arXiv subjects

Karin M. Rabe

Publications and source records attributed to Karin M. Rabe.

At least 37 records · Page 2Linked to original sources

arXiv2021

High-temperature phonon-mediated superconductivity in monolayer Mg2B4C2

A new two-dimensional material { Mg2B4C2, belonging to the family of the conventional superconductor MgB2, is theoretically predicted to exhibit superconductivity with critical temperature Tc estimated in the 47-48 K range (predicted using the McMillian-Allen-Dynes formula) without any tuning of external parameters such as doping, strain, or substrate-induced effects. The origin of such a high intrinsic Tc is ascribed to the presence of strong electron-phonon coupling and topological Dirac states (which are absent in MgB2) yielding a large density of states at the Fermi level. This material also features a nontrivial electronic band topology exhibiting Dirac points, practically gapless Dirac nodal lines, and topological nontrivial edge states. Consequently, it is a potential candidate for realization of topological superconductivity in 2D. This system is obtained after replacing the chemically active boron layers in MgB2 by chemically inactive boron-carbon layers. Hence, the surfaces of this material are inert. Our calculations confirm the stability of 2D Mg2B4C2. We also find that the key features of this material remain essentially unchanged when its thickness is increased by modestly increasing the number of inner MgB2 layers.

cond-mat.supr-con↗

arXiv2021

Polarization switching mechanism in HfO$_2$ from first-principles lattice mode analysis

In this work, we carry out first-principles calculations and lattice mode analysis to investigate the polarization switching mechanism in HfO$_2$. Because the stability of the polar orthorhombic $Pca2_1$ phase of HfO$_2$ arises from a trilinear coupling, polarization switching requires the flipping of not only the polar $Γ_{15}^Z$ mode, but also at least one zone-boundary anti-polar mode. The coupling between the polar and anti-polar modes thus leads to substantial differences among different polarization switching paths. Specifically, our lattice-mode-coupling analysis shows that paths in which the $X_2^-$ mode is reversed involve a large activation energy, which because the $X_2^-$ mode is nonpolar cannot be directly overcome by applying an electric field. Our results show that the anti-polar $Pbca$ phase, whose structure is locally quite similar to that of the $Pca2_1$ phase, similarly cannot be transformed to this phase by an electric field as this would require local reversal of the $X_2^-$ mode pattern. Moreover, for the domain wall structure most widely considered, propagation also requires the reversal of the $X_2^-$ mode, leading to a much larger activation energy compared with that for the propagation of domain wall structures with a single sign for the $X_2^-$ mode. Finally, these first-principles results for domain wall propagation in HfO$_2$ have implications to many experimental observations, such as sluggish domain wall motion and robust ferroelectricity in thin films, and lattice mode analysis deepens our understanding of these distinctive properties of ferroelectric HfO$_2$.

cond-mat.mtrl-sci↗

arXiv2021

Lattice dynamics and magnetic exchange interactions in GeCo2O4, a spinel with S = 1/2 pyrochlore lattice

GeCo$_2$O$_4$ is a unique system in the family of cobalt spinels ACo$_2$O$_4$ (A= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of GeCo$_2$O$_4$ spinel has remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel GeCo$_2$O$_4$, which exhibits an AFM ordering below the Néel temperature $T_N$ ~21 K, followed by a cubic ($Fd{\bar 3}m$) to tetragonal ($I4_{1}/amd$) structural phase transition at $T_S$ ~16 K. Our density-functional theory (DFT+U) calculations reveal that one needs to consider magnetic-exchange interactions up to the third nearest neighbors to get an accurate description of the low-temperature AFM order in GeCo$_2$O$_4$. At room temperature three distinct IR-active modes ($T_{1u}$) are observed at frequencies 680, 413, and 325 cm$^{-1}$ along with four Raman-active modes $A_{1g}$, $T_{2g}(1)$, $T_{2g}(2)$, and $E_{g}$ at frequencies 760, 647, 550, and 308 cm$^{-1}$, respectively, which match reasonably well with our DFT+U calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes, is also discussed. Noticeable changes around $T_N$ and $T_S$ are observed in the Raman line parameters of the $E_{g}$ and $T_{2g}$ modes, which are associated with the modulation of the Co-O bonds in CoO$_6$ octahedra during the excitations of these modes.

cond-mat.mtrl-sci↗

arXiv2021

Epitaxy, exfoliation, and strain-induced magnetism in rippled Heusler membranes

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al$_2$O$_3$ substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetixm and magnetostriction) and strain gradients (flexomagnetism).

cond-mat.mtrl-sci↗

arXiv2021

Resonant Band Engineering of Ferroelectric Tunnel Junctions

We propose energy band engineering to enhance tunneling electroresistance (TER) in ferroelectric tunnel junctions (FTJs). We predict that an ultrathin dielectric layer with a smaller band gap, embedded into a ferroelectric barrier layer, acts as a switch controlling high and low conductance states of an FTJ depending on polarization orientation. Using first-principles modeling based on density functional theory, we investigate this phenomenon for a prototypical SrRuO3/BaTiO3/SrRuO3 FTJ with a BaSnO3 monolayer embedded in the BaTiO3 barrier. We show that in such a composite-barrier FTJ, ferroelectric polarization of BaTiO3 shifts the conduction band minimum of the BaSnO3 monolayer above or below the Fermi energy depending on polarization orientation. The resulting switching between direct and resonant tunneling leads to a TER effect with a giant ON/OFF conductance ratio. The proposed resonant band engineering of FTJs can serve as a viable tool to enhance their performance useful for device application.

cond-mat.mtrl-sci↗

arXiv2020

Kinetically-stabilized Ferroelectricity in Bulk Singlecrystalline HfO2:Y without Wake-up Effects

HfO2, a simple binary oxide, holds ultra-scalable ferroelectricity integrable into silicon technology. Polar orthorhombic (Pbc21) form in ultra-thin-films ascribes as the plausible root-cause of the astonishing ferroelectricity, which has thought not attainable in bulk crystals. Though, perplexities remain primarily due to the polymorphic nature and the characterization challenges at small-length scales. Herein, utilizing a state-of-the-art Laser-Diode-heated Floating Zone technique, we report ferroelectricity in bulk single-crystalline HfO2:Y as well as the presence of anti-polar Pbca phase at different Y concentrations. Neutron diffraction and atomic imaging demonstrate (anti-)polar crystallographic signatures and abundant 90o/180o ferroelectric domains in addition to the switchable polarization with little wake-up effects. Density-functional theory calculations suggest that the Yttrium doping and rapid cooling are the key factors for the desired phase. Our observations provide new insights into the polymorphic nature and phase controlling of HfO2, remove the upper size limit for ferroelectricity, and also pave a new road toward the next-generation ferroelectric devices.

cond-mat.mtrl-sci↗

arXiv2020

Controlling ferroelectric hysteresis offsets in PbTiO$_{3}$ based superlattices

Ferroelectric materials are characterized by degenerate ground states with multiple polarization directions. In a ferroelectric capacitor this should manifest as equally favourable up and down polarization states. However, this ideal behavior is rarely observed in ferroelectric thin films and superlattice devices, which generally exhibit a built-in bias which favors one polarization state over the other. Often this polarization asymmetry can be attributed to the electrodes. In this study we examine bias in PbTiO$_3$-based ferroelectric superlattices that is not due to the electrodes, but rather to the nature of the defects that form at the interfaces during growth. Using a combination of experiments and first-principles simulations, we are able to explain the sign of the observed built-in bias and its evolution with composition. Our insights allow us to design devices with zero built-in bias by controlling the composition and periodicity of the superlattices.

cond-mat.mtrl-sci↗

arXiv2020

Phase Competition in HfO$_2$ with Applied Electric Field from First Principles

In this work, the results of first-principles density-functional-theory calculations are used to construct the energy landscapes of HfO$_2$ and its Y and Zr substituted derivatives as a function of symmetry-adapted lattice-mode amplitudes. These complex energy landscapes possess multiple local minima, corresponding to the tetragonal, oIII ($Pca2_1$), and oIV ($Pmn2_1$) phases. We find that the energy barrier between the non-polar tetragonal phase and the ferroelectric oIII phase can be lowered by Y and Zr substitution. In Hf$_{0.5}$Zr$_{0.5}$O$_2$ with an ordered cation arrangement, Zr substitution makes the oIV phase unstable, and it become an intermediate state in the tetragonal to oIII phase transition. Using these energy landscapes, we interpret the structural transformations and hysteresis loops computed for electric-field cycles with various choices of field direction. The implications of these results for interpreting experimental observations, such as the wake-up and split-up effects, are also discussed. These results and analysis deepen our understanding of the origin of ferroelectricity and field cycling behaviors in HfO$_2$-based films, and allow us to propose strategies for improving their functional properties.

cond-mat.mtrl-sci↗

arXiv2020

Stabilization of competing ferroelectric phases of HfO$_2$ under epitaxial strain

Hafnia (HfO$_2$)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO$_2$ has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO$_2$ have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO$_2$ are used to discover that imposing an in-plane shear strain on the tetragonal phase induces a nonpolar to polar phase transition. This in-plane shear-induced polar phase is shown to be an epitaxial distortion of a known metastable ferroelectric $Pnm2_1$ phase of HfO$_2$. It is proposed that this ferroelectric $Pnm2_1$ phase can account for the recently observed ferroelectricity in the (111)-oriented HfO$_2$-based thin film [Nature Materials 17, 1095-1100 (2018)]. Further investigation of this second functional ferroelectric phase in HfO$_2$ could potentially improve the performances of HfO$_2$-based films in logic and memory devices.

cond-mat.mtrl-sci↗

arXiv2020

Engineering Weyl phases and nonlinear Hall effects in T$_d$-MoTe$_2$

MoTe$_2$ has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there has been debate on the intriguing structural phase transitions among various observed phases of MoTe$_2$, and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory (DFT+U) calculations, we investigate the structural phase transition between the polar T$_d$ and nonpolar 1T$'$ phases of MoTe$_2$ in reference to a hypothetical high-symmetry T$_0$ phase that exhibits higher-order topological features. In the T$_d$ phase we obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a polar phonon mode. We also report the existence of a tunable nonlinear Hall effect in T$_d$-MoTe$_2$, and propose the use of this effect as a probe for the detection of polarity orientation in polar (semi)metals. By studying the role of dimensionality, we identify a configuration in which a nonlinear surface response current emerges. The potential technological applications of the tunable Weyl phase and the nonlinear Hall effect are discussed.

cond-mat.mtrl-sci↗

arXiv2020

Berry Flux Diagonalization: Application to Electric Polarization

The switching polarization of a ferroelectric is a characterization of the current that flows due to changes in polarization when the system is switched between two states. Computation of this change in polarization in crystal systems has been enabled by the modern theory of polarization, where it is expressed in terms of a change in Berry phase as the material switches. It is straightforward to compute this change of phase, but only modulo $2π$, requiring a branch choice from among a lattice of values separated by $2π$. The measured switching polarization depends on the actual path along which the material switches, which in general involves nucleation and growth of domains and is therefore quite complex. In this work, we present a physically motivated approach for predicting the experimentally measured switching polarization that involves separating the change in phase between two states into as many gauge-invariant smaller phase changes as possible. As long as the magnitudes of these smaller phase changes remain smaller than $π$, their sum forms a phase change which corresponds to the change one would find along any path involving minimal evolution of the atomic and electronic structure. We show that for typical ferroelectrics, including those that would have otherwise required a densely sampled path, this technique allows the switching polarization to be computed without any need for intermediate sampling between oppositely polarized states.

cond-mat.mtrl-sci↗

arXiv2020

"Fraternal-twin" ferroelectricity: competing polar states in hydrogen-doped samarium nickelate from first principles

Reversible intercalation of hydrogen into samarium nickelate, SmNiO$_3$ (SNO), has been of recent interest. Upon entering SNO, the hydrogen dissociates: the H$^+$ binds to an oxygen and the valence electron localizes on a nearby NiO$_6$ octahedron, resulting in a local dipole moment. In this work, we use first-principles calculations to explore the polar states of hydrogen-doped SNO at a concentration of 1/4 hydrogen per Ni. The inherent tilt pattern of SNO and the presence of the interstitial hydrogen present an insurmountable energy barrier to switch these polar states to their symmetry-related states under inversion. We find a sufficiently low barrier to move the localized electron to a neighboring NiO$_6$ octahedron, a state unrelated by symmetry but equal in energy under epitaxial strain, resulting in a large change in polarization. We term this unconventional ferroelectric a "fraternal-twin" ferroelectric.

cond-mat.mtrl-sci↗

arXiv2020

Control of polymorphism during epitaxial growth of hyperferroelectric candidate LiZnSb on GaSb (111)B

A major challenge for ferroelectric devices is the depolarization field, which competes with and often destroys long-range polar order in the limit of ultrathin films. Recent theoretical predictions suggest a new class of materials, termed hyperferroelectics, that should be robust against the depolarization field and enable ferroelectricity down to the monolayer limit. Here we demonstrate the epitaxial growth of hexagonal LiZnSb, one of the hyperferroelectric candidate materials, by molecular-beam epitaxy on GaSb (111)B substrates. Due to the high volatility of all three atomic species, we find that LiZnSb can be grown in an adsorption-controlled window, using an excess zinc flux. Within this window, the desired polar hexagonal phase is stabilized with respect to a competing cubic polymorph, as revealed by X-ray diffraction and transmission electron microscopy measurements. First-principles calculations show that for moderate amounts of epitaxial strain and moderate concentrations of Li vacancies, the cubic LiZnSb phase is lower in formation energy than the hexagonal phase, but only by a few meV per formula unit. Therefore we suggest that kinetics plays a role in stabilizing the desired hexagonal phase at low temperatures. Our results provide a path towards experimentally demonstrating ferroelectricity and hyperferroelectricity in a new class of ternary intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2019

High-density electron doping of SmNiO$_3$ from first principles

Recent experimental work has realized a new insulating state of samarium nickelate (SmNiO$_3$), accessible in a reversible manner via high-density electron doping. To elucidate this behavior, we use the first-principles density functional theory (DFT) + U method to study the effect of added electrons on the crystal and electronic structure of SmNiO$_3$. First, we track the changes in the crystal and electronic structure with added electrons compensated by a uniform positive background charge at concentrations of $\frac{1}{4}$, $\frac{1}{2}$, $\frac{3}{4}$, and 1 electrons per Ni. The change in electron concentration does not rigidly shift the Fermi energy; rather, the added electrons localize on NiO$_6$ octahedra causing an on-site Mott transition and a change in the density of states resulting in a large gap between the occupied and unoccupied Ni $e_g$ orbitals at full doping. This evolution of the density of states is essentially unchanged when the added electrons are introduced by doping with interstitial H or Li ions.

cond-mat.mes-hall↗

arXiv2019

Influence of magnetic ordering on the spectral properties of binary transition metal oxides

Using the ab initio embedded DMFT (eDMFT) approach, we study the effect of long-range magnetic ordering on the spectral properties in the binary transition metal oxides, and find that the most significant changes appear in the momentum resolved spectral functions, which sharpen into quite well-defined bands in the antiferromagnetic (AFM) phase. The strongest change across the transition is found at the topmost valence band edge (VBE), which is commonly associated with the Zhang-Rice bound state. This VBE state strengthens in the AFM phase, but only for the minority spin component, which is subject to stronger fluctuations. A similar hybridized VBE state also appears in the DFT single-particle description of the AFM phase, but gets much stronger and acquires a well-defined energy in the eDMFT description.

cond-mat.str-el↗

arXiv2019

Polar and phase domain walls with conducting interfacial states in a Weyl semimetal MoTe2

Much of the dramatic growth in research on topological materials has focused on topologically protected surface states. While the domain walls of topological materials such as Weyl semimetals with broken inversion or time-reversal symmetry can provide a hunting ground for exploring topological interfacial states, such investigations have received little attention to date. Here, utilizing in-situ cryogenic transmission electron microscopy combined with first-principles calculations, we discover intriguing domain-wall structures in MoTe2, both between polar variants of the low-temperature(T) Weyl phase, and between this and the high-T high-order topological phase. We demonstrate how polar domain walls can be manipulated with electron beams and show that phase domain walls tend to form superlattice-like structures along the c axis. Scanning tunneling microscopy indicates a possible signature of a conducting hinge state at phase domain walls. Our results open avenues for investigating topological interfacial states and unveiling multifunctional aspects of domain walls in topological materials.

cond-mat.mtrl-sci↗

arXiv2019

Systematic beyond-DFT study of binary transition metal oxides

Various methods going beyond density-functional theory (DFT), such as DFT+U, hybrid functionals, meta-GGAs, GW, and DFT-embedded dynamical mean field theory (eDMFT), have been developed to describe the electronic structure of correlated materials, but it is unclear how accurate these methods can be expected to be when applied to a given strongly correlated solid. It is thus of pressing interest to compare their accuracy as they apply to different categories of materials. Here, we introduce a novel paradigm in which a chosen set of beyond-DFT methods is systematically and uniformly tested on a chosen class of materials. For a first application, we choose the target materials to be the binary transition-metal oxides FeO, CoO, MnO, and NiO in their antiferromagnetic phase and present a head-to-head comparison of spectral properties as computed using the various methods. We also compare with available experimental angle-resolved photoemission spectroscopy (ARPES), inverse-photoemission spectroscopy, and with optical absorption. We find both B3LYP and eDMFT can reproduce the experiments quite well, with eDMFT doing best in particular when comparing with the ARPES data.

cond-mat.str-el↗

arXiv2019

Negative piezoelectric response of van der Waals layered bismuth tellurohalides

The polarization and piezoelectric response of the BiTe$X$ ($X$=Cl, Br, and I) layered tellurohalides are computed from first principles. The results confirm a mixed ionic-covalent character of the bonding, and demonstrate that the internal structure within each triple layer is only weakly affected by the external stress, while the changes in the charge distribution with stress produce a substantial negative piezoelectric response. This suggests a new mechanism for negative piezoelectric response that should remain robust even in ultra-thin film form in this class of materials.

cond-mat.mtrl-sci↗