Spontaneous symmetry breaking is ubiquitous in physics. Its spectroscopic signature consists in the softening of a specific mode upon approaching the transition from the high-symmetry side and its subsequent splitting into a zero-frequency Goldstone mode and a nonzero-frequency Higgs mode. Although they determine the whole system dynamics, these features are difficult to address in practice because of their vanishing coupling to most experimental probes and/or their strong interaction with other fluctuations. In this paper, we consider a periodic magnetic modulation occurring in a ferromagnetic film with perpendicular-to-plane magnetic anisotropy and observe its Goldstone and Higgs spin-wave modes at room temperature using microwave and optical techniques. This simple system constitutes a particularly convenient platform for further exploring the dynamics of symmetry breaking.

We study optical responses of Leggett modes in multiband superconductors in the linear response regime. The Leggett mode is a collective mode unique to multiband superconductors that arises from relative phase fluctuations of superconducting orders for different bands. We use the Ginzburg-Landau (GL) description to study the collective modes in multiband systems. We find that multiband superconductors generally allow a linear coupling between the Leggett mode and external electric fields due to the presence of a cross term between different components of superconducting orders in the GL theory. The presence of a linear coupling for the Leggett mode is in sharp contrast with the absence of that for Higgs (amplitude) modes in single-band superconductors that only support nonlinear optical responses such as third harmonic generation (THG). We further confirm such a linear coupling in multiband superconductors by a more microscopic description based on a diagrammatic approach. We study the collective modes within the random phase approximation (RPA) and compute their contribution to the linear optical conductivity. These findings suggest a new route to observe the Leggett mode by optical absorption.

We study the evolution of temperature-dependent resistivity with added pointlike disorder induced by 2.5 MeV electron irradiation in stoichiometric compositions of the “3-4-13” stannides, ${\left(\text{Ca,Sr}\right)}_3{\left(\text{Ir,Rh}\right)}_4{\text{Sn}}_{13}$. Three of these cubic compounds exhibit a proposed microscopic coexistence of charge density wave (CDW) order and superconductivity (SC), while ${\text{Ca}}_3{\text{Rh}}_4{\text{Sn}}_{13}$ does not develop CDW order. As expected, the CDW transition temperature $T_{\text{CDW}}$ is universally suppressed by irradiation in all three compositions. The superconducting transition temperature, $T_c$, behaves in a more complex manner. In ${\text{Sr}}_3{\text{Rh}}_4{\text{Sn}}_{13}$, it increases initially in a way consistent with a direct competition of CDW and SC, but quickly saturates at higher irradiation doses. In the other three compounds, $T_c$ is monotonically suppressed by irradiation. The strongest suppression is found in ${\text{Ca}}_3{\text{Rh}}_4{\text{Sn}}_{13}$, which does not have CDW order. We further examine this composition by measuring the London penetration depth $\lambda \left(T\right)$, from which we derive the superfluid density. The result unambiguously points to a weak-coupling, full single gap, isotropic superconducting state. Therefore we must explain two seemingly incompatible experimental observations: a single isotropic superconducting gap and a significant suppression of $T_c$ by nonmagnetic disorder. We conduct a quantitative theoretical analysis based on a generalized Anderson theorem which points to an unconventional multiband $s^{+-}$-pairing state where the sign of the order parameter is different on one (or a small subset) of the smaller Fermi surface sheets but remains isotropic and overall fully gapped.

We study coherent backscattering (CBS) of light from a magnetoactive medium doped by Mie particles. A novel version of the CBS diffusion theory is developed, which takes into account both the Faraday effect and the effect of circular polarization memory specific to Mie scattering. The theory is based on a system of coupled diffusion equations for two slowly decaying cooperon modes arising from interference of waves with coinciding helicities. The impact of a magnetic field on CBS is shown to be controlled by the ratio of the helicity-flip scattering cross section to the transport scattering one. If this ratio is small, the CBS can exhibit unusual features first found experimentally by R. Lenke, R. Lehner, and G. Maret [Europhys. Lett. 52, 620 (2000)]. In the magnetic field parallel to the sample surface, the peak of coherent backscattering for circularly polarized light is shifted from the exact backward direction, while, for linearly polarized light, it splits in two ones for both co- and cross-polarization channels, and the backscattered waves acquire circular polarization. Saturation of the magnetic field dependence of the CBS cone occurs in the magnetic field normal to the surface. If the above ratio is close to unity (Rayleigh scattering) all these features disappear, and the effect of the magnetic field on the CBS angular profile is reduced to the universal law studied previously. The results obtained are in good quantitative agreement with the available Monte Carlo simulation and experimental data.

Recently emergent current-induced orbital torque is considered more effective for magnetization switching than the well-established spin-orbit torque. However, long-range orbital transport in polycrystalline films and the theory on orbital transport in polycrystalline heterostructures remain elusive. Here we report a large torque effect in $\mathrm{CoFeB}/\mathrm{Ru}/{\mathrm{Al}}_2{\mathrm{O}}_3$ polycrystalline stacks. The unfilled $d$-shell and small spin-orbit coupling in Ru provide an ideal platform for orbital generation and transport. The orbital current from the $\mathrm{Ru}/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interface can go through a thick Ru layer, with a peak value at 7-nm-thick Ru, and then induces a strong torque effect in CoFeB. The torque efficiency increases unprecedentedly with increasing CoFeB layer thickness, leveling off at $\sim 0.3$ for 12-nm-thick CoFeB. Theoretical analysis shows that the orbital transport in polycrystalline materials exhibits a unique random precession behavior, leading to a more efficient orbital transport than that in single crystals. Besides the fundamental significance, our findings advance the development of practical orbital torque devices.

We study the ground-state properties of ferromagnetic quasi-one-dimensional quantum wires using the quantum Monte Carlo (QMC) method for various wire widths $b$ and density parameters $r_{\text{s}}$. The correlation energy, pair-correlation function, static structure factor, and momentum density are calculated at high density. It is observed that the peak in the static structure factor at $k=2k_{\text{F}}$ grows as the wire width decreases. We obtain the Tomonaga-Luttinger liquid parameter $K_{\rho }$ from the momentum density. It is found that $K_{\rho }$ increases by about 10% between wire widths $b=0.01$ and $b=0.5$. We also obtain ground-state properties of finite-thickness wires theoretically using the first-order random phase approximation (RPA) with exchange and self-energy contributions, which is exact in the high-density limit. Analytical expressions for the static structure factor and correlation energy are derived for $b\ll r_{\text{s}}<1$. It is found that the correlation energy varies as $b^2$ for $b\ll r_{\text{s}}$ from its value for an infinitely thin wire. It is observed that the correlation energy depends significantly on the wire model used (harmonic versus cylindrical confinement). The first-order RPA expressions for the structure factor, pair-correlation function, and correlation energy are numerically evaluated for several values of $b$ and $r_{\text{s}}\le 1$. These are compared with the QMC results in the range of applicability of the theory.

A major hurdle in understanding the phase diagram of twisted bilayer graphene is the roles of lattice relaxation and electronic structure on isolated band flattening near magic twist angles. In this work, the authors develop an accurate local environment tight-binding model fit to tight-binding parameters computed from ab initio density-functional theory calculations across many atomic configurations. With the accurate parametrization, it is found that the magic angle shifts to slightly lower angles than often quoted, from around $1.{05}^{\circ }$ to around $0.{99}^{\circ }$, and that isolated flat bands appear for rigidly rotated graphene layers, with enhancement of the flat bands when the layers are allowed to distort. Study of the orbital localization supports the emergence of fragile topology in the isolated flat bands without the need for lattice relaxation.

Authors of recent studies have established the significance of fourth-order anharmonicity in the linewidth of zone-center optical phonons, while it is unknown whether the fifth- and even higher-order phonon scattering is important. In this paper, we estimate the convergence of phonon scattering entropy with respect to perturbation orders. Using density functional perturbation theory, we calculate the three- and four-phonon linewidths for zone-center optical phonons in a series of zinc-blende III–V compounds including InP, c-GaN, BN, AlSb, GaP, InSb, AlAs, InAs, GaSb, and AlP. Our results show that, although the agreement between theory and experiment is greatly improved by incorporating four-phonon scattering, considerable discrepancies still exist, especially at high temperatures. We reveal that, on average, the phonon scattering entropy converges well at the eighth order, and the fifth- and higher-order phonon scattering entropy is about 37% of that of four-phonon scattering at Debye temperature and increases with temperature. With four-, five-, and higher-order phonon scattering included, the linewidth deviates largely from the linear scaling with temperature. In this paper, we provide evidence of the higher-than-fourth-order lattice anharmonicity in zone-center optical phonon linewidths as well as Raman and infrared spectra.

We introduce a method to investigate excitonic spin flips in a neutral quantum dot (QD) that is driven nonresonantly. By inserting the QD in an anisotropic photonic structure, one creates an imbalance between the radiative decay rates of the two bright excitons. Direct spin flips between the bright excitons as well as indirect ones (via a dark exciton) mix the level populations and profoundly affect the degree of linear polarization of the excitonic emission. Measuring this quantity under continuous wave optical excitation yields the spin-flip rate over a broad range of excitation powers. Additional time-resolved experiments allow disentangling the contributions of bright-bright and dark-bright spin flips in the low-excitation regime. After providing theoretical background, we demonstrate the method on a self-assembled InAs QD embedded in a GaAs photonic wire featuring an elliptical cross section. For low-excitation power and at $T=5\mathrm{K}$, bright-bright spin flips are much slower than dark-bright spin flips, which, in turn, remain much slower than the radiative decays. Upon increasing the temperature, we observe a superlinear increase in the bright-bright spin-flip rate which completely reverses the rate hierarchy above $T=50\mathrm{K}$. Moreover, polarization measurements reveal a dramatic increase in the spin-flip rate with the pumping power. Our findings are relevant to spontaneous emission control by anisotropic photonic structures and to the spectral coherence of QD-based quantum light sources.

Motivated by recent experiments that report the almost-generic large-conductance peaks without very extensive fine-tuning, we propose an alternative mechanism through direct theoretical simulations that can explain the large zero-bias conductance peaks being generated on-demand in the nontopological regime in Majorana nanowires by satisfying the following three sufficient conditions: (i) strong potential disorder in the bulk of the nanowire, suppressing the topological regime; (ii) strong suppression of the disorder near the nanowire ends connecting to the tunneling leads, perhaps because of screening by the metallic leads and gates; and (iii) low tunnel barrier strength leading to large tunneling amplitude. The third condition is typically achieved experimentally by fine-tuning the tunnel barrier and the first condition is generic in all existing nanowires by virtue of considerable sample disorder induced by unintentional random quenched charged impurities. The second condition is likely to apply to many samples since the disorder potential would be typically screened more strongly at the wire ends because of the large metallic tunnel pads used experimentally. We show that the resultant tunneling conductance manifests large trivial zero-bias peaks almost on demand, and such peaks could be $\sim 2e^2/h$, when appropriately fine-tuned by the tunnel barrier strength and the temperature, as reported experimentally. Our work not only solves the mystery in recent experiments that the observations of the large zero-bias conductance peaks are generic by proposing a theoretically possible mechanism but also explains why these hypothesized conditions are naturally satisfied in experiments.

Recently, two-dimensional (2D) moiré superlattices have been extensively studied, and many interesting physical phenomena have been observed. However, their one-dimensional (1D) counterpart—1D moiré superlattices—have been rarely explored yet. Here, we performed theoretical calculations of low-energy bands of 1D moiré superlattices of single-walled carbon nanotubes (CNTs) on hexagonal boron nitride (hBN) using a newly developed low-energy effective continuum model for 1D superlattices. We observed moiré-induced opening of small band gaps ranging from a few meV to a few tens of meV, which depends sensitively on the CNT chirality. The observed band-gap opening can be well understood by considering the coupling of electronic states between CNTs and the hBN using the effective continuum model. The results have been confirmed by the density functional theory calculations.

We introduce a generalized Hamiltonian describing not only all topological phases observed experimentally in Kekulé graphene (KekGr) but predicting also new ones. These phases show features like a quadratic band crossing point, valley splitting, or the crossing of conduction bands, typically induced by Rashba spin-orbit interactions or Zeeman fields. The electrons in KekGr behave as Dirac fermions and follow pseudorelativistic dispersion relations with Fermi velocities, rest masses, and valley-dependent self-gating. Transitions between the topological phases can be induced by tuning these parameters. The model is applied to study the current flow in KekGr $pn$ junctions evidencing a cooperative transport phenomenon, where Klein tunneling goes along with a valley flip. These junctions act as perfect filters and polarizers of massive Dirac fermions, which are the essential devices for valleytronics. The plethora of different topological phases in KekGr may also help to establish phenomena from spintronics.

We study the resistive collapse of the Mott insulator state in the dimer Hubbard model. This minimal model has been used to describe the physics of ${\mathrm{VO}}_2$, and should be relevant to other strongly correlated materials. It incorporates the physics of correlated dimers and the explicit competition between on-site Coulomb repulsion and magnetic exchange interactions. Our results unveil that between the Mott insulator at half filling and the Fermi liquid metal at high doping there is an intermediate bad metallic phase with exotic features such as a pseudogap, orbital selectivity, and a first-order metal-metal transition. The model is solved within dynamical mean field theory by means of quantum Monte Carlo, which provides the numerically exact solution of the model in the limit of large lattice dimensionality. This model can be considered as a minimal one that captures exotic phenomena associated to the physics of a doped Mott insulator, shading light on their basic physical mechanism.

Computational crystal-field models have provided consistent models of both electronic and Zeeman-hyperfine structure for several rare-earth ions. However, a computational crystal-field calculation of ${\mathrm{Eu}}^{3+}$incorporating the lattice electric quadrupole and nuclear Zeeman interactions has not been performed. Here, we include these terms in a computational model to fit the crystal-field levels and the Zeeman-hyperfine structure of the ${}^{7}F_0$ and ${}^{5}D_0$ states in three ${\mathrm{Eu}}^{3+}$sites: the ${\mathrm{C}}_{4v}$ and ${\mathrm{C}}_{3v}$ sites in ${\mathrm{CaF}}_2$and the ${\mathrm{C}}_2$ site in ${\mathrm{EuCl}}_3·6{\mathrm{H}}_2\mathrm{O}$. Close fits are obtained for all three sites which are used to resolve ambiguities in previously published parameters, including quantifying the anomalously large crystal-field-induced state mixing in the ${\mathrm{C}}_{3v}$ site and determining the signs of Zeeman-hyperfine parameters in all three sites. We show that this model allows accurate prediction of properties for ${\mathrm{Eu}}^{3+}$important for quantum information applications of these ions, such as relative transition strengths. The model could be used to improve crystal-field calculations for other non-Kramers singlet states. We also present a spin Hamiltonian formalism without the normal assumption of no $J$ mixing, suitable for other rare-earth ion energy levels where this effect is important.

We argue that the bonding or antibonding character of electronic states of a substrate can influence significantly the observed scanning tunneling microscopy (STM) spectra. In particular, the antibonding states formed by nonorthogonal orbitals of the substrate present much stronger extension in the vacuum compared to corresponding bonding combinations and are therefore much more visible in STM experiments. A clear example is provided by black phosphorus (BP), which is one of the most interesting two-dimensional layered materials nowadays. The pronounced asymmetry in its conductance spectra, with significant increase at positive voltage, can be attributed to the antibonding nature of the conduction band compared to the bonding valence band, despite the same $p_z$-orbital character. Furthermore, this asymmetry can be at the origin of different broadenings of frontier molecular orbitals observed experimentally for molecules on BP.

We study nonreciprocal nonlinear Schrödinger systems. As a prototype we analyze the Hatano-Nelson model together with a typical nonlinear term introduced and its generalization to two dimensions. We employ the quench dynamics, where a pulse is given to one site and its time evolution is analyzed. It is found that the skin state is always formed due to the nonreciprocal hopping and hence that the system is topological. However, the structure of the skin state is essentially modified by the nonlinear interaction because it favors a self-trapping. Four typically different states emerge as an interplay between these two interactions, depending how the pulse is trapped to the initial site. They are the skin, trap-skin, shifted-trap-skin, and embedded-trap-skin states, forming four phases in the one-dimensional model. The phase boundary is determined by a gap in terms of certain phase indicators. On the other hand, we find three phases with the shifted-trap-skin phase being absent in the two-dimensional model.

Domain switching takes a substantial role in the macroscopic electrical properties of perovskite-type ferroelectric materials. However, the attainable domain alignment in polycrystalline states is often far away from the saturation due to the constraint of long-range crystallographic symmetry. Herein, we report a field-induced saturated polarization alignment, facilitated by local monoclinic polarization rotation in conventional rhombohedral $\mathrm{Pb}\left({\mathrm{Zr}}_{0.54}{\mathrm{Ti}}_{0.46}\right){\mathrm{O}}_3$ via advanced electrical basing in situ synchrotron x-ray diffraction and pair distribution function measurements. It is found that the polarization alignment can reach the upper limit of theoretically calculated value in rhombohedral ceramic during high electric-field poling, and such a state can be maintained upon removal of electric field. In situ pair distribution function analysis demonstrates the occurrence of monoclinic distortion at local length $(<10\AA )$, where the electric-field driven polarization rotation emerges, and promoting the alignment of polarization variations. Concurrently, large local piezoelectric strain $\left(1000\mathrm{pm}/\mathrm{V}\right)$ and intrinsic lattice piezoelectric response $\left(1400\mathrm{pm}/\mathrm{V}\right)$ during poling are found to be generated, while large remanent strains are also observed. These results offer fresh insight into the role of local structure in domain switching behavior and have potential for the theoretical investigation of domain alignment in ferroelectrics.

High precision skyrmion pinning by an intentionally created magnetic structure is important in skyrmion manipulation. Here, we consider skyrmion pinning by various types of disks. Among other findings, we clarify that, in terms of pinning position and pinning potential landscape, one needs to distinguish thin-wall skyrmions from thick-wall skyrmions because of fundamental differences. A skyrmion wall prefers areas with a weaker exchange stiffness, a larger DMI constant, or a smaller magnetic anisotropy while a skyrmion core experiences only the magnetic anisotropy, not exchange stiffness and DMI. Depending on disk type, skyrmion type, and the relative size of the disk (to skyrmion), a skyrmion can be pinned at the symmetric (center) or asymmetric (off-center) point of a disk. In case a skyrmion is pinned by a local energy minimum, thermal agitation can depin the skyrmion, and the pinning lifetime follows an Arrhenius law. Interestingly, when the disk size is comparable to the skyrmion size, the skyrmion deforms greatly such that the skyrmion will shrink or expand to fill the whole disk. These findings should be important for skyrmion manipulations.

We investigate entanglement dynamics in the antiferromagnetic Heisenberg model in two dimensions following a spatially anisotropic quench of the exchange interactions. Opposed to established results in one dimension, the magnon quasiparticles show an initial growth of entanglement dynamics that does not depend on the system size and is governed by the oscillation period of the exchange interaction. We ascribe this to the dominance of the intrinsic entanglement of short wavelength nonpropagating magnon pairs, which also leads to a competition between area-law and volume-law contribution in the entanglement dynamics. Furthermore, by adopting the neural-network quantum states, we provide numerical evidence that this behavior survives even in the presence of strong magnon-magnon interactions, suggesting intriguing avenues for manipulating entanglement dynamics in quantum materials.