skip to main content

Title: Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO2, whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO2from 1.86 to 2.44 on the band structure and insulator–metal transitions are presented for the first time. Results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO2results in the formation of eand h+pairs that lead to delocalized polaronic V3+and V5+cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V3+electrons at V5+sites, which results in the formation of V4+–V4+dimers, and removal of$$\pi^{*}$$πscreening electrons. It is shown that the nature of phase transitions is linked to the lattice V3+/V5+concentrations of stoichiometric VO2and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition.

more » « less
Award ID(s):
Author(s) / Creator(s):
; ;
Publisher / Repository:
Nature Publishing Group
Date Published:
Journal Name:
Scientific Reports
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive$$\rho ^0$$ρ0meson muoproduction at COMPASS using 160 GeV/cpolarised$$ \mu ^{+}$$μ+and$$ \mu ^{-}$$μ-beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/$$c^2$$c2$$< W<$$<W<17.0 GeV/$$c^2$$c2, 1.0 (GeV/c)$$^2$$2$$< Q^2<$$<Q2<10.0 (GeV/c)$$^2$$2and 0.01 (GeV/c)$$^2$$2$$< p_{\textrm{T}}^2<$$<pT2<0.5 (GeV/c)$$^2$$2. Here,Wdenotes the mass of the final hadronic system,$$Q^2$$Q2the virtuality of the exchanged photon, and$$p_{\textrm{T}}$$pTthe transverse momentum of the$$\rho ^0$$ρ0meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ($$\gamma ^*_T \rightarrow V^{ }_L$$γTVL) indicate a violation ofs-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive$$\rho ^0$$ρ0production.

    more » « less
  2. Abstract

    The elliptic flow$$(v_2)$$(v2)of$${\textrm{D}}^{0}$$D0mesons from beauty-hadron decays (non-prompt$${\textrm{D}}^{0})$$D0)was measured in midcentral (30–50%) Pb–Pb collisions at a centre-of-mass energy per nucleon pair$$\sqrt{s_{\textrm{NN}}} = 5.02$$sNN=5.02 TeV with the ALICE detector at the LHC. The$${\textrm{D}}^{0}$$D0mesons were reconstructed at midrapidity$$(|y|<0.8)$$(|y|<0.8)from their hadronic decay$$\mathrm {D^0 \rightarrow K^-\uppi ^+}$$D0K-π+, in the transverse momentum interval$$2< p_{\textrm{T}} < 12$$2<pT<12 GeV/c. The result indicates a positive$$v_2$$v2for non-prompt$${{\textrm{D}}^{0}}$$D0mesons with a significance of 2.7$$\sigma $$σ. The non-prompt$${{\textrm{D}}^{0}}$$D0-meson$$v_2$$v2is lower than that of prompt non-strange D mesons with 3.2$$\sigma $$σsignificance in$$2< p_\textrm{T} < 8~\textrm{GeV}/c$$2<pT<8GeV/c, and compatible with the$$v_2$$v2of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.

    more » « less
  3. Abstract

    We present a proof of concept for a spectrally selective thermal mid-IR source based on nanopatterned graphene (NPG) with a typical mobility of CVD-grown graphene (up to 3000$$\hbox {cm}^2\,\hbox {V}^{-1}\,\hbox {s}^{-1}$$cm2V-1s-1), ensuring scalability to large areas. For that, we solve the electrostatic problem of a conducting hyperboloid with an elliptical wormhole in the presence of anin-planeelectric field. The localized surface plasmons (LSPs) on the NPG sheet, partially hybridized with graphene phonons and surface phonons of the neighboring materials, allow for the control and tuning of the thermal emission spectrum in the wavelength regime from$$\lambda =3$$λ=3to 12$$\upmu$$μm by adjusting the size of and distance between the circular holes in a hexagonal or square lattice structure. Most importantly, the LSPs along with an optical cavity increase the emittance of graphene from about 2.3% for pristine graphene to 80% for NPG, thereby outperforming state-of-the-art pristine graphene light sources operating in the near-infrared by at least a factor of 100. According to our COMSOL calculations, a maximum emission power per area of$$11\times 10^3$$11×103W/$$\hbox {m}^2$$m2at$$T=2000$$T=2000K for a bias voltage of$$V=23$$V=23V is achieved by controlling the temperature of the hot electrons through the Joule heating. By generalizing Planck’s theory to any grey body and deriving the completely general nonlocal fluctuation-dissipation theorem with nonlocal response of surface plasmons in the random phase approximation, we show that the coherence length of the graphene plasmons and the thermally emitted photons can be as large as 13$$\upmu$$μm and 150$$\upmu$$μm, respectively, providing the opportunity to create phased arrays made of nanoantennas represented by the holes in NPG. The spatial phase variation of the coherence allows for beamsteering of the thermal emission in the range between$$12^\circ$$12and$$80^\circ$$80by tuning the Fermi energy between$$E_F=1.0$$EF=1.0eV and$$E_F=0.25$$EF=0.25eV through the gate voltage. Our analysis of the nonlocal hydrodynamic response leads to the conjecture that the diffusion length and viscosity in graphene are frequency-dependent. Using finite-difference time domain calculations, coupled mode theory, and RPA, we develop the model of a mid-IR light source based on NPG, which will pave the way to graphene-based optical mid-IR communication, mid-IR color displays, mid-IR spectroscopy, and virus detection.

    more » « less
  4. Abstract

    Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy center ($${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VB) in hexagonal boron nitride (hBN), we grow isotopically purified h10B15N crystals. Compared to$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VBin hBN with the natural distribution of isotopes, we observe substantially narrower and less crowded$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VBspin transitions as well as extended coherence timeT2and relaxation timeT1. For quantum sensing,$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VBcenters in our h10B15N samples exhibit a factor of 4 (2) enhancement in DC (AC) magnetic field sensitivity. For additional quantum resources, the individual addressability of the$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VBhyperfine levels enables the dynamical polarization and coherent control of the three nearest-neighbor15N nuclear spins. Our results demonstrate the power of isotope engineering for enhancing the properties of quantum spin defects in hBN, and can be readily extended to improving spin qubits in a broad family of van der Waals materials.

    more » « less
  5. Abstract

    Given a suitable solutionV(tx) to the Korteweg–de Vries equation on the real line, we prove global well-posedness for initial data$$u(0,x) \in V(0,x) + H^{-1}(\mathbb {R})$$u(0,x)V(0,x)+H-1(R). Our conditions onVdo include regularity but do not impose any assumptions on spatial asymptotics. We show that periodic profiles$$V(0,x)\in H^5(\mathbb {R}/\mathbb {Z})$$V(0,x)H5(R/Z)satisfy our hypotheses. In particular, we can treat localized perturbations of the much-studied periodic traveling wave solutions (cnoidal waves) of KdV. In the companion paper Laurens (Nonlinearity. 35(1):343–387, 2022. we show that smooth step-like initial data also satisfy our hypotheses. We employ the method of commuting flows introduced in Killip and Vişan (Ann. Math. (2) 190(1):249–305, 2019. where$$V\equiv 0$$V0. In that setting, it is known that$$H^{-1}(\mathbb {R})$$H-1(R)is sharp in the class of$$H^s(\mathbb {R})$$Hs(R)spaces.

    more » « less