compared to previous ones. Using the method, we experimentally generate high-fidelity phase eigenstates under various Hilbert-space dimensions and squeezed states, which are useful for quantum walk and high-precision measurements.

quant-ph↗
arXiv2016

Entangled states of two quantum dots mediated by Majorana fermions

With the assistance of a pair of Majorana fermions, we propose schemes to entangle two quantum dots by Lyapunov control in the charge and spin degrees of freedom. Four different schemes are considered, i.e., the teleportation scheme, the crossed Andreev reflection scheme, the intradot spin flip scheme, and the scheme beyond the intradot spin flip. We demonstrate that the entanglement can be generated by modulating the chemical potential of quantum dots with square pulses, which is easily realized in practice. In contrast to Lyapunov control, the preparation of entangled states by adiabatic passage is also discussed. There are two advantages in the scheme by Lyapunov control, i.e., it is flexible to choose a control Hamiltonian, and the control time is much shorter with respect to the scheme by adiabatic passage. Furthermore, we find that the results are quite different by different adiabatic passages in the scheme beyond the intradot spin flip, which can be understood as an effect of quantum destructive interference.

quant-ph↗
arXiv2016

Rotational evolution of magnetars in the presence of a fallback disk

Magnetars may have strong surface dipole field. Observationally, two magnetars may have passive fallback disks. In the presence of a fallback disk, the rotational evolution of magnetars may be changed. In the self-similar fallback disk model, it is found that: (1) When the disk mass is significantly smaller than $10^{-6} \,\rm M_{\odot}$, the magnetar is unaffected by the fallback disk and it will be a normal magnetar. (2) When the disk mass is large, but the magnetar's surface dipole field is about or below $10^{14} \,\rm G$, the magnetar will also be a normal magnetar. A magnetar plus a passive fallback disk system is expected. This may correspond to the observations of magnetars 4U 0142$+$61, and 1E 2259$+$586. (3) When the disk mass is large, and the magnetar's surface dipole field is as high as $4\times 10^{15} \,\rm G$, the magnetar will evolve from the ejector phase to the propeller phase, and then enter rotational equilibrium. The magnetar will be slowed down quickly in the propeller phase. The final rotational period can be as high $2\times 10^4 \,\rm s$. This may correspond to the super-slow magnetar in the supernova remnant RCW 103. Therefore, the three kinds of magnetars can be understood in a unified way.

astro-ph.HE↗
arXiv2016

Population transfer driven by far-off-resonant fields

For a two-level system, it is believed that a far-off-resonant driving can not help coherent population transfer between the states. In this work, we propose a scheme to implement the coherent transfer with far-off-resonant driving. The scheme works well with both constant driving and Gaussian driving. The total time to finish population transfer is also minimized by optimizing the detuning and coupling constants. We find that the scheme is sensitive to spontaneous emission much more than dephasing. It might find potential applications in X-ray quantum optics and population transfer in Rydberg atoms as well.

quant-ph↗