We perform classical Monte Carlo and stochastic Landau-Lifshitz-Gilbert simulations to study the temperature-dependent magnetism of the kagome antiferromagnet Weyl metal ${\text{Mn}}_3\text{Ge}$, and we find that a long-range chiral order sets in at a transition temperature well below the Néel temperature ($T_N$). Based on the crystalline symmetries imposed by the chiral magnetic order, we argue for the presence of multiple isoenergetic Weyl nodes (nodes that are at the same energy and with a congruent Fermi surface around them) near the chemical potential. Using the semiclassical Boltzmann equations, we show that the combined contribution to the net longitudinal magnetoconductance (LMC) and the planar Hall conductance (PHC) from tilted Weyl nodes can lead to signatures that are qualitatively distinct from that of a single pair of Weyl nodes. In particular, we show that magnetic orders with different chiralities can give rise to different periods in LMC and PHC as a function of the in-plane magnetic field direction. This is ultimately related to differences in the symmetry-imposed constraints on the Weyl nodes.

We investigate the intraband nonlinear dynamics in doped bilayer graphene in the presence of strong, linearly polarized, in-plane terahertz fields. We perform degenerate pump-probe experiments with 3.4-THz fields on doped bilayer graphene at low temperature ($12\mathrm{K}$) and find that when the pump is copolarized with the probe beam, the differential pump-probe signal is almost double that found in the cross-polarized case. We show that the origin of this pump-induced anisotropy is the difference in the average electron effective mass in the probe direction when carriers are displaced in $k$ space by the pump either parallel or perpendicular to the direction of the probe polarization. We model the system using both a simple semiclassical model and a Boltzmann equation simulation of the electron dynamics with phenomenological scattering and find good qualitative agreement with experimental results.

We study the thermal transport through a Majorana island connected to multiple external quantum wires. In the presence of a large charging energy, we find that the Wiedemann-Franz law is nontrivially violated at low temperature, contrarily to what happens for the overscreened Kondo effect and for nontopological junctions. For three wires, we find that the Lorenz ratio is rescaled by a universal factor $2/3$ and we show that this behavior is due to the presence of localized Majorana modes on the island.

Unconventional computing schemes based on bistable systems (“Ising spins”) may supersede conventional computing paradigms. The “magnon parametron” is an Ising spin that forms as a result of parametric excitation of a ferromagnetic particle by microwaves beyond a certain threshold. This Ising spin becomes unstable at a second threshold power at which a high-frequency telegraph noise emerges. We explain the experimentally observed stochastic switching (“p-bit” characteristics) at room temperature by the Suhl instability of the uniform magnetization precession.

We calculate the frequency-dependent admittance of a phase-biased Josephson junction spanning a magnetic impurity or a spinful Coulomb-blockaded quantum dot. The local magnetic moment gives rise to Yu-Shiba-Rusinov bound states, which govern the subgap absorption as well as the inductive response. We model the system by a superconducting spin-polarized exchange-cotunnel junction and calculate the linear current response to an ac bias voltage, including its dependence on phase bias as well as particle-hole and source-drain coupling asymmetry. The corresponding inductive admittance is analyzed and compared to results of a zero bandwidth, as well as an infinite-gap approximation to the superconducting Anderson model. All three approaches capture the interaction-induced $\mathbf{0}–\mathit{\pi }$ transition, which is reflected as a discontinuity in the adiabatic inductive response.

Combined resistance noise and muon-spin relaxation ($\mu \mathrm{SR}$) measurements of the ferromagnetic semiconductor ${\mathrm{HgCr}}_2{\mathrm{Se}}_4$ suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency $1/f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95–98\mathrm{K}$ in the vicinity of the magnetic ordering temperature $T_{\mathrm{C}}\sim 105–107\mathrm{K}$. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$ noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in $\mu \mathrm{SR}$ and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.

The Majorana edge states of the Kitaev chain model have attracted extensive attention on their stability and experimental realization. One of the interesting aspects is finding the exotic proximity effect, which guarantees the presence of the Majorana modes, further enables efficient braidings between them. In this paper, we explore the superconducting proximity effect for quasiperiodic quantum wires and discuss how quasiperiodic patterns affect the stability of the Majorana modes. Considering the Kitaev chain model of the one-dimensional quasiperiodic system, we discuss the pattern-dependent proximity effects. First, we argue that the presence of quasiperiodic hoppings energetically induces the $p$-wave pairing also to be quasiperiodic rather than uniform pairing. More interestingly, when the normal metallic wire is adjacent to the quasiperiodic superconducting wire, we have found that the Majorana edge modes are transferred to the edge of the normal metallic side with enhanced stability. Finally, we discover the proximity effect on the strengths of the quasiperiodicities with a general power-law relationship, whose power depends on the tiling pattern. Our results show how quasiperiodic patterns play a crucial role in the Kitaev chain and the stabilization of the Majorana mode.

Owing to the advances in computational techniques and the increase in computational power, atomistic simulations of materials can simulate large systems with higher accuracy. Complex phenomena can be observed in such state-of-the-art atomistic simulations. However, it has become increasingly difficult to understand what is actually happening and mechanisms, for example, in molecular dynamics (MD) simulations. We propose an unsupervised machine learning method to analyze the local structure around a target atom. The proposed method, which uses the two-step locality preserving projections (TS-LPP), can find a low-dimensional space wherein the distributions of data points for each atom or groups of atoms can be properly captured. We demonstrate that the method is effective for analyzing the MD simulations of crystalline, liquid, and amorphous states and the melt-quench process from the perspective of local structures. The proposed method is demonstrated on a silicon single-component system, a silicon-germanium binary system, and a copper single-component system.

The topological Hall effect (THE) is an emergent magnetotransport phenomena in noncollinear itinerant magnets, and has been extensively studied in magnetic materials hosting skyrmion lattices and geometrically frustrated lattices. To explore THE in materials with new nontrivial spin textures, we investigate the anisotropic Hall effect in a longitudinal conical-ordered magnet ${\mathrm{CeMn}}_2{\mathrm{Ge}}_2$ crystallized in ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure, which has a spiral magnetic propagation vector along the $c$ axis and a helical component in the $ab$ plane. Magnetic susceptibility reveals it undergoes two magnetic transitions at ∼394 and ∼318 K; the latter corresponds to the antiferromagnetic to conical spin order transition in the magnetic Mn lattices. For applied field $\left({\mu }_{0}H\right)$ along the $c$ axis, ${\mathrm{CeMn}}_2{\mathrm{Ge}}_2$ exhibits conventional anomalous Hall effect with large intrinsic Hall conductivity ${\sigma }_{xy}^{\mathrm{int}}\sim 276{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ but no THE responses. A large THE is realized as ${\mu }_{0}H$ is oriented within the $ab$ plane due to a field-induced noncoplanar spin texture with scalar spin chirality. Moreover, room temperature (RT) angle-dependent Hall effect measurements reveal the THE emerges when the field is applied nonparallel to the $c$ axis and it reaches maximal value for ${\mu }_{0}H$ when it is tilted close to the $ab$ plane with a canting angle of ∼15°, at which large topological Hall conductivity of $\sim 43{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ is detected. Our results demonstrate a RT anisotropic THE and its tunability by the application of a tilted magnetic field in conical-ordered magnets, making a step forward for designing spintronic devices based on THE.

Topological materials, possessing spin-momentum locked topological surface states (TSS), have attracted much interest due to their potential applications in spintronics. α-phase Sn (α-Sn), being one of them, displays enriched topological phases via band-gap engineering through a strain or confinement effect. In this work, we investigated the band evolution of in-plane compressively strained α-Sn(001) thin films on InSb(001) in a wide range of thickness from 3 bilayers (BL) to 370 BL by combining angle-resolved photoemission spectra and first-principles calculations. Gapped surface states evolved to a linearly dispersive TSS at a critical thickness of 6 BL, indicating that the system undergoes a phase transition from topologically trivial to nontrivial. For films thicker than 30 BL, additional Rashba-like surface states (RSS) were identified. These RSS served as preformed TSS in another strain-induced topological phase transition. In thick films, 370-BL α-Sn(001), so as to preclude the confinement effect in thin films, our results were consistent with a Dirac semimetal phase with Dirac nodes located along $\mathrm{\Gamma }–Z$. This thickness-dependent band-structure study deepens our understanding of topological phase transitions and the evolution of Dirac states. Furthermore, the coexistence of TSS and RSS in a Dirac semimetal α-Sn might significantly enhance the potential for spintronic applications.

Flat bands are of extreme interest in a broad spectrum of fields since, given their high degeneracy, a small perturbation introduced in the system is able to push the ground state in the direction of an ordered phase of interest. Hence, the flat-band engineering in real materials attracts huge attention. However, manufacturing a flat band represents a difficult task because its appearance in a real system is connected to rigid mathematical conditions relating a part of Hamiltonian parameters. Consequently, whenever a flat band is to be manufactured, these Hamiltonian parameters must be tuned exactly to the values fixed by these rigid mathematical conditions. Here we demonstrate that taking the many-body spin-orbit interaction into account, which can be continuously tuned, e.g., by external electric fields, these rigid mathematical conditions can be substantially relaxed. Consequently, we show that a $\sim 20\%–30\%$ variation in the Hamiltonian parameters rigidly fixed by the flat-band conditions can also lead to flat bands in the same, or in a bit displaced, position on the energy axis. This percentage can even increase to $\sim 80\%$ in the presence of an external magnetic field. This study is made for the case of conducting polymers. These systems are relevant not only because they have broad application possibilities, but also because they can be used to present the mathematical background of the flat-band conditions in full generality, in a concise, clear, and understandable manner applicable everywhere in itinerant systems.

We study the effective action of hydrostatic response to torsion in the absence of spin connections in gapped $(2+1)$-dimensional topological phases at low temperatures. In previous studies, a torsional Chern-Simons term with a temperature-squared ($T^2$) coefficient was proposed as an alternative action to describe the thermal Hall effect with the idea of balancing the diffusion of heat by a torsional field. However, the question remains whether this action leads to a local bulk thermal response that is not suppressed by the gap. In our hydrostatic effective action, we show that the $T^2$ bulk term is invariant under variations up to boundary terms considering the geometry dependence of local temperature, which precisely describes the edge thermal current. Furthermore, there are no local boundary diffeomorphism anomalies or bulk inflow thermal currents at equilibrium, and also there is no edge-to-edge adiabatic thermal current pumping upon changing the gravitational background. These results are consistent with exponentially suppressed thermal current for gapped phases.

We propose an improved tensor renormalization-group (TRG) algorithm, the bond-weighted TRG (BTRG). In BTRG, we generalize the conventional TRG by introducing bond weights on the edges of the tensor network. We show that BTRG outperforms the conventional TRG and the higher-order tensor renormalization group with the same bond dimension, whereas its computation time is almost the same as that of TRG. Furthermore, BTRG can have nontrivial fixed-point tensors at an optimal hyperparameter. We demonstrate that the singular value spectrum obtained by BTRG is invariant under the renormalization procedure in the case of the two-dimensional Ising model at the critical point. This property indicates that BTRG performs the tensor contraction with high accuracy whereas keeping the scale-invariant structure of tensors.

The edges of quantum Hall phases give rise to a multitude of exotic modes supporting quasiparticles of different values of charge and quantum statistics. Among these are neutralons (chargeless anyons with semion statistics), which were found to be ubiquitous in fractional quantum Hall matter. Studying and manipulating the neutral sector is an intriguing and interesting challenge, all the more so since these particles are accessible experimentally. Here, we address the limit of strongly interacting neutralons giving rise to neutralon superconductivity, where pairing is replaced by a quarteting mechanism. We discuss several manifestations of this effect, realizable in existing experimental platforms. Furthermore, this superconducting gapping mechanism may be exploited to facilitate the observation of interference of the accompanying charged anyons.

Cubic boron nitride (cBN) is an ultrawide band gap, superhard material with potential for extreme-temperature and -pressure applications. A proof-of-principle p-n junction using cBN was demonstrated almost three decades ago. However, to date, there remain two unresolved challenges that prevent its practical use in technologies: (i) it is difficult to produce high-quality cBN films and (ii) it is difficult to controllably $n$ and $p$ dope its matrix. In this theoretical work, we study the reasons for doping limitations, which is an acute issue in realizing cBN-based electronics. In particular, we find that different unintentionally present intrinsic and extrinsic defects act as compensating defects and/or introduce trap states. In turn, the presence of these defects and their complexes affect the incorporation, as well as the electronic structure properties, of shallow dopants (silicon and beryllium), which are introduced intentionally to n- and p-doped cBN. Our analysis of doping-limitations provides a path toward finding solutions for controllably $n$ and $p$ doping cBN.

The spin-orbital-lattice coupling in spinel ${\mathrm{MnV}}_2{\mathrm{O}}_4$ single crystal has been investigated by static field magnetization (field sweep rate of 50 Oe/s), pulsed field magnetization (field sweep rate $\sim {10}^5$ kOe/s), dielectric permittivity, and magnetostriction. Experimental results show that the magnetization and magnetostriction are related to the field sweep rate. The analysis of the magnetization, dielectric permittivity and magnetostriction in static magnetic field shows that the collinear (CL) and non-CL (NCL) spin configurations coexist between $T_N\sim 56$ K and $T^{*}\sim 52$ K, and the NCL spin configuration can be induced by applied magnetic field. In particular, the magnetization and magnetostriction steps associated with CL and NCL transitions observed in the static magnetic field becomes smooth in the pulsed magnetic field, indicating the slow relaxation characteristic of these phase transitions. Based on the orbital state of ${\mathrm{MnV}}_2{\mathrm{O}}_4$ and the Kugel-Khomskii model, we propose an quasiadiabatic spin-orbital correlation mechanism in which the interaction between the spin and the $t_{2g}$ orbital of ${\mathrm{V}}^{3+}$ ions is rather weak; this weak interaction results in the magnetic properties of ${\mathrm{MnV}}_2{\mathrm{O}}_4$ is related to the field sweep rate.

We conclusively show that a pseudogap state can arise at $T>T_c$, for reasonable pairing interaction strength, from order parameter fluctuations in a two-dimensional minimal model of $d$-wave superconductivity. The occurrence of the pseudogap requires neither strong correlation nor the presence of competing order. We study a model with attractive nearest-neighbor interaction and establish our result using a combination of a cluster-based Monte Carlo method for the order parameter field and a twisted-boundary scheme to compute the momentum-resolved spectral function. Apart from a dip in the density of states that characterizes the pseudogap, the momentum and frequency resolution on our effective lattice size $\sim 160\times 160$ allows two major conclusions: (i) at $T<T_c$, despite the presence of thermal phase fluctuations, the superconductor has only nodal Fermi points while all non-nodal points on the normal state Fermi surface show a two-peak spectral function with a dip at $\omega =0$, and (ii) for $T>T_c$, the Fermi points develop into arcs, characterized by a single quasiparticle peak, and the arcs connect up to recover the normal state Fermi surface at a temperature $T^{*}>T_c$. We show the variation of $T_c$ and $T^{*}$ with coupling strength and provide detailed spectral results at a coupling where $T^{*}\sim 1.5T_c$.