We report second-principles simulations on the structural and energetic properties of domains in ${\left({\mathrm{PbTiO}}_3\right)}_n/{\left({\mathrm{SrTiO}}_3\right)}_n$ superlattices. For the explored layer thickness ($n$ ranging between 8 and 16 unit cells) and lateral sizes of the domains, the most stable configuration corresponds to polar domains separated by a sequence of counter-rotating vortices (clockwise/counterclockwise) perpendicular to the stacking direction and acting as domain walls. The balance between the domain wall energy and the electrostatic energy yields to an optimal domain period $\omega$ that is proportional to the square root of the thickness of the ${\mathrm{PbTiO}}_3$ layer, following the Kittel law. For a given lateral size of the simulation box, suboptimal domain structures (with a width larger than the one predicted by the Kittel law) can be obtained in a metastable form. However, at finite temperature, molecular dynamics simulations show the spontaneous change of periodicity, which implies the formation of new domains whose generation is initiated by the nucleation of vortices and antivortices at the interface between the ${\mathrm{SrTiO}}_3$ and the ${\mathrm{PbTiO}}_3$ layers. The vortices progressively elongate and eventually annihilate with the antivortices yielding the formation of new domains to comply with the Kittel law via defect recombination.

In this article, we present scattering and localization phenomena in a thin elastic plate comprising an aperiodic arrangement of scatterers. By analyzing the form factor of the scattering cluster, we sample the reciprocal space which shows strong scattering points in reciprocal space associated with nontrivial dynamic response. Specifically, we identify attenuating frequency regimes and bands where strong localization is predicted and experimentally observed. We show that both localization and attenuation are Bragg scattering phenomena, induced by the long-range aperiodic patterns. Illustrative comparisons are drawn with a periodic counterpart having the same density of scatterers. The novel findings are corroborated by analytical estimates, numerical finite-element predictions, and vibrometric experiments. The results are relevant for the research community interested in extending phononic crystal phenomena to lower frequencies.

Hexagonal manganites with coexisting antiferromagnetism and ferroelectricity have driven numerous research activities uncovering their potential in novel electronic devices. An antiferromagnetic order lowers the translational symmetry of the lattice; the structure of domain walls spatially separating distinctly ordered antiferromagnetic and structural states is quite rich and needs to be understood fundamentally. Here, we construct a model Hamiltonian to capture the low energy physics of coupled spins and phonons in hexagonal multiferroic ${\mathrm{YMnO}}_3$ derived from first-principles density functional theory and determine its temperature dependent behavior using Monte Carlo simulations. We demonstrate a weakly first order or close to being second order Néel transition accompanied by a $giant$ magnetoelastic effect observed experimentally in ${\mathrm{YMnO}}_3$ and show how it originates from the coupling between the in-plane ordering of spins with symmetry of ${\mathrm{\Gamma }}_3$ irreducible representation and collective atomic displacements with symmetry of ${\mathrm{\Gamma }}_1$ irreducible representation. We reveal the intriguing magnetic structure with the symmetry of ${\mathrm{\Gamma }}_1$ irreducible representation at the ${180}^{\circ }$ antiferromagnetic domain wall and predict a linear magnetoelectric coupling which can be confirmed using the piezoresponse force microscopy. Finally, we show that a stable magnetic vortex forms along a line of intersection of six ${60}^{\circ }$ antiferromagnetic domain walls, with energy comparable to that of dislocations in metals.

The introduction of effective spin-orbit couplings into magnon systems provides the opportunities to discover exotic and fascinating topological phases. In this work, we theoretically study the magnon spin-orbit coupling due to the natural dipole-dipole interaction in a layered collinear easy-axis ferrimagnet with two types of ferromagnetic sublayers coupled via antiferromagnetic interlayer exchange interaction, where we predict a nodal-loop semimetal phase and a Weyl semimetal phase in perpendicularly and laterally magnetized geometries, respectively. By driving the spin texture into a spin-canting phase with a magnetic field perpendicular to the easy axis, the nodal loop with a normal easy axis reduces to two Weyl points in the noncollinear spin configuration. Magnon arcs in the Weyl semimetal phases in both geometries are predicted in ferrimagnetic film with finite layers. Our results reveal the role of noncollinear spin texture on magnon topology, which may enrich the investigations in topological ferrimagnonics.

Two-dimensional (2D) topological half metals, namely, empowering nontrivial topological states with 100% spin-polarized 2D systems, may find important applications in low-dissipation spintronics. However, exploration of such intriguing materials is still very challenging. The recently discovered 2D magnetic materials provide unprecedented opportunities to realize 2D topological half-metallic state. Using first-principles calculations, we first find that the ferromagnetism of $\mathrm{Cr}{\mathrm{Cl}}_3$ monolayer can be well reserved by doping alkali metals, in fact, whose Curie temperature can be enhanced from 23 to 66 K after doping K. Then, we notice that semiconducting to Dirac half-metallic phase transition is achieved via Na and K doping, where the linear Dirac dispersion originates from the honeycomb lattice formed by Cr atoms. Most strikingly, Na- and K doped $\mathrm{Cr}{\mathrm{Cl}}_3$ exhibit quantum anomalous Hall effect (QAHE) with Chern number $C=1$, while the Li-doped one harbors high Chern number QAHE with $C=-2$. Thus, our work provides a theoretical guideline to realize 2D topological half-metallic state, rendering it as a promising platform for future applications.

We have studied the high-pressure vibrational and structural behavior of bulk graphite and graphene nanoplatelets at room temperature by means of high-pressure Raman spectroscopic and x-ray diffraction probes. We have detected a clear pressure-induced structural transition in both materials, evidenced by the appearance of new Bragg peaks and Raman features, deviating from the starting hexagonal graphitic structure. The high-pressure phase is identified as a partially disordered orthorhombic structure, consisting of mixed $s{p}^2$- and $s{p}^3$-type bonding. Our experimental findings serve as direct evidence for the existence of a metastable transient modification in cold compressed carbon, lying between the $s{p}^2$-type graphite and ${sp}^3$-type diamond allotropes.

We investigate coherent lattice dynamics in optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ driven by ultrashort ($\sim 12$ fs) near-infrared and near-ultraviolet pulses. Transient reflectivity experiments, performed at room temperature and under moderate ($<0.1$ mJ/${\mathrm{cm}}^2$) excitation fluence, reveal $A_g$-symmetry phonon modes related to the O(2,3) bending in the ${\mathrm{CuO}}_2$ planes and to the apical O(4) stretching at frequencies between 10 and 15 THz, in addition to the previously reported Ba and Cu(2) vibrations at 3.5 and 4.5 THz. The relative coherent phonon amplitudes are in stark contrast to the relative phonon intensities in the spontaneous Raman scattering spectrum excited at the same wavelength. This contrast indicates mode-dependent contributions of the Raman and non-Raman mechanisms to the coherent phonon generation. We show that the particularly intense coherent Cu(2) phonon, together with its initial phase, supports its generation predominately via a displacive mechanism, possibly involving the charge transfer within the ${\mathrm{CuO}}_2$ planes. The small amplitude of the coherent out-of-phase O(2,3) bending mode at 10 THz also suggests the involvement of non-Raman generation mechanism. The generation of the other coherent phonons can in principle be explained within the framework of Raman mechanism. When the pump light has the polarization component perpendicular to the ${\mathrm{CuO}}_2$ plane, the coherent O(4) mode at 15 THz is strongly enhanced compared to the in-plane excitation, corresponding to the large polarizability component associated with the hopping between the apical and the chain oxygens, O(4) and O(1).

The relaxation of observables to their nonequilibrium steady states in a disordered XX chain subjected to dephasing at every site has been intensely studied in recent years. We comprehensively analyze the relaxation of staggered magnetization, i.e., imbalance, in such a system, starting from the Néel initial state. We analytically predict emergence of several timescales in the system and extract results which match with large-system numerics without any extra fitting parameter until a universal timescale. An often reported stretched exponential decay is just one of the regimes which holds in a finite window of time and is therefore in fact not a true stretched exponential decay. Subsequently, the asymptotic decay of imbalance is governed by a power law irrespective of the disorder. We show that this emerges from the continuum limit of the low magnitude eigenspectrum of the Liouvillian. However, for finite systems, due to discreteness of the spectrum, the final phase of relaxation is governed by the relevant smallest Liouvillian gap.

The realization of the spin-glass (SG) state in Heusler alloys is very rare despite the presence of inherent structural and elemental disorder in those compounds. Although a few half- and full-Heusler alloys are known to exhibit the SG state, there is hardly any manifestation of the same in cases of quaternary Heusler compounds. Here we report the observation of a SG state in a highly disordered equiatomic quaternary Heusler compound: FeRuMnGa where the SG state is in between the canonical SG and the cluster glass. Different intricate features of the SG state including nonequilibrium magnetic dynamics at low temperatures in the compound are unveiled through our comprehensive magnetic, heat capacity, and neutron-diffraction studies. The structural disorder in the sample is neither conventional $A2$- nor $B2$ -type whereas those two types are commonly observed for Heusler compounds. The presence of disorder also plays a significant role in electron transport properties of the alloy, which is reflected in its exhibition of semimetallic behavior and anomalous Hall effect at low temperatures.

Metallic disks engineered on the 100 nm scale have an internal magnetic texture which varies from a fully magnetized state to a vortex state with zero moment. The interplay between this internal structure and the interdisk interactions is studied in magnetic metamaterials made of square arrays of the magnetic disks. The texture is modeled by a mesospin of varying length with $O\left(2\right)$ symmetry and the interdisk interaction by a nearest-neighbor coupling between mesospins. The thermodynamic properties of the model are studied numerically and an ordering transition is found which varies from Kosterlitz-Thouless to first order via an apparent tricritical point. The effective critical exponent characterizing the finite-size magnetization evolves from the value for the 2D XY model to less than half this value at the tricritical point. The consequences for future experiments both in and out of equilibrium are discussed.

We performed thermoelectric Hall conductivity ${\alpha }_{xy}$ measurements on single-crystal graphite in the quantum limit up to 13 T. Both electrical and thermoelectric transport measurements were performed on the same crystal to extract pure ${\alpha }_{xy}$, avoiding any sample quality dependence. The ${\alpha }_{xy}$ converges to a plateau in the quantum limit with a linear dependence on temperature. This behavior is analogous to the quantized thermoelectric Hall effect (QTHE) observed in three-dimensional Dirac/Weyl nodal-point semimetals, and experimentally confirms a theoretical proposal on the QTHE in semimetals with nodal lines as in graphite.

Identifying novel topological properties of topological quantum states of matter, such as exemplified by the quantized Hall conductance, is a valuable step towards realizing materials with attractive topological attributes that guarantee their imperviousness to realistic imperfections, disorder, and environmental disturbances. Is the gravitational coupling coefficient of topological quantum states of matter a promising candidate? Substantially building on well established results for quantum Hall states, using disclinations as tools for controlled creation of pristine spatial curvature free of undesirable artifacts such as would interfere with the electronic motion of interest, herein we report that a large class of lattice topological states of matter exhibits gravitational response, i.e., charge response to intrinsic spatial curvature. This phenomenon is characterized by a topologically quantized coupling constant. Remarkably, the charge-gravity relationship remains linear in the curvature, up to the maximum curvature achievable on the lattice, demonstrating an absence of higher order nonlinear response. Our findings facilitate articulating the physical principles underlying the topological quantization of the gravitational coupling constant, in analogy with the insights offered by the Chern number description of the quantized Hall conductance.

The chiral vortical effect is a chiral anomaly-induced transport phenomenon characterized by an axial current in a uniformly rotating chiral fluid. It is well understood for Weyl fermions in high energy physics, but its realization in condensed matter band structures, including those of Weyl semimetals, has been controversial. In this work, we develop the Kubo response theory for electrons in a general band structure subject to space- and time-dependent rotation or vorticity relative to the background lattice. For continuum Hamiltonians, we recover the chiral vortical effect in the static limit and the transport or uniform limit when the fluid, strictly, is not a Fermi liquid. In the transport limit of a Fermi liquid, we describe an effect that we dub the gyrotropic vortical effect. The latter is governed by Berry curvature of the occupied bands while the former contains an additional contribution from the magnetic moment of electrons on the Fermi surface. The two vortical effects can be understood as kinematic analogs of the well-known chiral and gyrotropic magnetic effects in chiral band structures. We address recent controversies in the field and conclude by describing device geometries that exploit Ohmic or Seebeck transport to drive the vortical effects.

The topological electronic structure of a single twin boundary and coherent twinning superlattices (TSLs) based on the SnTe class of materials is calculated and discussed within a supercell implementation. The superlattices consist of two twin planes (TPs) in the supercell arranged in such a way that each of the boundaries forms a mirror plane for the entire structure. Two types of TP boundary, cationic and anionic, can exist, and so three types of supercells can be constructed. We study the topological phases of each twinning configuration using the tight-binding approximation and calculating the topological invariants. We show that they differ by topological properties and find that all-cationic TSLs are topologically distinct from the anionic case due to the opposite sign of the Berry curvature around the $\mathrm{\Gamma }$ point of the TSL's Brillouin zone. Our findings are consistent with a complementary analysis of (111)-oriented slabs with a single twin boundary in the presence of a Zeeman field. They are also consistent with the calculated number of spin-polarized Dirac-like edge states of both superlattices and slabs. We conclude that each type of TP forms a two-dimensional mirror-plane-protected topological crystalline insulator.

Although the one-dimensional repulsive Fermi-Hubbard model has been intensively studied over many decades, a rigorous understanding of many aspects of the model is still lacking. In this work, based on the solutions to the thermodynamic Bethe ansatz equations, we provide a rigorous study on the following. (1) We calculate the fractional excitations of the system in various phases, from which we identify the parameter regime featuring the spin-incoherent Luttinger liquid (SILL). We investigate the universal properties and the asymptotic of correlation functions of the SILL. (2) We study the interaction-driven phase transition and the associated criticality, and build up an essential connection between the contact susceptibilities and the variations of density, magnetization, and entropy with respect to the interaction strength. As an application of these concepts, which hold true for higher-dimensional systems, we propose a quantum cooling scheme based on the interaction-driven refrigeration cycle.

Experiments have observed superconductivity in atomically thin metallic layers deposited on semiconducting substrates. As in any superconductor, it is important to determine the structure of the superconducting pairing function in order to reveal the mechanism responsible for superconductivity. To that end, we study the possible superconducting states of two-dimensional triangular lattices. We calculate the quasiparticle interference (QPI) patterns which would result from various nearest-neighbor pairing order parameters, and show how the QPI can be used to distinguish between those order parameters. The QPI patterns are the momentum-space representations of real-space local density-of-states fluctuations: the QPI signal at momentum $q$ reveals the strength of scattering processes at that momentum transfer. We show how characteristic differences between scattering from charge disorder (i.e., impurities) and from order-parameter disorder (i.e., vortices) can be used to identify the angular momentum of the superconducting pairs.