It has long been a challenging task to find compounds with similar crystal and electronic structures as cuprate superconductors with low dimensionality and strong antiferromagnetic fluctuations. The parent compounds of cuprate superconductors are Mott insulators with strong in-plane antiferromagnetic exchange interactions between Cu moments. Here, we show, based on first-principles density functional calculations, that $A{\mathrm{FeSe}}_2$ ($A=\text{Tl}$, K, Rb, or Cs) exhibit many of the physical properties common to the cuprate parent compounds: (1) The ${\mathrm{FeSe}}_2$ layer in $A{\mathrm{FeSe}}_2$ is similar in crystalline and electronic structures to the ${\mathrm{CuO}}_2$ plane in cuprates, although Se atoms are not coplanar to the square Fe lattice; (2) they are antiferromagnetic insulators, but with relatively small charge excitation gaps; (3) their ground states are Néel antiferromagnetic ordered, similar as in cuprates; and (4) the antiferromagnetic exchange interactions between Fe moments are larger than in other iron-based superconducting materials, but comparable to those in cuprates. As cuprates, these compounds may become high-$T_c$ superconductors upon doping of charge carriers either by chemical substitution or intercalation or by liquid or solid gating.

Using real-space generalized gradient approximation plus dynamical mean-field theory ($\mathrm{GGA}+\mathrm{DMFT}$) we explore the effect of uniaxial strain on the correlated electronic structure reconstruction of atomistic carbon patterned out in carbon-benzene single chains. Firstly, based on GGA we show how strain can trigger a valley electronic structure with similarities akin to narrow-band semiconductors. The correlated electronic structure we derive is promising in the sense that it enables the observation of emergent Kondo quasiparticle resonances for future valleytronics based on low-dimensional organic conductors. Our proposal is a key step to iteratively understand the intricate and interdependent changes in orbital and electronic degrees of freedom at the atomistic level.

In this study we systematically study the temperature dependence of interface energies in W using first principles. To that purpose, we compute interface free energies and consider different contributions, i.e., from lattice expansion, vibrational contribution from harmonic and quasiharmonic approximation with explicit phonon calculations, and electronic contribution. We find that interface energies decrease with temperature and that in most cases the harmonic approximation to the free energy is sufficient. The relative decrease of the interface energy at 2000 K for the surfaces varies from $-20$% to $-30$%, while for grain boundaries, the interface energies decrease from $-30$% to $-55$%. Our results are compared to models on temperature dependence of interface energies available in literature. We find that they are in qualitative agreement also exhibiting a general decrease with higher temperatures. However, most models neglect anisotropy effects and are not in quantitative agreement. The equilibrium crystal shape matches with experimentally observed particle and pore shapes.

Magnetoelectric coupling in metal/oxide heterostructures has opened up the possibility of controlling magnetization by voltage, i.e., electric field. However, the electric-field excitation of magnetization dynamics in perfectly in-plane and out-of-plane magnetized films have not been demonstrated so far due to zero electric-field torque originating from voltage control of perpendicular magnetic anisotropy. This limits the application of voltage-controlled magnetic anisotropy in magnetic field free control of magnetization dynamics. Here we show that magnetic annealing can induce an interfacial in-plane magnetic anisotropy of CoFeB/MgO junctions, thereby controlling the symmetry of interfacial magnetic anisotropy. The magnetic anisotropy is modulated by applying voltage: a negative bias voltage increases perpendicular magnetic anisotropy, while a positive bias voltage decreases perpendicular magnetic anisotropy and increases the in-plane magnetic anisotropy. Such a control of symmetry of the interfacial magnetic anisotropy by magnetic annealing and its tunability by electric fields is useful for developing purely voltage-controlled spintronic devices.

We discuss the boundary critical behaviors of two-dimensional (2D) quantum phase transitions with fractionalized degrees of freedom in the bulk, motivated by the fact that usually it is the one-dimensional boundary that is exposed and can be conveniently probed in many experimental platforms. In particular, we mainly discuss boundary criticality of two examples: (i) the quantum phase transition between a 2D $Z_2$ topological order and an ordered phase with spontaneous symmetry breaking; (ii) the continuous quantum phase transition between metal and a particular type of Mott insulator [$\mathrm{U}\left(1\right)$ spin liquid]. In particular, we obtain the critical exponents and scaling laws of these exotic quantum phase transitions when the systems are probed from the boundary in proposed experimental setup. These critical exponents obtained are significantly different from those one would see through bulk measurements. This theoretical study could be relevant to many purely 2D systems, where recent experiments have found correlated insulator, superconductor, and metal in the same phase diagram.

We study spontaneous dimerization and emergent criticality in a spin-$\frac{3}{2}$ chain with antiferromagnetic nearest-neighbor $J_1$, next-nearest-neighbor $J_2$, and three-site $J_3$ interactions. In the absence of three-site interaction $J_3$, we provide evidence that the model undergoes a remarkable sequence of three phase transitions as a function of $J_2/J_1$, going successively through a critical-commensurate phase, a partially dimerized gapped phase, a critical floating phase with quasi-long-range incommensurate order, to end up in a fully dimerized phase at very large $J_2/J_1$. In the field-theory language, this implies that the coupling constant of the marginal operator responsible for dimerization changes sign three times. For large enough $J_3$, the fully dimerized phase is stabilized for all $J_2$, and the phase transitions between the critical phases and this phase are both Wess-Zumino-Witten (WZW) SU(2)${}_3$ along part of the boundary and turn first order at some point due to the presence of a marginal operator in the WZW SU(2)${}_3$ model. By contrast, the transition between the two-dimerized phase is always first order, and the phase transitions between the partially dimerized phase and the critical phases are Kosterlitz-Thouless. Finally, we discuss the intriguing spin-$\frac{1}{2}$ edge states that emerge in the partially dimerized phase for even chains. Unlike their counterparts in the spin-1 chain, they are not confined and disappear upon increasing $J_2$ in favor of a reorganization of the dimerization pattern.

The effect of the modulation of a high-frequency ac magnetic field parallel to the dc one (longitudinal pumping) by the lower-frequency ac field acting on quantum spin systems is considered for closed and open systems. For most magnetically ordered spin systems, the modulation causes the oscillation of the threshold of the parametric instability of spin waves as a function of the amplitude and the frequency of the modulation. On the other hand, our results for quantum spin systems with fermionic eigenstates imply that the modulating field causes the combined resonance and the modulation of the magnitude of time-dependent oscillations of the magnetization and the absorbed by the system power of the ac field for the closed system and for the open system in the dynamical and the steady-state regimes.

The reversal of polarization in a ferroelectric material involves overcoming an energy barrier, and has been previously proposed and found to yield transient negative capacitance (NC) in the intermediate states of the switching process. Homogeneous switching was assumed to interpret the experimental results of NC; however, inhomogeneous switching is a more abundant mechanism than homogeneous switching, but its possible effect on NC is basically unknown. Here, we use first-principles-based techniques to investigate the occurrence of NC during these two types of switching processes in the supertetragonal phase of ${\mathrm{BiFeO}}_3$. We find NC in both cases, but with the magnitude of the inverse of the capacitance being drastically larger in the inhomogeneous (nucleation limited) switching, as compared to the homogeneous switching. We further analyze the origin of the different NC effects, and point to the different energetic trajectories between these two representative switching mechanisms.

We describe the crossover from generalized to conventional hydrodynamics in nearly integrable systems. Integrable systems have infinitely many conserved quantities, which spread ballistically, in general. When integrability is broken, only a few of these conserved quantities survive. The remaining conserved quantities are generically transported diffusively; we derive a compact and general diffusion equation for these. The diffusion constant depends on the matrix elements of the integrability-breaking perturbation; for a certain class of integrability-breaking perturbations, including long-range interactions, the diffusion constant can be expressed entirely in terms of generalized hydrodynamic data.

It is now well established that the Kitaev honeycomb model in a magnetic field along the [111] direction harbors an intermediate gapless quantum spin liquid (QSL) phase sandwiched between a gapped non-Abelian QSL at low fields $H<H_{c1}$ and a partially polarized phase at high fields $H>H_{c2}$. Here, we analyze the low-field and high-field phases and phase transitions in terms of single- and two-magnon excitations using exact diagonalization and density matrix renormalization group methods. We find that the energy to create a bound state of two-magnon ${\mathrm{\Delta }}_p$ becomes lower than the energy to create a single spin flip ${\mathrm{\Delta }}_s$ near $H_{c2}$. In the entire Kitaev spin liquid, ${\mathrm{\Delta }}_p<{\mathrm{\Delta }}_s$ and both gaps vanish at $H_{c2}$. We make testable predictions for magnon pairing that could be observable in Raman scattering measurements on Kitaev QSL candidate materials.

Unveiling the driving force for a phase transition is normally difficult when multiple degrees of freedom are strongly coupled. One example is the nematic phase transition in iron-based superconductors. Its mechanism remains controversial due to a complex intertwining among different degrees of freedom. In this Rapid Communication, we report a method for measuring the nematic susceptibly of ${\mathrm{FeSe}}_{0.9}{\mathrm{S}}_{0.1}$ using angle-resolved photoemission spectroscopy and an in situ strain-tuning device. The nematic susceptibility is characterized as an energy shift of band induced by a tunable uniaxial strain. We found that the temperature dependence of the nematic susceptibility is strongly momentum dependent. As the temperature approaches the nematic transition temperature from the high temperature side, the nematic susceptibility remains weak at the Brillouin zone center while showing divergent behavior at the Brillouin zone corner. Our results highlight the complexity of the nematic order parameter in momentum space, which provides crucial clues to the driving mechanism of the nematic phase transition. Our experimental method which can directly probe the electronic susceptibly in momentum space provides a way to study the complex phase transitions in various materials.

We study the magnetic and superconducting proximity effects in a semiconducting nanowire (NW) attached to superconducting leads and a ferromagnetic insulator (FI). We show that a sizable equilibrium spin polarization arises in the NW due to the interplay between the superconducting correlations and the exchange field in the FI. The resulting magnetization has a nonlocal contribution that spreads in the NW over the superconducting coherence length and is opposite in sign to the local spin polarization induced by the magnetic proximity effect in the normal state. For a Josephson-junction setup, we show that the nonlocal magnetization can be controlled by the superconducting phase bias across the junction. Our findings are relevant for the implementation of Majorana bound states in state-of-the-art hybrid structures.

One account of two-dimensional (2D) structural transformations in 2D ferroelectrics predicts an evolution from a structure with ${\mathrm{Pnm}2}_1$ symmetry into a structure with square P4/nmm symmetry and is consistent with experimental evidence, while another argues for a transformation into a structure with rectangular Pnmm symmetry. An analysis of the assumptions made in these models is provided here, and six fundamental results concerning these transformations are contributed as follows: (i) Softened phonon modes produce rotational modes in these materials. (ii) The transformation to a structure with P4/nmm symmetry occurs at the lowest critical temperature $T_c$. (iii) The hypothesis that one unidirectional optical vibrational mode underpins the 2D transformation is unwarranted. (iv) Being successively more constrained, a succession of critical temperatures $\left(T_c<T_c^{\prime }<T_c^{″}\right)$ occurs in going from molecular dynamics calculations with the NPT and NVT ensembles onto models with unidirectional oscillations. (v) The choice of exchange-correlation functional impacts the estimate of the critical temperature. (vi) Crucially, the correct physical picture of these transformations is one in which rotational modes confer a topological character to the 2D transformation via the proliferation of vortices.

Topological insulators (TIs) hold great promise for new spin-related phenomena and applications thanks to the spin texture of their surface states. However, a versatile platform allowing for the exploitation of these assets is still lacking due to the difficult integration of these materials with the mainstream Si-based technology. Here, we exploit germanium as a substrate for the growth of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, a prototypical TI. We probe the spin properties of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/Ge pristine interface by investigating the spin-to-charge conversion taking place in the interface states by means of a nonlocal detection method. The spin population is generated by optical orientation in Ge and diffuses toward the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, which acts as a spin detector. We compare the spin-to-charge conversion in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/Ge with the one taking place in Pt in the same experimental conditions. Notably, the sign of the spin-to-charge conversion given by the TI detector is reversed compared to the Pt one, while the efficiency is comparable. By exploiting first-principles calculations, we ascribe the sign reversal to the hybridization of the topological surface states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with the Ge bands. These results pave the way for the implementation of highly efficient spin detection in TI-based architectures compatible with semiconductor-based platforms.

We observed spin-wave transmission through an air gap that works as a prohibited region. The spin waves were excited by circularly polarized pump pulses via the inverse Faraday effect, and their spatial propagation was detected through the Faraday effect of probe pulses using a pump-probe imaging technique. The experimentally observed spin-wave transmission was reproduced using numerical calculations with a Green's function method and micromagnetic simulation. We found that the amplitude of the spin waves decays exponentially in the air gap, which indicates the existence of evanescent spin waves in the magnetic dipole regime. This finding will pave the way for controllable amplitudes and phases of spin waves propagating through an artificial magnonic crystal.

We present theoretical modeling of the quantum Hall phase and chiral edge states in a gyroelectric photonic crystal. The theoretical framework represented here is based on a coupled dipole method, which is a direct counterpart of the tight-binding approximation. We confirm that gyrotropic coupling produces a band gap characterized by a nonzero Chern number. Gapless edge states appear at an interface between a gyroeletric photonic crystal and a vacuum. At a domain wall between media of positive and negative gyrotropy, topological edge states that are analogous to the Jackiw-Rebbi states are observed. Propagation of the chiral edge states is also simulated in real space under excitation of an external dipole source. This analysis provides a toy model to describe the quantum Hall phase in a purely optical manner, without using a mapping or an analogy of the electron wave function in the quantum Hall effect.

We present an extension of a framework for simulating single quasiparticle or collective excitations on top of strongly correlated quantum many-body ground states using infinite projected entangled pair states, a tensor network ansatz for two-dimensional wave functions in the thermodynamic limit. Our approach performs a systematic summation of locally perturbed states in order to obtain excited eigenstates localized in momentum space, using the corner transfer matrix method, and generalizes the framework to arbitrary unit cell sizes, the implementation of global Abelian symmetries and fermionic systems. Results for several test cases are presented, including the transverse Ising model, the spin-$1/2$ Heisenberg model, and a free fermionic model, to demonstrate the capability of the method to accurately capture dispersions. We also provide insight into the nature of excitations at the $k=(\pi ,0)$ point of the Heisenberg model.

Magnetization plateaus are some of the most striking manifestations of frustration in low-dimensional spin systems. We present numerical studies of magnetization plateaus in the fascinating spin-1/2 skewed ladder system obtained by alternately fusing five- and seven-membered rings. This system exhibits three significant plateaus at $m=1/4$, 1/2, and 3/4, consistent with the Oshikawa-Yamanaka-Affleck condition. Our numerical as well as perturbative analysis shows that the ground state can be approximated by three weakly coupled singlet dimers and two free spins, in the absence of a magnetic field. With increasing applied magnetic field, the dimers progressively become triplets with large energy gaps to excited states, giving rise to stable magnetization plateaus. Finite-temperature studies show that $m=1/4$ and 1/2 plateaus are robust and survive thermal fluctuations while the $m=3/4$ plateau shrinks rapidly due to thermal noise. The cusps at the ends of a plateau follow the algebraic square-root dependence on $B$.

Two-dimensional (2D) band crossing semimetals (BCSMs) could be used to build a range of novel nanoscale devices, such as superlenses and transistors. We find that symmorphic symmetry can protect a new type of robust 2D BCSM, unlike the previously proposed 2D essential BCSMs protected by nonsymmorphic symmetry [Young and Kane, Phys. Rev. Lett. 115, 126803 (2015)]. This type of symmorphic symmetry-protected (SSP) 2D essential BCSM cannot be pair annihilated without destroying the crystalline symmetries as opposed to the 2D BCSM caused by the accidental band crossing. Through group-theory analysis, we find that 2D SSP BCSMs can only exist at the $K$(K′) point of the Brillouin zone of four-layer groups and identify nonmagnetic 2D $\mathrm{Fe}{\mathrm{B}}_2$ as a candidate. Interestingly, nonmagnetic 2D SSP BCSMs can host a single pair of band crossing points (BCPs), whereas nonmagnetic three-dimensional Weyl semimetals have, at least, two pairs of band crossing Weyl points. It is found that the single pair of BCPs is robust against any kind of strain. Furthermore, our calculation suggests that essential 2D SSP BCSMs can be used to realize electric-field control of spin texture and, thus, are promising candidates for spintronic devices.