Complex systems with strongly entangled microscopic degrees of freedom often host a plethora of exotic properties. Herein, we report the unconventional and interesting multiferroic behavior of a layered noncentrosymmetric ferrimagnetic spin-1/2 system, ${\mathrm{Bi}}_2{\mathrm{Cu}}_5{\mathrm{B}}_4{\mathrm{O}}_{14}$. Neutron powder-diffraction study deciphers a robust ferrimagnetic ($\uparrow \downarrow \downarrow \downarrow \downarrow$) ordering below a temperature $T_C=25\mathrm{K}$, which remains unaltered even under a high magnetic field of $H=10\mathrm{T}$. Interestingly, two successive ferroelectric transitions are observed, one of which is triggered by the magnetostriction effect and emerges concomitantly with the ferrimagnetic ordering at $T_C=T_{E2}=25\mathrm{K}$. This is accompanied by a pronounced dielectric peak and a prominent magnetodielectric coupling at $T_C$. By contrast, other ferroelectric transition commences in the paramagnetic state at $T_{E1}=32\mathrm{K}$, which is solely caused by a prominent structural distortion. This strong structural distortion is revealed by the synchrotron x-ray-diffraction study, which is further supported by a concurrent, sharp, anomalous phonon softening observed in the Raman spectra.

Two ferromagnetic phases, FM1 and FM2, were first proposed to exist in ${\mathrm{LaCrGe}}_3$ based on a broad maximum in the temperature derivative of resistivity resembling that of the superconducting ferromagnet ${\mathrm{UGe}}_2$ where FM1 and FM2 are well-established. While evidence for two FM phases can be found in certain additional probes, corresponding anomalies in magnetization have not been recognized until now. Our spatially resolved images of the magnetic domains show a substantial change in the domain structure between the higher temperature FM1 phase and the lower temperature FM2 phase. Furthermore, our measurements of the coercive field and virgin magnetization curves reveal an unconventional magnetic domain pinning region in the FM1 phase, followed by a depinning region at lower temperatures where the system is reported to crossover into the FM2 phase. We incorporate this discovery into a simple domain magnetization model that demystifies the magnetization curve seen in all previous studies. Finally, we find that the unusual domain behavior can be explained by a change in the magnetic moment and the presence of two ferromagnetic exchange constants, both of which are consistent with the existence of two FM phases. This revelation may help explain a range of anomalous behaviors observed in ${\mathrm{LaCrGe}}_3$ and rekindles the discussion about the prevalence of multiple FM phases in fragile FM systems.

When topology meets chirality, it is of great interest to search for possible topological properties in chiral crystals, thereby achieving the effect of making $1+1$ greater than 2. Here, we accomplish a complete list of topological charge-2 (C-2) Dirac fermions based on 65 Söhncke space groups. Such emergent fermions occurring in chiral crystals enjoy a higher topological charge number and exhibit more unique characteristics compared to C-0 cases, including multiple Fermi arcs extending to most of the Brillouin zone. For material realization, we propose that an ultralightweight chiral crystal ${\mathrm{N}}_4\mathrm{Cl}$ is a high-quality C-2 Dirac semimetal, where the seductive nodal points reside at the Fermi level and exhibit a large linear energy range to overcome the interruption of irrelevant bands. More interestingly, the doubly degenerate Landau levels are described by a Kramers-like degeneracy in such a C-2 Dirac fermion, featuring a doubly degenerate zero mode. Furthermore, the topological phase transitions from Dirac to Weyl states of ${\mathrm{N}}_4\mathrm{Cl}$ can be manipulated by applying shear strains. These fascinating findings will certainly evoke continued exploration of unconventional Dirac semimetals, in turn leading to extensive application prospects and uncommon insights.

A short-ranged, rotationally symmetric multi-Landau-level model Hamiltonian for strongly interacting electrons in a magnetic field was proposed [A. Anand et al., Phys. Rev. Lett. 126, 136601 (2021)] with the key feature that it allows exact many-body eigenfunctions on the disk not just for quasiholes but for all charged and neutral excitations of the entire Jain sequence filling fractions. We extend this to geometries without full rotational symmetry, namely, the torus and cylinder geometries, and present their spectra. Exact diagonalization of the interaction on the torus produces the low-energy spectra at filling fraction $\nu =n/(2pn+1)$ that is identical, up to a topological ($2pn+1$)-fold multiplicity, to that of the integer quantum Hall spectra at $\nu =n$, for the incompressible state as well as all excitations. While the ansatz eigenfunctions in the disk geometry cannot be generalized to closed geometries such as torus or sphere, we show how to extend them to cylinder geometry. Meanwhile, we show eigenfunctions for charged excitations at filling fractions between $\frac{1}{3}$ and $\frac{2}{5}$ can be written on the torus and the spherical geometries.

We propose a state described by the second Landau level (SLL) projection of a generalized Moore-Read Pfaffian wave function with an antiholomorphic pairing component. Unlike the PH-Pfaffian state which is described by the lowest Landau level (LLL) projection of the same antiholomorphic Pfaffian wave function, we find the proposed state, which we call the SLL PH-Pfaffian, represents a gapped, incompressible phase, and provides an excellent description for the exact ground state of finite-size SLL Coulomb-interacting electrons over a range of the short-distance interaction strength. We also find the SLL PH-Pfaffian, when mapped to the LLL, has a very large overlap and the same low-energy orbital entanglement spectrum structures with the anti-Pfaffian state. We discuss possible interpretations of our results on the nature of the topological order of both states.

Two-dimensional (2D) materials are being explored as a novel multiferroic platform. One of the most studied magnetoelectric multiferroic 2D materials are antiferromagnetically coupled (AFM) ${\mathrm{CrI}}_3$ bilayers. Neglecting magnetism, those bilayers possess a crystalline point of inversion, which is only removed by the antiparallel spin configuration among its two constituent monolayers. The resulting intrinsic electric dipole on those bilayers has a magnitude no larger than 0.04 pC/m, it points out of plane, and it reverts direction when the Ising-like-chromium spins are flipped (toward opposite layers versus away from opposite layers). The combined presence of antiferromagnetism and a weak intrinsic electric dipole makes this material a two-dimensional magnetoelectric multiferroic. Here, we remove the crystalline center of inversion of the bilayer by a relative ${60}^{\circ }$ rotation of its constituent monolayers. This process enhances the out-of-plane intrinsic electric dipole tenfold with respect to its magnitude in the nonrotated AFM bilayer and also creates an even stronger and switchable in-plane intrinsic electric dipole. The ability to create a three-dimensional electric dipole is important because it enhances the magnetoelectric coupling on this experimentally accessible 2D material, which is explicitly calculated here as well.

Two-dimensional systems such as quantum spin liquids or fractional quantum Hall systems exhibit anyonic excitations that possess more general statistics than bosons or fermions. This exotic statistics makes it challenging to solve even a many-body system of non-interacting anyons. We introduce an algorithm that allows to simulate anyonic tight-binding Hamiltonians on two-dimensional lattices. The algorithm is directly derived from the low energy topological quantum field theory and is suited for general Abelian and non-Abelian anyon models. As concrete examples, we apply the algorithm to study the energy level spacing statistics, which reveals level repulsion for free semions, Fibonacci anyons, and Ising anyons. Additionally, we simulate nonequilibrium quench dynamics, where we observe that the density distribution becomes homogeneous for large times—indicating thermalization.

We derive an arbitrary-gauge criterion under which condensed matter within an electromagnetic field may transition to a photon condensed phase. Previous results are recovered by selecting the Coulomb gauge wherein photon condensation can only occur for a spatially varying field and can be interpreted as a magnetic instability. We demonstrate the gauge invariance of our description directly, but since matter and photons are gauge-relative concepts we find more generally that photon condensation can occur within a spatially uniform field and that the relative extent to which the instability is both magnetic and electric versus purely magnetic or purely electric depends on the gauge. We use our results to propose a method to detect the photon condensed phase.

Fermi surfaces can undergo sharp transitions under smooth changes of parameters. Such transitions can have a topological character, as is the case when a higher-order singularity, one that requires cubic or higher-order terms to describe the electronic dispersion near the singularity, develops at the transition. When time-reversal and inversion symmetries are present, odd singularities can only appear in pairs within the Brillouin zone. In this case, the combination of the enhanced density of states that accompanies these singularities and the nesting between the pairs of singularities leads to interaction-driven instabilities. We present examples of single $n=3$ (monkey-saddle) singularities when time-reversal and inversion symmetries are broken. We then turn to the question of what instabilities are possible when the singularities are isolated. For spinful electrons, we find that the inclusion of repulsive interactions destroys any isolated monkey-saddle singularity present in the noninteracting spectrum by developing Stoner or Lifshitz instabilities. In contrast, for spinless electrons and at the mean-field level, we show that an isolated monkey-saddle singularity can be stabilized in the presence of short-range repulsive interactions.

In pump-probe spectroscopy, two laser pulses are employed to garner dynamical information from the sample of interest. The pump initiates the optical process by exciting a portion of the sample from the electronic ground state to an accessible electronic excited state, an exciton. Thereafter, the probe interacts with the already excited sample. The change in the absorbance after the pump provides information on transitions between the excited states and their dynamics. In this work, we study these exciton-exciton transitions by means of an ab initio real-time propagation scheme based on dynamical Berry phase formulation. The results are then analyzed taking advantage of a Fermi golden rule approach formulated in the excitonic basis-set and in terms of the symmetries of the excitonic states. Using bulk LiF and two-dimensional hBN as two prototype materials, we discuss the selection rules for transitions involving strongly bound Frenkel excitons, for which the hydrogen model cannot be used.

Reduced exciton mass, polarizability, and dielectric constant of the surrounding medium are essential properties for semiconducting materials, and they have been extracted recently from the magnetoexciton energies. However, the acceptable accuracy of the suggested method requires very high magnetic intensity. Therefore, in the present paper, we propose an alternative method of extracting these material properties from recently available experimental magnetoexciton $s$-state energies in monolayer transition metal dichalcogenides (TMDCs). The method is based on the high sensitivity of exciton energies to the material parameters in the Rytova-Keldysh model. It allows us to vary the considered material parameters to get the best fit of the theoretical calculation to the experimental exciton energies for the $1s, 2s$, and $3s$ states. This procedure gives values of the exciton reduced mass and 2D polarizability. Then, the experimental magnetoexciton spectra compared to the theoretical calculation also determine the average dielectric constant. Concrete applications are presented only for monolayers ${\mathrm{WSe}}_2$ and ${\mathrm{WS}}_2$ from the recently available experimental data; however, the presented approach is universal and can be applied to other monolayer TMDCs. The mentioned fitting procedure requires a fast and effective method of solving the Schrödinger equation of an exciton in monolayer TMDCs with a magnetic field. Therefore, we also develop such a method in this paper for highly accurate magnetoexciton energies.

The discovery of topological quantum states in two-dimensional (2D) systems is one of the most promising advancements in condensed matter physics. Linear Weyl point (LWP) phonons have been theoretically investigated in some 2D materials. Especially, Jin, Wang, and Xu [Nano Lett. 18, 7755 (2018)] proposed in 2018 that the candidates with threefold rotational symmetry at the corners of the hexagonal Brillouin zone can host LWP phonons with a quantized valley Berry phase. Note that all the candidates with hexagonal lattices may not host LWP phonons at $K$ ($K^{\prime }$) high-symmetry points (HSPs). Hence a strategy for narrowing the search range for LWP phonons in 2D is highly required. This work provides an exhaustive list of valley LWP phonons at HSPs in 2D by searching the entire 80 layer groups (LGs). We found that the valley LWP phonons can be obtained at HSPs in 11 of the 80 LGs. Guided by the symmetry analysis, we also contributed to realizing the ideal 2D material with valley LWP phonons. We identified the existence of the valley LWP phonons in 11 2D material candidates with 11 LGs. This work offers a method to search for valley LWPs in 2D phononic systems and proposes 2D material candidates to obtain the valley LWP phonons.

Linearly dispersing Rarita-Schwinger-Weyl (RSW) fermions featuring two Fermi velocities are the key constituents of itinerant spin-3/2 quantum materials. When doped, RSW metals sustain two Fermi surfaces (FSs), around which one fully gapped $s$ wave and five mixed-parity local pairings can take place. The intraband components of four mixed-parity pairings support point nodes at the poles of two FSs, only around which long-lived quasiparticles live. For weak (strong) pairing amplitudes ($\mathrm{\Delta }$), gapless north and south poles belonging to the same (different) FS(s) get connected by polar hairs, one-dimensional line nodes occupying the region between two FSs. The remaining one, by contrast, supports four nodal rings in between two FSs, symmetrically placed about their equators, but only when $\mathrm{\Delta }$ is small. For large $\mathrm{\Delta }$, this paired state becomes fully gapped. The transition temperature and pairing amplitudes follow the BCS scaling. We explicitly showcase these outcomes for a rotationally symmetric RSW metal and contrast our findings when the system possesses an enlarged Lorentz symmetry and with those in spin-3/2 Luttinger materials.

A predominant ubiquitous feature of strongly correlated oxides is the possible presence of oxygen vacancies, which has recently been shown to have profound effects on their electronic phase transitions. Here, we formulate a comprehensive phase-field model of intercoupled insulator-metal transitions and oxygen vacancy redox reactions, taking into account the valence electron state of oxygen vacancies. We use the model to study the voltage self-oscillation phenomenon in a prototypical strongly correlated oxide, ${\text{VO}}_{\text{2}}$, and discover the mutual activation of the insulator-metal transition and oxygen vacancy redox reactions leading to systematic enhancement of the oscillation frequency. The established methodology and the mutual activation mechanism are generally applicable to understanding any insulator-metal transition dynamics in oxygen-deficient correlated oxides and improving the performance of voltage self-oscillation-based artificial neurons.

Shift current and ballistic current have been proposed to explain the bulk photovoltaic effect (BPVE), and there have been experiments designed to separate the two mechanisms. These experiments are based on the assumption that under magnetic field, ballistic current can have a Hall effect while the shift current cannot, which is from some energy-scale arguments and has never been proven. A recent work [D. Hornung and R. von Baltz, Phys. Rev. B 103, 195203 (2021)] using quantum transport formalism achieves a conclusion that shift current indeed has a Hall current, seemingly contradicting the previous assumption and making the situation more confusing. Moreover, the behavior of BPVE under strong magnetic field is still unexplored. In this Letter, using a minimal two-dimensional tight-binding model, we carry out a systematic numerical study of the BPVE under weak and strong magnetic field by treating the field in a nonperturbative way. Our model clearly shows the appearance of the magnetically induced ballistic current along the transverse direction, which agrees with the previous predictions, and interestingly a sizable longitudinal response of the shift current is also observed, a phenomenon that is not captured by any existing theories where the magnetic field is treated perturbatively. More surprisingly, drastically different shift current is found in the strong-field regime, and the evolution from weak to strong field resembles a phase transition. We hope that our work could resolve the debate over the behavior of BPVE under magnetic field, and the strong-field behavior of shift current is expected to inspire more studies on the relation between nonlinear optics and quantum geometry.

In this paper, a non-Hermitian Aubry-André-Harper model with power law hoppings ($1/s^a$) and quasiperiodic parameter $\beta$ is studied, where $a$ is the power law index, $s$ is the hopping distance, and $\beta$ is a member of the metallic mean family. We find that the number of the quasiperiodic parameter $\beta$-dependent regimes depends on the strength of the non-Hermiticity. Under a particularly weak non-Hermitian effect, there preserves $P_{\ell =1,2,3,4}$ regimes where the fraction of ergodic eigenstates is $\beta$ dependent as ${\beta }^{\ell }L$ ($L$ is the system size), similar to those in the Hermitian case. However, $P_{\ell }$ regimes are ruined by the strong non-Hermitian effect. Moreover, by analyzing the fractal dimension, we find that there are two types of edges aroused by the power law index $a$ in the single-particle spectrum, i.e., an ergodic-to-multifractal edge for the long-range hopping case ($a<1$), and an ergodic-to-localized edge for the short-range hopping case ($a>1$). Meanwhile, the existence of these two types of edges is found to be robust against the non-Hermitian effect. By employing the Simon-Spence theory, we analyzed the absence of the localized states for $a<1$. For the short-range hopping case, with the Avila's global theory and the Sarnak method, we consider a specific example with $a=2$ to reveal the presence of the intermediate phase and to analytically locate the intermediate regime and the ergodic-to-localized edge, which are self-consistent with the numerically results.

The configurational entropy of supercooled liquids extrapolates to zero at the Kauzmann temperature, causing a crisis called the Kauzmann paradox. Here, using a class of multicomponent lattice glass models, we study a resolution of the paradox characterized by a sudden but smooth turn in the entropy as temperature goes sufficiently low. A scalar variant of the models reproduces the Kauzmann paradox with thermodynamic properties at very low temperatures dominated by correlations. An exactly solvable vector variant without correlation illustrates that a sudden entropy turn occurs when discrete local excitations are largely suppressed. Despite being disordered and infinitely degenerate, the ground states have zero entropy per particle.

We experimentally demonstrate spin-torque generation using a compositional gradient at the interface between titanium and tungsten thin films. The width of the compositional gradient interface (CGI) between films is varied from 1.4 to 2.0 nm via sputtering. Spin-torque ferromagnetic resonance is observed in the ferromagnetic ${\mathrm{Ni}}_{95}{\mathrm{Cu}}_5$ alloy fabricated on a Ti/W bilayer with the CGI. The positive spin torque increases with decreasing CGI width, but a negative spin torque is superimposed owing to the negative spin Hall effect in bulk tungsten. A structural undulation in the CGI eliminates this variation in positive spin torque. The CGI width dependence of the spin torque is associated with the generation of spin and/or orbital angular momentum flow at the CGI. Spin-torque generation using a CGI expands the range of material choice for magnetic nonvolatile memory applications.

NiMn is a collinear antiferromagnet with high magnetocrystalline anisotropy ($K=-9.7\times {10}^5\text{J}{\text{m}}^{-3}$). Through magnetic annealing of NiMn with excess Ni, strongly pinned magnetic moments emerge due to an imbalance in the distribution of Ni in the antiferromagnetic Mn sublattices. The results are explained with a model of magnetic field biased diffusion, supported by ab initio calculations. Another observation is the oxidation of Mn at the surface, causing an enrichment of Ni in the subsurface region. This leads to an additional ferromagnetic response appearing in the magnetization measurements, which can be removed by surface polishing.

Ultrafast demagnetization induced by femtosecond laser pulses in thin metallic layers is caused by the outflow of spin-polarized hot-electron currents describable by the superdiffusive transport model. These laser-generated spin currents can cross the interface into another magnetic layer and give rise to magnetization dynamics in magnetic spin valves with noncollinear magnetizations. To describe ultrafast transport and spin dynamics in such nanostructures, we develop here the superdiffusive theory for general noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru multilayer system with noncollinear Ni and Fe magnetic moments and analyze how the ultrafast demagnetization and spin-transfer torque depend on the noncollinearity. We employ ab initio calculations to compute the spin- and energy-dependent transmissions of hot electrons at the interfaces of the multilayer. Taking into account multiple electron scattering at interfaces and spin mixing in the spacer layer, we find that the laser-induced demagnetization of the Ni layer and the magnetization change of the Fe layer strongly depend on the angle between their magnetizations. Similarly, the spin-transfer torques on the Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer are found to vary markedly with the amount of noncollinearity. These results suggest that by changing the amount of noncollinearity in magnetic multilayers, one can efficiently control the hot-electron spin transport, which may open a way toward achieving fast, laser-driven spintronic devices.