Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Geometric phase analysis (GPA) is a widely used technique for extracting displacement and strain fields from scanning probe images. Here, we demonstrate that GPA should be implemented with caution when several fundamental lattices contribute to the image, in particular in twisted heterostructures featuring moiré patterns. We find that in this case, GPA is likely to suggest the presence of chiral displacement and periodic strain fields, even if the structure is completely relaxed and without distortions. These delusive fields are subject to change with varying twist angles, which could mislead the interpretation of twist angle-dependent properties.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Mermin–Wagner theorem forbids spontaneous symmetry breaking of spins in one/two-dimensional (2D) systems at a finite temperature and rules out the stabilization of this ordered state. However, it does not apply to all types of phase transitions in low dimensions, such as the topologically ordered phase rigorously shown by Berezinskii–Kosterlitz–Thouless (BKT) and experimentally realized in very limited systems such as superfluids and superconducting thin films. Quasi-2D van der Waals magnets provide an ideal platform to investigate the fundamentals of low-dimensional magnetism. We explored the quasi-2D honeycomb antiferromagnetic single crystals of (Ni<jats:italic> <jats:sub>x</jats:sub> </jats:italic>Fe<jats:sub>1−<jats:italic>x</jats:italic> </jats:sub>)<jats:sub>2</jats:sub>P<jats:sub>2</jats:sub>S<jats:sub>6</jats:sub> (<jats:italic>x</jats:italic> = 1, 0.7, 0.5, 0.3, and 0) using in-depth temperature-dependent Raman measurements supported by first-principles calculations of the phonon frequencies. Quite surprisingly, we observed renormalization of the phonon modes much below the long-range magnetic ordered temperature attributed to the topological ordered state, namely the BKT phase, which is also found to change as a function of doping. The extracted critical exponent of the order-parameter (spin–spin correlation length, <jats:inline-formula> <jats:tex-math/> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>ξ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>T</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad3e0aieqn1.gif" xlink:type="simple"/> </jats:inline-formula>) evinces the signature of a topologically active state driven by vortex–antivortex excitations. As a function of doping, a tunable transition from paramagnetic to antiferromagnetic ordering is shown via phonons reflected in the strong renormalization of the self-energy parameters of the Raman active phonon modes. The extracted exchange parameter (<jats:italic>J</jats:italic>) is found to vary by ∼100% by increasing the value of doping, ranging from ∼6 meV (for <jats:italic>x</jats:italic> = 0.3) to 13 meV (for <jats:italic>x</jats:italic> = 1).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Ferroelectricity with out-of-plane polarization has so far been found in several two-dimensional (2D) materials, including monolayers comprising three to five planes of atoms, e.g. <jats:italic>α</jats:italic>-In<jats:sub>2</jats:sub>Se<jats:sub>3</jats:sub> and MoTe<jats:sub>2</jats:sub>. Here, we explore the generation of out-of-plane polarization within a one-atom-thick monolayer material, namely hexagonal boron nitride. We performed density-functional-theory calculations to explore inducing ferroelectric-like distortions through incorporation of isovalent substitutional impurities that are larger than the host atoms. This disparity in bond lengths causes a buckling of the h-BN, either up or down, which amounts to a dipole with two equivalent energies and opposing orientations. We tested several impurities to explore the magnitude of the induced dipole and the switching energy barrier for dipole inversion. The effects of strain, dipole–dipole interactions, and vertical heterostructures with graphene are further explored. Our results suggest a highly-tunable system with ground state antiferroelectricity and metastable ferroelectricity. We expect that this work will help foster new ways to include functionality in layered 2D-material-based applications.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Hybrid structures consisting of two-dimensional (2D) magnets and semiconductors have exhibited extensive functionalities in spintronics and opto-spintronics. In this work, we have fabricated WSe<jats:sub>2</jats:sub>/Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub> van der Waals heterostructures and investigated proximity effects on 2D magnetism. Through reflective magnetic circular dichroism, we have observed a temperature-dependent modulation of magnetic order in the heterostructure. For temperatures above <jats:inline-formula> <jats:tex-math/> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>40</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad3b12ieqn1.gif" xlink:type="simple"/> </jats:inline-formula>, WSe<jats:sub>2</jats:sub>-covered Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub> exhibits a larger coercive field than that observed in bare Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub>, accompanied by a noticeable enhancement of the Curie temperature by <jats:inline-formula> <jats:tex-math/> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>21</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad3b12ieqn2.gif" xlink:type="simple"/> </jats:inline-formula>. This strengthening suggests an increase in magnetic anisotropy in the interfacial Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub> layer, which can be attributed to the spin–orbit coupling (SOC) proximity effect induced by the adjacent WSe<jats:sub>2</jats:sub> layers. However, at much lower temperatures (<jats:inline-formula> <jats:tex-math/> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>&lt;</mml:mo> <mml:mn>20</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad3b12ieqn3.gif" xlink:type="simple"/> </jats:inline-formula>), a non-monotonic modification of the coercive field is observed, showing both reduction and enhancement, which depends on the thickness of the WSe<jats:sub>2</jats:sub> and Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub> layers. Moreover, an unconventional two-step magnetization process emerges in the heterostructure, indicating the short-range nature of SOC proximity effects. Our findings on proximity coupling may shed light on the design of future spintronic and memory devices based on 2D magnetic heterostructures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Graphene-based electrodes have been extensively investigated for supercapacitor applications. However, their ion diffusion efficiency is often hindered by the graphene restacking phenomenon. Even though holey graphene (hG) is fabricated to address this issue by providing ion transport channels, those channels could still be blocked by densely stacked graphene nanosheets. To tackle this challenge, this research aims at improving the ion diffusion efficiency of microwave-synthesized hG films by tuning the water interlayer spacer towards the improved supercapacitor performance. By controlling the vacuum filtration during graphene-based electrode fabrication, we obtain dry films with dense packing and wet films with sparse packing. The SEM images reveal that 20 times larger interlayer distance is constructed in the wet film compared to that in the dry counterpart. The hG wet film delivers a specific capacitance of 239 F g<jats:sup>−1</jats:sup>, ∼82% enhancement over the dry film (131 F g<jats:sup>−1</jats:sup>). By an integrated experimental and computational study, we quantitatively show that the interlayer spacing in combination with the nanoholes in the basal plane dominates the ion diffusion rate in hG-based electrodes. Our study concludes that novel hierarchical structures should be further considered even in hG thin films to fully exploit the superior advantages of graphene-based supercapacitors.</jats:p>