This work reports on the pressure dependence of the octahedral tilts and mean Fe-O bond lengths in $R\mathrm{Fe}{\mathrm{O}}_3$ ($R=\mathrm{Nd}$, Sm, Eu, Gd, Tb, and Dy), determined through synchrotron x-ray diffraction and Raman scattering, and their role on the pressure-induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedral tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). $\mathrm{EuFe}{\mathrm{O}}_3$ stands at the borderline, with nearly pressure-independent tilt angles. For the compounds where the tilts increase with pressure, the $\mathrm{Fe}{\mathrm{O}}_6$ octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed in relation to the general rules proposed from different theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, points out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedral compressibility and critical pressure is ascertained.

Using ab initio molecular dynamics simulations, we calculate the physical properties of MgO at conditions extending from the ones encountered in the Earth mantle up to the ones anticipated in giant planet interiors such as Jupiter. We pay particular attention to the high-pressure melting temperature throughout this large density range as this is a key ingredient for building accurate planetary interior models with a realistic description of their inner cores. We compare our simulation results with previous ab initio calculations that have been so far limited to the pressure range corresponding to the Earth mantle and the B1-B2 transition around 6 Mbar. We provide our results for both the equation of state and high-pressure melting curve in parametric forms for direct use in planetary models. Finally, we compare our predictions of the high-pressure melting temperature with various planetary interior profiles to deduce the state of differentiated layer within the core made of MgO in differentiated cores of various types of planets and exoplanets.

We develop and/or use a combination of first-principles density functional theory and first-principles-based effective Hamiltonian approaches to investigate phase evolution in $\mathrm{Ba}({\mathrm{Ti}}_{1-x},{\mathrm{Zr}}_x){\mathrm{O}}_3$ ferroelectric relaxor. Our simulations reveal two competing effects, which are associated with substitution of Ti with Zr and primarily responsible for the unusual phase evolution and the properties of this family of solid solutions. They are the negative chemical pressure that Zr exerts on the ${\mathrm{BaTiO}}_3$ matrix and the ferroelectric “inactivity” of Zr itself. While the former has a stabilizing effect on ferroelectricity, the latter disrupts the ferroelectric cooperation. These competing effects are responsible for the so-called pinched phase transition, where the three phases of parent ${\mathrm{BaTiO}}_3$ merge together, and the loss of ferroelectricity at the onset of relaxor behavior. The origin of the controversial diffuse phase transition is attributed to the coexistence of the three ferroelectric phases. In the region of the diffuse phase transition, we detect polar nanoregions, which often exhibit unusual nanopillar geometry.

Using density-functional-theory-based calculations, we explore the factors that influence the phase stability of Ni and Mn excess off-stoichiometric ${\mathrm{Ni}}_2\mathrm{MnSb}$ alloys which have lately been proven to possess properties important for state-of-the art technology. Our calculations and subsequent analysis of the electronic structures pinpoint the origin of the behavior of phase stability in Ni-Mn-Sb systems. We find that the site occupancy patterns and subsequent magnetic structures are the key toward effecting a martensitic phase transformation. Our results demonstrate that apart from ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1+x}{\mathrm{Sb}}_{1-x}$ systems which have been experimentally observed to possess magnetic shape memory effect, ${\mathrm{Ni}}_{2+x}{\mathrm{MnSb}}_{1-x}$ too are potential shape memory alloys where an unexpected site occupancy configuration leading to reduction of total energy drives the martensitic phase transformation. This, thus, opens more possibilities for the experimentalists to explore new materials for magnetic shape memory and associated functional effects. The systems, irrespective of composition, are found to be ductile, a necessity for practical applications.

We report an investigation of Raman spectroscopy of the mixed-phase ${\mathrm{BiFeO}}_3$ film grown onto ${\mathrm{LaAlO}}_3$(001) by ${90}^{\circ }$ off-axis magnetron sputtering. The polarized Raman spectra unveil different sets of phonon vibrations, which can be assigned to the modes of the tetragonal($T$)-like polymorph, the rhombohedral($R$)-like polymorph, and the bridging phase ($S_{\mathrm{tilt}}^{\prime }$ and $T_{\mathrm{tilt}}^{\prime }$) in between. Furthermore, the unpolarized Raman scattering as function of temperature unravels multiple anomalies in the phonon behavior of the $T$-like phase not only at its multiferroic transition ($\sim 370\mathrm{K}$) but also at the Polomska transition ($\sim 450\mathrm{K}$) and the antiferromagnetic transition of the $R$-like phase ($\sim 640\mathrm{K}$). The unusual phonon behavior indicates the presence of spin-phonon coupling and also long-ranged strain coupling between the $T$-like and $R$-like polymorphs, which provides new insights into hybrid systems at the morphotropic phase boundaries.

Tilted off-plane magnetic anisotropy induces two unusual characteristic magnetotransport phenomena: extraordinary Hall effect in the presence of an in-plane magnetic field, and nonmonotonic anisotropic magnetoresistance in the presence of a field normal to the sample plane. We show experimentally that these effects are generic, appearing in multiple ferromagnetic systems with tilted anisotropy introduced either by oblique deposition from a single source or in binary systems codeposited from separate sources. We present a theoretical model demonstrating that these observations are natural results of the standard extraordinary Hall effect and anisotropic magnetoresistance, when the tilted anisotropy is properly accounted for. Such a scenario may help explain various previous intriguing measurements by other groups.

The present work reports the magnetodielectric (MD), ferroelectric, magnetoelectric (ME), low-temperature (LT) Raman, low-temperature high magnetic field (LTHM) ${}^{57}\mathrm{Fe}$ Mössbauer and x-ray diffraction (XRD) studies of polycrystalline multiferroic ${\mathrm{GdFeO}}_3$. Ferroelectric ordering as a result of magnetic ordering of ${\mathrm{Gd}}^{3+}$ ions ($T_{N,\mathrm{Gd}}$) is confirmed from the pyrocurrent measurements carried out in conventional and bias-electric field protocols. Coupling between magnetic and electric orderings, viz., magneto-electric (ME) coupling, is confirmed by measuring the dielectric constant and pyrocurrent in the presence of magnetic field. A huge magnetodielectric (MD) coefficient ($\approx -12\%$) is observed below the ${\mathrm{Gd}}^{3+}$ ordering temperature, reaching a saturation value with a field of about 3 Tesla. From LT Raman data across $T_{N,\mathrm{Gd}}$, the presence of spin-phonon coupling and the development of new modes indicating the symmetry reduction are observed. The average displacement of cations obtained from LTHM XRD measurement corroborates the ferroelectric polarization measurements. Signatures of a transferred hyperfine field are observed from LTHM ${}^{57}\mathrm{Fe}$ Mössbauer data. The variation of the effective internal magnetic field obtained from in-field Mössbauer data gives evidence for the predicted spin structures of ${\mathrm{GdFeO}}_3$ with magnetic fields below $T_{N,\mathrm{Gd}}$. The observed anomalous variation of the center shift and resonance area from ${}^{57}\mathrm{Fe}$ Mössbauer data across $T_{N,\mathrm{Gd}}$ ordering is explained in terms of phonon mode softening. Observed MD and ME effects are explained in terms of spin-phonon coupling as evidenced from LTHM ${}^{57}\mathrm{Fe}$ Mössbauer and LT Raman data.

Voltage-induced magnetization switching can lead to a new paradigm enabling ultralow-power and high density instant-on nonvolatile magnetoelectric random access memory (MeRAM) devices. Two major challenges for future MeRAM devices are to achieve large perpendicular magnetic anisotropy (PMA) and high voltage-controlled magnetic anisotropy (VCMA) coefficient of heavy-metal/ferromagnet/insulator heterostructures (HM/FM/I). Employing ab initio electronic structure calculations we have investigated the effect of epitaxial strain and thickness of both FeCo and Ir layers as design parameters to optimize the PMA and VCMA of Ir/FeCo/MgO. We predict that the Ir cap layer can induce both large PMA and colossal VCMA efficiency which depend on the strain. More importantly, we predict that a single Ir cap monolayer gives rise to a VCMA efficiency one order of magnitude higher than that of thicker Ir layers. The underlying mechanism is the synergistic effects of the emergence of Ir local moments, the large Ir spin-orbit coupling (SOC), and the large modulation of the magnetic anisotropy at the Ir/vacuum interface. These results provide useful guiding rules in the design of the next generation of high performance MeRAM memory devices.

We study the nonequilibrium dynamics of a one-dimensional topological Kondo insulator, modelled by a $p$-wave Anderson lattice model, following a quantum quench of the on-site interaction strength. Our goal is to examine how the quench influences the topological properties of the system, and therefore our main focus is the time evolution of the string order parameter, entanglement spectrum, and the topologically protected edge states. We point out that postquench local observables can be well captured by a thermal ensemble up to a certain interaction strength. Our results demonstrate that the topological properties after the interaction quench are preserved. Though the absolute value of the string order parameter decays in time, the analysis of the entanglement spectrum, Loschmidt echo and the edge states indicates the robustness of the topological properties in the time-evolved state. These predictions could be directly tested in state-of-the-art cold-atom experiments.

Padé approximants to the many-body Green's function can be built by rearranging terms of its perturbative expansion. The hypothesis that the best use of a finite number of terms of such an expansion is given by the subclass of diagonal Padé approximants is here tested, and largely confirmed, on a solvable model system, namely the Hubbard ring for a variety of site numbers, fillings, and interaction strengths.

Circularly polarized light (CPL) can induce an asymmetry between the number of left- and right-handed chiral quasiparticles in Dirac and Weyl semimetals. We show that if the photoresponse of the material is dominated by chiral quasiparticles, the total chiral charge induced in the material by CPL can be evaluated in a model-independent way through the chiral anomaly. In the presence of an external magnetic field perpendicular to the incident CPL, this allows us to predict the linear density of the induced photocurrent resulting from the chiral magnetic effect. The predicted effect should exist in any kind of Dirac or Weyl materials, with both symmetric and asymmetric band structure. An estimate of the resulting chiral magnetic photocurrent in a typical Dirac semimetal irradiated by an infrared laser of intensity $\simeq 5\times {10}^6\mathrm{W}/{\mathrm{m}}^2$ and a wavelength of $\lambda \simeq 10\mu \mathrm{m}$ in an external magnetic field $B\simeq 2\mathrm{T}$ yields a current $J\simeq 50\mathrm{nA}$ in the laser spot of size $\simeq 50\mu \mathrm{m}$. This current scales linearly with the magnetic field and wavelength, opening up possibilities for applications in photonics, optoelectronics, and THz sensing.

Crystallographic shear phases of niobium oxide form an interesting family of compounds that have received attention both for their unusual electronic and magnetic properties, as well as their performance as intercalation electrode materials for lithium-ion batteries. Here we present a first-principles density-functional theory study of the electronic structure and magnetism of H-${\mathrm{Nb}}_2{\mathrm{O}}_5$, ${\mathrm{Nb}}_{25}{\mathrm{O}}_{62}$, ${\mathrm{Nb}}_{47}{\mathrm{O}}_{116}$, ${\mathrm{Nb}}_{22}{\mathrm{O}}_{54}$, and ${\mathrm{Nb}}_{12}{\mathrm{O}}_{29}$. These compounds feature blocks of niobium-oxygen octahedra as structural units, and we show that this block structure leads to a coexistence of flat and dispersive energy bands, corresponding to localized and delocalized electronic states. Electrons localize in orbitals spanning multiple niobium sites in the plane of the blocks. Localized and delocalized electronic states are both effectively one-dimensional and are partitioned between different types of niobium sites. Flat bands associated with localized electrons are present even at the GGA level, but a correct description of the localization requires the use of $\mathrm{GGA}+U$ or hybrid functionals. We discuss the experimentally observed electrical and magnetic properties of niobium suboxides in light of our results, and argue that their behavior is similar to that of $n$-doped semiconductors, but with a limited capacity for localized electrons. When a threshold of one electron per block is exceeded, metallic electrons are added to existing localized electrons. We propose that this behavior of shear phases is general for any type of $n$-doping, and should transfer to doping by alkali metal (lithium) ions during operation of niobium oxide-based battery electrodes. Future directions for theory and experiment on mixed-metal shear phases are suggested.

We introduce Rényi entropy of a subsystem energy as a natural quantity which closely mimics the behavior of the entanglement entropy and can be defined for all quantum many body systems. In other words, consider a quantum chain in its ground state and then take a subdomain of this system with natural truncated Hamiltonian. Since the total Hamiltonian does not commute with the truncated Hamiltonian, the subsystem can be in one of its eigenenergies with different probabilities. Using the fact that the global energy eigenstates are locally close to diagonal in the local energy eigenbasis, we argue that the Rényi entropy of these probabilities follows an area law for the gapped systems. When the system is at the critical point, the Rényi entropy follows a logarithmic behavior with a universal coefficient. Consequently, our quantity not only detects the phase transition but also determines the universality class of the critical point. Moreover we show that the largest defined probabilities are very close to the biggest Schmidt coefficients. We quantify this by defining a truncated Shannon entropy which its value is almost indistinguishable from the truncated von Neumann entanglement entropy. Compared to the entanglement entropy, our quantity has the advantage of being associated to a natural observable (subsystem Hamiltonian) that can be defined (and probably measured) easily for all the short-range interacting systems. We support our arguments by detailed numerical calculations performed on the transverse field XY chain.

We propose insulating titanium oxynitrides ${\mathrm{Ti}}_n{\mathrm{N}}_2{\mathrm{O}}_{2n-3}$ as promising water-splitting photocatalytic materials in the visible light range. Using first-principles many-body perturbation theory based on the GW approximation, we show that corundum-type ${\mathrm{Ti}}_2{\mathrm{N}}_2\mathrm{O}$ (an example ${\mathrm{Ti}}_n{\mathrm{N}}_2{\mathrm{O}}_{2n-3}$ compound with $n=2)$ has a smaller band gap of about 2.5 eV, which is more suitable to absorb visible light, compared to other Ti-based oxides such as ${\mathrm{TiO}}_2$ and ${\mathrm{SrTiO}}_3$ with a band gap of more than 3 eV. Band-gap reduction in ${\mathrm{Ti}}_2{\mathrm{N}}_2\mathrm{O}$ is caused by an upward shift of the valence band (negative shift to the oxidation potential of ${\mathrm{H}}_2\mathrm{O}$ to ${\mathrm{O}}_2)$ due to the presence of nitrogen $2p$ states. The conduction band is dominated by Ti $3d$ states and the conduction-band minimum is nearly unchanged. As a result, the band-edge potentials of ${\mathrm{Ti}}_2{\mathrm{N}}_2\mathrm{O}$ are better aligned to the water reduction and oxidation levels. Our theoretical predictions provide useful insights for the discovery of efficient visible-light-driven photocatalysts for water splitting.

A formulation of the Kondo lattice Hamiltonian in terms of bond particles is derived and solved in two different approximations. The bond particles correspond to the eigenstates of a single unit cell and are bosons for states with an even electron number and fermions for states with an odd electron number. As a check, various physical quantities are calculated for the 1D Kondo insulator and good agreement with numerical results is obtained for $J/t>1$.

The trigonal structure of ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ is an anomaly in the tetragonal 122-type pnictide family. We report a detailed investigation of the underlying magnetic correlations in single crystal ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ using high-resolution elastic neutron-scattering measurements. The system undergoes two successive antiferromagnetic transitions at $T=135\mathrm{K}$ and 14.4 K, respectively. Numerical modeling of the experimental data reveals the long-range antiferromagnetic correlation of Mn-ions in the $a-b$ plane below $T_{N1}=135\mathrm{K}$. Mn spins are aligned closer to the diagonal axis of the unit cell. The lower temperature transition, below $T_{N2}=14.4\mathrm{K}$, is found to arise due to the long-range antiferromagnetic correlation of Eu spins that are rotated by $\theta ={55}^{\circ }$ from the $c$ axis of the unit cell.

The honeycomb Kitaev model in a magnetic field is a source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. We present experimental results for the thermal Hall effect of the material $\alpha \text{−}{\mathrm{RuCl}}_3$, which recently emerged as a prime candidate for realizing such physics. At temperatures above long-range magnetic ordering $T\gtrsim T_N\approx 8\mathrm{K}$, we observe with an applied magnetic field $B$ perpendicular to the honeycomb layers a sizable positive transversal heat conductivity ${\kappa }_{xy}$ which increases linearly with $B$. Upon raising the temperature, ${\kappa }_{xy}\left(T\right)$ increases strongly, exhibits a broad maximum at around 30 K, and eventually becomes negligible at $T\gtrsim 125\mathrm{K}$. Remarkably, the longitudinal heat conductivity ${\kappa }_{xx}\left(T\right)$ exhibits a sizable positive thermal magnetoresistance effect. Thus, our findings suggest significant longitudinal and transverse magnetic heat transport, underpinning the unconventional nature of the quasiparticles in the paramagnetic phase of $\alpha \text{−}{\mathrm{RuCl}}_3$.

The dependence of the magnetic ordering temperature $T_{\text{o}}$ of Sm metal was determined through four-point electrical resistivity measurements to pressures as high as 150 GPa. A strong increase in $T_{\text{o}}$ with pressure is observed above 85 GPa. In this pressure range, Sm ions alloyed in dilute concentration with superconducting Y exhibit giant Kondo pair breaking. Taken together, these results suggest that for pressures above $\sim 85\mathrm{GPa}\mathrm{Sm}$ is in a highly correlated electron state, like a Kondo lattice, with an unusually high value of $T_{\text{o}}.$ A detailed comparison is made with similar results obtained earlier on Nd, Tb, and Dy and their dilute magnetic alloys with superconducting Y.