Catálogo de publicaciones - revistas

<jats:p>Semiconductivity and superconductivity are remarkable quantum phenomena that have immense impact on science and technology, and materials that can be tuned, usually by pressure or doping, to host both types of quantum states are of great fundamental and practical significance. Here we show by first-principles calculations a distinct route for tuning semiconductors into superconductors by diverse large-range elastic shear strains, as demonstrated in exemplary cases of silicon and silicon carbide. Analysis of strain driven evolution of bonding structure, electronic states, lattice vibration, and electron-phonon coupling unveils robust pervading deformation induced mechanisms auspicious for modulating semiconducting and superconducting states under versatile material conditions. This finding opens vast untapped structural configurations for rational exploration of tunable emergence and transition of these intricate quantum phenomena in a broad range of materials.</jats:p>

<jats:p>The heavy fermion ferromagnet CeRh<jats:sub>6</jats:sub>Ge<jats:sub>4</jats:sub> is the first example of a clean stoichiometric system where the ferromagnetic transition can be continuously suppressed by hydrostatic pressure to a quantum critical point. In order to reveal the outcome when the magnetic lattice of CeRh<jats:sub>6</jats:sub>Ge<jats:sub>4</jats:sub> is diluted with non-magnetic atoms, this study reports comprehensive measurements of the physical properties of both single crystal and polycrystalline samples of La<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Ce<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>Rh<jats:sub>6</jats:sub>Ge<jats:sub>4</jats:sub>. With increasing <jats:italic>x</jats:italic>, the Curie temperature decreases, and no transition is observed for <jats:italic>x</jats:italic> &gt; 0.25, while the system evolves from exhibiting coherent Kondo lattice behaviors at low <jats:italic>x</jats:italic> to the Kondo impurity scenario at large <jats:italic>x</jats:italic>. Moreover, non-Fermi liquid behavior is observed over a wide doping range, which agrees well with the disordered Kondo model for 0.52 ≤ <jats:italic>x</jats:italic> ≤ 0.66, while strange metal behavior is revealed in the vicinity of <jats:italic>x</jats:italic> <jats:sub>c</jats:sub> = 0.26.</jats:p>

<jats:p>The competition between the RKKY interaction and the Kondo effect leads to a magnetic phase transition, which occurs ubiquitously in heavy fermion materials. However, there are more and more experimental evidences indicating that the valence fluctuation plays an essential role in the Ce- and Y-based compounds. We study an extended periodic Anderson model (EPAM) which includes the onsite Coulomb repulsion <jats:italic>U<jats:sub>cf</jats:sub> </jats:italic> between the localized electrons and conduction electrons. By employing the density matrix embedding theory, we investigate the EPAM in the symmetric case at half filling. By fixing the onsite Coulomb repulsion <jats:italic>U</jats:italic> of the localized electrons to an intermediate value, the interplay between the RKKY interaction, the Kondo effect and the Coulomb repulsion <jats:italic>U<jats:sub>cf</jats:sub> </jats:italic> brings rich physics. We find three different phases, the antiferromagnetic phase, the charge order phase and paramagnetic phase. When the hybridization strength <jats:italic>V</jats:italic> between the localized orbital and the conduction orbital is small, the Kondo effect is weak so that the AF phase and the CO phase are present. The phase transition between the two long-range ordered phase is of first order. We also find a coexistence region between the two phases. As <jats:italic>V</jats:italic> increases, the Kondo effect becomes stronger, and the paramagnetic phase appears between the other two phases.</jats:p>

<jats:p>Enhancing the dopability of semiconductors via strain engineering is critical to improving their functionalities, which is, however, largely hindered by the lack of basic rules. In this study, for the first time, we develop a universal theory to understand the total energy changes of point defects (or dopants) with different charge states under strains, which can exhibit either parabolic or superlinear behaviors, determined by the size of defect-induced local volume change (Δ <jats:italic>V</jats:italic>). In general, Δ <jats:italic>V</jats:italic> increases (decreases) when an electron is added (removed) to (from) the defect site. Consequently, in terms of this universal theory, three basic rules can be obtained to further understand or predict the diverse strain-dependent doping behaviors, i.e., defect formation energies, charge-state transition levels, and Fermi pinning levels, in semiconductors. These three basic rules could be generally applied to improve the doping performance or overcome the doping bottlenecks in various semiconductors.</jats:p>

<jats:p>Depositing magnetic insulators on graphene has been a promising route to introduce magnetism via exchange proximity interaction in graphene for future spintronics applications. Molecule-based magnets may offer unique opportunities because of their synthesis versatility. Here, we investigate the magnetic proximity effect of epitaxial iron phthalocyanine (FePc) molecules on high-quality monolayer and bilayer graphene devices on hexagonal boron nitride substrates by probing the local and nonlocal transport. Although the FePc molecules introduce large hole doping effects combined with mobility degradation, the magnetic proximity gives rise to a canted antiferromagnetic state under a magnetic field in the monolayer graphene. On bilayer graphene and FePc heterostructure devices, the nonlocal transport reveals a pronounced Zeeman spin-Hall effect. Further analysis of the scattering mechanism in the bilayer shows a dominated long-range scattering. Our findings in graphene/organic magnetic insulator heterostructure provide a new insight for use of molecule-based magnets in two-dimensional spintronic devices.</jats:p>

<jats:p>The angular-dependent magnetoresistance (AMR) of the <jats:italic>ab</jats:italic> plane is measured on the single crystals of iron-chalcogenide FeSe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>S<jats:sub> <jats:italic>x</jats:italic> </jats:sub> (<jats:italic>x</jats:italic> = 0, 0.07, 0.13 and 1) and FeSe<jats:sub>1–<jats:italic>y</jats:italic> </jats:sub>Te<jats:sub> <jats:italic>y</jats:italic> </jats:sub> (<jats:italic>y</jats:italic> = 0.06, 0.61 and 1) at various temperatures under fields up to 9 T. A pronounced twofold-anisotropic carrier-scattering effect is identified by AMR, and attributed to a magnetic-field-induced spin nematicity that emerges from the tetragonal normal-state regime below a characteristic temperature <jats:italic>T</jats:italic> <jats:sub>sn</jats:sub>. This magnetically polarized spin nematicity is found to be ubiquitous in the isoelectronic FeSe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>S<jats:sub> <jats:italic>x</jats:italic> </jats:sub> and FeSe<jats:sub>1–<jats:italic>y</jats:italic> </jats:sub>Te<jats:sub> <jats:italic>y</jats:italic> </jats:sub> systems, no matter whether the sample shows an electronic nematic order at <jats:italic>T</jats:italic> <jats:sub>s</jats:sub> ≲ <jats:italic>T</jats:italic> <jats:sub>sn</jats:sub>, or an antiferromagnetic order at <jats:italic>T</jats:italic> <jats:sub>N</jats:sub> &lt; <jats:italic>T</jats:italic> <jats:sub>sn</jats:sub>, or neither order. Importantly, we find that the induced spin nematicity shows a very different response to sulfur substitution from the spontaneous electronic nematicity: The spin-nematic <jats:italic>T</jats:italic> <jats:sub>sn</jats:sub> is not suppressed but even enhanced by the substitution, whereas the electronic-nematic <jats:italic>T</jats:italic> <jats:sub>s</jats:sub> is rapidly suppressed, in the FeSe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>S<jats:sub> <jats:italic>x</jats:italic> </jats:sub> system. Furthermore, we find that the superconductivity is significantly suppressed with the enhancement of the induced spin nematicity in both FeSe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>S<jats:sub> <jats:italic>x</jats:italic> </jats:sub> and FeSe<jats:sub>1–<jats:italic>y</jats:italic> </jats:sub>Te<jats:sub> <jats:italic>y</jats:italic> </jats:sub> samples.</jats:p>

<jats:p>Compounds with the A15 structure have attracted extensive attention due to their superconductivity and nontrivial topological band structures. We have successfully grown Nb<jats:sub>3</jats:sub>Sb single crystals with the A15 structure and systematically measured the longitudinal resistivity, Hall resistivity and quantum oscillations in magnetization. Similar to other topological trivial/nontrivial semimetals, Nb<jats:sub>3</jats:sub>Sb exhibits large magnetoresistance (MR) at low temperatures (717%, 2 K and 9 T), unsaturating quadratic field dependence of MR and up-turn behavior in <jats:italic>ρ<jats:sub>xx</jats:sub> </jats:italic>(<jats:italic>T</jats:italic>) curves under magnetic field, which is considered to result from a perfect hole-electron compensation, as evidenced by the Hall resistivity measurements. The nonzero Berry phase obtained from the de-Hass van Alphen (dHvA) oscillations demonstrates that Nb<jats:sub>3</jats:sub>Sb is topologically nontrivial. These results indicate that Nb<jats:sub>3</jats:sub>Sb superconductor is also a semimetal with large MR and nontrivial Berry phase. This indicates that Nb<jats:sub>3</jats:sub>Sb may be another platform to search for the Majorana zero-energy mode.</jats:p>

<jats:p>Interface engineering is an effective and feasible method to regulate the magnetic anisotropy of films by altering interfacial states between films. Using the technique of pulsed laser deposition, we prepared La<jats:sub>0.67</jats:sub>Sr<jats:sub>0.33</jats:sub>MnO<jats:sub>3</jats:sub> (LSMO) and La<jats:sub>0.67</jats:sub>Sr<jats:sub>0.33</jats:sub>MnO<jats:sub>3</jats:sub>/SrCoO<jats:sub>2.5</jats:sub> (LSMO/SCO) films on (110)-oriented La<jats:sub>0.3</jats:sub>Sr<jats:sub>0.7</jats:sub>Al<jats:sub>0.65</jats:sub>Ta<jats:sub>0.35</jats:sub>O<jats:sub>3</jats:sub> substrates. By covering the SCO film above the LSMO film, we transformed the easy magnetization axis of LSMO from the [001] axis to the [<jats:inline-formula> <jats:tex-math><?CDATA $1\bar{1}0$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mover accent="true"> <mml:mn>1</mml:mn> <mml:mo>¯</mml:mo> </mml:mover> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpl_38_8_087502_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>] axis in the film plane. Based on statistical analyses, we find that the corresponding Mn–Mn ionic distances are different in the two types of LSMO films, causing different distortions of Mn–O octahedron in LSMO. In addition, it also induces diverse electronic occupation states in Mn<jats:sup>3+</jats:sup> ions. The <jats:italic>e</jats:italic> <jats:sub>g</jats:sub> electron of Mn<jats:sup>3+</jats:sup> occupies 3<jats:italic>z</jats:italic> <jats:sup>2</jats:sup> – <jats:italic>r</jats:italic> <jats:sup>2</jats:sup> and <jats:italic>x</jats:italic> <jats:sup>2</jats:sup> – <jats:italic>y</jats:italic> <jats:sup>2</jats:sup> orbitals in the LSMO and LSMO/SCO, respectively. We conclude that the electronic spin reorientation leads to the transformation of the easy magnetization axis in the LSMO films.</jats:p>

<jats:p>Ferroelectric materials are spontaneous symmetry breaking systems that are characterized by ordered electric polarizations. Similar to its ferromagnetic counterpart, a ferroelectric domain wall can be regarded as a soft interface separating two different ferroelectric domains. Here we show that two bound state excitations of electric polarization (polar wave), or the vibration and breathing modes, can be hosted and propagate within the ferroelectric domain wall. In particular, the vibration polar wave has zero frequency gap, thus is constricted deeply inside ferroelectric domain wall, and can even propagate in the presence of local pinnings. The ferroelectric domain wall waveguide as demonstrated here offers a new paradigm in developing ferroelectric information processing units.</jats:p>

<jats:p>The persistent spin helix (PSH) system is considered to have promising applications in energy-conservation spintronics because it supports an extraordinarily long spin lifetime of carriers. Here, we predict that the existence of PSH state in two-dimensional (2D) ferroelectric NbOI<jats:sub>2</jats:sub> monolayers. Our first-principles calculation results show that there exists Dresselhaus-type spin-orbit coupling (SOC) band splitting near the conduction-band minimum (CBM) of the NbOI<jats:sub>2</jats:sub> monolayer. It is revealed that the spin splitting near CBM merely refers to out-of-plane spin configuration in the wave vector space, which gives rise to a long-lived PSH state that can be controlled by reversible ferroelectric polarization. We believe that the coupling characteristics of ferroelectric polarization and spin texture in NbOI<jats:sub>2</jats:sub> provide a platform for the realization of fully electric controlled spintronic devices.</jats:p>