arXiv subjects

Hugo Aramberri

Publications and source records attributed to Hugo Aramberri.

28 records · Page 2Linked to original sources

arXiv2020

Novel Family of Topological Semimetals with Butterfly-like Nodal Lines

In recent years, the exotic properties of topological semimetals (TSMs) have attracted great attention and significant efforts have been made in seeking for new topological phases and material realization. In this work, we propose a new family of TSMs which harbors an unprecedented nodal line (NL) landscape consisting of a pair of concentric intersecting coplanar ellipses (CICE) at half-filling. Meanwhile, the CICE at half-filling guarantees the presence of a second pair of CICE beyond half-filling. Both CICEs are linked at four-fold degenerate points (FDPs) at zone boundaries. In addition, we identify the generic criteria for the existence of the CICE in a time reversal invariant {\it spinless} fermion system or a spinfull system with negligible spin-orbital coupling (SOC). Consequently, 9 out of 230 space groups (SGs) are feasible for hosting CICE whose location centers in the first Brillouin zone (BZ) are identified. We provide a simplest model with SG $Pbam$ (No. 55) which exhibits CICE, and the exotic intertwined drumhead surface states, induced by double-band-inversions. Finally, we propose a series of material candidates that host butterfly-like CICE NLs, such as, ZrX$_2$ (X=P,As), GeTe$_5$Tl$_2$, CYB$_2$ and Al$_2$Y$_3$.

cond-mat.mtrl-sci↗

arXiv2020

Antiferroelectricity in a family of pyroxene-like oxides with rich polymorphism

Antiferroelectrics have potential applications in energy conversion and storage, but are scarce, particularly among oxides that otherwise display rich ferroic behaviours. Are we overlooking potential antiferroelectrics, simply because we have not discovered their corresponding ferroelectric phase yet? Here we report a first-principles study suggesting this is the case of a family ABO$_3$ pyroxene-like materials, characterized by chains of corner-sharing BO$_4$ tetrahedra, a well-known member being KVO$_3$. The irregular tetrahedra have an electric dipole associated to them. In the most stable polymorph, the dipoles display an antipolar pattern with zero net moment. However, upon application of an electric field, half of the tetrahedra rotate, flipping the corresponding dipoles and reaching a ferroelectric state. We discuss the unique possibilities for tuning and optimization these antiferroelectrics offer. We argue that the structural features enabling this antiferroelectric behaviour are also present in other all-important mineral families.

cond-mat.mtrl-sci↗

arXiv2020

Effect of dopant ordering on the stability of ferroelectric hafnia

Films of all-important compound hafnia (HfO2) can be prepared in an orthorhombic ferroelectric (FE) state that is ideal for applications, e.g. in memories or negative-capacitance field-effect transistors. The origin of this FE state remains a mystery, though, as none of the proposed mechanisms for its stabilization -- from surface and size effects to formation kinetics -- is fully convincing. Interestingly, it is known that doping HfO2 with various cations favors the occurrence of the FE polymorph; however, existing first-principles works suggest that doping by itself is not sufficient to stabilize the polar phase over the usual non-polar monoclinic ground state. Here we use first-principles methods to reexamine this question. We consider two representative isovalent substitutional dopants, Si and Zr, and study their preferred arrangement within the HfO2 lattice. Our results reveal that small atoms like Si can adopt very stable configurations (forming layers within specific crystallographic planes) in the FE orthorhombic phase of HfO2, but comparatively less so in the non-polar monoclinic one. Further, we find that, at low concentrations, such a dopant ordering yields a FE ground state, the usual paraelectric phase becoming a higher-energy metastable polymorph. We discuss the implications of our findings, which constitute a definite step forward towards understanding ferroelectricity in HfO2.

cond-mat.mtrl-sci↗

arXiv2019

Archetypal soft-mode driven antipolar transition in francisite Cu3Bi(SeO3)2O2Cl

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)2O2Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at hightemperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic X-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this softmode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behaviour over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

cond-mat.mtrl-sci↗

arXiv2019

Termination Dependent Topological Surface States in Nodal Loop Semimetal HfP2

Symmetry plays a major role in all disciplines of physics. Within the field of topological materials there is a great interest in understanding how the mechanics of crystalline and internal symmetries protect crossings between the conduction and valence bands. Additionally, exploring this direction can lead to a deeper understanding on the topological properties of crystals hosting a variety of symmetries. For the first time, we report the experimental observation of topological surface states in the nodal loop semimetal HfP2 using angle resolved photoemission spectroscopy (ARPES) which is supported by our first principles calculations. Our study shows termination dependent surface states in this compound. Our experimental data reveal surface states linked to three unique nodal loops confirmed by theoretical calculation to be topologically non-trivial. This work demonstrates that transition metal dipnictides provide a good platform to study non-trivial topological states protected by nonsymmorphic symmetry.

cond-mat.mes-hall↗

arXiv2018

Electric control of the heat flux through electrophononic effects

We demonstrate a fully electric control of the heat flux, which can be continuously modulated by an externally applied electric field in PbTiO$_3$, a prototypical ferroelectric perovskite, revealing the mechanisms by which experimentally accessible fields can be used to tune the thermal conductivity by as much as 50% at room temperature.

cond-mat.mes-hall↗

arXiv2017

Thermal conductivity changes across a structural phase transition: the case of high-pressure silica

By means of first-principles calculations, we investigate the thermal properties of silica as it evolves, under hydrostatic compression, from a stishovite phase into a CaCl$_2$-type structure. We compute the thermal conductivity tensor by solving the linearized Boltzmann transport equation iteratively in a wide temperature range, using for this the pressure-dependent harmonic and anharmonic interatomic couplings obtained from first principles. Most remarkably, we find that, at low temperatures, SiO$_2$ displays a large peak in the in-plane thermal conductivity and a highly anisotropic behavior close to the structural transformation. We trace back the origin of these features by analyzing the phonon contributions to the conductivity. We discuss the implications of our results in the general context of continuous structural transformations in solids, as well as the potential geological interest of our results for silica.

cond-mat.mtrl-sci↗

arXiv2017

Strain Effects in Topological Insulators: Topological Order and the Emergence of Switchable Topological Interface States in Sb$_2$Te$_3$/Bi$_2$Te$_3$ Heterojunctions

Strain can induce a topological phase transition in bismuth dichalcogenides. We present the phase diagram for 3D topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$, Bi$_2$Se$_3$ and Sb$_2$Se$_3$ with uniaxial and biaxal strain, which show metallic and insulating phases, both topologically trivial and non-trivial. In particular, uniaxial tension can drive Sb$_2$Te$_3$ into a topologically trivial insulating phase. Thus, we propose a Sb$_2$Te$_3$/Bi$_2$Te$_3$ heterojunction in which a topological interface state arises in the common gap of this topological insulator-normal insulator heterojunction that can be switched on or off by means of uniaxial strain. This interface state is confined in the Sb$_2$Te$_3$ subsystem and is physically protected from ambient impurities. Therefore, Sb$_2$Te$_3$/Bi$_2$Te$_3$ heterojunctions can host robust helical interface states with promising spintronic applications.

cond-mat.mes-hall↗

arXiv2017

Gap and spin texture engineering of Dirac topological states at the Cr-Bi$_2$Se$_3$ interface

The presence of an exchange field in topological insulators reveals novel spin related phenomena derived from the combination of topology and magnetism. In the present work we show the controlled occurrence of either metallic or gapped topological Dirac states at the interface between ultrathin Cr films and the Bi$_2$Se$_3$ surface. The opening and closing of the gap at the Dirac point is caused by the spin reorientation transitions arising in the Cr films. We find that atom thin layers of Cr adhered to Bi$_2$Se$_3$ surfaces present a magnetic ground state with ferromagnetic planes coupled antiferromagnetically. As the thickness of the Cr film increases stepwise from one to three atomic layers, the direction of the magnetization changes twice from out-of-plane to in-plane and to out-of-plane again. The out of plane magnetization drives the gap opening and the topological surface states acquire a circular meron spin structure. Therefore, the Cr spin reorientation leads to the metal/insulator transition in the Bi$_2$Se$_3$ surface and to the correlated modification of the surface state spin texture. Consequently, the thickness of the Cr film provides an effective and controllable mechanism to modify the metallic or gapped nature, as well as the spin texture of the topological Dirac states.

cond-mat.mtrl-sci↗

arXiv2014

Proposal for an ac spin current source

We propose an ac current source that can be tuned from a pure charge to a pure spin current source. The device consists of two mesoscopic capacitors attached to a two-dimensional strip of a topological insulator. The change from charge to spin current is controlled by an offset in the top gate potentials that drive the capacitors. In addition to this setup, which anticipates the experimental realisation of quantum point contacts in topological insulators, we propose an analogous source in the quantum Hall regime which only relies on presently available building blocks. To this end, we calculate the band structure of a topological insulator in a magnetic field. The intrinsic spin-orbit coupling, together with a split gate, allows for an analogous source, where charge and spin current can be manipulated. The realisation of the device as well as the detection of the ac spin current are within reach of present experimental technology.

cond-mat.mes-hall↗