Antiferroelectric perovskites form an important family of functional electric materials, which have high potential in energy storage and conversion applications. However, a full understanding of their crystal structural formation is still lacking. ${\mathrm{PbZrO}}_3$-based materials can serve as a model system for investigation, not only because ${\mathrm{PbZrO}}_3$ was the first discovered antiferroelectric, but also because it undergoes a typical phase transition sequence from a high-temperature paraelectric to the low-temperature antiferroelectric phase, passing through a possible intermediate phase that is poorly understood. Here we employ a combination of optical and scattering experiments and theoretical calculations to reveal the nature of the intermediate state. Evidence is found that this peculiar state consists of multiple short-range and long-range structural components, and their competition is crucial in stabilizing the antiferroelectric phase. External stimuli such as temperature change or chemical substitution can easily alter each component's energy landscape and thereby change the materials' electrical properties. These findings provide insights into understanding antiferroelectric-ferroelectric competition and can be useful in designing alternative antiferroelectric materials.

Tunneling two-level systems (TLSs), generic to amorphous solids, dictate the low-temperature properties of amorphous solids and dominate noise and decoherence in quantum nanodevices. The properties of the TLSs are generally described by the phenomenological standard tunneling model. Yet, significant deviations from the predictions of this model found experimentally suggest the need for a more precise model in describing TLSs. Here, we show that the temperature dependence of the sound velocity, dielectric constant, specific heat, and thermal conductivity can be explained using an energy-dependent TLS density of states reduced at low energies due to TLS-TLS interactions. This reduction is determined by the ratio between the strengths of the TLS-TLS interactions and the random potential, which is enhanced in systems with dominant electric dipolar interactions.

We use density functional theory to study the structural, magnetic, and electronic structures of the organometallic quantum magnet ${\mathrm{NiCl}}_2\text{−}4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$ (DTN). Recent work has demonstrated the quasi one-dimensional nature of the molecular crystal and studied its quantum phase transitions at low temperatures. The system includes a magnetoelectric (ME) coupling and, when doped with Br, the presence of an exotic Bose-glass state. Using the generalized gradient approximation with inclusion of a van der Waals term to account for weak intermolecular forces and by introducing a Hubbard $U$ term to the total energy, we systematically show that our calculations reproduce the magnetic anisotropy, the intermolecular exchange coupling strength, and the magnetoelectric effect in DTN, which have been observed in previous experiments. Further analysis of the electronic structure gives insight into the underlying magnetic interactions, including what mechanisms may be causing the ME effect. Using this computationally efficient model, we predict what effect applying an electric field might have on the magnetic properties of this quantum magnet.

The current-induced motion of transverse magnetic domain walls (DWs) in a multisegmented Co/Ni nanowire is investigated numerically. We find that the phase diagram current pulse length magnitude presents a rare diversity of behaviors depending on the segment's length and material parameters. We show that by changing only the pulse shape, in a range of parameters we obtain the controlled motion of the DW with or without polarity change. The polarity change arises in the simplest case from the birth and propagation of an antivortex along the width of the nanowire. The antivortex can be displaced over long distances depending on the pulse characteristics and boundary conditions. The systematic motion of the DW with polarity flip is found to be stable at room temperature. Moreover, by modifying the material parameters through alloying, the phase diagram can be engineered, decreasing the depinning current and paving the way for storage or logic applications.

The paper presents ab initio studies into the relative stability of the crystalline structures of copper, silver, and platinum up to high pressures at $T\ge 0$ K. Our calculations in quasiharmonic approximation suggest that not the fcc structure of Cu and Ag, but the body-centered cubic one, is thermodynamically most favorable at $P\gtrsim 100$ GPa and $T>3$ kK. The shock Hugoniot of Cu and Ag crosses the fcc-bcc phase boundary and the calculated transition pressures agree well with the result of recent laser shock experiments by Sharma et al. The advantage of the bcc structure comes from its softer low-frequency phonon modes and the smaller contribution of lattice vibrations to free energy at high temperatures, as compared to close-packed structures. The compression of platinum crystal also causes the $\mathrm{fcc}\to \mathrm{bcc}$ transition but at much higher pressures, $P>1.4$ TPa.

The strain glassy phase is produced by doping a small percentage of impurity in a martensitic alloy. Its ground state is conceived to consist of martensitic nanodomains spatially separated from each other by a defect phase. The present study, by probing the local structure around the Ni and Ti in martensitic and strain glassy compositions of ${\mathrm{Ni}}_{50+x}{\mathrm{Ti}}_{50-x}$, identifies the defect phase that is responsible for inhibiting the long range ordering of the elastic strain vector leading to the formation of the strain glassy phase.

Two-dimensional (2D) materials possessing intrinsic superconductivity with high transition temperature and unconventional pairing are highly desired, but their realizations are few and far between. Recently, ${\mathrm{W}}_2{\mathrm{N}}_3$ nanosheets down to three layers were successfully prepared [Yu et al., Adv. Mater. 30, 1805655 (2018)]. By performing solid ab initio calculations based on the anisotropic Migdal-Eliashberg theory, we predict that monolayer ${\mathrm{W}}_2{\mathrm{N}}_3$ is an unexplored intrinsic (without the assistance of external gating, strain, or a special substrate) 2D superconductor with large electron-phonon coupling (EPC) and high critical temperature $T_c=38\mathrm{K}$ accompanied with a single and broad superconducting gap ∼7.5 meV. The ratio between the gap and the critical temperature is much larger than the value derived from Bardeen-Cooper-Schrieffer (BCS) theory, further confirming the strong coupling feature of monolayer ${\mathrm{W}}_2{\mathrm{N}}_3$. The extremely strong EPC originates from the large deformation potential of low-frequency acoustic phonons rather than Fermi-surface nesting. Due to the partially filled $d$ orbitals, electron-electron correlation leads to remarkable enhancement of EPC based on frozen phonon analysis. Here, $T_c$ can be further enhanced via hydrogen passivation based on the McMillian-Allen-Dynes formula. In addition, the symmetry-restricted spin-orbit coupling (SOC) brings forth exotic type-I Ising pairing whose in-plane upper critical field is far beyond the Pauli paramagnetic limit. Our predictions provide a fascinating and highly feasible platform for realizing high-temperature 2D superconductivity and studying the interplay between electron-phonon coupling, electron-electron correlation, and SOCs.

Two-dimensional (2D) magnetoelectric multiferroics are promising multifunctional materials for miniaturized logic and memory devices. Herein, we explore the effectiveness of strain engineering for tuning the properties of a recently predicted 2D antiferromagnetic-ferroelectric, ${\mathrm{VOCl}}_2$ monolayer. Interestingly, we find that magnetic ordering and electric polarization can be tuned independently using uniaxial tensile strain along different in-plane lattice vectors. A 4% tensile strain along lattice vector $b$ induces a transition from an antiferromagnetic ground state with an out-of-plane magnetization to a ferromagnetic ground state with in-plane magnetization. On the other hand, tensile strain along lattice vector $a$ enhances spontaneous electric polarization, without affecting the magnetic ordering. The monolayers remain dynamically stable under tensile strain, which further helps to raise the Curie temperature of ferromagnetism, as well as ferroelectricity. Such a strain-tunable multiferroic material holds great promise for future generation nanoelectronic devices.

Excitonic resonance and binding energies can be altered by controlling the environmental screening of the attractive Coulomb potential. Although this screening response is often assumed to be static, the time evolution of the excitonic quasiparticles manifests a frequency dependence in its Coulomb screening efficacy. In this paper, we investigate a ground (1s) and first excited exciton state (2s) in a multilayered transition metal dichalcogenide $\left({\mathrm{MoS}}_2\right)$ upon ultrafast photoexcitation. We explore the dynamic screening effects on the latter and show its resonance frequency is the relevant frequency at which screening from the smaller-sized 1s counterparts is effective. Our finding sheds light on new avenues of external tuning on excitonic properties.

Recent investigation has connected topology with magnetization, which has been attracting great research interest. Here, based on symmetry analysis and first-principle calculations, we reveal rich band crossings in ferromagnetic quasi-one-dimensional compounds ${XYZ}_3$ ($X=\mathrm{Cs},\mathrm{Rb}$, $Y=\mathrm{Cr},\mathrm{Cu}$, $Z=\mathrm{Cl},\mathrm{I}$). We show that these materials host hourglass Weyl loops, nodal surfaces, and Dirac points in the absence of spin-orbital coupling. Notably, these topological phases only occur in a single spin channel, leading to fully spin-polarized phases. Based on symmetry analysis, we present that the hourglass-like Weyl loop is enforced by nonsymmorphic symmetries, which give rise to drumhead surface states. Particularly, the Weyl loop can persist when a magnetic direction is normal to the plane where it lies on. Additionally, we also find the nodal surfaces protected by the combination of screw rotation and time reversal symmetry. Besides, there exists a Dirac point enabled by nonsymmorphic symmetry and time reversal symmetry. Notably, this Dirac point shows a linear dispersion in one direction and quadratic dispersion in the normal plane, showing two Fermi arcs in its surface energy spectrum. By tuning the direction of magnetization, the Dirac point can be reduced to a pair of Weyl points. Our work suggests a concrete platform for exploring the fascinating physics associated with nonsymmorphic band crossings in quasi-one-dimensional ferromagnetic systems.

Weyl semimetals, featuring massless linearly dispersing chiral fermions in three dimensions, provide an excellent platform for studying the interplay of electronic interactions and topology, and exploring new correlated states of matter. Here, we examine the effect of a local repulsive interaction on an inversion-symmetry breaking Weyl semimetal model, using cluster dynamical mean-field theory and variational cluster approximation methods. Our analysis reveals a continuous transition from the gapless Weyl semimetal phase to a gapped spin density wave ordered phase at a critical value of the interaction, which is determined by the band structure parameters. Further, we introduce a finite tilt in the linear dispersion and examine the corresponding behavior for a type-II Weyl semimetal model, where the critical interaction strength is found to be significantly diminished, indicating a greater susceptibility towards interactions. The behavior of different physical quantities, such as the double occupancy, the spectral function, and the Berry curvature, associated with the Weyl nodes, is obtained in both the semimetallic and the magnetically ordered states. Finally, we provide an interaction-induced phase diagram for the Weyl semimetal model, as a function of the tilt parameter.

Two-dimensional hole gases (2DHGs) in semiconductor quantum wells are promising platforms for spintronics and quantum computation, but suffer from the lack of the $\mathbf{k}$-linear term in the Rashba spin-orbit coupling (SOC), which is essential for spin manipulations without magnetism. The Rashba SOC in 2DHGs is commonly believed to be a $\mathbf{k}$-cubic term as the lowest order. Here, contrary to conventional wisdom, we taking Ge/Si system as an example uncover a strong and tunable $\mathbf{k}$-linear Rashba SOC in 2DHGs of semiconductor quantum wells (QWs) by performing atomistic pseudopotential calculations in conjunction with theoretical analysis based on the effective model Hamiltonian approach. We illustrate that this emergent $\mathbf{k}$-linear Rashba SOC is a first-order direct Rashba effect, originating from a combination of heavy-hole-light-hole mixing and direct dipolar coupling to the external electric field. The enhanced interband mixing renders [110]-oriented Ge/Si QWs a much stronger linear Rashba SOC than [001]-oriented counterpart with the maximal strength exceeding 120 meVÅ, comparable to the highest values reported in two-dimensional electron gases made by narrow band-gap III-V semiconductors, which suffers from short spin lifetime due to the presence of nuclear spin. These findings confirm Ge-based 2DHGs to be an excellent platform for large-scale quantum computation.

We theoretically investigate ultrafast and nonlinear optical properties of graphite thin films based on first-principles time-dependent density functional theory. We first calculate electron dynamics in a unit cell of graphite under a strong pulsed electric field and explore the transient optical properties of graphite. The optical response of graphite shows a sudden change from the conducting to the insulating phase at a certain intensity range of the applied electric field. It also appears to be a saturable absorption (SA) in the energy transfer from the electric field to electrons. We next investigate a light propagation in graphite thin films by solving the coupled dynamics of the electrons and the electromagnetic fields simultaneously. It is observed that the SA manifests in the propagation with small attenuation in the spatial region where the electric field amplitude is about $4\times {10}^{-2}\text{to}7\times {10}^{-2}\mathrm{V}/\AA$.

We use the $\mathbf{k}·\mathbf{p}$ theory and the envelope function approach to evaluate the Rashba spin-orbit coupling induced in a semiconductor nanowire by a magnetic field at different orientations, taking explicitly into account the prismatic symmetry of typical nanocrystals. We make the case for the strongly spin-orbit-coupled InAs semiconductor nanowires and investigate the anisotropy of the spin-orbit constant with respect to the field direction. At sufficiently high magnetic fields perpendicular to the nanowire, a sixfold anisotropy results from the interplay between the orbital effect of field and the prismatic symmetry of the nanowire. A backgate potential, breaking the native symmetry of the nanocrystal, couples to the magnetic field inducing a twofold anisotropy, with the spin-orbit coupling being maximized or minimized depending on the relative orientation of the two fields. We also investigate in-wire field configurations, which shows a trivial twofold symmetry when the field is rotated off the axis. However, isotropic spin-orbit coupling is restored if a sufficiently high gate potential is applied. Our calculations are shown to agree with recent experimental analysis of the vectorial character of the spin-orbit coupling for the same nanomaterial, providing a microscopic interpretation of the latter.

Due to the lack of a net magnetization both at the interface and in the bulk, antiferromagnets with compensated interfaces may appear incapable of influencing the phase transition in an adjacent superconductor via the spin degree of freedom. We here demonstrate that such an assertion is incorrect by showing that proximity coupling a compensated antiferromagnetic layer to a superconductor-ferromagnet heterostructure introduces the possibility of controlling the superconducting phase transition. The superconducting critical temperature can in fact be modulated by rotating the magnetization of the single ferromagnetic layer within the plane of the interface, although the system is invariant under rotations of the magnetization in the absence of the antiferromagnetic layer. Moreover, we predict that the superconducting phase transition can trigger a reorientation of the ground state magnetization. Our results show that a compensated antiferromagnetic interface is in fact able to distinguish between different spin-polarizations of triplet Cooper pairs.

High critical temperature superconductivity often occurs in systems where an antiferromagnetic order is brought near $T=0$ K by slightly modifying pressure or doping. ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ is a superconducting, stoichiometric iron-pnictide compound showing optimal superconducting critical temperature with $T_c$ as large as 35 K. Doping with Ni induces a decrease in $T_c$ and the onset of spin-vortex crystal (SVC) antiferromagnetic order, which consists of spins pointing inwards to or outwards from alternating As sites on the diagonals of the in-plane square Fe lattice. Here we study the band structure of $\mathrm{CaK}{\left({\mathrm{Fe}}_{0.95}{\mathrm{Ni}}_{0.05}\right)}_4{\mathrm{As}}_4$ $(T_c=10 \mathrm{K},$ $T_{SVC}=50 \mathrm{K})$ using quasiparticle interference with a scanning tunneling microscope and show how the SVC modifies the band structure and induces a fourfold superconducting gap anisotropy.

We have studied the electrocaloric response of the archetypal antiferroelectric $\mathrm{PbZr}{\mathrm{O}}_3$ as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of −3.5 K to an electroheating of $+5.5\mathrm{K}$, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics.

Inelastic neutron scattering (INS) is one powerful technique to study the low-energy single-spin dynamics of magnetic materials. A variety of quantum magnets show novel magnetic correlations such as quantum spin liquids. These novel magnetic correlations are beyond the direct detection of INS. In this paper we propose a coincidence technique, coincidence inelastic neutron scattering (cINS), which can detect the two-spin magnetic correlations of the magnetic materials. In cINS there are two neutron sources and two neutron detectors with an additional coincidence detector. Two neutrons from the two neutron sources are incident on the target magnetic material, and they are scattered by the electron spins of the magnetic material. The two scattered neutrons are detected by the two neutron detectors in coincidence with the coincidence probability described by a two-spin Bethe-Salpeter wave function. Since the two-spin Bethe-Salpeter wave function defines the momentum-resolved dynamical wave function with two spins excited, cINS can explicitly detect the two-spin magnetic correlations of the magnetic material. Thus, it can be introduced to study the various spin valence bond states of the quantum magnets.

We compare the ground-state features of alternating ferrimagnetic chains $(1/2,S)$ with $S=1,3/2,2,5/2$ in a magnetic field and the corresponding Holstein-Primakoff bosonic models up to order $\sqrt{s/S}$, with $s=1/2$, considering the fully polarized magnetization as the boson vacuum. The single-particle Hamiltonian is a Rice-Mele model with uniform hopping and modified boundaries, while the interactions have a correlated (density-dependent) hopping term and magnon-magnon repulsion. The magnon-magnon repulsion increases the many-magnon energy and the density-dependent hopping decreases the kinetic energy. We use density matrix renormalization group calculations to investigate the effects of these two interaction terms in the bosonic model and display the quantitative agreement between the results from the spin model and the full bosonic approximation. In particular, we verify the good accordance in the behavior of the edge states, associated with the ferrimagnetic plateau, from the spin and from the bosonic models. Furthermore, we show that the boundary magnon density strongly depends on the interactions and particle statistics.

We investigate the geometric, electronic, and magnetic properties of ${\mathrm{TM}}_2\left(\mathrm{PAH}\right)$ complexes [transition metal (TM) = Fe, Co, Ni; $\text{polycyclic-aromatic}\text{hydrocarbon}\text{(PAH)}={\mathrm{C}}_{10}{\mathrm{H}}_8, {\mathrm{C}}_{16}{\mathrm{H}}_{10}, {\mathrm{C}}_{24}{\mathrm{H}}_{12}, {\mathrm{C}}_{32}{\mathrm{H}}_{14}]$ as well as the optically induced ultrafast spin dynamics from a first-principles study. Geometrically, the magnetic dimer of each of the complexes [except for ${\mathrm{Ni}}_2({\mathrm{C}}_{16}{\mathrm{H}}_{10}$)] turns out to prefer to adsorb above the hollow site of the outer carbon ring of the PAH molecule. The PAH-size effect and TM-element effect on the energy levels and spin localizations are analyzed. It is found that for the structures with the same PAH molecules, the level bands generally get wider from Fe via Co to Ni, while the number of spin-localized states overall decreases in the same order. For the structures with the same TM species [except for the two complexes ${\mathrm{Ni}}_2({\mathrm{C}}_{16}{\mathrm{H}}_{10}$) and ${\mathrm{Fe}}_2({\mathrm{C}}_{32}{\mathrm{H}}_{14}$)], the low-lying levels are hardly affected by the size of the PAH molecules. Among all the calculated levels for each complex [except for ${\mathrm{Ni}}_2({\mathrm{C}}_{16}{\mathrm{H}}_{10}$)], there are always more states with spin localized on the remote magnetic center than those localized on the near center. Driven by nonchirped subpicosecond laser pulses, a series of ultrafast spin-flip and spin-transfer scenarios on these structures are predicted and analyzed, based on which the laser chirp effect is further explored and some rules of thumb about the chirp tolerance and sensitivity are obtained. The results demonstrated in this paper are believed to enrich our understanding of the size- and structure-dependent electronic and magnetic properties of the carbon-based magnetic molecular structures, and further to promote the relevant experimental realization of the spin dynamics and their potential applications in future molecular spin devices.