Two-dimensional (2D) materials naturally form moiré patterns with other crystalline layers, such as other 2D materials or the surface of a substrate. These patterns add a nanoscale characteristic length in the form of a superlattice: the moiré wavelength. Understanding the origin and characteristics of these patterns is crucial to design and interpret moiré-induced physical properties. Here, we use a mixed continuum mechanics plus atomistic model to study two experimentally relevant epitaxial 2D materials—graphene on Ir(111) and ${\mathrm{MoS}}_2$ on Au(111)—under extrinsic and intrinsic strains. We consider three different scenarios that substantially affect the lattice constant of the 2D materials, the wavelength, and the corrugation of the moiré pattern. (i) Under the influence of the interaction with the substrate, bending energy produces nontrivial variations of the moiré properties, even when the strain is small. (ii) When locked on a progressively strained substrate via the valleys of the moiré, the membranes' nanorippling amplitude goes through several jumps related to relatively smaller jumps in the interatomic distance of the 2D materials. (iii) Finally, with an increasing zero-deformation value of this interatomic distance (possibly controllable with temperature or illumination in experiments), the moiré wavelength can either increase or decrease.

We present a numerically exact study of self-trapped (also referred to as soliton) states of electrons that form in materials with strong quadratic coupling to the phonon coordinates. Previous studies failed to observe predictions based on the variational approach in continuum space because soliton states form only when system parameters are taken to the extreme limit. At the variational level, we establish that finite-radius solitons emerge through the weak first-order transition as the coupling strength is increased, and subsequently collapse to the single-site state through a strong first-order transition. Both transitions transform into smooth crossovers between the light and heavy polaron states in the full quantum treatment. The most surprising effect not observed in other polaron models is the nonmonotonic dependence of the soliton effective mass and the residue at strong coupling.

We perform the energy minimization of the paired composite fermion (CF) wave functions, proposed by Möller and Simon (MS) [Phys. Rev. B 77, 075319 (2008)] and extended by Yutushui and Mross (YM) [Phys. Rev. B 102, 195153 (2020)], where the energy is minimized by varying the CF pairing function, in the case of an approximate model of the Coulomb interaction in the second Landau level for pairing channels $\ell =-1,3,1$, which are expected to be in the Pfaffian, anti-Pfaffian, and particle-hole symmetric (PH) Pfaffian phases, respectively. It is found that the energy of the $\ell =-1$ MS wave function can be reduced substantially below that of the Moore-Read wave function at small system sizes; however, in the $\ell =3$ case the energy cannot be reduced much below that of the YM trial wave function. Nonetheless, both our optimized and unoptimized wave functions with $\ell =-1,3$ extrapolate to roughly the same energy per particle in the thermodynamic limit. For the $\ell =1$ case, the optimization makes no qualitative difference and these PH-Pfaffian wave functions are still energetically unfavorable. The effective CF pairing is analyzed in the resulting wave functions, where the effective pairing for the $\ell =-1,3$ channels is found to be well approximated by a weak-pairing BCS ansatz and the $\ell =1$ wave functions show no sign of emergent CF pairing.

Efficient generation and manipulation of spin signals in a given material without invoking external magnetism remain one of the challenges in spintronics. The spin Hall effect (SHE) and Rashba-Edelstein effect (REE) are well-known mechanisms to electrically generate spin accumulation in materials with strong spin-orbit coupling (SOC), but the exact role of the strength and type of SOC, especially in crystals with low symmetry, has yet to be explained. In this study, we investigate REE in three different families of nonmagnetic chiral materials, elemental semiconductors (Te and Se), metal silicides (FeSi and OsSi), and semimetal disilicides (${\mathrm{TaSi}}_2$ and ${\mathrm{NbSi}}_2$), using an approach based on density functional theory. By analyzing spin textures across the full Brillouin zones and comparing them with REE magnitudes calculated as a function of chemical potential, we link specific features in the electronic structure with the efficiency of the induced spin accumulation. Our findings show that magnitudes of REE can be increased by (i) the presence of one particular type of spin-orbit field, for example, Weyl-type SOC that yields a radial spin texture, (ii) high spin polarization of bands along one specific crystallographic direction, and (iii) low band velocities. By comparing materials possessing the same crystal structures but different strengths of SOC, we conclude that larger SOC may indirectly contribute to the enhancement of REE. It yields greater spin splitting of bands along specific crystallographic directions, which prevents canceling the contributions from the oppositely spin-polarized bands over wider energy regions and helps maintain larger REE magnitudes. Surprisingly, however, for some materials, the velocity of electronic states plays an even more important role in regulating the REE magnitudes than SOC. Additionally, these magnitudes are strongly influenced by changes in band topology. We believe that our results will be useful for designing spintronics devices and may aid further computational studies searching for efficient REE in materials with different symmetries and SOC strengths.

Transition out of a topological phase is typically characterized by discontinuous changes in topological invariants along with bulk gap closings. However, as a clean system is geometrically punctured, it is natural to ask the fate of an underlying topological phase. To understand this physics we introduce and study both short- and long-ranged toy models where a one-dimensional topological phase is subjected to bond percolation protocols. We find that non-trivial boundary phenomena follow competing energy scales even while the global topological response is governed via geometrical properties of the percolated lattice. Using numerical, analytical, and appropriate mean-field studies we uncover the rich phenomenology and the various crossover regimes of these systems. In particular, we discuss the emergence of the “fractured topological region” where an overall trivial system contains a macroscopic number of topological clusters. Our study shows the interesting physics that can arise from an interplay of geometrical disorder within a topological phase.

Topological magnets host two sets of gauge fields: that of native Maxwell electromagnetism, owing to the magnetic dipole moment of its constituent microscopic moments, and that of the emergent gauge theory describing the topological phase. Here, we show that in quantum spin ice, the emergent magnetic charges of the latter carry native electric charge of the former. We both provide a general symmetry-based analysis underpinning this result, and discuss a microscopic mechanism which binds a native electric charge to the emergent magnetic one. This has important ramifications. First and foremost, an applied electric field gives rise to an emergent magnetic field. This in turn exerts an “inverted” Lorentz force on moving emergent electric/native magnetic charges. This can be probed via what we term a magnetic Nernst effect: Applying an electric field perpendicular to a temperature gradient yields a magnetization perpendicular to both. Finally, and importantly as a further potential experimental signature, a thermal gas of emergent magnetic charges will make an activated contribution to the optical conductivity at low temperatures.

Motivated by recent experiments observing the nonlinear planar Hall effect (NPHE) in nonmagnetic topological materials, we employ the density matrix method to consider all the intraband and interband transitions. This gives a deeper insight for the different mechanisms of NPHE on the same footing beyond the semiclassical theory. Under broken time-reversal symmetry, besides the usual Berry curvature dipole (BCD) contribution, there exists the quantum metric (QM) induced NPHE, which includes the intrinsic and extrinsic components, and exists even within the band gap. This QM term extends the Berry-connection polarizability (BCP) theory which captures only the intrinsic contribution and cannot be applied to the case with finite scattering and nonzero frequency. Moreover, we reveal that the underlying physics of BCP originates essentially from the combination of three interband transitions (injection, shift, and anomalous), very differently from the BCD which is only contributed by an anomalous mechanism. We compare different mechanisms by calculating the NPHE on the surface states of topological insulators and find that the NPHE from different mechanisms exhibits different dependence on the in-plane magnetic field and the chemical potential. Our theory provides an alternative perspective to understand the complicated lineshapes of the NPHE observed near the Dirac point.

The Rashba-Edelstein effect (REE), which generates interfacial spin polarization and subsequent spin current, is a compelling spin-charge conversion mechanism for spintronic applications, since it's not limited by the elemental spin-orbit couplings. In this work we demonstrate REE at Pt/ferroelectric interfaces by showing a positive correlation between polarization and effective spin Hall angle in the recently elucidated spin Hall Hanle effects (SHHE), in which a Larmor precession of spin polarization in a diffusion process from the interface manifest as magnetoresistance and Hall effect. We show that REE leads to a large enhancement of the effective spin Hall angle of ferroelectric interface $\mathrm{Pt}/\mathrm{h}\text{−}\mathrm{LuFe}{\mathrm{O}}_3$ compared with that of $\mathrm{Pt}/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, without obvious differences in the spin relaxation time. Modeling using SHHEs involving REE as an additional source of interfacial polarization suggests that REE can lead to an interfacial spin Hall angle ($0.4\pm 0.1$) in $\mathrm{Pt}/\mathrm{h}\text{−}\mathrm{LuFe}{\mathrm{O}}_3$ that is one order of magnitude larger than the bulk value of Pt. Our results demonstrate that a ferroelectric interface can produce large spin-charge conversion and that SHHEs are a sensitive tool for characterizing interfacial spin-transport properties.

Generating and probing the magnon squeezing is an important challenge in the field of quantum magnonics. In this work, we propose a cavity magnonics setup with an easy-axis ferromagnet to address this challenge. To this end, we first establish a mechanism for the generation of magnon squeezing in the easy-axis ferromagnet and show that the magnon squeezing can be critically enhanced by tuning an external magnetic field near the Ising phase transition point. When the magnet is coupled to the cavity field, the effective cavity-magnon interaction becomes proportional to the magnon squeezing, allowing one to enhance the cavity-magnon coupling strength using a static field. We demonstrate that the magnon squeezing can be probed by measuring the frequency shift of the cavity field. Moreover, a magnonic superradiant phase transition can be observed in our setup by tuning the static magnetic field, overcoming the challenge that the magnetic interaction between the cavity and the magnet is typically too weak to drive the superradiant transition. Our work paves the way to develop unique capabilities of cavity magnonics that goe beyond the conventional cavity QED physics by harnessing the intrinsic property of a magnet.

Optically active defects in solid-state systems have many applications in quantum information and sensing. However, unlike free atoms, which have fixed optical transition frequencies, the inhomogeneous broadening of the transitions in solid-state environments limit their use as identical scatterers for such applications. Here we show that crystals of argon and neon prepared in a closed-cycle cryostat doped with thulium atoms at cryogenic temperatures are an exception. High resolution absorption and emission spectroscopy show that the 1140 nm magnetic dipole transition is split into multiple components. The origin of this splitting is likely a combination of different classes of trapping sites, crystal field effects within each site, and hyperfine interactions. The individual lines have ensemble widths as small as $0.6\text{GHz}$, which temperature dependence and pump-probe spectroscopy indicate is likely a homogeneous effect, suggesting inhomogeneity is well below the GHz scale.

Repeated quantum measurements can generate effective new nonequilibrium dynamics in matter. Here we combine such a measurement driven system with disorder. In particular, we investigate the diffusive behavior in the system and the effect of various types of disorder on the measurement induced chiral transport protocol. We begin by characterizing the diffusive behavior produced by the measurements themselves in a clean system. We then examine the edge flow of particles per measurement cycle for three different types of disorder: site dilution, lattice distortion, and disorder in on-site chemical potential. In the quantum Zeno limit, the effective descriptions for the disordered measurement system with lattice distortions and random on-site potential can be modeled as a classical stochastic model, and the overall effect of increasing these disorders induces a crossover from perfect flow to zero transport. On the other hand if vacancies are present in the lattice the flow of particles per measurement cycle undergoes a percolation phase transition from unity to zero with percolation threshold $p_c\approx 0.26$, with critical exponent $\nu \approx 1.35$. We also present numerical results away from Zeno limit and note that the overall effect of moving away from the Zeno effect is to reduce particle flow per cycle when the measurement frequency in our protocol is reduced.

In magnetic memories, the state of a ferromagnet is encoded in the orientation of its magnetization. The energy of the system is minimized when the magnetization is parallel or antiparallel to a preferred (easy) axis. These two stable directions define the logical bit. Under an external perturbation, the direction of magnetization can be controllably reversed and thus the bit flipped. Here, we theoretically design a topological analog of the magnetic bit in the Su-Schrieffer-Heeger-Holstein model, where we show that a transient external perturbation can lead to a permanent change in the electronic band topology.

The microscopic properties of the superconducting cubic skutterudite-like material ${\mathrm{Y}}_3{\mathrm{Ru}}_4{\mathrm{Ge}}_{13}$ are investigated using muon spin relaxation and rotation ($\mu \mathrm{SR}$) measurements. Zero-field $\mu \mathrm{SR}$ measurements reveal the presence of a spontaneous internal field with a magnitude of $\simeq 0.18$ mT below the superconducting transition temperature, indicating broken time-reversal symmetry in the ground state. In line with previous experiments, transverse-field $\mu \mathrm{SR}$ measurements are consistent with a fully developed superconductivity gap in ${\mathrm{Y}}_3{\mathrm{Ru}}_4{\mathrm{Ge}}_{13}$. Our observations point towards the relevance of electronic correlations beyond electron-phonon coupling as the superconducting origin and indicate that spin-orbit coupling is likely not the key driving force behind the spontaneous breaking of time-reversal symmetry in this system.

Several unconventional superconductors show indications of zero-energy excitations in the superconducting state consistent with the existence of a so-called Bogoliubov-Fermi surface (BFS). In particular, FeSe isovalently substituted with S seems to acquire a nonzero density of states at zero energy at low temperatures as the system goes into the tetragonal phase, consistent with a previously proposed phenomenological theory assuming an anisotropic spin singlet pairing gap coexisting with a nonunitary interband triplet component. Here we search for a microscopic model that can support the coexistence of singlet pairing with other orders, including interband nonunitary triplet pairing with magnetization, and discuss several candidates that indeed stabilize ground states with BFS. We show that with proper choice of the coupling strength of the various orders in our model, spontaneous breaking of $C_4$ rotational symmetry is realized at low temperatures. This feature resembles the findings of recent angle-resolved photoemission experiments in Fe(Se,S) in the tetragonal phase.

An epitaxial semiconductor-superconductor nanowire is a superconducting system with a complex level structure originating from hybridization: in addition to a dense set of higher-energy states derived predominantly from the metallic superconducting shell above the bulk gap $\mathrm{\Delta }$, there is a smaller number of lower-energy proximitized states from the semiconducting core that define the induced gap ${\mathrm{\Delta }}^{*}$. Nanostructures built from such nanowires can furthermore incorporate quantum dots in order to obtain localized spins for storing and manipulating quantum information. We discuss the magnetic field dependence in three devices with different combinations of embedded quantum dots and superconducting islands. For strong fields, they show pinning of excitation energies to a uniform spacing, even if for weak fields they have nonuniversal properties with different behaviors for even and odd numbers of confined electrons. We propose a quantum impurity model for hybrid devices that incorporates all relevant physical effects. We show that the model accounts for the key observations and permits unambiguous interpretation in terms of many-particle states. In particular, we study the replicas of the Yu-Shiba-Rusinov states in the hybrid gap, their collapse and oscillation around zero bias with increasing field, and the strong smoothing effect of the spin-orbit coupling (SOC) on these oscillations. We propose that the SOC-induced mixing of many-body states is a generic mechanism and that magnetic pinning is likely to be a ubiquitous feature in hybrid nanowires.

The discovery of high critical temperature $T_c$ in compressed ${\mathrm{H}}_3\mathrm{S}$ has been followed by the prediction and experimental confirmation of even higher superconducting temperatures with $T_c$ approaching room temperature in ${\mathrm{LaH}}_{10}$ and ${\mathrm{CaH}}_6$. These works established the mechanism of the electron-phonon interaction and the dominant role of hydrogen in these materials. In the present work we focus on ${\mathrm{CaH}}_6$ and we follow the classic McMillan paper, which writes the electron-phonon coupling parameter $\lambda$ as a ratio of an electronic contribution $\eta$ over a force constant $k=M\langle {\omega }^2\rangle$ which contains the phonon contribution. First the numerator of McMillan's expression, the Hopfield-McMillan parameter $\eta$, is computed using the theory of Gaspari and Gyorffy (GG), and the force constants in the denominator are obtained from the paper of Quan et al. The resulting $\lambda$ is used in the Allen-Dynes equation to calculate $T_c$. We present an analysis of the different terms of the GG equation and conclude that the $sp$ channel of hydrogen has the most important contribution to obtain high values of $T_c$, as in the other hydrogenated materials. In addition, we further separate the three terms of the GG expression and assess the role of the phase shifts term versus the partial densities of states and free scatterers. We compare these quantities in ${\mathrm{CaH}}_6$ to those in ${\mathrm{SH}}_3$ and ${\mathrm{LaH}}_{10}$ and, in contrast to what is expected for high values of $\lambda$, we conclude that in ${\mathrm{CaH}}_6$ the coupling parameter $\lambda$ is the better indicator of high $T_c$ than $\eta$. However, we have found that in the strong coupling limit of large $\lambda$ the high values of $T_c$ are strongly dependent on the parameter $\eta$ and make $T_c$ a decreasing function of $\eta$. Unfortunately most of the researchers of these materials have ignored the importance of the parameter $\eta$ partly because the computational packages they are using do not separately compute $\eta$. Our view is that the parameters $\eta , \kappa$, and $\lambda$ must be examined on an equal footing as we study how to achieve high $T_c$ at low pressures. Finally, we present the following findings: (a) using the virtual crystal approximation and an extension of the Allen-Dynes finding that at the strong coupling limit $T_c$ depends on $\eta$, we predict that the alloy ${\mathrm{Ca}}_{1-x}{\mathrm{Sc}}_x{\mathrm{H}}_6$ can reach higher $T_c$ than ${\mathrm{CaH}}_6$. (b) If the higher hydrogen content material ${\mathrm{CaH}}_{10}$ can be made in the $Fm\overline{3}m$ structure it would have significantly higher $T_c$ than ${\mathrm{CaH}}_6$.

The stability of a quantum critical point in the $O\left(N\right)$ universality class with respect to an elastic coupling, which preserves $O\left(N\right)$ symmetry, is investigated for isotropic elasticity in the framework of the renormalization group (RG) close to the upper critical dimension $d=3-\epsilon$. With respect to the Wilson-Fisher fixed point, we find that the elastic coupling is relevant in the RG sense for $1\le N\le 4$, and the crystal becomes microscopically unstable, i.e., a sound velocity vanishes at a finite value of the correlation length $\xi$. For $N>4$, an additional fixed point emerges that is located at a finite value of the dimensionless elastic coupling. This fixed point is repulsive and separates the flow to weak and strong elastic coupling. As the fixed point is approached, the sound velocity is found to vanish only asymptotically as $\xi \to \infty$, such that the crystal remains microscopically stable for any finite value of $\xi$. The fixed point structure we find for the quantum problem is distinct from the classical counterpart in $d=4-\epsilon$, where the crystal always remains microscopically stable for finite $\xi$.