Catálogo de publicaciones - revistas

<jats:p>Wide bandgap semiconductors are crucially significant for optoelectronic and thermoelectric device applications. Metal nitride is a class of semiconductor material with great potential. Under high pressure, the bandgap of magnesium nitride was predicted to grow. Raman spectra, ultra-violet-visible (UV-Vis) absorption spectra, and first-principles calculations were employed in this study to analyze the bandgap evolution of Mg<jats:sub>3</jats:sub>N<jats:sub>2</jats:sub>. The widening of the bandgap has been first detected experimentally, with the gap increasing from 2.05 eV at 3 GPa to 2.88 eV at 47 GPa. According to the calculation results, the enhanced covalent component is responsible for the bandgap widening.</jats:p>

<jats:p>The control of thermal expansion is essential in applications where thermal stability is required from fiber optics coatings, high performance fuel cell cathodes to tooth fillings. Negative thermal expansion (NTE) materials, although rare, are fundamental for this purpose. This work focuses on studying tetracyanidoborate salt CuB(CN)<jats:sub>4</jats:sub>, an interesting cubic-structure material that displays large isotropic NTE. A joint study of synchrotron x-ray diffraction, temperature-dependent Raman spectroscopy, and lattice dynamics calculations was conducted, showing that not only low-frequency optical modes (transverse thermal vibrations of N and C atoms) but also the acoustic modes (the vibrations of Cu atoms as a collective torsion of the neighboring atoms), contribute to NTE. As a result, new insights were gained into the NTE mechanism of CuB(CN)<jats:sub>4</jats:sub> and related framework materials.</jats:p>

<jats:p>High mobility quasi two-dimensional electron gas (2DEG) found at the CaZrO<jats:sub>3</jats:sub>/SrTiO<jats:sub>3</jats:sub> nonpolar heterointerface is attractive and provides a platform for the development of functional devices and nanoelectronics. Here we report that the carrier density and mobility at low temperature can be tuned by gate voltage at the CaZrO<jats:sub>3</jats:sub>/SrTiO<jats:sub>3</jats:sub> interface. Furthermore, the magnitude of Rashba spin–orbit interaction can be modulated and increases with the gate voltage. Remarkably, the diffusion constant and the spin–orbit relaxation time can be strongly tuned by gate voltage. The diffusion constant increases by a factor of ∼ 19.98 and the relaxation time is reduced by a factor of over three orders of magnitude while the gate voltage is swept from –50 V to 100 V. These findings not only lay a foundation for further understanding the underlying mechanism of Rashba spin–orbit coupling, but also have great significance in developing various oxide functional devices.</jats:p>

<jats:p>By using scanning tunneling microscopy, we investigated the electronic evolution of <jats:italic>T</jats:italic> <jats:sub>d</jats:sub>-WTe<jats:sub>2</jats:sub> via <jats:italic>in-situ</jats:italic> surface alkali K atoms deposition. The <jats:italic>T</jats:italic> <jats:sub>d</jats:sub>-WTe<jats:sub>2</jats:sub> surface is electron doped upon K deposition, and as the K coverage increases, two gaps are sequentially opened near Fermi energy, which probably indicates that two phase transitions concomitantly occur during electron doping. The two gaps both show a dome-like dependence on the K coverage. While the bigger gap shows no prominent dependence on the magnetic field, the smaller one can be well suppressed and thus possibly corresponds to the superconducting transition. This work indicates that <jats:italic>T</jats:italic> <jats:sub>d</jats:sub>-WTe<jats:sub>2</jats:sub> exhibits rich quantum states closely related to the carrier concentration.</jats:p>

<jats:p>Van der Waals (VDW) heterostructures have attracted significant research interest due to their tunable interfacial properties and potential applications in many areas such as electronics, optoelectronic, and heterocatalysis. In this work, the influences of interfacial defects on the electronic structures and photocatalytic properties of hBN/<jats:italic>MX</jats:italic> <jats:sub>2</jats:sub> (<jats:italic>M</jats:italic> = Mo, W, and <jats:italic>X</jats:italic> = S, Se) are studied using density functional theory calculations. The results reveal that the band alignment of hBN/<jats:italic>MX</jats:italic> <jats:sub>2</jats:sub> can be adjusted by introducing vacancies and atomic doping. The type-I band alignment of the host structure is maintained in the heterostructure with n-type doping in the hBN sublayer. Interestingly, the band alignment changed into the type-II heterostructrue due to V<jats:sub>B</jats:sub> defect and p-type doping is introduced into the hBN sublayer. This can conduce to the separation of photo-generated electron–hole pairs at the interfaces, which is highly desired for heterostructure photocatalysis. In addition, two Z-type heterostructures including hBN(Be<jats:sub>B</jats:sub>)/MoS<jats:sub>2</jats:sub>, hBN(Be<jats:sub>B</jats:sub>)/MoSe<jats:sub>2</jats:sub>, and hBN(V<jats:sub>N</jats:sub>)/MoSe<jats:sub>2</jats:sub> are achieved, showing the decreasing of band gap and ideal redox potential for water splitting. Our results reveal the possibility of engineering the interfacial and photocatalysis properties of hBN/<jats:italic>MX</jats:italic> <jats:sub>2</jats:sub> heterostructures via interfacial defects.</jats:p>

<jats:p>Quantum critical phenomena in the quasi-one-dimensional limit remain an open issue. We report the uniaxial stress effect on the quasi-one-dimensional Kondo lattice CeCo<jats:sub>2</jats:sub>Ga<jats:sub>8</jats:sub> by electric transport and AC heat capacity measurements. CeCo<jats:sub>2</jats:sub>Ga<jats:sub>8</jats:sub> is speculated to sit in close vicinity but on the quantum-disordered side of a quantum critical point. Upon compressing the <jats:italic>c</jats:italic> axis, parallel to the Ce–Ce chain, the onset of coherent Kondo effect is enhanced. In contrast, the electronic specific heat diverges more rapidly at low temperature when the intra-chain distance is elongated by compressions along <jats:italic>a</jats:italic> or <jats:italic>b</jats:italic> axis. These results suggest that a tensile intra-chain strain (<jats:italic>ε<jats:sub>c</jats:sub> </jats:italic> &gt; 0) pushes CeCo<jats:sub>2</jats:sub>Ga<jats:sub>8</jats:sub> closer to the quantum critical point, while a compressive intra-chain strain (<jats:italic>ε<jats:sub>c</jats:sub> </jats:italic> &gt; 0) likely causes departure. Our work provides a rare paradigm of manipulation near a quantum critical point in a quasi-1D Kondo lattice by uniaxial stress, and paves the way for further investigations on the unique feature of quantum criticality in the quasi-1D limit.</jats:p>

<jats:p>Geometrical frustration in low-dimensional magnetic systems has been an intriguing research aspect, where the suppression of conventional magnetic order may lead to exotic ground states such as spin glass or spin liquid. In this work we report the synthesis and magnetism study of the monocrystalline Mn<jats:sub>2</jats:sub>Ga<jats:sub>2</jats:sub>S<jats:sub>5</jats:sub>, featuring both the van der Waals structure and a bilayered triangular Mn lattice. Magnetic susceptibility reveals a significant antiferromagnetic interaction with a Curie–Weiss temperature <jats:italic>θ</jats:italic> <jats:sub>w</jats:sub> ∼ –260 K and a high spin <jats:italic>S</jats:italic> = 5/2 Mn<jats:sup>2+</jats:sup> state. However, no long range magnetic order has been found down to 2 K, and a spin freezing transition is found to occur at around 12 K well below its <jats:italic>θ</jats:italic> <jats:sub>w</jats:sub>. This yields a frustration index of <jats:italic>f</jats:italic> = –<jats:italic>θ</jats:italic> <jats:sub>w</jats:sub>/<jats:italic>T</jats:italic> <jats:sub>f</jats:sub> ≈ 22, an indication that the system is highly frustrated. The absence of a double-peak structure in magnetic specific heat compared with the <jats:italic>TM</jats:italic> <jats:sub>2</jats:sub>S<jats:sub>4</jats:sub> compounds implies that the spin freezing behavior in Mn<jats:sub>2</jats:sub>Ga<jats:sub>2</jats:sub>S<jats:sub>5</jats:sub> is a result of the competition between exchange interactions and the 2D crystalline structure. Our results suggest that the layered Mn<jats:sub>2</jats:sub>Ga<jats:sub>2</jats:sub>S<jats:sub>5</jats:sub> would be an excellent candidate for investigating the physics of 2D magnetism and spin disordered state.</jats:p>

<jats:p>Unusual quantum phenomena usually emerge upon doping Mott insulators. Using a molecular beam epitaxy system integrated with cryogenic scanning tunneling microscope, we investigate the electronic structure of a modulation-doped Mott insulator Sn/Si(111)-(<jats:inline-formula> <jats:tex-math> <?CDATA $\sqrt{3}\times \sqrt{3}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msqrt> <mml:mn>3</mml:mn> </mml:msqrt> <mml:mo>×</mml:mo> <mml:msqrt> <mml:mn>3</mml:mn> </mml:msqrt> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_31_6_067401_ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>)<jats:italic>R</jats:italic>30°. In underdoped regions, we observe a universal pseudogap opening around the Fermi level, which changes little with the applied magnetic field and the occurrence of Sn vacancies. The pseudogap gets smeared out at elevated temperatures and alters in size with the spatial confinement of the Mott insulating phase. Our findings, along with the previously observed superconductivity at a higher doping level, are highly reminiscent of the electronic phase diagram in the doped copper oxide compounds.</jats:p>

<jats:p>High performance of the generation, stabilization and manipulation of magnetic skyrmions prompts the application of topological multilayers in spintronic devices. Skyrmions in synthetic antiferromagnets (SAF) have been considered as a promising alternative to overcome the limitations of ferromagnetic skyrmions, such as the skyrmion Hall effect and stray magnetic field. Here, by using the Lorentz transmission electron microscopy, the interconversion between the single domain, labyrinth domain and skyrmion state can be observed by the combined manipulation of electric current and magnetic field in a Hall balance (a SAF with the core structure of [Co/Pt]<jats:sub>4</jats:sub>/NiO/[Co/Pt]<jats:sub>4</jats:sub> showing perpendicular magnetic anisotropy). Furthermore, high-density room temperature skyrmions can be stabilized at zero field while the external stimulus is removed and the skyrmion density is tunable. The generation and manipulation method of skyrmions in Hall balance in this study opens up a promising way to engineer SAF-skyrmion-based memory devices.</jats:p>

<jats:p>The (1 – <jats:italic>x</jats:italic>)NiCo<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub>/<jats:italic>x</jats:italic>PbS (0 ≤ <jats:italic>x</jats:italic> ≤ 0.2) nanocomposite samples are synthesized using the hydrothermal and thermolysis procedures. The different phases developed in the obtained nanocomposite samples are accurately determined using the x-ray diffraction technique equipped with a line-detector. The percentage of the formed phases (NiCo<jats:sub>2</jats:sub>O<jats:sub>4</jats:sub> (NCO), PbS, PbSO<jats:sub>4</jats:sub>), structural and microstructure parameters are determined using Rietveld quantitative phase analysis. The transmission electron microscope (TEM) images and Rietveld analysis reveal almost isotropic particle size in the nano range with a very narrow size distribution. The obtained phase percentage of PbS and PbSO<jats:sub>4</jats:sub> are smaller than nominated values (<jats:italic>x</jats:italic>) suggesting dissolving of some Pb and S ions into NCO which is then confirmed by the analysis of Fourier-transform infrared (FTIR) spectra of nanocomposite samples. The absorption spectra are modified upon doping NCO with PbS. The optical band gaps of the nanocomposites increase as the amount of PbS augments. The effect of alloying on extinction coefficient, refractive index, dielectric constant, optical conductivity, the intensity, and emitted color from the photoluminescence of the nanocomposite samples are also studied. The refractive index value of NCO and NCO–PbS nanocomposite samples exhibit normal dispersions. The photoluminescent measurements reveal that the NCO–PbS nanocomposites can emit a violet color. The improvement in the values of the nonlinear optical (NLO) parameters of pristine NCO at high frequencies or the nanocomposite samples at low frequencies, are made them used in NLO photonic devices.</jats:p>