We investigate a spin-$\frac{1}{2}$ antiferromagnet, ${\mathrm{CuBr}}_2$, which has quasi-one-dimensional structural motifs. The system has previously been observed to exhibit unusual Raman modes possibly due to a locally deformed crystal structure driven by the low-dimensional magnetism. Using hard x-ray scattering and neutron total scattering, here we aim to verify a specific form of tetramerizing deformation proposed in the previous study. Apart from diffuse scattering signals, which we can reproduce by performing a thorough modeling of the lattice's thermal vibrations, we do not observe evidence for a tetramerized lattice structure within our detection sensitivity. Consequently, we consider it unlikely that the unusual Raman modes in ${\mathrm{CuBr}}_2$ arise from quantum spin-singlet-driven lattice deformations.

Spontaneous symmetry breaking in topological matter has emerged as a pivotal concept in describing phase transitions within condensed-matter physics. Of particular interest is the Berezinskii-Kosterlitz-Thouless (BKT) transition observed in two-dimensional superconductors. Here, we present comprehensive transport experiments on a nanoflake of the topological nodal-line semimetal ${\mathrm{SnTaS}}_2$ with a thickness of about 37 nm, revealing its hosting of a superconducting state with an onset transition temperature of 2.7 K. Temperature-dependent upper critical fields reveal an anisotropic superconductivity. Angle-dependent magnetotransport and the BKT transition confirm quasi-two-dimensional superconductivity in the ${\mathrm{SnTaS}}_2$ nanoflake. Given the presence of spin-polarized surface states in ${\mathrm{SnTaS}}_2$, these findings establish a novel platform for exploring topological superconductivity and Majorana modes.

Measurements of the magnetoresistance and Hall effect on transparent films of single-wall nanotubes before and after their doping with iodine, in the temperature range from 3–5 to 40 K and magnetic fields up to 16 T, are reported. The measured magnetoresistance is fairly well approximated by the sum of two contributions: negative, described in the framework of two-dimensional theory of weak localization (2D-WL), and linear in field positive, unsaturated in strong magnetic fields. The linear temperature dependence of $L_{\varphi }^{-2}\left(T\right)$ ($L_{\varphi }$ is the phase decoherence length) indicates electron-electron scattering as the main mechanism of dephasing, and the coincidence of these linear dependencies for the pristine and doped films suggests that the state of static defects and the nature of electron-electron scattering do not change when doping nanotubes. The values of the two-dimensional density of charge carriers determined from Hall measurements, being, apparently, highly overestimated, increase more than twice after iodine doping. This increase is clearly correlated with the magnetoresistance data processed in 2D-WL theory. The linear in field positive contribution to magnetoresistance is explained by the inhomogeneous distribution of the local density of charge carriers.

Su-Schrieffer-Heeger (SSH) lattices have helped to understand key concepts of topological insulators. In this paper, we study the transmission properties of $pn$ junctions of bearded SSH lattices forming a bipartite structure. From a tight-binding approach, we found the condition on the hopping parameters of the Bloch Hamiltonian to depict the electron behavior as a massive pseudospin-one particle in quasi-one-dimensional systems. It is possible to design the band structure of these lattices by tuning the on-site energy and hopping parameters. The characteristic flat band of pseudospin-one systems can be bent by the staggered potential of the bottom atom in the bearded edge. Transmissions in bipartite bearded SSH lattices are obtained from exact matching conditions by considering the full Bloch Hamiltonian of tight-binding to nearest neighbors. We explore interband transmissions from the bent flat band to the valence one of a bipartite structure, where Klein tunneling can be obtained depending on the hopping parameter associated with the bearded edge. Identifying Klein tunneling signatures is feasible in artificial lattices such as photonic lattices, elastic resonators, topolectric circuits, or quantum dot arrangements. On the other hand, the study of Klein tunneling in polyatomic molecules may contribute to advance in one-dimensional transistors.

Recent works have predicted monolayer gold selenide to be stable and easily exfoliable, with excellent potential optoelectronic applications. However, the presence of an inversion center prevents it from showing any spin-orbit splitting, limiting its use in the field of spintronics. In this study, we sulfurize one of the selenium layers of AuSe to construct the Janus ${\mathrm{Au}}_2\mathrm{SSe}$ monolayer with a low $C_s$ point group symmetry to tackle this problem. Unlike the conventional Janus structures, the two chalcogen atoms of ${\mathrm{Au}}_2\mathrm{SSe}$ are not mirror-symmetric, giving rise to both in-plane and out-of-plane spin polarizations. These spin polarizations, in turn, generate diverse spin-orbit coupling effects at different high-symmetry points in the Brillouin zone, namely, the linear-in-$k$ anisotropic Dresselhaus effect at the $\mathrm{\Gamma }$ point, the Zeeman-like effect at the conduction band minima, and the symmetry-induced persistent spin texture at the $X$ point. The observed spin-orbit effects have been explained by considering the variation in the strength of the SOC parameters in a $C_s$ symmetric spin-orbit coupling Hamiltonian. Additionally, these effects can be further influenced by changing the chalcogen atoms in the Janus structures without changing the system symmetry. Both the Zeeman-like and persistent spin splittings consist of momentum-independent canted spins in the $S_x\text{−}{S}_z$ plane and can be tuned using in-plane and out-of-plane strains, making the Janus ${\mathrm{Au}}_2\mathrm{SSe}$ structure ideal for realizing spin-based applications.

The mystery of the infamous sign problem in quantum Monte Carlo simulations mightily restricts applications of the method in fermionic and frustrated systems. A recent work [Science 375, 418 (2022)] made a remarkable breakthrough in the sign problem by pointing out that the sign can be used to probe phase transition. In this work, we proposed a general argument based on the definition of the sign that is related to the difference in free energy between the original and reference systems to clarify that the sign problem and phase transition cannot always be strictly related. The sign can exactly probe phase transition only if the free energy in the reference system is flat under variable parameters, which is almost impossible to design. Generally speaking, the conclusion that the sign can probe phase transition is survivorship bias without universality. To solve this problem, we define a modified sign that excludes the influence of the reference system, which can probe the phase transition strictly. The work gives an unbiased solution for detecting phase transition by the new modified sign.

Motivated by recent experiments demonstrating the creation of atomically sharp interfaces between hexagonal sapphire and cubic ${\mathrm{SrTiO}}_3$ with finite twist, we here develop and study a general electronic band theory for this novel class of moiré heterostructures. We take into account the three-dimensional nature of the two crystals, allow for arbitrary combinations of Bravais lattices, finite twist angles, and different locations in momentum space of the low-energy electronic bands of the constituent materials. We analyze the general condition for a well-defined crystalline limit in the interface electron system and classify the associated “crystalline reference points.'' We discuss this in detail for the example of the two-dimensional lattice planes being square and triangular lattices on the two sides of the interface; this reveals nontrivial reference points at finite twist angle and lattice mismatch, leading to a unique form of magic angles, which we refer to as “geometric magic angles.” We further show that band structures of mixed dimensionality naturally emerge, where quasi-one- and two-dimensional pockets coexist. Explicit computations for different bulk Bloch Hamiltonians yield a collection of interesting features, such as isolated bands localized at interfaces of nontopological insulators, Dirac cones, van Hove singularities, a nontrivial evolution of the band structures with Zeeman field, and topological interface bands. Our work illustrates the potential of these heterostructures and is anticipated to provide the foundation for moiré interface design and for the analysis of correlated physics in these systems.

For two-dimensional ferromagnetic semiconductors, robust ferromagnetic couplings and high Curie temperatures are highly desirable. The alloying strategy has been proposed to be able to strengthen ferromagnetic superexchange interactions and enhance Curie temperatures. This strategy has been applied and examined in ordered alloy structures in many recent studies. However, the efficacy of the alloying strategy in the other two distinct types of alloy structures (i.e., partially ordered and random) has not been studied yet. In addition, the effects of cation disorder on the strength of ferromagnetic superexchange interactions and the Curie temperatures in two-dimensional ferromagnetic semiconductor alloys remain to be clarified. Exemplified by the ${\mathrm{CrMoSe}}_2{\mathrm{Br}}_2$ alloy, we computationally study the ferromagnetic superexchange interactions in the ordered, partially ordered, and random alloy structures. For the ${\mathrm{CrMoSe}}_2{\mathrm{Br}}_2$ alloy structure, a local motif is defined to be consisting of eight nearest-neighboring cations surrounding a given cation site. The large variations in the Curie temperatures for the ordered and random alloy structures are attributed to the different types of local motifs. In partially ordered alloy structures, the strength of superexchange interactions and the Curie temperatures show monotonic behaviors as functions of the degree of cation disorder. This work validates the efficacy of the alloying strategy in strengthening ferromagnetic superexchange interactions and enhancing Curie temperatures in all three kinds of alloy structures and demonstrates that cation disorder engineering can be a general approach to tune ferromagnetic superexchange interactions in two-dimensional semiconductor alloys.

Hot-carrier solar cells offer potential for enhancing the energy-conversion efficiency of photovoltaic devices, but their design and operation require a good assessment of carrier temperatures. Electrons and holes may have different temperatures, for instance, because of their effective mass mismatch in III-V compounds. We propose a purely optical method which allows the direct and distinct estimation of electron and hole temperatures in the steady state. This technique, based on photoluminescence, relies on the precise determination of the band-filling signature. We apply this technique to an InGaAsP single quantum well. The electron temperature surpasses 1000 K at largest excitation intensity, while holes remain colder, close to lattice temperature. Nonetheless, the increase in hole temperature is too large to be explained purely by photon absorption, which demonstrates an energy transfer from electrons to holes.

The presence of random disorder in a metallic system accounts for the localization of extended states in general. On the contrary, the presence of disorder can induce topological phases hosting metallic boundary states out of a nontopological system, giving birth to the topological Anderson insulator phase. In this context, we theoretically investigate the generation of an out of equilibrium higher-order topological Anderson phase in the presence of disorder potential in a time-periodic dynamical background. In particular, the time-dependent drive and the disorder potential concomitantly render the generation of Floquet higher-order topological Anderson insulator (FHOTAI) phase, while the clean, undriven system is topologically trivial. We showcase the generation of FHOTAI hosting both 0 and $\pi$ modes. Most importantly, we develop the real space topological invariant: a winding number based on chiral symmetry to characterize the Floquet 0 and $\pi$ modes distinctly. This chiral winding number serves the purpose of the indicator for the topological phase transition in the presence of drive as well as disorder and appropriately characterizes the FHOTAI.

The kagome lattice, characterized by its flat band and Dirac electrons, continues to attract enduring attention in condensed matter physics as an ideal structure for studying electron correlation and various quantum phases. Despite recent experimental advances, two-dimensional (2D) kagome-structured materials with significant flat-band characteristics still remain rare. Based on first-principles calculations, we predict that nitrogen atoms can assemble into a single-layer double kagome lattice (DKL), which possesses the characteristics of an intrinsic direct band gap semiconductor, boasting a substantial band gap of 3.460 eV. The DKL structure results in a flat valence band with a high effective mass and a conduction band with a small effective mass that comes from Dirac electrons. These distinctive band edges lead to a significant disparity in carrier mobilities, with electron mobility being four orders of magnitude higher than that of holes. The presence of a flat band in DKL-nitrogene can be further discerned through the enhanced optical absorption and correlated effects as exemplified by hole-induced ferromagnetism. Interestingly, DKL-nitrogene exhibits inherent second-order topological states, confirmed by a nontrivial second Stiefel-Whitney number and the presence of 1D floating edge states and 0D corner states within the bulk band gap. Additionally, the robust N-N bonds and the lattice's bending structure ensure thermodynamic stability and mechanical stiffness. These attributes make it exceptionally stable for potential applications in nanodevices.

We show that rhombohedral multilayer graphene supports topological frequency conversion, whereby a fraction of electrons transfer energy between two monochromatic light sources at a quantized rate. The pristine nature and gate tunability of these materials, along with a Berry curvature that directly couples to electric fields, make them ideal platforms for the experimental realization of topological frequency conversion. Among the rhombohedral family, we find that Bernal bilayer graphene appears most promising for THz-scale applications due to lower dissipation. We discuss strategies to circumvent cancellations between the two valleys of graphene and to minimize dissipative losses using commensurate frequencies, thus opening a potential pathway for net amplification.

Odd-parity superconductivity is a fundamentally interesting but rare state of matter with a potential for applications in topological quantum computing. Crystals with staggered locally noncentrosymmetric structures have been proposed as platforms where a magnetic field can induce a transition between even- and odd-parity superconducting (SC) states. The strongly correlated superconductor ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, with the critical temperature $T_{\mathrm{c}}\approx 0.4\mathrm{K}$, is likely the first example material showing such a phase transition, which occurs at the magnetic field ${\mu }_0{H}^{*}=4\mathrm{T}$ applied along the crystallographic $c$ axis. ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ also undergoes a phase transition of an unknown origin at $T_0=0.5\mathrm{K}$. By subjecting ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ to hydrostatic pressure and mapping the resultant changes to the SC phase diagrams we investigated how the lattice compression and changes to the electronic correlations affect the stability and relative balance of the two SC states. The abnormally high in-plane upper critical field becomes even higher close to a quantum critical point of the $T_{\text{0}}$ order. Remarkably, the SC phase-switching field $H^{*}$ is drastically reduced under pressure, dropping to 0.3 T at 2.7 GPa. This result signals an apparent strengthening of the local noncentrosymmetricity and forecasts a possible stabilization of the putative odd-parity state down to zero field, hitherto not considered by theoretical models.

Discovering mixed state quantum orders is an ongoing issue. Recently, it has been recognized that there are (at least) two kinds of symmetries in the mixed state: strong and weak symmetries. Under symmetry-respective decoherence, spontaneous strong-to-weak symmetry breaking (SSSB) can occur. This paper provides a scheme to describe SSSB and other decoherence phenomena in the mixed state by employing the stabilizer formalism and the efficient numerical algorithm of Clifford circuits. We present two systematic numerical studies. In a two-dimensional circuit with a stochastic Ising type decoherence, an SSSB phase transition is clearly observed and its criticality is elucidated by the numerical methods. In particular, we calculate Rényi-2 correlations and estimate critical exponents of the SSSB transition. For the second system, we introduce an idea of subgroup SSSB. As an example, we study a system with symmetry-protected-topological order provided by both one-form and zero-form symmetries, and observe how the system evolves under decoherence. After displaying numerical results, we show that viewpoint of percolation is quite useful to understand the SSSB transition, which is applicable for a wide range of decohered states. Finally, we comment on SSSB of one-form-symmetry exemplifying toric code.

Optical rectification of intense, circularly polarized light penetrating a material generates a static magnetization through the inverse Faraday effect and, therefore, a magnetic field aligned with the light's direction of propagation. Recent ultrafast experiments have unveiled a substantial, orders-of-magnitude gap between the observed effective field and theoretical predictions. In this study, we show that the discrepancy arises due to a missing factor on the order of ${\alpha }^{-2}\approx 2\times {10}^4$, where α is the fine-structure constant. We demonstrate that alongside Maxwellian magnetization, circular polarization creates large non-Maxwellian fields that disrupt time reversal symmetry, effectively mimicking authentic magnetic fields within the material while eluding detection externally. These unconventional fields, reaching effective magnitudes as high as 100 T, lead to phenomena akin to Faraday rotation and robustly interact with magnons in magnetically ordered materials.

Magnon band structures in altermagnets are characterized by an energy splitting of modes with opposite chirality, even in the absence of applied external fields and relativistic effects, because of an anisotropy in the Heisenberg exchange interactions. We perform quantitative atomistic spin dynamics simulations based on ab initio electronic structure calculations on rutile ${\mathrm{RuO}}_2$, a prototypical “d-wave” altermagnet, to study magnon currents generated by thermal gradients. We report substantial spin Seebeck and spin Nernst effects, i.e., longitudinal or transverse spin currents, depending on the propagation direction of the magnons with respect to the crystal, together with a finite spin accumulation associated with nonlinearities in the temperature profile. Our findings are consistent with the altermagnetic spin-group symmetry, as well as predictions from linear spin-wave theory and semiclassical Boltzmann transport theory.