Topological defects are of fundamental interest to wide branches of physics. Exploiting the structural evolutions and behaviors of topological defects across the phase transition is vital in understanding complicated condensed matter. Via presetting different initial azimuthal orientations of a square radial-alignment lattice in a hybrid cell, the evolution of liquid-crystal topological defects across the nematic-smectic phase transition is investigated. Distinct species of disclination lines are achieved in nematic phase via varying the initial azimuthal orientations. After cooling, various deformed toric focal conic domain arrays are generated as a result of the unique symmetry restriction and the suppression of twisted deformation in smectic phase. Different metastable crossed walls are formed due to the inheritance of orientational order when reheating to nematic phase. Transformations among these textures are observed under a microscope. The Landau–de Gennes theory is adopted to analyze the corresponding director distributions and defect evolutions. The underlying mechanism and laws of defect-species variation along with the change in azimuthal orientation are clarified and summarized. This study enriches the knowledge on self-organized ordered systems and may inspire intriguing applications.

Skyrmions are localized swirling noncoplanar spin textures offering a promising revolution in future spintronic applications. These topologically nontrivial spin textures lead to an additional contribution to the Hall effect, called the topological Hall effect. Here, we investigate the origin of the topological Hall effect—a trademark of skyrmions—in a centrosymmetric shape memory Heusler alloy (SMHA) ${\mathrm{Mn}}_2\mathrm{NiGa}$. The magnetization measurement unveils the presence of austenite to martensite transition in the studied system. The topological Hall effect (THE) in the present system is examined experimentally and theoretically. The presence of a large THE in the austenite (cubic) phase of the system strongly suggests that the observed THE in ${\mathrm{Mn}}_2\mathrm{NiGa}$ cannot be attributed to the antiskyrmions stabilized by $D_{2d}$ symmetry as reported earlier. To comprehend the underlying mechanism behind the origin of THE, we have performed micromagnetic simulations for a range of magnetic field with a small value of DMI (local DMI) to consider the possible impact of earlier reported atomic disorder in the centrosymmetric SMHA ${\mathrm{Mn}}_2\mathrm{NiGa}$. The results showed the stabilization of Néel-type skyrmions, which can be assigned to the expected local symmetry breaking at the interface of disorder originated ferromagnetic nanoclusters and ferrimagnetic lattice of the system. A theoretical calculation of topological Hall resistivity by utilizing micromagnetic simulations is performed, which is of the same order as the experimentally obtained values in the both martensite and austenite phases.

Tiling models can reveal unexpected ways in which local constraints give rise to exotic long-range spatial structure. The recently discovered hat monotile (and its mirror image) has been shown to be aperiodic [Smith et al., arXiv:2303.10798]; it can tile the plane with no holes or overlaps, but cannot do so periodically. We show that the structure enforced by the local space-filling constraints is quasiperiodic with hexagonal (C6) rotational symmetry. Although this symmetry is compatible with periodicity, the incommensurate ratio characterizing the quasiperiodicity stays locked to the golden mean as the tile parameters are continuously varied. We analyze a modification of the metatiles introduced by Smith et al. that yields a set of key tiles that can be constructed as projections of a subset of six-dimensional hypercubic lattice points onto the two-dimensional tiling plane. We analytically compute the diffraction pattern of a set of unit masses placed at the tiling vertices, establishing the quasiperiodic nature of the tiling. We point out several unusual features of the family of key tilings and associated hat tilings, including the tile rearrangements associated with the phason degree of freedom associated with incommensurate density waves, which exhibit novel features that may influence the elastic properties of a material in which atoms or larger particles spontaneously exhibit the symmetries of the hat tiling.

The collective magnetic behaviors of dipole-coupled magnetic nanoparticles organized in periodic two-dimensional kagome and honeycomb lattices are investigated theoretically. The extended nanostructures are modeled by considering finite unit cells containing nanoparticles with periodic boundary conditions. The energy landscapes (ELs) of the ensembles are systematically explored as a function of the orientation of all the NP moments by calculating the local minima and first-order saddle points connecting them. The low-lying magnetic orders and elementary reorientation transitions are identified. The thus obtained kinetic networks and disconnectivity graphs of the ELs reveal profound differences in the topology of the networks of stationary states. One observes that the honeycomb nanostructures are typically good structure seekers with starlike kinetic networks and palm-tree-like disconnectivity graphs. They have a continuously degenerate ground state, which is the center of the network and directly connected to all excited metastable states. In contrast, kagome nanostructures show a particular form of bad structure seeker, which is known as the latticelike stationary-point network. In this case, from a local perspective, the degree distribution among the LM is extremely homogeneous, with no centers or hubs through which relaxation can be funneled. Furthermore, from an energy perspective, no hierarchy among the low-energy metastable states can be identified.

Exploring anisotropy and diverse magnetization dynamics in specimens with vanishing magnetic moments presents a significant challenge using traditional magnetometry, as the low resolution of existing techniques hinders the ability to obtain accurate results. In this study, we delve deeper into the examination of magnetic anisotropy and quasistatic magnetization evolution in ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ (MRG) thin films, as an example of a compensated ferrimagnetic half-metal, by employing anomalous Hall effect measurements within a tetragonal crystal lattice system. Our research proposes an innovative approach to accurately determine the complete set of anisotropy constants of these MRG thin films. To achieve this, we perform anomalous Hall voltage curve fitting, using torque models under the macrospin approximation, which allow us to obtain room-temperature out-of-plane anisotropy constants $K_1=4.0\times {10}^4\mathrm{J}{\mathrm{m}}^{-3}$ ($K_1/M=0.655\mathrm{T}$) and $K_2=2.54\times {10}^4\mathrm{J}{\mathrm{m}}^{-3}$ ($K_2/M=0.416\mathrm{T}$), along with a weaker in-plane anisotropy constant $K_3=3.48\times {10}^3\mathrm{J}{\mathrm{m}}^{-3}$ ($K_3/M=0.057\mathrm{T}$). By additionally employing first-order reversal curves and classical Preisach hysteresis (hysterons) models, we are able to validate the efficacy of the macrospin model in capturing the magnetic behavior of MRG thin films. Furthermore, our investigation substantiates that the complex steady-state magnetization behavior of MRG thin films can be effectively modeled using a combination of hysteronic and torque models. This approach facilitates the exploration of both linear and nonlinear steady-state magnetization evolution, in the presence of an external magnetic field and/or current-induced effective fields, generated by the spin-orbit torque and spin transfer torque mechanisms. The detailed understanding of the quasiequilibrium magnetization behavior is a key prerequisite for the exploitation of in-phase and out-of-phase resonance modes in this material class, for high-bandwidth modulators/demodulators, filters, and oscillators for the high-GHz and low-THz frequency bands.

The synchronized observation of a quantum magnetization plateau and multiferroicity is of particular interest in frustrated antiferromagnets, but the modulation of two different fascinating phenomena is greatly challenging and urgently desired. Here, we report an efficient approach with chemical substitutions to achieve flexible control of a half quantum magnetization plateau and high-field ferroelectricity in the $S=1$ skew-chain antiferromagnet ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$. In Zn-doped cases, the half quantum magnetization plateau is remarkably broadened with an increasing concentration on dopants, such as a giant variation reaching 69.3% at $x=0.7$, suggesting the existence of a larger spin gap than before. In contrast, the width of magnetization plateaus for Mn-doped samples pronouncedly shrinks about $-37.4\%$ at $x=0.2$, and then tends to stabilization upon further increasing $x$. Intriguingly, we find that the end of half magnetization plateaus locates in the same position between two groups of samples. In particular, the nontrivial magnetoelectric coupling related with the high-field ferroelectricity performs an identical evolution for ${\mathrm{Ni}}_{2\text{−}x}{T}_x{\mathrm{V}}_2{\mathrm{O}}_7 (T=\mathrm{Zn} \text{or} \mathrm{Mn})$ by sweeping magnetic fields. As a consequence, a close correlation between the half magnetization plateau and ferroelectricity is confirmed, indicating that the high-field polarization is derived from peculiar magnetic moments in ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$. Possible origins for the Zn- and Mn-doping effects on the quantum magnetization plateau and multiferroicity are discussed. In addition, high-field phase diagrams with the plateau and ferroelectricity are constructed for these doped compounds. Therefore, these experimental findings open an additional avenue to tune the quantum magnetization plateau and magnetically induced ferroelectricity, simultaneously.

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the millikelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material ${\mathrm{NaYbGeO}}_4$ which exhibits a distorted square lattice arrangement of ${\mathrm{YbO}}_6$ magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10 K, with a Curie-Weiss temperature of only 15 mK, that can be polarized by magnetic fields of order 1 T. For the ADR performance test, we start the demagnetization from 5 T at a temperature of $\sim 2\mathrm{K}$ and reach a minimum temperature of 150 mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210 mK, implying only weak magnetic frustration. The entropy density of $S_{\mathrm{GS}}\simeq 101 \mathrm{mJ}{\mathrm{K}}^{-1}{\mathrm{cm}}^{-3}$ and hold time below 2 K of 220 min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions.

By performing first-principles calculations in conjunction with Monte Carlo simulations, we systematically investigated the frustrated magnetic states induced by in-plane compressive strain in ${\mathrm{LiCrTe}}_2$. Our calculations support the idea that the magnetic ground state of the ${\mathrm{LiCrTe}}_2$ crystal is $A$-type antiferromagnetic (AFM) with an in-plane ferromagnetic (FM) state and interlayer AFM coupling. Furthermore, we find that compressive strain can significantly alter the magnetic interactions, giving rise to a transition from an in-plane FM state to an AFM state, undergoing a helimagnetic phase. Remarkably, a highly frustrated helimagnetic state with disordered spin spirals under moderate strain arises from the competition between spiral propagation modes along distinct directions. In addition, various topological spin defects emerge in this frustrated helimagnetic phase which are assembled from various domain wall units. These topological defects can be further tuned with external magnetic fields. Our calculations not only uncover the origin of exotic frustrated magnetism in triangular lattice magnetic systems but also offer a promising route to engineer the frustrated and topological magnetic state, which is significant for both fundamental research and technological applications.

We observe nearly vanishing Hall resistances for integer filling factors in a counterflow experiment on a density-balanced two-dimensional (2D) bilayer system. Filling-factor-dependent equilibration lengths demonstrate enhanced 1D coupling via edge channels. Due to the narrow barrier the edge modes of the two 2D electron gases are in close proximity allowing for exciton formation at the sample edges. Electron drag measurements confirm the observed quantum-state-selective coupling between the layers.

The exchange-correlation potential formalism previously introduced and applied to the one-dimensional Hubbard model has been extended to spin systems and applied to the case of the one-dimensional antiferromagnetic spin-$\frac{1}{2}$ Heisenberg model. Within the spin exchange-correlation potential formulation, a sum rule for spin systems is derived. The exchange-correlation potential for the Heisenberg model is extrapolated from exact diagonalization results of small antiferromagnetic Heisenberg clusters. This procedure is also employed to revisit and computationally improve the previous investigation of the exchange-correlation potential of the half-filled Hubbard model, which was based on the exchange-correlation potential of the dimer. Numerical comparisons with exact benchmark calculations for both the Heisenberg and the Hubbard models indicate that, starting from the exchange-correlation potential of a finite cluster, the extrapolation procedure yields a one-particle spectral function with favorable accuracy at a relatively low computational cost. In addition, a comparison between the ground-state energies for the one-dimensional Hubbard and Heisenberg models displays how the well-known similarity in behavior of the two models at large interactions manifests itself within the exchange-correlation potential formalism.

The calculation of the full counting statistics of the charge within a finite interval of an interacting one-dimensional system of electrons is a fundamental, yet as of now, unresolved problem. Even in the noninteracting case, charge counting turns out to be more difficult than anticipated because it necessitates the calculation of a nontrivial determinant and requires regularization. Moreover, interactions in a one-dimensional system are best described using bosonization. However, this technique rests on a long-wavelength approximation and is a priori inapplicable for charge counting due to the sharp boundaries of the counting interval. To mitigate these problems, we investigate the counting statistics using several complementary approaches. To treat interactions, we develop a diagrammatic approach in the fermionic basis, which makes it possible to obtain the cumulant generating function up to arbitrary order in the interaction strength. Importantly, our formalism preserves charge quantization in every perturbative order. We derive an exact expression for the noise and analyze its interaction-dependent logarithmic cutoff. We compare our fermionic formalism with the results obtained by other methods, such as the Wigner crystal approach and numerical calculations using the density-matrix renormalization group. Surprisingly, we show good qualitative agreement with the Wigner crystal for weak interactions, where the latter is in principle not expected to apply.

Employing a largely unbiased variational exact diagonalization technique, we analyze the consequences of longer-ranged electron hopping and electron-phonon interaction on polaron formation in one dimension. Having at our disposal the accurate ground-state energy and wave function, we calculate and discuss various physical quantities, such as the renormalized band structure, effective mass, wave-function renormalization factor, phonon dressing, and Drude weight, characterizing the properties of the polaronic quasiparticle. We demonstrate that the electron-phonon coupling affects the relative strength of the nearest neighbor (NN) and next nearest neighbor (NNN) hopping processes in a dynamic way. Most notably we observe that the minimum of the polaron band, occurring at a finite momentum for large negative ratio between NN and NNN transfer, jumps to zero momentum as the electron-phonon coupling exceeds a critical one, thereby causing a rather sharp polaron transition in the one-dimensional extended Holstein model. The signatures of this transition are seen in the effective mass and polaron mobility, and therefore should be easily detectable by transport measurements.

Motivated by the recently discovered incompressible insulating phase in the bilayer graphene exciton experiment [Zeng et al., arXiv:2306.16995], we study using bosonization two Coulomb-coupled spinless quantum wires and examine the possibility of realizing the similar phenomenology in one dimension. We explore the possible phases as functions of $k_F$'s and interactions. We show that an incompressible insulating phase can arise for two lightly doped electron-hole quantum wires (i.e., $k_{F1}=-k_{F2}$ and small $|k_{F1}|$) due to strong interwire interactions. Such an insulating phase forms a parity-even wire-antisymmetric charge density wave without interwire phase coherence, which melts to a phase allowing for a perfect negative drag upon heating. The finite-temperature response is qualitatively consistent with the “exciton solid” phenomenology in the bilayer graphene exciton experiment.

The phonon thermal transport properties of twisted bilayer graphene are investigated using lattice dynamics and the Boltzmann transport equation. The thermal conductivities of $13.2^{\circ }$ and $21.8^{\circ }$ twisted configurations are 56 and 36% lower than the untwisted configuration, which has a room temperature thermal conductivity of 2260 W/m K. The reason for this lower thermal conductivity is unraveled from phonon mode-level analysis made possible through the untwisting of layers. Due to a large commensurate unit cell of twisted configuration, the Brillouin zone is folded but this folding has no bearing on the phonon scattering phase space. The major impact of twisting is felt by flexural phonons with out-of-plane vibrations via the change in the strength of flexural interatomic interactions, and since these flexural phonons carry the majority of the heat in bilayer graphene (65% at room temperature) the thermal conductivity is sensitive to layer twisting. Our paper suggests that twisting will affect thermal transport only for those materials that have a major contribution from flexural phonon modes.

Zintl-phase compounds garner attention as promising thermoelectric materials due to observations of phonon-glass electron-crystal (PGEC) behavior, in combination with tunability that allows optimization of properties and doping. However, this is very much dependent on the specific materials, and understanding the factors that lead to PGEC behavior in some Zintl compounds but not others is an important challenge. Here, we investigate KSrBi and SrLiBi. KSrBi exhibits a significantly lower lattice thermal conductivity than SrLiBi, with the 300 K thermal conductivity of KSrBi (0.7 W/mK) being only one third of that in SrLiBi (2.2 W/mK). We find that pronounced rattling behavior of the K atoms in KSrBi leads to strong anharmonicity. The behavior of the two compounds is distinct due to the presence of Sr atoms within the cagelike structure formed by Li and Bi atoms in SrLiBi. The resulting enhanced bonding interactions between Li and Bi weaken the rattling vibrations of Li atoms in SrLiBi, hence influencing its thermal conductivity. Conversely, in KSrBi, K atoms reside within a framework formed by Sr and Bi atoms, exhibiting significant rattling vibrational behavior within this framework. This behavior results in strong scattering of heat-carrying phonons, and in particular large anharmonic scattering rates. Additionally, we find an antibonding electronic state involving the Bi $6p$ orbital and Sr $4p$ orbitals around the valence band edge in KSrBi but not SrLiBi. These antibonding states significantly weaken the bonding, resulting in a softer lattice and reduced sound velocity. Consequently, the combined effects of the strong rattling vibrations of K atoms and the presence of occupied antibonding states near the valence band maximum contribute to a lower lattice thermal conductivity in KSrBi relative to SrLiBi.

We investigate transport properties of stable gate-defined quantum dots formed in an ${\mathrm{InSb}}_{0.87}{\mathrm{As}}_{0.13}$ quantum well. High $g$ factor and strong spin-orbit coupling make ${\mathrm{InSb}}_x{\mathrm{As}}_{1-x}$ a promising platform for exploration of topological superconductivity and spin-based devices. We extract a nearly isotropic in-plane effective $\mathit{g}$ factor by studying the evolution of Coulomb blockade peaks and differential conductance as a function of the magnitude and direction of magnetic field. The in-plane $\mathit{g}$ factors, $|g_{\left[1\overline{1}0\right]}^{*}|$ and $|g_{\left[110\right]}^{*}|$, range from 49 to 58. Interestingly, this $\mathit{g}$ factor is higher than that found in quantum dots fabricated from pure InSb quantum wells. We demonstrate tunable spin-orbit coupling by tracking a spin-orbit coupling mediated avoided level crossing between the ground state and an excited state in magnetic field. By increasing the electron density, we observed an increase in an avoided crossing separation, ${\mathrm{\Delta }}_{SO}$. The maximum energy separation extracted is ${\mathrm{\Delta }}_{SO}\sim 100\mu \mathrm{eV}$.

A conductance zero that results from the destructive quantum interference of the electron states in quantum transport between two given sites of a molecular system persists or disappears depending on the location of an externally applied perturbation. The a priori knowledge of the perturbation site that destroys or preserves a zero is the basis of an algorithm that outlines the creation of logic gates having external perturbations as inputs and a given conductance as output. Using a graph of the possible conductance paths between the various sites, we showcase the several different scenarios that correspond to AND/OR/XOR logical functions for a given set of contacts. This setup is shown to be independent of the strength of the coupling to the leads and magnitude of the perturbation. We illustrate this approach in the case of bipartite and nonbipartite single carbon cycle molecules (fulvene and benzene) and double carbon cycle molecules (naphthalene and biphenyl).

Multicritical Ising models and their perturbations are paradigmatic models of statistical mechanics. In two space-time dimensions, these models provide a fertile testbed for investigation of numerous nonperturbative problems in strongly-interacting quantum field theories. In this work, analog superconducting quantum electronic circuit simulators are described for the realization of these multicritical Ising models. The latter arise as perturbations of the quantum sine-Gordon model with $p$-fold degenerate minima, $p=2,3,4,...$. The corresponding quantum circuits are constructed with Josephson junctions with $cos(n\varphi +{\delta }_n)$ potential with $1\le n\le p$ and ${\delta }_n\in [-\pi ,\pi ]$. The simplest case, $p=2$, corresponds to the quantum Ising model and can be realized using conventional Josephson junctions and the so-called $0\text{−}\pi$ qubits. The lattice models for the Ising and tricritical Ising models are analyzed numerically using the density matrix renormalization group technique. Evidence for the multicritical phenomena are obtained from computation of entanglement entropy of a subsystem and correlation functions of relevant lattice operators. The proposed quantum circuits provide a systematic approach for controlled numerical and experimental investigation of a wide range of nonperturbative phenomena occurring in low-dimensional quantum field theories.