Wentzcovitch<i>et al.</i>Reply

Wentzcovitch, Renata M.; Schulz, Werner; Allen, Philip B.

doi:10.1103/physrevlett.73.3043

Cited by 81 publications

(103 citation statements)

References 3 publications

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“…And it could be noticed that the gap of the hysteresis turned narrow with the increase of the doping amount of tungsten. Although there are still some controversies about the physical mechanism 19) about the phase transition of VO 2 (R), it is very clear that the principal crystallographic feature of the transition is the formation and dissociation of V 4+ V 4+ bonding. Figure 5 showed the formation of V 4+ V 4+ pairing in the phase transition from VO 2 (R) to VO 2 (M) and VO 2 (A)(HT) to VO 2 (A)(LT).…”

Section: Resultsmentioning

confidence: 99%

Synthesis and phase transition behavior of w-doped VO2(A) nanorods

Zhao

Feng

et al. 2010

J. Ceram. Soc. Japan

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In this paper, pure and w-doped VO 2 (A) nanorods were synthesized via the reduction of V 2 O 5 with oxalic acid through hydrothermal treatment and characterized by XRD, SEM and DSC. The study about the influence of w-doping on its phase transition and stability properties was carried out for the first time on the best of our knowledge. The doping of tungsten decreased the phase transition temperature of VO 2 (A) just as it does in the case of rutile VO 2 (R). The doping limitation of tungsten for pure VO 2 (A) was estimated to be about 0.50.75 at. %, whereas over doping of tungsten led to the destabilization of VO 2 (A) phase. Based on the experimental results, the solution reaction process should be such that VO 2 (B) was first formed at lower temperature or shorter time, and then transformed to VO 2 (A) or VO 2 (R) during hydrothermal treatment. Furthermore, inhomogeneous doping of tungsten in VO 2 during the crystallization from the solution was revealed by DSC measurements.

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Section: Resultsmentioning

confidence: 99%

Synthesis and phase transition behavior of w-doped VO2(A) nanorods

Zhao

Feng

et al. 2010

J. Ceram. Soc. Japan

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“…4,5 Recently, a theoretical study by Wentzcovitch et al has revived attention into this four-decade-long debate, 6 suggesting that the former mechanism may be dominant, i.e., the low-T phase may be bandlike and the transition structurally driven. New controversy has resulted 7,8 and the problem is yet to be settled experimentally.…”

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Evidence for a structurally-driven insulator-to-metal transition inVO2: A view from the ultrafast timescale

et al. 2004

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We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly correlated electron system. We discuss the case of the model system VO 2 , a prototypical nonmagnetic compound that exhibits cell doubling, charge localization, and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-T insulator. The insulator-to-metal transition is, however, delayed with respect to hole injection, exhibiting a bottleneck time scale, associated with the phonon connecting the two crystallographic phases. This structural bottleneck is observed despite faster depletion of the d bands and is indicative of important bandlike character for this controversial insulator. Correlated electron materials exhibit remarkable effects, ranging from metal-insulator transitions to nonconventional (high temperature) superconductivity. The subtle interplay between atomic structure, charge, spin, and orbital dynamics is responsible for many of the critical phenomena observed. 1 Importantly, because "simultaneous" changes in more than one degree of freedom are often observed as chemical doping or external parameters are tuned across critical values, time-integrated spectroscopies are unable to uniquely assign cause-effect relationships.Here, we demonstrate that time-resolved spectroscopy can instead be applied to overcome such ambiguities. We study the case of nonmagnetic VO 2 , a controversial, strongly correlated compound that exhibits cell doubling in "concomitance" with electron localization and a metal-insulator transition below 340 K 2 (see Fig. 1). The issue is whether the insulating behavior in the low-T phase derives directly from the Peierls distortion 3 or from electron localization and the consequent increase in electron-electron repulsion. 4,5 Recently, a theoretical study by Wentzcovitch et al. has revived attention into this four-decade-long debate, 6 suggesting that the former mechanism may be dominant, i.e., the low-T phase may be bandlike and the transition structurally driven. New controversy has resulted 7,8 and the problem is yet to be settled experimentally.Previous time-resolved optical 9 and x-ray diffraction 10 experiments in this compound demonstrated that impulsive photoexcitation of the low-T monoclinic insulator causes an ultrafast transition in both the electronic properties and the atomic structural arrangement. However, it was not clear whether the system becomes metallic due to the change in symmetry of the unit cell or to the prompt creation of holes, causing the closure of a Mott gap. We have now performed optical experiments with 15 fs resolution, and we report evidence of a limiting structural time scale for the formation of the metallic phase. This delay is observed despite much faster hole doping into the correlated d band. Such bottleneck time originates from the coherent optical-phonon distortions in the excited state of the system, mapping onto the crystal...

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“…The metal-insulator transition arises as a Peierls instability of the d band in an embedding background of e π g electrons. This basic mechanism should also apply to VO2, where, however, electronic correlations are expected to play a greater role due to stronger localization of the 3d electrons.PACS numbers: 71.30.+h, 72.15.Nj Despite intense work over decades the metal-insulator transition (MIT) of VO 2 has remained a matter of controversy [1,2,3,4,5,6,7]. This is related to the simultaneous occurrence of a structural transformation from the high-temperature rutile phase to a distorted monoclinic structure, which is characterized by (i) pairing of the metal atoms within chains parallel to the rutile c axis and (ii) their lateral zigzag-like displacement [8].…”

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“…Dispute centers about the question, whether the d splitting is caused by metal dimerization or by increased electronic correlations resulting from the reduced screening by the e π g electrons [3,4]. State of the art band calculations gave strong hints of a structural instability but were not able to reproduce the insulating gap due to the shortcomings of the local density approximation (LDA) [5]. Interestingly, only very few studies have dealt with the neighbouring dioxides, which display related phenomena.…”

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The metal-insulator transition of NbO ₂ : An embedded Peierls instability

Eyert

2002

Europhys. Lett.

101

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Results of first principles augmented spherical wave electronic structure calculations for niobium dioxide are presented. Both metallic rutile and insulating low-temperature NbO2, which crystallizes in a distorted rutile structure, are correctly described within density functional theory and the local density approximation. Metallic conductivity is carried to equal amounts by metal t2g orbitals, which fall into the one-dimensional d band and the isotropically dispersing e π g bands. Hybridization of both types of bands is almost negligible outside narrow rods along the line X-R. In the lowtemperature phase splitting of the d band due to metal-metal dimerization as well as upshift of the e π g bands due to increased p-d overlap remove the Fermi surface and open an optical band gap of about 0.1 eV. The metal-insulator transition arises as a Peierls instability of the d band in an embedding background of e π g electrons. This basic mechanism should also apply to VO2, where, however, electronic correlations are expected to play a greater role due to stronger localization of the 3d electrons.PACS numbers: 71.30.+h, 72.15.Nj Despite intense work over decades the metal-insulator transition (MIT) of VO 2 has remained a matter of controversy [1,2,3,4,5,6,7]. This is related to the simultaneous occurrence of a structural transformation from the high-temperature rutile phase to a distorted monoclinic structure, which is characterized by (i) pairing of the metal atoms within chains parallel to the rutile c axis and (ii) their lateral zigzag-like displacement [8]. Electronic states near the Fermi energy are of mainly V 3d t 2g character. They separate into the d band, which mediates V-V overlap along the metal chains, and the remaining e π g bands [3]. At the transition, splitting of the d band and upshift of the e π g bands due to increased metal-oxygen overlap produce a finite band gap. Dispute centers about the question, whether the d splitting is caused by metal dimerization or by increased electronic correlations resulting from the reduced screening by the e π g electrons [3,4]. State of the art band calculations gave strong hints of a structural instability but were not able to reproduce the insulating gap due to the shortcomings of the local density approximation (LDA) [5]. Interestingly, only very few studies have dealt with the neighbouring dioxides, which display related phenomena. NbO 2 likewise undergoes a MIT (at 1081 K) and a simultaneous structural transition from rutile to a distorted variant having a body-centered tetragonal (bct) lattice [9,10]. Despite the differences in long range order, local deviations from rutile are the same as in VO 2 , i.e. niobium atoms dimerize and experience lateral displacements at the transition [11,12,13]. In contrast, the metallic oxides MoO 2 , WO 2 , TcO 2 , and α-ReO 2 all crystallize in the same monoclinic structure as VO 2 [14]. No doubt the phase transitions of VO 2 and NbO 2 as well as the destabilization of the rutile structure in all these dioxides call for a uni...

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Wentzcovitchet al.Reply

Cited by 81 publications

References 3 publications

Synthesis and phase transition behavior of w-doped VO2(A) nanorods

Synthesis and phase transition behavior of w-doped VO2(A) nanorods

Evidence for a structurally-driven insulator-to-metal transition inVO2: A view from the ultrafast timescale

The metal-insulator transition of NbO ₂ : An embedded Peierls instability

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Wentzcovitchet al.Reply

Cited by 81 publications

References 3 publications

Synthesis and phase transition behavior of w-doped VO2(A) nanorods

Synthesis and phase transition behavior of w-doped VO2(A) nanorods

Evidence for a structurally-driven insulator-to-metal transition inVO2: A view from the ultrafast timescale

The metal-insulator transition of NbO 2 : An embedded Peierls instability

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The metal-insulator transition of NbO ₂ : An embedded Peierls instability