Understanding the mechanism of W-doping induced reduction of critical temperature (TC) for VO2 metal-insulator transition (MIT) is crucial for both fundamental study and technological application. Here, using synchrotron radiation X-ray absorption spectroscopy combined with first-principles calculations, we unveil the atomic structure evolutions of W dopant and its role in tailoring the TC of VO2 MIT. We find that the local structure around W atom is intrinsically symmetric with a tetragonal-like structure, exhibiting a concentration-dependent evolution involving the initial distortion, further repulsion, and final stabilization due to the strong interaction between doped W atoms and VO2 lattices across the MIT. These results directly give the experimental evidence that the symmetric W core drives the detwisting of the nearby asymmetric monoclinic VO2 lattice to form rutile-like VO2 nuclei, and the propagations of these W-encampassed nuclei through the matrix lower the thermal energy barrier for phase transition.

These results reveal that strain engineering can tune emergent functionality towards proximal macroscopic states to enable dynamic ultrafast optical phase switching and control.Precision tuning of the local environment in complex materials provides a route to control macroscopic functionality thereby offering a glimpse into how microscopic interactions conspire toward emergent behavior. Figure 1ashows resistivity measurements for a strained 30 nm LCMO film. In zero applied magnetic field, the film remains insulating at all temperatures (due to strain-enhanced orthorombicity 22 23 as depicted in Fig. 1d). For fields above 3 T the insulating phase collapses becoming a ferromagnetic metal at low temperatures. Figure 1b details the phase diagram of strained LCMO as determined from the field-dependent transport measurements 22 23 . The FM and AFM phases coexist over the range from 0-3 T, depending on temperature. To characterize the time-integrated electrodynamic response of the strained films, the optical conductivity was measured from 100 meV to 5 eV using spectroscopic ellipsometry (Fig. 1c). With decreasing temperature, the film displays spectral weight transfer from a small polaron (~1.5 eV) peak to a sharp well- Fig. 3a).As we now show, photoexcitation recovers the hidden FM phase of the strain- The photoinduced THz conductivity (PTC) is stable as long as the temperature is maintained. Further, the maximum conductivity is the same value as obtained with a strong magnetic field. This is clear from the blue dots of Fig. 1a, where the PTC from pulses, the resistivity continues to decrease by an additional order of magnitude, reaching a minimum after ~20 pulses. Figure 3b plots the photoinduced conductivity versus shot number for different fluences. The 4 mJ/cm 2 (same data as Fig. 3a) saturates at 800 Ω -1 cm -1 , while at lower fluences the conductivity saturates at a lower value. This is important, showing that the conductivity change does not simply arise from the absorbed number of photons. If there were a simple dependence on the number of absorbed photons, the data at lower fluences would saturate to the same conductivity value as the higher fluence data after a sufficient number of pulses. This is clearly not the case and indicates a cooperative process with a photon-absorption threshold. Figure 3c plots the conductivity plateaus from Measuring the conductivity dynamics of the pristine AFI state following singlepulse excitation would provide insight into the photoinduced IMT but is not experimentally feasible on a shot-to-shot basis. Instead, we employed an all-optical single-shot ultrafast spectroscopic method 28 which faithfully represents the conductivity dynamics because of the aforementioned spectral weight transfer. Figure 3d shows the results of single-shot photoinduced reflectivity dynamics (R/R) probing at the peak of the intersite transition (1.7 eV) following 1.55 eV excitation. There is a decrease in R/R consistent with dynamic spectral weight transfer to THz frequencies and the IMT dy...

The electric-field-modulated resistance switching in VO 2 thin films grown on piezoelectric (111)-0.68Pb(Mg 1/3 Nb 2/3 )O 3 -0.32PbTiO 3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO 2 /PMN-PT (111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO 2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resistive states result from the different stable remnant strain states of substrate, which is related to the rearrangements of ferroelectric domain structures in PMN-PT(111) substrate. The resistance switching tuned by electric field in our work may have potential applications for novel electronic devices.2

Nanoscale materials with size smaller than the characteristic domain size could simplify the domain structure and uncover the intrinsic properties in detail. Herein, a ultrafast open space calcination pathway is first put forward to synthesize high-quality single-domain VO(2)(M) nanocrystals and an in situ variable-temperature IR spectroscopy is first proposed to identify the size-dependent MIT behaviors in VO(2)(M) below single-domain size. The variable-temperature IR spectroscopy clearly reveals that these single-domain VO(2)(M) nanocrystals exhibit new size-dependent MIT behaviors, while the IR analysis further suggests that the size-related defect density and scattering efficiency could be used to account for their novel size-dependent MIT behaviors. This new characterization strategy of in situ variable-temperature IR spectroscopy holds great promise for extending to other systems to gain valuable insight into their intrinsic phase transition behaviors. Also, this ultrafast open space calcination pathway sets forth a new avenue in fabricating high-quality functional nanocrystals and paves the way for constructing intelligent nanodevices in the near future.

Exfoliation of layered bulk VO2(B) with strong covalent bonding by a convenient room‐temperature intercalation‐deintercalation strategy is proposed to obtain ultrathin sheets with atomic thickness of ∼0.62 nm. This VO2(B) single layer possesses a more symmetric atomic structure with slight lattice expansion that widens the band gap Energy by ΔEg = 0.19 eV, providing more possibilities for energy‐level engineering in photovoltaics.

The commensurate La 0.67 Ca 0.33 MnO 3 films, of which the bulk is a ferromagnetic-metal (FM), can show strikingly different ground states when grown on NdGaO 3 substrates of various orientations. With a larger orthorhombic lattice distortion in the substrates, the (110) films can grow essentially strain-free, but the (100) and (001) films are anisotropically strained, accompanied by an increased orthorhombicity. As a result, while the (110) films show constantly a bulklike FM ground state, the others behave dramatically different, i.e., they show a tunable antiferromagnetic-insulator (AFI) phase below ∼250 K with strong phase instability in the wide temperature range and at low magnetic fields. Considering that in La 1−x Ca x MnO 3 system the AFI phase is highly involved with the orthorhombic lattice distortion, the different ground state in (100) and (001) films could be ascribed to the epitaxial anisotropic strain, which enhances the orthorhombicity of the films via increasing the rotation and deformation of the MnO 6 octahedra.

NdNiO3 (NNO) films were grown by pulsed laser deposition on orthorhombic (110)-, (001)-, and (100)-oriented NdGaO3 substrates. It is found that all the films are tensile-strained but show dramatically different metal-insulator transition (MIT) temperatures (TMI) (160–280 K), as compared with the NNO bulk (∼200 K). A high resemblance in the sharpness of MIT and lattice variation across the MIT was observed. The TMI is highly dependent on the magnitude of the orthorhombic distortion induced by the different substrate surface plane and tends to recover the bulk value after annealing. Our results suggest that the anisotropic epitaxial strain can effectively tune the MIT of NNO films, and the NiO6 octahedra rotation and deformation involved in accommodating the tensile strain might cause the different TMI.

Understanding the key factor controlling phase transitions of correlated oxides by chemical doping is becoming of fundamental and technological interest. Here, we report vanadium chain symmetry as an effective means of mediating the structural phase transition (SPT) temperature in hydrothermal Cr-doped VO2. In-situ X-ray absorption fine structure spectroscopy unveils that high structural symmetry with equidistant 2.93 Å V–V zigzag chains, along with the dimerized V–V straight chain induced by Cr-doping, can stabilize VO2 at elevated temperature, thus raising the critical temperature of the VO2 phase transition. The Cr-doped VO2 system exhibited a SPT process involving lattice expansion, accompanied by V–V chain reconstruction into the rutile phase. These findings provide novel insights and guidance in chemically tailoring the phase transition of VO2.