We theoretically explore the Floquet generation of Majorana end modes (MEMs; both regular 0 modes and anomalous $\pi$ modes) implementing a periodic sinusoidal modulation in the chemical potential in an experimentally feasible setup based on a one-dimensional chain of magnetic impurity atoms having spin spiral configuration (out-of-plane Néel type) fabricated on the surface of a common bulk $s$-wave superconductor. We obtain a rich phase diagram in the parameter space, highlighting the possibility of generating multiple 0- and $\pi$-MEMs localized at the end of the chain. We also study the real-time evolution of these emergent MEMs, especially when they start to appear in the time domain. These MEMs are topologically characterized by employing the dynamical winding number. We observe that the existing perturbative analysis is unable to explain the numerical findings, indicating the complex mechanism behind the formation of the Floquet Shiba minigap, which is characteristically distinct from other setups, e.g., the Rashba nanowire model. We also discuss the possible experimental parameters in connection to our model. Our work paves the way to realize Floquet MEMs in a magnet-superconductor heterostructure.

At $x\approx 0.25{\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ undergoes a novel first-order transition from a fourfold symmetric double-$Q$ magnetic phase to a twofold symmetric single-$Q$ phase, which was argued to occur simultaneously with the onset of superconductivity [Böhmer et al., Nat. Commun. 6, 7911 (2015)]. Here, by applying magnetic fields up to 10 T, we investigate in more detail the interplay of superconductivity with this magnetostructural transition using a combination of high-resolution thermal-expansion and heat-capacity measurements. We find that a magnetic field suppresses the reentrance of the single-$Q$ orthorhombic phase more strongly than the superconducting transition, resulting in a splitting of the zero-field first-order transition. The suppression rate of the orthorhombic reentrance transition is stronger for out-of-plane than for in-plane fields and scales with the anisotropy of the superconducting state. These effects are captured within a phenomenological Ginzburg-Landau model, strongly suggesting that the suppression of the reentrant orthorhombic single-$Q$ phase is primarily linked to the field-induced weakening of the superconducting order. Not captured by this model is, however, a strong reduction in the orthorhombic distortion for out-of-plane fields, which deserves further theoretical attention.

We present an improved model for studying the interactions between dipole moments of water molecules confined in beryl crystals inspired by recent NMR experiments. Our model is based on a local crystal potential with dihexagonal symmetry for the rotations of water dipole moments, leading to deflection from the $ab$ hexagonal crystallographic plane. This potential shape has significant implications for dipole ordering, which is linked to the nonzero projection of the dipole moment on the hexagonal $c$ axis. To reveal the tendency toward equilibrium-ordered states, we used a variational mean-field approximation, Monte Carlo simulations, and quantum tunneling. Our analysis reveals three types of equilibrium-ordered states: a purely planar dipole order with an antiparallel arrangement in the adjacent $ab$ planes, a configuration with deflected dipole moments ordered in antiparallel directions, and a helical structure of the dipoles twisting along the $c$ axis.

Precessional solitons, referred to as magnon drop solitons driven by spin torque, are known to exist in ferromagnetic free layers of nanocontact devices. To increase the operating frequency of nanoscale devices, it has been proposed that transition metal–rare earth ferromagnetic alloys are good candidates. In addition, these alloys can exhibit angular momentum compensation providing another parameter controlling frequency. In this article magnon drop solitons in ferrimagnets are studied by a combination of analytical and numerical solutions of the Landau-Lifshitz equation for a two-sublattice model. For such magnets, two magnon modes exist, but only one of them, the mode with the lower absolute value of the precession frequency, is involved in the formation of the droplike soliton. The precession frequency for the soliton depends strongly on the angular momentum compensation parameter; it grows significantly in the vicinity of the angular momentum compensation point. These solitons are shown to be stable and a minimum sustainable compensation-dependent spin torque is obtained.

The (2+1)-dimensional continuum Lifshitz theory of a free compact scalar field plays a prominent role in a variety of quantum systems in condensed matter physics and high energy physics. It is known that in compact space, it has an infinite ground-state degeneracy. In order to understand this theory better, we consider two candidate lattice regularizations of it using the modified Villain formalism. We show that these two lattice theories have significantly different global symmetries (including a dipole global symmetry), anomalies, ground-state degeneracies, and dualities. In particular, one of them is self-dual. Given these theories and their global symmetries, we can couple them to corresponding gauge theories. These are two different $\text{U}\left(1\right)$ tensor gauge theories. The resulting models have excitations with restricted mobility, i.e., fractons. Finally, we give an exact lattice realization of the fracton and lineon-elasticity dualities for the Lifshitz theory, and scalar and vector charge gauge theories.

Rare Earth (RE) ion doped nanomaterials are promising candidates for a range of quantum technology applications. Among RE ions, the so-called Kramers' ions possess spin transitions in the GHz range at low magnetic fields, which allows for high-bandwidth multimode quantum storage, fast qubit operations as well as interfacing with superconducting circuits. They also present relevant optical transitions in the infrared. In particular, ${\mathrm{Er}}^{3+}$ has an optical transition in the telecom band, while ${\mathrm{Nd}}^{3+}$ presents a high-emission-rate transition close to 890 nm. In this paper, we measure spectroscopic properties that are of relevance to using these materials in quantum technology applications. We find the inhomogeneous linewidth to be 10.7 GHz for ${\mathrm{Er}}^{3+}$ and 8.2 GHz for ${\mathrm{Nd}}^{3+}$, and the excited state lifetime $T_1$ to be 13.68 ms for ${\mathrm{Er}}^{3+}$ and $540\mu \mathrm{s}$ for ${\mathrm{Nd}}^{3+}$. We study the dependence of homogeneous linewidth on temperature for both samples, with the narrowest linewidth being 379 kHz ($T_2=839$ ns) for ${\mathrm{Er}}^{3+}$ measured at 3 K, and 62 kHz ($T_2=5.14\mu \mathrm{s}$) for ${\mathrm{Nd}}^{3+}$ measured at 1.6 K. Further, we investigate time-dependent homogeneous linewidth broadening due to spectral diffusion and the dependence of the homogeneous linewidth on magnetic field to get additional clarity of mechanisms that can influence the coherence time. In light of our results, we discuss two applications: single qubit-state readout and a Fourier-limited single photon source.

The Hatano-Nelson model is one of the most prototypical non-Hermitian models that exhibit the intrinsic non-Hermitian topological phases and the concomitant skin effect. These phenomena unique to non-Hermitian topological systems originate from the Galilean transformation. Here, we extend such an idea to a broader range of systems based on an imaginary boost deformation and identify the corresponding energy-twisted boundary conditions. This imaginary boost deformation complexifies spectral parameters of integrable models and can be implemented by the coordinate Bethe ansatz. We apply the imaginary boost deformation to several typical integrable models, including free fermions, the Calogero-Sutherland model, and the XXZ model. We find the complex-spectral winding in free fermion models under the periodic boundary conditions and the non-Hermitian skin effect under the open boundary conditions. The interaction effect is also shown in the two-particle spectrum of the XXZ model.

The twisted hexagonal boron nitride (hBN) bilayer has demonstrated exceptional properties, particularly the existence of electronic flat bands without needing a magic angle, suggesting strong excitonic effects. Therefore, a systematic approach is presented to study the excitonic properties of twisted ${\mathrm{AA}}^{\prime }$ hBN using the Bethe-Salpeter equation based on single-particle tight-binding wave functions. These are provided by a one-particle Hamiltonian that is parameterized to describe the main features of ab initio calculations. The Bethe-Salpeter equation is then solved in the so-called excitonic transition representation, which significantly reduces the problem dimensionality by exploiting the system's symmetries. Consequently, the excitonic energies and the excitonic wave functions are obtained from the direct diagonalization of the effective two-particle Hamiltonian of the Bethe-Salpeter equation. We have studied rotation angles as low as $7.{34}^{\circ }$. The model allows the study of commensurate and incommensurate moiré patterns at much lower computational cost than the ab initio version of the Bethe-Salpeter equation. Here, using the model and effective screening of the Keldysh type, we could obtain the absorption spectra and characterize the excitonic properties of twisted hBN bilayers for different rotation angles, demonstrating how this property affects the excitonic energies and localizations of their wave functions.

We study the open quantum dynamics of the Sachdev-Ye-Kitaev (SYK) model described by the Lindblad master equation, where the SYK model is coupled to Markovian reservoirs with jump operators that are either linear or quadratic in the Majorana fermion operators. Of particular interest for us is the time evolution of the dissipative form factor, which quantifies the average overlap between the initial and time-evolved density matrices as an open quantum generalization of the Loschmidt echo. We find that the dissipative form factor exhibits dynamical quantum phase transitions. We analytically demonstrate a discontinuous dynamical phase transition in the limit of a large number of fermion flavors, which is formally akin to the thermal phase transition in the two-coupled SYK model between the black-hole and wormhole phases. We also find continuous dynamical phase transitions that do not have counterparts in the two-coupled SYK model. While the phase transitions are sharp in the limit of a large number of fermion flavors, their qualitative signatures are present even for a finite number of fermion flavors, as we show numerically.

We investigate the energy band structure and energy levels of a heterojunction composed of two antiferromagnetic graphene nanoflakes with opposite in-plane antiferromagnetic orderings, in which the modified Kane-Mele model is employed. Before forming an antiferromagnetic graphene heterojunction, the energy gap of helical edge states in each isolated graphene nanoflake are opened by the antiferromagnetic ordering and there is no the in-gap corner state. We find that when two opposite antiferromagnetic graphenes are coupled to form a heterojunction nanoflake, topologically protected zero-dimensional in-gap states can be induced. In addition, we demonstrate that the in-gap states locate at the end of the interface and are robust against magnetic disorder, Anderson disorder, and interfacial magnetic defects. The position and number of the in-gap interface end states in the heterojunction sample can be precise quantum controlled.

It has long been known that quantum particles moving in a periodic lattice and subject to a constant force field undergo an oscillatory motion that is referred to as Bloch oscillations (BOs). However, it is also known that, under quite general conditions, a biased mesoscopic system connected to leads should settle in a steady-state regime characterized by a constant electric current (described by the Landauer formula). These two observations naturally lead to the question: do BOs survive in some manner in mesoscopic devices in the presence of a constant electric field? To answer this question, we explore the interface between these two regimes in two-terminal devices and demonstrate theoretically that BOs can actually be observed in such apparatuses as a transient phenomenon, which relaxes for long times to a steady-state current that agrees with the Landauer formula. Furthermore, we also combine analytical and numerical time-evolution results for a one-dimensional tight-binding model of a biased two-terminal mesoscopic system, in order to characterize the decay times of the transient BOs and establish the conditions under which they can occur.

We present a general computational framework to investigate ground-state properties of quantum spin models on infinite two-dimensional lattices using automatic differentiation-based gradient optimization of infinite projected entangled-pair states. The approach exploits the variational uniform matrix product states to contract infinite tensor networks with unit cell structure and incorporates automatic differentiation to optimize the local tensors. We applied this framework to the Kitaev-type model, which involves complex interactions and competing ground states. To evaluate the accuracy of this method, we compared the results with exact solutions for the Kitaev model and found that it has a better agreement for various observables compared to previous tensor network calculations based on imaginary-time projection. Additionally, by finding out the ground state with lower variational energy compared to previous studies, we provided convincing evidence for the existence of nematic paramagnetic phases and 18-site configuration in the phase diagram of the $K\text{−}\mathrm{\Gamma }$ model. Furthermore, in the case of the realistic $K\text{−}J\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$ model for the Kitaev material $\alpha \text{−}{\mathrm{RuCl}}_3$, we discovered a noncollinear zigzag ground state. Lastly, we also find that the strength of the critical out-of-plane magnetic field that suppresses such a zigzag state has a lower transition field value than the previous finite-cylinder calculations. The framework is versatile and will be useful for a quick scan of phase diagrams for a broad class of quantum spin models.

We study the interplay of two distinct non-Hermitian parameters: directional coupling and on-site gain and loss, together with topology, in coupled one-dimensional non-Hermitian Su-Schrieffer-Heeger (SSH) chains. The SSH model represents one of the simplest two-band models that features boundary-localized topological modes. Our study shows how the hybridization between two topological modes can lead to a striking spectral feature of non-Hermitian systems, namely, exceptional points (EPs). We reveal the existence of an EP as a singularity in the parameter space of non-Hermitian couplings carrying a half-integer topological charge. We also demonstrate two different localization behaviors observed in the bulk and hybridized modes. While the bulk states and individual topological modes remain localized at the boundaries due to the skin effect, the competition between the constituent non-Hermitian parameters can overcome the strength of the skin effect and lead to the complete delocalization of these hybridized modes. We obtain explicit analytic solutions for the eigenfunctions and the eigenenergies of the hybridized modes, which exactly match the numerical results and successfully reveal the underlying cause of delocalization and the emergence of EPs.

We demonstrate field-induced single-ion magnetic anisotropy resulting from the multiorbital Kondo effect on the diluted ytterbium alloy $({\mathrm{Lu}}_{1-x}{\mathrm{Yb}}_x){\mathrm{Rh}}_2{\mathrm{Zn}}_{20}$. Single-ion anisotropic metamagnetic behavior is revealed in low-temperature regions where the local Fermi-liquid state is formed. Specific heat, low-field magnetic susceptibility, and resistivity indicate reproduction of the ground-state properties by the $\mathrm{SU}(N=8)$ Kondo model with a relatively large $c\text{−}f$ hybridization of $T_{\mathrm{K}}=60.9\mathrm{K}$. Dynamical susceptibility measurements on ${\mathrm{YbRh}}_2{\mathrm{Zn}}_{20}$ support realizing the multiorbital Kondo ground state in $({\mathrm{Lu}}_{1-x}{\mathrm{Yb}}_x){\mathrm{Rh}}_2{\mathrm{Zn}}_{20}$. The single-ion magnetic anisotropy becomes evident above $\sim 5\mathrm{T}$, which is lower than the isotropic Kondo crossover field of 22.7 T, verifying blurred low-lying crystal field states through the multiorbital Kondo effect.

We analyze the possible dynamical chiral symmetry-breaking patterns taking place within Weyl type of materials. Here, these systems are modeled by the $(2+1)$-dimensional Gross-Neveu model with a tilt in the Dirac cone. The optimized perturbation theory (OPT) is employed in order to evaluate the effective potential at finite temperatures and chemical potentials beyond the traditional large-$N$ limit. The nonperturbative finite-$N$ corrections generated by the OPT method and its associated variational procedure show that a first-order phase transition boundary, missed at large $N$, exists in the regime of low temperatures and large chemical potentials. This result, which represents our main finding, implies that one should hit a region of mixed phases when exploring the low-temperature range. The associated first-order transition line, which starts at $T=0$, terminates at a tricritical point such that the transitions taking place at high $T$ are of the second kind. In particular, we discuss how the tilt in the Dirac cone affects the position of the tricritical point as well as the values of critical temperature and coexistence chemical potential among other quantities. Some experimental implications and predictions are also briefly discussed.

Nonlinear optical responses are becoming increasingly relevant for characterizing the symmetries and quantum geometry of electronic phases in materials. Here, we develop an expanded diagrammatic scheme for calculating spatially dispersive corrections to nonlinear optical conductivities, which we expect to enhance or even dominate even-order responses in materials of recent interest. Building upon previous work that enforces gauge invariance of spatially uniform nonlinear optical responses, we review the cancellation of diagrams required to ensure the equivalence between velocity gauge and length gauge formulations, and provide a simple vertex rule for extending optical responses to first order in the light wave vector $\mathbf{q}$. We then demonstrate the method with calculations on a prototypical centrosymmetric model where spatial dispersion admits anomalous second-harmonic generation, a response that is symmetry forbidden under the dipole approximation.

Plasma echo is a dramatic manifestation of plasma damping process reversibility. In this paper we calculate temporal and spatial plasma echoes in graphene in the acoustic plasmon regime when echoes dominate over plasmon emission. We show an extremely strong spatial echo response and discuss how electron collisions reduce the echo. We also discuss differences between various electron dispersions and differences between semiclassical and quantum model of echoes.

A recent work by Zhao et al. [Nature (London) 616, 61 (2023)] reported the realization of a synthetic Kondo lattice in a gate-tunable Moiré transition metal dichalcogenide bilayer system. The observation of a Kondo lattice is supported by a plateau (or dip, depending on filling) in the temperature dependence of the resistivity $\rho \left(T\right)$ around $T^{*}\sim 40$ K, which is interpreted as the Kondo temperature scale, and an apparent enhancement of carrier mass extracted from the low-temperature resistivity data, indicating the emergence of “heavy fermions.” The latter observation is crucially based on the assumption that the primary resistive scattering mechanism is electron-electron scattering in the underlying Fermi liquid. In this work, we analyze the experimental data under the assumption that the primary resistive scattering mechanism is not electron-electron scattering, but Coulomb scattering by random quenched charged impurities and phonon scattering. We show that a combination of impurity and phonon scattering is a plausible alternative explanation for the observed resistivity that can describe the key features of the experimental data, even if no Kondo lattice has formed, indicating that further theoretical and experimental work is needed to conclusively verify the formation of a Kondo lattice in the work by Zhao et al.